Transpression in an Archean greenstone belt, northern Minnesota

1988 ◽  
Vol 25 (7) ◽  
pp. 1060-1068 ◽  
Author(s):  
P. J. Hudleston ◽  
D. Schultz-Ela ◽  
D. L. Southwick
Keyword(s):  
Volcanic Rocks ◽  
Shear Bands ◽  
Shear Zones ◽  
The North ◽  
Strain Pattern ◽  
Ductile Shear Zones ◽  
Continuous Shear ◽  
Measured Strain ◽  
Archean Greenstone Belt ◽  
North And South

Weakly metamorphosed Archean sedimentary and volcanic rocks of the Vermilion district, northern Minnesota, occupy an east–west-trending belt between gneisses of the Vermilion granitic complex to the north and the Giants Range batholith to the south. All the measured strain, a foliation, and a mineral lineation in this belt are attributed to the "main" phase of deformation (D2). Foliation strikes parallel to the belt and dips steeply, and the mineral lineation plunges moderately to steeply east or west and is parallel to the maximum stretching direction, X, and subparallel to fold hinges. An earlier, possibly nappe-forming, event (D1) left little evidence of fabric in the Vermilion district.A number of features indicate that the D2 deformation involved a significant component of dextral strike-slip shear in addition to north–south compression. They include ductile shear zones with sigmoidal foliation patterns, shear bands, asymmetric pressure shadows, and the fact that the asymmetry of the F2 folds is predominantly Z. Other features are more simply explained by a deformation involving simple shear. The S2 cleavage is locally folded, and a new spaced cleavage developed in an orientation similar to that of the old cleavage away from the folds. We consider this the result of a process of continuous shear, with perturbations of flow resulting in folding of S2 and the development of a new foliation axial planar to the folds. The same type of perturbation can lead to the juxtaposition of zones of constrictional and flattening strains, a distinctive feature of the rocks of the Vermilion district otherwise hard to account for. The strain pattern requires a north–south component of shortening in addition to shear. The D2 deformation in the Vermilion district can therefore be characterized as one of transpression: oblique compression between two more rigid lithospheric blocks to the north and south.

Strain-induced mineral growth in ductile shear zones and a preliminary study of ductile shearing in western Newfoundland

1988 ◽  
Vol 25 (12) ◽  
pp. 2118-2129 ◽  
Author(s):  
M. A. J. Piasecki
Keyword(s):  
Growth Characteristic ◽  
Shear Zones ◽  
The North ◽  
Mineral Growth ◽  
Inclined Surfaces ◽  
Ductile Shear Zones ◽  
Ductile Shearing ◽  
Preliminary Study ◽  
Ductile Shear ◽  
North And South

In the Fleur de Lys and the Central Gneiss terranes the presence of strain-induced mineral growth characteristic of ductile shear zones within zones of rocks with mylonitic fabrics indicates the existence of major belts of layer-parallel ductile shearing with complex evolutionary sequences. Kinematic markers in several of these shear belts indicate that shearing movements on initially probably gently inclined surfaces, directed not normal to the axial trend of the orogen but parallel to it, are tectonically important in western Newfoundland. The shear belts are in excess of 1 km thick, and one well-exposed example exhibits a pattern in which zones of the highest strain anastomose on the map scale.The base of the Fleur de Lys Supergroup is marked by one such major zone of shearing (décollement) in which kinematic indicators record movements directed to the north and to the south, before the regional attitude of the rocks was steepened. Along the Baie Verte Line, earlier north- and south-directed movements in the Fleur de Lys were succeeded by reverse movements towards the east, over the Dunnage Terrane.

scholarly journals Mise En Évidence De Nouvelles Structures Géologiques Dans La Région De Brobo (Centre De La Côte d’Ivoire). Aide À La Compréhension De La Tectonique Du Paléoprotérozoïque Du Craton Ouest Africain

2018 ◽  
Vol 14 (18) ◽  
pp. 305
Author(s):  
Daï Bi Seydou Mathurin ◽  
Ouattara Gbele ◽  
Koffi Gnammytchet Barthélémy ◽  
Gnanzou Allou ◽  
Coulibaly Inza
Keyword(s):  
Cote D'ivoire ◽  
Field Data ◽  
Côte D’Ivoire ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Cote D’Ivoire ◽  
Metasedimentary Rocks ◽  
Large Structures ◽  
Côte D'ivoire ◽  
Ductile Shear Zones

The lithological and structural observations of the region of Brobo (Central Côte d'Ivoire) indicate a succession of metasedimentary rocks (micaschists with cordierite, silstones, graphitic sediments, sandstones with amphibole-garnet, etc.) intermixed with volcanic rocks (rhyolite, dacite, andesite, basalt and the volcanoclastics). The whole is intruded by granites with one or two micas, sometimes porphyries, granodiorites, gabbros, and granite gneisses. Interpretations of Landsat ETM+ , RadarSat-1 and SRTM remote sensing imageries, as well as field data, revealed several lineament directions which, after field control, correspond to major faults and shear zones. These large structures show the N-S, NE-SW, NNE-SSW, E-W, NWSE, and NNW-SSE orientations. The field data also made it possible to describe several structures and to propose a preliminary geodynamic model for the setting and structuring of the formations of this region. This model suggests that the geodynamic took place in three stages: distension with a deformation of basement formations generating a gneissocity (D1), as well as deposits of sediments in the basins; followed by a NW-SE to E-W convergence generating a cleavage in the volcanogenic series (D2). This phase of deformation continues while creating, locally, a strain slip cleavage or a transposed schistosity. The third cleavage affects the volcanogenic series (fractures cleavages, D3) and ends in large corridors of ductile shear zones and associated faults.

scholarly journals Outline of the Nagssugtoqidian mobile belt of East Greenland

1979 ◽  
Vol 89 ◽  
pp. 9-18
Author(s):  
D Bridgwater ◽  
J.S Myers
Keyword(s):  
Shear Zones ◽  
Granulite Facies ◽  
Tectonic Activity ◽  
Mobile Belt ◽  
East Greenland ◽  
The North ◽  
Marginal Areas ◽  
High Level ◽  
North And South ◽  
Wide Zone

The Nagssugtoqidian mobile belt is a 240 km wide zone of deformation and plutonic activity which cuts across the Archaean craton of East Greenland. The belt was established 2600 m.y. ago by the formation of vertical E-W shear zones and the syntectonic intrusion of basic dykes. Tectonic activity along the E-W shear zones was followed by the emplacement of tonalitic intrusions, the Blokken gneisses, 2350 m.y. ago in the central parts of the mobile belt. The emplacement of the Blokken gneisses was accompanied and followed by further emplacement of basic dykes. These are synplutonic in the centre of the mobile belt but are emplaced into more rigid crust in the marginal areas of the belt and in the Archaean craton to the north and south. During a second major tectonic and thermal episode circa 1900 m.y. ago, the region was deformed by thrusting from the north. In the southem part of the mobile belt the earlier steep shear zones are cut by shear zones dipping gently northwards in which rocks are downgraded to greenschist facies. The grade of metamorphism increases northwards and shear zones are replaced by open folds with axial surfaces which dip gently northwards. The increasing ductility in the centre of and northem part of the belt is associated with the intrusion of charnockitic plutons and their granulite facies aureoles. Regional uplift occurred before the intrusion of high level post-tectonic plutons of diorite and granite 1550 m.y. ago.

Chapter 8 Outboard-migrating accretionary orogeny at 1.9–1.8 Ga (Svecokarelian) along a margin to the continent Fennoscandia

2020 ◽  
Vol 50 (1) ◽  
pp. 237-250 ◽  
Author(s):  
Michael B. Stephens
Keyword(s):  
Base Metal ◽  
Regional Scale ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Strike Slip ◽  
Base Metal Sulphide ◽  
Metal Sulphide ◽  
Time Period ◽  
Ductile Shear Zones ◽  
Strike Slip Movement

AbstractAn intimate lithostratigraphic and lithodemic connection between syn-orogenic rock masses inside the different lithotectonic units of the 2.0–1.8 Ga (Svecokarelian) orogen, Sweden, is proposed. A repetitive cyclic tectonic evolution occurred during the time period c. 1.91–1.75 Ga, each cycle lasting about 50–55 million years. Volcanic rocks (c. 1.91–1.88 Ga) belonging to the earliest cycle are host to most of the base metal sulphide and Fe oxide deposits inside the orogen. Preservation of relict trails of continental magmatic arcs and intra-arc basins is inferred, with differences in the depth of basin deposition controlling, for example, contrasting types of base metal sulphide deposits along different trails. The segmented geometry of these continental magmatic arcs and intra-arc basins is related to strike-slip movement along ductile shear zones during transpressive events around and after 1.88 Ga; late orogenic folding also disturbed their orientation on a regional scale. A linear northwesterly orogenic trend is suggested prior to this structural overprint, the strike-slip movement being mainly parallel to the orogen. A solely accretionary orogenic model along an active margin to the continent Fennoscandia, without any trace of a terminal continent–continent collision, is preferred. Alternating retreating and advancing subduction modes that migrated progressively outboard and southwestwards in time account for the tectonic cycles.

Neoproterozoic arc volcanic rocks of the Nanping-Ninghua tectonic belt, South China: Implications for the collision between the North and South Wuyi blocks

2018 ◽  
Vol 54 (4) ◽  
pp. 2679-2692 ◽  
Author(s):  
Yang Jiang ◽  
Xilin Zhao ◽  
Yanjie Zhang ◽  
Guangfu Xing ◽  
Mincheng Xu ◽  
...  
Keyword(s):  
South China ◽  
Volcanic Rocks ◽  
The North ◽  
Tectonic Belt ◽  
North And South

Chapter 3 Archean (>2.6 Ga) and Paleoproterozoic (2.5–1.8 Ga), pre- and syn-orogenic magmatism, sedimentation and mineralization in the Norrbotten and Överkalix lithotectonic units, Svecokarelian orogen

2020 ◽  
Vol 50 (1) ◽  
pp. 27-81 ◽  
Author(s):  
Stefan Bergman ◽  
Pär Weihed
Keyword(s):  
Variable Temperature ◽  
Tectonic Setting ◽  
Active Continental Margin ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Fe Oxide ◽  
Ductile Shear Zones ◽  
Three Stages ◽  
Felsic Volcanic

AbstractTwo lithotectonic units (the Norrbotten and Överkalix units) occur inside the Paleoproterozoic (2.0–1.8 Ga) Svecokarelian orogen in northernmost Sweden. Archean (2.8–2.6 Ga and possibly older) basement, affected by a relict Neoarchean tectonometamorphic event, and early Paleoproterozoic (2.5–2.0 Ga) cover rocks constitute the pre-orogenic components in the orogen that are unique in Sweden. Siliciclastic sedimentary rocks, predominantly felsic volcanic rocks, and both spatially and temporally linked intrusive rock suites, deposited and emplaced at 1.9–1.8 Ga, form the syn-orogenic component. These magmatic suites evolved from magnesian and calc-alkaline to alkali–calcic compositions to ferroan and alkali–calcic varieties in a subduction-related tectonic setting. Apatite–Fe oxide, including the world's two largest underground Fe ore mines (Kiruna and Malmberget), skarn-related Fe oxide, base metal sulphide, and epigenetic Cu–Au and Au deposits occur in the Norrbotten lithotectonic unit. Low- to medium-pressure and variable temperature metamorphic conditions and polyphase Svecokarelian ductile deformation prevailed. The general northwesterly or north-northeasterly structural grain is controlled by ductile shear zones. The Paleotectonic evolution after the Neoarchean involved three stages: (1) intracratonic rifting prior to 2.0 Ga; (2) tectonic juxtaposition of the lithotectonic units during crustal shortening prior to 1.89 Ga; and (3) accretionary tectonic evolution along an active continental margin at 1.9–1.8 Ga.

scholarly journals Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann Hills, East Antarctica

1995 ◽  
Vol 132 (2) ◽  
pp. 151-170 ◽  
Author(s):  
C. J. Carson ◽  
P. G. H. M. Dirks ◽  
M. Hand ◽  
J. P. Sims ◽  
C. J. L. Wilson
Keyword(s):  
High Strain ◽  
Shear Bands ◽  
Shear Zones ◽  
The South ◽  
Medium Pressure ◽  
Larsemann Hills ◽  
The North ◽  
Shear Sense ◽  
Low Pressures ◽  
East West

AbstractMeta-sediments in the Larsemann Hills that preserve a coherent stratigraphy, form a cover sequence deposited upon basement of mafic–felsic granulite. Their outcrop pattern defines a 10 kilometre wide east–west trending synclinal trough structure in which basement–cover contacts differ in the north and the south, suggesting tectonic interleaving during a prograde, D1 thickening event. Subsequent conditions reached low-medium pressure granulite grade, and structures can be divided into two groups, D2 and D3, each defined by a unique lineation direction and shear sense. D2 structures which are associated with the dominant gneissic foliation in much of the Larsemann Hills, contain a moderately east-plunging lineation indicative of west-directed thrusting. D2 comprises a colinear fold sequence that evolved from early intrafolial folds to late upright folds. D3 structures are associated with a high-strain zone, to the south of the Larsemann Hills, where S3 is the dominant gneissic layering and folds sequences resemble D2 folding. Outside the D3 high-strain zone occurs a low-strain D3 window, preserving low-strain D3 structures (minor shear bands and upright folds) that partly re-orient D2 structures. All structures are truncated by a series of planar pegmatites and parallel D4 mylonite zones, recording extensional dextral displacements.D2 assemblages include coexisting garnet–orthopyroxene pairs recording peak conditions of ∼ 7 kbar and ∼ 780°C. Subsequent retrograde decompression textures partly evolved during both D2 and D3 when conditions of ∼ 4–5 kbar and ∼ 750°C were attained. This is followed by D4 shear zones which formed around 3 kbar and ∼ 550°C.It is tempting to combine D2–4 structures in one tectonic cycle involving prograde thrusting and thickening followed by retrograde extension and uplift. The available geochronological data, however, present a number of interpretations. For example, D2 was possibly associated with a clockwise P–T path at medium pressures around ∼ 1000 Ma, by correlation with similar structures developed in the Rauer Group, whilst D3 and D4 events occurred in response to extension and heating at low pressures at ∼ 550 Ma, associated with the emplacement of numerous granitoid bodies. Thus, decompression textures typical for the Larsemann Hills granulites maybe the combined effect of two separate events.

Geological evolution of the northwestern Superior Province: Clues from geology, kinematics, and geochronology in the Gods Lake Narrows area, Oxford–Stull terrane, Manitoba

2006 ◽  
Vol 43 (7) ◽  
pp. 749-765 ◽  
Author(s):  
S Lin ◽  
D W Davis ◽  
E Rotenberg ◽  
M T Corkery ◽  
A H Bailes
Keyword(s):  
Large Scale ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Superior Province ◽  
Continental Margins ◽  
The North ◽  
Geological Evolution ◽  
Southern Half ◽  
Andean Type ◽  
Tectonic Interpretation

The study of lithology, geochronology, and structure in the Oxford–Stull terrane, in particular in the Gods Lake Narrows area, has led to the recognition of three distinct supracrustal sequences: ~2.8–2.9 Ga volcanic rocks; a ~2720 Ma fault-bounded package of volcanics and sandstones; and ~2705 Ma conglomerate and alkaline volcanic rocks of the Oxford Lake Group. Detrital zircon as old as 3647 Ma is present in the Oxford Lake Group. An early generation of folding and shearing occurred prior to deposition of the Oxford Lake Group and was probably synchronous with emplace ment of 2721 Ma tonalite dykes. The second generation of deformation caused south-over-north thrusting of volcanic rocks over the Oxford Lake Group. The youngest fabric resulted from east-southeast-trending, dextral, south-over-north shearing. The youngest rock dated in the area is the 2668 ± 1 Ma Magill Lake pluton, which records crustal melting following deformation. The pattern of sedimentation and deformation in this area is similar to but slightly older than that found in the southern half of the Superior Province, which shows a southward-younging diachroneity. The south-dipping north-vergent shear zones observed in the area contrast with dominantly north-dipping south-vergent structures observed and interpreted south of the North Caribou superterrane (NCS). The limited size of the study area precludes any strongly based large-scale tectonic interpretation; however, data and observations from the Gods Lake Narrows area are most easily accommodated in a model where the NCS served as a nucleus onto which other terranes were accreted and both the northern and southern margins of the NCS were Andean-type continental margins with opposite subduction polarities.

Chapter 4 Paleoproterozoic (2.0–1.8 Ga) syn-orogenic sedimentation, magmatism and mineralization in the Bothnia–Skellefteå lithotectonic unit, Svecokarelian orogen

2020 ◽  
Vol 50 (1) ◽  
pp. 83-130 ◽  
Author(s):  
Pietari Skyttä ◽  
Pär Weihed ◽  
Karin Högdahl ◽  
Stefan Bergman ◽  
Michael B. Stephens
Keyword(s):  
Volcanic Rocks ◽  
Shear Zones ◽  
Low Grade ◽  
Lower Grade ◽  
Magmatic Arc ◽  
The North ◽  
Magmatic Rocks ◽  
Structural Style ◽  
Volcanogenic Massive Sulphide ◽  
Felsic Volcanic

AbstractThe Bothnia–Skellefteå lithotectonic unit is dominated by turbiditic wacke and argillite (Bothnian basin), deposited at 1.96 (or older)–1.86 Ga, metamorphosed generally under high-grade conditions and intruded by successive plutonic suites at 1.95–1.93, 1.90–1.88, 1.87–1.85 and 1.81–1.76 Ga. In the northern part, low-grade and low-strain, 1.90–1.86 Ga predominantly magmatic rocks (the Skellefte–Arvidsjaur magmatic province) are enclosed by the basinal components. Subduction-related processes in intra-arc basin and magmatic arc settings, respectively, are inferred. Changes in the metamorphic grade and the relative timing of deformation and structural style across the magmatic province are linked to major shear zones trending roughly north–south and, close to the southern margin, WNW–ESE. Zones trending WNW–ESE and ENE–WSW dominate southwards. Slip along the north–south zones in an extensional setting initiated synchronously with magmatic activity at 1.90–1.88 Ga. Tectonic inversion steered by accretion to a craton to the east, involving crustal shortening, ductile strain and crustal melting, occurred at 1.88–1.85 Ga. Deformation along shear zones under lower-grade conditions continued at c. 1.8 Ga. Felsic volcanic rocks (1.90–1.88 Ga) host exhalative and replacement-type volcanogenic massive sulphide deposits (the metallogenic Skellefte district). Other deposits include orogenic Au, particularly along the ‘gold line’ SW of this district, porphyry Cu–Au–Mo, and magmatic Ni–Cu along the ‘nickel line’ SE of the ‘gold line’.

Development of C- shear bands in brittle-ductile shear zones: Insights from analogue and numerical models.

2020 ◽  
Author(s):  
Arnab Roy ◽  
Nandan Roy ◽  
Puspendu Saha ◽  
Nibir Mandal
Keyword(s):  
Shear Zone ◽  
Numerical Models ◽  
Shear Bands ◽  
Shear Zones ◽  
Rheological Parameters ◽  
Comprehensive Understanding ◽  
Experimental Conditions ◽  
Ductile Shear Zones ◽  
Analogue Models ◽  
Ductile Shear

<p>Development of brittle and brittle-ductile shear zones involve partitioning of large shear strains in bands, called C-shear bands (C-SB) nearly parallel to the shear zone boundaries. Our present work aims to provide a comprehensive understanding of the rheological factors in controlling such SB growth in meter scale natural brittle- ductile shear zones observed in in Singbhum and Chotonagpur mobile belts.  The shear zones show C- SB at an angle of 0°- 5° with the shear zone boundary. We used analogue models, based on Coulomb and Viscoplastic rheology to reproduce them in experimental conditions.</p><p>These models produce dominantly Riedel (R) shear bands. We show a transition from R-shearing in conjugate to single sets at angles of ~15<sup>o</sup> by changing model materials. However, none of the analogue models produced C-SB, as observed in the field. To reconcile the experimental and field findings, numeral models have been used to better constrain the geometrical and rheological parameters. We simulate model shear zones replicating those observed in the field, which display two distinct zones: drag zone where the viscous strains dominate  and the core zone, where both viscous and plastic strains come into play.  Numerical model results suggest the formation of  C- SB for a specific rheological condition. We also show varying shear band patterns as a function of the thickness ratio between drag and core zones.</p>

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