New Mexico Geological Society Annual Spring Meeting — Abstracts

Numerous low-to high-angle normal faults offset Tertiary volcanics and older sedimentary sequences of the Joyita Hills (north-central Socorro County). Regional bedding strikes northnortheast and dips 25°-30° west-northwest. Comparative analysis of fault orientations as both pre-tilt and post-tilt structures indicates that most faults were originally pre-tilt, high angle (60°-65°) normal faults. These faults form three distinct fault systems, which respectively strike to the north-northeast, northwest and east-northeast.

Other normal faults were not originally high-angle. These faults occur as shallow (0°-15°), east-dipping faults. Consideration of these faults as pre-tilt structures indicates that they could not have been steeper than 30°-45°. Some of these faults exhibit microbrecciated fault surfaces (4-5 cm thick) and are overlain by 0-30 m of tectonic melange, as along the southern exposure of the East Joyita fault. These faults are interpreted as extensional detachment faults that accommodated rotation and developed contemporaneously with rotation. Rotation of bedding and original high-angle faults produced domino-style faulting within extensionally detached terrain.

Allochthonous blocks, composed of Oligocene volcanic strata and older rocks form the upper plates with respect to these low-angle detachment faults. Bedding, fault and fault striae orientations indicate that the hanging wall displacement direction was to the east and east-southeast. Detachment faults, allochthonous blocks and tilted and eroded volcanic strata as young as the South Canyon tuff (27 Ma) are overlain by unfaulted, subhorizontal basin fill deposits of the Sierra Ladrones Formation of the Santa Fe Group (approximately 5 Ma).
Therefore, this (perhaps dominant) phase of Rio Grande rift tectonism is broadly constrained as an Oligocene-Miocene event.