INTERNAL STRUCTURE OF A SPREADING SEGMENT OF MESOZOIC OCEANIC-CRUST

We define the crustal architecture and internal structure of a 70-km-l ong segment of Mesozoic oceanic crust. Combined reflection and wide-an gle refraction profiles show the relations between reflectivity and se ismic velocity within the crust. The crust thins by approximately 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 coincid es 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 di stinct, approximately orthogonal sets of dipping reflectors within the crust. Ridge-normal reflectors dip at typically 25-degrees in both di rections. The majority dip west toward the spreading center, and their common association with basement steps suggests that they represent n ormal faults. They terminate above or near the base of the crust. Ridg e-parallel reflectors are shallower dipping (averaging 15-degrees) and planar. Many are restricted to the middle crust, and we speculate tha t they represent magmatic rather than tectonic features. A prominent d ipping reflector beneath a fracture zone cuts at 20-degrees into the u pper mantle. Similar reflectors are found elsewhere beneath fracture z ones in the North Atlantic and may be caused by thermal stresses gener ated as the lithosphere cooled and moved away from the spreading axis.