A discussion concerning the floor of the northwest Indian Ocean - Crustal structure between Kenya and the Seychelles

Abstract

A series of seismic refraction profiles has been shot between Kenya and the Seychelles Bank and in the neighbourhood of the Bank itself. Thick sediments have been observed for 300 to 400 km from the African coast. Near Kenya, great thicknesses of material of about 4.8 km/s velocity match closely the 9 to 10 km of Karroo beds expected on the coast at Lamu. The Mohorovicic discontinuity has been traced from 100 km off the African coast to the Seychelles Bank. West of the Bank the mantle is unusually shallow, rising to only 8.5 km below the surface, and the 6.8 km/s crustal layer unusually thin or absent. The absence of a gravity anomaly associated with this very shallow mantle raises a problem which has yet to be resolved.

Structure of the western Somali Basin

“…However, comparison of the two shows that the depth of the deepest observed reflector of the stratified sequence of the rise is comparable to that of the top of the 2.5-km/sec velocity layer as identified by Francis et al [1966]. DISCUSSION …”

Leg 25, Results in Relation to East African Coastal Stratigraphy

“…At Site 241, at the foot of the continental rise, the drill penetration amounted to 1174 meters, but the sedimentary column is believed to be 3.7 km thick overall (Francis et al, 1966). With an early Late Cretaceous date for the bottom of the hole, this implies a sedimentary history probably extending back into the Jurassic, with a fair presumption that true oceanic conditions were established by Late Jurassic times.…”

“…Site 240 lies to the east, 500 miles offshore. These sites are near the refraction traverse of Francis et al (1966) which demonstrated progressive easterly thinning of the coastal sedimentary column with apparent continuity of continental-type basement across the basin as far east as the Seychelles.…”

The structural and sedimentological evolution of the Somali Basin : paleoceanographic interpretations

“…If an isostatic model which assumes a negligible density contrast at the base of the lithosphere is used, the equivalent depth from sediment loading is: To translate these elevations into ages and determine tne accuracy of the method several data sets must be compared as in figure 14. The axis of present day spreading forms the comparison of crustal sections in the Somali Basin (Francis et al, 1966) 2055'S, 47°02'E and Gulf of Mexico (Menard, 1967). Other Somali Basin sections nearer A frica show up to 4 kms of sediments (Francis et a 1., 1966).…”

“…(1972) and Francis et al (1966) While the extent of the fracture zone may be extrapolated to 7. 50S its relationship with sea floor and continental structures at the northern tip of Madagascar (120 S)cannot be defined on the basis of this data alone.…”