The Russians' Findings

Formerly Emerging Crustal Blocks In The Equatorial Atlantic

The above heading is the same as the title of a paper published in 1981 in the journal Tectonophysics. Enrico Bonatti, of Columbia University and the Marine Geology Laboratory in Bologna, Italy, and Andy Chermak, of the University of Miami, are the authors. The National Science Foundation (NSF) and the Office of Naval Research (ONR) sponsored their work, and it was carried out on the Romanche fracture zone shown on Figure 2.

Fig. 2. Schematic representation of the structural pattern associated with the Romanche and other equatorial Atlantic fracture zones. SPSP = St. Peter/St. Paul islets. DSDP 25 = Deep Sea Drilling Project, sea-floor boring no. 25. Solid black triangle indicates sites where shallow water limestones were recovered. Modified from Fig. 1 of Bonatti and Chermak.

Among the results of their study, Bonatti and Chermak identified a strip of sea floor over 300 miles in length along the Romanche fracture zone (FZ) that is anomalously shallow by at least six-tenths of a mile relative to predicted depths. They suggest that the sea-floor high spots are capped by carbonate banks representing stages when they reached close to, and possibly above, sea level. They then estimate that the rate of uplift and sinking of the anomalous crustal blocks was about twice as fast as other crust of equivalent age.

The authors note further that at the Deep Sea Drilling Project (DSDP) hole 25 (Fig. 2), Eocene age shallow water limestones were recovered. They suggest that the emergent crust “may even have provided ‘land bridges’ for faunal migrations between Africa and South America in early Cenozoic times after the two continents had already been separated.” It is here that they bow to the hypothesis of sea-floor spreading and plate tectonics to explain their findings. That was in 1981. Only ten years later, however, the authors’ sea-floor spreading interpretation was to be disputed.