Cenozoic magmatism and extension in western Mexico: Linking the Sierra Madre Occidental silicic large igneous province and the Comondú Group with the Gulf of California rift

Abstract

A new view on the genesis of, and links between, the Sierra Madre Occidental silicic large igneous province, the Comondú Group of Baja California, and the Gulf of California rift has been emerging over the past decade. 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 Comondú Group had been widely regarded as supra-subduction volcanism, with the Comondú 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 the 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 no 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 5Ma from across western Mexico reveals a significant transition period between 50 and 40Ma where relatively low-volume magmatism was established across a broad area up to 800km wide and extended up to 1000km in the board of the paleotrench. Since 40Ma, magma fluxes greatly increased across this broad belt, and compositions were initially silicic-dominated but quickly became bimodal by ~30Ma. 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 500km inboard of the paleo-plate boundary. By the end of Oligocene, crustal extension had affected a wide region (250km 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 (SCORBA) suite] 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 ~19Ma, orthogonal extension became focused between the western side of the SMO and eastern Baja California in a ~80–100-km-wide belt. Consequences of this switch to a narrow rift mode were a general abandonment of volcanism across much of the wide rift zone and volcanism becoming more effusive and intermediate in composition (the Comondú Group), being concentrated within rapidly extending but narrow tectonic depressions along the future site of the Gulf of California. By ~12Ma, the crust had thinned to half its original thickness along the axis of the Gulf. Since the late Miocene, right-lateral transtensional deformation associated with the dragging of Baja California by the Pacific Plate was then able to quickly complete lithospheric rupture to form the modern Gulf of California.The tectono-magmatic evolution of western Mexico and, by extension, southwestern USA can be interpreted in the frame of recent plate reconstructions and seismic tomography models. The removal of the subducted slab began in the early Eocene with the separation of the Vancouver plate from the rest of the Farallon plate, whose diffuse plate boundary intersected Mexico at the latitude of the Sonora–Sinaloa border and moved progressively North. Since then, a slab window/slab-free area began to grow by rollback and slab fragmentation and detachment promoting the melting of the ascending asthenosphere, the mantle lithosphere, and crust, which are variably observed in the igneous records of the SMO and Comondú Group. The ascent of buoyant and hot mantle also favored crustal extension in a broad area that progressively focused in the Gulf of California region.