David Elliott

Abstract The total energy involved in emplacing a thrust sheet is expended in initiation and growth of the thrust surface, slip along this surface, and deformation within the main mass of the sheet. This total energy can be determined from potential energy considerations knowing the initial and final geometry from balanced cross sections after defining the thrust’s thermodynamic system boundaries. Emplacement of the McConnell thrust in the Canadian Rockies involved ca. 1019 J of gravitational work, an order of magnitude greater than any possible work by longitudinal compressive surface forces. A new theory for the initiation and growth of thrusts as ductile fractures is based on a demonstration that thrust displacement is linearly related to thrust map length and that fold complexes at the ends of thrusts are constant in size for a given metamorphic grade. Much of the total work is dissipated within the body of the sheet. Field observations show which mechanisms of dissipation are most important at various positions within the thrust sheet, and it is found that only the top 5 km of the McConnell was dominated by frictional sliding. A novel type of sliding along discrete surfaces is pressure solution slip, in which obstacles are by-passed by diffusive mass transfer. Fibres and pressure solution grooves are diagnostic features of this sliding law, in which slip velocity is linearly related to shear stress. Pressure solution slip is widespread at depths greater than about 5 km, but at this depth penetrative whole rock deformation by pressure solution becomes dominant - marked by cleavage and stretching directions - and accounts for much of the finite strain within the thrust sheet. The McConnell thrust has an outer layer which deformed by frictional sliding and this overlies a massive linearly viscous core responsible for much of the energy dissipation and gross mechanical behaviour.

The regional average basal shear stress τ of a thrust sheet of thickness H is equal to the down-surface slope stress ρgHα. Thrusts always move in the direction of surface slope α, even if they are moving up the dip β of the base. The sole thrust beneath the Canadian Rockies had τ of the order of 5×106 Pa (50 bars). Listric normal faults in the main ranges coincided with a reversed sense of τ. A dimensionless number gives the relative magnitude of compressive surface to gravitational forces in the formation of a thrust. The general strength of the rock imposes severe restrictions on the magnitude of surface forces, and gravitational forces dominate in the emplacement of entire thrust sheets, although compressive surface forces are important in the toes. If gravitational forces are dominant, continent-continent collision is not required to produce an orogeny, but significant surface slopes are. An important cause of such slopes is rapidly uplifted magmatic arcs. Piggyback stacks of imbricates can be interpreted in terms of the evolution of paleoslope. Ophiolite thrust sheets have affinities with thrusts made of shelf and slope sediments and may be treated geometrically and mechanically in similar ways.

ABSTRACT Four balanced cross sections, supported by longitudinal sections, structure contour maps, stratigraphic separation diagrams and hangingwall sequence diagrams are keys to this interpretation of the Moine thrust, which forms the western margin of the Caledonides in NW Scotland. New basement and cover correlations between foreland and thrust belt give new slip estimates for the Moine thrust (∼ 77 km), the Loch More klippe (≥ 43 km), Glencoul sheet (20–25 km), Ben More sheet (∼28 km), Achall and Dundonnell ‘sheet II’ (∼28 km). Like other major thrusts the Moine thrust moved in a smooth or rough fashion at different places and times, and many structures are a footwall response to its passage. Widely developed duplexes vary in thickness so that the roof thrust is folded and occasionally faulted; many late Caledonian folds in the Moine metasediments are of this origin. The presence of igneous bodies with contact aureoles increased the propensity to rough slip and this, by causing thickening in the footwall to the Moine thrust, is partly responsible for the Assynt culmination. The previously accepted sequence of thrusting from foreland to hinterland, which has been deduced from the concept of ‘overstep’ of the Moine thrust across lower thrusts, is considered to be a misconception of thrust geometry. Instead, a ‘piggy-back’ sequence of thrusts, from higher to lower, is proposed.

Research Article| August 01, 1973 Diffusion Flow Laws in Metamorphic Rocks DAVID ELLIOTT DAVID ELLIOTT 1Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar Author and Article Information DAVID ELLIOTT 1Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (8): 2645–2664. https://doi.org/10.1130/0016-7606(1973)84<2645:DFLIMR>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation DAVID ELLIOTT; Diffusion Flow Laws in Metamorphic Rocks. GSA Bulletin 1973;; 84 (8): 2645–2664. doi: https://doi.org/10.1130/0016-7606(1973)84<2645:DFLIMR>2.0.CO;2 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 Rocks which deform by pressure solution obey a diffusion flow law with a linear viscous or Newtonian stress to strain-rate relation. Undeformed relics of original grains preserved within newly grown crystals at grain boundaries under tension and presolved surfaces, together with accumulation of inert particles at grain boundaries under compression, are diagnostic evidence of a diffusion flow law. At a given stress, strain-rate is inversely proportional to the grain size to a power of two or three. A geologically useful plot has inverse temperature versus the logarithm of grain size as coordinates. Such a graph is separated into fields by three boundaries which meet at a triple point; within each field, either lattice diffusion, grain-boundary diffusion, or a dislocation flow law is predominant. It may be possible to calibrate this graph from naturally deformed rocks. Photomicrographs of isoclinally folded greenschist-grade quartzites and rhyolitic flows from the South Mountain–Blue Ridge area in Maryland demonstrate a diffusive mass transfer deformation mechanism, but estimates of effective diffusion coefficient compared to currently available laboratory diffusion data are insufficient to identify the diffusion path with certainty. However, the comparatively low ratio of metamorphic temperature to melting temperature and the physical nature of grain boundaries in metamorphic rocks, particularly concentrations of low-density impurities at grain boundaries, suggest the grain-boundary diffusion flow law. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Research Article| September 01, 1972 Deformation Paths in Structural Geology DAVID ELLIOTT DAVID ELLIOTT Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar Author and Article Information DAVID ELLIOTT Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Publisher: Geological Society of America Received: 01 Sep 1971 Revision Received: 13 Dec 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1972, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1972) 83 (9): 2621–2638. https://doi.org/10.1130/0016-7606(1972)83[2621:DPISG]2.0.CO;2 Article history Received: 01 Sep 1971 Revision Received: 13 Dec 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DAVID ELLIOTT; Deformation Paths in Structural Geology. GSA Bulletin 1972;; 83 (9): 2621–2638. doi: https://doi.org/10.1130/0016-7606(1972)83[2621:DPISG]2.0.CO;2 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 evolution of folds and the origin of mineral orientations and schistosity are analytically related to the deformation paths or histories in deforming rocks. The several different finite and incremental methods of representing deformation paths are connected by matrix algebra, and graphical presentation in natural strain space is found to be particularly clear. New methods permit calculation of various components of deformation paths from boudinage, pressure shadows, and inclusion trails in naturally deformed metamorphic rocks. In coaxially accumulating deformation paths the principal strains remain parallel to the same material lines in the rock and have been referred to inappropriately as irrotational. Straight inclusion trails in metamorphic minerals may be produced by synkinematic grain growth during a coaxially accumulating path and do not necessarily indicate postkinematic grain growth. Synkinematic inclusion trails can be produced by growth of the grain through its pressure shadow rather than matrix schistosity, and the quartz protected from re-crystallization by being included in snowball garnets may sometimes form this way. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.