Thermochronological insights into the intracontinental orogeny of the Chinese western Tianshan orogen

Abstract

The Chinese western Tianshan orogen (CWTS) experienced multiple tectono-thermal events since the late Paleozoic. In order to better understand the intracontinental orogeny, we integrate new thermochronologic data to reconstruct the long-term cooling history of the CWTS. Our new data reveal that individual tectonic units within the Tianshan orogen show spatial/temporal disparities in their cooling histories. The CWTS experienced a rapid cooling episode during a late Paleozoic orogeny. The Cimmerian collisional orogeny (i.e., the Qiangtang-Kunlun and Lhasa-Qiangtang collision) and the closure of the Mongol-Okhotsk ocean were not strictly coincident with the exhumation events in the CWTS, and thus these distal events were likely not the sole orogenic drivers. The thermal curves reflect that lower crustal rocks had exhumed to middle-upper crustal levels in the late Permian-Triassic, and a rapid cooling phase during this period records the collapse of the CWTS. Furthermore, magmatism related to the melting of subducted Paleozoic plates was probably a crucial factor in the Mesozoic thermal evolution. The age-elevation relationship produced here emphasizes the importance of faulting during intracontinental reactivation and implies that the CWTS has expanded from its central part to both the south and north sides since the late Mesozoic. Low exhumation rates and sedimentological analysis indicate that the relief gradually decreased in the Jurassic-early Cretaceous. The initial reactivation time of the CWTS could be as old as ~38 Ma based on a rapid exhumation event and sedimentary evidence. In the Cenozoic, the crust-mantle interaction, tectonic compression, and continuous northward pushing of the Indian plate rather than the India-Eurasia collision are interpreted as the driving forces that have rebuilt the CWTS. The variable tectono-thermal evolution of individual tectonic units ascribes to the discrepant thermal-rheological properties, complex tectonic boundaries, and the various components of the Tianshan orogen.