Geological studies of the eastern part of the Romanche transform (equatorial Atlantic): a first report.

The Mid Atlantic Ridge is intersected by a number of long-offset transforms in the equatorial region. The longest is the Romanche (offset 950 km) located close to the equator. The St. Paul transform (offset ~ 400 km) is located about 180 km to the north, the Chain transform (offset ~ 300 Km) about 180 km to the south. Multibeam and magnetometric surveys as well as high resolution multichannel seismic reflection experiments and rock and sediment sampling were carried out during a recent expedition to the eastern part of the Romanche transform. This field work is part of PRIMAR (Russian Italian Mid Atlantic Ridge Project), a collaboration for the study of mid ocean ridges between the Russian Academy of Sciences and the Italian CNR (National Research Council). A detailed morphobathymetric survey of a limited area at the eastem ridge/transform intersection (RTI) suggests that the axial segment of spreading approaching the transform loses a well delined morphotectonic signature. An overall oblique trend prevails, probably resulting from a number of short en-enchelon ridge segments. No well defined nodal deep was observed. An aseismic rift valley was observed about 80 km west of the present RTI suggesting a ridge jump to the east sometimes within the last 5 my. A markedly alkalic magmatism has been recently active near the RTI. These data suggest that the axial system of spreading approaching the Romanche transform from the south is sluggish and not well established, possibly due to a relatively "cold" upper mantle thermal regime below. Major positive topographic anomalies, reaching over 4 km above the predicted thermal contraction level of the crust, are found on the transverse ridge opposite the eastern RTI. Seismic reflection profiles and bottom samples indicate that shallow reliefs on the crest of the transverse ridge are wave-eroded blocks of oceanic lithosphere that formed islands between 10 and 5 my ago and subsided since then at rapid (~ 0.2 mm/y) rates. Their summits are now covered by originally shallow water reef-lagunar carbonate caps, whose thickness ranges from 200 to 400 m. An aseismic valley is observed to the north of and subparallel to the presently active transform valley. The active and inactive valleys merge near the eastern RTI. The inactive valley can be traced westwards as a continuous feature up to ~ 150 km from the western RTI. It is probably the trace of a former location of the Romanche transform boundary, that became inactive between 8 and 10 my ago. It appears, therefore, that the Romanche ridge/transform geometry has changed significantly through time, with ridge jumps that have increased the length of the transform offset and migration and the reorientation of the transform boundary. Transpression and transtension due to changes in the ridge/transform geometry and to a non-straight trans form boundary are probably the major cause of vertical crustal motion responsible for the topographic anomalies of this area.