Gravity and aeromagnetic expression of tectonic and volcanic elements of the southern San Luis Basin, New Mexico and Colorado
— V. J. S. Grauch and G. R. Keller

The San Luis Basin has been characterized as an east-tilted, northerly trending rift basin containing a narrow deep graben on the east and a largely buried, intrarift basement horst on the west. We refine these concepts for the southern San Luis Basin by revisiting existing gravity and aeromagnetic data using modern methods. In the New Mexico portion of the basin, the narrow (6-12 km) Taos graben follows a general north-south trend from Costilla near the state line to the Embudo fault at the southern end of the basin. West of the Taos graben, the intrarift basement horst changes orientation from north south in Colorado to southwestward south of the state line, merging with the Tusas Mountains in a complicated manner that is not entirely understood. At the state line, the Taos graben widens northward to reach 25-30 km wide in the Culebra reentrant in Colorado, accompanied by en echelon stepping of the western graben border to the west. For nearly the entire length of the graben, the gravity-defined western border coincides with Quaternary faults and is followed or paralleled by the course of the Rio Grande. In the southernmost basin, the intrarift horst is absent. Instead, a broad semi-circular gravity low (Tres Orejas gravity low) of ambiguous origin lies west of the geophysically defined Taos graben. Along the southern border of the San Luis Basin, a strong gravity gradient follows the Embudo fault farther westward than previously proposed for a scissors fault configuration. South of the Embudo fault, north-south gradients in both gravity and aeromagnetic data correspond to the Picuris-Pecos fault system and suggest a related, parallel fault exists about 20 km to the west. Prominent circular aeromagnetic anomalies in the Taos Plateau can be associated with individual volcanoes and are used to infer the remanent magnetic polarities the rocks acquired when they cooled. A circular aeromagnetic anomaly near the Gorge bridge suggests a previously unknown volcano lies beneath the Servilleta Basalt. This information can be used to aid future geologic mapping of the Plateau and, in certain circumstances, can help constrain uncertain or multiple age dates for the volcanoes.