The role of rheology and slab shape on rapid mantle flow: Three-dimensional numerical models of the Alaska slab edge

摘要

Away from subduction zones, the surface motion of oceanic plates is well correlated with mantle flow direction, as inferred from seismic anisotropy. However, this correlation breaks down near subduction zones, where shear wave splitting studies suggest the mantle flow direction is spatially variable and commonly non-parallel to plate motions. This implies local decoupling of mantle flow from surface plate motions, yet the magnitude of this decoupling is poorly constrained. We use 3D numerical models of the eastern Alaska subduction-transform plate boundary system to further explore this decoupling, in terms of both direction and magnitude. Specifically, we investigate the role of the slab geometry and rheology on the mantle flow velocity at a slab edge. The subducting plate geometry is based on Wadati-Benioff zone seismicity and tomography, and the 3D thermal structure for both the subducting and overriding plates, is constrained by geologic and geophysical observations. In models using the composite viscosity, a laterally variable mantle viscosity emerges as a consequence of the lateral variations in the mantle flow and strain rate. Spatially variable mantle velocity magnitudes are predicted, with localized fast velocities (greater than 80 cm/yr) close to the slab where the negative buoyancy of the slab drives the flow. The same models produce surface plate motions of less than 10 cm/yr, comparable to observed plate motions. These results show a power law rheology, i.e., one that includes the effects of the dislocation creep deformation mechanism, can explain both observations of seismic anisotropy and decoupling of mantle flow from surface motion.