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

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

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

anisotropy of magnetic susceptibility of heated rocks abstract heating produces changes which does not always correspond to simple enhancement of the magnetic fabric two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating the first diagonalizes the tensor from the difference between each tensor term before and after heating the second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment when the same magnetic fabric is obtained from several thermal steps it cannot be related to randomly oriented ferrimagnetic minerals instead the newly formed fabric must be related to characteristics of the pre existing rock by comparing this ferrimagnetic minerals fabric with the initial whole rock fabric we can distinguish cases where heating simply enhances pre existing fabric from those where thermal treatment induces a different fabric relative to the pre heating fabric this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction related to petrostructural elements other than those defining the initial fabric

internal dynamics of a paleoaccretionary wedge insights from combined isotope tectonochronology and sandbox modelling of the south central chilean forearc forearc accretionary wedges are cyclic systems in which material is frontally and or basally accreted material cycling involves underthrusting subduction underplating exhumation erosion transfer to the trench and underthrusting again in this study we present a novel tectonochronologic approach to constrain long term exhumation rates of basally accreted wedge complexes based on isotopic dating of structural features on petrological data and sandbox analogue simulations congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics for the present day surface material of the paleoaccretionary wedge of south central chile valdivia area 40 s published u pb ages of detrital zircon place a maximum age of 278 ma for subduction prograde metamorphism at transitional greenschist to blueschist facies conditions 420 c 8 9 kbar was immediately followed by progressive penetrative deformation associated with basal accretion dated at 250 245 ma using rb sr internal mineral isochrons the accretion process involved duplex tectonics and antiformal stacking with formation of near horizontal mylonitic shear zones at around 241 ma continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels both semi ductile small scale extensional shear zones and post kinematic vein mineralizations yield rb sr ages of 235 ma tension gashes representing the latest isotopically dateable stage of structural evolution were formed at 210 ma at conditions of 230 c at 1 5 3 kbar as constrained by fluid inclusion data zircon fission track data indicate final cooling to below 200 c at 186 24 ma the results suggest continuous basal accretion for at least 50 ma with long term average exhumation rates of 0 6 0 2 mm a most probably outbalanced by similar long term average erosion rates changing plate boundary conditions at about 210 200 ma terminated the accretion process as evident from a dramatic decrease of exhumation rates at that time since then the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the andean active margin the tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high temperature increments of p t trajectories

determination of fabric and strain ellipsoids from measured sectional ellipses implementation and applications abstract geologists examine fabrics to constrain models of formation or of deformation of rocks and it is often convenient to summarise the results by a fabric ellipsoid as fabric data are commonly collected on planar sections through the rock estimating a fabric ellipsoid from sectional ellipses often with arbitrary orientations is an important task an algebraic method to calculate such an ellipsoid presented in an earlier paper has been implemented with the program ellipsoid it is used here on examples that illustrate questions and issues that arise when collecting selecting and processing sectional fabric data and when assessing the results the quality of fit of the ellipsoid to the data is assessed in all cases examples include a case in which the average sizes of markers on individual sections can be used in the determination of the ellipsoid and other cases in which such sizes are not useful a case in which sectional ellipses are not obtained from closed markers and a case in which data scatter and insufficient coverage of section orientations lead to a hyperboloid instead of an ellipsoid

geology of proximal small volume trachyte trachyandesite pyroclastic flows and associated surge deposits roccamonfina volcano italy abstract this paper describes the 232 ka b p mttt trachyte trachyandesite pyroclastic succession of roccamonfina volcano this small volume proximal sequence crops out along mulino di sotto paratone and pisciariello ravines in the southwest sector of the central caldera and covers a minimum extent of 3 5 km2 area it is made up of seven pyroclastic flows and pyroclastic surge units consisting of trachytic ash matrix containing juvenile trachyandesitic scoria and dense lava fragments pumice clasts of uncertain trachyandesite and a foreign trachyandesitic lithic facies two stratigraphic markers allow correlation of the units no paleosoils and plinian fallout have been observed at the base and within the succession some lateral grading of scoria and lithic clasts suggests that mttt derived from three distinct source vents the sequence consists of a basal ash flow passing laterally to laminated surge deposits unit a this is overlain by a reversely graded scoria and pumice lapilli flow unit b which is in turn overlain by a thinly cross stratified scoria lapilli surge unit c unit c is capped by a prominent ash and scoria flow unit d a ground layer marker mk1 divides unit d from a massive ignimbrite which grades upcurrent to sand wave surge deposits unit e another ground layer marker mk2 separates unit e from unit f this unit consists of a basal ignimbrite passing laterally to bedded surge deposits with convolute structures subunit fl and grading upcurrent to a subhorizontally plane laminated ash cloud subunit f2 containing near the top a layer of millimetric lithic clasts embedded in fine ash the succession is closed by the pyroclastic flow unit g surge unit c can be interpreted in terms of vertical gradients in turbulence particle concentration and velocity during flowage whereas the bedded surge parts present in the massive deposits of units a and e f1 can be related to abrupt changes of velocity down the steep slopes of ravines reverse grading in unit b is probably due to grain dispersive pressures the convolute structures within fl are related to zones of diagenetic cementation associated with groundwater finally the laminated fine grained nature of subunit f2 is interpreted as due to ash clouds elutriated from the basal part of unit f stratigraphic markers mk1 mk2 are ground layer breccias formed by settling of lithic and scoria clasts from overlying units e and f respectively vesiculation and morphologies of glass shards of the mttt succession suggest that eruptions were essentially driven by magmatic explosions which had an appreciable hydromagmatic component

magma flow within dykes in submarine hyaloclastite environments an ams study of the miocene cabo de gata volcanic units this research has been funded by projects cgl2005 03511 bte hi2006 0073 prin2009 prin 2009h37m59 and prin 2010 11 2010tt22sc and 2010tt22sc 003

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

magmatic fabric acquisition mechanisms in a syenite results of a combined anisotropy of magnetic susceptibility and image analysis study the structural significance of the anisotropy of magnetic susceptibility ams and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the archean lebel syenite intrusion ontario canada bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples intensities shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding ams ellipsoids differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre full crystallization textures and structural overprinting in samples deformed after crystallization subfabrics of clinopyroxene and magnetite in samples showing pre full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes the resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite which is similar to the overall petrofabric of the samples crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains analogous to a logjam in a river the final characteristics of the ams and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma rather than transient late features

anomalous magnetic fabrics of dikes in the stable single domain superparamagnetic threshold this research has been partly funded by project cgl2008 04264 bte and by grant prx14 00303 to carles soriano

post emplacement melt flow induced by thermal stresses implications for differentiation in sills abstract we present the first steps of a new explanation model for differentiation in sills using a combination of geochemical data and field observations numerical modeling and dimensional analysis geochemical data from a saucer shaped dolerite sill intruded into the karoo basin south africa reveal a differentiation process which causes d shaped profiles the geometry name is based on the variation in whole rock mg number mg mg mg fe from floor to roof in a sill the d shaped geochemical profiles represent sheet intrusions with the most primitive composition i e high mg in its center and progressively more evolved composition i e low mg towards the upper and lower margins the differentiation is reversed compared to the normal differentiation produced by fractional crystallization c shaped profiles c shaped profiles are believed to be formed by segregation of crystals from the magma we propose that the opposite the d shaped profile may result from melt segregation from the crystal mush this is achieved by porous melt flow through a consolidated crystal network after the main phase of emplacement and before complete solidification we show that a significant flow is feasible under natural occurring conditions an underpressure of magnitude 108 pa develops at the cooling margins due to volume reduction of the crystallizing porous melt the resulting pressure gradient is the driving force for the melt flow towards cooling margins considered in this work as a result the margins will be enriched in the incompatible elements associated with the melt phase while the center will be depleted we show that the amount of flow is primarily a function of viscosity of the melt and permeability of the crystal network which in turn is a transient phenomenon dependent on a number of parameters diagrams have been constructed to evaluate the feasibility of substantial melt extraction in terms of these poorly constrained parameters data from the golden valley sill and many other natural occurrences of d and i shaped geochemical profiles show a reasonable agreement with our predictions of melt flow potential and are thus well explained by the presented model we conclude that in order to fully understand differentiation processes occurring in sheet intrusions we need to account for post emplacement segregation of melt from crystals and not only segregation of crystals from melt

emplacement of the early miocene pinto peak intrusion southwest utah usa 1 in this contribution we report rock magnetic petrographic and anisotropy of magnetic susceptibility ams data from the pinto peak intrusion all of which bear on volcanic construction rock magnetic data indicate that the dominant magnetic mineral phase is low ti titanomagnetite of multidomain grain size the composition of which varies spatially across the intrusion the intrusion is a porphyritic andesite dominated by ca rich plagioclase 60 as well as biotite amphibole olivine and opaque minerals reflected light petrography reveals mostly euhedral subhedral titano magnetite crystals that often form clustered glomerocrysts and stringers of equant crystals without exhibiting a consistent mineral alignment fabric moderate to shallow plunging prolate magnetic susceptibility ellipsoids dominate the northern part of the intrusion while steeply dipping plunging magnetic susceptibility ellipsoids are generally restricted to the southern part of the intrusion the vent facies rocks yield moderate to steep oblate susceptibility ellipsoids we interpret the flow pattern in the north to reflect subhorizontal flow of magma filling a tabular sheet like body associated with propagation of the intrusion to the north we argue that the southern part of the intrusion represents the ascent site of the magma rising to shallow crustal levels along a steep feeder system the oblate magnetic fabrics in the vent area plausibly represent flattening against the conduit walls as evidenced by a weak planar flow foliation observed in the vent conduit rocks on reaching shallow crustal levels the magma deformed and uplifted the roof rocks leading to gravitational instability as the slide mass released from the roof an explosive eruption ensued resulting in the emplacement of the rocks of paradise tuff and associated effusive lava flows following eruption magma pressure decreased and the magma drained northward forming the northern intrusive phase

assessing spo techniques to constrain magma flow examples from sills of the karoo igneous province south africa abstract shape ellipsoids that define the petrofabrics of plagioclase in jurassic karoo dolerite sills in kwazulu natal south africa are rigorously constrained using the long axis lengths of plagioclase crystals and ellipse incompatibility this has been undertaken in order to determine the most effective technique to determine petrofabrics when using the spo 2003 programme launeau and robin 2005 the technique of segmenting an image for analysis is scrutinised and as a process is found redundant a grain size threshold is defined to assist with the varying grain sizes observed within and between sills where grains exceed the 0 2 mm size threshold images should be acquired at a high magnification i e 10 magnification petrofabrics are determined using the foliation and the lineation of the ellipsoid as defined by the maximum and minimum principal axes respectively of the resultant ellipsoid samples with strongly prolate fabrics are isolated allowing further constraint on the petrofabric to be made once the efficacy of the petrofabric determination process has been determined the resultant foliations and lineations then elucidate the most accurate petrofabric attainable the most accurate petrofabrics will be determined by using the correct magnification when the images are obtained and to run the analyses without segmenting the image the fabrics of the upper and lower contacts of the karoo dolerite sills are analysed in detail using these techniques and the fabrics are used as a proxy for magma flow

40ar 39ar and k ar dating of k rich rocks from the roccamonfina volcano roman comagmatic region italy abstract roccamonfina is the northernmost volcano of the campanian area of the k rich roman comagmatic region of italy it erupted a huge amount of pyroclastics and lavas belonging to both the leucite basanite and leucitite series lbls and the shoshonite series ss spread over an area of about 300 km2 the above series correspond to the high k series hks and low k series lks of appleton 1972 respectively 40ar 39ar and k ar dating of samples from both series gave ages ranging from 0 656 to 0 096 ma for the ss and from 1 03 to 0 053 ma for the lbls these results indicate that the products of the two series were outpoured together at least between 0 7 and 0 1 ma ago i e during both the so called pre caldera phase and the post caldera phase of activity the latest products of the volcanism at roccamonfina were erupted just before the deposition of the grey campanian ignimbrite which erupted from vents located about 50 km to the south in the phlegrean fields near naples and has an age of about 33 000 years taking into account all the available radiometric data we conclude that roccamonfina was active between 1 5 and 0 05 ma ago in excellent agreement with the stratigraphie evidence in this same time span is concentrated the activity of all the centers of the roman region north of naples

origin of the calderas and evolution of roccamonfina volcano roman region italy abstract the roccamonfina volcano consists of a volcanic fluvial succession dated 549 ka bp paleoauruncus overlain by a 546 ka bp stratacone main cone formed by leucite bearing lava and tephra the main cone was dissected by four nested calderas a ring fracture concentric with and outside of the central caldera is also present detailed mapping of the volcano inspection of landsat satellite images and radiometric dating establish that the gli stagli and northern calderas subsided between respectively 546 474 and 403 374 ka bp and that the summit part collapsed between 546 and 446 ka bp to form the central caldera a chronologically intermediate state dated in the range 376 323 ka bp was identified between the 549 374 ka bp leucite bearing high k series hks stage i and the 317 96 ka bp leucite free low k series lks stage ii this stage is made up of concurrent hks lks rocks stage ics the central gli stagli and northern calderas are older than blt and wtt as supported by the strong discrepancies between calculated volumes of these pyroclastic rocks and the volume of the caldera also caldera collapses due to sector sliding are improbable as tested by the absence of rock slide avalanche deposits the collapses occurred incrementally through repetitive draining of the magma chamber s resulting from a succession of moderate sized volcanic events gli stagli caldera was infilled by a lake early after collapse so that sediments alternated with leucite tephrite lava flows and tephra within the lake the sequence was overlain by a thick succession of young pyroclastic rocks downfaulting downsagging cut the flat lying gli stagli filling sequence resulting in the old valle caldera successive pyroclastic and debris flows derived by updoming of the central caldera deposits infilled the last structure and downfaulted part of the valle fill producing the young valle caldera

proglacial sediment instability features on the scotian slope at 63 w abstract a small area of the scotian slope has been investigated in detail as a type example of a high latitude continental slope adjacent to a major ice outlet the geologic framework has been defined by airgun seismic profiles which show that late tertiary shales have irregular dips but overlie undisturbed strata these shales form a zone of diapirs at around 1200 m water depth which have ponded the prograding quaternary sediments upslope high resolution sparker profiles seamarc i mid range sidescan imagery and piston cores have been used to investigate the late quaternary sedimentary sequence a prominent gullied horizon about 40 m subbottom that is overlain by acoustically unstratified sediments sands is correlated with the occurrence of an early middle wisconsinan grounded icesheet at the top of the continental slope it is overlain by a thick sequence of proglacial muds and a thin veneer of holocene sediments much of the late wisconsinan sequence has been dissected by a series of closely spaced gullies both sidescan and subbottom profiler data show that slumping is common in the late wisconsinan sequence as a result of high sedimentation rates and local gully erosion piston cores provide evidence for debris flows and turbidity currents depositional sediment mounds in the diapir zone have accumulated sediment at three times the regional sedimentation rate suggesting that large amounts of fine sediment reached the slope in suspension and that much of the proglacial sediment by passed the continental slope

segmentation of the 2010 maule chile earthquake rupture from a joint analysis of uplifted marine terraces and seismic cycle deformation patterns abstract the segmentation of major fault systems in subduction zones controls earthquake magnitude and location but the causes for the existence of segment boundaries and the relationships between long term deformation and the extent of earthquake rupture are poorly understood we compare permanent and seismic cycle deformation patterns along the rupture zone of the 2010 maule earthquake m8 8 which ruptured 500 km of the chile subduction margin we analyzed the morphology of mis 5 marine terraces using lidar topography and established their chronology and coeval origin with twelve luminescence ages stratigraphy and geomorphic correlation obtaining a virtually continuous distribution of uplift rates along the entire rupture zone the mean uplift rate for these terraces is 0 5 m ka this value is exceeded in three areas which have experienced rapid emergence of up to 1 6 m ka they are located at the northern central and southern sectors of the rupture zone referred to as topocalma carranza and arauco respectively the three sectors correlate with boundaries of eight great earthquakes dating back to 1730 the topocalma and arauco sectors located at the boundaries of the 2010 rupture consist of broad zones of crustal warping with wavelengths of 60 and 90 km respectively these two regions coincide with the axes of oroclinal bending of the entire andean margin and correlate with changes in curvature of the plate interface rapid uplift at carranza in turn is of shorter wavelength and associated with footwall flexure of three crustal scale normal faults the uplift rate at carranza is inversely correlated with plate coupling as well as with coseismic slip suggesting permanent deformation may accumulate interseismically we propose that the zones of upwarping at arauco and topocalma reflect changes in frictional properties of the megathrust resulting in barriers to the propagation of great earthquakes slip during the 1960 m9 5 and 2010 events overlapped with the 90 km long zone of rapid uplift at arauco similarly slip in 2010 and 1906 extended across the 60 km long section of the megathrust at topocalma but this area was completely breached by the 1730 m 9 event which propagated southward until carranza both arauco and topocalma show evidence of sustained rapid uplift since at least the middle pleistocene these two sectors might thus constitute discrete seismotectonic boundaries restraining most but not all great earthquake ruptures based on our observations such barriers might be breached during multi segment super cycle events

patterns of magma flow in segmented silicic dikes at summer coon volcano colorado ams and thin section analysis abstract a complex pattern of magma flow is found in two silicic dikes of a radial swarm at summer coon an eroded stratovolcano in southern colorado the two intrusions are broken into multiple segments that suggest vertical dike propagation however anisotropy of magnetic susceptibility ams measurements and thin section observations suggest that magma flow was often subhorizontal and away from the center of the volcano segments that are proximal to the central intrusion are characterized by magma that flowed steeply upward at the proximal segment extremity then laterally along the segment and finally downward at the distal end of the segment magma flow in offset segment tips located far from the volcano center was subhorizontal towards the adjacent segment implying lateral propagation of segment tips towards one another this observation suggests relatively high driving pressure in distal dike segments as supported by dike thickening with radial distance from the center of the volcano the present study indicates that radial dike evolution at stratovolcanoes is dominated by lateral flow of magma and dike segmentation is a poor magma flow indicator a horizontally propagating radial dike has the potential to cause an eruption low on the flank of a composite cone which poses a significant yet largely unrecognized hazard to population centers and infrastructure that may surround the volcano

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

solidification and turbulence non laminar during magma ascent insights from 2d and 3d analyses of bubbles and minerals in an etnean dyke

a 100 m y long window onto mass flow processes in the patagonian mesozoic subduction zone diego de almagro island chile diego de almagro island was formed by the subduction and accretion of several seafloor derived tectonic slices with very heterogeneous ages and pressure temperature time p t t paths the highest element of the pile the lazaro unit evidences subduction in the high p granulite field 1 3 gpa 750 c at ca 163 ma below it a thin tectonic sliver the garnet amphibolite unit preserves eclogite facies remnants 570 c and 1 7 gpa formed at ca 131 ma in situ u pb zircon rim ages peak assemblages were nearly fully amphibolitized during decompression down to 1 2 gpa and 600 c at 125 120 ma rb sr multimineral dating the underlying blueschist unit has 50 m y younger metamorphic ages and exhibits slightly cooler peak burial conditions 520 c 1 7 gpa ca 80 ma in situ white mica ar ar ages and multimineral rb sr dating and is devoid of amphibolitization the mylonites from the sinistral strike slip seno arcabuz shear zone bounding diego de almagro island to the east also exhibit amphibolite facies 620 c and 0 9 gpa deformation at ca 117 ma multimineral rb sr ages in situ white mica ar ar dating and multimineral rb sr dating of low t mylonites 450 c along the base of the lazaro unit reveal partial resetting of high t assemblages during tectonic displacement between 115 and 72 ma and exhumation of the slice stack detrital zircon u th pb ages indicate that the material accreted on diego de almagro island has been mostly recycled from a permian triassic accretionary wedge madre de dios accretionary complex exposed along the subduction buttress geological and geochronological constraints suggest that the rocks of the seno arcabuz shear zone and the lazaro unit were tectonically eroded from the buttress while the underlying garnet amphibolite and blueschist units instead derive from the subducted oceanic basin with increasingly younger maximum depositional ages the very long residence time of the rocks 90 m y for the lazaro unit along the hanging wall of the subduction interface recorded long term cooling along the patagonian subduction zone during the mesozoic diego de almagro island therefore represents a unique window onto long term tectonic processes such as subduction interface down stepping tectonic erosion and episodic underplating near the base of an accretionary wedge 40 50 km

structural contacts in subduction complexes and their tectonic significance the late palaeozoic coastal accretionary wedge of central chile understanding the contact between the very low grade metagreywacke of the eastern series and high pressure metamorphosed schist of the western series in the late palaeozoic accretionary wedge of central chile is fundamental for the understanding of the evolution of ancient accretionary wedges we show the progressive development of structures and finite strain from the least deformed rocks in the eastern part of the eastern series of the accretionary wedge to high pressure schist of the western series at the pacific coast upright chevron folds of sedimentary layering are associated with an axial plane foliation s 1 as the f 1 folds became slightly overturned to the west s 1 was folded about west vergent open f 2 folds and an s 2 axial plane foliation developed near the contact between the western and eastern series s 2 represents a penetrative subhorizontal transposition foliation towards the structurally deepest units in the west the transposition foliation becomes progressively flattened finite strain data as obtained by r f analysis in metagreywacke and x ray texture goniometry in phyllosilicate rich rocks show a smooth and gradual increase in strain magnitude from east to west overturned folds and other shear sense indicators show a uniform top to the west shear sense in moderately deformed rocks whereas the shear sense is alternating top to the west and top to the east in the strongly flattened high pressure rocks of the western series near the pacific coast we interpret the progressive structural and strain evolution across the contact between the two series to reflect a continuous change in the mode of accretion in the subduction wedge initially the rocks of the eastern series were frontally accreted to the pre andean margin before c 300 ma frontal accretion caused horizontal shortening and upright folds and subvertical axial plane foliations developed at c 300 ma the mode of accretion changed and the rocks of the western series were underplated below the andean margin this basal accretion caused a major change in the flow field within the wedge and gave rise to vertical shortening and the development of the penetrative subhorizontal transposition foliation subsequent differential exhumation was resolved gradually over a wide region implying that exhumation was not tectonically controlled

when an inverse fabric is not inverse an integrated ams spo study in morb like dykes terra nova 23 49 55 2011 abstract magma flow direction is recorded in the macroscopic mineral petrofabric and magnetic fabric of the morb like rooi rand dyke swarm the bulk magnetic fabric is sub parallel to the dyke plane resulting from preferred orientation of grains during magma flow this is referred to as type a fabric a study of mineral shape preferred orientation reveals an additional fabric which is orthogonal to the average dyke orientation in 30 of the data this fabric is carried by both plagioclase and opaque grains and would typically be interpreted as inverse it is referred to here as type b fabric however because it is carried by the shape preferred orientation of macroscopic grains and is in most cases not coaxial to the anisotropy of magnetic susceptibility fabric it is interpreted as having been acquired during increased grain interaction during late stage magma flow associated with decreasing magma pressure

mesozoic magnetic anomalies in the southern natal valley the recognition of mesozoic magnetic anomalies m0 m12 within the submarine natal valley south west indian ocean provides a further constraint on the relative motion between south america falkland plateau and africa during the early opening phase of west gondwanaland fig 1 colinear with m12 magnetic anomaly g ref 1 is delineated north of the south tugela ridge2 an acoustic basement high which is suspected to have been emplaced within continental crust immediately north of the continental oceanic crust boundary cob during the initial rifting phase starting from a revised west gondwana reconstruction3 and using palaeo poles of rotation1 3 the falkland plateau can be rotated into the natal valley superposing the corresponding georgia basin and natal valley magnetic anomalies this achieves an optimum continuity of inter continental structural lineaments3 by overlapping of an anomaly intermediate to m10 and m11 m10n4 this suggests that the natal valley cob is located between anomalies m10 and m11 implying that in this zone of south west gondwana seafloor spreading commenced at 125 myr

devonian magmatism in the accretionary complex of southern chile supposed or potential devonian igneous rocks in the accretionary complex of southern chile were investigated using sensitive high resolution ion microprobe u pb dating of zircon with hf and o isotope analyses of selected grains ages of 384 3 and 382 2 ma are confirmed for two igneous bodies another having been previously dated at 397 1 ma detrital zircon ages in the host rocks some associated with devonian marine fossils indicate maximum possible sedimentation ages of c 330 385 ma devonian ages of 391 10 and 374 3 ma for plutonic rocks at the western edge of the north patagonian massif are somewhat older than those of orthogneisses in the western flank of the andes near chaiten 361 7 and 364 2 ma o and hf isotopes indicate that the devonian intrusions in the accretionary complex crystallized from mantle derived magmas whereas those in the north patagonian massif show a strong crustal influence corresponding to oceanic and continental margin subduction environments of magma genesis respectively devonian zircon provenance in the accretionary complex was from the north patagonian massif and not from the mantle derived intrusions suggesting that the accretionary complex formed an integral part of the gondwana margin during devonian carboniferous times supplementary material description of analytical methods and tables of isotope analytical data are available at http doi org 10 6084 m9 figshare c 2336728

upward and downward flow in a camptonite dike as recorded by deformed vesicles and the anisotropy of magnetic susceptibility ams abstract a 1 6 m wide cretaceous camptonite dike on higby mountain connecticut contains deformed vesicles that record an initial brief period of steeply upward intrusion followed by a prolonged vertical downward flow of magma prior to solidification the camptonite is unusual in that it is almost aphyric but it does contain abundant skeletal microphenocrysts 0 3 mm of magnetite which give the rock a strong magnetic susceptibility 127 10 3 si these crystals are slightly elongate and their long axes became preferentially aligned parallel to the dike walls but perpendicular to the magma flow direction which is indicated by the stretched vesicles consequently the rock has a strong anisotropy of magnetic susceptibility ams with the mean value of p being 1 03 the ams ellipsoid is everywhere prolate t 0 9 to 0 02 with the maximum susceptibility axis being almost parallel to the strike of the dike that is parallel to the walls of the dike but perpendicular to the magma flow direction in the outer part of the dike where the deformed imbricate vesicles indicate upward flow at an inclination of 70 75 the maximum susceptibility direction plunges 11 perpendicular to the flow direction in the interior of the dike where the vesicles indicate almost straight downward flow the maximum susceptibility direction is almost horizontal and again is perpendicular to the flow in many but not all samples the sense of flow i e up or down is indicated by imbrication of the intermediate magnetic susceptibility axis although the major magnetic susceptibility axis is commonly assumed to parallel the direction of flow of magma in dikes the present study indicates that at least in this camptonite dike the maximum susceptibility axis is perpendicular to the flow of magma

inversion tectonics and magnetic fabrics in mesozoic basins of the western tethys a review abstract positive tectonic inversion of sedimentary basins has been recognized as one of the primary mechanisms of mountain building and intraplate deformation reconstructing the tectonic history of basins is relatively easy for the inversion stage but becomes more difficult for the basinal stage especially when strong deformation involving cleavage development is associated with the subsequent compressional tectonics since tectonic markers for the extensional episodes are not commonly well developed anisotropy of magnetic susceptibility ams has provided recently a tool for analyzing early stages in the evolution of sedimentary basins even in the absence of other outcrop scale mesostructures here we expose and discuss the applicability of magnetic fabrics by means of ams to different types of intra plate sedimentary basins in the western tethys region formed under extensional or transtensional regimes and which underwent different inversion styles total or partial inversion with or without cleavage development forming part of compressional thrust sheets etc owing to specific particular p t conditions and structural controls factors such as lithology magnetic mineralogy position within the sedimentary pile and deformation intensity are key to interpret the obtained magnetic fabrics in terms of tectonic evolution a basin classification is proposed according to inversion styles and magnetic fabrics where inversion did not involve cleavage development magnetic lineation is parallel to the stretching direction corresponding to the extensional stage the transition between non cleaved to inversion related cleaved units is marked by the switch of magnetic lineations from parallel to extension to parallel to the intersection lineation between cleavage and bedding these relationships are enhanced when extension and compression are roughly coaxial then favoring the clustering of axes of the magnetic ellipsoid even when extreme inversion occurs and the early extensional fabric is obliterated magnetic fabrics provide information about the interaction between preferred deformation directions associated with the main stages in basin evolution

an assessment of 40 ar 39 ar dating of incompletely degassed xenoliths the possibility of measuring the age of eruption of pleistocene lavas by 40 ar 39 ar analysis of entrapped ancient potassic xenoliths is demonstrated by a study of model systems upon inclusion in the hot magma such xenoliths are commonly only partially degassed of radiogenic 40 ar which has accumulated in them since their original crystallization the residual 40 ar will increase the apparent k ar age of the xenolith however if a xenolith is of cretaceous age or younger then a plateau in its 40 ar 39 ar age spectrum giving the age of eruption is expected to extend over 25 50 of the total 39ar released if degassing of the xenolith in the magma exceeded 90 and if the phases in the xenolith are characterized by sufficiently different diffusion dimensions or activation energies if diffusion was from a bimodal population of spheres then the radii must differ by a factor of 10 or more or the diffusion coefficients by a factor of 100 or more or if the spheres were equal in size and in diffusion coefficients then the activation energies must differ by a factor of at least 1 5 that such requirements may be realized in real xenoliths containing k feldspars is expected from published activation energies for microcline and from data determined on a granitic xenolith which was degassed in an early pleistocene basalt flow the experimental results appear to establish that old xenoliths may contain ar in distinctive phases which degas at sufficiently different temperatures as to permit determination of the age of degassing or eruption

tectonostratigraphic evolution of a mesozoic graben border system the chachil depocentre southern neuquen basin argentina the chachil depocentre is one of a number of early mesozoic extensional basins that form the early depocentres of the southern neuquen basin in argentina the synrift volcanic fill is composed of andesites rhyolites and volcaniclastic deposits coarse grained non marine facies dominate the sedimentary fill mainly in the form of sediment gravity flow deposits stream flow deposits and minor non marine carbonates are also locally present the evolution of the graben border system was mainly controlled by subsidence along the main boundary fault the chihuido bayo fault system and recurrent volcanic activity marked changes in the thickness of the synrift megasequence indicate that episodic normal faulting in the hanging wall was also important the integration of structural magmatic and sedimentary data from the study area has led to the definition of three stages in the evolution of the synrift succession the early rift stage is defined by the interplay between bimodal volcanism and gravity driven sedimentation the mid rift stage is marked by the transition to acidic magmatism rhyolitic and pyroclastic flows also associated with coarse grained non marine deposition the late rift stage is dominated by fine grained turbidites and pyroclastic falls related to the first marine sedimentation in the neuquen basin

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 ams study of magma transport and emplacement mechanisms in mafic dykes from the etendeka province namibia abstract the henties bay outjo dyke swarm hod in nw namibia is part of the early cretaceous parana etendeka large igneous province the dykes are dominantly doleritic compositionally equivalent to the erupted lava series and thus the hod provides a look at the feeder systems of a flood basalt province the subvertical dykes mostly strike ne sw and minor nw se parallel or perpendicular to the damara belt in which they intruded we present a magnetic fabric study using the anisotropies of low field magnetic susceptibility ams and of anhysteretic remanent magnetization aarm with the aim to derive magma flow directions and better constrain emplacement mechanisms within the dyke swarm magnetic susceptibility and its anisotropy in the dykes is mainly controlled by distribution anisotropy of titanomagnetite that mimics the flow oriented silicate fabric the anisotropy is low in most samples supporting a magmatic origin in 66 of 110 investigated samples the ams fabric is normal with the max axis inferred flow orientation within the dyke plane most samples yielded vertical to subvertical flow orientations regardless of location near or distant from the former rifted margin the anomalous magnetic fabrics where max is inclined to the dyke plane are attributed to two mechanisms one is the single domain effect of titanomagnetite which was found by unequal orientations of ams and aarm fabrics the other case anomalous fabric is rotation of the ams axes by shear within the magma producing symmetric imbrication of ams fabric on opposite dyke walls or more commonly asymmetric magnetic fabrics which we relate to tectonic shear at the dyke walls during emplacement field support for syn emplacement shear is given by dyke segmentation geometries including locally curved segment tips en echelon arrangements and left right stepping displacements regionally syn emplacement shear is consistent with the observed reactivation during gondwana breakup of transcrustal shear zones in the damara mobile belt

significance of magnetic and petrofabric in karoo feeder dykes northern lebombo abstract the orientation of magnetic and petrofabric in 14 mafic dykes of the northern lebombo dyke swarm nlds has been determined using the anisotropy of magnetic susceptibility ams and mineral shape preferred orientation spo in order to constrain the magma flow direction during dyke emplacement the n s striking nlds intruded the n s trending 700 km long lebombo faulted monocline between 182 and 178 ma the lebombo faulted monocline which forms the eastern most sub province of the karoo large igneous province lip is essentially a volcanic rifted margin with a protracted magmatic and tectonic history related to the break up of southern gondwana it also forms the southern limb of the karoo triple rift system which has been hypothesised to have formed by a mantle plume centred on the triple junction locus the other two arms of the rift which converge on the triple junction are the wnw trending okavango dyke swarm ods and ne trending save limpopo dyke swarm slds in the nlds it is found that the magnetic fabric which is of magmatic origin in 50 of the samples is carried predominantly by stoichiometric magnetite there is remarkable agreement between the orientations of the ams and spo fabrics and at least two dykes show coaxial ams and mineral spo fabric consistent with lateral magma flow from the north this direction is supported by the imbrication of the foliations of the well defined bulk ams and plagioclase spo fabric when viewed in a dyke co ordinate system although steeply plunging maximum axes occur in the spo fabric their possible use as magma flow vectors is questioned because they do not necessarily mimic the elongation of plagioclase grains visible in orientated thin sections the magnetic and petrofabric of the nlds is interpreted to have developed during lateral magma flow from the locus of the karoo triple junction

40ar 39ar and rb sr age determinations on quaternary volcanic rocks 40 ar 39 ar and rb sr ages have been measured on separated minerals from the potassic volcanics of the roman comagmatic region to test the ability of these methods to accurately data quaternary geological events the very high k and rb contents of the roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides 40 k and 87 rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages six leucite separates contained ar with very high bulk 40 36 ratios above 1000 and in which the 40 ar and the 39 ar were very well correlated yielding well defined ages averaging 3 38 0 08 10 5 years two leucites contained ar with lower bulk 40 36 ratios 400 and in at least two release steps from these leucites the 40 ar 36 ar ratio was significantly lower than atmospheric despite the uncertainty in the composition of the trapped component these two leucites have ages that do not differ significantly from the ages of the other leucites for the biotites it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic 40 ar from trapped 40 ar and therefore the calculated ages are not as precise as those of the leucites in three cases the biotite age agrees with the age of the cogenetic leucite but in the remaining two cases discordant ages are obtained suggesting caution when using biotites as quaternary age indicators rb sr measurements on leucite biotite and pyroxene separates hand picked from each of three tuff samples yielded a dispersion in 87 sr 86 sr as large as 16 parts in 10 4 and 87 rb 86 sr as high as 218 for leucites and permitted the determination of internal isochron ages the ages obtained range from 3 8 0 2 10 5 to 3 3 0 2 10 5 years and are in good agreement with the 40 ar 39 ar ages on the leucites the data for each tuff sample yield a well defined uniform initial 87 sr 86 sr however different tuffs show small differences in initial 87 sr 86 sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs these measurements demonstrate the possibility of dating quaternary materials by both the 40 ar 39 ar method and the rb sr method the observation of concordant ages with a precision of a few percent represents a powerful tool in quaternary stratigraphy

an integrated stratigraphical approach to the middle pleistocene succession of the sessano basin molise italy abstract an integrated stratigraphical approach was applied to the sessano sedimentary succession molise investigated through an 80 m core and an 11 m trench section the lithostratigraphical study allowed reconstruction of the palaeoenvironmental conditions during the existence of a palaeo marsh based on tephrostratigraphical analysis the significant volcanoclastic inputs recorded along the sessano succession were correlated to the explosive activity of the nearby roccamonfina volcano and in particular to the high k series hks period of activity lasted from 546 to 364 ka the 40 ar 39 ar age of 437 9 1 9 ka obtained at the base of the trench section which stratigraphically overlaps the core succession confirms the tephrostratigraphical model and indicates that the marsh dried up during the glacial phase of marine isotope stage mis 12 on this basis the glacial interglacial cycle recognised in the core through pollen analysis was ascribed to mis 14 and 13 while the interglacial phase identified at the core base was tentatively ascribed to mis 15 the duration of the marshy phase was estimated between 130 and 150 ka the floral composition of the pollen spectra shows a more modern character with respect to coeval pollen records from campania confirming that the heterochronous disappearance of the last subtropical elements from italy had a latitudinal character

pre andean orogenesis in the coast ranges of central chile an intense structural metamorphic event of probable permian triassic age is inferred from petrologic and field observations of low grade schists from the coast ranges of central chile between latitudes 38 s and 43 s relict almandine rich garnet biotite and aluminous amphibole in mafic schists are replaced by a pervasive assemblage consisting of albite chlorite epidote actinolite sphene quartz stilpnomelane white mica iron oxide chemographic analysis of phase compatibilities based on electron microprobe analyses suggests a regional retrogradation from earlier upper greenschist and or epidote amphibolite to lower greenschist facies a regional northwest southeast trending fabric within the terrane was imposed during the lower greenschist metamorphism which involved widespread reintroduction of fluids into the system this major orogenic event is thought to be the culminating episode in a subduction related accretionary process along the late paleozoic margin of south america convergence may have been markedly oblique to the present north south trending continental margin

magma emplacement into the lemptegy scoria cone chaine des puys france explored with structural anisotropy of magnetic susceptibility and paleomagnetic data the lemptegy volcano is a small monogenetic scoria cone located in the chaine des puys auvergne france which erupted about 32 000 years ago a first edifice lemptegy 1 formed during a trachybasalt eruption as a group of satellite vents of the puy de gouttes scoria cone a second trachyandesitic edifice lemptegy 2 formed soon after and completely covered lemptegy 1 with an 80 m high breached cone since 1946 the lemptegy volcano has been quarried for scoria and today offers unprecedented three dimensional exposure of the subvolcanic plumbing system to map the internal flow architecture of the plumbing system and to study the subvolcanic deformation of lemptegy 2 structural mapping petrographic observations anisotropy of magnetic susceptibility ams rock magnetic and paleomagnetic data were collected field structural mapping and thin section study of tension gashes riedel shears striations as well as ductile shear zones and bubbles allow the direction and sense of the magma flow to be determined twenty ams sites were established in ten dikes one to four sites in each dike with 504 specimens analyzed and 479 specimens used to infer magma flow patterns structural data the maximum susceptibility axis k 1 and the imbrication of the magnetic foliation k 1 k 2 planes indicate both upward and downward sense of flow as well as flow toward and away from the central vent rock magnetic experiments reveal that a cubic fe ti oxide phase likely low ti titanomagnetite is the principal magnetic phase carrying both the remanence and anisotropy paleomagnetic data from some sites yield statistically distinct at the 95 confidence level remanence directions while at other sites the data are indistinguishable at the 95 confidence level the paleomagnetic results observed steeply tilted scoria layers internal unconformities and faults show that as each dike was emplaced it displaced earlier dikes evidencing subvolcanic deformation the lemptegy 2 volcano shares similarities in terms of inferred eruption style and structures with other scoria cones such as cerro negro nicaragua and thus provides an excellent field laboratory to investigate active scoria cones world wide

evolution of the late paleozoic accretionary complex and overlying forearc magmatic arc south central chile 38 41 s constraints for the tectonic setting along the southwestern margin of gondwana stratigraphic structural metamorphic and geochronologic studies of basement rocks in the andean foothills and coast ranges of south central chile 39 41 s suggest a protracted late paleozoic to middle mesozoic deformational and metamorphic history that imposes important constraints on the tectonic development of the southwestern gondwana margin in the study area the late paleozoic paired metamorphic belt coeval magmatic arc and overlying triassic sedimentary units preserve a record of late carboniferous to early permian subduction and arc magmatism subsequent deep exhumation of the western series subduction complex and diminished uplift and erosion of the eastern series arc forearc region by the late triassic late paleozoic structural elements and metamorphic assemblages formed during early subduction and arc magmatism collectively referred to as dl are largely erased in the western series by the dominant d2 schistosity and lower greenschist grade metamorphism d1 structural features as well as original sedimentary textures are relatively well preserved in the less penetratively deformed eastern series the regional distribution of late paleozoic arc magmatism suggests that the late paleozoic convergent margin deviated from a n s trend north of this area to a nw se trend near this latitude and faced an open marine environment to the southwest a transition from f2 isoclinal folding to more open larger scale f3 folds interpreted as change in ductility during differential uplift of the western series is not apparent in the eastern series despite a lesser degree of uplift during the main exhumational d2 event delineation of unconformities and u pb dating of detrital zircons and intrusions into the eastern series allow tighter constraints to be placed on timing of uplift and denudation of the eastern series than on that in the western series a regional unconformity exposed in the lake district that separates more highly deformed eastern series lithologies from late triassic shallow marine to continental deposits suggests that substantial uplift also affected the inner forearc and magmatic arc region during the d2 event we propose that dextral oblique convergence initiated during the middle permian along this segment of the gondwana margin resulted in the transpressional uplift and juxtaposition of high pressure temperature p t western series against low p t eastern series lithologies and culminated with deposition of late triassic continental to shallow marine coarse clastic sedimentary rocks in fault bounded strike slip basins adjacent to the exhumed western series large scale dextral transpression and northward displacement of the accretionary complex during late permian to late triassic time along the chilean margin of gondwana are synchronous and kinematically compatible with widespread regional transpression extension and silicic magmatism inboard of the southern gondwana margin at this time we thank c mpodozis m gardeweg and j munoz of the servicio de geologia y mineria de chile sernageomin for their support of this work fruitful discussions with n blanco f herve h moreno c mpodozis and f munizaga have aided in our understanding of the geology of the region the hard work by the staff of sernageomin s puerto varas office is graciously appreciated we thank j d walker and w r van schmus at the university of kansas for allowing mwm use of their u pb and mass spectrometer facilities and j vargas and the staff of sernageomin s geochemistry laboratory for their assistance in this project f munizaga allowed us to cite an unpublished 40ar 39ar date we thank g ya nez for access to aeromagnetic data t kato wishes to thank w g ernst comments by i dalziel s kay and v ramos helped clarify ideas presented in this paper and are greatly appreciated this work is dedicated to our friend and colleague alberto campos c who died in a climbing accident on calbuco volcano 1996

mineralogy and magnetic behavior of pyrrhotite from a 260 c section at the ktb drilling site germany the ultradeep bore hole of the german continental deep drilling program ktb reached a depth of 9100 m and in situ temperatures of about 260 c offering an unique opportunity to study natural pyrrhotite an integrative approach using optical methods electron microprobe analysis x ray diffraction transmission electron microscopy see posfai et al 2000 and temperature dependent magnetic susceptibility measurements were used to characterize pyrrhotite types as a function of lithology and depth we found a lithology controlled distribution of pyrrhotite types to a depth of 8080 m with ferrimagnetic monoclinic 4c pyrrhotite metal content 46 0 to 47 2 at as the dominant magnetic phase in gneisses and metabasic rocks in the gneisses a second pyrrhotite type with higher metal concentrations 46 9 to 48 2 at and antiferromagnetic behavior also occurs at depths greater than 8080 m in situ temperature 230 c antiferromagnetic pyrrhotite predominates in all lithologies that 4c pyrrhotite does not occur below 8080 m suggests that 4c is unstable above 230 c in these rocks instead of 4c a 5c type with a ferrimagnetic structure occurs below 8080 m thermomagnetic experiments indicate that the metal poor weiss type pyrrhotite is stabilized by oxygen that causes the formation of magnetite during heating from our observations on natural pyrrhotites we suggest that the magnetic transition is related to the growth of ordered na pyrrhotite domains to single domain size

magma flow revealed by magnetic fabric in the okavango giant dyke swarm karoo igneous province northern botswana to determine the magma flow direction of the giant 179 ma okavango dyke swarm of northern botswana we measured the anisotropy of magnetic susceptibility ams of 23 dykes dykes are located in two sections shashe and thune rivers which are about 300 km and 400 km from the presumed magma source respectively the nuanetsi triple point we collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction about half of the magnetic fabric in the dykes is inverse i e with the magnetic foliation perpendicular to the dyke plane lateral flow to the west and vertical flow is in evidence in the shashe section however the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the shashe section across the thune section a poorly defined imbricated magnetic foliation also suggests lateral flow to the west

comparing magnetic and magmatic fabrics to constrain the magma flow record in la gloria pluton central chile abstract this contribution illustrates a case study of a pluton la gloria pluton lgp where magnetic and magmatic fabrics are locally decoupled we compare the magmatic fabric with the available magnetic fabric data to explore their abilities and elucidate the magma flow record of lgp results indicate that magnetic controlled by multi domain magnetite and magmatic fabrics are generally consistent throughout lgp foliations define an axisymmetric pattern that gradually changes from vertical near lateral margins to less steep in the pluton interior whereas lineations are subhorizontal following the elongation direction of the pluton however samples at the pluton center show marked differences between both fabrics magnetic fabrics indicate subhorizontal magnetic lineations and foliations and magmatic fabrics indicate subvertical lineations and foliations both magnetic and magmatic fabrics are interpreted to record strain caused by magma flow during thermal convection and lateral magma propagation at the transition between low and high crystallinity stages we suggest that fabrics acquisition and consistency were determined by shear conditions pure simple shear rates ratio and the orientation of the magma flow direction with respect to a rigid boundary critical crystalline region of the pluton magmatic fabric differs at the center of the pluton because pure shear is dominant and ascendant flows are orthogonal to the horizontal rigid boundary lgp represents a whole scale partly molten magma reservoir where both thermal convection and lateral propagation of the magma are recorded simultaneously this study highlights the importance of characterizing both fabrics to properly interpret magma flow recorded in plutons

composite magmatic magnetic fabrics evidences late ams in syn tectonic dikes in the monteiro sume plutonic volcanic complex ne brazil abstract interpretations of flow fabrics in dikes using anisotropy of magnetic susceptibility ams are based on the orientation of the magnetic fabric recorded next to the dike walls however uncertainties remain when ams is sensitive to parameters such as grain size and composition of the magnetic carriers and deformation of the magma we study subvolcanic andesite to dacite dikes associated with granite plutons that intrude the basement rocks the dikes have magnetic susceptibilities mostly between 10 3 and 10 2 si carried by fe ti oxides ams shows fabrics that would be consistent with a lateral movement of the magma however the anisotropy of anhysteretic remanence aar and silicate shape preferred orientation spo indicate that the melt moved upward based on the steeply plunging lineations detected by aar and spo furthermore the obliquity of the aar foliations in the dike margins indicates a component of transtension resolved in the dike walls as there is no evidence for internal deformation of titanomagnetite ams would record its late growth in the tectonic extension direction the magnetic subfabric concealed by late ams therefore documents an earlier tectonic episode and evidence the limits of using just ams directions to constrain the emplacement of syntectonic magmas

late paleozoic serpentinites and mafic schists from the coast range accretionary complex central chile their relation to aeromagnetic anomalies regional structural trends within the poorly exposed metamorphic basement complex of central chile between 39 30 and 41 30 s latitudes are evident on total intensity aeromagnetic maps field and laboratory measurements of magnetic susceptibility indicate that anomalies are related to relic magnetite and rare ilmenite phase concentrations in serpentinites and lower greenschist facies mafic schists

inception of a devonian subduction zone along the southwestern gondwana margin 40ar 39ar dating of eclogite amphibolite assemblages in blueschist boulders from the coastal range of chile 41 s coarsely crystalline residual blueschist boulders from the coastal range of south central chile 41 s contain relict eclogite amphibolite assemblages that provide evidence of pre carboniferous hig

discerning magmatic flow patterns in shallow level basaltic dykes from the ne rift zone of tenerife spain using the anisotropy of magnetic susceptibility ams technique discerning magmatic flow patterns in shallow level basaltic dykes from the north east rift zone of tenerife spain using the anisotropy of magnetic susceptibility ams technique

internal flow structures in columnar jointed basalt from hreppholar iceland ii magnetic anisotropy and rock magnetic properties internal flow structures in columnar jointed basalt from hreppholar iceland ii magnetic anisotropy and rock magnetic properties

characterisation of grain size shape and orientation of plagioclase in the rooi rand dyke swarm south africa abstract magmatic type a fabric co exists with dyke orthogonal type b fabric in both the plagioclase and opaque grain fractions in dykes of the rooi rand dyke swarm rrds we present new data from the rrds pertaining to the size shape texture and orientation of plagioclase texturally the samples range from intersertal to sub ophitic and phenocrystic plagioclase phyric more than 90 of plagioclase grains are

rb sr and k ar ages on minerals at temperatures of 300 400 c from deep wells in the larderello geothermal field italy rb sr and k ar ages have been obtained on six biotites two muscovites and one hornblende from samples of micaschist gneiss and amphibolite of lower paleozoic to precambrian age at a depth exceeding 2 000 m in basement rocks of the larderello travale geothermal region most of the data cluster in the range 2 5 3 7 ma revealing the existence of a pliocene thermal event to which the origin of the field may be attributed the resulting duration of the larderello geothermal field is unexpectedly long in the basement levels of the two wells examined unstabilized minimum temperatures of 290 and 380 c were measured all the biotites show almost complete 40ar and 87sr retention at the measured well temperatures petrologic evidence stilpnomelane stability and experimental data activation energies and diffusion coefficients also favour a closure temperature above 400 c for rb sr and k ar in biotites in agreement with recent direct experimental determinations

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

recurrence of volcanic activity along the roman comagmatic province tyrrhenian margin of italy and its tectonic significance 1 the roman comagmatic province rcp developed along the nw se striking extensional tyrrhenian margin of italy since middle pleistocene and includes ultrapotassic small eruptive centers and large volcanic districts by means of a review and statistical analysis of a large data set of radiometric ages we demonstrate that in the rcp major eruptive cycles occurred at different volcanic districts almost simultaneously and with an almost constant recurrence time our analysis started with the alban hills volcanic district where a detailed 40ar 39ar chronology of the eruptive cycles has been recently achieved and then extended to include a database of 264 radiometric ages of the rcp volcanics published since 1980 statistical treatment of the data set shows a marked non poissonian behavior of volcanic activity through the whole magmatic province with climax of each major eruptive cycle occurring simultaneously at the different districts approximately every 48 kyr the concomitant eruption of many volcanic districts spreading over a 250 km long chain implies a control mechanism acting at a regional scale after discussing the possible relationship between tectonics sea level changes and volcanic activity we hypothesize that constant rates of slab retreat during middle pleistocene could have been a key factor controlling the rate of magma upwelling and the major cycles of recharge of the magma chambers as a working hypothesis we propose that the duration of the cycles may be related to 1 progressive weakening of the crust due to magma underplating or 2 cyclic injection of convective plumes into the mantle wedge

systematic retrograde metamorphism of sillimanite staurolite schists new salem area massachusetts the new salem retrograde metamorphic zone occupies an approximately triangular area covering 26 km 2 in sillimanite staurolite grade rocks largely pelitic schists of the devonian littleton formation acadian prograde metamorphism at 6 kbar and 600 c produced the common assemblage quartz muscovite biotite garnet staurolite iunenite albite graphite sillimanite pyrrhotite systematic hydration of the prograde assemblages by an influx of h 2 o produced the retrograde assemblages chlorite k feldspar celadonitic musco vite sphene anatase pyrite common and chlorite chloritoid musco vite anatase sphene rare retrograde sillimanite out staurolite out and garnet out isograds have been mapped representing the completion of important retrograde hydration reactions these isograds plus the thicknesses of chlorite rims on garnets 0 and 0 2 mm can be used to subdivide the area into six concentric retrograde metamorphic zones designated r1 to r6 during retrograde metamorphism the exchange reactions k na mn fe mg and fe mg proceeded while more or less consistent tie lines between the sheet silicates and ilmenite were maintained biotite and chlorite became more fe rich owing to staurolite garnet and ilmenite breakdown biotite chlorite and ilmenite became more mn rich owing to garnet breakdown coexisting biotite and muscovite became more k rich during biotite breakdown muscovite also became progressively more celadonite rich chlorite biotite and ilmenite are homogeneous in single thin sections muscovite is zoned with respect to m 2 m 2 al iv and k k na ratios which are higher in muscovite rims there is no evidence for re equilibration of staurolite garnet or albite during retrograde metamorphism temperature and pressure estimates for retrograde metamorphism are poorly constrained to 3 5 kbar and 280 c each kilometre of vertical extent of the retrograde zone required an influx of 0 26 km 3 of h 2 o at stp water may have been derived from a large volume of surrounding rock and concentrated in the study area by a pressure shadow effect around the north end of the prescott intrusive complex

a 90 000 200 000 yrs marine tephra record of italian volcanic activity in the central mediterranean sea abstract a detailed tephrochronological study was undertaken in three deep sea cores collected in the tyrrhenian and ionian seas the age and the origin of the marine tephra were inferred from oxygen isotope records of foraminifera and from major element compositions of glass shards seventy one eruptions were detected in the time interval 90 000 200 000 yrs during which the volcanoes of the roman and campanian regions and of the southern italy were in activity this is attested by the consistency of the geochemical compositions of both marine and terrestrial deposits most of the marine tephra consisted in trachytes and phonolites characterizing a roman and campanian origin several tephra were proposed as key horizons for proximal and distal sediments among them one tephra originating from mount etna 149 300 yrs and five tephra from pantelleria island 130 000 yrs 163 600 yrs 192 500 yrs 197 400 yrs and 198 400 yrs were northerly dispersed several other key horizons originated from the campanian or roman provinces were detected as far as 1000 km from the vents

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

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