The magma-poor Somalian continental margin: Lower crustal boudinage and mantle exhumation

Abstract

Combined interpretation of 2D reflection seismic data and 2D gravity modelling reveals an Iberian-type magma-poor margin across the Obbia Basin in offshore central Somalia. The configuration of the syn-rift and post-rift sediments as well as the upper crust, the lower crust and the Moho delineates four distinct marginal domains. The proximal domain is characterized by thickest crust and narrow underfilled basins, bounded by normal faults piercing the low-reflective upper crust. On top of the upper crust, there is a thick package of Karoo-aged pre-rift sediments (>2 s two-way time (TWT)). A major normal fault, with a throw of 4 s (TWT) dissects the sediments, the crystalline crust and likely also the Moho and represents the necking point, the oceanward limit of the proximal domain and the transition to the necking domain. The necking domain hosts deep half-grabens filled with wedge-shaped syn-rift sediments of likely Early Jurassic age. Crustal thinning was predominantly accommodated by ductile extension of the high-reflective lower crust, resulting locally in lower crustal boudins. With ongoing extension, the upper crust may have become completely detached from the lower crust and rests on the upper mantle at present-day. Within the subsequent hyperextended domain, the Moho shallows significantly and reaches the top basement level in the seaward portion of the line. Normal faults commonly penetrate through the entire crust and bound minor isolated syn-rift deposits. The termination of the hyperextended domain is marked by a major, shallow-dipping, concave downward fault that apparently connects the Moho with the top basement further seaward. Several small continental blocks, so-called extensional allochthones may be present in the transitional area between the coupling point and the exhumation point. The absence of Moho reflections in the east of the profile indicates that crust may be absent and the upper mantle is exhumed.