Thomas, Martin Vaughan; (1990) A geophysical study of the Romanche Transform and surrounding areas of the Mid-Atlantic Ridge. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Using a combination of GLORIA long range side-scan sonar, bathymetry, seismic reflection profiles, free-air gravity and total magnetic field data, this study looks at the morphology and structure of an area centred on the Romanche Transform in the Equatorial Atlantic. It also includes portions of both the active and inactive sections of the Saint Paul Fracture Zone, together with adjacent seafloor areas. The main tectonic and structural features are mapped from GLORIA sonographs. In particular, sonar records enable precise mapping of the plate boundary separating the African and South American plates. The identification of the Romanche Transform Fault allows a direct comparison between the direction of local plate motion and that predicted by global plate tectonic models. The overall trend of the Romanche Transform Fault is about 3o north of that predicted from the recent global models. Gravity data, after correction for topography, sediment thickness and a crustal layer of average thickness and density, show regional anomalies of up to 100mgal across the Romanche Fracture Zone. This phenomenon is attributed to the different ages and hence contrasting thermal and density structures of juxtaposed lithospheric blocks in fracture zones. These regional fields are isolated and removed using the method of upward continuation. Residual anomalies are modelled by local changes in the thickness and density of crust and reductions in upper mantle density using the results of seismic refraction experiments over fracture zones as constraints on possible solutions. Modelling indicates a lowering of crustal and, probably, upper mantle density beneath the Romanche Transform Valley. It also suggests that the crustal, and upper mantle structure, is generally different beneath the Saint Paul Fracture Zone fracture valleys and Romanche Transform Valley. Factors affecting magnetic anomalies associated with fracture zones located within the study area are examined. Two-dimensional modelling of total field magnetic profiles indicates that the intensity of magnetisation of crustal source rocks is inhomogeneous within the Romanche Fracture Zone. This modelling also indicates that crustal source rocks are weakly magnetised within the southern fracture valley of the Saint Paul Fracture Zone.

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