Strain partitioning in a pluton during emplacement in transpressional regime: the example of the Néouvielle granite (Pyrenees) (original) (raw)
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Geodinamica Acta, 2006
This work deals with the magnetic susceptibility and its anisotropy (AMS) in the Variscan Millares pluton in the Central Pyrenees. The zonation of low-field magnetic susceptibility is consistent with the concentric arrangement of rock-types, with more basic compositions at the external areas. Magnetic foliations defined from AMS strike NE-SW and dip gently towards the NW. Magnetic foliations are mainly perpendicular and oblique to the elongation of the pluton in map view (NW-SE) and show a concentric pattern at the central part, where the more acid rocks crop out. Magnetic lineations are scattered between NW-SE and NE-SW and plunge shallowly to the N. In map view magnetic lineations are distributed in domains normal to the elongation of the pluton. The contours of P' (degree of magnetic anisotropy) are oriented NE-SW and bands of oblate and prolate ellipsoids alternate perpendicular to the elongation of the pluton in map view. P' is between 1.009 and 1.055 in 93% of the specimens. Such low values are currently recorded in granites having magmatic fabrics and for which the anisotropy is mainly carried by biotite. The attitude of the magnetic foliation and the magnetic lineation, the geometry of the pluton, and their relationship with the host-rock structure suggest an intrusion contemporary with a transpressional regime, syntectonic with the late stages of the Variscan orogeny.
Journal of Structural Geology, 2004
In the French Massif Central, the Rouergue-Albigeois area consists of three tectonic units stacked during the Hercynian orogeny. The structural analysis of the units and particularly the quartz < c> axis, textural and structural observations in the syntectonic Pinet-type plutons allow us to determine the deformation history. A first thrusting event, D1, is responsible for a southwestward emplacement of the high-grade Lévézou nappe, above the Lower Gneiss Unit itself overlying the para-autochthonous micaschists. In Late Devonian-Early Carboniferous, this stack of nappes experienced a second event, D2, characterized by a top-to-the-NW emplacement under medium pressure/medium temperature conditions of the para-autochtonous unit upon the Lower Gneiss Unit. During D2, the Pinet-type plutons were deformed by a shearing consistent with the kinematics recorded in the country rocks. In contrast to previous works, which considered the Pinet-type plutons emplacement to be coeval with the D2 event, we argue that the emplacement of the plutons occurred during the top-to-the-SW D1 nappe stacking. This work emphasizes the importance of the quartz < c> axis and microstructure analysis for the identification of the early structural history in polyphase deformed terranes.
Tectonics, 2015
We decipher late-orogenic crustal flow characterized by feedback relations between partial melting and deformation in the Variscan Montagne Noire gneiss dome. The dome shape and finite strain pattern of the Montagne Noire Axial Zone (MNAZ) result from the superimposition of three deformations (D1, D2 and D3). The early flat-lying S1 foliation is folded by D2 upright ENE-WSW folds and transposed in the central and southern part of the MNAZ into steep D2 high-strain zones consistent with D2 NW-SE horizontal shortening, in bulk contractional coaxial deformation regime that progressively evolved to noncoaxial dextral transpression. The D2 event occurred under metamorphic conditions that culminated at 0.65 ± 0.05 GPa and 720 ± 20°C. Along the anatectic front S1 and S2 foliations are transposed into a flat-lying S3 foliation with top-toNE and top-to-SW shearing in the NE and SW dome terminations, respectively. These structures define a D3 transition zone related to vertical shortening during coaxial thinning with a preferential NE-SW to E-W directed stretching. Depending on structural level, the metamorphic conditions associated with D3 deformation range from partial melting conditions in the dome core to subsolidus conditions above the D3 transition zone. We suggest that D2 and D3 deformation events were active at the same time and resulted from strain partitioning on both sides of the anatectic front that may correspond to a major rheological boundary within the crust.
Tectonics, 2013
Field relationships combined with new U-Pb zircon geochronology suggest that the shallow-level Krkonoše-Jizera plutonic complex, northern Bohemian Massif, was assembled successively from bottom to top, starting with emplacement of the separately evolved S-type Tanvald granite (317.3 ± 2.1 Ma), followed by at least two voluminous batches of the I-type porphyritic Liberec (319.5 ± 2.3 Ma) and Jizera (320.1 ± 3.0 Ma and 319.3 ± 3.7 Ma) granites. The intrusive sequence was completed by uppermost, minor intrusions of the equigranular Harrachov (315.0 ± 2.7 Ma) and Krkonoše granites. The I-type granites exhibit an unusually complex pattern of superposed feldspar phenocryst and magnetic fabrics as revealed from the anisotropy of magnetic susceptibility (AMS). The outer Liberec granite preserves margin-parallel foliations and lineations, interpreted to record emplacement-related strain captured by cooling from the pluton floor and walls. In contrast, the inner Jizera, Harrachov, and Krkonoše granites were overprinted by synmagmatic strain resulting from dextral movements along regional strike-slip faults cutting the opposite ends of the plutonic complex. Late-stage felsic dikes in the Liberec and Jizera granites reorient from horizontal to vertical (lineation-perpendicular) attitude in response to changing the least principal stress direction, whereas mafic schlieren do not do so, representing only randomly oriented small-scale thermal-mechanical instabilities in the phenocryst framework. In general, this case example challenges the common approach of inferring pluton-wide magma flow from interpolated foliation, lineation, and schlieren patterns. More likely, magmatic fabrics in large plutons record complex temporal succession of superposed strains resulting from diverse processes at multiple scales.
Strain partitioning in banded and/or anisotropic rocks: Implications for inferring tectonic regimes
Journal of Structural Geology, 2013
Among the many concepts that have contributed to the development of structural geology in the last half-century, two are particularly important. These are: (i) the link between the symmetry of the structure, the vorticity of the associated deformation process and the tectonic regime, and (ii) the widespread partitioning of strain. However, a clear understanding of these concepts and the links between them are not always apparent in structural analyses carried out to determine the prevailing tectonic regime of an area through the use of structural symmetries and kinematic indicators. This contribution, based on field studies from Cap de Creus, experimental data and theoretical concepts, highlights some of the problems encountered when attempting to deduce the type of strain and associated regional tectonic regime from field structures. The relation between the symmetry of a structure and its associated kinematics is not unique. For example, it is demonstrated that in mechanically anisotropic materials, symmetry variations do not necessarily reflect marked vorticity variations, but rather the angular variations between the kinematic framework and the anisotropy planes. Because of widespread strain partitioning, local and general kinematics do not generally match. This mismatch is especially accentuated in rheologically heterogeneous rocks. Nevertheless, correct interpretations are possible by performing continuous multi-scale structural analyses in which the impact of strain partitioning and of the presence of material anisotropies and heterogeneities are considered.
To deepening the understanding of the interactions between tectonics and magmatism in the mid to deep crust, a detailed petrostructural analysis has been performed in the southern area of Punta dels Farallons-Volt Andrau (Cap de Creus, Eastern Pyrenees). In this area, schists under high-grade conditions suffered partial melting while a sequence of intermediate to acid magmatic rocks were emplaced during the Variscan D2 transpressive deformation event. Both regional tectonics and rheological features controlled the way deformation localized in the various rocks of the migmatitic complex. Schists and migmatites have a penetrative sub-vertical foliation (S2). Strain measurements of deformed late veins and dykes have allowed us to determine the regional deformation post-dating their emplacement. Dextral transpression was associated to N-S sub-horizontal shortening and the principal extension direction switched from sub-vertical to sub-horizontal under bulk constriction to plane strain.
Journal of Structural Geology, 2004
A structural and magnetic fabric study of the Bielsa granite (Axial Zone of the Pyrenees) provides new data indicating that this pluton was emplaced during the main Variscan event recognized in the Pyrenees. We argue that the later post-Triassic deformation was localized along mylonitic bands reactivating earlier Variscan shear bands. The Bielsa granite intrudes Cambro-Ordovician metasediments at its northern border, and the pluton is unconformably overlain by sediments Triassic in age to the south and east. The main structures in the igneous body are magmatic foliations and lineations defined by feldspar phenocrysts and biotite and dextral shear bands with dominant WNW -ESE trends. The WNW -ESE striking magmatic foliations have variable dips, and the magmatic lineation a subhorizontal WNW -ESE trend. The zonation of low-field magnetic susceptibility, coarsely related to mineral content, indicates a layer arrangement of rock-types, with more basic compositions at the northern border. The magnetic ellipsoids are prolate along a WNW -ESE central band, and oblate at the pluton borders, where flattening is more important. All these data allow us to interpret the Bielsa intrusion as being caused by coeval NNE shortening and dextral shear. The presence of shear bands indicates that strain localization under dextral transpression continued during late-Variscan times. q
Geodinamica Acta, 2003
The Bielsa thrust sheet is a south-verging unit of the Axial zone in the central Pyrenees. The Bielsa thrust sheet consists predominantly of a Variscan granite unconformably overlain by a thin cover of Triassic and Cretaceous deposits. During the Eocene-Oligocene, Pyrenean compression, displacement of the Bielsa thrust sheet generated a large-scale south-verging monocline. Low temperature deformation of the Bielsa thrust sheet resulted in the development of: (1) E-W trending, asymmetric folds in the Triassic cover with amplitudes up to 1.5 km; these folds of the cover are related with normal and reverse faults in the granite and with rigid-body block rotations. (2) Pervasive fracturing within the Bielsa granite is also attributed to Pyrenean deformation and is consistent with a NNE to ENE shortening direction; two main, conjugate fault systems are associated with this direction of shortening, as is a subvertical strike-slip system with shallow-plunging slickenside lineations and a moderately dipping fault system with reverse movement; and (3) in addition, we recognise strike-slip and reverse shear bands, associated with sericitisation and brittle deformation of quartz and feldspar in the granite, that enclose Triassic rocks. Basement deformation within the Bielsa thrust sheet can be related to movement of faults developed to accommodate internal deformation of the hanging wall. Several models are proposed to account for this deformation during the southward displacement of the thrust.
Tectonophysics, 1999
Granitic intrusions typically have large aspect ratios, with an horizontal major axis about 4-8 times the vertical one. Combined structural and geophysical (gravity) data allows distinction between two main types of plutons. Flat-floored plutons (the most represented) are rather thin (3-4 km) and extend in every horizontal direction with a gently dipping floor toward several root zones. These contrast with the thick (>10 km) wedge-shaped plutons, more elongated in one direction with a few root zones. We interpret the shape of flat-floored plutons as the result of a switch in the stress pattern caused by the emplacement of magma. Magma is preferentially emplaced into the plane (¦ 1 -¦ 2 ) perpendicular to the least principal stress component (¦ 3 ). This plane is initially vertical, except for compressional conditions. This dilation causes a local re-organization of the stress field, by increasing the minor and intermediate principal stress components. When they overcome the lithostatic load, a drastic change in the orientation of the opening plane results, switching from vertical to horizontal. This constitutes a change from vertically-oriented, dike-shaped intrusions to sub-horizontal laccoliths. Crustal anisotropies (vertical faults or horizontal zones of different rheology) also contribute to modify the shape of the intrusions. The continual exchange of the intermediate stress components also explains the rounded shape of intrusions in strongly extended regions. Our analysis suggests that the concept of magmas rising to a level of neutral buoyancy is not applicable to many settings. Rather, we suggest that the feedback between displacements resulting from magma intrusion and the local stress pattern controls the geometry of magma emplacement.