RESEARCH Nature of the Moho Transition in NW

Total Page:16

File Type:pdf, Size:1020Kb

RESEARCH Nature of the Moho Transition in NW RESEARCH Nature of the Moho transition in NW Canada from combined near-vertical and wide-angle seismic- refl ection studies J. Oueity1,* and R.M. Clowes1 1DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD, VANCOUVER, BC V6T 1Z4, CANADA ABSTRACT Although the year 2009 marked one century since the discovery of the Moho, or crust-mantle boundary, the exact nature of that boundary and the manner in which it formed remain major uncertainties in lithospheric studies. In northwestern Canada, sharp Moho refl ections at both near-vertical and wide-angle incidence have been imaged beneath the Great Bear magmatic arc. They show a remarkably fl at Moho and do not refl ect the complex tectonic history of the Wopmay orogen, of which the Great Bear arc is one part. In order to understand the origin of these refl ections and the nature of the Moho, we calculated near-vertical and wide-angle synthetic seismograms for a number of crust-mantle transition models using one- and two-dimensional wave propagation algorithms. Only laterally and vertically heterogeneous models can properly simulate the observed seismic signature recorded on both near-vertical and wide-angle refl ection data. The heteroge- neity is achieved by either laterally discontinuous layering or a lamellae structure with randomly distributed ellipses. These models suggest that the Moho represents a thermal/metamorphic front, a regional décollement, or both. LITHOSPHERE; v. 2; no. 5; p. 377–396. doi: 10.1130/L103.1 INTRODUCTION granulite, gabbro, and amphibolite and an upper mantle composed of sev- eral varieties of peridotite with some eclogite. The crust-mantle boundary, or the Moho, is one of the most distinct Despite all the geologic studies of the Moho by means of exposed sec- manifestations of a differentiated Earth. Major changes in petrology, min- tions (ophiolites) or xenolith samples, seismic data still remain the main eralogy, chemistry, seismic wave velocity, density, and rheology occur source for our understanding of the structure of the Moho and its develop- across it (Jarchow and Thompson, 1989). Originally, the Moho was defi ned ment. As techniques applied to lithospheric studies have improved con- on the basis of seismic head waves (Mohorovičić, 1910). It was described siderably in terms of seismic data acquisition technologies (e.g., Meiss- as the depth at which the compressional P-wave velocity increases rapidly ner, 1986; Mooney, 1987; Mjelde et al., 1997; Panea et al., 2005) and or discontinuously to 7.6–8.6 km/s (Steinhart, 1967). This suggested a pet- computational capabilities (e.g., Fuchs and Müller, 1971; Spence et al., rologic interpretation of the Moho as a fi rst-order (i.e., zero thickness) dis- 1984; Levander and Holliger, 1992; Zelt and Smith, 1992; Carbonell et al., continuity in rock composition from predominantly mafi c lower-crustal 2002), it has become apparent that the Moho is typically not a uniform, rocks to predominantly ultramafi c upper-mantle rocks. Ophiolite studies sharp, fi rst-order discontinuity, but rather a complex and variable transi- indicate that the lithologic sequence of the oceanic Moho includes pelagic tion zone. From the seismic perspective, a frequently cited model for the sediments, pillow basalts, massive and layered gabbro, and residual ultra- Moho is characterized by a variable-thickness transition zone composed mafi c tectonites of the upper mantle (Casey et al., 1981; Karson et al., of anastomosing layers progressing with depth from mafi c to ultramafi c 1984; Boudier and Nicolas, 1995; Jousselin and Nicolas, 2000; Dilek et rocks (Jarchow and Thompson, 1989; Braile and Chiang, 1986). Alterna- al., 2008). These rock cumulates form laterally discontinuous lenses with tively, the Moho transition may represent a change in scale of vertical lay- thicknesses ranging from 1 cm to several tens of meters and aspect ratios ering and horizontal extent of inhomogeneities (Tittgemeyer et al., 1999; ranging from 10:1 to 100:1. Prominent refl ections from within the oceanic Hurich, 2003; Carpentier and Roy-Chowdhury, 2007). Moho transition zone obtained by seismic-refl ection methods also support Although the Moho is well imaged by near-vertical incidence (NVI) a geologically complex boundary (e.g., Hasselgren and Clowes, 1995; and refraction/wide-angle refl ection (R/WAR) data, the exact nature and Nedimović et al., 2005). characteristics of the transition are not well understood. The wave fi eld Unlike abundant ophiolites, which allow direct observation of the oce- (i.e., waveform, duration, and amplitude) of a seismic signal (refl ected or anic Moho, exposed sections of the continental Moho are rare (Fountain refracted) from the lower crust and upper mantle may contain signifi cant and Salisbury, 1981). Nevertheless, important constraints on the nature information about the structure of the Moho transition zone, its fabric, and of the continental Moho are provided by the small continental data set scale of heterogeneities. Only a few studies have specifi cally addressed of xenoliths (McGetchin and Silver, 1972; Debari et al., 1987; Kopylova this issue. These studies can be grouped into two categories, qualitative et al., 1998). Xenolith samples indicate a lower crust composed of mafi c and quantitative. Examples of qualitative analyses of refl ection patterns near the Moho are those of Gibbs (1986) on Consortium for Continen- *Corresponding author: +1-604-822-5703; fax: +1-604-822-6088; e-mail: joueity@ tal Refl ection Profi ling (COCORP) seismic-refl ection profi les, Meissner eos.ubc.ca. (2000) using Deutsches Kontinentales Refl exionsseismisches Programm LITHOSPHEREFor permission to| Volumecopy, contact 2 | Number [email protected] 5 | www.gsapubs.org | © 2010 Geological Society of America 377 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/5/377/3050505/377.pdf by guest on 26 September 2021 OUEITY ET AL. (DEKORP) data, and Hammer and Clowes (1997), Cook (2002), and teristics of Moho refl ections by calculating the synthetic seismic signature Eaton (2006) on Lithoprobe refl ection transects that sample regions of for both near-vertical and wide-angle data of a number of crust-mantle diverse age and tectonic history. From these studies, refl ection patterns transition models using one- and two-dimensional wave propagation algo- appear to be divided into three primary classes: (1) a distinct, multicyclic rithms. The models range from a simple fi rst-order discontinuity to more band of refl ectors that are laterally continuous or piece-wise continuous complex, laterally and vertically heterogeneous structures. Unraveling over tens of kilometers, (2) fading out of crustal refl ectivity with depth, the detailed structure of the crust-mantle boundary and the way in which and (3) no clear refl ections in the vicinity of the Moho. These studies did it forms can help us understand some major tectonic processes, such as not involve wide-angle data. Although the comparisons have not clearly crustal growth, accretion, and delamination (Morozov et al., 2001; Car- linked a specifi c tectonic process with a distinct refl ection pattern, sharp bonell et al., 2002). Moho refl ections have been associated with lower-crustal deformational processes including compression, extension, and intrusive features. TECTONIC BACKGROUND Quantitative studies use synthetic modeling to construct the seismic response of the continental Moho using seismic velocity data (laboratory The Wopmay orogen in northwest Canada (Fig. 1) is a Paleoprotero- seismic-refraction results) together with petrologic information derived zoic assembly of domains that developed during the Calderian orogeny from exposures of the crust-mantle boundary when available. In earlier between 1.92 Ga and 1.84 Ga (Hoffman, 1989). The orogen is divided into studies, the Moho was modeled as a fi nite-thickness laminated zone of four major north-south–trending domains: the Coronation Supergroup, the alternating high- and low-velocity layers (Clowes and Kanasewich, 1970; Great Bear magmatic arc, the Hottah terrane, and the Fort Simpson ter- Hale and Thompson, 1982; Fountain, 1986; Braile and Chiang, 1986). rane. Rifting to the west of the Slave Province at ca. 1.90 Ga initiated More recently, the Moho has been considered as a laterally and vertically deposition of the Coronation Supergroup, a west-facing shelf rise and heterogeneous transition where the heterogeneities are represented by ran- foredeep succeeding prism, which shortly thereafter was intruded by a domly distributed ellipsoids (Carbonell et al., 2002; Tittgemeyer et al., suite of plutons and translated eastward by the Hottah collision to form a 2003). Another representation of the Moho follows a statistical approach, foreland fold-and-thrust belt (Hoffman, 1988). in which the transition zone is characterized by a stochastic heterogeneity The Hottah terrane formed as a magmatic arc and collided with the distribution (Hurich, 2003; Nielsen and Thybo, 2006; Carpentier and Roy- western Slave craton ca. 1.89–1.88 Ga (Hoffman and Bowring, 1984; Hil- Chowdhury, 2007). Only a few of these studies included consideration of debrand and Bowring, 1999). The collision caused compression, shorten- wide-angle data (Carbonell et al., 2002; Tittgemeyer et al., 2003; Nielsen ing, and eastward thrusting of the Coronation Supergroup onto the Slave and Thybo, 2006). craton (Hoffman and Bowring, 1984). The lack of coeval arc magmas on In this paper, we analyzed the near-vertical incidence and wide-angle the western Slave craton indicates that the accretion of the Hottah ter- seismic data acquired along line 1 of Lithoprobe’s Slave–Northern Cordil- rane onto the craton was the result of west-dipping subduction of an lera Lithospheric Evolution (SNORCLE) transect in the Paleoproterozoic– ocean basin. The Hottah terrane consists of Paleoproterozoic sedimentary Archean domains of the Northwest Territories, Canada (Cook et al., 1999; and intermediate volcanic rocks metamorphosed to amphibolite grade Fernández Viejo and Clowes, 2003). We investigated the dynamic charac- and intruded by calc-alkaline granitic plutons during the period 1.940– Cordillera Wopmay Slave 1100 CS N 64° Figure 1.
Recommended publications
  • Asthenosphere–Lithospheric Mantle Interaction in an Extensional Regime
    Chemical Geology 233 (2006) 309–327 www.elsevier.com/locate/chemgeo Asthenosphere–lithospheric mantle interaction in an extensional regime: Implication from the geochemistry of Cenozoic basalts from Taihang Mountains, North China Craton ⁎ Yan-Jie Tang , Hong-Fu Zhang, Ji-Feng Ying State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China Received 25 July 2005; received in revised form 27 March 2006; accepted 30 March 2006 Abstract Compositions of Cenozoic basalts from the Fansi (26.3–24.3 Ma), Xiyang–Pingding (7.9–7.3 Ma) and Zuoquan (∼5.6 Ma) volcanic fields in the Taihang Mountains provide insight into the nature of their mantle sources and evidence for asthenosphere– lithospheric mantle interaction beneath the North China Craton. These basalts are mainly alkaline (SiO2 =44–50 wt.%, Na2O+ K2O=3.9–6.0 wt.%) and have OIB-like characteristics, as shown in trace element distribution patterns, incompatible elemental (Ba/Nb=6–22, La/Nb=0.5–1.0, Ce/Pb=15–30, Nb/U=29–50) and isotopic ratios (87Sr/86Sr=0.7038–0.7054, 143Nd/ 144 Nd=0.5124–0.5129). Based on TiO2 contents, the Fansi lavas can be classified into two groups: high-Ti and low-Ti. The Fansi high-Ti and Xiyang–Pingding basalts were dominantly derived from an asthenospheric source, while the Zuoquan and Fansi low-Ti basalts show isotopic imprints (higher 87Sr/86Sr and lower 143Nd/144Nd ratios) compatible with some contributions of sub- continental lithospheric mantle. The variation in geochemical compositions of these basalts resulted from the low degree partial melting of asthenosphere and the interaction of asthenosphere-derived magma with old heterogeneous lithospheric mantle in an extensional regime, possibly related to the far effect of the India–Eurasia collision.
    [Show full text]
  • Seismic Velocity Structure of the Continental Lithosphere from Controlled Source Data
    Seismic Velocity Structure of the Continental Lithosphere from Controlled Source Data Walter D. Mooney US Geological Survey, Menlo Park, CA, USA Claus Prodehl University of Karlsruhe, Karlsruhe, Germany Nina I. Pavlenkova RAS Institute of the Physics of the Earth, Moscow, Russia 1. Introduction Year Authors Areas covered J/A/B a The purpose of this chapter is to provide a summary of the seismic velocity structure of the continental lithosphere, 1971 Heacock N-America B 1973 Meissner World J i.e., the crust and uppermost mantle. We define the crust as 1973 Mueller World B the outer layer of the Earth that is separated from the under- 1975 Makris E-Africa, Iceland A lying mantle by the Mohorovi6i6 discontinuity (Moho). We 1977 Bamford and Prodehl Europe, N-America J adopted the usual convention of defining the seismic Moho 1977 Heacock Europe, N-America B as the level in the Earth where the seismic compressional- 1977 Mueller Europe, N-America A 1977 Prodehl Europe, N-America A wave (P-wave) velocity increases rapidly or gradually to 1978 Mueller World A a value greater than or equal to 7.6 km sec -1 (Steinhart, 1967), 1980 Zverev and Kosminskaya Europe, Asia B defined in the data by the so-called "Pn" phase (P-normal). 1982 Soller et al. World J Here we use the term uppermost mantle to refer to the 50- 1984 Prodehl World A 200+ km thick lithospheric mantle that forms the root of the 1986 Meissner Continents B 1987 Orcutt Oceans J continents and that is attached to the crust (i.e., moves with the 1989 Mooney and Braile N-America A continental plates).
    [Show full text]
  • The Distribution of Water in the Continental Lithospheric Mantle and Its Implications for the Stability of Continents
    Invited Review Geology November 2013 Vol.58 No.32: 38793889 doi: 10.1007/s11434-013-5949-1 The distribution of water in the continental lithospheric mantle and its implications for the stability of continents XIA QunKe* & HAO YanTao CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China Received January 14, 2013; accepted May 14, 2013; published online July 11, 2013 The lithospheric mantle is one of the key layers controlling the stability of continents. Even a small amount of water can influence many chemical and physical properties of rocks and minerals. Consequently, it is a pivotal task to study the distribution of water in the continental lithosphere. This paper presents a brief overview of the current state of knowledge about (1) the occurrence of water in the continental lithospheric mantle, (2) the spatial and temporal variations of the water content in the continental litho- spheric mantle, and (3) the relationship between water content and continent stability. Additionally, suggestions for future re- search directions are briefly discussed. water, nominally anhydrous minerals, continental lithospheric mantle, continent stability Citation: Xia Q K, Hao Y T. The distribution of water in the continental lithospheric mantle and its implications for the stability of continents. Chin Sci Bull, 2013, 58: 38793889, doi: 10.1007/s11434-013-5949-1 The lithospheric mantle is the lowermost part of continental continental lithospheric mantle is tightly related to its vis- plates; its viscosity contrast with the underlying astheno- cosity and stability [23,25–28].
    [Show full text]
  • Plate Tectonics
    Plate tectonics tive motion determines the type of boundary; convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative move- ment of the plates typically varies from zero to 100 mm annually.[2] Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This predic- The tectonic plates of the world were mapped in the second half of the 20th century. tion of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some sup- porters) propose gradual shrinking (contraction) or grad- ual expansion of the globe.[3] Tectonic plates are able to move because the Earth’s lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topog- raphy and density of the crust, which result in differences in gravitational forces) and drag, with downward suction, at the subduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the tidal forces of the Sun and Moon. The relative im- portance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.
    [Show full text]
  • The Mid-Lithospheric Discontinuity Caused by Channel Flow in the Cratonic Lithosphere
    The Mid-Lithospheric Discontinuity caused by channel flow in the cratonic lithosphere Haibin Yang *a, b, Irina M. Artemieva c,d,e, Hans Thybo e,f,g a. School of Earth Sciences, University of Melbourne, Melbourne, Australia b. Research School of Earth Sciences, Australian National University, Canberra, Australia c. Department of Geophysics, Stanford University, Stanford, CA 94305, USA d. Section of Oceanic Dynamics, GEOMAR Helmholtz Center for Ocean Research, Kiel 24148, Germany e. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, China f. Eurasia Institute of Earth Science, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey g. Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Blindern, 0316 Oslo, Norway Corresponding author: Haibin Yang Email: [email protected] This is a non-peer reviewed preprint submitted to EarthArXiv. 1 Abstract Stable cratons with a thick (> 200 km) and cold lithosphere form rheologically strong plates that move atop a ductile asthenospheric mantle. Various types of seismic observations show the presence of a potentially rheologically weak zone at depths of ca. 80 – 150 km termed the Mid-Lithosphere Discontinuity (MLD). While various mechanisms may explain the MLD, the dynamic processes leading to the seismic observations are unclear. We propose that the MLD can be caused by channel flow in the lower lithosphere, triggered by negative Rayleigh-Taylor instabilities at cratonic margins in the Archean, when the mantle was hotter than at present. Presence of a chemically distinct, low-density cratonic lithospheric root is required to initiate the process. Numerical modeling shows that the top of the channel flow creates a shear zone at a depth comparable to the globally observed seismic MLD.
    [Show full text]
  • I a Petrographic, Geochemical and Isotopic Study of the 780 Ma
    A petrographic, geochemical and isotopic study of the 780 Ma Gunbarrel Large Igneous Province, western North America By Alana J. Mackinder A thesis submitted to the Faculty of Graduate and Postdoctoral Affairs In partial fulfillment of the requirements for the degree of Master of Science In Earth Sciences Carleton University Ottawa, Ontario © 2014 Alana J. Mackinder i Abstract On the western margin of North America lie dykes, sills and volcanic rocks which define a magmatic episode at 780 Ma. These units have been collectively termed the Gunbarrel Large Igneous Province (LIP) and share remarkably similar petrographical, geochemical and isotopic signatures indicating a single homogeneous source. The Irene and Huckleberry metavolcanic rocks of Washington State were correlated to the Gunbarrel LIP based on similar ages, but were found to not be related based on geochemistry and isotopic analysis. The timing of the Gunbarrel LIP coincides with the break-up of Rodinia, and Gunbarrel samples were compared to coeval magmatism from South China to test the “Missing Link” reconstruction model. While some mafic dykes have similar Nd isotopic ratios, no definitive correlations could be found between the different magmatic events. Finally, LIPs play a key role in housing various economic deposits and the Gunbarrel magmas show potential for being a Ni-Cu-PGE target. ii Acknowledgements First and foremost I would like to thank my co-supervisors Dr. Brian Cousens and Dr. Richard Ernst for providing me with this amazing project and helping me work through it and gain new knowledge throughout my 2 years as a graduate student. I would like to thank the NSERC CRDPJ 419503-11 grant which funded my research work and the Large Igneous Provinces – Supercontinent Reconstruction Group (www.supercontinent.org) funded by our industry sponsors for opportunities to present my work and with which this project originated.
    [Show full text]
  • Origin and Evolution of Asthenospheric Layers
    Two mechanisms of formation of asthenospheric layers L. Czechowski and M. Grad Institute of Geophysics, Faculty of Physics, University of Warsaw Ul. Pasteura 5, 02-093 Warszawa, Poland Phone: +48 22 5532003 E-mail: [email protected] Corresponding author: Leszek Czechowski: [email protected] The theory of plate tectonics describes some basic global tectonic processes as a result of motion of lithospheric plates. The boundary between lithosphere and asthenosphere (LAB) is defined by a difference in response to stress. Position of LAB is determined by: (i) the ratio (melting temperature)/( temperature) and (ii) an invariant of the stress tensor. We consider the role of these both factors for origin and decay of asthenosphere. We find that the asthenosphere of shear stress origin could be a transient, time-dependent feature. Key words: asthenosphere, evolution, LAB, origin of asthenosphere 1. Introduction The plate tectonics theory describes some of the basic tectonic processes on the Earth as motion of lithospheric plates. The plates are moved by large-scale thermal convection in the mantle. The lithosphere is mechanically resistant. Its thickness varies from ~50 to ~250 km. The lithosphere is underlain by the asthenosphere. The boundary between the lithosphere and the asthenosphere (LAB) is defined by a difference in response to stress: the asthenosphere deforms viscously. The bulk of asthenosphere is not melted (1), but at the deformation rate typical for the mantle convection (about 10-14 s-1) it behaves as a fluid with the viscosity η of about 5 1019 kg m-1 s-1. The mantle below has higher effective viscosity (e.g.
    [Show full text]
  • Lithospheric Strength Profiles
    21 LITHOSPHERIC STRENGTH PROFILES To study the mechanical response of the lithosphere to various types of forces, one has to take into account its rheology, which means knowing how it flows. As a scientific discipline, rheology describes the interactions between strain, stress and time. Strain and stress depend on the thermal structure, the fluid content, the thickness of compositional layers and various boundary conditions. The amount of time during which the load is applied is an important factor. - At the time scale of seismic waves, up to hundreds of seconds, the sub-crustal mantle behaves elastically down deep within the asthenosphere. - Over a few to thousands of years (e.g. load of ice cap), the mantle flows like a viscous fluid. - On long geological times (more than 1 million years), the upper crust and the upper mantle behave also as thin elastic and plastic plates that overlie an inviscid (i.e. with no viscosity) substratum. The dimensionless Deborah number D, summarized as natural response time/experimental observation time, is a measure of the influence of time on flow properties. Elasticity, plastic yielding, and viscous creep are therefore ingredients of the mechanical behavior of Earth materials. Each of these three modes will be considered in assessing flow processes in the lithosphere; these mechanical attributes are expressed in terms of lithospheric strength. This strength is estimated by integrating yield stress with depth. The current state of knowledge of rock rheology is sufficient to provide broad general outlines of mechanical behavior but also has important limitations. Two very thorny problems involve the scaling of rock properties with long periods and for very large length scales.
    [Show full text]
  • The Calderian Orogeny in Wopmay Orogen (1.9 Ga), Northwestern Canadian Shield
    The Calderian orogeny in Wopmay orogen (1.9 Ga), northwestern Canadian Shield Robert S. Hildebrand1,†, Paul F. Hoffman2,§, and Samuel A. Bowring3 1Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA 2Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA 3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Building 54-1126, Cambridge, Massachusetts 02139, USA ABSTRACT continent collisions, perhaps because the evolutionary model that incorporates progres- attenuated arc crust, coupled with the ex- sive rollback of the lower plate and extension of The Wopmay orogen is a Paleoproterozoic tended crust of the lower plate, never gen- the arc terrane on the upper plate, emplacement orogenic belt formed in part by the accre- erates severely overthickened crust. Rise of of the leading edge of the extended continental tion of Hottah terrane, an east-facing conti- asthenospheric mantle due to slab breakoff arc upon the western margin of Slave craton, nental magmatic arc, to the western margin generates rapid uplift, erosion, and collapse failure of the subducting plate during the colli- of the Archean Slave craton at ca. 1.88 Ga. of the mountains built during collision. sion, and the subsequent development of a new The arc-continent collision was responsible subduction zone, or possible propagation of an for the Calderian orogeny. Just prior to the INTRODUCTION older one, of opposite polarity outboard of the collision, arc volcanism of the Hottah terrane amalgamated collision zone—all within about had migrated trenchward and changed in Located in the northwest part of the Canadian 10 m.y.
    [Show full text]
  • The Mohorovicic Discontinuity Beneath the Continental Crust
    The Mohoroviˇci´cDiscontinuity Beneath the Continental Crust: An Overview of Seismic Constraints Ramon Carbonella,b, Alan Levanderc, Rainer Kindd aEarthquake Research Institute, Tokyo University, Tokyo, Japan bCSIC-Inst. Ci`enciesde la Terra Jaume Almera, Barcelona, Spain cRice Univ. Houston, TX, USA dGFZ, Potsdam, Germany Abstract The seismic signature of the Moho from which geologic and tectonic evo- lution hypothesis are derived are to a large degree a result of the seismic methodology which has been used to obtain the image. Seismic data of different types, passive source (earthquake) broad-band recordings, and con- trolled source seismic refraction, densely recorded wide-angle deep seismic reflection, and normal incidence reflection (using VibroseisTM, explosives, or airguns), have contributed to the description of the Moho as a relatively complex transition zone. Of critical importance for the quality and resolu- tion of the seismic image are the acquisition parameters, used in the imaging experiments. A variety of signatures have been obtained for the Moho at different scales generally dependent upon bandwidth of the seismic source. This variety prevents the development of a single universally applicable in- terpretation. In this way source frequency content, and source and sensor spacing determine the vertical and lateral resolution of the images, respec- tively. In most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the Moho, and its relationship to a petrologic crust-mantle transition. In regional seismic studies carried out using passive source recordings the Moho is a relatively well defined structure with marked lateral continuity. The characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area.
    [Show full text]
  • Coronation Loop Resurrected: Oscillatory Apparent Polar Wander of Orosirian (2.05–1.8 Ga) Paleomagnetic Poles from Slave Craton
    Precambrian Research 179 (2010) 121–134 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Coronation loop resurrected: Oscillatory apparent polar wander of Orosirian (2.05–1.8 Ga) paleomagnetic poles from Slave craton Ross N. Mitchell a,∗, Paul F. Hoffman b,c, David A.D. Evans a a Department of Geology & Geophysics, Yale University, 210 Whitney Ave, New Haven, CT 06511, USA b Department of Earth & Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA c School of Earth and Ocean Sciences, University of Victoria, Box 1700, Victoria, BC, Canada V8W 2Y2 article info abstract Article history: The Coronation loop is a 110◦ arcuate sweep of 15 paleomagnetic poles with ages of ca. 1950–1850 Ma, Received 20 September 2009 derived from contemporaneous basins on the western (Coronation), southern (Great Slave) and eastern Received in revised form 9 February 2010 (Kilohigok) margins of the Slave craton in the northwestern Canadian shield. Although the paleomag- Accepted 11 February 2010 netic results are either demonstrated as primary or most parsimoniously interpreted as such, it is likely they were subsequently rotated shortly after deposition during conjugate transcurrent faulting along the conjugate McDonald (Great Slave) and Bathurst (Kilohigok) strike-slip fault systems. No rotation is Keywords: expected of poles from the epicratonic Coronation margin. Previous analyses have debated the amounts Coronation loop Paleomagnetism of local rotations in the other basins, with one end-member view that the spread in paleomagnetic poles is Slave craton entirely due to local rotations. Here we propose that, relative to the principal axis of compression for con- Paleoproterozoic jugate faulting, the far-field Bathurst and McDonald fault systems have rotated (equally and oppositely) ◦ ◦ ◦ Great Slave Supergroup 12 to widen an original 60 geometry to the present-day 84 angle.
    [Show full text]
  • Constraints on the Composition of the Continental Lithospheric Mantle
    Earth and Planetary Science Letters, 101 (1990) 1-18 1 Elsevier Science Publishers B.V., Amsterdam [FB] Constraints on the composition of the continental lithospheric mantle W.F. McDonough Research School of Earth Sciences, The Australian National University, G.P.O. Box 4, Canberra, A. C.T. 2601, Australia and Max-Planck-Institutfiir Chemie, Abteilung Geochemie, Postfach 3060, Saarstrafle, D-6500 Mainz, F.R.G. Received December 12, 1988; revised version accepted June 19, 1990 ABSTRACT Major and trace element data for 375 spinel lherzolite and harzburgite xenoliths from continental basalts are used to constrain the nature and composition of the continental lithospheric mantle (CLM). The major and compatible trace elements (e.g., Ni, Co) have similar average and median values, whereas median values of the highly incompatible trace elements (e.g., LREE, Rb) are systematically lower than their average values. Both the average and median compositions have LREE-en- riched patterns, with La abundances at about 10 and 3 times C1 chondrites, respectively, and a relatively flat HREE pattern at about 2 times C1 chondrites. Relative to the primitive mantle, highly incompatible trace elements are enriched in the average and median, whereas the mildly incompatible elements (e.g., the middle and heavy REE, Hf, Ti, V, Sc, A1, Ca, etc.) are depleted. The enrichment pattern of incompatible elements in these peridotites is comparable to that in ocean island basalts, although at lower absolute abundances. For most elements the average composition provides an estimate of the bulk composition of the CLM, however for those elements with a strong degree of skewedness in their data the median composition may provide a more reasonable estimate.
    [Show full text]