Based on TM remote-sensing image interpretation and field observation of slip vectors of faults,coupled with the thermal isotope and radioactive isotope chronological studies of younger geologic bodies,this paper mainly describes the morpho-structural features in the southern segment of the Taihang uplift,defines the phases of neotectonic deformation and determines the neotectonic stress field and its transition history.Studies indicate that in the Neogene the southern segment of the Taihang Mountains experienced two major phases of extension.In the mid-late Miocene the area was subjected to nearly NE-SW extension,concomitantly with extensional eruption of basic volcanic rocks in North China.The extension was concentrated in fault zones on the eastern and southern margins of the area.During the Pliocene to early Pleistocene strong NW-SE crustal extension resulted in disintegration of the planation geomorphology and formation of a graben basin in the area.Since the mid-late Pleistocene the southern Taihang Mountain marginal fault zone has become a main boundary zone of neotectonic deformation.Analyses of fault plane slip vectors and displacement features of younger piedmont alluvial fans and small gullies along the fault suggest that the fault zone is an oblique-extensional,sinistral strike-slip boundary fault znoe,with the extension oriented in a NW-SE to NNW-SSE direction.In the tectonic context,the NE-SW extension in eastern China in the Miocene is related to the dynamic process of back-arc spreading resulting from westward subduction of the Pacific plate to the east,while the neotectonic deformation since the Pliocene is related to the rapid uplift of the Qinghai-Tibet Plateau and eastward tectonic extrusion due to the late-stage India-Eurasia collision.
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