CENOZOIC EXTENSION AND MAGMATISM IN WESTERN MEXICO: LINKING THE SIERRA MADRE OCCIDENTAL SILICIC LARGE IGNEOUS PROVINCE AND THE COMONDÚ GROUP WITH THE GULF OF CALIFORNIA RIFT

We present a new interpretation on the genesis of, and links between, the Sierra Madre Occidental silicic large igneous province (SMO), the Comondú Group of Baja California (CG) and the Gulf of California (GoC) rift. Previously, the SMO and CG had been regarded as supra-subduction arcs, pre-dating the opening of the GoC. In contrast, we conclude that this continental magmatism, occurring during the last phase of subduction of the Farallon Plate between ~40 and 13 Ma, shows little to no petrogenetic connection to the active plate boundary and is more strongly linked to the progressive thinning of the upper plate and the establishment of a shallow asthenospheric mantle beneath western Mexico induced by the piecemeal removal of the subducted slab. A database of 4255 ages and chemical analyses for igneous rocks from 100 to 5 Ma reveals a significant transition period between 50 and 40 Ma where relatively low volume magmatism spreads in an 800 km wide belt, up to 1000 km from the paleotrench. Since 40 Ma, magma fluxes greatly increased and compositions were initially silicic-dominated but became bimodal by ~30 Ma. Since the Eocene extensional basins developed in the easternmost part of the SMO, ~500 km in board of the paleo-plate boundary. By the end of Oligocene, crustal extension had affected a 250 km wide region from the eastern SMO to the the future GoC. Concomitant with this extension was: 1) a widespread invasion of the crust by mafic magmas associated with melting of the lithospheric mantle (SCORBA suite), and more limited erupted volumes of asthenosphere-derived within-plate lavas, and 2) crustal melting producing voluminous pulses of silicic ignimbrites (the SMO SLIP) with a ferroan (dry) and a transitional within-plate signature. At ~19 Ma, orthogonal extension focused between the western side of the SMO and eastern Baja California in a narrow ~80-100 km wide belt. Volcanism became more effusive and intermediate in composition (the CG), and concentrated within rapidly extending, tectonic depressions along the future site of the GoC. By ~12 Ma, the crust had thinned to half of its original thickness along the axis of the Gulf. Since Late Miocene right-lateral transtensional deformation associated with the dragging of Baja California by the Pacific Plate was then able to quickly finish the lithosphere rupture to form the modern GoC.