Paper No. 10
Presentation Time: 3:50 PM
PROTRACTED DEPOSITIONAL AND PROVENANCE RECORD OF CENOZOIC OROGENESIS IN NORTHERN TIBET: NEW EVIDENCE FROM THE QAIDAM BASIN
Integrated analysis of the sedimentology, stratigraphy and provenance record of the Qaidam basin on the northern Tibetan plateau reveals a long-term record of Cenozoic deformation associated with faulting and structural reactivation in response to the Indo-Asian collision. Multiple, independent lines of evidence point to a major shift in source composition is detected between Eocene and Miocene strata. The shift is seen in paleocurrent measurements (clast imbrication and trough cross-stratification), detrital zircon U-Pb geochronology, conglomerate clast counts, and sandstone petrography. Depositional systems of the >6 km-thick Xitieshan section (37°32' N 95°10' E) (include sand-dominated meandering rivers with extensive pedogenically modified alluvial plains, marginal lacustrine, and gravelly braided river systems. The section shows two major upward coarsening sequences from mudrock-dominated lithofacies to >1 km of conglomeratic facies, with smaller-scale internal shifts between various alluvial systems. Sedimentological and stratigraphic patterns reveal at least two phases of enhanced exhumation and sediment delivery during the Cenozoic.
Changing depositional systems and source areas are interpreted as a response to the reactivation of Paleozoic suture zones in the Qilian Shan (to the north) and Kunlun Shan (to the south), which served as the circum-basin catchment areas and primary sources of extrabasinal grains throughout the Cenozoic. Strike-slip deformation along the Altyn Tagh fault system (to the west) help promote a general eastward migration of the Qaidam basin depocenter. This study demonstrates that deposition in the Qaidam basin has been maintained throughout the Cenozoic largely by topographic isolation through uplift of resistant barriers. Possible geodynamic mechanisms for such long-term stability of the Qaidam basin as a topographically closed basin with high-relief walls include (i) deep crustal/ lithospheric viscous processes (crustal flow, mantle convection, injection) caused by Moho topography and/or (ii) heterogeneous and anisotropic rheological conditions associated with inherited terranes and resultant changes in elastic thickness of the crust (rigidity).