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Abstract.We have analyzed surface wave data ibr 245 events oc-{'•n'ring in Antarctica and surrounding oceans during yea. rs 1991-1999. We first apply dispersion analysis to t, he fimdamental mode of Rayleigh waves for peri-()ds ranging from 20 to 150 sec. We then invert pathaveraged dispersion curves to obtain 2D maps of group velocity for different periods, represented by splines on a, grid with 250 km spacing, with considerably more detail than that achieved by global models. We finally use gr6up velocity maps in a nonlinear inversion for the regional 3D v•v structure of the upper mantle. The resulting model shows thick continental roots under East Al•tarctica, reaching depths below 200 km; low velocities •mder oceanic ridges, mostly confined to the upper 150 kill; and a slow area under the Ross Sea volcanic (:entres, extending all the way down to 200 km, beyond which our data loose resolving power.

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S U M M A R YThe deformation and stress fields accompanying dyke injection in rift zones are described in terms of a vertical crack opening in response to internal overpressure, in plane-strain configuration. Previous numerical computations of the displacement field induced by shallow dykes are found to be affected, in some cases, by significant distortion, mainly due to the limited extension of the computational domain, difficulties in handling the singularities in the proximity of the free surface and an incorrect choice of the reference frame; in particular, it is found that no subsidence can be ascribed to the mere opening of a tensile crack in a homogeneous, elastic half-space. If analytical constant-dislocation models are employed, surface displacements compare reasonably well with crack-model solutions if the upper tip is not too shallow; however, constant dislocation solutions present unphysical singularities along the tips, which distort significantly the near-field stress pattern with respect to crack solutions. If the principal stress axes are computed from crack models, a broad region is found on both sides of the dyke where the induced stress has a nearly vertical intermediate axis while the compressive axis is normal to the dyke plane. Strike-slip earthquakes are expected to prevail in this region. Above the upper crack tip, a small region is present where the tensile stress is dominant and the intermediate stress is still vertical. In the proximity of the free surface, typically within a few tens of metres of it, induced tensile stresses are greater than the lithostatic pressure: open fissures might then develop in cohesionless soil or pre-faulted rock. The induced pressure in the host rock is found to be negative (suction) in the proximity of the ground surface and positive at greater depth: fluid flow within the aquifers can be significantly altered by this induced overpressure and by the anisotropic modification of the pre-existing permeability. According to the modified Coulomb failure criterion, in the short term the shallower region, characterized by suction, is strengthened, while the deeper, pressurized region is weakened. These results can explain in a straightforward way the abundance of strike-slip focal mechanisms in volcanic areas, the switch between tensional and compressional axes inferred from focal mechanisms of earthquakes in connection with a dyke injection episode on Mount Etna, en echelon fracture systems observed at Piton de la Fournaise above the feeding dyke and precursory geochemical anomalies.

Northern Victoria Land is located at the boundary between an extended, presumably hot, region (West Antarctic Rift System) and the thick, possibly cold, East Antarctic craton. The style and timing of Tertiary deformation along with relationships with the magmatic activity are still unclear, and contrasting models have been proposed. We performed structural and morphotectonic analyses at the NE termination of northern Victoria Land in the Admiralty Mountains area, where the relationship between topography, tectonics, and magmatism is expected to be well pronounced. We found evidence of two subsequent episodes of faulting, occurring concurrently with the Neogene McMurdo volcanism. The first episode is associated with dextral transtension, and it is overprinted by extensional tectonics during the emplacement of large shield alkaline volcanoes. Upper mantle seismic tomography shows that the extensional regime is limited to regions overlying a low‐velocity anomaly. We interpret this anomaly to be of thermal origin, and have tested the role of large‐scale upwelling on lithosphere deformation in the area. The results of this integrated analysis suggest that the morphotectonic setting of the region and the magmatism is likely the result of upwelling flow at the boundary between the cold cratonic and the hot stretched province (WARS), at work until recent time in this portion of the northern Victoria Land.