As part of the suite of multidisciplinary investigations undertaken by the Ridge 2000 Program, new multichannel seismic studies of crustal structure were conducted at the East Pacific Rise (EPR) 8°20’–10°10’N and Endeavour Segment of the Juan de Fuca Ridge. These studies provide important insights into magmatic systems and hydrothermal flow in these regions, with broader implications for fast- and intermediate-spreading mid-ocean ridges. A mid-crust magma body is imaged beneath Endeavour Segment underlying all known vent fields, suggesting that prior notions of a tectonically driven hydrothermal system at this site can be ruled out. There is evidence at both sites that the axial magma body is segmented on a similar 5–20 km length scale, with implications for the geometry of high-temperature axial hydrothermal flow and for lava geochemistry. The new data provide the first seismic reflection images of magma sills in the crust away from the axial melt lens. These off-axis magma reservoirs are the likely source of more-evolved lavas typically sampled on the ridge flanks and may be associated with off-axis hydrothermal venting, which has recently been discovered within the EPR site. Clusters of seismic reflection events at the base of the crust are observed, and localized regions of thick Moho Transition Zone, with frozen or partially molten gabbro lenses embedded within mantle rocks, are inferred. Studies of the upper crust on the flanks of Endeavour Segment provide new insights into the low-temperature hydrothermal flow that continues long after crustal formation. Precipitation of alteration minerals due to fluid flow leads to changes in P-wave velocities within seismic Layer 2A (the uppermost layer of the oceanic crust) that vary markedly with extent of sediment blanketing the crust. In addition, intermediate-scale variations in the structure of Layers 2A and 2B with local topography are observed that may result from topographically driven fluid upflow and downflow on the ridge flanks.

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