seismic image quality beneath strongly scattering structures and implications for lower crustal imaging numerical simulations summary the aim of this paper is to analyse how the scattered wavefield in a highly heterogeneous structure influences the image quality of structures below it and how the velocity control during processing of data from highly heterogeneous areas is important in the imaging problem statistical 2 d two layer models are built based on deep borehole data the layers are characterized by a fractal dimension hurst exponent and by the standard deviation of the velocity fluctuations over a constant background velocity value shot gathers are generated in these models with a 2 d elastic and viscoelastic finite difference simulator the synthetic data are first stacked and migrated using an average background velocity function as would be done in a realistic situation during which only a smooth version of the velocity field can be resolved second the full heterogeneous velocity models are applied to the migration scheme the final sections are compared using a semblance based technique to quantify the amount of contamination from an upper heterogeneous structure of a lower heterogeneous structure in data that are not properly migrated multiple scattering in the upper layer seems to dominate the lower part of the section this contamination is controlled by the standard deviation of the velocity fluctuations and it is reduced when viscoelasticity is introduced in the simulations counterintuitively a low q value may improve deep imaging when the seismograms are migrated with the real complex velocity field this contamination is reduced to acceptable levels residual contamination cannot be corrected completely because of the single scattering assumption inherent to all migration techniques

imaging and interpreting the shallow crystalline crust abstract in the past many tests of high resolution seismic imaging methods utilizing off the shelf technology from hydrocarbon exploration have had only limited success some problems associated with the application of seismic profiling in the shallow crystalline crust differ from those encountered in sedimentary basins perhaps the most fundamental distinction stems from differences in statistical properties of the reflection coefficient time series analysis of power spectra derived from borehole data indicates that the reflection coefficients in the crystalline crust are depleted in the seismic bandwidth relative to sedimentary basins this fundamental difference must be considered in the design of shallow crustal seismic surveys in order to achieve success to a first approximation acoustic impedance variations in sedimentary basins are controlled by velocity changes whereas velocity and density can vary independently in the crystalline crust and changes in density may be relatively more important in controlling reflectivity in certain instances density fluctuations outweigh velocity fluctuations for example physical rock property data from mining districts in the canadian shield suggest that massive fe oxide or sulphide bodies will generate strong seismic reflections

heterogeneity spectrum and scale anisotropy in the upper crust revealed by the german continental deep drilling ktb holes crustal heterogeneities and their statistical characteristics bear important information about the dynamic processes and evolution of the crust the velocity well log data from the german continental deep drilling project ktb offer a rare opportunity to measure directly the properties of crustal heterogeneities in this study we first estimated the power spectrum of crustal heterogeneities from the p velocity well logs of the two holes for the first time a power law spectrum of crustal heterogeneities was directly observed in the spatial wavelength range from a few meters to a few kilometers the slope of the vertical 1d power spectrum of the random velocity perturbations is about 1 1 corresponding to a flicker noise random medium the scale anisotropy ratio aspect ratio and the horizontal spectral slope of crustal heterogeneities were also estimated by cross correlation analysis between these two holes to be about 1 8 and 2 0 respectively

metamorphism of the witwatersrand gold fields conditions during peak metamorphism pelitic assemblages from all major witwatersrand gold fields record metamorphic conditions of the greenschist facies with minimal regional grade changes over at least 200 km strike length diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites the actively exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons bulk rock composition has been a major control on assemblage development key metapelitic assemblages include pyrophyllite chloritoid chlorite and muscovite in each gold field with less common metamorphic biotite accessory minerals are pyrite tourmaline rutile and zircon the abundance of chloritoid and pyrophyllite in thin shaly units together with their minor but widespread distribution in quartzites and conglomerates indicate that metamorphic temperatures reached 350 c 50 c in all the gold fields pressures are less well constrained 1 2 kbar being inferred outside the gold fields higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions the temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to re mobilize gold and may explain its secondary textural features

structure and seismic response of extended continental crust stochastic analysis of the strona ceneri and ivrea zones italy a two dimensional stochastic seismic model of the crustal cross section exposed in the strona ceneri and ivrea zones northern italy was derived from geologic maps and petrophysical data with techniques used in microfabric analysis the model has a layered self affine structure which agrees with the intrinsic scale independence of geologic phenomena synthetic seismograms from this model replicate many pertinent features observed in deep seismic reflection data and allow insight into crustal structure 1 reflectivity depends on the lithological populations present and their distribution within a crustal unit 2 transparent zones within the reflective lower crust can arise from metamorphic reactions 3 the transition from upper to lower crust may be a change in small scale seismic fabric rather than a first order seismic discontinuity

approach to chemical equilibrium in diagenetic chlorites electron microprobe analyses were made on diagenetic chlorites in sandstones and mudstones from two deep wells according to the petrographic character of the chlorite occurrence as pseudomorphic phases rims on quartz or glauconite or as distinct phases in the clay matrix chlorite compositions do not depend upon crystallization site reacting phases making it apparent that new chlorites can form in an approach to chemical equilibrium at or near the surface 40 c 1 km depth comparison of this data with that for late diagenetic and early metamorphic chlorites indicates that the compositional range for different grains in the same thin section is similar for the samples throughout the 40 270 c temperature span compositional range decreases upon further metamorphism

geochemistry and age of ivory coast tektites and microtektites abstract ivory coast tektites were first reported in 1934 from a geographically restricted area at ivory coast west africa although some additional specimens have been found later the total number remains small a few hundred the bosumtwi impact crater in ghana is most likely the source crater for the ivory coast tektites based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions in addition to tektites on land microtektites were found in so far eleven deep sea cores off the west african coast between about 9 n and 11 s and 0 and 23 w defining the extent of the ivory coast tektite strewn field in this study we analyzed eleven ivory coast tektites for their major and trace element composition studied their petrographical characteristics provided major element data for 111 microtektites and major and trace element data for four microtektites we determined the 40ar 39ar step heating age of five ivory coast tektites and four microtektites and obtained fission track dates for ten tektites and one bosumtwi impact glass the tektites have very small intersample and intrasample variations of their major and trace element composition 111 ivory coast microtektites from eleven cores were analyzed for their major element compositional range their compositional range is significantly wider than that of the ivory coast tektites but the majority of all microtektites have compositions very similar to those of the tektites within a factor of 1 2 trace element compositions of the tektites also show little variation between samples the samples do not show any distinct eu anomaly in the ree patterns this characteristic as well as the high absolute ree abundances and la nybn ratios of about 8 indicate that archean rocks are plausible source rocks the major and trace element contents of four individually analyzed ivory coast microtektites show compositions that are very similar to those of the ivory coast tektites however the microtektites contain 20 rel higher abundances of some of the lithophile and siderophile trace elements such as sc cr co ni sr zr ba hf ta th and the rees these differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite forming melt the ivory coast microtektites also have a uniform internal composition duplicate 40ar 39ar step heating age analyses were performed on five tektites the best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs resulting in an age of 1 1 0 05 ma we also tried to date four microtektites by 40ar 39ar age analyses but their young age and small sample size makes it impossible to assign a reliable age to the microtektites one run yielded satisfactory results that were similar to the tektite age in addition we determined the fission track ages for ten individual ivory coast tektite samples and for one impact glass sample from the bosumtwi crater the track size corrected ages for the ivory coast tektites ranged from 0 91 to 1 18 ma resulting in an average fission track age of 1 05 0 11 ma this age is within errors identical to that of the bosumtwi impact glass at 1 03 0 11 ma and to the 40ar 39ar age of 1 1 0 05 ma the preferred age of the ivory coast tektite event is 1 07 ma

polytypism of cookeite in low grade metapelites of the cameros basin spain lack of correlation of well ordered polytypes with pressure cookeite occurs in low grade chloritoid bearing metapelites of the cameros basin northern iberian range spain as thin coatings on poikiloblastic pyrite crystals 25 cm on edge textural relations imply formation via hydrothermal fluids in a syn to late regional metamorphic environment selected area electron diffraction patterns of cookeite display a wide range of stacking order disorder from patterns with streaking parallel to c in k 3 n reflections to sharply defined reflections having one two and greater than two layer periodicities well ordered polytypes in crystals up to 0 1 m thick are of type ia as determined by powder x ray diffraction coexisting chamosite likewise displays a range of order in polytypism from complete disorder to the well ordered iib polytype which is typical of low grade metamorphism contrary to assertions that ordered cookeite polytypes are indicators of medium to high pressure metamorphism the occurrence of ordered stacking sequences is direct evidence that although ordered cookeite may occur in medium to high p environments it is equally indicative of low p environments it can therefore not be used as an indicator of high pressure the occurrence of several ordered and disordered polytypes in the same sample is compatible with the generally accepted notion that free energy differences between polytypes are too small to be indicators of specific stable equilibrium p t conditions the general occurrence of semirandom stacking is attributed to a lack of equilibrium in structure that is generally characteristic of phyllosilicates in low p t environments of formation

clay mineral authigenesis in coal and shale from the anthracite region pennsylvania textural chemical and mineralogical analyses of authigenic clays in anthracite rank coal and associated shale from eastern pennsylvania have allowed a better understanding of the parameters controlling diagenesis and perhaps coalification in this region minerals in anthracite occur in distinct assemblages associated with the following microenvironments coal matrix two orthogonal joint sets termed systematic and nonsystematic cleat and a third joint set mineralogical differences among microenvironments allow inferences about clay mineral origins and the parameters that controlled mineral authigenesis kaolinite occurs in the shale and all microenvironments within the coal seam authigenic minerals that replaced preexisting kaolinite during the latest stage ofcoalification anthracitization at t 200 c include nhu rich illite pyrophyllite and the following minerals which are primarily restricted to the systematic cleat set tosudite rl ordered dioctahedral mixed layer chlorite smectite sudoite di trioctahedral chlorite and rectorite rlordered mixedjayer paragonite smectite alteration of smectite to illite may be responsible for formation of authigenic illite and na bearing illite which are present only in the shale and coal matrix and fe al rich chlorite and quartz in the third joint set the chemical components for authigenesis appear to have multiple sources ai and si from preexisting kaolinite and quartz n from local organic matter mg and na for tosudite sudoite and rectorite largely from metasomatic hydrothermal fluids fe and mg for fe al rich chlorite from smectite illitization minerals in the shale the coal matrix and the nonsystematic cleat set are interpreted to represent authigenesis in a low permeability environment closed system alteration however the assemblage sudoite tosudite rectorite in the systematic cleat set is interpreted to be the result ofone or two stages ofhydrothermal alteration open system alteration we suggest hat differences in minerals between the two nearly perpendicular cleat sets are the result of permeability differences which were maintained by an anisotropic lateral stress field created by plate convergence during the alleghanian orogeny hydrothermal alteration may be related to large scale basinal flow induced by alleghanianage uplift such migrating fluids could also have transported heat from depth and thereby significantly increased the rate and rank ofcoalification in this region

chloritoid pyrophyllite rectorite facies rocks from brittany france electron microprobe analyses of minerals in low grade metamorphic sandy black shales of middle paleozoic age indicate that chemical equilibrium was closely approached in authigenic ferro magnesian or alkali bearing phases while aluminum silicate minerals appear to have reacted at a slower rate the k for mg fe in chlorite chloritoid pairs is 0 19 the latter phase being iron rich the pseudomorphism of detrital white micas produces kaolinite pyrophyllite and intimate mixtures of muscoviteparagonite new phases forming in other sites include chlorite chloritoid and rectorite which is an ordered paragonitebeidellite mixed layered phase these minerals were formed at temperatures below 280 c this is just above the stability of the illite smectite mixed layered minerals which have an isii ordering sequence 85 to 95 illite it was noted that ferromagnesian minerals attain chemical equilibrium on a thin section scale while the aluminous phases often seem to respond to chemical potentials operative on a submillimetric scale

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

heterogeneity and seismic scattering in exploration environments abstract seismic scattering and the corresponding reduction in signal to noise ratio is a severe limitation to seismic exploration in crystalline environments in this paper we summarize recent elastic numerical modeling of scattering regimes and discuss the implications for seismic exploration in shallow hardrock environments the resulting synthetic seismograms range from relatively transparent to highly reflective depending on the ratio of seismic wavelength to heterogeneity scale length we note that the archean canadian shield is characterized by large scale lengths typical for crystalline environments which translates to moderately reflective seismic sections exhibiting scattering features that are coherent over large distances in contrast the highly fractured footwall of the sudbury impact structure in the canadian shield translates to a medium with small to moderate scale lengths resulting in seismic sections that can in places be highly reflective or relatively transparent from the perspective of mineral exploration in sudbury we find that the possible detection of massive sulfide targets is dependent not only on the shape and size of the deposit but on the scattering nature of the background as well with information about the petrophysical properties and the distribution of heterogeneity within an exploration area acquisition parameters such as source frequencies and minimum receiver spacing can be adjusted in order to survey within the scattering regime that will produce the least amount of noise

glaucophane chloritoid bearing assemblages from ne oman petrologic significance and a petrogenetic grid for high p metapelites pelitic layers and lenses interbedded with blueschists and eclogites in saih hatat ne oman contain chloritoid and sodic amphibole bearing mineral assemblages that are useful for reconstructing the p t history of the area textural and mineral chemical relations suggest that coexisting glaucophane gln and chloritoid ctd formed at the expense of chlorite chl paragonite pg and later broke down to garnet gt pg during prograde metamorphism according to the reaction gln ctd qz gt pg h2o during retrogression gln and chl first formed at the expense of gt and pg followed by the breakdown of ctd and gt to chl the final stages of retrogression are marked by the breakdown of gln to an aggregate of chl albite ab a projection from quartz qz h2o and phengite ph on the al2o3 fe2o3 feo mgo na2o plane in the system nfmash is best suited for the representation of the phase relations in high p metapelites petrogenetic grids for the model systems nmash and nfash were calculated using program geo calc berman et al 1987 and its database berman 1988 after the retrieval of s i o and h f o for gln and ctd by mathematical programming and calculating all possible reactions among gln ctd chl jadeite jd ab gt pg talc tc pyrophyllite prl and kyanite ky the calculated petrogenetic grid for the system nfash shows that fe ctd and ferroglaucophane coexist at p 6 5 kbar and t 6 kbar this grid is consistent with the p t estimates for high p metapelites from oman new caledonia seward peninsula ile de groix sifnos and peloponnese where gln ctd bearing units are interbedded with cofacial mafic blueschists and eclogites the grid also explains the observed textural relations in the metapelites of oman and is consistent with the clockwise p t path proposed for this area but differs from the grids of guiraud et al 1990 in showing a larger stability field for gln ctd that overlaps with the stability fields of jd qz as well as ab the petrogenetic grid calculated for the system nmash shows that mg ctd gln bearing assemblages require unusually low geothermal gradients to form in metapelites it also shows that mg rich ctd tc coexist at high pressures over a wider p t range than predicted by guiraud et al 1990 this grid can therefore be applied to high p metamorphic assemblages from the eastern and western alps

stochastic analysis of sonic logs from the upper crystalline crust methodology to relate local fluctuations observed in sonic logs to small scale velocity fabric along boreholes both filtering effects and noise introduced by the logging procedure must be taken into account sonic log velocities are represented as a time series consisting of a large scale deterministic and a small scale stochastic component the deterministic trend approximated by a low order polynomial best fit contains information on the average velocity structure whereas the small scale stochastic variations consist of noise plus in situ velocity variations convolved with the logging system response the velocity fluctuations of the sonic data considered here are zero mean and have quasi gaussian probability density functions therefore they are well characterised by their second statistical moment i e their autocovariance function tests on synthetic data indicate that the autocovariance function corresponding to this data model may be used to extract information on the second order statistics of the in situ velocity variations along the borehole and to constrain the level of white noise in sonic logs ignoring the presence of filtering effects and noise in sonic logs may result in seriously flawed estimates of the second order statistics of the actual velocity structure assuming a von karman autocovariance function for the in situ velocity variations this model provides a good match to the autocovariance functions of sonic log data from the siljan ring sweden and sudbury areas canada although differing significantly in their noise content these two data sets yield similar results for the small scale velocity structure which is modelled as a bandlimited self affine time series for the siljan ring borehole we found a close relation between small scale variations of the borehole diameter as determined from caliper logs and the level of uncorrelated noise present in the sonic log data

a late ordovician phreatomagmatic complex in marine soft substrate environment the crozon volcanic system armorican massif france the mafic lavas and the diabases of crozon armorican massif france belong to an anorogenic ordovician volcanic complex emplaced on a rifted passive margin in north gondwana magma passed through syn volcanic soft sedimentary substrate which is today mostly composed of alternating sandstones and mudstones from llanvirn to ashgill in age field observations together with microscopic studies and geochemical analyses of magmatic rocks lead us to propose a model of volcano formation which combines hydromagmatic processes peperitic intrusions a shallow submarine tephra settling eruption fed turbidity currents and a pillow lava effusion the crozon outcrops can be used to reconstruct a complete cross section from the root of the volcanic complex to the lavas and breccias emplaced on the sea floor the sites expose i a hypabyssal breccia containing mud chunks and coarse grained diabase clasts with amoeboidal fine grained magmatic material ii bulbous peperitic sills and pillow like lobes bearing a great quantity of sediment derived enclaves of fluidal morphology iii volcaniclastic breccias containing near spherical magmatic clasts that resulted from the complete fragmentation of sills in the ductile regime iv a rhythmic peperitic breccia interpreted as the product of mingling between thin lava flows and soft calcareous sediment the crozon volcanic form resulting from explosive interaction with subsurface surface water was probably a subaqueous collapsed tuff cone this upper part of the system is synchronous with an ashgill carbonate sedimentation which overlies an ordovician siliciclastic succession deposited in shelf environments

pronounced central uplift identified in the bosumtwi impact structure ghana using multichannel seismic reflection data the lake bosumtwi impact structure is the youngest and best preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform no significant backwash processes have modified the crater morphology not only may bosumtwi contain the best preserved central uplift structure on earth but it is the most accessible relatively large young crater in the solar system generated in a large gravity field there is a well established link between the lake bosumtwi impact structure and the ivory coast tektite field and the lacustrine sediments within the crater contain a unique 1 m y record of paleoclimate in the continental tropics south of the sahel eight profiles of marine type multichannel seismic reflection mcs data were acquired from the 8 km diameter similar to75 m deep lake that fills much of the crater these were augmented by wide angle seismic data acquired with ocean bottom hydrophones mcs data reveal a well defined central uplift near the northwest central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m the central uplift structure has a diameter of 1 9 km and a maximum height of 130 m above the annular moat inside the crater an intermediate velocity layer 3200 m s beneath the lacustrine sediment is interpreted as fallback breccia or a breccia melt horizon the measured apparent depth of the crater d a is 500 m implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships the bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws perhaps because of the effects of a large gravity field

first report of pan african sm nd and rb sr mineral isochron ages from regional charnockites of southern india mineral and whole rock isotope data for a massive charnockite from kottaram in the nagercoil block at the southern tip of peninsular india yield sm nd and rb sr ages of 517 26 ma and 484 15 ma respectively the nd model age calculated for the charnockite is c 2100 ma our study reports the first pan african mineral isochron ages from regional charnockites of peninsular india which are in good agreement with the recently obtained ages of incipient charnockites in the adjacent blocks as well as alkaline plutons within the same block our results indicate that the pan african tectonothermal event in the granulite blocks south of the palghat cauvery shear zone was regional with terrain wide rejuvenation these results correlate with similar pan african tectono thermal events reported from sri lanka and east antarctica and have an important bearing on gondwana reconstructions

paramagnetic metamorphic mineral assemblages controlling ams in low grade deformed metasediments and the implications with respect to the use of ams as a strain marker a regional analysis of the anisotropy of magnetic susceptibility ams has been performed on low grade metamorphic deformed homogeneous siltstone beds hsbs of the plougastel formation in the central armorican domain together with exhaustive compositional analyses of the studied specimens despite sampling a single horizon different paramagnetic minerals are controlling the ams in the crozon peninsula sites white mica and chlorite and in the inland sites of the central armorican domain white mica chloritoid and some chlorite both the crozon peninsula and inland datasets show a hockey stick shaped pattern on a plot of the shape parameter t versus the corrected degree of anisotropy p j although the whole pattern is shifted to higher p j values for the inland dataset high field ams indicates that the low field p j and t values from two inland sites are slightly enhanced by a small ferromagnetic sensu lato contribution furthermore variation in p j and t values within single sites can be attributed to an effect of the observed quartz white mica ratio as quartz grains disrupt the petrofabric intensity our findings clearly demonstrate that the ams of the hsbs in the central armorican domain is strongly influenced by compositional variations and does not merely indicate tectonic strain even in very similar tectonostratigraphic settings supplementary material outcrop coordinates with information on the hsbs and samples and the summarized low field room temperature ams data are available at www geolsoc org uk sup18578

sudoite a rock forming mineral in verrucano of the northern apennines italy and the sudoite chloritoid pyrophyllite assemblage in prograde metamorphism sudoite the di trioctahedral chlorite with ideal composition mg2al al2 si3al o10 oh 8 is a widespread rock forming mineral in meta siltstones and psammites of the verrucano sequence of the northern apennines sub ellipsoidal aggregates probably derived from muscovite clasts consisting of sudoite pyrophyllite and muscovite are common sudoite may also occur as thin blades in the rock matrix the co existence of sudoite ferich chloritoid and pyrophyllite reported here for the first time has been observed in specimens from the m argentario and monticiano roccastrada areas this three phase assemblage diagnostic of a specific metamorphic facies may be a tool for detailed zonation of low grade terranes

towards an improved seismic imaging technique for crustal structures the lithoprobe sudbury experiment under the canadian lithoprobe program conventional low 10 55 hz and high frequency 30 140 hz vibroseis reflection surveys were conducted across the sudbury structure the main objective of the study was to evaluate the performance of seismic exploration techniques in a complex mainly crystalline geological setting the sudbury experiment demonstrated that high input frequencies can be preserved through appropriate acquisition and processing procedures consequently the high frequency seismic data proved capable of imaging complex shallow crustal structures not seen by the conventional low frequency seismic survey in crystalline environments improved high frequency seismic exploration technology will provide a better link between seismic images and surface geology

the crust as a heterogeneous optical medium or crocodiles in the mist abstract based on petrophysical data geologic maps and a well log we present statistical descriptions of likely upper middle and lower crustal rocks to characterize the fine scale heterogeneity observed in crustal exposures and inferred from deep crustal seismic data the statistical models developed for granitic and metamorphic upper crust and for an extended metamorphic lower crust are used to construct whole crustal models of seismic velocity heterogenity we present finite difference synthetic cmp data from several models which compare favorably with field data the statistical models also permit classification of the seismic reflection experiment and the crustal heterogeneity according to scattering regime the optical or scattering properties of importance for classification are the velocity fluctuation intensity the horizontal and vertical correlation lengths of the medium the correlation function of the medium and the velocity population function for the crustal properties we measured the bandwidth of a typical deep crustal experiment overlaps from the weak to the strong scattering regime with implications for crustal seismic data processing and imaging notably deep crustal signals are likely to have experienced multiple scattering making common seismic imaging techniques of questionable value moreover the details of the unmigrated cmp stacked section bears little resemblance to the underlying medium

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

upper crustal seismic velocity heterogeneity as derived from a variety of p wave sonic logs sonic log measurements provide detailed 1 d information on the distribution of elastic properties within the upper crystalline crust at scales from about one metre to several kilometres 10 p wave sonic logs from six upper crustal drill sites in europe and north america have been analysed for their second order statistics the penetrated lithological sequences comprise archean volcanic sequences proterozoic mafic layered intrusions and precambrian to phanerozoic gneisses and granites despite this variability in geological setting tectonic history and petrological composition there are notable similarities between the various data sets after removing a large scale deterministic component from the observed velocity depth function the residual velocity fluctuations of all data sets can be described by autocovariance functions corresponding to band limited self affine stochastic processes with quasi gaussian probability density functions depending on the maximum spatial wavelength present in the stochastic part of the data the deterministic trend can be approximated either by a low order polynomial best fit or by a moving average of the original sonic log data the choice of the trend has a significant impact on the correlation length and on the standard deviation of the residual stochastic component but does not affect the hurst number for trends defined by low order polynomial best fits correlation lengths were found to range from 60 to 160 m whereas for trends defined by a moving average the correlation lengths are dominated by the upper cut off wavenumber of the corresponding filter regardless of the trend removed the autocovariance functions of all data sets are characterised by low hurst numbers of around 0 1 0 2 or equivalently by power spectra decaying as 1 k a possible explanation of this statistical uniformity is that sonic log fluctuations are more sensitive to the physical state in particular to the distribution of cracks than to the petrological composition of the probed rocks

relationships between strain and quartz crystallographic fabrics in the roche maurice quartzites of plougastel western brittany abstract an integrated microstructural and petrofabric study of the plastically deformed and partially recrystallized roche maurice quartzites of plougastel western brittany has revealed a clear correlation between the pattern of c axis fabrics displayed by detrital quartz grains and the symmetry of the calculated strain ellipsoid in specimens with flattening k 0 strains c axes lie on a small circle girdle opening angle 28 42 centred about the principal finite shortening direction z for specimens that exhibit approximate plane strain k 1 cross girdle c axis fabrics consisting of a small circle girdle centred about z and connected through the intermediate principal extension direction y were detected within individual specimens c axis fabrics of syntectonically recrystallized new quartz grains within the matrix are similar to those of detrital quartz grains c axes of new grains located within the relatively undeformed sections of the host detrital grains are commonly orientated at angles between 10 and 40 to the host c axis and are in addition statistically orientated at a higher angle to z than their host c axes these relationships are interpreted as indicating that both host grain control and the local strain and or stress field may have influenced the process of recrystallization the relative influence of these factors is however unknown microstructural and petrofabric studies indicate that the roche maurice quartzites have been subjected to essentially coaxial strain histories the role of syntectonic recrystallization in facilitating continued plastic deformation in quartzites subjected to such strain histories is considered

onaping fault system age constraints on deformation of the kapuskasing structural zone and units underlying the sudbury structure the deformation of the matachewan dyke swarm of the superior province which includes a 60 km right lateral offset and moderate uplift localized along the kapuskasing structural zone represents an

metamorphism of mineral matter in coal from the bukit asam deposit south sumatra indonesia abstract the coal of the miocene bukit asam deposit in south sumatra is mostly sub bituminous in rank consistent with regional trends due to burial processes however effects associated with plio pleistocene igneous intrusions have produced coal with vitrinite reflectance up to at least 4 17 anthracite in different parts of the deposit the un metamorphosed to slightly metamorphosed coals with rvmax values of 0 45 0 65 contain a mineral assemblage made up almost entirely of well ordered kaolinite and quartz the more strongly heat affected coals with rvmax values of more than 1 0 are dominated by irregularly and regularly interstratified illite smectite poorly crystallized kaolinite and paragonite na mica with chlorite in some of the anthracite materials kaolinite is abundant in the partings of the lower rank coals but is absent from the partings in the higher rank areas even at similar horizons in the same coal seam regularly interstratified illite smectite which is totally absent from the partings in the lower rank coals dominates the mineralogy in the partings associated with the higher rank coal beds a number of reactions involving the alteration of silicate minerals appear to have occurred in both the coal and the associated non coal lithologies during the thermal metamorphism generated by the intrusions the most prominent involve the disappearance of kaolinite the appearance of irregularly interstratified illite smectite and the formation of regular i s paragonite and chlorite although regular i s is identified in all of the non coal partings associated with the higher rank coals illite smectite with an ordered structure is only recognised in the coal samples collected from near the bases of the seams the i s in the coal samples adjacent to the floor of the highest rank seam also appears to have a greater proportion of illitic components the availability of sodium and other non mineral inorganic elements in the original coal to interact with the kaolinite under different thermal and geochemical conditions appears to be the significant factor in the formation of these new minerals and distinguishes the mineralogical changes at bukit asam from those developed more generally with rank increases due to burial and from the effects of intrusions into coals that were already at higher rank levels

calibration and testing of an empirical chloritoid chlorite mg fe exchange thermometer and thermodynamic data for daphnite a compilation of literature data on the fe mg composition of coexisting chlorite and chloritoid from metapelites metamorphosed at various p t conditions shows that the logarithm of the fe mg partitioning lnkd varies linearly with the inverse of temperature from about 2 4 at 300 c to about 1 3 at 600 c in contrast no trend was observed with pressure and the molar volumes of mg and fe chlorite end members suggest that the pressure dependence of lnkd is not significant therefore the chloritoid chlorite mg fe exchange reaction is a potential thermometer and has been empirically calibrated using the analyses of 112 chloritoid chlorite pairs from 28 different localities temperatures estimated using the chl cld thermometer were checked against independent estimates for 20 samples not involved in the calibration beni mzala window morocco and the results are in fair agreement with independent temperature estimates however the analytical uncertainties and errors are too large to obtain reliable temperature estimates for extremely mg rich or fe rich compositions the chl cld thermometer is unreliable at xmg cld 0 8 at 700 c and xmg cld 0 9 at 300 c using the results of the empirical calibration we calculated new thermodynamic data for daphnite implementing these data it becomes possible to estimate t and p conditions of metamorphism for the invariant chlorite chloritoid quartz aluminosilicate assemblage that is widespread in low grade metapelites these estimates appear to be relevant only in the stability field of kyanite whereas the uncertainties on the calculated pressure conditions are very large in the stability field of kaolinite and pyrophyllite

the mineralogy and metamorphic geology of low grade metasediments northern range trinidad forty one low grade metasediments from the northern range of trinidad and from the neighbouring island of tobago west indies were investigated by microscopic x ray powder diffraction guinier camera technique and electron microprobe methods the following minerals were encountered in order of decreasing abundance quartz muscovite chlorite paragonite muscovite mixed layer paragonite organic material calcite pyrophyllite dolomite albite hematite chloritoid pyrite rutile titanite rectorite stilpnomelane the chemical composition of muscovite is dependent on the mineral assemblage with si 6 1 6 2 per formula unit in two high alumina assemblages versus si 6 6 in a low alumina assemblage microprobe analyses indicate that muscovite paragonite and pyrophyllite are sometimes intergrown on a micrometre scale chlorite composition estimated from x ray data shows good correspondence with microprobe data for two al poor samples but is less satisfactory for an al rich sample observed mineral assemblages in akna are indicative of the pyrophyllite paragonite albite fades for the first time chemical data for coexisting pyrophyllite chlorite m mgo mgo feo 0 377 and chloritoid m 0 110 are given this assemblage is indicative of the low or medium pressure range of the lower greenschist facies pressure conditions during metamorphism are not well constrained but a minimum pressure of 2 kbar corresponding to 5 6 km of overburden can be estimated from the na m4 content of actinolite temperatures derived from the calcite dolomite geothermometer range between 300 and 350 c and are consistent with the presence of the assemblage pyrophyllite calcite quartz

a stochastic view of lower crustal fabric based on evidence from the ivrea zone despite its complicated history the ivrea zone is considered to be a representative surface exposure of extended continental crust we have digitized two standard 1 25 000 geological maps from this area and evaluated their structural statistics because of the subvertical orientation of the ivrea zone these maps can be considered as small scale cross sections through the lower continental crust the autocorrelation functions of the digitized lithologies measured from these maps show a clear self similar or fractal rather than a gaussian or deterministic trend we found that an anisotropic von karman correlation function with an aspect ratio around 4 and a hurst number of 0 3 corresponding to a fractal dimension of 2 7 matches the observed data our results represent an explicit confirmation of previous indirect evidence for the fractal nature of lithospheric heterogeneities and provide the means to construct realistic crustal scale seismic models of ivrea type lower continental crust