Southward migration of volcanic activity in the central Mexican Volcanic Belt: asymmetric extension within a two-layer crustal stretching model

Abstract

Magmatic activity in the Miocene to present Mexican Volcanic Belt (MVB) is migrating to the south. Volcanic activity is associated with extension, which is being accommodated along a major, 1000-km-long, E–W-oriented continental rift. In turn, the MVB is cross-cut by older, though active N–S- to NNW-oriented major extensional faults, along which the polygenetic volcanoes are aligned. A southward migration pattern is also observed for the monogenetic volcanism, which however appears to be related to E–W- and N60°E-oriented extensional faults. We suggest that the southward migration of volcanic activity can be explained in terms of a two-layer crustal stretching model (brittle and ductile domains). The layers would be separated at an upper crustal level by a zone of simple-shear decoupling, at the brittle–ductile transition zone (BDTZ). The overall movement above the BDTZ is southward-directed, the only direction to which the Central MVB can extend and grow. Our model suggests that the magmas that feed the volcanism become stored at the BDTZ. Evidence supporting this assumption is provided by the Al-in-hornblende thermobarometer and the Ol–Pl–SiOr pseudoternary diagram, which indicate average pressures between 2.5 and 4kbar. The magmas feeding the monogenetic volcanism ascend rapidly along active E–W and N60°E extensional faults (large strain rates), i.e. they do not have enough time to form large magma chambers. The magmas feeding the polygenetic volcanism are emplaced along N–S to NNW faults (lower strain rates). These magmas remain stored for longer periods at the BDTZ of the N–S to NNW faults, and therefore form large magmatic chambers, shaping vertical overshoots of several kilometers of height. The results from geobarometry indicating magma emplacement depth at around 8km for the polygenetic volcanism, and 12km for the monogenetic volcanism, are in good agreement with the rheological constraints of a BDTZ at about 10km of depth. We envisage a feedback mechanism regarding magma storage and shallowing of the BDTZ, i.e. magma emplacement shallows the BDTZ; in turn, this shallowing controls the new zones for magma emplacement, a southward directed process.