MEXICAN TERRANES: 20 YEARS ON

In the twenty years since terrane mapping was first applied to Mexico, many of the basic boundaries are similar, however the following advances have been made: (i) subdivision of the composite terranes; (ii) delimitation of the terranes in time; (iii) tectonic interpretation of the geological records; (iv) provision of better constraints on the provenance; and (v) reconstruction of actualistic palinspastic maps from the Mesoproterozoic to the present. In the Precambrian and Paleozoic terranes belong either to North America or Middle America (including Maya, Oaxaquia and Honduras), the latter lying adjacent to Amazonia off Venezuela and Colombia. Outboard of Oaxaquia, lay a Cambro-Ordovician, continental rise prism (Petlalcingo Group) and oceanic lithosphere (Acateco Group). The latter was obducted over the former in the Ordovician and/or Silurian and unconformably overlain by the Tecomate overstep sequence to form the Mixteco terrane. The Sierra Madre terrane lay west of Middle America to which it was accreted in the Ordovician (pebbles of the Sierra Madre occur in Wenlockian strata in Oaxaquia). The Coahuiltecano and Tarahumara terranes represent units lying between North and Middle America. All of these terranes were amalgamated into the supercontinent, Pangea, in the Late Permian. Middle Jurassic breakup of Pangea resulted in two continental blocks, Yucatán and Chortís that were surrounded by small ocean basins: Gulf of Mexico, Caribbean Sea, Juárez and Motagua terranes. In western Mexico, the Guerrero composite terrane consists of oceanic arc and periarc complexes, and equivalent terranes, is transitional into continental lithosphere in southern California. Amalgamation of individual terranes into the Guerrero composite terrane in the mid-Cretaceous may have resulted from collision of Baja British Columbia with the trench. An increased convergence rate between the Farallon and North American plates in the Late Cretaceous and early Tertiary resulted in obduction of the Guerrero composite terrane, the Laramide orogeny, inboard migration of the arc, and amalgamation of all the terranes with North America. Neogene propagation of the East Pacific Rise into the North American margin has led to separation of the Baja California block.