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.
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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.