Late Cenozoic evolution of the Lunggar extensional basin, Tibet: Implications for basin growth and exhumation in hinterland plateaus.

North-trending rifts and associated strike-slip faults in the Tibetan Plateau suggest Cenozoic east-west extension, but the dominant modes of distributed extensional deformation and basin formation are unclear. The Lunggar basin in west-central Tibet is bounded by a <40° low-angle detachment fault, contains active high-angle normal faults, and displays elevated topography toward the central segment of the basin with axial fluvial drainage toward the northern and southern basin terminations. Structural and stratigraphic features are consistent with a high-angle extensional system that evolved into a low-angle fault and supradetachment basin during progressive extension. This study seeks to constrain the depositional and exhumational history of the Lunggar basin and bounding fault system by assessing the sedimentologic, structural, and thermochronologic record of basin fill. Upper Cenozoic facies include alluvial-fan conglomerates and fluviolacustrine sandstones and siltstones. Sandstone petrographic data, conglomerate clast compositions, and detrital zircon U-Pb ages indicate systematic unroofing of the western footwall (including Jurassic-Cretaceous and Miocene granites that intruded Permian-Cretaceous strata). Paleocurrents are orthogonal or opposite to the current dispersal pattern, suggesting that growth of a modern intrabasin high in the central segment of the basin has modified the original basin configuration. As a proxy for footwall cooling histories, four basin-fill sandstones and 11 leucogranite boulders from proximal hanging-wall strata were sampled for low-temperature thermochronometry. Apatite and zircon (U-Th)/He results suggest that extension was underway by 10-8 Ma, with late Miocene-Pliocene exhumation rates of roughly 1 km/Myr. This rapid exhumation generated a conglomeratic unroofing sequence and promoted hanging-wall rebound and erosional recycling of range-front basin fill along the central segment of the detachment fault. The collective results support a model of rift evolution that invokes upper-crustal thinning, supra detachment basin subsidence, and subsequent isostatic rebound along the more-evolved central segments of Tibetan extensional systems. [ABSTRACT FROM AUTHOR]