EMPLACEMENT OF DEEP CRUSTAL AND MANTLE ROCKS ON THE WEST MEDIAN VALLEY WALL OF THE MARK AREA (MAR, 23-DEGREES-N)

Exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow-spreading Mid-Atlantic Ridge. Nine dives of the submersible Nautile explored the western axial valley wall in the northern cell of the MARK area (Mid-Atlantic Ridge/Kane fracture zone), where periodotite and gabbro outcrops had been previously reported, in order to constrain the structure and determine emplacement mechanisms. The ridge/transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl, locally overlain by metabasalts and metadolerites, capped by slightly weathered basalts. The morphology is controlled by ridge-parallel faults, dipping moderately (40-65-degrees) to the east. Transform-parallel scarps, present in the northernmost dives, become rare toward the south. Brittle deformation, along moderately dipping fault scarps, produced a dense microcrack network filled with greenschist facies minerals (chlorite, actinolite, epidote, quartz), locally overprinting high-temperature ductile deformation fabrics. The small hill located at 23-degrees 20' in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl. Above 3100 mbsl, the summit of the hill is composed of pillow-basalts and sediments. The periodotite outcrops display a strong schistosity dipping 20-40-degrees to the east, parallel to striated normal fault planes. Some steeper east-facing fault scarps truncate the lower-dipping fault surfaces. The serpentinites, deriving from a harzburgitic protolith, are cut by rare dikelets of highly differentiated gabbro. These new data, combined with previous results of Alvin dives, are used to draw a generalized geological map of the western axial valley wall. This map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the MARK area: the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets. Low-angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south.