Southern Tibetan rifting since late Miocene enabled by basal shear of the underthrusting Indian lithosphere

Syncontractional extension is prominent in present-day Tibet, but its origin remains vigorously debated. Several deep-seated geodynamic processes (e.g., Indian underthrusting, horizontal flow, and mantle upwelling) have been linked to Tibetan rifting. Indian underthrusting is a good candidate because it can well explain why surface rifts are more prominent south of the Bangong-Nujiang suture; however, how Indian underthrusting causes extension is not well understood and lacks observational constraints. Seismic anisotropy, measured by exploiting the birefringence effect of shear waves, can be indicative of the deformation styles within the crust. Here, we unveil the dominant convergence-parallel alignment of anisotropic fabrics in the deep crust of the southern Tibetan rifts using seismic recordings collected from our recently deployed and existing seismic stations. This finding suggests that the strong north-directed shearing exerted by the underthrusting Indian plate is key to enabling present-day extension in southern Tibet.

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.

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.