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

Emerging over the past decade has been a new view on the genesis of, and links between, the Sierra Madre Occidental silicic large igneous province, the Comondu Group of Baja California and the Gulf of California rift. Underpinning this has been a wealth of new data from both margins of the Gulf of California including offshore sampling, and marine geophysical data, in part seeded by the NSF Margins program where the Gulf of California was a principal focus site. Previously, the Sierra Madre Occidental silicic large igneous province and Comondu Group had been widely regarded as supra - subduction volcanism with the Comondu Group in particular, defining the location of the early to mid - Miocene supra - subduction zone volcanic arc, and therefore acting as both a spatial and temporal barrier to when rifting of the Gulf of California could begin. More broadly, this continental magmatism occurring during the last phase of subduction of the Farallon Plate between the Late Eocene and the Middle Miocene, shows little to n o petrogenetic connection to the active plate boundary and is more strongly linked to the progressive thinning of the upper plate and establishment of a shallow asthenospheric mantle beneath western Mexico. A database developed for this study of 4255 ages and chemical analyses for igneous rocks from 100 to 5 Ma from across western Mexico, reveals a significant transition period between 50 and 40 Ma where relatively low - volume magmatism was established across a broad area up to 800 km wide and extended up to 1000 km in board of the paleotrench. Since 40 Ma, magma fluxes greatly increased across this broad belt and compositions were initially silicic - dominated but quickly became bimodal by ~30 Ma. The space - time pattern of crustal extension is constrained in 39 areas, for which the approximate age of extension can be established on the basis of geologic relations or thermochronology. The onset of continental extension is constrained to the Eocene when extensional basins developed across the Central Plateau and the easternmost part of the Sierra Madre Occidental, approximately 500 km in board of the paleo - plate boundary. By the end of Oligocene, crustal extension had affected a wide region (250 km width) from the eastern Sierra Madre Occidental to the site of the future Gulf of California (wide rift mode). Concomitant with this extension was: 1) a widespread invasion of the mid to upper crust by mafic magmas with lithospheric signatures (the southern cordillera orogenic basaltic andesite suite or SCORBA), and lesser erupted volumes of uncontaminated asthenosphere - derived within - plate lavas, and; 2) crustal melting producing voluminous pulses of silicic ignimbrite eruptions (the SMO SLIP) with a ferroan (dry) and transitional within - plate signature. At ~19 Ma, ortho gonal extension became focused between the western side of the SMO and eastern Baja California in a ~80 - 100 km wide belt.