Guatemala is cut by several transcurrent faults. The northernmost of those is the Cuilco-Chixoy-Polochic system, a left-lateral fault zone which extends into southern Mexico. It represents the present boundary between the Caribbean and Americas plates, and has been active for the past 10 m.y.
Quaternary volcanoes north and west of the Polochic fault differ from those in the Guatemalan front. They occur in a NW trending belt and consist of small volume sphene bearing domes with intermediate silica content of the Chiapas volcanic belt. The volcanic centers of this belt (including Nicolas Ruiz, Tzomtehuitz and El Chichón) apparently result from oblique subduction of the Cocos plate under the Americas plate. (Capaul, 1987)
Burkhart and Self (1985) divided the volcanic front into four morphotectonic zones based on block rotation about the transcurrent faults. Tacaná lies on their Zone I, on basement rock, and is some 30 km south of the Polochic fault. In their model, Zone I is locked into place and unaffected by extensional tectonics.
Guatemala is an uplifted block of continental crust of at least Paleozoic age. The continental crust of Guatemala is characterized by metamorphic rocks, anatexites, and plutonites of granitic ocmposition, overlain by upper Paleozoic to Tertiary sediments. The Tertiary of Guatemala is characterized by violently explosive volcanism of cal-alkaline affinity, very similar to the Quaternary volcanism. The Quaternary vents lie mostly to the south of the Tertiary rocks.
