Paleomagnetic investigations of a Mesozoic limestone sequence around the Gibraltar arc show that there have been large systematic rotations about a vertical axis and imply that there must have been a significant component of westward motion within the Betic-Rif orogenic belt. Rotations of the Late Jurassic and Late Cretaceous limestones in the Betic Cordillera of southern Spain are clockwise, with the exception of the unrotated Sierra Gorda, whereas the Jurassic sites in the Rif Mountains of Morocco are rotated counterclockwise, except in the area around Tetuan. These data are generally consistent with formation of the arc either by a westward-moving Alboran microplate or by extensional collapse of a collisional mountain chain. The former model, however, fails to recognize the nonrigid, nonplate-like character of the Alboran domain and lacks a definable driving mechanism. A model of a collapsing east-west-elongated ridge that takes into account the obliquity of the convergence along the active margin may provide mechanisms both for the observed rotations and for the large westward component of motion, and therefore can explain the geometry of the Gibraltar are.

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