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Coseismic landslides pose immediate and prolonged hazards to mountainous communities, and provide a rare opportunity to study the effect of large earthquakes on erosion and sediment budgets. By mapping landslides using high-resolution satellite imagery, we find that the 25 April 2015 Mw7.8 Gorkha earthquake and aftershock sequence produced at least 25,000 landslides throughout the steep Himalayan Mountains in central Nepal. Despite early reports claiming lower than expected landslide activity, our results show that the total number, area, and volume of landslides associated with the Gorkha event are consistent with expectations, when compared to prior landslide-triggering earthquakes around the world. The extent of landsliding mimics the extent of fault rupture along the east-west trace of the Main Himalayan Thrust and increases eastward following the progression of rupture. In this event, maximum modeled Peak Ground Acceleration (PGA) and the steepest topographic slopes of the High Himalaya are not spatially coincident, so it is not surprising that landslide density correlates neither with PGA nor steepest slopes on their own. Instead, we find that the highest landslide density is located at the confluence of steep slopes, high mean annual precipitation, and proximity to the deepest part of the fault rupture from which 0.5–2 Hz seismic energy originated. We suggest that landslide density was determined by a combination of earthquake source characteristics, slope distributions, and the influence of precipitation on rock strength via weathering and changes in vegetation cover. Determining the relative contribution of each factor will require further modeling and better constrained seismic parameters, both of which are likely to be developed in the coming few years as post-event studies evolve. Landslide mobility, in terms of the ratio of runout distance to fall height, is comparable to small volume landslides in other settings, and landslide volume-runout scaling is consistent with compilations of data on larger slope failures. In general, the size ratios of landslide source area to full landslide area are smaller than global averages, and hillslope length seems to largely control runout distance, which we propose reflects a topographic control on landslide mobility in this setting. We find that landslide size dictates runout distance and that more than half of the landslide debris was deposited in direct connection with stream channels. Connectivity, which is defined as the spatial proximity of landslides to fluvial channels, is greatest for larger landslides in the high-relief part of the High Himalaya. Although these failures are less abundant than those at lower elevations, they may have a disproportionate impact on sediment dynamics and cascading hazards, such as landslide reactivation by monsoon rainfall and landslide dams that lead to outburst floods. The overall high fluvial connectivity of coseismic landsliding in the Gorkha event suggests coupling between the earthquake cycle and sediment/geochemical budgets of fluvial systems in the Himalaya., Geomorphology, 301, ISSN:0169-555x, ISSN:1872-695X

Thousands of landslides occurred during the April 2015 Gorkha earthquake in Nepal. Previous work using satellite imagery mapped nearly 25,000 coseismic landslides. In this study, the satellite-based mapping was analyzed in three areas where field deployment was also conducted—the Budhi Gandaki, Trishuli, and Indrawati river valleys—to better characterize the landslides. Unmanned aerial vehicles (UAVs) were deployed to map the three-dimensional (3-D) geometry of failed slopes using photogrammetry, as well as to characterize rock structure and strength. The majority of landslides were rock slides along the ridges and the steeper portions of the basins primarily involving the weathered rock zone. Additional landslides included rock falls and soil failures. Satellite imagery analysis indicated that landsliding was concentrated north of the physiographic transition, in steep areas, and in close proximity to the major rivers. The Trishuli area experienced the lowest landslide density in terms of number of landslides compared to the Budhi Gandaki and Indrawati areas, although all three areas had similar density in terms of total landslide area and other landslide statistics.

The 25 April 2015 M w 7.8 Gorkha earthquake produced strong ground motions across an approximately 250 km by 100 km swath in central Nepal. To assist disaster response activities, we modified an existing earthquake-triggered landslide model based on a Newmark sliding block analysis to estimate the extent and intensity of landsliding and landslide dam hazard. Landslide hazard maps were produced using Shuttle Radar Topography Mission (SRTM) digital topography, peak ground acceleration (PGA) information from the U.S. Geological Survey (USGS) ShakeMap program, and assumptions about the regional rock strength based on end-member values from previous studies. The instrumental record of seismicity in Nepal is poor, so PGA estimates were based on empirical Ground Motion Prediction Equations (GMPEs) constrained by teleseismic data and felt reports. We demonstrate a non-linear dependence of modeled landsliding on aggregate rock strength, where the number of landslides decreases exponentially with increasing rock strength. Model estimates are less sensitive to PGA at steep slopes (> 60°) compared to moderate slopes (30–60°). We compare forward model results to an inventory of landslides triggered by the Gorkha earthquake. We show that moderate rock strength inputs over estimate landsliding in regions beyond the main slip patch, which may in part be related to poorly constrained PGA estimates for this event at far distances from the source area. Directly above the main slip patch, however, the moderate strength model accurately estimates the total number of landslides within the resolution of the model (landslides ≥ 0.0162 km 2 ; observed n = 2214, modeled n = 2987), but the pattern of landsliding differs from observations. This discrepancy is likely due to the unaccounted for effects of variable material strength and local topographic amplification of strong ground motion, as well as other simplifying assumptions about source characteristics and their relationship to landsliding.

Earthquakes cause widespread landslides that can increase erosional fluxes observed over years to decades. However, the impact of earthquakes on denudation over the longer timescales relevant to orogenic evolution remains elusive. Here we assess erosion associated with earthquake-triggered landslides in the Longmen Shan range at the eastern margin of the Tibetan Plateau. We use the M w 7.9 2008 Wenchuan and M w 6.6 2013 Lushan earthquakes to evaluate how seismicity contributes to the erosional budget from short timescales (annual to decadal, as recorded by sediment fluxes) to long timescales (kyr to Myr, from cosmogenic nuclides and low temperature thermochronology). Over this wide range of timescales, the highest rates of denudation in the Longmen Shan coincide spatially with the region of most intense landsliding during the Wenchuan earthquake. Across sixteen gauged river catchments, sediment flux-derived denudation rates following the Wenchuan earthquake are closely correlated with seismic ground motion and the associated volume of Wenchuan-triggered landslides ( r 2 > 0.6 ), and to a lesser extent with the frequency of high intensity runoff events ( r 2 = 0.36 ). To assess whether earthquake-induced landsliding can contribute importantly to denudation over longer timescales, we model the total volume of landslides triggered by earthquakes of various magnitudes over multiple earthquake cycles. We combine models that predict the volumes of landslides triggered by earthquakes, calibrated against the Wenchuan and Lushan events, with an earthquake magnitude–frequency distribution. The long-term, landslide-sustained “seismic erosion rate” is similar in magnitude to regional long-term denudation rates (∼0.5–1 mm yr−1). The similar magnitude and spatial coincidence suggest that earthquake-triggered landslides are a primary mechanism of long-term denudation in the frontal Longmen Shan. We propose that the location and intensity of seismogenic faulting can contribute to focused denudation along a high-relief plateau margin.

We evaluate the mass balance of the Indo-Asian orogen by reconstructing the Indian and Asian margins prior to collision using recently published paleomagnetic and surface shortening constraints, and subtracting modern crustal volumes derived from gravity inversions and deep seismic soundings. Results show a ~30% deficit between original and modern orogen volumes if the average global crustal thickness of 41 km is assumed prior to collision, even once eastward extrusion and crustal flow are considered. Such a large discrepancy requires crustal recycling of a magnitude that is greater than one half of the modern orogenic mass, as others have previously suggested. Proposals for extensive high elevations prior to or soon after the collision further exacerbate this mismatch and dramatically increase the volume of material necessary to be placed into the mantle. However, we show that this discrepancy can be eliminated with a 23–29 km thick crust within the orogen prior to collision along with a thick southern Tibet margin (the Lhasa and Qiangtang terranes). Because of the relatively low magnitude of surface shortening in Asia, an initially thin crust would require underplating of Indian crust in southern Tibet and displacement of a highly mobile lower crust to the north and east in order to explain modern crustal thicknesses. The contrast between a proposed thinner Asian interior and older and thicker lithosphere of the North China block may have defined the distal extent of deformation at the time of collision and since.

The Tibetan Plateau is a prime example of a collisional orogen with widespread strike-slip faults whose age and tectonic significance remain controversial. We present new low-temperature thermochronometry to date periods of exhumation associated with Kunlun and Haiyuan faulting, two major strike-slip faults within the northeastern margin of Tibet. Apatite and zircon (U-Th)/He and apatite fission-track ages, which record exhumation from ~2 to 6 km crustal depths, provide minimum bounds on fault timing. Results from Kunlun samples show increased exhumation rates along the western fault segment at circa 12–8 Ma with a possible earlier phase of motion from ~30–20 Ma, along the central fault segment at circa 20–15 Ma, and along the eastern fault segment at circa 8–5 Ma. Combined with previous studies, our results suggest that motion along the Haiyuan fault may have occurred as early as ~15 Ma along the western/central fault segment before initiating at least by 10–8 Ma along the eastern fault tip. We relate an ~250 km wide zone of transpressional shear to synchronous Kunlun and Haiyuan fault motion and suggest that the present-day configuration of active faults along the northeastern margin of Tibet was likely established since middle Miocene time. We interpret the onset of transpression to relate to the progressive confinement of Tibet against rigid crustal blocks to the north and expansion of crustal thickening to the east during the later stages of orogen development.

Temporal variations in the orientation of Cenozoic range growth in northeastern Tibet define two modes by which India-Asia convergence was accommodated. Thermochronological age-elevation transects from the hanging walls of two major thrust-fault systems reveal diachronous Miocene exhumation of the Laji-Jishi Shan in northeastern Tibet. Whereas accelerated growth of the WNW-trending eastern Laji Shan began ca. 22 Ma, rapid growth of the adjacent, north-trending Jishi Shan did not commence until ca. 13 Ma. This change in thrust-fault orientation refl ects a Middle Miocene change in the kinematic style of plateau growth, from long-standing NNE-SSW contraction that mimicked the plate convergence direction to the inclusion of new structures accommodating east-west motion. This kinematic shift in northeastern Tibet coincides with expansion of the plateau margin in southeastern Tibet, the onset of normal faulting in central Tibet, and accelerated shortening in northern Tibet. Together these phenomena suggest a plateau-wide reorganization of deformation.

Models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary. Yet a growing body of evidence in the Tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen's history and thus our current mechanical understanding of orogenic plateau development is incomplete. New apatite (U–Th)/He ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures. Accelerated erosion is used as a proxy of fault timing, and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins. Helium results are synthesized with published geologic, thermochronometric, and sedimentologic data from which a growing picture of regional compressional deformation in Middle to Late Eocene time in northern Tibet emerges. We relate the early Cenozoic period of deformation to the initiation of collision between India and Eurasia, despite the fact that the plate boundary was located > 3000 km to the south. Regardless of whether or not high topography was built simultaneously as a result of this deformation, early Cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time.

Structural patterns, global positioning system (GPS) velocities, and Quaternary fault slip rates in northeastern Tibet indicate a transfer of left-lateral slip from the Kunlun fault northeast to the Haiyuan fault and minor crustal shortening and rotation within a 200-km-wide stepover zone. Related deformation also continues at least a few hundred kilometers north of the Haiyuan fault into a region of diffuse extensional(?) shear or rotation underlain by average thickness crust. Fast, localized slip along the central Kunlun fault transforms into distributed deformation across a 500-km-wide zone where the lower crust is weak. The distribution of fault-parallel GPS velocities across this region suggests a decrease in fault slip toward eastern fault tips and progressive dissipation of slip to the north rather than east of the Tibetan Plateau as previously suggested.

A common feature of convergent plate boundaries is the self-organization of strain, exhumation and topography along discrete, arcuate boundaries. Deviations from this geometry can represent first-order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen. The simplicity of the Himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km-long Shillong Plateau has developed. This change in strain partitioning affects nearly 25% of the arc and has not previously been considered to be important to the orogen's development. New low-temperature thermochronometry data suggest this structure initiated in mid to late Miocene time, significantly earlier than was previously estimated from the sedimentary record alone. Development of the Shillong Plateau may be linked to a number of kinematic changes within the Himalayan and Burman collision zones that occur at the same time. These events include the onset of E–W extension in central Tibet, eastward expansion of high topography of the Tibetan Plateau, onset of rotation of crustal fragments in southeastern Tibet, and re-establishment of eastward subduction beneath the Indo-Burman ranges. We suggest that the coincidence of these tectonic events is related to the ‘dismemberment’ of the eastern Himalayan arc, signifying a change in regional stress applied along the India–Eurasia–Burma plate boundaries. Discrepancies between vertical long-term faulting rates and geodetically derived far-field convergence rates suggest that the collisional boundary in the eastern Himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary. The introduction of dense material into the plate boundary late in the orogen's history may explain regional changes in the strain field that affect not only the Himalaya, but also the deformation field more than 1000 km into the Tibetan Plateau.

Recent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10-20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north-south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present-day Tibetan Plateau, deformed, and its north-south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since ~15 Ma and a less precisely dated change in orientation of crustal shortening since ~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE-SSW in northern Tibet to more NE-SW and even ENE-WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation. Key Points The north-south limits of Tibet were set by lateral variations in strength Roughly 15 million years ago, deformation of NE Tibet accelerated Since 20-15 million years ago, the orientation of shortening rotated eastward ©2013. American Geophysical Union. All Rights Reserved.

Field observations from Gonto La (southern Tibet), a pass through the high Himalaya, reveal a continuous, planar, ∼10°N-dipping detachment horizon (the Gonto La detachment). The detachment juxtaposes Tethyan dark slates over a footwall of extensive leucogranite of the Khula Kangri pluton, intruded into an injection complex layer regarded as an early Southern Tibet Detachment System (STDS) horizon. The leucogranite emplacement is protracted, and overlaps the STDS development. It is observed to intrude the earlier horizon of the STDS, which is deformed, partially cut by the pluton, and, in the southern part, rotated to a present south dip. Evidence for large scale folding and plutonism is also found east of Khula Kangri at Lhozag-La Kang, where the earlier STDS horizon is inferred. Here it excises the entire Palaeozoic Tethyan sedimentary sequence and is similarly folded. The Gonto La detachment, which cuts the Middle-Upper Miocene Khula Kangri pluton is, in turn, cut by the more steeply N-dipping Dzong Chu fault, demonstrating later N-S extension in this area. We interpret known outliers of Tethyan sequences in Bhutan as klippen, underlain by an early STDS horizon, which provide a more regional illustration of early extension on the STDS. In the Khula Kangri area, early extension was followed by plutonism and local relative uplift, and further N-S extension.

[1] Crustal deformation along the present-day northern margin of the Tibetan Plateau has occurred since mid to late Eocene time, soon after India collided with Eurasia. Assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet, we show that far-field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins. When a strong region analogous to cratonic lithosphere like that of the Tarim Basin is included in the calculations, shear and vertical strain rates concentrate along the southern edge of the strong region, resulting in enhanced deformation there. In calculations scaled to the dimensions of India and the Tibetan Plateau, shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins. Crustal thickening accelerates later in the calculations. We propose that the apparent increased exhumation (and perhaps surface uplift) around 10 Ma may signal the arrival of the thickening front.

INDEPTH seismic reflection profiling shows that the decollement beneath which Indian lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the Himalayan deformation front to a depth of approximately 50 kilometers. Prominent reflections appear at depths of 15 to 18 kilometers near where the decollement reflector apparently terminates. These reflections extend north of the Zangbo suture to the Damxung graben of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright spots) and coincident negative polarities implying that they are produced by fluids in the crust. The presence of geothermal activity and high heat flow in the regions of these reflections and the tectonic setting suggest that the bright spots mark granitic magmas derived by partial melting of the tectonically thickened crust.