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

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.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.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.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).References: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.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.Lebensohn, R.A., Rollett, A.D. 2020. Spectral methods for full-field micromechanical modelling of polycrystalline materials.&#160;Computational Materials Science,&#160;173, 109336.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.

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Strain localisation in mechanically Layered Rocks, insights from numerical modelling

Solid Earth Discussions ◽

10.5194/sed-4-1165-2012 ◽

2012 ◽

Vol 4 (2) ◽

pp. 1165-1204 ◽

Cited By ~ 1

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.

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Influence of initial preferred orientations on strain localisation and fold patterns in non-linear viscous anisotropic materials

10.5194/egusphere-egu2020-13160 ◽

2020 ◽

Author(s):

Tamara de Riese ◽

Paul D. Bons ◽

Enrique Gomez-Rivas ◽

Albert Griera ◽

Maria-Gema Llorens ◽

...

Keyword(s):

Simple Shear ◽

Shear Bands ◽

Shear Plane ◽

Slip Systems ◽

Full Field ◽

Deformation Structures ◽

Single Maximum ◽

Non Linear ◽

Very High ◽

Shear Localisation

Deformation localisation in rocks can lead to a variety of structures, such as shear zones and shear bands that can range from grain to crustal scale, from discrete and isolated zones to anastomosing networks. The heterogeneous strain field can furthermore result in a wide range of highly diverse fold geometries.We present a series of numerical simulations of the simple-shear deformation of an intrinsically anisotropic non-linear viscous material with a single maximum crystal preferred orientation (CPO) in dextral simple shear. We use the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code (e.g. Lebensohn & Rollett, 2020) coupled with the modelling platform ELLE (http://elle.ws) to achieve very high strains. The VPFFT-approach simulates viscoplastic deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. The approach is well suited for strongly non-linear anisotropic materials (de Riese et al., 2019). We vary the anisotropic behaviour of the material from isotropic to highly anisotropic (according to the relative critical resolved shear stress required to activate the different slip systems), as well as the orientation of the initial single maximum orientation, which we vary from parallel to perpendicular to the shear plane. To visualize deformation structures, we use passive markers, for which we also systematically vary the initial orientation.At relatively low strains the amount of strain rate localisation and resulting deformation structures highly depend on the initial single maximum orientation in the material in all anisotropic models. Three regimes can be recognised: distributed shear localisation, synthetic shear bands and antithetic shear bands. However, at very high strains localisation behaviour always tends to converge to a similar state, independent of the initial orientation of the anisotropy.In rocks, shear localisation is often detected by the deflection and/or folding of layers, which may be parallel to the anisotropy (e.g. cleavage formed by aligned mica), or by deflection/deformation of passive layering, such as original sedimentary layers. The resulting fold patterns vary strongly, depending on the original orientation of layering relative to the deformation field. This can even result in misleading structures that seem to indicate the opposite sense of shear. Most distinct deformation structures tend to form when the layering is originally parallel to the shear plane.&#160;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. J. Struct. Geol. 124, 81-90.Lebensohn, R.A., Rollett, A.D. 2020. Spectral methods for full-field micromechanical modelling of polycrystalline materials. Computational Mat. Sci. 173, 109336.

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Tropical Waves and the Quasi-Biennial Oscillation in a 7-km Global Climate Simulation

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0350.1 ◽

2016 ◽

Vol 73 (9) ◽

pp. 3771-3783 ◽

Cited By ~ 26

Author(s):

Laura A. Holt ◽

M. Joan Alexander ◽

Lawrence Coy ◽

Andrea Molod ◽

William Putman ◽

...

Keyword(s):

General Circulation ◽

Shear Zones ◽

Horizontal Resolution ◽

Small Scale ◽

Healthy Population ◽

Quasi Biennial Oscillation ◽

Wave Forcing ◽

Tropical Waves ◽

Biennial Oscillation ◽

Very High

Abstract This study investigates tropical waves and their role in driving a quasi-biennial oscillation (QBO)-like signal in stratospheric winds in a global 7-km-horizontal-resolution atmospheric general circulation model. The Nature Run (NR) is a 2-yr global mesoscale simulation of the Goddard Earth Observing System Model, version 5 (GEOS-5). In the tropics, there is evidence that the NR supports a broad range of convectively generated waves. The NR precipitation spectrum resembles the observed spectrum in many aspects, including the preference for westward-propagating waves. However, even with very high horizontal resolution and a healthy population of resolved waves, the zonal force provided by the resolved waves is still too low in the QBO region and parameterized gravity wave drag is the main driver of the NR QBO-like oscillation (NR-QBO). The authors suggest that causes include coarse vertical resolution and excessive dissipation. Nevertheless, the very-high-resolution NR provides an opportunity to analyze the resolved wave forcing of the NR-QBO. In agreement with previous studies, large-scale Kelvin and small-scale waves contribute to the NR-QBO driving in eastward shear zones and small-scale waves dominate the NR-QBO driving in westward shear zones. Waves with zonal wavelength < 1000 km account for up to half of the small-scale (<3300 km) resolved wave forcing in eastward shear zones and up to 70% of the small-scale resolved wave forcing in westward shear zones of the NR-QBO.

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Stress and strain rate variations and strain localisation in ice: Another complexity to be considered apart from a simple enhancement factor

10.5194/egusphere-egu2020-17864 ◽

2020 ◽

Author(s):

Paul D. Bons ◽

Tamara de Riese ◽

Enrique Gomez-Rivas ◽

Albert Griera ◽

Maria-Gema Llorens ◽

...

Keyword(s):

Strain Rate ◽

Ice Sheets ◽

Shear Zones ◽

Strain Localisation ◽

Polar Ice ◽

Differential Stress ◽

Full Field ◽

Polar Ice Sheets ◽

Slip Planes ◽

Flow Law

To describe the rheology of ice, it is customary to employ a flow law that relates the (differential) stress to the strain rate, typically as a function of temperature. The flow law thus predicts a single strain rate for a given stress and temperature. However, ice 1h is highly anisotropic when deforming by dislocation creep as is usually assumed to be the case in glaciers and polar ice sheets. Ice is effectively much softer in shearing parallel to the basal plane compared to deformation that requires activation of the non-basal crystallographic slip planes. Numerical simulation of ice deformation with the full-field crystal plasticity code (VPFFT, Lebensohn & Rollett, 2020) coupled with the numerical simulation platform Elle (Llorens et al., 2016) show that deformation in aggregates of ice grains is highly heterogeneous and typically shows strong strain heterogeneity and strain localisation in shear zones. This localisation remains when lattice rotation has resulted in a strong crystallographic preferred orientation (CPO) with basal planes all oriented approximately parallel to the shear plane in simple-shear deformation.&#160;Plots of the differential stress versus strain rate of all points of the full field model at one point in time show a wide scatter within the polycrystal. Although most basal planes have an orientation close to optimal for slip along this plane, few, if any material points actually show a stress-strain rate state close to the one predicted by the flow law for basal glide. On the contrary, the hard non-basal slip planes contribute significantly to the overall deformation. Shear zones show a stronger alignment of basal planes than the surrounding material. However, differential stress tends to be highest inside these shear zones, suggesting that shear zones are not simply the result of the presence of "soft" ice.&#160;The results give insight in the highly complex behaviour of the strongly anisotropic material ice. This complexity is insufficiently described with a simple enhancement factor. We discuss how this complexity may help explain variations in grain size and apparent strength found in deep drill cores in the polar ice sheets.&#160;Lebensohn, R.A., Rollett, A.D. 2020. Spectral methods for full-field micromechanical modelling of polycrystalline materials. Computational Materials Science 173, 109336.Llorens, G.-M., Griera, A., Bons, P.D., Lebensohn, R.A., Evans, L.A., Jansen, D., Weikusat, I. 2016. Full-field predictions of ice dynamic recrystallisation under simple shear conditions. Earth and Planetary Science Letters 450, 233-242.

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Synsedimentary submarine slope failure and tectonic deformation in deep-water carbonates, Cow Head Group, western Newfoundland

Canadian Journal of Earth Sciences ◽

10.1139/e86-050 ◽

1986 ◽

Vol 23 (4) ◽

pp. 476-490 ◽

Cited By ~ 20

Author(s):

Mario Coniglio

Keyword(s):

Large Scale ◽

Slope Failure ◽

Shear Zones ◽

Small Scale ◽

Tectonic Deformation ◽

Head Group ◽

Shallow Subsurface ◽

Deformation Structures ◽

Lenticular Bedding ◽

Basin Morphology

The Cow Head Group, interpreted as a southeast-dipping base-of-slope carbonate apron, contains intraformational truncation surfaces and slide masses. Synsedimentary shear zones are formed (1) below intraformational truncation surfaces; (2) in the basal parts of slide masses; and (3) in the shallow subsurface because of downslope creep. Shear zones are characterized by a variety of synsedimentary deformation structures. Limestones are subject to folding, brecciation, rotation of fragmented beds, and the development of fitted-lenticular bedding. In the interbedded shales, there is both disruption of fine laminations and small-scale isoclinal folding and faulting. Outcrops characterized by these features and the lack of truncation surfaces or slide masses may reflect minor downslope creep. The presence of truncation surfaces, slide masses, and shear zones indicates deposition on an unstable sloping surface.The recognition of intraformational truncation surfaces and slide masses usually requires extensive strike exposure, which when lacking, (e.g., drill cores), limits the potential of these large-scale features as useful indicators of slope deposition. In the Cow Head Group, the recognition and proper interpretation of the common, small-scale deformation structures of synsedimentary shear zones provides evidence for slope deposition that is independent of other sedimentologic, stratigraphic, and regional data.In some parts of the Cow Head Group, "wrinkled" limestones characterized by a prominent dome-and-basin morphology reflect layer-parallel shortening related to tectonic deformation. The deformation of these limestones was previously considered to be synsedimentary, but their association with late-diagenetic precipitates and tectonic stylolites, in conjunction with their continuity and regularity, distinguishes these folds from those produced during synsedimentary deformation.

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

Solid Earth ◽

10.5194/se-12-2369-2021 ◽

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.

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

Canadian Journal of Earth Sciences ◽

10.1139/e94-112 ◽

1994 ◽

Vol 31 (8) ◽

pp. 1301-1308 ◽

Cited By ~ 1

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.

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

10.5194/se-2021-30 ◽

2021 ◽

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 modelling 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 microstrutural 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.

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DEVELOPMENT OF A LAND DEGRADATION ASSESSMENT MODEL BASED ON FIELD INDICATORS ASSESSMENT AND PREDICTION METHODS IN WAI SARI, SUB-WATERSHED KAIRATU DISTRICT, WESTERN SERAM REGENCY, MALUKU PROVINCE, INDONESIA

SCIENCE NATURE ◽

10.30598/snvol2iss1pp071-084year2019 ◽

2019 ◽

Vol 2 (1) ◽

pp. 071-084

Author(s):

Silwanus M. Talakua ◽

Rafael M. Osok

Keyword(s):

Land Degradation ◽

Soil Loss ◽

Assessment Model ◽

Small Scale ◽

Small Islands ◽

Topographic Map ◽

Field Assessment ◽

Field Indicators ◽

Degradation Measurement ◽

Very High

The study was conducted in Wai Sari sub-watershed, Western Seram Regency Maluku to develop an accurate land degradation assessment model for tropical small islands. The Stocking’s field land degradation measurement and RUSLE methods were applied to estimate soil loss by erosion and the results of both methods were statistically tested in order to obtain a correction factor. Field indicators and prediction data were measured on 95 slope units derived from the topographic map. The rates of soil loss were calculated according to both methods, and the results were used to classify the degree of land degradation. The results show that the degree of land degradation based on the field assessment ranges from none-slight (4.04 - 17.565 t/ha/yr) to very high (235.44 - 404.00 t/ha/yr), while the RUSLE method ranges from none-slight (0.04-4.59 t/ha/yr) to very high 203.90 - 518.13 t/ha/yr. However, the RUSLE method shows much higher in average soil loss (133.4 t/ha/yr) than the field assessment (33.9 t/ha/yr). The best regression equation of logD/RP = - 0.594 + 1.0 logK + 1.0 logLS + 1.0 logC or D = 0.2547xRxKxLSx CxP was found to be a more suitable land degradation assessment model for a small-scale catchment area in the tropical small islands.

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