THE THERMOCHRONOMETRIC RECORD OF LANDSCAPE EVOLUTION IN THE LHASA TERRANE, SOUTHERN TIBET (Invited Presentation)

At the southern margin of the Tibetan plateau, the Lhasa terrane marks the transition between the internally drained, slowly eroding plateau interior and the rapidly deforming and exhuming edges of the Himalaya–Tibet orogen. Constraining the spatiotemporal patterns of surface processes and erosion rates across this transition region is critical to understanding the larger scale dynamics of tectonics and topographic growth during India–Asia collision. To this end, we are collecting a large thermochronometric dataset from exposures of the Gangdese batholith in the eastern Lhasa terrane. This dataset consists of more than one hundred bedrock samples collected over a 10⁵ km² area, including nine sample transects spanning 1 km or more in elevation, and utilizes four thermochronometric systems with temperature sensitivities ranging from ~30 to ~350 °C. A pervasive signature of this dataset recorded by the lowest-temperature thermochronometers is a rapid shutdown in exhumation at ~10 Ma. Prior to this, exhumation rates in the Lhasa terrane were comparable to recent exhumation rates observed in the Himalaya south of the modern topographic divide. While a rapid shutdown in erosion at ~10 Ma is consistent with other thermochronometric data from the central and western Lhasa terrane as well as the northern Tethyan Himalaya, apatite ⁴He/³He thermochronometry are required to resolve the timing and magnitude of this change. Notably, apatite ⁴He/³He thermochronometry excludes the possibility of significant relief development across the Lhasa terrane since ~10 Ma, indicating that the majority of the 1–2 km of relief characterizing the region today existed at that time. Here, we pair geologic observations and landscape evolution modeling to explore possible tectono-geomorphic scenarios that could explain this shutdown in erosion, and suggest avenues of future research to test these scenarios.