Brittle–ductile–brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps)
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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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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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