ASSESSING THE ROLE OF WATER IN ALASKAN FLAT-SLAB SUBDUCTION USING THERMAL AND PHASE EQUILIBRIA MODELING APPROACHES

The mechanisms that initiate, drive, and sustain flat-slab subduction are not well understood. Within all subduction zone systems, metamorphic dehydration reactions are important for influencing processes such as seismicity and arc volcanism. Additionally, hydration may play an important role in controlling the geometry of the subducting slab. When water is introduced to the slab crust and mantle, it is incorporated into hydrous phases, which results in lowering the rock density to values less than the parent rocks. The net effect of this process is an increase in the buoyancy of the downgoing oceanic lithosphere. To better understand the role of water in low-angle subduction settings, we conducted a study of flat-slab subduction in the area of southern Alaska, where the 20-km thick Yakutat oceanic plateau is subducted beneath the Alaskan continental lithosphere. In this work, we calculated the thermal conditions in the subducting slab lithosphere. We used Perple_X to calculate stable mineral assemblages within the slab crust and slab mantle using varying degrees of hydration. We then calculated slab density for several hydrated scenarios. The results show that crustal hydration is necessary for a slab to remain neutrally buoyant and sustain flat-slab subduction in Alaska. Within the region, a shallow subduction angle is maintained in Alaska for several hundred kilometers inboard from the trench. At 400 km inland from the trench, the slab reaches the necessary pressure-temperature conditions for dehydration to occur in the crust and lithospheric mantle, which is where slab dip increases. These models suggest that water is an important driver in initiating, sustaining, and ending flat-slab subduction in Alaska. These findings may extend to other flat-slab regions globally.