lower devonian platform carbonates from kasaan island southeastern alaska alexander terrane facies representing shallow subtidal conditions an open lagoon or shelf offshore biostromal banks and a restricted lagoon or shelf form the lower devonian emsian sequence of rocks exposed on k

lacustrine deposits in the karheen formation fortify links between alaska s alexander terrane and the old red sandstone continent in the late silurian early devonian the karheen formation of southeastern alaska alexander terrane is an 1800 m thick redbed molasse sequence that formed during the late silurian early devonian in the wake of the klakas orogeny previous research interpreted thin platy limestone in the karheen formation as marine in origin we describe new evidence including a lack of marine fossils which indicates that the rhythmically laminated limestone formed in deep stratified lakes intercalated siliciclastic lake margin facies record cycles of lake expansion and contraction comparative analysis with coeval lake orcadie deposits in scotland shows that the karheen formation is strikingly similar in age thickness lithology paleontologic characteristics and tectonic setting to the old red sandstone together with other published geologic data from southeastern alaska the karheen s lacustrine deposits strengthen paleogeographic connections between the alexander terrane and the n atlantic caledonide region in the mid paleozoic

enhancing weak magnetic fabrics using field impressed anisotropy application to the chinese loess summary the anisotropy of the low field magnetic susceptibility ams alternating field treated ams af ams and anisotropy of anhysteretic remanent magnetization aarm have been systematically examined for a sandwich sequence of loess palaeosols crossing the upper loess unit l2 marine oxygen isotope stage 6 mis6 to the last interglacial maximum palaeosol unit s1s3 mis5e from the yuanbao section northwestern china results show that a weak but detectable magnetic lineation is controlled by coarse grained aeolian fe3o4 magnetite because the long axes of the coarse grained magnetites are statistically parallel to the palaeowind directions the declination of the maximum susceptibility principal axis of ams can be used as an indicator of palaeowind direction in contrast fine grained pedogenic magnetic particles are responsible for the magnetic foliation we found that af ams can indicate the domain state of the magnetic particles which is consistent with potter stephenson s earlier model we also found that af demagnetization can significantly alter the ams in conclusion the angular dependence of af ams can enhance the weak ams fabrics

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

the anisotropy of magnetic susceptibility in biotite muscovite and chlorite single crystals the anisotropy of magnetic susceptibility ams of single crystals of biotite muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock forming minerals the low field ams and the high field paramagnetic susceptibility are defined for the high field values it is necessary to combine the paramagnetic deviatoric tensor obtained from the high field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals this leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice the ellipsoid of paramagnetic susceptibility which was obtained for the three phyllosilicates is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c axes the anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements the degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite is smaller than previously published values the shape of the chlorite ams ellipsoid for all the samples is near perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks d 2003 elsevier science b v all rights reserved

footwall rotation in an oceanic core complex quantified using reoriented integrated ocean drilling program core samples oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large slip detachment faults the common occurrence of these structures in slow and ultra slow spread oceanic crust suggests that they accommodate a significant component of plate divergence however the subsurface geometry of detachment faults in oceanic core complexes remains unclear competing models involve either a displacement on planar low angle faults with little tectonic rotation or b progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading the rolling hinge model we address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1 4 km long footwall section of gabbroic rocks recovered by integrated ocean drilling program iodp sampling at atlantis massif oceanic core complex on the mid atlantic ridge mar these rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons c1r 2r c1r 1n jaramillo and c1r 1r for the first time we have independently reoriented drill core samples of lower crustal gabbros that were initially azimuthally unconstrained to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall this allows reorientation of the palaeomagnetic data placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data analysis of the reoriented high temperature reversed component of magnetization indicates a 46 6 anticlockwise rotation of the footwall around a mar parallel horizontal axis trending 011 6 reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the jaramillo chron 0 99 ma the data provide unequivocal confirmation of the key prediction of flexural rolling hinge models for oceanic core complexes whereby oceanic detachment faults initiate at higher dips and rotate to their present day low angle geometries as displacement increases

quantitative correlation of texture and magnetic anisotropy of compacted calcite muscovite aggregates abstract deformation of carbonate rocks leads to crystallographic preferred orientations cpos of calcite which in turn generates an anisotropy of magnetic susceptibility ams the use of the diamagnetic anisotropy which arises from the preferential orientation of calcite for texture characterization is usually hindered by 1 the weakness of the anisotropy and 2 overlapping contributions from paramagnetic and ferromagnetic minerals to the ams in this study the diamagnetic paramagnetic and ferromagnetic sub fabrics of synthetic calcite muscovite samples have been separated using high field torque measurements the cpo of calcite and muscovite has been determined by neutron diffraction and the magnetic sub fabrics are compared to the ams that was re calculated from the cpo the isolated diamagnetic sub fabric shows a good correlation with the calcite texture for calcite concentrations above 70 the paramagnetic sub fabric is shown to be a reliable texture parameter for muscovite even at a concentration of 5 the results demonstrate the capability of the ams method as a texture parameter in impure carbonate rocks

coseismic deformation from earthquake faulting on a layered spherical earth summary a method for calculating the static displacement field following earthquake faulting in a layered spherical earth is presented at shallow levels the earth s layering is characterized by sharp jumps in bulk and shear moduli at the conrad discontinuity and the moho and is therefore important to consider when evaluating crustal deformation the solution to the equations of static equilibrium is represented as a superposition of spheroidal and toroidal components that each depend on spherical harmonic degree and the moment tensor a method that has recently been applied to the problem of wave propagation on a layered spherical earth is here applied to the static deformation field by representing the point source in terms of discontinuities in the displacement stress vector the green s function for a particular source geometry is derived directly numerical tests are presented to verify the accuracy of the method and to illustrate the effects of sphericity and layering on the calculated deformation fields the effect of sphericity is generally less than about 2 per cent of maximum deformation within 100 km of an earthquake source at crustal depths comparisons between the deformation calculated on a spherical homogeneous earth and spherical layered earth show that up to 20 per cent errors would be introduced if the earth s layered structure were ignored the effect of layering is strongest for sources with a strong horizontal slip component

field impressed anisotropies of magnetic susceptibility and remanence in minerals the application of alternating af or direct df magnetic fields to samples containing dispersed particles of magnetite titanomagnetite or maghemite has been found to alter significantly the measured low field susceptibility anisotropy even though the particle orientation is unchanged the acquisition of isothermal remanent magnetization irm is also strongly dependent on the previous af treatment given to the samples a method of quantifying the changes brought about by application of the fields is described the samples studied were intrinsically isotropic or only weakly anisotropic and the computed impressed susceptibility or irm ellipsoids produced by the applied fields were ellipsoids of revolution with the unique axis aligned with the applied field axis the size and shape of the field impressed ellipsoids were strongly dependent upon particle size these effects might thus form the basis of a rapid nondestructive means of estimating particle size in rocks the results also have important implications for magnetic fabric analysis and the static af demagnetization of rocks in particular to ensure that the intrinsic magnetic fabric is measured without any field impressed component of anisotropy samples should not be subjected to static af demagnetization prior to the determination of either low field susceptibility anisotropy or irm anisotropy samples should also not carry a significant remanence such as an irm acquired in a field greater than about 5 mt prior to low field susceptibility anisotropy analysis

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

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

anisotropy of magnetic susceptibility in alkali feldspar and plagioclase feldspars are the most abundant rock forming minerals in the earth s crust but their magnetic properties have not been rigorously studied this work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions because feldspar is often twinned or shows exsolution textures measurements were performed on twinned and exsolved samples as well as single crystals the anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes the most negative or minimum susceptibility is parallel to 010 and the maximum least negative is close to the crystallographic 001 axis however the magnetic anisotropy is weak when compared to other rock forming minerals 1 53 10 9 m3 kg 1 at maximum therefore lower abundance minerals such as augite hornblende or biotite often dominate the bulk paramagnetic anisotropy of a rock ferromagnetic anisotropy is not significant in most samples in the few samples that do show ferromagnetic anisotropy the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice these results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice

transtension to transpression a study of strain evolution in andaman islands based on gps measurements following great sumatra andaman earthquake 2004 we use campaign mode and continuous gps data from 2005 to 2008 to examine the evolving strain pattern in andaman islands after the great sumatra andaman earthquake two and half years after the earthquake the post seismic period changes to inter seismic period at port blair post seismic displacement rate reduced to about 95 with clockwise rotation of azimuths and towards north andaman velocity vector gradually rotates towards north with drop in magnitude the inter seismic displacement was dominated by trench parallel movement strain calculations on inclined transpession model shows deformations at andaman were highly heterogeneous both in post and inter seismic periods with dominant strike slip and dip slip shearing along with layer parallel shortening and extension post seismic and inter seismic periods had suffered simultaneous subduction perpendicular extension and contraction leading to transtension and transpression deformational environments respectively occurrence of earthquakes during post and inter seismic periods were results of interplay between simple shear and pure shear components of the strain matrix

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

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

seismology speed and size of the sumatra earthquake we now have a clearer picture of the seismic features of last year s gigantic event our seismological results reveal that indonesia s devastating sumatra andaman earthquake on 26 december 2004 was 2 5 times larger than initial reports suggested second only to the 1960 chilean earthquake in recorded magnitude they indicate that it slowly released its energy by slip along a 1 200 km fault generating a long rupture that contributed to the subsequent tsunami now that the entire rupture zone has slipped the strain accumulated from the subduction of the indian plate beneath the burma microplate has been released and there is no immediate danger of a similar tsunami being generated on this part of the plate boundary although large earthquakes on segments to the south still present a threat

control of magnetic rock fabrics by mica preferred orientation a quantitative approach magnetic anisotropy analysis represents a well established and frequently used method in structural geology several attempts were made to relate the anisotropy of magnetic susceptibility ams to rock fabrics and to discover the sources of the ams in gneissic rocks paramagnetic phyllosilicates with lattice dependent magnetic properties are assumed to control the whole rock ams i e it is dominated by mica lattice preferred orientation texture in this study ams was modelled on the basis of the mica texture the rock composition and compared to the experimentally determined ams the orientations of the modelled and experimental tensors agree quite well whereas all the other characteristic parameters shape anisotropy mean susceptibility display large differences it has been concluded that accessory high susceptible phases as well as the diamagnetic rock constituents may have significant influence on ams consequently mica pole figure measurements cannot be replaced by ams measurements without control of the sources of ams it was also concluded that the march model to evaluate strain from mica preferred orientations is not valid for highly strained rocks beside the fact that the obtained strains are too low the deformation is generally inhomogeneous which leads to an overprint of the mica texture since the microstructural fabric elements which originate in different strain regimes prolate or oblate may create similar modifications of the mica orientation patterns it is obvious that the relationship between ams and deformation is rather complex

coseismic slip and afterslip of the great mw 9 15 sumatra andaman earthquake of 2004 we determine coseismic and the first month postseismic deformation associated with the sumatra andaman earthquake of 26 december 2004 from near field global positioning system gps surveys in northwestern sumatra and along the nicobar andaman islands continuous and campaign gps measurements from thailand and malaysia and in situ and remotely sensed observations of the vertical motion of coral reefs the coseismic model shows that the sunda subduction mega thrust ruptured over a distance of about 1500 km and a width of less than 150 km releasing a total moment of 6 7 7 0 10 22 n m equivalent to a magnitude m w 9 15 the latitudinal distribution of released moment in our model has three distinct peaks at about 4 n 7 n and 9 n which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated t waves our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very long period surface waves the tsunami predicted from this model fits rela tively well the altimetric measurements made by the jason and topex satellites neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave the near field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500 sec long seismic rupture the postseismic geodetic moment of about 2 4 10 22 nm mw 8 8 is equal to about 30 5 of the coseismic moment release evolution of postseismic deformation is consistent with rate strengthening frictional afterslip online material summary of geodetic data used in this study

rupture process of the 2004 sumatra andaman earthquake the 26 december 2004 sumatra andaman earthquake initiated slowly with small slip and a slow rupture speed for the first 40 to 60 seconds then the rupture expanded at a speed of about 2 5 kilometers per second toward the north northwest extending 1200 to 1300 kilometers along the andaman trough peak displacements reached 15 meters along a 600 kilometer segment of the plate boundary offshore of northwestern sumatra and the southern nicobar islands slip was less in the northern 400 to 500 kilometers of the aftershock zone and at least some slip in that region may have occurred on a time scale beyond the seismic band

fabric of the rio ceara mirim mafic dike swarm northeastern brazil determined by anisotropy of magnetic susceptibility and image analysis 1 anisotropy of magnetic susceptibility ams and petrofabric studies of the early cretaceous rio ceara mirim dike swarm reveal that shape preferred orientation of opaque grains controls the magnetic fabric the 350 km long swarm is characterized by relatively thick dikes of the order of 20 m up to 150 m in width the strong magnetic susceptibility of these rocks around 5 10 2 si is attributed to magnetite with low ti content the magnetite usually forms euhedral to subhedral equant to skeletal grains the resulting ams has a large proportion of abnormal fabric types as observed in 58 of the 50 dikes studied the remaining 42 of the dikes show a normal fabric regionally characterized by steep magnetic foliations and subhorizontal lineations in the central eastern part of the swarm however the lineation plunges downdip suggesting a magmatic feeder zone the shape alignment of the plagioclase supports the inferred regional flow pattern indicating a primary origin for the normal ams inverse to highly imbricate magnetic fabrics have lineations perpendicular to or at a high angle to the dike wall a shape lineation of opaque grains lies generally close to the magnetic lineation while the foliation is provided by the oblate shaped plagioclase fabric petrofabric studies indicate that the abnormal magnetic fabric is unrelated to flow competition of interstitial magnetite grains crystallizing either along as well as perpendicular to the magma stretching direction seems to account for the weak magnetic anisotropy and the irregular distribution of the ams types on the swarm

the magnetic properties of natural and synthetic fex mg1 x 2 sio4 olivines article i nfo olivine is an abundant orthosilicate in the solar system in the upper mantle of rocky planets in meteorites and interstellar dust the magnetic properties of fex m g1 x 2 sio4 olivines result from the silicate matrix and its iron rich inclusions and have not always been separated in previous studies the properties of the matrix are important to understand mantle rocks anisotropy and their deformation both in xenoliths and peridotite massifs while the inclusions are potential paleomagnetic and paleointensity recorders in this study we performed new measurements on 7 natural and 23 synthetic ferromagnesian olivines covering the whole range from forsterite mg2sio4 fo100 to fayalite fe2sio4 fo0 and from 4 to 310 k many of our specimens contain ferromagnetic inclusions magnetite or maghemite with magnetic sizes ranging from super paramagnetic tomultidomain the respectivecontributions of the matrix and the inclusions are systematically isolated using magnetic fields large enough to saturate the ferromagnetic component due to inclusions at room temperature as predicted by molecular field theory while the forsterite end member is diamagnetic xhf 6 8 10 10 m 3 kg and the fayalite end member is paramagnetic xhf 1 10 10 6 m 3 kg their magnetic properties do not vary linearly with iron content these olivines also exhibit one or two magnetic transitions at composition dependent low temperatures at the neel temperature tn olivines exhibit a first magnetic transition from paramagnetic to antiferromagnetic behavior indicated by negative paramagnetic curie temperatures a second transition tt occurs at lower temperatures for specimens with x 0 1 and could be attributed to a change from collinear to canted antiferromagnetic state in the range fo83 fo93 corresponding to the earth s upper mantle the anisotropy of magnetic susceptibility ams is directionally consistent with the ams principal axes k1 k2 and k3 respectively corresponding to the a c and b crystallographic axes at room temperature the degree of anisotropy increases from 1 028 to 1 302 with decreasing iron content the presence of small and submicroscopic iron rich inclusions significantly complicates the investigation of olivine physical properties but also constitutes opportunities to record paleomagnetic field intensities

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

early devonian conodonts from prince of wales island southeastern alaska early devonian conodonts from 11 localities on prince of wales island and adjacent small islands southeastern alaska include pandorinellina optima moskalenko pandorinellina exigua philipi klapper pandorinellina exigua exigua philip pandorinellina boucoti klapper pelekysgnathus serratus jentzsch pelekysgnathus cf p klamathensis savage pelekysgnathus n sp eognathodus sulcatus sulcatus philip eognathodus sulcatus kindlei lane and ormiston eognathodus sulcatus n ssp a eognathodus sulcatus n ssp b ozarkodina eberleini savage kimognathus alexeii mashkova kimognathus n sp a and kimognathus n sp b the occurrences indicate that lower devonian deposits are widespread on prince of wales island on wadleigh island zlichovian and probably older deposits underlie eifelian beds of the wadleigh limestone previously thought to include limestone only of middle and late devonian age the early devonian faunas of this part of southeastern alaska contain species characteristic of cordilleran north america there are strong affinities with nevada eastern australia and central asia but lesser affinities with central and eastern europe affinities with eastern north america and western europe are relatively weak regional variations in conodont color alteration indices suggest that the southern half of prince of wales island has a different post devonian thermal history from the northern half

relative paleointensity estimates from magnetic anisotropy proof of concept relative paleointensity data from sedimentary rocks play an important role to decipher the workings of the geodynamo and to correct for atmospheric cosmogenic radionucleide production so it is important to understand how sediments acquire remanent magnetizations and to better assess the quality of relative paleointensity data we present experimental results from sediments deposited in controlled magnetic fields to observe the changes in magnetic anisotropy as a function of applied field strength going from near earth like values to almost full saturation relative paleointensity values followed a very well defined power law through the entire range of applied field intensities magnetic remanence fabrics evolved from oblate with maximum anisotropy axes in the sedimentary plane at low field strengths to prolate with maximum anisotropy axes parallel to the applied field direction at high fields anisotropy of magnetic susceptibility also evolved with field strength but in a much less coherent manner than anisotropy of magnetic remanence the experiments used well characterized natural sediments containing single domain magnetite which made it possible to numerically model the data the model matches the field dependency of both relative paleointensity and magnetic fabric development using a simple assumption that a large proportion similar to 80 of the remanence carriers in the sediments are unable to align with the magnetic field while a small fraction are free to align anisotropy of magnetic remanence thus holds promise to improve and assess relative paleointensity estimates and helps improve theoretical treatment of magnetic recording in sediments c 2019 elsevier b v all rights reserved

earth s free oscillations excited by the 26 december 2004 sumatra andaman earthquake at periods greater than 1000 seconds earth s seismic free oscillations have anomalously large amplitude when referenced to the harvard centroid moment tensor fault mechanism which is estimated from 300 to 500 second surface waves by using more realistic rupture models on a steeper fault derived from seismic body and surface waves we approximated free oscillation amplitudes with a seismic moment 6 5 10 22 newton meters that corresponds to a moment magnitude of 9 15 with a rupture duration of 600 seconds the fault rupture models represent seismic observations adequately but underpredict geodetic displacements that argue for slow fault motion beneath the nicobar and andaman islands

remanent and induced magnetic anomalies over a layered intrusion effects from crystal fractionation and magma recharge abstract the bjerkreim sokndal bks norite quartz mangerite layered intrusion is part of the early neoproterozoic rogaland anorthosite province intruded into the fennoscandian shield in south norway at 930 ma the bks is exposed over an area of 230 km 2 with a thickness of 7000 m and is of economic interest for ilmenite magnetite and apatite deposits from the point of view of magnetic minerals in the course of fractional crystallization and magma evolution the ilmenite becomes less fe 3 rich reflected by a change from ilmenite with hematite exsolution to nearly pure ilmenite magnetite starts to crystallize relatively late in the intrusive history but its crystallization is interrupted by influxes of more primitive magma the variations in aeromagnetic and ground magnetic anomalies measured over the bks can be explained in terms of the measured magnetic properties of nrm susceptibility and hysteresis presented here and in terms of mineralogy early layers in the intrusion contain hemo ilmenite as the magma evolved and magnetite started to crystallize this caused a distinct change over the layering from remanence controlled negative anomalies to induced positive anomalies when new more primitive magma was injected into the system hemo ilmenite returned as the major oxide and the resulting magnetic anomalies are again negative the most dramatic change in the magnetic signature is in the upper part of the intrusion in mcu ive where magnetite became a well established cumulate phase as indicated by susceptibility but its induced magnetization is overcome by large nrms associated either with hemo ilmenite or with hemo ilmenite and magnetite exsolved from pyroxenes the average natural remanent magnetizations change from 3 a m in mcu ivd to 15 a m in mcu ive and back to 2 a m in the overlying mcu ivf producing a strong negative remanent anomaly that has been followed along strike for at least 20 km by ground magnetic measurements the highly varied magnetic properties of this intrusion caused by varied magmatic crystallization of combinations of opaque minerals illustrate some of the possibilities to be considered in evaluating crustal magnetic anomalies

post seismic relaxation following the great 2004 sumatra andaman earthquake on a compressible self gravitating earth summary the mw 9 0 2004 december 26 sumatra andaman and mw 8 7 2005 march 28 nias earthquakes which collectively ruptured approximately 1800 km of the andaman and sunda subduction zones are expected to be followed by vigorous viscoelastic relaxation involving both the upper and lower mantle because of these large spatial dimensions it is desirable to fully account for gravitational coupling effects in the relaxation process we present a stable method of computing relaxation of a spherically stratified compressible and self gravitating viscoelastic earth following an impulsive moment release event the solution is cast in terms of a spherical harmonic expansion of viscoelastic normal modes for simple layered viscoelastic models which include a low viscosity oceanic asthenosphere we predict substantial post seismic effects over a region several 100s of km wide surrounding the eastern indian ocean we compare observed gps time series from ten regional sites mostly in thailand and indonesia beginning in 2004 december with synthetic time series that include the coseismic and post seismic effects of the 2004 december 26 and 2005 march 28 earthquakes a viscosity structure involving a biviscous burgers body rheology in the asthenosphere explains the pattern and amplitude of post seismic offsets remarkably well

magnetite grain shape fabric and distribution anisotropy vs rock magnetic fabric a three dimensional case study abstract shape fabric and distribution anisotropy of magnetite grains in a highly ferromagnetic s l syenite are determined for three mutually perpendicular planes on about 600 magnetite grains the grains are lenticular with an average aspect ratio of about 2 0 their preferred orientation determined by the inertia tensor method is consistent with the principal directions of the magnetic susceptibility ellipsoid and the axial ratios of the magnetic fabric ellipses are very close to those of the shape fabric ellipses e g 1 41 and 1 39 in the section plane perpendicular to the magnetic foliation and parallel to the magnetic lineation the anisotropic wavelet transform can detect and quantify the anisotropic distribution of the magnetite grains which are located interstitially along the feldspar grain boundaries this anisotropic distribution has no noticeable effect on the rock s anisotropy of magnetic susceptibility ams since only a few percent of grains are close enough to interact magnetically therefore it is realistic to consider that the ams of ferromagnetic granitic rocks originates mainly from the shape fabric of anisotropic magnetite grains and a close correlation between the magnetic fabric and the magnetite grain shape fabric is expected

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

influence of static alternating field demagnetization on anisotropy of magnetic susceptibility experiments and implications anisotropy of magnetic susceptibility ams indicates the preferred orientation of a rock s constituent minerals however other factors can influence the ams e g domain wall pinning or domain alignment in ferromagnetic minerals therefore it is controversial whether samples should be alternating field af demagnetized prior to ams characterization this may remove the influence of natural remanent magnetization nrm or domain wall pinning on ams however it may also result in field induced anisotropy this study investigates the influence of stepwise af and low temperature demagnetization on mean susceptibility principal susceptibility directions ams degree and shape for sedimentary metamorphic and igneous rocks alternating fields up to 200 mt were applied along the sample x y and z axes rotating the order for each step to characterize the relationship between ams principal directions and the last af orientation the changes in anisotropy defined by the mean deviatoric susceptibility of the difference tensors are between 2 and 270 of the ams in nrm state variations in ams parameters range from small changes in shape to complete reorientation of principal susceptibility axes with the maximum susceptibility becoming parallel to the last af direction this is most prevalent in samples with low degrees of anisotropy in the nrm state no clear correlations were found between field induced anisotropy and hysteresis properties therefore we propose that future studies check any samples whose ams is carried by ferromagnetic minerals and low anisotropy degrees for af induced artifacts these results highlight the need for understanding the ams sources and carriers prior to any structural interpretation

magnetic fabrics crystallographic preferred orientation and strain of progressively metamorphosed pelites in the helvetic zone of the central alps quartenschiefer formation we investigate the relationships between the anisotropy of magnetic susceptibility ams mineral preferred orientation macroscopic structures and finite strain in a section of increasing metamorphism in the helvetic zone of the central alps switzerland the upper triassic quartenschiefer formation an initially hematite bearing red bed sedimentary rock can be traced from unmetamophosed shale to amphibolite facies schists the magnetic fabrics of 805 specimens agree well in orientation with the foliation normal to kmin kmax kint kmin are principal susceptibilities and the mineral stretching lineation parallel kmax in all metamorphic grades the maximum susceptibility axes are in an approximately n s orientation and were acquired during the miocene north directed nappe transport the effects of the tectonic position and the metamorphic grade on the ams ellipsoid suggests that deformation has a stronger influence on its shape than the metamorphic grade the identification and quantification of the sources of magnetic susceptibility was done with 1 a model calculation 2 the high field low field method and 3 low temperature heating curves the ams of low grade samples is dominated by the paramagnetic minerals mainly mica and the ams of higher susceptible medium grade samples by magnetite the paramagnetic dominated magnetic fabrics correlate well with mica 002 lattice preferred orientations obtained from x ray texture goniometry a one to one correlation between march strains and a magnetic parameter mi ln ki kmaxkintkmin is predicted by a numerical model and verified empirically the magnetic fabrics of well characterized samples can thus be used to quantify the degree of mica preferred orientation using relatively simple magnetic anisotropy equipment

the magnetic anisotropy of mantle peridotites example from the twin sisters dunite washington abstract the twin sisters dunite massif washington state provides unweathered peridotites for magnetic analysis these rocks have been chosen to test a new magnetic fabric based on measuring the anisotropy of magnetic susceptibility ams in high field with a vibrating sample magnetometer although the dunites are fresh they host small quantities of secondary magnetite formed along cracks under low field conditions magnetite masks the magnetic fabric of olivine and thus the magnetic fabric does not reflect mantle flow deformation above the magnetic saturation at a field of 1 t the ferrimagnetic component is removed and the high field slope represents the paramagnetic ams only this high field ams originates from the magnetocrystalline anisotropy of olivine the high field ams are compared with lattice preferred orientation lpo fabric data obtained by electron backscatter diffraction lpo point distributions are formed by high temperature plastic flow and match the high field measurements in contrast girdle lpos present notable departures from the high field ams axes

phyllosilicate preferred orientation as a control of magnetic fabric evidence from neutron texture goniometry and low and high field magnetic anisotropy se rhenohercynian zone of bohemian massif

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

anisotropy of magnetic susceptibility in natural olivine single crystals mantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties to interpret anisotropy in mantle rocks it is necessary to understand the anisotropy of olivine single crystals we determined anisotropy of magnetic susceptibility ams for natural olivine crystals high field ams allows for the isolation of the anisotropy due to olivine alone the orientations of the principal susceptibility axes are related to the olivine s crystallographic structure as soon as it contains 3 wt feo the maximum susceptibility is parallel to the c axis both at room temperature rt and at 77 k the orientation of the minimum axis at rt depends on iron content it is generally parallel to the a axis in crystals with 3 5 wt feo and along b in samples with 6 10 wt feo the ams ellipsoid is prolate and the standard deviatoric susceptibility k0 is on the order of 8 10210 m3 kg for the samples with 1wt feo and ranges from 3 1 1029 m3 kg to 5 7 1029 m3 kg for samples with 3 10 wt feo at 77 k the minimum susceptibility is along b independent of iron content the shape of the ams ellipsoid is prolate for samples with 5 wt feo but can be prolate or oblate for higher iron content the degree of anisotropy increases at 77 k with p0 7757 160 5 the results from this study will allow ams fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content

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

aeromagnetic anomalies magnetic petrology and rock magnetism of hemo ilmenite and magnetite rich cumulate rocks from the sokndal region south rogaland norway aeromagnetic maps of the egersund mid proterozoic igneous province show a spectacular range of positive and negative magnetic anomalies with a contrast up to 15 600 nt the positive magnetic anomalies are over magnetite norites and overlying mangerites and quartz mangerites of the bjerkreim sokndal layered intrusion these rocks are dominated by multi domain md magnetite the negative magnetic anomalies are over ilmenite rich norites of the bjerkreim sokndal layered intrusion the tellnes ilmenite norite ore deposit and massif anorthosites these rocks are dominated by hemo ilmenite and or by silicates containing fine grained oxide exsolution lamellae electron microprobe analyzes of coexisting fe ti oxides in the layered intrusion confirm earlier observations that oxides in early magmatic rocks are dominated by hemo ilmenite with minor end member magnetite followed by more reduced oxides dominated by titanomagnetite with minor near end member ilmenite what is not fully understood is the property of ilmenite with hematite exsolution lamellae or even more striking hematite with ilmenite lamellae to produce strong remanent magnetization of high coercivity and with a neel temperature equal to or above the curie temperature of magnetite this property makes the rhombohedral oxides an important candidate to explain some high amplitude deep crustal anomalies on earth or strong remanent magnetization on other planets a remarkable feature in the egersund province is that primitive magmas produced rocks rich in hemo ilmenite causing negative magnetic anomalies related to magnetic remanence and more evolved magmas produced rocks rich in magnetite related to positive induced magnetic anomalies all in the course of crystallization differentiation

neotectonics of the sumatran fault indonesia the 1900 km long trench parallel sumatran fault accommodates a significant amount of the right lateral component of oblique convergence between the eurasian and indian australian plates from 10 n to 7 s our detailed map of the fault compiled from topographic maps and stereographic aerial photographs shows that unlike many other great strike slip faults the sumatran fault is highly segmented cross strike width of step overs between the 19 major subaerial segments is commonly many kilometers the influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry geomorphic offsets along the fault range as high as 20 km and may represent the total offset across the fault if this is so other structures must have accommodated much of the dextral component of oblique convergence during the past few million years our analysis of stretching of the forearc region near the southern tip of sumatra constrains the combined dextral slip on the sumatran and mentawai faults to be no more than 100 km in the past few million years the shape and location of the sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere nonetheless active volcanic centers of the sumatran volcanic arc have not influenced noticeably the geometry of the active sumatran fault on the basis of its geologic history and pattern of deformation we divide the sumatran plate margin into northern central and southern domains we support previous proposals that the geometry and character of the subducting investigator fracture zone are affecting the shape and evolution of the sumatran fault system within the central domain the southern domain is the most regular the sumatran fault there comprises six right stepping segments this pattern indicates that the overall trend of the fault deviates 4 clockwise from the slip vector between the two blocks it separates the regularity of this section and its association with the portion of the subduction zone that generated the giant m w 9 earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike slip faulting and unusually large subduction earthquakes

magnetic anisotropy in clinopyroxene and orthopyroxene single crystals pyroxenes constitute an important component in mafic igneous and metamorphic rocks they often possess a prismatic habit and their long axis the crystallographic c axis helps define a lineation in a textured rock anisotropy of magnetic susceptibility ams serves as a fabric indicator in igneous and metamorphic rocks if a rock s ams is carried by pyroxenes it can be related to their crystallographic preferred orientation and degree of alignment this requires knowing the intrinsic ams of pyroxene single crystals this study provides a comprehensive low field and high field ams investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure chemical composition oxidation state of fe and the possible presence of ferromagnetic inclusions the paramagnetic anisotropy extracted from high field data shows clear relationships to crystallographic directions and fe concentration both in clinopyroxene and orthopyroxene in the diopside augite series the intermediate susceptibility is parallel to b and the maximum is at 45 to the c axis in aegirine the intermediate axis remains parallel to b while the maximum susceptibility is parallel to c the ams of spodumene depends on fe concentration in enstatite the maximum susceptibility aligns with c and the minimum with b and in the case of hypersthene the maximum susceptibility is normal to the exsolution lamellae magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities which dominates the low field anisotropy these results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene bearing rocks

changes in mean magnetic susceptibility and its anisotropy of rock samples as a result of alternating field demagnetization measurements of low field magnetic susceptibility k and its anisotropy ams on different rock types during stepwise alternating field af demagnetization in increasing fields revealed not only significant changes of the ams principal susceptibilities but also an increase of the mean magnetic susceptibility km studied collections of loess paleosol samples from different sections in belgium bulgaria china siberia and tadjikistan and diorites granites and gneisses from antarctica show systematic km increase between 2 and 27 as compared to the initial values after af demagnetization up to 100 or 200 mt maximum amplitude the relationships between magnetic susceptibility increase and magnetic hysteresis parameters and their ratios indicate that the km increase is due to changes in magnetic domain configuration of the initial natural remanent magnetization nrm state of the remanence carriers during af treatment the obtained linear relationship between k increase and the degree of anisotropy p for strongly anisotropic gneiss samples suggests that magnetostatic interactions also play a role in the observed af effect on km

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

a new inclination shallowing correction of the mauch chunk formation of pennsylvania based on high field air results implications for the carboniferous north american apw path and pangea reconstructions abstract a new magnetic anisotropy study was performed on samples of the lower carboniferous mauch chunk formation of pennsylvania these red beds had been sampled for an inclination shallowing study by tan and kodama 2002 however application of a high field anisotropy of isothermal remanence magnetization hf air technique specifically designed to measure the anisotropy of hematite provides considerably different results from those previously reported the newly measured fabric has smaller anisotropy 9 17 as opposed to 25 40 and shows a pronounced ene wsw magnetic lineation that is sub parallel to the trend of the appalachians and interpretable as a hematite intersection lineation that occurred during local nnw directed shortening the measured magnetic fabric yields a new inclination correction with a corrected paleopole that is in better agreement with recently corrected carboniferous paleopoles than the previously corrected mauch chunk paleopole defining a more consistent apw path the corrected paleopoles allow calculation of new mean early 325 ma and late 312 ma carboniferous inclination corrected paleopoles for north america which can be compared to coeval but uncorrected paleopoles from gondwana results suggest a pangea b assemblage unless inclination shallowing is considered for gondwana estimating an inclination correction for gondwana sedimentary rock derived paleopoles permits a pangea a type assemblage at higher southern latitudes than previous reconstructions which we term pangea a3

stepwise and continuous low temperature demagnetization 1 magnetites with sizes from 1 m to 135 m were cooled in zero field and their magnetizations m t measured continuously m t changed reversibly in cooling from t0 300 k to 200 k and in subsidiary warming cooling cycles ti t0 ti for any ti changes in m t in cooling from 200 k to 130 k were largely irreversible due to decreasing magnetocrystalline anisotropy which promotes wall unpinning and domain nucleation low temperature demagnetization ltd is almost complete by 130 k in 20 135 m magnetites but in 1 14 m magnetites further ltd occurs on cooling to 120 k as magnetocrystalline easy axes change and domains reorganize at the verwey transition the observed irreversible changes are the basis of stepwise ltd as a method of paleomagnetic cleaning decrements m in remanence due to cooling are most accurately measured at t0 requiring a set of warming cooling cycles ti t0 ti a less accurate method continuous ltd measures decrements m ti m ti 1 from the main cooling curve below 200 k without intermediate warming cooling cycles this requires remanence measurements at ti t0 stepwise or continuous ltd curves m ti discriminate among remanence types and grain sizes the signal of finer psd grains is enhanced compared to coarser md grains analogous to the lowrie and fuller 1971 test the inverse thermoremanence itrm of 1 14 m grains is harder to stepwise ltd than saturation remanence sirm while anhysteretic remanence arm is harder than either for 20 135 m multidomain grains itrm is softer than sirm

plate boundary deformation associated with the great sumatra andaman earthquake the sumatra andaman earthquake of 26 december 2004 is the first giant earthquake moment magnitude m w 9 0 to have occurred since the advent of modern space based geodesy and broadband seismology it therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events here we report estimates of the ground displacement associated with this event using near field global positioning system gps surveys in northwestern sumatra combined with in situ and remote observations of the vertical motion of coral reefs these data show that the earthquake was generated by rupture of the sunda subduction megathrust over a distance of 1 500 kilometres and a width of 150 kilometres megathrust slip exceeded 20 metres offshore northern sumatra mostly at depths shallower than 30 kilometres comparison of the geodetically and seismically inferred slip distribution indicates that 30 per cent additional fault slip accrued in the 1 5 months following the 500 second long seismic rupture both seismic and aseismic slip before our re occupation of gps sites occurred on the shallow portion of the megathrust where the large aceh tsunami originated slip tapers off abruptly along strike beneath simeulue island at the southeastern edge of the rupture where the earthquake nucleated and where an m w 7 2 earthquake occurred in late 2002 this edge also abuts the northern limit of slip in the 28 march 2005 m w 8 7 nias simeulue earthquake

ams nrm interferences in the deccan basalts toward an improved understanding of magnetic fabrics in flood basalts the evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies a statistically significant sample set of titanomagnetite bearing lava flows from the malwa plateau the northern part of the deccan traps in india has been investigated for a possible interference of induced and natural remanent magnetization nrm the nrm alters the scalar anisotropy of magnetic susceptibility ams parameter and the orientations of the ams principal magnetic axes which are crucial for the evaluation of the flow direction for cleaning of the nrm component the lava samples have been demagnetized by use of an alternating field af tumbling demagnetizer peak fields of 100 mt as previous studies have shown that static af demagnetization can bias the results samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization the evaluated flow directions show a more differentiated flow pattern of the malwa area which seems to fit better into the regional geological setting in samples with inverse magnetic fabrics carrying a higher portion of single domain particles ams principal axes remain unchanged after the demagnetization indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations according to these results we propose that the ams measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers at least for samples with normal magnetic fabrics and therefore a more precise and better reflection of the actual mineral fabric

low temperature magnetic anisotropy in micas and chlorite abstract phyllosilicates such as micas and chlorite are common rock forming minerals and often show preferred orientation in deformed rocks in combination with single crystal anisotropy this leads to anisotropy of physical properties in the rock such as magnetic susceptibility in order to effectively use the magnetic anisotropy to understand a rock fabric it is necessary to identify the minerals responsible for the magnetic anisotropy techniques have been developed to separate contributions of the ferrimagnetic antiferromagnetic paramagnetic and diamagnetic susceptibilities to the anisotropy of magnetic susceptibility because diamagnetic and paramagnetic susceptibility are both linearly dependent on field separation of the anisotropic contributions requires understanding how the degree of anisotropy of the paramagnetic susceptibility changes as a function of temperature note that diamagnetic susceptibility is not dependent on temperature the increase in paramagnetic anisotropy at low temperature is used to separate the paramagnetic and diamagnetic subfabrics and can be expressed by the p77 factor in this study we determined p77 which is the change in the degree of anisotropy k between room temperature 298 k and liquid nitrogen temperature 77 k for a series of micas and chlorite the paramagnetic susceptibility ellipsoid is highly oblate with the minimum principal susceptibility normal to the silicate layers at both 77 k and rt the degree of anisotropy k increases by a factor of approximately 6 3 8 7 for individual samples of muscovite phlogopite and chlorite on cooling from rt to 77 k and between 11 2 and 12 4 for biotite a decrease in temperature enhances the paramagnetic anisotropy in a mineral biotite exhibits a relatively stronger enhancement due to the onset of magnetic ordering below 100 k this can have important implications for interpreting low temperature anisotropy in mudstones mica schists and gneisses

magnetic anisotropy in natural amphibole crystals anisotropy of magnetic susceptibility ams is often used as a proxy for mineral fabric in deformed rocks to do so quantitatively it is necessary to quantify the intrinsic magnetic anisotropy of single crystals of rock forming minerals amphiboles are common in mafic igneous and metamorphic rocks and often define rock texture due to their general prismatic crystal habits amphiboles may dominate the magnetic anisotropy in intermediate to felsic igneous rocks and in some metamorphic rock types because they have a high fe concentration and they can develop a strong crystallographic preferred orientation in this study the ams is characterized in 28 single crystals and 1 crystal aggregate of compositionally diverse clino and ortho amphiboles high field methods were used to isolate the paramagnetic component of the anisotropy which is unaffected by ferromagnetic inclusions that often occur in amphibole crystals laue imaging laser ablation inductively coupled plasma mass spectrometry and mossbauer spectroscopy were performed to relate the magnetic anisotropy to crystal structure and fe concentration the minimum susceptibility is parallel to the crystallographic a axis and the maximum susceptibility is generally parallel to the crystallographic b axis in tremolite actinolite and hornblende gedrite has its minimum susceptibility along the a axis and maximum susceptibility aligned with c in richterite however the intermediate susceptibility is parallel to the b axis and the minimum and maximum susceptibility directions are distributed in the a c plane the degree of anisotropy k increases generally with fe concentration following a linear trend k 1 61 10 9 fe 1 17 10 9 m3 kg additionally it may depend on the fe2 fe3 ratio for most samples the degree of anisotropy increases by a factor of approximately 8 upon cooling from room temperature to 77 k ferroactinolite one pargasite crystal and riebeckite show a larger increase which is related to the onset of local ferromagnetic s l interactions below about 100 k this comprehensive data set increases our understanding of the magnetic structure of amphiboles and it is central to interpreting magnetic fabrics of rocks whose ams is controlled by amphibole minerals

constraints on 2004 sumatra andaman earthquake rupture from gps measurements in andaman nicobar islands the 26 december 2004 sumatra andaman earthquake mw 9 0 9 3 is the greatest earthquake of the modern seismological era the rupture characteristics of the earthquake particularly in the andaman nicobar region are not well resolved from seismological or far field geodetic data here in this article we present campaign mode global positioning system gps measurements of coseismic displacements at 13 sites in the andaman nicobar islands before and after the 2004 sumatra andaman earthquake these measurements provide improved estimates of rupture characteristics in the region coseismic horizontal ground displacement of 1 5 6 5 m towards the southwest and coseismic vertical displacement mostly subsidence of 0 5 2 8 m occurred along the andaman nicobar islands with maximum displacements in the nicobar islands we estimate coseismic slip under the andaman and nicobar islands as 3 8 7 9 m and 11 15 m respectively the length of the rupture is estimated to be about 1500 km with a width varying from 120 km under middle andaman island to 160 km under great nicobar island gps measurements during january 11 22 2005 from port blair suggest rapid afterslip in the postseismic period limited gps data available from 1995 measurements at two sites in andaman provide evidence of strain accumulation that varied significantly in the 10 yr preceding the earthquake d 2005 elsevier b v all rights reserved

deformation and slip along the sunda megathrust in the great 2005 nias simeulue earthquake seismic rupture produced spectacular tectonic deformation above a 400 kilometer strip of the sunda megathrust offshore northern sumatra in march 2005 measurements from coral microatolls and global positioning system stations reveal trench parallel belts of uplift up to 3 meters high on the outer arc islands above the rupture and a 1 meter deep subsidence trough farther from the trench surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward a saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 sumatra andaman rupture the southeastern edge abuts a predominantly aseismic section of the megathrust near the equator

coseismic slip distributions of the 26 december 2004 sumatra andaman and 28 march 2005 nias earthquakes from gps static offsets static offsets produced by the 26 december 2004 m 9 sumatra andaman earthquake as measured by global positioning system gps reveal a large amount of slip along the entire 1300 km long rupture most seismic slip inversions place little slip on the andaman segment whereas both near field and far field gps offsets demand large slip on the andaman segment we compile available datasets of the static offset to render a more detailed picture of the static slip distribution we construct geodetic offsets such that postearthquake positions of continuous gps sites are reckoned to a time 1 day after the earthquake and campaign gps sites are similarly corrected for postseismic motions the newly revised slip distribution m w 9 22 reveals substantial segmentation of slip along the andaman islands with the southern quarter slipping 15 m in unison with the adjacent nicobar and northern sumatran segments of length 700 km we infer a small excess of geodetic moment relative to the seismic moment a similar compilation of gps offsets from the 28 march 2005 nias earthquake is well explained with dip slip averaging several meters mw 8 66 distributed primarily at depths greater than 20 km online material offset estimation procedure comparison of results to other stud ies and simpler solutions and tables of coseismic offsets

magnetic anisotropy rock fabrics and finite strain in deformed sediments of sw sardinia italy abstract although some empirical relationships have been established between the anisotropy of magnetic susceptibility ams and the state of finite strain in studies from specific geologic areas the mechanisms governing correlations are not well understood a comparative study has been made in order to elucidate the ams strain relationship by investigating mineral preferred orientations and the responsible microstructural deformation processes detailed analyses are presented from low grade metamorphic slates of a palaeozoic sequence exposed in the variscan of sw sardinia rock magnetic experiments suggest that paramagnetic minerals dominate the ams in these rocks excellent agreement was found between magnetic fabrics and lattice preferred orientations 001 of mica and chlorite determined from x ray texture goniometry measurements depending on the oblate or prolate ellipsoidal shape of anisotropy the minimum principal axes of ams and x ray textures can coincide with the poles to slaty cleavage or the maximum principal axes can coincide with the bedding cleavage intersection respectively the measured ams and x ray textures only partially reflect the finite strain determined from deformed micro pebbles and reduction spots axial orientations of magnetic fabrics mica and chlorite preferred orientations and finite strain ellipsoids are generally in good agreement depending on development of oblate or prolate fabric ellipsoidal shapes linear correlations can be established for magnitudes of minimum and intermediate axes of the ams and finite strain ellipsoids but shape comparisons are complex microstructural studies by scanning electron microscopy sem indicate that the preferred orientation of the phyllosilicates is caused by different grain scale deformation mechanisms such as kinking micro folding and preferential growth thus magnetic fabrics and mineral preferred orientations evidently result from different deformation mechanisms than the finite strain they reflect the heterogeneous deformation due to micro mechanical reorientation processes of mineral grains while finite strain reflects the homogeneous deformation accommodated cumulatively by the strain markers

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

magnetic anisotropy of chloritoid 1 the magnetocrystalline anisotropy of monoclinic chloritoid a relatively common mineral in aluminum rich metapelitic rocks has been determined for the first time by measuring the high field anisotropy of magnetic susceptibility hf ams using two independent approaches i e a directional magnetic hysteresis measurements and b torque magnetometry on a collection of single crystals collected from different tectonometamorphic settings worldwide magnetic remanence experiments show that all specimens contain ferromagnetic s l impurities being mainly magnetite the determined hf ams ellipsoids have a highly oblate shape with the minimum susceptibility direction subparallel to the crystallographic c axis of chloritoid in the basal plane of chloritoid though the hf ams can be considered isotropic the degree of anisotropy is found to be 1 47 which is significantly higher than the anisotropy of most paramagnetic silicates and even well above the frequently used upper limit i e 1 35 for the paramagnetic contribution to the ams of siliciclastic rocks the obtained values for the paramagnetic curie temperature parallel and perpendicular to the basal plane indicate that this pronounced magnetocrystalline anisotropy is related to strong antiferromagnetic exchange interactions in the direction of the crystallographic c axis 0 as a consequence chloritoid bearing metapelites with a pronounced mineral alignment can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic s l minerals the newly discovered magnetocrystalline anisotropy of chloritoid thus calls for a revised approach of magnetic fabric interpretations in chloritoid bearing rocks

insight into the 2004 sumatra andaman earthquake from gps measurements in southeast asia data collected at 60 global positioning system gps sites in southeast asia show the crustal deformation caused by the 26 december 2004 sumatra andaman earthquake at an unprecedented large scale small but significant co seismic jumps are clearly detected more than 3 000 km from the earthquake epicentre the nearest sites still more than 400 km away show displacements of 10 cm or more here we show that the rupture plane for this earthquake must have been at least 1 000 km long and that non homogeneous slip is required to fit the large displacement gradients revealed by the gps measurements our kinematic analysis of the gps recordings indicates that the centroid of released deformation is located at least 200 km north of the seismological epicentre it also provides evidence that the rupture propagated northward sufficiently fast for stations in northern thailand to have reached their final positions less than 10 min after the earthquake hence ruling out the hypothesis of a silent slow aseismic rupture data culled from gps stations in indonesia malaysia singapore thailand india and china have answered more questions relating to the 26 december sumatra andaman megathrust earthquake permanent displacements were seen 3 000 km away final displacements occurred within 10 minutes of the earthquake ruling out slow aseismic slip on the fault

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

uplift and subsidence associated with the great aceh andaman earthquake of 2004 rupture of the sunda megathrust on 26 december 2004 produced broad regions of uplift and subsidence we define the pivot line separating these regions as a first step in defining the lateral extent and the downdip limit of rupture during that great m w 9 2 earthquake in the region of the andaman and nicobar islands we rely exclusively on the interpretation of satellite imagery and a tidal model at the southern limit of the great rupture we rely principally on field measurements of emerged coral microatolls uplift extends from the middle of simeulue island sumatra at 2 5 n to preparis island myanmar burma at 14 9 n thus the rupture is 1600 km long the distance from the pivot line to the trench varies appreciably the northern and western andaman islands rose whereas the southern and eastern portion of the islands subsided the nicobar islands and the west coast of aceh province sumatra subsided tilt at the southern end of the rupture is steep the distance from 1 5 m of uplift to the pivot line is just 60 km our method of using satellite imagery to recognize changes in elevation relative to sea surface height and of using a tidal model to place quantitative bounds on coseismic uplift or subsidence is a novel approach that can be adapted to other forms of remote sensing and can be applied to other subduction zones in tropical regions

mathematical simulations of anisotropy of magnetic susceptibility on composite fabrics 1 the anisotropy of magnetic susceptibility ams is an integral measure of the preferred orientation of all minerals present in a rock when the ams is carried by paramagnetic minerals alone the principal directions of the susceptibility ellipsoid should reflect the crystallographic orientation of the minerals the relationship between the ams and deformation depends on several factors which control the development of lattice preferred orientation lpo in a rock a mathematical model is presented that simulates the magnetic susceptibility ellipsoid for samples composed of more than one mineral phase measurements of the ams are compared with fabric based anisotropy models for black slates from the navia alto sil slate belt in northern spain the ams is carried by paramagnetic minerals as has been confirmed by high field torque magnetometry low temperature ams and magnetization curves the lpo of mica and chlorite has been determined by x ray texture goniometry pole figures and orientation ellipsoids show the changes in the degree of alignment of the phyllosilicates the models have been tested in samples displaying three types of pole figures a high intensity point maximum a medium intensity elliptical maximum and a girdle shaped distribution at one site displaying kink bands the lpo shows variations between these types at the outcrop scale the case studies illustrate the success in modeling different lpo types

teleseismic relocation and assessment of seismicity 1918 2005 in the region of the 2004 mw 9 0 sumatra andaman and 2005 mw 8 6 nias island great earthquakes the m w 9 0 2004 sumatra andaman islands and m w 8 6 nias island great earthquake sequences have generated over 5000 catalog reported earthquakes along 1700 km of the sumatra andaman and western sunda regions studies of prior regional seismicity have been limited to global catalog locations that often have poorly constrained epicenters and depths approximately 3650 teleseismically well recorded earthquakes occurring in this region during the period 1918 2005 are relocated with special attention to focal depth reduced uncertainties of epicenters and depths in the region on the order of 15 and 10 km respectively foster interpretation of focal mechanism data and provide additional details about the subducting indian and australian plates the revised earthquake dataset reveals a sharp delineation between aftershocks of the 2004 and 2005 earthquakes near simeulue island and a steepening in slab dip from south to north the downdip width of the aftershock zone of the 2004 m w 9 0 earthquake varies from 200 km at its northern end to 275 km at its southern end and events located between 35 and 70 km focal depth occur more frequently in the southernmost section of this aftershock zone outer rise and near trench normal and strike slip faulting earthquakes also increase in frequency following the 2004 and 2005 earthquakes earthquake swarms triggered along the andaman backarc spreading center both north of sumatra and near siberut island 100 km south of the nias island aftershock sequence illustrate the complex and variable nature of seismicity following these great earthquakes

a method for the separation of paramagnetic ferrimagnetic and haematite magnetic subfabrics using high field torque magnetometry summary in this study the contribution of the paramagnetic ferrimagnetic and haematite components to the magnetic anisotropy is separated by means of high field torque magnetometry torque measurements at different fields which are high enough to saturate the ferrimagnetic minerals however still low enough that the torque resulting from the haematite is linear with field allow for the separation of the three magnetic anisotropy components the method has been applied to haematite single crystals in which no paramagnetic or ferrimagnetic components have been found contribute to the torque signal the mean direction of the poles to the crystallographic basal plane in the haematite single crystals is subparallel to the minimum susceptibility direction measured in low field the separation analysis has also been applied to highly deformed red beds from the lower glarus nappe complex switzerland no ferrimagnetic phases are present in the rocks and therefore they cannot contribute to the anisotropy of magnetic susceptibility the magnetic fabric arises from a paramagnetic subfabric carried by the phyllosilicate minerals and haematite in which the basal planes of both phases are in the cleavage plane the measured magnetic lineation seen in low field anisotropy of magnetic susceptibility appears to be an apparent lineation that arises from a weak girdling of haematite and the paramagnetic minerals conforming the rock

alternating field impressed ams in rocks we studied the evolution of anisotropy of magnetic susceptibility ams during stepwise alternating fields af demagnetization on various kinds of rock samples loess and palaeosols diorite granite gneiss with very different degree of magnetic anisotropy the variation of the magnetic fabric appears to be related to both the magnetic fabric before af demagnetization and to the direction of af application the anisotropy change is mainly controlled by the initial magnetic fabric the more anisotropic is the initial magnetic fabric the less is the effect of the direction of field application this can be clearly shown by determination of the difference of the susceptibility ellipsoids after and before af application even for rocks with weak magnetic anisotropy the effect of the initial ams is significant the difference ellipsoids allow in particular cases to point out that the initial magnetic fabric is composite

the size and duration of the sumatra andaman earthquake from far field static offsets the 26 december 2004 sumatra earthquake produced static offsets at continuously operating gps stations at distances of up to 4500 kilometers from the epicenter we used these displacements to model the earthquake and include consideration of the earth s shape and depth varying rigidity the results imply that the average slip was 5 meters along the full length of the rupture including the approximately 650 kilometer long andaman segment comparison of the source derived from the far field static offsets with seismically derived estimates suggests that 25 to 35 of the total moment release occurred at periods greater than 1 hour taking into consideration the strong dip dependence of moment estimates the magnitude of the earthquake did not exceed mw 9 2

correlation between magnetic anisotropy and fabric for devonian shales on the appalachian plateau the magnetic anisotropy of devonian black shale samples was measured from two cores drilled in the appalachian plateau the mineralogy of the shales is predominantly clay with small quantities of quartz and minor amounts of opaques and chlorite magnetite is the predominant ferromagnetic mineral present in the samples the magnetic fabric was measured at both room temperature and liquid nitrogen temperature and is dominated by a well defined bedding vertical compaction and a lesser defined magnetic lineation measurements of the anisotropy of magnetic susceptibility ams at liquid nitrogen temperature which enhances the paramagnetic contribution in the rock showed a strong increase in both the bulk susceptibility and susceptibility differences this increase suggests that the ams is controlled by the paramagnetic minerals particularly the clays and chlorite strain was measured from the orientation of basal planes of the chlorite crystals by texture goniometry good correlations have been found 1 between the orientation of the magnetic lineation and the long axes of the chlorite crystals and 2 between the degree of magnetic foliation and the amount of vertical compaction the magnetic lineation also agrees well with the direction of seismic anisotropy over the plateau the anisotropy of the anhysteretic remanence which expresses the anisotropy due to the ferromagnetic component in the rocks shows a weaker correlation with the amount of vertical compaction a weak magnetic lineation suggests that the magnetite grains were aligned during a deformation phase which post dates the main alleghanian orogeny the magnetic anisotropy of the devonian shales mirrors the compaction and tectonic fabric on the appalachian plateau

magnetic fabric interpretation complicated by inclusions in mafic silicates abstract the expected relationships between anisotropy of low field magnetic susceptibility ams and crystal symmetry of mafic silicates are disturbed by the presence of magnetite inclusions measurements of ams anisotropy of anhysteretic remanence aarm and theoretically predicted bulk susceptibilities from chemical composition all favour the exercise of great caution in the interpretation of preferred orientation distributions of silicates from a rock s ams these results pertain mainly to the mafic silicates of lower crustal rocks pyroxene orthopyroxene amphibole and some of their low grade metamorphic alterations serpentine epidote

magnetic fabric and its significance in the 1400 ma mealy diabase dykes of labrador canada an anistropy of magnetic susceptibility ams study of the relatively fresh mealy diabase dyke swarm indicates a correlation between the ams and the deformational effects in the rocks several ams components were revealed during experimental work on 211 samples that had been heated to 640 c during a paleomagnetic study and 22 fresh samples progressively treated in alternating fields seventeen sites with slight or no deformation have a magnetic fabric composed of two major components the stable component attributed to a primary shape anisotropy has minimum and maximum principal susceptibility directions preferentially oriented vertically and along dyke strike n 60 e respectively this configuration is consistent with an origin due to a post emplacement stress field whereby horizontal extension acted on a vertical sheet of partially crystallized magma the orientation of this component with respect to the present horizontal suggests that the dykes have not been tilted since their emplacement the unstable component removed by fields up to 250 mt is attributed to induced domain anisotropy produced by mild tectonic stress during the grenville orogeny the approximate orientation of its principal components as found by the method of substitutional ellipsoids suggests an east west directed maximum stress four other sites with noticeable deformation have magnetic fabric that could not be reliably analyzed but which may be largely composed of domain and stress induced anisotropies developed during the grenville orogeny