Mantle upflow tectonics in the Canadian Cordillera

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Abstract
A wide variety of geophysical, geological, and petrological results from the Cordillera of western Canada can be related to the history and present tectonics of the region. Southwest of the Rocky Mountains Belt, which is cratonic, much of the region has been accreted to North America within the last 200 m.y. High heat flux, high electrical conductance, low density, and a thick seismic low‐velocity layer characterize the uppermost mantle beneath the Omineca and Intermontane belts and together present strong evidence of high temperatures and partial melting. The crust in these tectonic provinces is only about 30 km thick, and the lithosphere little or no thicker, as shown both by surface wave dispersion and by lithosphere flexure after glacial unloading. In many characteristics the region strongly resembles the Basin and Range tectonic province in southwestern United States. The present accumulation of partial melt appears to be derived from the mantle, rather than from subducted lithosphere. This is shown by a change of composition from calc‐alkaline volcanics to alkaline olivine basalt about 25 m.y. B.P., a simultaneous change of tectonic style from rightlateral strike slip to normal faulting, the uniform composition of the basalts across the Cordillera, and the uniformly high heat flux across the Omineca and Intermontane Belts. The Basin and Range volcanics changed similarly at the same time, with the appearance of extensional tectonics. Mantle upflow tectonics dominates both regions. The continuing subduction of the Juan de Fuca plate, from 42° to 50°N, does not conflict with a mantle upcurrent in the central Cordillera, because the convergence is slow and the juvenile Juan de Fuca plate thin. North of 49°N, earthquake activity extends no further east than the Coast Plutonic Complex, and focal depths are less than 40 km. The seismicity along the convergent margin has no significant overlap with the area underlain by partial melt. There is no Benioff zone, and the subducted plate appears to lose its identity before it reaches the Intermontane Belt. An upflow in the mantle appears to lie beneath western North America from the Gulf of California to northern latitudes in Canada and to be continuous with an upcurrent beneath the East Pacific Rise of the south Pacific. Some implications for plate dynamics are noted.