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Simon Hanmer - One of the best experts on this subject based on the ideXlab platform.
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Initiation of cataclastic flow in a Mylonite zone
Journal of Structural Geology, 2003Co-Authors: Simon HanmerAbstract:Abstract At the Mylonite-cataclasite transition in the homogeneous quartzo-feldspathic Mylonites of the Great Slave Lake shear zone, northwest Canadian Shield, arrays of distributed macroscopic fractures evolved into zones of cataclastic flow. Fracture arrays occupy elliptical areas (metres by 10s of centimetres) aligned in the Mylonite foliation plane. Fracture segments rotated either forwards or backwards in response to deformation. Where fractures rotated backwards, they remained planar and cataclasite did not develop. In the case of forward-rotation, only the central part of each fracture rotated. The rotated fracture segments are notably shorter and more closely spaced (1–2 mm) than those which have not rotated and the fracture array has a sigmoidal geometry. Cataclasite developed by the further deformation of such sigmoidal, forward-rotated fracture arrays. Back-rotated fractures were initially planar, regularly spaced (2–10+ cm) and oriented at 70° to the Mylonite foliation; they correspond to extensional faults. Forward-rotated fractures were initially planar, regularly spaced (2 cm) and oriented perpendicular to the Mylonite foliation; they do not correspond to classical arrays of extensional or contractional faults. Both the initial orientation of the fractures and their subsequent rotational behaviour were constrained by the ability/inability of the wallrock to extend and by the ability/inability of the fractures to dilate. Within the forward-rotated fracture arrays, cataclasite was produced after the rotated fracture segments had locked. It is proposed that a second perpendicular fracture set cut the earlier sigmoidal fractures resulting in stubby fragments of Mylonite. The angular fragments acted as stress raisers within the otherwise homogeneous Mylonite. This allowed the transition from distributed macroscopic fracture to impingement-induced localized microfracture and reduction of the fragment size with further strain. When a sufficient volume of interconnected fine material had formed, significant displacements could be accommodated by cataclastic flow, leading eventually to the development of thick (1–100s m) zones of cataclasite. Models of cataclasis developed for porous and carbonate media predict initial grain-scale micro-fracture, slip and fragment rotation leading to frictional wear. The observations made in the Great Slave Lake shear zone suggest that such models may not be directly applicable to the initiation of cataclastic flow developed within non-porous, homogeneous, quartzo-feldspathic Mylonite belts in crustal-scale fault zones, at the elevated confining pressures typical of the transition from the plastic to the brittle deformation regimes.
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Matrix mosaics, brittle deformation, and elongate porphyroclasts: granulite facies microstructures in the Striding–Athabasca Mylonite zone, western Canada
Journal of Structural Geology, 2000Co-Authors: Simon HanmerAbstract:Abstract The Late Archean Striding–Athabasca Mylonite zone, western Canadian Shield, is a 400-km-long, linked system of granulite facies Mylonite belts developed in the continental crust. The granulite facies Mylonites (800–1000°C at 0.8–1.1 GPa) developed in subsets of the anhydrous mineral assemblage clinopyroxene–orthopyroxene–garnet–plagioclase–quartz±hornblende. Comparison of the microstructures of the Striding–Athabasca Mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the Striding–Athabasca Mylonites. In addition to dislocation creep and dynamic recrystallisation, extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism. The microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain-scale and aggregate-scale diffusion, grain boundary mobility, and mass transfer, at relatively slow strain rates. Despite their annealed appearance, the granoblastic matrix mosaics developed synkinematically; deformation-induced dilation may enhance metamorphic reactions where products are more voluminous than reactants. In the porphyroclast population, highly elongate (>20:1) monocrystalline orthopyroxenes appear to be fragments of dismembered, kinked parent grains, rather than stretched porphyroclasts. Granulite facies Mylonites should not be treated as direct analogues of greenschist facies Mylonites. In particular, it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained, high-temperature shear zone rocks.
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matrix mosaics brittle deformation and elongate porphyroclasts granulite facies microstructures in the striding athabasca Mylonite zone western canada
Journal of Structural Geology, 2000Co-Authors: Simon HanmerAbstract:Abstract The Late Archean Striding–Athabasca Mylonite zone, western Canadian Shield, is a 400-km-long, linked system of granulite facies Mylonite belts developed in the continental crust. The granulite facies Mylonites (800–1000°C at 0.8–1.1 GPa) developed in subsets of the anhydrous mineral assemblage clinopyroxene–orthopyroxene–garnet–plagioclase–quartz±hornblende. Comparison of the microstructures of the Striding–Athabasca Mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the Striding–Athabasca Mylonites. In addition to dislocation creep and dynamic recrystallisation, extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism. The microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain-scale and aggregate-scale diffusion, grain boundary mobility, and mass transfer, at relatively slow strain rates. Despite their annealed appearance, the granoblastic matrix mosaics developed synkinematically; deformation-induced dilation may enhance metamorphic reactions where products are more voluminous than reactants. In the porphyroclast population, highly elongate (>20:1) monocrystalline orthopyroxenes appear to be fragments of dismembered, kinked parent grains, rather than stretched porphyroclasts. Granulite facies Mylonites should not be treated as direct analogues of greenschist facies Mylonites. In particular, it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained, high-temperature shear zone rocks.
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modest movements spectacular fabrics in an intracontinental deep crustal strike slip fault striding athabasca Mylonite zone nw canadian shield
Journal of Structural Geology, 1995Co-Authors: Simon Hanmer, Michael L Williams, C KopfAbstract:Geometry and strain partitioning within lower-crustal intraplate strike-slip shear zones can be extremely complex, compared with analogous structural levels of interplate strike-slip shear zones sited at plate margins. Striding-Athabasca Mylonite zone, Canadian Shield, is a spectacular ca. 500 km long granulite facies continental intraplate shear zone. The shear zone is composed of Middle Archean granulite facies annealed Mylonites (ca. 3.13 Ga) and Late Archean (ca. 2.62-2.60 Ga) granulite facies ribbon Mylonite belts, which thread a sinuous course along a chain of crustal-scale ‘lozenges’ cored by relatively stiff rocks of mafic to intermediate composition. To the northeast, the Mylonites form a N-S-trending, 5–10 km thick, dextral strike-slip belt. To the southwest, this bifurcates into a pair of conjugate strike-slip shear zones, overlain by a contemporaneous dip-slip shear zone. Striding-Athabasca Mylonite zone was kinematically inefficient as a strike-slip fault and cannot have accommodated large wallrock displacements. Nevertheless, spectacular granulite facies ribbon Mylonites were formed throughout the shear zone, reflecting the very high temperatures (ca. 850–1000 °C), high recrystallization rate/strain rate ratios, and the transpressive nature of the deformation (Wk < 1), possibly accommodated by significant volume loss by magma migration.
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Striding-Athabasca Mylonite zone: Complex Archean deep-crustal deformation in the East Athabasca Mylonite triangle, northern Saskatchewan
Canadian Journal of Earth Sciences, 1994Co-Authors: Simon Hanmer, Michael L Williams, Randy Parrish, C KopfAbstract:The geophysically defined Snowbird tectonic zone is manifested in northernmost Saskatchewan as a deep-crustal, multistage mylonitic structure, the East Athabasca Mylonite triangle. The triangle, located at the northeastern apex of a stiff, crustal-scale "lozenge," is composed of mid-Archean annealed Mylonites and late Archean ribbon Mylonites, formed during two granulite facies events (850–1000 °C, 1.0 GPa). The flow pattern in the Mylonites is geometrically and kinematically complex, and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis. The late Archean Mylonites are divided into an upper structural deck, entirely occupied by a dip-slip shear zone, and an underlying lower deck. The latter is divided into two upright conjugate strike-slip shear zones, separated by a low-strain septum, which deformed by progressive coaxial flow. The flow pattern in the mid-Archean Mylonites is compatible with that of ...
Jurgen E Streit - One of the best experts on this subject based on the ideXlab platform.
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fluid infiltration and volume change during mid crustal mylonitization of proterozoic granite king island tasmania
Journal of Metamorphic Geology, 1998Co-Authors: Jurgen E StreitAbstract:The development of 10–30 m wide Mylonite zones at mid-crustal depths in late Proterozoic granitoids on King Island, Tasmania, was associated with pervasive infiltration of low δ18O-fluids (+5 to +7) on the scale of the shear zones. Syndeformational fluid–rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite. In a shear zone at Cape Wickham with a normal slip component, changes in whole-rock chemistry between granite and Mylonites indicate a gain of Ca, and losses of K and Na during deformation, which was nearly isovolumetric. Notable losses of K, Rb and Si occurred during partial retrograde alteration of Mylonites near the western margin of this shear zone. The alteration suggests a component of up-temperature fluid flow. In contrast, 3 km to the south east, in a strike-slip shear zone at Disappointment Bay, complete albitization of plagioclase was associated with Na-gain and Ca-loss. Deformation also involved losses of Mg and Fe. Up to 60% volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization. The substantial silica-gain in this, as well as in two Mylonite zones further south east, is interpreted to have been associated with upward flow of aqueous fluids along these shear zones. On the basis of a gradient reaction model, minimum time-integrated fluid-fluxes of 106 m3/m2 are estimated for the Disappointment Bay (West) Shear Zone.
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Fluid infiltration and volume change during mid‐crustal mylonitization of Proterozoic granite, King Island, Tasmania
Journal of Metamorphic Geology, 1998Co-Authors: Jurgen E StreitAbstract:The development of 10–30 m wide Mylonite zones at mid-crustal depths in late Proterozoic granitoids on King Island, Tasmania, was associated with pervasive infiltration of low δ18O-fluids (+5 to +7) on the scale of the shear zones. Syndeformational fluid–rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite. In a shear zone at Cape Wickham with a normal slip component, changes in whole-rock chemistry between granite and Mylonites indicate a gain of Ca, and losses of K and Na during deformation, which was nearly isovolumetric. Notable losses of K, Rb and Si occurred during partial retrograde alteration of Mylonites near the western margin of this shear zone. The alteration suggests a component of up-temperature fluid flow. In contrast, 3 km to the south east, in a strike-slip shear zone at Disappointment Bay, complete albitization of plagioclase was associated with Na-gain and Ca-loss. Deformation also involved losses of Mg and Fe. Up to 60% volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization. The substantial silica-gain in this, as well as in two Mylonite zones further south east, is interpreted to have been associated with upward flow of aqueous fluids along these shear zones. On the basis of a gradient reaction model, minimum time-integrated fluid-fluxes of 106 m3/m2 are estimated for the Disappointment Bay (West) Shear Zone.
Jacques Précigout - One of the best experts on this subject based on the ideXlab platform.
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garnet lherzolite and garnet spinel Mylonite in the ronda peridotite vestiges of oligocene backarc mantle lithospheric extension in the western mediterranean
Geology, 2011Co-Authors: C.j. Garrido, Frédéric Gueydan, Jacques Précigout, Guillermo Boothrea, Karoly Hidas, Jose Alberto Padronnavarta, Claudio MarchesiAbstract:Uplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel Mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel Mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4–2.7 GPa and 1020–1100 °C, demonstrating that the Ronda peridotite equilibrated at ∼85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800–900 °C and 1.95–2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel Mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel Mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust.
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Garnet lherzolite and garnet-spinel Mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean
Geology, 2011Co-Authors: C.j. Garrido, Frédéric Gueydan, G. Booth-rea, Jacques PrécigoutAbstract:Uplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel Mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel Mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4-2.7 GPa and 1020-1100 degrees C, demonstrating that the Ronda peridotite equilibrated at similar to 85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800-900 degrees C and 1.95-2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel Mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel Mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust.
C.j. Garrido - One of the best experts on this subject based on the ideXlab platform.
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garnet lherzolite and garnet spinel Mylonite in the ronda peridotite vestiges of oligocene backarc mantle lithospheric extension in the western mediterranean
Geology, 2011Co-Authors: C.j. Garrido, Frédéric Gueydan, Jacques Précigout, Guillermo Boothrea, Karoly Hidas, Jose Alberto Padronnavarta, Claudio MarchesiAbstract:Uplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel Mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel Mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4–2.7 GPa and 1020–1100 °C, demonstrating that the Ronda peridotite equilibrated at ∼85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800–900 °C and 1.95–2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel Mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel Mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust.
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Garnet lherzolite and garnet-spinel Mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean
Geology, 2011Co-Authors: C.j. Garrido, Frédéric Gueydan, G. Booth-rea, Jacques PrécigoutAbstract:Uplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel Mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel Mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4-2.7 GPa and 1020-1100 degrees C, demonstrating that the Ronda peridotite equilibrated at similar to 85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800-900 degrees C and 1.95-2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel Mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel Mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust.
Claudio Marchesi - One of the best experts on this subject based on the ideXlab platform.
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garnet lherzolite and garnet spinel Mylonite in the ronda peridotite vestiges of oligocene backarc mantle lithospheric extension in the western mediterranean
Geology, 2011Co-Authors: C.j. Garrido, Frédéric Gueydan, Jacques Précigout, Guillermo Boothrea, Karoly Hidas, Jose Alberto Padronnavarta, Claudio MarchesiAbstract:Uplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel Mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel Mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4–2.7 GPa and 1020–1100 °C, demonstrating that the Ronda peridotite equilibrated at ∼85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800–900 °C and 1.95–2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel Mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel Mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust.
