magnetic fabric and microstructural evidence for a tectono thermal overprint of the early proterozoic murray pluton central ontario canada abstract the 2 38 ga old murray granite pluton intruded the southern footwall of the sudbury igneous complex and was subjected to repeated ductile deformation its nw and se domains display differences in strain state ams signature and the nature and distribution of fe oxide minerals the magnetic foliation is parallel to the visible foliation in the nw domain but discordant to it in the se domain microstructural data show that the magnetic fabric in the nw domain coincides with a foliation formed at high temperature probably in response to thermal softening during the emplacement of the adjacent sudbury igneous complex this tectono thermal overprint affected pre existing shape fabrics and was facilitated by incipient melting of the granite near the intrusive contact with the sudbury igneous complex rocks of the se domain are only partially overprinted and two mineral subfabrics are preserved prolate susceptibility ellipsoids and clusters of maximum susceptibility directions at the intersections of the two planar fabrics suggest oblique superposition of strain increments increases in bulk magnetic susceptibility and magnetic anisotropy degree towards the intrusive contact with the sudbury igneous complex can be explained by respective increases in the proportion of magnetite over ilmenite and strain intensity in the murray pluton

magnetic versus crystallographic fabrics in a basaltic lava flow combined anisotropy of magnetic susceptibility ams and crystallographic studies were performed within a quaternary lava flow for which we have a thorough knowledge of the flow direction microscope examinations microprobe analyses and studies of rock magnetic properties show that ams is carried by homogeneous multidomain ti rich titanomagnetite xc0 6 this mineral forming either subhedral isolate individuals or grouped in various elongated aggregates represents a late crystallized interstitial phase ams determined from samples collected in the lower the middle and the upper part of the flow is characterized by a well defined magnetic foliation whereas magnetic lineation is generally poorly grouped and therefore meaningless the foliation plane of the lower and upper part of the flow show an obliquity which is interpreted as an opposite imbrication indicating a westward flow in agreements with the known flow direction in the lower part of the lava flow the tight correlation between ams and plagioclase lattice preferred orientation suggests that a silicate framework control the titanomagnetite crystallization and thus the orientation of the ams ellipsoid d 2005 elsevier b v all rights reserved

magnetic susceptibility anisotropy as a predictive exploration tool of metasomatic iron oxide deposits example from the panj kuh iron ore body ne iran abstract the panj kuh iron deposit hosted by late eocene igneous rocks in north of central iran is associated with a syenitic intrusive body magnetic fabric investigations indicate that the panj kuh pluton was emplaced within a sinistral shear zone corridors of sheared and metasomatized rocks show that the shear zone was active after emplacement of the pluton and in the final steps of crystallization the sheared rocks allowed for circulation of hydrothermal fluids during cooling of the magma and caused the pervasive na ca alteration the ferrimagnetic phase in fresh syenite is mostly euhedral or subhedral magnetite and is observed adjacent to or as inclusion in biotite and pyroxene anisotropy of magnetic susceptibility ams in the fresh syenite is due to preferred shape orientation of magnetite grains in na ca altered syenites magnetite has reduced or has been totally absorbed the alteration results in decreased bulk susceptibilities km and higher anisotropy degrees p accompanied by changes in the shape of magnetic susceptibility anisotropy ellipsoid this study of the panj kuh pluton suggests that ams can be used for mapping lithology and hydrothermal alteration types since various types of hydrothermal alteration create transform or destroy ferrimagnetic minerals in a mineralized system susceptibility measurements aid mapping of igneous rock type alteration zoning and mineralization integrating petrography magnetic mineralogy bulk magnetic susceptibility and preferred orientations of principal axes of the ams in fresh and altered rocks can be used to generate predictive magnetic exploration models for metasomatic iron oxide deposits

using frequency dependent avo inversion to predict the sweet spots of shale gas reservoirs abstract shale gas is an unconventional hydrocarbon resource with huge potential using geophysical methods to predict sweet spots is the key to commercial exploration of shale gas at present avo technology is one of most widely used geophysical techniques to find shale gas reservoirs with high abundance however the effects of the dispersion of actual mediums are neglected by conventional avo inversion recent experiments have shown that in gas bearing strata the dispersion may be rather remarkable in this paper we develop a frequency dependent avo inversion scheme which includes seismic dispersion in real rock medium to predict the sweet spots of shale gas reservoirs application of the inversion has been made to the shale gas in the south of jiaoshiba area to test the validity comparing the results of the frequency dependent avo inversion and well production we find that regions with high dispersion gradients correlate well with places with high production of shale gas therefore we conclude that the frequency dependent avo may be a workable approach to detect the sweet spots of shale gas reservoirs

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

magnetic susceptibility magnetic mineralogy and magnetic fabrics in a late archean granitoid gneiss belt abstract the opatica belt is a late archean granitoid gneiss terrane situated within the northern abitibi subprovince along the northern margin of the abitibi greenstone belt quebec this belt is composed of several suites of tonalite granodiorite granite composition variably deformed under epidote amphibolite to upper amphibolite grade conditions specimens from 87 sampling sites within the opatica belt show a wide range in variation of magnetic susceptibility k the large variations are due essentially to varying proportions of ferromagnetic trace minerals detailed study of the opaque mineralogy by reflected light petrography electron microprobe analysis and sem observations as well as alternating field and thermal demagnetization experiments confirm that multi domain very pure magnetite is the dominant ferromagnetic phase which along with ferrosilicates contribute to the magnetic susceptibility in some cases hematite may also make a significant contribution to k in rocks with low k the susceptibility may be dominated by the ferrosilicates hornblende and biotite intensity of anisotropy of magnetic susceptibility ams is also highly variable and appears to be controlled by the mineralogy of the rock rather than by strain intensity or deformation state the shapes of ams ellipsoids are also controlled by mineralogy detailed comparisons are made of ams fabrics with field measurements of magmatic and metamorphic foliations and mineral and stretching lineations and with mineral lineations determined by image analysis of polished slabs there is very good agreement between the average orientations of poles to structural foliations and the average orientation of the k3 principal axes of the susceptibility ellipsoids in all outcrops in most cases there is also good agreement between average structural lineations and the average k1 axis in quartz monzodiorites disagreement between the average lineation and average k1 may be due to complex textural occurrences of magnetite and the presence of abundant hematite in gneisses broad scatter of the orientations of k1 for individual samples from an outcrop quantified by statistical confidence regions about the principal axes locally occurs within the foliation plane and may be attributed to the partial overprinting of earlier fabrics by subsequent deformation

early jurassic schizosphaerellid crisis in cantabria spain implications for calcification rates and phytoplankton evolution across the toarcian oceanic anoxic event 1 the toarcian oceanic anoxic event 183 myr ago represents a global perturbation marked by increasing organic carbon burial and a general decrease in calcium carbonate production likely triggered by elevated carbon dioxide levels in the atmosphere here we present quantitative analyses of calcareous nannofossil diversity and abundance from the castillo de pedroso section in cantabria northern spain we compare these data with geochemical data c and o isotopes obtained from biogenic and bulk carbonate records in order to highlight the response of calcareous phytoplankton to major climatic and paleoceanographic changes the pliensbachian toarcian boundary is characterized by an abrupt decrease in abundance of schizosphaerella punctulata the most important lithogenic contributor to hemi pelagic carbonates in the early jurassic the early toarcian nannofloral assemblages show an increase in abundance of mitrolithus jansae and small sized r selected taxa and a progressive decrease in s punctulata percentages the deep dwellers m jansae and s punctulata experienced a major crisis slightly prior to the deposition of the toarcian black shales that are characterized by high abundances of eutrophic taxa such as lotharingius spp and biscutum spp the return of s punctulata associated with lower percentages of eutrophic taxa was observed just above the toarcian black shales the toarcian episode reveals that high co2 levels and increasing primary productivity probably triggered a shift in abundance from highly calcified nannoliths such as s punctulata and m jansae to small sized r selected coccoliths that overall record a biocalcification crisis at the onset and during the toarcian episode

calcareous nannofossils and milankovitch cycles the example of the albian gault clay formation southern england a quantitative study of calcareous nannofossil assemblages was carried out on the albian gault clay formation england this unit consists of grey marly claystones and marlstones with phosphatic nodules and glauconitic layers as minor lithologies two cores were analyzed in detail to focus on changes in nannofloral assemblages and consider their significance preservation of calcareous nannofossils is excellent and therefore diagenesis cannot be invoked to explain not even partially the changes registered in assemblages fast fourier spectral analysis was performed on percentages of selected nannofossil species fluctuations in abundance of both biscutum constans and watznaueria barnesae regarded as fertility and non fertility indices respectively appear to be orbitally driven because spectral analysis showed periodicities of approximately 41 kyr obliquity cycle and 100 kyr short eccentricity cycle in all the spectra the axial obliquityc cycle is a strong signal as expected at these high latitude sites from the boreal realm the same pattern was observed in another fertility controlled taxon zygodiscus cf erectus but cross correlation shows that its abundance plot is displaced with respect to that of b constans orbital perturbations induce variations in climate and ocean circulation because they influence the earth s insolation and seasonality during times of high seasonality strong gradients cause vigorous oceanic circulation a better cycling of nutrients and higher fertility whereas during times of low seasonality low gradients induce more sluggish circulation and lower fertility the carbonate content of the gault clay cores is not correlatable to the higher fertility indices as the abundance curves of b constans w barnesae and z cf erectus are displaced with respect to caco3 content variations this also means that cyclicity in nannofossil assemblages is not a result of diagenesis but an original signal abundance fluctuations of repagulum parvidentatum and combined parhabdolithus asper p splendens colder and warmer water indicators respectively provide evidence for orbital periodicities suggesting that also cyclic variations of surface water temperature might have influenced the nannofossil assemblage patterns within the gault clay

calcareous nannofossil changes during the late callovian early oxfordian cooling phase calcareous nannofossil quantitative and biostratigraphic analyses integrated with carbon and oxygen stable isotopes were carried out on the core andra agence nationale pour la gestion des dechets radio actifs france htm 102 across the callovian oxfordian boundary drilled at cirfontaines en ornois department de haute marne eastern france calcareous nannofossil assemblages at the callovian oxfordian transition are dominated by the genus watznaueria an increase in abundance of biscutum spp and a group which consists of axopodorhabdus spp a atavus a rahla and a cylindratus podorhabdus grassei octopodorhabdus decussatus hexapodorhabdus cuvillieri family axopodorhabdaceae and triscutum spp correlates with a significantpositiveexcursionin 18 osuggestingthatthesegroupswereprobablyadaptedtocoolersurfacewaters apositiveincrease in 13 c values is coupled with high abundances of eutrophic taxa such as zeughrabdotus erectus biscutum spp and small sized watznaueria britannica and a decrease in abundance of the big and oligotrophic taxa schizosphaerella punctulata and watznaueria manivitae climate cooling across the callovian oxfordian boundary probably triggered a breakdown in stratification of surface waters leading to more intense nutrient recycling and higher primary productivity that favoured the shift in abundance of small sized eutrophic taxa in the east paris basin

late quaternary nannofossil indicators of climate change in two deep sea cores associated with the leeuwin current off western australia the stratigraphic variations in nannoflora observed in the two deep sea cores recovered from the offshore western australia revealed a sequence of paleoceanographic changes for the last 250 kyr the dominance of small placolith taxa in the upper photic flora indicates continuous mild upwelling for the isotope stages 7 5 in this region the most important controlling factor for the abundance variation of florisphaera profunda in this region is considered to be the lower photic temperature rather than the stability of water column or the water turbidity the significant reduction in the abundance of f profunda combined with the stable in the northern core rs96gc21 and reduced in the southern core rs9 150 abundance of the small taxa suggest an intensified upwelling for the penultimate glaciation the sharp increase in f profunda abundance indicates a weakening of upwelling at the last interglacial climax lic offshore western australia after isotope stage 5 the existence of upwelling is not clear except in stage 2 when it was not strong enough to produce blooms of emiliania huxleyi among the subordinate taxa calcidiscus leptoporus calciosolenia murrayi neosphaera coccolithomorpha and oolithotus fragilis provide meaningful paleoceanographic signals supporting the conclusions of previous studies a weakening or cessation of leeuwin current for the glacial periods the shift of dominance from small gephyrocapsa mostly g ericsonii to small reticulofenestra mostly r parvula can be an useful datum event offshore western australia the reduced diversity of nannoflora during the interglacial periods can be explained by a lessened competition within nannoplankton community caused by particularly favorable conditions for the opportunists such as e huxleyi f profunda and small placoliths the variations in nannofossil abundance suggests an increased aeolian flux at the stages 6 and 2 offshore north west cape as well as at stage 6 offshore southwestern australia

magnetic anisotropy and fabric of some progressively deformed ophiolitic gabbros the ophiolites considered remnants of oceanic seafloor provide a fruitful source of information on the physical and chemical behavior of the deeper uncored lithosphere a study of the gabbros in a rather well preserved ophiolite from the western alps montgenevre france shows strong evidence for intraoceanic plate deformation to characterize the different steps of deformation observed in the outcrops a magnetic fabric study was conducted using the anisotropy of initial magnetic susceptibility aims method the main conclusions are 1 the aims is controlled by the paramagnetic ferromagnesian minerals such as hornblende resulting from the ocean floor metamorphism close to the spreading ridge partial amphibolitization and 2 the magnetic lineation and foliation of the gabbros show a deformation path with increasing strain a good correspondence between petrofabric and magnetic anisotropies is observed both suggest that the deformation mechanism was simple shear

magnetic fabric constraints on the initial geometry of the sudbury igneous complex a folded sheet or a basin shaped igneous body abstract interpreted either as a folded igneous sheet or a basin shaped intrusion the origin of the palaeoproterozoic sudbury igneous complex sic ontario canada has been debated by geologists for over a century most recent proposals view the sic to have been emplaced as a horizontal impact melt and subsequently folded after solidification this requires the sic to have folded from an originally horizontal sheet to the present basinal geometry with dips of 30 in the north range and 70 in the east range the fold scenario would require the sharply curved juncture between the north and east ranges north lobe to be deformed in the solid state due to folding in order to test the fold model the rock fabric was investigated in the east and north ranges and the north lobe of sic at 256 sites with the aid of anisotropy of magnetic susceptibility ams a sampling transect through an undeformed portion of the sic in the western north range reveals that the gabbro norite exhibits sic contact parallel magnetic foliation and down dip magnetic lineation both of which respectively correlate to an igneous foliation and lineation the granophyre in contrast features spatially consistent magnetic lineation that is oriented orthogonal to the sic contact this magnetic lineation can be correlated to a mineral fabric characterised by aligned acicular crystals of plagioclase and is interpreted to be a wall orthogonal crystallisation texture the east and north ranges in the eastern sic reveal similar patterns of ams fabrics the magnetic lineation of the granophyre is orthogonal to the basal contact of the sic in the east and north ranges and in the fold like north lobe between the two ranges an axial planar magnetic foliation is developed in the granophyre as the north lobe is approached but the magnetic lineation is nevertheless contact orthogonal and well defined this ams fabric is interpreted to be a combination of an original igneous contact orthogonal lineation and a foliation resulting from tectonic overprint the preservation of the contact orthogonal magnetic lineation implies low levels of tectonic overprint conservative shortening strains estimated from ams numerical modelling suggest

geophysical and structural signatures of syntectonic batholith construction the south mountain batholith meguma terrane nova scotia summary the late devonian south mountain batholith is a very large 7000 km2 composite peraluminous granitoid complex situated within the meguma terrane of the northern appalachians it is made up of two suites of granodioritic to leucogranitic plutons emplaced at approximately 380 370 ma during the acadian orogeny i e during the collision of gondwana with the eastern margin of north america a significant geophysical and geological database makes the south mountain batholith a type example of a very large syntectonic batholith emplaced within a collisional orogen gravity models reveal the plutons have flat or gently dipping floors at approximately 7 0 km depth and aspect ratios 651 they are underlain by deeper 10 km elongate northeast southwest trending roots that may indicate magma feeder zones dyke transport of granitic magma and the progressive construction of plutons by sheet injections are supported by field observations and by mapping of the anisotropy of magnetic susceptibility at the pluton scale the very narrow deformation aureole within the country rocks suggests lateral spreading of the plutons was not the main space creation mechanism during emplacement space was mostly created by vertical displacements of country rocks the data are consistent with a laccolithic model for syntectonic batholith assembly the laccolithic plutons may have been emplaced at the base of the meguma supergroup metasedimentary rocks suggesting a maximum thickness of approximately 7 0 km for the supracrustal rocks in the meguma terrane

avo investigations of shallow marine sediments amplitude variation with offset avo analysis is based on the zoeppritz equations which enable the computation of reflection and transmission coefficients as a function of offset or angle of incidence high frequency up to 700 hz avo studies presented here have been used to determine the physical properties of sediments in a shallow marine environment 20 m water depth the properties that can be constrained are p and s wave velocities bulk density and acoustic attenuation the use of higher frequencies requires special analysis including careful geometry and source and receiver directivity corrections in the past marine sediments have been modelled as elastic materials however viscoelastic models which include absorption are more realistic at angles of incidence greater than 40 degrees avo functions derived from viscoelastic models differ from those with purely elastic properties in the absence of a critical angle of incidence the influence of s wave velocity on the reflection coefficient is small especially for low s wave velocities encountered at the sea floor thus it is difficult to extract the s wave parameter from avo trends on the other hand p wave velocity and density show a considerably stronger effect attenuation described by the quality factor q influences the reflection coefficient but could not be determined uniquely from the avo functions in order to measure the reflection coefficient in a seismogram the amplitudes of the direct wave and the sea floor reflection in a common midpoint cmp gather are determined and corrected for spherical divergence as well as source and streamer directivity at cmp locations showing the different avo characteristics of a mud and a boulder clay the sediment physical properties are determined by using a sequential quadratic programming sqp inversion technique the inverted sediment physical properties for the mud are p wave velocity alpha 1450 25 m s s wave velocity beta 90 35 m s density rho 1220 45 kg m 3 quality factor for p wave qp 15 200 quality factor for s wave qs 10 30 the inverted sediment physical properties for the boulder clay are alpha 1620 45 m s beta 360 200 m s rho 1380 85 kg m 3 qp 790 660 qs 25 10

deformation of the archean quetico shebandowan subprovince boundary in the canadian shield near kashabowie northern ontario the southern boundary of the quetico metasedimentary subprovince of the superior province of the canadian shield near kashabowie ontario is a vertical east west feature affected by dextral transpression that had a north northwest south southeast compressional component a synmetamorphic locally d1 microfabric and magnetic susceptibility fabric with an east directed extension lineation is kinematically compatible with this pattern it shows the same bedding cleavage relationship and the same direction of structural facing on d1 in both the quetico metasediments and the shebandowan greenstone subprovince on the south side of the quetico subprovince in the low grade rocks of the study area there is a single phase of penetrative deformation giving a nearly vertical schistosity at a consistent angle anticlockwise with respect to the now nearly vertical east west striking strata the absence of penetrative polyphase deformations may be due to the near parallelism of the subprovince boundary with the s

fabric studies within the cascade lake shear zone sierra nevada california abstract the cascade lake shear zone occurs on the eastern margin of the tuolumne intrusive suite sierra nevada batholith california foliation in the zone is nnw trending and subvertical and lineation is moderately south plunging deformation is syn tectonic with emplacement of the cathedral peak granodiorite a deformation gradient exists toward the ne margin of this pluton with higher strains and lower temperatures of deformation found near the contact we compare fabric data collected very densely in this shear zone using several techniques field fabrics 3d orientation of k feldspar megacrysts and ams anisotropy of magnetic susceptibility analysis in general the results from the three different methods are in agreement deformation in this shear zone is part of a larger pattern of deformation within the cathedral peak granodiorite as recorded by ams analysis and dextral shearing associated within the last stage of plutonism within the sierra nevada magmatic arc

anisotropy of magnetic susceptibility and magmatic structures in the guerande granite massif france a multidisciplinary approach has been used in order to map out the syntectonic magmatic structures of the guerande leucogranite in southern brittany western france on one hand extensive field work and microscopic observations and on the other a detailed magnetofabric investigation have been carried out the former showed a magmatic flow plane trending generally to the ene wsw with a moderate dip to the north and a flow direction slightly dipping toward nne these observations suggest a blade shaped body which is not deeply rooted the magnetofabric as defined by the anisotropy of magnetic susceptibility is mainly related to hematite ilmenite grains with a mean rock susceptibility of about 3 10 6 g oe there is an excellent agreement between the magnetic lineation kmax and the magmatic flow direction the agreement between the magnetic foliation kmax kint plane and the magmatic flow plane is not as good the quantitative analyses of the anisotropy allows one to group the data into two domains the one with planolinear ellipsoids corresponding to the central part of the granitic body where flow was mainly achieved on the viscous state and the other in the flattening domain related to the northern border area which exhibits incipient orthogneissification that is some deformation on the solid state such a favorable comparison with the classical structural method is encouraging for using the anisotropy of magnetic susceptibility technique more routinely for structural mapping of granitic bodies provided that the microstructural state of the rock is already well known

pliensbachian early jurassic calcareous nannofossils from the peniche section lusitanian basin portugal a clue for palaeoenvironmental reconstructions abstract quantitative analysis of early jurassic calcareous nannofossil assemblages from the peniche section in portugal have been performed in order to interpret palaeoenvironmental changes occurring in the lusitanian basin during the late pliensbachian davoei margaritatus and spinatum ammonite zones nannofossil data are compared to already published carbon and oxygen stable isotope data organic matter content wt total organic carbon toc and biomarker analysis a significant change in calcareous nannofossil assemblages and species diversity at the transition between the margaritatus and spinatum ammonite zones matches with the pattern shown by geochemical data this suggests that a profound change in environmental conditions occurred at that time in the davoei and margaritatus ammonite zones in a context of general sea level rise the lusitanian basin was characterized by water column stratification that favoured the sedimentation and preservation of organic matter biomarkers and oxygen isotope trends suggest that stratification of water masses occurred because of low salinity in surface waters the shallowest part of the water column characterized by oligotrophic conditions was inhabited by the probable calcareous dinocyst schizosphaerella spp while the deep dweller crepidolithus crassus flourished in the lower photic zone layers in the spinatum ammonite zone a regressive trend occurred and a salinity increase is inferred on the basis of oxygen isotope values water masses were probably less stratified at that time the upper photic zone nannofossil assemblages were still dominated by schizosphaerella spp whilst in the deep photic zone mitrolithus jansae a mediterranean taxon replaced c crassus a taxon with nw european affinity this pattern may indicate a change in palaeoceanographic conditions related to surface current circulation the sea level fall occurring during the spinatum ammonite zone may have resulted in the partial isolation of the lusitanian basin from the nw europe basins because of the creation of a threshold alternatively tectonic drowning of north african carbonate platforms in the late pliensbachian may have promoted better exchanges of nannoplankton between the lusitanian basin and the mediterranean tethys via the creation of new seaway connections

magnetic susceptibility and ams of the bushveld alkaline granites south africa the bushveld complex in south africa includes one of the world s largest anorogenic alkaline granite intrusions 66 000 km2 the granite forms a composite laccolith of 350 250 km in area and about 2 km in thickness which was emplaced at about 5 km depth into sediments overlying the kaapvaal craton at 2054 ma the bushveld granite and its roof rocks have long been mined for sn w and f the bushveld granites have high magnetic susceptibilities km from 1000 to 4000 si and a quantitative model is presented suggesting that susceptibility fabrics are primarily carried by ferromagnetic minerals the measured ams foliations coincide with observed subhorizontal mineral lineations and compositional layering magnetic lineation trends vary considerably within the horizontal plane the existence of a weak planar fabric and an almost absent linear component may reflect a laccolithic emplacement by roof uplift causing flattening magmatic fabrics or b emplacement of largely crystal free magma crystallizing in situ and developing horizontal compositional layering from thermal chemical diffusion fronts and gravity driven mechanisms weak magnetic fabrics like those identified in the bushveld granites require specific sampling schemes and procedures in addition to rigorous constraint of magnetic mineralogy and crystallization sequence

deep crustal structure and tectonic history of the northern kapuskasing uplift of ontario an integrated petrological geophysical study the northeast trending kapuskasing uplift transects the east west belts of the central superior province over a distance of some 500 km granulite to upper amphibolite facies rocks of the uplift form three distinct geological geophysical entities from south to north the chapleau groundhog river and fraserdale moosonee blocks uplift of the granulites along a moderately northwest dipping crustal scale thrust fault is attributed to an early proterozoic compressional event major northeast striking faults that bound the kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet orthopyroxene plagioclase quartz assemblages to determine vertical displacement granulites in the kapuskasing zone have 7 to 9 kbar signatures whereas those in the quetico belt to the west indicate metamorphic pressure of 4 6 kbar individual calibrations of the barometer yield consistent pressure differences of 2 3 kbar suggesting 7 10 km of west side down movement on the faults modelling of gravity and aeromagnetic gradients indicates westerly dips of 60 65 with west side down offset of up to 14 km these major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage differences in the configuration of individual blocks of the kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults thus the groundhog river and southern fraserdale moosonee blocks are perched thrust tips analogous to the sangre de cristo range of the laramide uplift province whereas the southern chapleau block is a tilted slab with similar configuration to the laramide wind river range pop up geometry deduced for the northern fraserdale moosonee block resembles the structure of the laramide uinta mountains a normal fault crosses the surface trace of the basal thrust fault between the groundhog river and fraserdale moosonee blocks and causes a 65 km gap without granulites this article contains supplementary material

evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

shape anisotropy versus magnetic interactions of magnetite grains experiments and application to ams in granitic rocks the magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains measurement of the low field magnetic anisotropy of single multi domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio interactions between two elongated grains were studied experimentally using two types of grain arrangement an aligned configuration and a side by side configuration for a distance between the grain centers equal to approximately twice the average grain size the magnetic susceptibility and its anisotropy are enhanced in both configurations and the direction of kmax the easiest magnetization axis is stable in the aligned configuration whereas it rotates toward an orthogonal direction in the side by side configuration depending on the distribution of the interacting magnetite grains magnetic interactions may therefore either increase the whole rock anisotropy magnitude or reduce it as in the given example of the granitic rocks from madagascar

lithology color coded seismic sections the calibration of avo crossplotting to rock properties in the late 1960s several oil companies noticed that in environments of young clastic sediments large seismic amplitudes were associated with gas saturated sands this method of correlating lithology to normal incidence ni reflectivities was appropriately named the bright spot technique however it quickly became apparent that not all large amplitudes were necessarily associated with gas reservoirs and much to our chagrin not all gas reservoirs had large amplitudes for the next decade geophysicists tried numerous techniques to resolve the ambiguity associated with lithologic identification by means of the seismic attribute ni this met with various degrees of success finally ostrander s work on amplitude variation with offset avo led to the development of another seismic attribute s which improved our discrimination of lithologies see plane wave reflection coefficients for gas sands at nonnormal angles of incidence geophysics 1984 but proper application of this new attribute along wit

magnetic vs structural fabrics whipple detachment structure whipple wash california abstract orientations of ellipsoids of anisotropy of magnetic susceptibility ams are compared with those of mesoscopic structural features for the breccia and upper mylonite zones for the whipple detachment zone in whipple wash whipple mountains eastern california the anisotropy of the breccia which ranges up to 5 is much less than that of the mylonitic gneiss which can exceed 100 slight differences in trend are evident between the ams ellipsoid elongations and those of mineral lineations maximum axes 1 of the ams ellipsoids for both breccia and mylonite trend generally ssw nne with shallow plunge whereas mineral stretching lineations in the mylonite gneiss have trends near ne sw with shallow plunge measured fold axes are close to the 1 axes ssw nne 1 axes in the breccia show more variation e w and ssw nne at different structural levels mesoscopic mineral lineations in the breccia were not discernible for the planar elements ams ellipsoid flattening is generally more closely parallel to foliation surfaces in the mylonite than in the breccia this is evident from overlapping minimum susceptibility axes 3 and foliation pole clusters 3 axes are more scattered than are poles of foliation the scatter in 3 is possibly the superimposed susceptibility effect of several structural surfaces of different ages joints small faults transposed metamorphic foliations and mylonitic foliation especially interesting are the discovery of good 1 definitions in the breccia the apparent lack of correlation of 1 axes with the very prominent stretching lineations in the mylonite and the similarity of the 1 axial trends with those of the fold fracture pole patterns

magmatic fabric acquisition mechanisms in a syenite results of a combined anisotropy of magnetic susceptibility and image analysis study the structural significance of the anisotropy of magnetic susceptibility ams and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the archean lebel syenite intrusion ontario canada bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples intensities shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding ams ellipsoids differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre full crystallization textures and structural overprinting in samples deformed after crystallization subfabrics of clinopyroxene and magnetite in samples showing pre full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes the resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite which is similar to the overall petrofabric of the samples crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains analogous to a logjam in a river the final characteristics of the ams and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma rather than transient late features

titanohematite lattice preferred orientation and magnetic anisotropy in high temperature mylonites magnetic and crystallographic fabric studies were performed in mylonitic granulites from a km wide strike slip shear zone in the ribeira belt southeastern brazil in these mylonites a strong compositional layering underlines the tectonic foliation and the elongation of titanohematite amphibole and orthopyroxene crystals defines a mineral stretching lineation magnetic fabric deduced from anisotropy of magnetic susceptibility ams measurements and tectonic fabric compare favorably rock magnetic studies show that both paramagnetic and ferromagnetic minerals can be carriers of ams the anisotropy of isothermal remanent magnetization which is due to the shape preferred orientation of magnetite grains is coaxial with ams lattice preferred orientation lpo measurements using the electron backscattered diffraction technique show that orthopyroxene amphibole biotite and titanohematite have a strong lpo tightly related to the tectonic fabric among these four minerals titanohematite is the only mineral present in relatively large proportions s 1 5 in all studied samples titanohematite lpo is characterized by a strong concentration of 0001 poles c axes sub perpendicular to the foliation and by a distribution of the poles of the 2110 and 1010 prism planes within the foliation with a maximum close to the lineation this characteristic lpo is interpreted as resulting from dislocation creep during the mylonitization magnetic fabrics and titanohematite lpo fit well the axis of minimum susceptibility is aligned with the c axis maximum and the axis of maximum susceptibility coincides with the maximum concentration of poles of the prism planes titanohematite lpo may provide a valuable constraint for the kinematic interpretation of the magnetic fabrics fl 2002 elsevier science b v all rights reserved

shape preferred orientation of object populations automatic analysis of digitized images abstract an automatic technique is presented for systematic shape preferred orientation spo analysis of crystal fabrics in rocks this technique is based on digitization of an image with a ccd camera and separation of an object population from its matrix the image is analysed globally using a rotating grid that is superimposed on the square pixel grid of the computer screen the parametric method is based on counting intercepts in all directions the interceps rose provides information on both the orientation and strength of the spo problems of hardware related counting noise anisotropy are avoided using an original method of filtering the intercept function with a linear sliding mask on the counting grid the technique was tested both on single objects of different shapes and on different populations of identical objects a very high degree of accuracy was obtained for orientation measurements fabric shape parameters are introduced which are specific to this intercept technique they compare favourably with shape parameters used in axial fabric analysis

subprovince accretion tectonics in the south central superior province development of tectonic subprovinces as shear bounded granite greenstone and sediment dominated terranes during the late archaean is reviewed and interpreted from relationships between portions of

geophysical evidence for gas hydrates in the deep water of the south caspian basin azerbaijan new 2 d seismic reflection data from the south caspian sea offshore azerbaijan document for the first time in the deep water up to 650 m of this area the presence of gas hydrates geophysical evidence for gas hydrates consists of a shallow 300 500 m below seafloor zone of pronounced high velocity 2 100 m s as compared with the surrounding sediments 1550 1600 m s this zone appears on the seismic data as a depth limited 200 m thick layer extending down the flank of an elongate structural high and displays seismic blanking effects on the sedimentary section a strong positive polarityorc 0 123u reflector marks the top of this velocity anomaly and is interpreted as the top of the gas hydrate layer similarly a high amplitudeorc 0 11u negative polarity reflector coincides with the base of the highvelocity layer and is interpreted as the base of the hydrate zone both the top and bottom of the hydrate layer approximately parallel the seafloor bathymetry and cut discordantly across the stratigraphic section suggesting that the two reflectors are thermobaric and not stratigraphic interfaces decreasing amplitude with offset at the base of the gas hydrate layer may indicate the accumulation of free gas beneath this interface these gas hydrates fall within the hydrate stability field predicted from thermobaric modeling for the south caspian basin but typically in thinner layers than would be expected from theoretical calculations the minimum predicted water depth that allows hydrate formation is 150 m and the maximum predicted thickness of the gas hydrate stability field is 1350 m q 2001 elsevier science ltd all rights reserved

structure and emplacement of granite plutons in the paleoproterozoic crust of eastern burkina faso rheological implications the fada n gourma area in burkina faso is underlain by paleoproterozoic rocks that make the northeastern west african craton this region is composed of ne trending volcano sedimentary belts and foliated tonalites affected by several shear zones a generation of younger 2100 ma old non foliated biotite bearing granites intrudes the former rock units we have investigated the younger granite pluton of kouare that was previously considered as forming a single body with the pluton of satenga to the west a pluton which likely belongs to the 20 ma more recent tenkodogo yamba batholith magnetic fabric measurements have been combined with microstructural observations and the analysis of field and aeromagnetic data the granite encloses angular enclaves of the host tonalites magmatic microstructures are preserved inside the pluton and solid state high temperature deformation features are ubiquitous at its periphery the presence of steeply plunging lineations in the pluton of kouare and its adjacent host rocks suggests that large volumes of granitic magmas became crystallized while they were ascending through the crust that was softened and steepened close to the contact around kouare the foliation in the host tonalites conforms with a map scale z shaped fold in between nne trending shear zones implying a bulk clockwise rotation of the material contained in between the shear zones including the emplacing pluton regionally the fada n gourma area is concluded to result from nw shortening associated with transcurrent shearing and vertical transfer of granitic magmas this study concludes that the 2200 myears old juvenile crust of burkina faso was brittle before the intrusion of the biotite granites became softened close to them and that gravity driven and regional scale wrench tectonics were active together

viability of shear wave amplitude versus offset studies in anisotropic media summary the study of shear wave splitting can yield information about crack strike crack density and other parameters of the cracks and aligned inclusions within the in situ rockmass such information is important for reservoir characterization and other hydrocarbon production applications crack density is usually inferred from the percentage shear wave anisotropy which is measured from time delays between the two split shear waves reservoirs may be too thin for discernible time delays to build up and the time delay and crack density in a reservoir layer may be unresolvable in conventional arrival time analysis of reflection surveys or vertical seismic profiles shear wave amplitude versus offset avo techniques are studied to see if they provide a more viable method of determining anisotropic parameters in thin reservoir layers the behaviour of reflected shear wave amplitudes with angle of incidence is investigated in simple two layer models the variations in shear wave avo are calculated for a range of percentage shear wave anisotropies for water filled cracks and dry cracks in the second layer representing a reservoir to see if characteristic information about reservoir properties can be extracted from the shear wave avo signatures the shear wave avo curves for thin cracks are sensitive to changes in anisotropy and crack content most of the information about crack content is contained in wide offset reflection data which suggests applications in crosshole monitoring of enhanced oil recovery the variations in shear wave avo may be distorted by the acquisition system most of the information about the percentage anisotropy is contained in the amplitudes of shear waves reflected at near vertical angles of incidence vertical incidence reflection amplitude methods are reviewed and a simple graphical procedure is suggested for determining their viability in different reservoir environments

the influence of abnormally high reservoir attenuation on the avo signature the direct detection of hydrocarbons with the seismic technique is one of the main goals of current geophysical research amongst the many direct hydrocarbon indicators which have been proposed the avo technique has received by far the greatest attention in the literature the technique has inspired confidence in large part through its ability to relate the observed phenomena directly to rock and fluid properties through robust seismic modeling and rock physics relationships

preserved magnetic fabrics vs annealed microstructures in the syntectonic recrystallised george granite south africa the saldanian basement of the cape fold belt of south africa outcrops in the kaaimans inlier with granite plutons intruded in low grade pelitic and quartzitic metasediments around 535 ma new field data support a ubiquitous saldanian top to the north thrust kinematics coeval with granite emplacement with no substantial cape tectonic overprint the granites and their contact aureoles display both synkinematic and post kinematic fabrics this and the high strain zone commonly observed all along the contact between the kaaimans inlier and the cape fold belt suggest a structural decoupling between the basement and its cover microstructures in the kaaimans inlier and in the george pluton establish a post kinematic pervasive and thermal overprint of saldanian age granites and country rocks record a medium temperature high strain deformation phase followed by a strong low temperature static recrystallisation two sets of andalusite porphyroblasts occur systematically in the contact aureoles of the studied plutons and cannot be explained by successive magmatic pulses the granites studied by the anisotropy of magnetic susceptibility ams technique are paramagnetic 20 km 300 si biotite is mostly at the origin of the bulk rock susceptibility although minor contributions of tourmaline or ferromagnetic phases may occur the contribution of biotite alone to the bulk magnetic susceptibility is supported by two quantitative models based respectively on whole rock compositions curie weiss law and on intrinsic mineral susceptibilities the magnetic foliations and lineations are homogeneous throughout the george pluton and are consistent with field structures the ams results mainly from the magneto crystalline anisotropy of biotite and from its lattice preferred orientation lpo in the rock the magnetic fabric reveals the biotite subfabrics that had been acquired before static recrystallisation and which was not modified by the subsequent thermal metamorphic event the magnetic fabric therefore preserves the emplacement related deformation fabric

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

structure of the high grade opatica belt and adjacent low grade abitibi subprovince canada an archaean mountain front plutonic gneisses in the archaean opatica belt in canada record crustal scale wsw vergent i e longitudinal high temperature d1 ductile shearing similar structures occur in adjacent greenstone belts but only in narrow amphibolite facies zones near the base metavolcanics in the greenschist facies portions do not record penetrative d1 structures the greenstone belts are interpreted as allochthonous on the gneisses they form the uppermost d1 thrust sheets sse vergent i e approximately transverse d2 thrusting imbricated the opatica belt and formed a large antiform either a culmination or an antiformal stack in the southern opatica the imbricated opatica belt wedged under the northern part of the abitibi subprovince and created a tight syncline with nnw vergent d2 thrusts interpreted as backthrusts on its northern limb the opatica belt abitibi subprovince contact therefore resembles some phanerozic mountain fronts further southwards propagating imbrication of the crust beneath the abitibi subprovince created a s vergent foreland fold and thrust belt in the overlying greenstones high grade opatica gneisses and adjacent low grade abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an archaean mountain belt similar in many respects to phanerozoic examples

amplitude versus offset variations in gas sands seismic reflections from gas sands exhibit a wide range of amplitude versus offset avo characteristics the two factors that most strongly determine the avo behavior of a gas sand reflection are the normal incidence reflection coefficient r0 and the contrast in poisson s ratio at the reflector of these two factors r0 is the least constrained based on their avo characteristics gas sand reflectors can be grouped into three classes defined in terms of r0 at the top of the gas sand class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for r0 class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of r0 near zero class 3 sands have lower impedance than the encasing shale with negative large magnitude values for r0 each of these sand classes has a distinct avo characteristic an example of a gas sand from each of the three classes is presented in the paper the class 1 example involves a hartshorn channel s

magnetic fabrics as constraints on the kinematic history of a pre tectonic granitoid intrusion kristineberg northern sweden abstract outcrop scale correlations of deformation fabrics with low field anisotropy of magnetic susceptibility ams measurements revealed a two stage structural evolution of the pre tectonic palaeoproterozoic viterliden intrusion in the skellefte district sweden the first deformation event reflected n s compression during basin inversion and comprised reverse dip slip shearing along major e w faults whereas the low strain lenses in between experienced penetrative deformation with a component of ne sw elongation along the main foliation this event is largely responsible for the present structural geometry regionally and locally and also for the magnetic fabric of the rocks in particular the sub vertical maximum principal susceptibility axes k max within the high strain zones are related to early dip slip deformation and were virtually unaffected by subsequent dextral strike slip reactivation which is recorded by sub horizontal rock lineations the strike slip deformation reflects e w bulk shortening and may regionally be correlated with reverse faulting along a n s trending major shear zone east of the study area

textural map units in quartzo feldspathic mylonitic rocks the classical macrotextural subdivision of quartzo feldspathic mylonitic rocks yields only three rock types protomylonite mylonite and ultramylonite this restriction impedes detailed mapping of the internal textural transitions common in wide deep seated crustal scale shear zones where such transitions may occur over kilometres and involve several clearly mappable textural types the introduction of two objectively defined field mapping terms homoclastic and heteroclastic describing the macroscopic grain size distribution within the porphyroclast population provides descriptive flexibility without changing the matrix porphyroclast basis of the established classification this allows the description of textural paths other than protomylonite mylonite ultramylonite and facilitates the consideration of textural paths in terms of strain partitioning between the constituent grains of the deforming aggregate rather than as a simple function of finite strain

middle cretaceous calcareous nannofossil biogeography and preservation in the atlantic and indian oceans implications for paleoceanography abstract the quantitative study of calcareous nannofossil preservation and biogeographic distribution puts constraints on paleoceanographic models for the atlantic and indian oceans during the middle cretaceous we establish a dissolution ranking of middle cretaceous calcareous nannofossils based on susceptibility to dissolution a positive correlation of organic carbon content of the sediments and the relative abundance of watznaueria indicates that much of the carbonate dissolution was caused by the release of carbon dioxide during oxidation of organic matter the depth distribution of the most dissolution resistant species watznaueria barnesae shows relative abundance maxima at the deepest sites and a maximum where intermediate waters impinge on slopes this reflects stagnant oxygen poor bottom waters and possibly a mid water oxygen minimum zone with increased deposition and catabolic breakdown of organic matter factor analyses are performed on various data sets for three different time slices ranging in age from late aptian to early albian middle albian and late albian to cenomanian the resulting factors can be related to paleoceanographic conditions such as colder water at high latitudes high fertility upwelling conditions and neritic conditions for all three time intervals a wide tropical to subtropical zone was flanked by boreal and austral zones upwelling conditions probably existed along the north african margin and over the falkland plateau zonal upwelling is tentatively postulated for the northwestern and northeastern margin of the north atlantic in the middle albian and for the eastern margin of the north atlantic off western europe for the late albian to cenomanian we speculate on surface water current patterns location of upwelling regions and haline driven deep circulation for the middle cretaceous

magnetic fabric of the barrington passage pluton meguma terrane nova scotia a two stage fabric history of syntectonic emplacement abstract structural analysis and the anisotropy of magnetic susceptibility ams are used to document the internal magmatic to high temperature subsolidus fabrics of the tonalitic late devonian barrington passage pluton the ams fabric orientations compare favorably with mesoscopic scale fabrics in general the k1 k2 principal plane of the ams is close to the observed mineral foliation defined by biotite and plagioclase and the k1 axis is aligned parallel to a mineral lineation defined by the same minerals and to a locally developed weak extension lineation defined by quartz measurement of the anisotropy of anhysteretic remanence aar and image analysis of biotite preferred orientations confirm that the orientation of the ams is principally controlled by biotite locally anomalous ams orientations are attributed to the influence of fine grained magnetite crystals the presence of which is revealed by measurements of hysteresis and coercivity properties a two stage fabric history is revealed first horizontal fabrics were formed foliation compositional banding during emplacement of a laccolith like pluton the second stage involved synmagmatic folding of the horizontal foliation and banding in response to regional contraction associated with the acadian orogeny with development of a lineation that records stretching parallel to the fold axis the results demonstrate that the barrington passage pluton was emplaced during the main phase of the acadian orogeny a model is proposed that depicts the barrington passage pluton as the folded base of a larger syntectonic pluton that is now largely eroded off

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

emplacement of the whistle dike the whistle embayment and hosted sulfides sudbury impact structure based on anisotropies of magnetic susceptibility and magnetic remanence the sudbury impact structure is one of the few multi ring meteorite impact structures recognized on earth the meteorite impact occurred ca 1850 ma resulting in the production of impact melts derived from the crust possibly with a mantle component the impact melts formed the sudbury igneous complex and also were emplaced as radial and concentric dikes within the target rocks the whistle dike one of the radial offset dikes in the sudbury impact structure is connected to the sudbury igneous complex by the whistle embayment structure which is composed of noritic cumulates we present a study of the emplacement flow patterns in the whistle dike and the embayment using magnetic anisotropy methods to measure the petrofabrics that record emplacement petrographic and scanning electron microscope observations combined with thermal demagnetization measurements and tests of the acquisition and demagnetization of anhysteretic magnetic remanence indicate that the primary sources of the magnetic signals in the sample suite are coarse grained soft coercivity magnetite and pyrrhotite that are interpreted to be primary igneous phases some samples also contain hard coercivity phases of possible metamorphic origin the anisotropy of magnetic susceptibility ams the anisotropy of an hysteretic remanent magnetization aarm and the anisotropy of partial anhysteretic remanent magnetization aparm were measured for the samples collected at 20 sites within the whistle dike and the embayment structure including an exposed chalcopyrite rich massive sulfide body the ams method provided well defined interpretable fabrics for many of the sample sites however the ams and aarm methods were unsuccessful in providing interpretable fabric measurements at some sample sites due to contributions from the hard coercivity ferromagnetic minerals for those samples the aparm method was successful in isolating the sub fabric defined by the primary magnetite pyrrhotite crystals the fabric maps are consistent with lateral horizontal injection of the whistle offset dike and subsequent sinking of molten massive sulfides within the still ductile dike sinking of the massive sulfides is recorded by steeply plunging magnetic lineations the magnetic fabrics in the cumulate norites of the embayment structure show that the norites were deposited in a dynamic environment that resulted in a well oriented lineation which may record flow of gravity or thermal currents channelled along the embayment axis in the embayment the lineations plunge toward a massive sulfide body suggesting the flow that led to the formation of the lineation may also have led to emplacement of the massive sulfide ore it is suggested that steeply plunging lineations elsewhere in the whistle offset dike and possibly in other sudbury offset dikes might indicate the trails of sunken massive sulfides in the subsurface it is also suggested that magnetic fabrics might be useful to locate massive sulfide bodies hosted in embayment structures

subtle stretching lineation revealed by magnetic fabric of callovian oxfordian black shales french alps abstract the anisotropy of magnetic susceptibility ams of callovian oxfordian black shales from the subalpine massifs of the french alps has been measured the magnetic susceptibility of these formations originates from ferromagnetic titanomagnetite and paramagnetic phyllosilicates illite and chlorite although the magnetic fabric is complicated by the interaction of planar and linear structural elements and the presence of variable amounts of minerals two types of lineation are preserved a stretching lineation which exhibits counterclockwise rotation as a result of deformation from south to north and an intersection lineation parallel to the fold axis this set of new structural data together with semiquantitative analysis of ams is in agreement with the geodynamic evolution of the western alps

the measurement of velocity dispersion and frequency dependent intrinsic attenuation in sedimentary rocks a series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the imperial college borehole test site these experiments fall into four categories vertical seismic profiles vsps within the frequency range 30 280 hz crosshole surveys 0 2 2 3 khz sonic logging 8 24 khz and laboratory measurements 300 900 khz the intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible velocity dispersion is observed for both compressional and shear waves the intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band the data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio the modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around 10 super 3 to 10 super 4

magnetic mineralogy of some granites from the french massif central origin of their low field susceptibility abstract with a view to a better understanding of the origin of the low field susceptibility in the variscan granitoids of the french massif central for which extensive structural mapping using the anisotropy of magnetic susceptibility ams technique is currently applied a detailed study of magnetic carriers has been undertaken origin of the susceptibility has been studied using high field and low temperature measurements in a cryogenic magnetometer together with standard rock magnetic techniques additional determinations of the opaque phases have been performed with the metallographic microscope x ray microdiffraction and electron microprobe techniques in almost all the granite types the magnetic susceptibility is essentially carried by the paramagnetic biotite crystals minor amounts of ubiquitous oxides ilmenite rutile titano haematite do not contribute significantly to the susceptibility this is also the case for rare occurrences of magnetite in some granitoids mainly the cordierite bearing anatexites sulphides are found to give a large contribution magnetic mineralogy identifies a mixture of ferromagnetic pyrrhotite and antiferromagnetic fe9s10 hexagonal pyrrhotite hpo the latter species is identified as forming the larger unaltered pyrrhotite grains the ferromagnetic pyrrhotite is identified to be mainly smythite fe9s11 a species formed by supergenic alteration of monoclinic pyrrhotite located along the basal cleavage planes the microfractures and the grain boundaries of the hpo this study which constitutes the first report of the magnetic properties of smythite in rocks indicates that magnetic carriers derived from low temperature alteration have to be taken into account for purposes of structural studies using ams

the response of marine phytoplankton and sedimentary organic matter to the early toarcian lower jurassic oceanic anoxic event in northern england abstract early toarcian organic rich sediments reflecting the lower jurassic oceanic anoxic event were investigated in the brown moor borehole north yorkshire northern england integrated micropalaeontological calcareous nannofossils and dinoflagellate cysts and geochemical rock eval pyrolysis analyses reveal a sequence of changes mainly driven by palaeoecological shifts these changes mainly involve the composition source and preservation rate of sedimentary organic matter as well as algal population dynamics a sequence of successive disappearances of individual species during the early toarcian dactylioceras semicelatum and harpoceras exaratum ammonite subzones affect the composition of the phytoplankton assemblages which become gradually dominated by opportunistic taxa this range top sequence is followed by the temporary disappearance of both calcareous nannofossils and dinoflagellate cysts the disappearance event which are replaced by tasmanites an increase in type ii kerogen marks this interval these modifications were related to a gradual water stratification and to the development of a stable pycnocline these factors controlled the development of an oxygen minimum zone within the water column and the distribution of nutrients in the surface waters the disappearance event is coincident with the maximum extent of the oxygen minimum zone a repopulation event and an increase in type iii kerogen mark the end of the phytoplankton crisis in the upper part of the h exaratum ammonite subzone preadapted survivors e g biscutum dubium biscutum finchii and crepidolithus crassus mainly characterise the repopulated calcareous nannofossil assemblages whereas a significant turnover affected the dinoflagellate cyst community the re established phytoplankton populations are a response to the gradual restoration of mixed marine waters characterised by meso eutrophic conditions at the surface and by sustained nutrient regeneration the integrated calcareous nannofossil and dinoflagellate cyst biostratigraphy provided a reliable biochronological framework for the brown moor borehole succession allowing correlation with lower toarcian anoxic strata in central italy the integrated micropalaeontological approach indicates some diachroneity of lower jurassic ammonite zones in the boreal and tethyan realms

evidence for major environmental perturbation prior to and during the toarcian early jurassic oceanic anoxic event from the lusitanian basin portugal the timing and causal relationships between the pronounced negative c isotope excursion and paleoenvironmental perturbations associated with the toarcian oceanic anoxic event early jurassic remain unclear particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main c isotope anomaly here we present a new quantification of late pliensbachian early toarcian calcareous nannofossils abundance and size from the peniche reference section portugal together with o and c isotope records of well preserved brachiopod shells from the same section the brachiopod shell 13c curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions close to the pliensbachian toarcian boundary 2 and during the toarcian oceanic anoxic event 3 5 our results indicate that both c isotope negative excursions were characteristic of benthic and shallow water environments suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures probably as a result of increased co2 levels and enhanced greenhouse conditions co2 induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems as evidenced by synchronous drops of platform derived carbonate accumulation and drastic reductions in size 3 m of the main pelagic carbonate producer schizosphaerella we suggest that the early toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the karoo ferrar province

overprinting of magnetic fabrics in granites by small strains numerical modelling abstract the anisotropy of magnetic susceptibility ams is used to determine weakly defined mineral preferred orientation fabrics in granites however the effects of small strains superimposed on the low intensity ams fabrics typical of these rocks is not well understood numerical simulations are presented which investigate the effects of small superimposed strains on the shapes magnitudes and orientations of pre existing ams fabrics which are similar to those reported in the literature for undeformed granites a computer generated biotite petrofabric is subjected to progressive strain using march s strain response model for pure shear plane strain axially symmetric shortening and simple shear regimes results indicate that a pre existing ams fabric might be significantly modified or overprinted by small strains 10 15 strain in the pure shear and axially symmetric shortening regimes 0 5 1 0 for simple shear a new ams fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric at low strains the evolution of the shape of the ams ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework the ams ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain this modelling suggests that small strains affecting a pluton following its emplacement could mask ams fabrics developed earlier the results also suggest that in simple shear the orientation of the ams ellipsoid may serve as a shear sense indicator for small strains even if the earlier fabric is not isotropic

orogen parallel and transverse shearing in the opatica belt quebec implications for the structure of the abitibi subprovince the late archean opatica granitoid gneiss belt is situated within the northern abitibi subprovince along the northern margin of the abitibi greenstone belt approximately 200 km of structural section was mapped along three traverses within the previously unstudied opatica belt the earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing d1 late d1 deformation was concentrated into kilometre scale ductile fault zones typically with l s tectonite fabrics two families of lineations are associated with d1 indicating shearing both parallel and transverse to the east northeast trend of the belt lineations trending east northeast or northwest southeast tend to be dominant within domains separated by major fault zones in light of the abundant evidence for early north south compression documented throughout southern superior province including the abitibi greenstone belt d1 is interpreted in terms of mid crustal thrusting prob

an intensive study of magnetic susceptibility anisotropy of amphibolite layers of the thompson belt north manitoba abstract in an examination of the magnetic susceptibility anisotropy of folded amphibolite layers in intensly deformed gneisses at the pipe 2 mine thompson belt north manitoba the magnetic fabric was found to agree well in orientation with a fold axial planar foliation and the stretching lineation in the gneisses the prolateness or oblateness of the magnetic fabric ellipsoid in this lithology depends primarily on the mineralogy of the sample and therefore cannot be used as a strain gauge the magnetic fabric is made up of two components one is a small approximately isotropic part due to iron bearing silicates and perhaps isolated equant magnetite particles the second is a variable anisotropic part carried by elongated particles of magnetite and has constant ratio p k max k min of 2 018 0 008 this constant ratio represents a characteristic value of the magnetic anisotropy of these rocks and may be related to the strain in this area

framework for avo gradient and intercept interpretation amplitude variation with offset avo interpretation may be facilitated by crossplotting the avo intercept a and gradient b under a variety of reasonable petrophysical assumptions brine saturated sandstones and shales follow a well defined background trend in the a b plane generally a and b are negatively correlated for background rocks but they may be positively correlated at very high vp vs ratios such as may occur in very soft shallow sediments thus even fully brine saturated shallow events with large reflection coefficients may exhibit large increases in avo deviations from the background trend may be indicative of hydrocarbons or lithologies with anomalous elastic properties however in contrast to the common assumptions that gas sand amplitude increases with offset or that the reflection coefficient becomes more negative with increasing offset gas sands may exhibit a variety of avo behaviors a classification of gas sands based on location in the a b plane rather than on normal

endemism as a palaeobiogeographic parameter of basin history illustrated by early and mid liassic peri tethyan ammonite faunas abstract episodes of endemism during sinemurian pliensbachian times are described from synthetic data publications and unpublished collections about ammonite faunas of the western reaches of the tethys the lusitanian sub betic and high atlas basins receive special attention the study shows that 1 endemism occurs principally in the lusitanian and high atlas basins which are the most confined palaeogeographic structures 2 it tends to occur synchronously in different basins but involving different taxa i e it is independent of phylogeny 3 it is not obviously correlated with relative sea level at any given time however the fact that episodes of endemism coincide with second order regressive phases suggests some connection between sea level falls independently of the sea level and the development of endemic faunas

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

brasiliano crustal extension and emplacement fabrics of the mangerite charnockite pluton of umarizal north east brazil abstract shear zone kinematics and anisotropy of magnetic susceptibility ams data have been used in a brasiliano pan african fayalite clinopyroxene orthopyroxene syenite of umarizal ne brazil in order to reveal its emplacement mechanisms the pluton is located at the intersection of two major shear zones a ne trending transcurrent portalegre shear zone psz and a nw to e trending frutuoso gomes shear zone fgsz the fgsz dips gently ne in its nw trending branch and n in the e trending branch asymmetric porphyroclasts and s c shear planes observed in mylonitic gneisses along the fgsz point to a top to the east extensional movement hysteresis and low field susceptibility measurements on mangerites and charnockites indicate that magnetic susceptibility is principally carried by multidomain magnetite in the core of the pluton the magnetic anisotropy is weaker p f ams ellipsoids the magnetic anisotropy increases south westward p 1 10 approaching the fgsz and the ams ellipsoid becomes dominantly disc shaped f l magnetic lineations plunge gently to nw in the western part of the pluton and to ene on the east the absence of steep lineations except around a large xenolith in the center of the pluton exclude the presence of magmatic feeding zones at the actual level of exposure foliations are roughly parallel to the pluton contact but most of them dip moderately towards north east in the sw border the magnetic foliation is parallel to the mylonitic foliation displayed in the fgsz although emplaced within migmatized host rocks the kinematics of the shear zones and the magmatic fabric pattern rule out a buoyancy driven diapiric emplacement of the pluton syn magmatic shear deformation associated with e w crustal stretching accounts for the fabrics of emplacement recorded in the umarizal pluton

magnetic mineralogy in an archean crustal cross section implications for crustal magnetization a traverse across precambrian rocks from the kapuskasing uplift in ontario provided 31 samples from a crustal cross section that is now exposed at the surface about half the samples are dominantly paramagnetic magnetite accompanied in a few samples by minor amounts of pyrrhotite is the only significant magnetic mineral in the other half magnetic susceptibilities of these samples are constant between room temperature and about 540 c above which temperature they drop rapidly to zero magnetic susceptibility does not depend on rock type apparent original depth in the crust or degree of metamorphism the average induced magnetization of the most magnetic units in this section is less than 1 a m which is much lower than the source requirements for the deep crust deduced from aeromagnetic and satellite magnetometer surveys

silicate versus trace mineral susceptibility in metamorphic rocks rates of change of magnetic susceptibility during leaching can characterize the presence of certain common types of magnetic minerals in metamorphic rocks in this study the results of leaching are confirmed by mineral separation optical microscopy scanning electron microscopy sem and microchemical analysis under sem leaching curves provide a simple rapid way of determining the relative roles of oxides sulfides and silicates in carrying the susceptibility in metamorphic rocks

transpression in slates along the margin of an archean gneiss belt northern ontario magnetic fabrics and petrofabrics archean slates along the northern boundary of the quetico subprovince of the superior province show marked variations of the structural facing direction within coplanar primary cleavage surfaces these are interpreted as being due to sheath folds near the faulted east west boundary of the slates with the metavolcanic wabigoon belt sheath folds with axial traces nearly parallel to the belt boundary are macroscopic primary and isoclinal and are believed to result from pervasive dextral transpression of the northern margin of the quetico subprovince magnetic fabrics confirm the presence of a single penetrative flattening tectonic microfabric and considerable north south shortening on the scale of hand specimens and outcrops the magnetic fabric is due to the alignment of metamorphic sheet silicates with a subordinate contribution due to the preferred dimensional orientation of detrital magnetite grains in some low strain environments at the hinges of sheath folds and at the sandy bases of graded beds r

alternating field characteristics of pseudo single domain 2 14 μm and multidomain magnetite abstract we report normalized af demagnetization curves of anhysteretic remanences arm s produced by 1 10 and 40 oe steady fields and of saturation isothermal remanence irm s in a suite of dispersed unannealed magnetite powders with median sizes of 2 4 6 10 and 14 m pseudo single domain or psd size range and 100 m multidomain or md size interpreted in the light of the domain structure test first proposed by lowrie and fuller 12 the relative stability trend of curves for the 2 m sample is of single domain sd type the 1 oe arm being most resistant to demagnetization followed by the 10 oe and 40 oe arm s and irm s for the 100 m sample the trend is exactly reversed and is of md type in the 4 14 m samples hitherto undescribed transitional trends between sd type and md type occur at 6 m 1 oe 10 oe and 40 oe arm s preserve an sd type trend but for all af s 75 oe irm s is more resistant than any of these remanences at 10 m this trend is unmistakable and only at 14 m do the 1 oe 10 oe and 40 oe arm curves merge we conclude 1 that the lowrie fuller test distinguishes between small md grains enhanced by psd remanence and large md grains lacking psd remanence rather than between sd and md structures per se and 2 that in the psd transition region from 6 to 14 m in magnetite irm s changes over to md type relative stability around 6 m whereas 10 oe and 40 oe arm s achieve an md type trend around 14 m in accord with the predicted field dependence of the psd threshold size our theoretical interpretation assumes that the intrinsic internal field coercive force spectra of weak field and strong field remanences are identical but that the observed external field spectrum is shifted to lower fields as a result of the internal demagnetizing field nj r of the remanence j r the effect is slight for weak field j r s but substantial for irm s since all coercivities high as well as low are shifted the result of the lowrie fuller test is determined simply by the shape of the intrinsic coercivity spectrum or the corresponding af demagnetization curve depending on the model of self demagnetization used either subexponential or sublinear af decay curves of weak field remanence will automatically lead to an md type trend whereas by either model the decay curves that characterize sd and psd remanences decaying slowly initially and then more rapidly will always produce and sd type trend

paleomagnetism of archean rocks from northwestern ontario iii rock magnetism of the shelley lake granite quetico subprovince the shelley lake granite of northwestern ontario contains five magnetic phases deuteric and post crystallization hematites which are relatively abundant but carry only 1 4 of the natural remanent magnetization nrm primary magnetite in coarse 50 500 m grains both optically homogeneous and subdivided by hematite lamellae micrometre size secondary magnetite in chloritized biotites and submicrometre size magnetite whose presence is inferred from low blocking temperatures in thermal decay curves of the nrm the nrm is a composite of type 1 and type 2 remanences which differ in direction by about 90 see companion paleomagnetic paper both nrm components occur in normal n and reverse r polarities type 1 remanences 1n 1r have the hallmarks of multidomain md behaviour high blocking temperatures but low coercivities exponential alternating field af decay curves generally md results of the lowrie fuller test and md to transitional values 0 3 10 of the koenigsberger qn ratio further

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

nannofacies analysis as a tool to reconstruct paleoenvironmental changes during the early toarcian anoxic event the black shales of lower toarcian schistes carton in france jet rock in england posidonia shale in germany are sediments formed by fine alternations of millimetric to sub millimetric clear and dark laminae a study of polished core surfaces dotternhausen core sw germany was carried out in sem in order to characterize the micro fabrics of different laminae and their content in nannofossils two main intervals were recognized where fabrics and nannofossil content are significantly different in the clay rich interval characterized by high wt toc and a large negative carbon isotope excursion both in carbonates and organic matter the dominant micro fabrics are well foliated or lumpy giving evidence of the absence or reduction of benthic life nannofossils are scarce and only concentrated in discrete laminae separated by large abiotic intervals assemblages are indicative of a deep nutricline but sporadic nutrient inputs occurred as demonstrated by fluctuation in abundance of specimens of the biscutaceae nutrients were probably delivered to the german basin via riverine influx different sized framboids of pyrite were recognized in this interval the smallest ones 2 m being probably precipitated in an anoxic and sulphidic water column the clay rich interval was likely deposited during low sea level in a restricted basin these conditions were followed by a sea level rise and better water mass circulation within the german basin sedimentation records an increase in carbonate deposition organic matter content decreased and carbon isotopes came back to pre excursion values micro fabrics became less markedly laminated pyrite framboids nearly disappeared and rich and diversified nannofossil assemblages are recorded sea water conditions were probably highly fluctuating as indicated by alternation of intervals dominated in turn by schizosphaerella spp flourishing under generally oligotrophic conditions where nutrients were temporarily available in surface waters due to storms and by crepidolithus spp a deep dweller the results of this work fit well with a silled basin model

southern abitibi greenstone belt archean crustal structure from seismic reflection profiles lithoprobe seismic reflection profiles from the 2 75 2 68 ga southern abitibi greenstone belt reveal subhorizontal reflection zones of regional extent in the upper 15 km of crust that contrast with the steep dips of rock units and structures at the surface the shallowest 6 12 km regional subhorizontal reflection zone is interpreted to be the base of the abitibi greenstone belt supracrustal assemblage which may be an intrusive and or tectonic contact shallowly dipping subparallel reflectors in the upper crust are interpreted as low angle faults consistent with out of sequence stratigraphy revealed by u pb geochronology truncations of reflectors suggest that regional steeply dipping gold mineralized fault zones penetrate to depths of 15 km the profiles are consistent with the kapuskasing crustal model of the central superior province wherein greenstone belts are underlain by middle crustal felsic intrusions and gneiss and a lower crustal zone of mafic gneiss and intrusions the profiles support tectonic models of deformation of the abitibi greenstone belt that invoke either folding and thrusting followed by regional steep faulting or folding and thrusting linked to the steep faults

the magnetic fabric relationship between sedimentary and basement nappes in the high tatra mountains n slovakia abstract in the high tatra mountains n slovakia the magnetic fabrics are deformational in origin in sedimentary rocks of the kri na nappe in sedimentary rocks covering the underlying crystalline complex and in granitoids and metamorphic rocks of this complex the patterns of the principal susceptibilities are similar in all these units their most important feature is a girdle in magnetic foliation poles oriented nnw sse to n s in sedimentary formations the magnetic fabric pattern is compatible with that predicted by mathematical modelling to develop during a complex deformation comprising lateral shortening and simple shear which is characteristic of nappe deformation consequently the magnetic fabric in the high tatra mountains probably resulted from deformation associated with the formation and movement of the nappes during which not only the sedimentary strata but also the crystalline complex were thrust over the north european platform

weighted stacking for rock property estimation and detection of gas amplitude versus offset concepts can be used to generate weighted stacking schemes here called geo stack which can be used in an otherwise standard seismic data processing sequence to display information about rock properties the zoeppritz equations can be simplified and several different approximations appear in the literature they describe the variation of p wave reflection coefficients with the angle of incidence of a p wave as a function of the p wave velocities the s wave velocities and the densities above and below an interface using a smooth representative interval velocity model from boreholes or velocity analyses and assuming no dip the angle of incidence can be found as a function of time and offset by iterative ray tracing in particular the angle of incidence can be computed for each sample in a normal moveout corrected cmp gather the approximated zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather and certain rock properties can be estimated the estimation of the rock properties is achieved by the application of time and offset variant weights to the data samples before stacking the properties which can be displayed by geo stack are p wave reflectivity or true zero offset reflectivity s wave reflectivity and the reflectivity of p wave velocity divided by s wave velocity or pseudo poisson s ratio reflectivity if assumptions are made about the relation between p wave velocity and s wave velocity for water bearing clastic silicate rocks then it is possible to create a display which highlights the presence of gas

inverse magnetic fabric in carbonate bearing rocks abstract the observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric pole of bedding or schistosity is referred to as an inverse magnetic fabric the investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates that c axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones a mineralogical model based on the property of single domain grains to have a zero susceptibility parallel to their long axis is proposed however more complex cases are also encountered

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

paleomagnetic and rock magnetic evidence for inverse zoning in the parguaza batholith southwestern venezuela and its implications about tectonics of the guyana shield abstract we report paleomagnetic and rock magnetic data from the rapakivi granites of the parguaza batholith guyana precambrian shield southwestern venezuela these results suggest that the pluton is inversely zoned with respect to the cooling ages in order to explain such an age pattern tentative structural settings are proposed placing the parguaza intrusions in a plate tectonic context six sites were sampled along a 200 km transect that cuts through the northern lobe of the batholith thermomagnetic curves x ray diffraction and fluorescence hysteresis loops thermal and alternating field af intensity plots transmission tem and scanning sem electron microscope analyses and konigsberger ratios qn values were used to identify the different magnetic mineralogies and their distribution of grain sizes magnetite titanomagnetite near magnetite in composition and deuteric hematite are the three carriers of natural remanent magnetizations nrms in these rocks magnetic granulometry indicators such as konigsberger ratios qn values suggest the dominant presence of single domain magnetites with average grain sizes grading from finer to coarser away from the center of the transect the paleomagnetic results reveal the existence of two primary thermoremanent and or thermochemical magnetizations trms tcrms for sites csp 3 pi 2 and pi 4 decl 328 incl 21 k 15 95 11 4 and site csp 2 pi 1 and pi 3 decl 16 incl 87 k 10 95 13 respectively there is also a poorly defined g3r2 magnetization decl 284 incl 86 k 6 95 27 found in sites csp 2 pi 1 and pi 3 the overlap of coercivities and the unblocking temperatures spectra probably resulting from the coexistence of primary single domain magnetic mineralogies with secondary exsolutions of single domain like ti poor fe rich regions almost pure magnetite in multidomain titanomagnetite grains in most cases precludes the complete resolution of hybrid g1 n r or g3 n r the relative ages for these components were determined using a map of rb sr model age chrontours g1 and g3 are the older and the younger trms tcrms respectively and were acquired at two discrete moments of the batholith s geological history the final map resulting from integrating the paleomagnetic and rb sr data shows an age pattern for this batholith and the rest of the intrusions that belong to the parguaza igneous complex that could be explained either as the effect of inverse cooling of a single intrusive body the sequential emplacement of magmas controlled by normal faulting or an internal tectonism resulting in a system of ne trending horsts and grabens cutting through the pluton because of its feasibility and agreement with the most recent theories about the tectonic evolution of the guyana shield we favor the latter hypothesis

magnetic fabric vs magnetite and biotite shape fabrics of the magnetite bearing granite pluton of gameleiras northeast brazil abstract a well defined orientation pattern has been evidenced by the magnetic fabric lineation and foliation of the granite pluton of gameleiras northeast brazil this magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves and by the planar arrangement of tabular k feldspar phenocrysts low field anisotropy of magnetic susceptibility properties as well as magnetic mineralogy studies and optical microscopy observations indicate that multidomian iron rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy to examine in more detail the relationships between the magnetic and magmatic fabrics measurements of two dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections angular departure has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen we found that the fabric of biotite correlates strongly with the magnetic fabric 7 8 the fabric of magnetite although not as well defined mainly because of its weak shape anisotropy still correlates with the magnetic fabric 12 18 origin of low field magnetic fabric in the pluton of gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains magnetic interactions between the grains of magnetite where they form clusters may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy concerning the orientation of the magnetic fabric as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals particularly biotite and titanite the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite however as no correlation exists between shapes or clustering of magnetites and strain undergone by the deforming magma the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity

magnetic properties of exposed deep crustal rocks from the superior province of manitoba abstract a traverse across precambrian rocks of the superior province in central manitoba provided 56 samples for magnetic study from the pikwitonei granulite domain and the adjacent cross lake subprovince these rocks represent a cross section of the continental crust presently exposed at the surface thirty one samples are dominantly paramagnetic and three contain pyrrhotite twenty two samples from silicic plutonic rocks which make up about 80 of the section by volume contain nearly pure magnetite and have susceptibilities that are almost constant between room temperature and 540 c averaging about 1 10 2 si units the mafic supracrustal rocks with much higher iron content are about an order of magnitude less magnetic because they contain little or no magnetite susceptibility is generally independent of metamorphic grade or apparent depth in the crust the discrepancy between induced magnetizations of measured lower crustal rocks and the source requirements of long wavelength magnetic anomalies may be due to a high temperature viscous magnetization in the deep crust

calcareous nannoplankton changes across the early toarcian oceanic anoxic event in the western tethys 1 calcareous nannoplankton were profoundly affected by environmental perturbations coincident with the early toarcian oceanic anoxic event t oae we quantify the abundance of nannofossils across the t oae at three locations in western europe where the event is marked by a prominent negative carbon isotope excursion cie data were treated by statistical analysis and the shannon diversity index was applied in order to describe nannofossil assemblage changes related to paleoenvironmental evolution in the basal toarcian before the t oae high proportions of taxa with an affinity for low temperature e g p liasicus t patulus bussonius occurred this observation is consistent with interpretations of published oxygen isotope records during the t oae the lowest abundance of nannofossils is observed but there is a peak of a coccolith calyculus from an organism that probably thrived in low saline surface waters depleted of nitrate at the end of the perturbation the lowest diversities of nannofossils occurred and assemblages are dominated by crepidolithus crassus a deep dweller this interval corresponded to progressive reoxygenation of deep water and the reoccupation of the deep photic zone by nannoplankton the highest abundance of nannofossils is recorded above the cie and testifies to the recovery of the entire nannoplankton community the t oae was widespread perhaps global and probably linked to major changes in the atmosphere and hydrosphere however the record of this event varies in the basins surrounding the western tethys suggesting regional imprints on the global signal the regional variability may attest to establishment of effective connections between the arctic and tethys oceans which allowed cool low saline water formed at high latitudes to stream toward the western tethys

p wave seismic attenuation by slow wave diffusion effects of inhomogeneous rock properties recent research has established that the dominant p wave attenuation mechanism in reservoir rocks at seismic frequencies is because of wave induced fluid flow mesoscopic loss the p wave induces a fluid pressure difference at mesoscopic scale inhomogeneities larger than the pore size but smaller than the wavelength typically tens of centimeters and generates fluid flow and slow diffusion biot waves continuity of pore pressure is achieved by energy conversion to slow p waves which diffuse away from the interfaces in this context we consider a periodically stratified medium and investigate the amount of attenuation and velocity dispersion caused by different types of heterogeneities in the rock properties namely porosity grain and frame moduli permeability and fluid properties the most effective loss mechanisms result from porosity variations and partial saturation where one of the fluids is very stiff and the other is very compliant such as a highly permeable sandstone at shallow depths saturated with small amounts of gas around 10 saturation and water grain and frame moduli variations are the next cause of attenuation the relaxation peak moves towards low frequencies as the background permeability decreases and the viscosity and thickness of the layers increase the analysis indicates in which cases the seismic band is in the relaxed regime and therefore when the gassmann equation can yield a good approximation to the wave velocity

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

spatial and temporal distribution of calcareous nannofossils along a proximal distal transect in the lower jurassic of the umbria marche basin central italy abstract three upper pliensbachian lower toarcian successions of the umbria marche basin were analysed for sedimentology and calcareous nannofossil content across a 120 km long transect from proximal to distal areas with respect to the shallow latium abruzzi platform the uppermost part of the corniola unit uppermost pliensbachian and the marne di monte serrone formation p p lower toarcian polymorphum to levisoni ammonite zones that contains organic matter rich levels were studied in the three localities the analysis of the sedimentary structures in the studied successions indicates that the basin was a shallow water system above storm wave base during the late pliensbachian early toarcian the stratigraphic evolution of the sedimentary structures recorded in the studied localities shows that the upper pliensbachian corresponded to a regressive sedimentary interval whereas a complete transgressive regressive relative sea level cycle occurred during the lower toarcian polymorphum zone a further transgression occurred in the lowermost levisoni zone lower toarcian changes from carbonate dominated to marl dominated sediments form decimetre to metre scale lithological units that can be correlated from succession to succession and may have resulted from short term sea level and or palaeoenvironmental variations the mean duration of these lithological units suggests that they formed in tune with the first orbital eccentricity cycle 100 ka the analysis of calcareous nannofossil quantities abundance per mm2 of the smear slide percentage mean abundance shows both spatial and temporal changes in the community structure schizosphaerella spp and crepidolithus crassus were more abundant in proximity to the shallow latium abruzzi platform these two taxa were likely affiliated to oligotrophic conditions in marine surface waters explaining why they were generally more abundant in carbonate rich intervals than in marls mitrolithus jansae is interpreted as being a deep dwelling species which was more abundant during rising sea level in the earliest toarcian species of the genera lotharingius biscutum and calyculus were more abundant when higher trophic conditions occurred and were thus more common during periods of marl deposition the stratigraphic distribution of calcareous nannoplankton and its interpretation in terms of palaeoecological affinities allow three main phases in the environmental evolution during the early toarcian anoxic event to be recognised the first phase corresponds to a long term transgression and high nannoplankton abundance in distal areas mainly lotharingius biscutum and calyculus species suggests high productivity that may have resulted from upwelling the beginning of accumulation and preservation of organic matter in the sediments occurred during this phase the scarcity of nannoplankton during the second phase may indicate oligotrophic conditions in surface waters possibly due to water stratification during the highest sea level stand a high abundance of schizosphaerella spp and c crassus is recorded in proximal areas probably resulting from sporadic nutrient input from continental or shallow platform areas the maximum of organic matter preservation is recorded during this phase during the third phase relative sea level fall and efficient water mixing by frequent storm events led to enhanced nannoplankton productivity in proximal and distal areas and to a decrease in organic matter accumulation in the umbria marche basin

coccolithophores in the equatorial atlantic ocean response to seasonal and late quaternary surface water variability abstract the present study was initiated to ascertain the significance of coccolithophores as a proxy for paleoceanographic and paleoproductivity studies in the equatorial atlantic data from a range of different samples from the plankton surface sediments as well as sediment cores are shown and compared with each other in general the living coccolithophores in the surface and subsurface waters show considerable variation in cell numbers and distribution patterns cell densities reached a maximum of up to 300 103 coccospheres l in the upwelling area of the equatorial atlantic here emiliania huxleyi is the dominant species with relatively high cell numbers whereas umbellosphaera irregularis and umbellosphaera tenuis are characteristic for oligotrophic surface waters although they are observed in high relative abundances these species only occur in low absolute numbers the lower photic zone is dominated by high abundances and considerable cell numbers of florisphaera profunda the geographical distribution pattern of coccoliths in surface sediments reflects the conditions of the overlying surface water masses however abundances of the oligotrophic species umbellosphaera irregularis and umbellosphaera tenuis are strongly diminished causing an increase in relative abundance of the lower photic zone taxa florisphaera profunda and gladiolithus flabellatus during the past 140 000 years the surface water circulation of the equatorial atlantic has changed drastically as can be seen from changes in the coccolithophore species composition absolute coccolith numbers as well as coccolith accumulation rates significant increases in coccolith numbers and accumulation rates is observed in the southern equatorial atlantic during the last glacial interval oxygen isotope stages 2 4 which we attribute to enhanced upwelling intensities and advection of cool nutrient rich waters at this site in the western equatorial atlantic we observe an opposite trend with decreasing numbers of coccoliths during glacial periods which probably is caused by a deepening of the thermocline

paleoenvironmental controls on the morphology and abundance of the coccolith watznaueria britannica late jurassic southern germany the coccolithophore species watznaueria britannica is dominant in middle upper jurassic calcareous nannofossil assemblages and presents morphological variation including different coccolith size shape and length of the central area and of the bridge six morphotypes can be recognized in the polarizing light microscope the aim of this work is to better understand the morphological variability of w britannica and determine if this variability is controlled by paleoecological factors in order to investigate the potential paleoecological controls on w britannica morphology and abundance we carried out a biometric study on a restricted temporal interval the late oxfordian in the swabian alb southern germany characterized by increasing carbonate production linked to climatic changes the balingen tieringen section where previous works on sedimentology nannofossil assemblage composition and 18o and 13c analyses were performed was selected for this study the variations in morphology and abundances ofw britannicawere studied on 40 samples of the balingen tieringen section presenting variable lithologies and calcium carbonate contents for each level seven biometric parameters coccolith length width and ellipticity central area length width and ellipticity and central area proportion with respect to the coccolith were measured or calculated on digitally captured images of the first 100 w britannica coccoliths observed in the light microscope the relationships between the different biometric variables were described using bivariate and principal component analyses biometric parameters and principal component factors extracted from nannofossil assemblages as well as other paleoenvironmental proxies were investigated using regression and their stratigraphic trends were compared principal component analysis of the six biometric variables 3938 measurements on w britannica coccoliths shows a reduced morphological variability compared to a significant size gradient an allometric trend recognized on the total placolith and on the central area within thew britannica assemblages suggests that the different morphotypesmay represent intra specific variability rather than different species the general trend through late oxfordian shows an increase in size ofw britannica coccoliths mainly driven by an increase in the contribution of the largemorphotypes increasing placolith size is associated with drier and warmer climatic conditions during the latest oxfordian

strain and magnetic fabric in the santa catalina and pinaleno mountains metamorphic core complex mylonite zones arizona anisotropy of magnetic susceptibility ams is capable of recording finite strain in weakly magnetized rocks ams was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history ams measurements were compared with finite strains determined from dike rotations and from foliation orientations in one zone the santa catalina mountains the orientations of susceptibility and finite strain ellipsoids are in excellent agreement and there is a logarithmic relationship between susceptibility difference ki ki k k and finite strain magnitude in the second zone the pinaleno mountains minimum susceptibility is perpendicular to the finite flattening plane but the maximum susceptibility does not parallel the maximum extension direction and there is no systematic relationship between susceptibility magnitude and strain magnitude oriented polished thin sections indicate that magnetite in the protolith of the santa catalina mylonite occurs as randomly oriented elongate grains with subsequent deformation the long axes are rotated into the maximum extension direction in the pinaleno mylonites both equant and elongate magnetite grains are present with deformation the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane ams in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric the final orientation of the elongate grains is a function of their initial orientation as well as the finite strain therefore despite similar deformational histories the two zones display different ams patterns due to the differences in occurrence initial orientation and shape of ferromagnetic grains

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

magnetite microexsolutions in silicate and magmatic flow fabric of the goyozan granitoid ne japan significance of partial remanence anisotropy 1 anisotropy of magnetic susceptibility ams has been widely used to infer magmatic flow patterns of granitoids where an appropriate ams axis is parallel to an alignment of mafic minerals or magnetite the magmatic flow fabric in cretaceous granitic plutons from northeastern japan was verified using an analysis of anisotropy of partial anhysteretic remanent magnetization aparm which further isolates the magnetite subfabrics according to magnetite grain size the preferred orientation of polysynthetic twins in plagioclase laths and clinopyroxene is discordant with the bulk ams fabric along outer marginal zones of the granitoid as shown by image analysis of microphotographs from thin sections cut in orthogonal planes this suggests that the uncorroborated use of bulk ams to detect flow fabric in granitoids has risks scanning electron microscopy sem reveals that submicroscopic needle shaped magnetite inclusions exsolved in anhedral plagioclase and clinopyroxene may explain such anomalous exceptions to the validity of ams fabrics our aparm measurements show that the aparm alignment of relatively high coercive submicroscopic magnetite inclusions is concordant to the linear orientation of anhedral plagioclase and clinopyroxene the combination of sem ams and aparm was required to confirm the magmatic and submagmatic flow pattern of granitoids in this study and is generally preferable to the use of ams alone