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Ke Zhang, Sun Yat-sen University (China)
Zhanwu Ma, Sun Yat-sen University (China)
Yukun Tang, Sun Yat-sen University (China)
Shining Liu, Zhonghai Petroleum (china) Co., Ltd. Zhanjiang Branch (China)
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The study area is located in a loess plateau between two active thrust and sinistral strike-slip faults, the Xiangshan Fault to the north and the Haiyuan Fault to the south, along the northeastern margin of Tibetan Plateau. Rivers transecting the Haiyuan Fault are sinistrally offset by amounts of A (up to 627 m), B (up to 60 m) and C (up to 18 m), i.e. from large and old to small and young rivers. However, there are still some NE flowing rivers crossing the fault that are much larger and older than those of A but show no apparent offset. The valley sides of these rivers are coloured yellow on their NW and red on their SE sides. On the SE sides, Neogene red beds covered by thin late Pleistocene loess are exposed, while the NW sides of the valleys are composed of thick late Pleistocene loess underlain by alluvial gravel that directly overlies Neogene red beds, i.e. pedestal terraces covered by loess, constituting asymmetrical valleys. These major rivers with larger erosional capability are not easily deflected or beheaded by faulting.
Sinistral displacement on the Haiyuan Fault causes the SE banks of the rivers downstream of the fault to undergo vigorous erosion resulting in a steep profile, whereas on the NW bank of the rivers, deposition forms a floodplain. Thus, the river channels remain in their original position and the valleys widen asymmetrically. With progressive fault displacement, the same process continues and the river valley downstream of the fault becomes progressively wider. The amount of horizontal displacement of the fault is approximately equal to the width of the asymmetrical valley. Asymmetrical valleys of the Yuan and Maying rivers are up to 7 km wide with the oldest paleomagnetic age of 1.04 Ma, but the Hebao River is only 2.1 km wide, implying that up to 7 km of sinistral displacement has occurred on the Haiyuan Fault since formation of the Yuan River. The age of the Hebao River is considered to be about 0.35 Ma, i.e. 1/3 the age of the Yuan and Maying Rivers. NE thrusting of the SW block of the Xiangshan Fault has resulted in three episodes of degradation and related pedestal terrace formation as the valleys widen asymmetrically. There are two rivers N of the Haiyuan Fault, that have developed symmetrically without the formation of pedestal terraces, implying that they are very young. All rivers, either asymmetrical or symmetrical N of the Haiyuan Fault, have developed depositional terraces along their main channels and tributaries. They join together and then cut across the Xiangshan Fault, forming a deep gorge. A probable earthquake-induced landslide caused by movement on the Xiangshan Fault in the late Pleistocene, blocked the gorge, resulting in aggradation along the whole river system with the deposition terraces forming after the landslide dam was breached.
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