MULTI-STAGE UPLIFT OF THE ROCKY MOUNTAINS: USING THERMOCHRONOLOGY DATA TO UNRAVEL MECHANISMS AND DISCRETE EPISODES OF UPLIFT AND INTERVENING TECTONIC QUIESCENCE

A regional compilation of apatite fission track (AFT) and (U-Th)/He AHe data through the Rocky Mountains from New Mexico to Wyoming reveals spatial and temporal patterns in the timing cooling through 110-50 °C that we infer to be associated with discrete uplift episodes, with differing exhumation rates and uplift mechanisms. 1) 70-45 Ma cooling ages are located in Laramide uplifts in the Wyoming Wind River, Medicine Bow and Park Ranges, Colorado Front and Park, and some Colorado Plateau monoclines, with apparent exhumation based on age- elevation transects of ~60m/Ma; spatial concentration of these ages to fault hangingwall blocks suggests fault-driven localized surface uplift and cooling. 2) There was relative tectonic quiescence from 45-35 Ma coincident with development of the Rocky Mountain erosion surface. 3) Oligocene 35-25 Ma cooling ages are found in the central Colorado Rockies and Great Plains topographic ramp, with apparent exhumation rates of ~100 m/Ma from 35-25 Ma based on age-elevation transects; this event is interpreted to be associated with the ignimbrite flare-up beneath the San Juan and Mogollon Datil volcanic fields and long wavelength uplift and erosion due to epeirogenic mantle upwelling. 4) Miocene 25-10 Ma cooling ages are found in the rift flanks of the Rio Grande rift from Wyoming to Mexico, with apparent exhumation of fault blocks of 100-150 m/Ma based on age elevation transects and cooling models; these shorter wavelength uplifts are interpreted to reflect isostatic adjustments to mantle buoyancy (uplift of rift flanks and collapse of basins) in the high topography of the Rockies expressed through heterogeneous crustal fault-block structure. 5) Neogene (last 10 Ma) cooling ages found at highest elevations and in drill cores show apparent exhumation rates >120m/Ma and are interpreted to reflect renewed mantle –driven uplift, perhaps due to small scale upper mantle convection. Our goal is to apply both AFT and AHe regionally to evaluate long term differential incision rates and connect these to similar data from dated river terraces to give a more complete history of exhumation of rocks from ~2- 3 km to the surface in the Colorado Plateau-Rocky Mountain region.