GRAVITY MODELING OF NORTH-NORTHEAST TRENDING LOW-GRAVITY FEATURES AT THE WESTERN AND EASTERN COLORADO PLATEAU MARGINS

The western and eastern margins of the Colorado Plateau are topographically higher than the interior Colorado Plateau and the adjacent Basin and Range and Rio Grande Rift provinces. Both margins are associated with 100-200 km wide, north-northeast trending low Bouguer gravity features 25-35 mgals in amplitude. High topography is commonly associated with low gravity if elevations are Airy compensated; however observed crustal thickness under these margins is 5-7 km thinner than required by Airy isostatic compensation. We present gravity models along five west-northwest trending profiles (>1500 km length) across the Colorado Plateau and adjacent regions. Our models focus on the low gravity features at the western and eastern margins of the Colorado Plateau and constrain contributions due to upper crustal density structures inferred from geology and crustal thickness variations inferred from available seismic data. We find that upper crustal density structures and Moho depth variations do not fully explain the north-northeast trending Bouguer gravity low at the eastern and western margins of the Colorado Plateau. To explain the residual anomaly, we explore alternative interpretations in terms of mid- to lower-crustal and upper mantle density variations. We compare our results with seismic tomography and xenolith data to validate our models and arrive at a preferred interpretation for the structure of the western and eastern margins of the Colorado Plateau.