Tensor CSAMT survey over the Sulphur Springs thermal area, Valles Caldera, New Mexico, United States of America, Part I: Implications for structure of the western caldera

Abstract

An extensive tensor controlled‐source audiomagnetotelluric (CSAMT) survey has been carried out over the Sulphur Springs geothermal area, Valles Caldera, New Mexico. Forty‐five sites were acquired using two crossed transmitter bipoles placed approximately 13 km south of the center of the survey. The soundings in the Sulphur Springs area were arranged in four profiles to cross major structural features. To curtail spatial aliasing, the electric bipoles along each profile were deployed contiguously. The frequency range of acquisition was 4096 Hz down to 1 Hz for the central line, but only down to 4 Hz for most sites of the other lines. CSAMT and magnetotelluric (MT) data taken outside Valles Caldera were constrained by drill logs and imply resistive Bandelier Tuff, underlain by conductive Paleozoic sediments, and further underlain by resistive, primarily Precambrian crystalline rocks. Model cross‐sections within the caldera were derived using 2-D parameterized inversion constrained by drilling, with layered‐earth inversion for starting models. Southeast of the Sulphur Creek fault, the upper 200 m of the section are of relatively low resistivity and correspond to unconsolidated land‐slide and debris flows. The Bandelier Tuff below exhibits higher but variable resistivities because of alteration controlled by local faulting. Beneath the Bandelier Tuff, the Paleozoic sedimentary layer is only moderately less resistive than it is outside the caldera, with the lowest values occuring northwest of Sulphur Creek. Its low resistivity per se does not necessarily represent a hydrothermal aquifer. The Sulphur Creek fault appears to be a locus of substantial change in structural relief; up‐throw of stratigraphy and basement to its west appears to be about 400–500 m. A major normal fault down to the southeast is located under the topographic expression of Freelove Canyon, which is up to 1 km farther southeast than suggested by previous geologic sections. High resistivities possibly corresponding to a vapor zone in the upper 500 m near VC-2B and VC-2A are not consistent with the CSAMT data.