paragenesis of sodic pyroxene bearing quartz schists implications for the p t history of the sanbagawa belt sodic pyroxene jadeite content x jd 0 1 0 3 occurs locally as small inclusions within albite porphyroblasts and in the matrix of hematite bearing quartz schists in the sanbagawa sambagawa metamorphic belt central shikoku japan the sodic pyroxene bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene subcalcic or sodic amphibole phengitic mica albite quartz hematite titanite epidote spessartine rich garnet occurs in mn rich samples sodic pyroxene in epidote bearing samples tends to be poorer in acmite content average x acm 0 26 0 50 than that in the epidote free samples x acm 0 45 0 47 x jd shows no systematic relationship to metamorphic grade and is different among the three sampling regions saruta gawa asemi gawa and bessi besshi the average x jd of the saruta gawa samples 0 21 0 29 is higher than that of the asemi gawa 0 13 0 17 and bessi 0 14 0 23 the p t conditions of the asemi gawa and bessi regions are estimated at 5 5 6 5 kbar 360 c in the chlorite zone 7 8 5 kbar 440 15 c in the garnet zone and 8 9 5 kbar 520 25 c in the albite biotite zone metamorphic pressure of the saruta gawa region is systematically 1 1 5 kbar higher than that of the asemi gawa and bessi regions and materials of the saruta gawa region have been subducted to a level 3 5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the asemi gawa and bessi regions pressure slightly increases toward the north structurally high levels through the sanbagawa belt of central shikoku two types of zonal structure were observed in relatively coarse grained sodic pyroxenes in the matrix one type is characterized by increasing x jd from core to rim the other type by decreasing x jd from core to rim both types of zoned pyroxenes show an increase in x fe 2 fe2 fe2 mg from core to rim the first type of zoning was observed in a sample from the chlorite zone of lowest grade whereas the latter occurs in the garnet and albite biotite zones of higher grade the contrast in zonal structure implies that dp dt during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher grade zones the estimated dp dt of the prograde stage of the chlorite zone is 3 2 kbar 100 c and that of the garnet and albite biotite zones is 1 8 to 0 9 kbar 100 c the variation of dp dt at shallow and deep levels of a subduction system probably reflects the difference of heating duration and or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle

strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

crustal shortening at the sierra pie de palo sierras pampeanas argentina near surface basement folding and thrusting the sierras pampeanas consist of basement uplifts that are mainly controlled by w verging crustal thrusts one of these the sierra pie de palo is controlled by a blind thrust whose kinematics is debated we analysed the reverse faults located along the eastern side of the range and show that these are characterized by small displacements few hundreds of metres of the neogene and quaternary deposits controlled by the inherited basement metamorphic foliation and shear zones at the sierra scale the inherited foliation pattern appears already folded before the cenozoic shortening forming a series of antiforms and synforms with various wavelengths we propose that this structure is amplified during the plio quaternary shortening resulting in a large asymmetrical basement cored anticline where foliation parallel slip can act as folding mechanism the overall kinematics is consistent with an e verging fault propagation fold this proposed kinematics is an alternative to the frequently suggested w verging fault bend fold model finally we propose a new alternative and speculative crustal and lithospheric structure for this region consisting of two e verging basement thrusts coeval to w verging pampean thrusts both rooting below the precordillera above an eclogitized lower crust

one kilometre thick ultramylonite sierra de quilmes sierras pampeanas nw argentina abstract we describe a 1 km thick ultramylonite forming the high strain base of the 3 5 km thick el pichao shear zone in the sierra de quilmes this shear zone thrusted granulite facies migmatites onto amphibolite facies rocks during the 470 ma famatinian orogeny strain grades upwards from ultramylonites to weakly sheared migmatites across the 3 5 km zone and the mylonitic rocks define a geochemical field narrower than the protolith suggesting they underwent mixing and homogenization through shearing ultramylonites this thick are uncommon the width of a shear zone in the absence of significant compositional rheological contrasts controlling strain localization is controlled by the balance between shear heat generation and diffusion under typical crustal conditions a strain rate of 10 12 s 1 is required to form a 1 km thick ultramylonite and this is achieved when large movement velocities are imposed across the shear zone we postulate that the el pichao shear zone and its thick ultramylonite accommodated a significant fraction of convergence velocities driving the orogeny and that the wide mylonitic shear zones characteristic of the cambrian ordovician deformation of the sierras pampeanas result from the convergent movement being taken up by only a few active major shear zones

seismically invisible fault zones laboratory insights into imaging faults in anisotropic rocks phyllosilicate rich rocks which commonly occur within fault zones cause seismic velocity anisotropy however anisotropy is not always taken into account in seismic imaging and the extent of the anisotropy is often unknown laboratory measurements of the velocity anisotropy of fault zone rocks and gouge from the carboneras fault zone in se spain indicate 10 15 velocity anisotropy in the gouge and 35 50 anisotropy in the mica schist protolith greater differences in velocity are observed between the fast and slow directions in the mica schist rock than between the gouge and the slow direction of the rock this implies that the orientation of the anisotropy with respect to the fault is key in imaging the fault seismically for example for fault parallel anisotropy a significantly greater velocity contrast between fault gouge and rock will occur along the fault than across it highlighting the importance of considering the foliation orientation in design of seismic experiments

frictional viscous flow of phyllosilicate bearing fault rock microphysical model and implications for crustal strength profiles it is widely believed that around the brittle ductile transition crustal faults can be significantly weaker than predicted by conventional two mechanism brittle ductile strength envelopes factors contributing to this weakness include the polyphase nature of natural rocks foliation development and the action of fluid assisted processes such as pressure solution recently ring shear experiments using halite kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional viscous behavior i e both normal stress and strain rate sensitive behavior involving the combined effects of pressure solution and phyllosilicates this behavior was accompanied by the development of a mylonitic microstructure a quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model in this paper a microphysical model for shear deformation of foliated phyllosilicate bearing fault rock by pressure solution accommodated sliding along phyllosilicate foliae is developed the model predicts purely frictional behavior at low and high shear strain rates and frictional viscous behavior at intermediate shear strain rates the mechanical data on wet halite kaolinite gouge compare favorably with the model when applied to crustal materials the model predicts major weakening with respect to conventional brittle ductile strength envelopes in particular around the brittle ductile transition the predicted strength profiles suggest that in numerical models of crustal deformation the strength of high strain regions could be approximated by an apparent friction coefficient of 0 25 0 35 down to depths of 15 20 km

the effect of fluid composition on the behavior of well cemented quartz rich sandstone during faulting abstract previous studies o kane a onasch c m farver j 2007 the role of fluids in low temperature fault related deformation of quartz arenite journal of structural geology 29 819 836 cook j dunne w m onasch c m 2006 development of a dilatant damage zone along a thrust relay in a low porosity quartz arenite journal of structural geology 28 776 792 found that quartz arenite within two fault zones in the appalachian foreland thrust belt displays very different structural styles and histories despite deforming at similar pressures and temperatures during the alleghanian orogeny a comparison of the grain scale deformation and fluid histories using transmitted and cathodoluminescence microscopy and fluid inclusion microthermometry shows that fluid composition was a controlling factor for causing these differences the cove fault zone deformed in the presence of aqueous fluids first a cacl 2 brine and then an iron rich fluid the precipitation of quartz cement from the brine kept pace with brittle deformation maintaining overall rock cohesion in the fault zone the introduction of iron rich fluids caused a switch from precipitation to dissolution of quartz along with precipitation of goethite in a damage zone along a backthrust in the cave mountain anticline early deformation occurred in the presence of an aqueous fluid from which quartz was precipitated the latest deformation however occurred in the presence of a methane rich fluid which inhibited the precipitation of quartz cement producing porous breccias and open fractures despite deformation at 6 km depth fluid composition not only affected cementation in the fault zones but also the selection of grain scale deformation mechanisms therefore it is a controlling factor in determining the behavior and strength of these fault zones

weak faults rotten cores earthquakes are triggered when crustal deformation produces displacements along fault zones the rock in these zones is found to be weaker than adjacent material but what are the mechanisms of this loss of strength in his perspective holdsworth 1 discusses how recent studies of exposed ancient fault zones combined with laboratory analyses are revealing that rock properties at the grain scale can determine the large scale behavior of crustal deformation 1 http www sciencemag org cgi content full 303 5655 181

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

a constitutive model for layer development in shear zones near the brittle ductile transition 1 the microstructure of ductile shear zones differs from that of surrounding wall rocks in particular compositional layering is a hallmark of shear zones as layered rocks are weaker than their isotropic protolith when loaded in simple shear layering may hold the key to explain localization of ductile deformation onto ductile shear zones i propose here a constitutive model for layer development a two level mixing theory allows the strength of the aggregate to be estimated at intermediate degrees of layering a probabilistic failure model is introduced to control how layers develop in a deforming aggregate this model captures one of the initial mechanism of phase interconnection identified experimentally by holyoke and tullis 2006a 2006b fracturing of load bearing grains this model reproduces the strength evolution of these experiments and can now be applied to tectonic modeling

fault zone structure and weakening processes in basin scale reverse faults the moonlight fault zone south island new zealand abstract the 200 km long moonlight fault zone mfz in southern new zealand was an oligocene basin bounding normal fault zone that reactivated in the miocene as a high angle reverse fault present dip angle 65 75 regional exhumation in the last c 5 ma has resulted in deep exposures of the mfz that present an opportunity to study the structure and deformation processes that were active in a basin scale reverse fault at basement depths syn rift sediments are preserved only as thin fault bound slivers the hanging wall and footwall of the mfz are mainly greenschist facies quartzofeldspathic schists that have a steeply dipping 55 75 foliation subparallel to the main fault trace in more fissile lithologies e g greyschists hanging wall deformation occurred by the development of foliation parallel breccia layers up to a few centimetres thick greyschists in the footwall deformed mainly by folding and formation of tabular foliation parallel breccias up to 1 m wide where the hanging wall contains more competent lithologies e g greenschist facies metabasite it is laced with networks of pseudotachylyte that formed parallel to the host rock foliation in a damage zone extending up to 500 m from the main fault trace the fault core contains an up to 20 m thick sequence of breccias cataclasites and foliated cataclasites preserving evidence for the progressive development of interconnected networks of partly authigenic chlorite and muscovite deformation in the fault core occurred by cataclasis of quartz and albite frictional sliding of chlorite and muscovite grains and dissolution precipitation combined with published friction and permeability data our observations suggest that 1 host rock lithology and anisotropy were the primary controls on the structure of the mfz at basement depths and 2 high angle reverse slip was facilitated by the low frictional strength of fault core materials restriction of pseudotachylyte networks to the hanging wall of the mfz further suggests that the wide phyllosilicate rich fault core acted as an efficient hydrological barrier resulting in a relatively hydrous footwall and fault core but a relatively dry hanging wall

feldspar influenced rock rheologies progressive fracture subsequent cataclasis and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz mica phyllonite during early tertiary deformation in a 20 30 m wide shear zone in southeastern arizona the ductile deformation proceeded at temperatures and pressures approximating middle to upper crustal levels most of the deformation was taken up by the feldspar and its alteration products quartz deformed by both brittle fracture and crystal plastic mechanisms these results show that under certain temperature pressure strain rate conditions feldspars are weaker than quartz the observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature activated plastic deformation in quartz nor of the frictional behavior of rocks as inferred in the familiar strength vs depth curves it is more likely that the transition is complex and depends on a variety of parameters one such factor delineated in this study is the fracture and fracture facilitated syntectonic alteration of feldspar under hydrated conditions

low temperature frictional characteristics of chlorite epidote amphibole assemblages implications for strength and seismic style of retrograde fault zones in retrograde faults exhuming mafic rocks shearing occurs in metamorphic and or hydrothermally altered mineral assemblages whose frictional properties are not well known here we present the results of laboratory shearing experiments on chlorite schist epidotite and hornblende dominated amphibolite and mixtures of these rocks and evaluate their frictional properties and microstructures the experiments were conducted on powdered rock samples with starting grain size of 125 m at room temperature under fluid saturated conditions and applied normal stress of 10 mpa the results show that chlorite schist is relatively weak friction coefficient of 0 36 whereas epidotite and amphibolite are strong friction coefficients of 0 63 and 0 67 respectively the friction of chlorite schist epidotite and chlorite schist amphibolite mixtures decreases nearly linearly with increasing chlorite content chlorite schist exhibits velocity strengthening behavior epidotite is velocity weakening and the amphibolite shows mostly velocity weakening friction mixtures show intermediate strength and velocity dependence of friction well developed striations formed on slip surfaces in samples with 50 chlorite schist the epidotite slip surface exhibits a mixture of very fine particles and coarser crystals amphibolite slip surfaces have less very fine grains and are composed of subhedral to euheral needles few intragranular fractures are preserved and we infer wear at contact asperities to be the likely cause of velocity weakening in our epidote gouges addition of chlorite to epidotite and amphibolite produces a striated slip surface and disrupts contacts between harder grains therefore retrograde chlorite growth is expected to facilitate frictional weakening and stable slip in higher grade mineral assemblages exhumed to low temperature conditions

internal structure of the median tectonic line fault zone sw japan revealed by borehole analysis abstract the internal structure of the median tectonic line mtl fault zone and the processes that prevailed at depths are described based on an analysis of a borehole the fault plane which defines the boundary between the ryoke and sanbagawa derived rocks dips at 56 to the north immediately beneath the boundary approximately 40 m thick fractured rocks form the major strand of the mtl fault zone the hanging wall above the boundary comprises variably deformed ryoke granitoids including several mylonite zones and cataclasite zones the fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200 c the mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300 c these mylonites and cataclasites represent therefore fault rocks that formed immediately below and above the brittle plastic transition respectively development of dissolution seams in these cataclasites suggests that the cataclasite has low strength the presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle plastic transition on the other hand the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle plastic transition it is therefore likely that the differential stresses immediately below the brittle plastic transition are much higher than those immediately above the transition formation of laumontite in the major strand of the mtl fault zone occurred at temperatures of around 200 c the central slip zone of the major strand is about 30 cm thick and is surrounded by thick gouge zones this situation is favourable for thermal pressurisation during earthquake slips

late stages of continent continent collision timing kinematic evolution and exhumation of the northern rim aar massif of the alps abstract the alpine front s external crystalline massifs represent exhumation of mid crustal basement during late stage continent continent collision we unravel the geodynamic evolution of the northalpine front aar massif by combining structural and low t thermochronometric data a crustal scale tectonic restoration neogene present and estimation of associated changes in exhumation rates become possible the european continental margin experienced multiple switches from horizontal to vertical and again to horizontal dominated tectonics early thrust related inversion of rift basins is followed by a switch to rapid vertical exhumation with deformation along steep reverse normal faults this vertical tectonics is related to a positive buoyancy force of the european crust induced by lower crustal delamination of the aar massif during an orogenic rollback process of the european lithosphere 22 20 ma differential rock uplift rates of up to 1 3 1 6 km myr resulted which decreased northwards to 0 1 km myr reducing buoyancy forces isostatic compensation led to a horizontal tectonic style again persisting compressional forces induced an orogen scale strain partitioning with dextral transpressive faults in the south and simultaneous crustal scale northwest directed thrusting in the north en bloc exhumation of the entire aar massif in the thrust s hanging wall occurred between 13 5 ma and 5 0 ma at rates of 0 5 0 9 km myr and 0 6 0 8 km myr respectively it is this late stage thrusting in combination with surface erosion which shaped today s prominent northalpine front switches between horizontal and vertical tectonics might generally be relevant for late continent continent collisional stages here buoyancy forces of non thinned continental crust entering the subduction zone cause a strong vertical tectonic deformation style instead of classical thrust tectonics

shearing during progressive retrogression in granitoids abrupt grain size reduction of quartz at the plastic brittle transition for feldspar the kashio shear zone chubu district japan affects the southeast margins of the ryoke granitic plutons as well as the ryoke metasedimentary rocks and is truncated by the median tectonic line near the shear zone centre coarse quartz grains within the granitic rocks have been dynamically recrystallized to quartz aggregates towards the pluton margins the grain size data show an abrupt reduction from various sizes 59 148 m to a steady state size around 35 m at approximately 400 m from the boundary of the granitic plutons without any tectonic boundary such as a fault being present this abrupt grain size reduction accompanies brittle deformation in feldspar that overprints earlier plastic deformation and develops strong mylonitic fabrics in contrast feldspar away from the pluton margins contains only weak microstructural evidence for plastic deformation and still locally preserves magmatic polycrystal structures amongst the inhomogeneously recrystallized quartz grains these features combined with an isochronological study suggest that the kashio shear zone began at high temperature 450 c as a ductile event that affected relatively wide regions in the ryoke granitic plutons before rapid cooling by ca 70 ma at this latter stage strong mylonitization at lower temperatures 300 c in a semiductile regime occurred in a narrow zone towards the shear zone centre here recrystallized quartz aggregates reached a steady state grain size by ca 60 ma that is cooling within the granitic plutons resulted in strain localization with the abrupt grain size reduction of quartz aggregates correlating with the plastic brittle transition of feldspar

implications of mylonitic microstructures for the geotectonic evolution of the median tectonic line central japan the median tectonic line mtl the most prominent onshore fault in japan demarcates the cretaceous hiji quartz dioritic gneiss of the ryoke belt on the west from the high p t type sambagawa metamorphic rocks on the east in the takato area toward the mtl the hiji gneiss grades into strongly mylonitized rocks characterized by grain size reduction of quartz in the mylonitic rocks the development of fluxion banding sm is remarkably influenced by the existence of k feldspar forming a myrmekitic intergrowth brittle microstructures indicative of truly cataclastic deformation are observed only in mylonitic rocks close to the mtl early deep level ductile deformation apparently gave way to shallower brittle deformation at a later stage the attitude of stretching lineations lm and asymmetric microstructures observed in the mylonites suggest that sinistral strike slip shearing with a subordinate component of vertical slip took place during mylonitization in mid cretaceous time the mylonitic rocks and their protolith supposed to have constituted the eastern limb of the shear zone were probably eroded out and lost by upheaval of the sambagawa belt relative to the ryoke belt

structural controls on orogenic au mineralization during transpression lupa goldfield southwestern tanzania au mineralization in the western lupa goldfield southwestern tanzania was associated with transpression and reverse sinistral slip along a network of steeply s dipping shear zones with non andersonian geometries slip was accommodated by 1 frictional failure and sliding during emplacement of quartz au bearing veins and 2 crystal plasticity and fluid assisted diffusive mass transfer the kenge mineral system is situated along a nw se trending shear zone and is characterized by 10 m thick au bearing fault fill veins hosted by well developed phyllosilicate rich mylonites the broadly contemporaneous porcupine mineral system is situated along an ene wsw to e w trending shear zone which is characterized by narrow discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith au mineralization at porcupine occurs within steeply dipping fault fill and subhorizontal extension oblique extension veins three dimensional frictional reactivation theory provides a self consistent explanation for the different vein styles at kenge and porcupine and extends the classic fault valve model to the general case of oblique slip along multiple arbitrarily oriented shear zones analysis of the differential stress required for frictional reactivation suggests the following 1 the kenge shear zone was intrinsically weaker than the porcupine shear zone consistent with the lack of well developed mylonites at porcupine and 2 frictional reactivation of the kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures whereas frictional reactivation of the porcupine shear zone occurred under near lithostatic fluid pressures we hypothesize that near lithostatic pore fluid pressures relieved effective normal stresses at grain grain contacts helping to preserve intragranular and fracture porosity at the porcupine orebody as such these pore spaces may be important microstructural sites for au mineralization low effective normal stresses can also explain the poorly developed phyllosilicate rich mylonites and limited degree of shear zone weakening at porcupine

seismology talc at fault the behaviour of the san andreas fault varies along its length it slips in some places and creeps in others the discovery of the ultrasoft mineral talc in rocks from deep inside the fault could help to explain why high rates of creep along parts of the san andreas fault have been attributed to low fault strength associated with serpentinized rocks this is problematic because the frictional strength of serpentine minerals is considered too high to account for any weakness diane moore and michael rymer now report that talc the soft magnesium silicate mineral familiar in its pure form as talcum powder may be behind the high creep rate they discovered talc in serpentinite samples collected during drilling of the safod san andreas fault observatory at depth main hole in 2005 the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep

deformation processes and weakening mechanisms within the frictional viscous transition zone of major crustal scale faults insights from the great glen fault zone scotland abstract the great glen fault zone ggfz scotland is a typical example of a crustal scale reactivated strike slip fault within the continental crust analysis of intensely strained fault rocks from the core of the ggfz near fort william provides a unique insight into the nature of deformation associated with the main phase of sinistral movements along the fault zone in this region an exhumed sequence of complex mid crustal deformation textures that developed in the region of the frictional viscous transition ca 8 15 km depth is preserved fault rock fabrics vary from mylonitic in quartzites to cataclastic in micaceous shear zones and feldspathic psammites protolith mineralogy exerted a strong control on the initial textural development and distribution of the fault rocks at lower strains crystal plastic deformation occurred in quartz dominated lithologies to produce mylonites simultaneously with widespread fracturing and cataclasis in feldspar and mica dominated rocks at higher strains shearing appears to increasingly localise into interconnected networks of cataclastic shear zones many of which are strongly foliated textures indicative of fluid assisted diffusive mass transfer mechanisms are widespread in such regions and suggest that a hydrous fluid assisted grainsize controlled switch in deformation behaviour followed the brittle comminution of grains the fault zone textural evolution implies that a strain induced fluid assisted shallowing and narrowing of the frictional viscous transition occurred with increasing strain it is proposed that this led to an overall weakening of the fault zone and that equivalent processes may occur along many other long lived crustal scale dislocations

the effect of fluid flow on the magnetic field in low porosity crystalline rock changes in the local geomagnetic field have been observed and documented in several seismically active areas prior to during and following earthquakes mizutani and ishido 1975 proposed that groundwater may diffuse through a hypocentral region and that the fluid flow can produce an electric potential by means of electrokinetic phenomena the resulting current flow in turn will generate a magnetic field fluid flow tests conducted on samples of westerly granite demonstrated that the diffusion of an electrolytic fluid causes a measurable magnetic field change at high fluid flow rates a magnetic field change was observed that was in agreement with theoretically predicted and numerically calculated values in addition the sign of the magnetic field changes as the fluid flow direction through the sample was reversed moreover the tests showed that the magnitude of the magnetic field change is dependent upon 1 the porosity of the rock 2 the pore wall surface area 3 the permeability of the rock and 4 zeta potential of the fluid pore wall system

frictional strength and healing behavior of phyllosilicate rich faults 1 we study the mechanisms of frictional strength recovery for tectonic faults with particular focus on fault gouge that contains phyllosilicate minerals we report laboratory and microstructural work from fault rocks associated with a regional low angle normal fault in central italy experiments were conducted in a biaxial deformation apparatus at room temperature and humidity nominally dry under constant normal stresses of 20 and 50 mpa and at a sliding velocity of 10 mm s our results for nominally dry conditions show good agreement with previous work conducted under controlled pore fluid pressure the phyllosilicate contents of our samples which include clay talc and chlorite range from 0 to 52 weight we study both intact rock samples sheared in their in situ geometry and powders made from the same rocks to address the role of fabric in fault healing we measured frictional healing dm using slide hold slide tests with hold periods ranging from 3 to 3000 s phyllosilicate free materials show friction values of m 0 6 and healing rates that are larger in powdered samples b 0 006 dm per decade in time s compared to intact wafers of fault rock b 0 004 for phyllosilicate bearing materials healing rates are low b 0 002 and independent of fabric phyllosilicate content and normal stress we observe that frictional strength decreases systematically with increasing phyllosilicate content intact phyllosilicate bearing fault rock is consistently weaker than its powdered equivalent 0 2 m 0 3 versus 0 4 m 0 5 respectively we compare our data to results from experiments conducted on a wide range of materials and conditions deformation microstructures show localized slipping along sub parallel shear planes we suggest that low values of frictional strength and near zero healing rates will combine to exacerbate the weakness of phyllosilicate bearing faults and promote stable aseismic creep

sources of solutes and heat in low enthalpy mineral waters and their relation to tectonic setting new zealand abstract approximately 90 of the low enthalpy spring systems in new zealand are distributed along the present day convergence of the pacific and australian plates which varies from subduction to oblique plate collision forearc subducted waters rising in the hikurangi accretionary prism consist of pore waters containing labile organic material water of hydration from clays and seawater these subducted waters are traceable in the north island axial ranges and the northern marlborough fault system in the south island albeit modified by temperature mixing with surface and heated meteoric waters and protracted interaction with the rock the signature of subducted marine sediments in the hikurangi trench is retained in the chemical and isotopic compositions of the taupo volcanic zone magmatic arc type waters the concentrations of b ba br ca cl i k li mg na rb and sr solutes found in high concentrations in forearc subducted waters increase from northeast to southwest towards the incipient subduction zone in the puysegur trench southwest of south island most thermal mineral waters in the south island are expelled along the alpine fault zone where plate collision results to rapid uplift of the southern alps elevated geothermal gradients and fault shearing of rocks in the alpine fault zone variations in temperatures 175 30 c and mineral water compositions are affected by elevation hydrological gradient intensity of tectonic activity permeability fluid volume and rate of throughput and rates of uplift as the main trace of the alpine fault zone is approached concentrations of cl and other solutes generally increase suggesting increasing contributions from ascending cl rich metamorphic waters outside the active convergent zone low enthalpy springs discharge diagenetically modified ancient forearc waters na cl sedimentary formation waters and heated seawater na hco 3 cl waters and na hco 3 deep circulating heated meteoric waters the total surface heat flow in the north island low enthalpy mineral waters is more than 10 higher than in the south island with projected subsurface temperatures as high as 250 c this is because of the widespread occurrence of shallow magmatic and mantle sources associated with pliocene to holocene volcanism and back arc continental rifting the sources of heat and solutes in low enthalpy mineral waters outside the taupo volcanic zone and ngawha are decoupled except for rapidly ascending hot pore waters in the hikurangi accretionary prism and low temperature serpentinization ca cl waters in fiordland

the role of precambrian mylonitic belts and present day stress field in the coseismic reactivation of the pernambuco lineament brazil abstract the pernambuco lineament is a steeply dipping continental scale ductile shear zone rooted within the precambrian lithosphere of intraplate northeastern brazil it was formed during the brasiliano orogeny 600 ma and reactivated during the pangea breakup in the cretaceous when it controlled fault propagation and sediment accumulation in several rift basins the region is now under an e w trending horizontal compression and n s trending extension and faulting occurs predominantly in a strike slip regime we investigated two aftershock sequences and the preexisting tectonic fabrics along the pernambuco lineament in order to evaluate the role of these fabrics in the coseismic reactivation of continental scale structures the lineaments consist of a main ene wsw trending mylonitic belt about 2 3 km wide and two secondary ne trending mylonitic belts about 100 m wide they both present steeply dipping mylonitic foliations and shallowing plunging stretching lineations the mylonites present granitoid protoliths and mineral parageneses that range from amphibolite to greenschist facies brittle deformation overprints the ductile fabric in all mylonitic belts in 1991 coseismic reactivation nucleated along the ene wsw striking 3 3 5 6 km deep normal fault of less than 1 km in length in the main mylonitic belt in 2002 seismicity migrated to a ne trending secondary mylonitic branch and moved as a right lateral strike slip 1 2 km long 3 8 4 9 km deep fault plane both fault segments reactivated the mylonitic foliation and form part of a major system we conclude that the interplay between the present day stress field and preexisting fabrics controls seismogenic fault location attitude and kinematics the pernambuco lineament is an example of a long lived continental scale structure where selective reactivation has occurred other shear zones in the region also show a long history of brittle reactivation and present similar orientation in relation to the present day stress axes they might be dormant structures prone for reactivation under the present day stress field

the role of fluids in low temperature fault related deformation of quartz arenite abstract the microstructural and fluid histories in quartz arenite deformed in a regional scale fault zone were investigated to determine the role of fluids in the selection of grain scale deformation mechanisms using cathodoluminescence fluid inclusion microthermometry and stable isotope geochemistry of quartz cements three chemically distinct aqueous fluids were identified that affected the rock at different times during deformation the earliest deformation microstructures are distributed cataclasites which were cemented by reddish brown luminescing quartz precipitated from an aqueous cacl 2 brine fluid i once cemented brittle deformation continued but localized along discrete shear surfaces in narrow zones 20 1000 m wide and on discrete microfractures microveins associated with this phase were cemented with bluish green luminescing quartz precipitated from an aqueous cacl 2 brine fluid ii water channelized along dissolution surfaces and in pervasive microfracture networks that formed at this time promoted hydrolytic weakening and crystal plastic deformation especially near fluid pathways temperatures during deformation associated with fluids i and ii were 225 235 c as determined from fluid inclusion data in the later stages of deformation an influx of iron bearing aqueous fluid fluid iii greatly accelerated the rate of quartz dissolution and filled fractures stylolites and vugs with goethite the progression of deformation mechanisms indicated by the microstructures and their relationship to the different fluids show that the selection of grain scale deformation processes is affected not only by the presence of fluids but also by their chemistry

reaction weakening and emplacement of crystalline thrusts diffusion control on reaction rate and strain rate abstract in the southern appalachians the blue ridge piedmont crystalline thrust sheet was emplaced onto low grade late precambrian and paleozoic sedimentary rocks in the footwall along a basal detachment consisting of phyllosilicate rich mylonites phyllonites the phyllonites developed first by mechanical breakdown of feldspar followed by chemical breakdown to white mica in the presence of a pore fluid diffusion of solute in the pore fluid is the rate limiting step in controlling reaction rate and also the strain rate assuming solute diffusion follows the stokes einstein equation the shear strain rate is given by t 2 n k t 5 r x 2 for shear stress 20 mpa where n is a constant is a geometric factor k is boltzmann s constant t is absolute temperature is water viscosity r is the atomic radius of the diffusing species and x is the diffusion distance a bulk diffusion coefficient in the range of 10 10 to 10 12 m 2 s over distances of 10 100 m results in strain rates of 10 14 to 10 13 s 1 in the temperature range 200 400 c it is concluded that greenschist grade crystalline thrust sheets develop on pre existing basement faults that become weak during reaction softening and localize into high strain phyllonite zones in which pore fluid diffusion controls reaction rate and strain rate

damage patterns stress rotations and pore fluid pressures in strike slip fault zones 1 active faults unfavorably oriented with respect to the regional maximum compressive stress have been labeled as weak the seismic hazards posed by these faults make understanding this apparent weakness a priority stress rotations in these fault zones together with an increase in mean stress could enable high pore fluid pressures to weaken a fault zone such a model requires a foundation in the physics and mechanics of damage this paper presents a new model for stress rotations in fault zones by combining the effective medium theory with anisotropic poroelasticity this approach enables the quantitative characterization of crack damage and the prediction of progressive changes in the elastic properties of rocks across the fault zone the processes of fault growth and wear will lead to distinct patterns of crack damage with different effects on the elastic properties elevated pore fluid pressures have long been known to change the effective normal and shear stresses of anisotropic rocks and this work incorporates these effects into a multilayer fault zone model it is shown that high pore fluid pressures in the anisotropic rocks of the core zone can generate large stress rotations i e more fault parallel and increases in mean stress sufficient to weaken the fault stress rotations in the damage zones of unfavorably oriented faults tend to be away from the fault i e more fault normal for likely combinations of damage patterns and pore fluid pressure

the role of fault zone fabric and lithification state on frictional strength constitutive behavior and deformation microstructure 1 we examine the frictional behavior of a range of lithified rocks used as analogs for fault rocks cataclasites and ultracataclasites at seismogenic depths and compare them with gouge powders commonly used in experimental studies of faults at normal stresses of 50 mpa the frictional strength of lithified isotropic hard rocks is generally higher than their powdered equivalents whereas foliated phyllosilicate rich fault rocks are generally weaker than powdered fault gouge depending on foliation intensity most samples exhibit velocity strengthening frictional behavior in which sliding friction increases with slip velocity with velocity weakening limited to phyllosilicate poor samples this suggests that lithification of phyllosilicate rich fault gouge alone is insufficient to allow earthquake nucleation microstructural observations show prominent throughgoing shear planes and grain comminution in the r1 riedel orientation and some evidence of boundary shear in phyllosilicate poor samples while more complicated anastomosing features at lower angles are common for phyllosilicate rich samples comparison between powdered gouges of differing thicknesses shows that higher riedel shear angles correlate with lower apparent coefficients of friction in thick fault zones this suggests that the difference between the measured apparent friction and the true internal friction depends on the orientation of internal deformation structures consistent with theoretical considerations of stress rotation

the sierra ballena shear zone in the southernmost dom feliciano belt uruguay evolution kinematics and deformation conditions the sierra ballena shear zone sbsz is part of a high strain transcurrent system that divides the neoproterozoic dom feliciano belt of south america into two different domains the basement on both sides of the sbsz shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization grain boundary migration and fluid assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation age estimate of this episode is 658 ma the second stage corresponds to the start of transpressional deformation and the nucleation and development of the sbsz during this stage pure shear dominates the deformation and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites this event dates to 658 600 ma based on the age of these intrusions the third stage was a second transpressional event at about 586 to 560 ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening deformation in the sbsz took place during the late stages under regional low grade conditions as indicated by the metamorphic paragenesis in the supracrustals of the country rocks granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar a transition from magmatic to solid state microstructures is also frequently observed in syntectonic granites mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites which resulted in shear zone weakening quartz microstructures characteristic of the transition between regime 2 and regime 3 grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions 550 400 c on the other hand the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high t solid state deformation during cooling of the dikes

shortening of the axial zone pyrenees shortening sequence upper crustal mylonites and crustal strength abstract the strength of the lithosphere may be constrained qualitatively by field observations on localized vs distributed modes of deformation and by the mineral assemblages formed during deformation the internal deformation of the bielsa basement unit of the pyrenean axial zone is investigated through structural microstructural and thermometric data in this area shortening is widely distributed as attested by the folded attitude of the interface between the basement and its sedimentary triassic cover shortening is estimated around 1 7 km 13 from a regional balanced cross section and should be considered in pre pyrenean reconstructions shortening probably occurred before strain localization on crustal ramps as suggested by zircon fission track analysis distributed shortening is characterized at small scale by low temperature mylonites and cataclasites in thin section feldspar originally present in the magmatic protolith is partially to totally sericitized this transformation led to significant weakening of the rock and took place in the 250 350 c temperature range sericitization is ubiquitous even in un deformed granodiorites this shows that the weakening effect of sericitization not only occurs in ultra mylonites ultra cataclasites and phyllonites but also more generally in the upper crust early during the shortening history with implications for the shortening style estimates of the geothermal gradient suggest that inherited thermicity may also have influenced the style of shortening we propose that the upper crust was very weak before or at the onset of its shortening due to high thermal gradients and fluid circulation that induced large scale sericitization in greenschist facies conditions this has strong implications on the rheological evolution of the upper crust submitted to metamorphic alteration in the greenschist facies and below

mechanisms of margin inversion in the external western alps implications for crustal rheology the kinematics of the collisional crustal wedge in the external western alps is discussed in the light of new cross sections of the whole external zone at the latitude of the belledonne grandes rousses oisans massifs external crystalline massifs ecm as well as a detailed analysis of the deformation of their basement and cover the cross sections were built from new field data and published geological maps and were restored through time oligo miocene times to unravel the successive stages of the dauphinois margin contractional deformation during oligocene times the variscan basement was deformed essentially by greenschist facies alpine distributed shear zones with no significant reactivation of the inherited jurassic normal faults or the variscan foliation however the inherited syn rift basins localized the deformation characterized by a thick skinned style and a cover disharmonically folded over basement shear zones with no major decollement in between shortening of 11 5 km 20 those early deformations progressively localized on the frontal crustal ramp and caused shortening in the folded belt 16 5 km of shortening 23 a similar sequence of deformation has been inferred in other alpine ecms this strongly suggests that the crust of dauphinois proximal passive margin was thin enough and its buoyancy low enough as a result of the liassic rifting to experience significant tectonic burial during the tertiary collision as a consequence of a weakening by both the presence of inherited basins and their weak syn rift sediments and the p t conditions greenschist facies the crust was shortened and the structural style was thick skinned

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

gently north dipping median tectonic line mtl revealed by recent seismic reflection studies southwest japan abstract the median tectonic line mtl with a length of more 1000 km is the most significant fault in japan it juxtaposes the high p t sambagawa metamorphic rocks against the low p t metamorphic rocks of the ryoke belt the mtl was probably formed in the cretaceous with many subsequent reactivations the western segment of the mtl is still active with an almost pure right lateral sense of motion although a great amount of geological information on the mtl has been accumulated information about the subsurface especially the deep seated structure of the mtl is still insufficient it has been generally assumed that the mtl is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion recently new geophysical data have suggested that the mtl dips gently northward at depth we have acquired a complementary set of geophysical profiles seismic reflection and refraction gravity and mt across the mtl in east shikoku our results confirm that the mtl dips northward at about 30 to 40 degrees from the surface to about 5 km depth where it becomes listric this fault geometry more reasonably explains the reactivation history of the mtl the motion has occurred on a listric type fault in so called oblique or lateral ramp manner

evolution of the median tectonic line fault zone sw japan during exhumation abstract like many crustal scale fault zones the median tectonic line mtl fault zone in japan preserves fault rocks that formed across a broad range of physical conditions we examined the architecture of the mtl at a large new outcrop in order to understand fault behaviours under different crustal levels the mtl here strikes almost e w dips to the north and juxtaposes the sanbagawa metamorphic rocks to the south against the izumi group sediments to the north the fault core consists mainly of sanbagawa derived fault gouges the fault zone can be divided into several structural units including two slip zones upper and lower slip zones where the lower slip zone is more conspicuous crosscutting relationships among structures and kinematics indicate that the fault zone records four stages of deformation microstructures and powder x ray diffraction xrd analyses indicate that the four stages of deformation occurred under different temperature conditions the oldest deformation stage 1 was widely distributed and had a top to the east dextral sense of slip at deep levels of the seismogenic zone deformation with the same sense of slip then became localised in the lower slip zone stage 2 subsequently the slip direction in the lower slip zone changed to top to the west sinistral normal stage 3 the final stage of deformation stage 4 involved top to the north normal faulting along the two slip zones within the shallow crust near the surface the widely distributed stage 1 damage zone characterises the deeper part of the seismogenic zone while the sets of localised principal slip zones and branching faults of stage 4 characterise shallow depths the fault zone architecture described in this paper leads us to suggest that fault zones display different behaviours at different crustal levels

the structure and rheological evolution of reactivated continental fault zones a review and case study abstract repeated reactivation of structures and reworking of crustal volumes are characteristic though not ubiquitous features of continental deformation reactivated faults and shear zones exposed in the deeply exhumed parts of ancient orogenic belts present opportunities to study processes that influence the mechanical properties of long lived fault zones at different palaeo depths ancient basement fault systems typically comprise heterogeneous superimposed assemblages of fault rocks formed at different times and depths for which down temperature thermal histories are most common several lithological and environmental factors influence the evolution of fault rock fabrics and rheology but most fault shear zone arrays appear to develop as self organized deformation systems once mature the kinematic and mechanical evolution of the system is strongly influenced by the rheological behaviour of the interconnected fault shear zone network a case study from the crustal scale great glen fault zone ggfz scotland reveals a complex evolution of mid to upper crustal deformation textures formed adjacent to the frictional viscous transition fluid influx in the mid crust has led to reaction softening of the rock aggregate as strong pre existing phases such as feldspar are replaced by fine grained strongly aligned aggregates of weak phyllosilicates in addition a grainsize controlled switch to fluid assisted diffusional creep occurs in the highest strain regions of the fault zone it is proposed that this led to a shallowing and narrowing of the frictional viscous transition and to long term overall weakening of the fault zone relative to the surrounding wall rocks cataclasis is particularly important in the deeper part of the frictional regime as it helps to promote retrograde metamorphism and changes in deformation regime by both reducing grainsize and promoting pervasive fluid influx along fault strands due to grain scale dilatancy equivalent processes are likely to occur along many other long lived crustal scale fault zones

brittle to ductile transition due to large strains along the white rock thrust wind river mountains wyoming abstract the deformed foreland of the rocky mountain cordilleras is characterized by major reverse faults that bring precambrian basement rocks to the surface the white rock thrust in the wind river mountains of wyoming is one such fault that developed at fairly shallow levels in the crust detailed study of grain size variations and textures within the fault zone provide a framework for discussion of the mechanics of movement along the fault textural criteria are suggested for distinguishing between dominantly brittle and dominantly ductile deformation a transition from brittle fracturing to ductile deformation is seen with increasing strain and resultant decrease in grain size along the fault zone diffusional creep in the final stages of deformation leads to strain softening and may allow large strains to take place along narrow zones at low deviatoric stresses this may be one way of obtaining large displacements along major faults at shallow levels in the crust

the nature and importance of phyllonite development in crustal scale fault cores an example from the median tectonic line japan abstract like many large crustal scale faults the median tectonic line mtl in sw japan has a long history of movement having been active predominantly as a strike slip fault since the mid cretaceous fault rock exposures in the core of the mtl preserve a history of deformation at a range of mid to shallow crustal depths ryoke mylonites 1 4 km north of the main contact record deeper level cretaceous top to the south sinistral movements the remainder of the fault zone core is surprisingly narrow exhibiting a wide variety of fault rocks that illustrate both the interaction and effects of syn tectonic fluid influx over a range of deformation conditions exposures within 50 m of the central slip zone display a progressive sequence in fault rock evolution from ultramylonite cataclasite foliated cataclasite phyllonite breccia gouge this sequence occurs because cataclasis in the vicinity of the fault core creates permeable pathways for the ingress of chemically active fluids into the fault zone this leads to the replacement of load bearing phases such as feldspar by fine grained foliated aggregates of intrinsically weaker phyllosilicates such as white mica and chlorite the grain size reduction associated with both cataclasis and mineral alteration creates conditions ideal for the operation of fluid assisted stress induced diffusive mass transfer mechanisms comparison with the findings of recent experimental studies suggest that the fault zone processes observed in the core of the mtl will lead to long term weakening provided the network of phyllosilicate rich fault rocks are able to form an interconnected thin layer of weak material on kilometre to tens of kilometre length scales

influence of shear heating on microstructurally defined plate boundary shear zones abstract in plate boundary scale ductile shear zones defined by microstructural weakening shear heating may lead to a temperature increase over 5 m y of up to 80 c just below the brittle ductile transition up to 120 c just below the moho and to thermal boundary zones tens of km wide on either side of the shear zone where rock strength is highest shear zones are narrow 1 km and thermal gradients within the shear zone itself are low so there is no tendency for increased localization heating results in thermal weakening but this is partly offset by grain growth related to the decrease in stress in shear zones of the order of 10 km width shear stress strain rate and hence heat generation are lower and thermal gradients are insufficient to cause additional strain localization temperature increases in the mid crust are of the order of 10 c insufficient to cause partial melting or an increase in metamorphic grade in the upper mantle shear zones may be 50 km or more wide and the temperature increase is less than 20 c in 5 m y but temperature differences between center and margin may enhance the strain rate at the center by up to 18

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

fault zone weakening and character of slip along low angle normal faults insights from the zuccale fault elba italy a seismically active low angle normal fault is recognized at depth in the northern apennines italy where recent exhumation has also exposed ancient examples at the surface notably the zuccale fault on elba field based and microstructural studies of the zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability promoting influx of co 2 rich hydrous fluids this triggered low grade alteration and the onset of stress induced dissolution precipitation processes e g pressure solution as the dominant grain scale deformation process in the pre existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine grained hydrous mineral phases these rocks exhibit ductile deformation textures very similar to those formed during pressure solution accommodated frictional viscous creep in experimental fault rock analogues the presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core which significantly enhanced the sealing capacity of the fault zone a slip model for low angle normal faults in the apennines is proposed in which aseismic frictional viscous creep occurs on a weak slow moving slip rate 1 fault interspersed with small seismic ruptures caused by cyclic hydrofracturing events our findings are potentially applicable to other examples of low angle normal faults in many tectonic settings

influence of grain boundary structural evolution on pressure solution creep rates intergranular pressure solution is a well known rock deformation mechanism in wet regions of the upper crust and has been widely studied especially in the framework of compaction of granular materials such as reservoir sandstones and fault rocks several analytical models exist that describe compaction creep by stress induced mass transport and the parameters involved are relatively well constrained by laboratory experiments while these models are capable of predicting compaction behavior observed at relatively high porosities they often overestimate compaction rates at porosities below 20 by up to several orders of magnitude this suggests that the microphysical processes operating at low porosities are different and are not captured well by existing models the implication is that available models cannot be extrapolated to describe compaction of sediments and fault rocks to the low porosities often reached under natural conditions to address this problem we propose a new thermodynamic model that describes the decline of pressure solution rates within individual grain contacts as a result of time averaged growth of asperities or islands and associated constriction of the grain boundary diffusion path here termed grain boundary evolution the resulting constitutive equations for single grain grain contacts are then combined and solved semianalytically the compaction rates predicted by the model are compared with those measured in high strain compaction experiments on wet granular halite a significant reduction in compaction rate is predicted when grain boundary evolution is considered which compares favorably with the experimental compaction data

the structure and kinematics of an imbricate stack of oceanic rocks in the jurassic accretionary complex of central japan an oblique subduction model the jurassic accretionary complex of the mino belt in the takayama area central japan includes a major unit characterized by abundant bedded chert and basalt slabs this unit has an imbricate structure represented by the repetition of stratigraphic layers composed of sandstone black mudstone felsic tuff bedded chert siliceous claystone limestone and basalts shear zones subparallel to the trend of the slabs occur in the siliceous claystones and basalts deformation structures in the shear zones suggest that phyllosilicate rich layers within the accretionary complex were preferentially weakened due to grain size reduction by cataclasis and pressure solution accommodated deformation restored shear directions in the siliceous claystones and basalts indicate dextral kinematics subparallel to the trench axis in contrast mesoscopic asymmetric folding of the bedded chert indicated from the restored shear was oblique or normal to the trench axis to interpret the two distinct shear directions a model is proposed in which the constituent rocks of this unit were obliquely subducted dextrally along the east asian continental margin due to the oblique subduction slip vectors were apparently partitioned into trench orthogonal components in the shallower part of the subduction zone and trench parallel components in the deeper part

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

origin and mechanical significance of foliated cataclastic rocks in the cores of crustal scale faults examples from the median tectonic line japan 1 the median tectonic line mtl is japan s largest onshore fault and has been active since the mid cretaceous foliated cataclastic fault rocks are exceptionally well exposed in the fault core at anko nagano prefecture following an early phase of mylonitization and exhumation during left lateral shearing brittle fracture and cataclasis occurred leading to the development of centimeter to submillimeter spaced fault zone parallel fracture systems these fracture systems established an initial architectural hierarchy that influenced the subsequent development of foliated cataclasites and gouge initially fracture systems coalesced to form interconnected zones of fine grained ultracataclasite fluid influx at the onset of grain scale brittle deformation led to precipitation of fibrous chlorite within the ultracataclasites ultimately leading to the development of an interconnected network of foliated phyllosilicate rich cataclasites and gouges in the core of the mtl the brittle reduction of grain size and ingress of a chemically active fluid phase simultaneously promoted reaction softening and diffusive mass transfer in the foliated ultracataclasites leading to rate dependent frictional viscous flow at sub byerlee friction values associated weakening is indicated by the preferential localization of deformation within the ultracataclasites a protracted sequence of carbonate mineralization and cementation events is also recognized during the fault rock evolution and suggests episodic periods of fluid overpressuring a crustal scale fault zone model is proposed suggesting that the foliated cataclasites gouges are weak in the long term and represent shallower crustal equivalents of phyllonitic fault rocks exposed in more deeply exhumed fault zones including other parts of the mtl

detailed dem analysis of a rockslide scar to characterize the basal sliding surface of active rockslides 1 the basal sliding surfaces in large rockslides are often composed of several surfaces and possess a complex geometry the exact morphology and location in three dimensions of the sliding surface remains generally unknown in spite of extensive field and subsurface investigations such as those at the aknes rockslide western norway this knowledge is crucial for volume estimations failure mechanisms and numerical slope stability modeling this paper focuses on the geomorphologic characterization of the basal sliding surface of a postglacial rockslide scar in the vicinity of aknes this scar displays a stepped basal sliding surface formed by dip slopes of the gneiss foliation linked together by steeply dipping fractures a detailed characterization of the rockslide scar by means of high resolution digital elevation models permits statistical parameters of dip angle spacing persistence and roughness of foliation surfaces and step fractures to be obtained the characteristics are used for stochastic simulations of stepped basal sliding surfaces at the aknes rockslide these findings are compared with previous models based on geophysical investigations this study discusses the investigation of rockslide scars and rock outcrops for a better understanding of potential rockslides this work identifies possible basal sliding surface locations which is a valuable input for volume estimates design and location of monitoring instrumentation and numerical slope stability modeling

fluid flow and metasomatic fault weakening in the moresby seamount detachment woodlark basin offshore papua new guinea low angle normal faults play a prominent role in discussions about fault strength as they require significant weakening to remain active at low angles the submerged moresby seamount detachment msd is arguably the best exposed active low angle detachment worldwide we analyzed dredged msd protoliths cataclasites and mylonites to investigate deformation mechanisms and fault weakening processes deformation is accompanied by important syntectonic fluid induced mass transfer controlling the rheological behavior of the msd while the mafic protolith behaves brittlely at the onset of deformation the metasomatic mineralogical and chemical changes cause a transition to plastic flow as the rock is progressively exhumed immobile elements provide a reference frame for total material gains and losses si ca and k are syntectonically enriched while fe ti mg and al are depleted mass increase is about 10 in the cataclasites and about 48 in the mylonites main mechanism is syntectonic veining causing enrichment in calcite and quartz thus making the mylonites capable to flow plastically minimum time integrated fluid flux is calculated as 3 105 m3 m 2 indicating that the msd is an important fluid conduit the fluids have a deep crustal source a bottom water temperature and turbidity anomaly suggests that the hydrothermal system is still active syntectonic veining in fault rocks and recent seismic activity both suggest that the msd is intermittently brittle implying a brittle plastic transition at unusually high temperature and low differential stress we conclude that fault zone metasomatism is crucial in forming weak detachments at passive margins and may be a prerequisite for successful crustal breakup

reduction of friction on geological faults by weak phase smearing most common crustal rock types display friction coefficients of 0 6 or higher but some faults must be frictionally weak as they slip when the stress state is unfavourably oriented i e the resolved shear stress is low for a given normal stress across the fault surface a role for low friction minerals and high pore fluid pressures either separately or in combination is frequently invoked to explain such slip but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening by means of mechanical tests on synthetic fault gouge and microstructural study of run products we show that the effective area of an embedded weak phase graphite on a slip plane can be substantially increased by mechanical smearing and that the enlarged area of the weak phase on the slip plane follows a linear mixing law this allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them

development of phyllonite from granodiorite mechanisms of grain size reduction in the santa rosa mylonite zone california abstract field and laboratory investigations indicate that phyllonite within the santa rosa mylonite zone developed from granodiorite without hydrous alteration of the original mineral assemblage initial grain size reduction within zones of phyllonite was accomplished through whole rock cataclasis subsequent deformation occurred through ductile flow the original cataclastic failure is recorded by sharp boundaries with the surrounding rock which are retained through the ductile overprint biotite in phyllonite has a mean grain size of 3 m and exhibits straight boundaries and a strong crystallographic preferred orientation trace analyses of hvem images indicate that the largest biotite grains are most strongly oriented and the grains are elongate parallel to lineation tem observations indicate that typical grains have irregularly spaced stacking faults and twins evidence for intracrystalline folding and cataclasis present in protomylonite and mylonite is absent bending or kinking of grains is rare these observations suggest that biotite in phyllonite accommodated deformation through dynamic recrystallization in concert with intracrystalline slip and mechanical rotation grain size reduction to form phyllonite is accompanied by macroscopically and microscopically visible changes in the character of foliations in the rock we suggest that these changes are linked to the deformation mechanisms operating within the fine grained phyllonite

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

the occurrence of graphite bearing fault rocks in the atotsugawa fault system japan origins and implications for fault creep abstract graphite in fault zones has received little attention even though it is a well known solid lubricant that could affect frictional properties of faults dramatically this paper reports the presence of abundant graphite in fault zones of the atotsugawa fault system central japan mesoscopic and microscopic observations of fault rocks revealed two processes of carbon enrichment in fault zones one is a pressure solution process or diffusive mass transfer in general which removes water soluble minerals such as quartz and carbonates from rocks resulting in the enrichment of insoluble minerals including carbon the other process is precipitation of graphite from a high temperature carbon rich fluid forming graphite filling fractures within cataclasitic fault zones the two processes have led to the concentration up to 12 wt of graphite in the atotsugawa fault zones compared to 0 to 3 wt of carbonaceous materials in the host rocks this concentration is high enough for graphite to affect frictional properties at wide range of slip rates the presence of graphite may provide an explanation for the low resistivity the patterns of microearthquakes and fault creep along the western part of the atotsugawa fault system graphite should receive more attention as a weakening and stabilizing agent of faults

strain distribution within a km scale mid crustal shear zone the kuckaus mylonite zone namibia the subvertical kuckaus mylonite zone kmz is a km wide crustal scale proterozoic dextral strike slip shear zone in the aus granulite terrain sw namibia the kmz was active under retrograde amphibolite to greenschist facies conditions and deformed felsic and minor mafic gneisses which had previously experienced granulite facies metamorphism during the namaqua orogeny lenses of pre to syn tectonic leucogranite bodies are also deformed in the shear zone pre kmz deformation d1 is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering shear strain within the kmz is heterogeneous and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges strain is localized along the edge of leucogranites and between gneissic lozenges preserving d1 migmatitic foliations strain localization appears controlled by pre existing foliations grain size and compositional anisotropy between leucogranite and granulite the local presence of retrograde minerals indicate that fluid infiltration occurred in places but most ultramylonite in the kmz is free of retrograde minerals in particular rock composition and d1 fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space with deformation localization along planes of rheological contrast and along pre existing foliations therefore the spatial distribution of strain in crustal scale ductile shear zones may be highly dependent on lithology and the orientation of pre existing fabric elements in addition foliation development and grain size reduction in high strain zones further localizes strain during progressive shear maintaining the anastomosing shear zone network established by the pre existing heterogeneity

principal slip zones precursors but not recorders of earthquake slip narrow highly comminuted shear localization features in faults known as principal slip zones pszs are commonly associated with large offset seismogenic faults in this study laboratory friction experiments were performed using shale and slate gouges where deformation was encouraged to localize at the gouge wall rock boundary the slate gouges develop a black narrow psz composed of densely packed submicron particles that appear sintered while the spectator gouge remains largely undeformed these pszs form at subseismic slip velocities of 10 5 m s and with a calculated temperature rise of 3 c instances of velocity weakening friction which is necessary for unstable fault slip are only observed for slate samples with a psz shale gouges however do not develop a psz and exhibit only velocity strengthening frictional behavior the development of a psz may therefore be a prerequisite for future earthquake slip to occur rather than unequivocal evidence of past earthquake slip

fault zone weakening processes along the reactivated outer hebrides fault zone scotland the outer hebrides fault zone is a major reactivated structure cutting amphibolite grade lewisian basement gneisses in nw scotland during a regionally important phase of sinistral strike slip movements the influx of chemically active hydrous fluids along the fault zone was associated with the formation of a network of greenschist facies phyllonitic shear zones later ese directed extensional strain was preferentially focused into these pre existing zones of weakness the syn tectonic alteration of a relatively strong feldspar hornblende dominated load bearing framework microstructure to an interconnected weak layer microstructure of fine grained strongly aligned phyllosilicate aggregates leads to the long term weakening in the fault zone comparison with experimental data suggests that this produces a shallowing of the frictional viscous creep 9brittle ductile9 transition and a substantial reduction in total crustal strength similar processes may account for the apparent weakness of many long lived fault zones

iron oxide precipitate in seepage of groundwater from a landslide slip zone orange precipitate was collected at the mouth of groundwater drainage tubes from the kumanashi landslide slip zone in toyama prefecture northwest japan data from xrf x ray diffraction and mossbauer spectroscopy determined the precipitate as hydrous ferric oxide hfo bearing multi elements such as phosphorous silica calcium etc the occurrence of hfo may indicate an oxidation of ferrous iron in the percolated groundwater from the slip zone moreover the precipitate iron should be mobilized with groundwater circulation from the slip zone whose reducing condition was determined by the iron speciation on the same type of landslide profiles within the study area in previous studies the hfo precipitate may be considered as a secondary reliable indicator to locate the seepage of a slip zone on surface especially for a landslide newly investigated under wet warm climate

towards an improved understanding of the mechanical properties and rheology of the lithosphere an introductory article to rock deformation from field experiments and theory a volume in honour of ernie rutter ernie rutter s influential contribution to our understanding of rock deformation spans the whole continental crust from deformation just under the grass e g rutter green 2011 to melt migration in the lower crust e g rutter neumann 1995 ernie has also worked on many aspects of the deformation of oceanic crust rutter brodie 1988 however here we reflect on the wide range of deformation conditions ernie has studied by presenting key aspects of the strength and rheology of the continental lithosphere as we understand it today with the aim of providing the context for the contributions included within this volume the continental crust forms just over a third of the earth s surface and it is different from the crust of any other planet in our solar system it is rich in those elements that partition in a silicate melt and its formation and evolution determined the composition of the mantle and that of the atmosphere hawkesworth 2006 thanks to its physical and chemical fingerprint the earth s crust has sustained life uninterrupted for 3 8 ga and it is the source of major natural resources such as hydrocarbons and mineral deposits and others such as geothermal energy the continental crust is less dense than its oceanic counterpart and is therefore more buoyant it forms a substantial part of the cold thermal layer of the earth and its rheology from greek r1v rhe flow and logia logia study the study of the flow of matter is a key factor controlling plate tectonic processes at the earth s surface and in its interior davies 2011 beneath the continental crust lies the lithospheric mantle and together they form the continental lithosphere considerable variations in structure thickness and composition of the continental lithosphere mean that despite its importance we currently have limited understanding of the rheology of this complex system thatcher pollitz 2008 significant advances in our knowledge of continental rheology have been possible through

recrystallization fabrics of sheared quartz veins with a strong pre existing crystallographic preferred orientation from a seismogenic shear zone abstract microstructural investigations were carried out on quartz veins in schist protomylonite and mylonite samples from an ancient seismogenic strike slip shear zone sandhill corner shear zone norumbega fault system maine usa we interpret complexities in the microstructural record to show that 1 pre existing crystallographic preferred orientations cpo in the host rock may persist in the new cpo patterns of the shear zone and 2 the inner and outer parts of the shear zone followed diverging paths of fabric development the host rocks bounding the shear zone contain asymmetrically folded quartz veins with a strong cpo these veins are increasingly deformed and recrystallized with proximity to the shear zone core matrix accommodated rotation and recrystallization may position an inherited c axis maximum in an orientation coincident with rhomb or basal slip this inherited cpo likely persists in the shear zone fabric as a higher concentration of poles in one hemisphere of the c axis pole figure leading to asymmetric crossed girdle or paired maxima c axis patterns about the foliation plane three observed quartz grain types indicate a general trend of localization with decreasing temperature 1 large 100 m low aspect ratio

structural evolution of orogenic wedges interplay between erosion and weak decollements orogenic wedges commonly display an inner wedge where crystalline units have been exhumed and an outer wedge formed by imbricated sedimentary units detached from the basement analog experiments have shown that similar structures can emerge naturally in the presence of weak decollements due to the interplay between erosion and deformation in this study we further investigate this hypothesis using two dimensional visco elasto plastic numerical models our experiments assume a basal and an intermediate decollement within the wedge experiments with a frictional strength of the basal decollement lower or equal to that of the intermediate decollement show a structural evolution of fold and thrust belts dominated by out of sequence thrusting conversely when the intermediate decollement is weaker than the basal decollement distinct outer and inner wedges are formed this process leads to episodic migration of midcrustal ramps tectonic underplating and antiformal stacking facilitated by erosion comparison between our models and the himalayan wedge suggests a low effective friction 0 10 which is probably due to dynamic weakening during large m8 himalayan earthquakes the deeper decollement along which the lower plate thrusts beneath the high himalaya must be a thermally activated ductile shear zone with an apparent friction of 0 18 fold and thrust belts worldwide exhibit various architectures in which different decollement levels might be activated thus our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise

experimental investigation of incipient shear failure in foliated rock abstract it has long been known that rock fabric plays a key role in dictating rock strength and rheology throughout earth s crust however the processes and conditions under which rock fabric impacts brittle failure and frictional strength are still under investigation here we report on laboratory experiments designed to analyze the effect of foliation orientation on the mechanical behavior and associated microstructures of simulated fault rock sheared at constant normal stress of 50 mpa intact samples of pennsylvania slate were sheared with a range of initial fabric orientations from 0 to 165 with respect to the imposed shear direction foliation orientations of 0 30 produce initial failure at the lowest shear stress while samples oriented at higher angles are less favorable resulting in higher peak failure strength in all samples post peak development of through going deformation zones in the r1 riedel orientation results in a residual strength that is lower than that observed for powdered gouge from the same material as shear strain increases all samples approach a residual apparent friction defined by the ratio of shear strength to normal stress of 0 4 however the final deformation microstructure depends strongly on the orientation of the pre existing foliation when fabric is oriented at low angles to the shear direction

formation of low temperature mylonites and phyllonites by alkali metasomatic weakening of felsic volcanic rocks during progressive subduction related deformation abstract dacitic to rhyolitic volcanic rocks of the spruce lake nappe experienced two phases of alkali metasomatism as a result of fluids channelling along shear zones the shear zones formed during a progressive thrust related deformation associated with underplating and incorporation of the volcanic rocks into the brunswick subduction complex of northern new brunswick the fluids mainly represent chemically and isotopically modified seawater released by dewatering of the associated underthrusted shaly sedimentary rocks both phases of metasomatism weakened the felsic rocks leading to strain localisation albitisation of felsic volcanic rocks as a result of na metasomatism during underthrusting facilitated formation of mylonites near peak high pressure metamorphism 330 370 c 600 800 mpa the mylonites are preferentially preserved in the roof thrust shear zone of the spruce lake nappe core mantle structures bulging and crystallographically preferred orientations indicate that albite behaved more ductilely than k feldspar the ductility of albite at these low temperatures is interpreted as a function of abundant intragranular fluids phengite rich phyllonites formed after peak high pressure metamorphism during uplift by out of sequence thrusting these phyllonites are generally characterised by a slight gain in k and loss of na and are best developed in the basal shear zones of the spruce lake nappe

fluid flow within the damage zone of the boccheggiano extensional fault larderello travale geothermal field central italy structures alteration and implications for hydrothermal mineralization in extensional settings the neogene extensional province of southern tuscany in central italy provides an outstanding example of fossil and active structurally controlled fluid flow and epithermal ore mineralization associated with post orogenic silicic magmatism characterization of the hydrodynamic regime leading to the genesis of the polysulphide deposit known as filone di boccheggiano hosted within the damage zone of the boccheggiano fault is a key target to assess modes of fossil hydrothermal fluid circulation in the region and more generally to provide inferences on fault controlled hydrothermal fluid flow in extensional settings we provide a detailed description of the fault zone architecture and alteration mineralization associated with the boccheggiano ore deposit and report the results of fluid inclusion and stable oxygen isotope studies this investigation shows that the boccheggiano ore consists of an adularia illite type epithermal deposit and that sulphide ore deposition was controlled by channelling of hydrothermal fluids of dominantly meteoric origin within the highly anisotropic permeability structure of the boccheggiano fault the low permeability structure of the fault core compartmentalized the fluid outflow preventing substantial cross fault flow with focused fluid flow occurring at the hangingwall of the fault controlled by fracture permeability fluid inclusion characteristics indicate that ore minerals were deposited between 280 and 350 c in the upper levels of the brittle extending crust lithostatic pressure in the order of 0 1 gpa abundant vapour rich inclusions in ore stage quartz are consistent with fluid immiscibility and boiling and quartz ore vein textures suggest that mineralization in the boccheggiano ore deposit occurred during cyclic fluid flow in a deformation regime regulated by transient and fluctuating fluid pressure conditions results from this study i predict a strongly anisotropic permeability structure of the fault damage zone during crustal extension and ii indicate the rate of secondary structural permeability creation and maintenance by active deformation in the hangingwall of extensional faults as the major factor leading to effective hydraulic transmissivity in extensional terranes these features intimately link ore grade mineralization in extensional settings to telescoping of hydrothermal flow along the hangingwall block s of major extensional fault zones

mesozoic tectonothermal development of the sambagawa mikabu and chichibu belts south west japan evidence from 40ar 39ar whole rock phyllite ages five whole rock 40ar 39ar plateau ages from low grade sectors of the sambagawa belt besshi nappe complex range between 87 and 97 ma two whole rock phyllite samples from the mikabu greenstone belt record well defined 40ar 39ar plateau ages of 96 and 98 ma together these ages suggest that a high pressure metamorphism occurred in both the sambagawa and mikabu belts at c 90 100 ma the northern chichibu sub belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north c 110 ma to south c 215 ma the northern chichibu sub belt is correlated with the kuma nappe complex sambagawa belt two whole rock phyllite samples from the kurosegawa terrane display markedly older metamorphic ages than either the sambagawa or the chichibu belts accretion of sambagawa chichibu protoliths began prior to the middle jurrasic depositional ages decrease from middle jurassic kuma chichibu nappe complex to c 100 ma oboke nappe complex toward lower tectonostratigraphic units the ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units the kurosegawa belt and the geological units to the south belong to distinctly different terrances than the sambagawa chichibu belts these have been juxtaposed as a result of transcurrent faulting during the cretaceous

structural and temporal evolution of a reactivated brittle ductile fault part i fault architecture strain localization mechanisms and deformation history abstract faults are by nature dynamic as their architecture and composition evolve progressively in space and through time steered by the interplay between strain weakening and hardening mechanisms this study combines structural analysis geochemistry and chlorite geothermometry to investigate deformation and strain localization mechanisms of the kvenklubben fault a paleozoic brittle ductile thrust in northern norway with the goal to constrain their temporal variations and the consequences thereof on fault architecture development and rheological behavior the fault evolved from an initially discrete brittle feature slipping mainly by seismogenic ruptures to a wide brittle ductile phyllonite deforming by aseismic creep the formation of mechanically weak phyllosilicates by decarbonation of footwall dolostones and carbonation of hanging wall metabasalts was the main weakening mechanism whereas partitioning of fluid flow and fracture sealing following transient high pore pressure driven embrittlement caused episodic and localized strain hardening the interplay between strain weakening and hardening mechanisms caused the fault core to widen we suggest that the ability for carbonate hosted faults to slip by seismogenic rupture is also a function of the faults structural evolutionary stage and that it decreases progressively with fault maturity this study demonstrates the importance of calibrating the present day fault anatomy against the dynamic character of faults which evolve geometrically compositionally and mechanically in space and through time

basement involved thin skinned and thick skinned tectonics in the alps the deformation of continental crust during continental collision by folding and thrusting follows three types of structural styles 1 in a true thin skinned style only cover rocks are involved 2 in the case of a thin skinned basement involved style thin slabs of crystalline basement rocks are piled up into a nappe stack 3 in a true thick skinned style the entire upper crust is involved in the deformation in the alps all three styles can be recognized the helvetic nappes and parts of the penninic nappes exhibit true thin skinned style tectonics triassic evaporites jurassic shales and cretaceous marls acted as detachment horizons basement involved thin skinned tectonics is typical for the penninic nappes in the core of the orogen the thickness of the basement thrust sheets is controlled by the effects of mesozoic rifting by deep burial and heating of the subducting crust and by the presence of late palaeozoic structures thick skinned style is observed in the more external parts of the orogen the external massifs and the southalpine nappe system it occurred in the late phase of collision and involved the entire upper crust the basal detachment occurred possibly along phyllonites generated by the breakdown of load bearing feldspar considering the alpine orogen as a whole the lower crust deformed seemingly independently from the upper crust the detachment of the cover units by thin skinned tectonics occurred prior to thrusting related to basement involved thin skinned tectonics thrust faults of both types were overprinted by post nappe folding

effective crustal permeability controls fault evolution an integrated structural mineralogical and isotopic study in granitic gneiss monte rosa northern italy two dextral faults within granitic gneiss in the monte rosa nappe northern italy reveal key differences in their evolution controlled by evolving permeability and water rock reactions the comparison reveals that identical host rock lithologies develop radically different mineralogies within the fault zones resulting in fundamentally different deformation histories oxygen and hydrogen isotope analyses coupled to microstructural characterisation show that infiltration of meteoric water occurred into both fault zones the smaller virgin fault shows evidence of periodic closed system behaviour which promoted the growth of hydrothermal k feldspar whilst the more open system behaviour of the adjacent ciao ciao fault generated a weaker muscovite rich fault core which promoted a step change in fault evolution effective crustal permeability is a vital control on fault evolution and coupled to the temperature i e depth at which key mineral transformations occur is probably a more significant factor than host rock strength in controlling fault development the study suggests that whether a fault in granitic basement grows into a large structure may be largely controlled by the initial hydrological properties of the host rocks small faults exposed at the surface may therefore be evolutionary dead ends that typically do not represent the early stages in the development of larger faults

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault

long term fluid circulation in extensional faults in the central catalan coastal ranges p t constraints from neoformed chlorite and k white mica the neoformation of chlorite and k white mica in fault rocks from two main faults of the central catalan coastal ranges the valles and the hospital faults has allowed us to constrain the p t conditions during fault evolution using thermodynamic modeling crystallization of m1 and m2 muscovite and microcline occured as result of deuteric alteration during the exhumation of the pluton 290 c t 370 c in the permian after that three tectonic events have been distinguished the first tectonic event attributed to the mesozoic rifting is characterized by precipitation of m3 and m4 phengite together with chlorite and calcite c1 at temperatures between 190 and 310 c the second tectonic event attributed to the paleogene compression has only been identified in the hospital fault with precipitation of low temperature calcite c2 the shortcut produced during inversion of the valles fault was probably the responsible for the lack of neoformed minerals within this fault finally the third tectonic event which is related to the neogene extension is characterized in the valles fault by a new generation of chlorite associated with calcite c4 and laumontite formed at temperatures between 125 and 190 c in the absence of k white mica differently the hospital fault is characterized by the precipitation of calcite c3 during the syn rift stage at temperatures around 150 c and by low temperature fluids precipitating calcites c5 c6 and pc1 during the post rift stage during the two extensional events mesozoic and neogene faults acted as conduits for hot fluids producing anomalous high geothermal gradients 50 c km minimum

fabric induced weakness of tectonic faults 1 mature fault zones appear to be weaker than predicted by both theory and experiment one explanation involves the presence of weak minerals such as talc however talc is only a minor constituent of most fault zones and thus the question arises what proportion of a weak mineral is needed to satisfy weak fault models existing studies of fault gouges indicate that 30 of the weak phase is necessary to weaken faults a proportion not supported by observations here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt talc provided the talc forms a critically aligned through going layer observations of foliated fault rocks in mature large offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults

on palaeozoic mesozoic brittle normal faults along the sw barents sea margin fault processes and implications for basement permeability and margin evolution palaeozoic mesozoic brittle normal faults onshore along the sw barents sea passive margin off northern norway give valuable insight into fault and fluid flow processes from the lower brittle crust microstructural evidence suggests that late permian early triassic faulting took place during multiple phases with initial fault movement at minimum p t conditions of c 300 c and c 240 mpa c 10 km depth followed by later fault movement at minimum p t conditions of c 275 c and c 220 mpa c 8 5 km depth the study shows that pore pressures locally reached lithostatic levels 240 mpa during faulting and that faulting came to a halt during early deep stages of rifting along the margin fault permeability has been controlled by healing and precipitation processes through time which have sealed off the core zone and eventually the damage zones after faulting a minimum average exhumation rate of c 40 m ma 1 since the late permian is estimated it implies that the debated late cenozoic uplift of the margin may be explained by increased erosion rates in the coastal regions owing to climate detoriation which caused subsequent isostatic recalibration and uplift of the marginal crust the studied faults may be used as analogues of basement involved fault complexes offshore revealing details about the offshore nature of faulting including past and present basement and fault zone permeability

analysis of continental midcrustal strain localization induced by microfracturing and reaction softening 1 low angle extensional shear zones which often characterize the brittle ductile transition of the continental crust are seen here to result from strain localization the potentially destabilizing deformation mechanism is assumed to be the progressive transformation of fractured coarse feldspar grains into white mica as observed in the east tenda shear zone alpine corsica the coupling between microfracturing and feldspar to mica reaction is coeval with strain localization that occurred in that field case at a depth close to 15 km this reaction is proposed as the main destabilizing factor responsible for the onset of localization with feldspar having a stationary dislocation creep flow stress larger than mica to test this hypothesis a rheological model is constructed based on the field observations for a mixture of three phases mica quartz and feldspar deforming at a common strain rate the phase concentrations change with time according to the feldspar to mica reaction which takes place only if feldspar grains are fractured a condition detected with the mohr coulomb criterion the tendency for the strain to localize is assessed by numerical means for the structure composed of an upper crust gliding rigidly over the lower crust which sustains an overall simple shear the onset of strain localization is defined by an increase of at least two orders of magnitude in strain rate over part of the lower crust the upper crust gliding velocity has to be increased by at least a factor of 5 for localization to occur the time lapse for this velocity change determines the depth of the shear zone 15 17 km the kinetics of the metamorphic reaction and the final amount of white mica control its width 1 4 km the time of the shear zone formation is less than half a million years

experimental deformation of muscovite shear zones at high temperatures under hydrothermal conditions and the strength of phyllosilicate bearing faults in nature abstract layers of fine grained muscovite were hot pressed then sheared between alumina sliders to shear strains up to 2 at temperatures between 300 and 700 c confining pressures of 206 mpa and various pore water pressures high pore water pressures helped suppress dehydroxylation of the mica and permitted testing at temperatures higher than previously used shear strain rates between 10 3 s 1 and 10 7 s 1 were accessed using constant shear strain rate and stress relaxation testing except for strain rates 5 s 1 at 700 c deformation was strain rate and temperature insensitive but effective normal stress sensitive with a friction coefficient at yield of 0 3 rising with strain to 0 5 steady state sliding was not attained from the mechanical data and microstructural study deformation was inferred to have occurred by a mixture of brittle frictional and crystal plastic processes at 700 c and low strain rates the shear strength falls rapidly with a linear viscous characteristic in a way not previously reported this is tentatively attributed to rate control by viscous glide of basal dislocations extrapolating these results to geological strain rates we expect mica rich fault zones will exhibit frictional behaviour with a low friction coefficient between 0 25 and 0 5 giving way at mid crustal conditions to a rapid strength drop as viscous creep supervenes thus mid to lower crustal mica dominated faults of any orientation and terrains of schistose metapelitic rocks in the cores of orogeneic belts are expected to be very weak supporting shear stresses in the range 1 10 mpa the frictional behaviour of mica rich faults in upper crustal regions is however expected to be too strong to account alone for the proposed weakness of some major fault zones