Detailed cooling history of the central Nepalese Himalaya (Annapurna region), from apatite fission track and (U-Th)/He analyses

The Nepalese Himalaya, which is perhaps the quintessential collisional orogenic belt, presents a unique opportunity to examine changes in denudation rates over the last few million years. We document the low-temperature cooling history along the Marsyandi river catchment in central Nepal with apatite fission track (AFT) and (U-Th)/He analyses, which are commonly used to estimate denudation rates and are especially powerful if combined on samples collected at different elevations along a vertical transect. The Marsyandi catchment has its headwaters at the edge of the Tibetan Plateau and traverses the High Himalaya to the east of Annapurna. In order to document the most recent denudation rates possible, we collected samples along the Marsyandi catchment (near river bed level), where ages should be the youngest, as well as on 3 nearly vertical transects on the western side of the Marsyandi catchment. The southernmost vertical transect (south of the High Himalaya) consisted of 7 samples, collected at elevations ranging from 3100 m to 1200 m. AFT ages from these samples ranged from 2.5 to 0.6 Ma and showed a direct correlation of elevation to age. The denudation rate calculated directly from the age/elevation relationship is 1 km/Ma, for the time interval spanned by the ages. This rate is surprisingly low for this region (rates commonly cited for the Himalaya over the last 5 m.y. are 3-5 km/Ma). Initial apatite (U-Th)/He analyses from the 5 lowest of these samples ranged in age from 0.7 Ma (2500 m) to 0.3 Ma (1200 m), and indicate a much higher denudation rate (3 km/Ma) from 0.7 to 0.3 Ma. AFT analyses from the other two transects are consistent with those from the southern vertical transect. The middle vertical transect consisted of 3 samples spanning a total of 1 km of elevation. These 3 samples yielded AFT ages of 1.6 to 1.0 Ma, which were similar (althought slightly younger) than the southernmost samples from equivalent elevations. The northernmost transect consisted of 4 samples spanning nearly 2 km of elevation (from 4100 to 2300 m) near the High Himalaya: ages from these samples ranged from 1.2 to 0 Ma. These ages are significantly younger than samples from equivalent elevations from the other two transects. These preliminary data indicate that: 1) denudation rates along the Marsyandi catchment were relatively low from 2.5 to 0.7 Ma (1 km/Ma) but accelerated significantly from 0.7 to 0.3 Ma (to 3 km/Ma), and 2) denudation rates appear to be higher in the High Himalaya region.