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

Abstract

Abstract. The Aden spreading ridge (Somalia/Arabia plate boundary) does not connect directly to the Red Sea spreading ridge. It propagates toward the East African Rift through the Afar depression, where the presence of a hot spot has been postulated from seismological and geochemical evidence. The spreading direction (N37øE) is highly oblique to the overall trend (N90øE) of the ridge. We present and interpret new geophysical data gathered during the Tadjouraden cruise (R/V L 91talante, 1995) in the Gulf of Aden west of 46øE. These data allow us to study the propagation of the ridge toward the Afar and to discuss the processes of the seafloor spreading initiation. We determine the lithospheric structure of the ridge using gravity data gathered during the cruise with the constraint of available refraction data. A striking Bouguer anomaly gradient together with the identification of magnetic anomalies defines the geographical extent of oceanic crust. The inversion of the Bouguer anomaly is performed in terms of variations of crustal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere, which should depend not only on the seafloor 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 44ø45'E the lithosphere displays an oceanic character (thermal subsidence recorded for the last 10 Ma and constant crustal thickness). Between 43ø30'E and 44ø10'E the lithosphere is of continental type but locally thinned beneath the axial valley. The central domain defined between 44ø10'E and 44ø45'E is characterized by a transitional lithosphere which can be seen as a stretched continental crust where thick blocks are mixed with thinned crust; it displays en echelon basins that are better interpreted as extension cells rather than accretion cells.

Large-scale phylogeny of chameleons suggests African origins and Eocene diversification

“…Of these peripherally distributed species, Chamaeleo monachus from Socotra Island is the oldest (ca 20 +5 Myr). Although sea floor spreading in the Western portion of the Gulf of Aden is recent, Socotra is located substantially eastward, where rifting was complete ca 20 Ma [56][57][58][59]. The timing of divergence for C. monachus overlaps with the timing of separation between Socotra and Africa, and therefore, neither the isolation owing to vicariance nor oceanic dispersal [60] can be ruled out.…”

The Afar volcanic province within the East African Rift System: introduction

“…Dunbar & Sawyer (1989) use numerical models to demonstrate the role of zones of pre-existing lithosphetic weakness in the localization of strain at break-up, with or without the development of lithospheric-scale detachments. Taylor et al (1999) andHebert et al (2001) document the along-axis propagation of oceanic tiffing into the continental lithosphere through the emplacement of discrete mantle upwellings and ridge jumps, while Kusznir & Tymms (2003) have recently introduced models of depth-dependent stretching in which upper crustal extension is much less than that of the lower crust and lithospheric mantle, to…”

Mixing and Entrainment in the Red Sea Outflow Plume. Part I: Plume Structure

“…The seafloor topography south of the Strait of Bab el Mandeb is unusual in that rather than showing a conventional continental shelf, the terrain slopes down southward to a depth of about 800 m until it drops precipitously into the tectonically active, up to 1600 m deep, Tadjura Rift. The topography along the NC and SC was compiled by S. Swift (2002, personal communication; see also Hébert et al 2001) from multibeam soundings during our own two cruises on the Knorr and the Maurice Ewing as well as from an earlier cruise of the L'Atalante. The depths along both channel axes are shown in Fig.…”