The slow-spreading Mid-Atlantic Ridge (MAR) consists of both symmetrical and asymmetrical rift valley segments or spreading cells that are linked end-to-end by transform faults and "non- transform offsets" or accommodation zones and is similar to the structure of many continental rifts. Of the various components of MAR architecture the most important faulting occurs at the major border faults of asymmetrical (half-graben) spreading cells and at the accommodation zones that link them to adjacent cells. Typical examples of these feature are present in the MARK Area (23"N on the Mid-Atlantic Ridge) and can be investigated by Alvin in 1992. A joint U.S./French (F.A.R.A.) program of 20 dives to will be attempted investigate the surface manifestations of extensional tectonics in the southern part of the MARK Area about 100 km south of the Kane Transform. Two major geophysically defined structures will be investigated. 1.) The primary target will be the zone teleseismic and microseismic activity along the steep eastern median valley wall at about 22'30'N (Toomey et al., 1985; 1988). This is interpreted as a seismically active fault zone and perhaps a juvenile detachment fault. 2.) The secondary target will be the accommodation zone at the north end of the same spreading cell. This lies at the surface intersection of a zone at the north end of the same spreading cell. This lies at the surface intersection of a major north-dipping intra-crustal reflector identified in recent multichannel seismic reflection studies that may be a low-angle oblique-slip fault (lateral ramp). The Alvin dives will permit detailed mapping and structural measurement of various features (fault surfaces, shear zones, foliations, lineations, dikes and flows, etc.) in outcrops of deformed basement rocks. The goal is to use the correlation between seismic and geologic data to help bridge the gap between tectonic processes and the creation of near-axial bathymetric features.
