Temporal constraints on the Longriba fault motion, Eastern Tibetan plateau

Abstract

The spatial and temporal evolution of active fault structures within the eastern Tibetan Plateau (ETP) helps constrain the tectonic and topographic history of that region. In this study, we focus on the tectonic evolution of the Longriba fault (LRF), located ∼170 km west of the Longmenshan thrust belt (LTB) in the interior of the plateau. Increasing geological and geophysical evidence indicates that the LRF, a prominent dextral strike-slip fault with minor thrust components, serves as a significant tectonic and topographic boundary within the ETP. The basement rocks deformed by the LRF experienced rapid cooling between ∼156 and 136 Ma based on three pairs of muscovite/biotite ⁴⁰Ar/³⁹Ar ages, while four illite K-Ar ages (∼37.8–35.6 Ma) from fault gouges from the fault indicate mineral growth during fault reactivation. These results, combined with regional chronology, document several thermotectonic stages in the ETP since the closure of the Paleo-Tethys in the Late Triassic. An early Cretaceous cooling event was identified, reflecting boundary fault reactivation and crustal thickening, due to the Lhasa-Qiangtang collision to the south. The ∼38 Ma illite K-Ar ages are coeval with the India-Asia ‘hard’ collision, indicating fault propagation into the interior of the ETP. This study concludes that far field effects of plate convergence may strongly control fault movement, reactivation and propagation within the ETP, as well as triggered several pulses of cooling. Therefore, episodic mountain building may be due to continued fault-induced crustal thickening and uplift in the ETP.