A STUDY OF THE LITHOSPHERIC STRUCTURE BENEATH THE LOWER YANGZI VALLEY BY USING THE DIRECTLY INVERSING METHOD OF MULTI-LAYERED VARIABLE DENSITY
CHEN SHENG-ZAO(Institute of Geophysics, Slate Setsmological Bureau, Beijing)
This paper deals with the lithospheric structure beneath the Lower Yangzi Valley as derived from invesing the multi-layered variable density, and also discusses its geotectonic significances, and its connection with the evolution. The gravity formula in the form of cubic-spline function of density contrast with depth was induced, and the main density interfaces G5, G6, and G7, which are respectively corresponding to the bottom surface of the low-density layer in the mid-crust, the Moho discontinuity, and the upper surface of low-density layer in the top part of the upper mantle, were found by using the optimization algorithm of nonlinear least squares. Based on the analyses of the lithospheric structure, it is concluded that a hidden deep fault of the bay of Hangzhou-Haiyan-Jiaxing-Changxing-Gaochun-Wuhu, which takes an important part in the tectonic development and crustal evolution in the area, is a main tectonic boundary characterised by NW trend.