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D. Findlay - One of the best experts on this subject based on the ideXlab platform.
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Boudinage; a key to an organizing principle for the formation of ore deposits
Economic Geology, 1998Co-Authors: D. FindlayAbstract:A number of ore deposits are juxtaposed to illustrate a common Boudinage signature. Examples are (1) Kalgoorlie (an epigenetic vein-type gold deposit from the Eastern gold fields of Western Australia; (2) Broken Hill (a stratiform sediment-hosted lead-zinc-silver deposit from New South Wales, Australia); (3) three hematite enrichment iron ore deposits, one from the Hamersley region of Western Australia, one from the Krivoi Rog region of Ukraine, and one from Nimba Range, Liberia; (4) Kambalda (an Archcan stratiform and strata-bound ultramafic-hosted nickel deposit in the Eastern Goldfields of Western Australia, with the same tectonic environment as Kalgoorlie); (5) the diamondiferous Argyle kimberlite pipe in the Kimberley region of Western Australia; and (6) typical oil and gas plays of western Europe. The first four of these deposits are the type, or close to the type for their class, emphasizing the importance of the linkage. A further Boudinage emphasis lies in the historical perspective for these first enumerated deposits, for which a Boudinage control was either explicitly or implicitly described by others. It is concluded that an important organizing principal in ore emplacement is probably being addressed, one which operates regardless of mineralization type or crustal level (tectonic environment), which remains unrecognized in the popular consensus on ore controls, which could usefully be a framework for other popular consensual models, and which could substantially advance understanding of controls on emplacement of mineralization and related tectonics generally.
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Boudinage on radial fractures: An alternative to magmatic models for the emplacement of nickel ores, Lunnon Shoot, Kambalda, Western Australia
Australian Journal of Earth Sciences, 1998Co-Authors: D. FindlayAbstract:Restoration of fault displacements on a section through the Lunnon Shoot is made in accordance with the general importance, noted by others, of flexural slip and ductile flow in the growth of the Kambalda Anticline. Coupled with the interpretation here that ‘normal’ and ‘reverse’ faults at Kambalda may simply be opposite walls of the same dilated fracture (one movement instead of two) this restoration allows the prism of anomalous stratigraphic sequence confined by faults and hosting the ores to be interpreted structurally‐metamorphically (‐metasomatically) rather than magmatically. The movement picture may be synoptically described as ‘Boudinage on radial fractures or axial‐plane cleavage’, the structure being close to that for which the term ‘Boudinage’ was originally coined. A model is proposed whereby flattening and commensurate pullapart due to tangential longitudinal strain between the footwall Lunnon Basalt and the overlying Upper (ultramafic) Sequence is focused within the contact zone occupied by...
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Diagenetic Boudinage, an analogue model for the control on hematite enrichment iron ores of the Hamersley Iron Province of Western Australia, and a comparison with Krivoi Rog of Ukraine, and Nimba Range, Liberia
Ore Geology Reviews, 1994Co-Authors: D. FindlayAbstract:Abstract A prima facie comparison is made between diagenetic, ‘sedimentary’ Boudinage structures at outcrop scales (scales of centimetres to tens of centimetres), and zones of localised stratigraphic thinning (on scales of tens of metres) in beds of the Marra Mamba and Brockman Iron Formations of the Hamersley Iron Province of Western Australia. If the comparison is valid, it suggests that some of the hematite enrichment ores of the province may be diagenetic ores located in necks of diagenetic Boudinage structures related to extensional disturbance of the basin when the sequence was only partly consolidated. This interpretation is seen as similar to the consensual supergene metasomatic replacement hypothesis for the origin of the ores in respect of mineral solution-precipitation mechanisms, but differs in respect of important aspects of bulk process, and in their implications for iron ore exploration. A prima facie comparison is also made with the structure locating some ores of the Krivoi Rog region of Ukraine for which a Boudinage control has been explicitly described, and with the structure controlling the Nimba Range deposit, Liberia. If such a comparison is valid, then Boudinage could account simultaneously for the Proterozoic age of the deposits, the localised stratigraphic thinning, the influx of iron, and the ‘removal’ of silica. Further, on the basis of self-similarity of Boudinage structure across scale, region and tectonic regime, and in conjunction with the recognition by others on different grounds that the examples described in the paper may be extrapolated world-wide, Boudinage may provide a partial framework within which existing models for the formation of enriched hematite ores of Proterozoic banded iron formations can be adapted. The paper is conceptual and provides no new data.
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Boudinage control on the emplacement of lodes of the Kalgoorlie goldfield
Australian Journal of Earth Sciences, 1994Co-Authors: D. FindlayAbstract:Literature on the Kalgoorlie goldfield is reviewed and Boudinage is shown to be an important aspect of the structure of the field, the lode distribution to be coincident with the principal necks, and the configuration of the lodes to match the characteristic fracture patterns of classical boudin necks. Boudinage is therefore interpreted to be an important control on the emplacement of the mineralization. Boudinage and the concomitant introduction of mineralization is related to flattening of the Boomerang Anticline, which is consistent with the general consensus that mineralization is emplaced as hydrothermal veins during late‐stage deformation. This interpretation is proposed as a simpler alternative to other more complex shear‐related models and may be useful in exploration for deposits of similar type.
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Geological note: Boudinage, a reinterpretation of the structural control on the mineralization at Broken Hill
Australian Journal of Earth Sciences, 1994Co-Authors: D. FindlayAbstract:Recognizing Broken Hill geology as a large‐scale Boudinage structure provides an aspect of the geology that has been missing from the previous controversial debate, and a possible explanation for the orebody which has been previously overlooked. Although the interpretation implies that the orebody is epigenetic in its location and configuration, and is largely post‐F2 deformation, it allows that the ore constituents may have been syngenetic, possibly as dewatering of metal‐rich brines. If true, this would easily reconcile opposing epigenetic and syngenetic views.
J. Zulauf - One of the best experts on this subject based on the ideXlab platform.
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Coeval Boudinage and folding of oblique single layers under coaxial plane strain: Layer rotation around the principal stretching axis (X)
Journal of Structural Geology, 2020Co-Authors: J. Zulauf, Gernold Zulauf, E. HattingenAbstract:Abstract We carried out scaled analogue modelling to show the influence of varying layer obliquity on the geometry of single-layer folds and boudins, which developed simultaneously under coaxial plane strain. The initial angle (θZ(i)) between the competent layer and the shortening axis (Z) was gradually changed by turning the layer around the principal stretching axis (X). Although oblique-layer rotation was similar fast like that of a corresponding passive plane, each layer was crossing different strain fields (shortening, reduced shortening, elongation) resulting in coeval Boudinage and F1-folding, the latter with fold axes parallel to X. A rise in θZ(i) led to increase of Boudinage and decrease of F1-folding. The amplitude of F1-folds increased with viscosity ratio between layer and matrix but was hardly affected by θZ(i). The arc- and wavelength of F1-folds, on the other hand, increased if θZ(i) > 45°. In cases of low viscosity ratio between layer and matrix, boudins were affected by F2-folds with axes subperpendicular to the layer. Both F1- and F2-folds show increasing interlimb angles at rising θZ(i). The structures of the present study are largely not cylindrical and require investigation in three dimensions. They are frequent in high-pressure/low-temperature metamorphic rocks of subduction-zone settings and in salt walls.
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Formation of chocolate-tablet boudins: Results from scaled analogue models
Journal of Structural Geology, 2014Co-Authors: J. Zulauf, Gernold Zulauf, J Göttlich, M. PeinlAbstract:Abstract We used power-law viscous plasticine as a rock analogue to simulate chocolate tablet Boudinage of rocks undergoing dislocation creep. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent two phases of plane strain in a weaker plasticine matrix, with the principal stretching axis, X , and the axis of no-change, Y , replacing each other from the first to the second phase. In each phase of plane strain, Boudinage was controlled by an initial phase of viscous necking followed by extension fracture along the neck domain. Increase in the magnitude of finite strain ( e ) and decrease in layer thickness ( H i ) result in a decrease in the boudin width ( W a ) and an increase in the number of boudins ( N ). Given the viscosity ratio between layer and matrix ( m ) is higher than ca. 5, the number of boudins decreases and the boudin width increases with increasing values of m . An unexpected result of the present study is that in each experiment, the number of boudins was significantly higher during the second phase of plane strain. This difference should be related to additional drag of the matrix plasticine on the stiff layer in the neck domains formed during the first phase of Boudinage. The aspect ratio of the second generation of boudins ( W d = W a / H i ) is compatible with aspect ratios of natural boudins and with aspect ratios calculated using analytical solutions.
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the origin of tablet Boudinage results from experiments using power law rock analogs
Tectonophysics, 2011Co-Authors: J. Zulauf, Gernold Zulauf, R. Kraus, Gabriel Gutierrezalonso, Friedhelm E. ZanellaAbstract:Abstract We used power–law viscous plasticine ( n = ca. 7) as a rock analog to simulate Boudinage of rocks undergoing dislocation creep and brittle fracture. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent bulk pure flattening inside a less competent plasticine matrix. Computer tomographic analyses of the deformed samples revealed that Boudinage results from an initial phase of viscous necking followed by tensile failure along the previously formed necks. The resulting boudins display a polygonal shape in plan-view and are referred to as ‘tablet boudins’ (in contrast to the square to rectangular shaped chocolate-tablet boudins). The ratio between the plan-view long and short axis, R , ranges from 1.2 to 2.6. The polygonal, non-isometric shape of the tablet boudins can be explained by the strong interaction of concentric and radial tensile fractures. With increasing layer thickness, H i , the mean diameter of the boudin tablets, W a , increases, while the number of boudins, N , decreases. Progressive finite strain results in a higher number of the boudins and a smaller mean diameter. The thickness of the boudins, H f , is almost the same as the initial layer thickness, H i , while the aspect ratio ( W d = W a / H f ) decreases with layer thickness and finite strain. The mean W d values obtained from all experiments span from ca. 4 to ca. 11. Tablet boudins, described in the present paper, have yet not been described from natural outcrops. The reasons might be that pure flattening strain is not common in nature, and the characterization and evaluation of tablet boudins requires geometrical analysis in three dimensions, which is a difficult task when such structures occur in nature.
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The origin of tablet Boudinage: Results from experiments using power–law rock analogs
Tectonophysics, 2011Co-Authors: J. Zulauf, Gernold Zulauf, R. Kraus, Gabriel Gutiérrez-alonso, Friedhelm E. ZanellaAbstract:Abstract We used power–law viscous plasticine ( n = ca. 7) as a rock analog to simulate Boudinage of rocks undergoing dislocation creep and brittle fracture. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent bulk pure flattening inside a less competent plasticine matrix. Computer tomographic analyses of the deformed samples revealed that Boudinage results from an initial phase of viscous necking followed by tensile failure along the previously formed necks. The resulting boudins display a polygonal shape in plan-view and are referred to as ‘tablet boudins’ (in contrast to the square to rectangular shaped chocolate-tablet boudins). The ratio between the plan-view long and short axis, R , ranges from 1.2 to 2.6. The polygonal, non-isometric shape of the tablet boudins can be explained by the strong interaction of concentric and radial tensile fractures. With increasing layer thickness, H i , the mean diameter of the boudin tablets, W a , increases, while the number of boudins, N , decreases. Progressive finite strain results in a higher number of the boudins and a smaller mean diameter. The thickness of the boudins, H f , is almost the same as the initial layer thickness, H i , while the aspect ratio ( W d = W a / H f ) decreases with layer thickness and finite strain. The mean W d values obtained from all experiments span from ca. 4 to ca. 11. Tablet boudins, described in the present paper, have yet not been described from natural outcrops. The reasons might be that pure flattening strain is not common in nature, and the characterization and evaluation of tablet boudins requires geometrical analysis in three dimensions, which is a difficult task when such structures occur in nature.
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Coeval folding and Boudinage in four dimensions
Journal of Structural Geology, 2005Co-Authors: J. Zulauf, Gernold ZulaufAbstract:Scaled analogue experiments have been carried out incrementally to demonstrate the growth of constrictional folds and boudins through space and time. Pure constriction acted on a single stiff layer that was embedded in a weak matrix, with the layer trending parallel to the X-axis of the constrictional strain ellipsoid. The viscosity ratio between non-linear viscous layer and matrix was set at ca. 13. Three-dimensional images of the incrementally deformed layer have been obtained using computer tomography. They yielded clear evidence that D1-folds and D1-boudins show striking interactions when growing simultaneously at low to moderate finite strain. The axes of D1-folds are subparallel and the axes of D1-boudins are subperpendicular to the X-axis of the constrictional strain ellipsoid. The normalized wavelengths of D1-folds and D1-boudins, and the amplitudes of D1-folds, are fixed already at low finite strain (−5%) and do not significantly change as deformation proceeds. The normalized wavelength of both structures is not so in line as suggested by theoretical studies. At higher degrees of finite strain, D1-boudins are affected by D2-folds, and rotated limbs of the latter show D2-Boudinage, resulting in complex deformation patterns that are difficult to identify in the field. The model results are important for the analysis and interpretation of deformation structures in rheologically stratified rocks undergoing dislocation creep under bulk constriction. Tectonic settings where constrictional folds and boudins may develop simultaneously are stems of salt diapirs, subduction zones or thermal plumes.
Gernold Zulauf - One of the best experts on this subject based on the ideXlab platform.
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Coeval Boudinage and folding of oblique single layers under coaxial plane strain: Layer rotation around the principal stretching axis (X)
Journal of Structural Geology, 2020Co-Authors: J. Zulauf, Gernold Zulauf, E. HattingenAbstract:Abstract We carried out scaled analogue modelling to show the influence of varying layer obliquity on the geometry of single-layer folds and boudins, which developed simultaneously under coaxial plane strain. The initial angle (θZ(i)) between the competent layer and the shortening axis (Z) was gradually changed by turning the layer around the principal stretching axis (X). Although oblique-layer rotation was similar fast like that of a corresponding passive plane, each layer was crossing different strain fields (shortening, reduced shortening, elongation) resulting in coeval Boudinage and F1-folding, the latter with fold axes parallel to X. A rise in θZ(i) led to increase of Boudinage and decrease of F1-folding. The amplitude of F1-folds increased with viscosity ratio between layer and matrix but was hardly affected by θZ(i). The arc- and wavelength of F1-folds, on the other hand, increased if θZ(i) > 45°. In cases of low viscosity ratio between layer and matrix, boudins were affected by F2-folds with axes subperpendicular to the layer. Both F1- and F2-folds show increasing interlimb angles at rising θZ(i). The structures of the present study are largely not cylindrical and require investigation in three dimensions. They are frequent in high-pressure/low-temperature metamorphic rocks of subduction-zone settings and in salt walls.
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Formation of chocolate-tablet boudins: Results from scaled analogue models
Journal of Structural Geology, 2014Co-Authors: J. Zulauf, Gernold Zulauf, J Göttlich, M. PeinlAbstract:Abstract We used power-law viscous plasticine as a rock analogue to simulate chocolate tablet Boudinage of rocks undergoing dislocation creep. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent two phases of plane strain in a weaker plasticine matrix, with the principal stretching axis, X , and the axis of no-change, Y , replacing each other from the first to the second phase. In each phase of plane strain, Boudinage was controlled by an initial phase of viscous necking followed by extension fracture along the neck domain. Increase in the magnitude of finite strain ( e ) and decrease in layer thickness ( H i ) result in a decrease in the boudin width ( W a ) and an increase in the number of boudins ( N ). Given the viscosity ratio between layer and matrix ( m ) is higher than ca. 5, the number of boudins decreases and the boudin width increases with increasing values of m . An unexpected result of the present study is that in each experiment, the number of boudins was significantly higher during the second phase of plane strain. This difference should be related to additional drag of the matrix plasticine on the stiff layer in the neck domains formed during the first phase of Boudinage. The aspect ratio of the second generation of boudins ( W d = W a / H i ) is compatible with aspect ratios of natural boudins and with aspect ratios calculated using analytical solutions.
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the origin of tablet Boudinage results from experiments using power law rock analogs
Tectonophysics, 2011Co-Authors: J. Zulauf, Gernold Zulauf, R. Kraus, Gabriel Gutierrezalonso, Friedhelm E. ZanellaAbstract:Abstract We used power–law viscous plasticine ( n = ca. 7) as a rock analog to simulate Boudinage of rocks undergoing dislocation creep and brittle fracture. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent bulk pure flattening inside a less competent plasticine matrix. Computer tomographic analyses of the deformed samples revealed that Boudinage results from an initial phase of viscous necking followed by tensile failure along the previously formed necks. The resulting boudins display a polygonal shape in plan-view and are referred to as ‘tablet boudins’ (in contrast to the square to rectangular shaped chocolate-tablet boudins). The ratio between the plan-view long and short axis, R , ranges from 1.2 to 2.6. The polygonal, non-isometric shape of the tablet boudins can be explained by the strong interaction of concentric and radial tensile fractures. With increasing layer thickness, H i , the mean diameter of the boudin tablets, W a , increases, while the number of boudins, N , decreases. Progressive finite strain results in a higher number of the boudins and a smaller mean diameter. The thickness of the boudins, H f , is almost the same as the initial layer thickness, H i , while the aspect ratio ( W d = W a / H f ) decreases with layer thickness and finite strain. The mean W d values obtained from all experiments span from ca. 4 to ca. 11. Tablet boudins, described in the present paper, have yet not been described from natural outcrops. The reasons might be that pure flattening strain is not common in nature, and the characterization and evaluation of tablet boudins requires geometrical analysis in three dimensions, which is a difficult task when such structures occur in nature.
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The origin of tablet Boudinage: Results from experiments using power–law rock analogs
Tectonophysics, 2011Co-Authors: J. Zulauf, Gernold Zulauf, R. Kraus, Gabriel Gutiérrez-alonso, Friedhelm E. ZanellaAbstract:Abstract We used power–law viscous plasticine ( n = ca. 7) as a rock analog to simulate Boudinage of rocks undergoing dislocation creep and brittle fracture. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent bulk pure flattening inside a less competent plasticine matrix. Computer tomographic analyses of the deformed samples revealed that Boudinage results from an initial phase of viscous necking followed by tensile failure along the previously formed necks. The resulting boudins display a polygonal shape in plan-view and are referred to as ‘tablet boudins’ (in contrast to the square to rectangular shaped chocolate-tablet boudins). The ratio between the plan-view long and short axis, R , ranges from 1.2 to 2.6. The polygonal, non-isometric shape of the tablet boudins can be explained by the strong interaction of concentric and radial tensile fractures. With increasing layer thickness, H i , the mean diameter of the boudin tablets, W a , increases, while the number of boudins, N , decreases. Progressive finite strain results in a higher number of the boudins and a smaller mean diameter. The thickness of the boudins, H f , is almost the same as the initial layer thickness, H i , while the aspect ratio ( W d = W a / H f ) decreases with layer thickness and finite strain. The mean W d values obtained from all experiments span from ca. 4 to ca. 11. Tablet boudins, described in the present paper, have yet not been described from natural outcrops. The reasons might be that pure flattening strain is not common in nature, and the characterization and evaluation of tablet boudins requires geometrical analysis in three dimensions, which is a difficult task when such structures occur in nature.
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Coeval folding and Boudinage in four dimensions
Journal of Structural Geology, 2005Co-Authors: J. Zulauf, Gernold ZulaufAbstract:Scaled analogue experiments have been carried out incrementally to demonstrate the growth of constrictional folds and boudins through space and time. Pure constriction acted on a single stiff layer that was embedded in a weak matrix, with the layer trending parallel to the X-axis of the constrictional strain ellipsoid. The viscosity ratio between non-linear viscous layer and matrix was set at ca. 13. Three-dimensional images of the incrementally deformed layer have been obtained using computer tomography. They yielded clear evidence that D1-folds and D1-boudins show striking interactions when growing simultaneously at low to moderate finite strain. The axes of D1-folds are subparallel and the axes of D1-boudins are subperpendicular to the X-axis of the constrictional strain ellipsoid. The normalized wavelengths of D1-folds and D1-boudins, and the amplitudes of D1-folds, are fixed already at low finite strain (−5%) and do not significantly change as deformation proceeds. The normalized wavelength of both structures is not so in line as suggested by theoretical studies. At higher degrees of finite strain, D1-boudins are affected by D2-folds, and rotated limbs of the latter show D2-Boudinage, resulting in complex deformation patterns that are difficult to identify in the field. The model results are important for the analysis and interpretation of deformation structures in rheologically stratified rocks undergoing dislocation creep under bulk constriction. Tectonic settings where constrictional folds and boudins may develop simultaneously are stems of salt diapirs, subduction zones or thermal plumes.
Christopher J. L. Wilson - One of the best experts on this subject based on the ideXlab platform.
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material
2016Co-Authors: Brett A. Marmo, Christopher J. L. WilsonAbstract:stress distribution related to the Boudinage of a visco-elasti
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The stress distribution related to the Boudinage of a visco-elastic material: examples from a polar outlet glacier
Geological Society London Special Publications, 2000Co-Authors: Brett A. Marmo, Christopher J. L. WilsonAbstract:Abstract Quantitative stress measurements related to the development of mesoscale structures in rock are difficult, if not impossible. A method for determining the stress distribution and history during the development of mesoscale Boudinage structures in ice is introduced here. Boudinage structures in fracture trace ice have been observed in the outlets glaciers of the Framnes Mountains, east Antarctica. Fracture traces are preserved when creasses fill with surface melt water, which freezes to form coarse-grained columnar ice. A 4.0 km flow-parallel traverse across the Central Ice Stream of the Framnes Mountains is presented to illustrate the Boudinage of fracture traces with a mean width of 0.30 m. Field-based measurement of the geometric evolution of Boudinage structures has been combined with surface flow rate measurements to quantitatively determine the strain rate at which the Boudinage structures formed. The strain rate measurements provide boundary constraints on several two-dimensional finite difference models that have been used to analyse the stress distribution related to the formation of Boudinage structures in a visco-elastic solid. The results reveal the development of pressure-shadows during the boundinage of layered rocks, and demonstrate the degree of refraction of stress across rheological boundaries, which have important ramifications for the analysis of planar and linear fabrics in rocks.
Bernhard Grasemann - One of the best experts on this subject based on the ideXlab platform.
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The emergence of asymmetric normal fault systems under symmetric boundary conditions
Journal of Structural Geology, 2017Co-Authors: Martin P. J. Schöpfer, Conrad Childs, Tom Manzocchi, John J. Walsh, Andrew Nicol, Bernhard GrasemannAbstract:Abstract Many normal fault systems and, on a smaller scale, fracture Boudinage often exhibit asymmetry with one fault dip direction dominating. It is a common belief that the formation of domino and shear band Boudinage with a monoclinic symmetry requires a component of layer parallel shearing. Moreover, domains of parallel faults are frequently used to infer the presence of a decollement. Using Distinct Element Method (DEM) modelling we show, that asymmetric fault systems can emerge under symmetric boundary conditions. A statistical analysis of DEM models suggests that the fault dip directions and system polarities can be explained using a random process if the strength contrast between the brittle layer and the surrounding material is high. The models indicate that domino and shear band Boudinage are unreliable shear-sense indicators. Moreover, the presence of a decollement should not be inferred on the basis of a domain of parallel faults alone.
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Domino Boudinage under layer-parallel simple shear
Journal of Structural Geology, 2014Co-Authors: Marcin Dabrowski, Bernhard GrasemannAbstract:Abstract The boudin segments of a torn competent layer experience synthetic rotation in layer-parallel simple shear. As long as the individual segments in a boudin train are constrained by their neighbors, even a highly viscous boudin deforms internally to create the necessary space for rotation. The rotation rate is then much smaller compared to the case of an isolated segment. Hence, a small tilt of boudin segments is not indicative of low strain. The rotation rate at this stage largely depends on the aspect ratio of the boudin segments and the scaled gap width. Once the tilted boudins are no longer constrained by their neighbors, the rotation rate greatly accelerates. In the case of a low viscosity ratio between the boudins and the host, the boudin segments develop complex shapes, which may give an impression of shear-band boudins forming under the opposite shear sense. We furthermore investigate the behavior of boudin trains of finite length. The terminal segments are displaced out of the shear plane, deforming into isoclinal folds, and separate into groups of boudin segments that rotate into the shear direction and eventually lead to an overall chaotic appearance of the structure. Natural examples of domino Boudinage from a high shear –strain detachment zone in the Western Cyclades (Greece) show many similarities with the modeled structures suggesting that, under simple shear deformation, the rotation and separation of boudin segments is an indicator for high shear strain.
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How Boudinage triggers the localization of low-angle normal faults
2011Co-Authors: Bernhard Grasemann, Cornelius Tschegg, Marcin DabrowskiAbstract:Initiation and subsequent growth of mid to upper crustal low-angle normal faults is commonly explained by unusual low fault friction caused by either high fluid pressure, weak fault-zone materials probably formed by reaction softening, movement along pre-existing anisotropies and/or aseismic deformation mechanisms. Interestingly, low-angle normal fault localize within a broad range of different host rocks and obviously not a single process but several processes, which probably interact, may lead to the formation of faults with shallow dip angles. Here we report a complex chemo-mechanical feedback process, which facilitated the formation of a crustal scale low-angle normal fault in the Cyclades. On the island of Serifos in the Western Cyclades (Greece) a brittle/ductile low-angle normal fault localized within a dolomite/calcite marble mylonite with numerous centimeter-thick mylonitic quartz layers. The quartz layers have been fractured perpendicular to the layering and record moderate synthetic rotation of the individual blocky boudin segments with generally low aspect ratios into the shear direction. Intuitively, the shear strain recorded by this domino type Boudinage is only low, conflicting with the fact that the low-angle normal fault is supposed to juxtapose different tectonic units with an offset of several tens of kilometers. Closer inspection of the Boudinage reveals that a reaction front developed between the dolomite/calcite marble and the quartz mylonite consisting of fluid assisted and mainly stress-induced breakdown of dolomite and nucleation of talc and secondary generation calcite. Locally further fluid activity triggered also tremolite formation. During synthetic rotation the boudins get totally coated by a continuous talc/cc-rich layer, which developed into low-angle sc and scc’-type shear zones. Comparison of the domino Boudinage with mechanical finite element modeling reveals that as long as the sides of the boudins slide antithetically against each other the individual objects rotate much slower than an isolated rigid particle as predicted by analytical solutions. In case of separation of the neighbouring boudins, the rotation rate accelerates. In conclusion, the fluid assisted reaction of host marble and quartz layer and the developing feedback between this chemical reaction and the Boudinage may lead to shear zones which can accommodate high shear strain facilitating the evolution of crustal scale low-angle normal faults.