the iceland research drilling project in relation to the geology of iceland the lava pile penetrated by the iceland research drilling project irdp hole formed in an axial rift zone that remained active for at least the last 13 m y the stratigraphic sequence is formed of subaerial volcanics and it is likely that volcanism was subaerial for several million years or even tens of millions of years prior to the formation of the irdp sequence the volcanics in eastern iceland are tholeiitic like all tertiary sequences investigated in iceland to date and the lava extrusion rates in eastern iceland are reported to be similar to those of tertiary sequences in northern and western iceland respectively the irdp hole was sited in a dyke swarm extending from the breiddalur central volcano in the south the irdp hole was sited in a regional thermal anomaly with a gradient of about 80 c km that was found by a series of 100 m thermal gradient wells in eastern iceland aquifers of 48 c were encountered at about 600 m depth below which the thermal gradient lowered considerably this suggests that the regional thermal anomaly is caused by the flow of warm water at relatively shallow depths

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

effect of mantle compressibility on the thermal and flow structures of the subduction zones 1 the heat generated by viscous dissipation is consistently evaluated using a 2 d compressible subduction model with variations of mantle rheology constant as well as pressure and temperature dependent viscosity dip age and velocity of the subducting slab for comparison we also conduct 2 d incompressible subduction calculations with the same conditions and parameters used in the compressible formulation the effect of compressibility on the thermal and flow structures of the subduction zones is relatively small and concentrated along the base of the mantle wedge with temperature differences 100 c and differences in kinematic energy of the mantle wedge 1 between compressible and incompressible models mantle rheology has a stronger effect on thermal and flow structures than mantle compressibility as well as the variations of dip age and velocity of the subducting slab the heat from viscous dissipation in the compressible model increases the slab temperatures over the incompressible model 70 c as a result of additional conduction across the slab surface constant viscosity and thinning of the thermal boundary layer caused by viscosity reduction pressure and temperature dependent viscosity

detailed susceptibility log of iceland research drilling project drill core reydarfjordur eastern iceland a detailed log of initial susceptibility based on 6135 measurements made on drill core with a hand held meter is presented for the 1919 m long lower part of the irdp reydarfjordur crustal section the log shows typical strong internal variation with a central maximum in flow susceptibility while dikes show little internal variation differences between dikes and flows and internal variations are explained in terms of variation in degree of deuteric and hydrothermal alteration grain size and abundance of iron titanium oxides general decrease of flow susceptibility with depth is related to depth variation in hydrothermal alteration this has the effect of removing iron from oxides probably for incorporation in nonmagnetic silicates this effect is intense toward the bottom of the drill core linear extrapolation of the decrease of flow susceptibility with depth indicates a zero value at about 4 km beneath the original surface of the crustal section whether or not magnetization of the crust as a whole reaches zero level will depend on the dike density at this depth

soil variability in the area of an archaeological site near sparta greece results are analysed from a late roman site one of 20 studied in the laconia rural sites project the aim was to consider the extent to which effects of human activity at the time the site was inhabited were still evident in the soil and the exent to which these could be distinguished from natural soil variations the site has not been excavated patterns of soil properties of artefacts on the ground surface and of those encountered in the topsoil whilst sampling are analysed for a 5 m interval grid soil samples from 20 cm depth were analysed for total p pb cu zn fe mn ni ca mg k and organic c mass specific and volume magnetic susceptibility secondary crystalline and amorphous fe ph caco3 particle size distribution and colour all variables are mapped and data are further analysed using cluster regression and principal components analyses relatively high values for soil p cu pb ca organic c magnetic susceptibility and clay content and low ph and darker soil colour are associated with areas of the grid where artefact counts were highest the distinctive character of the soil in these areas reinforced by cluster and principal components analyses is attributed to a combination of inherent soil and anthropogenic effects

reconstructing site organization in the eastern aleutian islands alaska using multi element chemical analysis of soils this study presents the results of multi element analysis of weak acid extractions of 953 soil samples collected by coring on and around pre historic village sites on two islands sanak and amaknak in the eastern aleutians concentrations of aluminum al barium ba calcium ca iron fe potassium k magnesium mg manganese mn phosphorus p strontium sr titanium ti and zinc zn were determined using an inductively coupled plasma mass spectrometer icp ms resultant elemental signatures allow the identification of site features and organization in the absence of large scale excavation all the archeological sites showed similar and distinct chemical signatures regardless of the several thousand year date range of the sites or the length of time a site was occupied however intensity of occupation does appear to affect concentrations of elements site features such as houses house berms house floors and middens had distinct anthropogenic signatures and could be distinguished from one another this approach may be used to identify sites that do not have distinct surface features and to target areas for excavation

the natural magnetization of a 3 kilometer section of icelandic crust the magnetization and magnetic properties of samples from a 3 km vertical section of icelandic crust the iceland research drilling project section are described the section is located at the head of reydarfjordur eastern iceland at 65 01 n 14 21 w and extends vertically from about 0 5 km to 3 5 km depth beneath the original surface the section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2 km drill core component consisting of 60 flows and minor volcaniclastics and 40 dikes the section is largely normally magnetized corresponding in age and polarity to seafloor spreading anomaly 5 geomagnetic polarity epoch 9 flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000 m thick equivalent sections which are exposed updip this great increase in downdip thickening is consistent with observations elsewhere in eastern iceland and models for icelandic crustal formation involving the combination of volcanic activity subsidence and spreading a combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section this overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2 8 km crustal original depth where moderate secondary magnetite development has occurred flows frequently show both normal and reverse stable polarities with the latter being the original secondary magnetite development is high where dike density in the epidote bearing flows is also high here very strong well defined pervasive normal magnetization characterizes the flows this also being the dominant polarity of the dikes magnetic overprinting of this type imposed shortly after crustal formation could occur at 800 to 1200 m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins the initial susceptibility of the flows shows the best defined depth trends over the section susceptibility increases with depth to about 2 km crustal depth and then decreases systematically to the bottom of the section the reason for the initial increase is not known but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates the decrease is sufficiently linear to warrant extrapolation to a zero value at 4 3 km crustal depth or just below the estimated level of onset of greenschist facies metamorphic conditions in typical oceanic crust the equivalent level is estimated to be at 1 2 km depth the net magnetization of the section the combined remanent and induced contributions when expressed as average values for 500 m depth intervals is everywhere positive that is is in the sense of the present field variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

alteration processes at deep sea drilling project ocean drilling program hole 504b at the costa rica rift implications for magnetization of oceanic crust magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504b during ocean drilling program leg 111 our results combined with those from previous studies show abrupt first order changes in magnetic properties at alteration boundaries within the 504b crustal section changes in style and degree of alteration fall near the boundaries of the three well defined lithologic units the extrusive basalts the transition zone and the sheeted dike complex this postemplacement alteration heavily influences magnetic properties and is observed to change in both style and degree with depth within each lithologic unit our results indicate that at 504b the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration the upper more permeable basalts above this critical depth have not experienced this reheating within the sheeted dike complex the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth the changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth overall these observations suggest that the effective penetration of water into layer 2c decreased with increasing depth and resulted in a lower rate of convective cooling magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration the upper dikes have become magnetically more stable than the lower dikes a review of all magnetic properties indicate that the dike section at 504b carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict we suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions despite a magnetization which is lower than expected it appears that the sheeted dike complex at hole 504b is capable of making a substantial contribution to the overlying marine magnetic anomaly because of the systematic decrease in hydrothermal alteration of magnetic minerals the ability of the dike section to contribute decreases as a function of depth

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

field characteristics of laterally emplaced dikes anatomy of an exhumed miocene dike swarm in reydarfjordur eastern iceland abstract many dikes of the tertiary breiddalur dike swarm in reydarfjordur have a great lateral extent but are vertically upward and or downward discontinuous field relationships indicate that they are not the feeders of the lava pile they intrude their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading krafla volcanic center of the neovolcanic zone average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other tertiary dike swarms in eastern iceland it is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures

magnetic and thermal effects of dike intrusions in iceland oriented core samples were collected from 18 lava dike contacts in iceland in locations of low to moderate 100 c regional hydrothermal alteration alternating field demagnetization of these samples shows that they generally carry a strong and stable remanence magnetic directions in the lavas follow a simple pattern demonstrating that the remanence of the contact zone has only been reset within very short distances from the contact surface on average this distance may be only one tenth of the thickness of a typical dike 4 m other magnetic properties remanence intensity susceptibility curie point of the contact zone are affected to a lesser or a less regular degree than the remanence direction it is concluded that for the crustal depth of this study thermal conduction models do not apply to temperature anomalies set up by dike intrusions or fissure eruptions in iceland presumably because of rapid advection of groundwater into the active fissures these results also indicate that the magnetic anomalies observed over iceland are only to a small extent caused by dikes

primary and secondary alkali and halogen element distribution in iceland research drilling project basalts from eastern iceland the degree of oxidation and hydration and the variation of fe mg li na k rb f cl and i are reported for hydrothermally altered basaltic lavas and dikes intersected in the iceland research drilling project irdp borehole sequence from eastern iceland the concentrations of water soluble na cl and i have also been determined and represent approximately 1 40 and 53 respectively of the total content of each element hydration and oxidation in both lavas and dikes generally increase downward in the total lava pile to maxima toward the base of the drilled section three lava groups and two dike groups are recognized at different depths on the basis of the relative distribution of zr and y each group exhibits a trend toward more primitive compositions upward in the pile within group and vertical chemical variations reflect both 1 primary chemical features such as parental magma composition and degree of crystal fractionation and 2 secondary mobilization during hydrothermal alteration the latter caused variable alkali and halogen element dispersion but initial differences between the chemical groups appear still to be discernible mobility sequences were rb k li na and cl f i although all the analyzed elements were mobilized at the relatively low levels of hydration few systematic trends with progressive hydration are observed e g k and rb were leached and li was sometimes enriched the main effect of alteration was a localized redistribution of mobile elements into stable secondary phases apparent depletions of mobile elements at the base of the sequence reflect primary variations rather than extensive leaching

bifurcation of the yellowstone plume driven by subduction induced mantle flow in the northwestern united states two neighbouring volcanic chains the snake river plain and high lava plains decrease in age in opposing directions laboratory experiments show that diverging volcanic chains can form where vigorous mantle circulation beneath a subduction zone causes an upwelling mantle plume to split into two

volcaniclastic rocks of the reydarfjordur drill hole eastern iceland 1 primary features a 1918 m deep bore hole in a late miocene shield basalt central volcano mixed sequence contains 58 stratigraphically distinct volcaniclastic interbeds constituting 8 8 of the extrusive part of the section textural mineral chemical and bulk chemical criteria are applied to identify the original nature composition and origin of these rocks which have been metamorphosed by hydrothermal alteration between 100 c top to ca 300 c base of the section units range from a few centimeters to 18 m excluding a 30 m thick ignimbrite with 75 of the units being less than 1 m thick formerly vitric to tachylitic shards and lapilli lithic fragments and crystals are the main components most thin volcaniclastic units are basaltic vitric to tachylitic tuffs thin felsic air fall tuffs a few millimeters to a few centimeters thick occur as separate layers and interbedded with basaltic tuffs nearly all thicker units are made up dominantly of felsic vitric to lithic clasts and crystals phenocrysts common only in felsic air fall tuffs are clinopyroxene ranging from augite to ferrohedenbergite and plagioclase ranging from an80 20 both of which span the entire range of compositions previously known from iceland bulk chemical analyses of major and 11 trace elements of 65 samples allow texturally indistinct rocks to be compositionally identified even though chemical changes due to mixing and alteration are pronounced in many rocks the rhyolitic ignimbrite cooling unit is composed of four flow units the upper most being slightly more mafic in composition because of admixture of a basaltic component the two central flow units show extensive high temperature devitrification with lensoid tridymite rimmed cavities now laumontite filled representing collapsed pumice lapilli the composition of the clastic units corresponds in a general way to that of the interbedded lava flows these data and evidence for weathering and reworking in some units are used to reconstruct the volcanic history of the area phase a 1918 1306 m dominantly mafic shield basalts contains only few and mostly basaltic tuffs except for sparse rhyolitic air fall tuff from distant central volcanoes phase b 1306 653 m composed dominantly of evolved basalts and icelandites culminating with the rhyolitic ignimbrite 920 950 m probably represents the eastern flank of a nearby central volcanic complex whose center may be situated less than 10 km westward downdip the upper extrusive 500 m of the section phase c are basalt lavas of average tholeiitic composition containing abundant chiefly felsic and reworked clastic units in the interval between 286 and 410m these are tentatively interpreted to reflect a phase of structural basin forming not compensated by extrusive volcanic activity allanite phenocrysts were found for the first time in icelandic rocks and the host pumice tuffs may constitute an important stratigraphic marker horizon that may help to tie the drilled section to the subaerial volcanic series in this part of eastern iceland the alteration of the clastic layers is discussed in a companion paper

deep crustal reflectors under reydarfjordur eastern iceland crustal accretion above the iceland mantle plume summary deep seismic reflection profiling on the faroe iceland ridge experiment fire imaged a sequence of dipping events in the upper to crust beneath the tertiary lava pile in reydarfjordur eastern iceland these events dip towards the spreading centre at which the crust was formed and continue down to 6 s two way time corresponding to depths down to 15 18 km similar crustal reflections have been observed previously within the crust in other areas of iceland although the reflective events located by previous surveys have not been interpreted rigorously in this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava le the synthesized lava pile sections are based on field measurements made in the reydarfjordur area the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions and some intercalated tuffaceous clastic horizons we compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas we conclude that the upper and mid crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface with the main reflections resulting from packets of flows of similar thicknesses the reflectors that we have imaged beneath iceland are similar to those defining the seaward dipping reflectors on the volcanic continental margins and we therefore suggest that the seaward dipping reflections commonly result from packets of lavas with similar distributions of thicknesses the occurrence of lavas down to 15 18 km beneath reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust although the volcanic processes forming the crust in iceland generally lead to advection of extrusive material to mid crustal levels the production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity the advection of frozen lava flows to mid crustal levels contributes to the maintenance of the subsolidus crust that is observed on iceland and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques

transform deformation of tertiary rocks along the tjornes fracture zone north central iceland significant fracture zone deformation has been imparted to tertiary lavas on the peninsula flateyjarskagi located between eyjafjordur and skjalfandi in north central iceland the region lies immediately south of the flatey fault one of a series of wnw trending left stepping faults comprising the right lateral oblique slip husavik flatey fault system hffs the hffs defines the present southern margin of the 70 to 80 km wide tjornes fracture zone tfz lava bedding dikes and faults in the western half of flateyjarskagi display a progressive 0 110 clockwise change of trend and steepening of lava dip angles to 15 45 over an 11 km wide zone nearest the flatey fault accompanying this structural curvature is an increase in fault and dike frequency and amygdule vein mineralization northeast striking faults develop predominant downthrows to the east approaching the tfz accommodating the steep lava dips via repetition of stratigraphic section and normal fault bounded block rotations although the possibility cannot yet be conclusively ruled out that structural trends formed in curved orientations the simplest interpretation suggests that the lava pile was tectonically rotated by heterogeneous simple shear maximum on land shear strain has been crudely estimated at 3 5 estimates of shear displacement suggest that the tfz was active for at least 2 m y sometime prior to 6 7 ma at a strain rate of approximately 5 5 10 14 s 1 antithetic faulting and associated flexural bending may have been the means by which a great part of the proposed simple shear rotations were accomplished wnw extension produced by rotational normal faulting is interpreted to have accommodated systematic variations in shear strain parallel to the direction of shear paleomagnetic and structural evidence suggest that rocks of tertiary age on tjornes peninsula immediately to the east also have been tectonically rotated implying an original shear zone width of at least 25 km

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

frequent shifts of the volcanic zone in iceland the heterogeneous and locally anomalously thick icelandic upper crust cannot be fully explained with current models of crustal construction a model that invokes frequent shifts of the volcanic zone accreting plate boundary is proposed as an alternative three important parameters which result from a complex history of shifting determine the upper crustal structure of iceland 1 the time interval between two successive shifts 2 the scale of lateral displacements of the volcanic zone and 3 the intensity of volcanism at each volcanic zone

a brief k ar age study of the irdp borehole reydarfjordur eastern iceland five core samples from a depth of 182 to 1899 m were studied in order to ascertain if any geologically sensible k ar ages could be determined on such highly altered material ages were obtained ranging from 8 9 to 10 9 ma seemingly consistent with previous k ar work on correlated updip surface exposures serious radiogenic argon depletions or total overprinting by the hydrothermal metamorphism were not observed

the mode of emplacement of neogene flood basalts in eastern iceland the plagioclase ultraphyric basalts in the graenavatn group abstract plagioclase ultraphyric basalt lava with high fraction of solids have a mode of emplacement that is poorly understood in this study we conduct detailed mapping of a pub group in eastern iceland namely the graenavatn group and assess the group architecture flow morphology and internal structure with additional constraints from petrography petrology and crystal size distribution to derive information on emplacement dynamics of plagioclase ultraphyric basalts we also derive information on the plumbing system of the group with reference to the source of the macrocysts the group is exposed in steep glacially carved fjords and can be traced for more than 70 km along strike the flows have mixed architecture of simple and compound flows individual flow lobes have thicknesses in the range of 1 24 m and many reach widths and lengths exceeding 1000 m the flows vary from rubbly to slabby pahoehoe but are predominantly of pahoehoe type the aspect ratio of the group and the nature of the flows indicate fissure fed eruptions the plagioclase macrocrysts 5 30 mm are an rich exhibit bimodal size distribution and the modal proportions within the group varies from 15 40 clinopyroxene macrocrysts are also present ranging from 1 6 the lowermost flow is thickest and carries the greatest crystal cargo load the morphology of the lava flows suggests low viscous behavior at odds with the high crystal content the very calcic plagioclase macrocrysts an80 85 are in disequilibrium with the groundmass and plagioclase microlaths therein an50 70 meaning that the crystal laden magmas quickly ascended from deeper crustal levels to the surface the flows with highest crystal content may have maintained high temperatures by heat exchange with the primitive macrocrysts in the flows and developed non newtonian behavior such as shear thinning such conditions would have enabled the flows to advance rapidly during episodes with high effusion rates forming the simple flows and subsequently maintained by insulated lobe by lobe emplacement occasionally surges disrupted the crust to form rubbly and slabby pahoehoe a herschel bulkley model is applied to the flows yielding effusion rates in the order of 103 104 m 3 s for the largest eruptions variation in crystal proportions within the group and individual flows is considered to be the result of magmas penetrating a crystal mush followed by disaggregation and mobilization sorting by flotation mixing of magma batches and transportation of heterogeneous crystal laden magmas to the surface with opportunities for staging en route

geophysical response of flood basalts from analysis of wire line logs ocean drilling program site 642 voring volcanic margin analysis of wire line logs at ocean drilling program site 642 the deepest scientific hole yet drilled in an offshore large igneous province lip reveals the main features of the extrusive complex constructed during formation of a volcanic margin a characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record flows are 0 6 18 5 m thick while most sediments are 2000 m focused resistivity kicks the large flow scale variations in physical properties show that representative core measurements must be densely sampled both on flow and sequence scales site 642 log responses appears typical for the north atlantic volcanic province and other lips but differs from logs in deep ocean crustal holes principal differences in log responses reflect mode of crustal emplacement and alteration hence site 642 forms a reference hole for extrusive constructions on oceanic lips

volcaniclastic rocks of the reydarfjordur drill hole eastern iceland 2 alteration volcaniclastic rocks of basaltic to rhyolitic composition interlayered with basaltic to icelanditic lava flows in a 1918 m drill hole in eastern iceland are altered to zeolite and pumpellyite prehnite facies assemblages major alteration minerals are chlorites illite chlorite mixed minerals illites k feldspar laumontite quartz and sphene others include julgoldite and secondary allanite smectites as well as mordenite and heulandite are restricted to the upper 900 m epidote and minor pumpellyite prehnite wairakite and anhydrite are restricted to the core below 900 m the boundary between zeolite and prehnite pumpellyite facies is marked by the appearance of chlorite epidote quartz assemblages at around 900 m the typical paragenesis prehnite pumpellyite epidote chlorite occurs near the base of the drill hole at 1855 m textures indicate generally prograde metamorphic conditions the broad compositional spectrum ranging from basalt to rhyolite abundance of vitric fragments and high initial porosity and permeability of the volcaniclastic rocks were major factors governing alteration the site of growth crystal shard vesicle pore and vein of a secondary phase strongly influences composition habit amount paragenesis and time of crystallization for example sphene and to a lesser degree epidote are less rich in fe and better crystallized in plagioclase than in the adjacent matrix the different mineral assemblages developed within plagioclase crystals through the section most clearly reflect the metamorphic changes the abundance of vitric rhyolitic clasts shards and pumice favored k al si enriched fluids from which phases like illite and adularia crystallized these phases were not found in interlayered basalt flows mehegan et al this issue low ca activity is also indicated by the occurrence of wairakite and pumpellyite rather than prehnite which is common in the basalts secular changes during metamorphism in pore fluid composition are shown by zonations in mineral compositions for example the depletion of epidote and juldoldite pumpellyite in fe with time appears to reflect decreasing o2 bulk rock chemical changes are more intense in the volcaniclastic rocks compared to the basalt lavas h2o fe3 fe2 and in part co2 values are higher in the upper part of the hole reflecting the higher porosity and glass content of the clastic rocks they decrease markedly downhole approaching values of the lava flows due to more pervasive metamorphic alteration and thus similar mineral assemblages and physical properties

magnetostratigrahy of the exposed lava section east of the irdp drill hole in reydarfjordur eastern iceland magnetic polarity directions vgp of 154 exposed miocene basalt lavas from reydarfjordur eastern iceland can be used successfully for correlation with the irdp drill core three partly synchronous lava sections east of the drill site are predominantly normally magnetized and are stratigraphically of the same age as lavas dated k ar at about 9 to 10 5 m y mcdougall et al 1976 the age range and magnetic properties of these lavas the landward equivalents to marine magnetic anomaly 5 epoch 9 suggest that several reversely magnetized lavas in one of the investigated sections were remagnetized by a 10 m thick dyke the data presented suggest that in fresh lavas with low curie temperatures relatively thick dykes about 10 m may cause thermal overprinting of the host rocks to a distance from the dyke equal to its thickness these results are in contrast with the generally accepted thesis that dykes affect host rocks only up to about 1 5 of their thickness no systematic decrease in magnetic intensity with depth was observed for a 0 9 km thick vertical lava section in holmatindur about 13 5 km east of the drill site it is suggested that the heterogeneity of the lava pile in terms of magnetic intensity reflects primarily the distinct geochemistry and or grain size of magnetic phases of individual lavas

the iceland research drilling project synthesis of results and implications for the nature of icelandic and oceanic crust study of a 3 km thick section of tertiary basalts in eastern iceland has resulted in the formulation of a model for crustal construction based on accumulation of lava flows on the flanks of large central volcanoes and has led to the development of alteration and physical property profiles in a crustal segment formed at the mid atlantic ridge deep drilling at reydarfjordur revealed the existence of subaerial lava flows and volcaniclastic rocks to a crustal depth of 3 5 km three major stratigraphic units composed largely of basalts are recognized each characterized by distinctive major and trace element patterns each of these units is believed to be the product of a separate episode of mantle melting and fractionation and probably reflects accumulation on the flanks of a central volcano extensive fractionation in shallow magma chambers led to intermediate and silicic lavas which mark the cores of the central volcanic complexes dike density does not increase downward below about 1 5 km crustal depth and the sampled dikes appear to have been intruded laterally along rifts on the flanks of the central volcanoes secondary mineral assemblages are dominated by smectite and zeolites near the top of the section and by epidote and quartz near the base in general the temperature of alteration increases downward but the distribution of individual minerals is irregular locally earlier secondary mineral assemblages are overprinted by contact metamorphism along dike margins the upper surface of greenschist facies metamorphism crops out at the surface in thingmuli volcano but it lies at an estimated crustal depth of about 4 km in the reydarfjordur borehole located only 3 km east of the edge of thingmuli thus the metamorphic zones clearly crosscut the lava stratigraphy physical property profiles correlate well with variations in secondary mineralogy and the top of layer 3 velocities corresponds closely with the projected upper boundary of amphibolite facies rocks at about 6 km crustal depth magnetic susceptibility decreases with depth and is extrapolated to zero at about 4 3 km crustal depth close to the estimated upper limit of greenschist facies metabasalts a much greater crustal thickness leading to the emplacement of high level magma chambers within the extrusive part of the crust is thought to account for many of the differences in crustal structure and evolution between iceland and the rest of the mid atlantic ridge

effect of time evolving age and convergence rate of the subducting plate on the cenozoic adakites and boninites partial melting of subducting oceanic crust expressed as high mg volcanic rocks such as adakites and boninites has been actively studied for decades and lee and king 2010 reported that time evolving subduction parameters such as the age and the subduction rate of the converging oceanic plate play important roles in transient partial melting of the subducting oceanic crust e g aleutians however few subduction model experiments have considered time evolving subduction parameters posing problems for studies of transient partial melting of subducting oceanic crust in many subduction zones therefore we constructed two dimensional kinematic dynamic subduction models for the izu bonin mariana northeast japan kuril tonga java sunda and aleutian subduction zones that account for the last 50 myr of their evolution the models include the time evolving age and convergence rate of the incoming oceanic plate so the effect of time evolving subduction parameters on transient partial melting of oceanic crust can be evaluated our model calculations revealed that adakites and boninites in the izu bonin and aleutian subduction zones resulted from transient partial melting of oceanic crust however the steady state subduction model using current subduction parameters did not produce any partial melting of oceanic crust in the aforementioned subduction zones indicating that time evolving subduction parameters are crucial for modeling transient eruption of adakites and boninites our model calculations confirm that other geological processes such as forearc extension back arc opening mantle plumes and ridge subduction are required for partial melting of the oceanic crust in the mariana northeast japan tonga and southeastern java sunda subduction zones

episodic volcanism during break up of the north atlantic evidence from the east greenland plateau basalts abstract the lower tertiary upper paleocene lower eocene plateau basalts in the scoresby sund region of east greenland formed during three distinct magmatic episodes the activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence lower sequence the activity of the second episode was centred to the southeast offshore the present atlantic coast and this also produced a voluminous regional basalt sequence upper sequence the third episode was centred on the atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting the lavas are all tholeiitic basalts after an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height from tio2 rich feti basalts to tio2 poor basalts and ending with a reversal to tio2 rich basalts each of these cyclic patterns apparently resulted from a temporary rifting episode and much of the compositional variation found can be explained by magma development in open magma chambers the dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts and their tio2 p2o5 ratios indicate formation at a spreading ridge centre the pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region and the volcanism itself is closely related to this process

stratigraphy and correlation of the region surrounding the irdp drill hole 1978 reydarfjordur eastern iceland the 1 9 km thick sequence intersected by the irdp 1978 drill hole at reydarfjordur eastern iceland is correlated with the westward dipping exposed volcanic pile on the basis of extensive field mapping and paleomagnetic studies the base of the landward extension of sea floor magnetic anomaly 5 epoch 9 can be located at the top of the grjota olivine tholeiite formation intersected at about 1 5 km in the core and exposed at sea level about 9 km east of the drill site other exposed marker formations can also be identified in the core the drill site is located within a relatively dense dyke swarm 10 crustal dilation previously interpreted as the feeder of the overlying volcanic formations field observations including vertically discontinuous dykes and the lack of lava feeder relationships suggest that the dykes were intruded by lateral injection from the south and are considerably younger than and thus could not have fed the exposed lava pile in the vicinity of the drill site it is proposed that about 10 3 m y ago roughly coinciding with the lower boundary of anomaly 5 the reydarfjordur volcanic center ceased to be active and volcanic activity shifted about 20 km to the west where the breiddalur and thingmuli volcanic centers later developed the exposed stratigraphic succession between thingmuli and reydarfjordur volcanic centers is composed of lavas deposited in flank areas relatively removed from a paleovolcanic zone which now probably lies buried to the west of the irdp drill site the lowest 450 m of the core may represent lavas related to activity in the reydarfjordur volcanic center 13 5 km to the east

a supervised machine learning approach towards geochemical predictive modelling in archaeology abstract in this work data fusion of multi element xrf results from archaeological feature soils and regional background soils was applied to assess the complementary value of geochemistry and machine learning on predictive modelling in archaeology our principal aim was to integrate multiple data sources train learning models for classification of archaeological soils and background soils and compare model predictions for three validation areas with current archaeological interpretation and established predictive models this was done using three supervised machine learning algorithms k nearest neighbors support vector machines and artificial neural networks which were trained cross validated and tested the validation areas included a high archaeological potential area n 247 samples the dutch province of zeeland n 261 samples and an excavated imprint of an ancient farmhouse n 38 samples the predictive models showed good overall performance and correctly classified about 95 of all test instances according to the learning models the first validation site has a top soil horizon that shows limited parallels with archaeological horizons used in model training whereas features of high archaeological probability become more apparent below this horizon this is in good correspondence with geochemical depth profiles and current archaeological interpretation as for the second validation site the models highlighted several archaeological hotspots that to some extend spatially coincide with areas of high archaeological potential as indicated by established predictive modelling reversely the classifiers attributed high archaeological potential status to the most southern region of zeeland thereby complementing established modelling results for the third validation site none of the instances were correctly classified and these results clearly show the limitations of geochemical predictive modelling of significantly different soil types fine to coarse sands compared to the training set clayey sands present proof of concept study shows that modelling of multiple source geochemical soil data using machine learning algorithms can be successfully accomplished and that model predictions nicely complement current interpretation and or established archeological predictive modelling of areas of archaeological interest limitations of our approach were found to reside in lithological differences between sites used for model training and prediction sites

identification of activity areas by multi element characterization of sediments from modern and archaeological house floors using inductively coupled plasma atomic emission spectroscopy abstract a pilot study on the chemical analysis of earthen house floors using both modern and archaeological localities was undertaken to determine the degree to which activities leave detectable chemical residues and whether these residues are preserved in archaeological contexts the goals were to develop techniques for the identification and interpretation of activity areas in archaeological structures and to determine which elements serve to distinguish activities one modern structure from oaxaca mexico and two archaeological house floors from british columbia canada and oaxaca mexico were studied soil samples were collected from floors at all three sites and analysed by inductively coupled plasma atomic emission spectroscopy icp aes for al ba ca fe k mg mn na p sr ti and zn preliminary results show that activity areas in the modern house compound can readily be distinguished on the basis of chemical residues in soils that archaeological features are chemically distinct from natural soils and that features such as floors and hearths can be distinguished from each other and from the natural prehistoric ground surface multi element characterization by icp aes offers an advantage over traditional single element analysis by yielding more useful data for less effort expense and time

lithology and structure of the volcanic sequence in eastern iceland the reydarfjordur drill hole penetrated 1919 m of section with a recovery of 99 7 subaerial lava flows comprise 54 of the core dikes 41 and clastic rocks about 5 in addition a 1 3 km thick section of lava flows and clastic rocks exposed above the drill site was measured and sampled the lava flows are chiefly low magnesia basalts and ferrobasalts with lesser amounts of icelandite basaltic andesite and olivine tholeiite in the drill core complete flows range from 1 2 to 19 7 m in vertical thickness and average 7 1 m in the exposed section flow thicknesses range from 1 to 30 m and average 5 2 m most of the flows are characterized by an upper scoriaceous zone 1 3 meters thick grading downward through a zone of fractured basalt into a massive flow interior some flows also have a thin basal breccia olivine tholeiites and some low magnesia basalts are commonly mottled whereas ferrobasalts and icelandites have well developed flow banding most of the flows are fine to medium grained aphyric to sparsely phyric and moderately vesicular a few are moderately plagioclase phyric others have scattered phenocrysts of clinopyroxene and olivine over 100 intrusive units are recognized in the core and they span approximately the same compositional range as the flows they are characterized by having steeply inclined chilled contacts a uniform massive aspect and an absence of vesicles or brecciation most are less altered than the intruded lava flows and contain secondary minerals only along sparse high angle fractures many of the dikes are also aphyric but some contain notable plagioclase or olivine phenocrysts groundmass textures range from fine grained intersertal to coarse grained ophitic clastic units occur typically as thin varicolored layers between lava flows rarely exceeding 0 5 m in thickness these are chiefly fine to coarse grained poorly to well bedded lithic tuffs showing varying degrees of reworking most are silicic to intermediate in composition but basaltic units are also present two rhyolitic ash flow tuffs occur in the section one of which is 30 m thick we interpret the lavas to be the flanking portions of broad low angled central volcanoes centered on silicic complexes some of the flows may have erupted from central vents but most probably represent flank eruptions from rift zones represented by the dike swarms most of the dikes were probably intruded laterally into the lava pile thus they do not represent direct vertical feeders from deep magma bodies

on inclined features in the iceland research drilling project core precise dip and direction of dip measurements are reported for 148 dike contacts 126 bedding features in volcaniclastics and 30 planar upper surfaces of secondary mineral infillings in vesicles in flows from a 1919 m drill core from reydarfjordur eastern iceland the drill core extends from about 1 6 to 3 6 km beneath the original surface of the crustal section lava flow dips and directions of dips are reported for a 750 m exposed section situated directly above and in continuity with the drill core section while dips and within unit differences in dip direction appear to be accurately represented by the measurements absolute directions are only known inaccurately this is a consequence of the use of the paleomagnetic method of obtaining dip direction for which both high site latitude and tectonic rotation contribute to poorly constrained results the main results of the study are the commonplace irregular form of dike contacts such that the drill hole often entered and exited from the same side of a dike evidence for the presence of only one population of minor intrusions and the ability to recognize where several sections of intrusives in the drill core belong to the same dike the irregular form of the dikes prevents a reliable estimate of change of dilation with depth from being obtained it is clear however that a sheeted dike complex does not occur in the upper 4 km of the crust in the area measurement of bedding features in the volcaniclastics shows them to be considerably dispersed particularly in dip direction tectonic tilt in the crustal section increases from 11 at sea level to an extrapolated value of close to 17 at 4 km crustal depth corresponding lava deposition rates increase from 1620 to 2580 m m y respectively tectonic tilting was probably complete by the time of deposition of the last components of the secondary mineralization event in the vesicles of lava flows the optimum conditions for this type of tectonic investigation where measurements made on vertical drill core are used are high speed diamond drilling and original magnetization at middle to low but not equatorial latitudes

paleomagnetism of the samail ophiolite oman 1 the sheeted dike complex at ibra we have studied the rock magnetism and paleomagnetism of 200 oriented samples from the middle cretaceous sheeted dike sequence of the samail ophiolite oman the samples are from 28 sites spanning a cross strike distance of 30 km most of the dikes have undergone green schist metamorphism the natural remanent magnetization nrm of the samples is about 1 a m and qn is also near 1 thermal and alternating field demagnetization treatment resolves both normal north and reversed south polarities about one third of the samples have low median destructive fields mdf less than 100 oe 7 96 103 a turn m these low mdf samples have scattered directions and are classified as undetermined polarities u hysteresis measurements show pseudo single domain carriers predominate in all samples although the low mdf samples are suggested to have the largest grains in the samples studied thermomagnetic curves show both magnetite and minor maghemite thermal demagnetization and microscopic study reveals that hematite is abundant and is the carrier of the r remanence in addition several dikes exhibit both n and r polarities these facts coupled with the degree of metamorphism indicate that part of the remanence is secondary and could be due to hydrothermal metamorphism below the seafloor we suggest that the n polarity is primary and resides in exsolved magnetite while the r polarity is secondary and resides primarily in metamorphic origin hematite several short r events observed in the albian through cenomanian magnetostratigraphic record are candidates for the r event in the ophiolite if the r polarity originated near the spreading center alternately nonreproducible self reversal is another possible origin for the r polarity regardless of whether the reversal originated at the spreading center secondary magnetization in the dikes considerably increases the directional dispersion another major factor affecting dispersion is viscous remanent magnetization vrm in the larger magnetite grains this extreme dispersion disallows the samail ophiolite dike layer as a significant source for marine magnetic anomalies regardless of the strength of the nrm or the value of q the stable n and r directions yield a paleomagnetic pole which is far sided suggesting northward movement of the ophiolite however the paleomagnetic pole is not as far sided as the contemporaneous africa pole which suggests a component of north south closure between the ophiolite and africa prior to northward movement

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

the story in the floors chemical signatures of ancient and modern maya activities at aguateca guatemala abstract soil chemical analysis has been one of the most active and promising areas among the recent innovations in household archaeology ancient inhabitants unintentionally left chemical imprints of daily activities providing important clues as to past practices and space use which are difficult to judge from artifactual data alone soil chemical testing in the maya region is particularly promising given the highly calcareous nature of the soils derived from the carbonate geology of the region as calcium ions and soil alkalinity render phosphorous iron and other metallic ions insoluble this paper reports the results of soil chemical analysis of modern and ancient residential structures at the archaeological park of aguateca guatemala an ethnoarchaeological study of the guards living quarters and archaeological data from rapidly abandoned classic period residences provided opportunities to refine an understanding of the relationship between human activities and soil chemical signatures both cases exhibited good correlations of high phosphorous concentrations in soils with food processing consumption and disposal high levels of heavy metals in the modern structures probably derived from the filing of machetes and the disposal of flashlight batteries whereas the use of mineral pigments and craft activities appear to have contributed to the concentrations of these elements in archaeological contexts

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

a pedological investigation of an archaeological site in ontario canada ii use of chemical data to discriminate features of the benson site abstract soil samples from the benson site a former huron indian village 1600 s were collected on a 1 m grid at a depth of 15 cm over a portion of the village the benson site is located about 90 km northeast of toronto canada discriminant analysis statistically distinguishes between groups of cases by the employment of descriminating variables this mathematical technique made it possible to rank the chemical attributes of soil materials as to their usefulness in discriminating between huron settlement features such as middens pits longhouses hearths posts and former pathways two discriminant analyses were performed in this study one utilized all the features of the village and the other analysis did not include the chemical data from soils in the midden or pits the midden and some pits were exceedingly rich in most elements which overshadowed the amounts found in other features this tended to complicate the analysis of the data by statistical methods it was found however that soils of the former paths can be separated from those of pits posts and longhouses on exchangeable magnesium levels in addition organic and inorganic phosphorus levels between the soils of the paths pits and houses were statistically distinct dolomite exchangeable calcium and potassium levels are statistically different enough to distinguish the soils in paths or walking trails from those of posts exchangeable magnesium was the most useful chemical constituent of the soil material analyzed at the benson site in terms of the number of settlement features that could be distinguished by this information alone exchangeable magnesium is followed by organic phosphorus dolomite inorganic phosphorus and calcite exchangeable calcium potassium and ph and organic carbon in its usefulness in separating huron settlement features on the basis of chemical attributes of the soils

investigations on geomagnetic reversals in icelandic lavas 1953 78 recent reviews of the development of palaeomagnetic techniques in geology have shown that palaeomagnetism has contributed significantly to the acceptance of new concepts in global tectonics the present paper extends frankel s 1987 review of palaeomagnetic work on the basalt lava sequences of iceland in the early 1950s these lavas provide some of the most complete series of reliable spot readings of the geomagnetic field available it is shown that research on icelandic basalts has continued to be of importance in improving our understanding of palaeomagnetism and its applications

effects of temporal plume slab interaction on the partial melting of the subducted oceanic crust abstract the plume slab interaction facilitates the partial melting of the subducted oceanic crust in subduction zones and explains the pulse like adakite eruptions in the abukuma region northeastern japan from approximately 16 to 14 ma as well as the southwest to northeast migration of the adakite eruptions from southeastern china to southwestern japan during the cretaceous despite the potential implication of the plume slab interaction to arc magmatism the effects of the plume parameters on the partial melting of the subducted oceanic crust have not been quantitatively evaluated thus we ran two dimensional kinematic dynamic numerical subduction experiments to evaluate the effect of the duration and thickness of the injected plume blob into the mantle wedge on the slab surface temperature along with the consideration of the injection of the plume blob we consider diverse convergence rates and ages of the subducting slab with slab dips of 30 and 45 our model calculations show that the plume slab interaction is a promising process that increases the slab surface temperature up to around 60 c resulting in the partial melting of the subducted oceanic crust even though the plate age is 100 my million years increases in the duration and thickness of the plume blob positively increase the peak temperatures of the slab surface increases in the convergence rate and age of the subducting slab almost systematically increase and decrease the slab surface temperature respectively which indicates that the net temperature contribution of the plume slab interaction is similar regardless of the subduction parameters we found that the contribution of time evolving subduction parameters on the plume slab interaction can be approximated using the subduction parameters of approximately 5 my before the time of the partial melting of the subducted oceanic crust our model calculations facilitate the first order analysis of the plume slab interaction in the subduction zones of which subduction parameters are not consistent with the observed adakite

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

variable dispersion of neogene geomagnetic field directions in iceland abstract analysis of the primary paleomagnetic directions in 4000 lava flows in iceland reveals that the magnitude of total virtual geomagnetic pole dispersion has decreased during the last 15 ma this dispersion which is independent of the polarity of the field is in part caused by the occurrence of erratically varying paleofield directions during intermittent periods possibly as long as 100 ka

stresses and displacements in layered rocks induced by inclined cone sheets abstract currently the sheet intrusion paths and geometries including the sheet opening thickness as well as the depth to sheet tip are commonly determined from geodetic surface data using elastic dislocation models these models assume the volcanic zone central volcano to be an elastic half space of uniform mechanical properties field observations however show that volcanic zones volcanoes are composed of numerous layers whose mechanical properties primarily young s modulus vary widely here we provide new numerical models on the effects of a typical variation in young s modulus in an active volcanic zone central volcano on the internal and surface stresses and displacements induced by a sheet intrusion whose tip is arrested at a depth below the surface of 100 m the sheet has a dip dimension height of 2 km its opening thickness depends on the magmatic overpressure sheet dimension and host rock young s modulus for the values used here sheet thickness would be in the range of 0 5 1 4 m similar to commonly measured sheet thicknesses in the field the only loading is internal magmatic overpressure in the sheet of 5 mpa the modelled crustal segment volcano consists of 5 layers all with the same poisson s ratio 0 25 each of the 4 uppermost layers is 10 m thick layer 1 the top or surface layer has a young s modulus of 3 gpa layer 2 a modulus of 20 gpa layer 3 a modulus of 30 gpa and layer or unit 5 a modulus of 40 gpa we vary the young s modulus or stiffness of the fourth layer from 10 gpa to 0 01 gpa while the dip of the sheet takes the following values 30 45 60 for an inclined sheet and 90 for a dike the resulting displacement and stresses are highly asymmetric across the sheet tip except for the dike with the main surface stresses and displacements being above the dipping sheet and highest for the 30 dipping sheet for comparison three elastic half space models of the same sheet configuration and loading but uniform young s modulus in each model 40gpa 20gpa and 10 gpa all yield much higher surface stresses and displacements than any of the layered models as the stiffness of layer 4 decreases the surface stresses gradually decrease while changes in vertical displacements are comparatively small but greater in horizontal displacements in particular as the stiffness of layer 4 decreases from 10 gpa to 0 01 gpa for the 30 dipping sheet the maximum surface shear stress decreases from about 6 6 mpa to 2 2 mpa and the maximum tensile stress from about 6 9 mpa to about 2 3 mpa thus even a single comparatively thin 10 m soft layer close to the surface of a central volcano volcanic zone where such layers are almost universal may cause a great change in the maximum sheet induced stresses at the surface and thereby in any sheet induced fracture pattern furthermore the stress peaks in the layered models do not coincide with the displacement peaks fracture formation is most likely at the location of the stress peaks the results have important implications for the correct interpretation of geodetic data and fracturing during unrest periods with magma chamber rupture and sheet injection

along axis structure and crustal construction processes of spreading segments in iceland implications for magmatic rifts magmatic rift systems are composed of discrete spreading segments defined by morphologic structural and volcanic features that vary systematically along strike in iceland structural features mapped in the glaciated and exhumed miocene age upper crust correlate with analogous features in the seismically and volcanically active neovolcanic zone integrating information from both the active rift zones and ancient crust provides a three dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments crustal exposures in the skagi region of northern iceland reveal significant along strike variations in geologic structure the upper crust at exhumed magmatic centers segment centers is characterized by a variety of intrusive rocks high temperature hydrothermal alteration and geologic evidence for kilometer scale subsidence in contrast the upper crust along segment limbs which extend along strike from magmatic centers is characterized by thick sections of gently dipping lava flows cut by varying proportions of subvertical dikes this structure implies relatively minor upper crustal subsidence and lateral dike intrusion the differing modes of subsidence beneath segment centers and segment limbs require along axis mass redistribution in the underlying upper middle and lower crust during crustal construction this along axis material transport is accomplished through lateral dike intrusion in the upper crust and by along axis flow of magmatic to high temperature solid state gabbroic material in the middle and lower crust these processes inferred from outcrop evidence in skagi are consistent with processes inferred to be important during active rifting in iceland and at analogous magmatic oceanic and continental rifts

the iceland research drilling project crustal section variation of magnetic properties with depth in icelandic type oceanic crust an attempt has been made to identify the processes that give rise to a number of depth trends in the magnetization of a 3 1 km vertical section of icelandic type oceanic crust and to assess the possibility that similar processes act and depth trends occur in typical oceanic crust the depth trends in the icelandic section consist of a general increase in saturation and induced magnetization to 2 km crustal depth below which flow magnetization decreases while dike magnetization remains constant and of large changes in flow magnetization that occur on a scale of a few hundred metres below 3 km crustal depth increase in saturation and induced magnetization with depth in the upper 2 km is thought to be the result of two processes a decrease in low temperature oxidation from the original lava surface to 700 800 m crustal depth thence an increase in hydrothermal alteration with depth this interpretation is based on oxide petrography and curie temperatures which show a weakly defined minimum in the 700 8

formation of dykes feeder dykes and the intrusion of dykes from magma chambers the paper discussed the formation of dykes and applies the results to iceland it is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows it is suggested that dykes form in magma pulses where each dyke is split in two by the next magma pulse and so on in iceland the estimated time between successive magma pulses is of the order of several hundred days statistical considerations indicate that in iceland the probability of seeing dykes end upwards is only 1 2 which agrees with observations it is concluded that many and probably the majority of dykes are non feeders this together with the low probability of finding the connection between feeder and lava flow explains the scarcity of observed feeder dykes it is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in iceland is usually smaller than the tensile strength of the host rock several mpa which thereby is the critical factor in dyke intrusion

crustal drift in iceland summary there are indications that tertiary and post tertiary volcanism in iceland has been confined to the present main belt of active volcanism the geological structure of the tertiary lava piles in the east and west of iceland and results of geophysical studies may be explained by invoking an appreciable amount of crustal drift since the beginning of the tertiary the rocks in the extreme east and west of the island may have been carried apart by 400 km or more the crustal drift is believed to result mainly from crustal extension through the injection of dykes the structure of iceland and possibly that of other flood basalt areas is believed to be closely related to the world wide rift system

the mode of emplacement of neogene flood basalts in eastern iceland facies architecture and structure of the holmar and grjota olivine basalt groups abstract holmar and grjota are two stratigraphically distinct transitional alkaline olivine basalt lava groups within the westward dipping neogene flood basalts of eastern iceland the holmar olivine basalt group separated from the overlying grjota olivine basalt group by only a few tholeiite flows can be traced over 80 km north south with thicknesses varying from 250 m where thickest to 30 m where thinnest the grjota group can be traced over 50 km also north south reaching thicknesses of 250 m and thinning down dip to 10 m in contrast to other groups in eastern iceland that thicken down dip the studied olivine basalt groups thicken up dip the groups filled topographic confinements and formed aprons around central volcanoes we have estimated the minimum volumes to be 119 km 3 for holmar and 86 km 3 for grjota scoria cones are found in the holmar group and two thick olivine dolerite sills cross cut the holmar group and probably belong to the plumbing system that fed the grjota group the architecture of the lava groups are near identical the architecture is compound with lobes stacked horizontally and vertically varying from 1 15 m thick and 2 200 m long but do also encompass a number of thicker 15 20 m and more extensive 1 km long lava lobe in the stacks filled lava tubes are commonly observed within the lava flows the constituent lobes of the flows are often directly emplaced or welded together suggesting rapid buildup but are also found interbedded with redbeds and thicker tuff deposits and occasionally preserve tree molds the internal structure follows the characteristics for lava lobe morphology in general with an upper vesicular crust forming half to one third of the total thickness a massive core with abundant vesicle cylinders and a thin basal vesicular crust flow tops are of the pahoehoe type seldom found with scoria or clinker inflation structures such as tumuli and inflation clefts were identified in the flows the eruptions were dominantly fissure fed with a few vents evolving to short lived point source volcanism the pre existing topography exerted control on the advancement of the flows and influenced the final architecture of the groups at each location it is evident that the large and dense spatial distribution of these groups including numerous thick lava units suggests large volcanic episodes together the groups imply a period with higher production of olivine basalts in iceland and form well defined stratigraphic markers the transitional alkaline olivine basalt groups are interpreted to delineate a fossil southward propagating rift flank volcanic zone located east of a now extinct and buried mature rift zone

chemical stratigraphy iceland research drilling project reydarfjordur eastern iceland major element data are reported for lavas and dikes from a 3 2 km section through the eastern iceland lava pile the section includes a 1 2 km subaerial exposure and a 1 9 km sequence drilled by the iceland research drilling project irdp alteration increases down section although primary magma chemistry may be interpreted from feo al2o3 tio2 p2o5 and to a lesser extent sio2 in addition to immobile trace elements lava compositions range from mgo rich tholeiite to sio2 rich icelandite while dikes range from tholeiite to tholeiitic andesite three stratigraphic divisions based on volcanologic and trace element criteria e g ce yb and zr y are confirmed by major elements upper and lower groups are predominantly basaltic while the middle group includes a high proportion of silicic lavas variation within groups conforms to simple models combining fractional crystallization and accumulation of phenocryst plagioclase during build up of the pile eruptive phases commenced with silica rich magma and frequently terminated with basaltic or intermediate plagioclase cumulates dike chemistry resembles that of upper group lavas and also the nearby breiddalur volcano middle group lavas probably derive from a buried silicic complex while the lower group may represent the flank of a more deeply buried complex chemical stratigrapy is consistent with iterative burial with spreading of lenticular basaltic silicic complexes magma supply fractionation and mixing are considered in the framework of such kinematic models although it is premature to draw direct analogies to the active neogene zone of iceland

accretionary volcanic processes and the crystal structure of iceland abstract the geology of the quaternary axial zones in iceland is dominated by the occurrence of well defined volcanic centres such as krafla 65 40 n 16 40 w marked by the occurrence of a caldera overlying a high level magma chamber the lateral injection of magma from the chamber during active volcanism and crustal extension results in the emplacement of a dyke swarm occasional fissure eruptions and associated normal faulting comparable relations are observable in the older tertiary sections of the lava pile where well defined swarms of basic dykes are focused on tertiary volcanic centres some of which have high level plutonic complexes the gently dipping tertiary lava sequences are characterised by a dip at sea level which is notably greater than the dip of the lavas at higher elevations the features of both the quaternary and tertiary areas are interpreted in terms of a model for crustal generation at the axial zone involving fissure volcanism dyke emplacement and listric faulting rotation of successive lavas along the curved fault surfaces produces the changes in the dips of the flows with elevation observed in the tertiary areas and a broad synclinal structure centred about the spreading axis the hypothesis that listric faulting is important in relation to crustal growth in iceland links that extensional regime with continental rift environments where listric faulting has been recognised as the dominant structural element

precise dating of the holmatindur cooling event in eastern iceland evidence for mid miocene bipolar glaciation a succession of basaltic lavas volcaniclastic sediments and lignite beds in the holmatindur region of eastern iceland provides the means for wing magnetic reversal stratigraphy and a record of major north atlantic cooling to a precise radiometric timescale a prominent part of the section is the holmatindur clastic bed which is up to 66 m thick and can be traced 80 km along strike the geological significance of this unit lies in its composition and associated thin lignite seams the volcaniclastic composition is basaltic in contrast to the common rhyolitic dacitic composition of tuff units in the area and hyaloclastic material is dominant hydroexplosive volcanism most likely produced the hyaloclastite through subglacial eruptions within the paleovolcanic zone pollen assemblages from the lignite beds indicate a dramatic climatic deterioration from subtropical to cool temperate conditions through these sediments feldspar crystals separated from the holmatindur clastic bed were determined to be from 10 72 0 16 ma from 4oar 39ar incremental heating experiments we correlate the holmatindur cooling event with a relative maximum in benthic foraminiferal bso zone mi6 of miller et al 1991 which has been interpreted as an episode of ice accumulation within the mid miocene cooling period as well as with pulses in ice rafted debris supplied to deep water sites in the north atlantic ocean a global sea level drop of 70 m and submarine canyon cutting all of which occurred within the early part of magnetic anomaly 5 normal c5n while strong evidence exists for mid miocene ice sheets in antarctica northern hemisphere glaciation is thought to have started much later the iceland sites studied here demonstrate that significant ice accumulation occurred albeit intermittently as early as late middle miocene time in the north atlantic ocean from radiometric dating of tertiary lavas that lie below and above the well established lower boundary c5n we conclude that the onset of this long normal polarity period occurred at 10 94 0 16 ma the new data are compatible with the cande and kent 1995 interpolated estimate for this boundary 10 95 ma but are significantly older or more precise than previous direct dating by conventional k ar methods

the geoarchaeological significance and spatial variability of a range of physical and chemical soil properties from a former habitation site isle of skye of the 34 major and trace elements investigated in this study enrichments of k th rb and to a lesser extent cs were associated with former habitation of the area whilst high levels of sr and to a lesser extent ca were associated with the adjacent field area this latter enrichment is thought to represent additions of shell sand and or bone and fish refuse phosphorus is a less reliable indicator of human habitation than k rb and th possibly because p enrichment may be caused by additions of animal manure as well as by human habitation of an area vanadium co zn pb ni ga sc mg cu and u all indicated negligible on site enrichment whilst cr and organic matter content were found to be depleted in the on site soils relative to the control samples consideration of the degree of variability in the soil parameters investigated has implications for the bulk or composite sampling of soils over sites of former habitation

generation of tectonic over pressure inside subducting oceanic lithosphere involving phase loop of olivine wadsleyite transition abstract we conducted a two dimensional numerical model to analyze the generation of tectonic over pressure which is a positive deviation from lithostatic pressure for deep slabs which are anchored at the 660 km phase boundary the formation of the ductile shear zone under a compressional tectonic setting induces tectonic over pressure we first propose that an apparent shear zone originated from an elastic heterogeneity in the phase loop which is the two phase i e olivine and wadsleyite coexistence interval around the 410 km boundary within subducting oceanic lithospheres can cause tectonic over pressure with a range from 0 3 to 1 5 gpa this over pressure significantly impacts the formation of the olivine wadsleyite phase transition the flattening of the olivine wadsleyite interface by over pressure is well resolved therefore we argue that the over pressure should be applied when analyzing the phase boundary within the subducting lithosphere our results provide a new insight on the interplay among the phase transition shear zone formation and tectonic over pressure

migration of igneous activities related to ridge subduction in southwest japan and the east asian continental margin from the mesozoic to the paleogene abstract in the inner zone of southwest japan the northern part of the median tectonic line mtl granites and their volcanic equivalents were formed in large quantities during the late cretaceous to early tertiary migration of magmatic activity related to ridge subduction is proposed to explain the widely distributed magmatism and its tectonics for this purpose the ages of granitic rocks were carefully inspected according to the criteria of the dating methods and procedures concerned and compiled with the sampling localities they clearly show a trend in rock ages along the mtl in agreement with the general direction of strikes in southwest japan from the trend of the ages and the cooling histories of the bodies it was concluded that the magmatism migrated eastward along the mtl with a rate of 30 km m y this migration concept is extended to the mesozoic igneous rocks of the east asian continental margin there two magmatic belts ranging in age from mesozoic to paleogene are found to become younger northeastward along the margins which is ascribed to the subduction of the farallon izanagi and kula pacific ridges

distributed deformation in an oceanic transform system applying statistical tools to structural and paleomagnetic data near the husavik flatey fault northern iceland

paleogeographic and paleotectonic evolution of a basement controlled archean supracrustal sequence chibougamau caopatina quebec the 7 14 km thick archean volcano sedimentary sequence in the chibougamau caopatina region ccr composed of two mafic to felsic cycles and two thick essentially epiclastic successions is divided into a southern caopatina segment scs and northern chibougamau segment ncs the sedimentary units which evolved after cycle 1 volcanic rocks in the scs and after the combined cycles 1 and 2 in the ncs document a diachronous sedimentary basin evolution this criterion has been employed to distinguish an early evolutionary stage in the south and a late evolutionary stage in the north in each segment the basin episode represents a unique extensional event volcanism preceded the major sedimentary rifting event in both segments composition of sediments and sedimentary facies correlated with the interpreted basin margin faults support the interpretation of two phases of crustal extension volcanic construction and sill intrusion were the prevalent factors governing emergence of volcanic islands in the cc

the mode of emplacement of neogene flood basalts in eastern iceland facies architecture and structure of simple aphyric basalt groups simple flows tabular in the neogene flood basalt sections of iceland are described and their mode of emplacement assessed the flows belong to three aphyric basalt groups the kumlafell group the holmatindur group and the hjalmadalur group the groups can be traced over 50 km and originate in the brei dalur thingmuli volcanic zone the groups have flow fields that display mixed volcanic facies architecture and can be classified after dominating type morphology the kumlafell and the holmatindur groups have predominantly simple flows of p hoehoe and rubbly p hoehoe morphologies with minor compound or lobate p hoehoe flows the hjalmadalur group has simple flows of rubbly p hoehoe but also includes minor compound or lobate flows of rubble and a simple flows are most common in the distal and medial areas from the vents while more lobate flows in proximal areas the simple flows are formed by extensive sheet lobes that are several kilometers long with plane parallel contacts some reaching thicknesses of 40 m aspect ratios 0 01 they have overlapping contacts and are free of tubes and inflation structures their internal structure consists generally of a simple upper vesicular crust a dense core and a thin basal vesicular zone the brecciated flow top is formed by clinker and crustal rubble the clinker often welded or agglutinated the simple flows erupted from seemingly short lived fissures and have the characteristics of cooling limited flows we estimate the effusion rates to be 105 m3 s for the simple flows of the kumlafell and holmatindur groups and 104 m3 s for the hjalmadalur group the longest flows advanced 15 20 km from the fissures with lava streams of fast propagating flows inducing tearing and brecciation of the chilled crust compound or lobate areas appear to reflect areas of low effusion rates or the interaction of the lava with topographic barriers or wetlands resulting in chaotic flowage slowing lobes with brecciated flow tops developed into a flows the groups interdigitated with lava groups from the rey arfjor ur volcanic zone to the east and the exhumed brei dalur thingmuli volcanic zone which appear to have formed as a flank lineament parallel the main rift zone flood basalt volcanism in flank areas may support a mantle anomaly more pronounced and or perhaps more widespread in the neogene of iceland than today eruptions of simple flows have not been observed in modern times and are significant for models of crustal accretion in iceland and other large igneous provinces

the trace element composition of the lavas and dikes from a 3 km vertical section through the lava pile of eastern iceland the tertiary lava succession of eastern iceland dips at shallow angles towards and beneath the axial quaternary volcanic zone a 3 km vertical section through part of this tertiary pile studied during the iceland research drilling project contains flows ranging in composition from basalt to icelandite this succession can be divided into three distinct stratigraphic groups upper and lower groups 1 7 and 0 6 km thick with zr y ratios of about 4 5 and a 0 7 km thick middle group with a zr y ratio of about 6 5 the middle group also has a significantly higher ce yb ratio and higher elemental sr abundances the data on within group variation are equally compatible with high level fractionation of a plagioclase olivine clinopyroxene assemblage with accessory apatite and opaque minerals or with open system fractionation models dikes occur throughout the 3 km section and are comparable in trace element composition to the lavas of the upper group the dikes are part of a n s trending swarm originating from the breiddalur volcanic center some 15 km to the south these dikes were injected laterally northward into the reydarfjordur lava pile at the time of the eruption of the upper group lavas the lower two major lava groups may be related to similar but unidentified discrete volcanic centers perhaps buried downdip the new data on this deeper section corroborate earlier studies which indicated that on a gross scale the eastern iceland lava pile becomes less mafic with depth this vertical change occurs because each major volcanic center erupted more silicic lavas close to the center and more magnesium rich compositions toward the periphery the formation of the lava pile occurred by tilting toward the spreading center and overlapping repetition of such major units which resulted in the more mafic edges of the lava groups forming the topographically higher parts of the crustal section

the distribution of gold in volcanic rocks of eastern iceland abstract gold contents are reported for basaltic lavas and dykes from the tertiary succession of eastern iceland in unsaturated olivine normative tholeiites au decreases in concentration with degree of differentiation however in saturated quartz normative and more differentiated varieties au shows a high degree of positive correlation with incompatible elements such as y and zr this behaviour suggests that in the latter rocks au is not held in early crystallized phases but is enriched in the residual melt so that it follows the trend of the incompatible elements thus basaltic andesites and silica rich icelandites contain twice as much au x 4 1 ng g 1 as basalts this general trend appears to follow an erratic but concomitant enrichment of s in the rocks oxidation and zeolite alteration have led to mobilization and to enrichment of au in volcaniclastic units and in the porous top of a lava flow it is suggested that in older volcanic piles of icelandic type the most suitable source rocks for au mineralization could be in the more evolved quartz normative tholeiites ferrobasalts and basaltic andesites and icelandites particularly dykes and volcaniclastic units loosely held au is readily mobilized from volcanic glass and sulfides due to weathering burial metamorphism and hydrothermal alteration therefore only the freshest glassy rocks should be expected to retain their primary au concentrations

palynological evidence for miocene climatic cooling in eastern iceland about 9 8 myr ago the processes which led to the pleistocene glaciations in the northern hemisphere are presently known only from fragmentary evidence of major tertiary climatic cooling events1 2 although there is good evidence3 for glacial intervals in alaska 10 9 myr most terrestrial records4 5 which suggest a corresponding high latitude climatic cooling in the north east atlantic are poorly dated and north atlantic palaeoceanographic records contain large erosional hiatuses2 6 we have obtained new palynological data from eastern iceland from clastic units between lava formations dated by k ar and palaeomagnetic methods7 8 we now report that these data record a floral succession which suggests that a temperature decrease of 10 c occurred in iceland between 10 and 9 5 myr

coherent intrusion complexes in large basaltic volcanoes a new structural model abstract highly concentrated coherent intrusion complexes consisting of thousands of small mafic intrusions occur in probably all major basaltic volcanoes and play an important role in volcano development magma excursions from the high level chamber travel laterally along a surface of neutral buoyancy at the margin of a complex and cause the complex to grow the limited distance however that narrow intrusions can propagate before becoming blocked causes complexes to be wedge like intrusive dike complexes underlie rift zones and asymmetric growth of the dike wedge causes rift zones of shield volcanoes to become non collinear and may initiate a third rift zone in the obtuse angle downbowing of stress trajectories across complexes causes dikes to be non vertical and results in axial subsidence intrusive sheet complexes form instead of dike complexes in volcanic systems that have a restricted ability to expand laterally and accommodate intrusions by expanding vertically instead downbowing of stress trajectories causes sheets to be non horizontal and this combined with subsidence increasing toward the thicker part of the sheet wedge produces the inward and inwardly increasing dip that characterizes cone sheet complexes this mechanism for cone sheets differs considerably from previously proposed mechanisms successive injections of sheets at the top of a sheet complex probably offers the most efficient means of powering a high intensity geothermal field such as the 5000 mw grimsvotn system in iceland it is inferred that similar mechanisms to those in major basaltic edifices operate in spreading ridges study of basaltic edifices has the potential to contribute significantly to the understanding of spreading ridges

small scale convection under the back arc occurring in the low viscosity wedge abstract water released from subducting slabs through a dehydration reaction may lower the viscosity of the mantle significantly thus we may expect a low viscosity wedge lvw above the subducting slabs the lvw coupled with a large scale flow induced by the subducting slabs may allow the existence of roll like small scale convection whose axis is normal to the strike of the plate boundary such a roll structure may explain the origin of along arc variations of mantle temperature proposed recently in northeast japan we study this possibility using both 2d and 3d models with without pressure and temperature dependent viscosity 2d models without pressure and temperature dependence of viscosity show that with a reasonable geometry of the lvw and subduction speed small scale convection is likely to occur when the viscosity of the lvw is less than 10 19 pa s corresponding 3d model studies reveal that the wavelength of rolls depends on the depth of the lvw the inclusion of temperature dependent viscosity requires the existence of further low viscosity in the lvw since temperature dependence suppresses the instability of the cold thermal boundary layer pressure i e depth dependence coupled with temperature dependence of the viscosity promotes short wavelength instabilities the model which shows a relatively moderate viscosity decrease in the lvw most of the lvw viscosity is 10 18 10 19 pa s and a wavelength of roll 80 km has a rather small activation energy and volume 130 kj mol and 4 cm 3 mol of the viscosity this small activation energy and volume may be possible if we regard them as an effective viscosity of non linear rheology

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

magnetic properties of hematite in lava flows from iceland response to hydrothermal alteration the present study investigates changes in the magnetic properties of hematite more precisely low ti titanohematite in icelandic subaerial basalts during low temperature metamorphism 300 c the results have implications for detection of secondary magnetization directions in basalts and perhaps for seafloor magnetic anomaly modeling samples were collected both from outcrops and from the iceland research drilling project irdp core and represent 3 6 km of crust the natural remanent magnetization nrm unblocked below 590 c during thermal demagnetization was assigned to magnetite and that unblocked above 610 c to hematite unblocking temperatures of hematite gradually increased in the laumontite zeolite zone from the original low range of 610 650 c to 650 680 c we attribute this increase to the migration of a small amount of initial ti out of the hematite lattice titanium migration is consistent with microprobe analyses the intensity of the hematite magnetization correlated with that of magnetite until the top of the laumontite zeolite zone 1 8 km depth below this depth the intensity of the magnetite magnetization decreased and correlation became poor due to alteration of magnetite to nonmagnetic minerals both magnetite and hematite appeared to retain their primary directions of magnetization to the bottom of the laumontite zeolite zone even though ti had migrated out of the lattices if field controlled chemical remanent magnetization is not produced during ti migration then seafloor basalts undergoing the same processes may also faithfully retain primary directions of remanence below 3 1 km depth in the epidote metamorphic zone large secondary components in some samples could not be completely removed by thermal demagnetization to 550 c and were attributable to a thermo viscous remanent magnetization thermo vrm in magnetite the durability of the hematite magnetization suggests that the identification of hematite components in subaerial basalts may help reveal subtle secondary components contained in magnetite directions of subaerial basalts