Graduate Thesis Or Dissertation
 

Tectonic evolution of the Walvis Ridge and West African margin, South Atlantic Ocean

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  • The opening of the South Atlantic between 140 and 90 m. y. B. P. occurred about two poles of rotation. The initial pole of rotation was maintained until Africa and South America were completely separated. The subsequent removal of restraints imposed by the pre-existing structure of Africa and South America on the early spreading direction permitted a northward migration of the pole of rotation and concomitant reorientation of spreading direction, The NNE trending Cameroon-Gabon and southern Angola coastlines which offset the generally SSE trend of the west African margin between 5°N and the Walvis Ridge are proposed as initial transform offsets of the South Atlantic proto-rift. Location of these initial offsets was controlled by lineations of the Precambrian/Early Paleozoic belts of thermotectonic activity which occur between older, stable cratonic nuclei of Gondwana. Shorter offsets of the continental margin south of the Walvis Ridge and the two major offsets of the west African coastline north of the Walvis Ridge define a pole of rotation at 5°N, 26°w for the initial South Atlantic opening. A set of magnetic anomaly lineations near the continental margins of Angola and southwest Africa is described and named the Benguela sequence. These anomalies were formed during the initial phase of spreading and are displaced right-laterally almost 1000 kilometers across an extension of the continental offset along the southern Angola margin. This offset is named the Benguela Fracture Zone. The Benguela anomalies are correlated with anomalies of the Lynch sequence in the western North Atlantic. The change in direction between the two pre-Cenozoic phases of South Atlantic spreading is dated at roughly 120 m. y. B. P. based upon an extrapolation of the ages of anomalies in the Lynch sequence to the time of reorientation in the South Atlantic. Formation of the South Atlantic quiet zones occurred by sea-floor spreading about a pole of rotation at 21. 5°N, 14. 0°W during the second phase of opening. The present structure of the Walvis Ridge is controlled by nearly orthogonal NE and NW trending faults shown by seismic reflection profiling. The NE trending set of faults approximate small circles of the pole of rotation at 21. 5°N, 14. 0°W and appear to offset the Walvis Ridge topography in a right-lateral sense. Reorientation of spreading would have produced extension across the Benguela Fracture Zone; development of short offset spreading centers along the Benguela Fracture Zone during this reorientation is proposed to explain the right-lateral offsets of the Walvis Ridge topography. Lack of geophysical information on the lower crustal structure prevents a direct explanation of the present elevation of the Walvis Ridge. However the Walvis Ridge is probably underlain by a low density root produced by alteration of the lower crust and upper mantle materials beneath the Benguela Fracture Zone which began during the spreading reorientation. Asymmetric spreading over the Walvis Ridge may have permitted the zone of crustal accretion to remain near the older Benguela Fracture Zone long enough to allow the creation of an anomalously broad low-density root which is responsible for the uplift of the Walvis Ridge.
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