By a detailed investigation of geometry and kinematics of the Shangma fault in Dabieshan,three different crust levels of extension movement have been recognized in sequence from the deep to the shallow: low-angle ductile detachment shearing with top to the NW; low-angle normal fault with top to the NW or NWW in brittle or brittle-ductile transition domain;high-angle brittle normal fault with top to the W or NWW. Two samples were chosen for zircon U-Pb age dating to constrain the activity age of the Shangma fault. A bedding intrusive granitoid pegmatite vein that is parallel to the foliation of the low-angle ductile detachment shear zone of the country rock exhibits a lotus-joint type of boudinage deformation,showing syn-tectonic emplacing at the end of the ductile deformation period and deformation in the brittle-ductile transition domain. The zircon U-Pb dating of this granitoid pegmatite vein gives an age of (125.9±4.2) Ma,which expresses the extension in the brittle-ductile transition domain of the Shangma fault. The other sample,which is collected from a granite pluton cutting the foliation of the low-angle ductile detachment shear zone,gives a zircon U-Pb age of (118.8±4.1) Ma,constraining the end of the ductile detachment shearing. Then the transformation age from ductile to brittle deformation can be constrained between 126–119 Ma. Combined with the previous researches,the formation of the Luotian dome,which is located to the east of the Shangma fault,can be constrained during 150–126 Ma. This study gives a new time constraint to the evolution of the Dabie orogenic belt.
Four intensive uplift periods, i.e., 60–35, 25–17 and 12–8 Ma (but 18–13 Ma in the Himalayas of the southern Tibet), and since about 5 Ma, can be determined on the Tibetan Plateau by synthetical analysis of low-temperature thermo-chronology data, sedimentary deposit records, and structural deformation records of different areas. The strong tectonic uplift periods in different areas on the Tibetan Plateau are penecontemporaneous, except for the Himalayan area of the southern Tibet, where a rapid uplift and exhumation period, controlled by the activity of the South Tibetan Detachment System faults, occurred during 18–13 Ma. These strong uplift and exhumation periods correspond well to intensive deformation activity periods, suggesting tectonically-controlled uplift and exhumation. The deposit records, such as the distribution of coarse clastic sediments, the distribution of tectonically-controlled basins, stratigraphic discontinuousness or unconformity, and fault-controlled geomorphologic evolution, also match well with the strong uplift and exhumation periods. Expanding processes of the plateau are also discussed.
WANG GuoCanCAO KaiZHANG KeXinWANG AnLIU ChaoMENG YanNingXU YaDong
