Clay-rich Fault Gouge Research Articles

How clay delamination supports aseismic slip

Abstract Aseismic slip is a stable fault slip, which allows strain to be relieved smoothly. Aseismic slip prevents the earthquake propagation, but it could nucleate an earthquake elsewhere. Understanding the mechanism of aseismic slip is promising in revealing the seismic cycle. Experimental evidence showed clay-rich fault gouge bears a low-friction strength, and the friction is strengthened with slip velocity (velocity-strengthening), which was thought to support aseismic slip. Clay minerals are comprised of platy crystalline layers with water intercalated between them, which may act as a lubricant. Sliding between clay layers was suspected to support aseismic slip but lacked a clarified mechanistic insight. We use non-equilibrium molecular dynamics simulations to show that shear-induced interlayer sliding is frictionally weak and velocity-strengthening, which evidences the role of clay minerals in aseismic slip. We find that interlayer water is a viscous fluid at most times, which explains the shear response of interlayer sliding. Depending on temperature and pressure conditions, intercalated water can be monolayer or bilayer, fluidic or ice like. Shear induces ice-like water to transform into fluidic water, which happens as a stick-slip phenomenon reflecting a first-order transition. Increased pore fluid pressure leads to the transformation from monolayer to bilayer intercalated water, resulting in a lower friction strength and enhanced velocity-strengthening behavior. Our work suggests that disclosing the hydration state of a clay mineral is preliminary when studying fault mechanics.

Eocene fault-controlled fluid flow and mineralization in the Paradox Basin, United States

Abstract Sedimentary rocks of the Paradox Basin of the Colorado Plateau (southwestern USA) record widespread manifestations of paleo–fluid flow and fluid-rock reactions including Cu, U-V, and Fe-Mn mineral deposits, Si and Ca metasomatism, hydrocarbon accumulations, and bleached sandstones. Many of these are spatially associated with faults. Here we show evidence for a widespread phase of fault-related fluid migration and mineralization at 41–48 Ma in the Paradox Basin. We measured K-Ar dates of multiple size fractions of clay-rich fault gouge, yielding statistically overlapping dates of authigenic (1Md) illite for the Salt Valley (47.0 ± 3.0 Ma), Kane Springs (47.7 ± 3.8 Ma), Cliffdweller (43.4 ± 4.6 Ma), Courthouse (41.9 ± 2.3 Ma), Lisbon Valley (45.3 ± 0.9 Ma), and GTO (48.1 ± 2.6 Ma) faults. The latter two have an illite Rb-Sr isochron age of 50.9 ± 3.5 Ma, and fault-adjacent bornite has a Re-Os isochron age of 47.5 ± 1.5 Ma. Authigenic illite from a paleo–oil reservoir near the Courthouse fault formed from the interaction of reduced fluids with oxidized red-bed sandstones at 41.1 ± 2.5 Ma. The Moab and Keystone faults have older authigenic illite ages of 59.1 ± 5.7 Ma and 65.2 ± 1.0 Ma, respectively. Our results show a close temporal relationship between fault gouge formation, red-bed bleaching, and Cu mineralization during an enigmatic time interval, raising questions about drivers of Eocene fluid flow.

Influence of the microstructure and roughness of weakness planes on the strength anisotropy of a foliated clay-rich fault gouge

Cataclastic rocks, as clay-rich fault gouges, are commonly present in brittle rock masses when fault zones appear during geological engineering projects. Highly deformed rocks that are of poor mechanical quality can lead to technical, safety, and economic problems in rock engineering. The aim of this study is to characterise the resistant behaviour of a highly deformed clay-rich gouge >40 m wide with a marked tectonic fabric that indicates strength anisotropy. We present the results of consolidated-undrained (CU) triaxial tests that were performed at low confining pressures (50, 150, and 300 kPa) on several sets of foliated gouge specimens with four different orientations in the tectonic fabric. Specimens were collected from the encapsulated rock cores of two research boreholes drilled through the Alhama de Murcia Fault (AMF), a main regional fault located in SE Spain. The strain–stress relationships and failure modes were established, indicating that the gouge behaves as hard soil or very soft rock. The test results were adjusted at each orientation using the non-linear Hoek and Brown criteria by considering the fault gouge as an intact material or as a tectonised rockmass. Here, we use the Geological Strength Index (GSI) as an indicator of the rockmass strength that depends on the direction of the tectonic fabric. However, the results from specimens with tectonic fabric that is oriented most favourably for failure were not the weakest in terms of rock strength. Such an anomalous result could be the result of asymmetry in the roughness of the weakness planes that is related to the original gouge microstructure characterised by the strong reorientation of clays in an S-C′ like tectonic fabric.Our results will be useful for practical applications that are related to the stability of slopes and/or shallow underground excavations in brittle fault zones, and provide an inexpensive and easy way to preliminarily evaluate the anisotropic behaviour of this type of brittle fault zones for future engineering projects.

Economic and fault stability analysis of geothermal field development in direct-use hydrothermal reservoirs

The installed capacity of geothermal systems for direct use of heat is increasing worldwide. As their number and density is increasing, the their interaction with subsurface faults becomes more important as they could lead to safety risks from induced seismicity. Assessment and management of such risks is essential for the further development and extension of geothermal energy for heating. At the same time, the economic output of geothermal systems can be marginal and is hence often supported by subsidy schemes. A combined assessment of fault stability and economic output could help operators to balance economic and safety aspects, but this is currently not common practice. In this study we present a methodology to assess field development plans based on fault stability and Net Present Value (NPV) using reservoir simulations of a fluvial, heterogeneous sandstone representative of the majority of direct-use Dutch geothermal systems. We find that the highest friction coefficient leading to exceedance of the Mohr–Coulomb failure criteria in this sandstone is 0.17; such values could be encountered in clay-rich fault gouges. Similar or lower fault permeability compared to the reservoir results in no changes and an increase respectively of both NPV and fault stability with larger Fault-to-Well Distance (FWD). Fault permeability higher than the reservoir permeability results in a minor increase in NPV with smaller FWD. Our results demonstrate that a combined analysis of thermal, hydraulic, mechanical and economic assessment supports a responsible and viable development of geothermal resources at a large scale. The importance of a high spatial density of supporting stress data will be essential for a better understanding and quantification of economic and fault stability effects of geothermal operations.

Effects of sample preparation on the microstructural signatures of faulting in clay-bearing fault gouge

Clay-rich fault gouge in the principal slip zones of faults stores abundant water within pores and between clay interlayers. During the preparation of thin-sections and rock chips for microstructural observations, fault-gouge samples are commonly air-dried at room temperature or in an oven. However, during the drying process, remnant liquid water between gouge grains produces an inter-particle adhesion force (liquid-bridge force), which rearranges the grain-to-grain structure, likely resulting in a disturbance of the original fabric. For this study, we prepared gouge samples from the Itozawa fault, northeastern Japan, using a t-butyl alcohol freeze-drying method that mitigates drying-induced fabric disturbance. We then compared the microstructure of our samples with those prepared using the conventional air-drying method. The freeze-dried samples preserve a smooth fault plane, clearly defined nanoparticles, and well-developed shear-sense indicators, including slickenlines and Riedel shear planes. In contrast, the air-dried samples underwent shrinkage during drying, which distorted the geometry of the fault plane. These air-dried samples lack nanoparticles and display only a weak shear fabric. We conclude that microstructural observations on samples prepared using the t-butyl alcohol freeze-drying method, compared with conventional air-drying, could preserve more evidence for the retrieval of fault information, including the kinematics, slip stability, and dynamic weakening mechanism of a fault.

Clay mineral dating of displacement on the Sronlairig Fault: implications for Mesozoic and Cenozoic tectonic evolution in northern Scotland

AbstractTemporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

Assessing Chemo-mechanical Behavior Induced by CO2-Water-rock Interactions in Clay-rich Fault Gouges

Ensuring sealing integrity is key for long-term (>10 000 years) storage of CO2. Fault reactivation and leakage can potentially be impacted by (often slow) CO2-water-rock interactions. We used the predictive power of geochemical modelling coupled to mechanical experiments on simulated fault gouge material to study the impact of long-term CO2-water-rock interactions on fault friction and stability. Of particular interest were the conditions (mineralogical composition, temperature, slip velocity) that may result in stable (velocity-strengthening) vs. unstable (velocity-weakening), and thus potentially seismogenic, behavior. Our preliminary results show that this approach poses a promising avenue for assessing fault stability.

Fibrous clay mineral authigenesis induced by fluid-rock interaction in the Galera fault zone (Betic Cordillera, SE Spain) and its influence on fault gouge frictional properties

This study presents a mineralogical and mechanical analysis of the clay-rich materials and structures identified in the Galera Fault Zone, southern Spain that formed as a consequence of active deformation processes affecting this seismically active region. Significant differences in clay mineral assemblages and chemical composition were identified in rocks from the Galera Fault Zone, through a series of analytical techniques including XRD, SEM, TEM and XRF. Three distinct mineral assemblages were identified: 1) wall-rock assemblages including white marls and dark lutite layers, with the latter also found in injection features. Their assemblage includes dolomite, gypsum, quartz, calcite and phyllosilicates 2) smectite- and palygorskite-rich fault gouges formed on materials from the upper part of the stratigraphic sequence at the NE area of the fault (Galera Village), and 3) sepiolite-rich gouges in areas of the lower part of the stratigraphic sequence at the central SW segment of the fault (Rambla de los Pilares). Fibrous clay-rich gouges were formed by hydrothermal alteration during periods of fluid-rock interaction that was concentrated in fault planes and fractures. Their mineralogy is dominated by authigenic Mg-rich fibrous clay minerals; sepiolite, precipitated directly from an Mg-rich fluid; and palygorskite as the product of the interaction of the fluid with the Al-rich host rock. Experimental data from frictional sliding experiments on these clay-rich fault gouges reveal strong differences in their mechanical properties. Towards the north-eastern areas of the fault, the smectite- and palygorskite-rich gouge has a low friction coefficient (0.17 wet and 0.59 under vacuum) and its values of the friction stability parameter (a–b) are always positive indicating a stable regime that could be related with creeping and stable-sliding processes. In the central-south-western segment, however, the gouge is rich in authigenic sepiolite and presents complete absence of smectite. The higher friction coefficient (0.47 wet and 0.68 under vacuum) and negative values of a–b for this gouge suggest a more neutral to unstable regime that could favour earthquake nucleation.

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

The Silltal Fault is the northern brittle continuation of the Brenner Fault Zone, marked by a narrow zone of cataclasis and, in three locations, clay-rich fault gouges. The clay mineral composition of these gouges is dominated by higher temperature 2M1 polytype illite/muscovite, with no 1M/1Md illite or mixed layer illite/smectite detected. Smectite is limited to the northern samples from the Stephansbrucke location, whereas chlorite is present in all samples. The K–Ar ages from the different sample size fractions (<0.1, <0.4, <2, 2–6, 6–10 μm, “whole rock gouge”) show a wide spread, from ca. 115 to 12 Ma, with ages consistently decreasing with grain size. Although the ranges overlap, ages from the northern Stephansbrucke samples are generally older (115–36 Ma) than those from the south near Matrei (55–12 Ma), possibly reflecting increasing regional metamorphic temperatures to the south. The well-defined linear relationship between apparent age and hydrogen stable isotope (δD) values establishes a direct correlation between rejuvenation of the K–Ar system and increased interaction with meteoric water introduced and focussed within the fault zone. The dependence of both apparent age and δD on grain size also indicates that radiogenic and stable isotope exchange was controlled by grain size, reflecting new 2M1 illite growth, mechanical grinding of protolith muscovite during cataclastic faulting, or both. The results demonstrate the advantages of combining radiogenic and stable isotope analysis in interpretation of K–Ar ages from clay fault gouges. The combined approach was necessary to establish the crucial influence on apparent K–Ar ages of meteoric water influx focussed on the fault zone and its interaction with clay-size-fraction grains.

Real-time drilling mud gas monitoring records seismic damage zone from the 2008 Mw 7.9 Wenchuan earthquake

We investigate the drilling mud gas from 70m to 1200m borehole depth of the Wenchuan earthquake Fault Scientific Drilling Hole-1 (WFSD-1) in the context of rapid response fault zone drilling to large earthquakes. Complete depth profiles are accomplished for CH4, He, H2, CO2, N2, Ar, O2 and N2/Ar. 222Rn is also available for a limited depth. The volcanic hanging wall generates high Rn and He, whereas the CH4 is low. The sedimentary footwall yields higher CH4 and CO2, with CH4 peaks up to 2.5vol.% in the intensely fractured shale and siltstone from 640m to 715m.The drilling mud gases distribute asymmetrically across the co-seismic slip plane at 589.2m, with the bellow 117m true depth manifests as a prominent gas anomaly zone. Gas concentrations in this zone are much higher and vary more violently than the upper 200m. The low P-wave velocity and resistivity imply that the strata are highly fractured, and gases intruded the borehole mainly by dissolving in water. We suggest that the gas distributions are partially lithology affected, but mostly fracture constrained according to the lithology variations, subsidiary fault zone and fracture distribution retrieved from the core. Considering the low deformation rate and seismicity in this region before the earthquake, together with the ductile behavior of the clay-rich fault gouge under slow tectonic loading, and the decreasing gas anomalies away from the slipping plane, we propose that the fractures were mostly generated during the Wenchuan earthquake. It indicates that the Wenchuan earthquake damaged the footwall more than the hanging wall, and the 117m depth marks the width of the fracturing zone. The differential mechanic property between the hanging wall and the footwall, and the asymmetric stress during the rupturing may contribute to the asymmetric fracture distribution.

Clay-smear continuity and normal fault zone geometry – First results from excavated sandbox models

The continuity of clay-rich fault gouge has a large effect on fluid transmissibility of faults in sand–clay sequences, but clay gouge continuity and composition in 3D are not well known. We report observations of 3D clay smear continuity in water-saturated sandbox experiments where the sheared clay layers were excavated after deformation. The experiments build on existing work on the evolution of clay gouge in similar 2D experiments where interpretations were made in profile view.We used well-known model materials (“Benchmark” sand and uncemented kaolinite–sand mixtures) that were further characterized using standardized geotechnical tests and triaxial compression experiments at effective pressures corresponding to the sandbox experiments. Results show a nonlinear failure envelope of the sand, in agreement with existing models. Unconfined compression experiments with the clay show cohesion around 50 Pa and brittle behavior.A sheared, ductile clay layer embedded in sand above a 70° dipping basement fault reveals a complex, natural-looking clay gouge architecture with relay ramps, breached relays and fault lenses. The clay gouge shows clear variations in composition and thickness and becomes locally discontinuous at throw-thickness ratios above 7, in contrast to our earlier 2D observations where discontinuous clay-gouge only formed in cemented clay layers. In addition to tectonic telescoping in the relays, the thin, continuous parts of the clay gouge were transformed from an initial pure clay by mechanical mixing of sand and clay.We also discuss the applicability of these results to the evolution of normal fault zones and deformation bands in sand–clay sequences at effective pressures below the onset of cataclasis and conclude that in fault zones a higher degree of internal segmentation reduces the probability of the formation of discontinuities.

Shear zones in clay-rich fault gouge: A laboratory study of fabric development and evolution

Clay-rich fault rocks have long been recognized to host distinctive fabric elements, and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime. Although the geometries of fabric elements in fault gouges have been described for almost a century, the genesis and evolution of these elements during shear, and their links to bulk mechanical properties, remain poorly understood. We characterize the development and evolution of fabric elements with increasing shear in a variety of clay-rich experimental gouges over shear strains of <1 to >20 and at normal stresses of 2–150 MPa in the double-direct shear configuration. In addition to SEM observations of experiment products at a variety of shear strains, we quantified clay fabric intensity and the degree of grain size reduction using X-ray Texture Goniometry (XTG) and particle size distribution (PSD) measurements. We also measured P- and S-wave velocities during shear to further probe the evolution of shear fabric and gouge properties. We find that clay fabric elements develop in a systematic manner regardless of the gouge material. Riedel shears in the R1 orientation and boundary-parallel shears are the dominant fabric elements. Riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress. Clay particles rotate into the P-orientation shortly after Riedels propagate through the layer. The Riedel shears are through-going, but are >10× thinner than similar zones observed in coarser granular materials. Our results suggest that the weakness of clay-rich fault gouge may be less a function of anisotropic crystal structure, as has been suggested previously, and more a consequence of very thin shear surfaces permitting deformation in clay-rich materials with minimal dilation or cataclasis. The very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal PSD's.

Spatial distribution and characteristics of fracture zones near a long-lived active fault: A field-based study for understanding changes in underground environment caused by long-term fault activities

Fracture zone development has a substantial impact on long-term changes in the underground environment because fracture zones are potential major groundwater flowpaths as well as a locus for mechanical disturbance by fracturing. To understand fracture zone development, we surveyed the spatial distribution and characteristics of fracture zones in areas around the Atotsugawa Fault, a long-lived active fault in central Japan. Within 500 m of the Atotsugawa Fault trace, the number of exposed fracture zones increases sharply and most fracture zones greater than 2 m width are concentrated. The width of the clay-rich fault gouge in the majority of fracture zones is less than 1 m. Fracture zones greater than 2 m width along the Atotsugawa Fault are composed mainly of a zone with numerous fractures and breccia, and are considered to act as a major conduit controlling regional groundwater flow. Based on rock features and deformation fabrics at meso- and microscopic scales, once fracture zones had formed, epigenetic deformation was concentrated in the older fracture zones. Fracture zones formed by fracturing along hydrothermal veins or fracturing and weathering that accompanied displacement along joints, schistosities or lithological boundaries are sparsely but widely distributed in the study area, though most of their width are less than 2 m. The existence of unfavorable hydraulic characteristics needs to be studied by engineered methods, for the purpose of site selection and design of underground facilities.

Reproduction of thermal pressurization and fluidization of clay-rich fault gouges by high-velocity friction experiments and implications for seismic slip in natural faults

Abstract We examine the frictional properties and microstructures of clay-rich fault gouges subjected to thermal pressurization and fluidization, generated in high-velocity friction experiments under dry and wet conditions. In the dry tests, slip weakening occurs by thermal pressurization, which is marked by a fault-gouge expansion associated with a water-phase transition from liquid to vapour. The water is derived from dehydration of clay minerals by frictional heating. The resulting microstructure in the gouge layer is a random distribution of spherical clay–clast aggregates in the matrix, and mixing of different gouge constituents without shear surfaces. In the wet tests, slip weakening is caused by pore-fluid pressurization resulting from shear-enhanced compaction of the water-saturated gouge and frictional heating. Compared to the dry tests, the wet tests show smaller dynamic stress drops and slip weakening distance. The steady-state shear stress in the wet tests is almost independent of normal stress, suggesting a fluid-like behaviour of the fault gouge during high-velocity shearing. The microstructures after the wet tests show that the foliated zone is accompanied by grain-size segregation in the gouge layer. The grain-size segregation is attributed to the Brazil-nut effect resulting from the difference in dispersive pressure in the granular-fluid shear flow at high shear rates, indicating a fluidization of fault gouge. Our results obtained at seismic slip rates imply that the propagation of an earthquake rupture can be enhanced by fluid pressurization and frictional heat, potentially leaving characteristic microstructures resulting from water vaporization by frictional heating or flow sorting at high slip rates.

Characterization of fault zones by analysis of aftershock waveform data

Large property contrasts between materials in a fault zone and the surrounding rock are often produced by repeating earthquakes. Fault zones are usually characterized by fluid concentration, clay-rich fault gouge, increased porosity, and dilatant cracks. Thus, fault zones are thought to have re- duced seismic velocities than the surrounding rocks. In this article, we first investigated the synthetic waveforms at a linear array across a vertical fault zone by using 3D finite difference simulation. Syn- thetic waveforms show that when sources are close to, inside, or below the fault zone, both arrival times and waveforms of P- and S-waves vary systematically across the fault zone due to reflections and transmissions from boundaries of the low-velocity fault zone. The arrival-time patterns and waveform characteristics can be used to determine the fault zone structure. Then, we applied this method to the aftershock waveform data of the 1992 Landers M7.4 and the 2008 Wenchuan (汶川) M8.0 earthquakes. Landers waveform data reveal a low-velocity zone with a width of approximately 270-370 m, and P- and S-wave velocity reductions relative to the host rock of approximately 35%-60%; Wenchuan wave- form data suggest a low-velocity zone with a width of approximately 220-300 m, and P- and S-wave ve- locities drop relative to the host rock of approximately 55%.

Quantification of fabrics in clay gouge from the Carboneras fault, Spain and implications for fault behavior

Clays in fault rocks have the potential to control fault behavior. The formation of frictionally-weak clays in fault rocks can lead to fault zone weakening, and the development of fabrics through authigenesis and mechanical rotation in clay-rich fault rocks can influence fault-zone permeability structures. The left lateral Carboneras Fault of southwestern Spain is a ca. 1 km wide fault zone with anastomosing layers of clay-rich gouge up to ~ 50 m thick. The gouge, derived largely from mica schist, is composed of neoformed illite and chlorite. Anisotropy of magnetic susceptibility (201 measurements) and X-ray pole figures (144 measurements) show comparable oblate clay fabrics, with the axis of maximum anisotropy sub-parallel to strike. This dimensional fabric contrasts with previously published maximum permeability results that are sub-vertical. This observation, coupled with the generally poor clay fabric measured in the samples indicates that the orientation of clays is not likely to be the primary control of the permeability anisotropy of clay-rich fault gouge, becoming important only when gouge zones are 10s of meters thick, which is very unusual. Even in a fault zone like that of the Carboneras fault, with thick, very clay-rich gouge zones with a visible foliations and slip surfaces, the degree of clay preferred orientation is low above the millimeter scale, showing that any strong clay fabric associated with the foliation are extremely localized phenomena. This suggests that processes operating below the millimeter scale may significantly influence fault behavior, a result compatible with earlier characterizations of faults in a variety of geologic settings, highlighting the importance of understanding how the formation and fabric of fault-related authigenic clays influence fault behavior.

Investigation for probabilistic prediction of shear strength properties of clay-rich fault gouge in the Austrian Alps

Geotechnical practice in the Austrian Alps determined that it is difficult to evaluate the shear strength properties of clay-rich fault gouge associated with thin shear zones because of a lack of undisturbed samples and the high normal stresses required for appropriate shear tests. The objective of this paper is to do a probabilistic prediction of the shear strength properties of clay-rich fault gouges through simple geotechnical parameters. The representative clay-rich fault gouge samples were selected from engineering projects. Their characteristics, i.e. water content and density, were simulated through high normal stress consolidation. Then they were sheared under different normal stress with low velocities. Shear test results show that there are four types of shear strength properties, which can be classified by geotechnical parameters. The significance of the parameters for the classification was tested. Using the Stepwise Discriminant Analysis, the best predictors were evaluated from basic soil classification parameters. The probabilistic prediction of shear strength properties was established by the combinations of the best predictors using Bayes's theorem. This makes the prediction of the shear strength properties of clay-rich fault gouge in the Austrian Alps easy and practical when sample availability is limited.

Timing of Alpine fault gouges

K–Ar ages from clay-rich fault gouges in the European Alps are consistent internally, with established field constraints and with fission track ages, demonstrating the applicability of this method for direct dating of brittle deformation. Illite grown by retrograde hydration of granitic and/or high-grade metamorphic protoliths is the major K-bearing mineral in the fractions that have been separated (<0.1 to 6–10 μm). The ages obtained are bracketed by apatite and zircon fission ages from adjacent localities. This, together with the interpreted cooling curve for one well-constrained sample from the Simplon Fault, indicates that illite grew (or the K–Ar isotopic system in illite closed) at temperatures in the range 120–150 °C, consistent with the stability of illite alone rather than illite–smectite. Average ages obtained for fractions finer than 2 μm were: (1) 5.2±2.6 Ma (2 σ) for 11 analyses from the detachment and footwall of the Simplon Fault; (2) 8.8±0.6 Ma for four analyses from the Centovalli Fault at Trontano; (3) 19.9±2.6 Ma for two analyses from a major Riedel fault related to the Periadriatic Fault, offsetting the Tertiary Bergell tonalite at Gesero; (4) 21.5±5.5 Ma for two analyses from a zone parallel to the Periadriatic Fault in the adjacent Southern Alps in Val Morobbia; and (5) 14.5±3.9 Ma for two analyses from a gouge zone on the northern rim of the Tertiary Mauls tonalite in the Eastern Alps. The ages from the Simplon, Centovalli and Periadriatic Faults are not identical and do not lend support to models linking these three faults as a single, coeval structure.

Porosity distribution in a clay gouge by image processing of 14C-PolyMethylMethAcrylate ( 14C-PMMA) autoradiographs: : Case study of the fault of St. Julien (Basin of Lodève, France)

Clay-rich fault gouge is a highly heterogeneous material composed of clasts of host rocks surrounded by a clay matrix. Moreover, this fault rock is characterised by different layers with varying properties: clast content, quantity of shear planes and lithology of rocks. Consequently, the accurate porosity characterisation inside this heterogeneous rock is not possible using usual techniques such as microscopy or bulk physical measurements. A specimen representative of the centimetric thickness of Saint Julien fault gouge has been impregnated by a 14C marked MethylMethAcrylate resin. From the micrometric to the centimetric scale, the spatial distribution of porosity (micro to macropores) is revealed on autoradiographs of different sample sections: clasts (sandstone or pelite) and illite-rich matrix composing the gouge are distinguishable according to their porosity contrast, and the layering is highlighted through the entire gouge thickness. Quantification of porosity by a specific image analysis software shows that clast porosity (ranging from 2.7% to 8.1%) is always lower than the porosity of the clay matrix (ranging from 10.5% to 15.3%). The porosity of these two gouge components always presents the same difference in all gouge layers. A detailed observation of porosity maps reveals the spatial relations linking clast to clay matrix porosities: porosity gradients around the clasts indicate that dissolution of carbonate cements by a recent fluid circulation into the clay part of the gouge is responsible of the actual porous network.

Discussion on Palaeogene dyke swarm, NW Wales: evidence for Cenozoic sinistral fault movement

R. H. Maddock , W. P. Aspinall , E. A. Hailwood , Ting Fung &amp; E. H. Rutter write: In a paper concerning the ages of fault movements in NW Wales, Bevins et al. 1996 cite palaeomagnetic dating by Hailwood et al. 1992 in support of their argument for large sinistral fault displacements of post-Tertiary dyke age. Bevins et al. misinterpret the observations and results published by Hailwood et al. We wish to correct this misinterpretation and to make further comments. Bevins et al. state To date the only evidence of Cenozoic faulting in North Wales is provided by Hailwood et al. 1992 who, from an investigation of palaeomagnetism and fabric analysis of a clay-rich fault gouge in northern Anglesey demonstrated 180 m of sinistral strike-slip or 215 m dip-slip movement during the last 20 million year period (with the minimum age for last movement as 0.73 Ma)'. In fact, the observations and results reported by Hailwood et al. support the contrary view that the greater part of the movement on the fault in question, the E-W-trending Porth-y-pistyll fault, took place well before the intrusion of a Tertiary dyke which cuts and intrudes the fault. Hailwood et al. noted the following: (1) The offset of the boundary between the (early Palaeozoic) Gwna and New Harbour Groups across the fault was consistent with end-member (total) displacements of 215 m normal dip-slip or 180 m sinistral strike-slip movement. (2) A sub-vertical Palaeozoic dyke showed a 10 m dextral strike-slip offset across the fault. Over the