TRANSITION BETWEEN A VALLEY AND A HIGH AT THE AXIS OF MIDOCEAN RIDGES

An examination of 339 topographic profiles crossing the world mid-ocea n ridge system indicates that the axial topographic signature is corre lated to both the local spreading rate and the absolute depth of the r idge axis. The spreading rate (which controls the temperature structur e at the ridge axis) and the depth of the ridge axis (which is an indi cator of crustal thickness) are both expected to influence the rheolog y of the axial lithosphere, which in turn should control the shape of the axial topographic signature. The topographic signature of the axis is determined by fitting an axial high or an axial valley on a trend that follows the thermally driven subsidence of the ridge flanks. At a given axial depth the transition between an axial valley and a high t akes place abruptly, over a relatively small change in spreading rate (approximately 15 mm/yr).