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Evidence of Miocene crustal shortening in the North Qilian Shan from Cenozoic stratigraphy of the western Hexi Corridor, Gansu Province, China

Paul M. Bovet^*, Bradley D. Ritts^**, George Gehrels^***, A. Oscar Abbink, Brian Darby and Jeremy Hourigan

^* Chevron Energy Technology Company, 1400 Smith Street, Earth Science Department, Houston, Texas 77002, USA
^** Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, California 94583 USA
^*** Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721 USA
TNO B&O, Geological Survey of The Netherlands, P.O. Box 80015, NL-3508 TA Utrecht, The Netherlands
ExxonMobil Exploration Company, Business Development, NBI Regional Studies Asia-Pacific Opportunity Capture CORP GP3-627, Houston, Texas 77060 USA
Earth and Marine Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064 USA

PBGB{at}chevron.com

New sedimentologic, stratigraphic, and compositional data from the Paleogene-Neogene stratigraphic succession exposed in the northwest Hexi Corridor and within the North Qilian Shan, provide evidence to suggest that crustal shortening in the North Qilian Shan fold-thrust belt initiated during the Miocene. The section is composed of four lithostratigraphic units: Oligocene-Miocene fine- to coarse-grained Unit 1, Miocene conglomeratic Unit 2, and Pliocene-Pleistocene conglomeratic Units 3 and 4. Unit 3 lies in angular unconformity over both Units 1 and 2, and Unit 4 contains a progressive unconformity. The onset of conglomerate deposition at the base of Unit 2 suggests an increase in depositional energy, which we interpret as the result of proximal orogenesis in the North Qilian Shan fold and thrust belt. Supporting evidence includes the appearance of strongly northeast-trending paleocurrents, indicating paleoflow away from the Qilian Shan, clast lithologies that match sources in the North Qilian Shan, and sandstone with detrital framework modes that indicate a recycled orogen source. In contrast, Unit 1 contains paleocurrent indicators that are variable but generally trend northward and sandstone and clast compositions which are more diagnostic of a continental block source. Detrital zircon age determinations from Unit 1 are also not consistent with a source in the North Qilian Shan; rather, they suggest a provenance in hinterland regions within the South Qilian Shan and North Qaidam terranes. In sum, these results are all consistent with initiation of proximal uplift of the North Qilian Shan during deposition of the gradational transition from Unit 1 to Unit 2, demonstrating shortening in the Qilian Shan before the late Miocene. This comprehensive study tightens our understanding of when far-field stress related to the India-Eurasia continent-continent collision reached the northeastern edge of the Tibetan Plateau.