Asymmetrical River Valleys in Response to Tectonic Tilting and Strike-Slip Faulting, NE Margin of Tibetan Plateau

Abstract:

The NE margin of the Tibetan Plateau, as a particularly important area to understand the mechanism of plateau formation, is characterized by large transpressional arcuate faults. There is debate on the amount of Quaternary sinistral displacement on the major Haiyuan Fault. Previously unrecognized systemic asymmetrical valleys have developed between the Haiyuan and Xiangshan faults. Southeast tilting and sinistral displacement on the NE side of the Haiyuan Fault resulted in SE migration of large rivers and asymmetrical widening of their valleys, leaving a systematic distribution of tilted strath terraces along the their NW sides. Where asymmetrical widening created by tilting kept pace with sinistral displacement, rivers have not been deflected, and the increase in valley width downstream from the fault should equate to total lateral displacement since river formation (e.g. Yuan River, a 7 km asymmetrical valley with a c. 2.2Ma paleomagnetic age). Where river deflection and asymmetrical valley growth are coeval, valley width is less than total horizontal displacement (e.g. Hebao River, a c. 2.1km asymmetrical valley with c. 2km deflection). All rivers north of the Haiyuan Fault converge to cut across the Xiangshan Mountains as a gorge. Northeast thrusting of the upthrown side of the Xiangshan Fault has resulted in degradation and related strath terrace formation as the valleys asymmetrically widened. A probable earthquake-induced landslide caused by movement on the Xiangshan Fault in latest Pleistocene blocked the gorge causing aggradation along all rivers and their tributaries. Deposition terraces were formed after the landslide dam was breached. Together with previous research on the Xiangshan Fault, it is concluded that there has been c.7km of Quaternary sinistral displacement on the Haiyuan and Xiangshan faults along the NE margin of the Tibetan Plateau since the formation of rivers that intersect them.