Mesoproterozoic tectonic evolution of southwestern North America: Protracted intracratonic deformation, sedimentation, and differential exhumation in Grand Canyon and the Rocky Mountain region

The Unkar Group of the Grand Canyon Supergroup is one of the best-preserved remnants of Mesoproterozoic sedimentary rocks in the southwestern United States. It provides an exceptional record of intracratonic basin formation and associated tectonics kinematically compatible with protracted "Grenville-age" NW-directed shortening. New U/Pb age determinations from tephra at the base of the Unkar Group date the onset of deposition at ca. 1254 Ma. 40 Ar/39Ar Kfeldspar thermochronology in Grand Canyon indicates that basement rocks cooled through 150°C between about 1300 and 1250 Ma, refining exhumation rates of basement rocks just prior to Unkar deposition. Abrupt thickness and facies changes in conglomerate and dolomite of the Bass Formation associated with NE-striking monoclinal flexures indicate NW-directed syn-sedimentary contraction at about 1250 Ma. A large disconformity (∼75 m.y. duration) is inferred between lower and upper Unkar Group and is located below the upper Hakatai Shale as documented by detrital zircons. A second style of Unkar Group deformation involved the development of half grabens and full grabens that record NE-SW extension on NW-striking, high-angle normal faults. Several observations indicate that NW-striking normal faulting was concurrent with upper Unkar deposition, mafic magmatism and early Nankoweap deposition: (1) Intraformational faulting in the Bass Formation, (2) Intraformational faulting in the 1.07 Ga (Rb/Sr date) Cardenas Basalt and lower Nankoweap Formation, (3) syntectonic relationships between Dox deposition and 1104 Ma (new Ar/Ar date) diabase intrusion, and (4) an angular unconformity between Unkar Group and Nankoweap strata. The two tectonic phases affecting the Unkar Group (∼1250 Ma and ∼1100 Ma) provide new insight into tectonics of southern Laurentia: (1) Laramide-style (monoclines) deformation in the continental interior at 1250 Ma records Grenville-age shortening; (2) 1.1 Ga detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate that an Unkar Group source in the Grenville highlands during development of NW-striking extensional basins. We conclude that far-field stresses related to Grenville orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimentary and tectonic regime of southwestern Laurentia from 1.25 to 1.1 Ga.; The dominant structural grains of the Ancestral Rockies and Laramide Rockies (N-S and NW-trending) represent the reactivation of Proterozoic fault systems that formed during regional intracratonic deformation in the Mesoproterozoic and Neoproterozoic. 40Ar/39Ar K-feldspar thermochronology provides a test of this hypothesis by resolving the thermal history of different crustal blocks in the 150°--300°C temperature range.