Evidence for constriction and Pliocene acceleration of east-west extension in the North Lunggar rift region of west central Tibet

Abstract

The active north trending North Lunggar rift in west central southern Tibet exposes an extensional metamorphic core complex bounded by an east dipping low-angle normal fault. Apatite and zircon (U-Th)/He thermochronology and thermal modeling of the North Lunggar rift document a minimum timing for rift inception at >10 Ma and rapid footwall exhumation between 5 and 2 Ma. Miocene footwall cooling and exhumation rates were initially slow to moderate at <50°C Ma−1 and <1 mm a−1, followed by increased Pliocene rates as high as >400°C Ma−1 and 4–10 mm a−1. Footwall isotherms were significantly compressed during rapid exhumation resulting in an elevated transient geothermal gradient between 50 and 90°C km−1. The minimum magnitude of horizontal extension for the North Lunggar rift is 8.1–12.8 km; maximum is 15–20 km, less in the south at ~10 km. Mean Pliocene extension rate is 1.2–2.4 mm a−1 in the ~120° direction. Results for the North Lunggar rift are similar in magnitude, rate, and orientation of slip to the kinematically linked Lamu Co dextral strike-slip fault to the north. This suggests a state of constrictional strain during Pliocene time along this stretch of the Bangong-Nujiang suture from which the Lamu Co fault emanates. The onset of extension in this region may be explained by crustal thickening and gravitational orogenic collapse, followed by accelerated rifting resulting from localized crustal stretching and increased magmatic activity, potentially driven by the position and northward extent of underthrusting Indian lithosphere.