Showing papers on "Fault (geology) published in 2007"

Seal bypass systems

Abstract: We present an interpretational framework for the analysis of a diverse set of geological structures that breach sealing sequences and allow fluids to flow vertically or subvertically across the seal. In so doing, they act as seal bypass systems (SBS). We define SBS as seismically resolvable geological features embedded within sealing sequences that promote cross-stratal fluid migration and allow fluids to bypass the pore network. If such bypass systems exist within a given seal sequence, then predictions of sealing capacity based exclusively on the flow properties (capillary entry pressure and hydraulic conductivity) of the bulk rock can potentially be negated by the capacity of the bypass system to breach the grain and pore network. We present a range of examples of SBS affecting contrasting types of sealing sequences using three-dimensional (3-D) seismic data. These examples show direct evidence of highly focused vertical or subvertical fluid flow from subsurface reservoirs up through the seal sequence, with leakage internally at higher levels or to the surface as seeps.We classify SBS into three main groups based on seismic interpretational criteria: (1) fault related, (2) intrusion related, and (3) pipe related. We show how each group exhibits different modes of behavior with different scaling relationships between flux and dimensions and different short- and long-term impacts on seal behavior.

Spontaneous and triggered aseismic deformation transients in a subduction fault model

Abstract: [1] Aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault-sliding experiments. When that constitutive formulation is applied to model subduction earthquake sequences, transients can arise spontaneously for certain effective stress () variations with depth. We show that if interstitial fluids are present and pore pressure is near-lithostatic around and downdip from the frictional stability transition, transients with recurrence intervals of ∼1 year are predicted on the basis of laboratory friction parameters and their temperature (hence depth) variations. The recurrence interval decreases with and reaches 14 months when is ∼2–3 MPa. Dimensional analysis and numerical studies show that the fault response primarily depends on a parameter W/h*. Here the high pore pressure zone extends distance W updip from the stability transition, and h* is the stable patch size for steady sliding. Evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed. Transient sequences can also be triggered by a modest, one-time, step-like interseismic stress perturbation on the subduction fault, due to nearby earthquakes, or to pore pressure changes, e.g., during episodes of metamorphic fluid release. Properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced, its relative location along the subduction fault, and its magnitude.

Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau

Abstract: The steep, high-relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic cooling and denudation yet shows little evidence for large-magnitude shortening or accommodation generation in the foreland basin. We address this paradox by using a variety of geomorphic observations to place constraints on the kinematics and slip rates of several large faults that parallel the plateau margin. The Beichuan and Pengguan faults are active, dominantly dextral-slip structures that can be traced continuously for up to 200 km along the plateau margin. Both faults offset fluvial fill terraces that yield inheritance-corrected, cosmogenic 10Be exposure ages of <15 kyr, indicating latest Pleistocene activity. The Pengguan fault appears to have been active in the Holocene at two sites along strike. Latest Quaternary apparent throw rates on both faults are variable along strike but are typically <1 mm yr−1. Rates of strike-slip displacement are likely to be several times higher, probably ∼1–10 mm yr−1 but remain poorly constrained. Late Quaternary folding and dextral strike-slip has also occurred along the western margin of the Sichuan Basin, particularly associated with the present-day mountain front. These observations support models for the formation and maintenance of the eastern plateau margin that do not involve major upper crustal shortening. They also suggest that activity on the margin-parallel faults in eastern Tibet may represent a significant seismic hazard to the densely populated Sichuan Basin.

Talc-bearing serpentinite and the creeping section of the San Andreas fault

Abstract: The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr(-1) (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone.

Tectonics of strike-slip restraining and releasing bends

Abstract: Restraining and releasing bends are common, but enigmatic features of strike-slip fault systems occurring in all crustal environments and at regional to microscopic scales of observation. Regional-scale restraining bends are sites of mountain building, transpressional deformation and basement exhumation, whereas releasing bends are sites of topographic subsidence, transtensional deformation, basin sedimentation and possible volcanism and economic mineralization. Because restraining and releasing bends often occur as singular self-contained domains of complex deformation, they are appealing natural laboratories for Earth scientists to study fault processes, earthquake seismology, active faulting and sedimentation, fault and fluid-flow relationships, links between tectonics and topography, tectonic and erosional controls on exhumation, and tectonic geomorphology. This volume addresses the tectonic complexity and diversity of strike-slip restraining and releasing bends with 18 contributions divided into four thematic sections: (1) a topical review of fault bends and their global distribution; (2) bends, sedimentary basins and earthquake hazards; (3) restraining bends, transpressional deformation and basement controls on development; (4) releasing bends, transtensional deformation and fluid flow.

Evolution of fault-surface roughness with slip

Abstract: Principal slip surfaces in fault zones accommodate most of the displacement during earthquakes. The topography of these surfaces is integral to earthquake and fault mechanics, but is practically unknown at the scale of earthquake slip. We use new laser-based methods to map exposed fault surfaces over scales of 10 µm to 120 m. These data provide the fi rst quantitative evidence that fault-surface roughness evolves with increasing slip. Thousands of profi les ranging from 10 µm to >100 m in length show that small-slip faults (slip <1 m) are rougher than large-slip faults (slip 10‐100 m or more) parallel to the slip direction. Surfaces of small-slip faults have asperities over the entire range of observed scales, while large-slip fault surfaces are polished, with RMS values of <3 mm on profi les as long as 1‐2 m. The large-slip surfaces show smooth, elongate, quasi-elliptical bumps that are meters long and as high as ~1 m. We infer that these bumps evolve during fault maturation. This difference in geometry implies that the nucleation, growth, and termination of earthquakes on evolved faults are fundamentally different than on new ones.

Multi-interferogram method for measuring interseismic deformation: Denali Fault, Alaska

Abstract: SUMMARY Studies of interseismic strain accumulation are crucial to our understanding of continental deformation, the earthquake cycle and seismic hazard. By mapping small amounts of ground deformation over large spatial areas, InSAR has the potential to produce continental-scale maps of strain accumulation on active faults. However, most InSAR studies to date have focused on areas where the coherence is relatively good (e.g. California, Tibet and Turkey) and most analysis techniques (stacking, small baseline subset algorithm, permanent scatterers, etc.) only include information from pixels which are coherent throughout the time-span of the study. In some areas, such as Alaska, where the deformation rate is small and coherence very variable, it is necessary to include information from pixels which are coherent in some but not all interferograms. We use a three-stage iterative algorithm based on distributed scatterer interferometry. We validate our method using synthetic data created using realistic parameters from a test site on the Denali Fault, Alaska, and present a preliminary result of 10.5 ± 5.0 mm yr −1 for the slip rate on the Denali Fault based on a single track of radar data from ERS1/2.

Present-day kinematics at the India-Asia collision zone

Abstract: The collision of the Indian subcontinent with Asia drives the growth and evolution of the greater Tibetan Plateau region. Fault slip rates resulting from the relative motion between crustal blocks can provide a kinematic description of the distribution of presentday deformation. I construct a three-dimensional, regional-scale elastic block model of the India-Asia collision zone that is consistent with geodetic observations of interseismic deforma tion, mapped fault system geometry, historical seismicity, and the mechanics of the earthquake cycle. This mechanical model of the elastic upper crust yields a set of kinematically consistent fault slip rates and block motions that may serve to constrain dynamic models of continental crustal dynamics.

Regional patterns of earthquake-triggered landslides and their relation to ground motion

Abstract: [1] We have documented patterns of landsliding associated with large earthquakes on three thrust faults: the Northridge earthquake in California, Chi-Chi earthquake in Taiwan, and two earthquakes on the Ramu-Markham fault bounding the Finisterre Mountains of Papua New Guinea. In each case, landslide densities are shown to be greatest in the area of strongest ground acceleration and to decay with distance from the epicenter. In California and Taiwan, the density of co-seismic landslides is linearly and highly correlated with both the vertical and horizontal components of measured peak ground acceleration. Based on this observation, we derive an expression for the spatial variation of landslide density analogous with regional seismic attenuation laws. In its general form, this expression applies to our three examples, and we determine best fit values for individual cases. Our findings open a window on the construction of shake maps from geomorphic observations for earthquakes in non-instrumented regions.

Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge

Abstract: Newly acquired seismic refraction and microearthquake data from the Trans-Atlantic Geotraverse (TAG) segment of the Mid-Atlantic Ridge at 26°N reveal for the first time the geometry and seismic character of an active oceanic detachment fault. Hypocenters from 19,232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an ∼15-km-long, dome-shaped fault surface that penetrates to depths >7 km below the seafloor on a steeply dipping (∼70°) interface. A tomographic model of compressional-wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall, which appears to roll over to a shallow dip of 20° ± 5° and become aseismic at a depth of ∼3 km. Outboard of the detachment the exhumed lithosphere is deformed by ridge-parallel, antithetical normal faulting. Our results suggest that hydrothermal fluids at the TAG field exploit the detachment fault to extract heat from a region near the crust-mantle interface over long periods of time.

Tectonic Geomorphology of Mountains: A New Approach to Paleoseismology

Abstract: Preface. 1 Scrunch and Stretch Bedrock Uplift. 1.1 Introduction. 1.2 Pure Uplift, Stretch and Scrunch Bedrock Uplift. 1.2.1 Isostatic and Tectonic Uplift. 1.2.2 Stretch and Scrunch Tectonics. 1.3 Landscape Responses to Regional Uplift. 2 Concepts for Studies of Rising Mountains. 2.1 Themes and Topics. 2.2 The Fundamental Control of Base Level. 2.2.1 Base Level. 2.2.2 Base Level Change. 2.2.3 The Base Level of Erosion. 2.2.4 The Changing Level of the Sea. 2.2.5 Spatial Decay of the Effects of Local Base Level Changes. 2.3 Threshold of Critical Power in Streams. 2.3.1 Relative Strengths of Stream Power and Resisting Power. 2.3.2 Threshold-Intersection Points. 2.4 Equilibrium in Streams. 2.4.1 Classification of Stream Terraces. 2.4.2 Feedback Mechanisms. 2.4.3 Dynamic and Static Equilibrium. 2.5 Time Lags of Response. 2.5.1 Responses to Pulses of Uplift. 2.5.2 Perturbations that Limit Continuity of Fluvial Systems. 2.5.3 Lithologic and Climatic Controls of Relaxation Times. 2.5.4 Time Spans Needed to Erode Landforms. 2.6 Tectonically-Induced Downcutting. 2.6.1 Straths, Stream-Gradient Indices, and Strath Terraces. 2.6.2 Modulation of Stream-Terrace Formation by Climatic Changes. 2.7 Nontectonic Base-Level Fall and Strath Terrace Formation. 2.8 Hydraulic Coordinates. 3 Mountain Fronts. 3.1 Introduction. 3.2 Tectonically Active Escarpments. 3.2.1 Faceted Spur Ridges. 3.2.2 Mountain-Piedmont Junctions. 3.2.3 Piedmont Forelands. 3.3 Fault Segmentation of Mountain Fronts. 3.3.1 Different Ways to Study Active Faults. 3.3.2 Segmentation Concepts and Classification. 3.3.3 Fault-Segment Boundaries. 3.3.4 Normal Fault Surface Ruptures. 3.3.5 Strike-Slip Fault Surface Ruptures. 3.4 Summary. 4 Tectonic Activity Classes of Mountain Fronts. 4.1 Tectonic Setting of the North America-Pacific Plate Boundary. 4.2 Appraisal of Regional Mountain Front Tectonic Activity. 4.2.1 Geomorphic Tools For Describing Relative Uplift Rates. 4.2.1.1 Mountain-Front Sinuosity. 4.2.1.2 Widths of Valleys. 4.2.1.3 Triangular Facets. 4.2.2 Diagnostic Landscape Classes of Relative Tectonic Activity. 4.2.3 Regional Assessments of Relative Tectonic Activity. 4.2.3.1 Response Time Complications and Strike-Slip Faulting. 4.2.3.2 Maps of Relative Uplift. 4.3 Summary. 5 Fault Scarps. 5.1 General Features. 5.2 Scarp Morphology Changes with Time. 5.2.1 Changes in Scarp Height. 5.2.2 Decreases in Maximum Scarp Slope. 5.2.3 Diffusion-Equation Modeling. 5.3 Climatic Controls of Fault-Scarp Morphology. 5.4 Lithologic Controls of Fault-Scarp Morphology. 5.4.1 Fault Rupture of Different Materials. 5.4.2 Lithologic Controls on an 1887 Fault Scarp. 5.4.2.1 Geomorphic Processes. 5.4.2.2 Scarp Materials. 5.4.2.3 Scarp Morphology. 5.5 Laser Swath Digital Elevation Models. 5.6 Dating Fault Scarps with Terrestrial Cosmogenic Nuclides. 5.6.1 Alluvium. 5.6.2 Bedrock. 5.7 Summary. 6 Analyses of Prehistoric Seismic Shaking. 6.1 Paleoseismology Goals. 6.2 Earthquake-Generated Regional Rockfall Events. 6.2.1 New Zealand Earthquakes. 6.2.1.1 Tectonic Setting. 6.2.1.2 Background and Procedures. 6.2.1.3 Diagnostic Lichen-Size Peaks. 6.2.1.3 Tree-Ring Analyses. 6.2.1.5 Alpine Fault Earthquakes. 6.2.1.6 Recent Marlborough Earthquakes. 6.2.2 California Earthquakes. 6.2.2.1 Calibration of Lichen Growth Rates. 6.2.2.2 Recent Cliff Collapse. 6.2.2.3 Rockfall Processes in Glaciated Valleys. 6.2.2.4 San Andreas Fault Earthquakes. 6.2.2.5 Lichenometry and Precise Radiocarbon Dating Methods. 6.3 Summary. References Cited. Index

Balancing the plate motion budget in the South Island, New Zealand using GPS, geological and seismological data

Abstract: SUMMARY The landmass of New Zealand exists as a consequence of transpressional collision between the Australian and Pacific plates, providing an excellent opportunity to quantify the kinematics of deformation at this type of tectonic boundary. We interpret GPS, geological and seismological data describing the active deformation in the South Island, New Zealand by using an elastic, rotating block approach that automatically balances the Pacific/Australia relative plate motion budget. The data in New Zealand are fit to within uncertainty when inverted simultaneously for angular velocities of rotating tectonic blocks and the degree of coupling on faults bounding the blocks. We find that most of the plate motion budget has been accounted for in previous geological studies, although we suggest that the Porter’s Pass/Amberley fault zone in North Canterbury, and a zone of faults in the foothills of the Southern Alps may have slip rates about twice that of the geological estimates. Up to 5 mm yr −1 of active deformation on faults distributed within the Southern Alps <100 km to the east of the Alpine Fault is possible. The role of tectonic block rotations in partitioning plate boundary deformation is less pronounced in the South Island compared to the North Island. Vertical axis rotation rates of tectonic blocks in the South Island are similar to that of the Pacific Plate, suggesting that edge forces dominate the block kinematics there. The southward migrating Chatham Rise exerts a major influence on the evolution of the New Zealand plate boundary; we discuss a model for the development of the Marlborough fault system and Hikurangi subduction zone in the context of this migration.

Long-Term Perspectives on Giant Earthquakes and Tsunamis at Subduction

Abstract: Histories of earthquakes and tsunamis, inferred from geological evidence, aid in anticipating future catastrophes. This natural warning system now influences building codes and tsunami planning in the United States, Canada, and Japan, particularly where geology demonstrates the past occurrence of earthquakes and tsunamis larger than those known from written and instrumental records. Under favorable circumstances, paleoseismology can thus provide long-term advisories of unusually large tsunamis. The extraordinary Indian Ocean tsunami of 2004 resulted from a fault rupture more than 1000 km in length that included and dwarfed fault patches that had broken historically during lesser shocks. Such variation in rupture mode, known from written history at a few subduction zones, is also characteristic of earthquake histories inferred from geology on the Pacific Rim.

Interpretation of landslide distribution triggered by the 2005 Northern Pakistan earthquake using SPOT 5 imagery

Abstract: The 2005 northern Pakistan earthquake (magnitude 7.6) of 8 October 2005 occurred in the northwestern part of the Himalayas. We interpreted landslides triggered by the earthquake using black-and-white 2.5-m-resolution System Pour l’Observation de la Terre 5 (SPOT 5) stereo images. As a result, the counts of 2,424 landslides were identified in the study area of 55 by 51 km. About 79% or 1,925 of the landslides were small (less than 0.5 ha in area), whereas 207 of the landslides (about 9%) were large (1 ha and more in area). Judging from our field survey, most of the small landslides are shallow rock falls and slides. However, the resolution and whitish image in the photos prevented interpreting the movement type and geomorphologic features of the landslide sites in detail. It is known that this earthquake took place along preexisting active reverse faults. The landslide distribution was mapped and superimposed on the crustal deformation detected by the environmental satellite/synthetic aperture radar (SAR) data, active faults map, geological map, and shuttle radar topography mission data. The landslide distribution showed the following characteristics: (1) Most of the landslides occurred on the hanging-wall side of the Balakot–Garhi fault; (2) greater than one third of the landslides occurred within 1 km from the active fault; (3) the greatest number of landslides (1,147 counts), landslide density (3.2 counts/km2), and landslide area ratio (2.3 ha/km2) was found within Miocene sandstone and siltstone, Precambrian schist and quartzite, and Eocene and Paleocene limestone and shale, respectively; (4) there was a slight trend that large landslides occurred on vertically convex slopes rather than on concave slopes; furthermore, large landslides occurred on steeper (30° and more) slopes than on gentler slopes; (5) many large landslides occurred on slopes facing S and SW directions, which is consistent with SAR-detected horizontal dominant direction of crustal deformation on the hanging wall.

Late Quaternary and present-day rates of slip along the Altyn Tagh Fault, northern margin of the Tibetan Plateau

Abstract: [1] Both Global Positioning System (GPS) measurements and studies of Late Quaternary faulting are consistent with a slip rate of ∼10 mm/yr along the central segment of the Altyn Tagh Fault and a systematic decrease in that rate toward the eastern end of the fault. Dates of terraces above and below laterally offset terrace risers yield bounds on Quaternary slip rates that range from those that agree with GPS measurements to values as much as three times faster. We argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace. In some cases, valleys upstream of the fault have been incised into bedrock, and few if any terrace risers can be seen within the valleys. Such streams debouch onto alluviated floodplains or fans that become incised, presumably during climate changes, to create terrace risers. The terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault, and thus the risers become protected from further incision. In such cases, dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces. Such dates yield slip rates of ∼10 mm/yr in the central segment of the fault and decreasing rates eastward. Although we cannot with certainty rule out the higher slip rates along the Altyn Tagh Fault, our analysis does show that viable interpretations consistent with GPS measurements are more likely, at least along some segments of the fault. Not only do these rates support the view that the Tibetan Plateau deforms internally by slip on a distributed network of faults in the shallow brittle crust, and hence behaves as a continuum at depth, but the gradual decrease toward the east also shows that the Altyn Tagh Fault does not separate two effectively rigid lithospheric plates. Correspondingly, the relatively low slip rate and the eastward decrease in slip rate suggest that the Altyn Tagh Fault does not transfer a significant portion of the convergence between India and Asia into northeastward extrusion of the Tibetan Plateau. Thus, large-scale extrusion of crustal material in India's path into Eurasia seems to be limited largely to the confines of the Tibetan Plateau.

Oceanic detachment faults focus very large volumes of black smoker fluids

Abstract: It is generally assumed that the seawater-derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of 1–3 km. We present new oxygen and strontium isotope data that show that fluids at black smoker temperatures of 300–400 °C were focused along a low-angle detachment fault at 15°45'N near the Mid-Atlantic Ridge. Isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures, indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system. Rare earth element mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids, not conductively heated seawater. The fault zone protolith was predominantly ultramafic, but also included mafic rocks, with metasomatic alteration to talc-tremolite-chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow. Fluids in equilibrium with this assemblage would be similar to ultramafic-hosted black smoker fluids. We present a new model in which hydrothermal circulation around detachment faults evolves from basalt hosted (TAG type), to footwall ultramafic hosted (Rainbow type), to low-temperature ultramafic hosted (Lost City type). Key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation, and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis.

Tectonic inheritance and continental rift architecture: Numerical and analogue models of the East African Rift system

Abstract: [1] The western branch of the East African Rift is composed of an arcuate succession of elongate asymmetric basins, which differ in terms of interaction geometry, fault architecture and kinematics, and patterns of uplift/subsidence and erosion/sedimentation. The basins are located within Proterozoic mobile belts at the edge of the strong Tanzanian craton; surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture, although other processes have been proposed. In this study, we use lithosphere-scale numerical models and crustal-scale analogue experiments to shed light on the relations between preexisting structures and rift architecture. Results illustrate that on a regional scale, rift localization within the mobile belts at the curved craton's western border results in an arcuate rift system, which implies that under a constant extensional stress field, part of the western branch experienced orthogonal extension and part oblique extension. Largest depocenters are predicted to form mostly orthogonal to the extension direction, and smaller depocenters will form along the oblique parts of the rift. The varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry, segmentation characteristics, and border fault architecture (trend, length, and kinematics). Analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension-related faults at a local scale. Models support that fabric reactivation is responsible for the oblique-slip kinematics on faults and for the development of Z-shaped or arcuate normal faults typically documented in nature.

Tectonic evolution of the active Hikurangi subduction margin, New Zealand, since the Oligocene

Abstract: [1] Deformation across the active Hikurangi subduction margin, New Zealand, including shortening, extension, vertical-axis rotations, and strike-slip faulting in the upper plate, has been estimated for the last ∼24 Myr using margin-normal seismic reflection lines and cross sections, strike-slip fault displacements, paleomagnetic declinations, bending of Mesozoic terranes, and seafloor spreading information. Post-Oligocene shortening in the upper plate increased southward, reaching a maximum rate of 3–8 mm/year in the southern North Island. Upper plate shortening is a small proportion of the rate of plate convergence, most of which (>80%) accrued on the subduction thrust. The uniformity of these shortening rates is consistent with the near-constant rate of displacement transfer (averaged over ≥5 Myr) from the subduction thrust into the upper plate. In contrast, the rates of clockwise vertical-axis rotations of the eastern Hikurangi Margin were temporally variable, with ∼3°/Myr since 10 Ma and ∼0°–1°/Myr prior to 10 Ma. Post 10 Ma, the rates of rotation decreased westward from the subduction thrust, which resulted in the bending of the North Island about an axis at the southern termination of subduction. With rotation of the margin and southward migration of the Pacific Plate Euler poles, the component of the margin-parallel relative plate motion increased to the present. Plate convergence dominated the Hikurangi Margin before ca. 15 Ma, with the rate of margin-parallel motion increasing markedly since 10 Ma. Vertical-axis rotations could accommodate all margin-parallel motion before 1–2 Ma, eliminating the requirement for large strike-slip displacements (for example, >50 km) in the upper plate since the Oligocene.

Late Paleozoic tectonic history of the Ertix Fault in the Chinese Altai and its implications for the development of the Central Asian Orogenic System

Abstract: The Central Asian Orogenic System (CAOS) is one of the largest Phanerozoic accretionary orogens in the world and may represent a signifi cant site of continental growth. Its origin has been explained by two competing models: syn-subduction strikeslip duplication of a single (>1000 km) long-lived arc (ca. 630‐360 Ma) or collision of multiple arcs and micro-continents. Central to the debate are the relative roles of syn-subduction strike-slip faulting versus thrusting. In both models, the Ertix fault fi gures prominently, either as a roof fault of a large strike-slip duplex system developed during oceanic subduction or as a suture of arc-continent or continent-continent collision. In order to differentiate between the above models, we conducted fi eld mapping, detailed kinematic analysis, and geochronological dating of the Ertix fault zone in the Chinese Altai. Our work indicates that the fault is a crustal-scale thrust that was active in the Permian. Its hanging wall records two pulses of magmatism ca. 450 Ma and ca. 280 Ma and experienced peak pressure and temperature of 6.2‐7.7 kbar and 560‐ 670 °C. Our geologic observations, together with the existing geologic information, favor a tectonic model that involves two episodes of subduction below the Altai arc: fi rst, in the Ordovician, along a south-dipping subduction zone; and second, in the late Carboniferous and early Permian along northdipping subduction of the Junggar ocean. It was during the latter event that a melange complex was underplated below the older Ordovician arc, metamorphosed at lower crustal depths, and then exhumed to the upper crust along the south-directed Ertix thrust zone.

Cenozoic evolution of the eastern Pamir: Implications for strain-accommodation mechanisms at the western end of the Himalayan-Tibetan orogen

Abstract: Detailed field mapping, geochronologic and thermochronologic analyses, and petrologic investigations conducted along the southern segment of the late Cenozoic Kongur Shan extensional system provide new information on the Cenozoic tectonic evolution of the eastern Pamir at the western end of the Himalayan-Tibetan orogen. Field relations and cooling-age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east-west extension along the southern Kongur Shan extensional system, from 20 km or less along the Muztaghata massif in the north, to l3 km along the Tashkorgan fault in the south. These results, in conjunction with previously published work on the northern segment of the Kongur Shan extensional system, show a general southward decrease in east-west extension along the entire length of the extensional system, consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the Pamir. Petrologic data, 40 Ar/ 39 Ar cooling ages, and monazite Th-Pb ages from schists and gneisses in the footwall of the southern Kongur Shan normal fault along the Muztaghata massif record two tectonic events that immediately preceded late Miocene initiation of east-west extension: (1) high-grade schists and gneisses experienced upper amphibolite facies metamorphic conditions (9–10 kbar, 700–750 °C) dated as late Oligocene to middle Miocene by in situ ion-microprobe analyses of monazite inclusions in garnet; and (2) high-grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late Miocene with 40 Ar/ 39 Ar biotite cooling ages of 7.5–9 Ma. Rapid exhumation was accommodated in part by the east-west–striking, south-dipping, Shenti normal fault. Field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the Central Pamir gneiss domes, and the antiformal Muztaghata massif is the eastward continuation of the Sares dome of the Central Pamir. These observations suggest that the antiformal gneiss domes of the Central Pamir have not been offset across the Karakorum right-slip fault from the Qiangtang anticlinorium in Tibet. Instead, we propose that the development of the Central Pamir gneiss domes may have been related to Oligocene-Miocene northward underthrusting and thickening of crust beneath the Pamir.

Architecture and mechanics of an active low‐angle normal fault: Alto Tiberina Fault, northern Apennines, Italy

Abstract: [1] We present seismological evidence for the existence of an actively slipping low-angle normal fault (15° dip) located in the northern Apennines of Italy. During a temporary seismic experiment, we recorded ∼2000 earthquakes with ML ≥ 3.1. The microseismicity defines a 500 to 1000 m thick fault zone that crosscuts the upper crust from 4 km down to 16 km depth. The fault coincides with the geometry and location of the Alto Tiberina Fault (ATF) as derived from geological observations and interpretation of depth-converted seismic reflection profiles. In the ATF hanging wall the seismicity distribution highlights minor synthetic and antithetic normal faults (4–5 km long) that sole into the detachment. The ATF-related seismicity shows a nearly constant rate of earthquake production, ∼3 events per day (ML ≤ 2.3), and a higher b value (1.06) with respect to the fault hanging wall (0.85) which is characterized by a higher rate of seismicity. In the ATF zone we also observe the presence of clusters of earthquakes occurring with relatively short time delays and rupturing the same fault patch. To explain movements on the ATF, oriented at high angles (∼75°) to the maximum vertical principal stress, we suggest an interpretative model in which crustal extension along the fault is mostly accommodated by aseismic slip in velocity strengthening areas while microearthquakes occur in velocity weakening patches. We propose that these short-lived frictional instabilities are triggered by fluid overpressures related to the buildup of CO2-rich fluids as documented by boreholes in the footwall of the ATF.

Fault growth at a nascent slow-spreading ridge: 2005 Dabbahu rifting episode, Afar

Abstract: SUMMARY We present a preliminary account of the near-field surface strain associated with a major magmatic rifting episode at a nascent slow spreading ridge in the Afar depression. Between 2005 September 14 and October 4, a volcanic eruption and 163 earthquakes (mb > 3.9), including seismic tremor, occurred within the ∼60-km-long Dabbahu magmatic segment. Results of the early response team demonstrated that ground deformation, derived from satellite radar data (InSAR), together with seismicity, is consistent with dyke-induced deformation along the entire length of the segment. We document the distribution of brittle strain associated with the early part of this rifting cycle to verify the predicted pattern of deformation and constrain a conceptual model for normal fault growth in Afar, with general application to other slow spreading divergent margins. Our field investigations concentrate on the northern half of the segment, which ruptured through to the surface over a length of >30 km and a width of ∼5 km, consistent with the pattern of microseismicity recorded using a network deployed ∼1 month after the initial onset of the rifting episode on September 14. Severe ground shaking during the event was more widespread; fresh rock fall is common across the entire magmatic segment, particularly at the intersections between faults. Recent ground breaks, in the form of reactivated or newly initiated normal faults and fissures, opened with horizontal displacements up to 3 m and vertical displacements locally up to 5 m, but commonly ∼2 m. These structures are generally subvertical and open along pre-existing cooling joints. Fault offset is greater than expected given the magnitude of earthquakes during the episode. The axial relief that developed consequent on fault and fissure initiation and reactivation during the 2005 Dabbahu episode is consistent with that of the entire magmatic segment. We therefore suggest that melt delivery is sufficiently frequent that favourable stress conditions for faulting are primarily achieved during dyke events.

Extreme dynamic weakening of faults during dehydration by coseismic shear heating

Abstract: The dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release. High velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0.6 to 0.15 as the slip velocity reaches 1.1 m/s at normal stresses up to 24.5 MPa. The extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding. The rapid heating at asperities causes serpentine dehydration. In impermeable fault zones in nature, serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening. This dynamic fault-weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the San Andreas.

Co-Registration of Optically Sensed Images and Correlation (COSI-Corr): an operational methodology for ground deformation measurements

Abstract: Recent methodological progress, co-registration of optically sensed images and correlation, outlined here, makes it possible to measure horizontal ground deformation from optical images on an operational basis, using the COSI-Corr software package. In particular, its sub-pixel capabilities allow for accurate mapping of surface ruptures and measurement of co-seismic offsets. We retrieved the fault rupture of the 2005 Mw 7.6 Kashmir earthquake from ASTER images, and we also present a dense mapping of the 1992 Mw 7.3 Landers earthquake of California, from the mosaicking of 30 pairs of aerial images.

Physical properties of carbonate fault rocks, fucino basin (Central Italy): implications for fault seal in platform carbonates

Abstract: We documented the porosity, permeability, pore geometry, pore type, textural anisotropy, and capillary pressure of carbonate rock samples collected along basin-bounding normal faults in central Italy. The study samples consist of one Mesozoic platform carbonate host rock with low porosity and permeability, four fractured host rocks of the damage zones, and four fault rocks of the fault cores. The four fractured samples have high secondary porosity, due to elongated, connected, soft pores that provide fluid pathways in the damage zone. We modeled this zone as an elastic cracked medium, and used the Budiansky–O'Connell correlation to compute its permeability from the measured elastic moduli. This correlation can be applied only to fractured rocks with large secondary porosity and high-aspect ratio pores. The four fault rock samples are made up of survivor clasts embedded in fine carbonate matrices and cements with sub-spherical, stiff pores. The low porosity and permeability of these rocks, and their high values of capillary pressure, are consistent with the fault core sealing as much as 77 and 140 m of gas and oil columns, respectively. We modeled the fault core as a granular medium, and used the Kozeny–Carmen correlation, assigning the value of 5 to the Kozeny constant, to compute its permeability from the measured porosities and pore radii. The permeability structure of the normal faults is composed of two main units with unique hydraulic characteristics: a granular fault core that acts as a seal to cross-fault fluid flow, and an elastic cracked damage zone that surrounds the core and forms a conduit for fluid flow. Transient pathways for along-fault fluid flow may form in the fault core during seismic faulting due to the formation of opening-mode fractures within the cemented fault rocks.

Channel flow and the Himalayan–Tibetan orogen: a critical review

Abstract: The movement of a low-viscosity crustal layer in response to topographic loading provides a potential mechanism for (1) eastward flow of the Asian lower crust causing the peripheral growth of the Tibetan Plateau and (2) southward flow of the Indian middle crust to be extruded along the Himalayan topographic front. Thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface. Isotopic constraints on the timing of fault movement, anatexis and thermobarometric evolution of the exhumed garnet- to sillimanite-grade metasedimentary rocks support mid-crustal channel flow during the Early to Mid-Miocene. Exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface, defined by the South Tibetan Detachment System, and strain softening along the base, bounded by the Main Central Thrust. Neotectonic extrusion, bounded by brittle Quaternary faults south of the Main Central Thrust, is positively correlated with the spatial distribution of precipitation across a north–south transect, suggesting climate–tectonic linkage over a million-year time scale. A proposed orogen-wide eastward increase in extrusion rate over 20 Ma reflects current precipitation patterns but climate–tectonic linkage over this time scale remains equivocal.