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Title: The geological structure and history of the Gambia Basin and Senegal Continental Margin.
Author: Spathopoulos, Phoebus.
ISNI:       0000 0001 3473 6655
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 1990
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This thesis presents the results of an investigation of the structure and evolution of the Senegal continental margin and the adjacent Gambia Basin, based on seismic, gravity, magnetic, bathymetric and borehole data. It establishes the relation between the structure of this region and the overall geological and tectonic history of the Eastern Atlantic Ocean. The study begins with a presentation of the general geological features of the area and a review of previous work close to the West African continental margin. New bathymetric information from the area between 2°-18°N and 7°-45°W is then presented, which provides a detailed picture of the seafloor relief, its structural grain and an indication of the depositional patterns. Long single-channel seismic reflection profiles running from the continental shelf through the Gambia Basin have been tied to deep-sea drill sites to establish a seismic stratigraphy. The extent of the main reflectors and the structural characteristics of inactive ocean fracture zones have been determined from approximately 3500 km of reflection profiles. Analysis of the basement depth reveals the presence of large depth anomalies in the region, which have been related to the uplift of the nearby Cape Verde Rise. The position of an important tectonic boundary between rough and smooth basement in the study area has been defined as lying between magnetic anomalies M21 and M16 (150-142 Ma). Seismic refraction lines shot in the Gambia Basin and in the region immediately to the south reveal an anomalous oceanic structure, associated with the formation of fracture zones under the sediments of the basin. Crust as thin as 1.4 km, without an oceanic Layer 3 has been found near 1l°N, 23°30'W. In view of the presence of several closely-spaced fracture zones, the crustal structure of much of the Gambia Basin is probably not of normal oceanic type. Possible mechanisms responsible for the anomalous accretion of ocean crust are discussed. Crustal structure has also been inferred from new free-air gravity anomaly measurements between 3°-1.8°N and 7°-26°W and from the Bouguer anomalies on land. Two-dimensional crustal models based on gravity anomalies and constrained by refraction and other data have been derived for several traverses across the West African continental margin and the Gambia Basin. These models demonstrate important differences in crustal thickness and lateral variations in the density of the crustal and mantle rocks across the area. The models indicate that neither the transitional crust under the Senegal Basin and the adjacent shelf and slope, nor the fracture zones under the Gambia Basin are isostatically compensated at the level of the Noho. Furthermore, a large part of the Senegal continental margin and the Senegal Basin is shown to be underlain by transitional, stretched continental crust on which a thick sedimentary cover has accumulated. The subsidence history and the structural models indicate that the Cenozoic sediments are not isostatically balanced by the mechanism of local Airy compensation. These studies reveal that the structure of the Senegal margin and the Gambia Basin exhibits important differences from that of adjacent areas to the north and south, contrasts which appear to be related to the early opening history of the equatorial Atlantic.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geology Geology Mineralogy Sedimentology