Structural setting of the westernmost Agulhas-Malvinas fracture zone at the intersection with the Argentinean Continental Shelf

GEOPHYSICAL DATA AND STATE OF THE ART. The northern flank of the Malvinas Plateau structurally coincides with the western termination of the Agulhas-Malvinas Fracture Zone (Figs.1 and 2), a huge oceanic fracture system which driven the tectonic development of the southern Atlantic Ocean. The Malvinas plateau is considered to be part of Argentinean continental platform elongated towards east, and is delimited by the Malvinas trough to the south and by the Malvinas escarpment to the north (i.e.: Rabinowitz and LaBreque, 1979; Del Ben and Mallardi, 2004). Some Authors, however, consider the Malvinas plateau linked to an oceanic sub-aerial spreading during the initial opening of the South Atlantic and driven by the Falkland/Malvinas microplate rotation, occurred during the separation of the southern Gondwana continent between South Africa and South America (i.e.: Barker, 1999; Macdonald et al., 2003). Interpretation of multichannel seismic reflection profile (Fig. 3a), identification of marine magnetic anomalies (Fig. 3b), and analyses of satellite-derived data of the northern margin of the Malvinas plateau, are here presented. Within the framework of the joint Project between Italian and Argentinean Institutions about 2762 km of multichannel seismic lines, 3000 km of magnetic line have been interpreted off the Atlantic Argentinean margin. A segment of one of the seismic and magnetic lines is presented and synthesized in this contribution. The methodology applied in the seismic analysis consisted in the identification of acoustic fabrics and seismic units. The seismic section was divided in discontinuity limited units based on both the reflection terminations and the nature of the discontinuity surfaces - discontinuities, correlative continuities, hiatus - (Mitchum et al., 1977; Brown and Fisher, 1980). Sixt main discontinuities were identified which allowed to recognize five main units overlying the acoustic basement. Because of the great distance of the wells DSDP, the seismostratigraphic interpretation has been realized with the bibliographical aid, among which Lorenzo & Mutter (1988) and Lorenzo and Wessel (1997). DISCUSION. Available geophysical information used in this study show that the ocean-continent transition is located along the east-west-trending Malvinas escarpment. The analysis of seismic profiles and recently acquired magnetic data, and the presence of a gravimetric lows in the studied area (Macdonald et al, 2003), allow to hypothesize that the internal part of the Malvinas escarpment could be still oceanic in nature, as indicated by the magnetic anomaly pattern (Petruccione in prep.), in disagreement with the crustal model based on magnetic end gravity data introduced by Lorenzo and Wessel (1997). Our hypothesis seems confirmed by the presence of dredge basalts and gabbros recovered on the top of the escarpment at the same location (Lorenzo and Mutter, 1988), and by the southern divergent configuration of sediment reflectors in seismic profiles, which overlay the dipping south faulted acoustic basement.