Initial stages of oceanic spreading in the Bransfield Rift from magnetic and gravity data analysis

Bransfield Basin, a 500-km-long and 100-km-wide extensional structure with a well-marked NE–SW orientation, is considered a back-arc basin developed since the Pliocene and associated with subduction of the former Phoenix Plate below the South Shetland Islands Block. Extension also occurs in this area as a consequence of the end of the sinistral fault zone that deforms the South Scotia Ridge. On the basis of potential field data from marine cruises, we provide new magnetic and Bouguer gravity maps of the area at sea level. We have characterized the central magnetic anomaly by using Euler deconvolution method, spectral analysis and forward modeling obtaining a thin (1.5 km) and shallow (4 km b.s.l.) layer, and a low total magnetization (2.6 A/m). The forward modeling was constrained on basis of previous seismic refraction studies. Our models show two situations. The first presents a uniform density values along the entire crust in the basin. This would be compatible with rifting in a more advanced stage, or even an oceanic crust in its earliest stages, while the second would support the existence of a stretched, thinned and altered crust through the injection of volcanic material. In the light of these models, analysis of the new potential field maps presented in this work and information from previous studies we consider that the Central Bransfield Basin is in a rifting in its latest stages or presents an incipient oceanic crust formed by recent oceanic spreading.