high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

high efficiency of natural lamellar remanent magnetisation in single grains of ilmeno hematite calculated using mossbauer spectroscopy rocks from large remanent magnetic anomalies on earth have been found to contain exsolved rhombohedral oxides which also have been proposed as earth analogues for the rocks creating the observed large remanent anomalies on mars theoretical considerations and previous case studies of natural rocks have shown that the natural magnetisation is carried by lamellar magnetism due to uncompensated moments at the interface between antiferromagnetic hematite and paramagnetic ilmenite here single grains 250 m of titanohematite with ferri ilmenite exsolution lamellae from mesoproterozoic metamorphic rock samples in southwest sweden and the adirondack mountains usa are studied using room temperature mossbauer spectroscopy to identify possible characteristic magnetic signatures of lamellar magnetism mossbauer spectra of synthetic samples of titanohematite fe0 95ti0 05o3 and fe0 9ti0 1o3 were collected for comparison and showed a dominant six line magnetic spectrum due to fe3 in titanohematite with a weak sextet due to fe2 fe3 charge transfer mossbauer spectra of the natural ilmeno hematite grains are similar to those for synthetic titanohematite but contain additionally two paramagnetic doublets corresponding to fe2 and fe3 in ilmenite however several grains also contain an additional weak broad magnetic component that we assign to iron in contact layers according to the lamellar magnetism model similar to the previous mossbauer results for natural hemo ilmenite there is no evidence for superparamagnetic behaviour of the nanoscale titanohematite lamellae contained within coarser ferri ilmenite lamellae and no evidence for single domain or superparamagnetic magnetite the compositions of titanohematite and ferri ilmenite in the individual ilmeno hematite grains calculated from the mossbauer area ratios show that compositions are closer to end member values than the compositions inferred from electron microprobe and transmission electron microscopy on the same grains consistent with observations that lamella thicknesses are as small as a few nm bulk compositions of ilmeno hematite grains calculated from the mossbauer data confirm that the swedish samples are significantly more ilmenite rich than the adirondack samples and agree with an estimation of bulk composition through point counting of electron backscatter images the mossbauer data allow a quantitative estimation of contact layer abundance which is used to determine the efficiency of lamellar natural remanent magnetisation acquisition because minerals with a high natural remanent magnetisation efficiency are expected to create larger remanent magnetic anomalies contact layer abundance determination by mossbauer spectroscopy provides a valuable new tool for mineral based magnetic anomaly interpretation on earth and other planetary bodies

magnetic properties and potential field modeling of the peculiar knob metamorphosed iron formation south australia an analog for the source of the intense martian magnetic anomalies an extremely intense 120 a m 1 remanence directed steeply upward this ancient remanence reinforces the local earth s field inclination 63 a simple geological model constrained by drilling and physical property measurements explains both the observed magnetic and gravity anomalies consistent with the poisson theorem koenigsberger ratios qs of 10 and greater as found here are rare in nature we postulate that acquisition of a thermoremanent magnetization trm by the ore during

decompressed eclogites in the sveconorwegian grenvillian orogen of sw sweden petrology and tectonic implications relict eclogites and associated high pressure rocks are present in the eastern segment of the sw swedish gneiss region the tectonic counterpart of the parautochthonous belt of the canadian grenville these rocks give evidence of sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions the best preserved eclogite relics suggest a clockwise p t t history beginning in the amphibolite facies progressing through the eclogite facies decompressing and partially reequilibrating through the high and medium pressure granulite facies before cooling through the amphibolite facies textures demonstrate the former coexistence of the plagioclase free assemblages garnet clinopyroxene quartz rutile ilmenite garnet clinopyroxene kyanite rutile and garnet kyanite quartz rutile the former existence of omphacite is evidenced by up to 45 vol plagioclase expelled as small grains within large clinopyroxene matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine grained granulite facies reaction domains formed during resorption of garnet and kyanite garnet shows preserved prograde growth zoning with rimward increasing pyrope content decreasing spessartine content and decreasing fe fe mg ratio but is partly resorbed and reequilibrated at the rims p t estimates from microdomains with clinopyroxene plagioclase quartz garnet indicate pressures of 9 5 12 kbar and temperatures of 705 795 c for a stage of the granulite facies decompression the preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years and chemical disequilibrium and frozen reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies together these features suggest a rapid tectonic exhumation the eclogite relics occur within a high grade deformation zone with wnw ese stretching and associated oblique normal sense top to the east sensu lato displacement suggesting that extension was a main cause for the decompression and exhumation probable tectonic scenarios for this deformation are sveconorwegian late orogenic gravitational collapse or overall wnw ese extension

evolution of amphibolite facies structural features and boundary conditions for deformation during exhumation of high and ultrahigh pressure rocks nordoyane western gneiss region norway 1 geologic mapping and structural analysis of baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high pressure rocks where strain partitioning has preserved evidence for interpreting the evolution of late structural features the earliest of these were extensional detachments juxtaposing eclogite facies rocks against overlying amphibolite facies rocks that show no evidence for eclogite facies metamorphism these early detachments are strongly overprinted and complexly folded and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels younger more localized mylonite zones formed synchronously with tubular sheath isoclinal tight and open folding that shows a progression from wnw to ene trends the earliest mylonite zones interpreted as originally subhorizontal range in strike through a 20 angle from 110 to 90 later steeply dipping mylonite zones formed under lower amphibolite facies conditions strike 75 and locally truncate earlier structures the youngest mylonite zones formed under lowest amphibolite conditions strike 50 and truncate all earlier structures folds developed during this progression show the range in orientation from wnw to ene reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing these changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation l s fabrics absence of axial planar foliation and chaotic orientations of axial surfaces of granulite to amphibolite facies folds indicate formation in a constrictional strain field this is also supported by estimates of the finite strain accumulated at 780 c and 45 km and similar observations by previous workers assuming a simple monoclinic deformation for transtension strain estimates and structural measurements indicate apparent transtensional angles of 9 11 that increased to greater than 20 and then decreased to less than 20 these changes in transtensional angle agree with changes in the orientation of the x y plane of strain during exhumation from 45 km to less than 20 km depth and appear to reflect changes in the boundary conditions of deformation this upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of hp and uhp metamorphic rocks

u pb zircon geochronology of eclogites from the scandian orogen northern western gneiss region norway 14 20 million years between eclogite crystallization and return to amphibolite facies conditionsthis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh reconstructing tectonic histories involving continental collision subduction and exhumation at plate tectonic rates of 1 cm year requires precise u pb zircon geochronology the western gneiss region has exceptional exposures of high pressure hp and ultra high pressure uhp rocks the strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation respectively the oldest eclogite sampled is 415 1 ma from layered probably uhp eclogite at tevik averoya also with a garnet hornblende assemblage at 410 1 ma the flem gabbro eclogite margin with implied uhp conditions is 410 2 ma hornblende eclogite at seth lepsoya never at uhp is 412 2 ma these compare to devonian ages of 401 1 ma for overgrowths on proterozoic baddeleyite in selnes gabbro 402 2 ma for coesite eclogite at hareidlandet 405 400 ma for coesite eclogite at flatraket and 405 2 ma for near uhp eclogite at hjelmeland

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

magnetic properties of anorthosites a forgotten source for planetary magnetic anomalies 1 anorthosites igneous rocks very rich in plagioclase rarely considered to be strongly magnetic are common on earth and the moon and inferred to be on other planets magnetic properties of anorthosites could be important in investigating associated mineral deposits and in studying magnetic anomalies especially on mars here we investigate three late proterozoic anorthosites in rogaland norway for magnetic and petrographic properties two of the anorthosites have large natural remanent magnetization nrm with intensities comparable to tertiary basalts susceptibility nrm and hysteresis properties provide information about the magnetic minerals present and their response to inducing fields microscopic observations show ubiquitous hemo ilmenite in the anorthosites whereas magnetite is common in the haland helleren but rare in the ana sira and egersund ogna bodies this study illustrates that anorthosites can be important sources of magnetic anomalies and can retain a remanent field over geologic time it also supports the recently described property of lamellar magnetization

mapping magnetic sources at the millimeter to micrometer scale in dunite and serpentinite by high resolution magnetic microscopy abstract rock samples can have wide range of magnetic properties depending on composition amount of ferromagnetic minerals grain sizes and microstructures here we used scanning magnetic microscopy a highly sensitive and high resolution magnetometric technique to map remanent magnetic fields over a planar surface of a rock sample the technique allows for the investigation of discrete magnetic mineral grains or magnetic textures and structures with submillimeter scale resolution here we present a case study of magnetic scans of pristine and serpentinized dunite thin sections from the reinfjord ultramafic complex in northern norway the magnetic mineralogy is characterized by electron microprobe scanning electron and optical microscopy and with rock magnetic methods in serpentinized samples the magnetic carrier is end member magnetite occurring as large discrete grains and small grains in micron scale veins by contrast the pristine dunite sample contains large cr spinel grains with very fine equant exsolutions ranging in composition from ferrichromite to end member magnetite forward and inverse modeling of the magnetic anomalies is used to determine the remanent magnetization directions and intensities of discrete magnetic sources observed in the scanning magnetic microscopy the fine scale magnetization of the rock sample is used to investigate the magnetic carriers and the effect of serpentinization on the magnetic properties of the dunite modeling shows that the dipolar magnetic anomalies that are mapped by scanning magnetic microscopy are caused by grains with heterogeneous magnetic sources the intensity of the magnetization and the amount of magnetic minerals are higher in the serpentinized sample than the pristine dunite sample consistent with the measured bulk magnetic properties furthermore the serpentinized samples show a larger variability in the direction of the magnetization and a stronger heterogeneity with respect to the pristine sample the ability to rigorously associate components of the bulk magnetic properties to individual mineral phases creates new possibilities for rock magnetic paleomagnetic and exploration applications

correlated shear and bulk moduli to 1400 km beneath the mediterranean region we present a new model for the shear and compressional wave velocity structure of the mediterranean plate boundary region the model is obtained by a combined inversion of s and p data where we solve in one single step for shear wave and bulk sound velocity variations the ratios between the types of seismic velocities inferred from the combined inversion constrain the origin of heterogeneity our model is dominated by fast anomalies from subducted oceanic lithosphere we found that only a slight amount of bulk sound velocity heterogeneity is required to explain the combined set of p and s data thus compressional wave velocity anomalies are dominated by shear modulus heterogeneity and the ratio between bulk sound and shear wave velocity heterogeneity suggests a predominantly thermal origin 2006 elsevier b v all rights reserved

gravity measurement amazing grace two satellites that will chase one another around the globe are poised to map the earth s gravitational field with unprecedented accuracy researchers are feeling the pull of the data says david adam

the relationship between exsolution and magnetic properties in hemo ilmenite insights from mossbauer spectroscopy with implications for planetary magnetic anomalies 1 remanence properties of ilmenites with exsolved hematite have recently been the object of much study because their high natural remanent magnetization nrm is greater than predicted for these two minerals x ray diffraction xrd and mossbauer spectroscopy in conjunction with electron microprobe and nrm measurements were used to explore the possibility that single domain sd hematite lamellae in the ilmenite host could explain the observed remanence and to determine if magnetite is present xrd results show the presence of ilmenite and hematite mossbauer data show that only species assigned to hematite ilmenite and a small amount of tetrahedral fe3 are present magnetic properties at high and low t also indicate that the only magnetic minerals are ilmenite and hematite magnetic data suggest that ultra fine hematite lamellae are magnetically ordered and their resultant remanent magnetic anomalies may contribute significantly to magnetism on terrestrial planets even those without present day magnetic fields

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

a closer look at remanence dominated aeromagnetic anomalies rock magnetic properties and magnetic mineralogy of the russell belt microcline sillimanite gneiss northwest adirondack mountains new york a large distinct negative aeromagnetic anomaly of over 2000 nt associated with microcline sillimanite quartz gneisses in the russell area northwest adirondack mountains was previously shown to be remanence dominated although the carriers of remanence were not well documented russell belt gneisses have a strong natural remanent magnetization with steep remanence directions d 263 i 58 an average intensity of 3 6 a m and typical susceptibilities of 10 4 si the remanence is thermochemical in origin acquired during cooling from peak metamorphic conditions of 650 750 c during the ottawan orogen 1050 1080 ma the reversed polarity of remanence reflects a reversed paleofield rather than self reversed contrary to earlier suggestions the gneisses contain up to 3 oxide predominantly metamorphic titanohematite which accounts for the low susceptibility values and highly stable remanence optical observations show titanohematite grains with multiple generations of ilmenite pyrophanite rutile and spinel exsolution lamellae microprobe analyses confirm titanohematite compositions ranging from 72 to 97 fe2o3 with hematite83 being most typical in rare samples inclusions of magnetite were identified the ubiquitous presence of titanohematite and the rare occurrence of magnetite is supported by thermal and alternating field demagnetization studies saturation magnetization measurements hysteresis properties temperature hysteresis studies and low temperature remanence measurements numerous crustal granulites have titanohematite as part of the oxide assemblage and this may contribute a strong remanent component to what have previously been considered to be solely induced anomalies

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

a seismic hazard scenario for the main cities of crete island greece summary a probabilistic seismic hazard analysis is applied to the sites of the main cities chania rethymno and heraklion of crete island greece to compute probabilities of exceedance of specified values of peak ground acceleration pga and to estimate maximum possible pga at each site the methodology allows the use of historical or instrumental data or a combination of both the instrumental part of the data can be divided into subcatalogues with each having an individual minimum threshold magnitude for completeness also incorporated into the procedure were recently published maximum possible magnitudes for each site 6 48 5 21 7 52 for chania rethymno and heraklion respectively and an attenuation law for shallow seismicity in greece ln pga 3 52 0 70m w 1 14 ln r 2 h 2 1 2 0 12s 0 70 where r and h0 are respectively the epicentral distance and focal depth in kilometres s is 0 1 or 2 depending on soil conditions and pga is in units of cm s 2 the maximum pga for heraklion varies from 0 130g s 0 rock to 0 165g s 2 soft all three sites are coastal and alluvium is the dominant soil type for which s 2 probabilities that a given pga will be exceeded at least once during time intervals of 1 5 25 and 50 yr were also computed additionally maximum pga values associated with design earthquakes were computed for earthquakes at epicentral distances of 10 5 km median values of maximum pga were 0 23g 0 09g and 0 42g at chania rethymno and heraklion respectively

adaptively parametrized surface wave tomography methodology and a new model of the european upper mantle summary in this study we aim to close the gap between regional and global traveltime tomography in the contextofsurfacewavetomographyoftheuppermantleimplementingtheprincipleofadaptive parametrization observations of seismic surface waves are a very powerful tool to constrain the3 dstructureoftheearth suppermantle includingitsanisotropy becausetheysamplethis volumeefficientlyduetotheirsensitivityoverawidedepthrangealongtheraypath onaglobal scale surface wave tomography models are often parametrized uniformly without accounting for inhomogeneities in data coverage and as a result in resolution that are caused by effective under or overparametrization in many areas if the local resolving power of seismic data is not taken into account when parametrizing the model features will be smeared and distorted in tomographic maps with subsequent misinterpretation parametrization density has to change locally formodelstoberobustlyconstrainedwithoutlosinganyaccurateinformationavailable in the best sampled regions we have implemented a new algorithm for upper mantle surface wave tomography based on adaptive voxel parametrization with voxel size defined by both the hit count number of observations sampling the voxel and azimuthal coverage how welldifferentazimuthswithrespecttothevoxelarecoveredbythesource stationdistribution high image resolution is achieved in regions with dense data coverage while lower image resolution is kept in regions where data coverage is poorer this way parametrization is everywhere tuned to optimal resolution minimizing both the computational costs and the non uniqueness of the solution the spacing of our global grid is locally as small as 50 km we apply our method to identify a new global model of vertically and horizontally polarized shear velocity with resolution particularly enhanced in the european lithosphere and upper mantle we find our new model to resolve lithospheric thickness and radial anisotropy better than earlier results based on the same data robust features of our model include for example the trans european suture zone the panonnian basin thinned lithosphere in the aegean and western mediterranean possible small scale mantle upwellings under iberia and massif central subduction under the aegean arc and a very deep cratonic root underneath southern finland

coesite micro inclusions and the u pb age of zircons from the hareidland eclogite in the western gneiss region of norway abstract we have identified by laser micro raman spectroscopy that inclusions of coesite occur together with other eclogite facies mineral phases within metamorphic zircons separated from the large eclogite body at ulsteinvik dimnoy on hareidland this is the first identification of coesite from this portion of the northwestern western gneiss region wgr and supports continuity of ultrahigh pressure uhp metamorphism between the documented coesite occurrences on stadlandet to the south and the microdiamond and coesite pseudomorph localities on fjortoft in the nordoyene to the north the zircons first analysed by u pb tims in 1973 have been re analysed and have yielded a much more precise age of 401 6 1 6 ma that overlaps with the previously determined age our discovery of coesite and the indication of a close to 402 ma formation age add to a growing number of mid late early devonian ages that signal that the uhp metamorphism in this part of west norway occurred relatively late in the caledonian orogenic cycle these observations should be incorporated in geodynamic models for the exhumation of these rocks and for the metastable preservation of eclogite facies mineralogies

sea level changes and δ18o record for the past 34 000 yr from mayotte reef indian ocean new sea level and 18o curves for the past 34 000 yr based on uranium thorium chronology are proposed for the southwestern part of the indian ocean the archives include cores drilled from onshore coral reefs and submersed samples from foreslope corals of mayotte in the comoro islands the mayotte sea level curve shows a lowstand of 145 5 m below the present level during the last glacial maximum dated at 18 400 yr this lowstand is supported by the maximum18o enrichment in the coral colonies the residual signal 18o controlled by sea surface temperature changes indicates that surface waters 18 400 yr ago were approximately 5 c cooler than present the deglacial sea level rise is clearly recorded with a mean rate of about 1 7 cm yr 1between 18 400 and 10 000 yr ago the deglaciation phase is characterized by a strong18o depletion marked by two pulses related to meltwater discharges into the north atlantic ocean but also characterized by responses specific to the tropical indian ocean

regional magnetic anomalies crustal strength and the location of the northern cordilleran fold and thrust belt the northern cordilleran fold and thrust belt in canada and alaska is at the boundary between the broad continental margin mobile belt and the stable north american craton the fold and thrust belt is marked by several significant changes in geometry cratonward extensions in the central yukon territory and northeastern alaska are separated by marginward re entrants these geometric features of the cordilleran mobile belt are controlled by relations between lithospheric strength and compressional tectonic forces developed along the continental margin regional magnetic anomalies indicate deep thermal and compositional characteristics that contribute to variations in crustal strength our detailed analysis of one such anomaly the north slope deep magnetic high helps to explain the geometry of the fold and thrust front in northern alaska this large magnetic anomaly is inferred to reflect voluminous mafic magmatism in an old devonian extensional domain the presence of massive amounts of mafic material in the lower crust implies geochemical depletion of the underlying upper mantle which serves to strengthen the lithosphere against thermal erosion by upper mantle convection we infer that deep source magnetic highs are an important indicator of strong lower crust and upper mantle this stronger lithosphere forms buttresses that play an important role in the structural development of the northern cordilleran fold and thrust belt

scattered wave imaging of the lithosphere asthenosphere boundary several recent studies have imaged discontinuities interpreted as the lithosphere asthenosphere boundary using receiver function methods these are exciting results since this boundary is fundamental and yet relatively elusive its location is not well imaged nor are its properties well understood the receiver function method provides high depth resolution and the multitude of studies using this method have increased lateral resolution a better global understanding of the location and nature of the boundary is developing which ultimately will have implications for mantle dynamics and planetary evolution however these new results do not all form a clear and consistent picture and in some cases introduce new complexities and questions some apparent contradictions may represent unmodeled earth structure overall interpretation and synthesis requires careful consideration of the approaches and sensitivities of individual studies

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

deformation enhanced metamorphic reactions and the rheology of high pressure shear zones western gneiss region norway microstructural and petrological analysis of samples with increasing strain in high pressure hp shear zones from the haram garnet corona gabbro give insights into the deformation mechanisms of minerals rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions scanning electron microscopy with electron backscattering diffraction sem ebsd compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization plagioclase with a shape preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the 0 0 1 1 0 0 slip system in omphacite crystallographic preferred orientations indicate slip on 1 0 0 0 0 1 and 1 1 0 0 0 1 during deformation fine grained garnet deformed by diffusion creep and grain boundary sliding ilmenite deformed by dislocation glide on the basal and at higher strains prism planes in the a direction relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500 650 c and 0 5 1 4 gpa and continued during prograde metamorphism to ultra high pressure uhp conditions both products and reactants show evidence of syn and post kinematic growth indicating that prograde reactions continued after strain was partitioned away the restriction of post kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away demonstrating that deformation played an important role in enhancing metamorphic reactions two important consequences of deformation observed in these shear zones are i the homogenization of chemical composition gradients occurred by mixing and grain boundary migration and ii composition changes in zoned metamorphic garnet by lengthening diffusion distances the application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100 150 mpa and lower limits for strain rates of 10 12 to 10 13 s 1 as deformation conditions for the shear zones in the haram gabbro that were produced during subduction of the baltica craton and resulted in the production of hp and uhp metamorphic rocks

petrophysical constraints on magnetic anomalies associated with metamorphic reactions in northern saskatchewan canada1 the western chipman domain of the east athabasca mylonite triangle in northern saskatchewan canada displays a large positive aeromagnetic anomaly that is the result of retrograde magnetite produc

subduction zone magnetic anomalies and implications for hydrated forearc mantle continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate magnetite is a significant byproduct of mantle hydration and forearc mantle cooled by subduction should contribute to long wavelength magnetic anomalies above subduction zones we test this hypothesis with a quantitative model of the cascadia convergent margin based on gravity and aeromagnetic anomalies and constrained by seismic velocities and find that hydrated mantle explains an important disparity in potential field anomalies of cascadia a comparison with aeromagnetic data thermal models and earthquakes of cascadia japan and southern alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

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

metamorphic zircon formation at the transition from gabbro to eclogite in trollheimen surnadalen norwegian caledonides a transition from gabbro to eclogite has been investigated at vinddoldalen in south central norway with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct u pb zircon age of the metamorphic process in the different rocks of the transition zone zircon occurs as i igneous prismatic grains ii metamorphic polycrystalline rims and pseudomorphs after baddeleyite and iii as tiny 10 mu m bead like zircon grains textural relations suggest that type ii zircon formed by breakdown of baddeleyite in the presence of silica whereas fe ti oxides were the main zr source for the type iii zircon subsolidus liberation of zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid assisted metamorphism and by resorption of fe ti oxide in rock domains that were completely recrystallized to eclogite sims secondary ion mass spectrometry and tims thermal ionization mass spectrometry dating provides comparable u pb ages of magmatic zircon and baddeleyite baddeleyite tims yielded an age of 1457 11 ma for the gabbro emplacement bead type metamorphic zircon from eclogite gave 425 10 ma tims dating the metamorphic transition from gabbro to eclogite in the upper basement of the lower allochthon in the south central scandinavian caledonides less

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

remanent and induced magnetic anomalies over a layered intrusion effects from crystal fractionation and magma recharge abstract the bjerkreim sokndal bks norite quartz mangerite layered intrusion is part of the early neoproterozoic rogaland anorthosite province intruded into the fennoscandian shield in south norway at 930 ma the bks is exposed over an area of 230 km 2 with a thickness of 7000 m and is of economic interest for ilmenite magnetite and apatite deposits from the point of view of magnetic minerals in the course of fractional crystallization and magma evolution the ilmenite becomes less fe 3 rich reflected by a change from ilmenite with hematite exsolution to nearly pure ilmenite magnetite starts to crystallize relatively late in the intrusive history but its crystallization is interrupted by influxes of more primitive magma the variations in aeromagnetic and ground magnetic anomalies measured over the bks can be explained in terms of the measured magnetic properties of nrm susceptibility and hysteresis presented here and in terms of mineralogy early layers in the intrusion contain hemo ilmenite as the magma evolved and magnetite started to crystallize this caused a distinct change over the layering from remanence controlled negative anomalies to induced positive anomalies when new more primitive magma was injected into the system hemo ilmenite returned as the major oxide and the resulting magnetic anomalies are again negative the most dramatic change in the magnetic signature is in the upper part of the intrusion in mcu ive where magnetite became a well established cumulate phase as indicated by susceptibility but its induced magnetization is overcome by large nrms associated either with hemo ilmenite or with hemo ilmenite and magnetite exsolved from pyroxenes the average natural remanent magnetizations change from 3 a m in mcu ivd to 15 a m in mcu ive and back to 2 a m in the overlying mcu ivf producing a strong negative remanent anomaly that has been followed along strike for at least 20 km by ground magnetic measurements the highly varied magnetic properties of this intrusion caused by varied magmatic crystallization of combinations of opaque minerals illustrate some of the possibilities to be considered in evaluating crustal magnetic anomalies

aeromagnetic anomalies magnetic petrology and rock magnetism of hemo ilmenite and magnetite rich cumulate rocks from the sokndal region south rogaland norway aeromagnetic maps of the egersund mid proterozoic igneous province show a spectacular range of positive and negative magnetic anomalies with a contrast up to 15 600 nt the positive magnetic anomalies are over magnetite norites and overlying mangerites and quartz mangerites of the bjerkreim sokndal layered intrusion these rocks are dominated by multi domain md magnetite the negative magnetic anomalies are over ilmenite rich norites of the bjerkreim sokndal layered intrusion the tellnes ilmenite norite ore deposit and massif anorthosites these rocks are dominated by hemo ilmenite and or by silicates containing fine grained oxide exsolution lamellae electron microprobe analyzes of coexisting fe ti oxides in the layered intrusion confirm earlier observations that oxides in early magmatic rocks are dominated by hemo ilmenite with minor end member magnetite followed by more reduced oxides dominated by titanomagnetite with minor near end member ilmenite what is not fully understood is the property of ilmenite with hematite exsolution lamellae or even more striking hematite with ilmenite lamellae to produce strong remanent magnetization of high coercivity and with a neel temperature equal to or above the curie temperature of magnetite this property makes the rhombohedral oxides an important candidate to explain some high amplitude deep crustal anomalies on earth or strong remanent magnetization on other planets a remarkable feature in the egersund province is that primitive magmas produced rocks rich in hemo ilmenite causing negative magnetic anomalies related to magnetic remanence and more evolved magmas produced rocks rich in magnetite related to positive induced magnetic anomalies all in the course of crystallization differentiation

fluid and deformation induced metamorphic processes around moho beneath continent collision zones examples from the exposed root zone of the caledonian mountain belt w norway abstract exposed high pressure hp and ultra high pressure uhp metamorphic terrains have been studied in order to assess the metamorphic processes and their role in changing petrophysical properties near moho depth in continental root zones the investigation points to a critical role of fluid and deformation in metamorphic transformation in the deep crust and upper mantle this applies to a formation of granulite facies areas b transformation of granulites to eclogites c retrogression of eclogite facies rocks to amphibolite and green schist facies rocks and d the spinel lherzolite to garnet lherzolite transition dry rocks both feldspar bearing and ultramafic remain with their pre hp and uhp structures and anhydrous mineralogy preserved while reactions occur where fluid has been introduced along deformation zones a mixture of metamorphic facies formed at widely variable times on metre to km scale will be present throughout the crust and upper mantle pseudotachylytes frictional melts or ultracomminuted material are observed in both ultramafic and feldspar bearing lithologies spatially associated with hp and uhp rocks suggesting that rock properties at moho depth allow earthquakes seismicity enhances the metamorphic and metasomatic transitions through fragmentation and by opening the rock for fluid influx ductile eclogite facies shear zones nucleate along the brittle structures these observations point to moho as a rock processing zone with the following facets 1 a metastable dry and strong lower crust and upper mantle 2 earthquakes and tremors result in fluid flow and hp metamorphism 3 a pronounced weakening of the hydrated and transformed rocks allows flow of material and the development of new fabrics lpo in the transformed rocks deep tremor and earthquakes at moho depth may record ongoing metamorphic transitions

magnetic petrology of variably retrogressed eclogites and amphibolites a case study from the hercynian basement of northern sardinia italy 1 this study constitutes the first attempt to integrate rock magnetism and metamorphic petrology in the hercynian basement of northern sardinia the investigation focused on the magnetic petrology of variably retrogressed eclogites and amphibolites from two suites of basic meta igneous rocks which occur along a major tectonic line posada asinara line within a medium grade mg and a high grade hg metamorphic complex consistent with petromagnetic results hg metabasites contain variable amounts of monoclinic pyrrhotite intergrown with rutile and titanomagnetite occurring as inclusions in garnet abundant ilmenite associated to secondary hornblende and with sphene low ti magnetite rims and rutile both as inclusions in ilmenite and as discrete grains in mg metabasites pyrrhotite is restricted to amphibolites of the posada valley area where it occurs as rare inclusions in garnet all samples are characterized by variable amounts of ilmenite rutile and sphene which show the same microstructural features described in hg rocks microstructural evidence and geothermobarometric data indicate that 1 pyrrhotite and titanomagnetite likely formed prior to and remained stable during the eclogite facies metamorphic peak and 2 the growth of ilmenite and sphene can be attributed to the amphibolite facies retrogression mainly due to model reactions such as garnet omphacite rutile h2o hornblende plagioclase ilmenite and amphibole ilmenite o2 sphene magnetite quartz h2o the results from our combined petrological and petromagnetic study corroborate the hypothesis that significant volumes of mafic ultramafic rocks similar to some of the investigated outcrops may account for the magnetic anomalies flanking the northeastern part of the posada asinara line

localized granulite and eclogite facies metamorphism at flatraket and krakeneset western gneiss region u pb data and tectonic implications

surface wave tomography in the european and mediterranean region summary we present a new surface wave tomographic study of the broad european and mediterranean region our goal is to enhance the resolution of previously published group velocity models using new data from european permanent seismic networks and a dense broad band array in northern apennines retreat we measure fundamental mode rayleigh and love wave group velocities from long period seismograms recorded at regional distance between 600 and 7000 km our measurement technique is based on iterative application of multiple filters and phase matched filters we accurately estimate dispersion curves for more than 1500 rayleigh wave and about 850 love wave paths in the period range 35 170 s consistency of measurements is evaluated by comparing ray clusters from sample earthquakes to closely spaced retreat stations in the whole data set measurement errors in group velocity decrease with increasing distance and show to be caused by inaccuracy in the estimate of group arrival time we calculate maps of love and rayleigh group velocity at selected periods by linear tomographic inversion accounting for group arrival time errors and evaluating a posteriori group slowness errors data coverage in this region is not uniform and it is highly influenced by the uneven distribution of earthquakes and seismic stations we therefore build a laterally heterogeneous reference model by inverting a global data set of group velocity derived from the phase velocity library of ekstr

superplasticity in garnet from eclogite facies shear zones in the haram gabbro haramsoya norway study of garnet adjacent to and within shear zones that underwent synkinematic eclogitization reveals microstructures and textures that for the first time indicate grain boundary sliding as the main deformation mechanism in garnet monomineralic garnet layers exhibit characteristic features of superplastic behavior and show little strength contrast to the surrounding minerals the low strength is caused by the synkinematic growth of garnet limiting the grain size to tens of micrometers this behavior may significantly influence rheologic properties of high and ultrahigh pressure shear zones in subduction environments and may play a role during the formation of garnet in the transition zone of the mantle

earth s lithospheric magnetic field determined to spherical harmonic degree 90 from champ satellite measurements summary the champ magnetic field mission is providing highly reliable measurements from which the global lithospheric magnetic field can be determined in unprecedented resolution and accuracy using almost 5 yr of data we derive our fourth generation lithospheric field model termed mf4 which is expanded to spherical harmonic degree and order 90 after subtracting from the full magnetic field observations predicted fields from an internal field model up to degree 15 an external field model up to degree two and the predicted magnetic field signatures for the eight dominant ocean tidal constituents we fit and remove remaining external fields and polar electrojet signatures in a track by track scheme from a subset of least disturbed tracks we estimate the mf4 model by least squares damping ill determined coefficients by regularization the resulting mf4 model provides a good representation of the lithospheric field down to an altitude of about 50 km at lower latitudes with reduced accuracy in the polar regions crustal features come out significantly sharper than in previous models in particular bands of magnetic anomalies along subduction zones become visible by satellite for the first time

protolith ages and exhumation histories of ultra high pressure rocks across the western gneiss region norway the timing of protolith formation ultrahigh pressure uhp subduction and subsequent exhumation for the ultrahigh pressure to high pressure units across the eastern part of the western gneiss region norway were assessed using u pb zircon th pb monazite and 40 ar 39 ar white mica ages u pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the baltica basement before the entire mass was subducted to ultra high pressure eclogites within the allochthons across the entire western gneiss region are caledonian and show a degree of zircon re crystallization that increases with peak pressure permitting the interpretation that the entire region underwent synchronous subduction 40 ar 39 ar white mica ages of 399 ma indicate that the eastern part of the western gneiss region had been exhumed to shallow crustal levels while uhp metamorphism was ongoing farther west indicating a westward dip to the slab the 40 ar 39 ar white mica ages also show a clear east to west gradient across the entire western gneiss region indicating that the western gneiss region rose diachronously to crustal levels from east to west between 399 and 390 ma

europe mediterranean tomography high correlation between new seismic data and independent geophysical observables 1 to improve the fidelity of european tomographic images we implement variable resolution parameterization finite frequency and ray theory and improved data coverage in new tomographic inversions we supplement an existing high quality database of global teleseismic phase velocity observations of both love and rayleigh waves with a set of new phase delay observations recorded by a dense array of stations in the region we invert these data to find global phase velocity maps characterized by a more densely parameterized europe and mediterranean we compare our seismic images with laterally variable independent geophysical observables including crustal thickness gravity and heat flow and find correlations between 0 25 and 0 6 while at most periods finite frequency and ray theory yield almost coincident results we find some possibly significant differences in the phase velocity of 150s rayleigh waves our results represent significant progress toward high resolution modeling of upper mantle s velocity structure with the possibility of also constraining seismic anisotropy

multiple resolution surface wave tomography the mediterranean basin summary from a large set of fundamental mode surface wave phase velocity observations we map the transversely isotropic lateral heterogeneities in the upper mantle shear velocity structure we design a multiple resolution inversion procedure which allows us to parametrize any selected region more finely than the rest of the globe we choose as a high resolution region the upper mantle underlying the mediterranean basin we formulate the inverse problem as in a

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

multimode surface wave tomography for the australian region using a three stage approach incorporating finite frequency effects 1 a three stage inversion technique for surface wave tomography is applied to the australian region the inversion procedure consists of three independent processes in the first stage path specific one dimensional 1 d shear velocity profiles are derived from multimode waveform inversion to provide dispersion information the information from all paths is then combined to produce multimode phase speed maps as a function of frequency the first version of these phase speed maps is derived from linearized inversion based on the assumption of surface wave propagation along great circle paths subsequently the 2 d phase speed maps are updated by including ray tracing and finite frequency effects through the influence zone around the surface wave paths over which the phase is coherent finally in the third stage the 3 d shear wave speed distribution is reconstructed from the set of updated multimode phase speed maps this three stage inversion of surface waves has significant benefits because it is possible to incorporate multimode dispersion off great circle propagation and finite frequency effects for surface waves in a common framework the final 3 d model which includes the effects of ray bending and finite frequency shows improvement in the definition of the model in regions with high gradients in shear velocity such as near tectonic boundaries especially in eastern australia despite the natural smoothing imposed by considering the influence zone around the surface wave paths the final models still require rapid change in shear wave properties in the neighborhood of the edge of the craton

isostatic constraints on the assembly stabilization and preservation of cratonic lithosphere cratonic uplift or subsidence as recorded geologically by erosion or deposition indicates the net change in lithospheric buoyancy density change is primarily controlled by temperature change such that the geologic record reveals the net lithospheric heating or cooling our modeling shows that thermal stabilization of lithosphere can occur in 100 m y largely determined by the time for crustal radioactivity to warm the entire lithosphere longer term thermal evolution 350 400 m y is controlled by the feedback between isostatic and thermal responses crustal heat production is the dominant factor for evolution of crustal geo therms whereas the detailed initial thermal structure is less important the fact that many cratons are eroded to only modest depths indicates that they have a history of warming slightly since their formation implying a rather cool 700 1000 c for initial lithospheric mantle this is consistent with the growth of cratonic lithosphere by progressive thickening during continental assembly and or subduction stacking of oceanic lithosphere relatively warm early lithosphere due to elevated crustal heat production or higher initial lithospheric mantle temperatures may have had a lower preservation potential due to preferential deformation and destruction study of the proterozoic orogen of the southwestern united states indicates that the distinct thermal and isostatic records of adjacent crustal blocks are compatible with observed differences in crustal heat production but also with similar initial lithospheric mantle temperatures

relating crustal magnetization and satellite altitude magnetic anomalies in the ungava peninsula northern quebec canada abstract a prominent satellite altitude magnetic anomaly 6 nt located over the ungava peninsula northern quebec is coincident with a region of intense aeromagnetic anomalies and high susceptibilities of exposed rocks inversion of a combined pogo magsat anomaly data set shows that the susceptibility levels required to satisfy the satellite altitude anomaly range from 0 03 to 0 04 si assuming a 40 km thick crust these values are well within the range of measured surface susceptibilities in the minto region and the anomaly can be explained if the measured values are representative of the average upper crustal magnetization therefore the ungava peninsula is one of the few areas on earth where a direct relation between surface rock properties and satellite magnetic anomalies can be made

pressure dependence of fe ti order in the ilmenite hematite solid solution implications for the origin of lower crustal magnetization the view the mathml source to view the mathml source cation ordering phase transition in the ilmenite fetio3 hematite fe2o3 solid solution has been studied at high temperature and pressure using in situ time of flight powder neutron diffraction experiments were performed on a synthetic sample of fetio3 0 65 fe2o3 0 35 at t 933 c and p 1 1 gpa using an internally heated paris edinburgh cell the degree of cation order is observed to increase significantly with pressure thermodynamic analysis reveals an increase in the transition temperature from 911 c at atmospheric pressure to 1010 c at p 0 64 gpa this increase is much larger than that predicted on the basis of the volume change for the transition indicating that the pressure dependence of cation ordering is dominated by enhanced cation cation interaction and not by volume strain we predict an increase in the eutectoid temperature of up to 150 c at 1 gpa this implies that intergrowths of antiferromagnetic hematite and paramagnetic ilmenite could be stable at temperatures up to 540 c at 1 gpa and cannot be ruled out as a potential source of lower crustal magnetization

magnetism of the lower crust observations from the chipman domain athabasca granulite terrain northern canada abstract magnetic properties of lower crustal rocks produce anomalies seen in satellite aeromagnetic and ground studies and are assumed to be responsible for observed long wave length anomalies lwa of 20 nt the soon to be launched swarm satellites will provide extensive data on the magnetization of the lower to middle crust in anticipation of this event we are investigating magnetic properties in a superbly exposed section of lower crust in northern saskatchewan the athabasca granulite terrain agt is a complex region of felsic and mafic lower crustal rocks part of the snowbird tectonic zone stretching ne sw across the canadian shield the agt is composed of a sequence of rocks identified as lower crustal in origin by their high pressure 1 0 gpa and high temperature 800 c metamorphism dated at 2 6 ga and 1 9 ga with uplift and exhumation at 1 85 1 80 ga the agt is characterized by low negative aeromagnetic anomalies with distinct large positive anomalies in the southern and central regions the chipman domain on the east side consists of tonalites mafic granulites and granite intruded by the chipman dike swarm at 1 9 ga where anomalies cut across mapped lithologic boundaries susceptibility measurements from both field and lab readings range over several orders of magnitude from 1 10 5 to 3 10 1 si with higher values related to both mafic granulite and some tonalite samples remanence values also show considerable variability from 0 1 ma m to 90 a m with the weakest magnetization found in the chipman dikes and the fehr granite forty samples out of 89 have koenigsberger ratios greater than 1 but low initial remanence limits its influence on anomalies hysteresis and low temperature measurements identify magnetite as the predominant iron oxide this section of lower crustal rocks has paramagnetic granites and dikes with ferromagnetic mafic granulites and bimodal tonalites defined by geographic location

weakly magnetic crust in the canadian cordillera abstract current models of continental crust favor an increase in magnetization with depth here we report a counter example from the canadian cordillera where almost a full thickness of non magnetic continental crust is suggested by joint interpretation of magnetic and seismic data the magnetic field over the cordillera is characterized by complex short wavelength 100 km portion of the cordilleran field is subdued and mainly featureless and suggests a lack of magnetic sources at greater depths seismic reflection and refraction data from three major transects in the yukon and british columbia canada support this interpretation and indicate that sedimentary like formations make up the majority of the crust the dominance of shallow upper crustal magnetization in the canadian cordillera contrasts with the generally held view that the lower continental crust is the primary source for long wavelength magnetic anomalies sources for these anomalies are often assumed to be located in the lower crust when surface magnetizations are insufficient to produce such anomalies or no correlation exists between the magnetic field and the mapped surface geology the canadian cordillera appears to be an example of a non magnetic lower crust overlain by a more magnetic upper crust that is however not magnetized strongly enough to produce significant long wavelength magnetic anomalies

radial anisotropy in the european mantle tomographic studies explored in terms of mantle flow 1 previous studies have shown that radial seismic anisotropy as estimated from flow models is in good agreement with results from tomography at global scale in particular underlying oceanic basins however the fit is typically poor at smaller scale lengths particularly in tectonically complex regions we conduct a comparative analysis of tomographically mapped and dynamically modeled radial anisotropy at the scale of europe and the mediterranean basin including three tomographic models based on different observations and or methods we find that adaptive grid surface wave tomography with parametrization density depending locally on the spatial and azimuthal density of data coverage leads to the seismic model closest to albeit still far from geodynamic predictions theability tomap regional scaleseismic anisotropy may provide a new constraint complementary to isotropic tomography to the nature of upper mantle flow citation schaefer j f l boschi t w becker and e kissling 2011 radial anisotropy in the european mantle tomographic studies explored in terms of mantle flow geophys res lett 38 l23304 doi 10 1029 2011gl049687

high pressure metamorphism and deep crustal seismicity evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas the metamorphic complex of the western gneiss region wgr norway constitutes the root of the caledonian mountain belt and experienced temperatures of 700 800 c and pressures in excess of 20 kbar during peak metamorphism mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures the krakeneset gabbro located on the island of vagsoy in the mixed hp uhp zone of the western wgr display evidence for extensive metastability through the entire prograde and retrograde p t histories eclogite constitutes less than a few percent of the total volume of the body and high pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration the gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones 50 cm wide transecting the gabbro body the pseudotachylytes contain m to mm scale amoeboid and dendrite like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene omphacite amphibole and dolomite suggesting rapid disequilibrium growth of minerals during high pressure conditions textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid state alteration of a microcrystalline pseudotachylyte matrix the pseudotachylyte lined fault zones are in close spatial association with numerous amphibole carbonate filled hydrofractures with conspicuous fracture parallel alteration zones defined by hydrous eclogite facies assemblages these eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high grade metamorphic conditions geothermobarometric estimates ca t 650 700 c p 20 kbar and petrological data imply that hydrofracturing pseudotachylyte crystallization and the subsequent pseudotachylyte alteration process must have occurred during high pressure metamorphism our observations are suggestive of a deep crustal earthquake scenario where a high pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition the formation of weak deep crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the krakeneset gabbro during its residence deep in the former caledonian collision zone

raman confirmation of microdiamond in the svartberget fe ti type garnet peridotite western gneiss region western norway introductionvestiges of ultra high pressure meta morphism uhpm havebeenfoundincontinental and oceanic rocks of dif ferentorogensworldwide smith 1988 coleman and wang 1995 carswelland compagnoni 2003 indirectuhpmevidencecomesfromtheappli cation of geothermobarometric tech niques lappin and smith 1978 ravna krogh and paquin 2003 ap plied to uhpm mineral assemblagesand directly from mineralogical indi cators such as coesite diamond andmajoritic garnet chopin 1984 smith 1984 sobolev and shatsky 1990 do brzhinetskaya et al 1995 van roer mund and drury 1998 nasdala andmassonne 2000 mposkos andkosto poulos 2001 van roermund et al 2002 godard et al 2003 neverthe less the mechanisms responsible forthe formation preservation and exhu mation of uhpm rocksin orogens arestill not fully understood the volume and spatial distributionof uhpm rocks within uhpm terr anes including the observed contrastin metamorphic grade between rocksthat contain uhpm minerals andadjacent country rocks that do notcontain them have led to di erentexplanations concerning the origin emplacement and preservation ofthese rocks to explain the occur rence volume and areal extent ofuhpm rocks in the western gneissregion wgr the following mecha nisms that assume a continental sub duction scenario have been proposed a the existence of major and orminor post uhpm tectonic contactssuch as nappes and or tectonic mel anges e g lappin and smith 1978 smith 1984 1988 wain 1997 cuth bert et al 2000 terry and robinson 2003 2004 or b the presence of uids and or deformation leading todi erential prograde or retrogradereactions and di erences in reaction kinetics between rock types austr heim et al 1997 cuthbert et al 2000 wain et al 2001 carswell andcuthbert 2003 cuthbert and car swell 2003 hypotheses that do notnecessarily involve continental sub duction may involve local regional tectonic overpressure in rocks smith 1984 mancktelow 1993 petrini and podladchikov 2000 it isconsidered here that the volume andareal extent of uhpm domains withina single metamorphic terrain are thusimportant parameters to determine asthey can modify or identify the loca tions of inferred or observed tectoniccontacts the relations between uidsand uhpm and or the scales atwhich continental crust subductionor tectonic overpressure might act here we report raman data thatcon rm the previously reported butnot proven presence of microdia monds at the svartberget uhpmlocality in the wgr of norway vrijmoed et al 2006 micro diamond con rmation is consistentwith the results of vrijmoed et al 2006 that the volume and arealextent of the northern uhpm domainof the wgr fig 1 are considerablyexpanded eastwards and followingthe reasoning in the paragraph above the new microdiamond nding willput important constraints on themodels described above ultra high pressure metamorphismin the western gneiss regionthe wgr represents a window inthe scandinavian caledonides wherescandian reworked high grade gneis sic rocks of proterozoic age presum ably belonging to the baltic plate are exposed in addition variableamounts of metadolerites amphi abstract

finite frequency effects in global surface wave tomography summary we compare traditional ray theoretical surface wave tomography with finite frequency tomography using 3 d born sensitivity kernels for long period fundamental mode dispersion measurements the 3 d kernels preserve sidelobes beyond the first fresnel zone and fully account for the directional dependence of surface wave scattering and the effects of timedomain tapering and seismic source radiation tomographic inversions of love and rayleigh phase delay measurements and synthetic checkerboard tests show that 1 small scale s wave velocity anomalies are better resolved using finite frequency sensitivity kernels especially in the lowermost upper mantle 2 the resulting upper mantle heterogeneities are generally stronger in amplitude than those recovered using ray theory and 3 finite frequency tomographic models fit long period dispersion data better than ray theoretical models of comparable roughness we also examine the reliability of 2 d phase velocity sensitivity kernels in global surface wave tomography and show that phase velocity kernels based upon a forwardscattering approximation or previously adopted geometrical simplifications do not reliably account for finite frequency wave propagation effects 3 d sensitivity kernels with full consideration of directional dependent seismic scattering are the preferred method of inverting long period dispersion data finally we derive 2 d boundary sensitivity kernels for lateral variations in crustal thickness and show that finite frequency crustal effects are not negligible in long period surface wave dispersion studies especially for paths along continent ocean boundaries unfortunately we also show that in global studies linear perturbation theory is not sufficiently accurate to make reliable crustal corrections due to the large difference in thickness between oceanic and continental crust

effect of fine scale microstructures in titanohematite on the acquisition and stability of natural remanent magnetization in granulite facies metamorphic rocks southwest sweden implications for crustal magnetism mid proterozoic granulites in sw sweden having opaque minerals hematiteilmenite with minor magnetite and occurring in an area with negative aeromagnetic anomalies have strong and stable reversed natural remanent magnetization 9 2 a m with 100 remaining after demagnetization to 100 mt samples were characterized by optical microscopy electron microprobe emp transmission electron microscopy tem and rock magnetic measurements earliest oxide equilibrium was between grains of titanohematite and ferri ilmenite at 650 600 c initial contacts were modified by many exsolution cycles hematite and ilmenite ilm hosts and lamellae by emp are ilm 24 25 ilm 88 93 like titanohematite and ilmenite above 520 c on burton s diagram 1991 finer hosts and lamellae by tem are ilm16 3 and ilm 88 4 like coexisting antiferromagnetically ordered af hematite and ilmenite below 520 c on burton s diagram this may be the first example of analytical identification in one sample of former hematite now finely exsolved and af hematite tem microstructures consist of gently curving semicoherent ilmenite lamellae within hematite flanked by precipitate free zones and abundant ilmenite disks down to unit cell scale 1 2 nm strain contrast of disks suggests full coherence with the host and probable formation at the reaction titanohematite af hematite ilmenite at 520 c magnetic properties are a consequence of chemical and magnetic evolution of hematite and ilmenite with bulk compositions ilmenite richer than ilm 28 that apparently exsolved without becoming magnetized down to 520 c where hematite broke down to af hematite plus ilmenite producing abundant af hematite below its neel temperature intensity of magnetization is greater than possible with hematite alone and tem work suggests that ultrafine ilmenite disks in af hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces

high resolution teleseismic tomography of upper mantle structure using an a priori three dimensional crustal model summary the effect of an a priori known 3 d crustal model in teleseismic tomography of upper mantle structure is investigated we developed a 3 d crustal p wave velocity model for the greater alpine region encompassing the central and western alps and the northern apennines to estimate the crustal contribution to teleseismic traveltimes the model is constructed by comparative use of published information from active and passive seismic surveys the model components are chosen to represent the present large scale alpine crustal structure and for their significant effect on the propagation of seismic wavefields they are first order structures such as the crust mantle boundary sedimentary basins and the high velocity ivrea body teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite difference technique to the iasp91 traveltime tables residuals are produced for a synthetic upper mantle model featuring two slab structures and the 3 d crustal model on top of it the crustal model produces traveltime residuals in the range between 0 7 and 1 5 s that vary strongly as a function of backazimuth and epicentral distance we find that the non linear inversion of the synthetic residuals without correcting for the 3 d crustal structure erroneously maps the crustal anomalies into the upper mantle correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle we conclude that with the increasing amount of high quality seismic traveltime data correction for near surface structure is essential for increasing resolution in tomographic images of upper mantle structure

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

a major synmetamorphic early devonian thrust and extensional fault system in the mid norway caledonides relevance to exhumation of hp and uhp rocks

magnetic petrology of high fe ti eclogites from the ccsd main hole implications for subduction zone magnetism 1 the high fe ti eclogites with exsolved lamellar in 530 600 m depths from the chinese continental scientific drilling ccsd main hole in the sulu ultra high pressure uhp metamorphic belt eastern china record an anomalously high susceptibility natural remanent magnetization nrm and koenigsberger ratio q nrm ji ji is induced magnetization this provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks in this paper we have measured systematically magnetic properties and mineral assemblage and structure for three special samples no 83 no 86 and no 89 results show that these fe ti rich eclogites are the result of the fine grained titano magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo ilmenite we found that the dominant fe bearing oxide minerals in samples studied are ilmenite hematite hem ilm up to 25 little titano magnetite and pyrite the ferromagnetic susceptibility is mainly controlled by fine grained titano magnetite and nrm is closely related to the exsolved lamellar structure we hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno hematite during cooling and decompression processes these fe ti rich eclogites might be one of the sources of high magnetic anomalies observed in the sulu subduction zone eastern china

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

magnetization of exsolution intergrowths of hematite and ilmenite mineral chemistry phase relations and magnetic properties of hemo ilmenite ores with micron to nanometer scale lamellae from allard lake quebec hemo ilmenite ores from allard lake quebec were first studied over 50 years ago interest was renewed in these coarsely exsolved oxides based on the theory of lamellar magnetism as an explanation for the high and stable natural remanent magnetizations nrms 32 to 120 a m reported here to understand the magnetism and evolution of the exsolution lamellae the microstructures and nanostructures were studied using scanning electron microscopy and transmission electron microscopy tem phase chemistry and relations between mineral chemistry and the hematite ilmenite phase diagram cycles of exsolution during slow cooling resulted in lamellae down to 1 2 nm thick combined electron microprobe tem and x ray diffraction xrd results indicate that hematite hosts reached a composition approximately ilmenite ilm 14 4 and ilmenite hosts ilm 98 the bulk of the very stable nrm which shows thermal unblocking 595 620 c was acquired during final exsolution in the two phase region canted antiferromagnetic r3c hematite r3 ilmenite hysteresis measurements show a very strong anisotropy with a stronger coercivity normal to than parallel to the basal plane orientation of the lamellae magnetic saturation m s values are up to 914 a m compared to 564 a m predicted for a modally equivalent spin canted hematite corrected for 15 r 2 tio 3 substitution low temperature hysteresis ac susceptibility measurements and mossbauer results indicate a neel temperature t n of the geikielite substituted ilmenite at 43 k the low temperature hysteresis and ac susceptibility measurements also show a cluster spin glass like transition near 20 k below t n of ilmenite an exchange bias occurs with a 40 mt shift at 10 k

nature and origin of lamellar magnetism in the hematite ilmenite series grains consisting of finely exsolved members of the hematite ilmenite solid solution series such as are present in some slowly cooled middle proterozoic igneous and metamorphic rocks impart unusually strong and stable remanent magnetization tem analysis shows multiple generations of ilmenite and hematite exsolution lamellae with lamellar widths ranging from millimeters to nanometers rock magnetic experiments suggest remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths yet is stronger than can be explained by canted antiferromagnetic caf hematite or coexisting paramagnetic pm fe ti ordered r 3 ilmenite alone in alternating field demagnetization to 100 mt many samples lose little remanence indicating that the nrm is stable over billions of years this feature has implications for understanding magnetism of deep rocks on earth or on planets like mars that no longer have a magnetic field atomic scale simulations of an r 3 ilmenite lamella in a caf hematite host based on empirical cation cation and spin spin pair interaction parameters show that contacts of the lamella are occupied by contact layers with a hybrid composition of fe ions intermediate between fe2 rich layers in ilmenite and fe3 rich layers in hematite structural configurations dictate that each lamella has two contact layers magnetically in phase with each other and out of phase with the magnetic moment of an odd non self canceling fe3 rich layer in the hematite host the two contact layers and the odd hematite layer form a magnetic substructure with opposite but unequal magnetic moments a lamellar ferrimagnetism made possible by the exsolution because it is confined to magnetic interaction involving the moments of just three ionic layers associated with each individual exsolution lamella lamellar magnetism is unique and quite distinct from conventional ferrimagnetism simulation cells indicate that the magnetic moments of contact layers are locked to the magnetic moments of adjacent af hematite layers and are parallel to the basal plane 001 thus lamellar magnetism is created at the temperature of chemical exsolution and is a chemical remanent rather than thermal remanent magnetization however in thermal demagnetization experiments too short for lamellar resorption demagnetization temperatures are those of the caf hematite considerably higher than temperatures of original lamellae formation internal crystal structure cannot dictate that the contact layers of different lamellae will form magnetically in phase with each other to give the highest net magnetic moment but magnetic moments of lamellae can be made to form in phase by the external force of the magnetizing field at the time of exsolution a thesis of this paper is that an external magnetic field can dictate the magnetic moments and hence the chemical location of ilmenite lamellae in a hematite host and that once in place neither the location nor the magnetic moment will be easily disturbed in an ilmenite host the external magnetic field cannot control the chemical location of a hematite lamella which is dictated by the enclosing ilmenite but once lamellae have formed the field can dictate their magnetic moments these moments however are not locked chemically to the host resulting in lower coercivity the effectiveness of the external force in single crystals is dictated by their orientation with respect to the magnetizing field in grains with 001 oriented parallel to the field it would be effective in producing in phase magnetic moments and very strong remanence in grains with 001 normal to the field the field would be less effective in producing in phase magnetic moments hence producing weak remanence the most intense lamellar magnetism per formula unit occurs with in phase magnetization high lamellar yields and the largest number of lamellae per unit volume i e smallest lamellar size compared to the magnetic moment per formula unit m pfu and magnetic moment per unit volume m v of end member magnetite m pfu 4 b m v 480 ka m and hematite m pfu 0 0115 b m v 2 1 ka m results for some atomic models reasonably tied to natural conditions are m pfu 0 46 1 36 b and m v 84 250 ka m

magnetic properties of rocks of the kapuskasing uplift ontario canada and origin of long wavelength magnetic anomalies summary the sources of long wavelength magnetic anomalies lwa in the crust are poorly understood we have measured remanent and induced magnetizations of 210 samples of anorthosite tonalite and mafic gneiss from the kapuskasing uplift an exposed partial crustal crosssection in northwestern ontario canada anorthosites have generally high q ratios of remanent induced magnetization in the range 0 3 60 and their natural remanent magnetization nrm is resistant to both thermal and alternating field af demagnetization however anorthositenrms 0 001 0 3am 1 aretooweaktoexplainlwaamplitudes maficgneisses also have relatively high q ratios peaking in the range 1 10 and the nrm is resistant to thermal and af demagnetization nrm and induced magnetization im intensities are in the ranges 0 01 2 and 0 01 0 6 a m 1 respectively tonalites have a bimodal distribution of magnetization the more strongly magnetic group has both nrm and im intensities in the range 0 1 5 a m 1 and wide ranging q values from 0 1 to 10 approximately some tonalites could be an lwa source although the long term stability of their nrms at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 c in general q values measured at surface temperatures overestimate remanence at depth in the deep crust im remains more or less constant but remanence decreases both reversibly and irreversibly leading to q ratios of 0 2 0 3 for multidomain grains and 1 3 for single domain grains near their blocking temperatures thermoviscous magnetization over the brunhes chron could add substantially to the effective induced magnetization typically induced thermoviscous magnetization in the direction of the present earth s field will outweigh remanence in the direction of an ancient field as a source of lwa originating in the deepest crust remanence may play a larger role for mid crustal sources where single domain grains are well below their blocking temperatures

mantle structure beneath indonesia inferred from high resolution tomographic imaging we investigated mantle structure beneath the indonesian region by means of tomographic inversions of traveltime residuals of direct p and the surface reflected depth phases pp and pwp the hypocentres and phase data used in the inversions were derived from the reprocessing of data reported to international data centres which include data from the australian skippy project we used more than 6 0 10 data from more than 50 000 globally distributed earthquakes recorded at a subset of nearly 3000 seismological stations worldwide the use of depth phases improved the sampling of the upper mantle structure in particular beneath the back arc regions the mapping of distant aspherical mantle structure into the study region was minimized by combining a high resolution regional with a low resolution global inversion the inversion was linearized around the ak135 reference velocity model and a combination of minimum norm and gradient damping was applied to constrain the solution inversion of different subsets of the data and test inversions of synthetic data indicate that resolution is sufficient to substantiate the conclusions pertinent to the complex large scale structure of the subducted slab beneath the indonesian archipelago the tomographic images confirm previous evidence for regional slab penetration into the upper part of the lower mantle but suggest a more complex trajectory of mantle flow beneath indonesia the implied lateral variation in the shape of the upper mantle slab is generally in good agreement with inferences from seismicity the data reveal a seismic anomaly beneath the sunda arc which is detected to at least 1500 km in depth and forms the eastern end of a deep anomaly associated with the past subduction of lithosphere of the mesozoic tethys ocean below java the lithospheric slab is probably continuous from the earth s surface to the lower mantle but beneath sumatra the deep slab seems to be detached from the seismogenic slab the high resolution images suggest that the subducted slab is deflected in the transition zone beneath the banda arc and that the descending slabs form a spoon shaped structure north of the banda sea we detect the two opposing subducted slabs of the molucca sea plate this is consistent with previous studies but our images suggest that the westward subduction has a significantly steeper dip than the eastward subduction slab structure beneath the banda and molucca seas is explained in the context of the continent arc collision between australia and the southeast indonesian island arc and the arc arc collision between sangihe and halmahera inversion of traveltime residuals of direct s phases a novelty for this region largely confirms the inferences from the p wave data even though less detail is resolved

estimating the crustal power spectrum from vector magsat data the earth s magnetic field can be subdivided into core and crustal components and we seek to characterize the crustal part through its spatial power spectrum rl we process vector magsat data to isolate the crustal field and then invert power spectral densities of flight local components along track for rl following o brien et al 1999 our model designated lppc is accurate up to approximately spherical harmonic degree 45 900 km this is the resolution limit of our data and suggests that global crustal anomaly maps constructed from vector magsat data should not contain features with wavelengths less than 900 km we find continental power spectra to be greater than oceanic ones and attribute this to the relative thicknesses of continental and oceanic crust

new images of the earth s upper mantle from measurements of surface wave phase velocity anomalies 1 the sensitivity of a surface wave to the lateral structure of the earth s mantle depends on the local structure and thickness of the crust particularly at short periods the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave sensitivity functions thus far this effect has not been quantified and tomographic studies of the upper mantle based on surface wave observations have not taken it into account on the basis of a jeffreys wentzel kramers brillouin jwkb description of surface wave propagation we find and discuss the lateral variations of the sensitivity of love and rayleigh waves to mantle seismic anomalies associated with a realistic model of the crust mooney et al 1998 we then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from love and rayleigh wave phase velocity measurements periods from 35 to 300 s in terms of large scale patterns of high and low velocity our model is similar to previously published ones confirming among other features the anomalous anisotropy underlying the central pacific ekstrom and dziewonski 1998 we find however that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies which should not be neglected as we attempt to obtain higher resolution images of the upper mantle