Evolution of melt-bearing shear zones during cooling within an upper crustal aureole: the Calamita Schists (Island of Elba, Italy)

2021 ◽  
Author(s):  
Samuele Papeschi ◽  
Giovanni Musumeci ◽  
Omar Bartoli ◽  
Bernardo Cesare ◽  
Hans-Joachim Massonne ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Strain Localization ◽  
High Strain ◽  
Shear Bands ◽  
Physical And Mechanical Properties ◽  
Shear Zones ◽  
High Grade ◽  
Tectonic Structures ◽  
Regional Deformation ◽  
Brittle Ductile Transition

<p>The Calamita Schists in the aurole of the Late Miocene Porto Azzurro pluton underwent partial melting and HT metamorphism at P < 0.2 – 0.3 GPa and T > 650 – 700 °C, coeval with regional deformation. Deformation produced a network of shear zones that evolved from melt-present conditions to the brittle-ductile transition. Shearing at high temperature in the presence of melt allowed deformation to remain relatively distributed in wide high-strain zones. As the thermal pulse associated with the intrusion progressively faded away, deformation localized into anastomosing, mylonitic greenschist-facies shear zones surrounding lozenges of high-grade migmatitic schist. Mylonitic shear zones formed at low-angle with respect to the well-established high grade foliation preserved as a relic, oblique foliation. We show that such an extreme strain localization was determined by strain hardening of the no longer melt-bearing quartz-feldspar schist, localized embrittlement on precursory shear bands, and fluid-enhanced reaction softening that caused the breakdown of Al-silicates and the development of phyllosilicate-rich mylonitic bands. Consequently, tectonic structures with different orientation developed under the same kinematic regime, as a result of the changing physical and mechanical properties of the cooling rock volume.</p>

On brittle-ductile strain localization

2020 ◽  
Author(s):  
Yury Podladchikov
Keyword(s):  
Strain Localization ◽  
Shear Bands ◽  
Shear Zones ◽  
Thermal Runaway ◽  
Spontaneous Generation ◽  
Principal Stresses ◽  
Earth Planet ◽  
Brittle Ductile Transition ◽  
Shear Heating

<p>The classification of the strain localization modes is attempted around brittle-ductile transition. The stresses are high. The are a number of suspects: earthquake-like thermal runaway (Braeck et al. 2009), stable sliding as shear heating zones oriented 45 degrees to the principal stresses (Kiss et al. 2019), brittle faults/shear bands oriented ca. 30 degrees to the maximum compressive principal stress and mode 1 fracture. The coupling to the porous fluid hydrology is accounted for.  High resolution numerical simulations are compared to classical and newly derived composite asymptotic solutions.</p><p><strong>References</strong></p><p>Braeck, S., Podladchikov, Y., & Medvedev, S., 2009. Spontaneous dissipation of elastic energy by self-localizing thermal runaway, Phys. Rev. E , 80, 046105, doi:10.1103/PhysRevE.80.046105.</p><p>Kiss, D., Podladchikov, Y., Duretz, T., & Schmalholz, S., 2019. Spontaneous generation of ductile</p><p>shear zones by thermal softening: Localization criterion, 1D to 3D modelling and application to the</p><p>lithosphere, Earth Planet. Sci. Lett., 519, 284–296, doi:10.1016/j.epsl.2019.05.026.</p>

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.

Rheological properties of the shallow subduction interface: insights from the Chugach Complex, Alaska

2020 ◽  
Author(s):  
Zoe Braden ◽  
Whitney Behr
Keyword(s):  
Fluid Flow ◽  
Rheological Properties ◽  
Shear Zone ◽  
Strain Localization ◽  
High Strain ◽  
Shear Zones ◽  
Fluid Injection ◽  
Structural Mapping ◽  
Wide Range ◽  
Shallow Subduction

<p>The plate interface in subduction zones accommodates a wide range of seismic styles over different depths as a function of pressure-temperature conditions, compositional and fluid-pressure heterogeneities, deformation mechanisms, and degrees of strain localization. The shallow subduction interface (i.e. ~2-10 km subduction depths), in particular, can exhibit either slow slip events (e.g. Hikurangi) or megathrust earthquakes (e.g. Tohoku). To evaluate the factors governing these different slip behaviors, we need better constraints on the rheological properties of the shallow interface. Here we focus on exhumed rocks within the Chugach Complex of southern Alaska, which represents the Jurassic to Cretaceous shallow subduction interface of the Kula and North American plates. The Chugach is ideal because it exhibits progressive variations in subducted rock types through time, minimal post-subduction overprinting, and extensive along-strike exposure (~250 km). Our aims are to use field structural mapping, geochronology, and microstructural analysis to examine a) how strain is localized in different subducted protoliths, and b) the deformation processes, role of fluids, and strain localization mechanisms within each high strain zone. We interpret these data in the context of the relative ‘strengths’ of different materials on the shallow interface and possible styles of seismicity.  </p><p>Thus far we have characterized deformation features along a 1.25-km-thick melange belt within the Turnagain Arm region southeast of Anchorage.  The westernmost melange unit is sediment poor and consists of deep marine rocks with more chert, shale and mafic rocks than units to the east. The melange fabric is variably developed (weakly to strongly) throughout the unit and is steeply (sub-vertical) west-dipping with down-dip lineations. Quartz-calcite-filled dilational cracks are oriented perpendicular to the main melange fabric.</p><p>Drone imaging and structural mapping reveals 3 major discrete shear zones and 6-7 minor shear zones within the melange belt, all of which exhibit thrust kinematics. Major shear zones show a significant and observable strain gradient into a wide (~1 m) region of high strain and deform large blocks while minor shear zones are generally developed in narrow zones (~10-15 cm) of high strain between larger blocks. One major shear zone is developed in basalt and has closely-spaced, polished slip surfaces that define a facoidal texture; the basalt shear zone is ~1 m thick. Preserved pillows are observable in lower strain areas on either side of the shear zone but are deformed and indistinguishable within the high strain zone. The other two major shear zones are developed in shale and are matrix-supported with wispy, closely-spaced foliation and rotated porphyroclasts of chert and basalt; the shale shear zones are ~0.5-2 m thick.  </p><p>Abundant quartz-calcite veins parallel to the melange fabric and within shale shear zones record multiple generations of fluid-flow; early veins appear to be more silicic and later fluid flow involved only calcite precipitation. At the west, trench-proximal end of the mélange unit there is a 5-10 m thick silicified zone of fluid injection that is bound on one side by the basalt shear zone. Fluid injection appears to pre-date or be synchronous with shearing.</p>

Strain localization mechanism of graphitic carbon-bearing rocks: Constraints from microstructure, texture and graphite geothermometry

2021 ◽  
Author(s):  
Meixia Lyu ◽  
Shuyun Cao
Keyword(s):  
Shear Zone ◽  
Strain Localization ◽  
Carbonaceous Material ◽  
Shear Zones ◽  
Graphitic Carbon ◽  
Maximum Temperature ◽  
Low Grade ◽  
High Grade ◽  
Grade Metamorphism ◽  
Average Temperature

<p><strong>Abstracts:</strong></p><p>Graphitic carbon-bearing rocks can occur in low- to high-grade metamorphic units. In low-grade matamorphic rocks, graphitic carbon is often associated with brittle fault gouge whereas in middle- to high-grade metamorphic rocks, graphitic carbon commonly occurs in marble, schist or paragneiss. Previous studies showed that carbonaceous material gradually ordered from the amorphous stage, e.g. graphitization, is mainly controlled by increasing thermal metamorphism and has a good correlation with the metamorphic temperature. Besides, this ordered process is irreversible and the resulting structure is not affected by late metamorphism. Subsequently, the degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hanging-walls unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperature determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium–grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower–grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip–localized micro–shear zone and laterally continuous or discontinuous slip planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back–scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal <a>, rhomb <a> to prism <a> slip systems, which indicate increasing deformation temperatures. According to the graphitic slip–planes, micro–shear zones and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine–grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.</p><p><strong>Key Words:</strong> graphitic carbon, strain localization, graphitic thermometry, slip–localized micro–shear zone, rheological weakening</p>

scholarly journals Improvement of sub-level caving mining methods during high-grade iron ore mining

2021 ◽  
pp. 19-25
Author(s):  
A.V. Kosenko
Keyword(s):  
Mechanical Properties ◽  
Ultimate Strength ◽  
Uniaxial Compression ◽  
Physical Simulation ◽  
Physical And Mechanical Properties ◽  
High Grade ◽  
Iron Ores ◽  
Qualitative And Quantitative ◽  
Mining Methods ◽  
Quantitative Recovery

Purpose. To improve of the sub-level caving mining methods during mining of deposits of high-grade iron ores by applying a rational mode and intensifying the ore drawing. Methodology. It included an analysis of scientific literature, design documentation and the practice of mining of deposits of high-grade iron-ores in complex geomechanical conditions of deep horizons of mines, to establish the formation of fundamentally new foundations of scientific-and-design solutions for the rational extraction of minerals; numerical (application a special computer software package PFC 3D) and physical (based on the application of volumetric physical models and equivalent materials) modeling of ore drawing, aimed at identifying regularities of recovery process depending on the mining-geological and mining conditions of the mining of deposits, and also the physical- and-mechanical properties of the loosened ore mass. Findings. Regularities of changes in the qualitative and quantitative recovery percentages depending on the intensity of the ore drawing and physical-and-mechanical properties of the ore have been established using numerical and physical simulation. The obtained regularities made it possible to substantiate the rational parameters of the structural elements of the technological scheme for drawing and delivery of ore. A linear-alternating condition of ore drawing was developed, which will ensure an increase in the extraction of pure ore up to 10% and, as a result, up to 4.6% of quantitative and up to 5.2% of qualitative indicators of ore percentages, as well as up to 1.5% of absolute quality of mined ore mass. Originality. Power-law dependencies of the change in the angle of ore tapping on the intensity of the ore drawing and the ultimate strength of the ore for uniaxial compression have been established as well as dependencies of the increase in the volume of the tapping figure, when a linear-alternating condition of ore drawing is applied, on the intensity of the ore drawing, the height of the collapsed ore layer and the ultimate strength of the ore for uniaxial compression. Practical value. A linear-alternating condition of ore drawing has been developed, the implementation of which in practice makes it possible to increase the qualitative and quantitative recovery percentages and eliminate the human factor while observing the ore drawing planograms.

scholarly journals Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

2016 ◽  
Author(s):  
Frances J. Cooper ◽  
John P. Platt ◽  
Whitney M. Behr
Keyword(s):  
High Strain ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Lower Zone ◽  
Middle Crust ◽  
Metamorphic Core Complexes ◽  
Brittle Ductile Transition ◽  
Ductile Shear ◽  
Metamorphic Core ◽  
Whipple Mountains

Abstract. High strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada) suggest the presence of two distinct rheological transitions in the middle crust. (1) The brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear. (2) The localized-distributed transition or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths. In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. In some examples (e.g. the Whipple Mountains) the lower zone is spatially distinct from the detachment, although high-strain rocks from the lower zone were subsequently exhumed along the detachment. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization.

scholarly journals Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Solid Earth ◽  
2017 ◽  
Vol 8 (1) ◽  
pp. 199-215 ◽  
Author(s):  
Frances J. Cooper ◽  
John P. Platt ◽  
Whitney M. Behr
Keyword(s):  
High Strain ◽  
High Stress ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Middle Crust ◽  
Metamorphic Core Complexes ◽  
Brittle Ductile Transition ◽  
Ductile Shear ◽  
Metamorphic Core ◽  
Whipple Mountains

Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

scholarly journals Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

Solid Earth ◽  
2016 ◽  
Vol 7 (2) ◽  
pp. 579-598 ◽  
Author(s):  
Francisco José Fernández ◽  
Sergio Llana-Fúnez ◽  
Pablo Valverde-Vaquero ◽  
Alberto Marcos ◽  
Pedro Castiñeiras
Keyword(s):  
Subduction Zones ◽  
Shear Bands ◽  
Shear Zones ◽  
Ductile Deformation ◽  
High Grade ◽  
Nw Iberia ◽  
Planar Shear ◽  
Lower Crustal ◽  
Grade Rock ◽  
Tectonic Exhumation

Abstract. High-grade, highly deformed gneisses crop out continuously along the Masanteo peninsula and constitute the upper part of the lower crustal section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the early Ordovician (ca. 480–488 Ma) injection of felsic dioritic/granodioritic dykes at the base of the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma), prior to its exhumation. A SE-vergent ductile thrust constitutes the base of quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within migmatitic gneisses. A NW-vergent detachment displaced metasedimentary qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The tectono-metamorphic relationships of the basal ductile thrust and the normal detachment bounding the top of the migmatites indicate that both discrete mechanical contacts were active before the recumbent folding affecting the sequence of gneisses during their final emplacement. The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP–HT) Cabo Ortegal nappe. The necessary strain was accommodated by the development of a widespread main foliation, dominated by flattening, that subsequently localized to a network of anastomosing shear bands that evolved to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites were exhumed at > 3 mm yr−1, and the exhumation path involved a cooling of  ∼  20 °C/100 MPa, These figures are comparable to currently active subduction zones, although exhumation P–T trajectory and ascent rates are at the hotter and slower end in comparison with currently active similar settings, suggesting an extremely ductile deformation environment during the exhumation of qz-fsp gneisses within a coherent Cabo Ortegal nappe.

Fluid-total pressure partitioning in shear banding poro-visco-elasto-plastic media

2021 ◽  
Author(s):  
Yury Alkhimenkov ◽  
Beatriz Quintal ◽  
Yury Podladchikov
Keyword(s):  
Pressure Distribution ◽  
Strain Localization ◽  
Induced Seismicity ◽  
Total Pressure ◽  
Initial Conditions ◽  
Shear Bands ◽  
Fluid Pressure ◽  
Shear Zones ◽  
Fluid Injection ◽  
Plastic Media

<p><span>Fluid injection is one of the main triggers of induced seismicity. Accurate numerical modeling of such processes is crucial for the safety of many affected regions. We propose a high-resolution numerical simulation of the strain localization in elasto-plastic and poro-visco-elasto-plastic media with a particular focus on the fluid pressure distribution. The resolution of our numerical model is 10000 by 10000 grid cells. The simulation is accelerated using graphical processing units (GPUs), thus, the total simulation time is in the order of a few minutes. We implement a pressure-dependent Mohr-Coulomb plastic law and study the influence of fluid pressure on the triggering of shear bands. Mean stress is partitioned between fluid pressure and total pressure. This study is particularly important since the effective stress law (the difference between fluid and total pressures) controls brittle failure. We vary viscosity and permeability as well as initial conditions for fluid pressure to explore the physics of shear bands nucleation. We show that fluid pressure in hydro-mechanically coupled media significantly affects the strain localization pattern compared to only elasto-plastic media. Permeability and viscosity are important parameters that control the fluid pressure distribution in the localized shear zones. This work is a preliminary study to model induced seismicity due to the fluid injection in fluid-saturated rocks described as fully coupled poro-visco-elasto-plastic media. </span></p>

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