Field Calibration Studies of Continuous Thermal Histories Derived From Multiple Diffusion Domain (MDD) Modeling of 40Ar/39Ar K–feldspar Analyses at the Grayback and Gold Butte Normal Fault Blocks, U.S. Basin and Range
Tuesday, 16 December 2014
Most thermochronometers provide point constraints on a temperature–time path. Thermochronometers that record a continuous thermal history over a range of temperatures can provide more robust constraints, but only a few have such potential. One such system is multiple diffusion domain (MDD) modeling of 40Ar/39Ar K–feldspar analyses, which is interpreted to record thermal histories between ~150–300°C. Although this approach has been applied in numerous studies, some workers have questioned its accuracy in some applications. This study tests whether MDD K-feldspar modeling produces an accurate thermal history that is calibrated to other thermochronometers. Samples were collected from the Grayback normal fault block in central Arizona and the Gold Butte block in Nevada. Both blocks were exhumed during Oligo–Miocene extension and the timing and pre-extensional paleogeothermal gradient are well constrained from prior work. At the Grayback block, MDD thermal histories from shallow paleodepths show rapid cooling from 55–65 Ma following ~75 Ma pluton emplacement, slow cooling from 27–55 Ma at middle paleodepths, and rapid cooling at deep structural levels after 27 Ma during tectonic exhumation. The start of rapid cooling and residence temperatures at 27 Ma from MDD models match expected results from prior fission track work at all structural levels. In the Gold Butte block, MDD thermal models at moderate paleodepths also record slow cooling throughout the Mesozoic to early Tertiary, with deeper structural levels recording rapid cooling at 17 Ma, identical to previous work. Samples from the deepest paleodepths yield MDD thermal histories that repeat this pattern, suggesting that the Gold Butte block is not intact but cut by at least one normal fault at relatively deep paleodepths, which was not evident from prior thermochronologic data. Taken as a whole, the results provide strong confirmation that MDD modeling of K–feldspar 40Ar/39Ar analyses can provide accurate continuous thermal histories that are calibrated to other thermochronometers. In addition, the identification of a major basement fault using MDD thermochronology highlights the utility of this approach over other thermochronometers in some applications.