Assessing the Role of Water in Cordilleran Flat Slab Subduction Regions

大洋板块（板块）与大陆板块碰撞，然后沉入地幔的俯冲系统，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，这强烈地影响了这些系统的行为。通常，板块以大约45度的角度俯冲到地幔中，然而，在某些情况下，这些板块呈现低倾角几何形状，它们沿着上覆大陆板块的底部滑动很长一段距离，然后以更典型的角度恢复俯冲。这种形式的俯冲，被称为平板或低角度俯冲，被认为在过去深刻地影响了美国西部的构造发展，目前正在阿拉斯加南部发生。人们认为，对这些系统的行为产生强烈影响的一个因素是水的存在与否。大洋板块在海底运动时吸收水分。当板块在高压和高温下到达深处时，板块会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融化温度，从而导致火山活动；它还削弱了岩石，使变形和造山活动得以发生。在这些低角度俯冲体系中，水的吸收和释放过程以及水对上板块变形的影响尚不清楚。为了更好地评估水在这些俯冲环境中的作用，该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建模相结合，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，该研讨会将向中学生和高中生介绍地质学和将地球物理学纳入课堂的机制。板块水化和随后的脱水对俯冲带有着深远的影响，因为从板块中释放的水已经被证明会影响板块、地幔楔和火山弧的行为。在平板俯冲区内，与典型的俯冲带相比，表现出明显不同的热状态，水的作用不太清楚，因为弧通常是关闭的，地幔楔可能很薄，甚至不存在。这就引出了关于水在这些区域的上下板块动力学中所起作用的几个问题。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存在位置。为了实现这一目标，该提案的PI将利用联合表面波和接收函数分析来调查阿拉斯加南部、哥斯达黎加、秘鲁和智利/阿根廷的四个平板区域，选择的依据是它们在平板几何形状上的差异、它们在北美和南美科迪勒拉的位置，以及这些地区从IRIS DMC数据库中获得的地震数据。这项工作将产生详细的剪切速度模型以及这些区域内地壳和上地幔各向异性的量化，这些模型将与使用热力学建模软件计算的地震速度和水化状态模型进行比较。这些数据和正演模型将用于更好地评估这些区域内的平板水化程度，平板水储存在何处，以及相对于沟槽，平板脱水在何处。此外，这项工作将使PI能够更好地确定从板块释放的水在覆盖的地壳和地幔中储存的位置，以及释放到覆盖板块的水是否影响了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，这将允许识别它们之间的共性。作为这项工作的一部分收集的数据将允许PI评估关于水在平板地区的作用的两个主要假设：平板水化和随后的脱水在控制浮力方面起着重要作用，而浮力是维持和结束平板俯冲所必需的。板块的脱水作用和相应的上覆岩石圈的水化作用影响了板块上部的变形和俯冲角度。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具有重要意义。此外，评估现代平板区域的水化作用，对于理解当平板回滚时观察到的变形和火山作用具有重要意义，假设一个更典型的俯冲几何形状。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.