Effective rheology of a two-phase subduction shear zone calculated by numerical simple shear experiments

The physical nature and the rheology of a subduction shear zone play an important role in the deformation and the degree of locking along its interface with the upper plate. Inspired from exhumed subduction shear zones that exhibit block-in-matrix characteristics (m&#233;langes), we create synthetic models with different proportions of strong clasts within a weak matrix and compare them to natural m&#233;lange outcrops. Using 2D Finite Element visco-plastic numerical simulations and simple shear kinematic conditions, we determine the effective rheological parameters of such a two-phase medium, comprising blocks of basalt embedded within a wet quartzitic matrix. We treat our models and their structures as scale-independent and self-similar and upscale published field geometries to km-scale models, compatible with large-scale far-field observations. Exhumed subduction m&#233;langes suggest that deformation is mainly taken up by dissolution-precipitation creep. However, such flow laws are neither well-established yet experimentally nor of ample use in numerical modelling studies. In order to make our results comparable to and usable by numerical studies, we assume dislocation creep as the governing flow law for both basalt and wet quartz and by using different pressures, temperatures and strain rates we provide effective rheological estimates for a natural subduction interface. Our results suggest that the block-in-matrix ratio affects deformation and strain localization, with the effective dislocation creep parameters varying between the values of the strong and the weak phase, in cases where deformation of both materials is purely viscous. As the contribution of brittle deformation of the strong blocks increases, however, the value of the stress exponent, n, can exceed that of the purely strong phase.

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Effective rheology of a two-phase subduction shear zone: insights from numerical simple shear experiments and implications for subduction zone interfaces

10.5194/egusphere-egu21-10396 ◽

2021 ◽

Author(s):

Paraskevi Io Ioannidi ◽

Laetitia Le Pourhiet ◽

Philippe Agard ◽

Samuel Angiboust ◽

Onno Oncken

Keyword(s):

Simple Shear ◽

Large Scale ◽

Confining Pressure ◽

Shear Zones ◽

Dislocation Creep ◽

Small Scale ◽

Two Phase ◽

Weak Phase ◽

Pure Phases ◽

Geodynamic Models

Exhumed subduction shear zones often exhibit block-in-matrix structures comprising strong clasts within a weak matrix (m&#233;langes). Inspired by such observations, we create synthetic models with different proportions of strong clasts and compare them to natural m&#233;lange outcrops. We use 2D Finite Element visco-plastic numerical simulations in simple shear kinematic conditions and we determine the effective rheology of a m&#233;lange with basaltic blocks embedded within a wet quartzitic matrix. Our models and their structures are scale-independent; this allows for upscaling published field geometries to km-scale models, compatible with large-scale far-field observations. By varying confining pressure, temperature and strain rate we evaluate effective rheological estimates for a natural subduction interface. Deformation and strain localization are affected by the block-in-matrix ratio. In models where both materials deform viscously, the effective dislocation creep parameters (A, n, and Q) vary between the values of the strong and the weak phase. Approaching the frictional-viscous transition, the m&#233;lange bulk rheology is effectively viscous creep but in the small scale parts of the blocks are frictional, leading to higher stresses. This results in an effective value of the stress exponent, n, greater than that of both pure phases, as well as an effective viscosity lower than the weak phase. Our effective rheology parameters may be used in large scale geodynamic models, as a proxy for a heterogeneous subduction interface, if an appropriate evolution law for the block concentration of a m&#233;lange is given.

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Regional setting and kinematic features of the Needle Falls Shear Zone, Trans-Hudson Orogen

Canadian Journal of Earth Sciences ◽

10.1139/e93-115 ◽

1993 ◽

Vol 30 (7) ◽

pp. 1338-1354 ◽

Cited By ~ 19

Author(s):

Mel R. Stauffer ◽

John F. Lewry

Keyword(s):

Shear Zone ◽

Simple Shear ◽

Country Rock ◽

Shear Zones ◽

Small Scale ◽

Lake Zone ◽

Shear Sense ◽

Regional Tectonic ◽

Ductile Shear ◽

Shear Sense Indicators

Needle Falls Shear Zone is the southern part of a major northeast-trending ductile shear system within the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan. Throughout its exposed length of ~400 km, the shear zone separates reworked Archean continental crust and infolded Paleoproterozoic supracrustals of the Cree Lake Zone, to the northwest, from mainly juvenile Paleoproterozoic arc terrains and granitoid plutons of the Reindeer Zone, to the southeast. It also defines the northwest margin of the ca. 1855 Ma Wathaman Batholith, which forms the main protolith to shear zone mylonites. Although not precisely dated, available age constraints suggest that the shear zone formed between ca. 1855 and 1800 Ma, toward the end of peak thermotectonism in this part of the orogen.In the Needle Falls study area, shear zone mylonites exhibit varied, sequentially developed, ductile to brittle fabric features, including C–S fabrics, winged porphyroclasts (especially delta type), small-scale compressional and extensional microfaults ranging from thin ductile shear zones to late brittle faults, early isoclinal and sheath folds, later asymmetric folds related to compressional microfaults, and variably rotated and (or) folded quartz veins. All ductile shear-sense indicators suggest dextral displacement, as do most later ductile–brittle transition and brittle features. In conjunction with a gently north–northeast-plunging extension lineation, such data indicate oblique east-side-up dextral movement across the shear zone. However, preexisting structures in country rock protoliths rotate into the shear zone in a sense contrary to that predicted by ideal dextral simple shear, a feature thought to reflect significant flattening across the shear zone. Other ductile to brittle fabric elements in the mylonites are consistent with general noncoaxial strain, rather than ideal simple shear. Amount of displacement cannot be measured but indirect estimates suggest approximately 40 ± 20 km.The Needle Falls Shear Zone is too small and has developed too late in regional tectonic history to be considered a crustal suture. Rather, it is interpreted as either a late-tectonic oblique collisional structure or as the result of counterclockwise oroclinal rotation of the southern part of the orogen.

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Geological and geochemical implications of the genesis of the Qolqoleh orogenic gold mineralisation, Kurdistan Province (Iran)

Geologos ◽

10.1515/logos-2015-0002 ◽

2015 ◽

Vol 21 (1) ◽

pp. 31-57 ◽

Cited By ~ 3

Author(s):

Batoul Taghipour ◽

Farhad Ahmadnejad

Keyword(s):

Shear Zone ◽

Early Stage ◽

Hanging Wall ◽

Regional Metamorphism ◽

Shear Zones ◽

Northwestern Part ◽

Type I ◽

Two Phase ◽

Mean Values ◽

Calcite Veins

Abstract The Qolqoleh gold deposit is located in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), within the NE-SW trending Qolqoleh shear zone. Oligocene granitoids, Cretaceous meta-limestones, schists and metavolcanics are the main lithological units. Chondrite-normalised REE patterns of the ore-hosting metavolcanics indicate REE enrichment relative to hanging wall (chlorite-sericite schist) and footwall (meta-limestone) rocks. The pattern also reflects an enrichment in LREE relative to HREE. It seems that the LREE enrichment is related to the circulation of SO42- and CO2-bearing fluids and regional metamorphism in the Qolqoleh shear zone. Both positive and negative Eu anomalies are observed in shear-zone metavolcanics. These anomalies are related to the degree of plagioclase alteration during gold mineralisation and hydrothermal alteration. In progressing from a metavolcanic protomylonite to an ultramylonite, significant changes occurred in the major/trace element and REE concentration. Utilising an Al-Fe-Ti isocon for the ore-hosting metavolcanics shows that Sc, Y, K, U, P, and M-HREE (except Eu) are relatively unchanged; S, As, Ag, Au, Ca, LOI, Rb and LREE are enriched, and Sr, Ba, Eu, Cr, Co and Ni decrease with an increasing degree of deformation. Based on geochemical features and comparison with other well-known shear zones in the world, the study area is best classified as an Isovolume-Gain (IVG) type shear zone and orogenic type gold mineralisation. Based on the number of phases observed at room temperature and their microthermometric behaviour, three fluid inclusion types have been recognised in quartz-sulphide and quartz-calcite veins: Type I monophase aqueous inclusions, Type II two-phase liquid-vapour (L-V) inclusions which are subdivided into two groups based on the homogenisation temperature (Th): a) L-V inclusions with Th from 205 to 255°C and melting temperature of last ice (Tm) from -3 to -9°C. b) L-V inclusions with higher Th from 335 to 385°C and Tm from -11 to -16°C. Type III three-phase carbonic-liquid inclusions (liquid water-liquid CO2-vapour CO2) with Th of 345-385°C. The mean values of the density of ore-forming fluids, pressure and depth of mineralisation have been calculated to be 0.79-0.96 gr/cm3, 2 kbar and 7 km, respectively. The δ18Owater and δD values of the gold-bearing quartz-sulphide veins vary from 7.2‰ to 8‰ and -40.24‰ to -35.28‰, respectively, which are indicative of an isotopically heavy crustal fluid and likely little involvement of meteoric fluid. The δ18Owater values of the quartz-calcite veins have a range of -5.31‰ to -3.35‰, and the δD values of -95.65‰ to -75.31‰, which are clearly lower than those of early-stage quartz-sulphide-gold veins, and are close to the meteoric water line. Based on comparisons of the D-O isotopic systematics, the Qolqoleh ore-mineralising fluids originated from metamorphic devolatilisation of Cretaceous volcano-sedimentary piles. Devolatilisation of these units occurred either synchronously with, or postdates, the development of penetrative (ductile) structures such as shear zones and during overprinting brittle deformation

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Two-phase late Paleozoic magmatism (~ 313–312 and ~ 299–298 Ma) in the Lusatian Block and its relation to large scale NW striking fault zones: evidence from zircon U–Pb CA–ID–TIMS geochronology, bulk rock- and zircon chemistry

International Journal of Earth Sciences ◽

10.1007/s00531-021-02092-y ◽

2021 ◽

Author(s):

A. Käßner ◽

M. Tichomirowa ◽

M. Lapp ◽

D. Leonhardt ◽

M. Whitehouse ◽

...

Keyword(s):

Fault Zone ◽

Large Scale ◽

Age Groups ◽

Spatial Relation ◽

Shear Zones ◽

Igneous Rocks ◽

Geochemical Data ◽

Late Paleozoic ◽

Two Phase ◽

Saxothuringian Zone

AbstractLate Paleozoic (Variscan) magmatism is widespread in Central Europe. The Lusatian Block is located in the NE Bohemian Massif and it is part of the Saxothuringian Zone of the Variscan orogen. It is bordered by two major NW-trending shear zones, the Intra-Sudetic Fault Zone towards NE and the Elbe Fault Zone towards SW. The scarce Variscan igneous rocks of the Lusatian Block are situated close to these faults. We investigated 19 samples from Variscan plutonic and volcanic rocks of the Lusatian Block, considering all petrological varieties (biotite-bearing granites from the Koenigshain and Stolpen plutons, amphibole-bearing granites from three boreholes, several volcanic dykes, and two volcanites from the intramontane Weissig basin). We applied whole-rock geochemistry (18 samples) and zircon evaporation dating (19 samples). From the evaporation data, we selected six representative samples for additional zircon SHRIMP and CA–ID–TIMS dating. For the Koenigshain pluton, possible protoliths were identified using whole-rock Nd-isotopes, and zircon Hf- and O-isotopes. The new age data allow a subdivision of Variscan igneous rocks in the Lusatian Block into two distinct magmatic episodes. The spatial relation of the two age groups to either the Elbe Fault Zone (298–299 Ma) or the Intra-Sudetic Fault Zone (312–313 Ma) together with reports on the fault-bound character of the dated intrusions suggests an interpretation as two major post-collisional faulting episodes. This assumption of two distinct magmatic periods is confirmed by a compilation of recently published zircon U–Pb CA–ID–TIMS data on further Variscan igneous rocks from the Saxothuringian Zone. New geochemical data allow us to exclude a dominant sedimentary protolith for the Koenigshain pluton as supposed by previous investigations. This conclusion is mainly based on new O- and Hf-isotope data on zircon and the scarcity of inherited zircons. Instead, acid or intermediate igneous rocks are supposed as the main source for these I-type granitoids from the Koenigshain pluton.

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Numerical models of sheath fold development in rheologically heterogeneous rocks of the Cima Lunga-Adula shear zone (Central Alps)

10.5194/egusphere-egu2020-18351 ◽

2020 ◽

Author(s):

Filippo Luca Schenker ◽

Marta Adamuszek ◽

Matteo Maino

Keyword(s):

Aspect Ratio ◽

Shear Zone ◽

Simple Shear ◽

Viscosity Ratio ◽

Numerical Models ◽

Shear Zones ◽

Ellipsoidal Inclusion ◽

Central Alps ◽

The Third ◽

The Matrix

Highly curvilinear folds develop during simple shear deformation due to perturbations in the velocity field around the inclusion heterogeneity. In the field, such structures may be recognized at the micro- and meso-scale within high-strain crustal-scale shear zones. However, at scarce outcrop conditions, fragments of these structures are often interpreted as generated by poly-phase deformation. The structural history becomes even more complex when the deformation within the inclusion is considered. In this inclusion-matrix deformation system, two end-member regimes has been already investigated: (i) a weak ellipsoidal inclusion that acts as a slip surface over which sheath folds develop and (ii) a rigid ellipsoidal inclusion that rotates within the matrix generating sheath folds in the back of the rotating ellipse in direction of the shearing. Between these two end-members, understanding the clast-matrix deformational regime is not trivial and the genesis of sheath fold is unexplored.We employed 3D numerical models to study fold structure evolution around an ellipsoidal inclusion within a matrix during simple shear. Both inclusion and matrix were homogeneous and isotropic, and had linear viscous rheologies. We tested models with different (i) initial inclusion aspect ratio, (ii) viscosity ratio between the inclusion and the matrix, and (iii) strain. We identified three main deformation regimes that are closely related with the behaviour of the inclusion. In the first regime, the inclusion experiences massive stretching. In the second regime, we observe oscillatory motion of the principal inclusion axes and the deformation of the material lines within inclusion periodically changes from shortening to stretching conditions. In the third regime, principal inclusion axes rotate. The material lines within inclusion, similar as in the second regime, experience cyclic stretching and shortening, however, the amount of extension and shortening is significantly smaller. The transition between regimes is dependent of both initial inclusion aspect ratio and viscosity ratio. The first regime is characteristic for inclusions with small viscosity ratio. With increasing viscosity ratio, the regime changes to the second and eventually to the third. The change occurs at lower viscosity ratio for models with larger initial inclusion aspect ratio than for smaller once. All the models developed sheath folds around the inclusions.The results of our simulations were compared with the deformation pattern derived from a main shear zone of the Cima-Lunga in the Central Alps. In the field, the elongated high-pressure ultramafic bodies are surrounded by folded amphibolite-facies paragneisses that locally depict sheath folds. The internal structures of ultramafic bodies are characterize by recumbent, sub-isoclinal folds and folded boudinaged mafic layers that suggest internal changes in stress direction. In a selected ultramafic body elongated sub-parallel to the shearing direction and with an aspect ratio a/c=3 and b/c=2, we estimate from a mafic boudinaged layer subparallel to the a/c axis a minimum stretching of 40%. This field data allowed us to establish that the viscosity ratio of the ultramafic body to the paragneisses at the time of the deformation of the shear zone was in the range of 4-11 and the strain was &#947;>13.

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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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Gas Flow Through Water-Saturated Shear Zones: Field and Laboratory Experiments and Their Interpretation

MRS Proceedings ◽

10.1557/proc-506-741 ◽

1997 ◽

Vol 506 ◽

Cited By ~ 1

Author(s):

P. Marschall ◽

J. Croisé ◽

U. Fischer ◽

R. Senger ◽

E. Wyss

Keyword(s):

Shear Zone ◽

Gas Flow ◽

Two Phase Flow ◽

Gas Permeability ◽

Numerical Models ◽

Shear Zones ◽

Parameter Estimates ◽

Phase Flow ◽

Two Phase ◽

Flow Processes

ABSTRACTGas threshold pressure tests and gas tracer tests have been performed at the Grimsel Test Site to study two-phase flow processes in a shear zone. In addition, capillary pressure and gas permeability measurements were carried out in the laboratory on drillcore samples. The laboratory investigations were complemented by assessing the pore structure of the shear zone material. The interpretation of the field tests with numerical models indicated that the structural and two-phase flow parameters to be determined are highly correlated with one another and, consequently, the parameter estimates can be rather uncertain. The joint interpretation of field and laboratory results, however, led to a more stringent description of the two-phase flow processes, expressed by a better overall fit of the test data and smaller uncertainty ranges of the estimated parameters. The results showed that the gas mobility in the shear zone was very high even at high water saturation and gas flow was limited to the narrow zones of brittle deformation along the shear zone.

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Effective rheology of a two-phase subduction shear zone: Insights from numerical simple shear experiments and implications for subduction zone interfaces

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2021.116913 ◽

2021 ◽

Vol 566 ◽

pp. 116913

Author(s):

Paraskevi Io Ioannidi ◽

Laetitia Le Pourhiet ◽

Philippe Agard ◽

Samuel Angiboust ◽

Onno Oncken

Keyword(s):

Subduction Zone ◽

Shear Zone ◽

Simple Shear ◽

Two Phase

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U–Pb and Rb–Sr geochronology of the Wedgeport granitoid pluton, southwestern Nova Scotia

Canadian Journal of Earth Sciences ◽

10.1139/e88-028 ◽

1988 ◽

Vol 25 (2) ◽

pp. 255-261 ◽

Cited By ~ 8

Author(s):

R. F. Cormier ◽

J. D. Keppie ◽

A. L. Odom

Keyword(s):

Nova Scotia ◽

Shear Zone ◽

Large Scale ◽

Shear Zones ◽

Potassium Feldspar ◽

Slow Cooling ◽

Northwestern Margin ◽

Meguma Terrane ◽

Tin Deposit ◽

Straight Lines

Zircons from biotite monzogranite of the Wedgeport Pluton, intrusive into deformed metasediments of the Cambrian(?) Goldenville Formation in the southwestern Meguma Terrane of Nova Scotia, yield concordant U–Pb ages of 316 ± 5 Ma. This is interpreted as the time of intrusion and crystallization. Within the error limits, the 323 ± 12 Ma Rb–Sr whole-rock isochron age is identical and gives an initial 87Sr/86Sr ratio of 0.7137 ± 0.0056. Rb–Sr analyses of mineral separates of biotite, potassium feldspar, and quartz–plagioclase from several samples yield subparallel, internal isochrons with an average age of 257 ± 8 Ma. Initial ratios of the internal isochrons range from 0.716 to 0.759. A slow-cooling model for these latter data is discarded because the mineral data fall on straight lines. Instead, a reheating event related to plutonism ca. 257 Ma ago, which was sufficient to cause local grain-to-grain migration and reequilibration of strontium and rubidium but not large-scale redistribution, is invoked. This reheating is also inferred to be responsible for the hydrothermal alteration and Sn–U mineralization concentrated along the northwestern margin of the pluton. A dextral northeast–southeast shear zone cutting the pluton is also inferred to be ca. 257 Ma old. It may be related to the last stages of westward obduction of the Meguma Terrane.These results provide a clear example of Permo-Carboniferous plutonism in the southwestern Meguma Terrane and suggest a similar interpretation may apply to other anomalously young ages recorded in this area. In light of these results, the Permo-Carboniferous age of the large East Kemptville tin deposit and its location in a dextral shear zone suggest that the association of younger plutonism and shear zones may be a significant factor for economic mineralization.

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Large slow rock-slope deformations affecting hydropower facilities

10.5194/egusphere-egu2020-8288 ◽

2020 ◽

Author(s):

Margherita C. Spreafico ◽

Federico Agliardi ◽

Matteo Andreozzi ◽

Alessandro Cossa ◽

Giovanni B. Crosta

Keyword(s):

Shear Zone ◽

Urban Areas ◽

Large Scale ◽

Rock Slope ◽

Shear Zones ◽

Seasonal Effects ◽

Evolutionary Stage ◽

3D Analysis ◽

Future Evolution

Large-scale creeping landslides are widespread in alpine areas. Associated long-term, slow deformations threaten urban settlement, railways, main roads and hydropower facilities, on which our society is strictly dependent. Over the next decades, the continuous growing of the global population, the resulting increase in the urbanization (also closer to hazard-prone areas), and the climate change (e.g. melting of alpine glaciers) will increase these interactions and the related risk. Nevertheless, assessing the vulnerability of different types of elements at risk to this kind of hazard is not obvious, especially when hydropower structures (including dams, tunnels, penstocks, etc.) are involved. Large rockslides complexity often results in a variety of different evolutionary trends, making their forecasting and risk reduction a challenge. While catastrophic collapse can cause huge instantaneous damages, slow movements along long periods may lead to progressive damage of structures and infrastructures. In the alpine and pre-alpine areas of Lombardia (Central Italian Alps), slow rock-slope deformations affect an area of 750 km2, threatening more than 10 km2 of urban areas and about 100 km of penstocks or tunnels related to hydropower facilities. Here we focus on the Mt. Palino slope (Valmalenco, Italian Central Alps), that is affected by a complex, apparently long-lived DSGSD (Deep seated Gravitational Slope Deformation) with a relief exceeding 1000 m. The slope hosts hydropower facilities and a tourist resort. In order to recognize dominant processes and their possible evolution (internal deformation, low-rate steady activity, progressive behaviour, seasonal effects) for better risk assessment and mitigation, we investigated the volume and depth of displaced rock mass and the possible localization of deformations along a basal shear zone.&#160; Geomechanical and geomorphological surveys, seismic tomography, deep borehole logs and monitoring data (borehole instrumentation, precise levelling, topographic and GB-InSAR) allowed recognizing different sectors with different evolutionary stage and activity degree. The DSGSD which affect the entire Mt. Palino was probably active before the last LGM (Last Glacial Maximum), while only the northern slope sector is now considered as active. We recognized multiple nested phenomena faster than the main mass, identified as large rockslides. They are suspended over the valley floor and may evolve into fast rock avalanches. One of them is located in correspondence with the hydropower penstock, causing differential deformation to the structure. Borehole evidence of localization along cataclastic shear zones was found, motivating a petrographic geomechanical characterization of both rock masses and shear zone samples. Integrated 3D analysis of different information permitted to reconstruct displacement patterns, long-term mechanisms and the controlling factors of possible future evolution.&#160;

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