Late Miocene mafic volcanism and intra-arc tectonics during the early development of the Trans-Mexican Volcanic Belt

The early stage of the Trans-Mexican Volcanic Belt (hereafter TMVB) is marked by widespread, mafic to intermediate, volcanism emplaced between 11 and 7 Ma from the Pacific coast to the longitude of Mexico City, to the north of the modern volcanic arc. Petrological and geochronological data support the hypothesis that this volcanism made up a unique late Miocenic central Mexican comagmatic province. Mafic lavas at the mouth of the Gulf of California and along the northwestern sector of the TMVB made up the Nayarit district, which includes calc-alkaline to transitional varieties. The central sector of the TMVB is characterized by two basaltic districts: the Jalisco–Guanajuato and the Queretaro–Hidalgo, which are distinguished from the westernmost ones by their lower Nb/La and generally lower HFSE/LILE values, as well as by spider diagrams characterized by larger negative spikes at Th, Ta, Nb, and Ti. The surface occurrence of the late Miocene basalts appears to be controlled by pre-existing zones of crustal weakness that channeled the mafic magmas. Field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the TMVB. Starting from ∼12 Ma the TMVB separates a northern tectonic domain, subject to the developing divergent Pacific–North America plate boundary, from a southern tectonic domain, characterized by oblique subduction of the Rivera and Cocos plates. Apparently, far field stresses related to these complex plate boundaries reactivated older suture zones, allowing rapid uprise of mantle-derived magmas. The subduction-related signature shown by Miocene mafic lavas of the Jalisco–Guanajuato district argues against the existence of mantle plumes beneath this sector of the North America plate. On the other hand, the occurrence in the western TMVB and in the Guadalajara region of a large volume of mafic magmas, which sometimes show characteristics transitional to Ocean Island Basalts, could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction-related magmas and continental lithosphere to produce the observed basaltic varieties. Subslab magmas may have flowed through slab-free areas along the northern and eastern edges of the subducting Rivera plate, which in the late Miocene was already detached from the Farallon plate remnants and diverging from the Cocos plate.