tectonic setting and timing of the final deccan flood basalt eruptions the role of extensional tectonics in the generation of basaltic melt and eruption of continental flood basalts is controversial and yet it remains crucial to understanding the origin and cause of these large volume eruptions establishing the timing of lithospheric extension continental rifting and the onset of continental flood basalt volcanism is therefore of importance to petrogenetic models detailed mapping along the mumbai coast india reveals three chemically distinct sets of dikes which together with high precision 40 ar 39 ar dating demonstrate the temporal relationship between lithospheric extension and deccan volcanism the east west extension evident along the west coast of india which led to the separation of the seychelles from the indian plate only began during the final phases of the basalt eruptions and cannot have initiated the large volume eruptions of the deccan flood basalt province

effect of hydrostatic pressure on viscous remanent magnetization in magnetite bearing specimens rocks at depth in the crust acquire a viscous i e time dependent magnetization under the pressure temperature conditions at which they reside there have been numerous studies on the effect of temperature on viscous magnetization but little work has been performed on the effect of hydrostatic pressure we have studied viscous remanent magnetization at 22 c in a 0 1 mt field at 0 1 and 100 mpa for natural and synthetic multidomain magnetite the viscous remanent magnetization was found to increase nearly linearly with log time at both pressures there was little change in the rate of viscous acquisition between 0 1 and 100 mpa over the multidomain grain size range studied thus for rocks buried at depth in the earth the enhancement of magnetic viscosity by thermal fluctuations will dominate over effects due to hydrostatic pressure

study of the snow melt freeze cycle using multi sensor data and snow modelling the melt cycle of snow is investigated by combining ground based microwave radiometric measurements with conventional and meteorological data and by using a hydrological snow model measurements at 2000 m a s l in the basin of the cor devole river in the eastern italian alps confirm the high sensitivity of microwave emission at 19 and 37 ghz to the snow melt freeze cycle while the brightness at 6 8 ghz is mostly related to underlying soil simulations of snowpack changes performed by means of hydrological and electromagnetic models driven with meteorological and snow data provide additional insight into these processes and contribute to the interpretation of the experimental data

curie point depths of bulgarian territory inferred from geomagnetic observations and its correlation with regional thermal structure and seismicity abstract the basal depth of the magnetic layer curie point depths of bulgarian territory was calculated using geomagnetic field observation and the variations of this border were mapped for the first time the adopted computational method transforms the spatial data into the frequency domain and provides a relationship between radially averaged power spectrum of the magnetic anomalies and the depths to the magnetic sources the magnetic layer bottom thus obtained has normal 28 32 km values in the moesian platform and central part of bulgaria which are the relatively most stable regions of the investigated territory in the moesian platform cpd and moho are very close but the magnetic layer s bottom is still marked as a thermal boundary in the southern part calculated depths gradually reach the minimal values of the derived curie isotherm up to 14 16 km with relatively high gradient according to the results from geothermal modeling and paleomagnetic investigations magnetic layer s bottom of the bulgarian territory corresponds to different curie temperatures they vary from 400 425 c in the regions of rila rhodope unit up to 550 600 c in the moesian platform west and east balkan unit the obtained results comprise useful information concerning regional estimates of the correlation between different geophysical and geological characteristics of the earth s crust of the bulgarian territory

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

comparison of traditional and optical grain size field measurements with snowpack simulations in a taiga snowpack knowledge of snow microstructure is relevant for modelling the physical properties of snow cover and for simulating the propagation of electromagnetic waves in remote sensing applications characterization of the microstructure in field conditions is however a challenging task due to the complex sintered and variable nature of natural snow cover a traditional measure applied as a proxy of snow microstructure which can also be determined in field conditions is the visually estimated snow grain size developing techniques also allow measurement for example of the specific surface area ssa of snow from which the optical equivalent grain size can be derived the physical snow model snowpack simulates evolution of snow parameters from meteorological forcing data in this study we compare an extensive experimental dataset of measurements of traditional grain size and ssa derived optical grain size with snowpack simulations of grain size parameters on average a scaling factor of 1 2 is required to match traditional grain size observations with the corresponding snowpack simulation a scaling factor of 2 1 was required for the optical equivalent grain size standard deviations of scaling factors for the winters of 2011 12 and 2012 13 were 0 36 and 0 42 respectively the largest scaling factor was needed in early winter and under melting conditions

magnetic mineralogy of the lower continental crust rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths as parts of high grade metamorphic and plutonic terrains and as obducted lower crustal cross sections a review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks thermodynamic considerations indicate that tipoor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust regardless of the primary magmatic composition of the spinel phase furthermore it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle thus magnetite in the deep crust is the most likely candidate for the source of the long wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys if so the entire crust should be magnetic in most continental areas including shields in areas of high heat flow the bottom of the magnetic crust may lie considerably above the moho

thermal investigation in the cappadocia region central anatolia turkey analyzing curie point depth geothermal gradient and heat flow maps from the aeromagnetic data in this study curie point depth cpd heat flow geothermal gradient and radiogenic heat production maps of the cappadocian region in central anatolia are presented to reveal the thermal structure from the aeromagnetic data the large circular pattern in these maps matches with previously determined shallow 2 km in average depression estimated cpds in this depression filled with loose volcano clastics and ignimbrite sheets of continental neogene units vary from 7 to 12 km while the geothermal gradient increases from 50 to 68 c km heat flows were calculated using two different conductivity coefficients of 2 3 and 2 7 wm 1 k 1 the radiogenic heat production was also obtained between 0 45 and 0 70 w m 3 in this area heat flow maps were compared with the previous regional heat flow map of turkey and significant differences were observed in contrast to linear heat flow increment through the northeast in the previous map in the literature produced maps in this study include a large caldera like circular depression between nevsehir aksaray nigde and yesilhisar cities indicating high geothermal gradient and higher heat flow values in addition active deformation is evident with young magmatism in the neogene and quaternary times and a large volcanic cover on the surface boundaries of volcanic eruption centers and buried large intrusions are surrounded with the maxspots of the horizontal gradients of magnetic anomalies analytic signal as map pointing out exact locations of causative bodies is also presented in this study circular region in the combined map of as and maxspots apparently indicates a possible caldera

a remanent and induced magnetization model of magsat vector anomalies over the west african craton scalar and vector magsat anomalies over the west african craton are analysed by forward and inverse models a forward model of the man shield is based on liberia induced magnetization contrasts due to sporadic iron formations and to regional metamorphic rocks and a contrast in remanent magnetization within the lower crust are included this combination reproduces the location magnitude and adopted local zero level of anomalies in the initial magsat maps an inverse model of the reguibat shield estimates the magnetization contrast of its lithosphere and when magnetism is restricted to shallower than 75 km both shields can be represented by a susceptibility contrast of 0 02 a residual anomaly between the shields involves a relative deficiency of induced magnetization along with other causes

long wavelength magnetic anomalies in the caribbean plate boundaries and allochthonous continental blocks vector magnetic anomaly maps are derived at satellite altitude from magsat data over the caribbean and middle america to minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of magsat measurements two maps are derived in two nonoverlapping altitude ranges 350 420 and 420 550 km a composite map of the vertical component of the anomaly field is then computed at ground level this map displays large positive anomalies associated with fast subduction zones these anomalies define discontinuities in the downgoing slab associated with seismic gaps positive anomalies are also consistently observed above allochthonous blocks of continental origin in the caribbean area and suggest that the average magnetization is larger in the continental lithosphere elongated magnetic lows are observed above transform boundaries of the caribbean plate the cayman trough anomaly is consistent with magnetization of the order of 1 a m in the upper mantle demagnetization of the lithosphere along the bocono el pilar fault system which bounds the caribbean plate to the south may be accounted for by frictional heating along shear zones the pattern of anomalies along the southern margin of the united states is related to its accretionary tectonic history various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of magsat anomalies

the role of hematite ilmenite solid solution in the production of magnetic anomalies in ground and satellite based data abstract remanent magnetization rm of rocks with hematite ilmenite solid solution hiss minerals at all crustal levels may be an important contribution to magnetic anomalies measured by ground and satellite altitude surveys the possibility that lower thermal gradient relatively deep in the crust can result in exsolution of hiss compositions with strong remanent magnetizations rm was studied for two bulk compositions within the hiss system samples from granulite terrane around wilson lake labrador canada contains titanohematite with exsolved ferrian ilmenite lamellae other samples from the anorthosite terrane of allard lake quebec canada contain ferrian ilmenite with exsolved titanohematite lamellae in both cases the final exsolved titanohematite has similar ti content and carries dominant magnetic remanence with rem nrm sirm where nrm is the natural remanent magnetization and sirm is the saturation isothermal remanent magnetization that is comparable to the ti free end member the rm was acquired prior to exsolution and the ilmeno hematite rich rock possesses thermal remanent magnetization trm whereas rocks with hemo ilmenite possess chemical remanent magnetization crm in both cases we found fairly large homogeneous grains with low demagnetizing energy that acquired intense rm the magnetism of the ilmeno hematite solid solution phases is not significantly perturbed by the continuous reaction ilmeno hematite titanohematite solid solution hence the occurrence of hiss in rocks that cooled slowly in a low intensity magnetic field will have an intense magnetic signature characterized by a large rem

magnetic properties opaque mineralogy and magnetic anisotropies of serpentinized peridotites from odp hole 670a near the mid atlantic ridge abstract magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites harzburgites from odp hole 670a are studied the degree of serpentinization was determined by a new method which combines saturation magnetization and whole rock iron content a unique curie temperature t c of 580 c corresponding to pure magnetite was observed microprobe analyses whole rock metal content and curie temperatures indicate the absence of awaruite ni 3 fe or native metals hysteresis parameters reveal the contribution of pseudo single domain psd grains in samples with a low degree of serpentinization and large multi domain md grains in highly serpentinized samples with j rs j s 0 08 natural remanent magnetization nrm is mainly unstable and possibly composite particularly in the highly serpentinized samples the intensity j a of anhysteretic remanent magnetization arm in the geomagnetic field is much higher than j n intensity of nrm with j a j n averaging 2 4 the koenigsberger ratio q is low and susceptibility k depends on degree of serpentinization ranging from 1 2 to 51 5 10 6 m 3 kg 1 nrm inclinations are scattered with some tendency to lie near the horizontal plane which is close to the foliation plane anisotropies of susceptibility of arm and of saturation remanent magnetization srm were studied and compared with spinel foliation determined for three less serpentinized samples in these samples magnetic anisotropies seem to be mimetic of high temperature deformation before serpentinization when the degree of serpentinization is high later magnetic fabrics probably related to magnetite veins dominate previous anisotropies the temperature dependence of susceptibility and of viscous remanent magnetization vrm were studied the thermal variation of susceptibility shows a shallow hopkinson effect at about 300 400 c time dependence of induced and remanent magnetization increases with temperature reaching maximum values at about 300 400 c based on these observations induced magnetization and vrm of lower crustal and upper mantle rocks and their contribution to marine magnetic anomalies are discussed

the ivrea zone as a model for the distribution of magnetization in the continental crust magnetic properties of the principal lithologies of the ivrea zone of northern italy have been measured maficultramafic granulite facies rocks and amphibolites from the granulite amphibolite facies transition zone are highly magnetic with susceptibilities greater than 0 5 10 3 cgs saturation magnetization values greater than 90 10 3 emu cm and curie points between 570 and 580 c the magnetic phase is magnetite all other lithologies are moderately to weakly magnetic the magnetic mafic granulites are thick and laterally continuous and thus would make an appropriate deep crustal source for long wavelength magnetic anomalies in regions typified by low to average geothermal gradients

magnetic mapping of the western alps compilation and geological implications abstract extracted from large surveys of france italy and switzerland airborne magnetic data covering the western alpine arc have been compiled into a single homogeneous map of magnetic anomalies at the constant altitude of 3000 m for this purpose each data set has been revised thoroughly and accurately to give a single coherent large scale pattern the magnetic contour map reflects the anomaly pattern over the entire length of the western alpine collision suture the distribution of polarities exhibits a large anomalous low located by reduction to the pole over the whole external part of the belt the observed anomaly suggests a large gap of magnetization between the adriatic microplate and the european crust the analysis of the waveband shows that the broadest wavelengths are produced in the lower crust close to the transition zone in the granulite facies this highly magnetic layer is used as a marker to describe the geometry of the european and adriatic deep seated crust the main results are presented on a composite synthetic profile showing the sloping side of the european slab and an important crustal thinning to the southeast of the adriatic slab this feature is emphasized on the magnetic contour map by a linear magnetic low attributed to major transcurrent fault this trend is called the sestri voghera trend and extends from the ligurian basin by the sestri voltaggio zone to the judicarian system sinistral movements can be recognized along the whole axis as well as possible uplift of rift shoulders the magnetic anomaly pattern over the complete length of the anomalous body of ivrea as well as the insubric canavese line limit the extension of the adriatic microplate by a well defined linear trend the symmetrical shears deduced from consecutive anomalies are used to propose a structural scheme

magnetization of lower crust and interpretation of regional magnetic anomalies example from lofoten and vesteralen norway the interpretation of regional magnetic anomalies commonly observed over continental crust in both satellite and near surface magnetic data has remained enigmatic in its requirement for laterally extensive thick highly magnetic and deep seated sources presented here are the results of a detailed field analytical and experimental study of the magnetic mineralogy and magnetic properties of rocks from lofoten and vesteralen norway a province of deep seated origin with associated regional magnetic and gravity anomalies these rocks have a bulk magnetic character of surprising uniformity with susceptibilities hysteresis and curie temperatures defined solely by coarse grained magnetite stable remanence is observed in samples from sw lofoten but is not of significant magnitude the thermal enhancement of magnetic susceptibility the hopkinson effect is not important rocks from the deepest part of the section have the largest average susceptibilities 6 10 2 si and magnitudes of natural remanent magnetization nrm 5 10 4 a m2 kg these values of susceptibility together with the oft observed viscous character of the nrm suggest that these granulite facies lithologies particularly those of sw lofoten are perfect examples of lower crustal sources of typical regional magnetic anomalies furthermore these intermediate pressure granulites define a unique magnetic petrology especially for mafic compositions at higher grades of metamorphism high pressure granulites and eclogites or lower grades greenschist and amphibolite facies such magnetite bearing assemblages will be unstable and their high or low grade equivalents will be only weakly magnetic this evidence and these conclusions may be used to constrain models of lower crustal sources of regional magnetic anomalies over stable cratons it portrays the lower crust as a highly magnetic unit defined by metamorphic grade with a curie isotherm of 575 c magnetite and magnetization parallel to the ambient geomagnetic field induced and viscous components

crustal magnetization and long wavelength aeromagnetic anomalies of the minto block quebec measured susceptibilities of 3500 samples from a 500 500 km area of the minto block superior province quebec have an average value of 0 03 si equivalent to induced magnetizations of 1 5 a m charnockitic lithologies igneous orthopyroxene bearing granodiorite granite diorite that constitute large parts of the minto region yield the highest susceptibilities the variations in susceptibility show a good correlation with the variations in the observed magnetic field including those at long wavelengths 100 km suggesting that charnockitic and related granitoid rocks could be responsible for the long wavelength anomalies modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the minto block supports this contention charnockitic lithologies are not usually considered a possible sources of long wavelength anomalies mafic rock types are generally favored the dominant minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron

the bangui magnetic anomaly its geological origin satellite magnetometer data have revealed a long wave length magnetic anomaly in the vicinity of bangui central african republic substantiation of the anomaly has been made through analysis of air and ground magnetic surveys a correlative gravity anomaly has also been observed in ground gravity data the anomaly in an area of highly metamorphosed precambrian rocks does not correlate with any surficial features but is definitely crustal in origin an interpretation consistent with all available geophysical and geological data is that the anomaly could be the result of an early intrusion into the crustal rocks of this region followed by subsidence and deformation

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

the magnetic properties of metaperidotitic rocks as a function of metamorphic grade implications for crustal magnetic anomalies seventeen samples from the malenco serpentinite in the swiss alps representing systematic prograde metamorphic conditions from prehnite pumpellyite to upper amphibolite are moderately to strongly magnetic average susceptibility of 6 61 10 2 si units due predominantly to the presence of a magnetite rich spinel some greenschist facies samples contain the metal alloy awaruite ni3fe these contain in addition much greater amounts of magnetite small amounts of pyrrhotite in about half the samples complete the contributions to the total magnetization susceptibilities generally decrease with increasing metamorphic grade because of the production of increasing amounts of chromerich spinel which dilutes the magnetite component these results indicate that significant magnetic anomalies should be generated in suture zones containing metamorphosed serpentinite and in areas where serpentinite is being actively subducted

the geodynamic province of transitional lithosphere adjacent to magma poor continental margins two different types of transitional lithosphere have been documented along magma poor rifted margins one consists of apparently subcontinental mantle that has been exhumed brit tlely deformed and serpentinized during late stages of rifting a second is thinned 10 km con tinental crust which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth dependent extension in both cases it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter and thus that transitional lithosphere has relatively limited width here three representa tive cases of transitional lithosphere are examined one in the newfoundland iberia rift and one at goban spur both exhumed mantle and one off the angola congo margin thin continental crust flanked seaward by apparently exhumed lower continental crust exhumed mantle considering the geological and geophysical evidence it appears that depth dependent extension riftward flow of weak lower continental crust and or upper mantle may be a common phenomenon on magma poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed transitional lithosphere in this wide zone may consist of subcontinental mantle lower continental crust or some combination thereof depending on the strength profile of the pre rift continental lithosphere transitional lithosphere ceases to be emplaced i e final break up occurs only when emplacement of heat and melt from the rising asthenosphere becomes domi nant over lateral flow of the weak lower lithosphere this model implies a two stage break up first the rupture of the brittle continental crust and second the eventual separation of the ductile sub continental lithosphere which is coincident with emplacement of normal oceanic crust well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpenti nized exhumed mantle and such anomalies therefore are not diagnostic of oceanic crust where present the anomalies can be helpful in interpreting and dating the rifting history

evolution of the specific surface area of snow during high temperature gradient metamorphism the structural evolution of low density snow under a high temperature gradient over a short period usually takes place in the surface layers on clear cold nights in this paper x ray computed microtomography microct was combined with numerical simulations to investigate the temperature gradient metamorphism tgm on different types of snow precipitation particles pp small rounded particles rgsr and large rounded particles rglr were each observed in high temperature gradients 100 500 k m 1 at a mean temperature of 4 c the specific surface area ssa was used to characterize the tgm which were influenced by both the magnitude of the temperature gradient and the initial snow structures pp samples experienced a logarithmic decrease of ssa with time and the depth hoar structures created under high tgm 500 k m 1 have higher ssa compared to those under lower tgm unlike previous observations for initial rounded and connected structures like rglr samples the ssa increased during tgm simulated normal vapor flux distributions for different snow types were used to help understand the structural evolution under tgm understanding the ssa increase is important in order to predict the enhanced uptake of chemical species by snow or increase in snow albedo

what is magnetic in the lower crust long wavelength and satellite magnetic anomalies require that some regions of the earth s crust contain minerals that are magnetic at lower crustal conditions studies of high grade metamorphic rocks emphased multi domain magnetite as a likely source members of the hematite ilmenite series were not considered because hematite commonly has a relatively weak magnetization and though ilmenite compositions in the range ilm 50 70 can be strong ferrimagnets their low curie t 260 c makes them an unlikely source however recent investigations have described hematite ilmenite magnetism and coercivity in terms of a new ferrimagnetic substructure related to interfaces between ilmenite and hematite in exsolution lamellae down to unit cell thickness in this lamellar magnetism ferrimagnetism is coupled to principal af moments of hematite and retains many properties of titanohematite including high coercivity and thermal stability if exsolution lamellae are the source of the magnetism a temperature above that predicted for lamellar resorption should destroy it to look at behaviour of lamellae at mid to lower crustal conditions we selected a hemo ilmenite sample ilm 84 on which detailed studies of microstructures mineral chemistry and magnetic properties had been made we designed experiments at 10 kbar as an analog for lower crustal conditions after initial 5 day runs at 540 685 and 950 c a temperature of 580 c was chosen for 8 and 21 day runs at one atmosphere this t for ilm 84 is above that predicted for complete resorption of all lamellae to produce a homogeneous paramagnetic r3 ilmenite in these experiments exsolution lamellae coarsened if these features were reproduced in the lower crust hemo ilmenite or ilmeno hematite would retain a magnetic moment and higher than expected coercivity due to the microstructures emp and tem analyses were used to understand the compositional differences between the natural sample products of the experiments and one atmosphere phase diagrams if the experimental changes are interpreted as moving the compositions closer to equilibrium at 580 c 10 kbar then a considerable widening of miscibility gaps as a function of pressure and the 20 geikielite component in the ilmenite as well as an increase in neel temperature of hematite is predicted an alternate is to consider the response as ostwald steps short of equilibrium experimental results indicate that exsolution microstructures play a key role in mineral magnetism and that such microstructures as well as coarse hematite may be preserved at pressures and temperatures of the lower crust

viscous remanent magnetization model for the broken ridge satellite magnetic anomaly a range of crustal models for the interpretation of the broken ridge satellite magnetic anomaly have been constructed from bathymetric data the source magnetizations were assumed to be constant in direction throughout the source region and parallel to the present earth s field at the center of the source region the model calculations were made at the observation points of a selected set of magsat investigator b passes with a variable magnetic field direction calculated from the mgst 4 81 field model the horizontal perimeter of all the models was defined by the departure of the broken ridge bathymetry from an average oceanic depth of 4 5 km a topographic model with an upper surface defined by the bathymetry of the ridge and a lower surface defined by the average oceanic depth requires a magnetization of 42 a m to account for the observed anomaly amplitude an airy type isostatic model with an upper surface defined by the bathymetry of the ridge and a lower surface predicted from the bathymetry requires a magnetization of 6 a m to account for the observed anomaly amplitude a midcrust model with an upper surface at the average oceanic depth and a lower surface at the average oceanic moho requires a magnetization of 13 a m to account for the observed anomaly amplitude a comparison of the locations of the model and observed anomaly peaks shows that the broken ridge anomaly can be modeled assuming present field directions there is not sufficient sensitivity in the data to distinguish between the models on the basis of anomaly location the model magnetizations vary from 6 a m where the whole crust is uniformly magnetized to 42 a m where the topographic expression of the ridge is the source of the anomaly the strength of the required magnetization contrasts cannot be accounted for by induced magnetization if normal oceanic crustal materials are assumed to be the source for the observed anomaly measured and modeled remanent magnetizations of seafloor rocks are typically within the lower part of the range of required values but the higher magnetizations are associated with very young rocks and dredge samples it is concluded that the source magnetizations have been enhanced by the development of a viscous remanence effect since the last reversal of the earth s field

magnetic susceptibility used in mapping of amphibolite facies recrystallisation in basic dykes abstract the dense tholeiitic koster dyke swarm west sweden has undergone progressive deformation under amphibolite facies metamorphic conditions the varying influence of this event on the dyke rock forms the basis for a regional division of the swarm into three sectors the sectors of igneous dolerites partly recrystallised metadolerites and lineated amphibolites respectively systematic measurements of the magnetic susceptibility in the dykes have demonstrated a correlation between the regional pattern of recrystallisation and the pattern of mean susceptibility given as mean susceptibility per 0 5 km2 the transformation of dolerites to metadolerites and finally to lineated amphibolites is accompanied by a marked decrease in the susceptibility which is related essentially to the degree of alteration of titanomagnetite to sphene fe ions incorporated in the silicates

potential field signatures of buried precambrian basement in the western canada sedimentary basin an internally consistent levelled compilation of magnetic data is derived for alberta and northeastern british columbia with bouguer gravity data this compilation is used to refine the definition of precambrian basement domains within the western canada sedimentary basin magnetic data are draped at a constant distance above the mapped basement surface to reduce the effects of varying magnetic source depths automated interpretation methods that effectively map outlines of magnetic sources are used to characterize the internal structure of the domains and to aid in their delineation the basement domain map thus derived differs from previous interpretations in the extension of domains further to the southwest due mainly to the availability of new public domain magnetic data and the more precise definition of domain boundaries based on the magnetic source location maps the nahanni hottah chinchaga thorsby vulcan and kiskatinaw domains are weakly magnetic and characterized by magnetic sources tha

model for dry snow metamorphism by interparticle vapor flux kinetic growth metamorphism lowers the mean number of bonds between the ice particles in snow during the beginning state of snow metamorphism at a large mean temperature gradient significant particle growth occurs only at preferred growth sites three dimensional approximations of the sink source geometry are introduced to model particle growth and shrink rates the calculations show that enhanced growth occurs at very small and at large sink source distances the consideration of growth rate enhancement by differences in radii of curvature between sink source particles accounts for metamorphism at very low mean temperature gradients final particle size distributions and the reduction of the particle number density by metamorphism are calculated and may be compared with measured distributions

elevated temperature viscous remanent magnetization of natural and synthetic multidomain magnetite abstract the time temperature relationship of magnetization is a subject of much interest and debate by paleomagnetists rock magnetists and magnetic anomaly modellers we have investigated this relationship by studying the viscous remanent magnetization vrm of coarse grained multidomain md magnetite our experiments covered the temperature range from 22 to 400 c times from minutes to months and included both australian granulite samples and multidomain magnetite samples synthesized by the glass ceramic method vrm acquisition was found to generally increase with temperature but not always at the rate predicted from classical thermal fluctuation theories thermal cycling between room temperature at which the measurements were made and the vrm acquisition temperature sharply decreased the temperature dependence of the vrm room temperature vrm acquisition accelerates with time when plotted on a semilog plot whereas at elevated temperature the curves are quasilinear against log time for both the natural and synthetic samples this change in behavior may suggest a variation in the vrm acquisition mechanism as a function of temperature for md magnetite the granulites have a nearly linear increase in vrm acquisition rate with temperature whereas the glass ceramics display little change in the acquisition rate between 22 and 200 c but increase by nearly a factor of 3 by 400 c the increase in vrm of the glass ceramics between 200 and 400 c is in general qualitative agreement with thermal fluctuation theory there was no systematic change in the rate of vrm acquisition with grain size for the multidomain magnetites used in this study elevated temperature e g 400 c vrm acquisition by the deep crustal granulites if extrapolated over the brunhes chron would produce a magnetization of several a m which if true is of the order required by models for the source of long wavelength magnetic anomalies

theory of metamorphism of dry snow the growth of ice particles in dry seasonal snow is caused by vapor diffusion among particles due to temperature gradients imposed on the snow cover the diffusion is calculated by using the potential field solutions for electrostatically charged particles the stereography of snow is represented by using a log normal distribution function for a geometrical enhancement factor defined here reasonable crystal growth rates and supersaturations are found the transition between the growth of highly faceted crystals and the growth of highly rounded crystals is determined by the critical supersaturation for the onset of dislocation aided growth thermal convection and the continuous movement of vapor around the particles due to the imposed temperature gradient are accommodated in the theory although both have little effect relative to the interparticle diffusion the growth of a layer of faceted crystals just below semi permeable crusts is explained by showing that crusts can cause the local supersaturation to exceed the critical value faceted crystals are shown to grow most rapidly in the lower warmer portions of the snow cover because of temperature effects on growth rate

a numerical model to simulate snow cover stratigraphy for operational avalanche forecasting laws of snow metamorphism have been introduced in a numerical model which simulates the evolution of temperature density and liquid water profiles of snow cover as a function of weather conditions to establish these laws the authors have summarized previous studies on temperature gradient and on wet snow metamorphism and they have also conducted metamorphism experiments on dry or wet fresh snow samples an original formalism was developed to allow a description of snow with parameters evolving continuously throughout time the introduction of laws of metamorphism has improved significantly the derivation of the settlement of internal layers and of snow covered albedo which depend on the simulated stratigraphy i e the type and size of snow grains of different layers of the snow cover the model was tested during a whole winter season without any re initialization comparison between the simulated characteristics of the snow cover and the observations made in the field are described in detail the model proved itself to be very efficient in simulating accurately the evolution of the snow cover stratigraphy throughout the whole winter season

mobilization of gold in the deep crust evidence from mafic intrusions in the bamble belt norway abstract the location of mesothermal gold deposits in the upper part of major transcurrent shear zones suggests that gold was derived from the deep wider portions of the shears the 30 km wide bamble belt south norway is a deep shear belt metamorphosed to upper amphibolite and granulite grade at about 1 1 ga preliminary studies showed that its rocks are strongly depleted in gold and other chalcophile elements mafic rocks were chosen for more detailed study of the mobilization of gold during metamorphism gold in mafic rocks largely partitions into the sulphide phase which is mainly comprised of pyrrhotite modification of this phase was required to liberate gold bamble has two main groups of mafic rock the first metabasites intruded before peak metamorphism are uniformly low in gold their o2 at peak metamorphism obtained from ilmenite orthopyroxene equilibria indicates a strongly oxidized mineral assemblage well above fmq pyrrhotite was stable at peak temperature of 800 c but on cooling below 600 c internal mineralogical buffering by the oxidized assemblage caused the o2 to cross into the stability field of pyrite following conversion of pyrrhotite to pyrite the latter was partly replaced by magnetite these serial changes in the mineralogical form of iron sulphide was conducive to the extraction of gold as was the oxidized nature of the mineral assemblage that would have buffered metamorphic fluids to high o2 the second group of mafic rocks hyperites intruded after peak metamorphism record intermediate steps in the extraction of gold these intrusions of coronitic gabbro were metamorphosed in whole or in part to amphibolite there is a decrease in gold and other chalcophile elements across a hyperite dyke from the pyrrhotite bearing coronitic gabbro interior to the pyrite bearing amphibolite margin

magnetic measurements aboard a stratospheric balloon abstract we have measured the total intensity of the earth s magnetic field above the western mediterranean along a 2000 km profile flown at an altitude of 40 km a proton magnetometer was aboard a cnes centre national d etudes spatiales stratospheric balloon the core field and transient external field contributions have been carefully determined and removed from the data the resulting profile reveals significant anomalies of 10 20 nt amplitudes with 100 1200 km wavelengths these anomalies have a clear crustal origin and can be related to heterogeneities in magnetisation in the oceanic algero provencal basin and to major faults of the spanish continental crust comparison of our new data with previous data obtained over the same area at about 1 km altitude aeromagnetic surveys and about 400 km altitude magsat demonstrates the potential of stratospheric balloon measurements in bridging the 200 1500 km gap that exists in the knowledge of crustal anomalies a gap that is unavoidable if only aeromagnetic and satellite like magsat data are used

can remanent magnetization in the deep crust contribute to long wavelength magnetic anomalies long wavelength aeromagnetic anomalies require sources in the deep crust that are far more magnetic than the measured induced magnetization of lower crustal rocks several scientists have suggested that remanence in the deep crust may provide the missing magnetization this is not likely either through stable and or viscous remanence bodies large enough to contribute to surface anomalies and carrying stable remanence probably cooled slowly enough that they are subdivided into zones of alternating polarity this subdivision sharply reduces the power in the long wavelength portion of the anomaly spectrum on the other hand if such a body carries a viscous remanence the viscous component will be acquired in such a way as to bring the total magnetization of the body into equilibrium with the earth s field this equilibrium total magnetization is only about 25 greater than the induced magnetization not enough to explain the discrepancy

field observations on spatial variability of surface hoar at the basin scale inclination and wind exposure within an area of about 3 km 2 four automatic weather stations were located within the study area one on level terrain and three across a ridge despite the good instrumentation the correlation between surface hoar growth and calculated sublimation rate was poor distinct spatial patterns of surface hoar growth were found surface hoar crystals were frequently larger at the ridge site than in the surroundings of the automatic weather station on level terrain the variation in surface hoar formation was mainly due to different prevailing wind regimes during the formation periods the surroundings of the automatic weather station on level terrain were under the influence of local katabatic winds that dried up the air so that growth conditions were locally less favorable our observations suggest that predicting surface hoar formation for complex alpine terrain on the basis of data from an automatic weather station the standard procedure in avalanche forecasting seems nearly impossible unless at least the local wind regime is known at high resolution 10 m for both surface hoar formation and surface hoar destruction observations suggest wind conditions to be most crucial for spatial variation

an energy and mass model of snow cover suitable for operational avalanche forecasting a numerical model has been developed to simulate energy and mass evolution of snow cover at a given location as a function of meteorological conditions precipitation air temperature humidity wind velocity and incoming short wave and long wave radiation this model named crocus was first tested on a well instrumented field during a whole winter showing its ability to simulate the important phenomena affecting the evolution of the snow layers high temperature gradients wetting compaction and melting freezing cycles a second test was conducted at two locations in the french network used for operational avalanche forecasting though the weather observations are made there only twice daily the snow profiles calculated by the model were very close to those obtained once a week by a pit observation crocus proved itself sufficient to be considered now as a useful objective tool for operational avalanche forecasting

time dependence of snow microstructure and associated effective thermal conductivity this paper presents a sequential evaluation of snow microstructure and its associated thermal conductivity under the influence of a temperature gradient temperature gradients from 28 to 45 k m 1 were applied to snow samples having a density range 180 320 kg m 3 the experiments were conducted inside a cold room in a specially designed heat flux apparatus for a period of 4 weeks a constant heat flux was applied at the base of the heat flux apparatus to produce a temperature gradient in the snow sample a steady state approach was used to estimate the effective thermal conductivity of snow horizontal and vertical thick sections were prepared on a weekly basis to obtain snow micrographs these micrographs were used to obtain snow microstructure using stereological tools the thermal conductivity was found to increase with increase in grain size bond size and grain and pore intercept lengths suggesting a possible correlation of thermal conductivity with snow microstructure thermal conductivity increased even though surface area and area fraction of ice were found to decrease the outcome suggests that changes in snow microstructure have significant control on thermal conductivity even at a constant density

vapor transport grain growth and depth hoar development in the subarctic snow measurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow the fundamental processes of dry snow metamorphism due to extreme temperature gradients the subarctic pack undergoes extensive depth hoar metamorphism by the end of the winter a five layered structure with a pronounced weak layer near the base of the snow evolves grain size increases by a factor of 2 3 while the number of grains per unit mass decreases by a factor of 10 observed growth rates require significant net inter particle vapor fluxes stable isotope ratios show that there are also significant net layer to layer vapor fluxes soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack end of winter density profiles for snow on the ground compared with snow on tables indicate the net layer to layer vapor flux averages 6 x 10 7 kg m 2 s 1 though detailed measurements show the net flux is episodic and varies with time and height in the pack with peak net fluxes ten times higher than average a model driven by observed temperature profiles reproduces the layer to layer flux pattern and predicts the observed weak layer at the base of the snow calculated layer to layer vapor fluxes are ten times higher than inter particle fluxes which implies that depth hoar grain growth is limited by factors other than the vapor supply this finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow and it explains why grains can metamorphose into different forms so readily

the origin of magnetic anomalies in lower crustal rocks labrador mid to lower crustal granulites near wilson lake in central labrador exhibit pronounced aeromagnetic anomalies areas characterized by high and low field strength correlate with the wilson lake allochthon and the surrounding autochthon respectively the induced magnetization in both units has mean values of 0 34 a m allochthon and 0 33 a m autochthon however the mean values of remanent magnetizations are 9 2 a m allochthon and 3 3 a m autochthon in directions contributing to the present geomagnetic field therefore the remanent magnetization plays the main role in generating the observed magnetic anomaly thermal demagnetizations indicate that titanhematite is stable to 585 600 c the absence of significant verwey transitions indicates that titanhematite is the predominant carrier in some rocks and a significant carrier along with magnetite in others any interpretation of the magnetic anomalies above these rocks must consider magnetic remanence as the principle anomaly source

uplift and cooling magnetisation record in the bamble and telemark terranes sveconorwegian orogenic belt se norway and the grenville sveconorwegian loop article i nfo the 1100 ma sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against and onto the south western margin of the fennoscandian shield to resolve the magnetic signature acquired during post orogenic uplift and cooling and evaluate wider correlations with the contemporaneous grenville belt of north america this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium high metamorphic grade bamble terrane 48 sites and 390 cores and the adjoining medium low grade telemark terrane 33 and 240 cores juxtaposed by an orogen parallel porsgrunn kristiansand fault zone with major vertical displacement magnetite and ilmeno hematites are magnetic carriers with the latter more significant in the higher metamorphic grades magnetic intensities are stronger in the higher grade terrane presumably due to the growth of metamorphic ferromagnets but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust anisotropy of magnetic susceptibility ams largely reflects the ne sw tectonic grain of the last stage of sveconorwegian ductile deformation the magnetisation record is filtered by excluding magnetisations possibly acquired during regional mesozoic dyke emplacement development of the permo carboniferous oslo rift and late proterozoic magmatism the remaining record is a dual polarity signature summarised by mean poles at 31 9 n 50 9 e n 191 components in the bamble terrane and at 34 2 n 58 9 e n 151 components in the telemark terrane however these means are non fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best defined parts

magsat data and curie depth below deccan flood basalts india ground and airborne magnetic data are severely disturbed due to random susceptibility variations in deccan flood basalts however magnetic satellite magsat data over the deccan flood basaltic region of the indian subcontinent exhibit filtering of surficial noise three passes over deccan traps show a low at about 20 n latitude and a high at about 23 n latitude spectral analysis of these passes and an available 2 d magsat vertical intensity map indicate a deep 40 4 km magnetic interface it is interesting to note that the determination of curie depth from magsat matches and confirms the geothermal data model the estimates correspond to the moho depth derived from gravity and deep seismic sounding studies the study suggests a continental shield like geothermal gradient of about 14 c km below the area

limitations of the long wavelength components of the north american magnetic anomaly map the magnetic anomaly map of north america and its related data set provide the opportunity not only to view the obvious short wavelength anomalies 2600 km are strongly affected by core field components that have not been completely removed from the north american data set furthermore the piecewise matching of the magnetic anomalies of adjacent survey areas in the map compilation has contaminated the intermediate wavelength anomalies 300 2600 km

curie temperature depth estimation using a self similar magnetization model summary the earth s crust is magnetized down to the curie temperature depth at about 10 to 50 km this limited depth extent of the crustal magnetization is discernible in the power spectra of magnetic maps of south africa and central asia at short wavelengths the power increases as rapidly towards longer wavelengths as expected for a self similar magnetized crust with unlimited depth extent above wavelengths of about 100 km the power starts increasing less rapidly indicating the absence of deep seated sources to quantify this effect we derive the theoretical power spectrum due to a slab carved out of a self similar magnetization distribution this model power spectrum matches the power spectra of south africa and central asia for a self similarity parameter of 4 and curie temperature depths of 15 to 20 km

magnetism at depth a view from an ancient continental subduction and collision zone recent sophisticated global data compilations and magnetic surveys have been used to investigate the nature of magnetization in the lower crust and upper mantle two approaches to constraining magnetizations are developed providing minimum 0 01 si and maximum 0 04 si susceptibility estimates given some assumed thickness 15 km here these values are higher than are found in many continental rocks are there rocks deeper in the crust or upper mantle that are more magnetic than expected or are the model assumptions incomplete what is the magnetic behavior of deep crustal and upper mantle rocks when slightly cooler than the curie or neel temperatures of their magnetic minerals after being exhumed from locations of high grade metamorphism at greater depth different sets of equilibrium metamorphic minerals can be considered that would form under different conditions results on 1501 samples from the western gneiss region wgr norway mainly from mafic and ultramafic bodies subducted to depths of 60 200 km and temperatures of 750 up to 950 c at the very highest pressures show that rocks did not fully equilibrate to the dominant metamorphic facies conditions there is a large variation in petrophysical properties oxide minerals and mineral assemblages in wgr samples though they cannot explain the broad high amplitude deep seated anomalies measured in this region the presence of magnetite and exsolved titanohematite and hemo ilmenite in samples shows those magnetic phases are preserved even at eclogite facies conditions in part because complete eclogite facies equilibrium was rarely achieved

geochemistry of precambrian basal gneisses in lofoten vesteralen northern norway abstract the basal gneiss complex of lofoten vesteralen consists of an older ca 2 7 ga migmatite complex believed to be largely supracrustal in origin and a younger ca 1 8 ga cover complex of clearly supracrustal origin the major element geochemistry of both groups is that of an igneous suite of generally calc alkaline character similarities with the volcanic rocks of modern continental margin environments extend to trace element contents and ree patterns a complex multistage evolution is inferred an orthopyroxene isograd bisects the area element fractionation between amphibolitefacies and granulite facies rocks appears to be limited to some trace elements ree patterns are not affected by the metamorphism

marine magnetic anomalies over the lord howe rise and the tasman sea implications for the magnetization of the lower continental crust abstract marine geophysical and geological data provide evidence for a continental origin of the lord howe rise a submarine plateau east of australia our marine geomagnetic data reveal a long wavelength anomaly associated with the western margin of the rise forward and inverse modeling shows that the anomalies can be best explained by strong magnetization of the lower crust of the lord howe rise a geomagnetic map of the northern tasman sea that includes dampier ridge another elongated piece of presumably continental crust shows positive anomalies over areas of continental origin and negative ones over the deep sea basin this indicates higher values of the product of magnetization times thickness for the continental crust than for the oceanic crust three dimensional model calculations show that the high magnetization in the lower crust of eastern australia is limited to a maximum depth of 15 20 km this is in accordance with geothermal results that indicate the curie isotherm is at a shallow depth there the anomalies of the tasman sea are negative due to the negative part of the edge anomaly on the seaward side of the continental margin it is not necessary to hypothesize strong reversed magnetization in the lower oceanic crust and upper mantle

an equivalent layer magnetization model for australia based on magsat data abstract an equivalent layer magnetization model for australia and adjacent oceanic areas is presented the model is obtained by linear inversion of magsat anomaly data measured in the altitude range 325 550 km the anomaly data set has been isolated from the raw data set by use of models of the core field and very long wavelength external fields and is internally consistent certain major structural features of the australian continent are geographically associated with magnetization anomalies a first order difference is seen between the tasman zone and the precambrian cratonic areas magnetization anomalies are much more subdued in the former possibly reflecting a shallowing of the curie isotherm within the crust a profile of the vertical integral of magnetization is presented for a crustal section extending from the gawler block to the southeast coast it is shown that the magnetization variations are probably due partly but not wholly to depth to curie isotherm variations gross magnetization variations among at least three distinct crustal units must be involved

grain size evolution of polar firn a new empirical grain growth parameterization based on x ray microcomputer tomography measurements firn microstructure properties from six different sites in greenland and antarctica are investigated by means of x ray microcomputer tomography the optical effective radius is calculated from the specific surface area ssa and used as a measure of grain size it is shown that the recently introduced spherical approximation of firn grains using the effective radius r eff is representative of grain size in the microwave frequency region the measured profiles show the well known increase of grain size with depth at all sites where the increase is largest at near surface depths a large variability in grain size on the decimeter to centimeter scale as a result of different grain properties of single layers is superimposed on the overall trend at each site a simple empirical parameterization of grain size evolution is developed which allows the rapid grain growth in the uppermost layers of the firn to be predicted the growth is driven by strong seasonal and diurnal temperature gradients the model can be used to simulate grain size profiles required by models of firn microwave interaction e g for retrieval of accumulation rates from satellite microwave sensors in a more realistic fashion