MULTIPLE TRIGGERS FOR MULTIPLE EPISODES OF SYNCONVERGENT EXTENSION IN THE SEVIER HINTERLAND

Reductions in horizontal compressive stress, rock strength, or increases in lateral contrasts in gravitational potential energy are commonly proposed to be triggers for synconvergent extension. These mechanisms commonly provide positive feedbacks and play alternate roles as necessary conditions and triggers for synconvergent extension during the progressive geometric, kinematic, and dynamic evolution of orogenic belts. The mid Cretaceous to Paleogene geologic record of the hinterland of the Sevier belt serves as an excellent natural laboratory for studying synconvergent extensional processes and provides evidence of all three factors acting as triggers for synconvergent extension. Mid Cretaceous orogen-parallel extension of crustal welts developed at basement buttresses, was triggered by thermal softening resulting from intrusions (New York Mountains, eastern Mojave) and thermal relaxation following crustal thickening (Raft River, Albion, and Grouse Creek Mountains, ID-UT). In contrast, Late Cretaceous orogen-perpendicular extension and anatexis, pervasive in the Mojave Desert and to a lesser extent in the Great Basin, may have been triggered by the thermal and gravitational potential energy changes resulting from delamination of mantle lithosphere. The reduction in plate convergence velocity, and the resultant foundering of the Farallon slab, whether by slab rollback, downbuckling or drip, triggered widespread Paleogene magmatism and orogen-perpendicular extension. These synconvergent exhumation events were followed by additional extensional exhumation in postconvergent Neogene time, leading to exposure of mid crustal rocks in the Cordilleran metamorphic core complexes.