Collaborative Research: Integrating Fluorspar Ages and Geophysical Models to Constrain the Timing and Mechanisms of the Collapse of the Cordillera in SW North America

大约6000万年前，北美西南部是一条山脉带，有喜马拉雅山脉那么长，有安第斯山脉那么高。这些山脉从大约3000万年前开始在自己的负荷下坍塌，最终形成了今天该地区存在的典型的盆地和山脉省。这一崩塌山带的地质史还与美国安全和维持经济福祉至关重要的矿藏的形成有关。研究小组将通过对地理上相关的萤石矿床进行测年，来检验这一假设，即流经地壳的流体是坍塌的关键。这些矿藏本身就是一种重要的矿产资源。这项研究将使用创新的测年方法来估计萤石及其伴生矿物的形成时间，并将其与高地坍塌的时间进行比较。这些沉积物的就位时间将为拟议的过程数值模拟提供关键信息。以前的地球物理研究已经为该地区提供了地球内部的三维成像，这些成像也将被用来为数值模拟提供信息和测试，这些地球物理研究将进行更新，以更好地帮助模拟。计算机模拟还将有助于更好地了解导致该地区地震的构造力的来源和强度，有助于更好地评估地震危险性。该项目涉及一项合作计划，以吸引不同的本科生和研究生群体，包括实验室、计算和实地工作的所有组成部分。关于北美西南科迪勒拉伸展塌陷所涉及的热机械过程，包括热输入、地壳流体和熔体的演化、地形变化以及板块构造和地幔流动演化，仍然存在悬而未决的问题。较低的上地幔地震波速度，加上活火山作用，不足以预测美国西南部岩石圈的快速应变率。相反，除了缓慢的上地幔波速和火山作用外，岩石圈还具有丰富的地热水，富含F和3He(指示地幔流体来源)，这预示着岩石圈的快速应变。因此，与高度伸展的带有关的萤石矿床带很可能是在快速伸展地壳应变期间侵位的古流体的沉淀。建议的工作将通过使用U/Pb和(U-Th)/He测年结合高分辨率随时间变化的热力学模型来验证这一假说。此外，对伴生绢云母和明矾石矿床的40Ar/39Ar测年将为萤石测年方法提供验证和信心。热机械模型的输出将与地质、热年代学、沉积学和地球物理观测进行比较。拟议的热化学模拟将1)通过模拟内华达-普莱诺和亚利桑那-普莱诺的坍塌来量化研究区地形变化的原因和后果；2)确定岩石圈减弱的机制，包括热和岩浆演化的作用；3)为现代盆地和山脉省内的变形和地震灾害提供直接的构造背景；以及4)通过结合岩石圈-软流圈系统的地震、热和成分信息，整合地球望远镜项目的最新地震成果。变质核杂岩的形成被认为与高地的坍塌有关，但到目前为止，还没有一个3-D模型直接解决由现实地形建立的体力的影响。为这一提议开发的方法准备在萤石测年所推断的流变性减弱时间的背景下，处理核心杂岩发展的物理以及盆地和山脉的结构演化。这种新的萤石测年方法和综合的热机械模拟方案，可以应用于世界上其他与伸展带有关的萤石矿床地区。拟议的工作包括一项合作计划，将多样化的本科生和研究生群体与实验室、计算和现场工作的所有组成部分整合在一起。来自萨福克社区学院、金斯伯勒社区学院、纽约州立大学奥斯威戈分校、哥伦比亚大学和石溪大学的同事和学生参与了这次合作。综合工作将吸引未被充分代表的少数族裔本科生，并将以令人兴奋的跨学科努力培养下一代地球科学家。学生将在每年夏天参加培训研讨会，并将参与实验室工作，使用Python和Jupyter笔记本进行计算地球动力学建模，以及现场准备、培训和现场工作。在GSA和AGU举办的关于向更广泛的受众传播科学的短期课程中，外联工作将达到高潮。这项合作工作探索了关键矿物形成和西南科迪勒动力学3-D演化之间的前沿联系。该项目由地球系统前沿研究计划和地球科学部共同资助，以支持各种机构类型提高研究能力、能力和基础设施的项目，如地球观测组织拥抱DCL所概述的。该奖项反映了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。

About 60 million years ago Southwestern North America was a mountain belt with the length of the Himalayas and height of the Andes. These mountains collapsed under their own load starting about 30 million years ago, ending in the classic Basin and Range province that exists in the region today. The geological history of this collapsing mountain belt is also connected with the formation of deposits of minerals critically necessary for the security and maintenance of the economic well-being of the United States. The research team will test the hypothesis that fluids flowing through the crust were key to the collapse through the dating of fluorspar deposits that are geographically associated. These deposits themselves are a critical mineral resource. The research will use innovative dating methods to estimate the time of formation of the fluorite and associated minerals and compare this with the time of collapse of the highlands. The timing for the emplacement of these deposits will provide key information for proposed numerical simulations of the processes. Previous geophysical studies have provided three-dimensional imaging of the Earth’s interior for the region that will also be used to inform and test the numerical simulations, and these geophysical studies will be updated to better aid the simulations. The computer simulations will also contribute to a better understanding of the sources and magnitudes of tectonic forces responsible for earthquakes in the region, contributing to a better seismic hazard assessment. The project involves a collaborative plan to engage a diverse undergraduate and graduate student population with all components of laboratory, computational, and field work. Outstanding questions remain about the thermomechanical processes involved in the extensional collapse of the North American Southwest Cordillera, including the evolution of thermal input, crustal fluids and melts, topographic change, and plate tectonic and mantle flow evolution. Low upper mantle seismic wave speeds, together with active volcanism, are insufficient to predict rapid lithospheric strain rates in the southwestern US. Instead, in addition to the slow upper mantle wave speeds and volcanism, the lithosphere is characterized by abundant geothermal waters, enriched in F and 3He (indicating mantle fluid sources), which predicts rapid lithospheric strain. Belts of fluorspar deposits, which are associated with highly extended zones, are therefore likely to be precipitates of paleofluids emplaced during times of rapid transtensional crustal strain. The proposed work will test this hypothesis by using U/Pb and (U–Th)/He dating combined with a high-resolution time-dependent thermomechanical model. Furthermore, 40Ar/39Ar dating of associated sericite and alunite deposits will provide validation and confidence in the fluorite dating methods. Thermomechanical model outputs will be compared with geologic, thermochronologic, sedimentologic, and geophysical observations. The proposed thermochemical modeling will 1) quantify the causes and consequences of the topographic changes of the study area by modeling the collapse of the Nevada-plano and Arizona-plano; 2) identify the mechanisms for lithospheric weakening, including the role of thermal and magmatic evolution; 3) provide direct tectonic context for deformation and seismic hazards within the modern Basin and Range Province; and 4) integrate the latest seismic results from the Earthscope project by incorporating seismic, thermal, and compositional information of the lithosphere-asthenosphere system. Metamorphic core complex formation is hypothesized to be linked to collapse of highlands, but to date no 3-D models have directly addressed the effects of body forces set up by realistic topography. Methods developed for this proposal are poised to address the physics of the development of the core complexes as well as the structural evolution of the Basin and Range in the context of the inferred timing of rheological weakening informed by fluorspar dating. The novel approach to dating fluorspar, together with the integrated thermomechanical modeling plan, can be applied to other regions of the world where fluorspar deposits are associated with extensional zones. The proposed work involves a collaborative plan to integrate a diverse undergraduate and graduate student population with all components of laboratory, computational, and field work. The collaboration involves colleagues and students from Suffolk Community College, Kingsborough Community College, SUNY Oswego, Columbia University, and Stony Brook University. The integrated work will engage under-represented minority undergraduate students and will train the next generation of geoscientists in an exciting interdisciplinary effort. Students will be engaged every summer in training seminars, and will be involved in laboratory work, computational geodynamic modeling using Python and Jupyter Notebooks, and field preparation, training, and field work. Outreach efforts will culminate in short courses at GSA and AGU on communicating science to a broader audience. This collaborative work explores the cutting-edge linkage between critical mineral formation and the 3-D evolution of the dynamics of Southwest Cordillera.This project is jointly funded by the Frontier Research in Earth Systems Program and the Division of Earth Sciences to support projects that increase research capabilities, capacity and infrastructure at a wide variety of institution types, as outlined in the GEO EMBRACE DCL.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.