Thermal State and Rheological Strength of the Lithosphere Beneath the Eastern China
WANG Yang and CHENG Suhua(School of Earth Science and Resources,China University of Geosciences,Beijing 100083,China)
Lithospheric temperatures beneath the eastern China were estimated by local isostasy equilibrium constrained geothermal calculation in this study. Maps of the lateral temperature variation at depths of 40,70,100km and Moho boundary are presented for the eastern China,and the 600℃ and 1100℃ isotherm depth maps are also given. Meanwhile,the thermal thickness of the lithosphere is calculated as the depth of 1350℃ isotherm depth. The results show that the lithospheric root of 160~180km thick exists under Sichuan basin in the western part of Yangtze platform,but absent beneath the entire North China craton. It is notable that the lithospheric thickness is only 130~140km beneath Ordos region in the western part of North China craton; accordingly,there is no lithospheric root beneath Ordos block. Furthermore,no "mantle plumes" exist in the region east of the Daxing'anling-Taihangshan-Wulingshan Gravity Gradient Belt. These facts do not support the "continental roots-plume tectonics" argued by some scholars. These geothermal modeling results are in good agreement with those of seismic tomography and author's previous geothermal studies. Lithospheric thinning beneath the northeastern part of China mainland (including North China craton) may be related to the volatile infiltration by dehydration of deep subducted Pacific slab; therefore,we favor the thermo-chemical erosion mechanism for the North China craton lithosphere destruction during Meso-Cenozoic. The temperatures in lower crust of the North China craton estimated are in range of 500~600℃. Accordingly,the lower crust composition of the North China craton should be more mafic than that of previously estimated based on much cooler geotherms. Using the wet mantle rheology to represent the mechanical behaviour of upper mantle,we found that the "crème brlée" model can approximate the lithospheric rheology for the most regions of eastern part of China,which is represented by strong crust portion but weak lithospheric mantle portion in vertical strength profile. On the other hand,the "jelly sandwich" model demonstrates the mechanical strength of Sichuan basin and that of northeastern and southeastern portions of the North China craton,which exhibits strong crust and strong lithospheric mantle portions in vertical strength profile.