Strain localization bands in fine-grained aggregates of germanate olivine and pyroxene deformed by a Griggs type apparatus

2021 ◽  
Vol 144 ◽  
pp. 104812
Author(s):  
Sando Sawa ◽  
Jun Muto ◽  
Nobuyoshi Miyajima ◽  
Rei Shiraishi ◽  
Masanori Kido ◽  
...  
Keyword(s):  
Strain Localization ◽  
Fine Grained ◽  
Type Apparatus

On the length scale of cyclic strain localization in fine-grained copper films

2010 ◽  
Vol 90 (1) ◽  
pp. 69-76 ◽  
Author(s):  
B. Zhang ◽  
K.H. Sun ◽  
Y. Liu ◽  
G.P. Zhang
Keyword(s):  
Strain Localization ◽  
Cyclic Strain ◽  
Length Scale ◽  
Copper Films ◽  
Fine Grained

Microstructures reveal brittle and viscous flow during exhumation of the high-temperature lower oceanic crust from Site U1473A, Atlantis Bank, Southwest Indian Ridge (IODP Expedition 360)

2021 ◽  
Author(s):  
Rhander Taufner ◽  
Gustavo Viegas ◽  
Frederico Faleiros ◽  
Paulo Castellan ◽  
Raylline Silva
Keyword(s):  
Strain Localization ◽  
Shear Zones ◽  
Thermodynamic Modelling ◽  
Oriented Growth ◽  
Rock Interaction ◽  
Fine Grained ◽  
Atlantis Bank ◽  
Polyphase Matrix ◽  
Indian Ridge ◽  
Detachment Faulting

<p>Detachment faulting has been hypothesized as the main process of tectonic spreading in mid-ocean ridges. The ongoing faulting leads to exhumation of oceanic core complexes (OCC) through large-scale normal faults, exposing heterogeneous sectors of the mylonitic lower crust, locally interlayered with pristine upper-mantle rocks. However, the mechanisms involved in this process – and the interplay between magmatism, deformation and fluid-rock interaction – are still debatable. To address these issues, we performed a quantitative microstructural analysis and thermodynamic modelling on mafic shear zones that occur in the lower section (≥ 600 meters below sea-floor) of Site U1473A (Atlantis Bank OCC, SW Indian Ridge), the target of IODP Expedition 360, to constrain deformation conditions and strain localization mechanisms during detachment faulting. The gabbroic shear zones consist of large (up to 5 mm in size) porphyroclasts of clinopyroxene, orthopyroxene, plagioclase and olivine embedded in a fine-grained (≤ 30 µm), polyphase matrix composed of plagioclase, clinopyroxene, orthopyroxene, amphibole, ilmenite, magnetite and olivine. Plagioclase-rich layers (~ 80 µm) are in abrupt contact with the fine-grained mixture, which define the mylonitic foliation. The porphyroclasts have undulose extinction, subgrains and are surrounded by fine-grained recrystallized grains (core-mantle structure) showing internal lattice distortion. Microfractures are common in orthopyroxene porphyroclasts. Amphibole replaces clinopyroxene and orthopyroxene porphyroclasts at their margins and fills cleavage planes. The plagioclase-rich layers show undulose extinction and subgrain boundaries in the larger grains within the layers. Mechanical twin lamellae occur in some grains regardless of grain size. Plagioclase grains show a weak shape preferred orientation with their long axes parallel to the main planar fabric of the shear zone. The grains in the polyphase matrix are mostly strain free. EBSD data in clinopyroxene clasts indicate activation of (010)[001] slip system and twinning along (001)[100]. Plagioclase-rich layers deforms by slip along the (010)[100] system. The polyphase matrix has a very weak but non-random CPO pattern. #Mg and Al content in the recrystallized clinopyroxene and orthopyroxene grains are lower compared to the porphyroclasts. Plagioclase has similar An content in both porphyroclasts and recrystallized grains. Amphibole has low concentrations of Cl and high content of F. The content of #Mg, Al and Si is similar in amphibole grains replacing pyroxene and in the polyphase matrix. Thermodynamic modelling indicates that the gabbroic shear zones formed at 820-870 °C and 2.0-2.8 kbar. Our results suggest that deformation in the porphyroclasts was accommodated by combined mechanical fragmentation and intracrystalline plasticity, which resulted in fractured grains of orthopyroxene, and clasts rimmed by recrystallized neoblasts. Plagioclase-rich layers formed mainly through dislocation creep. Phase mixing and weak CPO in the polyphase matrix point to oriented-growth during diffusion-assisted grain boundary sliding, mainly in the presence of melt, as evidenced by amphibole formed at the expense of pyroxene. Magmatic fluids are the possible source of reactant amphibole. Such mechanisms effectively resulted in strain localization in fine-grained, polyphase shear zones that contributed to the weakening of the ocean crust during detachment faulting and subsequent exhumation of the Atlantis Bank OCC.</p>

Development of interconnected fine-grained polyphase networks during progressive exhumation of a shear zone

2020 ◽  
Author(s):  
Alexander Lusk ◽  
John Platt
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Shear Zone ◽  
Strain Localization ◽  
Cooling System ◽  
Boundary Migration ◽  
Grain Boundary Sliding ◽  
Fine Grained ◽  
Subgrain Rotation ◽  
Boundary Sliding

<p>Present exposure of the ductile Caledonian retrowedge in northwestern Scotland records the evolution of a shear zone that was exhuming while actively deforming, providing a natural laboratory to study strain localization in a progressively cooling system. Examination of rocks from two detailed transects across this region consistently show a transition from microstructures that are dominated by interconnected phyllosilicate networks in a quartz-rich matrix with feldspar porphyroclasts, to interconnected fine-grained regions of mixed quartz + phyllosilicate + feldspar. These polyphase regions are demonstrably weaker than surrounding quartz layers and likely deform by grain-size sensitive mechanisms including diffusion-accommodated grain boundary sliding.</p><p>Microstructures characterized by a quartz-rich matrix and interconnected phyllosilicates undergo quartz recrystallization by high temperature grain boundary migration and are dominated by prism <em>a</em> slip. In contrast, fine-grained polyphase microstructures record quartz recrystallization dominated by subgrain rotation and activation of rhomb <em>a</em> and basal <em>a</em> slip systems. We propose transient hardening occurs in quartz-dominated regions as quartz with a strong Y-axis maximum undergoes the switch from prism <em>a</em> easy slip to basal <em>a</em> easy slip during cooling, and thus partitions strain into interconnected phyllosilicate layers. In response, interconnected phyllosilicate layers undergo mechanical comminution, becoming increasingly mixed by grain-size sensitive creep processes to form polyphase layers as they accommodate an increased proportion of strain. This transition from quartz-rich matrix with phyllosilicate interconnected weak layers to fine-grained, polyphase weak layers could be of first-order importance in strain localization within polyphase mylonitic and ultramylonitic rocks.</p>

scholarly journals Thermo-Structural Evolution of the Val Malenco (Italy) Peridotite: A Petrological, Geochemical and Microstructural Study

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 962
Author(s):  
Wenlong Liu ◽  
Yi Cao ◽  
Junfeng Zhang ◽  
Yanfei Zhang ◽  
Keqing Zong ◽  
...  
Keyword(s):  
Strain Localization ◽  
Structural Evolution ◽  
Coarse Grained ◽  
Prograde Metamorphism ◽  
Fine Grained ◽  
Tectonic History ◽  
Serpentinized Peridotite ◽  
Geochemical Study ◽  
Dissolution And Precipitation

The Val Malenco peridotite massif is one of the largest exposed ultramafic massifs in Alpine orogen. To better constrain its tectonic history, we have performed a comprehensive petro-structural and geochemical study. Our results show that the Val Malenco serpentinized peridotite recorded both pre-Alpine extension and Alpine convergence events. The pre-Alpine extension is recorded by microstructural and geochemical features preserved in clinopyroxene and olivine porphyroblasts, including partial melting and refertilisation, high-temperature (900–1000 °C) deformation and a cooling, and fluid-rock reaction. The following Alpine convergence in a supra-subduction zone setting is documented by subduction-related prograde metamorphism features preserved in the coarse-grained antigorite and olivine grains in the less-strained olivine-rich layers, and later low-temperature (<350 °C) serpentinization in the fine-grained antigorite in the more strained antigorite-rich layers. The strain shadow structure in the more strained antigorite-rich layer composed of dissolving clinopyroxene porphyroblast and the precipitated oriented diopside and olivine suggest dissolution and precipitation creep, while the consistency between the strain shadow structure and alternating less- and more-strained serpentinized domains highlights the increasing role of strain localization induced by the dissolution-precipitation creep with decreasing temperature during exhumation in Alpine convergence events.

scholarly journals Identification of strain heterogeneities arising during deformation of ice

2000 ◽  
Vol 30 ◽  
pp. 121-126 ◽  
Author(s):  
Philippe Mansuy ◽  
Armelle Philip ◽  
Jacques Meyssonnier
Keyword(s):  
Strain Localization ◽  
Homogeneous Deformation ◽  
Creep Tests ◽  
Isotropic Matrix ◽  
Circular Shape ◽  
Fine Grained ◽  
Regular Shape ◽  
Geometric Singularity ◽  
Polycrystalline Ice

AbstractCreep tests carried out on specimens of isotropic ice containing a monocrystalline inclusion allow us to observe some strain heterogeneities that develop during the deformation of polycrystalline ice. Different kinds of heterogeneities, some of them leading to strain localization, are observed and described, and mechanisms are proposed to explain how they arise. However, when the inclusion has a very regular shape with no geometric singularity (e.g. circular shape) and is embedded in a fine-grained isotropic matrix, the observations lead us to assume homogeneous deformation of the inclusion, with no strain localization except that associated with basal glide.

scholarly journals A new finite element approach to model microscale strain localization within olivine aggregates

Solid Earth ◽  
2021 ◽  
Vol 12 (10) ◽  
pp. 2369-2385
Author(s):  
Jean Furstoss ◽  
Carole Petit ◽  
Clément Ganino ◽  
Marc Bernacki ◽  
Daniel Pino-Muñoz
Keyword(s):  
Finite Element ◽  
Mechanical Behavior ◽  
Crystal Plasticity ◽  
Strain Localization ◽  
Boundary Migration ◽  
Shear Zones ◽  
Full Field ◽  
Dislocation Glide ◽  
Fine Grained ◽  
Element Approach

Abstract. This paper presents a new mesoscopic full field approach for the modeling of microstructural evolutions and mechanical behavior of olivine aggregates. The mechanical framework is based on a reduced crystal plasticity (CP) formulation which is adapted to account for non-dislocation glide strain-accommodating mechanisms in olivine polycrystals. This mechanical description is coupled with a mixed velocity–pressure finite element (FE) formulation through a classical crystal plasticity finite element method (CPFEM) approach. The microstructural evolutions, such as grain boundary migration and dynamic recrystallization, are also computed within a FE framework using an implicit description of the polycrystal through the level-set approach. This numerical framework is used to study the strain localization, at the polycrystal scale, on different types of pre-existing shear zones for thermomechanical conditions relevant to laboratory experiments. We show that both fine-grained and crystallographic textured pre-existing bands favor strain localization at the sample scale. The combination of both processes has a large effect on strain localization, which emphasizes the importance of these two microstructural characteristics (texture and grain size) on the mechanical behavior of the aggregate. Table 1 summarizes the list of the acronyms used in the following.

scholarly journals Brittle grain size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

2015 ◽  
Vol 7 (4) ◽  
pp. 2953-2998
Author(s):  
G. Viegas ◽  
L. Menegon ◽  
C. J. Archanjo
Keyword(s):  
Grain Size ◽  
Shear Zone ◽  
Strain Localization ◽  
Shear Bands ◽  
Preferred Orientation ◽  
Size Reduction ◽  
Diffusion Creep ◽  
Crystallographic Preferred Orientation ◽  
Fine Grained

Abstract. The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse (> 50 μm) grained feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (~ < 15 μm in size) localized in C' shear bands. Detailed microstructural observations and EBSD analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (~ 1–2 mm) monomineralic bands or as thin ribbons dispersed in the feldspathic mixture. The microstructure and c axis crystallographic preferred orientation are similar in the thick monomineralic band and in the thin ribbons, and suggest dominant subgrain rotation recrystallization and activity of prism ⟨a⟩ and rhomb ⟨a⟩ slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the transposed veins to the thin ribbons embedded in the feldspathic C' bands (14 μm vs. 5 μm, respectively). The fine-grained feldspar mixture has a weak crystallographic preferred orientation interpreted as the result of oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Assuming that the C' shear bands deformed under constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate one order of magnitude faster than the monophase quartz ribbons. Overall, our dataset indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing under relatively fluid-absent conditions which would trigger a switch to diffusion creep and further strain localization without a prominent role of metamorphic reactions.

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