OFF-AXIS CRUSTAL THICKNESS ACROSS AND ALONG THE EAST PACIFIC RISE WITHIN THE MELT AREA

Wide-angle seismic data along the Mantle Electromagnetic and Tomograph y (MELT) arrays show that the thickness of 0.5- to 1.5-million-year-ol d crust of the Nazca Plate is not resolvably different from that of th e Pacific Plate, despite an asymmetry in depth and gravity across this portion of the East Pacific Rise. Crustal thickness on similarly aged crust on the Nazca plate near a magmatically robust part of the East Pacific Rise at 17 degrees 15'S is slightly thinner (5.1 to 5.7 kilome ters) than at the 15 degrees 55'S overlapping spreading center (5.8 to 6.3 kilometers). This small north-south off-axis crustal thickness di fference may reflect along-axis temporal variations in magma supply, w hereas the across-axis asymmetry in depth and gravity must be caused b y density variations in the underlying mantle.