MULTISTAGE EXHUMATION OF OROCOPIA SCHIST AT THE NORTHERN PLOMOSA MOUNTAINS METAMORPHIC CORE COMPLEX, WEST-CENTRAL ARIZONA

The northern Plomosa Mountains are a Miocene metamorphic core complex in the lower Colorado River extensional corridor in west-central Arizona. The range is bound by the Plomosa detachment fault, which defines a single broad corrugation that plunges ~12° to the NE, parallel to its slip direction. Mylonitic fabrics comprise ~30 km² of the footwall and record consistent top-to-the-NE sense of shear. Three mylonitic units dominate the footwall of the Plomosa Mountains metamorphic core complex: 1) Orocopia Schist (~10.5 km²), 2) an early Miocene intrusive complex (~22-23 Ma), named the Northern Plomosa Intrusive Complex (~5 km²), and 3) gneiss. Orocopia Schist is juxtaposed against the gneiss along a moderately SE-dipping tectonic contact, which has been overprinted by Miocene mylonitization. The paleo-depth of the top of the Orocopia Schist near the inception of early Miocene detachment faulting was ~3–4 km, suggesting a major Paleogene exhumation event brought the schist to upper-crustal depths after it was subducted at 68–70 Ma. Three mylonitic extensional shear zones are recorded in the footwall: 1) a greenschist-facies Miocene shear zone parallel to the ESE-dipping limb of the detachment fault, 2) a detachment-discordant, upper greenschist- to amphibolite-facies Miocene shear zone that initiated with a gentle NE dip, and 3) an amphibolite-facies pre-Miocene (Paleogene?) shear zone that initiated with a moderate NE dip.

The Orocopia Schist of the northern Plomosa Mountains was exhumed within the Maria fold-and-thrust belt, where crust is thought to have been 50+ km thick following mid-Cretaceous shortening. However, Orocopia metabasalt lacks evidence of eclogite-facies metamorphism, suggesting the schist was never subducted to >45 km depths. The lack of high pressure metamorphism indicates that either: 1) there was no significant Cretaceous crustal welt, 2) the crustal welt was sheared off by the subducting Farallon plate, or 3) the Orocopia Schist relaminated and extruded into the middle crust before subducting beneath the welt. Paleogene exhumation of the Orocopia Schist in the northern Plomosa Mountains is consistent with extensional exhumation in the Gavilan Hills, which closely postdated schist subduction, suggesting subduction of schist may have triggered Paleogene extension in the region.