ELUCIDATING THE STRUCTURAL GEOMETRY AND MAJOR FAULTS OF THE SAN MARCIAL BASIN, SOCORRO COUNTY, USING COMPLETE BOUGUER GRAVITY ANOMALY DATA

We conducted a gravity survey in the San Marcial Basin, Socorro County, New Mexico, over the summer of 2021 to clarify its structural geometry and major faults. The San Marcial Basin is one of several extensional sedimentary basins formed by the Rio Grande rift. We measured 89 new gravity points, in tandem with GPS, relative to absolute gravity benchmarks. We interpret the mapped complete Bougeur gravity anomaly and an anomaly with regional bedrock trends removed as showing a 10–20 km wide region of deeper basin fill in the central basin, as well as a 3-5 km wide arm of deep basin fill extending northeastward under Mesa del Contadero. There is weaker evidence for two gravity-low, embayment-like features that extend north-ward from the wider central gravity-low. We found two linear gradients in the complete Bouguer gravity data and bedrock-detrended anomalies in the south-central and eastern part of the basin. The western gradient is correlated to the down-to-the-east Black Hill fault, which strikes NW and forms fault scarps on select middle Pleistocene surfaces. The eastern gradient is a pronounced west-down gravity gradient that strikes NNE, which we interpret as a west-down fault. This eastern gradient, here named the Lava fault, coincides with an escarpment east of the Rio Grande. The Bison Corral and Pope well indicate shallow Cretaceous bedrock near the lava fault support that this eastern gradient is due to vertical displacement of bedrock rather than juxtaposition of two different bedrock types. The Lava fault projects southwards to a location about 5 km east of the northern end of the Fra Cristobal Mountains, separating the shallow Cretaceous bedrock from a poorly constrained gravity low immediately east of the north end of the Fra Cristobal Mountains. The lack of noteworthy footwall uplifts along either the Black Hill or Lava faults, especially when compared to the prominent Fra Cristobal Mountain footwall uplift on the Walnut Springs fault, may possibly be due to a northward partitioning of extensional strain from the Walnut Springs fault to the Black Hill and Lava faults. If this hypothesis is correct, then it indicates that lowered vertical displacement rates on either fault resulted in low footwall uplift rates that could not outpace long-term erosion rates or burial by sedimentation.