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Shigenori Maruyama - One of the best experts on this subject based on the ideXlab platform.
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Large P–T gap between Ballantrae blueschist/garnet pyroxenite and surrounding ophiolite, southern Scotland, UK: Diapiric exhumation of a Caledonian serpentinite mélange
Lithos, 2008Co-Authors: T. Kawai, Brian F. Windley, Takazo Shibuya, Soichi Omori, Yusuke Sawaki, Shigenori MaruyamaAbstract:Abstract The Ballantrae ophiolite in southern Scotland includes a NEE–SWW-trending serpentinite melange that contains blocks of mafic blueschist and high-pressure, granulite Facies, metapyroxenite (Sm–Nd Metamorphic age: 576 ± 32 and 505 ± 11 Ma). Tectonic blocks of mafic schist are less than 3 × 3 m in size, and have greenschist, blueschist or epidote amphibolite Facies assemblages corresponding to the high-pressure intermediate-type Metamorphic Facies series. Adjacent rocks of the serpentinite melange are hydrothermally-altered MORB-like ophiolitic basalt (prehnite–pumpellyite Facies), dolerite (actinolite–oligoclase sub-Facies) and gabbro (amphibolite Facies), all with assemblages that are diagnostic of the low-pressure Metamorphic Facies series. The difference in Metamorphic Facies series and parageneses of minerals between the high-pressure mafic blocks and the adjacent, low-pressure ophiolitic meta-basic rocks suggests that the former were exhumed from > 25 km depth within a cold subducted slab, and were juxtaposed with the latter, the bottom of a MORB-like ophiolite in the hanging wall of a trench. An ENE–WSW-trending, 501 ± 12 Ma volcanic arc belt extends for 3 km south of the serpentinite melange. We suggest that ridge subduction associated with a slab window created arc-related gabbro (483 ± 4 Ma) at Byne Hill and within-plate gabbro (487 ± 8 Ma) at Millenderdale. Final continental collision created the duplex structure of the Ballantrae complex that includes the HP blocks and serpentinite melange. These relations define diapiric exhumation in the Caledonian orogen of SW Scotland.
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Re-interpretation of progressive metamorphism, Facies series, P-T-t path and exhumation model for the collisional orogenic belts
Himalayan Journal of Sciences, 2008Co-Authors: Shigenori MaruyamaAbstract:Recent discovery of ultra-high pressure (UHP) mineralogy, and descriptions of geology, petrology, geochronology and geochemistry of the Phanerozoic collisional orogens have changed the basic concepts of Metamorphic Facies series, P-Ttime path, progressive metamorphism, Barrovian-type metamorphism and tectonic model. The Himalayan orogen has served as a world standard for the collisional orogenic belt for a long time, characterized by Barrovian-type metamorphism (intermediate-P type metamorphism within the stability field of plagioclase), tectonic overlapping of double-continental crust, and thermal relaxation with buoyant unroofing (England and Thompson 1984). Recent discovery of coesite-bearing eclogite in the western Himalaya and eclogites from the central to eastern Himalaya can not be explained by the previous models. In the followings, summarizing the new constraints on orogenic process, I try to explain the collision orogeny including Himalaya.
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a revolutionary new interpretation of a regional metamorphism its exhumation and consequent mountain building
Journal of Geography, 2004Co-Authors: Shigenori Maruyama, Hideki Masago, Ikuo Katayama, Yasuyuki Iwase, Mitsuhiro ToriumiAbstract:The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolu-tionized the classic interpretation of (1) progressive and retrogressive metamorphism recorded on surface exposures of regional Metamorphic belts, (2) geochronology of that metamorphism, (3) origin of Metamorphic textures, (4) P-T-t path, (5) Metamorphic Facies series, (6) exhumation model, and (7) role of fluids during regional metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia, and the Franciscan and Sanbagawa belts, we point out or predict the following seven revolutionary paradigm shifts.So-called mineral isograds defined on the maps of regional Metamorphic belts were a misunderstanding of the progressive dehydration reaction during subduction, because extensive late-stage hydration has mostly obliterated the progressive minerals in pelitic-psammitic, and metabasic rocks. Progressively zoned garnet has survived as the sole progressive mineral that was unstable with the majority of matrix-forming minerals. The classic Barrovian isograds should be carefully re-examined.The well-documented SHRIMP chronology of spot-dating zoned zircons with index minerals from low-P in the core, through HP-UHP in the mantle to low-P on the rim clearly shows that the slow exhumation speed of 23-40 m.y. from mantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensive hydration of the UHP-HP unit occurred due to fluid infiltration underneath, when the UHP-HP unit intruded a low-grade to un-metamorphosed unit at a mid-crustal level.Most deformation textures such as mineral lineations, porphyroblasts, pull-apart or boudinaged amphiboles, formed during extensive hydration at the late-stage, hence do not indicate a progressive stress regime.The P-T-t path determined by thermobarometry using mineral inclusions in garnet and forward-modeling of garnet zoning, independent of the matrix minerals, indicates an anticlockwise path in the P-T space, and it follows an identical P-T change in the Metamorphic Facies series. This is consistent with the numerically calculated geotherm along the WadatiBenioff plane.Collision-type orogenic belts have long been regarded as being characterized by the intermediate-type Metamorphic Facies series. The kyanite-sillimanite-type is an apparent type Facies series formed by late-stage extensive hydration. In contrast, the original high-P to ultrahigh-P type Facies series with an anticlockwise kink-point at around 10 kb is a progressive type.A collision-type regional Metamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low-P or weakly metamorphosed units. The Metamorphic belt has an aspect ratio (thickness vs width) of 1 : 100, and it extends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantle extending a depth of from 100-200 km into the crustal rock unit. The underlying unit is thermally metamorphosed in the andalusite-sillimanite type Facies series.The major reason for the misunderstanding of the progressive metamorphism in collisiontype orogenic belts is the underestimation of the role of fluids derived from the underlying the low-grade Metamorphic unit, when juxtaposed at a mid-crustal level. The circulation of fluids at a plate boundary is more important than a P-T change.
K. Okumura - One of the best experts on this subject based on the ideXlab platform.
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Thermal modelling in shallow subduction: an application to low P/T metamorphism of the Cretaceous Shimanto Accretionary Complex, Japan
Journal of Metamorphic Geology, 2002Co-Authors: Kazuhiro Miyazaki, K. OkumuraAbstract:This paper presents the results of numerical modelling to investigate the regional occurrence of prehnite-bearing Metamorphic rocks at shallow levels in subduction zones. The modelling assumes a simple geometrical configuration in which the thermal structure in a prism is controlled by boundary conditions at the top and base of the prism. It is expected that the predominant Metamorphic Facies in a prism will change with decreasing age of the descending slab. The results of thermal modelling show that the Facies boundary between pumpellyite–actinolite and prehnite–actinolite Facies (including prehnite–pumpellyite Facies) overlaps with an array of P–T conditions in the prism when the age of a descending slab is younger than 10 Myr. This implies that the change of the predominant Metamorphic Facies from pumpellyite–actinolite to prehnite–actinolite Facies will switch drastically. The critical age of the switch depends on subduction parameters. In particular, the critical age of the descending slab is
T. Kawai - One of the best experts on this subject based on the ideXlab platform.
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Large P–T gap between Ballantrae blueschist/garnet pyroxenite and surrounding ophiolite, southern Scotland, UK: Diapiric exhumation of a Caledonian serpentinite mélange
Lithos, 2008Co-Authors: T. Kawai, Brian F. Windley, Takazo Shibuya, Soichi Omori, Yusuke Sawaki, Shigenori MaruyamaAbstract:Abstract The Ballantrae ophiolite in southern Scotland includes a NEE–SWW-trending serpentinite melange that contains blocks of mafic blueschist and high-pressure, granulite Facies, metapyroxenite (Sm–Nd Metamorphic age: 576 ± 32 and 505 ± 11 Ma). Tectonic blocks of mafic schist are less than 3 × 3 m in size, and have greenschist, blueschist or epidote amphibolite Facies assemblages corresponding to the high-pressure intermediate-type Metamorphic Facies series. Adjacent rocks of the serpentinite melange are hydrothermally-altered MORB-like ophiolitic basalt (prehnite–pumpellyite Facies), dolerite (actinolite–oligoclase sub-Facies) and gabbro (amphibolite Facies), all with assemblages that are diagnostic of the low-pressure Metamorphic Facies series. The difference in Metamorphic Facies series and parageneses of minerals between the high-pressure mafic blocks and the adjacent, low-pressure ophiolitic meta-basic rocks suggests that the former were exhumed from > 25 km depth within a cold subducted slab, and were juxtaposed with the latter, the bottom of a MORB-like ophiolite in the hanging wall of a trench. An ENE–WSW-trending, 501 ± 12 Ma volcanic arc belt extends for 3 km south of the serpentinite melange. We suggest that ridge subduction associated with a slab window created arc-related gabbro (483 ± 4 Ma) at Byne Hill and within-plate gabbro (487 ± 8 Ma) at Millenderdale. Final continental collision created the duplex structure of the Ballantrae complex that includes the HP blocks and serpentinite melange. These relations define diapiric exhumation in the Caledonian orogen of SW Scotland.
Kazuhiro Miyazaki - One of the best experts on this subject based on the ideXlab platform.
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Thermal modelling in shallow subduction: an application to low P/T metamorphism of the Cretaceous Shimanto Accretionary Complex, Japan
Journal of Metamorphic Geology, 2002Co-Authors: Kazuhiro Miyazaki, K. OkumuraAbstract:This paper presents the results of numerical modelling to investigate the regional occurrence of prehnite-bearing Metamorphic rocks at shallow levels in subduction zones. The modelling assumes a simple geometrical configuration in which the thermal structure in a prism is controlled by boundary conditions at the top and base of the prism. It is expected that the predominant Metamorphic Facies in a prism will change with decreasing age of the descending slab. The results of thermal modelling show that the Facies boundary between pumpellyite–actinolite and prehnite–actinolite Facies (including prehnite–pumpellyite Facies) overlaps with an array of P–T conditions in the prism when the age of a descending slab is younger than 10 Myr. This implies that the change of the predominant Metamorphic Facies from pumpellyite–actinolite to prehnite–actinolite Facies will switch drastically. The critical age of the switch depends on subduction parameters. In particular, the critical age of the descending slab is
Y. Kristofferson - One of the best experts on this subject based on the ideXlab platform.
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Seismic studies around the Kola Superdeep Borehole, Russia
Tectonophysics, 1998Co-Authors: Y.v. Ganchin, Scott B. Smithson, Igor B. Morozov, David Smythe, V. Z. Garipov, N. A. Karaev, Y. KristoffersonAbstract:Abstract The Kola Superdeep Borehole (SG-3) provided an ideal opportunity to test hypotheses on the origins of crustal reflections and on the presence and seismic expression of fluids in the upper crust. The alternative sources of crustal reflections include compositional changes, shear zones, fluids, and Metamorphic Facies changes, all of which are represented at the well. Both the 38-km-long CDP section and the borehole VSPs in the range 2.2–6.0 km demonstrate the presence of reflections from dipping compositional layering, shear zones, and fluid-filled zones. Subhorizontal reflectivity zones are interpreted as horizontal fluid-filled fracture-type reservoir rocks. Results suggest the presence of fluids down to a depth of at least 12 km in the upper crust; the presence of these fluids lowering seismic velocity causes estimates of upper crustal composition to be too felsic.