⁴⁰Ar/³⁹Ar ages of selected basalts in the Sierra Cuchillo and Mud Springs Mountains, Sierra and Socorro counties, New Mexico
— Virginia T. McLemore, Matthew Heizler, David W. Love, Colin Cikoski, and Daniel J. Koning

Basalt flows have been mapped throughout the Sierra Cuchillo and in the Mud Springs Mountains. These basalts are typically small volume flows that are interbedded with sedimentary rocks. Before this study, many of these basalts were inferred to be Pliocene, based mostly on appearance and mineralogy compared to that of Pliocene basalts dated by K/Ar methods. Ten basalt flows were dated by the ⁴⁰Ar/³⁹Ar method in an effort to understand the depositional and tectonic evolution of the Monticello and Winston grabens. The basalt flows in the Cañada Alamosa and Spring Canyon areas of the Sierra Cuchillo range in age between ~18.0 and 18.7 Ma, with typical errors of 0.05 to 0.3 Ma. The Priest Tank basalt flow is similar in age at 19.06 ± 0.05 Ma relative to the stratigraphically equivalent basalt flows in Cañada Alamosa and Spring Canyon to the north. These basalts, which are interbedded with Neogene sedimentary deposits, indicate that the Monticello graben was undergoing sedimentation at ~18-19 Ma, probably in conjunction with tectonic subsidence. The andesite of Winston graben also is ~18 Ma (K/Ar). Two separate eruptions formed the basalt cap on Tabletop Mountain near Winston at 4.23 ± 0.36 and 4.91 ± 0.03 Ma. The Tabletop Mountain basalt places a minimum age on the end of deposition of the Santa Fe Group sediments in the Winston graben. These ⁴⁰Ar/³⁹Ar results indicate two episodes of basalt magmatism in the Sierra Cuchillo, at 19-18 Ma and again at 4.9 - 4.2 Ma, with the first episode occurring during a time of sparse volcanism in the rift. The Mud Springs Mountains basalt flow is 5.55 ± 0.21 Ma. Chemical analyses of selected basalts indicate that they are alkaline basalts.