CAREER: The Fall of Mountains: Reconstructing Extensional Collapse in the North American Cordillera from the Surface Record

自从刘易斯和克拉克站在大陆分水岭上眺望落基山脉北部那连绵不绝的高山和深谷，就激发了游客们的好奇心。这项研究的目的是了解这种戏剧性的景观是何时形成的，以及山脉是如何分裂的，这一过程今天在美国西部仍然很活跃。首席研究员和她的学生们利用在火山灰中保存了数百万年的古代雨水，测量了过去5000万年里爱达荷州、蒙大拿州和怀俄明州的海拔高度，并结合了随着时间推移填满山谷的沉积物的年龄，重建了落基山脉北部、盆地和山脉的山脉形成和崩塌的历史。通过这个项目，研究人员的目标是增加各级学生对地球科学的了解。学生将受益于对研究生和本科生研究项目的直接支持，为本科生课程开发广泛可访问的动手调查谷歌地球活动，以及通过STEM Access和TRIO-Inspire项目招募的代表性不足群体的高中生暑期实习。该项目通过对早期职业女性地球科学家研究人员及其学生的直接研究支持，以及通过Tuff Talk播客（通过与地球科学领域成功女性的互动访谈，为广大受众提供远程指导、指导和专业发展），支持扩大STEM领域代表性不足群体的代表性。这项研究还使用了地球化学和年龄测年分析技术的新组合，目的是改进这些方法。高海拔地区对区域水文、全球气候、侵蚀和沉积物沉积以及许多其他地球过程具有巨大的影响。在地壳缩短或地幔剥离结束后，造山带可以很好地维持高地形，尽管岩石圈的重力位能很高，热作用减弱。虽然对隆升时间的估计最近变得很普遍，但我们缺乏造山带崩塌的精确地表记录，即使在研究得很好的北美科迪勒拉地区也是如此。测量地表下降的时间和速度，以及它对侵蚀和盆地沉积的影响，可以让我们深入了解这种扩张的驱动因素，以及原始造山带的成因和结构。本研究的总体目标是重建整个北美科迪勒拉地区的新生代地形演化，包括地表下降、砾岩沉积和流域重组的时间、规模和机制，从而形成地表记录中表达的造山带崩塌的整体模型。首席研究员和她的学生们通过结合一种创新的、广泛分布的稳定同位素替代材料和精确的辐射年龄来量化地表地形的变化，并使用同位素支持的全球气候模型来解释这些数据，以捕捉海拔随时间的变化。古高程测量与盆地记录的新物源、挖掘和沉积滞后时间数据相结合，以确定伸展的开始，并测试地表变形是否产生了广泛的砾岩沉积。研究结果用于区分造山崩塌的各种潜在机制，并为岩心复杂构造、板块回滚和岩浆作用之间的相互作用和表面表达建立了一个模型。通过对多个现场和分析数据集的结合以及对研究结果的整合，项目团队对北美科迪勒兰地表下降的过程以及开始大范围延伸和岩石圈变薄所需的条件有了新的认识。该提案部分得到了地球科学教育和人力资源计划部门的共同资助。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 seemingly endless chain of high mountains and deep valleys of the northern Rocky Mountains has sparked wonder in visitors since Lewis and Clark looked across it from the continental divide. The goal of this research is to understand when this dramatic landscape formed and how mountains fall apart, a process that is still active in the western U.S. today. The principal investigator and her students are using ancient rainwater, preserved for millions of years in volcanic ash, to measure elevations from the past 50 million years across Idaho, Montana, and Wyoming, combined with the ages of sediments that have filled the valleys over time, to reconstruct the history of mountain building and collapse in the northern Rocky Mountains and Basin and Range. Through this project, the researcher aims to increase accessibility to geoscience for students of all levels. Students benefit from direct support for graduate and undergraduate research projects, development of widely accessible, hands-on investigative Google Earth activities for undergraduate courses, and summer internships for high school students from underrepresented groups, recruited via STEM Access and TRIO-Inspire programs. The project supports to broadening of representation of underrepresented groups in STEM fields through direct research support of an early career female geoscientist researcher and her students, and through the Tuff Talk podcast, which provides distance mentorship, coaching, and professional development to a broad audience through interactive interviews with successful women in Earth science. This research also uses a novel combination of analytical techniques in geochemistry and age dating, with the goal of refining these methods.Elevated regions have an outsized influence on regional hydrology, global climate, erosion and deposition of sediment, and many other Earth processes. Orogens can maintain high topography well after the end of crustal shortening or mantle delamination, despite high gravitational potential energy and thermal weakening of the lithosphere. Although estimates of the timing of uplift have recently become common, we lack precise surface records of orogen collapse, even in the well-studied North American Cordillera. Measuring the timing and pace of surface lowering, and its effects on erosion and basin sedimentation, can provide us with insights into the drivers for that extension, and into the genesis and structure of the original orogen. The overall objective of this research is to reconstruct the Cenozoic evolution of topography across the North American Cordillera, including the timing, magnitude, and mechanisms of surface-lowering, conglomerate deposition, and drainage reorganization, thereby generating a holistic model of orogen collapse as expressed in the surface record. The principal investigator and her students quantify changes in surface topography by combining an innovative, widely distributed stable isotope proxy material with precise radiometric ages, and interpreting those data using an isotope-enabled global climate model to capture changes in elevation over time. Paleoelevation measurements are integrated with new provenance, exhumation, and sediment lag time data from the basin record to determine the onset of extension and test if surface deformation generated widespread conglomerate deposition. Results are used to discriminate between various potential mechanisms for orogenic collapse and to develop a model for the interactions between, and surface expressions of, core complex formation, slab rollback, and magmatism. Through the combination of multiple field and analytical datasets and the integration of findings, the project team is gaining a new understanding of the progression of North American Cordilleran surface lowering and the conditions necessary to initiate range-wide extension and lithospheric thinning. The proposal is supported in part by co-funding by the Division of Earth Sciences Education and Human Resources Program.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.