Seismic structure of the 2015 Mw7.8 Gorkha earthquake revealed by ambient seismic noise and teleseismic surface wave tomography

The destructive 2015 Mw7.8 Gorkha earthquake occurred in the Main Himalayan Thrust due to the collision of the Indian and Asian plates, which provides a unique opportunity to understand the deep dynamic processes and seismogenic mechanisms of strong earthquakes. We construct a regional-scale shear-wave velocity model of the crust and uppermost mantle using ambient seismic noise and teleseismic surface wave at periods of 5-100 s around the Gorkha earthquake region. The new shear-wave velocity model exhibits prominently lateral heterogeneities in the Gorkha earthquake areas. We observe a high-velocity (high-V) zone around the Gorkha main shock in the Main Himalayan Thrust, indicating the existence of a high-strength asperity that sustains the stress accumulating. The aftershocks are primarily located in the low-velocity (low-V) anomalies and enclosed by two high-V anomalies, which appear to act as structural barriers that influence the spread of the aftershocks. Prominent low-Vanomalies from the lower crust to the mantle lithosphere are observed along the north-south trending rifts, suggesting the hot materials upwelling due to the tearing of the northward subducting Indian lithosphere. These observations may indicate that seismic velocity heterogeneity could play an essential role in earthquake initiation and the rupture process.

Record of Crustal Thickening and Synconvergent Extension from the Dajiamang Tso Rift, Southern Tibet

North-trending rifts throughout south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present new data from the Dajiamang Tso rift, including quantitative crustal thickness estimates calculated from trace/rare earth element zircon data, U-Pb geochronology, and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70–42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter thrust are ~28 Ma, while Gangdese arc samples in the footwalls of the Dajiamang Tso rift are 16–8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Eurasia collision onset (58 Ma). Following initial collision, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Current crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E–W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by an underthrusting Indian lithosphere balanced by upper plate extension.

Normal Faulting in the 2020 Mw 6.2 Yutian Event: Implications for Ongoing E–W Thinning in Northern Tibet

Extensional earthquakes in the Tibetan Plateau play an important role in the plateau’s orogenic evolution and cause heavy seismic hazard, yet their mechanisms remain poorly known, in particular in harsh northern Tibet. On 25 June 2020, a Mw 6.2 earthquake struck Yutian, Xinjiang, offering us a rare chance to gain insights into its mechanism and implications in the Tibetan extension. We used both descending and ascending Sentinel-1 images to generate coseismic deformation associated with this event, which indicates a typical extensional mechanism with a maximum subsidence displacement of 25 cm and minor uplift. The causative fault constrained with interferometric synthetic aperture radar (InSAR) data based on a finite fault model suggests that the fault plane has a strike of 186.4° and westward dip of 64.8°, and the main rupture is concentrated at a depth of 3.6–10.8 km with a peak slip of 0.85 m. Our source model indicates that the 2020 Yutian event ruptured an unknown high-angle blind normal fault with N–S striking. The total released geodetic moment yields 2.69 × 1018 N·m, equivalent to Mw 6.23. We used dense interseismic global positioning system (GPS) measurements to reveal an approximate 7 mm/yr extensional motion in the Yutian region, but it still does not seem large enough to support high local seismicity for normal events within 12 years, i.e., Mw 7.1 in 2008, Mw 6.2 in 2012, and this event in 2020. Combined with Coulomb stress change modeling, we speculate that the seismicity in Yutian is related to the lower lithospheric dynamics.

Genetic Analysis of Geothermal Resources and Geothermal Geological Characteristics in Datong Basin, Northern China

Datong Basin is a Cenozoic fault basin located in the central part of the North China Block with strong tectonic activity. The unique geological environment of Datong Basin is believed to have good conditions for the formation of geothermal resources. Based on the research of the classification, genesis and geothermal geological characteristics of geothermal resources, the geological conditions, seismic activity, volcanic activity, geophysical exploration results, terrestrial heat flow and hot springs in Datong Basin are analyzed. The possibility of the occurrence of geothermal resources in Datong Basin is determined, and the genesis and occurrence mechanisms of geothermal resources in Datong Basin are judged. The results show that Datong Basin satisfies the geological geothermal conditions of the formation of geothermal resources and is of great research value. The formation of geothermal resources in the Datong Basin is affected by the uplift of the Qinghai–Tibet Plateau and the destruction of the North China Craton. The geothermal resources in Datong Basin are formed by the combination of modern volcanic activity and strong inner-plate tectonic activities. The geothermal system is a combination of convective hydrothermal systems and partial melt systems. At the same time, it is concluded that the key research areas for the occurrence of geothermal resources are mainly in the northeastern part of the basin. It is recommended to carry out detailed and comprehensive exploration of the northeastern part of Datong Basin.

Crustal movement and strain distribution in East Asia revealed by GPS observations

East Asia is bounded by the Indian plate to the southwest and the Pacific and Philippine plates to the east, and has undergone complex tectonic evolution since ~55 Ma. In this study, we collect and process three sources of GPS datasets, including GPS observations, GPS positioning time series, and published GPS velocities, to derive unified velocity and strain rate fields for East Asia. We observed southward movement and arc-parallel extension along the Ryukyu Arc and propose that the maximum principal stress axis (striking NEE) in North China could be mainly induced by westward subduction of the Pacific plate and the southward movement of the Ryukyu Arc. The large EW-trending sinistral shear zone that bounds North China has been created by eastward movement of South China to the south and westward subduction of the Pacific plate to the north. GPS velocity profiles and strain rates also demonstrate that crustal deformation in mainland China is controlled by northeastward collision of the Indian plate into Eurasia and westward subduction of the Pacific and Philippine Sea plates beneath Eurasia. In particular, the India-Eurasia continental collision has the most extensive impact, which can reach as far as the southern Lake Baikal. The viscous behavior of the subducting Pacific slab also drives interseismic deformation of North China. The crustal deformation caused by Philippine oceanic subduction is small and is limited to the region between the southeast coast of mainland China and Taiwan island. However, the principal compressional strain around eastern Taiwan is the largest in the region.

Normal faulting and viscous buckling in the Tibetan Plateau induced by a weak lower crust

Flow of weak lower crust has been invoked to reconcile observed topographic gradients, uniform elevations, slow seismic velocity, and high conductivity measured in the Tibetan Plateau, with viscosity estimates of 10¹⁶-10²¹ Pa·s. Here we investigate the dynamic response resulting from a range of lower crust viscosities in a 3-D lithospheric-scale geodynamic simulation of the India-Eurasia collision zone to determine bounds of physically viable lower crustal strengths. We show that thickening of the plateau is accommodated through viscous buckling of the upper crust in response to lower crustal flow for a lower crustal viscosity on the order of 10²⁰ Pa·s. This generates two east-west trending bands of surface subsidence and dilatation consistent with observed normal faulting and estimates of vertical velocity. These results suggest viscous buckling of the upper crust, induced by lower crustal flow from gravitational pressure gradients due to high topography, is responsible for the observed extension in Tibet.

Tearing of Indian mantle lithosphere from high-resolution seismic images and its implications for lithosphere coupling in southern Tibet

What happened to the Indian mantle lithosphere (IML) during the Indian-Eurasian collision and what role it has played on the plateau growth are fundamental questions that remain unanswered. Here, we show clear images of the IML from high-resolution P and S tomography, which suggest that the subducted IML is torn into at least four pieces with different angles and northern limits, shallower and extending further in the west and east sides while steeper in the middle. Intermediate-depth earthquakes in the lower crust and mantle are located almost exclusively in the high-velocity (and presumably strong) part of the Indian lithosphere. The tearing of the IML provides a unified mechanism for Late Miocene and Quaternary rifting, current crustal deformation, and intermediate-depth earthquakes in the southern and central Tibetan Plateau and suggests that the deformations of the crust and the mantle lithosphere are strongly coupled.

Geometry and Evolution of the Cangdong Sag in the Bohai Bay Basin, China: Implications for Subduction of the Pacific Plate

The Cangdong Sag is a complex Cenozoic rift basin at the center of the Bohai Bay Basin. Cenozoic structures in the Cangdong Sag can be subdivided into the Cangdong Fault System in the west and the Xuxi Fault System in the east. The geometry of the boundary faults varies along the axes of half-grabens. According to the cross-sectional strata geometry, unconformity and planar structural pattern, the Cenozoic structural evolution of the Cangdong Sag can be divided into four distinct stages: (1) major Paleocene initial rift, (2) latest Paleocene-early Eocene intensive rift, (3) late Eocene-Oligocene strike-slip superimposed rift, and (4) Neogene to present-day post-rift depression. The extensional deformation was mainly derived from horizontal stress induced by the upwelling of asthenosphere. The strike-slip structure of the Cangdong Sag provides important information related to the subduction of the Western Pacific Plate. It was found that the strike-slip movement of the southern Xuxi Fault Zone was activated during the deposition of the third member of Shahejie Formation to the Dongying Formation; therefore, ~43 Ma probably marks the time when the Western Pacific Plate initially changed its subduction direction from northwest to nearly west.

Rifting and strike-slip shear in central Tibet and the geometry, age and kinematics of upper crustal extension in Tibet

The youngest deformation structures on the Tibet Plateau are about NNE-trending grabens. We first combine remote-sensing structural and geomorphological studies with structural field observations and literature seismological data to study the Muga Purou rift that stretches at c. 86°E across central Tibet and highlight a complex deformation field. ENE-striking faults are dominated by sinistral strike–slip motion; NNE-striking faults have normal kinematics and outline a right-stepping en-echelon array of grabens, also suggesting sinistral strike–slip; along NW-striking fault sets, the arrangement of grabens may indicate a dextral strike–slip component. Thus, in central Tibet, rifts comprise mostly grabens connected to strike–slip fault zones or are arranged en-echelon to accommodate sinistral wrenching; overall strain geometry is constrictional, in which NNE–SSW and subvertical shortening is balanced by WNW–ESE extension. The overwhelmingly shallow earthquakes only locally outline active faults; clusters seem to trace linkage or propagation zones of know structures. The earthquake pattern, the neotectonic mapping, and the local fault–slip analyses emphasize a distributed, heterogeneous pattern of deformation within a developing regional structure and indicate that strain concentration is weak in the uppermost crust of central Tibet. Thus, the geometry of neotectonic deformation is different from that in southern Tibet. Next, we use structural and palaeomagnetic data along the Zagaya section of southern central Tibet to outline significant block rotation and sinistral strike–slip SE of the Muga Purou rift. Our analysis supports earlier interpretations of reactivation of the Bangong–Nujiang suture as a neotectonic strike–slip belt. Then, we review the existing and provide new geochronology on the onset of neotectonic deformation in Tibet and suggest that the currently active neotectonic deformation started c. 5 Ma ago. It was preceded by c. north–south shortening and c. east–west lengthening within a regime that comprises strike–slip and low-angle normal faults; these were active at c. 18–7 Ma. The c. east-striking, sinistral Damxung shear zone and the c. NE-trending Nyainqentanghla sinistral-normal detachment allow speculations about the nature of this deformation: the ductile, low-angle detachments may be part of or connect to a mid-crustal décollement layer in which the strike–slip zones root; they may be unrelated to crustal extension. Finally, we propose a kinematic model that traces neotectonic particle flow across Tibet and speculate on the origin of structural differences in southern and central Tibet. Particles accelerate and move eastwards from western Tibet. Flow lines first diverge as the plateau is widening. At c. 92°E, the flow lines start to converge and particles accelerate; this area is characterized by the appearance of the major though-going strike–slip faults of eastern-central Tibet. The flow lines turn southeastward and converge most between the Assam–Namche Barwa and Gongha syntaxes; here the particles reach their highest velocity. The flow lines diverge south of the cord between the syntaxes. This neotectonic kinematic pattern correlates well with the decade-long velocity field derived from GPS-geodesy. The difference between the structural geometries of the rifts in central and southern Tibet may be an effect of the basal shear associated with the subduction of the Indian plate. The boundary between the nearly pure extensional province of the southern Tibet and the strike–slip and normal faulting one of central Tibet runs obliquely across the Lhasa block. Published P-wave tomographic imaging showed that the distance over which Indian lithosphere has thrust under Tibet decreases from west to east; this suggests that the distinct spatial variation in the mantle structure along the collision zone is responsible for the surface distribution of rift structures in Tibet.

Supplementary material:

Containing supporting data is available at http://www.geolsoc.org.uk/SUP18446.

Evidence for the multiphase nature of the India-Asia collision from the Yarlung Tsangpo suture zone, Tibet

Recent investigations in southern Tibet enable the testing and refinement of existing models for India-Asia collision. Presently available data indicate that marine deposition continued in the southern central portion of Tibet until at least the end of the Eocene. Sub-duction-related magmatism continued until the Mid-Oligocene, after which rapid uplift of the plateau was initiated. Mass-wasting of sediments into molasse basins did not commence until the latest Oligocene. The implications are that existing models, based on less-precise age constraints, invoking India-Asia collision at 55 Ma, are either flawed, or collision began at a different time. Recent work has produced sufficient data to allow the recognition of two different collisional events along the suture between India and Asia. Features related to each event require separate interpretation, and no collisional continuum should be assumed. In southern Tibet, a collision between the northern margin of India and a southfacing intra-oceanic island arc occurred at around 55 Ma, whereas continent-continent collision between India and Asia did not occur until at least 20 million years later.