Assessing the Role of Water in Cordilleran Flat Slab Subduction Regions

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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.