Recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust, with total crustal thickness less than half that of normal oceanic crust. The horizontal extent of this anomalous crust is not well known. To map the lateral distribution of thin crust in a major fracture zone, we have constructed free-air anomaly, bathymetry, and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the Vema fracture zone with the Mid--Atlantic Ridge. We isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water-crust interface, the sediment layer, and the crust-mantle interface, assuming constant crustal thickness and density. These calculations are performed using a fully three-dimensional model.

The remaining mantle Bouguer anomaly is dominated by the effects of the cooling lithosphere. Variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three-dimensional conduction of heat. The attraction of this three-dimensional density structure is subtracted from the mantle Bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density. The residual gravity amonaly map indicates that there is thin crust along much of the length of the Vema fracture zone. Thin crust is concentrated primarily beneath the walls of the fracture zone, rather than beneath the center of the fracture zone valley, an interpretation confirmed by seismic refraction data. The degree of thinning along the length of the fracture zone is variable, ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km. The excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust. Other major topographic features, such as the median valley and the elevated transverse ridge along the south wall of the fracture zone, are also not locally compensated by changes in crustal thickness. ¿ American Geophysical Union 1988