FIELD CALIBRATION STUDIES OF CONTINUOUS THERMAL HISTORIES DERIVED FROM MULTIPLE DIFFUSION DOMAIN (MDD) MODELING OF 40AR/39AR K–FELDSPAR ANALYSES AT THE GRAYBACK (AZ) AND GOLD BUTTE (NV) NORMAL FAULT BLOCKS

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 may 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 AZ and the Gold Butte block in NV. 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 ca. 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 temps at 27 Ma from MDD models match closely to expected results from prior fission track work at all structural levels. In the Gold Butte block, MDD thermal models from the deepest (western) block show slow cooling from >35–17 Ma followed by rapid cooling at 17 Ma, identical to previous work. Although the form of the MDD thermal history matches the expected, the MDD models suggest the deep footwall resided at 230-270°C at 17 Ma; 80-100°C cooler than expected based on the assumed paleodepth and paleogeothermal gradient, which could reflect a problem with the absolute temperatures of these MDD model. However, we believe it is more likely that the paleodepth calculations in the western Gold Butte block are flawed due to incorrect tilt estimates or faulting within the block. Repetitions of MDD thermal histories across the block suggest that the block may not be as intact as previously believed, at least at deeper paleodepths. Taken as a whole, the results from both fault blocks provide strong confirmation that MDD modeling of K–feldspar analyses can provide meaningful and accurate continuous thermal histories.