AIRBORNE GRAVITY GRADIENT MAPPING OF CONCEALED RANGE FRONT STRUCTURE, GREAT SAND DUNES NATIONAL PARK, RIO GRANDE RIFT, COLORADO

Gravity methods have critical importance for studying the geometry and structure of rift basins. High-resolution, helicopter-borne gravity gradient data acquired over Great Sand Dunes National Park in 2012 allow for detailed gravity analysis of range-front structure at the eastern margin of the San Luis Basin and Rio Grande rift. In this area, low density sediments, including a high elevation and high relief sand dunefield, overlie relatively high density Precambrian rocks at the margin between the San Luis Basin and Sangre de Cristo Mountains. Densities of the dunefield and adjacent Precambrian rocks were estimated using Nettleton-style analysis of gravity gradient and elevation profiles, resulting in estimates of 1800 kg/m³ and 2700 kg/m³, respectively. Terrain correction of the gravity gradient data using a density of 1800 kg/m³ effectively isolates the expression of buried basement structure beneath the dunefield. Using forward modeling of the gravity gradient data and locations of faults inferred from gravity gradient and aeromagnetic lineaments, we define several interpreted tectonic elements adjacent to the range front. A Precambrian bench buried 500 m to 1 km underlies most of the high dunefield and is truncated on its southwest margin by a northwest-trending, down to southwest normal fault zone. The basin deepens to ~2 km or more southwest of the fault zone and northwest of the Precambrian bench. Near the southern margin of the park, another concealed, yet shallow (<100 m depth) Precambrian bench is bounded by the aeromagnetically defined Blanca Piedmont fault zone along its western margin. A northeast-trending, down to northwest normal fault zone with ~600 m of relief estimated from 2D forward modeling controls the basin margin in the vicinity of Medano Creek in the southeastern part of the park. This fault zone may be the northeast extension of the Blanca Piedmont fault zone. Modeling demonstrates that several gravity gradient and aeromagnetic lineaments within the basin cannot be explained solely by faulting of Precambrian rocks. Their causative physical property contrasts likely arise from within the sedimentary section.