Yangfan Deng

Transition from continental collision to tectonic escape? A geophysical perspective on lateral expansion of the northern Tibetan Plateau

A number of tectonic models have been proposed for the Tibetan Plateau, which origin, however, remains poorly understood. In this study, investigations of the shear wave velocity (Vs) and density (ρ) structures of the crust and upper mantle evidenced three remarkable features: (1) There are variations in Vs and ρ of the metasomatic mantle wedge in the hanging wall of the subduction beneath different tectonic blocks of Tibet, which may be inferred as related to the dehydration of the downgoing slab. (2) Sections depicting gravitational potential energy suggest that the subducted lithosphere is less dense than the ambient rocks, and thus, being buoyant, it cannot be driven by gravitational slab pull. The subduction process can be inferred by the faster SW-ward motion of Eurasia relative to India as indicated by the plate motions relative to the mantle. An opposite NE-ward mantle flow can be inferred beneath the Himalaya system, deviating E and SE-ward toward China along the tectonic equator. (3) The variation in the thickness of the metasomatic mantle wedge suggests that the leading edge of the subducting Indian slab reaches the Bangoin-Nujiang suture (BNS), and the metasomatic mantle wedge overlaps with a region with poor Sn-wave propagation in north Tibet. The metasomatic layer, north of the BNS, deforms in the E-W direction to accommodate lithosphere shortening in south Tibet.

Joint Inversion of Surface Wave Dispersions and Receiver Functions with P Velocity Constraints: Application to Southeastern Tibet

We introduced a P-velocity model into the traditional joint inversion of P receiver function (RF) and surface wave dispersions to reduce model ambiguity. The method was implemented using a global search-based algorithm and a flexible parameterization of a sedimentary layer and spline-based parameterization that can represent sharp discontinuities. We applied the method to a dense array in SE Tibet (longitude ~97.5 °E to 107 °E, latitude ~25.3 °N). Extensive tests using synthetic and real data suggest that the method is suitable and robust for a variety of velocity structures and Moho discontinuities, and can simultaneously provide the crustal Vp/Vs profile and better-constrained Moho depth. The flexibility of the parameterization and the inclusion of the Vp constraint are crucial in the improved model recovery. Artifacts may be created without including the sedimentary layer. Even when it is less perfect, a reasonable Vp model is valuable in such a joint inversion. We showed that crustal multiples in RFs may bias the traditional H-k results when the crust structure is complex and should be avoided in a joint inversion before appropriate corrections can be made. The results from the joint inversion show two low-velocity zones (LVZs) reported previously and were identified as channels of crustal flow. A prominent isolated LVZ is observed in the mid-lower crust under the Xiaojiang fault area, which correlates with anomalously high Vp/Vs ratios, indicating possible partial melting. However, the other LVZ is imaged to be in the brittle shallow upper crust without very high Vp/Vs ratios, which is likely associated with crustal fault zones rather than partial melting. We observe clear low-velocity structures in the mantle beneath the two crustal LVZs, which also correlate with zones of low resistivity. The crust-mantle correlation may suggest influence of mantle processes on crustal deformation.

Lithospheric strength variations in Mainland China: Tectonic implications

We present a new thermal and strength model for the lithosphere of Mainland China. To this purpose, we integrate a thermal model for the crust, using a 3-D steady state heat conduction equation, with estimates for the upper mantle thermal structure, obtained by inverting a S wave tomography model. With this new thermal model and assigning to the lithospheric layers a “soft” and “hard” rheology, respectively, we estimate integrated strength of the lithosphere. In the Ordos and the Sichuan basins, characterized by intermediate temperatures, strength is primarily concentrated in the crust, when the rheology is soft, and in both the crust and upper mantle, when the rheology is hard. In turn, the Tibetan Plateau and the Tarim basin have a weak and strong lithosphere mainly on account of their high and low temperatures, respectively. A comparison of temperatures, strength, and effective viscosity variations with earthquakes distribution and their seismic energy released indicates that both the deep part of the crust and the upper mantle of the Tibetan Plateau are weak and prone to flow toward adjacent areas. The high strength of some of the tectonic domains surrounding Tibet (Tarim, Ordos, and Sichuan basins) favors the flow toward the weak western part of South China block.

Lateral variation in seismic velocities and rheology beneath the Qinling-Dabie orogen

The Qinling-Dabie orogen is an important tectonic belt that trends east-west and divides continental China into northern and southern parts. Due to its strong deformation, complicated structure, multiphase structural superposition and the massive exposed high and ultrahigh metamorphic rocks, its tectonic formation and geodynamical evolution are hot research topics worldwide. Previous studies mainly focused on the regional geological or geochemical aspects, whereas the geophysical constraints are few and isolated, in particular on the orogenic scale. Here, we integrate the available P- and S-wave seismic and seismicity data, and construct the rheological structures along the Qinling-Dabie orogen. The results demonstrate that: (1) there are strong lateral variations in the crustal velocity between the western and eastern sections of the Qinling-Dabie orogen, indicating the different origin and tectonic evolution between these two parts; (2) the lateral variations are also manifested in the rheological structure. The rigid blocks, such as South China and Ordos basin (North China Craton), resist deformation and show low seismicity. The weak regions, such as the margin of Tibet and western Qinling-Dabie experience strong deformation and accumulated stress, thus show active seismicity; (3) in the lower crust of most of the HP/UHP terranes the values of P-wave velocity are higher than the global average ones; finally (4) low P- and S-wave velocities and low strength in the lower crust and lithospheric mantle beneath Dabie indicate lithospheric delamination, and/or high temperature, and partial melting condition.