Geophysics of the Rio Grande Basins
San Luis Basin Geophysics
Central San Luis Basin
This study used recently acquired geophysical data to interpret the configuration of tectonic elements present along an east-west transect through a portion of the San Luis Basin. The main goal of this study was to determine how the Rio Grande rift is manifested within the central San Luis Basin, including the following specific goals: estimate the thickness of rift-fill sediments that are critical hosts of groundwater supplies; determine what the rift-sediment thickness distribution indicates about basin structure and temporal pattern of rifting; evaluate the distribution of syn-rift volcanic rocks and implications for the history of rifting; interpret the extent and structural configuration of pre-rift rocks and their relationship to the rift; and better delineate rift-bounding structures than surface geologic mapping is able to do alone.
More detailed information on the Central San Luis Basin study can be found in Drenth, 2009.
Simplified geology of the central San Luis Basin (see version in text for data sources).
Isostatic residual gravity anomaly map of the central San Luis Basin. Selected linework from geologic map included. Location of profile A-A' shown. Abbreviations: MVG, Monte Vista graben; TP, Taos Plateau; SLH, San Luis Hills; CP, Costilla Plains; SV, Sunshine Valley; SDC, Sangre de Cristo Mountains; SPM, San Pedro Mesa; SG, Sanchez graben; SL, San Luis; CR, Culebra Range. Gravity data were used primarily to investigate basin depth because gravity lows are commonly associated with large thicknesses of low-density sediments.
Perspective view of central San Luis Basin and gravity inversion results. Lower image shows view of basin with Santa Fe Group (syn-rift) sediments removed.
Reduced-to-pole total field aeromagnetic anomalies over the central San Luis basin. Selected linework from geologic map included. Location of profile A-A' shown. Same abbreviations as shown on gravity map. Aeromagnetic anomalies primarily reflect the magnetic properties (including remanent polarity) of pre- and syn-rift volcanic rocks.
Simplified geologic model along profile A-A'. Upper panel: gravity; middle panel: aeromagnetics; lower panel: geologic model. Geologic units as shown on simplified geologic map. N/R refers to local interpretations of overall magnetic polarity of the Servilleta basalt. Abbreviations as in previous figures. Note greatest thickness of Santa Fe Group sediments in the Sanchez Graben, and smaller accumulations under the Costilla Plains and Monte Vista graben. The ridge of Precambrian rock under San Pedro Mesa separates the Sanchez graben from the Costilla Plains structural depression. Depths of Servilleta basalt taken from aeromagnetic depth estimates and well data.
Conclusions: Syn-rift Santa Fe Group sediments have a maximum thickness of ~2 km in the Sanchez graben near the eastern margin of the basin along the central Sangre de Cristo fault zone. Under the Costilla Plains, a range of thicknesses of 600 m to 2 km are geophysically reasonable. Santa Fe Group sediments also reach a thickness of nearly 1 km within the Monte Vista graben near the western basin margin along the San Juan Mountains. A narrow, north-south-trending structural high beneath San Pedro Mesa with about 2 km of positive relief with respect to the base of the Sanchez graben separates the graben from the structural depression beneath the Costilla Plains. Geophysical data provide new evidence that this high is rooted in the Precambrian basement. Major faults in the study area include significant vertical offsets (> 1 km) of Precambrian rocks along the central and southern zones of the Sangre de Cristo fault system. Other faults with similarly large offsets of the Santa Fe Group include a fault bounding the western margin of San Pedro Mesa, and other faults that bound the Monte Vista graben. A major north-south-trending structure interpreted to be a down-to-the-east normal fault or fault zone occurs at the boundary between the Costilla Plains and the San Luis Hills structural high. Aeromagnetic anomalies are interpreted to mainly reflect variations of remanent magnetic polarity and burial depth of the 5.3-3.7 Ma Servilleta basalt of the Taos Plateau volcanic field. Magnetic-source depth estimates indicate patterns of subsidence following eruption of the basalt and show that the Sanchez graben has been the site of maximum subsidence.