GEOLOGIC MAP OF THE TANGRA YUMCO RIFT, SOUTH-CENTRAL TIBET

Researchers interpret east-west extension of the Tibetan Plateau to have initiated synchronously during mid-Miocene time (~16-12 Ma), preceding an observed acceleration of extension rates by ~5-10 Myr. Most tectonic models can explain extension onset, but struggle to reconcile the delayed acceleration of rifting. The India underthrusting hypothesis links rift acceleration to the northward propagation of the Indian plate beneath Asia, but has been tested in only one locality. The Tangra Yumco (TYC) rift in south-central Tibet is one of the longest (~250 km) north-south striking rifts on the Plateau, providing an ideal natural laboratory to test dynamic models of extension through geologic mapping, (U-Th)/He thermochronology, and thermo-kinematic modeling.

We conducted geologic mapping over ~2,000 km² in the south portion of TYC rift and present a 1:100,000 scale map from this work, revealing previously undocumented geologic relationships. TYC is bounded by two approximately north striking high angle (~45-70°) active normal faults that alternate dominance along strike, exhuming ~2 vertical km of granodiorite and apparent migmatite in central portions of the rift footwall, potentially indicating low-angle rather than high-angle normal faulting. Hanging wall rocks consist of Quaternary alluvial fan deposits that display synthetic graben structures in the central rift, while in northern and southern rift portions, deposits are cut by a stepped sequence of high angle normal faults which likely sole into the main rift-bounding fault at depth. Footwall granodiorite shows pervasive northeast dipping foliation, with slip lineations and fault plane striations suggesting northeast directed oblique sinistral-normal slip. This geologic map provides unprecedented detail of the TYC rift and insight into the dynamic processes that control changes in crustal thickness and evolution of topography during orogenesis – particularly upper crustal response to inter-continental collision – demonstrating the significance of Tibetan rifting within the wider framework of the India-Asia collision. Tibet is often cited as a primary example of an orogen undergoing synconvergent extension; this study may illustrate how analogous structures evolved in other synconvergent settings such as the North American Cordillera.