An early metamorphic stage for the Variscan Ulten Zone gneiss (NE Italy): evidence from mineral inclusions in kyanite

2007 ◽  
Vol 71 (06) ◽  
pp. 691-702 ◽  
Author(s):  
R. Braga ◽  
H.-J. Massonne ◽  
L. Morten
Keyword(s):  
Continental Collision ◽  
Crustal Thickening ◽  
Variscan Orogeny ◽  
Thermal Decay ◽  
Metamorphic Stage ◽  
Mineral Inclusions ◽  
Ne Italy

Abstract The early P-T evolution of a garnet-kyanite gneiss from the Variscan Ulten Zone has been defined by detailed petrographic observations leading to the detection of chlorite-epidote- and staurolite-bearing assemblages enclosed in kyanite porphyroblasts. Calculations of P-T pseudosections in the system NaCaKFeMgAlSiHO allowed us to constrain the evolution of these relics to the earliest metamorphic stages. The overall path shows a P-T increase to a peak of 11–12 kbar and 600–650°C followed by decompressional heating to 720°C and 9–10 kbar and final cooling at 7 kbar, 550–600°C. This clockwise P-T path reflects crustal thickening and subsequent thermal decay related to the continental collision of the Variscan orogeny 330–340 Ma ago. Our study demonstrates that large kyanite porphyroblasts may preserve assemblages related to prograde metamorphic stages. As a result, the detection of mineral inclusions in kyanite can complement many similar studies on mineral suites hosted in garnet and zircon.

scholarly journals Magmatic flare-up causes crustal thickening at the transition from subduction to continental collision

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Carlos E. Ganade ◽  
Pierre Lanari ◽  
Daniela Rubatto ◽  
Joerg Hermann ◽  
Roberto F. Weinberg ◽  
...  
Keyword(s):  
Production Rate ◽  
Continental Margin ◽  
Subduction Zones ◽  
Continental Collision ◽  
Crustal Thickening ◽  
Time Intervals ◽  
West Gondwana ◽  
Continental Arc ◽  
Order Of Magnitude

AbstractAbove subduction zones, magma production rate and crustal generation can increase by an order of magnitude during narrow time intervals known as magmatic flare-ups. However, the consequences of these events in the deep arc environment remain poorly understood. Here we use petrological and in-situ zircon dating techniques to investigate the root of a continental arc within the collisional West Gondwana Orogen that is now exposed in the Kabyé Massif, Togo. We show that gabbros intruded 670 million years ago at 20–25 km depth were transformed to eclogites by 620 million years ago at 65–70 km depth. This was coeval with extensive magmatism at 20–40 km depth, indicative of a flare-up event which peaked just prior to the subduction of the continental margin. We propose that increased H2O flux from subduction of serpentinized mantle in the hyper-extended margin of the approaching continent was responsible for the increased magma productivity and crustal thickening.

scholarly journals An anticlockwise metamorphic P-T path and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

2018 ◽  
Author(s):  
Carly Faber ◽  
Holger Stünitz ◽  
Deta Gasser ◽  
Petr Jeřábek ◽  
Katrin Kraus ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Large Scale ◽  
Structural Data ◽  
Geothermal Gradient ◽  
Continental Collision ◽  
Crustal Thickening ◽  
Northern Norway ◽  
Nappe Complex ◽  
T Path

Abstract. This study investigates the Caledonian metamorphic and tectonic evolution in northern Norway, examining the structure and tectonostratigraphy of the Reisa Nappe Complex (RNC; from bottom to top, Vaddas, Kåfjord and Nordmannvik nappes). Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and P-T conditions of deformation and metamorphism that formed the nappes and facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating followed by thrusting. An early Caledonian S1 foliation in the Nordmannvik Nappe records kyanite-grade partial melting at ~ 760–790 °C and ~ 9.4–11 kbar. Leucosomes formed at 439 ± 2 Ma (U-Pb zircon) in fold axial planes in the Nordmannvik Nappe indicate that compressional deformation initiated while the rocks were still partially molten. This stage was followed by pervasive solid-state shearing as the rocks cooled and solidified, forming the S2 foliation at 680–730 °C and 9.5–10.9 kbar. Multistage titanite growth in the Nordmannvik Nappe records this extended metamorphism between 444 and 427 Ma. In the underlying Kåfjord Nappe, garnet cores record lower P-T (590–610 °C and 5.5–6.8 kbar) but a similar geothermal gradient as the S1 migmatitic event in the Nordmannvik Nappe, indicating formation at a higher relative position in the crust. S2 shearing in the Kåfjord Nappe occurred at 580–605 °C and 9.2–10.1 kbar, indicating a considerable pressure increase during nappe stacking. Gabbro intruded in the Vaddas Nappe at 439 ± 1 Ma, synchronously with migmatization in the Nordmannvik Nappe. In the Vaddas Nappe S2 shearing occurred at 630–640 ºC and 11.7–13 kbar. Titanite growth along the lower RNC boundary records S2-shearing at 432 ± 6 Ma. It emerges that early Silurian heating (~ 440 Ma), probably resulting from large-scale magma underplating, initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual nappe units. This tectonic style contrasts subduction of mechanically strong continental crust to great depths.

scholarly journals Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

Solid Earth ◽  
2019 ◽  
Vol 10 (1) ◽  
pp. 117-148 ◽  
Author(s):  
Carly Faber ◽  
Holger Stünitz ◽  
Deta Gasser ◽  
Petr Jeřábek ◽  
Katrin Kraus ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Large Scale ◽  
Structural Data ◽  
Continental Collision ◽  
Crustal Thickening ◽  
Western Gneiss Region ◽  
Nappe Complex ◽  
T Path ◽  
Tectonic Style

Abstract. This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439 Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780 ∘C and ∼9–11 kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600 ∘C, 6–7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 ∘C and ca. 6–9 kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 ∘C and 9–11 kbar in the partially molten Nordmannvik Nappe, ca. 600 ∘C and 9–10 kbar in the Kåfjord Nappe, and ca. 640 ∘C and 12–13 kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.

A lifetime of the Variscan orogenic plateau from uplift to collapse as recorded by the Prague Basin, Bohemian Massif

2017 ◽  
Vol 156 (3) ◽  
pp. 485-509 ◽  
Author(s):  
FRANTIŠEK VACEK ◽  
JIŘÍ ŽÁK
Keyword(s):  
Bohemian Massif ◽  
Three Dimensional ◽  
Plate Boundary ◽  
Early Carboniferous ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Variscan Orogeny ◽  
Prague Basin ◽  
Lateral Extrusion ◽  
Orogenic Plateau

AbstractThe Ordovician to Middle Devonian Prague Basin, Bohemian Massif, represents the shallowest crust of the Variscan orogen corresponding toc.1–4 km palaeodepth. The basin was inverted and multiply deformed during the Late Devonian to early Carboniferous Variscan orogeny, and its structural inventory provides an intriguing record of complex geodynamic processes that led to growth and collapse of a Tibetan-type orogenic plateau. The northeastern part of the Prague Basin is a simple syncline cross-cut by reverse/thrust faults and represents a doubly vergent compressional fan accommodatingc.10–19 % ~NW–SE shortening, only minor syncline axis-parallel extension and significant crustal thickening. The compressional structures were locally overprinted by vertical shortening, kinematically compatible with ductile normal shear zones that exhumed deep crust in the orogen's interior atc. 346–337 Ma. On a larger scale, the deformation history of the Prague Syncline is consistent with building significant palaeoelevation during Variscan plate convergence. Based on a synthesis of finite deformation parameters observed across the upper crust in the centre of the Bohemian Massif, we argue for a differentiated within-plateau palaeotopography consisting of domains of local thickening alternating with topographic depressions over lateral extrusion zones. The plateau growth, involving such complex three-dimensional internal deformations, was terminated by its collapse driven by multiple interlinked processes including gravity, voluminous magma emplacement and thermal softening in the hinterland, and far-field plate-boundary forces resulting from the relative dextral motion of Gondwana and Laurussia.

Variscan crustal thickening in the Maures-Tanneron massif (South Variscan belt, France): new in situ monazite U-Th-Pb chemical dating of high-grade rocks

2015 ◽  
Vol 186 (2-3) ◽  
pp. 145-169 ◽  
Author(s):  
Emilien Oliot ◽  
Jérémie Melleton ◽  
Julie Schneider ◽  
Michel Corsini ◽  
Véronique Gardien ◽  
...  
Keyword(s):  
Partial Melting ◽  
Large Scale ◽  
Crustal Thickening ◽  
Variscan Orogeny ◽  
High Grade ◽  
Variscan Belt ◽  
Massif Central ◽  
Internal Zone ◽  
Chemical Dating

AbstractAge constraints on the protoliths, deformation, metamorphism and melting events are key parameters when correlating different continental lithospheric remnants among each other and disentangling their evolution within large-scale orogens. In situ U-Th-Pb chemical dating on monazites using Electron Probe Micro-Analyser (EPMA) has been performed on eight samples throughout the Variscan Maures-Tanneron massif (SE France) in order to date the medium to high-tectonothermal events related to the Variscan orogeny.Results indicate a polyphased crustal evolution : (i) U-Th-Pb ages obtained in polygenetic monazite grain cores gave inherited Upper Ordovician (456 ± 11 Ma) age, highlighting the large scale occurrence of the Ordovician magmatic activity in the North Gondwanian margin. An Early Devonian (404 ± 10 Ma) age may date a protolith emplacement related to calc-alkaline supra-subduction magmatism or could be associated to an early medium-grade metamorphism, prior to collisional stage. (ii) The crustal thickening stage has been further recorded in prograde metamorphic monazites formed during the underthrusting and subsequent nappe stacking events, under amphibolite facies conditions. This stage is dated between 382 ± 11 (Middle Devonian) and 331 ± 5 Ma (Late Visean). (iii) An orogenic partial melting event took place during Middle Carboniferous and is accompanied by the crystallization of crustal peraluminous magmas (Plan-de-la-Tour granite, 329 ± 3 Ma).This contribution demonstrates the capacity of monazite to record the prograde path of rocks during increasing metamorphic conditions related to stages of crustal thickening, and the robustness of the U-Th-Pb chronometer in monazite despite the overprinting of high-grade thermal events, including partial melting. The age ranges of the different orogenic stages reported in this study are in good agreement with those reported in adjacent Variscan Corsica and Sardinia; while correlations with other nearest Variscan massifs like the Argentera massif in the southwestern Alps or the French Massif Central remain more hypothetic. The Internal Zone of the Maures-Tanneron massif, and more widely the Internal Zone of the Maures-Tanneron-Corsica-Sardinia segment, is part of the southern orogenic root system of the Variscan belt.

scholarly journals Recurrence of Fault Valve Behavior in a Continental Collision Area: Evidence From Tilt/Strain Measurements in Northern Adria

2021 ◽  
Vol 9 ◽  
Author(s):  
Giuliana Rossi ◽  
Alberto Pastorutti ◽  
Ildiko Nagy ◽  
Carla Braitenberg ◽  
Stefano Parolai
Keyword(s):  
Fluid Pressure ◽  
Close Relation ◽  
Satellite System ◽  
Continental Collision ◽  
Transient Signal ◽  
Slow Slip Events ◽  
Ne Italy ◽  
Fault Valve ◽  
Global Navigation Satellite ◽  
Fluid Diffusion

We analyzed the data recorded by the NE-Italy subsurface tilt and strainmeter network evidencing a coherent transient signal in the recordings of four tiltmeter sites in the 1984–1990 period that produced a tilt along the main fractures. Borrowing from classical seismology techniques, we used the uprise times to locate the transient signal source. The propagation velocity is compatible with a fluid diffusion process that starts from a source located close to the hypocenter of the February 10, 1983 Uccea earthquake, MD = 4.2 at the Italy-Slovenia border, at an estimated depth of 10.8 km. Our results add to the previous interpretation of a transient signal recorded by several global navigation satellite system (GNSS) stations in the 2006–2009 period in terms of fluid diffusion below the Bovec basin (Slovenia). That source was located upon continuation to the northwest of the Ravne fault, few kilometers to the northeast from the present one, and about 6 km from the July 12, 2004 Bovec–Krn earthquake, Mw = 5.1, depth ~6.1 km. These observations suggest that the area is subject to fault valve behavior episodes that released fluids trapped at depth to the surrounding region as pore-pressure bulges. The convergence between Alpine and External Dinarides structures in this area puts highly permeable dolomitic limestones in contact with low-permeable fine-grained limestones and flysch formations. Therefore, the conditions for overpressure generation can be created, whereas fault movements, from time to time, in close relation with seismic events, can enable fluid diffusion in the surroundings. We also estimated the possible fluid influx needed to maintain overpressure and possible discharge across both the faults. The study provides insights on pore–fluid pressure variations related to slow slip events from a context different from subduction or transform margins, i.e., in a continental collision area.

Role of the deep crustal scale geometry on Western Alps strain partitioning : Insights from S-wave velocity tomography

2021 ◽  
Author(s):  
Stéphane Schwartz ◽  
Ahmed Nouibat ◽  
Yann Rolland ◽  
Thierry Dumont ◽  
Anne Paul ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Shear Zones ◽  
Western Alps ◽  
Crustal Thickening ◽  
Variscan Orogeny ◽  
Western Boundary ◽  
S Wave ◽  
Structural Anisotropy ◽  
Velocity Tomography

<p>The recent S-wave velocity tomography undertaken at the scale of the Alps by Nouibat et al. (2021) allows a reappraisal of the deep structure of this mountain belt. These geophysical data highlight the role of crustal geometry in the strain field development observed in the Western Alps. The geophysical imagery shows a standard crustal thickness in the foreland, with slow velocities (<3.6 km.s<sup>-1</sup>) in the lower crust. The occurrence of a sharp Moho offset of 5-12 km is detected beneath the External Crystalline Massifs (ECMs). The ECMs do not show any significant crustal thickening in their frontal parts (<35 km), except for the Pelvoux ECM (35-40 km). Beneath the internal zones, east of the Penninic Frontal Thrust, the crustal geometry is more complex with the presence of an European continental slab subducting locally deeper than 80 km beneath the Adria plate. This slab is overlain by a high-pressure metamorphic orogenic prism. The lower part, corresponding to the Ivrea gravimetry anomaly, shows seismic signatures of serpentinized mantle (Vs between 3.8 and 4.3 km.s<sup>-1</sup>) whose upper limit is located at 10 km depth below the Dora Maira internal crystalline massif. This new crustal-scale image can be compared to the current deformation pattern, which appears highly partitioned at the scale of the Alpine arc. The internal zones show a transtensional deformation regime, whose activity is distributed along two major seismic lineaments (the ‘Piemontais’ and ‘Briançonnais’ ones). The Alpine European foreland shows a transpressional deformation that is more diffuse and associated with vertical displacements in the ECMs. Beneath the Po plain, the seismic activity is deeper (>40 km), and correlates with a transpressional deformation which is localized along sub-vertical lineaments. The deformation of the orogenic prism appears controlled by a deeper and rigid mantle indenter split in two units by a major subvertical serpentinized structure. The upper unit, which indents horizontally and vertically the crustal orogenic prism, is located between 20 and 45 km depth. The lower unit corresponds to the western boundary of the Adria mantle that pinches directly the European slab. The surface observations and geochronological data suggest that the Moho offstets are superposed on European crustal-scale faults trend inherited from the Variscan orogeny, following the East-Variscan strike-slip system. This structural anisotropy was reactivated during the Alpine orogeny as shear zones in a mainly transpressional regime since about 25-30 Ma, as documented by Ar-Ar data on syn-kinematic mica and U-Pb on monazite. The comparison of current seismicity with the kinematics of exhumed shear zones suggests a continuity of this regime since 25-30 Ma, in response to the Adria plate anticlockwise rotation.</p>

Geodynamic significance of the Variscan eclogites in the External Crystalline Massifs (Western Alps): marker of a subduction or crustal thickening?

2020 ◽  
Author(s):  
Jean-baptiste Jacob ◽  
Stéphane Guillot ◽  
Daniela Rubatto ◽  
Emilie Janots ◽  
Jérémie Melleton ◽  
...  
Keyword(s):  
High Pressure ◽  
Western Alps ◽  
Late Carboniferous ◽  
Crustal Thickening ◽  
Garnet Growth ◽  
Variscan Orogeny ◽  
High Grade ◽  
Pressure Stage ◽  
Hp Metamorphism ◽  
High Pressure Stage

<p><span>The Paleozoic basement exposed in the External Crystalline Massifs of the Western Alps (ECM) contains numerous relics of Variscan eclogites and high pressure granulites preserved in high grade migmatitic gneisses. </span><span>These relics are taken to indicate</span><span> that the </span><span>ECM</span><span> underwent an early HP metamorphic stage during the Variscan Orogeny. However, due to the scarcity of recent thermobarometric and geochronological data, the geodynamic significance of this high pressure metamorphism remains unclear. Based on petrological similarities with other eclogite-bearing formations in the European Variscides (especially the “leptyno-amphibolic compex” in the French </span><span>Variscides</span><span>), it has been suggested that the high pressure rocks from the ECM mark a mid-Devonian subduction cycle, preceding the main Carboniferous Variscan collisional stage </span><span>(Fr</span>éville et al., 2018; Guillot and Ménot, 2009)<span>. This interpretation mostly relies on one mid-Devonian U-Pb zircon age (395</span>±<span>2 Ma) obtained in eclogites from the massif of Belledonne (Paquette et al., 1989), which has been interpreted as the age of eclogitization. However, dating of high pressure granulites in the Argentera Massif (Rubatto et al., 2010) yielded a Carboniferous age (ca. 340 Ma) for the high pressure stage, questioning the previous geodynamical interpretation. </span>We present here the results of a detailed petrological and geochronological investigation of the high grade formation of the Lacs de la Tempête in NE Belledonne, where some of the eclogites dated by Paquette et al. (1989) were sampled. This area exposes mostly high-grade migmatitic metasediments with intercalated lenses of orthogneiss and garnet-bearing amphibolites, preserving locally eclogitic assemblages. Thermobarometric estimations coupling forward pseudosection modelling, Zr in rutile thermometry and garnet growth modelling constrain the minimal P conditions during the high pressure stage at ca. 1.4-1.6 GPa and 700 °C. The early HP assemblage was then strongly overprinted by granulite facies metamorphism at ca. 1.0-1.2 GPa and 750 °C, also recorded in the surrounding metasediments. U-Pb dating of zircon reveals that the eclogites derived from Ordovician protoliths. Zircon overgrowth in the eclogites and the surrounding metasediments constrain the age of HP metamorphism between ca. 350-305 Ma, with no evidence for a Devonian event. Rutile dating in the eclogites supports the late Carboniferous age of metamorphism. The middle-late Carboniferous corresponds to the main period of Variscan nappe stacking in the ECM, following a period of arc magmatism during late Devonian-Tournaisian (ca. 360-350 Ma, <span>Fr</span>éville et al., 2018). We therefore suggest that the 350-305 Ma ages recorded in the HP units of the ECM do not correspond to a Devonian subduction, but rather represent the equilibration of orogenic lower crust at HP-MT conditions during the Variscan nappe stacking events, followed by re-equilibration at lower P during late Carboniferous. This evolution presents striking similarities with the high pressure units of the Moldanubian zone in the Bohemian massif (Schulmann et al., 2009). However, deciphering the exact meaning of U-Pb ages in retrogressed eclogites remains a challenge, and further field and petrological investigation is required to produce a consistent history of the Variscan collision in the ECM.</p>

Emplacement, rapid burial, and exhumation of 90-Ma plutons in southeastern Alaska

2004 ◽  
Vol 41 (1) ◽  
pp. 87-102 ◽  
Author(s):  
Glen R Himmelberg ◽  
Peter J Haeussler ◽  
David A Brew
Keyword(s):  
Structural Analysis ◽  
Country Rock ◽  
Crustal Thickening ◽  
Thermal Decay ◽  
Pluton Emplacement ◽  
The North ◽  
Regional Deformation ◽  
Mineral Assemblages ◽  
Accreted Terranes ◽  
Gravina Belt

In southeastern Alaska, granodiorite–tonalite plutons of the Admiralty–Revillagigedo belt intruded the Jurassic–Cretaceous Gravina belt along the eastern side of the Alexander terrane around 90 Ma. These plutons postdate some deformation related to a major contractional event between the previously amalgamated Wrangellia and Alexander terranes and the previously accreted terranes of the North American margin. We studied the aureole mineral assemblages of these plutons near Petersburg, Alaska, determined pressure and temperature of equilibration, and examined structures that developed within and adjacent to these plutons. Parallelism of magmatic and submagmatic fabrics with fabrics in the country rock indicates synchroneity of pluton emplacement with regional deformation and suggests that magma transport to higher crustal levels was assisted by regional deformation. Replacement of andalusite by kyanite or sillimanite indicates crustal thickening soon after pluton emplacement. Regional structural analysis indicates the crustal thickening was accomplished by thrust burial. Thermobarometric analyses indicate the aureoles reached near-peak temperatures of 525 to 635 °C at pressures of 570 to 630 MPa. Consideration of the rate of thermal decay of the aureoles suggests that burial was rapid and occurred at rates around 5 to 8 mm/year. Structural observations indicate there was contractional deformation before, during, and after emplacement of the 90-Ma plutons. Initial exhumation of the Admiralty–Revillagigedo belt in the Petersburg area may have occurred along a thrust west of the pluton belt within the Gravina belt.

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