Abstract

Late Quaternary volcanism in central Mexico is related to the subduction of young ocean lithosphere at the Middle America. Trench. Along-arc variations in seismicity, volcano structure, and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab.

Morphological and petrographic characteristics of major volcanoes within the Trans-Mexican Volcanic Belt (TMVB) serve to distinguish two calc-alkaline subprovinces:

1. A western arc, averaging 60 km in width, associated with aseismic subduction of the Rivera plate. The main cones of this region are dominated by two-pyroxene andesites, comprise volumes ⩽70 km3, and stand less than 3,000 m above sea level.

2. A broad central and eastern arc related to subduction of a gently inclined segment of the Cocos plate bounded by the Rivera transform and the Tehuantepec Ridge. Major volcanic edifices possess summit elevations in the range 4,000 to 6,000 m, have appropriately larger volumes (typically > 200 km3) and are constructed with a high proportion of amphibole-bearing lavas.

The boundary between these subprovinces is marked by a north-south–oriented structural depression, the Colima Graben, and it coincides with a 100-km offset in the “volcanic front.” Extensional tectonism in the Colima Graben, accompanied by mixed calc-alkaline and alkaline volcanism of potassic affinity, is likely related to a hinge-type transform fault which marks the Cocos-Rivera plate juncture in the downgoing slab.

A third segment of ocean floor is presently interacting with continental lithosphere south of the Gulf of Tehuantepec, where Quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse Cocos-NOAM-Caribbean triple junction. The northern limit of this triple junction is defined by the seismically active Isthmus fault, which may be related to alkaline volcanic activity at San Andrés Tuxtla.

A tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate-depth earthquakes along the arc is directly dependent upon the age of the subducted slab. Ocean lithosphere younger than approximately 20 m.y. is subducted aseismically at convergence rates approaching 9 cm/yr. The length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m.y. The TMVB overlies the aseismic extension of this young ocean lithosphere.

Several aspects of this study have a bearing on the segmented nature of converging margins in general:

1. The tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction.

2. The complete record of volcanism in the TMVB over the past million years can be related to the present plate configuration.

3. Alkaline and calc-alkaline volcanism have developed contemporaneously at a converging plate margin.

4. Lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab.

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