[SCG69-10] Rheological effects on aqueous fluid migration near the subducting plate interface
★Invited Papers
キーワード：流体移動、沈み込み帯、動圧力、体積粘性率
At least in several subduction zones low surface heat flow has been reported in the forearc region, which suggests the existence of a cold fore-arc mantle. These observations have been explained by a thin low-viscosity layer (LVL) above the subducting slab that decouples the movement between the slab and overriding mantle wedge. Although the development and evolution of the LVL remain unclear, hydrous minerals including serpentine may play an essential role in its development. Little is known of aqueous fluid migration in and around the LVL. A previous study demonstrated that the fluid migrates up-dip through the LVL in 3D due to the combined effects of permeability anisotropy of serpentinite and complex slab geometry. In this presentation, an alternative mechanism for fluid migration through the LVL is proposed based on numerical modeling of two-phase flow.
A two-dimensional numerical model is constructed based on two-phase flow theory, which enables the movements of the solid and fluid phases to be treated simultaneously. The following two-step approach is taken. First, solid velocity, dynamic pressure gradient, and solid shear viscosity are calculated for a simple one-dimensional, two-layer channel flow model where the LVL is overlain by stiff overriding mantle wedge. These results are then used to solve for the porosity and compaction pressure in and around the LVL.
The results show that a large amount of the fluid that is released from the source is trapped in the LVL and migrates up-dip over time. This entrapment occurs because the fluid cannot easily migrate into the overriding mantle wedge with a high resistance to a bulk volume change (i.e., a high bulk viscosity). The migration rate shows a marked decrease when the effects of the dynamic pressure gradient are not taken into account, suggesting the importance of dynamic pressure gradient as the main driving force for the fluid migration. Spatial variations in porosity, which is a characteristic behavior of fluid migration through a viscously deformable and permeable solid, are also observed. The fluid tends to stay near the base of the LVL when a non-linear solid viscosity is applied, because a region of high solid shear (and bulk) viscosity forms in the central part of the LVL and acts as a barrier to vertical upward fluid migration. It suggests a mechanism that preserves the LVL above the subducting slab over geological timescales. The fluid migration mechanism proposed here may also be applicable to other tectonic settings, which include the mantle wedge and asthenosphere.
A two-dimensional numerical model is constructed based on two-phase flow theory, which enables the movements of the solid and fluid phases to be treated simultaneously. The following two-step approach is taken. First, solid velocity, dynamic pressure gradient, and solid shear viscosity are calculated for a simple one-dimensional, two-layer channel flow model where the LVL is overlain by stiff overriding mantle wedge. These results are then used to solve for the porosity and compaction pressure in and around the LVL.
The results show that a large amount of the fluid that is released from the source is trapped in the LVL and migrates up-dip over time. This entrapment occurs because the fluid cannot easily migrate into the overriding mantle wedge with a high resistance to a bulk volume change (i.e., a high bulk viscosity). The migration rate shows a marked decrease when the effects of the dynamic pressure gradient are not taken into account, suggesting the importance of dynamic pressure gradient as the main driving force for the fluid migration. Spatial variations in porosity, which is a characteristic behavior of fluid migration through a viscously deformable and permeable solid, are also observed. The fluid tends to stay near the base of the LVL when a non-linear solid viscosity is applied, because a region of high solid shear (and bulk) viscosity forms in the central part of the LVL and acts as a barrier to vertical upward fluid migration. It suggests a mechanism that preserves the LVL above the subducting slab over geological timescales. The fluid migration mechanism proposed here may also be applicable to other tectonic settings, which include the mantle wedge and asthenosphere.