USING DETRITAL 10BE TO IMAGE THE TRANSIENT INCISION OF AN UPLIFTED RELICT LANDSCAPE IN THE HIMALAYA OF WEST NEPAL

Field studies and analysis of digital topography have identified remnants of a low-relief plateau situated at ~3-4 km elevation within the Himalaya of western Nepal. This landscape stands in stark contrast to the threshold hill slopes and deep gorges that characterize most of the Greater Himalaya. The margins of the relict plateau are more deeply incised than its interior, suggesting that the study area is adjusting to a new relative base level and will soon (in geologic time) be replaced by deep canyon topography. Our conceptual model holds that this plateau is being uplifted above a recently activated mid-crustal ramp in the Main Himalayan Thrust. This setting thus presents an interesting opportunity to study the landscape response to progressive incision into a tectonically-forced disequilibrium landscape.

In order to define millennial-scale erosion rates in the interior and periphery of this transient landscape, we present a suite of AMS measurements of detrital ¹⁰Be in quartz sand from mid-size tributaries to the Tila and Karnali rivers, including some that lay entirely within the more subtle topography of the plateau and others that appear to be more deeply dissected and ‘adjusted’ to the regional base level. The latter drainages often feature large knickpoints at the boundary between adjusted and unadjusted portions of the landscape. AMS ¹⁰Be results will be presented that highlight the contrasts and complexity of this region.

These results will: 1) track the landscape response to progressive incision into an intramontane plateau; 2) provide a clearer picture of the sensitivity of erosion rate to topography, tectonic forcing, and climate within orogens; and 3) allow comparison of millennial-scale erosion rates with long-term exhumation rates for the same area derived from low-temperature thermochronology.