Rock Magnetism and Palaeomagnetism of Meteorite Impact Craters in India

ROCK MAGNETISM AND PALAEOMAGNETISM OF METEORITE IMPACT CRATERS IN INDIA

A THESIS SUBMITTED TO THE UNIVERSITY OF MUMBAI FOR THE PH. D. (SCIENCE) DEGREE IN PHYSICS

Submitted By M. D. ARIF

UNDER THE GUIDANCE OF PROF. NATHANI BASAVAIAH

INDIAN INSTITUTE OF GEOMAGNETISM PLOT NO. 5, SECTOR 18, NEW PANVEL (W), NAVI MUMBAI-410 218 MAHARASHTRA, INDIA APRIL 2013

STATEMENT BY THE CANDIDATE

As required by the University Ordinances 770, I wish to state that the work embodied in this thesis titled “Rock Magnetism and Palaeomagnetism of Meteorite Impact Craters in India" forms my own contribution to the research work carried out under the guidance of Prof. Nathani Basavaiah at the Indian Institute of Geomagnetism, New Panvel, Navi Mumbai. This work has not been submitted for any other degree of this or any other University. Wherever references have been made to previous works of others, it has been clearly indicated as such and included in the Bibliography.

Signature of Candidate Full Name: Md. Arif

Certified by

Signature of Guide Name: Prof. Nathani Basavaiah

Statement required under 0.770 Statement No. 1

I hereby declare that the work described in the thesis has not been submitted previously to this or any other University for Ph. D. or any other degree.

Statement required under 0.771 Statement No.2

“Whether the work is based on the discovery of new facts by the candidate or of new relations of facts observed by others, and how the work tends to the general advancement of knowledge.” This thesis introduces a new method for the observation, evaluation and interpretation of the obliquity impact at Lonar based on structural and anisotropy of magnetic susceptibility (AMS) evidence. Impact stress changes are determined using AMS and rock magnetic characteristics. Palaeomagnetic properties are used to determine whether the target basalt was experienced by the meteoritic impact, by which stress-dependent magnetic field directions and rock magnetic parameters are evaluated. For the first time, the thesis introduces shock-induced magnetic effects, which can be evaluated as a natural geomagnetic stress sensor to Indian asteroid impact craters. The rock magnetic and palaeomagnetic techniques have been applied to obtain data from the investigating areas of Indian asteroid impact craters of Lonar and Ramgarh. The important findings are: (1) obliquity impact with impactor hitting the pre-impact target basalts from the east direction at Lonar as evidenced from structural and AMS studies, (2) variations in rock and mineral magnetic properties of Lonar basalts with the E-W plane of impact and direction of impact, (3) determination of SRM, impact-generated plasma fields and mean palaeomagnetic pole positions, (4) geochemical variations in Lonar impact rocks, and (5) rock magnetic properties of Ramgarh target rocks to confirm the impact evidence. This thesis has contributed to the knowledge of the magnetic, mineralogical and structural inventory of impact craters in the Indian shield. It is expected that the results will find use and application in monitoring stress dislocation around impact crater sites, in iii

remote sensing studies of impact craters and in Global Positioning System to monitor stress accumulation at active seismic zones. The use of rock magnetic, mineral magnetic and palaeomagnetic methods as indicators for impact craters is suggested, giving insights into the cratering process and the target subsurface, as well as aiding the structural study of known terrestrial impact crater analogues and the identification of unknown ones like Luna in Gujarat.

Statement required under 0.771 Statement No. 3

“The source from which this information has been derived and to the extent to which he has based his work on the work of others, and shall indicate which portion or portions of his thesis he claims as original.” The information mentioned is derived by the candidate during the course of research reported in the thesis. The results of the publications are genuine, original and have been published as the following peer reviewed research papers. Papers published in peer reviewed Journals (1) Md. Arif, N. Basavaiah, S. Misra, and K. Deenadayalan (2012), Variations in magnetic properties of target basalts with the direction of asteroid impact: Example from Lonar crater, India: Meteoritics & Planetary Science 47, 1305-1323. (2) S. Misra, Md. Arif, N. Basavaiah, P. K. Srivastava, and A. Dube (2010), Structural and anisotropy of magnetic susceptibility (AMS) evidence for oblique impact on terrestrial basalt flows: Lonar crater, India: Geological Society of America Bulletin 122, 563-574. Papers presented at National and International conferences (1) Md. Arif, S. Misra, N. Basavaiah, and H. Newsom (2009), Distribution of impact- induced stress around Lonar crater, India: 72nd Annual Meteoritical Society Meetings, held 13-18 July 2009 in Nancy, France (abstract no.5397). (2) Md. Arif, S. Misra, and N. Basavaiah (2010), Rock magnetic characterization of target basalts at Lonar crater, India: 41st Lunar and Planetary Science Conference, held March 1-5, 2010 in The Woodlands, Texas. LPI Contribution No. 1533, p. 1571. (3) Md. Arif, K. Deenadayalan, N. Basaviah, and S. Misra (2011), Variation of primary magnetization of basaltic target rocks due do asteroid impact: example from Lonar iv

crater, India: 42nd Lunar and Planetary Science Conference, held March 7-11, 2011 in The Woodlands, Texas. LPI Contribution No. 1383. (4) Md. Arif, N. Basavaiah, S. Misra, and K. Deenadayalan (2011), Asteroid impact variations of NRM and REM of target basalts of Lonar crater, India: 74th Annual Meteoritical Society Meeting, held 08-12 August 2011 in London, UK (abstract no.5248). (5) Md. Arif, N. Basavaiah, and S. Misra (2012), Rock- and palaeomagnetic properties of randomly oriented basaltic blocks from Lonar crater ejecta, India: European Planetary Science Congress (EPSC), held 23-28 September 2012 in Madrid, Spain (abstract: EPSC2012-163).

Statement required under 0.771 Statement No. 4

“Where a candidate presents joint work, he shall clearly state the portion which is his own contribution as distinguished from the portion contributed by his collaborators.” All field work in the collection of rock samples in Lonar and Ramgarh impact craters, laboratory measurements, data analysis and interpretation were performed by the candidate under the supervision and assistance of the supervisor, Prof. Nathani Basavaiah. Saumitra Misra (Durban, South Africa) assisted in the interpretation of geological inputs and structural results of Lonar and Ramgarh craters. K. Deenadayalan (IIG) assisted in field work in Lonar and Ramgarh craters. P.K. Srivastava (ISRO, Hyderabad) provided the ASTER image of Lonar crater. A. Dube (Kolkata) contributed in structural data analysis of Lonar crater. H. Newsom (New Mexico, USA) helped in conference presentations. This thesis was funded by the in-house IIG project, Environmental Magnetism.

(Prof. Nathani Basavaiah) (Md. Arif) Guiding Teacher Candidate

ACKNOWLEDGEMENTS

I wish to express my deepest sense of gratitude and profound thanks to my supervisor Prof. Nathani Basavaiah, Head, Environmental Magnetism Laboratory, IIG, for his inspiring guidance, constant encouragement, and patiently supporting me with his knowledge and experience during the entire span of my doctoral research. He cultivated my curiosity, sharpened my thinking and inspired me to become a successful future researcher. I am fortunate to having been associated with him here in the institute. Besides introducing me to the field of Impact Cratering on Indian Shield and formulating the research problem, the laboratory facilities and collaborations he established over the years have helped me enormously. I owe my sincere appreciation to Saumitra Misra for his participation in field trips to Lonar and Ramgarh impact craters, when he was a PDF under my supervisor and for his dedicated involvement in the published research papers with our group members. I wish to thank all my group members K. Deenadayalan, K.V.V. Satyanarayana, B.V. Lakshmi, P.B. Gawali, J.L.V. Mahesh Babu and Dupinder Singh for their kind help rendered in field trips to crater sites, for maintaining a cheerful atmosphere in the laboratory for generating good data, swapping data sets among us and for patiently going through the draft versions of my thesis chapters; often exchanging ideas to fix problems that used to come across in laboratory equipments. Thanks are due to Prof. S.K. Arora of BARC, Mumbai for providing many comments and suggestions. Also unsaid blessings and homely support of Madamji Mrs. Nathani Koteswari to whom I sought suggestions and advices before submitting research related tasks. It’s now my pleasure to thank my dear friends Mahesh Narayan Srivastava, Ajeet Kumar Maurya, Devanandan S. and R. Selvakumaran who made my stay in IIG hostel pleasant and provided friendly atmosphere. I also wish to thank all other research scholars of IIG for helping me directly or indirectly during the tenure of my Ph.D. I would like to thank my parents and family members without whom I would not have achieved this goal. It is the constant sacrifice and ceaseless encouragements of my parents that made it possible for me to realize my goals and dreams. This work would not have been feasible but for the keen interest, love, affection, and unflinching support from them.

Finally thanks to the Almighty Creator ‘ALLAH’ without HIS blessings this would not be happened. It is HIS will that I would be able to successfully complete it. This thesis is my humble offering to HIM because anything that I am, I believe it is all part of a plan which has already been made for me. Last but not least, I cannot express by words to thank my dear friend and my contemporary colleague Prasanta Kumar Das for sharing everything and he is invaluable source of encouragement at different stages of my career at IIG beginning from joining in the same group and selecting laboratory experimental research.

Md. Arif Navi Mumbai, April 2013.

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Abbreviations

AF Alternating Field AMS Anisotropy of Magnetic Susceptibility APWP Apparent Polar Wander Path ARM Anhysteretic Remanent Magnetization ASTER Advanced Spaceborne Thermal Emission and Reflection Radiometer ChRM Characteristic Remanent Magnetization CRM Chemical Remanent Magnetization FWHM Full Width Half Maxima GAD Geocentric Axial Dipole HC_HT High Coercivity and High Temperature IRM Isothermal Remanent Magnetization LC_LT Low Coercivity and Low Temperature LF Lowrie-Fuller MAD Maximum Angular Deviation MD Multi-Domain MDF Median Destructive Field NRM Natural Remanent Magnetization PCA Principal Component Analysis PDF Present-Day Earth’s Magnetic Field PDFs Planar Deformation Features PFs Planar Fractures PGEs Platinum Group Elements PSD Pseudosingle-Domain REM NRM/SIRM SD Single-Domain SEM Scanning Electron Microscope SIRM Saturation Isothermal Remanent Magnetization SP Superparamagnetic SRM Shock Remanent Magnetization TH Thermal viii

TRM Thermo-Remanent Magnetization VGP Virtual Geomagnetic Pole VRM Viscous Remanent Magnetization VSM Vibrating Sample Magnetometer XRD X-ray Diffraction XRF X-ray Fluorescence χ Bulk Suseptibility

Ms Saturation Magnetization

Mrs Saturation Remanent Magnetization

Hc Coercivity

Hcr Coercivity of remanence

α95 95% confidence limit D Declination I Inclination

Tc Curie point GPa gigapascals k Fisher’s precision parameter ka kiloannum Ma Million years

K1 Maximum susceptibility axes

K2 Intermediate susceptibility axes

K3 Minimum susceptibility axes P/ Corrected degree of anisotropy T Shape parameter dc Direct current

Contents

Statement ii

Acknowledgements iii

Abbreviations vi

Chapter 1: Introduction 1

1.1 What is an impact crater? 1 1.2 Classification of impact craters 2 1.3 Criterion for identification of impact structures 3 1.4 The importance of impact craters 6 1.5 The impact cratering process 8 1.6 Impact cratering on Indian Shield 10 1.7 Geophysics of impact structures 19 1.8 Aims of the thesis 22

Chapter 2: Sampling and laboratory experimental methods 25

2.1 Field survey of meteorite impact craters on Indian Shield 25 Sample collection and field evidences at Lonar crater 25 Sample collection and field evidences at Ramgarh crater 28 2.2 Laboratory experimental methods 34 Anisotropy of Magnetic Susceptibility (AMS) 36 Rock and mineral magnetic measurements 40 Palaeomagnetic measurements 41 Demagnetization techniques 43 Analysis of remanent magnetization components 44 Determination of Palaeomagnetic direction and pole 47 2.3 Instrumentation details 48 2.4 Laboratory shock experimental studies 48

Chapter 3: Structural and Anisotropy of Magnetic Susceptibility (AMS) 50

3.1 Introduction 50 3.2 Geological setting of Lonar crater 50 3.3 How special is the Lonar crater? 54 Deccan Traps 55 3.4 Nature of ejecta distribution 56 Ejecta distribution at Lonar 57 ASTER image of Lonar region 57 3.5 Regional structural analysis of Lonar crater 62 3.6 Low-field AMS analysis of Lonar crater 65 3.7 Direction of Lonar asteroid impact 73 3.8 Distribution of impact-induced shock pressure 75 3.9 Conclusion 86

Chapter 4: Rock and mineral magnetic study of Lonar and Ramgarh craters 88

4.1 Introduction 88 4.2 Sample details 89 4.3 Previous studies on rock and mineral magnetism of Lonar basalts 89 4.4 Rock and mineral magnetic characterization of Lonar basalts 90 NRM/χ and REM 91 Low and high temperature dependence of magnetic susceptibility 102 Hysteresis loop 105 IRM acquisition and backfield SIRM dc demagnetization 108 AF demagnetization spectra of NRM 109 Lowrie-Fuller (LF) test 111 Thermal demagnetization of SIRM 115 4.5 Rock magnetism of magnetic particles–Ramgarh structure 115 4.6 NRM of Ramgarh target rocks 123 4.7 Conclusion 127

Chapter 5: Palaeomagnetic study of Lonar basalts 131

5.1 Introduction 131 5.2 Samples and laboratory treatment 132 5.3 Previous studies on palaeomagnetism of Lonar basalts 132 5.4 Palaeomagnetic measurements 133 5.5 Palaeomagnetic data of Lonar basalts 133 Nature of LC_LT component 134 Nature of HC_HT component 138 5.6 Summary on Lonar palaeomagnetic directions 139 5.7 Radiometric dating of Lonar impact melts 142 5.8 Palaeomagnetism of Lonar ejecta basalts 143

Chapter 6: Geochemistry and XRD of Lonar and Ramgarh impact craters 145

6.1 Introduction 145 6.2 Previous studies on geochemistry of Lonar crater 146 6.3 Sample details 149 6.4 XRF analysis of Lonar samples 150 6.5 XRD of Lonar samples 150 FWHM measurement 153 6.6 XRD of magnetic particles from Ramgarh structure 155 6.7 Conclusion 156

Chapter 7: Conclusions and future directions 158

Bibliography 169

Synopsis

Published articles

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CHAPTER 1

INTRODUCTION

1.1 WHAT IS AN IMPACT CRATER? Impact craters are geological structures formed when a large meteoroid, asteroid or comet smashes into a planet or a satellite with high velocity. Impact cratering is a simple process in which a large object (impactor) strikes an even larger one (target) at very high velocity (tens of km/s), locally releasing a huge amount of energy. These are surficial depressions with elliptical shapes of varying eccentricity, having bowl-shaped cross section with or without a central peak, and ejecta blanket of various shapes around the raised rim. The size and shape of the crater depends on: (1) impactor’s size, composition, velocity, and angle of approach (2) nature of the target rock (crystalline, sediments, water or ice), and (3) gravitational pull of the target planetary bodies. The recognition and study of different aspects of impact craters give birth to the field of ‘Impact Cratering’. Impact cratering is an important geologic process that has modified/shaped the surfaces of the planet ‘Earth’ and all planetary bodies of our Solar System with or without atmospheric covers. The Moon and all the terrestrial planets were resurfaced during a period of intense impact cratering that occurred between the time of their accretion, ~4.5 billion years ago (Ga) and ~3.85 Ga. The average impact velocity for the earth is suggested to be at ~17 km/s, based on the escape velocity from the Earth’s surface (11.2 km/s) plus the average differential component of cosmic velocity of the projectile relative to Earth (Melosh, 1989). In comparison with other endogenous geological processes, the impact cratering process involves (i) extreme temperature and pressure conditions [several thousands of °C and several hundreds of gigapascals (GPa)], (ii) concentrated nature of energy release at a single point on the Earth’s surface, (iii) virtually instantaneous nature of the impact process (e.g. seconds to minutes), and (iv) high strain rates (~104–106 s-1 for impacts versus 10-3–10-6 s-1 for endogenous tectonic and metamorphic processes). A typical large stony projectile has a diameter between 0.5–10 km, mass of 109– 1016 kg, velocity 20–40 km/s and kinetic energy of 1015–1020 J.

An understanding of terrestrial impact cratering is important as it addresses how the outer layer of the Earth has been modified due to impacts, and also its effect on the physical, chemical, and biological systems. Impact cratering processes can well be understood by integrating a host of multidisciplinary disciplines such as remote sensing, geological, geophysical (gravity, magnetic, seismic, and electric methods), petrographical, mineralogical, geochemical, geochronological, numerical modelling, and laboratory experimental studies. These studies are aimed at identification of possible new impact structures, verification of their origin, and detailed analysis of the geologic structure and rock deformation in such crater structures. They also help in understanding the surface, target material topography, ejecta emplacement and all other cratering mechanisms.

1.2 CLASSIFICATION OF IMPACT CRATERS Impact craters are found on nearly all solid surface planets and satellites. The size of the crater depends on the kinetic energy of the impactor and the impact angle; the kinetic energy is defined as: KE = 1/2 (mv2), where m and v are respectively the mass and velocity of the impactor. During impact, the impactor’s KE is transformed into the kinetic and internal energy of the target and impactor. The internal energy heats up both the impactor and the target; the residual KE is spent in displacing and ejecting the target and impactor material thereby producing a crater at the target surface. Morphologically impact craters are classified into simple and complex types. Simple bowl-shaped craters (<2–4 km on Earth) show a circular outline where the depression has depth-diameter ratios of about 1:5 to 1:7 with raised rims by an exterior ejecta blanket, whereas larger complex craters have a shallower depth-diameter ratio (1:10 to 1:20). They exhibit central structural uplifts, rim synclines, and outer concentric zones of normal faulting. The central uplift consists of strata, which have been uplifted above the preimpact level, and is surrounded by a ring depression (or rim syncline) filled with fragmented material and impact melt. The transition between simple and complex craters depends on planet’s surface gravity and target rock strength. As the crater size increases further, the central peak in a complex crater begins to break up and form an inner ring of mountains. In sufficiently large craters, the ring appears at about one-half the rim diameter, and these craters are called peak-ring craters. Some larger craters are called multi-ring basins which present several rings assumed to be formed just as surface waves in a fluid or by specific

faulting. Simple, complex and peak-ring type structures are generally found on Earth while large multi-ring basins structures are found on the Moon surface (Fig. 1.1).

Figure 1.1: Morphology of different crater types and sizes (Spray, 2002).

1.3 CRITERION FOR IDENTIFICATION OF IMPACT STRUCTURES Impact cratering process involves extreme pressures and temperatures that can vaporize, melt, shock metamorphose (metamorphism of rocks and minerals caused by shock wave compression and decompression due to impact of a solid body) and/or deform a substantial volume of the target surface. The transportation and mixing of impact-metamorphosed rocks and minerals during the excavation and formation of impact craters produce a wide variety of distinctive impactites that can be found within and around impact craters. The identification of an impact structure is done from petrographic or geochemical analysis of shock-metamorphic effects in minerals, which can preserve in target material for 106–109 years. The confirmation of an impact origin requires initial observation of geophysical or remote sensing observations of a circular feature. The only impact-diagnostic recognition criteria in rocks and minerals that are generally accepted for their confirmation are: (1) the presence of projectile remnants, (2) shatter cones - megascopic shock features which are 3

distinctive, striated and horse-tailed conical fractures ranging in size varying from few millimeters to tens of meters (Fig. 1.2a), (3) the detection of shock metamorphic effects e.g. micro-deformation effects such as planar deformation features [PDFs] and planar fractures [PFs] in quartz (Fig. 1.2b, e). When fresh, the PDFs are parallel planes of glass, with specific crystallographic orientations as a function of shock pressures of ~10–35 GPa.

(e)

Figure 1.2: Shock metamorphic effects in rocks and minerals. (a) Shatter cones in limestone form Haughton impact structure (penknife for scale) (b) PDFs in quartz (c) and (d) are Plane- and cross-polarized light photomicrographs of diaplectic quartz glass from Haughton impact structure (e) Planar Fractures (PFs) developed in a brecciated quartzite from the central uplift of the Aorunga (Chad) impact structure.

At relatively higher pressures, the shock wave can destroy the internal crystallographic order of feldspars and quartz and convert them to solid-state glasses, which still have the original crystal shapes. These are ‘diaplectic’ glasses (Fig. 1.2c, d), the required pressures being 30–45 GPa for plagioclase feldspar (also known as maskelynite) and 35–50 GPa for quartz, (4) the extremely rapid cycles of compression and then decompression results in transformation of minerals into high pressure polymorphs such as quartz to coesite and/or

stishovite, graphite to diamond, zircon to reidite [ZrSiO4], or mineral dissociation due to

high shock-induced temperatures (e.g., zircon to baddeleyite [ZrO2] and silica [SiO2]), and (5) the chemical and isotopic traces of the extraterrestrial component in impact breccias and sub-mm–sized spherules (i.e. enrichment of siderophile elements, mainly of PGEs, and more recently, chromium and osmium isotopic studies). Any meteorite fragments that survive the impact process and are deposited in and around the newly-formed impact structure will be unlikely to survive for geologically long periods of time before being destroyed by post-impact weathering. Only in some young impact structures that formed by an iron meteorite projectile (e.g. Meteor crater [Arizona], Henbury [Australia], Wolfe Creek [Australia]) can iron meteorite fragments be recovered for a few tens to hundreds of thousands of years after the impact–geologically short timescales. The formation of large and small bodies of melted target rocks (from mm-size to thousands of km3 in volume) is a common characteristic of meteorite impact events. During the initial stages of impact, the more intense shock waves (≥40 GPa) generated near the impact point produce near-instantaneous heating to extreme temperatures (≥2000°C) throughout a large volume of the surrounding target rock. The resulting impact melts may form small glassy bodies that are ejected from the developing crater that remain within the resulting structure. Smaller impact melt bodies, mm to cm in size and generally glassy, may be ejected from the impact crater, often to regional or global distances, as individual “splash-form” objects, e.g. spherules and microspherules, dumbbells, droplets, and other aerodynamically shaped forms. On deposition, these objects often accumulate in distinct layers or as widely-distributed strewn fields. Tektites and microtektites are the best known and most studied of these ejecta deposits, although a variety of other glass-rich ejecta deposits, of both Precambrian and Phanerozoic ages have also been identified. Because their existence as melt particles is not diagnostic for impact, their identification as impact

5 products depends chiefly on association with other definitely impact-produced features, e.g. coesite, quartz with PDFs. Intermediate-sized (cm to dm) bodies of impact-produced glass in some cases occur as individual objects composed either of dense glass or of scoriaceous and vesicular melt. Many of these glassy bodies are clearly associated with a definite or possible impact structure, e.g., Meteor Crater (Arizona) (Mittlefehldt et al., 2005), Lonar Crater (India) (Osae et al., 2005). The identification of shock effects can best be carried out in a series of steps involving increasing complexity, sophistication, and cost: (1) field studies and sample collection, including the examination of outcrops or core samples for shatter cones (2) petrographic and petrofabric studies to identify and verify such key features as diaplectic glasses or PDFs in quartz (3) mineralogical searches for high-pressure phases, using XRD, Raman, and related methods (4) chemical and isotopic analyses to identify the basic characteristics of the rocks and to search for signatures of extraterrestrial projectiles. Some of the latter methods are intricate and expensive, but the required equipment is widely available; no major facilities or specific instrumentation are required.

1.4 THE IMPORTANCE OF IMPACT CRATERS Impact cratering plays an important role throughout the Earth's history, shaping the geological landscape, affecting the evolution of life and producing economic benefits. The Earth’s impact record is the only source of three-dimensional lithological and structural ground-truth data on natural impacts and their consequences. Unlike the Moon, Mars, and Mercury, the active geological processes of erosion, volcanism and tectonics rapidly obliterate the cratering record on Earth. Despite this, the Earth’s surface is home to about 184 impact structures (Earth Impact Database, 2013) preserved to varying extents from Archaean to Quaternary (Fig. 1.3). The benefits of studying impact craters include: understanding evolution of the Earth’s surface, making analogues to other planets, linking them to extinctions, and understanding evolution of meteorites from asteroid parent bodies, planets, and moons from the solar system to give us more information about the origin of the solar system.

Figure 1.3: Distribution Map of confirmed meteorite impact structures on ‘Earth’ (Ferrière et al., 2011).

The ability to identify meteorite impact structures from endogenic geological processes tend to expand our knowledge to understand the complete geological evolution of Earth and the way impacts have affected and reshaped our planet through the times. Identification of terrestrial impact structures also provide better understanding of environmental effects of impact events and their role in the formation of life on the Earth. The link between 65 Ma old Chicxulub impact structure in Mexico and the K/T boundary (Cretaceous-Paleogene extinction event) shows the significance and consequences of an impact event. Statistical database of impact craters provide an estimate for impact rate on Earth and possibly offer explanation to e.g. large volumes of igneous rocks, important mineral and hydrocarbon deposits, and biological extinctions in geologic record as well as wide spread perturbations in the Earth’s crust. The oldest and largest impact structure on Earth is the Vredefort structure in South Africa which took place 2.02 Ga and the crater has a diameter of ~160 km. The latest impact event on earth happened in September 2007 when a Chondrite (H4-5) meteorite hit the ground in Peru creating a 14 m wide and 4.5 m deep crater called Carangas. Impact cratering is, thus, a current and on-going phenomenon while large and catastrophic events, e.g. comet Shoemaker-Levy 9 impacting on Jupiter in 1994 have become rare.

1.5 THE IMPACT CRATERING PROCESS: The impact of an object moving at very large speed (many km/s) with the surface of a planet initiates an orderly sequence of events that eventually produces an impact crater. The actual impact process is divided into three stages, based on the dominance of different physical phenomena at different times of the process (Melosh, 1989). These stages are contact and compression, excavation, and modification (Fig. 1.4; Osinski, 2008). Contact and compression stage: The contact and compression stage is the briefest of the three stages, lasting only a few times longer than the time required for the impacting object (referred to hereafter as the “projectile”) to traverse its own diameter, tcc ≈ L/vi, where tcc is the duration of contact and compression, L is the projectile diameter, and vi is the impact velocity. For instance, a 1 km diameter projectile travelling at 10 km/s would undergo a compression phase lasting only a tenth of a second. The contact and compression stage begins at the instant when the projectile, which could be an asteroid or comet, contacts the surface of the target. The projectile, travelling at velocities of ranging from ~10 to 75 km/s, penetrates no more than 1–2 times its diameter before transferring its kinetic energy into the target in the form of shock waves. These shock waves expand from the point of impact through both the target and the projectile, and can reach peak pressures of several hundred GPa. Numerical models have shown that when this reflected shock wave reaches the upper surface of the projectile, it is reflected back into the projectile as a rarefaction or tensional wave. This causes the projectile to rapidly decompress from high shock pressures resulting in the virtually complete melting and/or vaporization of the projectile itself. The passage of the initial shock wave and subsequent rarefaction wave through the target rocks also results in the melting and vaporization of a large volume of target material close to the point of impact. The duration of the pressure pulse is governed by the projectile diameter. Pressure release occurs as the shock wave is reflected from the back of the projectile in the form of a rarefaction wave. The unloading of the projectile from high pressure ends the contact and compression stage. Excavation stage: The transition from the initial contact and compression stage into the excavation stage is a continuum. It is during this stage that the actual impact crater is opened up by complex interactions between the expanding shock wave and the original ground surface (Melosh, 1989). The projectile itself plays no significant role in the excavation of the crater, having been unloaded, melted and/or vaporized during the initial contact and

compression stage. During the excavation stage, the shock wave expands hemispherically through the target rock and generally weakens as it progresses further outward. Complex interactions between the outward-directed shock waves and the downward-directed rarefaction waves generate a so called ‘transient crater’. The transient crater comprises an upper ‘excavated zone’ and a lower ‘displaced zone’: material in the excavated zone is ejected beyond the transient cavity rim (forming impact ejecta), while the material in the displaced zone remains within the transient cavity (forming crater-fill impactites). Excavation ceases when the shock and rarefaction waves can no longer excavate or displace the target rocks. This stage is longer than initial contact and compression stage, but for a 200 km wide crater, calculations suggest that the excavation stage requires ~90 s to complete.

Figure 1.4: Series of schematic cross-sections depicting the formation of impact craters. At small diameters (i.e. diameter <2–4 km), a simple crater forms. For diameters >2–4 km, the initial transient crater is unstable and a complex crater forms, which following erosion, is termed an impact structure (Osinski, 2008).

Modification stage: The modification stage is the third stage of crater formation. The effects of this stage are controlled by the size of transient cavity and the target rock lithologies. For relatively small impact events (<2–4 km on Earth), the transient cavity undergoes only minor modification, resulting in the formation of a simple bowl-shaped crater. For larger impact events, however, the transient cavity is gravitationally unstable and a ‘complex crater’ is formed. Two competing processes are at work during the modification stage: uplift of the transient crater floor, resulting in a central uplift, and collapse of the initially steep walls of the transient crater. The modification stage typically takes only a few minutes to complete, although readjustment of the crater associated with minor faulting and mass movement continues indefinitely. The modification of an impact crater also occurs through impact-associated hydrothermal activity, which can lead to substantial alteration and mineralization of impact-produced and altered target rocks.

1.6 IMPACT CRATERING ON INDIAN SHIELD Impacts are ubiquitous in our Solar System. The Earth is the most geologically active of the terrestrial planets and, therefore, most of its impact structures have been destroyed over geologic times. Nevertheless, the Earth’s impact record is the only source of three- dimensional lithological and structural ground-truth data on natural impacts and their consequences. For obvious reasons, natural impact phenomena are not fully amenable to experimental duplication. In the Indian subcontinent, four impact craters have occurred in different target rocks at different time intervals (Fig. 1.5). These are characterized as follows: 1. Lonar crater (~1.9 km), eastern Maharashtra (19°58'N, 76°31'E) 2. Ramgarh structure (~5.5 km), southeast Rajasthan (25°20'N, 76°37'E) 3. Dhala structure (~11 km), Madhya Pradesh (25°18'N, 78°09'E) 4. Luna impact crater (~1.2 km), western Gujarat (23°42'N, 69°15'E) The well-recognized and well-studied impact crater in India is ‘Lonar’, where glass fragments resulting from shock melting, shocked plagioclase (maskelyinite), etc. have been identified (Fredriksson et al., 1973). The other impact structures identified in India include the Ramgarh in Bhander group of Vindhyan Supergroup, Dhala in Bundelkhand craton, Luna in Banni plains of Kachchh region, and Shiva in off shore region of Mumbai (Sisodia et al., 2006a, Pati et al., 2008, Karanth, 2006, Chatterjee et al., 2006).

Ramgarh Dhala

Lonar

Figure 1.5: Meteorite impact structures in India – Lonar, Ramgarh, and Dhala.

Lonar impact crater: The Lonar crater (Fig. 1.6a, b) is a simple, bowl-shaped, near-circular impact crater (Kumar, 2005) with N-S and E-W diameters of ~1832 and 1790 m respectively with a circularity of ~0.95 and a depth of ~150 m (Fredriksson et al., 1973; Fudali et al., 1980; Misra et al., 2010). All around its circumference, except for a small sector in the NE, there is a continuous rim raised ~30 m above the adjacent plains, whereas the crater floor lies ~90 m below the pre-impact surface. The rim is surrounded in all the directions by a continuous ejecta blanket that extends outward with a gentle slope of 2°-6° to an average distance of ~700 m from the crater rim, except to the west where it extends for little more than a kilometer (Misra et al., 2010). The interior of the crater is occupied by a shallow saline lake; below the lake water a sequence of ~100 m thick unconsolidated sediment is reported that overlies the base of the crater made up of highly weathered Deccan Trap basalt (Nandy and Deo, 1961; Fudali et al., 1980). About 700 m north of the rim, there is another relatively shallow depression known as the Little Lonar (Fig. 1.6b), which has a diameter of ~300 m. However, drilling into this structure revealed no evidence of impact (Fredriksson, personal communication, 1999; Maloof et al., 2010). Recently, Jourdan et al. (2011) obtain a precise

and accurate radiometric (40Ar/39Ar) isotopic age of 570 ± 47 ka for the Lonar impact event from four basaltic impact melt rocks.

Figure 1.6: (a) Photograph of Lonar crater (b) Portion of a four-band (RGB + NIR) pan- sharpened (0.6 m resolution) Quickbird image showing Lonar crater (Maloof et al., 2010). The Ambar Lake (‘Little Lonar’) is a blue patch surrounded by a circular feature consisting of distal ejecta beneath the label. Distal ejecta are also well exposed in outcrops near the Kalapani Dam and in hand dug water wells SW of the crater.

The target rocks at Lonar crater are subhorizontal Deccan Trap basalt flows that overlie the Precambrian basement with a thickness of >350 m (Kumar, 2005). These basalts contain intertrappean sediments of small areal extent of fluviatile and lacustrine origin of varied thickness up to 3 m (Jhingran and Rao, 1958; Venkatesh, 1967; Krishnan, 1968). There are altogether six basalt flows of ~8 to 40 m thickness in and around the Lonar crater; of which the four bottom flows are only exposed along the crater wall (Ghosh and Bhaduri, 2003). The two topmost flows occured away from the crater and they do not show any impact- induced deformation. The flows are separated from one another by a discontinuous marker horizon like red and green paleosols, chilled and vesicular margins, and vugs filled with secondary minerals. Fresh basalts occur only in the upper ~50 m of the crater wall, whereas below this level the flows are heavily weathered and friable (Fudali et al., 1980). The pre- impact black, sticky, humus-rich soil of ~5 to 90 cm thickness is still preserved at places between flows and overlying ejecta (Ghosh and Bhaduri, 2003). All of these basalt flows have a common mineralogy and texture except some minor petrographic differences in the abundance of plagioclase phenocrysts, glass, and opaque minerals (Ghosh and Bhaduri, 2003; Osae et al., 2005). The basalts contain occasional phenocrysts of plagioclase and rare olivine set in a groundmass of plagioclase, augite, pigeonite, titanomagnetite, palagonite, and secondary minerals such as calcite, zeolite, chlorite, serpentine, and chlorophaeite (Ghosh and Bhaduri, 2003). Ramgarh impact structure: The complex ‘Ramgarh’ structure (~5.5 km diameter) is excavated through sandstone and shale along with minor limestone horizons of the Bhander group of Vindhyan Supergroup (Fig. 1.7). Evidences in favour of asteroid impact origin of this structure are: (1) rectangular shape structure and presence of shatter-cone in colluvium near the center of the feature (Crawford, 1972), (2) impact spherules, diaplectic glasses, and PDFs in quartz grains (Sisodia et al., 2006a), (3) highly magnetic pieces with characteristic pitting and polish from inside and outside of the structure (Ahmed et al., 1974), (4) presence of high Fe (up to 58 wt%), Ni (~4000 ppm) and Co (~7000 ppm) with high Ni/Cr (average ~4, range 0.06 to 32) and Co/Cr (~10, 0.06 - 58) ratios in mm-size magnetic particles/spherules from the soil inside the structure (Misra et al., 2008b). Very high abundance of Ni can be suspected to be of asteroid origin, because this element occurs in very low proportions in terrestrial rocks except in primary mantle-derived mafic and ultramafic rocks, which are absent in and around the Ramgarh structure.

Figure 1.7: Panchromatic band Landsat-7 grey shaded image of the Ramgarh structure. Different sets of fault transecting the crater’s rim are shown in black lines labeled 1, 2, 3 and 4 respectively. Black arrow in northeast shows paleochannel of Kul River. The main river Parbati is at north of the crater.

The Ramgarh ring structure has been a subject of controversy for a long period of time regarding its origin. Several views exist to explain whether it is a product of extraterrestrial or intraterrestrial forces. These include intrusions such as kimberlite, carbonatite or diapir (intrusion in which more mobile and deformable material is forced into brittle overlying rocks) and related subsidence, tectonism, mechanism of centripetal rheid flow of kaolin-rich shales, a combination of magmatism and tectonism, meteorite impact, etc. Based on the presence of shatter cone structure noticed in the colluvium near the centre, Crawford (1972) was probably the first to suggest the Ramgarh ring structure to a possible impact structure.

Rakshit (1973) discussed several possible theories for its origin viz. (i) presence of intrusive rock at depth, (ii) volcanic crater, (iii) folding/ faulting, (iv) subsidence, (v) diapiric intrusion at depth and (vi) impact theory. In the absence of favorable evidences Rakshit (1973) himself negated the first five theories for the origin of Ramgarh structure, and opined that this structure strongly resembles an ‘impact crater’. He also suggested a ‘near surface explosion of a meteorite’ for the formation of this structure. The field evidences he found in support include (i) gradual increase of dips (70°-80°) near centre, (ii) shattered nature of sandstone, (iii) shatter-cone like structure in small outcrop of sandstone in the central part and in sandstone of the peripheral ridge at places, (iv) radial to sub-radial pattern of fractures and faults; sometimes displaying crudely-defined circular or conical pattern and, (v) partial recrystallization, granulation, deformation of lamellae and brecciation of quartz grains showing wavy and uniform extinction. Balasundaram and Dube (1973), based on shear fracturing, granulation and anomalous birefringence in quartz grains, concurred with Crawford's suggestion of impact origin of the Ramgarh structure. Sharma (1973) suggested that the Ramgarh ring structure was formed due to combined phenomenon of magmatism and tectonism. Based on aerial view and map published in the Survey of India (196) School Atlas along with some field observations i.e. magnetic pieces from central part and magnetic spherules in clay from outside the structure, Ahmad et al. (1974) considered the Ramgarh structure as a ‘meteoritic crater’. Ramasamy (1981, 1987), based on detailed geological mapping and structural analysis of the area, however, considered the evolution of this curious geomorphic structure as a product of structural or tectonic deformation and called it as ‘Ramgarh dome’. Prasad (1984) in his memoir on Vindhyan Supergroup also considered it as an oblong anticline. Murali and Lulla (1992) identified the structure using IRS-1A images and considered it a potential impact crater not studied hitherto, particularly in relation to Deccan volcanism. Nayak (1997) opined that definitive meteoritic impact signatures are lacking in the Ramgarh structure and, at present, the structure should be considered as 'Ramgarh astrobleme'. Master and Pandit (1999) gathered more evidences in favour of its being an ‘impact crater’. These include identification of closely spaced-fractures and multiple-joint striated surfaces (MSJS) in quartzites, undulose extinction of quartz grains and deformation twin lamellae, PDFs in association with planar fluid inclusion trails under high magnification, all indicative of intense shock metamorphism. Sisodia et al. (2006a) identified Ni-Fe rich

rounded micro glassy objects, severely fractured quartz grains (with corroded margins possibly after high silica polymorphs), sand-sized glistening black and transparent glassy (microtektites) spherules, crystalline silica grains showing spherulites and mosaic of spherulites characteristic of diaplectic glass formed under high pressure, and planar deformation structures–evidences in favour of its being an ‘impact crater’. Reimold et al. (2006), however, did not agree with evidences cited and conclusions drawn by Sisodia et al. (2006a) in favour of meteoritic crater, and they suggested more detailed analytical studies for identification of shock metamorphism in the central part of the area. The field and laboratory investigations carried out so far suggest that the ring structure could have been formed by various processes. Further evidences are in favour of an extraterrestrial or meteoritic impact regarding its formation; the ring structure, therefore, may be called as Ramgarh crater. The stratigraphic sequence of the Bhander Group rocks exposed inside the Ramgarh structure is younging as follows: (a) Ganurgarh shale (~430 m thick), (b) Lower Bhander limestone (~18 m), (c) Samria shale (~36 m) and (d) Lower Bhander sandstone (~240 m) (cf. Sisodia et al., 2006a). The Ganurgarh shale is dark red to reddish brown, purple and grayish green in colour. It is a friable, splintery and very finely-laminated shale. These shales are often intercalated with calcareous material. The calcareous amount increases gradually towards the top. The Lower Bhander limestone is a thinly laminated, stromatolitic limestone. It is found in various colors (grey-blue, blue, pink and purple), the grey-blue being most common. In thin sections, the limestone shows fine-grained calcite with little amount of quartz and iron oxide. The upper shale sequence of the Bhander Group constitutes the Samria shale, which is very thinly laminated and red or green. This shale, in contrast to Ganurgarh shale, is free from quartzite bands making it distinct from the former. The Samria shale is intercalated with stromatolitic limestone, and it is conformably overlain by Lower Bhander sandstone, which is fine-grained, compact and massive in nature. It is also well jointed and is dull white to reddish brown in color. This sandstone is pure quartz sandstone, mostly free from cementing material. The Lower Bhander sandstone shows local shale intercalations. Kumar and Reddy (1984) investigated the structure by drilling during 1981−82. A total of ten vertical boreholes (nine shallow and one deep) were drilled in the area to know the subsurface behavior of rocks. The deepest borehole drilled in the central part of the structure

reached up to 452 m depth. Drill core study showed the presence of an uninterrupted sedimentary sequence belonging to Rewa and Kaimur Groups. No intrusive or volcanic rock was found to intersect in any borehole. Analytical results of core samples did not show chemical composition akin to known meteorite. The cores of the borehole drilled near Bandewara temple, however, displayed tight isoclinal folds with vertical axial plane surfaces, flow-folds, micro-faults and micro-thrust (low-angle) faults in the Jhiri shale- sandstone sequence. Dhala impact structure: The newly discovered complex ‘Dhala’ structure (Fig. 1.8) is an eroded remnant of a large Paleoproterozoic impact structure with an estimated present-day apparent diameter of ~11 km (Pati et al., 2008). It is excavated on Precambrian granitoids of ~2.5 Ga age, with minor 2.0-2.15 Ga mafic intrusive rocks, and overlain by post-impact sediments of >1.7 Ga. The age of the impact event is apparently bracketed by these two sequences. This complex structure has a well defined central uplift and largely eroded multiple breccia rings. The breccia rings are separated by crater-fill sediments and suevite deposits. Many lithic and mineral clasts within the voluminous melt breccia vein occurring on the inner side of the inner-most breccia ring contains diagnostic shock metamorphic features, such as multiple sets of PDFs in quartz and feldspar, ballen-textured quartz, occurrences of coesite, and feldspar with checkerboard texture, as well as various thermal alteration textures that are typically found in clasts of initially superheated impact melt. Most PDFs are of the decorated type; many shocked quartz grains appear toasted. The percentage of shocked quartz and feldspar grains in the impact melt breccia is variable (7 to 67 percent by volume). The impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts. The presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm Dhala to be an impact structure (Pati et al., 2008). In order to better constrain the age of the impact event, a geochronological study was undertaken on Dhala impact melt breccia, involving 40Ar/39Ar step-heating and sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating techniques by Pati et al. (2010). Their U-Pb data for two breccia samples yield ages of 2563 Ma and 2553 Ma, which indicate the age of the granitoid basement. The 40Ar/39Ar experiments resulted in partial plateau ages indicating that the Dhala impact melt rock was affected by a strong thermal/hydrothermal overprint at ca. 1 Ga. The SHRIMP U-Pb ages for two zircon

overgrowths indicate a ca. 530 Ma event that may have contributed to the post-impact resetting of the impact melt rock. However, the results lead to a common problem experienced when attempting to date small- to moderately sized impact events (Pati et al., 2010). No rock-magnetic and palaeomagnetic work is known to have been carried out till date on this crater.

Figure 1.8: Geological map of Dhala area displaying a roughly circular feature (Central Elevated Area, CEA), surrounded by a broad monomict granitoid breccia ring and subhorizontally disposed Vindhyan sediments in the environs of the CEA. The flat-lying area in between comprising sandy siltstone and siltstone shows asymmetric outcrop patterns similar to the occurrence of the breccias exposures (Pati et al., 2008).

Luna impact crater: Luna crater is a circular crater having about 3.5 m diameter and excavated through Deccan basalt sediments in Kachchh district of Gujarat (Karanth et al., 2006). The radar-generated data of the satellite imagery suggest that it spreads for over 5 km radius. The various impact products found at this site include: (a) metallic meteorites fragments with spherical cavities, (b) glassy objects comparable to tektites, and (c) high

pressure mineral polymorphs of SiO2 (stishovite and coesite). These products are indicative of its impact origin. The structure is not yet confirmed to be as an asteroid impact origin. 18

1.7 GEOPHYSICS OF IMPACT STRUCTURES Geophysical studies (seismic, gravity, magnetics) are extremely useful to delineate the impact sites. Terrestrial impact structures bear characteristic gravity and magnetic signatures. Observations on circular/semi-circular gravity and magnetic anomalies (~2.5 mGal and 550 nT respectively) over the Lonar lake suggest that the asteroid impact modified the magnetization vector and density of the target Deccan trap (Rajasekhar and Mishra, 2005). The impact remagnetized the Lonar crater site in the present-day Earth’s magnetic field, demagnetizing the remanent magnetization of the Deccan trap. This implies that the temperature on impact raised >550°C corresponding to the Curie point of magnetite. Part of the brecciated zone showing very high susceptibility of about 4x10-3 SI, suggesting concentration of magnetite in this part which may represent parts of meteorite embedded. Seismic profiling techniques can establish that the near surface structural deformation decreases and disappears with depth, an important characteristic of an exogenic meteorite impact. Gravity measurements eliminate the presence of anomalous high- or low-density rock at depth beneath the structure, providing evidence against alternative theories of origin by (respectively) igneous intrusion or salt-dome uplift. Seismic reflectance and refraction: These methods provide complimentary evidence to geological field investigation. Reflectance defines the morphology as also zones of fracturing and brecciation. Disturbance of reflectors is mostly evident in the central uplift zone of complex structures in sedimentary targets, which decreases downwards and away from this zone. Faults are delineated from which the amount of uplift in the central areas can be estimated. The transition between coherent and incoherent reflectors allows estimation of the dimensions of the transient cavity. Refraction seismic surveys are useful in identifying zones of reduced seismic velocity (low velocity zone: LVZ) due to fracturing and brecciation. At Barringer, USA, the LVZ extends out beyond the rim of the crater. At the Ries crater, Germany, the complex structure extends well below the floor (as in other complex structures). In the case of very large structures such as Vredefort in South Africa, the uplifted zone generally produces increased velocity. Gravity anomalies: Negative gravity anomalies are mainly due to fracturing and brecciation, low density infill of sedimentary material and the topographic depression characterize circular impact structures (Fig. 1.9). Post-impact processes may change the nature of the anomaly. For simple circular bowl-shaped structures (<4 km diameter), a

negative Bouguer anomaly is typical. For larger structures, the anomaly reaches a limiting value of −30 mGal. Below 8 km depth, the fractures tend to close in the case of such structures. Gravity surveys can be used to indicate the depth to which fracturing extends.

Figure 1.9: Cross-section of the Lonar crater. The outline of the crater beneath the crater floor is based on drill holes (Fredriksson et al., 1973). The shaded areas of reduced density are those used to fit gravity model for Lonar crater (Rajasekhar and Mishra, 2005). Note that the reduced density is restricted to the center portion of the mapped breccias deposits. The portion of the crater labeled ‘target density’ must contain materials with a density similar to the unshocked basement rocks.

Magnetic anomalies: The dominant magnetic effect over impact craters is a magnetic low. This is most apparent in crystalline targets by the disruption of the regional magnetic fabric. In large complex structures (diameter>40 km), the reduced magnetic field is complicated by localized short wavelength anomalies up to 1000 nT at the center of the structure. Several processes may operate in an impact event to change the magnetic properties of the target rocks. The shock effects, thermal effects or chemical effects can cause magnetic anomalies related to impact. Shock effects in impact structures can serve to increase or decrease magnetization levels. Thermal effects can result in the production of non-magnetic impact glasses or in resetting magnetic minerals through thermoremanent magnetization (TRM) in the direction of the Earth’s magnetic field at the time of impact. Chemical effects can result in the production of new magnetic phases through elevated residual temperatures and hydrothermal alteration leading to the acquisition of a chemical remanent magnetization (CRM) in the direction of the ambient field. Shock can produce demagnetization and remagnetization effects in rocks. Experimental studies have shown that shock pressures of the order of 1 GPa can remove existing remanent magnetizations (Pohl et al., 1975; Cisowski and Fuller, 1978). The effects of shock

metamorphism aid in the modification of magnetic carriers; for example, at pressures >40 GPa and T>1000°C, amphibole and biotite decompose to produce magnetite (Chao, 1968). At lower pressures, titanomagnetite can result from the breakdown of ilmenite (Chao, 1968). In addition to demagnetization, target rocks can also acquire a shock remanent magnetization (SRM) in the direction of the ambient Earth’s magnetic field at the time of impact. The intensity of SRM is proportional to the ambient field strength (Pohl et al., 1975), decreasing with distance from the point of impact (Cisowski and Fuller, 1978). Cisowski and Fuller (1978) detected a secondary component of magnetization probably acquired at the time of impact at Meteor crater, Arizona, and Lonar crater, India, while Halls (1979) has documented the existence of a secondary magnetization at Slate Islands in Canada. Halls (1979) found that the intensity of the SRM decreases away from the point of impact and is restricted to the low coercivity fraction. The remanence is also acquired rapidly (between impact and the formation of central uplift), as evidenced by a small directional scatter. Studies at Charlevoix in Canada suggest that the large reduction in remanence intensity in samples from the central uplift is shock related (Robertson and Roy, 1979). However, the high coercivity of the carrier phase (titanohematite) at Charlevoix explains why an acquired SRM is absent. SRM is most likely to occur in autochthonous target rocks experiencing pressures >1 GPa, but temperatures less than Curie points of the magnetic phases present. The thermal effects of impact can also lead to the acquisition of TRM in the direction of Earth’s magnetic field at the time of impact in the slowly cooled crystalline impact melt rocks. Magnetic resetting through TRM is known to have led to a number of palaeomagnetic dating studies at impact craters based on samples of impact melt rocks and breccias. A stable remanence low directional scatter appears to be characteristics of impact melt rocks that most probably reflect the rapid acquisition of the magnetization (Pohl and Soffel, 1971). In addition, the overall elevated post-shock temperatures and circulation of fluids can lead to the acquisition of a CRM in the direction of the ambient magnetic field. For example, the oxidation of magnetite to hematite leads to a high coercivity chemical remanent magnetization (CRM). Electrical methods: Brecciation and fracturing can lead to changes in electric properties, principally through variations in the amount and distribution of fluids. Lower than normal resistivities due to fracturing have been detected up to one crater diameter beyond the rim in

the case of some craters. Deeper electrical profiling, using magnetotelluric (MT) methods, has been carried out and anomalies, again believed to be due to impact-induced fracturing and fluid distribution, identified. Changes in dielectric properties of exterior ejecta blanket and the interior deposits at Meteor crater, Arizona with the distribution of fluids, are observed using Ground Probing Radar (GPR) technique.

1.8 AIMS OF THE THESIS The impact history of Lonar, Ramgarh and Dhala structures is evaluated by rock magnetic, palaeomagnetic, and geochemical data. The thesis aims are:  To find out evidences of impact generated magnetic indicators.  To examine the orientation of structural deformation to the direction of impact by Anisotropy of Magnetic Susceptibility (AMS) technique.  To study the rock and mineral magnetic properties of impact craters on Indian shield.  To test shock remanent magnetization (SRM) hypothesis, impact-generated magnetic fields and to determine the mean palaeomagnetic pole positions.  To develop palaeomagnetic technique as dating tool of impact crater sites.  To understand the geochemical variations of impact rocks. Organization of the Thesis: In line with the principle objectives of this thesis, the chapters are grouped into seven topics: significance of impact cratering mechanics of the Indian impact craters; field sampling and laboratory experimental techniques, geological structural and AMS technique to evaluate the direction of projectile; effects of shock pressures on rock and mineral magnetic properties of impact rocks; palaeomagnetism of impact rocks; geochemistry of impact rocks and their products and discussions and conclusions with some remarks and future directions on the prospects of impact cratering research in India. Chapter 2 concerns the details of sampling methodology carried out in the field work (Lonar, Ramgarh), laboratory experimental procedures and the methods for analysis of rock magnetic, palaeomagnetic, and AMS data. All magnetic and geochemical measurements were done at Environmental Magnetism Laboratory, IIG. The collection of shock metamorphosed products (impactites, impact spherules) and their laboratory treatment and data analysis is also included in this chapter. The purpose of AMS, rock magnetic and palaeomagnetic studies on terrestrial impact craters is also included in this chapter.

Chapter 3 deals with the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image, geological structure analysis and AMS technique for evaluating the angle and direction of impactor that struck the preimpact target at Lonar. It also discusses the distribution of impact-induced shock front just after the impact at Lonar. Specifically, the AMS data suggest that the target basalts occurring at ~2 km WSW of the

crater rim are highly shocked as indicated by the random orientation of their K3 susceptibility axes in comparison to the unshocked basalts at ~2 km ESE of the crater, showing a bimodal distribution of susceptibility axes typical of lava flows. Moderate to strong westward shifts of the K3 axes are seen for the majority of the shocked basalts on the crater rim and WSW of the crater, indicating an oblique impact from the east when compared with modeling and experiments. Variation in attitudes of the basalt flows on the Lonar crater rim shows a bilaterally symmetrical distribution about an E-W axial plane, which includes quaquaversal dips of the flows all around the crater rim, except to the west where overturned dips of the basalt flows are seen. It appears that oblique impact and the symmetry in structural variations around the crater rim have a relationship for Lonar crater (Misra et al., 2010). It is also showed that the impact stress could have branched out into the major SW and NW components in the downrange direction immediately after the impact, inferred from the relative displacements of K3 axes of shocked basalts. Chapter 4 describes the rock and mineral magnetic properties of Lonar target rocks to identify the magnetic remanence carriers and their variations with the plane and direction of impact. The mineralogical data suggest that Lonar basalts essentially contain psuedosingle- domain (PSD) grain size Ti-rich to Ti-poor titanomagnetite and their low temperature oxidized products as the magnetic carriers. Their rock-magnetic results show an increase of NRM/χ and REM (=NRM/SIRM ratio in %) values with the E-W plane of impact. More rigorous description of data and discussion of results obtained were discussed in this chapter. The rock magnetic study of Ramgarh structure partly confirms its origin by asteroid impact event. Chapter 5 provides palaeomagnetic results to understand the shock effects on the magnetic remanence carriers with the plane and direction of impact. The results suggest that shocked target basalts acquired a high coercivity and high temperature (HC_HT) magnetization component due to impact. The orientation of HC_HT components in the uprange direction is symmetrically disposed about E-W plane of impact, making an obtuse

angle with the direction of impact. The low coercivity and low temperature (LC_LT) magnetization component of unshocked and shocked basalts are statistically identical to the present-day Earth’s magnetic field (PDF) direction. The PDF may likely be acquired during the last 570±47 ka, subsequent to the formation of the Lonar crater. Here it is demonstrated that the high coercivity remanence is the product of single-domain (SD) magnetite from shock-induced decomposition of target rocks in titanomagnetite, as suggested by Cloete et al. (1999). Discussion of palaeomagentic data analysis and the hypothesis of shock remanent magnetization (SRM) and evidence of impact-generated plasma magnetic fields around impact structures is verified. The mean palaeomagentic pole is calculated and to be used for dating the impact structures. The palaeomagnetic data of shocked ejecta basalts from Lonar suggest that impact shock-induced magnetic field could have existed beyond the modification stage of crater formation, when the newly formed ejecta with the randomly deposited basaltic blocks had weakly remagnetized. Chapter 6 deals with the geochemical and X-ray diffraction (XRD) analysis of target rocks, impact melts and products. The comparisons of major oxides compositions of Lonar impact products with those of target rocks were done using X-ray fluorescence (XRF) spectroscopy. There are slight depletions in the Na2O, P2O5 contents, and enrichment in the

K2O content, in the impact melt rocks compared to the target rocks. This is in agreement with the findings of Osae et al. (2005). The enrichment of ‘K’ in impactites is a common phenomenon and termed the process “potassium metasomatism”, probably due to vaporization fractionation during the impact, or possibly an immediate post-impact alteration. The XRD analysis is carried out for mineralogical phase identification of minerals and shock-induced phase changes. Chapter 7 summarizes the results from the thesis and presents an outlook for future directions on impact cratering research in India.

CHAPTER 2

SAMPLING AND LABORATORY EXPERIMENTAL METHODS

2.1 FIELD SURVEY OF METEORITE IMPACT CRATERS ON INDIAN SHIELD Geological field trips were carried out during the years 2008 and 2012 at Lonar and Ramgarh meteorite impact structures for collecting drill core samples (~2.5 cm diameter) from rock outcrops using a portable gasoline-powered rock drill. Samples were oriented in the field using the orienting device to measure the azimuth and hade of the drilled core. The following materials have been studied with regard to their rock magnetic, palaeomagnetic and geochemical properties: Shocked and unshocked target rocks (basalt, sandstone, shale, and limestone), Shock-metamorphosed products (impact breccias, impact melt rocks, and impact spherules/particles). Sample collection and field evidences at Lonar crater: The shocked altered basalts, weathered basalts, impactites, and impact spherules from Lonar ejecta blanket (12 sites) were collected for geochemical and rock magnetic measurements in May 2012. Oriented drill core samples in the SE, southern, SW, NW cross-section of the Lonar crater (from topmost flow of the crater rim to the bottom flow; 141 specimens, 5 sites) were collected in October 2011 for palaeomagnetic measurements as negative inclination polarity is noticed in characteristic remanent magnetization (ChRM) component of SW crater rim shocked basalts in our earlier sample collection dataset. We suggest that the presence of negative polarity of inclination in ChRM component in this sector may be due to the relative displacement of basaltic blocks as a result of asteroid impact. Oriented drill cores of unshocked target basalts from far eastern (~4.6 km from crater center), southern (4 km from crater center) sites, and shocked basalts from southern and SW crater rim sector (53 specimens, 4 sites) were collected in February 2011 to increase the earlier dataset for understanding the distribution of impact-induced shock pressure with the direction of impact. Oriented shocked basaltic samples (210 specimens, 15 sites) were collected from the SW, western, and NW sectors of crater rim and farther away from the crater center (~2.5 km) in the western and NW

locations in April 2010 to evaluate the impact-induced stress distribution pattern and variations in rock magnetic properties with the direction of impact. To evaluate the angle and direction of projectile trajectory of Lonar asteroid impact, oriented target basalts (226 specimens, 23 sites), shocked ejecta basalts (41 specimens, 5 sites), impactites and impact spherules were collected from around the crater rim and adjoining areas in February 2009. To test the hypothesis of shock remanent magnetization (SRM) or evidence of impact- amplified plasma magnetic fields in shocked rocks through palaeomagnetic technique, oriented basaltic target rocks (313 specimens, 38 sites) from around the crater rim and unshocked target basalts at a distance of ~3 km from the crater center in the east and west were collected in May 2008. The geological field evidences in favor of the meteoritic impact origin of Lonar crater is shown in figure 2.1.

Figure 2.1: Photographs of impact evidences from Lonar crater. (a) Dipping of basalt flow at western sector rim, (b) Cleavaged basaltic block within the ejecta at northern part, (c) Impact melt rock in the ejecta blanket outside the southeastern part of the crater rim, (d & e) Basaltic impact glass spherules with a variety of splash forms [spheres, pancaketoroid, ellipsoids, teardrops, and dumbbells] recovered from undisturbed regions of crater rim and at Little Lonar, (f) Randomly oriented ejecta blocks on the NE Lonar crater rim with characteristically small fraction of matrix, (g) Black paleosol (histosol) section with white calcified root casts fills a natural depression in the pre-impact basalt landscape near Kalapani Dam (KPD) village; the light-brown layer on top of the histosol is unsorted impact ejecta, and (h) Red bole horizon with calcite veins showing basaltic flow separation (j) Orienting the drilled sample at the NW crater rim.

Sample collection and field evidences at Ramgarh crater: The Ramgarh structure (~5.5 km diameter) is located in the Baran district of Rajasthan, India and represents an anomalous structure situated in an extensively flat terrain of Neoproterozoic sandstone and shale of the Vindhyan Supergroup. Ramgarh structure appears as an annular feature with a depressed interior surrounded by a raised rim having steep inner flanks and shallow outer flanks. The crater rim appears conspicuously raised by ~250 m from the ground level. The area to the west of the crater is a pediplain consisting of alluvium concentrated by the Parbati river system, while area to the east is an alluvial plain followed by a plateau. Field observations suggest that the crater can be divided into three parts viz. rock-exposure, rock debris and buried-pediment (basinal part). The ratio of rock exposure to rock-debris is 20% to 80% indicating an uncommon geomorpholgical/geological feature denoting a sudden geological event (Lashkari and Sisodia, 2002). The outer flanks of the crater rim are constituted of sandstone or quartzitic sandstone with quaquaversal dips. The inner flanks of rim have relatively steeper slope. The crater has an opening, which consists of shale that is loose and sometimes fragmented. A lineament trending NE-SW runs across the crater. The geological field evidences in favor of meteoritic impact origin of Ramgarh crater is shown in figure 2.2. The exposed crater rocks are severely deformed and fractured. The strike of the beds shows continual swing along the rim ultimately producing a characteristic ring structure. The strata at the foothills are almost horizontal with dips of 1°-5° whereas at the rim crest they are subvertical to vertical and sometimes overturned. The crater is not closed, but has an opening in the southwest. At the southwest side of the crater, the beds are folded (Fig. 2.2e). The northern limb of this fold strikes E-W and dips 41° towards north while south limb dips 47° due south and strikes NE-SW. Local crushing and brecciation can be noted frequently. However a definite fault pattern is difficult to be located. The central part of the crater where bedrock is well exposed has vertical dips. The contact between the underlying rocks belonging to Jhiri and Rewa sequence exposed here shows complex folding. The core collected from a vertical borehole drilled by Geological Survey of India (GSI) consists of uplifted shale and siltstone with intricate disharmonic folding and brecciation (Ramasamy, 1987). Similar features are observed in Manicouagan and Charlevoix craters. Such features are produced due to rebound after the impact (Grieve, 1991).

Figure 2.2: Photographs of impact evidences from Ramgarh structure. (a) Fractured sandstone at central uplift of Ramgarh structure, (b) Samples taken from an insitu exposure on a stream channel at the eastern sector outer base of the structure, (c) Section of soil underlying sandstone on the foot of western rim flank showing presence of excessive angular blocks of sandstone within matrix, (d) Cap of recently formed conglomerate at Parbati river (~4 km west from the structure), (e) Photograph taken at the opening of the Ramgarh structure showing shock effects such as brecciation/pulverization of the incompatible bedrock, shales; and folding and faulting in compatible lower Bhander group sandstone, (f) Multiple–striated–joint-surfaces in Bhander sandstone in the crater, (g) Relict exposure of sandstone on the crater’s rim at north showing vertical dip, (h) View of ~6m central uplift, with an ancient temple on its top (shown by arrow), (i) Diagenetically altered (original glass replaced by iron oxide) spherules, (j) Large-size magnetic particles found at southeastern (SE) channel, (k) Impact melt rock showing highly vesicular surface, and (l) Impact melt rock showing ropy structures on surface.

Field survey at Ramgarh structure was carried out in June 2009 for the collection of oriented drill core target rocks from rock formations which include the Bhander group sandstone, shale and limestone inside and outside the structure and unshocked rocks from the adjoining areas in the east and west direction (Table 2.1; Fig. 2.3). The mm-sized magnetic spherules/particles were collected both from the soil inside the crater and from within the finer fraction of reworked debris lying outside the raised rim of this structure (Table 2.1). The spherules/particles are magnetic and a hand magnet was used to collect the samples. These spherules are glistening black or transparent glassy (microtektites) with perfectly spherical shapes. The surface of many of these spherules is dotted with small vesicles or transparent glassy patches. Many of these spherules have undergone diagenetic alteration but have still retained their spherical shapes. The diagenetically altered spherules mostly have original glass replaced by iron oxide (Fig. 2.2i). The two distinctive attributes of these spherules viz. the dominant perfectly spherical shape and the sand size but more importantly their occurrence in a geological structure that has a shape of an impact crater generally point towards an impact by an extraterrestrial body on the Earth’s surface. Such spherules can also form by volcanism but it is to be noted that Ramgarh structure is located in a vast sedimentary terrain that has no evidence of volcanism anywhere in the vicinity.

Figure 2.3: Google Earth Image of Ramgarh structure showing drill core sample collection sites. 31

Table 2.1: Drill core sample collection sites at Ramgarh structure

Site Location Lat. / Long. RJ – 1, RJ – 2 Central uplift N25°19'57.0"/ E76°37'28.1" RJ – 3, 4, 5 Entrance to crater rim (SW) N25°19'14.7"/ E76°36'55.9" RJ – 6, 7 Brahmapura village (S) N25°18'53.6"/ E76°37'09.6" RJ – 8, 9, 10 SW sector N25°19.368'/ E76°36.629' RJ – 11, 12 Western sector N25°20'04.7"/ E76°36'22.5" RJ – 13 Collection of magnetic spherules N25°20.869'/ E76°37.457' RJ – 14 Dam (Northern sector) N25°20'56.8"/ E76°37'27.9" RJ – 15 NE sector N25°20.574'/ E76°38.409' RJ – 16 Near Parbati river (~4.2 km W sector) N25°20.505'/ E76°33.970' RJ – 17 ENE Rocky sheet (outer Crater flank) N25°20.307'/ E76°38.582' RJ- 18 Parbati river (~4 km West sector) N25°20.813'/ E76°34.347' RJ -19 Kishangang village (~12.5km of SE) N25°12.568'/ E76°39.496' RJ – 20 Top of Krishnamata temple (East) N25°19.946'/ E76°38.146' RJ – 21 Large-size magnetic particles collection N25°19.398'/ E76°37.979' RJ – 22 Zone of magnetic particles N25°19.381'/ E76°37.823' RJ – 23 Asnawar village (Kalapathar, ~3 km N) N25°22.529'/ E76°37.706' RJ – 24 Magnetic particles from a stream channel N25°19.953'/ E76°37.523' RJ – 25 East sector (outer-base) N25°19.614'/ E76°38.431'

RJ – 26 Ashram village (W) Nala channel (large-size N25°20.149'/ E76°36.424' magnetic particles collection) RJ – 27 Kishangang bridge (~21 km SW of structure) N25°08.176'/ E76° 36.460'

The field observations that are important to evaluate the possible impact origin of complex Ramgarh structure are: (a) rectangular panoramic view of the structure with a continuous raised rim all along its periphery that raises ~250 m above the surroundings (b) shatter cone-like structure on the crater wall, (b) extremely fractured, sometimes pulverized, sandstone in the central uplift (~6 m) of the structure, and (c) occurrence of chilled melt-like material within the soil inside the structure and within the relict/reworked debris in the outer flank of the structural rim (Misra et al., 2008a, b). The shatter cone-like structure occurs in the inner wall of the NW rim. These are cm-sized conical surfaces in sandstone with distinctive striations radiating vertically downward (Fig. 2.4a). It may be noted that shatter cone-like structure was also reported by Crawford (1972) in colluviums near the center of the Ramgarh structure. Chilled melt pieces with vesicular or ropy surfaces (Fig. 2.4b) were found in soil/eroded debris at the base of the crater rim outside the Ramgarh structure at the west. Some pieces of melts engulfed angular pieces of sandstone (Fig. 2.4c), reminiscent of the coatings of impact melt found at the Wabar impact structure, Saudi Arabia. Pieces of sandstone having melted surfaces with chilled and vesicular appearances are also found (Fig. 2.4d). These melt-like objects are unlikely to be of anthropogenic origin because no earthen bricks were used in the construction of ancient temples and forts inside the Ramgarh structure. Occurrence of melt rocks in the present sedimentary terrain without any record of igneous activity is unlikely. The vesicular and ropy structures on the melts are indicative of their chilling in atmosphere. While shock is commonly called upon to produce melts in impacts, an alternative geological process that can produce these chilled melts in a crater-like structure like Ramgarh is atmospheric air- burst during a small impact (Newsom and Boslough, 2008). Another feature, which is conspicuous on the inner wall of the Ramgarh structure, is evidence of aqueous or hydrothermal activity leading to mobility of manganese (Mn) solutions along the extensively developed fracture planes in sandstone. Joints trending in all directions contain manganese veins, and manganese is found in wall rock sandstone adjacent to veins (Fig. 2.4e, f).

Figure 2.4: (a) A shatter cone-like structure on the inner wall of Ramgarh structure at NW side inside the structure (b) Chilled melt-like material with vesicular surface recovered from soil inside the structure (scale bar ~6 cm) (c) A chilled melt piece (m) engulfing angular pieces of sandstone (s) (size: ~6 cm) (d) A piece of sandstone with melted vesicular surface (size: ~6 cm) (e) Manganese (Mn) veins along fractures within sandstone (shown by arrows), the height of the section ~2 m (hammer at left is shown within circle) (f) Enlarged view of Mn veins showing absorption of Mn solution by the wall (hammer scale).

2.2 LABORATORY EXPERIMENTAL METHODS In the laboratory, each collected cylindrical core (~9−12 cm length) was cut into two to three standard specimens of diameter 2.5 cm and length of ~2.2 cm. All rock magnetic, palaeomagnetic and geochemical measurements were performed at Environmental Magnetism Laboratory, Indian Institute of Geomagnetism (IIG), Navi Mumbai, India. The details of laboratory measurements, information they provide, instrumentation used are given in Table 2.2.

Table 2.2: Summary of Rock Magnetic, Palaeomagnetic, and Geochemical measurements for Characterization of Meteorite Impact Crater Rocks

Measurement Information Instrument Natural Remanent Fossil remanent magnetization of rocks Molspin and Agico Magnetization JR-6A spinner (NRM) magnetometers Bulk Concentration of ferrimagnetic minerals Agico MFK1-FA Susceptibility (χ) depending on magnetic grain size and Kappabridge mineralogy Anisotropy of Directional variation of susceptibility within a Agico KLY-4S Magnetic sample; provides principal susceptibility axes Kappabridge Susceptibility (AMS) (K1, K2, and K3), degree of anisotropy (P') and – magnetic fabric shape parameter (T) of susceptibility ellipsoid; employed to know flow direction in igneous rocks, paleocurrents in deep-sea sediments, strain in deformed rocks, tectonic stress, and impact-induced stress effects in meteorite impact structures Anhysteretic Measure of fine grained stable single domain ASC scientific D2000 Remanent (SSD) magnetic particles, also sensitive to AF demagnetizer magnetization (ARM) grain interactions Saturation Isothermal Maximum obtainable IRM, depends on Molspin pulse Remanent concentration and particle size of magnetic magnetizer Magnetization minerals (SIRM) REM Estimate of palaeomagnetic field and a ratio of Molspin spinner (=NRM/SIRM)% ~1.5% indicates an Earth-strength magnetic magnetometer and field Molspin pulse magnetizer IRM acquisition and Indication on type of magnetic minerals and Molspin pulse dc demagnetization of on their coercivity spectrum magnetizer SIRM Alternating Field Relates to coercivities of magnetic minerals; ASC scientific D2000 (AF) demagnetization can be used to separate ChRM component out AF demagnetizer of NRM of probable secondary overprints Thermal (TH) Relates to unblocking temperature of magnetic Magnetic demagnetization of minerals; can be used to separate ChRM measurements thermal NRM component out of probable secondary demagnetizer overprints (MMTD) 80

Measurement Information Instrument Thermal Magnetic mineral carriers based on unblocking MMTD80 demagnetization of temperatures SIRM

Magnetic hysteresis Parameters Ms, Mrs, Hc, Hcr to define overall Molspin Nuvo domain state (SD, PSD, and MD) of the vibrating sample magnetic minerals magnetometer (VSM) Temperature Determination of magnetic mineralogy; Agico KLY-4S dependence of Verwey/Morin transitions and Curie Kappabridge attached susceptibility temperature with CS3/CSL (χ-T) apparatus Lowrie-Fuller (LF) Domain structure of magnetic minerals and Molspin spinner test origin of magnetization magnetometer, pulse magnetizer, and ASC scientific D2000 AF demagnetizer X-ray Fluorescence Major oxides and elements expressed in wt% Ametek Spectro (XRF) spectroscopy and parts per million (ppm) XEPOS XRF

X-ray Diffraction Mineralogical phase identification InXitu–BTX (XRD) Benchtop XRD/XRF

Anisotropy of Magnetic Susceptibility (AMS): Anisotropy of Magnetic Susceptibility (AMS) is a versatile petrofabric tool. AMS is a tensor which relates the intensity of the applied field (H)

to the acquired magnetization (M) of a material through the equation: Mi = kijHj, where Mi (i= 1,

2, 3) are the components of the magnetization vector, Hj (j= 1, 2, 3) are the components of the

intensity of magnetic field vector, and kij (kij = kji) are the components of the symmetric second- rank susceptibility tensor that reflects the directional variation of susceptibility in its shape. The components k11≥k22 ≥k33, often denoted as k1≥k2≥k3 or kmax≥kint≥ kmin, are called the principal susceptibilities (maximum, intermediate, and minimum susceptibilities) which has both magnitude and direction. The susceptibility tensor can be geometrically represented by the susceptibility ellipsoid. The axes of the AMS ellipsoid are parallel to the principal directions and the semi-axes lengths equal the principal susceptibilities. AMS principal directions can record palaeocurrent directions from sediment, flow-directions from magma, finite-strain directions from tectonized rocks, and impact-induced stress propagation direction in terrestrial meteorite impact structures.

Low-field (300 Am−1 at 875 Hz) AMS was carried out using an AGICO KLY-4S Kappabridge for each specimen with measurement in 64 directions on three mutually orthogonal planes using an automatic rotator sample holder (spinning specimen method). The software ‘SUFAR’ and ‘ANISOFT 4.2’ were used for acquisition and processing of the AMS data (AGICO, Czech Republic). The AMS data gives the orientation and magnitude of three

orthogonal principal axes of the AMS ellipsoid (K1, K2, and K3). The magnitudes of these principal directions are used to calculate different parameters, e.g. K (mean bulk susceptibility), P/ (corrected degree of magnetic anisotropy, which is a measure of the eccentricity of magnetic susceptibility ellipsoid), T (shape parameter that describes the oblate/prolate shape of the magnetic susceptibility ellipsoid) (Jelinek, 1981; also see Tarling and Hrouda, 1993). The typical dataset of AMS is given in Table 2.3. Recently the AMS technique was found to be a potential indicator to understand the impact- induced stress effects in shocked target rocks at terrestrial impact structures (Nishioka et al., 2007). This technique has been applied to the Vredefort crater, South Africa and it was found that the minimum susceptibility axes (K3) of AMS of the granitoid target rocks are randomly oriented in the site of impact (Carporzen et al., 2005). In the impact experiments on basaltic andesites, Central Japan and basalts from the Lonar crater, India, it was also found that the AMS directions are highly efficient in determining the direction of impact and perhaps the distribution of the shock metamorphic front around the crater (Nishioka et al., 2007; Nishioka and Funaki, 2008). In the high pressure range (>3 GPa) of experiment, the anisotropy degree was increased, the K3 axes was oriented toward the shock direction and the average susceptibility was decreased. At a relatively low pressure range (0.5−3

GPa), the maximum susceptibility axes (K1) was induced parallel to the shock direction and was superposed on the initial AMS data. Gattacceca et al. (2007) demonstrated that explosive-driven shocks of about 10 GPa on basalt and microdiorite acted to change their AMS. The degree of

AMS was increased near the shock surface, and the minimum principal susceptibilities (K3) were reoriented toward the shock direction. The result of the experiment indicates that target rocks from the meteorite impact craters might not exhibit the primary AMS fabric.

Table 2.3: Anisotropy of Magnetic Susceptibility (AMS) Dataset

Specimen Km K1 K2 K3 L F P P' T dK1geo iK1geo dK2geo iK2geo dK3geo iK3geo LN9.1.1 3.65E-02 3.66E-02 3.66E-02 3.63E-02 1.002 1.007 1.01 1.01 0.522 113.7 31.4 298.2 58.5 204.9 2 LN9.1.2 3.49E-02 3.50E-02 3.49E-02 3.47E-02 1.001 1.007 1.007 1.008 0.746 111.1 20.1 275.8 69.2 19.2 5.1 LN9.1.3 3.34E-02 3.35E-02 3.34E-02 3.32E-02 1.001 1.007 1.008 1.009 0.697 116.7 26.8 267.7 60 20.3 12.5 LN9.2.1 2.62E-02 2.64E-02 2.62E-02 2.60E-02 1.008 1.006 1.014 1.014 -0.116 115 6.9 329.9 81.5 205.6 4.8 LN9.2.2 2.76E-02 2.77E-02 2.77E-02 2.75E-02 1.001 1.006 1.007 1.008 0.846 125.7 23.6 334.2 63.6 220.7 11.2 LN9.3.1 3.35E-02 3.37E-02 3.36E-02 3.31E-02 1.003 1.013 1.016 1.017 0.606 97 18.5 337.4 55.9 197.1 27.5 LN9.3.2 3.22E-02 3.24E-02 3.23E-02 3.20E-02 1.003 1.01 1.013 1.013 0.557 114.1 7.8 6.2 66 207.3 22.5 LN9.3.3 3.13E-02 3.15E-02 3.14E-02 3.10E-02 1.003 1.012 1.015 1.016 0.661 105.8 17.2 345.9 58.1 204.5 26 LN9.4.1 3.04E-02 3.05E-02 3.05E-02 3.02E-02 1.003 1.007 1.01 1.01 0.443 151.5 5.7 246.1 38.5 54.3 50.9 LN9.4.2 2.93E-02 2.94E-02 2.93E-02 2.92E-02 1.004 1.005 1.009 1.009 0.195 140.7 55.6 41 6.6 306.6 33.5 LN9N.1.1 2.41E-02 2.42E-02 2.41E-02 2.40E-02 1.005 1.003 1.008 1.008 -0.167 117.1 10.1 326.3 78.4 208.1 5.5 LN9N.1.2 2.41E-02 2.42E-02 2.41E-02 2.40E-02 1.003 1.004 1.007 1.007 0.126 117.6 2.9 13.8 78 208.2 11.6 LN9N.1.3 2.28E-02 2.28E-02 2.28E-02 2.27E-02 1.003 1.005 1.008 1.008 0.267 115.9 7 10 65.8 208.9 23 LN9N.2.1 2.58E-02 2.59E-02 2.58E-02 2.57E-02 1.003 1.005 1.007 1.007 0.245 125.9 1.8 6.7 86.2 216 3.3 LN9N.2.2 2.45E-02 2.46E-02 2.45E-02 2.44E-02 1.003 1.005 1.008 1.008 0.254 106.9 4.4 9.2 60.3 199.4 29.3 LN9N.2.3 2.40E-02 2.41E-02 2.40E-02 2.39E-02 1.003 1.005 1.007 1.007 0.258 113.1 2.1 18.2 66.8 204 23.1 LN9N.2.4 2.48E-02 2.48E-02 2.48E-02 2.47E-02 1.002 1.004 1.006 1.006 0.294 106 12.3 351.9 62 201.8 24.8 LN9N.3.1 2.72E-02 2.73E-02 2.72E-02 2.71E-02 1.004 1.004 1.008 1.008 -0.02 130.8 8.2 357.7 78.1 222 8.6 LN9N.3.2 2.74E-02 2.75E-02 2.74E-02 2.73E-02 1.004 1.003 1.007 1.007 -0.059 126 2.4 27.3 74.8 216.6 15 LN9N.3.3 2.62E-02 2.63E-02 2.62E-02 2.61E-02 1.004 1.003 1.007 1.007 -0.119 123.1 16.4 269.1 70.5 30 10.3 LN9N.3.4 2.46E-02 2.47E-02 2.46E-02 2.46E-02 1.005 1.002 1.007 1.007 -0.473 105.6 7.2 260.6 82.1 15.2 3.3 LN9N.4.1 3.34E-02 3.35E-02 3.35E-02 3.32E-02 1.002 1.006 1.009 1.009 0.464 96.2 3.9 196 68.1 4.6 21.5 LN9N.4.2 3.30E-02 3.32E-02 3.31E-02 3.29E-02 1.003 1.005 1.009 1.009 0.232 107.2 2 215 83.6 17 6.1 LN9N.4.3 3.25E-02 3.27E-02 3.25E-02 3.24E-02 1.004 1.005 1.01 1.01 0.098 285.7 2.4 25.1 75.5 195.1 14.3 LN9N.4.4 3.17E-02 3.18E-02 3.17E-02 3.15E-02 1.002 1.005 1.008 1.008 0.361 290.9 16 52 60.9 193.7 23.6 LN9N.5.1 3.33E-02 3.34E-02 3.34E-02 3.32E-02 1.002 1.006 1.008 1.009 0.472 106.8 8.2 228.6 74.8 14.9 12.7 LN9N.5.2 3.36E-02 3.38E-02 3.37E-02 3.35E-02 1.003 1.005 1.008 1.008 0.257 106 5.1 227.7 80.4 15.3 8.1 LN9N.5.3 3.31E-02 3.32E-02 3.31E-02 3.30E-02 1.003 1.004 1.007 1.007 0.109 113 2 209 71.9 22.3 18

Specimen Km K1 K2 K3 L F P P' T dK1geo iK1geo dK2geo iK2geo dK3geo iK3geo LN9N.5.4 3.05E-02 3.06E-02 3.05E-02 3.04E-02 1.004 1.004 1.007 1.007 -0.007 286.5 3.2 180.2 78.6 17.1 10.9 LN9N.6.1 3.24E-02 3.25E-02 3.24E-02 3.22E-02 1.003 1.006 1.009 1.009 0.298 86.7 26.6 288.8 61.7 181.3 9.2 LN9N.6.3 3.03E-02 3.04E-02 3.04E-02 3.02E-02 1.002 1.007 1.008 1.009 0.621 92 8.5 341.4 67 185.3 21.2 LN9N.6.4 3.13E-02 3.14E-02 3.14E-02 3.12E-02 1.002 1.005 1.007 1.008 0.436 298.6 2 35.6 73.9 208 15.9 LN9N.7.1 3.08E-02 3.10E-02 3.08E-02 3.07E-02 1.005 1.004 1.01 1.01 -0.135 94.8 3.4 190.3 58.4 2.7 31.4 LN9N.7.2 2.88E-02 2.89E-02 2.89E-02 2.87E-02 1.003 1.004 1.007 1.007 0.105 283.7 3.2 188.8 56.9 15.8 32.9 LN9N.7.3 3.10E-02 3.11E-02 3.10E-02 3.08E-02 1.002 1.006 1.008 1.008 0.391 97.5 11.1 222.7 71.2 4.5 15 LN9N.8.1 3.01E-02 3.03E-02 3.01E-02 2.98E-02 1.006 1.008 1.014 1.014 0.146 117.7 13.3 357 65.2 212.8 20.6 LN9N.8.2 2.90E-02 2.92E-02 2.90E-02 2.88E-02 1.005 1.007 1.012 1.012 0.186 117.3 26.1 347.8 52.4 220.5 25 LN9N.8.3 2.92E-02 2.93E-02 2.92E-02 2.90E-02 1.005 1.009 1.014 1.014 0.322 112 20.7 351.9 53 214.1 29.2 LN9N.8.4 2.98E-02 2.99E-02 2.98E-02 2.96E-02 1.004 1.007 1.011 1.011 0.198 110.2 17.5 1 46.1 214.8 38.6 LN9N.8.5 2.85E-02 2.87E-02 2.86E-02 2.83E-02 1.005 1.009 1.013 1.014 0.281 101.1 17 346.2 54.1 201.5 30.6 LN9N.9.1 3.02E-02 3.03E-02 3.02E-02 2.99E-02 1.005 1.009 1.014 1.014 0.227 101.6 17.3 342.1 57.6 200.5 26.4 LN9N.9.2 3.10E-02 3.12E-02 3.10E-02 3.07E-02 1.004 1.01 1.013 1.014 0.44 99 16.5 343.2 55.8 198.5 29.1 LN9N.9.3 3.11E-02 3.13E-02 3.11E-02 3.09E-02 1.006 1.008 1.014 1.014 0.117 105.4 15.3 353.2 54.1 205.1 31.5

Notations:

 L (Magnetic lineation) = K1/K2

 F (Magnetic foliation) = K2/K3

 P (Degree of anisotropy) = K1/K3 ' 2 1/2  P (Corrected degree of anisotropy) = exp [2*Σ(ln Ki/Km) ] where i = 1 to 3 (Jelinek, 1981)

 T (Shape parameter) = [(2*ln K2 - ln K1 - ln K3)/(ln K1 - ln K3)]; K1, K2 and K3 are the maximum, intermediate and minimum

susceptibility axes; Km = (K1+ K2+ K3)/3 (Jelinek, 1981)

 dK1geo, iK1geo, dK2geo, iK2geo, dK3geo, iK3geo are the declination and inclination of the three principal susceptibility axes (K1,

K2, and K3) in geographic coordinate system

Although the Lonar crater has been known for more than one hundred years (Gilbert, 1896), no clear idea exists on the projectile path of this crater. Ghosh (2003), assuming a cometary impactor of ~90 m diameter and ~500 kiloton mass, suggested a direction of impact from the NW but provided no supporting data for this estimation. In order to evaluate the angle and direction of Lonar asteroid impact, the present work utilizes the low-field AMS technique combined with the analyses of satellite images (ASTER, Landsat) and the geological structure. Rock and mineral magnetic measurements: Rock magnetic properties of impact target rocks were undertaken in order to understand the shock-induced magnetization effects in magnetic minerals as impact cratering events involves extreme temperatures (>20,000°C) and pressures (>100 GPa) that will modify the magnetic minerals present in the target rocks. Impact increases (or decreases) magnetizations of the target rocks and causes variations to the magnetic field. The rock magnetic measurements carried out for the characterization of shocked and unshocked targets, impact products viz. impact melt rocks, impact spherules are NRM/χ, REM (=NRM/SIRM expressed in %), low and high temperature dependence of magnetic susceptibility (χ-T), magnetic hysteresis loop, IRM acquisition and backfield SIRM dc demagnetization, AF demagnetization spectra of NRM, Lowrie-Fuller (LF) test, and thermal demagnetization of SIRM (Table 2.2). The study of shock effects upon magnetic properties is necessary to determine how sensitive these magnetic properties are to transient stress, how unique to shock the magnetic effects are, and how natural materials vary in their magnetic responses to shock. It is necessary to establish the sensitivity of the NRM of rocks to transient stress to understand the palaeomagnetic record of shocked rocks. The results of laboratory shock experiments reveal that the primary effects of impact are demagnetization or remagnetization, and magnetic hardening (Gattacceca et al., 2007). Experimental studies have shown that shock pressures of the order of 1 GPa can remove the existing remanent magnetizations (Cisowski and Fuller, 1978). At pressures of >10 GPa, shocks can also permanently modify the intrinsic magnetic properties of rocks, including saturation isothermal remanent magnetization (SIRM), coercivity (Hc), susceptibility (χ), and anisotropy of susceptibility and remanence (Gattacceca et al. 2007; Gilder and Le Goff 2008; Nishioka et al., 2007). The effects of shock metamorphism can also aid in the production and modification of magnetic carriers; for example, at pressures of >40 GPa and T>1000°C,

amphibole and biotite decompose to produce magnetite. At lower pressures, titanomagnetite can result from the breakdown of ilmenite (Chao, 1968). Carporzen et al. (2005) showed that rocks such as the dykes and pseudotachylites from Vredefort structure, South Africa, carrying thermoremanent magnetization (TRM) are acquired during cooling after the impact was magnetized coherently with small scatter between samples. In contrast, shocked granitoid basement rocks are magnetized randomly. These shocked rocks also have anomalously high intensities of magnetization. This magnetization of the shocked rocks is interpreted to have been acquired in the intense fields of the plasmas generated by the shock event. The anisotropy of these samples was also determined and the minimum

susceptibility axes (K3) was found to be randomly oriented. In individual samples the anisotropy was related to the remanence direction, indicating that both were caused by the shock events. Recently, Carporzen et al. (2012) suggested that this unusual magnetization of Vredefort rocks are the products of recent lightning strikes (lightning induced remanent magnetization, LIRM) from palaeomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes. Louzada et al. (2008) observed subtle effects of shock hardening in titanomagnetite- bearing ejecta basalt at Lonar crater. The shocked and unshocked basalt samples from Lonar crater are very difficult to distinguish only by petrography except the presence of some fractures in plagioclase phenocrysts in the former. The Lonar rock magnetic properties were undertaken to observe changes in bulk-

coercivity, squareness of hysteresis (Mrs/Ms), and low and high temperature susceptibility (χ-T) measurements. The variations in rock magnetic properties of shocked and unshocked target rocks were characterized with reference to the direction of impact. The observed changes in the rock magnetic properties of shocked basalts are related to either sub-microscopic changes in the domain state of the titanomagnetite grains (movement from PSD towards SD state), or modifications in the crystalline structure of titanomagnetite grains (namely the microfractures, lattice defects or dislocations) due to impact shock. Palaeomagnetic measurements: Palaeomagnetism of meteorite impact structures is useful to understand the shock-induced magnetization (remanence) effects in magnetic minerals to (1) test the hypothesis of shock remanent magnetization (Rao and Bhalla, 1984), (2) find the evidence of impact-amplified and/or impact-generated magnetic fields (Srnka et al., 1979; Crawford and Schultz, 1988), (3) identify potential magnetic shock indicators (Cisowski and Fuller, 1978), 4) 41

determine the palaeomagnetic pole, and 5) check the multi-remanence magnetization components in rocks with the direction of impact. The involvement of high temperatures and pressures during the impact event results in melting and/or vaporization of the target, and the destruction of any primary remanence. Basically two mechanisms are responsible to demagnetize the target rocks at the impact site: (1) thermal demagnetization, and (2) shock demagnetization. Thermal demagnetization operates by heating the rocks above their Curie temperature. If the rocks are exposed to a magnetic field during their cooling, they get remagnetized in the direction of that field (NRM). On the other hand, shock pressures of ~1 GPa are enough to demagnetize any NRM in the rocks (shock demagnetization). However, if the shock is applied under the presence of a magnetic field, an additional component of remanence parallel to the ambient field is recorded (shock remanent magnetization, SRM). Near the impact point, the impact energy is high enough to melt or vaporize the crust, destroying any primary magnetic remanence in rocks. Here thermal demagnetization (or acquisition) effects likely dominate over pressure modification of magnetic remanence, in particular for ferrimagnetic minerals with low Curie temperatures. With increasing distance from the impact point, r, shock pressures (and temperatures) decrease as 1/r1.5 to 1/r3 (the decay constant depends on the impact velocity) (Melosh, 1989, p. 62). Beyond the crater rim, where the shock has decayed to below a few GPa, the stress wave is elastic and pressure decreases as 1/r. In this region, shock heating is no longer substantial enough to affect the magnetic remanence of the rocks. In the absence of a magnetic field, low shock pressures (≤ a few GPa) are known to demagnetize the magnetic rocks and minerals (e.g. Borradaile, 1993; Borradaile and Jackson, 1993). In the presence of an ambient field, compression at low pressures may result in the acquisition of SRM (Gattacceca et al., 2008; Srnka et al., 1979). The ambient field at the time of impact may be transiently produced (Crawford and Schultz, 1988) or amplified (Hood and Artemieva, 2008) by the impact itself which may lead to nonunidirectional SRM (Crawford and Schultz, 1988; Srnka et al., 1979). These transient fields are preserved only in much larger craters, where they record either in shock-produced grains or as a thermoremanent magnetization (TRM) in extensive melt sheets (Louzada et al., 2008). The efficiency of SRM is significantly less than that of TRM. SRM is also more susceptible to viscous decay, and may not be stable over geologic time (Gattacceca et al., 2007). 42

Demagnetization techniques: Demagnetization techniques are either alternating field (AF) or thermal (TH) demagnetization techniques, both of which can be used to resolve the multi- remanence components which is primary and which is secondary. These techniques were used because components acquired by differing mechanisms will usually have contrasting coercivity spectra and blocking temperature. AF treatment relates to the coercivities of magnetic minerals. In AF demagnetization technique, a specimen is exposed to progressively increasing alternating magnetic field. The waveform is sinusoidal with a linear decrease of magnitude with time. AF demagnetization can be used to erase NRM carried by grains with coercivities less than the used peak demagnetizing field. The AF technique is a rather fast, cleaning procedure compared to the thermal demagnetization technique. Magnetic grains can also be demagnetized also by thermal treatment. When performing stepwise thermal (TH) demagnetization the samples are heated to elevated temperatures below and around the Curie temperatures of ferromagnetic minerals in steps of 50°C or 100°C and then cooled back to room temperature in zero magnetic field. This causes all the magnetic grains with blocking temperatures (Tb) less than the applied temperature to lose that part of their NRM. After each temperature step, the remaining magnetization and also the susceptibility was measured. The advantage of using thermal treatment is that we can also obtain the magnetic mineral carrying the remanence by their unblocking temperature, which relates to the Curie temperature. The main disadvantage is that the magnetic grains might be oxidized during the heating process, which changes their magnetic properties and thus disturb the magnetic analysis. It is necessary to use both AF and TH techniques and compare the results. Some specimens will be progressively demagnetized using the AF while other specimens using only the TH technique. The overall objective is to reveal the NRM components which are carried by ferromagnetic grains within a particular interval of coercivity or blocking temperature spectra. The basic measurement of NRM yields the fossil remanent magnetization recorded in rocks (declination, inclination, and total intensity). The samples were then subjected to AF and thermal demagnetization cleaning techniques to isolate the stable magnetization (ChRM) components. In the present study, the samples were AF demagnetized in 14 steps following a sequence 2.5, 5, 7.5 10, 12.5, 15, 17.5, 20, 25, 30, 40, 60, 80, 100 mT respectively. The typical dataset of AF demagnetization is given in Table 2.4. 43

Table 2.4: Alternating field (AF) demagnetization dataset

Sample : LN10N.5.2 (Azimuth, Hade) : (305, 23) AF (mT) Dec (°) Inc (°) F (A/m) Norm. F NRM 98.7 46.4 4.32 1.00 2.5 99.0 46.3 4.30 1.00 5.0 100.8 43.6 4.24 0.98 7.5 102.9 41.3 4.14 0.95 10.0 106.9 39.1 4.07 0.94 12.5 107.6 37.4 3.99 0.92

15.0 111.3 36.8 3.91 0.91 17.5 114.0 35.3 3.84 0.89 20 114.6 34.9 3.79 0.88 25 118.3 34.4 3.61 0.84 30 119.3 33.6 3.41 0.79 40 123.9 32.4 2.99 0.69 60 123.2 30.4 2.24 0.52 80 124.6 29.2 1.66 0.38 100 123.4 28.1 1.24 0.29

The thermal demagnetization was done on some selected samples in a sequence of 50, 100, 150, 200, 225, 235, 250, 275, 300, 310, 325, 340, 355, 375, 400, 420, 440, 460, 480, 505, 520, 540, 560, 580, 600, 630, 650, and 680°C respectively. The typical dataset of thermal demagnetization is given in Table 2.5. Analysis of remanent magnetization components: Isolation of different remanence components were analyzed by principal component analysis (Kirschvink, 1980), guided by visual inspection of orthogonal demagnetization plot (Zijderveld, 1967). The demagnetization data (both AF and thermal) were analyzed with AGICO’s Remasoft3.0 program (Chadima and Hrouda 2006). The software plots the standard orthogonal vector plot of Zijderveld, equal area stereoplot of directions, and a decay of intensity in the course of demagnetization steps (Fig. 2.5). Observing the NRM intensity decay behavior is a simple way to approximately specify the magnetic minerals and their coercivities. Low coercivity minerals (e.g. MD magnetite) tend to decay to zero rapidly while high coercivity minerals (e.g. hematite, goethite) will not reach zero magnetization even with applied AF maximum field. Equal area stereographic projections plot declination and inclination of every demagnetization step showing the progress of magnetic directions and components.

Table 2.5: Thermal (TH) demagnetization dataset

Sample : LN16.4.2 (Azimuth, Hade) : (332,79)

T (°C) Dec (°) Inc (°) F (A/m) Norm. F NRM 127.9 50.2 3.21 1.01 50 130.5 50.3 3.16 0.98 100 128 47.5 3.31 1.03 150 132.3 42.1 3.84 1.20 200 134.3 39.5 4.08 1.27 225 133 38.7 4.18 1.30

235 133.3 38.1 3.97 1.24 250 132.6 37.9 3.75 1.17 275 133.9 38.1 3.11 0.97 300 132.7 38.5 1.78 0.55

310 132.8 39.8 1.42 0.44 325 131.8 40.4 1.33 0.41 340 131.8 40.3 1.25 0.39 355 130.8 39.1 1.27 0.40 375 131.8 38.9 1.29 0.40 400 132.2 39.7 1.27 0.40 420 133.2 38.4 1.26 0.39

440 131.8 39.2 1.23 0.38 460 134.3 39.1 1.18 0.37 480 132.2 38.6 1.14 0.36 505 132.6 39 0.96 0.30 520 131.9 38 0.96 0.30 540 131.7 39 0.78 0.24 560 131.9 40 0.48 0.15 580 128 40.5 0.38 0.12 600 131.3 38.9 0.38 0.12 630 132.1 40.4 0.10 0.03 650 129.5 43.2 0.08 0.02 680 313.3 -10.9 0.02 0.01

Figure 2.5: Example represents the display of magnetic directions of one of the selected pilot specimen (Ln10N-5-2) from the Lonar western sector (W) crater rim.

The Zijderveld plot is used to visualize a three dimensional demagnetization data on a set of two projections of the vectors. The power of Zijderveld plot is its ability to display directional and intensity information on a single diagram by projecting the vector onto two orthogonal planes. The Zijderveld diagram projects the end point of the magnetization vector onto two planes simultaneously. Two planes are necessary because the vector is oriented in 3D-space. Any two orthogonal planes would do, but commonly the (geographic) horizontal and vertical planes 46 are chosen, such that the horizontal plane shows a component corresponding to the declination and the vertical plane shows the vertical (upward or downward) component, which gives an indication of the inclination. The horizontal and vertical projections have one horizontal axis in common, optimally the one to which the declination is closet. The successive end points obtained during demagnetization reflect the intensity of magnetization in their distance from the origin. From Zijderveld diagram, it is possible to distinguish the primary and secondary components present in an NRM direction along with their individual intensities and directions. However such distinction is possible provided that demagnetization procedure gives rise to Zijderveld diagrams consisting of only straight line segments. On the other hand if the diagrams show curves it means that the direction undergoes a change at every demagnetization step, and difficult to pick out any stable direction. Observing a linear trajectory of the vector end point toward the origin is a key to recognizing that a high-stability NRM (ChRM) component has been isolated. Principal component analysis (Kirschvink, 1980) determines palaeomagnetic components as a best-fit line through scattered observation points, usually minimum of 3 steps with maximum angular deviation (MAD)<6°. Mean remanence directions of best-fit lines are then calculated using Fisher (1953) statistics for a one specimen. Site mean directions are achieved when minimum of 3 specimens has the same components. Calculations are continued to component level, where all the different sites show the same direction. Using this analysis method, different palaeomagnetic components can be determined from demagnetization data. Determination of palaeomagnetic direction and pole: A mean direction of ChRM of the specimens for a site is a record of the past geomagnetic field direction during time when it was acquired. The determined pole, through an analysis of specimens (Fisher, 1953) from a certain site, is subsequently called a virtual geomagnetic pole (VGP). VGP is the position of pole of a geocentric dipole that can account for the observed magnetic field direction at one location and one point of time. For each continent, the apparent polar wander path (APWP) has sequential positions of palaeomagnetic poles. Due to the fact that Earth’s magnetic field changes its polarity it is not possible to know whether the studied site area was in the northern or in the southern hemisphere.

2.3 INSTRUMENTATION DETAILS Low-field (300 Am−1 at 875 Hz) AMS was carried out using an AGICO (Czech Republic) KLY- 4S Kappabridge for each specimen with measurement in 64 directions on three mutually orthogonal planes using an automatic rotator sample holder (spinning specimen method). NRM and saturation isothermal remanent magnetization (SIRM) were measured with either Molspin or JR-6A spinner magnetometers (Table 2.2). For palaeomagnetic directional analysis, samples were subjected to an AF (up to 100 mT in ~15 discrete steps) and thermal (up to 680°C in ~26 discrete steps) demagnetization analyses using an ASC D2000 AF and MMTD80 thermal demagnetizer. The anhysteretic remanent magnetization (ARM) was induced in a 0.05 mT bias dc field and a 100 mT peak alternating field using a D-2000 alternating field demagnetizer (D- Tech. Inc., USA) with a decay rate of 0.01 mT/cycle. IRM was imparted in progressively increasing magnetic fields up to 1 T using a Molspin pulse magnetizer. The measurement of temperature dependence (-196°C to 700C) of magnetic susceptibility was carried out using an AGICO KLY-4S Kappabridge attached with CS-3/CS-L furnace system in an argon atmosphere to determine the magnetic transitions, such as the Verwey/Curie temperature of ferrimagnetic phases. Room temperature hysteresis loops were measured using a Molspin NUVO VSM in an alternating field cycling between 1 T. Major oxides and elements were determined using an Ametek SPECTRO-XEPOS X-ray fluorescence (XRF) spectrometer (Turboquant-powders method). X-ray diffraction patterns were obtained for ball-milled powdered samples using the inXitu BTX Benchtop X-ray diffractometer and diffractograms were analyzed with XPowder software using ICDD PDF2 database. Morphology of impact spherules were observed under a TESCAN VEGA3 SB scanning electron microscope (SEM).

2.4 LABORATORY SHOCK EXPERIMENTAL STUDIES Natural impact cratering processes were interpreted based on the observations of hypervelocity laboratory shock experimental studies. Shock waves generated during impacts can modify both intrinsic magnetic properties and remanent magnetization of rocks. Consequently, the magnetic record of solid bodies in the solar system, affected by impacts to different degrees, could have been erased or overprinted by shock events. Concerning the Earth, shock-induced changes in rock magnetic properties and magnetic remanence should be taken into consideration while studying the remanent magnetization of terrestrial impacts. 48

Different authors have carried out experimental investigations of shock demagnetization (remagnetization) of rocks and pure minerals in the 1–30 GPa peak pressure range. Different techniques have been used for shock waves generation: air or gas gun accelerating Al or Cu projectiles; high explosive and nuclear; free falling mass and pulsed laser. The main caveats of such experiments are the complexity of dynamic pressure calibration, the possible mechanical damages of investigated samples, and deciphering of the effect of deviatoric versus hydrostatic stresses. Indeed, it is known that remanent magnetization is more sensitive to non-hydrostatic (deviatoric) than hydrostatic stresses. Moreover, shock may permanently modify the intrinsic magnetic properties (e.g. coercivity) thus complicating the interpretation. Numerous parameters must be considered when studying the effect of shock on the magnetic remanence: shock intensity and duration, background magnetic field during the shock event, magnetic mineralogy, pre-shock magnetization and temperature. These parameters are often complicate the comprehension of shock effect on rock magnetic remanence. Static pressure experiments are well suited to tackle these problems, allowing better pressure calibration and can be non- destructive for samples as demonstrated by Bezaeva et al., (2010).

CHAPTER 3

STRUCTURAL AND ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS)

3.1 INTRODUCTION The chapter aims in evaluating the angle and trajectory (direction) of the chondritic projectile (Misra et al., 2009) that struck at Lonar at 570±47 ka ago (Jourdan et al., 2011). The task is accomplished with the study of geologic structural analysis and satellite images, observation of ejecta distribution pattern, and low-field Anisotropy of Magnetic Susceptibility (AMS) technique. The chapter also discusses the propagation of impact- induced stress with the direction of impact using the AMS technique. The chapter describes the geological setting of the Lonar crater, target rock and the number of Deccan flows exposed at Lonar, nature of the ejecta distribution, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat images, regional structural analysis, and low-field AMS technique to evaluate the angle and direction of projectile approach. The distribution of impact-induced shock pressure with the direction of impact is also evaluated using the AMS technique.

3.2 GEOLOGICAL SETTING OF LONAR CRATER The Lonar crater (19o58'N, 76o31'E) (Fig. 3.1a) is a simple, bowl-shaped, near-circular impact crater (Kumar, 2005) having maximum and minimum diameters of ~1875 and 1787 m respectively, with a circularity of ~0.90 and a depth of ~150 m (Fredriksson et al., 1973; Fudali et al., 1980). The crater is situated in central India in a semi-arid region of low annual rainfall of ~60-80 cm/yr (IMD, 2007). According to Fudali et al. (1980), around its circumference, except for a small sector in the NE, there is a continuous rim raised ~30 m above the adjacent plains, whereas the crater floor lies ~90 m below the preimpact surface. The rim is surrounded in all directions by a continuous ejecta deposit that extends outward with a very gentle slope of 2o–6o to an average distance of ~1350 m from the crater rim. The ejecta deposit consists of angular blocks of basalt of variable sizes within a matrix of fine ejecta; the maximum size of blocks is 5 m.

The interior of the crater is occupied by a shallow saline lake; below the lake water a sequence of ~100 m thick unconsolidated sediment is reported that overlies the base of the crater made up of highly weathered Deccan Trap basalt (Nandy and Deo, 1961; Fudali et al., 1980). About 700 m north of the rim, there is another relatively shallow depression known as the Little Lonar (Fig. 3.1a), which has a diameter of ~300 m and is surrounded by a raised rim only along its southern and western margins. Master (1999) suggested that the Lonar crater and the Little Lonar might have been formed together by near-simultaneous double impact of fragments of the same bolide but drilling into the structure revealed no evidence for this (Fredriksson, personal communication 1999; Maloof et al., 2010) and no meteorites have been found associated with the Little Lonar structure. The most recent comprehensive geological map of the Lonar crater was presented by Maloof et al. (2010). Recently, Jourdan et al. (2011) obtain a precise and accurate isotopic age (40Ar/39Ar) of 570 ± 47 ka for the Lonar impact event from four basaltic impact melt rocks. Earlier the fission track, thermoluminescence, and radiocarbon dating of impactites yielded a wide range of dates ranging from ca. 15 to ca. 62 ka (Sengupta et al., 1997; Storzer and Koeberl, 2004), thus illustrating the complexity of dating the Lonar impact crater. For example, radiocarbon dating of Lonar lake sediments yielded ages ranging from ca. 15 to 30 ka, but this range likely represents minimum ages due to carbon contamination (Sengupta et al., 1997). Fission track data yielded an apparent age of 15 ± 13 ka, most likely indicating the age of post-impact processes such as (1) a younger thermal event (Storzer and Koeberl, 2004), (2) alteration, or (3) thermal annealing due to high soil temperature induced by wildfires (or possibly sunlight exposure). The 14C dating of histosols containing the Lonar crater ejecta yielded apparent ages ranging from 1.8 ± 0.5 to 40.8 ± 1.1 ka (Maloof et al., 2010). These dates were reported as positively correlated with δ13C, where such correlation is generally interpreted as demonstrating modern C contamination. Maloof et al. (2010) proposed a maximum age estimate for Lonar of younger than 12 ka, based on the paleosols positions in relation to the Lonar ejecta.

(a) (b)

(b)

Figure 3.1: (a) Gray-scale Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image of the Lonar crater, India, at 15-m resolution. Note the circularity of the crater rim and presence of a continuous zone of high reflectivity around it, which represents the ejecta. Symbols: 1—Lonar village, 2—Lonar lake, 3—Little Lonar, 4—crater rim, 5—extension of continuous ejecta deposit, 6 and 6a—faults running across crater, 7—upper basalt flows having similar reflectivity to that of ejecta and small hillocks around crater, 8—artificial reservoir (Kalapani dam), 9—tarred roads, 10—black paleosol, 11—vegetation. The geographic location of crater center: 19°58′35.37″N, 76°30′29.37″E (source: Google Earth). (b) Topographic map of the Lonar crater prepared from Shuttle Radar Topographic Mission (SRTM) digital elevation model (DEM) image with a contour interval of 5 m and image in background. The darkest and lightest gray tones represent the lowest and highest elevation in this area (475, 560, 580, and 600 m contours shown by thick lines), legend as in Figure 3.1a; note deformation of contours in the NE crater rim due to faulting. (c) Geometric map of the Lonar crater based on the ASTER image and the aerial photograph. Abbreviations: c and c —centers of circle and ellipse drawn on the crater rim c e and maximum extension of the ejecta around the crater, respectively; F1F1′ and F2F2′— faults, Cc—circularity of crater rim (after Pike, 1974), Ce and Ee—circularity and eccentricities of ellipse at maximum extension of ejecta. (d) Sketch map of the Lonar crater showing position of drilling sites (filled gray circles) for samples for anisotropy of magnetic susceptibility (AMS) study; RH—PWD (Public Works Department) rest house; sampling site of I. Nishioka (2007) is also shown.

The target rocks at the Lonar crater are sub-horizontal Deccan basalt flows overlying a Precambrian basement (Fudali et al., 1980). The exact thickness of these basalt flows is not clearly known. Fudali et al. (1980) suggested a thickness of 600-700 m. A thickness of basalt flows more than 350 m in the Lonar region (Kumar, 2005) seems to be more reasonable. This is because drilling at the 1993 Latur earthquake site, which is located ~150 km south of the Lonar, suggests a thickness of the basalt sequence of ~350 m in this area that overlies on the Archaean granites and gneisses (Gupta et al., 1999). As several studies indicate a northward increase of thickness of Deccan basalt (Mitchell and Widdowson, 1991), an estimated thickness of basalt flows more than 350 m in the Lonar area is more reasonable. These basalts contain intertrappean sediments of small areal extent of fluviatile and lacustrine origin that consist of chert, sandstone and impure limestone of varied thickness from a few centimeters to 3 m (Jhingran and Rao, 1958; Venkatesh, 1967; Krishnan, 1968). Ghosh and Bhaduri (2003) worked on the stratigraphy of basalt flows exposed at and around the Lonar crater. There are six flows of ~8 to 40 m thickness, the four bottom flows of which are only exposed along the crater wall. All flows are described as ‘a’ā–type flows except the flow at the bottom, which is pahoehoe type. The two topmost flows occur away from the crater and do not show any impact-induced deformation. Each flow is separated from the next one by a discontinuous marker horizon with red and green paleosols, chilled and vesicular margins, and vugs filled with secondary chlorite, zeolite, quartz, and limonite. In addition, chilled bottoms and brecciated tops with vesicular fillings characterize most of the flows. Fresh, dense basalts occur only in the upper ~50 m of the crater wall whereas below this level the flows are heavily weathered and friable (Fudali et al., 1980). The preimpact black, sticky, humus-rich soil of ~5 to 90 cm thickness is still preserved at places between flows and below ejecta (Ghosh and Bhaduri, 2003). Though exposures of four basalt flows along the crater wall were described, we have found that individual flows are difficult to distinguish in many cases due to thick overburden or talus, vegetation and surface weathering features. The weathered surface along the crater wall forms a slope of, on average, 26o (Fudali et al., 1980) but the slope plane is mostly inaccessible now. All basalt flows have a common mineralogy and texture, except for some minor differences in abundance of plagioclase phenocrysts, glass, and opaques (Ghosh and Bhaduri, 2003). The basalts are porphyritic with occasional phenocrysts of plagioclase and rare olivine, which are set in a groundmass of plagioclase, augite, pigeonite, titanomagnetite,

palagonite and secondary minerals (calcite, zeolite, chlorite, serpentine and chlorophaeite). The Lonar basalts are relatively low-K tholeiitic within-plate basalts that are marginally enriched in Fe and Ca, and depleted in Mg and Al compared to the average tholeiites of Irvine and Baragar (1971); they show limited compositional variation, except for some alkali elements, Cr and Ba, and some volatile elements, which show wider variation (Osae et al., 2005). Early pioneering studies of the shock petrography of the Lonar crater basalts were conducted by Schaal (1975) and Kieffer et al. (1976). These workers noted that the main shock effects are the conversion of plagioclase to diaplectic glass (maskelynite; Nayak, 1993) and vesiculated feldspar glass, and undulatory extinction in pyroxene. Shock effects on the palaeomagnetism of Lonar Crater (Cisowski, 1975; Rao and Bhalla, 1984; Louzada et al., 2008) are subtle and consistent with low to moderate shock levels. At the surface, the most dramatic evidence for the impact shock is the presence of impact spherules. Around the eastern and western rim of Lonar crater, Nayak (1972) identified sculpted, vesicular impact glasses with diameters 0.1–4 cm and densities 1.32–2.65 gcm–3. Fredriksson et al. (1973) also found small (0.1–3 mm) flow-banded, teardrop-shaped spherules and larger (10- to 15- cm-diameter) pieces that wrap around underlying clasts like the Flädle of Ries crater, Germany. Sengupta et al. (1997) describe spherule-rich layers 5 cm below modern alluvium from trenches along the western and southeastern crater rim.

3.3 HOW SPECIAL IS THE LONAR CRATER? The Lonar crater (Fig. 3.1a) is always a special attraction to the planetary scientists because it is one of the few among ~184 known terrestrial asteroid impact craters (Earth Impact Database, 2013) that is completely excavated on the basaltic target rocks and fully accessible (Gilbert, 1896; La Fond and Dietz, 1964; Nayak, 1972; Fredriksson et al., 1973; Kieffer et al., 1976; Morgan, 1978; Stroube et al., 1978; Fredriksson et al., 1979; Fudali et al., 1980; Rao and Bhalla, 1984; Ghosh, 2003; Ghosh and Bhaduri, 2003; Hagerty and Newsom, 2003; Kumar, 2005; Osae et al., 2005; Son and Koeberl, 2007). The other one is the Logancha crater, Russia, which could be on the Siberian Trap basalts (Reichow et al., 2002) but little information is available on this crater (Feldman et al., 1983; Masaitis, 1999). The most recently known impact structure in a basaltic target is the Vista Alegre crater on the Paraná flood basalt (~133-132 Ma), Brazil (Crosta et al., 2010). The Lonar crater is,

therefore, one of the few known terrestrial analogues available for evaluating the consequences of hypervelocity asteroid impacts on planetary surfaces having basaltic crusts. Although the Lonar crater has been known for more than one hundred years (Gilbert, 1896), no clear idea exists either on the nature of impactor or on the projectile path of this crater. The most recent geochemical studies on siderophile elements (Cr, Fe, Co, Ni) of sub- mm−sized impact spherules recovered from the ejecta suggest that the possible impactor might be a chondrite (Misra et al., 2009). Studies of the geologic structure are important in understanding the stress, strain, and strain rate in and around the impact craters (Grieve and Robertson, 1976; Brandt and Reimold, 1995; Dressler and Sharpton, 1997; Reimold et al., 1998a; Bjǿrnerud, 1998; Christeson et al., 2001; Lemieux et al., 2003; Sagy et al., 2004); however, no systematic study on the rock deformations along the rim of the Lonar crater has been undertaken. The only structural analysis carried out by Kumar (2005) has documented impact-induced deformational structures on the upper 30 m of the Lonar crater wall. He has identified four fracture systems—i.e., flow-parallel fracture; radial fracture; and concentric and conical fractures. The plumose and slump structures are common on the fracture planes and on the inner crater wall, respectively. The uplift and tilting of the basalt sequence and formation of the fracture inside the crater are suggested to have formed by an impact event and are different from the pre-impact, cooling-related columnar joints and tectonic fractures of the target Deccan basalts. The deep notch that crosscuts the NE crater rim (Fig. 3.1a) is attributed to a listric faulting that displaced the flows in the inner wall. It is suggested that the impact structures of the Lonar crater are broadly similar to those of the other simple terrestrial craters in granites and clastic sedimentary rocks and to the small-scale experimental craters in gabbro targets (e.g., Polanskey and Ahrens, 1990). Deccan Traps: The Deccan Traps are one of the largest volcanic provinces in the world, consisting of more than 2-km-thick flat-lying basalt lava flows covering an area of nearly 500,000 km2 in west-central India; estimates of the original area covered by the lava flows are as high as 1.5 million km2, and the volume of basalt is estimated at ~0.5 million km3 (e.g., Mahoney, 1988; Cox and Hawkesworth, 1985; Widdowson et al., 2000). The target Deccan basalt of the Lonar crater erupted close to the Cretaceous-Tertiary (KT) boundary at 65±0.9 Ma (Hofmann et al., 2000; also see Courtillot et al., 2000) or close to 67.4 Ma (Pande et al., 2004), although some controversy exists regarding the duration of this flood basalt volcanism (Courtillot and Renne, 2003; Wignall, 2001). The more recent idea

suggests that the first extensive phase of the Deccan volcanism, which might have lasted only a few hundred thousand years, occurred at ca. 67.5 Ma at the northern half of the present Deccan outcrops and after ca. 2.5 Ma of quiescence, the second major phase of volcanism occurred at ca. 65 Ma (Chenet et al., 2007).

3.4 NATURE OF EJECTA DISTRIBUTION Impacts have created enormous scars on the surfaces of nearly all solar system bodies, prompting Shoemaker (1977) to state that “impact of solid bodies is the most fundamental process that has taken place on the terrestrial planets”. Impact cratering processes has been well understood in the past decades by both experimental and theoretical perspectives. Laboratory-scale experiments were carried out for clarifying the aspects of crater excavation and ejecta emplacement. Scaling laws were derived to describe impact crater size, shape, depth, ejecta, and melt deposits as functions of impact speed and impactor size and type. The one parameter that has often been neglected in the study of impact craters is the angle of impact. It is well known that impact events normally strike planetary surfaces at an angle from the surface, and probability theory indicates that, with the assumption of an isotropic flux of impactors, the most likely angle of impact is 45°, regardless of the body’s gravitational field. Vertical and grazing impacts are rare. Despite the circularity of crater rims, signatures of oblique impacts are most readily visible in the shape of the ejecta blanket. Experimental work carried out by Gault & Wedekind (1978) found that for angles <30°, steeper interior slopes formed on the uprange wall of the craters. The ejecta deposits exhibit axial symmetry for impact angles down to at least 45°. As the impact angle decreases below 45°, however, ejecta deposits become asymmetric, and “forbidden” azimuthal zones appear for impact angles less than about 30°, first uprange, and then downrange of the crater. A characteristic “butterfly wing” pattern of ejecta develops in very oblique (<5°) impacts, in which most of the ejecta is thrown out perpendicular to the projectile’s path. Recent experiments by Schultz (1999) on the ejecta distribution from oblique impacts in particulate targets at low impact velocities (1–1.5 km/s) show a focusing of high-velocity ejecta in the downrange direction for a 30° impact, whereas the low-velocity ejecta is distributed more evenly around the crater. Melt produced by the impact is affected by impact angle as well and shows a pronounced downrange focusing for angles less than 45°.

Since the work of Gault and Wedekind (1978), the characteristic bilateral symmetry of the ejecta around craters has become the diagnostic feature for the recognition of craters formed by oblique impacts on planetary surfaces; it also provides a criterion for determining the direction of approach of the impactor. More recently, Dahl and Schultz (2001) measured the asymmetry of shock waves in oblique impact experiments, revealing an elevation of shock pressures downrange. Ejecta distribution at Lonar: Different views also exist on the distribution of quaquaversal and overturned dips of basalt flows observed around the Lonar crater rim. It is known that basalt flows are either overturned near the rim crest (Fredriksson et al., 1973) or that the bedrock in most of the rim-wall dips gently (8o–20o) away from the crater, except in some patches where the bedrock is overturned and approximately parallel to the rim-wall slope (Fudali et al., 1980). The distribution of ejecta around the crater rim is informative regarding the direction and obliquity of the impact (Pierazzo and Melosh, 2000; Herrick and Forsberg- Taylor, 2003), but no unanimous opinion exists on the distribution of ejecta around the rim of the Lonar crater (Fredriksson et al., 1973; Fudali et al., 1980; Maloof et al., 2005, 2007; Kumar, 2005). Ghosh (2003), assuming a cometary impactor of ~90 m diameter and ~500 kiloton mass, suggested a direction of impact from the NW but provided no supporting data for this estimation. Therefore, in the present study we used an ASTER image (LPDAAC, 2007) of the crater and its surroundings in conjunction with field observation to evaluate ejecta distribution around the rim of Lonar crater and to investigate the deformation of basalt flows around the crater rim, aiming at identification of the possible path of the impactor. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image of Lonar region: Multi-spectral Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery, acquired on April 8, 2003 and January 21, 2004 at 11.09 a.m. (IST) (LPDAAC, 2007), was used to investigate the shape of the Lonar crater and distribution of ejecta around it (Fig. 3.1a). The ASTER is the Earth’s first spaceborne multispectral Thermal Infrared Radiometer (TIR) instrument that has recently been used to obtain and interpret high spatial resolution images of terrestrial geological features including asteroid impact craters (e.g., Rowan et al., 2005; Wright and Ramsay, 2006). The Lonar crater was also examined with a panchromatic Landsat 7 image of 15-m spatial resolution (Misra et al., 2006a). The advantage of the ASTER image is that it produces a better image in the same spatial resolution (15 m) in the Visible and Near-Infrared Radiometer (VNIR)

bands and better spectral resolution in the Short Wave Infrared Radiometer (SWIR) (6 bands) and Thermal Infrared Radiometer (TIR) bands (5 bands) as well. A higher number of SWIR and TIR bands also provide important information on mineral composition on land surfaces (Vaughan et al., 2005). Two scenes captured on different dates (see above) were obtained to minimize the effect of the sun angle and enhanced the shape of the Lonar crater. For better understanding of the ejecta distribution around the crater rim, a false-color composite Landsat 7 image (GLCF, 2007) was merged with the panchromatic band by the principal component resolution merging technique to produce a color composite image of 15-m spatial resolution. Additionally, a contour map of the Lonar crater in grey tone with maximum and minimum contours of 600 and 475 m was also produced using Shuttle Radar Topographic Mission (SRTM) data of 90-m spatial resolution with a contour interval of 5 m (Fig. 3.1b). The darkest and lightest grey tones on this map represent the lowest and highest elevations respectively. In general, the contours gently slope away from the crater rim in all directions. The 580- and 560-m contours are situated very close to the crater rim in the southwest and far from it in the northeast. Fudali et al. (1980) suggested that this configuration resulted during pre-impact erosion and has no relation to the asteroid impact. According to their observation, the pre-impact surface must have been very similar to the present surface beyond the crater’s ejecta deposits, which is a sub-horizontal plain with a few meters of rolling relief cut by gullies 2–3 m deep. The upper-most basalt flow in this region is overlain in most places by a dense, structureless, black, clayey soil up to 2 m thick. This soil is exposed beneath the outer portions of the ejecta blanket in several gullies south of the crater (this feature is also observed in the present study to the west of the crater; Fig. 3.2a). Thus it definitely predates the crater, demonstrating that the surface configuration has not changed much since the cratering event. The crater wall exhibits steep slopes all around the circumference of the crater (cf. Fudali et al., 1980). Study of false-color ASTER (Fig. 3.1a, gray scale image only shown) and Landsat images, and observations on aerial photographs (Fredriksson et al., 1973) show that the rim of the Lonar crater is almost circular (Fig. 3.1c) with a circularity (Pike, 1975) of ~0.95 (cf. Fudali et al., 1980). It has a relatively long N-S axis of ~1832 m and an E-W axis of ~1790 m. As the depth of the crater from its rim crest was estimated by Fredriksson et al. (1973) and Fudali et al. (1980) of ~222 m; the estimated depth/diameter ratio of this crater is ~0.12. The rim of the crater is surrounded by a wide continuous zone, which has a reflectivity that

is higher than that of the underlying black soil that rests on the Deccan basalt flows. The ground studies confirm that this bright zone represents the ejecta that are presently lying under the Lonar village in the northeast. As the reflectivities of both ejecta and topmost basalt flows are very similar in remote sensing images (Fig. 3.1a, see Misra et al., 2006a), field work was conducted to demarcate the extent of continuous ejecta around the crater rim. Field observation suggests that the high reflectivity areas to the southeast, east, and northeast of the crater, which are partly or completely separated from the continuous ejecta by black soil of low reflectivity (Fig. 3.1a, unit 7), are hillocks formed by basalt that do not contain any ejecta. Anthropogenic activity, such as construction, that can affect the ejecta distribution is restricted mostly to the northeast of the crater around the Lonar village. The area to the west and south are either barren land or covered by agricultural fields, both on the outer slope of the crater rim and on the surrounding lands. Although there are various opinions on actual extension of ejecta around the Lonar crater rim (Fredriksson et al., 1973; Fudali et al., 1980; Maloof et al., 2005, 2007; Kumar, 2005), most of the continuous ejecta can be enveloped by an ellipse with major and minor axes in the E-W and N-S directions respectively, and with a circularity of ~0.77 and an eccentricity of ~0.47 (Fig. 3.1c). Hence, the distribution of ejecta around the crater rim is not radially symmetrical. The reflectivity of the ejecta near its outer fringe in the west is relatively low due to vegetation cover. The ejecta covers an area of ~6.7 km2 and extends to a distance of ~700 m in all directions from the crater rim, except to the west where it extends to a little over 1 km. Both the crater rim and the enveloping ejecta ellipse have overlapping E-W axes, but the N-S axis of the latter is displaced by ~200 m to the west in comparison to the crater rim circle. Although it is possible to demarcate the distribution of the continuous ejecta around the Lonar crater rim, the question still arises whether this extension represents the original distribution of ejecta deposition produced by impact or is the final result of substantial modification by post-impact erosion. Fudali et al. (1980) established that the very gently outward sloping (2o−6o) ejecta had a somewhat hummocky surface, which appeared to be an original characteristic rather than the result of erosional modification. They also suggested that there had been remarkably little post-cratering erosion of the ejecta, presumably due to very gentle slopes and a stabilizing cover of vegetation. Grant (1999) concluded from his studies on the degradation signature of impact craters that the Lonar crater was close in morphology to the Arizona crater, USA, which retains a remarkably pristine form.

Figure 3.2: (a) Ejecta profile at Kalapani dam area at the SW of the Lonar crater. Soil horizon formed by erosion of ejecta at higher altitude [S] forms a cap on the distant ejecta [E] resting over paleosol [P]; chisel marking the boundary between top soil layer and ejecta cover; hammer head indicates the boundary between ejecta cover and underlying paleosol. The boundary between ejecta and underlying paleosol is less distinct due to CaCO3 veins leached from the ejecta cap during weathering. The height of exposure is ~1.6 m. (b) Sub-horizontal basalt flows at ~2 km ESE of the Lonar crater at Durga Tegri area showing semi-continuous flow-parallel fractures (shown by horizontal arrows) and less-common sub-vertical fractures (shown by vertical arrows pointing downward) in cross-section; total thickness of exposure is ~3.5 m. (c) Plan view of an exposure of shocked basalt on crater rim at SSE sector showing fracture cleavage; the attitude of cleavage is shown by a symbol, which dips towards the crater depression (note arrow). (d) Plan view of basalt on crater rim at the SSW sector showing a set of widely spaced fractures, arrow indicates direction of dip of fractures towards the crater depression; scale- hammer with length ~39 cm. (e) Cross-sectional view of a remnant basalt flow in the WSW sector showing overturned dip; hammer (~39 cm) showing attitude of flows; black arrow points to crater depression. (f) Basalt flow with flow-parallel cleavage at the WNW sector showing overturned dip (cross-sectional view); hammer is placed parallel to flow; arrow points to the crater depression. (g) Cross-sectional view of the ejecta at the north of the Lonar crater. Note an angular piece of shocked basalt within ejecta showing development of strong fracture cleavage due to impact (double-headed arrow shows trend of fracture). Hammer head indicates bottom of the section. (h) A piece of maskelynite from within ejecta showing development of a strong fracture cleavage (P) and a secondary weak cleavage (S) in cross section; coin (1.8 cm) scale.

In our present study, we have attempted to examine field evidence for erosion of ejecta around the Lonar crater. Because this crater is situated neither within the drainage area of any major river system nor in a zone of high rainfall (IMD, 2007), long-distance transportation of ejecta material away from crater rim is not very likely. To learn about the post-impact erosion, we also searched for those soil horizons that were formed by erosion of the ejecta and deposited along or near the periphery of ejecta blanket over the underlying paleosol. This soil horizon is only observed to the west of crater near Kalapani dam (19o57.856/N, 76o29.493/E) (Fig. 3.1a, unit 8). In this part of the study area, there is an exposure of a classic stratigraphic cross-section where a 1.47-m-thick, reddish-black paleosol, lying over the weathered trap basalt, is covered by an ~1 m thick layer of light- colored fine ejecta. This ejecta layer, containing a few cm-sized angular basalt pebbles, tapers away westward over a distance of ~7−10 m. On the continuation of this ejecta layer, there is a cover of a discontinuous layer of younger soil of maximum ~0.5 m thickness that has a similar reflectivity to that of ejecta and consists of eroded ejecta components only (Fig. 3.2a). A couple of meters farther west, the ejecta cover has tapered away, and the newly formed soil horizon directly overlies underlying paleosol. However, this stratigraphy is of local significance and does not persist farther to the west. The presence of a weaker soil horizon on ejecta deposits also suggests little erosion of ejecta in the post-impact regime. Therefore, the present extension of ejecta that is seen in the ASTER and Landsat images and on aerial photographs is thought to be close to original extent directly after impact. The deep vertical notch that transects the crater rim and the ejecta in northeastern sector of Lonar crater (Figs. 3.1a and 3.1b) has been suggested to be a normal fault where the dip of basalt flows in the foot wall is ~20o and in the hanging wall is 50o−70o (Kumar, 2005). A study of remote sensing images (also see Google Earth image) and aerial photograph (Fredriksson et al., 1973) together with field observations suggested that it was part of a NE- SW–trending lineament that is radial to the crater but crosscuts the crater rim both in the NE and SW (Figs. 3.1a and 3.1c). Interception of contours along the rim of crater at this lineament (Figs. 3.1a and 3.1b, Google Earth image) suggests post-impact movement along this fault. According to eye witnesses (S. Bugdani and also local people, 2005; personal communication) the movement along this lineament took place for the last time in October 1998. There is another less prominent E-W lineament that radially crosscuts the crater rim (Figs. 3.1a and 3.1c).

3.5 REGIONAL STRUCTURAL ANALYSIS OF LONAR CRATER For systematic structural analysis of the impact-induced tilts of basalt flows, the crater rim and adjoining areas were segmented into nine arbitrary sectors based on geographic directions (Fig. 3.3). The difference between the Lonar crater and other impact craters formed in sedimentary rocks (e.g., Meteor crater, USA, Shoemaker and Kieffer, 1974) is that the impact-induced tilt of target rock around the crater rim can easily be identified in the latter cases due to the presence of marker bedding planes. For the Lonar crater, the only markers that can be trusted for structural measurements are the subhorizontal flow layers in the undeformed target basalt. These layers are, however, less identifiable when the thickness of the layer is rather high (e.g., in the ~40 m thick fourth basalt flow, Ghosh and Bhaduri, 2003). Hence, besides flow layers, some additional criteria are needed for measuring the tilts of basalt flows.

Figure 3.3: Summary of orientations of basalt flows in different geographic sectors around the Lonar crater rim; RH—the PWD (Public Works Department) rest house; dashed hemispherical line in west shows zone of overturned dips around crater rim; structural attitudes written as follows: strike-and-dip amount, dip direction, n= number of data averaged.

The subhorizontal Deccan basalt flows around the Lonar crater have dips between 3° and 8° toward various directions from N to SW (also cf. Kumar, 2005). Observations on these undeformed target-basalt flows ~1–2 km away from the crater rim to the northeast, east- southeast, and west of the crater show that there occur occasionally semicontinuous, flow- parallel fractures and even less frequent subvertical fractures within these flows (Fig. 3.2b). The subvertical fractures mostly trend NW-SE to NNW-SSE, and a few trend NE-SW (Kumar, 2005). Our observation on unshocked Deccan Trap near Pimpalner dam at ~6.6 km southeast of crater rim shows presence of four sets of fractures trending N-S, E-W, NE-SW, and NW-SE in the target basalt. The flow-parallel fractures may become more prominent due to local weathering. These flow-parallel fractures in the basalt flows, which are also observed on the upper crater wall (cf. Kumar, 2005), are clearly pre-impact in origin and can be used as an additional criterion for determination of the impact-induced tilts of basalt flows along the rim of the Lonar crater. In most sectors, except in the WSW, W, and WNW (Fig. 3.3), the basalt flows show quaquaversal dips that show a range of variation between 17° and 36° on average (Figs. 3.4a–3.4c, 3.4e, 3.4m, and 3.4n). An additional structural feature seen on the top of the hillock in the SSE sector is a subvertical fracture cleavage spaced at <5 cm (Fig. 3.2c). This fracture cleavage shows a steep dip with an average value of ~53° toward the north (Fig. 3.4d). Besides cleavaged basalts, a set of widely spaced fractures is commonly observed on the top of the hillocks in the SSW sector (Fig. 3.2d); these fractures show overturned dip toward the crater. To the west of the SSW sector, basalt flows with both vertical and overturned dips are seen in a few sections (Fig. 3.2e). The overturned dips shown by the fractures have an average value of ~60° toward the north (Fig. 3.4f). The WSW sector is dominated by a hillock (at the highest contour of ~609 m, Fig. 3.3). In this sector two types of basalt occur: (1) basalts with flow structure, and (2) less common basalts with a prominent cleavage. Both (1) and (2) types of basalt show overturned dip. The flow layers show relatively gentler dip of ~51° toward the NE on average (Fig. 3.4g), the cleavaged basalt has a steeper average dip of ~73° toward the NE (Fig. 3.4h).

Figure 3.4: Orientations of basalt flows (solid gray square) and fracture cleavage (open square) occurring on rim of the Lonar crater in the lower hemisphere of the π-pole diagram.

The western sector is characterized by a relatively low altitude of crater rim, which may be originally produced during asteroid impact or due to relatively stronger erosion (Fig. 3.1b). Remnants of basalt flows with overturned dip are also seen in this sector. At least in two cases, prominent basalt flows with overturned dips are seen. The basalt flows on the crater rim have an average dip of ~56° toward the east (Fig. 3.4i). In the deeply eroded parts of this sector, however, the basalt flows dip away from the crater (quaquaversal dip), which has an average dip of 17° toward the west (Fig. 3.4j). In the WNW sector, the exposures of remnant basalt flows on the hilltop show well- developed cleavage parallel to the flow orientation. These cleavaged basalts either show overturned dip toward the crater or are subvertical in nature (Fig. 3.2f) and have an average dip of 46° toward the SE (Fig. 3.4k). In the eroded segments between the hills, the basalt flows on the crater rim have dips of 23° toward the NW on average (Fig. 3.4l) indicating the presence of overturned basalt flows only on the crater rim. In the NNE sector, a set of roughly E-W–trending, widely spaced vertical fractures (Fig. 3.4o) cut across the basalt flows that dip away from the crater at an average dip of 17° toward the north (Fig. 3.4n). Attempts have also been made to examine the variations in attitudes of basalt flows in vertical sections along the crater wall. In most cases these attempts were unsuccessful because of talus cover and the presence of exotic blocks of basalt on the crater wall. However, we were able to obtain a limited number of data along an eastern section below the rest house and along a western section just opposite (Fig. 3.3). The exposures along the eastern section were better; the basalt flows up to an altitude of ~550 m have gentle dips of 5°–10° toward the east. Above this height the attitudes of basalt flows are slightly steep with dips of 15°–18° toward the east. The western section has gentle dips of 9°–19° toward the west up to an altitude of ~580 m and above.

3.6 LOW-FIELD AMS ANALYSIS OF LONAR CRATER Studies of low-field Anisotropy of Magnetic Susceptibility (AMS) is an important petrofabric tool and is widely used in geological sciences. This technique has recently been applied to the Vredefort crater, South Africa and it was found that the minimum susceptibility axes (K3) of AMS of the granitoid target rocks are randomly oriented in the site of impact (Carporzen et al., 2005). In the impact experiments on basaltic andesites, Central Japan and basalts from the Lonar crater, India, it was also found that AMS directions

are highly efficient in determining the direction of impact and perhaps the distribution of the shock metamorphic front around the crater (Nishioka et al., 2007; Nishioka and Funaki, 2008). In the high pressure range (>3 GPa) of experiment, the anisotropy degree was

increased, the K3 axes was oriented toward the shock direction and average susceptibility was decreased. At a relatively low pressure range (0.5-3 GPa), the maximum susceptibility

axes (K1) was induced parallel to the shock direction and was superposed on the initial AMS data. Therefore, besides our structural investigation, we also carried out AMS study of basalt samples systematically collected from the Lonar crater rim, from its adjoining area and from a vertical cross-section along the crater wall to the east to understand the direction of impact and distribution of shock fronts around the crater. Oriented drill core samples of ~2.5 cm diameter were collected from all around the Lonar crater rim and adjoining areas (Fig. 3.1d) with a portable gasoline-powered rock drill during three field seasons between the years 2008 and 2010. No sampling was possible from the NE crater rim sector because of the absence of a rim in this part of the crater due to faulting (Fig. 3.1a). Each core was cut into two to three specimens of ~2.2 cm height in the laboratory. Low-field (300 Am−1 at 875 Hz) AMS was carried out using an AGICO (Czech Republic) KLY-4S Kappabridge for each specimen with measurement in 64 directions on three mutually orthogonal planes using an automatic rotator sample holder (spinning specimen method). These data give the orientation and magnitude of three orthogonal

principal axes of the magnetic susceptibility ellipsoid (K1, K2, and K3). The magnitudes of

these principal directions are used to calculate different parameters, e.g., Km (mean susceptibility), P/ (corrected degree of magnetic anisotropy, which is a measure of the eccentricity of magnetic susceptibility ellipsoid), and T (shape parameter that describes the oblate/prolate shape of the magnetic susceptibility ellipsoid) (after Tarling and Hrouda, 1993), using the ‘SUFAR’ software. Besides the studies on the orientations of AMS principal susceptibility axes, the following two mathematical parameters (after Jelinek, 1978) were also considered for interpreting the Lonar samples: / 2  Degree of anisotropy (P ) = exp [2*Σ (ln Ki/K) ]1/2 where i= 1 to 3, and

 Shape parameter (T) = [(2*ln K2 - ln K1 - ln K3)/(ln K1 - ln K3)], where ‘exp’ and ‘ln’ are

exponential and natural log respectively; K1, K2 and K3 are the maximum, intermediate,

and minimum susceptibility axes; and K (mean susceptibility) = (K1+ K2+ K3)/3. 66

Additionally, a parameter called ‘degree of anisotropy (A)’ that quantified the departure from the isotropic case (when all three principal susceptibilities are equal) (after Cañón- Tapia et al., 1997) was also used for our samples for further comparison, where

A= 100 * {1-[(K3+K2)/2K1]}; ‘A’ ranges between 0% (isotropic) to 100% (K1>>K2 and K3). The basaltic rocks occurring in and around the Lonar crater (Fig. 3.1d) can apparently be classified into two types, i.e. (1) unshocked basalts— those lying away from the crater rim but close to the crater, and (2) shocked basalts— those lying along the crater rim and wall. This classification for the magnetic study of target basalts is perhaps justiﬁable because the

basalt flows on the wall and in the surroundings of the Lonar crater were found to differ in Jn

(NRM), K (susceptibility), Qn (Köenigsberger ratio), and declination, of basaltic samples collected from the inner walls of the Lonar crater (Rao and Bhalla, 1984). The shocked basalts on the crater rim, however, do not show any physical evidence of shock metamorphism in thin section studies, except in a few cases where the development of well- spaced fractures oblique to the length of plagioclase megacrysts (Osae et al., 2005) were found. The groundmass plagioclase laths, as well as the titanomagnetite do not show any fracturing or other deformational features. These samples likely belong to the class-I shock- metamorphosed basalt as described by Kieffer et al. (1976). The shock pressure experienced by these basalts is not clearly known. Experimental studies of Kieffer et al. (1976) established a shock pressure for these samples at <20 GPa. In the compiled pressure- temperature diagram on shock metamorphism by French (1998), the fracturing or other deformational features of target rocks can be achieved in a shock pressure of ~7–10 GPa. The best exposed unshocked basalt flow that is stratigraphically equivalent to the fourth flow along the crater rim (thickness ~40 m, Ghosh and Bhaduri, 2003) was sampled from near the hillock Durga Tegri at ~2 km east-southeast of the crater rim and from farther east (Fig. 3.1d). These unshocked target basalts, which were sampled from close to the top of the fourth flow (that is, also exposed along the crater rim, and thus comparison can be made), characteristically show an oblate shape susceptibility ellipsoid with P/ values mostly varying between 1.02 and 1.04, although a few higher values up to 1.06 are also present; the average is close to 1.03 (Fig. 3.5a). The basalt samples from the Khini village, ~2 km west- southwest of the crater rim, however, show a drastic reduction of P/ values to ~1.015 on average that mostly range between ~1 and 1.017; the shape of the susceptibility ellipsoids of these basalts varies from oblate to prolate (Fig. 3.5b). When compared, the Khini village

samples are similar to the crater rim samples, which, in general, show variation in shape of the susceptibility ellipsoid from oblate to prolate type with P/ values of ~1.01 on average that range mostly between ~1.004 and <1.02, except in the eastern sector (Figs. 3.5d–3.5i). In the eastern sector of the crater rim, the P/ values show a wide variation between ~1.007 and 1.04 with an average of ~1.02 (Fig. 3.5c). Another anisotropy degree parameter, which is also used to interpret the recent lavas from Hawaii, is ‘A’ parameter (Cañón-Tapia et al., 1997). When the target basalts from the Lonar crater are plotted in P/ versus A plot, they show almost a linear relationship; the target basalts from Durga Tegri and farther east show a mean ‘A’ value of ~1.75% with a range mostly between ~1.5% and 2%, although some spread of values up to ~3.5% is seen (Fig. 3.5j). The ‘A’ values of the unshocked Deccan target basalt are comparable to the 'a'ā flows of Hawaii (Cañón-Tapia et al., 1997). The 'a'ā type nature of this flow is also described by Ghosh and Bhaduri (2003) based on the field observation. The shocked basalts from the crater rim except from the eastern sector, however, show a lower range of ‘A’ values, mostly between ~0.26% and 1.42% with an average of 0.65%. The average ‘A’ value for the target rocks from the eastern sector of the crater rim is relatively high with an average of ~1.5% and ranges between 0.4% and 2.7%. The target basalts from the Khini village also overlap with that of the shocked basalts and have a very low average ‘A’ value of ~0.72% with a range between ~0.32% and 2.18%.

Figure 3.5: P/–T plots for unshocked and shocked basalts from around the Lonar crater (sample locations are shown in fig. 3.1d). Note higher P/ value and oblate shape of susceptibility ellipsoid are the characteristics of unshocked target basalts (a), whereas shocked basalts have restricted and lower P/ values with the variation of shape of susceptibility ellipsoid from oblate to prolate (b–i). Relationship between P/ and A parameters (Cañón-Tapia et al., 1997) is shown in (j); abbreviations: DT: target basalt samples from Durga Tegri, CR-E: samples from eastern crater rim sector, CR-rest: samples from rest of crater rim, Khini: samples from Khini village.

In stereographic plots, the minimum susceptibility axes (K3) of the unshocked Lonar basalts from the Durga Tegri and farther east show clustering of data with moderate to subvertical dips (40°–70°) toward the east; the maximum (K1) and intermediate (K2) axes are distributed on subhorizontal, southwesterly to westerly dipping girdles describing bimodal distribution (cf. Cañón-Tapia et al., 1997) for the Lonar target basalts (Figs. 3.6a and 3.6b). The orientations of the K1axes define flow direction of basaltic lava toward the west (cf. Cañón-Tapia et al., 1997 and references therein) with a dip between 20° and 40°. The remaining plots of basalt from the Khini village and crater rim (Fig. 3.1d) show significant spread in orientations of the AMS axes in most of the cases. The target basalt from the Khini village shows the distribution of many of the K1 axes along a northerly dipping

girdle with significant scatter; their K3 axes show a major shift toward the west and the southwest (Fig. 3.6c). The shocked basalts from the eastern and the western sectors of the

crater rim have a wide dispersion in K1and K2 orientations; the K3 axes are subvertical to westerly dipping with moderate to significant westward shift compared to unshocked basalts (Figs. 3.6d and 3.6h). The shocked basalts from the SE and NW sectors of the crater rim show a strong westward shift in the orientation of K3 axes with subvertical to moderate dip (Figs. 3.6e and 3.6i). The basalts belonging to the N sector, however, show a moderate clus-

tering of K3 axes with subvertical to moderate dip and a moderate westward shift compared

to the unshocked basalts at Durga Tegri (Fig. 3.6j). The K1 and K2 axes show subhorizontal orientation with wide variation in dip directions ranging from the east through the south to the west. In the S and SW sectors, the shocked basalts show a strong southwestward shift of

K3 axes with a subhorizontal dip, and some clustering in orientations of K1 and K2 axes may be present (Figs. 3.6f and 3.6g).

Figure 3.6: Lower hemisphere stereographic projections of anisotropy of magnetic susceptibility (AMS) axes of unshocked and shocked basalts from around the Lonar crater (for sample locations, see Fig. 3.1d). Symbols: open square—maximum susceptibility axis (K1), gray triangle—intermediate susceptibility axis (K2), black dot—minimum susceptibility axis (K3). Note the bimodal distribution of the susceptibility axes is characteristic for unshocked basalts from Durga Tegri area and farther east (a, b); the shocked basalts show relatively random orientation of AMS susceptibility axes (c–j).

For examining the variation of AMS properties of shocked basalts in the crater cross section, samples were also collected from three sites at altitudes ~560 m (i.e., close to crater rim), ~535 m and ~524 m along the eastern crater wall below the PWD rest house (see Fig. 3.1d). The samples collected below this altitude were highly weathered and fragile and could not be studied at present. The basalts at the altitude of ~560 m of the crater rim show moderate clustering of AMS axes in stereographic plot with a relative shift of the K3 axes toward the west (Fig. 3.7a) compared to the unshocked basalt at the Durga Tegri. The AMS

Figure 3.7: Lower hemisphere stereographic projections of AMS susceptibility axes (symbols as in fig. 3.6) and P/-T plots of shocked basalts from the eastern cross-section of the Lonar crater below the rest house. Note relatively clustering of AMS susceptibility axes for basalts from higher altitude (~560 m) and distribution of K2 and K3 susceptibility axes of basalts from lower altitude (~535-524m) on a NW-SE vertical plane.

data also show very restricted values of P/ close to ~1.01 (ranges between 1.01 and 1.013) and have an oblate shape ellipsoid (Fig. 3.7b). The target basalt at the lower altitude between

~535 and 524 m, however, shows scattered distribution of AMS axes, and the K2 and K3 axes are mostly confined on a vertical girdle with a NW-SE trend (Fig. 3.7c). These basalts show relatively wide variation of the P/ parameter between 1.009 and 1.025 with an average of ~1.008; the shape of the AMS ellipsoid varies between oblate to prolate (Fig. 3.7d).

3.7 DIRECTION OF LONAR ASTEROID IMPACT It is understood that half of all asteroid impacts strike their target planetary bodies at an angle between 30° and 60°; however, very low angle (5°–10°) and vertical impacts are a rare phenomenon (Shoemaker, 1962; Pierazzo and Melosh, 2000). The important geological features discussed in the present investigation of the possible direction and angle of impact for a relatively young crater such as Lonar are: (1) near-circular shape of the crater rim, (2) continuity of ejecta all around the rim and the elliptical shape of the ejecta, (3) E-W bilateral symmetry around the major axis of the ejecta ellipse, and (4) major westward displacement of the elliptical continuous-ejecta deposit compared to a near-circular crater rim. Before making any conclusion on the Lonar crater based on ejecta distribution, a clear understanding of the pristine nature of the ejecta is necessary. Fudali et al. (1980), based on their field observations, argued that the ejecta around the Lonar crater was close to its original shape and was not modified much due to erosion. The studies made by Grant (1999) on remote sensing images, our present work on ASTER and Landsat images, and aerial photographs coupled with field studies also support this conclusion. Because the Lonar crater did not form on a major river basin, the only erosional agent that can modify the ejecta distribution around this crater is rainfall. The present semi-arid climatic condition around the Lonar crater in central India is a result of the position of the Indian subcontinent with respect to the major southwest monsoons. A monsoon is prevented from reaching the central part of India due to the presence of a N-S–trending hill range along the western coast of the country, known as the Western Ghats. The Indian plate, on which the Indian subcontinent is situated, is moving northward against the Eurasian plate during the past ~60 million years (Molnar, 1984) at a present rate of ~3.7 cm/a (Malaimani et al., 2000; Wang et al., 2001). At this rate, the Indian subcontinent could have traveled only ~1.85 km northward during the past ~50,000 years. Because the Indian subcontinent did not suffer any major 73

change in its position due to plate tectonics with respect to a southwest monsoon during the past ~50,000 years, it appears that the area around the Lonar crater must have remained in a semiarid condition since its formation. Hence, it can be concluded that post-impact climate does not have much effect in shaping the ejecta distribution in bulk around the Lonar crater. A comparison with experimental studies primarily suggests that the bilateral symmetry of the ejecta around the Lonar crater (Fig. 3.1c) may be a result of an oblique impact at <45°, and the projectile path was confined to an east-west vertical symmetry plane passing through the major axis of the enveloping ellipse drawn on the periphery of continuous ejecta extended all around the crater rim (Gault and Wedekind, 1978; Pierazzo and Melosh, 2000). The circularity of the crater rim and the continuous extension of the surrounding ejecta further constrain the lower limit of impact to >30° (Gault and Wedekind, 1978). This is because during impact at <30°, the crater shape becomes elongated along the projectile trajectory (Gault and Wedekind, 1978). A recent experiment by the National Aeronautic and Space Administration (NASA) on the Deep Impact probe collision on 9P Tempel 1 at an angle ~30° with the horizon shows a very different pattern of ejecta. It includes an uprange “zone of avoidance,” a heart-shaped ejecta ray system (cardioid pattern), and a conical (but asymmetric) ejecta curtain (Schultz et al., 2007). Another possible example of ejecta distribution due to low-angle impact is perhaps the Tycho crater on the Moon (Anderson and Schultz, 2006; Schultz et al., 2007). For the Lonar crater, the ejecta is present all around the rim where its minimum width is ~700 m in the northern, eastern, and southern directions (Figs. 3.1a and 3.1c), and no “zone of avoidance” is seen in ejecta distribution. Hence, experimental and natural examples suggest that the minimum angle of impact for the Lonar crater is >30°. It is observed from both the ASTER and Landsat images, and partly from aerial photographs (Fudali et al., 1980), that the continuous ejecta around the crater rim extends more toward the west (Figs. 3.1a and 3.1c). A comparison with experimental results suggests that this type of ejecta distribution is possible only due to an oblique impact when the majority of ejecta are distributed in the downrange direction (e.g., Gault and Wedekind, 1978; Schultz and Anderson, 1996; Pierazzo and Melosh, 2000; Schultz et al., 2007). NASA’s Deep Impact experiment shows that this downrange ejecta plume generated during oblique impact travels with a velocity slightly less than the initial impact velocity (Schultz et al., 2007).

The oblique impact model can further be verified by our AMS data (Figs. 3.5, 3.6 and 3.7). Recent work by Cañón-Tapia et al. (1997) and Cañón-Tapia and Coe (2002) showed that AMS parameters might show systematic variations in the cross-sectional thickness of lava flows, and it may be a possibility that the variations observed between unshocked and shocked basalts of the Lonar crater might be the original magmatic variations and were not the result of the asteroid impact. This possibility is perhaps the least for the Lonar crater because (1) shock-induced rock magnetic properties (Qn, Jn, K, and declination) of the target basalt along the wall of the Lonar crater are already understood, and these are different from those of the surrounding unshocked basalt (Rao and Bhalla, 1984); (2) our sampling of unshocked and shocked target basalts was only concentrated at the topmost part of the fourth flow, which was exposed both on the crater rim and at Dugra Tegri at the east (Fig. 3.1d), and therefore the complication due to variation of the AMS properties with the thickness of the lava flows had been minimized in our work; and (3) magmatic flows resulted in three

types of AMS axes distributions— i.e., trimodal, bimodal, and uniform types, with Kmax pointed toward the direction of flow (Cañón-Tapia et al., 1997 and references therein). Extremely scattered distributions of the AMS axes of the target basalts from around the crater rim (Figs. 3.6d–3.6j), therefore, cannot be attributed to any magmatic process but rather to asteroid impact (Carporzen et al., 2005). One of our important observations is that P/ values of the shocked basalts around the crater rim are low and close to ~1.01 compared to the unshocked basalt (~1.03) at the east (Fig. 3.1d; Fig. 3.5). So natural observation suggests that P/ of the target rock, in general, decreases due to impact. This observation is, however, in disagreement with the experimental findings of Nishioka and Funaki (2008), although the proper explanation is not known at this stage. In comparison to the unshocked basalt ~2 km east-southeast of the Lonar crater rim, the target basalts collected from the Khini village ~2 km west-southwest of the crater rim (Fig. 3.1d) show lower P/ values (~1.01) and highly scattered distribution of the AMS axes (Fig. 3.5b, 3.6c). These samples are well plotted within the domain of shocked basalts (Fig. 3.5j) and suggest that target basalts at the west of the Lonar crater are shocked compared to that in the east. The shocked nature of the target basalts at the west of the crater rim is also well documented by Nishioka (2007). His detailed work at the north of the Kalapani dam (Fig. 3.1d), which includes 46 specimens on drill cores collected from 2 seven nearby sites over a 0.25 km area, showed a random orientation of the K3 axes similar 75

to that observed at the Vredefort impact crater, South Africa (Carporzen et al., 2005). This uneven distribution of shock at ~2 km distance to the east-southeast and west-southwest of crater rim, when compared with 3D hydrocode simulation (Pierazzo and Melosh, 2000), further suggests that asteroid impact at the Lonar was oblique and the impactor came from the east. The inclined shock front, generated just after the impact from the east, affected the target basalts at the west of the Lonar crater. This conclusion is reasonable because when the shock front propagates through the target rocks, it appears to be symmetrical around the impact point, but the strength of the shock is asymmetric with the strongest shock in the downrange direction during oblique impact (Pierazzo and Melosh, 1999, 2000).

The shocked basalts in the Khini village show mostly a westward shift of the K3 axes on a stereographic plot (Fig. 3.6c). In the Kalapani dam area, west-southwest of the crater rim,

the K3 axes of the shocked basalt also show a shift and concentration toward the southwest

to west with a horizontal to moderate dip; their K1 axes are mostly oriented horizontally to subhorizontally to the northwest-southeast direction on a stereographic plot (Nishioka,

2007). This distribution of K3 axes is comparable to a high-pressure shock experiment (>3 GPa) (Nishioka et al., 2007; Nishioka and Funaki, 2008), which finally suggests that the target basalts occurring at ~2 km west-southwest of the Lonar crater rim are highly shocked with a westward shock component in comparison to the unshocked basalts occurring at an equal distance to the east-southeast (Fig. 3.1d) indicating an oblique impact at the Lonar area from the east. On the other hand, the basaltic rocks occurring along the crater rim (except at NE) can be compared with high-pressure (>3 GPa) experiments. These shocked basalts from the E, SE,

W, NW, and N sectors (Fig. 3.1d) show a moderate to strong westward shift of the K3 susceptibility axes (Figs. 3.6d, 3.6e, and 3.6h–3.6j) indicating an oblique impact direction from the east. Only in the S and SW sectors, however, the shock fronts had a strong southwestward component as indicated by major shift of the respective K3 axes (Figs. 3.6f and 3.6g); this shock component is perhaps also present in part in the target basalts at Kalapani dam area (Fig. 3.6c and Nishioka, 2007). Our preliminary investigation suggests that this local change in shock direction was perhaps related to the distribution of the shock front around the crater just after impact.

In the eastern cross section of the Lonar crater below the rest house (Fig. 3.1d), we find the basaltic rocks at higher altitudes (~560 m) on the crater rim are shocked as indicated by their low P/ values close to ~1.01 (Fig. 3.7b) compared to the unshocked basalts at Durga

Tegri (~1.03) (Fig. 3.5a). The K3 susceptibility axes are shifted to the west relative to the unshocked basalt in stereographic plot (Fig. 3.7a). Although some scatter exists, these shocked basalts still preserve more or less the trimodal distribution of the susceptibility axes (Fig. 3.7a), which is observed in the underformed lava (Cañón-Tapia et al., 1997). We interpret this feature to be due to relatively low shock in the uprange direction during the oblique impact from the east. Relatively low shocked nature of the target basalts in the eastern sector of the crater is also indicated by the eastern crater rim samples (Fig. 3.1d), which are comparable in their P/ and ‘A’ parameters to the unshocked basalts from the Durga Tegri area (Figs. 3.5c and 3.5j). Relatively high P/ values (average ~1.025) and a wide range of variation (1.009–1.025) in the target basalt from the lower half of the cross section between ~535 and 524 m height (Fig. 3.7d) suggest that these rocks were relatively less shocked compared to the top of this section. This conclusion is perhaps justiﬁable because the topmost layer of target rock is supposed to receive the maximum shock during

the impact in comparison to layers below. However, distribution of the K2 and K3 axes of this shocked basalt on a roughly vertical plane (Fig. 3.7c) is not understood with our present level of knowledge. In the deeply eroded complex impact structures of larger dimensions (~5–24 km) in sedimentary target rocks, bilateral symmetry of the distribution and the characteristic imbrications of thrust planes in the core of the central uplifts provide important information on direction of the oblique impact (Scherler et al., 2006). For a simple crater such as Lonar, the best suitable structural element that can be studied to get information on obliquity of impact is perhaps a systematic observation of the variation of attitudes of basalt ﬂows around the crater rim (Fig. 3.3; Fig. 3.4). It is seen that the attitudes of basalt flows now exposed along the Lonar crater rim are not random as thought earlier (cf. Fredriksson et al., 1973; Fudali et al., 1980) but show a systematic distribution. The summary of our observation is as follows: (1) flows show quaquaversal dips in most parts of the crater rim except in the WSW, W, and WNW sectors where overturned dips of basalt flows are seen, and (2) flows in the SSE, SSW, and NNE sectors show cleavage and/or fracture that also has vertical and/or overturned dip. Therefore, the structural deformations along the Lonar crater 77

rim are almost bilaterally symmetrical about an east-west vertical plane (Figs. 3.1c and 3.3). The high deformation of basalt flows in only a relatively small sector of the crater rim covering approximately one-fourth of its periphery toward the west perhaps can be best explained when compared with experiments. In their oblique impact experiments (90° to 15° to the horizontal), Dahl and Schultz (2001) showed that stress asymmetries occur at an impact angle of ~45°. This is a direct consequence of the way in which a projectile couples its energy into the target during oblique impact. Such asymmetries extend significant distances from the impact point and can be expressed by the asymmetries in target damage (Schultz and Anderson, 1996). As the projectile continues to couple its energy into the target, shock pressures are elevated downrange from the point of first contact. In their original experiment, Schultz and Anderson (1996) observed the asymmetry of fractures on target leucite, ice, and basalts generated during the oblique impact. In our present study, however, we must restrict ourselves to only observations of changes in attitudes of basalt flows around the crater rim because other deformations (such as fractures) on the target rocks around and at the outer part of the crater rim are not visible due to ejecta and/or paleosol cover. Higher tilts of basalts in the west and adjacent sectors definitely suggest that these rocks must have experienced more stress during impact as compared to basalts in other sectors of the crater rim. When compared with experiments (Dahl and Schultz, 2001), this finding also suggests that the impact at the Lonar crater was oblique and the impactor came from the east. There are reasons to believe that the cleavage observed in basalts around the crater rim (Fig. 3.2c) is most likely the result of impact. This is because these basalts with cleavage do not occur away from the crater rim. Isolated meter-size or smaller throw-out blocks of basalt of variable orientations within the ejecta around the crater rim show well-developed fracture cleavages (Fig. 3.2g), and the maskelynite-bearing shock basalt (class II shocked basalt of Kieffer et al., 1976) rarely occurring within the ejecta (Misra et al., 2007) shows well- developed cleavage in the cross section (Fig. 3.2h). The present structural analysis on the variations in attitudes of basalt flows on the Lonar crater rim together with the valuable observations of Kumar (2005) on the deformations in the mostly inaccessible upper ~30 m of the crater wall perhaps give a better idea of the impact-induced deformations when an impactor hits obliquely a thick basaltic sequence (>350 m, Gupta et al., 1999; Mitchell and Widdowson, 1991). Both datasets suggest upward

turning of basalt flows along the crater wall similar to the impact deformation of target rocks at other simple craters (Shoemaker, 1960; Shoemaker and Eggleton 1961; Brandt and Reimold, 1995; Koeberl et al., 1998; Reimold et al., 1998b). Kumar (2005) also suggested that the major faulting and slumping present at the NNE sector of the crater (Fig. 3.1a) took place during the crater modification stage and might have been controlled by the development of impact-generated concentric fractures. The present observations on the ASTER and Landsat images and aerial photographs (Fredriksson et al., 1973) coupled with field observations suggest that the faulting in the northeastern rim of the Lonar crater is a part of the northeast-southwest–trending lineament that crosscuts the crater rim (Fig. 3.1a). There is also another east-west–trending, less prominent fracture that crosscuts the crater rim. The crosscutting relationship of these fractures with the rim of the crater definitely suggests their reactivation in the post-impact period. According to an eyewitness (S. Bugdani), the reactivation is true for the northeast-southwest–trending lineament, which took place in the recent past on the northeastern part of the crater rim. The unshocked target Deccan basalts away from the crater rim have tectonic fractures that trend mostly in the NNW-SSE, NW-SE, WNW-ESE, and nearly E-W directions, and a less prominent component in the NE-SW direction (Kumar, 2005). Therefore, it is perhaps more reasonable that these fractures were originally present in the pre-impact target and reactivated in the post-impact period just to balance the inequilibrium in mass distribution produced by formation of a huge cavity by impact.

3.8 DISTRIBUTION OF IMPACT–INDUCED SHOCK PRESSURE A detailed AMS study on the target rocks established the oblique asteroid impact hypothesis from the east for the Lonar crater, which showed that target basalts from ~2 km west- southwest of the crater rim were highly shocked compared to the unshocked basalts at ~2 km east-southeast of the crater rim (Misra et al., 2010). However, the study was further extended to evaluate the distribution of impact-induced shock front around this hypervelocity impact crater through AMS technique. The basaltic rocks occurring in and around the Lonar crater (Fig. 3.8b) can apparently be classified into two types depending on our previous observation on impact-induced rock deformational features and AMS studies (Misra et al., 2010), viz., (a) unshocked basalts: those lying away from the crater rim but close to the crater to the east, and (b) shocked 79

basalts: lying along the crater rim and walls, and in the downrange direction to the west. The best exposed, thick, unshocked basalt flow that could be stratigraphically equivalent to the fourth flow along the crater rim (thickness ~40 m, Ghosh and Bhaduri, 2003) was sampled from the base of hillock Durga Tegri (DT/100°-2.93; meaning of the notation: sample site DT is located at 2.93 km distance from the crater centre along a direction of 100o from the geographic north in clockwise direction) (Fig. 3.8b). Additional samples of the fourth basalt flow were also collected from farther east of Durga Tegri (A5/102°-4.61).

Figure 3.8: Sketch maps of (a) India showing the location of Lonar crater, and (b) the Lonar crater showing most of the locations of drilling sites (filled black circles) for present study; Abbreviations: DT- Durga Tegri, SWT- Swraswati village, CRW- wall to the crater west, KHN- Khini village, KPD- Kalapani dam. The sampling locations from the ejecta blanket is shown in filled green circles.

It was observed that the Lonar unshocked basalts (n= 47) show an oblate shaped susceptibility ellipsoid with degree of anisotropy (P/) values mostly between ~1.02 and 1.04, although a few higher values up to ~1.06 are also present; the average is close to 1.033±0.008 (Misra et al., 2010) (Fig. 3.9a). The ellipsoids of shocked basalt samples from around the crater rim and farther west are, however, oblate to prolate in shape with low average P/ value of 1.01±0.006 that mostly lie between ~1.00 and 1.03. A more extensive set of samples of shocked basalts [number in total= 478, which was ~320% higher over Misra et al. (2010)] were sampled mostly from the top of the basalt flow exposed on the crater rim and the horizontal basalt flow at ~2 to 3 km west of the crater center in the downrange direction (Fig. 3.8b). None of the samples was collected from the possible folded basalt flows on the crater rim in the present study. The shocked basalt 80

samples from the eastern sector of the Lonar crater rim (092°-0.98, n= 30) show that the average P/ value (1.017±0.009) of the mostly oblate to prolate susceptibility ellipsoids is lower than earlier shown (~1.02, Misra et al., 2010), although the data show significant variation of P/ between ~1.006 and 1.042 (Fig. 3.9b) (Table 4.1). The mostly oblate to prolate shaped susceptibility ellipsoids from the southeastern sector of the crater rim (138°- 0.88, n= 54) show average P/ of 1.009±0.004 with a range between ~1.004 and 1.025 (Fig. 3.9c). The oblate to prolate shaped susceptibility ellipsoids from the southern sector of the crater rim (190°-0.86, n= 40) also have similarly low average P/ of 1.012±0.002 with a restricted range of variation between ~1.005 and 1.016 (Fig. 3.9d). The data from southwestern sector of the crater rim (229°-0.82, n= 44) show low average P/ of 1.01±0.003 for the oblate to prolate ellipsoids with a restricted range of variation between ~1.006 and 1.017 (Fig. 3.9e). Unlike our previous observation, the extensive data from western sector (272°-0.92) and northwestern sector (319°-0.92) of crater rim (n= 60 each) suggest strong variation of shape of ellipsoids from oblate to prolate with a low average P/ of 1.01±0.004 and range of variation between ~1.004 and 1.025 (Fig. 3.9h, 3.9j). The susceptibility ellipsoids from northern sector of crater rim (358°-0.92, n= 34) remain mostly oblate with low average P/ of 1.012±0.003 with a range between ~1.007 and 1.02 (Fig. 3.9k). The additional shocked basalt samples from distant locations at ~2 to 3 km west of the crater center in the downrange direction (cf. Misra et al., 2010) (Fig. 3.8b) are also investigated in the present study. Samples collected from the wall to the west of the crater rim (CRW/267°-0.92, n= 38) show oblate to prolate susceptibility ellipsoids with an average P/ of 1.015±0.003 and restricted variation of data between ~1.009 and 1.023 (Fig. 3.9g). The shocked basalts from the Kalapani dam (KPD/233°-2.17, n= 44), close to Khini village (KHN/251°-2.92) (Misra et al., 2010) (Fig. 3.8b), show mostly oblate to prolate shaped susceptibility ellipsoids that have significant variation of P/ between ~1.003 and 1.056 with an average P/ of 1.019±0.013 (Fig. 3.9f).The samples from the Swraswati village (SWT/300°-2.10, n= 74) show an equal distribution of susceptibility ellipsoids in the oblate and prolate fields with an average P/ of 1.013±0.006 (Fig. 3.9i). Most of the data show a restricted variation of P/ between ~1.004 and 1.019, although a few samples have values up to ~1.04.

Table 4.1: Corrected degree of anisotropy (P/) and shape parameter (T) variation of Lonar unshocked and shocked target basalts.

Site of investigation C N P/ T

Unshocked basalts 100°-2.93 & 102°-4.61 15 47 1.033±0.008a 0.673±0.165a [1.020–1.060]c [0.157–0.914]c

Shocked basalts at East 092°-0.98 (samples 14 30 1.017±0.009a 0.311±0.353a from crater rim) [1.006–1.042]c [-0.436–0.862]c

at Southeast 138°-0.88 (samples 20 54 1.009±0.004a 0.335±0.384a from crater rim) [1.004–1.025]c [-0.770–0.956]c

at South 190°-0.86 (samples 17 40 1.012±0.002a 0.269±0.408a from crater rim) [1.005–1.016]c [-0.878–0.867]c

at Southwest 229°-0.82 (samples 13 44 1.010±0.003a 0.265±0.269a from crater rim) [1.006–1.017]c [-0.473–0.846]c

233°-2.17 & 251°-2.92 (Samples from SW of 15 90 1.014±0.011a 0.122±0.412a crater rim) [1.003–1.056]c [-0.863–0.818]c

at West 272°-0.92 (samples 28 60 1.011±0.004a 0.170±0.432a from crater rim) [1.004–1.025]c [-0.695–0.935]c

267°-2.07 (samples 14 38 1.015±0.003a 0.186±0.322a from west of crater rim) [1.009–1.023]c [-0.575–0.810]c

at Northwest 319°-0.92 (samples 25 60 1.009±0.004a 0.160±0.431a from crater rim) [1.004–1.023]c [-0.826–0.902]c

Site of investigation C N P/ T

300°-2.10 (samples 35 74 1.013±0.006a 0.203±0.462a from NW of crater rim) [1.004–1.040]c [-0.738–0.952]c

at North 358°-0.92 (samples 18 34 1.012±0.003a 0.507±0.296a from crater rim) [1.007–1.020]c [-0.244–0.960]c

Abbreviations: C- number of cores drilled; N- number of specimens; P/- corrected degree of anisotropy; T- shape parameter (after Tarling and Hrouda, 1993). a –Average ±1σ standard deviation (important average in each group is shown in bold), c –range of variation of data. Shocked and unshocked basalts were defined on basis of earlier AMS study by Misra et al., 2010.

Figure 3.9: Degree of anisotropy (P/) vs. shape parameter (T) plot for unshocked and shocked basalts from around the Lonar crater rim and at ~2 to 3 km west of the crater center in the downrange direction (sample locations are shown in figure 3.8b). Note higher P/ value and oblate shape of susceptibility ellipsoid are characteristics of unshocked target basalts (a), whereas shocked basalts have restricted and lower P/ values with variation of shape of susceptibility ellipsoids from oblate to prolate type (b-k); P/ and T values are computed after Tarling and Hrouda (1993). Note: Each sampling site is represented by its distance from the crater center along a direction from geographic north in clockwise direction; abbreviations as in figure 3.8b.

In stereographic plots, the minimum susceptibility axes (K3) of the unshocked Lonar basalts from the base of Durga Tegri (DT/100°-2.93) and farther east (A5/102°-4.61) show clustering of data with moderate to subvertical dips (40o-70o) toward the east; the maximum

(K1) and intermediate (K2) susceptibility axes are distributed on subhorizontal, west- southwesterly dipping girdle describing a bimodal distribution (cf. Cañón-Tapia et al., 1997) for the Lonar target basalts (Fig. 3.10a). The average orientation of the K1 (maximum) axes on this girdle defines a flow direction of the Deccan lava at Lonar close toward the west (cf. Cañón-Tapia et al., 1997 and references therein) with a dip ~20°.

Figure 3.10: Stereographic projections of AMS susceptibility axes of (a) unshocked target, and (b-k) shocked basalts from around the rim of the Lonar crater and at ~2 to 3 km west of the crater center in the downrange direction (sample locations are shown in figure 3.8b); Note the bimodal distribution of the AMS axes is a characteristic for unshocked basalts from Durga Tegri (DT) and farther east (A5); most of the shocked basalts show triaxial distribution of AMS axes, except the samples from the north and west crater rim sectors, which show transitional type distribution.

The shocked basalts from the Lonar crater rim and adjoining area to the west, however, show different distribution of AMS axes in stereographic projections (cf. Misra et al., 2010). The susceptibility axes from the eastern sector of the crater rim (092°-0.98) show more or

less a triaxial distribution where the K1 and K2 axes are subhorizontal mostly oriented in E-

W and N-S directions respectively, although a moderate overlap of data exist (Fig. 3.10b,

also see Fig. 3.11 for displacement directions of K3 axes for all shocked basalts). The K3 axes show vertical to subvertical orientation, and are shifted mostly toward the northwest compared to those of the unshocked Lonar basalts (Fig. 3.10a). The target basalts from the

southeastern sector (138°-0.88) show a strong west- to west-southward shift of the K3 axes;

the susceptibility axes show a broad triaxial distribution where the orientations of K1 and K3 axes are similar to those of the shocked basalt from the eastern sector (Fig. 3.10c). The stereographic distribution of AMS axes of the shocked basalts from the southern sector (190°-0.86) shows a similar type of triaxial distribution as observed in the eastern sector of the crater rim, although inter-change in position of AMS axes is noticed (Fig. 3.10d). In this sector, the K1 axes are oriented vertical to subvertical; the K2 and K3 axes show horizontal to subhorizontal orientations mostly in E-W and N-S directions respectively. In comparison to

the unshocked target basalts, the K3 axes of these shocked basalts show south- and southwestward shift. The stereographic plots of the AMS axes of shocked target from the southwestern sector of the crater rim (229°-0.82) show a prominent triaxial distribution (Fig.

3.10e). The K1 axes are horizontal to subhorizontal in an ESE-WNW direction, whereas the

K2 axes are vertical to subvertical in orientation. The subhorizontal K3 axes show a southwestward shift in position compared to those of unshocked target basalts at Lonar. The shocked basalts from the western sector (272°-0.92) show significant northwestward shift of

K3 axes compared to the unshocked basalts (Fig. 3.10h). Most of the K1 and K2 susceptibility axes are broadly distributed on a subhorizontal girdle dipping toward the south-southwest, and the bimodal distribution of AMS axes as observed for the unshocked target is partly retained. The AMS axes of shocked basalts from the northwestern sector of

the crater rim (319°-0.92) show a triaxial distribution of axes; the K3 axes are vertical to subvertical in orientation and show displacement mostly towards the northwest in

comparison to the unshocked basalts; the K1 and K2 susceptibility axes are horizontal to subhorizontal and are mostly directed toward the SE-NW and SW-NE respectively (Fig.

3.10j). The K3 susceptibility axes of shocked basalts from the northern sector (358°-0.92) are oriented vertically to subvertically and show a dominant westward shift (Fig. 3.10k).

Most of the K1 and K2 axes are broadly distributed on a southwesterly dipping subhorizontal girdle, and the bimodal distribution of the AMS axes as observed for the unshocked basalts is partly retained.

The distribution of AMS axes of shocked basalts from the village Khini (KHN/233°- 2.17) at the southwest of Lonar crater (Fig. 3.8b) has been described in Misra et al. (2010),

which show a major shift of the K3 axes toward the southwest compared to the unshocked

target, and most of the K1 and K2 axes describe a subhorizontal northerly dipping girdle. The shocked basalts from the Kalapani dam (KPD/233°-2.17), which is close to the village

Khini, also show a southwestward shift of the K3 axes compared to the unshocked Lonar basalts, a broadly defined triaxial distribution of susceptibility axes is observed (Fig. 3.10f). The target basalts collected from a wall to the west of the Lonar crater rim (CRW/272°-0.92) show a well defined triaxial distribution of AMS axes (Fig. 3.10g). The K2 axes are mostly

subvertical in orientation, whereas the K1 and K3 axes are subhorizontal and oriented

towards the northwest and northeast respectively. The K3 axes of these shocked basalts are shifted towards the north compared to the Lonar unshocked basalts (Fig. 3.10a). The shocked target basalts from the village Swraswati at the northwest of the Lonar crater (SWT/300°-0.92) (Fig. 3.8b) also show a triaxial distribution of data (Fig. 3.10i). Most of

the K2 axes are vertical to subvertical; the K1 and K3 axes are mostly horizontal and oriented

towards the north and west respectively. The K3 axes of these target basalts are shifted towards the west and northwest compared to the Lonar unshocked target.

3.9 CONCLUSION Studies on the distribution of ejecta and structural deformations of basalt flows around the crater rim, together with AMS of unshocked and shocked basalts from around the Lonar crater suggest that the impactor asteroid most probably stuck the pre-impact surface from the east at an angle of between ~30 and 45o (Misra et al., 2010). The observation on elaborate AMS data for evaluating the distribution of impact-induced shock pressure confirms that the degree of anisotropy (P/) of the Lonar shocked basalts (collected from the crater rim and from ~2 to 3 km west of the crater center in the downrange direction) has reduced by ~2% in average when compared to the value for unshocked target basalt (~1.03); the shape of AMS ellipsoids also vary from oblate to prolate shape in comparison to the oblate shape for the unshocked target basalt (Fig. 3.9). In

stereographic projections, the distribution of AMS axes (K1, K2, K3) mostly changes from a biaxial distribution for the unshocked target basalt to triaxial for the shocked basalt, although scatter in data exists (Fig. 3.10). In a few cases, e.g., from the northern (358°-0.92)

and western (272°-0.92) sectors of the crater rim (Fig. 3.10k, 3.10h), a transitional type distribution of AMS axes between biaxial and triaxial types is also noticed. Besides the

general westward shift, the K3 axes also show either southwest-ward or northwest-ward shift for the shocked basalts lying mostly to the south and north of the east-west plane of impact respectively (Fig. 3.10, 3.11). As the displacement of K3 axes in the shocked basalts is sensitive to propagation of high impact stress (>3 GPa) (Nishioka and Funaki, 2008), it can be concluded that the impact stress could have branched out in the downrange direction into major southwest- and northwest components from the crater’s centre immediately after the impact making acute angle to the impact direction (Fig. 3.11).

Figure 3.11: A schematic

diagram showing probable

direction of shock pressure

propagation just after asteroid

impact at the Lonar crater, India.

E, SE, S, SW, W, NW and N

represent eastern, southeastern,

southern, southwestern, western,

northwestern and northern

sectors of the Lonar crater rim

respectively; abbreviations as in

figure 3.8b.

The distribution of K3 axes around the Lonar crater is perhaps very similar to the distribution of ejecta around the crater rim in oblique impact experiments. Gault and Wedekind (1978) showed that during oblique impact less than 45°, a forbidden zone in the ejecta distribution first appeared uprange from the crater, and then subsequently at shallower incidences a second zone appeared downrange, both extending from the crater rim with bilateral symmetry about the path of the projectile trajectory. If this ejecta distribution in the oblique impact experiment reflects the distribution of impact generated shock pressure around the crater, the AMS data from the Lonar crater are also in accordance with the bilateral branching of the impact-induced shock pressure at least in the downrange direction just after the low angle asteroid impact.

Chapter 4

ROCK AND MINERAL MAGNETIC STUDY OF LONAR AND RAMGARH IMPACT CRATERS

4.1 INTRODUCTION The chapter concerns the rock magnetic and mineral magnetic study of rocks from Lonar and Ramgarh impact structures for characterizing the shocked and unshocked nature. It deals with the study of magnetic shock indicator parameters viz. NRM/χ and REM (=NRM/SIRM expressed in %) of target rocks in order to understand their variations with the direction of asteroid impact. The magnetic mineralogy and grain size of target rocks and impact products were determined through the measurements of low and high temperature magnetic susceptibility (χ-T curves), magnetic hysteresis loop, IRM acquisition and backfield SIRM dc demagnetization. The chapter also provides some of the rock magnetic results on the magnetic particles/spherules collected from the Ramgarh structure. The effects of high pressures on rock magnetic properties are of interest in terms of gaining an understanding of magnetism of rocks subjected to strong stress waves, such as terrestrial impact structure target rocks. The study of shock effects upon magnetic properties is necessary to determine how sensitive these magnetic properties are to transient stress, how unique to shock the magnetic effects are, and how natural materials vary in their magnetic responses to shock. It is necessary to establish the sensitivity of NRM of rocks to transient stress to understand the palaeomagnetic record of shocked rocks. The results of laboratory shock experiments reveal that the primary effects of impact are demagnetization or remagnetization, and magnetic hardening (Gattacceca et al., 2007). Experimental studies have shown that shock pressures of the order of 1 GPa can remove existing remanent magnetizations (Pohl et al., 1975; Cisowski and Fuller, 1978). At pressures of >10 GPa, shocks can also permanently modify the intrinsic magnetic properties of rocks, including saturation remanent magnetization, coercivity, susceptibility, and anisotropy of susceptibility and remanence (Gattacceca et al., 2007; Gilder and Le Goff 2008; Nishioka et 88

al., 2007). The effects of shock metamorphism can also aid in the production and modification of magnetic carriers; for example, at pressures of >40 GPa and T>1000°C, amphibole and biotite decompose to produce magnetite. At lower pressures, titanomagnetite can result from the breakdown of ilmenite (Chao, 1968). The impact rocks results in creation of more single domain-sized grains from multi domain-sized grains, although there is no

apparent trend in squareness of hysteresis ratio (Mrs/Ms) data (Cloete et al., 1999).

4.2 SAMPLE DETAILS Oriented drill core samples and rock chips of both shocked and unshocked basalts, ejecta basalts, impact melt rocks and impact spherules collected from Lonar ejecta soil matrix were used for the basic rock and mineral magnetic measurements (Table 2.2, Fig. 3.8b). The samples of shocked ejecta basalts were collected from the N and NE (n= 3 sites), and S and SE directions (n= 2) of the crater at ~1 to 1.5 km distance from the crater center (green dots in fig. 3.8b). Impactites (glasses), which are mm-size, and in situ impact spherules, were recovered from trenches dug (~47 cm-deep pit) on ejecta blanket close to the SE part of the crater rim (GPS location: 19°58.356′N, 76°31.072′E). Contamination is unlikely as the site has no evidence of any development. The glasses were encountered at ~5 cm below the alluvium surface in the spherule-rich ejecta horizon. They are characteristically black, have vitreous luster and a highly vesicular surface and a variety of geometric shapes including rod, ellipsoidal, dumbbell, and tear-drop shapes (Fig. 2.1e, f; Fig. 6.1). Secondary infillings of quartz are sometimes found in the vesicles. In hand specimens, these impact melt rocks appear black in color, are vesicular, and show ropy or flow structures on the surface. Under the microscope, these glasses appear brown in color and also show flow structures. The brownish glasses usually contain unmelted fragments of basalt of various degree of shock deformation, fragments of clinopyroxene and plagioclase, and crystals of magnetite.

4.3 PREVIOUS STUDIES ON ROCK AND MINERAL MAGNETISM OF LONAR BASALTS

Rao and Bhalla (1984) reported that some magnetic parameters; viz. Jn (NRM), K

(susceptibility), Qn (Köenigsberger ratio), and declination, of basaltic samples collected from the inner walls of the Lonar crater showed systematic variations, whereas random variations were observed for the surrounding target rocks. The ferrimagnetic minerals in these basalts

were magnetite, ilmenite, and hematite (in some cases an alteration product of magnetite). The NRM and SIRM of the upper and lower flows respond differently to AF demagnetization as a function of their Hcr (remanence coercivity). They noticed that NRM carried by the lower flows is similar to that of IRM probably reset by asteroid impact

because of their low Hcr (remanence coercivity) values. Weiss et al. (2007) reported magnetic data from impact spherules and ﬂäden (0.01–1 cm in size) recovered from the ejecta blanket around the east and west of the crater rim of Lonar crater to test the hypothesis that these materials might have acquired a magnetization in an unusually strong paleointensity ﬁeld (Gattacceca and Rochette, 2004). Their hysteresis measurements indicated that these basaltic impact glasses were strongly magnetic (saturation remanence of ~2 A/m) having a squareness of 0.2, a ratio of coercivity of remanence to coercivity of 2, a ratio of initial susceptibility to saturation remanence of 0.007, and no significant remanence -3 anisotropy. Ratios of NRM to SIRM for the small glasses were only 0.5–1 × 10 , while the large glasses had ratios twice as large. These values were nearly an order of magnitude lower than those measured for the nearby Deccan basalts (Louzada et al., 2008). The low NRM/SIRM ratios of the Lonar glasses were interpreted to indicate the absence of any impact-generated paleofields substantially higher than several tens of micro Tesla (μT) at the Lonar crater (also Weiss et al., 2010), and the glasses slightly underestimated the intensity of the field in which they cooled, probably due to the effects of rotation during cooling.

4.4 ROCK AND MINERAL MAGNETIC CHARACTERIZATION OF LONAR BASALTS The rock magnetic characterization experiments on Lonar target rocks and their products include NRM/χ, REM (=NRM/SIRM expressed in %), low and high temperature magnetic susceptibility (χ-T) curves, magnetic hysteresis loop, IRM acquisition and backfield SIRM dc demagnetization, AF demagnetization spectra of NRM, Lowrie-Fuller test, and thermal demagnetization of SIRM. The NRM and saturation IRM were measured by a Molspin spinner magnetometer. The IRM was imparted in progressively increasing magnetic fields up to 1 T and their back field

application to the SIRM for evaluating remanent coercive force (Hcr) by a Molspin pulse magnetizer; magnetic remanence after each IRM step was measured by a Molspin spinner 90

magnetometer. The stepwise thermal demagnetization of SIRM was carried out in a MMTD80 furnace between 100 and 700°C at temperature increments of 50°C, and a Molspin pulse magnetizer with a maximum field of 1 T that was used for inducing IRM; magnetic remanence after each temperature step was measured by a Molspin spinner magnetometer. The measurement of temperature dependence (-196 to 700C) of magnetic susceptibility was carried out using an AGICO KLY-4S Kappabridge coupled with a CS- 3/CS-L furnace system in an argon atmosphere to determine the magnetic transitions, such as the Verwey/Curie temperature of ferrimagnetic phases. Room temperature hysteresis loops were measured using a Molspin NUVO vibrating sample magnetometer (VSM) in an alternating field cycling between 1 T to define their coercivities. Coercivities depend on the mineral composing the material, as well as on the grain size of the mineral itself. The Day plot of hysteresis shows the dependence of the hysteresis parameters on the grain size of magnetites (Day et al., 1977). The plot is divided into single domain (SD), pseudosingle domain (PSD), and multi domain (MD) regions. More recently, Dunlop (2002) generated a set of new regions that depend on the mixture of various magnetite grain sizes: the SD-MD mixing line, the SD-SP mixing line, and the SP (superparamagnetic) saturation envelope. The mineral magnetic carriers of Lonar target basalts were found to be titanomagnetite (Cisowski and Fuller, 1978). Micron-size Ti-rich exsolution lamellae divides ferrimagnetic Ti-poor titanomagnetite grains (tens of microns in size) into interacting single-domain needles of high coercivity, although some grains had poorly developed lamellae and low- coercivities. NRM/χ and REM: Different views exist on the possible relationship between the shock pressure due to asteroid impact and the resulting NRM intensities of target rocks. Some workers believed that the NRM of target rocks increased many times due to impact-induced shock pressure (Pesonen et al., 1997; Ugalde et al., 2005; Carporzen et al., 2005); others suggested that NRM decreased with shock pressure (Nishioka, 2007; Louzada et al., 2008). In the present research work, we carry out measurements of NRM/χ (expressed in units of Am-1) instead of NRM of both the unshocked and shocked basalt samples from around the Lonar crater (Table 4.1) [the shocked and unshocked basalts are defined based on AMS properties of the target rocks as suggested in Misra et al., 2010]. The detailed variations of NRM/χ within each group of samples are shown graphically in figure 4.1 and a summary on

Table 4.1: Rock magnetic data of unshocked and shocked basalts from the Lonar crater, India

-1 Site of investigation C N NRM/(Am ) R1 N REM% R2 N Mrs/Ms Hcr/Hc Unshocked basalts 100°-2.93 & 102°-4.61 15 44 40±23a (7)b 26 0.15±0.06a (3)b 11 0.16±0.02a 2.92±0.36a [19–74]c [0.08–0.19]c [0.13–0.21]c [2.44–3.72]c 116±23a (30)b 0.70±0.24a (19)b [89–160]c [0.32–1.29]c 248±56a (7)b 3.53±1.54a (4)b [181–331]c [1.86–4.94]c 125±68d (44)b 1.07±1.22d (26)b [19–331]c [0.08–4.94]c

Shocked basalts at East 14 27 47±16a (7)b 13 0.35±0.1a (8)b 8 0.19±0.10a 2.74±0.64a 092°-0.98 (samples [32–77]c [0.18–0.47]c [0.09–0.30]c [2.01–3.67]c from crater rim) 175±27a (18)b 2.19±1.01a (5)b [129–235]c [1.3–3.36]c 491±157a (2)b [380–602]c 165±116d (27)b 2.86:1 1.06±1.10d (13)b 0.63:1 [32–602]c [0.18–3.36]c

at Southeast 138°-0.88 (samples 20 54 203±33a (27)b 11 0.30±0.16 (8)a 9 0.24±0.02a 2.41±0.21a from crater rim) [167–321]c [0.11–0.55]b [0.22–0.27]c [2.14–2.42]c 626±165a (3)b 3.54±0.23 (3)a [438–751]c [3.35–3.80]c 1904±306a (19)b [1347–2600]c 8337±2721a (5)b [5249–10802]c 1578±2440d (54)b 54 (>116) 1.19±1.52a (11)b 0.38:1 [167–10802]c [0.11–3.80]c

Table 4.1: Continued

-1 Site of investigation C N NRM/(Am ) R1 N REM% R2 N Mrs/Ms Hcr/Hc Shocked basalts at South 17 40 161±57a (5)b 12 0.45±0.18a (4)b 9 0.22±0.04a 2.51±0.23a 190°‐0.86 (samples [73–213]c [0.19–0.58]c [0.14–0.26]c [0.28–2.98]c from crater rim) 571±169a (19)b 3.47±1.86a (7)b [284–826]c [1.82–6.52]c 1579±484a (7)b 30.00 (1)b [1079–2269]c 4746±792a (3)b [3977–5560]c 13106±859a (6)b [11943–14334]c 2890±4515d (40)b 39.00:1 4.68±8.22d (12)b 2.00:1 [73–14334]c [0.19–30]c at Southwest 13 44 168±25a (36)b 15 0.08 (1)b 7 0.22±0.03a 2.60±0.12a c a b c c 229°‐0.82 (samples [113–217] 0.34±0.13 (12) [0.19–0.28] [2.45–2.75] a b c from crater rim) 328±38 (8) [0.18–0.60] [254–367]c 1.03±0.09a (2)b [0.96‐1.09]c 198±68d (44)b 43.00:1 0.41±0.28d (15)b 0.15:1 [113–367]c [0.08–1.09]c 233°‐2.17 & 251°‐2.92 15 90 133±48a (44)b 30 0.16 (1)b 13 0.20±0.04a 2.94±0.49a c a b c c (Samples from SW of [75–241] 0.50±0.17 (13) [0.15–0.20] [2.21–3.96] crater rim) 772±340a (23)b [0.32–0.79]c [331‐1514]c 2.10±0.71a (12)b 2992±762a (21)b [1.14–3.02]c [1622–4261]c 5.34±1.41a (3)b 6508±387a (2)b [4.48–6.96]c [6234–6782]c 13.03 (1)b 1105±1470d (90)b 3.29:1 2.03±2.61d (30)b 1.50:1 [75–6782]c [0.16–13.03]c

Table 4.1: Continued

-1 Site of investigation C N NRM/(Am ) R1 N REM% R2 N Mrs/Ms Hcr/Hc Shocked basalts at West 28 57 43±20a (3)b 16 0.45±0.25a (12)b 6 0.22±0.03a 2.47±0.20a 272°‐0.92 (samples [27–65]c [0.19–1.10]c [0.17–0.24]c [2.19–2.70]c from crater rim) 152±45a (46)b 4.95±1.99a (3)b [88–277]c [3.35–7.18]c 442±77a (8)b 12.65a (1)b [342–587]c 187±118d (57)b 3.75:1 2.06±3.43d (16)b 0.45:1 [27–587]c [0.19–12.65]c

267°‐2.07 (samples 14 38 295±83a (19)b 14 0.23 (1)b 6 0.17±0.03a 3.06±0.85a from west of crater [179–418]c 0.82±0.35a (5)b [0.11–0.21]c [2.70–4.76]c rim) 1711±778a (14)b [0.40–1.16]c [737–2939]c 3.91±1.87a (5)b 3992±160a (3)b [2.44–7.00]c [3814–4124]c 14.52±4.23a (3)b 6463±26a (2)b [11.95–19.41]c [6444–6481]c 1433±1677d (38)b 38 (>116) 4.82±5.80d (14)b 3.67:1 [179–6481]c [0.23–19.41]c

at Northwest 25 60 69±15a (3)b 17 0.33±0.16a (13)b 6 0.24±0.03a 2.51±0.33a 319°‐0.92 (samples [52–80]c [0.12–0.54]c [0.22–0.30]c [2.12–2.99]c from crater rim) 170±22a (53)b 1.79±0.40a (4)b [115–237]c [1.32–2.23]c 644±252a (4)b [321–936]c 197±137d (60)b 14.00:1 0.67±0.67d (17)b 0.31:1 [52–936]c [0.12–2.23]c

Table 4.1: Continued

-1 Site of investigation C N NRM/(Am ) R1 N REM% R2 N Mrs/Ms Hcr/Hc

Shocked basalts a b a b a a 300°‐2.10 (samples 35 74 215±127 (57) 15 0.36±0.08 (6) 8 0.20±0.06 2.69±0.50 from NW of crater rim) [31–517]c [0.28–0.47]c [0.07–0.27]c [2.04–3.70]c 1337±777a (12)b 1.08±0.26a (4)b [608–2637]c [0.86–1.45]c 4415±318a (4)b 4.23±1.46a (4)b [4098–4802]c [2.51–6.03]c 10763 (1)b 29.1 (1)b 766±1580d (74)b 8.25:1 3.5±7.3d (15)b 1.50:1 [31–10763]c [0.28–29.1]c

at North 358°‐0.92 (samples 18 34 157±35a (22)b 14 0.62±0.24a (13)b 5 0.20±0.01a 2.67±0.04a from crater rim) [94–216]c [0.27–1.04]c [0.19–0.21]c [2.63–2.73]c 506±248a (10)b 2.19 (1)b [249–963]c 4449±655a (2)b [3986–4912]c 512±1027d (34)b 10.33:1 0.73±0.48d (14)b 0.56:1 [94–4912]c [0.27–2.19]c Abbreviations: C- number of cores drilled; N- number of specimens; NRM/χ- natural remanent magnetization/bulk susceptibility; REM%-[NRM/(saturation isothermal remanent magnetization, SIRM)]%; Mrs/Ms – saturation remanence/saturation magnetization; Hcr/Hc – coercivity of remanence/coercive force. a –Average ±1σ standard deviation (important average in each group is shown in bold), b –number of samples considered for the average value, c –range of variation of data, d –grand mean for each group of samples (in bold and italics). The shocked and unshocked basalts were defined on the basis of earlier AMS study by Misra et al., 2010.

R1 – ratios of number of target basalt samples from each location having NRM higher and lower than the average unshocked target basalts from Durga Tegri and farther east (i.e. 116±23 Am-1)

R2 – ratios of number of target basalt samples from each location having REM higher and lower than the average unshocked target basalts from Durga Tegri and farther east (i.e. 0.70±0.24%)

observation is shown in figure 4.2. The data shows that the NRM/χ values of the unshocked target basalts from Durga Tegri and farther east (sites: DT/100°-2.93 and A5/102°-4.61, Fig. 3.8b) [n=44] are ≤330 Am-1 where the majority of samples (~68) have a restricted range of variation (~89-160 Am-1) with an average of 116±23 Am-1 (Fig. 4.2a), which is similar to the grand mean of all the unshocked target basalts (Table 4.1). The shocked basalts collected from around the Lonar crater rim show a general increase of NRM/χ. The ratios of the number of target basalt samples having NRM/χ higher and

lower than the average unshocked target (R1) within each group of target basalts collected from the different sectors of the crater rim vary over a wide range between ~3:1 and >54:1 (Table 4.1). The lowest range of ratios (~3:1) are only observed for the samples collected from the eastern and western sectors of the crater rim. The higher ranges of ratios (~10:1 and >54:1) are only observed for those crater rim target basalts that are situated oblique or perpendicular to the east-west impact plane of the crater. The NRM/χ values are also found to be low for the majority of samples collected from the eastern and western sectors of the crater rim (Table 4.1, Fig. 4.2). The majority of samples (~67%) from the eastern sector [n=27] of the crater rim (092o-0.98) (Fig. 4.2b) have average NRM/χ ~1.5 times higher over the unshocked target, and the value is very similar when the entire range of dataset for this sector is considered. The most of the samples (~80%) of the shocked basalts collected from the western half of the crater rim including the southwestern (229o-0.82), western (272o-0.92), and northwestern (319o-0.92) sectors [n=161] show a restricted range of NRM/χ (between ~90 and 210 Am-1) and their average (159±28 Am-1) is only ~1.4 times higher over the unshocked target (Fig. 4.2e, h, j). The variation of NRM/χ of the shocked basalts from the southeastern sector [n=54] of crater rim (Fig. 3.8b) is very interesting (Fig. 4.2c). The basalts from this sector show three ranges of variation, viz. ~50% samples show relatively low and a restricted range of NRM/χ, which is ~1.8 times higher over the unshocked target in average; ~35% samples show very high NRM/χ, which is ~16 times higher over the unshocked target; and a small proportion (~9%) show the highest NRM/χ that are greater than ~5000 Am-1. Our experimental data on the shocked basalts from the southern sector [n=40] of crater rim is also similar type (Fig. 4.2d); ~48% samples show wide variation of NRM/χ and their average value is ~5 times higher over the unshocked target, a significant proportion of target samples (~18%) show very high NRM/χ between ~1000 and 2000 Am-1, and an equivalent proportion show the

Figure 4.1: (a–k) Overall variations of NRM/χ of unshocked and shocked basalts from around the Lonar crater and adjoining areas; sample locations are shown in figure 3.8b.

Figure 4.2: Bar diagrams showing the variation of NRM/χ (Am-1) [white color bar] and REM% [grey color bar] of (a) unshocked basalts from Durga Tegri (DT) and farther east (A5); and (b–k) shocked basalts from around the crater rim and at ~2 to 3 km west of the crater center in the downrange direction. Note that the data is represented in logarithmic scale for NRM/χ and in linear scale for REM.

highest NRM/χ greater than ~4000 Am-1. The shocked basalts from the northern sector [n=34] of crater rim also show a three-fold distribution of NRM/χ, viz., ~65% samples show low and restricted range of NRM/χ, which is ~1.4 times higher over the unshocked target in average, ~29% samples show higher NRM/χ values with a wide range of variation and their average is ~4 times higher over the unshocked target, and the rest ~6% samples show the highest NRM/χ greater than ~4000 Am-1 (Fig. 4.2k). The samples studied from the Kalapani dam and Khini village (KDP/233o-2.17, KHN/251o-2.92), wall to the west of the crater rim (CRW/272o-0.92) and village Swraswati (SWT/300o-2.10), which are at ~2 to 3 km west of the crater center in the downrange direction (Fig. 3.8b), also show both low and high NRM/χ values compared to that of the unshocked target (Table 4.1). The ratios of the number of target basalt samples with NRM/χ

higher and lower than the average unshocked target (R1) in the southwest and northwest directions of the crater rim are lower, and the values are extremely high for the target samples taken from the west of the crater rim. Nearly half of the samples from the Kalapani dam and Khini village [n=90] at southwest of the Lonar crater show low and very restricted NRM/χ (Fig. 4.2f) and their average value is very close to that of unshocked target, a subordinate proportion (~26%) show higher NRM/χ values with wide range of variation and their average is ~7 times higher over the unshocked basalt, the rest (23%) show still higher NRM/χ greater than ~1600 Am-1. Nearly 50% samples collected from the wall to the west of the crater rim [n=38] show low and restricted range of NRM/χ values (Fig. 4.2g) and their average is ~2.5 times higher over the unshocked target, ~37% samples show higher NRM/χ that vary over a wide range and their average is ~15 times higher over the unshocked basalt, the rest (~13%) show highest NRM/χ greater than ~3810 Am-1. The majority (~77%) of shocked basalts from the Swraswati village [n=74] at the northwest of crater has lower but considerably variable NRM/χ (Fig. 4.2i) and their average is ~1.9 times higher over the unshocked target, ~16% samples has higher NRM/χ with a wide range of variation, and the remaining samples show highest NRM/χ greater than ~4090 Am-1. The REM [NRM/SIRM ratio in percentage] provides an estimate of the paleomagnetic field (Kletetschka et al., 2003) and a ratio of ~1.5% indicates an Earth-strength (several tens of μT) field (Gattacceca and Rochette, 2004; Kletetschka et al., 2004; Yu, 2006). For basaltic lava flow samples, a restricted REM range of ~0.5 to 1.5% corresponds to normal thermoremanent magnetization (TRM) in magnetic fields comparable to the geomagnetic field (Parry, 1974). The low-field processes other than TRM (e.g., viscous or chemical remanent magnetization) yield lower REM values for the same paleofield (Fuller et al., 1988), whereas high-field processes (e.g., lightning-induced or artificial IRM or plasma- induced magnetization) yield REM values above 10% (Wasilewski and Dickinson, 2000). REM data has also been used for extraterrestrial materials to evaluate the magnetic field intensity of planetary bodies (cf. Yu, 2006, and references therein). The REM ratio of the most of the unshocked target basalts (~73% data) from the base of the Durga Tegri (DT/100o-2.93) and its farther east (A5/102°-4.61) (Fig. 3.8b) [n=26] are low (<1.29%), which is lower than the typical Earth strength field (~1.5%, often considerable up to 3% if NRM is carried by low-Ti titanomagnetite; Yu, 2006) and have an

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average of 0.70±0.24% (Table 4.1, Fig. 4a); only few samples (~15%) show REM ratio greater than 1.5%.

We have computed the ratios (R2) of number of basalt samples having REM higher and lower than that of the average unshocked basalt (~0.70%) from each sector of the crater rim (Table 4.1). It is observed that the number of basalt samples with REM higher than the average unshocked basalt is always lower than the number of samples with REM lower than the average unshocked basalt from all the sectors of the crater rim except to the southern sector where the ratio is 2:1. The majority of the crater rim target basalts (~67%) also show REM values lower than that of the average unshocked target basalts from Durga Tegri and farther east (~0.70%) with few exceptions (Table 4.1, Fig. 4.2). A subordinate population of target basalts (~26%) from the eastern, southeastern, western and northwestern sectors has higher REM between ~1.8 and 5% in average. However, majority of samples (~64%) from the southern sector of the crater rim only show higher average REM (~3.5%). Extensive samples of shocked basalt from ~2 to 3 km west of the crater center in the downrange direction from the Kalapani dam (KPD) and Khini village (KHN), wall to the west of crater rim (CRW), and Swraswati village (SWT) (Fig. 1b) [n=59] have been studied to evaluate shocked induced REM of target basalts. An observation on R2 ratios (Table 4.1) suggests that the number of shocked basalt samples with REM higher than that of the average unshocked basalt (~0.70%) always constitute a dominant portion of each sample

population, and the R2 ratio is found to be the highest (~3.7:1) for the samples collected from the west of the crater rim (CRW) (Fig. 4.2). Nearly half of the sample population (~57%) from the west of the crater to the downrange shows REM less than ~1.50% with an average comparable to the unshocked target. The rest of the samples, however, show higher REM; ~34% samples show REM (between ~1.7 and 7%) with an average that is ~5 times higher over the unshocked target, and a few samples (~9%) show the highest REM>12% (Fig. 4.2f, g, i). NRM/χ and REM of shocked ejecta basalts: Most of the shocked ejecta basalt samples (n= 23) had NRM/χ<400 Am-1 with an average of ~132 Am-1. The unshocked target basalts from ~3 km east of the Lonar crater center have an average NRM/χ of ~116 Am-1 (for sample locations, see fig. 3.8b). These samples also had low REM (<1%) with an average of ~0.38%. Only four (4) samples had NRM/χ between ~650 and 990 Am-1 with high REM between 1.5 and 7%. The rock magnetic study shows that the majority of samples (60%)

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from the ejecta basalt population have NRM/χ lower than that of average unshocked basalts. Leaving a few exceptions, these ejecta basalts, in general, also have low REM (<1%). Low and high temperature dependence of magnetic susceptibility: To determine the magnetic mineralogy of Lonar basalts, the variation of magnetic susceptibility with temperature (χ-T) (Fig. 4.3a-e) is carried out on a set of systematically collected target rock samples from around the Lonar crater, which includes a sample of unshocked target basalt [A4/100o-2.93], two samples of shocked basalts from around the crater rim (L5/092o-0.98, A18/272o-0.92), and additional two samples of shocked basalts from the downrange direction (A11/251o-2.92, A14/300o-2.10) (see fig. 3.8b). The χ-T curve of unshocked sample A4 (100°-2.93) has apparently two different thermomagnetic phases during heating

(Fig. 4.3a). The lower Tc (Curie temperature) ranges between ~280 and 350°C, while the higher one is at ~580°C. The cooling curve shows only a single phase, with a Tc close to that of magnetite (~585°C). Such irreversible χ-T curves could be due to the presence of titanomaghemite, which probably transformed into magnetite; their low temperature χ-peak at -155°C reflects the isotropic point of multi-domain (MD) magnetite (cf. Radhakrishnamurty et al., 1978). The χ-T curve of shocked basalt sample L5 (092°-0.98) from the crater rim shows a gradual increase of susceptibility with temperature until ~235°C followed by a sharp decrease up to ~580°C with variable slopes (Fig. 4.3b). An initial Tc

between ~235 and 325°C indicates Ti-rich titanomagnetite, followed by a final Tc of magnetite at ~580°C. For this sample, the low temperature variation of χ (warming from liquid nitrogen to room temperature) shows a smooth increase implying the presence of high Ti content in the sample, which shifts the Verwey transition (Carter-Stiglitz et al., 2006). This is also similar to the behavior with SD nature of magnetic carrier (Radhakrishnamurty et al., 1982). The χ-T curve of another shocked basalt sample A18 (272°-0.92) from the crater rim shows an increase in χ at ~300°C and a subsequent decrease at ~400°C, followed by a peak at ~500°C with a sharp drop at ~580°C, which indicates Tc due to magnetite (Fig.

4.3c). The low Tc (~300°C) indicates Ti-rich titanomagnetite, titanomaghemite with

intermediate Tc (~400°C), and finally high Tc (~580°C) of magnetite.

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Figure 4.3: Susceptibility verses temperature (χ-T) curves of representative Lonar unshocked [a] and shocked [b–e] basalt samples. The low temperature susceptibility behavior shows a suppressed χ-peak (-163°C). The possible presence of pyrrhotite in this sample (A18) could be excluded because in the first three progressive heating-cooling cycles i.e. upon heating to ~300°C, 350°C and 400°C (Fig. 4.4a-c), the χ-T curves show nearly reversible behavior with very small degrees of irreversibility. Upon further heating to 500°C, larger degrees of irreversibility are observed (Fig. 4.4d); and the final heating-cooling cycle to 600°C, titanomaghemite is completely inverted, probably forming an iron-rich spinel inversion product with Curie temperature (Tc)

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at ~585°C (Fig. 4.4e). Irreversibility of χ-T curves suggests the onset of titanomaghemite inversion at or slightly below ~400°C (i.e., in a 325–400°C temperature range). The magnetic susceptibility remains constant during gradual increase of the field from ~2 to 450 A/m (Fig. 4.4f), which indicates absence of pyrrhotite in sample A18 (cf. Hrouda et al., 2006). The χ-T curves of shocked basalt samples A11(251°-2.92) and A14 (300°-2.10) in the downrange show a single ferrimagnetic phase with Tc at 580°C corresponding to that of the Ti-poor titanomagnetite or magnetite (Fig. 4.3d, e); low temperature χ-peak at -155°C reflects the isotropic point of MD magnetite (cf. Radhakrishnamurty et al., 1978).

Figure 4.4: (a–e) χ-T curves of target basalt sample (A18/272°-0.92) in progressive heating- cooling runs; red line: heating run, blue line: cooling run; (f) ac field (~2 to 450 A/m) dependence of bulk susceptibility for the same representative specimen.

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The mineralogy of shocked Lonar ejecta basalts (green filled circles in Fig. 3.8b) are found be Ti-rich to Ti-poor titanomagnetite and its oxidized phase, estimated through the

range of Tc’s (between 205°C and 580°C) from χ-T curves (Fig. 4.5a). The low temperature (-192 to 0°C) χ-T curves of these basalts show (i) smooth increase implying presence of high Ti content, which shifts the Verwey transition (ii) shift in isotropic point of MD magnetite from -143°C to -170°C, and (iii) suppressed isotropic point (-170°C).

Figure 4.5: (a) χ-T curves of representative shocked ejecta basalts. (b) Drilling site in Little Lonar ejecta (hammer for scale).

Hysteresis loop: The hysteresis loop shows the irreversible, nonlinear response of a ferromagnet to a magnetic field. It reflects the arrangement of the magnetization in ferromagnetic domains. The shape of the hysteresis loop gives the nature of the magnetic domain state. The typical hysteresis loops of Lonar basalts is shown in figure 4.6. SD grains are characterized by square hysteresis loops; loops of PSD and MD grains are increasingly slender and have inclined slopes. The loop of MD magnetite (Fig. 4.6d, k) shows the typical

ramp-like shape and a quick saturation below 500 mT, low Hc (~5 mT), and small area

inside the loop. The high Hcr/Hc ratio is also the characteristic for MD particles. In basalts where titanomagnetite is transformed into titanomaghemite by low temperature oxidation often show almost perfect SD behavior (Fig. 4.6j). Most of the shocked basalts display

increase in squareness of hysteresis (Mrs/Ms) compared to the unshocked basalts (see Fig. 4.6a, b). The hysteresis data of Lonar basalts are plotted on the reappraisal of the Day plot of ratios between Mrs/Ms against Hcr/Hc (Day et al., 1977; modified after Dunlop, 2002) (Fig.

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4.7; Table 4.1). Most of the samples are plotted on a linear trend within the PSD field between the theoretical mixing curves of Dunlop (2002); the linear trend defined by the samples is directed toward the SD grain size.

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Figure 4.6: (a–l) Typical hysteresis loops of Lonar basalts

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Figure 4.7: Day plot of hysteresis (Day et al., 1977) of Lonar target basalts; dash curved lines are theoretical curves for mixtures of SD+MD grains and SD+SP grains after Dunlop (2002). SD, single-domain; PSD, psuedosingle-domain; MD, multi-domain; SP, superparamagnetic grains respectively.

IRM acquisition and backfield SIRM dc demagnetization: The IRM acquisition curves of unshocked target basalts from Durga Tegri and farther east in the uprange, and shocked basalts from around the Lonar crater rim and ~2-3 km west of the crater center in the downrange [total number of samples (n) = 33, Fig. 3.8b] saturate at low field of <200 mT (Fig. 4.8a) indicating low coercivity magnetic mineral as the main remanence carrier. The

IRM backfield curves (Fig. 4.8a in inset) indicate a broad range of remanent coercivity (Hcr) from 15 to 45 mT, revealing the grain size as SD to PSD type (Ciswoski, 1981; Dankers,

1981). The impact melts possess high Hcr values (up to 90 mT) than the target rocks (both from the crater rim and ejecta) indicating their impact shock hardening nature (Fig. 4.8b inset). The IRM acquisition curves of shocked ejecta basalts also saturate at low field of <300 mT (Fig. 4.8c) indicating low coercivity magnetic mineral as the remanence carrier.

Their IRM backfield curves (Fig. 4.8c in inset) indicate a broad range of Hcr from 10 to 60

mT. The elevated range of Hcr from 50-65 mT (shock hardening) is observed for ejecta basalts collected from little Lonar (A24) location (Fig. 3.8b).

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(b)

Applied field (mT)

(c)

Figure 4.8: Normalized IRM acquisition curves of Lonar (a) target basalts, (b) impact melts, and (c) ejecta basalts; their respective backfield IRM curves are shown in inset.

AF demagnetization spectra of NRM: Demagnetization curves of some Lonar shocked basalts are relatively flat compared to unshocked basalts, indicating that low coercivity fractions of these samples have been preferentially removed (demagnetized) due to impact

(Fig. 4.9). The median destructive field (MDF), the value of Hc for which one-half the initial remanence is randomized, can be used to obtain estimation on coercivity spectra. For unshocked Lonar basalts, collected from ~2 km east of the crater rim, the MDF value ranges between 54 and 58 mT. The shocked basalts from the E, NW, and SW crater rim are characterized by the highest MDF between 78 and 90 mT. But the crater rim samples from

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N, W and SE show relatively low MDF of 39 to 63 mT, whereas the southern rim samples show the lowest MDF between 10 to 15 mT. In the demagnetization data, the unshocked basalts possess a single component of magnetization, which is interpreted as the original TRM. Apart from the original TRM, some shocked basalts possess a more stable secondary component isolated below 10 mT, which is interpreted as a shock remanent magnetization (SRM) acquired in the ambient field direction present during the impact. The directions of the SRM are not closely related to the ambient field at the time of impact but lie within the plane defined by the pre-shock NRM and the ambient field.

Figure 4.9: AF demagnetization spectra of NRM of unshocked and shocked basalts.

The majority of the shocked basaltic blocks in ejecta showed either two or three NRM components or a stable single magnetization component (Fig. 4.10). Some of the samples (n= 4) from the randomly oriented basaltic blocks showed increase in NRM after AF demagnetization to 7.5 mT, which could be the shock-related remanence (SRM). Shocked-induced magnetization effects on magnetic minerals results in demagnetization and remagnetization of ejecta basalts at Lonar crater, which were absent in unshocked target basalts from ~2 km east of crater rim. Shock demagnetized ejecta blocks destructs the low coercivity grains and shocked generated fields may possibly resides in the high coercivity fractions. The shocked elevated Hcr is observed for ejecta basalts which are shocked

110 demagnetized. The minerals that are susceptible to SRM acquisition are Ti-rich titanomagneties carried by low Hcr values. Shock-induced effects results in permanent modification of the crystalline structure of the magnetite grains namely micro-fractures, lattice defects or dislocations.

Figure 4.10: AF demagnetization spectra of NRM of shocked ejecta basalts.

Lowrie-Fuller (LF) test: In the LF test, for SD grains, NRM or ARM are more resistant to AF than SIRM, and for MD grains the opposite occurs. The NRM decay curves are above the ARM and SIRM decay curves in the case of unshocked target from Durga Tegri area and ‘E’ crater rim sector basalts (Fig. 4.11a, b) (for sample locations see fig. 3.8b). The ARM and SIRM decay curves are coincident and are harder than NRM in the case of western sector crater rim basalts (Fig. 4.11c). In the case of CRW and SWT sites, NRM is similar to SIRM and was probably acquired due to meteorite impact (Fig. 4.11e, f). In the impactites, NRM and ARM decay curves are harder than SIRM decay curves (Fig. 4.12). For impact spherules, ARM is much harder than NRM and SIRM indicating that it contains SD magnetic carriers (Fig. 4.13). Thus the presence of SD grains is evident in the impact melts and impact spherules of Lonar crater indicating the meteoritic impact origin.

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Figure 4.11: (a–f) Representative LF test specimens of unshocked and shocked basalts.

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Figure 4.12: (a–f) Representative LF test specimens of impact melts.

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Figure 4.13: (a–f) Representative LF test specimens of impact spherules.

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Thermal demagnetization of SIRM: The thermal demagnetization of SIRM curves of Lonar basalts from crater rim and adjoining areas show a range of unblocking temperatures (Fig. 4.14) corresponding to titanomagnetite (ca. 200°C and 300°C) and magnetite (ca. 580°C) as the mineral magnetic carriers (for sample locations see fig. 3.8b).

Figure 4.14: Thermal demagnetization of SIRM spectra for the representative shocked basalt samples collected mostly from the crater rim and one from the downrange direction.

4.5 ROCK MAGNETISM OF MAGNETIC PARTICLES–RAMGARH STRUCTURE The representative samples of target rocks viz. the Bhander Group sandstone, shale and limestone were collected from inside and outside the Ramgarh structure (Fig. 2.4). The mm- sized magnetic spherules/particles were collected both from the soil inside the crater and from within the finer fraction of reworked debris lying outside the raised rim of this structure (Fig. 4.15). The spherules/particles are magnetic and a hand magnet was used to collect the samples.

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Ramgarh Structure

Figure 4.15: Panchromatic band Landsat-7 grey shaded image of the Ramgarh structure showing locations of samples. Abbreviations: Sst- sandstone, Lst- limestone, and sample R/D-4 was collected from ~250-300 m south of location R/D-2.

The rock magnetic studies were performed on two sets of magnetic particles, one collected from the rim of the structure and the other from the reworked debris from outside the crater (R-80, R-71, Fig. 4.15). They showed very high NRM intensity (~2-17 A/m) and REM ratio (~7-145%) (Das et al., 2009). More elaborate rock magnetic studies on additional samples collected from reworked debris from outside the crater (R-72, RD-2, RD-4, RD-9; Fig. sampling location) were carried out to compare with the previous observations. The collected magnetic particles are reddish-brown in colour, spheroidal to angular in shape, and of relatively small sizes ≤2 mm. All the samples have very high NRM intensities, varying from 1 to 80 Am-1 (0.1 to 5.9 Am2kg-1) with an average value of 15.2 Am-1 (Fig. 4.16a). These values are much higher compared to those for the Upper Bhander sandstone, which has NRM intensities between 0.008 and 0.02 Am-1 (Malone et al., 2008). Our measurements

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of NRM on four sandstone rock samples from within the Ramgarh structure are also much lower and lie between 0.004 and 0.008 Am-1. The NRM values of Ramgarh particles are comparable to the highly shocked granite target rocks of the Vredefort impact crater, South Africa (ranging between 2 and 17 Am-1, average 16.2 Am-1) (Carporzen et al., 2005) or experimentally shocked diabase samples at 4.5-35 GPa (Pesonen et al., 1997). The NRM/SIRM ratio, known as the REM, is indicative of the magnetic field that existed during the formation of the samples under investigation, whereby a ratio of ~1.5% is indicative of the Earth’s magnetic field (Gattacceca and Rochette, 2004). The magnetic particles show very high SIRM values between 1.5 and 235 Am-1, and REM ratios of 0.1 to 1.8 (i.e. 10-180%) for all the samples (Fig. 4.16b), which is much higher than those for the sedimentary rocks between 0.001 and 0.01 (SIRM between 0.06 and 1.2 Am-1). Although lightning was suggested as a possible cause for high REM values (above 10%) of the samples under study (Gattacceca and Rochette, 2004), the very high REM values of Ramgarh may probably be due to the existence of a strong impact-generated plasma magnetic fields during their formation. On the basis of NRM intensity and its response to AF demagnetization, the magnetic particles from the Ramgarh structure can be classified into four distinct groups, viz., (i) particles that have NRM intensities of <0.2 Am2kg-1 and decaying very gently against the applied increasing alternating magnetic field (Fig. 4.16c, f), (ii) particles with moderately higher NRM intensity range of ~0.5 and 1 Am2kg-1 and being stable up to a peak AF demagnetization of 5 mT followed by smooth decay with increasing magnetic field (Fig. 4.16e), (iii) particles having a much higher NRM intensity range of 1 to 1.6 Am2kg-1 and being stable up to a peak AF demagnetization of 5-15 mT and decaying sharply afterwards (Fig. 4.16c, e, f), (iv) particles with the highest NRM intensity (>2 Am2kg-1 ) that are stable up to a peak AF demagnetization of 10-20 mT (Fig. 4.16e), (v) finally specimens collected from the rim of the Ramgarh structure (R-80) at the southeast that acquired magnetization during the AF demagnetization of the NRM intensity up to a peak AF field of 30-45 mT and later sharply decaying (Fig. 4.16d), a property which is quite uncommon in most terrestrial materials.

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Figure 4.16: (a) Bar diagram showing variation of NRM intensity in Ramgarh magnetic particles. Most of the samples have very high NRM not expected for a sedimentary terrain like Vindhyan. (b) Bar diagram showing exceptionally high REM (NRM/SIRM) of magnetic particles indicating their formation under a field several order of magnitude higher than the present Earth’s magnetic field (~40 Am-1). (c-f) AF demagnetization curves for the Ramgarh particles that show four distinct group of samples: (i) particles with low NRM (~0.2 Am2kg-1) with very gentle decay pattern (Fig. 4.16e, f), (ii) particles with moderate NRM (~0.5 and 1 Am2kg-1) with beginning of NRM decay at ~5 mT (Fig. 4.16e), (iii) particles with high NRM (~1 to 1.6 Am2kg-1) showing beginning of NRM decay between 5 to 15 mT (Fig. 4.16c, e, f), (iv) particles with the highest NRM with beginning of NRM decay at 10-20 mT (Fig. 4.16e), and (v) particles showing a general increase of NRM up to 0.2 to 1.4 Am2kg-1 with increasing demagnetization level up to ~30-40 mT followed by a sharp decrease in NRM intensity (Fig. 4.16d). Figure 4.15 shows the sample locations of R-80, 71, 72, R/D-9,2,4. 118

The magnetic mineralogy of magnetic particles was found to be magnetite as evident from a drop in susceptibility at 585°C in the χ-T curve measurements. The sample R/80/6 shows a drop in χ at 605°C indicating the presence of Fe content (Fig. 4.17).

Figure 4.17: (a–d) χ-T curves of magnetic particles collected from Ramgarh structure.

The Zijderveld plot (Zijderveld, 1967) of magnetic particles show three distinct components of magnetization of the intensity vector for most of the magnetic particles (Fig. 4.18). One component, which is removed within 5-10 mT of the AF field, is a low coercivity component. Another component that is removed within 15-30 mT is a secondary component, and the third component is a stable primary component which could be acquired during the formation of these spherules within the impact-generated magnetic field.

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Figure 4.18: Zijderveld plots of Ramgarh magnetic particles.

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The IRM acquisition of magnetic particles show a very sharp increase in IRM intensity within a forward magnetic field of 80-100 mT that becomes saturated within 200 mT indicating the presence of low coercivity magnetic mineral contribution for most of the samples; the coercivity of remanence (Hcr) spectrum of these samples lies within 15-50 mT range (Fig. 4.19).

Figure 4.19: IRM acquisition curves for magnetic particles collected from surrounding soils of the Ramgarh structure; sample locations are shown in figure 4.15.

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The AF demagnetization of NRM and SIRM intensity of the spherule-like substances shows that the NRM intensity is relatively stable against the AF demagnetization and their decay pattern is quite similar suggesting that the magnetization has been caused by lightning or other high energy fields such as impact-generated plasma magnetic field (Carporzen et al., 2005). In the LF test, NRM decay curve is above the SIRM decay curve and ARM decay curve remains constant except for the sample RD/2/4 suggesting the presence of SD grains.

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Figure 4.20: (a–i) Lowrie-Fuller (LF) test of representative magnetic particles collected from the Ramgarh structure.

4.6 NRM of Ramgarh target rocks: The NRM of target rocks from Ramgarh structure were measured with an AGICO JR-6A spinner magnetometer (sampling locations are shown in figure 2.4). The NRM dataset is provided in Table 4.2. The samples are too weak in the NRM intensity to further carry out the AMS measurements. For palaeomagnetic measurements, a SQUID magnetometer is required.

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Table 4.2: NRM of Ramgarh target rocks F F Sample D I Sample D I (mA/m) (mA/m) RJ1.1 353 -14 7.43 RJ7.4.1 78 4 2.30 RJ1.2 346 -11 7.91 RJ7.4.2 135 10.7 1.49 RJ3.2 2 27 0.58 RJ7.5.1 347.6 4.9 3.78 RJ3.3 40 15 0.41 RJ7.5.2 347.5 -5.2 5.61 RJ4.1 159 68 203 RJ7.6.1 288 -30.7 1.41 RJ4.2 213 -10 64.78 RJ7.6.2 311.1 19.8 1.96 RJ4.3 203 -9 212 RJ8.1.1 322 1 2.83 RJ5.1 149 50 1.63 RJ8.2.1 359.5 32.4 2.06 RJ5.2 312 -11 1.51 RJ9.2.1 331.3 -23 0.60 RJ5.3 37 15 1.67 RJ10.1.1 146.9 46.8 3.55 RJ5.4.1 303 -10 2.29 RJ10.2.1 348.3 3.7 0.91 RJ5.4.2 279 16 11.46 RJ10.4.1 204.9 50.8 0.84 RJ5.5.1 224 61 1.85 RJ10.5.1 245.6 -39.5 8.23 RJ5.6.1 156 31 3.13 RJ11.1.1 31.7 19.5 1.74 RJ5.6.2 163 52 1.77 RJ11.2.1 42.8 46.2 0.50 RJ5.6.3 115 66 1.42 RJ11.2.2 62.7 21.6 0.66 RJ5.9.1 226 39 1.28 RJ11.3.1 26.7 -16.3 2.29 RJ6.1.1 187 -12 0.79 RJ11.3.2 38.1 -15.1 3.31 RJ6.2.1 350 63 0.44 RJ11.4.1 63.8 31.7 0.42 RJ6.3.1 116 44 0.65 RJ11.5.1 30.9 -27 3.07 RJ6.4.1 355 51 0.66 RJ11.5.2 38.1 -18.3 4.27 RJ7.1.1 29 42 2.91 RJ11.6.1 40.4 12.3 0.43 RJ7.1.2 4 22 2.48 RJ11.6.2 58.2 48.8 0.54 RJ7.1.3 354 -23 4.65 RJ11.7.1 28.5 -2.6 1.74 RJ7.2.1 29 -42 1.57 RJ11.8.1 14.7 -27.7 1.05 RJ7.2.2 23 -21 0.94 RJ11.8.2 66.1 -3.1 0.56 RJ7.3.1 318 -3 1.07 RJ11.9.1 29 -1.9 2.28 RJ7.3.2 130 13 1.42 RJ11.9.2 23.1 -8.8 2.82

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F F Sample D I Sample D I (mA/m) (mA/m) RJ11.10.1 33.3 -1.7 0.39 RJ15.6.1 339.8 -53.7 5.73 RJ11.10.2 30.1 23.2 0.60 RJ15.7.1 324.6 25.4 1.48 RJ11.11.1 57.5 23.8 1.91 RJ15.7.2 347 32.4 3.60 RJ12.1.1 75.6 27.6 1.06 RJ15.9.1 330 19.7 2.09 RJ12.1.2 13.9 -26.2 0.67 RJ15.11.1 303.7 26.7 2.04 RJ12.3.1 349.8 35 4.79 RJ16.1.1 55 -24.3 11.78 RJ12.3.2 334.7 -0.2 16.89 RJ16.2.2 221.6 45.3 7.21 RJ12.4.1 246.6 -48.6 1.12 RJ16.3.1 350.1 -11.8 5.30 RJ12.4.2 258.7 -13.5 4.81 RJ16.3.2 347.7 5.5 10.58 RJ14.1.1 9.1 30.6 0.65 RJ16.4.1 86.9 -16.4 7.78 RJ14.2.1 350.5 45.9 7.80 RJ16.4.2 108.7 -2 3.01 RJ14.3.1 335.8 37.8 0.37 RJ16.5.1 88.7 10.1 4.91 RJ14.4.1 112.4 24.7 0.80 RJ16.5.2 34.7 42.1 6.66 RJ14.5.1 21.1 49.5 0.39 RJ16.6.1 358 67.3 4.74 RJ14.7.1 3.7 26.7 0.35 RJ16.7.1 357.4 9.4 3.15 RJ14.8.1 335.8 26.7 1.33 RJ16.8.1 20.8 -10 13.24 RJ14.9.1 320 -14.4 0.22 RJ16.8.2 28.6 -19.7 4.26 RJ14.10.1 345.8 -7.6 1.14 RJ16.9.1 39.2 63.7 14.17 RJ14.11.1 329.7 44.5 0.90 RJ16.9.2 9.1 56.7 15.13 RH14.12.1 39.7 63.4 1.38 RJ16.10.1 267.4 -1.4 7.90 RJ14.13.1 354.8 39.5 0.61 RJ16.11.1 88.9 -9.5 9.61 RJ14.14.1 354.4 46.6 0.93 RJ16.12.1 244.8 56.2 8.02 RJ14.15.1 13.6 41.2 0.47 RJ16.12.2 348.6 37.7 16.33 RJ15.1.1 344.6 30.3 2.49 RJ16.13.1 344.8 10.4 21.84 RJ15.1.2 340.2 39 4.46 RJ16.14.1 106.8 51.4 7.68 RJ15.2.1 343.2 45.8 11.27 RJ16.14.2 348.5 47.2 22.82 RJ15.3.1 306.9 30.3 2.36 RJ16.15.1 106.7 48.4 7.43 RJ15.3.2 329.6 45.6 5.10 RJ16.15.2 356 45.2 21.98 RJ15.5.1 311.8 30.9 1.68 RJ17.1.1 32.8 -84.7 0.27

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F F Sample D I Sample D I (mA/m) (mA/m) RJ17.2.1 6.7 8.7 3.37 RJ19.2.1 179.2 -40.8 3.03 RJ17.3.1 8.3 -11.7 0.53 RJ19.3.2 174.9 -36.5 3.91 RJ17.4.1 6.8 -48.9 1.85 RJ19.4.1 205.3 21.7 3.09 RJ17.5.1 16.5 64.3 0.64 RJ19.4.2 206.4 9 2.21 RJ17.6.1 3.5 10.7 0.99 RJ19.4.3 184 -43.7 3.21 RJ17.7.2 333 28 0.29 RJ19.5.1 148.9 -72.8 1.10 RJ17.7.3 259.6 66.7 0.45 RJ19.5.2 180.6 -67.6 1.42 RJ17.8.1 333.6 32.9 0.37 RJ19.5.3 174.7 -38.2 3.04 RJ18.1.1 24.7 -59.3 3.19 RJ19.6.1 169.7 -47.1 2.79 RJ18.1.2 55.5 -39.7 2.85 RJ19.7.1 183.8 -38.1 0.94 RJ18.2.1 45.2 -44 2.61 RJ19.7.2 171.6 49 2.41 RJ18.2.2 29.5 -43.3 1.69 RJ19.7.3 176 -11.3 1.92 RJ18.3.1 35.4 -51.9 1.81 RJ19.8.1 210.5 -64.7 1.72 RJ18.4.1 6.8 -45.3 2.24 RJ19.8.2 180.7 -30.6 2.48 RJ18.4.2 21 -39.1 1.61 RJ19.9.1 178.6 -49.2 2.65 RJ18.5.1 119.7 -63 0.93 RJ20.2.1 157.9 -16.2 53.09 RJ18.6.1 37.1 -59.8 2.13 RJ20.4.1 161.3 -16.4 55.16 RJ18.6.2 39.8 -52.4 2.00 RJ20.5.2 239.3 33 3.05 RJ18.8.1 21.5 -62.7 1.87 RJ20.6.1 156.3 -5 53.60 RJ18.8.2 42.8 -66.8 1.57 RJ20.8.1 157.9 -3.1 55.60 RJ18.9.1 347 -57.4 1.00 RJ20.9.1 251.9 20.8 43.15 RJ18.10.1 54.3 -40 0.88 RJ20.11.1 255.2 13.2 33.89 RJ18.11.1 8.3 -27.9 1.25 RJ20.12.1 223.7 27.8 41.71 RJ18.12.1 86.1 -53.4 1.42 RJ23.1.1 24.6 -42.3 0.82 RJ18.12.2 130.4 -58 2.95 RJ23.2.1 227.8 -9.1 0.58 RJ18.13.1 12.6 -5 1.11 RJ23.2.2 260.3 3.8 0.68 RJ18.14.1 27.2 -44.1 1.93 RJ23.2.3 286.6 22.5 0.61 RJ18.15.1 23.8 -39.6 0.93 RJ23.3.1 10 -23.2 0.75 RJ19.1.1 181.9 -58.3 0.98 RJ23.4.1 348.7 -32.4 0.30

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F F Sample D I Sample D I (mA/m) (mA/m) RJ23.5.1 27.5 -44.1 0.59 RJ25.3.1 5.6 54.9 0.56 RJ23.5.2 5.5 -28.4 1.10 RJ25.4.1 103.3 -35.8 0.31 RJ23.6.1 317.8 -11 0.52 RJ26.1.1 237.8 27.9 0.52 RJ23.6.2 320.5 -8.8 1.11 RJ26.2.1 148 -10.2 0.44 RJ23.6.3 310.2 -46.8 1.10 RJ26.3.1 232.4 78.7 0.98 RJ23.7.1 42.6 -41.8 0.65 RJ26.4.1 328.8 38.4 0.47 RJ23.7.2 21.4 11.9 0.67 RJ26.5.1 276.4 67.8 0.78 RJ23.8.1 344.9 27.6 1.83 RJ27.1.1 338 28.9 0.54 RJ23.9.1 352 -24.3 0.64 RJ27.2.1 177 -61.9 0.51 RJ23.9.2 20.8 -12.4 0.81 RJ27.2.2 143.1 -79.2 1.04 RJ23.9.3 355.6 19.8 0.70 RJ27.2.3 132.9 -60.8 0.35 RJ23.10.1 3.3 37 3.05 RJ27.3.1 319.3 31.4 0.67 RJ25.1.1 30.5 40.3 0.62 RJ27.4.1 172.4 -74.6 0.63 RJ25.2.1 261.8 33.6 1.14 RJ27.4.2 141.6 -64.7 1.00

4.7 CONCLUSION The Lonar shocked basalts, in general, show an increase of NRM/χ in variable proportions compared to the unshocked basalts in the Lonar crater (Fig. 4.2). The distribution of NRM/χ in shocked basalts around the crater rim appears to be systematic with reference to the east- west plane of impact. The shocked basalts in the eastern sector of the crater rim in the uprange direction (Fig. 4.2b) and in the western half of the crater rim including the southwestern, western, and northwestern sectors in the downrange (Fig. 4.2e, h, j) show minimum increase in average NRM/χ (~1.5 times) compared to the unshocked target. These two zones on the crater rim in fact represent the forbidden zones of ejecta distribution around the crater rim in oblique impact experiments (Gault and Wedekind, 1978) and most likely represent the zones of low shock pressure. Our present observation on AMS also confirms major branching of the impact stress into the southwest- and northwest components (Fig. 3.10, 3.11) (see chapter 3 for more details) leaving the western sector crater rim in between these two major shock pressure components in the downrange as a low shock pressure zone during an oblique impact from the east. Although variable, the average NRM/χ of target basalts in the southeastern, southern, and northern sectors of the crater rim,

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which are oriented oblique to the east-west plane of impact and have experienced high shock pressure (Fig. 3.11), show a wide increment (~1.4 to 16.4 times) in comparison to the unshocked target (Fig. 4.2c, d, k). The distant target-rocks in the downrange direction at ~2 to 3 km west of the crater center, which could have experienced high shock pressure during oblique impact (cf. Pierrazo and Melosh, 2000), also show increase in NRM/χ (~1.1 to 25.8 times) over the unshocked target (Fig. 4.2f, g, i). So it could be concluded that the NRM/χ of the target rocks increases with shock pressure during impact, and its variations around the crater rim and in downrange are dependent on the distribution and variation in the intensity of the shock pressure during an oblique impact. It is observed in our present study that the REM values for the most of the unshocked basalt samples are lower than that of the typical Earth (~1.5%). The exact reason of this observation is not clearly known at our present stage of knowledge but it appears that it could be related to the complex evolution of continental flood basalts that is characterized by multiple eruptions and reheating of the stratigraphically older flows (cf. Vandamme et al., 1991). It was also argued that the high-field processes like asteroid impact could have resulted high REM ratio of the target rocks (Wasilewski and Dickinson, 2000; Gattacceca and Rochette, 2004), our observation on the Lonar crater is, however, different (Fig. 4, Table 1). The most of the shocked target basalt samples from around the crater rim (~67% of total 99 samples), where the important magnetization factor beside the variable shock pressure could be the impact-generated magnetic field (Weiss et al., 2010), show average REM (~0.37%) that is nearly half of the unshocked target. Although a subordinate proportion of samples (~31%) have REM (average ~2.7%) ~4 times higher than that of the average unshocked target basalt, they do not show any systematic distribution around the crater rim with reference to the east-west plane of impact at Lonar. Weiss et al. (2010) reported REM of Lonar impact spherules and impact-melts, which are mostly less that ~1% and these low REM values could be attributed to the weak impact-generated magnetic field (<100 µT) existed during the formation of the Lonar crater. So it appears that the weak impact-generated magnetic field could be one of the factors beside the variable impact stress around the rim of the Lonar crater that resulted overall low REM of the target basalts from around the crater rim, however, this aspect of magnetization of Lonar target rocks needs further investigation.

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On the other hand, nearly half of the target basalt samples from ~2-3 km west of the crater center in the downrange, where the impact-generated stress dominates in oblique impact (Pierazzo and Melosh, 2000) and is perhaps the only important factor controlling the impact-induced magnetization, have REMs ~5 times higher over the unshocked target basalt. So it can be concluded that the high impact stress (>3 GPa) could have also increased the REM of the target basalt, however, the variable distribution of shock pressure around the rim of the Lonar crater aided with some unknown impact induced magnetic field existed on the Lonar crater could have resulted low REM of the shocked basalts on the crater rim. The rock magnetic study shows that the majority of samples (60%) from the ejecta basalt population have NRM/χ lower than that of the average unshocked basalts (~116 Am-1). Leaving a few exceptions, these ejecta basalts, in general, also have low REM (<1%). So it can be concluded that the ejecta basalts, in general, are poorly magnetized due to impact. The rock magnetic measurements suggest that the Lonar target basalts essentially contain PSD grain size Ti-rich to Ti-poor titanomagnetite and their low temperature oxidized products as the magnetic carriers (cf. Basavaiah, 2011). The rock magnetic properties show that the unshocked and shocked basalts differ significantly in: (a) bulk-coercivity (MDF), (b) squareness of hysteresis (Mrs/Ms), and (c) low and high temperature susceptibility

measurements (Figs. 4.3, 4.6, 4.9). The impact melt rocks possess high Hcr values (up to 90 mT) than the target rocks (both from the crater rim and ejecta blanket) indicating their impact shock hardening nature (Fig. 4.8). Shocked-induced effects on magnetic minerals results in demagnetization and remagnetization of the ejecta basalts at Lonar crater, which were absent in unshocked target basalts from ~3 km east from the center of the crater rim (Fig. 4.10). Shock demagnetized ejecta blocks destructs the low coercivity grains and shocked generated fields may possibly resides in the high coercivity fractions. The shocked elevated Hcr is observed for ejecta basalts which are shocked demagnetized. The minerals

that are susceptible to SRM acquisition are Ti-rich titanomagneties carried by low Hcr values. Shock-induced effects on magnetic minerals results in permanent modification of the crystalline structure of the magnetite grains namely micro-fractures, lattice defects or dislocations. Preliminary observations on FeO-rich particles/spherules from the Ramgarh structure show that these spherules have very high NRM (1-80 Am-1), which are much higher compared to target sedimentary rocks (~0.004 - 0.008 Am-1), and REM ratio (10-180%)

129 indicating the presence of a high magnetic field during their formation, much higher than the ambient Earth’s magnetic field (~40 Am-1). The rock magnetic study of these magnetic particles thus suggests the evidence of impact-amplified magnetic field resulting from a plasma cloud generated by the impact interacting with Earth ambient magnetic field, or less likely, the shock magnetization.

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CHAPTER 5

PALAEOMAGNETIC STUDY OF LONAR BASALTS

5.1 INTRODUCTION The chapter deals with the palaeomagnetic study of the Lonar basalts to test the hypothesis of shock remanent magnetization (SRM), to find the evidence of impact-amplified plasma magnetic fields, and to determine the palaeomagnetic pole. The chapter also concerns to check whether there are any variations in low coercivity and low temperature (LC_LT) and/or high coercivity and high temperature (HC_HT) magnetization components with the E-W plane of asteroid impact. The palaeomagnetic measurements of randomly oriented shocked ejecta basalts were also discussed in this chapter. Shock can produce demagnetization and/or remagnetization effects in shocked target rocks which results in the acquisition of SRM in the direction of the Earth’s magnetic field at the time of impact. Cisowski and Fuller (1978) detected a secondary component of magnetization probably acquired at the time of impact at Meteor crater, Arizona, and Lonar crater, India, while Halls (1979) has documented the existence of a secondary magnetization at Slate Islands, Canada, which exhibits several properties in keeping with those of SRM determined experimentally (Cisowski and Fuller, 1978). Halls (1979) found that the intensity of the SRM decreases away from the point of impact and is restricted to the low coercivity fraction. The remanence is also acquired rapidly (between impact and the formation of central uplift), as evidenced by a small directional scatter. Studies at Charlevoix, Canada, suggest that the large reduction in remanence intensity in samples from the central uplift is shock related (Robertson and Roy, 1979). However, the high coercivity of the carrier phase (titanohematite) at Charlevoix explains why an acquired SRM is absent. SRM is most likely to occur in autochthonous target rocks experiencing pressures greater than 1 GPa, but temperatures less than Curie points of the magnetic phases present. The ambient field at the time of impact may also be transiently produced (Crawford and Schultz, 1988) or amplified (Hood and Artemieva, 2008) by the impact itself which may lead to nonunidirectional SRM (Crawford and Schultz, 1988; Srnka et al., 1979). These

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transient fields are preserved only in much larger craters, where they record either in shock- produced grains or as a TRM in extensive melt sheets (Louzada et al., 2008). The efficiency of SRM is significantly less than that of TRM. SRM is also more susceptible to viscous decay, and may not be stable over geologic time (Gattacceca et al., 2007).

5.2 SAMPLES AND LABORATORY TREATMENT The palaeomagnetic samples are collected with a portable gasoline-powered rock drill from rock outcrops. The sampling angles (azimuth and hade) are noted with an orienting device. The samples are then cut in the laboratory to prepare the specimens to be measured. The specimens are cylinders with diameter 25.4 mm and height 22 mm (for sampling locations, see figure 3.8b). The goal of the palaeomagnetic measurements on Lonar rock formation is to measure the remanent magnetization, isolating the various remanence components using AF and thermal demagnetization techniques and to test for the hypothesis of SRM or evidence of impact-amplified plasma magnetic fields as suggested over impact structures.

5.3 PREVIOUS STUDIES ON PALAEOMAGNETISM OF LONAR CRATER Rao and Bhalla (1984) carried out palaeomagnetic and rock magnetic investigations on oriented rock samples collected from the inner walls of Lonar lake and reported that some

magnetic parameters; viz., Jn (NRM), K (susceptibility), Qn (Köenigsberger ratio), and declination showed systematic variations, whereas random variations were observed for the surrounding target rocks. A soft secondary shock component (SRM) was also identified in their study that was acquired in the Earth’s present magnetic field, whereas the stable primary component is similar to that of Lower Tertiary period reverse magnetization. More recent work by Louzada et al. (2008), however, suggested that this secondary component (LC_LT) was not directly related to shock metamorphism but was replaced by a post-impact component acquired by viscous and/or chemical remanent magnetization (VRM and/or CRM). They observed slightly elevated coercivity in shocked ejecta basalts and suggested that paleomagnetism can provide a constraint on shock heating in the absence of petrographic evidence of shock (<187±15°C). Weiss et al. (2010) carried out palaeomagnetic study of Lonar basaltic impact glasses and found that these glasses contain a NRM whose properties depend strikingly on sample mass. Small (<0.5 g), splash-form samples demagnetize erratically and are inefficiently magnetized, while larger, irregularly shaped samples contain a stable component that is efficiently magnetized similar to the 132

Lonar basalts. They observe that the rock magnetic recording properties of these samples are uncorreleated with mass and conclude that the size dependence of the NRM reflects a difference in how the samples acquired thermoremanence. The splash forms of the smaller samples indicate they cooled during flight and therefore that they were magnetized while in motion, explaining their weak and unstable NRM. This motional NRM is a new manifestation of TRM not observed before in geologic samples. The ratios of NRM to saturation isothermal remanent magnetization (SIRM) for the small glasses reported by them were only 0.5-1×10-3, while the large glasses had ratios twice as large. These values were nearly an order of magnitude lower than those measured for the nearby Deccan basalts (Louzada et al., 2008). The low NRM/SIRM ratios of the Lonar glasses were interpreted to indicate the absence of any impact-generated paleofields substantially higher than several tens of micro Tesla (μT) at the Lonar crater (also Weiss et al., 2010), and the glasses slightly underestimated the intensity of the field in which they cooled, probably due to the effects of rotation during cooling.

5.4 PALAEOMAGENTIC MEASUREMENTS The NRM of Lonar basalts was measured with a Molspin spinner and Agico JR-6A spinner magnetometers. The measurement of NRM and low-field AMS of the unshocked and shocked basalts were performed before proceeding to AF and thermal demagnetization cleaning techniques. For palaeomagnetic directional analysis, selected specimens were subjected to an alternating field (up to 100 mT in ~15 discrete steps) and thermal (up to 680°C in ~26 discrete steps) demagnetization analyses using an ASC D2000 AF demagnetizer and MMTD80 thermal demagnetizer respectively; magnetic remanence after each demagnetization step was measured by an Agico JR-6A spinner magnetometer. The LC_LT and HC_HT magnetization components were derived from these data by principal component analysis (Kirschvink, 1980), guided by visual inspection of orthogonal demagnetization plots (Zijderveld, 1967).

5.5 PALAEOMAGNETIC DATA OF LONAR BASALTS The basaltic rocks occurring in and around the Lonar crater (Fig. 3.8b) can apparently be classified into two types depending on our previous observation on impact-induced rock

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deformational features and AMS studies (Misra et al., 2010), viz., (a) unshocked basalts: those lying away from the crater rim but close to the crater to the east, and (b) shocked basalts: lying along the crater rim and walls, and in the downrange direction to the west. The best exposed, thick, unshocked basalt flow that could be stratigraphically equivalent to the fourth flow along the crater rim (thickness ~40 m, Ghosh and Bhaduri, 2003) was sampled from the base of hillock Durga Tegri (DT/100°-2.93; meaning of the notation: sampling site DT is located at 2.93 km distance from the crater center along a direction of 100o from the geographic north in clockwise direction) (Fig. 3.8b). Additional samples of the fourth basalt flow were also collected from farther east of the Durga Tegri (A5/102°-4.61). Difference in opinion also exists on the status of LC_LT magnetization component of shocked basalts from the Lonar crater (see Rao and Bhalla, 1984; Louzada et al., 2008). To examine any possible relationship between HC_HT and LC_LT directions of shocked basalts with reference to the trajectory of asteroid impact, we have investigated the AF and thermal demagnetization behavior of NRM of unshocked and shocked basalts (Table 5.1). Nature of LC_LT component: Our observations show that the LC_LT component in both the unshocked and shocked Lonar basalts is easily erased by peak AF demagnetization between ~10 and 25 mT or by thermal demagnetization to less than ~300oC (Fig. 5.1). The LC_LT direction of the unshocked basalts from the base of Durga Tegri (DT/100°-2.93), and the east of this location (A5/102°-4.61) [see foot note below]*, and the apparently unshocked basalts from the far south of the Lonar crater (A1/188°-4.04) (not shown in figure) [n=25] shows high variation in declinations mostly within NW and NE, and inclinations of both reversed and normal polarities with a site-mean of D=348.6°, I=+56.2°,

(k=3.5, α95=32.8°) (Fig. 5.2b). The LC_LT direction of shocked basalts from all around the crater rim and at ~2 to 3 km west of the crater center in the downrange direction from the Kalapani dam (KPD/233°-2.17), Khini village (KHN/251°-2.92), wall to the west of crater rim (CRW/267°-2.07), and Swraswati village (SWT/300°-2.10) (Fig. 3.8b) [n=109] similarly show wide variation in declination between NW and NE, and inclinations of both

normal and reversal polarities with a site-mean of D=357.7°, I=+49.7° (k=3.2, α95=10.7°) (Fig. 5.2h). The LC_LT components of both the unshocked and shocked target basalts are statistically identical to present day field (PDF) direction (Dec=-0.6°, Inc=+28.8°; IGRF,

2011), with ~20° inclination bias that is in range of confidence circle of α95 of present study. * One more location (A6: 113°-7.30) ignored from this population because as this site was affected by dynamite blast during a dam construction, and this shock effect is found to have affected the AMS properties of the target from this site. 134

Table 5.1: Palaeomagnetic components of unshocked and shocked basalts from around the Lonar crater, India

(a) High coercivity and high temperature (HC_HT) magnetization component:

Location/Site name D I k α95 (S) (E) K A95 Reference

Lonar Crater—Unshocked Basalts 108.0 +47.4 154.6 5.9 5.3 133.5 ----- 6.2 Present study [site: 188°-4.04] (1) Lonar Crater—Unshocked Basalts 196.1 +68.9 702.4 13.4 16.2 66.4 324.2 19.7 Present study [sites: 100°-2.93, 102°-4.61] (2) Lonar Crater Rim Sector 120.5 +34.2 106.7 10.3 19.6 136.6 134.0 9.2 Present study [sites: 092°-0.98, 272°-0.92, 319°-0.92, and 358°-0.92] (4) Lonar Crater Rim Sector 88.8 +66.6 106.0 5.8 ------Present study [site: 138°-0.88] (1) Lonar Crater Rim Sector, 51.0 +15.6 12.5 16.1 ------Present study [site: 190°-0.86] (1) Lonar Crater Rim Sector 117.3 -27.4 52.1 7.1 ------Present study [site: 229°-0.82] (1) Lonar [sites: 233°-2.17, 251°-2.92, 120.0 +38.0 165.4 8.3 18.2 134.6 141.7 9.0 Present study 267°-2.07, 300-2.10] (4) Lonar—Background flows 137.9 +55.4 12.1 5.2 21.3 112.1 ------Louzada et al. (2008) Lonar—Crater Walls 136.0 +42.0 71.4 5.2 28.4 122.6 ------Rao & Bhalla (1984) Lonar—Crater Rim 128.0 +38.0 56.2 9.5 24.5 130.3 ------Pal & Ramana (1972) Paleo-Deccan direction at Lonar 157.6 +47.4 ----- 1.9 36.9 101.2 ------Vandamme et al. (1991) Aurangabad [19°51'N, 75°16'E] — 150.0 +48.0 26.0 5.5 33.2 106.8 ------Athavale & Anjaneyulu Deccan Basalt (1972) Jalna [19°51'N, 75°16'E] — 160.0 +46.0 32.0 3.8 39.0 98.9 ------Pal & Bhimasankaram Deccan Basalt (1971)

(b) Low coercivity and low temperature (LC_LT) magnetization component:

Location D I k α95 Reference Lonar Crater—Unshocked Basalts (3) 348.6 +56.2 3.5 32.8 Present study Lonar Crater—Shocked Basalts (11) 357.7 +49.7 3.2 10.7 Present study Lonar—Background flows 7.4 +30.7 11.2 5.5 Louzada et al. (2008) Lonar—Crater Walls 9.0 +47.0 47.6 6.4 Rao and Bhalla (1984) Lonar—Crater Rim -0.6 +28.8 ------IGRF, 2011 Abbreviations: D - I, declination - inclination of the site mean direction; k, K, the best estimate of the site mean

Fisher’s (1953) precision parameter, which is a measure of the dispersion of a population of directions; α95, radius of the cone of 95% confidence about the site mean;  - , latitude - longitude of the virtual geomagnetic pole (VGP) in °S and °E; dp - dm, the minor and major axes of the oval of 95% confidence for the VGP; A95 – radius of the 95% confidence circle about the calculated mean pole. Note: The number in the bracket indicates the number of sites considered for computation of site mean ChRM directions and VGPs.

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Figure 5.1: Representative Zijderveld plots of alternating field (AF) and thermal demagnetization data of (a) unshocked target basalts from Durga Tegri (DT), and (b–k) shocked basalts from around the Lonar crater rim and at ~2 to 3 km west of the crater center in the downrange direction; open and closed circle symbols represent projections in vertical and horizontal planes respectively. Note that the LC_LT component for all the samples erased between 10-25 mT alternating field or ~200-300°C unblocking temperature; sample locations are shown in figure 3.8b.

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Figure 5.2: Equal area stereographic plots of unshocked (a, b) and shocked Lonar basalts (c-h) from the Lonar crater: (a) high coercivity and high-temperature (HC_HT) component of the unshocked basalts from the far south of Lonar crater (A1/188°-4.04, cluster 1), and from Durga Tegri and its adjacent east (DT/100°-2.93, A5/102°-4.61, cluster 2); (b) low coercivity and low temperature (LC_LT) magnetization component of both cluster 1 and 2. Other figures show HC_HT magnetization components of shocked target basalts from (c) most of the crater rim, (d) the southeastern sector crater rim, (e) the southern sector crater rim, (f) the southwestern sector crater rim, and (g) at ~2 to 3 km west of crater center in the downrange direction. (h) LC_LT component of all shocked basalts from c–g; and (i) a summary diagram on figures c-g showing symmetrical disposition of HC_HT component of shocked basalts, with reference to the east-west plane of impact and impact direction; sample locations are shown infigure 3.8b. Note that data with positive inclinations are shown in solid circles, and with negative inclinations with open circles.

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Nature of HC_HT component: The main difficulty in evaluating any possible variation in HC_HT component of Lonar shocked basalts due to asteroid impact is the lack of any consistent orientation of HC_HT component of unshocked target basalts that can be used as a reference (cf. Louzada et al., 2008). To get a better solution, the palaeomagnetic data of unshocked target basalts from the Durga Tegri (DT/100°-2.93) and the east of this location (A5/102°-4.61); and apparently unshocked basalt samples from the far south of the Lonar crater (A1/188°-4.04) (not shown in figure 3.8b) are analyzed in the present study. Significantly, the unshocked basalts from the south of the Lonar crater [n=6] yield a point concentration of data with a site-mean HC_HT direction of D=108°, I=+47.4° (k=154.6,

α95=5.9°) (cluster 1 in Fig. 5.2a, Table 5.2), which is different from the mean paleo-Deccan

magnetization direction (D=157.6°, I=+47.4°, α95=1.9°, Vandamme et al., 1991, also see Pal and Bhimasankaram, 1971; Athavale and Anjaneyulu, 1972). The virtual geomagnetic pole

(VGP) of these unshocked basalts lies at 5.3°S, 133.5°E (A95=6.2°). The unshocked target basalts from the Durga Tegri (DT) and its adjacent east (A5) [n=19] also show point concentration of data, however, with a different site-mean HC_HT direction of D=196.1°,

I=+68.9° (k=702.4, α95=13.4°) [VGP: 16.2°S, 66.4°E (A95=19.7°)] (cluster 2 in Fig. 5.2a). The HC_HT component of most of the shocked basalts [n=43] from around the crater rim, except those from the southeastern (138°-0.88), southern (190°-0.86), and southwestern sectors (229°-0.82) (Fig. 3.8b), are oriented mostly towards the ESE to SSE with a site-mean o o of D=120.5 , I=+34.2 (k=106.7, α95=10.3°) (Fig. 5.2c). The HC_HT component of shocked basalts from southeastern sector (138°-0.88) [n=8] have a site-mean of D=88.8°, I=+66.6° o (k=106.0, α95=5.8 ) (Fig. 6d), those from the southern sector (190°-0.86) [n=9] show a o o different site-mean of D=51 , I=+15.6 (k=12.5, α95=16.1°) (Fig. 5.2e), and all those from the southwestern sector (229°-0.82) [n=10] yield reverse polarity inclination with a site-

mean of D=117.3°, I=-27.4° (k=52.1, α95=7.1°) (Fig. 5.2f). We exclude calculations on VGP on our studied shocked basalts in the present study because magnetization history of these samples could be different from other basaltic rocks that quenched under normal polarity of the Earth’s magnetic field. We have also examined the HC_HT component of shocked basalts from ~2 to 3 km west of the crater center in the downrange direction [n=39] from Kalapani dam (KPD/233°-2.17), village Khini (KHN/251°-2.92), wall to the west of the crater rim (CRW/267°-2.07), and from the village Swraswati (SWT/300°-2.10) (Fig. 3.8b). Like most of the shocked basalts 138

from around the Lonar crater rim (Fig. 5.2c), these target basalts show a site-mean of

D=120.0°, I=+38.0° (k=165.4, α95=8.3°) (Fig. 5.2g). The target basalt samples, occurring at ~2 to 3 km west of the crater center in the downrange and having high NRM/χ or REM values compared to those of the unshocked target, yield a LC_LT component and a HC_HT component (Fig. 5.1f, g). The HC_HT components of these shocked basalts are different from the characteristic direction of Deccan traps at Lonar (cf. Vandamme et al., 1991) but are similar to those obtained for the shocked basalts from most of the crater rim having normal NRM/χ or REM values comparable to those of the unshocked target. Some of these shocked basalts from the downrange only yield single magnetization HC_HT component (Fig. 5.1h, j). On the other hand, the shocked basalts from the southeastern, southern, southwestern crater rim sectors with both normal and high NRM/χ or REM values yield different HC_HT directions in comparison to those shocked basalts from most of the crater rim and ~2 to 3 km west of the crater center in the downrange (Fig. 5.1c-e).

5.5 SUMMARY ON LONAR PALAEOMAGNETIC DIRECTIONS Observations on circular/semi-circular gravity and magnetic anomalies (~2.5 mGal and 550 nT respectively) over the Lonar lake suggest that the asteroid impact could have modified the magnetization vector and density of the target (Deccan Traps) up to a depth of ~500-600 m below the surface (Rajasekhar and Mishra, 2005). The HC_HT component of shocked basalts of Louzada et al. (2008) perhaps support this idea because the topmost (fourth) basalt flow on the Lonar crater wall, which experienced the highest stress during the oblique asteroid impact, show a sudden change in HC_HT direction. While the second, third and fifth flows (the flows exposed on the Lonar crater wall are numbered from the base of the crater) show HC_HT direction close to the paleo-Deccan direction at Lonar (D=157.6o, o o I=+47.4 , α95=1.9 , Vandamme et al., 1991; also see Pal and Bhimasankaram, 1971; Athavale and Anjaneyulu, 1972); the fourth basalt flow has acquired a different orientation o o o of HC_HT magnetization of D=126.4 , I=+44.7 , α95=4.1 (Louzada et al., 2008). This sudden change in HC_HT direction between the second and fourth flows of Lonar target was also observed by Rao and Bhalla (1984). Our present strength of data on the fourth basalt flow from the crater rim, except the southeastern, southern, and southwestern sectors, o o o also show a similar site-mean of D=117.9 , I=+39.6 , α95=4.6 (Fig. 5.2c). The average 139

HC_HT direction in the shocked fourth basalt flow from around the crater rim is although similar to one of the two relatively less prominent orientations of unshocked basalt at Lonar (cluster 1 in Fig. 5.2a), considerable scatter in orientations of HC_HT component for this shocked fourth flow is seen (Fig. 5.2c). Additionally, the fourth flow on the crater rim from o the southeastern sector has easterly directed HC_HT direction (D=88.8°, I=+66.6°, α95=5.8 ) (Fig. 5.2d), southern sector has a northeasterly directed HC_HT component (D=51o, o o I=+15.6 , α95=16.1 ) (Fig. 5.2e), and those from the southwestern sector show a negative o o o inclination of HC_HT direction (D=117.3 , I=-27.4 , α95=7.1 ) (Fig. 5.2f), which are absent in the unshocked target basalt populations and could be acquired due to asteroid impact. The HC_HT direction of unshocked Deccan Trap flows at Lonar and nearby areas are variable

(Fig. 5.2a) and are different from the mean Deccan direction (D=157.6°, I=+47.4°, α95=1.9°, cf. Vandamme et al., 1991), and it is argued that significant scatter between site mean directions within a single Deccan flow may be due to remaining early or late overprints due to later reheating. However, our further observations on the AMS data show that unlike the unshocked target basalt samples from the Durga Tegri (DT/100o-2.93) and to its east (A5/102o-4.61), the apparently unshocked samples from the far south of the Lonar crater (A1/188o-4.04) show triaxial distribution of AMS axes and the AMS ellipsoids vary in shape from oblate to prolate type (Fig. 5.3), which indicate that the samples were shocked by some unknown process and can safely be excluded from the present discussion.

Figure 5.3: (a) Stereographic projection of AMS susceptibility axes, and (b) P/ vs. T plot of apparently unshocked target basalt samples from far south of Lonar crater (A1/188o-4.04).

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There could be two possible explanations on the above mentioned observations. The fourth target basalt flow on the Lonar crater rim did not remagnetized during the impact and could represent the primary Deccan magnetization component. This possibility was suggested by Louzada et al. (2008), although their dataset was spatially and temporally too limited to average out secular variation. Rao and Bhalla (1984) also estimated the HC_HT component of Lonar target basalt samples collected from the inner walls of the crater along two profiles in ENE-WSW direction (ten sites: six from eastern profile, four from western o o o profile). Their site-mean HC_HT direction (D=136 , I=+42 , K=71.4, α95=5.2 ), which was computed on data collected mostly from the fourth and second basalt flows, was also different from the mean palaeo-Deccan direction (Vandamme et al., 1991) and the average orientation observed for the unshocked target basalts from the east of the Lonar crater (Fig. 5.2a). So the existing idea that the observed HC_HT component of the fourth basalt flow around the Lonar crater rim could represent the primary Deccan magnetization component is not well supported by data. Alternatively, a more favourable idea is that the fourth basalt flow exposed on the Lonar crater rim could have acquired a HC_HT magnetization component during the impact. This is because (a) the HC_HT magnetization component of the fourth basalt flow is different from the average Deccan direction at Lonar (Vandamme et al., 1991) and that of the unshocked target from the east of the Lonar crater (Fig. 5.2a, cluster 2), (b) these are symmetrically disposed with reference to the east-west plane of impact, and (c) these are directed to the uprange direction and makes obtuse angle with the direction of impact (Fig.

5.2i). Observation on the systematic displacement of the K3 susceptibility axes (see Fig. 3.10) suggests that the Lonar target basalt around the crater rim could have experienced a shocked pressure >3 GPa particularly in the northern and southern sectors of the crater rim (cf. Nishioka, 2007; Nishioka et al., 2007; Nishioka and Funaki, 2008). This intense shock pressure was perhaps sufficient to remagnetize the HC_HT component of the Lonar target basalt within an impact-induced magnetic field (Cisowski and Fuller, 1978). The LC_LT magnetization components of both the unshocked and shocked target basalts are statistically identical to present-day Earth’s magnetic field (PDF) direction (Table 5.1) and it could be the CRM and/or VRM acquired during the last 570±47 ka subsequent to crater formation (cf. Louzada et al., 2008).

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It is understood that the shock heating of target basalts and ejecta blocks in and around the Lonar crater did not exceed ~200°C (Louzada et al., 2008). Our experiments on variation of magnetic susceptibility with temperature (Fig. 4.3b) and unblocking low temperature component in Zijderveld plots (Fig. 5.1b, f, k) also suggest that the shocked basalts around the crater rim and in downrange to the west had remagnetized at around 200-300°C. The mineral that mostly responds to remagnetization at low temperature is Ti-rich titanomagnetite with its low Curie point at around 235°C (Fig. 4.3b). Also our low temperature susceptibility observations have indicated that χ-peak can occur anywhere in the temperature range of -163 to -155°C (Fig. 4.3a, c, d, e), possibly arising from shifts in the isotropic points of magnetite grains because of low concentrations of Ti in them. All such cases represent the presence of MD grains of magnetite. The χ-peak susceptibility shown by MD grains is suppressed for the SD states due to shape anisotropy of the SD grains that possibly occurred during remagnetization. Hence, our present observation on AMS, NRM/χ, partly on REM and paleomagnetism of target basalts suggests that the impact-induced remagnetization of Lonar shocked basalts must have taken place under low temperature– high impact shock pressure conditions, where predominantly PSD and SD Ti-rich titanomagnetites perhaps were the magnetic remanence carriers. In summary, the Lonar basalts acquired a HC_HT component due to impact, which are mostly oriented in uprange, symmetrically disposed about E-W impact plane making an obtuse angle with impact direction. The LC_LT component in both the shocked and unshocked basalts are statistically identical to PDF direction and it could be CRM and/or VRM acquired during the last ~570 ± 47 ka.

5.7 RADIOMETRIC DATING OF IMPACT MELTS Recently, Jourdan et al. (2011) obtain a precise and accurate isotopic age (40Ar/39Ar) of 570 ± 47 ka for the Lonar impact event from four basaltic impact melt rocks. Earlier the fission track, thermoluminescence, and radiocarbon dating of impactites yielded a wide range of dates ranging from ca. 15 to ca. 62 ka (Sengupta et al., 1997; Storzer and Koeberl, 2004), thus illustrating the complexity of dating the Lonar impact crater. For example, radiocarbon dating of Lonar lake sediments yielded ages ranging from ca. 15 to 30 ka, but this range likely represents minimum ages due to carbon contamination (Sengupta et al., 1997). Fission track data yielded an apparent age of 15 ± 13 ka, most likely indicating the age of 142

postimpact processes such as (1) a younger thermal event (Storzer and Koeberl, 2004), (2) alteration, or (3) thermal annealing due to high soil temperature induced by wildfires (or possibly sunlight exposure). The 14C dating of histosols containing the Lonar crater ejecta yielded apparent ages ranging from 1.8 ± 0.5 to 40.8 ± 1.1 ka (Maloof et al., 2010). These dates were reported as positively correlated with δ13C, where such correlation is generally interpreted as demonstrating modern C contamination. Maloof et al. (2010) proposed a maximum age estimate for Lonar of younger than 12 ka, based on the paleosols positions in relation to the Lonar ejecta.

5.8 PALAEOMAGNETISM OF LONAR EJECTA BASALTS The samples of randomly oriented shocked ejecta basalts were collected from N and NE (n= 3 sites), and S and SE directions (n= 2) of the crater at ~1 to 1.5 km distance from the crater centre (green circles in Fig. 3.8b). For palaeomagnetic directional analysis, samples were subjected to alternating field (up to 100 mT in ~15 discrete steps) demagnetization analyses. The low coercivity (LC) and high coercivity (HC) magnetization components were derived from these data by principal component analysis coupled with orthogonal demagnetization plots (Zijderveld, 1967). The mean LC component (n= 12) of the shocked ejecta blocks was statistically identical to the PDF direction (D= 342.5°, I= 39.8°, α95= 38.7°), whereas their mean HC component, which includes both positive and negative inclinations, appears to be random (Fig. 5.4). The HC component of these shocked ejecta basalts is different from that of the unshocked target

(D=196.1°, I=+68.9°, α95=13.4°) and show a broad concentration around the average HC component of the shocked target from around the crater rim (D=120.5°, I=+34.2°,

α95=10.3°).

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Low Coercivity (LC) Component High Coercivity (HC) Component

0° 0°

270° 90°

180° 180°

Figure 5.4: Equal area stereographic plots of (a) LC, and (b) HC of shocked Lonar ejecta basalts, solid circle data with positive inclination, open circle- negative inclination, and ellipse in (b) indicates the zone of distribution of HC component of ejecta basalts.

This observation suggests that the impact shock-induced magnetic field could have existed beyond the modification stage of formation of Lonar crater (French, 1998) when the newly formed ejecta with the randomly deposited basaltic blocks had weekly remagnetized.

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Chapter 6

GEOCHEMISTRY AND XRD OF LONAR AND RAMGARH IMPACT CRATERS

6.1 INTRODUCTION Geochemistry is an extremely versatile tool for studying the impact events. Studies can range from simple major and trace element characterization of impact breccias, melt rocks, glasses, and target rocks to elaborate isotope investigations and oxidation state determinations. Geochemistry can also be used to search for extraterrestrial components and to identify projectiles in impactites and ejecta, or to determine noble gas abundances in minute minerals and the clay mineral composition of fracture fillings in impact breccias. Geochemical analyses are of crucial importance for establishing the impact origin of suspicious geological structures or stratigraphic units, and they contribute invaluable information about every part of the impact process. From nano- and microscale features to global processes, our understanding of impact as a geological phenomenon would be incomplete and lack quantification without geochemical data. The term ‘impactite’ comprises a large variety of rocks formed by the modification of crustal rocks due to impact processes. Impact processes produce brecciation, shock metamorphism, and melting and vaporization of the target rocks. The chemical composition of impactites provides important information that supplements petrological data. It depends on (1) the composition and spatial distribution of the target lithologies; (2) impact energy, which affects the size of the crater, the depth of material involved, and the volume of rocks vaporized or melted; (3) the emplacement and cooling history of impactites; (4) the admixture of projectile material; and (5) post-impact modifications by metamorphism and/or hydrous alteration (including weathering). The objectives of geochemical analysis are to discover how the impactites are related to the target rocks geochemically, and how much of the target rock and what mineral phases are contributing to the impactites. This study is essential in order to better constrain the impact processes and the formation of impact craters.

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Although projectile fragments rarely survive an impact event, detectable amounts of melted and recondensed projectile are often incorporated into impact-produced breccias and melt rocks during crater formation. This dispersed projectile (meteoritic) material can be conclusively identified by distinct chemical and isotopic signatures in the host rocks, thus providing reliable evidence for a meteorite impact event. During impact, original projectile material is diluted by mixing with a volume of vaporized, melted, and fragmented target rock that may be orders of magnitude larger than the volume of the projectile. As a result, the actual amount of projectile material incorporated into impact crater rocks is generally small, typically <1 wt%. Siderophile elements, such as Ni, Co, and the PGEs, i.e. Pt, Pd, Os, Ru, Rh, Ir occur at significantly higher concentrations in meteorites than in average crust. They also show interelement ratios distinct from those of crustal rocks and mantle melts. With target that has low siderophile element contents, it is possible to measure meteoritic contributions down to 0.1% using the PGEs (Huber et al., 2001; Simonson et al., 2009). Distinctly higher siderophile element contents in impact melts, compared to target-rock abundances, can be indicative of the presence of either a chondritic or an iron projectile. Achondritic projectiles are much more difficult to discern because they have significantly lower abundances of the key siderophile elements, and it is necessary to sample all possible target rocks to determine the so-called indigenous component (i.e. the siderophile element content of the impact melt rocks contributed by the target) and thus ascertain that no possibly siderophile element–rich mantle-derived target rock has remained undetected. So far, meteoritic components have been identified in about 45 out of the ca.180 currently known impact structures on Earth (cf. Koeberl, 2007).

6.2 PREVIOUS STUDIES ON GEOCHEMISTRY OF LONAR CRATER Relatively few studies on the geochemistry of Lonar basalts, impactites, and impact glasses (spherules) have been carried out. Morgan (1978) analyzed two Lonar target basalts and three impact-glasses with radiochemical neutron activation analysis and did not find any significant Iridium (Ir) enrichment. In his study, Lonar glasses were also found to be significantly depleted in Re (~7 times) and Se (~2.5 times) relative to parent basalts, which was attributed to a volatilization (elements with low vaporization temperature will vaporize) effect occurring during terrestrial impacts. The small depletions of Lonar impact glasses in

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Au (~27%) and Zn (~14%) were attributed to variations in target rock composition. Stroube et al. (1978) provided a study on the geochemistry (29 elements) of four target rocks and two impact melt rocks. They noted only some variation in the Cr content among the basalt and impact melt samples, and concluded that impact-induced chemical fractionation during formation of the glasses from basalt is minimal for many of the principal rock and mineral- forming elements. Ghosh and Bhaduri (2003), based on a detailed geochemical study of six target rock basalt and six impact-melt samples, noticed that the impact melts were only slightly enriched in Al, Fe, K, Co, and Sr, and slightly depleted in Ti, Mg, Cr, and Sc, compared to the basalt composition. Osae et al. (2005), based on an extensive geochemical study of target basalts and some impactites reinforced the conclusion that chemical fractionation between Lonar basalt and impact-melts was minimal. They also suggested a non-chondritic or Ir-poor impactor for Lonar crater because of the low level of meteoritic contamination in the Lonar impactites. Misra et al. (2006b) suggested that the depletion of

MgO, Na2O, and K2O, and enrichment of Rb and Cs in fine ejecta, compared to target basalts, is probably due to aqueous or hydrothermal alteration. Chakrabarti & Basu (2006) claimed to have found geochemical traces of Archaean basement in some Lonar impactites. This is very difficult to reconcile with the excavation depth expected for a crater of this size, which for a crater of this size would have been on the order of 150 to 300 m – about one third to half of the thickness of the Deccan basalts in the area. Recently, Son and Koeberl (2007) reported detail analyses of 67 impact-melt samples collected from the ejecta blanket around the crater rim, but no enrichment of any meteoritic component was seen. With this background, we attempt here to provide detailed geochemical analysis (XRF) of varieties of target basalts, impactites found in and around the Lonar crater and ejecta blanket. These data are used for a geochemical comparison between target and impactites, and also to provide data for the detailed classification of the Deccan basalts at Lonar. The evidence of the impactor that formed the Lonar crater has been identified within the impact spherules, which are ~0.3 to 1 mm in size and of different aerodynamic shapes including spheres, teardrops, cylinders, dumbbells and spindles by Misra et al., 2009 (Fig. 6.1). They were found in ejecta on the rim of the crater. The spherules show schlieren structure described by chains of tiny dendritic and octahedral-shaped magnetite crystals indicating their quenching from liquid droplets. Microprobe analyses show that, relative to

the target basalt compositions, the spherules have relatively high average Fe2O3 (by ~1.5

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wt%), MgO (~1 wt%), Mn (~200 ppm), Cr (~200 ppm), Co (~50 ppm), Ni (~1000 ppm) and

Zn (~70 ppm), and low Na2O (~1 wt%) and P2O5 (~0.2 wt%). Very high Ni contents, up to 14 times the average content of Lonar basalt, require the presence of a meteoritic component in these spherules. Misra et al., 2009 interpret the high Ni, Cr, and Co abundances in these spherules to indicate that the impactor of the Lonar crater was a chondrite, which is present in abundances of 12 to 20 percent by weight in these impact spherules. Relatively high Zn

yet low Na2O and P2O5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume.

Figure 6.1: Scanning electron microscopy (SEM) images of typical spherules from Lonar ejecta blanket. a, d, e) Spherical shaped particles. b) Spindle shaped particle. c) Dumbbell shaped particle with a prominent vesicle noted by an arrow. d, e, f) The attached welded droplets (d) and drapings (e, f) indicated by arrows provide evidence for low-velocity impacts between the particles entrained in an impact-generated plume (Misra et al., 2009).

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6.3 SAMPLE DETAILS X-ray fluorescence (XRF) spectroscopy and X-ray diffraction (XRD) analysis were carried out on Lonar target rocks and impact products. For this study, rock samples were collected from all around the crater rim, adjoining areas and in the ejecta blanket (for sampling locations see fig. 3.8b). Impactites (glasses), which are mm-size, and in situ impact spherules, were recovered from trenches dug (~47 cm-deep pit) on ejecta blanket close to the southeastern part of the crater rim (GPS location: 19°58.356′N, 76°31.072′E). Contamination is unlikely as the site has no evidence of any development. The glasses were encountered about 5 cm below the alluvium surface in the spherule-rich ejecta horizon. They are characteristically black, have vitreous luster and a highly vesicular surface and a variety of geometric shapes including rod, ellipsoidal, and tear-drop shapes (Figs. 6.1, 6.2a). Secondary infillings of quartz are sometimes found in the vesicles.

(b)

Figure 6.2: (a) Impact spherules (rod, teardrop shape) (b) glass-rich vesicular

impactites showing smooth flow-textured surface and vesicular interior; long

dimension of sample= ~16 cm (c) impact melt rock in the ejecta blanket outside SE

part of crater rim; diameter of coin= 2.5 cm.

Nayak (1972) also reported spherules of comparable dimension and morphology from the eastern and western sides of the crater rim, from the upper layer of ejecta-rich soil. Fredriksson et al. (1973) described the microscopic character of these spherules, which they retrieved from a trench dug to the east of the crater. In their description, these spherules were brown in color, although some colorless and darker brown schlieren and partly melted mineral inclusions were present. Flow banding was also reported. These samples are ellipsoidal in shape and have sizes in the centimeter range. In hand specimens, these impact

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melt rocks appear black in color, are vesicular, and show ropy or flow structures on the surface (Fig. 6.2b, c). Under the microscope, these glasses appear brown in color and also show flow structures. The brownish glasses usually contain unmelted fragments of basalt of various degree of shock deformation, fragments of clinopyroxene and plagioclase, and crystals of magnetite.

6.4 XRF ANALYSIS OF LONAR SAMPLES Major oxide contents were determined using XRF analysis on 4-5 g of powdered sample. The measurement was performed with a Spectro XEPOS XRF spectrometer using the

Turboquant-Powders method (www.ametek.com). The major oxides determined are SiO2,

TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, and K2O in wt% (Table 6.1).

Table 6.1: Major oxides content of unshocked and shocked basalts, shocked ejecta basalts, and impactites expressed in wt%.

Major Unshocked Shocked Basalts Ejecta Basalts Impactites Oxides Basalts (n = 19) (n =6) (n = 3) (wt%) (n = 6) AVG SD AVG SD AVG SD AVG SD SiO2 48.16 0.86 48.20 0.49 48.7 0.44 49.95 0.47 TiO2 2.71 0.25 2.97 0.14 2.87 0.10 2.67 0.04 Al2O3 14.34 0.54 13.85 0.40 14.07 0.67 13.04 0.65 Fe2O3 14.63 1.17 15.67 0.71 15.04 0.61 14.87 0.13 MnO 0.20 0.02 0.23 0.02 0.21 0.01 0.20 0.00 MgO 4.98 0.80 4.09 0.35 4.25 0.41 5.18 0.05 CaO 11.19 0.52 10.83 0.38 10.59 0.42 10.41 0.01 Na2O 3.27 0.36 3.49 0.24 3.56 0.22 2.87 0.05 K2O 0.22 0.10 0.30 0.07 0.39 0.14 0.58 0.02 P2O5 0.30 0.05 0.36 0.03 0.33 0.03 0.23 0.01

6.5 XRD OF LONAR SAMPLES XRD patterns were obtained for powdered samples of Lonar using the inXitu–BTX Benchtop XRD/XRF instrument. The instrument is capable of measuring the diffraction peaks between 5°–55° of 2θ angle at a resolution of 0.25° 2θ full width half maxima (FWHM). An ~20 mg of powdered sample with grain size <150μm is needed for each run. Copper (Cu) target is used in X-ray tube which gives the Kα₁ radiation at λ= 1.540598 and Kα₂ radiation at λ= 1.54433. The reflections due to Kα₂ can be stripped during processing.

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Powder diffraction is a non-destructive technique used for material characterization. It utilizes the diffraction of X-rays from the atomic planes. When X-rays are diffracted from the atomic planes the peak position depends upon interatomic spacing as defined by Bragg’s law. The position, intensity, and width of the peak contain important information about the diffracting material. XRD act as a fingerprint method for mineral identification. Every mineral has its own diffraction pattern which acts like a fingerprint for that mineral but when the mineral composition is complex it may be confusing to resolve the mineralogy. In such cases the data from other techniques like XRF can be combined with XRD for successful qualitative analysis. Further quantitative analysis can be done using the XPowder software. XPowder is a powerful commercial software used for qualitative and quantitative analysis of powder XRD data. XPowder uses PDF2, Difdata as well as user defined databases. Difdata is AMCSD (American Mineralogist Crystal Structure Database) which has been compiled by Bob Downs and Paul Heese of the University of Arizona. It includes all the crystal structure published in American Mineralogist, The Canadian Mineralogist and the European Journal of Mineralogy. This database is having 13,596 entries and is distributed freely. PDF2 is most popular database and is sold by International Center for Diffraction Data (ICDD). It contains the data from ICDD National Institute of Standards and Technology (NIST), Inorganic Crystal Structure Database (ICSD) which sums to 1,57,048 patterns. PDF2 contains all the data present in AMCSD database and Reference Intensity Ratio (RIR) measured with respect to corundum. XRD studies were done on Lonar target rocks and their impact products to know the mineralogy and signatures of shock metamorphism. Whereas the XRF data gives the elemental and oxide composition of the sample, the powder XRD is used to identify the mineralogical phase of powdered rock samples. The XRD study can further be extended to study the variation of crystallite size and residue strain in the crystals of shocked and unshocked samples (peak broadening effect). Magnetic extraction of some representative samples has been done and XRD studies are undertaken for any significant observation among the samples. The Quantification of amorphous content in Lonar samples has been done using the XPowder software. The samples for XRD analysis are classified into five types: shocked, unshocked, ejecta, impactites (melts) and impact spherules. Most of the shocked, unshocked and ejecta samples

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do not show any major difference between them, but melts show high background (Fig. 6.3). The XRD pattern of impact spherules is not clear.

Figure 6.3: Diffraction pattern of Lonar crater rim basalts (pink), ejecta basalts (red), impactites (blue), and impact spherules (dark green).

The amorphous content and FWHM were calculated from the diffractograms of Lonar samples. An attempt was made to analyze size-strain analysis using Williamson-Hall method, but was unsuccessful due to the absence of second order reflections. The amorphous content has been plotted for shocked, unshocked, ejecta, melt and spherules with respect to sample number (Fig. 6.4). Spherules and melt shows very high amount of amorphous content which is possibly due to high rate of cooling. Further the amorphous content of spherules in higher than that of melt. Spherules usually cool down at higher rates while falling from air, and thus show high amorphous content.

Sample Number vs. Amorphous Content

Figure 6.4: Estimation of amorphous content of Lonar samples (shocked and unshocked basalts, ejecta basalts, impact melts, and impact spherules).

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FWHM measurement: Peak broadening in XRD pattern is carried out to find the signature of shock metamorphism in Lonar samples and to understand their variations in peak-width with the direction of Lonar asteroid impact. Skala (2002) has shown the peak broadening as a result of shock metamorphism at Ries crater, Germany. FWHM was used as the measure

of peak-width. Peak broadening has been defined as the ratio 100 × (We – Ws)/Ws, where

We is the half width of the sample and Ws is the half width of the standard reference sample. Skala (2002) found the domain size as major factor in peak broadening. The unit cell refinement from the diffractogram of shocked calcite has shown a decrease in cell parameters and cell volume by about 0.2%, which shows that unit cell does not relax completely to the state before impact. However such shock evidences are not yet clear in the present study. The FWHM measurement was carried out on some selected profiles of diffractogram of shocked, unshocked, ejecta basalts, and melt samples. FWHM of selected peaks for all the samples was not measured due to peak mixing issue. Also it is not possible to include spherule samples due to very broad features in the diffractogram, which is possibly due to high amorphous content. FWHM has been plotted with respect to samples. The plot has been divided into shocked, unshocked, ejecta and melts sub-parts by vertical lines (Fig. 6.5). Theoretically the peak broadening should be more under the combined effect of crystallite size and non-uniform strain in shocked samples, but no clear evidence has been found. From the FWHM plot there is no clear difference between shocked and unshocked samples, but no clear evidence has been found. Magnetic extraction of some representative samples is carried out using ultrasound disaggregation. Three to five gram of powdered sample was dispersed in distilled water, from which the magnetic extract was taken out by a strong magnet. This process was repeated five times for each sample. After this each sample was put to ultrasound disaggregation for ~16 minutes to remove any non-magnetic grains attached to magnetic grains and again the magnetic particles were extracted by strong magnet edge. This process was repeated three times for each sample. The resultant magnetic extract was dark black probably due to the presence of magnetite.

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Figure 6.5: The first plot shows the seven peaks which have been used for FWHM measurement. The second plot shows the magnitude of FWHM (X-axis) with respect to samples (Y-axis). The plot is subdivided according to shocked, unshocked, ejecta and melts types. Note that some values are missing in the plot. This is due to merging of some peaks. It appears that FWHM will not be a reliable measure to compare the shocked and unshocked samples as there is too much variation between same sample types. The reason may be different mineralogy in them. This problem can be overcome by magnetic extraction of samples and then comparing those with XRD peaks.

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The XRD of magnetic extracts has been analyzed with XPowder software, which shows the presence of spinel minerals. The confirmation of exact minerals can be only done by having XRF data. XRF data along with XRD data shows the presence of magnetite, titanomagnetite and possibly ilmenite (Fig. 6.6).

Figure 6.6: XRD of magnetic extract of sample LN8-2-1 from the south crater rim. The sample is exactly matching with that of magnetite PDF2 database. The peak at 2θ~32.5° possibly belongs to ilmenite.

6.6 XRD OF MAGNETIC PARTICLES FROM RAMGARH STRUCTURE The XRD of magnetic particles/spherules collected from the soil inside the Ramgarh structure, show these materials are essentially composed of quartz (PDF no# 86-1561 or 83- 0542; JCPDS-ICDD, 1999) (Fig. 6.7). These quartz fragments are likely from the target sandstone of the Upper Bhander group. The additional varieties of SiO2 polymorphs that may also be present in these spherules are coesite (83-1832), tridymite (01-0378) and cristobalite (82-1232). The other minerals are Fe-Ni alloy (fe-ni), tanite (ta) and molybdenum (mo). The mineral coesite is the high pressure polymorph of quartz that forms at an impact pressure between ~2.8 and 10 Gpa (cf. Deer et al., 1992; French, 1998). The occurrence of high temperature polymorphs like tridymite and cristobalite beside coesite suggests definite rise of temperature in the Ramgarh structure, which could be between ~1470 to 1713oC (cf. Deer et al., 1992).

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Figure 6.7: XRD analysis of six spherule-like materials collected from the soil inside the Ramgarh structure; abbreviations: q- quartz, tr- tridymite, co- coesite, cris- cristobalite.

Note the presence of high pressure polymorphs of SiO2 (tridymite, coesite, cristobalite) indicate possible impact origin of the structure.

6.7 CONCLUSION The XRF analysis of Lonar samples shows that impact melts are depleted or enriched in

some oxides as compared to target rocks. They are depleted in Na2O, P2O5 and enriched in

K2O in comparison to the target rocks, which is in accordance with the findings of Son &

Koeberl (2007). The melt rocks have higher K2O/Na2O ratio than the target rocks. The reason could be selective elemental vaporization and condensation during melt and vapor formation, or hydrothermal alteration. The enrichment of ‘K’ in impactites is a common phenomenon and termed the process “potassium metasomatism”, probably due to vaporization fractionation during the impact, or possibly an immediate post-impact alteration (Puura et al., 2004). The XRD pattern of Lonar shocked, unshocked and ejecta samples do not show any major difference in mineralogical phases between them, but melts show high background effect. The XRD pattern of impact spherules is not clear. The amorphous content estimated from XRD suggest that spherules and melt have very high amount of amorphous content which is possibly due to high rate of cooling while falling from air. Peak broadening (FWHM) in XRD pattern suggests the signature of shock metamorphism but there is no

156 clear difference between shocked and unshocked samples is observed. It appears that FWHM is not a reliable measure to compare the shocked and unshocked samples as there is too much variation between same sample types and it may be due to the presence of different mineralogy in them. The XRD of Lonar magnetic extracts suggests the presence of magnetite, titanomagnetite and possibly ilmenite minerals in them. While the XRD of six spherule-like materials from Ramgarh structure shows the presence of high-pressure polymorphs of SiO2 (tridymite, coesite, cristobalite) indicating the possible asteroid impact origin of the structure.

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CHAPTER 7

CONCLUSIONS AND FUTURE DIRECTIONS

Meteorite/Asteroid impact is a common geological process that shapes the surfaces of rocky (or icy) planetary bodies in our Solar System. The hypervelocity (>11 km/s) impacts of asteroids create some circular to elliptical shape depressions on the target planetary bodies called ‘impact craters’. Most of the rocky planetary bodies in our Solar System have basaltic crusts. Out of the three known terrestrial impact craters that are excavated in the basaltic target rocks, the Lonar crater in India, is fully accessible and possibly the best studied crater; it can be taken as an example to evaluate planetary impact cratering processes. Impact cratering has been studied using mineralogy, geochemistry, geochronology, sedimentology, and stratigraphy. The present work provides an alternative palaeomagnetic and rock magnetic tool to study terrestrial impact craters (Lonar, Ramgarh) in India. It reports for the first time the magnetic properties of flows exposed in the crater wall, target rocks, ejecta clasts and magnetic particles/spherules samples. The benefits of studying impact craters include: (a) understanding the evolution of Earth’s surface, making analogues to other planets, (b) linking them to extinctions, and (c) understanding evolution of meteorites from asteroid parent bodies, planets, and Moons from the Solar System that give us more information about the origin of the Solar System. The impact crater record on the Earth is crucial to understand the only ground-truth data we have to base interpretations of impact craters on other planets and Moons. Meteorite impact structures are the rare features on the surface of the Earth and only 184 confirmed impact structures are currently known till date as shown in figure 1.3. The discovery of meteorite impact structures is based on various diagnostic petrographic and mesoscopic shock metamorphic features such as planar deformation features (PDFs) in the rock forming minerals, shatter cones, and/or fragments of the impacting projectile (e.g. Fig. 1.2). It is revealed that Lonar crater is an excellent analogue for Mars, in close agreement with general understanding. The Lonar crater is 1.8 km diameter across and is formed about

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~52±6 ka or 570±47 ka ago on the basaltic target rocks of Deccan Traps. The basaltic host rock is porphyritic texture with ground mass of plagioclase, augite, pigeonite, titanomagnetite and palagonite. The secondary minerals such as calcite, zeolite, chlorite, serpentine, and chlorophaeite are also present. Impact evidences of Lonar include maskelynite, PDFs in plagioclase and pyroxene including the presence of impact melt glasses/spherules, ejected melt breccias with shocked minerals, and subsurface breccias observed in drill cores below the crater floor (Fig. 2.1). The role of asteroid impact is to create a high magnetic field. The origin of high magnetic fields during impact has been studied for some time. An amplified magnetic field could result from a plasma cloud generated by the impact interacting with the Earth’s magnetic field, or less likely, shock magnetization. In India, the Lonar crater and Dhala structure are the confirmed impact structures; the Ramgarh structure is in a controversial stage (Fig. 1.5). Shock metamorphism and its effects on rocks and minerals were investigated by studying rock magnetic and palaeomagnetic properties of shocked target rocks. Because the intensity of the shock wave decreases radially from the point of impact, the target materials experience varied degrees of shock damage during an impact event. Signatures of the shock metamorphism are shown by damaged mineral crystals, including surface dislocations, formation of diaplectic glass, and partial melting along grain boundaries in the shocked target rocks. Magnetically, shock pressure is accompanied by damage (e.g., microfracturing of magnetic grains) resulting in permanent changes in the intrinsic magnetic properties (a more single-domain like behavior). This study investigated the degree of alteration caused by stress waves on target rocks and shock-induced magnetization effects on magnetic remanence properties. By numerous observations and laboratory studies, it is concluded that magnetic minerals can undergo rock magnetic changes of increasing coercivity as a result of shock (Figs. 4.8, 4.9). Further, it is concluded that the variability of remanence in the Deccan basalts from Lonar can be explained in terms of their differing response to shock as a function of coercivity (Fig. 4.11).The rock magnetic and palaeomagnetic studies on impact craters provide useful data for: (1) evaluation of the angle and direction of asteroid impactor hitting at the crater site (Figs. 3.1–3.6); (2) evaluation of the distribution of shock front with the direction of impact (Fig. 3.11); (3) characterization of shocked and unshocked target rocks (Figs. 4.3, 4.6, 4.7, 4.8, 4.9); (4) understanding the variations in magnetic properties with the direction 159

of impact (Fig. 4.2, Table 4.1); (5) test the hypothesis of shock remanent magnetization (SRM) and evidence of impact-amplified plasma magnetic fields (Fig. 5.2, Table 5.1); (6) identification of potential magnetic shock indicators (ejecta blocks) (Figs. 4.10, 5.4). The conclusions can be summarized as follows:

I. Structural and Anisotropy of Magnetic Susceptibility (AMS) evidence for oblique impact at Lonar: The present study successfully applied a new technique: the Anisotropy of Magnetic Susceptibility (AMS) technique, combined with the analyses of satellite images and the geological structure to evaluate the obliquity (angle) and direction of projectile approach of the Lonar asteroid impact. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image of the Lonar crater in the subhorizontal Deccan basalt shows that this simple, bowl- shaped impact crater has a near-circular rim with a circularity of ~0.95. Most of the highly reflecting, continuous-ejecta blanket around the crater rim can be enveloped with an ellipse whose major E-W axis is coincident with the diameter of the crater rim and minor N-S axis is relatively displaced toward the west by ~200 m (Fig. 3.1c). The present ejecta distribution, which appears to be close to its pristine shape, extends to a distance of ~700 m in all the directions from the crater rim except to the west where it extends to a distance of a little more than 1 km. The circular shape of the crater rim, the E-W bilateral symmetry of the enveloping ellipse on the ejecta, and the greater extension of the ejecta toward the west appear to be the result of an oblique impact from the east with an angle of incidence of 30°– 45° when compared with laboratory experiments. The AMS data suggest that the target basalts occurring at ~2 km west-southwest of the

crater rim are highly shocked, as indicated by the random orientation of their K3 (minimum) susceptibility axes in comparison to the unshocked basalts at ~2 km east-southeast of the crater; the unshocked basalts show a bimodal distribution of susceptibility axes typical of lava flows (Fig. 3.6). Moderate to strong westward shift of the K3 axes are seen for the majority of the shocked basalts on the crater rim and west-southwest of the crater; the shocked basalts also indicate an oblique impact from the east when compared with modeling and laboratory experiments. A general lowering of degree of anisotropy of the Lonar

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shocked basalts (~1.01) compared to the surrounding unshocked basalts (~1.03) is found to be a characteristic feature of impact crater target rocks (Fig. 3.5). Variation in attitudes of the basalt flows on the Lonar crater rim shows a bilaterally symmetrical distribution about an E-W axial plane, which includes quaquaversal dips of the flows all around the crater rim, except to the west where overturned dips of the basalt flows are seen (Fig. 3.3). It appears that oblique impact and the symmetry in structural variations around the crater rim have a relationship for a small crater like Lonar.

II. Distribution of impact-induced stress with the direction of Lonar impact: The distribution of impact-induced shock front resulting in branching of impact stress with the direction of Lonar asteroid impact is evaluated using the AMS technique. Specifically, the AMS data suggest that the target basalts occurring at ~2 to 3 km west of the crater center in the downrange are highly shocked as indicated by their major shift of K3 (minimum) axes toward the southwest, west, and northwest directions in comparison to the unshocked basalts

occurring at ~3 to 5 km east of the crater center, which show clustering of K3 axes with moderate to subvertical dips (40°-70°) toward the east (Fig. 3.10). This distribution of K3 axes is symmetrical about the E-W plane of impact and make acute angle with the impact direction in the downrange, suggesting a major branching of impact stress toward the southwest- and northwest components from the crater’s center immediately after the impact (Fig. 3.11).

III. Rock magnetic study of Lonar basalts: The rock magnetic (mineral magnetic) properties suggest that Lonar basalts essentially contain PSD grain size Ti-rich to Ti-poor titanomagnetite and their low temperature oxidized products of titanomaghemite as the main magnetic carriers (Figs. 4.3, 4.7, 4.8). The unshocked (UNS) and shocked (SH) basalt samples from Lonar crater are very difficult to distinguish by petrography except the presence of some fractures in plagioclase phenocrysts in the former. The rock magnetic properties show that the UNS and SH basalts differ significantly in: (a) bulk-coercivity, (b)

squareness of hysteresis (Mrs/Ms), and (c) low and high temperature susceptibility measurements (Figs. 4.3, 4.6, 4.9). The observed changes in rock magnetic properties of shocked basalts are related to either sub-microscopic changes in the domain state of 161

titanomagnetite grains (movement from PSD toward SD state), or modifications in the crystalline structure of titanomagnetite grains (namely microfractures, lattice defects or dislocations) due to impact shock. The amount of acquired magnetization due to asteroid impact in target rocks is characterized by NRM/χ and REM ratio i.e. NRM/SIRM expressed in % (Fig. 4.2). The NRM/χ (Am-1) values of the shocked basalts on the rim of the Lonar crater do not show much change in the uprange or downrange direction on and close to the E-W plane of impact, and the values are ~1.5 times higher in average over the unshocked basalts around the crater. However, the values become ~1.4 to 16.4 times higher for the shocked basalts on the crater rim, which occur obliquely to the plane of impact. The target basalts at ~2 to 3 km west of the crater center in the downrange also show a significant increase (up to ~26 times higher) in NRM/χ. The majority of the shocked basalt samples (~73%) from around the crater rim, in general, show a lowering of REM, except those from ~2 to 3 km west of the crater center in the downrange, where nearly half of the sample population shows a higher REM of ~3.63% in average.

IV. Rock magnetic study of Ramgarh magnetic particles: The rock magnetic properties of FeO-rich spherules from the Ramgarh structure show very high NRM (1–80 Am-1), which are much higher compared to the target sedimentary rocks (~0.004–0.008 Am-1), and REM ratio (10–180%) indicating the presence of a high magnetic field during their formation (Fig. 4.16a, b). The rock magnetic study thus suggests the evidence of impact-amplified magnetic field resulting from a plasma cloud generated by the impact interacting with the ambient Earth’s magnetic field, or less likely, shock metamorphism.

V. Palaeomagnetic study of Lonar basalts: The palaeomagnetic results suggest that shocked target basalts acquired a high coercivity and high temperature (HC_HT) magnetization component due to impact. The orientation of HC_HT component is symmetrical with reference to the E-W plane of impact and directed in the uprange direction making an obtuse angle with the impact direction (Fig. 5.2). The low coercivity and low temperature (LC_LT) magnetization component of unshocked and shocked basalts are statistically identical to the present-day Earth’s magnetic field (PDF) direction. The PDF 162

may likely be acquired during the last 570±47 ka, subsequent to the formation of the Lonar crater and could be the chemical and⁄or viscous remanent magnetization (CRM and/or VRM). Here it is demonstrated that the high coercivity remanence is the product of single- domain magnetite from shock-induced decomposition of target rocks in titanomagnetite, as suggested by Cloete et al. (1999). Discussion of palaeomagentic data analysis and the hypothesis of shock remanent magnetization (SRM) and impact-generated plasma magnetic fields around impact structures is verified and it suggests that the HC_HT component is symmetrically oriented with the direction of impact. The mean palaeomagnetic pole is calculated for each sector of the crater rim (Table 5.1). The lack of oriented ejecta basalts led to argue for the presence of SRM at Lonar by Rao and Bhalla (1984) and that the LC_LT component could be used to constrain the age of the Lonar impact event.

VI. Rock- and Palaeo-magnetic study of randomly oriented ejecta blocks from Lonar: The unshocked target basalts from ~3 km ESE of the Lonar crater center have an average NRM/χ of ~116 Am-1. The rock magnetic study shows that the majority of samples (60%) from the ejecta basalt population have NRM/χ lower than that of the average unshocked basalts. Leaving a few exceptions, these ejecta basalts, in general, also have low REM (<1%). So it is concluded that the ejecta basalts, in general, are poorly magnetized due to impact. The palaeomagnetic data shows that the high coercivity (HC) component of these ejecta basalts is random and different from that of the unshocked target (D=196.1°, I=+68.9°, α95=13.4°); the data show a broad concentration around the average HC component of the shocked target from around the crater rim (D=120.5°, I=+34.2°, α95=10.3°) (Fig. 5.4). This observation suggests that the impact shock induced magnetic field could have existed beyond the modification stage of Lonar crater formation, when the newly formed ejecta with the randomly deposited basaltic blocks had weakly remagnetized.

VII. Geochemistry of rocks from Lonar and Ramgarh crater: The geochemical and X- ray diffraction (XRD) analysis of target rocks and impact products are carried out to observe the geochemical variations and shock-induced mineralogical phase changes. The X-ray fluorescence spectroscopy (XRF) analysis shows that impact melts are depleted or enriched in some oxides as compared to the target rocks (Table 6.1). The impact melt rocks are 163

depleted in Na2O, P2O5 and enriched in K2O in comparison to the target rocks, which is in accordance with the findings of Son and Koeberl (2007). The melt rocks have higher

K2O/Na2O ratio than the target rocks. The reason could be selective elemental vaporization and condensation during melt and vapor formation, or hydrothermal alteration. The enrichment of ‘K’ in impactites is a common phenomenon and termed the process “potassium metasomatism”, probably due to vaporization fractionation during the impact, or possibly an immediate post-impact alteration. The XRD of shocked, unshocked and ejecta samples do not show any major difference in mineralogical phases between them, but melts show high background effect (Fig. 6.3). The XRD pattern of impact spherules is not clear. The amorphous content estimated from XRD suggest that spherules and melt rocks have very high amount of amorphous content which is possibly due to high rate of cooling while falling from air (Fig. 6.4). Peak broadening (FWHM) in XRD suggests the signature of shock metamorphism but there is no clear difference between shocked and unshocked samples is observed (Fig. 6.5). It appears that FWHM is not a reliable measure to compare the shocked and unshocked samples as there is too much variation between the same sample types and it may be due to the presence of different mineralogy in them. The XRD of Lonar magnetic extracts suggests the presence of magnetite, titanomagnetite and possibly ilmenite minerals in them (Fig. 6.6). The XRD of six FeO-rich spherules from Ramgarh structure shows the presence of high-

pressure polymorphs of SiO2 (tridymite, coesite, cristobalite) indicating the possible impact origin of the structure (Fig. 6.7).

IMPACT CRATERING RESEARCH IN INDIA AND FUTURE DIRECTIONS This study provides invaluable data in the investigation of natural impact craters in India; it suggests that Lonar and Ramgarh samples provide new data to study the magnetic field phenomena associated with the impact craters. From the investigation of magnetic study, it is inferred that a relationship exists between an oblique impact from the east at Lonar and geological structural variations around the crater rim. This work provides a useful tool in understanding the obliquity of impacts for small craters of terrestrial and extraterrestrial examples. Additionally, this work draws our attention on impact cratering research for searching new craters in the Indian shield. Incidentally, Radhakrishna (2005) proposed to re-

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examine circular to semi-circular morphology from impact cratering research view point in different parts of India: (i) the Cuddapah basin, (ii) the tectonic group of islands off the coast of Mumbai, and the islands of Saurashtra and Kachchh, (iii) the regions of Malani, and (iv) Simlipal complex, Odisha. The reported magnetization mechanism of shocked and unshocked basalts as a method of characterizing samples helped in advancement of knowledge in the field of impact cratering in order to understand the shock-induced magnetization effects on target lithologies and to investigate their structural deformation, emplacement mode and post-impact alterations. Initial rock magnetic and palaeomagnetic data of Ramgarh structure partly confirmed its origin by asteroid impact; it suggests that the Ramgarh impact crater is only weakly magnetic. This kind of geophysical magnetic investigation can be extended further to explore the Dhala and Luna impact crater structures in the Indian subcontinent. In such investigations, the present work is expected to be an initial platform to furtherance of a multi-disciplinary application to impact cratering processes using both magnetic and advanced techniques such as Fourier Transform Infrared (FTIR) spectroscopy, XRD, scanning electron microscope (SEM) and geochemical analysis. It is hoped that the impact cratering research will become popular among geoscientists in India, so that adequate trained manpower will be available for future space missions to the Moon and Mars.

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Bibliography

AGICO, 2004, KLY-4/KLY-4S/CS-3/CS-L USER’S GUIDE: Version 1, Brno, Czech Republic, 62 p.

Ahmed, N., Bhardwaj, B. D., Sajid, H. A., and Hasnain, I., 1974, Ramgarh Meteorite Crater: Current Science, v. 43, pp. 598.

Anderson, J. L. B., and Schultz, P. H., 2006, Flow-field center migration during oblique impacts: Implications for curved uprange ejecta rays: 37th Lunar and Planetary Science Conference, Houston, abstract no. 1726 (CD-ROM).

Arif, Md., Deenadayalan, K., Basavaiah, N., and Misra, S., 2011, Variation of primary magnetization of basaltic target rocks due to asteroid impact: example from Lonar crater, India: 42nd Lunar and Planetary Science Conference, Abstract no. 1751 (CD-ROM).

Athavale, R. N. and Anjaneyulu, G. R. 1972, Palaeomagnetic results on Deccan Trap lavas of the Aurangabad region and their tectonic significance: Tectonophysics, v. 14: p. 87–103.

Balasundaram, M. S., and Dube, A., 1973, Ramgarh structure, India: Nature, v. 242, pp. 40.

Basavaiah, N., 2011, Geomagnetism: Solid Earth and Upper Atmosphere Perspectives: Springer publisher, 486 p.

Bezaeva, N. S., Gattacceca, J., Rochette, P., Sadykov, R. A., and Trukhin, V. I., 2010, Demagnetization of terrestrial and extraterrestrial rocks under hydrostatic pressure up to 1.2 GPa: Physics of the Earth and Planetary Interiors, v. 179, p. 7–20.

Bjǿrnerud, M. G., 1998, Superimposed deformation in seconds: Breccias from impact structure at Kentland, Indiana (USA): Tectonophysics, v. 290, p. 259-269.

Borradaile, G. J., 1993, Strain and magnetic remanence: Journal of Structural Geology, v. 15, p. 383-390.

Borradaile, G. J., and Jackson, M., 1993, Changes in magnetic remanence during simulated deep sedimentary burial: Physics of the Earth and Planetary Interiors, v. 77, p. 315-327.

Borradaile, G. J., and Henry, B., 1997, Tectonic applications of magnetic susceptibility and its anisotropy: Earth Science Reviews, v. 42, p. 49-93.

Brandt, D., and Reimold, W. U., 1995, The geology of the Pretoria Saltpan impact structure and the surrounding area: South African Journal of Geology, v. 98, p. 287-303.

166

Cañón-Tapia, E., Walker, G. P. L., and Bervera, E. H., 1997, The internal structure of lava flows-insights from AMS measurements II: Hawaiian pahoehoe, toothpaste lava and ‘a’a: Journal of Volcanology and Geothermal Research, v. 76, p. 19-46.

Cañón-Tapia, E., and Coe, R., 2002, Rock magnetic evidence of inflation of a flood basalt lava flow: Bulletin of Volcanology, v. 64, p. 289-302.

Carporzen, L., Gilder, S. A., and Hart, R. J., 2005, Palaeomagnetism of the Vredefort meteorite crater and implications for craters on Mars: Nature, v. 435, p. 198-201.

Carporzen, L., Weiss, B. P., Gilder, S. A., Pommier, A., and Hart R. J., 2011, Lightning remagnetization of the Vredefort impact crater: No evidence for impact-generated magnetic fields: Journal of Geophysical Research, v. 117, E01007

Carter-Stiglitz, B., Moskowitz, B., Solheid, P., Berquó, T. S., Jackson, M., and Kosterov A., 2006, Low-temperature magnetic behavior of multidomain titanomagnetites: TM0, TM16, and TM35: Journal of Geophysical Research, v. 111, B12S05.

Chadima, M., and Hrouda, F., 2006, Remasoft 3.0 a user-friendly paleomagnetic data browser and analyzer: Travaux Géophysiques, XXVII, p. 20-21.

Chakrabarti, R. and Basu, A. R., 2006, Trace element and isotopic evidence for Archean basement in the Lonar crater impact breccia, Deccan volcanic province: Earth and Planetary Science Letters, v. 247, p. 197–211.

Chao, E. C. T., 1968, Pressure and temperature histories of impact metamorphosed rocks- based on petrographic observations, In: B. M. French and N. M. Short (Editors), Shock Metamorphism of Natural Materials, Mono, Baltimore, MD, p. 135-158.

Chatterjee, S., Guven, N., Yoshinobu, A., and Donofrio, R. 2006, Shiva structure: A possible KT boundary impact crater on the western shelf of India: Texas Technical University Natural Science Research Laboratory Special Publication, #50, 39 pages.

Chenet, A-L., Quidelleur, X., Fluteau, F., Courtillot, V., and Bajpai, S., 2007, 40K-40Ar dating of the Main Deccan large igneous province: Further evidence of KTB age and short duration: Earth and Planetary Science Letters, v. 263, p. 1-15.

Christeson, G. L., Nakamura, Y., Buffler, R. T., Morgan, J., and Warner, M., 2001, Deep crustal structure of the Chicxulub impact crater: Journal of Geophysical Research, v. 106, p. 21751- 21769.

Cisowski, S., 1975, The effect of shock on the magnetic moments and hysteresis properties of natural materials, with special emphasis on rocks from the Lonar meteorite crater, India, and their relationship to lunar paleomagnetism: Meteoritics, v. 10, p. 383.

Cisowski, S. M. and Fuller, M., 1978, The effect of shock on the magnetism of terrestrial rocks: Journal of Geophysical Research, v. 83, p. 3441–3458.

167

Cisowski, S., 1981, Interacting vs. non-interacting single-domain behavior in natural and synthetic samples: Physics of the Earth and Planetary Interiors, v.26, p. 56–62.

Cloete, M., Hart, R. J., Schmid, H. K., Drury, M., Demanet, C. M., and Vijaya, S. K., 1999, Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa: Contributions of Mineralogy and Petrology, v. 137, p. 232–245.

Courtillot, V., Gallet, Y., Rocchia, R., Féraud, G., Robin, E., Hofmann, C., Bhandari, N., and Ghevariya, Z. G., 2000, Cosmic markers, 40Ar/39Ar dating and paleomagnetism of the KT sections in the Anjar area of the Deccan large igneous province: Earth and Planetary Science Letters, v. 182, p. 137-156.

Courtillot, V. E., and Renne, P. R., 2003, On the ages of flood basalt events: Comptes Rendus Geoscience, v. 335, p. 113-140.

Cox, K. G., and Hawkesworth, C. J., 1985, Geochemical stratigraphy of the Deccan traps at Mahabaleshwar, Western Ghats, India, with implications for open system magmatic processes: Journal of Petrology, v. 26, p. 355–377.

Crawford, A. R., 1972, Possible impact structure in India: Nature, v. 237, p. 96.

Crawford, D. A., and Schultz, P. H., 1988, Laboratory observations of impact-generated magnetic fields: Nature, v. 336, p. 50-52.

Crawford, D. A., and Schultz, P. H., 1999, Electromagnetic properties of impact-generated plasma, vapor and debris: International Journal of Impact Engineering, v. 23, p. 169-180.

Crosta, A. P., Koeberl, C., Furuie, R. A., and Kazzuo-Vieira, C., 2010, The first description and confirmation of the Vista Alegre impact structure in the Paraná flood basalts of southern Brazil: Meteoritics and Planetary Science, v. 45, p. 181–194.

Dahl, J. M., and Schultz, P. H., 2001. Measurement of stress wave asymmetries in hypervelocity projectile impact experiments: International Journal of Impact Engineering, v. 26, p. 145-155.

Dankers, P., 1981, Relationship between medium destructive field and remanent coercive forces for dispersed natural magnetite, titanomagnetite and hematite: Geophysical Journal of the Royal Astronomical Society, v. 64, p. 447–461.

Das, P. K., Misra, S., Basavaiah, N., Newsom, H., and Dube, A., 2009, Rock magnetic evidence of asteroid impact origin of ramgarh structure, India: 40th Lunar and Planetary Science Conference, Abstract no. 1466 (CD-ROM).

Day, R., Fuller, M., and Schmidt, V. A., 1977, Hysteresis properties of titanomagnetites: Grain size and composition dependence: Physics of the Earth and Planetary Interiors, v. 13, p. 260–267.

168

Deer, W. A., Howie, R. A., and Zussman, J., 1992, An introduction to the rock forming minerals: Essex, Pearson Education Limited, 696 p.

Dressler, B. O., and Sharpton, V. L., 1997, Breccia formation at a complex impact crater, Slate Islands, Lake Superior, Ontario, Canada: Tectonophysics, v. 275, p. 285- 311.

Dunlop, D. J., 2002, Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1.Theoretical curves and tests using titanomagnetite data: Journal of Geophysical Research, v. 107, B2056.

Earth Impact Database, 2013 [www.passc.net/EarthImpactDatabase/index.html]

Feldman, V. I., Sazonova, L. V., Mironov, Y. V., and Kapustkina, I. G., 1983, Circular structure Logancha as possible meteorite crater in basalts of the Tunguska syncline (abstract): 14th Lunar and Planetary Science Conference, p. 191-192.

Ferrière, L., Lubala, F. R. T., Osinski, G. R., and Kaseti, P. K., 2011, The newly confirmed Luizi impact structure, Democratic Republic of Congo - Insights into central uplift formation and post-impact erosion: Geology, v. 39, p. 851-854

Fisher, R., 1953, Dispersion of a sphere: Proceedings of the Royal Society of London, v. 217, p. 295-305.

Fredriksson, K., Dube, A., Milton, D. J., and Balasundaram, M. S., 1973, Lonar lake, India: An impact crater in basalt: Science, v. 180, p. 862-864.

Fredriksson, K., Brenner, P., Dube, A., Milton, D., Mooring, C. and Nelen, J. A., 1979, Petrology, mineralogy, and distribution of Lonar (India) and lunar impact breccias and glasses: Smithsonian Contributions to the Earth Sciences, v. 22, p. 1-12.

French, B. M., 1998, Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures: Lunar and Planetary Institute, Houston, Contribution no. 954, 120 p.

French, B. M. and Koeberl, C., 2010, The convincing identification of terrestrial meteorite impact structures: What works, what doesn’t, and why: Earth Science Reviews, v. 98, p. 123-170.

Fudali, R. F., Milton, D. J., Fredriksson, K., and Dube, A., 1980, Morphology of Lonar Crater, India: Comparisons and implications: The Moon and the Planets, v. 23, p. 493- 515.

Fuller, M., Cisowski, S., Hart, M., Haston, R., Schmidtke, E., and Jarrard, R., 1988, NRM:IRM(S) demagnetization plots; an aid to the interpretation of natural remanent magnetization: Geophysical Research Letters, v. 15, p. 518–521.

Gattacceca, J., and Rochette, P., 2004, Toward a robust normalized magnetic paleointensity method applied to meteorites: Earth and Planetary Science Letters, v. 227, p. 377-393. 169

Gattacceca, J., Lamali, A., Rochette, P., Boustie, M., and Berthe, L., 2007, The effects of explosive-driven shocks on the natural remanent magnetization and the magnetic properties of rocks: Physics of the Earth and Planetary Interiors, v. 162, p. 85–98.

Gattacceca, J., Berthe, L., Boustie, M., Vadeboin, F., Rochette, P., and De Resseguier, T., 2008, On the efficiency of shock magnetization processes: Physics of the Earth and Planetary Interiors, v. 166, p.1-10.

Gault, D. E., and Wedekind, J. A., 1978, Experimental studies of oblique impact: Proceedings of the 9th Lunar and Planetary Science Conference, p. 3843-3875.

Ghosh, S., 2003, Is Lonar astrobleme an example of cometary impact?: Indian Minerals, v. 57, no. 3 and 4, p. 105-114.

Ghosh, S., and Bhaduri, S. K., 2003, Petrography and petrochemistry of impact melts from Lonar Crater, Buldana district, Maharashtra, India: Indian Minerals, v. 57, no. 1 and 2, p. 1-26.

Gilbert, G. K., 1896, The origin of hypotheses, illustrated by the discussion of a topographic problem: Science, v. 3, p. 1-13.

Gilder, S. A., and Le Goff, M., 2008, Systematic pressure enhancement of titanomagnetite magnetization: Geophysical Research Letters, v. 35, L10302, doi:10.1029/2008GL033325.

GLCF, 2007, Global Land Cover Facility. University of Maryland, USA, available online at http://glcf.umiacs.umd.edu (Last accessed on April 27, 2007).

Gold, T., and Soter, S., 1976, Cometary impact and the magnetization of the Moon: Planetary and Space Science, v. 24, p. 45-54.

Grant, J. A., 1999, Evaluating the evolution of process specific degradation signatures around impact craters: International Journal of Impact Engineering, v. 23, p. 331-340.

Grieve, R. A. F., and Robertson, P. B., 1976, Variation in shock deformation at the Slate Islands impact structure, Lake Superior, Canada: Contributions to Mineralogy and Petrology, v. 58, p. 37-49.

Grieve, R. A. F., 1991, Terrestrial impact: the record in the rocks: Meteoritics, v. 26, p. 175– 194.

Gupta, H. K., Rao, R. U. M., Srinivasan, R., Rao, G. V., Reddy, G. K., Dwivedy, K. K., Banerjee, D. C., Mohanty, R., and Satyasaradhi, Y. R., 1999, Anatomy of surface rupture zones of two stable continental region earthquakes, 1967 Koyna and 1993 Latur, India: Geophysical Research Letters, v. 26, p. 1985-1988.

170

Hagerty, J.J., and Newsom, H.E., 2003, Hydrothermal alteration at the Lonar Lake impact structure, India: Implications for impact cratering on Mars: Meteoritics and Planetary Science, v. 38, p. 365-381.

Halls, H. C., 1979, The Slate Islands meteorite impact site: a study of Shock Remanent Magnetization: Geophysical Journal of the Royal Astronomical Society, v. 59, p. 553– 591.

Herrick, R. R., and Forsberg-Taylor, N. K., 2003, The shape and appearance of craters formed by oblique impact on the Moon and Venus: Meteoritics and Planetary Science, v. 38, p. 1551-1578.

Hofmann, C., Féraud, G., and Courtillot, V., 2000, 40Ar/39Ar dating of mineral separates and whole rocks from the Western Ghats lava pile: further constraints on duration and age of the Deccan Traps: Earth and Planetary Science Letters, v. 180, p. 13- 27.

Hood, L. L., and Artemieva, N. A., 2008, Antipodal effects of lunar basin-forming impacts: Initial 3D simulations and comparisons with observations: Icarus, v. 193, p. 485-502.

Huber, H., Koeberl, C., Mc Donald, I., and Reimold, W. U., 2001, Geochemistry and petrology of Witwatersrand and Dwyka diamictites from South Africa: search for an extraterrestrial component: Geochimica et Cosmochimica Acta, v. 65, p. 2007-2016.

Hrouda, F., Chlupáčová, M., and Mrázová, Š., 2006, Low-field variation of magnetic susceptibility as a tool for magnetic mineralogy of rocks: Physics of the Earth and Planetary Interiors, v. 154, p. 323–336.

IGRF, International Geomagnetic Reference Field, 2011.

IMD, 2007, Indian Meteorological Department, Government of India, available online at http://www.imd.ernet.in, (last accessed on June 19, 2007).

Irvine, T. N., and Baragar, W. R. A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, p. 523- 548.

JCPDS-ICDD 1999, http://www.msm.cam.ac.uk/xray/

Jelinek, V., 1978, Statistical processing of anisotropy of magnetic susceptibility measured on groups of specimens: Studia Geophysica et Geodaetica, v. 22, p. 50-62.

Jhingran, A. G., and Rao, K. V., 1958, Lonar lake and its salinity: Records of Geological Survey of India, v. 85, no. 3, p. 313-334.

Jourdan, F., Moynier, F., Koeberl, C., and Eroglu, S., 2011, 40Ar/39Ar age of the Lonar crater and consequence for the geochronology of planetary impacts: Geology, v. 39, p. 671– 674.

171

Karanth, R. V., Thakker, P. S., and Gadhavi, M. S., 2006, A preliminary report on the possible impact crater of Kachchh, Current Science, v. 91, p. 877–879.

Kieffer, S. W., Schaal, R. B., Gibbons, R., Hörz, F., Milton, D. J., and Dube, A., 1976, Shocked basalt from Lonar impact crater, India and experimental analogues: Proceedings of 7th Lunar Science Conference, p. 1391-1412.

Kirschvink, J. L., 1980, The least-squares line and plane and the analysis of palaeomagnetic data: Geophysical Journal of the Royal Astronomical Society, v. 62, p. 699–718.

Kletetschka, G., Kohout, T., and Wasilewski, P. J., 2003, Magnetic remanence in the Murchison meteorite: Meteoritics and Planetary Science, v. 38, p. 399–405.

Kletetschka, G., Acuna, M. H., Kohout, T., Wasilewski, P. J., and Connerney, J. E. P., 2004, An empirical scaling law for acquisition of thermoremanent magnetization: Earth and Planetary Science Letters, v. 226, p. 521–528.

Khan, M. A., 1962, The anisotropy of magnetic susceptibility of some igneous and metamorphic rocks: Journal of Geophysical Research, v. 67, p. 2873-2885.

Koeberl, C., Reimold, W. U., and Shirey, S. B., 1998, The Aouelloul crater, Mauritania: On the problem of confirming the impact origin of a small crater: Meteoritics and Planetary Science, v. 33, p. 513- 517.

Koeberl, C., 2007, The geochemistry and cosmochemistry of impacts: In: Meteorites, comets, and planets, edited by Davis, A., Treatise on geochemistry, v. 1, Amsterdam: Elsevier, p.1-52. doi: 10.1016/B978-008043751-4/00228-5.

Krishnan, M. S., 1968, Geology of India and Burma: Higginbothams, Madras.

Kumar, P. S., 2005, Structural effects of meteorite impact on basalt: Evidence from Lonar crater, India: Journal of Geophysical Research, v. 110, B12402, doi:10.1029/2005JB003662.

Kumar, V. and Reddy, B. V. R., 1984, Report on the investigation of Ramgarh dome structure, Kota district, Rajasthan: Unpub. Geol. Surv. Ind. report for FS, 1981-82.

La Fond, E. C., and Dietz, R. S., 1964, Lonar Crater, India, a meteorite crater?: Meteoritics, v. 2, no. 2, p. 111-116.

Lashkari, G., and Sisodia, M. S., 2002, Morphotectonics of Ramgarh structure: National Space Science Symposium (NSSS), India, XII, p. 34.

Lemieux, Y., Tremblay, A., and Lavoie, D., 2003, Structural analysis of supracrustal faults in the Charlevoix area, Quebec: Relation to impact cratering and the St-Laurent fault system: Canadian Journal of Earth Sciences, v. 40, p. 221-235.

172

Louzada, K. L., Weiss, B. P., Maloof, A. C., Stewart, S. T., Swanson-Hysell, N. L., and Soule, S. A., 2008, Paleomagnetism of Lonar impact crater, India: Earth and Planetary Science Letters, v. 275, p. 308-319.

LPDAAC, 2007, Land Processes Distributed Active Archive Center, Accessed online http://elpdl03.cr.usgs.gov/pub/imswelcome/ (Last accessed on April 21, 2007).

Mahoney, J. J., 1988, Deccan traps. In Continental flood basalts, edited by Macdougall J. D. Dordrecht: Kluwer Academic Publishing, p. 151–194.

Malaimani, E. C., Campbell, J., Gorres, B., Kotthoff, H., and Smaritschnik, S., 2000, Indian plate kinematic studies by GPS-geodesy: Earth Planets Space, v. 52, p. 741-745.

Malone, S. J., Meert, J. G., Banerjee, D. M., Pandit, M. K., Tamrat, E., Kamenov, G. D., Pradhan, V. R., and Sohl, L. E., 2008, Paleomagnetism and detrital zircon geochronology of the Upper Vindhyan sequence, Son Vally and Rajasthan, India: A ca. 1000 Ma closure age for the Purana Basins: Precambrian Research, v. 164, p. 137-159.

Maloof, A. C., Louzada, K. L., Stewart, S. T., and Weiss, B. P., 2005, Geology of Lonar crater, India: An analog for Martian impact craters: 36th Lunar and Planetary Science Conference, Abstract no. 3046 (CD-ROM).

Maloof, A. C., Stewart, S. T., Swanson-Hysell, N., Louzada, K. L., Garrick-Bethell, I., Soule, S. A., and Weiss, B. P., 2007, Lonar crater, India: An analog for Martian impact craters: 38th Lunar and Planetary Science Conference, Abstract no. 2316 (CD-ROM).

Maloof, A. C., Stewart, S. T., Weiss, B. P., Soule, S. A., Swanson-Hysell, N. L., Louzada, K. L., Garrick-Bethell, I., and Poussart, P. M., 2010, Geology of Lonar Crater, India: Geological Society of America Bulletin, v. 122, p. 109–126.

Masaitis, V. L., 1999, Impact structures on northeastern Eurasia: The territories of Russia and adjacent countries: Meteoritics and Planetary Science, v. 34, p. 691- 711.

Master, S., 1999, Evidence for an impact origin of the Amber lake structure: a smaller companion crater to the Lonar impact crater, Maharashtra, India: Meteoritics and Planetary Science, v. 34, p. A78.

Master, S., and Pandit, M. K., 1999, New evidence for an impact origin of the Ramgarh structure: Meteoritics and Planetary Science (supplement), v. 34, p. 4.

Melosh, H. J., 1989, Impact cratering: A geologic process: Oxford Monographs on Geology and Geophysics, v. 11. Oxford University Press, New York, 253 p.

Misra, S., Srivastava, P. K., and Dube, A., 2006a, Lonar impact crater, India: Did it form by oblique impact: 37th Lunar and Planetary Science Conference, Abstract no. 1085 (CD- ROM).

173

Misra, S., Bose, T., Newsom, H. E., and Sengupta, D., 2006b, Geochemistry of impact ejecta from Lonar crater, India: More clues to crater evolution: 37th Lunar and Planetary Science Conference, Abstract no. 2123 (CD-ROM).

Misra, S., Newsom, H., Mukherjee, T., Dube, A., and Sengupta, D., 2007, No evidence of impact induced volatile loss from maskelynite of Lonar crater, India: 38th Lunar and Planetary Science Conference, Abstract no. 1672 (CD-ROM).

Misra, S., Dube, A., Srivastava, P. K., and Newsom, H. E., 2008a, Time of formation of Ramgarh crater, India - constrains from geological structures: 39th Lunar and Planetary Science Conference, Abstract no. 1502 (CD-ROM).

Misra, S., Lashkari, G., Panda, D., Dube, A., Sisodia, M. S., Newsom, H. E., and Sengupta, D., 2008b, Geochemical evidence for the meteorite impact origin of Ramgarh structure, India: 39th Lunar and Planetary Science Conference, Abstract no. 1499 (CD-ROM).

Misra, S., Newsom, H. E., Prasad, M. S., Geissman, J. W., Dube, A., and Sengupta, D., 2009, Geochemical identification of impactor for Lonar crater, India: Meteoritics & Planetary Sciences, v. 44, p. 1001–1018.

Misra, S., Arif, Md., Basavaiah, N., Srivastava, P. K., and Dube, A., 2010, Structural and anisotropy of magnetic susceptibility (AMS) evidence for oblique impact on terrestrial basalt flows: Lonar crater, India: Geological Society of America Bulletin, v. 122, p. 563–574.

Mitchell, C., and Widdowson, M., 1991, A geological map of the southern Deccan Traps, India and its structural implications: Journal of Geological Society of London, v. 148, p. 495-505.

Mittlefehldt, D. W., Hörz, F., See, T. H., Scott, E. R. D., Mertzman, S. A., 2005, Geochemistry of target rocks, impact-melt particles, and metallic spherules from Meteor Crater, Arizona: empirical evidence on the impact process. In: Kenkmann, T., Hörz, F., Deutsch, A. (Eds.), Large Meteorite Impacts III, Special Paper, v. 384. Geological Society of America, Boulder, CO, p. 367–390.

Molnar, P., 1984, Structure and tectonics of the Himalaya: constraints and implication of geophysical data: Annual Review of Earth and Planetary Sciences, v. 12, p. 489-516.

Morgan, J. W., 1978, Lonar crater glasses and high-magnesium australites: Trace element volatilization and meteoritic contamination: Proceedings of 9th Lunar Planetary Science Conference: p. 2713- 2730.

Murali, A. V., and Lulla, K. P., 1992, Ramgarh crater, Rajasthan, India: study of multispectral images obtained by Indian remote sensing satellite (IRS-IA): Geocarto International, v. 7 (3) p. 75-80.

Nandy N.C., and Deo V. B., 1961, Origin of the Lonar lake and its alkalinity: Journal of Tata Iron and Steel Company, v. 8, p. 1-12. 174

Nayak, V. K., 1972, Glassy objects (impactite glasses?): A possible new evidence for meteoritic origin of the Lonar Crater, Maharashtra state, India: Earth and Planetary Science Letters, v. 14, p. 1-6. Nayak, V. K., 1993, Maskelynite from the Indian impact crater at Lonar: Journal of the Geological Society of India, v. 41, p. 307–312.

Nayak V. K. 1997, The circular structure at Ramgarh, India: an astrobleme (?): Lunar and Planetary Institute, Contribution No. 922, p. 31

Newsom, H. E., and Boslough, M. B. E., 2008, Impact melt formation by low-altitude airbrust processes, evidence from small terrestrial craters and numerical modeling: 39th Lunar and Planetary Science Conference, contribution no. 1391, p. 1460.

Nishioka, I., 2007, Rock magnetic study of basalt at Lonar impact crater in India: Effects of stress waves on rock magnetic properties: The Graduate University for Advanced Studies, Japan (unpublished Ph.D. thesis), 70 p.

Nishioka, I., and Funaki, M., 2008, Irreversible changes in anisotropy of magnetic susceptibility: Study of basalts from Lonar crater and experimentally impacted basaltic andesite: 71st Annual Meteoritical Society Meeting (abstract no. 5207), p. A116.

Nishioka, I., Funaki, M., and Sekine, T., 2007, Shock-induced anisotropy of magnetic susceptibility: impact experiment on basaltic andesite: Earth Planets Space, v. 59, p. e45- e48.

Osae, S., Misra, S., Koeberl, C., Sengupta, D., and Ghosh, S., 2005, Target rocks, impact- glasses, and melt rocks from the Lonar impact crater, India: Petrography and geochemistry: Meteoritics and Planetary Science, v. 40, p. 1473-1492.

Osinski, G. R., 2008, Meteorite impact structure: The good and the bad. Geology Today, v. 24 (1), p. 13–19.

Pal, P. C., and Bhimasankaram, V. L. S., 1971, Palaeomagnetism of the Deccan Trap flows of Jalna, India: Earth and Planetary Science Letters, v. 11, p. 109–112.

Pal, P. C., and Ramana, C. V., 1972, Lonar Lake-Volcanic Crater or Astrobleme: C. E. G. Bulletin, p. 114–121.

Pande, K., Pattanayak, S. K., Subbarao, K. V., Navaneethakrishnan, P., and Venkatesan, T. R., 2004, 40Ar-39Ar age of a lava flow from the Bhimashankar Formation, Giravali Ghat, Deccan Traps: Proceedings of the Indian Academy of Sciences (Earth and Planetary Sciences), v. 113, no. 4, p. 755- 758.

Pati, J. K., Reimold, W. U., Koeberl, C., and Pati, P., 2008, The Dhala structure, Bundelkhand craton, Central India – Eroded remnant of a large paleoproterozoic impact structure: Meteoritics andPlanetary Science, v. 43, p. 1383–1398.

175

Pati, J. K., Jourdan, F., Armstrong, R. A., Reimold, W. U., and Prakash, K., 2010, First SHRIMP U-Pb and 40Ar/39Ar chronological results from impact melt breccia from the Paleoproterozoic Dhala impact structure, India.: Geological Society of America Special Paper, v. 465, p. 571–591. Parry, L. G., 1974, The diversity of magnetized particles in a single lava: Geophysical Journal of the Royal Astronomical Society, v. 38, p. 225–236.

Pesonen, L. J., Deutsch, A., Hornemann, U., and Langenhorst, F., 1997, Magnetic properties of diabase samples shocked experimentally in the 4.5 to 35 GPa range: 38th Lunar and Planetary Science Conference, Abstract no. 1370, p. 1087–1088.

Pierazzo, E., and Melosh, H. J., 1999, Hydrocode modeling of Chixulub as an oblique impact event: Earth and Planetary Science Letters, v. 165, p. 163-176.

Pierazzo, E., and Melosh, H. J., 2000, Understanding oblique impacts from experiments, observations, and modeling: Annual Review of Earth and Planetary Sciences, v. 28, p. 141-167.

Pike, R. J., 1975, Craters on Earth, Moon and Mars: Multivariate classification and mode of origin: Earth and Planetary Science Letters, v. 22, p. 245-255.

Pohl, J., and Soffel, H., 1971, Paleomagnetic age determination of the Rochechouart Impact Structure (France): Zeitschrift fur Geophysik, v. 37, p. 857-866.

Pohl, J., Bleil, U., and Hornemann, U., 1975, Shock Magnetization and Demagnetization of Basalt by Transient Stress up to 10 kbar: Journal of Geophysics, v. 41, p. 23-41.

Polanskey, C. A., and Ahrens, T. J., 1990, Impact spallation experiments: Fracture patterns and spall velocities: Icarus, v. 87, p. 140-155.

Prasad, B., 1984, Geology, sedimentation and palaeogeography of the Vindhyan Supergroup, southeastern Rajasthan: Geological Survey of India Memoir, v. 116, p. 1-107.

Puura, V., Huber, H., Kirs, J., Kärki, A., Suuroja, K., Kirsimäe, K., Kivisilla, J., Kleesment, A., Konsa, M., Preeden, U., Suuroja, S., and Koeberl, C. 2004, Geology, petrography, shock petrography, and geochemistry of impactites and target rocks from the Kärdla crater, Estonia: Meteoritics and Planetary Science, v. 39, p. 425–452.

Radhakrishna, B. P., 2004, Extra-terrestrial impacts in geological past: some reflections: Journal of Geological Society of India, v. 64, no. 2, p. 127-132.

Radhakrishna, B. P., 2005, Some thoughts on planetary impact structure in India and on the importance of their study: Journal of Geological Society of India, v. 66, p. 423–431.

Radhakrishnamurty, C., Likhite, S. D., Deutsch, E. R., and Murthy, G. S., 1978, Nature of magnetic grains in basalts and implications for palaeomagnetism.: Proceedings of the Indian Academy of Sciences, v. 87A, p. 235–243.

176

Radhakrishnamurty, C., Likhite, S. D., Deutsch, E. R., and Murthy, G. S., 1982, On the complex magnetic behaviour of titanomagnetites: Physics of the Earth and Planetary Interiors, v. 30, p. 281–290.

Rajasekhar, R. P., and Mishra, D. C., 2005, Analysis of gravity and magnetic anomalies over Lonar lake, India: An impact crater in a basalt province: Current Science, v. 88, p. 1836– 1840.

Rakshit, A. M., 1973, A short report on the ring structure near Ramgarh, Kota district, Rajasthan: Unpublished Geological Survey of India report for FS 1971-72.

Ramasamy, S. M., 1981, An interim report on the lithostratigraphy and structure of Ramgarh dome, Kota district, Rajasthan: Geol. Surv. Ind. report for FS 1979-80.

Ramasamy, S. M., 1987, Evolution of Ramgarh Dome, Rajasthan, India: Rec. Geol. Surv. India, v. 113(7), p. 13-22.

Rao, M. S., 2007, Special publication on the Shiva structure: Journal of Geological Society of India, v. 69, no. 1, p. 196.

Rao, G. V. S. P., and Bhalla, M. S., 1984, Lonar Lake: Palaeomagnetic evidence of shock origin: Geophysical Journal of the Royal Astronomical Society, v. 77, p. 847- 862.

Reichow, M. K., Saunders, A. D., White, R.V., Pringle, M. S., Al'Mukhamedov, A. I., Medvedev, A. I., and Kirda, N. P., 2002, 40Ar/39Ar dates from the West Siberian Basin: Siberian flood basalt province doubled.: Science, v. 296, p. 1846–1849.

Reimold, W. U., Brandt, D., and Koeberl, C., 1998a, Detailed structural analysis of the rim of a large, complex crater: Bosumtwi crater, Ghana: Geology, v. 26, p. 543- 546.

Reimold, W. U., Koeberl, C., and Reddering, J. S. V., 1998b, The 1992 drill core from the Kalkkop impact crater, Eastern Cape Province, South Africa: Stratigraphy, petrography, geochemistry and age: Journal of African Earth Sciences, v. 26, p. 573- 592.

Reimold, W. U., Trepmann, C., and Simonson, B., 2006, Comment on the paper “Impact origin of the Ramgarh structure, Rajasthan: Some new evidences” by Sisodia, M. S., Lashkari, G., and Bhandari, N., Journal of Geological Society of India, v. 67, p. 423- 431: Journal of Geological Society of India, v. 68, p. 561-563.

Richmond, N. C., Hood, L. L., Mitchell, D. L., Lin, R. P., Acuña, M. H., and Binder, A. B., 2005, Correlations between magnetic anomalies and surface geology antipodal to lunar impact basins: Journal of Geophysical Research, v. 110, E05011, doi:10.1029/2005JE002405

Robertson, P. B., and Roy, J. L., 1979, Shock-diminished paleomagnetic remanence at the Charlevoix impact structure, Quebec: Canadian Journal of Earth Sciences, v. 16, p. 1842-1856.

177

Rowan, L. C., Mar, J. C., and Simpson, C. J., 2005, Lithologic mapping of the Mordor, NT, Australia ultramafic complex by using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER): Remote Sensing of Environment, v. 99, p. 105−126.

Sagy, A., Fineberg, J., and Reches, Z., 2004, Shatter cones: Branched, rapid fractures formed by shock impact: Journal of Geophysical Research, v. 109, B10209, doi:10.1029/2004JB003016.

Schaal, R. B., 1975, Shock metamorphism in basalt from Lonar Crater, India and in six lunar microcraters [M.Sc. thesis]: Los Angeles, University of California, 143 p.

Scherler, D., Kenkmann, T., and Jahn, A., 2006, Structural record of an oblique impact: Earth and Planetary Science Letters, v. 248, p. 43-53.

Schultz, P. H., and Srnka, L. J., 1980, Cometary collisions on the Moon and Mercury: Nature, v. 284, p. 22–26.

Schultz, P. H., and Anderson, R. R., 1996, Asymmetry of the Manson impact structure: Evidence for impact angle and direction: Geological Society of America Special Publication, no. 302, p. 397-417.

Schultz, P. H. 1999, Ejecta distribution from oblique impacts into particulate targets: 30th Lunar Planetarey Science Conference, Abstract no. 1919 (CR-ROM).

Schultz, P. H., Eberhardy, C. A., Ernst, C. M., A’Hearn, M. F., Sunshine, J. M. and Lisse, C. M., 2007, The deep impact oblique impact cratering experiment: Icarus, v. 191, p. 84- 122.

Sen, K., and Mamtani, M. A., 2006, Magmatic fabric, shape preferred orientation and regional strain in granitic rocks: Journal of Structural Geology, v. 28, p. 1870-1882.

Sengupta, D., Bhandari, N., and Watanabe, S., 1997, Formation age of Lonar Meteor Crater, India: Revista de Fisica Aplicada e Instrumentacao, v. 12, p. 1-7.

Sharma, H. S., 1973, Ramgarh structure, India: Nature, v. 242, p. 39-40.

Shoemaker, E. M., 1960, Penetration mechanics of high velocity meteorites, illustrated by Meteor crater, Arizona: 21st International Geological Congress, Norway, p. 418- 434.

Shoemaker, E. M., 1962, Interpretation of lunar craters, in Kopal, Z., ed., Physics and Astronomy of the Moon: Academic press, New York, p. 283-351.

Shoemaker, E. M., 1977, Why study impact craters? in Roddy, D. J., Pepin, R. O., and Merrill, R. B., eds., Impact and explosion cratering: Planetary and terrestrial implications: Pergamon, New York, p. 1-10.

178

Shoemaker, E. M., and Eggleton, R. E., 1961, Terrestrial features of impact origin, in Milo, M. D., ed., Proceedings of the Geophysical Laboratory/Lawrence Radiation Cratering Symposium, Lawrence Berkeley National Laboratory, Berkeley, California, p. A1- A27.

Shoemaker, E. M., and Kieffer, S. W., 1974, Guidebook to the geology of Meteor Crater, Arizona: Arizona State University, Tempe, AZ, Centre of Meteorite studies, Publication no. 17, 66 p.

Simonson, B. M., McDonald, I., Shukolyukov, A., Koeberl, C., Reimold, W. U., Lugmair, G. W., 2009, Geochemistry of 2.63–2.49 Ga impact spherule layers and implications for stratigraphic correlations and impact processes: Precambrian Research, v. 175, p. 51-76.

Sisodia, M. S., Lashkari, G., and Bhandari, N., 2006a, Impact origin of the Ramgarh structure, Rajasthan: some new evidences: Journal of Geological Society of India, v. 67, p. 423-431.

Sisodia, M. S., Lashkari, G., and Bhandari, N., 2006b, Reply to the comment on paper ‘Impact origin of the Ramgarh structure, Rajasthan: some new evidences in Journal of Geological Society of India, v. 67, p. 423-431’: Journal of Geological Society of India, v. 68, p. 563- 565.

Skála, R., 2002, Shock-induced phenomena in limestones in the quarry near Ronheim, the Ries Crater, Germany: Bulletin of the Czech Geological Survey, v. 77, no. 4, p. 313– 320.

Son, T. H., and Koeberl, C., 2007, Chemical variation in Lonar impact glasses and impactites: GFF, v. 129, p. 161-176.

Spray, J. G., 2002, Planetary science: Impacts in the round: Nature, v. 418, p. 487-488.

Srnka, L. J., Martelli, G., Newton, G., Cisowski, S. M., Fuller, M. D., and Schaal R .B., 1979, Magnetic Field and Shock Effects and Remanent Magnetization in a Hypervelocity Experiment: Earth and Planetary Science Letters, v. 42, p. 127-137.

Storzer, D., and Koeberl, C., 2004, Age of the Lonar impact crater, India: First results from fission track dating: 35th Lunar and Planetary Science Conference, Abstract no. 1309 (CD-ROM).

Stroube, W. B. Jr., Grag, A. N., Ali, M. Z., and Ehmann, W. D., 1978, A chemical study of the impact glasses and basalts from Lonar crater, India: Meteoritics, v. 13, no. 2, p. 201- 208.

Tarling, D. H., and Hrouda, F., 1993. The magnetic anisotropy of rocks: Chapman and Hall, London, 212 p.

Ugalde, H. A., Artemieva, N., and Milkereit, B., 2005, Magnetization on impact structures- Constraints from numerical modeling and petrophysics: Geological Society of America Special Paper, v. 384, p. 25–42. 179

Urrutia-Fucugauchi, J. and Velasco-Villareal, M., 2008, Rock magnetism and magnetic fabrics of impact polymictic breccias and melt from the Chicxulub crater, Mexico (abstract #A11485): European Geosciences Union General Assembly 10.

Vandamme, D., Courtillot, V., Besse, J., and Montigny, R., 1991, Paleomagnetism and age determinations of the Deccan traps (India): results of a Nagpur–Bombay traverse and review of earlier work: Reviews of Geophysics, v. 29, p. 159–190.

Vaughan, R. G., Hook, S. J., Calvin, W. M., and Taranik, J. V., 2005, Surface mineral mapping at Steamboat Springs, Nevada, USA, with multi-wavelength thermal infrared images: Remote Sensing of Environment, v. 99, p. 140−158.

Venkatesh, V., 1967, Geology and origin of the Lonar Crater, Maharashtra: Record of Geological Survey of India, v. 97, p. 30-45.

Wang, Qi, Zhang Pei-Zhen, Freymueller, J. T., Bilham, R., Larson, M., Lai, X., You, X., Niu, Z., Wu, J., Li, Y., Liu, J., Yang, Z., and Chen, Q., 2001, Present-day crustal deformation in China constrained by Global positioning system measurements: Science, v. 294, p. 574-577.

Wasilewski, P. and Dickinson, T., 2000, Aspects of the validation of magnetic remanence in meteorites: Meteoritics and Planetary Science, v. 35, p. 537–544.

Weiss, B. P., Garrick-Bethell, I., Pederson, S., Maloof, A. C., Louzada, K. L. and Stewart, S. T., 2007, Paleomagnetism of impact glass and spherules from Lonar crater, India: 38th Lunar and Planetary Science Conference, Abstract no. 2360 (CD-ROM).

Weiss, B. P., Pederson, S., Garrick-Bethell, I., Stewart S. T., Louzada, K. L., Maloof, A. C., and Swanson-Hysell, N. L., 2010, Paleomagnetism of impact spherules from Lonar crater, India and a test for impact-generated fields: Earth and Planetary Science Letters, v. 298, p. 66–76.

Widdowson, M., and Mitchell, C., 1999, Large-scale stratigraphical, structural and geomorphological constraints for earthquakes in the southern Deccan Traps, India: The case for denudationally-driven seismicity, in Subbarao, K. V., ed., Deccan Volcanic Province: Memoir Geological Society of India, no. 43, p. 425-452.

Widdowson, M., Pringle, M. S., and Fernandez, O. A., 2000, A post KT boundary (early Palaeocene) age for Deccan-type feeder dykes, Goa, India: Journal of Petrology, v. 41, p. 1177–1194.

Wignall, P. B., 2001, Large igneous provinces and mass extinctions: Earth Science Reviews, v. 53, p. 1-33.

Wright, S. P., and Ramsey, M. S., 2006, Thermal infrared data analyses of Meteor Crater, Arizona: Implications for Mars spaceborne data from the Thermal Emission Imaging System: Journal of Geophysical Research, v. 111, E02004, doi:10.1029/2005JE002472. 180

www.ametek.com

Yu, Y. J., 2006, How accurately can NRM/SIRM determine the ancient planetary magnetic field intensity?: Earth and Planetary Science Letters, v. 250, p. 27–37.

Zijderveld, J. D. A., 1967, A.C. demagnetization of rocks: analysis of results. In: Methods in palaeomagnetism, edited by Collinson, D. W., Creer, K. M., and Runcorn, S. K., Amsterdam: Elsevier, p. 254–286.

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SYNOPSIS OF THE THESIS SUBMITTED TO THE UNIVERSITY OF MUMBAI FOR THE DEGREE OF DOCTOR OF PHILOSOPHY (PH.D) IN PHYSICS

NAME OF THE CANDIDATE : M. D. ARIF

TITLE OF THE THESIS : ROCK MAGNETISM AND PALAEOMAGENTISM OF METEORITE IMPACT CRATERS IN INDIA

DEGREE : DOCTOR OF PHILOSOPHY (PH.D)

SUBJECT : PHYSICS

NAME OF THE GUIDING TEACHER : PROF. NATHANI BASAVAIAH

INSTITUTE WHERE RESEARCH : INDIAN INSTITUTE OF GEOMAGNETISM WORK WAS DONE NEW PANVEL, NAVI MUMBAI – 410218, INDIA (RECOGNIZED BY UNIVERSITY OF MUMBAI)

NUMBER AND DATE OF REGISTRATION : IIG/NO. 10/12-10-2010

DATE OF SUBMISSION OF THE SYNOPSIS : 27-12-2012

SIGNATURE OF THE CANDIDATE :

SIGNATURE OF THE GUIDE :

ROCK MAGNETISM AND PALAEOMAGNETISM OF METEORITE IMPACT CRATERS IN INDIA

Impact Craters, Crater Formation and Shock Metamorphism: Geologic processes of volcanism, tectonism (e.g. earthquakes), gradation (erosion, etc.) and impact cratering often result in distinct landforms that can be identified based on their shape and form, or morphology. Among the four major geologic processes, the impact cratering occurs when meteoroids, asteroids, or comets impacts planetary surfaces. Given that impact cratering is the most common geological process in the solar system, study of this topic provides an excellent means of interplanetary comparison. Laboratory experiments, explosion craters, natural impact craters, and computer simulations have given us a general outline of the main stages that characterize the formation of an impact crater. An understanding of terrestrial impact cratering is important, as it addresses how the outer layer of the Earth has been modified due to impacts, and also its effect on the physical, chemical, and biological systems. Impact cratering processes can be well understood by integrating multidisciplinary disciplines such as remote sensing, geological, geophysical (gravity, magnetic, seismic, and electric methods), petrographical, mineralogical, geochemical, geochronological, numerical modelling, and laboratory experimental studies. These studies were aimed at identification of possible new impact structures, verification of their origin, and detailed analysis of the geologic structure and rock deformation in such crater structures. The study also helps in understanding the surface, target material topography, ejecta emplacement and all other cratering mechanisms. A well-planned multi-disciplinary approach is imperative to make a significant contribution to the Earth’s cratering history. The present thesis provides important support for the study of impact structures on Indian Shield region that otherwise be impossible without undertaking costly drilling ventures. The main focus here is the identification of the magnetic and mineralogical signatures of an impact crater, as well as the deduction of the direction of the incoming projectile. This finally helps in understanding the different stages (projectile characteristics, size of the resultant crater, nature and distribution of proximal ejecta) that characterize impact events in the Indian Shield region. The dating of the impact

event is attempted by application of palaeomagnetic studies because impact formed minerals are newly magnetized along the orientation of the Earth’s magnetic field. 1. Impact Crater Formation: Impact craters are found on nearly all solid surface planets and satellites. The size of the crater depends on the kinetic energy of the impactor and the impact angle; kinetic energy is defined as: KE = 1/2 (mv2), where m and v are the mass and velocity of the impactor. During impact, the impactor’s KE is transformed into the kinetic and internal energy of the target and impactor. The internal energy heats both the impactor and the target; the residual KE is spent displacing and ejecting target and impactor material, producing a crater in the target surface. Morphologically impact craters are grouped into simple and complex types. For simple bowl-shaped craters (<2−4 km on Earth) showing a circular outline, the depression has depth-diameter ratios of about 1:5 to 1:7 with raised rims by an exterior ejecta blanket, whereas larger complex craters show a shallower depth-diameter ratio (1:10 to 1:20). They exhibit central structural uplifts, rim synclines, and outer concentric zones of normal faulting. The central uplift consists of strata, which have been uplifted above the preimpact level, and is surrounded by a ring depression (or rim syncline) filled with fragmented material and impact melt. The transition between simple and complex craters depends on planet’s surface gravity and target-rock strength. As the crater size increases further, the central peak in a complex crater begins to break up and form an inner ring of mountains. In sufficiently large craters, the ring appears at about one-half the rim diameter and these craters are called peak-ring craters. 2. Terrestrial Impact Craters: The Indian Example: Impacts have created enormous scars on the surfaces of nearly all solar system bodies, prompting Shoemaker (1977) to state that “impact of solid bodies is the most fundamental geologic process that has taken place on the terrestrial planets”. Impact cratering processes has been well understood in the past decades by both experimental and theoretical perspectives. Laboratory-scale experiments were carried out for clarifying the aspects of crater excavation and ejecta emplacement. Scaling laws were derived to describe impact crater size, shape, depth, ejecta, and melt deposits as functions of impact speed and impactor’s size and type. The one parameter that has often been neglected in the study of impact craters is the angle of impact. It is well known that impact events normally strike planetary surfaces at an angle from the surface, 2

and probability theory indicates that, with the assumption of an isotropic flux of impactors, the most likely angle of impact is 45°, regardless of the body’s gravitational field. Vertical and grazing impacts are rare. Unlike the Moon, Mars, and Mercury, the active geological processes of erosion, volcanism and tectonics rapidly obliterate the cratering record on Earth. Despite this, the Earth’s surface is home to 183 impact structures preserved to varying extents (database of the University of New Brunswick, www.unb.ca.passc/ImpactDatabase). The confirmation of an impact origin requires the initial observation of geophysical or remote sensing observations of a circular feature. The only impact-diagnostic recognition criteria in rocks and minerals that are generally accepted for their confirmation are: (1) the presence of projectile remnants, (2) the detection of shock metamorphic effects (e.g., micro-deformation effects such as planar deformation features [PDFs] and planar fractures [PFs] in quartz), (3) high-pressure (diaplectic) mineral glasses (maskelynite), (4) transformation of minerals into high pressure polymorphs such as quartz to coesite and/or stishovite, graphite to diamond,

zircon to reidite [ZrSiO4], or mineral dissociation due to high shock-induced temperatures

(e.g., zircon to baddeleyite [ZrO2] and silica [SiO2]), (5) shatter cones (macroscopic feature), and (6) chemical and isotopic traces of the extraterrestrial component in impact breccias and sub-mm-sized spherules (enrichment of siderophile elements, mainly of platinum-group elements, and more recently, chromium and osmium isotopic studies) . Efforts have been made to investigate possible diagnostic impact indicators in the structural geological characteristics of craters here. Values of crater type, target rock type, impact age, diameter, depth, rim height, circularity, floor diameter etc. are listed for all four impact craters in Indian Shield region (Table 1). (i) Lonar crater is a well-preserved simple impact crater in basaltic target rocks (Deccan Traps) of Cretaceous-Tertiary (K-T) age (~65 Ma). It is unique in being the only terrestrial impact crater in basalts as the shock-metamorphic features of ‘Lonar’ basalts (Fredriksson et al., 1973; Schaal and Hörz, 1977) serve as an excellent terrestrial analog to impact craters formed on other planets and planetary bodies with basaltic crusts, such as Moon and Mars (e.g. Hagerty and Newsom, 2003). The nature of target basaltic rocks, various degrees of shock metamorphic signatures, nature of shocked and unshocked basalts, macro and micro breccias, glass spherules and impactites have provided definitive evidences for impact origin 3

of the Lonar crater. The characteristic shock metamorphic features of Lonar crater are compared and correlated with those of the lunar craters in a planetary context, and thus helping to unravel the extraterrestrial geoscientific mysteries of the planets and the evolution of the solar system. Table 1 Comparisons of important features of all four impact craters in India

Location, India Crater Crater Crater Target rim Impa- dia. Impact evidences References Lat. Long. height ctor (Type) (age) (km) (N) (E) (m)

Lonar 19°58' 76°31' Basalt 1.9 30 Stony Maskelynite Fredriksson et (Chon- al. 1973 Maharashtra (570±47 dm-size glass drite) Fudali et al. ka) bombs, mm & (Simple) 1980 sub-mm-size- Rao & Bhalla, spherules, clasts, 1984 shatter cones Louzada et al. high Cr (200 2008 ppm) Misra et al. Co (50 ppm) 2010 Ni (1000 ppm) Maloof et al. 2010 Jourdan et al. 2011 Arif et al. 2012 Coesite Ramgarh 25°20' 76°38' Sand 5.5 250 Iron Crawford, 1972 Tridymite stone, Ahmed et al. Rajasthan Cristobalite Shale, 1974 (Complex) Lime mm-size magnetic Sisodia et al. stone particles/spherules 2006 impact-melts with Misra et al. 2008 vesicular surfaces

shatter cones high Fe (58%) Ni (4000 ppm) Co (7000 ppm) Dhala 25°18' 76°08' Granite 11.0 295 - Coesite Pati et al. 2010 Madhya (~1.7- Granitoid melts Pradesh 2.5 Ga) PDFs in Quartz (Complex) Feldspar

Luna 23°42' 69°16' Basalt 1.2 3.5 - Stishovite, Karanth et al. sediment Coesite 2006 Gujarat Magnetic and (Simple) nonmagnetic glassy fragments

Studies were conducted at IIT Kharagpur to infer the nature of the impact event and the composition of impacting body based on major and trace element compositions of Lonar samples (sub-mm-sized impact spherules) that are products of the impact event (Misra et al., 2009). Based on magnetic studies carried out at IIG, it is shown that the distribution of ejecta and deformation of target rocks around the Lonar crater rim is symmetrical to the E– W plane of impact, and a chondritic asteroid struck the pre-impact target basalt from the east at an angle of between ~30 and 45° to the horizon (Misra et al., 2009, 2010).

Previous studies on rock magnetism and palaeomagnetism of Lonar crater: The existing rock magnetic and palaeomagnetic studies on Lonar crater are not adequate in the evaluation of the chain of events initiated by oblique impact, projectile trajectory, the orientation of structural deformation to the direction of impact inferred by the ejecta blanket, and the relationship of the shock front to the impact direction. An altitude dependence on shock demagnetization (Rao and Bhalla 1984; Nishioka et al., 2006) is explained by differing mineralogical content and extent of weathering between the lower and upper flows.

Rao and Bhalla (1984) reported systematic variations in some of the magnetic parameters [Jn

(NRM),  (susceptibility), Qn (Köenigsberger ratio) and declination] of basaltic samples collected from the inner walls of the Lonar crater, whereas random variations were observed for the surrounding target rocks. A soft secondary shock component was also identified in their study that was acquired in the present Earth’s magnetic field. However, Louzada et al. (2008) suggested that this low coercivity and low temperature (LC_LT) component was not directly related to shock metamorphism, but was replaced by viscous and/or chemical remanent magnetization after the impact. Weiss et al. (2007) also reported rock magnetic data of the impact glasses and spherules. The ratios of NRM to saturation isothermal remanent magnetization (SIRM) for the small glasses were only 0.5-1×10-3, while the large glasses had ratios twice as large. These values were nearly an order of magnitude lower than those measured for the nearby Deccan basalts (Louzada et al., 2008). The low NRM/SIRM ratios of the Lonar glasses were interpreted to indicate the absence of any impact-generated paleofields substantially higher than several tens of micro Tesla (μT) at the Lonar crater (Weiss et al., 2010), and the glasses slightly underestimated the intensity of the field in which they cooled, probably due to the effects of rotation during cooling.

(ii) The complex Ramgarh structure occurred into sedimentary target strata (sandstone, shale, limestone) of Late Proterozoic Bhander Group (1700 Ma) of rocks of the Vindhyan Supergroup (Sisodia et al., 2006). The origin of this unusual structure has been attributed to an impact event based on the following findings: (1) rectangular shape structure and presence of shatter cone in colluvium near the center of the feature (Crawford, 1972), (2) occurrences of impact spherules, diaplectic glasses, and planar deformation features (PDFs) in quartz grains (Sisodia et al., 2006), (3) occurrences of highly magnetic pieces with characteristic pitting and polish from inside and outside of the structure (Ahmed et al., 1974), (4) presence of high Fe (up to 58 wt%), Ni (~4000 ppm) and Co (~7000 ppm) with high Ni/Cr (average: ~4, range: 0.06 to 32) and Co/Cr (~10, 0.06 to 58) ratios in mm-size magnetic particles/spherules from the soil inside the structure (Misra et al., 2008). Very high abundance of Ni can be suspected to asteroid origin, because this element occurs in very low proportions in terrestrial rocks, except in primary mantle-derived mafic and ultramafic rocks, which are absent in and around the Ramgarh structure. In order to confirm the structure as asteroid/meteoritic impact origin, rock magnetic and palaeomagnetic sampling will be carried out to study the magnetic mineralogical constituents and shock remanent magnetization (SRM) acquired during shock compression in the presence of the ambient geomagnetic field. (iii) The complex Dhala structure is formed on Precambrian granitoids of ~2.5 Ga age, with minor 2.0-2.15 Ga mafic intrusive rocks, and overlain by post-impact sediments of >1.7 Ga. Based on textural and mineralogical studies and shock features present in samples of the Dhala structure within the Bundelkhand craton, carried out at the Allahabad University, a meteorite impact origin of this structure is now confirmed (Pati et al., 2008). The sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of two breccia samples yielded ages of 2563 Ma and 2553 Ma, indicating the age of the granitoid basement, whereas the 40Ar/39Ar dating resulted in partial plateau ages indicating that impact melt rock was affected by a strong thermal/hydrothermal overprint at ca. 1 Ga. The SHRIMP U-Pb dating of two zircons from impact melt rock indicates ca. 530 Ma event that could have contributed to the post-impact resetting (Pati et al., 2010). The upper age limit of 2.5 Ga may represent Dhala is the oldest impact structure found on Earth. No rock magnetic and

paleomagnetic study has been carried out till date on this crater, and the present thesis will propose for magnetic studies like Lonar and Ramgarh craters. (iv) Luna crater is a circular crater having about 3.5 m diameter and excavated in Deccan basalt sediments in Kachchh district of Gujarat (Karanth et al., 2006). The radar- generated data of the satellite imagery suggest that it spreads for over 5 km radius. The various impact products found at this site include: (a) metallic meteorites fragments with spherical cavities, (b) glassy objects comparable to tektites, and (c) high pressure mineral

polymorphs of SiO2, stishovite and coesite, and thus confirming its impact origin.

3. Effect of shock on the magnetic properties of target rocks: Shock metamorphism and its effects on rocks and minerals are investigated by studying magnetic properties of shocked rocks. Because the intensity of the shock wave decreases radially away from the point of impact, the target materials experience varied degrees of shock damage during an impact event. Signatures of the shock metamorphism are shown by damaged mineral crystals, including surface dislocations, formation of diaplectic glass, and partial melting along grain boundaries in the shocked target rocks. Magnetically, shock pressure is accompanied by damage (e.g., microfracturing of magnetic grains) resulting in permanent changes in the intrinsic magnetic properties (a more single-domain like behavior). Here, the degree of alteration caused by stress waves on target rocks and shock-induced magnetization effects on magnetic remanence properties will be investigated by rock magnetic and palaeomagnetic techniques. For example, rock magnetic measurements of Lonar basalts and Ramgarh rock samples were performed to study the magnetic mineralogical assemblages. Magnetic minerals can undergo rock magnetic changes of increasing coercivity as a result of shock (Cisowski and Fuller 1978; Pesonen et al., 1997; Gattacceca et al., 2007). Laboratory shock experiments reveal that the primary effects of shock on magnetic properties of target rocks results in demagnetization or remagnetization of magnetic minerals, and shock hardening. The shocked rocks are remagnetized and acquire a shock remanent magnetization (SRM) in the direction of the Earth’s magnetic field at the time of impact. Louzada et al. (2008) observed subtle effects of shock hardening in titanomagnetite- bearing ejecta basalt at Lonar crater. The efficiency of SRM is significantly less than that of thermoremanent magnetization (TRM), and it is more susceptible to viscous decay as SRM 7

is recorded in low coercivity magnetic grains indicating it may not be stable over geologic time. However, the directionality of magnetic remanence can be used to study impact tectonics and constrain an upper limit on shock heating. Transient impact-related magnetic fields may lead to nonunidirectional SRM, but are confined by the ejecta curtain and located near the surface, resulting in intense amplified magnetic fields around impact structures. Cisowski and Fuller (1978) concluded that the variability of remanence in the Deccan basalts from Lonar crater can be explained in terms of their differing response to shock as a function of coercivity.

4. Magnetic fabric studies on shocked rocks: The anisotropy of magnetic susceptibility (AMS) of rocks is usually controlled by the preferred orientation of magnetic minerals related to the primary formation of rock or later tectonic deformation. Here AMS results indicate that target rocks of meteorite impact crater possibly not exhibit the primary AMS

fabric, and the K3 susceptibility axes were found to be randomly oriented (Carporzen et al., 2005). Experiments on basaltic andesites from central Japan and basalts from the Lonar crater showed that the orientations of AMS axes could be an indicator of the propagation directions of stress waves generated in rocks at terrestrial impact structures (Nishioka, 2007; Nishioka et al., 2007; Nishioka and Funaki, 2008). In the high pressure range (>3 GPa) of

experiment, the anisotropy degree was increased, the K3 axes were oriented towards the shock direction and the average susceptibility was decreased. At a relatively low pressure range (0.5–3 GPa), the maximum susceptibility axes (K1) was induced parallel to the shock direction and was superposed on the initial AMS data. The present thesis uses AMS studies to investigate on structural deformation, emplacement mode and post-impact alterations at impact and target lithologies of Lonar and Ramgarh craters (Kontny et al., 2008; Misra et al., 2010). The AMS technique has been used on basaltic rock samples from Lonar crater to evaluate the direction and obliquity of meteoritic impact on this carter (Misra et al., 2010).

5. Thesis Objectives: The impact history of Lonar, Ramgarh and Dhala structures is evaluated by rock-magnetic, palaeomagnetic, and geochemical data. The thesis objectives are:  To find out evidences of impact generated magnetic indicators.  To examine the orientation of structural deformation to the direction of impact by AMS.  To study the rock and mineral magnetic properties of impact craters on Indian shield.  To test shock remanent magnetization (SRM) hypotheses, impact-generated magnetic fields and mean palaeomagnetic pole positions.  To develop palaeomagnetic technique as dating tool of impact crater sites.  To understand the geochemical variations of impact rocks. Organization of the Thesis: In line with the principle objectives of this thesis, the chapters are grouped into seven topics: significance of impact cratering mechanics of the Indian impact craters; field sampling and laboratory experimental techniques, geological structural and AMS technique to evaluate the direction of projectile; effects of shock pressures on rock and mineral magnetic properties of impact rocks; palaeomagnetism of impact rocks; geochemistry of impact rocks and their products and discussions and conclusions with some remarks on the prospects of impact crater research in India. Chapter 1 gives a brief overview on the study of impact craters, classification of impact craters and their recognition criterion, significance of terrestrial impact cratering processes, and its role in shaping the landscape and generating economic resources. It also discusses geologic overview about the Indian impact features (craters). It tries to discuss the use of rock magnetism and palaeomagnetism technique in the recognition of geology and geophysics of impact structures at Lonar, Ramgarh, and Dhala structures in the Indian Shield region. Chapter 2 concerns the details of sampling methodology carried out in the field work, laboratory experimental procedures and the methods for analysis of rock magnetic, palaeomagnetic, and AMS data. All magnetic and geochemical measurements were done at Environmental Magnetism Laboratory, IIG. The collection of shock metamorphosed products (impactites, impact spherules) and their laboratory treatment and data analysis is also included in this chapter. 9

Chapter 3 deals with the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image, geological structure analysis and AMS technique for evaluating the direction and angle of impactor that struck the preimpact target at Lonar. It also discusses the distribution of impact-induced shock front after the impact at Lonar. Specifically, the AMS data suggest that the target basalts occurring at ~2 km West-South- West (WSW) of the crater rim are highly shocked as indicated by the random orientation of

their K3 susceptibility axes in comparison to the unshocked basalts at ~2 km ESE of the crater, showing a bimodal distribution of susceptibility axes typical of lava flows. Moderate

to strong westward shifts of the K3 axes are seen for the majority of the shocked basalts on the crater rim and WSW of the crater, indicating an oblique impact from the east when compared with modeling and experiments. Variation in attitudes of the basalt flows on the Lonar crater rim shows a bilaterally symmetrical distribution about an E-W axial plane, which includes quaquaversal dips of the flows all around the crater rim, except to the west where overturned dips of the basalt flows are seen. It appears that oblique impact and the symmetry in structural variations around the crater rim have a relationship for Lonar crater (Misra et al., 2010). It is also showed that the impact stress could have branched out into the major SW and NW components in the downrange direction immediately after the impact, inferred from the relative displacements of K3 axes of shocked basalts (Arif et al., 2012). Chapter 4 describes the rock and mineral magnetic properties of Lonar target rocks to identify the magnetic remanence carriers and their variations with the plane and direction of impact. The mineralogical data suggest that Lonar basalts essentially contain PSD grain size Ti-rich to Ti-poor titanomagnetite and their low temperature oxidized products as the magnetic carriers. Their rock-magnetic results show an increase of NRM/χ and REM (=NRM/SIRM ratio in %) values with the E-W plane of impact. Rock magnetic and palaeomagnetic properties of shocked ejecta basalts suggest that impact shock-induced magnetic field could have existed beyond the modification stage of Lonar crater formation, when the newly formed ejecta with the randomly deposited basaltic blocks had weakly remagnetized. More rigorous description of data and discussion of results obtained were discussed in this chapter (Arif et al., 2012). Rock magnetic and palaeomagnetic study of Ramgarh structure confirmed its origin by asteroid impact.

Chapter 5 provides palaeomagnetic results to understand the shock effects on the magnetic remanence carriers with the plane and direction of impact. The results suggest that shocked target basalts acquired a high coercivity (HC) and/or high temperature (HT) magnetization component due to impact. The orientation of HC-HT components in the uprange direction is symmetrically disposed about E-W plane of impact, making an obtuse angle with the direction of impact. The low coercivity (LC) and/or low temperature (LT) components of unshocked and shocked basalts are statistically identical to the present day field (PDF) direction. The PDF may likely be acquired during the last 570±47 ka, subsequent to the formation of the Lonar crater. Here it is demonstrated that the high coercivity remanence is the product of single-domain magnetite from shock-induced decomposition of target rocks in titanomagnetite, as suggested by Cloete et al. (1999). Discussion of palaeomagentic data analysis and the presence or absence of shock remanent magnetization (SRM) and impact- generated plasma fields around impact structures is verified. The mean palaeomagentic pole is calculated and to be used for dating the impact structures. Chapter 6 deals with the geochemical and X-ray diffraction (XRD) analysis of target rocks, impact melts and products. The comparisons of major oxides compositions of Lonar impact products with those of target rocks were done using X-ray fluorescence spectroscopy (XRF).

There are slight depletions in the Na2O, P2O5 contents, and enrichment in the K2O content, in the impact melt rocks compared to the target rocks. This is in agreement with the findings of Osae et al. (2005). The enrichment of ‘K’ in impactites is a common phenomenon and termed the process “potassium metasomatism”, probably due to vaporization fractionation during the impact, or possibly an immediate post-impact alteration. The XRD analysis is carried out for phase identification of minerals and shock-induced phase changes. Chapter 7 summarizes the results from the thesis and presents an outlook for future impact crater research in India.

REFERENCES

Ahmed N., Bhardwaj B. D., Sajid H. A., and Hasnain I. 1974. Ramgarh Meteorite Crater: Current Science 43:598.

Arif Md., Basavaiah N., Misra S., and Deenadayalan K. 2012a. Variations in magnetic properties of target basalts with the direction of asteroid impact: Example from Lonar crater, India. Meteoritics & Planetary Science 47:1305–1323.

Arif Md., Basavaiah N., and Misra S. 2012b. Rock- and palaeomagnetic properties of randomly oriented basaltic blocks from Lonar crater ejecta, India. European Planetary Science Congress (EPSC), held 23-28 September 2012, Madrid, Spain (abstract#EPSC2012- 163).

Carporzen L., Gilder S. A., and Hart R. J. 2005. Palaeomagnetism of the Vredefort meteorite crater and implications for craters on Mars. Nature 435:198–201.

Cisowski S. M. and Fuller M. 1978. The effect of shock on the magnetism of terrestrial rocks. Journal of Geophysical Research 83:3441–3458.

Cloete M., Hart R. J., Schmid H. K., Drury M., Demanet C. M., and Vijaya S. K. 1999. Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa. Contributions of Mineralogy and Petrology 137: 232–245.

Crawford A. R. 1972. Possible impact structure in India. Nature 237:96.

Fredriksson K., Dube A., Milton D. J., and Balasundaram M. S. 1973. Lonar Lake, India: an impact crater in basalt. Science 180:862–864.

Gattacceca J., Lamali A., Rochette P., Boustie M., and Berthe L. 2007. The effects of explosive-driven shocks on the natural remanent magnetization and the magnetic properties of rocks. Physics of the earth and planetary interiors 162:85–98.

Hagerty J. J. and Newsom H. E. 2003. Hydrothermal alteration at the Lonar Lake impact structure, India: implications for impact cratering on Mars. Meteoritics & Planetary Science 38:365–381.

Herrick R. R. and Forsberg-Taylor N. K. 2003. The shape and appearance of craters formed by oblique impact on the Moon and Venus. Meteoritics & Planetary Science, 38:1551- 1578.

Jourdan F., Moynier F., Koeberl C., and Eroglu S. 2011. 40Ar/39Ar age of the Lonar crater and consequence for the geochronology of planetary impacts. Geology 39:671–674.

Karanth R. V., Thakker P. S., and Gadhavi M. S. 2006. A preliminary report on the possible impact crater of Kachchh. Current Science 91:877–879. Kieffer S.W., Schaal R. B., Gibbons R., Horz F., Milton D. J., and Dube A. 1976. Shocked basalt from Lonar Impact Crater, India, and experimental analogues. Proceedings of the 7th Lunar and Planetary Science Conference:1391–1412.

Kontny A., Fruhauf P., and Elbra T., 2008. Rock magnetic properties and magnetic fabric of impact and basement lithologies of the Chesapeake Bay impact structure. Geotectonic Research, 3:89–91.

Louzada K. L., Weiss B. P., Maloof A. C., Stewart S. T., Swanson-Hysell N. L., and Soule S. A. 2008. Paleomagnetism of Lonar impact crater, India. Earth and Planetary Science Letters 275:308–319.

Misra S., Lashkari G., Panda D., Dube A., Sisodia M. S., Newsom H. E., and Sengupta D. 2008. Geochemical evidence for the meteorite impact origin of Ramgarh structure, India: 39th Lunar and Planetary Science Conference, abstract #1499 (CD-ROM).

Misra S., Newsom H. E., Prasad M. S., Geissman J. W., Dube A., and Sengupta D. 2009. Geochemical identification of impactor for Lonar crater, India. Meteoritics & Planetary Science 44:1001–1018.

Misra S., Arif Md., Basavaiah N., Srivastava P. K., and Dube A. 2010. Structural and anisotropy of magnetic susceptibility (AMS) evidence for oblique impact on terrestrial basalt flows: Lonar crater, India. Geological Society of America Bulletin 122:563–574.

Nishioka I., Funaki M., and Rao V. 2006. Shock demagnetization of basalt from Lonar impact crater in India, AGU 2006 Fall Meeting, San Francisco, p. GP11B-0081. Nishioka I. 2007. Rock magnetic study of basalt at Lonar impact crater in India: Effects of stress waves on rock magnetic properties. The Graduate University for Advanced Studies, Japan (Unpublished Ph.D. thesis), 70 p.

Nishioka I., Funaki M., and Sekine T. 2007. Shock-induced anisotropy of magnetic susceptibility: impact experiment on basaltic andesite. Earth Planets Space 59:e45–e48.

Nishioka I. and Funaki M. 2008. Irreversible changes in anisotropy of magnetic susceptibility: Study of basalts from Lonar crater and experimentally impacted basaltic andesite. 71st Annual Meteoritical Society Meetings (abstract#5207).

Osae S., Misra S., Koeberl C., Sengupta D., and Ghosh S. 2005. Target rocks, impact glasses, and melt rocks from the Lonar impact crater, India: Petrography and geochemistry. Meteoritics & Planetary Science 40:1473–1492.

Pati J. K., Reimold W. U., Koeberl C., and Pati P. 2008. The Dhala structure, Bundelkhand craton, Central India – Eroded remnant of a large paleoproterozoic impact structure. Meteoritics & Planetary Science 43:1383–1398. 13

Pati J. K., Jourdan F., Armstrong R. A., Reimold W. U., and Prakash K. 2010. First SHRIMP U-Pb and 40Ar/39Ar chronological results from impact melt breccia from the Paleoproterozoic Dhala impact structure, India. Geological Society of America Special Papers 465:571–591.

Pesonen L. J., Deutsch A., Hornemann U., and F. Langenhorst F. 1997. Magnetic properties of diabase samples shocked experimentally in the 4.5 to 35 GPa range. 28th Lunar and Planetary Science Conference:1087–1088.

Pierazzo E. and Melosh H. J. 2000. Understanding oblique impacts from experiments, observations, and modeling. Annual Review of Earth and Planetary Sciences 28:141–167.

Rao G. V. S. P. and Bhalla M. S. 1984. Lonar lake: Palaeomagnetic evidence of shock origin. Geophysical Journal of the Royal Astronomical Society 77:847–862.

Schaal R. B. and Hörz F. 1977. Shock metamorphism of lunar and terrestrial basalts: Proceedings of the 8th Lunar Science Conference:1697–1729.

Sisodia M. S., Lashkari G., and Bhandari N. 2006. Impact origin of the Ramgarh structure, Rajasthan: some new evidences. Journal of Geological Society of India 67:423–431.

Shoemaker E. M. 1977. Why study impact craters? in Roddy D. J., Pepin R. O., and Merrill R. B., eds., Impact and explosion cratering: Planetary and terrestrial implications: Pergamon, New York, p. 1–10.

Weiss B. P., Garrick-Bethell I., Pederson S., Louzada K. L., Stewart S. T., and Maloof A. C. 2007. Paleomagnetism of impact glass from Lonar crater, India (abstract#2360). 38th Lunar and Planetary Science Conference (CD-ROM).

Weiss B. P., Pederson S., Garrick-Bethell I., Stewart S. T., Louzada K. L., Maloof A. C., and Swanson-Hysell N. L. 2010. Paleomagnetism of impact spherules from Lonar crater, India and a test for impact-generated fields. Earth and Planetary Science Letters 298:66–76.