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We examine three-dimensional seismic data from the Santos Basin, offshore Brazil, to determine how, where, and when radial faults grow near a salt diapir. We show roof stretching alone cannot account for the large heights and lengths of the kilometer-scale radial faults, suggesting stock widening ('stem push'), a mechanism implied in numerical models but not yet recognized in natural examples, played a pivotal role in fault formation. We suggest that, when a diapir is covered by a roof, radial faults form due to roof stretching, extending no further than the limit of the drape folding. The roof may then be shouldered aside and the faults buried along the stock flanks, exposing these strata to stem push-related stresses that may then re-activate preexisting, or form new, radial faults. We suggest the causal mechanism for radial fault formation will likely change as roof thickness varies during diapirism, with this reflecting the ratio between sedimentation rate and salt volumetric flux. [ABSTRACT FROM AUTHOR]

Initiation of subduction is rarely encountered in modern tectonic environments due to its ephemeral and destructive nature. We report the geological and geophysical evidence indicating a transitional phase from buckling to embryonic subduction along the eastern Korean margin. The transition appears to be caused by compressional reactivation of the strike-slip boundary between the continental (Korean Peninsula) and oceanic (Ulleung Basin) crusts since the Early Pliocene. Evidence for compressional reactivation includes (1) a west-dipping major thrust and coincident crustal buckling of the Ulleung Basin; (2) an east-west structural asymmetry inferred from the gravity anomaly and P-wave tomography; and (3) ongoing crustal uplift and high-angle faults along the eastern Korean margin. The juxtaposition of underthrusting and buckling of the crust in the Ulleung Basin, and its associated ubiquitous reverse faulting on the eastern Korean margin, imply the potential development of a new subduction system along the western margin of the East Sea (Japan Sea). We propose that the East Sea comprises two incipient subduction margins (i.e., the Korean and Japanese sides), which are now competing to reach a self-sustaining subduction stage. [ABSTRACT FROM AUTHOR]

Where short-term (10²-104 yr) tectonic deformation rates derived from paleoseismology and tectonic geomorphology differ from long-term (105-106 yr) rates derived from geologic and thermochronometric methods, opportunities arise to assess the causes of spatio-temporal variations in fault slip rates. Such discrepancies are germane to understanding seismic hazard and fault interactions in regions such as southern California, USA, that are underlain by complex fault systems prone to large earthquakes on blind thrust faults. We present thermochronometric results from two transects in the hanging wall of the Malibu Coast fault in the Santa Monica Mountains, on which limited evidence for recent earthquakes has been found. Apatite (U-Th-Sm)/He ages from these transects range from late Miocene to Pleistocene but are dominantly Pliocene. Zircon (U-Th)/He ages from Cretaceous-Paleocene strata are partially reset. Inverse thermal modeling of these data indicates that the most deeply buried samples reached peak temperatures of ~150 °C (~5 km depth) at 5-7 Ma, and were then exhumed at rates of ~1 mm/yr. These long-term exhumation rates are faster than short-term fault slip rates on which current hazard assessments are based. [ABSTRACT FROM AUTHOR]

The Weber Deep--a 7.2-km-deep forearc basin within the tightly curved Banda arc of eastern Indonesia--is the deepest point of the Earth's oceans not within a trench. Several models have been proposed to explain the tectonic evolution of the Banda arc in the context of the ongoing (ca. 23 Ma-present) Australia-Southeast Asia collision, but no model explicitly accounts for how the Weber Deep achieved its anomalous depth. Here we propose that the Weber Deep formed by forearc extension driven by eastward subduction rollback. Substantial lithospheric extension in the upper plate was accommodated by a major, previously unidentified, low-angle normal fault system we name the "Banda detachment." High-resolution bathymetry data reveal that the Banda detachment is exposed underwater over much of its 120 km down-dip and 450 km lateral extent, having produced the largest bathymetric expression of any fault discernable in the world's oceans. The Banda arc is a modern analogue for highly extended terranes preserved in the many regions that may similarly have "rolled open" behind migrating subduction zones. [ABSTRACT FROM AUTHOR]

Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 Mw 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a décollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide. [ABSTRACT FROM AUTHOR]

At the field scale, nearly all fault surfaces contain grooves generated as one side of the fault slips past the other. Grooves are so common that they are one of the key indicators of principal slip surfaces. Here, we show that at sufficiently small scales, grooves do not exist on fault surfaces. A transition to isotropic roughness occurs at 4-500 mm. Although the scale of the transition can vary even between locales on a single fault, the aspect ratio of the roughness at the transition is well defined for a given fault. We interpret the transition between grooved and ungrooved scales as a transition in deformation mode of asperities on the slip surface. Grooves can form when a hard indenter slides past a softer surface. At small scales, the asperities appear to yield plastically and therefore do not generate grooves as hard indenters. The plastic yielding can be a consequence of the high shear strains required to deform the surfaces at small scales where the aspect ratio (roughness) is high. The transition to plastic yielding is predicted to occur at a specific aspect ratio for each fault, as observed. The new observation both shows a limit to one of the most commonly observed features of faults and suggests a change in the mode of failure of faults as a function of scale. [ABSTRACT FROM AUTHOR]

The ongoing collision of India with Asia is partly accommodated by slip on the Main Himalayan Thrust (MHT). The 25 April 2015, Mw 7.8 Gorkha earthquake is the most recent major event to rupture the MHT, which dips gently northward beneath central Nepal. Although the geology of the range has been studied for decades, fundamental aspects of its deep structure remain disputed. Here, we develop a structural cross section and a three-dimensional, geologically informed model of the MHT that are consistent with seismic observations from the Gorkha earthquake. A comparison of our model to a detailed slip inversion data set shows that the slip patch closely matches an ovalshaped, gently dipping fault surface bounded on all sides by steeper ramps. The Gorkha earthquake rupture seems to have been limited by the geometry of that fault segment. This is a significant step forward in understanding the deep geometry of the MHT and its effect on earthquake nucleation and propagation. Published models of fault locking do not correlate with the slip patch or our fault model in the vicinity of the earthquake, further suggesting that fault geometry was the primary control on this event. Our result emphasizes the importance of adequately constraining subsurface fault geometry in megathrusts in order to better assess the sizes and locations of future earthquakes. [ABSTRACT FROM AUTHOR]

A strike-slip fault is present outboard and subparallel to the Wassuk Range front within the central Walker Lane (Nevada, USA). Recessional shorelines of pluvial Lake Lahontan that reached its highstand ca. 15,475 ± 720 cal. yr B.P. are displaced ~14 m and yield a right-lateral slip-rate estimate approaching 1 mm/yr. The strike-slip fault trace projects southeastward toward the eastern margin of Walker Lake, which is ~15 km to the southeast. The trace is obscured in this region by recessional shorelines features that record the historical dessication of the lake caused by upstream water diversion and consumption. High-resolution seismic CHIRP (compressed high intensity radar pulse) profiles acquired in Walker Lake reveal ~20 k.y. of stratigraphy that is tilted westward ~20-30 m to the Wassuk Range front, consistent with ~1.0-1.5 mm/yr (20-30 m/20 k.y.) of vertical displacement on the main rangebounding normal fault. Direct evidence of the northwest-trending right-lateral strikeslip fault is not observed, although a set of folds and faults trending N35°E, conjugate to the trend of the strike-slip fault observed to the north, is superimposed on the west-dipping strata. The pattern and trend of folding and faulting beneath the lake are not simply explained; they may record development of Riedel shears in a zone of northwest-directed strike slip. Regardless of their genesis, the faults and folds appear to have been inactive during the past ~10.5 k.y. These observations begin to reconcile what was a mismatch between geodetically predicted deformation rates and geological fault slip rate studies along the Wassuk Range front, and provide another example of strain partitioning between predominantly normal and strikeslip faults that occurs in regions of oblique extension such as the Walker Lane. [ABSTRACT FROM AUTHOR]

Stress field perturbations are known to follow earthquakes, but the length scales over which the stress field varies are still poorly understood. Most focal mechanism inversions are assumed to represent the bulk regional stresses, yet fault irregularity and observed small-scale stress changes in boreholes suggest a strong degree of local heterogeneity. We present geologic paleostress inversion results from a segment of the folded West Salton detachment fault located in a left stepover in the San Felipe dextral fault zone of Southern California (United States). Extension and dextral slip in this region were synchronous during latest detachment slip. The local (1-10 km scale) vertical stress field alternated from G1 during extension to σ3 during flex-slip folding of the detachment in a restraining bend of the San Felipe fault zone (where σ1, σ2, and σ3 are the maximum, intermediate, and minimum principal stress directions, respectively). Regionally (10-100 km scale), we infer that the vertical stress alternates from σ1 during extension to σ2 during dextral slip. Both are consistent with nearly complete earthquake stress drops. If the fault-slip data set records paleoseismic strains, the magnitudes of differential stress and stress drop must be similar. Both may be relatively small during tectonic transitions. [ABSTRACT FROM AUTHOR]

We compare the spatiotemporal progression, geometry, and earthquake source characteristics of a zone of anomalous swarm seismicity between the Maacama and Bartlett Springs faults (California, United States) within the northern San Andreas fault system to both laboratory studies of fracture initiation and structural field observations of fault formation. The similarities between laboratory and field studies of incipient faulting and the earthquake swarms suggest that the seismic lineament on which the swarms occur is a fault in an early stage of development. Kinematic descriptions of faulting and models of fault system development suggest that the ability of existing faults to accommodate deformation across plate boundaries is governed by the length scales over which they accommodate stress. Many of the characteristics of juvenile fault zones, such as segmentation, geometric complexity, and the depth extent of faulting, act to reduce this length scale; this requires the reactivation of existing faults or production of accessory faults to accommodate deformation across the plate boundary. [ABSTRACT FROM AUTHOR]

Late Cenozoic tectonic reconstructions of southern California (United States) have been hindered for decades by disagreement over the total displacement on the southern San Andreas fault; disparate estimates of dextral offset range from 160 km to 240 km. Prior estimates were based on purely translational models that neglect the effects of transrotational strain on the orientation and distribution of key geologic markers. We present a new reconstruction that integrates published lithologic, structural, and paleocurrent data with the effects of clockwise rotation in the eastern Transverse Ranges (ETR). This model yields 200 ± 14 km of cumulative offset on the southern San Andreas fault system in the Salton Trough. The disparity between our estimate and a lower estimate of 160 km for the Mojave segment can be explained by a combination of transtensional elongation along the southwest boundary of the ETR, transfer of strain to the eastern California shear zone, and diffuse crustal shortening in the Big Bend region. This reconstruction reconciles previously incompatible and long-debated models, and places an important new constraint on the tectonic evolution of the Pacific-North America plate boundary in southern California. [ABSTRACT FROM AUTHOR]

High-resolution lidar data reveal a prominent latest Pleistocene-Holocene scarp on the Sawtooth fault (central Idaho, United States). The fault scarp marks 55-65 km of the range front, and may comprise two segments. The scarp is 4-9 m high in latest Pleistocene glacial landforms (11-14 ka) and 2-3 m high in Holocene alluvial landforms, implying 2-3 postglacial rupture events. Patterns of fault scarp continuity, coupled with existing gravity data, suggest that active faulting may have migrated northward during Pleistocene time. Detailed comparisons of raw lidar digital elevation models (DEMs), bare-earth lidar DEMs, and field surveys indicate that the bare-earth lidar data document the fault scarp morphology accurately and allow for detailed fault analysis where field evaluation is difficult. The documentation of Holocene motion on the Sawtooth fault demonstrates that ENE-directed extension extends across central Idaho, and that the fault contributes to seismic hazards. [ABSTRACT FROM AUTHOR]

Direct dating of brittle fault activity is of fundamental importance to tectonic reconstructions and paleoseismic studies. One way to address this issue is by constraining the timing of fault striations, but this requires a better understanding of the striation formation mechanism and associated mineralization. We present results from a microstructural, geochemical, and geochronological study of calcite precipitates associated with striated fault planes from the Dead Sea fault zone in northern Israel. We recognize four types of coexisting calcite precipitates, including calcite cement in dilation breccia, calcite in striated groove morphology, calcite gouge associated with hydraulic fracturing and pressure solution, and calcite coating of the fault surface. Carbon-oxygen isotopes, 87Sr/86Sr ratios, and rare earth element and yttrium (REY) patterns indicate various precipitation mechanisms associated with formation of syntectonic (calcite cement and striations), coseismic (calcite gouge), and interseismic (calcite coating) precipitates in the fault zone. Using U-Th dating of samples from three adjacent fault planes, we delineate four well-defined deformation ages in the period from 220 to 60 ka. We conclude that these ages constrain the timing of activity along the Dead Sea fault zone in northern Israel, and argue that a similar methodological approach could potentially shed light on the timing of deformation in other brittle fault zones. [ABSTRACT FROM AUTHOR]

The question of whether coseismic ruptures along megasplay faults in accretionary prisms (i.e., large landward-dipping thrust faults branching from the plate boundary) reach the seafloor is critical for assessing the risk of tsunami disaster. However, samples from active megasplay faults have not previously been available. Here we present geochemical and mineralogical data of megasplay fault samples obtained from the shallow (<300 m below seafloor) portion of the megasplay fault that coincides with the rupture area of the A.D. 1944 Tonankai earthquake in the Nankai Trough. The megasplay fault zone is characterized by localized shear zones of brecciated host rocks. A prominent slip zone, here termed "dark gouge," was discovered within one of the shear zones. Compared to the surrounding breccias, the dark gouge is chemically enriched in Al and K, and depleted in Ca and Sr. It is also characterized by higher illite content in illite-smectite mixed-layer clays. These chemical and mineralogical features may reflect a transformation in clay mineralogy caused by frictional heating, and suggest that the seismic slip can propagate to very shallow levels along megasplay fault systems. [ABSTRACT FROM AUTHOR]

Seamount subduction is a common process in subduction zone tectonics. Contradicting a widely held expectation that subducting seamounts generate large earthquakes, seamounts subduct largely aseismically, producing numerous small earthquakes. On rare occasions when they do produce relatively large events, the ruptures tend to be complex, suggesting multiple rupture patches or faults. We explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault. The complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures. [ABSTRACT FROM AUTHOR]

The diffuse deformation zone in the central Indian Ocean is the classical example of distributed deformation of the oceanic lithosphere, where shortening between the Indian and Capricorn plates is manifest as reverse faulting (5-10-km-spaced faults) and long-wavelength (100-300 km) folding. The onset of this deformation is commonly regarded as a key far-field indicator for the start of major uplift of the Himalayas and Tibet, some 4000 km further to the north, due to increased deviatoric stresses within the wider India-Asia area. There has been disagreement concerning the likely timing for the onset of deformation between plate-motion inversions and seismic reflection-based studies. In the present study, fault displacement data from seismic-reflection profiles within the central Indian Ocean demonstrate that compressional activity started much earlier than previously thought, at around 15.4-13.9 Ma. From reconstructions of fault activity histories, we shows that 12% of the total reverse fault population had been activated, and 14% of the total strain accumulated, prior to a sharp increase in the deformation rate at 8.0-7.5 Ma. There is no evidence for any regional unconformity before 8.0-7.5 Ma; early shortening was accommodated by activity on single isolated fault blocks. Total strain estimates derived are more variable and complex than those predicted from plate inversion, and they do not show simple west to east increase. [ABSTRACT FROM AUTHOR]

Structural, geomorphic, and interferometric synthetic aperture radar (InSAR) surface displacement data show that uplift of the northernmost onshore Scandinavian passive margin is associated with active normal faulting and anomalous clusters of landslides. Regional-scale normal faults separate alpine footwall ranges with impressive range-front scarps from more gentle hanging-wall topography. In the Lyngen region, rockslides, sackung features (i.e., deep-seated slope gravitational deformation), and active faults cluster inside an ~1000 km2 area of hanging-wall subsidence parallel to an ~30-km-long, faceted range-front scarp. From postglacial to recent time, rockslides have been released along the main range front and other propagating scarps, both in the footwall range and in structurally controlled, glacially oversteepened valleys of the hanging wall. The active rockslides, moving at rates of >10 mm/yr, show a variety of relationships to the controlling structures, reflecting tectonic as well as gravitational faulting. Recent and present-day landscape evolution and large-volume rockslide distribution in Scandinavia should be viewed from the perspective of tectonic processes involving active normal faults. [ABSTRACT FROM AUTHOR]

Stresses released by coseismic faults during subduction toward lawsonite-eclogite facies conditions in the Alpine subduction complex of Corsica can be estimated based on the energy required to form pseudotachylyte fault veins where shear strain can be measured . Congruent peridotite melting at ambient conditions of 1.5 GPa and 470 °C requires a temperature increase of 1280 °C to 1750 °C. We assume that more than 95% of the work is converted to heat during faulting, hence that the stress drop is nearly proportional to the amount of melting and inversely proportional to shear strain. Minimum estimates of released stress are typically greater than 220 MPa and as high as 580 MPa. The abundance of pseudotachylyte on small faults in the studied peridotite suggests that melting is very common on intermediate and deep earthquakes and that shear heating is important for seismic faulting at depth. [ABSTRACT FROM AUTHOR]

In the Miocene, the European Eastern Alps extruded laterally along orogen-scale strikeslip faults due to both extensional gravitational collapse and compressional tectonic forcing. Horizontal extension is most prominently evidenced by detachments east and west of the Tauern Window; it is commonly explained by a retreating slab beneath the Carpathian arc hundreds of kilometers east of the orogen. Horizontal compression is shown by north-south shortening in the Tauern Window and the entire Eastern Alps in response to the convergence of the Adriatic plate with Europe. It is interesting that analogue and numerical models for the Eastern Alps designed to describe the east-directed lateral extrusion have failed to explain the extensional regime in the region of the Tauern Window. Using a numerical model for planview deformation that considers internal faults, we show here that orogen-scale strike-slip faults are mechanically required to cause extension during plate convergence in the Miocene Eastern Alps. We test the idea by coupling this model with a landscape evolution model and by comparing modeled and observed drainage system geometries. [ABSTRACT FROM AUTHOR]

x The manner in which calderas develop is a key question involcanology and has important implications for associated volcanogenic risk and geothermal and ore exploitation. To better understand the structural evolution of calderas, I use a discrete element model of a frictional cover undergoing piston-like subsidence at its base, simulating magma chamber deflation and cover collapse. These novel simulations capture not only the initiation of calderas but also much of the complexity of faulting during their later development. In all models, both normal and reverse faults accommodate deeper subsidence at higher structural levels. Curved, outward-dipping reverse faults are consistently the first structures to develop; subsequent caldera growth is mainly the result of movement on vertical or steeply inward-dipping normal faults. This may be followed by a later phase of lateral collapse on shallower normal faults. The roof aspect ratio (R = cover thickness/piston width) is seen to be an important factor: calderas with low aspect ratios (R≤1) exhibit a coherent central subsiding block, while those with higher aspect ratios have a complexly faulted internal structure. [ABSTRACT FROM AUTHOR]

A clay- or shale-rich fault gouge can significantly reduce fault permeability. Therefore, predictions of the volume of clay or shale that may be smeared along a fault trace are important for estimating the fluid connectivity of groundwater and hydrocarbon reservoir systems. Here, we show how fault smears develop spontaneously in layered soil systems with varying friction coefficients, and we present a quantitative dynamic model for such behavior. The model is based on Mohr-Coulomb failure theory, and using discrete element computations, we demonstrate how the model framework can predict the fault smear potential from soil friction angles and layer thicknesses. [ABSTRACT FROM AUTHOR]

We examine active deformation of the region surrounding the eastern South Caspian Sea using a combination of seismic, geological, geodetic, and geomorphological data. Global positioning system (GPS) velocities indicate a westward component of motion of the South Caspian basin, relative to Eurasia and central Iran. We identify major zones of shear (the Ashkabad and Shahrud fault zones) that accommodate this westward extrusion on either side of the South Caspian block. Estimates of total strike-slip motion could, at present-day slip rates derived from GPS observations, be achieved in ∼10 m.y. Therefore, the northwest extrusion of the South Caspian region, which is accommodated by subduction beneath the North Caspian region (stable Eurasia) along the Apsheron-Balkan sill, may also date from that time. This suggests that the onset of subduction may be older than previous estimates (ranging from 1.8 to 5.5 Ma). Our results are summarized in a new kinematic model that significantly clarifies the active tectonics of this complicated region. [ABSTRACT FROM AUTHOR]

The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the northwest margin of the Indo-Asian collision zone. To address the quantitative contribution of the earthquake to plate convergence, we performed paleoseismological trench excavations at Nisar Camp site near Muzaffarabad across the middle section of the 2005 surface rupture. The fault strands exposed in the trench cut late Holocene fluvial deposits and record evidence of both the 2005 and a penultimate event, supported by the presence of colluvial deposits and a downdip increase in displacement along the fault strands. The 2005 event produced a net slip of 5.4 m, and the penultimate earthquake exhibits a similar amount of slip. Radiocarbon ages and historical accounts loosely constrain the timing of the penultimate event between 500 and 2200 yr B.P.; however, the exposed section encompasses ~4 k.y. of stratigraphy, suggesting an average interevent interval of ~2 k.y. for the 2005 type events. We thus conclude that the 2005 event did not occur on the plate boundary megathrusts, but on intraplate active faults within the Sub-Himalaya. Consequently, the accumulated elastic strain around the complex northwestern margin of the Indo-Asian collision zone has not been significantly released by the 2005 earthquake. [ABSTRACT FROM AUTHOR]

Far-field compressional stresses resulting from arc collisions with the northeast coast of Africa-Arabia propagated across the African plate in the Santonian (ca. 84 Ma), and later similar pulses occurred through much of the Cenozoic. Waves of inversion-related features including folds, thrusts, and igneous activity developed during the compressional events. Structures generated were uneven in intensity and distribution, and were commonly concentrated in existing rifts. Because of the irregular shapes of the North African continental margin and associated inboard basins, Cyrenaica underwent the most severe shortening, although deformation extended eastward from Libya to the Levant. Areas south and southeast of Cyrenaica (the Sirt Basin and the far Western Desert of Egypt) underwent very little shortening because they occupied a regional strain shadow. The eastern region of the Western Desert and Sinai, which were not sheltered by the stress-absorbing Cyrenaica inversion, recorded strong compressional deformation synchronous with that in Cyrenaica. As closure of the Mediterranean continues, Cyrenaica will become an indenter comparable to those of other orogenic belts. The detailed tectonic evolution of individual northeast African basins has depended on their positions in relation to the Cyrenaican strain shadow. No single subsidence and paleostress history reflects the complete regional development of the converging Mediterranean-North African margin. [ABSTRACT FROM AUTHOR]

Intraplate orogeny remains an enigmatic process in terms of plate tectonics. Regional continental stress fields agree well with stress orientations predicted by the main forces driving and resisting plate motion. However, regional continental stress fields fail to explain major localized compressive plate deformation in continental interiors far away from plate boundaries. Global or plate-wide intraplate stress models typically assume a laterally homogeneous plate rheology and are confined to modeling the present, without addressing the antiquity of the current stress field. An understanding of intraplate stress and deformation requires the linking of spatial variations in continental rheology with time-dependent plate geometry and driving forces. Here we demonstrate how the complex interplay between juxtaposed weak and strong geological provinces and changes in far-field plate boundary forces has caused intraplate orogenesis and tectonic reactivation in southeastern Australia during the Tertiary. Our findings are contrary to the opinion that continental interiors are insensitive to compression at plate margins, and help to explain the mechanisms causing intraplate orogenesis. [ABSTRACT FROM AUTHOR]

Observations of precipitation fall speeds and precipitation patterns suggest that precipitation phase (rain versus snow) is a significant control on the relationship between precipitation patterns and topography, due to the potential for increased downwind advection of snow relative to rain. A coupled model of orographic precipitation and surface erosion shows that for a range of climate variables, steady-state precipitation patterns vary from nearly uniform and maximizing over the highest topography, to highly spatially variable, closely coupled to topography and reaching a maximum on low slopes. Precipitation patterns are a first-order control on modeled range scale and ridge-valley scale relief, channel concavity, and the position of the drainage divide. An association between cool climates, spatially uniform precipitation, and efficient erosion of high topography is indicated. The importance of precipitation phase to the evolution of precipitation patterns and topography further demonstrates the fundamental importance of the coupled climate, erosion, and tectonic system in the evolution of mountain topography. [ABSTRACT FROM AUTHOR]

After 120 years of unsuccessful endeavor, a paradigm shift is required before earthquakes can be predicted. The most sensitive diagnostic of low-level changes of stress in in situ rock, variations in microcrack geometry, can be monitored by analyzing shear-wave splitting. The suggested paradigm shift is that, instead of investigating the source zone, we monitor stress accumulation before earthquakes at, possibly, substantial distances from the source. Characteristic temporal variations of shear-wave time delays have been observed in retrospect before 14 earthquakes worldwide. On one occasion, when changes were recognized early enough, the time, magnitude, and fault break of an M = 5 earthquake in southwest Iceland were successfully stress-forecast in a narrow time-magnitude window. Such stress accumulation can be theoretically modeled and is believed to be at least partially understood. When sufficient shear-wave source earthquakes are available, increasing time delays also show an abrupt decrease shortly before the impending earthquake occurs. This is not fully understood but is thought to be caused by stress relaxation as microcracks coalesce onto the eventual fault break. The new result confirming these ideas, and justifying the paradigm shift, is that logarithms of the durations of both increases and decreases in time delays are found to be proportional (self-similar) to the magnitudes of impending earthquakes. [ABSTRACT FROM AUTHOR]

Pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip. We report from high-velocity friction experiments on siderite-bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte. Experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite, calcite, and quartz, initially at room temperature, under normal stresses of 0.6-1.3 MPa and at seismic slip rates of 1.3-2.0 m/s. In all cases, gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and CO2 gas, as confirmed by CO2 measurement, X-ray diffraction analyses, and transmission electron microscopy. The weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates. Magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude. Those changes can be recognized in natural fault zones, and black gouge in the Chelungpu fault zone in Taiwan is perhaps such an example. Thus our results suggest that thermal decomposition in shallow crustal faults can be an important co-seismic process not only for dynamic fault weakening, but also for leaving seismic slip records. [ABSTRACT FROM AUTHOR]

The strengths of mountain belts and major faults have been notoriously difficult to constrain and there is ongoing debate over the controlling mechanisms and stress magnitudes. Here we show that the strengths of active thrust-belt wedges and their basal detachments can be directly determined from the covariation of surface slope a with detachment dip β, without strong assumptions about the specific strength-controlling mechanisms. Even a single taper measurement (α,β) can strongly constrain the set of possible wedge and detachment strengths. This theory is tested with dry sand wedges and then applied to the Niger delta thrust belt, the active Taiwan mountain belt, and the thrust that slipped in the M = 7.6 Chi-Chi earthquake. Their basal detachments are shown to be exceedingly weak, with effective coefficients of friction (0.04-0.1) that are an order of magnitude less than most laboratory friction coefficients (0.6-0.85). In contrast, these wedges are moderately strong internally, within the range of pressure-dependent strengths in deep boreholes. These results confirm the existence of exceedingly weak faults and strong crust, which raises important causal questions. [ABSTRACT FROM AUTHOR]

The North Pacific Rim is a tectonically active plate boundary zone, parts of which may be characterized as a laterally moving orogenic stream. Crustal blocks are transported along large-magnitude strike-slip faults in western Canada and central Alaska toward the Aleutian-Bering Sea subduction zones. Throughout much of the Cenozoic, at and west of its Alaskan nexus, the North Pacific Rim orogenic Stream (NPRS) has undergone tectonic escape. During transport, relatively rigid blocks acquired paleomagnetic rotations and fault-juxtaposed boundaries while flowing differentially through the system, from their original point of accretion and entrainment toward the free face defined by the Aleutian-Bering Sea subduction zones. Built upon classical terrane tectonics, the NPRS model provides a new framework with which to view the mobilistic nature of the western North American plate boundary zone. [ABSTRACT FROM AUTHOR]

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 fl uid 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. [ABSTRACT FROM AUTHOR]

K2O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K2O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K2O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K2O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K2O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K2O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K2O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy. [ABSTRACT FROM AUTHOR]

Paleoseismological data suggest the occurrence of four bursts of seismic moment release in the Los Angeles region during the past 12,000 yr. The historic period appears to be part of an ongoing lull that has persisted for about the past 1000 yr. These periods of rapid seismic displacement in the Los Angeles region have occurred during the lulls between similar bursts of activity observed on the eastern California shear zone in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the greater San Andreas system suppress activity on faults in the eastern California shear zone, and vice versa, can explain the apparent switching of activity between the two fault networks. Combined with the observation that short-term geodetic and longer-term geologic rates co-vary on major southern California fault systems, this suggests that either (1) a temporal cluster of seismic displacements on upper-crustal faults increases ductile deformation on their downward extensions, or (2) rapid ductile slip in the lower crust beneath faults loads the upper crust, driving a seismic cluster. We suggest that alternating periods of rapid seismic displacement may be the expected mode of seismicity when two fault systems accommodate the same plate-boundary motion, and slip on one system suppresses slip on the other. [ABSTRACT FROM AUTHOR]

The displacement rate history of the Loreto fault, Gulf of California, is well documented; it is characterized by episodically accelerating displacement (10 k.y. frequency) superimposed upon a period of 200 k.y. of increasing fault displacement rate. A detailed conglomerate provenance analysis in the Loreto basin records the footwall unroofing history, which has been profoundly affected by increased fault displacement rate. Clast count data capture an immediate drainage response to a significant increase in slip rate, along with the occurrence and systematic increase in abundance of a new source rock type. Our data record an increase in the rate of incision and catchment expansion associated with this increase in displacement rate. A provenance signal from a shorter-term (100 k.y.) displacement rate increase has a 30–40 k.y. lag time. No change in provenance is recorded for high-frequency (10 k.y.) variations, which are filtered out by the system. [ABSTRACT FROM AUTHOR]

Evaluation of microstructures in unlithified near-surface sediments provides promising results for differentiating between earthquake rupture-related (coseismic) and creep-related (aseismic) structures formed by the San Andreas fault. Paleoseismic trenches excavated at two sites, Flook Ranch on the creeping section, and Alder Creek on the 1906 rupture trace, show contrasting fault-zone structures and microstructures in near-surface, late Holocene sand. At Alder Creek, the 1–2-m-wide fault zone consists of both faults and 2-10-mm-thick deformation bands. Deformation bands have preferred grain orientations ∼30° counterclockwise from the fault (viewed in the slip-parallel direction), broken and disaggregated grains, smaller than average grain size, and lower porosity than control samples. In contrast, at Flook Ranch, two 4–6-m-wide fault zones consist of multiple faults but lack deformation bands. Silty sand in the fault zone at Flook Ranch has preferred grain orientations ∼10° clockwise from the fault, lacks broken grains, and has comparable grain size but lower porosity than control samples. These microstructures record different deformation mechanisms in near-surface sediment: cataclasis at Alder Creek, and distributed deformation at Flook Ranch. Deformation bands on the 1906 rupture trace of the San Andreas fault at Alder Creek demonstrate that these structures, with their grain rotation, grain breakage, and localized porosity variations, can form coseismically in unlithified sediment. The grain bridge model accounts for fundamental microstructural characteristics of Alder Creek deformation bands, and it provides a connection between these microstructures and laboratory studies of stickslip instability. Deformation bands are easily recognizable in field and trench exposures and may be a useful indicator of coseismic slip. [ABSTRACT FROM AUTHOR]

Laser-ablation 40Ar/39Ar step-heating analyses of 20 pseudotachylyte veins from a single location along the exhumed central portion of the active Alpine fault of New Zealand yield total gas age values between 1 and 19 Ma. Evidence shows that they are genetically related and were formed during coeval episodes of seismogenic melting at shallow crustal depth, contrasting with a spread in formation ages. The total gas ages show an exponential decrease with increasing proportion of melt matrix and K content, reflecting incomplete degassing and mixtures of radiogenic Ar sources. Calculation of intercepts for all-melted matrix and all-clast end-member components indicate ca. 570 ka (Quaternary) friction-melting ages of ca. 332 Ma (Lower Carboniferous) source rock. Assuming an average exhumation rate of 6–9 mm/yr for uplift and erosion, these results imply that friction melts were generated during major slip episodes at ∼3.5–5 km crustal depth. We conclude that reliable dating of young pseudotachylyte can be accomplished by combining chronologic study with clast-matrix quantification of genetically related vein assemblages. [ABSTRACT FROM AUTHOR]

It is widely believed that cyclic sedimentation in active tectonic basins is caused by external forcing such as sea-level cycles, episodic tectonic events, or variation in sediment supply due to climate change. Here we present a case of cyclic sedimentation in an experiment subject to constant fault slip rate and sediment discharge with no base-level fluctuation. The experiment was designed to study sedimentation in a simplified extensional relay ramp system. Reorganization of the fluvial channel network in a cyclic manner caused local variation in sediment supply to the hanging-wall basin, where subsidence was maximized, and resulted in autogenic appearance and disappearance of a lake associated with 90° rotation of paleocurrent direction and delta formation. Comparing an early phase of the experiment using no external forcing (i.e., no tectonic movement or base-level change) with the following main phase of the experiment (i.e., including subsidence in an extensional relay ramp pattern) shows that the characteristic time scale for channel mobility was five times longer during the phase with subsidence than during the previous phase without subsidence. Active fault slip imposed a new, longer time scale for autogenic dynamics by establishing a linked tectonic-sedimentation system. Scaling the experimental results to field length and time scales suggests that comparable autogenic cycles would produce 10–20-m-thick strata on time scales of 105 yr, comparable to observed cases attributed to allogenic effects. [ABSTRACT FROM AUTHOR]

Exposures of gabbros and mantle-derived peridotites at slow-spreading oceanic ridges have been attributed to extension on long-lived, low-angle detachment faults, similar to those described in continental metamorphic core complexes. In continental settings, such detachments have been interpreted as having originated and remained active at shallow dips. Alternatively, currently shallow dipping fault surfaces may have originated at moderate to steep dips and been flattened by subsequent flexure and isostatic uplift. While the latter interpretation would be more consistent with Andersonian faulting theory, it predicts large footwall tilts that have not been observed in continental detachment faults. Here we use the magnetization of oceanic gabbro and peridotite samples exposed near the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge to demonstrate that substantial footwall rotations have occurred. Widespread rotations ranging from 50° to 80° indicate that original fault orientations dipped steeply toward the spreading axis. [ABSTRACT FROM AUTHOR]

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 pm to 120 m. These data provide the first quantitative evidence that fault-surface roughness evolves with increasing slip. Thousands of profiles ranging from 10 pm 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 profiles 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. [ABSTRACT FROM AUTHOR]

We present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates. We show that traditional hydraulic scaling laws break down in this situation, and that channel widths become decoupled from drainage area upstream of the fault. Unit stream powers are ~4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled. The loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing. We show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault-induced adjustment of channel geometries has reset hillslope gradients. The results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first-order control the fluvial system exerts on the locus and magnitude of sediment supply to basins. [ABSTRACT FROM AUTHOR]

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 present-day 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 deformation, 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. [ABSTRACT FROM AUTHOR]

The deformation in the Asal Rift (Djibouti) is characterized by magmatic inflation, diking, distributed extension, fissure opening, and normal faulting. An 8 yr time line of surface displacement maps covering the rift, constructed using radar interferometry data acquired by the Canadian satellite Radarsat between 1997 and 2005, reveals the aseismic behavior of faults and its relation with bursts of microseismicity. The observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults. Fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the Fieale volcanic center. Slip distribution along fault strike shows triangular sections, a pattern not explained by simple elastic dislocation theory. These observations suggest that the Asal Rift faults are in a critical failure state and respond instantly to small pressure changes in fluid-filled fractures connected to the faults, reducing the effective normal stress on their locked section at depth. [ABSTRACT FROM AUTHOR]

Several basins are developing near bends on strands of the North Anatolian transform fault in northwest Turkey. Oblique slip on these faults, rather than strain partitioning, accounts for trans-tension and subsidence. These basins are asymmetric, and tilt and subside most rapidly at their narrow ends near the bends. The turbidite surface marking the floor of the Cinarcik Basin (eastern Marmara Sea) was mostly abandoned at a sudden drop in sedimentation, which was likely coincident with the 14 ka lake-sea transition, and is now a warped reference surface from which we can measure strain and sedimentation. Subsidence and tilt are rapid, but do not require late Quaternary changes in regime. They are linked to transcurrent motion by slip parallel to an oblique bend on the North Anatolian fault and suggest tsunamogenic vertical motion in large Marmara Sea earthquakes. [ABSTRACT FROM AUTHOR]

The proposed national high-level nuclear waste repository at Yucca Mountain is close to Quaternary cinder cones and faults with Quaternary slip. Volcano eruption and earth. quake frequencies are low, with indications of spatial and temporal clustering, making probabilistic assessments difficult. In an effort to identify the most likely intrusion sites, we based a three-dimensional finite-element model on the expectation that faulting and basalt intrusions are sensitive to the magnitude and orientation of the least principal stress in extensional terranes. We found that in the absence of fault slip, variation in overburden pressure caused a stress state that preferentially favored intrusions at Crater Flat. However, when we allowed central Yucca Mountain faults to slip in the model, we found that magmatic clustering was not favored at Crater Flat or in the central Yucca Mountain block. Instead, we calculated that the stress field was most encouraging to intrusions near fault terminations, consistent with the location of the most recent volcanism at Yucca Mountain, the Lathrop Wells cone. We found this linked fault and magmatic system to be mutually reinforcing in the model in that Lathrop Wells feeder dike inflation favored renewed fault slip. [ABSTRACT FROM AUTHOR]

Seamount subduction along subduction-zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate. However, tectonic erosion processes have not been examined in detail with high-quality three-dimensional (3-D) seismic reflection imaging. In 1999 we acquired 3-D seismic reflection data from the Nankai Trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure. Fortuitously, these data reveal a small (to 1 km high) basement ridge that has subducted to 7 km subseafloor. Updip from the basement ridge, a 1-km-thick sequence of sediment from the base of the accretionary wedge appears to be missing. We interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction. Tectonic erosion has removed more than 25 km³ from the updip edge of the seismogenic zone and carried it down into the seismogenic zone. The tectonically eroded sediments are presumed to enhance fault-zone fluid content, potentially reducing fault-zone effective stress, and may temporarily inhibit earthquake rupture potential. After the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating. [ABSTRACT FROM AUTHOR]

Space is used as a proxy for time in determining the role of glaciation in shaping valley cross profiles in the Two Thumb Range (Southern Alps), New Zealand. The range is undergoing rapid tectonic transport and uplift as it is advected toward the Alpine fault. Repeated cycles of glacial erosion during the Quaternary have fashioned U-shaped valleys in the north of the range, close to the Main Divide, while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross profiles. Tectonic transport and uplift rates, coupled with an offshore oxygen isotope (δ18O) record and glacial-geological reconstructions, allow time constraints to be applied to valley development along the range. Valley width and depth measurements and power-law equations were used to quantify the shape of cross-profile transects in 37 valleys. Valleys evolved into a characteristic U-shaped cross profile formed over ≥400 k.y. of glacial occupancy. This, regardless of other models of glacial flattening of valley long profiles, suggests that alpine glaciation excavates greater volumes of rock, indicating that glaciers are a more effective erosional agent than fluvial activity in alpine mountain belts. [ABSTRACT FROM AUTHOR]

A major earthquake on the Seattle fault, Washington, Ca. A.D. 900-930 was first inferred from uplifted shorelines and tsunami deposits. Despite follow-up geophysical and geological investigations, the rupture parameters of the earthquake and the geometry of the fault are uncertain. Here we estimate the fault geometry, slip direction, and magnitude of the earthquake by modeling shoreline elevation change. The best fitting model geometry is a reverse fault with a shallow roof ramp consisting of at least two back thrusts. The best fitting rupture is a SW-NE oblique reverse slip with horizontal shortening of 15 m, rupture depth of 12.5 km, and magnitude Mw = 7.5. [ABSTRACT FROM AUTHOR]

High-resolution multibeam bathymetry data from the Sumatran subduction zone reveal the regional and local morphology, including small-scale fault-related features and landslides that may be linked to earthquakes in the recent geological past. The accretionary prism is steeply sloped and pervasively eroded, with evidence of unusual landward vergence (seaward fault dip) of the frontal thrusts. Small-scale (5-100 m height) fault scarps, folds, and troughs are common along the seaward edge of the frontal thrust at the deformation front. A model of back-thrust fault slip or bending moment folding during plate-boundary slip, such as during the 2004 M9.2 megathrust earthquake, can explain the position of these features on the seaward fold limb of a seaward-dipping thrust. We infer that in major Sumatran (and other similar settings) plate-boundary earthquakes, coseismic surface rupture may occur at the prism toe. [ABSTRACT FROM AUTHOR]

Analysis of force and torque balance on the Salton block, Southern California, yields estimates of time-averaged shear stress acting on the bounding faults and the base of the crust. Averaged over a depth of 30 km, the San Andreas and San Jacinto faults sustain time-averaged shear stress of ∼21-35 and 24-43 MPa, respectively. This implies that tectonic shear stress at seismogenic depths is greater than a typical 1-10 MPa earthquake stress drop and, with a corresponding effective friction coefficient of 0.10-0.21, lower than that predicted by laboratory experiments. Basal stress of 3-14 MPa also is required to drive the Salton block into the Transverse Ranges. Thus, the forces driving mountain building, basin formation, and the generation of earthquakes south of the Transverse Ranges in Southern California stem from stresses transmitted laterally across weak faults and from below. [ABSTRACT FROM AUTHOR]

We present analytical results from four dredge locations across the eastern Zealandia continental margin and adjacent ocean crust. The 115 Ma dacites dredged from the West Wishbone Ridge (WWR) are isotopically primitive, weakly adakitic, slab-derived lavas. The 97 Ma A-type granites and a basalt from the easternmost Chatham Rise enlarge the known area of postsubduction Gondwana magmatism. Amphibolite-grade schists from a fault block south of the Chatham Rise provide a critical bridge between the Zealandia and West Antarctica belts of Jurassic- Cretaceous accretionary prism rocks. The new recognition of the WWR as a remnant of a 115 Ma intraoceanic subduction system means that previous hypotheses of the WWR as a fracture zone or spreading ridge require modification. The dacite ages constrain the start of Osbourn Trough spreading, which caused breakup of the Hikurangi-Manihiki igneous plateau, to before 115 Ma. We speculate that, after 115 Ma, the WWR was rifted by an intraoceanic spreading center that developed along its southeast side. Impingement of this spreading center against the Gondwana margin led to widespread 95-100 Ma postsubduction magmatism, variable lithospheric stretching, and ultimately continental splitting of Zealandia and West Antarctica across basement trends. [ABSTRACT FROM AUTHOR]