THE ONSET AND EVOLUTION OF NEOGENE FLAT SUBDUCTION IN SOUTHERN MEXICO

The horizontal subduction of the Cocos plate between 100°30’ W and 97°00’ W in southern Mexico is one of the most enigmatic features of the Middle America subduction zone. The flat slab segment extends for about 300 km inland at ~50 km depth, and then sinks into the asthenosphere at a steep angle beneath the present volcanic front of the central Trans-Mexican volcanic belt (TMVB). Unlike in the central Andes, flat subduction in Mexico is not associated with upper plate shortening because the continental crust is decoupled from the flat slab by a <5 km thick ultra low velocity layer. The origin of the flat slab subduction is puzzling as it is not associated with the subduction of a ridge or plateau, a variation of plate age, or an over-thickened (150-200 km) continental lithosphere, as proposed in other regions. Based on the space-time evolution of volcanism and the geochemical variations of the central TMVB the flattening of the slab began at ca. 19 Ma, when the volcanic front starts migrating away from the trench. The contribution of subduction components (fluids, sediment melts) to the magmas decreased during the migration farther from the trench. At ca. 10-9 Ma the volcanic front reached its maximum distance from the trench (ca. 480 km) with the emplacement of adakitic volcanoes and domes. The volcanic front then migrated backward until its present position at 250 km from the trench, suggesting that the flat slab segment started to roll back since ca. 8 Ma following the detachment of the lower part of the slab. The onset of slab flattening in central Mexico may have been promoted by (1) a rapid increase in Pacific-Cocos spreading rate from ~4 cm/yr at 23 Ma to ~12 cm/yr at 12 Ma; (2) the active overriding of the North America plate over the Cocos plate, leading to trench roll back at 0.5-1 cm/yr; (3) the relatively young age of the subducted slab (14-17 Ma); (4) the fact that the continental crust in the central TMVB is ca. 10 km thicker than in the neighboring regions underlain by a ~45° slab; and (5) the decreasing input of slab-derived fluids to the mantle inferred from the volcanic record. The combination of these features may have induced and sustained flat subduction by reducing slab pull, decreasing the thickness and increasing the viscosity of the mantle wedge, thus increasing the suction forces above the slab interface.