Jurassic tectono-stratigraphic evolution of the Yarkand-Fergana Basin (Kyrgyz Tian Shan)
Résumé
Extending NW-SE from eastern Kazakhstan to western China, the Talas Fergana/Karatau fault is a major NW-SE orientated structure which has undergone multiple phases and styles of deformation from the Neoproterozoic to the Cenozoic. A peculiar period of transtensional tectonics affected this structure during the Early-Middle Jurassic leading to the formation of the South Turgay, Leontiev and Yarkand-Fergana basins. However, the kinematic of the fault and the geodynamic mechanism that led to the opening of these basins are still poorly constrained due to the lack of field data but are of major importance for the understanding of the evolution of Central Asia. The Yarkand-Fergana Basin is located at the south-western termination of the Talas Fergana/Karatau fault and contains up to 5 km of Jurassic sediments. Based on field work and on the description of several sedimentary sections we reconstructed the Jurassic tectono-sedimentary evolution of the basin. During the Early-Middle Jurassic, the Yarkand- Fergana Basin evolved in an extensional setting and was separated from the adjacent Fergana Basin by basement highs. Sedimentation consisted mainly of alluvial to lacustrine deposits settled under humid conditions. We propose that this transtensive deformation could have been mostly controlled by the far-field effects of the ending Qiangtang collision and by the subduction related extension affecting the Caspian – Turan domain. The Yarkand-Fergana was subsequently inverted during a Late Jurassic – Early Cretaceous period of transpressive reactivation of the Talas Fergana-Karatau fault leading to the deposition of extensive alluvial fan systems along the eastern margin of the Fergana Basin. We propose that this renewed transpressive deformation phase was mainly related to the 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.