Three-dimensional thermo-rheological structure of the lithosphere in the North China Craton determined by integrating multiple observations: Implications for the formation of rifts

The lithosphere of the North China Craton(NCC)has experienced significant destruction and deformation since the Mesozoic,a notable feature of which is the widespread extensional structure and lithospheric thinning in the eastern NCC.Since the thermo-rheological structure of the lithosphere is one of the main factors controlling these dynamic processes,a threedimensional thermo-rheological model of the present lithosphere in the NCC was developed based on a geophysical-petrological method using a variety of data,and its relationship with the extensional structures and the formation of rifts was further analyzed.Our results show that the western NCC is characterized by thick lithosphere,low Moho temperature(TMoho<600°C),as well as high lithospheric strength and mantle-crust strength ratio(Sm/Sc>1).The deformation of the western narrow rift is consistent with the localized deformation dominated by the strength of lithospheric mantle.On the other hand,the lithosphere in the eastern NCC is characterized by extensive thinning(with lithospheric thickness of about 80–110 km).However,the decrease of lithospheric strength is not uniform,with high strength(10×1012 Pa m)observed in some areas(such as the Bohai Bay Basin and Hehuai Basin).Most of the eastern lithosphere is characterized by high TMoho(600–750°C)and low Sm/Sc(<1),which is inconsistent with the widespread extensional structure in the eastern NCC.Incorporating results from palaeo-geothermal and petrological studies,we developed a thermo-rheological structure model of the lithosphere at different evolutionary stages of the NCC,and suggested that the eastern NCC had a significantly thinned and weakened lithosphere in the early stages of the formation of the rift,leading to a regional distributed extension deformation dominated by crustal strength,which eventually evolved into a series of wide rifts.However,the cooling and accretion of the lithosphere in the subsequent stages significantly increased the strength of the lithospheric mantle,resulting in the inconsistency between the present thermo-rheological structure of the lithosphere and the extensional structure formed in the past.

Geodynamic characteristics of tectonic extension in the northern margin of South China Sea

Based on the geothermal and gravitation methods, this paper investigated the rheological and thermal structure of the lithosphere under the northern margin of South China Sea. The result shows that the temperature of the upper crust is 150–300°C lower than that of the lower crust, and the viscous coefficient of the upper crust is 2–3 orders of magnitude larger than that of the lower crust. It reveals that the upper crust is characterized by brittle deformation while the lower crust by ductile deformation. A channel of lower-viscosity should be formed between the upper and lower crust when the lithosphere is scattered and spreads out toward ocean from northwest to southeast along the northern margin of South China Sea. And, a brittle deformation takes place in the upper part of the lithosphere while a ductile deformation takes place in the lower part of the lithosphere due to different viscous coefficients and temperature. The layered deformation leads the faulted blocks to rotate along the faulting and the marginal grabens to appear in the northern margin of South China Sea in Cenozoic tectonic expansion.