| Popis: |
Abstract 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, 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. |
