Petrologic data, U-Pb analyses of zircon, and 40Ar/39Ar analyses were used to document the tectonothermal history of two mylonite zones and the temporal relationship of mylonite formation to movement on detachment faults in the Pinaleño-Jackson Mountain metamorphic core complex of southeastern Arizona. Mylonite formation in the Pinaleño Mountains is dated at ca. 29 and ca. 19 Ma. The later event possibly is synchronous with, and structurally related to, the most recent movement on the Black Rock detachment fault (at <22 Ma) and the Eagle Pass detachment fault (at <25 Ma). The detachment surface on the northeast flank of the Pinaleño Mountains may have been active without significant changes in geometry for up to 10 m.y. Heterogeneous age distributions documented over relatively short distances in the lower plate of the Pinaleño-Jackson Mountain metamorphic core complex cannot be readily explained by a model of simple unroofing through a steady-state geotherm. Multiple recrystallization events during mylonite formation, and/or transient thermal events are required to explain the partial and complete isotopic resetting of thermochronometers in this area.

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