Ambient Noise Tomography of the Pampean Flat Slab Region

Ambient Noise Tomography of the Pampean Flat Slab Region, Figure 2 Figure 2.
Cross-section of shear wave velocities at 31 degrees S. (Location shown in Figure 1). The low velocities observed above the Moho at -66 to -68 degrees are likely due to uncertainties in Moho depth. Measurements are corrected for topography and zones with less than 100 km resolution are shaded out. Green dashed lines show surface locations of terrane boundaries. Topography is exaggerated 10x.
<p>
Ambient noise tomography is a recently developed seismic analysis technique that uses background noise to approximate the seismic velocities with a region. We apply this technique to a study of the Pampean flat slab region (Fig. 1) to better understand crustal features related to the convergence of the Nazca and South American Plates. Flat slab subduction has led to a shut- off of arc magmatism and a migration of deformation inboard from the plate margin. The region’s crust is composed of several terranes accreted on the Rio de la Plata craton with zones of thin-skinned deformation in the Precordillera and thick-skinned deformation in the Sierras Pampeanas. The purpose of this work is to use ambient noise to better understand the role these features play in the region’s tectonic evolution. Ambient noise tomography is based on the principal that the cross-correlation of seismic noise recorded at two seismic stations can approximate the Green’s function between the stations. This noise is generally associated with ocean waves colliding with the coast. We use the ambient noise to calculate Rayleigh wave dispersion curves and convert those measurements into shear wave velocities. A detailed description of the method is found in Benson et al. [2007]. Two advantages of ambient noise over traditional surface wave tomography are that it produces a higher quality signal at shorter periods and it does not require earthquakes at certain backazimuths and distances. Initial results from this work suggest that shear wave velocities in the upper crust are primarily controlled by the presence of (slow) basins and (fast) bedrock exposures and that these basins may extend as deep as ~10-12 km (Fig 2). Lower velocities are observed beneath the shutoff volcanic arc than in the Sierras Pampeanas suggesting that a) the presence of arc related rocks is retarding seismic velocities or b) that the differences in Moho depth is impacting the measured seismic velocities. We generally observe faster seismic velocities in areas of thick-skinned deformation than in thin-skinned areas. This is especially true beneath the Precodillera, which has accommo- dated 60–75% of the surface shortening since 10 Ma [Allmendinger, 1990], and exhibits relatively slow velocities down to 30 km, suggesting that deformation is preferentially focused in this low velocity region.
</p><p>References
</p><p>Allmendinger, R., D. Figueroa, D. Snyder, J. Beer, C. Mpodozis, and B. Isacks (1990), FORELAND SHORTENING AND CRUSTAL BALANCING IN THE ANDES AT 30°S LATITUDE, Tectonics, 9(4), 789-809.
</p><p>Anderson, M.L., Alvarado, P., Zandt, G., Beck, S. (2007), Geometry and brittle deformation of the subducting Nazca Plate, Central Chile and Argentina. Geophys. J. Int., 171, 419-434.
</p><p>Bensen, G.D., M.H. Ritzwoller, M.P. Barmin, A.L. Levshin, F. Lin, M.P. Moschetti, N.M. Shapiro, and Y. Yang (2007), Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements, Geophys. J. Int., 169, 1239-1260.
</p><p>Acknowledgements: This work was funded by NSF award #0510966</p>

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