Abstract

The Ceará Rise is an aseismic rise located in the western equatorial Atlantic between latitudes 3° and 8 °N, and longitudes 38 ° and 48 °W. An analogous feature, the Sierra Leone Rise, is located in the eastern equatorial Atlantic. These two rises on the two sides of the Atlantic are bounded by the same fracture zones and are approximately equidistant from the ridge axis. The northern boundary of the two rises is formed by the 8 °N fracture zone, whereas the southern boundary is formed by the 4 °N fracture zone.

Seismic-profiler records show that the acoustic basement under the Ceará Rise is smooth in areas where it is also the shallowest. Moreover, areas of smooth and shallow acoustic basement coincide with the presence of a seismic layer with a velocity of 3.5 km/sec overlying the oceanic layer 2. These observations indicate that volcanic extrusives probably account for the basement relief of the Ceará Rise.

On the basis of the drilling data and on the basis of its distance from the Mid-Atlantic Ridge, we suggest that the Ceará Rise originated approximately 80 m.y. ago. At that time, the region of the Mid-Atlantic Ridge located between the 4° and 8 °N fracture zones underwent a period of extensive volcanic extrusion. This volcanic pile reached a thickness of 2 km. As the spreading continued at the ridge, the volcanic pile was split into two segments. The western segment formed the Ceará Rise; the eastern segment formed the Sierra Leone Rise. The Ceará Rise has continuously subsided since its formation and has been covered by calcareous and siliceous pelagic sediments. However, a large influx of terrigenous sediments from the Brazilian margin, which coincided with the growth of the Amazon Cone since early Miocene time, has buried the western part of the rise under several kilometres of terrigenous sediments. The onlapping of terrigenous sediments over the flank of the rise has continued until at least the end of Pleistocene time.

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