1. SFB 574,
2. Leibniz Institute of Marine Sciences (IFM-GEOMAR), University of Kiel, Wischhofstrasse 1-3, Kiel 24148, Germany
3. Department of Geological Sciences, Brown University, Box 1846, Providence, Rhode Island 02912, USA
4. Boston University, Department of Earth Sciences, 675 Commonwealth Avenue, Boston, Massachusetts 02215, USA
5. Observatorio Sismológico y Vulcanológico de Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apdo. 10032-1000, Costa Rica
6. Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional, Apdo. 86-3000, Heredia, Costa Rica
7. Instituto Nicaragüense de Estudios Territoriales, Apdo. 2110, Managua, Nicaragua
8. Present address: Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA.

Correspondence to: Kaj Hoernle^1,2 Correspondence and requests for materials should be addressed to K.H. (Email: khoernle@ifm-geomar.de).

Abstract

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, 2, 3} or eroded fore-arc complexes⁴ 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 (63–190 mm yr^-1) and are comparable to the magnitude of subducting Cocos plate motion (85 mm yr^-1). Trench-parallel flow needs to be taken into account in models evaluating thermal and chemical structure and melt generation in subduction zones.

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