Lithospheric seismic fabrics of Sulu ultrahigh-pressure metamorphic belt

Calibration of seismic reflectors appearing in the crust of the Chinese continent scientific drilling site can be completed through the correlation studies between direct evidences, such as the drill cores, and geophysical signatures; therefore the interpretation of geophysical data could produce reliable results of crustal structure and composition. On the other hand, there are two Cenozoic volcanoes close to the scientific drilling site; analyzing composition of xenoliths existent in the volcanoes and evaluating their seismic velocities can also offer information about the mantle and lower crust. After the calibration via cores and well-logging data, the seismic reflectors appearing in the UHP belt can be caused by lithological changes within the UHP rock slice, ductile shearing rock-suites, and later fracture zones. Among these sources, ductile shearing resulted in displacement and detachment of original rock-sheets, producing some rock-interbeds of several hundred meters thick that are named the ductile shearing rock-suites. A suite consists of mylonized gneiss and eclogite slices that underwent shearing, becoming the major mechanism responsible to generate regional strong reflections. The UHP rock-slice is characterized by complicated structures and high density, high seismic velocity and high electrical resistivity, its thickness is usually less than 11 km. Velocity and density of the gneiss-layer beneath gradually tend to normal with increasing depth. Based on the xenoliths we can infer that the middle crust contains a lot of gneisses, and the lower crust consists of different granulites. The lithospheric mantle has multi-layer structures and consists mainly of spinal lherzolite and harzburgite, implying late Mesozoic lithospheric thinning. The seismic fabrics with different origins were possible products of different geodynamic processes. For instance, the UHP rock-slice was produced by the UHP metamorphic process and the exhumation of subducted supercrustal rocks after the Triassic collision between the Yangtze and Sino-Korean cratons; whilst the ductile shearing rock-suites resulted from shearing deformation processes during the subduction and exhumation. The normal velocity below the UHP rock-slice was correlated with Mesozoic extension processes in the area. Through careful calibration of seismic reflectors and analyzing xenoliths, one can find the relationship between the causes of seismic reflectors and corresponding geodynamic processes, offering a new basis for reconstruction of regional dynamic evolution history.

作 者： YANG Wencai Yang Wuyang JIN Zhenmin CHENG Zhenyan   作者單位： YANG Wencai,Yang Wuyang,CHENG Zhenyan(Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China)

JIN Zhenmin(China University of Geosciences, Wuhan 430074, China) 

刊 名： 中國科學D輯（英文版）  SCI 英文刊名： SCIENCE IN CHINA SERIES D(EARTH SCIENCES)  年，卷(期)： 2005 48(5)  分類號： P3  關鍵詞： deep seismic reflection   ultrahigh-pressure belt   Sulu China   calibration of reflectors   lithospheric fabrics  

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