EXHUMATION IN THE HIMALAYA OVER MILLENNIAL TO MILLION-YEAR TIMESCALES

Recent reports of structural, thermochronologic, and climatic data from central Nepal have led to varying hypotheses regarding the degree of climatic-tectonic coupling in this setting. These disparate interpretations underscore the need for exploiting data representing a variety of temporal scales to evaluate the persistence of perceived breaks in exhumation rates. Here we synthesize new and previously reported results from thermochronologic, cosmogenic and geomorphic analyses to shed light on the degree of climate-tectonic coupling in the central Nepal Himalaya. Detrital ⁴⁰Ar/³⁹Ar thermochronologic data from three trans-Himalayan drainages delineate a sharp discontinuity in cooling history between 10 and 20 km south of the Main Central Thrust (MCT). These data suggest a near step-function increase in long-term exhumation rates that does not correlate with previously mapped faults, and imply the existence of Pliocene or younger out-of sequence thrusting between the Main Central and Main Boundary thrust systems. Cosmogenic isotope data from the Burhi Gandaki transect suggest a co-located peak in erosion rates over millennial timescales, and the pattern of erosion rates in this transect closely mimics patterns in precipitation over decadal timescales. Finally, quantitative measures of landscape morphology are also consistent with an increase in landscape denudation rates from south to north. While the patterns of million-year exhumation and millennial erosion may differ in detail, the data all point to a locus of focused exhumation approximately 20 km south of the MCT, and suggest that precipitation may be an important driver of exhumation over a wide range of timescales. Explicit consideration of multiple timescales may begin to reconcile disparate hypotheses regarding the nature and degree of tectonic-climatic coupling in active orogens.