Combined analysis of high-resolution seismic images of the Alaska and Cascadia subduction zones reveals where metamorphic fluids are released. Both images show the subducted oceanic crust as a dipping low-velocity layer with a clear termination depth. However, in Alaska the crust is thicker (15-20 km compared to 8 km) and terminates at greater depth (120 km compared to 40 km) than in Cascadia. Based on metamorphic reaction estimates and geodynamic models, we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water-bearing minerals, and that the location of this reaction is dependent on the thermal structure of the subducted slab.
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