scholarly journals Fluid-mediated, brittle-ductile deformation at seismogenic depth: Part II – Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)

2019 ◽  
Author(s):  
Francesca Prando ◽  
Luca Menegon ◽  
Mark W. Anderson ◽  
Barbara Marchesini ◽  
Jussi Mattila ◽  
...  
Keyword(s):  
Nuclear Waste ◽  
Electron Backscatter Diffraction ◽  
Microstructural Analysis ◽  
Fluid Pressure ◽  
Deformation Mode ◽  
Mineral Chemistry ◽  
Ductile Deformation ◽  
Rheological Parameters ◽  
Nuclear Waste Repository ◽  
Brittle Ductile Transition

Abstract. Microstructural record of fault rocks active at the brittle ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode, and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile-brittle deformation, with a narrow (

scholarly journals Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 489-511 ◽  
Author(s):  
Francesca Prando ◽  
Luca Menegon ◽  
Mark Anderson ◽  
Barbara Marchesini ◽  
Jussi Mattila ◽  
...  
Keyword(s):  
Shear Zone ◽  
Nuclear Waste ◽  
Strain Localization ◽  
Microstructural Analysis ◽  
Fluid Pressure ◽  
Rheological Parameters ◽  
Nuclear Waste Repository ◽  
Brittle Deformation ◽  
Differential Stress ◽  
Brittle Ductile Transition

Abstract. The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (<20 cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.

scholarly journals Fluid-mediated, brittle-ductile deformation at seismogenic depth: Part I – Fluid record and deformation history of fault-veins in a nuclear waste repository (Olkiluoto Island, Finland)

2019 ◽  
Author(s):  
Barbara Marchesini ◽  
Paolo Stefano Garofalo ◽  
Luca Menegon ◽  
Jussi Mattila ◽  
Giulio Viola
Keyword(s):  
Structural Analysis ◽  
Fluid Inclusion ◽  
Strain Localization ◽  
Damage Zone ◽  
Fluid Pressure ◽  
Ductile Deformation ◽  
Nuclear Waste Repository ◽  
Homogeneous State ◽  
Deformation History ◽  
Brittle Ductile Transition

Abstract. The dynamic evolution of fault zones at the seismogenic brittle-ductile transition zone (BDTZ) expresses the delicate interplay of numerous physical and chemical processes that occur at the time of strain localization. Deformation and flow of aqueous fluids in these zones, in particular, are closely related and mutually dependent during cycles of repeating, transient frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the role of fluids in facilitating deformation in this zone, particularly with regard to the mechanics of broadly coeval brittle and ductile deformation. We combine here structural analysis, fluid inclusion data and mineral chemistry data from synkinematic and authigenic minerals to reconstruct the temporal variations in P, T and bulk composition of the fluids that mediated deformation and steered strain localization in a strike-slip fault from the BDTZ. This is a fault formed within the Paleoproterozoic migmatitic basement of southwestern Finland, hosting in its core two laterally continuous quartz veins formed by two texturally distinct quartz types – Qtz I and Qtz II, where Qtz I is demonstrably older than Qtz II. Veins within the diffuse damage zone of the fault are infilled by Qtz I. Multi-scalar structural analysis indicates recurrent cycles of mutually overprinting brittle and ductile deformation. Fluid inclusion microthermometry and mineral pair geothermometry indicate that both quartz types precipitated from a fluid that was in a homogeneous state during the recurrent cycles of faulting, and whose bulk salinity was in the 0–5 wt % NaCleq range. The temperature of the fluid phase involved with the various episodes of initial strain localization and later reactivation changed with time, from c. 240 °C in the damage zone to c. 350 °C in the core during Qtz I precipitation to < 200 °C at the time of Qtz II crystallization. Fluid pressure estimates show an oscillation in pore pressure comprised between 160 and 10 MPa during the fault activity stages. Our results suggest significant variability in the overall physical conditions during the fault deformation history, possibly reflecting the interaction of several batches of compositionally similar fluids ingressing the dilatant fault zone at different stages of its evolution, each with specific T and P conditions. Initial, fluid-mediated embrittlement of the faulted rock volume generated a diffuse network of joint and/or hybrid/shear fractures in the damage zone, whereas progressive strain localization led to more localized deformation within the fault core. Localization was guided by cyclically increasing fluid pressure and transient embrittlement of a system that was otherwise at overall ductile conditions. Our analysis implies that fluid overpressure at the brittle-ductile transition can play a key role in the initial embrittlment of the metamorphic basement and strain localization mechanisms.

scholarly journals Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 1: Fluid record and deformation history of fault veins in a nuclear waste repository (Olkiluoto Island, Finland)

Solid Earth ◽  
2019 ◽  
Vol 10 (3) ◽  
pp. 809-838 ◽  
Author(s):  
Barbara Marchesini ◽  
Paolo Stefano Garofalo ◽  
Luca Menegon ◽  
Jussi Mattila ◽  
Giulio Viola
Keyword(s):  
Fluid Inclusion ◽  
Strain Localization ◽  
Damage Zone ◽  
Fluid Pressure ◽  
Fault Reactivation ◽  
Ductile Deformation ◽  
Nuclear Waste Repository ◽  
Structural Level ◽  
Localized Deformation ◽  
Brittle Ductile Transition

Abstract. The dynamic evolution of fault zones at the seismogenic brittle–ductile transition zone (BDTZ) expresses the delicate interplay between numerous physical and chemical processes. Deformation and fluid flow at the BDTZ are closely related and mutually dependent during repeating and transient cycles of frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the exact role of fluids in facilitating broadly coeval brittle and ductile deformation at that structural level. We combine structural analysis, fluid inclusion, and mineral chemistry data from synkinematic and authigenic minerals to reconstruct the temporal variations in fluid pressure (Pf), temperature (T), and bulk composition (X) of the fluids that mediated deformation and steered strain localization along BFZ300, a strike–slip fault originally active at the BDTZ. BFZ300 deforms the Paleoproterozoic migmatitic basement of southwestern Finland and hosts in its core two laterally continuous quartz veins formed by two texturally distinct types of quartz – Qtz I and Qtz II, with Qtz I older than Qtz II. Veins within the damage zone are formed exclusively by Qtz I. Mesostructural and microstructural analysis combined with fluid compositional data indicate recurrent cycles of mutually overprinting brittle and ductile deformation triggered by oscillations of fluid pressure peaking at 210 MPa. Fluid inclusion microthermometry and mineral pair geothermometry indicate that the two documented quartz types precipitated from different fluid batches, with bulk salinities in the 1 wt % NaCleq–5 wt % NaCleq range for Qtz I and in the 6 wt % NaCleq–11 wt % NaCleq range for Qtz II. The temperature of the fluids involved with initial strain localization and later fault reactivation evolved through time from > 350 ∘C during Qtz I precipitation to < 300 ∘C at the time of Qtz II crystallization. The peak fluid pressure estimates constrain pore pressure oscillations between 80 and 210 MPa during the recorded faulting episodes. Our results suggest variability of the physico-chemical conditions of the fluids steering deformation (Pf, T, X), reflecting the ingress and effects of multiple batches of fluid in the fault zone. Initial fluid-mediated embrittlement generated a diffuse network of joints and/or hybrid–shear fractures in the damage zone; subsequent strain localization led to more localized deformation within the fault core. Localization was guided by cyclically increasing fluid pressure and transient embrittlement of a system that was otherwise under overall ductile conditions. Our analysis suggests that fluid overpressure at the BDTZ can play a key role in the initial embrittlement of the deforming rock and steer subsequent strain localization.

Brittle–ductile fabrics and P–T conditions of deformation in the East Pernambuco shear zone (Borborema Province, NE Brazil)

2020 ◽  
Vol 178 (1) ◽  
pp. jgs2020-109
Author(s):  
Paulo Castellan ◽  
Gustavo Viegas ◽  
Frederico M. Faleiros
Keyword(s):  
Shear Zone ◽  
Microstructural Analysis ◽  
Mineral Chemistry ◽  
Prism Slip ◽  
Content Type ◽  
Fine Grained ◽  
Brittle Ductile Transition ◽  
Epma Analysis ◽  
Borborema Province ◽  
Supplementary Material

Fabrics of the East Pernambuco shear zone (EPSZ) were studied via microstructural analysis, mineral chemistry and isochemical phase diagram modelling to constrain the pressure and temperature conditions of deformation during shearing. Granitic mylonites show fractured feldspar porphyroclasts embedded in a fine-grained, recrystallized quartzo-feldspathic matrix. These mylonites grade laterally into banded ultramylonites characterized by stretched feldspar clasts alternated with recrystallized quartz bands. Fractures in these ultramylonites are filled by phyllosilicates. The mineral chemistry of the feldspars points to systematic changes between porphyroclasts, grains within fractures and fine-grained mixtures. Quartz crystallographic fabrics in the mylonites suggest activation of prism slip, while the ultramylonites show the activation of both rhomb and basal slip systems. Thermodynamic modelling suggests that the mylonites were formed at 4.75 ± 0.25 kbar and 526 ± 9°C, while the ultramylonites yield conditions of 5.9 ± 1 kbar and 437 ± 17°C. These observations suggest that the EPSZ records a heterogeneous path of strain accommodation, marked by decreasing temperature from its western sector to its eastern termination. The differences in metamorphic conditions are consistent with a transitional, brittle–ductile strain regime. Such characteristics indicate that the EPSZ is a Neoproterozoic shear belt nucleated and heterogeneously exhumed at the brittle–ductile transition, possibly in an intracontinental setting.Supplementary Material: EPMA analysis of feldspars in Caruaru and Gravatá domains and T-X(O2) pseudosections are available at https://doi.org/10.6084/m9.figshare.c.5125957

scholarly journals Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

2017 ◽  
Author(s):  
Giancarlo Molli ◽  
Luca Menegon ◽  
Alessandro Malasoma
Keyword(s):  
Shear Zone ◽  
Continental Crust ◽  
Fluid Pressure ◽  
Deformation Mode ◽  
Shear Zones ◽  
Small Scale ◽  
Plastic Shear ◽  
Stick Slip ◽  
Brittle Ductile Transition

Abstract. The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle-ductile transition and seismogenesis. In subduction environment, switching in deformation mode and mechanisms may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). On the other hand, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in the localization of deformation and shear zone development, thus representing a case in which switching deformation mode and mechanisms interact and relate to each other. This contribution analyzes an example of a crystal plastic shear zone localized by brittle precursor formed within a host granitic-protomylonite during deformation in subduction-related environment. The studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations may be considered as a small scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow slip structures. Our case-study represents, therefore, a fossil example of association of fault structures related with stick-slip strain accomodation during subduction of continental crust.

The Fabrication and Characterization of Spark Plasma Sintered Nickel Based Binary Alloy at Different Heating Rate

2019 ◽  
Vol 7 (3) ◽  
Author(s):  
B. J. Babalola ◽  
M. B. Shongwe ◽  
A. L. Rominiyi ◽  
P. L. Lepele ◽  
A. P. I. Popoola
Keyword(s):  
Relative Density ◽  
Heating Rate ◽  
Binary Alloy ◽  
Microstructural Analysis ◽  
Deformation Mode ◽  
Xrd Analysis ◽  
Ductile Deformation ◽  
Indentation Hardness ◽  
Sintered Alloy ◽  
Spark Plasma

In this paper, the effect of heating rate was investigated on the densification and mechanical properties of Ni–19at %Cr binary alloy developed by spark plasma sintering technique. The sintered binary alloy at different heating rate was prepared metallurgically for microstructural analysis, nanoindentation, and X-ray diffraction (XRD) analysis. Nanoindentation technique at room temperature was utilized in investigating the elastic modulus, and hardness of the alloys. However, nanoindentation tests showed that the maximum indentation hardness was obtained by the alloy sintered at heating rate of 100 °C/min, the hardness value and density were observed to decreases as the heating rate increases beyond 100 °C/min. Similar trend was observed for the relative density of the alloys, with the relative density decreasing as heating rate increases beyond 100 °C/min. The fractography analysis of the sintered alloy at different heating rate was characterized by ductile deformation mode.

scholarly journals Frictional behaviour of carbonate bearing faults at brittle-ductile transition

2021 ◽  
Author(s):  
Francesco Figura ◽  
Carolina Giorgetti ◽  
Mathias Lebihain ◽  
Marie Violay
Keyword(s):  
Normal Stress ◽  
Principal Stress ◽  
Fluid Pressure ◽  
Confining Pressure ◽  
Shear Loading ◽  
Experimental Results ◽  
Ductile Deformation ◽  
Critical Conditions ◽  
Brittle Ductile Transition ◽  
Wide Range

&lt;p&gt;One of the most alarming recent findings in geo-science is the dramatic rise in the rate of human-induced earthquakes in the past decade. This is due to the fluid injection or extraction in deep reservoirs for hydrocarbon production, wastewater and CO&lt;sub&gt;2&lt;/sub&gt; storage and exploitation of geothermal resources which result in the reactivation of nearby faults. These reservoirs are often located 2-3 km depth (i.e. 30 MPa), and are hosted in or covered by sedimentary carbonate layers. As carbonate undergoes a brittle-ductile transition with increasing confining pressure from values of around 20 MPa, ductile deformation can play an important role on the nucleation and propagation of earthquakes on carbonate faults. Here, we investigate the role of increasing ductile behaviour on fault frictional parameters. The research is performed through the new biaxial apparatus installed at EPFL, the &lt;em&gt;HighSTEPS &lt;/em&gt;(High Strain TEmperature Pressure Speed) apparatus, able to measure frictional parameters in a wide range of shearing velocities (10&lt;sup&gt;-6&lt;/sup&gt; m/s &amp;#8211; 0.2 m/s) and under unique boundary conditions representative of the Earth&amp;#8217;s crust, i.e., normal stress up to 100 MPa, confining pressure up to 100 MPa, pore fluid pressure up to 100 MPa and temperature up to 120&amp;#176;. The induced stress state in bare surface samples was previously studied by a comparison between results of FEM numerical analyses and experimental ones. Under shear loading conditions, the principal stress &amp;#963;1 is oriented at about 25&amp;#176; to the vertical axis, and the confining pressure corresponds to the principal stress &amp;#963;2. Tests are performed under different values of applied confining pressure (1 - 60 MPa) and normal stress (1.5 &amp;#8211; 90 MPa) on the faults, keeping constant the ratio between &amp;#963;n/&amp;#963;3 around ~ 3, to mimic faults at different depth. We present experimental results mapping carbonate fault mechanical behaviour from low shearing velocity 10&lt;sup&gt;-6&lt;/sup&gt; m/s to high shearing velocity 10&lt;sup&gt;-1&lt;/sup&gt; m/s. Moreover, experimental results are modelled with rate-and-state friction laws (RSFLs) to define rate and state parameters related to the critical conditions for fault stability and its dependence on the presence of ductile deformation. These results shed new light on the nucleation and propagation of earthquake within the brittle-ductile transition in carbonate bearing rocks.&lt;/p&gt;

scholarly journals A dislocation density-based model for the work hardening and softening behaviors upon stress reversal

2021 ◽  
Vol 21 (2) ◽  
Author(s):  
Paulina Lisiecka-Graca ◽  
Krzysztof Bzowski ◽  
Janusz Majta ◽  
Krzysztof Muszka
Keyword(s):  
Dislocation Density ◽  
Electron Backscatter Diffraction ◽  
Microstructural Analysis ◽  
Back Stress ◽  
Carbon Steels ◽  
Internal Variables ◽  
Low Carbon ◽  
Average Dislocation Density ◽  
Calculation Results ◽  
Mechanical Behaviours

AbstractThe mechanical behaviours of microalloyed and low-carbon steels under strain reversal were modelled based on the average dislocation density taking into account its allocation between the cell walls and cell interiors. The proposed model reflects the effects of the dislocations displacement, generation of new dislocations and their annihilation during the metal-forming processes. The back stress is assumed as one of the internal variables. The value of the initial dislocation density was calculated using two different computational methods, i.e. the first one based on the dislocation density tensor and the second one based on the strain gradient model. The proposed methods of calculating the dislocation density were subjected to a comparative analysis. For the microstructural analysis, the high-resolution electron backscatter diffraction (EBSD) microscopy was utilized. The calculation results were compared with the results of forward/reverse torsion tests. As a result, good effectiveness of the applied computational methodology was demonstrated. Finally, the analysis of dislocation distributions as an effect of the strain path change was performed.

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