Elucidating the structural geometry and major faults of the San Marcial Basin, Socorro County, using total Bouguer gravity anomaly data
— Kyle K.. Gallant, Daniel J. Koning, Andrew P. Jochems, and Alex Rinehart

Gravity surveys are an effective way to aid in interpretation of sedimentary basins, allowing inferences to be made regarding subsurface bedrock depth and geologic structures. A gravity survey was conducted in the San Marcial Basin, Socorro County, New Mexico, over the summer of 2021 in order to clarify the structural geometry and major faults of this basin. The San Marcial Basin was formed as part of the Rio Grande rift, which produced several extensional sedimentary basins from southern Colorado through New Mexico. The topographic basin is covered by a wide blanket of Palomas Formation clastic sediment that is minimally faulted, so field mapping only gave hints of the basin’s structural geometry and potential the major geologic structures. In this study, 89 new gravity measurements were tied to National Geospatial-Intelligence Agency (NGA) benchmarks. At each location, gravity and GPS measurements were taken, with GPS coordinates measured to a resolution of 5 cm vertical and horizontal. Interpreting the processed data indicates a 10–20 km wide region of deeper basin fill (widening to the north) centered near the axial river of the rift, the Rio Grande. A 3–5 km wide arm of deep basin fill extends northeastward under Mesa del Contadero. There is weaker evidence for two gravity-low, embayment-like features that extend northward from the wider gravity-low centered on the Rio Grande. One of these gravity-low embayments extends northwards ~5 km between the southern Chupadera Mountains and southern Magdalena Mountains, corresponding to the southernmost Highland Springs topographic embayment, and the other appears to approximately follow NM State Road 107 for ~15 km northwest of its intersection with Interstate 25 (I-25). Analyzing the processed data also indicates two notable, linear, total Bouguer gravity anomalies in the south-central and eastern part of the basin. The western anomaly 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 anomaly is a pronounced west-down gravity gradient that strikes NNE, which we interpret as a west-down fault. This eastern anomaly, here named the Lava fault, coincides with a laid-back escarpment east of the Rio Grande. The Bison Corral well in the footwall of the fault and the subsurface stratigraphy of the Pope boring within the Lava fault zone, indicate shallow Cretaceous bedrock (15 m in the Bison Corral well) east of the Lava fault, providing 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 gravity low immediately east of the north end of the Fra Cristobal Mountains; more data are needed to constrain the location of the Lava fault in that area. 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 to the south, may possibly be due to a northward partitioning of extensional strain from the Walnut Springs fault to the Black Hill and Lava faults. In this hypothesis, which remains to be tested by numeric modeling, 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.