Collaborative Research: Fluid infiltration of the continental crust during Laramide flat-slab subduction: a unique tectonic setting

这项研究调查了一个古老的大陆边缘环境的例子，它涉及浅角或平板俯冲，即大洋板块以浅角度俯冲到大陆边缘之下。南加州和亚利桑那州保存着8000万至4000万年前活跃的古代岩石。将古代的例子与这种情况下的数学模拟进行比较，发现它们没有准确地模拟重要的方面。例如，研究表明，大洋板块顶部的岩石比模拟预测的要热得多，而在预测冷却的大洋板块上方，一些岩石融化了。研究人员将从这个古老环境中的岩石中收集数据，这些岩石曾经位于下降的大洋板块上方。这将导致改进模拟，并更好地了解类似的现代构造环境，其中许多对人类人口构成重大地震风险。该项目支持北亚利桑那大学和威斯康星大学麦迪逊分校的本科生参与这项研究。此外，该项目还支持四名早期职业博士，其中三人属于代表性不足的群体。该项目将调查加利福尼亚州东南部(大玛丽亚山脉)和亚利桑那州西部(墓地岭)拉莱姆平板俯冲过程中地壳深部流体渗透的条件、时间和流体来源。墓地山脊暴露出变质的大洋沉积物(OroCopia Sist)，其中包含通过拉莱姆低角度俯冲向内陆输送约300公里的地幔橄榄岩块。大玛利亚山脉暴露出超俯冲的地壳岩石，也被推测位于古海沟内侧约300公里处。这两个地区都显示出强烈的拉酰胺流体驱动的变质作用和交代作用的证据。与交代作用有关的化学变化将通过将交代岩石的块状岩石化学与未变质原岩进行比较来评估。利用石榴石弹性压力计、石榴石弹性压力计和热力学模拟，利用碳质材料的拉曼光谱、测温和石英研究了交代作用的压力-温度条件。流体渗入的时间将由钛铁矿、独居石和锆石的激光烧蚀分离流质谱确定。将使用氧和硼的稳定同位素来评估流体的来源。新的数据将为约束系统的数学模拟所需的模型参数(摩擦系数、俯冲角、超俯冲粘性加热和俯冲通道内热减弱的临界温度)提供基础。该项目将通过努力培养一支多元化的地球科学队伍，并提高公众科学素养，从而产生更广泛的影响。本科生参与者将在两个参与机构举办的研究研讨会上向公众介绍这项研究。此外，该项目支持四名早期职业博士的职业发展，其中三名是代表不足的群体的成员。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。

This study investigates an ancient example of a continental margin setting that involves shallow-angle or flat-slab subduction, that is, where an oceanic plate dives beneath a continental margin at a shallow angle. Rocks from an ancient example, which was active 80 to 40 million years ago, are preserved in southern California and Arizona. Comparison of the ancient example with mathematical simulations of this setting shows that they do not accurately simulate important aspects. For example, studies have shown that rocks at the top of the oceanic plate got much hotter than predicted by the simulations, and that above the oceanic plate, where cooling is predicted, some rocks melted. The investigators will collect data from rocks in the ancient setting that were once located just above the descending oceanic plate. This will lead to improved simulations and a better understanding of similar modern tectonic settings, many of which pose significant seismic risk to human populations. The project supports the participation of undergraduates in the research at Northern Arizona University and the University of Wisconsin at Madison. In addition, the project supports four early career PhDs, three of whom are members of an underrepresented group. This project will investigate the conditions, timing, and fluid sources of deep-crustal fluid infiltration during Laramide flat-slab subduction in southeastern California (Big Maria Mountains) and western Arizona (Cemetery Ridge). Cemetery Ridge exposes metamorphosed oceanic sediments (Orocopia Schist) containing blocks of mantle peridotite transported ~300 km inland by Laramide low-angle subduction. The Big Maria Mountains exposes suprasubduction crustal rocks, also inferred to have been located ~300 km inboard of the paleotrench. Both areas display strong evidence for Laramide fluid-driven metamorphism and metasomatism. Chemical changes associated with metasomatism will be evaluated by comparing the bulk-rock chemistry of metasomatised rocks to unmetamorphosed protoliths. The pressure-temperature conditions of metasomatism will be investigated using Raman spectroscopy of carbonaceous material thermometry and quartz in garnet elastic barometry and thermodynamic modeling. The timing of fluid infiltration will be determined by laser ablation split stream mass spectrometry of titanite, monazite, and zircon. The source of fluids will be evaluated using stable isotopes of oxygen and boron. The new data will provide a basis for constraining model parameters needed for mathematical simulation of the system for which independent estimates are unavailable (frictional coefficient, subduction angle, suprasubduction viscous heating, and critical temperature for thermal weakening within the subduction channel). The project will achieve broader impacts through efforts that target the development of a diverse workforce in the geosciences and increasing public scientific literacy. Presentations of the research to the public will be made by undergraduate participants at research symposia hosted at both participating institutions. In addition, the project supports career advancement for four early career PhDs, three of whom are members of an underrepresented group.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.