Introduction and background

The Equatorial region of the Atlantic Ocean is geologically peculiar. The axis of the Mid Atlantic Ridge (MAR) is broken in short ($<$ 200 Km), roughly N-S, segments that are separated by several long offset ($>$ 300 km), E-W transforms. This is clearly seen in satellite radar altimetiy elaborations (Fig. 1) or in maps of distribution of earthquake epicenters (Fig. 2). The largest of the transforms is the Romanche (offset $\sim$ 950 km) located very close to the equator. The St. Paul transform (offset $\sim$ 400 km) is located between 1N and 2N, roughly 180 km north of the Romanche. About 180 km south of the Romanche we find the Chain transform (offset $\sim$ 300 km). Each of these transforms has a strong topographic signature, generally with a deep transform valley parallelled by prominent transverse ridges. These transverse ridges constitute major topographic anomalies relative to the thermal subsidence curve of the oceanic lithosphere.

The topographic signature of each of these major transforms can be traced outside the ridge/ridge offset, and can be shown to extend as fracture zones from one side to the other of the Atlantic (Fig. 1). Given the above, the equatorial Atlantic may be considered a large latitudinal megashear zone that crosses the ocean from coast to coast. The Romanche transform is the longest active transform of the entire mid ocean ridge system HEEZEN1964. SEASAT gravity imagery HAXBY1987 shows that the Romanche fracture zone can be traced across the equatorial Atlantic from an offset of the Gulf of Guinea continental shelf to an E-W branch of the North Brazilian Ridge on the American side (Fig. 1). Thus, the Romanche probably originated as a continent-continent transform at the time of initial rifting of the protoAt1antic.

The Romanche is characterised by a deep, roughly E-W valley flanked on both sides by two prominent ridges and by a system of secondary parallel troughs and ridges HEEZEN1964, GORINI1977, CHERMAK1979. If we assume a constant ridge/transform geometry and an average spreading rate of 1.75 cm/year (one way) both N and S of the transform, the offset represents about 50my. The assumed spreading rate is derived from plate kinematic reconstructions of CANDE1988. The shallowest depths are reached in the narrow transverse ridge flanking the northern side of the transform, east of about 18W. This constitutes a major topographic anomaly that rises up to 4 km above the level predicted by the thermal contraction depth age law BONATTI1981, BONATTI1994. With the assumptions defined above the crust affected by the topographic anomaly ranges in age roughly from 35 to 55 million years. Previous work has indicated that the summit of the transverse ridge was above sea level up to about 5 my before present and subsided since then at a rate one order of magnitude faster than the plate thermal subsidence rate BONATTI1979, BONATTI1981.

We report in this paper the first results of an expedition carried out on the eastern part of the Romanche transform by the research ship Akademik Nikolay Strakhov of Moscow Geology Institute of the Russian Academy of Sciences. This expedition (S-13) was part of PRIMAR (Russian Italian Mid Atlantic Ridge Project), a long range collaboration between the Russian Academy of Sciences and the Italian CNR (National Research Council) for the study of mid ocean ridges. The expedition took place in the period November 1991-January 1992. The field work consisted in multibeam and magnetometric surveys of selected areas at the eastern ridge/transform intersection (RTI) as well as multichannel high-resolution seismic reflection experiments and rock and sediment sampling.

Figure 2: Distribution of earthquake epicenters(1970-1990) in the Central Atlantic (L-DGO data bank).