Assessing the Role of Water in Cordilleran Flat Slab Subduction Regions

俯冲系统是大洋板块（板块）与大陆板块碰撞，然后沉入地幔的地方，在地震、火山、变形甚至地球内部矿产资源的分散中起着重要作用。在全球范围内，下行板块的几何形状有很大的变化，这强烈影响了这些系统的行为。通常，板片以大约45度的角度俯冲到地幔中，然而，在某些情况下，这些板片呈现低倾角的几何形状，在以更典型的角度恢复俯冲之前，它们沿着覆盖大陆板块的底部滑动很长的距离。这种形式的俯冲，被称为平板或低角度俯冲，被认为在过去深刻影响了美国西部的构造发展，目前正在阿拉斯加南部发生。据信强烈影响这些系统的行为的一个因素是水的存在或不存在。大洋板块在海底移动时吸收水分。当板块到达具有高压和高温的深度时，板块然后将水释放到覆盖的地壳和地幔中。重要的是，这些水降低了岩石的熔化温度，从而导致火山活动;它也削弱了岩石，使变形和造山运动发生。在这些低角度俯冲系统中，水的吸收和释放过程以及水对上板块变形的影响仍然没有很好的了解。为了更好地评估水在这些俯冲环境中的作用，本项目将利用地震成像技术对四个现代低角度区域（阿拉斯加、哥斯达黎加、秘鲁和智利）的俯冲板块进行成像。这些地震成像结果将与热力学模型相结合，以进一步了解俯冲系统的行为。这最终将提高我们对美国西部和阿拉斯加的地质演变以及水如何影响这些地区地质的理解。此外，这项工作的结果将被纳入一个教学研讨会，将教育初中和高中学生的地质学和机制，将地球物理学纳入他们的课堂。板片水合作用和随后的脱水对俯冲带有深远的影响，因为从板片释放的水已经被证明会影响板片的行为，地幔楔和火山弧。在板块俯冲区，表现出显着不同的热制度比典型的俯冲带，水的作用是不太好理解，因为弧通常是关闭和地幔楔可能是薄的不存在。这导致了几个问题，水在这些地区的上下板块动力学中所起的作用。为了评估这一作用，第一步是更好地确定水在这些系统中的位置。为此，本建议书的PI将利用联合表面波和接收器功能分析，对阿拉斯加南部、哥斯达黎加、秘鲁和智利/阿根廷的四个平板区域进行勘测，根据平板几何形状的差异、其在南北美洲科迪勒拉山脉内的位置以及IRIS DMC数据库中这些区域的地震数据的可用性进行选择。这项工作将导致详细的剪切速度模型和量化的地壳和上地幔各向异性在这些地区，将地震速度和水化状态的模型计算使用热力学建模软件进行比较。这些数据和正演模型将用于更好地评估这些区域内的板水化程度，板水储存在这些区域内，以及相对于沟槽，板脱水的位置。此外，这项工作将使PI能够更好地确定从板块释放的水存储在覆盖地壳和地幔内，以及释放到覆盖板块的水是否影响下行板块的几何形状和覆盖岩石圈内的变形。通过系统地分析这四个区域，可以在研究区域之间建立直接的相关性，从而可以确定它们之间的共同点。作为这项工作的一部分，收集的数据将使PI评估两个主要假设，关于水在平板地区的作用：1。板块水化和随后的脱水在控制维持和随后结束板块俯冲所需的浮力方面起着重要作用。板块的脱水作用和与之相关的上覆岩石圈的水化作用影响了上板块的变形和俯冲角度。解决这些问题对于更好地理解平板俯冲的驱动机制和这些地区的行为是很重要的。此外，评估现代平板区域内的水合作用将对理解平板回滚以呈现更典型的俯冲几何形状时观察到的变形和火山作用具有重要意义。最终，这项工作将为更好地理解这些系统的行为提供有用的约束。

Subduction systems, where oceanic plates (slabs) collide with continental plates and then sink into the mantle, play an important role in the occurrence of earthquakes, volcanoes, deformation, and even the dispersal of mineral resources within the Earth. Globally, there is significant variation in the geometry of downgoing slabs, which strongly impacts the behavior of these systems. Typically, slabs subduct into the mantle at an angle of roughly 45 degrees, however, in some cases these slabs assume low-dip geometries where they slide along the base of the overriding continental plate for great distances before resuming subduction at a more typical angle. This form of subduction, known as flat-slab or low-angle subduction, is thought to have profoundly influenced the tectonic development of the western US in the past and is currently occurring in southern Alaska. One factor that is believed to strongly impact the behavior of these systems is the presence or absence of water. Oceanic plates absorb water as they travel beneath oceans. As the plates reach depths with high pressures and temperatures, the plates then release water into the overriding crust and mantle. Importantly, this water lowers the melting temperature of rock, which leads to volcanism; it also weakens rock, enabling deformation and mountain building to occur. Within these low-angle subduction systems, the process of water absorption and release, and the effect of water on upper plate deformation are still not well understood. In order to better assess the role of water in these subduction settings, this project will utilize seismic imaging techniques to image subducting slabs in four modern low-angle regions, Alaska, Costa Rica, Peru, and Chile. These seismic imaging results will be combined with thermodynamic modeling to further our understanding of how subduction systems behave. This ultimately will improve our understanding of how the western US and Alaska have evolved geologically and how water impacts the geology of these regions. Further, the results of this work will be incorporated into a teaching workshop that will educate middle and high-school students about geology and mechanisms for incorporating geophysics into their classrooms.Slab hydration and subsequent dehydration have a profound impact on subduction zones as water released from the slab has been shown to affect the behavior of the slab, the mantle wedge, and the volcanic arc. Within flat-slab subduction regions, which exhibit dramatically different thermal regimes than typical subduction zones, the role of water is less well understood, as the arc is commonly shut-off and the mantle wedge may be thin to non-existent. This leads to several questions about the role water plays in both upper and lower plate dynamics within these regions. In order assess this role, a first step is to better determine where water is present within these systems. To accomplish this, the PI of this proposal will utilize joint surface wave and receiver function analyses to survey four flat-slab regions in southern Alaska, Costa Rica, Peru, and Chile/Argentina, chosen based on their differences in slab geometry, their locations within the North and South American Cordillera, and the availability of seismic data from these regions from the IRIS DMC Database. This work will result in detailed shear velocity models and quantification of crustal and upper mantle anisotropy within these regions that will be compared to models of seismic velocity and hydration state calculated using thermodynamic modeling software. These data and forward models will be used to better assess the degree of slab hydration within these regions, where within the slab water is stored, and where, relative to the trench, the slab dewaters. Additionally, this work will allow the PI to better determine where water released from the slab is stored within the overriding crust and mantle and if water released into the overriding plate impacts the geometry of the downgoing slab and deformation within the overriding lithosphere. By analyzing these four regions in a systematic manner, direct correlations can be made between the study areas, which will allow for the identification of commonalities between them. The data collected as part of this work will allow the PI to evaluate two primary hypotheses regarding the role of water in flat-slab regions: 1. That slab hydration and subsequent dehydration play an important role in controlling the buoyancy necessary to maintain and subsequently end flat-slab subduction and 2. That dehydration of the slab and related hydration of the overriding lithosphere impact upper-plate deformation and the angle of subduction. Addressing these questions is important for better understanding the driving mechanisms for flat-slab subduction and the behavior of these regions. Additionally, assessing hydration within modern flat-slab regions will have important implications for understanding the deformation and volcanism observed when a flat slab rolls back to assume a more typical subduction geometry. Ultimately, this work will provide useful constraints for better understanding how these systems behave.