Tectonic and paleoelevation history of the Thakkhola Graben and implications for the evolution of the southern Tibetan Plateau

Abstract

Sediment accumulation in extensional basins in the Tibetan Plateau records tectonic processes and paleoenvironments on the plateau. It is generally assumed that extension on the plateau took place during uplift of the plateau. Based on this assumption, several studies have been aimed at determining the timing of extensional basin development as a proxy for the timing of uplift of the plateau. This dissertation documents the sedimentology of the N-trending Thakkhola graben in the southern Tibetan Plateau in an attempt to test various models for the timing and mechanisms of uplift of the plateau. Magnetostratigraphic and stable carbon isotopic age constraints indicate that deposition in the Thakkhola graben occurred during the Late Miocene (∼11 Ma) to Pliocene. The oxygen isotopic composition of carbonate rocks deposited in the basin records the isotopic composition of paleometeoric water that fell in the basin and in flanking drainages when the carbonate was precipitating. Carbonate oxygen isotopes indicate high-elevation rainfall in the basin, consistent with modern elevations since the onset of deposition in the basin. This implies that the average elevation in the Thakkhola graben has been >4,500 m since it began to form. Lateral facies changes, conglomerate provenance, and paleocurrent data document significant displacement on the western basin-bounding fault since deposition began in the basin. By Pliocene time, a large, southward axial drainage had developed that was similar in size to the modern Kali Gandaki River, which drains the southern plateau, through the Thakkhola graben and Himalayan fold-thrust belt to the south. Change in environments of deposition in the Thakkhola graben indicates trends toward an increasingly arid climate through time. This climate change is documented throughout south Asia and possibly Tibet between ∼8 and 7 Ma and have been assumed to reflect uplift of the plateau. However, high elevation in the Thakkhola graben since ∼11 Ma challenges these commonly held notions.