deformation processes at the down dip limit of the seismogenic zone the example of shimanto accretionary complex in order to constrain deformation processes close to the brittle ductile transition in seismogenic zone we have carried out a microstructural study in the shimanto accretionary complex japan the fossil equivalent of modern nankai accretionary prisms the hyuga tectonic melange was sheared along the plate interface at mean temperatures of 245 c 30 c as estimated by raman spectroscopy of carbonaceous material rscm it contains strongly elongated quartz ribbons characterized by very high fluid inclusions density as well as micro veins of quartz both fluid inclusion planes and micro veins are preferentially developed orthogonal to the stretching direction furthermore crystallographic preferred orientation cpo of quartz c axes in the ribbons has maxima parallel to the stretching direction recrystallization to a small grain size is restricted to rare deformation bands cutting across the ribbons in such recrystallized quartz domains cpo of quartz c axes are orthogonal to foliation plane the evolution of deformation micro processes with increasing temperature can be further analyzed using the foliated morotsuka a slightly higher grade metamorphic unit 342 30 c by rscm from the shimanto accretionary complex in this unit in contrast to hyuga tectonic melange recrystallization of quartz veins is penetrative cpo of quartz c axes is concentrated perpendicularly to foliation plane these variations in microstructures and quartz crystallographic fabric reflect a change in the dominant deformation mechanism with increasing temperatures above 300 c dislocation creep is dominant and results in intense quartz dynamic recrystallization in contrast below 300 c quartz plasticity is not totally activated and pressure solution is the major deformation process responsible for quartz ribbons growth in addition the geometry of the quartz ribbons with respect to the phyllosilicate rich shear zones shows that bulk rheology is controlled by quartz behavior consequently below 300 c the application of quartz pressure solution laws based on realistic geometry derived from hyuga microstructures results in strongly lowering the overall strength of the plate interface with respect to the classical brittle envelop

evolution of the raman spectrum of carbonaceous material in low grade metasediments of the glarus alps switzerland terra nova 22 354 360 2010 abstract the raman spectrum of carbonaceous material cm from advanced diagenesis 200 c to low grade metamorphism 320 c is documented in the helvetic flysch of the glarus alps switzerland the spectrum is complex with several defect bands at 1200 d4 1350 d1 1500 d3 and 1620 d2 cm 1 we document the evolution of these bands relative to the graphite g band with increasing metamorphic grade and we show that this qualitative evolution may be used as a proxy for temperature in the glarus alps we develop a robust peak fitting method and propose quantitative parameters that may be used as proxies for thermal metamorphism in this region further work in other geological contexts is needed to assess whether the spectral evolution observed in the glarus alps may be generalized and to critically assess the potential for calibrating a general empirical and quantitative thermometer based on the raman spectrum of cm in low grade rocks

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

water distribution in low grade siliceous metamorphic rocks by micro ftir and its relation to grain size a case from the kanto mountain region japan abstract infrared microspectroscopy reveals that liquid like water is always present in polycrystalline quartz grains from low grade metamorphic cherts and shale in the chichibu group and the mikabu greenrock complex of the kanto mountains area the water distribution in these rocks is heterogeneous and is related to rock textures in the microcrystalline quartz o h species due possibly to si oh are generally present the liquid like water is also present at grain boundaries and or in sub micron fluid inclusions in the microcrystalline parts on the other hand in quartz grains larger than 20 m in diameter within veins only the liquid like water is present possibly as fluid inclusions trapped during quartz vein formation in the microcrystalline cherty matrix of the metamorphic rocks a general decrease in liquid like water content is observed with increasing metamorphic grade associated with an increase in grain size two representative grain shapes a cube and a regular tetradecahedron having 14 planes are used to estimate the surface area between grains per unit volume with the grain size d m 3 d for cubic and 2 37 d for tetradecahedron grain boundary volumes were then calculated assuming grain boundary widths from 0 5 to 20 nm and normalized by unit volume of the rock the measured ir data on low grade metamorphic cherty rocks fall closely on these curves with grain boundary width of around 10 nm for the both models assuming saturation of h2o at grain boundary the observed decrease in water content with increase in grain size can be rigorously explained by the decrease in grain boundary volume per unit volume these results suggest that liquid like water occurs mainly between grain boundaries in the microcrystalline quartz of low grade metamorphic rocks although this grain boundary water model is the first approximation and requires further details of quantitative grain boundary textures the present model can provide a new approach to understand water distribution in polycrystalline rocks

stability of tunnels according to depth and variability of rock mass parameters abstract even with the most recent advances in rock mechanics the stability assessment of underground tunnels continues to involve many uncertainties in the parameters which can significantly affect the calculated factors of safety in the present work a straight characterization which can help to evaluate the risk of a tunnel collapse with respect to the depth of excavation is proposed following a consolidated limit analysis approach and by means of the hoek brown failure criterion the galleries are divided into shallow intermediate and deep according to the variability of the mechanical parameters of the rock mass with depth a solution to the problem of impending collapse in the case of variable rock properties is proposed and examples are presented and discussed both from an analytical and a numerical standpoint

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

apparent activation energy and rate limiting process estimation from natural shale deformed by pressure solution in shallow subduction zone pressure solution is one of the main deformation mechanisms for lithify and stress release in the rocks from shallow subduction zones we observed temperature dependent pressure solution development in naturally deformed shale in shimanto accretionary complex in southwest japan the pressure solution develops with shear dominated or co axial dominated deformation we evaluated apparent activation energy by applying the constitutive equation of pressure solution creep to the temperature dependent relations the activation energy of each deformation type was estimated as 18 kj mol 1 for shear dominated and 45 kj mol 1 for co axial dominated shale the energies enable us to speculate rate limiting processes of pressure solution i e dissolution diffusion and precipitation by comparing the energies obtained in this study with energies had been measured by experiments the lower activation energy estimated here was similar with that of diffusion the similarity indicates that possible rate limiting process of shale deformation in shallow subduction zone would be diffusion the difference of energy between deformation types can be explained by distinctive grain boundary structure

deformation and fluid flow of a major out of sequence thrust located at seismogenic depth in an accretionary complex nobeoka thrust in the shimanto belt kyushu japan 1 nobeoka thrust in kyushu southwest japan was investigated to understand the relationship between the seismogenic out of sequence thrust ost and fluid flow in accretionary prisms the nobeoka thrust is a fossilized ost being active at seismogenic depth the hanging wall exhibits a penetrative plastic deformation while a brittle cataclastic melange like occurrence characterizes the footwall although both of them have same shale and sandstone dominant protolith vitrinite reflectance analyses indicate that the maximum temperatures of the hanging wall and footwall are approximately 320 and 250 c respectively this thermal gap across the thrust corresponds to 8 6 14 4 km of displacement assuming a 28 47 c km geothermal gradient the brittle damage zone of the thrust is asymmetric only 2 m for hanging wall side and 100 m for footwall three types of mineral veins quartz and carbonate are well developed especially in the damaged footwall the tension crack filling vein the fault filling vein and postmelange one the first is harmonious with fabric perpendicular to the p surface fluid inclusion geothermobarometry indicates the p t of fluid in the intensively damaged zone of the footwall is 300 c 230 250 mpa higher than that from vitrinite reflectance which suggests that hydrothermal fluid flow is associated with deformation the same type vein in the lowest portion of the footwall damaged zone includes a much lower p t fluid this difference suggests that continuous underplating caused the damaged zone to propagate downward with cooling and shallowing which differs from faults characterized by shear localization and might be unique to aquiferous ost in accretionary complexes

long term evolution of an accretionary prism the case study of the shimanto belt kyushu japan the shimanto belt in sw japan is commonly described as a paleo accretionary prism whose structure is explained by continuous accretion like in modern accretionary prisms such as nankai we carried out a structural study of the cretaceous to miocene part of the shimanto belt on kyushu to test this hypothesis of continuous accretion most deformation structures observed on the field are top to the se thrusts fitting well the scheme of accretionary wedge growth by frontal accretion or underplating in particular the tectonic melange at the top of the hyuga group records a penetrative deformation reflecting burial within the subduction channel in contrast we documented two stages of extension that require modifying the traditional model of the belt as a simple giant accretionary wedge the first one in the early middle eocene is mostly ductile and localized in the foliated bases of the morotsuka and kitagawa groups the second one postdating the middle miocene is a brittle deformation spread over the whole belt on kyushu integrating these new tectonic features to existing data we propose 2 d reconstructions of the belt evolution leading to the following conclusions 1 erosion and extension of the margin in the early middle eocene resulted from the subduction of a trench parallel ridge 2 the late eocene to early miocene evolution is characterized by rapid growth of the prism followed by a middle miocene stage where large displacements occurred along low angle out of sequence thrusts such as the nobeoka tectonic line 3 from middle miocene the strain regime was extensional

dissolution and replacement creep a significant deformation mechanism in mid crustal rocks abstract zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and s c fabrics in these rocks were produced by dissolution precipitation and replacement processes even at epidote amphibolite facies metamorphic conditions the metamorphic fabric is defined by alternating layers and folia dominated by quartz feldspars and biotite epidote zoning patterns in most metamorphic plagioclase orthoclase epidote and sphene are truncated at boundaries normal to the shortening direction suggesting dissolution interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite epidote and myrmekitic intergrowths of plagioclase and quartz metamorphic plagioclase grains are also replaced by epidote we interpret these microstructures to reflect strain enhanced dissolution the cores of many grains show asymmetric overgrowths with at least two generations of beards all oriented on the ends of grains that face the extension direction we interpret these textures to reflect precipitation of components dissolved by deformation enhanced dissolution while biotite and quartz probably deformed by dislocation creep the overall deformation was accommodated by dissolution perpendicular to the shortening direction and precipitation parallel to it these chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks thus wet crust is likely to be weaker than calculated from these experimental studies

three dimensional roof collapse analysis in circular tunnels in rock abstract collapse of a roof in circular cross section tunnels is analyzed the kinematic approach of limit analysis is utilized with strength of the rock described by the hoek brown failure criterion the parametric form of the hoek brown function is used to avoid introduction of an alternative explicit form of the shear strength criterion three measures of safety are considered stability number the factor of safety and the support pressure needed to assure roof stability the shape of the rock block in the failure mechanism consists of a right elliptic cone with a piece wise linear generatrix and a prismatic section inserted between the two halves of the cone the complexity of the cross section of the block with the tunnel makes for an intricate integration of the rates of the dissipated work and the work of external forces all three measures of safety are strongly dependent on the quality of rock described in the geological strength index whereas the dependence on the rock type captured in coefficient mi is less distinct the length of the roof failure mechanism is subject to a constraint dependent on the spacing of the supporting ribs in the tunnel all measures of safety or stability are distinctly dependent on the length constraint the shorter the spacing between the ribs the safer the tunnel against roof collapse the 2d analysis yields the most conservative outcome

cohesive strengthening of fault zones during the interseismic period an experimental study 1 there is widespread evidence indicating that faults regain a portion of their strength during the interseismic period here we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid rock reactions in fault zones the triaxial experiments consisted of fracturing cores of fontainebleau sandstone under dry conditions forming a localized shear failure zone stage 1 the specimens were then reacted hydrothermally under isostatic conditions allowing the fault damage zone to compact consolidate and strengthen stage 2 following reaction the specimens were then reloaded to failure under nominally dry conditions so that the increase in cohesive strength of the fault could be measured stage 3 experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction after 6 hours of reaction at the highest temperatures 927 c and pore pressures 200 mpa cohesion increases by as much as 35 mpa microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed a theoretical treatment of the data was conducted using these experiments in combination with results on time dependent changes in fault cohesion presented by tenthorey et al 2003 we find that the rate controlling process in our experiments has an activation energy q of approximately 70 kj mol 1 we use this information to develop a model for time dependent cohesive strengthening in fault zones within the continental seismogenic regime we conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

roof stability in deep rock tunnels abstract a method is presented addressing quantitative assessment of tunnel roof stability based on the kinematic approach of limit analysis long tunnels with both rectangular flat ceiling and circular cross sections are considered the rock is governed by the hoek brown strength envelope and the normality flow rule and it is assumed to provide enough ductility at failure making plasticity theorems applicable a failing block in the collapse mechanism is separated from the stationary rock by a deformation band with a large gradient of velocity across its width the shape of the block in the critical mechanism is found from the requirement of the mechanism s kinematic admissibility and an optimization procedure consistent with respective measures of stability the stability number and the supporting pressure needed for tunnel stability are calculated first although less commonly used in rock engineering a procedure is developed for estimating the factor of safety defined as the ratio of the rock shear strength determined from the hoek brown criterion to the demand on the strength curiously for flat ceiling tunnels such definition of the factor of safety yields results equivalent to the ratio of a dimensionless group dependent on the uniaxial compressive strength and the size of the tunnel to the stability number such an equivalency does not hold for tunnels with ceilings of finite curvature not surprisingly all measures of tunnel roof stability are strongly dependent on the geological strength index that describes the quality of the rock

shear veins observed within anisotropic fabric at high angles to the maximum compressive stress some faults seem to slip at unusually high angles 45 relative to the orientation of the greatest principal compressive stress1 2 3 4 5 this implies that these faults are extremely weak compared with the surrounding rock6 laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre existing frictionally weak surface7 8 9 weakening from chemical reactions10 elevated fluid pressure11 12 13 or dissolution precipitation creep14 15 here we describe shear veins within the chrystalls beach accretionary melange new zealand the melange is a highly sheared assemblage of relatively competent rock within a cleaved anisotropic mudstone matrix the orientation of the shear veins compared with the direction of hydrothermal extension veins that formed contemporaneously indicates that they were active at an angle of 80 5 to the greatest principal compressive stress we show that the shear veins developed incrementally along the cleavage planes of the matrix thus we suggest that episodic slip was facilitated by the anisotropic internal fabric in a fluid overpressured heterogeneous shear zone a similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings we therefore conclude that incremental slip along a pre existing planar fabric coupled to high fluid pressure and dissolution precipitation creep may explain active slip on severely misoriented faults

time scales and trends in the central england temperature data 1659 1990 a wavelet analysis we have applied the standard wavelet and the adaptive wavelet transform algorithms to the record of the central england temperature cet from 1659 1990 peaks in the cet spectra include 7 5 1 0 yr 14 4 1 0 yr 23 5 2 0 yr as well as a previously unreported variation at 102 15 yr our wavelet analysis of cet agrees with previous results from singular spectrum analysis ssa by plaut et al 1995 and gives additional results of variability on longer timescales the interdecadal and century scale variability in cet is strongly dependent on the interval of analysis estimates of a data trend are also shown to be sensitive to the cutoff timescale of the filter a cooling of 0 3 c during 1659 1720 is found relative to the temperatures during the 1800s the complex time dependence of the actual data cautions against using model derived representations of natural variability on such long timescales

fluid circulation in the depths of accretionary prisms an example of the shimanto belt kyushu japan accretionary prisms constitute ideal targets to study fluid circulation and fluid rock interactions at depths beyond the reach of active margin deep drilling the highest grade rocks from the shimanto belt on kyushu were buried under 3 5 kbars at 300 c toriumi and teruya 1988 they contain abundant quartz veins formed throughout burial and exhumation and variably affected by brittle and ductile deformation cathodoluminescence cl reveals the existence of two distinct types of quartz characterized by a blue and brown color respectively cl blue quartz fills macro veins width 10 m while cl brown quartz is present in micro veins width 1 10 m and ductilely recrystallized domains on the basis of microstructures the fluids associated with the cl blue and cl brown quartz are interpreted as external and local respectively quartz growth rims of alternating cl colors as well as mutually cross cutting veins show that the two fluids cyclically wetted the host rock from fluid inclusions analysis the fluid associated with cl blue quartz has a salinity similar to seawater while the fluid associated with cl brown quartz is less saline in addition cl blue quartz is richer in aluminum than the cl brown one in contrast to the salinity aluminum signature the 18o isotopic signature of both quartz types is similar and buffered by host rock the difference between the preservation of the salinity signature of the fluid and the loss of its 18o signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration the local fluid associated with cl brown quartz reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals the external fluid associated with cl blue quartz is interpreted as seawater or pore water from shallow depth 1 2 km below seafloor sediments we propose that downward percolation of shallow water to depths 10km is a transient process associated with mega earthquakes

upper bound analysis of progressive failure mechanism of tunnel roofs in partly weathered stratified hoek brown rock masses abstract a reliable prediction of roof stability in stratiform or deteriorated rock banks remains a difficult problem in tunnel engineering while it is still one of the most classical engineering issues needed to be investigated so as to provide relevant reference to practical projects in previous research homogeneous geomaterials were adopted to determine the analytic solutions of collapse profile in hoek brown rock masses however when rock heterogeneities are considered in different rock layers the results gained from existing collapse mechanisms would underestimate the dimensions of falling blocks thus in this paper a new curved failure mechanism constructed with two continuous lines intersecting at the interface namely progressive failure mechanism is employed to derive the analytic solutions of the shape and region of roof collapse on the basis of upper bound theorem and variational approach the corresponding results calculated are in well accordance with the existing findings which shows sound agreement and validity of the proposed mechanism and from which one can notice that additional detaching blocks would occur apart from the initial collapsing block when there exist various rock layers with different mechanical parameters due to physical erosion or other factors such as weathering process to investigate the influence of different parameters on the range of roof failure special attention is given to some examples of square and circular tunnels and the results computed can give some instruction for practical engineering projects

geological analysis and stability assessment of a sea arch in palinuro southern italy abstract the study aims to describe the geological and geostructural layouts of an unstable sea arch located in a very attractive tourist area of southern italy on the basis of ad hoc field surveys and laboratory tests the problems of an in depth geo mechanical characterization of the rock mass are discussed in details and its main strength parameters are derived an indirect assessment of the possible geostatic stress field due to the neotectonic events which affected this sector of apennine range during the last 100 000 years is proposed and validated by means of numerical non linear fe analyses the results suggest that the presence of a lateral thrust acting into the promontory located at the back of the arch is very likely and its effects would agree well with the failures occurred in the past on the arch surfaces it is concluded that any possible intervention should be carefully designed on the basis of the highlighted factors and further investigations

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

the interplay between fluids folds and thrusts during the deformation of a sedimentary succession abstract in this paper field evidence is presented to support the idea that fluid pressures in excess of lithostatic pressure were operating during the initiation and reactivation of large thrusts and folds in the wenlock shales of llangollen north wales it is also argued that as major thrusts are reactivated and as major folds amplify into finite structures they in turn exert an influence on the subsequent migration and concentration of fluids within the deforming sedimentary succession

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

incrementally developed slickenfibers geological record of repeating low stress drop seismic events an accretionary melange of triassic age ocean floor sediments exposed in the chrystalls beach complex south island new zealand comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix deformed in a continuous discontinuous style at subgreenschist conditions deformation structures include a pervasive anastomosing fault fracture mesh of multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz calcite slickenfibers microstructural observations reveal slickenfiber growth by crack seal shear slip increments of 10 100 m with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip u to potential rupture length l predominantly lies within the range 10 6 u l 10 5 characteristic of microearthquakes obeying constant stress drop scaling with a low stress drop 30 kpa typical of low frequency earthquakes the host rock assemblage metamorphic environment inference of near lithostatic fluid overpressures low stress drop and mixed continuous discontinuous shearing resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms