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Large-scale detailed mapping plays a key role in revealing the rupture characteristics and mechanisms of strong earthquakes. Relatively few studies have been performed on the surface ruptures of large earthquakes in central and western Tibet due to its remote nature and high elevation. Based on high-resolution unmanned aerial vehicle (UAV) photography, we mapped the coseismic surface rupture of the 2014 Yutian Ms7.3 earthquake. Along the western Altyn Tagh fault system, the earthquake produced ≈37 km of surface rupture along the South Xor Kol fault (southern section S1), Xor Kol fault (central section S2) and Ashikule fault (northern section S3). Section S1 has a 16-km-long surface rupture with an average sinistral offset of 52±25 cm and a maximum offset of ~90 cm, while section S3 has a 14.2-km-long surface rupture with an average sinistral offset of 36±21 cm and a maximum offset of ~84 cm. A compilation of 5308 cracks yields an average crack width along the southern section of 85±71 cm and a maximum width of ~700 cm; the average width along the central section is 39±21 cm, and the maximum width is 243 cm; and the average width along the northern section is 61±44 cm with a maximum of ~340 cm. In addition, the average cumulative opening across rupture zone is 3.4±2.9 m along the southern section, with a maximum of ~17 m; 4.3±3.6 m along the central section, with a maximum of ~13 m; and 1.7±1.6 m along the northern section, with a maximum of ~6 m. Evidently, the average crack width and cumulative opening decrease towards bends and steps along the fault. A global synthesis of surface rupture distributions corresponding to strike-slip earthquakes indicates that the rupture zone is wider near the complex parts of fault geometries (such as bends, steps and fault bifurcations) than along straight sections, suggesting that the fault geometry has an obvious control on the surface rupture width. The widespread cracks at the intersection between the Xor Kol and South Xor Kol faults may indicate that an extensional regime is more likely to produce distributed offfault deformation, which provides an observational constraint for the numerical simulation of dynamic rupture on a fault. In addition to coseismic surface rupture, the Yutian earthquake also produced a large number of gravity-driven slides on alluvial fans with gentle slopes. The friction efficiency of the water-bearing salt layer beneath fans could decrease the sliding threshold and trigger instability under surface shaking. These distributed deformations and gravity-driven slides reflect the coupling between the rupture propagation and fault geometry and indicate that the rupture may have propagated in two directions along the Ashikule fault after passing through a step. Therefore, the investigation of coseismic surface rupture provides important observational constraints on the dynamic rupture process. [ABSTRACT FROM AUTHOR]

The topographic evolution of SE Tibet is essential for understanding the growth of this orogenic plateau margin and the mechanism of continental deformation in general. Stable isotopes have been used to construct the paleo-elevation of SE Tibet during the Cenozoic. However, this methodology's reliability for the region is controversial; the results require validation through other independent constraints. The Eocene-Oligocene adakite-like rocks exposed along the Jinshajiang Suture are mostly the production of melts from the lower crust, and thus their geochemical features are used to track ancient crustal thickness and plateau elevation of SE Tibet. The whole-rock [La/Yb] N ratios indicate a thickened SE Tibet. Meanwhile, the paleo-elevation based on the calculated crustal thickness shows that central and southeastern Tibet had reached modern heights by the early Oligocene, corroborating results by stable isotope paleoaltimetry. Synthesizing previous studies and our results, we suppose that the deformation caused by the India-Aisa collision had propagated to and thickened central and southeastern Tibet before the Eocene-Oligocene. The uplifted and low-gradient plateau margin of SE Tibet had already been formed by this time. The lateral extrusion has mainly accommodated the deformation with minor shortening since the late Eocene. • Eocene-Oligocene adakite-like rocks exposed in SE Tibet were derived from melts in the deep crust. • Whole-rock La/Yb ratio-derived Moho depth indicates a thickened SE Tibet during the late Eocene-early Oligocene. • The paleo-elevation of SE Tibet had reached modern heights before the early Oligocene. [ABSTRACT FROM AUTHOR]

Understanding the three‐dimensional structure, segmentation, and kinematics of complex fault systems is essential to assessing the size of potential earthquakes and related seismic hazards. The Danghe Nan Shan thrust, a major splay of the Altyn Tagh fault (ATF) in north Tibet, is one of these complex fault junctions. Near the town of Subei, the western Danghe Nan Shan thrust composes two left‐stepping faults outlined by fault scarps in front of folded and uplifted alluvial fans and terraces. Age constraints and 2D reconstructions of the accumulated slip above a transient base level of four terraces standing 7–60 m above the present stream bed yield shortening and vertical uplift rates of 0.5 ± 0.1 and 1.1 ± 0.3 mm/yr, respectively, over the last 130 ka on the southern thrust. Along the northern thrust, vertical terrace offsets of 1.5–3.6 m and horizontal slip of 4.5 m documented in a paleoseismological trench occurred after 12 ± 4 ka, constraining coeval rates of 0.3 ± 0.1 mm/yr for uplift and shortening. Overall, 1.4 ± 0.4 mm/yr terrace uplift and 0.8 ± 0.2 mm/yr shortening rates are determined, in agreement with late Miocene long‐term exhumation rate estimates. Our fault mapping and geomorphic and structural observations imply that the western Danghe Nan Shan thrust accommodates slip transfer from the ATF to the west to thrusting and shortening farther east in the Qilian Shan region. Considering the scarp sizes, their lateral extent, the geometry of the faults at depth, and their slip‐rate, we suggest the possible occurrence of Mw 7+ earthquakes near Subei. Plain Language Summary: How faults slip is essential to understanding continental deformation and seismic hazards. In the western Qilian Shan region, the Danghe Nan Shan is one of the largest ranges bounded by active thrust faults. West and south of Subei, these thrusts can be followed for several kilometers along meter‐high scarps and folded terraces. Quantifying the cumulative deformation of the folded and uplifted terraces and fans enables to constrain shortening rates of about 1 mm/yr over the last 130 ka. Evidence from surface geomorphology and sub‐surface trenching across the faults indicates the occurrence of large magnitude potentially up to Mw 7 seismic events in the last 16 ka. Integrating age‐controlled cumulative geological observations with field‐documented co‐seismic deformation increases the knowledge of slip rate and co‐seismic displacement for seismic hazards. Key Points: Combined cosmogenic nuclides, optically stimulated luminescence, and 14C dating methods constrain the deposition age of four folded inset terraces over the last 130 kaThe shortening rate of 0.8 ± 0.2 mm/yr across the western Danghe Nan Shan thrusts is coeval with decreasing left‐slip along the Altyn Tagh faultConcurrent rupture of thrusts and strike‐slip fault strands could result in a Mw 7+ earthquake southwest of Subei town [ABSTRACT FROM AUTHOR]

Repeating earthquakes have been found at many faults around the world. The Haiyuan fault is a major left‐lateral strike‐slip fault along the northeastern boundary of the Tibetan Plateau. Two great earthquakes (1920 Haiyuan and 1927 Gulang) have occurred on and round this fault system, but the section between the ruptures of the two earthquakes, also known as the Tianzhu seismic gap, remains unbroken. Shallow creep has been observed from geodetic data at the Laohushan section of the Haiyuan fault near the eastern end of the seismic gap. However, the driving mechanism and depth extent of shallow creep are not clear. Here we conduct a systematic search for repeating earthquakes in northeastern Tibet based on seismic data recorded by permanent stations in 10 years (2009–2018). Based on waveform cross‐correlations, we find several repeating earthquake clusters at the Laohushan section. This is consistent with the shallow creep inferred from the geodetic data, indicating repeating earthquakes can be driven by nearby aseismic slip. Approximately 300 repeaters were found within clusters of intense seismicity near the rupture zones of the 1927 M8.0 Gulang and 2016 M6.4 Menyuan earthquakes. Relocation of events in the cluster near the Gulang earthquake delineates two possible unmapped faults orthogonal to the Haiyuan fault. In addition, we also identify several repeating earthquakes generated by mining activities with different waveforms and occurrence patterns. Our study suggests that repeating earthquakes around the Haiyuan fault are likely driven by long‐term postseismic relaxation process associated with the 1920 Haiyuan and 1927 Gulang earthquakes. Plain Language Summary: Repeating earthquakes usually have a similar magnitude and occur at the same fault patch, and they generate very similar seismic waves recorded by seismic stations. Repeating earthquakes are mostly found in fast‐slipping faults near major plate boundary. In this study, we systematically search for repeating earthquakes along the Haiyuan Fault, a major left‐lateral fault along the northeastern edge of the Tibetan Plateau. In the past 100 years, two great earthquakes (the 1920 Haiyuan and 1927 Guhang) with magnitude around 8 occurred on and around this fault. We find repeating earthquakes at the Laohushan section of the Haiyuan fault, which mark the boundary between the creep and locked regions, and the western end of the fault that ruptured in the 1920 great Haiyuan earthquake. In addition, we find several clusters of repeating earthquakes around the aftershock zone of the 1927 Gulang earthquake. We suggest that those repeating events are likely driven by long‐term creep in the surrounding faults following both great earthquakes. Key Points: Repeating earthquakes are found at the creeping Laohushan section of the Haiyuan FaultIntense seismicity at the Gulang seismic zone reveals two possible hidden faultsRepeating earthquakes along the Haiyuan Fault are likely driven by postseismic processes of historic large earthquakes [ABSTRACT FROM AUTHOR]

The evolution of the Mekong River valley offers a unique record of the rock uplift and climate history of southeastern Tibet. Faster erosion rates along the valley than those on the flanks caused valley incision and local incision rates may reflect a mix of driving mechanisms, migrating knickpoints linked to a change in base level, and large-scale surface uplift. The thermochronometric-constrained erosion rates in the 2–3 km deep river channel compared to rates outside of the channels provide unique constraints on the driving mechanisms of the river incision. In particular, the onset of river incision at various points along the Mekong River helps identify whether exhumation signals are the result of knickpoint migration that would generate a younger incision upstream, or the outgrowth of the plateau, which would produce younger incision downstream. We analyze new and published apatite (U Th)/He ages collected along two elevation transects in the upper Mekong River: downstream at Tu'Er and upstream at Deqin. The data and thermokinematic modeling predict that the exhumation rates of the valley near Tu'Er and Deqin increased more rapidly than those of adjacent high-elevation, low-relief surfaces at ~11 Ma and ~ 4 Ma, respectively. Since then, exhumation variations have caused an increase of >2 km in the depth of the valley, indicating that the major incision of the Mekong River valley began at ~11 Ma near Tu'Er and approximately 4 Ma near Deqin. The results indicate that a major phase of the Mekong River valley incision may have developed and progressed upstream since the Miocene. • Thermochronometric study of the fluvial incision history of the Mekong River • Presents new and existing apatite (U Th)/He ages from two elevation transects • Incision onset at 11 Ma downstream near Tu'Er town and 4 Ma upstream near DeQin town • 2 km deep valley formed by progressive upstream erosion since the Miocene • Exhumation-development patterns of Tibetan Plateau low-relief surfaces are nonuniform. [ABSTRACT FROM AUTHOR]

High‐resolution imaging of fault zone structures is essential for understanding earthquake physics and fault mechanics. As a major left‐lateral strike‐slip fault in northeastern Tibet, fine structures of the damage zone in the creeping (Laohushan) section of the Haiyuan fault remain unclear. To resolve geometry and velocity reduction of the damage zone, we deployed a dense temporary network of 110 seismic stations around the creeping section of the Haiyuan fault. Travel time delays from teleseismic P arrivals suggest an approximately 1‐km‐wide low‐velocity zone, likely illuminating a broader damage zone around the Haiyuan fault. A catalog is constructed for local earthquakes based on phase picks identified from a machine learning technique. The amplification of waveforms from these local events and waveform modeling of fault zone trapped waves indicate a narrower inner damage zone with depth‐dependent width (ranging from 150 to 50 m) that extends to a depth of approximately 4 km. These values are generally consistent with those found on other noncreeping faults in California, suggesting that these damage zone properties are not affected by fault slip behaviors at shallow depth. In addition, a clear bi‐material velocity contrast across the fault is revealed by the analysis of teleseismic P arrivals. Assuming the contrast extends to a depth of 15 km, we find that P‐wave velocity is approximately 5% slower in the crustal block north of the fault. Our study shows that a temporary dense seismic network is effective in illuminating cross‐fault velocity contrast and fault geometry. Plain Language Summary: The Haiyuan fault is a major left‐lateral strike‐slip fault in northeastern Tibet and has hosted two destructive earthquakes (1920 Haiyuan and 1927 Gulang earthquake) in the past century. Along the Laohushan section of the Haiyuan fault, between rupture zones of these two earthquakes, shallow aseismic creep and repeating earthquakes have been detected using geodetic and seismic data. However, the structure of this creeping section is still poorly understood. Therefore, we deployed a dense array with 110 stations crossing the fault surface trace. A catalog with more local earthquakes during the 1‐month recording period is constructed and relocated using this dense array. Based on analyses of signals from nearby and distant earthquakes, we find an approximately 100‐m‐wide slower fault inner zone that can trap incoming seismic energy and a wider (approximately 1‐km wide) low‐velocity damage zone beneath the array. The inferred fault zone structure is consistent with those found along other noncreeping faults in California, suggesting damage zone properties are likely not controlled by fault slip behaviors at shallow depth. The resulting high‐resolution fault zone image also helps improve ground motion prediction and rupture simulation in the Laohushan section of the Haiyuan fault. Key Points: A dense network of 110 seismic stations was deployed around the creeping section of the Haiyuan faultTravel time delays of teleseismic events suggest an approximately 1‐km‐wide low velocity zone around the Haiyuan faultAmplification and waveform modeling of local events infer a flower‐shaped inner damage zone, ranging from 150 to 50 m [ABSTRACT FROM AUTHOR]

The Lancang River (Upper Mekong) is a key large river in Asia with a prominent knickzone over 500 km distance (27.5‐30.7°N), as it descends the southeastern margin of the Tibetan Plateau. To explore the role of tectonics in regulating the river downcutting at regional and local scales, we reconstructed the rate and spatial pattern of incision within and below the knickzone. We combined the investigation of fluvial terraces with tributary river profile analysis for a 300-km reach near Yunlong below the knickzone (25–27.5°N). We conducted field mapping and dating of strath terraces at the heights of 60–380 m above the trunk river. The overlying terrace deposits comprise fluvial conglomerates and overbank sands as well as tributary alluvial fan conglomerates. K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating results suggest terrace formation during the Middle Pleistocene and the incision rate varying at <0.59–0.22 mm/yr. The river profiles of most tributaries in the study area are in a transient state with a knickpoint separating an upstream relict graded profile and a downstream steepened reach. These tributary profiles suggest river incision of ~50–1800 m, showing a spatial trend of increase towards the north. Knickpoint elevation and channel steepness of tributaries show a similar spatial pattern of northward increase. We suggest that the spatial variation in river incision along the Lancang River near Yunlong is likely to have been regulated by differential rock uplift in southeastern Tibet. Using the K-feldspar pIRIR dating method, we rectified the incision rate within the knickzone to vary at <0.63–0.28 mm/yr. New dating results thus suggest that terrace-derived incision rates along the Lancang River are similar to regional erosion rates on million-year and millennial time scales in terms of magnitude and spatial pattern. Coupled erosion rates on various time scales along the Lancang River and spatial pattern of river incision near Yunlong supports a primary role of tectonic forcing in shaping the southeastern Tibetan landscape. • River terraces and tributary river profiles used to quantify Lancang River incision. • River terraces used to quantify incision amount and rate since the Middle Pleistocene. • Profiles show spatial gradient in knickpoint elevation, steepness and incision amount. • Incision rate magnitude and spatial patterns in-keeping with regional erosion rates. • Incision patterns controlled by tectonically driven differential rock uplift. [ABSTRACT FROM AUTHOR]

Abstract: This contribution provides new constraints on the timing of Tibetan glacial recessions recorded by the abandonment of moraines. We present cosmogenic radionuclide 10Be inventories at 17 sites in southern and western Tibet (32 crests, 249 samples) and infer the range of permissible emplacement ages based on these analyses. Individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for 10Be abundance. We consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages: 1) Erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age; and 2) Erosion progressively exposes large boulders with little prior exposure, and so the oldest sample age records the minimum moraine emplacement age. We found that depending on the scenario chosen, the moraine emplacement age can vary by > 50% for ∼100 ka-old samples. We consider two scaling models for estimating the production rates of 10Be in Tibet, which has an important, although lesser, effect on inferred moraine ages. While the data presented herein effectively increase the database of sample exposure ages from Tibet by ∼20%, we find that uncertainties related to the interpretation of the 10Be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies. A reanalysis of published data from moraines throughout Tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the Tibetan moraine age database. Thus, when viewed in a similar way with other samples collected from this area, uncertainties within moraine exposure ages obscure attribution of Tibetan glacial advances to temperature minima or precipitation maxima. Our work suggests that more reliable chronologies of Tibetan glaciations will come from improvements in production rate models for this portion of the world, as well as a better understanding of the processes that form and modify these geomorphic surfaces. [Copyright &y& Elsevier]