Laramide Crustal Shortening and Cenozoic Extensional Overprint in Southeastern Arizona: Insights from Structural Reconstructions and Relation to Porphyry Copper Systems

Abstract

The Laramide orogeny is poorly understood in southeastern Arizona, largely due to the effects of superimposed Cenozoic extension which has dismembered, titled, and concealed older reverse faults. This study combines previous work with new geologic mapping and structural analyses to determine the primary structural style, magnitude, timing, and structural evolution of Laramide shortening within the region. Areas that have been highly extended and others only affected by minor extension were analyzed, and structural reconstructions of Cenozoic deformation were required in most areas to determine the original geometry of reverse faults. These reconstructions also provide insight into the style of extension throughout the region. This is particularly relevant to the Catalina Core complex, which has historically been interpreted in terms of detachment faults. Detailed reconstructions were carried out in the Northern Tortilla, Galiuro, Santa Catalina, and Rincon Mountains. In all areas, crosscutting relations between normal faults and synextensional strata indicate that extension was accomplished via multiple generations of originally steeply dipping normal faults that progressively tilted the crust eastward. These field relations and a structural reconstruction indicate that detachment faults play no role in the formation of the Catalina core complex and possibly no role in similarly interpreted areas pending detailed reconstructions. Geologic maps and reconstructions indicate that reverse faults throughout the region were originally steeply dipping, contain fault propagation folds, and significantly involve crystalline basement. This supports the interpretation that thick-skinned basement-cored uplifts mainly characterized Laramide shortening in southeastern Arizona, not thin-skinned thrusts which have been interpreted in several locales. Erosion surface data were used to construct a paleogeologic map of the region, depicting a possible configuration of Laramide block uplifts before they were deformed by Cenozoic extension. The map suggests the presence of at least five major uplifts bounded by moderate-angle reverse faults. Total estimated crustal shortening across the region ranges from 5-15%, which is not sufficient to explain previously proposed thickened crust that characterized the region prior to extension. This suggests that other mechanisms such as lower crustal flow, underplating of subducted trench sediments, and/or transport of lower lithospheric material of the overriding plate may have played a role in thickening the crust. Finally, a compilation of reverse fault data and porphyry deposits throughout Arizona and New Mexico was carried out to determine the relation between shortening and porphyry generation. In most cases, reverse faults predate porphyry generation by 5-10 Ma. Given available data, reverse faults do not control the emplacement of porphyries, as most cut across faults where their interactions are uncommonly observed. Also, there is no convincing evidence that preexisting lineaments control the localization of porphyry systems. The compilation suggests that shortening may be genetically linked to porphyry systems, as andesitic magmatism typically predates compression, and porphyries postdate compression. Compression may inhibit magmatism and allow magma chambers to evolve to compositions that favor the production of porphyry deposits; however, more age dating and geologic observations are needed to verify this potential genetic link.