CRUSTAL STRUCTURE OF ENDEAVOR RIDGE SEGMENT, JUAN-DE-FUCA RIDGE, FROMA DETAILED SEISMIC REFRACTION SURVEY

A detailed seismic refraction survey was carried out over the Endeavou r Segment of the Juan de Fuca Ridge, a medium-rate spreading center wh ich lies off western North America, to investigate the creation and ev olution of oceanic crust. A bathymetric high and the presence of hydro thermal vents suggested that the study area was the most recent locus, of spreading. Travel time and amplitude data from 15 in-line air gun/ ocean bottom seismometer profiles were forward modeled using asymptoti c ray theory to obtain two-dimensional velocity models consisting of f our primary layers which correlate well with ClaSSiC ModelS of oceanic crust. Significant lateral variations in thicknesses and velocities o n the scale of a few to 10 km are superimposed on this basic velocity structure, but they appear to be random rather than distributed symmet rically about the ridge. We attribute them to fracturing which causes porosity changes, hydrothermal circulation which fills voids and fract ures with alteration products, and variations in magmatic and/or defor mational processes at the spreading center. Layer 2A is found to have low (2.6-2.8 km/s) velocities, to average 0.4 km in thickness with var iations up to 0.2 km, and to be bounded at its base by a sharp velocit y increase to 4.8 km/s. Along the axial ridge, velocities 0.4-0.6 km/s higher than average are interpreted for layers 2B and 2C, but these v alues are confined to a 2-km-wide zone centered below the ridge. Veloc ities along ridge-parallel lines offset 10 km are normal, indicating t hat maturation to off-ridge structure has occurred within at most 0.3 Ma. Layer 3 velocities decrease by 0.1-0.2 km/s for arrivals traveling along and under the axial ridge, perhaps caused by higher temperature s. However, we find no anomalously low velocities beneath the ridge, i ndicating that no large crustal magma chamber exists. On the basis of this study. we conclude that magmatic accretion is a fully three-dimen sional process within ridge segments such as Endeavour Ridge.