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

Abstract

ABSTRACTNorth-trending rifts and associated strike-slip faults in the Tibetan Plateau suggest Ceno zoic east-west extension, but the domi-nant modes of distributed extensional defor-ma tion 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 to-ward the central segment of the basin with axial fl uvial 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 depo-sitional and exhumational history of the Lunggar basin and bounding fault system by assessing the sedimentologic, structural, and thermochronologic record of basin fi ll. Upper Cenozoic facies include alluvial-fan conglomerates and fl uviolacustrine sand-stones and siltstones. Sandstone petrographic data, conglomerate clast compositions, and detrital zircon U-Pb ages indicate systematic unroofi ng 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 modi-fi ed the original basin confi guration. 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 thermo-chronometry. Apatite and zircon (U-Th)/He results suggest that extension was underway by 10–8 Ma, with late Miocene–Pliocene ex-humation rates of roughly 1 km/Myr. This rapid exhumation generated a conglomeratic unroofi ng sequence and promoted hanging-wall rebound and erosional recycling of range-front basin fi ll along the central seg-ment of the detachment fault. The collective results support a model of rift evolution that invokes upper-crustal thinning, supra detach-ment basin subsidence, and subsequent iso-static rebound along the more-evolved central segments of Tibetan extensional systems.INTRODUCTION