Testing models of metamorphic sole formation and early subduction evolution in the Easton Metamorphic Suite, Northwest Cascades, Washington

俯冲带发生在寒冷和致密的海洋地壳下降到温暖的浮力海洋或大陆地壳之下的地方。 活跃的俯冲带是地球大部分大地震和火山活动的原因。古代俯冲带提供了在活动俯冲带中无法观察到的深度很深的过程的重要信息，例如地壳随时间的成分变化、过去构造事件的历史和相互关系以及俯冲带过程的时间尺度。因此，了解俯冲带是如何形成和随时间变化的，对于了解地球以及地震和火山活动的过程至关重要。该项目将测试俯冲带如何形成和随时间变化的多种假设。该研究将通过以下几个方面促进科学的进步：1.通过招募和参与项目中代表性不足的本科生和研究生来增加STEM的代表性，2.通过新的实验室练习、新的高级课程和扩大的实地机会，改善本科生和研究生教育;通过在美国多个实验室的本科生和研究生实地研究和合作，开发增强的STEM专业知识。俯冲带启动和向稳定状态演变的机制尚未完全了解。 俯冲的开始通常保存在俯冲带上的蛇绿岩和变质底中，它们是新俯冲板块的残余物，在俯冲的前一千万年中，这些板块被上覆地幔加热，被刮掉，并增生到上板块。这项研究将解决与早期俯冲过程有关的几个问题，具体来说：俯冲是通过引力不稳定自发地开始的，还是由强迫收敛引起的？在变质的鞋底中，倒转的变质梯度是通过向下的热平流形成的，还是通过俯冲物质的逐渐增生形成的？俯冲带内的增生单元是通过热松弛、俯冲通道内的折返、还是底侵和增生而冷却的？这些问题将通过对位于华盛顿西北瀑布的伊斯顿变质套房的调查来解决。该地区包含侏罗纪蛇绿岩序列和侏罗纪-白垩纪变质鞋底在俯冲开始下北美西部形成。结合野外测绘、构造和显微构造分析、变质岩石学和地质年代学，将确定：1。超俯冲带蛇绿岩与变质鞋底形成的相对时间; 2.变质鞋底的每个单元内的变质的时间和条件;以及3.俯冲单元之间接触的结构关系。该奖项反映了NSF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。

Subduction zones occur where cold and dense oceanic crust descends beneath warmer buoyant oceanic or continental crust. Active subduction zones are responsible for most of Earth’s large earthquakes and volcanic activity. Ancient subduction zones provide important information on processes operating at great depths that are not observable in active subduction zones, such as compositional changes in the crust through time, the history and correlation of past tectonic events, and the timescales of subduction zone processes. Understanding how subduction zones form and change over time is therefore critical to understanding the Earth and the processes responsible for earthquakes and volcanism. This project will test multiple hypotheses for how subduction zones form and change with time. The research will promote the progress of science by: 1. increasing representation in STEM by recruiting and involving underrepresented undergraduate and graduate students in the project, 2. improving undergraduate and graduate education through new laboratory exercises, new advanced courses, and expanded field opportunities, and 3. developing enhanced STEM expertise through undergraduate and graduate field research and collaboration in multiple laboratories in the U.S.The mechanisms by which subduction zones initiate and evolve towards steady state are not fully understood. Subduction initiation is often preserved in suprasubduction zone ophiolites and metamorphic soles, remnants of a newly subducted plate that was heated by the overlying mantle, scraped off, and accreted to the upper plate during the first ten million years of subduction. This research will address several questions related to early subduction processes, specifically: Does subduction initiate spontaneously via a gravitational instability or is subduction induced by forced convergence? Do inverted metamorphic gradients in metamorphic soles form via downward heat advection or by the progressive accretion of subducted material? Do accreted units within subduction zones cool by thermal relaxation, exhumation within the subduction channel, or underplating and accretion? These questions will be resolved through an investigation of the Easton Metamorphic Suite in the Northwest Cascades of Washington. The region contains Jurassic ophiolite sequences and a Jurassic-Cretaceous metamorphic sole formed during subduction initiation beneath western North America. Combined field mapping, structural and microstructural analysis, metamorphic petrology, and geochronology will determine: 1. the relative timing of suprasubduction zone ophiolite and metamorphic sole formation, 2. the timing and conditions of metamorphism within each unit of the metamorphic sole, and 3. the structural relationship of contacts between subducted units. The results will establish a detailed reconstruction of the subduction history that can be used to test proposed models for early subduction evolution.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.