MESOZOIC HISTORY OF THE SOUTHEASTERN TIBETAN PLATEAU: SETTING THE STAGE FOR PLATEAU RISE ASSOCIATED WITH CENOZOIC HIMALAYAN OROGENESIS

Regional geologic investigations provide constraints on the Mesozoic lithosphere configuration of the SE Tibetan plateau, which is essential for understanding both Cenozoic Tibetan plateau rise and continental-scale escape tectonism in response to Himalayan orogenesis. Along the western margin of the Yidun arc, youngest detrital zircon age populations in Triassic turbidites are Late Permian-Early Triassic; whereas along eastern margin zircon, youngest ages decrease southward from ~260-250 Ma to 220-215 Ma. Inversion of this region was driven by impingement of the Zhongza terrane/Jomda-Weixi arc upon the Yidun arc in latest Permian/Early Triassic, followed by progressive NW-SE impingement of the Yidun arc upon the Songpan-Ganzi turbidite basin from latest Permian/Late Triassic. No substantial post-Triassic sedimentation is recorded north of the Jinsha suture; however to the south, the Qamdo basin contains ≥5 km of strata with basal Upper Triassic quartz-rich marine-marginal marine strata that reflect recycling Paleozoic strata uplifted by Triassic collision of the Qiangtang block with the Yidun/Zhongza/Jomda-Weixi complex. Well-developed paleosols in Lower Jurassic strata reflect a period of tectonic quiescence, and Middle-Upper Jurassic lacustrine deposits record influx of Bangong arc-derived material from the south. Rejuvenated tectonism along the Jinsha suture is indicated by Cretaceous alluvial strata that contain detrital zircons derived from the Jomda-Weixi arc. Similarly, the Ganzi basin(s) situated atop the Yidun arc contain alluvial strata locally sourced by erosion of Triassic turbidites and Yidun granitoids uplifted along faults within the Yidun arc complex. Cretaceous tectonic rejuvenation is likely related to collision of the Lhasa block with the Qiangtang block along the Bangong-Nujiang suture. Since this formative geodynamic period, the SE Tibet plateau region has undergone limited (<2 km) denudation, and localized sedimentary basin development immediately adjacent to major faults, consistent with the geodynamic and geomorphologic behavior of a plateau. Furthermore, Mesozoic tectonism of the SE Tibetan plateau region prior to the Cenozoic likely induced “crustal set-up”, or anisotropic upper crustal thickening causing viscous flow at lower crustal levels.