Structure of Atlantic Oceanic Crust Around Chron M16 from Deep Seismic Reflection Profiles

Abstract

Deep seismic images around isochron M16 (142 Ma) are presented from three different sites on both flanks of the North Atlantic ocean. The similarity of the acquisition parameters and of the reflective patterns, allows us to perform an identical post-stack processing sequence in order to interpret jointly the three data-sets. Despite the scattered energy produced at the contact between the sedimentary cover and the top of the igneous basement, a reasonable generalised velocity-depth function has been obtained with which to perform seismic migration. The final results provide clearer images of structures in the igneous crust than previously obtained.

The migrated sections show in general rather similar images on the two plates (North America and West Africa), some distinctions can also be deduced from the Individual profiles. A common feature on the three profiles is the blocky character of the top of the igneous basement, which is cut by a series of offsets ranging from 200 to 600 ms. Most of them can be extended down to the base of the upper crust as dipping reflections, and can be interpreted as normal faults. On the American profile, two steeply dipping events show continuity down to the lower crust, dying out as low-angle normal faults (detachment). On the American plate profile, the geometry of the faults suggests a domino-style faulting with most of the steeply dipping events striking ridgewards. On the African plate profiles, the majority of the dipping events also show a ridgeward dip. The upper crust is made up of short and gently dipping events mostly located beneath the basement ridges. An interesting feature is seen on the American profile, where a pair of almost parallel reflections located beneath a basement ridge show an intermittent continuity with a lateral extension of about 5 km. This event could be interpreted as magmatic in origin possibly produced during a high melt production period. The lower crust shows scarce reflectivity on the African profiles, whereas on the American ones it shows a great variety of events. In general the reflectivity is higher on the older side of the lines. From the available data-set it seems that the deep seismic images obtained on both sides of the Atlantic are fairly symmetrical. Some exceptions could he ascribed to small differential changes, between ridge segments, in the spreading rate