ACCELERATING SUBSIDENCE, ANOMALOUS INTRABASINAL DEFORMATION, AND RAPID POST-BURIAL EXHUMATION IN AN ENIGMATIC EOCENE LACUSTRINE BASIN IN CENTRAL TIBET

It has long been speculated that lithosphere removal contributed to the uplift of the Tibetan Plateau, and other orogenic plateaus across the planet. Determining the scale, timing, and style of such removal events (whether by wholesale delamination or piecemeal dripping) is difficult, and presents a critical obstacle to our understanding of orogenic mass balance. Sedimentary basins potentially record the surface response to deeper lithospheric dynamics, providing a window into ephemeral, deep-seated processes. Here we present new geologic mapping, measured stratigraphic sections, paleoenvironment interpretations, subsidence analysis, and structural observations from a previously undocumented Eocene basin in central Tibet (Qiangtang). We document anomalous patterns of intrabasinal deformation that are difficult to explain in a regional tectonic setting with strong N-S directed shortening due to active continental collision. A convex-up subsidence curve representing the accelerating growth of accommodation space over time also corresponds to an initially gradual transition from overfilled basin, to shallow lacustrine depositional environment, to finally a profundal lacustrine depositional environment in an underfilled basin. The late, rapid deepening of the basin does not correspond to conglomeratic alluvial fan facies, which are a dominant component of other Cenozoic deposystems proximal to thrust fronts elsewhere in Tibet. We consider the extent to which dynamic subsidence and rebound associated with lithosphere removal may be a plausible explanation for the enigmatic combination of observations. Could this basin provide a window into ephemeral, deep-seated processes at the base of the Tibetan Plateau lithosphere during Eocene time, at a critical moment after the “hard” collision of India and Asia?