TECTONIC HEREDITY IN THE GRAND CANYON AND IMPLICATIONS FOR TIBETAN-SCALE MESOPROTEROZOIC INTRA-CONTINENTAL DEFORMATION IN THE SOUTHWESTERN UNITED STATES

Detailed structural analysis and in situ monazite geochronology demonstrate significant Mesoproterozoic reactivation in Paleoproterozoic basement of the Grand Canyon. Ar⁴⁰/Ar³⁹ data reveal a major thermochronologic break near the 96-Mile shear zone, separating ~1.48-1.4 Ga muscovite apparent ages on the east from ~1.64-1.5 Ga ages on the west. A change in Pb isotopes near the break marks a major terrane boundary between the Mojave and Yavapai provinces. The break coincides with a ~500 m-wide ultramylonite that separates upper amphibolite-grade schists, amphibolites, pegmatite pods, and granitic dikes on the east (Trinity Creek Block) from upper greenschist-grade schists and meta-turbidite sequences on the west (Topaz Canyon Block). Early dip-slip movement occurred on steeply NW-dipping foliation planes with steeply plunging stretching lineations, consistent with sub-vertical extension during Paleoproterozoic NW-SE contraction. Later W-side down, sinistral, oblique- to strike-slip movement occurred on the same foliation along variable shallowly to moderately SSW-plunging lineations.

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.