CORRUGATED SLIP SURFACES FORMED AT RIDGE-TRANSFORM INTERSECTIONS ON THE MID-ATLANTIC RIDGE

The strips of ocean crust formed at the inside corners of both transfo rm and non-transform offsets on the Mid-Atlantic Ridge are punctuated by topographic highs-the 'inside-corner highs'(1-3)-where plutonic roc ks (including gabbros and peridotites) are frequently found(4,5). Curr ent tectonic models consider the inside-corner highs to be lower-crust and upper-mantle materials that have been exhumed by low-angle detach ment faults dipping away from the inside corner to beneath the ridge a xis(3,6-8). But much of the evidence for the existence of such faults has hitherto been circumstantial. Here we present sonar images of two ridge-transform intersections on the Mid-Atlantic Ridge (near 30 degre es N), which show that both active and 'fossil' inside-corner highs ar e capped by planar, dipping surfaces marked by corrugations and striat ions oriented parallel to the plate spreading direction. Although thes e surfaces may be the low-angle detachment faults envisaged by the mod els, they dip at much shallower angles than expected. This could be ex plained by the lubricating presence of serpentinized peridotite, fragm ents of which have been dredged from both surfaces. Alternatively, the se slip surfaces may instead represent failure surfaces in serpentine- lubricated landslide zones.