East African Margin Ocean-Continent Transition Structure and Location: Offshore and Onshore

We investigate the structure and location of the ocean-continental transition of the East African continental margin basins formed during mid-Jurassic breakup of Gondwanaland using combined seismic and gravity inversion methods. Moho depth, crustal basement thickness and ocean-continent transition location has been mapped from northern Mozambique to southern Somalia using a new gravity inversion method incorporating a lithosphere thermal gravity anomaly correction together with offshore sediment thicknesses derived from ION-GXT deep long-offset seismic reflection data. Onshore we use public domain sediment thickness data. Crustal thickness mapping suggests that the coastal region of southern Somalia is underlain by the ocean-continent transition and that substantially thinned continental extends to the north-west with the continental margin hinge located approximately 200 km from the coast. In contrast to the southern Somali rifted margin, its conjugate northern Madagascar (Comoros) margin shows a much sharper (narrower) transition from thick continental crust to thin oceanic crust. Results show that, offshore mid Tanzania, thin crust extends westwards from the Western Somalia Basin across the Davie and Sea Gap fracture zones. Variations of crustal thickness between the Davie and Sea Gap fracture zones suggest both transtensional and transpressional tectonics within oceanic or highly thinned continental crust. The nature of the East African margin appears to be more complex than that predicted by the dominantly transform fault kinematics predicted by simple plate kinematic models for the mid Jurassic formation of the Somali Basin. In addition the margin structure and geometry is influenced by Karoo and possibly earlier rifting, as well as Cenozoic fault reactivation. Moho depths determined by gravity inversion are validated by refraction seismology measurements.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California