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

Abstract

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.

USGS research on mineral resources, 1994: Part B, Guide book for field trips

“…In a nutshell, tuff erupted from the Silver Bell caldera (tuff of Confidence Peak) is present as megablocks within the younger Tucson Mountains caldera, and the welded tuff member of the Mount Lord Volcanics that caps the Silver Bell Mountains is now interpreted as outflow Cat Mountain Tuff from the Tucson Mountain caldera. These correlations are confirmed by distinctions in petrography, minorelement compositions, and paleomagnetic pole positions (table I; Hagstrom and Sawyer, 1989;Hagstrom and Lipman, 1991).…”

“…(Sawyer, 1987). The capping tuffs of the Silver Bell Mountains have at times been designated the Mount Lord Tuff (Ignimbrite), but correlation with the Cat Mountain Tuff (which has precedence as a name) is now frrmly based on stratigraphic position with respect to the tuff of Confidence Peak (erupted from the Silver Bell caldera), petrographic and chemical distinctions, and distinctive paleomagnetic pole positions (Hagstrom and Sawyer, 1989;Hagstrom and Lipman, 1991).…”

“…Detailed mapping and petrologic studies of three such areas of Laramide age near Tucson have now been largely completed, including the Silver Bell Mountains (Sawyer, 1987;Hagstrom and Sawyer, 1989), the Sierrita Mountains (Lipman and Fridrich, 1990), and the Tucson Mountains (Hagstrom and Lipman, 1991;Lipman, 1993), which are the focus of this field trip ( fig. 1).…”

A cratonic Middle Jurassic paleopole: Callovian‐Oxfordian stillstand (J‐2 cusp), rotation of the Colorado Plateau, and Jurassic North American apparent polar wander

“…Unfortunately, each one of those poles carries associated irresolvable uncertainties: igneous rock paleopoles including the Moat Volcanics possess a paleohorizontal uncertainty, whereas the Corral Canyon sites lie within a tectonic block which subsequently has been detached from the North American craton. Studies of other Basin and Range blocks in the geographic area have demonstrated that these blocks have been rotated relative to cratonic North America [e.g., Hagstrum and Lipman , 1991]. Thus without corroborating evidence, the Corral Canyon pole cannot be considered a reliable reflection of NA Jurassic APW.…”

Mineral resource potential and geology of Coronado National Forest, southeastern Arizona and southwestern New Mexico

“…Locally, the volcanic rocks include coarse and (or) exotic-breccia deposits. To the west, the rhyolite tuffs are known as the upper member of the Salero Formation (Drewes, 1971c) or the Cat Mountain Tuff (Cooper, 1973;Drewes, 198la;Hagstrum and Lipman, 1991).…”