East African Continental Margin Transect
M.F. Coffin, P.D. Rabinowitz, 1983. "East African Continental Margin Transect", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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From November 1980, through January 1981, R/V Vema cruises 3618 and 3619 were devoted to studying the evolution of the East African continental margin and the western Somali basin. Multi- and single-channel seismic reflection, sonobuoy reflection/refraction, gravity, magnetics and echo-sounding data were collected on these cruises. We show prominent features on one of the multichannel seismic lines (line 84V, location in Figure 1) which demonstrate important processes affecting the passive continental margin of southeastern Somalia and northeastern Kenya. These features include diapirs, a sediment slide, and a deep sea channel (Figures 2, 3 and 4). We are able to constrain the ages of the above structures by seismic correlation with a deep drill hole (DSDP Site 241, Leg 25; Simpson et al, 1974).
The regional setting of line 84V is well-constrained by paleomagnetic (ref. group 2), marine magnetic (ref. group 3), gravity (ref. group 4), physiographic (ref. group 5), and paleogeographic (ref. group 6) data. These data support a northerly fit of Madagascar in the Gondwana reconstruction, with Madagascar subsequently drifting south relative to Africa along the Davie Fracture Zone (ref. group 4; also see inset, Figure 1). Therefore, the continental margin of Tanzania and most of Kenya was created by transform motion between Madagascar and Africa; the continental margin of northeastern Kenya and southwestern Somalia was formed by rifting between Madagascar and Africa. Marine magnetic evidence (ref. group 3) indicates that Madagascar and Africa began to separate about 170 m.y. ago; relative motion between the two ceased at about 121 m.y. ago. Thus the age of this passive margin coincides with the initial breakup of Gondwana, and the separation between North America and Africa.
The Vema multichannel data were recorded at 4-ms intervals by a Texas Instruments DFS IV system receiving input from a 1.2 km (0.74 mi) long, 12-channel Seismic Engineering Streamer. Two synchronized Bolt Associates 466 cu in air guns, fired at 15 to 20 sec intervals, comprised the energy source. SSQ 41, 42 and 57 sonobuoys were deployed frequently along the lines to obtain wide-angle reflection and refraction velocities which supplemented semblance velocities for stacking.