Lithospheric structure of a nascent spreading ridge inferred from gravity data: The western Gulf of Aden

The Aden spreading ridge (Somalia/Arabia plate boundary) does not connect d irectly to the Red Sea spreading ridge. It propagates toward the East Afric an Rift through the Afar depression, where the presence of a hot spot has b een postulated from seismological and geochemical evidence. The spreading d irection (N37 degreesE) is highly oblique to the overall trend (N90 degrees E) of the ridge. We present and interpret new geophysical data gathered dur ing the Tadjouraden cruise (RN L'Atalante, 1995) in the Gulf of Aden west o f 46 degreesE. These data allow us to study the propagation of the ridge to ward the Afar and to discuss the processes of the seafloor spreading initia tion. We determine the lithospheric structure of the ridge using gravity da ta gathered during the cruise with the constraint of available refraction d ata. A striking Bouguer anomaly gradient together with the identification o f magnetic anomalies defines the geographical extent of oceanic crust. The inversion of the Bouguer anomaly is performed in terms of variations of cru stal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere, which should depend not only on the s eafloor spreading but also on the hot spot beneath East Africa. Our results allow us to define three distinct lithospheric domains in the western Gulf of Aden. East of 44 degrees 45'E the lithosphere displays an oceanic chara cter (thermal subsidence recorded for the last 10 Ma and constant crustal t hickness). Between 43 degrees 30'E and 44 degrees 10'E the lithosphere is o f continental type but locally thinned beneath the axial valley. The centra l domain defined between 44 degrees 10'E and 44 degrees 45'E is characteriz ed by a transitional lithosphere which can be seen as a stretched continent al crust where thick blocks are mixed with thinned crust; it displays en ec helon basins that are better interpreted as extension cells rather than acc retion cells.