A 1500 km2 Sea MARC I side scan sonar survey south of Axial Volcano investigated the geological structure and constitution of the South Axial Rift Zone (SARZ) and the northern 30 km of the Vance spreading segment. Relative age assignments based on structural relationships indicate that recent volcanism on the southern flank of Axial Volcano has been restricted to the SARZ. The surficial volcanic morphology of the SARZ changes downrift, from linear volcanic ridges (north) to small (1-km diameter) cratered cones (south), perhaps indicating a variation in eruptive vent geometry from fissures to point sources downrift. Recent lavas on the northern and central SARZ erupted along preexisting faults in the underlying crust. By acting as pathways for SARZ magmas, these faults may have cotrolled the orientation of SARZ volcanic ridges and perhaps the orientation of the entire SARZ edifice. The Vance spreading segment terminates at 45¿40'N and is not linked with a transform fault. The axis of spreading between 45¿40'N and 46¿00'N is offset west of the Vance segment and is constrained to lie between 129¿54'W and 130¿06'W. A discrete, structurally defined spreading axis, however, is not evident over the southern flank of Axial Volcano. The divergent plate boundary may now underlie the SARZ massif, although South Helium Basin (an embayment in the southeast flank of Axial Volcano) has undergone, and may continue to accommodate, crustal extension. A 48 km2 lava field discovered east of the SARZ is composed of lavas that are inferred to have erupted from the SARZ. These lavas can be traced eastward from the SARZ into the axial valley of the northern Vance segment and partially fill the valley with lavas up to 60 m thick. The eruption of these lavas, which occupy an estimated volume of 1.8 km3, may have contributed to the formation of Axial Volcano's summit caldera. ¿ American Geophysical Union 1990