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On the Age of the Xigeda Formation in Luding, Sichuan, and Its Neotectonic Significance

Jiang Fuchu,Wu Xihao,Xiao Huaguo and Zhao Zhizhong(Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, 100081)  
A sequence of fliuviolacustrine deposits, named the Xigeda Formatiom, with an approximate thickness of 411 m, is distributed on a valley shoulder about 920 m above the water level of the Dadu River in Haiziping, Luding, western Sichuan, on the inner side of the southeastern boundary belt of the West Sichuan Plateau. A preliminary study of the magnetostratigraphic column suggests that the upper and lower parts separated at a depth of 386. 0 m from the surface belong to the Gauss normal polarity chron and the Gilbert reverse polarity chron respectively. According to the Cande and Kent's polarty time scale, the age of the Xigeda Formation ranges from 4. 20 to 2. 60 Ma BP, which is assigned to the middle and late Pliocene. The Gauss/Gilbert boundary recorded by the Xigeda Formation in Luding and that in Hanyuau located outside the boundary belt of the plateau lies at altitudes of 2200 m and 1040 m above sea level respectively, with an altitude difference of 1180 m. This is possibly the result of the differential uplift of the blocks on both sides of the southeastern boundary belt of the West Sichuau Plateau since 2. 6 Ma BP.
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