Apatite (U-Th)/He (AHe) data from ∼60 km of exposed footwall in the Harcuvar Mountains metamorphic core complex, Arizona, reveal a higher-precision late-stage cooling history than previously reported for this complex and others in the region. Progressively younger AHe ages recorded with distance along the slip direction show two distinct trends separated by a sharp inflection. At structurally shallower levels, ages decrease from ca. 23 to 15 Ma toward the northeast, defining an estimated slip rate of 2.6 +0.1/–0.2 km/ m.y. Samples from the structurally deepest ∼35 km of footwall yield concordant ages of ca. 15 Ma (±2σ error limits). The effects of mineralizing fluids, advection of isotherms toward the surface, and flexure-induced erosion are considered, but are unlikely to have influenced the ages. Therefore, it is suggested that the fault slip rate must have increased ca. 15 Ma to a rate of ∼30 km/m.y. Evidence for an increased slip rate ca. 15 Ma has also been observed in other metamorphic core complexes nearby, and this increase is attributed to the thermal effects of a slab-free window on the extending lithosphere.

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