Geophysical study of the northwest African margin off Morocco

Abstract

Six geophysical profiles were taken over the northwestern African margin between the Canary Islands and Morocco. A magnetic smooth zone occupies the upper continental rise landward of lineated sea-floor spreading anomalies of Late Jurassic to Early Cretaceous age. This smooth zone is believed due in part to a Late Jurassic uniform polarity interval; its seaward boundary is correlated with a basement high and/or abrupt landward thickening of sediments. Spreading may have started over 200 × 10 6 yr ago at a rate of 0·55 cm/yr and accelerated 150 to 160 × 10 6 yr at the smooth zone boundary. No counterpart of the east coast magnetic slope anomaly was observed off Morocco while the distance between the shelf break and the smooth zone boundary is also less here than off northeastern America. This difference suggests a shift in the position of the accreting plate boundary from west to east, about 200 × 10 6 yr ago, after the formation of the slope anomaly. Gravity data show that the Moroccan margin is largely in isostatic equilibrium, and that the zone of transition from oceanic to continental crustal thickness is wider off the Canaries (400 km) than off Morocco (100–150 km). The sediment section of the seismic reflection profiles includes an upper acoustically transparent unit (usually restricted to the upper rise) a middle stratified unit, and a lower transparent unit. The upper unit is probably pelagic calcareous ooze. Other lithologies are unknown but the stratified unit may in part include an Early Tertiary unconformity. Sections of the shelf and slope exhibit deformation that have probably been caused by the Alpine orogeny.