Mafic volcanism gradually encroaching on the tectonically stable Colorado Plateau in the southwestern United States appears to originate from within Paleoproterozoic lithosphere. New Hf and Nd isotope data strengthen evidence for magma source heterogeneity; other geochemical signatures show these sources to consist dominantly of peridotite. Tomographic and receiver function analyses reveal that young volcanism occurs above or outboard of a pronounced shallowing in the lithosphere-asthenosphere boundary. Melt extraction extends to the base of lithosphere thinned to <75 km, with more shallowly derived melts characterized by higher degrees of partial melting. Accordingly, decompression melting of a reactivated chemical boundary layer ± asthenosphere, rather than in situ lithospheric melting or melting of lithospheric mantle drips/delaminations, appears to be responsible for recent Colorado Plateau magma generation.