THE ESPIRITU CANYON SHEAR ZONE IN THE FOOTWALL OF THE SAN PEDRO – CATALINA DETACHMENT FAULT EAST OF TUCSON, ARIZONA: AN EXHUMED, DEEP-SEATED SEGMENT OF THE SAN XAVIER DETACHMENT FAULT?

Geologic mapping in the northeastern Rincon Mountains east of Tucson, Arizona, revealed previously unidentified relationships between two major low-angle structures. The hanging-wall block of the San Pedro detachment fault contains fragments of a thrust fault and an Oligo-Miocene stratal sequence with bed dips that decrease up section. The footwall block contains the low-angle Espiritu Canyon shear zone, which places highly deformed strata of the Jurassic to Cretaceous Bisbee Group and upper Paleozoic carbonates over generally weakly deformed Proterozoic granite and Eocene leucogranite. At several excellent exposures of this shear zone, marble tectonites are smeared over crushed and chloritically altered granitoids. We interpret the Espiritu Canyon shear zone as a normal-sense, section-attenuating shear zone that developed under conditions in which carbonates deformed plastically whereas quartzofeldspathic rocks deformed brittlely. This shear zone was previously identified by Steve Lingrey (1982 U. Arizona Ph.D. dissertation) in areas to the southeast where it formed the base of the “mylonitic carapace,” a feature related to genesis of metamorphic core complexes by Davis and Coney (1979, GEOLOGY). New mapping in the Redington Pass area revealed that the Espiritu Canyon shear zone diverges from near the northwest-striking San Pedro detachment fault and passes westward into a north-dipping mylonitic shear zone entirely within Proterozoic granite and Eocene leucogranite. This shear zone, with north- to northeast-trending lineations and top-north or top-northeast shear sense, strikes westward into the Santa Catalina Mountains to become the Molino basin mylonitic shear zone, and from there projects westward beneath Tucson basin west of Sabino Canyon. Restoration of 25-30 km of displacement on the Catalina detachment fault places this mylonitic shear zone beneath the Tucson basin and the southeastern Tucson Mountains. At that position it projects up dip to the south toward the middle Tertiary San Xavier detachment fault which has at least 16 km of top-north displacement. We infer that the Espiritu Canyon shear zone is the down-dip continuation of the San Xavier fault that has been exhumed in the footwall of the cross-cutting and apparently younger, top-SW Santa Catalina – San Pedro detachment-fault system.