RUI: Field-testing the Multiple Diffusion Domain 40Ar/39Ar K-feldspar Thermochronometer at Tilted Normal Fault Blocks in the U.S. Basin and Range

地球内部的许多构造作用都会对周围的岩石产生热效应。因此，热年代学可以重建样品的热历史，可以提供对许多地质过程的时间、速率和空间模式的独特见解。热年代学的应用包括研究地貌演化、造山、大陆伸展和裂谷作用以及沉积盆地的热史，这对石油勘探具有重要意义。因此，热年代学是研究地球科学中各种过程的重要工具。然而，大多数热年代学系统只提供了通过单一关闭温度的冷却时间，因此只提供了沿温度/时间路径的点约束，这可能导致不准确的解释。一个理想的热年代学系统将保留样品在大范围温度范围内的连续热历史。40Ar/39Ar钾长石系统被解释为记录了样品从~350°C到150°C的连续热历史，从而使其成为一种潜在的强大的热年代学工具。然而，一些研究质疑该系统是否记录了具有地质意义的热史。这项研究的目的是在约束良好的地质背景下对40Ar/39Ar钾长石数据进行现场热历史测试，以评估该方法获得的热历史的地质意义。研究将在两个被大型正断层倾斜和剥离的断块上进行：内华达州的金巴特断块和亚利桑那州的格雷巴克断块。这些区块暴露出从近地表条件到10公里深的地壳横截面。根据先前的研究，这些地块具有已知的伸展前热结构，以及随着伸展开始从缓慢到快速冷却的独特热史。将从每个断块的大范围古深度采集样品，并对每个样品进行40Ar/39Ar钾长石数据的多扩散域(MDD)模拟。将根据先前的热年代学数据、已知的热结构和每个区块的热/构造历史来评估所产生的热历史。如果MDD得到的热历史在形式和绝对温度方面都符合预期行为，这项工作将提供重要的案例研究，证明40Ar/39Ar K长石热历史的地质意义及其对其他温度计时器的校准。将对结果偏离预期行为的样本进行评估，以评估为什么会得到有问题的结果(有问题的微观结构、成分、颗粒大小、扩散行为等)。这些结果将被用来评估目前用于评估MDD热历史有效性的方法，例如年龄谱和R/R0图之间的相关性。这些结果还可能提供对产生最可靠热历史的样品特征的洞察，从而为未来的研究提供指导。总体而言，这些结果将为MDD 40Ar/39Ar钾长石热历史的地质有效性、为什么一些样品产生有问题的结果以及当前用于识别不准确结果的方法提供新的见解。

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.