Crustal Channel Flow Research Articles

Contemporary crustal strain rates derived from GPS measurements in southwestern China

Surface strain rates in southwestern China can provide constraints on tectonic evolution of the Tibetan Plateau. Unfortunately, different strain rate fields were obtained from different authors although almost the same GPS data were exploited. Based on updated GPS data, in this paper, we calculate the strain rates in southwestern China using the method proposed by Zhu et al. (2005, 2006). The results confirm that the strain rates in the Sichuan Basin and the South China Block are very small, and the high values are largely concentrated along the Xianshuihe-Anninghe-Xiaojiang fault system and the Sagaing Fault. Furthermore, the highest principal strain rates are located around the eastern Himalayan syntaxis (EHS), with compressive orientations perpendicular to the strike of the Main Thrust Belt. The basic characteristic of the strain rate distribution is in accordance with the tectonic structures from the geological and geophysical investigations. In particular, the two calculated extensive deformation regions basically match the locations of normal-faulting earthquakes in the Sichuan-Yunnan Rhombic Block (SYRB), with one in N-S extension trending in the northern SYRB, and the other nearly E-W orientation in the southern SYRB. We suggest that the extensive deformation and the normal faulting earthquakes in southwestern China is mainly controlled by the lower crustal channel flow with a bend direction, rather than by gravitational spreading alone, as previous authors proposed.

Uncertainty Quantification in Radial Anisotropy Models Based on Transdimensional Bayesian Inversion of Receiver Functions and Surface-Wave Dispersion: Case Study Sri Lanka

ABSTRACT In geophysical inference problems, quantification of data uncertainties is required to balance the data-fitting ability of the model and its complexity. The transdimensional hierarchical Bayesian approach is a powerful tool to evaluate the level of uncertainty and determine the complexity of the model by treating data errors and model dimensions as unknown. In this article, we take account of the uncertainty through the whole procedure, thus developing a two-step fully Bayesian approach with coupled uncertainty propagation to estimate the crustal isotropic and radial anisotropy (RA) model based on Rayleigh and Love dispersion as well as receiver functions (RFs). First, 2D surface-wave tomography is applied to determine period-wise ambient noise phase velocity maps and their uncertainty for Rayleigh and Love waves. Probabilistic profiles of the isotropic average VS and RA as a function of depth are then derived at station sites by inverting the local surface-wave dispersion and model errors and RFs jointly. The workflow is applied to a temporary seismic broadband array covering all of Sri Lanka. The probabilistic results enable us to effectively quantify the uncertainty of the final RA model and provide robust inferences. The shear-wave velocity results show that the range of Moho depths is between 30 and 40 km, with the thickest crust (38–40 km) beneath the central Highland Complex. Positive RA (VSH>VSV) observed in the upper crust is attributed to subhorizontal alignment of metamorphic foliation and stretched layers resulting from deformation. Negative RA (VSV>VSH) in the midcrust of central Sri Lanka may indicate the existence of melt inclusions and could result from the uplift and folding process. The positive RA in the lower crust could be caused by crustal channel flow in a collision orogeny.

Thermal and Physical Properties of Barrovian Metamorphic Sequence Rocks in the Ailao Shan‐Red River Shear Zone, and Implications for Crustal Channel Flow

AbstractThe collisional history between Greater India and the Eurasian plate has been well constrained by the study of exhumed Barrovian metamorphic sequence (BMS) rocks in the Himalayan Range. However, in the southeastern Tibetan Plateau, the collisional records have been obscured by intense, regional‐scale strike‐slip overprinting and recrystallization. Here, in BMS rocks from the Ailao Shan–Red River shear zone (ARSZ), we report the first discovery of a >250 km long, high‐pressure (high‐P) granulite belt (>1.0 GPa), identified by the presence of relict kyanite and associated decompression reaction textures. Petrological phase equilibrium modeling showed that exposed micaschists in the region represent exhumed middle crust (20–25 km, 600–670°C), while the high‐P granulite rocks are remnants of thickened lower crust (45–55 km, 800–850°C). This indicates that the northeast edge of the ARSZ experienced an additional ∼25 km of uplift and exhumation compared to the southwest side, facilitated by brittle thrusting/imbrication along the Ailao Shan fault (micaschists) and ductile extrusion along the Red River fault (granulite). Geochronological study shows that the upper portion of the BMS preserves older metamorphic ages (52–34 Ma) than the lower portion (32–29 Ma), which was attributed to spatial variation in cooling rates. Using calculated P–T–t–d paths, we also examined variation in density and seismic wave speeds for BMS in the ARSZ. Our data correlate with fieldwork conducted elsewhere within the Himalayan Range indicating that the kyanite to sillimanite transition zone may serve as a “cap” for the horizontal migration of melt within the lower crust.

Crustal structure and deformation mechanism of the western northeast Tibetan Plateau

The collision between the Indian and Eurasian plates continues to drive significant deformation and uplift within the interior of the Tibetan Plateau, together with its outward expansion along the margins. In particular, the North Qilian Shan fold-thrust belt (NQLS) and the Hexi Corridor basins (HXBS) represent the northernmost region of the northeastern Tibetan Plateau. This area serves as a natural laboratory for deciphering mechanisms of crustal deformation and thickening along the plateau’s margins. Specifically, the northeastern Tibetan Plateau has been attributed to 1) southward underthrusting of the Asian lithosphere, 2) distributed shortening and crustal thickening, 3) vertical inflation of the Tibetan crust due to mid-lower crustal channel flow, and 4) intracontinental subduction facilitated by large-scale strike–slip faults. The exact mechanism underlying the most concentrated convergent stress in the western segment of NQLS–HXBS remains a subject of debate. To address this uncertainty, we gathered seismic data along a 130-km-long linear array that extends northward from NQLS, traversing the Jiuquan Basin and reaching the Huahai Basin. Our analysis, conducted through the receiver function method, reveals intriguing findings. The Moho depth deepens from 45–50 km beneath the Huahai Basin to 55–60 km beneath NQLS. Notably, a double Moho structure emerged, marked by a distinctive near-flat positive amplitude at a depth of 45–50 km beneath NQLS within a distance of 0–50 km. Our study presents a comprehensive analysis of the crust-scale deformation mechanism, shedding light on the following key aspects: 1) the development of a decollement at 12–20 km depth decoupling the upper and lower crust; 2) deformation of the upper crust occurring through south-dipping brittle thrust faults, while the lower crust features imbricate structures and duplexes; 3) evidence pointing to the underthrusting of the Beishan Block beneath NQLS, indicated by the double Moho beneath NQLS; and 4) the formation of a Moho ramp beneath the Jiuquan Basin, facilitating the transfer of shortening stress from beneath NQLS and HXBS to the north. In the context of the western segment of NQLS and HXBS, our speculation is that coupled distributed shortening and Beishan Block subduction beneath NQLS work in tandem to accommodate crustal deformation.

Crustal deformation in the southeastern margin of the Tibetan Plateau: insights from broad-band Pg-wave attenuation tomography

SUMMARY The deformation mechanism in southeastern Tibet since the continental collision between the Indian and Eurasian plates could be explained by several models, including two major classic end-member models, the rigid-block extrusion model and the crustal flow model. Crustal channel flow is likely an important tectonic regime for properly explaining a large number of geological and geophysical observations but remains in competition with the block extrusion model. Consequently, detecting ductile flow connectivity would play a key role in understanding the tectonic evolution of the southeastern Tibetan Plateau. Here, we established a high-resolution broad-band QPg model for the crust in SE Tibet by using a joint inversion tomography method based on both single- and two-station Pg data. We verified the stability of the QPg tomography by comparing the QPg values at 1 Hz between the joint inversion and the two-station method. Two low-QPg zones were observed, isolated by the high-QPg Emeishan large igneous province (ELIP). Strong Pg attenuation beneath the Songpan-Ganzi Block and Western Sichuan Block may indicate the presence of crustal material flow due to relatively weak rheological strength. Cooled basaltic magma remnants in the inner zone of the ELIP likely block the southeastward migration of crustal materials driven by the gravity and lateral pressure gradient, and restrict the flow to the Western Sichuan Block, resulting in surface uplift and crustal thickening. Strong Pg attenuation near the Xiaojiang Fault and the Red River Fault may result from mantle upwelling in this region. Our QPg model, combined with previous results, suggests that the tectonic deformation in the southeastern Tibetan Plateau has been mainly controlled by the effects of crustal channel flow and asthenospheric upwelling since the Late Miocene.

U-Pb ages of Gongga Shan granite, western Sichuan and its implication to the tectonic evolutions of eastern Tibet

The eastern margin of the Tibetan Plateau has given rise to much debate about mechanisms of plateau uplift and evolution and, in particular, the role of the lower crust in crustal thickening. Knowledge of the middle to lower crust conditions is critical for evaluating various models of crustal deformation, but data on crustal evolution through time are lacking. Here, we turn to the Gongga Shan granite, an intrusion along the Xianshuihe fault in easternmost Tibet that directly records local Cenozoic crustal conditions. We present 124 U-Pb samples from the Gongga Shan granite (GSG) that prove that the crust has been stepwise producing partial melt from 56 Ma to 4 Ma. According to the age distribution, the GSG can be separated into four major groups with ages of 4–10 Ma, 12–20 Ma, 25–40 Ma, and 43–56 Ma. Combining the timing information with geophysics and low-temperature thermochronology data, we suggest that events younger than 10 Ma may indicate the onset of recent crustal channel flow in the middle to lower crust. In contrast, the youngest 4 Ma ages indicate the ongoing partial melting of the middle crust. The 12–20 Ma events could be related to an earlier stage of crustal channel flow, consistent with the regional large-scale crustal channel flow in central Tibet.

Insights on the uplift mechanism of the eastern Tibetan Plateau from the flexural isostasy study

AbstractIt is generally believed that the rapid uplift of the eastern Tibetan Plateau is due to the continuous continental collision between the Indian and Eurasian plates. But the uplift mechanisms of the eastern Tibetan Plateau remain uncertain. The analysis of flexural isostasy can provide new insights into those uplifting mechanisms. In this study, the effective elastic thickness () and load ratio () of the lithosphere are estimated around the eastern Tibetan Plateau. The derived and are used to compute flexural isostatic anomalies (FIA) using the theoretical Bouguer admittance method. We find the eastern margin of the Tibetan Plateau is dominated by low , high , and significant negative FIA. Our results suggest that this area has a weak lithosphere with heavy subsurface loadings and crustal mass surpluses. This supports that a middle‐lower crustal channel flow probably underlies this region.

Deep crustal structure across northeastern Tibet from P receiver functions

The uplift mechanism and deep crustal deformation of the northeast of the Tibetan plateau in response to the Cenozoic India-Asian collision are still poorly constrained. Previous studies linked the uplift mechanism of the northeast of Tibet to the southward underthrusting of the Asian mantle lithosphere and the northward subduction of the Indian plate; yet, the details of crustal properties and thickening mechanism beneath the northeast of Tibet remain controversial. To shed light on the uplift mechanism of the northeast of the Tibetan plateau, we present a detailed image of the crustal structure beneath northeastern Tibet using teleseismic P receiver functions from 54 temporary broadband seismic stations. The Moho beneath the Kunlun Shan reaches ∼66 km but abruptly shallows to ∼40 km at the conjunction of the Kunlun and Qaidam, marking the boundary of northern and central Tibet. The Moho has a convex shape beneath the Qaidam basin and gently undulates northward beneath the Qilian Shan, and substantially shallows to ∼48–50 km under the Alashan block without any Moho offset. The undulating Moho and the absence of any Moho disturbance suggest that crustal thickening and shortening is the main driving mechanism for the northeastern plateau uplift. The average Vp/Vs ratio (∼1.75) beneath northeastern Tibet is indicative of a felsic to intermediate composition without a prominent low-velocity zone or low-viscosity channel in the middle to lower crust. Thus, the moderate crustal Vp/Vs ratio excludes the crustal channel flow as a primary uplift mechanism, implying that the horizontal crustal shortening and vertical thickening is responsible for creating the thick crust of northeastern Tibet.

Seismic Azimuthal Anisotropy of Northeastern Tibetan Plateau From Ambient Noise Double Beamforming Tomography: Implications for Crustal Deformation

AbstractIn this study, we obtain a three‐dimensional (3‐D) high resolution seismic anisotropy model of NE Tibet using ambient noise double beamforming (DBF) tomography with ChinArray‐Himalaya II data. DBF tomography technique allows for the simultaneous derivation of local phase velocity and azimuthal anisotropy from ambient noise cross‐correlations without performing tomographic inversion. Based on the DBF tomography results, we have constructed a high resolution 3‐D azimuthal anisotropic shear wave velocity model of NE Tibet. Our model indicates the presence of prominent low velocities and strong azimuthal anisotropy in the mid‐to lower crust beneath the Songpan‐Ganzi belt, while the mid‐to lower crust of the Qilian belt exhibits relatively weak low velocity and azimuthal anisotropy. We propose that crustal channel flow may be the primary mechanism driving crustal deformation in the Songpan‐Ganzi belt, which is in the mature part of the plateau. As the Qilian belt is at an early stage of plateau formation, it is characterized by crustal shortening with the development of a sub‐vertical foliation. Additionally, the misalignment between predicted and observed shear wave splitting reveals decoupled crust and upper mantle deformation west of the Ordos block where eastward mantle flow is deflected by the lithospheric keel of Ordos.

Characteristics of lower crustal channel flow and geomorphology of the Tibetan Plateau

Characteristics of lower crustal channel flow and geomorphology of the Tibetan Plateau

Cenozoic kinematics of the Wenchuan-Maoxian fault implies crustal stacking rather than channel flow extrusion at the eastern margin of Tibetan plateau (Longmen Shan)

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.

Middle–Late Cenozoic Stepwise Deformation Propagation in Eastern Tibet

AbstractThe uplift and deformation styles of the Tibetan Plateau have been long debated on stepwise growth and crustal channel flow. Here, we offer new insight into this issue by constraining the pulsed exhumation history of the Yalong thrust belt in eastern Tibet, using apatite and zircon (U–Th)/He data from three ∼1 km vertical transects. The results revealed two rapid cooling pulses in the Late Oligocene (∼24 Ma, 50°C/m.y.) and the Middle Miocene (17–14 Ma, 35 °C/m.y.), respectively, which we ascribe to the staged‐thrusting faulting of this belt. These thrust faulting events indicated upward and eastward growth of the Tibetan Plateau, further suggested that the high topographic relief across the Yalong thrust belt mainly formed (at least) since the Late Oligocene. The available documents from paleoaltimetry, basin sediment accumulation, and thermochronometers and our new results reveal a regional‐scale stepwise propagation pattern of tectonic deformation during the Middle–Late Cenozoic in eastern Tibet.

Crustal structure beneath the northern part of the southeastern Tibetan Plateau revealed by a seismic dense nodal array

The continental convergence between the Indian and Eurasian plates in the Cenozoic not only caused the formation of the Tibetan Plateau (TP) but also pushed a large volume of continental lithosphere out of the plateau southeastwards. It has been strongly debated for many years how to transport mass from the central plateau to the southeastern TP. In the area between the central plateau and its southeastern margin, we deployed a southwest-northeast-trending dense seismic nodal array to record teleseismic waveforms. Receiver function methods were used to estimate the crustal bulk properties and to image the crustal structure. Our results show that the crustal thickness is approximately 60 km and the crustal average Vp/Vs ratio is approximately 1.78, which is less than the predicted values of the crustal channel flow model. In the crustal structure image, the Moho beneath the southwestern part of the profile dips northeast and extends below the Moho beneath the northeastern part of the profile. Beneath the southwestern part of the profile, a lower crustal northeast-dipping interface is also imaged, which is likely linked with the Jinsha River suture. Our results suggest that the most significant crustal thickening in the southeastern TP was produced by the northwards or northeastwards subduction of the Qiangtang block, which occurred in the late Oligocene to early Miocene, as indicated by palaeo-altimetric studies. This finding implies that the modern landform in the southeastern TP resulted from southeastwards block extrusion along strike-slip faults and more monsoonal precipitation and seasonal climate variation since 10 Ma.

The 2022 Luding, Sichuan, China, M 6.8 earthquake: A fluid-related earthquake?

We determine a detailed 3-D crustal S-wave velocity structure in and around the Xianshuihe (XSH) fault zone using the direct ambient-noise surface-wave tomography. The continuous waveform data recorded at 22 portable seismic stations of our newly deployed around the XSH fault zone and 13 Chinese provincial stations are used in the data collection during September to December 2020. Our results show obvious low-velocity (low-V) anomalies under the Sichuan basin and high-velocity (high-V) anomalies under the Songpan-Ganzi block at 5–10 km depths but high-V anomalies under the Sichuan basin and low-V anomalies under the Chuandian block at 20–40 km depths. The XSH faut zone shows a clear boundary of low-V and high-V anomalies. Furthermore, there is a clear relationship between velocity structure and Luding mainshock. The Luding mainshock and its aftershocks occurred at the boundary of high-V anomalies in the upper crust but underlain by low-V anomalies in the mid-lower crust. These low-V anomalies indicate that the fluids have reached in the source area. These results suggest that fluids might reduce the friction force due to the decrease of both frictional coefficient and effective normal stress on the fault planes and trigger the strike-slip movement of the fault zone and the occurrence of the Luding earthquake. These fluids could be contained in the hot and wet mantle upwelling flow in the big mantle wedge caused by the deep subduction of the India slab down to the mantle transition zone, in addition to the crustal channel flow due to the Indo-Eurasian collision.

Imaging the Northeastern Crustal Boundary of the Tibetan Plateau With Radial Anisotropy

AbstractWe obtained a 3‐D crustal radial anisotropy model beneath northeastern (NE) Tibet by joint inversion of Rayleigh and Love dispersion curves from ambient noise tomography. Positive crustal radial anisotropy and significant low velocity are dominant beneath the Songpan‐Ganzi Terrane (SGT), indicating the presence of crustal channel flow. The Qilian orogen is characterized by negative radial anisotropy, which could be caused by folding and thrusting due to crustal shortening. This difference in radial anisotropy suggests that crustal shortening deformation may have occurred at a relatively early stage of the plateau evolution (the Qilian orogen) and crustal channel flow at the later stage (the SGT). The crustal radial anisotropy clearly defines the NE crustal boundary of the expanding Tibetan Plateau, which is roughly situated along the north Qilian frontal thrust from 97° to 103.5°E in the north and turns north‐south, passing the Zhuanglanghe Fault and terminating at the Kunlun Fault in the south.

Seismic imaging of the crust and uppermost mantle structure in the Qinling orogenic belt and its surroundings: Geodynamic implications

The Qinling orogenic belt and surrounding areas are a junction zone in Central China that connects the North China craton, Yangtze craton and northeastern Tibetan plateau. This area is in a key position within the more general framework where there is eastward extrusion of the Tibetan plateau and westward subduction of the Pacific plate. There is an ongoing scientific debate on how these two main processes are driving the geodynamic evolution of the Qinling orogenic belt and surrounding areas. Specific mechanisms have been proposed, such as lower crustal flow, crustal shortening and big mantle wedge tectonics. Knowledge of the seismic velocity structure helps us understand how these two driving forces affect the tectonic blocks of the study area, as well as test how well the proposed mechanisms to fit the regional velocity structure. For this purpose, a high-resolution P-wave velocity model of the crust and uppermost mantle was obtained by inverting arrival times from local earthquakes that occurred in the Qinling and surrounding region. Our three-dimensional (3D) model suggests that crustal channel flow cannot exist in the whole Qinling orogenic belt, and the localized low-velocity zone generated by the horizontal compressive stress caused by the collision of the Indian and Eurasian plates, is a possible explanation for the growth of the northeastern Tibetan plateau. The contrast in seismic velocities found under the east-Qinling and the west-Qinling orogenic belts implies a different response of these two terranes to the eastward extrusion of the Tibetan plateau. Our velocity model points to relatively stable structural features in the east-Qinling and the Weihe graben, while a low-velocity anomaly in the uppermost mantle of the southernmost Ordos plateau suggests that the lithosphere of the southern Ordos plateau is undergoing destruction.

Characteristics and Removal of Continuous Topographic Scattering in Dense Array Receiver Function Imaging

With dense nodal seismic arrays growing rapidly, receiver function (RF) imaging has revealed unprecedented small-scale features. However, whether these fine signals represent real intra-crustal structures needs to be verified. In this study, we take the RF profiles recorded by a dense array in Ruoergai basin in northeastern Tibet as an example and demonstrate that continuous dipping phases in receiver functions (RFs) could be generated from the interference between the incoming P-wave and dramatic topographic variations. Full-waveform modeling illustrates that when an incident P-wave encounters a topographic peak, it can induce scattered P- and Rayleigh waves that propagate in backward and forward directions, consistent with the observations. The scattered Rayleigh waves exhibit a cosine radiation pattern in R-components, which explains the azimuthal-dependent observation of dipping phases in RF profiles. We then propose a two-step separation method to suppress the continuous topographic scattered phases in RF imaging, which involves a time-domain adaptive matched filtering and a curvelet-based separation. The resulting common conversion point (CCP) images show that the method can effectively suppress the continuous scattered phases induced by topography. CCP images exhibit flat intra-crustal layers, suggesting a decoupled deformation style in the upper and lower crust in NE Tibet. The upper crustal layer may represent the basement of the décollement layer, and the intermittent lower crustal interfaces are likely to indicate the partial melt within the crustal channel flow. The separation method is applicable for dense nodal array RF imaging to reveal reliable, detailed intra-crustal structures in tectonically active regions that developed dramatic topography.

Deep Crustal Structure Beneath the Pamir–Tibetan Plateau: Insights From the Moho Depth and Vp/Vs Ratio Variation

The Cenozoic convergence between India and Asia has created Earth’s thickest crust in the Pamir–Tibetan plateau, leading to broadly distributed deformation and extensive crustal shortening; however, the crustal deformation of the high plateau is still poorly constrained. The variation of the Moho topography and crustal composition beneath the Pamir–Tibetan plateau has an important correlation with the major tectonic units. In this study, the results of the receiver functions have been reviewed and analyzed to observe variations in the Moho depth and crustal Vp/Vs ratio beneath the Pamir–Tibetan plateau. We found a notable SE–NW-oriented deep Moho interface that starts from the southeast of the Tibetan plateau and continues to the eastern Pamir with a northward dipping direction, which may indicate the northern frontier of the decoupled lower crust of northward underthrusting of the Indian plate. In contrast, the deepest Moho beneath the Pamir plateau has a southward dipping direction indicating the southward underthrusting Asian plate. In general, the average crustal Vp/Vs ratio is relatively low beneath the South-Central Pamir (∼1.70), while it is relatively higher (∼1.75) under the Himalaya–Lhasa terrane, suggesting more felsic to intermediate rock composition with locally high values indicating a low-velocity zone, possibly caused by partial melting. Elevated Vp/Vs ratios are observed beneath the northern Pamir (>1.77) and Qiangtang and Songpan–Ganze terranes (>1.80), which can be related to the high mafic rock content and upwelling hot materials from the upper mantle. The Vp/Vs ratio beneath the Pamir–Tibetan plateau presents complex north–south variations with a relatively low crustal Vp/Vs ratio in the south, while it gradually increases toward the north of the Pamir and central-northern Tibet, which is probably caused by the joint effects of the northward underthrusting Indian lower crust and southward subduction of the Asian plate, the low-velocity zones within the mid-upper crust, and substantial crustal shortening and thickening. The low to average crustal Vp/Vs ratio throughout the plateau (except the central Tibet) indicates a limited amount of hot materials to support the low crustal channel flow model, instead suggesting that crustal thickening and shortening is the main uplifting mechanism of the Pamir–Tibetan plateau.

Fluids Triggered the 2021 Mw 6.1 Yangbi Earthquake at an Unmapped Fault: Implications for the Tectonics at the Northern End of the Red River Fault

Abstract On 21 May 2021 a magnitude Mw 6.1 earthquake occurred in Yangbi region, Yunan, China, which was widely felt and caused heavy casualties. Imaging of the source region was conducted using our improved double-difference tomography method on the huge data set recorded by 107 temporary stations of ChinArray-I and 62 permanent stations. Pronounced structural heterogeneities across the rupture source region are discovered and locations of the hypocenters of the Yangbi earthquake sequence are significantly improved as the output of the inversion. The relocated Yangbi earthquake sequence is distributed at an unmapped fault that is almost parallel and adjacent (∼15 km distance) to the Tongdian–Weishan fault (TWF) at the northern end of the Red River fault zone. Our high-resolution 3D velocity models show significant high-velocity and low-VP/VS ratios in the upper crust of the rupture zone, suggesting the existence of an asperity for the event. More importantly, low-VS and high-VP/VS anomalies below 10 km depth are imaged underlying the source region, indicating the existence of fluids and potential melts at those depths. Upward migration of the fluids and potential melts into the rupture zone could have weakened the locked asperity and triggered the occurrence of the Yangbi earthquake. The triggering effect by upflow fluids could explain why the Yangbi earthquake did not occur at the adjacent TWF where a high-stress accumulation was expected. We speculate that the fluids and potential melts in the mid-to-lower crust might have originated either from crustal channel flow from the southeast Tibet or from local upwelling related to subduction of the Indian slab to the west.

Late Eocene palaeogeomorphology and uplift process of the Central Tibetan Plateau: Constraints from intermediate-felsic magmatic rocks

The Eocene palaeogeomorphology and uplift process of the Central Tibetan Plateau including the Lhasa and Qiangtang terranes have been controversial. In this paper, we study Eocene rhyolites in Wuma area of the Northern Lhasa terrane on the Central Tibetan Plateau by zircon U-Pb dating, geochemistry, zircon Lu-Hf isotopes, and Sr-Nd isotopes. The results show that the ages of Wuma rhyolites are 40-39 Ma, and the petrogenesis of them is related to partial melting of lower crust. The La/Yb, Dy/Yb, Sm/Yb and zircon Eu/Eu* of lower-crust-derived intermediate-felsic magmatic rocks can well reflect crustal thickness and Moho depth. Based on statistics of intermediate-felsic magmatic rocks in the Central Tibetan Plateau at 47–37 Ma, we find that lower-crust-derived intermediate-felsic magmatic rocks of the Northern Lhasa terrane have lower La/Yb, Dy/Yb, Sm/Yb and zircon Eu/Eu* than those of the Southern-Central Lhasa and Qiangtang terranes, indicating that Northern Lhasa terrane had thinner crust than Southern-Central Lhasa and Qiangtang terranes during late Eocene. Further, according to Airy isostatic compensation, in which thick crusts correspond to high altitudes and thin crusts correspond to low altitudes, we speculate that the Northern Lhasa terrane had lower altitude, while the Southern-Central Lhasa and Qiangtang terranes had higher altitude during late Eocene; that is, the Central Tibetan Plateau had the late Eocene palaeogeomorphology of two mountains and one basin. In addition, through statistics of intermediate-felsic magmatic rocks in the Northern Lhasa terrane and near its suture zones on both sides since 100 Ma, we find that La/Yb, Dy/Yb and Sm/Yb of these intermediate-felsic magmatic rocks increase at 40-34 Ma, indicating that the crust of the Northern Lhasa terrane have thickened and the surface may have been uplifted during late Eocene. Based on that there are abundant late Eocene lower-crust-derived intermediate-felsic magmatic rocks in the Central Tibetan Plateau and the late Eocene palaeogeomorphology of Central Tibetan Plateau is two mountains and one basin, and other evidences, we speculate that the late Eocene uplift of Northern Lhasa terrane may have been related to lower crustal channel flow.