Kinematic analysis of the southern Iberian shear zone and tectonic evolution of the Acebuches metabasites (SW Variscan Iberian Massif)
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Abstract:
The Acebuches metabasites constitute a slice of oceanic crust located at the Variscan suture between the Ossa Morena and the South Portuguese zones (Iberian Massif). The emplacement of the metabasites onto the accretionary prism was associated with the activity of the southern Iberian shear zone. Careful measurements of the fabrics and structures related to this shear zone show consistent kinematic criteria pointing to the presence of a simple shear component, with an oblique predominantly left‐lateral sense of displacement. The data are consistent with existing models of triclinic or oblique shear zones considering coeval simple shear and pure shear components. These models provide a satisfactory explanation of the observed spatial variations in the foliation, lineation and fold hinge data. The kinematics of other shear deformations, older and younger than the southern Iberian shear zone, have also been established. These results offer new insights into the tectonic evolution of this suture of the Variscan belt, and show how complex transpressional strains can arise during oblique crustal deformation associated with the motions of tectonic plates.Keywords:
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Accretionary wedge
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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Finite strain theory
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Mylonite
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Indian Shield
Charnockite
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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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Foliation (geology)
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Main Central Thrust
Mylonite
Muscovite
Tectonite
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