Miocene Uplift and Northeast Propagation of the Qilian Shan, Gansu Province, China

The Oligocene-Miocene stratigraphic succession in the Qilian Shan foreland contains three distinct lithostratigraphic units. Unit 1 is the oldest, most likely Oligocene to earliest Miocene, and has a maximum thickness of 1260 m. It consists of mostly red, often nodular, thick overbank mudstone and fine to very coarse channelized sandstone deposits. Paleocurrent indicators show a range of flow directions from east to west but generally north directed. Unit 1 coarsens upward into Unit 2, a Miocene pebble conglomerate with occasional fine-grained lenses, large boulders, and a maximum thickness of 860 m. It contains abundant imbrication indicating flow to the NE and clast composition of limestone, quartzite, marble, phyllite, and sandstone, matching sources in the North Qilian terrane. Unit 3 is a 437 m thick Miocene pebble to cobble conglomerate, with clast composition similar to Unit 2 and imbrication indicating NNW flow. Oligocene-Miocene strata are distributed within the northern Qilian Shan and Hexi Corridor. Unit 1 is the most regionally extensive, appearing in both the hanging wall and footwall of the North Qilian fault (NQF). Unit 2 appears in only the footwall of the NQF, while Unit 3 is further restricted to the footwall of a frontal imbricate thrust fault, and forms a progressive unconformity. The presence of Unit 1 in all sections suggests that during the Oligocene and earliest Miocene there was a laterally continuous basin with no relief in the present North Qilian Shan. The appearance of conglomerate, derived from North Qilian terrane sources, in Unit 2 is interpreted to mark the initiation of shortening and uplift in the North Qilian thrust belt in the Miocene. The absence of Unit 2 in Tertiary deposits from the hanging wall of the NQF suggests that Unit 2 may be the result of activity specifically along that structure. Propagation of shortening into the foreland is marked by initiation of faulting on the frontal imbricate, NE of the NQF, with associated deformation of Units 1 and 2, and deposition of Unit 3.