TRANSFORM MIGRATION AND VERTICAL TECTONICS AT THE ROMANCHE FRACTURE-ZONE, EQUATORIAL ATLANTIC

The Romanche transform offsets the Mid-Atlantic Ridge (MAR) axis by ab out 950 km in the equatorial Atlantic. Multibeam and high-resolution m ultichannel seismic reflection surveys as well as rock sampling were c arried out on the eastern part of the transform with the R/V Akademik Strakhov as part of the Russian-Italian Mid-Atlantic Ridge Project (PR IMAR). Morphobathymetric data show the existence on the northern side of the transform of a major 800-km-long aseismic valley oriented 10 de grees to 15 degrees from the active valley; it disappears about 150 km from the western MAR segment. The aseismic valley marks probably the former location of the Romanche transform (''PaleoRomanche'') that was active up to roughly 8-10 Ma, when the transform boundary migrated to its present position. A temporary microplate developed during the mig ration and reorientation of the transform. This microplate changed its sense of motion as it was transferred from the South American to the African plate. A prominent transverse ridge extends for several hundre d kilometers parallel to the transform on its northern side, reaching its shallowest part (shallower by over 4 Km than the predicted thermal contraction depth) in a zone opposite the eastern MAR axis/transform intersection (RTI). Flat-top peaks on the summit of the transverse rid ge are capped by acoustically transparent, weakly stratified, shallow water platform/laguna/reef limestones. This limestone unit is a few hu ndred meters thick and overlies igneous basement. Evaluation of the se ismic reflection data as well as study of samples of carbonates, venti fact basaltic pebbles and gabbroic, peridotitic and basaltic rocks rec overed at different sites on the transverse ridge, suggest that (1) th e summit of the transverse ridge was above sea level at and before abo ut 5 Ma; (2) the transverse ridge subsided since then at an average ra te 1 order of magnitude faster than the predicted thermal contraction rate; its summit was flattened by erosion at sea level during subsiden ce; (3) the transverse ridge is an uplifted sliver of lithosphere and not a volcanic constructional feature; and (4) transtensional and tran spressional tectonics have affected the transverse ridge. Hypotheses o n the origin of the Romanche transverse ridge include (1) lateral heat conduction across the RTI; (2) shear heating; (3) lithospheric flexur e due to thermal stresses in the cooling lithosphere; (4) viscoelastic deformation of the lithosphere; (5) hydration/dehydration of mantle p eridotites; and (6) longitudinal flow of melt and igneous activity acr oss the RTI. These processes cannot by themselves explain the transver se ridge, although some of them could contribute to its formation to a small extent. Vertical tectonics due to transpressional and transtens ional events related to a nonstraight transform boundary and to region al changes in ridge/transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recen t subsidence. Uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithosp heric plate against the former (PaleoRomanche) and the younger transfo rm boundaries, before and during the transition to the present boundar y. After migration of the transform boundary to its present position, transpression was replaced by transtension and by subsidence of the tr ansverse ridge. An aseismic axial rift valley impacting against the tr ansform valley about 80 km west of the present RTI suggests eastward r idge jumping that probably followed transform migration. Localized tra nstension or transpression due to bends in the orientation of the tran sform may have caused intense although localized vertical movements, s uch as those that formed an ultradeep (> 7800 m) pull-apart basin alon g the transform valley.