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The stability of drainage divides carries important tectonic and climatic information. However, there is still no consensus on the criterion for measuring the stability of drainage divides, which may lead to different conclusions. There are two different views on the stability of the divide at the Wulashan horst, northern margin of the Ordos block: The drainage divide is moving northward, according to the comparison of drainage-basin morphology (such as elbows of capture, knick points); The drainage divide remains stable, according to the comparison of Gilbert metrics. In this study, we used the χ-plot method to check both the stability of the Wulashan drainage divide and the reliability of the above methods. The analysis shows that the χ value at the top of the northern side is higher than that at the same elevation of the southern side if a lower baseline was set (1300 m a.s.l.); If a higher baseline (~1800 m a.s.l.) is set, the χ values on both sides of the divide are the same at the same elevation. Because the tilting has relatively less influence, the χ-plot with a higher baseline is more representative of the drainage divide stability. In summary, the result supports the view that the drainage divide at the Wulashan horst is at a steady state. Moreover, we discussed the limitations of the methods in measuring the stability of drainage divides.

The Chenghai Fault in the Chuan–Dian block terminates at the northwestern segment of the Red River Fault, and is a significant seismogenic structure. The kinematic evolution of this fault should be closely related to the regional tectonic deformation. However, it is difficult to obtain information on structural deformation of the Chenghai Fault due to the large amount of precipitation and well-developed vegetation. The Chenghai normal faulting may drive drainage reorganization in this region, which provides a new perspective for reconstructing and evaluating the tectonic history. High-resolution digital elevation models (DEM) obtained by remote sensing greatly facilitate the study of drainage evolution and active tectonics. We use two methods (χ-plot and Gilbert metrics) to measure the drainage divide stability based on the ALOS DEM (12.5 m resolution) and further reproduce the drainage evolution process in response to the asymmetric uplift by numerical modeling. The results show that the Chenghai–Jinsha drainage divide, hosted by the footwall block of the Chenghai Fault, is migrating westward (away from the Chenghai Fault) and will continue moving ~2.2–3.5 km to reach a steady state. Its migration is controlled by the Chenghai normal faulting. The Chenghai–Jinsha drainage divide formed close to the Chenghai Fault's surface trace and continues to migrate westward in response to the asymmetric uplift. It only took a few million years for the Chenghai–Jinsha drainage divide to migrate to its current location based on the numerical modeling. The restoration of the drainage reorganization implies that the Chenghai Fault initiated in the Pliocene, which probably results from kinematic reversal along the Red River Fault. [ABSTRACT FROM AUTHOR]

It is generally agreed that the channel‐head steady‐state elevations (zs) across a drainage divide are different when the drainage divide is moving. However, whether it is the hillslope or river channel that absorbs the cross‐divide difference in channel‐head steady‐state elevation (Δzs) remains unclear. These different views have consequences for both the methods used to measure drainage‐divide stability and tectonic reconstructions from channel profiles. Two methods for determining drainage‐divide stability include Gilbert metrics and χ‐plots, which emphasise the role of hillslopes and river channels, respectively. Here, we address this issue by deducing equations for estimating Δzs and identifying the absorbers of Δzs using numerical simulations and two natural cases. Our results show that both hillslopes and river channels absorb parts of Δzs in each case; however, the proportion absorbed varies from case to case. When the hillslope absorbs a greater proportion of Δzs, the river channel absorbs less, and vice versa. We suggest that both Gilbert metrics and χ‐plots should be applied when evaluating drainage‐divide stability; if either suggests the divide is unstable, then it is indeed unstable. Moreover, the river channel profiles on both sides of a drainage divide are in disequilibrium when the divide is moving, and the erosion rates are greater and less than the uplift rates at the expanding and shrinking sides, respectively. This underscores that drainage‐divide migration can significantly hinder the extraction of uplift history from channel profiles. [ABSTRACT FROM AUTHOR]

The Cenozoic India‐Asia collision has elevated the Tibetan Plateau and produced large strike‐slip faults in the interior and margins of the plateau, which profoundly influenced drainage reorganization and divide migration in Asia. Recent studies have revealed that the drainage divides between the major rivers in and around the Tibetan Plateau have been migrating for tens of millions of years, due to tectonic and/or climatic disturbance or river capture events. Drainage‐divide stability analysis can provide new, independent insights into the Cenozoic evolution of the river systems. In this study, we focus on the Hanzhong Basin and the adjacent Micang Shan (Shan means Mountain(s)) at the tail of the Qingchuan strike‐slip fault in the outskirts of eastern Tibet. We investigated the stability of the Micang Shan drainage divide, which separates the Han and Jialing rivers (two major tributaries of the Yangtze River), using two methods—χ‐plot and Gilbert metrics. The results show that most segments of the Micang Shan drainage divide are either moving south or stable. We further calculated the predicted stable divide location and identified the abandoned river channels and residual planation surfaces. Based on these analyses, we suggest that (1) the migration of the Micang Shan drainage divide is driven by the tectonic subsidence of the Hanzhong Basin; and (2) the upper reaches of the Han River flowed southward to the Sichuan Basin before basin subsidence. This study supports the hypothesis that the Palaeo‐Middle Yangtze River and its tributaries primarily flowed southward. Moreover, the flow direction of the Middle Yangtze River has been, and still is, transitioning from southward to eastward. The change in river network flow direction is driven by regional block tilting towards the east, surface deformation from strike‐slip faulting, regional extension east of the Tibetan Plateau and/or increased influence from the summer monsoon. [ABSTRACT FROM AUTHOR]

The lateral movement of drainage divides is co-influenced by tectonics, lithology, and climate and therefore archives a wealth of geologic and climatic information. It also has wide-ranging implications for topography, the sedimentary record, and biological evolution and thus has drawn much attention in recent years. Several methods have been proposed to determine drainage divides' migration state (direction and rate), including geochronological approaches (e.g., 10 Be) and topography-based approaches (e.g., χ plots or Gilbert metrics). A key object in these methods is the channel head, which separates the hillslope and channel. However, due to the limited resolution of topography data, the required channel-head parameters in the calculation often cannot be determined accurately, and empirical values are used in the calculation, which may induce uncertainties. Here, we propose two methods to calculate the migration rate of drainage divides based on the relatively accurate channel-head parameters derived from high-resolution topographic data. We then apply the methods to an active rift shoulder (Wutai Shan) in the Shanxi Rift and a tectonically stable area (Yingwang Shan) in the Loess Plateau, to illustrate how to calculate drainage-divide migration rates. Our results show that the Wutai Shan drainage divide is migrating northwestward at a rate between 0.21 and 0.27 mm yr -1 , whereas the migration rates at the Yingwang Shan are approximately zero. This study indicates that the drainage-divide stability can be determined more accurately using high-resolution topographic data. Furthermore, this study takes the cross-divide differences in the uplift rate of channel heads into account in the measurement of drainage-divide migration rate for the first time. [ABSTRACT FROM AUTHOR]

The lateral movement of drainage divides is co-influenced by tectonics, lithology, and climate, and therefore archives a wealth of geologic and climatic information. Several methods have been proposed to determine the direction of drainage-divide migration. However, how to quantify the migration rate of drainage divides remains challenging. Here, we propose a new approach to calculate the migration rate of drainage divides from high-resolution topographic data. The new method is based on the cross-divide comparison of channel-head parameters, including the critical upstream drainage area and the gradient of channel head, both of which are used to calculate the normalized channel steepness at the channel head. We then apply the new method to an active rift shoulder (Wutai Shan), and a tectonically stable area (a mountain range in the Loess Plateau) in North China, to illustrate the calculation of drainage-divide migration rates. The northward migration rates at the Wutai Shan range from 0.10 to 0.13 mm/yr. The migration rates are approximately zero at the mountain range in the Loess Plateau. This study demonstrates that the migration rate of drainage divides can be determined more accurately once the cross-divide differences in uplift rate are taken into account. [ABSTRACT FROM AUTHOR]

From 2017 to 2019, three destructive earthquakes (27 January 2017 Mw 4.7, 16 December 2018 Mw 5.2, and 3 January 2019 Mw 4.8) occurred in the Changning shale gas field in the southwest Sichuan Basin, China. Previous seismological studies attributed these events to hydraulic fracturing (HF), but were unable to identify the causative seismogenic faults and their slip behaviors. Here, we use Sentinel‐1 synthetic aperture radar data to measure surface deformation triggered by the three events and conduct geodetic inversions to characterize their rupture models. The resulting coseismic interferograms show prominent surface deformation with the maximum line‐of‐sight displacements of up to 4 cm. The inversion results show that all three earthquakes mainly ruptured sedimentary formations above the shale gas bed, in the upper 3 km of the crust, with slip magnitudes ranging from 8.5 to 15 cm, and stress drops ranging from ∼1.8 to ∼3.3 MPa. Their source faults intersect with horizontal HF wells, but do not root in the crystalline basement. Combined with the reported difficulty of increasing HF operation pressures prior to the three events, we argue that they were most likely induced by direct injection of pressurized fluids into the fault zones. Crustal deformation patterns inferred from regional topography and GPS velocities highlight that the Changning field is located within a triple junction region near the southeastern margin of the Tibetan Plateau with large deformation gradients; such conditions are not only favorable to the development of critically stressed faults, but also facilitate the occurrence of at least moderate magnitude earthquakes. Plain Language Summary: The number of earthquakes induced by the process of shale gas extraction through hydraulic fracturing (HF) has been increasing across the globe, with dozens of M > 4 events reported by the end of August 2021. Although most case studies on HF‐induced seismicity are from the United States and Canada, the southwestern Sichuan Basin (China), located to the southeast of the Tibetan Plateau, is another region with notable seismicity linked to HF operations, including dozens of moderate‐to‐large earthquakes. We here focus on the three largest Mw ≥ 4.7 earthquakes that occurred within the Changning shale gas field up to now. We use geodetic techniques to measure ground deformation caused by the three earthquakes and to further infer their ruptures at depth. Our results reveal that the three earthquakes occurred on shallow faults intersecting with horizontal HF wells, with little‐to‐no slip in the crystalline basement below the depth of 3 km. These questions the common theme of HF‐induced earthquakes having hypocenters proximal to basement rock, presumably associated with basement‐rooted faults, and suggests that the potential for ground shaking of HF‐induced earthquakes is more significant than previously understood. Key Points: The three events ruptured shallow faults within sedimentary formations above the shale gas bed rather than basement‐rooted faultsThe faults of the three events intersect with horizontal hydraulic fracturing wells and were likely reactivated by direct fault‐zone pressurizationHigh HF‐induced seismicity rate in the Changning area links to the proximity to a triple junction region with relatively high tectonic strain accumulation rates [ABSTRACT FROM AUTHOR]

The western North China Craton (NCC) entered a stage of craton thinning concentrated on its margin in the Cenozoic, but its Late‐Cretaceous conditions were previously deduced to be relatively quiescent. Therefore, whether the Late‐Cretaceous tectonic reactivation existed on the margin of this block is critical to understanding how the craton evolved into a thinned one. To investigate this possible margin reactivation in the Late Cretaceous, we apply zircon (U‐Th)/He thermochronology to Taibai Mountain on the southern margin of the western NCC. From an ∼3‐km‐high vertical profile with ages decreasing from 98 ± 5 Ma at the peak to 52 ± 7 Ma at the foot, the age‐elevation relationship suggests the rapid exhumation at ca. 78–71 Ma bracketed by two slow exhumation phases. This interpretation is confirmed by 1D half‐space modeling and QTQt inverse modeling, with the rapid exhumation stage having an exhumation rate of 0.1–0.6 km/Myr and a cooling rate of ∼10°C/Myr. Together with previously reported thermochronological data from the margin of the western NCC, the rapid exhumation at ca. 78–71 Ma implies a reactivated margin in the Late Cretaceous before widespread Cenozoic thinning of the craton margin. This Late‐Cretaceous reactivation likely resulted from WNW‐ESE extension expressed by thermo‐tectonic events, basin subsidence, and volcanism in East Asia. Excluding impacts from climate change, lithospheric delamination, and India subduction beneath Eurasia, we infer that the Late‐Cretaceous reactivation on the margin of the western NCC was driven by the far‐field effect of Izanagi Plate subduction beneath the Eurasian Plate during slab rollback of the subducted plate. Key Points: Rapid exhumation of Mt. Taibai at ca. 78–71 Ma is confirmed by the age‐elevation relationship, 1D half‐space modeling, and QTQt modelingSynchronous Late‐Cretaceous rapid exhumation suggests a reactivated margin of the western North China Craton before Cenozoic thinningThis reactivation was triggered by the far‐field effect of Izanagi Plate subduction beneath the Eurasian Plate [ABSTRACT FROM AUTHOR]

The Shanxi Rift System (SRS) is a prominent intracontinental rift in eastern Eurasia. However, its tectonic origin remains enigmatic, as the timing of rift initiation and its subsequent evolution is not well constrained. To evaluate the cooling history of rift flanks, we present joint apatite fission track (n = 15) and apatite (U‐Th‐Sm)/He (n = 62) thermochronological study across the Huo Shan and the Zhongtiao Shan in the central and southern SRS, respectively. Inverse modeling of the thermochronological data yields two episodes of enhanced exhumation during the Cenozoic. Both ranges record rapid cooling circa 50‐35 Ma, coeval with a phase of widespread rifting across entire North China. Data from the Zhongtiao Shan record renewed cooling from ∼8 Ma to the present, following a protracted near‐isothermal condition. Considering the SRS in the context of plate reconstructions, we propose that the Eocene rift initiation is triggered by the subduction of the trench‐parallel Izanagi‐Pacific mid‐ocean ridge followed by subduction of the Pacific plate. Tectonic quiescence along the Shanxi rift during Oligocene and Miocene time reflects slow thermal subsidence as the Pacific subduction regime was established. In late Miocene, faults associated with the SRS were reactivated in dextral transtension linked to fault systems that extend outward from the northeastern Tibetan Plateau. This kinematic reorganization implies a fundamental change in force balance throughout North China. Overall, our results reflect the changing influence of tectonic regimes along the eastern Eurasian plate boundary and intracontinental deformation associated with the India‐Eurasia collision. Key Points: The Shanxi Rift System initiated in the Eocene (∼50–35 Ma), triggered by the Izanagi‐Pacific ridge subductionThe rift was reactivated in the late Miocene (∼8 Ma), associated with India‐Eurasia and eastward extrusion of South ChinaPreexisting structures influenced the initiation, tectonic quiescence, and rejuvenation of the rift [ABSTRACT FROM AUTHOR]

Details of lithospheric structures in three‐dimensions (3‐D) are key to understanding the dynamics of crustal deformation and earthquakes in active orogenic systems. In this study, we develop a 3‐D model of the eastern Tibetan Plateau using the Skua‐Gocad software based on the latest Rayleigh wave tomography. We perform a quantitative modeling workflow to map the details of the Moho discontinuities and the high‐velocity anomalies. Then, we integrate the topography, major active faults, large earthquakes, and crustal Poisson's ratios to analyze the relationship between the shallow and deep structures of this region. Our study shows that the Moho is generally coupled with the topography. A steep Moho ramp exists under the plateau margin and its surface projection intersects the Longmen Shan (LMS) at a low oblique angle (~22°). Active deformation and large earthquakes are related to the steep Moho ramp under the plateau margin. Moreover, based on a 3‐D model of the Poisson's ratio perturbations, we find that deformation across the central LMS is characterized as a crocodile‐type wedge in the crust and lithospheric mantle, due to the resistance of the Yangtze craton and isostatic rebound. We suggest that a tectonic wedging model that contains both the upper‐crust thrusting and lower‐crustal thickening contribute to the LMS orogeny. Three‐dimensional visualization of the lithospheric structure is well suited to reveal the different levels of deformation and their relationships. Quantitative modeling methods provide effective constraints on the active blocks in the eastern Tibetan Plateau. Key Points: A quantitative modeling workflow is designed to map the crustal and lithospheric structure under the eastern Tibet Plateau in 3‐DSteep Moho ramp crosses the LMS area at an angle (~22 degrees) and causes the along‐strike variations in deformation and exhumationBoth upper‐crustal thrusting and crustal‐mantle wedging contribute to the LMS orogeny, as suggested by our integrated 3‐D models [ABSTRACT FROM AUTHOR]

Abstract Knowledge of how strain is partitioned and accommodated in crustal shortening provides fundamental constraints on evaluating tectonic models of orogenic plateaus. A number of tectonic models have been proposed for the central Longmen Shan along the eastern Tibetan Plateau margin, including mid-crustal channel flow, upper crustal shortening, and whole crust shearing. However, it remains controversial whether these models work in the Min Shan – northern Longmen Shan area, and if so, how the models function in three dimensions. One straightforward test is to evaluate the timing and rate of uplift on the principle crustal faults. Here we present 36 fission track dates from this region. Our data show that the southeastern Min Shan has experienced the most rapid exhumation in this area since the late Miocene, due to slip along the Huya thrust and coeval northwestward tilting of the Min Shan fault block. Further, we identify a "maximum exhumation belt" (MEB) along Min Shan and Longmen Shan that is characterized by the greatest amount of exhumation in the late Cenozoic and the formation of 5000+ m peaks along the eastern Tibetan Plateau margin. Interestingly, the MEB overlies the "Moho ramp" between the Tibetan Plateau and the Sichuan Basin. The parallelism between the MEB and the Moho ramp leads us to a crustal-scale tectonic model, in which the Tibetan upper crust undergoes shortening and thrusting and the lower crust undergoes pure-shear thickening. Moreover, a ∼15° angle exists between the trend of the MEB and that of the Longmen Shan, implying that the Longmenshan thrust belt is a reactivated structure, which explains the along-strike variations of fault kinematics and activity. We conclude that the eastern Tibetan crust is partially decoupled, and future studies need to quantify strain partitioning in both the upper and the lower crust during continental orogenesis. Highlights • Rapid exhumation (∼0.9 mm/yr) and northwestward tilting occurred in Min Shan. • A maximum exhumation belt (MEB) overlaps the Moho ramp along the plateau margin. • The parallelism implies deep crustal deformation influences surface exhumation. • The MEB has ∼15° angle with the Longmen Shan, indicative of tectonic reactivation. • Partially decoupled upper and lower crust and a fully coupled crust and mantle. [ABSTRACT FROM AUTHOR]

Whether external or internal forces of the Earth control the behaviors of upper‐crustal faults in a fold‐and‐thrust belt has been debated for decades. The Longmenshan thrust belt (LTB) along the eastern margin of the Tibetan Plateau may provide insights into such a debate, as the central segment of the LTB has relatively uniform shortening strains yet various fluvial incision capability along the strike. This tectonic setting enables a better assessment of the effects of external forces on fault activity. We analyzed the variations of topography, fluvial incision intensity, coseismic slips, and coseismic landslides along the central LTB. The Longmen subsegment in the northern half has higher elevation and lower fluvial incision intensity than the Hongkou subsegment in the south. We calculated the topographic stresses on the faults ruptured during the 2008 Wenchuan earthquake and found topographic introduced normal stress increase may explain the coseismic slip partitioning onto two subfaults along the Longmen subsegment. Our results indicate that fluvial incision may have produced the along‐strike variations of the topography, which may further produce the different rupture behavior. In addition, the mean hillslope angle along the central LTB appeared to be at the critical condition prior to the 2008 Wenchuan earthquake and reduced significantly by the earthquake, indicating that geomorphic indices may vary with different stages in an earthquake cycle. Therefore, scrutinizing the mean hillslope angle and other geomorphic indices may help identify potential seismic hazards in an active fault system. Key Points: The central Longmenshan is an ideal place to quantitatively understand the influences of external forces on orogenic processesThe fluvial incision is the fundamental driver for the along‐strike variation of fault activities in central LongmenshanThe geomorphic indices vary with the different stages in an earthquake cycle, which may shed light on seismic hazard assessment [ABSTRACT FROM AUTHOR]

Abstract: The deformation processes at work across the eastern margin of the Tibetan Plateau remain controversial. The interpretation of its tectonic history is often polarized between two deformation models: ductile flow in the lower crust and shortening and crustal thickening accommodated by brittle structures in the upper crust. Many geological investigations on this plateau margin focused on the Longmen Shan, at the western edge of the Sichuan Basin. However, the Longriba fault system (LFS) located 200 km northwest and parallel to the Longmen Shan structures provides an opportunity to understand the role of hinterland faults in eastern Tibet geodynamics. For this reason, we investigate the exhumation history of rocks across the LFS using (U‐Th)/He and fission track ages from apatite and zircon. Results show a significant contrast in cooling histories across the Maoergai fault, the southernmost fault of the LFS. South of the Maoergai fault, the bedrock records a rapid increase in exhumation rate since ~10–15 Ma. In contrast, the area north of the fault has experienced steady cooling since ~25–35 Ma. We attribute this cooling contrast to ~2 km of differential rock uplift across the Maoergai fault, providing the first evidence of activity of the LFS in the Late Cenozoic. Our results indicate that deformation of the eastern Tibetan margin has been partitioned into the LFS and the Longmen Shan over an ~200 km wide block, which should be incorporated in future studies on the region's deformation, and in both above‐mentioned deformation models. [ABSTRACT FROM AUTHOR]

On 20 April 2013, an unexpected M w 6.7 earthquake occurred in Lushan County at the southern Longmen Shan, the eastern margin of the Tibetan Plateau. A high-resolution seismic reflection profile was combined with near-surface geological data, earthquake relocation and geodetic measurements, and a recent deep artificial seismic reflection profile to identify the active fault and seismotectonics of this earthquake. Three-dimensional imaging of the aftershocks was used to identify two planar faults that together form a y shape (f1 and f2). Seismic interpretations suggest that fault f1 did not break through the overlying Mesozoic and Cenozoic rocks and is a typical blind fault. Geodetic measurements suggest that the coseismic deformation is consistent with the geometry and kinematics of shear fault-bend folding. Deep seismic data indicate the syndepositional nature of fault f1 a preexisting normal fault older than the Triassic, which underwent positive inversion tectonics during the Late Cenozoic. A thrust fault f3 converges with f1 at a depth of approximately 12 km with an accumulated slip 3.6 km. This 2013 Lushan earthquake triggered by blind faults is a hidden earthquake. Blind and reactivated faults increase the potential risk and uncertainty related to earthquakes in the eastern margin of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

We applied the geomorphic indices (hypsometry and stream length gradient) to evaluate the differential uplift of the central and southern Longmenshan, a mountain range characterized by rapid erosion, strong tectonic uplift, and devastating seismic hazards. The results of the geomorphic analysis indicate that the Beichuan-Yingxiu fault and the Shuangshi-Dachuan fault act as major tectonic boundaries separating areas experiencing rapid uplift from slow uplift. The results of the geomorphic analysis also suggest that the Beichuan-Yingxiu fault is the most active fault with the largest relative uplift rates compared to the rest of the faults in the Longmenshan fault system. We compared reflected relative uplift rates based on the hypsometry and stream length gradient indices with geological/geodetic absolute rates. Along-strike and across-strike variations in the hypsometry and stream length gradient correlate with the spatial patterns derived from the apatite fission track exhumation rates, the leveling-derived uplift rate, and coseismic vertical displacements during the 2008 Wenchuan earthquake. These data defined multiple fault relationships in a complex thrust zone and provided geomorphic evidence to evaluate the potential seismic hazards of the southern Longmenshan range. [ABSTRACT FROM AUTHOR]

On 3 July 2015, the Mw 6.5 Pishan earthquake occurred in the western Kunlun Mountains front, at the northern margin of the Tibetan Plateau. To reveal the sedimentary-tectonic framework of the seismically active structure, three high-resolution seismic reflection profiles and well drilling data were collected for seismic interpretation. The western Kunlun Mountains and Tarim Basin have two gypseous detachments and one basement detachment that control the tectonic framework and structural deformation. The upper gypseous detachment (D1) is in the lower Paleocene, and the middle gypseous detachment (D2) is in the Middle to Lower Cambrian. A Neogene shallow thrust system is developing above D1 and includes the Zepu fault (F2) and Mazar Tagh fault (F3). A deep thrust system is developing between D1 and D2 and forms a large-scale structural wedge beneath the western Kunlun Mountains front. The Pishan Mw 6.5 earthquake was triggered on a frontal blind fault of this deep thrust system. The lower detachment is in the Proterozoic basement (D3), which extends into the Tarim Basin and develops another deep thrust (F4) beneath the F3 belt. D1, D2, D3, and the Tiekelike fault (F1) merge together at depth. Crustal shortening of the western Kunlun Mountains front continues for approximately 54 km. Two tectonic evolutionary stages have occurred since the Miocene according to sedimentary unconformity, axial analysis, and fault interpretation. The results of this study indicate a regime of episodic growth of the western Kunlun Mountains and Tarim Basin during the Cenozoic. [ABSTRACT FROM AUTHOR]

In the epicenter of the Lushan M7.0 earthquake there are several imbricate active reverse faults lying from northwest to southeast, namely the Gengda-Longdong, Yanjing-Wulong, Shuangshi-Dachuan and Dayi faults. Emergency field investigations have indicated that no apparent earthquake surface rupture zones were located along these active faults or their adjacent areas. Only brittle compressive ruptures in the cement-covered pavements can be seen in Shuangshi, Taiping, Longxing and Longmen Townships, and these ruptures show that a local crustal shortening occurred in the region during the earthquake. Combining spatial distribution of the relocated aftershocks and focal mechanism solutions, it is inferred that the Lushan earthquake is classified as a typical blind reverse-fault earthquake, and it is advised that the relevant departments should pay great attention to other historically un-ruptured segments along the Longmenshan thrust belt and throughout its adjacent areas. [ABSTRACT FROM AUTHOR]

As revealed by field investigations, the co-seismic surface rupture zone of the 2010 MS7.1 Yushu earthquake, Qinghai is a characteristic sinistral strike-slip feature consisting of three distinct sinistral primary ruptures, with an overall strike of 310°--320° and a total length of 31 km. In addition, an approximately 2-km-long en-echelon tensile fissure zone was found east of Longbao Town; if this site is taken as the north end of the rupture zone, then the rupture had a total length of ~51 km. The surface rupture zone is composed of a series of fissures arranged in an en-echelon or alternating relationship between compressive bulges and tensile fissures, with a measured maximum horizontal displacement of 1.8 m. The surface rupture zone extends along the mapped Garzê-Yushu Fault, which implicates it as the seismogenic fault for this earthquake. Historically, a few earthquakes with a magnitude of about 7 have occurred along the fault, and additionally traces of paleoearthquakes are evident that characterize the short-period recurrence interval of large earthquakes here. Similar to the seismogenic process of the 2008 Wenchuan earthquake, the Yushu earthquake is also due to the stress accumulation and release on the block boundaries resulting from the eastward expansion of Qinghai-Tibet Plateau. However, in contrast with the Wenchuan earthquake, the Yushu earthquake had a sinistral strike-slip mechanism resulting from the uneven eastward extrusion of the Baryan Har and Sichuan-Yunnan fault blocks. [ABSTRACT FROM AUTHOR]

Mechanical models and analog experiments show that erosion plays an important role in controlling along-strike variations in thrust-belt deformation. Despite this fundamental insight, natural examples of focused erosion causing local variations of thrust-belt geometry, such as the formation of salients and recesses, remain underappreciated. In this study, we address this issue by integrating theoretical model predictions with structural, morphological, and erosion-pattern analyses along the thrust system defining the eastern margin of the Tibetan Plateau. Our work shows a nearly constant surface slope across and an overall cylindrical fault-related fold geometry along the thrust belt. These phenomena can be explained by the development of a critically-tapered thrust wedge along the plateau margin. A prominent recess exists at the intersection between the transverse Minjiang River and the range-bounding thrust. There, the surface trace displays a much higher curvature than the rest of the thrust belt and the local fold geometry is conical rather than cylindrical. We infer a warped passive-roof duplex under the recess. We build a classification diagram for recesses developing in critical-taper wedges, in which six independent variables lead to three morphotectonic scenarios regarding the effects of mass influx, mass efflux, and intrinsic properties of a thrust wedge. Formation of the Dujiangyan recess and its spatial correlation to the Minjiang River can be best attributed to localized fluvial erosion during forward propagation of the thrust wedge in eastern Tibet. This study sets a type example for map-view curves in thrust belts undergoing deformation-erosion interactions. • Conical-shaped folding and localized erosion in a critical-taper wedge in eastern Tibet. • Six wedge-related mechanisms lead to three morphotectonic scenarios for recess classification. • Deformation-erosion interactions govern the Dujiangyan recess formation. [ABSTRACT FROM AUTHOR]