Intersecting Mylonitic Shear Zones in the Footwall of the Catalina Detachment Fault, Santa Catalina Mountains, Southeastern Arizona

The forerange arch on the south flank of the Santa Catalina Mountains north of Tucson, Arizona, consists of a broadly arched, gently dipping mylonitic foliation developed within gneissic and granitic rocks as young as Eocene. The south side of the east-west trending arch is strongly mylonitic, with locally very well developed mylonitic lineations and inclined quartz c-axis petrofabrics. Foliation dips southward beneath the Tucson basin and beneath the Catalina detachment fault at the foot of the range. Lineation trend is 245° ± 5° and shear sense, as indicated by numerous asymmetric mylonitic petrofabrics, is top southwest. The north side of the arch is weakly to moderately mylonitic. Most shear-sense indicators are top northeast, but a significant fraction are top southwest, especially near the crest of the arch. Lineations trend 060° near the arch crest but are progressively more northerly trending farther north from the arch crest, reaching about 035° in the most northerly exposures in the Molino Canyon area. Two separate foliations are not distinguishable on the crest of the arch. However, the opposite shear sense on the two sides of the arch, as well as the gradual northward change in lineation trend on the north side of the arch, are interpreted as evidence for two separate shear zones in which foliations and lineations in the older north-dipping zone have been rotated into parallelism with the southern shear zone over a broad arch-crest region where the two shear zones intersect. Restoration of normal displacement on the Catalina detachment fault places the forerange arch beneath the Tucson Mountains west of Tucson. In this position, the north-dipping, top-northeast mylonitic shear zone projects up dip to the south toward the San Xavier detachment fault in the Sierrita Mountains. Correlation of the two is problematic, however, because displacement on the San Xavier fault is approximately along 350°, which is different by about 45° from the trend of the most northerly measured lineations on the north side of the forerange arch. However, it is possible that mylonitic lineations are northerly trending at sufficiently large distances north of the forerange arch, as suggested by reconnaissance mapping north of Reddington Pass, and that the mylonitic shear zone originally down-dip from the San Xavier detachment fault is preserved in the Santa Catalina Mountains.