magnetic fabrics of the miocene ignimbrites from west cameroon implications for pyroclastic flow source and sedimentation abstract the miocene ignimbrites of mounts bambouto and bamenda located in the central part of cameroon volcanic line are generally made of welded and non welded massive lapilli tuff and lithic breccias these discontinuous deposits cover a total area of 180 km 2 with thickness ranging from 25 to 200 m the different facies contain several lithic fragments of mainly trachytic nature the devitrified matrix of the welded ignimbrites is constituted by sanidine anorthoclase quartz plagioclase clinopyroxene biotite fe ti oxides and devitrified fiammes anisotropy of magnetic susceptibility ams is used to characterize magnetic fabrics and to provide an estimate of flow direction of each ignimbrite sheet magnetic mineralogy results from different flow units show that titanomagnetite titanohematite maghemite and goethite with grain size ranging from coarse md to very fine sp are the main magnetic carriers of these ignimbrites inferred transport directions based on the ams data and field indicators show that bambouto caldera is the source of main pyroclastic deposits of mount bambouto in southwestern mount bamenda santa mbu caldera or bambouto caldera constituted the probable emission center of mbengwi bamenda and mbu ignimbrite sheets whereas magnetic fabrics of bambili sabga and big babanki ignimbrites demonstrate that these deposits were emitted from a northeastern source namely oku vent in mount oku a small number of subvertical ams fabrics correspond to rocks possibly modified by an elutriation process

laccolithic as opposed to cauldron subsidence emplacement of the eastern mourne pluton n ireland evidence from anisotropy of magnetic susceptibility the structural evolution and emplacement of the eastern mourne pluton was investigated using anisotropy of magnetic susceptibility ams measurements carried out on 112 oriented block samples and structural data from the host rocks from these new data cauldron subsidence as the emplacement mechanism is disputed and evidence for an alternative laccolithic style model involving inflation is presented this includes deflection and uplift of host rock bedding close to contacts and the magnetic fabric pattern which has a gentle dome geometry even close to contacts the magnetic lineations usually plunge down dip near the external margins but otherwise have a general ssw nne trend that diverges northward this suggests a northward directed inflow direction the model for the emplacement of the eastern mourne pluton is a laterally fed laccolith emplaced south to north the eastern margin is interpreted as a faulted contact facilitating the inflation of an asymmetrical breached laccolith

shoreline response to submerged structures a review abstract submerged coastal structures are widely perceived to be capable of providing beach protection without the adverse impacts including loss of beach amenity and aesthetic considerations often associated with more conventional structures such as revetments and groynes in addition there is growing interest in the concept that the layout and cross section of submerged coastal protection structures can be optimised to also enhance local surfing conditions however as submerged structures have only rarely been adopted for beach protection the shoreline response to these structures is not well understood at present therefore this review of the available published literature was undertaken with the aim of investigating the environmental and structural parameters governing shoreline response to submerged structures gleaned from the results of field laboratory and numerical studies undertaken to date the review reveals that contrary to expectations a majority of the submerged structures constructed to date have resulted in shoreline erosion in their lee furthermore the key environmental and structural parameters governing the mode i e erosion or accretion and the magnitude i e size of salient of shoreline response to submerged structures are yet to be identified although submerged coastal structures offer the potential for low aesthetic impact incorporating multi function design until the response of the adjacent shoreline to submerged structures is better resolved their use is likely to remain relatively limited

magnetic fabrics and rock magnetism of proterozoic dike swarm from the southern sao francisco craton minas gerais state brazil magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a proterozoic dike swarm from the southern sao francisco craton sfc minas gerais state se brazil magnetic anisotropies were determined by applying anisotropy of low field magnetic susceptibility ams and anisotropy of remanent magnetization arm the latter was performed imposing both anhysteretic total aar and partial paar and isothermal remanence magnetizations airm partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site in most sites ams is dominantly carried by ferromagnetic minerals however in some sites the paramagnetic contribution exceeds 70 of bulk susceptibility rock magnetism and thin section analysis allow classifying the dikes as non hydrothermalized and hydrothermalized magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5 10 3 si ti poor titanomagnetites up to pure magnetite pseudo single domain psd grain sizes carry the majority of magnetic fabrics for non hydrothermalized dikes whereas coarse to fine grained ti poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes three primary ams fabrics are recognized which are coaxial with arm fabric except for two dikes from both non hydrothermalized and hydrothermalized dikes normal ams fabric surprisingly is not dominant 31 the parallelism between ams paar0 30 paar30 60 and paar60 90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse grained early crystallized ti poor titanomagnetites this fabric is interpreted as magma flow in which the analysis of kmax inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes kmax 30 intermediate ams fabric is the most important 41 in the investigated swarm it is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike arm determinations for these sites also remained intermediate except for two dikes in one of them airm fabric resulted in normal ams fabric while for the other aar fabric resulted in inverse ams fabric a combination of ams and arm fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event probably related to brasiliano orogenic processes after dike emplacement inverseinverse ams fabric is a minority four dikes arm determinations also remained inverse suggesting a primary origin for inverse ams fabric

extreme effusive eruptions palaeoflow data on an extensive felsic lava in the mesoproterozoic gawler range volcanics the gawler range volcanics are the eroded remnants of a mesoproterozoic dominantly felsic large igneous province the eucarro rhyolite is one of three voluminous 675 km3 units in the upper part of the succession eight stratigraphic sections through the eucarro rhyolite were sampled for anisotropy of magnetic susceptibility ams and petrofabric analysis in order to determine the source location the ams foliations kmax kint plane consistently strike northwest and most dip steeply at sites in the western and central parts of the unit there is a change from steep southwesterly dips near the base to vertical or northeasterly dips in the middle and near the top in the same area kmax axes plunge to the south or southwest in the lower parts and to the north in upper parts of the unit at sites in the eastern part of the unit the pattern is reversed ams foliations dip northerly near the base and southerly near the top and kmax axes plunge northerly near the base and southerly near the top such up section reversals in dip or plunge directions of ams parameters resemble the patterns shown by other lavas a dominance of very steep dips and an up section reversal in dip direction are also expected for flow bands in felsic lavas furthermore the ams pattern implies a northeasterly flow direction for the western and central parts of the unit and a southerly flow direction for the eastern part the opposing flow directions may indicate that the western and central parts of the unit versus the eastern part belong to different flow lobes or that they came from separate sources lineations defined by preferred alignment of elongate phenocryst intersections on gently dipping surfaces show a less consistent pattern and alone could not be used to define palaeoflow directions this study confirms the existence of felsic lavas that have dimensions comparable with the largest known mafic lavas

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

mineralogical control on the magnetic anisotropy of lavas and ignimbrites a case study in the caviahue copahue field argentina

lithofacies association and stratigraphy of the paredao volcano trindade island brazil abstract a stratigraphic framework can be constructed through lithofacies description and interpretation the application of this method in volcanic systems is a fundamental tool for understanding their evolution and eruptive dynamics and for establishing hazard assessments in active volcanic areas this work presents and discusses a detailed stratigraphy of the paredao volcano allowing the understanding of the lateral and vertical distribution of volcanic facies and the 3d geometry of the deposits generating a model for the volcanism and discussing the erupting mechanisms the paredao volcano is a scoria cone located in the southeast of trindade island and represents the youngest subaerial volcanic episode pleistocene recorded in brazil its stratigraphic organization indicates nephelinitic flows alternate with lapillistones and lapilli tuffs of same composition at the base and a pyroclastic cone with nearly 200 m high above the sea level through lithofacies description and association 4 stratigraphic columns were constructed in scale 1 50 three of them at the tartarugas beach trv 01 trv 02 trv 03 and the other one at the pyroclastic cone trv 04 the petrographic characterization of facies was carried through conventional optical microscopy the stratigraphic analysis resulted in the distinction of seven lithofacies divided into two coherent and five volcaniclastic one autoclastic and four pyroclastic the coherent lithofacies are of massive npm and vesicular porphyritic nephelinite npv texturally the lavas are holocrystalline and vesicular composed by olivine phenocrysts in a very fine matrix of clinopyroxene nepheline zeolite and titanomagnetite at times with phlogopite the autoclastic lithofacies of nephelinitic breccia nb is composed of fragments 2 mm 15 cm of vesicular nephelinite and the pyroclastic ones are composed mainly of lapillitic scoria fragments with subordinate bombs and volcanic ash the pyroclastic lithofacies are of massive lapillistone lm and lapilli breccia lbm planar stratified lapilli tuff ltp and reverse grading lapillistone lrg the lithofacies associations characterize a a rubbly pahoehoe and pahoehoe flow types distal and proximal pyroclastic deposits the interpretation of the vertical and lateral facies distribution leads to the conclusion that the volcanism occurred with an initial phreatomagmatic stage succeeded by a strombolian stage pyroclastic with primary fragments domain and a final hawaiian stage lava flows domain

magnetic fabric and its significance in the sills and lava flows from taimyr fold belt arctic siberia abstract magnetic fabric research on the emplacement of the taimyr fold belt is still absent therefore we have performed magnetic fabric studies on samples collected from 23 sites from mafic igneous rocks 75 n 100 e in south taimyr this study indicates a correlation between the anisotropy of magnetic susceptibility ams magma flow and fold related compression in the rocks ams measurements on 183 unheated and 122 heated samples reveal the magma flow directions the magma flow direction is mainly parallel to the ese wnw and secondarily parallel to the ssw from the nne in the mafic sills the unheated and heated basaltic flows reveal a ne trending flow vector in some of the sills ams shows a tight clustering of the maximum k1 axes close to the bedding pole which is not thought to define the flow direction corresponding ams measurements on igneous rocks allow us to infer the existence of magnetic fabrics of tectonic origin linked to the main folding episode that occurred subsequent to the mid triassic magmatic event in latest triassic early jurassic times in the sills the distribution of most maximum k1 axes is close to ns that corresponds to the maximum compressive stress or folding directions contrarily the minimum k3 axes have an elongated distribution along the ew direction or parallel to the fold axis before exchanging k1 and k3 axes for inverse fabrics but close to the ns stress direction after exchanging k1 and k3 axes for inverse fabrics in the basaltic flows the minimum k3 axes almost parallel to the ns folding direction the structural interpretation of all ams data taken from the igneous bodies is in accordance with a nnw sse stress related folding taking place around 198 ma in the taimyr peninsula arctic siberia the relationship of susceptibility axes to bedding surfaces and magnetic foliation planes is a criterion that permits differentiation of normal magnetic fabrics from inverse fabrics in the igneous samples single domain sd and small pseudo single domain psd magnetite crystals in the sill samples are the main carriers of the ams fabrics implying the abundance of inverse magnetic fabrics psd and multidomain md titanomagnetite tm grains as well as minor hematite are identified in the basalt flows suggesting the occurrence of more normal magnetic fabrics in the basalts than in the sills

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

monogenetic vulcanism in sierra chichinautzin mexico the variation in the activity patterns of the chichinautzin volcanic rocks is discussed this sequence of lavas and pyroclastic deposits is located in the central part of the mexican volcanic belt directly south of mexico city and is typical of its quaternary monogenetic vulcanism

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

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

age and archaeological implications of xitle volcano southwestern basin of mexico city the pedregal lavas are fresh well exposed basaltic flows erupted from the xitle scoria and cinder cone in the southwestern part of the basin of mexico these lavas cover an area of 70 km 2 and were emplaced over pyramids and other buildings e g cuicuilco and copilco archaeological sites today a part of mexico city including the national university is built on the flows initial strombolian activity produced an ash fallout layer which was immediately followed by effusive emplacement of lava flows the xitle cone grew on the north facing slope of ajusco volcano and lava flowed down to the n ne until it reached the basin floor more than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava and several ages for the eruption have been proposed from these dates most dated samples were not directly produced by xitle s eruption but instead are artifacts of human activity that predates the eruption thus these ages mostly about 2000 bp are older than the eruption a new age of 1670 35 years bp ad 245 315 obtained on charcoal samples collected just beneath the lavas is favored for the xitle eruption these samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra the new age is within the classic period of mesoamerican archaeology whereas the earlier reported ages are at the end of the preclassic the new age carries important implications for the timing of population shifts within the basin of mexico q 2000 elsevier science b v all rights reserved

magnetic fabrics and rock magnetism of archaean and proterozoic dike swarms in the southern sao francisco craton brazil abstract magnetic fabric and rock magnetism studies were performed on three mafic dike swarms total of 38 dikes from the southernmost part of the sao francisco craton sfc minas gerais state se brazil they cut archaean granite gneiss migmatite and paleoprototerozoic terranes these swarms are classified as basic noritic sm nd age 2 65 ga basic rb sr age 1 87 ga and metamorphic rb sr age 1 87 ga suites in which the second is the most important magnetic fabrics were determined by applying both anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm in most sites magnetic susceptibility is dominantly carried by ferromagnetic minerals however in some sites the paramagnetic contribution exceeds 70 of bulk susceptibility mainly coarse to fine grained ti poor titanomagnetite up to pure magnetite carry the magnetic fabrics three primary ams fabrics are recognized which are all coaxial with the aarm fabric normal ams fabric is dominant in the basic suite 16 of 20 analyzed dikes and occurs in 4 and 3 dikes from the basic noritic and metamorphic suites respectively this fabric is interpreted as a result of magma flow in which the analysis of k max inclination permitted to infer that the majority of dikes were fed by inclined flows 30 k max k max intermediate ams fabric was found in 50 of dikes from the basic noritic and metamorphic suites but in only 2 dikes from the basic suite it is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike inverse ams fabric is a minority 2 dikes from each suite the parallelism between ams and aarm tensors for dikes with abnormal fabrics suggests a primary origin for them gyroremanent magnetization grm effect was negligible for the majority of dikes but it was found in two dikes from the basic suite with normal ams fabric magnetic fabrics recognized for the three studied swarms do not depend on magnetic mineralogy geochemical composition dike strikes nor the age of the swarms since the same magnetic minerals and magnetic fabric types are found in dikes from all suites inclined and horizontal flows allow us to infer the relative position of at least three magma sources or magma chambers from which the dikes were fed

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

sampling strategies and the anisotropy of magnetic susceptibility of dykes abstract most studies of the anisotropy of magnetic susceptibility ams of dykes have assumed that susceptibility axes should define a unique orientation relative to the dyke walls and magma flow direction theoretical considerations however predict systematic variations of ams as a function of the amount of shear experienced by the flowing magma although this feature of ams might seem undesirable at first sight based in a theoretical model it has been suggested that actually we can take advantage of such variation in the orientation of the ams axes to infer magma flow directions more confidently than until now if proper attention is given to the scheme followed during sample collection in this work we report the results of a pilot study made to test those theoretical predictions we resampled two dykes from the ko olau dyke complex in o ahu and for which magma flow direction was previously inferred albeit with some uncertainty the resampling was made following the guidelines suggested by the theoretical work as closely as possible i e collecting more than one profile parallel to the dyke walls and across the dyke the results of our measurements provide the first empirical evidence supporting the occurrence of a cyclic fabric acquisition during the emplacement of at least some dykes the procedure described in this paper therefore constitutes a self contained approach that can be followed to test in a case by case basis the occurrence of such fabric behavior this in turn should provide enough elements to increase the reliability of the interpretation in the obtained results

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

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

on the reliability of the ams ellipsoid by statistical methods weak magnetic materials whose susceptibility values are close to the instrument s accuracy show very large errors in the direct evaluation of their ellipsoid parameters this may lead to misinterpretation of the magnetic fabric which is often used as a geological indicator in order to estimate the measurement uncertainties several statistical methods have been proposed within the available statistical methods the linear perturbation analysis hext 1963 and the non parametric bootstrap constable and tauxe 1990 technique have been widely used in this paper we make a complete study about these methods to estimate their limitations when applied to n measurements of a single sample we will analyze which method is better in terms of uncertainties we will determine when the methods do not provide reliable results and we will establish a measuring protocol for that we run simulations for the linear perturbation analysis and the non parametric bootstrap varying i the number of measurements ii the instrumental error and iii the shape parameter and the anisotropy degree of the ams ellipsoid the results show that both methods are not reliable when the difference between eigenvalues is too close in relation to the instrumental error but increasing the number of measurements can improve the results

stratigraphy paleomagnetism and magnetic fabric of the toba tuffs constraints on the sources and eruptive styles the toba depression in north central sumatra is a complex of several overlapping calderas resulting from three major ignimbrite forming eruptions within the depression the upland masses of samosir and northern uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment hitherto interpreted to be two parts of a single resurgent dome this study has demonstrated that the welded tuffs of samosir and uluan have different magnetic polarities and therefore at least two different ignimbrites are present the samosir uluan massif may consist of parts of two resurgent domes the first ignimbrite eruption occurred at 0 84 ma and produced a very thick 400 m densely welded unit having a reversed polarity anisotropy of magnetic susceptibility ams flow direction and lithic size data indicate that the source lies in the southern part of the toba depression and the thick deposit of uluan is thought to have ponded in a 40 km wide caldera the second ignimbrite is normally magnetized ams flow direction data indicate two separate source vents one to the north in the haranggaol caldera and another to the south the thick deposit at samosir is thought to have ponded in the southern caldera coarse sediments then accumulated over samosir and northern uluan and were capped by lacustrine deposits a renewed episode of resurgence then uplifted samosir island and possibly the northern part of uluan at approximately 0 075 ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the toba depression this ignimbrite is mostly nonwelded and normally magnetized part of the uluan dome was destroyed by collapse of the sibandung caldera and latung graben and concomitant with renewed subsidence of the haranggaol and porsea calderas

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

determination of flow directions by combining ams and thin section analyses implications for oligocene volcanism in the kerguelen archipelago southern indian ocean we report an anisotropy of magnetic susceptibility ams study carried out on seven oligocene volcanic sections widely distributed on the surface of the kerguelen archipelago the statistical results at flow scale and at section scale were checked with ams density diagrams and thin section analysis because of the axis permutations frequently observed for lava flows two possible flow directions were nicely defined for the majority of the sections we then used two alternative approaches imbrication of the minimum axis and symmetry of the ams distribution to infer a flow direction for each section these two methods and thin section analysis not being decisive for choosing between the two solutions geological and geomorphological observations were finally used to obtain confident interpretations the results of this study enable the supposed eruptive centres of the kerguelen archipelago to be located more precisely

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

microwave palaeointensities from a recent mexican lava flow baked sediments and reheated pottery microwave palaeointensity pi estimates have been produced from samples from the 1670 yr old xitle lava flow mexico seventeen out of 19 experiments were successful producing high quality estimates and a mean pi of 58 3 7 9 5 wt within errors of that expected from global data for the time period the dispersion is high but could be reduced to 5 5 wt by using stricter selection criteria previous data obtained by the thellier coe method using samples from the same lava flow were less successful and of much lower quality producing a higher mean of 67 9 7 9 8 wt 25 out of 65 samples this difference is most probably caused by the presence of multi domain particles producing concave up arai plots combined with alteration processes affecting the thellier coe experiments at higher temperatures pi calculations restricted to the initial part of an arai plot therefore tend to overestimate the correct values in addition to lava samples microwave pis were also determined from pottery fragments recovered from the contact zone between the lava flow and the underlying baked sediments which were also studied these materials also provided good quality results with a higher mean pi of 66 8 7 7 1 wt which is still statistically indistinct from that produced by the microwave analysis of the lava samples nevertheless these samples seem to be less suitable for pi determinations because they are characterised by larger magnetic grains and apparently more prone to thermally induced alteration than the lava samples we conclude that in the case of the xitle lava the microwave pi method is superior to the conventional thellier coe method in many respects and that the results produced by the latter method even when satisfying strict acceptance criteria may be unreliable however we also draw attention to the fact that microwave produced pi determinations though of high technical quality may still be prone to inaccuracy when rock magnetic parameters indicate that the material is likely to be a poor pi recorder as for the pottery sediment and some of the lava samples described here 2003 elsevier 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

strong neotectonic block rotations related to the africa eurasia convergence in northern algeria paleomagnetic evidence from the mitidja basin

vertical ams variation within basalt flow profiles from the xitle volcano mexico as indicator of heterogeneous strain in lava flows abstract the within flow vertical variation of anisotropy of the magnetic susceptibility ams of three basaltic flow profiles from the xitle volcano were investigated in relation to the lava flow induced shear strain rock magnetic properties and opaque microscopy studies have shown that the magnetic mineralogy is dominated by ti poor magnetite with subtle vertical variations in grain size distribution psd grains dominate in a thin bottommost zone and from base to top from psd md to psd sd grains are found the vertical variation of ams principal direction patterns permitted identification of two to three main lava zones some subdivided into subzones the lower zone is very similar in all profiles with the magnetic foliation dipping toward the flow source whereas the upper zone has magnetic foliation dipping toward the flow direction or alternates between dipping against and toward the flow direction the k 1 maximum ams axis directions tend to be mostly parallel to the flow direction in both zones the middle zone shows ams axes diverging among profiles we present heterogeneous strain ellipse distribution models for different flow velocities assuming similar viscosity to explain the ams directions and related parameters of each zone irregular vertical foliations and transverse to flow lineation of a few samples at the bottommost and topmost part of profiles suggest sd inverse fabric levels of intense friction or degassing effects in ams orientations

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

magma flow pattern of the 1 78 ga dyke swarm of the north china craton during the initial assembly of the supercontinent nuna columbia constraints from rock magnetic and anisotropy of magnetic susceptibility studies abstract the geodynamic origin of the 1 78 ga dyke swarm in the north china craton ncc is long debated it is considered either a post collision process or paleoplume related constraining its origin is essential for the reconstruction of the ncc position within the supercontinent nuna also referred to as columbia here we report on the rock magnetism and anisotropy of magnetic susceptibility ams of thirty three 1 78 ga dykes to constrain the magma center of the igneous event the samples were collected from the entire swarm across the ncc magnetite appears to be the dominant magnetic mineral in all the dykes occasionally with minor maghemite while the magnetite grain size within the dykes is generally increasing from south to north ams results of the dykes portray a magma flow related fabric over half of the studied dykes show a predominance of horizontal to subhorizontal magmatic flow while a few have been formed by vertical intrusion these observations imply that the 1 78 ga dyke swarm may have been generated by sustained magma flow from a magmatic source centered at the south margin of the ncc during dyke propagation some localized magma subchambers may have been formed they could be responsible for the less common vertically intruded dykes and the ew trending dykes therefore a plume related tectonic setting for the 1 78 ga dyke swarm is more compelling with the eruption center along the margin of the ncc it may serve as an essential criterion to search for possible counterparts in other tectonic units within nuna columbia to evaluate the proximity of the ncc with them

magnetic and shape fabrics of magnetite in simple shear flows abstract the magnetite fabrics measured by anisotropy of magnetic susceptibility ams and by shape preferred orientation spo optical methods are classically used as flow kinematics indicators in lava flows the development of magnetite fabrics during simple shear strains 20 was performed using a suspension of 1 volume fraction of multidomain magnetite randomly contained in a mixture of silicone and wax we measured ams fabric and spo ellipsoids by calculating a quadratic shape tensor from oriented thin sections for jeffery 1922 fabrics are usable to determine the flow kinematics including the amount of applied finite strain for 8 fabric elements foliation and lineation are stabilised closely parallel to the flow plane and the shear direction respectively two and three dimensional numerical simulations using measured aspect ratios of magnetite point out that the large scattering of aspect ratios and the initial orientation distribution of particles are together responsible for a wide ranging loss of periodicity the stable ams and spo fabrics observed at large strains in experiments are the result of these primary fabric properties combined to collisions between particles and possibly their complex three dimensional shapes in addition the constant angular relationship observed at large strains between fabrics and flow components is related to the transient collisions consequently the determination of the lava flow kinematics by using fabric properties measured either by ams or by spo analyses should be indubitably associated to a detailed study of the three dimensional shape of the solid carriers regularly shaped populations of low elongated particles will be capable to produce cyclic to oscillating fabrics while the fabric of elongated particles will be more sensitive to the shape parameters and collisions ultimately favouring stable fabrics at large strains

growth of intra caldera lava domes controlled by various modes of caldera collapse the stiavnica volcano plutonic complex western carpathians abstract the stiavnica volcano plutonic complex is an erosional relic of miocene caldera stratovolcano in the western carpathians the complex exposes a vertical section from the volcano basement through subvolcanic intrusions and a ring fault to volcanic edifice comprising mostly andesitic lava flows and domes this paper examines internal structure magnetic fabric as derived from the anisotropy of magnetic susceptibility ams and emplacement dynamics of three intra caldera andesite domes referred to as domes 1 3 located near the presumed ring fault magnetic fabrics carried by multi domain titanomagnetite and titanomaghemite are interpreted as recording various mechanisms of dome growth controlled by active caldera collapse dome 1 is explained as a lava coulee fed by conduits located along the ring fault with a long lava outflow down the sloping caldera floor dome 2 represents an elongated ring fault parallel dome wherein the lava flowed a short distance over a flat floor dome 3 is interpreted as a composite dome fed from multiple linear fissures opened at a high angle to the ring fault subsequently the dome was intruded by ring fault parallel dikes that may have potentially fed overlying now largely eroded lava domes and flows finally we suggest that all domes formed during collapse of the stiavnica caldera and the various mechanisms of their growth reflect different stages of the caldera evolution from piston dome 2 through trap door dome 1 to piecemeal dome 3

internal structure of basalt flows insights from magnetic and crystallographic fabrics of the la palisse volcanics french massif central we present a new interpretation of anisotropy of magnetic susceptibility ams fabrics in basaltic lava flows based on the detailed study of magnetic mineralogy and silicate crystallographic fabric of a quaternary lava flow from the french massif central la palisse we consider the model of ams fabric imbrication between magnetic foliation and flow surface as initially proposed for dykes at the two sampling sites the concordance between the flow direction deduced from the ams foliation and that deduced from field observations indicates that the imbrication model could apply to the lava flows however the flow senses inferred fromamsare systematically opposed between the two sampling sites suggesting permutations between k1 and k3 ams axes a configuration referred to as inverse fabric electron backscatter diffraction ebsd measurements show strong lattice preferred orientations lpo for plagioclase especially the 010 plagioclase plane which tends to be parallel to the flow clinopyroxene lpo remains less marked than plagioclase lpo whereas titanomagnetite does not display a significant lpo comparison between magnetic and crystallographic fabrics suggests that the ams fabric of the lava flow results from the distribution of titanomagnetite grains which is in turn controlled by the fabric of the silicate framework magnetic hysteresis parameters and anisotropy of remanent magnetization arm measurements exclude a significant contribution from single domain grains often called upon to explain inverse magnetic fabrics the origin of the observed inverse magnetic fabric may relate to the dip of the palaeosurface which is the only remarkable difference between the two sampling sites ams appears as a good tool to determine the direction of basaltic lava flows and coupling with local crystallographic fabric data provides a valuable control of relationships between magnetic fabrics and flow and thus contributes to better constrain the ams signature of lava flows

paleosols beneath a lava flow in the southern basin of mexico the effect of heat on the paleopedological record abstract volcanic activity affects past and present landscapes here we present a case study related to the heat impact of the xitle lava flow on the former soil cover the eruption occurred 1670 years b p in south mexico city the evaluated properties include those related to the soil memory used as tools of paleoenvironmental interpretation in volcanic paleosols macro and micromorphology color grain size distribution magnetic susceptibility and carbon and nitrogen contents changes of these properties by the heat influence are relevant for an adequate paleoenvironmental reconstruction two sites and three soil profiles are selected to show soil modifications by the volcanic activity the two sites represent the following 1 slightly transformed paleosol buried by the lava flow but separated from it by a 10 cm thick volcanic ash layer site cuicuilco enah 2 moderately and strongly transformed paleosols two profiles inside the copilco archeological zone one of the copilco profiles represents a buried paleosol with a very thin 3 cm ash layer on top at the contact with the lava the second profile is the most affected by the lava flow as it is directly beneath the lava all studied paleosols are sandy have a low amount of organic matter narrow c n ratios and very high values of magnetic susceptibility the strongest modifications are observed in the upper horizons which are in contact with the lava however the presence of the ash layer minimizes the heat effect despite the observed modifications it is possible to establish three cycles of soil formation in the paleolandscape the first and the oldest related to the development of a luvisol type soil indicates a longer time of landscape stability during the second cycle a pedosediment with a high concentration of artifacts was formed the last and the youngest cycle includes the ab and the aeb horizons present only in copilco they form by an andosol type pedogenesis which reflects a shorter time of soil development the presence of the aeb horizon is interpreted as the result of the degradation of the ab horizon either due to the heat effect of the lava or to cultivation and irrigation practiced by the pre hispanic communities

an ams study of different silicic units from the southern parana etendeka magmatic province in brazil implications for the identification of flow directions and local sources abstract in the southern portion of the parana etendeka magmatic province in brazil extensive silicic dacite rhyolite deposits occur at the top of a sequence of low ti pahoehoe to rubbly basalts the internal architecture of the silicic deposits and their eruptive style as well as the location of their sources are still unsatisfactorily known in an attempt to provide independent evidence for flow directions in deposits previously characterized as effusive and test the hypothesis of local sources we carried out anisotropy of magnetic susceptibility ams studies on the two main silicic units caxias do sul dacites and santa maria rhyolites with the best exposures in an area previously mapped in detail magnetic anisotropies were determined on oriented cylindrical specimens from a total of 28 sites rock magnetism properties indicate that pseudo single domain magnetite carries the fabrics and the remanence magnetic fabrics were determined by applying anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm both ams and aarm tensors are coaxial indicating that the ams fabric is not affected by the effect of magnetite single domain grains magnetic data from several dacitic coulees caxias do sul unit indicate flows from se to nw the location and spatial distribution of these lavas support the hypothesis of local sources aligned along a ne sw trend these data are in agreement with the alignments of structures dome shaped hills observed in field work and dem images on the other hand magnetic data obtained in santa maria rhyolites indicate that flow directions in two different areas are distinct towards nw ne and w suggesting that they derived from different emission centers so regarding the silicic volcanism in the studied region our data do not support the model which classifies the entire silicic volcanism of the province as extensive rheomorphic pyroclastic deposits released from a central conduit in contrast we propose the occurrence of local volcanic events implying in the existence of different sources possibly characterized by local emission centers

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

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

the significance of groundmass ilmenite and megacryst ilmenite in kimberlites criteria are suggested for distinguishing xenocrystic ilmenites from those indigenous to the host kimberlite for instance in contrast to groundmass grains ilmenite xenocrysts usually are larger have reaction rims of leucoxene and perovskite exhibit strong magnesium enrichment outward and sometimes have exsolution lamellae and deformation features most of the abundant ilmenite macrocrysts found in kimberlite appear to have been phenocrysts in a crystal mush unrelated to kimberlite on the other hand kimberlitic groundmass ilmenite is rare but consistently more magnesian than the cores of macrocrysts strong mg enrichment patterns evident in the ilmenite macrocrysts probably developed during their attempt to equilibrate with the more magnesian fractionating kimberlitic liquid the hypothesis of extensive reaction of ilmenite with kimberlite melt fluid has implications with regard to the following 1 the degree of differentiation of kimberlite melts 2 the genesis of mantle megacrysts 3 the reactivity of kimberlite and 4 the usefulness of groundmass ilmenite as a petrogenetic indicator

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

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

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

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

origin of vesicle layering and double imbrication by endogenous growth in the birkett basalt flow columbia river plateau abstract the 40 m thick birkett basalt pahoehoe flow at sentinel gap in the columbia river plateau has an unusually thick 15 m upper vesicular zone this zone includes a striking layering in which the layers have contrasted vesicle abundances and sizes most layers show a reverse grading of vesicle size and abundance the layering is interpreted to have grown endogenously by the cyclic injection of vesicular lava layers under the growing top crust accommodated by uplift of that crust grading of the layers resulted from vesicle growth and ascent each injection occurred at or near the boundary between vesicular and non vesicular lava of the preceding layer and split that layer into an upper vesicular part and a lower non vesicular part critical to this interpretation are 1 a pervasive foliation and lineation defined by the parallelism of strongly flattened and elongate vesicles transects the vesicle layers obliquely and 2 the magnetic fabric the anisotropy of magnetic susceptibility is oriented similarly to the vesicle foliation and also defines a cryptic foliation in the non vesicular zone having a dip opposed to that in the layered zone these foliations are interpreted to be opposed imbrications and indicate the flow azimuth of the lava they strongly support the concept of lava growth by successive thin sill like insertions of fresh vesicular lava between hot but static and effectively solid floor and roof

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

morphological structural and textural variations in the 1988 1990 andesite lava of lonquimay volcano chile the 1988 1990 eruption of lonquimay volcano chile 38 s formed a 10 2 km long andesite lava with a volume of 0 23 km 3 over a period of 13 months the lava extrusion rate decreased with time as chamber pressure and vent dimensions decreased the velocity of the flow front decreased exponentially with distance from vent as a consequence of cooling and the increase of apparent viscosity at the flow front the lava developed a central channel which decreased in width and depth with time three prominent lava levees were formed on each margin and resulted from abandonment as the channel decreased in width as a result of a rapid decrease of flow rate over the first 100 days of activity a fourth major levee developed in february during a brief period of flow rate increase down the main channel but its walls were gradually exposed as the lava depth again decreased due to declining flow rate the structure of lava levees depended on their age and longevity of the flow in the adjacent channel initial levees were formed in the first few days as the lava spread laterally and then retreated leaving levees of massive lava more mature rubble levees were formed during the next month by the lava pushing and then shearing aa and blocky breccia which formed on the cooling flow margin fragmentation and abrasion formed a characteristic zonation in the levees a basal zone consists of very poorly sorted matrix rich breccia with very rounded vesicular clasts and bimodal grain size distribution the basal breccia zone strongly resembles block and ash flow deposits this zone passes up into a zone of clast supported clinker breccia which becomes increasingly matrix poor and coarser with clasts becoming more angular upwards the crest of the levee is composed of large 10 100 cm angular to subangular blocks with no matrix the zoned levees form after the active lava channel suddenly narrows lava depth initially increases and breccias are deposited on the channel margins and acquire the zoned structure by progressive shearing and accretion of clinkery aa breccia the lava level then drops exposing the steep inner scarp of a levee the most mature levee type formed in a long lived channel over several months the outer wall of the levee consists of zoned breccia but the inner wall consists of a massive curving wall of strongly foliated lava with well developed horizontal striations and ductile reidel shears the massive foliated facies is a consequence of prolonged flow which coats strongly sheared lava onto the inner levee wall scanning electron microscopy shows that the aa clinker clasts and foliated lava from the levee walls form at low melt fractions 15 in the last three months of the eruption the flow front ceased to advance but thickened as lava drained from proximal regions and intruded into the interior of the distal lava the last stages of lava movement were characterized by updoming in the central channel a lava surface feature named here armadillo structure was formed by deformation of the cooler but still ductile lava crust the deformation caused by underflow produced reidel shears dipping upstream and doming of the lava due to rotation of the shear planes the study demonstrates that lava morphology structure and texture are strongly influenced by variations of effusion rate local flow rate channel topography and thermal maturity of the lava which is reflected in downstream changes in viscosity

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

time dependent profiles of lava flows this work investigates the combined effects of a time dependent effusion rate and a spatially varying viscosity on the thickness profile of a flowing lava we formulate relatively simple governing equations as a primitive dynamical model for these two influences on lava flow morphology free boundary solutions for the profile of the flow and its extent are presented and analyzed for different boundary conditions at the source of the flow and several models for the spatial dependence of the viscosity the results presented here suggest that time dependence in the flow depth at the source and the form of the viscosity variation are significant influences on the morphology and dimensions of lava flows moreover this analysis implies that time dependent source conditions may contribute to the scatter in plots of eruption rate versus flow length and disparities between field and laboratory estimates of lava viscosity

magnetic fabrics study and inferred flow directions of lavas of the old pali road o ahu hawaii abstract a section 600 700 m deep in the koolau tholeiitic lava shield volcano consists of mostly thin pahoehoe and aa flows cut by occasional dikes and outward dipping gravity driven intrusive sheets a magnetic fabric study of 250 specimens from 18 sites shows a tight clustering of k1 axes for some of the lava flows and is thought to fairly reliably define the lava flow direction a systematic plunge of the k1 axes appears to define an imbrication and yields a flow azimuth the azimuth differs by 60 from that anticipated but is readily explained by a change in shape of the koolau volcano during growth the tightest clustering is in massive aa lava flows 5 m thick samples collected from near the flow base give the most consistent orientations compound pahoehoe lavas with many small flow units give the least tightly clustered anisotropy of magnetic susceptibility ams axes and in some flows the orientation is random the gravity driven sheet has an ams fabric indicating a down dip magma flow direction after the koolau volcano was deeply eroded a cinder cone and lava flows of nephelinite were formed in the rejuvenation stage of volcanism the nephelinite is normally magnetized and the ams fabric indicates a generally downslope 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

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

application of ams for reconstruction of the geological evolution of recent volcanic systems case of deception island south shetland islands antarctica abstract the analyses of recent volcanic systems can encounter limitations due to recent eruptions hiding part of the geological history where the geomorphological expression can present non univocal interpretations or where modern relief and abrupt cliffs can decrease the accessibility to places that are suitable for detailed analysis in the case of deception island bransfield trough the recent tectonic and volcanic history and the interactions with glacial periglacial and marine activities make traditional analysis difficult anisotropy of magnetic susceptibility ams a petrofabric approach has allowed for the identification of a clear correlation between the anisotropy of the magnetic ellipsoid the axial distribution characteristics at the site scale the type of defined magnetic fabrics and the imbrication angle between the magnetic foliation and the emplacement surface however these aspects are independent of the dip of the emplacement surface in some of the analysed sites the shear plane the plane that contains kmax and kmin axes and the flow direction shows non suitable kinematic patterns to respect the dip of the emplacement surface a tilting process is needed to recover the expected original distribution in some cases affecting the analysed units through a concentric fracture system with respect to the main caldera or following the regional tectonic structures a detailed analysis of ams data permits us to constrain the recent evolution of a volcanic system through three stages 1 a radial distribution of volcanic deposits 2 a coaxial tilting of the analysed zones respect the caldera collapse and 3 a tectonic related fracturing system responsible for the tilting of the volcanic system

crystallization of groundmass spinel in kimberlite groundmass spinel grains in 46 kimberlite and related rocks have been analyzed and compared the majority of the spinel analyses are classified as high chromium chromite chr and magnesioulvo spinel magnetite mum and represent two significant stages of spinel growth there are also a significant number of spinel grains that are classified as xenocryst spinel xen pleonaste spinel ple and magnetite mag eight different spinel zoning trends are identified the majority of the chr spinel grains are interpreted as a primary phase that crystallized as small octahedra from kimberlite magma on the journey from the upper mantle to the final resting place in the upper crust three zoning trends lead directly away from primary chromite the major zoning trend trend 1 is from chromite to magnesio ulvo spinel magnetite this zoning trend is unique to spinel in kimberlite carbonatites and lamprophyres we suggest that this somewhat oxidizing and more magnesian trend was influenced by the high carbonate content of group i kimberlites and the rapid crystallization of the minerals during the evolution of volatiles the zoningtrend 2 involves increasing titanium and ferric iron as a function of increasing fe 2 fe 2 mg this trend is similar to the zoning of spinel in basalt and is thought to be due to co crystallization of magnesium and aluminum rich silicate minerals such as olivine and phlogopite in kimberlites or pyroxene and plagioclase in basalt zoning trend 3 in kimberlite leads away from primary chromite and towards an aluminous pleonaste ple spinel this trend is characterized by a large decease of cr cr al parallel to so called olivine spinel iso potential lines similar trends of lesser magnitude and cyclic al cr zoning have been identified in basaltic spinel this trend is thought to be due to very rapid crystallization under conditions of supersaturation where the crystallization of spinel affects the local environment ahead of the growing spinel crystal i e diffusion controlled crystallization the tendency for immiscibility between ferrite or titanate rich spinel and aluminate rich spinel pleonaste has a great influence on trends 1 and 3 zoning and also on atoll spinel formation very local conditions such as nucleation or lack of nucleation of other minerals can influence both the textural environment and composition of kimberlitic spinel