Tectonic evolution of the Qiangtang terrane and Bangong-Nujiang suture zone, central Tibet

Constraining the timing and mechanisms of Tibetan plateau formation is significant for understanding how continental lithosphere deforms during collision and for addressing potential relationships between tectonics, climate, and ocean-water chemistry. Models assume that the Tibetan crust is homogeneous and that it thickened northward during the Cenozoic Indo-Asian collision by the same mechanisms that are observed to be active along the plateau margins. Testing these assumptions is hindered by our poor geologic understanding of the Tibetan interior. The goal for my research has been to constrain the crustal structure and thickening history of central Tibet through a combination of field-based structural, geochronologic, and petrologic studies.; Deeply-exhumed rocks in the central Qiangtang provide windows into the deeper crust of central Tibet. They consist of blueschist-bearing melanges and occur in the footwalls of Late Triassic-Early Jurassic domal, low-angle normal faults. Thermochronologic studies coupled with application of a new titanite-rutile barometer suggest that the melanges were exhumed directly from >30 km depth by detachment faulting. As the melanges are structurally beneath Paleozoic-Mesozoic continental margin strata, they likely underplated central Tibet during early Mesozoic low-angle oceanic subduction and represent the central Tibetan deeper crust. This crustal structure contrasts sharply with that inferred for southern Tibet; this difference may help explain why Cenozoic crustal shortening and magmatism were localized >100 km north of the India-Asia suture zone, and why the central Tibetan crust exhibits anomalously high Poisson's ratios.; Geologic mapping and geochronologic studies elucidate the style, magnitude, and dominant mechanism of crustal thickening in central Tibet. The Bangong-Nujiang suture zone between the Lhasa and Qiangtang terranes was significantly modified by Cretaceous-Tertiary upper-crustal shortening subsequent to ocean closure. This shortening was coeval with growth of a >200-km-wide east-plunging anticlinorium in the central Qiangtang. A possibility is that the Qiangtang anticlinorium formed above a thrust ramp at depth, while the Lhasa terrane underthrust northward beneath the Qiangtang terrane along the reactivated Bangong-Nujiang suture. This underthrusting began before and continued during the Indo-Asian collision. This hypothesis raises the provocative possibility for Tibetan plateau formation from the “inside-out” and beginning prior to the Indo-Asian collision.