a comparison of archaeointensity results from chinese ceramics using thellier s and shaw s palaeointensity methods summary a collection of chinese ceramics covering the time period 2700 7500 bp has been investigated using adaptations of both the thellier thellier 1959 and shaw 1974 palaoeintensity techniques several rock magnetic techniques were used to investigate the magnetic properties of the samples and on the basis of these measurements some samples were rejected because of their poor magnetic behaviour anisotropy of magnetic susceptibility ams thermo remanent magnetization trm and anhysteretic remanent magnetization arm were investigated based on the relationship between arm and trm anisotropy modified arms were used for anisotropy correction twenty samples were studied using the shaw method and 34 were studied using the thellier method an acceptable agreement between the results from the two methods was found general features of the secular variation of geomagnetic intensity determined from the present study are consistent with those of the global model of mcelhinny senanayake 1982

tectonic evolution of the european variscan belt constrained by palaeomagnetic structural and anisotropy of magnetic susceptibility data from the northern vosges magmatic arc eastern france geochronology structural and anisotropy of magnetic susceptibility data from the northern vosges batholith which belongs to the northern vosges mid german crystalline rise arc show contrasting emplacement modes of southern granodiorites and northern granites the ene wsw trending fabrics of granodiorites 346 334 ma are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression the nnw sse trending fabrics of younger granitoids c 330 ma reveal an extensional emplacement mode associated with a normal shear zone separating the two magmatic suites palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation the 330 325 ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith finally the system is rotated clockwise without any structural overprint data from the early carboniferous northern vosges magmatic arc and remote bohemian massif suggest that this evolution is valid for the whole eastern branch of the european variscan belt for which the following model is proposed 1 late devonian early carboniferous east west shortening of the variscides above the rhenohercynian subduction zone 2 axial nnw sse shortening of the assembled variscan orogen associated with 3 internal sinistral rotation of inherited rhenohercynian transform faults and shortening of intervening blocks 4 east west extension and dextral external rotation of blocks between dextrally reactivated transforms supplementary materials analytical procedures for geochronology magnetism anisotropy of magnetic susceptibility and palaeomagnetism are available at www geolsoc org uk sup18635

paleomagnetism magnetic anisotropy and mid cretaceous paleolatitude of the duke island alaska ultramafic complex we report paleomagnetic results from layered igneous rocks that imply substantial post mid cretaceous poleward motion of the insular superterrane western canadian cordillera and southeast alaska relative to north america the samples studied are from the stratiform zoned ultramafic body at duke island which intruded rocks of the alexander terrane at the south end of the southeastern alaska archipelago at about 110 ma thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the curie temperature of magnetite this remanence is likely carried by low ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts the duke island intrusion exhibits a well developed gravitational layering that was deformed during initial cooling but below 540 c into folds that plunge moderately to the west southwest the characteristic remanence clearly predates this early folding and is therefore primary the fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site mean directions are corrected by restoring the layering to estimated paleohorizontal all samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion and we show that it has no significant or systematic effect on the site mean directions of remanence at least some of the anisotropy derives from secondary magnetite formed during partial serpentinization the mean paleomagnetic inclination 56 10 corroborates paleomagnetic results from five coeval silicic plutons of the canadian coast plutonic complex to the south and southeast and implies 3000 km 1300 km of poleward transport relative to the north american craton between mid cretaceous and middle eocene time the insular superterrane and coast plutonic complex shared a common paleolatitude history with more poleward transport than coeval inboard terranes

on the suitability of baked clay for archaeomagnetic studies as deduced from detailed rock magnetic studies summary extensive rock magnetic investigations have been carried out on baked clays from four kilns two from bulgaria and two from switzerland found in archaeological sites of different age knowledge of the magnetic characteristics of the grains responsible for the archaeomagnetic signal enables us to determine which baked clays have the stablest magnetization and why this is so this is important in directional studies but even more so in painstaking palaeointensity studies that require a very careful evaluation of the suitability of the burnt clay material the proposed rock magnetic experiments enable the identification of the carriers responsible for the remanence and an adequate interpretation of the experimental results connected with the palaeointensity evaluation the experimental methods employed are illustrated with the particular results obtained from each of the four kilns studied the preliminary elucidation of the magnetic mineralogy of the archaeological samples helps first by obtaining a more reliable palaeointensity result and secondly by explaining some of the discrepancies in the palaeodirectional results examples of successful and failed palaeointensity experiments are given in relation to the magnetic properties previously established for each oven the burnt clay materials in this present study satisfy the essential condition of carrying a thermoremanence in spite of that it is shown that there are many factors that can produce undesirable magnetic properties and thus restrict the suitability of these materials for archaeomagnetic analysis the most important factors influencing the magnetic behaviour during magneto diagnostic experiments are the degree of heating in antiquity the initial composition of the unbaked material and the burial conditions the large difference in heating temperatures within a particular archaeological feature is a major cause of variation in magnetic behaviour amongst individual specimens and so preventing a successful pre selection of specimens for palaeointensity experiments nevertheless the study has shown a very good coincidence between the determined rock magnetic characteristics and the success rate in palaeointensity evaluation

geophysical investigation of the honningsvag igneous complex scandinavian caledonides the honningsvag igneous complex within the mageroy nappe northern norway has marked magnetic and gravimetric signatures palaeomagnetic studies reveal a siluro devonian scandian dual polarity remanence ne and up sw and down mean declination 045 inclination 38 and a 95 8 4 rock magnetic and petrological studies indicate a pure magnetite host for the remanent magnetization component magnetic fabric ellipsoids delineate dl and d2 scandian structures affecting the area and demonstrate an internal reclined fold structure of the honningsvag igneous complex forward modelling demonstrates that a short wavelength pouitive aeromagnetic anomaly c 700 nt over the northwestern part of the complex is associated with an area of reverse remanent polarity sw and down conversely areas of normal remanence polarity ne and up most common are associated with magnetic lows or quiet zones the gravity field in the vicinity of the complex displays a symmetrical positive anomaly of approximately 200 g u wavelength 15 km this can be attributed to a density contrast of approximately 0 20 mg m 3 between the gabbros of the honningsvag complex 2 93 and surrounding metasediments 2 73 an optimal geophysical and geological model demonstrates that the complex has been tilted through 90 and now has the form of a steeply dipping reclined fold structure extending to a depth of at least 6 km palaeomagnetic remanence acquisition post dates emplacement of the mageroy nappe late dl and probably originated during post dl uplift brittle upper crust d2 deformation which continued into devonian times folded the honningsvag igneous complex and introduced a small but systematic dispersion in the palaeomagnetic remanence directions the magnetization is therefore post dl but pre d2 in age and can be termed syn scandian palaeomagnetic and isotopic evidence suggests a syn tectonic magnetization age of c 410 ma the palaeomagnetic pole n7 and e344 resembles that from the underlying kalak nappe complex it is therefore inferred that the scandian emplacement of the mageroy nappe was synchronous with a thermal event which affected the older and underlying finnmarkian defor med kalak nappe complex

using magnetic anisotropy to correct for paleomagnetic inclination shallowing in some magnetite bearing deep sea turbidites and limestones abstract in sediments remanence inclination i may be shallower than the inclination ih of the earth s field in which they were deposited use of magnetic anisotropy to correct for this paleomagnetic inclination shallowing is tested for quaternary turbidite muds from the scotian rise and for cretaceous pelagic limestones from the pacific plate both are dominated by pseudo single domain magnetite carrying a stable remanence whose inclination i determined by stepwise alternating field demagnetization is shallower than ih by an average of 12 for the turbidites and 17 for the limestones anhysteretic remanence arm applied identically to various axes is weakest armmin perpendicular to bedding and strongest armmax parallel to bedding with little anisotropy in the bedding plane and arm min arm max averaging 0 84 for the turbidites and 0 87 for the limestones magnetite also dominates susceptibility with minimum kmin and maximum kmax susceptibility axes perpendicular and parallel to bedding respectively and with k min k max averaging 0 91 for the turbidites and 0 95 for the limestones in theory we expect to find a correlation between tan i and arm min arm max for a suite of specimens with the correlation line s prediction of tan i when arm min arm max 1 yielding an ih estimate corrected for inclination shallowing this correction method is also expected to succeed if arm min arm max is replaced by k min k max provided susceptibility is dominated by multidomain or pseudo single domain magnetite in practice both anisotropies do succeed in correcting for inclination shallowing in three of the four turbidite cores both anisotropies would also likely succeed for the limestones if there were enough samples at any one site since correlating tan i tan i h in place of tan i for the five sites combined predicts tan i tan i h 1 when arm min arm max 1 or k min k max 1 hence we recommend routinely measuring anisotropy in paleomagnetic studies of magnetite bearing sediments and looking for a correlation between tan i and arm min arm max or k min k max remanence anisotropy is more laborious to measure but provides the more reliable estimate of inclination shallowing in cases where no significant correlation is found remanence anisotropy of the magnetite particles in the sediment may also have to be measured to correct for inclination shallowing

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

estimate of the magnetic anisotropy effect on the archaeomagnetic inclination of ancient bricks abstract the magnetic fabric of 59 bricks coming from 5 ancient kilns has been studied by measuring the anisotropy of magnetic susceptibility ams and the anisotropy of isothermal airm anhysteretic aarm and thermal atrm remanent magnetization the bricks are characterized by a well developed magnetic fabric that matches their flat shape the shape of the anisotropy ellipsoids is in almost all cases oblate with the maximum and intermediate axes lying parallel to the large face of the brick and the minimum axis perpendicular to it the directions of the principal axes are almost the same irrespectively of the type of anisotropy measured whereas the degree of anisotropy of the airm aarm and atrm is much higher than the ams as the bricks lie horizontally within the kiln the planar magnetic fabric results in an inclination shallowing of the archaeomagnetic direction with respect to that of the earth s magnetic field at the time of their last cooling estimation of this effect on the grounds of atrm measurements yields a shallowing that varies from 4 to 10 for individual samples such inclination difference may significantly bias archeomagnetic dating for the case of the canosa late roman kiln it leads to a dating error of more than two centuries

paleointensity of the geomagnetic field recovered on archaeomagnetic sites from france abstract twenty new paleointensity determinations have been obtained from archaeomagnetic sites from france time intervals investigated range from 65 to 380 ad and from 1370 to 1700 ad two hundred forty two samples of tiles and bricks were treated with the classical thellier method for paleointensity determination rock magnetic studies identify magnetite substituted maghemite and hematite as the main magnetic minerals as is usual with archaeomagnetic samples especially tiles magnetic anisotropy was observed in all samples the effect of thermoremanent magnetization trm anisotropy upon the paleointensity values was corrected after the determination of the tensor of trm anisotropy in each sample these determinations allow corrections of the field strength obtained which are usually lower than 20 the effect of the cooling rate on the trm intensity acquired was also determined and seems relatively low on our samples most frequently eight to 10 samples per site were used to calculate the mean intensity standard deviations are typically lower than 7 of the average value during the roman period small variations of the field intensity in paris are observed between 62 and 70 t while during the more recent period the intensity of the field decreases from 62 to 52 t an analysis of the paleointensity data available for the last 2000 years obtained from sites in western europe was performed using a weighing factor which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination this weighing factor clearly shows that most of the existing data might not be reliable the analysis performed indicates that more data is needed especially from the time interval 500 to 1400 ad before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in western europe

magnetic fabric studies of the nipissing sill province and senneterre dykes canadian shield and implications for emplacement the 2 21 ga nipissing diabase sills intrude the huronian supergroup of the southern province coeval senneterre dykes in the adjacent portion of the superior province are potential feeders for the sills anisotropy of magnetic susceptibility ams data were obtained from both of these units to study flow patterns and test emplacement models the sills exhibit three distinct paleomagnetic directions that are likely primary and represent a time correlation tool within the magmatic event two of the three directions are also found in senneterre dykes sites in the northern cobalt embayment carry an n1 reverse paleomagnetic signature they possess maximum ams axes that trend north northwest south southeast interpreted as the direction of magma emplacement in one area the data rule out a cone sheet emplacement mechanism the n3 paleomagnetic signature is restricted to a sill and or dyke like bodies near lake timiskaming where the ams axes are dominantly north northwest south southeast similar to that of the nearby n1 reverse sites sites in a geographically restricted area farther south carry an n1 normal paleomagnetic direction the ams patterns in these sites exhibit very poor within and between site consistency compatible with the magma having crystallized statically in this volumetrically minor magmatic pulse in the sudbury elliot lake area the nipissing diabase is deformed and outcrops exhibit east west elongation parallel to the fold axes of the enclosing huronian sedimentary rocks ams fabrics are interpreted as reflecting this deformation rather than primary magmatic flow three of eight senneterre dyke sites show horizontal flow patterns whereas the remainder have more complicated ams patterns

magnetic fidelity of lunar samples and implications for an ancient core dynamo abstract some lunar rocks have stable magnetizations that provide compelling evidence for an ancient lunar core dynamo however a longstanding problem has been interpreting the unstable alternating field af demagnetization behavior observed in the remaining majority of the apollo lunar sample suite similar unstable behavior has also been observed for many meteorites it is unclear whether the behavior of these samples indicates that they formed in the absence of an ancient magnetizing field or whether they simply have poor magnetic recording properties it is necessary to distinguish between these two possibilities in order to determine whether paleomagnetic fields were present on a sample s parent body to address this issue we analyzed five samples whose rock magnetic properties span the full suite of observed demagnetization behaviors mare basalts 15556 15016 12017 10020 and troctolite 76535 we demonstrate that the effects of spurious anhysteretic remanent magnetization arm during af demagnetization in combination with multidomain magnetic carriers and magnetic remanence anisotropy are likely responsible for the apparently poor magnetic behavior of many lunar samples therefore the unstable af demagnetization behavior observed for many lunar samples is not alone sufficient for ruling out the presence of an ancient lunar core dynamo nevertheless spurious arm may explain the observation of surprisingly high 1 t apollo era paleointensity measurements for samples thought to have formed after the cessation of a lunar core dynamo

differences in paleomagnetic interpretations due to the choice of statistical demagnetization and correction techniques kapuskasing structural zone northern ontario canada abstract the kapuskasing structural zone ksz reveals a section through the archean lower crustal granoblastic gneisses our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical demagnetization and field correction techniques first where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular the commonly used statistical model is invalid fisher r a proc r soc a217 1953 295 any tendency to form an elliptical distribution indicates that the sample is drawn from a bingham type population bingham c 1964 distributions on the sphere and on the projective plane phd thesis yale university fisher and bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic bingham distribution it seems prudent to apply both models wherever a non fisher population is suspected and that may be appropriate in any tectonized rocks non fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks more dispersed sampling across tectonic structures may be more productive second from the same specimens mean vectors isolated by thermal and alternating field af demagnetization differ which treatment gives more meaningful results is difficult to decipher especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events in this study thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles third of more parochial significance tilt corrections may be unnecessary in the ksz because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off at approximately 15 km depth with archean geothermal gradients primary remanences were blocked after the foliation was tilted to rise on the thrust ramp therefore the rocks were probably magnetized in their present orientation tilting largely or entirely predates magnetization

changes in direction of the remanence of rocks produced by stationary alternating field demagnetization summary a theoretical investigation of the way in which an isotropic rock containing single domain particles acquires both irm and arm or trm has indicated that stationary single axis alternating field af demagnetization with the af axis at an angle to the remanence vector should produce progressive angular changes in a single component remanence as demagnetization proceeds just before the remanence is completely removed it should lie at 90 to the af axis irrespective of the original orientation of the remanence apart from 0 experimental observations on a rock sample support these deductions this analysis has been extended to investigate the way in which arm or trm and irm are demagnetized by static three axis demagnetization methods which are used by some workers in palaeomagnetism theory in conjunction with the use of a numerical model predicts that an arm or trm should not undergo significant direction changes when these methods are applied but an irm should undergo progressive direction changes as demagnetization proceeds usually moving until it makes an angle of cos 1 1 3 with each of the three af axes just before it is removed confirmation that such changes do occur have been obtained by experiments on a rock sample the relative merits of static and tumbling af demagnetization methods are also discussed

correction for deviations due to magnetic anisotropy of palaeomagnetic directions from the mulcahy gabbro north west ontario summary the mulcahy gabbro is an archean layered intrusion in the wabigoon subprovince of north western ontario alternating field and thermal demagnetization of specimens from it have provided consistent within site characteristic remanence chrm directions in eight of 19 sites antipodal polarities of remanence are recovered and the overall in situ formation mean is d 240 i 51 k 21 this direction is unlike any reported archean direction from the superior province of north america the degree of anisotropy of low field magnetic susceptibility is high with k k3 values up to 1 3 cooling selected specimens in weak fields indicate that trm is deflected towards the magnetic fabric plane accordingly the in situ chrm was corrected for the effect of magnetic anisotropy two methods of correction give similar results however the resultant formation mean remains essentially unchanged as a consequence of sufficient variability in the attitudes of k k planes although uncertainty in the initial dip renders the tilt correction problematical partial tilt correction 36 per cent results in a formational mean with better precision than the in situ formation mean furthermore partial untilting of remanence directions results in a positive reversal test with a better classification than that obtained using in situ directions this partially tilt corrected result d 202 i 49 k 35 yields a pole latitude 62 n longitude 42 e which is archean in aspect

magnetic anisotropy as a correction for compaction caused palaeomagnetic inclination shallowing summary a theoretical model presented by jackson et al 1991 suggests that the anisotropy of anhysteretic remanence aar can be used to recognize and correct for compaction caused inclination shallowing in sediments and sedimentary rocks however we have conducted experiments in which inclination shallowing is more strongly dependent on magnetic anisotropy during compaction of synthetic clay rich sediments containing 0 45 m acicular magnetite than predicted by the jackson et al model this discrepancy can be explained if randomization or disturbance of subvertical magnetite particles is the cause of inclination shallowing early in the compaction process the principal experimental evidence for this effect is a decrease in magnetization intensity coincident with inclination shallowing an intensity decrease during inclination shallowing is also observed in some dsdp cores suggesting that this randomization process may also occur in nature after randomization magnetite particles become firmly attached to clay particles and both magnetic anisotropy and inclination shallowing show a regular increase during compaction consistent with the jackson et al model we suggest that models like that of jackson et al can be used to correct for inclination shallowing if they are used in conjunction with an experimentally derived relationship between inclination shallowing and magnetic anisotropy for a particular sediment

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

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

quantification of illite content in sedimentary rocks using magnetic susceptibility a rapid complement or alternative to x ray diffraction abstract the paramagnetic clay mineral illite can have important controls on fluid permeability and microporosity in sedimentary rocks increases in illite content of a few percent can reduce permeability by several orders of magnitude traditional x ray diffraction xrd techniques for quantifying illite content can be very time consuming requiring significant sample preparation and generally examine only a relatively small sample volume in contrast a technique based on magnetic susceptibility described here is very rapid cheap sensitive nondestructive and requires no extra preparation of the sample it is representative of a much larger sample volume and in the present paper utilizes standard 1 inch diameter cylindrical core plugs but it can be applied to other sample shapes and volumes the magnetic susceptibility derived estimates of illite content from core plugs correspond well with xrd results from small powdered samples of the same material in terms of both absolute values and overall trends with depth however neither the magnetic method nor xrd should be regarded as definitive each technique has merits and limitations and these help to explain some observed differences in the illite determinations by each method xrd may underestimate the illite content particularly in muddy rocks whereas the magnetic method theoretically provides an upper limit for the illite content the magnetic method is also capable of simultaneously estimating the quartz content in the present samples changes in the concentration and distribution of illite during core cleaning or core flooding experiments can also be potentially quantified by the magnetic technique the development of portable field sensors potentially allows the magnetic method to provide high resolution illite profiles on slabbed cores outcrops and unconsolidated samples without the need to cut core plugs the method could also be applied to whole core measurements and to downhole magnetic susceptibility data the processing of the magnetic susceptibility signal can also be extended to quantify other minerals in simple systems

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

rock magnetic and magnetic fabric variations across three thin alkaline dykes from sunnhordland western norway influence of initial mineralogy and secondary chemical alterations summary profiles of natural remanent magnetization nrm rock magnetic properties and anisotropy of magnetic susceptibility ams across three thin 26 to 109 cm alkaline dykes from sunnhordland western norway are presented the magnetic remanence is carried by pseudo single domain psd lowtemperature oxidized titanomagnetite with circa 6 per cent of cations substituted by aluminium systematic variations in magnetic properties across the dykes are caused by differences in titanomagnetite concentration as well as grain size lowtemperature alteration and laboratory oxidation by heating in air to a maximum temperature of 600 c do not homogenize the magnetic properties across the dykes fine grained regions near dyke margins carry the most stable remanent magnetization but are also most amenable to secondary chemical remanent magnetization crm resetting between dyke variations in magnetic properties are larger than within dyke variations a stable primary remanence is found in two of the dykes while the third is totally reset by a soft viscous remanent magnetization vrm despite similar optical grain sizes magnetic mineralogy and oxidation state the difference is attributed to a lower titanium content in the latter dyke preventing effective partitioning of magnetic grains during low temperature alteration laboratory heating affects anisotropy variations across the dykes resulting in more consistent axis orientations within each dyke and symmetric edge to edge variations in the degree of anisotropy ams directions are different from those in the country rock and vary systematically across the thinnest dyke before heating

directional petrological characterisation of deep marine sandstones using grain fabric and permeability anisotropy methodologies theory application and suggestions for integration abstract different methods for quantifying preferred grain orientation or grain shape fabric in sandstones are reviewed and their value for the reconstruction of palaeocurrent direction and preferred direction of pore fluid flow is reassessed critically based on existing knowledge from literature and new grain fabric data it is shown that petrographic analysis has distinct merits because grain fabric can be determined with low equipment costs and population statistics are based on the orientation of individual grains several recommendations are given how to standardise procedures for acquisition of grain fabric through microscopy these recommendations include 1 the careful selection of cross section orientations to acquire apparent long grain axes that are representative of true long grain axes 2 the preferred measurement of the coarsest grains in cross section and 3 a new statistically founded method to select a subset of grains that is representative of the total population of grains in a sandstone sample magnetic techniques that use anisotropy of magnetic susceptibility ams as a proxy for grain orientation require specialised equipment and are more restricted in statistical evaluation but these methods benefit greatly from being able to measure a large number of grains in a three dimensional space in a short amount of time and with a lower sensitivity to user bias of the two magnetic techniques discussed in this paper the new enhanced ams method in which rock magnetic properties are enhanced by introducing magnetite into the pores and as precipitates on grain surfaces is regarded to be an improvement over the traditional natural ams method in which only the natural assemblage of magnetite particles present in the rock is analysed enhanced ams measures the preferred orientation of essentially all grains in a rock sample thus providing the shape fabric of a hydrodynamically more accurate representation of framework grains than natural ams through a small technical modification the enhanced ams method can be used to obtain a magnetic cast of the pore network of a rock sample so that the resulting ams is controlled by the shape anisotropy of the individual pores it is commonly assumed that long axes of sand grains in sedimentary deposits parallel the direction of the flows that formed the deposits this concept is challenged by a new systematic study of grain fabric in deep marine turbidites it indeed appears that grains in ripple cross laminated divisions of turbidites dominantly attain a preferred orientation parallel to the palaeoflow direction but grains in plane parallel laminated divisions have an additional flow perpendicular mode which suggests a contribution of rolling grain transport three different fabric modes occur in massive structureless sandstones and massive divisions in turbidites apart from flow aligned and flow perpendicular modes there is a statistically significant but hydrodynamically enigmatic flow oblique mode with an average azimuthal deviation of 40 this large variation of fabric types suggests that a similar variation exists in preferred paths of pore fluid flow within turbidites controlled to at least some degree by depositional process this is confirmed by independent laboratory measurements of permeability anisotropy on samples for which grain fabric data are available and thus defines grain fabric analysis as a viable alternative to traditional permeametry techniques

tilting and transpression of an archaean anorthosite in northern ontario abstract an archaean anorthosite complex crops out over more than 300 km 2 at the junction between an archaean metasedimentary accretionary prism quetico belt and a volcanic greenstone terrain to the north wabigoon belt the anorthosite is feebly deformed and weakly foliated however anisotropies of low field susceptibility ams and anhysteretic remanence aarm define consistent magnetic fabric directions extending over subareas 25 km 2 in extent whereas magnetic fabrics cannot define strain magnitudes they normally provide reliable indications of finite strain orientations even where the fabrics developed by stress controlled nucleation during a progressive noncoaxial strain history the field schistosity s 1 being the oldest foliation rotated closest to the shear plane ams foliation due to preferred crystallographic orientation of silicates and pyrrhotite as well as preferred dimensional orientation of magnetite lies at a greater angle to the shear plane because it formed late then s 1 had less time to rotate the few measurements of the youngest aarm foliation caused by magnetite and pyrrhotite suggest that it may lie at a still greater angle to the shear plane thus s 1 and succeeding magnetic foliations provide a cryptic shear sense indicator they reveal relative motions along the terrain boundary that include transpression due to nw se shortening causing dextral e w shear with a north side up component palaeomagnetic data confirm a progressive northwards steepening of the anorthosite sills that is consistent with northwards subduction of the accretionary prism to the south

tectonic strain recorded by magnetic fabrics ams in plutons including mt kinabalu borneo a tool to explore past tectonic regimes and syn magmatic deformation tectonic strain commonly overprints magmatic fabrics in ams anisotropy of magnetic susceptibility data for plutonic rocks produced by both compressional and extensional regimes mt kinabalu borneo is a composite pluton with an exceptional vertical range of exposure and clearly defined internal contacts we show that tectonic fabrics are recorded pervasively throughout the intrusion even near contacts and present a workflow distinguishing compressive and extensional syn magmatic deformation at mt kinabalu this reveals a pervasive tectonic fabric indicating nw se miocene extension in borneo at 7 9 7 3 ma later than previously recognised oriented nw se at 319 13 1 comparing data from mt kinabalu with data from globally distributed studies shows that tectonic strain is commonly recorded by plutons therefore ams fabric can be used to identify the syn magmatic tectonic setting and combined with both geochronology and evidence for paleomagnetic rotation to provide a powerful tool for accurate determination of syn magmatic tectonic regimes and strain orientations within temporal frameworks

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

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

trm deviations in anisotropic assemblages of multidomain magnetite anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility ams which in turn induces deviations of thermo remanent magnetization trm from the field direction from the theories of multidomain trm acquisition it is shown that the trm anisotropy tensor has its eigenvalue ratios ti related to the principal weak field susceptibility ratios pi by the order of magnitude ti p2i this relation has been experimentally verified on two sets of highly anisotropic rock samples the exponent has been determined to be 1 94 in the samples from a peruvian gabbro and 1 81 in those from the granite of flamanville nw france accounting for experimental difficulties in determining the trm anisotropy tensors these exponents are judged to agree well with the expected one it is therefore stressed that ams measurements provide a good means of evaluating the magnetic field direction from deviated trm directions providing magnetic carriers are mainly multidomain magnetites

thick dyke emplacement and internal flow a structural and magnetic fabric study of the deep seated dolerite dyke of foum zguid southern morocco 1 knowledge on forced magma injection and magma flow in dykes is crucial for the understanding of how magmas migrate through the crust to the earth s surface because many questions still persist we used the long thick and deep seated foum zguid dyke morocco to investigate dyke emplacement and internal flow by means of magnetic methods structural analysis petrography and scanning electron microscopy we also investigated how the host rocks accommodated the intrusion regarding internal flow 1 important variations of the rock magnetic properties and magnetic fabric occur with distance from dyke wall 2 anisotropy of anhysteretic remanent magnetization reveals that anisotropy of magnetic susceptibility ams results mainly from the superposition of subfabrics with distinct coercivities and that the imbrication between magnetic foliation and dyke plane is more reliable to deduce flow than the orientation of the ams maximum principal axis and 3 a dominant upward flow near the margins can be inferred the magnetic fabric closest to the dyke wall likely records magma flow best due to fast cooling whereas in the core the magnetic properties have been affected by high temperature exsolution and metasomatic effects due to slow cooling regarding dyke emplacement this study shows that the thick forceful intrusion induced deformation by homogeneous flattening and or folding of the host sedimentary strata dewatering related to heat as recorded by thick quartz veins bordering the dyke in some localities may have also helped accommodating dyke intrusion the spatial arrangement of quartz veins and their geometrical relationship with the dyke indicate a preintrusive to synintrusive sinistral component of strike slip

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

magnetic fabric of ignimbrites a case study from the central anatolian volcanic province the magnetic fabric of the pliocene kizilkaya ignimbrite in the central anatolian volcanic province has been investigated by anisotropy of magnetic susceptibility ams and isothermal remanent magnetization airm seven sections were sampled at various stratigraphic heights within the devitrified portion of the ignimbrite the magnetic mineralogy is complex titanomagnetite occurs as magmatic grains and as inclusions in other phenocryst and glass shards an oxidized phase and hematite occur in deposit levels affected by alteration processes the disturbance produced by lithic and pumice clasts has been reduced by discarding the specimens that deviate more than 1 from the site mean value of the density the ams fabric varies along each individual section neither the ams magnetic lineation nor the magnetic foliation plunge clearly define a common area as the vent location the airm fabric of low coercivity minerals mainly represented by free titanomagnetite grains of magmatic origin is consistent between sites and the inferred flow directions converge on a region near derinkuyu in the nevsehir plateau previously reported as the kizilkaya ignimbrite source area this study shows that systematic use of the remanent fabric improves the results given by ams and aids the identification of the primary magnetic fabric related to the ignimbrite emplacement dynamics

can remanence anisotropy detect paleomagnetic inclination shallowing due to compaction a case study using cretaceous deep sea limestones we studied 35 cretaceous limestone specimens from five pacific plate deep sea drilling project sites inclination in of the natural remanence is on average 17 shallower than the average 44 expected paleofield inclination i anhysteretic remanence arm applied identically to various axes was found to be weakest armmin perpendicular to bedding and strongest armmax parallel to bedding the average armmin armmax of 0 87 as well as the inclination shallowing of 17 likely originated from sediment compaction rotating the long axes of magnetite grains toward the bedding plane this origin is theoretically and experimentally consistent with the average fractional compaction of 0 6 experienced by our sediments estimated from their porosity a compaction origin is also supported by the significant correlation found between tan in tan i and armmin armmax the correlation line s slope of 2 3 0 7 agrees with theory taking into account our observation that arm given perpendicular to the long axes of magnetite grains has on average 0 37 times the intensity of arm given axially these results suggest that compaction induced inclination shallowing may be detected in a suite of fine grained magnetite bearing sediments by looking for a correlation between tan in and armmin armmax having shown that arm anisotropy is foliated in the bedding plane this correlation line s prediction of in when armmin armmax 1 should estimate i corrected for inclination shallowing

structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements the western red hills granites skye scotland thulean igneous province the geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied but such is not the case for their internal fabric the tertiary western red hills epigranites of the isle of skye are a classic example of such intrusions using anisotropy of magnetic susceptibility measurements we present the first structural data of their internal magmatic fabric the magnetic foliations equated with the magmatic flow planes have strikes which roughly follow the walls of the different intrusions they dip steeply toward the convex wall of each intrusion the lineations or maximal magnetic susceptibility axes generally have shallow plunges except in the latest granite intrusion these structures appear to be related to the compressional deformation of each intrusion toward the end of its crystallization this shortening would be a consequence of a radial and compressive stress field acting after each injection of magma this radial stress field is interpreted as the effect of high magma pressures originating from the acid magma chamber underlying the ring dyke complex at a shallow depth

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

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

magnetic fabric welding texture and strain fabric in the nuraxi tuff sardinia italy anisotropy of magnetic susceptibility ams has been used to interpret flow directions in ignimbrites but no study has demonstrated that the ams fabric corresponds to the flow fabric in this paper we show that the ams and strain fabric coincide in a high grade ignimbrite the nuraxi tuff a miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well defined magnetic fabric natural remanent magnetization nrm data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by ti magnetite crystals comparison between the magnetic fabric and the deformation features along a representative section shows that ams and anisotropy of isothermal remanent magnetization airm fabric are coaxial with and reproduce the shape of the strain ellipsoid magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer size pseudo single domain magnetically interacting magnetite crystals microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff the fabric was thus acquired after deposition before late rheomorphic processes and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow

palaeomagnetism of welded pyroclastic fall scoriae at vulcano aeolian archipelago abstract this paper deals with the magnetic fabric and remanent magnetization of welded scoriae fall deposits four scoriae blankets were emplaced on vulcano aeolian islands from about 50 to 8 ka their detailed chronostratigraphic position has been debated in the literature and they have therefore been given a different interpretation in the reconstruction of the vulcano s eruptive history the aims of the study were a better understanding of the magnetic features of the scoriae fall deposits which have so far been given little attention and to set the chronostratigraphic relationships of the sampled deposits on the ground of their palaeomagnetic directions overall the magnetic properties of the four deposits are alike investigation of isothermal remanence and susceptibility versus temperature measurements point to low ti titanomagnetite as the main ferromagnetic mineral magnetic foliation is well developed moderately dispersed and in most cases close to horizontal lineation is either clustered or dispersed within the foliation plane stepwise thermal and alternating field demagnetization shows that secondary components are negligible in two units in the other two they are always completely removed in the first steps below 20 mt or 400 c thereafter the characteristic component chrm is clearly isolated and the within site dispersion of its direction is small 95 95 the palaeomagnetic results corroborate the hypothesis that the investigated scoriae deposits correspond to different volcanic phases related to recurrent arrival of mafic magma and show that those of spiaggia lunga and quadrara were produced during two distinct periods of volcanic activity with little time separation between them

global variability of the wavenumber spectrum of oceanic mesoscale turbulence abstractthe wavenumber spectra of sea surface height from satellite altimeter observations have revealed complex spatial variability that cannot be explained by a universal theory of mesoscale turbulence near the edge of the core regions of high eddy energy agreement is observed with the prediction of the surface quasigeostrophic sqg turbulence theory which has fundamental differences from that of the traditional quasigeostrophic qg turbulence theory in the core regions of high eddy energy the spectra are consistent with frontogenesis that is not fully accounted for by the sqg theory however the observations in the vast ocean interior of low eddy energy exhibit substantial differences from the predictions of existing theories of oceanic mesoscale turbulence the spectra in these regions may reflect the ocean s response to short scale atmospheric forcing and air sea interaction the observations presented in this paper serve as a test bed for new theories and ocean general circulation models

composite ferromagnetic fabrics magnetite greigite measured by ams and partial aarm in weakly strained sandstones from western makran iran summary we present examples of composite magnetic fabrics ams and partial aarm in five sites from the western makran accretionnary prism these rocks are weakly strained sandstones of mio pliocene age rock magnetic investigations show a large occurrence of magnetite and also some greigite at two sites partial anhysteretic remanent magnetization coercivity spectra reveal that grain size is not homogeneous suggesting several grain fractions ams foliations are close to bedding except at one greigite bearing site where ams foliation is almost perpendicular to bedding soft aarm 0 50 mt foliations are slightly oblique both to bedding and ams foliation surprisingly hard aarm 50 100 mt foliations are perpendicular to bedding comparison of two limbs of one fold suggests that ams foliations are pre tilting while hardaarm foliations are post tilting corrected degree of anisotropy parameters display similar trends for all sites p 1 04 p 1 11 p hardaarm 1 19 we suggest that ams soft aarm and hard aarm reflect distinct magnetic grain populations however a possible relationship between hard aarm and a component of natural remanent magnetization is possible our study shows that two orthogonal fabrics exist in these weakly strained rocks coarse grains are parallel to bedding while fine grains are aligned perpendicular to bedding

eight thousand years of geomagnetic field intensity variations in the eastern mediterranean 1 twenty new intensity determinations of the ancient geomagnetic field have been obtained from groups of potsherds and brick fragments from syria these artifacts archeologically well dated from 6000 b c to approximately a d 1200 have been analyzed using the thellier and thellier 1959 method as modified by coe 1967 intensity values have been corrected for the effects of anisotropy of thermal remanent magnetization and cooling rate our results indicate that field intensities were moderate in syria from 6000 b c to 3500 b c with values of 30 40 t there was a significant increase in intensity by a factor of 2 from 3500 b c to 700 b c which was interrupted by a moderate decrease between 2550 b c and 1750 b c during more recent periods our results show an intensity minimum approximately a d 200 and a maximum around the tenth century comparison with different data sets from the eastern mediterranean and central asia shows that geomagnetic field intensity variations were consistent at this large regional scale at least over the last 5 millennia

magnetic susceptibility and partial anhysteretic remanence anisotropies in the magnetite bearing granite pluton of tourao ne brazil abstract rock magnetism and magnetic fabric studies were performed in the tourao granite western part of serido belt ne brazil this pluton 350 km2 is composed of k rich calc alkalic monzogranites and was emplaced between a set of ne and e trending dextral strike slip shear zones electronic and optical microscopy thermomagnetic curves hysteresis data and coercivity spectra revealed that magnetite fine and coarse grained is the unique ferromagnetic mineral in these rocks anisotropy of low field magnetic susceptibility ams was measured in 83 sites and for many sites from central and southern parts of the intrusive body a large scatter was noticed anisotropy of partial anhysteretic remanence paar was determined for 29 sites in the 0 50 mt paar0 50 and 19 sites in the 50 90 mt paar50 90 windows to isolate the fabric contribution of the remanence bearing grains magnetite in most sites ams is dominantly carried by magnetite whereas in some sites the paramagnetic contribution exceeds 60 of the bulk susceptibility the paar0 50 is assumed to be related to coarse grained early crystallized magnetites while the paar50 90 is interpreted as due to fine needle shaped magnetites hosted in biotites the combined analysis of these magnetic techniques permits interpretation in terms of fabric formation in the tourao granite and the inference that at least two successive kinematic events were recorded in this intrusive body devoid of any solid state deformation overprint the ams and paar0 50 fabrics for most of the sites correlate with the early nne trending magmatic fabric whereas paar50 90 reveals a different fabric probably related to late partitioning of magmatic straining along the e trending shear zones in the southern part of the body a completely different paar0 50 fabric is correlated with the late intrusion of charnockite stocks

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

geochemical characterization of secondary microbial gas occurrence in the songliao basin ne china abstract a suite of natural gases from the northern songliao basin in ne china were characterized for their molecular and carbon isotopic composition gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation with very high dryness c1 c2 100 high c2 c3 and i c4 n c4 ratios high nitrogen content and variable carbon dioxide content isotopic values show wide range variations 13 c ch 4 from 79 5 to 45 0 13 c c 2 h 6 from 53 7 to 32 2 13 c c 3 h 8 from 36 5 to 20 1 13 c n c 4 h 10 from 32 7 to 24 5 and 13 c co 2 from 21 6 to 10 5 a variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures system openness also was considered for shallow biodegraded gas accumulations gases from the daqing anticline are relatively wet with 13c enriched methane and 13c depleted co2 representing typically thermogenic origin gases within the longhupao da an terrace have variable dryness 13c enriched methane and variable 13c of co2 suggesting dominant thermogenic origin and minor secondary microbial methane augment the puqian ao nan uplift contains relatively dry gas with 13c depleted methane and 13c enriched co2 typical for secondary microbial gas with a minor part of thermogenic methane gas accumulations in the western slope are very dry with low carbon dioxide concentrations some gases contain 13c depleted methane ethane and propane indicating low maturity primary microbial origin recognition of varying genetic gas types in the songliao basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems

clockwise rotation and implications for northward drift of the western transverse ranges from paleomagnetism of the piuma member sespe formation near malibu california 1 new paleomagnetic results from mid tertiary sedimentary beds in the santa monica mountains reinforce the evidence for large scale rotation of the western transverse ranges and anisotropy measurements indicate that compaction induced inclination flattening may resolve a long standing controversy regarding the original paleolatitude of the rotated block previously published paleomagnetic data indicate that post oligocene rotation amounts to 70 110 clockwise affecting the channel islands santa monica mountains and santa ynez mountains the sespe formation near malibu consists of a lower member dominated by nonmarine sandstone and conglomerate and an upper section the piuma member which consists of gray red sandstone and mudstone interbedded with minor tuff and limestone beds the piuma member has a paleomagnetic pole at 36 6 n 326 7 e a95min 5 0 a95max 9 6 obtained by thermal demagnetization of 34 oriented cores from oligocene and early miocene beds after correcting for plunge of the geologic structure the data are consistent with significant clockwise rotation 77 7 of the region relative to stable north america rotation of the western transverse ranges is generally viewed as a consequence of pacific north american plate interactions after 28 ma when east west subduction gave way to northwest transform motion in southern california inclinations from the piuma study indicate a paleolatitude anomaly of 11 7 and are consistent with a mean northward drift that exceeds generally accepted san andreas fault displacement by a factor of 3 however sedimentary inclination error may accentuate the anomaly anisotropy of isothermal remanent magnetization indicates inclination flattening of approximately 8 and correction for the effect reduces the paleolatitude anomaly to 5 3 5 8 compaction may explain the inclination flattening in these sedimentary rocks but the process does not adequately explain lower than expected inclinations found in previous studies of miocene volcanic rocks of the western transverse ranges