GEOPHYSICAL ANOMALIES AND NEW CONSTRAINTS ON BURIED STRUCTURES, EAST-CENTRAL SAN LUIS BASIN, COLORADO AND NEW MEXICO

Preliminary interpretations of recently collected high-resolution aeromagnetic data and ground-based gravity data are combined with subsurface constraints from magnetotelluric soundings, well log, and limited seismic data in the east-central San Luis basin. The region is characterized by widespread Miocene-Pliocene volcanic rocks interbedded with sediments of the Santa Fe Group, all related to Rio Grande rift extension. These units appear to lie directly on Precambrian basement, which is thought have been deformed along largely preexisting structures, leaving behind variably sized horst blocks within the basin. Depth estimates derived from aeromagnetic anomalies show discrete sub-regions with distinct structures, including a horst and graben near the eastern basin margin, extensions of surficial volcanic units into the subsurface, buried faults, and the general rift architecture (e.g. configuration of crystalline rocks and faults) of the shallower portion of the basin. Gravity models using new data and constraints from recently uncovered vintage seismic data reveal complex relations between modern horst blocks and older basement uplifts. A large N-S trending basalt-capped mesa near the Colorado-New Mexico border appears from gravity and magnetotelluric data to be partially cored by basement rocks and contains a strongly magnetized Miocene volcanic section overlain by Pliocene basalt flows that display variable magnetic polarity. Relations observed on the surface of this horst are extended into the subsurface of the basin in order to better understand the region's stratigraphy and deformational history.