Description
During the summers of 2000-2003, 79 magnetotelluric soundings were recorded during the Summer of Applied Geophysical Experience (SAGE) program in tbe Rio Grande rift spanning approximately 9.5 km in an east-west transect across the La Bajada Fault within the Santo Domingo Basin. The main purpose of studying this region of the Rio Grande rift was to obtain a better understanding of the shallow (upper ~3 km) geological structure along the eastern margin of the Santo Domingo Basin. The overall geoelectric strike direction obtained from magnetotellurics is 45 degrees east of north and is coincident with the trends of the Santa Anna accommodation zone and the large complete Bouguer anomaly gravity low prevalent throughout the Santo Domingo Basin. Mapped intrabasinal faults are mostly orthogonal to the northeastern trend seen in the magnetotellurics and gravity results. Faults not detected by surface mapping were inferred from aeromagnetic data as small linear anomalies. Two Tertiary monzonite intrusives appear as prominent resistive zones in the magnetotelluric results. The intrusives also correlate with gravity and magnetic anomalies, proprietary seismic data and borehole information. Detailed mapped sections within the Santa Fe River canyon reveal approximately 1.3 km of the stratigraphic sequence of Tertiary sediments while three regional exploratory water wells estimate less than 1.5 km of Paleozoic and Mesozoic sediments in the vicinity of the Santa Fe River canyon. The basin resolved by the magnetotellurics between the two monzonite intrusives shows definite thickening westward. By utilizing the point at which the rate of inverted resistivity change is greatest with depth, a trio of differing resistive boundaries is apparent delineating the offset of the La Bajada Fault and an unnamed intrabasinal fault of 200 and 1400 m, respectively. The distribution of resistivities along the MT profile within the basin reveals: (1) an uppermost, thickening westward wedge of relatively high resistivity (~ 10 – 40 Ω-m), (2) a deeper, more conductive zone of <5 – 10 Ω-m, and (3) an underlying resistive basement where values exceed 1000 Ω-m. The preponderance of geological and geophysical evidence supports the interpretation that these three zones represent: (1) a surficial wedge of Tertiary sands and gravels containing the freshest water and/or least amount of conductive clay (the maximum thickness of this aquifer is ~ 0.5 km at the western end of the MT profile), (2) a relatively poorer quality, water-saturated zone with salty water and/or high clay content composed of faulted Mesozoic and Paleozoic sedimentary rocks, and (3) the faulted, crystalline Precambrian basement complex with a maximum depth of ~ 2.5 km on the western end of the profile.
