Skylab photographs of Guatemala clearly show the prominent fault zones that have acted as the Caribbean-Americas plate boundary. The present boundary, the Cuilco-Chixoy-Polochic fault zone, can now be extended westward into Mexico where it apparently bifurcates into northwest-trending and southwest-trending forks. The southwest-trending fork separates regions of different seismicity and volcanism and is proposed as the present plate boundary. The eastern end of the Cuilco-Chixoy-Polochic fault zone abuts a pair of faults that trend N60°E and bound Lago de Izabal. These faults bound a graben that has the shape of a parallelogram. This shape is repeated along the eastern Motagua fault zone and offshore in the adjacent part of the Bartlett Trench.

The wedge of the Caribbean plate between the Americas and Cocos plates (southern Guatemala, El Salvador, and western Honduras) is being fragmented by east-west extension. Individual fragments are being slightly rotated counterclockwise as a result of a northward component of motion of eastern Honduras.

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