Stratigraphic uncertainty and errors in shortening from balanced sections in the North American Cordillera

Research Article| September 01, 2013 Stratigraphic uncertainty and errors in shortening from balanced sections in the North American Cordillera Richard W. Allmendinger; Richard W. Allmendinger † Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA †E-mail: rwa1@cornell.edu Search for other works by this author on: GSW Google Scholar Phoebe Judge Phoebe Judge Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Richard W. Allmendinger † Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA Phoebe Judge Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA †E-mail: rwa1@cornell.edu Publisher: Geological Society of America Received: 18 Feb 2013 Revision Received: 05 Jun 2013 Accepted: 02 Jul 2013 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2013 Geological Society of America GSA Bulletin (2013) 125 (9-10): 1569–1579. https://doi.org/10.1130/B30871.1 Article history Received: 18 Feb 2013 Revision Received: 05 Jun 2013 Accepted: 02 Jul 2013 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Richard W. Allmendinger, Phoebe Judge; Stratigraphic uncertainty and errors in shortening from balanced sections in the North American Cordillera. GSA Bulletin 2013;; 125 (9-10): 1569–1579. doi: https://doi.org/10.1130/B30871.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Shortening from balanced cross sections is typically cited as a minimum estimate because of the uncertainty in the position of eroded hanging-wall cutoffs. We show here using area balancing that the single most significant source of error is that associated with the shape and thickness of the initial stratigraphic wedge involved in the deformation. New methods and software are introduced that, in combination with freely available scans of virtually all geologic maps published by the U.S. Geological Survey (USGS) and Geological Survey of Canada (GSC), allow one to make hundreds of map thickness measurements in just a few hours. These methods are applied to geologic maps and cross sections from the western North American Cordillera to determine the uncertainty of shortening in iconic cross sections by R.A. Price and P.R. Fermor, and by F. Royse. One-sigma stratigraphic thickness uncertainty, measured in homoclinal dip packages of prominent Paleozoic units within single thrust plates (and single quadrangles), ranges from 20% to 30% of average thickness in the Costigan and Sulfur Mountain plates in southern Canada and can be as high as 37% in the Darby and Absaroka plates of the Idaho-Wyoming (United States) thrust belt. Assuming that all individual Paleozoic units in the cross sections have similar errors, the entire Paleozoic section used in the area balancing would have 8%–11% uncertainty, assuming a normal or Gaussian distribution. Because the stratigraphic wedges in southern Canada and Idaho-Wyoming do not have a uniform original taper, our basic area balance underestimates shortening. By balancing the non-uniform taper back to a uniform taper, we can account for this discrepancy. Total shortening determined in this way is ∼104 ± 17 km, which is identical to that obtained by Price and Fermor. The Idaho-Wyoming section of Royse (with maps by the USGS) yields similarly large errors in shortening magnitude: 56 ± 10 km for the three frontal thrust faults. Of those errors, uncertainty in the shape and thickness of the initial stratigraphic wedge accounts for 56% and 70% of the total in southern Canada and western Wyoming, respectively. Eroded hanging-wall cutoffs account for just 15% in both areas, with the remainder coming from uncertainty in the subsurface geometry, including the position of the décollement. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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