The Romanche transform offsets the Mid‐Atlantic Ridge (MAR) axis by about 950 km in the equatorial Atlantic. Multibeam and high‐resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the R/V Akademik Strakhov as part of the Russian‐Italian Mid‐Atlantic Ridge Project (PRIMAR). Morphobathymetric data show the existence on the northern side of the transform of a major 800‐km‐long aseismic valley oriented 10° to 15° 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 migration 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 hundred 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 ridge are capped by acoustically transparent, weakly stratified, shallow water platfonn/lagunal/reef limestones. This limestone unit is a few hundred meters thick and overlies igneous basement. Evaluation of the seismic reflection data as well as study of samples of carbonates, ventifact basaltic pebbles and gabbroic, peridotitic and basaltic rocks recovered at different sites on the transverse ridge, suggest that (1) the summit of the transverse ridge was above sea level at and before about 5 Ma; (2) the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate; its summit was flattened by erosion at sea level during subsidence; (3) the transverse ridge is an uplifted sliver of lithosphere and not a volcanic constructional feature; and (4) transtensional and transpressional tectonics have affected the transverse ridge. Hypotheses on the origin of the Romanche transverse ridge include (1) lateral heat conduction across the RTI; (2) shear heating; (3) lithospheric flexure due to thermal stresses in the cooling lithosphere; (4) viscoelastic deformation of the lithosphere; (5) hydration/dehydration of mantle peridotites; and (6) longitudinal flow of melt and igneous activity across the RTI. These processes cannot by themselves explain the transverse ridge, although some of them could contribute to its formation to a small extent. Vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge/transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence. Uplift may have been caused primarily by thrust faulting in...

Small‐scale variations in composition of mantle‐derived peridotites have been investigated in the 0°–15°N portion of the Mid‐Atlantic Ridge (MAR), thanks to a relatively close‐spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle‐equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a “cold” upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter‐Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (∼8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20′ FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°–3°N and 14°–15°N regions is associated spatially with light rare earth element enriched mid‐ocean ridge basalt showing a “hot spot”‐type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero‐age topographic highs in the 2°–3°N and 14°–15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O‐rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.

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The Romanche is a long offset (-950 km), slow slip (~ 1.7 cm/yr) transform; thus a hot ridge axis should meet a ~50-m.y.-old, thick and cold lithosphere at the ridge-transform intersection (RTI). A strong thermal/topographic "transform cold edge effect" is therefore predicted. A morphobathymetric, seismic reflection and petrologic study of the eastern Romanche RTI shows that as the Mid-Atlantic Ridge approaches the transform, a well-formed axial rift valley disappears about 80 km from the RTI and is substituted by short en echelon, poorly developed axial ridge segments; they too disappear about 30 km from the edge of the transform valley. The predicted gradual deepening of the ridge axis toward the transform was not observed. An active nodal deep and an "inside comer high" are also absent. These observations, and the distribution of earthquake epicenters, suggest a poorly developed, diffuse RTI. An inactive rift valley ~80 km west of the present RTI suggests ridge jumping within the last ~4 m.y. The present poorly developed RTI may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform. We infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle-derived serpentinized peridotites outcrop ubiquitously on the seafloor starting ~30 km from the edge of the transform valley. The unusually deep (~4 km below sea level) axial ridge segments, the lack of crust, and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region. Absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column. Mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform. These results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold RTIs and that ultracold RTIs are magma starved, short lived, and unstable in time and space. following generalizations [Fox and Gallo, 1984; Bender et al., 1984; Karson and Dick, 1983; Phipps Morgan and Forsyth, 1988; Blackman and Forsyth, 1989]: (1) the floor of the axial rift valley deepens and widens approaching a transform boundary; (2) a characteristic, areally limited topographic Paper number 95JB02249. 0148-0227/95/95JB-02249505.00 depression ("nodal basin") occurs as the rift valley enters the transform valley; (3) a large topographic high ("inside corner high") is observed commonly at the transform side of the rift valley as it approaches the transform valley; (4) morphostructural lineations oriented obliquely to both ridge axis and transform occur in the transform side corner but not in the non transform side corner; and (5) a systematic change in the geochemistry of axial rift basalts occurs approaching a transform. These features, and particularly the deepening of the axial valley as it approaches a transform, an...