Meso-Cenozoic exhumation of the Beishan, southern Central Asian Orogenic Belt: Insights from low-temperature thermochronology

Abstract

The Beishan Orogenic Collage (BOC) is located in the south of the Central Asian Orogenic Belt (CAOB) and formed during the final consumption of the Paleoasian Ocean in the Late-Permian to Middle-Triassic. This study applies low temperature thermochronology to constrain the Meso-Cenozoic exhumation history of the BOC. Apatite fission track and U-Th-Sm/He data obtained for granitoid samples along a north-south transect through the BOC suggest evidence for three distinct phases of exhumation during (1) the Late Triassic - Early Jurassic (~225 – 180 Ma), (2) Early - mid Cretaceous (~130 – 95 Ma) and (3) Late Cretaceous - Early Palaeogene (~75 – 60 Ma). Samples from northern Beishan reveal a more profound early to middle Cretaceous signal and a weaker Late Triassic - Early Jurassic signal than those in southern Beishan. A potential explanation for this discrepancy is the presence of the Xingxingxia fault in the northern BOC which is interpreted to have undergone repeated reactivation throughout the Mesozoic, exposing deeper exhumed sections of the BOC. The fault may thus have acted as a control on exhumation in the region. This pattern is consistent with results from elsewhere in the CAOB such as in the Tianshan and the Altai, where regional widespread exhumation occurred since the Early Cretaceous while major fault zones record localised exhumation during the Late Cretaceous – Early Palaeogene. Additional late Cretaceous – early Palaeogene cooling ages found only in the south of the BOC suggest that exhumation at that time was more localised and didn’t reach the northern margins of the study area.

Our results indicate that the Triassic - early Jurassic and early to middle Cretaceous exhumation events in Central Asia were more widespread than previously anticipated, extending to the northern margin of the Tarim Craton. This observation hence refines the existing tectonic history models for Central Asia.