A 5km swath-width SeaMARC I sidescan sonar survey, conducted over the zone of overlap between the southern rift zone of Axial Volcano and the northern tip of the Vance spreading segment on the Juan de Fuca Ridge (between 45°24'N and 45°50'N latitude), was analyzed to locate the present position of the Juan de Fuca spreading axis, and to determine the tectonic and volcanic structure of the seafloor. Sidescan data were processed in concert with the ship's Loran-C navigation to construct navigated, orthorectified mosaics of the sidescan imagery. In order to navigate the sidescan swaths, a simple numerical model was developed to describe the tracking behavior of the towed sidescan vehicle. Successive positions and orientations of the sidescan towfish were estimated, and were used to assign latitude/longitude values to individual sidescan pixels. Navigated sidescan pixels were mapped by computer onto an absolute (latitude/longitude) reference grid, and the resulting sidescan mosaic was compared directly to existing high-resolution SeaBeam bathymetry in order to discriminate the effects of large- and small-scale roughness on the observed backscatter distribution. The Juan de Fuca spreading axis between 45°25'N and 45°39'N is located within the axial valley of the Vance segment. Relative age relationships, based on crosscutting and superposition principles, indicate that the most recent volcanism within the axial valley has occurred along the valley's central ridge, and that the most recent resolvable extension within the axial valley has been concentrated between the central ridge and west valley wall. The Vance segment terminates at 45°39'N, and is not associated with a transform fault. The south rift zone of Axial volcano is a constructional volcanic feature that is not faulted, and a discrete axis of spreading over the south flank of Axial volcano is not resolvable in the sidescan imagery; however, the spreading locus north of 45°39'N is constrained to a zone between 130°06'W and 129°54'W. The lack of a well-defined spreading axis north of 45°39'N indicates that the physical manifestation of the divergent plate boundary has been modified or masked by hotspot volcanic processes associated with Axial volcano such that a definitive locus of spreading is not expressed in the surface morphology. / Graduation date: 1989