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

late cretaceous cenozoic intraplate contractional deformation at the norwegian continental shelf timing magnitude and regional implications abstract contractional structures large anticlines and synclines reverse faults and inverted centres of deposition of assumed late cretaceous cenozoic age are common in cretaceous tertiary basins of the northwestern european margin the similarities in style orientation and timing of these structures are striking the present detailed analysis of one anticline the ormen lange dome of the mid norwegian continental shelf indicates that the total contraction is moderate less than 2 3 and that the analysed anticline has been growing almost continuously since its initiation in eocene till present inversion in the barents sea started already in the late cretaceous this episode is suggested to be related to far field effects of active plate margin processes and transfer of stresses across the plate as a consequence of the sub hercynian and paleocene laramide event of the alpine orogeny the development of co axial structures was facilitated by stress focusing along pre existing high relief n s and ne sw trending fault complexes far field plate tectonic stresses originating mainly from the alpine orogeny seem to have been the most important cause of contractional deformation on the nw european shelf in addition ridge push from the north atlantic spreading may have contributed significantly particularly during the neogene

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

historic and ancient tsunamis uncovered on the jalisco colima pacific coast the mexican subduction zone research on extreme wave events such as tsunamis using the geological record in areas of infrequent and or small magnitude earthquakes can aid in extending the long term history and recurrence intervals of large events this information is valuable for risk management and community preparedness in coastal areas here we investigate tsunami deposits on the jalisco coast of mexico that overlies the subducting rivera plate under the north american plate an area due for a large thrust earthquake and potential tsunami here we apply a full battery of rock magnetic analyses that also include a detailed ams study and other typically applied proxies in tsunami deposits research we present evidence to demonstrate that anomalous sand units with sharp basal contacts at la manzanilla tenacatita bay and el tecuan shore sites on the jalisco coast may be the products of tsunamis generated by known historical ms 8 2 earthquake of 3 june 1932 and other earlier tsunamigenic earthquakes a sandy unit with a sharp basal contact flame structures at the base rip up clasts at la manzanilla and four sand units with sharp basal contact overlying buried soils at el tecuan together with other proxies such as magnetic properties and others suggest tsunami deposits 210pb dating of sediments slightly above the upper sand layer indicate an age a d 1935 11 at el tecuan historical accounts of tsunami inundation at both sites provide further evidence that this is most probably the result of the 3 june 1932 tsunami hence this study may provide the first evidence of a tsunami triggered by this earthquake and also of three probable predecessors further evidence of at least three earlier tsunamis that occurred since the fifteenth century is also evident in the stratigraphy these events may correspond to events listed in historical archives namely the 1563 1816 and or the 1818 events

a case study of the internal structures of gossans and weathering processes in the iberian pyrite belt using magnetic fabrics and paleomagnetic dating in the rio tinto district of the iberian pryrite belt of south spain the weathering of massive sulfide bodies form iron caps i e true gossans and their subsequent alteration and re sedimentation has resulted in iron terraces i e displaced gossans to study the stucture and evolution of both types of gossans magnetic investigations have been carried out with two foci 1 the characterisation and spatial distribution of magnetic fabrics in different mineralised settings including massive sulfides gossans and terraces and 2 paleomagnetic dating hematite has been identified as the suceptibility carrier in all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom with 1 a horizontal foliation refered to as mature fabric in the uppermost part of the primary gossans 2 highly inclined or vertical foliation interpreted as immature fabric between the uppermost and lowermost parts and 3 a vertical foliation interpreted to be inherited from hercynian deformation in the lowermost part of the profiles in terraces a horizontal foliation dominates and is interpreted to be a sedimentary fabric rock magnetic studies of gossan samples have identified goethite as the magnetic remanence carrier for the low temperature component showing either a single direction close to the present earth field pef direction or random directions maghemite hematite and occasionally magnetite are the remanence carriers for the stable high temperature component that is characterized by non pef directions with both normal and reversed magnetic polarities no reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples in gossans the polarity is reversed in the upper part and normal in the lower part this vertical distribution with a negative reversal test suggests remanence formation during two distinct periods remanence in the upper parts of the gossans is older than in the lower parts indicating that the alteration proceeded from top to bottom of the profiles in the upper part the older age and the horizontal mature fabric is interpreted to be a high maturation stage of massive sulfides alteration in the lower part the age is younger and the inherited imature vertical hercynian fabric indicates a weak maturation stage these two distinct periods may reflect changes of paleoclimate erosion and or tectonic motion

development of planar and linear fabrics in dauphinois shales and slates french alps studied by magnetic anisotropy and its mineralogical control the progressive development of planar and linear fabrics due to alpine deformation in dogger calcareous shales has been studied in the dauphinois zone of the french alps using measurements of the anisotropy of magnetic susceptibility the axes and shapes of magnetic ellipsoids and petrofabrics show good qualitative correlation through a series of different fabric types from a sedimentary to a highly deformed one together with various petrological techniques magnetization measurements in a variable field up to 5 t and under variable temperature with a cryogenic magnetometer were used to achieve a complete analysis of the magnetic mineralogy with its dia para and ferromagnetic s l contributions the effects on the anisotropy of the complex mineralogy and various geological processes including pressure solution metamorphism and weathering are discussed

the main variscan deformation event in the pyrenees new data from the structural study of the bielsa granite abstract a structural and magnetic fabric study of the bielsa granite axial zone of the pyrenees provides new data indicating that this pluton was emplaced during the main variscan event recognized in the pyrenees we argue that the later post triassic deformation was localized along mylonitic bands reactivating earlier variscan shear bands the bielsa granite intrudes cambro ordovician metasediments at its northern border and the pluton is unconformably overlain by sediments triassic in age to the south and east the main structures in the igneous body are magmatic foliations and lineations defined by feldspar phenocrysts and biotite and dextral shear bands with dominant wnw ese trends the wnw ese striking magmatic foliations have variable dips and the magmatic lineation a subhorizontal wnw ese trend the zonation of low field magnetic susceptibility coarsely related to mineral content indicates a layer arrangement of rock types with more basic compositions at the northern border the magnetic ellipsoids are prolate along a wnw ese central band and oblate at the pluton borders where flattening is more important all these data allow us to interpret the bielsa intrusion as being caused by coeval nne shortening and dextral shear the presence of shear bands indicates that strain localization under dextral transpression continued during late variscan times

the guitiriz granite variscan belt of northern spain extension controlled emplacement of magma during tectonic escape abstract the ollo de sapo domain of the northern part of the variscan belt of spain contains precambrian and ordovician metamorphic rocks intruded by the guitiriz granite the domain is bounded by two n s transcurrent shear zones detailed structural and anisotropy of magnetic susceptibility studies of the guitiriz pluton reveal a syntectonic emplacement relative to movement on the two n s trending bounding shear zones characterized by 1 development of n s trending structural and magnetic fabrics 2 concordant structures in granites and country rocks and 3 development of shear zones along the east and west granite margins gravity data show two roots trending 110 e extending down to 4 5 km from an otherwise thin

granites built by sheeting in a fault stepover the sanabria massifs variscan orogen nw spain the sanabria region central iberian zone variscan belt of spain shows an asymmetric thermal dome marked by migmatites accompanied by the ribadelago and the sotillo plutons these small plutons display pronounced mineralogical variations biotite granodiorites and tonalites prevail and granites and gabbros are common both plutons are confined in the releasing stepover of a transpressional shear zone that strikes 120 e and dips 70 sw most of the igneous rocks form sheets parallel to the shear zone magnetic foliations and lineations in the igneous rocks are parallel respectively to the shear bands and stretching lineations observed in the shear zone the formation of these igneous sheets at high angle to the main axis of the regional field stress is explained by a combination of the fault valve behaviour of the shear zone with the power of melt overpressure to open and ascend through previously formed planar structures like s or c planes

track of fluid paleocirculation in dolomite host rock at regional scale by the anisotropy of magnetic susceptibility ams an example from aptian carbonates of la florida northern spain the present study aims to apply the ams method anisotropy of magnetic susceptibility at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre existing dolomite host rock the urgonian formations hosting the zn pb mineralizations in la florida cantabria northern spain have been taken as target for this purpose sampling was carried out in addition to ferroan dolomite host rock enclosing the zn pb mineralizations in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks where fluid circulation occurred and their surrounding formations ams study was coupled with petrofabric analysis carried out by texture goniometry scanning electron microscopy sem observations and also shape preferred orientation spo statistics sem observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to fe enriched and pure dolomites spo statistics applied on four images from ferroan dolomite host rock give a well defined orientation of ribbons related to the intermediate axis of magnetic susceptibility k2 for ams data two magnetic fabrics are observed the first one is observed in ferroan dolomite host rock and characterised by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation the magnetic susceptibility carrier is fe rich dolomite these features are probably acquired during metasomatic fluid circulations in fe rich dolomite host rock c axes are vertical as a rule 0001 planes i e planes perpendicular to c axes are isotropic with respect to crystallographic properties so the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation this is confirmed by the texture observed using the sem consequently ams results show a dominant ne sw elongation interpreted as the global circulation direction and a nw se secondary elongation that we have considered as sinuosities of the fluid trajectory the second type of magnetic fabric is essentially observed in the limestone and characterised by an oblate form of the ellipsoid of magnetic susceptibility a horizontal magnetic foliation and mixed magnetic susceptibility carriers it is interpreted as a sedimentary fabric

magnetic mineralogy of some granites from the french massif central origin of their low field susceptibility abstract with a view to a better understanding of the origin of the low field susceptibility in the variscan granitoids of the french massif central for which extensive structural mapping using the anisotropy of magnetic susceptibility ams technique is currently applied a detailed study of magnetic carriers has been undertaken origin of the susceptibility has been studied using high field and low temperature measurements in a cryogenic magnetometer together with standard rock magnetic techniques additional determinations of the opaque phases have been performed with the metallographic microscope x ray microdiffraction and electron microprobe techniques in almost all the granite types the magnetic susceptibility is essentially carried by the paramagnetic biotite crystals minor amounts of ubiquitous oxides ilmenite rutile titano haematite do not contribute significantly to the susceptibility this is also the case for rare occurrences of magnetite in some granitoids mainly the cordierite bearing anatexites sulphides are found to give a large contribution magnetic mineralogy identifies a mixture of ferromagnetic pyrrhotite and antiferromagnetic fe9s10 hexagonal pyrrhotite hpo the latter species is identified as forming the larger unaltered pyrrhotite grains the ferromagnetic pyrrhotite is identified to be mainly smythite fe9s11 a species formed by supergenic alteration of monoclinic pyrrhotite located along the basal cleavage planes the microfractures and the grain boundaries of the hpo this study which constitutes the first report of the magnetic properties of smythite in rocks indicates that magnetic carriers derived from low temperature alteration have to be taken into account for purposes of structural studies using ams

magnetic properties of the high himalayan leucogranites structural implications the magnetic properties of the high himalayan leucogranites have been investigated on 527 specimens in three plutons everest makalu 6 sites and manaslu 40 sites in nepal and gangotri 43 sites in india susceptibility varies between 2 and 100 10 6 si with an anisotropy ratio up to 1 16 high field and low temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline the latter produces an inverse fabric i e with the minimum axis parallel to mineral lineation the magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement and its usefulness in semi quantitatively estimating petrofabric intensity is demonstrated for the biotite bearing facies natural remanent magnetization was measurable at only two sites in everest makalu where there are well defined reverse directions carried by titanomagnetite and pyrrhotite comparison of these preliminary results with predicted directions for stable india suggests northward tilting of about 10 and a small clockwise rotation of this massif

sulphide and oxide minerals from the ohmine granitic rocks in kii peninsula central japan and their primary paragenetic relations in the ohmine granitic rocks of kii peninsula central japan ilmenite rutile pyrrhotite pyrite and chalcopyrite commonly occur but no magnetite is present their primary paragenetic relations were confirmed by examining their mode of occurrences as inclusion species in major silicate minerals and the phase relations in the cu fe s system though their parageneses changed in cooling of the rocks this makes it possible to estimate the fugacities of oxygen and sulfur in the granitic rocks based upon their paragenetic relations and to discuss the nature of volatiles in the granitic rocks significant subsolidus reactions for sulfide minerals continued until the rocks cooled below 300 c and took place in two types of mineral grains in groundmass and as inclusions the removal of sulfur and copper through the silicate crystals was too easy to preserve the chemistry of sulfides included in silicates

shock experiments up to 30 gpa and their consequences on microstructures and magnetic properties in pyrrhotite 1 shock experiments with pressures ranging from 3 to 30 gpa have been conducted on a mixed assemblage of hexagonal and monoclinic pyrrhotite all samples were studied with respect to their particular shock induced microstructures and magnetic properties at high and low temperatures up to 8 gpa microstructures in shocked pyrrhotite are characterized by mechanical deformation producing a damage of the crystal structure at pressures of 20 gpa and upward amorphization and mechanical twinning are the dominant structural features induced by shock within the lower pressure range coercivity saturation isothermal remanent magnetization and coercivity of remanence increase with shock pressures in agreement with more single domain sd like behavior simultaneously the peak of hexagonal pyrrhotite decreases and the 34 k transition of monoclinic pyrrhotite broadens and is depressed magnetic hardening is triggered by grain size reduction but also by the formation of sd within discrete multidomain grains planar deformation features subdivide such multidomain grains into lath shaped domains with average sizes lying in the sd range the planar deformation features disappear at 20 gpa and irregular nanometer sized amorphous domains occur instead pressure release from 30 gpa finally triggers partial melting of pyrrhotite the sharp interfaces between molten and crystalline pyrrhotite document a rapid change of thermal conditions within molten pyrrhotite quenched iron crystals occur the presence of native iron strongly influences the magnetic properties depending on the particular amount in the studied sample and likely affects the magnetic properties of impact lithologies on earth and extraterrestrial material

magnetic iron nickel sulphides in the pliocene and pleistocene marine marls from the vrica section calabria italy abstract the rock magnetic properties of the late pliocene and early pleistocene open marine marls from the vrica section in calabria italy point to magnetic sulphide as the main magnetic mineral and remanence carrier the maximum blocking temperatures however are between 340 and 360 c which is too high for stoichiometric monoclinic pyrrhotite magnetic concentrates of the sediment are rich in iron sulphide grains and iron nickel sulphide grains microprobe observations show that most of the grains are of the order of 1 m in size and attached to iron bearing clay flakes microprobe analyses of the sulphides yield a large range of compositions ranging from pyrite through greigite pyrrhotite to sulphur deficient monosulphides and with ni contents varying from zero to a few atom percent in the pyrite grains and from zero to as much as 35 atm in the monosulphides the high ni content of many of the grains is extraordinary and has not been reported before in marine sediments most of the compositions cannot be directly connected with a known mineral phase the marine depositional environment of the marls imposes an authigenic origin for the sulphides and this is supported by several observations the great number of sulphide grains in the magnetic concentrates suggests that at least one of the sulphide compositions must have a ferrimagnetic structure possibly the sulphides with a metal to sulphur ratio close to that of monoclinic pyrrhotite and between 25 and 35 of the fe replaced by ni the ni substitution could possibly be the cause of the high blocking temperatures

sulfide petrology of four nakhlites northwest africa 817 northwest africa 998 nakhla and governador valadares the nakhlites martian meteorites contain small proportions of cu fe ni sulfide minerals we have studied these sulfides in northwest africa nwa 998 nakhla governador valadares and nwa 817 with optical microscopy scanning electron microscope and electron microprobe the meteorites studied are thought to derive from the bottom inner part and top of hypothetical nakhlite lava flow s respectively modal abundances of magmatic sulfides as estimated by image analysis on thin section are uniformly low 0 02 to 0 05 0 03 vol i e a factor 5 lower than in shergottites sulfides occur within the glassy mesostasis as composite two phase fe ti oxide sulfide grains intimately associated with interstitial grains or locally enclosed in postcumulus melt inclusions e g governador valadares in olivine they exhibit a uniform low ni monoclinic pyrrhotite composition chalcopyrite there is a gradation of sulfide grain sizes and textures across the nakhlites flow s droplets in nwa 817 resorbed blebs in governador valadares more massive true intercumulus blebs in nakhla and nwa 998 these nakhlites also show evidence for sulfide weathering hot desert finds e g nwa 998 and nwa 817 show a few percent fracture filling iron oxy hydroxides of likely terrestrial origin original sulfides are 50 altered in our nwa 998 section with iron oxy hydroxides at grain boundaries and as complete pseudomorphs the compositions of unaltered pyrrhotites are homogeneous close to that of the monoclinic endmember fe7s8 and are too sulfur rich to have been in chemical equilibrium with the late magmatic redox state fixed by the fayalite magnetite quartz buffer therefore the compositions of the pyrrhotites must have been altered during the later stages of magmatic crystallization by assimilation of s rich regolith and hydrothermal circulation

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

grain size dependence of initial susceptibility and saturation magnetization related parameters of four natural magnetites in the psd md range summary initial susceptibility coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain size fractions of a natural magnetite and titanomagnetite both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite both of which show exsolution phenomena it was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic fe rich inclusions these inclusions are magnetically screened in coarse grains only for the lower grain sizes studied they have an appreciable influence on the data the size of the fractions varied from 5 to 250 m the parameters and several parameter ratios are plotted as a function of grain size several of these parameters can be used as mineralogical and grain size indicators the varying degree of exsolution of the minerals is reflected in the gradients of the plots from plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size variation of the various parameters with grain size is explained in terms of a gradual transition from large psd grains to md grains without there being any sharp transition for titanomagnetite x 0 55 the transition takes place at grain sizes that are about 15 m larger than for magnetite

composite magnetic fabrics from s c mylonites magnetic anisotropy data are presented for samples of s c mylonites from a shear zone developed in the veiga granodiorite a late kinematic pluton located in the northern part of the iberian variscan belt spain the magnetic susceptibility is geometrically represented by an ellipsoid whose principal axes k 2 kint 1 kmi often show a one to one correlation with the principal directions x y and 2 f finite strain we report an exception to this rule since our results reflect composite magnetic fabrics arising from the contribution of both the c and s structures the magnetic foliation defined as the plane normal to k i shows an intermediate orientation between the c and s planes and rotates towards the orientation of the c planes as finite strain increases the magnetic lineation parallel to k and the stretching lineation present a similar trend but different values of plunge copyright 0 1996 elsevier science ltd

sulfide mineralogy and redox conditions in some shergottites magmatic sulfide mineralogy has been studied in 2 olivine phyric shergottites dag 476 and sau 005 and 4 basaltic shergottites zagami shergotty los angeles and nwa 480 modal abundances of magmatic sulfides as estimated by image analysis on thin section are high 0 16 to 0 53 area percent and correlate positively with abundances of fe ti oxides sulfides are mesostasis minerals being mostly interstitial grains or locally enclosed in post cumulus melt inclusions e g sau 005 in olivine sulfides in shergottites are composed of major pyrrhotite containing pentlandite exsolutions associated with minor amounts of cu sulfides chalcopyrite and or cubanite hot desert finds e g dag 476 show abundant fracture filling iron oxy hydroxides of probable terrestrial origin unaltered sulfides show metal rich hexagonal pyrrhotite compositions with metal sulfur m s ratio ranging between 0 936 0 005 and 0 962 0 01 this compositional range corresponds to the two phase structural domain 2c nc of the fe s system however the high temperature disordered hexagonal 1c pyrrhotite structure would be in better agreement with magnetic properties of shergottites ni contents in pyrrhotite increase from los angeles 0 05 at to shergotty zagami and nwa 480 0 2 0 5 at and dag 476 and sau 005 up to 3 at the higher ni values of pyrrhotite in olivine phyric shergottites correlate with the abundance of pentlandite exsolutions both reflecting more primitive ni rich sulfide liquids where abundant olivine crystallized this result and the strong correlation between sulfide abundances and fe ti oxides argue for a primary magmatic origin of these sulfides although they reproduce the trend of magmatic oxygen fugacity conditions determined from fe ti oxide pairs observed pyrrhotite compositions are systematically more metaldeficient compared to those calculated from the fe s o system this suggests post magmatic oxidation during cooling on mars followed by terrestrial weathering for hot desert finds

resolution of ferrimagnetic and paramagnetic anisotropies in rocks using combined low field and high field measurements summary a method of separation of ferrimagnetic and paramagnetic components of magnetic anisotropy from the total anisotropy is developed from the anisotropy measurement of a specimen in a low field susceptibility anisotropy and from measurement in two high fields stronger than the saturation field of the ferrimagnetic fraction present the following anisotropy components can be calculated susceptibility anisotropy field independent of the paramagnetic fraction low field susceptibility anisotropy of the ferrimagnetic fraction and high field anisotropy of the ferrimagnetic fraction errors possibly arising from imperfect saturation of the ferrimagnetic fraction in the field practically available are analysed

a decompressional p t path reinbolt hills east antarctica granulites exposed in the reinbolt hills east antarctica are part of the extensive late proterozoic granulite complex of east antarctica which includes the rauer group to the east and the northern prince charles mountains to the west the deformation history includes three pervasive deformation phases no chemical or mineralogical distinction between these phases has been detected and this is interpreted to be the result of complete re equilibration at the end of the third deformation phase two late deformation phases post date the metamorphism and record a medium temperature cooling path a short segment of the p t path of these rocks was inferred from mineral reactions that occurred during these late deformation phases the path passes from 800 c 7 kbar to 690 c 5 kbar indicating strong decompression which is typical of a thrust dominated crustal thickening followed by rapid erosion or extensional collapse

study on finite strain and its relationship with magnetic fabric along ailao shan red river shear zone 1 the finite strain along ailao shan red river shear zone has been studied the axial ratios of strain markers and shear strains vary greatly arising principally from deformation intensity and from contrasting competencies ellipticity of chloritic amygdales in basalt ranges from 6 79 to 10 47 corresponding shear strain gamma xz 2 22 2 93 and ellipticity of garnets from 1 56 to 2 95 gamma xz 0 45 1 14 in core gneiss the s c angle of the sheared rocks varies from 2degrees to 28degrees gamma sc 1 35 28 60 mostly in a range 2degrees to 7degrees optical observation thermal demagnetization and hysteresis curves were conducted to determine the nature of minerals and the contribution of paramagnetic and ferromagnetic minerals to magnetic susceptibility the result shows that ferromagnetic minerals and hornblende dominate the magnetic fabric of samples with high to very high susceptibility and play an important role in the magnetic fabric of samples with low to intermediate susceptibility the ferromagnetic minerals are mainly pseudo single domain and multidomain ti poor magnetite most samples have their magnetic lineation lying between the c and s surfaces in the remaining samples deviations from this common alignment principally resulted from the magnetic minerals being deflected by porphyroclasts mineral cleavage and cracks the axial ratio correlation is very poor between the magnetic and actual or imaginary strain ellipsoid calculated from the s c fabric this poor correlation suggests that the shape of the finite strain ellipsoid cannot be evaluated using the magnetic fabric when the latter is a combined result from multiple minerals and especially when the shape effect of multidomain titanomagnetite is important

anisotropy of magnetic susceptibility and magmatic structures in the guerande granite massif france a multidisciplinary approach has been used in order to map out the syntectonic magmatic structures of the guerande leucogranite in southern brittany western france on one hand extensive field work and microscopic observations and on the other a detailed magnetofabric investigation have been carried out the former showed a magmatic flow plane trending generally to the ene wsw with a moderate dip to the north and a flow direction slightly dipping toward nne these observations suggest a blade shaped body which is not deeply rooted the magnetofabric as defined by the anisotropy of magnetic susceptibility is mainly related to hematite ilmenite grains with a mean rock susceptibility of about 3 10 6 g oe there is an excellent agreement between the magnetic lineation kmax and the magmatic flow direction the agreement between the magnetic foliation kmax kint plane and the magmatic flow plane is not as good the quantitative analyses of the anisotropy allows one to group the data into two domains the one with planolinear ellipsoids corresponding to the central part of the granitic body where flow was mainly achieved on the viscous state and the other in the flattening domain related to the northern border area which exhibits incipient orthogneissification that is some deformation on the solid state such a favorable comparison with the classical structural method is encouraging for using the anisotropy of magnetic susceptibility technique more routinely for structural mapping of granitic bodies provided that the microstructural state of the rock is already well known

magnetic mineralogy and metamorphic zonation in the ardennes massif france belgium abstract in the ardennes area pelitic schists of cambrian to early devonian age have experienced a low grade polyphase metamorphism which can be characterised by changes in magnetic mineralogy using mainly rock magnetic tools we have defined various iron oxides and sulphide assemblages allowing a magnetic mineralogy zoneography of the ardennes metamorphism to be constructed low temperature and three axial irm measurements identify pyrrhotite in the southern part of the massif this mineral had not been previously recognised in the ardennes area in the devonian cover toward the metamorphic apex the appearance of coarse grained haematite is characterised by low coercivity values the trends of low field and anhysteretic susceptibilities allow a discrimination between the lithologic and metamorphic effects to be made additionally the relative contributions of an early metamorphic event and the classical metamorphism linked to the variscan shortening are discussed

from qualitative to quantitative magnetic anisotropy analysis the prospect of finite strain calibration abstract a method of separating low field susceptibility anisotropies due to ferrimagnetics and the matrix proposed hitherto in cases where the principal axis of these two components are merged is generalized in cases where the axes are distinct the method consists of analysing in specimens close in the same sample anisotropies the differences of which are due to the variation of respective percentages of the two components although the strain rate remains constant one thus uses the cause which itself renders global anisotropy unusable a first application is presented where a control using the anisotropy of isothermal remanent magnetization allows the validity of the method to be tested a discussion of the choice of the best ellipsoid relative either to the matrix or to the ferrimagnetics to give the best indication of relative finite strain rate is opened

kinematics and strain distribution in an orogen scale shear zone insights from structural analyses and magnetic fabrics in the gavarnie thrust pyrenees abstract this work aims to characterize the gavarnie thrust one of the large scale thrusts that define the alpine structure of the west central sector of the pyrenees a detailed comparison of structural analysis and magnetic fabrics is carried out for the paleozoic phyllonites of the gavarnie thrust in order to decipher strain distribution and transport direction the ams at room rt ams and low lt ams temperature and the airm can be correlated with the structural patterns kmin axes are mainly parallel to the pole of the s c or c planes and kmax axes are parallel to the transport direction and related to ductile s c structures furthermore the pj t changes across the shear zone characterize strain variations larger pj and t are found in the basal most deformed part of the shear zone and lower values are found where the interaction between alpine and variscan related petrofabrics is stronger we also interpret the reactivation of variscan inherited fabrics within the alpine shear zone in spite of the heterogeneous strain markers indicate a common top to the south n190e alpine transport direction which contrasts with the strong obliquity of the genetically related structures developed in the southern mesozoic sedimentary cover in this sense our data suggest a complete decoupling between basement and cover units during the alpine compression

analysis of the ferromagnetic contribution to the susceptibility by low field and high field methods in sedimentary rocks of the southern pyrenees and northern ebro foreland basin spain magnetic susceptibility in rocks is the sum of the contributions of different magnetic particles paramagnetic diamagnetic and ferromagnetic s l these contributions can be measured at variable temperatures or at variable fields hysteresis loops both are time consuming techniques that cannot be routinely used in magnetic fabrics analysis in this study we propose a simplified method to determine the ferro and paramagnetic contributions to the susceptibility based on the discrete measurement of susceptibility at two different fields near 0 and 2 5 t the results obtained in samples from the southern pyrenees and ebro basin indicate that the ferromagnetic contribution vary considerably within homogeneous susceptibility values and within the expected range of paramagnetic values standard bulk susceptibilities higher than 200 10 6 si contribute significantly to the ferromagnetic fraction 50 on average

metamorphic control of the magnetic mineralogy of black shales in the swiss alps toward the use of magnetic isogrades abstract the magnetic susceptibility of the helvetic jurassic black shales sampled around the aar and gothard massifs in the swiss alps is due to iron bearing silicates magnetite or pyrrhotite it exhibits a clear correlation with the alpine metamorphism from zeolite to amphibolite facies it firstly decreases due to the breakdown of original magnetite then sharply increases when pyrite is transformed into magnetic pyrrhotite using numerous field measurements of the susceptibility it was possible to carry out a fast and precise mapping of the pyrrhotite in isograde which appears to correspond closely to the stilpnomelane out isograde the matrix susceptibility obtained by means of high field measurements with a cryogenic magnetometer is due to iron bearing silicates and shows only smooth variations with metamorphism both monoclinic and hexagonal pyrrhotites are observed in the high grade area

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

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

low temperature ams and the quantification of subfabrics in deformed rocks abstract we evaluate the application and significance of low temperature anisotropy of magnetic susceptibility lt ams measurements in deformed mudrocks originally conceived as a way to enhance paramagnetic relative to ferromagnetic susceptibility lt ams studies offer significant potential in constraining the coexistence of subfabrics that are due to phyllosilicate grains with different preferred orientations in this study we report a detailed procedure to obtain such directional susceptibilities measuring samples in multiple orientations at liquid nitrogen temperatures in order to determine the lt ams due to unequal changes of magnetic susceptibility in micas at low temperature the enhancement of standard ams at low temperature better separates interacting fabrics in natural rocks particularly depositional fabrics versus deformational fabrics lt ams is a non destructive technique that readily offers an ability to separate ferromagnetic and paramagnetic fabrics and allows the characterization and quantification of multiple fabrics in natural rocks