TRENCH-PARALLEL FLOW BENEATH THE NAZCA PLATE FROM SEISMIC ANISOTROPY

Shear-wave splitting of S and SKS phases reveals the anisotropy and st rain field of the mantle beneath the subducting Nazca plate, Cocos pla te, and the Caribbean region. These observations can be used to test m odels of mantle flow. Two-dimensional entrained mantle flow beneath th e subducting Nazca slab is not consistent with the data. Rather, there is evidence for horizontal trench-parallel flow in the mantle beneath the Nazca plate along much of the Andean subduction zone. Trench-para llel flow is attributable to retrograde motion of the slab, the decoup ling of the slab and underlying mantle, and a partial barrier to flow at depth, resulting in lateral mantle flow beneath the slab. Such flow facilitates the transfer of material from the shrinking mantle reserv oir beneath the Pacific basin to the growing mantle reservoir beneath the Atlantic basin. Trench-parallel flow may explain the eastward moti ons of the Caribbean and Scotia sea plates, the anomalously shallow ba thymetry of the eastern Nazca plate, the long-wavelength geoid high ov er western South America, and it may contribute to the high elevation and intense deformation of the central Andes.