Emplacement and rheomorphic deformation of a large, lava-like rhyolitic ignimbrite: Grey's Landing, southern Idaho

Research Article| March 01, 2011 Emplacement and rheomorphic deformation of a large, lava-like rhyolitic ignimbrite: Grey's Landing, southern Idaho Graham D.M. Andrews; Graham D.M. Andrews † Department of Geology, University of Leicester, Leicester, LE1 7RH, UK †Geological Survey of Canada, 625 Robson Street, Vancouver V6B 5J3, Canada; Current address: Department of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-3003; e‑mail: gdma1977@gmail.com Search for other works by this author on: GSW Google Scholar Michael J. Branney Michael J. Branney Department of Geology, University of Leicester, Leicester, LE1 7RH, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Graham D.M. Andrews † Department of Geology, University of Leicester, Leicester, LE1 7RH, UK Michael J. Branney Department of Geology, University of Leicester, Leicester, LE1 7RH, UK †Geological Survey of Canada, 625 Robson Street, Vancouver V6B 5J3, Canada; Current address: Department of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-3003; e‑mail: gdma1977@gmail.com Publisher: Geological Society of America Received: 06 Sep 2009 Revision Received: 07 Dec 2009 Accepted: 27 Jan 2010 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2011 Geological Society of America GSA Bulletin (2011) 123 (3-4): 725–743. https://doi.org/10.1130/B30167.1 Article history Received: 06 Sep 2009 Revision Received: 07 Dec 2009 Accepted: 27 Jan 2010 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Graham D.M. Andrews, Michael J. Branney; Emplacement and rheomorphic deformation of a large, lava-like rhyolitic ignimbrite: Grey's Landing, southern Idaho. GSA Bulletin 2011;; 123 (3-4): 725–743. doi: https://doi.org/10.1130/B30167.1 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 The Miocene Grey's Landing ignimbrite reaches 70 m thick and covers at least 400 km2 in the central region of the Snake River Plain. It shows particularly intense welding and rheomorphic deformation, and although parts are eutaxitic, most is lava-like with flow-banding and no fiamme. A near-ubiquitous penetrative flow lamination, associated with a well-developed elongation lineation, is folded into small intrafolial tight to isoclinal oblique and sheath folds, which are refolded by larger folds in the upper parts. Structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot (≤1000 °C), high-mass-flux pyroclastic density current that flowed westward across a graben-faulted landscape. As hot particles were deposited, they rapidly agglutinated and coalesced, and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current-deposit interface. The shear zone is interpreted to have been less than 2 m thick. It produced and deformed the rheomorphic fabric, and it migrated upward with the rising current-deposit interface during aggradation, so that it transiently affected all levels of the resultant thick ignimbrite. Deformation was progressive, and after the density current had dissipated, viscous spreading and downslope flow continued and involved an increasingly thick portion of the sheet. This folded the flow banding and F1 intrafolial isoclines into larger sheath folds, and into more upright periclines near the top of the ignimbrite. We demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.