The Winston graben is located on the western margin of the Rio Grande rift in south-central New Mexico in an area of intense Neogene faulting. It is a symmetrical graben, 5 to 10 km wide, approximately 56 km long, and trends north-northwest to north-northeast. The structure is bounded by high-angle normal faults with about 2 km of stratigraphic separation across both margins. Most of its eastern boundary fault is believed to be a reactivated Eocene strike-slip fault. Northeast-trending accommodation zones (structural highs) terminate the graben at both north and south ends. Rocks exposed within the graben include Pennsylvanian and Permian sedimentary formations, several Eocene-Oligocene volcanic and volcaniclastic units, the upper Oligocene-Quaternary Santa Fe Group, a Miocene andesite flow that is intercalated with the Santa Fe Group, and a Pliocene basalt flow. Some of the volcanic units occur only within the Winston graben. Initial development of the graben began in the late Oligocene, but most of its growth was in the Neogene. Boundary faults of the Winston graben have been inactive for at least the past 4.8 m.y. When placed in a tectonic setting with surrounding structures of the Rio Grande rift, the Winston graben is interpreted as having formed over a deep detachment surface.