Structural and temporal evolution of a reactivated brittle–ductile fault – Part I: Fault architecture, strain localization mechanisms and deformation history

Torgersen, Espen; Viola, Giulio

doi:10.1016/j.epsl.2014.09.019

Cited by 35 publications

(46 citation statements)

References 62 publications

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“…However, it has also been clearly demonstrated in a range of natural examples that authigenic 2M 1 illite can form in fault gouges derived from metamorphic or magmatic protoliths that were initially devoid of such illite (Zwingmann and Mancktelow 2004;Zwingmann et al 2010a;Viola et al 2013;Bense et al 2014;Song et al 2014;Torgersen and Viola 2014;Torgersen et al 2014a). In particular, Viola et al (2013) have recently described a clay fault gouge in the Palaeoproterozoic basement of Finland that contained almost exclusively 2M 1 illite/muscovite as authigenic clay, developed from a protolith gneiss that contained no illite and\4 % protolith muscovite.…”

Section: Discussionmentioning

confidence: 99%

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

et al. 2015

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The Silltal Fault is the northern brittle continuation of the Brenner Fault Zone, marked by a narrow zone of cataclasis and, in three locations, clay-rich fault gouges. The clay mineral composition of these gouges is dominated by higher temperature 2M 1 polytype illite/muscovite, with no 1M/1M d illite or mixed layer illite/ smectite detected. Smectite is limited to the northern samples from the Stephansbrücke location, whereas chlorite is present in all samples. The K-Ar ages from the different sample size fractions (\0.1, \0.4, \2, 2-6, 6-10 lm, ''whole rock gouge'') show a wide spread, from ca. 115 to 12 Ma, with ages consistently decreasing with grain size. Although the ranges overlap, ages from the northern Stephansbrücke samples are generally older (115-36 Ma) than those from the south near Matrei (55-12 Ma), possibly reflecting increasing regional metamorphic temperatures to the south. The well-defined linear relationship between apparent age and hydrogen stable isotope (dD) values establishes a direct correlation between rejuvenation of the K-Ar system and increased interaction with meteoric water introduced and focussed within the fault zone. The dependence of both apparent age and dD on grain size also indicates that radiogenic and stable isotope exchange was controlled by grain size, reflecting new 2M 1 illite growth, mechanical grinding of protolith muscovite during cataclastic faulting, or both. The results demonstrate the advantages of combining radiogenic and stable isotope analysis in interpretation of K-Ar ages from clay fault gouges. The combined approach was necessary to establish the crucial influence on apparent K-Ar ages of meteoric water influx focussed on the fault zone and its interaction with clay-size-fraction grains.

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Section: Discussionmentioning

confidence: 99%

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

et al. 2015

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“…Development of volumetrically small gouges along the fracture planes is also envisaged (Stünitz et al, 2003; Fig. 11c, insert).…”

Section: Strain Accommodation History Within the Different Domainsmentioning

confidence: 99%

Brittle–viscous deformation of vein quartz under fluid-rich lower greenschist facies conditions

et al. 2015

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Abstract.We studied by Electron BackScatter Diffraction (EBSD) and optical microscopy a coarse-grained (ca. 0.5-6 mm) quartz vein embedded in a phyllonitic matrix to gain insights into the recrystallization mechanisms and the processes of strain localization in quartz deformed under lower greenschist facies conditions, broadly coincident with the brittle-viscous transition. The vein deformed during faulting along a phyllonitic thrust of Caledonian age within the Porsa Imbricate Stack in the Paleoproterozoic Repparfjord Tectonic Window in northern Norway. The phyllonite hosting the vein formed at the expense of a metabasaltic protolith through feldspar breakdown to form interconnected layers of fine, synkinematic phyllosilicates. In the mechanically weak framework of the phyllonite, the quartz vein acted as a relatively rigid body. Viscous deformation in the vein was initially accommodated by quartz basal slip. Under the prevailing deformation conditions, however, dislocation glide-and possibly creep-accommodated deformation of quartz was inefficient, and this resulted in localized strain hardening. In response to the (1) hardening, (2) progressive and cyclic increase of the fluid pressure, and (3) increasing competence contrast between the vein and the weakly foliated host phyllonite, vein quartz crystals began to deform by brittle processes along specific, suitably oriented lattice planes, creating microgouges along microfractures. Nucleated new grains rapidly sealed these fractures as fluids penetrated the actively deforming system. The grains grew initially by solution precipitation and later by grain boundary migration. We suggest that the different initial orientation of the vein crystals led to strain accommodation by different mechanisms in the individual crystals, generating remarkably different microstructures. Crystals suitably oriented for basal slip, for example, accommodated strain mainly viscously and experienced only minor fracturing. Instead, crystals misoriented for basal slip hardened and deformed predominantly by domainal fracturing. This study indicates the importance of considering shear zones as dynamic systems wherein the activated deformation mechanisms may vary through time in response to the complex temporal and spatial evolution of the shear zone, often in a cyclic fashion.

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“…In the western part of the window, the Lomvatna Formation is in a parautochthonous position, tightly to iso clinally folded and is characterised by the complete rejuve nation of mica K-Ar ages (at approximately 430-410 Ma; Dallmeyer et al, 1988). Although the Palaeoproterozoic rocks of the RTW are gene rally little affected by Cale donian deformation, recording subgreenschist-facies conditions (Dallmeyer et al, 1988) and still preserve primary sedimentary structures, significant Palaeozoic localised de formation was accommodated in the northwestern part of the window (Torgersen & Viola, 2014;Kjøll et al, 2015). Here, a series of discrete thrusts developed on the limbs of Palaeo proterozoic folds (Pharaoh et al, 1983;Gayer et al, 1987), defining a large compressional imbricate stack, the Porsa Imbricate Stack (Torgersen & Viola, 2014;Torgersen et al, 2014b; the Porsa Duplex of Gayer et al, 1987).…”

Section: Basaltic Lavasmentioning

confidence: 99%

“…Although the Palaeoproterozoic rocks of the RTW are gene rally little affected by Cale donian deformation, recording subgreenschist-facies conditions (Dallmeyer et al, 1988) and still preserve primary sedimentary structures, significant Palaeozoic localised de formation was accommodated in the northwestern part of the window (Torgersen & Viola, 2014;Kjøll et al, 2015). Here, a series of discrete thrusts developed on the limbs of Palaeo proterozoic folds (Pharaoh et al, 1983;Gayer et al, 1987), defining a large compressional imbricate stack, the Porsa Imbricate Stack (Torgersen & Viola, 2014;Torgersen et al, 2014b; the Porsa Duplex of Gayer et al, 1987). The impact of the Cale donian orogeny on the RTW may, however, have been more significant than commonly assumed if the suggestion that the RTW is a far-travelled allochthonous basement horse within the Caledonian nappes is correct (e.g., Gayer et al, 1987;Rice, 2013).…”

Section: Basaltic Lavasmentioning

confidence: 99%

“…White stippled lines represent major lithological and structural contacts. Viola, 2014). In the fold hinges, however, a basal polymict conglomerate is locally preserved between the Nussir and Porsa groups, indicating that the Porsa sediments were possibly deposited on an erosional surface on top of the in the Repparfjord volcano-sedimentary succession (Fig.…”

Section: Geological Background and The Classic Stratigraphy Of The Rementioning

confidence: 99%

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Revised structure and stratigraphy of the northwestern Repparfjord Tectonic Window, northern Norway

Torgersen¹,

Viola²,

Sandstad³

2016

NJG

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The Repparfjord Tectonic Window (RTW) consists of a ~8 km-thick low-grade, weakly deformed Early Palaeoproterozoic volcano-sedimentary succession that hosts abundant Cu mineralisations. Two major, sediment-hosted, Cu-deposits occur in the Saltvatnet Group, a <6 km-thick sequence of mainly coarse-grained clastic rocks that forms the core subject of the present study. Results from new mapping, structural analysis and whole-rock geochemistry from the northwestern part of the RTW are presented and used in association with high-resolution geophysical data to propose a revised stratigraphy of the Repparfjord volcano-sedimentary succession and its tectonic and geodynamic settings. The Saltvatnet Group is better characterised as containing four formations, now including also the uppermost volcaniclastic siltstones and dolostone members of the herein newly defined Gorahatjohka Formation. Field oberservations and structural analysis show that the current geometry of the Saltvatnet Group is that of a km-scale, upright, NE-SW-trending anticline, the Ulveryggen anticline, cored by the Saltvatnet Group, which thus represents the lowermost exposed rocks of the RTW. New and existing geochemical data are used to demonstrate that the two volcanic-dominated Nussir and Holmvatnet groups, which crop out on either side of the Ulveryggen anticline, have similar geochemical character and can be interpreted as temporal and lateral correlatives. The basaltic Ulveryggen intrusions within the Saltvatnet Group are geochemically different from any other igneous rocks in the RTW, suggesting that these intruded prior to the volcanic-dominated groups in the RTW. The revised stratigraphy of the RTW is interpreted as reflecting deposition and volcanism in an evolving continental back-arc, which contrasts with the purely intraplate rift setting inferred for most parts of the Fennoscandian Shield in the Early Palaeoproterozoic.

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Structural and temporal evolution of a reactivated brittle–ductile fault – Part I: Fault architecture, strain localization mechanisms and deformation history

Cited by 35 publications

References 62 publications

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

Brittle–viscous deformation of vein quartz under fluid-rich lower greenschist facies conditions

Revised structure and stratigraphy of the northwestern Repparfjord Tectonic Window, northern Norway

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