The Experts below are selected from a list of 4845 Experts worldwide ranked by ideXlab platform
Emmanuel Masini - One of the best experts on this subject based on the ideXlab platform.
-
The Angola-Gabon rifted margin: reappraisal of the upper- and lower-plate concept
2015Co-Authors: Gwenn Peron-pinvidic, Emilie Sutra, Gianreto Manatschal, Emmanuel Masini, Jean Marie Flament, Isabelle Haupert, Patrick UnternehrAbstract:In this contribution we summarize observations from the South Atlantic Angola-Gabon rifted margin. Our study is based on interpretation of a selection of deep penetration depth migrated seismic reflection profiles. We describe the dip architecture of the margin under five structural domains (proximal, necking, distal, outer and oceanic), listing their characteristics. We further explain the necking domain and discuss the architecture of the distal domain as a combination of hyper-Extended Crust and exhumed mantle. The mapping and characterization of these domains permit to illustrate the along strike structural and stratigraphic variability of the margin. We interpret this variability as the result of a shift from an upper-plate setting (central segment, South Congo to North Angola) to lower-plate settings (southward with the inner Kwanza Basin, and northward with the Gabon Basin). The transfer from one setting to the other is either sharp, typified by a major regional normal fault on the northern flank of a (residual) H-block, identified offshore Cabinda-Zaire, or more diffuse southward. First order screening of conjugate profiles confirmed the segmentation and the structural characteristics of the transfer zones. The studied dataset also permitted identifying key sections that can be considered as type-examples of upper-plate and lower-plate settings, what permits us reviewing the characteristics of upper- and lower-plate rifted margins.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Briançonnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction melange dynamics or deposition of sedimentary melanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Brianconnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Necking of continental Crust in magma‐poor rifted margins: Evidence from the fossil Alpine Tethys margins
Tectonics, 2012Co-Authors: Geoffroy Mohn, Gianreto Manatschal, Marco Beltrando, Emmanuel Masini, Nick KusznirAbstract:[1] Studies conducted in present-day magma-poor rifted margins reveal that the transition from weakly thinned continental Crust (∼30 km) in proximal margins to hyper-Extended Crust (≤10 km) in distal margins occurs within a narrow zone, referred to as the necking zone. We have identified relics of a necking zone and of the adjacent distal margin in the Campo, Grosina and Bernina units of the fossil Alpine Tethys margins and investigated the deformation and sedimentary processes associated with extreme Crustal thinning during rifting. Within the basement rocks of the necking zone, we show that: (1) Grosina basement represents pre-rift upper/middle Crust, while the underlying Campo unit consists of pre-rift middle/lower Crust that was exhumed and cooled below ∼300°C by ca. 180 Ma, when rifting started to localize within the future distal margin; (2) the juxtaposition of the Campo and Grosina units was accommodated by the Eita shear zone, which is interpreted as a decollement/decoupling horizon active at mid-Crustal depth at 180–205 Ma; (3) the Grosina unit hosts a large-scale brittle detachment fault. Our observations suggest that Crustal thinning, accommodated through the necking zone, is the result of the interplay between detachment faulting in the brittle layers and decoupling and thinning in ductile quartzo-feldspatic middle Crustal levels along localized ductile decollements. The excision of ductile mid-Crustal layers and the progressive embrittlement of the Crust enables major detachment faults to cut into the underlying mantle, exhuming it to the seafloor. This structural evolution can explain the first-order Crustal architecture of many present-day rifted margins.
-
The tectono-sedimentary evolution of a supradetachment rift basin at a deep-watermagma-poor riftedmargin: the example of the Samedan Basin preserved in the Err nappe in SE Switzerland
Basin Research, 2011Co-Authors: Emmanuel Masini, Gianreto Manatschal, Geoffroy Mohn, Jean-françois Ghienne, François LafontAbstract:We describe the tectono- sedimentary evolution of aMiddle Jurassic, rift-related supra-detachment basin of the ancientAlpineTethysmargin exposed in theCentralAlps (SE Switzerland). Based on pre- Alpine restoration, we demonstrate that the rift basin developed over a detachment system that is traced over more than 40 km from thinned continental Crust to exhumed mantle.The detachment faults are overlain by extensional allochthons consisting of upper Crustal rocks and pre-rift sediments up to several kilometres long and several hundreds of metres thick, compartmentalizing the distal margin into sub-basins.We mapped and restored one of these sub-basins, the Samedan Basin. It consists of aV- shape geometry in map view,which is con¢ned by extensional allochthons and £oored by a detachment fault. It can be restored over a minimum distance of 11km along and about 4 km perpendicular to the basin axis. Its sedimentary in¢ll can be subdivided into basal (initial), intermediate (widening) and top (post-tectonic) facies tracts.These tracts document (1) formation of the basin initially bounded by high-angle faults and developing into low-angle detachment faults, (2) widening of the basin and (3)migration of deformation further outboard.The basal facies tract is made of locally derived, poorly sorted gravity £owdeposits that show a progressive change from hangingwall to footwall-derived lithologies.Upsection the sediments develop into turbidity current deposits that show retrogradation (intermediate facies tract) and starvation of the sedimentary system (post-tectonic facies tract). On the scale of the distal margin, the syn-tectonic record documents a thinning- and ¢ning-upward sequence related to the back stepping of the tectonically derived sediment source, progressive starvation of the sedimentary system and migration of deformation resulting in exhumation and progressive delamination of the thinned Crust during ¢nal rifting.This study provides valuable insights into the tectono- sedimentary evolution and stratigraphic architecture of a supra-detachment basin formed over hyper- Extended Crust.
Gianreto Manatschal - One of the best experts on this subject based on the ideXlab platform.
-
The Angola-Gabon rifted margin: reappraisal of the upper- and lower-plate concept
2015Co-Authors: Gwenn Peron-pinvidic, Emilie Sutra, Gianreto Manatschal, Emmanuel Masini, Jean Marie Flament, Isabelle Haupert, Patrick UnternehrAbstract:In this contribution we summarize observations from the South Atlantic Angola-Gabon rifted margin. Our study is based on interpretation of a selection of deep penetration depth migrated seismic reflection profiles. We describe the dip architecture of the margin under five structural domains (proximal, necking, distal, outer and oceanic), listing their characteristics. We further explain the necking domain and discuss the architecture of the distal domain as a combination of hyper-Extended Crust and exhumed mantle. The mapping and characterization of these domains permit to illustrate the along strike structural and stratigraphic variability of the margin. We interpret this variability as the result of a shift from an upper-plate setting (central segment, South Congo to North Angola) to lower-plate settings (southward with the inner Kwanza Basin, and northward with the Gabon Basin). The transfer from one setting to the other is either sharp, typified by a major regional normal fault on the northern flank of a (residual) H-block, identified offshore Cabinda-Zaire, or more diffuse southward. First order screening of conjugate profiles confirmed the segmentation and the structural characteristics of the transfer zones. The studied dataset also permitted identifying key sections that can be considered as type-examples of upper-plate and lower-plate settings, what permits us reviewing the characteristics of upper- and lower-plate rifted margins.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Briançonnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction melange dynamics or deposition of sedimentary melanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Brianconnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Necking of continental Crust in magma‐poor rifted margins: Evidence from the fossil Alpine Tethys margins
Tectonics, 2012Co-Authors: Geoffroy Mohn, Gianreto Manatschal, Marco Beltrando, Emmanuel Masini, Nick KusznirAbstract:[1] Studies conducted in present-day magma-poor rifted margins reveal that the transition from weakly thinned continental Crust (∼30 km) in proximal margins to hyper-Extended Crust (≤10 km) in distal margins occurs within a narrow zone, referred to as the necking zone. We have identified relics of a necking zone and of the adjacent distal margin in the Campo, Grosina and Bernina units of the fossil Alpine Tethys margins and investigated the deformation and sedimentary processes associated with extreme Crustal thinning during rifting. Within the basement rocks of the necking zone, we show that: (1) Grosina basement represents pre-rift upper/middle Crust, while the underlying Campo unit consists of pre-rift middle/lower Crust that was exhumed and cooled below ∼300°C by ca. 180 Ma, when rifting started to localize within the future distal margin; (2) the juxtaposition of the Campo and Grosina units was accommodated by the Eita shear zone, which is interpreted as a decollement/decoupling horizon active at mid-Crustal depth at 180–205 Ma; (3) the Grosina unit hosts a large-scale brittle detachment fault. Our observations suggest that Crustal thinning, accommodated through the necking zone, is the result of the interplay between detachment faulting in the brittle layers and decoupling and thinning in ductile quartzo-feldspatic middle Crustal levels along localized ductile decollements. The excision of ductile mid-Crustal layers and the progressive embrittlement of the Crust enables major detachment faults to cut into the underlying mantle, exhuming it to the seafloor. This structural evolution can explain the first-order Crustal architecture of many present-day rifted margins.
-
The tectono-sedimentary evolution of a supradetachment rift basin at a deep-watermagma-poor riftedmargin: the example of the Samedan Basin preserved in the Err nappe in SE Switzerland
Basin Research, 2011Co-Authors: Emmanuel Masini, Gianreto Manatschal, Geoffroy Mohn, Jean-françois Ghienne, François LafontAbstract:We describe the tectono- sedimentary evolution of aMiddle Jurassic, rift-related supra-detachment basin of the ancientAlpineTethysmargin exposed in theCentralAlps (SE Switzerland). Based on pre- Alpine restoration, we demonstrate that the rift basin developed over a detachment system that is traced over more than 40 km from thinned continental Crust to exhumed mantle.The detachment faults are overlain by extensional allochthons consisting of upper Crustal rocks and pre-rift sediments up to several kilometres long and several hundreds of metres thick, compartmentalizing the distal margin into sub-basins.We mapped and restored one of these sub-basins, the Samedan Basin. It consists of aV- shape geometry in map view,which is con¢ned by extensional allochthons and £oored by a detachment fault. It can be restored over a minimum distance of 11km along and about 4 km perpendicular to the basin axis. Its sedimentary in¢ll can be subdivided into basal (initial), intermediate (widening) and top (post-tectonic) facies tracts.These tracts document (1) formation of the basin initially bounded by high-angle faults and developing into low-angle detachment faults, (2) widening of the basin and (3)migration of deformation further outboard.The basal facies tract is made of locally derived, poorly sorted gravity £owdeposits that show a progressive change from hangingwall to footwall-derived lithologies.Upsection the sediments develop into turbidity current deposits that show retrogradation (intermediate facies tract) and starvation of the sedimentary system (post-tectonic facies tract). On the scale of the distal margin, the syn-tectonic record documents a thinning- and ¢ning-upward sequence related to the back stepping of the tectonically derived sediment source, progressive starvation of the sedimentary system and migration of deformation resulting in exhumation and progressive delamination of the thinned Crust during ¢nal rifting.This study provides valuable insights into the tectono- sedimentary evolution and stratigraphic architecture of a supra-detachment basin formed over hyper- Extended Crust.
Geoffroy Mohn - One of the best experts on this subject based on the ideXlab platform.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Briançonnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction melange dynamics or deposition of sedimentary melanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Brianconnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Necking of continental Crust in magma‐poor rifted margins: Evidence from the fossil Alpine Tethys margins
Tectonics, 2012Co-Authors: Geoffroy Mohn, Gianreto Manatschal, Marco Beltrando, Emmanuel Masini, Nick KusznirAbstract:[1] Studies conducted in present-day magma-poor rifted margins reveal that the transition from weakly thinned continental Crust (∼30 km) in proximal margins to hyper-Extended Crust (≤10 km) in distal margins occurs within a narrow zone, referred to as the necking zone. We have identified relics of a necking zone and of the adjacent distal margin in the Campo, Grosina and Bernina units of the fossil Alpine Tethys margins and investigated the deformation and sedimentary processes associated with extreme Crustal thinning during rifting. Within the basement rocks of the necking zone, we show that: (1) Grosina basement represents pre-rift upper/middle Crust, while the underlying Campo unit consists of pre-rift middle/lower Crust that was exhumed and cooled below ∼300°C by ca. 180 Ma, when rifting started to localize within the future distal margin; (2) the juxtaposition of the Campo and Grosina units was accommodated by the Eita shear zone, which is interpreted as a decollement/decoupling horizon active at mid-Crustal depth at 180–205 Ma; (3) the Grosina unit hosts a large-scale brittle detachment fault. Our observations suggest that Crustal thinning, accommodated through the necking zone, is the result of the interplay between detachment faulting in the brittle layers and decoupling and thinning in ductile quartzo-feldspatic middle Crustal levels along localized ductile decollements. The excision of ductile mid-Crustal layers and the progressive embrittlement of the Crust enables major detachment faults to cut into the underlying mantle, exhuming it to the seafloor. This structural evolution can explain the first-order Crustal architecture of many present-day rifted margins.
-
The tectono-sedimentary evolution of a supradetachment rift basin at a deep-watermagma-poor riftedmargin: the example of the Samedan Basin preserved in the Err nappe in SE Switzerland
Basin Research, 2011Co-Authors: Emmanuel Masini, Gianreto Manatschal, Geoffroy Mohn, Jean-françois Ghienne, François LafontAbstract:We describe the tectono- sedimentary evolution of aMiddle Jurassic, rift-related supra-detachment basin of the ancientAlpineTethysmargin exposed in theCentralAlps (SE Switzerland). Based on pre- Alpine restoration, we demonstrate that the rift basin developed over a detachment system that is traced over more than 40 km from thinned continental Crust to exhumed mantle.The detachment faults are overlain by extensional allochthons consisting of upper Crustal rocks and pre-rift sediments up to several kilometres long and several hundreds of metres thick, compartmentalizing the distal margin into sub-basins.We mapped and restored one of these sub-basins, the Samedan Basin. It consists of aV- shape geometry in map view,which is con¢ned by extensional allochthons and £oored by a detachment fault. It can be restored over a minimum distance of 11km along and about 4 km perpendicular to the basin axis. Its sedimentary in¢ll can be subdivided into basal (initial), intermediate (widening) and top (post-tectonic) facies tracts.These tracts document (1) formation of the basin initially bounded by high-angle faults and developing into low-angle detachment faults, (2) widening of the basin and (3)migration of deformation further outboard.The basal facies tract is made of locally derived, poorly sorted gravity £owdeposits that show a progressive change from hangingwall to footwall-derived lithologies.Upsection the sediments develop into turbidity current deposits that show retrogradation (intermediate facies tract) and starvation of the sedimentary system (post-tectonic facies tract). On the scale of the distal margin, the syn-tectonic record documents a thinning- and ¢ning-upward sequence related to the back stepping of the tectonically derived sediment source, progressive starvation of the sedimentary system and migration of deformation resulting in exhumation and progressive delamination of the thinned Crust during ¢nal rifting.This study provides valuable insights into the tectono- sedimentary evolution and stratigraphic architecture of a supra-detachment basin formed over hyper- Extended Crust.
-
Unravelling the interaction between tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins
International Journal of Earth Sciences, 2010Co-Authors: Geoffroy Mohn, Gianreto Manatschal, Marco Beltrando, O. Muntener, Emmanuel MasiniAbstract:The discovery of exhumed continental mantle and hyper-Extended Crust in present-day magma-poor rifted margins is at the origin of a paradigm shift within the research field of deep-water rifted margins. It opened new questions about the strain history of rifted margins and the nature and composition of sedimentary, Crustal and mantle rocks in rifted margins. Thanks to the benefit of more than one century of work in the Alps and access to world-class outcrops preserving the primary relationships between sediments and Crustal and mantle rocks from the fossil Alpine Tethys margins, it is possible to link the subsidence history and syn-rift sedimentary evolution with the strain distribution observed in the Crust and mantle rocks exposed in the distal rifted margins. In this paper, we will focus on the transition from early to late rifting that is associated with considerable Crustal thinning and a reorganization of the rift system. Crustal thinning is at the origin of a major change in the style of deformation from high-angle to low-angle normal faulting which controls basin-architecture, sedimentary sources and processes and the nature of basement rocks exhumed along the detachment faults in the distal margin. Stratigraphic and isotopic ages indicate that this major change occurred in late Sinemurian time, involving a shift of the syn-rift sedimentation toward the distal domain associated with a major reorganization of the Crustal structure with exhumation of lower and middle Crust. These changes may be triggered by mantle processes, as indicated by the infiltration of MOR-type magmas in the lithospheric mantle, and the uplift of the Briançonnais domain. Thinning and exhumation of the Crust and lithosphere also resulted in the creation of new paleogeographic domains, the Proto Valais and Liguria-Piemonte domains. These basins show a complex, 3D temporal and spatial evolution that might have evolved, at least in the case of the Liguria-Piemonte basin, in the formation of an embryonic oceanic Crust. The re-interpretation of the rift evolution and the architecture of the distal rifted margins in the Alps have important implications for the understanding of rifted margins worldwide, but also for the paleogeographic reconstruction of the Alpine domain and its subsequent Alpine compressional overprint.
Marco Beltrando - One of the best experts on this subject based on the ideXlab platform.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Briançonnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study
Earth-Science Reviews, 2014Co-Authors: Marco Beltrando, Gianreto Manatschal, Geoffroy Mohn, Giorgio Vittorio Dal Piaz, Alberto Vitale Brovarone, Emmanuel MasiniAbstract:Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-Extended Crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-Extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments fromthe hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn- to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high- to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction melange dynamics or deposition of sedimentary melanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboringmeta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that theywere juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement–cover relationships in the North-Western Alps following this approach, combined with published studies on exhumedmantle domains sampled in the rest of theWestern Alps, indicates that several tectono-metamorphic units fromthemost deformed/metamorphosed part of the belt, between the Canavese Line and the Penninic Front, sample hyper-Extended lithosphere related to the Jurassic opening of the Western Tethys. Relative plate motion during Cretaceous–Tertiary basin inversion was largely accommodated at the transition between areas floored by hyper-Extended Crust or hydrated subcontinental mantle and domains consisting of thicker continental Crust. As a result, distal margins were preferentially subducted, whereas the proximal domains and the Brianconnais paleo-high underwent relatively minor deformation and metamorphism. The high-pressure Alpine tectono-metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present-day distal margins. The recognition of preserved pre-Alpine relationships between continental basement, post-rift sediments and/or serpentinized ultramafic rocks calls for a re-assessment of the relative role of subduction and rifting dynamics in establishing the present-day orogen architecture.
-
Necking of continental Crust in magma‐poor rifted margins: Evidence from the fossil Alpine Tethys margins
Tectonics, 2012Co-Authors: Geoffroy Mohn, Gianreto Manatschal, Marco Beltrando, Emmanuel Masini, Nick KusznirAbstract:[1] Studies conducted in present-day magma-poor rifted margins reveal that the transition from weakly thinned continental Crust (∼30 km) in proximal margins to hyper-Extended Crust (≤10 km) in distal margins occurs within a narrow zone, referred to as the necking zone. We have identified relics of a necking zone and of the adjacent distal margin in the Campo, Grosina and Bernina units of the fossil Alpine Tethys margins and investigated the deformation and sedimentary processes associated with extreme Crustal thinning during rifting. Within the basement rocks of the necking zone, we show that: (1) Grosina basement represents pre-rift upper/middle Crust, while the underlying Campo unit consists of pre-rift middle/lower Crust that was exhumed and cooled below ∼300°C by ca. 180 Ma, when rifting started to localize within the future distal margin; (2) the juxtaposition of the Campo and Grosina units was accommodated by the Eita shear zone, which is interpreted as a decollement/decoupling horizon active at mid-Crustal depth at 180–205 Ma; (3) the Grosina unit hosts a large-scale brittle detachment fault. Our observations suggest that Crustal thinning, accommodated through the necking zone, is the result of the interplay between detachment faulting in the brittle layers and decoupling and thinning in ductile quartzo-feldspatic middle Crustal levels along localized ductile decollements. The excision of ductile mid-Crustal layers and the progressive embrittlement of the Crust enables major detachment faults to cut into the underlying mantle, exhuming it to the seafloor. This structural evolution can explain the first-order Crustal architecture of many present-day rifted margins.
-
Unravelling the interaction between tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins
International Journal of Earth Sciences, 2010Co-Authors: Geoffroy Mohn, Gianreto Manatschal, Marco Beltrando, O. Muntener, Emmanuel MasiniAbstract:The discovery of exhumed continental mantle and hyper-Extended Crust in present-day magma-poor rifted margins is at the origin of a paradigm shift within the research field of deep-water rifted margins. It opened new questions about the strain history of rifted margins and the nature and composition of sedimentary, Crustal and mantle rocks in rifted margins. Thanks to the benefit of more than one century of work in the Alps and access to world-class outcrops preserving the primary relationships between sediments and Crustal and mantle rocks from the fossil Alpine Tethys margins, it is possible to link the subsidence history and syn-rift sedimentary evolution with the strain distribution observed in the Crust and mantle rocks exposed in the distal rifted margins. In this paper, we will focus on the transition from early to late rifting that is associated with considerable Crustal thinning and a reorganization of the rift system. Crustal thinning is at the origin of a major change in the style of deformation from high-angle to low-angle normal faulting which controls basin-architecture, sedimentary sources and processes and the nature of basement rocks exhumed along the detachment faults in the distal margin. Stratigraphic and isotopic ages indicate that this major change occurred in late Sinemurian time, involving a shift of the syn-rift sedimentation toward the distal domain associated with a major reorganization of the Crustal structure with exhumation of lower and middle Crust. These changes may be triggered by mantle processes, as indicated by the infiltration of MOR-type magmas in the lithospheric mantle, and the uplift of the Briançonnais domain. Thinning and exhumation of the Crust and lithosphere also resulted in the creation of new paleogeographic domains, the Proto Valais and Liguria-Piemonte domains. These basins show a complex, 3D temporal and spatial evolution that might have evolved, at least in the case of the Liguria-Piemonte basin, in the formation of an embryonic oceanic Crust. The re-interpretation of the rift evolution and the architecture of the distal rifted margins in the Alps have important implications for the understanding of rifted margins worldwide, but also for the paleogeographic reconstruction of the Alpine domain and its subsequent Alpine compressional overprint.
-
From passive margins to orogens: The link between ocean-continent transition zones and (ultra)high-pressure metamorphism
Geology, 2010Co-Authors: Marco Beltrando, Daniela Rubatto, Gianreto ManatschalAbstract:A lithostratigraphic association consisting of serpentinized mantle rocks, continent-derived allochthons, mid-oceanic ridge gabbros of Jurassic age and post-rift sediments, typical of an ocean-continent transition, is found in the eclogitic Piemonte units, in the Western Alps. In situ U-Pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement, in contact with serpentinites. Primary magmatic zircons of Permian age were overgrown by a second generation of zircon at ca. 166–150 Ma, likely related to melt infiltration associated with the intrusion of the underlying gabbroic body. This indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of hyper-Extended Crust related to the opening of the Tethys. Therefore, the complex lithological association described here, which is also characteristic of several (ultra)high-pressure melange zones worldwide, was acquired prior to the orogenic event, during which it was only partly reworked. Ocean-continent transitions are in positions favorable to reach (ultra)high-pressure conditions, following negatively buoyant oceanic lithosphere into subduction, and then being accreted to the orogen, in response to the arrival of more buoyant continental lithosphere, resisting subduction. The ocean-continent transition is now found in the immediate footwall of a 500-m-thick shear zone, which accommodated multiple episodes of deformation during Eocene–Oligocene time, suggesting an important link between Alpine deformation and rift-related structures.
Sridhar Anandakrishnan - One of the best experts on this subject based on the ideXlab platform.
-
Mapping Crustal Shear Wave Velocity Structure and Radial Anisotropy Beneath West Antarctica Using Seismic Ambient Noise
Geochemistry Geophysics Geosystems, 2019Co-Authors: J. P. O'donnell, Alex Brisbourne, Graham Stuart, C. K. Dunham, Yingjie Yang, Grace A. Nield, Pippa L. Whitehouse, Andrew A. Nyblade, Douglas A. Wiens, Sridhar AnandakrishnanAbstract:Using 8‐25s period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3D shear wave velocity structure of the West Antarctic Crust and (ii) map variations in Crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of Crust ~26‐30km thick extending south from Marie Byrd Land separates domains of more Extended Crust (~22km thick) in the Ross and Amundsen Sea Embayments, suggesting along‐strike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS is defined along the southern Transantarctic Mountains (TAM) and Haag Nunataks‐Ellsworth Whitmore Mountains (HEW) block by a sharp Crustal thickness gradient. Crust ~35‐40km is modelled beneath the Haag Nunataks‐Ellsworth Mountains, decreasing to ~30‐32km km thick beneath the Whitmore Mountains, reflecting distinct structural domains within the composite HEW block. Our analysis suggests that the lower Crust and potentially the mid Crust is positively radially anisotropic (VSH > VSV) across West Antarctica. The strongest anisotropic signature is observed in the HEW block, emphasising its unique provenance amongst West Antarctica's Crustal units, and conceivably reflects a ~13km thick metasedimentary succession atop Precambrian metamorphic basement. Positive radial anisotropy in the WARS Crust is consistent with observations in extensional settings, and likely reflects the lattice‐preferred orientation of minerals such as mica and amphibole by extensional deformation. Our observations support a contention that anisotropy may be ubiquitous in continental Crust.
