MAPPING SUBSURFACE STRUCTURE AND AQUIFERS IN THE RIO GRANDE RIFT— AEROMAGNETIC AND GRAVITY INTERPRETATIONS OF THE SOUTHERN ESPAÑOLA BASIN, NORTH-CENTRAL NEW MEXICO

The Española basin is located between the right-stepping, en echelon Albuquerque and San Luis segments of the Rio Grande rift in north-central New Mexico. Between the Santa Fe Range on the east and the Jemez Mountains on the west, the southern Española basin is represented by a westward-thickening wedge of rift sediments that serve as aquifers for the city of Santa Fe and surrounding areas. Regional gravity data show that the deepest parts (>2 km) of the Española basin are farther west under rocks related to the syn-rift Jemez volcanic field. To better understand ground-water resources in areas of critical need, detailed aeromagnetic surveys were flown over the southern Española basin and adjoining Santa Fe embayment on the southeast side. In the embayment, magnetic pre-rift and early rift volcanic and volcaniclastic rocks directly underlie less magnetic rift sediments. Aeromagnetic data indicate an extensive, shallowly buried (<75 m) bedrock platform, interrupted by two paleochannels(?) up to 150 m deep. The buried platform helps explain why water wells in this area commonly have poor productivity. In a few exceptional areas the magnetic rocks appear to be absent, suggesting a direct hydrologic connection between syn-rift and pre-rift sedimentary aquifers at depth. Southwest of Santa Fe, magnetic analysis indicates that aquifer thickness increases dramatically north of the platform across a curvilinear hinge line, where the magnetic rocks drop from ~600 m to ~2 km depth across a distance of ~4 km. Below this north-dipping decline, seismic interpretations by other workers indicate that southward-dipping Paleozoic-Mesozoic sedimentary units are unconformably truncated. Gravity modeling suggests these truncations follow a northwest trend, possibly marking the southernmost limit of Laramide uplift. North of Santa Fe, the buried basement surface undulates from moderate westward dip (~20^o) at the mountain front, to shallow dip (~5^o), and back to moderate dip 10-12 km west of the front. Aeromagnetic data, supported by geologic mapping and seismic data, indicate that faults cutting the basin sediments form a NNW-trending zone that is partly coincident with steepening of the underlying basement. Drops in water level in nearby wells suggest that parts of this deformation zone may act as a barrier to ground-water flow.