DOCSLIB.ORG

5.10 Geomagnetic Excursions C

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
5.10 Geomagnetic Excursions C 5.10 Geomagnetic Excursions C. Laj, Laboratoire des Sciences du Climat et de l’Environment, Unite´ Mixte CEA-CNRS-UVSQ, Gif-sur-Yvette, France J. E. T. Channell, University of Florida, Gainesville, FL, USA ª 2007 Elsevier B.V. All rights reserved. 5.10.1 Introduction 373 5.10.1.1 History of the Polarity Timescale and Excursions 373 5.10.1.2 Nomenclature for Excursions and Polarity Intervals 375 5.10.2 Geomagnetic Excursions in the Brunhes Chron 376 5.10.2.1 Introduction 376 5.10.2.2 The Laschamp Excursion 379 5.10.2.3 The Mono Lake Excursion 383 5.10.2.4 The Blake Excursion 385 5.10.2.5 The Iceland Basin Excursion 387 5.10.2.6 The Pringle Falls Excursion 389 5.10.2.7 Excursions in the Early Brunhes Chron 391 5.10.3 Geomagnetic Excursions in the Matuyama Chron 393 5.10.3.1 Background 393 5.10.3.2 Excursions between the Gauss–Matuyama Boundary and the Re´ union Subchron 396 5.10.3.3 Huckleberry Ridge 396 5.10.3.4 Gilsa 396 5.10.3.5 Gardar and Bjorn 397 5.10.3.6 Cobb Mountain 397 5.10.3.7 Punaruu 397 5.10.3.8 Intra-Jaramillo Excursion (1r.1n.1r) 397 5.10.3.9 Santa Rosa 397 5.10.3.10 Kamikatsura 398 5.10.4 Geomagnetic Excursions in Pre-Matuyama Time 398 5.10.4.1 C5n.2n (Late Miocene) 400 5.10.4.2 Other Miocene Excursions/Subchrons 401 5.10.4.3 Oligocene and Eocene 401 5.10.4.4 Middle Cretaceous 402 5.10.5 Duration of Geomagnetic Excursions 402 5.10.6 Excursional Field Geometry 404 5.10.7 Concluding Remarks 405 References 407 5.10.1 Introduction Miocene lavas from central France (Puy de Dome). In so doing, Brunhes (1906) made first use of a field 5.10.1.1 History of the Polarity Timescale test for primary thermal remanent magnetization and Excursions (TRM) that is now referred to as the ‘baked contact’ David (1904) and Brunhes (1906) were the first to test (see Laj et al. (2002) for an account of Brunhes’ measure magnetization directions in rocks that were work). Matuyama (1929) was the first to attribute approximately antiparallel to the present Earth’s reverse magnetizations in (volcanic) rocks from field. Brunhes (1906) recorded magnetizations in Japan and China to reversal of geomagnetic polarity, baked sedimentary rocks that were aligned with and to differentiate mainly Pleistocene lavas from reverse magnetization directions in overlying mainly Pliocene lavas based on the polarity of the 373 374 Geomagnetic Excursions magnetization. In this respect, Matuyama (1929) was Proterozoic Torridonian Sandstones in Scotland, and the first person to use magnetic stratigraphy as a in rocks of Devonian and Triassic age. For the means of ordering rock sequences. Torridonian Sandstones, normal and reverse magne- The modern era of paleomagnetic studies began tizations were observed from multiple outcrops, and with the studies of Hospers (1951, 1953–54) in Iceland, an attempt was made to arrange the observed polarity and Roche (1950, 1951, 1956) in the Massif Central of zones in stratigraphic sequence. Meanwhile, Khramov France. The work of Hospers on Icelandic lavas was (1958) published magnetic polarity stratigraphies in augmented by Rutten and Wensink (1960) and Pliocene–Pleistocene sediments from western Wensink (1964) who subdivided Pliocene–Pleistocene Turkmenia (central Asia), and made chronostrati- lavas on Iceland into three polarity zones from young to graphic interpretations based on equal duration of old: N–R–N. Magnetic remanence measurements on polarity intervals. Early magnetostratigraphic studies basaltic lavas combined with K/Ar dating, pioneered by were carried out on Triassic red sandstones of the Cox et al. (1963) and McDougall and Tarling (1963a, Chugwater Formation (Picard, 1964), on the 1963b, 1964), resulted in the beginning of development European Triassic Bundsandstein (Burek, 1967, of the modern geomagnetic polarity timescale (GPTS). 1970), and on the Lower Triassic Moenkopi These studies, and those that followed in the mid- Formation (Helsley, 1969). The above studies were 1960s, established that rocks of the same age carry the conducted on poorly fossiliferous mainly continental same magnetization polarity, at least for the last few (red) sandstones and siltstones; therefore, the correla- million years. The basalt sampling sites were scattered tions of polarity zones did not have support from over the globe. Polarity zones were linked by their K/ biostratigraphic correlations. It was the early magne- Ar ages, and were usually not in stratigraphic super- tostratigraphic studies of Plio-Pleistocene marine position. Doell and Dalrymple (1966) designated the sediments, recovered by piston coring in high south- long intervals of geomagnetic polarity of the last 5 My ern latitudes (Opdyke et al., 1966; Ninkovitch et al., as magnetic epochs, and named them after pioneers of 1966; Hays and Opdyke, 1967) and in the equatorial geomagnetism (Brunhes, Matuyama, Gauss, and Pacific Ocean (Hays et al., 1969), that mark the begin- Gilbert). The shorter polarity intervals (events) were ning of modern magnetic stratigraphy. These studies named after localities: e.g., Jaramillo (Doell and combined magnetic polarity stratigraphy and biostra- Dalrymple, 1966), Olduvai (Gromme´ and Hay, 1963, tigraphy and in so doing refined and extended the 1971), Kaena and Reunion (McDougall and GPTS derived from paleomagnetic studies of basaltic Chamalaun, 1966), Mammoth (Cox et al., 1963), and outcrops (e.g., Doell et al., 1966). Nunivak (Hoare et al., 1968). The nomenclature for Heirtzler et al. (1968) produced a polarity timescale excursions has continued this trend by naming excur- for the last 80 My using South Atlantic MMA record sions after the localities from where the earliest records (V-20) as the polarity template. They assumed constant were derived (e.g., Laschamps in France). spreading rate, and extrapolated ages of polarity chrons The fit of the land-derived polarity timescale, using an age of 3.35 Ma for the Gilbert/Gauss polarity from paleomagnetic and K/Ar studies of exposed chron boundary. This was a dramatic step forward that basalts, with the polarity record emerging from mar- extended the GPTS to 80 Ma, from 5Mabasedon ine magnetic anomalies (MMAs) (Vine and magnetostratigraphic records available at the time (e.g., Matthews, 1963; Vine, 1966; Pitman and Heirtzler, Hays and Opdyke, 1967). Heirtzler et al. (1968) made 1966; Heirtzler et al., 1968) resulted in a convincing the inspired choice of a particular South Atlantic MMA argument for synchronous global geomagnetic polar- record (V-20). Several iterations of the polarity time- ity reversals, thereby attributing them to the main scale (e.g., Labrecque et al., 1977) culminated with the axial dipole. The results relegated self-reversal, work of Cande and Kent (1992a) that re-evaluated the detected in the 1950s in the Haruna dacite from the MMArecord,andusedtheSouthAtlanticasthefun- Kwa district of Japan (Nagata, 1952; Nagata et al., damental template with inserts from faster spreading 1957; Uyeda, 1958), to a rock-magnetic curiosity centers in the Indian and Pacific Oceans. They then rather than to a process generally applicable to interpolated between nine numerical age estimates volcanic rocks. that could be linked to polarity chrons over the last The first magnetic stratigraphies in sedimentary 84 My. Eight of these ages were based on ‘high- rocks may be attributed to Creer et al. (1954) and temperature’ radiometric ages (no glauconite ages Irving and Runcorn (1957) who documented normal were included), and the youngest age (2.60 Ma) was and reverse polarities at 13 locations of the the astrochronological age estimate for the Gauss/ Geomagnetic Excursions 375 Matuyama boundary from Shackleton et al. (1990). In magnetic stratigraphy because the high sedimentation the subsequent version of their timescale, hereafter rate sequences, required to record brief (few kiloyears referred to as CK95, Cande and Kent (1995) adopted duration) magnetic excursions, were generally not tar- astrochronological age estimates for all Pliocene– geted during conventional piston coring expeditions. Pleistocene polarity reversals (Shackleton et al., 1990; Indeed, at the time, low sedimentation rate sequences Hilgen, 1991a, 1991b) and modified the age tie-point at were often preferred as they allowed the record to be the Cretaceous–Tertiary boundary from 66 to 65 Ma. pushed further back in time. The development of The astrochronological estimates for Pliocene– hydraulic piston coring (HPC) techniques, first used Pleistocene polarity chrons (Shackleton et al., 1990; in early 1979 during the Deep Sea Drilling Project Hilgen, 1991a, 1991b), incorporated in CK95, have (DSDP) Leg 64, brought high sedimentation rate not undergone major modification in the last 15 years, sequences within reach by increasing penetration by a and indicate the way forward for timescale calibration factor of 15, from 20 m for conventional piston further back in time. Subsequent to CK95, astrochro- coring to 300 m for the HPC. nological estimates of polarity chron ages have been extended into the Miocene, Paleogene, and Cretaceous 5.10.1.2 Nomenclature for Excursions and (Krijgsman et al., 1994, 1995; Hilgen et al., 1995, 2000; Polarity Intervals Abdul-Aziz, 2000, 2003, 2004). Many of these advances are included in the timescale of Lourens et al. (2004). At The International Stratigraphic Commission (ISC) present, more or less continuous astrochronologies tied has guided the use of magnetostratigraphic units to polarity chrons are available back to the Oligocene (polarity zones), their time equivalents (polarity (e.g., Billups et al., 2004). chrons), and chronostratigraphic units (polarity Recognition of brief polarity excursions as an inte- chronozones) (see Anonymous, 1977; Opdyke and gral part of the Earth’s paleomagnetic field behavior Channell, 1996). The revelation in the last 20 years has developed alongside astrochronological calibration of numerous short-lived excursions within the of the polarity timescale in the last 20 years.
Load more

Recommended publications
  • Morrison Formation 37 Cretaceous System 48 Cloverly Formation 48 Sykes Mountain Formation 51 Thermopolis Shale 55 Mowry Shale 56
    THE STRUCTURAL AND STRATIGRAPHIC FRAMEWORK OF THE WARM SPRINGS RANCH AREA, HOT SPRINGS COUNTY, WYOMING By CHRISTOPHER JAY CARSON Bachelor of Science Oklahoma State University 1998 Submitted to the Faculty of the Graduate College of the Oklahoma State University in partial fulfillment of the requirements for the Degree of MASTER OF SCIENCE July, 2000 THE STRUCTURAL AND STRATIGRAPHIC FRAMEWORK OF THE WARM SPRINGS RANCH AREA, HOT SPRINGS COUNTY, WYOMING Thesis Approved: Thesis Advisor ~~L. ... ~. ----'-"'-....D~e~e:.-g-e----- II ACKNOWLEDGEMENTS I wish to express appreciation to my advisor Dr. Arthur Cleaves for providing me with the opportunity to compile this thesis, and his help carrying out the fieldwork portion of the thesis. My sincere appreciation is extended to my advisory committee members: Dr. Stan Paxton, Dr. Gary Stewart, and Mr. David Schmude. I wish to thank Mr. Schmude especially for the great deal of personal effort he put forth toward the completion of this thesis. His efforts included financial, and time contributions, along with invaluable injections of enthusiasm, advice, and friendship. I extend my most sincere thank you to Dr. Burkhard Pohl, The Big Hom Basin Foundation, and the Wyoming Dinosaur Center. Without whose input and financial support this thesis would not have been possible. In conjunction I would like to thank the staff of the Wyoming Dinosaur Center for the great deal of help that I received during my stay in Thermopolis. Finally I wish to thank my friends and family. To my friends who have pursued this process before me, and with me; thank you very much.
    [Show full text]
  • Is Earth's Magnetic Field Reversing? ⁎ Catherine Constable A, , Monika Korte B
    Earth and Planetary Science Letters 246 (2006) 1–16 www.elsevier.com/locate/epsl Frontiers Is Earth's magnetic field reversing? ⁎ Catherine Constable a, , Monika Korte b a Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0225, USA b GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Received 7 October 2005; received in revised form 21 March 2006; accepted 23 March 2006 Editor: A.N. Halliday Abstract Earth's dipole field has been diminishing in strength since the first systematic observations of field intensity were made in the mid nineteenth century. This has led to speculation that the geomagnetic field might now be in the early stages of a reversal. In the longer term context of paleomagnetic observations it is found that for the current reversal rate and expected statistical variability in polarity interval length an interval as long as the ongoing 0.78 Myr Brunhes polarity interval is to be expected with a probability of less than 0.15, and the preferred probability estimates range from 0.06 to 0.08. These rather low odds might be used to infer that the next reversal is overdue, but the assessment is limited by the statistical treatment of reversals as point processes. Recent paleofield observations combined with insights derived from field modeling and numerical geodynamo simulations suggest that a reversal is not imminent. The current value of the dipole moment remains high compared with the average throughout the ongoing 0.78 Myr Brunhes polarity interval; the present rate of change in Earth's dipole strength is not anomalous compared with rates of change for the past 7 kyr; furthermore there is evidence that the field has been stronger on average during the Brunhes than for the past 160 Ma, and that high average field values are associated with longer polarity chrons.
    [Show full text]
  • 39. Paleomagnetic Results from Early Tertiary/Late Cretaceous Sediments of Site 384
    39. PALEOMAGNETIC RESULTS FROM EARLY TERTIARY/LATE CRETACEOUS SEDIMENTS OF SITE 384 P. A. Larson and N. D. Opdyke, Lamont-Doherty Geological Observatory, Palisades, New York ABSTRACT Magnetic measurements were made on samples taken from the region of the Tertiary/Cretaceous boundary at Site 384. We had hoped that the magnetic stratigraphy of this section could be used to independently substantiate the magnetic stratigraphy of a section of the same age at Gubbio, Italy. Although an attempt has been made to correlate the two sections, the quality of the magnetic data is too poor either to confirm or deny the Gubbio results. INTRODUCTION inclination for this section of Site 384 should lie be¬ tween 46° and 58°. This range of inclinations is indi¬ A type section for the Late Cretaceous-Paleocene cated by the two stippled bands on the plot in Figure portion of the geomagnetic reversal time scale has been 2. As can seen in the figure, the actual inclination val¬ proposed recently from studies of the pelagic limestone ues are considerably scattered. The normally magnet¬ sequence at Gubbio, Italy (Alvarez et al., 1977; Lowrie ized specimens have an average inclination of 43.3° and Alvarez, 1977; Roggenthen and Napoleone, (standard deviation = 19.6°), while the reversely 1977). The upper Maestrichtian-Danian sediments magnetized ones average -24.9° (standard deviation from Site 384 were studied to see whether they would = 22.1°). These averages exclude specimens which fall yield a magnetic stratigraphy correlative to the section within the two hachured zones in the polarity diagram at Gubbio. of Figure 2.
    [Show full text]
  • Late Neogene Chronology: New Perspectives in High-Resolution Stratigraphy
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Columbia University Academic Commons Late Neogene chronology: New perspectives in high-resolution stratigraphy W. A. Berggren Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 F. J. Hilgen Institute of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands C. G. Langereis } D. V. Kent Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 J. D. Obradovich Isotope Geology Branch, U.S. Geological Survey, Denver, Colorado 80225 Isabella Raffi Facolta di Scienze MM.FF.NN, Universita ‘‘G. D’Annunzio’’, ‘‘Chieti’’, Italy M. E. Raymo Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 N. J. Shackleton Godwin Laboratory of Quaternary Research, Free School Lane, Cambridge University, Cambridge CB2 3RS, United Kingdom ABSTRACT (Calabria, Italy), is located near the top of working group with the task of investigat- the Olduvai (C2n) Magnetic Polarity Sub- ing and resolving the age disagreements in We present an integrated geochronology chronozone with an estimated age of 1.81 the then-nascent late Neogene chronologic for late Neogene time (Pliocene, Pleisto- Ma. The 13 calcareous nannoplankton schemes being developed by means of as- cene, and Holocene Epochs) based on an and 48 planktonic foraminiferal datum tronomical/climatic proxies (Hilgen, 1987; analysis of data from stable isotopes, mag- events for the Pliocene, and 12 calcareous Hilgen and Langereis, 1988, 1989; Shackle- netostratigraphy, radiochronology, and cal- nannoplankton and 10 planktonic foram- ton et al., 1990) and the classical radiometric careous plankton biostratigraphy.
    [Show full text]
  • Appendix 1 – Environmental Predictor Data
    APPENDIX 1 – ENVIRONMENTAL PREDICTOR DATA CONTENTS Overview ..................................................................................................................................................................................... 2 Climate ......................................................................................................................................................................................... 2 Hydrology ................................................................................................................................................................................... 3 Land Use and Land Cover ..................................................................................................................................................... 3 Soils and Substrate .................................................................................................................................................................. 5 Topography .............................................................................................................................................................................. 10 References ................................................................................................................................................................................ 12 1 OVERVIEW A set of 94 potential predictor layers compiled to use in distribution modeling for the target taxa. Many of these layers derive from previous modeling work by WYNDD1, 2, but a
    [Show full text]
  • Constraining in Situ Cosmogenic Nuclide Paleo-Production Rates Using Sequential Lava flows During a Paleomagnetic field Strength Low
    Journal Pre-proof Constraining in situ cosmogenic nuclide paleo-production rates using sequential lava flows during a paleomagnetic field strength low A.M. Balbas, K.A. Farley PII: S0009-2541(19)30484-X DOI: https://doi.org/10.1016/j.chemgeo.2019.119355 Reference: CHEMGE 119355 To appear in: Received Date: 6 August 2019 Revised Date: 26 September 2019 Accepted Date: 31 October 2019 Please cite this article as: Balbas AM, Farley KA, Constraining in situ cosmogenic nuclide paleo-production rates using sequential lava flows during a paleomagnetic field strength low, Chemical Geology (2019), doi: https://doi.org/10.1016/j.chemgeo.2019.119355 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier. 1 Constraining in situ cosmogenic nuclide paleo-production rates using sequential lava flows during a paleomagnetic field strength low A.M. Balbas1,2*, K.A. Farley2 1 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA 2Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA * Corresponding author: [email protected] Abstract The geomagnetic field prevents a portion of incoming cosmic rays from reaching Earth’s atmosphere.
    [Show full text]
  • Collision Orogeny
    Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 PROCESSES OF COLLISION OROGENY Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 Shortening of continental lithosphere: the neotectonics of Eastern Anatolia a young collision zone J.F. Dewey, M.R. Hempton, W.S.F. Kidd, F. Saroglu & A.M.C. ~eng6r SUMMARY: We use the tectonics of Eastern Anatolia to exemplify many of the different aspects of collision tectonics, namely the formation of plateaux, thrust belts, foreland flexures, widespread foreland/hinterland deformation zones and orogenic collapse/distension zones. Eastern Anatolia is a 2 km high plateau bounded to the S by the southward-verging Bitlis Thrust Zone and to the N by the Pontide/Minor Caucasus Zone. It has developed as the surface expression of a zone of progressively thickening crust beginning about 12 Ma in the medial Miocene and has resulted from the squeezing and shortening of Eastern Anatolia between the Arabian and European Plates following the Serravallian demise of the last oceanic or quasi- oceanic tract between Arabia and Eurasia. Thickening of the crust to about 52 km has been accompanied by major strike-slip faulting on the rightqateral N Anatolian Transform Fault (NATF) and the left-lateral E Anatolian Transform Fault (EATF) which approximately bound an Anatolian Wedge that is being driven westwards to override the oceanic lithosphere of the Mediterranean along subduction zones from Cephalonia to Crete, and Rhodes to Cyprus. This neotectonic regime began about 12 Ma in Late Serravallian times with uplift from wide- spread littoral/neritic marine conditions to open seasonal wooded savanna with coiluvial, fluvial and limnic environments, and the deposition of the thick Tortonian Kythrean Flysch in the Eastern Mediterranean.
    [Show full text]
  • A Preliminary Assessment of Paleontological Resources at Bighorn Canyon National Recreation Area, Montana and Wyoming
    A PRELIMINARY ASSESSMENT OF PALEONTOLOGICAL RESOURCES AT BIGHORN CANYON NATIONAL RECREATION AREA, MONTANA AND WYOMING Vincent L. Santucci1, David Hays2, James Staebler2 And Michael Milstein3 1National Park Service, P.O. Box 592, Kemmerer, WY 83101 2Bighorn Canyon National Recreation Area, P.O. Box 7458, Fort Smith, MT 59035 3P.O. Box 821, Cody, WY 82414 ____________________ ABSTRACT - Paleontological resources occur throughout the Paleozoic and Mesozoic formations exposed in Bighorn Canyon National Recreation Area. Isolated research on specific geologic units within Bighorn Canyon has yielded data on a wide diversity of fossil forms. A comprehensive paleonotological survey has not been previously undertaken at Bighorn Canyon. Preliminary paleontologic resource data is presented in this report as an effort to establish baseline data. ____________________ INTRODUCTION ighorn Canyon National Recreation Area (BICA) consists of approximately 120,000 acres within the Bighorn Mountains of north-central Wyoming and south-central Montana B (Figure 1). The northwestern trending Bighorn Mountains consist of over 9,000 feet of sedimentary rock. The predominantly marine and near shore sedimentary units range from the Cambrian through the Lower Cretaceous. Many of these formations are extremely fossiliferous. The Bighorn Mountains were uplifted during the Laramide Orogeny beginning approximately 70 million years ago. Large volumes of sediments, rich in early Tertiary paleontological resources, were deposited in the adjoining basins. This report provides a preliminary assessment of paleontological resources identified at Bighorn Canyon National Recreation Area. STRATIGRAPHY The stratigraphic record at Bighorn Canyon National Recreation Area extends from the Cambrian through the Cretaceous (Figure 2). The only time period during this interval that is not represented is the Silurian.
    [Show full text]
  • Re-Examining the Evidence for Cycles in Magnetic Reversal Rate
    Does the Planetary Dynamo Go Cycling On? Re-examining the Evidence for Cycles in Magnetic Reversal Rate Adrian L. Melott1, Anthony Pivarunas2, Joseph G. Meert2, Bruce S. Lieberman3 1University of Kansas, Department of Physics and Astronomy, Lawrence, KS, 66045; [email protected]; 785-864-3037 2University of Florida, Department of Geological Sciences, 241 Williamson Hall, Gainesville, FL 32611 3University of Kansas, Department of Ecology & Evolutionary Biology and Biodiversity Institute, 1345 Jayhawk Blvd., Lawrence, KS 66045 1 Abstract The record of reversals of the geomagnetic field has played an integral role in the development of plate tectonic theory. Statistical analyses of the reversal record are aimed at detailing patterns and linking those patterns to core-mantle processes. The geomagnetic polarity timescale is a dynamic record and new paleomagnetic and geochronologic data provide additional detail. In this paper, we examine the periodicity revealed in the reversal record back to 375 million years ago (Ma) using Fourier analysis. Four significant peaks were found in the reversal power spectra within the 16-40-million-year range (Myr). Plotting the function constructed from the sum of the frequencies of the proximal peaks yield a transient 26 Myr periodicity, suggesting chaotic motion with a periodic attractor. The possible 16 Myr periodicity, a previously recognized result, may be correlated with ‘pulsation’ of mantle plumes and perhaps; more tentatively, with core-mantle dynamics originating near the large low shear velocity layers in the Pacific and Africa. Planetary magnetic fields shield against charged particles which can give rise to radiation at the surface and ionize the atmosphere, which is a loss mechanism particularly relevant to M stars.
    [Show full text]
  • Fault Zone Architecture Within Miocene–Pliocene Syn-Rift Sediments, Northwestern Red Sea, Egypt
    J. Earth Syst. Sci. (2017) 126: 37 c Indian Academy of Sciences DOI 10.1007/s12040-017-0812-4 Fault zone architecture within Miocene–Pliocene syn-rift sediments, Northwestern Red Sea, Egypt Khairy S Zaky Department of Geology, Minia University, El Minia 61519, Egypt. e-mail: [email protected] The present study focusses on field description of small normal fault zones in Upper Miocene–Pliocene sedimentary rocks on the northwestern side of the Red Sea, Egypt. The trend of these fault zones is mainly NW–SE. Paleostress analysis of 17 fault planes and slickenlines indicate that the tension direction is NE–SW. The minimum (σ3) and intermediate (σ2) paleostress axes are generally sub-horizontal and the maximum paleostress axis (σ1) is sub-vertical. The fault zones are composed of damage zones and fault core. The damage zone is characterized by subsidiary faults and fractures that are asymmetrically developed on the hanging wall and footwall of the main fault. The width of the damage zone varies for each fault depending on the lithology, amount of displacement and irregularity of the fault trace. The average ratio between the hanging wall and the footwall damage zones width is about 3:1. The fault core consists of fault gouge and breccia. It is generally concentrated in a narrow zone of ∼0.5 to ∼8cmwidth. The overall pattern of the fault core indicates that the width increases with increasing displacement. The faults with displacement <1 m have fault cores ranging from 0.5 to 4.0 cm, while the faults with displacements of >2 m have fault cores ranging from 4.0 to 8.0 cm.
    [Show full text]
  • The Relative Stabilities of the Reverse and Normal Polarity States of the Earth's Magnetic Field
    Earth and Planetary Science Letters, 82 (1987) 373 383 373 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands [51 The relative stabilities of the reverse and normal polarity states of the earth's magnetic field P.L. McFadden 1, R.T. Merrill 2, William Lowrie 3 and Dennis V. Kent 4 t Division of Geophysics, Bureau of Mineral Resources, G.P.O. Box 378, Canberra, A. C. 72 2601 (Australia) : Geophysics Program, University of Washington, Seattle, WA 98195 (U.S.A.) 3 Institut fiir Geophysik, E. 72 H. -Hfnggerberg, CH-8093 Ziirich (Switzerland) 4 Lamont-Dohertv Geological Observatory, and Department of Geological Sciences, Palisades, N Y 10964 (U.S.A.) Received August 22, 1986; revised version received December 16, 1986 Recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states. The main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the Cenozoic. These additional events occur predominantly during reverse polarity time, but it is unlikely that all of them represent true reversal events. Nevertheless the question of the relative stabilities of the polarity states is examined in detail, both for the case when all 57 "events" are included in the reversal chronology and when they are all excluded. It is found that there is not a discernible difference between the stabilities of the two polarity states in either case. Inclusion of these short events does, however, change the structure of the non-stationarity in reversal rate, but still allows a smooth non-stationarity.
    [Show full text]
  • Triassic-Jurassic 'Red Beds' of the Rocky Mountain Region": a Discussion
    University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey 1929 "Triassic-Jurassic 'Red Beds' of the Rocky Mountain Region": A Discussion John B. Reeside Jr. U.S. Geological Survey Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Earth Sciences Commons Reeside, John B. Jr., ""Triassic-Jurassic 'Red Beds' of the Rocky Mountain Region": A Discussion" (1929). USGS Staff -- Published Research. 498. https://digitalcommons.unl.edu/usgsstaffpub/498 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. The Journal of Geology, Vol. 37, No. 1 (Jan. - Feb., 1929), pp. 47-63 "TRIASSIC-JURASSIC 'RED BEDS' OF THE ROCKY MOUNTAIN REGION": A DISCUSSION' JOHN B. REESIDE, JR. U.S. Geological Survey ABSTRACT The conclusion drawn by Professor E. B. Branson in a recent paper on the "Red Beds" of the Rocky Mountain region that parts of these beds are marine is considered likely. The conclusion that there exists no basis for subdivision of the beds is not accepted, and a division of the Mesozoic part into Lower Triassic, Upper Triassic, and Jurassic units is advocated. The conclusion that no eolian deposits are present likewise is not accepted, and the interpretation of important parts of the Jurassic unit as eolian is advocated. Disagreement is expressed with much of Professor Branson's correlation table, and a substitute is offered.
    [Show full text]