Geophysical and Geochemical Constraints on Neogene‐Recent Volcanism in the North American Cordillera.

Widespread recent volcanic rocks occur across the Cordillera landward of the current/recent volcanic arc Mexico to Alaska and most other subduction backarcs. We conclude that most are produced by partial melt in the upper asthenosphere where two conditions are met: (1) thin lithosphere, shallow hot asthenosphere. Most of the Cordillera is uniformly hot with thin lithosphere such that the hot asthenosphere extends up to a sufficiently shallow depth to intersects the wet solidus; (2) wet upper asthenosphere. There is substantial water in the upper asthenosphere that reduces the solidus sufficiently for partial melting. We integrate geochemical analyses that constrain the partial melt source temperature and depth, seismic velocities that define the upper mantle temperature and partial melt zones, and seismic receiver functions that define the lithosphere‐asthenosphere boundary (LAB). Geochemical data for the Canadian Cordillera are integrated with data from the western United States. Upper mantle xenoliths indicate a dry strong lithosphere, <50 ppm H2O. A wet asthenosphere source, >250 ppm, is indicated by the volcanics, facilitating small‐scale convection. Geochemical equilibration averages ∼1,350°C, at ∼65 km. Receiver functions also define the LAB at ∼65 km for the western Cordillera, deeper in the eastern US Cordillera, and asthenosphere tomography velocities indicate ∼1,350°C. Low velocities above ∼150 km suggest a few percent partial melt that percolates upward and ponds at the base of the lithosphere until enough accumulates to locally penetrate upward. The 65‐km depth may be controlled by the spinel‐garnet phase transition. Mechanisms are discussed for the spatial distribution of recent volcanics. Plain Language Summary: There are widespread recent volcanic rocks in most of the North American Cordillera from Mexico to Alaska and most 200–1,000 km wide global subduction zone backarcs. We interpret them to result mainly from partial melting in upper wet asthenosphere where there is small‐scale solid‐state convection. There is substantial geochemical modification and differentiation in transit to the surface through the dry strong lithosphere. Partial melt is inferred to pond and chemically equilibrate at the lithosphere‐asthenosphere boundary which is near 65 km for most of the Cordillera from geochemical and seismic structure constraints. This depth may be stabilized by the spinel‐garnet phase transformation. The boundary in most areas is at about 1,350°C which strongly constrains upper mantle and crust temperatures, with the Cordillera Moho generally at 800°C–850°C. Local concentrations of volcanics may result from upward movement into the asthenosphere melt window. Key Points: Similar mafic backarc volcanics are widespread in the Cordillera from Mexico to AlaskaMost of Cordillera is uniformly hot with base of the lithosphere at ∼1,350°C and ∼65 km from geochemical equilibria, seismic tomography, and receiver functionsVolcanics produced mainly by partial melting in wet asthenosphere, ponding at base of dry lithosphere; depth controlled by the spinel‐garnet phase change [ABSTRACT FROM AUTHOR]