The late Precambrian "sparagmites" of southern Norway; a major Caledonian allochthon; the Osen-Roa nappe complex
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Northeastern and northwestern margins of Baltica in Neoproterozoic time: evidence from the Timanian and Caledonian OrogensKeywords:
Allochthon
Baltica
Allochthon
Massif
Plutonism
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Abstract. A stacking of nappes forms the Parautochthonous Domain located beneath the Allochthonous Complexes of the NW Iberian Massif. Lower Paleozoic metasedimentary and felsic metavolcanic rocks, including some riebeckite gneiss, form the uppermost nappe of the Parautochthon. Micaschists of this nappe may contain albite porphyroblasts with aligned mineral microinclusions defining a relic internal schistosity. Thermobarometric estimates on the relic mineral assemblage suggest an early M1 stage of High-Pressure, Low-Temperature (HP-LT) metamorphism (11–14 kbar; 450–500 °C), probably related to the same continental subduction process that affected the overlying Lower Allochthon nappes or Basal Units. This uppermost nappe was emplaced onto its relative autochthon by a broad, ductile shear zone that shows top-to-the-E/ENE shear-sense and whose exposure has lateral continuity through to the basal thrust of the Lower Allochthons in the Bragança Complex. The HP-LT metamorphism and its hanging wall position relative to the basal thrust of the Allochthonous Complexes, suggest a re-interpretation of this uppermost parautochthonous nappe as another nappe of the Lower Allochthon. In turn, the other parautochthonous nappes comprise both, Lower Paleozoic pre-orogenic rocks, with no M1 relics, and Upper Paleozoic syn-orogenic rocks. Shear-sense criteria related to the current juxtaposition of the Parautochthon Domain onto the Central Iberian autochthon show contrasting top-to-the-S/SE kinematics. The whole orogenic section was affected by an M2 episode of recrystallization under intermediate P/T conditions during the first stages of collision. P-T estimates on kyanite-bearing schists from the upper sections of the autochthon (9.0 kbar; 425–450 °C) are consistent with a geothermal gradient somewhat lower than the classical Barrovian one at the beginning of collision. Thermobarometric calculations on schists from the lower parautochthonous nappes yield peak metamorphic conditions around 7.5 Kbar and 600–700 °C, constraining the original thickness of the allochthonous/parautochthonous pile to about 22.5–27.0 km. Schists sampled from deeper sections of the autochthon yield M2 conditions around 11–12 kbar and 700–725 °C, matching those registered by correlative regions from Central Iberia. Subsequent M3 syn-collisional recrystallization under High-Temperature and Low-Pressure conditions is associated with top-to-the-N/NW and top-to-the-S/SE extensional flow. The early metamorphic evolution of the autochthons, Parautochthon and Allochthonous Complexes of NW Iberia recorded a transition in P/T regimes and appears as a model case of a change from subduction to collision.
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Massif
Mylonite
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Baltica
Allochthon
Orogeny
Baltic Shield
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Flysch
Syncline
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Terra Nova, 24, 363–372, 2012 Abstract Over 100 new Nd model ages are presented for orthogneisses from the Lac Dumoine region, western Quebec, interpreted as a thrust sheet of the Grenvillian Allochthonous Polycyclic Belt. The thrust sheet has an isotope signature consistent with other terranes of the allochthonous belt, with TDM ages <1.8 Ga in the main allochthon, but ages of 1.8–2.0 Ga in an underlying tectonic unit, itself thrust over parautochthonous Archean crust. However, the new data also reveal a half‐window of Palaeoproterozoic crust that divides the allochthon into two lobes, showing them to be relatively thin‐skinned nappes originating from the Baskatong crustal‐scale ramp. When combined with new Nd isotope mapping for Algonquin Park, Ontario, these results suggest that the main ramp of the Allochthon Boundary Thrust is located further to the SE than previously thought, with a strong SW‐NE linear trend that coincides with dipping reflectors imaged on Lithoprobe seismic line 32 south of Algonquin Park.
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Imbrication
Thrust fault
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Abstract The Revsegg Nappe is the uppermost tectonic unit of the Hardanger–Ryfylke Nappe Complex in the SW-Scandinavian Caledonides. The Revsegg Nappe consists mainly of two-mica schists with boudinaged lenses of sandstone, amphibolite, granodiorite and pegmatite. Analyses by U–Pb isotope dilution thermal ionization mass spectrometry on metamorphic zircon in a mafic lens and on rutile in a metasandstone reveal Ordovician ages of 471±18 and 480±15 Ma, respectively. Pegmatites intruded at 433±1 Ma, contemporaneously with rutile formation at the bottom of the nappe (434±6 Ma). Granodiorite intruded at 428±1 Ma, during or shortly before deformation of the biotite schists at c. 428–424 Ma. The specific lithology and the Ordovician imprint distinguish the Revsegg Nappe from the underlying crystalline nappes. The latter have characteristic indicative of a Baltic derivation whereas the Revsegg Nappe correlates better with nappes of Iapetan affinity, such as the Jæren Nappe south of Stavanger.
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Norwegian
Baltica
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Baltica
Apparent polar wander
Riphean
Rodinia
Polar wander
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Thrust fault
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