Architecture of a magma poor passive margin - Insights from the Somali margin

The architecture of magma-poor continental margins, from break-up to the establishment of an oceanic spreading centre has received significant recent interest. A general consensus as to their characteristics exists and although it has been applied to margins globally is still dominated by the Iberian-Newfoundland type margin. Additionally, many studies rely upon sparsely distributed 2D seismic reflection profiles, or single seismic sections. Questions remain as to the validity of the model across other magma-poor margins, in particular the role of magmatic additions during the rifting phase and their relationship to the creation of true oceanic crust at the onset of sea-floor spreading. Our data comprise a 2D seismic reflection survey from the Obbia Basin, northern Somalia passive margin. The data are extremely well imaged to over 13 s two-way time, providing insights into the crustal structure. Here, we present 4 representative sections from this larger survey, which combined with gravity modelling, describe the character and crustal architecture along the margin from the proximal rifted continental crust to the oceanic domain. We find that the Somali margin is a poorly understood example of a magma-poor margin exhibiting Iberian-type architecture. The margin comprises a stretched proximal domain that passes seaward into a strongly thinned necking zone across which the lower crust is completely thinned. We identify a high amplitude reflection (HAR) that corresponds to the decoupling level, and into which crustal faults sole out. Seaward, the HAR merges with the top of the exhumed mantle directly underlying post-rift sediments. In contrast to other magma-poor margins, the Somali margin has a narrow necking zone and a wide zone of exhumed mantle (in excess of 100 km). On most sections true oceanic crust is never encountered, with only the distal portion of one section exhibiting oceanic crustal character. We place the continent-ocean transition zone in excess of 200 km east of the shoreline. In the north, the basin is affected by transform faults; where this occurs we note a presence of volcanics within the crust and post-rift sediments. Gravity modelling supports the addition of magmatic material in these regions. This addition of volcanic material, in contrast to that proposed at transform faults in other passive margins, is spatially restricted to the transform faults, emplaced post-kinematically, and is not associated with oceanic crust formation. The results of this study not only provide new insights into the architecture on nontypical non-magmatic margins but also reveal the importance of incorporating these findings into plate-reconstructions.