strain localization due to a positive feedback of deformation and myrmekite forming reaction in granite and aplite mylonites along the hatagawa shear zone of ne japan abstract with increasing mylonitization of aplite veins and their adjacent granite in the hatagawa shear zone of ne japan the modal content and grain size of feldspar porphyroclasts decrease while the modal content of fine grained 50 m polymineralic aggregate of plagioclase k feldspar quartz and biotite increases up to 60 80 in the granite and aplite ultramylonites deformation induced myrmekite lobes are ubiquitously developed around k feldspar porphyroclasts the occurrence and chemical composition of plagioclase and k feldspar in fine grained polymineralic aggregates indicate that the plagioclase is mostly derived from myrmekite while the interstitial k feldspar originates from isolated porphyroclast rims between growing myrmekite lobes and or precipitations from solution modal and bulk chemical compositions of granite and aplite indicate that the mylonites are preferentially developed in aplite veins richer in k feldspar than the surrounding granite deformation induced k feldspar replacement by myrmekite is then more favored during deformation to produce more abundant fine grained polymineralic aggregates subsequent reaction softening leads to more enhanced deformation which promotes further myrmekite forming reaction such a positive feedback of deformation and myrmekite forming reaction in aplite veins and their adjacent granite may be responsible for strain localization in the granite and aplite mylonites along the hatagawa shear zone

an assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz bearing mylonite zones abstract recrystallized grainsize has been measured in quartz and in some cases feldspar in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings 1 1 supracrustal low temperature shallow dipping thrusts 2 2 deeper level major 100 km thrusts in high grade metamorphic rocks and 3 3 steeply dipping fault zones in granite within a low grade palaeozoic fold belt the concept of a steady state dynamically recrystallized grainsize largely unaffected by post creep processes e g static grain growth receives strong support especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones however the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize the available theoretical and empirical stress grainsize relationships do not account for these effects the concept of a steady state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation predicted deviatoric stresses have been computed using twiss theoretical model for comparison with other crustal stress estimates and most samples fall in the range from 20 to 150 mpa in a few cases where discrete high strain mylonite zones occur within contemporaneously regionally deformed rocks the recrystallized grainsize indicates that stresses were 1 5 to 2 times higher within the zones there is a clear need for improved theoretical and or empirical stress grainsize relationships for quartz and other minerals before extensive further application of this technique

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

seismic evidence for late proterozoic orogenic structures below the phanerozoic sedimentary cover in the kattegat area sw scandinavia 1 commercial seismic reflection profiles from sw scandinavia image sveconorwegian orogenic structures in the late precambrian basement beneath the thin phanerozoic cover we interpret 1 pronounced west to wnw dipping reflections as sveconorwegian localized ductile shear which overprint 2 weaker east dipping reflections interpreted as gothian or early sveconorwegian compressional structures folded and laminar seismic fabrics may represent f and c type crystalline thrust sheets emplaced during the gothian and sveconorwegian events some of the dipping reflections are observed to sole into 3 an interval of subhorizontal reflections at 2 5 4 5 s two way travel time which we interpret as a fossil ductile to brittle transition that was unroofed during the gothian orogeny 4 nw se striking thrusts cutting the sveconorwegian structures may indicate a shift in the late sveconorwegian stress field they may represent an early stage in the subsequent phanerozoic basin evolution in the danish area

viscoplasticity of polycrystalline olivine experimentally deformed at high pressure and 900 c abstract we have performed tri axial compression experiments on olivine aggregates at 900 c and at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus deformation at two different constant strain rates 1 1 10 5 s 1 and 3 4 10 4 s 1 yields continuous hardening reaching maximal differential stresses of 930 and 1076 mpa respectively before sample failure at ca 10 strain the deformed samples were characterized by electron backscatter diffraction ebsd and transmission electron microscopy tem crystallographic preferred orientations cpo are weak but reveal an alignment of the 010 axes around the compression direction and some concentration of 001 and 100 axes normal to it tem observations confirm plastic deformation by dislocation glide with both 001 and 100 burgers vectors electron tomography performed for the first time on deformed fined grained olivine shows that 001 dislocations glide extensively on 110 and 130 planes in complement to the 010 and 100 glide planes often reported at high temperatures 100 dislocations glide mostly on 010 but 011 and 041 planes are also activated the experimental cpo and relative activity of slip systems are well reproduced by viscoplastic self consistent simulations in which all 100 and 001 slip systems have similar strengths they are also consistent with olivine cpo from natural samples deformed at low temperature comparison of the present mechanical results to those obtained for olivine single crystals at similar conditions shows that at low temperature below 1100 c and high stress 300 mpa polycrystalline olivine has a more complex mechanical behavior than single crystals however both datasets advocate for a much weaker dry mantle lithosphere than previously thought

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

the 10a phase a high pressure expandable sheet silicate stable during subduction of hydrated lithosphere h2o storage and release in deep subducting lithosphere is controlled by complex reaction suites involving a variety of hydrous phases as a result of its relatively large thermal stability and intermediate composition the 10a o phase mg3si4o10 oh 2wnh2o has been regarded as a relevant h2o reservoir in a wide range of rock compositions and mineral assemblages high pressure syntheses of the 10a o phase were carried out at 6 7 gpa and 650 c under fluidsaturated conditions in a walker type multi anvil apparatus from 5 min to 430 h x ray powder diffraction of large platy hexagonal crystals of the 10a o phase up to 100 wm were indexed on the basis of a trioctahedral type structure long term run products s 110 h reveal sensitivity of the 10a o phase to treatment with acetone leading to the appearance of diffractions at greater d spacings 10 2 11 6 a o with respect to the basal peak of the 10a o phase 9 64 10 07 a o this swelling behavior is strongly related to synthesis run duration the raman spectrum of the 10a o phase at frequencies less than 800 cm 31 shows a strong similarity to talc in the si o stretching region 800 1100 cm 31 the 10a o phase exhibits three modes 909 992 and 1058 cm 31 as compared to two in talc the bending mode of water x2 i s found at 1593 cm 31 in the oh stretching region peaks at 3593 3622 and 3668 cm 31 were observed the acetone treated sample shows a c h stretching mode at 2923 cm 31 while the double bond cno signal is absent the swelling behavior of the 10a o phase is interpreted as due to intercalation of acetone with pre existing interlayer water the efficiency of this process is dependent on the amount of the interlayer water which in turn depends on run duration the relation between the response to acetone treatment and run duration is therefore interpreted as a time dependent hydration of the 10a o phase the fractions transformed from non expandable to expandable fractions was fitted to the avrami empirical law which suggests that kinetics are mainly controlled by diffusion rather than phase boundary reactions the ability to accommodate variable amounts of h2o makes the 10a o phase a major h2o sink whenever a hydrous phase such as chlorite and serpentine breaks down during prograde transformations in the subducted lithosphere under h2o saturated conditions a fully hydrated 10a o phase occurs when h2o undersaturated conditions prevail a h2o deficient 10a o phase incorporates the volatile component available the exchange capacity of interlayer molecules in the 10a o phase structure opens new scenarios on the control of fluid compositions escaping from subducted

kinematics nature of deformation and tectonic setting of the taxaquara shear zone a major transpressional zone of the ribeira belt se brazil abstract in this paper we have investigated the taxaquara shear zone tsz a major strike slip structure within the ribeira belt rb se brazil in order to assess its kinematics the nature of deformation of rocks affected by the tsz and its tectonic setting the tsz presents a vertical ne trending mylonitic foliation and ene to wsw stretching lineation with variable plunge granitic rocks are intensely deformed giving rise to mylonites with newformed matrix assemblage quartz oligoclase k feldspar biotite muscovite rutile magnetite this assemblage within a pseudosection couple with muscovite biotite thermometer helps to constrain a metamorphic condition during deformation which spans 513 525 c and 3 9 4 4 kbar with mean peak at 520 c and 4 0 kbar quartz accommodates deformation through dislocation creep achieving subgrain rotation with minor contribution of grain boundary migration at a mean strain rate of 10 13 10 12 s 1 feldspar porphyroclasts record brittle deformation microfaults and evidences of dislocation creep e g kink twinning the cpos for monomineralic quartz ribbons display monoclinic pattern and activation of basal rhomb slip systems during deformation however the cpos for quartz and feldspar from fine grained polyphase matrix show a switch in deformation mechanism from dislocation creep to diffusion creep the vorticity and three dimensional finite strain data results suggest that the tsz was developed under a moderate to high strain conditions and a transpressional sub simple shear environment with predominance of simple over pure shear regional correlation with other structures from the ribeira belt and african belts based on finite strain quantification support that a major transpressional system was mainly responsible for the shear zones development during late stages of braziliano pan african orogeny

cataclastic rocks of the san gabriel fault an expression of deformation at deeper crustal levels in the san andreas fault zone the san gabriel fault a deeply eroded late oligocene to middle pliocene precursor to the san andreas was chosen for petrologic study to provide information regarding intrafault material representative of deeper crustal levels cataclastic rocks exposed along the present trace of the san andreas in this area are exclusively a variety of fault gouge that is essentially a rock flour with a quartz feldspar biotite chlorite amphibole epidote and fe ti oxide mineralogy representing the milled down equivalent of the original rock anderson and osborne 1979 anderson et al 1980 likewise fault gouge and associated breccia are common along the san gabriel fault but only where the zone of cataclasis is several tens of meters wide at several localities the zone is extremely narrow several centimeters and the cataclastic rock type is cataclasite a dark aphanitic and highly comminuted and indurated rock the cataclastic rocks along the san gabriel fault exhibit more comminution than that observed for gouge along the san andreas the average grain diameter for the san andreas gouge ranges from 0 01 to 0 06 mm for the san gabriel cataclastic rocks it ranges from 0 0001 to 0 007 mm whereas the san andreas gouge remains particulate to the smallest grain size the ultra fine grain matrix of the san gabriel cataclasite is composed of a mosaic of equidimensional interlocking grains the cataclastic rocks along the san gabriel fault also show more mineralogiec changes compared to gouge from the san andreas fault at the expense of biotite amphibole and feldspar there is some growth of new albite chlorite sericite laumontite analcime mordenite and calcite the highest grade of metamorphism is laumontite chlorite zone zeolite facies mineral assemblages and constrained uplift rates allow temperature and depth estimates of 200 30 c and 2 5 km thus suggesting an approximate geothermal gradient of 50 c km such elevated temperatures imply a moderate to high stress regime for the san andreas which is consistent with experimental rock failure studies moreover these results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress much of the mineralogy of the cataclastic rocks is still relict from the earlier igneous or metamorphic history of the protolith porphyroclasts even in the most deformed rocks consist of relict plagioclase oligoclase to andesine alkali feldspar quartz biotite amphibole epidote allanite and fe ti oxides ilmenite and magnetite we have found no significant development of any clay minerals illite kaolinite or montmorillonite for many sites the compositions of these minerals directly correspond to the mineral compositions in rock types on one or both sides of the fault whole rock major and trace element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform and that originally micaceous foliated or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material as previously found for the gouge along the san andreas chemical mobility is not a major factor in the formation of cataclastic rocks of the san gabriel fault we see only minor changes for si and alkalies however there is a marked mobility of li which is a probable result of the alteration and formation of new mica minerals the gouge of the san andreas and san gabriel faults probably formed by cataclastic flow there is some indication presently not well constrained that the fine grained matrix of the cataclasite of from the san gabriel fault formed in response to superplastic flow

phyllosilicates in hydrothermally altered basalts from dsdp hole 504b leg 83 a tem and aem study phyllosilicates occurring as replacements of olivine clinopyroxene and interstitial materials and as veins or fracture fillings in hydrothermally altered basalts from dsdp hole 504b leg 83 have been studied using transmission and analytical electron microscopy the parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration which in turn is related to permeability of basalts saponite and some mixed layer chlorite smectite are the dominant phyllosilicates at the top of the transition zone chlorite corrensite and mixed layer chlorite corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone chlorite talc and mixed layer talc chlorite are the major phyllosilicates in the sheeted dike zone although replacement of talc or ohvine by saponite is observed the phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from 100 a to a few hundred a the packets of saponite layers are much smaller or less well defined than those of chlorite corrensite and talc indicating poorer crystal linity of saponite by contrast chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of a thick the si si al ratio of these trioctahedral phyllosilicates increases and fe fe mg decreases in the order chlorite corrensite saponite and talc these relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets variations in composition of hydrothermal fluids and precursor minerals especially in si si al and fe fe mg ratios are thus important factors in controlling the parageneses of phyllosilicates the phyllosilicates are generally well crystallized discrete phases rather than mixed layered phases where they have been affected by relatively high fluid rock ratios as in high permeability basalts in veins or areas adjacent to veins intense alteration in basalts with high permeability indicating high fluid rock ratios is characterized by pervasive albitization and zeolitization minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite mixed layer chlorite smectite in the low temperature alteration zone and mixed layer chlorite corrensite or mixed layer talc chlorite in the high temperature alteration zone textural non equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers the approach to textural equilibrium was controlled largely by the availability of fluid or permeability

slip and recrystallization of halite gouge in experimental shear zones abstract cylindrical specimens of tennessee sandstone with dry crushed halite between 35 precut surfaces are deformed at 70 mpa confining pressure at displacement rates from 10 2 to 10 6 cm s at 25 c and the shear strains of 16 0 under these conditions peak differential stress is attained at shear strains between 3 and 5 followed by quasi steady state behavior with subtle but possibly significant oscillations over the complete variation of conditions steady state stress levels range from 146 to 169 mpa at shear strains 6 0 the fabric of the halite gouge 0 2 to 1 0 mm thick changes with increasing shear strain from 1 a homogeneously elongated grains up to peak stress indicative of dislocation glide to 2 b a bimodal mosaic of recrystallized grains consisting of very fine grained neoblasts and relatively large euhedral porphyroblasts that is associated with a small stress drop to 3 c a mosaic of uniformly sized anhedral neoblasts produced from the coalescence of porphyroblasts grain size of the very fine grained neoblasts remain reasonably constant with increasing but that of the porphyroclasts and of the resulting mosaic decrease with increasing 100 of the porphyroblasts are strongly and conspicuously oriented at 45 to the shear zone boundary at 4 4 and progressively rotate with the finite strain axes so that at 9 4 100 are inclined at 70 80 and 10 20 to the shear zone it is argued that fabrics b and c are consistent with dynamic recrystallization by grain boundary migration from favorably oriented nuclei grains a ghost structure of aligned grain boundaries and impurities pervades all the shear zones and is oriented at progressively smaller angles to the shear zone boundary as shear strain increases there is a one to one correspondence between these angles and those calculated assuming simple shear this fact along with the constant thickness of the gouge with increasing shear strain indicates that the deformation is modeled by progressive homogeneous simple shear

geotechnical assessment and engineering classification of the antalya tufa rock southern turkey abstract this study mineralogically and geotechnically assesses the antalya tufa rock types which are highly variable in nature geological observations geotechnical site investigations and in situ and laboratory tests have been performed to determine the parameters i e organic matter content rock fabric apparent porosity unit weight uniaxial compressive strength young s modulus tensile strength cohesion and angle of internal friction that may have a significant influence on tufa rock behavior and to develop a comprehensive geotechnical database for the antalya tufa rock the geotechnical database was utilized for the engineering classification of the antalya tufa rocks using the gsi classification system

chemical changes associated with deformation of granites under greenschist facies conditions the example of the zawidow granodiorite se lusatian granodiorite complex poland abstract small scale subvertical retrograde shear zones are developed in the zawidow granodiorite located in the eastern part of the lusatian massif southwest poland heterogeneous deformation under conditions of greenschist facies metamorphism has produced mylonite zones that show a broad range of microstructural development three fabric types in the granodiorite deformation have been distinguished a weakly deformed protolith b moderately deformed in lens shaped domains c strongly foliated in fabric types b and c whole rock analyses show a decrease in cao mno cr sr pb whereas na2o and sio2 increase slightly overall al2o3 tio2 and zr remain essentially constant this pattern of element variation is consistent with the observed feldspar breakdown and increase of sericite and also with the transformation of biotite into chlorite mass balance calculations based on major and trace element geochemistry indicate constant volume or less than 10 volume loss from this study it is apparent that a low grade metamorphic deformation event does not have to have a profound influence on whole rock geochemical composition

structural analysis of the central and southwestern sudbury structure southern province canadian shield the sudbury structure ss is an unusual crater structure which acquired its present oval surface shape during northwest directed ductile thrusting lower amphibolite facies metamorphism accompanied the thrusting which generated a major reverse shear zone at least 50 km long the south range shear zone srsz transects the south range of the sudbury structure and exhumes a low level of the sudbury igneous complex sic assuming heterogeneous simple shear in the northwest southeast vertical plane on northeasterly striking glide surfaces minimal estimates of net displacement across the srsz exceed 8 km this displacement magnitude and the map pattern of the sic require the southwest closure of the ss to be steeply plunging in accord with a hypothetical funnel shape of the sic the rocks of the metasedimentary core of the ss are deformed into a family of second order buckle folds the tangent surface of which forms an upright open flexure within the first order structure of the sudbury synclinorium the

flow stresses from microstructures in mylonitic quartzites of the moine thrust zone assynt area scotland recrystallized grain sizes subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy tem the samples come from the moine thrust zone in the assynt district scotland they had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures the stresses determined 43 244 mpa from recrystallized grain size 9 13 mpa from etched subgrain size 50 95 mpa from tem scale subgrain size and 75 147 mpa from dislocation density are not constant in individual samples simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 mpa a later dynamic recovery event reset subgrain sizes and dislocation densities to a constant value for each microstructure throughout the area there is insufficient empirical information available on the flow stress subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density the effect of chemical environment on the behaviour of all the microstructures is still unknown

new geochronological information by rb sr and u pb investigations from the pre alpine basement of grande kabylie algeria this study presents the first geochronological results apart from previously obtained mica ages from metamorphic basement complex occurring within the internal zone of the alpine chain of algeria the rb sr whole rock and u pb zircon data reveal the existence of an episode of anatectic magmatism around 510 ma ago that corresponds to late pan african orogenic events a deep crustal fault is thought to represent a major phase of shearing associated with hercynian blastomylonitic metamorphism and the production of granitic remelts at 271 12 ma reworking of an ancient basement is demonstrated by the presence of inherited radiogenic lead in zircons from lower palaeozoic orthogneisses and metarhyolites upper intercepts on concordia plots yield ages of 1950 100 and 1321 30 ma respectively apart from the dating results which may provide new insight into the geological history of the region this study also provides general implications for the interpretation of geochronological data in metamorphic and

shear heating associated with movement along the alpine fault new zealand abstract geothermometry based on the fe mg partitioning between biotite and garnet has revealed a temperature increase of circa 100 c associated with the development of schist derived mylonites that presently crop out within the alpine fault zone in new zealand calculations show that the temperature increase is compatible with a short shear heating event 1 5 10 5 years it has been tentatively associated with a possible high stress deformation 1 2 kbar which occurred when the then existent mylonites were unable to accommodate imposed strain rates because of the misorientation of their fabric elements during the change in the relative slip vectors between the pacific and australian plates in the late miocene early pliocene the high stress deformation resulted in the present mylonites which have their fabric elements orientated to accommodate the present oblique reverse slip movement between the two plates

see through deformation experiments on brittle viscous norcamphor at controlled temperature strain rate and applied confining pressure abstract an integrated experimental analytical deformational system allows the observation of evolving microstructures in rock analogues under an optical macroscope at controlled confining pressure p c 0 001 0 3 mpa temperature t 10 150 c and strain rate 10 2 10 6 s 1 this system is ideally suited for studying grain scale processes across the complete spectrum of deformational behaviour from brittle cataclastic to fully viscous mylonitic flow the experimental system comprises two main elements 1 a modified urai means deformation rig and 2 a steel pressure vessel that accommodates the deformation rig yet is small enough to fit under an optical microscope first results of controlled confining pressure experiments on fluid bearing polycrystalline norcamphor an organic quartz analogue indicate that deformation involves a combination of dislocation creep and intergranular hydrofracturing leading to the formation of discrete shear surfaces comparable to c planes in mylonite at applied confining pressure the relative activity of brittle and viscous deformation mechanisms varies across the sample this in turn affects the distribution of strain on the sample scale the pressure dependence of strain localization in these experiments is interpreted to reflect a combination of local variations in the effective stresses and strain incompatibilities associated with the experimental simple shear configuration

formation mechanisms of illite chlorite and mixed layer illite chlorite in triassic volcanogenic sediments from the southland syncline new zealand clay minerals from the three principal kinds of zeolitic sediments from the type area for zeolite facies alteration the triassic murihiku supergroup southland new zealand have been studied by tem bentonitic tuff consists largely of smectite and heulandite with minor illite they occur as replacements of glass shards and are inferred to be direct alteration products of tuff alteration both analcime and laumontite rich tuffs contain chlorite illite and mixed layer illite chlorite including 1 1 mixed layer sequences subhedral to euhedral phyllosilicate crystal shapes and other textural features imply that phyllosilicates crystallized from solution derived in part by dissolution of precursor smectite intralayer transitions involving illite and chlorite are inferred to be products of crystallization rather than direct alteration and replacement petrographically similar bentonitic and analcimized tuffs overlap each other in the stratigraphic section supporting earlier observations that there is no systematic change in smectite relative to the illite plus chlorite derived from smectite in sections up to 8 5 km thick the data imply that smectite may be metastable relative to illite plus chlorite permeability and fluid chemistry are inferred to be as significant as temperature in promoting reactions in clay minerals as well as zeolites during burial metamorphism

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

nature of mixed layer clays and mechanisms of their formation and alteration abstract mixed layer clay minerals are intermediate products of reactions involving pure end member clays they come from natural environments ranging from surface to low grade metamorphic and hydrothermal conditions most often mixed layering is essentially two component but more complicated interstratifications have also been documented variable tendency to form regular 1 1 interstratifications has been observed and explanations of this phenomenon have been proposed mixed layer clays are either di or trioctahedral di trioctahedral interstratifications are rare most mixed layer clays contain smectite or vermiculte as a swelling component exceptions are all trioctahedral serpentine chlorite in low temperature environments and mica chlorite and talc chlorite at high temperatures solid state transformation and dissolution crystallization are the two mechanisms responsible for the formation of different mixed layer clays in general the weathering reactions that produce mixed layering are revers

classification and environmental models of cool freshwater tufas abstract tufa deposits are common in the quaternary record facies developments are dependent upon localised carbonate precipitation generally these take the form of phytoherm build ups together with associated detrital deposits derived from disagragation of the former phytoherm cements differ from speleothems and floe calcite accumulations in being biologically mediated or modified this paper recognises the following five types perched spring line cascade fluviatile paludal and lacustrine facies each type is composed of distinctive constructional and detrital sediment associations these unique sediment combinations are the product of an interplay between angle of slope water discharge rate standing water bodies phytal and microbial colonization and ambient temperature the proposed models will be of value in the interpretation of ancient and partially exposed tufas

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

local shear zone pattern and bulk deformation in the gran paradiso metagranite nw italian alps the gran paradiso nappe of the northwestern alps mostly consists of augen gneisses derived from the alpine deformation of permian granitoids the regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top to west sense of shear the granitoid protolith is preserved in the kilometre scale low strain domain of the piantonetto valley and mainly consists of a porphyritic metagranite including joints leucocratic dykes and biotite rich schlieren in this low strain domain the alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite the shear zones mostly dip towards s se in a shallow shear zones1 to steep inclination shear zones2 the shear zones show typical features that can be explained by reactivation of pre existing joints and planar compositional heterogeneities palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening the kinematics of individual shear zones depends on the orientation of the original heterogeneities acting as nucleation planes and by partitioning of strain components at the kilometre scale with concentration of the flattening component to the piantonetto low strain domain the strain geometry and the kinematics of individual shear zones within piantonetto are not directly connected to the top to west sense of tectonic transport observed elsewhere in the gran paradiso nappe however the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the gran paradiso massif we conclude that the shear zone network of the piantonetto valley is representative of the incipient stages of ductile deformation of a granite nappe even if its architecture is determined by the arrangement of pre existing structural and compositional heterogeneities aspects of the large scale bulk strain can be derived from this local shear zone pattern

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

kulkeite a new metamorphic phyllosilicate mineral ordered 1 1 chlorite talc mixed layer kulkeite occurs as platy colorless porphyroblastic single crystals up to 2 mm in size in a low grade dolomite rock associated with a triassic meta evaporite series at derrag tell atlas algeria it is associated with sodian aluminian talc unusual chlorite polytypes and both k and na phlogopite kulkeite is optically biaxial negative nx 1 552 ny 1 5605 nz 1 5610 2vz 24 obs based on microprobe analysis the empirical formula is na0 38k0 01ca0 01 mg8 02al0 99 al1 43si6 57o20 oh 10 with some variation in na si and tetrahedral al the crystals are monoclinic with a 5 319 1 b 9 195 2 c 23 897 10 a 97 1 3 z 2 the calculated density is 2 70 g cm 3 the four strongest lines in the x ray powder pattern are d i hkl 7 90 100 003 1 533 100 060 7 42 80 002 3 38 80 007 the 001 reflection with 23 7 a has intensity 10

role of regional extension and uplift in the plio pleistocene evolution of the aksu basin sw turkey the aksu basin within the isparta angle area of sw turkey documents plio pleistocene crustal processes at the interface between the tauride mountains and the eastern mediterranean sea basin sedimentation began in the late miocene following an earlier history of emplacement of mesozoic allochthonous units antalya complex miocene basin development and localized late miocene compression aksu phase late miocene early pliocene transtension deformed the aksu basin exploiting pre existing zones of structural weakness during early mid pliocene muddy sediments accumulated in a shallow shelf sea 2 and locally thick up to 250 m cool water antalya tufa deposits then followed prior to glacial times initial localized lacustrine carbonate deposition was followed by development of mainly algal carbonates that are thickest in the west the terraced nature of the tufa terraces at 300 250 100 m above present sea level is explained in terms of fluvial processes regional comparisons suggest a two stage history for the aksu basin first late miocene early pliocene transtension and subsidence secondly late pliocene early pleistocene rifting and marginal uplift the overall tectonic setting reflects interplay between supra subduction zone extension and tectonic escape of anatolia westwards towards the aegean

grain size reduction and foliation development in a deformed granitoid batholith abstract the predominantly peraluminous wologorong batholith in eastern new south wales australia has undergone heterogeneous deformation it always has at least one gneissic foliation but in strongly deformed zones it has two foliations one of these is a gneissic foliation s 1 consisting of relatively discontinuous fine grained micaceous and feldspathic folia anastomosing between feldspar relics and elongate aggregates of recrystallized quartz the other foliation s 2 is more continuous than s 1 and the folia are fine grained and commonly laminated as in mylonites the two foliations could have formed siimultaneously or sequentially during a single deformation s 2 formed oblique up to 30 to s 1 in order to accommodate an imposed shear component of the strain in strongly deformed parts of the batholith s 2 folia appear to have initiated on suitably oriented parts of anastomosing s 1 folia continued deformation locally reduced the angle between s 1 and s 2 and resulted in the development of mylonite zones grain size reduction by recrystallization and or neocrystallization occurred in all minerals during the deformation but quartz appears to have attained a steady state grain size of around 0 15 mm which was not reduced during more intense deformation deformation mechanisms are difficult to determine but microfracturing associated with offsetting and microboudinage intragranular slip and solution transfer of material may have operated in the earlier stages of deformation and grain boundary sliding and or solution transfer of material may have dominated the later stages the material transfer may have been associated with chemical and mineralogical changes that accompanied neocrystallization

corrensite and mixed layer chlorite corrensite in metabasalt from northern taiwan tem aem empa xrd and optical studies many chloritic minerals in low grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties expand slightly upon glycolation expandable chlorite and or have excess alvi relative to aliv as well as significant ca k and na contents chloritic minerals with these properties fill vesicles and interstitial void space in low grade metabasalt from northern taiwan and have been studied with a combination of tem aem empa xrd and optical microscopy the chloritic minerals include corrensite which is an ordered 1 1 mixed layer chlorite smectite and expandable chlorite which is shown to be a mixed layer chlorite corrensite corrensite and some mixed layer chlorite corrensite occur as rims of vesicles and other cavities while later formed mixed layer chlorite corrensite occupies the vesicle cores the tem observations show that the mixed layer chlorite corrensite has ca 20 and the corrensite has ca 50 expandable smectite like layers consistent with xrd observations and with their abnormal optical properties the aem analyses show that high si and ca contents high alvi aliv and low fevi fe mg vi ratios of chlorites are correlated with interstratification of corrensite or smectite like layers in chlorite the aem analyses obtained from 200 500 a thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low aliv siiv and low fevi fe mg vi while chlorite has high aliv siiv and high fevi fe mg vi this implies that the trioctahedral smectite like component of corrensite has significantly lower aliv siiv and fevi fe mg vi the ratios of fevi fe mg vi and aliv siiv thus decrease in the order chlorite corrensite smectite the proportions of corrensite or smectite like layers relative to chlorite layers in low grade rocks are inferred to be controlled principally by fe mg ratio in the fluid or the bulk rock and by temperature compositional variations of chlorites in low grade rocks which appear to correlate with temperature or metamorphic grade more likely reflect variable proportions of mixed layered components the assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases such as chlorite corrensite corrensite smectite or chlorite corrensite smectite a model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1 1 ordered mixed layer chlorite smectite

local variations in quartz c axis orientations in non coaxial regimes and their significance for the mechanics of s c fabrics abstract the quartz c axis orientations of quartz bearing s c mylonites vary on the mm scale two specimens from different tectonic settings the variscan schwarzwald west germany and the eastern alps austria are investigated in the first specimen prism glide along a predominates and in the second specimen basal glide along a predominates in both specimens the pattern of c axis variations is similar it is suggested to be characteristic of s c mylonites on the local scale there are two patterns of preferred orientation 1 a crossed girdle type i which is inclined in a sense opposite to the inferred overall sense of shear this occurs in moderately strained domains 2 a single girdle which is slightly inclined in the same sense as the inferred overall sense of shear this occurs in domains that suffered higher strain both types are considered to reflect stable orientations of the easy glide direction crystallographic a direction at different stages of the deformational history two kinematic interpretations are discussed quartz c axis fabric variations reflect a spatial and possibly also temporal partitioning in flow path between domains that underwent either 1 spinning coaxial flow and non spinning non coaxial flow or 2 spinning non coaxial flow and non spinning non coaxial flow associated with variations in parameters of the deformation such as magnitude of strain and strain rate and perhaps also rate of recrystallization and recovery in either interpretation in moderately strained domains oblique c axis girdles are generated that cannot be used as shear sense indicators

measurement and interpretation of strain in the syntectonic solis de mataojo granitic complex uruguay the neoproterozoic solis de mataojo granitic complex is an intrusive body elongate north south emplaced in the sarandi del yi arroyo solis grande shear zone in the present work a quantification of the magmatic strain has been made by analysis of fabric enclave geometry and minor structures structural evidence indicates that deformation began in the magmatic state and continued through the sub solidus stage the observed distribution of magmatic foliations and lineations the fabric the geometry of the enclaves and the late to post magmatic structures are related to a non coaxial flattening regime that took place after the emplacement and late in the magmatic history these structures reveal that magmatic strain involved a shortening of the order of 80 with a simple shear component between 3 and 4 5 the solid state deformation records an additional mean of 2 7

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

dynamic recrystallization in deformed plagioclase during progressive shear deformation grain size reduction of plagioclase accompanied with progressive ductile deformation was investigated in a centimeter scale small shear zone collected from the hatagawa shear zone northeast japan the angle between the foliation and the shear zone boundary decreases from the margin to the center of the shear zone indicating an increase of shear strain toward the center the volume fraction of fine plagioclase grains xpl diameter 15 m increases with progressive shear strain the value of xpl increases steeply from 30 to 60 between the angles 23 and 20 and then it gradually increases from 60 to 90 toward 0 the fine plagioclase grains have a variable dislocation density and a crystallographic preferred orientation suggesting that recrystallization accommodated dislocation creep transmission electron microscopy tem observations revealed that strain induced grain boundary bulging occurred along the grain boundaries of plagioclase porphyroclasts thus the grain size of plagioclase was reduced by strain induced grain boundary bulging the kinetic theory of dynamic recrystallization approximately predicts the formation of the above mentioned xpl curve and suggests that the inflection point of xpl at 23 20 is probably due to site saturation in which all of the original grain boundaries are covered by the recrystallized grains the kinetic theory of dynamic recrystallization also gives a physical basis for the classification of mylonite on the basis of the volume fraction of fine grains

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

high temperature microstructures and rheology of deformed granite erzgebirge bohemian massif abstract three stages of granite deformation were recognized in porphyric granitoids sheared under amphibolite facies conditions in the erzgebirge mts 1 weakly deformed granite in lozenge shaped pods 2 s c orthogneiss and 3 thin zones of banded mylonites weakly deformed granite forms a load bearing framework lbf structure in which quartz and feldspar aggregates exhibit similar strain intensities and shapes k feldspar deforms by fracturing and onset of dynamic recrystallization along clast margins plagioclase recrystallizes completely and quartz shows effects of grain boundary migration recrystallization and activity of basal a slip s c orthogneiss is interpreted as an interconnected weak layer iwl structure with high viscosity contrast between quartz and both feldspars concentration of deformation into thin quartz layers is indicated by intense strain of quartz grain size variations of recrystallized quartz and dominant prism a slip feldspars show well annealed structures and slip on 010 001 banded mylonite is marked by similar strain intensity of quartz and feldspar aggregates and low viscosity contrast between constituent phases quartz shows evidence for grain size increase and rhomb a c slip fully recrystallized feldspars show evidence for 010 100 slip the whole microstructural sequence and the transitions from lbf structure to iwl structures show the dependence of microstructural evolution of different materials on the bulk strain intensity

study on finite strain and its relationship with magnetic fabric along ailao shan red river shear zone 1 the finite strain along ailao shan red river shear zone has been studied the axial ratios of strain markers and shear strains vary greatly arising principally from deformation intensity and from contrasting competencies ellipticity of chloritic amygdales in basalt ranges from 6 79 to 10 47 corresponding shear strain gamma xz 2 22 2 93 and ellipticity of garnets from 1 56 to 2 95 gamma xz 0 45 1 14 in core gneiss the s c angle of the sheared rocks varies from 2degrees to 28degrees gamma sc 1 35 28 60 mostly in a range 2degrees to 7degrees optical observation thermal demagnetization and hysteresis curves were conducted to determine the nature of minerals and the contribution of paramagnetic and ferromagnetic minerals to magnetic susceptibility the result shows that ferromagnetic minerals and hornblende dominate the magnetic fabric of samples with high to very high susceptibility and play an important role in the magnetic fabric of samples with low to intermediate susceptibility the ferromagnetic minerals are mainly pseudo single domain and multidomain ti poor magnetite most samples have their magnetic lineation lying between the c and s surfaces in the remaining samples deviations from this common alignment principally resulted from the magnetic minerals being deflected by porphyroclasts mineral cleavage and cracks the axial ratio correlation is very poor between the magnetic and actual or imaginary strain ellipsoid calculated from the s c fabric this poor correlation suggests that the shape of the finite strain ellipsoid cannot be evaluated using the magnetic fabric when the latter is a combined result from multiple minerals and especially when the shape effect of multidomain titanomagnetite is important

the development of discrete shear zones in a granite stress strain and changes in deformation mechanisms abstract the development of millimetre scale continuous shear zones in the sidi ali bou nab algeria granite and their progressive transformation into discrete shear zones are presented increases in strain stress and strain rate magnitudes as well as changes of deformation mechanisms are suggested by an evolution of the microstructure across these narrow shear zones temperatures are assumed to be constant from boundaries to core the most striking aspects of this evolution are 1 1 a decrease in the angle between the foliation and the shear zone 2 2 a decrease in the recrystallized grain size of quartz together with the development of a crystallographic orientation characterized by a strong concentration of c axes around the y axis of the finite strain ellipsoid 3 3 the development of very fine grained feldspathic aggregates or layers probably through dynamic recrystallization of original large crystals and 4 4 the replacement of original biotite by muscovite and a new generation of biotite in the continuous shear zones the deformation is controlled by the behaviour of quartz this mineral undergoes extensive dynamic recrystallization which probably favours a higher strain rate and a change in the active slip system from a dominant basal one in the outermost part of the structures to a dominant prismatic one in its core paleopiezometry and strain computations show a shear strain up to 10 and an inferred steady state differential stress to 100 130 mpa respectively the high stress level is suspect because the estimates are derived from recrystallized grain size which may be sensitive to strain and variations of chemical environment discrete shear zones develop in relation with dynamic recrystallization of feldspars in the core of continuous shear zones dynamic recrystallization of feldspars produces very fine grained aggregates grain size of 6 7 m which coalesce and form a continuous layer at the central part of the shear zones this fabric may result from 1 1 a change in the driving flow mechanism from dislocation creep of quartz to dislocation creep or and grain boundary sliding of feldspar neoblasts and 2 2 a large strain softening that would favour strain concentration in the fine grained matrix and the development of sharp shear zone boundaries

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

stratal extension in thrust footwalls makran accretionary prism implications for thrust tectonics small scale stratal extension is commonly associated with thrust faults in the makran accretionary prism of southwest pakistan low angle normal faults dipping in the direction of transport relative to the thrust are common in gouge zones and duplexes and may also cut the footwall and hanging wall where they cause stratal extension these faults are probably riedel shears we have studied a broad zone of more complex extensional deformation beneath one bedding parallel thrust the zone disrupts earlier compressional structures and affects a significant volume of the footwall normal faults initiating in a rearward sequence branch off the thrust and cut down into the footwall merging at a lower stratigraphic level part of the thrust displacement was thereby transfe red to that level and fragments of the footwall were detached and transferred southward beneath the hanging wall the latter remained undeformed and accommodation structures are confined to the footwall these effects may have been caused by variations in sliding resistance along the fault if a thrust transfers part or all of its displacement down section along normal faults in this way before eventually cutting back up toward the surface it will detach a rootless fragment of footwall rocks from a position forward of the initial ramp through that horizon this process can be referred to as footwall plucking it can explain rootless basement slices along thrusts for example and it has important implications for section restoration techniques and the interpretation of thrust belts

stibnite microstructures as imprints of fault movement the natural deformation of stibnite in relation to the kinematic history of a regional fault is resolved using microstructural analysis of massive stibnite along the biards fault in the massif central france stibnite underwent intra crystalline deformation associated with a strong linear and planar anisotropy lepidoblastic textures undulatory extinction of oriented grains growth of antimony between these grains and a colour change of stibnite from white to black result from dynamic recrystallisation a range of asymmetric microstructures indicate non coaxial deformation and a left lateral sense of shear related to the movement of the fault zone the computed displacement corresponds to the geological offsets measured along the fault we conclude that stibnite has microstructurally recorded the bulk kinematics of its host fault zone

s c cataclasite in granitic rock abstract foliated cataclasites in granitic rocks along the rokko awaji fault zone japan display s c fabrics similar to those of mylonites the s c cataclasites have s surfaces defined by shape preferred orientation of biotite fragments and aggregates of quartz and feldspar fragments and c surfaces defined by microshears all quartz and feldspar fragments show mainly brittle deformational microstructures the biotite fragments however have some fabric characteristics like those observed in s c mylonites such as biotite fish cleavage steps bending and folding these differences in microfabric between biotite quartz and feldspar suggest that there are marked differences in their deformational behavior under the same deformational conditions biotite deformed by a combination of brittle plastic shearing processes and quartz and feldspar were brittlely damaged one of the most significant microstructural differences between s c cataclasites and s c mylonites is the absence of dynamically recrystallized grains in the s c cataclasites the microstructural characteristics and the geological evidence suggest that the s c cataclasites in the rokko awaji fault zone formed at temperatures between 150 and 250 c corresponding to depths of 5 8 km at a continental geothermal gradient of 30 c km before being uplifted and exposed this study of cataclastically deformed rocks shows that the fault zone rheology of the upper 5 10 km of crust is greatly influenced by deformational processes in s c cataclasites

foundation evaluation of underground metro rail station using geophysical and geotechnical investigations abstract excavations for underground space developments in a thickly populated urban area are sometimes hindered by unexpected formidable consequences like formation of sinkholes or uneven settlement of foundations such troublesome incidences occasionally damage structures in the vicinity of excavations although adequate design guidelines and codes of practices are adopted the disturbance of subsurface regime due to excavation or construction activities often leads to undesired stability issues with resultant appearance of weak zones the reinforcement of weak zones by grouting is one of the common remedies without demarcating the weak or the affected zones becomes counter productive excavation at a location in chennai mega city for metro rail was severely affected by soil piping with development of sinkholes and weakening of the surrounding strata subsequently development of cracks in the adjacent buildings and caused tilting of the nearby taller structures the mandated the geophysical and geotechnical investigations were carried out to demarcate the affected area to avoid further deterioration of the site cross hole seismic p and s wave velocities were estimated between10 pairs of boreholes up to 20 m depth on both the sides of the excavation two electrical resistivity tomography ert profiles each of 141 m in length were carried out in the same location cross hole s wave velocity profiles and resistivity tomograms clearly indicate the presence of weak zones and possible sinkhole locations site specific empirical relationship was established between s wave velocity and standard penetration test spt n value this relationship was further used for determination of spt n values from the measured vs values to compare the subsurface conditions identified under pre and post excavations the results confirmed the presence of weak zones which are responsible for the weakening of the surrounding strata in the foundation geophysical and geotechnical investigations similar to present investigation can be useful for designing metro rail projects in a subsurface environment comprising alluvium sand clay mixed with silt pebbles and boulders lying near a region of ground water pocket in a thickly populated urban area

high temperature deformation in the neoproterozoic transpressional ribeira belt southeast brazil abstract the neoproterozoic ribeira belt is subdivided in two domains with contrasting tectonic characteristics the northern domain is dominated by shallowly dipping foliations and orogen normal thrust tectonics the southern domain is characterized by a 1000 km long network of anastomosing transcurrent shear zones parallel to the belt this contrast is interpreted as reflecting continent continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain the central transitional domain of the ribeira belt displays the northern termination of the transcurrent shear zone network the alem paraiba padua shear zone system appss the 250 km long alem paraiba padua system involves granulites facies mylonites deformed through transpression a detailed study of the microstructure and lattice preferred orientation lpo of the rock forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust plagioclase crystals are plastically deformed they display curved twins and cleavages mechanical twins and evidence of dynamic recrystallization lpo of plagioclase is consistent with activation of the 010 100 and 010 001 slip systems lpo of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the 100 001 slip system quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration the lpo of quartz is therefore the result of a static transformation of an initial syn kinematic lpo and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations

origin and incremental evolution of brittle ductile shear zones in granitic rocks natural examples from the southern abitibi belt canada abstract metre scale shear zones developed in the mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation growth and termination the subvertical e w brittle ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well developed down dip mineral lineations they are exposed on a single large and flat outcrop perpendicular to the mineral lineation these brittle ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated joints fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation the deformation was accompanied by the influx of a metasomatic fluid which altered original fe mg bearing minerals and plagioclase into quartz epidote chlorite sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones we recognize two different types of lateral terminations i shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction and ii curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction in the latter case sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures this demonstrates that shear zones tended to grow towards each other and coalesce to form a well developed anastomosing network we propose that narrow dilatational fractures and joints acted as paleo weakness planes along which fluid rock interaction and resulting reaction softening occurred the deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system dilatancy related fracturing and reaction softening lead to deformation under brittle to plastic conditions and to shear strain localization