MIDDLE TO LATE MIOCENE SLAB FLATTENING AND ARC MIGRATION IN THE EARLY TRANS-MEXICAN VOLCANIC BELT

The Trans-Mexican Volcanic Belt (TMVB) was established as an independent province in Middle Miocene, as a result of the counterclockwise reorientation of the Sierra Madre Occidental magmatic arc. This transition is marked by a change in the style and composition of the dominant volcanism from silicic explosive to intermediate effusive. Nevertheless, the conditions that prevailed during the establishment and initial evolution of the arc are poorly known. We conducted field, geochemical and geochronologic studies of rocks emplaced during the early evolutionary stages of the TMVB along a north-south transect located between 101º15’ and 98º45’ W. New U-Pb and Ar-Ar ages, along with published data, indicate that the locus of volcanism migrated to the north and northeast (farther from the present volcanic front and trench) between ca. 22 and 9 Ma. Also, since ~16.5 Ma the arc significantly broadens (from ~100 to ~200 km) within a short time span.

In the studied areas, lavas of intermediate composition are dominant, whereas rhyolitic lavas are scarce. Rhyolitic tuffs and ignimbrites have been recognized only in the oldest units (>16 Ma). Analyzed samples have compositions that range from andesite to dacite (SiO₂: 58.5-66.2 wt. %), and all have the geochemical features of subduction related rocks. Geochemical data also indicate that the contribution of subduction components (mainly fluids) to the magmas continuously decreases toward the north and northeast, a behavior that abruptly changed with the emplacement of adakite-like volcanoes at 12 - 9 Ma, that also mark the end of the arc migration away from the trench. The origin of such adakite-like rocks has been related to slab melting in response to prolonged flat subduction.

The observed migration and broadening of the arc, the decrease in the contribution of subduction fluids to the magmas with increasing distance from the trench, and the final generation of adakitic rocks, are consistent with a process of slab flattening between middle and late Miocene. We also found that the generation of adakite-like magmas occurred within a restricted time span and only in the northernmost areas, and was thus not a process that evolved gradually.