DOCSLIB.ORG

Fluid Overpressures and Strength of the Sedimentary Upper Crust

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Fluid Overpressures and Strength of the Sedimentary Upper Crust Journal of Structural Geology 69 (2014) 481e492 Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Fluid overpressures and strength of the sedimentary upper crust John Suppe Department of Geosciences, National Taiwan University, Taipei 106, Taiwan article info abstract Article history: The classic crustal strength-depth profile based on rock mechanics predicts a brittle strength s1 À s3 ¼ Available online 1 August 2014 kðr À Þ gz Pf that increases linearly with depth as a consequence of [1] the intrinsic brittle pressure dependence k plus [2] an assumption of hydrostatic pore-fluid pressure, Pf ¼ rwgz. Many deep borehole Keywords: stress data agree with a critical state of failure of this form. In contrast, fluid pressures greater than Crustal strength hydrostatic rgz > P > r gz are normally observed in clastic continental margins and shale-rich mountain fl f w Pore- uid overpressures belts. Therefore we explore the predicted shapes of strength-depth profiles using data from over- Fluid-retention depth pressured regions, especially those dominated by the widespread disequilibrium-compaction mecha- Critical-taper wedge mechanics fl fl Jointing nism, in which uid pressures are hydrostatic above the uid-retention depth zFRD and overpressured r Strength-depth profile below, increasing parallel to the lithostatic gradient gz. Both brittle crustal strength and frictional fault ðr À Þ strength below the zFRD must be constant with depth because effective stress gz Pf is constant, in contrast with the classic linearly increasing profile. Borehole stress and fluid-pressure measurements in several overpressured deforming continental margins agree with this constant-strength prediction, with the same pressure-dependence k as the overlying hydrostatic strata. The role of zFRD in critical-taper wedge mechanics and jointing is illustrated. The constant-strength approximation is more appropriate for overpressured crust than classic linearly increasing models. © 2014 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 1. Introduction 1.1. Classic brittle strength-depth relationship The classic Brace and Kohlstedt (1980) strength-depth graph, The predicted linear increase in brittle strength s1 À s3 with based on experimental rock mechanics, predicts an initial linear depth is a consequence of [1] the intrinsic pressure dependence k of increase in brittle frictional strength with increasing pressure, brittle strength, plus [2] an assumption that pore-fluid pressure is followed by an exponential decay in ductile strength with hydrostatic and therefore linearly increasing Pf ¼ rwgz. Ignoring increasing temperature (Fig. 1A). This first-order prediction has cohesion we approximate brittle crustal strength very simply as the r À k been remarkably successful, for example in predicting the tem- vertical effective stress gz Pf , times the pressure dependence . perature of the brittle-ductile transition in common rock types. s À s ¼ k r À ; The intricacies of this graph in the lower crust and upper mantle 1 3 gz Pf (1a) continue to be widely discussed and are even controversial in the case of the proposed “jelly sandwich” of weak lower continental (Suppe, 1985, p. 185), where r is the mean bulk density of the crust above a presumed stronger mantle (e.g. Chen and Molnar, overlying rock. Eq. (1) also can be written in terms of the classic fl l ¼ =r 1983; Suppe, 1985,p.188e189; Jackson, 2002; Burov and Hubbert-Rubey (1959) uid-pressure ratio Pf gz Watts, 2006; Bürgmann and Dresen, 2008). In contrast, the s À s ¼ kð1 À lÞrgz; (1b) predicted linear increase in brittle strength is generally assumed 1 3 without discussion and agrees with deep borehole stress data in where (1 À l) may be considered the fractional fluid-pressure crystalline rock, showing that the crust is commonly close to a weakening. In submarine cases l is measured with respect to the critical state of brittle failure (e.g. Townend and Zoback, 2000; sea bottom (Davis et al., 1983). For the hydrostatic case (1a) Zoback, 2007), for example the deep KTB borehole in Germany becomes. (Fig. 1B). s À s ¼ kðr À r Þ : 1 3 w gz (2) These crustal-strength equations express the fact that brittle E-mail address: [email protected]. pressure-dependent strength is a function of effective stress http://dx.doi.org/10.1016/j.jsg.2014.07.009 0191-8141/© 2014 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 482 J. Suppe / Journal of Structural Geology 69 (2014) 481e492 A.σ −σ B. in situ stress σ −σ faulting, off-fault fracturing, and folding processes, some of which strength 1 3 1 3 0 500MPa 0 200MPa may be coseismic and may be controlled by dynamic frictional 0 0 processes in earthquakes (e.g. Di Toro et al., 2011). possible stress states 4 2 2. Implications of fluid overpressures for regional strength hydrostatic In contrast with the classic view of linearly increasing crustal 8 brittle strength 4 strength dominated by hydrostatic pore-fluid pressures, it is well established from petroleum boreholes and seismic velocity analysis that fluids are overpressured in deeper parts of thick, fine-grained 12 6 clastic sedimentary basins and in deforming shale-rich plate- boundary mountain belts, largely due to disequilibrium compac- tion from stratigraphic and tectonic loading, but with additional 16 8 effects including vertical and lateral pressure redistribution and gas plastic strength Germany generation (Bredehoeft and Hanshaw, 1968; Fertl, 1976; Magara, KTB borehole 1978; Hart et al., 1995; Swarbrick and Osborne, 1996; Yardley and 20km 10km Swarbrick, 2000; Tingay et al., 2009a). The basins of most interest for crustal strength and large-scale tectonics are continental mar- Fig. 1. (A) The classic strength-depth graph with linearly increasing brittle strength, gins built on oceanic or highly-thinned continental crust that have which is based on the assumption of hydrostatic pore-fluid pressures (Eq. (2)). (B) This fi brittle-hydrostatic model agrees well with stress data from the deep KTB borehole in vast deforming clastic-rich volumes that approach a signi cant Germany where fluid pressures are hydrostatic (Brudy et al., 1997; Zoback and Harjes, fraction of crustal thickness, even spanning the brittle-ductile 1997; Grawinkel and Stockhert,€ 1997), as well as data elsewhere (see Townend and transition (e.g. Gulf of Mexico, Niger delta, Bangladesh/Myanmar, Zoback, 2000; Zoback, 2007). Sumatra, Makran, Gulf of Alaska, New Zealand, Nankai trough, Barbados/Trinidad, and offshore southwestern Taiwan). Here we illustrate typical observed fluid-pressure/depth relationships from r À r boreholes. We then compute the expected strength-depth profiles gz Pf and not the solid pressure gz or even depth, which is a distinction that becomes more important in the case of fluid (Eq. (1a)) and compare them with borehole stress data in actively overpressures. deforming regions, where stress is expected to be close to regional The pressure dependence k can be expressed in terms of the strength, or at least limited by Eq. (1a) as an upper bound. familiar Coulomb internal friction (m ¼ tan4) and varies between kc ¼ 2sin4 (1 À sin4) in pure-thrust compression and ke¼2sin4 2.1. Disequilibrium-compaction mechanism and crustal strength (1 þ sin4) in pure normal-fault extension, with strike-slip stress states (s2 vertical) lying between these limits, as can be shown from The fluid-pressure depth relationship for the Yinggehai basin the Mohr diagram. offshore south China (Fig. 2A) (Luo et al., 2003)istypicalofthe disequilibrium-compaction mechanism (e.g. Swarbrick and 1.2. Success of the classic brittle-hydrostatic assumption Osborne, 1996; Magara, 1978), which has also been called “leaky overpressure” (Crans and Mandl, 1980; Mandl and Crans, Brace and Kohlstedt (1980) assumed that brittle crustal strength 1981). Pressures are hydrostatic down to a critical depth of is controlled by an optimally oriented static fault friction of ~1.8 km, the fluid-retention depth z , below which fluid pres- m ¼ 0.6e0.85 based on lab static friction measurements from a wide FRD sures increase parallel to the lithostatic gradient (Fig. 2A), variety of rock types (Byerlee, 1978)(k ¼ 2.1e3.7, k ¼ 0.68e0.79). c e because permeability becomes sufficiently low that the incre- Townend and Zoback (2000) showed that deep borehole stresses mental increases in sedimentary or tectonic load are supported are typically close to a critical state of failure consistent with by the pore fluid, rather than by incremental compaction of the m ¼ 0.55e1(k ¼ 1.9e4.8, k ¼ 0.65e0.83) and an assumed hydro- c e solid-grain framework. Fluid pressures under the disequilibrium- static fluid pressure, implying that stress z regional strength in compaction mechanism are a simple function of the fluid- many regions (Zoback, 2007). Therefore the Brace and Kohlstedt retention depth, representing the sum of the hydrostatic and (1980) brittle-hydrostatic proposal (Eq. (2)) is in good agreement lithostatic contributions (Fig. 2A). with available observations, as is illustrated by stress data from the deep KTB borehole in Germany (Fig. 1B). ¼ r Pf wgz for z zFRD (3a) We have expressed brittle crustal strength (Eq. (1a)) in terms of the vertical effective stress because this quantity can be directly ¼ r þ r ð À Þ calculated from commonly available borehole data or estimated Pf wgzFRD g z zFRD for z zFRD (3b) from seismic velocities in clastic sedimentary sections that have not It follows that the Hubbert-Rubey fractional fluid-pressure undergone uplift and erosion (e.g. Fertl, 1976; Magara, 1978; weakening (1 À l) (see Eq. (1b)) is a simple function of the fluid- Swarbrick et al., 2002; Zoback, 2007).
Load more

Recommended publications
  • Analog Experiments and Mechanical Analysis Applied to the Alaskan Accretionary Wedge Marc-André Gutscher, Nina Kukowski, Jacques Malavieille, Serge Lallemand
    Analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge Marc-André Gutscher, Nina Kukowski, Jacques Malavieille, Serge Lallemand To cite this version: Marc-André Gutscher, Nina Kukowski, Jacques Malavieille, Serge Lallemand. Analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge. Journal of Geophysical Research, American Geophysical Union, 1998, 103 (B5), pp.10161-10176. hal-01261538 HAL Id: hal-01261538 https://hal.archives-ouvertes.fr/hal-01261538 Submitted on 26 Jan 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. B5, PAGES 10,161-10,176,MAY 10, 1998 Episodic imbricate thrusting and underthrusting' Analogexperiments and mechanicalanalysis applied to the Alaskan Accretionary Wedge Marc-Andrd Gu•scher • and Nina Kukowski GEOMAR, Kiel, Germany JacquesMalavieille and SergeLallemand Laboratoire de G•ophysique et Tectonique, Universit• de Montpellier II, Montpellier, France Abstract. Seismic reflection profiles from the sediment rich Alaska subduction zone image short, frontally accreted, imbricate thrust slices and repeated se- quencesof long, underthrust sheets. Rapid landward increasesin wedgethickness, backthrusting,and uplift of the forearc are observed,suggesting underthrusting beneaththe wedge.These features and a widely varyingfrontal wedgemorphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations.
    [Show full text]
  • Deep Sea Drilling Project Initial Reports Volume
    30. THE COMPRESSIVE MECHANICS OF ACCRETIONARY WEDGES APPLIED TO THE LEG 78A STUDY AREA NEAR BARBADOS1 Dan M. Davis, Department of Earth and Planetary Sciences, Massachusetts Institute of Technology2 ABSTRACT Sites 541 and 542 of DSDP Leg 78A are very near the front of a large wedge of accreted sediments at the eastern edge of the Caribbean Plate where it overthrusts the downgoing crust of the Altantic Ocean. The mechanics of this wedge are considered in light of the Coulomb-wedge hypothesis of Davis et al. (1983), which regards the sediments of the wedge as analogous to soil being pushed by a bulldozer. The wedge deforms in order to attain and maintain its criti- cal taper, at which it could stably ride over the downgoing plate. If it were allowed to maintain that taper without any erosion or accretion, then no significant internal deformation would be required for the wedge to ride over its décolle- ment. The magnitude of this critical taper is a function of fluid pressures and the mechanical properties of the wedge and décollement. The narrow taper of the Barbados wedge is consistent with the evidence of high fluid pressures for the region, suggesting that its behavior is dominated by horizontal compression resulting from plate convergence. The hy- pothesis that the wedge is at or near compressive failure permits a description of the state of stress in the wedge. The borehole stability problems experienced at Sites 541 and 542 are consistent with the view (Hottman et al., 1979) that boreholes act as stress concentrators. The resulting stress concentration can initiate compressive failure of the borehole wall, and may have contributed to the borehole instability observed on Leg 78A.
    [Show full text]
  • Growth and Erosion of Fold-And-Thrust Belts with an Application to the Aconcagua Fold-And-Thrust Belt, Argentina G
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B01410, doi:10.1029/2002JB002282, 2004 Growth and erosion of fold-and-thrust belts with an application to the Aconcagua fold-and-thrust belt, Argentina G. E. Hilley1 and M. R. Strecker Institut fu¨r Geowissenschaften, Universita¨t Potsdam, Potsdam, Germany V. A. Ramos Department de Geologia, Universidad de Buenos Aires, Buenos Aires, Argentina Received 1 November 2002; revised 26 August 2003; accepted 11 September 2003; published 23 January 2004. [1] The development of topography within and erosional removal of material from an orogen exerts a primary control on its structure. We develop a model that describes the temporal development of a frontally accreting, critically growing Coulomb wedge whose topography is largely limited by bedrock fluvial incision. We present general results for arbitrary initial critical wedge geometries and investigate the temporal development of a critical wedge with no initial topography. Increasing rock erodibility and/or precipitation, decreasing mass flux accreting to the wedge front, increasing wedge sole-out depth, decreasing wedge and basal decollement overpressure, and increasing basal decollement friction lead to narrow wedges. Large power law exponent values cause the wedge geometry to quickly reach a condition in which all material accreted to the front of the wedge is removed by erosion. We apply our model to the Aconcagua fold-and-thrust belt in the central Andes of Argentina where wedge development over time is well constrained. We solve for the erosional coefficient K that is required to recreate the field-constrained wedge growth history, and these values are within the range of independently determined values in analogous rock types.
    [Show full text]
  • Critical Taper Model of Fold-And-Thrust Belts and Accretionary Wedges
    AmIU. Rev. Earth Planet. Copyright © by AnnualSci. Reviews]990. 18:55-99 Inc. All rights reserved 1990 CRITICAL TAPER MODEL OF FOLD-AND-THRUST BELTS AND ACCRETIONARY WEDGES F. A. Dahlen Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 INTRODUCTION The fold-and-thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the Earth's upper crust. Although there is considerable natural variation among the many fold-and-thrust belts and accretionary wedges that have been recognized and explored, several features appear to be universal . In cross section, fold-and-thrust belts and accretionary wedges occupy a wedge-shaped deformed region overlying a basal detachment or decollement fault; the rocks or sediments beneath this fault show very little deformation. The decollement fault characteristically dips toward the interior of the mountain belt or, in the case of a submarine wedge, toward the island arc; the topography, in contrast, slopes toward the toe or deformation front of the wedge. Deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault-bend folding. Access provided by University of Chicago Libraries on 01/11/17. For personal use only. Annu. Rev. Earth Planet. Sci. 1990.18:55-99. Downloaded from www.annualreviews.org Two North American fold-and-thrust belts that exhibit these features are shown in Figure 1. Neither of these two examples is tectonically active today; the southern Canadian fold-and-thrust belt was active during the late Jurassic and Cretaceous (150-100 Ma), whereas the southern Appa­ lachians were deformed during the late Carboniferous to Permian Alle­ gh enian orogeny (300-250 Ma).
    [Show full text]
  • Geodynamics of Wedges
    1 DRIVING MECHANISMS OF THRUST SYSTEMS Dynamics of accretionary, orogenic wedges Tens of kilometres long displacements are reported for only a few thousands meters thick thrust sheets. With accepted values of frictional resistance, stresses necessary to push a thrust sheet on a horizontal or slightly up-inclined basal thrust exceed the failure strength of intact rocks. Accordingly, hinterlands should deform and crush before fronts of thrust sheets would move. This mechanical paradox has triggered formulation of a few working hypotheses. The question is still not fully resolved, possibly because all envisioned mechanisms are involved on a scale-dependent importance. Concepts and definitions Thrust systems are common in converging settings, resulting from compressional deformation of rocks once the shear strength along weak layers or planes is exceeded. In convergent plate boundaries, thrusts cutting through accretionary wedges are virtually splays of the subduction plane. This mechanism, where the accretionary hanging wall is deformed while the subducting footwall plate descends undeformed, is typical of thin-skinned tectonics depicted in mountain systems. Therefore, mechanics developed from accretionary wedges are often applied to foreland fold-and-thrust belts and even to orogenic belts (see lecture on thrust systems). Geometry A thrust sheet (or nappe; distinguish from fold nappe = large recumbent fold) is an allochthonous unit that has been transported and lies almost horizontally, away from its original position, on an autochthonous substratum. The bottom boundary is an originally shallow-dipping thrust fault or shear zone along which most of the displacement has taken place. The frontal part in the direction of movement is the leading edge.
    [Show full text]
  • Ocean Trenches
    This article was originally published in Encyclopedia of Geology, second edition published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author’s institution, for non-commercial research and educational use, including without limitation, use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation, commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: https://www.elsevier.com/about/policies/copyright/permissions Stern Robert J. (2021) Ocean Trenches. In: Alderton, David; Elias, Scott A. (eds.) Encyclopedia of Geology, 2nd edition. vol. 3, pp. 845-854. United Kingdom: Academic Press. dx.doi.org/10.1016/B978-0-08-102908-4.00099-0 © 2021 Elsevier Ltd. All rights reserved. Author's personal copy Ocean Trenches☆ Robert J Stern, Department of Geosciences, The University of Texas at Dallas, Richardson, TX, United States © 2021 Elsevier Ltd. All rights reserved. Introduction 845 Early Years of Study 845 Plate Tectonic Significance 846 Geographical Distribution 846 Morphological Expression 848 Filled Trenches 849 Accretionary Prisms and Sediment Transport 850 Empty Trenches and Subduction Erosion 851 Outer Trench Swell 852 Controls on Trench Depth 852 Fluids Released From the Fore Arc 853 Biosphere 854 Further Reading 854 Introduction An oceanic trench is a long, narrow, and generally very deep depression of the seafloor.
    [Show full text]
  • Ocean Trenches Trolled Depression in the Rocky Mountains
    Article Number: GEOL: 00141 a0005 EARTH PROCESSES after the war ended to describe a structurally con- AU:1 Ocean Trenches trolled depression in the Rocky Mountains. Johnstone, AU:6 in his 1923 textbook An Introduction to Ocean- ography, first used the term in its modern sense R J Stern, The University of Texas at Dallas, to describe a marked elongate depression of the Richardson, TX, USA seafloor. ß 2004, Elsevier Ltd. All Rights Reserved. During the 1920s and 1930s, Vening Meinesz p0015 developed a unique gravimeter that could measure gravity in the stable environment of a submarine s0005 Introduction AU:3 and used it to measure gravity over ocean trenches. p0005 An oceanic trench is a long, narrow, and generally His gravity measurements revealed that trenches are very deep depression of the seafloor. Oceanic trenches sites of downwelling in the solid Earth. The concept are the deepest places on the Earth’s solid surface and of downwelling at trenches was characterized by range down to 11 km below sea-level. These tremen- Griggs in 1939 as the tectogene hypothesis, for AU:7 dous depths mark fundamental breaks in the Earth’s which he developed an analogous model using a lithosphere, the great plates that we all ride on. If pair of rotating drums. The war in the Pacific led to AU:8 mid-ocean ridges are where the Earth turns itself great improvements in bathymetry, especially in the inside out, trenches are where the Earth swallows its western and northern Pacific, and the linear nature of skin. The asymmetry of trenches reflects a deeper the trenches became clear.
    [Show full text]
  • Accretionary Prisms in Subduction Earthquake Cycles: the Theory Of
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B06410, doi:10.1029/2005JB004094, 2006 Click Here for Full Article Accretionary prisms in subduction earthquake cycles: The theory of dynamic Coulomb wedge Kelin Wang1,2 and Yan Hu2 Received 7 October 2005; revised 3 March 2006; accepted 17 March 2006; published 27 June 2006. [1] We expand the theory of critically tapered Coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles. Building on the Coulomb plasticity of the classical theory, we assume an elastic–perfectly Coulomb plastic rheology and derive exact stress solutions for stable and critical wedges. The new theory postulates that the actively deforming, most seaward part of an accretionary prism (the outer wedge) overlies the updip velocity-strengthening part of the subduction fault, and the less deformed inner wedge overlies the velocity-weakening part (the seismogenic zone). During great earthquakes, the outer wedge is pushed into a compressively critical state, with an increase in basal and internal stresses and pore fluid pressure. After the earthquake, the outer wedge returns to a stable state. The outer wedge geometry is controlled by the peak stress of the updip velocity-strengthening part of the subduction fault achieved in largest earthquakes. The inner wedge generally stays in the stable regime throughout earthquake cycles, acting as an apparent backstop and providing a stable environment for the formation of forearc basins. The new theory has important implications for the studies of the updip limit of the seismogenic zone, the evolution of accretionary prisms and forearc basins, activation of splay faults and tsunami generation, evolution of the fluid regime, and mechanics of frontal prisms at margins dominated by tectonic erosion.
    [Show full text]
  • 22.2 the CASCADIA SUBDUCTION WEDGE: the ROLE of ACCRETION, UPLIFT, and EROSION—An Essay by Mark T. Brandon1
    2917-CH22.pdf 11/20/03 5:26 PM Page 566 Brandon, M.T., 2004, The Cascadia subduction wedge: the role of accretion, uplift, and erosion. in In: "Earth Structure, An Introduction to Structural Geology and Tectonics", by B.A. van der Pluijm and S. Marshak, Second Edition, WCB/McGraw Hill Press, p. 566-574 22.2 THE CASCADIA SUBDUCTION WEDGE: THE ROLE OF ACCRETION, UPLIFT, AND EROSION—An essay by Mark T. Brandon1 22.2.1 Introduction 566 22.2.6 The Cascadia Subduction Zone 569 22.2.2 Accretionary Flux 566 22.2.7 Comparison between the Cascadia 22.2.3 Wedges, Taper, and Stability 567 and Alpine Wedges 574 22.2.4 Double-Sided Wedges 567 Additional Reading 574 22.2.5 Subduction Polarity and Pro-Side Accretion 568 22.2.1 Introduction the Mariana system, to as much as 7 km at ocean- continent subduction zones, such as the Makran mar- Given the constant surface area of Earth, there must be gin of southwest Pakistan. There is evidence from a balance between the amount of plate created at modern subduction zones that not all of the incoming spreading centers and the amount consumed at sub- sedimentary section is accreted. The global rate of sed- duction zones and other sites of plate convergence. iment subduction has been estimated in one study to be Today, 80% of this return flow occurs at subduction equivalent to an average thickness of 300–500 m of zones, and the other 20% at continent-continent colli- compacted sedimentary rock along all subduction sion zones and diffuse oceanic convergent zones.
    [Show full text]
  • Part 4: Thrust Tectonics
    12.113 Structural Geology Part 4: Thrust tectonics Fall 2005 Contents 1 Reading assignment 1 2 Jargon 1 3 Geometry, general characteristics 2 4 Restoration of balanced cross­sections 3 5 Sandboxes and critical taper theory 3 5.1 . 3 5.2 What sets the angle of the wedge? . 4 5.3 If wedges grow self­similarly, can they grow forever? . 4 5.4 What is the backstop anyways? . 5 6 Mechanical paradox of overthrusts 5 7 Review questions 8 1 Reading assignment Chapter 6 in Twiss and Moores is the relevant chapter. Make sure you understand and are familiar (i.e. you could write a caption and label them) with the figures in the chapter. Figures 6.11, 12, 15, and 19 are particularly germane to what you need to take away. Viz.: you should be familiar with the general geometry of thrust faults and their associated folds; the geometry of thrust belts; how that geometry is associated with various tectonic settings. The material covered in the lab on fold and thrusts is obviously relevant. 2 Jargon Make sure all these are familiar to you! autochthonous – allochthonous – klippe – fenster/window – decollement – thick­ skinned vs. thin­skinned thrusts – duplexes – vergence – tear faults – fault­propagation folds – fault bend folds – ramps – flats – foreland – hinterland – vergence – nappe – backthrust – admissible cross­section – accretionary prism – admissible and retrode­ formable cross­sections. 1 3 Geometry, general characteristics A thrust or a reverse fault is a dipping fault whose hanging­wall is translated up­dip. Generally, when the fault dips less than 45◦, it’s called a thrust fault, steeper faults are called reverse faults.
    [Show full text]
  • Thrust Wedges with Décollement Levels and Syntectonic Erosion: a View from Analog Models
    Tectonophysics 502 (2011) 336–350 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Thrust wedges with décollement levels and syntectonic erosion: A view from analog models E. Konstantinovskaya a,⁎, J. Malavieille b a Institut national de la recherche scientifique, Centre Eau, Terre et Environnement (INRS-ETE), 490, de la Couronne, Quebec City (QC), Canada G1K 9A9 b Géosciences Montpellier, CNRS UMR 5243, Université Montpellier 2, 34095 Montpellier, Cedex 5, France article info abstract Article history: Analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with Received 19 August 2010 one and two décollement levels. Different accretion mechanisms are activated depending on interactions Received in revised form 7 January 2011 between surface processes and wedge mechanics: frontal accretion, backthrusting, underthrusting and Accepted 14 January 2011 underplating due to décollement induced duplex formation at depth. These mechanisms may function Available online 15 February 2011 simultaneously, being located at different parts across the wedge. For all the experiments, a high friction is Keywords: imposed at the base of models and the volume of eroded material remains equal to the volume of newly Thrust wedge accreted material, maintaining a constant surface slope during the shortening. Erosion limits the forward Antiformal stack propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation. Analog modeling Erosion promotes development of major backthrusts in the thrust wedges without or with one décollement, Erosion but no backthrusts was formed in the wedges with two décollements. Slow erosion allows lower extent of Exhumation basal underthrusting in comparison with regular-rate erosion.
    [Show full text]
  • Growth of the South Pyrenean Orogenic Wedge 241
    TECTONICS, VOL. 16, NO. 2, PAGES 239-258, APRIL 1997 Growthof the South Pyrenean orogenicwedge AndrewJ. Meigs DivisionofGeological and Planetary Sciences, Calif•a Instituteof Technology,Pasadena DouglasW. Burbank DepartmentofEarth Sciences, University.of Southern California, Los Angeles Abstract. A six-stepreconstruction of the South Pyrenean DeCelles et al., 1993; DeCettes, 1994; DeCe!!es and Mitra, forelandfold-and-thrust belt in Spaindelineates the topographic 1995]. Othershave argued, in contrast,that critical-taper models slope,basal dtcollement angie, internal deformation, and thrust- have limited applicabilityto fold-and-thrustbelts becauseof front advance from the Early Eocene until the end of mechanicaland deformationalincompatibilities between the contractionaldeformation in the Late Oligocene. Style of thrust- geologicrecord and the model [Woodward, 1987; Price, 1988; frontadvance, dip of the basaldtcollement, slope of the upper Meigsand Burbank, 1993; Bombolakis, !994]. surface,and internal deformationare decoupledand not simply This paper is a retreat to geologic data from the South related. Internal deformation increased, decreased, and Pyreneanforeland fold-and-thrust belt in Spain. A uniquelyhigh- maintainedsurface slope angle at differentstages. From the onset resolutionrecord of deformation and topography,contained to the cessationof deformation, the basal dtcollement angle within sedimentsdeposited during thrusting, enablesdetailed decreasedoverall suggestingtranslation of the thrust belt onto reconstructionof the structuraland
    [Show full text]