mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

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

flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

the use of palaeomagnetism and rock magnetism to understand volcanic processes introduction this special publication provides a snapshot of our understanding of volcanic processes through the use of palaeomagnetic and rock magnetic techniques here we provide a context for the book placing individual chapters within the milieu of previous work including some magnetic techniques that were not used in the particular studies described herein thermoremanent magnetization is a powerful tool to understand processes related to heating and cooling of rocks including estimating the temperature of emplacement of pyroclastic deposits which may allow us to better understand the rates of cooling during eruption and transport anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics and allow the interpretation of flow dynamics in dykes lava flows and pyroclastic deposits as well as the location of the eruptive vents rock magnetic characteristics can help in the correlation of volcanic deposits but also provide means to date volcanic deposits and to better understand the processes of cooling of the deposits as the magnetic minerals can change with temperature in addition volcanic rocks may be key recorders of past magnetic fields allowing a better understanding of changes in field intensity and perhaps providing clues of how the magnetic field is formed over the past several decades palaeomagnetic and rock magnetic techniques have been applied to numerous problems in volcanology since aramaki akimoto s pioneering study in 1957 which used natural remanent magnetization nrm data to qualitatively estimate the temperature of emplacement of selected pyroclastic deposits in japan aramaki akimoto 1957 many studies have utilized and refined the approach of using progressive thermal demagnetization data to estimate emplacement temperatures of pyroclastic deposits and have demonstrated the utility of this approach in many settings another extensively utilized technique is that of anisotropy of magnetic susceptibility ams which was first applied to pyroclastic volcanic rocks by ellwood 1982 on ignimbrites in the san juan mountains sw colorado usa to estimate flow fabrics and in that study to infer source locations through triangulation it is staggering to think of the number and diversity of ams studies on volcanic rocks that have been undertaken since that time in a bit more than three decades remanent magnetization characteristics have long been used to correlate volcanic rocks and such data together with rock magnetic properties have been employed to study crystallization and cooling processes e g schlinger et al 1988 wells hillhouse 1989 as techniques have been refined and the sensitivity precision reliability and or speed of measurements for an array of modern instrumentation all have improved workers have been able to study volcanic problems in more detail pulling out details of the processes that have not previously been considered in addition the types of problems addressed have diversified considerably we intend this special publication to provide a representative suite of contributions that help to summarize the current array of studies that apply palaeomagnetism and rock magnetism to volcanic processes and to stimulate further research in this exciting science this book is a result of two sessions on volcanic processes and palaeomagnetism held at the international union of geodesy and geophysics congress in melbourne australia in july 2011 and at the american geophysical union annual meeting in san francisco usa in december 2011 from ort m h porreca m geissman j w eds 2015 the use of palaeomagnetism and rock magnetism to understand volcanic processes geological society london special publications 396 1 11 first published online january 20 2015 http dx doi org 10 1144 sp396 17 2015 the geological society of london for permissions http www geolsoc org uk permissions publishing disclaimer www geolsoc org uk pub ethics angharad hills at the geological society of london graciously and tactfully guided this book and the editors through the entire process from chapter solicitation to publication thermoremanent magnetization in volcanic rocks and emplacement temperature

anatomy of an extinct magmatic system along a divergent plate boundary alftafjordur iceland recent rifting episodes highlight the role of magmatic systems with propagating dikes on crustal spreading however our knowledge of magmatic systems is usually limited to surface observations and geophysical data eastern iceland allows direct access to extinct and eroded deeper magmatic systems here we collected field structural and ams anisotropy of magnetic susceptibility data on 187 and 19 dikes respectively in the 10 12 ma old alftafjordur magmatic system at a paleodepth of 1 5 km the extension due to diking is at least 1 2 orders of magnitude larger than that induced by regional tectonics confirming magmatism as the key mechanism for crustal spreading this magma induced extension inferred from the aspect ratio of the magmatic system was of 8 mm yr lower than the present one ams data suggest that most of dikes have geometrically normal fabric at least at the margins consistent with prevalent subvertical magma flow and propagation

magnetic fabrics of dike swarms from se bahia state brazil their significance and implications for mesoproterozoic basic magmatism in the sao francisco craton abstract magnetic fabrics were determined by applying two techniques anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence aar ams was performed in 81 dikes from ilheus olivenca and itaju do colonia swarms located in se bahia the dikes intrude high grade metamorphic terrains as old as 3 2 ga dikes are vertical or subvertical in dip and trend mainly e w they are tholeiitic in composition and are classified into two groups characterized by high tio 2 2 and incompatible elements contents and low tio 2 and incompatible elements contents respectively thicknesses vary from a few centimeters to ca 20 m average of 3 m 40 ar 39 ar data indicate ages of 1 012 ilheus dikes and 1 078 ga olivenca dikes rock magnetism shows that ti poor titanomagnetites up to pure magnetites pseudo single domain multidomain grain sizes carry the magnetic fabrics three types of ams fabric are recognized i normal fabric 64 dikes defined by the clustering of k 1 k 2 axes on the dike plane whereas k 3 axes are nearly perpendicular to it this fabric is interpreted as due to magma flow ii intermediate fabric seven dikes characterized by k 1 k 3 axes clustering close to dike plane and k 2 axes perpendicular to the dike wall it was interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike direction and iii inverse fabric nine dikes defined by k 2 k 3 plane parallel to the dike plane and k 1 perpendicular to dike wall aar measurements were performed in three dikes two of them with inverse and one with normal ams fabrics aar was coaxial for normal ams fabric and it resulted in intermediate and normal for inverse ams fabrics a combination of ams and aar fabrics suggests that the magmatic fabric for these dikes that should be flow was overprinted by some local event probably related to brasiliano orogeny processes after dike emplacement inverse ams fabric would record this event normal fabric is the most important one found for the studied swarms the analysis of k 1 inclination permitted one to infer that the dikes from ilheus were fed dominantly by horizontal fluxes k 1 k 1 60 dikes from itaju do colonia region have the steepest k 1 suggesting that this region could be closer to a magma source on the other hand a magma source near to olivenca area could be also inferred since the majority of the dikes were fed by inclined flow the existence of more than one source is supported by geochemical geochronological and paleomagnetic data which have shown that contemporaneous dikes were fed by compositionally distinct magmas the ams data also suggest that the majority of dikes were fed from upward flow coming from west to east and only a few of them were fed from upward flow from east to west

palaeomagnetism and magnetic fabrics of the late palaeozoic volcanism in the castejon laspaules basin central pyrenees implications for palaeoflow directions and basin configuration the authors thank cristina garcia and teresa roman for her help during field and laboratory work and manuel tricas for the preparation of thin sections this work forms part of the research projects cgl2009 08969 and cgl2006 05817 spanish ministry of education and is included in the objectives of the pre doctoral grant ap 2009 0554 esther izquierdo programa de formacion de profesorado universitario spanish ministry of education funding from the instituto de estudios altoaragoneses is also acknowledged bou acknowledges the jaedoc postdoctoral programme financed with esf the authors acknowledge the careful and constructive revisions by b henry and an anonymous reviewer

turbulent transport and deposition of the ito pyroclastic flow determinations using anisotropy of magnetic susceptibility the ito pyroclastic flow erupted about 22 000 years ago from aira caldera in southern kyushu japan flow directions were determined by anisotropy of magnetic susceptibility ams which measured the preferential alignment of magnetite microphenocrysts usually 0 25 mm in diameter the microphenocrysts are aligned with the long axis of the clasts during strain and fracture in the vent the grains are then deposited parallel to the flow direction with an imbrication there is no evidence of rolling of clasts or nonflow parallel lineations and thus ams can be used to determine flow directions that occurred immediately before deposition beyond 30 km from the center of the caldera flow directions were predominantly down paleogradient indicating expanded flow and that the depositional system was gravity driven and largely decoupled from the transport system within 30 km measured flow directions are random indicating that sedimentation occurred from a depositional system that was closely coupled to the turbulent transport system individual flow directions at all sites except one varied more than the analytical error demonstrating that a variety of flow directions existed even within a small area of the flow this implies that the depositional system was turbulent

anisotropy of magnetic susceptibility investigations of the st malo dyke swarm brittany france emplacement mechanism of doleritic intrusions a high quality aeromagnetic survey of northern brittany line spacing 250 m flight elevation 150 m has been used to delineate the lower carboniferous st malo dyke swarm in detail the dyke swarm fans at its northern and southern ends and is affected by n60 sinistral transcurrent faults after restoration of these offsets the full structure trends in a n s direction small dykes are not imaged and only one third of the swarm is evidenced by magnetism gravity and magnetic modelling shows that the swarm overlies a single n s elongated magma chamber the distribution of kmax inclinations of anisotropy of magnetic susceptibility ams suggests that the dykes display a fanning magma flow in section computed kmax inclinations are usually shallower than the measured geological dips probably because the flow becomes more disturbed as the dyke becomes shallower we observe that the mean susceptibility values increase when the magma flow is steeper than about 70 a detailed cross section of st briac dyke which is part of the st malo dyke swarm reveals that the main carrier of magnetization is magnetite in the centre of the dyke and magnetite maghaemite on its rims the emplacement of the st malo dyke swarm could have been contemporaneous with the n60 shearing which displaced the dyke swarm by about 20 km the dyke swarm is cut at its southern end by an e w orientated fault which probably acted during late carboniferous times

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

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

interpretation of magnetic fabrics in the early proterozoic diabase dykes of keuruu central finland abstract magnetic fabrics petrophysics petrography and chemical compositions are described for 14 diabase dykes located in the keuruu area central finland these early proterozoic 1880 ma dykes sharply cut both low density granodiorites and high density diorites and gabbros the diabase dykes with a nw se strike and an almost vertical dip have a maximum length of a few kilometers and widths of about 1 m strongly developed magnetic fabrics of deformation type anisotropy degree up to 1 4 were observed in six of the dykes and weakly developed fabrics of flow type anisotropy degree below 1 1 in eight dykes no systematic differences in the dyke widths mean densities palaeomagnetic poles mineral compositions degrees of alteration or chemical characteristics were observed between the two types of dykes suggesting that the dykes had a common magma source all the fabrics of deformation type are dominantly ferrimagnetic with tightly aligned horizontal foliations and well grouped lineations which in most cases are parallel to the dyke trends it is suggested that the deformation fabrics were generated as tholeiitic magma was forced to intrude laterally along its level of neutral buoyancy lnb which may have been close to the present erosion level pel in the low density granodiorite environment ferri and paramagnetic fabrics of flow type characterize the dykes located in the denser mafic parts of the crust in these parts the lnb was possibly somewhat higher in the crust so that magma flow passed the pel and formed weakly developed flow fabrics with gently plunging lineations and vertical or variable magnetic foliations

palaeomagnetism of the 1 89 ga boonadgin dykes of the yilgarn craton possible connection with india abstract a palaeomagnetic study was carried out on the newly identified 1888 9 ma boonadgin dyke swarm of the yilgarn craton in western australia the bonnadgin dykes yield a mean direction of magnetisation of d 143 i 13 k 37 and 95 8 based on samples from 10 diabase dykes with a corresponding palaeopole at 47 s 235 e a95 6 a positive baked contact test establishes the primary nature of the magnetisation the ca 1 89 ga palaeopole suggests that the yilgarn craton was near the equator at this time and the boonadgin dyke swarm can be interpreted to represent an arm of a radiating dyke swarm that shared the same plume centre with coeval mafic dykes in the dharwar and bastar cratons of southern india we therefore propose that the west australian craton wac consisting of the yilgarn and pilbara cratons and the south indian block sib consisting of the dharwar bastar and singhbhum cratons were connected ca 1 89 ga globally available high quality palaeopoles of similar age allow the west australian craton to be placed northwest of proto laurentia during the assembly of the supercontinent nuna

the sonju lake layered intrusion northeast minnesota internal structure and emplacement history inferred from magnetic fabrics abstract the sonju lake intrusion sli in northeastern minnesota is a layered mafic complex of keweenawan age 1096 1 0 8 ma related to the midcontinent rift the cumulate paragenesis of the intrusion is recognized as broadly similar to the skaergaard intrusion a classic example of closed system differentiation of a tholeiitic mafic magma the sli represents nearly closed system differentiation through bottom up fractional crystallization geochemical studies have identified the presence of a stratabound 50 100 m thick zone anomalously enriched in au pge similar to the pge reefs of the skaergaard intrusion this pge enriched zone is hosted within oxide gabbro cumulates about two third of the way up from the base of the intrusion we present a petrofabric study using the anisotropy of magnetic susceptibility ams to investigate the emplacement and flow patterns within the sonju lake intrusion petrographic and electron microprobe studies combined with ams and hysteresis measurements indicate the primary source of the magnetic signal is pseudo single domain psd magnetite or titanomagnetite low field ams was measured at 32 sites within the sonju lake intrusion which provided information about primary igneous fabrics the magnetic fabrics in the layered series of the sonju lake intrusion are consistent with sub horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment well aligned magnetic lineations consistently plunging shallowly toward the southwest indicate the source of the magma is a vertical sill like feeder presumably located beneath the finland granite the finland granite acted as a density trap for the sonju lake magmas forcing lateral flow of magma to the northeast the strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush

local and regional controls on the lateral emplacement of the ben hiant dolerite intrusion ardnamurchan nw scotland abstract sub volcanic and related satellite intrusions record the delivery storage and accommodation of magma before eruption however when several volcanic centres are in close proximity the relationship between centre and satellites may be ambiguous here we examine the structure of the ben hiant dolerite satellite intrusion discuss its relationship with the ardnamurchan central complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre the mull central complex 35 km to the southeast structural field observations and anisotropy of magnetic susceptibility ams fabric analyses reveal that the ben hiant dolerite was emplaced as a series of lobes in a sequence of stacked sheets the ams fabric data further indicate that the ben hiant dolerite intruded laterally and may have been sourced from either the ardnamurchan central complex or the mull central complex testing either model and discriminating between these two possible source reservoirs remains an outstanding challenge however the potential implications of the lateral movement of magma over many 10 s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically

dike surface lineations as magma flow indicators within the sheeted dike complex of the troodos ophiolite cyprus mesoscopic flow lineations and anisotropy of magnetic susceptibility ams have been measured for dikes within the cretaceous age troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge and groove structures termed hot slickenlines found throughout the complex a unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back flow of magma during the waning stages of intrusion surface lineations generally lie subparallel to the direction of flow inferred from ams determinations on cores within 5 cm of dike margins surface lineations also lie subparallel to the long axis e1 of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from ams cores correlation of surface lineations with interior indicators of flow ams plagioclase trachytic texture indicate that the surface features are good proxies for grain scale magma flow directions during dike propagation in troodos dikes orientations of surface flow features in the dikes of the troodos ophiolite indicate an approximately equal mix of subhorizontal to near vertical magma flow contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest

magma flow directions of shallow dykes from the east greenland volcanic margin inferred from magnetic fabric studies abstract the role played by plume generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the east greenland volcanic margin we investigate the magmatic texture using anisotropy of magnetic susceptibility for eight dykes of tholeitic affinity belonging to a dyke swarm associated with the tertiary opening of the north atlantic ocean the thickness of the sampled dykes ranges from 3 to 37 m the dykes are of doleritic texture and contain up to 12 of opaque minerals and 35 of plagioclase laths dykes showing a magnetic foliation plane within the dyke plane i e a magnetic fabric usually attributed to magmatic processes represent 40 of the samples from the studied swarm these dykes have a low degree of anisotropy and their ellipsoid of magnetic susceptibility is strongly oblate inverse magnetic fabrics where the maximum principal susceptibility axis lies near the pole to the dyke account for 60 of the data we interpret these inverse fabrics either as alteration minerals with highly prolate ellipsoids of magnetic susceptibility or primary titanomagnetites with oblate ellipsoid of low anisotropy the flow direction is inferred for normal fabric dykes using the mirror imbrication of magnetic lineation analysis of thin sections shows a good agreement between magnetic fabric directions and phenocryst preferred orientations the inferred flow directions are predominantly horizontal throwing a new light on volcanic margin development

shallow intrusive directions of sheeted dikes in the troodos ophiolite anisotropy of magnetic susceptibility and structural data sheeted dikes play a central role in the formation of oceanic crust it is commonly assumed that sheeted dikes intrude vertically upward from elongated mid ocean ridge mor magma chambers but there are no direct observational data bearing on this hypothesis this assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes the authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust the troodos ophiolite structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow 20 degree to nearly vertical upward magma transport directions these data suggest that horizontal emplacement has to be considered for sheeted dikes at mors implying more centralized mor plumbing systems than previously thought such plumbing systems provide ample opportunity for complex mixing fractionation and contamination of mor lavas in magma chambers and tabular magma storage volumes whether the mor magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems

magnetic fabric and its significance in the florianopolis dyke swarm southern brazil summary magnetic fabric was determined by applying the anisotropy from the low field magnetic susceptibility ams technique in 62 mafic dykes from the mesozoic florianopolis santa catarina island dyke swarm southern brazil these dykes cut the crystalline basement rocks which are mainly proterozoic they are vertical or subvertical in dip and trend mainly ne although nw trending dykes are also found dykes are tholeiitic in composition and are geochemically similar to those from the ponta grossa swarm thicknesses vary from 0 3 to 60 m polished sections show that titanomagnetites carry the ams in these dykes hysteresis parameters show that the magnetic minerals fall in the psd range two types of magnetic fabric are recognized type i is characterized by k1 k2 parallel to the dyke wall representing magma flow within the dykes type ii with k1 k3 parallel to the dyke wall was found in four dykes type i is found in 94 per cent of the dykes and approximately 20 per cent of these have k1 inclinations of less than 30 suggesting horizontal or subhorizontal flow about 80 per cent have k1 inclinations of greater than 30 due to inclined to vertical flow the comparison of ams studies from both the florianopolis and the ponta grossa dykes suggests a source position closer to santa catarina island than the ponta grossa arch

the relation between magnetite and silicate fabric in granitoids of the adamello batholith abstract the link between the macroscopic silicate fabric and the magnetite controlled ams anisotropy of magnetic susceptibility fabric in ferromagnetic rocks was investigated through a comprehensive comparison between different fabric measurement techniques sample lithologies include tonalites and granodiorites from the lago della vacca complex adamello batholith italy the datasets used to assess the link between subfabrics and the coherence between methods include 1 macroscopic silicate fabric measured directly in the field 2 macroscopic silicate fabric derived from image analysis ia of outcrop pictures and sample pictures 3 shape preferred orientations spo of mafic silicates 4 spo of magnetite and 5 calculated distribution of magnetite grains from computer assisted high resolution x ray tomography x ray ct images 6 fabrics derived from the ams macroscopic mineral fabrics measured in the field agree with the ia results and with the spo of mafic silicates obtained from the x ray ct imaging the x ray ct results show that the spo of the magnetite grains are consistent with the ams data whereas the spatial distribution of the magnetite grains is less compatible with the ams fabric this implies that the ams signal is mainly controlled by the shape of the magnetic carrier mineral rather than by the spatial arrangement of the magnetite grains an exception is the presence of magnetite clusters furthermore the spo of mafic silicates and the spo of the magnetite grains are consistent with the ams data another finding of this study is that the magnetic susceptibility correlates linearly with the amount of magnetite in the samples the coherent results obtained from a variety of methods reinforce the application of both ams measurements and ia as robust tools to analyse fabrics in granitic intrusions

mechanical models for dikes a third school of thought abstract geological and geophysical data from continental volcanic centers and giant radial swarms and from oceanic shield volcanoes and rift zones indicate that dikes propagate laterally for distances that can be 10 to over 100 times their height what traps dikes within the shallow lithosphere and promotes these highly eccentric shapes gravity induced stress gradients in the surrounding rock and pressure gradients in the magma are the primary loading mechanisms pressure gradients due to magma flow are secondary to insignificant because the flow direction is dominantly horizontal this configuration of vertical blade shaped dikes with horizontal dike propagation and magma flow is fundamentally different from the two dike model configurations described in a recent review paper as two schools of thought for mechanical models of dikes in school i a dike is disconnected from its source and ascends under the influence of buoyancy in school ii a dike is connected to a magma reservoir and is driven upward by magma flux from the source we review the geological and geophysical data supporting the vertical dike horizontal flow propagation configuration and suggest the abundance and veracity of these data in many different geological settings and the modeling results that address this physical process warrant adding this as a third school of thought a new analytical solution for the boundary value problem of a homogeneous isotropic and linear elastic solid with a vertical fluid filled crack is used to investigate the effects of gravitationally induced stress and pressure gradients on the aperture distribution dike tip stress intensity and stable height model results indicate that in a homogeneous crust dikes can achieve stable heights greater than a kilometer only if the host rock fracture toughness k ic 100 mpa m 1 2 however density stratification of the crust is an effective mechanism for trapping kilometer scale dikes even if the host rock is very weak k ic 0 this analysis may explain why vertical dikes propagate laterally for great distances but reside within a narrow range of depths in the crust

variations in magnetic anisotropy and opaque mineralogy along a kilometer deep profile within a vertical dyke of the syenogranite porphyry at cinovec czech republic abstract vertical variations in magnetic fabric and paragenesis of the ferrimagnetic minerals for a virtually upright dyke of the altenberg syenogranite porphyry were investigated using profile data of borehole e 16 to a depth of 922 7 m eastern krusne hory mts nw bohemia it was revealed that this dyke likely consists of two magma pulses indicated both by magnetic fabric and by opaque mineralogy in most of the profile the magnetite grains are oriented parallel to the sub vertical dyke only at the base of the upper magma pulse in a depth interval of 200 400 m are these grains oriented sub horizontally being thus perpendicular to the dyke walls this pattern can be interpreted as a consequence of a static vertical compaction of the magma of the basal portions of the upper pulse due to the pressure of the ascending lower pulse the large planes of the ferrimagnetic minerals are oriented perpendicular to the shortening direction

anisotropy of magnetic susceptibility in the ponta grossa dyke swarm brazil and its relationship with magma flow direction abstract measurements of anisotropy of magnetic susceptibility ams in 95 mafic dykes mainly tholeiites 10 200 m in width from the mesozoic ponta grossa swarm southern brazil revealed two main types of magnetic fabric type i fabric plane k 1 k 2 parallel to the dyke plane represents magma flow within the dykes whereas type ii plane k 1 k 3 parallel to the dyke plane is compatible with a fabric pattern reflecting vertical compaction of the magma column fabric type i dominates 51 of the dykes within the swarm whereas type ii 38 of the dykes concentrates mainly in the western region where the dykes intrude sediments considering the dykes with type i fabrics it is concluded that 58 of the dykes were fed by horizontal or sub horizontal k 1 inclinations less than 30 magmatic flow and 42 were fed by inclined to vertical k 1 inclinations more than 30 magma flow the latter are more frequent in the southeastern part of the swarm suggesting a magma source close to this area although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found

unravelling textural heterogeneity in obsidian shear induced outgassing in the rocche rosse flow abstract obsidian flow emplacement is a complex and understudied aspect of silicic volcanism of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow using an array of methods we study the extreme textural heterogeneity of the rocche rosse obsidian flow in lipari a 2 km long 100 m thick 800 year old lava flow with respect to outgassing and emplacement mechanisms 2d and 3d vesicle analyses and density measurements are used to classify the lava into four textural types glassy obsidian 40 vesicles high aspect ratio shear banded lava 20 40 vesicles and low aspect ratio frothy obsidian with 30 60 vesicles textural heterogeneity is observed on all scales m to m and occurs as the result of strongly localised strain magnetic fabric described by oblate and prolate susceptibility ellipsoids records high and variable degrees of shearing throughout the flow total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary magmatic and secondary meteoric water glass water contents are between 0 08 0 25 wt water analysis also reveals an increase in water content from glassy obsidian bands towards frothy bands of 0 06 0 08 wt reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content we present an outgassing model that reconciles textural volatile and magnetic data to indicate that obsidian is generated from multiple shear induced outgassing cycles whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian which then re vesiculates to produce dry vesicular magma repetition of this cycle throughout magma ascent results in the low water contents of the rocche rosse lavas and the final stage in the degassing cycle determines final lava porosity heterogeneities in lava rheology vesicularity water content microlite content viscosity play a vital role in the structural evolution of an obsidian flow and overprint flow scale morphology post emplacement hydration also depends heavily on local strain whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water a process previously correlated to lava vesicularity alone

the magnetic fabric relationship between sedimentary and basement nappes in the high tatra mountains n slovakia abstract in the high tatra mountains n slovakia the magnetic fabrics are deformational in origin in sedimentary rocks of the kri na nappe in sedimentary rocks covering the underlying crystalline complex and in granitoids and metamorphic rocks of this complex the patterns of the principal susceptibilities are similar in all these units their most important feature is a girdle in magnetic foliation poles oriented nnw sse to n s in sedimentary formations the magnetic fabric pattern is compatible with that predicted by mathematical modelling to develop during a complex deformation comprising lateral shortening and simple shear which is characteristic of nappe deformation consequently the magnetic fabric in the high tatra mountains probably resulted from deformation associated with the formation and movement of the nappes during which not only the sedimentary strata but also the crystalline complex were thrust over the north european platform

magnetic and plagioclase linear fabric discrepancy in dykes a new way to define the flow vector using magnetic foliation in basaltic dykes the magnetic lineation k1 maximum magnetic susceptibility axis is generally taken to indicate the flow direction during solidification of the magma this assumption was tested in tertiary basaltic dykes from greenland displaying independent evidence of subhorizontal flow the digital processing of microphotographs from thin sections cut in k1 k2 planes yields the preferred linear orientation of plagioclase which apparently marks the magma flow lineation in up to 60 of cases the angular separation between k1 and the assumed flow direction is greater than 45 this suggests that the uncorroborated use of magnetic lineations in dykes is risky a simple geometrical method is proposed to infer the flow vector from ams in dykes based solely on magnetic foliations

shape fabrics of particles in low concentration suspensions 2d analogue experiments and application to tiling in magma abstract jeffery s equations ascribe a theoretical cyclic nature to the shape fabric of non interacting rigid particles immersed in a viscous fluid undergoing simple shear flow this theoretical behaviour is confirmed at low shear strains 5 first experiment 134 identical particles or 1 second experiment 178 particles with two size classes then become stable at 17 first experiment and at more than 50 second experiment of the population of particles due to the contribution of the tiled particles the shape fabric becomes asymmetrical in its orientation distribution with a maximum lying above the shear plane the latter result provides a new shear sense indicator in addition to the statistical determination of the tiled features the study also suggests that crystalline fabrics in magmas could be acquired at high melt fractions i e early in the crystallization history of the magma

paleomagnetic study of french guyana early jurassic dolerites hypothesis of a multistage magmatic event a detailed paleomagnetic and anisotropy of magnetic susceptibility ams study was carried out on 34 sites of early jurassic dolerite dykes from french guyana which formed during the initial opening of the central atlantic ocean four types of ams fabrics are recognized i normal fabric 21 dykes defined by clustering of k1 k2 axes on the dyke plane whereas the k3 axis is nearly perpendicular to it this fabric is interpreted as due to magma flow the subhorizontal inclination of the k1 axis permitted to suggest that the french guyana dykes could be fed by horizontal magma fluxes from a distant magma source ii reversal fabric 8 dykes is characterized by the k2 k3 plane close to the dyke plane and the k1 perpendicular to dyke orientation such fabric was attributed to the local shearing stress iii intermediate fabric 1 dyke is defined by k1 k3 axes close to the dyke plane and k2 axis is perpendicular to this plane it was interpreted as due to vertical compaction of a static magma column iiii other fabric 4 dykes does not show any preferential orientation scanning electronic microscope and susceptibility versus temperature experiments show that minerals of the titanomagnetite family are main magnetic remanence carriers two magnetic components were isolated ages of magnetic remanences are estimated at 198 3 2 0 ma to 192 3 1 5 ma their virtual geomagnetic poles are calculated pole a v 73 2 n p 15 3 e k 288 8 a95 3 4 n 8 and pole b v 81 6 n p 89 1 e k 69 8 a95 4 2 n 18 these two groups probably correspond to two distinct magmatic events which occurred in a short period this hypothesis is consistent with published 40 ar 39 ar radiometric ages though with mini plateau spectra these paleomagnetic results suggest the presence of magmatic pulses led to the construction of the central atlantic magmatic province in french guyana during the early jurassic fl 2000 elsevier science b v all rights reserved

magma flow inferred from anisotropy of magnetic susceptibility in the coastal parana etendeka igneous province evidence for rifting before flood volcanism the parana etendeka igneous province is one of the largest flood volcanic provinces in the world peak magmatic activity at 132 ma is believed to have occurred about 5 m y before the birth of south atlantic sea floor and development of rift basins along the brazilian coastal margin anisotropy of magnetic susceptibility ams measurements on 283 samples 28 flows and 3 sills from the etendeka igneous province of namibia and 180 samples 21 flows from the parana province in brazil reveal remarkably consistent fabric orientations with maximum susceptibility k1 axes subhorizontal and parallel to the rifted margin the ams results are most likely due to shape anisotropy reflecting magma flow directions suggesting that lava flows and intrusive conduits near the eventual rifted margin were controlled by structures having topographic expression in existence at the time of peak flood volcanism these results imply that rifting preceded flood volcanism at least in the portion of the magmatic province within 100 km of the nascent mid atlantic ridge

palaeomagnetic study of oligocene 24 30 ma lava flows from the kerguelen archipelago southern indian ocean directional analysis and magnetostratigraphy we report palaeomagnetic determinations in order to estimate the palaeosecular variation of the geomagnetic field for the upper oligocene period from a southern site we measured the anisotropy of magnetic susceptibility ams of basalts from five sections to check the reliability of applying tectonic correction and concluded that no stratigraphic corrections were needed a positive reversal test implies that secondary components were sufficiently removed from the characteristic remanent magnetization also the sampling was sufficiently random to accurately reflect upper oligocene palaeosecular variation calculations of the between flow angular standard deviation sb from the geographic pole show values between 21 2 and 21 8 this palaeosecular variation estimate agrees with model g for palaeosecular variation j geophys res 96 b3 1991 3923 which is axisymmetric and assumes an equatorial symmetry of the geomagnetic field the virtual geomagnetic poles vgps from the mont des ruches section seem to exhibit two polarity changes reverse normal r n first and in more detail a second normal reverse n r one finally assuming that no polarity chrons are missed by any of the sections we compare the magnetostratigraphy we defined to the global geomagnetic polarity scale of geophys j int 129 1 1997 176 using previously obtained radiometric dates to correlate the sections j petrol 39 4 1998 711 earth planet sci lett 174 2000 313 j petrol 43 7 2002 1341 the precise age constraints furnished by these correlations allow an estimation of eruption rates from 0 30 to at least 1 31 km my with wide variations from section to section and an increase of eruption rate for younger sections however due to the non uniqueness of the correlations the age limits of the sections and consequently the extrusion rate estimations can vary moreover assuming the mont des ruches section that has probably recorded all the magnetic polarity intervals the mont de la rabouillere and mont de la tourmente intermediate vgp directions may indicate unrecorded polarity chrons the eruption rates calculated can thus be overestimated 2002 elsevier science b v all rights reserved

magnetic fabric and palaeomagnetic analyses of the plio quaternary calc alkaline series of aegina island south aegean volcanic arc greece abstract the south aegean volcanic arc represents the magmatic expression of active subduction of the african plate beneath eurasia in the region of greece plio quaternary calc alkaline dacite and andesite lava flows exposed on the island of aegina have been sampled for palaeomagnetic analyses to determine whether the western arc area has experienced vertical axis rotations in addition these units allow an assessment of the use of magnetic fabric anisotropy of magnetic susceptibility ams data as indicators of lava flow directions in silicic extrusive rocks providing a complement to the existing literature on ams of basaltic flows the magnetic mineralogy of both dacites and andesites is dominated by fine grained magnetite ti poor titanomagnetite sampled flows are characterised by oblate magnetic fabrics at site level which are co axial to flow surfaces although most sites also contain specimens with prolate ams ellipsoids where magnetic lineations are developed they are systematically aligned either parallel or orthogonal to average exposure scale lava flow directions magnetic lineations in these rocks therefore may be interpreted in terms of flow directions only when the latter are known a priori observed variability in the directions of remanent magnetisation recorded by the aegina lava flows may be explained wholly in terms of geomagnetic variations several sites record spot readings of the geomagnetic field during polarity transitions scatter of directions at the remaining sites is consistent with that expected from secular variation post eruption tilting of magnetisation vectors is shown to be limited the in situ mean direction of magnetisation of the non transitional sites is indistinguishable from the geocentric dipolar field used as a reference direction for these young

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

inverse magnetic fabric in carbonate bearing rocks abstract the observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric pole of bedding or schistosity is referred to as an inverse magnetic fabric the investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates that c axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones a mineralogical model based on the property of single domain grains to have a zero susceptibility parallel to their long axis is proposed however more complex cases are also encountered

applicability of ams technique as a flow fabric indicator in dykes insight from nandurbar dhule deccan dyke swarm determination of the nature of magma flow vertical or lateral from rock fabrics in a dyke swarm holds significant importance in deciphering its emplacement mechanism depth of magma chamber and its association with mantle plumes nandurbar dhule dyke swarm from western india consists of 210 mappable tholeiitic large linear dykes intruding the country rocks of deccan basalt we discuss here the development of flow controlled rock fabrics in these dykes as a function of their thicknesses magnetic fabric obtained from anisotropy of magnetic susceptibility ams analysis of the samples collected from these dykes was used as a proxy for rock flow fabric samples from the central parts of the thicker dykes strike perpendicular thickness 10 m show random and inconsistent development of magnetic fabrics than their marginal counterparts petrography suggests the development of relatively clear shape preferred orientations of the elongated mineral grains only at the margin of a thick dyke thinner dykes strike perpendicular thickness 10 m show both consistent magnetic fabrics and clear shape preferred orientations we propose that the center of a thick dyke fails to provide due to high flow velocity slower cooling rate lack of friction induced shearing and draining back of magma a favorable condition where a flow fabric can efficiently develop and b magnetic fabric can effectively mimic the flow fabric thinner dykes and the margins of the thicker dykes are hence better candidates for observing flow fabrics represented by magnetic fabrics this work is also motivated by the reservations expressed by earlier researchers in using the ams technique for magma flow characterization especially for giant dyke swarms we have critically evaluated their reservations in light of our results and commented on the applicability of the ams technique on such studies

correlation of deposits and vent locations of the proximal campanian ignimbrite deposits campi flegrei italy based on natural remanent magnetization and anisotropy of magnetic susceptibility characteristics abstract correlation of the distal deposits of the campanian ignimbrite with their proximal equivalents in the campi flegrei caldera is complicated by a lack of medial exposures complex and limited proximal stratigraphic sections and large lateral facies changes paleomagnetic data from 10 sites in and near the campi flegrei yield natural remanent magnetizations nrm that are statistically indistinguishable from the distal campanian ignimbrite in addition their virtual geomagnetic pole vgp yields a possible correlation with lac du bouchet france secular variation data that indicate an age of approximately 32 850 14 c years the secular variation curve at this age was only briefly at this vgp and did not return to it for 10 000 years so the paleomagnetic correlation of proximal and distal deposits is unique and robust the date is consistent with 14 c dates from the campanian ignimbrite but younger than 39 ar 40 ar dates for the same rocks this suggests that a better correction factor for cosmic flux for this time period is needed to calibrate older 14 c dates anisotropy of magnetic susceptibility ams data show that the proximal deposits have an oblate disk shaped poorly lineated magnetic fabric the distal deposits are much better lineated the difference may be due to chaotic depositional currents in the proximal areas in which particles were not well aligned with greater distance of travel and loss of energy particles within the flow became aligned and developed stronger ams lineations early eruptions of the piperno tuff were from a central vent north of pozzuoli whereas later tuffs that underlie the breccia museo may have been emplaced by flows associated with ring vents located on the northern and southern caldera margins

rock texture and magnetic lineation in dykes a simple analytical model abstract we develop two simple models for simulating the combination of magnetic sub fabrics related to magma flow in dykes the basic assumptions are i the petrofabric is representative of the flow fabric and ii the petrofabric is composed of s c type structures related to flow the first model consists of summing the magnetic tensors of two identical sub fabrics differing solely by their relative rotation this model accounts for the possible change of the macroscopic magnetic lineation from a flow related fabric to a lineation situated at the geometric intersection between the two sub fabrics such a result is obtained in the case of oblate to highly oblate sub fabric ellipsoids the second model integrates the effect of very oblate grains of variable orientations into calculating the shape controlled magnetic tensor of each sub fabric and emphasizes the possible under estimation of fabric superposition due to microscopic disordering the magma fluxes along the east greenland volcanic margin are illustrated by the flow pattern within the major dyke swarm the magmatic flow vectors inferred from the imbrication of magnetic foliation at the dyke margins are primarily horizontal the classic use of magnetic lineation can lead to contradictory results giving flow vectors perpendicular to the flow directions the magnetic lineation is situated close to the zone axis of magnetic foliation planes over a wide range of scales throughout the dyke swarm suggesting that the contradiction may arise from the association of several textural domains at the sample scale forward modelling of macroscopic magnetic fabrics using the first model yields good agreement with the measured magnetic fabric of the east greenland dykes our results which are applicable to strained sedimentary rocks highlight the possible misuse of the magnetic lineation due to combination of magnetic textures the exchange between a microscopic lineation i e mineralogical lineation and a macroscopic lineation i e intersection lineation is particularly expected for dykes that generally bear oblate magnetic textures

flow direction and magnetic mineralogy of lava flows from the central parts of the peninsula of baja california mexico magnetic properties of two apparently unrelated lava suites one tholeiitic and the other alkalic coexisting in the central parts of the peninsula of baja california mexico were measured in this study macroscopic indicators and measurements of anisotropy of magnetic susceptibility ams were combined to infer the flow direction on those lavas these measurements were used to investigate the likelihood of the existence of an abnormally long tholeiitic lava flow in this part of the peninsula the obtained results indicate that the tholeiitic flows in the region constitute regular length flows and are likely to have been issued through vents located within the area of study additionally comparison of the magnetic signature of tholeiitic and alkalic lavas reveals a systematic difference in some of the magnetic parameters bulk susceptibility and curie temperatures in particular it is proposed that the narrow range of curie temperatures characteristic of each lava type can be used as a reliable proxy for the identification of each lava type in the region all of these findings show that the alkalic and tholeiitic lavas of the region are more closely related to each other than previously suspected

magnetic fabric and flow direction in basaltic pahoehoe lava of xitle volcano mexico abstract we sampled five basaltic lava flow units from xitle volcano mexico city to study the variation of anisotropy of magnetic susceptibility within their cooling boundaries we find that the mean maximum susceptibility parallels the geologically inferred flow direction in the units that were emplaced on a steeper slope whereas for those on a negligible slope the mean intermediate susceptibility points in the flow direction we propose however that the maximum susceptibility always points in the direction of local movement and that a change in slope produces a deviation of the local motion from that of the unit as a whole the axis of susceptibility closest to the geologically inferred flow direction usually plunges upflow in the basal part of the flow unit comprising an imbrication which clearly marks the flow azimuth of the lava thus the scenario of emplacement may influence the results in a predictable way we suggest that the degree of anisotropy could bear a direct relationship to either the viscosity of the lava the morphology of the flows or both based on a comparison with lavas from azufre argentina and ko olau o ahu volcanoes also we suggest that the shape of the susceptibility ellipsoid may be related to the degree of internal deformation of the lava flows we also compare the two methods currently available to calculate regions of confidence around the mean principal susceptibilities

magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy

magma flow inferred from ams fabrics in a layered mafic sill insizwa south africa the insizwa sill is a 25 km diameter 1000 m thick layered mafic intrusion part of the karoo igneous province in south africa the peridotitic and gabbronoritic rocks are undeformed and mineral fabrics demonstrably result from magma flow a horizontal centimeter scale model layering is visible in numerous outcrops plagioclase crystals are both tabular and elongated their preferred orientation parallel to the layering forms a foliation and a nw se lineation respectively interpreted as the magma flow plane and flow direction throughout the 78 stations of this study 699 specimens magnetic susceptibilities km range from 750 to 10 000 10 6 si the magnetic anisotropy pj ranges from 1 03 to 1 08 magnetic ellipsoids are both prolate and oblate average tjc0 anisotropy of magnetic susceptibility ams fabrics are dominated by multidomain to pseudo single domain magnetite high field magnetic experiments indicate that the paramagnetic contribution from the mafic silicates is less than 50 for low susceptibility rock types the anisotropy results from magnetite grain shape solely as shown by no significant increase in pj with increasing km the magnetic lineation 305j 0 5j is consistent throughout the sill at various scales and coincides with the mineral lineation in average in contrast the magnetic foliation 125j ne 10j is generally perpendicular to the mineral foliation and to the layering several explanations for this odd configuration are discussed the variations of magnetic parameters across the layering and field observations point to a multiple injection the magnetic lineation is consistent with the presence of a single feeder dike situated to the se of the sill d 2002 elsevier science b v all rights reserved

the internal structure of lava flows insights from ams measurements ii hawaiian pahoehoe toothpaste lava and a ā abstract we studied the anisotropy of magnetic susceptibility ams of 22 basaltic flow units including s type pahoehoe p type pahoehoe toothpaste lava and a emplaced over different slopes in two hawaiian islands systematic differences occur in several aspects of ams mean susceptibility degree of anisotropy magnetic fabric and orientation of the principal susceptibilities among the morphological types that can be related to different modes of lava emplacement ams also detects systematic changes in the rate of shear with position in a unit allowing us to infer local flow direction and some other aspects of the velocity field of each unit a flows are subject to stronger deformation than pahoehoe and also their internal parts behave more like a unit according to ams the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes probably resulting from endogenous growth

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

an alternative emplacement model for the classic ardnamurchan cone sheet swarm nw scotland involving lateral magma supply via regional dykes abstract sub volcanic intrusive networks of which cone sheets are recognised as a major constituent partially control volcano growth and eruption style the accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip parallel magma flow however the emplacement mechanism of cone sheets remains poorly understood the 58 ma cone sheet swarm of ardnamurchan nw scotland offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets through studying magma flow and their importance in volcanic edifice construction structural measurements and anisotropy of magnetic susceptibility ams analyses have constrained a lateral magma flow pattern consistently oriented nw se in the majority of the ardnamurchan cone sheets field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry our observations suggest cone sheet formation may be linked to laterally propagating nw se trending regional dykes sourced from laterally adjacent magmatic systems likely the palaeogene mull central complex which are deflected around the central complex by stress field interference implicitly edifice construction and potential eruption precursors observed at a volcano may instigate or result from magmatic activity in laterally adjacent volcanic systems

rock magnetic evidence of inflation of a flood basalt lava flow the anisotropy of magnetic susceptibility ams of lava flows is an innovative method which has been proved to be directly related to the shear history of lava one of the advantages of this method is that it can be used in the absence of other morphological features commonly employed to study the mechanism of emplacement of lava flows this feature of the ams method makes it very attractive to gain insight into the mechanism of emplacement of massive relatively featureless long lava flows such as those forming flood basalt provinces in this work we report the results of the measurement of ams as a function of vertical position within the birkett lava flow one of the columbia river basalt group flows the observed variation of ams allows us to identify at least 16 discrete events of lava injection and to estimate the thickness of individual injection events the ams estimated thickness of each injection event in the range of 0 5 4 0 m coincides with the range inferred for injected lava pulses in modern hawaiian lava flows thus the evidence provided by the ams method supports the notion that at least some flood basalt lava flows were emplaced by the same mechanism as many present day inflated pahoehoe flows regarding the orientation of the principal susceptibilities in the central part of the flow they define a preferred orientation along an e w trend whereas in the outer parts of the flow they have a nne ssw trend this difference in the orientation of the principal susceptibilities is interpreted as the result of a change of flow direction of the lava as emplacement progressed electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00445 002 0203 8

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

magnetic anisotropy produced by magma flow theoretical model and experimental data from ferrar dolerite sills antarctica summary volcanic rocks forming sills dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement we model a sill as a steadystate flow of a bingham fluid driven by a pressure gradient in a horizontal conduit the magma velocity as a function of depth is calculated from the motion and constitutive equations vorticity and strain rate are determined for a reference system moving with the fluid the angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations it is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction and that the magnetic foliation plane changes periodically from horizontal to vertical the results are compared with the magnetic fabric of ferrar dolerite sills victoria land east antarctica derived from low field susceptibility measurements

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

factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments abstract the effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility ams of igneous rocks despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks on this work a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non interactive fractions in a rock results of the model show that a variable proportion of magnetically interactive grains the relative values of the bulk susceptibility degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock ams experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported and supports the model predictions both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains the contribution to the ams of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed

paleomagnetism and magnetic fabric of middle jurassic dykes from western patagonia argentina abstract jurassic volcanism in patagonia is widespread although associated dyke swarms are conspicuous they have been almost neglected in previous geologic studies of this region radiometric paleomagnetic and magnetic anisotropy studies are reported from a middle jurassic basic to intermediate dyke swarm exposed in the sierra de mamil choique western patagonia in an area of over 350 km2 two whole rock k ar determinations indicate that these dykes were intruded at around 170 ma an anisotropy of magnetic susceptibility ams study on 15 dykes 74 block samples shows that they carry different kinds of magnetic fabric both normal and inverse apparently governed by compositional differences a paleomagnetic study of these samples suggests that ti poor titanomagnetite is the probable carrier of the characteristic remanence mean site characteristic directions pass a reversal test grade c a paleomagnetic pole computed by averaging vgps from each individual dyke is situated at 70 2 s 190 4 e n 13 95 9 7 not substantially apart from other middle jurassic poles from south america further refinement of the jurassic south american apparent polar wander path is needed to establish whether or not the mamil choique dykes were affected by a small tectonic rotation

applicability of magnetic fabrics in rocks associated with the emplacement of salt structures the bicorb quesa and navarres salt walls prebetics se spain abstract this work assesses the applicability of the analysis of the anisotropy of magnetic susceptibility ams to rocks associated with salt structures magnetic fabrics from 16 sites sampled on upper triassic clays keuper facies interbedded within the salt layers of the bicorb quesa and navarres salt walls prebetic zone se spain are mostly characterized by oblate magnetic ellipsoids with their magnetic foliation parallel to the bedding and magnetic lineation contained in the bedding plane but without a constant relationship with dip direction to evaluate the quality and significance of the low field ams several experiments were carried out to characterize the magnetic mineralogy of representative samples and enable separation of the contributions from paramagnetic and ferromagnetic s l minerals the comparison between the orientation of the observed magnetic lineation and that of the structural elements characterizing the internal geometry of the studied salt structures antiforms minor folds faults and shear zones and flanks indicates that the magnetic fabric is controlled by deformation processes affecting the salt walls during their growth and or evolution samples were also analyzed from sixteen sites from miocene rocks outcropping in several syn diapiric miocene half grabens related to the salt walls studied their magnetic fabric is interpreted to reflect the dominant stretching direction at the syn diapiric half grabens during and or shortly after deposition our results indicate that caution is required in interpreting magnetic lineations related to salt flow mechanics from rocks associated with salt structures

conflicting structural and geochronological data from the ibituruna quartz syenite se brazil effect of protracted hot orogeny and slow cooling rate the ibituruna quartz syenite was emplaced as a sill in the ribeira aracuai neoproterozoic belt southeastern brazil during the last stages of the gondwana supercontinent amalgamation we have measured the anisotropy of magnetic susceptibility ams in samples from the ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric a large magnetic anisotropy dominantly due to magnetite and a consistent magnetic fabric have been determined over the entire ibituruna massif the magmatic foliation and lineation are strikingly parallel to the solid state mylonitic foliation and lineation measured in the country rock altogether these observations suggest that the ibituruna sill was emplaced during the high temperature similar to 750 degrees c regional deformation and was deformed before full solidification coherently with its country rock unexpectedly geochronological data suggest a rather different conclusion la icp ms and shrimp ages of zircons from the ibituruna quartz syenite are in the range 530 535 ma and la icp ms ages of zircons and monazites from synkinematic leucocratic veins in the country rocks suggest a crystallization at similar to 570 580 ma i e an ht deformation 35my older than the emplacement of the ibituruna quartz syenite conclusions from the structural and the geochronological studies are therefore conflicting a possible explanation arises from ar 40 ar 39 thermochronology we have dated amphiboles from the quartz syenite and amphiboles and biotites from the country rock together with the ages of monazites and zircons in the country rock ar 40 ar 39 mineral ages suggest a very low cooling rate 3 degrees c my between 570 and similar to 500 ma and similar to 5 degrees c my between 500 and 460 ma assuming a protracted regional deformation consistent over tens of my under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time this may be a characteristic of slow cooling hot orogens that rocks deformed at significantly different periods during the orogeny but under roughly unchanged temperature conditions may display almost indiscernible microstructure and fabric

magnetic fabric and inferred flow direction of dikes conesheets and sill swarms isle of skye scotland abstract we have measured the ams of 16 dikes 35 conesheets and three sills associated with the cuillin hills magmatic center of the isle of skye in the inner hebrides of scotland and of nine dikes located in the regional dike swarm of skye sixty three intrusives totalling 734 samples were studied to determine the plumbing of the skye volcanic system low field susceptibility versus temperature k t identified three different mineral phases in the area namely ti rich magnetite pyrrhotite and titanomaghemite the petrofabrics of the 63 intrusives yielded coherent flow azimuths regardless of their time of emplacement three main types of magnetic fabric a c were found fabric type a plane kmax kint parallel to the dike plane represents the magma flow direction within the intrusives and is the dominant fabric 55 of all the intrusives within the cuillin hills magmatic center and its regional dike swarm the kmax inclinations show that 55 of the intrusives were fed by horizontal to sub horizontal ams inclination of the kmax axis is equal or less than 30 magma fluxes and the rest of them were fed by inclined to vertical fluxes horizontal magma flow means lateral magma injection inside fractures and becomes more probable as the source is located further away

mantle potential temperature estimates of basalt from the surface of venus abstract the crater density and distribution of venus indicates the average surface age is younger 1 ga than most terrestrial planets and satellites in the solar system the type and rate i e equilibrium catastrophic or differential of volcanism associated with the stagnant lid tectonic system of venus is a first order problem that has yet to be resolved but is directly related to the thermal conditions of the mantle the calculated primary melt composition of basalt at the venera 14 landing site is high mg basalt to picrite with a mantle potential temperature estimate similar to terrestrial ambient mantle 1370 70 c the calculated accumulated fractional melting curves indicate the olivine compositions from the melt have mg of 89 91 the results show that the thermal regime required to generate the primary melt composition of the venera 14 basalt was not anomalously high i e mantle plume system but rather consistent with a lithospheric tensional rift system the juxtaposition of high thermal regime structures e g beta regio and ambient mantle potential temperature estimates of the venera 14 basalt suggests that the relatively young surface of venus is the result of volcanism from a combination of thermal systems that resurfaced the planet at variable rates

magnetic fabric in the umvimeela dyke satellite of the great dyke zimbabwe abstract anisotropy of magnetic susceptibility ams has been determined for 22 locations from the 500 km long gabbroic umvimeela dyke of zimbabwe ams is controlled by the presence of multi domain magnetite grains which appear to have preserved a variety of magnetic fabrics related to magma movement at the time of dike emplacement three locations along the umvimeela dyke have been identified as feeder points at which magma flowed into the dike from below lateral flow away from these points supplied the rest of the dike system the presence of several foci of igneous activity along the umvimeela dyke parallels the development of the neighbouring contemporaneous great dyke layered complex which consists of a line of contiguous subchambers individual feeder points to the umvimeela dyke may be directly related to activity in adjacent subchambers

ropy flow structures a neglected indicator of magma flow direction in sills and dikes ropy flow structures that form in dikes and sills are very similar to ropy lava structures pahoehoe but are of intrusive origin they are unusual among magma flow indicators in that they provide a frozen record of the magma flow our observations suggest that they occur in large vesicles which form as a result of local repeated pressure drops during propagation and emplacement of the magma adiabatic gas expansion causes a ductile rim to form around a vesicle this rim is then sheared by the underlying flow anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures but the results from deeper within the body both locally and regionally yield different yet consistent flow orientations this discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes

interpreting anomalous magnetic fabrics in ophiolite dikes abstract anisotropy of magnetic susceptibility ams may reveal mineral orientation distributions defining magmatic flow axes in igneous dikes the mafic silicates are the best indication of magmatic flow but fe ti accessories may contribute more to the bulk susceptibility if the orientation distributions of the two subfabrics are incongruent anomalous fabrics will occur that do not reflect magma flow axes for ophiolite dikes ocean floor metamorphism changes the mineralogy producing new fe oxides by retrogression and exsolution from mafic silicates and by the oxidation of primary oxides incompatibly oriented ferro magnetic subfabrics may be isolated by anisotropy of anhysteretic remanence aarm anomalous ams fabrics in ophiolites elsewhere have been attributed to inverse fabric contributions from single domain magnetite in varying combinations however in ophiolite dikes from the troodos ophiolite of cyprus anomalous fabrics arise from ocean floor metamorphism extensively or completely replacing the original magnetite and titanomagnetite accessory phases with titanomagnetite fe 2 4 ti 0 6 o 4 tm60 and its oxidised versions titanomaghemite to varying degrees according to depth beneath the ocean floor distance from spreading axis and proximity to transform faults at best the oxide orientation distribution defined by aarm could only be indirectly related to magma flow if its nucleation orientation controlled by a host lattice however more commonly the topotactic lattice reorganization produces weaker arm fabric anisotropies although recrystallized oxidised tm60 dominates the bulk low field susceptibility its anisotropy is generally too feeble to compete with the flow fabric defined by the ams contribution from paramagnetic mafic silicates

the problem of the lower boundary of the pleistocene in eastern lithuania abstract a sequence of lacustrine and alluvial sediments represented by stratified sand silt and rarely clay over 20 m thick is widespread in the anyksciai and vilnius districts in eastern and south eastern lithuania respectively these sediments are exposed only in eastern lithuania in the sventoji river valley and its tributaries here they overlie devonian bedrock and are overlain by the pleistocene glacial deposits the results of different geological investigations palaeobotanical mineralogical geochemical etc indicate that these sediments probably formed in the basins and rivers that date from the neogene to the early pleistocene stratigraphical subdivision and correlation of sediments from the transition zone between the neogene and quaternary are problematic for several reasons this article presents new results of palaeomagnetic investigations of these sediments from five outcrops accompanied by lithological studies the sediments were laid down during a relatively long and variable period of palaeomagnetic reversals comparable to the matuyama and gauss polarity chrons the boundary between the brunhes and matuyama polarity chrons has been traced in the upper part of the sections the stratigraphical subdivision and correlation of the entire sequence as well as the identification of the neogene quaternary boundary in these sediments remain undetermined

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

ams measurements on obsidian from the inyo domes ca a comparison of magnetic and mineral preferred orientation fabrics abstract measurements of the anisotropy of magnetic susceptibility ams on lavas can yield information concerning flow directions deformation regimes and relative shear rates here we report results of measurements made on samples of rhyolitic obsidian collected from obsidian dome ca comparisons of ams measurements with previously determined 3d microlite orientation distributions in these samples serve as a calibration of the ams technique despite the low values of bulk susceptibility 1 06 7 04 10 3 si found in the dome relative to typical values in basalts 10 2 si the degree of anisotropy is extremely high in all specimens 3 4 10 7 furthermore susceptibility tensors exhibit a wide range of shapes varying from prolate to triaxial and flattened ellipsoids b parameter from 8 6 to 6 4 corresponding to various degrees of microlite dispersion our results also indicate that despite the tight clustering of principal susceptibility axes obtained on each site bulk flow directions inferred from the orientations of k max axes may be misleading because the orientation of the local extension changes from place to place within the dome however it is shown that the ams of each specimen provides a good measure of the direction of local extension consequently it is possible to interpret spatial variations in flow regime from systematic changes in the orientation of principal susceptibility axes as the proposed association between the local deformation regime and ams is strongly dependent on the aspect ratio of the crystals embedded in the flowing lava some caution must be exerted when interpreting ams results from other flows and a direct comparison between different flows may not be granted in many occasions

ams detection of inverse fabrics without aarm in ophiolite dikes inverse fabric contributions from single domain sd magnetite complicate interpretation of magmatic flow from anisotropy of magnetic susceptibility ams in the s troodos ophiolite dike swarm flow directions and magma chamber dimensions may only be inferred correctly if magnetic fabrics correspond to the orientation distribution od of minerals magnetite shape fabrics were isolated by anisotropy of anhysteretic remanence aarm but ams ambiguously merges magnetite and silicate fabrics however with the retrospective benefit of aarm in this area we recommend avoiding sd complications by excluding magnetite rich samples for which the bulk low field susceptibility 27 500 si magma flow was steeply upwards to the sw proximal to an axial magma chamber

crustal structure of the lomonosov ridge and the fram and makarov basins near the north pole reversed refraction surveys were conducted along and across the lomonosov ridge as part of the 1979 lomonosov ridge experiment interpretation of the strike profiles indicates a 5 km thick upper crustal layer with a velocity of 4 7 km s overlying a 15 to 20 km thick layer of 6 6 km s material an upper mantle velocity of 8 3 km s is indicated by a few pn arrivals high amplitude reflection events recorded from this boundary can be successfully modeled by a transition zone of rapidly changing velocity over a depth interval of 5 km ray trace modeling of the dip profiles suggests a root structure extending to about 28 km depth flanked by crust thinning to a depth near 13 km beneath the makarov basin and a more gradual thinning to near 16 km beneath the fram basin the similarity between the crust of the lomonosov ridge and that beneath the barents and kara seas supports the suggestion that the ridge is a slice rifted from the baltic shelf

anisotropy of magnetic susceptibility in welded tuffs application to a welded tuff dyke in the tertiary trans pecos texas volcanic province usa consideration of published anisotropy of magnetic susceptibility ams studies on welded ignimbrites suggests that ams fabrics are controlled by groundmass microlites distributed within the existing tuff fabric the sum result of directional fabrics imposed by primary flow lineation welding and if relevant rheomorphism ams is a more sensitive indicator of fabric elements within welded tuffs than conventional methods and usually yields primary flow azimuth estimates detailed study of a single densely welded tuff sample demonstrates that the overall ams fabric is insensitive to the relative abundances of fiamme matrix and lithics within individual drilled cores ams determinations on a welded tuff dyke occurring in a choked vent in the trans pecos texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width the dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit followed by residual squeezing between the failed block and in situ wallrock irrespective of the precise mechanism widespread occurrence of both welded tuff dykes and point welded aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process

orientation distribution of microlites in obsidian abstract the shape and three dimensional orientation distribution of microlites are measured in obsidian from little glass mountain ca measurements are made from thin sections using an image series of high magnification digital micrographs taken serially through different focal depths these measurements agree well with the theoretically predicted and experimentally measured distribution of long slender rods in a newtonian fluid undergoing simple shear flow in this type of flow rods in a dilute suspension rotate periodically spending most of the time aligned with the flow measurements of the detailed orientation distribution integrated with theoretical models provide a tool for inferring flow dynamics and the timing of magmatic processes

evidence of sheared sills related to flank destabilization in a basaltic volcano piton des neiges basaltic volcano la reunion has been deeply dissected by erosion exposing large volumes of debris avalanche deposits to shed light on the factors that led to volcano flank destabilizations we studied the structure the crystallographic and magnetic fabrics of the substratum of a debris avalanche unit this substratum is a complex of n50 seaward dipping sills that has been exposed by the avalanche structural observations show that the sill plane in contact with the avalanche is one of the latest intrusions in the sill complex in this uppermost sill the anisotropy of magnetic susceptibility ams is correlated to the crystallographic preferred orientation of magmatic silicate minerals allowing us to use ams as a proxy to infer the magmatic flow the ams fabric across the intrusion is strongly asymmetric which reveals that the contact sill was emplaced with a normal shear displacement of its hanging wall the shear displacement and the magma flow in the intrusion are both directed toward the nne i e toward the sea which is also the direction of the slope and of the debris avalanche runout because all the sills in the intrusion complex have a similar dip and dip direction it is likely that several of them also underwent a cointrusive slip toward the nne we conclude that this cointrusive normal slip repeated over many intrusions of the sill complex increased the flank instability of the volcano this incre mental instability may have ended up into the observed debris avalanche deposit at piton de la fournaise the active volcano of la reunion sill intrusion and cointrusive flank displacement have been inferred from geophys ical studies for the april 2007 eruption by providing direct evidence of sheared sills our study substantiates the idea that repeated sill intrusions may eventually trigger flank destabilizations in basaltic volcanoes

rotation of uniaxial ellipsoidal particles during simple shear revisited the influence of elongation ratio initial distribution of a multiparticle system and amount of shear in the acquisition of a stable orientation abstract the rotation of uniaxial ellipsoidal particles has provided the base for many studies of shape preferred orientations of mineral grains in a large variety of rock types in this paper we present reports of a study of the evolution of a multiparticle system as a function of the elongation ratio of the particles and of the initial particle distribution our model reveals that both factors are important in controlling the acquisition of a stable orientation upon deformation the more elongated particles r short axis long axis 0 5 will define a stable fabric in almost any situation of geologic interest shear deformation 0 5 can also be considered to have achieved a stable orientation depending on the total amount of shear experienced by the rock however in systems with the less elongated particles r 0 8 it is possible to find mean orientations that are perpendicular to the direction of shear although the occurrence of such behavior constitutes only a minor fraction of the studied examples in any case it is suggested that a systematic study of mineral fabrics in samples from the same rock can yield enough information to evaluate whether the mean shape preferred orientation of the minerals is parallel to flow direction

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

significance of anisotropy of magnetic susceptibility fabrics in proterozoic mafic dykes hopedale block labrador abstract anisotropy of magnetic susceptibility fabrics have been studied in a suite of mafic early proterozoic kikkertavak dykes and two mid proterozoic kokkorvik sills in the hopedale block of labrador two sets of en echelon dyke arrays from the kikkertavak swarm are considered to represent two linear dyke intrusions at depth three main fabrics were revealed in the kikkertavak dykes in the first the maximum susceptibility axis is sub horizontal or gently inclined and parallel to the dyke walls and is interpreted as the primary magma flow direction a second fabric affects only dykes along strike from a much younger dyke and is interpreted as the result of local stress anisotropy the third fabric with significantly lower susceptibility is only locally developed in the southeast near the mid proterozoic kanairiktok shear zone and is also interpreted as the result of local superimposed tectonic stress the kokkorvik sheets preserve a single flow induced fabric sub parallel to the measured crack propagation lineation indicating that the magma flow was parallel to the crack propagation direction the gently inclined magma flow direction inferred for the kikkertavak dykes is believed to reflect later stage magmatic emplacement when consolidation had already commenced at this stage the buoyancy contrast between magma and host rock had been reduced to the point when the initially vertical magma flow was no longer sustainable and was replaced by lateral flow

egan range volcanic complex nevada geochronology paleomagnetism and magnetic fabrics abstract in a relatively brief interval near the eocene oligocene boundary trachydacite lavas were erupted over an area of about 300 km2 to form the egan range volcanic complex in the southern butte mountains and adjacent egan range of east central nevada the mean radiometric age for this volcanic episode is 34 1 0 2 ma based on whole rock and biotite k ar ages detailed comparisons were made between anisotropy of magnetic susceptibility ams and the macroscopic petrofabric at 31 sites great variation in flow structures is attributed to complex flow in viscous lava over a surface of considerable relief the magnetic fabric as expressed by ams axes agrees well with the flow petrofabric typically flow lineations are parallel to k1 and flow foliation is perpendicular to k3 as expected extension cracks and joints are normal to k1 late stage deformation of pipe vesicles also impressed a measurable magnetic fabric because of significant initial dip and complexity of flow fabric paleohorizontal is not generally demonstrable paleomagnetic studies indicate an exclusively normal polarity the mean in situ characteristic direction is relatively well grouped d 350 i 52 95 8 for 28 selected sites and provides a mean pole at 124 e 81 n which is not significantly different from the coeval cratonic reference pole these results indicate that large oligocene or younger rotations have not affected this sector of the basin and range province the combination of age and magnetic polarity suggests a correlation with a normal polarity subchron of chron 13 or 15

preferred orientation developed by rigid markers in two dimensional simple shear strain a theoretical and experimental study abstract in a homogeneous two dimensional deformation linear markers embedded in a viscous matrix develop a preferred orientation along the principal axis of the strain ellipse theoretical analysis of marker rotation under two dimensional simple shear is used to derive an orientation distribution function this function shows that the relative density maximum increases with progressive strain its value being equal to the major minor strain ellipse axis ratio moreover the density is equal to the unity along the lines of no finite longitudinal strain experimentally observed preferred orientation is in good agreement with the expected theoretical distribution

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

the anisotropy of magnetic susceptibility of lava flows an experimental approach measurements of the anisotropy of magnetic susceptibility ams of natural lavas have shown that ams varies with depth within a lava flow we have investigated the reasons for such variation by studying the effects of temperature and strain rate on the ams of recent lava in the laboratory samples of lava from kilauea were melted and subjected to a range of strain rate and cooling histories the results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample high degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation additionally lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it these results lead to the conclusion that only the last phase of deformation is detectable using ams and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time the relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time consequently flow directions can be inferred confidently with this type of measurements

post emplacement tilting of lava flows inferred from magnetic fabric study the example of oligocene lavas in the jeanne d arc peninsula kerguelen islands abstract statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results a difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement for 2 successive series of flows on the jeanne d arc peninsula presently nnw dipping this method shows lava emplacement along a ssw nne direction this indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago additionally the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow