electrical conductivity of the continental crust geophysical measurements indicate that the earth s continental lower crust has a high electrical conductivity for which no simple cause has been found explanation usually relies on either saline fluids saturating the pores or interconnected highly conducting minerals such as graphite fe ti oxides and sulphides providing conducting pathways attempts in the laboratory to clarify the problem have hitherto been unable to recreate conditions likely to be present at depth by controlling the confining pressure and pore fluid pressure applied to a rock saturated with saline fluids at temperatures between 270 c and 1000 c here we report conductivity data obtained using a cell designed to make such measurements on rocks saturated with saline fluids our results show that the conductivity of saturated samples of acidic rocks is explicable entirely in terms of conduction through the pore fluid whereas the conductivity of saturated basic rocks requires the presence of an additional conduction mechanism s we have used the experimental data to construct electrical conductivity depth profiles for the continental crust which when compared with profiles obtained from magnetotelluric observations demonstrate that a mid to lower crust composed of amphibolite saturated with 0 5 m nacl shows electrical conductivities sufficient to explain conductivity depth profiles for the continental crust inferred from geophysical measurements

on the existence of a geothermal anomaly in southern portugal abstract a geothermal anomaly for which the heat flow density hfd values are more than three times the normal average value for the earth has been reported to exist in hercynian structures in southern portugal in the area of the intersection of the messejana fault and the ferreira ficalho overthrust such high hfd values would imply that the thermal regime of the crust is different from that found in other hercynian regions to try to identify the geothermal anomaly in geoelectrical terms and to understand its origin and relation to the regional geological framework a magnetotelluric survey in the frequency range 0 031 128 hz was carried out in the area that includes the zone of high hfd values the results of the survey are discussed in conjunction with magnetic gravimetric and seismic information as well as synthetic calculations of temperature distributions within the crust it is concluded that the geothermal anomaly is probably a shallow feature and does not represent the regional thermal regime of the crust in southern portugal

genesis and mineralogical classification of ni laterites oman mountains abstract the laterite profiles investigated in the present study developed after the emplacement of a slab of oceanic crust and upper mantle sequence the semail ophiolite onto the east arabian margin during the late cretaceous the laterites formed as a result of prolonged weathering of the ophiolitic assemblage under tropical to subtropical conditions nine laterite profiles have been examined for their ni potential along a nw se segment of the oman mountains the profiles show a vertical succession from bedrock protolith through saprolite oxide laterite to clay laterite a significant enrichment in ni and co occurred as a result of the lateritization process ni concentrations average 0 63 ibra 0 72 3 profiles east ibra 0 67 3 profiles al russayl and 0 33 2 profiles tiwi electron microprobe analyses showed that goethite chlorite kaolinite serpentine and talc are the main ni bearing minerals this study showed that the ibra east ibra and tiwi laterites are similar to the other hydrous mg silicates type a ni deposits where nickel bearing talc willemseite or chlorite and serpentine nepouite chrysotile and antigorite are the main constituents al russayl laterite is however similar to the oxide type type c where most likely the nickel is included in the oxyhydroxides mainly goethite minerals

manganese authigenesis in the landsort deep baltic sea abstract manganese mineralogy because of its redox sensitivity is a valuable parameter for interpreting water column anoxia and sediment diagenesis in the baltic and especially the landsort deep the relatively widespread occurrence of mn carbonates and the unique observations of authigenic mn sulphides in sedimentary environments need to be accounted for within the models proposed for mn authigenesis in diverse settings in this study stratigraphic changes in the geochemistry and mineralogy are documented in two gravity cores using petrographic microscopy xrd sem eds analyses of c and s content 13 c isotopes icp ms cation determinations and electron microprobe element profiling the lamination in the bottom sediments of the landsort deep suggests essentially continuous anoxic conditions at the sediment surface during the last ca 4500 14 c years the most striking variations in the sediments are related to the occurrence of two mn ii precipitate rich sapropel intervals recent and 1390 1990 14 c years bp sapropel deposition is associated with relatively slow detrital sedimentation allowing the concentration of organic matter the sediment c org content is influenced by the variations in the primary production as well as the supply of terrestrial c org but a major control appears to be detrital dilution the precipitation of mn carbonate as individual aggregates of spherical particles dispersed within clayey sediment matrix and as laminae with semi seasonal cyclicity is believed to be largely induced by the high alkalinity in connection with organic matter deposition and degradation where h 2 s has exceeded fe availability mns has also precipitated most commonly as well developed hexagonal crystals but also as the cubic variety alabandite which has not previously been reported from a sedimentary environment an expansion of the anoxic bottom water conditions during sapropel deposition is interpreted by correlation with laminated intervals on the slope of the deep basin budget calculations for the supply of mn to the basin during mn rich sapropel formation can be better balanced by the addition of mn from dissolution of oxides formed prior to the anoxic expansion the importance of alkalinity for the precipitation of mn carbonates and sulphides does not exclude the need for considerable mn concentrations but the model argued in several recently published studies stressing mn availability as the major control does not seem appropriate for the landsort deep and may be too restrictive in many other settings as well

biogenic components and elements in sediments of the central baltic and their redistribution abstract the content and distribution of biogenic components caco 3 and sio 2am and elements c org n and p and some metals fe and mn were investigated in the marine holocene sediments of the baltic sea including the curonian and vistula lagoons and the gulf of riga the average contents of the studied components and elements are increasing in the order sand coarse silt fine silty mud mud pelitic clayey mud the sand and the pelitic mud contain in average in caco 3 1 30 and 8 46 c org 0 38 and 3 27 sio 2am 1 32 and 3 11 p 0 05 and 0 10 fe 1 49 and 4 67 mn 0 03 and 0 04 respectively maps of the areal distributions were compiled and show that areas of elevated values are located mainly in the central parts of lagoons or deep areas based on the sediment thickness it is proposed that strong near bottom currents beneath the halocline level exist in the eastern gotland deep the thin mud layer or the absence of mud and low sedimentation rates may suggest that non deposition and resuspension of bulk sediments and associated nutrients occur in many peripheral and central parts of the eastern gotland deep at depths of more than 90 220 m

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

rapid crustal accretion and magma assimilation in the oman u a e ophiolite high precision u pb zircon geochronology of the gabbroic crust 1 new high precision u pb zircon geochronology from the oman united arab emirates u a e ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust the new data come from a well preserved and exposed crustal section in the center of the wadi tayin massif single grain and grain fragment 206pb 238u dates from upper level gabbros tonalites trondhjemites and gabbroic pegmatites corrected for initial th exclusion range from 112 55 0 21 to 95 50 0 17 ma with most data clustered between 96 40 0 17 to 95 50 0 17 ma zircon dates from upper level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half rates of 50 100 km ma dates from tonalites trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper level gabbros suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates three of the dated upper level gabbros and a single gabbroic pegmatite from the base of the crust have 1 ma intrasample variability in single grain dates suggesting assimilation of older crust during the formation or crystallization of the magmas whole rock end t of seven samples including the upper level gabbros with variable zircon dates have tightly clustered initial values ranging from end 96 ma 7 59 0 23 to 8 28 0 31 the end values are similar to those from other gabbros within the ophiolite suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas the new dates suggest that the studied section formed at a fast spreading mid ocean ridge between 96 4 95 5 ma the large intrasample variability in zircon dates in some samples is unexpected in this setting and may be related to propagation of a younger ridge into older oceanic lithosphere

a 200 300 year cyclicity in sediment deposition in the gotland basin baltic sea as deduced from geochemical evidence abstract during the eu funded project basys baltic sea system study short niemisto type and long box and piston cores sediment cores were taken which cover sedimentation during the past 8 ka the uppermost part of the sedimentary sequence was chosen for a detailed geochemical study and freeze dried samples were analysed for about 20 elements but only the elements mn and ca are discussed an age model was constructed using radiometric dating results by 210 pb 137 cs and 14 c ams significant correlation exists along the cores between very high mn and moderately high ca due to occrrences of the mineral rhodochrosite kuthnahorite a complex mn ca carbonate this mineral is thought to be produced when salt water meets the pool of dissolved mn at the bottom of the gotland basin during favourable hydrographic conditions e g strong northwesterly winds salt water from the north sea invades even the deepest parts of the central baltic mn 2 which is produced mainly by the dissolution of ferromanganse oxides oxyhydroxides in the water colum and in the course of destruction of organic matter in the sediments combines with hco 3 2 and ca 2 in the seawater to form rhodochrosite after burial this mineral stays in the sediment and is seen as light coloured layers a certain cyclicity in the upper 1 5 m of the cores was observed in that about 200 300 a periods of elevated mn ca are followed by periods with lower mn ca of similar duration an explanation for the observed cyclicity may be sea level variations during rising sea level transgression more and more saline water is pushed into the deep basin of the baltic sea and if conditions are favourable high dissolved mn the mineral rhodochrosite is precipitated

early diagenetic processes during mn carbonate formation evidence from the isotopic composition of authigenic ca rhodochrosites of the baltic sea the formation of authigenic ca rich rhodochrosite acr in sapropelic sediments of the gotland basin baltic sea is governed by deepwater renewal processes whereby saline water from the north atlantic flushes the brackish anoxic baltic deeps the carbon and oxygen isotopic compositions of these mn carbonates suggest that acr formation takes place just below the sediment surface and that dissolved compounds from the deepwater column such as water and bicarbonate molecules were incorporated in acr during authigenesis porewaters near the sediment surface display 18 o values of 5 4 vsmow and are generally depleted in 18 o compared to the oxygen isotopic composition of water in equilibrium with mn carbonate solid solutions acr 18 o values are 4 6 this suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during mn carbonate formation the reduction of sulfate having 18 o values of 8 4 accounts for a permanent enrichment of porewater 18 o and observed 18 o values at depth equal to 4 6 however this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and acr close to the sediment surface where mn carbonate formation takes place based on isotopic mass balance calculations we suggest that mno2 with 18 o values of 8 9 released oxygen enriched in 18 o into the anoxic porewaters close below the sediment surface this process should occur after oxygenation events during deepwater renewal when mno2 accumulates at the surface of anoxic sediments manganese carbonates formed in these waters display 18 o values of 1 0 heavier than values expected solely from the initial deepwater composition this quantitatively explains the discrepancy between paleosalinities calculated from acr 18 o based on mn carbonate water isotopic equilibrium fractionation and direct observations for the same period our results emphasize the important role of microbial mno2 reduction during rhodochrosite authigenesis and suggest that mn ii activity rather than alkalinity is the limiting component for sedimentary mn carbonate formation copyright 2002 elsevier science ltd

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

characterization of paralic paleoenvironments using benthic foraminifera from lower cretaceous deposits scotian shelf canada abstract the foraminifera from a lower cretaceous cored interval of cohasset a 52 well located on the scotian shelf were used to interpret the depositional environments of these sediments paleoenvironmental interpretation was based on the analysis on 41 samples from four cored intervals of the well collected between 2069 and 2219 m below seafloor the interval consists of 25 m of interbedded gray black shale mudstone and sandstone belonging to the cree member of the logan canyon formation the foraminiferal association recorded in the studied samples is comprised mainly of agglutinated specimens of trochammina haplophragmoides ammobaculites and verneuilinoides a scattered occurrence of calcareous benthic foraminifera and thecamoebians is also recorded a comparison of these microfauna with modern foraminiferal associations from paralic environments suggests that these sediments were deposited in a cretaceous marsh estuarine environment

tectonic evolution of the oman mountains an introduction it is now more than 20 years since the geologicalsociety published the geology and tectonics ofthe oman region robertson et al 1990 as oneof its early special publications that volume the blue book as it became fondly known to multiplegenerations of undergraduates has been a majorreference on the geology of oman for many whowork in the region this present volume moremodest in its size compared to its illustrious prede cessor is the fruit of a conference on the geologyof the arabian plate and the oman mountainsheld in january 2012 at sultan qaboos universityin muscat organized by professor sobhi nasirwith colleagues from sultan qaboos university the oman government and the geological societyof oman since the rst comprehensive study ofthe geology of the oman mountains during thelate 1960s and early 1970s by ken glennie and histeam from shell the oman mountains have beenknown to contain some of the most spectacularand best exposed geology not only in the middleeast but across the world a summary of the geo logy and the lithostratigraphy is given in figure 1 the publications of glennie et al 1973 1974 building on many earlier shell geologists internalreports notably d m morton r h tschopp h h wilson b m reinhardt and m w hughes clarke set out the major stratigraphic frameworkfor the whole area de ned the major structuresand critically interpreted the semail ophiolite as athrust sheet of oceanic crust and upper mantleemplaced onto the previously passive continentalmargin of arabia since the glennie et al 1974 memoir was published oil companies have carriedout increasingly more intensive work comparingwell sections in the interior with the permian mesozoic shelf carbonates so beautifully exposedalong numerous wadis cutting through the jebelal akhdar and saih hatat massifs during the late 1970s and 1980s two majorresearchteamsfromtheusgs ledbybobcolemanand cliff hopson and the open university uk led by ian gass john smewing and steve lip pard conducted detailed geological surveys acrossthe ophiolite in the muscat ibra transect usgs and northern ophiolite ou respectively theirresults were published in a special issue of thejournal of geophysical research volume 86 editors coleman hopson 1981 and the geologi cal society of london memoir no 11 lippard etal 1986 major systematic mapping of the omanmountains as well as the interior batain coast anddhofar region has been carried out by the bureaude recherches ge ologiques et minieres brgm france the geological survey of japan and theuniversity of berne switzerland see for examplepeters et al 1991 and the whole uae part of thenorthern oman mountains was recently mapped bythe british geological survey uae ministry ofenergy petroleum mineral resources 2012 during the last 20 years major research groupsfrom france japan usa and the uk have con ducted research projects in the country detailed eld studies of the oman ophiolite have beencombined with geochemical and isotopic studiesto determine the tectonic setting and evolution ofthe late cretaceous oceanic crust and upper mantle boudier juteau 2000 field based studies havealso been linked to ocean drilling sites in particularthe east paci c rise to compare the ophiolite toregions of active fast spreading key to the obduc tion emplacement story are the amphibolite andgreenschist facies rocks in the metamorphic solethat record an inverted metamorphic eld gradientalong the base of the ophiolite the ophiolite hasbeen the source of recent detailed studies involvingthe possible sequestration of co

the application of an indirect electromagnetic geothermometer to temperature extrapolation in depth we study the feasibility of the application of an indirect em geothermometer developed recently to the temperature extrapolation in depth using magnetotelluric data collected in the seismically active northern tien shan faulted area bishkek geodynamic test site kyrgyzstan and hengill geothermal zone iceland the approach used is based on the artificial neural network technique which does not imply the prior knowledge of the electrical conductivity mechanisms on the one hand and provides temperature estimates based on the analysis of the implicit conductivity temperature relations on the other the samples for neuronet teaching consisted in the well temperature records and electrical conductivity values determined for the same depths from the magnetotelluric data measured in the vicinities of eight boreholes in each testing area the testing of the taught neuronets was carried out using the temperature records not involved in the teaching process the results indicate that the temperature extrapolation accuracy essentially depends on the ratio between the well length and the extrapolation depth in particular in extrapolation to a depth twice as large as the well depth the relative error is 5 6 and in case of threefold excess the error is around 20 this result makes it possible to increase significantly the depth of indirect temperature estimation in the earth s interior in particular for geothermal exploration based on the available temperature logs the practical application of an indirect electromagnetic geothermometer could provide the following facilities 1 more exact temperature estimation in the extrapolation mode 2 remote temperature estimates in the boreholes in areas characterized by extreme conditions for conventional geothermometers

petrology of lavas from the puu oo eruption of kilauea volcano iii the kupaianaha episode 1986 1992 the puu oo eruption has been remarkable in the historical record of kilauea volcano for its duration over 13 years volume 1 km3 and compositional variation 5 7 10 wt mgo during the summer of 1986 the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser like fountaining at puu oo to virtually continuous relatively quiescent effusion at the kupaianaha vent this paper examines this next chapter in the puu oo eruption episodes 48 and 49 and presents new icp ms trace element and pb sr and nd isotope data for the entire eruption 1983 1994 nearly aphyric to weakly olivine phyric lavas were erupted during episodes 48 and 49 the variation in mgo content of kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of kilauea both of which probably were controlled by variations in kilauea s magma supply rate these lavas contain euhedral olivines which generally are in equilibrium with whole rock compositions although some of the more mafic lavas which erupted during 1990 a period of frequent pauses in the eruption accumulated 2 4 vol olivine the highest forsterite content of olivines 85 in kupaianaha lavas indicates that the parental magmas for these lavas had mgo contents of 10 wt which equals the highest observed value for lavas during this eruption the composition of the puu oo lavas has progressively changed during the eruption since early 1985 episode 30 when mixing between an evolved rift zone magma and a more mafic summit reservoir derived magma ended the normalized to 10 wt mgo abundances of highly incompatible elements and cao have systematically decreased with time whereas ratios of these trace elements and pb sr and nd isotopes and the abundances of y and yb have remained relatively unchanged these results indicate that the hawaiian plume source for puu oo magmas must be relatively homogeneous on a scale of 10 20 km3 assuming 5 10 partial melting and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued the rate of variation of highly incompatible elements in puu oo lavas is much greater than that observed for kilauea historical summit lavas e g ba y 0 09 a 1 vs 0 03 a 1 this rapid change indicates that puu oo magmas did not mix thoroughly with magma in the summit reservoir thus except for variable amounts of olivine fractionation the geochemical variation in these lavas is predominantly controlled by mantle processes

greenhouse crises of the past 300 million years proxies of past co 2 and climate over the past 300 m y now reveal multiple global climate change events in unprecedented detail evidence for past co 2 spikes comes from expanded and refined data on stomatal index of fossil ginkgo and related leaves new evidence for synchronous climatic change comes from paleosols in montana utah and neighboring states each co 2 spike was coeval with unusually clayey red and decalcified paleosols which can be traced throughout the colorado plateau spikes in atmospheric co 2 also were coeval with increases in paleosol alkali depletion as an indication of high temperature and spikes in paleosol base depletion and depth to calcic horizons as indications of high precipitation in the colorado plateau times of warmer climate were also more humid perhaps due to greater moisture potential of warmer air seasonality of climate did not increase during warm wet spikes the mesozoic greenhouse was not persistently hot with cool spells but warm with hot flashes these data furnish power laws predicting the sensitivity and magnitude of change in mean annual temperature mat and mean annual precipitation map due to rising co 2 in a mid latitude mid continental region the magnitude of the coming anthropogenic greenhouse pales in comparison with past greenhouse spikes at times of global mass extinctions

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

crustal temperature and mantle heat flow in europe abstract the map of the surface heat flow in europe was supplemented by the crustal thickness pattern and data on crustal radioactivity and transformed to the mantle moho heat flow distribution using a model of radioactivity decreasing exponentially with depth a z a 0 exp z d with a0 2 wm 3 and d 10km we computed generalized one dimensional steady state conductive temperature depth curves for the major tectonic units of europe the available information on the heat flow tectonic age heat flow heat generation and heat flow crustal thickness relationships were summarized and discussed in terms of crustal structure the mohorovicic discontinuity is clearly not an isothermal surface neither is the outflow of heat from the upper mantle constant but both parameters may vary within broad limits the moho temperature can be as low as 280 c in the shield areas however it can exceed 900 c in some young intrarnontane depressions or rift structures regional variations of the moho heat flow can be as large as 30 40 mwm 2 the value of the moho temperature reflects primarily the tectonic stabilization of the crust as a result of the deep seated processes in the upper mantle

near surface diagenesis of ophiolite derived conglomerates of the barzaman formation united arab emirates a natural analogue for permanent co2 sequestration via mineral carbonation of ultramafic rocks carbon capture and storage by mineralization is a potential method for storing anthropogenic co2 emissions and is based on the reaction between mg silicate and co2 to form mg carbonate the conglomerates of the barzaman formation exposed in the eastern united arab emirates represent an excellent natural analogue of this process these conglomerates were deposited as a series of alluvial fans along the western margin of the hajar mountains part of the oman uae ophiolite and are composed largely of ultramafic and lesser mafic clasts the clasts and matrix have been extensively altered to dolomite during diagenetic processes analysis and interpretation of rock textures provide evidence for the various factors that influenced the diagenetic processes and shed light on the silicate carbonate transformation process all the reactions have taken place in the near surface environment the silicate carbonate conversion reaction is exothermic and occurs spontaneously at near ambient pressure and temperature probably no greater than 50 c estimates of the amount of co2 stored in this way can be obtained from considerations of outcrop area formation thickness and percentage of dolomite replacement and show that c 150 billion tonnes equivalent to about 4 years of worldwide co2 emissions at current rates are stored

temperature profiles in the earth of importance to deep electrical conductivity models deep in the earth the electrical conductivity of geological material is extremely dependent on temperature the knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models the measured values of the terrestrial heat flow radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere various methods of deep temperature calculations are presented and discussed characteristic geotherms are proposed for major tectonic provinces of europe and it is shown that the existing temperatures on the crust upper mantle boundary may vary in a broad interval of 350 1 000 c the present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200 1 000 c it is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity versus temperature data into the conductivity versus depth data

geochemical variability of the oman ophiolite lavas relationship with spatial distribution and paleomagnetic directions 1 the multielemental composition of 31 lavas sampled in the extrusive section of the oman ophiolite was determined by inductively coupled plasma source mass spectrometry icp ms this study allowed us to define clear geochemical criteria to characterize the different lavas types in oman most of the oman ophiolite extrusive sequence is composed of lavas of composition similar to present day morb with the exception of slight nb ta negative anomalies v1 magmatism v1 lavas display ree patterns and zr hf ratios in the range of morb zr hf 33 15 38 7 in contrast the overlaying v2 lavas which outcrop only in the northern part of the ophiolite are ree depleted relative to v1 and display low zr hf ratios 23 6 30 5 v2 lavas may be further classified into two sub groups v2 type i lavas display a continuous decrease from hree to lree the upper v2 type ii lavas are more depleted in hree and mree but they are enriched in lree they are also distinguished by their low nb ta ratios 10 53 11 65 relative to other oman basalts v1 12 5 14 v2 type i 12 35 14 4 as for n morb melting of an upwelling mantle beneath an oceanic spreading centre formed v1 lavas v2 resulted from fluid enhanced melting of previously depleted mantle residual after v1 extraction this process probably occurred at different melt rock ratios thus resulting in the two v2 sub groups and was enhanced in the northern part of the ophiolite v3 lavas overlie the v1 v2 sequence in the salahi area they display incompatible element rich patterns lree mree hree and a high nb ta ratio typical of within plate basalts in the different studied areas the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data

recycling of manganese from anoxic sediments in stagnant basins by seawater inflow a study of surface sediments from the gotland basin baltic sea abstract the occurrence of mn carbonates in sapropelic sediments has been proposed to indicate that the host sediment originally accumulated under oxygenated bottomwaters in the central baltic sea ca rich rhodochrosite layers in sapropelic sediments have been related to inflows of oxic seawater from the north sea into the brackish predominantly anoxic deeps of the central baltic this study attempts to verify the model of authigenic mn carbonate formation by comparing oceanographic records directly with the composition of surficial sediments of the gotland basin surface sediments of the gotland deep sampled in 1997 displayed significant ca rich rhodochrosite enrichments at 6 5 11 and 15 cm sediment depth which reflect periods of intense seawater inflows in 1969 76 1948 56 and 1931 39 respectively however the latest major seawater inflow detected in 1993 was not reflected in the surface sediment the topmost 6 cm of the sediment was totally depleted in mn and 210pb data implied a disturbed top layer down to 5 cm calculations of saturation indices indicate that the porewaters were undersaturated with respect to mn and ca carbonate phases in the uppermost 8 cm in 1997 at greater depths the porewaters were close to equilibrium with respect to calcite and rhodochrosite the ca rhodochrosite layers corresponded well to a sediment core sampled at the same location in 1994 but that core displayed a further enrichment of ca rich rhodochrosite close to the sediment surface which can be related to a major inflow of north sea water in 1993 the significant decrease of about 2 3 mol m2 mn in the uppermost 6 cm of a disturbed sediment surface from 1994 to 1997 corresponds to an enrichment of about 20 mol l of dissolved mn within the 90 m thick anoxic water column in 1997 we conclude that several inflows of dense seawater into the gotland deep monitored between 1994 and 1997 led to a resuspension of the unstable fluffy surface sediment layer authigenic ca rich rhodochrosite which had formed immediately after the major inflow of oxic seawater in 1993 was therefore redissolved in undersaturated bottomwaters and mn was recycled from surficial sediments into the water column this in turn means that in contrast to the established model of ca rhodochrosite formation as a result of seawater inflow the occurrence of mixed mn depleted layers does not prove that seawater inflow has not occurred during the deposition of these sediments

new heat flow density data from southern portugal a geothermal anomaly revisited abstract previous geothermal work has indicated that a geothermal anomaly with heat flow density values in excess of 200 mw m 2 exists in southern portugal other geological and geophysical data from the area show no evidence of such an anomaly to determine whether or not a geothermal anomaly occurs there the published geothermal data and some new temperature data obtained from newly available wells as well as from some of the previously used wells were reprocessed the reprocessing of the published data took into consideration thermal and hydrodynamic equilibrium criteria and so some of the wells that were previously used to draw the heat flow density map were rejected reprocessing of the data together with the new measurements indicate that a geothermal anomaly does not exist in the area and that it is a normal geothermal area with heat flow density values ranging from 50 to 90 mw m 2 these values are similar to those obtained for other hercynian regions in europe

the death of an accretion zone as evidenced by the magmatic history of the sumail ophiolite oman abstract the ophiolitic crustal sequence of the salahi block in the northern part of oman shows a complex magmatic history the oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates isotropic gabbros a sheeted dike complex and a first tholeiitic lava series v1 depleted in compatible elements the second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite v2 directly linked as can be seen from field evidence to more differentiated high level intrusive plutons this volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements a third volcanic event v3 exhibiting a transitional character enriched in lree s caps the ophiolite these v3 lavas are separated from the v2 unit by a 15 m thick pelagic sedimentary section a geotectonic evolution of the sumail ophiolite is proposed on the basis of field mineralogical and geochemical data after the oceanic accretion phase that gave birth to the main ophiolitic sequence a second stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode such a model accounts well for the depleted character of the v2 lavas the compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second stage melts which are then emplaced in the crust and partly extruded the v3 volcanism postdates the intraoceanic detachment of the ophiolite but precedes its obduction onto the arabian continental margin it is interpreted as an intraplate seamount type magmatism

geology and petrology of the plutonic complexes in the wadi fizh area multiple magmatic events and segment structure in the northern oman ophiolite 1 multiple magmatic events are recorded in the gabbroic unit in the fizh area of the northern oman ophiolite gabbroic blocks intruded by sheeted dike complex and upper gabbros of the main crustal sequence show the oldest event gabbronorite sills in the gabbroic blocks are nearly coeval with the host gabbro wehrlitic intrusions wehrlite i mark the third event of magmatism these three magmatic events occurred at the retreating dying ridge axis because all these rocks are intruded by dolerite dike swarm which is generally regarded as a precursor of advancing ridge axis the next stage of magmatism is a main phase of oceanic crust generation in this area wehrlite ii and then gabbronorite dikes intrude the still hot main gabbro unit all of these above rocks have similar signatures with respect to clinopyroxene compositions and covariations between plagioclase and mafic minerals though slight differences are present in the compositional ranges and clinopyroxene compositions of each unit after considerable cooling of the main gabbro unit primitive basalt dikes intrude the main gabbro unit which may correspond to the lasail unit finally the fizh south complex intrudes into considerably cooled crustal sequence being below the brittle plastic transition temperatures the fizh south complex which was regarded as a common wehrlitic intrusion is significantly different from all of the above mentioned rocks with respect to the covariation between plagioclase and associating mafic minerals crystallization order and clinopyroxene compositions the clinopyroxenes are characterized by extremely low ti and na contents comparable with those of the v2 unit alley volcanics suggesting that the fizh south complex correlates with the plutonic facies of the v2 unit during arc stage layered gabbros in the wadi zabin area about 10 km north of the fizh area may be a northern extension of the gabbro blocks of the fizh area because they are intruded by numerous dolerite dikes on the other hand basin like structure of the main gabbroic unit in the northern end of the fizh area may demonstrate a fossilized magma chamber beneath the advancing ridge axis due to the ceasing of magmatic crustal accretion on the basis of these lines of evidence we propose that the fizh area indicates the northward propagation tip of the ridge axis

a late early albian marine transgressive pulse over northeastern australia precursor to epeiric basin anoxia foraminiferal evidence abstract a major transgressive pulse occurred during the late early albian in the western papuan basin and totally changed the character of foraminiferal faunas in the basin from impoverished agglutinated dominated assemblages to diverse calcareous assemblages a similar change is reflected in coeval deposits of other basins on the northeastern margin of the australian continent laura basin and northern carpentaria basin and to a lesser extent in the intracratonic eromanga and surat basins this pulse preceded the condensed section of the toolebuc formation in the eromanga basin of late middle albian to early late albian age and succeeded deposition of the first radiolarian rich units late aptian in all basins a rapid marine regression took place in the western papuan basin immediately after the latest albian 95 ma and reflected the final retreat of marine conditions in the intracratonic basins to the south foraminiferal biofacies indicate that the late early albian transgressive pulse resulted in peak sea levels in all basins between 200 m and 500 m deep along the northern edge of the cretaceous continent northern papuan basin decreasing to about 150 m in the northern carpentaria basin and the laura basin and to 50 100 m in the southeastern carpentaria basin northeastern eromanga basin and southwestern eromanga basin and to around 50 m in the surat basin the change in bathymetry during the transgressive pulse was greatest in the northern marginal basins possibly a deepening of over 100 m and decreased to the southeast about 10 20 m where increased sand deposition occurred although the transgressive pulse bought slightly more saline and more oxygenated bottom water far into the basin a barrier still existed in bottom waters across the central carpentaria basin between slightly brackish dysaerobic conditions to the south and normal marine conditions to the north after peak sea level was reached bottom water circulation within the southern carpentaria and eromanga sea decreased and this was accompanied by a decrease in sediment influx during the late middle albian through early late albian anoxic conditions existed in central parts of the epeiric basins and a condensed section of black shales represented by the toolebuc formation was deposited the late early albian sea level rise was partly controlled by eustatism but may also have been influenced in different areas by varying amounts of continental uplift and subsidence associated with tectonism in the volcanic arc along the eastern australian margin the transgressive pulse broadly coincides with albian transgressions reported widely from passive margins and epeiric basins elsewhere but precise correlation is difficult the consistently high sea level during the middle and early late albian and the regression at the end of the albian are out of phase with events on other continents and probably resulted from increasing tectonism along the eastern continental margin

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

sub seafloor epidosite alteration timing depth and stratigraphic distribution in the semail ophiolite oman abstract pervasive epidotization of igneous rocks is a common feature in the ophiolite record of hydrothermally altered oceanic crust current genetic models view epidosites as markers of focussed upflow of hydrothermal fluid beneath oceanic spreading ridges the epidosites are envisaged to form at the base of the sheeted dike complex sdc during active plate spreading our mapping of the semail ophiolite in oman has revealed abundant epidosites in the volcanic sequence some exceeding 1 km2 in extent they are more frequent and far larger than the mineralogically identical epidosites in the sdc we have also found epidosites that traverse the entire sdc from bottom to top thus rather than being restricted to the base of the sdc as implied by current models epidosites in fact occur throughout the sdc and dominantly within the overlying volcanic pile we report the occurrence of 19 epidosite bodies and their crosscutting relations with respect to host lava units dikes intrusive stocks and also seafloor umbers the volcanostratigraphic affiliation of the dikes is identified by their whole rock and clinopyroxene compositions the relations set constraints on the timing of epidotization with respect to igneous activity in the ophiolite at least one of the epidosites in the sdc formed during lasail off axis volcanism another epidosite in the sdc and many in the volcanic units formed later during post spreading alley and boninitic alley supra subduction zone volcanism only permissive not compelling evidence allows just two of the epidosites to have formed within the main stage sdc during or shortly after its emplacement we conclude that epidotization of the oceanic crust is not necessarily coupled to spreading ridges and that it can occur during fore arc volcanism this finding is consistent with evidence from the modern seafloor and it requires a different hydrothermal environment to that traditionally associated with alteration beneath spreading axes the timing and stratigraphic constraints show that some of the epidosites formed within the top 1 4 km of the crust others between 1 2 and 1 7 km depth whereas the deepest formed between 2 6 and 3 8 km below the paleo seafloor finally the timing constraints do not prescribe a strict temporal relationship between epidotization and the formation of the numerous seafloor massive sulphide vms deposits known in the ophiolite however a role for epidosites as source rocks for the metals in the deposits remains permissible

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

natural dissolution of modern shallow water benthic foraminifera taphonomic effects on the palaeoecological record comparison of the living and dead assemblages of benthic foraminifera from the coastal areas of the skagerrak kattegat reveals that although there are considerable similarities in terms of the presence absence of species there are major differences in relative abundance at species level furthermore the relative abundance of calcareous tests in the living assemblages is considerably higher than that of the dead assemblages average values of 67 and 30 respectively since the environments are microtidal there is little transport of foraminiferal tests due to tidal currents so there are no introduced exotic species although there is local transport caused by waves and wave induced currents however the major taphonomic process appears to be dissolution of calcareous tests this is manifested through etching and breakage of test walls leading to complete decalcification which leaves a residue of organic linings of certain taxa e g ammonia beccarii and a marked increase in abundance of agglutinated tests in the dead assemblages although dissolution is normal in marsh settings throughout the world the skagerrak kattegat area is unusual in the intensity of dissolution in the subtidal zone whereas most of the calcareous species occur throughout the investigated depth range except marsh two of the subtidal agglutinate species eggerelloides scaberand ammotium cassis show upper depth limits consequently the agglutinated foraminifera are better indicators than calcareous foraminifera of upper depth limits in these shallow subtidal environments 1999 elsevier science b v all rights reserved

quantitative analysis of the tremor wavefield at etna volcano italy abstract the properties of volcanic tremor wavefield at mt etna volcano italy are investigated using data from two dense small aperture arrays of short period seismometers deployed on the north and south flank of the volcano spectral analysis shows that most of the seismic energy is associated to several narrow spectral peaks spanning the 1 5 hz frequency band analysis of simultaneous recordings evidences that most of these peaks are common to different sites thus suggesting a source effect as the origin of this energy frequency slowness analyses evidence a complex wavefield where body and surface waves alternatively dominate depending on the frequency band and component of motion taken into account surface waves are found to dominate at frequencies below 1 hz and above 3 hz conversely the 0 8 2 3 hz vertical and radial component wavefields at both arrays exhibit a nondispersive nature with apparent velocities spanning the 1 2 s km range particle motion analysis suggests these arrivals are associated to both p and sv waves inciding at shallow angles at the northern array back azimuths of these waves encompass the whole summit crater area at the southern array back azimuths are instead clustered around a direction pointing about 500 m east of the se crater at frequency around 4 hz the dominant direction of wave propagation at the southern site shifts about 30 w pointing to the bocca nuova voragine craters and concordance of location is found with the source imaged by the northern array the 0 8 2 3 hz transverse component of motion depicts velocities of about 0 5 km s a value which is about three times lower than those associated to the vertical and radial components results from polarization analyses at the two array sites depict the dominance of horizontal linear particle motion oriented transversally with respect to the source direction polarization ellipsoids at the stations of the sparse network all depict a quasi horizontal setting with two exceptions the direction of particle motion is always oriented tangentially to the summit volcanic edifice the origin of the large transverse motion observed at the two array sites is thus attributed to sh waves generated by free surface interaction of waves impinging the concave topography the correlation method is used to derive the dispersion properties of short period 0 5 5 hz rayleigh waves from which the shallow shear wave velocity structures are derived for beneath the two semicircular arrays using a probabilistic approach we invert slowness data measured at the two dense arrays for retrieving source location and extent the joint inversion of slowness data from the two arrays point to different sources this observation is interpreted in terms of ray bending associated to lateral heterogeneity and or strong topographic effects on wave propagation once the propagation effects are taken into account the most probable source locations are associated to a shallow region encompassing the summit craters and the eruptive fissures active at the time of the experiment september 1999

late paleozoic middle latitude gondwana environment stable isotope records from western australia abstract one hundred and twelve permian fossil brachiopod samples from two western australian basins carnarvon and canning basins have been analyzed for oxygen and carbon isotopes to study the middle paleolatitude environment in the permian all samples were thin sectioned and examined using petrography and cathodoluminescence microscopy for evaluating shell preservation a total of 249 isotopic analyses from the best preserved portion non luminescent nl of shells were found useful for inferring paleoenvironments for the carnarvon basin average 13 c values of nl shells were between 4 2 and 4 5 for the callytharra formation n 123 late sakmarian early artinskian jimba jimba calcarenite n 7 early artinskian coyrie madeline formations n 71 both late artinskian early kungurian and wandagee formation n 18 late kungurian average 13 c values of nl shells were 5 0 0 4 n 9 for the quinnanie shale cundlego formations both kungurian for the canning basin mean 13 c values of nl shells were 5 1 1 1 n 11 5 3 0 2 n 4 and 4 8 0 5 n 6 for the noonkanbah formation artinskian kungurian lightjack formation roadian capitanian and hardman formation wuchiapingian respectively within the uncertainty of the stratigraphical correlation the carbon isotope values spanning late sakmarian to kungurian in middle latitude western australia are comparable to those of coeval low latitude urals average 18 o values of nl shells were about 0 0 1 to 0 1 for the callytharra formation jimba jimba calcarenite coyrie madeline formations quinnanie shale cundlego formations noonkanbah formation and hardman formation and were 0 5 0 7 and 0 3 0 7 for the wandagee formation and lightjack formation respectively these oxygen isotope values are overall higher than the coeval values reported for low latitude regions and for eastern australia the positive oxygen isotope values are here interpreted to indicate a cool temperature and or a possibly moderately high salinity condition for the two studied basins in western australia during the permian

electrical conductivity of dry lower crustal rocks this study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings we present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500 c to 1000 c we observe statistically significant differences between rock types mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group aphanitic fine grained rocks have higher conductivity than phaneritic coarse grained ones best fitting curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity because the laboratory rocks are dry their conductivities are lower at a given temperature than they would be if fluids or volatiles were present hence any temperatures inferred from

late artinskian early kungurian early permian warming and maximum marine flooding in the east gondwana interior rift timor and western australia and comparisons across east gondwana abstract substantial new information is presented on upper artinskian kungurian deposits in timor leste and in the canning southern carnarvon and northern perth basins of western australia these basins situated between about 35 s and 55 s palaeolatitude formed part of the east gondwana interior rift a precursor to the rift that 100 my later formed the indian ocean in this region timor lay near the main axis of the east gondwana interior rift whereas the western australian basins were marginal splays from the rift axis the main depocentres developed as a result of faulting that was initiated during the late pennsylvanian detailed lithostratigraphic and biostratigraphic analyses have been made on the newly recognized bua bai limestone and the type cribas group in timor the noonkanbah formation in the canning basin the byro group in the merlinleigh sub basin of the southern carnarvon basin and the carynginia formation in the northern perth basin in timor the succession which is highly disrupted by faulting was deposited under open marine conditions probably in a shelf basin setting restricted very shallow water seas flooded the canning basin and the merlinleigh byro irwin sub basins of the southern carnarvon and northern perth basins and had highly variable oxygen levels and salinities typical of estuarine environments a similar pattern of warming and bathymetric change is recognized in all studied basins during the early part of the late artinskian cool conditions prevailed with water temperatures 0 4 c forming sea ice in the merlinleigh byro irwin rift rapid warming during the latter part of the late artinskian was accompanied by maximum marine flooding close to the artinskian kungurian boundary climatic and bathymetric conditions then allowed carbonate mounds with larger fusulines and a variety of algae to develop in the northern part of the rift system and tubiphytes conodonts and brachiopods with tethyan affinities to migrate into the marginal rift basins despite the generally adverse water quality at these depositional sites comparison between the stratigraphic record from the east gondwana interior rift and coeval records from lhasa and sibumasu indicate a similar pattern of climate change during the carboniferous to end cisuralian similar trends probably are present in eastern australia although there is confusion over the correlation of some units

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

geochemical evidence in clinopyroxenes from gabbroic sequence for two distinct magmatisms in the oman ophiolite abstract in the oman ophiolite one of the best preserved and most studied ophiolites in the world two distinct petrogenetic suites of gabbroic rocks from the layered gabbro sequence of the wadi haymiliyah section is established using trace element chemistry of ca rich clinopyroxenes the earlier gb1 suite is characterized by plagioclase with lower an ca ca na content and clinopyroxene with low large ion lithophile elements lile concentrations the later dwgb2 suite contains plagioclase with rather high an content and clinopyroxene with high lile this difference in clinopyroxene chemistry can be extended to the extrusive rocks in this section lower earlier hv1 suite with low lile clinopyroxene and upper later hv2 suite with high lile clinopyroxene difference in lile concentration of clinopyroxenes is essentially due to geochemical difference in parental magmas the gb1 hv1 suites formed at fast spreading mor setting and dwgb2 hv2 suites at ssz setting supporting a model of transition from mid oceanic ridge to supra subduction zone settings of the oman ophiolite our results indicate that geochemical signature of clinopyroxene is a very strong tool for identification of tectonic setting of ophiolites

carboniferous permian palynostratigraphy of west australian marine rift basins resolving tectonic and eustatic controls during gondwanan glaciations abstract the longest interval of cold climate in earth s phanerozoic history spanned some 55 million years from the namurian late carboniferous to the kungurian early permian when glaciation affected much of high latitude gondwana uncertainty surrounds the timing of glaciation s and the frequency of interglacial episodes ice volumes and the location of ice centres this is because the glacial record is preserved predominantly within marine strata deposited in intracontinental rift basins marginal to onshore ice centres on uplifted basement highs the direct stratigraphic record of glaciation is meagre on adjoining basement highs and extremely difficult to date many basins contain broadly similar threefold lithostratigraphic successions of lowermost glaciomarine diamictite facies middle shales and uppermost carbonate and sandstone rich commonly coal bearing deltaic strata in the absence of absolute age control and infrequent presence of macrofossils such deglaciation sequences are widely assumed to be correlative from basin to basin recording gondwanan deglaciation s and a world wide glacioeustatic sea level rise a newly compiled biostratigraphic database for west australia using first appearance datums of spore pollen types allows correlation of late palaeozoic siliciclastic successions between seven basins bonaparte canning carnarvon collie gunbarrel northern and southern perth basins these extend some 3800 km along australia s western continental margin adjacent to the west australian shield all but the gunbarrel basin contain tripartite glacially influenced successions diamictite shale sandstone biostratigraphic data reveal sharp differences in ages and stratigraphic distributions and marked variations in the thickness of coeval units diamictite shale sandstone successions accumulated diachronously from basin to basin recording a dominant tectonic control on subsidence and relative sea level middle shale units are not exact correlatives between basins and thus are unlikely to simply represent widespread climatically controlled glacioeustatic events instead they are argued to record high relative sea levels during times of maximum synrift subsidence this tectonostratigraphic model can be applied to other intracontinental rift basin successions across gondwana

al khlata glacial deposits in the oman mountains and their implications a thick succession of al khlata glacio lacustrine deposits including diamictites crops out in the wadi daiqa inlier and there are other possible al khlata outcrops in wadi amdeh and wadi al arabiyin these outcrops in the oman mountains are 100 km north of where the al khlata had been thought to pinch out by non deposition or erosion fossil spores and pollen from wadi daiqa are highly carbonized having been subject to greenschist facies metamorphism but are still clearly identifiable as taxa from the late carboniferous 2159a zone of the oil producing areas of interioroman this northernmost al khlata is sand rich and interpretedto be glacio lacus trine previously the sand dominated al khlata successions north of the central oman high have been consideredto be glacio fluvial outwash largely based on their context the al khlata deposits in wadi daiqa and the underlying several kilometre thick amdeh succession are preserved in saih hatat probably in a continuation of the ghaba salt basin that itself overlies an accreted terrane from the pan african orogeny gold open access this article is published under the terms of the cc by 3 0 license al khlata glacial deposits occur widely in the sub surface of interior oman they are absent along the easternmost coastal area and were believed absent from northern oman and the oman moun tains due to non deposition here we describe a c 300 m succession of al khlata p9 deposits from wadi daiqa and other possible outcrops of al khlata lithologies in the saih hatat area the wadi daiqa sections are important in being outcrop analogues of sand prone oil producing intervals in the subsurface of southern oman unfortunately the most easily accessible section was flooded with the filling of the wadi daiqa water reservoir in 2009 the better known outcrops at al khlata in the huqf area are of younger p5 and p1 age and are predominantly of non reservoir facies

holocene history of the baltic sea as recorded in a sediment core from the gotland deep a 4 m long sediment core from the gotland deep baltic sea was investigated by means of biostratigraphical and chemical parameters and c 14 datings this multidisciplinary approach allows us to ev

geothermal model of the earth s crust in the pannonian basin abstract a geothermal model of the hyperthermal zone of the pannonian basin is constructed on the basis of results of seismic measurements along five deep seismic sounding profiles on the territory of the basin and the surrounding areas and also of measurements of heat flow heat production by radioactive elements and thermal conductivity of rocks the variation of temperature with depth and maps of mono temperatures and heat flux through this surface are calculated and constructed respectively it is shown by numerical model calculations that the heat anomaly of the pannonian basin indicated by a number of surface measurements is mainly of mantle origin inhomogeneities of the heat flow increase with depth down to the upper mantle and the temperature on the moho surface below the hyperthermal zone has values on average 400 500 c more than those in the surrounding areas heat flux through the moho under the pannonian basin is also higher by about 40 50 mw m 2 on the basis of the present calculations it can be suggested that the upper mantle is probably partially molten beneath the annonian basin as a most reasonable source mechanism of formation of this heat anomaly the frictional heating arising in areas of induced secondary convection that probably has proceeded also beneath the basin from the triassic to the miocene is suggested here

boninitic volcanism in the oman ophiolite implications for thermal condition during transition from spreading ridge to arc the discovery of boninite a typical high mgo andesite in the oman ophiolite is reported the boninites in the oman ophiolite occur as lavas and dikes of the alley volcanic sequence that overlie or crosscut the spreading ridge derived lavas geotimes volcanic sequence and sheeted dikes the phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the izu mariana forearc boninites indicating that the alley boninites represent primitive melt generated by partial melting of hydrous peridotite the occurrence of boninite provides strong thermal and chemical constraints on the formation of the oman ophiolite that require hot hydrous shallow mantle 1250 c at 30 km depth to have underlain the proto oman ophiolite at the time of boninite generation the initiation of subduction of the young hot oceanic lithosphere and obduction of the future oman ophiolite near the spreading ridge and the resultant melting of the highly depleted shallow mantle wedge metasomatized by slab derived fluid represent the most favorable mechanism for the genesis of the alley boninites

taphonomic experiments on marginal marine foraminiferal assemblages how much ecological information is preserved the most important taphonomic processes affecting the composition of benthic foraminiferal assemblages are transport and destruction of tests the latter includes dissolution of calcareous tests we have previously simulated nature experimentally by dissolving mainly shelf to deep sea samples in weak acid to give acid treated assemblages ata in this study we sampled the marginal marine environments of the skagerrak kattegat and found that the main taphonomic process there is dissolution of calcareous tests consequently the taphonomic processes occurring in this area represents a perfect natural analogue of what we suspect to be the main mechanism for the formation of some fossil agglutinated assemblages the evidence for this is that some living assemblages dominated by calcareous forms become original dead assemblages oda often consisting entirely of agglutinated species and equivalent to atas the principal ata is dominated by miliammina fusca and this ata can be derived from 10 different types of oda including those that are strongly dominated by various calcareous taxa this residual m fusca assemblage represents a broad range of intertidal to shallow subtidal environments on the other hand marsh living assemblages are completely agglutinated and obviously give rise to agglutinated oda atas but not necessarily dominated by the same taxa all the atas faithfully record the low species diversity characteristic of marginal marine environments the principal conclusion is that in spite of the severe taphonomic loss the preserved assemblages record much ecological detail this is in accordance with similar investigations of deeper water assemblages and has important consequences for palaeoecology this study demonstrates unequivocally that it is always essential to compare data on living and dead assemblages in order to determine the pathways to fossilisation the use of total living plus dead assemblages in this area would give unrepresentative and misleading results

formation of submarine flat topped volcanic cones in hawai i high resolution bathymetric mapping has shown that submarine flat topped volcanic cones morphologically similar to ones on the deep sea floor and near mid ocean ridges are common on or near submarine rift zones of kilauea kohala or mauna kea mahukona and haleakala volcanoes four flat topped cones on kohala were explored and sampled with the pisces v submersible in october 1998 samples show that flat topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage similarly shaped flat topped cones on the northwest submarine flank of ni ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage submarine postshield stage eruptions on hilo ridge mahukona hana ridge and offshore ni ihau form pointed cones of alkalic basalt and hawaiite the shield stage flat topped cones have steep 25 sides remarkably flat horizontal tops basal diameters of 1 3 km and heights 300 m the flat tops commonly have either a low mound or a deep crater in the center the rejuvenated stage flat topped cones have the same shape with steep sides and flat horizontal tops but are much larger with basal diameters up to 5 5 km and heights commonly greater than 200 m the flat tops have a central low mound shallow crater or levees that surrounded lava ponds as large as 1 km across most of the rejuvenated stage flat topped cones formed on slopes 10 and formed adjacent semicircular steps down the flank of ni ihau rather than circular structures all the flat topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades these and other submarine volcanic cones of similar size and shape apparently form as continuously overflowing submarine lava ponds a lava pond surrounded by a levee forms above a sea floor vent as lava continues to flow into the pond the lava flow surface rises and overflows the lowest point on the levee forming elongate pillow lava flows that simultaneously build the rim outward and upward but also dam and fill in the low point on the rim the process repeats at the new lowest point forming a circular structure with a flat horizontal top and steep pillowed margins there is a delicate balance between lava heat supply to the pond and cooling and thickening of the floating crust factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents moderate to high confining pressure at moderate to great ocean depth long lived steady eruption years to decades moderate effusion rates probably ca 0 1 km3 year and low but not necessarily flat slopes with higher effusion rates sheet flows flood the slope with lower effusion rates pillow mounds form hawaiian shield stage eruptions begin as fissure eruptions if the eruption is too brief it will not consolidate activity at a point and fissure fed flows will form a pond with irregular levees the pond will solidify between eruptive pulses if the eruption is not steady lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones as occurs during the post shield stage the steady effusion of lava on land constructs lava shields which are probably the subaerial analogs to submarine flat topped cones but formed under different cooling conditions

relations between deep temperatures and other geophysical characteristics in central honshu japan abstract the area of interest central honshu represents a complicated knot of triple junction of the north american plate the eurasian plate and the izu bonin arc of the philippine sea plate due to complicated geological structural characteristics of the crust and specific plate tectonic features of the lithosphere this region has been extensively investigated by different geophysical methods for decades beside seismic studies yielding detailed information on the crustal structure and providing models for seismic velocity perturbations and seismic wave attenuation in the area particularly measurements of three component geomagnetic variations magnetic anomalies studies of seismicity etc were made this paper presents the results of a search for correlations between the thermal field in the region and other geophysical parameters to specify the thermal regime of the crust and the uppermost mantle a quantitative geothermal model of central honshu for six seismic profiles with a total length of some 1500 km was used no correlation is indicated between curie point depth from magnetic data and that inferred by thermal modeling moderate but significant correlations are found between heat flow and depths to the conrad and the moho discontinuities strong correlations were found between temperature and seismic wave attenuation and also between fractional velocity perturbations and temperature in the uppermost mantle again the correlation is strong and highly significant between electrical conductivity and temperature in the lower crust upper mantle

the subsurface temperature assessment by means of an indirect electromagnetic geothermometer abstractadvances have been made in the subsurface temperature estimation basing on the indirect electromagnetic geothermometer the approach used was based on the artificial neural network technique which contrary to other available approaches does not imply the prior knowledge of the electrical conductivity mechanisms and rock properties application of the indirect em geothermometer to the interwell space interpolation in three areas tien shan kyrgyzstan soultz sous forets france and hengill iceland characterized by different geologic environments indicated that the temperature estimation errors are controlled by four factors faulting distance between the em site and the area where the temperature is estimated meteoric and groundwater flows and lateral geologic inhomogeneity although the latter factor is less restrictive if appropriate em inversion tools are used it was demonstrated that the extrapolation errors depend on two factors spacing between the em site and the borehole and ra

a latest carboniferous warming spike recorded by a fusulinid rich bioherm in timor leste implications for east gondwana deglaciation abstract during the late paleozoic and early mesozoic timor lay in the northern part of the north south east gondwana rift system along which the western margin of australia later developed discovery of a latest gzhelian bioherm in the central highlands of timor leste has implications for latest carboniferous earliest permian climate history and deglaciation in basins further south in the rift system limestone outcrop of the maubisse formation near the village of kulau is recognized as a bioherm with a massive lower unit including reef framework at the base and a bedded grainstone upper unit the bioherm developed on a basalt substrate in warm shallow water as indicated by photozoan assemblages in the massive lower unit foraminifera belonging to 17 genera are recorded from the bioherm these include representatives of the families biseriamminidae biwaellidae bradyinidae cornuspiridae lasiodiscidae palaeotextulariidae pseudotaxidae ozawainellidae schubertellidae schwagerinidae staffellidae and textrataxidae twenty one species have been referred to known types and 12 species are left in open nomenclature the assemblage probably belongs within the uppermost gzhelian schwagerina robusta ultradaixina bosbytauensis zone although a possible lowest asselian correlation cannot be excluded the bioherm is the oldest carbonate unit so far recorded from the maubisse formation and the oldest sedimentary unit biostratigraphically dated in timor the dominantly heterozoan composition of the skeletal component of the limestone except for the basal photozoan assemblage and the taxonomic diversity of the larger foraminifera suggest a subtropical environment consistent with a paleolatitude of about 40 s the late pennsylvanian was a time of glaciation that in australia is represented by a significant stratigraphic hiatus in basins to the south of timor in the east gondwana rift system the development of the kulau bioherm during the latest gzhelian may have coincided with a global warming spike that led to rapid melting of continental ice sheets and a substantial influx of glacigene sediment alternating diamictite and mudstone in the southern basins

palaeobathymetric zonation of foraminifera from lower permian shale deposits of a high latitude southern interior sea abstract foraminifera are documented from the type section of the quinnanie shale in order to interpret their bathymetric distributions in a shallow high latitude interior sea during the kungurian the marine setting was a narrow elongate half graben merlinleigh sub basin of the southern carnarvon basin in the western australian portion of eastern gondwana the quinnanie shale is part of the byro group of formations that display pronounced shale sand cyclicity recording frequent changes in bathymetry the type section of the quinnanie shale shows an overall progradational pattern in lithofacies and consists of five cycles each culminating in a prominent sandstone bed foraminifera are abundant in the shale and are almost entirely siliceous organic cemented agglutinated types that probably dominated the original fauna of the interior sea hierarchical cluster analysis of samples taken every meter through the 162 m thick type section is used to distinguish ten biofacies each defined by a different set of dominant agglutinated species although biofacies frequently change up section there is an overall trend that is related to the progradational trend suggested by the lithofacies based on comparisons between lithofacies and biofacies a palaeobathymetric zonation is established for the foraminifera this zonation the sparse macrofauna and the lithofacies suggest that the interior sea was stratified in terms of salinity and dissolved oxygen levels and the water was generally hyposaline most of the agglutinated foraminiferal species have analogous morphotypes present in modern confined estuaries and interior seas and this points to great conservatism in the evolutionary and ecological development of this component of interior sea faunas aaptotoichus quinnaniensis sp nov an organic cemented agglutinated foraminifer is described from the quinnanie shale type section

geochemistry and palaeo oceanography of metalliferous and pelagic sediments from the late cretaceous oman ophiolite abstract metalliferous and pelagic sediments are exposed within and above the extrusive successions of the upper cretaceous oman ophiolite which on the basis of mostly geochemical evidence is believed to have formed in an incipient marginal basin setting located above a ne dipping subduction zone the ophiolitic extrusives document various volcano tectonic settings which include the axial zones of a spreading ridge fault controlled seamounts and off axis volcanic edifices most of the fe mn and trace metal enriched sediments studied are interpreted as precipitates formed by oxidation of solutions derived from high temperature sulphide precipitating vents the trace element content e g ree and sr was largely scavenged from seawater the sediments are similar to the dispersed metalliferous sediments on the flanks of modern spreading ridges and the basal sediments of dsdp wells and of other ophiolite complexes e g troodos cyprus distinctive mound structures located low in the lavas are attributed to percolation of sulphide rich solutions into already deposited metalliferous oxide sediments the resulting iron silica rock was probably originally precipitated as ferruginous silicates major massive sulphides formed off axis at the base of intermediate basic edifices of volcanic arc affinities fe mn and trace metal enrichment in the sediment cover of a flat topped seamount of axial lavas is interpreted as a dispersion halo around the largest massive sulphide orebody which is situated 5 km away lasail small massive sulphide bodies are common in the axial lavas particularly along major seafloor fault zones the metalliferous sediments locally precipitated near these vents are ferromanganiferous but trace metal depleted the metalliferous and pelagic sediment cover of the extrusive successions generally documents waning hydrothermal input after volcanism ended in the area a model is discussed in which the ophiolite was created at a spreading axis above a subduction zone dipping away from the arabian continental margin with progressive subduction this crust approached the margin initially calcareous sediment accumulated above the calcite compensation depth ccd but then non calcareous radiolarites were deposited as the ophiolitic crust approached the continental margin where the ccd was higher and marginal upwelling possibly enhanced productivity as the edge of the arabian continental margin entered the trench the over riding ophiolite was regionally uplifted allowing short lived chalk accumulation above the ccd this was followed by volcaniclastic deposition related to the tectonic emplacement

postglacial early permian late sakmarian early artinskian shallow marine carbonate deposition along a 2000 km transect from timor to west australia abstract late sakmarian to early artinskian early permian carbonate deposition was widespread in the marine intracratonic rift basins that extended into the interior of eastern gondwana from timor in the north to the northern perth basin in the south these basins spanned about 20 of paleolatitude approximately 35 s to 55 s this study describes the type section of the maubisse limestone in timor leste and compares this unit with carbonate sections in the canning basin nura nura member of the poole sandstone the southern carnarvon basin callytharra formation and the northern perth basin fossil cliff member of the holmwood shale the carbonate units have no glacial influence and formed part of a major depositional cycle that in the southern basins overlies glacially influenced strata and lies a short distance below mudstone containing marine fossils and scattered dropstones perhaps indicative of sea ice in the south marine conditions became more restricted and were replaced by coal measures at the top of the depositional sequence in the north the carbonate deposits are possibly bryozoan crinoidal mounds whereas in the southern basins they form laterally continuous relatively thin beds deposited on a very low gradient seafloor at the tops of shale limestone parasequences that thicken upward in parasequence sets all marine deposition within the sequence took place under very shallow inner neritic conditions and the limestones have similar grain composition bryozoan and crinoidal debris dominate the grain assemblages and brachiopod shell fragments foraminifera and ostracod valves are usually common tubiphytes ranged as far south as the southern carnarvon basin albeit rarely but is more common to the north gastropod and bivalve shell debris echinoid spines solitary rugose corals and trilobite carapace elements are rare the uniformity of the grain assemblage and the lack of tropical elements such as larger fusulinid foraminifera colonial corals or dasycladacean algae indicate temperate marine conditions with only a small increase in temperature to the north the depositional cycle containing the studied carbonate deposits represents a warmer phase than the preceding glacially influenced asselian to early sakmarian interval and the subsequent cool phase of the mid artinskian that is followed by significant warming during the late artinskian early kungurian the timing of cooler and warmer intervals in the west australian basins seems out of phase with the eastern australian succession but this may be a problem of chronostratigraphic miscorrelation due to endemic faunas and palynofloras

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

downward continuation of heat flow data by means of the least squares method abstract an analysis of the steady state heat conduction model for the earth s crust and the upper mantle revealed that the main difficulties in the downward continuation of subsurface geothermal measurements q were connected with an estimation of the heat flow density qm at the moho boundary and of the source distribution solving the steady state heat conduction equation numerically by means of finite differences finite elements we can describe the relation between q qm and by a linear algebraic system to overcome the non uniqueness and instability a least squares approach is suggested to solve these systems bounds for the square means of the unknown parameters qm and are introduced whereby it is possible to find solutions with appropriate mean properties a two dimensional geothermal profile crossing central europe is used to demonstrate this method the difficulties in estimating reasonable mantle heat flow density and optimal heat source models from geothermal measurements only are pointed out

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

early permian sediment provenance and paleogeographic reconstructions in southeastern gondwana using detrital zircon geochronology northern perth basin western australia abstract the perth basin western australia forms part of a north trending belt of intracratonic rift basins that initiated in southeastern gondwana in the late paleozoic existing provenance studies have highlighted the dominance of proterozoic zircons derived from southerly terranes such as the albany fraser orogen leeuwin complex and more distant provinces in east antarctica with limited input from the adjacent archean western australian craton new u pb geochronology data were acquired using a sensitive high resolution ion microprobe shrimp ii on detrital zircons from lower permian formations exposed in the northeastern perth basin in contrast to previous studies archean zircons showing fine oscillatory zoning dominate the age spectra with up to 90 grains ranging between 3 0 and 2 6 ga indicating that granitic parent rocks of the adjacent craton were a major sediment source during the early permian a minor paleoproterozoic age population from the gascoyne complex of the capricorn orogen was likely transported southward and dispersed in the basin by a combination of longshore drift and tidal currents meso and neoproterozoic detritus became prominent only from the middle artinskian and may relate to the establishment of large northward flowing river systems draining the albany fraser province the prydz leeuwin belt and or the wilkes land in east antarctica this study shows that integrating depositional processes and stratigraphic evolution of sedimentary formations with provenance analysis can enhance reconstruction of dispersal pathways to and within basins

organic carbon burial rates and carbon and sulfur relationships in coastal sediments of the southern baltic sea abstract organic c burial rates and c s relationships were investigated in the holocene sediment sequences of 3 shallow polymictic coastal lagoons in the southern baltic sea to better understand the biogeochemical cycling of c and s in these environmental systems the results show that these lagoons may have a considerable influence on the environmental status of the southern baltic sea area in having the potential to act as a temporary sink or source for heavy metals high organic c accumulation rates corg ar can be observed in the sediments due to a high organic matter supply from land and a high productivity of the water bodies as a result of eutrophication however organic c burial does not increase as a result of increasing sediment accumulation rates sar even when high sedimentation rates do occur there appears to be a thorough recycling and resuspension of the sediment enhancing the biological decay of organic matter before burial or the removal of organic matter from the system by transport that is why high sar in the coastal lagoons do not enhance pyrite formation and thereby permanent fixing of heavy metals in the sediments to the extent that could be expected from their magnitude initially there is a high potential for a temporary binding of heavy metals but the latter are likely to be subject to mobilization and redistribution within the sediments and the water column the patterns of burial of organic and mineral matter are different from those observed in the present day baltic proper implying possible important links in deposition between the central and coastal areas of the baltic sea and implications for c cycling in the ecosystem of the baltic sea