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

在过去的数千万年里，内华达州的盆地和山脉地貌是由地壳伸展形成的。这一延伸是由广泛的断层和同步岩浆活动所容纳的，这产生了危险的地震，导致了许多与有价值的矿床相关的岩体（即结晶岩浆体），并导致了地壳的高温，为整个州提供了地热能源。在这项研究中，主要研究人员正在研究地壳伸展的样式和幅度以及断裂与岩体侵位的关系。特别是不同类型的地壳变形影响了初热岩体的冷却和区域地温梯度。地壳冷却的速度直接影响到地壳的伸展方式、地壳的整体强度以及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这部分盆地和山脉地壳的构造和热历史，该岩体提供了地壳垂直成分和古热剖面的独特暴露。这项工作对认识盆地和山脉的构造和岩浆演化、局部地震的地壳强度和深度以及构造和岩浆活动与地壳高温和地热能之间的联系具有重要意义。除了项目的研究目标之外，该奖项的重要社会成果包括支持在重要的科学，技术，工程和数学（STEM）学科中培养研究生。它还推动了两位早期职业研究者的研究工作。该项目的参与者还参与了内华达大学里诺分校的公共推广项目，从而为提高公众科学素养和公众对STEM的参与做出了贡献。这些努力包括通过外展活动向公众传播前沿研究成果，包括在内华达州矿业和地质局举办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对可能有身体残疾的学生实施适应性教育，这些学生可能会影响传统的现场技术和教育，从而有助于残疾人充分参与STEM教育。最后，本研究呼应了最近社区对构造研究未来挑战和机遇的看法，并促进了社区对构造地质学和构造学知识的发展。该项目量化了拉斯维加斯附近科罗拉多伸展走廊中新世探照灯岩体在侵位和随后冷却期间的构造和热历史，以更好地了解伸展断裂过程。pi表征了岩体内部的结构和结构，以验证“岩体在单剪切伸展过程中经历了流状下盘旋转”的假设。通过综合构造、数值模拟和热年代学研究，对观察到的构造的性质、岩体和围岩的变形运动学以及冷却历史进行约束，这种伸展机制做出了具体的预测。工作假设是，在简单剪切伸展期间，下盘旋转比纯剪切伸展模式是一种更有效的地壳冷却机制，因为旋转将冥王星较热的部分平流到地球表面冷却。快速冷却增加了上地壳的流变强度，并可以作为负反馈来阻止通过脆性断裂进一步快速扩展。本研究中使用了各种互补方法，包括(1)数字场测绘来表征织物图案、局部剪切感和探照灯岩体内的接触；(2)通过电子背散射衍射进行高级显微研究，以确定织物的性质、温度、样式和运动学；(3)建立和完善下盘倾斜和相关地壳冷却的解析/数值热运动学模型，通过热年代学来预测冷却历史；(4)磷灰石和锆石(U-Th)/He热年代学来约束冷却历史。该项目揭示了地壳的瞬态热流变演化与从颗粒到露头再到区域尺度的变形机制之间的构造/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。推导出的冷却速率，加上详细的构造分析，限制了盆地和山脉的地质应变速率，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、矿石形成和热液系统之间的联系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.