Meso-Cenozoic growth of the eastern Qilian Shan, northeastern Tibetan Plateau margin: Insight from borehole apatite fission-track thermochronology in the Xiji Basin

Abstract

The surface uplift of the Tibetan Plateau (TP) is among the most important geological events in Earth's history, but the growth of the eastern TP margin that yielded the thickened crust and abrupt topography remains controversial. In this paper, six sample apatite fission-track (AFT) analyses of three boreholes in the Xiji Basin are applied to constrain the Mesozoic-Cenozoic uplift and exhumation history of the eastern Qilian Shan, northeastern TP. Most of the AFT ages range from 134 ± 8 Ma to 117 ± 6 Ma, except for the shallowest sample, with a younger age of 65.9 ± 3.2 Ma. Thermal history modeling indicates that the eastern Qilian Shan experienced a three-phase differential cooling history: (1) widespread rapid cooling during the Middle Jurassic-Early Cretaceous (ca. 174-120 Ma, Stage 1), (2) regional cooling during the Late Cretaceous-Paleocene (ca. 80-60 Ma, Stage 2) and (3) widespread cooling during the Eocene-early Miocene (ca. 40-20 Ma, Stage 3). We conclude that the Middle Jurassic-Early Cretaceous was the main stage of the growth and thickening of the northeastern TP, which was related to the upper-crustal horizontal shortening of the eastern Qilian Shan. Thermal history modeling of the youngest sample and seismic profile analysis imply significant reactivation of the Liupanshan fault zone during ca. 80-60 Ma. This locally intense uplift and deformation on the eastern margin of the Qilian Shan during the Late Cretaceous was likely induced by the closure of the Neo-Tethys and the continued shortening of the Lhasa-Qiangtang block. Late Eocene-Early Miocene (ca. 40-20 Ma) cooling of the eastern margin of the Qilian Shan records the coeval crustal extension and exhumation of the northeastern margin of the TP as the far-field effect of subduction of the western Pacific plate. Our AFT data detect no intense late Cenozoic reactivation information for the Xiji region, which indicates that the ca. 15-7 Ma rapid uplift triggered by fault reactivation was only located in the Haiyuan-Liupanshan fault zone, along the eastern margin of the Qilian Shan, northeastern TP. This multi-phase exhumation history is not beneficial for regional hydrocarbon exploration.