USING GEODYNAMIC AND THERMODYNAMIC MODELING APPROACHES TO ASSESS THE ROLE OF WATER IN FLAT SLAB SUBDUCTION

Although plate tectonic theory is well established in the geosciences, the mechanisms and details of various plate-tectonics related phenomena are not always well understood. In some (~10%) convergent plate boundaries, subduction of downgoing oceanic plates is characterized by low angle to horizontal geometries and is termed “flat slab subduction.” The mechanism(s) driving this form of subduction are not well understood. The goal of this study is to explore the role that water plays in these flat slab subduction settings. This is important not only for providing a better understanding of the behavior of these systems, but also for understanding volcanic hazards associated with both subduction and slab rollback. During the Laramide, flat-slab subduction of the Farallon slab underneath modern day North America influenced various tectonic processes, and rollback of the Farallon plate triggered region-wide volcanism. The goal of this study is to better understand the influence of hydration on initiating, sustaining, and ending flat-slab subduction. This study evaluates two primary hypotheses on the role of water in flat-slab subduction settings: (1) Slab hydration and dehydration help control slab buoyancy that determines whether flat-slab subduction will be maintained or ended. (2) Slab hydration/dehydration of the overlying lithosphere impact both deformation of the upper plate and the angle of subduction. To test this, we generated 2d numerical thermal models of flat-slab subduction using Thermod8, and used the outputs of these models to calculate pseudosections that predict equilibrium mineral assemblages using the thermodynamic modeling program Perple_X. We constructed models to simulate the upper mantle and lithosphere to 150 km depth along a transect in Alaska where flat-slab subduction is currently ongoing. Preliminary results show that mineral reactions associated with slab dehydration occur where the subduction transitions from flat-slab to a higher angle in the Alaskan region. The spatial correlation suggests that the increase in subduction angle is due to the increase in density that results from dehydration. This indicates that dehydration reactions within the slab have an important influence in sustaining and ending flat-slab subduction.