GRAVITY CONSTRAINTS ON THE DISTRIBUTION OF EXTENSION AND VOLCANISM IN THE NORTHERNMOST RIO GRANDE RIFT, NORTHWEST COLORADO

The Rio Grande Rift (RGR) is a north-trending belt of Cenozoic extensional basins in the western United States that extends from southern New Mexico to Colorado. The physiographic expression of the RGR ends in Leadville (central Colorado) but extensional features can also be found in north-central and northwestern Colorado as far north as the southern edge of the Wyoming craton near the Wyoming state border. Late Cenozoic faulting, sedimentary strata filling shallow extensional basins, and minor volcanism distributed throughout northwestern Colorado coincide roughly in age with RGR development further south. In northwestern Colorado, these include the Middle and North Park basins, which lie between the Rocky Mountain Front Range to the east and the Park Range to the west. Middle and North Park are separated by the Rabbit Ears range, a late tertiary intermediate to mafic volcanic complex. Volcanism is more diffuse west of the Park range, consisting mostly of small volcanic centers. Previous studies of the RGR crustal-scale structure have focused primarily on the well-developed rift basins that form the physiographic rift in southern Colorado and New Mexico. In this work, previously collected gravity data in northern Colorado is complemented by a new gravity survey west of the Park Range. This is an area of diffuse extensional tectonism with scattered small mafic and intermediate volcanic centers. These gravity data fill gaps in the existing dataset with a nominal station separation of 2.5 km. The combined dataset is used to create a new free air gravity grid covering northwestern Colorado. The gravity data are used to construct profiles that constrain the amount and spatial distribution of extension in the region, how that is accommodated by crust and lithosphere thinning, the volume and spatial distribution of syn-rift igneous rocks, and major lateral density variations occurring in the upper mantle.