An allochthonous cover in northern Vestranden, Western Gneiss Region, central Norway
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Abstract Abstract The present studies in the northwesternmost part of the Western Gneiss Region, Norway, have demonstrated the existence of previously unknown metasediments and metavolcanics in the Caledonian nappe units. Tectonic and metamorphic breaks as well as lithological differences separate these units from the underlying migmatized gneissic granites and granodiorites. Structural concordance between basement and cover developed as a result of high strain induced by nappe translation. An intense interfolding of basement and cover units postdated the thrusting event. Metamorphic and lithological contrasts in the area are generally preserved. Garnet-biotite thermometry data suggest that the temperatures during post-thrusting metamorphism did not exceed 600°C. Later movements were cataclastic in character and often occurred parallel to older thrust planes. Key Words: Allochthonous coverWestern Gneiss RegionNorwaystructural concordanceCaledonidesinigmatized granitoidsUlPbagecalc-silicatesmarblerootless dykesmylonitesamphibolite faciesgeothermometrylow-grade metamorphismKeywords:
Basement
Cataclastic rock
Mylonite
Analyses of intact orientated samples of incompetent fault rocks from thrusts within the Magura nappe in the Beskid Wyspowy Mountains indicate an arrangement of clay mineral plates parallel to numerous shears present in the rocks. Reconstruction of the development of the shears suggests several phases of formation. Maximum palaeotemperatures in the range 80 - 160°C were reconstructed from the fault zones.
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Recent field survey in eastern Dabie Mountain has revealed extensive existence of pseudotachylite. The pseudotachylite tends to occur in the NE-SW trending fracture zones or shear zones which parallel the Tan-Lu fault zone and cut the early-Cretaceous granites. Mylonitic bands are developed in some pseudotachylites. This paper mainly discusses the characteristics of microstructures between the pseudotachylites and their host rocks through observed mainly by optical microscope and SEM imaging. The microstructure characteristics of pseudotachylites and their respective wall rocks indicate that the pseudotachylites were formed mainly by ultracataclasis of wall rocks. Nevertheless, a small amount of corrosion borders along the margins of some porphyroid crystals of feldspar, plagioclase and pyrite from the SEM imaging of the sample Loc.1 suggest that there might have somewhat existed melt du-ring the pseudotachylite generation. The macro-mylonitic quartz bands have been proved to be the mylonitic foliation developed in early mylonitic breccias. The superimposition of microstructures shows the multi-stage development of the pseudotachylite-bearing fault zone and the pseudotachylite itself. In addition, the veins show features of progressive fracturing of the wall rocks interspersed with many rock fragments, which strongly suggests that antecedent cataclasis or pre-existing tectonic soften belt was a reworking and forming the pseudotachylite, so, pseudotachylites were formed in which they occurred. The deformation succession between the pseudotachylite and its wall rock generally exhibits the evolutionary process from ductile through ductile-brittle to brittle, which shows that the pseudotachylites were generally formed during the uplift of the orogenic belt instead of during the early subduction.
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Breccia
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Cataclastic rock
Mylonite
Lineation
Fault gouge
Breccia
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Detailed lithostratigraphic and structural mapping on the northwestern part of the island of Kea (Western Cyclades, Greece) revealed a major low-angle normal fault system dominated by both high- and low-angle extensional faults ‐ the Otzias Bay Detachment. It forms parts of a dome-shaped low-angle normal fault system which is characterised by SW/SSW-sense of shear and bends over the whole island, thus representing a Miocene extensional event. An early stage extension-related deformation phase encompasses ductile mylonitic processes within the metabasitic/calcitic footwall and ductile-brittle cataclastic conditions dominating the calcite/dolomite hosting fault-rock zone. As recorded by samples taken along a profile across the Otzias Bay Detachment structures give evidence for a variety of transitions between the mylonitic and cataclastic end-members such as mylonitic to cataclastic fabrics and also cataclastic fabrics overprinted by mylonitic SCC’-foliations. Within the cataclasite of the fault-rock zone fabric evidence for viscous /frictional interaction is observed. Due to a variation in frictional behavior broken-up components form angular fragments of various sizes and zones of very fine-grained material of intense micro-fracturing, as almost to a fault gouge. Some fragments act as clasts and become rounded components, in some parts they are even enveloped in a rim of fine-grained phyllosilicate. Different vein generations can be observed as ductilely rotated and folded as well as cataclastically deformed together with its host rocks. In later exhumation multiple low-angle cataclastic fault zones formed within, and (sub) parallel to a regional mylonitic ductile foliation. A system of minor and major sets of high-angle cross-cutting steep normal-faults is present. Acting as conduits for hydrothermal fluid infiltration, some of these host remnants of high fluid pressure events such as healed up joints of angular breccia and fluidised cataclastic injection veins. Different deformation mechanisms within the microstructural record along the Otzias Bay Detachment and the observed cross cutting relationship between frictional dominated and viscous dominated deformation suggests, that it acted as a sub-horizontal extensional fault within the brittle/ductile transition zone with a switch between a velocity strengthening and velocity weakening regime occurring several times during ongoing extension.
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Abstract A microstructural investigation of cataclastic fault rock evolution from a quartzite with an original mylonitic microstructure is reported. The fault rocks produced range from clast dominated microbreccias to matrix dominated ultracataclasites. The recrystallized grain size and the sub-grain size in the original mylonite appear to control the development of the fine-grained matrix in the microbreccias and cataclasites by focusing fracture along sub-grain and grain boundaries. The ultracataclasite generation involves further grain size reduction which is dominated by transgranular fracturing. The host rock clasts present in the fault zones show a significant increase in dislocation density indicating that a component of low temperature crystal plasticity is associated with the faulting. In addition the fault rocks show evidence of partial cementation by the growth of quartz and carbonate cements. The evolution of the fault rocks studied in terms of the clast size and the clast/fine-grained matrix ratios are not a simple function of the displacement magnitude.
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Cataclastic rock
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