HR: 0800h
AN: V31B-1434    [Abstracts]
TI: Shear Wave Splitting Beneath the Galapagos Archipelago
AU: * Fontaine, F R
EM: fontaine@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique, ISTEEM, CNRS/Universit‚ Montpellier II, Place Eugene Bataillon, Montpellier, 34095 France
AU: Burkett, P G
EM: burkett@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington DC, 20015 United States
AU: Hooft, E E
EM: emilie@newberry.uoregon.edu
AF: Department of Geological Sciences, University of Oregon, 1272 Cascade Hall, Eugene, OR 97403 United States
AU: Toomey, D R
EM: drt@newberry.uoregon.edu
AF: Department of Geological Sciences, University of Oregon, 1272 Cascade Hall, Eugene, OR 97403 United States
AU: Solomon, S C
EM: scs@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington DC, 20015 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington DC, 20015 United States
AB: We report measurements of teleseismic shear wave splitting in the Gal\'{a}pagos Archipelago. The inferred lateral variations in azimuthal anisotropy allow us to examine the dynamics of an evolving hotspot-ridge system. The data are from SKS and SKKS phases, as well as S waves from deep sources, recorded by a relatively dense network of 10 portable broadband seismometers deployed from 1999 to 2003 for the IGUANA (Imaging Gal\'{a}pagos Upwelling and Neotectonics of the Archipelago) experiment and from the GSN broadband station in Santa Cruz (PAYG). We find a delay time between fast and slow shear waves of 0.4 to 0.9 s and fast polarization directions of N85-$90\deg$E beneath five stations at the leading and southern edge of the archipelago. Despite clear seismic signals, we did not find any anisotropy at the six stations located in the interior of the archipelago. For those stations that show shear wave splitting, there is an increase in the delay time toward the expected location of the Gal\'{a}pagos hotspot at the western edge of the archipelago. With the exception of Espa\~{n}ola, fast polarization directions (N85-$90\deg$E) are close to the current direction of absolute plate motion of the overlying Nazca plate (N$91\deg$E). The lack of azimuthal anisotropy in the interior of the archipelago is interpreted as an absence of strongly oriented mantle fabric beneath these stations. The apparent isotropy in this dynamic region, where we expect considerable mantle strain, is surprising. It is not likely that the olivine {\it a}-axis is oriented vertically beneath the interior of the archipelago as the Gal\'{a}pagos plume is thought to lie at the western edge. It is also unlikely that there are two layers of perpendicularly-oriented anisotropy which are solely confined to the center of the archipelago. However, there appears to be some correlation between the region of apparent isotropy and a zone of anomalously low upper mantle velocities imaged beneath Santiago and Marchena from surface waves by Villagomez and others, though the low-velocity region is spatially more confined. This pattern suggests that the presence of melt in the upper mantle may weaken the effects of fabric on shear wave splitting, as suggested by Holtzman and others. An alternative explanation is that the flow field in the near ridge setting is complex, resulting in apparent isotropy. Due to the very young lithospheric age and to the effect of both the neighboring ridge and Gal\'{a}pagos hotspot, the lithosphere thickness is likely thinner than 40 km. Therefore no more than about 40% of the recorded delay time could originate from the lithosphere. We propose that the splitting pattern in the Gal\'{a}pagos Archipelago may be the result of plate drag and frozen lithospheric anisotropy which, beneath the center of the archipelago, is weakened by the presence of melt in the upper mantle.
DE: 8434 Magma migration
DE: 7220 Oceanic crust
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting