GEOLOGIC INFORMATION SYSTEM OF THE TRANS-MEXICAN VOLCANIC BELT AND ADJOINING AREAS: A TOOL TO TRACE THE EVOLUTION OF NEOGENE VOLCANISM IN CENTRAL MEXICO

We present the first version of the digital geologic map of the Trans-Mexican Volcanic Belt (TMVB). The basic structure of this electronic document consists of three databases: geochronologic (over 1,300 ages), geochemical (over 2,600 analyses) and geologic (36 geologic units, 5 types of faults, vents and other volcanic features) that are superposed on a geographic frame using ArcGIS. This evolving document is now available at the website: http://satori.geociencias.unam.mx/digital_geosciences/. We used chronostratigraphic and lithologic geologic units designed to emphasize the main volcanic episodes that took place since the Oligocene. Volcanism belonging to the Sierra Madre Occidental consists of two main pulses: one during early Oligocene (34 – 27 Ma) and another during early Miocene (24 – 20 Ma). The first episode of the TMVB dates back to middle Miocene (19 to 11 Ma). It formed a broad arc of large polygenetic volcanoes of intermediate composition (partly adakitic) emplaced from western Michoacán (Long. 102°) up to the Gulf of Mexico. In late Miocene (11 to 6 Ma) a voluminous mafic episode is recognized across the whole central Mexico to the north of the previous arc with ages progressively younger from west to east. This episode is thought to trace the lateral propagation of a slab detachment event. Large calderas and silicic dome complexes developed in latest Miocene and early Pliocene (7.5 to 3.5 Ma). Lavas with an intra-plate (OIB) affinity were emplaced since early Pliocene. In the last 3.5 Ma many segments of the arc broaden by a southward migration of the volcanic front but volcanism remain active in the rear part of the arc. The analysis of the geochemical data reveals several interesting features: 1) Silica contents and Mg# in rocks younger than 7.5 Ma correlate with distance from the trench in the western TMVB but are more homogeneously distributed in the central TMVB. Late Miocene rocks (7.5 – 11 Ma) show a constant mafic character. 2) On average, rocks younger than 7.5 Ma display a negative correlation between Ba/Nb and distance from the trench. Rocks far from the trench span a larger Ba/Nb range. In 7.5 – 11 Ma rocks Ba/Nb does not correlate with distance from the trench, suggesting a genesis unrelated to flux melting of the mantle. 3) Max value of Sr/Y increase along arc toward the west where the subducting plate is younger and dips steeper.