8 Cenozoic Cordilleran Tectonics, Magmatism, and Crustal Evolution: Perspectives from Mexico and Western North America (Posters)

Tectonic reconstructions of the southwestern North American plate boundary provide an excellent framework for understanding Cenozoic volcanism (arc, rift, slab window, seafloor) in this largely extensional and transtensional setting. Geologic, geochemical and geophysical submissions encouraged.

Wednesday, 22 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Broadening the Scope of Magma Genesis During the Oligocene-Miocene Ignimbrite Flare-Up

Paper number: 8-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The western margin of North America experienced convergence for most of the last 200 Myrs including the addition of terranes. A major controversy associated with these terranes is whether substantial lateral displacement occurred (Tikoff et al., 2023) during collisional episodes that may have also triggered slab break-off (Humphreys, 2009; Kant et al., 2018; Wells et al., 2014). The result of such collisional events would be the resetting of subduction instead of the continuous presence of a subduction system that simply changed its geometry (regular vs. flat slab subduction) over time. A key period in western North America that has been classically interpreted via flat-slab subduction and subsequent slab roll back is the voluminous magmatism of the Oligocene to Miocene that has been labeled as the ignimbrite 'flare-up' where progressive eruptions from multiple calderas are correlated in time and space (older NE to younger SW) (Henry and John, 2013). As the Farallon plate shallowly subducted under the North American plate, it began to rollback like a curtain, allowing for the hot asthenosphere to rapidly melt the enriched mantle. There are inconsistencies when considering the flat slab rollback model with the lateral terrane displacement model. Petrologic and geochemical constraints for the Mesozoic plutons of the Western US suggest that magmatism was driven by slab failure (Hildebrand and Whalen, 2017). Involving slab failure may reconcile some of the inconsistencies of the competing models. Slab-failure magmatism leads to rapid heating of the removed slab with its increased surface area by hot asthenosphere which triggers melting of garnet bearing slab lithology. Such melts are depleted in heavy rare earth elements (HREE) that strongly partition into garnet, yielding a high La/Yb ratio. So far, whole rock geochemistry analyses from ‘flare-up’ volcanics from the Great Basin have been obtained using the Earthchem database. The La/Yb ratios exhibit significant variability, however, majority of them exceed 30, suggestive of the involvement of garnet during magma genesis. To further elucidate whether slab-failure instead of slab-rollback was the main driver for the Oligocene-Miocene ignimbrite ‘flare-up’, trace elements in early crystallizing minerals may provide additional constraints isolating the primitive magma signatures from other differentiation processes.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-411051

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