INSIGHTS INTO CENOZOIC LANDSCAPE EVOLUTION OF THE ROCKY MOUNTAIN-HIGH PLAINS REGION BASED ON THERMOCHRONOMETRIC DATA FROM SOUTHWESTERN COLORADO

Regional-scale thermochronometric and geologic studies define four episodes of erosion and related tectonic uplift in the Rocky Mountain region. Laramide compressional deformation (75-45 Ma) produced localized uplifts, basins, and fault systems over a broad area that had a profound influence on subsequent landscape evolution. Most geologic data (except leaf physiognomy) point to average basin-floor elevations of 300-500 m during this timeframe. The two erosional events that followed Laramide deformation affected smaller areas. An Eocene (42-37 Ma) exhumation focused on the southern margin of the Archean craton in Wyoming, Colorado, and Montana was associated with rebound after Laramide dynamic subsidence. Late Oligocene to early Miocene (27-15 Ma) exhumation of the southern part of the region was likely caused by increased mantle buoyancy during and following the ignimbrite flareup. The final episode of exhumation started < 10 Ma and appears to be driven by hot, buoyant mantle that underlies the central Colorado Plateau, the Rocky Mountains, and the Jemez lineament. This latest event has resulted in ~ 1 km of uplift, perhaps half of which is due to isostatic compensation. AFT and AHe data from the West Elk Mountains, Sawatch Range, and San Juan Mountains provide new constraints on Laramide, Oligocene to Miocene, and late Cenozoic cooling and exhumation. AFT and AHe data from Oligocene plutons in the West Elk Mountains, from Huron Peak in the Sawatch, and from Molas Pass in the San Juan Mountains record steady average rates of exhumation during Oligocene to Miocene time. AHe dates derived from deep canyons near 8 to 15 Ma intrusions and mineralization zones indicate an acceleration of cooling/exhumation during the last 10 Ma. The presence of several small 8 to 14 Ma intrusions in southwestern Colorado may be surface manifestations of melt-flux related to the Neogene component of mantle reorganization and related surface uplift. Thus, thermochronologic data suggest that the low wave speed mantle beneath the Colorado Rockies likely has a composite Mid-Cenozoic and Neogene origin.