Neotectonic structures and Late Quaternary tectonics in the slowly deforming Ejina Basin

The collision of the Indian and Eurasian tectonic plates leads - next to the orogeny of the Tibetan Plateau, to far-field deformations as far as the intracontinental Gobi Altai mountains. The northeastern margin of the Tibetan Plateau is constituted by the alpine ranges of the Qilian Shan, which are uplifted along the Qilian Shan Thrust. The sedimentary foreland basins of the Hexi Corridor, are also shaped by thrusts parallel to the Qilian Shan and by eastern continuations of the sinistral Altyn Tagh Fault, which delimits the Tibetan Plateau to the north. Also the northeastern boundary of the Hexi Corridor, the foreland mountain ranges of the Heli Shan and Longshou Shan are overprinted tectonically. The instrumental seismicity at the margin of the Tibetan Plateau and in the Gobi Altai documents frequent moderate earthquakes with magnitudes up to M5. Sporadic earthquakes with higher magnitudes are also documented in the historic catalogue. The Ejina Basin, situated between the Heli Shan and the Gobi Altai, exhibits only very low seismicity (M<4) and was considered as tectonically stable and seismically quiescent for a long time. However, significant lineaments were observed during the last two decades and a tectonic formation was ascribed to several landforms within the basin. The Ejina Basin constitutes the terminal sink of the Heihe (also Hei River), which forms the Ejina Basin by its sedimentary input and which crosses three tectonic zones Qilian Shan, Hexi Corridor and the Ejina Basin. Hence a morphotectonic approach was chosen to characterize the tectonic overprinting of the landscape and to investigate lineaments and escarpments. Moreover, geophysical methods like HTS TEM, HVSR microtremor recordings and ground-penetrating radar were chosen to examine relevant lineaments and to trace identified structures from the base of the Quaternary sediments to the shallow subsurface. A previously suspected lineament connecting the Gaxun Nur (lake) in the northwest with the Juyanze Palaeolake, could not definitely be characterized as a strike-slip fault. A W dipping, N-S striking normal fault west of the Ejina oasis could be discovered and a palaeocliff in the eastern basin could be confirmed as normal fault. Together with the outcropping Tienehu Fault and the so-called inverted channels, a tectonic subsidence of the Western Juyanze Palaeolake Basin is beyond question. The temporal context of the neotectonic activity can be defined by the maximal age of the Tienehu Fault to < 36 ka BP. Within the Western Juyanze Palaeolake outcropping seismites with scattered Holocene ages confirm the recent seismicity within the region. Since seismites were only found within the Basin, a local seismite-generating fault is favoured