DETAILED GEOLOGIC MAPPING OF MYLONITIC UNITS IN THE FOOTWALL OF THE PLOMOSA DETACHMENT FAULT, WEST-CENTRAL ARIZONA

New 1-10,000-scale geologic mapping reveals that the footwall of the Plomosa detachment fault is dominated by early Miocene mylonitic fabrics associated with detachment faulting, demonstrating that the northern Plomosa Mountains is a metamorphic core complex similar to adjacent core complexes in the region. Mylonitic fabrics have systematic NE-SW-trending lineations (average T/P = [220/09]), and record a top-NE sense of shear. The Plomosa detachment fault dips ~10–30° ENE along most of the range, but dips shallowly N along its northern extent, defining a broad NE-trending corrugation parallel to its slip direction. NW-striking normal faults, joints, and dikes throughout the footwall record NE-SW extension consistent with that of the mylonitic fabrics and detachment fault.

Three mylonitic units dominate the footwall of the Plomosa Mountains core complex: 1) Orocopia Schist, 2) an early Miocene Intrusive Complex, and 3) layered gneiss. The quartzofeldspathic Orocopia Schist encompasses 10.5 km² of the northern Plomosa Mountains, and locally contains graphitic plagioclase porphyroblasts. We mapped 24 occurrences of course-grained green actinolite pods up to 1.5 m-wide within the Schist. These pods are high in Mg, Cr, and Ni, suggesting derivation from an ultramafic protolith. A laterally continuous unit of hornblende amphibolite (~3.5 km long, 10–150 m thick), interpreted as Orocopia metabasalt, is localized along a moderately SE-dipping contact between the Schist and the gneiss, and contains metachert layers 3–30 cm thick. L>S mylonitic fabrics are common throughout the Schist and gneiss, and a zone of L-tectonites is localized along their contact. The Intrusive Complex, which is dominated by biotite granodiorite and hornblende diorite, parallels the detachment fault along its eastern margin.

Late-stage dextral- and sinistral-normal faults strike N and ENE, respectively, and associated barite and carbonate veins strike NE. These faults locally cut moderately-consolidated colluvium deposits, and veins cut NW-striking joints and faults associated with detachment faulting. Together these structures record a minor phase of NW-SE extension, which we attribute to regional post-middle Miocene dextral faulting, with the northern Plomosa Mountains having occupied a transtensional releasing bend.