Geodynamic shift drives two-stage Cenozoic deformation of the Tianshan: New thermochronologic constraints from the Tomor Peak (southern Chinese Tianshan)

The Tianshan orogenic belt provides an ideal laboratory for investigating continental lithosphere deformation far from the plate boundary. Here, we integrate multiple thermochronometers [apatite/zircon (U-Th)/He, zircon fission track, and biotite 40Ar/39Ar] from a 1.2 km transect in the Tomor Peak region to constrain the Cenozoic exhumation history of the southern Tianshan, China. We found an exhumation signal much stronger than previously reported, manifested in Neogene zircon (U-Th)/He and fission track ages that are significantly younger than published thermochronological data. This allows us to reliably identify two pulses of accelerated exhumation, the first beginning no later than 20 Ma and the second at ca. 12−10 Ma, and to reveal stepwise intensification of tectonic deformation in the Tianshan. Synthesis of regional data shows that the Cenozoic deformation in the Tianshan is characterized by both spatial heterogeneity and temporal clustering, with two prominent deformation epochs at ca. 30−20 Ma and ca. 12−10 Ma. We suggest that the deformation characteristics reflect a geodynamic shift from crust-dominated processes (30−12 Ma) driven by gravitational potential energy associated with the Tibetan Plateau uplift to mantle-dominated processes (post-ca. 12 Ma) triggered by direct contact between the Indian and Tarim lithospheric mantles. This transition is facilitated by pre-existing Moho offsets and a weak mid-lower crust. The two-stage tectonic evolution model effectively explains the spatiotemporal variability of intracontinental orogenesis, highlighting the interplay between regional geodynamics and inherited lithospheric architecture in modulating far-field stress propagation.