NEW AEROMAGNETIC DATA FOR THE CENTRAL SAN LUIS BASIN GIVE INSIGHTS FOR IMPROVING REGIONAL GROUND-WATER FLOW MODELS, SOUTHERN COLORADO AND NORTHERN NEW MEXICO

High-resolution aeromagnetic data recently collected over the central San Luis basin within the Rio Grande rift provide new insights on the locations of faults, subsurface composition of aquifers, and presence of buried igneous rocks. These insights will significantly improve regional ground-water flow models that are used to manage water resources in the southern San Luis Valley in southern CO and the Taos Plateau in northern NM. In late 2004, aeromagnetic data were acquired over 4400 sq km of the basin between Alamosa, CO, and Arroyo Hondo, NM. The survey was flown along east-west lines spaced 250 m apart with a nominal terrain clearance of 150 m. The new data in CO support the existence of a long-suspected, major NE-SW fault that generally traverses the basin along the NW margin of the San Luis Hills. However, rather than a discrete trace, the fault is imaged as irregular steps dividing a fundamental difference in subsurface aquifer composition, from primarily porous sediments and minor basalt on the NW, to primarily fracture-prone crystalline rocks on the SE. The aeromagnetic map also reflects differences in subsurface character within the latter area, inferred as (1) uplifted Precambrian basement and Tertiary intrusions underlying pre-rift volcanic deposits in the western San Luis Hills, (2) mostly buried, presumably syn-rift lava flows interbedded with sediments on the SE, and (3) primarily pre-rift volcanic and volcaniclastic deposits in the NE San Luis Hills and partially buried syn-rift flows farther east. Northwest of the San Luis Hills, the aeromagnetic data indicate that syn-rift Servilleta basalt has shallow NNE dip, extending from exposures near Antonito below younger sediments nearly to Alamosa. A previously unrecognized N-S fault bounds the basalt on the west. Other exposed N-S faults within and east of the San Luis Hills coincide with several long N-S aeromagnetic lineaments. The new data in NM display negative and positive anomalies that reflect the variable ages and magnetic polarities of the syn-rift volcanoes of the Taos Plateau. N-striking Quaternary faults in Sunshine Valley correspond to differences in aeromagnetic character that can be explained by the subsurface juxtaposition of volcanic rocks on the west against fine-grained sediments on the east, a major hydrogeologic contrast.