An outline of shear-sense analysis in high-grade rocks
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Abstract A 3‐4 km wide shear zone, here named the Anita Shear Zone, runs parallel to the west coast of northern Fiordland and is marked by amphibolite facies mylonites and intensely deformed gneisses. This shear zone marks a tectonic boundary between the metasediments and meta‐granites of the Tuhua Sequence and the gneisses of the Arthur River Complex and Western Fiordland Orthogneiss. Detailed structural analysis indicates that the Anita Shear Zone has been folded and reoriented into its current steep northeast trend by late folds associated with steeply dipping retrograde shear zones. Removing the effects of the overprinting folds reveals that the Anita Shear Zone was originally subhorizontal or shallowly dipping. The Anita Shear Zone is very similar in many aspects to a middle Cretaceous shear zone in Doubtful Sound which has formed along the same lithological boundary. Both shear zones separate the Tuhua Sequence from the Western Fiordland Orthogneiss, indicating that there is no intrusive contact between these two units as has been previously suggested. Keywords: structuremetamorphismmyloniteshear zoneCretaceousJagged GneissAnita UltramafitesTuhua SequenceWestern Fiordland OrthogneissArthur River ComplexFiordland
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"A 1.7 Ga old thrust and Precambrian low angle faults; their implications for Caledonian tectonics." Geologiska Föreningen i Stockholm Förhandlingar, 110(4), p. 419
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Crustal-scale faults have been argued to represent relative and absolute zones of weakness in
comparison to the intact continental crust due to their preferential reactivation and accumulation of
strain. In order to understand the long term deformation behaviour of crustal-scale faults, it is
important to study how deformation processes in the mid-crustal frictional-viscous transition zone
(FVTZ) can alter the frictional strength of the crust. Exhumed fault rocks from the Dover Fault
Shear Zone (DFSZ) record evidence of long-term weakening mechanisms. The DFSZ in north
eastern Newfoundland represents a major Appalachian terrane boundary that separates highly
metamorphosed gneisses in the Gander Zone from deformed volcanics in the Avalon Zone.
Analysis of field data, hand specimens and microstructures revealed a series of progressively lower
temperature, overprinting deformation phases in increasingly narrower, localised shear zones. The
fault rocks show increasing strain towards the boundary as grain size reduces, fabric intensifies and
folds tighten and become progressively curvilinear. Evidence of fluid influx during deformation
includes microstructures that are indicative of fluid assisted diffusive mass transfer (DMT) and a
high degree of phyllonitization of the fault rocks.
Increasing strain and structural overprinting towards the centre of the shear zone is indicative of
strain weakening and the later brittle faulting that has reactivated the DFZ is evidence of this long
term weakening. The most important weakening mechanisms to have affected the DFSZ arose from
the syn-tectonic influx of fluids, including both hydrous fluids and magmas, as this led to
production of phyllosilicates in reaction softening, the development of interconnected weak layers
and thermal perturbations in the fault zone. These processes produced a highly localised network of
shear zones whose frictional strengths were permanently reduced, thus impacting the long-term
strength and behaviour of the fault in the upper crust.
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Abstract On the northern shore of Lake Vänern, southern Värmland, southwestern Sweden, various orthogneisses are affected by a major, steeply dipping shear zone, the Hammarö Shear Zone. It is several kilometres wide and comprises a network of individual shear zones (less than 100 m wide), which strike approximately E‐W across the Eastern Segment of the Southwest Scandinavian Domain in the Fennoscandian Shield, from the major Mylonite Zone in the west to the northeastern shore of Lake Vänern. Three fold phases, extensive dynamic and static recrystallisation, and a conspicuous, gently plunging lineation are the main features of the Hammarö Shear Zone. Early deformation occurred under amphibolite‐facies conditions, but narrow zones with a green‐schist‐facies assemblage demonstrate a lower grade of metamorphism during the late stages. Orthogneisses are variably overprinted by the shear deformation. No indisputable supracrustal rocks are found adjacent to the zone, and within the zone there is an extensive tectonic overprinting on primary and pre‐shear zone structures. The shear zone is intimately related to large‐scale regional folding (F4) which also occurs south of Lake Vänern. The steep mylonitic foliation and the horizontal stretching lineation are interpreted to be the result of the final stages of horizontal E‐W extension of the crust during the Mesoproterozoic D4 event. Berglund, J., Larson, S.Å. & Vinnefors, A., 1997: Sveconorwegian extension‐parallel deformation structures; an example from the Hammarö Shear Zone, SW Sweden. GFF, Vol. 119 (Pt. 2, June), pp. 169–180. Stockholm. ISSN 1103–5897.
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