Evolução tectônica da Zona de Cisalhamento Caxambu, MG
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The NE-SW trending Caxambu Shear Zone (CSZ) is an example of a group of similar shear zones that crop out in southern Minas Gerais and adjacent Rio de Janeiro states. They represent the final stage of the Neoproterozoic tectonic activity related to the Brasiliano Orogeny in this region. It is a dextral shear zone with predominantly ductile behavior. The CSZ affects metaigneous and metasedimentary rocks of a Paleoproterozoic basement with its Neoproterozoic cover denominated Andrelândia Megasequence (AMS). Six mapping units were recognized: orthogneiss (basement), banded biotite gneiss (AMS), quartzite (AMS), biotite schist (AMS), garnet muscovite schist (AMS) and a plug of alkaline intrusive rocks of probable Cretaceous age. Two groups of structures were recognized in the area. The first group, attributed to deformation phase DB, related to the Brasília Belt, includes the main foliation and associated lineation and extensive nappe structures with tectonic transport top to the east. The second group, related to the Ribeira Belt and labeled DR, includes the Caxambu Shear Zone. The CSZ is more then 100 km long and about 2.5 km wide; its dextral horizontal displacement in the studied area is estimated at about 14.5 km. Including published estimates from adjacent areas an average value of 17.6 km is obtained, resulting in an approximate average value for γ of simple shear of seven. Associated to the CSZ mylonitic rocks were formed containing the following shear sense indicators: mica fish and other minerals with similar shape and asymmetry, oblique foliation, S-C structures, mantled porphyroclasts and asymmetric microfolds. The metamorphic conditions during mylonitization are estimated at lower amphibolite facies.Keywords:
Mylonite
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Pure shear
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Mylonite
Tectonite
Lineation
Main Central Thrust
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Previous studies (Drewes, 1974; Lingrey, 1982), as well as this study, included measurements of various foliations and lineations in the penetratively deformed rocks of the eastern Rincon Mountains (see also Davis, 1980). All resulting data were compiled into a Microsoft Excel spreadsheet and plotted on equal-area, lower hemisphere stereonets using GEOrient graphing and statistical analysis software developed by Rod Holcombe. San Pedro detachment fault. Mylonitic foliation and lineation are associated with the San Pedro detachment fault only near the western edge of the Happy Valley 7 ½' Quadrangle. This is interpreted as a consequence of an east-to-west increase in paleodepth and corresponding paleotemperature during initial extensional shearing along the San Pedro detachment fault and its down-dip continuation as a mylonitic shear zone. Foliation and lineation measurements (n=10 and 11, respectively; Fig. 1A) indicate early extension oriented 4° 075°. However, restoration of presumed regional eastward tilting associated with isostatic rebound of the fault footwall during Oligo-Miocene tectonic exhumation (e.g., Spencer, 1984) would result in gentle westward rather than eastward plunge for the lineations at the time of mylonitic shearing.
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Mylonite
Quadrangle
Detachment fault
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Lineation
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The region discussed in this paper lies in Chester County, Pa., and is included in the eastern half of the Coatesville quadrangle. (See fig. 3.) It is within the belt of crystal-line schists and gneisses of the Piedmont Plateau. The northern half of the area, which will be called the Doe Run region, from the village of that name (see Fig. 4, p. 15), has been surveyed by Eleanora F. Bliss in connection with the problem of the relation of the Wissahickon mica gneiss to the Octoraro schist.
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ABSTRACT In Central Asia, thrusts and shear zones resulting from Palaeozoic accretional events were reworked by E–W‐trending ductile strike‐slip faults during late Palaeozoic–early Mesozoic time. In the Tianshan range, microstructures and quartz C‐axis fabrics show a main dextral shearing associated with sinistral localized shear zones. The relationship between these conjugate structures indicates a NNW–SSE‐trending bulk shortening. In the Chinese Altay mountains, the existence of δ‐type microstructures in an important sinistral mylonitic zone infers high rates of deformation. This shear zone is bordered by a late dextral ductile fault synchronous with a granite emplacement. Field evidence and datings from the literature provide chronological constraints. In the late Carboniferous, the sinistral mylonitic deformation took place in the Erqishi–Irtysh shear zone in the northeastern part of Xinjiang and in Kazakhstan. During the Early Permian, a regional dextral event occurred in the Tianshan range and under the whole of northern Xinjiang.
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Mr. Geo. G. Holmes and others have observed the remarkable and highly suggestive fact that not only do the schists (which contain in many places interlaminated beds of quartzites and massive crystalline limestone, e.g. in the Dwarsberg on the Magalakwin River in the Northern Transvaal) appear interbedded in the gneisses, but that the strike foliation and planes of schistosity of these old schists and of the gneisses seem invariably to be parallel.
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Echelon formation
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Lineation
Mylonite
Muscovite
Foliation (geology)
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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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Foliation (geology)
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