Many tectonic processes within the Earth impart thermal effects on surrounding rocks. As a result, thermochronology, which allows the thermal history of a sample to be reconstructed, can provide unique insights into the timing, rates, and spatial patterns of many geologic processes. Applications of thermochronology include investigating landscape evolution, mountain building, continental extension and rifting, and the thermal histories of sedimentary basins, which are important to petroleum exploration. As such, thermochronology is an essential tool to investigate a wide variety of processes within the Earth Sciences. However, most thermochronologic systems only provide the time of cooling through a single closure temperature and thus only provide a point constraint along a temperature/time path, which can result in inaccurate interpretations. An ideal thermochronologic system would instead retain the continuous thermal history of a sample through a large range of temperatures. The 40Ar/39Ar K-feldspar system has been interpreted to record the continuous thermal history of a sample from ~350 to 150°C, thus making it a potentially powerful thermochronologic tool. However, a number of studies have questioned whether this system records geologically meaningful thermal histories. The goal of this study is to field test the thermal histories derived from 40Ar/39Ar K-feldspar data in a well constrained geologic context in order to evaluate the geologic significance of the thermal histories derived by this approach.
The study will be conducted at two fault blocks that have been tilted and exhumed by large normal faults; the Gold Butte block, NV and the Grayback fault block, AZ. These blocks expose crustal cross sections that range from near surface conditions to >10 km depth. These blocks have a known pre-extensional thermal structure based on prior studies as well as a distinct thermal history that evolved from slow to rapid cooling as extension began. Samples will be collected from a wide range of paleo-depths at each fault block and multiple diffusion domain (MDD) modeling of 40Ar/39Ar K-feldspar data will be conducted on each sample. The resultant thermal histories will be evaluated against the prior thermochronologic data, the known thermal structure and thermal/tectonic history of each block. If MDD derived thermal histories match the expected behavior with regard to both the form and absolute temperatures, this work will provide important case studies that document the geologic significance of 40Ar/39Ar K-feldspar thermal histories and their calibration to other thermochronometers. Samples whose results depart from the expected behavior will be evaluated to assess why problematic results were obtained (problematic microtextures, compositions, grain sizes, diffusion behavior, etc). These results will be used to evaluate approaches currently used to assess the validity of MDD thermal histories, such as the correlation between the age spectrum and the R/R0 plot. These results may also provide insight into sample characteristics that yield the most reliable thermal histories, thus providing a guide for future studies. As a whole, these results will provide new insight into the geologic validity of MDD 40Ar/39Ar K-feldspar thermal histories, why some samples yield problematic results, and the methods currently used to identify inaccurate results.
