Strain localization along a detachment system: Deformation of natural dolomitic and calcitic mylonites (Mt. Hymittos, Attica, Greece)
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<p>Carbonate rocks can be thick, mineralogically-homogeneous packages, which accomodate strain in orogenic belts. Despite its contribution to rock strength, the deformation of dolomite as a major rock forming mineral is understudied in comparison to calcite, quartz, and feldspar. We use field, petrographic, and electron back scatter diffraction (EBSD) analyses of dolomitic and calcitic marbles to investigate the response of these rocks to different degrees of strain under greenschist facies. Mt. Hymittos, Attica, Greece, preserves a pair of Miocene top-SSW ductile-then-brittle low-angle normal faults dividing a tripartite tectonostratigraphy. The bedrock of the massif comprises sub-greenschist facies phyllites and marbles in the uppermost hanging wall unit, and high-pressure greenschist facies schists and marbles of the Cycladic Blueschist Unit in the lower two packages. Ductile mylonites in the footwalls of both detachments grade into brittle-ductile mylonites and finally into cataclastic fault cores. The dolomitic and calcitic marbles of the lower units deformed under greenschist facies conditions and their fabrics reflect the relative differences in strengths between these two minerals. In the middle tectonostratigraphic unit, dolomitic rocks are brittlely deformed and calcitic marbles are mylonitic to ultramylonitic with recrystallized grain sizes ranging from 55 to 8 &#956;m. Within the lower package, dolomitic and calcitic rocks are both mylonitic to ultramylonitic with previous P-T data suggesting metamorphism at ~470 &#176;C and 0.8 GPa. EBSD analysis of six dolomitic marbles of the lower unit reveals a progressive fabric evolution from mylonites to ultramylonites reflecting the magnitude of strain and decreasing temperature of deformation. In mylonitic domains, average grain diameters range from 70 to 25 &#956;m. The mylonitic dolomite exhibits low-angle grain boundaries, internal misorientation zones and textures suggestive of subgrain-rotation recrystallization. This mylonitic fabric is crosscut by ultramylonite bands of dolomite with grain diameters of 15 to 5 &#956;m, which overlaps with the dominant grain size of the subgrains formed within the mylonitic domains. In samples closer to the fault core, the ultramylonite fabric is predominant though boudinaged veins, and relict mylonite zones with coarser grains may still be observed. Uniformly ultramylonitic dolomitic marbles exhibit grain diameters of 40 to 5 &#956;m; the majority of grain diameters are less than 15 &#956;m. The ultramylonite bands have low degrees of internal misorientation and an absence of low-angle grain boundaries that, along with correlated misorientation diagrams, suggest the ultramylonitic dolomite grains are randomly oriented and deforming via grain-boundary sliding. Interstitial calcite grains within these samples may reflect creep-cavitation processes interpreted to have occurred syn-kinematically with grain-boundary sliding. The change from subgrain-rotation recrystallization to grain-boundary sliding is interpreted to reflect the interplay of grain-size sensitive and insensitive processes. Following grain size reduction, subsequent deformation was dominantly accommodated by grain boundary sliding. The dolomitic marbles of the lower unit deformed on the retrograde path from the high-pressure, mid-temperature portion of the greenschist facies. The position of the dolomitic ultramylonites immediately below the cataclastic detachment fault suggest these ultramylonites were deforming very close to the brittle-ductile transition suggesting ductile deformation at lower temperatures than might be predicted by deformation experiments.</p>Keywords:
Mylonite
Greenschist
Cataclastic rock
Blueschist
Protolith
Abstract Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure–temperature ( P–T ) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 ± 16 to 619 ± 32°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c . 32 Ma, with one sample showing rejuvenation at c . 27 Ma. We conclude that the high‐ P rocks from south Naxos were exhumed to upper mid‐crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c . 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid‐controlled metamorphic re‐equilibration, and is unrelated to the strong high‐ T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.
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New exposures of Monian schists in SE Anglesey reveal a transition from weakly foliated, coarse, porphyroblastic actinolite greenschist to intensely foliated, fine-grained blueschist dominated by epidote and sodic amphibole. Textures are interpreted as recording an initial, near-static greenschist recrystallisation of a basic plutonic igneous rock, followed by deformation during continuing metamorphism at greenschist, then blueschist, grades. The sodic amphibole ranges in composition from glaucophane to ferro-glaucophane and crossite. A balanced reaction expressing the greenschist to blue-schist transition is:
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Greenschist
Actinolite
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Greenschist
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Cataclastic rock
Mylonite
Lineation
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Breccia
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Greenschist
CYCLADES
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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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Detachment fault
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Greenschist
Phengite
Protolith
Amphibole
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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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Abstract The Cascine Parasi Mélange (CPM) of the high-pressure, meta-ophiolitic Voltri Massif (Ligurian Western Alps), consists of a foliated chlorite-actinolite greenschist matrix enclosing lenses of metabasites and metasediments. The surrounding units consist of serpentinites not enclosing these metamorphic rocks. The matrix records three sets of folds: (i) Dm1/Dm2 (blueschist to greenschist-facies conditions), which can be correlated to folds in the metasedimentary blocks; (ii) Dm3, which are the most obvious in the field and which partially re-orient the previous structures. The metabasite lenses preserve internal High-Pressure (HP) schistosities unrelated to the matrix foliation. The lenses equilibrated at different peak metamorphic conditions (ranging from eclogite- to blueschist-facies) and some recorded the prograde transition from lawsonite-bearing assemblages to garnet blueschists. Individual lenses display different segments of typical subduction PT paths which apparently converge in the blueschist facies. A late stage greenschist-facies re-equilibration is particularly widespread at the rims of the HP lenses. These structural and metamorphic features suggest that the mélange was active during early phases of the structural evolution of the area, at least through the exhumation and emplacement of the HP blocks into shallower crustal levels at conditions transitional from blueschist- to greenschist-facies; the older history is only preserved inside the blocks. Keywords: tectonic mélangehigh-pressure metamorphismLigurian Alpsexhumation
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