Arrested tectonic development: Preservation of high-temperature fabrics in the sinistral Caiçara shear zone (Borborema province, northeastern Brazil)
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Mylonite
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Shearing (physics)
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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A complex mylonite zone contains rock types of varied mineralogy and a range in mylonitic textures, depending on rock type and strain history. The field structural geologists working in a complex mylonite zone is interested in (1) internal deformation in the zone and (2) regional displacement (tectonic transport) predicted by structures within the zone. It is common to determine a principal elongation direction or to determine a general shear sense from one or more of the numerous small-scale criteria available. Of interest here are the relationships among stretched grains, shear sense, regional displacement, and shear bands in button schist. Potential elongations between shear bands in button schist from the Brevard zones in North Carolina include: (1) chlorite tails on retrograded garnets, (2) pulled-apart grains, particularly epidote, garnet, pyrite, and rare feldspar, and (3) linear aggregates of quartz grains. Stereonet orientations of these lineations are dispersed along the average great circle for mylonite foliation orientation because of crenulations associated wit the shear bands but, in general, the lineations are concentrated near the strike of the foliation. In this case, finite elongations and shear sense criteria agree in giving a regional strike-slip style of tectonic transport and the local shear direction ismore » contained in a plane normal to both the mylonite foliation and the shear bands. The shear direction thus determined is a simple shear component of th actual particle displacement in the shear zone because a component of pure shear may be involved in the formation of shear bands.« less
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Mylonite zones are generally characterized by abrupt and very large strain transitions, which commonly result in excessively anastomosing schistosities on a wide range of scales when compared with non-mylonitic foliations. This geometry is very susceptible to remodification during progressive mylonitization, resulting in unusual and complex fold, lineation, and foliation geometries and interrelationships. Open folds of the mylonitic foliation with axes parallel to the stretching lineation in the surrounding mylonite cannot have formed by the rotation of fold axes through a large angle within their axial planes, as has been usually proposed for isoclinal and sheath folds in mylonitic zones. Open folds initiate with axes parallel or close to the stretching lineation due to the geometric effects of folding a mylonitic foliation, which anastomoses around an ellipsoidal pod of less deformed material. This initial geometry also allows the generation of fold axes curved within their axial plane through 180° about the stretching lineation at the time of nucleation. Successive mylonitic foliations develop during this folding and refolding process with boundaries that truncate and isolate earlier fold hinges and portions of fold limbs. As a consequence, stretching and intersection lineations can vary from plane to plane through the mylonite zone, although careful examination often reveals a weak overprinting stretching lineation parallel to the bulk movement direction for the whole zone. Fold asymmetry in mylonite zones is a potential indicator of shear sense across a zone, if the fold axes lie at an angle to the bulk stretching lineation direction. In such circumstances, however, a single asymmetry projected onto a plane perpendicular to the mylonitic foliation and containing the bulk stretching lineation can indicate either sense-of-shear depending on a variety of factors. These include whether the foliation folded is primary or mylonitic, and in the latter case whether the mylonite zone formed with a steep-dip and horizontal stretching lineation or in some other orientation. The most satisfactory sense-of-shear indicator is the asymmetry of S and C planes.
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