274 Tectonics of Western North America: Late Paleozoic and through Time

Wednesday, 28 September 2016
8:00 AM - 12:00 PM
Mile High Ballroom 2C

WERE CORE COMPLEX COVER ROCKS IN NE NEVADA BURIED TO GREAT DEPTH DURING METAMORPHISM? – I DON’T THINK SO!!

Paper number: 274-14
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM

Authors

  • Charles Thorman Presenting Author
    Scientist Emeritus, U.S. Geological Survey, Denver, CO

Abstract

The depth of burial during metamorphism of the Paleozoic-Mesozoic Cover Rocks (CR) of the Ruby-East Humboldt Metamorphic Core Complex (REHMCC) area has been the subject of much discussion. Temperature and depth estimates of ~620-800°K and ~79,000’-65,000’ (~15-12.3 mi/~24.5-20km), based on the mineral assemblage gr-bio-mus-ky-st of the Middle Cambrian meta-Dunderberg Shale using the Gibbs method, have been put forth as the conditions for metamorphism. The following factors argue strongly against such an interpretation.
  • Metamorphosed (up to amphibolite facies) Cambro-Ordovician rocks at several locales grade upward in stratigraphically continuous normal sections to nonmetamorphosed Triassic-Jurassic strata.
  • Regional total Late Precambrian to Jurassic thickness was ~+40,000’ (~7.6mi/12.7 km) and the regional depth to the Dunderberg Shale in the Jurassic was ~25,000’ (~4.7 mi/~7.6 km).
  • Penn-Perm strata have CAIs of 1 to 2 (< 140°C), indicating that the entire Paleozoic-Mesozoic CR section was never buried to more than normal stratigraphic section.
  • Documented thicknesses of Cretaceous &/or Cenozoic rocks do not support erosion &/or displacement of the 40,000’ of strata required to have buried the metamorphosed Dunderberg Shale to the 65,000’.
  • Wheeler (2014) demonstrates that applying the Gibbs method to metamorphic temperature and pressure is not valid. “For indicator mineral assemblage reactions the apparent lithostatic pressure can be offset 500 MPa (5 kb) for a 50 MPa (0.5 kb) differential stress....,” which translates to inaccurate depth estimates by as much as ~15 km.

These observations suggest that invoking burial of the CRs of the REHMCC to ~65,000’ is not plausible. Based on the above data, the CRs were at normal stratigraphic depths throughout their Paleozoic, Mesozoic, and Cenozoic history. Accordingly, a significant upward decreasing thermal profile must have prevailed during regional igneous & synkinematic metamorphic events that contributed to the Late Jurassic, Cretaceous, and Cenozoic geologic evolution. This has a direct impact on the existing concept regarding rapid deep burial and subsequent rapid uplift of the REHMCC terrain. The deep burial and rapid uplift concept has to be revisited. The burial and subsequent uplift values may be less than half of what is presently considered.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-286239

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