LARGE MAGNITUDE EXTENSION IN THE CATALINA-RINCON CORE COMPLEX, SOUTHEASTERN ARIZONA

Abstract

Warren Hamilton was a pioneer in identifying evidence for large-magnitude extension in the Basin and Range province and suggested in 1966 that extension was as much as 100%. Recent field mapping in the Rincon Mountains east of Tucson, Arizona, indicate that Cenozoic extension was greater than suggested by previous studies. The southwest side of the Rincon and Santa Catalina Mountains are moderately to strongly affected by Oligocene to lower Miocene mylonitization that occurred during tectonic extension, with an approximately 245° normal displacement direction indicated by mylonitic fabrics at the foot of the ranges. Displacement on the Catalina – San Pedro detachment fault resulted in uplift and exhumation of the mylonitic footwall, which consists largely of Eocene leucogranites. An upper-plate fault block within a synformal fault groove on the west side of the Rincon Mountains contains a thrust fault that is interpreted as displaced 34-38 km westward from an original position adjacent to a similar thrust in the footwall of the San Pedro detachment fault. The axis of Tucson basin southwest of the Rincon Mountains is interpreted as the trailing edge of the main mass of the detachment-fault upper plate, which includes the Sierrita Mountains to the southwest and the Tucson Mountains to the west. Restoration of displacement in cross-section view suggests that this trailing edge is displaced 50-60 km from an original position adjacent to the northwestern Johnny Lyon Hills east of the Rincon Mountains. In the Santa Catalina Mountains area, restoration of the Catalina – San Pedro detachment fault by ~50 km places the Amole granite (72.5 +/- 0.1 Ma) and border phase granodiorite (73.4 +/- 0.2 Ma) in the northern Tucson Mountains above the Leatherwood granodiorite (69.8 +/- 1.4 Ma) in the central Santa Catalina Mountains. These intrusions are possibly parts of the same intrusive suite that was dismembered by detachment faulting. At exposed levels both plutons intrude Paleozoic strata, which is consistent with the concept that crustal sections were duplicated by a pre-intrusion, NE-vergent thrust fault and that greenschist-grade metamorphism in the high Santa Catalina Mountains occurred after thrust burial. This hypothetical, ~SE-striking thrust fault would be buried under Tucson basin.