Geochemical evidence suggests that in low porosity rocks, fluid flow is almost always focussed into faults and shear zones (McCaig, 1997). Estimates of fluxes indicate time-averaged permeability enhancement by around three orders of magnitude. This paper concentrates on mechanisms of deformation-induced permeability enhancement in ductile and brittle-ductile shear zones, emphasising the microstructural and microchemical signatures of mass-transfer in fault rocks.
The main micro-scale evidence for mass-transfer in deformed rocks is normally chemical heterogeneity in the form of veins, growth zones in minerals or variations in mineral composition. This can only develop if fluid flow results in chemical change, which generally means fluid is out of equilibrium with the rock. In brittle-ductile shear zones veins and fluid flow channelways may be overprinted by dynamic recrystallisation with diffusive redistribution of material. Such cryptic fluid flow pathways can sometimes be revealed by detailed microchemical studies. Hence the majority of mass-transfer generally relates to cyclic brittle fracture events, but the majority of microstructures form through essentially diffusive processes, and permeability in shear zones probably cycles through many orders of magnitude.
A key question in microchemical studies of ancient fluid flow in fault rocks is distinguishing the effects of diffusive and advective mass-transfer (Knipe and McCaig, 1994). New experiments on tracer diffusion in calcite graphically illustrate the effects of diffusion even under dry conditions. Grain boundaries provide rapid diffusion pathways allowing penetration of geochemical anomalies 2 to 3 orders of magnitude further than through grain interiors. If free water is present diffusion distances will be increased still further. The volume of rock affected by diffusion is, however, restricted by the slow rate of volume diffusion unless grain boundary migration occurs, in which case the diffusing tracer can be incorporated as zones into the volumes swept by the grain boundaries. Sites of grain growth can be extremely heterogeneous within a deforming rock, leading to apparent bypassing of large volumes of rock which might be interpreted in terms of advective mass-transfer. Normally the main distinguishing feature of diffusive mass-transfer is symmetry about initial heterogeneities and consistency of direction of chemical change within adjacent mineral growth sites. In contrast, advection is more likely to be unidirectional, but may be heterogeneous in its effects if microcrack-controlled channelways switch position within a chemically-heterogeneous rock.
Lithology has a profound effect on the mechanical behaviour of fault rocks, and hence on fluid flow. For example, on the Gavarnie Thrust in the Pyrenees fluid flow was focussed into highly fractured dolomite layers while ductile strain was partitioned into adjacent calcite mylonites. Advective flow events in the mylonites are reflected in deformed and recrystallised veins, but the micro-chemistry of the mylonites appears to be dominated by diffusive mass-transfer.
Knipe RJ & McCaig AM, Geol. Soc. Spec. Publ., 78, 99-111, (1994).
McCaig AM, Min. Soc. Series, 8, 227-260, (1997).
Sr, O, Nd and Hf isotopic studies have been performed across a meter-size ductile shear zone located in the NW French Massif Central. It belongs to the Chabanne leucogranitic belt and run parallel to the "La Marche" transcurrent fault. A strong deformation gradient results in little deformed leucogranites at the margins to mylonites at the core. The aim of this study is to detect the effects of deformation and the nature of the associated fluid phase on isotopic systems.
The eleven studied samples corresponding to undeformed and deformed facies have been collected every 10 cm. Large variations in Rb/Sr and associated 87Sr/86Sr values define an isochron at 318±26 Ma. This hercynien age is in agreement with the age of the surrounding undeformed Guérêt granite. By contrast, restricted ranges in both 147Sm/144Nd (0.1519-0.1750) and 176Lu/177Hf (0.0051-0.0058) ratios associated to identical within errors 143Nd/144Nd (0.51199-0.512046) and 176Hf/177Hf (0.282397-0.282429) ratios preclude any meaningfull isochrons. In addition, samples plot far below the 318 Ma reference isochron. Negative initial <epsilon> Nd and <epsilon> Hf at 318 Ma of -10.6 and -9.7, respectively, and old Nd model ages in the range of 2-2.2 Ga similar to those calculated from lower-crust granulitic xenoliths (Downes and Leyreloup, 1986) favor a metasedimentary origin. The younger Hf model age of 0.8 Ga are in agreement with the occurence of garnet in these granulites. O isotopes analyses were performed on both whole rock samples and mineral separates (Qtz+Musc), assuming a complete recrystallization of these minerals during deformation. Whole rock 18O values range between 7.4 and 10.0‰. A well-defined negative correlation is observed between Sr and O isotopes, with the deformed samples having high 87Sr/86Sr and low 18O. However, the sample from the highest strain zone exhibits the highest 87Sr/86Sr ratios and 18O, and does not fit in with the Sr-O correlation. This suggests rock interaction with a fluid component during deformation. Assuming T=400-450°C from mineralogical assemblages, the calculated O isotopic ratios of the fluid in equilibrium with newly-formed minerals range between 6.9 and 8.0‰ SMOW. The fluid in equilibrium with quartz from the highest strain zone is 18O enriched (18O=7.9-8.9‰). This can indicate O isotopes exchange during water/rock interaction. Nd and Hf isotopic systems record the pre-hercynian metasedimentary granulitic lower-crust. They remain undisturbed during greenschist-facies metamorphism and deformation associated to the emplacement and genesis of hercynian granitoids by partial melting of the lower-crust. Deformation which is subcontemporaneous with emplacement has completely reset the Rb-Sr geochronometer at 318 Ma. O isotopes are a tracer of fluid circulation through the most deformed zone.
A quantitative appraisal of timing of fault activity and cessation is important not only for tectonic analysis but also for evaluation of traps in petroleum basins. Applications of thermochronological techniques for dating of faults are based on two approaches: (1) Comparison of time-temperature pathways (cooling histories) of hangingwall and footwall rocks; (2) dating of fault rock. This study illustrates the latter approach by the application of fission-track and 40Ar/39Ar techniques to two faults in the Himalaya. The Main Boundary Thrust (MBT) is a major fault boundary between the Proterozoic rocks of the Lesser Himalaya and the Tertiary Siwalik molasse sediments of the Sub-Himalaya. In the central Himalaya (Kumaun region, India) a Siwalik sandstone sample was collected at the fault contact with the MBT. The sandstone, which has hardened and become cataclastic, belongs to the Lower Siwaliks (with depositional age of 14-11 Ma). Detrital apatites were separated and dated by the FT technique. Three analyses yielded an FT age of 5.3 Ma. The FT is younger than the depositional age; it indicates the reheating (hence resetting) of the sample at least up to ~110 degrees C probably by frictional heating along the MBT. Length measurement of fission tracks (>14 microns) indicates rapid surficial cooling of the sample and support this the shear-heating fault interpretation. Another case study comes from the 40Ar/39Ar dating of biotite and muscovite from a fault gouge in the Higher Himalaya in India. This fault (the Jhala Fault) is a post-metamorphic brittle thrust within the Higher Himalayan metamorphic complex. The 40Ar/39Ar plateau ages of micas from rocks collected away from the fault are 18 Ma; however, 40Ar/39Ar ages of biotite and muscovite from the fault gouge both give distrusted spectra with a minimum age of ~8 Ma for the first fractions of the argon release spectra. The disturbance at ~8 Ma probably indicates the fault activity at that time. The application of thermochronological techniques to fault rocks is an emerging field and is expected to draw more attention in the coming years.
A major problem often encountered in the study of collisional belts is not knowing the absolute ages of multiple deformation phases. In many cases successive deformation events can be observed on thin section. Thus dating individual phases of deformation generally requires high-spatial resolution geochronology from microstructures that are defined from metamorphic and deformation studies. In this study we use a high-spatial resolution 40Ar/39Ar UV laserprobe to date different phases of deformation of the Roffna metarhyolite within the Suretta nappe.
The Suretta nappe belongs to the upper Pennine domain in the eastern part of the Central Alps. The early Permian Roffna metarhyolite intruded the older basement and has been deformed only during Alpine tectonics. Two main episodes of ductile deformation producing different generations of shear zones are recognised. The geometry and associated kinematics permit a distinction between D1 and D2 shear zones in the field. D1 and D2 microstructures show different amounts of phengitic substitution in white micas.
Phengite and quartz in shear zones belonging to the first heterogeneous ductile deformation phase have been analyzed by in-situ 40Ar/39Ar UV laserprobe. Microstructural sites corresponding to the schistosity and shear bands show different mineral ages. Ages from zones of finely intergrown quartz and phengites defining the schistosity range from 26 to 48 Ma, whereas shear band phengites intergrown with quartz range in age from 15 to 34 Ma. While the fine-grained and intergrown nature of the phengite and quartz prevents the exclusive analysis of phengite, these data do indicate a younger age for the shear bands. The range in ages observed for the schistosity and shear band phengites can be interpreted as a result or combination of neoformation during deformation, cooling or mixed ages. Measurable argon is observed in quartz and thus extraneous argon must also be considered as having a possible influence on the observed ages.
A considerable number of geological age constraints have been published for this part of the Pennine domain. A late Eocene to middle Oligocene age could be postulated for the D1-D2 deformations. The youngest sedimentary rocks affected by D1 deformation are Valaisan flysch, which are biostratigraphically dated as late Eocene in age (Bagnoud, in press). The Turba Mylonite Zone (TMZ) is truncated by the Bergell granodiorite (30 Ma, von Blanckenburg, 1992). Assuming that D2 is contemporaneous with the TMZ, D2 is older than 30 Ma. Previous isotopic studies in this area indicate ages ranging from 45 to 30 Ma, however without always separating the D1 and D2 deformations.
Our 40Ar/39Ar UV laserprobe results agree with the geological and field constraints for the deformation of the Suretta nappe. The significance of this type of microscale analysis is discussed.
Bagnoud A, Wernli R & Sartori M, Eclogae Geol. Helv, (in press).
von Blanckenburg F, Chem. Geol, 100, 19-40, (1992).
Shearing of gneisses in the early Mesozoic Pogallo Shear Zone in the southern Alps led to mass transfer during viscous and brittle deformation. Steeply dipping shear planes define the km wide shear zone along the southern margin of the Ivrea Zone, near the Lago Maggiore. A 30 to 40 m wide mylonite belt at the southern margin of this shear zone is overprinted by brittle deformation and includes a 10 cm wide cataclasite zone. Microstructures indicate oblique slip during mylonitization and cataclasis. The coexistence of muscovite + biotite in mylonites indicates minimum temperatures of 400 - 475°C in a metamorphic pressure range of 300 - 600 MPa during early mylonitization. Lower temperatures prevailed during the syntectonic growth of prehnite + chlorite + epidote along foliation planes and fractures in cataclastically overprinted mylonites. The presence of mylonitized and cataclastically overprinted prehnite veins indicate repeated episodes of vein formation during shearing. The gradual transition from upper to lower greenschist facies mineral assemblages together with a transition from dominantly mylonitic to dominantly cataclastic deformation suggest progressive deformation during uplift. A comparison of the whole rock chemical composition of gneissic host rocks and of mylonites reveals minor losses of SiO2 and K2O, and slight gains of Na2O and CaO during deformation. Mass losses of 5 to 10% within the mylonites are due to dissolution of quartz. Gains of Na and Ca correlate with the albitization of plagioclase and the growth of prehnite. In contrast to the alteration in the mylonites, reactivated cataclasites have gained up to 120% SiO2, as well as K2O, Ba and Rb. Mass gains of 10 to 130% are mainly due to the mineralization of quartz + alkali feldspar. High mass gains, as well as different alteration patterns in some reactivated cataclasites indicate mass transfer via fluid advection. The precipitation of quartz + alkali feldspar suggests a down-temperature and down-pressure fluid flow path. During early mylonitization mass transfer was pervasive as indicated by the albitization of plagioclase. Evidence for localized fluid flow along meso-scale fracture permeability during brittle overprinting of mylonites is provided by the presence of prehnite veins, and the intense alteration in cataclasites. Intensive veining suggests that fluid pressures were at least transiently suprahydrostatic. Strain localization has evidently enhanced mass transfer in this crustal-scale shear zone. The difference in distribution and intensity of alteration in the mylonites and cataclastically overprinted mylonites demonstrates how the transition from viscous to brittle deformation in the mid-crust leads to different mass and fluid transport properties between the mid and upper crustal portions of shear zones.
This contribution presents results of ongoing microstructural investigations on Lower Liassic marbles from the Alpi Apuane, NW Tuscany (Italy). This region represents a former crustal-scale shear zone developed at mid-crustal levels (greenschist facies) during the Tertiary evolution of the Northern Apennine thrust system (D1), later involved in exhumation and extensional collapse (D2) (Carmignani & Kligfield, 1990). The partitioning of strain and the development of different generations of structures formed during thermal perturbation allowed the preservation of a variety of microstructures associated with both the thermal peak recrystallization and the retrograde history. The microstructural features and calcite/dolomite thermometry will be presented with emphasis to the geometrical and structural position within the polyphasic structural building. Two main groups of microstructures can be distinguished: a) the static microstructures, characterized by the typical granoblastic polygonal "foam" grain shape and by a systematic increase in grain size from approximately 0.08-0.1 mm to 0.15-0.3 mm in the eastern and in the western parts of the Alpi Apuane metamorphic complex, respectively. Calcite/dolomite investigations yield increasing temperatures from 360-390°C in the east to 430-440°C in the west suggesting a direct control of the temperature on the grain size; b) the dynamic microstructures, characterized by shape and crystallographic preferred orientations, mechanical twinning and dynamic recrystallization features. These microstructures can be found in different tectonic settings: i) associated with decametre-thick late-D1 shear zones in the eastern part of the Alpi Apuane; ii) within millimetre- to decimetre-thick D2 shear zones in the western part. Microstructures associated with the decametre-thick shear zones show a coarse grain size (0.1-0.2 mm) and calcite/dolomite temperature of 380-390°C, while the microstructures in the millimetre- to decimetre-thick D2 shear zones show exterme grain size reduction (0.02-0.05 mm) associated with lower temperatures (about 350°C). Though commonly considered homogeneous and statically recrystallized, the Alpi Apuane marbles reveal a wide variability-in-time of the relationships among deformation mechanisms, thermal evolution and fabric development. Such a variabilty occurred during the thermo-mechanical history of the Alpi Apuane metamorphic complex during the Apennine Tertiary orogeny.
Carmignani L & Kligfield R, Tectonics, 9, 1275-1303, (1990).
The Larra thrust, in the west-central Pyrenees, is a bedding-parallel decollement which we interpret to have moved by a cyclic process of slip on dilated bedding-parallel cracks, alternating with cementation of the cracks and ductile shear.
The fault zone is located within a limestone succession and consists of a stack of calcite veins that are dominantly parallel to the fault zone boundaries. The frequency of veins increases towards a meter-thick fault zone centre, where interleaving with host-rock slivers imparts a strongly foliated aspect to the fault rock. The veins are cemented by blocky calcite crystals, indicating cavity-type infilling of open fractures, and exhibit variable degrees of internal deformation by twinning and of recrystallization (twin-boundary migration and subgrain rotation at twin and grain boundary regions).
Cleaner, less deformed veins progressively successively crosscut older veins with more advanced stages of recrystallization, suggesting cyclic repetition of brittle and ductile behaviour during deformation. Given a background behaviour dominated by cleavage-forming ductile deformation, transient brittleness and fracture could only be induced by fluid pressure build-ups, which were channelled along the fault zone. Dominance of twins as a deformation mechanism and absence of shape fabrics and microscopic kinematic indicators in the fault rock suggest that ductile shear is insufficient to account for the kilometric displacement of the Larra thrust. Displacement must have been mostly accommodated by slip on water sills constituted by linked, open cracks preceding their cementation into veins, leaving little fabric in the fault rock.
Episodic movement along the fault zone is interpreted to result from repeated cycles of stress reorientation related to cycles of differential contraction, shear and relaxation in the hangingwall and footwall. These cycles produced alternating sequences of bedding-parallel contraction, leading to crack dilation, and bedding-parallel shear, leading to decollement slip.
Sometimes fault planes show intriguing shining surfaces. A limestone sample bounded by such a fault has been investigated using a SEM equipped with an EDS analysis sytem. The normal fault have a vertical downthrow of about 7 m, it dips 66° west and strikes N164°E. It affects Oxfordian reefal and chalky limestones from the "Revoi" quarry (Pagny-sur-Meuse, NE France), where it is the only example showing such a polished surface. The host-rock adjacent to the fault is brecciated on, at least, 50 cm wide and the striations indicate a dip-slip movement. In the sampling area about 30% of the displacement occurred with the movement taking place within the chalky Oxfordian whereas the other 70% of movement occurred with these limestones in contact with the upper "Sequanian" (Late Oxfordian) clays and intercalated limestone beds.
The electron microscope observations were made in the topographic- and backscattered-electron beam mode. The topographic mode shows that the glossy surface is made of linear glossy zones nearly as perfectly smooth as a man-made polished surface. They are affected by strike-slip scratches. In contrast, images in the backscattered-electron beam mode don't look smooth but intensely striated. The lines are parallel and distributed in one, two, even three sets at low angle. This mode of observation gives an image of the chemical composition, and an insight into the upper few microns below the surface.
The analytical weight percent results on the glossy surface are low, (except CaO), but consistent and greater than the error bar of the EDS analysis. The results can be summarised as follows: (1), the glossy part constitutes a micron-scale thin film chemically different from the underlying crystals; (2), its composition is not constant from place to place; (3), in all cases the composition is richer in alumina and magnesium than in the underlying crystals; (4), in some cases, silica, potash, iron, manganese and strontium are significant elements in the glossy part.
The chemical concentrations observed on the glossy surfaces are indicative of pressure-solution phenomenas implying the occurrence of a fluid and a slow movement. Common stylolitic striations are lacking on this surface and it could be a consequence of a friction limestone/clay rathan than limestone/limestone. The scratches may indicate co-seismic strike-slip reactivation of this fault.
In the northern sector of Ligurian Alps, oceanic units (Voltri Group, consisting in metabasites, serpentinites and calcschists) and continental crust slices (Valosio Crystalline unit, made up of polimetamorphic calc-micaschists and orthogneisses) were stacked by Eocene mesoalpine thrusts. Successively, since Early Oligocene, these coupled units were northward overthrusted onto the adjoining Neoalpine episutural basin (Tertiary Piemonte Basin, Alto Monferrato domain) by Neoalpine (Apennines-related) tectonics which led to the uplifting of the whole stacked units. These different metamorphic and sedimentary units are coupled by km-thick brittle-ductile shear zones in the later stages of the exhumation, when the development of penetrative brittle fabrics was superimposed on sin-metamorphic folds and foliations.
These deformations, in which carbonate-rich and methane rich fluids played an important role, gave origin to several fault-rock types:
- in the metamorphic rocks, pseudo-tachylyte and cataclasites are observed to affect older metamorphic fabrics. Fault rocks often show very distinct features and are separated from the parent rock by individual shear planes or shear zones where intense fluid circulation occurred.
- in the sedimentary rocks, cohesive fault rocks did not developed, since the deformation mainly consisted in flexural slip mechanisms giving origin to cleavages and fractures within different-size shear zones. Cataclastic rocks were produced only where sin-diagenetic methane-rich fluids previously recemented the sandstones or marl levels.
Complex vein patterns displace the two distinct fault-rock groups. The relations between the cementation of the fault rocks and the carbonate infillings of these veins are discussed, while their geochemical isotope compositions are compared.
The Parry Sound shear zone occurs in the Grenville Province of the Canadian Precambrian Shield between the predominantly amphibolite facies gneisses of the Britt Domain and the granulite facies mafic rocks and migmatites of the Parry Sound Domain of the Central Gneiss Belt. The shear zone transects the Whitestone Anorthosite, located along the western edge of the Parry Sound Domain, and produces a range of new textures and paragenesis.
Two types of mylonites occur along the intersection between the Whitestone Anorthosite and the Parry Sound shear zone. Away from the mylonites, anorthosite exhibiting well-defined schistosity, and anorthosite exhibiting relict igneous textures and ill-defined foliation occur. Minor planar and anastomosing shear zones are superimposed on these modified anorthosites.
Scapolite-bearing planar shear zones and scapolite-bearing mylonites display a mineral assemblage of plagioclase (An34-60), pargasite, scapolite (Me68-75), and almandine garnet which suggests middle-upper amphibolite facies conditions of metamorphism. They display small chemical modifications compared to the undeformed Whitestone Anorthosite; CO2 is the most prominently enhanced component. Quartz-bearing anastomosing shear zones and quartz-bearing mylonites consist of plagioclase (An20-47), hornblende/pargasite, scapolite (Me57-65) and almandine garnet which suggest middle-lower amphibolite facies. They display significant chemical modifications; SiO2 and K2O are the most dramatically increased components.
Under amphibolite and higher metamorphic conditions, plagioclase in the modified anorthosites underwent syn-tectonic recrystallization which produced non-random crystallographic orientation patterns. Quartz present in ribbons in quartz rich mylonites and anastomosing shear zones, deformed by dislocation creep resulting in the development of two different preferred crystallographic orientations. Isolated quartz and recrystallized plagioclase in the matrix deformed by the grain boundary process which produced random crystal orientation.
The textural and mineralogical characteristics of the ductile fault rocks in the Parry Sound shear zone suggest a history of deformation that extended from granulite facies to lower amphibolite facies conditions. The observed sequence of chemical modifications suggests an evolutionary fluid history resulting from the inverted metamorphic gradient of an overthrust. The different orientation patterns observed in quartz grains within the quartz ribbons suggest the occurrence of different regimes during the late stage of deformation. The reported zircon U-Pb ages of syn- and late tectonic pegmatites indicate this progressive deformation occurred from about 1160 Ma to at least 1120 Ma.
In the Santa-Rosa mylonite zone (Southern California) and in the Ivrea-Verbano zone (Northern Italy) pseudotachylites occur in rocks of medium to high grade metamorphism. They have been generated along fault planes during palaeoseismic events. In both areas, individual pseudotachylite veins cut mylonites, indicating, that the formation of pseudotachylites took place in the transition zone between ductile and brittle deformation after the ductile deformation. The host rocks of the pseudotachylites are mainly granodiorites and metabasites in the Ivrea-Verbano zone and in the Santa Rosa mylonite zone granodiorites and granites. The homogeneous matrix of the pseudotachylites shows an intense fracturing and a small amount of selective, frictional melting part. Chemical studies characterize the directly neighboring rocks as the host rocks of the pseudotachylites. After their formation, the pseudotachylites in both areas became weakly altered at temperature conditions around 300°C.
The former melting of minerals in pseudotachylites is often only visible using electron optical methods. An amount of frictional melting about 20 to 30 volume percentage, depending on the lithology of the host rocks, has been calculated from SEM observations. Recently, new electron optical observations (TEM and AEM) suggest, that the amount of melting is lower. Only small volume parts below 10 volume -% have been melted. In some samples these former amorphous areas are devitrified and crystalline.
The Atomic Force Microsocopy (AFM) provides the possibility, to quantify the characteristics of glass surfaces at the nanometer scale like, e.g., roughnesses and the topography of inhomogenities (Rädlein and Frischat, 1997). The roughness values (rms-values) have been measured with the AFM at air on randomly orientated and freshly fractured pseudotachylite surfaces. It can be demonstrated, that even lower amounts of typical glass occur in the matrix, showing roughnesses (rms-values) about 1 nm.
It is concluded , that in the Santa Rosa Mylonite zone and the Ivrea-Verbano zone the homogeneous matrix of pseudotachylite veins mainly comprises the strongly fractured host rock with sizes of individual fragments of <10 mm, which are mixed with a small part of former frictional melt of around 5 volume-%.
Rädlein E & Frischat G-H, J. of Non-Cryst. Solids, 222, 69 - 82, (1997)
The permeability of fault zones depends on the deformation affecting them, and on fault gouge interaction with fluids. For example, fault sealing can occur by a combination of physical (e.g. cataclasis) and chemical (e.g. pressure solution) processes. These mechanisms operate individually, but often occur as combined processes, depending on the pressure and temperature regimes. To date, experimental and theoretical work combining these mechanisms have only considered ideal spherical particles. Here, we integrate these processes into a single study, using more realistic rough-surfaced gouge particles. Synthetic gouges will be tested in the laboratory, under various pressure and temperature conditions, both in standard batch tests (for the calibration of pure chemical effects), and in static flow experiments within artificial fault zones. Both experiments utilise size-fractionated as well as bulk mixtures of quartz-feldsparsandstone powders, from which contaminant fines have been removed using a new technique. This allows a more accurate measurement of gouge surface area and reaction rates. Rates of precipitation and dissolution of the synthetic fault gouges are monitored by analysing for silica in the fluids for a range of realistic particle size distributions, using High Performance Liquid Chromatography. Permeability changes are also monitoredusing pulse decay techniques in association with the static flow tests. The results show a strong grain size dependence, and also point to the need for more careful fines removal (washing) techniques prior to experimentation.
In the Eastern Helvetic Alps, Permian Verrucano siltstones are thrust over Mesoszoic carbonates in the south and Tertiary flysch in the north, along the Glarus overthrust, with an intermediate thin layer (1-5 m thick) of intensively deformed calc-mylonite that accomodated a minimum of 35 km translation. This calc-mylonite is characterized by a a strong depletion in , compared to presumed marine Helvetic carbonate protoliths (O=25.4‰±2). In a previous study, Burkhard and Kerrich have shown that this depletion increases, following a trend, from north to south, which is due to externally derived fluids advected northwards along the thrust fault. They also showed an heterogenous O composition of the calc-mylonite on the cm scale, suggesting a partially veiny origin of the mylonite.
Further field investigations and observations on thin sections have led to a better understanding of the formation of the mylonite. In the south, where Verrucano is thrust over Mesozoic carbonates, the calc-mylonite has a smooth and planar appearance. In the North, where Verrucano is thrust over Tertiary flysch, the calc-mylonite has a very chaotic appearance underlined by the disrupted and refolded alternation of dark (styloliths) and white layers. The latter can still be recognized as progressively deformed veinlets including some coarser twinned calcite grains. This suggests that the contribution of veins to the bulk calc-mylonite formation is more important in the north than in the south, where the calc-mylonite is mostly an extremely sheared Mesozoïc carbonate.
Optical microscopy shows that plastic deformation mechanisms such as dislocation glide have played a role during calc-mylonite formation as prooved by the presence of shape fabric of grains and LPO. These ductile deformation mechanims were accompanied by recovery and dynamic recrystallisation leading to the formation of subgrains or new grains in veins composed of initially coarse and intensely twinned grains. The omnipresence of ductilely overprinted "ghost" veins, visible in cathodoluminescence, is consistent with a high fluid pressure leading to intermittant britlle deformation probably by repeated seismic failure.
The calc-mylonite associated with the Glarus overthrust contact has formed by the alternative activity of ductile and brittle deformation mechanisms, the latter being related to high fluid pressure related to a fluid flow inducing an massive 18O depletion of the calc-mylonite.
We have conducted triaxial shearing experiments on a synthetic mud at effective pressures equivalent to those at crustal depths near 5 km. The synthetic mud consists of 10 wt% montmorillonite, 40 wt% illite, and about 50 wt% silt-sized quartz. A layer of water-saturated mud was placed between forcing blocks with the layer oriented at 45° to the maximum principal stress. Shear strains > 5 were achieved. Distilled water was used as pore fluid. Permeability parallel and across the mud layer was measured intermittently during shear deformation. All specimens were first loaded isostatically to an effective pressure of about 90 MPa in such a way that confining pressure and pore fluid pressure approximately follow the lithostatic and hydrostatic pressure gradient respectively. In two experiments, shear sliding was begun at an effective pressure of 90 MPa and continued after effective pressure was reduced (by raising fluid pressure) to 30 MPa and then 20 MPa. In two other experiments, shear deformation was not conducted until the effective pressure was reduced from 90 to 30 MPa. Both permeability and permeability anisotropy depend on deformation history. We find that, at an effective pressure of 90 MPa, permeability decreased continuously with increasing shear displacement; whereas at effective pressures of 30 and 20 MPa, permeability increased significantly (up to 2 orders of magnitude) with increasing displacement. In the case where shear deformation commenced at 90 MPa, permeabilities parallel and across to the mud layer are similar and remain so during shear deformation at lower effective pressures. In the case where shear deformation commenced at effective pressures much lower than the initial isostatic loading pressure, permeability parallel to the shear direction is one order of magnitude higher than that across it. Our results confirm that the relative timing of overpressure and shear deformation is a critical factor affecting fluid flow through shear zones in wet sediments. The results suggest that high permeability and channelised fluid migration along shear zones can occur if the fluid pressure within the shear zones is significantly higher than in the surrounding sediments, even though porosity in the shear zone is much lower than in the wall-rocks.
This contribution presents the results of a structural investigation of high angle faults in the Alpi Apuane (NW Italy). The studied structures are developed in homogeneous Jurassic marbles (the well known Carrara marble) where a gradual transition in fabric type can be observed across the faults: from undeformed marble through a zone containing widely (0,5-1 m) spaced fractures grading towards a transitional zone in which the density of fractures increases, ending up with the main fault zone. This is characterized by cataclasites in which angular/subangular clasts of marble (decimetric to millimetric in size) are floating within an orange to brownish matrix. At microscopic scale this matrix appears to be formed by fine grained (micrometric in size) calcite with angular to sub-angular fragments of marble, showing evidence of crystal plastic deformation (thin twinning, undulose extinction, bending and kinking) and suggesting that the cataclastic process, responsible for rock failure at the boundary of the structure, was associated within the fault zone by other deformational mechanisms (dislocation glide and creep) possibly enhanced by fluids. In order to verify the involvement of fluids during deformation, trace element and stable isotope analyses were performed on calcite troughout the transition from undeformed marble to cataclasite. The results of analyses, common to all the structures investigated, indicate that: i) the 18O values decrease considerably from perifery toward the main fault zone, due to increasing water-rock interaction; ii) the 13C values behave similarly, but the range of variation is narrower and iii) the base metal contents display a reversed trend, decreasing from cataclasite to undeformed marble. Although the studied structures are observable within marbles, the isotopic and chemical data concur to suggest that fluids from the Paleozoic basement may have been involved during the deformation in keeping with the occurrence in the basement of positive metal content anomalies and 13C-depleted carbonate rocks and carbonaceous matter. In conclusion fluids appear to be largely involved within the fault zone, explaining the observed microstructural features in the fault rocks.
Faults in silicoclastic formations are generally associated to clay-gouge. The great amount of clay minerals can be explained by three mecanisms: 1) the ductile shearing of clay layers (Jev and al, 1993); 2) the pressure-solution processes (Gratier, 1984) leading to the residual concentration of detrital phyllosilicates; 3) the hydrothermal processes in which fluid discharge or fluid/rock interactions lead to cristallisation of clay mineral (Beaufort and al, 1995). In the first case, the gouge material resulting from the shearing do not show any significant chemical variation with the original sedimentary clay layers. On the opposite, the two other processes induce chemical variations. The gouge material resulting from cataclasis and the pressure-solution processes has a chemical compositions deriving from that other parent rocks. The clay material formed by hydrothermal processes has a chemical composition controled by infiltraring fluids.
A detailed study of clay gouges from a set of normal faults has been performed in the Permian basin from Lodève (France). Several analyses (petrographic observations, X-ray diffraction, atomic absorption spectrophotometry, electron microprobe analysis) have been used in order to better understand the origin of the clay minerals presently observed in the gouge material. In most of the cases studied in the bassin of Lodève, the gouges results from a selective dissolution of feldspars and with of quartz during pressure-solution processes. This induces a drastic chemical variation of the whole rocks, a significant volume loss (up to 50% for 100 meters throwed faults) and an increasing concentration of micas and illitics minerals in the faults. The clay shearing are also observed in the vicinity of rich clay layers. The last mecanism observed, in the Lodève basin, is the hydrothermal processes. The three mecanisms described above are acted at different geological periods and overprinting currently occured. This phenomenon probably explain why clay shearing which is so rarely observed presently. Indeed, in the Saint Julien fault (113 meters throwed fault), the predominant mecanism is the pressure solution with important mass transport followed by a late hydrothermalism episod parallel to the fault plan.
The spatial organisation of the gouge material controls the very low transversal permeability of the sedimentary formations and constrains vertical circulations of present day fluid in a longitudinal drain parallel to the fault plane.
Jev BI, Kaars-Sijpesteijn CH, Peters MPAM, Watts NL & Wilkie JT, Bull. Amer. Assoc. Petroleum. Geol, 77, 1389-1404, (1993).
Gratier JP, Thèse, Uninersité de Grenoble
Beaufort D, PApapanagiotou P, Patrier P & traineau H, Bull Elf Acquitaine, 19 n°1, 267-299, (1995).
Physical properties of faults are of a first order interest in petroleum research. Physical and chemical processes that occur during the development of faults in silicic clastic sediments lead to the formation of gouges. Clay-rich gouges provide efficient seal across fault zones. Wether fluids can migrate through a fault, depends on the possible break of this seal. Here we show that seal breaking may results from fault geometry, itself controlled by the rheological coupling of the different silicic clastic layers. When the sedimentary pile contents clay-rich layers interbedded within sandstones, downdip displacement along faults within sandstones is accomodated by layer-parallel movement within clay-rich levels. These small-scale décollements are responsible, in a first time, for the "in staircase" geometry of faults. Following displacement results in the enlargement of fault zones that recover a more rectilinear geometry. In such case, enlargement of fault zone is less due to increasing displacement rather than to deformation partioning. Friction within enlarged zones is reduced so that development of mature gouge is less favoured than in rectlinear fault segments. Consequently, sealing properties of fault rocks within enlarged zones are strongly reduced so that fluids may pass trough. In conclusion, sealing efficiency of faults is dependent on the rheology of the sedimentary pile in which they develop.
The Glarus Thrust is one of the major late structures of the Helvetic nappes, emplacing Verrucano siltstones northwards over Mesozoic carbonates and Tertiary flysch. It is remarkable for the Lochseiten mylonite, a layer as little as 1 m thick which is present below the thrust plane for tens of km both along and across strike. The Lochseiten mylonite could be derived from Mesozoic carbonates or may be mainly deformed vein material. The fault rock is fine grained and often contains only a weak crystallographic fabric, leading to suggestions that deformation in the mylonite was dominated by grain boundary sliding. Prominent within the mylonite at many localities is a thin planar "septum" containing ultrafine grained calcite. The Glarus Thrust was also the locus of syndeformational fluid flow leading to systematic shifts in oxygen and strontium isotopic ratios compared with footwall carbonates (Burkhard et al, 1992).
An albite-bearing dolomitic layer up to 20 cm thick is present within the Lochseiten mylonite at many localities, although this has not been emphasised by previous authors. It is generally located immediately below the Verrucano and above the septum, and systematic changes related to distance from the contact suggest it formed by reaction of the calcite mylonite with the Verrucano siltstones. The dolomite layer thickens into late folds in the thrust contact and is cut by numerous shear fractures, suggesting a syntectonic origin. Mm-scale shear veins containing dolomite and albite also cut the calcite mylonites.
Backscatter SEM studies show that at Piz Dolf the dolomite is extremely fine-grained (< 1 micron) and contains fragments of calcite. This fine-grained matrix contains irregular patches of undeformed zoned dolomite and euhedral albite up to 100 microns across, which appear to filling porosity. Both the fine matrix and the patches are cut by late calcite veins, some of which formed in pull-aparts on discrete faults. At Pic Segnas euhedral dolomite and albite appear to be growing within the fine calcite mylonite on the edges of veins containing intensely twinned coarse (20 micron) calcite. Albite also decorates conjugate shears which cut the mylonitic foliation at a high angle.
The ultrafine dolomite/calcite matrix at Piz Dolf is interpreted to result from cataclastic deformation along the thrust, probably at high pore pressures. This may have graded mechanistically into diffusion-accomodated grain boundary sliding. Instabilities in the flow allowed fluid-filled porosity to open, and as the strain-rate decreased this was filled by dolomite and albite derived by predominantly diffusive mass-transfer from the overlying Verrucano. At present it is not clear how this diffusive mass-transfer interacted with the advection of fluid along the thrust revealed by isotopic studies.
Burkhard M, Kerrich R, Maas R & Fyfe WS, Contrib Mineral Petrol, 112, 293-311, (1992).
The eastern part of the Tonale line in the Val di Sole (Trento region, Northern Italy) has been contact-metamorphosed by the Adamello intrusion. In the contact aureole, the deformation along the Tonale line has produced an 800 m wide mylonitic zone, while further to the west of the magmatic contact the deformation resulted in cataclasis only (Werling, 1992). The deformation temperature in the mylonites ranges from upper amphibolite to lower greenschist facies conditions. All three dynamic recrystallization regimes including the brittle-ductile transition of quartz can be observed. A correlation of the critical mineral assemblages with the quartz recrystallization microstructures yields temperature ranges for the dominant recrystallization processes: bulge nucleation recrystallization up to 370°C, subgrain rotation recrystallization from 370°C to 500°C and grain boundary migration recrystallization above 500°C. The microstructural evolution can be described as a function of temperature, i. e. the recrystallized grain size increases from 5 mm up to more than 5 mm from the lowest to the highest deformation temperature conditions. Measurements of grain size have been carried out by the autocorrelation function (Panozzo Heilbronner, 1992) in the regime of bulge nucleation recrystallization and by image analysis combined with orientation imaging (CIP; Panozzo Heilbronner & Pauli, 1993) in the regimes of subgrain rotation and grain boundary migration recrystallization. From digital image analysis a solid statistical base of microstructural data is obtained, e. g. aspect ratios, shape fabrics, grain sizes.
The extrapolation of experimental constitutive equations to natural conditions requires three sets of data (Paterson, 1987): knowledge of the flow mechanism, knowledge of the flow law and knowledge of the internal and external variables (microstructure of the starting material, Cfluid, T, p, t, (sum) , <epsilon> ). Deformation under natural conditions can be interpreted from the finite state only, and requires these three sets of data to infer the rheology. In the investigated samples crystal plasticity is the dominant deformation mechanism. In the case of the eastern Tonale line the external variables, e. g. temperature, pressure, time of deformation, are quite well constrained. The amount of accumulated strain is high but impossible to quantify. From rough estimates of the external variables certain upper and lower limits of stress and strain rate can be obtained.
Panozzo Heilbronner, R, Tectonophysics, 212, 351-370, (1992).
Heilbronner, R & Pauli, C, J. Struct. Geol, 15, 369-382, (1993).
Paterson, MS, Tectonophysics, 133, 33-43, (1987).
Werling, E, unpubl. PhD thesis, ETH Zürich, 9923, 1-276, (1992).
The northern Menderes Massif, northwest Anatolia, is a good example of forming a metamorphic core complex. The detachment fault separates lower plate that have been deformed ductilely from an upper plate rocks. The lower plate consists of Precambrian migmatites and intruding granites of Miocene (?) age. Non-mylonitic granitic rocks grade into mylonites displaying variations in the intensity of mylonitazition. A detailed microstructure analysis on deformed grains was carried out in the granitic mylonites of lower plate in northwest of Simav (Kütahya). Conditions of mylonitization in the northern Menderes Massif are based on the microstructures of minerals associated with the mylonitic deformation.
In mylonitic deformation, quartz shows evidence of crystal-plastic deformation, including undulose extinction, deformation bands, subgrains and recrystallized grains. Most quartz is completely recrystallized showing a strong crystallographic preferred orientation. In some samples strongly flattened old crystals are characteristic. Feldspars in mylonitic rocks deformed mainly in a brittle manner but also behave in plastic fashion. In granitoids in lower plate affected by mylonitic deformation, feldspars are commonly fractured. Deformation twins, undulose extinction, deformation bands and kink bands occur typically in plagioclase. In some samples, core-and-mantle structure that is characterized by large feldspar core surrounded by a mantle of fine recrystallized grains is very typical. Strain-related myrmekite is ubiquitous on the long sides of K-feldspar porphyroclasts. Both biotite and muscovite are mostly recrystallized or mechanically rotated minerals. Microstructures most commonly observed are undulose extinction, bending and kinking. Microstructures in amphiboles are fracturing along cleavage planes and undulose extinction that is attributed to brittle mechanisms.
The microstructures in quartz, feldspar and micas, the predominant minerals in the lower plate of granitic mylonites in northern Menders Massif, constrain temperature during mylonitic fabric formation to between 250°C and 500°C. In a conclusion, the mylonitic fabric formed under greenschist to lower amphibolite facies-grade conditions.
We investigate the Punchbowl fault, southern California, and small-displacement strike-slip faults in the Sierra Nevada, California faults to characterize the structure, geochemistry and petrology of fault-related rocks and to determine the deformation processes in seismogenic faults. The Punchbowl fault has 44 km of right lateral slip and exhibits an extremely narrow, planar, central zone of ultracataclasis bounded by sheared and foliated cataclasite. Most of the slip has accumulated on the central ultracatalcasite core. Foliated cataclasites are 3-5 m thick, and consists of sheared lozenges of brittly deformed and highly altered rocks. Adjacent to the fault core is a zone of highly fractured, veined, and faulted rock which extends up to ~ 100 m away from either side of the fault core. These damaged zones contain fractured and fault-bounded blocks of altered protolith. Deformation mechanisms are brittle fracture, faulting, and cataclasis at the grain scale in the damaged zone and parts of the foliated fault core. Foliated cataclasites in the fault cores exhibit also layer-parallel slip in clay and clay-vein layers. The ultracataclasites are black and aphanitic; grain size is typically less than 20 microns, and are high in clay and zeolite with fine-grained rounded quartz grains scattered in the matrix. Small faults in the Sierra Nevada with < 3 m of slip exhibit similar fine-grained fault cores, suggesting fluid-assisted process of slip, milling and/or flow of very-fined grained clay-or mica - zeolite at very low slip form very early in the development of some faults. By doing so faults may develop as a stable sliding or strain-softening zone over much of its history, with continual narrow core development occurring only on a few principal slip surfaces. Whole-rock geochemical analyses indicate that the faults had moderate to significant volume loss. This suggests that disequilibrium fluids move through the fault zones, and result in compaction of the fault core by removal of a fraction of the original rock volume. The degree of fluid-rock interaction varies along strike. Our work indicates that geophysical imaging of fault zones depicts the damaged zone, in which a damaged zone induced low-velocity acts as an effective wave guide. Not imaged with most geophysical techniques are the extremely narrow fault core zones that appear to have accommodated most of the slip. These slip zones are likely to be extremely narrow and embedded within the imaged fault zone, and probably would go undetected by most methods.
In the Kandalaksha-Kolvitsa-Umba area on the northern coast of White Sea 2.72-2.74 Ga TTG gneisses are tectonically overlain by the Kandalaksha sequence of amphibolites, clastic metasediments and minor intermediate metavolcanites.. These are separated from the Poriya Guba series of granulite facies rocks of mainly metavolcanic origin by the ca 2.46 Ga gabbro-anorthosite massif which is cut by intrusions of gabbro-diorites, several generations of ca 2.42-2.39 Ga mafic dykes. The easternmost contact between the Poriya Guba series and the ca 1.9 granulite facies metasediments of the Umba block is a tectonic melange which developed high-pressure granulite facies assemblages. All the units are strongly deformed and metamorphosed under amphibolite and granulite facies. The massive lithologies of gabbro-anorthosites and mafic dykes are coronitic showing development of several types of multilayered coronas at the contacts between igneous Fe-Mg minerals and Pl. Several stages of corona development can be suggested. During the earliest ones the gabbro-anorthosites has experienced cooling under pressure increasing from P=ca 9 kbar to P= 11.0 kbar. Coronas of the next stages developed during the cooling of gabbro-anorthosites followed by retrograde decompression to P=6.65-6.80 kbar. Development of earliest ca 2.43 shear zones led to recrystallisation of coronitic gabbro-anorthozites to mylonites under garnulite facies. P-T of equilibrium between coronitic minerals in the mafic dykes fit with conditions of the granulite shear fabric development in the gabbro-anothosites. Development of mylonitic textures in marginal parts of the mafic dykes is apparently related to next phase of the shear deformation. P-T calculations fix the visible decompression which is similar to that in gabbro-anorthosites. Comparison of these data allow to suggest that the emplacement of the dykes took place at great depth corresponding to P=10-11 kbar and marks a deep-level extension of crust.
Index of EUG 10 Volume
Further EUG 10 Information
Index of the Journal of Conference Abstracts
Cambridge Publications Home Page
Last Updated on Wednesday, March 17, 1999.
© 1997 Cambridge Publications