TIMING OF DEFORMATION AT FOUR PEAKS, CENTRAL ARIZONA

The ca. 1.65 Ga Mazatzal orogeny has long been considered one of the major accretionary events that contributed to the southward growth of Laurentia in the Paleoproterozoic. Recent geochronology (especially detrital zircon data) indicate that at least the upper parts of Proterozoic metasedimentary successions across the Southwest are younger (ca. 1.6-1.5 Ga) than previously thought, which precludes the possibility of their deformation during the 1.65-1.60 Ga Mazatzal orogeny (Doe et al., 2012, Jones et al., 2011). This suggests that the ca. 1.45 Ga tectonic event (Picuris orogeny per Daniels et al., 2013) may have been the dominant deformational event in some regions and implies that some of the classic structures that constrain the Mazatzal orogeny need to be reexamined. The Four Peaks in the southern Mazatzal Mountains of central Arizona exhibit a large synclinal sheath fold of Proterozoic metasediments, which has been considered a classic example of deformation related to the Mazatzal orogeny. Recent field work confirms the presence of several zones of intense shearing south and east of the Four Peaks syncline. More detailed geochronology is needed to 1) determine whether the shearing and folding at Four Peaks can be ascribed to ca. 1.65 Ga tectonism and 2) discern the significance of the ca. 1.45 Ga event in central Arizona. Monazite geochronology is underway to temporally constrain fabric formation, folding and metamorphic history at Four Peaks. Detrital zircon geochronology is underway to place constraints on the timing of sedimentation and refine correlations between the Four Peaks metasediments and other metasediments around the Tonto Basin. If the deformation at Four Peaks proves to be part of the younger 1.45 Ga event, then ca. 1.4 Ga tectonism was much more significant than previously thought and the overall style and character of the Mazatzal orogeny must be reevaluated.