TECTONIC HEREDITY IN THE GRAND CANYON AND IMPLICATIONS FOR TIBETAN-SCALE MESOPROTEROZOIC INTRA-CONTINENTAL DEFORMATION IN THE SOUTHWESTERN UNITED STATES
High-resolution BSE-imaging, X-ray mapping, and EMP monazite geochronology on schists in and near the 96-Mile shear zone reveal several populations of kinematically significant 10-150 µm monazite grains: 1) anhedral sulfur-rich monazite that fills or overgrows fractures in association with apatite, 2) subhedral monazite aligned with shear bands and foliation, and 3) poikiloblastic monazite cut by synthetic and antithetic micro-faults and fractures. Preliminary microprobe dates on low Y-low Th rim domains, interpreted to have precipitated syn- to post-kinematically, are 1505 +/- 24 Ma.
The 96-Mile shear zone records two distinct tectonic episodes: 1) post-peak metamorphic juxtaposition during the Yavapai-Mazatzal Orogeny, and 2) Mesoproterozoic reactivation in the vicinity of a crustal-scale suture (~3.5 km SE of the Crystal shear zone). We suggest the second episode documents an early, intra-continental (100s of km from the active margin, i.e. Tibetan-scale) response to the onset of ~1.5-1.35 Ga tectonism. The reactivation may have been localized at deeper crustal levels along an early accretionary boundary.