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The Haast Schist of South Island, New Zealand, consisting of the Otago, Alpine, and Marlborough Schist components, is the most deeply buried and strongly metamorphosed part of an accretionary complex above a westward-dipping subduction zone on the eastern margin of Gondwana. Conventional wisdom is that the schist formed in the Jurassic (ca. 170 Ma) by collision between Carboniferous to Triassic graywacke sequences of the exotic eastern Torlesse composite terrane and the western Caples terrane. However, extensive areas of Alpine Schist in the west, rich in metabasite, and in part overlying a metamorphosed ophiolitic sequence, have anomalously young, mid-Cretaceous, ca. 108 Ma, detrital zircon ages. Their deposition and subsequent deformation occurred approximately synchronous with the cessation of westward-directed subduction in the Eastern Province of New Zealand, the onset of regional extension, and deposition of terrestrial and volcanic sediments on the exhumed erosion surface of the eastern schist basement. These Cretaceous schist protoliths are interpreted to form the exotic Pounamu terrane, accreted and interfolded with Otago Schist on the western margin of the Haast Schist. The Alpine Schist metamorphism is dated from zircon overgrowths on detrital grains at ca. 70 Ma (although over an extended period, 98-64 Ma, regionally). This thermal event is unknown further east. The Haast Schist is therefore a polygenetic unit formed by the amalgamation of metamorphic components accreted from both the east and west sides of the SE Zealandia microplate. Existence of the Pounamu terrane requires a major revision to the Cretaceous tectonic history of southern New Zealand. [ABSTRACT FROM AUTHOR]

Knowledge of the last reactivation age of faults in the western Basin and Range province (western North America) is key to understanding the recent evolution of the province in the context of the Pacific-North America plate boundary region. The Dixie Valley fault system (DVFS; Nevada, USA) is an important active normal fault in this region, but its slip history is not well known. Our goal is to determine the age of faulting initiation and the history of vertical displacement at the million-year time scale. We used well data from the Dixie Valley geothermal field and geologic maps to calculate offsets of geologic units. Tectonic events and their ages were determined from knickpoint analysis in the Stillwater Range. We then matched basin unconformities and knickpoints to obtain a vertical displacement history. The age of inception of the modern DVFS is likely between 13 and 7 Ma. The total vertical displacement of the geologic units is between 3 and 3.5 km, and most of it occurred in the last 2-3 Ma. Assuming an erosion rate between 0.03 and 0.07 m/ka, the maximum vertical displacement rate (~1-2 mm/a) occurred between 3 and 0.5 Ma, and resulted in ~2 km of vertical displacement. Finally, the age-versusdistance profile of the Stillwater Range crest is tapered at both ends, as expected for a propagating range-bounding normal fault, but it also has two maxima matching the deepest parts of the Dixie Valley basin. This indicates that the present-day DVFS may have started out as two separate strands that connected within 2-3 Ma of inception. Our work shows that the combination of stratigraphic and structural data from the basin with geomorphological data from the adjacent mountain range is suitable for placing constraints on the displacement history at the million-year time scale of an active range-bounding normal fault. The resulting displacement-time paths for normal faults may not be as uniform as commonly assumed. [ABSTRACT FROM AUTHOR]

Two outcrops in the Asturian Basin (northwest Iberian Peninsula) composed of Lower-Middle Jurassic alternations of limestones and marls, including black shales proven to be hydrocarbon source rocks, were analyzed from a structural point of view. In both outcrops, an inversion tectonic event of Cenozoic age caused contractional folds and faults, as well as a change in the structural style up section, superimposed on previous Mesozoic extensional structures. However, the mode and kinematic evolution of the inversion tectonics were very different; folding predominated in one of the outcrops, whereas faulting is the most important phenomenon in the other one. The characteristics of the two coastal outcrops (irregular surface, inaccessible portions, accessibility restricted to low tide periods, good quality of rock exposure and scale) led us to build virtual outcrop models using photogrammetry (structure from motion) and interpret them in a computer-assisted virtual environment (CAVE). The analysis of both field data and the virtual outcrop model-derived results (three-dimensional models and cross sections) allowed us to conclude that mechanical stratigraphy and the type, characteristics, and distribution of previous structures were the key factors that controlled the inversion tectonic mode. Awareness of their influence should help when trying to understand inversion structures in similar geological settings. [ABSTRACT FROM AUTHOR]

Many recent thrust fault earthquakes have involved coseismic surface faulting and surface folding strains. This multifaceted nature of active thrust sheet deformation can make attempts to quantify slip and slip rates from surface strains challenging and uncertain. We present new methods for integrating records of surface deformation, subsurface structure, and geochronology to investigate active deformation over multiple rupture cycles across the Southern Junggar Thrust in the southern Junggar Basin, NW China, from ~225 ka to present. Fluvial terraces preserve records of surface faulting as a prominent fault scarp where the Southern Junggar Thrust is surface-emergent. Terraces also exhibit progressive folding strains across fold scarps that are spatially coincident with subsurface fault-bend folds--constrained by seismic reflection data--along the Southern Junggar Thrust. We quantify the fault slip at depth implied by fold scarp relief along Holocene-aged terraces, and the results are corroborated by independent estimates of slip implied by fault scarp relief for the same terraces. Older terraces exhibit a distinct fanning of dips across fold scarps, suggesting active fault-bend folding kinematics involving a component of limb rotation. We developed quantitative relations for fault-bend folds between fault slip and fold dip using a mechanical, forward modeling approach. Using this novel method, we show how Southern Junggar Thrust slip rate has decelerated markedly, from ~4.1 mm/yr in the middle Quaternary to ~1.2 mm/yr throughout the Holocene. These results provide new insight into natural fault-bend folding kinematics and define innovative methods for elucidating accurate estimates of fault slip and slip rates from terrace folds in active thrust sheets. [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]

Various methods have been proposed to measure fracture aperture distributions, including X-ray computed tomography (CT) imaging, which has the advantage that it can be combined with dynamic flow experiments. In this paper, we present a calibration-free missing CT attenuation (CFMA) imaging method for measuring fracture apertures that avoids time-consuming calibration. In addition, this model does not assume a homogeneous matrix and thus provides a good estimate of fracture apertures even when rock properties are heterogeneous. The validity of the CFMA model is established by four approaches: comparing apertures calculated with the conventional calibration-based method; evaluating model predictability at different scanner voxel sizes; comparing with calibration coefficients in the literature from a number of experiments with different rocks and X-ray scanners; and comparing aperture measurements for dry and wet scans. We analyze the systematic error and the random error introduced by rock heterogeneities and CT scanning and show that by averaging 5 replicate scans, we reduce the aperture measurement error to ~22 µm. [ABSTRACT FROM AUTHOR]

The combination of structural data from the source area and descriptive data from the deposit's carapace, as well as remote sensing and statistical analysis, allows a better understanding of the collapse, fragmentation, and propagation processes of the Frank Slide rock avalanche. The in situ observed conservation of the stratigraphic sequence of the Turtle Mountain anticline's normal limb in the deposit is interpreted as the consequence of the collapse mode, involving simple shearing of the mass accompanied by a rotational movement, i.e., a "simple shear" parallel to the topography, and a breakage of the hinge, followed by overthrusting of the normal limb on the inverse limb. Coherence between the block size distributions of the source area and the deposit carapace is interpreted as a demonstration of the primary control of preexisting fracturing on the fragmentation processes. Remote-sensing indexes allow us to uncover a priori hidden morphological features preserved on the surface of the deposit, i.e., longitudinal and compressional features, as well as lobes. Their location on the carapace provides evidence of lateral heterogeneity in the propagation, highlighted by three different types of propagation behaviors. This comprehensive study not only provides elements that contribute to an understanding of the Frank Slide rock avalanche, but it also provides insight into essential parameters to take into account in further modeling of these types of phenomena. [ABSTRACT FROM AUTHOR]

The study of shock pressure indicators can provide important clues for understanding the cratering process, though the estimation of shock pressures in weakly shocked rocks is commonly difficult. In this study, we selected a very young and well-preserved impact structure, the Lonar crater in India. The crater, devoid of any tectonic overprint, can be assumed as pristine. We used a combination of rock magnetic and microfracture studies to estimate shock pressure in the crater rim. On the basis of present results, the magnetic fabrics are interpreted to be of magmatic origin related to the Deccan basalt emplacement. The high-coercivity component of the natural remnant magnetization in the crater rim basalt is similar to that in the unshocked basalt. The lack of any shock-related magnetic overprint on the crater rim basalt is, therefore, evident in the Lonar crater. In contrast, radial and concentric microfractures observed in basalts at the crater rim and farther away show symmetric distribution with respect to the crater. The concentric microfractures consistently overprint the radial microfractures. We infer that the radial and concentric microfractures were developed during propagation of the early compressional and the late decompressional shock wave components, respectively. The results of our rock magnetic and microfracture studies, when interpreted in light of published experimental and numerical simulation studies on the Lonar basalt, reveal that the shock pressure in the Lonar crater rim was less than 0.5 GPa but greater than 0.2 GPa. This shock pressure was high enough to produce fractures but too low to affect the magnetic fabrics. These results give new information on the relationship between shock pressure and resulting microfractures. [ABSTRACT FROM AUTHOR]

New mapping of the southern McDoogle pluton, an early intrusion in the John Muir suite in the central Sierra Nevada, California, has resolved important details of incremental assembly by magmatic crack-seal. The tabular McDoogle pluton concordantly intruded the steeply dipping mylonitic Sawmill Lake shear zone around 95 Ma. The pluton was previously divided into three phases based on wall-rock screen abundance and lithology. The earliest central phase consists of steep dikes separated by abundant wall-rock screens. These screens are largely absent from border phases located on either side of the central phase. The inferred relative ages of intrusive phases and the wall-rock screen distribution signify a change from antitaxial (addition at the margins of the central phase) to syntaxial (addition at the center of each border phase) assembly during pluton growth. Interaction among the effects of preexisting foliation, older nondeformed intrusions, and synintrusive static recovery of shear zone rocks likely caused the transition from antitaxial to syntaxial growth. The trend from antitaxial to syntaxial growth during emplacement has been observed in other intrusions and intrusive suites and may be a general pattern of incremental intrusion associated with the feedback between preexisting deformation and thermal addition in magmatic arcs. [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) profi les 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]

The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24-25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely reflecting a subcritically tapered orogenic wedge. The first eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12-10 Ma, the thrust front jumped ~75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of shortening at this latitude is ~142 km, which is moderate in magnitude compared to the 250-350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can influence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities. [ABSTRACT FROM AUTHOR]

The Andean fold-and-thrust belts of west-central Argentina (33°S and 36°S), above the normal subduction segment, present important along-strike variations in mean topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. To analyze the controlling factors of these latitudinal changes, we compare these parameters and the chronology of deformation along 11 balanced crustal cross sections across the thrust belts between 70°W and 69°W, where the majority of the upper-crustal deformation is concentrated, and reconstruct the Moho geometry along the transects. We propose two models of crustal deformation: a 33º40'S model, where the locus of upper-crustal shortening is aligned with respect to the maximum crustal thickness, and a 35º40'S model, where the upper-crustal shortening is uncoupled from the lower-crustal deformation and thickening. This degree of coupling between brittle upper crust and ductile lower crust deformation has strong influence on mean topographic elevation. In the northern sector of the study area, an initial thick and felsic crust favors the coupling model, while in the southern sector, a thin and mafic lower crust allows the uncoupling model. Our results indicate that interplate dynamics may control the overall pattern of tectonic shortening; however, local variations in mean topographic elevation, deformation styles, and crustal root geometry are not fully explained and are more likely to be due to upper-plate lithospheric strength variations. [ABSTRACT FROM AUTHOR]

The active dextral-reverse Wairarapa fault in the North Island, New Zealand, cuts ca. 12 ka, post-Last Glacial Maximum (post-LGM) fluvial gravels with a highly segmented fault trace. This <350-m-wide fault zone displaces the gravels laterally by ∼130 m and warps them into elongate deformational bulges in the left-stepover regions between overlapping synthetic fault strands. Aerial photography, field observations, and global positioning system surveying were used to map the structural geology of a central section of this major oblique-slip fault zone. Topographic data collected at two particularly well-expressed and exposed pressure bulges of contrasting size were used to undertake a kinematic-volumetric analysis and to calculate the likely vertical extent of the bulge-related distributed deformation. In the near-surface, the Wairarapa fault zone is characterized by a two-order hierarchy of en echelon fault segments. Type A segments are the longest (2-7 km) and are separated by the largest stepover widths (250-350 m). Volumetric calculations suggest that these splays converge downward into a master fault plane at depths of ∼100-260 m, well below the base of the post-LGM gravels. Accordingly, we infer that type A faults and the bulges that form in their stepovers had already developed in the lithified basement rock prior to deposition of the surficial gravels. Type B segments are shorter (500 m to ∼4 km long) and are separated by 30-150-m-wide stepovers. We calculate that these splays converge at shallow depths of <20 m near the base of the poorly consolidated gravels, where they developed in response to distributed deformation of that surficial cover during the past ∼12 k.y. The segmented surface trace of the Wairarapa fault in some ways resembles that produced in analogue models of deformation above a strike-slip fault in basement rock; however, the synthetically slipping fault segments of the Wairarapa fault zone strike at a smaller angle (2°-18°) to the overall strike of the fault than do most analogue models (typically, 10°-30°), raising questions about the angles at which "R-shears" may develop along natural strike-slip faults that slip with a convergent component. Also, the natural fault zone is narrower (350 m wide) than equivalent scaled widths of most analogue models (typically 1-2 km). These differences are interpreted to reflect the shallow depth of splaying of the natural Wairarapa fault zone in comparison to thickly sedimented analogue models. Based on our field data, we present a general model for (1) the relationship between an oblique-slip fault and the asymmetry of its near-surface synthetic fault splays and pressure bulges that deform gravel, and (2) the along-strike partitioning of strike- and dip-slip components of slip between adjacent splays in the near-surface in relation to the oblique slip accumulating on the main fault at depth. [ABSTRACT FROM AUTHOR]

We use continuous measurements of GPS sites from across the Rio Grande Rift, Great Plains, and Colorado Plateau to estimate present-day surface velocities and strain rates. Velocity gradients from five east-west profiles suggest an average of ~1.2 nanostrains/yr east-west extensional strain rate across these three physiographic provinces. The extensional deformation is not concentrated in a narrow zone centered on the Rio Grande Rift but rather is distributed broadly from the western edge of the Colorado Plateau well into the western Great Plains. This unexpected pattern of broadly distributed deformation at the surface has important implications for our understanding of how low strain-rate deformation within continental interiors is accommodated. [ABSTRACT FROM AUTHOR]

The article evaluates the hypothesis that porous sedimentary rocks subjected to high strain may not form deformation band zones (DBDZs). The accessible faults planes at the Upheaval Dome impact structure in southeastern Utah reportedly yielded no DBDZ formation evidence and instead pulverized quartz grains in the adjacent Navajo structure were found. The article states that strain rates at Upheaval Dome are consistent with seismic slip rates along faults like the San Andreas fault.

The article discusses the late Miocene to early Pleistocene sedimentary rocks in the Fish Creek-Vallecito basin in southern California that records late Cenozoic subsidence and basin filling due to deformation in the San Andreas fault system. Data reportedly show that the section ranges in age from circa 8.0-0.4 million years (Ma). The article states that the onset of sedimentation at circa 8.0-0.4 million years (Ma) preserves the earliest extension or transtension in the area.

The article reports on the development of a crenulation cleavage in a Mooselookmeguntic igneous complex roof due to intrusion resulting to mechanical thinning by complex thermal, chemical and mechanical interaction. This deformation gradient may reportedly mirror the steeply dipping of the pre-emplacement foliation that might be expected for nonlinear creep mechanisms. The article states that the cleavage may be important in the spatial distribution of localized strain in evolving orogens.

The article presents a study on the interactions of tectonic deformation, climate, eustasy and isostasy on forearc basin sedimentation in the Pleistocene sedimentary record of Hawke Bay, New Zealand. Eleven depositional sequences that developed during the last 1.1 million years are reportedly preserved into four controlled basins. The article states that the major uncorformity S5 separates megasequences M1 and M2 and that an overall retrogradational pattern of sequences were identified.

The article presents a case study of the four structural deformation stages in the Western Hills of Beijing in China during the Mesozoic time. These include the East-West-trending folds, North-East trending, Fangshan granitic pluton intrusion and East-dipping normal faulting in 70-60 million years (Ma). The article states that the structure of the hills in eastern China are related to complex imprinting of tectonic regimes that involved time-space deformation and various tectonic stresses.

This paper presents a detailed investigation of the structure and evolution of the Precordillera southern sector (Argentina). We document the development and successive reactivation of regional and discrete structural grain through time, and discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. Our kinematic studies indicate that the Permian orogeny generated a doubly vergent foldand- thrust belt of transpressive deformation, where strain was partitioned into two different types of deformation domains. The westvergent western domain was characterized by partitioned transpression with shortening dominating, and a strike-slip-dominated subdomain. The east-vergent eastern domain was characterized by pure contractional deformation. Our model for the Late Permian to Early Triassic evolution of the Pre cordillera involves a north-northwest-trending weakness zone affected by north-northeast- directed extension, generating an area with transtensional deformation during the Choiyoi volcanism development. Later, during the Triassic generation of the Cuyana rift basin, the northeast stretching direction was orthogonal to the rift trend, indicating pure extensional deformation. We propose a model where the clear parallelism between the distribution of an inferred early Paleozoic suture zone, a northnorthwest- trending late Paleozoic belt, and Permian-Triassic rift-related magmatism indicates the reactivation of a northnorthwest- trending long-lived lithospheric weakness zone. [ABSTRACT FROM AUTHOR]

This paper addresses the contribution of extension in the upper crust during syncollisional exhumation of subducted material. We present structural data based on the analysis of ductile, brittle-ductile, and brittle deformation features in meta-pelitic schists and meta-basalts from the Schistes lustrés nappe stack in the Piedmont zone of the western Alps. To link the structures with synkinematic pressure and temperature estimates, geothermobarometry was performed using fluid inclusion investigation in quartz together with temperature estimations based on metamorphic chlorite composition. Samples used for geothermobarometry were collected taking into account the structural framework within veins associated with extensional structures observed at the regional scale. The early generation of veins associated with boudinage occurred at 415-345 °C and 6-2.5 kbar, and subsequent incipient normal faulting occurred at temperatures of 315-235 °C and pressures <1.7 kbar. The transition from ductile to brittle extension occurred between ca. 27 Ma and ca. 10 Ma. These results indicate a continuous extension in the upper crust during exhumation and continuous deformation associated with regional-scale tilting of the exhumed Schistes lustrés tectonic pile. The extensional tectonic regime in greenschist-facies conditions remains active today in most of the internal Alpine zones as the Alpine collision continues. [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]

Recent seismic imaging across the Rio Grande rift, western United States, revealed unexpected structures in the underlying mantle. Low seismic wave velocity anomalies below the Rio Grande rift have been interpreted as being partially of melt origin, and high-velocity structures below the western Great Plains have been proposed to be the result of small-scale convection, i.e., cold downwelling lithospheric material with probably a compositional contribution. We perform a dynamic test of these interpretations using a passive rift model for isochemical convection. The models self-consistently produce a rift localized at approximately the right distance from the border to the nearby thicker Great Plains lithosphere. With realistic upper mantle rheologies, small-scale convection forms, aided by the lithospheric step. The resulting thermal anomalies produce seismic low-velocity anomalies below the rift of amplitudes similar to those imaged seismically, requiring the presence of only small amounts of melt. The lateral extent of the observed low velocities below the Rio Grande rift is as in the models, where it is controlled by the spacing between downwelling limbs of the small-scale convection. The fast velocity structure below the western Great Plains can be produced by cold downwelling lithosphere. The thermal rifting models can predict the amplitudes and size of the main seismic anomalies; compositional heterogeneity may contribute to some of the smaller features observed. [ABSTRACT FROM AUTHOR]

Apatite (U-Th)/He dating from the St. Elias orogen in southern Alaska illustrates a potential association between long-term denudation and glacier sliding. Cooling ages as young as 0.4 Ma (exhu mation ~4-5 mm/yr) are concentrated in a narrow band near the glacier equilibrium line altitude (ELA) front, where mean Quaternary ELA intersects the windward flank of the orogen. This band of denudation is not correlated with individual faults, structural trends, or known concentrations of precipitation, and we propose that it is produced by focused glacier sliding at or near the ELA front. This implies that long-term glacial erosion is a maximum at ELA, which corroborates model predictions that glaciers and climate can control the pattern of crust removal from orogens. Denudation rates do not covary with fluctuations in glacier size along the ELA front, suggesting that small glaciers are capable of keeping pace with incision by larger ones and tectonic rock uplift. Ice discharge may thus play a critical, but complex, role in excavating glaciated orogens. [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]

The recent dramatic increase in seismic anisotropy and surface global positioning system (GPS) data for central Asia permits a comprehensive examination of the mantle's role in mountain building. A joint analysis of 178 shear-wave-splitting and 2000 GPS observations using a new technique reveals that the crust and lithospheric mantle deform coherently, arguing for crust-mantle mechanical coupling during deformation. The observed spatial variations in anisotropy reflect the large-scale pattern of lithospheric deformation, as well as a change in deformational style from simple shear on the Tibetan Plateau transitioning to pure shear in surrounding regions. [ABSTRACT FROM AUTHOR]

The Macquarie Ridge Complex portion of the Australia-Pacific plate boundary south of New Zealand provides a unique, complete record of a 60°-90° change in spreading direction since 40 Ma that resulted in the transition from a spreading center to a transform plate boundary. Marine geophysical data show that during reorientation, most ridge segments completely disappeared and all shortened. Additionally, modification of newly created crust caused differences in widths of correlative spreading segment corridors on the two plates. We propose two models for ridge reorientation that explain the observed spreading fabric and arcuate fracture zone relationships. Nonrigid plate deformation was accommodated by failing and propagating spreading ridge segments (rifts) and transfer of crust between plates during the gradual reorientation of the spreading axes. [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]

A high-resolution Compressed High-Intensity Radar Pulse (CHTRP) survey reveals shore-parallel variations in the Holocene sediment thickness offshore La Jolla, California. Sediment thicknesses decrease from >20 m in the south near Scripps Canyon to zero in the north approaching Torrey Pines. In addition to the south-to-north variation in sediment thickness, the transgressive surface observed in seismic lines shoals from Scripps Canyon to the north. Despite these dramatic shore-parallel subsurface changes, the nearshore bathymetry exhibits little to no change along strike. A left jog (i.e., a constraining bend) along the Rose Canyon fault causes local uplift in the region and appears to explain the northward shoaling of the transgressive surface, the decrease in relief on the transgressive surface away from the left jog, and the Holocene sediment thickness variation. This tectonic deformation is shore parallel, and thus the accommodation can be separated into its tectonic and eustatic components. [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]

Recent observations of cleavage patterns, strain histories, and kinematics across the Taiwan mountain belt depict systematic orogen-scale variations with respect to the synorogenic divide and suggest that the pattern of cleavage development is a predictable consequence of orogen stresses and kinematics. In Taiwan, continental crust within the collision is accreted in the prowedge facing Asia, but is advected eastward into the east-verging retrowedge, where the most deeply exhumed rocks are exposed. Wedge mechanics predict a reversal in the direction of plunge of the principal compressive stress at the topographic divide between the opposing wedges. The observation of a single cleavage in western Taiwan suggests that the cleavage in the prowedge remains stable with respect to the stress orientation. In contrast, the existence of a second crenulation cleavage in the retrowedge is evidence for an abrupt change in stress orientation and unstable buckling of preexisting prowedge fabrics. Advection of a fabric across a topographic divide in a doubly vergent wedge provides an explanation for the occurrence of cleavage fronts and fans in natural systems such as Taiwan. [ABSTRACT FROM AUTHOR]

Surface deformation in Morocco, derived from five years of global positioning system (GPS) survey observations of a 22-station network, four continuously recording GPS (CGPS) stations, and four International GNSS Service (IGS) stations in Iberia, indicates roughly southward motion (∼3 mm/yr) of the Rif Mountains, Morocco, relative to stable Africa. Motion of the Rif is approximately normal to the direction of Africa-Eurasia relative motion, which is predominantly strike slip, and results in shortening of the Rif and subsequent crustal extension of the adjacent Atboran Sea region. The sense, and the N-S asymmetry of the observed deformation (i.e., no evidence for north-directed shortening in the Betic Mountains north of the Alboran Sea) cannot be easily explained in terms of crustal plate interactions, suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western Mediterranean. The model that best fits the observations involves delamination and southward rollback of the African lithospheric mantle under the Alboran and Rif domains. [ABSTRACT FROM AUTHOR]

Recently collected multibeam bathymetry and hydroacoustic earthquake data are used to investigate the recent tectonics of the Explorer-Juan de Fuca Ridge systems, the boundaries between the Pacific and northern Juan de Fuca plates. The bathymetric and seismic evidence presented is consistent with a zone of shear extending well south of the Sovanco Fracture Zone to include the Heck and Heckle seamounts, and potentially as far south as the Springfield Seamounts and Cobb offset along the Juan de Fuca Ridge. This indicates that the triple junction between the Pacific-Juan de Fuca-North American plates may be reorganizing southward to establish at the Cobb offset. [ABSTRACT FROM AUTHOR]

The processes that initiate and maintain prograde ductile shear zones are not well understood. We have performed shear experiments (γ = 1 to 3) on a fine-grained (100- 150 μm) gneiss (59% quartz, 28% plagioclase, 13% aligned but not interconnected biotite) to determine the evolution of deformation mechanisms that produce dramatic strain weak- ening and localization, at conditions (1.5 GPa, 800 °C, ... = 2 × 10-5/s) where pure quartz aggregates deform homogeneously by dislocation creep. Initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase, allowing local biotite interconnection by slip on (001) and initiating strain weakening. After yield, the interconnected biotites kink and thus strengthen, but the highly strained parts of grains react to a fine-grained, mixed-phase assemblage which deforms by grain-size-sensitive creep, allowing further strain weakening and localization. At γ = 3.3, the strain and strain rate in the ∼100-μm-thick shear zone are 100 times that of the enclosing host rock, similar to the localization observed in natural shear zones. Thus, the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone, and once formed, a shear zone may be permanently weakened. [ABSTRACT FROM AUTHOR]

The 40-km-wide Araguainha impact structure in central Brazil provides extensive out- crops to study the structural evolution of all parts of a complex crater, including the central uplift, annular trough, and crater rim. While most craters of comparable size are buried by impact-related or postimpact sedimentary deposits, Araguainha is deeply eroded and it exposes in detail outcrop-scale structural features that can be used to understand the structural evolution of large impact craters. This study explores evidence from structural features across the entire impact structure in order to provide constraints on the target rock movement during the crater collapse. Most of the structural features described here are consistent with folding and bedding-parallel shearing during several kilometers of lateral inward movement of the target rocks. Vertical movement was, in contrast, restricted to distances of less than a few hundred meters along radial and concentric fault zones around the crater rim. [ABSTRACT FROM AUTHOR]

Our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation. The significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level. The over- lap region between mid-ocean ridge segments at 9°N on the East Pacific Rise is characterized by aligned cracks that are structurally more closely related to the propagating-ridge segment. These aligned cracks, which are approximately parallel to the ridge segments, provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers, especially those with overlap basins covered by volcanic edifices. The aligned cracks of the 9°03′N overlapping spreading center are more similar to the ridge-parallel lineaments observed between overlapping axial-summit collapse troughs than those found at larger overlapping spreading centers. Our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related. [ABSTRACT FROM AUTHOR]

New continuous and differential global positioning system (GPS) measurements of recent slip rates and 30 yr alignment-array offsets from the central creeping segment of the San Andreas fault yield a maximum right-lateral slip rate of 25 ± 1 mm/yr. This slip rate is 20% slower than the 30 mm/yr slip rate accepted for this segment of the fault and 35% slower than the 39 mm/yr slip rate predicted between the Sierra Nevada-Great Valley block and the Pacific plate. New continuous GPS measurements between pairs of sites that flank the creeping segment at intersite distances of 1.0 km and 70 km give relative fault-parallel slip rates of 28 ± 2 and 30 ± 2 mm/yr, respectively. These observations indicate that right-lateral deformation rates increase with distance from the fault. Possible explanations for the gradient observed in the geodetic data are elastic strain accumulation along the creeping segment or significant distributed deformation on off-fault structures. [ABSTRACT FROM AUTHOR]

A new type of oh vine fabric was found by high-strain simple-shear deformation experiments in the presence of trace amounts of water at -0.5-2.2 GPa and --1470-1570 K. In this fabric, called E-type fabric, the olivine [100] axis is subparallel to the shear direction, and the (001) plane is parallel to the shear plane; this geometry suggests that the [100](001) slip system makes the dominant contribution to total strain. This fabric is dominant at a modest water content, 200 < COH C 1000 HJ106Si at low stresses and high temperatures. Some mylonites from peridotite massifs show this type of olivine fabric, which suggests the presence of water during the shear localization. The seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric (A type), but the magnitudes of anisotropy are different between the two types: for horizontal flow, the amplitude of VSH/ Vsv anisotropy is weaker, but the amplitude of shear-wave splitting is stronger for the E-type fabric than for the A-type dry fabric. Seismic anisotropy in the oceanic upper mantle may be due to the olivine E-type fabric. [ABSTRACT FROM AUTHOR]

A lithospheric model scaled for density and viscosity was shortened obliquely in front of an indenter whose shape and orientation match those of the South Alpine indenter. Deformation was initially localized into subparallel sets of conjugate transpressive thrusts. These structures evolved into folds, which were strongly amplified and converged closer together during ongoing shortening, finally forming a series of tight antiforms. At the same time, this tightly folded structure locally accommodated significant orogen-parallel extension, attaining 100% in front of the leading edge of the indenter, but only 15% on the scale of the entire model. The internal architecture and fault pattern of the model in map view are very similar to those of the Tauern Window in the Eastern Alps, emphasizing the importance of localization of folding and shortening rather than orogen-parallel extension for its formation. [ABSTRACT FROM AUTHOR]

This study addresses some aspects of the kinematics and structural setting of the Mediterranean Ridge accretionary complex, particularly those aspects related to the occurrence and distribution of Messinian evaporites. The approach is by (1) identification of the geometry of deformation through reprocessing of existing multichannel seismic reflection data from the Levantine Basin, aimed at the definition of the velocity field and depth imaging; (2) physical modeling in the laboratory, aimed at the study of the kinematics of shortening of sedimentary sequences detaching on viscous deco-elements; and (3) comparison between the results of the two approaches. We conclude that the thick Messinian sequence of the Levantine Basin is composed primarily of salt, so that the entire evaporitic sequence can be expected to behave as a viscous material. It follows that the main detachment of the post-Messinian wedge is located at the base of the Messinian salt layer. A viscous (salt) décollement would produce thrusting trending normal to the shortening direction, but boundary conditions affect structural trends even more than stress and/or movement direction. Both strain partitioning and the formation of major strike-slip faults within the post-Messinian wedge are prevented by the high angle of convergence between the African and Aegean plates as well as by the Iow intraplate friction. In addition, we show that curved and anastomosing thrust fronts are reflected in the cobblestone topography of the Mediterranean Ridge. We finally postulate that extension may occur in the central Mediterranean Ridge as a result of the geometry of the plate boundaries. This extension is considered as a possible cause of mud volcanism and mud diapirism. [ABSTRACT FROM AUTHOR]

Most structural/kinematic models are inherently two-dimensional; even several recent three-dimensional models are "pseudo-three-dimensional" in that they consist of a series of parallel two-dimensional cross sections. Lack of a true three-dimensional formulation hampers our abilities to simulate three-dimensional structures such as oblique- and strike-slip faulting and displacement gradients perpendicular to the slip vector. The mathematical formulation of trishear deformation using incompressibility of flow is well suited to a solution in three dimensions. We derive one plausible velocity field for true three-dimensional flow in a triangular shear zone. This formulation allows us to simulate the deformation in oblique-slip deformation zones as well as flower structures associated with strike-slip fault zones. The strain distribution in flower structures combined with some simple mechanical assumptions suggests that faults in these zones would have a helicoidal geometry. The results of the kinematic model compare well to well-described structures in the Colorado Plateau, Andaman Sea, and Death Valley, as well as to data from analogue experiments. [ABSTRACT FROM AUTHOR]

Discusses the reactivation of Proterozoic structures during Laramide tectonism in Grand Canyon, Arizona. Geologic setting; Age of the Grand Canyon Supergroup; Tilting and normal faulting of the Unkar Group; Unconformities and intraformational faults in the Nankoweap Formation; Geometry and style of deformation.

Presents a structural map and crustal-scale cross sections of the Sulu area. Argument that the boundary between the North and South China blocks must be to the north of the Sulu area; Tectonic map of the Jiaodong Peninsula; Structural units of slate-sandstone; High-pressure gneiss-quartzite; Foliated migmatite; Marble-amphibolite; Geometry and kinematics of the deformation phases.

Examines a natural experiment in river incision into bedrock produced by the 1912 eruption that inundated the Valley of Ten Thousand Smokes in Katmai National Park, Alaska. Influence of the rates and patterns of bedrock channel incision on landscape evolution and long-term interactions among climate, tectonics and erosion; Incision rate data.