Extension rate variations across the South Tibetan rifts – New data from the northern Xainza-Dinggye Graben

Abstract

The active ∼NS-trending rifts in southern Tibet reflect ongoing ∼EW extension alongside ∼NS shortening from the India-Asia collision. Quantifying late Quaternary extension rates is essential to comprehend Tibetan deformation and evolution. Here, we test whether extension rates across the Xainza-Dinggye rift (XDR) are on the same order as those along the Yadong-Gulu rift (YGR), and whether a similar northward increase in extension rate exists along the XDR due to interaction with the dextral Gyaring Co fault to its north as exists along the YGR due to interaction with the dextral Beng Co fault to the north, where rates increase from 0.8 to 1.3 to 3–6 mm/yr from south to north. We use ¹⁰Be cosmogenic dating and topographic surveying at three sites along the northernmost XDR, where geomorphic surfaces are vertically offset by several normal faults bounding the western side of the graben. Our results suggest extension rates <1 mm/yr, i.e., much smaller than those across the YGR, except where surfaces are younger than the Last Glacial Maximum (LGM). This suggests a post-LGM rate acceleration following glacier melting and post-glacial rebound due to fault unloading. Therefore, we may need to reconsider the previous assumption of a constant extension rate of 1.3 mm/yr across each of the seven main rifts yielding a total extension rate of 9 ± 2 mm/yr across southern Tibet. We infer that only the larger and more continuous rifts may absorb significant horizontal extension, compared to smaller rifts which may absorb less. Importantly, that extension rates do not increase northwards along the XDR may be due to its particular orientation perpendicular to the Main Frontal Thrust (MFT), compared to other rifts that are more oblique to it. This reinforces earlier inferences that the rifts are related to divergent, orthogonal thrusting along the curved Himalayan MFT.