Using Detrital Zircon (U-Th)/He Thermochronology From the Sutlej River Valley in the NW Indian Himalaya to Examine Erosion Distribution During the Early Holocene Monsoon

The Himalaya and Tibet are an unrivaled example of continent-continent collision. This extensive orogenic system influences regional climate and is characterized by rapid erosion and exhumation. The interplay between climate-driven erosion and rock uplift is key in understanding the geomorphic evolution of the orogen. Recent studies using detrital zircon fission track data, combined with geomorphic models informed by Tropical Rainfall Measurement Mission (TRMM) [Bookhagen et al. 2005] data and thermal-kinematic models [Brewer and Burbank 2006] show a strong correlation between regions of high precipitation rates and rapid erosion in the modern. However, paleoclimate records indicate evidence for a strengthened monsoon during the early Holocene [e.g., Fleitmann et al. 2003, Gupta et al. 2003]. It is believed that in addition to an increase of the monsoon's intensity precipitation penetrated farther into the Himalayan valleys [Bookhagen et al. 2005; Goodbred and Kuehl, 2000]. This study examines this correlation using detrital (U-Th)/He analyses in both modern and paleo-fluvial fill terrace sediments. A preliminary data set of sixty (U-Th)/He zircon grain-ages show the modern and paleo-fluvial terrace sediments reflect different population distributions, or probability density functions (PDF), of grain-ages. The slightly older (3 Ma higher) peak age of the paleo-fluvial terrace sample grain-age population is interpreted to correlate with a northward shift in spatial erosion, due to a strengthened early Holocene monsoon. Eighty more (U-Th)/He zircon grain-ages are presently being analyzed in order to achieve two statistically significant (n= 70) PDFs to make the comparison between modern-day and paleo-fluvial fill populations more robust. In addition, this study synthesizes TRMM-imaged precipitation and geomorphic and thermal-kinematic models to yield a synthetic, 'predicted' detrital grain-age population; a PDF for the modern-day fluvial system. This approach allows us to (i) test the thermal-kinematic model with a low closure temperature chronometer and (ii) characterize the correlation between the locus of maximum rainfall from the TRMM data and the predicted locus of maximum erosion. Finally, in order to ground truth the exhumation rates and zircon (U-Th)/He grain-ages predicted by our thermal-kinematic model, (U-Th)/He grain-ages from ten bedrock samples spaced along the river valley bottom will be produced.