LITHOSPHERIC STRUCTURE OF THE COLORADO ROCKY MOUNTAINS: IMPLICATIONS FOR TOPOGRAPHIC SUPPORT AND CENOZOIC EVOLUTION

The structure of the crust and upper-most mantle beneath the Colorado Rockies is investigated using seismic data collected by the Colorado Rocky Mountain (CRM) Experiment and Seismic Transects (CREST) and the Earthscope Transportable Array (TA). P wave receiver function results show that this region exhibits thicker than average crust (48 km). However, shallower Moho depths (< 45 km) are observed beneath much of the highest elevations and therefore, variations in crustal thickness cannot account for the excess topography in this region. The Colorado Rocky Mountains are rootless. Greatest Moho depths (45–51 km) occur in NW Colorado and in a SW trending lineament near the New Mexico border. We speculate that these structures were inherited from the Proterozoic assembly of the continent, specifically the formation of the Cheyenne Belt and the Yavapai-Mazatzal suture.

Shear wave velocities from surface wave tomography are mapped to density employing empirical velocity-to-density relations in the crust and temperature modeling in the mantle. Predicted elastic plate flexure and gravity fields derived from the resulting density model agree with observed long-wavelength topography and Bouguer gravity indicating that low density crust and mantle are sufficient to support much of the CRM topography. The lowest mean crustal velocities and reduced crustal thicknesses are correlated with centers of Oligocene volcanism, e.g., the San Juan Mountains, suggesting that magmatic modification has strongly influenced the modern crustal structure and surface topography.

S wave receiver function imaging suggests that the lithosphere-asthenosphere boundary (LAB) is located at 100–150 km depth beneath the Colorado Plateau and increases to 150–200 km depth beneath the High Plains. At 73 km depth, the CRM display regions of elevated temperature and are clearly distinct from the cooler adjacent Colorado Plateau and Great Plains. Reduced mantle velocities are spatially correlated with a broad negative arrival imaged by the S wave receiver functions. An intermediate CRM geotherm and the composition of Cenozoic basalts and ultra potassic rocks suggest that this feature is internal to the lithosphere and may be the result of metasomatisation and/or thermal modification of old lithosphere rather than a shallow CRM LAB.