Collaborative Research: Timing of slip along the Sierra Nevada frontal fault zone, California: A thermochronologic study

南内华达山脉的东侧，加利福尼亚州，形成了一个令人印象深刻的地形悬崖，在美国的一些最高海拔的山峰。沿欧文斯谷的断崖在构造上受活动的内华达山脉锋面断裂带控制。本研究项目的重点是太平洋-北美板块相互作用过程与扩展的盆地和山脉相交的区域。因此，内华达山脉的东部边缘长期以来一直是构造界的兴趣所在，该地区断层和隆起的时间仍然存在争议。该研究项目聚焦于南内华达山脉锋面断裂带，将揭示内华达山脉锋面周围正断层的长期结构演变，并解决几个基本问题：山脉锋面是什么时候形成的？内华达山脉锋面断裂带是晚新生代一次或两次明显的正断层滑动的产物吗？在这些正常的滑动中，南内华达山脉的海拔是下降了还是上升了？该项目通过以下方式促进预期的社会成果：(a)通过本科生（包括少数民族学生）和研究生参与研究的各个方面来培养下一代科学家；(b)通过研究生和本科生培训，培养一支多元化、具有全球竞争力的STEM劳动力队伍；(c)促进国际合作；(d)通过在内华达山脉为本科新生和转学学生制定一个基于实地的外展计划来提高科学素养（科学人才扩展计划，该计划以传统上代表性不足的学生的科学培训为中心）。本研究将验证两个主要假设：(1)南内华达山脉锋面断裂带记录了两次晚新生代正断层滑动；(2)南内华达山脉在第一次正断层滑动期间高度下降或上升，在第二次正断层滑动期间高度上升。新的实地研究（地质制图、构造和运动学）和低温热年代学（磷灰石裂变轨迹、(U-Th)/He和4He/3He）与已发表的地质数据相结合，将用于正演和逆热运动学景观演化建模，以检验这些假设。根据模式结果对晚新生代正常滑动的时间和地表隆起历史的数据进行检查，将有助于评估不同地球动力学过程的有效性(例如，板块窗的发展、太平洋-北美板块运动相对速率的增加、弯曲-均衡模型、以及下沉的致密岩石圈被软流圈取代)被认为是内华达山脉锋面断裂带断裂和内华达山脉平均地表高度变化的驱动因素。内华达山脉是在中生代晚期隆升，然后在整个新生代没有变化或降低海拔高度，还是在中生代晚期隆升，然后在中生代中期降低海拔高度，然后在晚新生代出现第二次隆升，长期以来一直存在分歧，促使了本研究的开展。研究过程中获得的数据——沿内华达山脉锋面断裂带的正断层滑动运动学、滑动历史和滑动速率——将与已发表的侵蚀速率、热气压测量和其他地质约束结果相结合，形成正运动学和逆运动学景观演化模型，以表征内华达山脉南部的正断层滑动和晚新生代隆升历史。并评价所提出的驱动正断层滑动和隆升的地球动力学过程的合理性。本研究首次将低温热时学研究直接应用于内华达山脉锋面断裂带的整个断裂带，并利用这种调查工具的组合来表征内华达山脉锋面断裂带的晚新生代滑动历史和内华达山脉的隆升历史。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 eastern flank of the southern Sierra Nevada, California, forms an impressive topographic escarpment with some of the highest elevation peaks in the conterminous United States. The escarpment along Owens Valley is structurally controlled by the active Sierra Nevada frontal fault zone. This research project focuses this zone where Pacific-North America plate interaction processes intersect with the extending Basin and Range. Thus, the eastern margin of the Sierra Nevada has been of long-standing interest to the tectonics community and the timing of faulting and uplift of the region remains under debate. This research project focused on the southern Sierra Nevada frontal fault zone will shed light on the long-term structural evolution of the normal faults bounding the impressive Sierra Nevada mountain front and address several fundamental questions: When did the range front form? Is the Sierra Nevada frontal fault zone the product of a single or two distinct episodes of late Cenozoic normal fault slip? And did the southern Sierra Nevada lose or gain elevation during these episodes of normal slip? The project advances desired societal outcomes by: (a) training the next generation of scientists via involvement of undergraduates, including minority students, and graduate students in all aspects of the research; (b) developing a diverse, globally competitive STEM workforce through graduate and undergraduate student training; (c) fostering international collaboration and (d) enhancing scientific literacy by developing a field-based outreach program in the Sierra Nevada for undergraduate freshman and transfer STEP students (Science Talent Expansion Program, which is centered on scientific training of traditionally underrepresented students).This research project will test two primary hypotheses: (1) the southern Sierra Nevada frontal fault zone records two episodes of late Cenozoic normal fault slip and (2) the southern Sierra Nevada either lost or gained elevation during the first episode of normal slip and gained elevation during the second episode of normal slip. Integration of new field studies (geologic mapping, structural, and kinematic) and low-temperature thermochronology (apatite fission track, (U-Th)/He, and 4He/3He), with published geologic data, in forward and inverse thermokinematic-landscape evolution modeling will be used to test these hypotheses. Examination of data on the timing of late Cenozoic normal slip and the surface uplift histories in light of model results will allow assessment of the validity of different geodynamic processes (e.g. development of a slab window, increase in relative rate of Pacific-North America plate motion, a flexural-isostatic model, and replacement of foundering dense lithosphere with asthenosphere) postulated as driving both faulting along the Sierra Nevada frontal fault zone and changes in mean surface elevation of the Sierra Nevada. This research is motivated by the long-standing disagreement whether the Sierra Nevada records uplift in the late Mesozoic followed by no change or a decrease in elevation throughout the Cenozoic, or uplift in the late Mesozoic followed by a decrease in elevation during the middle Cenozoic and then a second pulse of uplift in the late Cenozoic. Data acquired during this research - normal fault slip kinematics, slip histories, and slip rates along the Sierra Nevada frontal fault zone - will be combined with published results on erosion rates, thermobarometry, and other geologic constraints into forward and inverse thermokinematic-landscape evolution models to characterize the normal fault slip and late Cenozoic uplift histories of the southern Sierra Nevada, and to assess the soundness of proposed geodynamic processes driving normal fault slip and uplift. This research is the first to apply low-temperature thermochronometry studies directly to the entire Sierra Nevada frontal fault zone escarpment and to use this combination of investigative tools to characterize the late Cenozoic slip history along the Sierra Nevada frontal fault zone and uplift history of the Sierra Nevada.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.