New Mexico Geological Society Annual Spring Meeting — Abstracts

Exposures of Proterozoic rocks within the core of the Joyita Hills display a multitude of planar fabrics, including ductile mylonite zones, amphibolite dikes, pegmatites and aplite dikes. Mylonite zones are high-angle to vertical, and form three common trends: North, northwest, and east-northeast. North-striking and east-northeast-striking mylonites are most common. Brittle reactivation of mylonite trends is either known to have occurred or can be logically concluded to have occurred during each major Phanerozoic orogenic event.

Ancestral Rocky Mountain deformation resulted in two normal fault trends. Through direct field observation, north-striking brittle faults are known to be directly superimposed along north-striking mylonites. Northwest-striking brittle faults are also thought to repreI sent reactivation of northwest-striking mylonites, in that both brittle faults and mylonites are comparably oriented.

Orientations of Laramide wrench faults also show similar attitudes with respect to the ductile basement fabrics. North-striking (synthetic) and east-northeast-striking (antithetic) strike-slip faults define a north-trending, dextral wrench fault system. Although less wellI defined, northwest-striking strike-slip faults have also been observed, and are thought to represent the antithetic shear orientation associated with right-lateral (synthetic) displacement along the northeast-trending Montosa fault zone.

Mylonite fabrics are also thought to have been influential during the Tertiary development of the Rio Grande rift. A well-developed set of conjugate normal faults and mafic dikes strike to the northwest. A less well-developed set of conjugate normal faults and mafic dikes strike to the east-northeast. North-striking mylonites have been observed along the trace of the East Joyita fault and apparently influenced both the location and orientation of this major, down-to-the-east normal fault (-3 km stratigraphic throw).