magnetic susceptibility anisotropy strain and progressive deformation in permian sediments from the maritime alps france abstract the relationships among magnetic susceptibility anisotropy finite strain and progressive deformation have been studied in permian red shales and slates of the maritime alps southeastern france these rocks contain deformed reduction spots which serve as finite strain indicators the magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements the magnetic fabric changes progressively from oblate to prolate and back to oblate as deformation increases the quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined they differ between the less and more deformed areas perhaps due to a change in deformation mechanism accompanying an increase in metamorphism the relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control

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

origin of late archean granite geochemical evidence from the vermilion granitic complex of northern minnesota the 2 700 ma vermilion granitic complex of northern minnesota is a granite migmatite terrane composed of supracrustal metasedimentary rocks mafic rocks tonalitic and granodioritic plutonic rocks and granite the metasedimentary rocks are predominantly graywacke which has been regionally metamorphosed to garnet sillimanite muscovite bearing biotite schist and has locally undergone anatexis the mafic rocks form early phases within the complex and are of two types 1 basaltic amphibolite and 2 monzodiorite and essexite rich in large ion lithophile elements lile the members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks producing an early migmatite the granite is of two distinct varieties 1 white garnet muscovite biotite leucogranite s type chappell and white 1974 and 2 grayish pink biotite magnetite lac la croix granite i type the leucogranite occurs in the early migmatite and in paragneissic portions of the complex whereas the lac la croix granite is a late stage intrusive phase that invades the early migmatite and metasediment producing a late migmatite and forms a batholith this study focuses specifically on the origin of granite in the vermilion granitic complex chemical mass balance calculations suggest that the s type two mica leucogranite had a metagraywacke source and that the i type lac la croix granite formed via partial fusion of calc alkaline tonalitic material which may have been similar to rocks of the early plutonic suite this model is satisfactory for petrogenesis of similar late archean post kinematic granites throughout the canadian shield

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

interpretation of satellite elevation magnetic anomalies the magnetic field satellite magsat obtained total field data over almost the whole of the earth we have studied these data in two different ways the first way was to use spherical harmonic coefficients it is often believed that these coefficients can be separated into those originating within the core of the earth and those from the crust at about degree 13 because of the change in slope of the power spectrum using coefficients above degree 13 in two different representations we have shown that there is little latitudinal variation of the source function and that the continent to ocean source function ratio is almost unity we suggest that these unlikely observations are in reality due to the possibility that much of the signal carried in coefficients above degree 13 may originate within the core of the earth if this is true it is then possible to keep the energy external to the core of the field generated within the core at a constant even though the dipole field has varied in strength by a factor of 3 over the past 104 years we have also produced models of crustal magnetization using the 2 2 average anomalous field and have shown how it is possible to obtain a physically meaningful susceptibility model in which magnetizations are always positive this is done by using an annihilator which is a magnetization distribution within a spherical shell which produces no external field the use of an annihilator is also appropriate for many problems connected with oceanic basins since present day oceanic crust is everywhere less than 180 ma the average magnetization seen at the altitude of a satellite is either zero for crust which is produced during times of rapid field reversal or positive for crust produced during times of constant normal polarity some results from continental and oceanic areas are discussed in the light of applying an annihilator

a comparison of satellite and observatory estimates of geomagnetic secular variation a comparison is made between secular variation values computed at the positions of 168 magnetic observatories from two spherical harmonic models based almost entirely on the 7 month spread of magsat data and observed secular variation values derived from the observatory annual mean values the two models differ from each other rms difference in z 27 8 nt yr and from the observed values rms differences in z of 22 1 and 30 6 nt yr it appears that a time span of more than 7 months is needed to produce accurate models of the secular variation from satellite data

compositional control of anisotropy of remanent and induced magnetization in synthetic samples anisotropy of remanent and induced magnetization was found to be compositionally controlled in synthetic samples of varying proportions of chlorite a platy paramagnet manganese oxide an isotropic paramagnet and magnetite an anisotropic ferrimagnet grains were magnetically aligned in a 100 mt field then uniaxially compressed at 27 6 mpa to produce an l s fabric anisotropy of anhysteretic susceptibility aas and anisotropy of magnetic low field susceptibility ams are coaxial and anhysteretic susceptibility is consistently more anisotropic than low field susceptibility ams increases with the addition of oblate magnetite but decreases with addition of isotropic mn2o3 to chlorite these experiments demonstrate that magnetic anisotropy depends on the shape relative magnetic intensity and proportion of the magnetic minerals

building archean cratons from hadean mafic crust geologic processing of earth s surface has removed most of the evidence concerning the nature of earth s first crust one region of ancient crust is the hudson bay terrane of northeastern canada which is mainly composed of neoarchean felsic crust and forms the nucleus of the northeastern superior province new data show these 2 7 billion year old rocks to be the youngest to yield variability in neodymium 142 142nd the decay product of short lived samarium 146 146sm combined 146 147sm 142 143nd data reveal that this large block of archean crust formed by reworking of much older 4 2 billion year old mafic crust over a 1 5 billion year interval of early earth history thus unlike on modern earth mafic crust apparently could survive for more than 1 billion years to form an important source rock for archean crustal genesis

anisotropy of magnetic susceptibility of heated rocks abstract heating produces changes which does not always correspond to simple enhancement of the magnetic fabric two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating the first diagonalizes the tensor from the difference between each tensor term before and after heating the second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment when the same magnetic fabric is obtained from several thermal steps it cannot be related to randomly oriented ferrimagnetic minerals instead the newly formed fabric must be related to characteristics of the pre existing rock by comparing this ferrimagnetic minerals fabric with the initial whole rock fabric we can distinguish cases where heating simply enhances pre existing fabric from those where thermal treatment induces a different fabric relative to the pre heating fabric this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction related to petrostructural elements other than those defining the initial fabric

missing eu anomaly and archean high grade granites granites emplaced in low grade granite greenstone archean terranes typically exhibit negative eu anomalies and such eu anomalies have been transmitted to post archean cratonic sediments by weathering and erosion because average continental crust does not exhibit an eu anomaly a complementary reservoir with a positive eu anomaly must exist to balance the low grade granites neither the upper mantle nor average archean lower crust appears to represent this reservoir archean lower crustal rocks which are dominantly plutonic rocks of tonalitic composition have ree distributions indicative of a mafic parentage however archean high grade granites in southern india exhibit the required positive eu anomalies and may be representative of a widespread positive eu anomaly reservoir in the lower crust geochemical model studies indicate that archean low and high grade granites may represent residual liquid and cumulate respectively from the fractional crystallization of a granite parent magma produced by partial melting of tonalitic rocks in the lower crust such melting may occur at the wave front of a co 2 rich fluid that dehydrates the lower crust and produces granulites

high resolution magnetic field modeling application to magsat and orsted data abstract launched on 23rd february 1999 the orsted satellite opened the decade of geopotential field research this is the first satellite to measure the three components of the earth s magnetic field since magsat 1979 1980 orsted orbital parameters are very similar to those of magsat allowing a first order comparison of the 1979 and 2000 magnetic fields using the available vector and scalar data over the first 14 months of the orsted mission and applying classical selection criteria local time external magnetic activity we compute a 29 degree order main field model and a 13 degree order secular variation model for the period 1999 2000 the applied method and the accuracy of the derived model are discussed we compare the resulting main field model to a similar one derived from magsat data results of this comparison are presented such as i morphology and energy spectrum of the secular variation and ii morphology of the crustal magnetic field at magsat and orsted epochs

reply to discussion of magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes borradaile and gauthier 2005

experimental shear zones and magnetic fabrics abstract magnetic fabric analysis has been used as a non destructive means of detecting petrofabric development during experimentally produced multi stage transpressive deformations in shear zones artificial magnetic bearing silicate sands and calcite sands bonded with portland cement were deformed at room temperature and at 100 and 150 mpa confining pressure the slip rate for the shear zone walls was 0 73 10 4 mm s 1 and the maximum shear strains were about 0 38 across zones that were initially about 5 mm thick the magnetic fabric ellipsoid rapidly spins so that the maximum and intermediate susceptibilities tend to become parallel to the shear zone walls throughout the sheared zone the ellipsoid becomes increasingly oblate with progressive deformation however in all cases the anisotropy is strongly influenced by the pre deformation magnetic fabric during deformation the cement gel collapses so that cataclasis of the mineral grains is suppressed in the quartz feldspar aggregates the magnetite s alignment is accommodated by particulate flow intergranular displacements of the grains in the calcite aggregates stronger magnetic fabrics develop due to plastic deformation of calcite grains as well as particulate flow however the calcite grain fabrics are somewhat linear l s whereas the magnetic fabrics are planar s l the preferred dimensional orientations of magnetite are weak and it is possible that the magnetic fabrics are due to intragranular rearrangements of magnetic domains the transpressive shear zones are much more efficient than axial symmetric shortening in the increase of anisotropy of the magnetic fabrics especially in the case of the calcite aggregates this suggests that flow laws derived for axial symmetric shortening experiments may not be appropriate for non coaxial strain histories such as those of shear zones

magnetic and petrofabric studies in the multiply deformed thomson formation east central minnesota abstract two contrasting zones of deformation have been previously recognized in the proterozoic thomson formation of east central minnesota the northern zone consists of open upright e w trending folds with a well developed steeply south dipping axial planar cleavage the southern zone is marked by a pervasive subhorizontal cleavage s 1 crenulated by a second cleavage that is vertical to steeply south dipping s 2 paleomagnetic results and anisotropy of anhysteretic remanent magnetization aarm in these rocks indicate that the ferrimagnetic particles preserve a record of the field during deposition and primary compaction neither nrm nor aarm were much affected by tectonic deformation by contrast the low field magnetic susceptibility which is dominated by paramagnetic minerals particularly chlorite reflects the tectonic fabric of the rock anisotropy of magnetic susceptibility ams is characterized by minima normal to the cleavage in the northern zone and to the s 1 cleavage in the southern zone thus the s 2 deformation did not significantly affect the ams fabric march strains were obtained by measuring the 002 lattice preferred orientation of chlorite using an x ray pole figure goniometer the principal orientations of march strain match well those of ams and ams and march strain ellipsoids for selected samples in both northern and southern zones plot in the flattening field of a flinn diagram based on ams signature it is possible that the deformation in the northern zone occurred at the same time as the first deformation in the southern zone this could have involved simple shear on subhorizontal planes giving way to pure shear with horizontal shortening towards the north in this scenario the second deformation the result of regional late stage shortening was only marked by a cleavage s 2 in the southern zone due to the favorable orientation there of s 1

comparison of strain and magnetic fabrics in dalradian rocks from the southwest highlands of scotland abstract the magnetic fabrics of 235 samples from 31 localities in argyllshire scotland were determined to study the development of the caledonian tectonic fabric in the southwest highlands of scotland the regional fabric indicates a strong ne sw compressional foliation due to the primary deformational phases which in parts has been overprinted by secondary deformations a detailed comparison of the anisotropy data and the available strain data shows that the two fabric ellipsoids are co axial and that their axial mean ratios seem to be related by an empirical power relationship of the type xi xj li lj a for i 1 2 3 j 1 2 3 and i j where i and j are orthogonal principal axes and li and lj are the corresponding orthogonal principal strain axes the exponent a for the sites from scotland is 0 088 0 017 compared with 0 142 0 001 and 0 145 0 005 found in the caledonian slates of the english lake district and the welsh slate belt

magnetic susceptibility anisotropy of mylonites from the brevard zone north carolina u s a abstract this study presents the results of magnetic susceptibility anistropy measurements on mylonites from the brevard zone of the southern appalachians the brevard zone is a major crustal mylonite zone and the samples analyzed for this study represent the progressive increase of mylonitization decrease in grain size of the henderson gneiss with increasing proximity to the brevard zone our results suggest that magnetic fabric measurements in mylonites are at least qualitatively significant with respect to the strain history of mylonites the orientations of the principal susceptibilities are parallel to rock fabric elements indicating that the magnetic fabric is a deformationally induced phenomenon although the henderson gneiss appears to undergo a smooth progressive mylonitization with increasing proximity to the brevard fault zone our magnetic measurements suggest that strain states do not increase smoothly as the brevard zone is approached however the magnetic measurements do suggest that at some sites the strain is very high the highest site mean anisotropy max min mean 100 we have measured is 120 cgs which could based on strain to magnetic susceptibility anisotropy msa correlations by other workers indicate strain ratios 1 e 1 1 e 3 of between 186 and 1 4 1017 because we cannot correlate our magnetic fabric measurements with strain in these rocks we cannot prove that such high strain states in fact exist in the brevard mylonites however these appear to the highest magnetic anisotropies yet recorded our data also indicate that the susceptibility ellipsoid is severely flattened with k values commonly lower than than 0 1 we also observe an inverse relationship between k values and anistropy similar relationships have been observed in other shear zones and suggest that mylonites follow a strain path of increasing departure from plane strain k 1

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

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

observatory crustal magnetic biases during magsat and orsted satellite missions 1 computing main field models derived from magsat and orsted satellite magnetic measurements the crustal influence on satellite data can be treated as random noise satellite data can thus be used in conjunction with magnetic observatory measurements to isolate the non core field at the observatory locations crustal biases for the horizontal northward x eastward y and vertical downward z components are computed for all magnetic observatories where data are available for magsat 1979 1980 and orsted 1999 2000 epochs to study their correlation over twenty years for a set of observatories installed after 1979 new crustal biases are computed

experimental deformation of synthetic magnetite bearing calcite sandstones effects on remanence bulk magnetic properties and magnetic anisotropy we have quantified effects of experimental deformation on the magnetic properties of a set of synthetic calcite sandstone samples containing magnetite the deformation was carried out in a microcomputer controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 5 s 1 most samples were deformed under dry conditions but a few were deformed with a pore fluid present the samples deformed under dry conditions required substantially higher differential stresses macroscopically ductile shortening strains of up to 25 produced the following irreversible changes in magnetic properties 1 increased bulk coercivity remanence coercivity and mean anhysteretic remanence susceptibility 2 decreased mean low field susceptibility 3 decreases in the component of remanence parallel to shortening 4 smaller decreases for most samples in the component normal to shortening resulting in a net rotation of the remanence away from the shortening axis 5 larger decreases in the normal component in a few samples resulting in a net rotation of the remanence towards the shortening axis 6 increased magnetic anisotropy and 7 increased deformation of initial magnetic ellipsoids a comparison of data for samples deformed under dry and wet conditions higher and lower differential stresses respectively indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain i e rotation and displacement of particles whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress

anisotropy of magnetic susceptibility in diamagnetic limestones reveals deflection of the strain field near the dead sea fault northern israel abstract to exploit the potential of anisotropy of magnetic susceptibility ams as a tool to estimate the strain field around major faults we measured the ams of calcite bearing diamagnetic rocks that crop out next to the dead sea fault dsf in northern israel through integrated magnetic and geochemical methods we found that the rocks are almost pure calcite rocks and therefore the magnetic fabric is primarily controlled by preferred crystallographic orientation pco with the minimum principal ams axes k 3 parallel to calcite c axes we applied a separation procedure in several samples with high fe content in order to calculate the ams anisotropy parameters and compare them to pure diamagnetic rocks aarm thermo susceptibility curves and irm were used to characterize the magnetic phases we found that for fe content below 500 ppm the ams is mostly controlled by the diamagnetic phase and showed that differences in the degree of anisotropy p up to 3 p 1 005 to 1 023 and in anisotropy difference k up to 0 25 10 6 si in diamagnetic rocks are related to differences of strain magnitudes the spatial distribution of the magnetic fabrics indicates n s maximum shortening parallel to the strike of the hula western border fault hwbf one of the main strands of the dsf in northern israel the anisotropy parameters suggest that the strain magnitudes increase eastward with the proximity to the hwbf these results suggest that the strain field near the hwbf is locally deflected as a consequence of the dsf activity in light of the fault weakness model and geological setting of the study area we suggest that the area accommodates dominant transtension during the pleistocene the present study demonstrates the useful application of ams measurements in iron free limestones as recorders of the strain field near plate boundaries

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

correlation of strain and the anisotropy of magnetic susceptibility in the onaping formation evidence for a near circular origin of the sudbury basin abstract measurements of finite strain the anisotropy of magnetic susceptibility the magnetic carrier of the susceptibility and fieldwork have been carried out in the onaping formation of the sudbury basin ontario the magnetic fabric indicates that changes in the magnitude and type of magnetic susceptibility ellipsoids closely mirror the changing pattern of deformation from northwest to southeast across the onaping formation outcrops a quantitative correlation has been established between finite strain measurements and the magnetic susceptibility results and this allows the number of available strain measurements to be extended by more than 500 samples the finite strain results suggest a deformation path in which the onaping formation was first affected by nw se directed horizontal layer parallel shortening which produced an upright tectonic cleavage subsequently components of overthrust nw directed simple shear were superimposed upon the already deformed rocks of the onaping formation the result of removing the strain according to the sequence of superimposed deformations revealed by the deformation path suggests that the sudbury basin was nearly circular in shape prior to its deformation consequently a major objection to the meteorite impact theory for origin of the sudbury structure is eliminated

secular variation crustal contributions and tectonic activity in california 1976 1984 five to ten years of data from an array of 34 total field magnetometers are used to define the temporal and spatial characteristics of secular variation throughout central and southern california for this period well determined rates of secular variation are obtained at each site these rates are temporally linear but spatially variable ranging from 45 nt a near san francisco to 54 nt a near the mexican border least squares analysis of all data indicates secular variation decreases in a general southeasterly direction according to f k1 k2 k where f is in nanoteslas per year and are the geographic latitude and longitude and k1 k2 and k are 1 66 0 13 nt a deg 0 13 0 10 nt a deg and 123 2 0 2 nt a respectively deviations of as much as 1 nt a occur on scales of a few tens of kilometers these apparent small scale secular variation anomalies result in part from differences in local induction and remanent magnetization and may be reduced by determination of a site transfer function a planar surface fit to the corrected data has the form f k1 k2 k where k1 k2 and k are now 1 50 0 08 nt a deg 0 23 0 06 nt a deg and 129 2 0 1 nt a respectively residual field variations obtained after correction of all data for secular variation are most apparent on the san andreas fault in southern california between palm springs and the salton sea and marginally so along the recent coyote ml 5 9 of august 6 1979 and morgan hill ml 5 8 of april 24 1984 aftershock zones these residuals could be explained by stress localization in these regions and particularly in the case of the southern san andreas anomaly may indicate the location of a future damaging earthquake incomplete correction for complex site effects may be an alternative explanation in the first large scale test of global secular variation models we find that the models for this region do not predict either the amplitudes or the mean isogram directions of these data to better than several tens of nanoteslas per year and several tens of degrees respectively this may result from a failure to correct for site response effects at some observatories before using the data in global spherical harmonic expansions local magnetization response can therefore bias estimates of secular variation and yield apparent impulsive behavior when external fields are perturbed

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

episodic mantle melting crustal reworking in the late neoarchean of the northwestern north china craton zircon ages of magmatic and metamorphic rocks from the yinshan block abstract the yinshan block in the northwestern north china craton comprises late neoarchean greenstones high grade metamorphic rocks granulite charnockite enderbite and amphibole gneiss diorites sanukitoid and mg adakite and granitoids predominantly trondhjemite tonalite a greenstone terrane comprises a volcanic sequence of predominant basalt and dacite with minor andesite zircons from a high mg andesite and an andesite have weighted mean 207pb 206pb ages of 2533 5 ma and 2510 7 ma respectively which reflect the timing of two distinct volcanic episodes these two either predated or postdated a major volcanic episode that formed a basalt dacite bimodal suite during 2515 10 basalt to 2516 10 ma dacite magmatic zircons from the high grade metamorphic rocks yielded weighted mean 207pb 206pb ages of 2544 5 ma for a mafic granulite 2521 6 ma for a garnetiferous gneiss 2535 5 ma for a charnockite and 2526 5 ma for an amphibole gneiss these ages reflect the minimum emplacement time of rocks that make up the protoliths of the high grade metamorphic complex the metamorphic ages of the high grade complex were determined at 2515 18 ma for mafic granulite and 2503 12 ma for garnetiferous gneiss zircons from a high mg diorite i e sanukitoid have a weighted mean age of 2520 6 ma which is significantly younger than the 2556 14 ma age of a magnesian diorite mg adakite from the same diorite massif these ages reflect distinctive magmatic pulses that generated the diorite massif a tonalite has a zircon age of 2516 7 ma indistinguishable from the 2520 9 ma age of a trondhjemite from the same trondhjemite tonalite dominated batholith a granitic compositional layer in the 2516 7 ma tonalite has zircon ages of 2515 6 ma high u zircon domains and 2512 22 ma low u zircon domains which reflect an intracrustal melting event the presence of an older crustal remnant ca 2600 ma prior to the formation of the yinshan block is indicated by a 2594 4 ma leucogranite xenolith in the diorite massif and by the widespread presence of ca 2583 3485 ma mostly ca 2700 ma zircon xenocrysts in a variety of volcanic and plutonic rocks an undeformed pegmatite dike in high mg andesite has an emplacement age of 2508 17 ma zircon an undeformed trondhjemite dike that cuts the supracrustal greenstone sequence yielded a zircon age of 2502 6 ma similar to that of the pegmatite dike and representing the latest archean magmatic event in the yinshan block these ages of undeformed granitoid dikes delimit the termination of thermo tectonic development of the yinshan block based on these new ages and published geological and geochronological data we conclude that the yinshan block is a typical neoarchean cratonic block not a segment of the so called paleoproterozoic inner mongolia north hebei orogen episodic mantle melting and resultant crustal reworking account for the thermo tectonic development of the yinshan block in a brief period ca 2556 2502 ma in the late neoarchean and based on their regional chronological correlations we argue for their craton scale significance

palaeomagnetism of the permian volcanic rocks of moissey french jura implications for the palaeofield and tectonic evolution rocks from the massif de la serre in the french jura latitude 47 3 n longitude 5 6 e belonging to an ignimbritic assemblage dominated by vitrophyric rhyolites and whose age of formation is probably permian autunian to saxonian have been studied by applying thermal and alternating field demagnetization the characteristic magnetization has a mean direction derived from 89 samples of d 170 i 16 k 26 2 95 3 and a corresponding north palaeopole at 41 n 172 e a95 5 the pole which is very close to the permian european poles can thus be considered as a new contribution some samples are found to carry a unique normal polarity magnetization others carry both normal and reverse polarities it therefore seems that similar to permian series in the ussr these west european rocks have registered a normal event in the kiaman interval from a structural point of view we may conclude that during the alpine tectonic phases the massif de la serre has not been subjected to substantial rotation

magnetic fabrics in the bjerkreim sokndal layered intrusion rogaland southern norway mineral sources and geological significance abstract magnetic anisotropy can provide important information about mineral fabrics and thus magmatic processes particularly when it is known how multiple mineral species contribute to the anisotropy it may also affect the direction of induced or remanent magnetization with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies here we aim at describing the magnetic fabrics in the bjerkreim sokndal layered intrusion and identifying their carriers anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the bjerkreim sokndal layered intrusion the intrusion is characterized by magmatic layering and has a synform structure with strong foliation on the limbs detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial but the minimum susceptibility and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering the minimum susceptibility and anhysteretic remanence are associated with pyroxene 100 axes and the maximum susceptibility and anhysteretic remanence are sub parallel to the pyroxene 001 axes in layers mcu ivc and mcu ive for which electron backscatter data are available however the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy we propose that the magnetic anisotropy of magnetite free specimens is carried by hemo ilmenite exsolutions within pyroxene in addition to pyroxene itself when present multi domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence due to shape preferred orientation and distribution anisotropy the orientation of the magnetic fabric appears independent of carrier due to their common deformation history but the degree of anisotropy is stronger for magnetite bearing specimens the results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection

compositional control of magnetic anisotropy in the thomson formation east central minnesota abstract anisotropy of magnetic low field susceptibility ams and anisotropy of anhysteretic susceptibility aas of the middle precambrian thomson formation east central minnesota generally record structural features but orientations and axial ratios of the susceptibility ellipsoids depend on local magnetic mineralogy the northern structural zone of the thomson formation is characterized by an upright east west slaty cleavage the susceptibility ellipsoids plot in the flattening field of a flinn diagram with minimum axes normal to the cleavage aas ellipsoids are coaxial with ams ellipsoids but are generally more prolate anisotropy increases with mean susceptibility at one outcrop interpreted as mixing of weakly anisotropic paramagnetic chlorite with more strongly anisotropic magnetite a second outcrop shows the opposite pattern anisotropy decreases with increasing mean susceptibility suggesting that here the magnetite may be relatively isotropic or possibly post deformational the southern structural zone of the thomson formation is characterized by a subhorizontal foliation s 1 and an upright crenulation cleavage s 2 here the magnetic susceptibility minima are coincident with poles to s 1 and maxima are parallel to the s 1 s 2 intersection correlation between directional susceptibilities and mean susceptibility technique of henry and daly 1983 suggests that paramagnetic minima are poles to the early foliation while possibly interdeformational magnetite minima record the second deformation this interpretation however is not supported by the aas fabric by thin section observations or by high field measurements of paramagnetic susceptibility alternatively both directional and mean susceptibilities may be controlled by syndeformational chlorite

a comparative study of magnetic anisotropy measurement techniques in relation to rock magnetic properties abstract magnetic anisotropy measurements are becoming increasingly common to many studies within the different disciplines of geology involving sedimentary igneous and metamorphic rocks a plethora of techniques exists for measuring magnetic anisotropy of rocks some are rapid and non destructive while others are more labor intensive or may result in alteration of the magnetic minerals all however have the potential of revealing a wealth of information when measured and interpreted correctly in broad terms anisotropy techniques subdivide into measurements of susceptibility remanence and torque here we consider the first two of these anisotropy of magnetic susceptibility ams is by far the most utilized and measures composite fabrics magnetic susceptibilities in high fields and low temperatures however are being increasingly used to isolate the paramagnetic contribution to the fabrics when distinguishing between fabrics carried by different ferromagnetic phases or to separate these from the diamagnetic and paramagnetic contributions to the fabric then remanence anisotropy techniques become necessary anisotropies of thermal remanence atrms of anhysteretic remanence aarm and of isothermal remanence airm are the most common examples remanence anisotropy may be measured over the full spectrum of magnetic coercivities or over a targeted range e g partial or aparm moreover anisotropies may be calculated using only the resolved field parallel component of the vector in which case a minimum of six different orientations is necessary to obtain a complete symmetric tensor or using the three components full vector of the measured magnetic vectors e g avarm in which case three orthogonal applied magnetizations are the minimum requirement in this study we utilize a variety of magnetic remanence room temperature techniques to measure remanence anisotropy of selected coarse and finer grained gneiss granulitic specimens with well pronounced fabrics results are compared to room temperature ams and are interpreted in terms of the applicability of instrumentation technique to specific rock magnetic properties

magnetic anisotropy of some phyllosilicates magnetic susceptibility anisotropy of susceptibility and hysteresis of single microcrystals of chlorite biotite phlogopite muscovite zinnwaldite and fuchsite were measured in low and high magnetic fields with an alternating gradient force magnetometer micromag their properties are sufficient to account for the low field susceptibility ams of most micaceous rocks nearly all samples show some ferromagnetic contribution at low fields due to inclusions of pseudosingle domain and multidomain magnetite the paramagnetic contribution isolated at high fields usually exceeds the ferromagnetic contribution the paramagnetic susceptibility is intrinsic to the silicate lattice and agrees with values predicted from chemical composition within the limits of error the minimum susceptibility is nearly parallel to c another axis is parallel to b and the third susceptibility usually the maximum is close to a the paramagnetic susceptibility has a disk shaped magnitude ellipsoid with strong anisotropy p 2 the ferromagnetic contributions at low fields have more variably shaped ellipsoids with greater eccentricity p 5 the silicate lattice does not constrain their orientation our technique cannot determine the principal axes of the ferromagnetic component however its principal values usually correspond with the paramagnetic principal susceptibilities in order of magnitude thus the combined paramagnetic ferromagnetic anisotropy recognised in routine studies of ams should faithfully represent the petrofabric of most micaceous rocks nevertheless nearly 10 of our samples have incompatible anisotropy ellipsoids for the silicate host and magnetite inclusions these yield a net inverse ams that does not correctly represent the orientation of the silicate lattice therefore some caution is necessary in petrofabric ams studies of micaceous rocks

anisotropies of partial anhysteretic remanence and susceptibility in compacted black shales grainsize and composition dependent magnetic fabric we have quantified the magnetic fabric of the highly compacted heebner shale pennsylvanian of kansas in terms of the anisotropies of partial anhysteretic remanent magnetization parm and low field susceptibility hysteresis loops exhibit almost purely paramagnetic behavior and the ratio of high field linear susceptibility to low field susceptibility is very close to one the susceptibility anisotropy therefore reflects the fabric of the paramagnetic matrix while the remanent anisotropies are due to the preferred orientations of ferrimagnetic grains both the paramagnetic and ferrimagnetic fabrics are dominated by a horizontal foliation with a weak northeasterly lineation but the parm anisotropies are generally much larger than the susceptibility anisotropy the degree of foliation measured with parm s decreases markedly with increasing coercivities indicating stronger foliation in the coarser grainsize fraction in contrast the parm mean lineations are grainsize independent suggesting a post compactional origin related to tectonic compression the susceptibility lineation appears to have a slightly different azimuth and may therefore be due to paleocurrents

synthetic magnetic fabrics in a plasticene medium mixtures of ferrimagnetic grains with a supporting medium of paramagnetic plasticene permit hypotheses concerning the interpretation of assemblages of different minerals to be confirmed the deformation of such plasticene mixtures in pure shear transpression and simple shear emphasize the dominance of the initial anisotropy over the end result for low strains 25 shortening in pure shear for minor amounts of multidomainal ferrimagnetic grains the deformation experiments indicate that the magnetic lineation rotates rapidly into the maximum extension direction for the three strain histories tested although the magnetic fabric directions are reliable kinematic indicators after modest strain in simple shear the symmetry of the fabric is not a plane strain fabric great discrepancies occur in the case of assemblages single domain femmagnetic grains in plasticene in pure shear these produce inverse fabrics in which the magnetic lineation spins towards parallelism with the maximum shortening direction

interpretation quantitative de l anisotropie de susceptibilite magnetique abstract we propose a model in which the magnetic susceptibility anisotropy is considered as resulting from the distinct properties of two rock components this model suggests that differences in the susceptibility ellipsoid can be principally related to mineralogical variations thus some difficulties in the quantitative interpretation of anisotropy data may be explained this model is consistent with a detailed analysis carried on dalradian rocks from scotland a new methodology of anisotropy analysis is derived it is based on the fact that in specimens from a same sample the susceptibility ellipsoid often varies but relations nevertheless exist between values of the principal and mean susceptibilities for each sample measurements are made on 6 12 specimens the study of the relations between principal and mean susceptibilities permits separation of the anisotropies of the rock components the anisotropy determination appearing the best for the high susceptibility component

small scale features in the earth s magnetic field observed by magsat a spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of magsat data global maps representing the contribution due to terms of the expansion above n 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar even in polar regions an expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high order field structure at a constant altitude than do component maps derived independently the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 008 2 50 payment must accompany order

magnetic fabrics of the morcles nappe complex abstract anisotropy of magnetic susceptibility ams measured with high fields at ambient and low temperatures 77 k has been used to separate ferromagnetic sensu lato paramagnetic and diamagnetic sub fabrics for deformed limestones located in a nappe structure from southwest switzerland the magnetic fabrics are dominantly controlled by paramagnetic and diamagnetic minerals and ferromagnetic minerals contribute significantly only at two of the eight study locations separation of the magnetic sub fabrics is possible in the root zone and overturned limb of the morcles nappe which contains a high ratio of diamagnetic matrix calcite to paramagnetic secondary phases each of the sub fabrics displays a relationship with structural cleavage and the crystallographic preferred orientation cpo of calcite in the root zone and overturned limb the maximum axis of the paramagnetic ams ellipsoid kmax lies close to the pole to cleavage with the intermediate kint and minimum kmin axes near the cleavage plane conversely the diamagnetic kmin lies close to the pole to cleavage with kmax and kint axes near the cleavage plane slight offsets in the poles to cleavage and in the principal ams directions are observed they are interpreted to be the consequence of a second phase deformation event affecting only the cpo but not the macroscopic structural elements the diamagnetic sub fabric reflects the cpo of calcite produced through recrystallization during nappe deformation iron rich calcite appears to control the paramagnetic sub fabric susceptibility ellipsoids approach oblate shapes for the diamagnetic sub fabric with the most extreme shapes reaching susceptibility differences k kmax kmin close to that of single crystal calcite whereas the paramagnetic sub fabric is represented mainly by prolate shape ellipsoids the low field ams ellipsoids show mixed oblate and prolate shapes without consistency within or between sites it is evident that when the magnetic sub fabrics of the root zone and overturned limb are separated their individual shapes and distribution of principal directions mirror the developed cpo

new evidence of a meridional current system in the equatorial ionosphere an interesting variation in the geomagnetic d component has been found by analysis of the magsat data characteristics of this variation are that it appears everyday on the low latitude duskside and is antisymmetric about the dip equator negative in the northern hemisphere that the magnitude from 5 to 25 nt depends upon the geographic longitude and altitude and that no corresponding change can be seen on the ground magnetic data the change suggests a meridional current system in the equatorial ionosphere produced in association with the electrojet current along the dip equator this effect should be taken into account in the study of geomagnetic anomaly of internal origin with the magsat data

geochemical characteristics and possible origins of the southern closepet batholith south india the polyphase closepet batholith in south india intrudes archean tonalitic and granitic gneisses in a terrane that grades from amphibolite to granulite grade at the southern end of the batholith two granite and two granodioritic suites are recognized in the batholith one granite suite from the southern terminus of the batholith has high sr ba la yb and eu eu and low sc y nb ta and total ree and is similar to other granites from along the amphibolite granulite transition zone that have the characteristics of a fractionally crystallized cumulate the other granite suite has lower sr and ba higher sc and y and no distinctive eu anomaly this latter group which is widespread in the southern closepet batholith can be derived by 20 equilibrium partial melting of a tonalite source containing hornblende and garnet that is compositionally similar to south india tonalites near the amphibolite granulite transition the two granite groups may have been produced by separate episodes of co 2 metam

mathematical model relationship between the paramagnetic anisotropy and strain in slates abstract a simple mathematical model is used to investigate the relationship between the paramagnetic anisotropy and strain in slates the relationship found is similar to the relationship of whole rock magnetic anisotropy to strain in weakly magnetic slates the anisotropy is probably mostly due to paramagnetic phyllosilicates and the whole rock relationship reflects the paramagnetic fraction

the near earth magnetic field at 1980 determined from magsat data data from the magsat spacecraft for november 1979 through april 1980 and from 91 magnetic observatories for 1978 through 1982 are used to derive a spherical harmonic model of the earth s main magnetic field and its secular variation at epoch 1980 0 the model is called gsfc 12 83 constant coefficients are determined through degree and order 13 and secular variation coefficients through degree and order 10 the first degree external terms and corresponding induced internal terms are given as a function of dst preliminary modeling using separate data sets at dawn and dusk local time showed that the dusk data contain a substantial field contribution from the equatorial electrojet current the final data set was therefore selected first from dawn data and then augmented by dusk data to achieve a good geographic data distribution for each of three time periods 1 november december 1979 2 january february 1980 and 3 march april 1980 a correction for the effects of the equatorial electrojet was applied to the dusk data utilized the solution included calculation of fixed biases or anomalies for the observatory data although similar in many respects gsfc 12 83 differs from international geomagnetic reference field 1980 by 3 6 nt in the g10 term and shows a slightly negative b in the northern polar region as well as other differences in secular variation pattern

geochemistry and origin of archean granites from the black hills south dakota the little elk granite 2549 ma and granite at bear mountain bmg 2 5 ga of the black hills formed as a result of a collisional event along the eastern margin of the wyoming province during th

evidence for non coaxiality of ferrimagnetic and paramagnetic fabrics developed during magma flow and cooling in a thick mafic dyke abstract a detailed analysis of fabrics of the chilled margin of a thick dolerite dyke foum zguid dyke southern morocco was performed in order to better understand the development of sub fabrics during dyke emplacement and cooling ams data were complemented with measurements of paramagnetic and ferrimagnetic fabrics measured with high field torque magnetometer neutron texture and microstructural analyses the ferrimagnetic and ams fabrics are similar indicating that the ferrimagnetic minerals dominate the ams signal the paramagnetic fabric is different from the previous ones based on the crystallization timing of the different mineralogical phases the paramagnetic fabric appears related to the upward flow while the ferrimagnetic fabric rather reflects the late stage of dyke emplacement and cooling stresses

magnetic anisotropy in the trenton limestone results of a new technique anisotropy of anhysteretic susceptibility a new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described the magnetic fabric of a collection of trenton limestone specimens has been determined using this method as well as by conventional anisotropy of magnetic susceptibility the results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the trenton in which the bulk magnetic susceptibilities are low a model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses the original fabric is inferred to have been isotropic the anisotropy resides in secondary magnetite of late paleozoic age it is argued that the observed magnetic fabric must therefore be alleghenian or younger in age our method has the potential to determine paleostress directions in carbonates elsewhere provided our assumptions are correct

plate tectonics 2 5 billion years ago evidence at kolar south india the archean kolar schist belt south india is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources the amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago ma along a zone marked by the kolar schist belt this history of accretion of discrete crustal terranes resembles those of phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 ma

was the 1970 geomagnetic jerk of internal or external origin annual mean data from worldwide magnetic observatories show that there was a jerk1 step change in the second time derivative in the geomagnetic field which took place over an interval of 2 yr around 1970 if such a short lived phenomenon originated in the earth s core and were still detectable after passing through the mantle to the surface this would imply a much lower conductivity for the lower mantle than had been believed previously here we show by an objective test that most of the jerk has an internal origin

on the origin of the magnetic fabric in purple cambrian slates of north wales anisotropy of magnetic susceptibility ams in samples of cambrian slate from penrhynn wales is surprisingly weak maximum minimum 1 1 to 1 2 in view of their large finite strain x z 3 5 to 4 5 and their hematite dominated magnetic mineralogy several lines of evidence suggest that this is a result of the presence of two or more generations of hematite sem observations show a somewhat bimodal size distribution of hematite particles with modal diameters of approximately 1 3 m and 10 20 m the larger grains tend to have ragged subequant shapes and contain titaniferous laminae they are probably of igneous detrital origin the finer grains have crystallographically controlled shapes are ti free and are probably authigenic or metamorphic initially chemical leaching of deeply incised solid specimens preferentially removes the more soluble particles and results in significant changes in ams further leaching attacks the remaining grains on exposed surfaces and the ams ellipsoid reverts to its initial shape isothermal remanent magnetization irm is significantly more anisotropic than low field susceptibility the variation of irm anisotropy with applied field indicates relatively strong fabric development in the fine grained hematite but weak fabric in the coarser fraction grain boundaries were digitized from sem photos in order to calculate flinn fabric ratios by the rf method the x z ratios thus obtained for quartz and fine hematite grains are 3 14 and 1 70 respectively it is therefore clear that the hematite fabric does not reflect the total finite strain

resolution of ferrimagnetic and paramagnetic anisotropies in rocks using combined low field and high field measurements summary a method of separation of ferrimagnetic and paramagnetic components of magnetic anisotropy from the total anisotropy is developed from the anisotropy measurement of a specimen in a low field susceptibility anisotropy and from measurement in two high fields stronger than the saturation field of the ferrimagnetic fraction present the following anisotropy components can be calculated susceptibility anisotropy field independent of the paramagnetic fraction low field susceptibility anisotropy of the ferrimagnetic fraction and high field anisotropy of the ferrimagnetic fraction errors possibly arising from imperfect saturation of the ferrimagnetic fraction in the field practically available are analysed

the use of magsat data to determine secular variation the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 004 2 50 payment must accompany order a combined spatial and secular variation model of the geomagnetic field labeled m061581 is derived from a selection of magsat data secular variation sv data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model the sv patterns from this new model are compared with those from the 1980 igrf and with those for 1970 computed by the dgrf and with the 1960 patterns computed using the gsfc 12 66 model most of the features of the m061581 are identical in location and level with those of the 1980 igrf together they confirm that the reversals in sign of field change seen over asia and north america between 1965 and 1975 are reverting to the pre 1965 states the m061581 model gives 32 nt yr for the dipole decay rate larger than the 70 increase already reported since 1965 this abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger 100 nt yr drop in field over the polar regions not indicated by the 1980 igrf this north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric counterclockwise afternoon sqp cell increasing in intensity from the beginning november 1979 to the end june 1980 of the magsat data collection period coupled with an enhancement of its effect as the orbit lowered from the 350 550 km initial altitudes to near 200 km just prior to burnup this experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that annual snapshots of the field by a satellite would allow easy and accurate models of secular change without the use of any surface data

frequency dependent ams of rocks a tool for the investigation of the fabric of ultrafine magnetic particles abstract in some geological processes new very fine grained magnetic minerals may originate the variation in content of these minerals is routinely investigated by frequency dependent magnetic susceptibility which is traditionally interpreted in terms of the presence of viscous superparamagnetic sp particles in addition to stable single domain ssd and multidomain md magnetic particles in addition the fabric of these grains can be investigated through the frequency dependent ams through standard ams measurement at different frequencies one can evaluate the contribution of sp particles to the whole rock ams appropriate methods were developed various rocks soils and ceramics showing frequency dependent magnetic susceptibility were investigated measurable changes of ams with operating frequency were revealed and attempts are made of their fabric interpretation

spherical cap harmonic analysis of geomagnetic secular variation over canada 1960 1983 the new technique of spherical cap harmonic anaylsis has enabled a model of secular variation to be constructed that applies to an area of over 30 million square kilometers and a time interval of over 20 years this technique is a regional analog of the ordinary method of spherical harmonic analysis and ensures that the curl and divergence of the secular variation vector are zero by making each harmonic coefficient a polynomial in time the model provides both spatial and temporal continuity the secular variation data from 1960 to 1983 comprise 1227 first central differences of annual mean values from 24 observatories in canada united states greenland and iceland and 939 first central differences from 138 canadian and american repeat stations divided by their respective time intervals the secular variation model includes terms up to spatial index 4 and temporal degree 2 although such a model allows for 75 coefficients only 39 were found to be statistically significant the temporal behavior of the model at observatories and repeat stations is compared on time plots to that of the secular variation data itself the scatter about the regression model or standard error of estimate was 11 8 nt yr the model is required both to update aeromagnetic survey data for producing canadian geomagnetic main field charts at 1985 0 and to provide annual change information for the extrapolation of those charts up to 1990 the average secular variations derived from the definitive dgrf and provisional pgrf international geomagnetic reference fields agree well with the model but the average secular variation for 1980 1985 from the predictive international geomagnetic reference field igrf does not

current challenges and future developments in magnetic fabric research abstract magnetic fabrics are used extensively as proxies for mineral fabrics and to correct paleomagnetic data and have been proposed to quantify pore fabrics today s understanding and interpretation of magnetic anisotropy benefits from decades of carefully observing anisotropic mineral and rock properties establishing and testing empirical relationships between magnetic and mineral fabrics and developing models to quantify aspects of anisotropy each advance was preceded by some puzzling observations i e data that could not be explained based on the models available at the time e g oblique or inverse fabrics or lithology dependent anisotropy strain relationships these observations led to numerous experimental and numerical techniques designed to characterize magnetic fabrics and determine their origin despite the successful application of magnetic fabrics in many structural and tectonic problems there are still phenomena and measurements that cannot be explained based on today s magnetic fabric theory with the purpose of fostering future development i will touch on three main areas where i see challenges in magnetic fabric research 1 experimental characterization of magnetic anisotropy including the non uniqueness of reported tensors and non linearity 2 anisotropy modelling and defining the carrier minerals and origin of the magnetic fabric 3 selection of adequate sets of anisotropy tensors to correct paleomagnetic data the description of the challenges presented here will hopefully help define directions for future research inform those that are new to anisotropy and advance our field

archaeomagnetic dating and palaeochannels sediments data from the mediaeval channel fills at hemington leicestershire this paper illustrates how archaeomagnetic dating can be used for palaeochannel fills within floodplain sequences the sites investigated are palaeochannels associated with the mediaeval braided anastomosing channels of an unstable reach of the river trent at hemington north west leicestershire this reach has revealed abundant archaeological remains including fish weirs anchor stones a norman mill and three mediaeval bridges the studies used here show good agreement between archaeomagnetic dating and other evidence from archaeology dendrochronology and radiocarbon dating fabric and mineral analyses of the sediments are used to identify the source of the remanent magnetization and it is believed that this is predominantly the result of the biogenic synthesis of magnetic minerals in high ph but anoxic conditions this study illustrates the type of alluvial sediments that are most suitable for archaeomagnetic dating

the time varying geomagnetic field the scope of this review is limited to papers on the large scale geomagnetic field of deep internal origin and its secular variation published by u s authors and their colleagues during the 1983 1986 quadrennial the many fine papers on this broad topic by authors outside the u s are regretfully excluded fortunately most of them are cited by the u s authors listed in the reference section of this review labreque and mayhew this issue review papers treating the smaller scale fields usually attributed to visco inductively and remanently magnetized crustal geologic structures review articles dealing with external fields are surveyed by m schulz in the magnetosphere section of this u s national report booker and chave this issue review papers concerning the use of more rapidly fluctuating externally induced internal fields to probe the electrical properties of the crust and upper mantle lund and olson this issue review papers concerning paleo secular variation and core geodynamo theory

geochemistry and origin of the archaean beit bridge complex limpopo belt south africa the beit bridge complex bbc in the central zone of the limpopo belt in southern africa includes metasedimentary rocks of archaean age chiefly metapelites quartzites and carbonates amphibolites chiefly mafic dykes and sills and granitoids which comprise 50 of the complex the amphibolites are tholeiites with relatively unfractionated ree patterns and pronounced subduction zone geochemical signatures in terms of relatively immobile incompatible elements tonalites and granites which also exhibit such a signature can be formed by batch melting of a mafic source with residual garnet or amphibole most granites have negative eu anomalies and appear to represent residual liquids bbc metapelites have negative eu anomalies and other immobile element distributions similar to phanerozoic shales one group of metapelites that shows significant depletion in ca na and sr appears to have been derived from intensely weathered sources the negative eu anomalies in bbc metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of southern africa by about 3000 ma bbc quartzites are quartz rich sandstones that are depleted in hfs elements ba and sr relative to phanerozoic counterparts a feature that probably reflects less recycling during the early archaean bbc banded iron formation bif is similar to other archaean bif and shows large positive eu anomalies bbc carbonates are dolostones depleted in ree hfs elements u th ba sr ni sc v and y and enriched in fe mn na rb and cs relative to phanerozoic marine carbonates

from qualitative to quantitative magnetic anisotropy analysis the prospect of finite strain calibration abstract a method of separating low field susceptibility anisotropies due to ferrimagnetics and the matrix proposed hitherto in cases where the principal axis of these two components are merged is generalized in cases where the axes are distinct the method consists of analysing in specimens close in the same sample anisotropies the differences of which are due to the variation of respective percentages of the two components although the strain rate remains constant one thus uses the cause which itself renders global anisotropy unusable a first application is presented where a control using the anisotropy of isothermal remanent magnetization allows the validity of the method to be tested a discussion of the choice of the best ellipsoid relative either to the matrix or to the ferrimagnetics to give the best indication of relative finite strain rate is opened

comparative petrogenesis and tectonics of paleoproterozoic malanjkhand and dongargarh granitoids central india abstract the malanjkhand granitoid mg and two units of dongargarh granitoids dgs represent contemporaneous episodes of paleoproterozoic felsic magmatism in the bastar craton central india a comparative geochemical study is carried out between the mineralized malanjkhand granitoid mg and barren dongargarh granitoids dgs that occur adjacent to each other separated by the dongargarh group of rocks major element oxides geochemistry reveals that mg is granite granodiorite whereas dg is strictly granite in composition and both plutons show calc alkaline affinity geochemical discrimination indicates that mg is an i type whereas an a type affinity can only be suspected in case of dg and both can be labeled as peraluminous trace element data from these paleoproterozoic granitoids indicate that they evolved in a tectonic environment similar to continental rift setting hfse ratios suggest variable degree of fractionation of highly differentiated granitic magma was a very complex crystallization process results in the formation of these two granitoids by common mechanism possibly fluid separation might be involved highly fractionated ree and depleted mree patterns with eu anomaly absent in mg and negative in dg inferred that the parental magma derived by low degree partial melting of heterogeneous mafic source probably amphibolitic in nature followed by progressive differentiation zircon saturation temperature and zr content reveals that mg crystallized from wet melt whereas dg crystallized from relatively dry melt but both are classify as hot granites thermobarometric calculations indicates that mg has emplaced at upper level whereas dg at relatively deeper or middle upper level in the continental crust the origin of palaeoproterozoic granitoids in central india has attributed to the magma mixing process between crustal derived felsic and mafic magma contributed by basaltic underplating in a continental rift environment

band limited global scalar magnetic anomaly map of the earth derived from magsat data the magnetometer satellite magsat data are separated into two distinct sets the dawn and the dusk sets the two sets are analyzed independently because of the dawn dusk asymmetry of the external magnetic field components of the data two scalar magnetic anomaly maps the dawn and the dusk maps are derived using the spherical harmonic expressions of the respective data set the significant correlation of the two maps in the harmonic range of 18 41 indicates the repeatability of the magsat data in these harmonics the shortest repeatable wavelength is about 920 km a band limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps the map is then downward continued to about 10 km altitude the downward continued magnetic anomalies are relatively more localized the main features of the anomalies are as follows 1 continents have relatively stronger magnetic anomalies than oceans especially the younger oceans 2 many circum pacific subduction zones have magnetic signatures and 3 there is no overall correlation between anomalies and major shields

modelling the relationship between magnetic fabric and strain in polymineralic rocks abstract we present a model that is applicable to the relatively frequent case of rocks in which the magnetic fabric is dominated by uniaxial paramagnetic minerals and in which the deformation pure shear corresponds to the march fernandez model borradaile et al s procedure for the isolation of the magnetic fabric of monocrystals of anisotropic components allows us to obtain the magnetic properties equivalent to those of all the component minerals if they should be artificially aligned in the case of real component minerals with similar shape parameters dependent on dimension ratio these properties will correspond to those of a theoretical equivalent mineral therefore finite strain and orientation tensors may be determined from magnetic fabric measurements of such polymineralic rocks

anisotropy of magnetic susceptibility constraints on variscan obduction processes in the braganca massif ne portugal abstract in this paper we present an anisotropy of magnetic susceptibility ams and structural study of high grade metamorphic and ophiolitic rocks from the braganca massif trying to contribute to the debate concerning the identification of transport directions during the closure of the variscan ocean the study area comprises high grade metamorphic rocks of the continental allochthonous terrane cat and the northern ophiolite terrane not in the braganca massif mineral parageneses and fabrics were characterized through conventional petrographic and microstructural studies electron microprobe and high field and thermomagnetic analysis with very few exceptions the magnetic lineations and foliations were coincident within limited error with the observed mesoscopic mylonitic and metamorphic foliations and with the mineral and stretching lineations in places of the studied major tectonic contacts where the mineral and stretching lineations were obliterated by post kinematic recrystallization ams data revealed a magnetic lineation ams results confirmed the nnw sse to n s amphibolite facies lineation common to cat and not and revealed an e w greenschist facies lineation in not ams results combined with detailed structural and metamorphic data from previous work showed that there are two e w lineations separated in time 1 an e w mesoscopic and magnetic lineation in competent granulites that is older than the n s magnetic lineation in more ductile gneisses of a major shear zone within the cat 2 an e w magnetic lineation in incompetent greenschists of the not younger than the nnw sse to n s mesoscopic and magnetic lineations found in more competent amphibolites of cat and not the nnw sse to n s lineations can be interpreted as the result of the uppermost allochthonous terranes transport to the nnw over the iberian terrane dated with 40 ar 39 ar at ca 390 ma