The thermal history of the crust in west central Arizona, as documented by 40Ar.39Ar step-heating analysis of mineral suites from crystalline nappes of the Maria fold and thrust belt, is characterized by three distinct stages for Late Cretaceous and Tertiary time. Regional heating of the crust occurred during basement-involeve folding and thrust faulting in middle Late Cretaceous time (~80--90 Ma) and represents the earliest phase recorded by the 40Ar/39Ar data. The second stage was dominated by uplift and slow cooling (5--10¿C/m.y.) of the crust throughout latest Cretaceous and early Tertiary time. Tectonic denudation in late Oligocene to early Miocene time resulted in rapid uplift and cooling of midcrustal rocks in the footwall of the Whipple-Buckskin-Rawhide detachment system, marking the final thermal signature in the region. Evidence of variable argon loss in the hornblende spectra implies exposure of differing structural levels within the thrust belt and/or significant thermal heterogeneity in the crust during Late Cretaceous orogenesis. Partially outgassed hornblende in the northern Granite Wash Mountains reflects temperatures of ~450¿C, whereas complete resetting of hornblende at Mesquite Mountain suggests that this area attained temperatures in excess of 500¿C.

In contrast, data from the northern Plomosa Mountains indicate that this area did not attain temperatures sufficient to completely degas biotite or K-feldspar (>350 ¿C) during Late Cretaceous time. The range of early Tertiary muscovite and biotite ages in the region reflects slow cooling, presumably associated with uplift and erosion following crustal thickening. K-feldspars yield saddle-shaped age spectra with minimum apparent ages ranging from 22--28 Ma, and reflect quenching due to rapid exhumation of midcrustal to upper crustal rocks in the detachment terrain. Collectively, the data are consistent with either of two models for the thermal evolution of the crust in middle to late Tertiary time. Either (1) lower plate rocks were still sufficiently hot following Late Cretaceous heating the argon clocks were set simply due to rapid cooling or (2) the area experienced a renewed thermal input associated with extension, causing argon loss from previously cool rocks. Based on consideration of diffusion domain behavior in K-feldspars, we infer that a renewed thermal pulse was associated with the early stages of extension and was related to the cause for crustal weakening. ¿American Geophysical Union 1990