sources of magnetic susceptibility in a slate abstract magnetic susceptibility and its anisotropy in the borrowdale volcanic slates at kentmere in the english lake district are attributed largely to preferred orientation of a paramagnetic chlorite of diabantite ripidolite composition in units of 10 6 cgs g the principal susceptibilities for the slates are 9 61 9 42 8 69 and for the chlorite grains the minimum anisotropy is represented by principal susceptibilities of 11 57 11 22 and 9 15 because the magnetic susceptibility is carried by a tightly packed matrix forming mineral that has recrystallised during the deformation it is not possible to imagine simple grain rotation as being responsible for the anisotropy of susceptibility

magnetic susceptibility fabrics in slates structural mineralogical and lithological influences abstract a single penetrative tectonic deformation of archean greywackes accompanied by low grade metamorphism generated low field magnetic susceptibility fabrics that are coaxial with the rock s schistosity although there are multiple sources of magnetic susceptibility pre metamorphic magnetite and the metamorphic minerals pyrrhotite biotite and chlorite variety thuringite daphnite aphrosiderite are responsible for the anisotropy of susceptibility these minerals grew with preferred crystallographic orientations influenced by the prevailing stress regime on its own rotation cannot be invoked to explain the alignment of the metamorphic minerals there are varying proportions of at least two of these minerals in most specimens for 153 specimens the degree of anisotropy increases as the mean susceptibility increases over the range from 100 to 700 in units of 10 6 s i theory shows that this is to be expected either due to increasing the traces of strongly oriented ferrimagnetics or due to increasing the proportion of chlorite in the latter case this simultaneously increased its preferred orientation by closer packing stress influenced crystallization of chlorite rapidly achieves saturation alignment and a limiting magnetic anisotropy although strain may continue to increase samples taken through individual graded beds show that the susceptibility ellipsoid varies from oblate in slaty rocks to a neutral shape in sandier rocks however its orientation is always structurally controlled with the minimum susceptibility axis perpendicular to schistosity

magnetic susceptibility anisotropy of rocks saturated with ferrofluid a new method to study pore fabric abstract the shape effect of ferrofluid filled cavities on the anisotropy of magnetic susceptibility ams is examined for different geometries principal directions of magnetic susceptibility correspond to principal axes of the shape of the cavity elongation directions and axial ratios are reflected in the ams response which for simple geometries is explained by the anisotropy of the demagnetizing factor the experiments can be carried further for the analysis of pore structure in natural rock samples techniques developed for the injection of fractionated ferrofluid into a porous sandstone successfully lead to a homogeneous saturation of the sample the method potentially provides a new means to determine three dimensional pore fabric anisotropy and to relate it to the anisotropy of various petrophysical parameters

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

direct observations of velocity and transport in the passages between the intra americas sea and the atlantic ocean 1984 1996 shipboard acoustic doppler current profiler observations of the velocity in the upper 200 m of the water column collected during 1984 1996 using the national oceanic and atmospheric administration r v malcolm baldrige are used to examine the velocity structure and transport in the passages between the atlantic ocean and the intra americas sea ias data were collected during 23 cruises along the following sections across the straits of florida in the northwest providence channel nwpc across the northern passages into the caribbean sea windward mona and anegada across the eastern caribbean along 63 30 w thereby forming a closed quadrangle and in the grenada passage the florida current the eastern caribbean and the grenada passage share a similar mean velocity structure characterized by high velocity surface intensified flows with strong vertical and horizontal shears the northern caribbean passages nwpc windward mona and anegada share a different common mean velocity structure with subsurface velocity maxima directed into the ias and surface intensified counterflows along one side of each passage on average there is a transport balance in the upper 200 m between waters entering and exiting the ias with the 16 5 2 4 sv 1 sv 106 m3 s transport of the florida current at 27 n comprised of 0 4 0 8 sv from the nwpc 2 2 1 5 sv from the windward passage 2 8 2 1 and 2 4 2 8 sv from the mona and anegada passages respectively and 9 5 4 7 sv across the eastern caribbean for a total of 17 3 sv the four passages north of 17 n from nwpc to anegada passage have a combined transport of 7 8 sv nearly half of the transport of the florida current in the upper 200 m of the 9 5 sv flowing through the eastern caribbean between 11 n and 17 n 4 9 2 6 sv or more than half come from the grenada passage this is significant to the subject of cross equatorial exchange of mass heat and salt as the grenada passage is where the highest transport of waters originating in the southern hemisphere is thought to enter the caribbean

transition from extensional to compressional magnetic fabrics in the cretaceous cabuerniga basin north spain abstract the transition between extensional and compressional driven magnetic fabrics in sedimentary rocks is explored in this paper through the study of an example of the basque cantabrian basin in the area where extensional structures prevail and no superimposed deformation is observed except for gentle large scale folds the magnetic fabric is interpreted as extensional in consistency with mesostructural tension gashes and macrostructural large scale faults data compressional tectonic fabrics are unequivocally interpreted in the area with cleavage development related to the buttressing of the syn rift sequence against faults located near the northern basin margin in this area k max is oriented according either to the intersection lineation or the dip direction of cleavage planes in the area located in between where no macroscopic evidence of either compression or extension exist there is a transitional fabric between compressional resulting from the modification during inversion of a previous sedimentary or extensional fabric and extensional inherited from the extensional stage magnetic fabrics that correlate with subtle evidences at the microscopic scale pressure shadows deformation and re orientation of nodules therefore the magnetic fabric is revealed as an exceptionally sensitive marker of deformation in sedimentary rocks this transition in the magnetic fabric occurs within a length of 6 25 km along the cross section that correlates with a thickness of 200 m of the stratigraphic pile these results indicate that even in the absence of clear structural markers of compressional deformation extensional magnetic fabrics can be only interpreted when there is a minimum separation in the vertical or the horizontal to the cleavage front

separation of a component of tectonic deformation from a complex magnetic fabric abstract a method has been developed for separation of the deformational component from a complex rock fabric for the separation data on the final measured fabric and the primary pre deformational fabric are necessary as input the primary fabric data are obtained either from measurements in terrains where gradual transitions from primary to deformational fabrics occur or in the case of magnetic anisotropy this may be obtained from data in the literature on individual rock types examples of the use of the method are presented for rocks from the nizk jesenik mountains of the ne bohemian massif

development of planar and linear fabrics in dauphinois shales and slates french alps studied by magnetic anisotropy and its mineralogical control the progressive development of planar and linear fabrics due to alpine deformation in dogger calcareous shales has been studied in the dauphinois zone of the french alps using measurements of the anisotropy of magnetic susceptibility the axes and shapes of magnetic ellipsoids and petrofabrics show good qualitative correlation through a series of different fabric types from a sedimentary to a highly deformed one together with various petrological techniques magnetization measurements in a variable field up to 5 t and under variable temperature with a cryogenic magnetometer were used to achieve a complete analysis of the magnetic mineralogy with its dia para and ferromagnetic s l contributions the effects on the anisotropy of the complex mineralogy and various geological processes including pressure solution metamorphism and weathering are discussed

open system constant volume development of slaty cleavage and strain induced replacement reactions in the martinsburg formation lehigh gap pennsylvania no important changes in the volume of mudrocks occur across the mudstone to slate transition in the martinsburg formation at lehigh gap pennsylvania mass balance calculations based on chemical analyses and specific gravity measurements of 48 mudstones and 26 graywackes from the martinsburg formation across this 130 m strain gradient show constant ratios of al2o3 tio2 feot mgo mno y v and zr this reflects the relatively low differential mobility of these components during diagenesis and slaty cleavage development as well as the uniformity of compositions of the mudstones and graywackes at deposition using minimum strain samples and al2o3 as references the calculations show no loss in sio2 but large losses of cao na2o ba and sr from the outcrop that are directly proportional to strain and reflect an open system on the scale of kilometers analyses of graywacke mudstone metagraywacke slate pairs show that losses in sio2 na2o and volume and gains in k2o and ba in the graywacke metagraywacke are balanced by opposite changes in the adjacent mudstone slate this documents a local mobility of sio2 on the scale of centimeters the inertness of most major components over the 100 m outcrop especially sio2 however requires 1 that the composition of the fluid passing through the rocks was closely buffered by the quartz albite muscovite chlorite assemblage 2 that the volume of the fluid was small and 3 that the p t gradient down which it traveled was gentle the changes in chemical composition identified can be explained by the inferred replacement reaction plag 1m musc k 2m musc qz ca 2 na sr 2 ba 2 the strong correlation of minor element chemistry and strain indicates that this reaction was strain induced and that changes in bulk chemical composition are defined by the progress of this reaction

thermal history of uht metamorphism in the napier complex east antarctica insights from zircon monazite and garnet ages abstract high grade gneisses from mt riiser larsen east antarctica have been dated by whole rock mineral sm nd and shrimp zircon and monazite u pb to help define the thermal history of ultrahigh temperature uht metamorphism in the napier complex both the monazite and youngest zircon yield a range of apparent ages 2 51 2 47 ga consistent with crystallization during an extended period of metamorphism some zircon also preserves an isotopic record of earlier events placing an upper limit of a few million years on the duration of peak metamorphic conditions the similarity of the monazite and zircon u pb ages implies rapid initial postpeak cooling to below the blocking temperature of these minerals 900 c consistently lower sm nd whole rock mineral isochron ages 2 38 ga indicate that cooling slowed before the temperature reached 650 c the history of the uht metamorphism is interpreted to be 1 protracted high temperature 800 c conditions 2 51 2 47 ga 2 peak conditions up to 1100 c

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

early proterozoic tectonothermal events in the napier complex east antarctica implications for the formation of east gondwana abstract sm nd internal isochron ages involving retrograde garnet determined from three ultrahigh temperature uht gneisses taken from tonagh island the western part of the napier complex east antarctica gave 1870 82 ma 1897 50 ma and 1557 35 ma these ages are younger than the late archaean timing of uht metamorphism in the napier complex the ca 1900 ma age is considered to reflect an important tectonothermal event in the napier complex including a tholeiite dyke intrusion on the other hand the ca 1600 ma age represents a thermal modification lacking signs of deformational events and separates from the ca 1900 ma event the related east gondwana fragments such as the rayner complex in antarctica and the eastern ghats belt in india record extensive tectonothermal event of ca 1400 1600 ma and rare indications of ca 1900 2000 ma it is stressed that the assembly of east gondwana including the napier complex the rayner complex and the eastern ghats belt if it existed should be before ca 1600 ma and may be traced back to ca 2000 ma of the supercontinent columbia era

the geochemistry and sm nd isotopic systematics of precambrian mafic dykes and sills in the southern prince charles mountains east antarctica

the influence of deformation mechanisms on magnetic fabrics in weakly deformed rocks abstract magnetic fabrics of weakly deformed rocks are not simply correlated with the principal finite strain directions depositional magnetic fabrics persist at 30 shortening where the rocks are deformed by particulate flow grain boundary sliding rocks with a spaced stylolitic cleavage believed to be formed by pressure solution and shortened by 20 50 have maximum susceptibility parallel to the intersection of cleavage and bedding this type of magnetic fabric is attributed to the interference of a depositional magnetic fabric between cleavage surfaces with a tectonic magnetic fabric in the cleavage surfaces

slaty cleavage development and magnetic anisotropy fabrics the shale to slate transition preserved in the ordovician martinsburg formation at the lehigh water gap pennsylvania provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage our previous work has indicated that anisotropy of magnetic susceptibility ams does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of ehlorite additional ams data presented in this paper lend further support to this conclusion conversely anhysteretie remanent magnetization anisotropy arma which is not affected by paramagnetie ehlorite accurately reflects the strain induced rock fabrics associated with cleavage formation arma results show that magnetite dimensional orientations vary from bedding parallel in shale samples to cleavage parallel in samples with well developed slaty cleavage samples with weak and pencil cleavage display scattered arma orientations which lie in between bedding and cleavage these intermediate orientations may be due to either passive rotation of magnetite from bedding parallel to cleavage parallel or re crystallization of magnetite if rotation occurred grain rotation was highly heterogeneous in the samples with incipient cleavage the intermediate arma orientations may also reflect the varying contribution of two magnetite preferred orientations a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage

a transtensional basin model for the organya basin central southern pyrenees based on magnetic fabric and brittle structures summary the purpose of this paper is to explain the kinematics of the inverted extensional organy basin centralsouthernpyrenees duringitsextensionalevolution thestudyoftheanisotropy of magnetic susceptibility ams and the study of brittle mesostructures are the instruments used to decipher the extensional kinematics separation of magnetic subfabrics analyses of ams at low temperature and anisotropy of the anhysteretic remanence aarm has been also done to properly interpret the obtained magnetic fabric data the combination of all magnetic fabric and brittle mesostructures analyses has provided new information to clarify thestructuralevolutionofthebasin whichsupportanewtectonicmodelfortheaptian albian basinal stage 20 sites for the magnetic fabric analysis were sampled in aptian lower albian marls the ams ellipsoids obtained at room temperature show their kmin axes perpendicular to bedding and kmax axes oriented nw se in 70 per cent of the samples n s in 15 per cent and ne sw in 15 per cent analyses of ams at low temperature show a better clustering of the axes of the magnetic ellipsoid pointing out that phyllosilicates constitute the carriers of the ams whereas the analyses of the aarm confirm the scattering of the ferromagnetic grains these results suggest that the obtained ams in the organy a basin was acquired during or shortly after deposition of the aptian lower albian marls without any subsequent overprint the direction of the magnetic lineation obtained from the ams at room and low temperatures agrees with the orientation of the stress axes obtained from the analysis of brittle mesostructures mainly faults and calcite filled tension gashes indicating its tectonic origin we interpret the orientation of the magnetic ellipsoids related to the transtensional movements linked to the formation of the organy a basin during the aptian albian interval which are consistent with the eastward displacement and rotation of iberia with respect to europe analysis oftensiongashesalsoindicatesamainnw seextensiondirection whereaspalaeostressanalysis of striated faults shows a more n s trending 3 the transtensional model proposed from magnetic fabric and brittle mesostructures for the organy a basin during the aptian albian is consistent with the creation of a strongly subsiding area south of the axial zone during the cretaceous and implies that rotation and separation of iberia respect to europe took place simultaneously

palaeoproterozoic assembly of the napier complex southern india and western australia implications for the evolution of the cuddapah basin abstract a detailed examination of lithostratigraphic tectonothermal geochronologic and palaeomagnetic database of the dharwar bastar cratons of south india the yilgarn craton of western australia and the east antarctic shield has shown broad similarity two palaeomagnetic key poles of 2400 ma age from the dharwar and yilgarn cratons suggest near neighbor positions matching patterns of craton outlines orientation of mafic dyke swarms and palaeo north direction for 2400 ma have established a continental assembly of siwa an acronym for south india si and western australia wa at 2400 ma in this assembly the yilgarn craton fits against the bastar craton and the singhbhum craton the available geological data from the napier complex of the eastern antarctica and the dharwar craton of south india were used to prepare the barcode style reconstruction of archaean tectonostratigraphic events of both the cratons the method of matching continental outlines the internal features of two continental blocks and the palaeomagnetic data demonstrate that the napier complex was also a part of siwa during the period 2200 1900 ma and was located at the position of the cuddapah basin of india adjacent to the dharwar craton and the yilgarn craton the separation of the napier complex from the south indian block led to the development of the cuddapah basin at 1950 ma

shrimp and electron microprobe chronology of uht metamorphism in the napier complex east antarctica implications for zircon growth at 1 000 c zircons in ultra high temperature uht metamorphosed paragneisses from mt riiser larsen in the napier complex east antarctica were dated by using ion microprobe shrimp and electron microprobe emp both shrimp and emp analyses yield consistent 2520 2460 ma age populations for garnet orthopyroxene bearing paragneiss and leucosomes enclosed within the peak uht event was dated at 2480 ma by shrimp analyses on metamorphic zircons from the garnet orthopyroxene paragneiss and those on magmatic zircons from the leucosomes which are interpreted to be formed at syn uht as obtained by shrimp the uht metamorphic event was terminated no later than 2460 ma minor 2520 ma shrimp age suggests either the onset of prograde metamorphism or another high grade metamorphic event unrelated to the uht emp analyses on metamorphic zircons from sapphirine quartz and osumilite bearing magnesian paragneisses give c 2500 2450 ma ages inherited igneous zircon cores of the magnesian paragneisses yield relatively scattered emp ages ranging over c 3000 2650 ma suggesting that igneous materials of these ages sourced the protoliths of the paragneisses and that they were deposited during the interval c 2650 2520 ma

early to middle proterozoic dykes in the mt riiser larsen area of the napier complex east antarctica tectonic implications as deduced from geochemical studies

strain anisotropy of anhysteretic remanence and anisotropy of magnetic susceptibility in a slaty tuff finite strain data for the borrowdale slaty tuffs compare variably with the anisotropy of magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm finite strain determined from lapilli rims shows that slaty cleavage was formed by coaxial flattening with x y z in the ratio 1 74 1 21 and 0 48 aarm was measured in different coercivity windows to isolate contributions from magnetite of different grain sizes a 0 3 mt for multi domain md b 3 15 mt for pseudo single domain psd and c 15 60 mt for single domain sd ams combines petrofabric contributions from silicates as well as magnetite magnetite grains may grow recrystalize or rearrange domains after or during metamorphism and postdate or overlap with the silicate s fabric evolution ams foliation defined by paramagnetic chlorites is parallel to slaty cleavage aarm foliation for sd magnetites is offset clockwise from ams foliation which may reflect late crystallization or domain rearrangement of magnetites in response to a latter noncoaxial increment ams fabric shape consistently corresponds to strain ellipsoids and indicates that the strain induced ams fabric is susceptible to the change of oblateness rather than strain intensity furthermore investigation of the different aarm subfabrics and finite strain shows that only sd magnetite s aarm correlates with finite strain and weakly at that 2001 elsevier science b v all rights reserved

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

the rayner complex of east antarctica complex isotopic systematics within a proterozoic mobile belt new isotopic rb sr u pb zircon and sm nd and petrological data are presented for part of an extensive proterozoic mobile belt locally known as the rayner complex in east antarctica much of the belt is the product of mid proterozoic 1800 2000 ma juvenile crustal formation melting of this crust at about 1500 ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived deformation and transitional granulite amphibolite facies conditions which peaked at 750 50 c and 7 8 kbar 0 7 0 8 gpa produced open to tight folding about e w axes and syn tectonic granitoids about 960 ma ago subsequent felsic magmatism occurred at about 770 ma and not as has been widely advocated at 500 550 ma which appears to have been a time of widespread upper greenschist facies 400 500 c metamorphism localized shearing and faulting sm nd model ages of 1 65 2 18 ga disprove a previously favoured hypothesis that the rayner complex mostly represents reworked archaean rocks from the neighbouring craton napier complex models that involve rehydration of the napier complex are no longer required since the rayner complex was its own source of water two episodes of proterozoic crustal growth are identified the later of which occurred between about 1200 ma and 1000 ma and was relatively minor sedimentation took place only shortly before late proterozoic orogenesis the multiphase history of the rayner complex has resulted in complex isotopic behaviour three temporally discrete episodes of pb loss from zircon have been identified the earliest two of which are responses to the c 960 ma and 540 ma tectonothermal events fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth pb loss during the high grade event was probably governed by the same process or by lattice annealing some zircon suites also document recent pb loss most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete pb loss during two or more secondary events whereas all zircon separates from the orthogneisses produce u pb isotopic alignments zircons from the only analysed paragneiss produce scattered data in part reflecting a range of provenance the 960 ma event was also associated with the growth of a characteristically low u zircon 300 g g in rocks of inferred high zr content there is ubiquitous evidence for the resetting of rb sr total rock isochrons even samples separated by up to 10 km fail to produce igneous crystallization ages minor mineralogical changes produced by the 540 ma upper greenschist facies metamorphism were sufficient to almost completely reset some rb sr isochrons and to produce open system conditions on outcrop scale at least in one location

feldspar thermometry in ultrahigh temperature metamorphic rocks evidence of crustal metamorphism attaining 1100 c in the archean napier complex east antarctica ultrahigh metamorphic temperatures attained in the mid to lower crust have been assessed by examining the mineral chemistry of ternary feldspars with relatively coarse exsolution lamellae from the archean napier complex east antarctica chemical compositions of re integrated perthitic mesoperthitic and antiperthitic feldspars are calculated from the modal proportions and the chemical analyses of host and lamellar domains formed through exsolution based on ternary feldspar solvus models re integrated compositions of feldspars from a variety of rock types yield the minimum equilibrium temperatures ranging from 1000 to 1110 c 0 8 gpa these data confirm the suggestion that the regional thermal conditions of the napier complex reached or exceeded 1100 c as feldspar is one of the common constituents of the crustal rocks this approach could be applicable to a wide variety of rocks in which feldspar represents exsolution textures

a view of the canary basin thermocline circulation in winter during january and february 1989 the recirculation of the subtropical gyre in the eastern north atlantic was surveyed with a three ship experiment the analysis of hydrographic measurements and velocity data from a shipboard acoustic doppler current profiler reveals the synoptic scale circulation patterns and water mass distributions in the canary basin the geostrophic transport stream function estimated with a horizontally varying reference level of no motion highlights the major currents in three layers representing the vertical structure of the horizontal circulation the classical circulation scheme is shown by the stream function in the upper 200 m the azores canary and north equatorial currents unlike the deep penetrating azores current the canary current and the north equatorial current are restricted to the upper 200 m both carry north atlantic central water along the water mass boundary with south atlantic central water south atlantic central water flows through the passage between the cape verde archipelago and africa via narrow currents into the area north of 14 5 n at the southern edge of the subtropical gyre we identify an eastward flow of antarctic intermediate water between 700 and 1200 m

chlorite control of correlations between strain and anisotropy of magnetic susceptibility abstract anisotropy of magnetic susceptibility ams has in the past been used as an indicator of finite strain mathematical relationships between the measured ams ellipsoid and the finite strain ellipsoid in slates are numerous the ordovician martinsburg formation at lehigh gap pennsylvania provides a unique opportunity to test this correlation as a complete shale to slate transition is preserved magnitudes of the principal susceptibility axes appear to reflect a strain gradient at this location apparent strain values derived from ams using equations from rathore and hrouda have x y z ratios from 2 79 1 87 0 19 shale to 2 92 0 82 0 42 slate the orientations of the susceptibility axes characterize two fabrics i k int k max planes at 20 to bedding and ii k int k max planes parallel to cleavage anisotropy of magnetic susceptibility fails to show any intermediate orientations of the principal susceptibility axes in the zone in which cleavage is developed comparison with phyllosilicate crystallographic orientations indicates that the ams intermediate and maximum susceptibility axes are parallel to chlorite basal planes because chlorite reoriented via dissolution and new growth as cleavage developed in this location and because models correlating ams and strain instead assume that strain is reflected by the rotation of the magnetic susceptibility carriers the apparent strain values calculated from ams reflect only the degree of dissolution and neocrystallization in these rocks and not the finite strain

u pb zircon dating of mafic dykes and its application to the proterozoic geological history of the vestfold hills east antarctica the vestfold hills one of several archaean cratonic blocks within the east antarctic shield comprises a high grade metamorphic basement complex intruded by at least nine generations of early to middle proterozoic mafic dykes extensive u pb ion microprobe shrimp analyses of zircons derived predominantly from late stage felsic differentiates of the mafic dykes provide precise crystallisation ages for several dyke generations these new ages enable constraints to be placed on both the history of mafic magmatism in the vestfold hills and the timing of the various interspersed proterozoic deformation events in addition to demonstrating the utility of zircons derived from felsic late stage differentiates for the dating of co genetic mafic dykes this study also places doubt on previous wholerock rb sr dating of mafic dyke suites in this and other areas of east antarctica the 207pb 206pb zircon ages of 2241 4 ma and 2238 7 ma for the homogeneous and mottled norites respectively provide a younger emplacement age for associated group 2 high mg tholeiite dykes than the whole rock rb sr date 2424 72 ma originally interpreted as the age of all high mg intrusives in the vestfold hills zircon ages of 1754 16 ma and 1832 72 ma confirm the previously defined rb sr age of the group 2 fe rich tholeiites two later dyke generations the group 3 and 4 fe rich tholeiites are distinguished on the basis of field orientations and cross cutting relationships and yield zircon emplacement ages of 1380 7 ma and 1241 5 ma which also define minimum ages for two suites of lamprophyre dykes xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478 5 ma to 2740 ma indicating the presence of archaean crustal source rocks of this antiquity beneath the vestfold hills

u pb geochronology from tonagh island east antarctica implications for the timing of ultra high temperature metamorphism of the napier complex abstract ion microprobe u pb zircon geochronology of an orthopyroxene bearing felsic orthogneiss from central tonagh island enderby land east antarctica provides insight into the chronological metamorphic evolution of the archaean napier complex the details of which have been the source of debate for over two decades the orthogneiss crystallised at 2626 28 ma predating peak ultra high temperature uht metamorphism and development of an intense regional s1 gneissosity two subsequent episodes of zircon growth resetting can be identified a minor period of zircon growth occurred at 2546 13 ma the regional significance and geological nature of which is unclear this was followed by an episode of abundant zircon growth as mantles on 2626 ma cores and as anhedral grains partly characterised by high th u 1 2 at 2450 2480 ma this age coincides with both lower and upper concordia intercept ages from other u pb zircon studies and several rb sr and sm nd whole rock isochron ages from the napier complex we conclude that uht metamorphism occurred at 2450 2480 ma and find no compelling evidence that uht occurred much earlier as has been postulated the zircon u pb data from this study also indicates a lower intercept age of 500 ma which coincides with the emplacement of early palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the napier complex

in situ u pb geochronology and geochemistry of a 1 13 ga mafic dyke suite at bunger hills east antarctica the end of the albany fraser orogeny abstract antarctica contains continental fragments of australian indian and african affinities and is one of the key elements in the reconstruction of nuna rodinia and gondwana the bunger hills region in east antarctica is widely interpreted as a remnant of the mesoproterozoic albany fraser orogen which formed during collision between the west australian and mawson cratons and is linked with the assembly of rodinia previous studies have suggested that several generations of mafic dyke suites are present at bunger hills but an understanding of their origin and tectonic context is limited by the lack of precise age constraints new in situ shrimp u pb zircon and baddeleyite dates of respectively 1134 9 ma and 1131 16 ma confirm an earlier rb sr whole rock age estimate of ca 1140 ma for emplacement of a major mafic dyke suite in the area existing and new geochemical data suggest that the source of the dyke involved an emorb like source reservoir that was contaminated by a lower crust like component the new age constraint indicates that the dykes post date the last known phase of plutonism at bunger hills by ca 20 million years and were emplaced at the end of stage 2 of the albany fraser orogeny in current models post orogenic uplift and progressive tectonic thinning of the lithosphere were associated with melting and reworking of lower and middle crust that produced abundant plutonic rocks at bunger hills a major episode of mafic dyke emplacement following uplift cooling and plutonic activity with increasing mantle input suggests that the dykes mark the end of a prolonged interval of thermal weakening of the lithosphere that may have been associated with continued mafic underplating during orogenic collapse if the undated olivine gabbro dykes with similar trend geochemistry and petrology at windmill islands are coeval with the ca 1134 ma dyke at bunger hills this would suggest the presence of a major dyke swarm at least 400 km in extent in such case the dykes could have been emplaced laterally from a much more distant mantle source possibly a plume and interacted with the locally heterogeneous and variably metasomatised lithosphere

geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica evidence for mantle metasomatism two episodes of tholeiite dyke emplacement have been identified in archaean high grade metamorphics of the napier complex in enderby land middle proterozoic amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low pressure crystal fractionation group i amundsen tholeiites were derived from a relatively homogeneous source region 1 190 200 m y ago whereas that of the group ii amundsen tholeiites was chemically and isotopically heterogeneous group ii dykes have various degrees of enrichment in incompatible elements and commonly show normalised trace element abundance patterns with negative nb anomalies these features imply variable metasomatism of the source region by a volatile rich fluid phase rather than a melt of any observed igneous composition enriched in k rb ba th and possibly la and ce early proterozoic 2 350 48 m y tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite facies metamorphism and include a high mg suite of possible komatiitic affinity ranging in composition from hypersthene rich tholeiite norite to quartz rich tholeiite they tend to have higher ratios of highly to moderately incompatible elements e g k zr k ce and larger nb anomalies i e higher k nb compared with middle proterozoic tholeiites suggesting derivation from more enriched source regions isotopic data are not compatible with significant crustal contamination but constrain source metasomatism to a time immediately before emplacement metasomatism of the source region of the much younger group i tholeiites may have been contemporaneous with that of the high mg suite

the difficulties of dating mafic dykes an antarctic example archaen gneisses of the vestfold hills of east antarctica are transected by several compositionally discrete suites of tholeiitic dykes a representative of one of those suites which has been dated in the present study shows that not only rb sr whole rock isochrons but also u pb zircon techniques if not properly applied can produce erroneous crystallisation ages two zircon populations were recovered from the mafic dyke itself one of which is 2 483 9 ma in age and clearly of xenocrystic origin the other yields an age of 1 025 56 ma which is not ascribed to the magmatic crystallisation of the dyke but rather to the time that it underwent metamorphic metasomatic alteration as a response to high grade tectonism in the adjacent mobile belt it is presumed that the zircon in question formed by the breakdown of another mineral or minerals possibly magmatic baddeleyite due either to ingress of a siliceous fluid or more probably by the release of silica from the breakdown of pyroxene to amphibole a cogenetic 1 2 cm wide felsic vein of late magmatic early hydrothermal origin also contains two zircon populations again most of the grains therein which are interpreted as of xenocrystic origin grew at 2 483 9 ma however a few euhedral zircons with very high u and th contents grew at 1 248 4 ma which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke

composite magnetic anisotropy fabrics experiments numerical models and implications for the quantification of rock fabrics housen b a richter c and van der pluijm b a 1993 composite magnetic anisotropy fabrics experiments numerical models and implications for the quantification of rock fabrics tectonophysics 220 1 12 magnetic fabrics from rocks with multiple mineral preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and or orientations of the individual mineral fabrics and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks these criteria include 1 the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics and 2 the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases composite magnetic fabrics are observed in the shales and slates of the martinsburg formation lehigh gap pennsylvania modeling of the ams anisotropy of magnetic susceptibility and arma anhysteretic remanent magnetization anisotropy behavior constrains the relative degree of anisotropy of the bedding parallel and cleavage parallel fabrics in particular arma model results allow a good estimate of magnetite fabric strength we conclude that in the presence of composite magnetic fabrics quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric such determinations require knowledge of the mineral s that are responsible for the measured magnetic fabric and their behavior during deformation

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

chemical th u total pb dating by electron microprobe analysis of monazite xenotime and zircon from the archean napier complex east antarctica evidence for ultra high temperature metamorphism at 2400 ma abstract the chemical th u total pb isochron method chime of dating was carried out on monazite xenotime zircon and polycrase in six samples of ultra high temperature uht granulites and one of pegmatite from five localities in the napier complex enderby land east antarctica despite the chronological heterogeneity in many grains the chime ages overall show strong evidence for a major event near 2420 ma analyzed monazite and xenotime grains except monazite from mt riiser larsen have a narrow range of chime ages 2412 42 ma from beaver island and 2429 36 2421 18 and 2431 22 ma from reference peak these ages are indistinguishable from chime ages on zircon 2436 17 and 2420 20 ma and on central older domains of monazite 2404 54 ma from mt riiser larsen and on a zircon rim 2430 15 ma from mt cronus we conclude that the ca 2420 ma ages relate to the uht metamorphism but we do not exclude the possibility that lead loss has lowered the ages by some 50 ma this interpretation is consistent with a preponderance of ages obtained by other methods on the napier complex particularly the ca 2475 25 ma age for a generation of pegmatites coeval with the uht event and containing the uht mineral assemblage beryllian sapphirine khmaralite quartz events prior to 2500 ma and after 2420 ma have also left their imprint on the chime ages for example a 3646 ma chime age suggests that a zircon core is inherited whereas ca 2200 and 2000 ma chime ages in zoned monazite could be due to further lead loss during post uht activity the much younger chime age of 1094 67 ma for monazite from a pegmatite at zircon point casey bay confirms 1000 1100 ma lead loss ages reported for this region and could result from fluid activity in an outlier of rayner activity near the boundary between the rayner and napier complexes approximately 2500 ma ages have been reported from granulite facies but not uht rocks in the madras and nilgiri blocks in south india these blocks and the napier complex could have constituted a single structural unit by late archean time

shipboard acoustic doppler current profiling in the eastern caribbean sea 1985 1986 procedures to collect and reduce shipboard acoustic doppler current profiles adcp for application to quantitative estimates of transport variation on a synoptic scale are described transport estimates based on data from five consecutive cruises across the eastern caribbean are discussed in terms of ensemble averaging and vertical smoothing of adcp records and satellite navigation procedures comparison of smoothed adcp with pegasus dropsonde current profiles indicates an error of 4 cm s 1 for earth referenced adcp eastward velocities comparisons with geostrophic current estimates in an area of significant and irregular ageostrophic effects indicate shipboard adcp as a more accurate and reliable method for establishing seasonal and spatial variability of transport within instrument specific depth limitations

kinematics and strain distribution in an orogen scale shear zone insights from structural analyses and magnetic fabrics in the gavarnie thrust pyrenees abstract this work aims to characterize the gavarnie thrust one of the large scale thrusts that define the alpine structure of the west central sector of the pyrenees a detailed comparison of structural analysis and magnetic fabrics is carried out for the paleozoic phyllonites of the gavarnie thrust in order to decipher strain distribution and transport direction the ams at room rt ams and low lt ams temperature and the airm can be correlated with the structural patterns kmin axes are mainly parallel to the pole of the s c or c planes and kmax axes are parallel to the transport direction and related to ductile s c structures furthermore the pj t changes across the shear zone characterize strain variations larger pj and t are found in the basal most deformed part of the shear zone and lower values are found where the interaction between alpine and variscan related petrofabrics is stronger we also interpret the reactivation of variscan inherited fabrics within the alpine shear zone in spite of the heterogeneous strain markers indicate a common top to the south n190e alpine transport direction which contrasts with the strong obliquity of the genetically related structures developed in the southern mesozoic sedimentary cover in this sense our data suggest a complete decoupling between basement and cover units during the alpine compression

geochemistry and petrogenesis of the proterozoic dykes in tamil nadu southern india another example of the archaean lithospheric mantle source approximately 1650 ma old nw se and ne sw trending dolerite dykes in the tiruvannamalai tnm area and approximately 1800 ma old nw se trending dolerite dykes in the dharmapuri dp area constitute major proterozoic dyke swarms in the high grade granulite region of tamil nadu southern india the nw and ne trending tnm dykes are compositionally very similar and can be regarded as having been formed during a single magmatic episode the dp dykes may relate to an earlier similar magmatic episode the dolerites are fe rich tholeiites and most of the elemental variations can be explained in terms of fractional crystallisation clinopyroxene and olivine are the inferred ferromagnesian fractionation phases followed by plagioclase during the late fractionation stages all the studied dykes have similar to many continental flood basalts cfb large ion lithophile element lile and light rare earth element lree enrichment and nb and ta depletion the incompatible element abundance patterns are comparable to the patterns of many other proterozoic dykes in india and antarctica to the late archaean 2 72 ga dominion volcanics in south africa and to the early proterozoic 2 0 ga scourie dykes of scotland the geochemical characteristics of the tnm and dp dykes cannot be explained by crustal contamination alone instead they are consistent with derivation from an enriched lithospheric mantle source which appears to have been developed much earlier than the dyke intrusions during a major crustal building event in the archaean the dyke magmas may have been formed by dehydration melting induced by decompression and lithospheric attenuation or plume impingement at the base of the lithosphere these magmas compared with cfb appear to be the minor partial melts from plume heads of smaller diameter and of shallow origin 650 km therefore the proterozoic thermal events could induce crustal attenuation and dyke intrusions in contrast to the extensive cfb volcanism and continental rifting generally associated with the phanerozoic plumes of larger head diameter 1000 km and of deeper origin at crust mantle boundary

magma flow in the east greenland dyke swarm inferred from study of anisotropy of magnetic susceptibility magmatic growth of a volcanic margin summary volcanic passive margins vpms are characterized by large volumes of melt emplaced within the lithosphere during break up processes several data and a recently proposed conceptual model of volcanic margin development suggest that vpms are fed from localized crustal zones of magma storage underlying large polygenetic volcanoes localized above diapir like instabilities of the asthenosphere we investigated the magma flow pattern within the coastparallel dyke swarm of the east greenland vpm which is the only outcropping vpm over a distance of 125 km the 44 sampled dykes are representative of the successive families of intrusions igneous petrofabrics are constrained by the measurements of the anisotropy of magnetic susceptibility the magnetic fabrics are of medium to low anisotropy p 0 flow related fabrics are recorded in 75 per cent of the sampled dykes we infer the flow directions from the imbrication geometry of the magnetic foliation planes at the dyke margins and check the results by measuring the preferred orientation of plagioclase in thin sections cut in the magnetic principal planes due to probable fabric superposition the magnetic lineation represents the zone axis for the distribution of magnetic foliation plane we obtained 23 reliable flow directions that are predominantly horizontal and directed away from identified crustal reservoirs this flow pattern supports the proposed model of vpm growth and emphasizes the localized nature of the magma sources in the mantle the entire flood basalt sequence appears to have been fed by a restricted number of crustal reservoirs and associated dyke swarms

noritic dykes of southern west greenland early proterozoic boninitic magmatism two major swarms of early proterozoic ca 2 1 ga basic dykes occur within the archaean craton of southern west greenland one swarm comprises ophitic and sub ophitic tholeiitic dolerites while the other the bn dyke swarm constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts the close geochemical similarity between a quenched norite and the coarser grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation the bn dykes are geochemically distinctive most having high mgo ca 16 cr and ni contents in conjunction with relatively high sio2 light rare earth ree and large ion lithophile lil element concentrations the texture mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas the bn dykes also correspond to proposed parental liquids of the bushveld complex and other major layered basic igneous intrusions the two dyke swarms are petrogenetically distinct the tholeiitic dolerites were derived from a relatively undepleted primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source the wide spread distribution of similar proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world wide scale at this time

caribbean current and eddies as observed by surface drifters abstract recent satellite tracked surface drifter trajectories were analyzed to describe the mean currents and eddies in the caribbean sea the structure of the caribbean current and its variability were determined from high resolution 1 2 degree maps of the mean velocity and eddy kinetic energy looping drifter trajectories were used to identify discrete cyclones and anticyclones and their characteristics were described and related to the structure of the mean flow the translation rate of eddies in different areas was found to be similar to the mean velocity of the local background flow fields suggesting that the eddies were largely advected by the background flow ten energetic anticyclones translated westward at 13 cm s in the venezuela and colombia basins these anticyclones tended to lie in two bands centered near 15 n and 17 n coinciding with two jets of the caribbean current the northern weaker jet contains water primarily from the north atlantic the southern stronger jet contains water from the tropical and south atlantic the anticyclones are thought to have formed in the eastern caribbean from the anticyclonic vorticity derived from north brazil current rings the ring vorticity enters the eastern caribbean through island passages and is probably amplified by the anticyclonic shear on the northern side of the jets southwest of cuba a cyclone anticyclone pair was observed to translate slowly 2 cm s westward into the yucatan current the cyclone was tracked for 10 5 months with four drifters making it the longest tracked of the caribbean eddies