Structural evolution and timing of deformation along the Proterozoic Spring Creek shear zone of the northern Tusas Mountains
— Peter Davis, Mike Williams, and Karl Karlstrom

The Spring Creek shear zone is a newly mapped structural feature that closely parallels a lithologic discontinuity in the north-central Tusas Mountains. Both strike east-west and dip steeply to the south. The lithologic discontinuity juxtaposes island arc mafic metavolcanic rocks and metasedimentary rocks of the Moppin Complex to the north to northeast with metarhyolites and quartzites of the Vadito and Hondo Groups to the south to southwest. The shear zone juxtaposes amphibolite facies rocks to the south and greenschist facies rocks to the north. The Moppin Complex is typical of Yavapai Province rocks; the southern domain is typical of the Yavapai-Mazatzal transitional domain in New Mexico. Preliminary U-Pb dates on two granites near the boundary help constrain the timing of deformation. The Tres Piedras metagranite (orthogneiss) is 1700-1693 Ma and is interpreted to truncate the lithologic discontinuity on the eastern end. It contains high-temperature solid-state to possibly magmatic flow foliation that parallels a low-temperature fabric in the host rock. Closed to isoclinal similar folds that contain little to no axial planar cleavage, as well as localized reorientation of F2 folds suggest that strain partitioned during cooling of the pluton. The 1693 Ma Tusas Mountain orthogneiss appears to be part of the Tres Piedras orthogneiss in time and composition. It intrudes greenschist-grade Moppin Series north of the Spring Creek shear zone. It lacks folds, but contains possible syn-tectonic magmatic flow structures with little evidence for solid-state deformation. D₁ and D₂ are interrupted to have occurred in the 1700-1690 Ma window during the Yavapai to Mazatzal orogenic transition and were probably a progressive event. These early fabrics were variably reactivated during later Mesoproterozoic 1450-1350 Ma tectonism, with increasing degrees of reactivation south of the Spring Creek shear zone. The reverse fault offset of D₂ structures and amphibolite to greenschist facies juxtaposition that define the Spring Creek shear zone post dates D₂ and is probably a Mesoproterozoic structure.