Late Cretaceous paleomagnetism of the Tucson Mountains: Implications for vertical axis rotations in south central Arizona

The Tucson Mountains of southern Arizona are the site of an Upper Cretaceous caldera from which the rhyolitic Cat Mountain Tuff was erupted at about 72 Ma. Two magnetic units within the Cat Mountain Tuff are distinguished by paleomagnetic data in both the northern and southern Tucson Mountains. The resurgent Amole pluton (≃72 Ma) in the northern Tucson Mountains was emplaced soon after eruption of the Cat Mountain Tuff but cooled and was magnetized after northeastward tilting (50°–85°) of the adjacent caldera-fill sequence. Petrologic and paleomagnetic data indicate that the lower magnetic unit of the Cat Mountain Tuff caps the Silver Bell Mountains to the northwest. A previous paleomagnetic investigation (N = 34) indicates that the Silver Bell Mountains have been rotated clockwise 30°±16° (95% confidence level) about a vertical axis relative to cratonic North America. A similar paleomagnetic study of Upper Cretaceous volcanic, volcaniclastic, and intrusive units in the Tucson Mountains (N = 26) indicates that these rocks have been rotated 7°±14° clockwise relative to stable North America. A direct comparison of paleomagnetic directions for the lower unit of the Cat Mountain Tuff shows a 17°±10° clockwise rotation between the Silver Bell Mountains and the Tucson Mountains which supports the relative accuracy of the absolute rotations determined for these two mountain ranges. Preliminary paleomagnetic directions for middle Tertiary units from the Silver Bell and Tucson Mountains are consistent with clockwise rotation having occurred prior to deposition of these rocks. Clockwise rotation of crustal blocks in southern Arizona likely was associated with strike-slip movement on major northwest trending faults in the region, and this movement may have been related to oblique subduction of oceanic plates along the western continental margin in Late Cretaceous and early Tertiary time. The available paleomagnetic data indicate that rocks in southern Arizona have not remained unrotated with respect to North America since Late Cretaceous time and that vertical axis rotations may have played an important role in the region during Laramide deformation.