Seismic properties of layer 2A in the Pacific

Abstract

Many new sonobuoy solutions have been combined with published data and incorporated into a revised plot of igneous layer 2A thickness as a function of plate age at the East Pacific Rise. Each solution was redated on the basis of regional magnetic anomaly maps to improve the accuracy of crustal dating. Results show that layer 2A becomes unobservable by seismic methods on crust older than about 15 m.y. Although igneous materials must make up layer 2A at the East Pacific Rise, low-velocity acoustic basement exists elsewhere in the Pacific. Near seamounts and seamount chains, volcanic aprons with the seismic properties of layer 2A thin distally away from eruptive centers and seem to persist indefinitely without being affected by processes that normally increase the sound velocity at the ridge crests; This is apparently due to a lack of fracturing, which prevents hydrothermal solutions from percolating through the volcanic rubble. At active spreading centers, percolation through fractures provides minerals that could fill voids and increase the sound velocity of the basaltic rubble. Statistical studies show that the velocity in layers 2B and 2C is independent of plate age, but layer 3 velocities peak up significantly on the East Pacific Rise. In the older parts of the Pacific, low-velocity acoustic basement is sedimentary or a mixture of sediment with volcanics. An unusually thick sequence (1 km) of material that is probably mostly sediment occurs in the Nauru and Mariana basins.