Jurassic paleogeography of the Tian Shan: An evolution driven by far-field tectonics and climate
Résumé
The strongly intracontinental Tian Shan region, in Central Asia represents a key area to understand the long term evolution of continents in general and Asia in particular. If its Paleozoic and Cenozoic geodynamics are well understood, its Mesozoic evolution remains poorly constrained. In order to decipher the paleogeographic and large-scale tectonic evolution of the Tian Shan area during the Jurassic, we compiled, detailed field analyses of sedimentary rocks acquired within and around the Chinese Tian Shan region together with previously published data. We present three paleogeographical maps corresponding to the late Early – early Middle Jurassic, late Middle – early Late Jurassic and Late Jurassic - Early Cretaceous transition periods. We provide a large - scale picture of the Jurassic paleogeographic and climatic evolution of the Tian Shan region and discuss the geological evolution of the range together with the possible driving mechanisms. During the Early to early Middle Jurassic, the topographic evolution of the Tian Shan Range was dominated by progressive planation of late Paleozoic to early Mesozoic relief, locally interrupted by short-lived tectonic uplift. Throughout the region, contemporaneous sedimentation was characterized by alluvial to lacustrine strata deposited under humid conditions. During this period, recurrent limited deformation events associated with strike-slip and compressive tectonics occurred that cannot be explained by far field effect of the Qiangtang collision but could instead be associated to the coeval subduction-related extension affecting the Caspian - Turan domains. During the late Middle to early Late Jurassic, the planation of the Paleozoic – early Mesozoic Tian Shan Range then continued. A shift to more semi-arid conditions during the Late Jurassic is also recorded in the sedimentary series all over the region. At that time, few evidences of deformation exists in the Tian Shan or within the Caspian – Turan domains. We propose that the late Middle – early Late Jurassic corresponded to a period of relative tectonic quiescence in the area. Finally, the Late Jurassic – Early Cretaceous transition was marked by a tectonic reactivation leading to the inversion of the Yarkand – Fergana Basin and to localized relief building in the Tian Shan. This renewed transpressive deformation phase could be mainly related to the coeval accretion of the Helmand block to the south-west, and possibly to the onset of the accretion of the Lhasa Block along the southwestern margin of Eurasia. Finally, this period was also characterized by the climax of aridification which played a major role on the emplacement of extensive alluvial fan systems in the basins surrounding the range.
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