LARAMIDE MAGMATISM AND EXTENSIONAL EXHUMATION RECORDED IN MIOCENE METAMORPHIC CORE COMPLEXES IN WEST-CENTRAL ARIZONA

Pioneering field studies by Jon Spencer and colleagues at the AZGS established west-central Arizona as one of the world’s type localities for detachment faulting and large-magnitude extension. These studies formed a foundation for numerous subsequent research projects, including our own. Here we present results from geologic mapping, geo/thermochronology, and microstructural analysis to address the latest Cretaceous to Paleocene (Laramide) magmatic, metamorphic, and structural evolution of crystalline rocks in the footwall of Miocene detachment faults in west-central Arizona. Miocene greenschist-facies shear zones exposed in the Buckskin-Rawhide, Harcuvar, and northern Plomosa Mountains metamorphic core complexes overprint amphibolite-facies shear zones with the same geometry and top-NE sense of shear. Based on systematic Paleocene to Eocene 40Ar/39Ar hornblende cooling ages and a ~63 Ma U-Pb zircon age of a synkinematic dike, we interpret these amphibolite-facies mylonitic fabrics to record Laramide-age extension. Laramide fabrics are spatially associated with 74–62 Ma leucogranite, whereas Miocene fabrics are mostly localized in early Miocene plutons and within a <300 m-thick zone below the detachment fault. Laramide proto-core complex extension is consistent with significant pre-Miocene exhumation inferred from: a) the presence of Orocopia Schist at a paleodepth of ~3-4 km below the ~21 Ma surface in the northern Plomosa Mountains, and b) ~77-65 Ma lower crustal metamorphism in the central Harcuvar Mountains recorded by monazite. In the Harquahala Mountains core complex, ~68-71 Ma U-Pb ages of synkinematic titanite directly date top-SW/dextral shear along a SE-dipping shear zone in ~76 Ma leucogranite. This dextral shear could represent a transfer zone at the lateral margin of the top-NE Laramide shear zone that is well developed in the adjacent Harcuvar Mountains. Leucogranite magmatism across the west Arizona core complex belt peaked at ~68-70 Ma, which corresponds to subduction of Orocopia Schist now exposed in the northern Plomosa Mountains. Farallon plate subduction beneath thickened crust of the Maria fold-thrust belt may have triggered dehydration of the schist and slab, resulting in widespread crustal melting and gravitational collapse of the region during the Laramide orogeny.