Spatial variation in Meso-Cenozoic exhumation history of the Longmen Shan thrust belt (eastern Tibetan Plateau) and the adjacent western Sichuan basin: Constraints from fission track thermochronology

Abstract

The NE-trending Longmen Shan thrust belt, marking the eastern margin of the Tibetan Plateau adjacent to the Sichuan basin, is characterized by the paradox of high topographic gradients but low convergence rates, and thus critical for understanding of geodynamic processes involved in the eastward growth of the plateau. Many low-temperature thermochronological studies focused on the central and southern portions of the Longmen Shan and western Sichuan basin to investigate the Cenozoic exhumation history of the eastern Tibetan Plateau, but little work has been done on their northern portions. Furthermore, the spatial variation in both Cenozoic and Mesozoic thermal histories of this region has been poorly documented. In this paper, zircon (ZFT) and apatite (AFT) fission track analyses for samples collected from three transects and three boreholes covering the northern, central and southern Longmen Shan and the western Sichuan basin provide new data demonstrating several cooling events in this region since the Indosinian deformation. They occurred during the late Triassic (∼200Ma), at the end of early Cretaceous (∼100Ma), during the early Cenozoic (60–40Ma), the early Miocene (20–25Ma) and the late Miocene (9–14Ma), respectively. On regional scale the Longmen Shan experienced a slow cooling between the Mesozoic and early Cenozoic, followed by a rapid cooling during the late Cenozoic, when the rapid regional exhumation began at ∼10Ma, with a sharp increase in exhumation rate from <0.1mm/yr to >0.2mm/yr, up to 0.9mm/yr locally. Comparison of ZFT and AFT ages shows that during the Mesozoic the northern segment of the Longmen Shan belt cooled more rapidly than the central and southern segments, while younger AFT ages indicate more rapid exhumation in central and southern segments of the belt during the late Cenozoic. Similar trend is demonstrated within the western Sichuan basin, where the onset of exhumation changes from ∼45Ma in the northern portion to 20–25Ma in the central and southern portions of the basin. The thermochronological data reveal that the Yingxiu-Beichuan and Guanxian-Anxian faults have accommodated significant differential exhumation by thrusting activity and associated denudation, with a greater amount of differential exhumation from north to south. We propose that the major cause for the rise in topography and rapid exhumation in the Longmen Shan thrust belt during the late Cenozoic is the thrusting activities integrated with the regional uplift of the eastern Tibetan Plateau, a process unrelated to the channel flow within the lower crust.