Crustal and upper mantle S-wave velocity structure beneath the Bransfield Strait (West Antarctica) from regional surface wave tomography A. Vuan a, * , S.D. Robertson Maurice b , D.A. Wiens c , G.F. Panza d,e a Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, 34010, Sgonico, Trieste, Italy b 12450 Greenspoint Drive, Suite 811, Houston, TX 77060, USA c Dept. of Earth and Planetary Sciences, Washington University of Saint Louis, St. Louis, MO., USA d Dipartimento di Scienze della Terra, Universita ` degli Studi di Trieste, v. E. Weiss 4, 34126 Trieste, Italy e Abdus Salam International Centre for Theoretical Physics, Strada Costiera, Trieste, Italy Received 4 March 2004; accepted 7 December 2004 Available online 25 January 2005 Abstract Regional surface wave tomography in the sub-Antarctic Scotia Sea is helpful in revealing the nature of the crust and the S- wave seismic velocity profile beneath the Bransfield Strait. The joint use of our regional network, global seismographic network stations and local temporary arrays provide better lateral resolution than that obtained in our previous studies concerning the Scotia Sea region. Tomographic analysis of data obtained using 10 broad band seismic stations and more than 300 regional events, shows that the Bransfield Basin is characterised by a strong group velocity reduction of 8% with respect to the surrounding areas, in the period range from 15 s to 50 s. The crustal and upper mantle models of the eastern, central and western Bransfield Basin are obtained by joint inversion of Rayleigh and Love local dispersion curves from 15 s to 50 s. In addition our data set is expanded to a broader period interval (1–80 s), in central Bransfield Strait in order to better constrain the upper mantle and shallow crust. The main results can be summarized as follows: (a) the crust thins distinctly from W toward E; the variation is consistent with the type of volcanism, earthquake distribution and bathymetric observations, (b) low upper mantle velocities (soft lid) extend down to depths exceeding 70 km as a consequence of elevated temperatures, (c) the crust beneath the central Bransfield Basin displays continental characteristics with a gradually increasing S-wave velocity distribution versus depth analogous to the East African Rift structure of Kenya, (d) negative velocity gradients are present in the lower crust beneath 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.12.011 * Corresponding author. Dipartimento bCentro di Ricerche SismologicheQ, CRS, Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Borgo Grotta Gigante 42c, 34010, Sgonico (Trieste), Italy. Tel.: +39 040 2140370; fax: +39 040 327307. E-mail address: avuan@inogs.it (A. Vuan). Tectonophysics 397 (2005) 241– 259 www.elsevier.com/locate/tecto