Brittle Deformation Mechanisms Research Articles

Age, tectonic evolution and origin of the Aswa Shear Zone in Uganda: Activation of an oblique ramp during convergence in the East African Orogen

The Aswa Shear Zone (ASZ) is a major NW–SE trending structure of over 1000 km length in East Africa. In Uganda, the ASZ is a steeply NE-dipping, up to 11 km wide mylonitic shear zone that shows multiple stage brittle reactivation. On outcrop-scale, the fabric in the ASZ is characterized by a well-developed NW–SE striking and subvertical or steeply NE or SW dipping mylonitic foliation and a subhorizontal to moderately NW- or SE-plunging stretching lineation. Sinistral kinematics and fabric are very consistent along strike. The strain is heterogeneously distributed and partitioned into lens-shaped lower strain zones dominated by folding and characterized by pure shear, which are surrounded by high strain zones, some of them thick ultramylonites, with intense simple shear combined with flattening and strong transposition of pre-existing fabrics. Ductile shearing occurred during bulk E–W shortening, commenced at amphibolite facies conditions and continued with similar kinematics at greenschist and even lower grade conditions. A number of (sub-)parallel shear zones occur to the NE and SW of the main zone at a distance of up to 20–45 km. They show similar fabrics and kinematics and are thus related to activity along ASZ reflecting strain partitioning into simple shear and pure shear domains on a regional scale.Samples of mylonitic gneisses from the shear zone have been analyzed with U–Pb LA-MC-ICPMS and show Neoarchaean crystallisation ages between 2.66 and 2.61 Ga. Timing of ductile sinistral shearing is poorly constrained by lower intercept ages of 686 ± 62 and 640 ± 44 Ma. The fabric and structural relationship of the ca. 660 Ma Adjumani Granite exposed in the northern segment of ASZ suggest that the age of shear activity can be further limited to ca. 685 and 655 Ma.The Aswa Shear Zone is interpreted as an intra-cratonic, crustal-scale structure close to the northeastern margin of the Congo Craton, possibly inherited from previous continental extension. Early Aswa Shear Zone activation is linked to underthrusting of the Congo Craton and coeval high-grade metamorphism and intense deformation in the orogen interior. During E–W convergence between ca. 690 and 650 Ma, the NE-dipping ASZ was activated as an oblique ramp leading to deflection of the transport direction and concentration of non-coaxial strain and sinistral shear along the shear zone system. During progressive convergence, between ca. 645 and 620 Ma, sinistral shearing along ASZ changed to ductile–brittle deformation mechanisms, while thrusting took place in Pan-African belts in eastern and western Uganda. Late-orogenic brittle sinistral reactivation of the ASZ can be regarded as the result of continent collision and closure of the Mozambique ocean further to the east, that potentially caused lateral escape manifested in NW–SE striking sinistral shear zones in Kenya and the southern Arabina-Nubian Shield between 620 and 570 Ma.

Excess Coalbed Methane Production Mechanism in the Process of Coal Tectonic Deformation

Source and occurrence of Excess Coalbed Methane is a long-term concern research topic in Coal Geology and Structural Geology. Since it is essential to understand the outburst mechanism of coal gas, and to support the coalbed methane development projects as the theoretical basis. We found in the study that, huge imparity is behind the evolutionary trend on molecular structure and the mechanism of influence from different deformation. The thesis demonstrates its probable routes of gas evolution according to distinct deformation mechanisms of coal. In the role of brittle deformation mechanism, a rapidly formed advantage rupture surface along with sliding motion from which has worked on coal. As another result, mechanical energy has transformed into friction and kinetic energy during the process. Kinetic energy increases simultaneously, which brings some results, that the new generated gas molecule. While the chemical structure of coal remains in a steady-state and do not react easily an outburst with gas. Mechanical energy turns into strain energy through its ductile deformation mechanism. The dislocation or lamellar slip made disordered between the constitutional units of aromatic rings and aromatic lamellas, as soon as secondary structural defects created. On another hand, molecular motion accelerates and splits off the small molecular on the side chain, due to the dissociation of aromatic nucleus; CH4 gas molecular was generated and placed in the secondary structural defect of coal, along with a great deal of strain energy in non-steady-state. By breaking away the balance maintaining terms, huge strain energy releases suddenly, small moleculars are free from the secondary structure defect, react outburst with gas. Furthermore to extend the discussion of the conventional physical ideas on coal absorb gas, according to the phenomenon of exceeded CBM, the gas molecular has a significant chance existing in a low bond energy of chemical bonds of coal structure.

Upper–greenschist facies intragrain deformation of albite in mylonitic meta–pegmatite and the influence of crystallographic anisotropy on microstructure formation

We studied the deformation behaviour of albite from Permian meta–pegmatite in Cretaceous upper–greenschist facies shear zones from the Austroalpine Matsch Unit in the Eastern Alps (Italy). Sodium–feldspars from these rocks provide excellent natural examples for studying mechanisms of intragranular deformation under mid–crustal conditions in grains with different angular relations between their (010) planes and the kinematic frame. The studied rocks were deformed at c. 500 °C in localized shear zones with well characterized top–W shear kinematics supposedly during the Cretaceous upper–greenschist facies tectonometamorphic event. Microstructural and chemical data suggest that crystallographic anisotropies in albite exert a strong control on microstructure formation and that albite primarily deformed by a combination of brittle fracturing, dissolution–precipitation and incipient crystal plasticity as a function of the orientation of the crystallographic anisotropy relative to the supposed shortening direction. Dissolution along discontinuities forming stylolites perpendicular to the shortening direction is associated with the precipitation of fine–grained albite with some compositional variability (Ab96–98 and Ab89–91) in cracks. New albite precipitates form aggregates with straight segments of high angle grain boundaries, nearly 120° dihedral angles and only a poor or no orientation relation to the hosting clast. Intragranular kinking is related to continuous lattice rotation of up to 15° by a misorientation axis close to albite [100] and the formation of subgrain boundaries with maximum misorientations of 7°. Synthetic microshear zones supposedly nucleated on pre–existing cracks, and are associated with formation of subgrain boundaries in shortening quadrants and cracks together with precipitates of potassium feldspar in extensional quadrants adjacent to the microshear zone. New microstructural and textural data from mylonitic Permian meta–pegmatites document various closely linked crystal plastic and brittle deformation mechanisms and highlight the role of crystallographic anisotropies and their orientation with respect to the kinematic frame in microstructure formation.

Scratch deformations of poly (ether ether ketone) composites

The experimental results obtained from scratching a semicrystalline poly (ether ether ketone) surfaces, (PEEK), and its composites are presented in this paper. A semicrystalline PEEK and a carbon fiber oriented PEEK were scratched using conical indenters on a pendulum sclerometer. The carbon fiber oriented PEEK composites were scratched in the parallel, the orthogonal and the transverse direction to the fiber orientation. Subsequent deformations of the surfaces were assessed through subjective evaluation of the images obtained from a scanning electron microscope (SEM). The semicrystalline PEEK samples were found to be deformed by ductile ploughing and brittle deformation mechanism. In addition a fibrillation of the crystalline lamella of the polymer was also seen to be formed in case of severe brittle deformations. Fiber matrix debonding, matrix material debris formation, and fiber breakage were observed to be the dominant deformation mechanisms of the carbon fiber oriented composites. The scratch deformations of fiber oriented polymers were found to be highly dependent on fiber orientation angle relative to the scratching direction.

Terrestrial planet evolution in the stagnant-lid regime: Size effects and the formation of self-destabilizing crust

The ongoing discovery of terrestrial exoplanets accentuates the importance of studying planetary evolution for a wide range of initial conditions. We perform thermal evolution simulations for generic terrestrial planets with masses ranging from that of Mars to 10M⊕ in the stagnant-lid regime, the most natural mode of convection with strongly temperature-dependent viscosity. Given considerable uncertainty surrounding the dependency of mantle rheology on pressure, we choose to focus on the end-member case of pressure-independent potential viscosity, where viscosity does not change with depth along an adiabatic temperature gradient. We employ principal component analysis and linear regression to capture the first-order systematics of possible evolutionary scenarios from a large number of simulation runs. With increased planetary mass, crustal thickness and the degree of mantle processing are both predicted to decrease, and such size effects can also be derived with simple scaling analyses. The likelihood of plate tectonics is quantified using a mantle rheology that takes into account both ductile and brittle deformation mechanisms. Confirming earlier scaling analyses, the effects of lithosphere hydration dominate the effects of planetary mass. The possibility of basalt-eclogite phase transition in the planetary crust is found to increase with planetary mass, and we suggest that massive terrestrial planets may escape the stagnant-lid regime through the formation of a self-destabilizing dense eclogite layer.

Surface roughness evolution on experimentally simulated faults

To investigate the physical processes operating in active fault zones, we conduct analogue laboratory experiments where we track the morphological and mechanical evolution of an interface during slip. Our laboratory friction experiments consist of a halite (NaCl) slider held under constant normal load that is dragged across a coarse sandpaper substrate. This set-up is a surrogate for a fault surface, where brittle and plastic deformation mechanisms operate simultaneously during sliding. Surface morphology evolution, frictional resistance and infra-red emission are recorded with cumulative slip. After experiments, we characterize the roughness developed on slid surfaces, to nanometer resolution, using white light interferometry. We directly observe the formation of deformation features, such as slip parallel linear striations, as well as deformation products or gouge. The striations are often associated with marginal ridges of positive relief suggesting sideways transport of gouge products in the plane of the slip surface in a snow-plough-like fashion. Deeper striations are commonly bounded by triangular brittle fractures that fragment the salt surface and efficiently generate a breccia or gouge. Experiments with an abundance of gouge at the sliding interface have reduced shear resistance compared to bare surfaces and we show that friction is reduced with cumulative slip as gouge accumulates from initially bare surfaces. The relative importance of these deformation mechanisms may influence gouge production rate, fault surface roughness evolution, as well as mechanical behavior. Finally, our experimental results are linked to Nature by comparing the experimental surfaces to an actual fault surface, whose striated morphology has been characterized to centimeter resolution using a laser scanner. It is observed that both the stress field and the energy dissipation are heterogeneous at all scales during the maturation of the interface with cumulative slip. Importantly, we show that the formation of striations on fault planes by mechanical abrasion involves transport of gouge products in the fault plane not only along the slip direction, but also perpendicular to it.

Brittle-plastic deformation in initially dry rocks at fluid-present conditions: transient behaviour of feldspar at mid-crustal levels

We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones, constituting Stage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands (Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability, volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing.

Brittle plus plastic deformation of gypsum aggregates experimentally deformed in torsion to high strains: quantitative microstructural and texture analysis from optical and diffraction data

Abstract This contribution presents a quantitative microstructural analysis of a polycrystalline aggregate of gypsum, deformed in torsion ( T =70–90 °C) at γ (shear strain) ranging from 0 to 4.82. Quantitative microstructural analysis is used to compare the evolution of microstructures observed by optical microscope with those obtained from analysis of X-ray and neutron diffraction data. This analysis shows that during experimental deformation, gypsum accommodated strain by brittle and plastic deformation mechanisms, developing Riedel-like microfaults with plastic foliations and crystallographic preferred orientation (CPO). The relations of microstructures show that with increasing strain, the Riedel systems start from R planes with an angle of ≈30° to the Imposed Shear Plane. This angle decreases (5°–15°) when strain increases, and Y planes develop. Quantitative texture analysis (QTA) shows that S-foliations start developing at low γ and maintain their orientation up to high γ, and that the most active slip system is the (010) along normal to (100) and the [001]-axis. Shape preferred orientation (SPO) of gypsum does not coincide with the theoretical orientation as it does not decrease with increasing strain. This discrepancy is explained by the role of the brittle shear planes that impose a back rotation to gypsum. No brittle to plastic transition occurs. But both plastic and brittle structures contemporaneously accommodate and localize strain.

A multidisciplinary approach to the study of the fault networkin the internal Cottian Alps (Western Alps)

The architecture of the fault network and the late- to post-metamorphic structural evolution in the internal Cottian Alps have been studied by an integrated approach combining morphostructural analysis, field mapping and detailed mesostructural analysis between the Susa and Pellice Valleys. Available apatite fission track data indicate that this sector of the Western Alps reached shallow crustal levels, where brittle deformation mechanisms prevail, by Late Oligocene times. The analysed area comprises two main N-S striking regional, polyphasic semi-brittle to brittle deformation zones: the Colle delle Finestre Fault Zone (CFFZ) and the Col del Lis-Trana Deformation Zone (LTZ). The activity of those long-lived structures evolved, respectively, from dextral and dextral-reverse to extensional. In the block bounded by these two regional structures, the late- to post-metamorphic tectonic evolution is characterized by two faulting stages: (a) the development of an E-W left-lateral fault zone with a minor normal component of movement; at the regional scale, this fault zone may be considered as an antithetical shear of the major LTZ and the CFFZ; (b) the extensional reactivation and development of N-S normal faults, coeval with those recorded both by the LTZ and the CFFZ. The geometry of the fault and fracture systems has been compared with the remote sensed lineaments pattern, characterized by four main systems: Ln1 (N0°-30°E), Ln2 (N45°-70°E), Ln3 (N80°-100°E) and Ln4 (N100°-120°E). The lineament systems display similarities in terms of orientation, geometry and length with the fault systems observed at different scales. These similarities allowed us to extrapolate the structural model outlined in the mapped area to the adjoining sectors. Comparison between the different data suggests that the pattern of the lineaments detected between the LTZ and the CFFZ agrees with the architecture of a strike-slip fault network, which accommodated the internal deformation of the block bounded by these two major N-S transcurrent deformation zones. The transcurrent activity of these faults is here interpreted as coeval with the late dextral movements along the Canavese Line and with the propagation, in the Late Oligocene-Early Miocene, of the roughly E-vergent South-Alpine Thrusts beneath the Tertiary Piemonte Basin. The extensional event that followed may be related to the uplift with the subsequent isostatic readjustment of the chain which may have also induced the normal reactivation of the Penninic Frontal Thrust. This extensional regime is also in agreement with focal mechanisms of recent earthquakes.

Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures

Alpine antigorite serpentinites associated with eclogites were investigated to determine if they can be used as indicators of the tectono-metamorphic conditions during subduction and exhumation processes. The detailed petrology of serpentinites sampled in the Monviso massif (Western Alps, Italy) was combined with a transmission electron microscopy study. Alpine serpentinites display a degree of serpentinization close to 100%. Antigorite is the main mineral present, forming non-pseudomorphic textures in the various studied samples and exhibiting a homogeneous chemical composition with limited cationic substitutions. Considering its oceanic origin, the Alpine serpentinite in the Monviso massif formed a lizardite + chrysotile assemblage that recrystallized under greenschist-facies conditions into poorly ordered antigorite, with a modulation wavelength showing significant variations at the crystal scale. Under blueschist-facies conditions, the modulation wavelength of antigorite becomes regular. Thus, periodic antigorites can be related to high-grade conditions, while poorly ordered antigorites characterize lower metamorphic grade. In the present study, we failed to observe any elimination of structural defects with increasing metamorphic grade. While around 50% of the antigorite crystals are highly ordered, it seems that this ordering is at least partly obliterated by retrogressive deformation. Antigorite displays strong evidence of deformation-sensitivity, and the observed microstructures can be directly related to the mechanical behaviour of serpentinites in subduction zones. We investigated the deformation-induced microstructures in serpentinites collected in the Erro-Tobbio eclogitic unit (Ligurian Alps, Italy), which appear to preserve prograde and retrograde structures formed during subduction. According to the microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. Collected samples were fractured at different scales (cm to nm) and have a well-developed schistosity characterized by a strong crystallographic fabric. With increasing metamorphic grade, the brittle behaviour gives way to pressure-solution, which persists up to eclogite-facies conditions. The common obliteration of high-grade microstructures in antigorite, as observed in the Monviso serpentinites, results from continuous recrystallization of this mineral during retrogressive deformation.

Grain-size-sensitive flow and shear-stress enhancement at the brittle–ductile transition of the continental crust

Localized shear zones along low-angle normal faults have been identified in regions of extension at the brittle–ductile transition of the continental crust. The possibility of the strain localizing at a depth of 10 km is interpreted here as a consequence of an increase in the equivalent shear stress applied to the flow of the lower crust. This enhancement of the flow stress is seen as a prerequisite for the triggering of brittle deformation mechanisms leading to strain localization. The lower crust rheology used to examine this stress increase is strain-rate, temperature and grain-size dependent, due to the coupling of dislocation and diffusion creep. The model structure proposed consists of a top layer, the upper crust, gliding rigidly above a bottom layer, the lower crust, which deforms in simple shear. During a short time interval (1400 years), the equivalent shear stress is found to increase by a factor of up to 3 (67 MPa for anorthite and 17 MPa for quartz). For anorthite, this stress could explain the activation of a Mohr-Coulomb failure with a friction coefficient of 0.2, which is reasonable at the depth of 10 km. Dislocation creep is activated during a rapid change in the prescribed velocity, whereas diffusion creep dominates if the velocity is held constant, highlighting the importance of grain-size sensitivity for lower crustal rheology.

Deformation partitioning during folding of banded iron formation

Domainal electron and optical microscopy and c-axis fabric analysis are utilized to document microstructures and the associated deformation partitioning between crystal plastic, brittle and fluid-assisted deformation mechanisms during folding of Proterozoic banded iron formation (a hematite–quartz–calcite multilayer) from the Quadrilátero Ferrı́fero granite–greenstone terrain (southeastern Brazil). The operation of different mechanisms was partially determined by the contrasting rheologic response of these three minerals at greenschist facies metamorphic conditions. Fracturing was the main deformation process in hematite, while solution-transfer accounted for part of the deformation in both calcite and quartz. Notably, there is strong interaction among the different deformation mechanisms, which have influenced each other in various ways. Fracturing of hematite in hinge zones of folds caused opening of gaps, which were sealed by direct precipitation of silica from the fluid phase. As a consequence, a chemical potential gradient between crystals and fluid phase was produced, and quartz was dissolved to restore the thermodynamical crystal–fluid equilibrium. Thus, brittle deformation of hematite partially controlled solution-precipitation creep in quartz. Heterogeneous access of fluid into the deforming medium also affected the deformation processes. Inhomogeneous deformation in the quartz–calcite aggregates generated intergranular porosity and increased fluid access, with solution-transfer becoming dominant in these domains. In contrast, the relatively more homogeneous deformation in the pure quartz aggregates served to maintain well-adjusted grain boundaries and reduced fluid access into the intergranular space, such that these domains deformed uniformly by crystal plastic processes, at relatively dry conditions.

Brittle–ductile deformation in the Glarus thrust Lochseiten (LK) calc‐mylonite

The Glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1‐m layer of ‘Lochseiten’ (LK) calc‐mylonite. This layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture. Lattice‐ and shape‐preferred orientations are observed within the fine‐grained, recrystallized matrix. These features indicate the alternate activity of brittle and ductile deformation mechanisms. In contrast to the classical view that grain boundary sliding (superplasticity) is the dominant deformation mechanism, it is advocated that fluids, derived from the footwall and expelled along the thrust, are responsible for hydrofracturing and cataclastic deformation. In periods between fracture events, deformation was ductile. In this new interpretation, a substantial amount of the total thrust displacement was accommodated by numerous short‐lived and strongly localized fracture events at the base of the Verrucano thrust sheet, rather than a permanently weak décollement lithology.

‘See-through’ deformation experiments on brittle–viscous norcamphor at controlled temperature, strain rate and applied confining pressure

An integrated experimental–analytical deformational system allows the observation of evolving microstructures in rock analogues under an optical macroscope at controlled confining pressure ( P c=0.001–0.3 MPa), temperature ( T=−10–150°C) and strain rate (10 −2–10 −6 s −1). This system is ideally suited for studying grain-scale processes across the complete spectrum of deformational behaviour from brittle (cataclastic) to fully viscous (mylonitic) flow. The experimental system comprises two main elements: (1) a modified Urai–Means deformation rig, and (2) a steel pressure vessel that accommodates the deformation rig, yet is small enough to fit under an optical microscope. First results of controlled confining pressure experiments on fluid-bearing polycrystalline norcamphor, an organic quartz analogue, indicate that deformation involves a combination of dislocation creep and intergranular hydrofracturing leading to the formation of discrete shear surfaces comparable to C′ planes in mylonite. At applied confining pressure, the relative activity of brittle and viscous deformation mechanisms varies across the sample. This in turn affects the distribution of strain on the sample scale. The pressure-dependence of strain localization in these experiments is interpreted to reflect a combination of local variations in the effective stresses and strain incompatibilities associated with the experimental, simple-shear configuration.

Cataclastic solution creep of very soluble brittle salt as a rock analogue

Until about the late 1960s, macroscopically ductile deformation of quartz was seen as a microscopically cataclastic process by most geologists (cf. the origin of the name `mylonite'). Undulatory extinction, subgrains, recrystallised grains and even crystallographic preferred orientations were interpreted as due to water-assisted brittle deformation processes. Nowadays, by contrast, the occurrence of these optical microstructures is considered as conclusive and unequivocal evidence for dislocation creep. The abundance of these microstructures in naturally deformed rocks lead to the conclusion that dislocation creep is the most important ductile deformation mechanism within the Earth's crust. We studied whether a water-assisted brittle deformation mechanism could, in principle, be able to produce apparent `crystal plastic' microstructures, and how. To this end we performed a long-term deformation experiment using soluble brittle salt (NaClO 3) as an analogue for quartz. A single crystal of NaClO 3 was uniaxially stressed for 44 days at room temperature in the presence of a saturated NaClO 3 solution under atmospheric pressure. The crystal was microscopically studied during the experiment. It slowly deformed by cataclastic creep. First, irregular free face dissolution structures developed at highly stressed portions of the crystal. These locally developed into very fine channel-like structures, mostly oriented sub-parallel to crystallographic planes. Most of the channels developed into slits and finally into fractures. Fracture walls migrated by solution transfer leading to changes in shape of the crystal fragments. The resulting polygonal deformation microstructure could easily be mistaken for a dynamically recrystallised `crystal plastic' microstructure. Therefore, care should be taken to use such optical microstructures as conclusive evidence for dislocation creep. This is important, because the occurrence of these microstructures is the strongest argument for dislocation creep in crustal rocks.

Fluid impregnation and development of fault breccias in the Tavetsch basement rocks (Sedrun, Central Swiss Alps)

Microstructural, mineralogical and microthermometric studies provide an insight into the brittle deformation mechanisms and the P– T– t– x evolution of fault breccias from the Tavetsch massif. The fault breccias are overprinted by several pre- to early Alpine deformation events (summarized as deformation stage I), and at least five Alpine to late Alpine deformation stages (II to VI). Stages III to VI were responsible for the generation of cohesionless fault breccias. The predominant loss of cohesion occurred at a depth of 9–6 km, 280–190°C and fluid pressures of 2.3–0.6 kbar. These conditions prevailed between deformation stages IV and V. The loss of cohesion is interpreted to be the result of pore space interconnection induced by brittle deformation. This process was accompanied by a decrease in fluid pressure towards hydrostatic conditions. Finally stage VI generated fault gouge and further fault breccias due to small displacements (millimetre to centimetre range) along discrete faults and fractures. The fluid evolution shows a retrograde trend to lower salinities, indicating a meteoric influence during hydrostatic conditions.

Kinematics of postseismic relaxation from aftershock focal mechanisms of the 1994 Northridge, California, earthquake

Geodetic observations of surface deformation associated with the 1994 Northridge, southern California, earthquake generally are reproduced by simple models of a large‐scale elastic dislocation on a blind or buried thrust fault. The smaller‐scale aftershocks of the Northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock. These aftershocks, averaged over volumes that are large relative to their rupture radii, reflect a distributed, permanent deformation that is accommodated by local brittle fracture. We use a micropolar continuum model to invert the aftershocks in such volumes for the average incremental strain, and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain. Aftershock deformation that occurred at depths below about 6 km, and which is associated with the primary rupture zone, is consistent with slow continuation of the southwest‐side‐up reverse slip on the blind Northridge thrust fault. In contrast, aftershock deformation from the upper 5–7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal NE‐SW shortening and horizontal NW‐SE (i.e., fault‐parallel) extension. This pattern of deformation is similar to that associated with the mainshock, as observed geodetically and as calculated from the elastic dislocation model. We interpret that the aftershock activity in the hanging wall represents the quasi‐ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block. The aftershocks along the mainshock rupture zone are interpreted as resulting from either (1) the time‐dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or (2) the continued slip in the weakened fault zone driven by the deformation of a ductile‐elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust. In either case, the aftershock activity suggests that the crust undergoes quasi‐ductile flow as a brittle‐elastic material, and is not a strictly elastic material.

Strength characteristics and shear acoustic anisotropy of rock core subjected to true triaxial compression

Results are presented from an initial experimental programme aimed towards evaluating the capabilities of a new true triaxial cell, designed to apply independent and unequal principal stresses to the curved surfaces of cylindrical core plugs. A series of discrete failure tests on dry specimens from two sandstone lithologies exhibiting different deformation, strength and poroperm characteristics, were conducted under azimuthal stress anisotropy (σ2>σ3) with σ1 being applied axially. The true triaxial cell consistently orientates induced brittle shear fractures so that they strike parallel to the direction of σ2, and slip against the direction of least confinement, σ3. Both peak (fracture) and residual (friction) strengths are shown to be strongly dependent on the magnitude of the applied σ2, as well as on that of σ3. Results from multi-failure state testing using the conventional “triaxial” compression configuration are contrasted with discrete failure tests conducted in the true triaxial cell, by means of the familiar von Mises and extended 3-D Griffith criteria. Digitised records of shear-waves obtained at 40, 60 and 80% of peak failure strength during true triaxial testing, show clear evidence of progressively increasing stress-induced “splitting” or birefringence between the arrival of the faster S1(∥σ2) and the slower S2(∥σ3) shear-wave. Microseismic data and macroscopic observations from discrete failure tests performed within the true triaxial cell, are thus supportive of a brittle deformation mechanism involving stress-induced dilatant microcracks extending parallel to σ2 and opening against σ3, progressively coalescing with increasing σ1 to form a pervasive fault also oriented by the applied 3-D stress field.

Seismic shear demand of ductile cantilever walls: a Canadian code perspective

During severe earthquakes, ductile flexural walls are expected to exhibit inelastic flexural behaviour while other brittle deformation mechanisms, such as shear, should remain elastic. The philosophy of the Canadian seismic provisions for flexural walls is based on the assumption that the force reduction factor is applicable to both flexure and shear. If the bending moments are limited because of the flexural strength of a wall, then the shear forces are considered to be limited by the same ratio. Recent case studies have not confirmed this philosophy. Brittle shear failures in walls are still possible even if their shear strengths are established by the Canadian standards. This paper presents an analytical investigation on the shear demand of ductile flexural walls designed for three different seismic zones in Canada. For each zone, an ensemble of code compatible historical earthquake ground motions is identified. The shear demand of each structure, under each earthquake record, is obtained by nonlinear time-history dynamic analyses. In 77% of the cases, the computed dynamic shear demand is higher than the current code shear strength. To address this issue, a force modification factor for shear, different from the one for flexure, is suggested for the Canadian code. Key words: earthquake, seismic response, shear walls.

Effects of differential reactivity of minerals on the development of brittle to semi-brittle structures in granitic rocks: Textural and oxygen isotope evidence

Microstructural studies of fault zones in two post-tectonic granitic plutons indicate that deformation was profoundly influenced by the differential reactivity of quartz, microcline and plagioclase. In rocks affected by faulting at 200–350°C, plagioclase undergoes intragranular fracturing and syntectonic alteration to various hydrous mineral aggregates. In contrast, microcline and quartz show little or no alteration, and define cataclastic microstructures. Elongated remnants of plagioclase and its alteration products wrap around porphyroclasts of microcline and quartz to form an anastomosing foliation. The contrast in style of deformation between plagioclase and that of microcline and quartz may be due to different reaction softening rates, controlled by the relative stability of microcline and plagioclase under the ambient temperatures of 200–350°C, and K H + and Na H + activities of pore fluids. Fracturing and preferential alteration of plagioclase, under these conditions, is a strain softening feedback process in which pore fluids enhance reaction, and fracture propagation increases permeability and fluid infiltration. The relative reaction softening rates, plagioclase > microcline > quartz, deduced in this study conform to the mineral dissolution rates previously published by Lasaga. Fault zone tectonites are generally 18O-enriched ( δ 18 O = + 8.9 to + 9.2‰ ) or depleted ( δ 18 O = + 8.2 to + 8.9‰ ) relative to undeformed granitic protoliths, signifying open-system infiltration of fluids during deformation. Relative to undeformed granite where δ 18 O plagioclase = + 8.3‰ , microcline = + 7.3‰ and quartz = + 8.8‰ ; δ 18O-values in tectonites for plagioclase (+ 7.1 to + 9.6‰) are more scattered than those for microcline (+ 7.7 to + 8.8‰); and quartz is rarely 18O-shifted. These results confirm the reactivity order plagioclase > microcline > quartz deduced from textural relationships. The observed deformational characteristics imply that, in addition to environmental factors such as temperature, confining pressure, strain rate and differential stress, the stability or alteration rates of minerals, and open- vs. closed-system conditions, also influence the brittle and/or ductile deformation mechanisms of quartzo-feldspathic rocks in macroscopic, brittle and semi-brittle faults.