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Abstract

The composition and spatial distribution of Quaternary volcanism in the Trans-Mexican Volcanic Belt (TMVB) exhibit some remarkable correlations with the seismicity, age, and structure of ocean lithosphere being consumed at the Middle America Trench. In the west, the TMVB is related to aseismic subduction of the Rivera plate (2 cm/yr) and in the east to a moderately dipping (20-30°) rapidly subducting Cocos plate (6-9 cm/yr). These contrasting arc segments are bounded by the Colima Graben, a zone of high-angle faulting and contemporaneous alkaline/calc-alkaline volcanism, situated above a sinistral transform fault (4 cm/yr) developed in the downgoing slab at the Cocos/Rivera juncture. Geologic mapping and K-Ar dating of the lavas of Iztaccíhuatl, a major calc-alkaline volcano in the TMVB, have established two main phases of eruptive activity that began prior to 0.9 Ma. The substructure of Iztaccíhuatl (>0.6 Ma) is composed principally of two-pyroxene andesites and dacites (300 km³) erupted from Llano Grande and Ancestral Pies volcanoes. The second stage of cone construction (