Thesis defense of Julien MORIN

The Jurassic evolution of the Tian Shan region (Central Asia) : Geodynamic context and paleogeographic consequences

Abstract

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 and characterized by a succession of orogenesis driven by accretion of continental blocks, its Mesozoic evolution remains poorly constrained. Within this largely compressive geodynamic setting, the Jurassic period corresponds to a peculiar time span dominated by widespread extension within the Caspian – Turan domain to the west and within the Siberian/Mongolian domain to the east. However, the Jurassic paleogeographic and kinematic evolution of the probable relay zone corresponding to the Tian Shan region is yet to be fully understood.

To do so, we conducted sedimentological analyses within several basins associated to the Tian Shan Range and compiled previously published sedimentological data in order to characterize the evolution of the depositional environments through time as well as to document climate conditions. In parallel, we conducted geomorphological analyses and compiled both detrital geochronology and low - temperature thermochronology data to describe the paleotopographical evolution of the Tian Shan area, especially constraining the location and the timing of relief building in the range.

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 strikeslip and compressive tectonics occurred. To the west of the Tian Shan, the Early Jurassic Sinemurian (?) – Pliensbachian marks the onset of sedimentation, at least in the northern Yarkand-Fergana Basin. At that time, renewed activity along the Talas- Fergana/Karatau fault led to the opening of the Yarkand-Fergana Basin as an half-graben. Continuous opening of this basin occurred during the late Early – Middle Jurassic. These episodes of deformation and their timing cannot be related to the far-field effects of the Qiangtang collision but could instead, be associated to the coeval subduction-related extension affecting the Caspian - Turan domains to the west of the Tian Shan area. We propose that this extensional stress-field, induced by the Neo-Tethys subduction, played a major role in driving the late Early to early Middle Jurassic tectonic and topographic evolution of the Tian Shan region. During the late Middle to early Late Jurassic, few evidences of deformation exist 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.