Dike orientations, fault-block rotations, and the construction of slow spreading oceanic crust at 22°40′N on the Mid-Atlantic Ridge

The first paleomagnetic results from oriented dike samples collected on the Mid-Atlantic Ridge shed new light on the complex interplay between magmatic accretion and mechanical extension at a slow spreading ridge segment. An upper crustal section about 1.5 km thick is exposed along a west-dipping normal fault zone that defines the eastern median valley wall of the southern segment of the Mid-Atlantic Ridge south of the Kane fracture zone (MARK area). Two distinct groups of dikes are differentiated on the basis of orientation and paleomagnetic characteristics. One group, on the basis of the paleomagnetic data, appears to be in its original intrusion orientation. This group includes both ridge-parallel, vertical dikes as well as dikes in other orientations, calling into question assumptions about uniform dike orientations at oceanic spreading centers. The second group consists of dikes that have paleomagnetic directions that are distinct from the predicted dipole direction, and we interpret them to have been tectonically rotated. These also occur in many orientations. The spatial relations between rotated and nonrotated dikes indicate that intrusion, faulting, and block rotation were contemporaneous beneath the median valley floor. Nonrotated dikes exposed on the eastern median valley wall indicate that there has been no net rotation of this upper crustal assemblage since magmatic construction ceased. Hence slip and associated uplift probably occurred in the fault zones' present orientation. These results provide the basis for a general model of mechanical extension and dike intrusion for this segment of the Mid-Atlantic Ridge. Initially, a portion of crust forms beneath the median valley by synkinematic dike intrusion into laterally discontinuous fault blocks. Slip and associated uplift along a cataclastic normal fault zone later exposes this crustal section on the valley margin. As spreading continues, this valley-bounding cataclastic normal fault zone is abandoned in favor of a new fault system thus passively moving the exposed crustal section away from the median valley.