SOME ARIZONA EXAMPLES OF THE VERTICAL INTEGRATION OF FAULT ZONE BEHAVIOR

Crustal evolution creates rheological boundaries and shear zones that tend to be weaker than the lithotectonic assemblage(s) they cross. As zones of slip/strain localization they are dynamically exploited and linked through renewed and/or redirected stress and the catalyst of fluid pressures. Mesh-like shear-zone linkages between neofaulting and reactivated faulting create physical/kinematic connectivity of upper-crustal and mid-crustal shear zones. Deeply penetrating fault zones tend to be hybrid composites, whose development is enmeshed in total tectonic history. We present three examples that support this view. The first is hard linkage between late Mesoproterozoic and Neoproterozoic shear zone deformation that reactivated to accommodate Laramide-style uplift. This is pieced together along a 500+ km ‘geologic walk’, starting in the Paleo- and Mesoproterozoic basement of the Mogollon Rim region, proceeding through the Neoproterozoic basement of the Grand Canyon, and moving upward through Cambrian-Paleocene strata along the trace of the East Kaibab monocline. The second example is a softer linkage in the Basin and Range of southeastern Arizona. Tracking this example requires ‘mountain-range hopping’ through which we identify Precambrian shear zones reactivated by Jurassic oblique-slip faulting; Jurassic faults inverted as reverse faults; and reverse faults negatively inverted in the Neogene. The third is a metamorphic core complex example (Rincon-Catalina Mountains) in which early deep development of ultramylonite established a strain-localization vulnerability that progressively focused more brittle shear upward from basement to the Miocene land surface. Does ‘hybrid composite’ shear zone behavior and linkage descend into the lower-crust as well? Investigations of the rheological behavior of lower crust conclude that even lower- crust shear zones are weak relative to the low-viscosity crust they inhabit, and this favors the development of broad anastomosing mesh-like networks. Thus the premise of interconnectivity in time/space/orientation may hold up throughout the crust, with shear zone networks- in the language of Burgmann and Dresen (2008, p. 554) - “typically display[ing] slip transfer between cooperative shears and partitioning of strain between network structures.”