Jian‐Hua Liu

The subducted slab of Yangtze Continental block beneath the Tethyan orogen in western Yunnan

The western Yunnan area is a natural laboratory with fully developed and best preserved Tethyan orogen in the world. Seismic tomography reveals a slab-like high velocity anomaly down to 250 km beneath the western Yunnan Tethyan orogen, to its west there is a low-velocity column about 300 km wide. In the region from Lancangjiang to Mojiang an obvious low velocity in the lower crust and uppermost mantle overlies on the slab. Synthesizing the available geological and geochemical results, the present paper demonstrates that this slab-like high velocity anomaly is a part of the subducted plate of Yangtze continental segment after the closure of Paleotethys. The collision of India and Eurasia continent starting from 50–60 MaBP might trigger thermal disturbance in the upper mantle and cause the uprising of asthenosphere, in that case the subducted Yangtze plate could be broken off, causing Cenozoic magmatic activities and underplating in the Lancangjiang-Mojiang region.

Study of seismic tomography in Panxi paleorift area of southwestern China

Structural features of the typical continental paleorift in Panxi area are revealed by seismic tomography. (1) In the profile along the minor axis of Panxi paleorift, we found alternating high and low-velocity strips existing at different depths in the crust, presenting itself as a “sandwich” structure. The existence of these high and low-velocity anomaly strips is related to the basal lithology in the rift area. (2) An addition layer with velocity values of 7.1-7.5 km/s and 7.8 km/s exists from the base of lower crust to uppermost mantle and its thickness is about 20 km. Some study results indicate that the addition layer results from the invasion of mantle material. (3) A lens-shaped high-velocity body surrounded by relatively low-velocity material is observed at depths of 110-160 km between Huaping and Huidong in the axis of the paleorift. This is the first time to discover it in the upper mantle of the paleorift. Based on the results of geology, petrology and geochemistry, we infer that the formation of the addition layer and the lens-shaped high-velocity body in the upper mantle are related to the deep geodynamic process of generation, development and termination of the rift. On the one hand, the upwelling of asthenosphere mantle caused partial melting, and then the basaltic magma from the partial melted material further resulted in underplating and formed the crustal addition layer. On the other hand, the high-density content of mineral facies was increased in the residual melted mass of intensely depleted upper mantle, formed by basalt withdrawing. The solid-melt medium in the depleted upper mantle was mainly an accumulation of garnet and peridotite because the heating effect of lithosphere was relatively weakened in the later riftogenesis, so that a lens-shaped high-density and high-velocity zone was produced in the upper mantle. The results indicate that the energy and material exchange between asthenosphere and lithosphere and remarkable underplating would have an important effect on the material state and propagation of seismic wave in the lower crust, crust-mantle interface, asthenosphere and lithosphere. This process possibly is an important mechanism on the growth of continental crust and the evolution of deep mantle.