the infiltration of fine matrices into coarse grained alluvial sediments and its implications for stratigraphical interpretation traps designed to simulate the framework gravel of natural streams and installed in a coarse grained channel allow the identification and differentiation of the processes governing the infiltration of fines into bed interstices vertical patterns of matrix distribution reflect the relative importance of variables acting at both the surface of the channel bed and within the bed sediments differences in grain size and therefore pore size of the framework exercise a strong controlling influence an armour layer of coarse particles superimposed on comparatively finer sub surface bed material common to many coarse grained rivers is shown to encourage the clogging of near surface pores with matrix fines below the obstruction interstices may be protected from incursion and the gravel framework is often left open this is the reason for patchy concentrations of matrices often reported in the stratigraphical literature in contrast a uniform size grading of the bed sediment or an upwards fining leads to a more uniform packing of the interstices with matrix flood history is shown to be an important determinant of both the size distribution and the spatial distribution of matrices and is useful in providing a dynamic explanation of the variability of matrices observed from place to place in ancient deposits

sediment supply and relative size distribution effects on fine sediment infiltration into immobile gravels 1 we present results and analyses from flume experiments investigating the infiltration of sand into immobile clean gravel deposits three runs were conducted each successive run with the same total sediment feed volume but a 10 fold increase in sand feed rate the highest sand feed rate produced less sand infiltration into the subsurface deposits than the other two runs which had approximately equivalent amounts of sand infiltration experimental data combined with simple geometric relations and physical principles are used to derive two relations describing the saturated fine sediment fraction in a gravel deposit and the vertical fine sediment fraction profile resulting from fine sediment infiltration the vertical fine sediment fraction profile relation suggests that significant sand infiltration occurs only to a depth equivalent to a few median grain diameters of the bed material

the relative contribution of near bed vs intragravel horizontal transport to fine sediment accumulation processes in river gravel beds understanding flow sediment interactions is important for comprehending river functioning fine sediment accumulation processes in particular have key implications for ecosystem health however the amount of fines generated by intragravel flows and later accumulated in gravel streambeds may have been underestimated as the hydraulic related driving transport mechanisms in play are not clearly identified specifically the relative contribution of fines from upper vs lower sediment layers in gravel beds is not well understood by recreating flooded and dewatered conditions in an experimental flume filled with natural sediment we estimated such contributions by observing and collecting intragravel transported fines that were later accumulated into a void in the middle of the sediment matrix near bed transport in the upper sediment layers named brinkman load during flooded conditions accounted for most 90 of the accumulated fines intragravel transport in the lower sediment layers named interstitial load was the sole source of transport and accumulation during dewatered conditions with steeper hydraulic gradients interstitial load accounted for 10 of the total transport during flooded conditions although small such estimations demonstrate that hydraulic gradient transport in the lower sediment layers occurs in spite of the contradicting analytical assessments we provide a case study to challenge the traditional approaches of assessing intragravel transport and a useful framework to understand the origin and relative contribution of fine sediment accumulation in gravel beds such knowledge will be highly useful for the design of monitoring programs aiding river management particularly in regulated rivers

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

effects of experimental deformation on the remanent magnetization of sediments abstract changes in direction and in intensity of the remanent magnetization of sediments were studied during experimental deformation leading to an axial shortening of 20 30 the remanence was produced before shortening by anhysteretic magnetization in an alternating field of 90 mt superimposed on a parallel steady field of 1 mt at various inclinations with respect to the sample the samples were sandstones marls and volcanic sediments the results show that the direction of magnetization is stable during the deformation within the experimental precision the intensity of magnetization decreases during shortening by an amount which depends on the type of rock about 30 for sandstones and about 5 for marls and volcanic sediments the march model does not account for the data and a discontinuous deformation must be invoked the evolution of the young modulus of sandstones during cyclic loading and unloading shows first a decrease in relation to crack density growth then an increase showing a localization of the fracturation and finally a deformation by displacement of microblocks an interpretation is given after a discussion of the results obtained by other authors with plasticine and artificial clays the amplitude of the decrease and of the rotation of the remanent magnetization could be a function of the size of the magnetic grains compared with the scale of the fracturation during non homogeneous processes of deformation

strain factorization by identical finite increments and its use abstract progressive deformation of layered rocks up to their present finite strain and bedding attitudes is simulated by an identical increment path model a deformation matrix connecting the initial and deformed states is formulated with the conceptual help of a hypothetical three step deformation path two sequential non coaxial rigid body rotations bring an original horizontal bed to a pre strain attitude a pure shear follows the rotations thus the deformation matrix is calculated by premultiplying the matrices representing the three stages of deformation sixteen identical tectonic deformation increments are then obtained by four times sequentially extracting the square root of that deformation matrix the method has been applied to the campbell block a late jurassic fault bounded turbidite sliver in the northern sierra nevada california the block was deformed into a train of folds during the nevadan orogeny fourteen oriented samples were collected for march strain measurements based on the strains and bedding attitudes the block was divided into 11 deformation bands of three types by applying the proposed model to the campbell block it was found that the deformation style of the block consists of a non coaxial deformation path the regional strain is accommodated among the several deformation bands by strain partitioning the collective deformation indicated by this model is transpressional

the rotation of magnetic grains abstract it has been questioned whether magnetic remanence rotates as a rigid marker or as a passive marker with no material properties during tectonic strain the remanence of a rock is actually the sum of the moments of individual grains so we must first understand their rotation simple shear provides a simple strain history which may be used to distinguish between the two extreme possibilities a passive marker cannot rotate through the shear plane but a rigid marker can this is a useful criterion to distinguish between the two extreme models however for reasonable strains 10 the passive model is successful in explaining the rotation of these grains even though their behaviour is mechanistically closer to that of a rigid marker this explains the success of field studies in which the remanence of redbeds has been de strained using the hypothesis of passive behaviour notwithstanding the reality that the natural iron oxide grains do not rotate in that manner

reply to comment on magnetic fabrics crystallographic preferred orientation and strain of progressively deformed metamorphosed pelites in the helvetic zone of the central alps quartenschiefer formation by carl richter lothar ratschbacher and wolfgang frisch

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

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

diachronous folding and cleavage in an intraplate setting central high atlas morocco determined through the study of remagnetizations abstract remagnetizations are common in intraplate basins when remagnetizations occur at an intermediate stage between different tectonic processes they can be used for paleo geometrical reconstructions and relative dating of different structures this has a particular interest in geological frameworks where other geological time markers are absent in order to apply this methodology it is necessary to calculate the regional remagnetization direction and subsequently to use this reference direction to restore the attitude of the beds at the moment of remagnetization acquisition in this work we use this methodology for dating a pervasive cleavage whose time of formation is controversial and the associated structures in the central high atlas morocco paleomagnetic directions from 64 sites were used to calculate the remagnetization direction d 330 9 i 35 1 a n 6 107 which is coincident with the albian cenomanian ca 100 m a expected direction for nw africa this direction was used to restore the mesozoic paleo geometry of beds allowing us to analyze bedding orientation cleavage and folding relationships between the present day and the cretaceous geometry after restoration we conclude that the development of cleavage post dates remagnetization being in relation with cenozoic basin inversion however the paleo geometry shows incipient folds at cretaceous times which can be related to an intra mesozoic compressional event

structure composition and bulk properties of upland stream gravels upland gravel bedded streams in the u k have received only scant attention from both hydrologists and sedimentologists but are worthy of further investigation the sedimentology of three small streams in teesdale in the pennines has been examined in detail grain size characteristics bedforms structure composition and packing characteristics of these deposits are described and compared where appropriate with published information it is argued that a fuller appreciation of gravel bed composition and morphology should eventually contribute to an improved understanding of sediment transport and deposition mechanisms and hence to improved accuracy in sediment transport and deposition estimates

vertical gradational variability of fines deposited in a gravel framework vertical gradational structures develop as sand infiltrates into static gravel beds understanding the vertical distribution of interstitial sand deposits will improve predictions of ecological suitability and hyporheic hydrodynamics a series of flume experiments was performed to investigate fine infiltration processes four sand distributions were introduced into flows over gravel beds after each experiment bed cores were extracted and analysed in vertical layers to examine the gradational trends with depth vertical trends of fine content were highly sensitive to the relative grain size distributions of the gravel bed and the introduced sand for experiments with d15gravel d85sand ratios 15 4 and larger unimpeded static percolation was observed where sand filled the voids relatively uniformly from the bottom of the gravel layer to the top experiments with ratios 10 6 and smaller bridged sand clogged a thin layer of gravel pores near the bed surface precluding subsequent infiltration interstitial sand deposits fined with depth of penetration for all experiments which was the result of three distinct but overlapping processes i granular sorting as particles fell through the substrate smaller material preferentially passed through the voids deeper into the gravel ii bed load sorting size segregation occurs in the wake of the leading bed form as smaller particles saltate further and settle first iii hydraulic sorting smaller sand was transported preferentially as suspended load filling the deep voids of the furthest flume positions downstream finally when the experiments that formed a bridge layer were replicated with higher bed shear stresses less interstitial sand deposition was observed higher shear stresses transported coarse particles downstream more efficiently causing bridge layers to form earlier and allowing less time for suspended load to settle into the deeper substrate pores before the pathways were closed

tectonic strain and paleomagnetism experimental investigation a porous clay rich sandstone with a natural remanent magnetisation shows complex changes in the orientation of the remanence vector when the sandstone is subjected to experimental deformation at 100 mpa room temperature and constant natural strain rates of 1 10 5 s 1 the paleomagnetic vector does not simply rotate away from the axis of compression when the rock is shortened perpendicular to bedding the remanence vector rotates toward the bedding as expected but the remanence also rotates toward bedding when the rock is shortened parallel to bedding moreover in some cases the remanence vector changes azimuth within the specimen during experimental deformation thus bedding anisotropy and the accompanying mineral fabric anisotropy are more influential than strain in controlling deflections of the paleomagnetic vector in this study this anisotropy also influences the post deformational behaviour of remanence 4 months after deformation some cores shortened perpendicular to bedding relaxed their remanence vectors back towards the pre deformational attitude whereas some other cores continued to show progressive changes intragranular recoverable elastic strains in the magnetic grains may be partly responsible for the experimentally induced deflections of remanence

deformation of remanent magnetism in a synthetic aggregate with hematite a hematite calcite aggregate bonded with portland cement is deformed triaxially at 200 mpa confining pressure and at a constant strain rate of 10 5 s 1 in some cases with a pore fluid pressure of 120 mpa this aggregate deforms in a ductile fashion at room temperature and is a good textural analog of naturally penetratively strained hematite bearing rocks the changes in inclination of an isothermal remanent magnetisation are indistinguishable from the behaviour expected of a homogeneously strained non material line rotating according to wettstein s equation this is encouraging for paleomagnetists who might wish to restore remanence in tectonically strained hematite bearing rocks an unknown combination of rigid grain rotation and intracrystalline stress must be responsible for rotation of the grains that carry remanence and thus for the changes in inclination of remanence the reason that wettstein s equation is approximately valid may be because the large aspect ratios of rigid hematite grains cause them to rotate nearly as quickly as a passive line p hydrostatic compaction wet deformation and dry deformation in that order are progressively more effective in reducing the intensity of remanence however wet deformation is more effective than dry deformation in reducing the intensity of the most coercive components presumably by spinning the finest grains in the cement matrix and scattering their individual moments p where multiple direction components of remanence are present deformation reduces the components with higher coercivities 70 mt more than those with lower coercivities in previous experiments on magnetite bearing samples the lower coercivity components were most reduced thus the natural tectonic deformation of an nrm may be expected to preserve selectively low coercivity components in hematite and high coercivity components in magnetite this may be used to predict the success of preservation of a characteristic nrm and conversely the success of overprinting by syntectonic remagnetisation

compaction induced inclination shallowing in synthetic and natural clay rich sediments experiments using single clay slurries consisting of acicular or equidimensional magnetite mixed with either kaolinite illite montmorillonite or chlorite were conducted to determine the effect of compaction on inclination a natural marine sediment and a reconstructed marine sediment made up of the clays used in our single clay experiments were also studied a laboratory postdepositional remanence imparted to the slurries by stirring had its inclination shallowed by compaction for both the single clay samples as seen previously anson and kodama 1987 and the natural and reconstructed marine sediments suggesting that our single clay sediments are valid analogues of natural sediments magnetic intensity also decreased by 30 50 during compaction the greatest decrease occurring for samples with the steepest initial inclination altering the ph of a slurry so that the magnetite would have a negative surface charge and be repelled by clay particles did not affect a slurry s inclination shallowing behavior suggesting that electrostatic attraction between magnetite and clay is probably not a major contributor to inclination shallowing however most of the inclination shallowing observed in our experiments occurs at low pressures when void ratios are high comparison with a clay microfabric compaction study mcconnachie 1974 suggests that volume loss at these pressures is accommodated by reorientation of clay particles or clay particle aggregates clay domains since pore spaces are large at these low pressures inclination shallowing is probably caused by magnetite particles either being attached to clay particles perhaps by van der waals forces or having been incorporated into clay domains alternating field demagnetization of compacted samples which contain magnetite with a range in grain sizes supports this model indicating that the smallest magnetic grains are slightly more shallowed by compaction than larger grains our results suggest that a sediment s clay content may be an important contributor to inclination shallowing

methods and applications of magnetic anisotropy a special issue in recognition of the career of graham j borradaile methods and applications of magnetic anisotropy a special issue in recognition of the career of graham j borradaile

remanence stability and magnetic fabric development in synthetic shear zones deformed at 500 c shear experiments were performed on magnetite bearing calcite aggregates to examine magnetic fabric development and remanence stability in a deforming system using elevated temperature and pressure to encourage deformation by crystal plastic processes samples composed of 1 wt pseudo single domain magnetite 1 2 m in a calcite matrix were created with either strong or weak initial fabrics and deformed in coaxial simple shear to strains up to 1 5 at constant strain rates between 6 10 5 and 1 10 4 s 1 at 500 c and confining pressure of 300 mpa samples were given weak field thermal remanent magnetizations prior to deformation demagnetization of postdeformation remanence reveals that a primary remanent magnetization can withstand deformation at pressures and temperatures approximately equivalent to greenschist facies metamorphic conditions on laboratory time scales but this stability is found to depend on the character of the predeformation fabric the origin of secondary remanence components acquired during deformation is uncertain but is likely to partially result from thermal viscous remagnetization complete postdeformation remagnetization in initially anisotropic samples appears to involve a stress softening or piezoremanent magnetization mechanism postdeformation anisotropy measurements show progressive changes in magnetic fabric strength with strain in the absence of a strong initial magnetic anisotropy magnetic fabric intensity increases linearly as a function of strain however deformation that overprinted an existing fabric results in an apparent decrease of the initial anisotropy at low strains followed by rapid increases in magnetic fabric strength with increasing strain our results underscore the important role that initial fabric can play in determining the character of deformation fabrics

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

simple shear experiments on magnetized wax hematite samples c a e s s laboratoire de geophysique interne uniuersite de rennes 1 f 35042 rennes francereceived july 1 1991 revision accepted may 28 1992abstractwe present the results of a series of simple shear experiments on hematite bearing paraffin wax samples homogeneousand continuous deformation was obtained up to a shear strain value of y 1 4 by deforming the samples in atemperature controlled bath and at a constant strain rate of 9x10 5 s r during deformation hematite plateletsprogressively rotate which has been checked by measuring the anisotropy of magnetic susceptibility ams principalsusceptibility directions tend to parallel the corresponding principal strain directions and ams intensity increases withincreasing strain this supports the idea that the hematite population develops a preferred orientation by progressive rigidrotation within the paraffin matrix before each deformation step an isothermal remanent magnetization irm was given to the samples within ara finite strain plane the containing the shear direction and normal to the shear plane with various initial angles to theshear direction ranging from 0 to 180 after each deformation step the irm was measured it showed no deflectiontowards the a finite strain direction in contrast it did show a systematic rotation within the ara plane always in the samesense as the rotation of strain axes e g counterclockwise in sinistral shear furthermore irm deviation is of the sameorder of magnitude for each initial direction from 0 to 180 this contradicts the passive behaviour model that haspreviously been shown to hold for irm in coaxially deformed samples finally we show that in the shear strain range of ourexperiments y 0 to 1 4 the deviation of irm depends only on the shear value and not on its initial direction and thatthis deviation is equal to the rigid rotation angle of the shear strain tensor

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

differences in paleomagnetic interpretations due to the choice of statistical demagnetization and correction techniques kapuskasing structural zone northern ontario canada abstract the kapuskasing structural zone ksz reveals a section through the archean lower crustal granoblastic gneisses our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical demagnetization and field correction techniques first where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular the commonly used statistical model is invalid fisher r a proc r soc a217 1953 295 any tendency to form an elliptical distribution indicates that the sample is drawn from a bingham type population bingham c 1964 distributions on the sphere and on the projective plane phd thesis yale university fisher and bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic bingham distribution it seems prudent to apply both models wherever a non fisher population is suspected and that may be appropriate in any tectonized rocks non fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks more dispersed sampling across tectonic structures may be more productive second from the same specimens mean vectors isolated by thermal and alternating field af demagnetization differ which treatment gives more meaningful results is difficult to decipher especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events in this study thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles third of more parochial significance tilt corrections may be unnecessary in the ksz because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off at approximately 15 km depth with archean geothermal gradients primary remanences were blocked after the foliation was tilted to rise on the thrust ramp therefore the rocks were probably magnetized in their present orientation tilting largely or entirely predates magnetization

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

remagnetisation of a rock analogue during experimental triaxial deformation abstract synthetic specimens containing pseudo single domain magnetite were given a univectorial anhysteretic remanent magnetisation they were deformed in a ductile manner at room temperature under a confining pressure of 150 mpa 1 5 kbar the strain rate was 10 5 s 1 and the maximum strain achieved was 23 shortening a weak 0 035 mt magnetic field was present in the pressure vessel during deformation hydrostatic compaction alone produced pressure demagnetisation of low coercivity components of remanence but did not change remanence directions macroscopic differential stress was needed to deform the specimens and this also permitted syndeformational remagnetisation for values of more than 20 mpa 200 bars this added new components of piezo remanent magnetisation prm approximately parallel to the direction of the magnetic field in the pressure vessel they were acquired in all coercivity intervals but the largest components were added below 35 mt the final remanence orientation and its spatial distribution of vector components were dictated by the relative orientations of the remagnetising field and the initial remanence prm contributions almost completely masked the changes related to strain induced grain rotation at these low strains the directions of the externally imposed stress strain axes may not significantly influence the direction of new remanence components prm acquisition occurs when differential stress exceeds a threshold value that probably depends on the previous stress history of the magnetite grains in part this may explain the origin of syntectonic remagnetisation in some naturally deformed rocks where the remanence is not reset by recrystallisation or thermal or chemical processes

effective radium 226 concentration in meteorites the analysis of noble gases in meteorites provides constraints on the early solar system and the pre solar nebula this requires a better characterization and understanding of the capture production and release of noble gases in meteorites the knowledge of transfer properties of noble gases for each individual meteorite could benefit from using radon 222 radioactive daughter of radium 226 the radon 222 emanating power is commonly quantified by the effective radium 226 concentration ecra the product of the bulk radium 226 concentration and of the emanation coefficient e which represents the probability of one decaying radium 226 to inject one radon 222 into the free porous network owing to a non destructive high sensitivity accumulation method based on long photomultiplier counting sessions we are now able to measure ecra of meteorite samples which usually have mass smaller than 15 g and ecra 0 5 bq kg 1 we report here the results obtained from 41 different meteorites based on 129 measurements on 70 samples using two variants of our method showing satisfactory repeatability and a detection limit below 10 2 bq kg 1 for a sample mass of 1 g while two meteorites remain below detection level we obtain for 39 meteorites heterogeneous ecra values with mean min max range of ca 0 1 0 018 1 30 bq kg 1 carbonaceous chondrites exhibit the largest ecra values and eucrites the smallest such values are smaller than typical values from most terrestrial rocks but comparable with those from archean rocks mean of ca 0 18 bq kg 1 an end member of terrestrial rocks using uranium concentration from the literature e is inferred from ecra for all the meteorite samples values of e for meteorites mean 40 4 are higher than e values for archean rocks and reported values for lunar and martian soils exceptionally large e values likely suggest that the 238u 226ra pair would not be at equilibrium in most meteorites and that uranium and or radium are most likely not uniformly distributed ecra of meteorites is correlated with e and seems to mainly reflect the gas permeability of the meteorite which could be one important property preserved in the meteorite of its parent body characterizing its history in space possibly modified by alteration shock metamorphism and eventually weathering on earth larger radon emanation values are associated with larger concentrations of the heaviest noble gases argon krypton xenon and larger 20ne 22ne and 36ar 38ar ratios suggesting earth s atmosphere contamination or solar wind implantation and probably a similar carrier phase such as q phase an unclear correlation is observed with 40ar which may rule out a purely radiogenic effect on radon emanation thus larger radon emanation suggests a larger capacity of collecting solar and terrestrial gases which should imply higher loss of gases generated in the meteorite and larger dispersion of pb u ratios for age determination this study provides the first quantification of natural radon 222 loss from meteorites and opens promising perspectives to quantify the relationship between pore space connectivity and the transfer properties for noble gases in meteorites and other extraterrestrial bodies

geomorphic controls on fine sediment reinfiltration into salmonid spawning gravels and the implications for spawning habitat rehabilitation abstract anthropogenic activities often increase the flux of fine sediment to fluvial environments in gravel bed streams the extent to which augmented fines loading causes the degradation of vital interstitial habitats is determined by factors controlling fines infiltration into channel substratum previous research suggests that substrate pore constriction size intensity of upwelling interstitial flow and the quantity of fines transported across the bed surface i e exposure dose are three important factors controlling substrate fines content few field studies have investigated the interactive effects of these physical factors we constructed 17 experimental redds in brook trout spawning microhabitats in a boreal forest stream in quebec canada to investigate the role of pore constriction size hyporheic flow and exposure dose on substrate fines content to simulate the effect of spawning in coarsening the substrates redds were partially cleaned of coarse sand and of all fine sediment

biases of submerged bulk and freeze core samples freeze coring and bulk sampling are routine methods used to sample subsurface spawning gravel under shallow water both methods have limitations freeze coring is not believed to representatively sample coarse grain sizes and the sample volumes are relatively small conversely when bulk sampling even within an enclosure some fine sediment is suspended and washed away from the sample this paper assesses the biases in sampling performance between the two methods and determines whether the loss of fines that occurs when bulk sampling could be predicted and thus corrected for at six riffles the spawning substrate was sampled under approximately 50 cm of water with a bulk sample and three adjacent freeze cores for each riffle data from the two samples were combined using the method of fripp and diplas 1993 and the resultant composite sample was compared with the original freeze core and bulk samples to assess the relative precision and biases of the two techniques on average the d50 of the bulk samples was 4 mm larger and a one third loss of the 16 mm compared with composite samples based on six samples taken from six riffles the amount of sediment finer than 0 5 mm lost using our bulk sampling technique with an enclosure appears to be predictable and correctable copyright 2005 john wiley sons ltd

structural implications of paleomagnetic data from pelvoux belledonne area french alps abstract paleomagnetic studies were carried out in triassic lias lavas from five new sites of the external zone of the french alps pelvoux belledonne area and compared with previous data in two sites magnetization seems to have been disturbed during alpine deformation but the paleomagnetic directions obtained in the other sites allow one to define the eastern boundary of the anticlockwise rotated taillefer block and the importance of local rotation along the aspres les corps faults the major anticlockwise rotation of taillefer is probably of ante nummulitic age but other rotations in the pelvoux belledonne area could have occurred during this same period or be of more recent age

re computing palaeopoles for the effects of tectonic finite strain abstract the pre messinian limestone cover 58 8 ma to the troodos ophiolite 88 ma of southern cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and in many sites stylolitic cleavage these define a gently n dipping foliation and an n plunging extension south vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south disturbed by faulting the magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de strained assuming continuum behaviour the optimum de straining 30 40 shortening in a flattening strain is compatible with the stylolitic cleavage development restores bedding to the near horizontal and restores the characteristic remanent magnetization vectors chrms to concentrated symmetrical fisherian distributions the strain corrected chrms yield more reasonable palaeopole locations for the lefkara and pakhna limestone and more uniform micro plate rotation rates corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the troodos plate since the creation of the late cretaceous 88 ma ocean lithosphere it did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at 1 5 ma 1 since 58 ma

experimental deformation of two component irm in magnetite bearing limestone a model for the behaviour of nrm during natural deformation abstract experimental room temperature triaxial deformation at 200 mpa 2 kbar and at a controlled strain rate of 10 5 s 1 is performed on three limestones with magnetic behaviour dominated by single domain magnetite permanent deformation is achieved mainly by twinning of calcite isothermal remanent magnetisation irm in the samples is reduced in magnitude especially when parallel to compressive stress the decrease in magnitude correlates slightly better with peak stress than with strain in accord with previous work in uniaxial tests indicating the role of stress in deformational demagnetisation remanences have their vector plots condensed after deformation particularly in the range 0 30 mt a two component irm is used as a simple model of a multi component natural remanent magnetisation nrm the resultant vector is deflected by the action of stress towards whichever components are least reduced by deformation in the case of magnetite bearing rocks this appears to be the most coercive components in the case of a natural deformation it is predicted that the effects of stress at any stage should be to remove progressively any low coercivity components added to an nrm by remagnetisation during tectonism especially those parallel to compressive stress

experimental stress remagnetization of magnetite abstract pseudo single domain psd and multidomain md magnetite grains remagnetize in weak magnetic field 30 t during experimental triaxial deformation the magnetite is supported in a calcite cement matrix minor remagnetization occurs with hydrostatic stress of 100 mpa significant remagnetization requires hydrostatic pressure of 150 mpa with differential stress of 5 mpa superposed on the sample intergranular differential stresses must be much higher due to amplification at grain asperities stress remagnetization does not need chemical or thermal energy new components of magnetic remanence are added parallel to the remagnetizing field however this only happens in grains or parts of grains with coercivities of remanence 60 mt grains with coercivities of 20 55 mt remember the primary magnetization and are not stress magnetized these coercivity limits do not depend on the differential stress or strain rate of the experiment the spatial distribution of vector components of remanence was isolated by af demagnetization after deforming a magnetized sample the components of remanence spread along a partial great circle between the initial remanence and the direction of the remagnetizing field the directions of the original magnetization and the remagnetizing field are the only factors controlling the course of the remagnetization path triaxial deformation shortened these samples by if the experimental results are transferable to nature it is possible that a pulse of excess crustal stress 25 mpa could partially remagnetize low dislocation density magnetite the experiments show that the directions of the remagnetizing field and the primary magnetization are the only variables that affect the demagnetization vector path we do not need to know the strain history coaxial or noncoaxial finite strains or principal strain directions to detect the paleofield directions unlike paleomagnetic vectors affected by large strains permanent changes in magnetic structure occur as a result of experimental deformation regardless of the differential stress the proportion of grains with remanence coercivities

block rotations along the strike slip finlay ingenika fault north central british columbia implications for paleomagnetic and tectonic studies structure developed in association with oblique motions on the dextral strike slip finlay ingenika fault fif in the mcconnell creek area north central british columbia is dominated by subvertical to vertical strike slip faults some of the faults cut the area into discrete fault bounded blocks several kilometres wide stress tensors for 24 sites were inverted from the regional cleavage which predates the block bounding faults the site mean stress tensors indicate that the fault bounded blocks were rotated clockwise about a subvertical axis during progressive oblique motions on the fif rotation varies systematically being maximal 58 7 3 3 close to the fif and minimal 0 0 1 6 about 20 km away paleomagnetic samples were collected from 13 sites in the widespread early jurassic to cretaceous plutonic rocks interpretable magnetic components of presumed late cretaceous age were obtained from six sites the observed paleopole is significantly different from the late cretaceous reference pole for cratonal north america cna after corrections for the local block rotations the precision is much improved k increased from 48 to 383 and the paleopole moves closer to the reference pole but is not coincident with it rotation about the eulerian pole for the best fitting small circle to the tintina trench and northern rocky mountain trench fault zone however brings the observed pole into coincidence with that for cna and requires 670 km of dextral displacement on the fault zone it is evident therefore that local structures associated with large dextral strike slip faults could account for at least part of the paleomagnetic disparity between some western parts of the canadian cordillera and cna

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

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

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

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

uranium in the silicate inclusions of stony iron and iron meteorites abstract the microdistribution of u has been studied using fission track techniques in eleven mesosiderites seven pallasites and four iron meteorites with silicate inclusions when concentrated u is usually found in phosphates merrillite and or chlorapatite as in stony meteorites the u concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also exhibit variations in the same meteorite uranium is found to be concentrated in merrillite 0 25 to 1 43 ppm in all the mesosiderites except bondoc where none was observed no u rich phase was identified in six of the seven pallasites in the seventh marjalahti there are merrillite grains with concentrations ranging from 0 06 to 0 14 ppm where observed the phosphates from silicate inclusions in the irons appear to have u concentrations similar to the mesosiderites

remanent magnetism and ductile deformation in an experimentally deformed magnetite bearing limestone abstract a limestone with pseudo single domain psd pure magnetite is given an isothermal remanent magnetization irm samples are deformed triaxially at 200 mpa 2 kbar confining pressure at room temperature and at a strain rate of 10 5 s 1 the limestone deforms in a ductile manner by twinning to produce sufficiently homogeneously strained specimens up to about 20 shortening the uniform saturation irm rotates away from the shortening axis approximately by an amount expected for the homogeneous strain of a non material line marker hydrostatic compaction alone shows that this is time dependent and compatible with progressive damage of the ferrimagnetic grains strain produces an increase in coercivity of the magnetite an increase in its saturation remanence and an increase in its anisotropy of magnetization with respect to irm these observations are compatible with reduction in effective grain size and change of shape of magnetite experimental deformation reduces the intensity of magnetisation chiefly by the removal of low coercivity components of remanence when there is a pure irm vector

using multiple bed load measurements toward the identification of bed dilation and contraction in gravel bed rivers 1 this study examines bed load transport processes in a small gravel bed river beard creek quebec using three complementary methods bed elevation changes between successive floods bed activity surveys using tags inserted into the bed and bed load transport rates from bed load traps the analysis of 20 flood events capable of mobilizing bed material led to the identification of divergent results among the methods in particular bed elevation changes were not consistent with the bed activity surveys in many cases bed elevation changes were significant 1 to 2 times the d50 even if the bed surface had not been activated during the flood leading to the identification of processes of bed dilation and contraction that occurred over 10 to 40 of the bed surface these dynamics of the river bed prevent accurate derivation of bed load transport rates from topographic changes especially for low magnitude floods this paper discusses the mechanisms that could explain the dilation and contraction of particles within the bed and their implications in fluvial dynamics bed contraction seems to be the result of the winnowing of the fine sediments under very low gravel transport bed dilation seems to occur on patches of the bed at the threshold of motion where various processes such as fine sediment infiltration lead to the maintenance of a larger sediment framework volume both processes are also influenced by flood history and the initial local bed state and in turn may have a significant impact on sediment transport and morphological changes in gravel bed rivers

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

further palaeomagnetic data from chitral eastern hindukush evidence for an early india asia contact the eastern hindukush forms part of an elongate belt central domain collage of cimmerian microcontinents that encircles the northern part of the indian subcontinent a gondwanan origin is commonly assumed for this belt but a laurasian origin for the chitral region has been argued on palaeontological talent and mawson 1979 and palaeomagnetic klootwijk and conaghan 1979 grounds the laurasian view was based on a pilot study we undertook of upper devonian pisolitic ironstones from a thrust sheet at kur gh spur in chitral preliminary results showed a characteristic magnetization component d 318 i 6 5 n 7 block samples k 14 95 16 5 indicating an equatorial palaeoposition this component was thought to be of primary origin and was interpreted in terms of a late devonian laurasian affinity of the kur gh spur rocks this controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the resh n kur gh b ni region of chitral and the primary origin of the characteristic magnetization component refuted thermal demagnetization of 333 block samples from middle to upper devonian variegated sediments permian quartz flysch permo triassic carbonates and mid cretaceous redbeds showed two interpretable components a softer component of recent origin a and a harder characteristic component b of both normal and reverse polarity whose mean direction d 314 1 i 6 0 n 4 thrust sheets k 198 2 95 6 5 is comparable to the characteristic component observed in our preliminary study however the universal presence of this component throughout the thrust pile proves its overprint origin which we attribute to initial india asia contact palaeomagnetic information pertinent to the controversy of a laurasian versus a gondwanan origin of the chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt hence we retract herewith our original conclusion of a late devonian laurasian affinity of the chitral region on the basis of the palaeomagnetic evidence the secondary component b comprises a suite of secondary magnetizations acquired at equatorial to low northern palaeolatitudes and is attributed to initial contact between greater india and southern asia component b has been observed previously in the himalayan tibetan region both north and south of the indus tsangpo suture zone identification is herein extended to the hindukush region north of the northern kohistan or shyok suture zone which is a western continuation of the indus tsangpo suture comparison of this suite of collision at tributed equatorial palaeolatitude data from the india asia convergence zone with new palaeolatitude constraints from the ninetyeast ridge on the northward movement of the indian plate constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of greater india indicates that initial contact between northwestern greater india and southern asia was established at or before the cretaceous tertiary boundary the overprint origin of component b at about this time is further supported by observations by zeitler 1985 on rocks from the sampled area in chitral of partially reset zircon fission track ages around 68 55 ma the nw se declination axis of component b indicates a 60 70 counterclockwise rotation of the sampled thrust pile with respect to eurasia and a counterclockwise rotation between 10 and 30 with respect to india some of the recent field components a show a comparable rotation and indicate that the tectonic activity that led to the formation of the hindukush pamir karakorum syntaxial zone has continued into recent times

tilting and transpression of an archaean anorthosite in northern ontario abstract an archaean anorthosite complex crops out over more than 300 km 2 at the junction between an archaean metasedimentary accretionary prism quetico belt and a volcanic greenstone terrain to the north wabigoon belt the anorthosite is feebly deformed and weakly foliated however anisotropies of low field susceptibility ams and anhysteretic remanence aarm define consistent magnetic fabric directions extending over subareas 25 km 2 in extent whereas magnetic fabrics cannot define strain magnitudes they normally provide reliable indications of finite strain orientations even where the fabrics developed by stress controlled nucleation during a progressive noncoaxial strain history the field schistosity s 1 being the oldest foliation rotated closest to the shear plane ams foliation due to preferred crystallographic orientation of silicates and pyrrhotite as well as preferred dimensional orientation of magnetite lies at a greater angle to the shear plane because it formed late then s 1 had less time to rotate the few measurements of the youngest aarm foliation caused by magnetite and pyrrhotite suggest that it may lie at a still greater angle to the shear plane thus s 1 and succeeding magnetic foliations provide a cryptic shear sense indicator they reveal relative motions along the terrain boundary that include transpression due to nw se shortening causing dextral e w shear with a north side up component palaeomagnetic data confirm a progressive northwards steepening of the anorthosite sills that is consistent with northwards subduction of the accretionary prism to the south

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

folding strain and graham s fold test in palaeomagnetic investigations summary graham s fold test is an important test of stability of magnet ization and is widely used in palaeomagnetic studies the change in dispersion of palaeomagnetic directions achieved by returning layers to their original orientation can give an indication of the relationship between the time of deformation and of magnetization acquisition however mcelhinny s criteria for application of the test are regarded by mcfadden jones to be inappro priate additionally simple rotation of directions about the strike of the layer may not be a sufficient procedure the rock type and style and amount of deformation may often appreciably affect the orientation of a vector or magnetic direction during deformation hence detailed observations are necessary to provide the basis of the steps used in applying the fold test apparent failure of the test may not indicate the absence of pre deformation stable magnetization in a rock although graham s fold test is important in palaeomagnetic and related tectonic investigations of deformed rocks application of the test may be very complex

experimental strain of isothermal remanent magnetization in ductile sandstone abstract a macroscopically ductile sandstone to which a homogeneous isothermal remanent magnetization irm was applied has been deformed at 150 mpa confining pressure and constant strain rate of 10 5 s 1 hydrostatic compaction does not produce a deflection of the irm vector although it is reduced in intensity pure shear producing shortening in the range 2 to 35 steadily reduces the intensity of magnetization but also homogeneously rotates the remanence vector toward the plane of flattening the amount of rotation is slightly less than that expected for a non material line undergoing homogeneous strain deformation selectively removes weakly coercive components of remanence as revealed by alternating field af demagnetization during deformation a weak deformational viscous remanent magnetization dvrm is acquired from the pressure vessel this is different from a conventional viscous remanent magnetization vrm in that it is not acquired when the specimen is subject to hydrostatic confining pressure alone even for periods three times longer than the longest deformation test

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

paleomagnetism and cross folding in a key area of the asturian arc spain in the northwest corner of spain paleozoic and older rocks outcrop in an arcuate pattern asturian arc concave toward the east the arc may be primary before sedimentation or secondary after sedimentation supporting a secondary origin are many structural features including two sets of folds one radial and one tangential to the arc the villasecino area was selected for a pilot study of the relationships between paleomagnetism and folding detailed structural mapping has shown that radial folds postdate tangential folds interference of the two sets has produced the doubly plunging villasecino anticline forty two oriented paleomagnetic cores were drilled at six sites distributed around the anticline all within red limestones griotte of the alba formation lower carboniferous the typical sample contains 95 by volume of fine grained calcite each grain being rimmed by very fine grained goethite and other iron bearing minerals the natural remanent magnetism nrm is of moderate intensity about 3 10 3 a m 1 readily measured and very homogeneous for each site within site scatter of vectors is low but the site mean varies significantly from site to site the angle between bedding and magnetization vector is nearly constant these data suggest that magnetization predated folding thirteen samples from six sites were subjected to alternating field demagnetization up to 50 mt and this showed the nrm to be very stable thermal demagnetization indicated a unique stable component with blocking temperature in the range 300 600 c chemical demagnetization was attempted using sodium sulphite results suggest that the carriers of magnetization are not iron hydroxides but probably hematite even after demagnetization a simple bedding correction does not yield a good grouping of magnetization vectors we have therefore tested five other more complex models of unfolding based on structural observations the statistically best grouping is produced by a model involving two successive fold phases where first folds have horizontal e w axes and second folds have vertical axes this is compatible with structural evidence after unfolding we obtain d 101 8 i 13 1 95 1 5 for the orientation of the earth s magnetic field in the area if we compare with the direction for the carboniferous of stable europe we get a difference in declination of 50 to 65 according to the model used for closing the bay of biscay possibly in association with tectonic evolution of the asturian arc

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

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

the volcano sedimentary evolution of a post variscan intramontane basin in the swiss alps glarus verrucano as revealed by zircon u pb age dating and hf isotope geochemistry the late palaeozoic glarus verrucano basin gvb glarus alps eastern switzerland formed as an intramontane graben in the aftermath of the variscan orogeny its fill the glarus verrucano consists of immature alluvial fan and playa lake deposits with intercalated bimodal volcanics basalts and rhyolites despite its importance for local and regional geology no modern sedimentologic or stratigraphic studies on the gvb exist by means of sedimentologic and geochronologic studies we reconstruct the volcano sedimentary evolution of the gvb it developed at the carboniferous permian boundary and experienced a first bimodal volcanic phase around 285 ma for the same time indications for temporarily humid climate in the otherwise rather arid early permian are demonstrated e g pyrite bearing sandstones during the middle and early late permian increasing aridity is indicated by playa deposits fanglomerates and subaerial ignimbrites which mark a second silicic volcanic phase at 268 ma the detrital zircon age spectra are dominated by late variscan ages and thus demonstrate that older sedimentary and metamorphic rocks once forming the variscan nappe edifice were already mostly eroded at that time finally some larger scale speculations are given which could indicate a causal connection between the widespread tectono magmatic mid permian episode and the local development of the glarus verrucano basin

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

key hydraulic drivers and patterns of fine sediment accumulation in gravel streambeds a conceptual framework illustrated with a case study from the kiewa river australia fine sediment processes in gravel beds may have significant impacts to overall river ecosystem function in addition to gravitational deposition horizontal intragravel transport has been recognized to influence fine sediment accumulation however the specific hydraulic mechanisms and origin of fine sediment movement are not clearly identified the purpose of this study was to investigate key hydraulic drivers and patterns of fine sediment accumulation using a conceptual framework to set the scene we implemented an experimental setup in a gravel lateral bar subject to irregular flow fluctuations in the kiewa river australia we installed nine sets of sediment collector pairs and piezometers into the gravel each pair included one horizontally and one horizontally vertically perforated collector mid range rather than peak flows covering the site in water drove fine sediment deposited in the collectors we estimated horizontal contribution to final deposition as 59 such contribution resulted from shear stresses 3 n m 2 promoting streamwise near bed turbulence at the water sediment interface during flooded conditions despite high subsurface hydraulic gradients intragravel transport in the lower sediment layers via darcy flow did not show any influence to fine sediment deposition our findings contribute to an improved understanding of fine sediment accumulation processes key for overall river ecosystem functioning particularly in regulated rivers

deformation and magnetic fabrics in ductile shear zones a review abstract the anisotropy of magnetic susceptibility ams is a well established petrofabric tool for indicating relative strain and microstructural character and has been validated on various rock types and different structural settings the magnetic susceptibility of a rock k depends primarily on the nature and abundance of magnetic minerals the physical arrangement and lattice preferred orientation of these magnetic minerals give rise to magnetic anisotropy the ams scalar parameters most commonly used to constrain strain include the corrected degree of anisotropy p 1 a proxy for fabric intensity and the shape factor 1 t 1 an indicator of the magnetic fabric symmetry prolate vs oblate a number of studies have shown that a positive correlation generally exists between p and strain thus the ams shows a great potential as a tool for examining deformation in geologic structures characterized by large strain gradients such as shear zones however a number of caveats exist i the increase of p with strain cannot be solely attributed to deformation because p also increases with k regardless of deformation ii strain across shear zones is typically heterogeneous and is often localized in units of different lithology thus making the separation of the lithological and strain controls on ams difficult also deformation is commonly accompanied by mineral segregation or fluid rock interaction that induces changes in magnetic mineralogy iii even if the undeformed lithology was uniform across a shear zone variations in strain rate or temperature may result in different deformation mechanisms hence the relationship between p and strain depends strongly on both the mineral carriers of ams and on deformation mechanisms and iv the ams is unable to resolve composite fabrics such as those resulting from s c structures where minerals on the c and s planes respectively contribute to ams

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

paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff southern nevada implications for welding temperatures two voluminous rhyolitic to quartz latitic ash flow sheets the tiva canyon and topopah spring members of the paintbrush tuff were erupted from the miocene claim canyon cauldron in southern nevada although lithologically similar these units differ greatly in their recording of the ancient geomagnetic field the reversely magnetized tiva canyon member yields remanent directions that are well grouped both within and between sites the normally magnetized topopah spring member however yields remanent directions that vary both laterally and vertically remanent directions from a 100 m thick section of nonwelded to partially welded topopah spring are well grouped precision parameter k 200 and give a mean direction d 293 i 69 that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling in three 300 m thick sections of more densely welded tuff inclinations from the upper parts of the topopah spring are steep 70 but flatten with depth to about 20 and then steepen abruptly near the base of the unit in general the directional dispersion increases with the thickness of the section k 30 for sections that are about 300 m thick much of the directional variation in the topopah spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding the possibility of subblocking temperature deformation in an ash flow sheet is enhanced by 1 high blocking temperature spectra of the primary magnetic phases 2 relatively low emplacement temperatures and 3 great stratigraphic thickness thermal demagnetization results from the topopah spring member indicate that welding may have continued to temperatures as low as 475 c in the basal vitrophyre of the ash flow sheet remanent directions from the tiva canyon member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the topopah spring

paleomagnetism of the hervey group central new south wales and its tectonic implications paleomagnetic work on the hervey group in southeast australia reveals a predeformational magnetization as suggested by the retention of a predeformational compaction produced magnetic fabric and the effect of deformation on both the magnetic fabric and the direction of the magnetic remanence in cleaved rocks the formation mean direction d 29 2 i 19 3 with 95 15 5 gives a latest devonian early carboniferous palaeomagnetic pole at 54 4 s 24 1 e with dp 8 4 dm 16 2 comparing this pole with existing poles from australia and north america it is suggested that 1 the lachlan fold belt of southeast australia has been part of cratonic australia since at least the late devonian and consequently the late palaeozoic palaeomagnetic data from the lfb may be applied to the whole of gondwanaland 2 cratonic australia and the new england fold belt were all in an equatorial position during the early carboniferous 3 there was probably a rapid anticlockwise rotation of gondwanaland during the late devonian around an axis close to australia 4 a v shaped ocean existed between laurussia and the northern margin of western gondwanaland during the late devonian early carboniferous the appalachian hercynian orogenic belt is probably the result of the closure of this ocean

theory of fine sediment infiltration into immobile gravel bed a theoretical model is developed to describe the process of fine sediment infiltration into immobile coarse sediment deposits the governing equations are derived from mass conservation and the assumption that the amount of fine sediment deposition per unit vertical travel distance into the deposit is either constant or increases with increasing fine sediment fraction model results demonstrate that fine sediment accumulation decreases rapidly with depth into coarse substrate initially void of fine sediment which is consistent with experimental observations that significant fine sediment infiltration occurs to only a shallow depth comparisons of the theory with flume data indicate that the model adequately reproduced the weighted averaged fine sediment fraction values from experiments an early model developed by sakthivadivel and einstein for fine sediment infiltration is in part based on the generally incorrect assumption that intragravel flow remains constant following fine sediment infiltration applying a correction to the sakthivadivel and einstein model based on alternate hypothesis that introgravel flow is driven by a constant head gives similar results as the proposed model

the impact of biological bedforms on near bed and subsurface flow a laboratory evaluated numerical study of flow in the vicinity of pits and mounds the complex surface topography of river substrates controls near bed hydraulics and drives the exchange of subsurface and surface flow in rivers the topographic structures that are studied are usually formed by the flow but it is known that many animals also create biogenic bedforms such as pits and mounds here a large eddy simulation les model of flow over a pit and a mound is evaluated with flume experiments the model includes actual bedform topography and the topographic complexity of the surrounding bed surface subsurface grains are organized in a body centered cubic packing arrangement model evaluation showed strong agreement between experimental and modelling results for velocity r2 0 8 and good agreement for reynolds stresses r2 0 7 which is comparable to other similar studies simulation of the pit shows that the length of the downwelling region is smaller than the upwelling region and that the velocity magnitude is higher in the downwelling region simulation of the mound reveals that the flow is forced into the bed upstream of the mound and re emerges near the top of the mound the recirculation zone is limited at the leeside of the mound with increasing reynolds number the depth of the upwelling region at the leeside of the mound increases the analysis of shear stress indicates that sediments on the upstream edge of the pit and on the downstream face of the mound are relatively unstable these results demonstrate the effect of biogenic structures on the near bed flow field hyporheic exchange and sediment stability

late proterozoic reconstructions of north west scotland and central canada magnetic fabrics paleomagnetism and tectonics red beds dominate the stoer group and the unconformably overlying torridon group having accumulated between 1400 and 1000 ma in a rift of the hebridean foreland the sequences were weakly strained and shortened e w and each underwent successive diagenetic changes which altered their magnetic properties and caused their different characteristic post depositional magnetizations anisotropy of magnetic susceptibility ams reveals an n s vertical tectonic foliation superimposed on the bedding planar ams sub fabric the tectonic ams sub fabric was isolated by comparing normalized and non normalized mean tensors of multiple specimens the n s vertical ams tectonic foliation postdates warps of bedding thus the high susceptibility minerals including those carrying palaeomagnetic signals re oriented or recrystallized after deposition and diagenesis thus characteristic remanence vectors chrm were acquired long after deposition first in the stoer group then in the torridon chrms were isolated for 143 stoer specimens and 94 torridon specimens using two cycles of low temperature demagnetization followed by at least 12 steps of thermal demagnetization for the stoer group two structurally integral groups of sites yield mean magnetizations of 317 0 43 1 95 11 3 n 49 and 309 9 52 1 95 10 6 n 23 some stoer specimens bear a torridon age overprint 133 3 525 95 9 6 n 19 the younger torridon group yields 138 4 52 2 95 7 4 n 42 134 9 45 5 95 16 0 n 15 and 134 4 55 1 95 11 3 n 19 from three structurally integral clusters of sites these late chemical magnetizations postdate bedding warps faults and tectonic ams fabrics they do not warrant local rigid body tilt corrections however we restored the chrm directions for the regional post cambrian eastwards tilt which affected the entire foreland from the lewisian basement to the overlying proterozoic strata paleopoles calculated from the strata s restored vectors were rotated into laurentian coordinates for comparison with canadian proterozoic palaeopoles the stoer group was magnetized in a normal polarity geomagnetic field whereas the torridon group was magnetized in a reversed polarity epoch however both sequences include at least one polarity transition indicating long duration chemical magnetizations successfully averaging secular variation at a minimum red bed magnetization processes exceeded 0 1 ma and may have encompassed several million years during stoer group magnetization the hebridean craton was at paleolatitudes of 26 44 n when the torridon group magnetized it was at 23 27 s accepting published ages this defines a minimum southward latitudinal displacement rate of 27 40 km ma restored stoer group paleopoles lie in the present day north central pacific ocean 190e 25n corresponding to the laurentian apwp dated at between 1090 and 1109 ma torridon group paleopoles lie in the south central pacific 210e 25s near the poorly documented 900 ma southernmost part of the laurentian apwp

sediment transport and resulting deposition in spawning gravels north coastal california incubating salmonid eggs in streambeds are often threatened by deposition of fine sediment within the gravel to relate sedimentation of spawning gravel beds to sediment transport infiltration of fine sediment 2 mm in diameter into clean gravel beds bed material size distributions scour fill depths and sediment transport during 10 storm flow events were measured in three streams of north coastal california although suspended sediment comprised most 75 94 of the clastic load during storm flows bed load material 0 25 2 mm accounted for most 70 78 of the fine sediment accumulated in experimental gravel implanted in the streambeds sand trapped in the interstices of the top several centimeters formed a seal that impeded deeper deposition of very fine sand and finer material the seal was responsible at least in part for a decrease in the rate of fine sediment accumulation with increasing cumulative bed load transport areas of the streambeds commonly scoured or filled 0 1 m or more during storm flows and thus scour and fill commonly created a sandy layer at least as thick as the seal formed by sediment infiltration scour could erode eggs laid in the bed and expose deeper levels of the bed to infiltration by fine sediment but at the same time could allow fine sediment to be winnowed away great temporal and spatial variation in sedimentation in these streams suggests that individual storms of moderate size pose a threat to eggs in many but not all areas selected by fish for spawning

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

thermal history of mesosiderites revisited abstract the results of a fission track study of uranium rich merrillite crystals in a number of mesosiderites are clearly incompatible with cooling rates as low as 0 1 c myr which have previously been inferred for these meteorites classical metallographic techniques thus appear to give erroneous results when applied to this type of meteorites there appears to be no reason to advocate exotic scenarios to explain the thermal history of mesosiderites

strain and magnetic remanence abstract experimental data may be compatible with the hypothesis that a single direction of magnetic remanence rotates as a rigid marker with strains up to 40 shortening in coaxial perfect flattening x y z detailed agreement with the passive line model is relatively poor for the specimens in which remanance is carried by magnetite however for this range of strains the differences with the passive line model wettstein s equation are so slight that the latter model may be more easily employed to de strain or restore deformed remanance to its original attitude in the case of hematite bearing remanences the differences between the passive line and rigid marker model are even smaller because of the higher aspect ratios of grains of hematite therefore it is suggested that wettstein s equation may be safely used to restore remanence after even higher strains where the remanence is carried by hematite

strain partitioning in the helvetic thrust belt of eastern switzerland from the leading edge to the internal zone abstract the strain and strain partitioning in the helvetic thrust belt are analysed in order to determine their variation across the belt from the external thin skinned portion to the internal thick skinned portion and in order to begin the detailed mechanical analysis of the entire fold and thrust belt coarse grained limestones of jurassic and cretaceous age were chosen for analysis because they can be traced from the external to the internal zones the strain within a bed is partitioned into three components transgranular pressure solution microfaults veins twinning granular and intragranular exclusive of twinning crystal plasticity associated with recrystallization transgranular strain was very important in the early history of the helvetic nappes giving way to twinning at a later stage under tectonic burial twinning strain was early in the infrahelvetic complex giving way to intracrystalline deformation at the later stage of burial the transition from transgranular to intragranular dominated strain occurred at a vitrinite reflectance value of 3 5 significantly below the greenschist grade of metamorphism in a large scale fold at the leading edge of the helvetic nappes transgranular strains were associated with the buckling stage of the fold and twinning occurred during fold tightening in more internal parts of the helvetic nappes twinning strains are also late with regard to transgranular strains but here their pattern corresponds to the regional finite strain pattern in the internal zone the infrahelvetic complex twinning seems to have been an early event and records an initial layer parallel shortening followed by regional internal deformation with intragranular strains

discriminating between pore filling load and bed structure load a new porosity based method exemplified for the river rhine sediments contained in the river bed do not necessarily contribute to morphological change the finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level the discrimination between pore filling load and bed structure load therefore is of practical importance for morphological predictions in this study a new method is proposed to estimate the cut off grain size that forms the boundary between pore filling load and bed structure load the method evaluates the pore structure of the river bed geometrically only detailed grain size distributions of the river bed are required as input to the method a preliminary validation shows that the calculated porosity and cut off size values agree well with experimental data application of the new cut off size method to the river rhine demonstrates that the estimated cut off size decreases in a downstream direction from about 2 to 0 05 mm covariant with the downstream fining of bed sediments grain size fractions that are pore filling load in the upstream part of the river thus gradually become bed structure load in the downstream part the estimated mass percentage of pore filling load in the river bed ranges from 0 in areas with a unimodal river bed to about 22 in reaches with a bimodal sand gravel bed the estimated bed porosity varies between 0 15 and 0 35 which is considerably less than the often used standard value of 0 40 the predicted cut off size between pore filling load and bed structure load dc p is fundamentally different from the cut off size between wash load and bed material load dc w irrespective of the method used to determine dc p or dc w dc w values are in the order of 10 1 mm and mainly dependent on the flow characteristics whereas dc p values are generally much larger about 100 mm in gravel bed rivers and dependent on the bed composition knowledge of dc w is important for the prediction of the total sediment transport in a river including suspended fines that do not interact with the bed whereas knowledge of dc p helps to improve morphological predictions especially if spatial variations in dc p are taken into account an alternative to using a spatially variable value of dc p in morphological models is to use a spatially variable bed porosity which can also be predicted with the new method in addition to the morphological benefits the new method also has sedimentological applications the possibility to determine quickly whether a sediment mixture is clast supported or matrix supported may help to better understand downstream fining trends sediment entrainment thresholds and variations in hydraulic conductivity