Porphyroblast inclusion‐trail orientation data: eppure non son girate*!
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Abstract:
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non‐coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens.Keywords:
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The subject of the present contribution is the analysis of deformation in the Fellos ductile shear zone, which crops out in the island of Andros. Fellos shear zone (FSZ) is an outcrop scale NNE –striking structure with a total structural thickness of, approximately, 200 m. The central main domain of the shear zone is defined by mylonitized metapelitic rocks, bounded by strongly deformed meta-basic and meta–ultrabasic rocks both on top and bottom. Mylonitic rocks in the shear zone can be classified as S to S – L tectonites. Structural mapping revealed that the Fellos shear zone resulted from the complete transposition of a gently inclined synform the axial plane of which is orientated sub – parallel to the mylonitic foliation. The rheological contrasts between the core of the shear zone and its margins in conjunction with the structural framework of a transposed synform are first order influences to the localization of deformation. The map scale pattern of the stretching lineations in the shear zone shows that the lineation swings from a NNE orientation, trending parallel to the strike of the shear zone, to an ESE orientation. This variation shows that transport orientation in the shear zone is spatially partitioned into strike parallel and strike normal movements. This kinematic partitioning in the Fellos shear zone is a characteristic feature of transpressional high strain shear zones. The recognized partitioning was further investigated by examining the pattern of quartz [c]-axes fabrics in quartz veins oriented parallel to the mylonitic foliation as well as in quartz –rich mylonites. Quartz [c] axes fabric diagrams shows variations in their topology at different structural levels of the shear zone and distinct differences with the [c] axes patterns of monoclinic strain symmetry. Distinct feature, especially at the uppermost structural levels of the shear zone, is the point maxima clustering of the [c] axes in peripheral position of the crystallographic diagrams. At the base of the shear zone, in the rheological interface of metapelites with metabasites, the fabric diagrams from neighboring quartz vein specimens shows consistently small circle girdle pattern. This observation denotes localization in the geometry of deformation (flattening) at this lithological contact. Finite strain analysis shows that Rxz varies from 2.1 to 6.45, while the Flinn parameter range between 0.01 and 0.93, supporting quantitatively the flattening geometry of strain. Flattening strain is another diagnostic criterion of transpressional shear zones.
The means to constrain the compositional features of the mylonitic rocks was mineral chemistry and petrologic analysis. Petrographic examination, specifically, of a metabasite outcrop from the base of the shear zone revealed, in thin section scale, two lithologic types : (a) Garnet blueschist and (b) Epidote blueschist. The mineral chemistry of zoned garnets and amphiboles from these petrographic types has been examined using SEM/EDS. The microchemical analysis revealed three different zoning patterns in the amphiboles which are characterized by (a) dark green Ferro – hornblende cores with crossite rims (b) Amphiboles with compositional zoning and alternations of blue (Ferro – glaucophane) and green amphiboles (Ferro – barroisite) and (c) Isolated grains with crossite cores and riebeckite rims. Compositional mapping of zoned garnets witnessed fractionation of manganese to the cores, which is the Prima facie evidence of growth zoning. In addition, the almandine rims of the zoned garnets are in textural equilibrium with blue amphiboles, observation which implies that these garnets nucleated during the HP event.
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The Cloncurry Fault Zone is a north-northwest-trending zone of complex deformation over 100 km long and up to 7 km wide near the eastern edge of the Mt Isa Inlier. The zone includes mylonites in an anastomosing shear-zone system with variably plunging mineral lineations within a north-northwest subvertical girdle that formed synchronously with north-northwest- and south-southeast-plunging folds. No clear overprinting of lineations in different orientations is observed, and the complexity of the penetrative fabrics can be attributed to strain partitioning during east-northeast contraction, rather than requiring a more complex history of overprinting relations. The mylonites formed at temperatures of 350–500°C, below the ∼650°C metamorphic peak that occurred during the regional D2 event, and they are superimposed on Maramungee-aged granites (1555–1545 Ma), implying that the majority of fabrics formed during D3. D3 was followed by the development of a D4 sinistral Riedel strike-slip fault system involving east-southeast contraction, which was coincident with massive Na–Ca alteration and brecciation within the zone. Reactivation with a normal component of movement occurred some time after the Jurassic. The Cloncurry Fault Zone is a crustal-scale feature of the Mt Isa inlier that records strain partitioning and a deformation history lasting over 1.5 Ga.
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Abstract Differences in structural overprinting of rocks east and west of a Sveconorwegian shear zone, the Mylonite Zone, Värmland County, SW Sweden, reveal that this part of the Southwest Scandinavian Domain should be subdivided into different structural domains. We here distinguish the Southwest Scandinavian Domain of eastern Värmland to be affected by Sveconorwegian ductile deformation mainly, where the most prominent structural elements are a foliation and mineral lineations, one of which is associated with open, roughly upright, N–S trending folds. Rocks of western Värmland have a pre-Sveconorwegian deformation history characterized by stromatic veining and isoclinal folding. Subsequent Sveconorwegian thrusting overprints these structures in localised shear zones. One of these is the Mylonite Zone, which roughly marks the eastern border of Gothian folding in this area.
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