UNIFORM ACCRETION OF OCEANIC-CRUST SOUTH OF THE GARRETT TRANSFORM AT 14-DEGREES-15'S ON THE EAST PACIFIC RISE

Using migrated common depth point reflection profiles, we find the str uctural differences along the ultrafast spreading (> 150 mm/yr) East P acific Rise south of the Garrett fracture zone are second-order, sugge sting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (< 1.0 km) me lt lens which shows great along-strike continuity. The depth of the ax ial melt sill is approximately 1200 m beneath the seafloor which is ab out 400 m shallower than along the slower spreading East Pacific Rise at 9-degrees-30'N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate d ependent. Melt sill width, however, shows little variation along the E ast Pacific Rise, suggesting no dependence of magma chamber size on sp reading rate. The melt reservoir decreases in width toward/across the 14-degrees-27'S ridge axis discontinuity by a modest 250-300 m and app ears to be continuous across this feature. Given the small aspect rati o (approximately 1.0 km by approximately 50 m by tens of kilometers) o f the axial melt lens, the previously recorded jump in MgO content acr oss the 14-degrees-27'S offset is likely the result of a mixing bounda ry which is sustained through an along-strike impedance in convection. Wide-angle reflections originating at the base of seismic layer 2A, a ssumed to coincide with the extrusive layer, reveal a twofold to three fold increase (200-250 to 500-600 m) in thickness within 1-2 km of the rise axis. The pattern of extrusive thickening imaged south of the Ga rrett transform is similar to that observed along the slower spreading (110-120 mm/yr) East Pacific Rise at 9-degrees-N. Outside of the neov olcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along-strike magma supply when integrated over the 1 0 kyr that corresponds to the width of the neovolcanic zone. The infer red uniformity of off-axis mean extrusive thickness is inconsistent wi th the conjecture that decreases in axial volume toward the 14-degrees -27'S discontinuity are caused by long-term reductions in magma supply . Second-order differences in the style of extrusive thickening may be related to structural differences within the low-velocity zone underl ying the rise axis and/or changes within the stress field in the overl ying carapace which results in the diffuse emplacement of lavas near t he rise axis. Images of Moho on cross-axis profiles may be traced to w ithin approximately 1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal s ection through the advection of material down and outward from the axi al melt lens rather than through cumulate deposition at the base of a large magma chamber.