CAN THE SCALE OF OBSERVATION HIDE COMPLEXITIES IN THE DEFORMATION HISTORY OF A TERRANE? AN EXAMPLE FROM THE BALMUCCIA PERIDOTITE MASSIF, IVREA ZONE (NW ITALY)

Crystal fabric evolution in lava flows: results from numerical simulations

Numerical simulation of deformation microstructures and folds in polar ice and ductile rocks

Porosity localizing instability in a compacting porous layer in a pure shear flow and the evolution of porosity band wavelength

Streamlines around single spheres and trajectories of pairs of spheres in two-dimensional creeping flows

<p>Much of our understanding of the strength of the continental crust is based on flow laws derived from homogeneous mono-mineralic aggregates (quartzites).  However, crystal plastic deformation of rocks in the middle to lower continental crust during orogenic events forms foliations, lineations and lattice preferred orientations (LPOs) which produce physical and viscous anisotropies in rocks.  In some of these orogenic events, such as in the Appalachian mountains, multiple deformation events form different, cross-cutting foliations and overprint existing LPOs.  In order to determine the effects foliation/lineation and preexisting LPO have on the strength of rocks in the middle crust, we deformed a natural quartzite with a cross-girdle LPO from the Moine Thrust in Scotland with the compressive stress at six different primary orientations relative to the foliation and lineation. This quartzite has aligned but distributed fine-grained muscovite which defines a foliation and lineation.  The cores were deformed at the same temperature (800°C), pressure (1500 MPa) and strain rate (1.6*10<sup>-6</sup>/s) to similar strains (50-58%), leaving the foliation/lineation orientation as the only difference between experiments.  Peak stresses occur at strains of 10-20% and are lowest for the sample with foliation at 45<sup>o</sup> to the compression direction (400 MPa, the weak orientation).  All other cores (hard orientations) have peak strengths of 600 to 1100 MPa and highest for the cores with lineation perpendicular to the compression direction (1100 MPa). These cores in hard orientations all strain weaken to a similar stress (~500 MPa), but are still ~100 MPa stronger than the core with both foliation and lineation initially oriented at 45 degrees to the compression direction.  Optical microstructures include undulatory extinction, deformation lamellae, and at high strain (58%), the quartzite is more than 50% recrystallized. Scanning electron microscope electron backscatter diffraction analyses indicate that recrystallized grains in all cores reflect the deformation conditions of the experiment and original grains retain their initial LPO.  Strength anisotropy at low strains is due to placing the foliation and lineation at non-ideal (hard) orientations relative to the compression direction and is greatest in cores with the lineation perpendicular to the compression direction.  The evolution to a similar strength at high strains indicates that dynamic recrystallization creates new grains oriented for easy slip in the second (experimental) deformation event. These results suggest that differences in lineation and foliation orientations and a pre-existing LPO may cause strength anisotropy in rocks in the mid to lower continental crust, but this anisotropy may be transient and unlikely to exist to high strains.</p>

Research Article| May 01, 2004 Late Paleozoic tectonism in Nevada: Timing, kinematics, and tectonic significance James H. Trexler, Jr.; James H. Trexler, Jr. 1Department of Geological Sciences, University of Nevada, Reno, Nevada 89557-0180, USA Search for other works by this author on: GSW Google Scholar Patricia H. Cashman; Patricia H. Cashman 1Department of Geological Sciences, University of Nevada, Reno, Nevada 89557-0180, USA Search for other works by this author on: GSW Google Scholar Walter S. Snyder; Walter S. Snyder 2Department of Geology and Geophysics, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA Search for other works by this author on: GSW Google Scholar Vladimir I. Davydov Vladimir I. Davydov 2Department of Geology and Geophysics, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2004) 116 (5-6): 525–538. https://doi.org/10.1130/B25295.1 Article history received: 14 Nov 2002 rev-recd: 03 Jun 2003 accepted: 14 Aug 2003 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 James H. Trexler, Patricia H. Cashman, Walter S. Snyder, Vladimir I. Davydov; Late Paleozoic tectonism in Nevada: Timing, kinematics, and tectonic significance. GSA Bulletin 2004;; 116 (5-6): 525–538. doi: https://doi.org/10.1130/B25295.1 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 Three late Paleozoic, angular unconformities, each tightly constrained in age by biostratigraphy, are exposed in Carlin Canyon, Nevada. These record deformation as well as erosion. Folding associated with these deformation events is roughly coaxial; all three sets of fold axes trend northeast. Each unconformity represents tectonic disruption of the middle part of the western North American margin between the times of the initiation of the Antler orogeny (Late Devonian–Early Mississippian) and the Permian–Triassic Sonoma orogeny. This paper focuses on one of these unconformities in the Middle Pennsylvanian—the C6 unconformity—and the deformation and age constraints associated with it.Our data from Carlin Canyon yield detailed glimpses of how the Antler foreland evolved tectonically in Mississippian and Pennsylvanian time. Middle Pennsylvanian (Desmoinesian) northwest-southeast contraction resulted in thin-skinned folding and faulting, uplift, and erosion. These data require reinterpretation of the tectonic setting at the time of the Ancestral Rocky Mountains orogeny and suggest that plate convergence on the west side of the continent played a significant role in late Paleozoic tectonics of the North American continent. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lattice preferred orientations and microstructures of deformed Cordilleran marbles: correlation of shear indicators and determination of strain path

Lattice preferred orientations (LPO) of quartz in gneiss domes of the D'Entrecasteaux Islands, Woodlark Rift shed insight into exhumation of the world's youngest (~5–7 Ma) eclogite‐bearing terrane at cm/yr rates. We focus on deformation that affected the terrane as it transited between lower crustal depths and the surface, including: (1) grain‐scale deformation mechanisms; and (2) style of flow and mode of emplacement of the domes. Electron‐backscatter diffraction was used to analyze microstructure and LPOs of 37 quartzofeldspathic gneiss samples that enclose meter‐scale mafic blocks preserving original eclogite‐facies assemblages. During exhumation of the ultrahigh‐pressure (UHP) terrane, gneisses were retrogressed in the amphibolite facies at lower crustal depths. The LPOs change from dome cores to carapaces, consistent with decreasing deformational temperatures. In the relatively chilled outer carapaces of the domes, the quartz LPOs consist of mostly crossed‐girdle [c]‐axis patterns, with some cleft‐girdle and small‐circle LPOs, and record dislocation creep accommodated by mixed‐ < a > slip. In the cores of the migmatitic domes, a chessboard pattern of subgrains is common, and quartz LPOs primarily record prism‐[c] slip, probably at >630 °C. Other microstructures indicate recovery by high‐temperature grain‐boundary migration. Grain‐boundary mobility was anisotropic, leading to strong grain‐shape fabrics oblique to foliation, but not obviously relatable to shear sense. Evidence for melt‐present deformation is abundant, and microstructures (including partially dissolved feldspar grains) indicate some deformation by fluid‐assisted grain‐boundary diffusion creep. LPOs in carapace rocks are symmetrical, recording flow that was dominantly coaxial. We interpret the gneiss domes to have been emplaced into the rift as partially molten diapirs.

Mantle‐derived xenoliths associated with continental rifting can provide important information about the mantle structure and the physicochemical properties of deformation processes in the upper mantle. Metasomatized spinel peridotites from Adam's Diggings (AD) at a rift shoulder and Elephant Butte (EB) at the rift axis in the Rio Grande rift (RGR) were investigated to understand the deformation processes and seismic anisotropy occurring in the upper mantle. As determined through analysis of the lattice preferred orientation (LPO) of olivine by using a scanning electron microscope equipped with electron backscatter diffraction (SEM/EBSD), AD peridotites exhibited C‐type LPO of olivine indicating a dominant slip system of (100)[001] at the rift shoulder, whereas EB peridotites exhibited A‐type LPO indicating a dominant slip system of (010)[100] at the rift axis. Both geochemical data and microstructural observations indicate that the localized mantle enrichment processes, including melts with hydrous fluids, controlled multiple mantle metasomatisms and deformation of rocks under wet conditions (with olivine C‐type LPO) at the rift shoulder (AD), whereas mantle depletion by decompression partial melting caused deformation of rocks under dry conditions (with olivine A‐type LPO) at the rift axis (EB). These observations provide evidence for localized hydration and physicochemical heterogeneity of the upper mantle in the RGR zone. Seismic anisotropy observed beneath this zone can be attributed to the transtensional rupture, such as inhomogeneous stretching, and the petrofabrics of olivine beneath the study area.

Upper mantle tectonics: three-dimensional deformation, olivine crystallographic fabrics and seismic properties

Does cation ordering in omphacite influence development of lattice-preferred orientation?

Stream functions and complex potentials: implications for development of rock fabric and the continuum assumption

The anisotropy of magnetic susceptibility (AMS) is an integral measure of the preferred orientation of all minerals present in a rock. When the AMS is carried by paramagnetic minerals alone, the principal directions of the susceptibility ellipsoid should reflect the crystallographic orientation of the minerals. The relationship between the AMS and deformation depends on several factors, which control the development of lattice‐preferred orientation (LPO) in a rock. A mathematical model is presented that simulates the magnetic susceptibility ellipsoid for samples composed of more than one mineral phase. Measurements of the AMS are compared with fabric‐based anisotropy models for black slates from the Navia‐Alto Sil slate belt in northern Spain. The AMS is carried by paramagnetic minerals, as has been confirmed by high‐field torque magnetometry, low‐temperature AMS and magnetization curves. The LPO of mica and chlorite has been determined by X‐ray texture goniometry. Pole figures and orientation ellipsoids show the changes in the degree of alignment of the phyllosilicates. The models have been tested in samples displaying three types of pole figures: a high‐intensity point maximum, a medium‐intensity elliptical maximum, and a girdle‐shaped distribution. At one site displaying kink bands the LPO shows variations between these types at the outcrop scale. The case studies illustrate the success in modeling different LPO types.

We report a new observation of the olivine B-type lattice-preferred orientation (LPO), from the garnet peridotite at Cima di Gagnone, Switzerland. The olivine B-type fabric forms at low temperatures and/or high stress in the presence of water, and is of particular interest because it may be used to explain the trench- parallel shear-wave splitting that is often observed at subduction zones. In conjunction with the olivine B-type fabric, we have found strong orthopyroxene LPO that is identical to those formed under water-free conditions. This suggests that water may not have a significant effect on orthopyroxene fabric. From the olivine microstruc- ture, we determine that a stress of 22 ± 8 MPa was applied during the deformation event that formed the olivine LPO. Using an olivine flow-law, and assuming geological strain-rates, we determine the temperature of deformation to be 800 ± 175C. This does not preclude an ultra-deep origin for the ultramafic rocks at Cima di Gagnone, but indicates that much of the deformation recorded in the microstructure occurred at modest temperatures.

Summary A 2-D model for the development of lattice preferred orientation (LPO) in aggregates of crystals (such as high T/Tm olivine) which deform with a single dominant slip system is presented. In two dimensions, an arbitrary LPO can be described by an orientation distribution function (ODF) g(o, t), such that g(o, t)do represents the fraction of crystals for which the orientation of the slip plane lies between o and o+ do at time t. A differential equation which describes the evolution of the ODF during an arbitrary deformation history is described. This evolution is controlled by the vorticity number λ(t) = Ω/e of the deformation, where 2Ω(t) is the vorticity and e(t) is the strain rate. For λ = O (Uniaxial compression or pure shear), the ODF of an initially isotropic aggregate consists of two growing peaks oriented symmetrically about the extensional axis. For |λ|= 1 (simple shear), the ODF consists of two unequal peaks which migrate relative to the extensional axis, and which eventually merge into a single peak centred on the shear plane orientation. If |λ| exceeds a critical value ˜1.15, the ODF periodically returns to its initial isotropic state. the theory gives an excellent fit to data from olivine aggregates deformed in uniaxial compression, and an acceptable fit to data from ice aggregates deformed in simple shear.