HR: 0800h
AN: T31A-0274    [Abstracts]
TI: Mantle flow and lithospheric detachment beneath the Puna plateau: Insights from seismic anisotropy.
AU: * Sanders, K
EM: glkes@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, BS8 1RJ, United Kingdom
AU: Kendall, J
EM: gljmk@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, BS8 1RJ, United Kingdom
AU: Rietbrock, A
EM: A.Rietbrock@liverpool.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, Liverpool, L69 3GP, United Kingdom
AB: Collision of the Nazca and South American plates in the central Andes has generated the Altiplano-Puna plateau, the second largest continental plateau on Earth. A range of mechanisms has been proposed to explain this feature, including magmatic addition, tectonic shortening and lithospheric thinning. Recent deployments of temporary seismic networks in the region are providing seismic images that help constrain the mechanism. Beneath the Puna plateau and between 23°S and 24°S, travel-time and attenuation tomography have illuminated a near-vertical high-velocity and high-Q_p structure, which has been interpreted as detached continental lithosphere. Here we use observations of shear-wave splitting in both local and teleseismic phases to constrain the style of anisotropy beneath the region and help interpret deformation in the upper-mantle wedge. Splitting measurements are made at nearly 70 stations using 7 teleseismic (SKS) events and 16 local events, which range in depth from 90~km to 250~km. The magnitude of the splitting is highly variable throughout the region, ranging from 0.1secs to 1.4secs in the local data and ranging from 0.4 to 2.2 secs in the SKS data. In general the magnitude of the splitting for the local events is much larger than that seen in other subduction regions and can explain most of the SKS splitting. The polarization of the fast shear-wave is generally N-S beneath the Eastern Cordillera and to some extent also beneath the Western Cordillera. Beneath the central volcanic region the polarizations are more variable. Between 23°S and 24°S there is significant deviation in the splitting to a more E-W polarization in the fast shear-wave. We interpret this is a being due to a perturbation in the mantle flow around the region of detached lithosphere. A lack of obvious depth dependence in the splitting suggests that deformation is most pronounced in the region between the detached lithosphere and crust. Anisotropy in the thickened lower crust may also contribute to the observed splitting. It would seem that detachment has disturbed the convective regime of the upper-mantle wedge and the signature of this is revealed in the pattern of shear- wave splitting.
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting