关键词: 塔尔湾断裂, 地貌陡坎, 逆断裂

Abstract: Many NW-trending faults have been developed on the north of the eastern segment of Altyn Tagh Fault. The Tarwan Fault,about 10km long and striking NW on the whole,is the western segment of the largest Tarwan-Dengdengshan-Chijiaciwo Fault among these faults. The fault appears as a straight linear scarp in the satellite image and a geomorphic scarp of dozens of centimeters to 5 meters high,topographically. The scarp dips NE and is composed mainly of beds of early Pleistocene conglomerate and Holocene aeolian sandy soil. As revealed by a measured topographic profile,the scarp composed of Holocene aeolian sandy soil is about 5m high,and that of early Pleistoscene conglomerate is about 1m high. Field investigation and trenches excavated on the vertical scarp have revealed the Tarwan Fault is a thrust fault,striking NW and dipping SW.The Geogene mudstone is thrust over the early Pleistocene conglomerate,with a throw of 0.5m. The Holocene aeolian sand and late Pleistocene gravel layers overlying the fault are not dislocated. The hanging wall of the fault is Geogene mudstone with rich groundwater and well-developed vegetation. Due to the protection and control of sand movement with vegetation,aeolian sand was accumulated constantly and preserved,and as a result,the aeolian sand layer became higher gradually. The foot wall of the fault consists of a Gobi gravel layer of a few centimeters thick on the surface and hard cemented conglomerate of early Pleistocene under it,with groundwater and vegetation being undeveloped. Therefore,Holocene aeolian sand is only developed on the hanging wall of the fault,and there is no Holocene stratum developed in the footwall. The height of the scarp formed on the early Pleistocene conglomerate is far lower than that on the Holocene aeolian sand. These findings indicate that the topographic scarp composed of Holocene aeolian sand was produced by external dynamic process rather than faulting,and that the Tarwan Fault is an early-middle Pleistocene thrust fault.

Key words: Tarwan Fault, geomorphic scarp, thrust fault