THE NATURE AND PROCESSES OF CORDILLERA BACKARC VOLCANISM: GEOPHYSICAL AND GEOCHEMISTRY CONSTRAINTS

The North American Cordillera backarc from Mexico to Alaska has wide-spread small-volume recent volcanism that extends over 800 km wide, generally bounded at the edge of the craton. Most volcanism appears to be generated as partial melt in the asthenosphere, but commonly modified in its upward transit. The basalts are generally wet (1-3% H2O), indicating a wet asthenosphere source, in contrast to the dry lithosphere indicated by mantle xenoliths. Geophysical data, especially seismic receiver functions show that the base of the Cordillera lithosphere is at a depth of 66±5 km and surface wave tomography velocity data indicate a temperature of 1300-1400 C. The Cordillera lithosphere-asthenosphere boundary (LAB) has a strong 5-10% seismic velocity decrease downward with a sharp top and underlying gradient, explained by ponding of a few percent partial melt rising from greater depth. Complementary basalt geochemical data mainly for western USA indicate partial melt ponding and chemical equilibration over a similar narrow range of depth and temperature, ~66±5 km and ~1350±25 C, likely at the base of the lithosphere. An interpretive model has partial melting starting at ~120 km where the mantle adiabat temperature intersects the wet solidus, and extending up to the LAB. The lithosphere is nominally dry so has a high solidus temperature and remains stable. The base of the lithosphere is defined by the intersection of the mantle adiabat with the dry solidus at about 1350 C and 66 km. In the Snake River Plain, Yellowstone hotspot trace, the adiabat is at a slightly higher temperature and the LAB consequently deeper. A surprisingly inference from the constant LAB depth and temperature, is that most of the Cordillera is remarkably uniformly hot with Moho temperatures of 800-850 C.