DIKE ORIENTATIONS, FAULT-BLOCK ROTATIONS, AND THE CONSTRUCTION OF SLOW-SPREADING OCEANIC-CRUST AT 22-DEGREES-40'N ON THE MID-ATLANTIC RIDGE

The first paleomagnetic results from oriented dike samples collected o n the Mid-Atlantic Ridge shed new light on the: complex interplay betw een magmatic accretion and mechanical extension at a slow spreading ri dge segment. An upper crustal section about 1.5 km thick is exposed al ong a west-dipping normal fault zone that defines the eastern median v alley wall of the southern segment of the Mid-Atlantic Ridge south of the Kane fracture zone (MARK area). Two distinct groups of dikes are d ifferentiated on the basis of orientation and paleomagnetic characteri stics. One group, on the basis of the paleomagnetic data, appears to b e in its original intrusion orientation. This group includes both ridg e-parallel, vertical dikes as well as dikes in other orientations, cal ling into question assumptions about uniform dike orientations at ocea nic spreading centers. The second group consists of dikes that have pa leomagnetic directions that are distinct from the predicted dipole dir ection, and we interpret them to have been tectonically rotated. These also occur in many orientations. The spatial relations between rotate d and nonrotated dikes indicate that intrusion, faulting, and block ro tation were contemporaneous beneath the median valley floor. Nonrotate d dikes exposed on the eastern median valley wall indicate that there has been no net rotation of this upper crustal assemblage since magmat ic construction ceased. Hence slip and associated uplift probably occu rred in the fault zones' present orientation. These results provide th e basis for a general model of mechanical extension and dike intrusion for this segment of the Mid-Atlantic Ridge, Initially, a portion of c rust forms beneath the median valley by synkinematic dike intrusion in to laterally discontinuous fault blocks. Slip and associated uplift al ong a cataclastic normal fault zone later exposes this crustal section on the valley margin. As spreading continues, this valley-bounding ca taclastic normal fault zone is abandoned in favor of a new fault syste m thus passively moving the exposed crustal section away from the medi an valley.