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J. Pablo Canales,* Robert S. Detrick, Sara Bazin, Alistair J. Harding, John A. Orcutt
Wide-angle seismic data along the Mantle Electromagnetic and Tomography (MELT) arrays show that the thickness of 0.5- to 1.5-million-year-old crust of the Nazca Plate is not resolvably different from that of the 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(deg)15'S is slightly thinner (5.1 to 5.7 kilometers) than at the 15(deg)55'S overlapping spreading center (5.8 to 6.3 kilometers). This small north-south off-axis crustal thickness difference may reflect along-axis temporal variations in magma supply, whereas the across-axis asymmetry in depth and gravity must be caused by density variations in the underlying mantle.
The MELT area between 15(deg) to 19(deg)S (17) shows a pronounced asymmetry across the East Pacific Rise (EPR) in several characteristics such as spreading rate (faster to the east) (5, 8), subsidence rate (slower on the west) (6), and gravity anomaly [a less pronounced increase of the mantle Bouguer anomaly (MBA) away from the ridge axis on the Pacific Plate than on the Nazca Plate] (7). This asymmetry in MBA across the EPR in the MELT area can be interpreted in terms of variations in crustal thickness or in crustal or mantle density. The lower MBA and shallower seafloor on the Pacific Plate ~100 km west of the rise axis (7) could mean that the oceanic crust is ~1 km thicker than similarly aged crust east of the rise axis or that the mantle there is hotter and less dense (9). We used wideangle seismic data collected as part of the MELT experiment to determine the crustal thickness variations across and along the EPR to evaluate the crustal contribution to the asymmetry in regional depth and gravity anomalies across the EPR in this area.
The data were recorded by 15 ocean-bottom seismometers (OBS) (10) deployed along three seismic refraction lines (Fig. 1). The northern line along the secondary MELT array was 360 km long and crossed the EPR a few kilometers north of the 15(deg)55'S overlapping spreading center (OSC), and included five instruments (OBS sites 3, 4, 5,...