ULTRAMAFIC AND GABBROIC EXPOSURES AT THE MID-ATLANTIC RIDGE - GEOLOGICAL MAPPING IN THE 15-DEGREES-N REGION

The outcrops of mantle-derived ultramafic rocks in the 15 degrees N re gion are the most extensive yet reported for the Mid-Atlantic Ridge. N orth of the Fifteen Twenty fracture zone, these outcrops form a belt a t least 20 km long along the west wall of the axial valley and also cr op out on the east axial valley wall, Ultramafic rocks also crop out e xtensively south of the Fifteen Twenty fracture zone. Based on dredgin g and on a morphological analysis of the bathymetric map, we propose t hat ultramafic outcrops may be common in the crust formed between 14 d egrees 30'N and 15 degrees 50'N during at least the past 2.4 m.y. Mode rately dipping fault planes and large expanses of tectonic breccia hav e been observed during dives on the ultramafic outcrops. Diving observ ations also show that the ultramafic rocks are capped, in stratigraphi c contact, by a thin layer of basalt. This suggests that these rocks w ere tectonically emplaced at the axial seafloor, or very close to it, then uplifted in the footwall of the faults that bound the axial valle y, The occurrence of ultramafic rocks on both walls of the axial valle y may be due to frequent changes of faulting polarity in the axial reg ion: instead of one master shear zone, there would be a complex array of cross-cutting conjugate faults and shear zones that could jump inwa rd in the axial domain as spreading proceeds.