Slab window opening beneath southwestern North America and its relationship to regional magmatism: Insights from numerical modeling

The formation of a slab window beneath southwestern North America was an inevitable consequence of major Cenozoic geodynamic events, namely the separation of the Vancouver plate from the Farallon plate by 52 Ma, and the collision of the East Pacific Rise (EPR) with the North American trench by 30 Ma. Igneous records along northwestern Mexico and southwestern U.S. are linked to mantle upwelling episodes following the formation and expansion of this slab window. Here we present a 3D model of Farallon plate subduction and fragmentation beneath North America from 60 Ma to the present, using reconstructed plate kinematics and seafloor age as boundary conditions. Our predicted slab geometry shows good agreement with subduction-related high P-wave velocity anomalies in the upper mantle. We show that the waning of subduction started between 40 and 30 Ma, as the consumption of progressively younger oceanic lithosphere straddling the nearest segment of the EPR to the trench caused the slab to thin and flatten around this region, leading to ridge abandonment and a first detachment at ∼26 Ma. A slab window then propagated northwards and southwards, facilitating pulses of upward flux below the slab-free area across northwestern Mexico and southwestern U.S. Mafic volcanic occurrences show a close spatial correspondence with the predicted slab window through time, consistent with asthenospheric decompression melting above the slab-free area. Early magmatism in southwestern North America silicic provinces predates EPR-trench collision, implying additional mechanisms –such as earlier Farallon-Vancouver boundary subduction– also contributed to the complete volcanic record of this region.