the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

effects of experimental deformation on the remanent magnetization of sediments abstract changes in direction and in intensity of the remanent magnetization of sediments were studied during experimental deformation leading to an axial shortening of 20 30 the remanence was produced before shortening by anhysteretic magnetization in an alternating field of 90 mt superimposed on a parallel steady field of 1 mt at various inclinations with respect to the sample the samples were sandstones marls and volcanic sediments the results show that the direction of magnetization is stable during the deformation within the experimental precision the intensity of magnetization decreases during shortening by an amount which depends on the type of rock about 30 for sandstones and about 5 for marls and volcanic sediments the march model does not account for the data and a discontinuous deformation must be invoked the evolution of the young modulus of sandstones during cyclic loading and unloading shows first a decrease in relation to crack density growth then an increase showing a localization of the fracturation and finally a deformation by displacement of microblocks an interpretation is given after a discussion of the results obtained by other authors with plasticine and artificial clays the amplitude of the decrease and of the rotation of the remanent magnetization could be a function of the size of the magnetic grains compared with the scale of the fracturation during non homogeneous processes of deformation

coercive forces and coercivity spectra of submicron magnetites abstract remanent coercivity spectra have been determined for four dispersions of magnetite with mean particle sizes of 0 04 0 22 m using four different methods 1 acquisition of isothermal remanent magnetization irm in continuous fields 2 stepwise reversal of saturation irm by opposite polarity continuous fields 3 stepwise alternating field af demagnetization of saturation irm and 4 af demagnetization of weak field thermoremanent magnetization trm alternating field methods gave narrower distributions with lower median coercivities than continuous field cf methods the two cf spectra are offset the remanent coercive force hr which is the median coercivity determined by cf reversal of saturation remanence is always less than the median coercivity h r determined from irm acquisition the median destructive field of saturation irm h 1 2 the median coercivity determined by af demagnetization is less than eitherhr orh r the relation reported by dankers 1 h r h 1 2 2hr is obeyed for these samples and for a companion suite of single domain magnetites and maghemites ordinary coercive force hc is substantially affected by reversible magnetization and is a poor indicator of the median coercivity of remanence hc is much smaller thanh 1 2 hr orh r and varies in a different way with particle size

strain factorization by identical finite increments and its use abstract progressive deformation of layered rocks up to their present finite strain and bedding attitudes is simulated by an identical increment path model a deformation matrix connecting the initial and deformed states is formulated with the conceptual help of a hypothetical three step deformation path two sequential non coaxial rigid body rotations bring an original horizontal bed to a pre strain attitude a pure shear follows the rotations thus the deformation matrix is calculated by premultiplying the matrices representing the three stages of deformation sixteen identical tectonic deformation increments are then obtained by four times sequentially extracting the square root of that deformation matrix the method has been applied to the campbell block a late jurassic fault bounded turbidite sliver in the northern sierra nevada california the block was deformed into a train of folds during the nevadan orogeny fourteen oriented samples were collected for march strain measurements based on the strains and bedding attitudes the block was divided into 11 deformation bands of three types by applying the proposed model to the campbell block it was found that the deformation style of the block consists of a non coaxial deformation path the regional strain is accommodated among the several deformation bands by strain partitioning the collective deformation indicated by this model is transpressional

analysis of strained sedimentary fabrics review and tests accurate analysis of the homogeneous strain of ellipsoidal objects requires that certain premises are met these are 1 coaxiality of strain increments 2 accurate definition of finite strain axes and 3 perfectly passive behaviour of the objects with respect to the matrix the equations provide a mathematical model for this idealized scenario in practice the analysis is limited to data from plane surfaces and its success depends upon the type of initial fabric of the objects the frequency distribution of their initial shapes and errors of measurement of the final shapes rf1 2 and of final orientations together with the difficulty in locating the finite strain axes these factors greatly affect the outcome of the analysis and their combined effects have not been studied previously detailed observations on the orientations of strain and of the orientation distribution of the objects long axes sometimes indicate that the model is incompatible with the data strain analysis is then not possible

tectonic strain and paleomagnetism experimental investigation a porous clay rich sandstone with a natural remanent magnetisation shows complex changes in the orientation of the remanence vector when the sandstone is subjected to experimental deformation at 100 mpa room temperature and constant natural strain rates of 1 10 5 s 1 the paleomagnetic vector does not simply rotate away from the axis of compression when the rock is shortened perpendicular to bedding the remanence vector rotates toward the bedding as expected but the remanence also rotates toward bedding when the rock is shortened parallel to bedding moreover in some cases the remanence vector changes azimuth within the specimen during experimental deformation thus bedding anisotropy and the accompanying mineral fabric anisotropy are more influential than strain in controlling deflections of the paleomagnetic vector in this study this anisotropy also influences the post deformational behaviour of remanence 4 months after deformation some cores shortened perpendicular to bedding relaxed their remanence vectors back towards the pre deformational attitude whereas some other cores continued to show progressive changes intragranular recoverable elastic strains in the magnetic grains may be partly responsible for the experimentally induced deflections of remanence

deformation of remanent magnetism in a synthetic aggregate with hematite a hematite calcite aggregate bonded with portland cement is deformed triaxially at 200 mpa confining pressure and at a constant strain rate of 10 5 s 1 in some cases with a pore fluid pressure of 120 mpa this aggregate deforms in a ductile fashion at room temperature and is a good textural analog of naturally penetratively strained hematite bearing rocks the changes in inclination of an isothermal remanent magnetisation are indistinguishable from the behaviour expected of a homogeneously strained non material line rotating according to wettstein s equation this is encouraging for paleomagnetists who might wish to restore remanence in tectonically strained hematite bearing rocks an unknown combination of rigid grain rotation and intracrystalline stress must be responsible for rotation of the grains that carry remanence and thus for the changes in inclination of remanence the reason that wettstein s equation is approximately valid may be because the large aspect ratios of rigid hematite grains cause them to rotate nearly as quickly as a passive line p hydrostatic compaction wet deformation and dry deformation in that order are progressively more effective in reducing the intensity of remanence however wet deformation is more effective than dry deformation in reducing the intensity of the most coercive components presumably by spinning the finest grains in the cement matrix and scattering their individual moments p where multiple direction components of remanence are present deformation reduces the components with higher coercivities 70 mt more than those with lower coercivities in previous experiments on magnetite bearing samples the lower coercivity components were most reduced thus the natural tectonic deformation of an nrm may be expected to preserve selectively low coercivity components in hematite and high coercivity components in magnetite this may be used to predict the success of preservation of a characteristic nrm and conversely the success of overprinting by syntectonic remagnetisation

magnetic characterization using a three dimensional hysteresis projection illustrated with a study of limestones summary limestones provide an important source of palaeomagnetic information despite their low content of submicroscopic remanence bearing minerals the chief sources of these minerals are thought to be clastic volcanic magnetite and titanomagnetite and organic magnetite the latter mostly from bacterial sources chemically remagnetized limestones carry magnetite or pyrrhotite three hysteresis properties prove useful in identifying and characterizing these mineralogical influences on limestones the ratio of zero field maximum remanence to saturation remanence m r m s in an applied field coercivity of remanence b cr and coercivity b c to a lesser extent k f m s may be useful where k f is the ferrimagnetic susceptibility traditionally these have been plotted on a combination of 2 d graphs that of necessity only preserve two variables day et al 1977 wasilewski 1973 however we found that magnetic discrimination and characterization of the limestones was much easier on a three axis hysteresis projection that preserves the values of b cr b c and m r m s as independent variables using logarithmic scales the regression surfaces through the data become almost planar and distinguish pelagic shallow marine shelf and remagnetized limestones on the basis of the slope and intercept of the associated regression surface clearly there are sensitive sedimentological geochemical or organic influences that dictate the magnetic mineralogy through sedimentary environment moreover the 3 d plot of hysteresis criteria avords easy recognition of remagnetized limestones and may permit the rejection of material unsuitable for palaeomagnetic study the 3 d hysteresis projection may be useful for the characterization of other rocks and magnetic materials

the geon 14 arc related mafic rocks from the central grenville province the late paleoproterozoic to mesoproterozoic ca 1 7 1 2 ga evolution of the active southeastern margin of laurentia terminated with the grenvillian continental collision and the development of a large hot long duration orogen at ca 1 09 0 98 ga as a result much of the hinterland of the grenville province consists of paleoproterozoic and mesoproterozoic rocks mostly preserved as an imbricate stack of high grade gneisses that represent a potential repository of active margin processes this study presents geochronologic geochemical and isotopic analyses of two granulite facies suites of ca 1 45 1 40 ga mafic tholeiites from the canyon domain manicouagan area central grenville province one suite consists of 1439 76 68 ma high feti mafic sills with end values of 0 4 tdm 2 57 2 72 ga indicate derivation from variably depleted to enriched morb type mantle sources probably in an extensional back arc setting before intrusion in a ca 1 5 ga supracrustal metasedimentary sequence the other

re computing palaeopoles for the effects of tectonic finite strain abstract the pre messinian limestone cover 58 8 ma to the troodos ophiolite 88 ma of southern cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and in many sites stylolitic cleavage these define a gently n dipping foliation and an n plunging extension south vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south disturbed by faulting the magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de strained assuming continuum behaviour the optimum de straining 30 40 shortening in a flattening strain is compatible with the stylolitic cleavage development restores bedding to the near horizontal and restores the characteristic remanent magnetization vectors chrms to concentrated symmetrical fisherian distributions the strain corrected chrms yield more reasonable palaeopole locations for the lefkara and pakhna limestone and more uniform micro plate rotation rates corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the troodos plate since the creation of the late cretaceous 88 ma ocean lithosphere it did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at 1 5 ma 1 since 58 ma

experimental stress remagnetization of magnetite abstract pseudo single domain psd and multidomain md magnetite grains remagnetize in weak magnetic field 30 t during experimental triaxial deformation the magnetite is supported in a calcite cement matrix minor remagnetization occurs with hydrostatic stress of 100 mpa significant remagnetization requires hydrostatic pressure of 150 mpa with differential stress of 5 mpa superposed on the sample intergranular differential stresses must be much higher due to amplification at grain asperities stress remagnetization does not need chemical or thermal energy new components of magnetic remanence are added parallel to the remagnetizing field however this only happens in grains or parts of grains with coercivities of remanence 60 mt grains with coercivities of 20 55 mt remember the primary magnetization and are not stress magnetized these coercivity limits do not depend on the differential stress or strain rate of the experiment the spatial distribution of vector components of remanence was isolated by af demagnetization after deforming a magnetized sample the components of remanence spread along a partial great circle between the initial remanence and the direction of the remagnetizing field the directions of the original magnetization and the remagnetizing field are the only factors controlling the course of the remagnetization path triaxial deformation shortened these samples by if the experimental results are transferable to nature it is possible that a pulse of excess crustal stress 25 mpa could partially remagnetize low dislocation density magnetite the experiments show that the directions of the remagnetizing field and the primary magnetization are the only variables that affect the demagnetization vector path we do not need to know the strain history coaxial or noncoaxial finite strains or principal strain directions to detect the paleofield directions unlike paleomagnetic vectors affected by large strains permanent changes in magnetic structure occur as a result of experimental deformation regardless of the differential stress the proportion of grains with remanence coercivities

experimental and numerical modeling of irm rotation in deformed synthetic samples abstract the effect of strain upon isothermal remanent magnetization irm carried by hematite particles embedded in a plasticine matrix has been investigated magnetized artificial hand samples were deformed by simple coaxial shortening after deformation the hand samples were cut into 8 15 specimens and each specimen s irm measured hence a mean irm direction was obtained for each shortening step the main results are 1 when the initial direction of irm is parallel to the shortening axis 3 the hand sample mean irm stays parallel to 3 during deformation however individual magnetization directions are deflected towards the flattening plane and thus the specimen directions are scattered by strain this is accompanied by a significant decrease of magnetization intensity 2 in contrast when the initial direction of irm is perpendicular to 3 the shortening results in a clustering of specimen magnetization directions the mean direction being unchanged this is accompanied by a slight increase of magnetization intensity 3 in all other cases where initial irm direction makes an intermediate angle with shortening direction deformation induces a deviation of each specimen irm toward the flattening plane this results in an overall deflection of hand sample mean irm the amount of rotation increases with strain intensity changes of grouping and intensity depend on the initial angle between irm and 3 defining a continuum in behavior between irm directions being either parallel to 3 or perpendicular to 3 finally it is shown that all the experimental changes in irm due to deformation direction grouping intensity can be modeled by a passive marker rotation of hematite planar particles

effect of progressive deformation on remanent magnetization of permian redbeds from the alpes maritimes france abstract the relationships among the anisotropy of magnetic susceptibility the finite strain and the characteristic remanent magnetization chrm have been investigated in deformed red sediments from the alpes maritimes in southeast france the magnetic properties of these rocks are due to hematite the finite strain is known from field studies of the shapes and orientations of green reduction spots and from optical microscopy and x ray texture goniometry studies of chlorite and illite the strain data permit a classification of the magnetic results according to four main stages of progressive deformation in the compaction stage magnetic anisotropy is flattened in the bedding due to compactional loading the chrm directions approximate to a fisherian distribution about a mean value close to the known permian direction for the region as deformation proceeds pencil structure first develops and the anisotropy is characterized by a prolate pattern with maximum axis parallel to the subhorizontal azimuth of the pencil structure and fold axes with increasing deformation the magnetic susceptibility ellipsoid becomes oblate and finally triaxial at these stages of high deformation the anisotropy patterns show that the hematite grain orientations are strongly influenced by the cleavage the remanence directions in deformed rocks do not correspond to a fisher distribution and eventually deviate strongly from permian directions these results show that paleomagnetic vectors are affected by the distortional strains which accomodate shape change in the rock simple tilt correction without compensation for the strain would yield incorrect paleopole positions an attempt has been made to compensate for the effects of deformation by regarding the remanent vector as a passive marker march 1932 equations for strain removal were applied to the remanence in two ways 1 a using strain data directly obtained in the field for each site and 2 b by converting the magnetic susceptibility anisotropy of each sample to a corresponding finite strain value correcting for the finite strain by both methods improves the within site grouping of paleomagnetic vectors in strongly deformed rocks strain removal assuming the march model results in overcorrection of the remanent magnetization data in low strain regions distortion may produce mechanical rotation of the hematite particles but recrystallization may alter the deformation mechanism in regions of high strain the limits of applicability of the passive marker treatment could not be tested adequately in the deformed red sediments of the alpes maritimes

remanent magnetism and ductile deformation in an experimentally deformed magnetite bearing limestone abstract a limestone with pseudo single domain psd pure magnetite is given an isothermal remanent magnetization irm samples are deformed triaxially at 200 mpa 2 kbar confining pressure at room temperature and at a strain rate of 10 5 s 1 the limestone deforms in a ductile manner by twinning to produce sufficiently homogeneously strained specimens up to about 20 shortening the uniform saturation irm rotates away from the shortening axis approximately by an amount expected for the homogeneous strain of a non material line marker hydrostatic compaction alone shows that this is time dependent and compatible with progressive damage of the ferrimagnetic grains strain produces an increase in coercivity of the magnetite an increase in its saturation remanence and an increase in its anisotropy of magnetization with respect to irm these observations are compatible with reduction in effective grain size and change of shape of magnetite experimental deformation reduces the intensity of magnetisation chiefly by the removal of low coercivity components of remanence when there is a pure irm vector

on pressure demagnetization the effects of pressure on the ramanent magnetization of rocks may well be important in regions of low geothermal gradients for example subduction zones or at localities where a magnetization is associated with a meteorite impact in a series of experiments undertaken to determine the effects of hydrostatic pressure on the saturation magnetic remanent moment in a variety of materials a pressure demagnetization has been observed from these experiments we conclude that there is a relationship between coercivity and the extent of this pressure demagnetization for rocks bearing magnetite and titanomagnetites

eocene 46 44 ma onset of australia pacific plate motion in the southwest pacific inferred from stratigraphy in new caledonia and new zealand

strain removal applied to paleomagnetic directions in an orogenic belt the permian red slates of the alpes maritimes france abstract detailed investigations of relationships between strain and directions of characteristic remanent magnetization chrm have been conducted on the permian red series of the alpes maritimes france the analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations in most sites the mean direction of the strain shortening axis is close to the expected permian paleomagnetic direction in this case we observe a large scatter of the chrm directions within each site which is interpreted as an increase of an initial dispersion due to strain moreover this dispersion more or less increases with strain intensity in other cases where the angle between the shortening axis and the paleomagnetic direction is high the whole population is deflected towards the cleavage xy plane these observations lead us to assume a material line like behaviour of the chrm vectors and consequently an attempt has been made to recalculate these directions by strain removal these calculations give positive results at two levels 1 a statistically significant improvement in the clustering of the within site distributions and 2 after tilt correction using unstrained bedding planes a significant improvement in the clustering of the between site distribution the formation mean direction after strain removal is in good agreement with known permian results for stable europe

experimental strain of isothermal remanent magnetization in ductile sandstone abstract a macroscopically ductile sandstone to which a homogeneous isothermal remanent magnetization irm was applied has been deformed at 150 mpa confining pressure and constant strain rate of 10 5 s 1 hydrostatic compaction does not produce a deflection of the irm vector although it is reduced in intensity pure shear producing shortening in the range 2 to 35 steadily reduces the intensity of magnetization but also homogeneously rotates the remanence vector toward the plane of flattening the amount of rotation is slightly less than that expected for a non material line undergoing homogeneous strain deformation selectively removes weakly coercive components of remanence as revealed by alternating field af demagnetization during deformation a weak deformational viscous remanent magnetization dvrm is acquired from the pressure vessel this is different from a conventional viscous remanent magnetization vrm in that it is not acquired when the specimen is subject to hydrostatic confining pressure alone even for periods three times longer than the longest deformation test

magnetite dissolution and neocrystallization during cleavage formation paleomagnetic study of the martinsburg formation lehigh gap pennsylvania paleomagnetic study of the shale to slate transition in the ordovician martinsburg formation at lehigh gap pennsylvania reveals that the remanence of the relatively undeformed shales predates cleavage formation and that the slates were remagnetized during cleavage development the shales display two components of remanence component 1 c1 was the only magnetization direction in the majority of the shales and the first removed component in 10 shale specimens component 2 c2 is observed as a single component remanence with a low 300 400 c unblocking temperature range in samples from one shale station and as a second removed component in 10 shale specimens the mean directions are declination inclination 347 04 346 47 in situ tilt corrected for c1 and 80 63 135 40 for c2 the pencil slates and well cleaved slates both display single component remanences the mean directions are 198 12 201 26 for the pencil slates and 199 24 225 54 for the well cleaved slates using the relationship between cleavage development and remagnetization in the slates the timing of cleavage development and folding and the regional geology we can constrain the relative ages of the remanences as follows 1 the shale c1 direction is most likely prefolding and represents a primary upper ordovician direction 2 the slate directions are younger in age than the shale directions and are probably of permo carboniferous alleghenian age these results show that remagnetization occurred during development of slaty cleavage in these rocks we interpret the mechanism for this remagnetization to have been strain induced dissolution and neocrystallization of magnetite the additional difference in the characteristic directions of the pencil slates and well cleaved slates may be attributed to strain reorientation of the pencil slate remanence after new growth of magnetite the unknown temporal relationship between dissolution and neocrystallization and strain precludes any quantitative attempt to correct remanence for strain in these samples our study shows that paleomagnetic studies of deformed sediments must consider the possibility of dissolution and neocrystallization of remanence carriers in addition to strain induced rotation

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

effects of tectonic deformation on the remanent magnetization of rocks paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis however the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known anisotropy of magnetic susceptibility ams provides a quantitative basis for strain determination the principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid a linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types if the correlation method used is valid ams observations can be used to establish the magnitudes and directions of the principal strains strain modifies bedding planes and realigns the grains that carry magnetic remanence compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain the effects of strain on bedding can be corrected using passive marker theory attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive during deformation linear and planar elements deflect away from the axis of maximum shortening the passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions the passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix

late proterozoic reconstructions of north west scotland and central canada magnetic fabrics paleomagnetism and tectonics red beds dominate the stoer group and the unconformably overlying torridon group having accumulated between 1400 and 1000 ma in a rift of the hebridean foreland the sequences were weakly strained and shortened e w and each underwent successive diagenetic changes which altered their magnetic properties and caused their different characteristic post depositional magnetizations anisotropy of magnetic susceptibility ams reveals an n s vertical tectonic foliation superimposed on the bedding planar ams sub fabric the tectonic ams sub fabric was isolated by comparing normalized and non normalized mean tensors of multiple specimens the n s vertical ams tectonic foliation postdates warps of bedding thus the high susceptibility minerals including those carrying palaeomagnetic signals re oriented or recrystallized after deposition and diagenesis thus characteristic remanence vectors chrm were acquired long after deposition first in the stoer group then in the torridon chrms were isolated for 143 stoer specimens and 94 torridon specimens using two cycles of low temperature demagnetization followed by at least 12 steps of thermal demagnetization for the stoer group two structurally integral groups of sites yield mean magnetizations of 317 0 43 1 95 11 3 n 49 and 309 9 52 1 95 10 6 n 23 some stoer specimens bear a torridon age overprint 133 3 525 95 9 6 n 19 the younger torridon group yields 138 4 52 2 95 7 4 n 42 134 9 45 5 95 16 0 n 15 and 134 4 55 1 95 11 3 n 19 from three structurally integral clusters of sites these late chemical magnetizations postdate bedding warps faults and tectonic ams fabrics they do not warrant local rigid body tilt corrections however we restored the chrm directions for the regional post cambrian eastwards tilt which affected the entire foreland from the lewisian basement to the overlying proterozoic strata paleopoles calculated from the strata s restored vectors were rotated into laurentian coordinates for comparison with canadian proterozoic palaeopoles the stoer group was magnetized in a normal polarity geomagnetic field whereas the torridon group was magnetized in a reversed polarity epoch however both sequences include at least one polarity transition indicating long duration chemical magnetizations successfully averaging secular variation at a minimum red bed magnetization processes exceeded 0 1 ma and may have encompassed several million years during stoer group magnetization the hebridean craton was at paleolatitudes of 26 44 n when the torridon group magnetized it was at 23 27 s accepting published ages this defines a minimum southward latitudinal displacement rate of 27 40 km ma restored stoer group paleopoles lie in the present day north central pacific ocean 190e 25n corresponding to the laurentian apwp dated at between 1090 and 1109 ma torridon group paleopoles lie in the south central pacific 210e 25s near the poorly documented 900 ma southernmost part of the laurentian apwp

complex trichomes associated with foliar odontopteris schlotheimii brongniart manebach type locality germany implication for taxonomy abstract reported for the first time is a physical association of complex structured trichomes with foliar odontopteris schlotheimii brongniart 1831 from the type manebach formation germany this is based on examining rock matrices dissolved by hydrofluoric acid of bipinnate and pinnate material these trichomes and their inferred dense distribution most significantly are directly comparable with those of specimens of odontopteris cantabrica wagner 1969 from the canadian sydney coalfield used as index fossil to establish the cantabrian substage this result is one strong argument that invites comparison with o schlotheimii in addition to the acknowledged common pinnule morphologies of o cantabrica and o schlotheimii additionally trichomes are not observed in samples of the rock matrix of a fragmentary axis associated with lesquereux specimen of o schlotheimii southern anthracite field eastern pennsylvania usa results of the trichomic comparison and taxonomic implication thereof will be discussed in a future paper

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