Activity History of the Gangga Graben in the Southern Segment of Kung Co Rift in Southern Tibet Constrained by ESR and U-Series Dating

Abstract

Characterising the spatial and temporal distribution of the S-N-trending rift in southern Tibet is crucial for elucidating the dynamics of E-W extension within the Tibetan Plateau since the Miocene. The Kung Co–Tangra Yumco rift, located in the central part of the rift system, was initiated at the peak of rifting development. While the initiation of rifting has been ascertained through low-temperature thermochronology, direct timing constraints remain absent for the Gangga Graben in the southernmost region. Utilising the quartz electron spin resonance (ESR) dating technique, we ascertained the onset age of the eastern boundary fault of the Gangga Graben to be 12.00 ± 1.80 Ma, with an accelerated activity phase at 9.17 ± 1.19 Ma, and an activity age for the western boundary fault at 2.22 ± 0.24 Ma. U-series dating conducted on the western boundary disclosed a time span of 20–13 ka for hot spring fissure activity. Seismic evidence, inclusive of those from the Gongdapu Horst, indicates persistent activity of the Gangga Graben since the Pleistocene. Comparative analysis of age data from the Kung Co–Tangra Yumco rift suggests that it commenced almost synchronously along its strike at ~14.5–12 Ma. Considering the rifts trending from west to east in southern Tibet and the genetic mechanism of leucogranite, we propose an E-W extension mechanism in southern Tibet. At ~26–14 Ma, the lithosphere experienced weakened delamination and asthenosphere upwelling, leading to a series of magmatic activities and onset of E-W extension in southern Tibet. At ~17–7 Ma, as the basal shearing of the underthrusting Indian Plate increased, it marked the peak phase of rifting in southern Tibet.