CHARACTERISTICS OF CRUST-MANTLE TRANSITION ZONE AND ITS TECTONIC SIGNIFICANCE IN THE CONTINENT OF CHINA

摘要

The crust-mantle transition zone (CMTZ) is an important site for mass and energy exchange between the lower crust and upper mantle. Several kinds of CMTZ exist beneath the continent of China, which show different seismic reflection characteristics and are composed of different rock associations. In this paper, we identify three types of CMTZ in the continent of China. (a) The CMTZ beneath the Tibet Plateau exhibits a grid-shaped seismic reflection characterized by random and reticular high and low seismic velocity lamellae. It is about 30 km thick, comprising both mafic granulites of lower crust and ultramafic rocks of upper mantle. Such lithological association and seismic velocity structure were inherited from the crustal overthrust and overlapping during the Cenozoic collision between the Indian and Euro-Asian continents; The corresponding crust movement is still very intense in this region. (b) The CMTZ underneath North China Block is usually composed of a thinner strong positive velocity gradient layer with dual properties in the rock associations. This suggests that the crust is still active in the North China Block after significant lithospheric thinning and thermal accretion. (c) The CMTZ in the remaining platform basins of China is characterized by a relatively sharp seismic discontinuity and a clear lithological boundary between the low crust and upper mantle. Such features of the CMTZ beneath Ordos basin correspond to weak crustal movement after a long period of isolation from thermo-tectonic overprinting.