WHERE DID THE ARIZONAPLANO GO? MIDDLE TO LOWER CRUSTAL PROCESSES REQUIRED

Metamorphic core complexes (MCCs) are regions of high-magnitude extension where the middle crust is exhumed along low-angle detachment faults. MCCs, observed within many global orogenic systems, represent a fundamental mechanism for redistributing unstable over-thickened crust. Despite their prevalence and significance for crustal redistribution, it remains unclear if upper-crustal tectonic exhumation alone is sufficient to adequately thin the crust. The Catalina MCC, in southern Arizona, USA, is a type example of a Cordilleran MCC and part of a discontinuous belt of MCCs which extend from northern Mexico to southern Canada as part of the North American Cordillera. The Cenozoic exhumation of the Catalina MCC reflects partial redistribution of an over-thickened orogenic plateau (~58 km thick “Arizonaplano”) coinciding with the roll-back of the Farallon plate, following the Late Cretaceous-Paleogene Laramide Orogeny. Thus, the Catalina MCC represents an excellent locality to investigate the relative contribution of upper-crustal removal to crustal thinning.

In this study, we integrate 59 apatite and zircon fission-track and apatite and zircon (U-Th-Sm)/He dates across the Catalina MCC to constrain its upper crustal cooling history. These data identify three phases of cooling. A minor, early phase of pre-MCC cooling at ~40 Ma. The major phase of cooling at 19-26 Ma is related to detachment faulting and exhumation of the Catalina MCC. Before finally, a young phase of cooling at 11-17 Ma which is associated with Basin and Range extension. When comparing thermochronometric data to whole rock Sr/Y crustal thickness estimates for southern Arizona, we find that the Basin and Range and the MCC detachment faulting events are contemporaneous with a decrease in crustal thickness estimates. Although only cryptically recorded in the thermochronometric data, the pre-MCC cooling event also corresponds to a sharp drop in crustal thicknesses. Thus, an additional mechanism is required to thin the crust prior to detachment faulting. We suggest that lower-crustal flow at 45-35 Ma may have thinned the “Arizonaplano" prior to MCC detachment faulting and Basin and Range extension.