Surface-wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the Chaco and Paran¿ Basins of central South America. The upper-mantle velocity structure beneath the Paran¿ Basin is cratonic in character, whereas that beneath the Chaco Basin is tectonic or asthenospheric in character. The surface-wave analysis used broadband recordings from a subset of a 14-station array deployed in a roughly east-west sawtooth arrangement along 20 ¿S latitude, with a total E-W aperture of ~1,400 km. Results from receiver-function analysis, as well as direct P-wave regional travel-time data, were used in the inversions to help constrain Moho depths and crust and upper-mantle velocities. S-wave structure for the intracratonic Paran¿ Basin was determined using interstation phase and group velocities for Rayleigh waves (fundamental and first higher mode) and Love waves (fundamental mode only) based on seven events with paths which traverse the eastern Paran¿ Basin and one event with a path across the western Paran¿ Basin. The average Moho depth in the eastern Paran¿ Basin is ~42 km. The high-velocity upper-mantle lid has a maximum S-wave velocity of 4.7 km/s, with no resolvable low-velocity zone to at least 200 km depth. This cratonic velocity structure indicates the presence of a lithospheric root beneath the Paran¿ Basin despite emplacement of the Paran¿ plume. The limited data from the western Paran¿ Basin are consistent with a homogeneous upper-mantle structure throughout the Paran¿ Basin. Waveform inversion of fundamental-mode and first-higher-mode Rayleigh waves from a single subandean event was used to obtain estimates for pure-path dispersion along propagation paths through the Chaco Basin and the western half of the Paran¿ Basin. The data were partitioned to isolate the partial-path contribution of the phase and group velocities for the Chaco Basin. The phase and group velocities from this somewhat sparse data set were inverted to obtain a velocity-depth model for the Chaco Basin. The distinguishing features of the Chaco model consist of a rather shallow Moho depth, 32 km, and low (asthenospheric) upper-mantle S-wave velocities, about 4.2 km/s, with velocity increasing only slightly to about 4.3 km/s at 150 km depth.¿ 1997 American Geophysical Union