NEW INSIGHTS INTO BASIN GEOMETRY AND RANGE-FRONT FAULTING FROM RECENT GEOPHYSICAL SURVEYS IN NORTHEASTERN SAN LUIS BASIN, RIO GRANDE RIFT, COLORADO

New aeromagnetic data combined with geologic mapping, gravity modeling, and LiDAR data provide insights into rift structure where the San Luis Basin constricts to a narrow valley on the north and along the master Sangre de Cristo fault system on the east. Geophysical evidence suggests that a major, buried NE-striking fault system contributes to the narrowing of the northern San Luis Basin and cross-cuts the N-S trending intra-basin Alamosa horst at its northern end. The fault system in turn is cross-cut by more northerly striking rift faults at Mineral Hot Springs.

The Sangre de Cristo range front can be generally divided on the basis of structural, geomorphologic, and geophysical character into three sections from south to north between Great Sand Dunes National Park and Poncha Pass. In the northern section, both aeromagnetic and LiDAR data show multiple NNW- to NW-striking strands of the Villa Grove fault zone that cross-cut more northerly, linearly extensive faults. The LiDAR data indicate the faults offset Pleistocene fans; the aeromagnetic data suggest the faults also displace older basin-fill deposits. In the central section, the aeromagnetic data show isolated highs along the range front that correspond to exposures of Proterozoic basement within the upper plates of Laramide thrusts. In detail, the aeromagnetic character varies abruptly at the mapped range front fault, but large vertical displacement is not indicated. Instead, geophysical data suggest that the basin floor slopes less than 45 degrees into the basin for about 4-5 km before dropping more steeply into the deeper basin. It is unclear whether the gentle slope represents a low-angle detachment or an older thrust surface that is gradually down-dropped into the basin. Enigmatic NE-striking aeromagnetic linear features perpendicular to the range front may provide clarity after further geologic and geophysical investigation. In the southern segment, a separate Laramide thrust package is exposed at the range front that is dominated by WNW-striking structural trends that are well expressed in the aeromagnetic data. The Laramide structures extend into the basin but are cut by normal faults to form an apparent segmented, 2-3-km-wide structural bench rather than low-angle fault surfaces as previously proposed.