Abstract:

The Sichuan (Chuan)-Guizhou (Qian)-Hunan (Xiang) tectonic belt (CQXTB) can be divided into four different sub-belts. The Xuefengshan tectonic belt (XTB), one of the four sub-belts, is the most particular and hotly debated on its genetic mechanism, because it’s just located in the gravity gradient belt of South China block, and the thickness of lithosphere changes greatly along the both sides of the XTB. The structural styles of CQCTB are analyzed. The geological basement of XTB shows a flower style and constitutes a thick-skinned structure together with Chuan-Qian fold belt, as a whole. The XTB is characterized by compressive-twisting features and its basement uplifts to the maximum height in the Yuanma basin. The lithosphere thickness of South China is also transformed with time. Referring to the deep seismic reflection profile, the authors draw the deep and shallow structure profile maps of the QXTB. Based on the different coupling styles between middle-upper crust and lower crust, the authors discussed the deformation process of the lower crust. The lower crust’s downward bidirectional subduction under the XTB forms a root without subduction into the mantle. Meanwhile, the lithosphere is bended due to the thickening of crust. After then, the lower crust and upper mantle probably depressed due to Rayleigh-Taylor instability of dense subcrustal lithosphere, where the lower crust and upper mantle sank into the asthenosphere mantle. Finally, under the extensional tectonic background, the asthenosphere rose, which resulted in the delamination of lithosphere at the east of XTB during the Late Mesozoic Era.

Key words: Xuefengshan tectonic belt, Jiangnan orogen, deep structure, lithosphere fold, dynamics