Shear wave anisotropy beneath Nicaragua and Costa Rica: Implications for flow in the mantle wedge

Abstract

We present new shear wave splitting data from local events in Costa Rica and Nicaragua recorded by the temporary (July 2004 to March 2006) 48‐station TUCAN broadband seismic array. Observed fast polarization directions in the fore arc, arc, and back arc range from arc‐parallel to arc‐normal over very short distances (<5 km when plotted at raypath midpoints) making the direct interpretation of individual splitting measurements in terms of flow tenuous, even when considering variations in the relationship between lattice‐preferred orientation and deformation (e.g., B‐type dislocation creep in olivine). Therefore, we tomographically invert the splitting measurements to find a three‐dimensional model of crystallographic orientation in the wedge. We assume the elastic constants of olivine and orthopyroxene with hexagonal symmetry and use a damped, iterative least squares approach to account for the nonlinear behavior of splitting when considering three‐dimensional ray propagation and distributions of anisotropy. The best fitting model contains roughly horizontal, arc‐parallel olivine [100] axes in the mantle wedge down to at least 125 km beneath the back arc and arc, which we interpret to indicate along‐arc flow in the mantle wedge. Pb and Nd isotopic ratios in arc lavas provide additional evidence for arc‐parallel flow and also constrain the direction (northwest, from Costa Rica to Nicaragua) and minimum flow rate (63–190 mm/a). With only slightly oblique subduction at 85 mm/a of the relatively planar Cocos Plate, the most likely mechanism for driving along‐arc transport is toroidal flow around the edge of the slab in southern Costa Rica, generated by greater slab rollback in Nicaragua. Two important implications of this arc‐parallel flow are the progressive depletion of the mantle source for arc lavas from Costa Rica to Nicaragua and the possible need for significant decoupling between the wedge and downgoing plate.