Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

Abstract

The outcrops of mantle-derived ultramafic rocks in the 15°N region are the most extensive yet reported for the Mid-Atlantic Ridge. North of the Fifteen Twenty fracture zone, these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall. Ultramafic rocks also crop out extensively south of the Fifteen Twenty fracture zone. Based on dredging and on a morphological analysis of the bathymetric map, we propose that ultramafic outcrops may be common in the crust formed between 14°30′N and 15°50′N during at least the past 2.4 m.y. Moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops. Diving observations also show that the ultramafic rocks are capped, in stratigraphic contact, by a thin layer of basalt. This suggests that these rocks were tectonically emplaced at the axial seafloor, or very close to it, then uplifted in the footwall of the faults that bound the axial valley. The occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region: instead of one master shear zone, there would be a complex array of cross-cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds.