INTRUSION AND UNDERPLATING OF MAFIC MAGMAS - THERMAL-RHEOLOGICAL EFFECTS AND IMPLICATIONS FOR TERTIARY TECTONOMAGMATISM IN THE NORTH-AMERICAN CORDILLERA

Intrusion and underplating of mafic magmas tend to facilitate continen tal extension by thermally weakening the lithosphere. However, adding rheologically hard mafic material to the crust also strengthens the li thosphere. We have investigated the time-dependent competing effects o f thermal weakening and rheological hardening using a simple numerical model, and compared the results with the spatial and temporal develop ments of Tertiary tectonomagmatism in the North American Cordillera. T he close temporal-spatial correlation between magmatism and formation of metamorphic core complexes in the Cordillera was consistent with mo del predictions when a relatively cold lithospheric mantle was assumed . In addition to thermally weakening the lithosphere, magma intrusion may have facilitated core-complex formation by reducing the effective viscosity of the lower crust, allowing crustal collapse to be decouple d from the mantle and to occur at relatively low stress levels. The co mplicated spatial and temporal patterns of tectonomagmatism in the Gre at Basin since the mid-Tertiary were predictable with intrusion and un derplating of mafic magmas in conjunction with significant lithospheri c thining. Depending on the thermal structure of the lithosphere and t he nature of mafic intrusion, the minimum lithospheric strength may la g the peak volcanism by a few million years; and the center of major v olcanic fields may become relatively stronger than the surrounding are as because of rheological hardening.