A Low-Velocity Zone Within the Layer 3 Region of 118 Myr Old Oceanic Crust In the Western North Atlantic

Abstract

Summary During the North Atlantic transect (NAT) experiment 11 expanding spread profiles (ESPs) were acquired using a 3300 cubic inch, 30-element tuned airgun array (S/V Prospekta) and a 48-channel seismic streamer (R/V Moore). Extremely high-density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter (tau-p) with high precision. Velocity structures have been obtained by tau-sum inversion, ray tracing, and synthetic WKBJ and reflectivity seismogram methods both in tau-p and x-t domains. A detailed analysis of ESP 5 suggests the presence of a low-velocity zone within the lower oceanic crust at the location of magnetic anomaly MO (118 Ma). It is apparently 2.86km thick, has a p-wave velocity of 6.5 km s-1, S-wave velocity of 3.6 km s-1, and occupies much of the depth interval normally associated with oceanic layer 3. Converted shear wave arrivals have been used to obtain the shear velocity structure. We find that the Vp/Vs ratio in the uppermost part of the crust is about 1.9, and much of the crustal section has a Vp/Vsratio greater than √3. Attenuation of P- and S-waves has been studied by amplitude analysis, and we find that in most of the lower crust Qs is equal to Qp. Comparison with solutions from ESPs in younger and older parts of the basin suggests that ESP 5 is unique in displaying a low-velocity zone. Despite this the total thickness at ESP 5 is not significantly different than at other locations. We suggest that this provides strong evidence that the low-velocity zone occurs within the lower crust, and does not result from serpentinization of the upper mantle beneath thin oceanic crust.