Melt Segregation and Strain Partitioning: Implications for Seismic Anisotropy and Mantle Flow

Abstract

One of the principal means of understanding upper mantle dynamics involves inferring mantle flow directions from seismic anisotropy under the assumption that the seismic fast direction (olivine a axis) parallels the regional flow direction. We demonstrate that (i) the presence of melt weakens the alignment of a axes and (ii) when melt segregates and forms networks of weak shear zones, strain partitions between weak and strong zones, resulting in an alignment of a axes 90 degrees from the shear direction in three-dimensional deformation. This orientation of a axes provides a new means of interpreting mantle flow from seismic anisotropy in partially molten deforming regions of Earth.

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

“…Although the geometry of melt distribution on which the Hammond and Humphreys estimate is based has been challenged (Wark et al, 2003), disequilibrium melt distribution produced during recrystallization (Waite et al, 2003) or localization of melt into bands during deformation (Holtzman et al, 2003) could produce the flat melt pockets needed to strongly affect shear velocities with only small melt concentrations (Schmeling, 1985).…”

Patterns in Galápagos Magmatism Arising from the Upper Mantle Dynamics of Plume‐Ridge Interaction

“…A stronger effect may be achieved with anisotropic melt percolation. Melt channels are known to form in response to mantle shear and are expected to be angled toward a ridge axis in the areas where the rate of horizontal mantle flow away from the ridge axis decreases with depth [Holtzman et al, 2003;Katz et al, 2006]. This sense of shear is expected to develop due to mantle flow driven passively by plate-spreading and would be further enhanced by northward flow of plume material toward the GSC.…”

Upper mantle anisotropy beneath Japan from shear wave splitting

“…There is a diverse range of processes that could potentially affect the three-dimensional anisotropic structure of subduction zones, including corner flow in the mantle wedge induced by viscous coupling between the slab and wedge material, trench-parallel flow in the mantle wedge (due to transpression) or beneath the slab (due to slab rollback), formation and migration of melt, flow around the slab edge, frozen lithospheric anisotropy in the slab, and anisotropic structure in the overriding plate. Additionally, new experimental studies suggest that the relationships between strain and resulting LPO can be dramatically altered by the presence of a small amount of water or other volatiles (Jung and Karato, 2001;Karato, 2004) or by the presence of melt (Holtzman et al, 2003). Subduction-associated processes, therefore, are likely to lead to anisotropic structures that are more complicated than the simple single-layer anisotropic models that are often assumed when interpreting splitting measurements.…”

“…It is worth noting that both trench-perpendicular and trench-parallel fast directions associated with subduction zone anisotropy have been observed in different parts of the world (e.g., Fouch and Fischer, 1996;Smith et al, 2001;Levin et al, 2004). Three possible mechanisms have been suggested to generate trench-parallel fast directions above a subduction zone: trench-parallel flow above the slab, due to transpression of the overlying mantle wedge (Mehl et al, 2003), trench-parallel flow beneath the slab due to slab rollback or a similar mechanism (Peyton et al, 2001) or corner flow in the mantle wedge induced by viscous coupling with the downgoing slab, along with the presence of an "exotic" deformation-induced LPO pattern (Jung and Karato, 2001;Holtzman et al, 2003).…”