Abstract

In the Transmexican volcanic belt, polygenetic and monogenetic volcanism has taken place concurrently with extensional deformation since the late Miocene. At a regional scale, the deformation is manifested by two groups of faults. The dominant group consists of normal faults nearly parallel to the arc. In the other group are north-northwest–trending normal faults that cross the arc and, in places, form the boundaries of crustal blocks. The larger stratovolcanoes of the Transmexican volcanic belt are aligned in north-south volcanic chains along some of these faults, whereas monogenetic volcanoes are usually located along arc-parallel normal fault systems. Because the arc-parallel faults are 15° oblique to the subduction plate boundary, and assuming stretching perpendicular to the trench, the extensional deformation field facilitates activation of both arc-parallel and arc-transverse structures, the former having a higher displacement rate than the latter. We observe that in the Transmexican volcanic belt polygenetic volcanoes develop along faults having small strain rate and monogenetic volcanoes are emplaced along faults having higher strain rate. The agreement with the theoretical model in which monogenetic or polygenetic volcanism depends on the magmatic input rate and the regional stress is true only assuming a linear relation between regional differential stress and local strain rate, as in a continuous and homogeneous medium. We propose that the local strain rate rather than the regional stress field controls the coexistence of both types of volcanism in the Transmexican volcanic belt.

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