Abstract
Laboratory experiments and geodynamic simulations demonstrate that poloidal- and toroidal-mode mantle flows develop around subduction zones. Here, we use a new 3-D azimuthal anisotropy model constructed by full waveform inversion, to infer deep subduction-induced mantle flows underneath Middle America. At depths shallower than 150 km, poloidal-mode flow is perpendicular to the trajectory of the Middle American Trench. From 300 to 450 km depth, return flows surround the edges of the Rivera and Atlantic slabs, while escape flows are inferred through slab windows beneath Panama and central Mexico. Furthermore, at 700 km depth, the study region is dominated by the Farallon anomaly, with fast axes perpendicular to its strike, suggesting the development of lattice-preferred orientations by substantial stress. These observations provide depth-dependent seismic anisotropy for future mantle flow simulations, and call for further investigations about the deformation mechanisms and elasticity of minerals in the transition zone and uppermost lower mantle.
Highlights
Laboratory experiments and geodynamic simulations demonstrate that poloidal- and toroidalmode mantle flows develop around subduction zones
The first end-member suggests that the downdip movement of slabs entrains mantle flows to great depths, causing 2-D corner flows perpendicular to the trench in the mantle wedge and beneath the slabs, known as the poloidal-mode flow[3]
The strain-induced lattice-preferred orientation (LPO) of olivine leads to the direction and polarization dependence of seismic wavespeeds[15], which can be detected using shear wave splitting and surface wave tomography
Summary
Flows surrounding the deep keel of the North American lithosphere[45], which reaches to around 250 km (Supplementary Fig. 12). There is a sharp transition in the mantle wedge from the trench-parallel pattern at 50 km to the trenchperpendicular pattern at greater depths These complex, depthdependent anisotropic fabrics suggest the development of toroidal-mode flows in the sub-slab region, in response to fast slab rollback (25 mm/yr) of the MAT5. In contrast to the Rivera and Cocos slabs, the WadatiBenioff zone beneath the LAVA reflects subduction from the surface down to 200 km, with a slow anomaly imaged in the mantle wedge underneath the LAVA. By pushing mantle materials in the sub-slab region, the fast northwestward rollback of the Caribbean slab (22 mm/yr)[55] might contribute to the observed northeast-southwest oriented flow field underneath the Caribbean Plate at depths ranging from 300 to 500 km (Fig. 5). This process results in large stresses due to a significant viscosity jump across the 660-km discontinuity[56], and might produce detectable straininduced seismic anisotropy
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