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

• Published in: Geophysical Journal International
• Main Authors: Mithal, Rakesh, Mutter, John C.
• Format: Text
• Language: English
• Published: Oxford University Press 1989
• Subjects: Articles; North Atlantic
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/97/2/275; https://doi.org/10.1111/j.1365-246X.1989.tb00501.x

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 2.86 km 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 V p / V s ratio in the uppermost part of the crust is about 1.9, and much of the crustal section has a V p / V s ratio 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 Q s is equal to Q p . 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.