FIELD CALIBRATION OF 40AR/39AR K-FELDSPAR MULTIPLE DIFFUSION DOMAIN (MDD) THERMAL MODELS AT THE GRAYBACK NORMAL FAULT BLOCK, ARIZONA

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 can result in an accurate thermal history that is calibrated to other thermochronometers. We chose the Grayback normal fault block in central Arizona as a case study, as the block was tilted 90° during Oligo–Miocene extension, which allows sampling of a range of different paleo-depths. In addition, the block experienced rapid cooling during extension and the timing of extension and pre–extensional thermal structure of the block are well constrained from prior thermochronology. As a result, this block provides a well–known thermal history against which MDD models can be rigorously compared.

Samples were collected primarily from the Late Cretaceous (~75 Ma) Tea Cup pluton. MDD thermal histories from shallow paleodepths of the pluton yield the oldest ages and indicate rapid cooling below 150°C from 65–55 Ma, consistent with pluton emplacement at relatively shallow levels. Samples from middle paleodepths remained hotter and MDD models indicate very slow cooling from 55–30 Ma, consistent with geologic and thermochronologic studies indicating tectonic quiescence during this period. MDD models from the deepest paleodepths (~12.5 km) record a transition from slow to rapid cooling at 27 Ma, consistent with the known inception of rapid extension from prior thermochronology. MDD models also indicate that samples at this paleodepth resided at 250 ± 25 °C just prior to extension, consistent with prior work. 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 well calibrated to other thermochronometers.