Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua

Resolving flow geometry in the mantle wedge is central to understanding the thermal and chemical structure of subduction zones, subducting plate dehydration, and melting that leads to arc volcanism, which can threaten large populations and alter climate through gas and particle emission. Here we show that isotope geochemistry and seismic velocity anisotropy provide strong evidence for trench-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua. This finding contradicts classical models, which predict trench-normal flow owing to the overlying wedge mantle being dragged downwards by the subducting plate. The isotopic signature of central Costa Rican volcanic rocks is not consistent with its derivation from the mantle wedge 1–3 or eroded fore-arc complexes 4 but instead from seamounts of the Galapagos hotspot track on the subducting Cocos plate. This isotopic signature decreases continuously from central Costa Rica to northwestern Nicaragua. As the age of the isotopic signature beneath Costa Rica can be constrained and its transport distance is known, minimum northwestward flow rates can be estimated (63–190mmyr 21 ) and are comparable to the magnitude of subducting Cocos plate motion ( 85mmyr 21 ). Trench-parallel flow needs to be taken into account in models evaluating thermal and chemical structure and melt generation in subduction zones. Preferential alignment of the minerals olivine and orthopyroxene occurs as a result of deformation, and produces anisotropy in seismic-wave velocities within the upper mantle. Assuming classical trench-normal ‘corner flow’ and standard models of crystallographic fabric development, the direction of fast shear-wave polarization will be normal to the arc in the warmer wedge beneath the arc and backarc, although the fast shear-wave polarization may be parallel to the arc in the cold corner of the wedge beneath the fore-arc 5,6 . Whereas this pattern of seismic anisotropy is observed in some subduction zones 7,8 , many display more variable fast directions further from the trenchbeneath thearcandback-arc,often witharoughlyarc-parallel trend 9–12 . The origin of this arc-parallel fast anisotropy, in particular whether it is related to along-arc flow within the mantle wedge or