Lower plate rocks exposed in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, underwent a pronounced increase in cooling rate 7–12 m.y. after the onset of extensional tectonic denudation at ca. 27 Ma. A detailed 40Ar/39Ar study of lower plate rocks exposed near Planet Peak, in the southwest part of the core complex, indicates that the highest-level mylonitic rocks exposed in the complex cooled below 350 °C prior to 20 ± 1 Ma, at an average rate of ≤15 °C/m.y. Thereafter, the cooling rate of these rocks increased progressively, peaking at 280–80 °C/m.y. prior to initial exposure of the core between 15.1 and 13.3 Ma. Published K-Ar, 40Ar/39Ar, and fission-track ages for the most recently unroofed lower plate rocks, exposed at the northeastern end of the core complex, are consistent with a similar cooling history, although the abrupt increase in cooling rate occurred as many as 5 m.y. later in this area.

Because the increase in the cooling rate appears to have occurred progressively later toward the northeast, it is unlikely to reflect changes in slip rate. It appears to have coincided with cooling below 350–300 °C, and thus is interpreted to reflect steepening of the detachment system above the brittle-ductile transition. Thermochronological data from lower plate rocks in the Planet Peak area, together with constraints on their initial exposure at the surface, are consistent with an increase in the dip of the detachment from le;5° at midcrustal depths (where it was represented by a shear zone) to <24° in the upper crust.

At temperatures below 200–150 °C, lower plate rocks at both ends of the core complex probably cooled at rates >130 °C/m.y., which can not be reconciled with the average dip (le;25°) and slip rate (6–12 mm/yr) for the detachment fault, given the likely geothermal gradient (∼25 °C/km) during the early stages of extension. This implies that cooling did not keep pace with denudation at upper crustal levels. The very high cooling rates are interpreted to have primarily reflected the development of a pronounced thermal discontinuity across the detachment fault (possibly >100 °C), as hot lower plate rocks were transported to shallow crustal levels. Rapid cooling of the lower plate at these levels may have been enhanced by interaction with fluids derived from the upper plate, consistent with models that link the shallow-level (<2–3 km), moderate-temperature (<330 °C) hydrothermal mineralization along detachment faults to the rapid uplift of hot lower plate rocks.

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