TRIGGER FOR LARAMIDE MAGMATISM: NORMAL SUBDUCTION, SHALLOW-ANGLE SUBDUCTION, OR STRUCTURAL INHERITANCE?

Late Cretaceous - Paleogene Laramide deformation consisted of basement-cored uplifts and associated sedimentary basins as much as 1000 inboard of the active continental margin throughout western North America. Deformation was accompanied by magmatism in three regions: Montana/Idaho/Wyoming, the Colorado Mineral Belt, and the Southern Laramide Province, which occupies the southern parts of California, Arizona, New Mexico, and northern Mexico. The association of magmatism with Laramide deformation is problematic. First, shallow-angle subduction, one of the leading models for Laramide deformation, precludes magmatism. In shallow-angle models, the subducted oceanic slab heats and dehydrates slowly along the base of the continental lithosphere, so that volatiles are not available to trigger flux melting in hot asthenosphere. Second, syn-deformational magmatism occurred only in three regions throughout the orogenic belt, i.e., the processes that caused shortening did not lead to ubiquitous magmatism.

We propose that magmatism in the three regions had different causes and that the pre-Laramide thermal and structural states of the continental lithosphere were the critical factors that permitted syn-deformational magmatism in the Colorado Mineral Belt and the Southern Laramide Province. The Colorado Mineral Belt coincides with a complex, leaky suture between Precambrian terranes (Karlstrom et al., 2005); the belt became magmatically active during Late Cretaceous and Cenozoic tectonic events (Wilson and Sims, 2002). Structural inheritance of a different style caused magmatism in Southern Laramide Province. During Late Jurassic through Early Cretaceous time, the region experienced extension, which thinned the continental lithosphere to form the Border rift. This unique lithospheric geometry allowed the Farallon plate to dehydrate as it passed into the welt of hot asthenosphere along its subhorizontal path, creating a broad band of Laramide magmas with arc affinities.