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Subduction Zone Processes

Melting processes and mantle flow in the Central American subduction zone: From Nicaragua to Costa Rica, seismic imaging has revealed strong along-arc variations in the velocity and attenuation structure of downgoing plate and mantle wedge. These results correlate with geochemical data and indicate along-arc changes in the extent of melting and the role of water fluxed from the subducting lithospheric slab.

To better understand melting processes in the Nicaragua-Costa Rica subduction zone, and in subduction zones worldwide, we deployed 48 IRIS/PASSCAL broadband seismometers across the volcanos of these countries. This project was a collaboration with Geoff Abers and Terry Plank (LDEO, Columbia University), Marino Protti and Victor Gonzalez (OVSICORI, Universidad Nacional, Costa Rica), Wilfried Strauch and Pedro Perez (INETER, Nicaragua) and an excellent team of students. The array operated from July/August, 2004 to March, 2006.

Velocity and attenuation tomography using data recorded by the TUCAN Broadband Seismic Experiment resolve a high velocity, low attenuation subducting slab, a shallow wedge corner with intermediate attenuation, and a slower, more highly attenuating mantle wedge beneath the arc and back-arc. However, velocity and attenuation structures also manifest strong along-arc variations. The subducting slab beneath Nicaragua contains a shallow layer of slow velocities and is more attenuating than the slab beneath Costa Rica, consistent with greater slab hydration. Continuing the along-arc trend, attenuation in the Nicaraguan mantle wedge at depths of 60-100 km is significantly higher than in Costa Rica, suggesting that enhanced slab hydration beneath Nicaragua results in a more hydrated wedge. Moving to the arc, olivine melt inclusions indicate that Nicaraguan magmas are more water-rich on average than those erupted in the Costa Rican arc within the region best-imaged by the TUCAN array. Assuming that these wet melts equilibrated with olivine in the mantle wedge, then the inferred decrease in wedge hydration from Nicaragua to Costa Rica can explain the along-arc variation in wedge attenuation. A roughly vertical column of high Vp/Vs extends from the slab surface to the arc in Nicaragua, but this feature dies away along the arc to Costa Rica. This anomaly could reflect larger melt fractions in the Nicaraguan wedge, possibly produced by the apparently greater wedge hydration.

Seismic imaging with TUCAN array data has also placed constraints on the geometry of flow in the mantle wedge of the Nicaragua-Costa subduction zone. In three-dimensional models of anisotropy obtained by tomographically inverting shear-wave splitting measurements from local events, olivine a-axes are predominantly arc-parallel in the mantle wedge beneath the arc and back-arc at depths of 50 to 150 km. The arc-parallel a-axes extend into mantle wedge well beyond the cold, shallow wedge corner where B-type olivine fabric may occur. The observed anisotropy cannot be explained by simple two-dimensional arc-normal corner flow, and instead suggests significant arc-parallel flow. This interpretation is supported by isotopic variations in arc lavas that suggest NW arc-parallel transport of wedge material at rate comparable to downgoing plate motion.

Several other analyses have also been completed or are underway: Rayleigh wave tomography to image lateral variations in shear velocity and azimuthal anisotropy in the upper plate and underlying mantle wedge; ambient noise tomography; receiver function analysis of crustal thickness and Vp/Vs; and double-difference relocation of the aftershocks of a large strike-slip earthquake to test models of upper plate deformation.

Related Publications:
Abt, D. L., K. M. Fischer, G. A. Abers, J. M. Protti, V. González, and W. Strauch, Constraints on upper mantle anisotropy surrounding the Cocos Slab from SK(K)S splitting, J. Geophys. Res., 115, B06316, doi:10.1029/2009JB006710, 2010b.

MacKenzie, L. S., G. A. Abers, S. Rondenay, K. M. Fischer, Imaging a steeply dipping subducting slab in southern Central America, Earth. Planet. Sci. Lett., 296, 459-468, doi:10.1016/j.epsl.2010.05.033, 2010.

French, S.W., L. M. Warren, K. M. Fischer, G. A. Abers, W. Strauch, J. M. Protti, and V. González, Constraints on upper-plate deformation in the Nicaraguan subduction zone from earthquake relocation and directivity analysis, Geochem. Geophys. Geosyst., 11, Q03S20, doi:10.1029/2009GC002841, 2010.

Abt, D. L., K. M. Fischer, G. A. Abers, W. Strauch, J. M. Protti, and V. González, Shear wave anisotropy beneath Nicaragua and Costa Rica: Implications for flow in the mantle wedge, Geochem. Geophys. Geosyst., 10, Q05S15, doi:10.1029/2009GC002375, 2009.



Abt, D. L., K. M. Fischer, G. A. Abers, J. M. Protti, V. González, and W. Strauch, Constraints on upper mantle anisotropy surrounding the Cocos Slab from SK(K)S splitting, J. Geophys. Res., submitted, 2009.

Abt, D. L., K. M. Fischer, G. A. Abers, W. Strauch, J. M. Protti, and V. González, Shear wave anisotropy beneath Nicaragua and Costa Rica: Implications for flow in the mantle wedge, Geochem. Geophys. Geosyst., 10, Q05S15, doi:10.1029/2009GC002375, 2009.

Hoernle K., D.L. Abt, K.M. Fischer, H. Nichols, F. Hauff, G. Abers, P. van den Bogaard, G. Alvarado, M. Protti, W. Strauch, Geochemical and geophysical evidence for arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua, Nature, 451, 1094-1098, doi:10.1038/nature06550, 2008.

Abt, D., and K. M. Fischer, Resolving three-dimensional anisotropic structure with shear-wave splitting tomography, Geophys. J. Int., doi: 10.1111/j.1365-246X.2008.03757.x, 2008.

Rychert, C. A., K. M. Fischer, G. A. Abers, T. Plank, E. Syracuse, J. M. Protti, V. Gonzalez,W. Strauch, Strong Along-Arc Variations in Attenuation in the Mantle Wedge beneath Costa Rica and Nicaragua, Geochem. Geophys. Geosys. 9, Q10S10, doi:
10.1029/2008GC002040, 2008.

MacKenzie, L. G., A. Abers, K. M. Fischer, E. M. Syracuse, J. M. Protti, V. Gonzalez, W. Strauch, Crustal Structure Along the Central American Volcanic Front, Geochem. Geophys. Geosys., 9, Q08S09, doi:10.1029/2008GC001991, 2008.

Syracuse, E. M., G., A. Abers, K. M. Fischer, L. G. MacKenzie, C. A. Rychert, J. M. Protti, V. Gonzalez, W. Strauch, Seismic Tomography and Earthquake Locations in the Nicaraguan and Costa Rican Upper Mantle, Geochem. Geophys. Geosys., 9, Q07S08, doi:10.1029/2008GC001963, 2008.

Harmon, N., P. Gerstoft, C. A. Rychert, G. A. Abers, M. Salas de la Cruz, and K. M. Fischer, Phase velocities from seismic noise using beamforming and cross correlation in Costa Rica and Nicaragua, Geophys. Res. Lett., 35, L19303, doi:10.1029/2008GL035387, 2008.

Abers, G. A., K. M. Fischer, M. Protti, W. Strauch, The TUCAN broadband seismometer experiment, IRIS Newsletter, 15, 10-12, 2007.

Funding for this research was provided by the MARGINS Program of the National Science Foundation.

Brown faculty collaborators:

None

Other project collaborators:

G. Abers, T. Plank, K. Hoernle, J. M. Protti, V. Gonzalez, W.
Strauch, D.L. Abt, C. A. Rychert, M. Salas de la Cruz, E. Syracuse, L. G. MacKenzie, N. Harmon, P. Gerstoft, H. Nichols, F. Hauff, P. van den Bogaard, G. Alvarado

TUCAN stations shown in yellow

Alexis Walker (left) and Catherine Rychert working at a station near Miravalles volcano in Costa Rica

(l-r): Alexis, Catherine, Karen Fischer and a local friend at a station near Rincon de la Vieja volcano in Costa Rica

Alexis, Laura Auger (Boston University), and Pedro Perez (INETER) installing the northern-most station in the Nicaraguan back-arc

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