VOLCANIC EPISODES IN THE TRANSMEXICAN VOLCANIC BELT: IMPLICATIONS FOR SUBDUCTION DYNAMICS

The first, GIS-based, geologic map of the Trans-Mexican Volcanic Belt (TMVB), incorporating over 1,000 ages and 2,200 geochemical data, has been recently completed. The resulting picture shows that the TMVB is a composite arc made by the superposition of 4 episodes that correlate with slab loss, changes in slab geometry and subduction rate: (1) The initial TMVB consists of a broad arc of mafic to intermediate polygenetic volcanoes that extends from western Michoacán (Long. 102°) to Gulf of Mexico from 17 to 10 Ma. During this period volcanism migrated NE-ward and many centers have an adakitic signature. The location and geochemistry of this arc indicates that the subducted slab was already flat to moderately dipping. Accordingly, part of the mantle lithosphere beneath the TMVB must have been removed at this time. (2) From 11 to 6 Ma voluminous mafic lava flows were emplaced across the whole central Mexico through major extensional fault systems. They show ages progressively younger from west to east. This episode is related to a slab detachment event triggered by the end of subduction off southern Baja California. (3) Large calderas and silicic dome complexes developed from 7.5 to 3.5 Ma between Tepic and Mexico City and at the eastern border of the altiplano (Hidalgo state). From ~9 and 3.5 Ma volcanism is absent between Mexico City and Pico de Orizaba. Mafic lavas with OIB affinity are emplaced shortly after the silicic pulse together with the dominant calcalkaline lavas. This episode marks the reorganization of subduction after detachment. In the west silicic volcanism reflects the decline in subduction rate following the decrease in slab pull after the loss of the deep slab. In the central TMVB it indicates that the slab began to rollback exposing the base of the crust to higher T. The concurrent emplacement of OIB and arc lavas point to a heterogeneous mantle resulting from the influx of pristine asthenosphere into the wedge. The volcanic lull occurs where subduction is still flat and the crust thicker. (4) The modern arc consists of a whole range of products emplaced from the Gulf of California to the Gulf of Mexico since 3.5 Ma. Many segment of the arc broaden by a southward migration of the volcanic front but volcanism remains active in the rear part. This episode indicates that the leading edge of the slab keep rolling back re-creating a "normal" mantle wedge along the entire TMVB.