2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 8
Presentation Time: 9:50 AM


TIMMONS, J. Michael, New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, FLETCHER, Kathryn, Department of Earth and Environmental Sciences, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, BLOCH, John D., Dept. of Earth & Planetary Sciences, Univ. of New Mexico, Northrop Hall, Albuquerque, NM 87131, KARLSTROM, Karl, Earth and Planetary Sciences, University of New Mexico, Northrop Hall, MSCO3-2040; 1 University of New Mexico, Albuquerque, NM 87131-0001, HEIZLER, Matthew T., New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining & Technology, Socorro, NM 87801, GEHRELS, George, Geosciences, Univ of Arizona, Tucson, AZ 85721, BOWRING, Sam, Dept. of Earth, Atmospheric, and Planetary Sciences, Mass Inst. of Technology, 77 Massachusetts Ave, Cambridge, MS 02139 and CROSSEY, Laura, Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, mtimmons@nmbg.nmt.edu

The Grand Canyon Unkar Group is one of the best-preserved remnants of a Mesoproterozoic sedimentary basin in southwestern Laurentia. Unkar Group deposits record protracted basin formation and intracratonic deformation compatible with NW-directed shortening during Grenville-age orogenesis. A previously reported age determination from the Unkar Group suggested a lower depositional age of 1255 Ma (U/Pb), however, new detrital zircon analyses suggest the lower Bass Formation may be as young as 1200 Ma. A new date from a dike in the Unkar Group confines the upper depositional age of the sequence at 1104 Ma (Ar/Ar). Abrupt thickness changes in conglomerate and dolomite facies of the Bass Formation of the lower Unkar Group associated with NW-vergent monoclinal flexures indicate a regime of NW-directed syn-sedimentary contraction during early Unkar Group time. A second style of Unkar Group deformation involved the development of half grabens and full grabens, which record NE-SW extension on NW-striking high-angle normal faults. Field relationships suggest extensional deformation was concurrent with contractional deformation but likely outpaced monoclinal development. The structural regime that affected the Unkar Group provides new insight into the tectonics of southern Laurentia: 1) There was Laramide-style monoclinal deformation in the continental interior at ca. 1200 Ma that records Grenville-age shortening, 2) Orogen-perpendicular rift basins were likely developing in the continental interior during contraction, and 3) Paleocurrent analysis, 1.1 Ga detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate that sedimentary rocks of the Unkar Group were deposited between about 1200 and 1100 Ma and have a provenance within the Grenville highlands. 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 about 1250 to 1100 Ma.