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Relationship between a syntectonic granitic intrusion and a shear zone in the Southern Carpathian-Balkan area (Alm aj Mountains, Romania): Implications for late Variscan kinematics and Cherbelezu granitoid emplacement

Journal of Structural Geology (Impact Factor: 2.29). 06/2012; 39:83-102. DOI: 10.1016/j.jsg.2012.03.004.

ABSTRACT a b s t r a c t The Carboniferous Cherbelezu batholith (Alm aj Mountains, Romania) is a well-preserved but poorly studied intrusion belonging to the Upper Danubian Alpine Nappe. This pluton crops out along a pre-existing major verticalized formation, the Corbu Mylonitic Zone (CMZ). Our study investigates the role of the CMZ on the deformation recorded during the mush emplacement and cooling. A detailed microstructural study of this granitic body, coupled with investigations on both Anisotropy of Magnetic Susceptibility (AMS) and Shape Preferred Orientation (SPO) of biotite subfabric, has been performed. The surrounding rocks preserve evidence that the CMZ has been reactivated as a sinistral strike-slip fault before the pluton emplacement. Microstructural investigations of the granitic facies indicate that the pluton has undergone superimposed deformations during its cooling, from submagmatic to LT condi-tions. Foliation and lineation patterns obtained by AMS and SPO e both methods giving similar results e reflect either magmatic/submagmatic or solid-state flows. Magmatic flow, preserved in the western and southern parts of the pluton, is characterized by concentric foliation pattern with both divergent and parallel lineations, the latter pointing to an early transcurrent regime. Subsequently, a solid-state deformation, recorded during the pluton cooling and restricted to its eastern and northern parts, argues for the concomitant CMZ activity under a sinistral transpressive regime. This is supported by the P 0 and T parameter distributions, especially for SPO results, this technique showing clear advantages for the interpretation of the fabric scalar parameters.

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    ABSTRACT: The Variscides of Europe and N-Africa are the result of the convergence of the plates of Gondwana and Laurussia in the Paleozoic. This orogen is characterized by the juxtaposition of blocks of continental crust that are little affected by the Variscan orogeny. These low strain domains principally consist of Neoproterozoic/Cambrian Cadomian basement overlain by volcano-sedimentary successions of an extended peri-Gondwana shelf. These Cadomian blocks are separated by high strain zones containing the record of subduction-related processes. Traditionally the high strain zones are interpreted as sutures between one or more postulated lithospheric microplates sandwiched between the two major plates. Paleobio-geographic constraints in combination with geochemical and isotopic fingerprints of the protoliths, however, imply that the Variscides are the result of the exclusive interaction of the two plates of Gondwana and Laurussia. Here we explain the Variscan orogen in a two plate scenario, reasoning that the complexity of the Variscan orogen (multitude of high-grade metamorphic belts, compositional diversity of coeval magmatism, and arrangement of foreland basins) is the result of the distribution of crustal domains of contrasting rheological properties. Post-Cadomian rifting along the Cadomian–Avalonian belt, which culminated in the opening of the Rheic Ocean, resulted in vast coeval intracontinental extension and the formation of extended peri-Gondwana shelf areas, namely the Avalonian shelf and the Armorican Spur to the north and south of the evolving Rheic Ocean, respectively. Both shelf areas affected by heterogeneous extension consist of stable continental blocks separated by zones of thinner continental crust. During Variscan collisional tectonics the continental blocks behave as unsubductable crust, whereas the thinner continental crust was subductable and came to constitute the high strain domains of the orogen. The variable interplay between both crustal types in space and time is seen as the principal cause for the observed sequence of orogenic processes. The first collisional contact along the convergent Gondwana–Laurussia plate boundary occurred between Brittany and the Midland microcraton causing the early Devonian deformation along the Anglo-Brabant Fold Belt. This process is coeval with the initiation of continental subduction along the Armorican Spur of the Gondwana plate and the formation of back arc and transtensional basins to both sides of the Armorican Spur (e.g., Lizard, Rheno-Hercynian, Careón, Sleza) on the Laurussia plate. As further subduction along this collision zone is blocked, the plate boundary zone between the Gondwana and Laurentia plates is reorganized, leading to a flip of the subduction polarity and a subduction zone jump outboard of the already accreted blocks. The following Devonian–Early Carboniferous subduction accretion process is responsible for the juxtaposition of additional Cadomian blocks against Laurussia and a second suite of high-pressure rocks. The final collision between Gondwana and Laurussia is marked by an intracontinental subduction event affecting the entire internal zone of the orogen. Subduction stopped at 340 Ma and the following isothermal exhumation of the deeply subducted continental crust is primarily responsible for Late Variscan high-temperature metamorphism and cogenetic voluminous granitic magmatism. During this final transpressional stage the irregular shape of the Variscan orogen was established by the highly oblique motion of the decoupled lithospheric blocks (e.g. Iberia and Saxo-Thuringia). Rapid overfilling of synorogenic marine basins in the foreland and subsequent folding of these deposits along vast external fold and thrust belts finally shaped the Variscides, feigning a relatively simple architecture.In terms of plate tectonics, the model places the opening of the Paleotethys in the Devonian with a rotational axis of the spreading center just east of the Variscan orogen. The movement of Gondwana relative to Laurussia follows small circle paths about this axis from 370 to 300 Ma. As a consequence of the incomplete closure of the Rheic Ocean after the termination of the Variscan orogeny, Gondwana decoupled from the European Variscides along the dextral Gibraltar Fault Zone. The relative motion between Gondwana and Laurussia after 300 Ma is associated with a shift of the rotational axis to a position close to the Oslo Rift, and is related to the opening of the Neotethys and the evolution of the Central European Extensional Province. The Permian convergence of Gondwana and Laurussia led to the final Permian collisional tectonics along the Mauritanides/Alleghanides. The assembly of the “Wegenerian” Pangea is complete by the end of the Paleozoic.
    Gondwana Research 07/2013; 24(1):298-329. · 7.40 Impact Factor

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