The Northern Somali Basin, located between Chain Ridge and the Horn of Africa north of 4 ¿N, is characterized by a distinct 5-m geoid low and by large negative gravity anomalies. The boundaries of the basin are marked by steep gradients in both gravity and geoid. Basement in Northern Somali Basin is 1--2 km deeper than on the Carlsberg Ridge flank to the southeast or on the Sheba Ridge flank to the north with a sharp discontinuity across the boundary of the basin. The Western Somali Basin, to the south, was created in the Late Jurassic and Early Cretaceous by the movement of Madagascar away from Africa. Reinterpretation of the magnetic anomalies in the Western Somali Basin shows that they record both limbs of a mid-ocean ridge that was active by M22 time (Kimmeridgian) and died soon after MO (Aptian). Magnetic and gravity data allow the relic ridge crest to be traced from Davie Ridge near the African coast to the Dhow-VLCC-ARS fracture zone complex at 50¿E. Davie Ridge is a transform fault connecting the Western Somali Basin spreading center with a similar age spreading center in the Mozambique Basin.

The Dhow-VLCC-ARS complex can be shown to continue north of the 4 ¿N bathymetric high separating the Northern and Western Somali Basins and to intersect the African margin near 7 ¿N. The Northern Somali Basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between East and West Gondwanaland. The original Northern Somali Basin was split apart by the northward motion of India in the Late Cretaceous and Chain Ridge formed along the new boundary. Thermal and gravity modeling shows that the flexure resulting from differential subsidence across Chain Ridge combined with the difference in lithospheric thermal structure (Late Jurassic vs. Early Tertiary) on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across Chain Ridge. The geoid step up from the basin up to the African coast can be modeled as an edge effect between the juxtaposed Jurassic oceanic and African continental crust. Thus, the geoid and gravity low over the Northern Somali Basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly. ¿ American Geophysical Union 1988