DIFFUSE IMPACT OF THE MID-ATLANTIC RIDGE WITH THE ROMANCHE TRANSFORM - AN ULTRACOLD RIDGE-TRANSFORM INTERSECTION

The Romanche is a long offset (similar to 950 km), slow slip (similar to 1.7 cm/yr) transform; thus a hot ridge axis should meet a similar t o 50-m.y.-old, thick and cold lithosphere at the ridge-transform inter section (RTI). A strong thermal/topographic ''transform cold edge effe ct'' is therefore predicted. A morphobathymetric, seismic reflection a nd petrologic study of the eastern Romanche RTI shows that as the Mid- Atlantic Ridge approaches the transform, a well-formed axial rift vall ey disappears about 80 km from the RTI and is substituted by short en echelon, poorly developed axial ridge segments; they too disappear abo ut 30 lan 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 corner high'' are also absent. Thes e observations, and the distribution of earthquake epicenters, suggest a poorly developed, diffuse RTI. An inactive rift valley similar to 8 0 km west of the present RTI suggests ridge jumping within the last si milar to 4 m.y. The present poorly developed RTI may reflect the attem pts of an embryonic spreading axis to become established and to propag ate toward the transform. We infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle-derived serpentinized pe ridotites outcrop ubiquitously on the seafloor starting similar to 30 km from the edge of the transform valley. The unusually deep (similar to 4 km below sea level) axial ridge segments, the lack of crust, and the chemistry of the peridotites suggest a prevalently amagmatic regim e due to an ultracold upper mantle in this region. Absence of basaltic crust would favor massive serpentinization of a several kilometers th ick peridotite column. Malls balance modeling suggests that the decrea se of density and volume expansion resulting from serpentinization cou ld explain the absence of the predicted deepening of the seafloor as i t approaches the transform. These results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultrac old RTIs and that ultracold RTIs are magma starved, short lived, and u nstable in time and space.