Confirmation of a low pre-extensional geothermal gradient in the Grayback normal fault block, Arizona: Structural and AHe thermochronologic evidence.

Previous work at the Grayback normal fault block in Arizona suggested that a low geothermal gradient (~17 °C/km) existed just prior to crustal extension in the late Oligocene to early Miocene. We present new structural data and (U-Th)/He apatite (AHe) thermo-chronometry that robustly support this conclusion. New 40Ar/39Ar geochronology and structural analysis of dikes support a uniform tilting across the width of the block of ~80° east as a result of extension, removing an important uncertainty in calculating the paleodepth. New AHe results document decreasing ages at deeper paleodepths until ~4-6 km, below which preferred AHe ages are essentially invariant between 22 and 17 Ma. We interpret this inflection in the age-paleodepth trend as the base of the fossil Oligocene AHe partial retention zone (~70 °C), which yields an estimated paleogeothermal gradient of 14 ± 7 °C/km at the inception of extension, strongly confirming earlier estimates. Recent studies suggest that the crust across much of the Basin and Range was quite cold prior to major extension, which has important implications for estimating the degree of fault block tilt from thermochronologic data and for assessing the role of crustal heating in triggering continental extension. [ABSTRACT FROM AUTHOR]