Very different crustal response to extreme extension in the southern Basin and Range and Colorado Plateau transition

Clustered about the southwest edge of the Colorado Plateau lie many highly extended terranes. Among these are metamorphic core complexes, distinguished by low-angle normal faults with sufficient offset to expose middle crustal rocks at higher elevation relative to the surrounding areas. About 150 km to the southwest, strong extension in the Salton Trough manifests itself very differently; high-angle normal faults form deep basins, with the strongest extension causing subsidence beneath sea level. We ask why strong extension takes such different forms, and, to help answer the question, we take advantage of seismic and gravity profiles that cross through the Salton Trough, metamorphic core complexes, and the Colorado Plateau. We propose that the relative rate of extension vs. intrusion of crust by magmatism and crustal flow controls the extensional style. We further show that conditions may have been ideal for core complex formation along the edges of the Colorado Plateau because its thick crust and high elevation provided a source of crustal flow and the pressure gradient to drive the flow.