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Reconnaissance mapping indicates that parts of nine mountain ranges previously considered to be Precambrian basement are instead variations of Tertiary metamorphic core complexes. From southeast to northwest, these ranges include the Pinaleno, Picacho, South Mountains, parts of the Buckeye, White Tank, Harquahala, Harcuvar, Buckskin, and Rawhide Mountains. Together with the already recognized Santa Catalina–Rincon–Tortolita complex, these ranges define a broad northwest-trending belt through Arizona.

The northeast-trending Buckskin-Harcuvar-Harquahala Mountains are transverse foliation arches, the latest expression of a huge, northwest-elongated metamorphic area herein named the Harcuvar metamorphic core complex. This Tertiary phenomenon is superimposed on an ill-defined center of late Mesozoic metamorphism. Traverses into the complex from its unmetamorphosed southwestern margin reveal progressive Cretaceous conversion of Mesozoic sedimentary rocks into migmatites. Metamorphism just preceded intrusion of the Tank Pass batholith, an Upper Cretaceous pluton which itself became foliated and involved in early Tertiary migmatization and intrusion in the Harcuvar Mountains.

A marginal zone of penetrative mylonitization, capped by a more brittlely deformed dislocation surface, flanks the Harcuvar complex on its upper and broadly arcuate northeastern margins. Resting on this tectonic surface are highly tilted, unmetamorphosed, layered rocks (Paleozoic to Tertiary). Geochronologic and geologic data place the time of mylonitization as Tertiary, perhaps as recently as 25 to 20 m.y. B.P. This deformation (flattening and northeast-southeast extension) was closely followed by development of chlorite breccia, the dislocation surface, thick wedges of coarse clastic sediment, and listric faulting. Finally, the core complex was arched and uplifted.

A model for this sequence of events is predicated on mobile northeast-directed extension of a flat upper-crustal layer facilitated by intense mid-Tertiary plutonism in an actively tensile stress field. Tectonism in a brittle, surficial upper plate is governed by listric faulting and detachment as the plate fragments and extends “piggyback” style upon subjacent, ductilely stretched layer.

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