EFFECT OF STRAIN-RATE IN THE DISTRIBUTION OF MONOGENETIC AND POLYGENETIC VOLCANISM IN THE TRANSMEXICAN VOLCANIC BELT

In the Transmexican volcanic belt, polygenetic and monogenetic volcani sm has taken place concurrently,vith extensional deformation since the late Miocene. At a regional scale, the deformation is manifested by t wo groups of faults. The dominant group consists of normal faults near ly parallel to the are, In the other group are north-northwest-trendin g normal faults that cross the are and, in places, form the boundaries of crustal blocks. The larger stratovolcanoes of the Transmexican vol canic belt are aligned in north-south volcanic chains along some of th ese faults, whereas monogenetic volcanoes are usually located along ar e-parallel normal fault systems, Because the are-parallel faults are 1 5 degrees oblique to the subduction plate boundary, and assuming stret ching perpendicular to the trench, the extensional deformation field f acilitates activation of both are-parallel and are-transverse structur es, the former having a higher displacement rate than the latter. We o bserve that in the Transmexican volcanic belt polygenetic volcanoes de velop along faults having small strain rate and monogenetic volcanoes are emplaced along faults having higher strain rate, The agreement wit h the theoretical model in which monogenetic or polygenetic volcanism depends on the magmatic input rate and the regional stress is true onl y assuming a linear relation between regional differential stress and local strain rate, as in a continuous and homogeneous medium. We propo se that the local strain rate rather than the regional stress field co ntrols the coexistence of both types of volcanism in the Transmexican volcanic belt.