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, Karl E. Karlstrom, Matthew T. Heizler, Samuel A. Bowring, George E Gehrels, Laura J. Crossey

Research output: Contribution to journalArticle

42 Citations (Scopus)

Abstract

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 airfall 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.

Original languageEnglish (US)
Pages (from-to)1573-1595
Number of pages23
JournalBulletin of the Geological Society of America
Volume117
Issue number11-12
DOIs
StatePublished - Nov 2005

Fingerprint

basin evolution
orogeny
canyon
Laurentia
tectonics
faulting
basement rock
zircon
intracratonic basin
continental interior
monocline
thermochronology
flexure
age determination
tephra
diabase
muscovite
exhumation
unconformity
normal fault

Keywords

  • Gran Canyon
  • Grenville
  • Intracratonic rifting
  • Nankoweap Formation
  • Precambrian monocline
  • Unkar Group

ASJC Scopus subject areas

  • Earth and Planetary Sciences (miscellaneous)

Cite this

Tectonic inferences from the ca. 1255-1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon : Intracratonic deformation and basin formation during protracted Grenville orogenesis. / Timmons, J. Michael; Karlstrom, Karl E.; Heizler, Matthew T.; Bowring, Samuel A.; Gehrels, George E; Crossey, Laura J.

In: Bulletin of the Geological Society of America, Vol. 117, No. 11-12, 11.2005, p. 1573-1595.

Research output: Contribution to journalArticle

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abstract = "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 airfall 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.",
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T1 - Tectonic inferences from the ca. 1255-1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon

T2 - Intracratonic deformation and basin formation during protracted Grenville orogenesis

AU - Timmons, J. Michael

AU - Karlstrom, Karl E.

AU - Heizler, Matthew T.

AU - Bowring, Samuel A.

AU - Gehrels, George E

AU - Crossey, Laura J.

PY - 2005/11

Y1 - 2005/11

N2 - 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 airfall 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.

AB - 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 airfall 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.

KW - Gran Canyon

KW - Grenville

KW - Intracratonic rifting

KW - Nankoweap Formation

KW - Precambrian monocline

KW - Unkar Group

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