REVEALING THE STRUCTURAL AND VOLCANIC FRAMEWORK FOR UNDERSTANDING GROUND WATER RESOURCES IN THE NORTHERN RIO GRANDE RIFT: PRELIMINARY RESULTS OF A HIGH-RESOLUTION AEROMAGNETIC SURVEY OVER TAOS, NEW MEXICO

High-resolution aeromagnetic surveys have been used successfully to reveal faults and buried basalts that are important for understanding geologic controls on ground water resources in several basins of the north-central Rio Grande rift. Following on this success, high-resolution aeromagnetic data were recently collected in an area surrounding the Town of Taos, New Mexico, to ultimately aid ground-water resource management. The survey area is located in a populated area of the Taos embayment, a structurally complex reentrant in the southeastern portion of the San Luis Basin segment of the northern Rio Grande rift. Unlike previous surveys, the Taos data were collected from helicopter rather than fixed-wing aircraft to target the range-front fault system, which required some low flying over mountainous terrain. Flight lines were oriented east-west with nominal terrain clearance of 150 m and spaced 200 m apart. Preliminary interpretation of the data provides an initial view of the subsurface configuration of basement structural blocks and buried basalts. Basement blocks step down into the basin along faults that parallel the general NE to N strikes of range-front faults that form the Taos embayment. Servilleta Basalt, a series of geochemically unique flows related to the Pliocene Taos Plateau volcanic field to the west, is shallowly buried just north and west of Taos. Aeromagnetic patterns indicate that basalt nearest the surface carries strong, reversed-polarity remanence, which has implications for correlation of basalt flows encountered in wells. Faults interpreted to cut basalt follow the same N and NE strikes as those that offset basement. Major streams follow this pattern as well. The intersection of the N- and NE-striking faults coincides with a spring discharge area. Fault displacements within basalt on the west and within basement on the east are interpreted as east-down and west-down, respectively. These interpretations suggest that sediment-filled grabens generally follow major stream valleys to the southwest, west, and north of Taos.