WHAT THE AMAGMATIC CORRIDOR IN THE CENTRAL BASIN AND RANGE REVEALS ABOUT THE RELATIONSHIP BETWEEN MAGMATISM AND REGIONAL NEOGENE EXTENSION

Between 25 and 12 Ma, lateral migration of magmatism in the central and southern Basin and Range was approximately convergent N-S, but the migration was arrested and never joined, leaving a 100 km-wide amagmatic E-W corridor between the arrest lines. The N and S shoulders of the corridor are highly extended, but the central part is little extended, resulting in strong strain gradients herein interpreted as resulting from thermal weakening directly adjacent to the edge of the arrested magmatism. Extensional style differs dramatically in the marginal shoulders; detachment-fault-dominated in the Mormon Mountains transect on the north and tectonic rafting in the Frenchman Mountain transect on the south. A wide array of geophysical and geologic data shows SSW-directed tectonic displacement of the eastern Basin and Range between central Utah and the Lake Mead area. Displacements were approximately parallel to the Colorado Plateau margin and occurred during the final stages of, and following, laterally migrating magmatism. Throughout most of that area, displacements on left-slip faults are coupled with easterly trending folds, deep basins, enormous steep-axis rotations, and zones of large tectonic escape; reflecting strains that greatly exceed extensional strain associated with normal faulting. A growing body of structural data suggests a weak extension-normal dextral strain signature in the lower Colorado River extensional corridor. Displacement is northerly relative to the Colorado Plateau. Extension-normal flow of mid-crust is conjectured as the cause of the magmatic migrations as well as the extension-normal displacements, and the highly extended margins of the corridor are conjectured as westerly deflection of that flow against the amagmatic corridor. If valid, the model defines a close regional genetic tie between mid-crustal flow, magmatism, and extension and no obvious tie with plate-boundary tectonics such as toroidal flow resulting from slab rollback. That model calls for northerly migration of magmatism and northerly tectonic displacement in the central Basin and Range.