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

regional retrograde metamorphism of a high grade terrain the willyama complex broken hill australia abstract retrograde metamorphism has been a major influence in the development of the presently observed lithologies of the willyama complex broken hill two broad types of retrogression are distinguished pseudomorphous and kinematic retrogression the former type of retrogression involves replacement of prograde phases without complete loss of the high grade fabric hence the prograde assemblage can frequently be inferred kinematic retrogression involves the development of a new schistose fabric and like pseudomorphous retrogression is commonly related to f3 deformation retrogression was initiated during the waning stage of prograde metamorphism and was accompanied by an influx of aqueous fluid at similar pressure but lower temperature conditions than prograde metamorphism the source of the water is believed to be the crystallization of cooling partial melts the regional nature of this willyama style of retrogression is attributed to the metasediment rich nature of the sequence the high geothermal gradient and the considerable amount of in situ partial melting

a quaternary solution model for white micas based on natural coexisting phengite paragonite pairs a thermodynamic model for the quaternary white mica solid solution with end members muscovite mg celadonite paragonite feceladonite ms mgcel pg fecel is presented the interaction energies for the mgcel pg join the fecel pg join and the ternary interactions were obtained from natural coexisting phengite paragonite pairs phengite paragonite pairs were selected based on the criteria that their chemical compositions may be represented as a linear combination of the model end member compositions and that the respective formation conditions 350 650 c 4 21kbar are accurately known previously published excess free energy expressions were used for the ms pg ms mgcel and ms fecel binaries the suggested mixing model was tested by calculating multicomponent equilibrium phase diagrams this proved to be particularly well suited to reproduce compositional variations of white micas from amphibolite facies metapelites

retrieval of p t t d paths in a syn metamorphic shear zone implications for p t calculations and the permian triassic orogeny on the korean peninsula abstract crustal scale shear zones developed on the boundaries of the precambrian gyeonggi massif gm can play key role on understanding collisional orogeny during the late paleozoic to early mesozoic in the korean peninsula garnet schist formed on the boundary between the gm and okcheon metamorphic belt omb experienced contractional dn and extensional dn 1 shearing combined with a clockwise pressure temperature deformation p t d path two distinct mineral assemblages were found in the schists 1 garnet pre and or syn dn staurolite syn dn and or inter dn and dn 1 and syn dn 1 andalusite syn dn 1 bearing and 2 garnet pre and or syn dn epidote bearing assemblages depending on bulk rock compositions various compositions an10 40 in the former assemblage and an45 70 in the latter one and texture of plagioclase resulted from bulk rock compositions heterogeneous strain and dissolution and precipitation during the multiple shearing the p t conditions estimated for the early contractional shearing top to the north reverse shear are 560 c 600 c and 5 5 7 0 kbar based on the intersections between compositional isopleths for pre and or syn dn garnet and plagioclase an30 40 an45 55 calculated in the mno na2o cao k2o feo mgo al2o3 sio2 h2o and mno na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 systems subsequent extensional shearing top to the south normal shear was characterized by recrystallization of fine grained syn dn 1 staurolite with growth of syn dn 1 andalusite porphyroblasts during cooling and decompression to 560 c and 3 5 kbar the contractional and extensional shearing probably occurred at the permian and the late triassic to early jurassic respectively based on the previous geochronological data consequently inferred p t t d paths on the gm omb boundary probably suggest not only crustal thickening followed by thinning but also multiple amalgamations of several terranes

microstructures mineral chemistry and geochronology of white micas along a retrograde evolution an example from the aar massif central alps switzerland abstract this study investigates gneissic tectonites of different mechanical origins and aims at unravelling the link between microstructures white mica chemistry and k ar data the highest temperature mylonitic deformation which is investigated is dominated by dislocation creep in monomineralic quartz and viscous granular flow in polymineralic domains these mylonites show homogenous mineral chemistry resulting in robust and consistent k ar age data the lowest temperature deformation investigated is that of a retrograde brittle deformation producing cohesionless fault gouges within the cohesionless fault gouges frictional granular flow is considered as the main deformation mechanism the k bearing phases inside the fault gouges record no chemical resetting during this brittle overprint but contain mechanically fragmented k bearing micas and significant amounts of newly produced smectite this yields k ar ages inside of a gouge that are in the range of adjacent mylonite ages and may not represent the timing of gouge formation in an effort to understand these uncertainties in k ar ages detailed observations of the microstructural context of k bearing white mica were made microstructural criteria in combination with quantitative element mapping allowed for the discrimination of five important mica related processes 1 fracturing recrystallization of white mica 2 re precipitation of new fine grained white mica inside the deforming rock matrix 3 pseudomorphic replacement of precursor minerals by white mica and chlorite 4 pseudomorphic replacement of pre existing white mica without grain size reduction and 5 remnant grains the combination of these different processes led to a high variability in mineral chemistry and isotope data for tectonites which relates directly to variations in the size of the equilibrium volumes despite the samples being highly deformed in the presence of fluids inherited sheet silicates may survive if inherited sheet silicates are present they contribute to an increase in both the chemical heterogeneity and the apparent k ar ages of a sample

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

determining ages of multiple muscovite bearing foliations in phyllonites using the 40ar 39ar step heating method applications to the alleghanian orogeny in central new england

metamorphic thermal and tectonic evolution of central new england mountain skitchewaug and big staurolite nappes reactivation of a new detailed tectonic model is presented for the acadian orogenic this fabric during thrusting is recorded in some rocks of the big belt of central new england vermont and new hampshire that staurolite nappe by rotated garnets that grew during near isothermal accounts for a wide range of petrological and structural observations loading only the sillimanite isograd crosses the fall mountain three belts are considered the eastern vermont merrimack and skitchewaug nappe boundary metamorphic breaks across the skitintervening bronson hill belts specific observations in eastern chewaug big staurolite nappe boundary at the base of the big vermont that are accounted for in the model include the following staurolite nappe and at the margin of the keene and alstead domes p t paths are clockwise with maximum pressures near the athens require post metamorphic thrusting when p t conditions were in chester and strafford domes of 8 11 kbar but with maximum the greenschist facies these observations can be explained by a pressures decreasing to 3 5 kbar at the boundary with the bronson relatively simple model involving in sequence thrusting from east to hill belt differential exhumation of the vermont domes relative to west commencing in central new hampshire at 400 410 ma the rocks in easternmost vermont is required by the recorded differences preservation of the low grade belt along the vermont new hampin maximum pressure 5 6 kbar 15 20 km and the present day shire border requires that crustal thickening in vermont was not geographical separation 7 10 km specific observations in new caused by emplacement of new hampshire nappes onto eastern hampshire that are explained include the following p t paths in vermont and that the nappes of western new hampshire had time the merrimack belt are counter clockwise with maximum pressures to cool before final juxtaposition against the low grade belt cooling of 4 5 kbar and are related to high regional heat flow and heat ages constrain this final juxtaposition to have occurred in the transfer by early acadian plutons p t paths in the bronson hill carboniferous suggesting that the acadian was a prolonged event belt are intimately associated with structural position an early spanning as much as 100 myr contact metamorphism is evidenced in the skitchewaug and fall mountain nappes near contacts with the early acadian bethlehem gneiss 400 410 ma peak metamorphic temperature rises upwards in the nappe sequence an inverted metamorphic sequence

hf radar comparisons with moored estimates of current speed and direction expected differences and implications the validation of estimates of ocean surface current speed and direction from high frequency hf doppler radars can be obtained through comparisons with measurements from moored near surface current meters acoustic doppler current profilers or drifters expected differences between current meter cm and hf radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations sampling characteristics mooring response and geophysical variability we classify these sources of errors and differences as being associated exclusively with the current meter as being associated exclusively with the hf radar or as a result of differing methodologies in which current meters and hf radars sample the spatially and temporally varying ocean surface current vector field in this latter context we consider three geophysical processes namely the stokes drift ekman drift and baroclinicity which contribute to the differences between surface and near surface vector current measurements the performance of the hf radar is evaluated on the basis of these expected differences vector currents were collected during the high resolution remote sensing experiment ii off the coast of cape hatteras north carolina in june 1993 the results of this analysis suggest that 40 60 of the observed differences between near surface cm and hf radar velocity measurements can be explained in terms of contributions from instrument noise collocation and concurrence differences and geophysical processes the rms magnitude difference ranged from 11 to 20 cm s 1 at the four mooring sites the average angular difference ranged between 15 o and 25 o of which about 10 o is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams

evidence for syntectonic crystallization for the mudstone to slate transition at lehigh gap pennsylvania u s a data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well developed slaty cleavage at lehigh gap pennsylvania and for slates from quarries in the same area samples were studied by optical microscopy powder x ray diffraction and electron microprobe analysis with emphasis on transmission and analytical electron microscopy mineral grains are virtually free of deformation induced strain concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed 1 the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage 2 crystal imperfections as expressed in layer terminations low angle grain boundary like features and other defects decrease in density 3 complex mixed layering is replaced by homogeneous packets of layers of single phases and 4 illite transforms to muscovite with increase in k a1 and change from a lmd to 2m polytype slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding mass transport of components and crystallization to form new grains parallel to cleavage it reflects transitions from imperfect metastable phases toward ordered stable phases in a low temperature 225 c metamorphic environment

the effect of dislocation density on the aqueous solubility of quartz and some geologic implications a theoretical approach the internal energy associated with dislocations is calculated in quartz as a function of dislocation density these calculations show that at dislocation densities of 109 1010 1011 and 1012 cm dislocation cm3 the energy stored per mole of quartz is 2 2 19 7 170 and 1426 j respectively the density of free dislocations in natural quartz rarely exceeds 109 line cm cm3 and the resulting increase in free energy is probably too small to affect any geochemical processes significantly this applies to the overwhelming volume of rocks in the earth s crust the density of dislocations in tangles may exceed 1011 line cm 2 which is significant but the volume of quartz containing tangles is a very small fraction of most quartz grains nevertheless where fluid rock ratios are low the aqueous activity of sio2 may be significantly increased by the dissolution of quartz containing dislocation tangles such dissolution could be geologically significant in active greenschist facies fault zones and mylonites where repeated cold working would produce a sustained source and large population of dislocation tangles in such a setting the locally high internal energies of quartz containing dislocation tangles would lead to strain solution the resulting high activity of aqueous silica would enhance the stability of feldspars and would establish chemical potential gradients of sio2 which would drive the diffusive mass transfer of sio2 out of the fault zone

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

reaction localization and softening of texturally hardened mylonites in a reactivated fault zone central argentina the tres arboles ductile fault zone in the eastern sierras pampeanas central argentina experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes much of the fault zone is characterized by a d2 ultramylonite composed of fine grained biotite plagioclase that lacks a well defined preferred orientation the d2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep these ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase sillimanite and garnet inherited from the gneissic and migmatitic protolith d1 of the hangingwall the assemblage of garnet sillimanite biotite suggests that d1 related fabrics developed under upper amphibolite facies conditions and the persistence of biotite garnet sillimanite plagioclase suggests that the ultramylonite of d2 developed under middle amphibolite facies conditions greenschist facies mylonitic shear bands d3 locally overprint d2 ultramylonites fine grained folia of muscovite chlorite biotite truncate earlier biotite plagioclase textures and coarser grained muscovite partially replaces relic sillimanite grains anorthite content of shear band d3 plagioclase is c an30 distinct from d1 and d2 plagioclase c an35 the anorthite content of d3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite biotite is partially replaced by muscovite and or chlorite particularly in areas of inferred high strain quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric all d3 reactions require the introduction of h and or h2o indicating an open system and typically result in a volume decrease syntectonic d3 muscovite quartz chlorite preferentially grew in an orientation favourable for strain localization which produced a strong textural softening strain localization occurred only where reactions progressed with the infiltration of aqueous fluids on a scale of hundreds of micrometre local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids however the predominant greenschist facies deformation d3 along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system

age and setting of the bronson hill magmatic arc a re evaluation based on u pb zircon ages in southern new england new u pb zircon ages of selected rocks from the bronson hill anticlinorium in southern new england are as follows microcline gneiss of the dry hill gneiss in the pelham dome believed to be metamorphosed alkali rhyolite is confirmed as late proterozoic yielding an age of 613 3 ma another sample shows strong evidence of zircon inheritance with a minimum mean age of 1402 ma plagioclase rich gneisses with associated amphibolites swanzey pauchaug monson and fourmile gneisses now believed to represent metamorphosed intrusive igneous rocks yield ages from 454 to 442 3 2 ma thus ranging from early late through latest ordovician consistent with other absolute ages of the oliverian plutonic series in the region quartz phyric rhyolite from the upper member of the ammonoosuc volcanics and rhyolite tuff from the partridge formation are dated at 453 2 ma and 449 3 2 ma respectively indicating that eruption of felsic lavas in the overlying volcanic section spanned an interval of approximately 4 m y additional zircon ages are reported from a crosscutting but deformed tonalite dike in the swanzey gneiss 381 3 ma and a coarse grained gabbro from the prescott intrusive complex 407 3 2 ma intrusive into the partridge formation metamorphic overprinting of the region during both the acadian and alleghanian orogenies is indicated by u pb titanite ages of 357 5 ma and 292 5 ma from a tonalite gneiss on prescott peninsula and the dry hill gneiss respectively the above ages and other zircon ages as well as petrography and geochemistry of the fourmile monson swanzey pauchaug and other plagioclase rich gneisses of the bronson hill anticlinorium suggest that they represent the plutonic root of a calc alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in late ordovician time ca 454 442 ma the physically overlying cover sequence of chemically different tholeiitic arc volcanic and sedimentary rocks of the ammonoosuc volcanics and partridge formation previously thought to rest unconformably on the plagioclase rich plutonic gneisses is now shown to have had a magmatic history overlapping with them the zircon ages in the plutonic gneisses and the cover sequence indicate that both are the same age or slightly younger than the late ordovician caradocian emplacement of the giddings brook slice of the taconian allochthons and too young to have formed above an early and medial ordovician subduction zone

porphyroblast nucleation growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development in regional metamorphic rocks the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain rate gradients across the boundaries between the partitioned zones these generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain because of their uniquely layered crystal structure however most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients and hence are susceptible to dissolution and solution transfer as a consequence zones of progressive shearing become zones of dissolution of most minerals and of concentration of phyllosilicates and graphite exceptions are mylonites where strain rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing as they would be dissolved by the effects of shearing strain on their boundaries however they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation this includes a build up of stored strain energy relative to zones of progressive shearing in which dissolution is occuring porphyroblast growth is generally syndeformational and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account porphyroblasts in a contact aureole do not grow statically either as microfracturing associated with emplacement allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening the criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning reactivation of deforming foliations and a general lack of rotation of porphyroblasts with the spectacular exception of genuinely spiralling garnet porphyroblasts

monazite geochronology in central new england evidence for a fundamental terrane boundary monazite crystallization ages have been measured in situ using sims and emp analysis of samples from the bronson hill anticlinorium in central new england in west central new hampshire each major tectonic unit nappe displays a distinctive p t path and metamorphic history that requires significant post metamorphic faulting to place them in their current juxtaposition and monazite ages were determined to constrain the timing of metamorphism and nappe assembly monazite ages from the low pressure high temperature fall mountain nappe range from c 455 to 355 ma and y zoning indicates that these ages comprise three to four distinct age domains similar to that found in the overlying chesham pond nappe the underlying skitchewaug nappe contains monazite ages that range from c 417 to 307 ma 40 ar 39 ar ages indicate rapid cooling of the chesham pond and fall mountain nappes after 350 ma which is believed to represent the time of emplacement of the high level chesham pond and fall mountain nappes onto rocks of the underlying skitchewaug nappe garnet zone rocks from western new hampshire contain monazite that display a range of ages c 430 340 ma both the metamorphic style and monazite ages suggest that the low grade belt in western new hampshire is continuous with the vermont sequence to the west rocks of the big staurolite nappe in western new hampshire contain monazite that crystallized between c 370 and 290 ma and the same unit along strike in northern new hampshire and central connecticut records ages of c 257 300 ma conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks these monazite ages confirm that the metamorphism observed in the big staurolite nappe occurred significantly later than that in the units structurally above and below these data support the hypothesis that the big staurolite nappe represents a major tectonic boundary along which rocks of the new hampshire metamorphic series were juxtaposed against rocks of the vermont series during the alleghanian

scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite grade phyllites se vermont detailed electron microprobe analyses of phyllosilicates in crenulated phyllites from south eastern vermont show that grain scale zoning is common and sympathetic zoning in adjacent minerals is nearly universal we interpret this to reflect a pressure solution mechanism for cleavage development where precipitation from a very small fluid reservoir fractionated that fluid multiple analyses along single muscovite biotite and chlorite grains 30 200 m in length show zoning patterns indicating tschermakitic substitutions in muscovite and both tschermakitic and di trioctahedral substitutions in biotite and chlorite using cross cutting relationships and mineral chemistry it is shown that these patterns persist in cleavages produced at metamorphic conditions of chlorite grade chlorite grade overprinted by biotite grade and biotite grade zoning patterns are comparable in all three settings requiring a similar cleavage forming mechanism independent of metamorphic grade moreover the use of 40ar 39ar geochronology demonstrates this is true regardless of age furthermore samples with chlorite grade cleavages overprinted by biotite porphyroblasts suggest the closure temperatures for the diffusion of al si mg and fe ions are greater than the temperature of the biotite isograd 400 c parallel and smoothly fanning tie lines produced by coexisting muscovite chlorite and muscovite biotite pairs on compositional diagrams demonstrate effectively instantaneous chemical equilibrium and probably indicate simultaneous crystallization these results do not support theories suggesting cleavages form in fluid dominated systems if crenulation cleavages formed in systems in which the chemical potentials of all major components are fixed by an external reservoir then the compositions of individual grains defining these cleavages would be uniform on the contrary the fine scale chemical zoning observed probably reflects a grain scale process consistent with a pressure solution mechanism in which the aqueous activities of major components are defined by local dissolution and precipitation thus the role of fluids was probably limited to one of catalysing pressure solution and fluids apparently did not drive cleavage development

uncertainties in reservoir production forecasts being aware of uncertainties in forecasts of production characteristics is important for reservoir management decisions concerning further appraisal drilling flexibility in development plans and selection among reservoir prospects all require that uncertainties are taken into account this study addresses the problem of quantifying uncertainties due to incomplete knowledge of the initial reservoir characteristics with emphasis on the difference between heterogeneity modeling and assessment of uncertainty in this study we outline a formalism for uncertainty modeling in a bayesian framework and present an extensive case study of a north sea brent group reservoir the results are obtained by computer software implemented such that no human interference is required once the stochastic reservoir model is established once the stochastic reservoir model is established multiple realizations can be generated rescaled and fed into a fluid flow simulator to forecast production characteristics and quantify uncertainty associated with response parameters such as cumulative oil production recovery factors and water cuts the results demonstrate that uncertainty in model parameters contributes significantly to the overall uncertainty the most influential parameters in our case study include the sealing capacity of major faults seismic velocities used in depth conversion and average porosities and shale continuity within the main reservoir sandstone

ocean surface features and currents measured with synthetic aperture radar interferometry and hf radar this paper describes the results of the first quantitative comparison between high resolution ocean surface current fields extracted from interferometric synthetic aperture radar insar measurements and those from a high frequency hf ocean surface current radar oscr system data from each of these radar systems along with supporting measurements from shipboard and buoy mounted sensors were collected during the high resolution remote sensing experiment high res on june 20 1993 on the continental shelf off the coast of cape hatteras north carolina both components of the surface current were obtained from the insar system at roughly 10 m resolution from two orthogonal flight legs over the region separated in time by about 30 min the oscr system measured two dimensional surface current vectors at about 1 km resolution over this same region while the usns bartlett was collecting hydrographic samples and near surface current measurements two dimensional wave spectra as well as meteorological and additional current measurements were collected at two buoys within the experimental area we discuss in the paper two techniques for eliminating the effect of surface wave motion on the insar current estimates one method relies on some knowledge of the local wind and wave field and the use of a microwave scattering model the other method makes use of a few in situ current measurements spaced at different range locations across the insar image using either of these techniques we find that the agreement between the insar and oscr current estimates is generally very good furthermore the insar current and magnitude imagery show the presence of undulating surface features where abrupt changes in the current speed and direction occurred the ship surveys indicate that these features were caused by the collision of water masses of different density we show for the first time in this paper a high resolution area extensive vector surface current map derived from the insar of the two dimensional flow in the vicinity of these features our results demonstrate convincingly that high resolution oceanic surface current vectors can be derived from insar current measurements and that these measurements may be very beneficial for detailed studies of the dynamics of small scale surface features in regions of strong current divergences or shears

dating polygenetic metamorphic assemblages along a transect across the western alps multichronometric analyses were performed on samples from a transect in the french italian western alps crossing nappes derived from the brianconnais terrane and the piemonte liguria ocean in an endeavour to date both high pressure hp metamorphism and retrogression history twelve samples of white mica were analysed by 39ar 40ar stepwise heating complemented by two samples from the monte rosa nappe 100 km to the ne and also attributed to the brianconnais terrane one sm nd and three lu hf garnet ages from eclogites were also obtained white mica ages decrease from c 300ma in the westernmost samples zone houille re reaching c 3008c during alpine metamorphism to 548ma in the internal units to the east which reached c 5008c during the alpine orogeny the spatial pattern of eocene k ar ages demonstrates that si rich hp white mica records the age of crystallization at 47 48ma and retains ar at temperatures of around 5008c paleocene early eocene lu hf and sm nd ages recording prograde garnet growth before the hp peak confirm eclogitization in eocene times petrological and microstructural features reveal important mineralogical differences along the transect all samples contain mixtures of detrital syn d1 and syn d2 mica and retrogression phases d3 in greatly varying proportions according to local variations in the evolution of pressure temperature fluid activity deformation p t a d conditions samples from the zone houille re mostly contain detrital mica the abundance of white mica with si46 45 atoms per formula unit increases eastward across the whole traverse phengitic mica grown during hp metamorphism defines the d1 foliation syn d2 mica is more si poor and associated with nappe stacking exhumation and hydrous retrogression under greenschist facies conditions syn d1 phengite is very often corroded overgrown by or intergrown with syn d2 muscovite most importantly syn d2 recrystallization is not limited to s2 schistosity domains micrometrescale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals which could be mistaken for syn d1 mica based on microstructural arguments alone the cl k ratio in white mica is a useful discriminator as d2 retrogression was associated with a less saline fluid than eclogitization as petrology exerts the main control on the isotope record constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data described in both the present study and the literature our approach which ties geochronology to detailed geochemical petrological and microstructural investigations identifies 47 48ma as the age of hp formation of syn d1 mica along the studied transect and in the monte rosa area cretaceous apparent mica ages which were proposed to date eclogitization by earlier studies based on conventional thermochronology are due to ar inheritance in incompletely recrystallized detrital mica grains the inferred age of the probably locally diachronous greenschist facies low si syn d2 mica ranges from 39 to 43ma coexistence of d1 and d2 ages and the constancy of non reset d1 ages along the entire transect provides strong evidence that the d1 white mica ages closely approximate formation ages volume diffusion of ar in white mica activation energy e 250 kj mol 1 pressure adjusted diffusion coefficient d 050 03 cm2 s 1 has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples

40ar 39ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations abstract artificial mixtures of two age populations of muscovite and of biotite were prepared at relative weight percents of 3 1 1 1 and 1 3 and analyzed by the 40ar 39ar vacuum furnace step heating method the original micas are late jurassic and late cretaceous samples that yield flat age spectra with plateau ages defined by 97 100 of the gas release and no evidence for excess argon the age spectra from mica mixtures yield patterns that systematically decrease with decreasing jurassic mica content and increasing cretaceous component however the mixed muscovite spectra are relatively flat consistent with the two original samples showing similar degassing patterns during vacuum furnace heating in contrast the mixed biotite spectra are highly discordant with an overall decrease in age during the step heat analysis this is consistent with the degassing patterns of the original mica samples which indicate the jurassic biotite high fe mg outgases at lower temperatures and the cretaceous biotite low fe mg remains retentive until the higher temperature steps the individual degassing patterns and compositions show consistencies with known compositional controls on argon diffusivity indicating that micas may maintain argon retention characteristics rooted in crystal chemistry during vacuum furnace heating all but one of the mixed samples failed to yield ages reflecting the age of the original micas these results imply that complex age spectra obtained for multiply deformed rocks with multiple mica populations may define geologically meaningless ages as a result of simultaneous degassing i e mixing of sample reservoirs during heating comparison of laboratory degassing rates and crystal chemistry indicates compositional controls on argon retention are likely preserved during vacuum furnace step heating and micas may degas retain argon in a predictable manner these conclusions support assertions that recovery of fossil age gradients in slowly cooled samples may be possible using furnace step heating

the fine structure of nearshore tidal and residual circulations revealed by h f radar surface current measurements abstract using the ocean surface current radar oscr developed by the rutherford appleton laboratory uk 30 days of synoptic hourly surface current vectors were obtained for 84 locations within a nearshore region some 18 kms square tidal analyses of these data show that the currents associated with the predominant m2 constituent sweep smoothly and regularly through the area unaffected by the finer topographic features moreover contours of the amplitude of the m2 semi major axis are mutually consistent to a precision of better than 0 5 cm s 1 statistical analyses of these data indicate that the standard error of oscr current measurements is less than 4 cm s 1 by contrast the major higher harmonic constituent m4 shows pronounced but ordered spatial variability relating the observed distributions for m4 and m2 is foreseen as an instructive modeling problem that should advance our knowledge of shallow water tidal interaction processes standard relationships between residual surface currents and

probing the depths of oliverian magmas implications for paleozoic tectonics in the northeastern united states oliverian magma series gneisses 442 454 ma in the cores of domes in southwestern new hampshire have an igneous mineralogy that crystallized at pressures p of 9 11 kbar and temperatures t of 650 775 c whereas the cover ammonoosuc volcanics has the same formation age 453 2 ma yet is clearly extrusive a significant crustal section 30 km thick must have originally separated the two units a recognition that demands tectonic reinterpretation of this classic orogen acadian age ca 400 ma metamorphism at p 4 6 kbar t 625 c is well documented in the cover but the absence of expected diffusional reequilibration in oliverian garnets indicates that they never exceeded 550 c after formation juxtaposition of the oliverian magma series gneisses and their cover must have occurred after the cover had cooled below 550 c and was likely via either neo acadian or alleghanian thrusting

dating slate belts using 40 ar 39 ar geochronology and zircon ages from crosscutting plutons a case study from east central maine usa abstract we report the ages of cleavage development in a normally intractable lower greenschist facies slate belt the central maine aroostook matapedia belt in east central maine we have attacked this problem by identifying the minimum ages of muscovite in a regional acadian cleavage s 1 and in a local ductile fault zone cleavage s 2 using 40 ar 39 ar geochronology and the ages of crosscutting plutons our success stems from the regional low grade metamorphism of the rocks in which each crystallization event preserves a 40 ar 39 ar crystallization age and not a cooling age evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital authigenic and in some rocks s 1 muscovite and chlorite grains by new cleavage forming muscovite and chlorite grains low blank furnace age spectra from meta arkosic and slaty rocks climb from moderate temperature devonian age steps dominated by cleavage forming muscovite to ordovician age steps dominated by a detrital muscovite component s 1 and s 2 cleaved rocks were hornfelsed by granitoids of 407 and 377 ma respectively the combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage forming events 410 ma s 1 and 380 ma s 2 these two events coincide in time with a change in the plate convergence kinematics from the arrival of the avalon terrane acadian orogeny to a right lateral transpression arrival of the meguma terrane in the neoacadian orogeny

replacement reactions and deformation by dissolution and precipitation processes in amphibolites the deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition fluid content pressure and temperature these mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation here we show that fracturing and fluid flow triggered coupled dissolution precipitation and dissolution precipitation creep processes which were responsible for the formation of a mylonitic microstructure in amphibolites this fabric is developed over a crustal thickness of 500 m in the lower seve nappe scandinavian caledonides amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence the albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the 001 cleavage planes and display lobate edges with embayments and peninsular features two albite oligoclase generations bright in cathodoluminescence resorbed and overgrew the porphyroclasts sealing the fractures electron backscattered diffraction shows that the two albite oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them these two albite oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation the amphibole crystals experienced a similar microstructural evolution as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c axis no crystal plasticity is observed in either mineral species plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies 600 c 0 75 0 97 gpa our results demonstrate that at middle to lower crustal levels the presence of h2o rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution precipitation and favours deformation by dissolution precipitation creep over dislocation creep in plagioclase and amphibole

rapid exhumation of cretaceous arc rocks along the blue mountains restraining bend of the enriquillo plantain garden fault jamaica using thermochronometry from multiple closure systems abstract the effect of rapid erosion on kinematic partitioning along transpressional plate margins is not well understood particularly in highly erosive climates the blue mountains restraining bend bmrb of eastern jamaica bound to the south by the left lateral enriquillo plantain garden fault epgf offers an opportunity to test the effects of highly erosive climatic conditions on a 30 km wide restraining bend system no previous thermochronometric data exists in jamaica to describe the spatial or temporal pattern of rock uplift and how oblique 20 plate motion is partitioned into vertical strain to define the exhumation history we measured apatite n 10 and zircon n 6 u th he ages 40ar 39ar n 2 amphibole and k spar ages and u pb zircon n 2 crystallization ages late cretaceous u pb and 40ar 39ar ages 74 68 ma indicate rapid cooling following shallow emplacement of plutons during north south subduction along the great caribbean arc early to middle miocene zircon helium ages 19 14 ma along a vertical transect suggest exhumation and island emergence at 0 2 mm yr older zircon ages 10 15 km to the north 44 35 ma imply less rock uplift apatite helium ages are young 6 1 ma across the entire orogen suggesting rapid exhumation of the bmrb since the late miocene these constraints are consistent with previous reports of restraining bend formation and early emergence of eastern jamaica an age elevation relationship from a vertical transect implies an exhumation rate of 0 8 mm yr while calculated closure depths and thermal modeling suggests exhumation as rapid as 2 mm yr the rapid rock uplift rates in jamaica are comparable to the most intense transpressive zones worldwide despite the relatively slow 5 7 mm yr strike slip rate we hypothesize highly erosive conditions in jamaica enable a higher fraction of plate motion to be accommodated by vertical deformation thus strike slip restraining bends may evolve differently depending on erosivity and local climate

contrasting tectonothermal domains and faulting in the potomac terrane virginia maryland discrimination by 40ar 39ar and fission track thermochronology new 4 0 ar 3 9 ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the potomac terrane in northern virginia thus confirming previous interpretations that it is a composite terrane the rocks of the potomac terrane were examined along the potomac river where it has been previously subdivided into three units the mather gorge sykesville and laurel formations in the mather gorge formation at least two metamorphic thermal domains were identified the blockhouse point and bear island domains separated by a fault active in the late devonian early ordovician ca 475 ma cooling ages of amphibole in the bear island domain reflect cooling from taconic metamorphism whereas the blockhouse point domain was first metamorphosed in the devonian the 4 0 ar 3 9 ar data from muscovites in a third eastern domain within the mather gorge formation the stubblefield falls domain record thrusting of the sykesville formation over the mather gorge formation on the plummers island fault in the devonian the existence of two distinctly different thermal domains separated by a tectonic boundary within the mather gorge argues against its status as a formation hornblende cooling ages in the sykesville formation are early devonian ca 400 ma reflecting cooling from taconic and acadian metamorphism the ages of retrograde and overprinting muscovite in phyllonites from domain bounding faults are late devonian acadian and late pennsylvanian alleghanian marking the time of assembly of these domains and subsequent movement on the plummers island fault our data indicate that net vertical motion between the bear island domain of the mather gorge complex and the sykesville formation across the plummers island fault is east side up zircon fission track cooling ages demonstrate thermal equilibrium across the potomac terrane in the early permian and apatite fission track cooling ages record tilting of the potomac terrane in the cretaceous or later with the west side up at least 1 km

geochronologic studies in central new england i evidence for pre acadian metamorphism in eastern vermont sup 40 ar sup 39 ar geochronology on hornblendes from central new england reveals a grenvillian cooling age 1026 plus minus 12 ma along the eastern margin of the green mountain massif acadian ages in east central vermont 350 397 ma and along the bronson hill anticlinorium in new hampshire and massachusetts 407 280 ma and late ordovician to silurian ages 440 451 ma along a belt in eastern vermont and central massachusetts these ages indicate a minimal acadian overprint in eastern vermont the highest degree of acadian overprint being located along the axis of the domes in the connecticut valley synclinorium these data suggest that the formation of the domes occurred in the acadian moreover the late ordovician to silurian cooling ages in eastern vermont suggest that much of the metamorphic mineralogy and fabric observed in the connecticut valley synclinorium may be pre silurian in age

high resolution compositional mapping of matrix phases implications for mass transfer during crenulation cleavage development in the moretown formation western massachusetts abstract high resolution compositional maps provide a new tool for investigating mass transfer during cleavage formation the moretown formation of western massachusetts contains a well developed crenulation cleavage with alternating mica rich crenulation limbs and mica poor crenulation hinges compositional mapping shows two generations of plagioclase the second of which was synchronous with the crenulation cleavage a significant amount of the syn crenulation plagioclase 10 20 modally grew in hinge domains a small amount of syn crenulation plagioclase 1 and a large amount of phengitic muscovite grew in limb domains the maps also show that uncrenulated domains experienced mass transfer and reactivation of older cleavages and thus cannot be used as undeformed reference domains for comparison with crenulated regions compositional mapping facilitates a new degree of integration between petrologic and structural analysis knowledge of the structural context of compositional domains allows better selection of phases and compositions for interpreting metamorphic reactions and linking metamorphism to deformational stages knowledge of syntectonic reactions provides new insights into mass transfer and volume change during deformation in the moretown formation plagioclase and phengite producing reactions play a large role in controlling the nature and magnitude of mass transfer but microstructures control the location of reactants and products within the evolving fabric

phase relations and chemical composition of phengite and paragonite in pelitic schists during decompression a case study from the monte rosa nappe and camughera moncucco unit western alps the metamorphic evolution of metapelites from the eastern part of the monte rosa nappe and the camughera moncucco unit both situated in the upper penninic units sw of the simplon line were investigated using microstructural relationships and equilibrium phase diagrams the units under consideration experienced prealpine amphibolite facies conditions and underwent a complex metamorphic evolution during the alpine orogeny peak pressures during an early alpine high pressure stage of 12 5 16 kbar were similar in the monte rosa nappe and camughera moncucco unit a pronounced thermal gradient is indicated during decompression leading to an amphibolite facies overprint as the decompression paths went through the chlorite biotite and plagioclase stability fields in most of the monte rosa nappe through the staurolite field in the easternmost monte rosa nappe and in the camughera moncucco unit and through the sillimanite field in the easternmost camughera moncucco unit in high al metapelites the initial formation of staurolite is related to continuous paragonite breakdown and associated formation of biotite in the course of this reaction phengite becomes successively sodium enriched in low al metapelites in contrast the initial staurolite formation occurs via the continuous breakdown of sodium rich phengite in both low and high al metapelites the largest volume of staurolite is formed during the continuous breakdown of sodium rich phengite below p t conditions of about 9 5 kbar at 600 650 c during this reaction phengite becomes successively potassium enriched as sodium from phengite is used to form the albite component in plagioclase for normal pelitic chemistries phengite becomes na enriched during decompression through the breakdown of paragonite along a nearisothermal decompression path the na content in phengite reaches its maximum when paragonite is entirely consumed during further decompression the paragonite component in phengite decreases again because na is preferentially incorporated into the albite component of plagioclase

reaction softening by dissolution precipitation creep in a retrograde greenschist facies ductile shear zone new hampshire usa we describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host littleton schist near claremont new hampshire here na poor muscovite and chlorite progressively replace first staurolite then garnet and finally biotite porphyroblasts as the core of the fault zone in approached across the transect higher grade fabric forming na rich muscovite is also progressively replaced by fabric forming na poor muscovite the mineralogy of the new phyllonitic fault rock produced is dominated by na poor muscovite and chlorite together with late albite porphyroblasts the replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement in contrast element mapping shows that fabric forming na rich muscovite is selectively replaced at high strain microstructural sites indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite the replacement of strong high grade porphyroblasts by weaker na poor muscovite and chlorite constitutes reaction softening the crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated na rich muscovite defined foliation destroys not only the metastable high grade mineralogy but also its stronger geometry this process constitutes both reaction and microstructural softening the deformation mechanism here was thus one of dissolution precipitation creep activated at considerably lower stresses than might be predicted in quartzo feldspathic rocks at the same lower greenschist facies conditions this article is protected by copyright all rights reserved

controls on metamorphic equilibration the importance of intergranular solubilities mediated by fluid composition a dramatic demonstration of the role of intergranular solubility in promoting chemical equilibration during metamorphism is found in the unusual zoning of garnet in pelitic schist exposed at harpswell neck maine usa many garnet crystals have irregular patchy distributions of mn cr fe and mg in their inclusion rich interiors transitioning to smooth concentric zoning in their inclusion poor outer rims in contrast zoning of ca and y is comparatively smooth and concentric throughout we re assess the disputed origin of these zoning features by examining garnet growth in the context of the thermal and structural history of the rocks and by evaluating the record of fluid rock interaction revealed in outcrop scale veining and fluid inclusion assemblages the transition in the character of garnet zoning correlates with the onset of a synkinematic simple shear dominated phase of garnet growth and with a shift in the composition of the intergranular fluid from co2 rich to h2o rich compositional variations in garnet are therefore best explained by a two stage growth history in which intergranular diffusive fluxes reflect differences in the concentration of dissolved species in these two contrasting fluids interiors of garnet crystals grew in the presence of a co2 rich fluid in which limited solubility for mn and cr and perhaps fe and mg produced patchy disequilibrium overprint zoning while appreciable solubility for ca and y permitted their rock wide equilibration rims grew in the presence of an h2o rich fluid in which high intergranular concentrations for all elements except cr enabled diffusion over length scales sufficient for rock wide equilibration this striking example of partial chemical equilibrium during reaction and porphyroblast growth implies that thermal effects may commonly be subsidiary in importance to solubilities in the intergranular medium as determinants of length scales for metamorphic equilibration