Strain variation associated with the development of a major recumbent curvilinear fold in the Dalradian rocks of NW Achill Island, western Ireland

ABSTRACTThe regionally metamorphosed, Riphean–Cambrian Argyll Group Dalradian rocks of NW Achill Island, western Ireland are disposed in a large-scale, regionally west-facing, tight, recumbent F2 curvilinear fold, with which two ductile shear zones are associated. Clasts in conglomerates within the Dalradian sequence that are deformed by the shear zones preserve evidence for a constrictional overprint of earlier plane strain as the fold became curvilinear, while stretched clasts maintained a constant orientation as the hinge curvilinearity developed. During the constrictional overprint a crenulation fabric, S2b, overprinted a penetrative foliation, S2a, in the shear zones. The S2b has an orientation that varies systematically with that of the fold hinge. It is inferred that, although the S2b surfaces initiated as a dip-slip fabric, there was an increasing degree of strike slip on these surfaces as the fold hinge approached parallelism with the direction of tectonic transport. It is possible that many curvilinear folds have an early history involving plane strain, but that increasing constrictional strain is intrinsic to the later stages of their development.

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Strain-partitioned dextral transpression in the Great Boundary Fault Zone around Chittaurgarh, NW Indian Shield

Geological Magazine ◽

10.1017/s0016756821000157 ◽

2021 ◽

pp. 1-15

Author(s):

Deepak C. Srivastava ◽

Ajanta Goswami ◽

Amit Sahay

Keyword(s):

Fault Zone ◽

Large Scale ◽

Damage Zone ◽

Shear Zones ◽

Strike Slip ◽

Indian Shield ◽

Boundary Fault ◽

The Core ◽

Ductile Shear Zones ◽

Ductile Shear

Abstract Delimiting the Aravalli mountain range in the east, the Great Boundary Fault (GBF) occurs as a crustal-scale tectonic lineament in the NW Indian Shield. The structural and tectonic characteristics of the GBF are, as yet, not well-understood. We attempt to fill this gap by using a combination of satellite image processing, high-resolution outcrop mapping and structural analysis around Chittaurgarh. The study area exposes the core and damage zone of the GBF. Three successive phases of folding, F1, F2 and F3, are associated with deformation in the GBF. The large-scale structural characteristics of the GBF core are: (i) a non-coaxial refolding of F1 folds by F2 folds; and (ii) the parallelism between the GBF and F2 axial traces. In addition, numerous metre-scale ductile shear zones cut through the rocks in the GBF core. The damage zone is characterized by the large-scale F1 folds and the mesoscopic-scale strike-slip faults, thrusts and brittle-ductile shear zones. Several lines of evidence, such as the inconsistent overprinting relationship between the strike-slip faults and thrusts, the occurrence of en échelon folds and the palaeostress directions suggest that the GBF is a dextral transpression fault zone. Structural geometry and kinematic indicators imply a wrench- and contraction-dominated deformation in the core and damage zone, respectively. We infer that the GBF is a strain-partitioned dextral transpression zone.

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Architecture of the continental lithosphere

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1986.0020 ◽

1986 ◽

Vol 317 (1539) ◽

pp. 5-12 ◽

Cited By ~ 18

Keyword(s):

Large Scale ◽

Shear Zones ◽

Continental Lithosphere ◽

Fault Rocks ◽

Plate Motions ◽

First Approximation ◽

Fracture Systems ◽

Ductile Shear Zones ◽

Lithospheric Plates ◽

Internal Deformation

The continental lithospheric plates are traversed by numerous large-scale dislocations, many of which date back to late Archean or Proterozoic times. These major dislocations, often extending for 1000 km or more, are commonly characterized not only by the presence of fault rocks and by significant misfits between the adjacent crustal blocks, but also by the presence of localized basins of deposition, or by anomalous igneous suites, or mineral deposits. Although to a first approximation they may be regarded as narrow linear features, most consist of complex arrays of faults, ductile shear zones and fractures; some with associated deformation spread over tens and hundreds of kilometres. Lineaments that originated as within-plate transcurrent shear zones or rift systems are generally steep and many of these probably extend down through the base of the crust into lithospheric mantle. Lineaments that originated as sutures along which formerly separated crustal blocks have been welded together are, on the other hand, sometimes expressions of low-angle dislocations not continuous with mantle structures directly beneath the lineament observed at the surface. Geological records show that displacements or magmatic activity (or both) along many lineaments were repeated over long periods spanning successive changes in the global tectonic regime. Modern lineament movements can be fitted to plate tectonic hypotheses. However pre-Phanerozoic movements are more conjectural: for example, early Proterozoic lineaments appear to be associated with more extensive internal deformation within large crustal blocks. The siting of fracture systems activated in response to regional plate motions has, on occasion, been determined by the occurrence of pre-existing deep dislocations. The scale and longevity of the deep dislocations raise many interesting general questions with respect to the coordination of mantle and crustal activities and to the evolution of the continental lithosphere.

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Chapter 16 Polyphase (1.6–1.5 and 1.1–1.0 Ga) deformation and metamorphism of Proterozoic (1.7–1.1 Ga) continental crust, Idefjorden terrane, Sveconorwegian orogen

Geological Society London Memoirs ◽

10.1144/m50-2018-34 ◽

2020 ◽

Vol 50 (1) ◽

pp. 397-434 ◽

Cited By ~ 9

Author(s):

Ulf Bergström ◽

Michael B. Stephens ◽

Carl-Henric Wahlgren

Keyword(s):

Large Scale ◽

Mineral Resources ◽

Shear Zones ◽

Granulite Facies ◽

Crustal Extension ◽

Crustal Component ◽

Ductile Shear Zones ◽

High Pressure Granulite ◽

Western Segment ◽

Sveconorwegian Orogen

AbstractCrust generated during an accretionary orogeny at 1.66–1.52 Ga (Gothian), and later during crustal extension at c. 1.51–1.49, c. 1.46, c. 1.34–1.30 Ga and after c. 1.33 Ga, dominate the Idefjorden terrane. Metamorphism under greenschist to, locally, high-pressure granulite facies, emplacement of syn-orogenic pegmatite and granite, and polyphase deformation followed at 1.05–1.02 Ga (Agder tectonothermal phase, Sveconorwegian orogeny). Sinistral transpressive deformation, including foreland-directed thrusting, preceded top-to-the-west movement and large-scale open folding along north–south axial trends during the younger orogeny. Crustal extension with emplacement of dolerite and lamprophyre dykes, norite–anorthosite, and a batholithic granite took place at c. 0.95–0.92 Ga (Dalane phase, Sveconorwegian orogeny). Ductile shear zones divide the Idefjorden terrane into segments distinguished by the character of the Gothian crustal component. Orthogneisses with c. 1.66 and c. 1.63–1.59 Ga protoliths occur in the Median segment; c. 1.59–1.52 Ga gneissic intrusive rocks and 1.6 Ga paragneisses with relicts of Gothian deformation and migmatization at c. 1.59 Ga and at c. 1.56–1.55 Ga occur in the Western segment. Mineral resources include stratabound Cu–Fe sulphides hosted by sandstone deposited after c. 1.33 Ga, and polymetallic quartz vein mineralization locally containing Au.

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Planar dispersion of folds in ductile shear zones and kinematic interpretation of fold-hinge girdles

Journal of Structural Geology ◽

10.1016/0191-8141(91)90129-7 ◽

1991 ◽

Vol 13 (3) ◽

pp. 281-297 ◽

Cited By ~ 14

Author(s):

Jonathan W. Mies

Keyword(s):

Shear Zones ◽

Fold Hinge ◽

Ductile Shear Zones ◽

Ductile Shear

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Structural correlation within the Kapuskasing uplift

Canadian Journal of Earth Sciences ◽

10.1139/e94-097 ◽

1994 ◽

Vol 31 (7) ◽

pp. 1081-1095 ◽

Cited By ~ 14

Author(s):

J. T. Bursnall ◽

A. D. Leclair ◽

D. E. Moser ◽

J. A. Percival

Keyword(s):

Fault Zone ◽

Large Scale ◽

Shear Zones ◽

Transitional Zone ◽

Metamorphic History ◽

Ductile Shear Zones ◽

Supracrustal Belt ◽

Ductile Shear ◽

High Level ◽

Multiple Stages

Comparison of progressive deformation and metamorphic history within and between the tectonic domains of the Kapuskasing uplift indicates significant variation in age and style of deformation across this large segment of the central Superior Province; multiple stages of tonalite and granitoid intrusion, melt generation, polyphase diachronous deformation, and likely rapid deep burial of supracrustal rocks collectively produced the complex character of this example of Archean mid to deep crust. At least four Archean deformation phases are recognized, although not all are of regional extent. Dated structural chronology suggests that the locus of the earliest recorded deformations migrated to deeper crustal levels with time. Pre-2680 Ma deformation (local D1–D2) within high-level tonalites is correlated with deformation in the Michipicoten supracrustal belt. The apparent earliest deformational fabrics at deeper crustal levels in the granulite terrane of the Kapuskasing structural zone occurred between 2660 and 2640 Ma. Archean third and fourth phase deformation phases (~ 2667 to ~ 2629 Ma) are present at mid-crustal and deeper levels and deform post-2667 Ma metaconglomerate; these resulted in large-scale folding and subhorizontal ductile shear zones, which seem to represent an important transitional zone that separated a passive upper crust from continued ductile strain at deeper levels.Subsequent uplift of the high-grade rocks was accomplished in multiple stages, initiated prior to 2.45 Ga and likely culminated around 1.9 Ga, although continued movement occurred as late as 1.14 Ga. The Ivanhoe Lake fault zone, along which much of the uplift must have occurred, exhibits some evidence of ductile deep-thrust-related fabrics, but most of the observed structures are brittle to brittle–ductile and steeply inclined. A broad zone of pervasive cataclasis and brittle–ductile shear zones is a characteristic feature of the fault zone throughout its length, and both dextral and sinistral offset are locally present. Clear ground evidence for major transcurrent or thrust displacements, however, has not been recognized.

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Meso- and micro-structural analysis of the Briançonnais Front in the Grand Saint Bernard area (Aosta Valley - Italy, and Valais - Switzerland)

10.5194/egusphere-egu2020-22548 ◽

2020 ◽

Author(s):

Daniele Pini ◽

Gloria Arienti ◽

Matteo Pozzi ◽

Bruno Monopoli ◽

Andrea Bistacchi

Keyword(s):

Large Scale ◽

Shear Bands ◽

Structural Evolution ◽

Shear Zones ◽

Pressure Solution ◽

Low Grade ◽

Fine Grained ◽

Ductile Shear Zones ◽

Type 3 ◽

Saint Bernard

We present preliminary results on the meso- and micro-structural evolution of high-strain rocks of the Houill&#232;re Zone and Pierre-Avoi Unit outcropping along the Swiss-Italy boundary ridge, to the west of the Grand Saint Bernard Pass.The stack of Middle and External Pennidic units is folded by polyphasic folds, developed at least partly under low-grade metamorphic conditions. Different generations of folds show isoclinal to open geometries. Fold axes are subhorizontal, trending NE-SW, and the overall fold interference pattern can be generally classified as a type 3 (Ramsay). At the microscale, an important deformation mechanism is pressure solution cleavage, consistent with relatively low-temperature conditions.Brittle-ductile shear zones, characterized by anastomosing bands of very fine-grained fault rocks, with pressure solution seams and SCC&#8217; shear bands, exploit the weak and strongly anisotropic phyllosilicate-rich layers, particularly in the black schists of the Houill&#232;re Zone.Brittle high-angle faults crosscut ductile and semi-brittle features and show an oblique-normal kinematics. These faults are particularly well developed in the more competent rocks of the Pierre-Avoi Unit (e.g. massive carbonates, metaconglomerates and metasandstones).A continuous horizon, a few metres thick, with a high density of quartz veins, can be followed in the internal and upper part of the Houill&#232;re Zone. This horizon is folded, at least by the younger open folds, and constitutes a major marker to study the large-scale structure of this unit.

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