melt viscosity temperature and transport processes troodos ophiolite cyprus abstract the lava section in the troodos ophiolite cyprus is chemically stratified and divided into a shallow lava sequence with low tio 2 content and a deeper lava sequence with high tio 2 content we calculate the viscosity at magmatic temperature based on major element chemistry of lavas in cyprus crustal study project ccsp holes cy 1 and 1a we find that typical shallow low ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high ti lavas this implies that after eruption on axis troodos low ti lavas would have been able to flow down the same slope faster and farther than high ti lavas the calculated lava viscosity increases systematically from the lava sediment interface to the bottom of the composite hole cy 1 1a this suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo troodos rise may have been responsible for the chemical stratification in the troodos lava pile calculated magmatic temperature and molar mg mg fe or mg decrease systematically down section while sio 2 content increases correlation of mg in the lavas with mg in the underlying lower crustal plutonic rocks sampled by ccsp hole cy 4 shows that the shallow lavas came from a high temperature lower crustal magma reservoir which is now represented by high mg pyroxenite cumulates while the deeper lavas were erupted from a lower temperature mid crustal reservoir which is now represented by gabbroic cumulates with lower mg

constructional features of the troodos ophiolite and implications for the distribution of orebodies and the generation of oceanic crust the map pattern of the troodos ophiolite in cyprus is an expression of a simple elongate anticlinal structure the cross sectional form of the anticline can be described by a sinusoidal dip model on the northern flank of the anticline the thickness of various units number of gossans etc were measured along profiles at 2 5 km intervals normal to strike using existing maps supplemented by new observations profiles are approximately perpendicular to the spreading direction during crustal construction and uncertainties in unit thickness are smaller than the range of unit thicknesses data on the profiles were analyzed quantitatively by harmonic analysis using the fourier transform method the harmonic analysis discloses cyclic variation in the thickness of major units number of gossans etc that provides a link with the regular spacing of five groups of cu fe zn orebodies groups of orebodies occur preferentially where sheeted dikes are at a shallow crustal depth sheet and massive flows and dike

multiple tectonic rotations and transform tectonism in an intraoceanic suture zone sw cyprus abstract new palaeomagnetic data from the late cretaceous neotethyan troodos ophiolite and related units in sw cyprus provide compelling evidence that transform tectonism is recorded outside the main outcrop of the fossil southern troodos transform fault zone sttfz high level intrusive and extrusive sequences were sampled along the sw margin of the troodos massif and in ophiolitic slivers preserved along arcuate fault lineaments further to the south and west these units show cross cutting relationships with corresponding differences in remanent magnetisation directions characteristic of syn magmatic rotation in an active transform setting net tectonic rotation analyses allow decomposition of the total rotation at these sites into early and late components early transform related rotations are consistently clockwise in agreement with all other studies of rotations associated with the sttfz further to the east late rotations and the net rotations derived at localities where cross cutting relationships were not present are regarded as composite in origin the composite rotation axes closely relate to the orientation of observed regional structures in these circumstances additional constraints are used to attempt geologically viable interpretations the overall distribution of transform related rotations steeply and shallowly plunging rotation axes and spreading axis related rotations sub horizontal rotation axes is comparable with that along the sttfz and adjacent parts of the troodos massif this is most readily explained by simple along strike extension of the primary spreading axis and transform fabrics into sw cyprus with minor late disruption by high angle dextral shear zones and neotectonic graben systems

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

the notch peak contact metamorphic aureole utah petrology of the big horse limestone member of the orr formation the upper cambrian big horse limestone member of the orr formation is contact metamorphosed by the jurassic notch peak quartz monzonite in the central portion of the house range utah two lithologic types were sampled from specific stratigraphic horizons within the member over a lateral distance of 6 km at locations reflecting a range in metamorphic grades the rocks were metamorphosed at 2 kbar on the basis of estimates of stratigraphic overburden at the time of intrusion the lithologies are relatively pure dolomitic limestones and impure argillaceous limestones argillites the progressive metamorphism has resulted in the successive appearance of talc tremolite scapolite diopside and forsterite in the meta limestones and the appearance of biotite tremolite diopside plagioclase scapolite vesuvianite grossular and wollastonite in the argillites mineral assemblages of the limestones compared with isobaric 2 kbar phase equilibria in the cao mgo sio 2 h 2 o co 2 system suggest that 1 fluid buffering by metamorphic reactions has occurred 2 domains of equilibrium are small and 3 the limestones behaved as relatively closed systems during metamorphism maximum temperatures and x co 2 composition consistent with assemblages in the limestone are see pdf for table temperatures estimated by the calcite dolomite geothermometer are consistent with these temperature estimates temperatures estimated from limestone mineral assemblages are used to establish the sequence of prograde reactions in the multi component argillite system compositions of fluids in equilibrium with low grade argillites are poorly constrained but may have reached a maximum x co 2 of 0 75 fluid compositions in equilibrium with medium to high grade argillites were more h 2 o rich than x co 2 0 20 indicated primarily by the presence of wollastonite at temperatures between 475 and 600 c therefore the argillites were either initially more water rich than limestones at the same grade or they were more open to h 2 o rich fluids during metamorphism

chlorine 36 fallout in the summit greenland ice core project ice core the natural cosmogenic radioisotope 36cl has been measured in the greenland ice core project ice core from summit greenland generally a good correlation between 36cl and 18o was found even in glacial periods with strong 18o oscillations the 36cl flux is calculated assuming an exponential relationship between accumulation rate and 18o a prominent excursion is detected between 35 and 40 kyr b p which coincides with a similar 10be peak measured in the same samples this sheds light on possible causes of this excursion in the lowest part of the ice core the flux decreases smoothly this can be interpreted as radioactive decay of 36cl t1 2 300 kyr

carboniferous submarine volcanism at herbornseelbach lahn dill area germany a 100 m thick sequence of carboniferous submarine dominantly basaltic volcanic rocks at herbornseelbach lahn dill area is subdivided into six units two pillow volcanoes show a characteristic change from basal megapillows up to 5 m across through normal pillows 0 5 to 0 8 m across to a top facies of small pillows 0 5 m across and or breccias of scoriaceous clasts and discontinuous lava stringers sheet flows 0 5 to 4 m thick can be distinguished from sills by asymmetric cooling structures and textures vesicle concentrations near the top basal flow wrinkles and ponding of sediments on irregular flow tops

mid ocean ridge or marginal basin origin of the east taiwan ophiolite chemical and isotopic evidence the east taiwan ophiolite eto occurs as allochthonous fragments embedded in the plio pleistocene lichi melange in the coastal range of eastern taiwan it is probably the youngest among the known ophiolitic complexes of the world the overall geochemical and isotopic characteristics of the representative rock types are very oceanic and strongly argue for a mid ocean or marginal basin origin of the eto the best criteria include 1 major element compositions 2 ree distribution patterns 3 nd isotopic ratios some have nd 12 4 pb isotopic ratios and 5 the presence and nature of the red shale this conclusion is supported by previous studies of metamorphic evolution of the eto

electrical conductivity measurements from the gisp2 and grip greenland ice cores the direct current electrical conductivity of glacial ice depends on its acidity1 3 and can also indicate changes in climate as ice formed in cold dusty periods has a high concentration of alkaline dust1 4 5 which significantly reduces the conductivity6 7 compared to warmer less dusty periods here we present electrical conductivity records for the greenland ice sheet project 2 gisp2 and greenland ice core project grip ice cores drilled 28 km apart to enable direct comparison of the results the upper parts of both records are consistent with previous evidence from other greenland cores4 8 12 for a stable greenland climate during the holocene and a series of warm events punctuating the last glacial period however there is a significant discrepancy between the two records in the bottom 10 of the cores calling into question recent reports of climate variability in the last interglacial4 8 and the penultimate glaciation8 at this stage it is too early to say what exactly is causing the discrepancy although ice flow may have introduced some discontinuities into the records further work will be necessary to establish how much climatic information it will eventually be possible to extract from the lower parts of the two cores

large variations in holocene solar activity constraints from 10be in the greenland ice core project ice core 1 cosmogenic radionuclides extracted from ice cores hold a unique potential for reconstructing past solar activity changes beyond the direct instrumental period taking the geomagnetic modulation into account the solar activity in terms of the heliospheric modulation function can quantitatively be reconstructed in high resolution throughout the holocene for this period our results reveal changes in heliospheric modulation of galactic cosmic rays significantly larger than the variations reconstructed on the basis of neutron monitor measurements of galactic cosmic rays for the last 50 years moreover the 10be data from the greenland ice core project ice core as well as 14c support a high current solar activity however although the reconstruction of solar activity on long timescales is difficult our result suggests that the modern activity state of the sun is not that exceptional regarding the entire holocene this extended solar activity record provides the basis for further detailed investigations on solar and cosmic ray physics as well as on solar forcing of the earth s climate whose importance is suggested by increasing paleoclimatic evidences

boron isotope systematics of a fossil hydrothermal system from the troodos ophiolite cyprus water rock interactions in the oceanic crust and subseafloor ore deposits abstract we determined concentrations and isotopic compositions of boron in a complete section of the hydrothermally altered cretaceous oceanic crust of the troodos ophiolite the boron content and 11 b value for each lithological section are pillow lava 3 8 206 8 g g 63 g g average 11 b 0 17 to 15 6 8 1 average sheeted dike complex 0 6 18 0 g g 4 0 g g average 11 b 3 3 to 10 6 6 0 average and plutonic complex 0 3 8 4 g g 1 7 g g average 11 b 1 7 to 18 5 4 5 average these boron contents are higher than the estimated original igneous values throughout the oceanic crust indicating uptake of boron from seawater and hydrothermal fluid at temperatures ranging from 50 c to 300 c although our boron data for the troodos ophiolite are generally consistent with those for the oman ophiolite of similar age the distinctly low 11 b values of the lower gabbro section in the troodos ophiolite 11 b depleted fluid at a very small water rock ratio the boron content of the bulk oceanic crust 12 3 g g estimated for the troodos ophiolite is relatively high as a result of strong boron enrichment in the pillow lava section which underwent prolonged seafloor weathering despite these differences the weighted average 11 b value of the bulk oceanic crust 7 6 is similar to that of the oman ophiolite 7 9 we also analyzed the boron isotope geochemistry of a subseafloor hydrothermal stockwork sulfide deposit in the troodos ophiolite to investigate its formation processes in contrast to the normal upper oceanic crust the 11 b values of the rocks below the ore body decrease with increasing depth and have large negative values 6 in the highly altered uppermost dike section these low 11 b values are coupled with high boron contents 2 5 17 g g and high and uniform 87 sr 86 sr ratios 0 7064 average and are unlikely to have resulted from interactions with fluids at a small water rock ratio these characteristics are better explained by interaction of ore forming hydrothermal fluids with oceanic crust that had previously been enriched in boron through hydrothermal alteration at low temperatures these observations demonstrate that boron and boron isotopes are useful for quantitative evaluation of fluid related processes with multiple stages including petrogenesis of hydrothermal ore deposits

paleomagnetic and petrographic investigation of the taum sauk limestone southeast missouri paleomagnetic results from 17 sites in one quarry of the cambrian dresbachian stage taum sauk limestone member bonneterre formation reveal two components of magnetization on the basis of thermal demagnetization coercivity spectrum analysis and petrographic evidence both components reside in hematite sites can be divided into three groups based on behavior during thermal demagnetization and petrographic criteria the first group consists of massive pink limestones and contains a component with declination 270 inclination 11 corresponding to a cambrian pole at 175 6 e 3 6 s the second group of sampling sites consists of specimens with numerous burrows filled with hematite intergrown with calcite the sites are dominated by the second component of magnetization which has declination 147 and inclination 10 and corresponds to a reversed pole position at 133 1 e 37 3 n a significant fraction of this component which is interpreted to be carboniferous in age resides in hematite intergrown with calcite in the burrows petrographic relationships indicate that the calcite and hematite were acquired before dolomite which places a maximum age on dolomitization the third group consists of massive pink limestones with variable amounts of burrowing and contains both directions of magnetization in roughly equal proportions with closely overlapping blocking temperature spectra the presence of two stable directions of magnetization from one location with neither being dominant or even present throughout the quarry indicates the significant effect that diagenetic and sedimentologic processes may have on the magnetization of sedimentary units

a better radiocarbon clock reliable radiocarbon dating is essential in many fields from geology and archaeology to geochemistry and paleoclimatology but because the atmosphericc c ratio has changed over time obtaining accurate dating is difficult in their perspective bard et al discuss efforts to improve the accuracy of radiocarbon dating presented at a recent conference in new zealand stratigraphic results from ocean sediments off the coast of venezuela reported by hughen et al were presented at this meeting and are put into context by bard et al taken together these and other findings give hope that reliable radiocarbon calibration curves will be ready for the next such meeting in 2006

the hozameen fault system and related coquihalla serpentine belt of southwestern british columbia the hozameen fault of southwestern british columbia is associated with the coquihalla serpentine belt and separates two distinct crustal units northeast of the fault are greenstones of the early t

chlorine 36 evidence for the mono lake event in the summit grip ice core abstract a distinct peak has been discovered in the 36 cl data from the grip ice core between the dansgaard oeschger d o events 6 and 7 at approximately 32 kyr bp this peak can be attributed to a minimum of the geomagnetic dipole field associated with the mono lake event since the 36 cl peak reflects a higher production rate of all cosmogenic radionuclides it has an impact on the 14 c dating of the last ice age furthermore it provides an additional time marker similar to a peak found earlier corresponding to the laschamp event at approximately 39 kyr bp

beryllium 10 in the greenland ice core project ice core at summit greenland concentrations of the cosmogenic isotope 10be have been measured in more than 1350 samples from the greenland ice core project grip ice core drilled at summit greenland although a dust associated component of 10be retained by 0 45 m filters in some of the samples complicates the interpretations the results confirm that the first order origin of 10be concentration variations is changes in precipitation rate associated with different climate regimes this effect is seen not only between glacial and interglacial periods but also during the shorter dansgaard oeschger interstadials by contrast the 10be data do not support the interpretation of rapidly varying accumulation i e climate during the last interglacial they can however be used to help place limits on the origin of the ice in these events after taking into account variable snow accumulation effects variations in the 10be flux are observed probably caused by solar and geomagnetic modulation but possibly also by primary cosmic ray variations the most dramatic is a 10be peak 40 000 years ago similar to that found in the vostok ice core thus permitting a very precise correlation between climate records from arctic and antarctic ice cores the 36cl 10be ratio considering either total or only ice associated 10be shows significant variability over the whole core depth thus confirming the difficulty in using this parameter for dating ice cores

the genesis of refractory melts in the formation of oceanic crust refractory primary liquids arising in various oceanic plate tectonic settings are characterized by high mgo sio2 ca na low tio2 and generally low incompatible element abundances relative to primary liquids parental to morb we propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to morb a compositional range in the second stage melts is expected depending on the extent of previous depletion of the peridotite the temperature and pressure of melt segregation and the possible influence of volatile phases c h o present during melting

fine scale seismic structure of young upper crust at 17 20 s on the fast spreading east pacific rise 1 the detailed upper crustal structure of the east pacific rise epr at 17 20 s is examined by applying a genetic algorithm based waveform inversion to five multichannel seismic lines one on axis and two to either side along 42 and 85 kyr old crust on axis a double stepped velocity pattern is recorded beneath 70 100 m of low velocity extrusives 2 1 2 4 km s 1 we define the upper velocity contrast as the base of seismic layer 2a due to its severity and continuity along and across axis the more subdued and intermittent lower velocity step is not observed off axis material between the two high gradient intervals is proposed to represent the pillow dike transition bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and or deformation extrusive velocities increase quite rapidly in this area with velocities 3 km s 1 common in crust 85 kyr old this plus a rapid 300 400 m thickening of layer 2a observed within 1 4 km of the rise axis indicates that this segment is undergoing focused melt delivery 500 m wide dike intrusion zone and elevated hydrothermal activity these findings demonstrate the ability of single ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information

enhanced cosmic ray production of 10be coincident with the mono lake and laschamp geomagnetic excursions the cosmogenic isotope 10 be total be and al were measured in partly varved sediments from the upper 50 m of core 480 leg 64 dsdp gulf of california the concentration of 10 be from 1 to 50 kyr is in general agreement with estimates of the geomagnetic dipole moment obtained from archaeomagnetic and marine core research 10 be anomalies were also found at 32 kyr and 43 kyr contemporaneous with the mono lake and laschamp excursions respectively the production of 10 be required to explain these anomalies is too high particularly for the mono lake excursion to be produced by a combination of decreased geomagnetic field and unprecedented long term solar activity we conclude that the cause is a change in the galactic cosmic ray flux consistent with a supernova event the coincidence with the two excursions remains a paradox

the late cretaceous palaeolatitude of the neotethyan spreading axis in the eastern mediterranean region abstract a compilation of available palaeomagnetic data from the troodos cyprus and baer bassit syria ophiolitic terranes of the eastern mediterranean tethyan orogenic belt is presented the ophiolites represent fragments of oceanic lithosphere generated at a neotethyan spreading axis in the late cretaceous although debate continues over the tectonic setting of this spreading axis and its position within the eastern mediterranean palaeogeography two types of model reconstructions have been proposed type 1 the ophiolites formed in a southerly neotethyan basin by spreading above an oceanic subduction zone the baer bassit ophiolite was then emplaced a relatively short distance tens of kilometers southwards on to the arabian continental margin leaving the troodos ophiolite isolated in an intra oceanic setting to the west and type 2 the ophiolites formed in a northerly neotethyan basin by spreading at a normal oceanic ridge with subsequent large scale thrusting hundreds of kilometers to the south of emplaced ophiolites over microcontinental fragments to reach their present positions palaeomagnetic determination of the palaeolatitude of the neotethyan spreading axis is therefore of considerable interest previous palaeomagnetic analyses have demonstrated the presence of significant and in some cases extreme relative tectonic rotations of a variety of origins in both ophiolites to allow palaeomagnetic data from these rotated units to be combined an inclination only formulation of the palaeomagnetic tilt test is employed this provides unequivocal evidence that both ophiolites retain pre deformational remanent magnetizations which are interpreted as original ocean floor magnetizations acquired close to the time of crustal formation in the late cretaceous the mean inclinations of 37 0 2 6 for the troodos terrane and 41 1 3 4 for the baer bassit terrane indicate respective palaeolatitudes for the spreading axes of 20 6 n 1 8 and 23 6 n 2 5 consistent with a late cretaceous position between the arabian and eurasian margins these data together with a well defined palaeolatitude of 25 5 n 4 5 for the eastern pontides previously reported in the literature provide constraints which must be incorporated in any successful tectonic reconstruction of the eastern mediterranean tethys the implications of these constraints for type 1 and 2 models are discussed using a series of plate tectonic cross sections constructed along a line extending northwards from the arabian continental margin in the absence of palaeomagnetic data from late cretaceous rocks of the eastern taurides however it is presently impossible to use these palaeolatitudinal constraints to resolve the root zone debate on a purely palaeomagnetic basis solutions which satisfy the constraints may be found for both types of model reconstruction additional published field based geological considerations however strongly support models in which the troodos and baer bassit and other southerly ophiolites were generated in a southern neotethyan basin rather than those involving generation in a northerly basin and subsequent large scale thrust displacement to the south

geochemistry and tectonic environment of basaltic rocks from the misis ophiolitic melange south turkey abstract the miocene misis tectonosedimentary melange was formed during the closing stages of the neotethys in what is now the adana basin of southern turkey a major part is characterized by large disorientated limestone blocks set in a predominantly volcanogenic matrix of bedded and chaotic units comprising basaltic pillow lavas associated hyaloclastites serpentinites and various acidic tuffs all the basaltic rocks are mildly metamorphosed zeolite facies variably vesicular and differentially altered to smectite chlorite fe oxides zeolites and carbonate petrographic and chemical data on the least altered pillow lavas indicate they were originally sparsely olivine plagioclase clinopyroxene phyric tholeiites and minor amphibole phyric basaltic andesites the basaltic clasts in the melange matrix are genetically related to the pillow lava sequences with the observed variation throughout the suite being governed by low pressure fractional crystallization as a group the basalts are characterized by low zr y 2 4 relatively high th yb 0 5 0 7 and la nb 1 5 2 5 ratios and moderate chondrite normalized light ree enrichment la yb n 3 patterns the basalts have a mildly enriched subduction related chemistry and were possibly formed initially in a back arc basin setting the supra subduction zone characteristics of the misis basaltic units are similar to other tethyan basalts in eastern mediterranean ophiolites

paleomagnetism of the late cambriancrepicephalus aphelaspis trilobite zone boundary in north america divergent poles from isochronous strata paleomagnetic samples from the nolichucky formation late cambrian sampled at two sites in the valley and ridge province of east tennessee yield a possibly penecontemporaneous characteristic magnetization that appears to reside in detrital magnetite the paleomagnetic pole positions are paleozoic but differ site i lat 41 n long 109 e dp 1 dm 2 site ii lat 39 n long 131 e dp 4 dm 7 the difference in poles reflects a significant difference in declination between the site mean directions and this declination difference probably reflects relative tectonic rotation as the sites are in different thrust sheets the paleontologic age of both sections is exceptionally well constrained as they are sampled across an abrupt biomere boundary between contrasting trilobite faunas comparison of these results with paleomagnetic data from coeval strata elsewhere in north america reveals gross discrepancies so that at least some of the published data must reflect remagnetization and or tectonic rotation

remagnetized cratonic cambrian strata from southern nevada stratigraphic sections of cambrian strata in southeastern nevada are akin to thin cratonic facies exposed in the grand canyon their structural setting is much more complicated however from mesozoic and cenozoic tectonism paleomagnetic samples from two sections through these strata appear to have been completely remagnetized coarse grained lightcolored sandstone from the tapeats sandstone yields scattered magnetizations residing in hematite that appear to reflect protracted diagenetic acquisition of remanence a hematitic sandstone in the bright angel shale yields relatively consistent paleozoic directions of magnetization but petrographic study shows that the hematite results from diagenetic oxidation and stratigraphic arguments suggest that the oxidation was not penecontemporaneous gray limestones of the jangle and muav limestones yield a magnetization residing in magnetite that may reflect late tertiary remagnetization being imposed during uplift related to the onset of basin and range deformation in any case this magnetization differs greatly in direction from a hematite magnetization reported from slightly younger muav limestone in the grand canyon both the sampling sites also appear to have been tectonically rotated but whether this rotation is true or a geometric artifact of the tilt correction cannot be determined from the present data these results suggest that paleomagnetic data from rocks as old as cambrian must be scrutinized very carefully before their magnetizations can be accepted as penecontemporaneous and such scrutiny must include geologic data

paleomagnetism and geochronology of sills of the doherty mountain area southwestern montana implications for the timing of fold and thrust belt deformation and vertical axis rotations along the southern margin of the helena salient paleomagnetic and 40ar 39ar data from intermediate composition sills exposed along the southern margin of the helena salient in southwestern montana help refine the age of late cretaceous contraction in this part of the north american cordillera the structures in the doherty mountain fold complex deform cambrian through mississippian strata in the hanging wall of the jefferson canyon lombard fault system sills exposed within these strata which are complexly deformed into northeast to northwest plunging anticlines and a northwest plunging syncline give well defined remanent magnetizations and after correction for fold plunge followed by bedding tilt paleomagnetic data from 20 of 21 sites from four folds yield positive fold tests at 100 percent unfolding the magnetizations from these sites are interpreted to be primary remanent magnetizations acquired during sill emplacement prior to fold and thrust deformation although pre folding remagnetization due to widespread latest cretaceous magmatism in the area cannot be ruled out one site yields a magnetization that appears to postdate folding this site has either been remagnetized or is in a distinct intrusive phase emplaced subsequent to folding biotite mineral separates from sills from two folds yield identical 40ar 39ar plateau dates of 77 ma although the group mean directions from sills in each of the folds may represent incomplete samplings of the geomagnetic field and the number of independent readings is low their grand mean direction declination dec 347 inclination inc 66 k 71 9 95 9 n 5 independent observations based on results from 21 sites is similar to the expected late cretaceous reference direction for the study area based on results from the essentially coeval adel mountain volcanics therefore these data suggest that deformation began after 77 ma campanian maestrichtian and that upper plate strata in the lombard thrust sheet of the helena salient in this area have not experienced significant local vertical axis rotation due to thrust movement and impingement with foreland structures along the southwest montana transverse zone the absence of appreciable vertical axis rotations is consistent with data obtained from other paleomagnetic studies in the region

on the age of the laschamp geomagnetic excursion abstract the laschamp geomagnetic excursion is a critical tie point found directly in deep sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice and to providing an independent calibration of the radiocarbon calendar its timing remains poorly known previous k ar 40 ar 39 ar and u th isochron determinations from lava flows at the type locality in the massif central france vary widely are imprecise and suggest a mean age of about 46 2 2 5 ka 2 results of 6 new unspiked k ar and 13 40 ar 39 ar incremental heating experiments on subsamples from three sites on the laschamp and olby flows are concordant and give a weighted mean age of 40 4 1 1 ka 2 uncertainty including analytical sources only that is 10 younger than the previous estimates considering that the 40 k 40 ar decay constant is not known to a precision better than 2 4 the most probable radioisotopic age for the laschamp excursion is 40 4 2 0 ka 2 analytical plus decay constant uncertainties this new age for the laschamp excursion agrees precisely with that deduced from the napis 75 deep sea sediment paleointensity stack when calibrated against the gisp2 ice core chronology using the o isotopes in ice and the magnetic properties of the marine cores

remagnetization and tectonic rotation of upper precambrian and lower paleozoic strata from the desert range southern nevada in the desert range of southern nevada miogeoclinal sedimentary rocks mostly shallow water limestones of latest precambrian through early ordovician age yield three components of magnetization having different blocking temperature spectra 1 a low blocking temperature component near the direction of the present axial dipole field 2 an intermediate blocking temperature component with northerly declination and inclination 60 and 3 a characteristic component with southeasterly declination and inclination 20 combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite the intermediate magnetization probably reflects a viscous partial thermoremanent magnetization vptrm imposed between the late cretaceous and mid cenozoic it was probably acquired when the strata were dipping slightly eastward the characteristic magnetization is probably a vptrm imposed during regional uplift in the late permian the degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the ordovician rocks additionally the characteristic magnetization in those rocks is younger than bedding disruption caused by major late stylolitization red purple mudstones from the middle member of the wood canyon formation in which a directionally similar characteristic magnetization resides in fine grained hematite also appear to have been remagnetized in these rocks the remagnetization probably reflects partial recrystallization as the blocking temperatures are too high to have been reset by burial heating the sampled sections have undergone relative tectonic rotation about a vertical axis consistent with late tertiary oroflexural bending that had been proposed on independent geologic evidence the characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations as the regional geology suggests the desert range strata were essentially horizontal throughout the paleozoic the total observed relative rotation is 44 5 representing absolute counterclockwise rotation 27 4 of the northern part of the desert range and absolute clockwise rotation 17 5 of the southern part the only unit in the desert range sequence that may retain a primary magnetization is the late precambrian rainstorm member of the johnnie formation the characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite after correction for 18 of counterclockwise rotation the resulting pole 5 n 151 e is near other late hadrynian poles from north america

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

accelerator mass spectrometry in geologic research the ability of accelerator mass spectrometry ams to measure very small concentrations of the nuclides 10 be 14 c 26 al 36 cl and 129 i has led to many innovative applications in geologic research to take advantage of this opportunity in the geosciences it is important to understand how ams works how these nuclides are produced and how they can be applied to geologic problems we first discuss the basics of ams explaining what gives the method its ability to count small numbers of these nuclides we review how these nuclides are produced and transported in the atmosphere hydrosphere and lithosphere we then explain the ways that ams is being used to solve a wide range of problems in geologic research by discussing specific applications in areas such as geomorphology tectonics climatology hydrology and geochronology

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

sedimentation and volcanism in a permian arc related basin southern new zealand an exceptionally well exposed ancient intra arc basin in the permian takitimu group of new zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition these are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation the takitimu oceanic arc basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres volcanism probably included 1 moderate to deep water extrusion of lava and deposition of hyaloclastite 2 extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin and 3 plinian and phreato plinian eruptions from more distant subaerial vents along the arc much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows slumping and sliding hemipelagic sedimentation predominated on the outer margin of the basin infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites turbidite sedimentation prevailed in the remainder of the basin producing a thick prograding volcaniclastic apron adjacent to the arc the volcaniclastic strata closely resemble classic turbidite deposits and show similar lateral facies variations to submarine fan deposits study of such sequences provides insight into poorly understood processes in modern arc related basins

the notch peak contact metamorphic aureole utah paleomagnetism of the metasedimentary rocks and the quartz monzonite stock the notch peak pluton a late jurassic granitic stock intrudes early paleozoic miogeoclinal strata in the house range western utah as part of a detailed study to determine textural and chemical changes related to the intrusion a paleomagnetic reconnaissance has been made of the pluton and its contact aureole steep scattered magnetizations with both polarities present are found in the pluton upon alternating field af demagnetization and they may reflect an original thermoremanence in magnetite that was acquired over at least one polarity reversal limestones and argillites in the big horse canyon member of the orr formation late cambrian were sampled at four stratigraphically controlled sites ranging from unaltered country rock to marble the site farthest from the pluton is well behaved upon progressive thermal demagnetization and yields a low inclination reversed paleozoic direction by 400 c the sites nearer the stock generally have a diffusely defined two polarity high inclination magnetization that reflects remagnetization by the pluton this magnetization is removed above 300 c and af demagnetization suggests that it resides in fine grained hematite of these sites one appeared nearly unaltered in thin section whereas the others range from hornfels to skarn the remagnetization resulted from a late oxidation event related to the intrusion perhaps from mobilization of groundwater thus the country rock has been remagnetized chemically rather than thermally these results suggest that paleomagnetism may be a useful tool for detecting altered zones around plutons in addition no consistent high blocking temperature magnetization was found in the aureole rocks the magnetization of the hornfels was easily measurable at higher steps but directions changed erratically and this behavior in part results from the presence of pyrrhotite

volcanic evolution of oceanic crust in a late ordovician back arc basin the solund stavfjord ophiolite complex west norway 1 the stratigraphy of the well preserved solund stavfjord ophiolite complex in the west norwegian caledonides documents the volcanic evolution of a spreading center in a late ordovician back arc basin basaltic sheet flows pillow lavas and volcanic breccias are the main components of the 470 800 m thick extrusive sequence and are organized stratigraphically in a cyclic manner cyclic units vary in thickness from 5 m to 225 m and are typically composed of basal sheet flows or lava flows with large pillows that are succeeded by flows with progressively smaller pillows and volcanic breccias thick independent breccia units also occur in the stratigraphy in sheet flow dominated parts of the sequence the cyclic units are thicker average 85 m than in pillow dominated parts average 20 m detailed logging of closely spaced profiles 1 km or less apart shows that the proportions of sheet flows pillow lavas and volcanic breccias varies laterally along an axial segment of less than 10 km the volcanic products change from predominantly sheet flows reflecting robust volcanic centers to pillow lavas to volcanic breccias sheet flow dominated volcanic centers seem to be spaced at intervals of 25 30 km and we tentatively interpret their regularity as an expression of volcanic segmentation at an intermediate to fast spreading center observations of modern ocean crust suggest that sheet flows dominate at fast spreading ridges while pillow lavas dominate at slow spreading ridges volcanic breccias are apparently rare in both of these environments these features contrast with the stratigraphy of the solund stavfjord ophiolite complex where the proportion of different volcanic products varies laterally and volcanic breccias are common we emphasize the importance of detailed studies of the volcanic stratigraphy of ophiolites as complements to those of in situ oceanic crust in order to provide a more complete picture of volcanic evolution of oceanic crust in different spreading regimes

reorganization of geological sciences and particularly of metamorphic geology by the advent of plate tectonics a personal view abstract plate tectonics having being formulated in geophysics in the 1960 s was introduced into and caused reorganization of geological sciences in about 1968 1975 an anticipated system of reorganized geology was formulated around 1970 by a few pioneering geologists with a strong background of global scale regional geology pre existing dutch school tectonics with an explicit uniformitarian tendency played an important role in the beginning development and acceptance of plate tectonic viewpoints global scale views in metamorphic geology that had begun prior to the advent of plate tectonics were developed into tectonic models of regional metamorphism as well as discussions of the p t t paths and of secular changes in the type of regional metamorphism

volcanic belts as markers of the mesozoic cenozoic active margin of eurasia abstract from the triassic to the present linear belts of calc alkaline volcanism developed along the southern margin of eurasia in the triassic the calc alkaline belt can be traced in turan fore caucasus crimea and dobrogea its development is related to the closure of the paleo tethys when fragments of gondwana iran afghanistan etc were approaching eurasia since the early jurassic calc alkaline volcanism has been related to subduction of the oceanic crust of the mesozoic tethys five stages of calc alkaline volcanic activity can be distinguished early jurassic middle jurassic early cretaceous late cretaceous paleogene and late miocene quaternary the configuration of the volcanic belts the intensity of the volcanism and the chemistry of the erupted products change considerably from stage to stage mesozoic belts were most pronounced in the pontian transcaucasian segment of the margin starting from the late cretaceous and especially in the cenozoic the volcanic belts can be continuously traced from the balkans to afghanistan most of the belts were actually island arcs formed on the fragments of the hercynian of baikalian basement and in this respect similar to the island arc of japan only the incipient island arcs of senonian age formed close to the passive margin of gondwana are the exceptions they were built on oceanic crust and are geochemically close to the mariana arc at certain times early jurassic eocene parallel chains of island arcs existed in general the situation resembled the present day convergent boundary in the western pacific as closure of the tethys proceeded island arcs gave way to marginal continental volcanic belts with wide spread development of high potassium particularly shoshonitic series peaks of volcanic activity in the bajocian late cretaceous eocene and late miocene present alternated with quiet periods and corresponded to the times when the convergence rate between africa and eurasia increased the opening of the back arc basins is reflected in outbursts of basaltic rift type volcanism these outbursts occurred in the toarcian aalenian bathonian late jurassic aptian turonian and early middle eocene

geochemistry and intrusive directions in sheeted dikes in the troodos ophiolite implications for mid ocean ridge spreading centers 1 sheeted dikes at mid ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity as such they are crucial for the interpretation of many observations at mid ocean ridges or other volcanoes with pronounced rift zones including topography hydrothermal systems petrology and geochemistry we carried out a structural magnetic and chemical investigation of a 4 10 km sheeted dike section in the troodos ophiolite cyprus on the basis of major and trace element geochemistry we distinguish dikes that may be correlated with the basal high ti series hts lavas from those of the overlying low ti series lts lavas all dikes studied are nearly parallel to each other with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center anisotropy of magnetic susceptibility measurements suggests that the hts and lts dikes intrude in fundamentally different ways hts dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber lts dikes show a bimodal orthogonal set of intrusive directions one shallow and one near vertical near lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system and near vertical dike propagation directions are probably associated with feeder dikes to down rift surface flows our study suggests that the types of dike intrusive behavior in the troodos ophiolite may also be typical for normal mid ocean ridges or other major shield volcanoes with well developed rift zones

tectono stratigraphic evolution of the early proterozoic wisconsin magmatic terranes of the penokean orogen the early proterozoic penokean orogen developed along the southern margin of the archean superior craton the orogen consists of a northern deformed continental margin prism overlying an archean basement and a southern assemblage of oceanic arcs the wisconsin magmatic terranes the south dipping niagara fault suture zone separates the south facing continental margin from the accreted arc terranes the suture zone contains a dismembered ophiolite the wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure the northern pembine wausau terrane contains a major succession of tholeiitic and calc alkaline volcanic rocks deposited in the interval 1860 1889 ma and a more restricted succession of calc alkaline volcanic rocks deposited about 1835 1845 ma granitoid rocks ranging in age from about 1870 to 1760 ma intrude the volcanic rocks the older succession was generated as island arcs and or closed back arc basins above the south dipping subducti

reconstruction of the paleoaccumulation rate of central greenland during the last 75 kyr using the cosmogenic radionuclides 36cl and 10be and geomagnetic field intensity data abstract the accumulation rate is one of the most fundamental climate parameters to be derived from ice cores in addition to its climatic importance the accumulation rate provides a crucial constraint on depth age scales where annual layer counting is not possible so far there is no consistent picture of the accumulation rate for the last ice age in central greenland therefore we have derived the accumulation rate for the time interval of about 3 75 kyr bp by applying a method based on the cosmogenic radionuclides 36cl and 10be in the summit ice cores and on geomagnetic field data the main difference between our approach and the methods applied previously is that the accumulation rate based on 36cl and 10be does not depend on an ice flow model estimating the original thickness of annual layers our new reconstruction agrees well with the accumulation rate published by johnsen et al tellus 47b 1995 624 629 until 60 kyr bp and differs significantly from that of cuffey and clow j geophys res 102 1997 26383 26393

pervasive late paleozoic triassic remagnetization of miogeoclinal carbonate rocks in the basin and range and vicinity sw usa regional results and possible tectonic implications abstract relatively unmetamorphosed paleozoic miogeoclinal carbonate rocks in the basin and range of e nevada sw nevada and adjacent california and w utah yield low inclination magnetizations that reflect pervasive regional remagnetization around the close of the paleozoic the rocks range in age from mid cambrian through pennsylvanian and lie generally in a broad belt between the mid paleozoic roberts mountain thrust and the late cretaceous sevier thrusts most of the magnetizations reside in magnetite but at one site the magnetization is evidently carried by pyrrhotite preliminary rock magnetic data suggest samples with magnetite borne remanences have wasp waisted hysteresis curves typical of remagnetized carbonates the origin of the remagnetization is problematic and probably polygenetic both the permo triassic sonoma orogeny and deformation associated with the ancestral rockies seem too spatially limited but magnetite from smectite destruction seems difficult to reconcile with the great stratigraphic extent of late paleozoic remagnetization unless combined with thermal resetting of the lowermost units a number of sites also appear to have undergone some vertical axis rotation and the sense and magnitude of these rotations are grossly consistent with independent geologic evidence however the probably large age range of the low inclination components complicates their use for resolving tectonic rotations younger intermediate stability components of magnetization probably of cretaceous or cenozoic age are also found in many sites and also probably have multiple origins at sites farther w the late paleozoic component is not found which probably reflects its destruction by later mesozoic or cenozoic heating at sites farther e on and near the colorado plateau gray carbonates yield only cenozoic magnetizations some reddish oxidized carbonates there locally contain a hematite borne magnetization of late paleozoic age however it is probably related to the development of thick continental redbed sequences in overlying strata on the plateau rather than to the remagnetization process es in the miogeocline

ion exchange experiments and rbsr dating on celadonites from the troodos ophiolite cyprus abstract rb sr dating of celadonites from several geographic stratigraphic locations from the troodos ophiolite cyprus yield crystallization ages varying between 57 9 3 9 and 89 0 1 9 ma the oldest celadonite age 89 0 1 9 ma agrees well with the crystallization age of the troodos igneous complex suggesting that low temperature chemical exchange with seawater started immediately after formation of the ophiolite drillcore samples suggest a duration of low temperature alteration of ca 10 ma whereas outcrop samples yield much longer durations of 30 ma the exact reason for this discrepancy is not fully understood but may represent the difference in localized downwelling in different parts of the ophiolite or may be related to the extensive weathering of the upper extrusive suite additionally in nearly all cases rb sr age estimates are older than k ar age data from the same samples to evaluate the potential disturbances of rb sr isotope systematics from exchangeable interlayer and basal surface positions a series of ion exchange experiments was carried out these data show that significant amounts of rb and sr can be in exchangeable positions and therefore a fraction of the parent and daughter inventory of these clays is vulnerable to late stage chemical exchange however the overall agreement between the data of the whole mineral and ion exchanged clay separates suggests that the influence of this exchange on the rb sr isotopic system is minor the post crystallization chemical behavior of exchangeable k and ar was not evaluated but the loss of radiogenic argon from exchangeable sites could result in k ar ages younger than rb sr ages of the same sample

evidence for enhanced 10be deposition in mediterranean sediments 35 kyr bp in this letter we report the profiles 160 samples of 10be 9be zn in a mediterranean sediment core spanning the last 60 kyr we show the existence of a 10be peak whose absolute age is estimated to be 34 3 kyr bp based on its stratigraphic position between two tephra layers originating from two volcanic eruptions campanian ignimbrite and citara dated by k ar 40ar 39ar and 14c methods this peak is similar to the increase in the 10be concentration observed in vostok and dome c ice cores at the same age the increase with approximately similar amplitude in both ice and sediment cores at different latitudes and hemispheres cannot be explained by changes in the archives the present result supports the view of a well defined global enhancement of the 10be flux related to an increase of the 10be production rate and not to a redistribution of the 10be fallout between different latitudes

petrogenesis of a tholeiite boninite sequence from ayios mamas troodos ophiolite evidence for splitting of a volcanic arc lavas and intrusives from ayios mamas at the western end of the limassol forest complex south of the arakapas fault belt troodos comprise three groups a a basal sequence of pillow lavas intruded by dikes and sills with the phenocryst assemblage plag cpx mt ol b a middle sequence of pillows and massive flows cpx plag ol and c an upper sequence mostly of pillows and breccia ol opx cpx sp the lower group is tholeiitic and the upper two groups boninitic group c whole rock and glass mg numbers exceed 72 and range up to 78 with corresponding olivine compositions of fo92 they are strongly depleted in high field strength elements and on the basis of cr y variation and major element mass balances are interpreted to reflect partial melting of a basalt depleted cpx harzburgite source group a tholeiites in contrast are interpreted to derive from a more fertile lherzolite source magmaphile and compatible trace element variation is best modelled after an open magma system involving periodic replenishment tapping and fractionation although neither tholeiites nor boninites represent steady state compositions the boninites belong to a cao rich sio2 poor variant resembling those from guam but contrasting with those from bonin and cape vogel morb normalized enrichment of low field strength elements k sr ba and rb in all groups indicates subduction derived source contamination it is proposed that the ayios mamas section reflects splitting of a volcanic arc prior to back arc spreading in an environment analogous to that of the present day andaman sea

patterns and processes of alteration in the lavas and dykes of the troodos ophiolite cyprus alteration patterns in the lavas and dykes of the troodos ophiolite cyprus record a complex history of axial hydrothermal alteration crustal aging and subsequent uplift and emplacement of the ophiolite field mapping shows that distribution of five alteration zones each with distinct mineralogical geochemical and hydrologie characteristics is influenced by igneous stratigraphy structure and the nature and thickness of the overlying sediments paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off axis along spreading axes low temperatures 50 c were maintained by the rapid flow of seawater in and out of the lavas and only minimal alteration took place in contrast lower water rock ratios and higher temperatures 200 c in the dykes promoted extensive seawater rock interaction although the sharp rise in temperature between the two regimes generally coincides with the lava dyke transition late stage intrusions or hydrothermal upwelling zones locally cause high temperature alteration to extend upward into the lavas as a segment of crust moved off axis temperatures remained low in the lavas and progressively decreased from 250 to 80 c in the dykes high permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and thus the original seafloor morphology although field relations show that alteration has a consistent vertical pattern in troodos the alteration zones are not laterally continuous and the stratigraphie depth of their boundaries varies considerably

submarine volcaniclastic rocks in this chapter we are concerned with underwater volcaniclastic eruptions and products as well as pyroclastic flows and debris generated on land and transported by mass flowage into the sea submarine fallout deposits derived from land based eruptions are treated in chapter 7 clastic materials redistributed as mass flows or turbidity currents originating from the rapid build up of tephra and epiclastic volcanic debris along coastal parts of volcanoes are also discussed in various places throughout the text e g chap 13

calibration of the 14c time scale to 40 ka by 234u 230th dating of lake lisan sediments last glacial dead sea a new comparison of 14c dates with 234u 230th ages is presented of aragonites from lake lisan the last glacial dead sea between 20 52 cal ka bp the lisan data are coincident with the coral based 14c calendar age calibration through the continuous portion of the curve to 23 5 cal ka bp and with the additional checkpoints at 30 and 40 cal ka bp the agreement with the corals provides evidence for the accuracy of the u th and 14c ages and indicates that lisan aragonites can potentially be used to generate a nearly continuous record of the atmospheric 14c variations through this crucial time interval the lisan data are compared with other records older than 25 cal ka bp from lake suigetsu japan north atlantic foraminifera south african cave deposits and tufa from spain over some age intervals the records show broad agreement over other intervals they diverge all agree that 14c ages were 2 5 0 5 ka younger than calendar ages between 20 32 cal ka bp for ages 32 cal ka bp the lake suigetsu data indicate small differences between 14c ages and calendar ages of less than 1 5 ka the other records broadly agree that 14c ages are too young by 3 1 ka from 32 39 cal ka bp at 40 cal ka bp the foraminifera show equal 14c and calendar ages while the corals lisan aragonites and the spanish tufa indicate a large age difference of 4 5 ka a recent paleomagnetic study of the lisan formation indicates that the high calendar 14c age difference at 40 cal ka bp may be associated with the laschamp paleomagnetic excursion

spreading induced faulting and fracturing of oceanic crust examples from the sheeted dyke complex of the troodos ophiolite cyprus

a view into the cretaceous geomagnetic field from analysis of gabbros and submarine glasses the nature of the geomagnetic field during the cretaceous normal polarity superchron cns has been a matter of debate for several decades numerical geodynamo simulations predict higher intensities but comparable variability during times of few reversals than times with frequent reversals published geomagnetic paleointensity data from the cns are highly scattered suggesting that additional studies are required here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the troodos ophiolite cyprus 92 1 ma old together with recently published data from the troodos upper crust we obtain three independent paleointensity time series these sequences reveal quasi cyclic variations of intensities about a mean value of 54 20 z am 2 providing insight into the fluctuating nature of the cretaceous magnetic field our data suggest the cns field was both weaker and more variable than predicted by geodynamo simulations the large amplitudes of these variations may explain the wide range of dipole moments previously determined from the cns 2007 elsevier b v all rights reserved

spatial and temporal variability in crystallization of celadonites within the troodos ophiolite cyprus implications for low temperature alteration of the oceanic crust celadonite a low temperature 30 c hydrothermal alteration mineral is a predominant fracture and void filling phase within the volcanic rocks of the troodos ophiolite the combined chemical and structural properties of celadonite i e high k2o and ar retentive along with its common occurrence provide a valuable tracer for studying the temporal and spatial variability of low temperature hydrothermal fluid circulation and alteration within this ancient oceanic crust some 54 new k ar age determinations of celadonites from various geographic and stratigraphic locations within the extrusive rocks of troodos yield crystallization ages ranging from 90 9 1 0 to 49 8 0 5 ma with the oldest in close agreement with the estimated 91 92 ma crystallization age of troodos igneous rocks the youngest age indicates that low temperature water rock chemical exchange continued for at least 40 m y after crustal formation this represents a 100 increase over previous estimates based on limited numbers of k ar dates of celadonite of the duration of low temperature mineral precipitation in troodos correlation between celadonite ages and field relationships suggests the following 1 there is no apparent relationship between celadonite crystallization and stratigraphic depth 2 celadonite crystallization occurs homogeneously on an outcrop scale with adjacent samples from a single flow unit yielding equivalent k ar ages 3 flow units with relatively high primary permeabilities e g pillows and breccias yield celadonites with younger k ar ages 4 celadonite precipitation at distinct locations occurred rapidly with little or no age difference between rim and core sections of single large deposits the above correlations suggest that low temperature hydrothermal fluid circulation and secondary mineral precipitation are controlled by the local alteration conditions such as intrinsic permeability degree of fracturing and water rock ratios and are independent of time and space integration of these local heterogeneities over the entire extrusive sequence suggests that the upper oceanic crust undergoes relatively homogeneous closure to hydrothermal fluid circulation no evidence is found to support a progressive upward sealing of the oceanic crust

tectonic evolution of the troodos ophiolite within the tethyan framework a new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the troodos ophiolite a well preserved remnant of neotethyan oceanic crust grabens and normal faults within the sheeted dike complex and the extrusive sequence of the troodos ophiolite resemble those of oceanic spreading centers diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along and across strike variation in volcanic and tectonic activity during development of oceanic crust coupled with the existence of the arakapas transform fault to the south these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault the arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue however for generation of part of the ophiolite within a subduction zone environment regional reconstructions suggest that the mesozoic neotethys may have evolved as a marginal basin both to the afro arabian continent and the paleotethyan ocean over an active or recently active south dipping subduction zone the troodos ophiolite and other eastern mediterranean ophiolites whose magma compositions were affected by the subducted paleotethyan slab may have formed along east west trending spreading centers separated by north south trending transform faults within this marginal basin a rapid change in relative plate motion in late cretaceous time between eurasia and afro arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the neotethyan realm and in initiation of north dipping subduction zone s beneath the troodos and other ophiolites in the region the apparent forearc setting of the troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation

geomagnetic modulation of the 36cl flux in the grip ice core greenland geomagnetic field strength is expected to affect the production rate of cosmogenic isotopes such as beryllium 10 carbon 14 or chlorine 36 chlorine 36 data from the greenland ice core project grip ice core agree well with a production rate calculation based on a paleomagnetic reconstruction for the past 100 000 years over both long and short term variations a chlorine 36 peak at 38 000 years ago previously found in the beryllium 10 record from the vostok ice core can be explained by a period of low geomagnetic field intensity

structural and geophysical expression of the solea graben troodos ophiolite cyprus field studies show that extensional structures in the north central part of the troodos ophiolite including steep normal faults grabens dikes and low angle detachment faults are related to e w present coordinates spreading the solea graben directly north of the mafic ultramafic rock outcrops around mount olympus consists of a 10 to 15 km wide western portion that is underlain by a low angle detachment fault between the sheeted dike complex and the plutonic section structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40 or more about subhorizontal axes the eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross cutting ophiolitic dikes and hydrothermal mineralization timing of the uplift of the central part of the troodos ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low angle normal faulting the solea graben spreading center provides a model for the structure of modern mid ocean ridges that are spreading amagmatically

cosmogenic nuclides during isotope stages 2 and 3 abstract changes in the concentration of cosmogenic nuclides measured in natural archives are related mainly to changes in the cosmic ray induced production rate in the atmosphere but also to the subsequent transport and deposition processes which depend on the geochemical properties of the nuclides a comparison of 10 be and 36 cl data from ice cores from greenland ice cores and 14 c data from tree rings and other archives provides the following information for the isotope stages 2 and 3 1 changes in the cosmogenic nuclide production rate on time scales longer than approximately 2 kyr agree well with paleomagnetic data derived from deep sea sediments in particular the geomagnetic excursions leading to weak fields around 40 kyr bp laschamp and 32 kyr bp mono lake can be clearly seen 2 production variations on shorter time scales contain information on solar variability as an example the 205 yr devries cycle already known from the analysis of 14 c tree ring data during the holocene is also present in the 10 be data of the time period 25 55 kyr bp 3 a comparison of 14 c calculated using 10 be data with the measured 14 c record points to a reduction of the global deep water formation during the younger dryas cold event 4 using information available on the production rates of cosmogenic nuclides in the past derived from paleomagnetic data the paleoprecipitation rate can be calculated this is crucial for establishing precise time scales

petrological and chemical evolution of the northeastern troodos extrusive series cyprus four magma series are distinguished in the northeastern troodos extrusive series a a lower low ti series lo lts of basaltic andesites b a high ti series hts of basaltic andesites to rhyodacites c a low ti series lts and d a depleted low ti series dlts the last two being of basaltic andesite trace element characteristics vary systematically from series a to d and are interpreted in terms of a variable contribution of three major source components scs lile enriched water rich fluids sci derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge increasing lile hfse and lile ree ratios and decreasing absolute hfse and ree concentrations from series a to d indicate progressive depletion of the actual mantle source scii a negative ta anomaly in the lavas decreases from series a to d and is interpreted to have resulted from partial melting of the lower crust scii where ta nb ti may be fractionated by ti rich accessory phases the contribution of sciii decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases preventing partial melting of the lower crust chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series a to d lo lts and hts lavas are intrepreted to be directly related by open system fractional crystallization in crustal magma chambers removal of observed phenocryst phases clinopyroxene orthopyroxene plagioclase and magnetite and repeated subsequent mixing of dacitic to rhyodacitic magmas with batches of replenishing basaltic andesites are the major processes possibly induced by vesicle formation in the mafic layer after a period of some crystallization lts and dlts magmas were directly fed to the surface without stagnating at crustal levels with feeder dykes positioned marginal to the central rift zone and thus by passing the central magma chambers these magmas apparently experienced only limited fractional crystallization of 10 15 wt olivine clinopyroxene chromite probably at the mantle crust boundary

biogenic alteration of volcanic glass from the troodos ophiolite cyprus alteration of basaltic glass through the entire volcanic sequence of the troodos ophiolite is partly bio mediated the following observations support this conclusion alteration textures resembling microbes in form and size altered glass locally shows high carbon concentrations at the alteration front and organic remains the absence of dna suggests that the bio alteration is a fossil process temperatures calculated from 18 o of carbonate assumed to have been in equilibrium with seawater yield 27 65 c and 18 o silicate versus h 2 o relationships indicate seawater alteration these data are consistent with biogenic alteration during an early stage of ocean floor alteration of the troodos oceanic crust some 70 90 ma ago

10be in polar ice data reflect changes in cosmic ray flux or polar meteorology we have theoretically estimated the expected changes in the global cosmic ray production of 10be in the atmosphere with changes in solar activity and the consequent variations in its fallout in the polar regions the global 10be production rate is found to be about 20 higher during periods of very low solar activity compared to the average solar modulation level observed during the past 3 solar cycles the stratospheric 10be fallout pattern has been derived using the fallout data for 90sr as an analog this fallout shows an amplitude attenuation by a factor of about three at 70 the higher the latitude the higher the attenuation the results have been compared with the long time series available for 10be in polar ice in greenland and in antarctica 70 78 latitude it is concluded that the observed variations in 10be concentrations in ice cores are primarily due to climatic changes for both short and long period variations thus 10be data can be used as a proxy for climate induced meteorological changes in the polar region

radiocarbon calibration beyond 20 000 14c yr b p by means of planktonic foraminifera of the iberian margin we present a new set of 14c ages obtained by accelerator mass spectrometry ams on planktonic foraminifera from a deep sea core collected off the iberian margin md952042 this site at 37 n is distant from the high latitude zones where 14c reservoir age is large and variable many independent proxies alkenones magnetic susceptibility ice rafted debris foraminifera stable isotopes abundances of foraminifera pollen and dinoflagellates show abrupt changes correlative with dansgaard oeschger and heinrich events of the last glacial period the good stratigraphic agreement of all proxies from the fine to the coarse size fractions indicates that the foraminifera 14c ages are representative of the different sediment fractions to obtain reliable 14c ages of foraminifera beyond 20 000 14c yr b p we leached the shells prior to carbonate hydrolysis and subsequent analysis for a calendar age scale we matched the iberian margin u37k profile with that of greenland summit 18o both are proxies for temperature which in models varies synchronously in the two areas the match creates no spurious jumps in sedimentation rate and requires only a limited number of tie points except for ages older than 40 000 14c yr b p greenland s gisp2 and grip records yield similar calendars the 14c and imported calendar ages of the iberian margin record are then compared to data from lacustrine annual varves and from corals and speleothems dated by u th previously used to extend the calibration beyond 20 000 14c yr b p the new record follows a smooth pattern between 23 000 and 50 000 cal yr b p we find good agreement with the previous data sets between 23 000 and 31 000 cal yr b p in the interval between 33 000 and 41 000 cal yr b p for which previous records disagree by up to 5000 cal yr the iberian margin record closely follows the polynomial curve that was previously defined by an interpolation of the coral ages and runs between the lake suigetsu and the bahamian speleothem data sets

paleomagnetism of the notch peak contact metamorphic aureole revisited pyrrhotite from magnetite pyrite under submetamorphic conditions 1 on the basis of new demagnetization and rock magnetic experiments a scattered two polarity remagnetization extending throughout the contact metamorphic aureole surrounding the jurassic notch peak granitic stock in western utah is now thought to reside in metamorphic pyrrhotite rather than in fine grained hematite in part the pyrrhotite formed by in situ reduction of magnetite by pyrite and organic compounds it could not have been formed by the large scale introduction of reducing fluids as isotopic evidence indicates that these metalimestones remained impermeable at higher grades some pyrrhotite probably also formed by the reaction of pyrite and fe bearing silicates as magnetite destruction and pyrrhotite formation extends into rocks that are not obviously metamorphosed the reactions responsible could proceed at very low less than 200 c temperatures and the feasibility of such low temperature reactions in a closed system is shown by detailed thermodynamic calculations this study demonstrates that under reducing conditions in the presence of sufficient pyrite pyrrhotite can form at the expense of magnetite under very mild conditions indeed magnetite may be destroyed merely by deep burial such a mechanism may help explain the paucity of primary magnetite in ancient sedimentary rocks particularly since the single domain and pseudo single domain grains capable of retaining a stable paleomagnetic signature over geologic time are extremely fine grained and hence particularly vulnerable to destruction

geochemistry of volcanic rocks from the naga hills ophiolites northeast india and their inferred tectonic setting highly disrupted and deformed slices of ophiolitic rocks occurring along a linear belt in nagaland and manipur states ne india are known as the naga hills ophiolites nho the principal rock types include dunite harzburgite lherzolite wehrlite pyroxenite and mafic volcanics the volcanic rocks of nho have been classified into low ti and high ti groups with 2 wt tio 2 as the boundary which is corroborated by the differences in their immobile trace element and rare earth element characteristics the two groups are probably not cogenetic their chemical differences reflect variable source composition within group variation of incompatible elements such as la ta in the high ti group may be related to varying degrees of partial melting the ti v ratios and ree distribution pattern of low ti group show overlapping mid ocean ridge basalt and island arc like characteristics and possibly suggest back arc basin setting the high ti group on the other hand has similarities with within plate basalts erupted at off axis sea mounts and ocean islands the sea mounts and ocean islands brought to the subduction zone acted as barriers within the imbricate fault slices are components from the oceanic crust which were accreted to the continental margin and which represent clipped off portions from these barriers

evidence for two intervals of enhanced 10 be deposition in antarctic ice during the last glacial period we have previously reported concentration profiles of cosmic ray produced cosmogenic 10be in deep ice cores from dome c and vostok antarctica1 2 in both these cores we found a concentration of 10be approximately two times larger in ice from the late glacial period than in the holocene ice we interpreted this as probably resulting from a lower precipitation rate on the antarctic plateau during glacial periods compared to interglacial periods in the vostok profile there was one sample corresponding to 60 000 yr bp which gave an unusually large 10be concentration not correlated with any obvious climatic event we suggested that this sample might be reflecting increased 10be production as for example during a period of reduced solar modulation2 we have now measured a much more detailed concentration profile for 10be in the vostok core the results confirm a 10be peak lasting 1 000 2 000 years at 60 000 yr bp and show another similar peak at 35 000 yr bp we have also observed the latter peak in the dome c core we discuss possible sources for these peaks and their potential as stratigraphic markers

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

palaeomagnetic insights into the evolution of neotethyan oceanic crust in the eastern mediterranean abstract a synopsis of palaeomagnetic data from three late cretaceous eastern mediter ranean tethyan ophiolites troodos hatay and baer bassit and their sedimentary coversequences is presented these data provide valuable insights into the role of regional andlocal scale tectonic rotations in the geodynamic evolution of neotethyan oceanic crust thegeologically earliest phases of tectonic rotation are documented in the troodos ophiolite where rotations around both subvertical and subhorizontal axes are readily related to thedevelopment of the spreading fabrics and structures during crustal genesis subsequent c 74 anticlockwise intra oceanic rotation of a troodos microplate has been quantified throughanalysis of the in situ sedimentary cover of the troodos ophiolite results indicate that bulkanticlockwise rotation began soon after the cessation of spreading and ended by the end ofthe eocene with c 50 60 of microplate rotation being over by the maastrichtian the timeat which ophiolite thrust sheets were emplaced onto the arabian continental margin to theeast of troodos recent results from the emplaced structurally dismembered baer bassitophiolite indicate extreme anticlockwise rotations of ophiolitic thrust sheets varying on akilometre scale new data from the post emplacement sedimentary cover confirm that onlya small component of these rotations is due to post emplacement tectonism baer bassitrepresents the leading edge of the emplaced ophiolitic sheet new data from the morecoherent section preserved in the hatay ophiolite to the north demonstrate significantanticlockwise rotation this is equivalent to the rotation of the most northerly part of thebaer bassit units to the south and is of the same sense and magnitude as the pre maastrichtian phase of microplate rotation documented in the troodos this suggests acommon intra oceanic origin for the majority of the troodos and hatay rotations and asignificant component of the more variable rotations observed in baer bassit overall therefore the data support a model involving 1 intra oceanic rotation of a coherent regionof crust within the southern neotethyan basin this rotated unit is more areally extensivethan has previously been inferred from consideration of data from the troodos ophiolitealone 2 emplacement of part of the rotated unit onto the arabian platform 3 subsequentlocalized post emplacement modification related to the development of the current plateconfiguration

troodos extrusive series of cyprus a comparison with oceanic crust the extrusive section on the northern flank of the troodos ophiolite is 1 000 to 1 500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia some pillowed units which average 100 to 200 m in thickness vary systematically from lava tubes up to 6 m in diameter at the base through common pillows less than 1 m in diameter to small pillows and extrusive breccias at the top these assemblages appear to be individual volcanoes suggesting episodic volcanic accretion the upper 100 to 200 m of the section is pervasively altered and oxidized but fresh volcanic glass is preserved throughout most of the extrusive section the lavas are subdivided on geochemical evidence into a lower differentiated andesite to dacitic andesite series resembling evolved island arc tholeiites and an upper series of picrite mafic basalt and basaltic andesite very low in tio2 k2o p2o5 nb rb and ba the highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process the occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion located above a subduction zone

paleomagnetism of tertiary tuffs from the lava dam southern house range utah at lava dam utah the tunnel spring tuff yields a reversed polarity characteristic magnetization two units within the tuff yield similar but distinct directions of magnetization lower d 156 i 73 95 2 upper d 137 i 73 95 2 no bedding corrections applied if this difference reflects paleosecular variation it suggests that the two units reflect different eruptions perhaps a few tens of years apart the cottonwood wash tuff member d 348 i 50 95 2 and the wah wah springs tuff member d 344 i 47 95 3 of the needles range formation yield normal polarity magnetizations the polarity of the cottonwood wash is consistent with the results of other workers but the wah wah springs has been reported to have reversed polarity whether the normal polarity at lava dam is due to pervasive remagnetization of that section or to miscorrelation of the wah wah springs cannot be determined from the present data but miscorrelation seems more likely

distribution of porosity in a section of upper oceanic crust exposed in the troodos ophiolite the porosity structure of a section of old oceanic crust was characterized by a detailed study of the volcanic sequence of the troodos ophiolite the distribution and abundance of porosity was compared to the amount of secondary minerals in order to calculate how porosity created by crustal accretion is modified by alteration as a section of crust ages eight sites with one dominant lithology and contrasting alteration histories were studied two types of porosity were quantified macroscopic macrofractures vesicles vugs interpillow zones and total macroscopic and laboratory measured microscopic porosity crustal accretion produced a vertically zoned porosity structure due to differences between pillows that formed during the main and waning stages of crustal construction mean initial macroscopic porosity ranges from 14 to 17 in uppermost pillows and from 6 to 10 in the underlying pillows and massive flows macroscopic porosity was reduced by 10 11 where alteration was most pervasive i e 250 m of the paleoseafloor beneath this zone pillow and flow outcrops showed decreases of 1 3 and 2 5 respectively the combined effects of volcanic morphology and alteration resulted in no systematic variation with depth in the final macroscopic porosity 4 8 the greatest decrease in macroscopic porosity was localized where abyssal hill topography or volcanic edifices and sedimentation rates facilitated the growth of the seafloor weathering zone which comprises 10 of the volcanic pile thus in modern ocean basins alteration would have the greatest effect on crustal porosity in areas of rough topography where crests of abyssal hills remain unsedimented for long periods of time porosity data derived from deep sea drilling project ocean drilling program cores and borehole logs show similar relationships which implies that the evolution of upper crustal porosity is controlled by the local environment and that the evolution of upper crustal porosity cannot be uniquely described by seismic data

geomagnetic field behavior during the iceland basin and laschamp geomagnetic excursions a simple transitional field geometry we present four new records of the iceland basin excursion ibe and five new records of the laschamp excursion le obtained from rapidly deposited marine sediments in the north atlantic ocean the nordic seas the gulf of mexico the south china sea and the southern indian ocean marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records the virtual geomagnetic pole vgp paths of the four ibe records are all similar the vgps move southward over europe and africa reaching the southern hemisphere three reach antarctica and then move to more eastern longitudes before returning northward over australia and east asia describing a large counterclockwise loop the same vgp pattern is observed in other published records the vgp paths observed for the le are similar to those of the ibe however they loop clockwise instead of counterclockwise despite the different sense of looping the marked similarity among the paths for the two excursions suggests that a similar relatively simple geometry dominated the transitional field during both the ibe and the le similar dynamo mechanisms must therefore have been active in the earth s core for both excursions the duration of the excursions is estimated at 2 000 years which supports the suggestion that a difference exists between the mechanisms for excursions and reversals however the coincidence of the longitudinal bands for vgps associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions

a comparative analysis of potential biosignatures in basalt glass by fib tem abstract bulk analyses of fresh and altered subseafloor basalts have elucidated the large scale geochemical fluxes that result from water rock interactions which ultimately affect both seawater and mantle compositions but these analyses do not provide insights into the physical and chemical processes occurring on the scale of microorganisms micron sized tubules that may represent microbial boring into basalt glass are ubiquitous throughout the seafloor and can potentially be preserved over geological time these putative biosignatures give clues into the history of life on earth and may be significant for the search for life on other planets however their formation and mineralization are still poorly understood we have used a combination of focused ion beam fib milling and transmission electron microscopy tem to analyze the textural and geochemical characteristics of tubular alteration features from four basalt glass samples representing different sample locations collection depths and ages including one sample from the cretaceous troodos ophiolite high resolution tem imaging consistently identified platy layers of partially crystalline material infilling the tubules the lattice spacings and diffraction patterns showed that this material comprises a mix of two layer and three layer silicates closer inspection of the contact point between the tubules and the surrounding basalt glass commonly revealed a thin leached rim around the tubules electron energy loss spectroscopy mapping demonstrated that these rims are depleted in everything but si and o which implies that they are formed by a leaching process likely the incongruent dissolution of the glass during formation of the tubule the infilling material is depleted in ca and enriched in k and in most cases also enriched in fe which is consistent with their identification as fe bearing phyllosilicates these observations are consistent across the samples which implies that the processes that form and mineralize tubular glass alteration features are similar throughout the oceanic subsurface and through time

a 50 000 year record of climate oscillations from florida and its temporal correlation with the heinrich events oscillations of pinus pine pollen in a 50 000 year sequence from lake tulane florida indicate that there were major vegetation shifts during the last glacial cycle episodes of abundant pinus populations indicate a climate that was more wet than intervening phases dominated by quercus oak and ambrosia type ragweed and marsh elder the pinus episodes seem to be temporally correlated with the north atlantic heinrich events which were massive periodic advances of ice streams from the eastern margin of the laurentide ice sheet possible links between the tulane pinus and heinrich events include hemispheric cooling the influences of mississippi meltwater on sea surface temperatures in the gulf of mexico and the effects of north atlantic thermohaline circulation on currents in the gulf