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

加利福尼亚州内华达山脉南部的东侧形成了一个令人印象深刻的地形悬崖，拥有美国毗邻的一些海拔最高的山峰。欧文斯河谷的陡坡在构造上受活动的内华达山脉前缘断裂带控制。这一研究项目聚焦于太平洋-北美板块相互作用过程与伸展盆地和山脉相交的区域。因此，内华达山脉东缘长期以来一直是构造界感兴趣的地区，该地区的断层和隆起时间仍在争论中。这项专注于内华达山脉南部前缘断裂带的研究项目将揭示环绕着令人印象深刻的内华达山脉前缘的正断层的长期构造演化，并解决几个基本问题：山脉前锋是什么时候形成的？内华达山脉前缘断裂带是晚新生代正断层滑动的一段或两段截然不同的产物？在这些正常滑动的时期，内华达山脉南部是否失去或上升了海拔？该项目通过以下方式推动预期的社会成果：(A)通过本科生，包括少数族裔学生和研究生参与研究的各个方面，培训下一代科学家；(B)通过研究生和本科生培训，发展一支多样化的、具有全球竞争力的STEM劳动力队伍；(C)促进国际合作和(D)通过在内华达山脉为本科生和转学学生制定实地推广方案(科学人才扩展方案，其中心是对传统上代表性不足的学生进行科学培训)来提高科学素养。这项研究项目将检验两个基本假设：(1)内华达山脉南部前缘断裂带记录了两幕晚新生代正断层滑动；(2)内华达山脉南部在第一次正常滑动期间失去或上升了海拔，并在第二次正常滑动期间上升了。新的野外研究(地质填图、构造和运动学)和低温热年代学(磷灰石裂变径迹、(U-Th)/He和4He/3He)与已公布的地质数据相结合，将在正反向热运动学-地貌演化模拟中用于验证这些假说。根据模型结果对晚新生代正常滑动的时间和地表隆升历史的数据进行检验，将有助于评估不同地球动力学过程(例如，板条窗口的发展、太平洋-北美板块运动的相对速率增加、挠曲-均衡模型和用软流圈取代下沉的致密岩石圈)的有效性，这些过程被认为既推动了内华达山脉前缘断裂带的断裂活动，也推动了内华达山脉表面平均高度的变化。内华达山脉是在晚中生代隆升之后整个新生代海拔没有变化或下降，还是在中生代晚期隆升后中新生代海拔下降，然后在晚新生代第二次隆升，这一问题一直存在分歧。在这项研究中获得的数据--沿着内华达山脉前缘断裂带的正常断层滑动运动学、滑动历史和滑动速率--将与已发表的关于侵蚀速率、温压测量和其他地质约束的结果结合在一起，形成正向和反向热运动学-地貌演化模型，以表征内华达山脉南部的正常断层滑动和晚新生代隆起历史，并评估所提出的驱动正常断层滑动和隆起的地球动力学过程的可靠性。这项研究是首次将低温热计时研究直接应用于整个内华达山脉前缘断裂带的悬崖，并使用这种调查工具的组合来表征内华达山脉前缘断裂带的晚新生代滑移历史和内华达山脉的隆起历史。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 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.