We use multichannel seismic reflection data to image the upper crustal structure of 0 - 620 ka crust along the southern Juan de Fuca Ridge. The study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow ( Cleft) and wide (Vance) axial summit grabens (ASG). Along most of the axis of both segments we image the top of an axial magma chamber (AMC). The AMC along Cleft deepens from south to north, from 2.0 km beneath the RIDGE Cleft Observatory and hydrothermal vents near the southern end of the segment to 2.3 km at the northern end near the site of the 1980s eruptive event. Along the Vance segment, the AMC also deepens from south to north, from 2.4 to 2.7 km. Seismic layer 2A, interpreted as the basaltic extrusive layer, is 250 - 300 m thick at the ridge axis along the Cleft segment and 300 - 350 m thick along the axis of the Vance segment. However, off-axis layer 2A is similar in both segments ( 500 - 600 m), indicating similar to 90% and similar to 60% off-axis thickening at the Cleft and Vance segments, respectively. Half of the thickening occurs sharply at the walls of the ASG, with the remaining thickening occurring within 3 - 4 km of the ASG. Along the full length of both segments, layer 2A is thinner within the ASG, compared to the ridge flanks. Previous studies argued that the ASG is a cyclic feature formed by alternating periods of magmatism and tectonic extension. Our observations agree with the evolving nature of the ASG. However, we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply. Thus the ASG, rather than being formed by excess volcanism, is a rifted flexural axial high. The changes in axial morphology affect the distribution of lava flows along the ridge flanks, as indicated by the pattern of layer 2A thickness. The fluctuations in magma supply may occur at all spreading rates, but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges.
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