ABSTRACT

Compilation and analysis of geophysical and geological data indicate that the Cocos plate consists of three segments that have individual poles of rotation and independent motion vectors.

Contoured heat-flow and gravity maps of the region delineate the boundaries of the segments within the Cocos plate. These segments have different focal-plane solutions along the Middle America Trench and different sedimentary-basin configurations within the Central America-Mexico island arc. Recent studies of seismic data from the region also have suggested that the subducted Cocos plate consists of three segments.

The proposed northern and central segments are separated by the northeast-trending Siqueros-Tehuantepec Ridge fracture zone. The proposed central and southern segments are separated by the northeast-trending Costa Rica fracture zone that is located just northwest of the Cocos Ridge and extends from the Galapagos rift to the central valley of Costa Rica. Poles of rotation and relative motion vectors have been calculated with respect to the Caribbean plate for each segment. The northern segment is moving N75°E, oblique to the trench; the central segment is moving N50°E, perpendicular to the trench; the southern segment is moving north, perpendicular to the trench.

The Siqueros-Tehuantepec and Costa Rica fracture zones appear to join with “tectonized” zones that dissect the Central America-Mexico island arc and extend across the Caribbean plate, suggesting that it too is segmented. Structural and stratigraphic data from the sedimentary basins of the island arc suggest that these fracture zones have existed throughout the Tertiary history of the region.

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