Abstract

We define the crustal architecture and internal structure of a 70-km-long segment of Mesozoic oceanic crust. Combined reflection and wide-angle refraction profiles show the relations between reflectivity and seismic velocity within the crust. The crust thins by ∼4 km from the segment center to the bounding fracture zones. A 2-km-thick lens at the base of the crust in the segment center exhibits enhanced reflectivity in the otherwise mainly opaque lower crust, which coincides with abnormally high seismic velocities of >7.5 km/s: we attribute these characteristics to the presence of a melt injection region. The reflection profiles in the 4-km-spaced grid show that there are two distinct, approximately orthogonal sets of dipping reflectors within the crust. Ridge-normal reflectors dip at typically 25° in both directions. The majority dip west toward the spreading center, and their common association with basement steps suggests that they represent normal faults. They terminate above or near the base of the crust. Ridge-parallel reflectors are shallower dipping (averaging 15°) and planar. Many are restricted to the middle crust, and we speculate that they represent magmatic rather than tectonic features. A prominent dipping reflector beneath a fracture zone cuts at 20° into the upper mantle. Similar reflectors are found elsewhere beneath fracture zones in the North Atlantic and may be caused by thermal stresses generated as the lithosphere cooled and moved away from the spreading axis.

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