magnetic susceptibility anisotropy strain and progressive deformation in permian sediments from the maritime alps france abstract the relationships among magnetic susceptibility anisotropy finite strain and progressive deformation have been studied in permian red shales and slates of the maritime alps southeastern france these rocks contain deformed reduction spots which serve as finite strain indicators the magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements the magnetic fabric changes progressively from oblate to prolate and back to oblate as deformation increases the quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined they differ between the less and more deformed areas perhaps due to a change in deformation mechanism accompanying an increase in metamorphism the relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control

palaeomagnetism of the ordovician dolerites of the crozon peninsula france summary in order to obtain a lower palaeozoic pole for the armorican massif and to test the origin of the ibero armorican arc the ordovician dolerites of the crozon peninsula have been palaeomagnetically studied the samples show a multicomponent magnetization which has been revealed by af and thermal demagnetization and thoroughly investigated with rock magnetic experiments polished section examinations and k ar dating four groups of directions have been recognized often superimposed on each other in an individual sample one component d has always the lowest blocking temperatures and coercivities and is considered to be of viscous origin acquired recently in situ or in the laboratory during storage two components a and b are interpreted to be of secondary origin and to correspond to the observed k ar age distribution between 300 and 190myr these ages represent the time interval between two regional thermo tectonic events associated with the hercynian orogeny and the intrusion of dykes related to the early opening of the central atlantic ocean and the bay of biscay a fourth component c could be of ordovician or younger palaeozoic age it is not clear whether the age of the magnetization is pre or post folding but a prefolding age would yield a direction of magnetization similar to ordovician results from the iberian peninsula the latter interpretation suggests a fairly high palaeolatitude which is in agreement with a glacio marine postulated for sediments overlying the dolerite sills

oxygen isotopic composition of individual olivine grains from the allende meteorite abstract the oxygen isotopic compositions of individual olivine grains from the allende meteorite were determined by ion microprobe techniques a variety of olivine grains were analysed refractory forsterite grains 26 wt rims and twenty eight individual matrix olivine grains 21 41 wt feo in an oxygen three isotope plot refractory forsterite grains 17o 11 18 o 8 fall on the allende mixing line but are significantly less enriched in 16o than spinel and pyroxene from ca al rich inclusions cais possibly indicating a later formation of forsteritic olivine cores of isolated olivine grains with somewhat higher feo contents iogs have oxygen isotopic compositions similar to those of the refractory forsterites 17 o 12 and 18 o 9 both types of olivine grains have oxygen isotopic compositions significantly different from those of chondrules this clearly demonstrates that these olivine grains are not the result of crystallization of chondrule melts fayalite rich rims of the iogs are less enriched in 16o 17 o 3 18 o 0 than the forsteritic cores arguments are presented that this supports a condensation origin of the rims from a gas phase less enriched in 16o than the gas phase from which the forsteritic cores had formed matrix olivines have oxygen isotopic compositions similar to fayalite rich rims however there are variations in the oxygen isotopic composition of matrix olivines that exceed analytical uncertainties suggesting a more complex origin

the palaeozoic variscan oceans revisited abstract geological evidence supported by biogeographical data and in accord with palaeomagnetic constraints indicates that one ocean models for the variscides should be discarded and confirms instead the existence of three gondwana derived microcontinents which were involved in the variscan collision avalonia north armorica franconia and thuringia subdivided by a failed vesser rift and south armorica central iberia armorica bohemia all divided by small oceans in addition parts of south eastern europe including adria and apulia are combined here under the new name of palaeo adria which was also peri gondwanan in the early palaeozoic oceanic separations were formed by the break up of the northern gondwana margin from the late cambrian onwards most of the oceans or seaways remained narrow but much like the alpine cenozoic oceans gave birth to orogenic belts with hp uhp metamorphism and extensive allochthons the saxo thuringian ocean between north and south armorica and the galicia moldanubian ocean between south armorica and palaeo adria only the rheic ocean between avalonia and peri gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism and its north western arm closed before or during the emsian in europe ridge subduction under the northernmost part of armorica in the emsian created the narrow and short lived rheno hercynian ocean it is that ocean and not the rheic whose opening and closure controlled the evolution of the rheno hercynian foldbelt in south west iberia south west england germany and moravia czech republic devonian magmatism and sedimentation set within belts of early variscan deformation and metamorphism are probably strike slip related the first arrival of flysch on the forelands and or the age of deformation of foreland sequences constrains the sequential closure of the variscan seaways galicia moldanubian in the givetian saxo thuringian in the early famennian rheno hercynian in the tournaisian additional mid to late devonian and partly early carboniferous magmatism and extension in the rheno hercynian saxo thuringian and galicia moldanubian basins overlapped with variscan geodynamics as strictly defined the early carboniferous episode was the start of episodic anorogenic heating which lasted until the permian and probably relates to tethys rifting

paleomagnetism and magnetic fabric of the deformed redbeds of the cap de la chevre formation brittany france abstract we present the results of a palaeomagnetism and magnetic fabric study in six sampling sites of the lower ordovician redbeds of the cap de la chevre formation crozon peninsula france measurement of anisotropy of magnetic susceptibility ams shows that these beds are affected by a weak but significant tectonic strain during the hercynian folding of the armorican palaeozoic cover samples containing small elliptical reduction spots allowed us to check the control of strain on ams site mean ams data were then used to provide strain estimates at each site analysis of remanent magnetization through thermal and chemical stepwise demagnetization procedures allowed us to isolate three magnetic components component a defined in the lower unblocking temperature range is a recent overprinting which clusters around the present day dipole field direction components b and c defined respectively in the intermediate and higher unblocking temperature ranges appear to be pretectonic in age the fold test however is inconclusive for both because of the significant dispersion still remaining after the simple tilt correction assuming that at least a part of this dispersion is due to strain we have applied the strain removal technique using ams derived strains at each site this results in a clustering of component c from four sites allowing recovery of the primary ordovician paleomagnetic direction d 261 i 71 95 11 in contrast unstrained component b shows a small circle distribution containing the formation mean of unstrained component c the hypothesis that component b and c distributions arise from beds being tilted during a long term magnetization acquisition in the ordovician field is discussed taking into account the extensional geodynamic context during deposition of lower palaeozoic redbeds in brittany

relationship among o mg isotopes and the petrography of two spinel bearing compound chondrules the petrological properties and o and al mg isotopic compositions of two spinel bearing chondrules from the allende cv chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry a coarse spinel grain in a barred olivine bo chondrule is less enriched in 16o 17o 5 17o 17o 0 52 18o whereas smaller spinel grains in a plagioclase rich chondrule member of a compound chondrule are extremely 16o rich 17o 17 and the spinels have a strongly serrated character the petrological features and 16o enrichments of the spinels in the plagioclase rich chondrule indicate that spinels originating in coarse grained ca al rich inclusions cais were incorporated into chondrule precursors and survived the chondrule forming event the degree of 16o excesses among minerals within each chondrule is correlated to the crystallization sequences this evidence suggests that the o isotopic variation among minerals may have resulted from incomplete exchange of o isotopes between 16o rich chondrule melt and 16o poor nebular gas aqueous alteration also has changed the o isotope compositions in the mesostasis the feldspathic mesostasis in the bo chondrule shows a disturbed mg al isochron indicating that the bo chondrule experienced secondary alteration while plagioclase in the plagioclase rich chondrule member of the compound chondrule shows slight 26mg excesses corresponding to 26al 27al 0 4 6 4 0 2 10 6 nepheline formed by secondary alteration shows no detectable excess the al mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the allende parent body

paleomagnetic direction obtained by strain removal in the pyrenean permian redbeds at the col du somport france abstract in order to investigate the possibility and limitations of paleomagnetic works within strained regions a paleomagnetic study related with strain analysis has been conducted in the deformed pyrenean permian redbeds in the col du somport area paleomagnetic sampling together with strain estimates have been conducted in 6 sites through a fold the results obtained by measuring the orientation and axial ratios of elliptical reduction spots show that 1 the shale beds have undergone a penetrative strain 2 the sandy beds can be regarded as tectonically unstrained with reduction spots flattened in the bedding showing that they recorded the compaction it is shown that the total strain recorded in the slaty beds probably results from the superimposition of tectonic strain upon the compaction fabric the paleomagnetic study shows that the primary pretectonic magnetization is widely overprinted by a secondary syn or post tectonic magnetic component as both components appear to be carried by hematite pigment their separation using classical demagnetization procedures has been difficult a characteristic remanent magnetization chrm has however been determined when possible as the hardest component in demagnetization curves then the chrm direction distributions are represented in stereographic density plots although these chrm directions exhibit a clear tendency towards se declinations and shallow inclinations characteristic of permian paleomagnetic field direction for the iberian plate the tilt correction does not induce a clustering of these directions strain is inferred to be responsible for this situation assuming that both pretectonic magnetization directions and bedding planes closely follow the material plane and line strain response model of march 1 an attempt has been made to remove the effect of strain upon the remanent magnetization it is shown that when using a reconstructed tectonic strain tensor i e the total strain tensor as measured in the field corrected for an estimated compaction we obtain a significant clustering of chrm directions the computation of the relevant vgp gives a pole position 210 5 e 42 0 n compatible with the reference apwp for the iberian plate it is therefore inferred that the strain removal technique is a usable tool in order to obtain paleomagnetic results within such strained rocks

mathematical removal of regional ductile strains in central brittany evidence for wrench tectonics abstract hercynian deformation in central brittany is responsible for folding of paleozoic and earlier sediments with concurrent development of cleavage essentially e w and vertical and stretching lineation essentially horizontal in the past the regional motion was inferred to be a n s shortening more recent studies emphasize e w transcurrent motions associated with major crustal faults values and orientations of ductile strain at 29 localities in central brittany show that the regional deformation is basically a horizontal plane strain with local three dimensional perturbations we first remove the perturbations rendering the problem two dimensional then a selected domain is divided into an array of finite elements parallel to strain trajectories using strain values extant at element centroids each element is unstrained independently of its neighbours resulting gaps and overlaps are minimized using a least squares technique which allows elements to translate and rotate rigidly thus we obtain the configuration of the domain in the undeformed state to within an unknown but constant rigid rotation the domain has undergone a heterogeneous dextral simple shear with vertical shearing plane striking n125 and horizontal shear direction a band of low strain running through guer is flanked by two zones of more intense strain the banding is approximately parallel to a major transcurrent fault in the area the south armorican shear zone

26al 26mg chronology and oxygen isotope distributions of multiple melting for a type c cai from allende abstract disequilibrium oxygen isotopic distributions of ca al rich inclusions cais correspond to multiple melting events in the solar nebula 26al 26mg systematics may be applicable for age differences among such melting events we have carried out a coordinated study of detailed petrographic observations and in situ oxygen and magnesium isotope measurements for a type c cai ek1 04 2 from the allende cv3 meteorite to determine the melting events and their ages the cai consists mainly of spinel anorthite olivine and pyroxene and has a core and mantle structure petrography of the core suggests that the crystallization sequence of the core minerals is from spinel anorthite olivine and to pyroxene the mantle has the same mineral assemblage as the core and shows incomplete melting and solidification textures oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral ccam line 18o 44 to 9 which indicates to preserve a chemical disequilibrium status in the cai spinel shows a 16o rich signature 18o 43 while anorthite is 16o poor 18o 8 olivine and pyroxene in the core have the same oxygen isotopic composition 18o 15 which indicates their equilibrium olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16o 18o 13 to 4 compared with the same minerals in the core the 26al 26mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes spinel is plotted on a line of 26al 27al 0 3 5 0 2 10 5 anorthite is plotted on a line of 1 5 10 7 and olivine and pyroxene in the core are plotted on a line of 1 7 10 6 plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core this study indicates that the ek1 04 2 type c cai underwent multiple heating events after the formation of its cai precursor the precursor cai was formed 0 4 myr after the formation of the solar system defined by canonical cai formation at least 1 6 myr after the precursor cai formation the cai was partially melted and the melt exchanged oxygen isotopes with surrounding 16o poor nebular gas 16o poor olivine and pyroxene in the core crystallized from the melt subsequently al rich chondrules accreted onto the cai and the cai experienced partial melting again and recrystallized to form the mantle the oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the allende parent body our study reveals that the cai had been retained in the solar nebula for at least 1 6 myr and underwent multiple melting events in the nebula and oxygen and 26al 26mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body

origin of plagioclase olivine inclusions in carbonaceous chondrites plagioclase olivine inclusions pois are an abundant group of chondrule like objects with igneous textures found in carbonaceous chondrites pois consist of plagioclase olivine pyroxene and spinel and cover a wide range of compositions between type c ca al rich inclusions cais and ferromagnesian chondrules pois are distinguished from cais by the absence of melilite lack of refractory siderophile rich opaque assemblages more sodic plagioclase and abundance of olivine and aluminousenstatite rare accessory minerals including armalcolite zirconolite rutile and sapphirine are found in several pois the petrographie and chemical properties of pois indicate that they are not condensates or evaporative residues but formed by melting or partial melting of pre existing solids seven of fourteen pois contain isotopically fractionated mg and despite their textures these pois are not isotopically homogeneous spinel is the major carrier of fractionated mg in six pois the magnitude of mg isotopic fractionation fmg of spinel ranges from 8 to 11 amu among the pois but f mg is always either positive or negative within an individual pol within a single inclusion f mg of coexisting spinel is not constant but varies by up to 7 amu isotopic fractionation of mg in silicates is less common and always positive f mg 0 pyroxenes in two pois and plagioclase in a third contain isotopically heavy mg radiogenic 26 mg is rare in pois as only two inclusions show evidence of excess 26 mg with 26 mg 27 al 4 10 6 the preservation of isotopic heterogeneity in objects whose igneous textures suggest crystallization from a homogeneous melt implies that melting was incomplete allowing survival of relict phases a comparison of the essential characteristics of pois and cais suggests that the major processes leading to formation of pois including condensation dust gas fractionation aggregation of chemically and isotopically disparate materials and partial melting are common to most cais and chondrules we present a scenario for the formation of these objects and conclude that the homogeneity of the final assemblage cai poi or chondrule is primarily a reflection of the thermal history rather than the nature of precursor materials

compaction induced inclination shallowing of the post depositional remanent magnetization in a synthetic sediment summary a synthetic sediment comprised of kaolinite distilled water and either equidimensional or acicular magnetite was given a post depositional remanent magnetization pdrm by being stirred in a magnetic field this sediment was compacted under pressures which varied continuously from 0 to 0 14 mpa in a water tank consolidometer and to higher pressure steps 2 53 mpa in a standard soil consolidometer compaction took place in the same magnetic field in which the sample was given its pdrm the compaction caused shallowing of the sample s magnetic inclination this shallowing was found to be a function of the sample s initial magnetic inclination and the degree of sample compaction tan i 1 aav tan lo where ir is the remanent inclination after compaction av is the volume change i is the initial magnetic inclination and a is an empirically derived constant the data show a maximum inclination shallowing of 12 for an initial inclination of 54 in good agreement with the maximum inclination shallowing predicted by the above equation we propose an electrostatic mechanism to be the cause of the inclination shallowing in this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain s surface as the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post compaction nrms implying that the smaller higher coercivity grains are most affected by the compaction process these data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected

grain scale deformation and the fold test evaluation of syn folding remagnetization the paleomagnetic fold test examines for pre syn or post folding re magnetization generally passive limb rotation is used to restore folded units to their original position and the position of the fisherian maximum deter mines the relationship between folding and magnetization in a more complete analysis the reorientation of material lines in different fold models is used however the magnetic vec tor should not be considered a material line as it represents the sum of vectors in individual magnetic carriers and does not cross grain boundaries therefore the role of grain scale deformation mechanisms was examined using two end mem bers rigid body rotation spin and homogeneous strain from this study it is concluded that 1 the fisherian maximum will in general occur before complete unfolding 2 the sign of the inclination may change during unfolding and consequently cross over unfolding is not representati ve of syn folding remagnetization 3 statistical analysis will not necessarily produce significant results a number of recommendations are given which include stepwise unfol ding and unplunging should be carried out routinely local rather than regional fold tests should be performed both limbs and the hinge of a single folded layer should be exa mined in order to determine the active deformation mecha

ca al rich inclusions amoeboid olivine aggregates and al rich chondrules from the unique carbonaceous chondrite acfer 094 i mineralogy and petrology based on their mineralogy and petrography 200 refractory inclusions studied in the unique carbonaceous chondrite acfer 094 can be divided into corundum rich 0 5 hibonite rich 1 1 grossite rich 8 5 compact and fluffy type a spinel melilite rich 50 3 pyroxene anorthite rich 7 4 and type c pyroxene anorthite rich with igneous textures 1 6 ca al rich inclusions cais pyroxene hibonite spherules 0 5 and amoeboid olivine aggregates aoas 30 2 melilite in some cais is replaced by spinel and al diopside and or by anorthite whereas spinel pyroxene assemblages in cais and aoas appear to be replaced by anorthite forsterite grains in several aoas are replaced by low ca pyroxene none of the cais or aoas show evidence for fe alkali metasomatic or aqueous alteration the mineralogy textures and bulk chemistry of most acfer 094 refractory inclusions are consistent with their origin by gas solid condensation and may reflect continuous interaction with sio and mg of the cooling nebula gas it appears that only a few cais experienced subsequent melting the al rich chondrules arcs 10 wt bulk al2o3 consist of forsteritic olivine and low ca pyroxene phenocrysts pigeonite augite anorthitic plagioclase spinel feni metal and crystalline mesostasis composed of plagioclase augite and a silica phase most arcs are spherical and mineralogically uniform but some are irregular in shape and heterogeneous in mineralogy with distinct ferromagnesian and aluminous domains the ferromagnesian domains tend to form chondrule mantles and are dominated by low ca pyroxene and forsteritic olivine anorthitic mesostasis and fe ni metal nodules the aluminous domains are dominated by anorthite high ca pyroxene and spinel occasionally with inclusions of perovskite have no or little feni metal and tend to form cores of the heterogeneous chondrules the cores are enriched in bulk ca and al and apparently formed from melting of cai like precursor material that did not mix completely with adjacent ferromagnesian melt the inferred presence of cai like material among precursors for al rich chondrules is in apparent conflict with lack of evidence for melting of cais that occur outside chondrules suggesting that these cais were largely absent from chondrule forming region s at the time of chondrule formation this may imply that there are several populations of cais in acfer 094 and that mixing of normal cais that occur outside chondrules and chondrules that accreted into the acfer 094 parent asteroid took place after chondrule formation alternatively there may have been an overlap in the cai and chondrule forming regions where the least refractory cais were mixed with fe mg chondrule precursors this hypothesis is difficult to reconcile with the lack of evidence of melting of aoas which represent aggregates of the least refractory cais and forsterite grains

palaeomagnetism of the ordovician moulin de chateaupanne formation vendee western france summary a palaeomagnetic study of the moulin de chateaupanne formation reveals two characteristic components of magnetization the first one isolated by thermal treatment at intermediate temperatures is strongly resistant to chemical leaching its direction d 217 i 25 ag5 5 5 and the corresponding pole position lat 21 s long 320 e are typical of the hercynian overprints in the armorican massif the second component carried by fine grained haematite presents a steep downward direction d 228 i 81 ag5 6 pole position lat 34 n long 343 e already recognized in other ordovician rock units from the same area it is resistant to high temperature demagnetization treatment and is believed to be of chemical origin as assessed by a conglomerate test a mutual consistency test with the nearby thouars massif suggests that the tilt of the formation has a sedimentary origin and that the thouars massif has not suffered significant tilt during the hercynian orogenic events

snow and ice facies variability and ice layer formation on canadian arctic ice caps 1999 2005 1 time series of enhanced resolution data from the seawinds scatterometer aboard quikscat were used to map the distribution of snow and ice surface facies and ice layer formation in the percolation zone on ice caps in the queen elizabeth islands during the period 1999 2005 iterative self organizing data analysis classification of the mean postfreeze up biweekly average 0 signal for the 7 year period resulted in the delineation of four snow and ice surface facies interpreted as the percolation saturation superimposed ice and glacier ice zones analysis with national centers for environmental prediction national center for atmospheric research reanalysis reveals that changes in geopotential height in the troposphere 700 500 and 300 hpa and air temperature 700 hpa level are positively negatively correlated with area changes in the glacier ice percolation and saturation zones and changes in facies boundary elevation the change in biweekly averaged backscatter following the freeze up periods between successive autumns was used to map changes in the distribution of ice layers formed by meltwater percolation and refreezing in the snowpack within the percolation zone strongly positive air temperature anomalies at the 700 hpa level in 2001 and 2005 are consistent with extensive ice layer formation in the percolation zones of all ice caps such large interannual changes in ice layer formation are likely associated with large changes to the density profile of the snowpack and may be associated with surface elevation changes that are unrelated to changes in surface mass balance

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

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

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

a paleomagnetic study of mesozoic sediments from the junggar and turfan basins northwestern china mesozoic sedimentary rocks have been collected from the karamay area northwestern junggar block and in the turfan basin northeastern tarim block for a paleomagnetic study with the aim of testing the rigid nature of these blocks and to better constrain their paleogeographic relationships within central asia prior to the collision with india the results from the karamay area are disappointing lower cretaceous grey sandstones having apparently been recently remagnetized this remagnetization is carried by magnetite and yields highly clustered normal polarity directions both a medium mtc and a high htc unblocking temperature component are isolated from flat lying or weakly tilted jurassic red sandstones the mtc could be carried by maghemite and the htc is carried by hematite the two components are statistically identical and seem to correspond to an ancient magnetization however there is no fold test and comparison with previous data from junggar and the reference apwp of siberia does not allow us to decide between a middle jurassic primary magnetization and an upper jurassic or cretaceous remagnetization stronger constraints are obtained from upper jurassic to lower eocene fine grained red sandstones from anticlines that have been thrust southwards in the turfan area of south tien shan after the elimination of a recent overprint that is probably related to weathering a high coercivity high unblocking temperature component carried essentially by hematite is isolated all directions from the jurassic to the lower eocene which yield positive fold and reversal tests are statistically indistinguishable this is compatible with a primary origin for these magnetizations similar to what has previously been found in junggar tarim and western tibet the corresponding turfan pole is not statistically distinct from that of tarim for the same period in accordance with the notion of a large quasi rigid tarim block remaining attached to eurasia in a rather constant position and at a constant latitude throughout the second half of the mesozoic this new datum can therefore be included to derive a new mean pole for the tarim that is valid for the period from the middle jurassic to the end of the cretaceous 68 4 n 224 5 e a95 3 1

passive rotation of hematite during deformation a comparison of simulated and natural redbeds fabrics abstract a simulation model is used to generate pole figures assuming strain induced passive rotation of hematite crystals comparison of simulated and natural hematite fabrics allows us to distinguish between dominant passive rotation and other grain scale deformation mechanisms two natural examples are discussed redbeds from the french maritime alps and pyrenean redbeds fabrics indicate that passive rotation is the dominant reorientation mechanism of primary hematite grains during progressive deformation of redbeds

the effect of magnetic anisotropy on paleomagnetic directions in high grade metamorphic rocks from the juiz de fora complex se brazil the juiz de fora complex is mainly composed of granulites and granodioritic migmatite gneisses and is a cratonic basement of the ribeira fold belt which was formed during the last stage of brasiliano orogeny samples widely distributed over the studied region se brazil rio de janeiro state were collected for paleomagnetic and magnetic anisotropy determinations after measurement of anisotropy of low field magnetic susceptibility ams the same specimens were submitted to both alternating field for anisotropy of remanent magnetization arm determinations and thermal demagnetization for paleomagnetic analyses demagnetization processes allowed isolating an eastern steep positive inclination characteristic remanent magnetization chrm direction from the rocks the studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse grained titano magnetite and either fine grained titano magnetite or titanohematite grains respectively these magnetic minerals are responsible for both remanent direction and magnetic anisotropies even though for some specimens paramagnetic minerals are the carriers of ams fabric the chrm direction found for the rocks should represent the cooling phase of the brasiliano event in the area and it was acquired after the magnetic fabric was formed the ams and arm fabrics are coaxial and tectonic in origin and compare favorably with the mesoscopic scale fabrics in the adjacent areas however arm measurements have reached only low coercivity grains and it could be determined in few specimens then only an estimate of arm effect on chrms correction is shown the rocks are strongly magnetically anisotropic 50 and foliated the paleomagnetic directions from each specimen within a site tend to approach its ams or arm foliation kmax kint or armmax armmin plane these directions were corrected specimen by specimen from the sites taking into account the pole of ams or arm foliation kmin or armmin and the value of the degree of anisotropy p results show that even for specimens with high p the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens our data set suggests that ams is a powerful tool to correct the chrm deviation from paleofields in rocks with complex magnetic mineralogy such as those from the juiz de fora complex

micro distribution of oxygen isotopes in a refractory inclusion from the allende meteorite abstract isotope ratios of oxygen have been measured on a fassaite crystal and the surrounding area of a coarse grained cai from the allende meteorite using secondary ion mass spectrometry values of 18 o smow for fassaite spinel anorthite grains and alteration products are 27 5 38 2 5 6 and 4 respectively which are consistent with the results obtained on mineral separates the o isotopic distributions are homogeneous either within individual single crystals of fassaite and anorthite or among the spinel grains within the analytical error no gradual variation in o isotopic composition was detected even at the boundary between different phases or at cracks inside a phase with a spatial resolution of 10 m these observations strongly suggest that the partial exchange of o isotopes in the minerals in the cai with the surrounding gas through diffusion is an unlikely origin for the observed inter mineral heterogeneity of o isotopes in the allende cais

a paleomagnetic study of tectonically deformed red beds of the lower glarus nappe complex eastern switzerland a paleomagnetic study at 15 sites 315 samples in permo triassic red beds of the helvetic nappes of eastern switzerland isolated stable characteristic and secondary remanent directions thermal demagnetization and isothermal remanent magnetization experiments suggest pigmentary hematite carries the characteristic direction secondary directions are scattered but may have been acquired during the alpine deformation in the oligocene and miocene the characteristic directions also show large between site scatter anisotropy of magnetic susceptibility indicates that the hematite grains are flattened within the slaty cleavage and that there is a north south lineation this anisotropy pattern implies that the magnetic grains were realigned by the alpine deformation it is not possible to explain the characteristic remanent magnetization deviations on a site by site basis but the site mean directions are streaked along a great circle the down dip direction of which is approximately parallel to the regional fold axes 60 20 this spread could be explained by a penetrative simple shear deformation associated with the nappe internal deformation

strain magnetic fabric and paleomagnetism of the deformed red beds of the pont rean formation brittany france a joint study of strain anisotropy of magnetic susceptibility ams and paleomagnetism has been conducted in the deformed red beds of the ordovician pont rean formation brittany france analysis of results from 13 sampling sites shows that homogeneous strain estimated from the shape of elliptical reduction spots in each site controls the development of a magnetic fabric during the hercynian folding of this formation the good correlation between strain and ams tensors is interpreted as mainly arising from a strain induced progressive rotation of planar hematite that tends to align within cleavage plane this rotation of magnetic carriers in turn induces a rotation of net measurable pretectonic remanent magnetization toward cleavage because of the high angle between the initially subvertical ordovician remanent magnetization and the regionally subhorizontal shortening direction this deviation of the pretectonic magnetization is achieved without any scattering but with a probable clustering effect upon within site initial dispersions for the same reason the overall distribution of mean site pretectonic magnetization vectors is clustered at the in situ deformed state this results in an anomalous behavior of the classical fold test that could erroneously lead to assignment of a syntectonic to post tectonic age to the magnetizations finally the strain removal technique based upon the passive behavior hypothesis is applied to the deviated vectors it allows recovery of the initial direction of these pretectonic ordovician magnetizations d 235 i 75 95 6 5

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

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

ordovician palaeogeography with new palaeomagnetic data from the montagne noire southern france a joint palaeomagnetic and ar 40 ar 39 study has been performed on two olistolithic blocks from the cabrieres wildflysch in the montagne noire region of the massif central in france there andesitic volcanic and volcaniclastic rocks of llanvirn early caradoc age ca 470 458 ma occur despite extensive secondary alteration destruction of the dominant magnetic mineral phase and ar 40 ar 39 whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss a positive fold test and the presence of dual polarities suggest that a primary ordovician magnetisation has mostly survived this is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small relatively unaltered magnetite grains probably hosted in silicates still carry the original in this case ordovician remanence the new data show that the montagne noire region was located at high southerly latitudes 68degrees 19 15 during the mid ordovician this latitude represents the location for nw gondwana of which the massif central was a part palaeomagnetic data from all the central european massifs and terranes demonstrate a close link to the gondwana margin during the lower and middle ordovician

paleoremanence dispersal across a transpressed archean terrain deflection by anisotropy or by late compression the quetico belt is an archean metasedimentary terrain sandwiched between greenstone subprovinces a single regional schistosity is subparallel to magnetic foliation defined by anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence aarm aligned micas are the primary cause of ams aligned pyrrhotite is the sole cause of aarm each magnetic subfabric is due to minerals that grew in overlapping intervals in the strain history their slight obliquity may be due to crystallization during noncoaxial strain magnetic fabric ellipsoids suggest varying severity of flattening very strong flattening on the northern boundary less severe on the southern boundary and least severe in the interior the interior and the southern boundary of the belt stretched horizontally and the shortening direction varies gradually from ns in the north to nnw in the south the paleomagnetic record carried by pyrrhotite shows characteristic remanences dispersed around a great circle subparallel to the regional schistosity and magnetic foliations late stress effects rotated paleoremanences into this pattern we discount deflection of the paleofield by anisotropy the pattern of dispersal indicates uniaxial oblate flattening of the region with nnw sse shortening late in the metamorphic cooling history 300 c this postdates schistosity ams and aarm fabrics but it is controlled by the same kinematic pattern

identification of ordovician block tilting in the hercynian fold belt of central brittany france field evidence and computer models abstract the significance of the brioverian lower ordovician unconformity in central brittany has been discussed by many authors a new interpretation in terms of extensional tectonics is presented based on structural sedimentological and stratigraphical data however because sedimentary formations have been strongly folded by hercynian deformation extensional structures are obscured and difficult to recognize to identify the normal faults the geological map is compared with computer models of the folded tilted blocks it is shown that major normal faults had a mean n70 trend dipping toward the se with an almost regular spacing of 7 8 km

remanence rotation due to rock strain during folding and the stepwise application of the fold test numerical modeling was conducted to determine the effects of simple models of folding strain on prefolding fisherian distributions of magnetization i found that the constant volume simple shear associated with flexural slip flexural flow folding can rotate a prefolding magnetization to have a maximum precision parameter k value at partial unfolding and thus appear to be synfolding in age if the magnetization is treated as a passive line marker this behavior only occurs for steep initial magnetizations the initial homogeneity of the fisherian precision parameters from each limb is degraded by this type of deformation and may be used to detect a strain modified remanence if the magnetization is carried by rigid mechanically active ellipsoidal or spherical particles a maximum k value at partial unfolding will occur for any initial inclination with the special case that rigid spheres will be rotated to cause a maximum k value at 50 unfolding for any initial inclination homogeneity of limb precisions is not affected in this case a discontinuous model of strain during folding in which pressure solution removes material at an axial planar spaced cleavage and causes limb dips steeper than bedding dips could also produce a maximum k value at partial unfolding strain and remanence studies of flexural slip folding in the allentown formation dolomite and the bloomsburg formation red sandstones and mudstones suggest that 1 the rigid grain model may be a more realistic model of magnetic particle behavior during shear strain and 2 about one quarter to one third of the theoretically predicted simple shear strain occurs in these rocks this amount of strain cannot explain the synfolding behavior observed in the allentown formation but could explain the maximum k values at 80 90 unfolding seen in the bloomsburg formation and perhaps other appalachian clastic rocks

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

origins of al rich chondrules clues from a compound al rich chondrule in the dar al gani 978 carbonaceous chondrite abstract aluminum rich chondrules are one of the most interesting components of primitive chondrites because they have characteristics that are similar to both ca al rich inclusions cais and ferromagnesian chondrules however their precursor and formation history remain poorly constrained especially with respect to their oxygen isotopic distributions in this study we report on the petrography mineralogy oxygen isotope ratios and rare earth element compositions of a sapphirine bearing al rich chondrule sarc in the ungrouped chondrite dar al gani dag 978 the sarc has a complex core mantle rim texture while both the core and the mantle are mainly composed of al rich enstatite and anorthite with minor amounts of mesostasis these regions are distinguished by the presence of fe rich spinel and sapphirine in the core and their absence in the mantle the rim of the sarc consists mainly of fe rich olivine enstatite and fe ni metal spinel and some olivine grains in the sarc are 16o rich with 17o values down to 20 and 23 respectively enstatite sapphirine and most olivine grains have similar 17o values 7 which are lower than those of anorthite and the mesostasis including augite therein 17o 3 mesostasis from both the core and mantle have group ii rare earth element ree patterns however the core mesostasis has higher ree concentrations than the mantle mesostasis these observations provide a strong indication that the sarc formed by the melting and crystallization of a mixture of materials from group ii cais and ferromagnesian chondrules both spinel and olivine with 16o rich features could be of relict origin the 16o poor isotopic compositions of most components in al rich chondrules can be explained by oxygen isotopic exchange between the melt and 16o poor nebular gas 17o 7 during melting in chondrule forming regions whereas the anorthite and mesostasis could have experienced further oxygen isotopic exchange with a relatively 16o poor reservoir 17o 3 on the parent body likely during fluid assisted thermal metamorphism during the same thermal metamorphism event spinel olivine some enstatite and the mesostasis experienced mg fe exchange to various extents

oxygen isotope evidence regarding the formation of spinel bearing chondrules oxygen isotope compositions of individual mineral grains in two spinel bearing chondrules have been determined by secondary ion mass spectrometry o isotope ratios of spinel grains in a plagioclase rich chondrule were extremely 16 o rich d 18 o d 17 o 35 indicating close relationship to spinels in ca al rich inclusions cais whereas o isotope ratios of a spinel grain in barred olivine chondrule were less enriched in 16 o d 18 o d 17 o 10 the degree of 16 o excesses among minerals within each chondrule was parallel to the crystallization sequences these measurements suggest that some cai spinel grains were incorporated into the chondrule precursors and that the o isotopic variation among minerals may result in incomplete exchange of o isotopes between 16 o rich chondrule melt and 16 o poor ambient nebular gas the mesostasis having an o isotopic composition near the terrestrial fractionation line may have been somewhat affected by the aqueous alteration in the parent body 1999 elsevier science b v all rights reserved

origin of magnetite in oxidized cv chondrites in situ measurement of oxygen isotope compositions of allende magnetite and olivine magnetite in the oxidized cv chondrite allende mainly occurs as spherical nodules in porphyritic olivine po chondrules where it is associated with ni rich metal and or sulfides to help constrain the origin of the magnetite we measured oxygen isotopic compositions of magnetite and coexisting olivine grains in po chondrules of allende by an in situ ion microprobe technique five magnetite nodules form a relatively tight cluster in oxygen isotopic composition with delta 18o values from 4 8 to 7 1 and delta 17o values from 2 9 to 6 3 seven coexisting olivine grains have oxygen isotopic compositions from 0 9 to 6 3 in delta 18o and from 4 6 to 7 9 in delta 17o the delta 17o values of the magnetite and coexisting olivine do not overlap they range from 0 4 to 2 6 and from 4 0 to 5 7 respectively thus the magnetite is not in isotopic equilibrium with the olivine in po chondrules implying that it formed after the chondrule formation the delta 17o of the magnetite is somewhat more negative than estimates for the ambient solar nebula gas we infer that the magnetite formed on the parent asteroid by oxidation of metal by h2o which had previously experienced minor o isotope exchange with fine grained silicates

the early palaeozoic break up of northern gondwana new palaeomagnetic and geochronological data from the saxothuringian basin germany early palaeozoic volcanic and sedimentary rocks from the saxothuringian basin franconian forest northern bavaria have been subjected to detailed radiometric and palaeomagnetic studies in order to determine the tectonic environment and geographic setting in which they were deposited two hand samples were collected from the as yet undated pyroclastic flow deposits for 207pb 206pb age dating radiometric results for these samples obtained by the single zircon evaporation technique are identical within error and the mean age of all measured grains is 478 2 1 8 ma n 11 this age is considered to be primary and firmly constrains the eruption of the ignimbrites and formation of the subaqueous pyroclastic flows as having occurred in early ordovician arenig times palaeomagnetic studies were carried out on these early ordovician volcanic rocks and also on the biostratigraphically dated late ordovician ashgillian dobra sandstones the volcanic rocks carry up to three directions of magnetisation the poorly defined low and intermediate unblocking temperature directions are thought to represent secondary overprint directions of post ordovician age the high temperature component however identified at temperatures of up to 580 c is of mixed polarity and passes the fold test with 99 confidence the overall mean direction after bedding correction is 189 76 95 11 6 k 44 7 25 samples five sites and is considered to be primary and early ordovician in origin it yields a palaeo south pole at 24 n and 007 e which translates into palaeolatitudes of 63 21 7 17 3 s for the saxothuringian terrane samples from the late ordovician dobra sandstone are generally very weakly magnetised a high temperature d component of magnetisation can be identified in some samples and yields a mean direction of 030 58 95 18 5 k 25 7 15 samples four sites after bedding correction the arenig palaeomagnetic results indicate high palaeolatitudes but separation from northern gondwana this is in basic agreement with data from elsewhere in the armorican terrane assemblage all of which suggest high southerly palaeolatitudes in the early ordovician the geochemical signatures of these rocks indicate emplacement in an extensional environment these new data therefore are interpreted as marking the onset of rifting of saxothuringia from the north african margin of gondwana and the start of the relative northward migration of the saxothuringian terrane although the late ordovician palaeomagnetic results presented here are only poorly constrained they suggest an intermediate palaeolatitude for saxothuringia in ashgillian times in good agreement with late ordovician palaeomagnetic data from the barrandian

strain reorientation of hematite and its bearing upon remanent magnetization using a computer based nume rical model we suggest that passive reorientation of hematite planar particles induced by strain perturb significantly the remanent magnetization carried by the hematite the evolution of its direction and dispersion in a progressively incre asing strain can be correlated with the evolution of passive 1ines in real rocks the amount of reorientation can be appro ched by measuring one of the thr ee follo wing tensors strain tensor orientation tensor of some hematite cristallographic axes or magnetic susceptibility tensor from examples taken in the maritime alps and in brittany we emphasize the possible correlations between these tensors orien tation of principal axes and axial ratios fina1ly we use our best estimate of strain to correct the paleomagnetic results by the strain removal technique we have already used in the maritime aips cogne and perroud 1985 when applied to the preliminary paleom gnetic results ob tained from ordovician deformed redbeds of brittany it allows to recover a conclu sive primary magnetization direction although the characteristic magnetizations

paleomagnetism of the late ordovician thouars massif vendee province france a paleomagnetic study of the late ordovician thouars massif reveals very steep components of magnetization directed upward and downward and with thermally discrete blocking temperatures of 550 to 580 c this characteristic magnetization yields a mean direction with declination inclination 340 83 with k 27 and 95 7 based on 15 sites the south paleopole is located at 34 n 5 e a partial magnetic overprint of hercynian age is observed in sites close to the south armorican shear zone the 15 sites are spread over some 20 by 30 km and the internal consistency of the site mean directions indicates that differential tilting of parts of the massif has not occurred since it appears to be unlikely that the entire massif was tilted we infer that the characteristic direction is primary and that the paleopole adequately represents the local magnetic field in the late ordovician the paleopole for the thouars massif supports previously obtained results from other ordovician rocks in hercynian europe but it is the first result to pass modern reliability criteria the high ordovician paleolatitudes of the armorican massif other areas in hercynian europe and north western africa indicate that gondwana and armorica were adjacent until late ordovician time low paleolatitudes for late devonian time suggest that armorica underwent rapid northward motion at a rate of about 10 cm yr during the silurian and earliest devonian armorica is thought to have collided with north america and the baltic shield russian platform before or during the early devonian forming the old red continent the collisions produced the caledonian and acadian orogenic belts