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We report remote sensing evidence and reconnaissance field observations for an actively evolving transpressional duplex 150 km north of Tibet, in the southern Beishan region of western China. We analyze Landsat-8, Google Earth and SRTM-30 data to reveal active fault traces, kinematics, offset contacts and landforms, and the overall duplex geometry in an area of limited recorded seismicity compared to surrounding regions. Eight NE-striking faults transfer sinistral displacement between E-W bounding faults and accommodate N-S shortening by sinistral-reverse displacements that have tilted the internal duplex blocks. The potential moment magnitude of a surface-rupturing event on one fault, that displays a twenty km-long, 1–3 m-high surface scarp, is estimated to be 6.6. Our study adds to the growing body of evidence that suggests the Beishan is a region of subtly expressed, but widespread, continental interior, strike-slip reactivation at low strain rates - challenging the long-held view that the Beishan has anomalous crustal stability within the huge Indo-Eurasia deformation field.

The distribution of landslides induced by the 2008 M w 7.9 Wenchuan earthquake was investigated following the method of Meunier et al. (2007) to examine the relationships to peak ground acceleration (PGA). Results suggest that landslide densities are quantifiably related to PGA. Thresholds of PGAcr for landslide failure are ![Graphic][1] for average horizontal PGA and ![Graphic][2] for average vertical PGA. As with other earthquakes, the distribution of the Wenchuan earthquake‐triggered landslides can be described by two equations, based on area and source densities, respectively. The similar damping factors of ∼20 in these two equations suggest that both density distribution patterns of landslides reflect the dissipation of seismic energy. Landslide‐derived PGA equations were then produced for the hanging‐wall region of the earthquake‐affected area. The results indicate that it should be possible to produce an earthquake shake map from the distribution of earthquake‐triggered landslides in an area where there are few or no seismic instruments but many landslides. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Mountain ranges that are actively forming around the western and northern perimeter of the Indo-Eurasia collisional deformational field, such as the Mongolian Altai, comprise a unique class of intracontinental intraplate transpressional orogen with structural and basinal elements that are distinct from contractional and extensional orogens. Late Cenozoic uplift and mountain building in the Mongolian Altai is dominated by regional-scale dextral strike–slip faults that link with thrust and oblique–slip faults within a 300-km-wide deforming belt sandwiched between the more rigid Junggar Basin block and Hangay Precambrian craton. Dominant orogenic elements in the Mongolian Altai include double restraining bends, terminal restraining bends, partial restraining bends, single thrust ridges, thrust ridges linked by strike–slip faults, and triangular block uplifts in areas of conjugate strike–slip faults. The overall pattern is similar to a regional strike–slip duplex array; however, the significant amount of contractional and oblique–slip displacement within the range and large number of historical oblique–slip seismic events renders the term “transpressional duplex” more accurate. Intramontane and range flanking basins can be classified as ramp basins, half-ramp basins, open-sided thrust basins, pull-apart basins, and strike–slip basins. Neither a classic fold-and-thrust orogenic wedge geometry nor a bounding foredeep exists. The manner in which upper crustal transpressional deformation is balanced in the lower crust is unknown; however, crustal thickening by lower crustal inflation and northward outflow of lower crustal material are consistent with existing geological and geodetic data and could account for late Cenozoic regional epeirogenic uplift in the Russian Altai and Sayan regions.

The Sutai Range is a structural and topographic culmination at the southeastern end of the Mongolian Altai and a world-class example of an actively forming restraining bend. The range occurs at a major stepover zone along the Tonhil dextral strike-slip fault within a wider region dominated by late Cenozoic transpressional deformation. Analysis of satellite imagery and the results of field investigations reveal that the range is structurally asymmetric with an overall NE tilt due to several major SE-directed thrusts within the core of the range and along its SW margin. The distribution of alluvial sediments shed from the range and stream length asymmetries also indicate a regional NE tilt for the range. Faults that splay off of the main Tonhil Fault bound discrete uplifted blocks that define an oblique-slip duplex at the surface and asymmetric flower structure in cross-section. Outward growth of the range is partly accommodated by growth of foreberg thrust ridges within the adjacent Dariv Basin. Thrust blocks within the centre of the range expose basement schists and mylonitic granite suggesting that the greatest uplift and crustal exhumation has occurred within the core of the restraining bend, although much of the exhumation is likely to be due to older Palaeozoic structural events. The southeast and northwest ends of the range are characterized by smooth unbroken surface ramps (“gangplanks”) that are upwarped towards the centre of the range where maximum Cenozoic uplift has occurred. The geometry and evolution of Sutai Uul and other intracontinental and intraplate restraining bends is fundamentally influenced by the initial width of the stepover zone, the attitude of regional basement structures and extent of brittle reactivation, the direction of SHmax relative to basement structures, and progressive fault and block rotation which may change the kinematics along faults or lead to their abandonment.

The Gobi Altai is the easternmost extension of the Mongolian Altai and consists of topographically discontinuous E-W-trending ranges with peaks averaging 2000–3000 m in elevation. The region is seismically active and characterized by prominent E-W left-lateral strike-slip faults that localize transpressional deformation and uplift along their lengths and at stepover zones. This report summarizes structural field investigations made in the easternmost Gobi Altai to document the structural geometry and style of late Cenozoic transpressional deformation in the region in order to better understand processes of intracontinental mountain building and the distant intracontinental strain response to the Indo-Eurasian collision. The Artsa Bogd range marks the northeastern terminus of the Gobi Altai and is topographically asymmetric with a high northern margin marked by N-vergent thrust faults and left-lateral oblique-slip faults. The northern side of the range is also bounded by a foreland basin that contains N-vergent thrust faults and folds that deform Quaternary sediments. The southern margin of Artsa Bogd appears tectonically inactive but contains S-vergent thrust faults and left-lateral wrench zones. The range appears to have a flower structure cross-sectional geometry that may reflect transpressional inversion of a Mesozoic basin. The isolated, high and narrow Tsost Uul range south of Artsa Bogd occupies a restraining bend position along the left-lateral Tsost Uul strike-slip fault system. Major faults within the range define a half-flower structure cross-sectional geometry. To the south of the Tsost Uul range, the Gobi Bulag left-lateral strike-slip fault system is marked by small push-up ridges and one major restraining bend mountain where the fault steps to the right near its western end. Throughout the region, Late Cretaceous-Tertiary basalts and Tertiary and Quaternary sediments are deformed by the major fault systems indicating late Cenozoic fault activity. These fault systems and the ranges formed along them occur at fairly regular intervals (approximately 20 km) between the North Gobi Altai fault system and the Gobi Tien Shan fault system, two major left-lateral strike-slip faults that cut across southern Mongolia. Together the faults define a parallel array of discrete linear belts of Cenozoic E-W left-lateral transpressional deformation south of the Hangay Dome. The regular spacing of the fault systems may suggest more uniform distributed left-lateral flow at depth. Eastward-directed lower crustal and lithospheric mantle flow is suggested by existing seismic anisotropy data for the eastern Gobi Altai and is believed to be the driving force for the upper crustal deformation.

The Gobi Altai region of southwestern Mongolia is a natural laboratory for studying processes of active, transpressional, intracontinental mountain building at different stages of development. The region is structurally dominated by several major E—W left-lateral strike-slip fault systems. The North Gobi Altai fault system is a seismically active, right-stepping, left-lateral, strike-slip fault system that can be traced along the surface for over 350 km. The eastern two-thirds of the fault system ruptured during a major earthquake (M = 8.3) in 1957, whereas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the Quaternary. The highest mountains in the Gobi Altai are restraining bend uplifts along the length of the fault system. Detailed transects across two of the restraining bends indicate that they have asymmetric flower structure cross-sectional geometries, with thrust faults rooting into oblique-slip and strike-slip master faults. Continued NE-directed convergence across the fault system, coupled with left-lateral strike-slip displacements, will lead to growth and coalescence of the restraining bends into a continuous sublinear range, possibly obscuring the original strike-slip fault system; this may be a common mountain building process. The largely unknown Gobi-Tien Shan fault system is a major left-lateral strike-slip fault system (1200 km + long) that links the southern ranges of the Gobi Altai with the Barkol Tagh and Bogda Shan of the easternmost Tien Shan in China. Active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section, indicating Quaternary activity. The total displacement is unknown, but small parallel splays have apparent offsets of 20 + km, suggesting that the main fault zone has experienced significantly more displacement. Field investigations conducted at two locations in southwestern Mongolia indicate that late Cenozoic transpressional uplift is still active along the fault system. The spatial relationship between topography and active faults in the Barkol Tagh and Bogda Shan strongly suggests that these ranges are large, coalescing, restraining bends that have accommodated the fault's left-lateral motion by thrusting, oblique-slip displacement and uplift. Thus, from a Mongolian perspective, the easternmost Tien Shan formed where it is because it lies at the western termination zone of the Gobi-Tien Shan fault system. The Gobi-Tien Shan fault system is one of the longest fault systems in central Asia and, together with the North Gobi Altai and other, smaller, subparallel fault systems, is accommodating the eastward translation of south Mongolia relative to the Hangay Dome and Siberia. These displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topographically low, E–W trending corridor between the northern edge of the Tibetan Plateau and the Hangay Dome, presumably in response to the Indo-Eurasian collision 2500 km to the south.

We have attempted to quantify the relative motion history between southernmost South America (SSA) and the Antarctic Peninsula (AP) by calculating and comparing SSA-Africa, AP-Africa and SSA-AP synthetic flow lines for 84–0 Ma. The flow lines were created using published poles of rotation and plate reconstruction software. The results indicate that since 84 Ma, SSA and AP have moved approximately westward relative to a fixed Africa; however, SSA's rate of westerly motion in that reference frame has been significantly more rapid than AP's rate. Approximately 1320 km of east-west, left-lateral strike-slip displacement and 490 km of north-south, divergent displacement have occurred between the southern tip of SSA and the northern tip of AP since 84 Ma. Increased rates of SSA-AP interplate separation and a change in the angle of plate divergence at approximately 55–40 Ma marked the onset of accelerated continental separation that eventually led to seafloor spreading in the western Scotia Sea at 30 Ma and the development of the Scotia Arc. Increased separation rates between SSA and AP at 55–40 Ma may be related to a global Eocene plate reorganization event. The northeast-southwest oriented western Scotia Sea spreading centers appear to have accommodated all of the SSA-AP interplate motion between 30 and 9 Ma. We suggest that prior to 30 Ma and the opening of Drake Passage, components of interplate strike-slip and divergent motion were accommodated by intracontinental deformation that included strike-slip faulting, counterclockwise tectonic rotation, and continental extension in the southernmost Andes. The results indicate that the opening of the Scotia Sea was caused by plate-scale motions as SSA and AP drifted away from Africa at different velocities along different, nonparallel trajectories. Subduction retreat along the South Scotia Ridge and South Sandwich arc and back arc spreading in the Scotia Sea contributed to the width of separation between SSA and AP across Drake Passage. The results place limits on how SSA-AP relative motion has been temporally and spatially partitioned in the Scotia Arc region.

[1] Spectacular refolded folds clearly visible from space deform Permian turbidite successions in a 180 km2 fault-bounded compartment within the Beishan region of northwestern China. The folds formed after final terrane amalgamation of the Central Asian Orogenic Belt and record an important sequence of intraplate deformation events in central Asia during the Late Permian-Jurassic. The fold interference patterns are dominantly Type 2 refolds with kilometer-scale wavelengths. They are bounded to the north and south by regional E-W faults with multiple displacement histories as revealed by crosscutting ductile and brittle fabrics and kinematic indicators indicating opposing senses of motion. The proximity of the refolded folds to the main bounding faults strongly suggests that the fold interference structures were caused by strike-slip fault reactivation and shear related bending of F1 folds combined with components of N-S compression across the compartment. Faulting within the folded compartment is limited to an array of right-lateral strike-slip faults which appear to have helped accommodate room problems during the refolding event. The driving force for the deformation was likely to be distant—including regional block collision to the south, large-scale rotations between the Siberian and North China Cratons and plate boundary convergence along the eastern Chinese margin. Our study demonstrates that continental interior regions characterized by terrane collages are susceptible to reactivation by distant plate margin effects and that polyphase reactivation of major strike-slip and oblique-slip faults can generate refolding within upper crustal basinal compartments. In addition, the fault-bounded basin containing the refolded folds in the Beishan region is surrounded by older more complexly deformed crystalline basement. Therefore, the deformed basin sequence represents a unique compartmental archive of the Permian-Triassic history of intraplate deformation north of Tibet.

The Patagonian orocline is the 90° bend in the southernmost Andes between 50°S and 56°S. Paleomagnetic and structural data indicate that the orocline is, at least in part, the product of tectonic rotation. Recent field work in the Beagle Channel region of southernmost Chile provides evidence for widespread left-lateral strike-slip faulting in the internal zones of the mountain belt. Both arms of the Beagle Channel are interpreted to be left-lateral strike-slip faults based on detailed study of mesoscale strike-slip faults (Riedel shears) observed in coastal outcrops. Although much of the evidence indicates Cenozoic brittle strike-slip faulting, other fabric data, including vertical foliation zones containing horizontal quartz stretching lineations and ductile left-lateral kinematic indicators, suggest that Mesozoic ductile strike-slip or oblique-slip shearing also occurred. The implication is that the mid-Cretaceous Andean orogeny involved the transpressional inversion of the Rocas Verdes marginal basin and that transpression has been the dominant deformational regime in the region for the last 120 Ma. Regional left-lateral strike-slip faults are now recognized in all lithotectonic provinces of the southernmost Andes. A statistical study of regional lineament trends using aerial photographs and satellite imagery suggests that many unstudied lineaments are also strike-slip faults. A new model is proposed that integrates the development of strike-slip faulting and the structural evolution and uplift of the southernmost Andes with the rotational development of the orocline. The Patagonian orocline appears to be the product of broad interplate shearing accommodated by strike-slip faulting, block rotation, and contraction and is probably continuing to evolve today.

Results are presented of the first airborne LiDAR survey ever flown in Europe for the purpose of mapping the surface expression of earthquake-prone faults. Detailed topographic images derived from LiDAR data of the Idrija and Ravne strike-slip faults in NW Slovenia reveal geomorphological and structural features that shed light on the overall architecture and kinematic history of both fault systems. The 1998 Mw = 5.6, and 2004 Mw = 5.2 Ravne Fault earthquakes and the historically devastating 1511 M = 6.8 Idrija earthquake indicate that both systems pose a serious seismic hazard in the region. Because both fault systems occur within forested terrain, a tree removal algorithm was applied to the data; the resulting images reveal surface scarps and tectonic landforms in unprecedented detail. Importantly, two sites were discovered to be potentially suitable for fault trenching and palaeo-seismological analysis. This study highlights the potential contribution of LiDAR surveying in both low-relief valley terrain and high-relief mountainous terrain to a regional seismic hazard assessment programme. Geoscientists working in other tectonically active regions of the world where earthquake-prone faults are obscured by forest cover would also benefit from LiDAR maps that have been processed to remove the canopy return and reveal the forest floor topography.

Most geoscientists acknowledge the fundamental importance of field-based research. Many of us have listened to lectures involving numerical modeling of some geological process and concluded that the speaker should see more outcrops. Some scientists appear to have forgotten that geological and geophysical field data have fueled many, if not most, major developments in Earth science. Christopher Scholz celebrates that point in Fieldwork: A Geologist's Memoir of the Kalahari, a highly readable and entertaining account of a microearthquake survey carried out 23 years ago in the Kalahari region of Botswana to investigate the propagating tip of the southwestern arm of the East African rift system and the effects of ground displacements on the hydrogeology of the Okavango Delta.

The Hardy Formation is a 1300-m-thick succession of Upper Jurassic-Lower Cretaceous volcaniclastic sedimentary rocks interbedded with lava flows on Hoste Island at the southernmost tip of South America (55.5°S, 291.8°E). The strata are gently folded and metamorphosed to the prehnite-pumpellyite grade. A well-defined characteristic direction of magnetization, carried by magnetite, was readily identified in 95 samples from seven sites. At a given site, the directions group slightly better without structural correction. However, the means of the seven sites cluster better without tilt correction at the 99% significance level, implying that the magnetization postdates the folding event. It is most likely that the magnetization was acquired during the mid- to Late Cretaceous Andean orogeny that involved the folding and emplacement of the Patagonian Batholith. The fact that all samples are normally magnetized supports this age assignment. The pole position of 42.9°N, 156.6°E, α95=3.3° implies that the sampling area has rotated counterclockwise relative to cratonic South America by 90.1±11.9° with no significant flattening of inclination (F=1.9 ± 3.7°). Geologic considerations indicate that the rotation involved the entire Andean magmatic arc in Tierra Del Fuego. The results support interpretation of the Hardy Formation as part of the Andean magmatic arc deposited on the Pacific side of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. Oroclinal bending of the arc in southernmost South America accompanied inversion of the marginal basin and the development of a Late Cretaceous-Cenozoic left-lateral transform system (South America-Antarctica) that later developed into the North Scotia Ridge.