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Marc Fournier - One of the best experts on this subject based on the ideXlab platform.
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pseudotachylyte in the monte maggiore ophiolitic unit alpine corsica a possible lateral extension of the cima di gratera intermediate depth wadati benioff paleo seismic zone
Bulletin De La Societe Geologique De France, 2018Co-Authors: Olivier Fabbri, Remi Magott, Marc Fournier, Lucas EtienneAbstract:At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustres). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.
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Pseudotachylyte in the Monte Maggiore ophiolitic unit (Alpine Corsica): a possible lateral extension of the Cima di Gratera intermediate-depth Wadati-Benioff paleo-seismic zone
Bulletin de la Société Géologique de France, 2018Co-Authors: Olivier Fabbri, Remi Magott, Marc Fournier, Lucas EtienneAbstract:At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustrés). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.
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polyphase ductile brittle deformation along a major tectonic boundary in an ophiolitic nappe alpine corsica insights on subduction zone intermediate depth asperities
Journal of Structural Geology, 2017Co-Authors: Remi Magott, Olivier Fabbri, Marc FournierAbstract:In an ophiolitic nappe of Alpine Corsica, a major fault zone superimposes metagabbro over serpentinite and peridotite. Ductile and brittle deformation structures are observed in the fault damage zones. In the metagabbro damage zone, early deformation culminates in blueschist or eclogite facies conditions and consists of west-verging mylonitization alternating with pseudotachylyte-forming faulting with undetermined vergence. This early deformation is likely coeval with west-verging seismic (pseudotachylyte-forming) reverse faulting in the footwall peridotite or with aseismic distributed cataclastic deformation of footwall serpentinite. These early events (aseismic mylonitization or distributed cataclasis and seismic faulting) are interpreted as reverse faulting/shear in an east-dipping subducting oceanic lithosphere in Cretaceous to Eocene times. Late deformation events consist of ductile shear and seismic faulting having occurred under retrograde greenschist conditions. Kinematics of the ductile shear is top-to-the-east. These events are interpreted as the result of syn-to post-collision extension of Alpine Corsica in Eocene to Miocene times. The heterogeneous distribution of pseudotachylyte veins along the fault zone (abundant at peridotite-metagabbro interfaces, rare or absent at serpentinite-metagabbro interfaces) is interpreted as the consequence of contrasted frictional properties of the rocks in contact. High-friction peridotite-metagabbro contacts could correspond to asperities whereas low-friction serpentinite-metagabbro contacts could correspond to creeping zones.
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subduction zone intermediate depth seismicity insights from the structural analysis of alpine high pressure ophiolite hosted pseudotachylyte corsica france
Journal of Structural Geology, 2016Co-Authors: Remi Magott, Olivier Fabbri, Marc FournierAbstract:Pseudotachylyte in the Cima di Gratera ophiolite, Alpine Corsica, is distributed in the peridotite unit and in the overlying metagabbro unit and was formed under blueschist to eclogite metamorphic facies conditions, corresponding to a 60–90 km depth range. Peridotite pseudotachylyte is clustered in fault zones either beneath the tectonic contact with overlying metagabbros or at short distance from it. Fault zones are either parallel to the contact or make an angle of 55° to it. Displacement sense criteria associated with fault veins indicate top-to-the-west or top-to-the-northwest reverse senses. Cataclasite flanking most veins was formed before or coevally with frictional melting and likely mechanically weakened the peridotite, facilitating subsequent seismic rupture. In the basal part of the metagabbro unit, post-mylonitization pseudotachylyte can be distinguished from pre-mylonitization pseudotachylyte formed earlier. In the equant metagabbro above the mylonitic sole, only one episode of pseudotachylyte formation can be identified. Kinematics associated with metagabbro pseudotachylyte remain unknown. The geometry and kinematics of the pseudotachylyte veins from the peridotite unit and to a lesser extent from the metagabbro unit are similar to modern seismic ruptures of the upper parts of the Wadati-Benioff zones such as in the Pacific plate beneath NE Japan.
Olivier Fabbri - One of the best experts on this subject based on the ideXlab platform.
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pseudotachylyte in the monte maggiore ophiolitic unit alpine corsica a possible lateral extension of the cima di gratera intermediate depth wadati benioff paleo seismic zone
Bulletin De La Societe Geologique De France, 2018Co-Authors: Olivier Fabbri, Remi Magott, Marc Fournier, Lucas EtienneAbstract:At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustres). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.
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Pseudotachylyte in the Monte Maggiore ophiolitic unit (Alpine Corsica): a possible lateral extension of the Cima di Gratera intermediate-depth Wadati-Benioff paleo-seismic zone
Bulletin de la Société Géologique de France, 2018Co-Authors: Olivier Fabbri, Remi Magott, Marc Fournier, Lucas EtienneAbstract:At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustrés). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.
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polyphase ductile brittle deformation along a major tectonic boundary in an ophiolitic nappe alpine corsica insights on subduction zone intermediate depth asperities
Journal of Structural Geology, 2017Co-Authors: Remi Magott, Olivier Fabbri, Marc FournierAbstract:In an ophiolitic nappe of Alpine Corsica, a major fault zone superimposes metagabbro over serpentinite and peridotite. Ductile and brittle deformation structures are observed in the fault damage zones. In the metagabbro damage zone, early deformation culminates in blueschist or eclogite facies conditions and consists of west-verging mylonitization alternating with pseudotachylyte-forming faulting with undetermined vergence. This early deformation is likely coeval with west-verging seismic (pseudotachylyte-forming) reverse faulting in the footwall peridotite or with aseismic distributed cataclastic deformation of footwall serpentinite. These early events (aseismic mylonitization or distributed cataclasis and seismic faulting) are interpreted as reverse faulting/shear in an east-dipping subducting oceanic lithosphere in Cretaceous to Eocene times. Late deformation events consist of ductile shear and seismic faulting having occurred under retrograde greenschist conditions. Kinematics of the ductile shear is top-to-the-east. These events are interpreted as the result of syn-to post-collision extension of Alpine Corsica in Eocene to Miocene times. The heterogeneous distribution of pseudotachylyte veins along the fault zone (abundant at peridotite-metagabbro interfaces, rare or absent at serpentinite-metagabbro interfaces) is interpreted as the consequence of contrasted frictional properties of the rocks in contact. High-friction peridotite-metagabbro contacts could correspond to asperities whereas low-friction serpentinite-metagabbro contacts could correspond to creeping zones.
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subduction zone intermediate depth seismicity insights from the structural analysis of alpine high pressure ophiolite hosted pseudotachylyte corsica france
Journal of Structural Geology, 2016Co-Authors: Remi Magott, Olivier Fabbri, Marc FournierAbstract:Pseudotachylyte in the Cima di Gratera ophiolite, Alpine Corsica, is distributed in the peridotite unit and in the overlying metagabbro unit and was formed under blueschist to eclogite metamorphic facies conditions, corresponding to a 60–90 km depth range. Peridotite pseudotachylyte is clustered in fault zones either beneath the tectonic contact with overlying metagabbros or at short distance from it. Fault zones are either parallel to the contact or make an angle of 55° to it. Displacement sense criteria associated with fault veins indicate top-to-the-west or top-to-the-northwest reverse senses. Cataclasite flanking most veins was formed before or coevally with frictional melting and likely mechanically weakened the peridotite, facilitating subsequent seismic rupture. In the basal part of the metagabbro unit, post-mylonitization pseudotachylyte can be distinguished from pre-mylonitization pseudotachylyte formed earlier. In the equant metagabbro above the mylonitic sole, only one episode of pseudotachylyte formation can be identified. Kinematics associated with metagabbro pseudotachylyte remain unknown. The geometry and kinematics of the pseudotachylyte veins from the peridotite unit and to a lesser extent from the metagabbro unit are similar to modern seismic ruptures of the upper parts of the Wadati-Benioff zones such as in the Pacific plate beneath NE Japan.
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Segmentation of the southern part of the Outer Hebrides Fault Zone (NW Scotland). Implications on the seismotectonic behaviour of large-scale crustal faults
2008Co-Authors: François Souquière, Olivier FabbriAbstract:The crustal-scale Outer Hebrides Fault Zone (OHFZ) in NW Scotland is a complex fault zone which cuts Precambrian gneisses and which was active, with varied kinematics, at several epochs between the Proterozoic and the Mesozoic. It is outlined by a series of fault rocks formed under ductile to brittle conditions. The southern part of the Outer Hebrides Fault Zone, exposed in South Uist, Eriskay and Barra, is outlined by pseudotachylytes and cataclasites presumably formed by reverse motion during the brittle stages of the Caledonian orogeny about 430 Ma ago. Analysis of published geological maps and detailed field mapping between latitudes N56°54' and N57°17' show that the OHFZ consists of two segments. Between N57°07' and N57°17', the northern segment emplaces a banded granulite-textured metadiorite, forming the so-called Corodale gneiss, upon Lewisian gneiss. This fault segment is characterised by moderate deformation in the footwall. The hanging-wall consists of a pseudotachylyte sole whose thickness is comprised between 2 and 10 m and which is overlain by a cataclastic zone whose thickness ranges from a few meters to more than 50 m, locally reaching 250 m. Between N56°54' and N57°07', the hanging-wall and footwall of the southern segment both consist of Lewisian gneiss but with different metamorphic grades. Unlike the northern segment, the southern segment is characterized by numerous pseudotachylyte-bearing thrust surfaces along which the pseudotachylyte thickness seldom exceeds 0.5 m. In summary, seismic deformation along the northern segment appears localized and is associated with significant cataclasis, whereas along the southern segment, seismic deformation is essentially distributed, with limited formation of pseudotachylyte and very little cataclasis. The segmentation likely reflects the contrasted mineralogical compositions and the texture of the rocks from the hanging-wall. The Corodale gneiss mainly consists of an assemblage of minerals (opx-cpx-gt-pl) whose friction melting susceptibilities are higher than the mineralogical assemblage of the Lewisian gneiss (qz-pl-kf- bi and minor hbl-px). Similarly, unlike the Lewisian gneiss, the Corodale gneiss is devoid of any planar fabric. These differences may account for the preferred development of cataclasites in Corodale gneiss. The segmentation is further discussed by comparison with modern earthquake ruptures.
Remi Magott - One of the best experts on this subject based on the ideXlab platform.
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pseudotachylyte in the monte maggiore ophiolitic unit alpine corsica a possible lateral extension of the cima di gratera intermediate depth wadati benioff paleo seismic zone
Bulletin De La Societe Geologique De France, 2018Co-Authors: Olivier Fabbri, Remi Magott, Marc Fournier, Lucas EtienneAbstract:At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustres). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.
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Pseudotachylyte in the Monte Maggiore ophiolitic unit (Alpine Corsica): a possible lateral extension of the Cima di Gratera intermediate-depth Wadati-Benioff paleo-seismic zone
Bulletin de la Société Géologique de France, 2018Co-Authors: Olivier Fabbri, Remi Magott, Marc Fournier, Lucas EtienneAbstract:At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustrés). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.
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polyphase ductile brittle deformation along a major tectonic boundary in an ophiolitic nappe alpine corsica insights on subduction zone intermediate depth asperities
Journal of Structural Geology, 2017Co-Authors: Remi Magott, Olivier Fabbri, Marc FournierAbstract:In an ophiolitic nappe of Alpine Corsica, a major fault zone superimposes metagabbro over serpentinite and peridotite. Ductile and brittle deformation structures are observed in the fault damage zones. In the metagabbro damage zone, early deformation culminates in blueschist or eclogite facies conditions and consists of west-verging mylonitization alternating with pseudotachylyte-forming faulting with undetermined vergence. This early deformation is likely coeval with west-verging seismic (pseudotachylyte-forming) reverse faulting in the footwall peridotite or with aseismic distributed cataclastic deformation of footwall serpentinite. These early events (aseismic mylonitization or distributed cataclasis and seismic faulting) are interpreted as reverse faulting/shear in an east-dipping subducting oceanic lithosphere in Cretaceous to Eocene times. Late deformation events consist of ductile shear and seismic faulting having occurred under retrograde greenschist conditions. Kinematics of the ductile shear is top-to-the-east. These events are interpreted as the result of syn-to post-collision extension of Alpine Corsica in Eocene to Miocene times. The heterogeneous distribution of pseudotachylyte veins along the fault zone (abundant at peridotite-metagabbro interfaces, rare or absent at serpentinite-metagabbro interfaces) is interpreted as the consequence of contrasted frictional properties of the rocks in contact. High-friction peridotite-metagabbro contacts could correspond to asperities whereas low-friction serpentinite-metagabbro contacts could correspond to creeping zones.
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subduction zone intermediate depth seismicity insights from the structural analysis of alpine high pressure ophiolite hosted pseudotachylyte corsica france
Journal of Structural Geology, 2016Co-Authors: Remi Magott, Olivier Fabbri, Marc FournierAbstract:Pseudotachylyte in the Cima di Gratera ophiolite, Alpine Corsica, is distributed in the peridotite unit and in the overlying metagabbro unit and was formed under blueschist to eclogite metamorphic facies conditions, corresponding to a 60–90 km depth range. Peridotite pseudotachylyte is clustered in fault zones either beneath the tectonic contact with overlying metagabbros or at short distance from it. Fault zones are either parallel to the contact or make an angle of 55° to it. Displacement sense criteria associated with fault veins indicate top-to-the-west or top-to-the-northwest reverse senses. Cataclasite flanking most veins was formed before or coevally with frictional melting and likely mechanically weakened the peridotite, facilitating subsequent seismic rupture. In the basal part of the metagabbro unit, post-mylonitization pseudotachylyte can be distinguished from pre-mylonitization pseudotachylyte formed earlier. In the equant metagabbro above the mylonitic sole, only one episode of pseudotachylyte formation can be identified. Kinematics associated with metagabbro pseudotachylyte remain unknown. The geometry and kinematics of the pseudotachylyte veins from the peridotite unit and to a lesser extent from the metagabbro unit are similar to modern seismic ruptures of the upper parts of the Wadati-Benioff zones such as in the Pacific plate beneath NE Japan.
Yanick Ricard - One of the best experts on this subject based on the ideXlab platform.
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intermediate depth earthquake generation and shear zone formation caused by grain size reduction and shear heating
Geology, 2015Co-Authors: Marcel Thielmann, Boris J. P. Kaus, Antoine Rozel, Yanick RicardAbstract:The underlying physics of intermediate-depth earthquakes have been an enigmatic topic; several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation. Here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and Pseudotachylite formation. Our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway, making it more likely to result in intermediate-depth earthquakes at shallower depths. Furthermore, grain size is reduced in and around the shear zone, which agrees with field and laboratory observations where Pseudotachylites are embedded in a simultaneously formed mylonite matrix. The decrease in critical stress to initialize localization has important implications for large-scale geodynamics, as this mechanism might induce lithosphere-scale shear zones and subduction initiation. We suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate-depth earthquakes and the formation of large-scale shear zones.
S P Kelley - One of the best experts on this subject based on the ideXlab platform.
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an 40ar 39ar laser probe study of Pseudotachylites in charnockite gneisses from the cauvery shear zone system south india
Gondwana Research, 2006Co-Authors: Y Bhaskar J Rao, S P Kelley, Nigel Harris, B L Narayana, C SrikantappaAbstract:The age of Pseudotachylite formation in the crustal-scale Cauvery Shear Zone system of the Precambrian Southern Granulite Terrain (South India) has been analyzed by laser-probe Ar-40-Ar-39 dating. Laser spot analyses from a Pseudotachylite from the Salem-Attur shear zone have yielded ages ranging from 1214 to 904 Ma. Some evidence for the presence of excess Ar is indicated by the scatter of ages from this locality. The host gneiss preserves Palaeoproterozoic Rb-Sr whole rock-biotite ages (2350 +/- 11 to 2241 +/- 11 Ma). A mylonite in the Koorg shear, ca. 200 km to the north, yielded an age of 895 17 Ma the consistency of the age distribution from spot analyses precludes the presence of significant excess Ar. Despite published evidence for the growth of high-grade minerals within some components of the Cauvery Shear Zone during the Pan-African event (700-550 Ma), the Pseudotachylites in this study provide no evidence for Pan-African formation. Instead they document the first evidence for Mesoproterozoic tectonism in the Cauvery Shear Zone system, thus prompting a review of the correlation between the Cauvery Shear Zone system and the large-scale shear zones located elsewhere in eastern Gondwana. (c) 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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laser probe argon 40 argon 39 dating of coesite and stishovite bearing pseudotachylytes and the age of the vredefort impact event
Meteoritics, 1995Co-Authors: John G Spray, S P Kelley, Uwe W ReimoldAbstract:— Age determinations have been made on pseudotachylytic rocks from the controversial Vredefort structure of South Africa using the laser microprobe 40Ar/39Ar dating technique. Coesite- and stishovitebearing veins in a quartzite from the Central Rand Group of the collar rocks were dated using a 10-μm diameter focused ultra-violet laser beam. These yielded a weighted mean age of 2027 ± 18 Ma (2σ). Six pseudotachylytes, sampled from four different locations within the Outer Granite Gneiss of the core, were dated using an 50–100-μm diameter focused infrared laser beam. These pseudotachylytes exhibit altered vein margins with apparent ages considerably younger than ages obtained from the fresher centres of veins. The best weighted mean pseudotachylyte matrix age obtained was 2018 ± 14 Ma (2σ). Most of the clasts within the pseudotachylyte matrices retain significantly older (e.g., Archean) ages, indicative of their parent rock history. Our results show that five of the seven dated samples possess matrix ages of ∼2000 Ma, similar to the age of the Granophyre (Walraven et al., 1990), a supposed impact melt rock (French and Nielsen, 1990). The dating of coesite- and stishovite-bearing veins equates the shock event with pseudotachylyte formation, generation of the Granophyre and creation of the Vredefort structure. The results affirm that the Vredefort Dome is a meteorite impact structure and show that it formed at 2018 ± 14 Ma (2σ).
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laser probe 40ar 39ar investigation of a pseudotachylyte and its host rock from the outer isles thrust scotland
Geology, 1994Co-Authors: S P Kelley, Steven M Reddy, R MaddockAbstract:The formation of a friction melt or pseudotachylyte in the footwall of the Outer Isles thrust, Scotland, during thrust movement in the early Paleozoic Caledonian orogeny caused argon loss from biotites in the host gneiss. Mean 40 Ar/ 39 Ar ages for biotite grains decrease from 1450 to 923 Ma in a zone 730°C in the host gneiss; these high temperatures may have caused catastrophic argon loss rather than loss by volume diffusion. An 40 Ar/ 39 Ar age traverse across the pseudotachylyte vein revealed old ages adjacent to the margin, reflecting the incorporation of partially outgassed host-rock clasts. Apparent ages for the pseudotachylyte decrease unevenly toward the center of the vein. A weighted mean age of 430 ±6 Ma (2σ) obtained in the center corresponds closely to movement ages derived for the associated Moine thrust zone.
