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: 2.88). 06/2012; 39:83-102. DOI: 10.1016/j.jsg.2012.03.004.


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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    • "Rocks deforming in ductile shear zones commonly undergo grain-size reduction, a process that decreases the mechanical strength of rocks (Hobbs et al., 1990; Poirier, 1980), and thus further localizes deformation. A fundamental correlation between finite strain and magnetic anisotropy has often been proposed (Fig. 2; e.g., Benn, 1994; Borradaile; 1987, 1988; Cogne and Perroud, 1988; Henry and Daly, 1983; Hirt et al., 1993; Housen et al., 1995; Hrouda, 1987, 1993; Kligfield et al., 1977, 1981; Kontny et al., 2012; Lüneburg et al., 1999; Plissart et al., 2012; Rathore, 1979; Tikoff et al., 2005; Tripathy et al., 2009). This correlation would apply to both the magnitude of strain and magnetic anisotropy , and to the principal directions of the strain and magnetic anisotropy tensors. "
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    ABSTRACT: The Anisotropy of Magnetic Susceptibility (AMS) is a well-established petrofabric tool for indicating relative strain and microstructural character and has been validated on various rock types and different structural settings. The magnetic susceptibility of a rock (K) depends primarily on the nature and abundance of magnetic minerals. The physical arrangement and lattice-preferred orientation of these magnetic minerals give rise to magnetic anisotropy. The AMS scalar parameters most commonly used to constrain strain include the corrected degree of anisotropy (P’ > 1), a proxy for fabric intensity, and the shape factor (− 1 ≤ T ≤ + 1), an indicator of the magnetic fabric symmetry (prolate vs oblate). A number of studies have shown that a positive correlation generally exists between P’ and strain. Thus, the AMS shows a great potential as a tool for examining deformation in geologic structures characterized by large strain gradients such as shear zones. However, a number of caveats exist: (i) The increase of P’ with strain cannot be solely attributed to deformation because P’ also increases with K regardless of deformation; (ii) Strain across shear zones is typically heterogeneous and is often localized in units of different lithology, thus making the separation of the lithological and strain controls on AMS difficult; also, deformation is commonly accompanied by mineral segregation or fluid-rock interaction that induces changes in magnetic mineralogy; (iii) Even if the undeformed lithology was uniform across a shear zone, variations in strain rate or temperature may result in different deformation mechanisms; hence, the relationship between P’ and strain depends strongly on both the mineral carriers of AMS and on deformation mechanisms; (iv) The AMS is unable to resolve composite fabrics, such as those resulting from S-C structures, where minerals on the C and S planes respectively contribute to AMS.
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    No preview · Article · Jan 2013 · Gondwana Research
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