the effect of low temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites a case for acquisition of chemical remanent magnetization the effect of hydrothermal alteration on the thermoremanent magnetization trm of synthetic titanomagnetite tm40 fe2 6ti0 4o4 has been studied to simulate the alteration that occurs in the oceanic crust pseudo single domain titanomagnetite grains similar in size to those often found in oceanic basalts were dispersed in a permeable but rigid glass matrix this resulted in a trm in the sample which was subsequently oxidized in acidic solutions while a magnetic field 0 1 mt was applied perpendicular to the trm direction the experiments were conducted in a non magnetic stainless steel pressure vessel at 150 c in solutions of acidity varying from ph 2 to ph 7 in addition to being time and temperature dependent the acquired chemical remanent magnetization crm was also found to be very ph dependent the degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions long term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating it was found that during the alteration of tm40 to titanomaghemite the crm is along the ambient field direction from the onset and not partly or wholly along the trm direction as has been found in previous air oxidation experiments this has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples

observed effects of mechanical grain size reduction on the domain structure of pyrrhotite abstract the magnetic mineral grains that carry natural remanent magnetization nrm in rocks can undergo changes in volume chemical composition or both because domain structure is a function of particle size composition and the dominant anisotropy a change in these properties could trigger changes in the number of domains drive domain walls to new positions and thus reset natural remanence the manner in which active grain size reduction alone affects domain state has not been studied however to address this problem we have observed bitter patterns on two mutually perpendicular surfaces of natural pyrrhotite grains whose sizes were reduced mechanically in the laboratory both one dimensional and two dimensional thinning experiments were performed each grain was thinned until at least one dimension was approximately 20 50 of its original size domain widths the overall positions of surviving walls and even many small scale details in the shapes of curved walls were remarkably insensitive to thinning these results lead to three conclusions first local energy minimum lem domain states in pyrrhotite could be stable across a wide range of grain sizes and shapes second defects that pin domain walls could inhibit lem lem transitions by impeding the wall motions required for such transitions third in pyrrhotite the defects which prevent walls from adjusting to thinning induced changes in demagnetizing field appear to be densely and rather homogeneously distributed across a wall s surface rather than being volumetrically rare when viewed together these results strongly suggest that an nrm direction originally carried by two domain grains would survive grain size attrition as the two domain parents were reduced to stable single domain daughters

the palaeomagnetic record of the santiago volcanics republic of cape verde multiphase magnetization and age consideration abstract the palaeomagnetism of the older volcanic complex of santiago constitutes a multicomponent structure and magnetomineralogical evidence suggests that the various remanence vectors are carried principally by titanomaghaemites when viewed in the context of the african palaeomagnetic reference frame the predominant magnetization is consistent with an upper tertiary age but two older magnetizations of inferred lower tertiary age are also in evidence the demonstrable secondary nature of all three palaeomagnetic axes each of them represented by both polarities makes it likely that the actual emplacement of the older complex is of pre tertiary age being probably an integral component of the pronounced tectonomagmatic uplift phase that affected the entire northwest african margin in upper cretaceous time the older complex of santiago can probably be correlated with the igneous core of the neighbouring island of maio which apparently formed during turonian time 90 ma ago some younger lavas of upper miocene age the flamengos formation show a much simpler palaeomagnetic history than rocks of the older complex yielding only the upper tertiary palaeomagnetic field axis

magnetic properties and ferromagnetic mineralogy of oceanic basalts summary rockmagnetic properties of a number of oceanic basalt samples collected by the dsdp odp drillings after leg 45 as well as one dredge haul were investigated in conjunction with palaeomagnetic characteristics of the same samples the electron probe micro analysis epma study indicated that chemical composition of the primary titanomagnetites has a variety particularly in minor constituent elements such as al2o3 mgo mno and v2o3 the variation in chemical composition in the primary phase of titanomagnetites seems to be controlled by changes in temperature oxygen fugacity and volatile pressure during the crystallization as well as chemistry of residual liquid hi magma general crystallization trend is found that is the later crystallized titanomagnetites have higher ulvospinel component higher titanium oxide and lower iron oxide due to decrease in temperature and oxygen fugacity it is established that minor constituent elements are good indicators of crystallization sequence of titanomagnetites in the contrast of variation in major constituent elements in the later stage al2o3 and mgo spinel component and v2o3 decrease whereas mno increases although there is a reversed crystallization trend with a few elements in several cases this reversed trend may suggest that there are discontinuous changes of environmental condition in magma to control titanomagnetite chemistry during a crystallization sequence the primary variation of chemistry shows different trends of the secondary variation due to oxidation oxidation process of titanomagnetites contained in oceanic basalts was examined in detail by means of an improved epma technique being able to directly determine the absolote contents of oxygen in materials the result of the quantitative oxygen measurement shows that the oxidation process of titanomagnetites is not a simple process by changes of ferrous ions to ferric ones with acrretion of oxygen but associated with migration of iron ions of the titanomagnetite lattice this is consistent with the results of recent studies on oxidation processes of titanomagnetites in oceanic basalts

vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

parameters of tsunami source versus earthquake magnitude this study employs an empirical analytical approach in combination with monte carlo method to establish relationships between earthquake moment magnitude and upper limits of tsunami source parameters double amplitude of vertical bottom deformation displaced water volume and potential energy of initial elevation the approach is based on the okada solution for a finite rectangular fault and empirical scaling laws for earthquake sources results are compared to empirical and theoretical relationships published previously parameters of some recent tsunami sources those for which usgs provides slip distribution data are considered in light of the established relationships

long term polarity stability and strength of the geomagnetic dipole palaeomagnetically observed reversals so far provide the strongest evidence1 that reversals originate from processes of core magnetohydrodynamics mhd thus furnishing a crucial constraint for geomagnetic dynamo modelling the long term variations in reversal frequency favouring2 independent mechanisms for the dynamo and its reversals provide an added constraint here we present evidence for a correlation between polarity superchrons and palaeointensity data and also marine magnetic and mags at anomalies our results indicate that the dipole moment is correlated to the reversal frequency being appreciably greater during the fixed polarity superchrons this new information derived from the reversal record provides fresh insight into the workings of the geodynamo and also explains why the oceanic crust associated with the cretaceous quiet zone carries such a strong thermoremanence

magma flow exsolution processes and rock metasomatism in the great messejana plasencia dyke iberian peninsula magma flow in dykes is still not well understood some reported magnetic fabrics are contradictory and the potential effects of exsolution and metasomatism processes on the magnetic properties are issues open to debate therefore a long dyke made of segments with different thickness which record distinct degrees of metasomatism the messejana plasencia dyke mpd was studied oriented dolerite samples were collected along several cross sections and characterized by means of microscopy and magnetic analyses the results obtained show that the effects of metasomatism on rock mineralogy are important and that the metasomatic processes can greatly influence anisotropy degree and mean susceptibility only when rocks are strongly affected by metasomatism petrography scanning electron microscopy sem and bulk magnetic analyses show a high temperature oxidation exsolution event experienced by the very early ti spinels during the early stages of magma cooling which was mostly observed in central domains of the thick dyke segments exsolution reduced the grain size of the magnetic carrier multidomain to single domain transformation thus producing composite fabrics involving inverse fabrics these are likely responsible for a significant number of the abnormal fabrics which make the interpretation of magma flow much more complex by choosing to use only the normal fabric for magma flow determination we have reduced by 50 per cent the number of relevant sites in these sites the imbrication angle of the magnetic foliation relative to dyke wall strongly suggests flow with end members indicating vertical dominated flow seven sites and horizontal dominated flow three sites

petrogenesis of a basalt rhyolite tephra from the west central afar ethiopia the cindery tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption it contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage brown glasses are ferrobasaltic in composition and are similar to the voluminous pliocene tholeiites of the surrounding west central afar volcanic field both are enriched in the light rare earth and incompatible elements and possess higher 87sr 86sr and lower 143nd 144nd than morb rhyolitic glasses are subalkaline and compared to the basaltic glasses are strongly depleted in the compatible elements and enriched in the incompatible elements both glass types have similar incompatible element and isotopic ratios and with the rhyolite glass showing a 2 fold parallel enrichment in rare earth element abundances over the basaltic glass these observations suggest that the two glasses are genetically related

origin of the absence of magnetic lineations in the yamato basin of the japan sea magnetic properties of mafic rocks from ocean drilling program hole 794 d we investigated magnetic properties of mafic rocks recovered from ocean drilling program hole 794 d in the yamato basin of the japan sea in contrast to other back arc basins in the western pacific lineated magnetic anomaly patterns have not been recognized in the yamato basin our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks the dominant magnetic mineral of hole 794 d is titanomaghemite as commonly observed for typical oceanic basalts but the grain size is much larger greater than several tens of microns remanence properties i e low natural remanent magnetization intensities 1 5 a m low koenigsberger ratios 1 0 high viscous remanent magnetization and mixed magnetic polarities indicate that the mafic rocks cannot be a source of lineated magnetic anomalies while curie temperature saturation magnetization and susceptibility values are not significantly different from those of typical oceanic basalts magnetic properties related to stability are greatly reduced the large grain size of the mafic rocks from hole 794 d plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts and is caused by the formation of sill sediment complexes beneath the yamato basin sill sediment complexes rather than extrusive basalts in typical oceanic crust are responsible for the absence of lineated magnetic anomalies in the yamato basin

importance of environment on the order of mineral weathering in olivine basalts hawaii examination of weathering rinds from lava flows on hawaii with backscatter electron microscopy and electron micro probe analysis reveals that olivine weathers more slowly than adjacent clinopyroxene and plagioclase in environments with a paucity of organic acids yet when weathering rinds are in contact with abundant organic acids secreted by lichens olivine weathers before clinopyroxene and plagioclase weathers last the exception to goldich s widely accepted mineral stability series in organic poor environments runs counter to a thermodynamics explanation for the order of mineral weathering and illustrates the importance of the biogeochemical environment

saturation magnetostriction and its low temperature variation inferred for natural titanomaghemites implications for internal stress control of coercivity in oceanic basalts summary highly oxidized titanomaghemite in oceanic basalts often carries remanent magnetization of high coercivity stability helping preserve the oceanic magnetic anomaly pattern we study the source of this high coercivity in four oceanic basalts from odp sites 238 572d 470a and 556 containing highly oxidized titanomaghemite titanium content parameter x 0 55 and oxidation parameter z 0 9 on average most of the titanomaghemite is likely in singledomain grains with uniaxial anisotropy because the ratio of saturation remanence j rs to saturation magnetization j s approaches 0 50 j rs j s 0 46 on average we show that the uniaxial anisotropy is very likely magnetostrictively controlled through internal stresses i in the titanomaghemite grains this allows us to use a novel indirect method to estimate the saturation magnetostriction s of the titanomaghemite a saturation remanence j rs is given along the axis of a cylindrical sample of each basalt then a small compression is applied

magnetic effects of maghemitization of oceanic crust both theoretical considerations and available experimental results indicate that magnetic effects of maghemitization are strongly dependent on the grain size of the originally unoxidized titanomagnetite maghemitization of single domain titanomagnetite results in a decrease in coercivity an increase in susceptibility and a large decrease in q ratio maghemitization of multidomain titanomagnetite results in an increase in coercivity a decrease in susceptibility and no large changes in q ratio single domain titanomagnetite is probably resistant to the development of a chemical remanent magnetization crm whereas multidomain titanomagnetite can acquire a crm during maghemitization the behavior of pseudo single domain titanomagnetite which is the main carrier of remanence in submarine extrusive rocks is investigated by comparing the magnetic properties of the french american mid ocean undersea study famous less than 0 1 m y old and the leg 37 3 5 m y old pillow basalts recovered from the mid atlantic ridge near 37 n combining electron microprobe analyses curie temperature measurements and cell edge determinations we find that the famous rocks are already oxidized z 0 38 possibly as a result of some high temperature maghemitization during cooling of the magma comparison with the more highly oxidized z 0 7 leg 37 pillow basalts indicates that low temperature maghemitization of such rocks does not result in appreciable changes of coercivity and susceptibility although the q ratio does decrease and crm seems to be acquired such a crm could account for the anomalously low magnetic inclinations observed at most of the leg 37 sites

magnetism in rocks rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks sediments soils and organisms its applications include environmental magnetism magnetic anisotropy sources of continental and oceanic magnetic anomalies records of geomagnetic field variations and polarity reversals and the paleomagnetic record of plate motions and the wilson cycle this paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas pseudo single domain behavior magnetic domains and micromagnetic structures diagnostic tests of the type and stability of remanent magnetization magnetic microanalysis thermoviscous remagnetization and chemical remanent magnetization other areas including sediment and soil magnetism are covered in a companion paper by verosub and roberts on environmental magnetism

magnetic properties of variably oxidized pillow basalt in our study of changes in magnetic properties during low temperature oxidation of oceanic pillows we were able to essentially eliminate variations in grain size which plagued many previous studies this was accomplished by keeping the cores parallel to the outer glassy margin of the pillow while drilling the variably oxidized portions of the basalt curie temperature tc measurements indicated a large range in the degree of low temperature oxidation although most were in the early stages of maghemitization tc 300 c in samples with hysteresis parameters showing pseudo single domain behavior the natural remanent magnetization jnrm and susceptibility both decreased substantially with low temperature oxidation thus the koenigsberger ratio jnrm jinduced did not change the reduction in jnrm was mirrored by comparable decreases in saturation magnetization and saturation remanence low temperature oxidation also increased the bulk coercivity and the coercivity of remanence in the oxidized basalt relative to the less oxidized these later results are consistent with the theory that low temperature oxidation generally stabilizes the remanent magnetization of seafloor pillow basalts the increase in coercivity with oxidation in these pillow basalts contrasts will synthetic samples which show an overall decrease the behavior of natural samples reflects processes which occur during low temperature oxidation that do not normally occur in synthetic samples specifically the loss of iron from the grain and perhaps grain cracking

magnetic petrology of eastern north america diabases i olivine normative dikes from western south carolina abstract the oxide mineralogy and magnetic properties were examined in a suite of fifteen olivine normative diabase dike samples from western south carolina in an attempt to elucidate their magnetic petrology titanomagnetite 1 2 vol is the dominant fe ti oxide mineral ilmenite and secondary magnetite are generally present in very minor amounts chromite constitutes up to 0 5 vol its abundance and composition correlate with bulk rock cr various types of fine scale microstructure are evident in titanomagnetite crystals the most important are patterned anisotropism and the development of trellis type ilmenite lamellae microprobe analyses indicate 1 titanomagnetite compositions x are mostly between 0 4 and 0 55 and 2 low analytical totals are characteristic of most titanomagnetites curie temperatures of the diabases are 500 540 c which are several hundred degrees higher than predicted from the observed titanomagnetite x s 150 300 c we attribute these higher curie temperatures to oxidation of the titanomagnetites which has produced titanomaghemites having visible microstructure and yielding low analyses because they are cation deficient natural remanence magnetization and rem ratio of natural remanence to saturation remanence vary between 4 and 100 10 4 a m2 kg 1 and 0 0019 and 0 032 respectively these properties inversely correlate with cr content and demonstrably contrast cr rich and cr poor samples initial susceptibility saturation magnetization and coercivity values show a two to three fold range variations in initial susceptibility and coercivities appear to be largely related to the type and extent of oxidation induced microstructure in the titanomagnetites

weathering and abrasion of fe ti oxides during rock degradation and fluvial transport implications for sedimentary provenance studies physical and chemical degradation of individual grains of titanomagnetite and ilmenite during rock weathering and fluvial transport processes in the river eden catchment eastern scotland have been traced using backscattered electron images and electron probe microanalysis magnetic characteristics have been used to detect differences in magnetic mineralogy concentration paragenesis and grain size between rock samples showing different stages of weathering and between different sources and sediment samples results have shown higher physical and chemical durability of ilmenite during this segment of the geological cycle than titanomagnetite which is rapidly altered during progressive subaerial weathering of a dolerite the increasingly intensive and extensive oxidation and development of internal fractures in titanomagnetite are observed however under subaqueous conditions titanomagnetite was found to be progressively replaced by titanite and ti impoverished magnetite due to hydrocirculation neither the total physical nor the total compositional degradation of titanomagnetite is achieved under rock weathering and fluvial transport processes thus the importance of titanomagnetite as a provenance indicator remains despite its degradation during the geological processes studied in the river eden catchment

recent advances in fine particle magnetism implications for paleomagnetism it is becoming increasingly obvious in paleomagnetism that the older practice of using large numbers of samples to improve statistical measures of accuracy is often not adequate to reveal the signal of greatest geological importance blanket demagnetization treatments are now largely outdated except for the most ideally behaved samples a rapidly vanishing phenomenon in many locales variations in characteristic remanent directions between neighboring and supposedly contemporaneous but lithogically distinct rock units can no longer be simply treated as noise we now recognize that such differences may actually reflect rock magnetic effects rather than such factors as the short term behavior of the geomagnetic field or local variations in structure multiple overprints often the result of moderate reheating and or chemical alteration can result in an uncertain relationship between the timing of emplacement and the acquisition of remanence in the past rocks which presented such problems could often be avoided in favor of more straightforward units much of that geological cream has been skimmed however further progress in many areas particularly the determination of the apw path and reversal stratigraphy in the paleozoic will require a better understanding of magnetization processes in order to resolve the above mentioned problems and produce accurate and reliable results

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

evidence for deuteric magnetization in hydrothermally altered mesozoic basaltic rocks from east antarctica abstract mesozoic basaltic lavas and dykes from vestfjella east antarctica are dominated by well grouped single component palaeomagnetic directions of normal polarity reversed magnetizations heavily overprinted by normal polarity components were encountered in a few lava flows and dykes only thermal demagnetization was successful in separating tight distributions of reversed polarity directions interpreted to represent a deuteric magnetization the region has been exposed to regional hydrothermal alteration to the epidote prehnite metamorphic facies t 300 c titanomagnetite grains in high temperature oxidation classes ii iii show alteration features typical for hydrothermally altered basalts partial to complete replacement of both ilmenite and magnetite to sphene is common in the most altered rocks which also exhibit varying degrees of decomposition of the opaque minerals the lavas display two major but distinctly different thermomagnetic curves the kink type previously reported from rocks exposed to the epidote metamorphic facies and curves dominated by a paramagnetic contribution defining a magnetite curie point it is concluded that the basalts have retained palaeomagnetic directions acquired during the deuteric cooling phase implying that granulation sphene formation and fe depletion of decomposed titanomagnetite grains in classes ii iii do not necessarily cause complete remagnetization or the acquisition of significant secondary remanent magnetizations

effects of the core shell structure on the magnetic properties of partially oxidized magnetite grains experimental and micromagnetic investigations the relationship between magnetic hysteresis parameters and the degree of oxidation of ultrafine magnetite particles is examined by both experimental measurements distributed particle assemblage with median grain size of 80 nm and standard deviation 0 43 and micromagnetic simulations single particles from 40 nm to 140 nm experimental results show that both coercivity bc and the ratio of saturation remanence to saturation magnetization mrs ms increase slowly as the oxidation parameter z increases from 0 to 0 9 thereafter both parameters decrease sharply as magnetite becomes completely oxidized to maghemite numerical simulations of hysteresis loop and microstructure using a micromagnetic model with a core shell geometry a stoichiometric core surrounded by an oxidized shell show three categories of behavior for magnetic grains during oxidation first the coercivity of sd particles decreases as oxidation proceeds but their remanence magnetization remains in a uniform state second for psd sized particles near the critical sd boundary 80 nm to 100 nm the initial vortex domain structure changes to a sd as oxidation occurs and returns to a vortex state upon complete maghemitization resulting in an initial rise and then fall of bc and mrs finally larger psd grains remain a vortex state throughout the maghemitization with less variations of bc and mrs the predicted magnetic properties exhibit good agreement with experimental observations and suggest that the domain arrangement is likely to be dominated by a core shell structure with strong exchange coupling at their interface overall the partially oxidized magnetite in sd psd range can reliably record palaeomagnetic signals

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

an investigation of ulvospinel composition and cation migration during maghemitization in deep sea drilling project leg 61 titanomagnetites titanomagnetites of basalts and diorites from an off axis volcanic episode in the western pacific ocean leg 61 deep sea drilling project were studied for their ulvospinel composition and for changes in the iron titanium ratio with progressive low temperature oxidation thermomagnetic analysis and microscopy indicate that these fe ti spinels are relatively little low temperature oxidized despite their cretaceous age microprobe investigation shows that the fe ti spinels have a wide range of ulvospinel contents and that the range directly reflects the range of parent rock chemistries this is an important demonstration that ulvospinel contents are not constant but vary within a restricted range of compositions namely at least x 0 50 to x 0 75 fe3 xtixo4 recognition of this fact is important in attempting to elucidate changes in cation ratio as a result of progressive low temperature oxidation examination of samples of different oxidation states for cation ratio changes associated with low temperature oxidation displayed lower titanium in the more oxidized samples and ti enrichment in the less oxidized ones this however appears to be a reflection of changing x values with progressive crystallization of individual sills rather than a consequence of low temperature oxidation in the more than 100 grains analyzed no evidence was found for a more oxidized shell on the exteriors of grains suggesting that the oxidation process did not proceed uniformly with respect to grain geometry

steady and non steady state magma chambers below the east pacific rise volcanic rocks collected on flowline traverses on the flanks of the east pacific rise epr document changes in axial magma chemistry with time providing a record of the thermal history of axial magma chambers amc we present data for a sample set of closely spaced 1 2 km samples along epr flowlines at three localities out to 800 ka showing both steady constant average temperature and non steady state behavior of magma chambers on time scales of 200 500 ka though based on only three symmetrical traverses so far it appears that magmatically robust ridge locations 11 20 n and 9 30 n have steady state chambers whereas a magmatically starved axis 10 30 n shows large temperature changes with time these observations provide a new petrologic perspective to the ongoing debate regarding the significance and causes of morphologic variations along the axis of the east pacific rise

electron probe microanalysis study on processes of low temperature oxidation of titanomagnetite abstract partially oxidized titanomagnetite grains in various kinds of volcanic rocks were investigated by electron probe microanalyzer epma in order to clarify the process of oxidation at low temperature the following results were obtained by the present investigation 1 primary composition of titanomagnetite is homogeneous in individual grains although variation in composition among different grains is observed on each thin section 2 migration of fe cations during low temperature oxidation is clearly seen in all oxidized grains some other constituent cations are also bleached and consequently the relative content of the remaining cations becomes large 3 the detailed internal structures of titanomagnetite grains are observed as backscattered electron images bei with an electronprobe microanalyzer and it seems likely that the structures depend upon the degree of low temperature oxidation 4 the chemical and physical properties of oxidized titanomagnetites imply that low temperature oxidation is not a simple process but a complex one such an oxidation process is correlatable to both the mobility of cation and the oxidation condition such as a circulation of some active hydrothermal materials at low temperature

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

relationship of tsunami intensity to source earthquake magnitude as retrieved from historical data operational prediction of near field tsunamis in all existing tsunami warning systems twss is based on fast determination of the position and size of submarine earthquakes exceedance of earthquake magnitude above some established threshold value which can vary over different tsunamigenic zones results in issuing a warning signal usually a warning message has several from 2 to 5 grades reflecting the degree of tsunami danger and sometimes contains expected wave heights at the coast current operational methodology is based on two main assumptions 1 submarine earthquakes above some threshold magnitude can generate dangerous tsunamis and 2 the height of a resultant tsunami is in general proportional to the earthquake magnitude while both assumptions are physically reasonable and generally correct statistics of issued warnings are far from being satisfactory for the last 55 years up to 75 of warnings for regional tsunamis have turned out to be false while each tws has had at least a few cases of missing dangerous tsunamis this paper presents the results of investigating the actual dependence of tsunami intensity on earthquake magnitude as it can be retrieved from historical observations and discusses the degree of correspondence of the above assumptions to real observations tsunami intensity based on the soloviev imamura scale is used as a measure of tsunami size its correlation with the ms and mw magnitudes is investigated based on historical data available for the instrumental period of observations from 1900 to present

magnetization during low temperature oxidation of seafloor basalts no large scale chemical remagnetization difficulty in removing the effects of grain size dependence have hampered interpretations of magnetic property changes during low temperature oxidation of titanomaghemite for the present study samples were collected perpendicular to cracks in seafloor basalts to produce subsamples of essentially constant grain size but varying degree of low temperature oxidation within a few centimeters coarse grained samples and those showing evidence of high temperature oxidation were excluded increasing curie temperature was used as the most reliable measure of increasing maghemitization intensity of natural remanent magnetization jnrm consistently decreased with increasing low temperature oxidation indicating that any increase in jnrm with age of oceanic crust must be due to other factors coercive force hc increased until curie temperatures reached 250 c and then in samples exhibiting single domain behavior decreased drastically because coercive force depended on degree of oxidation in this complex manner chemical remanent magnetization crm in even slightly oxidized grains should have a different coercivity than the original thermal remanent magnetization trm most deep sea drilling project samples in this study have experienced tectonic rotations or field reversals within 1 0 m y of extrusion thus field controlled crm if present should have different coercivities and a discrete direction and be discernible during alternating field af demagnetization alternating field demagnetizations showed no evidence for significant acquisition of field controlled crm viscous remanent magnetization vrm was the dominant secondary magnetization in samples with several components of remanence in very fine grained basalts vrm was difficult to remove by af demagnetization and may be the source of increased dispersion in highly oxidized oceanic basalts mechanisms which may in part be responsible for coercivity and vrm changes are 1 an increase in the critical size of domain transitions during low temperature oxidation and 2 incipient phase separation into fe rich and ti rich areas the fe rich areas being superparamagnetic the latter mechanism explains the increase in saturation magnetization js and concomitant decrease in jnrm when curie temperatures are above 360 c

displaced water volume potential energy of initial elevation and tsunami intensity analysis of recent tsunami events we consider recent ocean bottom earthquakes for which detailed slip distribution data are available using these data and the okada formulae we calculate the vector fields of co seismic bottom deformations which allow us to determine the displaced water volume and the potential energy of initial elevation of the tsunami source it is shown that in the majority of cases the horizontal components of bottom deformation provide an additional contribution to the displaced water volume and virtually never diminish the contribution of the vertical component the absolute value of the relative contribution of the horizontal components of bottom deformation to the displaced volume varies from 0 07 to 55 on average amounting to 14 the displaced volume and the energy of initial elevation tsunami energy are examined as functions of the moment magnitude and the relevant regressions least squares fits are derived the obtained relationships exhibit good correspondence with the theoretical upper limits that had been obtained under the assumption of uniform slip distribution along a rectangular fault tsunami energy calculated on the basis of finite fault model data is compared with the earthquake energy determined from the energy magnitude relationship by kanamori it is shown that tsunami takes from 0 001 to 0 34 of the earthquake energy and on average 0 04 finally we analyze the soloviev imamura tsunami intensity as a function of the following three quantities 1 the moment magnitude 2 the decimal logarithm of the absolute value of displaced volume and 3 the decimal logarithm of the potential energy of initial elevation the first dependence exhibits rather poor correlation whereas the second and third dependences demonstrate noticeably higher correlation coefficients this gives us grounds to suggest considering the displaced volume and the energy of initial elevation as measures of the tsunamigenic potential of an earthquake

low temperature oxidation in morb of titanomagnetite to titanomaghemite a gradual process with implications for marine magnetic anomaly amplitudes mid ocean ridge basalt morb samples varying in age from recent to jurassic were selected for electron microscopic and rock magnetic studies our observations indicate that the degree of oxidation of titanomagnetite in morb increases only gradually with sample age the titanomagnetite in recent morb 100 ka both electron microscopic observations and rock magnetic data support the notion that low temperature oxidation of titanomagnetite to titanomaghemite in morb is a gradual process moreover the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales including regional oceanic crustal structures lithological features grain size and surrounding matrices pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows owing to higher porosity and permeability of the former in contrast interstitial glass can protect fine titanomagnetite grains from alteration the natural remanent magnetization nrm intensity of morb varies as a function of age magnetic granulometry concentration of magnetic materials and the degree of alteration fine grained morb samples typically have higher nrm intensity the nrm intensity appears to decrease substantially with increasing degree of maghemitization and hence with age the envelope of the smoothed magnetic anomaly amplitudes resembles the change in nrm intensity for morb samples of the last 30 million years but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported maghemitization of the micrometer sized and larger grains is probably only partly responsible whereas a significant contribution to nrm intensities is inferred from those submicrometer sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass

magnetization of axial lavas from the southern east pacific rise 14 23 s geochemical controls on magnetic properties although the spatial association of iron rich lavas and high amplitude magnetic anomalies is well documented a causal link between enhanced iron content and high remanent magnetization has been difficult to establish here we report magnetic data from approximately 250 samples with 8 16 feo total iron as feo from the southern east pacific rise epr that provide strong support for the presumed geochemical dependence of remanent intensity the limited age range 0 6 ka of axial lavas from this ultrafast spreading ridge 150 mm yr full rate effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low temperature alteration systematic sampling relative to the chilled margin illustrates that substantial grain size related variations in magnetic properties occur on a centimeter scale both microprobe data and curie temperatures suggest that the average groundmass titanomagnetite composition in the southern epr samples is approximately constant modal modified ulv6spinel content 0 67 over a wide range of lava compositions saturation magnetization and saturation remanence are highly correlated with feo r 0 73 and 0 83 respectively indicating that more iron rich lavas have higher abundances of otherwise similar titanomagnetite we show that there is a good correlation between natural remanent magnetization nrm and feo provided that sufficient specimens are used to determine the average nrm of a sample r 0 63 because the range of iron contents in mid ocean ridge basalts is limited the best fit slope 4 44 a m per feo in an ambient field of 0 030 mt should provide reasonable bounds on the equatorial magnetization of submarine lavas 10 a m at 8 5 feo and 50 a m at 16 feo finally we demonstrate that along axis variations in nrm closely parallel geochemical changes along the southern epr where magnetization values deviate significantly from those predicted from the range of measured feo contents these discrepancies may reflect additional unrecognized geochemical variability

a unified source model for the 2011 tohoku earthquake abstract the devastating 2011 tohoku earthquake was observed by dense networks of geophysical instruments a unified source model was constructed through joint inversion of teleseismic strong motion and geodetic datasets the result indicates that the earthquake consists of three main ruptures after small rupture in the initial 50 s the first rupture expanded at a slow speed of 1 8 km s to the northeast and east the second rupture began 20 s later at the slowest speed of 1 5 km s and became dominant with the largest slip of 36 m the third rupture then played the leading role propagating southward at a speed of 2 5 km s only the tsunami inversion requires an extension of the source fault to a shallow part between the first rupture area and the japan trench recovering tsunamigenic slips larger than 40 m the slow rupture speed and tsunamigenic slips can explain the features of the disaster by the earthquake

cooling rates of pyroclastic deposits inferred from mineral magnetic investigations a case study from the pleistocene mýtina maar czech republic tephra layers of the m tina maar czech republic contain ferrimagnetic mg al rich titanomagnetite which is suggested to originate from a fractionated alkaline co 2 rich lithospheric mantle melt we investigated the magnetic mineralogy and curie temperature t c from tephra deposits of two drill cores 9 m depth t c calculated 208 14 c from chemical composition fe 2 0 8 mg 0 5 fe 3 1 1 al 0 3 ti 0 3 o 4 is in accordance with tc retrieved from cooling curves of temperature dependent magnetic susceptibility measurements 195 232 c however thermomagnetic curves are irreversible either with lower type i or higher type ii tc in the heating curve all curves show transition temperatures above ca 390 c indicating maghemitization we interpret the irreversibility of tc t c in terms of different degrees of cation ordering overprinted or masked by different degrees of maghemitization which is a low temperature phenomenon negative t c indicates that original deposited titanomagnetite has cooled faster and therefore has stored a lower degree of cation ordering compared to heating cooling rate of 11 c min in the kappabridge type ii with positive t c indicates higher degree of cation ordering and therefore slower cooling rate the central part of this deposit shows most severe maghemitization indicating rather wet emplacement we therefore suggest different eruption styles for deposition of type i pyroclastics with more phreatomagmatic and type ii pyroclastics with more phreato strombolian eruption styles our study is a new approach to discriminate different cooling histories in maar deposits using the curie temperature of titanomagnetite we suggest that this method has the potential to discriminate different emplacement modes resulting from different eruption styles

crustal magnetization in the south atlantic from inversion of magnetic anomalies a method for inverting magnetic anomalies in the presence of topography parker and huestis 1974 is used to analyze and describe crustal magnetization along 12 profiles in the south atlantic the profiles are approximately 2000 km long and are almost perpendicular to the trend of the magnetic anomalies a bandpass filter is used to remove long wavelength anomalies unassociated with seafloor spreading processes a filter that passes only wavelengths between 200 and 3 km produces a magnetization that approximates the positive and negative aspects of the geomagnetic reversal sequence the positive aspects originating from normally magnetized portions of the crustal source and negative aspects originating from reversely magnetized portions of the crustal source a 6 km thick crust is assumed in order to produce magnetizations consistent with dredged oceanic rocks a long wavelength pattern is evident in the magnetization solutions the highest magnetizations are associated with the ridge crest 1 35 1 74 a m within the western section of the study area a sharp decrease in magnetization occurs within 100 km of the ridge crest 4 5 ma a minimum in magnetization is reached at 400 km from the axis or at 18 ma 0 51 0 57 a m the magnetization then gradually increases out to 1100 km from the ridge axis or 50 ma 0 91 0 93 a m and dramatically decreases to 1900 km or 86 ma 0 34 0 43 a m within the eastern section of the study area a sharp decrease in magnetization occurs within the first 100 km of the ridge crest 5 ma a minimum in the magnetization occurs at approximately 400 km from the ridge axis or at 20 ma 0 54 0 56 a m the magnetization then gradually increases out to 1200 km from the ridge axis or to 60 ma 1 15 1 24 a m an overall pronounced decrease occurs to 1900 km from the ridge axis or to 95 ma 0 36 0 40 a m in order to explain this behavior the field intensity may have changed with a time scale of several millions of years alternatively the higher magnetizations at 50 60 ma could be explained by a uniform intensity of magnetization in a thicker crust

generating random earthquake events for probabilistic tsunami hazard assessment to perform probabilistic tsunami hazard assessment for subduction zone earthquakes it is necessary to start with a catalog of possible future events along with the annual probability of occurrence or a probability distribution of such events that can be easily sampled for near field events the distribution of slip on the fault can have a significant effect on the resulting tsunami we present an approach to defining a probability distribution based on subdividing the fault geometry into many subfaults and prescribing a desired covariance matrix relating slip on one subfault to slip on any other subfault the eigenvalues and eigenvectors of this matrix are then used to define a karhunen loeve expansion for random slip patterns this is similar to a spectral representation of random slip based on fourier series but conforms to a general fault geometry we show that only a few terms in this series are needed to represent the features of the slip distribution that are most important in tsunami generation first with a simple one dimensional example where slip varies only in the down dip direction and then on a portion of the cascadia subduction zone

theoretical effects of mechanical grain size reduction on gem domain states in pyrrhotite abstract recent laboratory experiments by halgedahl and ye 1999 show that domain widths in pyrrhotite change very little or not at all as a grain is mechanically thinned along one or two directions in their experiments particles were initially demagnetized in an alternating field until a global energy minimum gem domain state was attained surprisingly the overall positions of surviving walls and many small scale details in the shapes of curved walls were remarkably insensitive to thinning thus domain states that survived thinning were interpreted to be local energy minimum lem states as a first step toward providing a theoretical reference frame for the thinning results gem domain widths in pyrrhotite have been calculated here as grains are thinned to one fourth or less of their original size nine models assume one dimensional 1d thinning which greatly changes both particle size and shape two other models address the effects of three dimensional 3d thinning in which particles retain a cubic shape as their sizes are reduced if a particle can maintain a gem state while it is thinned seven of the nine 1d models and both 3d models predict that domain widths will adjust by amounts that are readily detected experimentally thus results of these calculations support the interpretation that lem states in pyrrhotite can be stable over a broad range of grain sizes and shapes the primary origin of this stability remains an unsolved problem however if this stability is intrinsic to the pure material then future micromagnetic models for pyrrhotite are required to investigate lem states and their stability as functions of grain size and grain shape on the other hand this stability could originate from the pinning of preexisting walls by defects halgedahl and ye 1999 whatever their origins the energy barriers that inhibit lem lem transitions could play a significant role in the acquisition of remanence and the temporal stability of the paleomagnetic signal in rocks

low temperature alteration and magnetic changes of variably altered pillow basalts summary pillow basalt fragments from the east pacific rise dredged during the phoenix expedition often show discoloured rims due to alteration a suite of nine pillow basalts with such discoloured rims and ranging in age between 200 and 820 ka has been characterized in terms of their fe ti oxide mineralogy and rock magnetic properties these large pillow fragments show relatively unaltered grey interiors surrounded by darker concentric halos which vary in thickness as measured from glassy pillow rims and surfaces caused by large cracks penetrating into the original pillow interior the discoloured zones are characterized by precipitation of abundant secondary minerals such as fe 3 rich clays that filled vesicle spaces fe ti oxides in subsamples from discoloured rims and grey interiors have been investigated with electron microscopy and rock magnetic techniques the subsamples come from traverses that are parallel to the outer glassy pillow rims allowing us to study the low temperature alteration effects and rock magnetic properties without having to take variable grain size into account not surprisingly titanomaghemites in discoloured rims are in a general sense oxidized to a higher degree z typically 0 55 than those in the relatively unaltered grey interior z typically 0 6 within 200 000 yr the difference between the oxidation states of titanomaghemite within the grey pillow interior and the discoloured rims gradually diminishes with increasing age so that for samples with ages of 800 ka the oxidation state of titanomaghemites in the grey interior approaches that of the discoloured rim our study demonstrates that visible discolouration of pillow basalts is not a suitable proxy for z because average ti content can vary from sample to sample curie temperatures are also inaccurate proxies for z i fone wants to study possible correlations between z and rock magnetic parameters the best technique is to determine z for each subsample by using trans

determination of lattice parameter oxidation state and composition of individual titanomagnetite titanomaghemite grains by transmission electron microscopy a transmission electron microscopy tem method higher order laue zone holz diffraction has been applied to measurement of accurate lattice parameters of individual titanomagnetite and titanomaghemite grains lattice parameters and cation composition data are obtained with tem analytical electron microscopy aem for individual grains smaller than 0 1 m in diameter a suite of ocean drilling program deep sea drilling project ocean floor basalts containing titanomagnetite titanomaghemite with a wide range of oxidation degree z were employed to establish the relationship between lattice parameter a z and ti content x for naturally occurring samples standardization was accomplished with measurements on grains sufficiently large to allow the determination of absolute oxygen contents by electron microprobe analyses which directly yielded z values the probe data confirm that the maghemitization process in ocean floor basalts is one involving loss of iron and indicate a nearly linear relationship between lattice parameters and oxidation degrees for a given x value except for the smallest and largest values of z the standardized relation permits determination of z for individual submicrometer sized grains using only measured a values and cation compositions as determined by holz diffraction and aem

variable ti content and grain size of titanomagnetite as a function of cooling rate in very young morb transmission electron microscopy observations and rock magnetic measurements on a pillow from the new flow extruded in 1993 at the juan de fuca ridge demonstrate that ti content in large up to 40 m titanomagnetite grains varies as a function of the cooling rate large grains in the interior of the pillow have a narrow composition range of approximately x 0 6 where x represents the ulvospinel content in the solid solution series with magnetite in contrast the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range average x 0 45 within 0 5 cm of the rim a significant portion of the grains is single domain sd to superparamagnetic sp and appears to have little or no ti this lower ti content provides a ready explanation for the higher curie temperature near the glassy margin of the pillow moreover determination of the oxidation state of the titanomagnetite does not show low temperature oxidation anywhere within the pillow indicating no rapid alteration to titanomaghemite as previously suggested for this very young mid ocean ridge basalt morb in order to explain the curie temperatures submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values these grains make contributions to susceptibility that show up as high up to 580 c curie temperatures in thermomagnetic analyses and they appear to be sd to sp the variation of x values for the titanomagnetite in the morb is consistent with thermodynamic and kinetic considerations because crystallization of titanomagnetite in the morb is controlled by processes depending on cooling rate and crystal melt fractionation

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

a preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements 2 experimental testing 1 this paper reports an empirical test of a new nonheating preisach based protocol for determining the absolute ancient magnetic field intensity paleointensity using a selection of synthetic samples and a large suite of modern lavas and pyroclastic lithic samples generally the preisach paleointensity estimates compare favorably with the expected field intensities 1 for the synthetic samples displaying single domain like behavior the method returned the correct result for the known field while multidomain samples yielded an underestimate and 2 averaging 168 post 1850 a d lavas yielded a value 6 within the expected field value these preisach paleointensity estimates are also compared with paleointensity determinations made on the same suite of samples using standard thellier type heating protocols and the nonheating remanence rem method the preisach paleointensity estimates compare favorably for samples that yielded correct thellier type determinations including the synthetic single domain like samples in addition the preisach method produced estimates for cases where the thellier type estimates failed a possible selection criterion was identified median destructive field which was found to improve the paleointensity estimates in some sample suites for the investigated sample set the preisach method was found to be much more accurate than the rem method the importance of cooling rate on the preisach paleointensity estimate is also examined

long term variations in magnetization intensity with crustal age in the northeast pacific atlantic and southeast indian oceans the magnetization intensity of oceanic crust in the northeast pacific atlantic and southeast indian oceans was calculated from marine magnetic anomaly data to investigate long term variations in magnetization intensity with crustal age the magnetization intensity was derived by two dimensional inversion of the upward continued marine magnetic anomalies the principal magnetization intensity variations versus age are summarized as follows 1 in all the oceans a high magnetization intensity exists at the ridge crest followed by a sharp decrease within 8 my and a minimum is reached at 8 20 ma 2 in the northeast pacific and atlantic oceans the magnetization intensity gradually increases up to at least 70 ma this variation pattern is consistent with the previous results of nrm measurements of drilled rocks and inversion of marine magnetic anomalies in the south atlantic

preservation of pristine titanomagnetite in older ocean floor basalts and its significance for paleointensity studies transmission electron microscopy observations and rock magnetic measurements reveal that alteration of fine and large grained iron titanium oxides can occur at different rates fine grained titanomagnetite occurs as a crystallization product within interstitial glass that originated as an immiscible liquid within a fully differentiated melt in several samples with ages to 32 ma it displays very little or no oxidation z 0 in contrast samples with ages of 10 ma or older are observed to also contain highly oxidized z 0 66 large grained titanomaghemite these large grains having originated by direct crystallization from melt are associated with pore space such pore space can serve as a conduit for fluids that promote alteration whereas fine grains may have been armored against alteration by the glass matrix in which they are embedded apparently alteration of oceanic crust is a heterogeneous process on a microscopic scale the existence of pristine fine grained titanomagnetite in the interstitial glass of older ocean floor basalts that have undergone significant alteration implies that such glassy material is capable of carrying original thermal remanent magnetization and may be suitable for paleointensity determinations

low intensity of the geomagnetic field in early jurassic time from a large collection of jurassic continental tholeiites cropping out in europe and africa we selected 90 samples for paleointensity determinations the samples were carefully selected to avoid any secondary magnetizations especially viscous magnetization use of the thellier method reveals that magnetic property changes due to heating begin often at quite low temperatures but fortunately without modifying noticeably their natural remanent magnetization thermoremanent magnetization ratio twenty eight well clustered paleointensity estimates were obtained from two european dikes that were emplaced during early jurassic time the kerforne dike at brenterc h in brittany northwestern france and the messejana dike on the iberian peninsula spain and portugal virtual dipole moments calculated from both magmatic units are similar and only about one third of present day values these new data lend support to the recently postulated low dipole moment of the mesozoic geomagnetic field

thermoviscous remanent magnetization of oceanic lithosphere inferred from its thermal evolution remanent magnetization patterns of the oceanic lithosphere determined on the basis of the thermal evolution of the lithosphere while taking into account both thermal and viscous magnetizations give rise to magnetic anomalies similar to those observed by magsat over the cretaceous quiet zones of the atlantic ocean and by marine magnetic surveys over anomaly 33 viscous magnetization acquired during a given core field polarity period is not removed completely by that acquired in the next period of opposite polarity because of cooling of the lithosphere and the strong temperature dependence of viscous magnetization the magnetization pattern of layer 2a resembles that of the standard seafloor spreading magnetic model consisting of normally and reversely magnetized zones separated by vertical boundaries and produces symmetric magnetic anomalies the magnetization pattern of the middle crust is similar to that of layer 2a for high magnetic blocking temperature models but it is slightly skewed with respect to layer 2a for low magnetic blocking temperature models the magnetization pattern of the lower crust is strongly skewed relative to that of layer 2a and has diffuse boundaries with pronounced slopes between normally and reversely magnetized zones it produces magnetic anomalies with appreciable skewness suggesting that observed skewness of marine magnetic anomaly 33 arises from the magnetization of this layer the magnetization pattern of the upper mantle is appreciably skewed and almost devoid of small scale variations it produces strongly skewed magnetic anomalies at satellite altitudes which may explain the observed skewness of magsat anomalies over the cretaceous quiet zones of the atlantic ocean

chemical remanent magnetization of oceanic crust the effects of chemical remanent magnetization crm of oceanic crust on the anomalous skewness of sea floor spreading magnetic anomalies are investigated considering a realistic constraint that the actual magnetization at anomaly m0 is reversed the crm of layer 2a basalts fails to explain the anomalous skewness of the magnetic anomalies the crm of the deeper layers does contribute to the anomalous skewness of anomalies 33 34 but the major contribution comes from thermal remanent magnetization

a 20 km diameter multi ringed impact structure in the north sea most craters found on earth are highly eroded poorly preserved and only exposed on land here we describe a multi ringed impact structure discovered in the north sea from the analysis of three dimensional seismic reflection data the structure is 20 km in diameter and has at least ten distinctive concentric rings located between 2 and 10 km from the crater centre the structure affects cretaceous chalk and jurassic shales and is well preserved below several hundred metres of post impact tertiary strata which constrains its age to be 60 65 myr old the formation of concentric ringed impact structures at this relatively small scale had not previously been thought possible especially on the terrestrial planets we have mapped the ring structures at a resolution of tens of metres both laterally and in depth and show that the rings are fault bounded graben structures similar to fault arrays formed in low strain rate detachment tectonic settings strata deeper than 500 m palaeodepth appear unfaulted and we infer that the concentric ring structures may have accommodated post impact extension towards the excavated crater through detachment on weak layers within the chalk

a theoretical comparison of tsunamis from dislocations and landslides we use simple physical models to evaluate and compare the orders of magnitude of the energy generated into a tsunami wave by seismic dislocations and underwater slumps we conclude that the two sources can generate tsunamis of comparable total energy however the slumping source is shown to be fundamentally dipolar in nature which results in a low frequency deficiency in the far field these simple conclusions corroborate the interpretation of the 1998 papua new guinea tsunami as being generated by an underwater slump

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

ages and magnetic structures of the south china sea constrained by deep tow magnetic surveys and iodp expedition 349 combined analyses of deep tow magnetic anomalies and international ocean discovery program expedition 349 cores show that initial seafloor spreading started around 33 ma in the northeastern south china sea scs but varied slightly by 1 2 myr along the northern continent ocean boundary cob a southward ridge jump of approximate to 20 km occurred around 23 6 ma in the east subbasin this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the southwest subbasin which propagated for about 400 km southwestward from approximate to 23 6 to approximate to 21 5 ma the terminal age of seafloor spreading is approximate to 15 ma in the east subbasin and approximate to 16 ma in the southwest subbasin the full spreading rate in the east subbasin varied largely from approximate to 20 to approximate to 80 km myr but mostly decreased with time except for the period between approximate to 26 0 ma and the ridge jump approximate to 23 6 ma within which the rate was the fastest at approximate to 70 km myr on average the spreading rates are not correlated in most cases to magnetic anomaly amplitudes that reflect basement magnetization contrasts shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers in addition to large vertical intensity variations these complexities are caused by late stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0 5 which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization the primary contribution to magnetic anomalies of the scs is not in the top 100 m of the igneous basement

revisiting a domain state independent method of palaeointensity determination abstract the use of a domain state independent method the wilson method of acquiring absolute palaeointensities is tested using three sets of historical lavas from mexico italy and iceland the wilson method works by plotting continuous thermal demagnetisation curve data for the original natural remanent magnetisation against those for a laboratory induced thermoremanence by comparing the predicted intensities with the known fields a new empirically derived rejection criterion is proposed that accounts for chemical alteration and improves the accuracy of the palaeointensity estimates the wilson palaeointensity estimates are compared with palaeointensity determinations made on sister samples using standard thellier type heating generally the wilson palaeointensity estimates compare favourably for samples that yielded technically correct thellier type determinations however at one locality the wilson method did not yield any estimates due to severe chemical alteration in contrast it was still possible to determine palaeointensity estimates from the lower temperature thellier data to assess the wilson palaeointensity protocol s ability to return a correct estimate for a true multidomain system a synthetic multidomain magnetite sample was imparted with a thermoremanent magnetisation the wilson protocol returned the correct palaeointensity within error in contrast the thellier type protocol failed as the wilson method is significantly faster than thellier type methods it is suggested that the wilson method should be tried as a first approach in palaeointensity studies or for samples dominated by multidomain material

modification of the magnetic mineralogy in basalts due to fluid rock interactions in a high temperature geothermal system krafla iceland summary active high temperature 150 c geothermal areas like the krafla caldera ne iceland often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity the main alteration processes in such an environment are low temperature oxidation 350 c maghemitization and fluid rock interactions we investigated the rock magnetic properties natural remanent magnetization nrm magnetic susceptibility and their temperature and field variation and the mineralogy using x ray diffraction microscopic methods and electron microprobe analyses of two drill cores kh1 and kh3 from the rim of the krafla caldera the drill cores have distinctly lower nrm values average 3 a m 1 compared to younger surface basalts average 20 a m 1 along with a large variation in magnetic susceptibility 1 3 10 7 4 9 10 5 m3 kg 1 the secondary mineral assemblage sulphides sphene rutile and chlorite indicates an alteration within the chlorite smectite zone for both cores without depth zoning optical miscroscopy in combination with the bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite 1 titanomaghemite with intermediate titanium concentration and probably high vacancy concentration and 2 titanomaghemite with low titanium concentration and low vacancy concentration the mineral assemblages textures and magnetic properties deduced from the mentioned magnetic measurements indicate two stage transformation mechanism 1 dissolution of titanium at low ph under oxidizing conditions the ulvospinel component of titanomagnetite and ilmenite forms rutile or sphene and fe2 migrates out of the spinel lattice forming titanomaghemite 2 formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions the latter mechanism produces ghost textures all titanomaghemite is transformed and only their former grain shapes are preserved with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved this mechanism could explain the significant magnetization loss which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like iceland or hawaii the production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of krafla

lituya bay landslide impact generated mega tsunami 50th anniversary on july 10 1958 an earthquake mw 8 3 along the fairweather fault triggered a major subaerial landslide into gilbert inlet at the head of lituya bay on the southern coast of alaska the landslide impacted the water at high speed generating a giant tsunami and the highest wave runup in recorded history the mega tsunami runup to an elevation of 524 m caused total forest destruction and erosion down to bedrock on a spur ridge in direct prolongation of the slide axis a cross section of gilbert inlet was rebuilt at 1 675 scale in a two dimensional physical laboratory model based on the generalized froude similarity a pneumatic landslide tsunami generator was used to generate a high speed granular slide with controlled impact characteristics state of the art laser measurement techniques such as particle image velocimetry piv and laser distance sensors lds were applied to the decisive initial phase with landslide impact and wave generation as well as the runup on the headland piv provided instantaneous velocity vector fields in a large area of interest and gave insight into kinematics of wave generation and runup the entire process of a high speed granular landslide impact may be subdivided into two main stages a landslide impact and penetration with flow separation cavity formation and wave generation and b air cavity collapse with landslide run out and debris detrainment causing massive phase mixing formation of a large air cavity similar to an asteroid impact in the back of the landslide is highlighted a three dimenional pneumatic landslide tsunami generator was designed constructed and successfully deployed in the tsunami wave basin at osu the lituya bay landslide was reproduced in a three dimensional physical model at 1 400 scale the landslide surface velocities distribution was measured with piv the measured tsunami amplitude and runup heights serve as benchmark for analytical and numerical models

time and space distribution of coseismic slip of the 2011 tohoku earthquake as inferred from tsunami waveform data a multiple time window inversion of 53 high sampling tsunami waveforms on ocean bottom pressure global positioning system coastal wave and tide gauges shows a temporal and spatial slip distribution during the 2011 tohoku earthquake the fault rupture started near the hypocenter and propagated into both deep and shallow parts of the plate interface a very large slip approximately 25 m in the deep part off miyagi at a location similar to the previous 869 jogan earthquake model was responsible for the initial rise of tsunami waveforms and the recorded tsunami inundation in the sendai and ishinomaki plains a huge slip up to 69 m occurred in the shallow part near the trench axis 3 min after the rupture initiation this delayed shallow rupture extended for 400 km with more than a 10 m slip at a location similar to the 1896 sanriku tsunami earthquake and was responsible for the peak amplitudes of the tsunami waveforms and the maximum tsunami heights measured on the northern sanriku coast 100 km north of the largest slip the average slip on the entire fault was 9 5 m and the total seismic moment was 4 2 1022 n m m w 9 0 the large horizontal displacement of seafloor slope was responsible for 20 40 of tsunami amplitudes the 2011 deep slip alone could reproduce the distribution of the 869 tsunami deposits indicating that the 869 jogan earthquake source could be similar to the 2011 earthquake at least in the deep plate interface the large tsunami at the fukushima nuclear power station is due to either the combination of a deep and shallow slip or a triggering of a shallow slip by a deep slip which was not accounted for in the previous tsunami hazard assessments online material table of estimated slip for all subfaults at 0 5 min invervals

the corossol structure a possible impact crater on the seafloor of the northwestern gulf of st lawrence eastern canada we report on a 4 1 0 2 km diameter and 185 m deep circular submarine structure exposed on the seabed in 40 m water depths in the northwestern gulf of st lawrence eastern canada from the analysis of high resolution multibeam bathymetric and seismic data the presence of a circular form characterized by a central uplift and concentric rings resembles the morphology and geometry of complex meteorite impact structures also other origins such as kimberlites intrusions karsts or diapirs can be eliminated on geological criteria a single 4 cm long breccia fragment recovered from the central uplift has numerous glassy droplets of fluorapatite composition assumed to be impact melts and a single quartz grain with planar intersection features thought to be shock induced planar deformation features pdfs the absolute age of this possible impact structure is unknown but its geological setting indicates that it was formed long after the mid ordovician and before regional pre quaternary sea level lowstands present results outline the need for further examination to confirm an impact origin and to precisely date the formation of the structure

magnetic telechemistry and magmatic segmentation on the southern east pacific rise results from axial dredges and a profile inversion of magnetic anomaly data along the axis of the east pacific rise epr at 13 23os provide an estimate of the average degree of fractionation for the extrusive layer at this ultrafast spreading 145 mm yr full rate ridge we find a high correlation rd 0 81 between dredge mean feo total iron as feo and natural remanence for 34 axial dredges with multiple samples having coincident geochemical and magnetic data we attribute this good correlation to detailed sampling spanning the full range of cooling related magnetization changes within a flow and to the young age 0 6 ka of these axial samples which effectively minimizes time dependent magnetization changes due to geomagnetic intensity or alteration a composite axial magnetic anomaly profile shows large amplitude up to 400 nt fluctuations with wavelengths of 50 200 km which theoretical considerations suggest can reliably be related to the magnetization directly beneath the ship for much of the southern epr seismic data provide independent limits on the axial thickness 259 55 m and the pattern of off axis thickening of the extrusive magnetic source layer these data also provide evidence for an axial magma lens that effectively eliminates anomaly contributions from deeper magnetic sources inversion of the axial magnetic anomaly data utilizing these geophysical constraints yields a magnetization solution which through use of the regression relating feo and natural remanence may be related to the average degree of differentiation of the extrusive source layer the magnetic data reveal a pattern of magmatic segmentation that closely parallels the tectonic segmentation of the ridge suggesting that magma supply may be an important control on the average degree of differentiation of the extrusive layer 1998 elsevier science b v all rights reserved

tsunami generation by horizontal displacement of ocean bottom tsunami generation by an earthquake is generally modeled by water surface displacement identical to the vertical deformation of ocean bottom due to faulting the effect of horizontal deformation is usually neglected however when the tsunami source is on a steep slope and the horizontal displacement is large relative to the vertical displacement the effect becomes significant we show this for two recent earthquakes which generated much larger tsunamis than expected from seismic waves in the case of the 1994 june 2 java indonesia earthquake the focal mechanism was a very shallow dipping thrust and the source was near a very steep trench slope in the case of the 1994 nov 14 mindoro philippines earthquake strike slip faulting extended from ocean to land perpendicular to the coast line in both cases we found that the horizontal motion of slope had an important contribution to the tsunami generation

tem study of meteorite impact glass at new zealand cretaceous tertiary sites evidence for multiple impacts or differentiation during global circulation abstract study by transmission electron microscopy of samples from the cretaceous tertiary k t boundary clay at flaxbourne river and woodside creek new zealand has revealed the occurrence of nanometer sized meteorite impact derived glass the average glass composition is exceptionally ca rich and is distinct from other glass found on earth apart from glass inferred to be of impact origin at mexican and haitian k t sites the glass shards are partially altered to montmorillonite like smectite with the dominant interlayer cation ca reflecting the composition of the parent glass the data imply a heterogeneous global distribution in composition of k t boundary impact glass si rich and ca rich in mexico and haiti si rich in denmark and ca rich in new zealand this heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the chicxulub impact site however recent discovery of an impact crater of k t boundary age in ukraine raises the possibility of impact clusters which produce material of heterogeneous composition

effects of weathering on single domain magnetite in early pliocene marine marls rock magnetic parameters are often used to recognize variations in the original magnetic mineralogy and for normalizing purposes in palaeointensity studies incipient weathering however is shown to have a profound but partly reversible influence on the rock magnetic properties of the marls of the early pliocene trubi formation in southern sicily italy the remanence in the marls resides in single domain sd magnetite grains but the remanent coercive force hcr shows a strong variation and most values observed are anomalously high hcr range 36 188 mt the enhanced coercivities are attributed to stress in the magnetite grains induced by surface oxidation at low temperature upon heating to 150 c a reduction of coercivities occurs that can be explained by a stress reduction as a result of a reduction of fe2 gradient due to a higher diffusion rate at elevated temperature after heating to 150 c coercivities are quite uniform throughout the outcrop and the values are characteristic of sd magnetite hcr range 30 38 mt the bulk susceptibility increases by 4 24 per cent and the isothermal remanent magnetization irm decreases by 5 11 per cent the increase in anhysteretic remanent magnetization arm is large 20 242 per cent the magnitude of the changes is related to the degree of weathering another effect of heating the marl samples to 150 c is a substantial reduction of the coercivities of the secondary overprint in the natural remanent magnetization after heating separation of the secondary and primary components by alternating field demagnetization is more efficient the usual difficulties of thermal demagnetization above 300 c may thus be avoided by a combination of moderate heating to 150 c and subsequent alternating field demagnetization

tsunami energy ocean bottom pressure and hydrodynamic force from stochastic bottom displacement tsunami generation and propagation due to a randomly fluctuating of submarine earthquake modeled by vertical time dependent of a stochastic bottom displacement are investigated the increase in oscillations and amplitude in the free surface elevation are controlled by the noise intensity parameter of the stochastic bottom displacement evolution of kinetic and potential energy of the resulting waves by the stochastic bottom displacement is examined exchange between potential and kinetic energy was achieved in the propagation process the dynamic ocean bottom pressure during tsunami generation is investigated as the vertical displacement of the stochastic bottom increases the peak amplitude of the ocean bottom pressure increases through the dynamic effect time series of the maximum tsunami wave amplitude kinetic and potential energy wave and ocean bottom pressure gauges and the hydrodynamic force caused by the stochastic source model under the effect of the water depth of the ocean are investigated

observation of shrinkage cracks in ocean floor titanomagnetites abstract a characteristic of low temperature ocean floor titanomagnetite oxidation is the formation of irregular curved cracks which are caused by shrinkage of the spinel lattice the spatial distribution of the shrinkage cracks within individual titanomagnetite grains magnetically extracted from different ocean floor basalts was studied by scanning electron microscopy the cracks do not develop in grains smaller than approximately 5 microns in diameter under ocean floor conditions the critical titanomagnetite oxidation state z for the onset of cracking is between z 0 23 and z 0 37 shortly before the onset of cracking internal stress reaches a maximum which if reflected in the magnetic coercivity of the material

horizontal motions of water in the vicinity of a tsunami source because it is based on an initial seismic analysis and preset criteria tsunami forecast often fails in assessment of tsunami danger the level of danger can be determined more or less reliably only when observed sea level data became available along with the sea level data i e vertical motions of free water surface we suggest considering horizontal motions of water that accompany the formation and the propagation of a tsunami the amplitude of horizontal motions is normally much higher than the amplitude of the vertical motions detection of the horizontal motions may provide tsunami warning centers with additional in situ data that can be used for estimation of tsunami strength in this study taking the 2011 tohoku oki event as an example horizontal motions of water in the vicinity of the tsunami source are theoretically examined by means of dynamic and static numerical models developed within the framework of linear shallow water approximation it is shown that in the vicinity of the tsunami source within a wide area of about 0 5 million square kilometers the amplitude of horizontal motions exceeded 10 m whereas in some shallow water areas the amplitude amounted to hundreds of meters possible methods of in situ detection of the horizontal motions are discussed

optimal initial conditions for simulation of seismotectonic tsunamis numerical simulation of seismotectonic tsunamis usually starts with specification of the initial elevation of the water surface in the tsunami source the initial elevation is traditionally set equal to the vertical residual bottom deformation resulting from earthquakes we discuss the imperfectness of the traditional approach and suggest an improved practical method of calculating the initial elevation from the solution of the 3d problem in the framework of potential theory the method takes into account horizontal and vertical components of bottom deformation and bathymetry in the source area within the assumption of instant tsunami generation the suggested method represents the optimal way to specify the initial condition in the tsunami propagation problem the tsunamis in the central kuril islands on 15 november 2006 and 13 january 2007 are taken as examples to demonstrate the efficiency of the new method

a volcanomagnetic observation on mount st helens washington total magnetic fields were recorded at five sites on mt st helens between 23 october and 11 november 1981 an interval which included an extrusive dome building eruption of the volcano two of the magnetometers located in the volcano s crater measured reversible magnetic changes which correspond to fluctuations in tilt measured nearby however the relationship is highly nonlinear electric fields were measured on the east flank of the volcano near its summit to search for electrokinetic effects they show no correlation with the magnetic changes and in the long term are uncorrelated with eruptive activity our favored interpretation of the magnetic changes is that they result from stress induced changes in the magnetization of the volcano magnetic field values returned to pre anomaly values this reversibility rules out pressure induced magnetization as the dominant mechanism and places an upper limit of 300 bars on the stress changes the limited spatial extent of the magnetic anomaly field places the source of stress at shallow depth beneath the crater floor consistent with models based on strain data

impacts in the round a well preserved crater has been identified deep beneath the bed of the north sea it may well have been produced by the impact of an extraterrestrial body some 60 million years ago