Using structural fabrics and thermochronology to document extensional footwall rotation--Insights from the tilted Miocene Searchlight pluton, NV

内华达州的盆地和山脉景观在过去的数千万年里是由地壳伸展形成的。这种扩张受到广泛的断裂和同步岩浆活动的影响，这种活动产生了危险的地震，产生了许多与有价值的矿藏有关的深成岩浆(即结晶岩浆体)，并促成了全州提供地热能潜力的高地壳温度。在这项研究中，主要研究人员正在调查地壳伸展的方式和强度，以及断裂作用与岩体侵位的关系。特别是，不同类型的地壳形变影响着最初热的深成岩的冷却和区域地温梯度。地壳冷却的速度直接影响着地壳的伸展样式、地壳的整体强度和未来地震的潜在分布。这项工作的目标是通过研究一个倾斜的岩体来重建盆地和山脉地壳这一部分的构造和热史，该岩体提供了地壳垂直成分和古热剖面的独特暴露。这项工作对了解盆地和山脉的构造和岩浆演化、当地地震的地壳强度和深度以及构造和岩浆活动与高地壳温度和地热能之间的联系具有重要意义。除了该项目的研究目标外，该奖项的重要社会成果包括支持对一名研究生进行重要的科学、技术、工程和数学(STEM)学科的培训。它还在推进两名职业生涯早期调查人员的研究工作。该项目的参与者还参与了内华达大学雷诺分校的公共推广计划，从而促进了公众科学素养的提高和公众对STEM的参与。这些努力包括通过外联活动向公众传播尖端研究成果，包括在内华达州矿产和地质局举行的年度地球科学周活动。开发数字测绘技术，并通过虚拟现实测绘为可能具有损害传统实地技术和教育的肢体残疾的学生实施适应性教育，从而促进残疾人充分参与STEM教育。最后，这项研究呼应了社区最近对未来构造研究挑战和机遇的愿景，并促进了社区推动构造地质和构造知识的努力。该项目量化了拉斯维加斯附近科罗拉多伸展走廊中新世Searchlight岩体侵位和随后冷却期间的构造和热史，以更好地了解伸展断裂过程。PI表征了岩体内部的结构和组构，以检验“在简单剪切伸展过程中，岩体经历了类似流动的下盘旋转”的假说。这种伸展机制通过综合结构、数值模拟和热年代学研究来约束观察到的组构的性质、岩体和围岩内的变形运动学以及冷却历史，从而做出具体的预测以进行验证。工作假设是，在简单剪切伸展过程中的下盘旋转是一种比纯剪切伸展模式更有效的地壳冷却机制，因为旋转使岩体的较热部分向地球表面平移以冷却。快速冷却增加了上地壳的流变强度，并可起到负反馈的作用，阻止通过脆性断层进一步快速伸展。在这项研究中使用了各种互补的方法，包括(1)数字场图来表征Searchlight岩体内的组构模式、局部剪切感和接触，(2)通过电子背散射衍射进行先进的微观研究，以确定组构的性质、温度、风格和运动学，(3)最先进的计算模拟，以建立和完善分析/数值模拟下盘倾斜和相关的地壳冷却的热运动学模型，将预测将通过热年代学测试的冷却历史，以及(4)磷灰石和锆石(U-Th)/He热力学，以约束冷却历史。该项目揭示了将地壳的瞬时热和流变演化与从颗粒到露头再到区域尺度的变形机制联系起来的构造/构造过程。热年代学和模拟研究了伸展过程中的热演化及其对地壳流变学的影响。得出的冷却速率，加上详细的构造分析，限制了盆地和山脉的地质应变率，并对科罗拉多伸展走廊中新世伸展的时间和速率进行了新的可靠估计。这项研究还揭示了地表热通量、成矿作用和热液系统之间的联系。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The Basin and Range landscape of Nevada has been shaped by crustal extension over the past tens of millions of years. This extension is accommodated by widespread faulting and synchronous magmatism, which has generated hazardous earthquakes, resulted in many plutons (i.e., crystalized magma bodies) associated with valuable ore deposits, and contributed to the high crustal temperatures that provide geothermal energy potential across the state. In this study, the principal investigators are investigating the styles and magnitudes of the crustal stretching and the relationship between faulting and pluton emplacement. In particular, different styles of crustal deformation affect the cooling of initially hot plutons and the regional geothermal gradient. The rate of crustal cooling directly impacts the extensional style, the overall strength of the crust, and the potential distribution of future earthquakes. The goal of this work is to reconstruct the tectonic and thermal histories of this part of the Basin and Range crust by investigating a tilted pluton that provides a unique exposure of the vertical compositional and paleo-thermal profile of the crust. This work has implications for understanding the tectonic and magmatic evolution of the Basin and Range, the crustal strength and depth of local earthquakes, and the linkage between tectonic and magmatic activities to high crustal temperatures and geothermal energy. In addition to the research goals of the project, important societal outcomes of the award including supporting the training of a graduate student in an important science, technology, engineering and mathematics (STEM) discipline. It is also advancing the research efforts of two early-career investigators. Participants of this project are also involved in public outreach programs at the University of Nevada, Reno thereby contributing to increased public science literacy and public engagement with STEM. These efforts include dissemination of cutting-edge research results with the public via outreach events including the annual Earth Science Week event at the Nevada Bureau of Mines and Geology. The development of digital mapping techniques, and the implementation of adaptive education via virtual reality mapping for students who may have physical disabilities that impair traditional field techniques and education, thereby contributing to the full participation of persons with disabilities in STEM education. Finally, this research echoes the recent community vision on the future challenges and opportunities in tectonics research and promotes the community effort to advance the knowledge of structural geology and tectonicsThe project quantifies tectonic and thermal histories during the emplacement and subsequent cooling of the Miocene Searchlight pluton in the Colorado extensional corridor near Las Vegas to better understand the extensional faulting process. The PIs characterize structures and fabrics within the pluton to test the hypothesis that "the pluton experienced flow-like footwall rotation during simple-shear extension." This extensional mechanism makes specific predictions to be tested by constraining the nature of the observed fabrics, deformation kinematics within the pluton and wallrock, and cooling history through an integrated structural, numerical modeling, and thermochronology study. The working hypothesis is that the footwall rotation during simple-shear extension is a more effective crustal cooling mechanism than pure-shear modes of extension because rotation advects the hotter parts of the pluton toward the Earth's surface to cool. Rapid cooling increases the rheological strength of the upper crust and can act as a negative feedback to cease further rapid extension via brittle faulting. Various complementary methods are used in this study including (1) digital field mapping to characterize the fabric patterns, local shear sense, and contacts within the Searchlight pluton, (2) advanced microscopic study by electron backscatter diffraction to determine the nature, temperature, style, and kinematics of fabrics, (3) state-in-art computational simulations to construct and refine analytical/numerical thermo-kinematic models of footwall tilting and associated crustal cooling that will predict cooling histories to be tested via thermochronology, and (4) apatite and zircon (U-Th)/He thermochronometry to constrain the cooling history. This project sheds light on structural/tectonic processes linking transient thermal and rheological evolution of the crust with deformation mechanisms from grain to outcrop to regional scales. Thermochronology and modeling explore the thermal evolution during the extensional process and impacts on crustal rheology. The derived cooling rates, together with detailed structural analysis, constrain the geologic strain rates in the Basin and Range and new robust estimates for the timing and rates of Miocene extension in the Colorado extensional corridor. This study also sheds light on linkages between surface heat flux, ore formation, and hydrothermal systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.