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

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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Proterozoic geological evolution of the northern Vestfold Hills, Antarctica

Geological Magazine ◽

10.1017/s0016756800017581 ◽

1991 ◽

Vol 128 (4) ◽

pp. 307-318 ◽

Cited By ~ 20

Author(s):

C. W. Passchier ◽

R. F. Bekendam ◽

J. D. Hoek ◽

P. G. H. M. Dirks ◽

H. de Boorder

Keyword(s):

Large Scale ◽

Structural Evolution ◽

Shear Zones ◽

Granulite Facies ◽

Strike Slip ◽

Amphibolite Facies ◽

Tectonic Event ◽

Vestfold Hills ◽

Two Phases ◽

Brittle Faulting

AbstractThe presence of polyphase shear zones transected by several suites of dolerite dykes in Archaean basement of the Vestfold Hills, East Antarctica, allows a detailed reconstruction of the local structural evolution. Archaean and early Proterozoic deformation at granulite facies conditions was followed by two phases of dolerite intrusion and mylonite generation in strike-slip zones at amphibolite facies conditions. A subsequent middle Proterozoic phase of brittle normal faulting led to the development of pseudotachylite, predating intrusion of the major swarm of dolerite dykes around 1250 Ma. During the later stages and following this event, pseudotachylite veins were reactivated as ductile, mylonitic thrusts under prograde conditions, culminating in amphibolite facies metamorphism around 1000–1100 Ma. This is possibly part of a large-scale tectonic event during which the Vestfold block was overthrust from the south. In a final phase of strike-slip deformation, several pulses of pseudotachylite-generating brittle faulting alternated with ductile reactivation of pseudotachylite.

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Evolution of brittle structures in plagioclase-rich rocks at high-grade metamorphic conditions – Linking laboratory results to field observations

10.5194/egusphere-egu2020-8367 ◽

2020 ◽

Author(s):

Sarah Incel ◽

Jörg Renner ◽

Bjørn Jamtveit

Keyword(s):

Structural Evolution ◽

Confining Pressure ◽

Shear Zones ◽

Ductile Shear Zones ◽

Laboratory Results ◽

Experimental Laboratory ◽

Eclogite Facies ◽

Ductile Shear ◽

Host Rocks ◽

Lower Crustal

Plagioclase-rich lower crustal granulites exposed on the Lofoten archipelago, N Norway, display pseudotachylytes, reflecting brittle deformation, as well as ductile shear zones, highlighting plastic deformation. Pristine pseudotachylytes often show no or very little difference in mineral assemblage to their host-rocks that exhibit limited, if any, metamorphic alteration. In contrast, host-rock volumes that developed ductile shear zones exhibit significant hydration towards amphibolite or eclogite-facies assemblages within and near the shear zones. We combine experimental laboratory results and observations from the field to characterize the structural evolution of brittle faults in plagioclase-rich rocks at lower crustal conditions. We performed a series of deformation experiments on intact granulite samples at 2.5 GPa confining pressure,&#160; a strain rate of 5&#215;10-5 s-1,&#160; temperatures of 700 and 900 &#176;C, and total strains of either ~7-8 % or ~33-36 %. Samples were either deformed &#8216;as-is&#8217;, i.e. natural samples without any treatment, or with ~2.5 wt.% H2O added. Striking similarities between the experimental and natural microstructures suggest that the transformation of precursory brittle structures into ductile shear zones at eclogite-facies conditions is most effective when hydrous fluids are available in excess.

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U–Pb geochronology of Late Proterozoic rocks of the eastern Cobequid Highlands, Avalon Composite Terrane, Nova Scotia

Canadian Journal of Earth Sciences ◽

10.1139/e91-044 ◽

1991 ◽

Vol 28 (4) ◽

pp. 504-511 ◽

Cited By ~ 20

Author(s):

Ronald Doig ◽

J. Brendan Murphy ◽

R. Damian Nance

Keyword(s):

Nova Scotia ◽

Shear Zones ◽

Low Grade ◽

Geochronological Data ◽

Magmatic Arc ◽

Late Precambrian ◽

Ductile Shear Zones ◽

Ductile Shear ◽

Orogenic Cycle ◽

High Level

In the Cobequid Highlands of Nova Scotia, low-grade late Precambrian arc-related volcano-sedimentry rocks typical of the Avalon Composite Terrane overlie platformal metasedimentry rocks and are spatially associated with gneisses previously considered to be basement to both these units. U–Pb zircon dates of 580–587 Ma from an orthogneiss and an amphibolite are similar to the U–Pb zircon dates of 580–610 Ma from both syntectonic granites in ductile shear zones and high-level posttectonic plutons that intruded the Avalonian successions. Hence, the gneisses do not represent basement but are an integral part of the Avalonian orogenic cycle. The geochronological data indicate that penetrative fabrics in the gneisses, syntectonic granites, and volcano-sedimentary successions are penecontemporaneous (ca. 580–620 Ma) and not sequential, as previously interpreted. The gneisses have a metamorphic fabric (S1a), crystallized under amphibolite-facies conditions, and may represent the deeper roots of a late Precambrian magmatic arc. Fabrics within the deformed granite gneisses (S1b) are interpreted as reflecting crystallization within active ductile shear zones associated with intra-arc transtension and basin development. Fabrics in the volcano-sedimentary successions (S1c) are associated with deformation of the basin.

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Relationship between microstructures and resistance in mafic assemblages that deform and transform

Solid Earth ◽

10.5194/se-11-2141-2020 ◽

2020 ◽

Vol 11 (6) ◽

pp. 2141-2167

Author(s):

Nicolas Mansard ◽

Holger Stünitz ◽

Hugues Raimbourg ◽

Jacques Précigout ◽

Alexis Plunder ◽

...

Keyword(s):

Grain Size ◽

Shear Strain ◽

Phase Distribution ◽

Shear Bands ◽

Constant Strain Rate ◽

Peak Stress ◽

Confining Pressure ◽

Shear Zones ◽

Reaction Products ◽

Fine Grained

Abstract. Syn-kinematic mineral reactions play an important role for the mechanical properties of polymineralic rocks. Mineral reactions (i.e., nucleation of new phases) may lead to grain size reduction, producing fine-grained polymineralic mixtures, which have a strongly reduced viscosity because of the activation of grain-size-sensitive deformation processes. In order to study the effect of deformation–reaction feedback(s) on sample strength, we performed rock deformation experiments on “wet” assemblages of mafic compositions in a Griggs-type solid-medium deformation apparatus. Shear strain was applied at constant strain rate (10−5 s−1) and constant confining pressure (1 GPa) with temperatures ranging from 800 to 900 ∘C. At low shear strain, the assemblages that react faster are significantly weaker than the ones that react more slowly, demonstrating that reaction progress has a first-order control on rock strength. With increasing strain, we document two contrasting microstructural scenarios: (1) the development of a single throughgoing high-strain zone of well-mixed, fine-grained aggregates, associated with a significant weakening after peak stress, and (2) the development of partially connected, nearly monomineralic shear bands without major weakening. The lack of weakening is caused by the absence of interconnected well-mixed aggregates of fine-grained reaction products. The nature of the reaction products, and hence the intensity of the mechanical weakening, is controlled by the microstructures of the reaction products to a large extent, e.g., the amount of amphibole and the phase distribution of reaction products. The samples with the largest amount of amphibole exhibit a larger grain size and show less weakening. In addition to their implications for the deformation of natural shear zones, our findings demonstrate that the feedback between deformation and mineral reactions can lead to large differences in mechanical strength, even at relatively small initial differences in mineral composition.

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A large-scale detachment system in the central Eastern Alps (Upper Austroalpine Unit, Austria)

10.5194/egusphere-egu2020-19144 ◽

2020 ◽

Author(s):

Manuel Werdenich ◽

Christoph Iglseder ◽

Bernhard Grasemann ◽

Gerd Rantitsch ◽

Benjamin Huet

Keyword(s):

Large Scale ◽

Hanging Wall ◽

Shear Bands ◽

Carbonaceous Material ◽

Eastern Alps ◽

Low Grade ◽

Fold Axis ◽

Wall Unit ◽

Detachment Zone ◽

Austroalpine Unit

Based on new structural field data and Raman micro-spectroscopy on carbonaceous material a major detachment juxtaposing Drauzug-Gurktal Nappe System (DGN) against the transgressive Permo-Mesozoic cover sequence of the &#214;tztal-Bundschuh Nappe System (BN, Stangalm Mesozoic s. str.) in the area SE of Flattnitz (Carinthia, Austria). An Eo-alpine top-SE kinematic has been identified.The hanging wall unit comprise lithologies of the DGN phyllites, conglomerates and graphite schists (Stolzalpe nappe), which have experienced only low grade greenschist deformation. Raman constrains 350&#176;C&#177;40&#176;C.The footwall unit consists of dolomitic ultra-mylonites, calcitic marble mylonites, meta-conglomerates and quarzites (Stangalm Mesozoic and Kuster nappe), which have experienced at least four main deformation phases. The oldest structures (D1) corresponding to Eo-Alpine nappe stacking are overprinted by (D2) isoclinal recumbent folds with E-W oriented shallow dipping fold axis and an axial plane schistosity, dipping shallowly to WSW. Ductile to brittle-ductile top to the E shearing (D3) is indicated by ESE-trending stretching lineation, C-type shear bands, stylolites, crystal- and shape preferred orientations of mineral grains. Late brittle deformation (D4) is recorded in steep joint sets with dip-directions to NW. Raman constrains 480&#176;C&#177;40&#176;C.The detachment zone comprises a complicate zone of high strain including units from DGN folded together within the Stangalm Mesozoic, which have experienced the same deformation as the BN.

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870 Ma age of South Delhi Orogeny: A study on Geochronology of the granites and the metasediments, NW India.

10.5194/egusphere-egu2020-3259 ◽

2020 ◽

Author(s):

Subhash Singh ◽

Tapas Kumar Biswal

Keyword(s):

Interference Pattern ◽

Boundary Migration ◽

Shear Zones ◽

High Temperature Deformation ◽

Temperature Deformation ◽

Subgrain Rotation ◽

Ductile Shear Zones ◽

Biotite Schist ◽

Magmatic Fabric ◽

Type 3

South Delhi orogeny is constrained by correlating the deformational fabric with geochronology of the granites and metasediments around Beawar- Rupnagar-Babra, Rajasthan, NW India. The area consists of metaconglomerate, calcareous schist, mica schist and amphibolite. These were deformed by three stages of deformation(D1-3) and intruded by four types of granite plutons (G1-4). The D1 deformation produced F1, reclined/recumbent folds with S1 axial planar fabric in greenschist facies metamorphic condition. The D2 deformation produced NE-SW trending F2 folds coaxial with F1(type 3 interference pattern), crenulations and F2-axial parallel ductile shear zones. &#160;The D3 deformation produced NW-SE F3 folds, which superimposed on F1 and F2 to create type 1 and 2 interference pattern. Granites carry pervasive S1 fabric. In G1-3 granites, the S1 is characterized by low temperature deformation fabric marked by bulging recrystallization of quartz. The G4 granite (namely Sewariya granite) contains magmatic to submagmatic fabric and the S1 fabric in it is a high temperature deformation fabric and lies parallel to magmatic fabric in the rock. Plagioclase is dynamically recrystallized by subgrain rotation and grain boundary migration and quartz shows chess board twinning. We interpret that the G4 granite is syntectonic and G1-3 were pre-tectonic to D1 deformation. &#160;U-Pb data (SHRIMP method) of G1, G2 and G4 granites yield Concordia age calculated with 206Pb/238U and 207Pb/235U ratio at ~982 Ma, ~992 Ma and ~878 Ma respectively. Thus the South Delhi orogeny is constrained by the age of G4 granite at ~ 878 Ma (~ 870 Ma).&#160; The G1-3 granites are pre- Delhi orogeny and probably constrain the age of rifting of the Delhi basin. EPMA Th-U-total Pb monazite geochronology of the garnet-staurolite-quartz-feldspar-biotite schist from the basal conglomerate zone shows three distinct ages, ca. 1611 Ma, 864 Ma and 718 Ma. &#160;Correlating with granite SHRIMP age, the ~ 864 Ma corresponds to Delhi metamorphism and D1 deformation (~ 870 Ma). The event ca. 1611 Ga probably belongs to pre-Delhi age, which is observed in nearby pre-Delhi localities like Sandmata terrane.Keywords: Deformational fabric, geochronology, metaconglomerate, granite and geochronology.

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

10.5194/egusphere-egu2020-960 ◽

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

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.&#160; The shear zones show C- SB at an angle of 0&#176;- 5&#176; with the shear zone boundary. We used analogue models, based on Coulomb and Viscoplastic rheology to reproduce them in experimental conditions.These models produce dominantly Riedel (R) shear bands. We show a transition from R-shearing in conjugate to single sets at angles of ~15o 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 &#160;and the core zone, where both viscous and plastic strains come into play. &#160;Numerical model results suggest the formation of &#160;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.

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Structural and metamorphic evolution of the Ambin massif (western Alps): toward a new alternative exhumation model for the Briançonnais domain

BSGF - Earth Sciences Bulletin ◽

10.2113/gssgfbull.178.6.437 ◽

2007 ◽

Vol 178 (6) ◽

pp. 437-458 ◽

Cited By ~ 13

Author(s):

Jerome Ganne ◽

Jean-Michel Bertrand ◽

Serge Fudral ◽

Didier Marquer ◽

Olivier Vidal

Keyword(s):

Tectonic Evolution ◽

Large Scale ◽

Regional Scale ◽

Shear Bands ◽

Shear Zones ◽

Western Alps ◽

Ductile Deformation ◽

Movement Direction ◽

Lower Grade ◽

Structural Investigations

Abstract The basement domes of the central part of western Alps may result either from a multistage tectonic evolution with a dominant horizontal shortening component, an extensional behaviour, or both. The Ambin massif belongs to the “Briançonnais” domain and is located within the HP metamorphic zone. It was chosen for a reappraisal of the tectonic evolution of the Internal Alps in its western segment. Structural investigations have shown that Alpine HP rocks were exhumed in three successive stages. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds to a non-coaxial regime with dominant horizontal shortening and north movement direction. Petrological observations and P-T estimates show that the exhumation process was initiated during D1, the corresponding mechanism being still poorly understood. The D2 stage took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive shear-zones at all scales that locally define major tectonic contacts. D2 shear zones show a top-to-east movement direction and correspond actually to large-scale detachment faults responsible for the juxtaposition of less metamorphic units above the Ambin basement and thus to a large part of the exhumation of HP rocks toward the surface. D2 shear zones were subsequently deformed by D3 open folds, large antiforms (e.g. the Ambin dome) and associated brittle-ductile D3 shear-bands. The D1 to D3 P-T conditions and P-T path of the blueschists occurring in the deepest part of the Ambin dome, was estimated by using the multi-equilibrium thermobarometric method of the Tweeq and Thermocalc softwares. Peak pressure conditions, estimated at about 14–16 Kb, 500oC, are followed by a nearly-isothermal decompression that occurred concurrently with the major D1–D2 change in the ductile deformation regime. Eastwards, the Schistes Lustrés units exhibit a similar geometry on top of the Gran Paradiso dome but exhibit opposite D2 movement direction. Lower-grade units are lying above higher-grade units, the shear zones occurring in between being similar to Ambin’s D2 detachments. Thus at regional scale, the D2 detachments seem to form together with the Ambin shear-zones, a network of conjugate detachments. Such a pattern suggests that the exhumation history is mostly controlled by a D2+D3 crustal-scale vertical shortening resulting in the thinning of the previous tectonic pile formed during D1. The slab-break off hypothesis may explain such an extensional behaviour within the western Pennine domain. It is suggested that the thermo-mechanical rebound of the residual European slab initiated between 35 and 32 Ma the fast exhumation of the previously thickened orogenic wedge (stack of D1 HP slices). It was immediately followed by a collapse of the wedge that may correspond to the E-W Oligocene extensional event responsible for the opening of rifts in the West European platform.

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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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The ephemeral development of C' shear bands

10.5194/egusphere-egu21-752 ◽

2021 ◽

Author(s):

Melanie Finch ◽

Paul Bons ◽

Florian Steinbach ◽

Albert Griera ◽

Maria-Gema Llorens ◽

...

Keyword(s):

Strain Rate ◽

Numerical Models ◽

Shear Bands ◽

Shear Zones ◽

Stress And Strain ◽

High Mechanical Strength ◽

Weak Phase ◽

Ductile Shear Zones ◽

Stress And Strain Rate ◽

Stress And Strain Distribution

C' shear bands are common structures in ductile shear zones but their development is poorly understood. They occur in rocks with a high mechanical strength contrast so we used numerical models of viscoplastic deformation to study the effect of the proportion of weak phase and the phase strength contrast on C' shear band development. We employed simple shear to a finite strain of 18 in 900 steps and recorded the microstructure, stress and strain distribution at each step. We found that C' shear bands form in models with &#8805;5% weak phase when there is a moderate or high phase strength contrast, and they occur in all models with weak phase proportions &#8805;15%. Contrary to previous research, we find that C' shear bands form when layers of weak phase parallel to the shear zone boundary rotate forwards. This occurs due to mechanical instabilities that are a result of heterogeneous distributions of stress and strain rate. C' shear bands form on planes of low strain rate and stress, not in sites of maximum strain rate as has previously been suggested. C' shear bands are ephemeral and they either rotate backwards to the C plane once they are inactive or rotate into the field of shortening and thicken to form X- and triangle- shaped structures.

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