Kinematic evolution of metre-scale shear zones in foliated low-grade tectonites: an example from the Bousquet gold district, Quebec

1994 ◽  
Vol 31 (8) ◽  
pp. 1301-1308 ◽  
Author(s):  
Ghislain Tourigny ◽  
Francis Chartrand
Keyword(s):  
Shear Bands ◽  
Shear Zones ◽  
Acute Angle ◽  
Small Scale ◽  
Low Grade ◽  
Plan View ◽  
Kinematic Indicators ◽  
Kinematic Evolution ◽  
Dextral Shear ◽  
Scale Shear

Small-scale subvertical shear zones developed parallel to a regional preexisting S2 schistosity exhibit evidence of a complex shearing history recorded by conflicting kinematic indicators in both crosssection and plan view. The concordant schistosity internal to the shear zones contains a steeply plunging stretching lineation. Coexisting kinematic indicators of non-coaxial deformation parallel to this lineation are compatible with reverse dip-slip. This earliest shearing event was characterized by (1) the development of several shear discontinuities along selected preexisting S2 foliation surfaces, (2) subvertical transposition of both bedding and the oldest (S1) flat-lying foliation, and (3) by the emplacement of shear veins along the S2 foliation planes. The youngest shearing event reactivated the foliation-parallel shear discontinuities as dextral shear planes, thereby causing concomitant subhorizontal retransposition, east–west subhorizontal stretching, and emplacement of en echelon extension veins. A single set of shear bands occurring at a clockwise acute angle to the slipping foliation indicates that small-scale shear zones were transpressional during the late dextral shearing.

scholarly journals Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

Solid Earth ◽  
2017 ◽  
Vol 8 (4) ◽  
pp. 767-788 ◽  
Author(s):  
Giancarlo Molli ◽  
Luca Menegon ◽  
Alessandro Malasoma
Keyword(s):  
Shear Zone ◽  
Continental Crust ◽  
Ambient Pressure ◽  
Deformation Mode ◽  
Shear Zones ◽  
Small Scale ◽  
Low Grade ◽  
Stick Slip ◽  
Brittle Ductile Transition

Abstract. The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle–ductile transition and seismogenesis. In a subduction environment, switching in deformation mode and mechanisms and scale of localization may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). However, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in some cases for the localization of deformation and shear zone development, thus representing a case in which switching deformation mechanisms and scale and style of localization (deformation mode) interact and relate to each other. This contribution analyses an example of a millimetre-scale shear zone localized by brittle precursor formed within a host granitic protomylonite. The studied structures, developed in ambient pressure–temperature (P–T) conditions of low-grade blueschist facies (temperature T of ca. 300 °C and pressure P ≥ 0. 70 GPa) during involvement of Corsican continental crust in the Alpine subduction. We used a multidisciplinary approach by combining detailed microstructural and petrographic analyses, crystallographic preferred orientation by electron backscatter diffraction (EBSD), and palaeopiezometric studies on a selected sample to support an evolutionary model and deformation path for subducted continental crust. We infer that the studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations, may be considered as a small-scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow-slip structures. Our case study represents, therefore, a fossil example of association of fault structures related to stick-slip strain accommodation during subduction of continental crust.

Styles and regimes of orogenic thickening in the Peloritani Mountains (Sicily, Italy): new constraints on the tectono-metamorphic evolution of the Apennine belt

2008 ◽  
Vol 145 (4) ◽  
pp. 552-569 ◽  
Author(s):  
GIANLUCA VIGNAROLI ◽  
FEDERICO ROSSETTI ◽  
THOMAS THEYE ◽  
CLAUDIO FACCENNA
Keyword(s):  
Shear Bands ◽  
Shear Zones ◽  
Geothermal Gradient ◽  
Brittle Deformation ◽  
Metamorphic Evolution ◽  
Constituent Element ◽  
Shear Sense ◽  
Tectonic Transport ◽  
History Of ◽  
Scale Shear

AbstractThe Peloritani Mountains constitute the Sicilian portion of the Calabria–Peloritani Arc (Italy), a tectono-metamorphic edifice recording the history of the subduction–exhumation cycle during Tertiary convergence between the African and European plates. Here, we describe the kinematic and the petrological characteristics of the major shear zones bounding the lowermost continental-derived metamorphic units cropping out in the eastern portion of the Peloritani Mountains. Both meso- and micro-scale shear sense criteria indicate a top-to-the-SSE tectonic transport, during a general evolution from ductile to brittle deformation conditions. Quantitative thermobarometry on texturally equilibrated phengite–chlorite pairs crystallized along the shear bands indicates pressure of 6–8 kbar at temperatures of 360–440 °C for the structurally highest units and 3–4 kbar at 380–440 °C for the lowest ones. This documents an overall inverse-type nappe arrangement within the tectonic edifice and a transition from an Alpine- (13–18 °C km−1) to a Barrovian-type (28–36 °C km−1) geothermal gradient during the progress of the Alpine orogenic metamorphism in the Peloritani Mountains. The integration of these results allows the Peloritani Mountains to be considered as a constituent element of the Apennine orogenic domain formed during the progressive space–time transition from oceanic to continental subduction at the active convergent margin.

scholarly journals Strain localisation in mechanically Layered Rocks, insights from numerical modelling

2012 ◽  
Vol 4 (2) ◽  
pp. 1165-1204 ◽  
Author(s):  
L. Le Pourhiet ◽  
B. Huet ◽  
P. Agard ◽  
L. Labrousse ◽  
L. Jolivet ◽  
...  
Keyword(s):  
Large Scale ◽  
Shear Bands ◽  
Structural Diversity ◽  
Shear Zones ◽  
Small Scale ◽  
Lithostatic Pressure ◽  
Shear Direction ◽  
Strain Localisation ◽  
Initial Anisotropy ◽  
Micro Structures

Abstract. Small scale deformation in stratified rocks displays a large diversity of micro-structures, from the microscopic scale to the scale of orogens. We have designed a series of fully dynamic numerical simulations aimed at assessing which parameters control this structural diversity and which underlying mechanisms lead to strain localisation. The influence of stratification orientation on the occurrence and mode of strain localisation is tested by varying the initial dip of inherited layering versus the large scale imposed simple shear. The detailed study of the models indicates that (1) the results are length-scale independent, (2) the new shear zones are always compatible with the kinematics imposed at the boundary (3) micro-structures formed encompass the full diversity of micro-structures observed in the field and chiefly depend on the direction of the initial anisotropy versus shear direction, (4) depending on the orientation of the anisotropy, the layers may deform along subtractive or additive shear bands, (5) the deformation in anisotropic media results in non-lithostatic pressure values that are on the order of the deviatoric stress in the strong layers and (6) the introduction of brittle rheology is necessary to form localised shear bands in the ductile regime.

Meso- and micro-structural analysis of the Briançonnais Front in the Grand Saint Bernard area (Aosta Valley - Italy, and Valais - Switzerland)

2020 ◽  
Author(s):  
Daniele Pini ◽  
Gloria Arienti ◽  
Matteo Pozzi ◽  
Bruno Monopoli ◽  
Andrea Bistacchi
Keyword(s):  
Large Scale ◽  
Shear Bands ◽  
Structural Evolution ◽  
Shear Zones ◽  
Pressure Solution ◽  
Low Grade ◽  
Fine Grained ◽  
Ductile Shear Zones ◽  
Type 3 ◽  
Saint Bernard

<p>We present preliminary results on the meso- and micro-structural evolution of high-strain rocks of the Houillère Zone and Pierre-Avoi Unit outcropping along the Swiss-Italy boundary ridge, to the west of the Grand Saint Bernard Pass.</p><p>The stack of Middle and External Pennidic units is folded by polyphasic folds, developed at least partly under low-grade metamorphic conditions. Different generations of folds show isoclinal to open geometries. Fold axes are subhorizontal, trending NE-SW, and the overall fold interference pattern can be generally classified as a type 3 (Ramsay). At the microscale, an important deformation mechanism is pressure solution cleavage, consistent with relatively low-temperature conditions.</p><p>Brittle-ductile shear zones, characterized by anastomosing bands of very fine-grained fault rocks, with pressure solution seams and SCC’ shear bands, exploit the weak and strongly anisotropic phyllosilicate-rich layers, particularly in the black schists of the Houillère Zone.</p><p>Brittle high-angle faults crosscut ductile and semi-brittle features and show an oblique-normal kinematics. These faults are particularly well developed in the more competent rocks of the Pierre-Avoi Unit (e.g. massive carbonates, metaconglomerates and metasandstones).</p><p>A continuous horizon, a few metres thick, with a high density of quartz veins, can be followed in the internal and upper part of the Houillère Zone. This horizon is folded, at least by the younger open folds, and constitutes a major marker to study the large-scale structure of this unit.</p>

Fluid-rock reactions and resulting change in rheological behavior of a composite granitoid: the Archean Mooshla stock, Canada

1998 ◽  
Vol 35 (2) ◽  
pp. 131-146 ◽  
Author(s):  
Abdelhay Belkabir ◽  
Claude Hubert ◽  
Larry Hoy
Keyword(s):  
Shear Zones ◽  
Small Scale ◽  
Mafic Rocks ◽  
Physical Conditions ◽  
Ductile Shear Zones ◽  
Chemical Studies ◽  
High Quartz ◽  
Scale Shear ◽  
Lithological Heterogeneity

The composite Mooshla stock displays clear evidence of variations in style and intensity of strain that are closely related to its internal lithological heterogeneity. Gabbro-diorite, quartz diorite, and tonalite rocks are weakly foliated and characterized by brittle and brittle-ductile small-scale shear zones, whereas leucotonalitic rocks are strongly foliated and transected by numerous wide and extensive ductile shear zones. Increasing degrees of penetrative deformation and marked changes of strain style in the pluton, from the more mafic rocks to the more felsic ones, are interpreted to reflect metamorphism-related rheological contrasts, rather than differences in the physical conditions of deformation. Metamorphism of the stock is characterized by an intensive hydration of the igneous rocks that has greatly enhanced their original heterogeneities. Petrographic, microstructural, and chemical studies show that the least deformed rocks are characterized by abundant albite-oligoclase (65-80%) with a matrix of minor quartz (5-10%) and actinolitic amphibole. The resistant plagioclase laths, although altered and replaced, form a stress-supporting framework that has protected the interstitial weak minerals, such as quartz, chlorite, and biotite, from deformation. However, the least deformed leucotonalites are characterized by low albite (35-45%) and high quartz contents (up to 65%). Extensive metamorphic hydration of these rocks produces quartz and phyllitic minerals that had enhanced significantly the ductility of the leucotonalites. Characterization of the chemical changes and the thermochemical conditions of the fluid, using microstructure and measurement of stable isotopes, indicates that fluid-rock interactions during metamorphism and syntectonic hydrothermal alteration have played an important role in creating the contrasting deformation of the composite granitoid.

scholarly journals Deformation structures resulting from anisotropy during high-strain deformation of ice 1h with initial CPO

2021 ◽  
Author(s):  
Tamara de Riese ◽  
Paul D. Bons ◽  
Enrique Gomez-Rivas ◽  
Albert Griera ◽  
Maria-Gema Llorens ◽  
...  
Keyword(s):  
Steady State ◽  
Structural Geology ◽  
Shear Bands ◽  
Shear Zones ◽  
Small Scale ◽  
Strain Localisation ◽  
Full Field ◽  
Dislocation Glide ◽  
Deformation Structures ◽  
Very High

<p>Ice 1h shows a strong viscoplastic anisotropy, as the resistance to activate dislocation glide on basal planes is at least one order of magnitude smaller than on the other slip planes. During flow the viscoplastic anisotropy leads to the development of a crystallographic preferred orientation (CPO). The anisotropic behaviour of flowing ice can lead to strain localisation. Only when the ice is layered (e.g. due to cloudy bands) it may be possible to identify localisation structures, as ice otherwise has no readily recognisable strain markers.</p><p>We use the Viscoplastic Full-Field Transform (VPFFT; Lebensohn and Rollett, 2020) crystal plasticity code coupled with the modelling platform ELLE (http://www.elle.ws; Piazolo et al., 2019) to simulate the deformation of intrinsically anisotropic ice 1h with an initial single maximum CPO in dextral simple shear up to very high strains. The VPFFT-approach simulates deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. We use an anisotropy similar to that of ice 1h, systematically vary the orientation of the initial CPO, and use passive markers/layers to visualise deformation structures.</p><p>The localisation behaviour strongly depends on the initial CPO, but reaches a consistent steady state after very high shear strains of about 30. The fabric and stress evolution reach a steady-state situation as well. The orientation of the CPO controls the style of deformation, which varies from (1) synthetic shear zones with a stable shear-direction parallel orientation and that widen with ongoing strain to unstable, (2) rotating antithetic shear bands, (3) initial formation of antithetic shear bands and subsequent development of synthetic shear bands and (4) distributed localisation. Furthermore, evolving visual structures depend on the presence and orientation of a visual layering in the material. However, at very high strains, the material is almost always strongly mixed and any original layering would be destroyed.</p><p>Our results highlight the challenge to identify strain localisation in ice, yet they can help the ice community to identify and interpret deformation structures in large ice masses (e.g. the Greenland ice sheet). As strain localisation in anisotropic materials behaves scale independent (de Riese et al., 2019), large-scale equivalents may occur of the observed small-scale structures (Jansen et al., 2016).</p><p>References:</p><p>de Riese, T., Evans, L., Gomez-Rivas, E., Griera, A., Lebensohn, R.A., Llorens, M.G., Ran, H., Sachau, T., Weikusat, I., Bons, P.D. 2019. Shear localisation in anisotropic, non-linear viscous materials that develop a CPO: A numerical study. Journal of Structural Geology, 124, 81-90.</p><p>Jansen, D., Llorens, M.-G, Westhoff, J., Steinbach, F., Kipfstuhl, S., Bons, P.D., Griera, A., Weikusat, I. 2016. Small-scale disturbances in the stratigraphy of the NEEM ice core: observations and numerical model simulations. The Cryosphere 10, 359-370.</p><p>Lebensohn, R.A., Rollett, A.D. 2020. Spectral methods for full-field micromechanical modelling of polycrystalline materials. Computational Materials Science, 173, 109336.</p><p>Piazolo, S., Bons, P.D., Griera, A., Llorens, M.G., Gomez-Rivas, E., Koehn, D., ... Jessell, M.W. 2019. A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future. Journal of Structural Geology, 125, 111-123.</p>

Characteristics of small-scale shear layers in a temporally evolving turbulent planar jet

2021 ◽  
Vol 920 ◽  
Author(s):  
Masato Hayashi ◽  
Tomoaki Watanabe ◽  
Koji Nagata
Keyword(s):  
Shear Layers ◽  
Small Scale ◽  
Planar Jet ◽  
Scale Shear

Abstract

Effective rheology of a two-phase subduction shear zone: insights from numerical simple shear experiments and implications for subduction zone interfaces

2021 ◽  
Author(s):  
Paraskevi Io Ioannidi ◽  
Laetitia Le Pourhiet ◽  
Philippe Agard ◽  
Samuel Angiboust ◽  
Onno Oncken
Keyword(s):  
Simple Shear ◽  
Large Scale ◽  
Confining Pressure ◽  
Shear Zones ◽  
Dislocation Creep ◽  
Small Scale ◽  
Two Phase ◽  
Weak Phase ◽  
Pure Phases ◽  
Geodynamic Models

<p>Exhumed subduction shear zones often exhibit block-in-matrix structures comprising strong clasts within a weak matrix (mélanges). Inspired by such observations, we create synthetic models with different proportions of strong clasts and compare them to natural mélange outcrops. We use 2D Finite Element visco-plastic numerical simulations in simple shear kinematic conditions and we determine the effective rheology of a mélange with basaltic blocks embedded within a wet quartzitic matrix. Our models and their structures are scale-independent; this allows for upscaling published field geometries to km-scale models, compatible with large-scale far-field observations. By varying confining pressure, temperature and strain rate we evaluate effective rheological estimates for a natural subduction interface. Deformation and strain localization are affected by the block-in-matrix ratio. In models where both materials deform viscously, the effective dislocation creep parameters (A, n, and Q) vary between the values of the strong and the weak phase. Approaching the frictional-viscous transition, the mélange bulk rheology is effectively viscous creep but in the small scale parts of the blocks are frictional, leading to higher stresses. This results in an effective value of the stress exponent, n, greater than that of both pure phases, as well as an effective viscosity lower than the weak phase. Our effective rheology parameters may be used in large scale geodynamic models, as a proxy for a heterogeneous subduction interface, if an appropriate evolution law for the block concentration of a mélange is given.</p>

1-D Model Analysis of Tesla Turbine for Small Scale Organic Rankine Cycle (ORC) System

2017 ◽  
Author(s):  
Jian Song ◽  
Chun-wei Gu
Keyword(s):  
Large Scale ◽  
Low Cost ◽  
Organic Rankine Cycle ◽  
Axial Flow ◽  
Rankine Cycle ◽  
Expansion Ratio ◽  
Working Fluid ◽  
Small Scale ◽  
Low Grade ◽  
D Model

Energy shortage and environmental deterioration are two crucial issues that the developing world has to face. In order to solve these problems, conversion of low grade energy is attracting broad attention. Among all of the existing technologies, Organic Rankine Cycle (ORC) has been proven to be one of the most effective methods for the utilization of low grade heat sources. Turbine is a key component in ORC system and it plays an important role in system performance. Traditional turbine expanders, the axial flow turbine and the radial inflow turbine are typically selected in large scale ORC systems. However, in small and micro scale systems, traditional turbine expanders are not suitable due to large flow loss and high rotation speed. In this case, Tesla turbine allows a low-cost and reliable design for the organic expander that could be an attractive option for small scale ORC systems. A 1-D model of Tesla turbine is presented in this paper, which mainly focuses on the flow characteristics and the momentum transfer. This study improves the 1-D model, taking the nozzle limit expansion ratio into consideration, which is related to the installation angle of the nozzle and the specific heat ratio of the working fluid. The improved model is used to analyze Tesla turbine performance and predict turbine efficiency. Thermodynamic analysis is conducted for a small scale ORC system. The simulation results reveal that the ORC system can generate a considerable net power output. Therefore, Tesla turbine can be regarded as a potential choice to be applied in small scale ORC systems.

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