Initiation of transform continental margins: the Cretaceous margins of the Demerara plateau

Abstract

During the end of the lower Cretaceous, the connection between the South and the Central Atlantic accretionary axis led to the oblique opening of the Equatorial Atlantic, and to the separation of Africa and South America by alternating transform and rift margins. At the western end of the Equatorial Atlantic, we investigate the structure of the Cretaceous margins surrounding the Demerara plateau, north of French Guiana and Suri-name. These margins were previously described as transform northward and divergent eastward. From the bathymetry and deep structures, we propose to divide the northern transform into three margin segments, with two transform segments separated by a divergent one. These two transform margins are very different, the northwestern one being linear and associated with a steep and erosive continental slope, the northeastern one consisting of several faults and ridges en echelon disposed. In between, the divergent margin appears to be a pull-apart basin localized by structures inherited from the previous Jurassic rifting. Additionally, the eastern divergent margin may have been localized by a thermal anomaly tentatively related to a hotspot. It is proposed that the deformation was first localized in divergent (rift) basins, subsequently connected by transform faults. The structure of the transform fault varies with the offset between adjacent rift basins: a large offset forms a linear transform and a short (less than 200 km) offset forms en echelon structures.