review of magnetic and electric field effects near active faults and volcanoes in the u s a abstract synchronized measurements of geomagnetic field have been recorded along 800 km of the san andreas fault and in the long valley caldera since 1974 and during eruptions on mount st helens since 1980 for shorter periods of time continuous measurements of geoelectric field measurements have been made on mount st helens and near the san andreas fault where moderate seismicity and fault slip frequently occurs significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include 1 two moderate earthquakes m l 5 8 for which magnetometers were close enough to expect observable signals about three source lengths 2 one moderate earthquake m s 7 3 for which magnetometers were installed as massive fluid outflow occurred during the post seismic phase 3 numerous fault creep events and moderate seismicity 4 a major explosive volcanic eruption and numerous minor extrusive eruptions and 5 an episode of aseismic uplift for one of the two earthquakes with m l 5 8 seismomagnetic effects of 1 3 and 0 3 nt were observed for this event magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a m l 5 2 earthquake longer term changes were recorded in one region in southern california where a moderate m l 5 9 earthquake has since occurred surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision 0 25 nt and 0 01 respectively and are consistent with near surface fault failure models of these events longer term creep is sometimes associated with corresponding longer term magnetic field perturbations correlated changes in gravity magnetic field areal strain and uplift occurred during episodes of aseismic deformation in southern california primarily between 1979 and 1983 because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models the preferred explanation appeals to short term strain episodes independently detected in each data set an unknown source of meteorologically generated noise in the strain gravity and uplift data and an unknown but correlated disturbance in the absolute magnetic data might also explain the data no clear observations of seismoelectric or tectonoelectric effects have yet been reported the eruption of mount st helens generated large oscillatory fields and 9 2 nt offset on the only surviving magnetometer a large scale traveling magnetic disturbance passed through the san andreas array from 1 to 2 h after the eruption subsequent extrusive eruptions generated small precursory magnetic changes in some cases these data are consistent with a simple volcanomagnetic model magneto gas dynamic effects and a blast excited traveling ionospheric disturbance traveling ionospheric disturbances tids also generated by earthquake related atmospheric pressure waves may explain many electromagnetic disturbances apparently associated with earthquakes local near fault magnetic field transients rarely exceed a few nt at periods of a few minutes and longer

geochemistry of cumulates from the bjerkreim sokndal layered intrusion s norway part ii ree and the trapped liquid fraction rare earth elements in bulk cumulates and in separated minerals plagioclase apatite ca poor and ca rich pyroxenes ilmenite and magnetite from the bjerkreim sokndal layered intrusion rogaland anorthosite province sw norway are investigated to better define the proportion of trapped liquid and its influence on bulk cumulate composition in leuconoritic rocks made up of plagioclase ca poor pyroxene ilmenite fmagnetite folivine where apatite is an intercumulus phase even a small fraction of trapped liquid significantly affects the ree pattern of the bulk cumulate together with cumulus minerals proportion and composition contrastingly in gabbronoritic cumulates characterized by the presence of cumulus ca rich pyroxene and apatite cumulus apatite buffers the ree content la sm and eu eu vs p2o5 variations in leuconorites display mixing trends between a pure adcumulate and the composition of the trapped liquid assumed to be similar to the parental magma assessment of the trapped liquid fraction in leuconorites ranges from 2 to 25 and is systematically higher in the north eastern part of the intrusion the likely reason for this wide range of tlf is different cooling rates in different parts of the intrusion depending on the distance to the gneissic margins the ree patterns of liquids in equilibrium with primitive cumulates are calculated with mass balance equations major elements modelling duchesne j c charlier b 2005 geochemistry of cumulates from the bjerkreim sokndal layered intrusion s norway part i constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent lithos 83 299 254 permits calculation of the ree content of melt in equilibrium with gabbronorites partition coefficients for ree between cumulus minerals and a jotunitic liquid are then calculated calculated liquids from the most primitive cumulates are similar to a primitive jotunite representing the parental magma of the intrusion taking into account the trapped liquid fraction calculated from the p2o5 content consistent results demonstrate the reliability of liquid compositions calculated from bulk cumulates and confirm the hypothesis that the trapped liquid has crystallized as a closed system without subsequent mobility of ree in a migrating interstitial liquid d 2005 elsevier b v all rights reserved

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

the j m platinum palladium reef of the stillwater complex montana ii origin by double diffusive convective magma mixing and implications for the bushveld complex sequences of cumulates form concurrently by downdip accretion from a column of liquid layers that are primarily separated by diffusive interfaces the low cumulates grow in advance of the upper cumulates in accord with their higher crystallization temperatures an important observation is that mixing of the two parent liquids lowers their crystallization temperatures and consequently should produce unsaturated hybrids that could dissolve earlier cumulates the repetition of cyclic units above the j m reef is explained by having periodic additional injections of dense magma beneath the subsiding column of crystallizing liquid layers neither the j m reef nor the bushveld ore zones should ever have been continuous across the floor of their respective magma chambers and for bushveld the circumstances resulted in zones that are actually minable modified journal abstract

review of stratigraphy structure and hydrocarbon potential of bass basin australia the intracratonic bass basin is the central basin of three that underlie bass strait in southeastern australia the basin was formed by extensional tectonics during the late jurassic to early cretaceous it contains up to 7 km 23 000 ft of nonmarine sediments of late jurassic to late eocene age and up to 3 5 km 11 500 ft of upper eocene to holocene marine sediments volcanics occur in places the sedimentary history of the basin is interpreted to have been dominated initially by cretaceous flood plain alluvial fan and lacustrine deposition then late cretaceous and early tertiary flood plain deposition and finally cenozoic shallow marine deposition seismic reflection studies show that significant structural and stratigraphic prospects for petroleum exploration occur at paleocene cretaceous and jurassic levels source rock studies combined with depositional models for the upper cretaceous to paleocene nonmarine fill suggest that suitable source and reservoir rocks occur at these levels present day maturation levels of organic matter in sediments of the basin suggest that mature levels for petroleum generation occur at 3 5 km 10 000 16 000 ft depth which generally corresponds to upper cretaceous to paleocene sediments an analysis of the maturation history of the basin suggests that the lower levels of the upper cretaceous sediments had peak petroleum generation potential ro 0 7 by about 40 ma whereas the upper levels of the paleocene sediments have attained this level of maturity only in the last 5 m y furthermore th relative lack of exploration success at shallower immature levels of the basin argues against well developed vertical migration paths and suggests that petroleum may have been trapped mainly at mature levels considerable unexplored petroleum potential exists in the bass basin the best potential for petroleum exploration appears to be at mature levels within the upper cretaceous to paleocene eastern view coal measures

magmatic metasomatism and formation of the merensky reef bushveld complex the rare earth element ree contents of pyroxenes and other minerals from the merensky reef and stratigraphically adjacent rocks of the atok section bushveld complex have been determined with the ion microprobe merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the ree than their cumulus counterparts in rocks several meters below the reef chondrite normalized merensky clinopyroxene ce contents vary from 10 to 90 for ce and from 4 to 17 for yb they also possess deep negative eu anomalies the eu anomalics being deeper for crystals having high ree contents and relatively shallow for pyroxenes with low ree contents similar compositional characteristics are displayed by cl rich apatite which is an accessory phase in the rocks interstitial pyroxene in cumulates above and below the reef also tends to have elevated ree contents and in general is not in equilibrium with coexisting cumulus minerals the melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts that from which merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in ree than any normal tholeiitic or alkalic basalt despite their highly evolved nature in terms of the ree the merensky reef pyroxenes are not evolved in terms of major elements the decoupling of incompatible trace and major elements is best explained by a metasomatic process it is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile the fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed

the lower zone critical zone transition of the bushveld complex a quantitative textural study the lower zone critical zone boundary of the bushveld complex is an intrusion wide major stratigraphic transition from ultramafic harzburgite and pyroxenite in the lower zone to increasingly plagioclase richpyroxenitesandnoritesinthecriticalzone quantitative textural and compositional data for 29 samples through this transition show the following lower zone orthopyroxene grains are larger have higher aspect ratios are better foliated and have a lower trapped liquid component than those of the critical zone the larger grain size of the lower zone results in crystal size distribution plots that are rotated to lower slopes and intercepts relative to those in the critical zone although all rocks show differing amountsoffoliation mineral lineations are weak to absent these data are consistent with significant compaction driven recrystallization in the study section numerical modeling of concurrent compaction and crystallization provides a quantitative model of how the lower zone critical zone transition may have formed plagioclase is rare in the lower zone because compaction removes interstitial liquid before it reaches plagioclasesaturation however asthecrystalpilegrows plagioclase saturation is reached in the interstitial liquid before compaction is complete in more evolved pyroxenites producing more abundant but still modest amounts of plagioclase characteristic of the lower critical zone it is concluded that both the textures and the modal mineralogy are largely controlled by compaction and compactiondriven recrystallization primary magmatic textures are not preserved

decrease of intensity of elf vlf waves observed in the upper ionosphere close to earthquakes a statistical study 1 we present results of a systematic study of intensity of vlf electromagnetic waves observed by the demeter spacecraft in the upper ionosphere altitude 700 km we focus on the detailed analysis of the previously reported decrease of wave intensity shortly before the main shock during the nighttime using a larger set of data more than 3 5 years of measurements and a newly developed data processing method we confirm the existence of a very small but statistically significant decrease of wave intensity 0 4 hours before the time of the main shock at frequencies of about 1 7 khz it is shown that the decrease does not occur directly above the earthquake epicenter but is shifted about 2 in the westward direction moreover it is demonstrated that the decrease occurs more often close to shallower earthquakes and close to earthquakes with larger magnitudes as it is intuitively expected representing an additional proof of validity of the obtained results finally no dependence has been found on the occurrence of the earthquake below the ocean or below the continents

a search for elf vlf activity associated with earthquakes using isis satellite data for more than 10 years researchers using satellite observations have reported anomalous phenomena associated with seismic activity increases in elf vlf filter amplitudes have been associated with preshock and postshock events near satellites in a number of studies however an examination of the wideband elf vlf data from the low altitude isis satellites shows no significant changes in electromagnetic em activity associated with earthquakes in the analysis of wideband data from 164 satellite passes above 39 earthquakes no unique or familiar previously observed seismo active emission can be linked to the occurrence of a nearby earthquake the electromagnetic activity levels in elf vlf spectra observed by the isis 2 satellite before and after 37 shocks is examined and compared with a set of selected control orbits comparisons are made of em activity changes between earthquake passes and control passes over the epicenter location no increase in the activity for earthquake passes is found these results do not change when various constraints are placed on the data in an attempt to test for dependencies on different criteria no precursory coseismic or postseismic effects are found no significant evidence is found for differences between the data sets which would suggest any seismo active activity

evidence of electro seismic phenomena abstract ground based observations of electromagnetic precursors to large earthquakes have been reported in japan iran chile greece and the soviet union ionospheric disturbances measured by soviet satellites have been claimed to be seismic precursors in addition observations of very low frequency anomalies measured by geos satellites have been reported by french researchers to confirm reports of electromagnetic precursors to earthquakes a series of low frequency broadband radio frequency receivers were deployed along the san andreas fault system this system of monitors has recorded electromagnetic anomalies occurring just before earthquakes between 1983 and 1986 these anomalies are of two types and are presumably caused by separate effects both of which may be related to stress induced changes in the crust just before fault rupture the first of these types of anomalies appears to be an interruption in the propagation of radio waves radio depressions lasting for many hours before an earthquake the second type is similar to those in previously published reports and consists of short duration broadband radio emissions given the results reported by other researchers and the results from our own experiments we believe that electromagnetic data may be useful as a tool for predicting earthquakes

subsurface exploration via satellite structure visible in seasat images of north sea atlantic continental margin and australia satellite observations of the sea surface height are sensitive to crustal structure overcoming the inability of previous remote sensing techniques to penetrate the earth s surface seasat gravity images have been compared with the structure of offshore sedimentary basins in northwestern europe seasat images display primary structural features e g the anglo dutch and smaller basins down to the scale of the manx furness basin and numerous secondary structural features e g the halibut horst additionally because of their synoptic viewpoint seasat images reveal features hard to distinguish i cite as an example a basement ridge extending from scotland to norway in position to have controlled marine circulation during deposition of the principal source rock wh ch is equivalent to the kimmeridge clay in north america and south america seasat images have been used to examine the relation of the atlantic fracture zones to the structure of the continental margin of the united states and brazil in australia known basins of the northwest continental margin and the bass strait have been compared with seasat projections of shallow structure in comparison with reconnaissance seismic magnetics and ship borne gravity surveys seasat images are valuable in discovering the presence of thick sediment accumulations seasat images are potentially more valuable than seismic and magnetics in the presence of basement masking by intrasectional volcanics or massive carbonates in known basins seasat data deliver a uniform synoptic view permitting the identification of large structural features in this form these images give the structural geologist a tool like that given the stratigrapher by regional seismic sections in remote unexplored regions seasat images provide a means of optimizing the layout of reflection seismic the gravity data from seasat offer the first of increasingly accurate structural images to be expected from altimetric satellites

the upper critical zone of the eastern bushveld complex precursor of the merensky reef the upper critical zone in the central sector of the eastern bushveld complex consists predominantly of norites although bronzitites anorthosites chromitites and other chromitic rocks appear at intervals in the sequence gabbronorites appear near the middle of the zone the rock sequence is divided into 14 principal units plus the series from the base of the merensky reef to the top of the bastard reef mineralogical compositions of successive units have been determined from modal analyses and compositions of bronzite and plagioclase in the various rock units have been determined by microprobe analysis the results indicate that although parts of the zone show normal fractionation the zone as a whole is not the product of progressive fractionation of a magma crystal settling appears to have played a relatively minor role in evolution of the zone successive pulses of parent magma into the chamber may account for some of the mineralogical and chemical variations but features of certain thick norite units suggest stratification of the magma in the chamber the study suggests that the merensky reef registers a sudden event in the bushveld magma chamber an event that altered the chemistry of the system and triggered precipitation of sulfides and platinoid minerals the alternative hypothesis that the reef is the culmination of a long period of magma fractionation is not supported by the evidence presented chromite occurs in a variety of associations with plagioclase and bronzite the associations are consistent with phase relations in the system mg 2 sio 4 iron oxide caal 2 si 2 o 8 sio 2 at elevated pressure

sill intrusion into pyroxenitic mush and the development of the lower upper critical zone boundary of the bushveld complex implications for the origin of stratiform anorthosites and chromitites in layered intrusions

a lower crustal origin for massif type anorthosites the origin of batholith sized masses of plagioclase rich crustal rocks mainly proterozoic in age has mostly been ascribed to aluminous magmas of mantle derivation here we propose that such masses are derived by partial melting of an aluminous lower continental crust which has developed following extraction of a granitic upper crust by intra crustal melting this segregation of upper and lower crusts occurred principally in the late archaean as the lower crust was depleted in heat producing elements during the initial melting to produce granites an external source of heat is required to produce proterozoic massif type anorthosites we speculate that this heat resulted from the continental crust moving over a mantle hotspot or line plume

electromagnetic emissions during dilating fracture of a rock abstract a lot of experiments on electromagnetic emissions emes have been reported under axial compressive fracture shear fracture indentation fracture and stick slip friction in lab and blasting in situ but there are rare reports on the in lab experimental work on emes during dilating fracture of a rock which is helpful in studying and understanding emes related to slow earthquakes and the earthquakes due to volcanic activities and water level changes of reservoirs therefore in the present paper in order to check whether there are detectable emes during dilating fracture of a rock in lab dilating fracture experiments were conducted the dry cuboid specimens of initially intact granodiorite and limestone were tested inside magnetic field free space mffs at room temperature we arranged evenly 20 eme antennas whose resonance frequencies range from 2 5 khz to 540 khz close to rock specimens our experimental results strongly indicate that detectable emes could generate during dilating fracture of a rock they were recorded only associated with some but not all phases of fracture their waveforms often took on the trend that a peak arrived at first and then attenuated sharply and followed by a series of low amplitude oscillations the electromagnetic em signals after eliminating the effects of eme antennae via deconvolution had the maximum peak to peak amplitudes of about 80 0 mv and 40 5 mv for granodiorite and limestone respectively their main spectral components often concentrated in the band of several khz to 60 khz and of several khz to 280 khz the emission of electrons and charged particles from fracture surfaces and or micro fracture electrification could be possible mechanisms for our experimental results

the petrogenesis of platinum group element reefs in the upper main zone of the northern lobe of the bushveld complex on the farm moorddrift south africa boreholes have intersected the interval hosting the pyroxenite marker at the base of the upper main zone on the farm moorddrift in the northern bushveld complex the concentrations of the major and compatible trace elements in the rocks are in the range of other pyroxenite marker intervals elsewhere in the bushveld complex mgo up to 22 wt cr up to 300 ppm over a stratigraphic height of some 300 m the interval is characterized by a broad but irregular reversal in mg number and cr v analogous to stratigraphically equivalent horizons elsewhere in the bushveld complex the rocks have distinctly higher concentrations in incompatible trace elements particularly large ion lithophile elements lile and light rare earth elements lree than other main zone rocks and thus appear to contain an elevated crustal component the interval hosts several thick meters to 10s of meters but mostly relatively low grade and laterally discontinuous pge reefs that have been delineated over a strike length of ca 1 km pge contents locally reach approximately 16 ppm pt pd au in rocks containing up to approximately 4 to 5 percent sulfides the pge tenors of most sulfides are 100 ppm pt pd ratios are above unity and pd ir is 200 based on well defined positive correlations between pt and pd with ir we interpret the mineralization as being largely of primary magmatic origin most previous models interpreted the pyroxenite marker to have formed from a major replenishment to the bushveld magma chamber with relatively primitive magma the new magma would have been emplaced on top of an approximately 4 km thick cumulate package consisting of the ultramafic mafic lower and critical zones and the lower to central portions of the gabronoritic main zone here we propose that the upper main zone including the pyroxenite marker did not result from a magma replenishment instead it crystallized from the s saturated residual magma of the upper critical zone cumulates and originally directly overlaid the latter the approximately 1 to 2 km thick lower central main zone package formed from a crystal charged magma emplaced at a relatively late stage in the solidification history of the bushveld chamber between the largely consolidated upper critical and upper main zones we envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite perhaps in response to the evacuation of a putative lower staging chamber from which the bushveld complex was fed semiconsolidated portions of the cumulate package sagged and slumped toward the center of the complex and the flowing crystal mushes underwent mechanical unmixing of light relatively buoyant plagioclase from denser pyroxenes oxides and sulfides this resulted in the formation of an interlayered sequence of relatively melanocratic sulfide enriched with relatively leucocratic sulfide poor layers reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the main zone magma

composition of the ultramafic mafic contact interval of the great dyke of zimbabwe at ngezi mine comparisons to the bushveld complex and implications for the origin of the pge reefs the great dyke contains the world s second largest platinum resource after the bushveld complex isotopic and trace element data from the interval straddling the contact between the ultramafic and mafic sequences of the great dyke indicate a less enriched composition than in the bushveld complex great dyke sri 0 7024 0 7028 end mostly 1 to 1 ce sm 2 6 bushveld sri 705 0 709 end 5 to 7 ce sm 5 15 these data are interpreted to reflect relatively moderate amounts of contamination of the great dyke parent magma all analyzed isotopes show little variation across the main sulfide zone and the ultramafic mafic contact this corroborates earlier work by other researchers that the great dyke crystallized from a single magma type mixing of compositionally distinct magmas proposed to have caused sulfide melt saturation in the bushveld complex seemingly played little or no role in the formation of the pge mineralization in the main sulfide zone and neither did enhanced crustal contamination of specific magma batches instead sulfide melt saturation of the magma was likely triggered by silicate fractionation the mechanism of concentration of the sulfide melt remains uncertain but theoretical considerations suggest that phase sorting in response to slumping of crystal mushes possibly caused by chamber subsidence played an important role this model is consistent with the highly irregular undulating nature of the contact between the mafic and ultramafic zones of the intrusion in the hanging wall of the main sulfide zone

magnetic field observations in the near field the 28 june 1992 mw 7 3 landers california earthquake recent reports suggest that large magnetic field changes occur prior to and during large earthquakes two continuously operating proton magne tometers lsbm and ochm at distances of 17 3 and 24 2 km respectively from the epicenter of the 28 june 1992 mw 7 3 landers earthquake recorded data through the earthquake and its aftershocks these two stations are part of a differentially connected array of proton magnetometers that has been operated along the san andreas fault since 1976 the instruments have a sensitivity of 0 25 nt or better and transmit data every 10 min through the goes satellite to the usgs headquarters in menlo park california seismomagnetic offsets of 1 2 0 6 and 0 7 0 7 nt were observed at these sites in comparison offsets of 0 3 0 2 and 1 3 0 2 nt were observed during the 8 july 1986 mt 5 9 north palm springs earthquake which occurred directly beneath the ochm magnetometer site the observations are generally consistent with seismomagnetic models of the earthquake in which fault geometry and slip have the same form as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake in these models right lateral rupture occurs on connected fault seg ments in a homogeneous medium with average magnetization of 2 a m the fault slip distribution has roughly the same form as the observed surface rupture and the total moment release is 1 1 1020 nm there is no indication of dif fusion like character to the magnetic field offsets that might indicate these ef fects result from fluid flow phenomena it thus seems unlikely that these earth quake generated offsets and those produced by the north palm springs earthquake were generated by electrokinetic effects also there are no indications of en hanced low frequency magnetic noise before the earthquake at frequencies be low 0 001 hz

antarctic marine gravity field from high density satellite altimetry summary closely spaced satellite altimeter profiles 5 km collected during the geosat geodetic mission geosat gm and those planned for the extended ers 1 mission are easily converted to grids of vertical gravity gradient and gravity anomaly as profile spacing decreases it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross track gradients if one is only interested in the horizontal and vertical derivatives of the gravitational potential however adjustment of the profile is unnecessary the long wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along track derivative of each profile ascending and descending slope profiles are then interpolated onto separate uniform grids these two grids are summed and differenced to form comparable grids of east and north vertical deflection using laplace s equation the vertical gravity gradient is calculated directly from the vertical deflection grids fourier analysis is required to construct gravity anomalies from the two vertical deflection grids these techniques are applied to high density 2 km profile spacing geosat gm profiles in antarctic waters 60 s to 72 s gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean the resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles after adjustment of a dc shift in the shipboard gravity profiles 5 mgal the rms difference between the ship and satellite gravity is 5 5 mgal many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large leaky transform along the pacific antarctic rise

experimental measurement of electromagnetic emissions possibly related to earthquakes in japan recent observations in the ussr appear to suggest that wide band electromagnetic radiation occurs just prior to earthquakes to apply this phenomenon to earthquake prediction measurements have been carried out at the sugadaira space radiowave observatory in japan under the ussr japan cooperation program in 1980 the recorded noise level at 81 khz is comparatively quiet throughout the day and night however about one half hour before the main shock of a magnitude 7 earthquake at 0733 ut on march 31 1980 the instrument recorded an anomalous amplitude increase to 15 db higher than the normal level vlf data recorded synoptically at sugadaira suggest that unusual impulsive radiation at frequencies below 1 5 khz also occurred shortly before the earthquake similar 81 khz emissions were observed prior to magnitude 5 and 6 earthquakes on september 25 1980 and january 28 1981

application of pattern recognition techniques to earthquake catalogs generated by model of segmented fault systems in three dimensional elastic solids techniques and seismicity parameters described by eneva and ben zion 1997 are used to examine synthetic earthquake catalogs generated by ben zion 1996 for precursory patterns of large model events different model realizations represent various levels of fault zone disorder these include models with uniform properties u a parkfield type asperity a fractal brittle properties f and multi size scale heterogeneities m the seismicity parameters used are based on information contained in typical earthquake catalogs reflecting earthquake distribution in space time and size the analysis highlights the complexity of the information content of the synthetic earthquake catalogs simple repetitive precursory signals have not been found however local extrema in the examined parameters are found to have significant association in time with large events thus our techniques and parameters may be useful for intermediate term earthquake prediction especially when parameters are used in combinations some analysis results are the same for all model realizations and some depend on the model features characterizing all catalogs are as follows 1 large model events are statistically predictable on the basis of patterns in the distribution of smaller events 2 for a given parameter the type of precursory extrema maxima or minima is the same for all models 3 the interparameter correlation for any parameter pair has the same sign positive or negative in all models 4 the large events are neither slip nor time predictable based on previous large events results that differ from model to model include the following 1 the degree of predictability of large events correlates with the degree of regularity in the assumed fault properties following the order u f a and m 2 there is no one to one correlation between type of precursory extrema maxima or minima and type of precursory trends increase or decrease this produces great variations in observable trends for any given parameter both from model to model and for different events in the same model 3 the interparameter correlations vary among models with the highest correlations in model f most discussed patterns are in agreement with observations from seismically active zones laboratory models and mining induced seismicity

intermediate wavelength magnetic anomaly field of the north pacific and possible source distributions the objective of this study is to investigate the feasibility of extracting intermediate wavelength magnetic anomalies of wavelengths between 4000 and 400 km from total field data acquired in marine magnetic surveys the intention is to develop a second data set which may be used in conjunction with the satellite magnetic field to determine anomaly and source distributions we apply our technique to the recovery of intermediate wavelength total intensity anomalies over the north pacific from marine survey data the technique also provides a record of secular variation during the period 1960 to 1980 in areas of high track density the sea surface data can be used to refine the magsat data processing techniques and also provide further constraints on source models in areas of reduced sea surface track density the magsat field provides higher resolution the magsat data set also records vector field data as will be shown the filtered sea surface data set provides a higher resolution field than the magsat data due to proximity to lithospheric source bodies the sea surface data also provide an independent check of the magsat field since the two fields should be equivalent after upward continuation of the sea surface field we notice however that for the central pacific the upward continued sea surface field is approximately 2 times the amplitude of the magsat field the spatial correlation of the two anomaly fields is very strong

a detailed view of the south pacific geoid from satellite altimetry images of sea surface undulations in the south pacific have been constructed from geos 3 and seasat altimeter data height discrepancies at crossover points associated with long wavelength radial orbit error were suppressed by taking along track derivatives of the ascending and descending profiles these geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical finally the results are displayed by using the hill shading technique where gray tone images represent the innner product of the deflection vector with an assigned sun vector less apparent sea surface undulations can be enhanced by varying the sun s zenith and azimuth shorterwavelength sea surface undulations reflect seafloor topography for instance fracture zones fz s appear as elongated sharp steps in the sea surface while seamounts produce circular bumps since large areas of the south pacific are unsurveyed many previously undetected features appear on the images comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to easter island s expression the dominant features in the images however are the large age offset fz s such as the eltanin and udintsev fz s the images reveal that the eltanin fz is connected to the louisville ridge combined they produce a continuous geoid signature across most of the south pacific this supports the hypothesis of hayes and ewing 1968 that the louisville ridge is the northwest extension of the eltanin fz

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

geochemistry and mineralogy of the platreef and critical zone of the northern lobe of the bushveld complex south africa implications for bushveld stratigraphy and the development of pge mineralisation the northern lobe of the bushveld complex is currently a highly active area for platinum group element pge exploration this lobe hosts the platreef a 10 300 m thick package of pge rich pyroxenites and gabbros that crops out along the base of the lobe to the north of mokopane formerly potgietersrus and is amenable to large scale open pit mining along some portions of its strike an early account of the geology of the deposit was produced by percy wagner where he suggested that the platreef was an equivalent pge rich layer to the merensky reef that had already been traced throughout the eastern and western lobes of the bushveld complex wagner s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe this correlates the platreef and an associated cumulate sequence that includes a chromitite layer known as the grasvally norite pyroxenite anorthosite gnpa member directly with the sequence between the ug2 chromitite and the merensky reef as it is developed in the upper critical zone of the eastern and western bushveld implicit in this view of the magmatic stratigraphy is that similar critical zone magma was present in all three lobes prior to the development of the merensky reef and the platreef however when this assumed correlation is examined in detail it is obvious that there are significant differences in lithologies mineral textures and chemistries mg of orthopyroxene and olivine and the geochemistry of both rare earth elements ree and pge between the two sequences this suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct the critical zone of the northern lobe cannot be correlated with the critical zone in the rest of the complex and the simplest explanation is that the gnpa platreef sequence formed from a separate magma or mixture of magmas chilled margins of the gnpa member match the estimated initial composition of tholeiitic main zone type magma rather than a critical zone magma composition where the gnpa member is developed over the ultramafic lower zone hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid this style of interaction and the resulting rock sequences are unique to the northern lobe the gnpa member contains at least seven sulphide rich horizons with elevated pge concentrations some of these are hosted by pyroxenites with similar mineralogy crystallisation sequences and pd rich pge signatures to the platreef chill zones are preserved in the lowest main zone rocks above the gnpa member and the platreef and this suggests that both units were terminated by a new influx of main zone magma this opens the possibility that the platreef and gnpa member merge laterally into one another and that both formed in a series of mixing quenching events involving tholeiitic and ultramafic magmas prior to the main influx of tholeiitic magma that formed the main zone

origin of phlogopite orthopyroxene inclusions in chromites from the merensky reef of the bushveld complex south africa about 30 of the chromite grains of variable sizes in a chromitite seam at the base of the merensky reef of the bushveld complex on the farm vlakfontein contain abundant composite mineral inclusions the inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite they vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains na phlogopite and orthopyroxene are most abundant in the inclusions edenitic hornblende k phlogopite oligoclase and quartz are less abundant cl rich apatite rutile zircon and chalcopyrite are present at trace levels na phlogopite is unique to the inclusions it has not been found elsewhere in the bushveld complex other minerals in the inclusions are also present in the matrix of the chromitite seam but their compositions are different the mg mg fe2 ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix k phlogopite in the inclusions contains more na than in the matrix the average compositions of the inclusions are characterized by high mgo 26 wt na2o 2 4 wt and h2o 2 6 wt and low cao 1 1 wt and feo 4 4 wt the 18o value of the trapped melt estimated by analysis of inclusion rich and inclusion poor chromites is 7 this value is consistent with the previous estimates for the bushveld magma and with the 18o values of silicate minerals throughout the reef the textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile rich melts during chromite crystallization the volatile rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and na k rich fluids expelled from the underlying crystal pile the addition of fluid to the magma is thought to have caused dissolution of orthpyroxene leaving the system saturated only in chromite both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution most of the cr and al in the inclusions was contributed through wall dissolution of the host chromite dissolution of minor rutile trapped along with orthopyroxene provided most of the ti in the inclusions the na and k rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile rich melts

the determination of geoid undulations and gravity anomalies from seasat altimeter data the global seasat altimeter data set has been edited and adjusted using a crossing arc procedure fixing one long arc to provide control in doing this a set of 168 master arcs were formed from repeat and near repeat arcs the adjustment carried out by d rowlands was done in a global net and four regional areas the average crossover discrepancy before the adjustment was 1 5 m while after the adjustment solving for a bias and trend it was 28 cm the examination of the crossover discrepancies by area indicated larger than normal discrepancies in certain areas such as the hudson bay the mediterranean and the north and baltic seas some of these discrepancies are caused by invalid tide values applied to the altimeter data in these areas the adjusted seasat sea surface heights were used to predict point values at 1 1 intersections in 10 different geographic regions for comparison with corresponding geos 3 values a mean difference geos 3 minus seasat of 1 3 0 67 m was found the mean difference being caused by different ellipsoid parameters the adjusted data were also used to determine mean surface heights and gravity anomalies in 37 9051 1 and 1178 5 equal area blocks with some of these values being on land these estimations were done using least squares collocation using covariances functions scaled to each 5 region the average predicted standard deviation of the 1 1 anomalies was 5 1 mgal and 2 7 mgal for the 5 blocks seasat values were compared to the geos 3 values finding a difference of 7 8 mgal and 0 87 m for the 1 1 data and 2 2 mgal and 0 76 m for the 5 data comparisons with terrestrial data showed no significant difference between the geos 3 seasat implied anomalies although a number of significant discrepancies have been resolved with the seasat data

field relations and strontium isotope systematics of the marginal rocks of the eastern bushveld complex the well exposed basal contact of the eastern bushveld complex was mapped at scales of 1 10 000 and 1 20 000 along 130 km of strike and through a 9 km vertical section field relations petrography and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups a broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone a gabbroic group occurs above this level each group consists of a number of closely related rock types most of which have sill analogs rocks of the first group b1 include pyroxenites norites and peridotites all of which may have quench textures all contain orthopyroxene en 78 plagioclase an 65 and chromite with subordinate clinopyroxene and biotite rare earth element patterns are mostly strongly fractionated ce n yb n approximately 10 and have 40x chondritic abundances at ce their initial strontium isotope ratios at 2 05 b y fall in the range 0 7032 to 0 7057 and rb sr ratios approximate 0 2 the gabbroic group consists of microcrystalline two pyroxene gabbros b2n and b3n and their coarser grained equivalents b2 and b3 that border the upper critical and main zones respectively mineral assemblages are dominated by plagioclase approximately an 55 clinopyroxene and orthopyroxene approximately en 63 with abundant magnetite and traces of ilmenite and biotite their rare earth element patterns are less fractionated than b1 rocks the b2n and b2 family have ce n yb n approximately 6 and 30x chondritic abundances at ce the b3n and b3 rocks have ce n yb n approximately 3 6x chondritic abundances at ce and pronounced positive eu anomalies strontium isotope initial ratios at 2 05 b y fall in the range 0 706 to 0 708 and the rb sr ratios are approximately 0 02 whole rock and sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial sr isotope ratio r 0 similar to that of the transition zone 0 7056 a fraction of such liquid enriched in cumulus olivine crystals when mixed with up to 30 percent of a pyroxenitic liquid with low r 0 0 7034 yields a product having the isotopic and compositional characteristics of the peridotite sills a continuum exists in the initial sr ratios through the pyroxenitic and gabbroic marginal rocks suggesting that these represent mixtures of at least two sources with distinct initial sr isotope ratio characteristics it is argued that neither magma aging nor crustal contamination adequately accounts for the sr isotope characteristics of the marginal rocks or their derived cumulates the unusual chemistry of the pyroxenitic early liquids sio 2 approximately 55 mgo approximately 13 coupled with high concentrations of incompatible elements should be considered primary

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

depositional wedge cycles on evolving plate margin western and northwestern australia the barrow province lies at the transition between the western and northwestern rift domains of the australian continental margin as these domains evolved during the mesozoic five depositional wedge sequences developed in the eastern half of the province each sequence possesses distinct stratigraphic and sedimentological characteristics which differs from the others as a function of proximity direction and style of rift related diastrophism along the evolving margin the first depositional sequence developed during the triassic as part of a regional tethyan facing passive margin clastic wedge this event was followed during the early and middle jurassic by a second more provincial sequence in response to early syn rift fault related subsidence by the callovian a deep anastomosing rift complex had formed and deposition of the third late jurassic cycle was initially very localized confined by the new graben topography however stratigraphic continuity returned during post breakup subsidence of the northwestern margin as subregional depositional systems began to bury the fragmented borderland this cycle was terminated at the close of the jurassic in response to renewed rifting between western australia and greater india sediment stripped from t e uplifted rift shoulders fed the northerly prograding barrow delta system to form a fourth depositional sequence during the late jurassic early neocomian the final cycle commenced as greater india uncoupled from australia during the middle neocomian post breakup subsidence is reflected by a major regional transgression and the development of a second margin wide depositional wedge during the later cretaceous the close temporal and spatial relationship that exists between these cycles and the evolution of the western and northwestern australian continental margin emphasizes the importance of tectonic control upon sedimentation

early opening history of the north atlantic i structure and origin of the faeroe shetland escarpment summary marine geophysical surveys show that the escarpment is the buried feather edge of a thick pile of flood basalts of early eocene age overlying a thinner more widespread layer of basalts of late palaeocene age the escarpment does not therefore define the continent ocean boundary in the southern norwegian sea but instead marks the contemporary shoreline separating terrestrially erupted basalt flows in the north from a restricted shallow water shelf to the south the basalts overlie 5 6 km of mesozoic sediments which have completely buried a large conical flat topped seamount of similar dimensions to anton dohrn and rosemary bank we call this newly postulated body the brendan seamount the mesozoic sediments are at least as old as early cretaceous in age therefore precluding the possibility that the mid cretaceous seafloor spreading episode supposed by some to have created the rockall trough could have also created the thin crust inferred to underlie the faeroe shetland trough and mbre basin

evidence for magma mixing processes within the critical and lower zones of the northwestern bushveld complex south africa abstract during deposition of the lower lz lower critical lcz and upper critical ucz zones the course of crystallization was dominated by repeated emplacement of primitive mafic magma evidence for this is provided by textural chemical and isotopic data emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends i e progressive increase in mg mg fe 2 ratios and enrichment of cr in orthopyroxenes through intervals of 350 m in the lz 100 200 m in the lcz and 10 80 m within the lower part of the ucz reversals culminated with the deposition of olivine cumulates upon pyroxenite within the lz and lcz but a change in the style of reversals led ultimately to ultramafic cumulates with or without olivine resting directly upon anorthositic cumulates at the top of the ucz merensky and bastard units these incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the ucz whereas the cumulus feldspar bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident residual liquids there is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the ucz wide variations of initial sr isotope ratios exist throughout the entire section within individual units and within individual members of units isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples and between individual feldspar grains of different textural habit the first appearance of cumulus feldspar bearing cumulates of the ucz is presaged by the appearance of small partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the lcz this texture is conspicuous throughout the lowermost 400 m of the ucz but is still sporadically developed within the bastard unit at the top of the ucz anorthositic rocks of the lower part of the ucz are held to have been derived from residual liquids evolved during crystallization of earlier cumulates but anorthosites with a main zone isotopic signature cap the merensky and bastard units

the role of magma mixing in the genesis of pge mineralization in the bushveld complex thermodynamic calculations and new interpretations the elegant magma mixing model for the formation of the platinum group elements pge mineralization associated with chromite and sulfide in the bushveld complex proposed by naldrett and von gruenewaldt 1989 has recently been questioned by cawthorn 1999 based on his own calculation cawthorn concluded that the concave upward sulfur solubility curve of a fractionated basaltic magma which is central to the model does not exist cawthorn 1999 pointed out that both the hanging wall and footwall sequences of the merensky reef formed from sulfide undersaturated magmas instead of sulfide saturated magmas as predicted by the model cawthorn argued that there is a mass balance problem with the model if these rocks did not contribute pge to the sulfides in the reef our thermodynamic calculations indicate that the concave upward curve of s solubility though different in detail from the original is very much in evidence our calculations also indicate that the content of s dissolved in the residual magma may fall below the s solubility curve when the rate of s increase in the magma due to the removal of silicate cumulates is less than the rate of increase in its s solubility due to the change in its temperature and composition during fractional crystallization mixing of such fractionated and yet sulfide undersaturated magma with appropriate amounts of a primitive magma is still capable of achieving sulfide oversaturation in the hybrid once the immiscible sulfides settle out of the magma the content of s dissolved in the hybrid may fall below its solubility curve again during further crystallization thus no sulfide segregation is expected in the rocks crystallized subsequently r factor assessment needs to consider the consequence of sulfide suspension in magma small quantities of sulfide droplets formed during the normal course of fractional crystallization may remain suspended in magma until the next replenishment and in this case they can be redissolved in the hybrid when the proportion of a new input is large enough because there is no early sulfide liquid segregation both the residual and new magmas will not be depleted in pge and are equally important for contributing pge to the sulfide liquid segregated during a subsequent magma mixing event

geochemistry and pge of the lower mineralized zone of the waterberg project south africa the waterberg deposit is located north of the northern lobe of the bushveld complex in south africa and represents a large high grade new platinum group element pge discovery the first comprehensive study of the lower ultramafic section of the area has been completed and whole rock chemical analyses and pge data are presented the section studied comprises mineralized harzburgite and marginal orthopyroxenite overlain by troctolite grading into gabbroic rocks whole rock analyses show geochemical variations typical of differentiated assemblages of cumulus olivine plagioclase and pyroxene normalized trace element data display hree depletion strong positive eu and sr anomalies and lree enrichment the negative anomalies for th rb nb and ta are typical for rocks of the bushveld complex normalized pge distributions are fractionated pd ir 177 pdenriched and au poor emplacement of the magmas is believed to have commenced with west east trending finger like intrusions followed by lateral dilation and emplacement of sulfide droplet bearing ultramafic magmas this was followed by a second phase of intrusions characterized by sheet like bodies of troctolite fractionation of these magmas led to the development of gabbroic rocks that make up the top of the succession the waterberg project is located in the southern marginal zone of the limpopo belt this position in a structurally active area may have facilitated the creation of space for initial magmas it is argued that the mafic to ultramafic succession of the waterberg project does not represent a simple marginal extension of the northern lobe nor does it directly correlate with the platreef it shares geological features but represents a separate magmatic basin

sulfur saturation and second stage melts application to the bushveld platinum metal deposits a model is presented for the formation of platinum group element ores by precipitation from sulfur deficient platinum group element rich second stage magmas numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption mantle derived peridotites residual from midocean ridge basalt magma genesis have pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation they imply the presence of an accessory immiscible sulfide melt to host the platinum group elements the very high sulfide melt silicate melt partition coefficients of the platinum group elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event the extreme platinum group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing pd and pt abundances at the low ppb level keays et al 1981 magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation these first stage magmas become impoverished in platinum group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase e g midocean ridge basalt glasses average

strontium isotope evidence for preserved density stratification in the main zone of the bushveld complex south africa the bushveld complex formed from two isotopically distinct magmas magma addition and mixing can be tracked by measuring initial strontium isotope ratios of cumulates rocks with ratios of 0 7065 lie below the lower 2 2 km of the main zone ratio 0 7085 whereas those with ratios of 0 7073 lie above it this unit represents a liquid layer intruded into the density stratified magma pile near the base of the chamber the merensky reef was produced by mixing of the pile with underlying trapped liquid enriched in precious metals

elf vlf recordings during the 11 march 2011 japanese tohoku earthquake 1 broadband elf vlf radio recordings 0 2 40 khz were made at a site in onagawa japan located 102 km from the epicenter of the m9 0 11 march 2011 earthquake the fifth most powerful earthquake in recorded history the receiver operated for about two minutes after the start of the earthquake after which the receiver lost power examination of the vlf data shows no radio emissions preceding or coincident with the onset of the earthquake however once the secondary seismic waves reached the receiver a number of impulses and diffuse noise bands arose which may result from the entire power grid shaking or from radio emissions from compressing rocks examination of the elf data 0 2 1 khz show no precursor effect in the hours preceding the earthquake examination of vlf data from narrowband transmitters shows no anomalous activity in the days or weeks preceding the earthquake our instrument low frequency cut off is 200 hz and thus we cannot comment on emissions in the ulf range 10 hz

depth dependent δ 13 c trends in platform and slope settings of the campbellrand malmani carbonate platform and possible implications for early earth oxygenation highlights carbon cycle of neoarchean carbonate platform and potential oxygen oasis carbon isotopes reveal a shift to aerobic biosphere and increasing oxidation state rare earth element patterns reveal decrease in open ocean water influx rimmed margin architecture was crucial for evolution of aerobic ecosystems abstract the evolution of oxygenic photosynthesis is widely seen as the major biological factor for the profound shift from reducing to slightly oxidizing conditions in earth s atmosphere during the archean proterozoic transition period the delay from the first biogenic production of oxygen and the permanent oxidation of earth s atmosphere during the early paleoproteorozoic great oxidation event goe indicates that significant environmental modifications were necessary for an effective accumulation of metabolically produced oxygen here we report a distinct temporal shift to heavier carbon isotope signatures in lagoonal and intertidal carbonates 13ccarb from 1 6 to 0 2 relative to vpdb and organic matter 13corg from about 40 to 25 relative to vpdb from the 2 58 2 50 gy old shallow marine campbellrand malmani carbonate platform south africa this indicates an increase in the burial rate of organic matter caused by enhanced primary production as well as a change from an anaerobic to an aerobic ecosystem trace element data indicate limited influx of reducing species from deep open ocean water into the platform and an increased supply of nutrients from the continent both supporting primary production and an increasing oxidation state of the platform interior these restricted conditions allowed that the dissolved inorganic carbon dic pool in the platform interior developed differently than the open ocean this is supported by coeval carbonates from the marginal slope setting which had a higher interaction with open ocean water and do not record a comparable shift in 13ccarb throughout the sequence we propose that the emergence of stable shallow water carbonate platforms in the neoarchean provided ideal conditions for the evolution of early aerobic ecosystems which finally led to the full oxidation of earth s atmosphere during the goe

geochemistry of cumulates from the bjerkreim sokndal layered intrusion s norway part i constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent whole rock major element compositions are investigated in 99 cumulates from the proterozoic bjerkreim sokndal layered intrusion rogaland anorthosite province sw norway which results from the crystallization of a jotunite fe ti p rich hypersthene monzodiorite parental magma the scattering of cumulate compositions covers three types of cumulates 1 ilmenite leuconorite with plagioclase ilmenite and ca poor pyroxene as cumulus minerals 2 magnetite leuconorite with the same minerals plus magnetite and 3 gabbronorite made up of plagioclase ca poor and ca rich pyroxenes ilmenite ti magnetite and apatite each type of cumulate displays a linear trend in variation diagrams one pole of the linear trends is represented by plagioclase and the other by a mixture of the mafic minerals in constant proportion the mafic minerals were not sorted during cumulate formation though they display large density differences this suggests that crystal settling did not operate during cumulate formation and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism the trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition each linear trend is a locus for the cotectic composition of the cumulates this property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent starting from a primitive jotunite the tjfrn parental magma another type of cumulate called jotunite cumulate and defined by the mineral association from the transition zone of the intrusion has to be subtracted to simulate the most evolved part of the liquid line of descent the proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large n 40 the model however does not account for the k2o evolution suggesting that the system was open to contamination by roof melts the liquid line of descent corresponding to the bjerkreim sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most ti p and fe rich melts evolved jotunite are lacking the constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a

large scale magmatic layering in the main zone of the bushveld complex and episodic downward magma infiltration the bellevue drillcore intersects 3 km of main and upper zone cumulates in the northern limb of the bushveld complex main zone cumulates are predominately gabbronorites with localized layers of pyroxenite and anorthosite some previous workers using bulk rock major trace and isotopic compositions have suggested that the main zone crystallized predominantly from a single pulse of magma however density measurements throughout the bellevue drillcore reveal intervals that show up section increases in bulk rock density which are difficult to explain by crystallization from a single batch of magma wavelet analysis of the density data suggests that these intervals occur on length scales of 40 to 170 m thus defining a scale of layering not previously described in the bushveld complex upward increases in density in the main zone correspond to upward increases in modal pyroxene producing intervals that grade from a basal anorthosite with 5 pyroxene to gabbronorite with 30 40 pyroxene we examined the textures and mineral compositions of a 40 m thick interval showing upwardly increasing density to establish how this type of layering formed plagioclase generally forms euhedral laths while orthopyroxene is interstitial in texture and commonly envelops finer grained and embayed plagioclase grains minor interstitial clinopyroxene was the final phase to crystallize from the magma plagioclase compositions show negligible change up section average an62 with local reverse zoning at the rims of cumulus laths average increase of 2 mol in contrast interstitial orthopyroxene compositions become more primitive up section from mg 57 to mg 63 clinopyroxene similarly shows an up section increase in mg pyroxene compositions record the primary magmatic signature of the melt at the time of crystallization and are not an artefact of the trapped liquid shift effect combined the textures and decoupled mineral compositions indicate that the upward density increase is produced by the downward infiltration of noritic magma into a previously emplaced plagioclase rich crystal mush fresh noritic magma soaked down into the crystallizing anorthositic mush partially dissolving plagioclase laths and assimilating fe enriched pore melt the presence of multiple cycles showing upward increases in density in the bellevue drillcore suggests that downward magma infiltration occurred episodically during crystallization of the main zone

some statistical results on very low frequency radiowave emissions in the upper ionosphere over earthquake zones abstract an anomalous increase in the intensity of low frequency 0 1 16 khz radiowave emissions was detected using intercosmos 19 data when the satellite was nearly over the epicentre of an earthquake zone ample statistical data are the basis for a determination of the probable spatial temporal dimensions of the region of the upper ionosphere in justify which elf vlf emissions were detected the reliability of the detected phenomenon is 0 8 0 9 the ionospheric regions with the low frequency emissions observed by intercosmos 19 have been found to be formations noise belts extended along 60 of geographic longitude and 2 of geographic latitude the spectral characteristics of elf vlf emissions near seismic areas are specified the earthquake depth and location under a continent or under an ocean has an effect on the results of the satellite observations of earthquakes possible mechanisms of the phenomenon are discussed

the effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions a series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates the calculation assumes local equilibrium between phases and that the system remains chemically closed during crystallization of the trapped liquid the latter assumption is held to be valid on a scale of tens to hundreds of centimeters it is not necessary to know the composition of the trapped liquid as the calculation only requires an estimate of feo content and trapping temperature the change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the trapped liquid shift its magnitude depends on the modal proportions of cumulus phases and the initial porosity and is only weakly dependent on initial phase compositions trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization crystallization of 30 trapped liquid gives rise to shifts of up to 10 mol percent in mg number of olivine or pyroxene the size of the shift becomes greater when the initial cumulus assemblage has a lower total feo mgo content and vice versa as a result of the relationship between trapped liquid shift and cumulus mode mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals for example in a cyclic unit grading from a pyroxenitic base to an anorthositic top crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle as has been observed in the upper critical zone of the bushveld complex

a statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric vlf lf propagation and earthquakes 1 the subionospheric vlf lf propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes an extensive period of data over 7 yr from january 2001 to december 2007 and a combination of different propagation paths in and around japan are used to examine the statistical correlation between the vlf lf propagation anomaly average nighttime amplitude dispersion and nighttime fluctuation and earthquakes with magnitude 6 0 it is then found that the propagation anomaly exceeding the 2 standard deviation criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth 40 km finally the mechanism of seismoionospheric perturbations is discussed

bathymetry estimates in the southern oceans from seasat altimetry a mean sea surface height based on seasat altimeter data has been high pass filtered to reveal a pattern of geoid anomalies which are highly correlated with sea floor topography the technique is used to locate new bathymetric features in the poorly surveyed southern oceans

trace element stratigraphy of the bellevue core northern bushveld multiple magma injections obscured by diffusive processes the 3 km long bellevue core was recovered from the upper zone uz and main zone mz of the northern limb of the bushveld complex the largest known layered mafic intrusion we present abundance data for 57 trace elements hosted in plagioclase clinopyroxene low ca pyroxene and olivine obtained by laser ablation inductively coupled plasma mass spectrometry these data permit the first comprehensive investigation of trace element distribution in the bushveld complex on a mineral by mineral basis and show that the cr content of pyroxene is the only proxy that records three cryptic magma injections coincident with two pyroxenite horizons and the uz mz boundary on the basis of cr abundance we infer that an anomalous troctolitic horizon at the base of the bellevue core was related to injection of relatively primitive magma analogous to the lower or critical zone magmas in the eastern and western limbs of the bushveld complex this horizon defines the greatest inconsistency of magmatic stratigraphy between the northern limb and the eastern and western limbs of the complex and is prospective for platinum group element exploration however the most significant finding of this study is that plagioclase pyroxene and olivine have undergone extensive diffusive modification the major and trace element geochemistry of these minerals records sub solidus equilibration during the prolonged cooling of the bushveld complex controlled by diffusion rates and local partitioning between adjacent minerals by analogy texturally mature cumulates in any layered intrusion are also likely to be chemically re equilibrated

subionospheric vlf signal perturbations possibly related to earthquakes a likely vlf subionospheric signal effect related to seismic activity was first reported by hayakawa et al 1996a b in association with the great kobe earthquake we have analyzed similar data during periods around 10 other great earthquakes magnitude m 6 in order to understand the main features of such an effect the following characteristics emerged from our analysis the effect appears as a transient oscillation with a 5 to 10 day period which is initiated a few days before a large earthquake and decays over a few days to weeks after it it is mainly related to crustal earthquakes it appears when resonant atmospheric oscillations with periods in a range of 5 11 days exist before the earthquake the seismic influence on the vlf signal is probably explained by the generation of long period gravity waves during the earthquake process and their intensification at heights of 70 90 km

low frequency magnetic field measurements near the epicenter of the ms 7 1 loma prieta earthquake the authors report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the m sub s 7 1 loma prieta earthquake of 17 october 1989 their measurements cover 25 narrow frequency bands in the more than six decade frequency range 0 01 hz 32 khz with a time resolution varying from a half hour in the ulf range 0 01 10 hz to one second in the elf vlf range 10 hz 32 khz the ulf system is located near corralitos about 7 km from the epicenter the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf system is located on the stanford campus about 52 km from the epicenter analysis of the elf vlf data has revealed no precursor activity that they can identify at this time however the ulf data have some distinctive and anomalous features first a narrow band signal appeared in the range 0 05 0 2 hz around september 12 and persisted until the appearance of the second anomalous feature which consisted of a substantial increase in the noise background starting on 5 october and covering almost the entire frequency range of the ulf system third there was an anomalousmore dip in the noise background in the range 0 2 5 hz starting one day ahead of the earthquake finally and perhaps most compelling there was an increase to an exceptionally high level of activity in the range 0 01 0 5 hz starting approximately three hours before the earthquake there do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase further while the systems are sensitive to motion seismic measurements indicate that there were no significant shocks preceding the quake thus the various anomalous features in the data and in particular the large amplitude increase in activity starting three hours before the quake may have been magnetic precursors less

study of seismic influence on the ionosphere by super long wave probing of the earth ionosphere waveguide abstract investigation of the earth ionosphere waveguide by super long waves has been suggested for study of ionospheric sources related to seismic activity the omega system of phase radionavigation is used as the transmitting apparatus daily phase and amplitude variations have been analysed along seismo active profiles deviations of daily phase and amplitude variations were detected before a number of earthquakes

the chronology of magma influxes to the eastern compartment of the bushveld complex as exemplified by its marginal border groups the floor contact of the eastern compartment of the bushveld complex has been mapped in detail for 130 km field relations petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated these represent the 4 main magma influxes which formed the malic phase of the bushveld complex and are in chronological order 1 a contaminated magnesian basalt b1 which gave rise to a saucer shaped laminated marginal zone at the base of the complex with diverse related sills 2 a harrisitic peridotite representing quenched lower zone magma 3 a border group to the more evolved probably basaltic critical zone magma which was intruded in two phases an early quenched norite facies n being followed by a chilled bulk critical zone facies b2 which forms a skin to the whole critical zone finally 4 a border group to the main and upper zone b3 which represents an early magnesian pulse of this last immense volume of tholeiitic liquid there is also evidence for minor influxes of magma within the critical zone and main zone

investigating seismoionospheric effects on a long subionospheric path we examine the possibility of earthquake precursors influencing the subionospheric propagation of vlf transmissions we consider the long 12 mm path from northeastern united states to faraday antarctica 65 s 64 w during 1990 1995 and investigate the subionospheric amplitude variation of signals from the naa communication transmitter 24 0 khz 1 mw in cutler maine with particular emphasis on possible changes induced by seismic events occurring in south america we have analyzed the changes in timing of modal minima generated by the passage of the sunrise terminator over the andes i e the vlf terminator time tt method the anomalous variations in timing throughout the year are of a size and occurrence frequency similar to those previously reported i e 0 5 1 hour and 1 2 per month however we find that in these anomalous cases the time of the sunrise modal minimum does not change significantly but rather the minimum becomes insufficiently deep to be detected and the time of the next nearest minimum is logged our analysis indicates that the occurrence rate of successful earthquake predictions using the tt method cannot be distinguished from that of chance additionally the level of false earthquake prediction using the tt method is high

geophysical applications of satellite altimetry publications related to geophysical applications of seasat and geosat altimetry are reviewed for the period 1987 1990 problems discussed include geoid and gravity errors regional geoid heights and gravity anomalies local gravity field flexure plate tectonics and gridded geoid heights gravity anomalies 99 refs

concentrations of rare earth elements in silicate rocks of the lower critical and main zones of the bushveld complex concentrations of rare earth elements in the lower critical and main zones of the bushveld complex at union section show distinct variation with stratigraphic height in that lower and lower critical zone cumulates are lree and th enriched ce sm 10 25 ce smn 3 65 th smn 6 17 over main zone cumulates ce sm 4 10 ce smn approximately 2 th smn 1 57 the upper critical zone constitutes a transitional interval in terms of ree concentrations ce sm 9 17 this pattern is broadly coupled albeit in a contraposed manner to that in initial sr isotopic ratio and confirms earlier studies that the bushveld complex crystallized from at least two compositionally distinct parental magmas mixing between the two magmas occurred as early as in the lower zone and is sensitively reflected by variation in ce sm th sm modal plagioclase and chromite while ree contents of the lower and critical zones are approximately 15 those of the corresponding b1 and b2 marginal rocks the main zone cumulates have similar ree contents to the b3 marginal rocks this suggests that the main zone rocks either crystallized from a highly viscous crystal mush without appreciable fractionation or from magma strongly ree enriched over the marginal rocks which would imply that the main zone is not represented in the suite of marginal rocks

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

platinum group element distribution in the main zone and upper zone of the bushveld complex south africa abstract the platinum group element pge contents of the upper portions of the bushveld complex were investigated with three questions in mind a in natural systems does magnetite concentrate os ir ru and rh ipge as has been observed in experimental systems b is there a au pd enriched layer present such as observed in the upper parts of the skaergaard intrusion c can changes in metal ratios be used for prospecting for pge deposits in the sulfide poor main magnetite layer of the eastern bushveld ir and rh are enriched relative to pt pd and au and this could be because magnetite preferentially concentrated ir and rh over pt pd and au however in most other magnetite layers no enrichment was observed this could be because most magnetite layers contain approximately 1 sulfides and the pge budget is dominated by the sulfides these sulfides obscured the effects of magnetite collecting ipge because sulfides collect all the pge and the partition coefficients for the pge into a sulfide liquid are much greater than the partition coefficient for ipge into magnetite the weighted average of the platinum group elements pge and au over the 2000 m of sampled stratigraphy is au 2 1 ppb pd 1 7 ppb pt 1 7 ppb rh 0 18 ppb ru comparison of the composition of the marginal rocks of the bushveld with a weighted average for the complete 6 km of the bushveld cumulates shows that the cumulate pile is much richer in compatible elements ir rh cr and poorer in incompatible elements sm and hf than the marginal rocks two possible solutions to this are a the magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or b fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres

marine gravity of the southern ocean and antarctic margin from geosat in november of 1986 the u s navy satellite geosat began collecting unclassified gravity altimeter data as part of its exact repeat mission erm for national security reasons the geosat orbit was arranged so that it closely follows the seasat satellite altimeter ground track however there are two advantages of the geosat data over the seasat data first because of improvements in altimeter design geosat profiles are about 3 times more precise than seasat profiles this corresponds to an accuracy of 2 3 rad i e 2 3 mgal for wavelengths greater than 20 km second the geosat altimeter data were collected when the antarctic ice coverage was minimal february 1987 to march 1987 while seasat was only active during an antarctic winter june 1978 to september 1978 these new data reveal many previously uncharted seamounts and fracture zones in the extreme southern ocean areas adjacent to antarctica seven large age offset fracture zones apparent in the geosat data record the early breakup of gondwana finally the new data reveal the detailed gravity signatures of the passive and active continental margins of antarctica these data are an important reconnaissance tool for future studies of these remote ocean areas

a comparison of satellite and shipboard gravity measurements in the gulf of mexico satellite altimeters have mapped the marine geoid over virtually all of the world s oceans these geoid height measurements may be used to compute free air gravity anomalies in areas where shipboard measurements are scarce two dimensional 2 d transformations of geoid height to gravity are limited by currently available satellite track spacing and usually sacrifice short wavelength resolution full resolution may be retained along widely spaced satellite tracks if a one dimensional 1 d transformation is used although the 1 d transform retains full resolution it assumes that the gravity field is lineated perpendicular to the profile and is therefore limited by the orientation of the profile relative to the field we investigate the resolution and accuracy of the 1 d transform method in the northern gulf of mexico by comparing satellite gravity profiles with high quality shipboard data provided by edcon inc the long wavelength components of the gravity field are constrained by a low degree reference

origin of the pegmatitic pyroxenite in the merensky unit bushveld complex south africa the genesis of the pegmatitic pyroxenite that often forms the base of the merensky unit in the bushveld complex is re examined large 1 cm orthopyroxene grains contain tricuspidate inclusions of plagioclase and chains and rings of chromite grains which are interpreted to have grown by reaction between small primary orthopyroxene grains and superheated liquid this superheated liquid may have been an added magma or be due to a pressure reduction as a result of lateral expansion of the chamber there would then have been a period of non accumulation of grains permitting prolonged interaction with the crystal mush at the crystal liquid interface crystal ageing and grain enlargement of original orthopyroxene grains would ensue only after the pegmatitic pyroxenite had developed did another layer of chromite and pyroxenite with normal grain size accumulate above it immiscible sulphide liquids formed with the second pyroxenite but percolated down as a result of their density contrast even as far as the footwall anorthosite in some cases whole rock abundances of incompatible trace elements in the pegmatitic pyroxenite are comparable with or lower than those of the overlying pyroxenite and so there is no evidence for addition and or trapping of large proportions of interstitial liquid or of an incompatible element enriched liquid or fluid in the production of the pegmatitic rock because of the coarse grained nature of the rock modal analysis especially for minor minerals is unreliable annealing has destroyed primary textures such that petrographic studies should not be used in isolation to distinguish cumulus and intercumulus components geochemical data suggest that the merensky pyroxenite both pegmatitic and non pegmatitic typically consists of about 70 80 cumulus orthopyroxene and 10 20 cumulus plagioclase with a further 10 of intercumulus minerals and could be considered to be a heteradcumulate

new experimental approach to understanding igneous texture thin sections containing a magma like mixture of three crystalline ammonium compounds and intercrystalline melt can be prepared and observed microscopically during crystallization melting and deformation the aim of such observations is to establish links between active processes in the crystal melt mixture and the textural signatures of the processes the hope is to improve the basis for process interpretation of textures in rocks although the value of the experimental system in this regard is still unknown three unexpected processes are seen creation of growth centers for later blocky grains by isothermal decomposition of early dendritic grains migration of grain boundaries early in the crystallization history and growth of porphyroblast like grains by phase boundary migration with abundant melt nearby textural metamorphism begins in this material well before its melt present history is complete

cu pd and cu pt of silicate rocks in the bushveld complex implications for platinum group element exploration the bushveld complex contains the bulk of the world s reserves of pge a total of 89 samples were analysed for cu pd and pt the stratigraphy sampling and analytical methods are reviewed and the results are discussed the data are tabulated plotted and discussed

patterns of basement structure and reactivation along the ne atlantic margin a lineament pattern on the ne atlantic margin is discussed illustrated by gravity and magnetic images in the norwegian sea and reviewed in the context of onshore field evidence while most possible fault trends exist three major sets predominate a ne sw left stepping lineament set defines the gross geometry of the margin while interposing northerly trends impose a rhomboidal geometry at a variety of scales the margin is segmented by nw se transfer zones sometimes involving significant offsets the principal trends are primarily a function of mesozoic cenozoic plate wide extensional stress fields certain proterozoic and caledonian lineaments were however opportunistically reactivated according to the extension direction caledonian ne sw orogen oblique shears typified by the more trondelag fault zone were reactivated via jurassic strike slip or oblique slip and were further exploited during cretaceous early cenozoic extensional episodes leading to continental break up jurassic e w extension may also have reactivated n s faults existing in the basement or generated in duplex systems between the ne sw shears precambrian and caledonian basement lineaments striking at a low angle to the extension direction probably predisposed the formation of major transfer zones

3 d modelling of seamount topography from satellite altimetry we develop a complete set of algorithms to perform 3d modelling of seamount bathymery from satellite altimetry the first stage of the data processing consists in gridding the geoid to account for the long wavelength errors geoid heights are first bias adjusted at cross overs then a collocation on a regular grid is performed accounting for the altimeter errors in a second stage geoid heights are converted into bathymetry no simplifying assumption on the shape and location of the bathymetry highs is necessary bathymetric uncertainties due to the data sampling and the parameters of the mechanical and crustal models are evaluated

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

physical properties of the variations of the electric field of the earth preceding earthquakes i abstract in an earlier paper we described the physical properties of the preseismic transient variations in the electrotelluric field the present letter clarifies a few points that were not properly emphasized in our previous publications furthermore recent results like that of the destructive kalamata earthquake on september 13 1986 indicate that the duration of a preseismic electric signal may reach considerably high values lasting up to several hours

vlf emissions associated with earthquakes and observed in the ionosphere and the magnetosphere abstract several electromagnetic phenomena recorded at the time of earthquakes were recorded independently by the geostationary satellite geos 2 above africa longitude 37 e and the polar orbiting satellite aureol 3 apogee 2012 km perigee 408 km inclination 82 5 period 109 5 min the observations were made with magnetic and electric antennae in the frequency range 10 hz 15 khz characteristics of the emissions and differences between low and high altitude observations are presented some geos 2 very low frequency data can be correlated with ionospheric measurements made on ground at the djibouti station latitude 11 n longitude 42 e where increases of the critical frequency f 0 e s of the sporadic e s layer are observed when earthquakes occur nearby time differences between disturbance at ionospheric level and at the height of geos 2 seem to indicate that earthquakes cause two distinct types of electromagnetic disturbance an electromagnetic wave in the ionosphere and a density increase of the e s layer

precursory analysis of gps time series for seismic hazard assessment we discuss the use of fractal analysis of geophysical big data as a precursory method for the estimation of current seismic hazard we begin by analyzing gps time series of land surface displacements recorded in various regions of the planet exhibiting different degrees of seismic activity we show how the quantitative measure of chaoticity based on a modified detrended fluctuation analysis dfa method can be used to distinguish critical and noncritical regimes of evolution of the earth s crust having established the reference values for the measure of chaoticity we then choose california as the case study region and apply modified dfa to over 1200 time series for determining local areas currently exposed to higher seismic hazard we comprehensively analyze the results obtained and compare them with those produced by conventional risk assessment methods we demonstrate consistency of our results and yet discover that the suggested approach allows us to quantify two qualitatively different mechanisms involved in the growth of seismic hazards we propose a hypothesis which relates these mechanisms to the known mechanisms of earthquake generation that evince a growth of collective behavior in the earth s crust on the eve of an earthquake in order to verify the hypothesis we analyze the same gps data by an independent statistical method canonical coherence analysis cca which permits us to quantify the degree of synchronized behavior of constituents of the earth s crust the cca method confirms our previously obtained estimates thus supporting fractal analysis of gps time series as a suitable precursory method for current seismic hazard assessment

interpretation of aeromagnetic data from the norwegian sector of the north sea total magnetic intensity contour maps for the study region between 2 e to 10 e and 56 n to 60 n were digitized and converted to a regular grid of 285 285 points the study area measures approximately 444 km 444 km and the grid spacing is thus 1 56 km the international geomagnetic reference field for 1975 was gridded for the above used net and from the two data sets a further grid of the t field was generated a large number of profiles were constructed which were suitable for depth determinations the regular grid t data is also convenient for the computation of the second vertical derivative using the method of vertical prisms of vacquier et al 1963 a large suite of curvature depth indices was measured to complement the depths obtained from the intensity slopes and from boreholes which reach the crystalline basement the depth to the magnetic basement has been contoured and the resulting map is shown to be in good agreement with what is known about the deeper geology of the study area the work reported here is part of a research project supported by amoco norway bp petroleum development ltd elf aquitaine esso exploration and production norwegian gulf norsk hydro mobil exploration norway norwegian petroleum directorate royal norwegian council for scientific and industrial research ntnf norske shell and statoil

the platinum group element and gold contents of the marginal rocks and sills of the bushveld complex previous work has defined three suites of sills and marginal rocks adjacent to and below the rustenburg layered suite of the bushveld complex namely an ultramafic mgo approximately 30 a magnesian basaltic mgo approximately 12 and a tholeiitic suite mgo approximately 7 these three suites are thought to represent magmas which have intruded and been incorporated into the rustenburg layered suite at various times during its formation the elements ir ru rh pd pt and au have been analyzed in 17 samples representative of the three suites the ultramafic sills have the highest total noble metal content 46 ppb followed by the magnesian basaltic suite 37 ppb and the tholeiitic suite 21 ppb the ultramafic sills are characterized by a relatively fiat chondrite normalized platinum group element pattern with an average pt ir ratio of 3 52 and a pronounced rh anomaly the magnesian basaltic and tholeiitic suites both have steep positive chondrite normalized patterns with average pt ir ratios of 16 14 and 40 5 respectively and no rh anomaly modeling done using incompatible trace elements and petrographic mixing suggests the ultramafic sills represent crystal mushes of cumulate olivine and chromite that were intruded into the floor during the crystallization of the lower portion of the rustenburg layered suite the enrichment of ir ru and rh in the ultramafic sills appears to be associated with the cumulate enrichment of olivine or chromite in these sills the chondrite normalized patterns found for the merensky reef and the platreef are similar to those of the magnesian basaltic magma the pattern found for the ug 2 is much flatter and in some respects very similar to that found for the ultramafic sills on a worldwide basis the platinum group element contents of the magnesian basaltic and tholeiitic magmas generally fall in the range of basalts and chill zones of other layered suites the magnesian basaltic magma does have one of the highest pt contents measured for a magma

the 25 april 2015 nepal earthquake investigation of precursor in vlf subionospheric signal we present a critical analysis of the observations and interpretation of vlf sub ionospheric measurements related to the main nepal gorkha earthquake which occurred on april 25 2015 mw7 8 and its major aftershock on may 12 2015 mw7 3 the vlf narrowband signal used is from nwc 19 8 khz vlf transmitter located in australia and recorded at allahabad lat 25 41 n long 81 93 e allahabad is located very close 360 km to these earthquake eq epicenters two widely used analysis viz 1 terminator time and 2 night time fluctuation techniques are applied to extract seismic related effects in the nwc narrowband vlf data the terminator time analysis yields statistically significant shifts of 45 and 26 minutes respectively in evening terminator time in the nwc vlf amplitude signal one day before both the earthquakes the nighttime fluctuation method shows a consistent statistically significant increase in three parameters one day before the earthquake the observed terminator time and nighttime fluctuation shifts were associated with these earthquakes only after scrutinizing possible contributions from other potential sources such as solar activity other earthquakes on the signal path meteorological disturbances such as lightning activity wind speed temperature etc along the transmitter receiver great circle path the vlf sub ionospheric signal analysis results unambiguously point towards the presence of seismically excited atmospheric gravity waves agws during these major earthquakes and their important role in providing the coupling between the seismic source region and overlying ionosphere

genetic aspects of the granophyric rocks of the bushveld complex this study of the granophyric rocks of the bushveld complex was undertaken in order to distinguish the different types of granophyre and to determine their probable origin aspects of the granophyre and associated rock types that were studied include their field relations geochemistry and rb sr isotope chemistry thermal modeling of causal mechanisms was also carried out the results obtained demonstrate that a number of genetically different types of granophyre exist in the bushveld complex two types the diepkloof granophyre and the zwartbank pseudogranophyre were formed by metamorphic processes related to the intrusion of the bushveld basic rocks a third type the stavoren granophyre was formed by magmatic processes it is also concluded that the stavoren granophyre is genetically related to the volcanic pre bushveld rooiberg group and forms the shallow intrusive equivalent thereof

vlf observations of ionospheric disturbances in association with tles from the eurosprite 2007 campaign 1 two very low frequency vlf awesome remote sensing systems located at algiers algeria 36 45 n 3 28 e and sebha libya 27 02 n 14 26 e monitor vlf signal perturbations for evidence of ionospheric disturbances during the eurosprite 2007 campaign a number of transient luminous events tles were captured over the mediterranean sea by cameras at pic du midi 42 94 n 0 14 e and at centre de recherches atmospheriques cra in southwestern france 43 13 n 0 37 e the cameras observations are compared to collected vlf awesome data we consider early vlf perturbations observed on 12 13 17 18 october and 17 18 december 2007 the data from the two vlf receivers confirm the association between tles and early vlf signal perturbations with the perturbations amplitudes dependent on the observation configuration i e whether the tle is near the receiver near the transmitter or far from both and the scattering process the results also reveal that the early vlf perturbations can occur in the absence of a tle

platinum group elements in silicate rocks of the lower critical and main zones at union section western bushveld complex concentration patterns of platinum group elements pge in the lower critical and main zones of the bushveld complex are modelled by a process of metal collection by segregating sulphide melt followed by fractional crystallization of monosulphide solid solution mss separation of os ir ru rh enriched mss cumulate ore and pt pd enriched residual sulphide melt is inferred to have occurred by upward percolation of interstitial silicate melt entraining sulphide melt whereas mss crystals are entrapped by early crystallizing chromite this process resulted in os ir ru rh enrichment of the chromitites and an upward increasing trend of pt pd os ir ru crystallization of platinum group minerals pgm from s undersaturated sulphide melt or from coalescing noble metal clusters are alternative possibilities to explain the observed pge patterns but remain difficult to model at present remobilization of pt and pd by percolating late magmatic hydrous fluids could have resulted in metal enrichment of pre existing magmatic sulphides but this model is not supported by the metal budget of the reefs and the field data there is no evidence for solid substitution of pge into oxides and silicates