ABSTRACT

The Motagua fault zone in Guatemala is the landward extension of the Cayman-Bartlett Trough in the Caribbean Sea and represents part of the southern boundary of the North American Plate. Large masses of serpentinite have been emplaced along portions of the fault zone during the Late Cretaceous and Tertiary periods. Structural fabric is poorly developed in the serpentinites, though some discharmonic folding does occur in these rocks. Near the fault zone the foliation is steeply inclined. Farther to the north the foliation becomes subhorizontal and is related to massive thrust sheets.

Within and adjacent to the serpentinite are extensive outcroppings of a tremolite amphibolite formation. The origin of the amphibolite is complex but is due in part to metamorphism of basic igneous rocks and partial metsomatism of pre-existing Chuacus metamorphic rocks. Foliation is well developed in the amphiholites, and indicates that most of this formation has experienced at least two phases of folding. The structural fabric in the amphibolites is a distinctly different pattern and orientation than found in the Paleozoic Chuacus Group indicating that it has a different tectonic origin. The structural fabric in the amphibolites suggests that these features are related to the mode and direction of serpentinite emplacement through and over surrounding country rock. A scissors type of opening and closing strike-slip movement along the Motagua fault zone is proposed for the mechanism responsible for the emplacement of serpentinite thrust sheets.