ALBERT

Description: Movements within early Gondwana dispersal are poorly constrained and there is uncertainty about the position and structural style of the continent-ocean transition and the timing and directions of the rifting and earliest seafloor spreading phases. In this paper, we present a combined structural interpretation of multichannel reflection seismic profiles from offshore northern Mozambique (East Africa), and the conjugate Riiser Larsen Sea (Antarctica). We find similar structural styles at the margins of both basins. At certain positions at the foot of the continental slope, the basement is intensely deformed and fractured, a structural style very untypical for rifted continental margins. Sediments overlying the deformation zone are deformed and reveal toplap and onlap geometries, implying a post-breakup deformation phase. We propose this unique deformation zone as tie-point for Gondwana reconstructions. Accordingly, we interpret the western flank of Gunnerus Ridge, Antarctica as a transform margin, similar to Davie Ridge, East Africa, implying that they are conjugate features. We consider it likely that a first phase of rifting and early seafloor spreading in NE-SW direction was subsequently replaced by a N-S directed transform deformation phase, overprinting the continent-ocean transition. This change of the spreading directions from NW-SE to N-S is suggested to have occurred by the Late Middle Jurassic, around magnetic anomaly M38n.2n (~ 164 Ma). We suggest that the second phase of deformation corresponds to the strike-slip movement of Madagascar and Antarctica and discuss implications for Gondwana breakup.

Description: Movements within early East Gondwana dispersal are poorly constrained, and there is debate about conjugate geologic structures and the timing and directions of the rifting and earliest seafloor spreading phases. We present a combined structural interpretation of multichannel reflection seismic profiles from offshore of northern Mozambique (East Africa) and the conjugate Riiser-Larsen Sea (Antarctica). We find similar structural styles at the margins of both basins. At certain positions at the foot of the continental slope close to the continent–ocean transition, the basement is intensely deformed and fractured, a structural style very untypical for rifted continental margins. Sediments overlying the fractured basement are deformed and reveal toplap and onlap geometries, indicating a post-breakup deformation phase. We propose this unique deformation zone as a tie point for Gondwana reconstructions. Accordingly, we interpret the western flank of Gunnerus Ridge, Antarctica as a transform margin similar to the Davie Ridge offshore of Madagascar, implying that they are conjugate features. As the continental slope deformation is post-rift, we propose a two-phase opening scenario. A first phase of rifting and early seafloor spreading, likely in NW–SE direction, was subsequently replaced by a N–S-directed transform deformation phase overprinting the continent–ocean transition. From previously identified magnetic chrons and the sediment stratigraphy, this change in the spreading directions from NW–SE to N–S is suggested to have occurred by the late Middle Jurassic. We suggest that the second phase of deformation corresponds to the strike-slip movement of Madagascar and Antarctica and discuss implications for Gondwana breakup.