EVIDENCE FOR RAPID DISPLACEMENT ON HIMALAYAN NORMAL FAULTS AND THE IMPORTANCE OF TECTONIC DENUDATION IN THE EVOLUTION OF MOUNTAIN-RANGES

East-striking, low-angle normal faults of the South Tibetan detachment system have played an important role in exposing the high-grade metam orphic core of the Himalayan orogen In the Mount Everest region of sou thern Tibet, granites both pre-and postdate an important fault of the system, the Qomolangma detachment. New U-Pb and Ar-40/Ar-39 geochronol ogic data for these rocks constrain the age of brittle faulting to bet ween 16.67 +/- 0.04 and 16.37 +/- 0.40 Ma, significantly expanding the known age range for extension in the central Himalaya (widely regarde d as ca. 20-22 Ma). More importantly, they indicate an average displac ement rate of greater than or equal to 47 mm/yr and a consequent tecto nic unroofing rate of greater than or equal to 8.2 mm/yr. Such unroofi ng is faster than all but the highest estimates of combined physical a nd chemical erosion rates in mountainous regions, suggesting that larg e-displacement normal faulting can be an extremely efficient agent of mass redistribution in orogenic systems.