Tectonic Faults:Agents of Change on a Dynamic Earth

Cover Image Copyright Year: 2007
Author(s): Mark R. Handy; Greg Hirth; Niels Hovius
Book Type: MIT Press
Content Type : Books & eBooks
Topics: Geoscience
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Abstract

Tectonic faults are sites of localized motion, both at the Earth's surface and within its dynamic interior. Faulting is directly linked to a wide range of global phenomena, including long-term climate change and the evolution of hominids, the opening and closure of oceans, and the rise and fall of mountain ranges. In Tectonic Faults, scientists from a variety of disciplines explore the connections between faulting and the processes of the Earth's atmosphere, surface, and interior. They consider faults and faulting from many different vantage points--including those of surface analysts, geochemists, material scientists, and physicists--and in all scales, from seismic fault slip to moving tectonic plates. They address basic issues, including the imaging of faults from Earth's surface to the base of the lithosphere and deeper, the structure and rheology of fault rocks, and the role of fluids and melt on the physical properties of deforming rock. They suggest strategies for understanding the interaction of faulting with topography and climate, predicting fault behavior, and interpreting the impacts on the rock record and the human environment. Using an Earth Systems approach, Tectonic Faults provides a new understanding of feedback between faulting and Earth's atmospheric, surface, and interior processes, and recommends new approaches for advancing knowledge of tectonic faults as an integral part of our dynamic planet.

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      Front Matter

      Page(s): i - xiii
      Copyright Year: 2007

      MIT Press eBook Chapters

      This chapter contains sections titled: Half Title, Report of the 95th Dahlem Workshopx, Title, Copyright, Contents, Dahlem Konferenzen®, List of Participants View full abstract»

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      Tectonic Faults

      Page(s): 1 - 8
      Copyright Year: 2007

      MIT Press eBook Chapters

      This chapter contains sections titled: What are Faults and Why Should we Study Them?, The Workshop, What Was Learned?, Recommendations for Future Research, Acknowledgments View full abstract»

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      Fault Zones from Top to Bottom

      Page(s): 9 - 46
      Copyright Year: 2007

      MIT Press eBook Chapters

      We review recent geophysical insights into the physical properties of fault zones at all depths in the crust and subcrustal lithosphere. The fault core zone, where slip occurs, is thin (tens of centimeters) and can mainly be studied in trenches and in borehole well logs. The fault damage zone is wider (tens to hundred of meters) and can be measured by the analysis of fault zone-trapped waves. Such studies indicate that the damage zone extends to a depth of at least 3–5 km, but there is no agreement on the maximum depth limit. The damage zone exhibits a seismic velocity reduction (with respect to the neighboring country rock) as high as 20–50%. Significantly, this velocity reduction appears to have a temporal component, with a maximum reduction after a large rupture. The fault damage zone then undergoes a slow healing process that appears to be related to fluid-rock interactions that leads to dissolution of grain contacts and recrystallization. Deep seismic reflection profiles and teleseismic receiver functions provide excellent images of faults throughout the crust. In extensional environments these profiles show normal faulting in the upper crust and ductile extension in the lower crust. In compressional environments, large-scale low-angle nappes are evident. These are commonly multiply faulted. The very thin damage zones for these low angle faults are indicative of high pore-fluid pressures that appear to counteract the normal stresses, thereby facilitating thrusting. The presence of fluids within fault zones is also evidenced by geo-electrical studies in such diverse environments as the Himalayan and Andean orogens, the San Andreas fault, and the Dead Sea Transform. Such studies show that the fault can act as a fluid conduit, barrier, or combined conduit-barrier system depending on the physical properties of the fault core zone and damage zone. The ge ometry of active fault zones at depth is revealed by precise microearthquake hypocentral locations. There is considerable geometric diversity, with some strike-slip faults showing a very thin (less then 75 m wide) fault plane and others showing wider, segmented planes and/or parallel strands of faulting. A new discovery is slip-parallel, subhorizontal streaks of seismicity that have been identified on some faults. Such streaks may be due to boundaries between locked and slipping parts of the fault or lithologic variations on the fault surface. Measurements of seismic anisotropy across strike-slip faults are consistent with localized fault-parallel shear deformation in the uppermost mantle, with a width that varies between 20 and 100 km. In addition to shear deformation zones, seismic reflection profiles have imaged discrete faults in the uppermost mantle, mainly associated with paleo-continent/continent collisions. Looking deeper, the lithosphere-asthenosphere boundary may be considered as a major shear zone, considering the horizontal movement of lithospheric plates. This shear zone can be imaged with newly developed seismic receiver function methods. View full abstract»

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      Strain Localization within Fault Arrays over Timescales of 100-107 Years

      Page(s): 47 - 78
      Copyright Year: 2007

      MIT Press eBook Chapters

      Statistical characterization of fault networks, combined with an analysis of geodetic data and the location of historical earthquakes, is a method commonly used to quantify the degree of strain localization in a given tectonic setting. However, such analyses do not address the fundamental questions of why, how, and when (i.e., after what percent total strain) does localization occur on a lithospheric scale. Many studies suggest that the initial phase of crustal deformation is characterized by distributed strain accumulation and structural complexity and that the system evolves towards highly localized deformation on a small number of discrete fault zones. What controls the transition from one regime to the other within a rheologically layered lithosphere? Observations of the evolution of fault networks over a range of spatial and temporal scales may help us to understand the underlying controls on the localization process. Dip-slip faults, especially moderate to high-angle extensional structures, inherently provide the best conditions for preserving such temporal information over geological time because they generate adjacent sedimentary depocenters that usually remain undeformed by subsequent movement on the fault. The aim of this paper is (a) to review recent observations of extensional fault growth, (b) to summarize conclusions drawn from these observations concerning the underlying controls on strain localization in extensional settings, and (c) to discuss the relevance of these observations to other tectonic settings. In particular, several of the ideas that have been derived from studies of strain localization in extensional settings are used to reexamine existing theories concerning strike-slip fault evolution. View full abstract»

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      Group Report: Nucleation and Growth of Fault Systems

      Page(s): 79 - 98
      Copyright Year: 2007

      MIT Press eBook Chapters

      This chapter contains sections titled: Introduction, Timescales of Fault Development, Localization of Faults Throughout the Lithosphere, Interaction of Fault Zones with Lithospheric and Asthenospheric Layering, Summary, References View full abstract»

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      Seismic Fault Rheology and Earthquake Dynamics

      Page(s): 99 - 137
      Copyright Year: 2007

      MIT Press eBook Chapters

      As preparation for this Dahlem Workshop on The Dynamics of Fault Zones, specifically on the subtopic “Rheology of Fault Rocks and Their Surroundings,” we addressed critical research issues for understanding the seismic response of fault zones in terms of the constitutive response of fault materials. This requires new concepts and a host of new observations and experiments to document material response, to understand the shear localization process and the inception of earthquake instability, and especially to understand the mechanisms of fault weakening and dynamics of rupture tip propagation and arrest during rapid, possibly large, slip in natural events. We examine in turn the geological structure of fault zones and its relation to earthquake dynamics, the description of rate and state friction at slow rates appropriate to the interseismic period and earthquake nucleation, and the dynamics of fault weakening during rapid slip. The last topic gets special attention in view of the important recent advances in theoretical concepts and experiments to probe the range of slip rates prevailing during earthquakes. We then address the assembly of the constitutive framework into viable, but necessarily simplified, conceptual and computational models for description of the dynamics of crustal earthquake rupture. This is done principally in the slip-weakening framework, and we examine some of the uncertainties in doing so, and issues of how new understanding of the rapid large slip range will be integrated to model the traction evolution and the weakening process during large slip episodes. View full abstract»

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      Continental Fault Structure and Rheology from the Frictional-to-Viscous Transition Downward

      Page(s): 139 - 182
      Copyright Year: 2007

      MIT Press eBook Chapters

      Faulting is an expression of the interaction between rock rheology, kinematic boundary conditions, and associated stress fields. The structure and rheology of faults vary with depth, such that pressure-dependent frictional behavior predominating in the upper, brittle part of the crust is transitional to strongly temperature- and rate-dependent behavior in the lower part of the crust and mantle. This frictional-to-viscous transition (FVT) is characterized by changes in rock structure, rheology, and fluid activity that are closely tied to the earthquake cycle. As such, the FVT is a first-order decoupling zone, whose depth and lateral extent vary in time. Brittle, sometimes seismic, instabilities perturb the ambient stress field within the lithosphere on timescales ranging from seconds to years. These instabilities are measurable as transient motions of the Earth's surface and are manifest both at, and below, the FVT by the development of structural anisotropies (fractures, foliations). Surface motion studies of plate-boundary strike-slip faults indicate that shearing below the FVT is more localized in the lower crust than in the upper mantle. Structural investigations of exhumed shear zones reveal that this localization involves the nucleation of fractures at the FVT, as well as the buckling and rotation of existing foliations below the FVT. In some cases, rotation of these surfaces can initiate transient deformation, transferring stress upward and potentially triggering earthquakes. The networking of shear zones on several length scales allows them to function as decoupling horizons that partition three-dimensional strain within the lithosphere. The simplification of fault geometry with progressive strain lends justification to the use of laboratory-derived flow laws to estimate the bulk rock rheology on length scales at which strain is homogeneous. In general, the longer the timeand length scales of faulting considered, the greater the potential influence of the kinematic and thermal history on the rheology of the fault system. Taken together, studies suggest that future fault modeling must include parameters that quantify the thermal and structural aspects of rock history, as well as the fluid activity in and around faults. View full abstract»

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      Group Report: Rheology of Fault Rocks and Their Surroundings

      Page(s): 183 - 204
      Copyright Year: 2007

      MIT Press eBook Chapters

      This chapter contains sections titled: Overview, Large-Scale View of the Earthquake Machine, Seismic Cycle: Nucleation Leading to Unstable Slip, Seismic Cycle: Dynamic Rupture, Fault Zone Restrengthening During Co-, Post-, and Inter-Seismic Periods, Fault Geometry and the Significance of Distributed Damage, Subduction Thrusts, Stress on Faults, Summary, References View full abstract»

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      Topography, Denudation, and Deformation

      Page(s): 205 - 230
      Copyright Year: 2007

      MIT Press eBook Chapters

      Topography formed during active orogenesis and the elevated denudation rates established on that topography fundamentally influence the location and rates of interplate deformation. Stress concentration through slope-generated shear and normal stresses reduces the amount of stress of tectonic origin required to reach failure by variable amounts, depending upon topographic wavelengths, orientation, magnitude, and material property. In areas of extreme topography associated with plate convergence, this stress concentration can lead to significant strain concentration into valleys and away from loads generated by large mountains. This strain concentration then leads to crustal weakening via advection of hot lower crust into the upper crust resulting in significant reduction in integrated strength of the crust. Together, these mechanisms produce a highly heterogeneous lithosphere reflecting the distribution and intensity of surface processes that controls numerous crustal phenomena including the degree of metamorphism, decompression melting, and strain partitioning within oblique orogens over time frames of ˜106 yr. Although three-dimensional topography produces stress states with large spatial variability, the general form of these stress states is readily approximated, and they should not be avoided when calculating the mechanical evolution of crustal fault zones. The magnitude of these stresses may be sufficient to influence earthquake nucleation or propagation. View full abstract»

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      Constraining the Denudational Response to Faulting

      Page(s): 231 - 272
      Copyright Year: 2007

      MIT Press eBook Chapters

      Denudation links tectonics with climate by changing topographic loads and promoting the drawdown of CO2. Measurements of denudation are a key to understanding this link. In particular, they are required to test and calibrate geodynamic models, to evaluate the tectonic control on landscape evolution, to quantify the geomorphic impact of faulting and seismicity, and to assess the role of tectonically driven denudation in stabilizing Earth's climate. We review techniques used to measure denudation, and weathering on timescales relevant to faulting and the dynamics of fault zones, with particular attention paid to the use of hydrometric data and cosmogenic isotopes. Using selected examples, we illustrate the application of these techniques to problems ranging from soil formation and coseismic erosion of earthquake epicentral areas to the erosion of orogens and estimation of catchment-scale erosion and weathering fluxes. The examples show that faulting is the Earth's premier erosion and weathering engine. Globally, erosion scales with tectonic forcing. Locally, fluvial incision and landscape lowering are correlated with faulting and seismic activity. Thus, tectonically active areas yield disproportionate amounts of sediment. Erosion refreshes rock surfaces in these areas, thereby enhancing chemical weathering rates and CO2 consumption. The effects of climate variability and change are evident in the patterns and rates of erosion and weathering. However, they are almost always superimposed on a stronger tectonic signal. We highlight the potential of cosmogenic nuclides to quantify present and past rates and patterns of denudation associated with faulting. Finally, we identify outstanding challenges for future work: (a) to characterize crustal deformation, climate, and denudation over their full range of time and length scales; (b) to analyze the geomorphic imp act and stratigraphic record of recent earthquakes; (c) to identify the processes, thresholds, and feedback mechanisms that control global weathering and regulate the long-term climate; and (d) to provide constraints that help to mitigate the risks associated with geomorphic processes triggered by earthquakes. Constraining the denudational response to faulting will help to meet these challenges. View full abstract»

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      Group Report: Surface Environmental Effects on and of Faulting

      Page(s): 273 - 294
      Copyright Year: 2007

      MIT Press eBook Chapters

      This chapter contains sections titled: Overview, Topic 1: Surface Processes and Strain Localization, Topic 2: Topographic Response to Coupled Erosion, Weathering, and Tectonics, Topic 3: Impact of Climate and Climate Change on Surface Deformation, Topic 4: The Fidelity of the Geomorphic and Geologic Records of Changes in Rates of Surface Deformation and Erosion/Sedimentation, Topic 5: Impact of Faulting on the Human Environment, Summary, References View full abstract»

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      Fluid Processes in Deep Crustal Fault Zones

      Page(s): 295 - 318
      Copyright Year: 2007

      MIT Press eBook Chapters

      Fluid as a C-O-H dominated phase is widespread, but not ubiquitous, in the Earth's crust. The presence or absence of fluid is in large part a function of thermal history, at least up to the onset of melting. Rocks containing relatively low-temperature assemblages that are subject to further heating release fluid and so are commonly saturated, while rocks undergoing cooling resorb fluid into hydrous minerals and so are dry. Fluid may be introduced from external sources during faulting or magmatic activity, and the degree to which it persists depends on the interplay between injection rates and reaction rates. Where fluids do occur in the crust, fluxes are generally low, so that many aspects of fluid chemistry are dictated by saturation with rock-forming minerals. These mineralogical controls on fluid chemistry and activities of volatile species further affect the rheology of the crust by determining whether or not deformation can be fluxed by fluid processes. It is argued that rocks undergoing progressive metamorphism are wet and experience widespread deformation, while rocks that are cooling are strong and deformation is localized into zones, particularly during times of fluid infiltration. The transition between brittle and ductile behavior may therefore reflect changes in the availability of water rather than changes in temperature. Faults themselves are important loci of fluid flow, but it is often difficult to identify the sources of fluid, because geochemical tracers are mainly reset in a rock-dominated environment. Nevertheless, it is unlikely that faults are commonly effective drains of fluid being released by prograde metamorphism, because the very low permeability of such rocks (inferred from evidence for strong overpressuring) means that fluid cannot easily drain into fractures, even where a strong gradient in hydraulic head exists. View full abstract»

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      Deformation in the Presence of Fluids and Mineral Reactions

      Page(s): 319 - 356
      Copyright Year: 2007

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      Natural and experimental deformation of fault rocks show that fluid flow and mineral reactions are linked to fracturing in a nonlinear feedback relationship that potentially affects the displacement and stress histories of large faults. These interactions spawn instabilities that are expressed as episodic seismic events involving cataclasis, which alternate with slow, aseismic deformation involving pressure-solution creep, as well as healing and sealing by fluid-assisted mass transfer. This chapter focuses on the timescale of these processes during the earthquake cycle, with special emphasis on the evolution of rheological and transport properties of fault rock during the interseismic period. Fracturing weakens faults dramatically by enhancing the kinetics of pressure-solution creep and of mineral reactions. Therefore, during the postseismic period and initial part of the interseismic period, weakening is faster than fault strengthening by healing and sealing of fractures. During the interseismic period, mass transfer associated with fluid-assisted chemical reactions smoothes asperities on fault surfaces, heals fractures and enhances the formation of a foliation parallel to the fault plane, and decreases permeability. If advective fluid inflow is significant, this can increase pore-fluid pressure and reduce effective shear strength, at least locally within the fault. In the long term, however, the combined effect of fracturing, pressure-solution creep, and sealing is to restore the rheological and transport properties of the fault during the interseismic period, setting the stage for renewed stress build-up and seismicity. We demonstrate the salient characteristics of fluid-assisted fault weakening and strengthening with a one-dimensional model of an idealized fault zone undergoing simple shear at constant velocity. The model shows that the kinetics of the weakening a nd strengthening processes determine the relative rates of shear stress decrease and increase during the interseismic period. The kinetics of dissolution precipitation and mineral reactions are therefore expected to exert an important control on the recurrence time of earthquakes. View full abstract»

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      Effects of Melting on Faulting and Continental Deformation

      Page(s): 357 - 402
      Copyright Year: 2007

      MIT Press eBook Chapters

      The presence of melt is closely related to the localization of deformation in faults and shear zones in a variety of tectonic settings. This relationship is observed on length scales from the outcrop to plate boundary faults to orogens. However, the question of whether melting induces localization, or localization creates a pathway for melts, can rarely be answered from field observations alone. Experimental studies show that rock strength decreases exponentially with increasing volume percentage of melt. This suggests that melting facilitates strain localization where deformation would be homogeneous in the absence of melt. Yet, the extrapolation of experimental relationships between rock strength and melt content to natural conditions at depth in the lithosphere remains speculative, largely because the grain-scale processes underlying dramatic weakening at small amounts of melt have yet to be investigated in crustal rocks. New geochronological methods for dating minerals that crystallized during deformation in the presence of melt have the potential to constrain the time lag between the onset of melting and deformation in naturally deformed anatectic rocks. An indirect, but clear answer to the question of whether melting induces strain localization on a regional scale comes from numerical models of orogenesis which can be run in the presence or absence of low-viscosity domains that approximate the mechanical behavior of partially melted rock. These models show that melting induces lateral flow of anatectic crust within horizontal channels usually situated at the base of the continental crust. These channels have strong vertical strain gradients, especially at their boundaries where shear zones accommodate lateral extrusion of the anatectic rock in between. Together with their bounding shear zones, these flow channels form a new class of faults, which we term “ ;extrusional faults.” Extrusional faults containing long-lived melt (tens of millions of years) can support large, broadly distributed topographic loads such as orogenic plateaus and can exhume deeply buried rocks from beneath orogens. In contrast, strike-slip and oblique-slip faults serve as steep conduits for the rapid ascent, differentiation, and crystallization of melt. The relatively short residence time of melts in such moderately to steeply dipping fault systems can lead to episodic motion, with long periods of creep punctuated by shorter periods of melt veining, magmatic activity, and/or faster slip. View full abstract»

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      Group Report: Fluids, Geochemical Cycles, and Mass Transport in Fault Zones

      Page(s): 403 - 425
      Copyright Year: 2007

      MIT Press eBook Chapters

      This chapter contains sections titled: Overview, Theme 1: What are The Controls on Fluid-Rock Chemical Interaction in and Adjacent to Fault Zones?, Theme 2: How Does Fluid Flow Change Before, During, and After Earthquakes?, Theme 3: What are the Magnitudes of Fluid Flux Throughout the Lithosphere in Different Tectonic Environments?, Summary, References View full abstract»

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      Author Index

      Page(s): 427
      Copyright Year: 2007

      MIT Press eBook Chapters

      Tectonic faults are sites of localized motion, both at the Earth's surface and within its dynamic interior. Faulting is directly linked to a wide range of global phenomena, including long-term climate change and the evolution of hominids, the opening and closure of oceans, and the rise and fall of mountain ranges. In Tectonic Faults, scientists from a variety of disciplines explore the connections between faulting and the processes of the Earth's atmosphere, surface, and interior. They consider faults and faulting from many different vantage points--including those of surface analysts, geochemists, material scientists, and physicists--and in all scales, from seismic fault slip to moving tectonic plates. They address basic issues, including the imaging of faults from Earth's surface to the base of the lithosphere and deeper, the structure and rheology of fault rocks, and the role of fluids and melt on the physical properties of deforming rock. They suggest strategies for understanding the interaction of faulting with topography and climate, predicting fault behavior, and interpreting the impacts on the rock record and the human environment. Using an Earth Systems approach, Tectonic Faults provides a new understanding of feedback between faulting and Earth's atmospheric, surface, and interior processes, and recommends new approaches for advancing knowledge of tectonic faults as an integral part of our dynamic planet. View full abstract»

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      Subject Index

      Page(s): 429 - 446
      Copyright Year: 2007

      MIT Press eBook Chapters

      Tectonic faults are sites of localized motion, both at the Earth's surface and within its dynamic interior. Faulting is directly linked to a wide range of global phenomena, including long-term climate change and the evolution of hominids, the opening and closure of oceans, and the rise and fall of mountain ranges. In Tectonic Faults, scientists from a variety of disciplines explore the connections between faulting and the processes of the Earth's atmosphere, surface, and interior. They consider faults and faulting from many different vantage points--including those of surface analysts, geochemists, material scientists, and physicists--and in all scales, from seismic fault slip to moving tectonic plates. They address basic issues, including the imaging of faults from Earth's surface to the base of the lithosphere and deeper, the structure and rheology of fault rocks, and the role of fluids and melt on the physical properties of deforming rock. They suggest strategies for understanding the interaction of faulting with topography and climate, predicting fault behavior, and interpreting the impacts on the rock record and the human environment. Using an Earth Systems approach, Tectonic Faults provides a new understanding of feedback between faulting and Earth's atmospheric, surface, and interior processes, and recommends new approaches for advancing knowledge of tectonic faults as an integral part of our dynamic planet. View full abstract»