Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Timmons, J. M. Articles by Crossey, L. J. Search for Related Content GeoRef GeoRef Citation

Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis

J. Michael Timmons^,1, Karl E. Karlstrom^,2, Matthew. T. Heizler^,3, Samuel A. Bowring^#,4, George E. Gehrels^,5 and Laura J. Crossey^,6

¹ New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, New Mexico 87801, USA
² Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
³ New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, New Mexico 87801, USA
⁴ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
⁵ Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
⁶ Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

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 an air-fall tephra at the base of the Unkar Group dates the onset of deposition at ca. 1255 Ma, and ⁴⁰Ar/³⁹Ar K-feldspar thermochronology in the Grand Canyon indicates that basement rocks cooled through 150 °C between ca. 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 (lower Unkar Group) associated with NE-striking monoclinal flexures indicate NW-directed synsedimentary contraction at ca. 1250 Ma. A large disconformity (75 m.y. duration) is inferred between the 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 1070 Ma (old 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 (ca. 1250 Ma and ca. 1100 Ma) provide new insight into tectonics of southern Laurentia: (1) Laramide-style (monoclines) deformation in the continental interior at ca. 1250 Ma records Grenville-age shortening; and (2) ca. 1100 Ma detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate an Unkar Group source in the Grenville-age highlands of southwestern Laurentia during development of NW-striking extensional basins. We conclude that far-field stresses related to Grenville-age orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimentary and tectonic regime of southwestern Laurentia from 1250 to 1100 Ma.

Key Words: Grenville • Unkar Group • Nankoweap Formation • Precambrian monocline • intracratonic rifting • Grand Canyon

This article has been cited by other articles:

				W. R. Dickinson and G. E. Gehrels U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment Geological Society of America Bulletin, March 1, 2009; 121(3-4): 408 - 433. [Abstract] [Full Text] [PDF]

				P. A. Mueller, D. A. Foster, D. W. Mogk, J. L. Wooden, G. D. Kamenov, and J. J. Vogl Detrital mineral chronology of the Uinta Mountain Group: Implications for the Grenville flood in southwestern Laurentia Geology, May 1, 2007; 35(5): 431 - 434. [Abstract] [Full Text] [PDF]

				W. R. Dickinson Geotectonic evolution of the Great Basin Geosphere, December 1, 2006; 2(7): 353 - 368. [Abstract] [Full Text] [PDF]

				R. E. Sanders, M. T. Heizler, and L. B. Goodwin 40Ar/39Ar thermochronology constraints on the timing of Proterozoic basement exhumation and fault ancestry, southern Sangre de Cristo Range, New Mexico Geological Society of America Bulletin, November 1, 2006; 118(11-12): 1489 - 1506. [Abstract] [Full Text] [PDF]

				S.M. Cather, K.E. Karlstrom, J.M. Timmons, and M.T. Heizler Palinspastic reconstruction of Proterozoic basement-related aeromagnetic features in north-central New Mexico: Implications for Mesoproterozoic to late Cenozoic tectonism Geosphere, October 1, 2006; 2(6): 299 - 323. [Abstract] [Full Text] [PDF]

				J. D. Bloch, J. M. Timmons, L. J. Crossey, G. E. Gehrels, and K. E. Karlstrom Mudstone Petrology of the Mesoproterozoic Unkar Group, Grand Canyon, U.S.A.: Provenance, Weathering, and Sediment Transport on Intracratonic Rodinia Journal of Sedimentary Research, September 1, 2006; 76(9): 1106 - 1119. [Abstract] [Full Text] [PDF]