Mantle Flow associated with the Peruvian Flat Slab and Subducting Nazca Ridge as inferred from Seismic Anisotropy

Abstract:

The Peruvian flat slab is the largest region of flat slab subduction in the world today and therefore represents an excellent location to study the dynamics and evolution of shallow subduction. We aim to provide insight on current open questions on the topic such as what is the response of the mantle to flat slabs? What is the degree of coupling between a flat slab and the upper plate? What role does slab buoyancy and the subduction of thickened oceanic crust play, such as for the Nazca Ridge? We address these questions by investigating seismic anisotropy across the region using stations belonging to PULSE (PerU Lithosphere and Slab Experiment). We analyze shear wave splitting using a variety of different phases (e.g. local S, SKS, sSKS, and PKS), and also investigate anisotropy observed from the transverse component of receiver functions. We find that mantle flow is modified by flat subduction both below and above the slab. Laterally and along strike there are also sharp boundaries in the anisotropic structure, revealed in both the local S and *KS splitting, that correlate with the present day location of the Nazca Ridge. We infer that the thin mantle layer above the flat slab has been primarily deformed by the southwards migration of the Nazca Ridge across the flat slab region, with trench-parallel deformation trailing the ridge and heterogenous deformation directly in front of it. This supra-slab mantle heterogeneity to the south of the ridge appears to result in a large region of null *KS splitting despite indications that the sub-slab mantle below is anisotropic. Our results are consistent with trench normal flow in the mantle beneath the flat slab and that the slab dip exerts a control on the orientation of mantle flow.