THE SIERRA MADRE OCCIDENTAL: SYN-EXTENSIONAL MAGMATISM AND CRUSTAL MELTING RELATED TO OPENING OF THE GULF OF CALIFORNIA

The Sierra Madre Occidental (SMO) is the largest silicic igneous province in North America and the most recent of such events on Earth. Voluminous silicic ignimbrites, making up 85-90% of its total erupted volume (~400,000 km³), cluster in two main “flare-ups” at ~34-28 Ma and ~24-18 Ma. The SMO magmatism has been assumed to be a supra-subduction zone volcanic arc principally because of its proximity to an active continental margin, the calc-alkaline chemistry, relatively primitive isotopic compositions and association with some andesite. However, it strongly contrasts with modern-day non-extending volcanic arcs in terms of dominant erupted composition (rhyolite-high silica rhyolite), eruptive volume and rates, and volcanic architecture. This contrast extends to succeeding Middle Miocene volcanism surrounding the Gulf of California (GoC) that has also historically been interpreted to be non-extensional supra-subduction arc volcanism. To address these issues, we have taken a multi-disciplinary approach that: 1) uses zircons as probes of crustal sources involved in SMO magmatism; 2) focuses on basaltic geochemistry and age to constrain mantle source regions and changes; 3) couples stratigraphic, structural and geochronologic studies to constrain the timing of extension; and 4) integrate offshore geological data from the GoC with SMO onshore geology. Key outcomes that have emerged from our studies are: 1) generation of SMO rhyolites requires a substantial contribution from crustal partial melting. Many Early Miocene rhyolites were generated by remelting of silicic plutonic rock formed during earlier phases of SMO magmatism; 2) whole-rock chemical and isotopic signatures of the rhyolites have been inherited from the crust and provide no constraint on tectonic setting; 3) asthenospheric mafic magmas since the Oligocene were important in providing a long-lived heat and material flux into the crust, resulting in the remelting and recycling of older crust and newly formed igneous materials; 4) invasion of the mid to upper crust by mafic magmas was facilitated by syn-volcanic extension, particularly after 30 Ma; and 5) crustal extension and magmatism were intrinsically linked - extension changed from wide- (<30 Ma) to narrow- (~18 Ma) rift modes when it became focused along the western side of the SMO and in the GoC.