Abstract:

The northward convergence of India into Asia over the past 50 m.y. has led to the development and lateral extrusion of thickened crust. On the basis of velocity field constrained by GPS measurements, the Chuandian active crustal block adjusts the crustal lateral extrusion movement by clockwise rotation on a background of southeastward movement. The block exhibits a relatively homogenous movement at rates 19 mm/a and decreases to be 13-14 mm/a across the NE-striking faults such as Lijiang fault. The fact suggests that NE-striking faults have prevented the movement of active crustal block.Field observations collected recently show that the Zhoucheng-Qingshui fault zone, located in the interior of the Chuandian active crustal block, is an important component of the active tectonics. The fact that the early fault trace well preserved in riverine-lacustrine deposit and covered by the Xigeda Group deposited during the Pliocene suggests that the faulting onset in Early Pliocene. Many phenomena indicate the Zhoucheng-Qingshui fault zone is characterized by sinistral strike-slip faults with the component of normal or thrust along the fault zone striking NE45°during Quaternary. Age results of Thermoluminescence (TL) and Electronic Spin Resonance (ESR) indicate that the fault zone reactive during Mid-Pleistocene, and its activity has become weak since Late Pleistocene. The fault prevented southeastward movement of active crustal block. A long-term slip-rate of 0.1 mm/a of deformation on this fault, derived from the cumulative offset and Pliocene lacustrine deposit, is much slower than that of the Chuandian active crustal block, which suggests that the Zhoucheng-Qingshui fault zone isn't a significant factor in accommodating of the continued southward movement of the active crustal block.

Key words: northwest Yunnan, Zhoucheng-Qingshui fault zone, Late Cenozoic, active fault