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.

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.

The role of fluids on strain localization at seismogenic depth: a case study from brittle-ductile faults from Olkiluoto Island, SW Finland

2020 ◽  
Author(s):  
Barbara Marchesini ◽  
Giulio Viola ◽  
Luca Menegon ◽  
Jussi Mattila ◽  
Gunnar Schwarz ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Strain Localization ◽  
Crustal Deformation ◽  
Inductively Coupled Plasma ◽  
Damage Zone ◽  
Fluid Pressure ◽  
Temporal Variations ◽  
Fault System ◽  
Brittle Ductile Transition ◽  
Fracture Development

&lt;p&gt;Fluids play a key role in weakening rocks, controlling crustal deformation from early fracture development to mature strain localization, fault nucleation and propagation through cumulative slip. In particular, at the brittle-ductile transition zone crustal deformation and fluid flow are mutually interconnected by repeating cycles of transient frictional and viscous deformation. Uncertainties remain, however, on the details of the micromechanical and chemical influence of fluids in facilitating strain localization processes.&lt;/p&gt;&lt;p&gt;N-S to NW-SE sub-vertical brittle-ductile faults cut across the Paleoproterozoic migmatitic basement of southwestern Finland on the island of Olkiluoto, where the Finnish authorities plan the construction of a deep repository for high-grade nuclear waste. The faults are characterized by a brittle&amp;#8211;ductile to fully brittle deformation style resulting from transient fluid pressurization. We investigated a representative fault by combining field and microstructural observations with fluid inclusion and mineral chemistry analysis on synkinematic and authigenic minerals in order to reconstruct the temporal variations of pressure, temperature, composition and salinity of the synkinematic fluids that controlled strain localization. Combined laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) and electron back-scattered diffraction analysis (EBSD) were also applied on authigenic sulphides to gain insights into their role upon strain accommodation and deformation-induced elemental transport and distribution at the microscopic scale.&lt;/p&gt;&lt;p&gt;Initial embrittlement of the Olkiluoto basement occurred under a first event of fluid overpressure conditions (&gt; 210 MPa) with formation of a diffuse network of joints and/or hybrid&amp;#8211;shear fractures in a volume that corresponds to the fault damage zone. Subsequent deformation was caused by repeated hydrofracturing induced by fluid pressure up to 210 MPa. Brittle ruptures affected a system that was otherwise under overall ductile conditions, as demonstrated by mutually overprinting veining, cataclasis and plastic deformation.&lt;/p&gt;&lt;p&gt;Later exhumation and cooling of the fault system to fully brittle conditions was aided by reactivation triggered by a distinct fluid ingress at lower pressure (140-180 MPa) and temperature (&amp;#8804; 300&amp;#176; C). Deformation was accommodated at that stage by the interplay of brittle fracturing and low-temperature crystal-plastic in sulphides. Strain and fluid flow created high diffusivity pathways within the pyrite crystal lattices contributing to- and enhancing the net transport of a significant range of heavy elements (e.g. Co, Ni, Cu, Sn, Ag, As, Sb, Pb). These data indicate that the studied fault zone acted as a chemically open system and fault valve.&lt;/p&gt;

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 (

Fluid flow within the damage zone of the Boccheggiano extensional fault (Larderello–Travale geothermal field, central Italy): structures, alteration and implications for hydrothermal mineralization in extensional settings

2010 ◽  
Vol 148 (4) ◽  
pp. 558-579 ◽  
Author(s):  
FEDERICO ROSSETTI ◽  
LUCA ALDEGA ◽  
FRANCESCA TECCE ◽  
FABRIZIO BALSAMO ◽  
ANDREA BILLI ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Fluid Inclusion ◽  
Hydrothermal Fluid ◽  
Damage Zone ◽  
Central Italy ◽  
Fluid Pressure ◽  
Lithostatic Pressure ◽  
Anisotropic Permeability ◽  
Ore Deposit ◽  
Permeability Structure

AbstractThe Neogene extensional province of southern Tuscany in central Italy provides an outstanding example of fossil and active structurally controlled fluid flow and epithermal ore mineralization associated with post-orogenic silicic magmatism. Characterization of the hydrodynamic regime leading to the genesis of the polysulphide deposit (known as Filone di Boccheggiano) hosted within the damage zone of the Boccheggiano Fault is a key target to assess modes of fossil hydrothermal fluid circulation in the region and, more generally, to provide inferences on fault-controlled hydrothermal fluid flow in extensional settings. We provide a detailed description of the fault zone architecture and alteration/mineralization associated with the Boccheggiano ore deposit and report the results of fluid inclusion and stable oxygen isotope studies. This investigation shows that the Boccheggiano ore consists of an adularia/illite-type epithermal deposit and that sulphide ore deposition was controlled by channelling of hydrothermal fluids of dominantly meteoric origin within the highly anisotropic permeability structure of the Boccheggiano Fault. The low permeability structure of the fault core compartmentalized the fluid outflow preventing substantial cross-fault flow, with focused fluid flow occurring at the hangingwall of the fault controlled by fracture permeability. Fluid inclusion characteristics indicate that ore minerals were deposited between 280° and 350°C in the upper levels of the brittle extending crust (lithostatic pressure in the order of 0.1 GPa). Abundant vapour-rich inclusions in ore-stage quartz are consistent with fluid immiscibility and boiling, and quartz ore vein textures suggest that mineralization in the Boccheggiano ore deposit occurred during cyclic fluid flow in a deformation regime regulated by transient and fluctuating fluid pressure conditions. Results from this study (i) predict a strongly anisotropic permeability structure of the fault damage zone during crustal extension, and (ii) indicate the rate of secondary (structural) permeability creation and maintenance by active deformation in the hangingwall of extensional faults as the major factor leading to effective hydraulic transmissivity in extensional terranes. These features intimately link ore-grade mineralization in extensional settings to telescoping of hydrothermal flow along the hangingwall block(s) of major extensional fault zones.

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 Fault reactivation and strain partitioning across the brittle-ductile transition

Geology ◽  
2019 ◽  
Vol 47 (12) ◽  
pp. 1127-1130 ◽  
Author(s):  
Gabriel G. Meyer ◽  
Nicolas Brantut ◽  
Thomas M. Mitchell ◽  
Philip G. Meredith
Keyword(s):  
Fault Slip ◽  
Fault Reactivation ◽  
Ductile Deformation ◽  
Tectonic Deformation ◽  
Gradual Change ◽  
Bulk Rock ◽  
Total Deformation ◽  
Bulk Strain ◽  
Brittle Ductile Transition ◽  
Ductile Flow

Abstract The so-called “brittle-ductile transition” is thought to be the strongest part of the lithosphere, and defines the lower limit of the seismogenic zone. It is characterized not only by a transition from localized to distributed (ductile) deformation, but also by a gradual change in microscale deformation mechanism, from microcracking to crystal plasticity. These two transitions can occur separately under different conditions. The threshold conditions bounding the transitions are expected to control how deformation is partitioned between localized fault slip and bulk ductile deformation. Here, we report results from triaxial deformation experiments on pre-faulted cores of Carrara marble over a range of confining pressures, and determine the relative partitioning of the total deformation between bulk strain and on-fault slip. We find that the transition initiates when fault strength (σf) exceeds the yield stress (σy) of the bulk rock, and terminates when it exceeds its ductile flow stress (σflow). In this domain, yield in the bulk rock occurs first, and fault slip is reactivated as a result of bulk strain hardening. The contribution of fault slip to the total deformation is proportional to the ratio (σf − σy)/(σflow − σy). We propose an updated crustal strength profile extending the localized-ductile transition toward shallower regions where the strength of the crust would be limited by fault friction, but significant proportions of tectonic deformation could be accommodated simultaneously by distributed ductile flow.

Parental Melts and Magma Storage of a Large-volume Dacite Eruption at Vetrovoy Isthmus (Iturup Island, Southern Kuril Islands): Insights into the Genesis of Subduction-zone Dacites

2019 ◽  
Vol 60 (7) ◽  
pp. 1349-1370
Author(s):  
S Z Smirnov ◽  
A V Rybin ◽  
N N Kruk ◽  
T Yu Timina ◽  
E N Sokolova ◽  
...  
Keyword(s):  
Fluid Inclusion ◽  
Large Volume ◽  
Peritectic Reaction ◽  
Fluid Pressure ◽  
Kuril Islands ◽  
Strong Increase ◽  
Magma Storage ◽  
Ti Oxides ◽  
Partial Melts ◽  
Parental Melts

Abstract Detailed mineralogical and melt and fluid inclusion constraints on magma storage, and the evolution of melts, are presented for the large-volume caldera eruption on the Vetrovoy Isthmus on Itutrup Island (Kuril Islands, Russia). The shallow magma reservoir beneath the Vetrovoy Isthmus is composed of a mush of plagio-rhyolitic melt, phenocrysts and the products of peritectic reaction(s). The melt appears to have formed as a result of partial melting of previously erupted rocks, which probably had andesitic to basaltic compositions and were metamorphosed into amphibole-bearing assemblages. The breakdown of amphibole in the partially melted precursor rocks led to the formation of early Mg-rich clino- and orthopyroxene, along with plagioclase and Fe–Ti oxides, and the release of aqueous fluids. Variations in fluid pressure are recorded by a strong increase of An contents in plagioclase. Crystallization took place at around 850°C with pressure ranging from 0·9 to 3 kbar. This study demonstrates that dacitic magmas erupted during the course of a 20 kyr voluminous eruption were the result of mixing between plagio-rhyolitic partial melts and the breakdown reaction minerals (i.e. pyroxenes, plagioclase and Fe–Ti oxides). Plagioclase and quartz were the last minerals to crystallize from these melts prior to eruption.

A Generalized Elasto-Plastic Micro-Polar Constitutive Model

2015 ◽  
Vol 798 ◽  
pp. 505-509 ◽  
Author(s):  
Lapo Gori ◽  
Roque Luiz da Silva Pitangueira ◽  
Samuel Silva Penna ◽  
Jamile Salim Fuina
Keyword(s):  
Structural Analysis ◽  
Constitutive Model ◽  
Numerical Method ◽  
Strain Localization ◽  
Continuum Model ◽  
Constitutive Models ◽  
Localization Problem ◽  
Polar Model ◽  
Basic Characteristics ◽  
Analysis Environment

This paper summarizes the implementation of an elasto-plastic constitutive model for a micro-polar continuum in the constitutive models framework of the software INSANE (INteractive Structural ANalysis Environment). Such an implementation is based on the tensorial format of a unified constitutive models formulation, that allows to implement different constitutive models independently on the peculiar numerical method adopted for the solution of the problem. The basic characteristics of the micro-polar continuum model and of the unified formulation of constitutive models are briefly recalled. A generalization of the micro-polar model is then introduced in order to include this model in the existent tensor-based formulation. Finally, an enhanced version of the general closest-point algorithm, ables to manage the generalized micro-polar formulation, is derived. A strain localization problem modeling illustrates the implementation.

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