East-west extension and Miocene environmental change in the southern Tibetan plateau: Thakkhola graben, central Nepal

摘要

East-west extensional basins are distributed across the southern half of the Tibetan plateau at an elevation of 4 km. These basins have generated much interest because of their potential implications for the regional tectonics and force distribution in the plateau. This study documents the sedimentology of the Miocene–Pliocene Thakkhola graben fill in order to reconstruct basin evolution and paleoenvironment. Analysis of depositional systems, paleodrainage patterns, and conglomerate clast provenance of the >1-km-thick graben fill sets limits on the timing of activity of the basin-bounding faults and the development of southward axial drainage in the basin. During the deposition of the oldest basin fill (Tetang Formation, ca. 11–9.6 Ma), probably in a restricted basin, minor motion occurred on the basin-bounding fault systems. An angular unconformity separates the Tetang and overlying Thakkhola Formations, where this contact can be observed in the southern part of the basin. Southward axial drainage was established by ca. 7 Ma with the onset of deposition of the Thakkhola Formation. Several episodes of damming of this drainage system are recorded by widespread lacustrine deposits in the southern part of the basin. Facies distribution and the progressive rotation of strata in the Thakkhola Formation indicate that the Dangardzong fault on the western edge of the basin was active at this time, and drainage ponding may have been related to displacement on normal faults associated with the South Tibetan detachment system to the south of Thakkhola graben.