27 Cordilleran Core Complexes and Arizona Geology II: A Celebration of Jon Spencer's Career

Wednesday, 16 May 2018
10:20 AM - 12:05 PM
Canyon Room

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

Paper number: 27-4
Program: Symposia Sessions
Time: 11:05 AM – 11:25 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-314196

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