Assessing the Role of Water in Alaskan Flat-Slab Subduction Using Thermodynamic and Phase Equilibria Modeling Approaches
Abstract
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 overthickened Yakutat block is subducted beneath the Alaskan continental lithosphere. In this work, we calculated the thermal conditions and stable mineral assemblages in the slab crust and mantle from shallow depths down to 150 km. Varying degrees of hydration for both peridotite and basalt compositions are assumed in the models in order to assess the role that water plays in altering the density of the subducting slab. 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. Slab hydration may occur preferentially at aseismic ridges and oceanic plateaus due to increased hydrothermal activity related to volcanism which, in conjunction with increased crustal thickness, can substantially reduce the density of the oceanic lithosphere enabling flat-slab subduction. 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.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T31G0387R
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 7240 Subduction zones;
- SEISMOLOGYDE: 8045 Role of fluids;
- STRUCTURAL GEOLOGYDE: 8170 Subduction zone processes;
- TECTONOPHYSICS