SEGMENTATION OF MIDOCEAN RIDGES WITH AN AXIAL VALLEY INDUCED BY SMALL-SCALE MANTLE CONVECTION

THE small-scale segmentation of mid-ocean ridges with an axial rise ha s been modelled by considering each ridge segment as a giant crack in the lithosphere with a tip propagating along the ridge axis(1,2). For ridges with an axial valley, however, this type of model fails because the lithosphere is too thick to tear(3). Yet such ridges are clearly segmented, as defined by morphology, gravity and structure at the 50-1 00 km length scale. The-ridge offsets are large(4,5), and vary dramati cally with time. This type of segmentation is commonly related to a th ree-dimensional, small-scale mantle flow occurring in the partially mo lten asthenosphere below the ridge(6-8). Here we propose a model for s egmentation in such ridges, in which the convective flow below the rid ge favours an asymmetrical breaking of the axial-valley lithosphere. T his leads to the development and separation of ridge segments in a pat tern that mimics the observed geometry and temporal evolution of the s egmentation of most ridges,vith an axial valley, If our model is corre ct, it implies that coupling of oceanic lithosphere to small-scale con vection controls the dynamics of mid-ocean ridges.