Abstract

Two classes of fracture zones are distinguished on the basis of their orientations with respect to a spreading axis and length of associated ridge-ridge transform offset. (1) Minor fracture zones associated with transform faults of short offset (<30 km; minitransforms); the minor fracture zones may exhibit an asymmetric V-shaped configuration with respect to a spreading axis at variance with small circles about poles of plate rotation. (2) Major fracture zones associated with transform faults of long offset (>50 km); the major fracture zones exhibit a symmetric configuration with respect to a spreading axis following small circles about poles of plate rotation. Recognition of the coexistence of the two classes of fracture zones with different orientations on a slow-spreading oceanic ridge raises questions regarding the significance of the different orientations and nature of the intervening structural transition.

A systematic narrow-beam bathymetric and magnetic investigation was performed to answer these questions in an area encompassing the change from minor fracture zones asymmetric with respect to the axis of the rift valley of the Mid-Atlantic Ridge at lat 26°N between major fracture zones, and a major fracture zone, the Kane, symmetric with respect to the axis of the rift valley at lat 24°N. The investigation delineated an intervening transitional region where structural features have continuously undergone geometric adjustments that have accommodated the discrepancy in orientation between the two classes of fracture zones for at least the past 6 m.y.

A hypothesis of differential structural stability determined by thickness of lithosphere within transform offsets is advanced to explain the observed differences in behavior of the two classes of fracture zones. The orientation of a major fracture zone is constrained to follow small circles along a trajectory of relative plate motion by a long section of thick lithosphere in the associated transform fault. The orientation of a minor fracture zone is susceptible to reorientation in the short section of thin lithosphere in the associated transform fault; the reorientation reflects response to intra-plate and interplate stresses. The geometric adjustments that occur as a consequence of differential structural stability continuously accommodate any discrepancy in orientation that may develop between coexistent major and minor fracture zones, so that an ocean basin such as the Atlantic can open symmetrically.

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