Seismic Reflection Crustal Structure and 3-D Geometry of Cordilleran Metamorphic Core Complexes in West-Central Arizona

A grid of over 1000 km of 2-D oil industry reflection seismic data in west-central Arizona have been reprocessed and interpreted in order to develop a 3-D picture of crustal structures associated with Cordilleran metamorphic core complexes and highly extended terrains. Detachment faults, tilted upper plate strata, and uplifted zones of crustal reflectivity interpreted to have originated in the middle crust are imaged on these seismic lines. These reflections are correlated to detachment faults, tilted Tertiary volcanics/sediments, and mylonite zones at the surface. Numerous sub-parallel reflections within the lower crust abruptly die out at the Moho. The grid of reprocessed seismic data show widespread middle and lower crustal reflectivity that is domed upward beneath the subsurface continuation of the Buckskin-Rawhide, Harcuvar, Harquahala, and White Tank core complexes. The top of this reflective crust is interpreted as one or more mylonite zones that are exposed in the adjacent core complexes. However, most of the reflectivity is probably due to either widespread sub- horizontal intrusions and cumulates in the middle and lower crust, anastomosing mylonitic shear zones, partial melts or other fluids, ductile fabrics that stretch out pre-existing compositional variations, or a combination of these. Although this reflective fabric is widespread, it varies in amplitude and continuity, and in some areas is localized in layers or pods within the middle and lower crust. This suggests that intrusions, ductile deformation, or other pre-existing compositional variations are not evenly spaced throughout the middle and lower crust. Elongated domes of crustal reflectivity occur beneath the breakaway zones of detachment faults associated with the exposed core complexes. These zones of upwarped crustal reflectivity are interpreted to have been uplifted by deeper detachment faults that have breakaway zones farther southwest. These reflective crustal upwarps do not crop out at the surface, but have a northeastward elongated antiformal top similar to those imaged beneath exposed core complexes to the northeast. Therefore, these buried upwarps are interpreted as incipient core complexes that have not experienced enough extension along the overlying detachment faults to bring mylonite zones to the surface.