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Assessing fault activity in regions lacking Quaternary sedimentary constraints remains a global challenge. In this study, we used channel slope distribution to examine variations in rock uplift along faults. By comparing channel steepness with published low-temperature thermochronology and paleo-seismic data, we identified deformation changes both perpendicular to and along the Longmen Shan at various time scales. Our data revealed distinct fault segments displaying distinct thrust activities along the Longmen Shan's strike. In the southern segment, the Dachuan fault exhibited the highest activity, and its movement had persisted for millions of years. In the central segment, the Wenchuan fault was active during theearly Quaternary but has become dormant since the late Pleistocene. Within the past millions of years, the Yingxiu and Pengguan faults displayed significant vertical displacement. Fault activity in the northern Longmen Shan was relatively weak, with the Qingchuan fault transitioning from thrust movement during the Neogene to pure strike-slip activity since the Pleistocene. Overall, the Dachuan and Huya faults exhibited deformation patterns similar to the Yingxiu fault during the Quaternary. Similar to the Yingxiu fault, which triggered the Wenchuan earthquake, the Dachuan and Huya faults possess the capacity to produce significant earthquakes in the future. The variations in deformation perpendicular to and along the Longmen Shan fault system underscore the importance of upper crustal shortening in shaping the rock uplift patterns and topography of the eastern Tibetan Plateau margin. [ABSTRACT FROM AUTHOR]

Assessing fault activity in regions lacking Quaternary sedimentary constraints remains a global challenge. In this study, we used channel slope distribution to examine variations in rock uplift along faults. By comparing channel steepness with published low-temperature thermochronology and paleo-seismic data, we identified deformation changes both perpendicular to and along the Longmen Shan at various time scales. Our data revealed distinct fault segments displaying distinct thrust activities along the Longmen Shan’s strike. In the southern segment, the Dachuan fault exhibited the highest activity, and its movement had persisted for millions of years. In the central segment, the Wenchuan fault was active during theearly Quaternary but has become dormant since the late Pleistocene. Within the past millions of years, the Yingxiu and Pengguan faults displayed significant vertical displacement. Fault activity in the northern Longmen Shan was relatively weak, with the Qingchuan fault transitioning from thrust movement during the Neogene to pure strike-slip activity since the Pleistocene. Overall, the Dachuan and Huya faults exhibited deformation patterns similar to the Yingxiu fault during the Quaternary. Similar to the Yingxiu fault, which triggered the Wenchuan earthquake, the Dachuan and Huya faults possess the capacity to produce significant earthquakes in the future. The variations in deformation perpendicular to and along the Longmen Shan fault system underscore the importance of upper crustal shortening in shaping the rock uplift patterns and topography of the eastern Tibetan Plateau margin.

Highlights: What are the main findings? The Xiongpo anticline is characterized by a superimposed fault-fold belt in the central seg-ment, with a simple shallow fault-related fold in its two plunging edge zones. A three-stage growth of the Xiongpo anticline has been proposed on the basis of a 3D fault model. What is the implication of the main finding? Three stress fields have been deduced for the Xiongpo anticline evolution. The Xiongpo fault-fold belt shows prominent NE, ENE- and ~N–S-trending relief, which resulted from multi-stage upper crustal shortening in the Longmen Shan piedmont during the eastward growth of the eastern Tibetan Plateau. Previous studies have determined its 2D structural configurations from seismic profiles and field-based geological cross-sections. Here, we extend this analysis into the entire belt to explore the 3D structural evolution of this complex fault-fold belt and have built a 3D regional fault model. The results reveal along-strike variation of subsurface structural architecture of the Xiongpo fault-fold belt, which is characterized by transformation from a complex superimposition of a deep fault-bend fold beneath a shallow structural wedge in the center segment to a simple shallow fault-bend fold on both ends of the structure, and then to a trishear fault propagation fold on the plunging edges. This structural transformation determines the contrast between the NE-striking relief of the central segment, and the ENE- and ~N-S-striking relief in the two plunging zones. We combine our results with published low-temperature thermochronology and growth strata results to propose a three-stage evolution for the Xiongpo fault-fold belt that closely relates with regional stress field changes, including a NE-striking fault under the NW–SE compression between 40–25 Ma and 15–10 Ma, lateral propagation of the NE-striking fault and initiation of ENE-striking fault by WNW–ESE compression from ~5–2 Ma, ~N–S fault under ~E–W compression until the present. This work enhances our understanding of the stress field changes of eastern Tibet since the Late Eocene. It also can serve as a typical case study deciphering 3D fault-fold growth using seismic and geological imaging, which is helpful to understand 3D structural and landscape evolutions of other complex fault-fold belts worldwide. [ABSTRACT FROM AUTHOR]

We investigated interactions between structural deformation and surface processes in the Longmen Shan fold‐and‐thrust belt and the adjacent western Sichuan foreland basin (WSFB) in the eastern margin of the Tibetan Plateau. The discrete‐element modeling (DEM) method was used to study the influences of the various mechanical properties of the detachments and syn‐tectonic erosion/deposition on the structural evolution of the Longmen Shan and WSFB. DEM simulations demonstrated two stages of surface processes during the Late Cenozoic profoundly influenced thrusting sequences and strain localization in the hinterland and foreland portions of the Longmen Shan. Models indicate that the fold‐and‐thrust belt lacking surface processes propagate in a forward‐breaking manner, whereas those with surface processes develop following an out‐of‐sequence thrusting pattern. We infer that large‐scale erosion propagation from the Sichuan Basin westward to the Tibetan Plateau since Late Cenozoic caused the Longmen Shan hinterland to reach a subcritical wedge state. Tectonic activity retreats to the edge of Plateau, enhancing the rapid uplift of the Longmen Shan and inhibiting the propagation of substantial shortening deformation to the foreland basin. The foreland thrust belt slides stably along the shallow detachment, causing the initiation and growth of the Longquan fault in the leading front. These results explain why both the Longmen Shan hinterland and the western Sichuan foreland thrust belts are currently in a state of simultaneous seismic activity. Our findings offer important implications regarding the seismic potentials of other fold‐and‐thrust belts that interact with dynamic surface processes. Plain Language Summary: The Longmen Shan is located between the high‐altitude Tibetan Plateau and the Sichuan Basin, the steepest topographic margin around the Tibetan Plateau. This region was the site of devastating earthquake events including the 2008 magnitude 7.9 Wenchuan earthquake and the 2013 magnitude 6.6 Lushan earthquake, which caused severe casualties. Geologists differ in opinion regarding how this high mountain range was uplifted during the Late Cenozoic, and the roles of surface processes are often ignored. To address this issue, we used a numerical sandbox‐like simulation method to explore the effects of erosion and sedimentation on the uplift and lateral propagation of the Longmen Shan during the Late Cenozoic. Most fold‐and‐thrust belts models traditionally show a forward‐breaking thrusting sequence, where the youngest and most active thrust fault is located at the toe of the fold‐and‐thrust belt. However, we found that surface processes significantly changed this thrusting sequence and that the fault activity of the Longmen Shan hinterland was reactivated. During the Late Cenozoic, large‐scale erosion propagating from the Sichuan Basin toward the Tibetan Plateau destabilized the mountain front and increased the activities of the hinterland faults, leading to the rapid uplift of the Longmen Shan and inhibited shortening deformation of the foreland basin. Key Points: Numerical simulation indicated surface processes affecting strain localization and seismic activity in the Longmen Shan fold‐and‐thrust beltLarge‐scale erosion propagated from the Sichuan Basin toward the Tibetan Plateau caused the activation of faults in the Longmen Shan hinterlandSurface processes destabilized the edge of Plateau, caused rapid uplift of the Longmen Shan and inhibited shortening of the foreland basin [ABSTRACT FROM AUTHOR]

Classic L2-norm-based waveform tomography is often plagued by insurmountable cycle skipping problems; as a result, the iterative inversion falls into local minima, yielding erroneous images. According to the optimal transportation theory, we adopt a novel geometry-preserving misfit function based on the quadratic Wasserstein metric (W2-norm), which improves the stability and convexity of the inverse problem. Numerical experiments illustrate that W2-norm-based full-waveform tomography has a larger convergence radius and a faster convergence rate than the L2-norm and can effectively mitigate cycle skipping issues. We apply this method to the Longmen Shan area and obtain a reliable lithospheric velocity model. Our tomographic results indicate that the crystalline crust underlying the Sichuan Basin wedges into the crustal interior of the Tibetan Plateau, and the mid-lower crust of the eastern Tibetan Plateau is characterized by low shear-wave velocities, indicating that ductile crustal flow and strong interactions between terranes jointly dominate the uplift behavior of the Longmen Shan. Furthermore, we find that large earthquakes (e.g., the Wenchuan and Lushan events) occur not only at the junction between high- and low-velocity regions but also in the transition zone from positive to negative radial anisotropy. These findings improve our understanding of the mechanism responsible for large earthquakes in this region. [ABSTRACT FROM AUTHOR]

The MS8.0 Wenchuan earthquake occurred on May 12th, 2008 on the steeply reverse fault of the Longmen Shan fault zone, Western China. Catastrophic failure and rupture occurred along the listric sliding surface. Several numerical simulations have been performed to reconfigure the dynamic processes of this strong earthquake. However, few scientific studies on the stick-slip of the listric fault have been conducted. In this study, finite element method is utilized to simulate the instant stick-slip fault responses at the triggering moment. The fault surface is a non-continuous and frictional contact interface. The simulation results demonstrated that the sliding friction and sticking friction interact spatially. This interaction occurred temporarily on swarms of slip patches, rather than the entire fault slipping surface. Stick-slip causes serrated energy release, and transient aseismic slip can occur spontaneously. These tremors propagate from the epicenter upwards to the ground surface. Therefore, they may provide important details on earthquake onset rupture.

Copyright of Acta Geoscientica Sinica is the property of Acta Geoscientica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Xiongpo fault-fold belt shows prominent NE, ENE- and ~N–S-trending relief, which resulted from multi-stage upper crustal shortening in the Longmen Shan piedmont during the eastward growth of the eastern Tibetan Plateau. Previous studies have determined its 2D structural configurations from seismic profiles and field-based geological cross-sections. Here, we extend this analysis into the entire belt to explore the 3D structural evolution of this complex fault-fold belt and have built a 3D regional fault model. The results reveal along-strike variation of subsurface structural architecture of the Xiongpo fault-fold belt, which is characterized by transformation from a complex superimposition of a deep fault-bend fold beneath a shallow structural wedge in the center segment to a simple shallow fault-bend fold on both ends of the structure, and then to a trishear fault propagation fold on the plunging edges. This structural transformation determines the contrast between the NE-striking relief of the central segment, and the ENE- and ~N-S-striking relief in the two plunging zones. We combine our results with published low-temperature thermochronology and growth strata results to propose a three-stage evolution for the Xiongpo fault-fold belt that closely relates with regional stress field changes, including a NE-striking fault under the NW–SE compression between 40–25 Ma and 15–10 Ma, lateral propagation of the NE-striking fault and initiation of ENE-striking fault by WNW–ESE compression from ~5–2 Ma, ~N–S fault under ~E–W compression until the present. This work enhances our understanding of the stress field changes of eastern Tibet since the Late Eocene. It also can serve as a typical case study deciphering 3D fault-fold growth using seismic and geological imaging, which is helpful to understand 3D structural and landscape evolutions of other complex fault-fold belts worldwide.

The utilization of thermal-chronological data to constrain mountain building processes exploits the links among rock uplift, exhumation, and cooling during orogenesis. Conceptually, periods of rapid uplift and associated denudation will lead to cooling of rocks as they approach Earth’s surface. The linkage between uplift and exhumation can be complex, but in practice exhumation is often assumed to directly track uplift. The reconstruction of temperature-time histories via thermochronologic systems provides a proxy method to relate the cooling of rock as it is exhumed toward the surface to orogenesis. For the rapid exhumation rates that can occur in active orogenic systems the thermal history will be complex as a result of heat advection, rates of propagation of thermal perturbations, and other processes that affect the cooling behavior. These effects become amplified as exhumation rates increase, and in regions experiencing exhumation rates greater than ∼0.2–0.3 mm/yr (0.2–0.3 km/Ma) simple assumptions of cooling through a constant geotherm will bias the subsequent interpretation. Here we explore, through a suite of generalized models, the impact of exhumation rate and duration on the resulting thermal history and apparent age results. We then apply lessons from these simple exhumation systems to data sets from the high-relief ranges along the eastern margin of the Tibetan Plateau to determine exhumation histories constrained by those data. The resulting exhumation histories provide constraints on the onset of Cenozoic exhumation, the subsequent pace of exhumation, and on the tectonic history of one of the major fault systems in the central Longmen Shan.

The Longmen Shan range, located on the eastern margin of the Tibetan Plateau, is characterized by steep topography and a shortening rate of <3 mm/yr. This peculiar configuration is a source of controversy and questions about the topographic evolution and dynamics of this orogenic plateau margin. Investigating the variations in surface denudation over different spatial and temporal scales is important for a better understanding of topographic evolution, but there is still a lack of erosion‐rate data averaged over millennial timescales along the frontal range of the Longmen Shan, especially in its southern part. We present 25 new catchment‐wide denudation rates derived from 10Be concentrations in river sediments across the southern Longmen Shan. Our results show that average denudation rates increase from <0.15 mm/yr near Ya'an, located in the Sichuan Basin, northwestward to >0.50 mm/yr inside the southern Longmen Shan. The denudation rates correlate with slope gradient, relief, channel steepness, and specific stream power but exhibit a fair degree of scattering at high values. In combination with previous 10Be and low‐temperature thermochronology analyses, we found the denudation rates over kyr‐ to Myr‐timescales to be roughly consistent across the southern Longmen Shan, suggesting this part of the range is close to an exhumational steady state. In terms of spatial distribution, high rates of exhumation and denudation are localized in the hanging walls of major thrust faults, highlighting the role of tectonic structures in regulating the pattern of denudation and topography across the Longmen Shan. Our results favor the "brittle crustal shortening" model in which rock uplift pattern across the Longmen Shan is largely controlled by upper crustal shortening. Along‐strike variations in the distribution of denudation rates and topography can probably be attributed to segmentation of subsurface fault structures. Key Points: 10Be‐derived catchment‐wide denudation rates are positively correlated with topographic indices in the southern Longmen ShanErosion rates are consistent over short‐ and long‐term timescales across the Longmen Shan, suggesting an exhumational steady stateRock uplift pattern and topographic evolution across the Longmen Shan have been largely controlled by upper crustal shortening and fault geometry [ABSTRACT FROM AUTHOR]

In 2008, the Mw 7.9 Wenchuan earthquake killed >80,000 and injured more than 370,000 people in the province of Sichuan, China. About 5 years later, Mw 6.6 Lushan earthquake occurred on 20 April 2013, only ~90 km further south. More recently, in 2017, a third event, Mw 6.5 Jiuzhaigou earthquake, occurred ~250 km north of the Wenchuan earthquake epicenter. These events were all along the Longmen Shan thrust belt in eastern Tibet. The questions posed by these events still reverberate in the Chinese and international geoscience communities. In this special section, we summarize and report new data sets, observations, and modeling results of the eastern Tibet during the last decade. It covers the tectonics of eastern Tibet, the seismology, and the surface response of the earthquake. All these contributions will provide new insights into the geodynamics of the eastern Tibet. We further highlight a number of outstanding questions that remain to be addressed. Key Points: We summarize and report new data sets, observations, and modeling results of the eastern Tibet during the last decade.All these contributions will provide new insights into the geodynamics of the eastern Tibet.We further highlight a number of outstanding questions that remain to be addressed. [ABSTRACT FROM AUTHOR]

• Xiongpo structure is a superimposed deep fault-bend fold and shallow structural wedge. • Xiongpo structure is accommodated a horizontal shortening rate of 0.2 mm/yr. • LMS front is partitioned low shortening rates (<0.5 mm/yr) on individual structures. Cenozoic crustal shortening across the Longmen Shan (LMS) fold-and-thrust belt records the eastward growth of the Tibetan Plateau. Although the structural geometries of individual structures in the region have been studied, the kinematics and rates of deformation across these structures remain unclear. In this study, we construct a geological cross section to derive the subsurface structural architecture of the Xiongpo structure in the LMS piedmont using one seismic reflection profile, and data from two boreholes. The structure displays a superimposed deep fault-bend fold and shallow structural wedge with three splay thrust faults. Using two-dimensional kinematic models, based on fault-bend folding algorithm, we reconstruct three episodes of upper crustal shortening, with 7.0 ± 0.3 km of total shortening. Combining with previously published low-temperature thermochronology data, we further estimate that the structure has experienced an average shortening rate of ∼ 0.2 mm/yr since 40–25 Ma. We infer that the structure acted as the deformation front of the LMS fold-and-thrust belt between 40 and 25 Ma and 15–10 Ma driven by the eastward growth of the Tibetan Plateau. Finally, we review the shortening rates of other five structures within the LMS piedmont, and find that these six structures (including the Xiongpo structure) individually have undergone an average shortening rate < 0.5 mm/yr on each fault, suggesting that strain is partitioned along several structures within the LMS fold-and-thrust belt. We also show that the southern LMS piedmont has absorbed more upper crustal shortening than the central LMS, ∼0.5–0.7 mm/yr, since ∼2.6 Ma. [ABSTRACT FROM AUTHOR]

• The Lushan earthquake occurred at the boundary zone between the high- and low-resistivity bodies. • The crustal structure of the southern Longmen Shan thrust belt matches the multiunit combination seismogenic model. • The seismogenic potential of the southern segment of Longmen Shan is weaker than that of the central segment. Since the Ms 7.0 earthquake occurred in Lushan on April 20, 2013, five years after the Wenchuan Ms 8.0 earthquake, the seismogenic potential of southern Longmen Shan has been a considerable concern. Comparative studies of active faults and deep structures of the Lushan and Wenchuan earthquakes are important to evaluate Longmen Shan potential. Terrace measurements, geophysical surveys and borehole drilling reveal that late Quaternary fault activity in the southern segment of the Longmen Shan thrust belt is weaker than that along the central segment. Magnetotelluric data across the southern segment of the Longmen Shan thrust belt show that the central fault and the front-range fault are the electrical boundaries of the crust and that the Longmen Shan thrust belt is a high-resistivity body. The Lushan earthquake occurred at the boundary zone between the high- and low-resistivity bodies, and the crustal structure of the southern Longmen Shan thrust belt matches the multiunit combination seismogenic model, indicating a potential large earthquake in Longmen Shan. Compared to conditions along the central segment, the low-resistivity layer of the upper and lower crust on the western side of the Songpan-Ganzi block develops along the Xianshuihe fault, which may reduce the strain accumulated from the Songpan-Ganzi block to the southern Longmen Shan thrust belt. This interpretation suggests that the main crustal shortening is accommodated by foreland thin fold structures, whereas the structural deformation accommodated by the thrust belt accounts for only a small proportion; thus, the seismogenic potential of southern Longmen Shan is weaker than that of the central segment. [ABSTRACT FROM AUTHOR]

Previous studies have proposed that the Eastern Tibetan Plateau underwent ∼E–W oblique crustal shortening, due to the eastward extrusion of the plateau since ∼5–2 Ma to present. However, the effects of the oblique thrusting on the three-dimensional (3D) fold morphology and kinematics in the front of the Longmen Shan fold-and-thrust belt are still poorly understood. We use 3D seismic data to produce five 3D geometric models and compare our results with structural models of the Yanjinggou anticline in the Longmen Shan fold-and-thrust belt, to investigate the effects of oblique thrusting on 3D fold evolution. The results show that regional oblique thrusting occurred within the LMS fold-and-thrust belt, resulting in rotation and a curved structural morphology, and suggest that this process may be applicable globally to both small- and regional-scale structures, as well as larger-scale orogenic belts in obliquely convergent settings. • The effects of oblique thrusting on 3D fold geometry and kinematics are investigated. • The Yanjinggou anticline developed as a trishear fault-propagation fold with 1.2 km shortening. • The Yanjinggou anticline was affected by oblique thrusting at an angle of 55°. Three-dimensional (3D) subsurface seismic data acquisition and modeling are rarely undertaken; consequently, the complex effects of oblique thrusting on the 3D morphology and kinematics of folds are still poorly understood. We use 3D seismic data to produce five 3D geometric models and compare our results with structural models of the Yanjinggou anticline in the Longmen Shan fold-and-thrust belt, to investigate the effects of oblique thrusting on 3D fold evolution. 3D geometric modeling results reveal that oblique thrusting may cause fold asymmetry, vertical axis rotation, and a curved fold axis. 3D seismic interpretations and structural modeling suggest that the Yanjinggou anticline developed as a trishear fault-propagation fold and experienced a maximum shortening amount of 1.2 ± 0.1 km. The 3D structural and geometric models are consistent with paleomagnetic results, suggesting that the anticline was affected by oblique thrusting at an angle of 55° above a listric thrust fault with three fault ramps (19°, 30°, and 40°), causing the fold to rotate and develop a curved morphology. The integrated method used in this study to investigate the effects of oblique thrusting on 3D fold morphology and kinematics can be applied to regional oblique thrusting in the Longmen Shan and other fold-and-thrust belts worldwide. [ABSTRACT FROM AUTHOR]

The Longmen Shan orogenic belt is the landform boundary between the eastern Tibetan Plateau and the Sichuan Basin. However, there are significant differences in lithospheric deformation across its southern and northern segments. We established numerical models to investigate the effect of rheological heterogeneity on lithospheric deformation across the eastern plateau margin. The results show that the rheological heterogeneity of the Longmen Shan controls the type of lithospheric deformation and the strain partitioning. When the Longmen Shan is weak or narrow, high strain is predicted between the plateau and the Sichuan Basin along the Longmen Shan, which helps to explain the lack of a Cenozoic foreland basin along the southwestern part of the Sichuan Basin. However, when the upper crust of the Sichuan Basin is weak, the deformation should extend into the interior of the basin, such as the Longmen Shan‐Longquan Shan thrust system, which provides sedimentary space for the Cenozoic sediments. In contrast, when the Longmen Shan is strong or wide, the deformation is diffuse over a broad transition zone. The strain is mainly localized along the boundary between the Songpan‐Ganzi terrane and the Longmen Shan. This result can be used to understand the uplift of the Min Shan. Compared to the observations, the high strength of the Longmen Shan corresponds to the distribution of the Precambrian crystalline basement. The rheological heterogeneity of the Longmen Shan is from the result of the amalgamation of ancient continents, which illustrates the control of the ancient continental margin on recent tectonics. Key Points: The rheological heterogeneity controls the type of lithospheric deformation and strain partitioning across the Longmen ShanHigh strain is localized along the uplift of the Precambrian basement of the Longmen ShanThe thickening of the mantle lithosphere also contributes to the uplift of the Longmen Shan [ABSTRACT FROM AUTHOR]

Abstract Erosion of organic carbon from the terrestrial biosphere and sedimentary rocks plays an important role in the global carbon cycle across a range of timescales. Over geological timescales (>104 years), erosion and burial of particulate organic carbon (POC) from the terrestrial biosphere (POC biosphere) is an important CO 2 sink, while oxidation of organic carbon derived from sedimentary rocks (petrogenic, POC petro) releases CO 2 to the atmosphere. Over decadal to millennial timescales, the balance between POC biosphere production and degradation affects atmospheric CO 2 concentrations. To better constrain the controls on erosional carbon transfers, here we quantify POC biosphere and POC petro fluxes in a mountain range with relatively low runoff, the Longmen Shan, which drains the eastern margin of the Tibetan Plateau. We measure total organic carbon content ([OC total ]) and the carbon isotopic compositions (13C/12C expressed as δ13C; 14C/12C expressed as fraction modern or F mod) of organic matter in suspended sediments collected from six gauging stations on the Min Jiang, a tributary of the Yangtze River, from 2005 to 2012. We find that POC petro has a large range of δ13C, from −26.2‰ to −13.2‰. This POC petro mixes with POC biosphere to set the δ13C of POC in river sediments. Binary mixing models reveal the possibility of aged POC biosphere at two gauging stations which drain the high elevations of the eastern Tibetan Plateau, with modelled biospheric F mod values of 0.82 ± 0.09 and 0.84 ± 0.08. This is consistent with prior suggestions of aged biospheric carbon being eroded from the plateau. The annual POC petro yields range from 0.04 ± 0.02 tC km−2 yr−1 to 1.69 ± 0.56 tC km−2 yr−1 across the five study catchments, with basin average yield that appears to be linked to catchment average slope as a likely proxy for erosion rate. Here, the variability in the petrogenic organic carbon content of rocks masks the signal of the weathering and oxidation of this rock-derived organic carbon. The annual POC biosphere yields range from 0.21 ± 0.04 tC km−2 yr−1 to 3.33 ± 0.57 tC km−2 yr−1. These values are towards the lower end of those measured in mountain ranges around the world, which we suggest not only reflects the relatively low erosion rates of the Longman Shan, but also the low annual runoff (<1 m yr−1). Across this region, the river POC biosphere discharge is related to the intensity of runoff events. Our data suggest that a wetter (and/or stormier) climate could increase the erosional export of POC biosphere in this tectonically-active mountain range. Depending on the fate of POC biosphere downstream in larger river systems, this could act as carbon-cycle climate feedback over geological timescales. [ABSTRACT FROM AUTHOR]

Pseudotachylytes, produced by frictional heating during seismic slip, convey information that is critical to understanding the physics of earthquakes, their geochemical features triggered by frictional melting shed light on the seismic processes and frictional strength as well. However, whether their geochemical properties changed during exhumed process is still uncertain. To provide insight to this topic, we conducted a case study of pseudotachylytes in the Longmen Shan thrust belt, eastern Tibet. Pseudotachylyte samples drilled from the WFSD boreholes at deep depth and collected at surface outcrops were analyzed by μXRF core scanner to estimate their geochemical variations. Macro- and micro-structural observations suggest that the pseudotachylytes are from melt-origin. Comparative analyses of the geochemical characteristics between surface and deep pseudotachylytes show that the former are K- and Ti-depleted in an oxidation environment, while the latter have low Si, high K, Fe, Ti and Mn in a reductive environment. Besides the selective melting process, another reasonable explanation for these differences is that the geochemical properties of the surface pseudotachylytes have changed due to the prolonged fluid influence during the long-term exhumation processes, which cannot exactly reflect the nature of the initial frictional melt. Therefore, we conclude that primary pseudotachylytes are characterized by enrichment of K, Fe, Ti, Mn and depletion of Si. The frictional melt was generated in high-temperature reductive environment, and fluids have influenced their chemical composition during the exhumation process. These results are of great significance for correctly understanding earthquake mechanics and fault behavior. [ABSTRACT FROM AUTHOR]

Abstract As the eastern margin of the Tibetan Plateau, the Longmen Shan fault zone (LMSFZ) is characterized by a complex structural style and strong seismicity, such as the 2008 Wenchuan Ms8.0 and 2013 Lushan Ms7.0 earthquakes. In order to better understand the subsurface geometry of the LMSFZ we inverted 391 focal mechanism solutions (FMS) of earthquakes (M ≥ 3.5) that occurred between 2009 and 2016 in the Longmen Shan region using seismic waveforms. Subsequently, we calculated the stress field based on the FMSs. The results show that the LMSFZ is dominated by a thrust faulting stress regime with subhorizontal NW-SE-trending maximum principal stress σ 1 and subvertical minimum principal stress σ 3 , indicating a lateral compression due to the southeastward movement of the Bayan Har block. Changes in fault dips at ~ 10 km depth are inferred as the result of a shallow level detachment layer, which may also promote shortening of the upper crust and uplift of the Longmen Shan. Based on differences in subsurface fault geometry, stress field and geomorphological evidence, we divide the LMSFZ into two segments by Beichuan County: the southwestern segment has a listric fault geometry, while the northeastern segment is dominated by subvertical transpressional strike-slip faults. According to the FMSs results, we suggest that the Lixian fault is a tear fault accommodating different thrusting rates in the eastern margin of the Bayan Har block. The normal faulting earthquakes in the Lixian area are caused by local extension resulting from the sinistral shear movement of Lixian fault in a strike-slip faulting stress regime. It is suggested that under the continuous southeastward movement of the Bayan Har block and the obstruction of the rigid Sichuan basin, stress is accumulating along the southwestern segment of the LMSFZ, to which accordingly a high seismic hazard is ascribed and which should be carefully monitored. Graphical abstract Unlabelled Image Highlights • 391 Focal mechanism solutions in the Longmen Shan fault zone are presented. • A shallow detachment layer at ~10 km depth is revealed by changes in fault dips. • Normal faulting earthquakes in the Lixian area are caused by local extension. • The Longmen Shan fault zone should be divided into two segments. • Seismic hazard in the southwestern segment is potentially high. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Strain transfer is mainly affected by thickness of shallow detachment. • Seismic gap is ascribed to ductile rocks and enhanced strain transfer. Abstract Southeastward strain transfer in the middle-southern Longmen Shan (LMS) thrust belt not only can promote the development of fault-related folds and earthquakes in the piedmont, but also affect the seismogenic capacity of the middle-southern LMS thrust belt. However, the detailed transfer process remains elusive. In this work, we combine structural profile re-analysis, sandbox modeling and seismic statistics to illustrate the strain transfer process. Our main results indicate that: (1) the thickness of the shallow detachment (e.g. Middle-Lower Triassic gypsum-salt strata) is probably the main factor for differences in southeastward strain transfer within the domain between the middle-southern LMS thrust belt and Weiyuan-Moxi anticline. This is also supported by our sandbox model; (2) the seismogenic capacity in the southern LMS thrust belt is probably proportional to the corresponding shortening of the Range Front Blind Thrust (RFBT). Thus, seismogenic capacity in the seismic gap zone is weaker than those of other segments of the southern LMS thrust belt, which is probably < Ms 7.0. (3) The weak seismogenic capacity (

Copyright of Acta Geoscientica Sinica is the property of Acta Geoscientica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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Copyright of Acta Geoscientica Sinica is the property of Acta Geoscientica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We document the geological fault zone geometry and internal architecture of the Yingxiu‐Beichuan fault (YBF) in the Longmen Shan, eastern Tibetan Plateau, which hosted the catastrophic 2008 Wenchuan earthquake. The YBF has been repeatedly reactivated during its long‐term activity and contains widespread brittle fault rocks. Pseudotachylytes are recognized in the Pengguan Complex both from the Wenchuan earthquake Fault Scientific Drilling (WFSD) cores and at a surface outcrop. Microstructural evidence of embayed and rounded fragments, flow structures, vesicles, and microlites of various morphologies indicate that the pseudotachylytes were generated by frictional melting during earthquakes. Cataclasite and fault gouge with fragments of angular shapes and varied sizes observed within the fault zones are also considered as evidence of paleoearthquakes. These imply that the main fault zones in the WFSD cores record ancient seismic slip events. Geological field mapping at the surface and in WFSD cores shows that the YBF is a listric reverse fault, with a 155–290‐m thick fault zone that dips 73°–68° at depths <700 m and 30° at depths >700 m. Three faulted sets of alternating Neoproterozoic Pengguan Complex overthrusting upon the Late Triassic Xujiahe Formation in the WFSD‐2 core imply that the main tectonic framework of Longmen Shan consists of a series of imbricated thrust sheets that have accommodated strong crustal shortening. These suggest that the accumulated crustal shortening caused by imbricated thrusting and long‐term seismic activity along the YBF and deep concealed faults is an important process responsible for the rapid uplift of the Longmen Shan. Key Points: The YBF is a typical listric thrust fault that dips ~70° at depths <700 m and ~30° at depths >700 m revealed from WFSD coresFault rocks in the YBF at the surface and in WFSD cores contain pseudotachylytes that record paleoseismic slip eventsUplift of the Longmen Shan was accommodated by imbricated thrusting of the Pengguan Complex over the Xujiahe Formation [ABSTRACT FROM AUTHOR]

The Longmen Shan belt (eastern border of the Tibetan plateau) constitutes a tectonically active region as demonstrated by the occurrence of the unexpected 2008 Mw 7.9 Wenchuan and 2013 Mw 6.6 Lushan earthquakes in the central and southern parts of the belt, respectively. These events revealed the necessity of a better understanding of the long‐term geological evolution of the belt and its effect on the present dynamics and crustal structure. New structural and thermobarometric data offer a comprehensive data set of the paleotemperatures across the belt and P‐T estimates for low‐grade metamorphic domains. In the central Longmen Shan, two metamorphic jumps of 150–200 °C, 5–6 kbar and ~50 °C, 3–5 kbar acquired during the Early Mesozoic are observed across the Wenchuan and Beichuan faults, respectively, attesting to their thrusting movement and unrevealing a major decollement between the allochtonous Songpan‐Garze metasedimentary cover (at T > 500 °C) and the autochtonous units and the basement (T < 400 °C). In the southern Longmen Shan, the only greenschist facies metamorphism is observed both in the basement (360 ± 30 °C, 6 ± 2 kbar) and in the metasedimentary cover (350 ± 30 °C, 3 ± 1 kbar). Peak conditions were reached at ca. 80–60 Ma in the basement and ca. 55–33 Ma in the cover, ca. 50 Ma after the greenschist facies metamorphic overprint observed in the central Longmen Shan (ca. 150–120 Ma). This along‐strike metamorphic segmentation coincides well with the present fault segmentation and reveals that the central and southern Longmen Shan experienced different tectonometamorphic histories since the Mesozoic. Key Points: Metamorphic jumps of ~150 °C, 5 kbar and ~50 °C, 3 kbar are observed across the Wenchuan and Beichuan faults, respectivelyP‐T‐t conditions for the southern Longmen Shan, lower than for central Longmen Shan, do not exceed 395 °C and 6 ± 2 kbar (at 80–33 Ma)Different segments of the Longmen Shan underwent different tectonometamorphic evolution since the Mesozoic [ABSTRACT FROM AUTHOR]

Abstract: Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim 40Ar/39Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology. [ABSTRACT FROM AUTHOR]

Abstract: Crustal active faults can slip either steadily by aseismic creep, or abruptly by earthquake rupture. Creep can continuously relax the stress and reduce the magnitude of earthquakes. Identifying the slip behavior of active faults plays a crucial role in predicting and preventing earthquake disasters. Here we carried out multiscale structural analyses of fault rocks from the Guanxian‐Anxian fault zone (GAF) surface rupture of the 2008 Mw 7.9 Wenchuan earthquake (Longmen Shan thrust belt, China), and from the 3rd pilot borehole of the Wenchuan earthquake Fault Scientific Drilling project (WFSD‐3P). Results revealed that distributed R1 shears, pressure solution seams, partly dissolved clasts and mineral grains (e.g., quartz and albite), and newly formed phyllosilicate minerals all prevail in the clay‐rich fault rocks at different depths. Microstructural observations combined with X‐ray diffraction analysis show that the majority of clay minerals in the fault gouges were newly formed by pressure solution. The growth of new phyllosilicates prevents soluble grains from coalescing, thereby maintaining fast diffusive paths along solution seams, which promotes the formation of pressure solution. Both weak phyllosilicate minerals and pressure solution can control aseismic creep. Therefore, the slip behavior of the GAF is dominated by long‐term aseismic creep due to the interplay between new phyllosilicate minerals and pressure solution. Based on P wave velocity and strata distribution, we propose a model in which the GAF is creeping at shallow depths and locked at greater depths, as a possible explanation for the Wenchuan earthquake rupture mechanism. [ABSTRACT FROM AUTHOR]

Abstract: Eastern Tibet is an important example of oblique convergence and associated strain partitioning, as suggested by recent 2‐D and 3‐D structural interpretations, yet the nature and evolution of oblique strain partitioning of this region remain poorly known. Here we use seismic reflection profiles, borehole data, and field investigations in the Longmen Shan piedmont to determine the subsurface structural architecture, and we observe several nearly N‐S striking thrusts and reactivation of NE striking preexisting faults. We interpret that this behavior is due to a regional principal compressional stress oriented in the E‐W direction, oblique to the NE striking Longmen Shan. Using the records of fault activity and related Late Pliocene and Quaternary foreland sediments and growth strata, as well as the coseismic rupture of the 2008 Mw 7.9 Wenchuan earthquake, we demonstrate that the Longmen Shan has experienced E‐W crustal shortening and oblique motion since ~5–2 Ma. We present two strain partitioning models arising from oblique thrusting in eastern Tibet and suggest that the eastward extrusion from Tibet is mainly accommodated on the strike‐slip Longriba fault and the dip‐slip Longmen Shan‐Min Shan fault zones. These results enhance our understanding of the tectonic relationship between the Songpan‐Ganzi terrane and the Sichuan basin and provide additional constraints for studies of the geodynamic response of eastern Tibet to the ongoing India‐Eurasia collision. [ABSTRACT FROM AUTHOR]

Quantifying the removal of co-seismic landslide material after a large-magnitude earthquake is central to our understanding of geomorphic recovery from seismic events and the topographic evolution of tectonically active mountain ranges. In order to gain more insight into the fluvial erosion response to co-seismic landslides, we focus on the sediment fluxes of rivers flowing through the rupture zone of the 2008 Mw 7.9 Wenchuan earthquake in the Longmen Shan of the eastern Tibetan Plateau. Over the post-seismic period of 2008-2013, we annually collected river sediment samples (0.25-1 mm) at 19 locations and measured the concentration of cosmogenic 10Be in quartz. When compared with published pre-earthquake data, the 10Be concentrations declined dramatically after the earthquake at all sampling sites, but with significant spatial differences in the amplitude of this decrease, and were starting to increase toward pre-earthquake level in several basins over the 5-year survey. Our analysis shows that the amplitude of 10Be decrease is controlled by the amount of landslides directly connected to the river network. Calculations based on 10Be mixing budgets indicate that the sediment flux of the 0.25-1 mm size fraction increased up to sixfold following the Wenchuan earthquake. Our results also suggest that fluvial erosion became supply limited shortly after the earthquake, and predict that it could take a few years to several decades for fluvial sediment fluxes to go back to pre-earthquake characteristics, depending on catchment properties. We also estimate that it will take at least decades and possibly up to thousands of years to remove the co-seismic landslide materials from the catchments in the Longmen Shan. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Longmen Shan, eastern and steepest margin of the Tibetan plateau, is often seen as the archetype example of an orogenic system built by crustal channel flow extrusion since the Miocene. This model is controversial as other studies propose an accretionary prism mechanism. A key difference between these models resides in the kinematics proposed for the Wenchuan - Maoxian (WM) fault zone, a major tectonic structure of the Longmen Shan. We constrain the Cenozoic kinematics of the WM fault zone by combining structural observations, fault gouge K/Ar dating and 40Ar/39Ar dating of syn -kinematic white mica. Normal / right lateral ductile deformation occurred at 28.0 ± 0.9 Ma while top-to-the-east reverse deformation at 15.4 ± 0.2 Ma. K/Ar ages of authigenic illite from two fault gouges show that brittle right-lateral / reverse deformation was active at 6.9 ± 2.9 Ma. These ages are consistent with the relative vertical motions across the fault zone deduced from thermochronology. Three deformation phases can be identified: right-lateral / normal in the Middle Oligocene (∼30–25 Ma), reverse in the middle Miocene (25–15 Ma), and right-lateral / reverse since the upper Miocene (since ∼6 Ma). The WM fault zone never experienced pure normal motion, and only shortening since the Oligocene, in contrast to predictions of lower crustal channel-flow extrusion models. These results are in favor of a crustal accretionary prism model for the Cenozoic building of the eastern Tibetan plateau. [Display omitted] • First account of three Cenozoic tectonic phases along the Wenchuan - Maoxian fault. • Direct deformation dating results coherent with thermochronology. • No pure normal motion on the fault in Cenozoic, no normal component since 25 Ma. • Fault Kinematics dismiss the crustal channel-flow extrusion model in Longmen Shan. [ABSTRACT FROM AUTHOR]

Competing hypotheses have been proposed to explain the seismic hazard (i.e. whether earthquakes with M ≥ 7 occur) of the southern Longmen Shan (LMS). This region did not rupture during the 2008 M w 7.9 Wenchuan earthquake, but later generated the 2013 M w 6.6 Lushan earthquake. Currently, the maximum possible earthquake magnitude, its average recurrence interval, and the seismogenic structure of the southern LMS, remain poorly documented. This study aims to re-evaluate seismogenic structures and seismic hazard along the southern LMS. We first describe the sub-surface structural geometry, as well as the total slip and Quaternary activity of the Range Front blind thrust (RFBT), using high-resolution seismic reflection profiles, borehole data, and intensity-derived macroscopic epicenters. This thrust, which generated the 1970 M s 6.2 Dayi and 2013 M w 6.6 Lushan earthquakes, extends for > 250 km along the LMS range front. Integrating new evidence of active faulting and folding and previous quantitative horizontal shortening rate results, we estimate that the Quaternary slip rate of the RFBT is nearly 1 mm/yr, with a minimum total slip of 5 km since 8–5 Ma. Furthermore, we calculate the accumulation rate of seismic moment, 8.04 (± 2.09) × 10 17 N·m/yr, for the main active thrusts on the southern LMS, to compare with the moment release rate for earthquakes in the region. When we combine this with the Gutenberg–Richter distribution obtained from historical and instrumental catalogs, we estimate that the potential maximum moment magnitude of an earthquake in the southern LMS is 7.7. Finally, we conclude that the entire southern LMS is capable of generating much larger earthquakes ( M w 7.3–7.7) than seen recently, with an average frequency of once every 1000–1400 years. Therefore, our findings confirm that there is potential for large earthquakes in the southern LMS, especially on the RFBT, which extends beneath the densely populated Chengdu Plain. [ABSTRACT FROM AUTHOR]

The Cenozoic orogenic process of the Longmen Shan (LMS) and the kinematics of major faults along the LMS are crucial for understanding the growth history and mechanism of the eastern Tibetan Plateau. Three major faults, from west to east, are present in the central segment of the LMS: the Wenchuan-Maoxian Fault (WMF), the Beichuan-Yingxiu Fault (BYF), and the Jiangyou-Guanxian Fault (JGF). Previous researchers have placed great impetus on the Pengguan Massif, between the WMF and BYF. However, limited low-temperature thermochronology data coverage in other areas prevents us from fully delineating the tectonic history of the LMS. In this study, we collect 22 samples from vertical profiles in the Xuelongbao Massif and the range frontal area located at the hanging walls of the WMF and JGF respectively, and conduct apatite and zircon fission track analyses. New fission track data reveal that the Xuelongbao Massif has been undergoing rapid exhumation with an average rate of ~ 0.7–0.9 mm/yr since ~ 11 Ma, and the range frontal area began rapid exhumation at ~ 7.5 Ma with total exhumation of ~ 2.5–4.5 km. The exhumation histories indicate that the three major faults (WMF, BYF and JGF) in the central LMS are all reverse faults, and show a basinward in-sequence propagation from middle Miocene to present-day. Such a pattern further implies that upper crustal shortening is the primary driver for the LMS' uplift during the Late Cenozoic. Nevertheless, middle-lower crustal deformation is difficult to be constrained by the exhumation histories, and its contribution to LMS' uplift cannot be ruled out. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Present-day convergence within the Longmen Shan fold-and-thrust belt (LSFTB) was released by the 2008 M w 7.9 Wenchuan earthquake, which ruptured multiple thrust ramps beneath the range front structures. However, it is still unclear whether fault slip propagated eastward into the foreland, closer to densely populated areas around Chengdu. In this study, we provide constraints on the 3-D subsurface structure, fault activity, and seismic hazards of the Longquan fault located in the central Sichuan basin, ~100 km east of the range front structures of the LSFTB. We develop a seismotectonic model and assess the activity of the Longquan fault by a combination of methods, including interpretation of seismic reflection profiles, analysis of satellite images, field mapping, trench logging, and radiocarbon dating. Our survey reveals that at least two surface-rupturing events occurred on the Longquan fault. Our 3-D structural model and evidence for Holocene faulting events on the Longquan fault reveals that upper crustal shortening in the Sichuan basin is accommodated by a frontal thrust system that is linked to the recently active range front blind structures by a shallow detachment. We suggest that dynamic weakening following fault activity at the Longmen Shan range front might unlock the updip portion of this shallow detachment, sending slip eastward into the foreland and to the surface along the Longquan thrust ramps. Our results have important implications for seismic hazards assessment of active frontal thrusts linked by upper crustal detachments in active fold-and-thrust belts around the world. [ABSTRACT FROM AUTHOR]

The deep structure and deformation mechanisms of the Longmen Shan thrust belt (Sichuan, China), at the eastern border of the Tibetan plateau, were largely debated after the devastating Mw 7.9 Wenchuan earthquake (2008). Recent geophysical studies and field investigations have been focused on the active motion of the major Beichuan fault, which ruptured during the earthquake. However, the total exhumation across the fault still remains unclear. In the hanging wall of the Beichuan fault, the South China block is exhumed in the Pengguan massif. Close to the Beichuan fault, the rocks of the Pengguan massif underwent greenschist facies metamorphism associated with brittle-ductile deformation. No metamorphism is observed in the footwall of the fault. In this study, we characterize and date the metamorphic history recorded in the hanging wall of the Beichuan fault in order to constrain the depth and timing of exhumation of the rocks of the Pengguan massif along the fault. A high-resolution petrological approach involving chemical analyses and X-ray maps was used to analyze the micrometric metamorphic minerals. The P-T conditions of the greenschist facies metamorphic event were estimated by an inverse multi-equilibrium thermodynamic approach. The results, 280 ± 30 °C and 7 ± 1 kbar, suggest that the rocks of the Pengguan massif were exhumed from ca. 20 km depth. Our results underline the importance of the thrusting component in the long-term behavior of the Beichuan fault and provide a minimal depth at which the fault is rooted. In situ laser ablation 40 Ar/ 39 Ar dating of metamorphic white mica revealed that the greenschist overprint occurred at 135–140 Ma. The Pengguan massif was therefore partially thrusted along the Beichuan fault during the Lower Cretaceous, long before the Eocene-Miocene exhumation phase which is well-constrained by low-temperature thermochronology. Our results provide the first independent depth information for the exhumation history of the Pengguan massif and reveal a previously undocumented Lower Cretaceous tectonic event that marks the onset of the thick-skinned deformation in the external domain of the Longmen Shan (East of the Wenchuan Fault). [ABSTRACT FROM AUTHOR]

Pressure-temperature-deformation (P-T-ε) paths for metamorphic rocks that experienced a complex deformation history commonly rely on the link between successive metamorphic assemblages and the different microstructures preserved at the sample scale. However, with changing P-T conditions, metamorphic minerals in a specific microstructure can chemically re-equilibrate. The direct link between deformation phases and mineral compositions for thermobarometry purposes can therefore be distorted. This study focuses on a series of garnet-biotite metapelites from the Longmen Shan (Sichuan, China) that preserve muscovite of different chemistry in distinct microstructures. To quantify the degree of re-equilibration of muscovite, a microstructural study was coupled with high-resolution chemical mapping. The chemical evolution of muscovite was modeled along a P-T loop previously constrained with phase equilibria calculations, semi-empirical and empirical thermobarometry. Results show that metapelites experienced a three-stage metamorphic history: (1) heating and burial up to 11 ± 2 kbar, 530 ± 20 °C, (2) minor decompression and heating up to 6.5 ± 1 kbar, 575 ± 10 °C and (3) decompression and cooling down to 4 ± 1 kbar, 380–450 °C. Muscovite partially re-equilibrated by pseudomorphic replacement during the three metamorphic stages, although it is mainly observed in prograde microstructures preceding the pressure peak. The main factors controlling the re-equilibration are the intensity of the deformation and the fluid availability during metamorphism. The P-T conditions of metamorphic assemblages thus reflect pulses of fluids release that enhanced mineral resorption and local replacement. Such micro-chemical behavior is probably relatively common in metapelites and questions the reliability of the P-T predictions based on relict phase chemistry, apparently preserved in microstructures that might have been affected by later re-equilibration. [ABSTRACT FROM AUTHOR]

Reliable estimates of Quaternary or Cenozoic upper crustal shortening in the Longmen Shan fold-and-thrust belt are rare. In this paper, we report on our use of high-resolution 2-D and 3-D seismic reflection profiles at various scales, together with borehole data, to investigate the structural geometry of the Longmen Shan piedmont. The results reveal a thrust system beneath the Longmen Shan, termed the range front thrust system, which consists of the range front blind thrust and its upward splay faults. Moreover, on these faults we identified growth strata that provide an excellent opportunity for assessing the activity of this thrust system. Analyses of the growth strata reveal early to late Pleistocene activity on the range front blind thrust, with minimum dip slip and horizontal shortening rates of 1.1 ± 0.2 mm/yr and ~1 mm/yr. Accordingly, the maximum accumulated slip on the range front blind thrust is calculated to be 7.5 ± 0.3 km in the Longmen Shan. Using the new horizontal shortening rate and other published data, we also estimated that the long-term shortening rate across the Longmen Shan fold-and-thrust belt is 1-3 mm/yr, which is comparable to the short-term GPS rate. The similarity of these rates suggests that the Longmen Shan attained a steady state condition over the past 2 Myr. An additional highlight of our results is that we show Quaternary activity around the Tibetan Plateau to have been nearly synchronous in different regions, including in the Longmen Shan, the Himalayas, the western Kunlun Shan, and the northern Qilian Shan. [ABSTRACT FROM AUTHOR]

Pseudotachylyte is an ideal target to directly date ancient earthquake associated with regional faulting. Here we perform step-heating 40Ar/39Ar, clay mineral K-Ar, and zircon fission track (ZFT) analyses on the pseudotachylyte samples collected from the Yingxiu-Beichuan coseismic rupture of the Longmen Shan fault belt (LSFB) to provide time constraints for the tectonic evolution of the LSFB during the Indosinian orogeny. 40Ar/39Ar results from the matrix show that the frictional melting occurred 226-235 Ma ago. Combined with mylonite dating of the host rock, the age of the ancient earthquakes is constrained at 231-238 Ma, with a formation depth of 10-14 km. As a response to the earthquakes, a series of soft-sediment deformation structures are widely preserved in the Middle and Late Triassic strata along the Yingxiu-Beichuan fault (YBF), indicating that the LSFB was seismically active since the Late Anisian and persisted episodically until the end of the Indosinian orogeny. Both clay mineral K-Ar and ZFT analyses record a younger deformation or alteration with an age of approximately 195 Ma, corresponding to a postcollisional orogeny. These new data represent the first direct evidence of the regional thrusting (YBF) in the central LSFB during the Indosinian orogeny, concurrent with the initial ductile deformation of the western boundary fault. Tectonic inheritance then strongly influenced the evolution of the LSFB as most of the Mesozoic faults are reactivated by major Tertiary tectonic deformations. [ABSTRACT FROM AUTHOR]

Drainage divides can become unstable due to tectonic disturbances and therefore can be used to constrain past tectonic activity. However, many studies have focused mainly on the influence of vertical motion with less attention on the drainage divide response to strike-slip faulting. How the drainage divide response to strike-slip faulting is still uninvestigated. Here we analyze the drainage divide stability along the Beichuan-Yingxiu Fault, an active dextral strike-slip fault with a reverse component in the Longmen Shan, on the eastern margin of the Tibetan Plateau. Based on activity of the Beichuan-Yingxiu Fault and response of the local landscape, we have identified a site in the Guixi area where a river capture event may occur in the near future, and a site in the Leigu area where a river capture event may have occurred. We used the χ-plot and Gilbert metrics methods to measure drainage divide stability at these two sites. The results show that (1) drainage divides near active strike-slip faults will be unstable before and after a capture event in response to strike-slip faulting and (2) before a capture event, the drainage divide on opposing sides of the strike-slip fault will migrate in opposite directions. This study systematically reveals the drainage divide migration generated in response to strike-slip faulting. We suggest that the drainage divide stability has the potential to be used to identify strike-slip fault activity and slip sense. • A model for drainage divide migration in response to strike-slip faulting. • Divide migration in response to strike-slip faulting in the northern Longmen Shan. • χ-plot and Gilbert metrics methods are used to reveal the drainage divide stability. • Divide migrates before and after a capture event caused by strike-slip faulting. • Divide migration can be used as evidence for strike-slip fault activity. [ABSTRACT FROM AUTHOR]

This study presents structural and 40Ar/39Ar geochronological data from the southern part of the Longmen Shan fold-and-thrust belt that forms the eastern margin of the Tibetan Plateau. Investigations focused on hinterland ductile top-to-the-WNW shear deformation, which has been linked previously to late Cenozoic lower crustal flow. Consistent with previous studies, the sense of deformation is mapped as top-to-the-WNW in the Longmen Shan hinterland. The timing of the deformation is constrained by 40Ar/39Ar geochronological data of recrystallized minerals aligned along the shear foliation as Late Cretaceous-earliest Paleogene, thus predating the inferred late Cenozoic crustal flow. This deformation is contemporaneous with SE verging thrusting and loading along the Longmen Shan front, which formed a coeval ~2-3 km thick foredeep sequence along the southwestern margin of the Sichuan Basin. In the context of the regional geology, this tectonic configuration could result from either extrusion of a crustal wedge or back thrust in a duplex. Compared to other orogens, where similar crustal configurations have been reported, it is speculated that the eastern Tibetan Plateau margin acquired thickened crust and highly elevated topography in Late Cretaceous-earliest Paleogene time. [ABSTRACT FROM AUTHOR]

Research on the complex structure of the Longmen Shan foreland is of great significance for understanding the tectonism of the eastern Tibetan Plateau. Therefore, using field survey of abandoned alluvial fans that developed during the middle Pleistocene and the terraces of the modern Qingyi River, a geomorphological surface sequence for the foreland region was established to study the tectonic surface processes. We know that the deformations of river terraces can serve as foundations for the study of tectonic activity. Because the Qingyi River ran through the foreland region in the south range of Longmen Shan, it is an appropriate research area and was adopted to solve these problems. However, in the humid temperate region, the terraces are strongly eroded and hardly retain continuous morphological surfaces. In addition, no marker horizons are available that can be utilized to restrain the corresponding relationships among terraces at the same level. To solve these problems, high-precision field measurements of the terraces and alluvial fan were made, and a series of long cross sections were acquired to determine the spatial relationships between the geomorphological surfaces; moreover, based on major element tests and grain size analyses, we found that the sediments of the geomorphological surfaces at all levels had favorable corresponding relationships. Using those specific analyses of geomorphological surfaces and sediments, a geomorphological surface sequence was derived for the foreland region. The surface sequence can be employed to study the tectonism of the foreland region over larger spatial and temporal ranges rather than using the limited modern terraces. In addition, after the ages of the geomorphological surfaces at various levels were further tested, the evolution of Qingyi River especially its two migrations since the middle Pleistocene in the foreland was determined. [ABSTRACT FROM AUTHOR]

An active source measurement of shallow resistivity using fixed-electrode quasi-Schlumberger arrays has been conducted at Pixian, Jiangyou and Wudu stations on the Longmen Shan tectonic zone in western China, with the hope of detecting earthquake-associated changes. For the duration of the monitoring experiment, a gradual decrease of apparent resistivity of up to 6.7% several years prior to the 12 May 2008 M 8.0 Wenchuan earthquake had been recorded clearly at Pixian station, approximately 35 km from the epicenter. The change of apparent resistivity was monitored with a fixed Schlumberger array of AB/MN spacings of 736 m/226 m in the direction of N57.5°E, giving precisions in measured daily averages of 0.16% or less. A coseismic resistivity drop of up to 5.3% was observed at Jiangyou station, using a Schlumberger array of AB/MN spacings of 710 m/90 m in the direction of N10°E. No fluctuation of resistivity was detected at Wudu station at the time of the Wenchuan mainshock. While the focus of this paper is on monitoring or tracking resistivity variations prior to, during, and after the Wenchuan earthquake, we also aim to compare resistivity records of the Wenchuan earthquake to those of the M 7.8 Tangshan and M 7.2 Songpan earthquakes of 1976. Attempts to explain the observed resistivity variations have been made. The results show that the resistivity variations observed at all three stations are in approximate agreement with resistivity–stress behavior deduced from in situ experiments, focal mechanisms, a simplified dynamical model, static stress analyses, and field investigations from along the Longmen Shan fault zone. [ABSTRACT FROM AUTHOR]

The coseismic surface uplift of the Longmen Shan (LMS) created an instantaneous topographic load over the western margin of the Sichuan Basin, where surface subsidence, decreasing eastward, has been measured using several methods, such as GPS, SAR and levelling. Using an elastic flexural model, we aim to interpret the coseismic surface uplift and subsidence, and constrain the effective lithospheric elastic thickness ( Te) of the Sichuan Basin. Using different effective elastic thickness values for the Sichuan Basin, a series of subsidence curves were computed by the elastic flexure model equation for a broken elastic plate. The curves, produced by models using an effective elastic thickness of 30-40 km, provided the best fit to the general pattern of observed coseismic subsidence of the Sichuan Basin. However, the calculated subsidence (∼40-70 cm) at the front of the LMS is evidently lower than the observed values (∼100 cm), suggesting that the effective elastic thickness therein should be lower. These results indicate that the lithospheric strength may decrease westward from the Sichuan Basin to the LMS. [ABSTRACT FROM AUTHOR]

This study focuses on the upper–middle crust (UMC) deformation in the central Longmen Shan (LMS). The results of this study are constrained by the surface geology, typical seismic reflection profiles, previously available thermochronology, and deep geophysical data. Regional seismic profiles demonstrate a strata dip of about 2°, northwest trending, from the Sichuan Basin (SB) to LMS. The interpretation of shallow artificial seismic reflection data indicates that an unknown basement structure lead to the uplifting of the sedimentary cover by 3–4 km. A long and wide-angle reflection seismic profile presents evidence that the middle crustal-scale structure is involved in the deformation. Geophysical data support that there is an upper detachment (D1) at the depth of ∼20 km. The other lower detachment (D2) could be generated at approximately 30–40 km depth in the low velocity zone. The ductile middle crust between the D1 and D2, has shortening and forming a wedge tip beneath the transition zone of the LMS and the SB. The deformation of the LMS frontal monocline belt is related to this crustal-scale wedging. Two different tectonic stages are distinguished in the Cenozoic through the axial surface analysis and chronological data. During the first stage, the crustal-scale tectonic wedge was developed between the upper and lower detachment, resulting in the uplift of the UMC. During the second stage, the middle crust could hardly be extruding and uplifting. The brittle upper crust was rapidly uplifted and shortened by the shallow major thrusts, which were developed on the D1. The D1 and D2 controlled the uplifting and shortening in the southeastern margin of the Tibetan Plateau. The lower crust (LC) may be decoupled from the D2 and subducted due to the resistance by the stable craton underlying the SB. The structural model manifests the importance of multi-detachments and the superimposed deformation in the LMS thrust belt. However, we emphasize that this crustal-scale tectonic wedging only discovered in the central LMS. The tectonic wedge is an important uplifting pattern that should be not neglected. [ABSTRACT FROM AUTHOR]

The mechanism for uplift of the eastern Tibetan Plateau is still a matter of debate. There are two main models: extrusion and crustal flow. These models have been tested by surface observations, but questions about the uplift remain. In addition, the devastating 2008 Mw 7.9 Wenchuan earthquake along the Longmen Shan fault zone (LMSFZ) reminds us that the tectonic activity within eastern Tibet is complex and poses a major natural hazard. This activity is accompanied by dramatic uplift along the LMSFZ, but only minor convergence (<4 mm year–1) against the Sichuan basin is observed. In order to investigate the mechanism for uplift of Longmen Shan (LMS) area, we explored the lithospheric structure across the Songpan–Ganzi terrane (SGT), LMS, and western Sichuan basin by undertaking an integrated analysis of a variety of data including new, logistically challenging controlled-source seismic profiling (reflection and refraction) results, receiver function estimates of crustal thickness, gravity and magnetic data, GPS data, and geologic constraints. Our analysis of crustal structure indicates that the crust is not thick enough to support its current elevation and that the crust is essentially composed of three layers of similar thickness. Thus, based on our crustal structure model, 2D numerical modelling was conducted to investigate uplift mechanisms. The modelling results indicate that the middle crust beneath the SGT is the most ductile layer, which is the key factor responsible for the crustal-scale faulting, earthquake behaviour, and periods of uplift. In addition, the modelling results indicate that the strong Sichuan block acts as a backstop for the thrusting along the LMS and crustal thickening to the west. [ABSTRACT FROM AUTHOR]