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

Abstract

[1] 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.Components: 15,269 words, 15 figures.

Igneous Rock Associations 13. Focusing on the Central American Subduction Zone

“…suggest that trench-parallel mantle flow, perhaps coupled with oblique subduction, causes diffusion of this Galapagos signature from central Costa Rica northwestward into Nicaragua. Trench-parallel mantle flow is consistent with recent seismic anisotropy data for the southern portion of the CASZ Abt et al 2009Abt et al , 2010. Gazel et al (2009Gazel et al ( , 2011 show that the appearance of the Galapagos signature in the central Costa Rican volcanic front correlates with the production of magmas having primitive andesitic/adakitic compositions.…”

Seismic anisotropy tomography: New insight into subduction dynamics

“…Therefore, surface and deep deformation patterns are poorly correlated and most likely decoupled. Active N-S convergence in the Transverse Ranges results in strongly coherent E-W alignment of olivine fast (Abt et al, 2009). All well-resolved model a axes of olivine (color bars) are projected onto an along-arc, an across-arc, and a horizontal plane.…”

“…To account for the non-linear properties of SWS, the inversion is applied iteratively and partial derivatives are recalculated after each iteration. Abt et al (2009) applied this SWS tomography method to determine a 3-D model of crystallographic orientation in the mantle wedge beneath Costa Rica and Nicaragua (Fig. 13).…”

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

“…Progress in this area has been (and will continue to be) enabled by both the increasing availability of dense array data and by the theoretical development of frameworks for splitting tomography. Both local ray theoretical tomography Abt et al 2009) and teleseismic splitting intensity tomography using finite-frequency kernels (Chevrot 2006;) each have pros and cons and have different data and computational requirements, but both are likely to find a variety of useful applications in the future.…”

“…One of these Abt et al 2009) has focused on carrying out local splitting tomography using a ray theoretical approach, with application to data sets acquired in subduction zone settings using earthquakes located in the subducting slab to obtain a set of crossing rays that sample the mantle wedge. The inversion approach described by Abt and Fischer (2008) utilizes measurements made by the transverse component minimization method (Silver and Chan 1991) and the model space is described using a 3-D block parameterization which allows for axes of symmetry that are described by an azimuth and a dip.…”