Fragmentation of the Farallon Plate by Pivoting Subduction

Abstract

At the beginning of Cenozoic time a large region between the Pacific and American plates consisted of a continuous Farallon plate. Beginning 55 m.y. ago, the Farallon plate has been complexly fragmented by the formation of ridge-trench transforms, paired ridges, leaky transforms and transverse spreading centers as well as by ridge jumps, and ridge and fracture zone reorganization. Many of these phenomena can be related to pivoting of fragments of the plate around points near ridge-trench-fault triple junctions. Pivoting appears to be a consequence of the shape of the subducted part of the plate which narrows to a point near the oblique intersection of a ridge and trench. Pivoting fragmentation produces two types of phenomena where plates converge. The rate of convergence varies with distance from the pivot and this determines many aspects of topography, structure, sedimentation, vulcanism, and metamorphism. Transverse phenomena which penetrate North America for hundreds of kilometers are associated with the pivoting Cocos plate. They lie over the tapering edge of the subducting plate.