| 1. |
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| 2. |
- Bender, Hagen, et al.
(författare)
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Absolute timing of Caledonian orogenic wedge assembly, Central Sweden, constrained by Rb-Sr multi-mineral isochron data
- 2019
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 344, s. 339-359
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Tidskriftsartikel (refereegranskat)abstract
- Multi-mineral Rb-Sr isochron ages for mylonites of the Caledonian nappe pile (Jamtland, Sweden) constrain the orogenic evolution of the central Scandinavian Caledonides. An age of 437.8 +/- 3.9 Ma from a metamorphic leucosome in the central nappe stack dates crystallization of migmatized gneiss. Thirteen isotopic ages derived from mylonites across all structural levels are between 434.6 +/- 6.9 Ma and 426.3 +/- 3.1 Ma. These results, complemented by a dense network of kinematic field data, are interpreted to reflect the timing of protracted top-to-the-ESE general shear under decreasing amphibolite- to upper-greenschist-facies conditions across all nappes. Three samples, derived from the top, middle and bottom of the nappe pile, yield additional deformation ages between 416.0 +/- 3.6 Ma and 410.1 +/- 3.0 Ma. These ages are interpreted to represent post-assembly imbrication of the nappe stack under decreasing metamorphic conditions. The new tectonochronologic data are consistent with a tectonic model that explains nappe stacking in Jamtland by three stages of ESE-directed in- and out-of-sequence thrusting. In the first stage, postdating earlier subduction-exhumation of the high-grade metamorphic Seve Nappe Complex, the latter was emplaced in-sequence in the lower/mid-crust above nappes in its footwall (foreland) at c. 430 Ma. This event is related to the collision of Baltica with an island arc. Concurrently at c. 430 Ma, determined by paleogeography and structurally unrelated to the first stage, second-stage out-of-sequence thrusting emplaced the Kali Nappe Complex atop the developing nappe stack during final Baltica-Laurentia collision. Eventually, imbrication and exhumation of the composite Jamtland nappe stack in the uppermost middle crust during the third stage at c. 414 Ma was caused by continuous underthrusting of Baltica beneath Laurentia. This three-stage thrust model reflects the complex interactions of Baltica-arc-Laurentia collisions that were accommodated by two merging subduction zones.
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| 3. |
- Glodny, Johannes, et al.
(författare)
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The Cycladic Blueschist Unit of the Hellenic subduction orogen : Protracted high-pressure metamorphism, decompression and reimbrication of a diachronous nappe stack
- 2022
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Ingår i: Earth-Science Reviews. - : Elsevier BV. - 0012-8252 .- 1872-6828. ; 224
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the assembly and tectonochronology of the Cycladic Blueschist Unit (CBU) in the central Aegean Sea region, Greece, as a composite, subduction-related, high-pressure (blueschist to eclogite facies) stack of nappes. Our work is based on a review of geochronologic data from the Cycladic islands of Tinos, Syros and Sifnos. In addition, we present a large set of 34 new, internally consistent Rb-Sr multimineral isochron age data from these islands that can be linked to distinct metamorphic stages and episodes of ductile deformation. The reviewed and new ages provide remarkably similar results indicating that the age data and their interpretation is robust. We find that three distinct, significantly diachronous, CBU high-pressure nappes were sequentially accreted between ~55 and ~30 Ma, with a fourth ~24-22 Ma old high-pressure belt, the Basal Unit, below the CBU. Each CBU nappe was exhumed soon after its high-pressure imprint and experienced greenschist-facies overprinting with partial metamorphic reworking ~10 Ma after high-pressure accretion. At the time the structurally higher high-pressure nappe was exhumed, an underlying nappe was accreted and metamorphosed at high-pressure conditions. Our work suggests that the high-pressure nappes were considerably exhumed as rather coherent units during sustained lithospheric shortening in a subduction channel by return flow. The early Oligocene underthrusting of the External Hellenides to the south of the Cyclades, and tectonically below the CBU, is interpreted to have reduced the taper of the Hellenic subduction wedge, which caused significant shortening across the Cyclades region and an important phase of out-of-sequence thrusting that emplaced the non-high-pressure upper units above the CBU. Our analysis shows that this shortening event also caused significant reimbrication within the CBU along the recently discovered Trans Cycladic Thrust.
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| 4. |
- Hansman, Reuben J., et al.
(författare)
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Structural architecture and Late Cretaceous exhumation history of the Saih Hatat Dome (Oman), a review based on existing data and semi-restorable cross-sections
- 2021
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Ingår i: Earth-Science Reviews. - : Elsevier BV. - 0012-8252 .- 1872-6828. ; 217
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Forskningsöversikt (refereegranskat)abstract
- The Saih Hatat Dome in the Al Hajar Mountains provides an outstanding opportunity to study subduction/exhumation processes coeval with obduction of the Semail Ophiolite. The exceptionally good outcrop conditions offer a unique opportunity to constrain the geometry of this subduction/obduction complex. In this review, the metamorphic, structural, and tectonic evolution of the Oman high-pressure complex in the Saih Hatat Dome is discussed. New structural cross-sections are developed and are used to interpret a geometrically feasible tectonic model for the Saih Hatat Dome. Our review highlights the importance of two major tectonic boundaries: (1) The As Sheik Shear Zone which separates the high pressure rocks of the As Sifah Unit (1.7–2.3 GPa and 510–550 °C) from the overlying Hulw Unit (1.0–1.2 GPa and 250–300 °C), and was active at ~79–76 Ma; and (2) the Upper Plate–Lower Plate Discontinuity, which forms a major surface in the landscape and developed by ~76–74 Ma, cutting through structures of the HP rocks in the lower plate (footwall). This discontinuity is associated with a pronounced strain gradient, notably in its upper plate (hanging wall), and separates rocks that have markedly different deformation geometry. The Upper Plate–Lower Plate Discontinuity initiated with a modest dip angle, making it a neutral structure in terms of crustal shortening vs extension. As a result, there is no discernable break in P-T conditions across it. The upper plate is dominated by the Saih Hatat Fold Nappe, forming between ~76 and 70 Ma. Subsequently, the upper plate has been dissected by a number of NNE-dipping thrusts at ~70–66 Ma, followed by normal faults at <~66 Ma. Our review and tectonic model indicate that the Oman high-pressure rocks were exhumed in a contractional tectonic setting that was possibly driven by forced return flow assisted by buoyancy forces. During this exhumation, when the rocks reached the greenschist-facies middle crust the Upper Plate–Lower Plate Discontinuity formed, as a shallow, south-dipping backthrust. Final exhumation of the high-P rocks was achieved by late normal faults after ~66 Ma.
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| 5. |
- Peillod, Alexandre, et al.
(författare)
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An Eocene/Oligocene blueschist-/greenschist facies P-T loop from the Cycladic Blueschist Unit on Naxos Island, Greece : Deformation-related re-equilibration vs. thermal relaxation
- 2017
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Ingår i: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 35:7, s. 805-830
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Tidskriftsartikel (refereegranskat)abstract
- Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P-T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 +/- 16 to 619 +/- 32 degrees C and 15.5 +/- 0.5 to 16.3 +/- 0.9kbar from a subsequent greenschist facies overprint at 384 +/- 30 degrees C and 3.8 +/- 1.1kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist facies assemblages between 40.5 +/- 1.0 and 38.3 +/- 0.5Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32Ma, with one sample showing rejuvenation at c. 27Ma. We conclude that the high-P rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 +/- 4.6km/Ma, completing a full blueschist-/greenschist facies metamorphic cycle soon after subduction within c. 8Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic re-equilibration, and is unrelated to the strong high-T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.
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| 6. |
- Peillod, Alexandre, 1998-
(författare)
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How were high-pressure rocks exhumed in Naxos, Greece?
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Combining petrological and geochronological data we were able to show that rocks in south Naxos completed a full blueschist-/greenschist-facies metamorphic loop in about 10 Myr, distinctly faster than hitherto believed. We show that the high-pressure rocks reached peak pressure (16.3±0.9 kbar - 619 ±32°C) between 40.5±1.0 and 38.3±0.5 Ma and were re-equilibrated in the middle crust (3.8±1.1 kbar - 384±30 °C) under greenschist-facies metamorphism during in the Oligocene ~32 Ma. Our data indicate that this exhumation occurred at rates of 7.4±4.6 km Ma-1. Therefore, the Oligocene greenschist-facies overprint of the blueschist-facies rocks from south Naxos is unrelated to the Miocene amphibolite-/greenschist-facies metamorphism observed in Naxos.
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| 7. |
- Ring, Uwe, et al.
(författare)
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Fault-gouge dating in the Southern Alps, New Zealand
- 2017
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 717, s. 321-338
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Tidskriftsartikel (refereegranskat)abstract
- We report two Ar-40/Ar-39 illite ages from fault gouge directly above the current trace of the Alpine Fault in New Zealand at Gaunt Creek (1.36 +/- 0.27 Ma) and Harold Creek (1.18 +/- 0.47 Ma), and one Ar-40/Ar-39 illite age from fault gouge from the Two Thumbs Fault on the east side of the Southern Alps. Metamorphic muscovite clasts inherited into the Alpine Fault gouge yielded Ar-40/Ar-39 ages of 2.04 +/- 0.3 Ma at Gaunt Creek and 11.46 +/- 0.47 Ma at Harold Creek. We also report Rb-Sr muscovite-based multimineral ages of Alpine Schist mylonite adjacent to the dated fault gouge at Harold Creek (13.1 +/- 43 Ma) and Gaunt Creek (8.9 +/- 3.2 Ma). Ar-40/Ar-39 muscovite ages from the Gaunt Creek mylonite yielded plateau ages of 1.47 +/- 0.08 Ma and 1.57 +/- 0.15 Ma. Finally, we report zircon fission track (0.79 +/- 0.11 and 0.81 +/- 0.17 Ma) and zircon (U-Th)/He ages (0.35 +/- 0.03 and 0.4 +/- 0.06 Ma) from Harold Creek.& para;& para;We interpret the fault gouge ages to date growth of newly formed illite during gouge formation at temperatures of similar to 300-350 degrees C towards the base of the seismogenic zone. Simple backcalculation using current uplift/exhumation and convergence rates, and dip angles of 45-60 degrees at the Alpine Fault support that interpretation. We infer that the fault gouge ages record faulting and gouge formation as the rocks passed very rapidly through the brittle-ductile transition zone on their way to the surface. Rb-Sr and Ar-40/Ar-39 ages on muscovite from Alpine Schist mylonite date muscovite growth at similar to 11 Ma together with a younger phase of cooling/shearing at similar to 1.5-2 Ma. Our ages from the Alpine Schist indicate extremely rapid cooling exceeding 200 degrees C/Ma. The fault gouge age from the Two Thumbs Fault is significantly too old to have formed as part of the late Neogene/Quaternary Southern Alps evolution.
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| 8. |
- Ring, Uwe, et al.
(författare)
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Forced Return Flow Deep in the Subduction Channel, Syros, Greece
- 2020
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Ingår i: Tectonics. - 0278-7407 .- 1944-9194. ; 39:1
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of a detailed structural study in the Cycladic Blueschist Unit at Fabrika on Syros Island, Greece, and discuss their significance for tectonic processes at the subduction interface. Some samples record top-to-the-west shear reflecting prograde (burial), peak high-pressure (HP) and initial decompression (exhumation) conditions. Other nearby samples record top-to-the-east shear during HP metamorphism and exhumation. Some rocks re-equilibrated at greenschist-facies conditions and record top-to-the-west shear. Greenschist-facies top-to-the-west shear is also found at the base of non-HP upper units above the Fabrika HP sequence. We interpret the HP structures to reflect forced return flow and incipient formation of an extrusion wedge in the subduction channel. The HP top-to-the-west structures resulted from thrusting along the base of the wedge and started to form during burial before the rocks reached their deepest point. The HP top-to-the-east structures reflect deformation near the top of the developing extrusion wedge. After considerable exhumation during ongoing subduction, out-of-sequence, top-to-the-west thrusts emplaced the non-HP upper units above the exhuming extrusion wedge 10 Myr after the wedge initially formed. Our work suggests that the HP rocks were considerably exhumed during sustained lithospheric shortening in the subduction channel by forced return flow. Because return flow is controlled by the velocity of the subducting slab, it may explain why HP rocks can be exhumed at subduction rates. On the regional scale we find that four distinct HP belts were sequentially accreted and exhumed between 50 and 20 Ma suggesting continuous subduction-channel return flow in the Hellenic subduction zone.
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| 9. |
- Ring, Uwe, et al.
(författare)
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Geometry and Kinematics of Bivergent Extension in the Southern Cycladic Archipelago : Constraining an Extensional Hinge Zone on Sikinos Island, Aegean Sea, Greece
- 2021
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Ingår i: Tectonics. - 0278-7407 .- 1944-9194. ; 40:6
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Tidskriftsartikel (refereegranskat)abstract
- We report the results of a field study on Sikinos Island in the Aegean extensional province of Greece and propose a hinge zone controlling incipient bivergent extension in the southern Cyclades. A first deformation event led to top-S thrusting of the Cycladic Blueschist Unit (CBU) onto the Cycladic basement in the Oligocene. The mean kinematic vorticity number (Wm) during this event is between 0.56 and 0.63 in the CBU, and 0.72 to 0.84 in the basement, indicating general-shear deformation with about equal components of pure and simple shear. The strain geometry was close to plane strain. Subsequent lower-greenschist-facies extensional shearing was also by general-shear deformation; however, the pure-shear component was distinctly greater (Wm = 0.3–0.41). The degree of subvertical pure-shear flattening increases structurally upward and explains alternating top-N and top-S shear senses over large parts of the island. Along with an increased coaxial deformation component, the strain geometry became oblate. Published quantitative data from nearby Ios Island are similar and both data sets define an extensional hinge zone between top-N extensional deformation across large parts of the central and northern Cyclades and top-S extensional deformation at the southern and western fringe of the archipelago. This extensional hinge zone is an important large-scale structure forming early in the history of lithospheric extension due to southward retreat of the Hellenic slab.
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| 10. |
- Ring, Uwe, 1960-, et al.
(författare)
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Some Like It Cold : The “Conundrum of Samail” Revisited
- 2023
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Ingår i: Tectonics. - 0278-7407 .- 1944-9194. ; 42:1
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Tidskriftsartikel (refereegranskat)abstract
- Most tectonic models consider that the “Samail subduction zone” was the only subduction zone at the mid-Cretaceous convergent Arabian margin. We report four new Rb-Sr multimineral isochron ages from high-pressure (HP) rocks and a major shear zone of the uppermost Ruwi-Yiti Unit of the Saih Hatat window in the Oman Mountains of NE Arabia. These ages demand a reassessment of the intraoceanic suprasubduction-zone evolution that formed the Samail Ophiolite and its metamorphic sole in the Samail subduction zone. Our new ages constrain waning HP metamorphism of the Ruwi subunit at ∼99-96 Ma and associated deformation in the Yenkit shear zone between ∼104 and 93 Ma. Our ages for late stages of deformation and HP metamorphism (thermal gradients of ∼8–10°C km−1) overlap with published ages of ∼105-102 Ma for Samail-subduction-zone prograde-to-peak metamorphism (thermal gradients of ∼20–25°C km−1), subsequent decompressional partial melting of the metamorphic sole and suprasubduction-zone crystallization of the Samail Ophiolite (thermal gradients of ∼30°C km−1) between ∼100 and 93 Ma. Thermal considerations demand that two subduction zones existed at the mid-Cretaceous Arabian margin. High-pressure metamorphism of the Ruwi-Yiti rocks occurred in a mature, thermally equilibrated “Ruwi subduction zone” that formed at ∼110 Ma. Initiation of the infant, thermally unequilibrated and, thus, immature, outboard intraoceanic Samail subduction zone occurred at ∼105 Ma. The Samail Ophiolite and its metamorphic sole were then thrust over the exhuming Ruwi-Yiti HP rocks and onto the Arabian margin after ∼92 Ma, while the bulk of the Saih Hatat HP rocks below the Ruwi-Yiti Unit started to be underthrust in the Ruwi subduction zone.
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