CREST- COLORADO ROCKIES EXPERIMENT AND SEISMIC TRANSECTS: TIME-SPACE PATTERNS OF NEOGENE UPLIFT AND THEIR CORRESPONDENCE TO THE ASPEN ANOMALY

A 60-station passive IRIS-PASSCAL array was installed in August 2008 and data collected for 14 months, ending October, 2009. The array was configured and temporally synchronized with 30 USArray stations. Processing is still ongoing, but the emerging geophysical findings are: 1) mantle velocity structure beneath Colorado involves extreme and high-amplitude spatial heterogeneity suggestive of a complex and convectively turbulent mantle extending from the lithosphere-asthenosphere boundary to the 660 km lower mantle transition zone; 2) crustal thickness variations of >12 km over short lateral distance are observed from receiver function data, with a large zone of relatively shallow Moho (42 km) beneath the highest topography and lowest velocity mantle of the region. This necessitates mantle support for topography which we term the “rootless Rockies”. Geomorphic analysis of the Colorado River suggests Neogene (since 10 Ma ) surface uplift in both the Rockies and the western Colorado Plateau. Epeirogenic uplift of the Rockies relative to the central Colorado Plateau is manifested by the Colorado River system having higher normalized channel gradients, discharge, topographic roughness, and higher incision rates in a region with a positive geoid anomaly, lower mantle velocity, thinner crust, lower crustal seismic attenuation (Q) than the Green River. Presence of mantle ³He in CO₂-rich hot springs in the Colorado Rockies reflects ongoing mantle tectonism. Mantle driving forces are manifested by lowest velocity mantle under steepest river segments, the Neogene change from lithospheric- to asthenospheric-sourced basalts in the western CP, geodynamic models, analysis of the geoid, and residual tectonic uplift resolved from calculations of isostatic response to denudation. Apatite fission track thermochronology shows rapid exhumation in the Rockies starting 10-6 Ma at modern elevations ranging from 1.5 to 3 km, requiring Neogene tectonic uplift. Our data support the hypothesis that complex upper mantle convection and resulting buoyancy variations are driving 400-800 m of dynamic uplift in the region and actively re-shaping the physiographic characteristics of western U.S. tectonic provinces.