Oligocene-Miocene tectonics and sedimentation along the Altyn Tagh Fault, northern Tibetan Plateau: Analysis of the Xorkol, Subei, and Aksay basins

Tertiary basins adjacent to the northern edge of the Tibetan Plateau include both giant intracontinental sedimentary basins such as Tarim and Qaidam and smaller basins directly related to the evolution of the Altyn Tagh strike-slip fault system and nearby fold-thrust belts. Three examples of these smaller basins that formed during early evolution of the Altyn Tagh fault system are the Xorkol, Aksay, and Subei basins. The Xorkol basin was the site of nonmarine siliciclastic sedimentation in alluvial fan, playa, and fluvial environments from at least the mid-Oligocene through the Miocene. Sediment was delivered to the basin from source areas uplifted by basin-bounding thrust faults and from the opposite side of the Altyn Tagh fault. The age of the Aksay basin is more poorly constrained, but it also contains Oligocene(?)-Miocene(?) nonmarine clastic strata that were derived from the southern side of the Altyn Tagh fault and adjacent thrust-fault-bounded uplifts. The Subei basin is characterized by similar Oligocene-Miocene fluvial, playa, and alluvial fan deposits; however, in this basin, sediment was derived only from adjacent reverse-fault-bounded highlands rather than from across the Altyn Tagh fault. Differences in the lithostratigraphy, depositional environments, clast composition, and paleocurrent directions demonstrate that these basins formed as individual entities and do not represent remnants of a regionally extensive basin that was later dislocated by strike-slip faulting. Thus, these basins do not represent piercing point pairs, as was previously suggested. However, because they are syn-slip basins, they do record the relative timing of strike-slip and reverse faulting and the surface response to deformation, and they provide fault offset data in the provenance relationships between sediment and source terranes on opposite sides of the fault.