a new high precision 40ar 39ar age for the rochechouart impact structure at least 5 ma older than the triassic jurassic boundary the rochechourt impact structure in south central france with maximum diameter of 40 50 km has previously been dated to within 1 uncertainty of the triassic jurassic boundary at which time 30 of global genera became extinct to evaluate the temporal relationship between the impact and the triassic jurassic boundary at high precision we have re examined the structure s age using multicollector argus v 40ar 39ar mass spectrometry results from four aliquots of impact melt are highly reproducible and yield an age of 206 92 0 20 0 32 ma 2 full analytical external uncertainties thus the rochechouart impact structure predates the triassic jurassic boundary by 5 6 0 4 ma and so is not temporally linked to the mass extinction rochechouart has formerly been proposed to be part of a multiple impact event but when compared with new ages from the other purported paired structures the results provide no evidence for synchronous impacts in the late triassic the widespread central atlantic magmatic province flood basalts remain the most likely cause of the triassic jurassic mass extinction

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

origin ore forming fluid evolution and timing of the logrosan sn w ore deposits central iberian zone spain the logrosan sn w ore deposits in the metallogenic sn w province of the european variscan belt consist of endo and exogranitic greisen type and quartz cassiterite veins associated with a s type granite mineral characterization fluid inclusion study isotope geochemistry and ar ar geochronology have been combined in order to reconstruct the conditions for sn w mineralization the endo and exogranitic mineralization must have been developed in a relatively long lived system 308 303 ma during or soon after the emplacement of the logrosan related granite at ca 308 ma the mineralizing fluids are characterized by complex aqueous and volatile h2o n2 co2 ch4 nacl fluid inclusions microthermometry and raman analyses indicate that fluid composition evolved from n2 ch4 to n2 rich followed by co2 rich fluids with varying amounts of h2o the presence of n2 and ch4 suggests the interaction with fluids derived from the nearby metasedimentary host rocks a model of host rock interaction assimilation and mixing of metamorphic and magmatic fluids resulting in change of the redox conditions is proposed for tin deposition later sulfide minerals were precipitated as a result of pressure and temperature release

40ar 39ar dating of quaternary lavas in northwest iran constraints on the landscape evolution and incision rates of the turkish iranian plateau summary we report five new 40 ar 39 ar ages for basaltic lavas in the maku region of northwest iran between ca 1 87 and 0 40 ma which help constrain the tectonic and landscape evolution of this part of the turkish iranian plateau flows originated from the composite volcanoes ararat agridagi tend urekandyigitdagi ineasternturkey anatolia thesevolcanoesarewithin the turkish iranian plateau which is a consequence of the arabia eurasia collision but has a poorly constrained evolution and surface uplift history current plateau elevations are typically 1 5 2 km and relief between non volcanic summits and basins is typically on the scale of 1 km samples are from flows that passed along pre existing river valleys gorges were cut by re established rivers after the eruptions but the great majority of the local relief 95 per cent lies above the sampled flows and so most likely pre dates the volcanism gorge depths and lava ages allow local quaternary fluvial incision rates to be calculated which are 0 01 to 0 05 mm yr 1 these rates imply slow surface uplift of this part of the turkish iranian plateau during the quaternary we therefore constrain the generation of the great majority of relief in the study area to be pre quaternary and caused by the tectonic construction of the plateau rather than a subcrustal origin related to the quaternary magmatism

automated velocity model building with wavepath tomography wavefield continuation migration generally is recognized as superior to kirchhoff methods in complex velocity models such as below rugose salt bodies it accounts for multipathing sharp velocity contrasts and the limited bandwidth of seismic wave propagation wavepath tomography builds the velocity model in a way that is consistent with the wavefield migration operator traveltime residuals are back projected along a wavepath instead of rays the actual wavefield continuation operator is used to represent the wave propagation between surface source receiver pairs and subsurface reflection points a wavepath is obtained by multiplying impulse responses from a surface location and a reflection point the inversion matrix is kept to a manageable size by restricting the wavepath to the first fresnel zone the considerable expense of computing a single wavepath kernel in comparison to ray tomography is partially offset by the smaller number of back projections necessary to sample the velocity model adequately we have used wavepath tomography to build subsalt velocity models using 2d synthetic data wavepath tomography was implemented in 3d

a high precision 40ar 39ar age for hydrated impact glass from the dellen impact sweden the dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic however if the maximum potential of the terrestrial impact crater record is to be realized accurate and precise ages for crater forming events are critical here we report a high precision 40ar 39ar age for the dellen impact structure sweden previous radio isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach a detailed petrographical and mineralogical study provides a solid foundation for interpretation of step heating 40ar 39ar data culminating in a statistically robust age of 140 82 0 51 ma 2 full external precision for the dellen impact event for which data disfavour an inherited argon component primary hydration of the impact melt during cooling quenching and entrapment of molecular water promoted rapid loss of inherited 40ar from the impact melt of rhyolitic composition duplicate analyses of the water content and d of the glass give similar values for the former 1 9 0 1 mol mg 1 but unexpectedly low values for the latter 159 8 with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity this study highlights that the 40ar 39ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events less

geochemical correlation of three large volume ignimbrites from the yellowstone hotspot track idaho usa three voluminous rhyolitic ignimbrites have been identified along the southern margin of the central snake river plain as a result of wide scale correlations new volume estimates can be made for these deposits 350 km3 for the steer basin tuff and cougar point tuff xi and 1 000 km3 for cougar point tuff xiii these volumes exclude any associated regional ashfalls and correlation across to the north side of the plain which has yet to be attempted each correlation was achieved using a combination of methods including field logging whole rock and mineral chemistry magnetic polarity oxygen isotope signature and high precision 40ar 39ar geochronology the steer basin tuff cougar point tuff xi and cougar point tuff xiii have deposit characteristics typical of snake river sr type volcanism they are very dense intensely welded and rheomorphic unusually well sorted with scarce pumice and lithic lapilli these features differ significantly from those of deposits from the better known younger eruptions of yellowstone the ignimbrites also exhibit marked depletion in 18o which is known to characterise the sr type rhyolites of the central snake river plain and cumulatively represent 1 700 km3 of low 18o rhyolitic magma feldspar values 2 3 2 9 erupted within 800 000 years our work reduces the total number of ignimbrites recognised in the central snake river plain by 6 improves the link with the ashfall record of yellowstone hotspot volcanism and suggests that more large volume ignimbrites await discovery through detailed correlation work amidst the vast ignimbrite record of volcanism in this bimodal large igneous province

geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin 1 we report natural remanent magnetization nrm directions and geomagnetic paleointensity proxies for part of the matuyama chron 0 9 2 2 ma interval from two sites located on sediment drifts in the iceland basin at ocean drilling program sites 983 and 984 mean sedimentation rates in the late matuyama chron are 15 9 and 11 5 cm kyr 1 respectively for the older part of the record 1 2 ma oxygen isotope data are too sparse to provide the sole basis for age model construction the resemblance of the volume susceptibility record and a reference 18o record led us to match the two records to derive the age models this match based on site 983 984 susceptibility is consistent with available site 983 984 benthic 18o data paleointensity proxies were derived from the slope of the nrm versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30 60 mt peak field range paleointensity lows correspond to polarity reversals at the limits of the jaramillo olduvai cobb mountain and reunion subchrons and to seven excursions in nrm component directions magnetic excursions defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator are observed at 932 1048 1115 1190 1215 cobb mountain subchron 1255 1472 1480 1567 1575 gilsa subchron and 1977 ka the results indicate that geomagnetic directional excursions associated with paleointensity minima are a characteristic of the matuyama chron and probably of polarity chrons in general

in situ determination of k ar ages from minerals and rocks using simultaneous laser induced plasma spectroscopy and noble gas mass spectrometry abstract geochronology is one of the foundations of earth sciences and is evolving constantly towards higher reliability and usefulness currently ar ar and u pb dating are the geochronological techniques most used however the classical k ar technique can be modified to measure the age of single minerals or rock sections in situ this new technique combines the use of laser induced plasma spectroscopy for the determination of potassium with noble gas mass spectrometry for the determination of argon both extracted simultaneously by laser ablation this work constitutes both a proof of concept and a test of this method on 13 samples 53 analyses with ages in the range 950 70 ma deviations from the conventional k ar age are lower than 5 for most samples the method is characterized by easy analytical procedures and relatively low uncertainty with the equipment used this method is excellent for exploratory chronology of the earth and extraterrestrial bodies owing to the simple sample preparation and low turnaround time for the analysis

re evaluation of the ages of 40ar 39ar sanidine standards and supereruptions in the western u s using a noblesse multi collector mass spectrometer abstract the rewards from resolving the tempo of magmatic paleobiologic paleoclimatic and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision accuracy and calibration of several chronometers including the 40 ar 39 ar variant of the k ar clock a new generation of multi collector mass spectrometers for 40 ar 39 ar geochronology provides the means to address these issues as well as to evaluate the ages of 40 ar 39 ar standards and key marker horizons deposited by large volume caldera forming supereruptions in the western u s at unprecedented temporal resolution we highlight the utility of a 5 collector noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36 ar h 35 cl from a 36 ar h 35 cl 12 c 3 sum peak at m e 36 maintains a remarkably low and stable background and thus yields 40 ar 39 ar dates of exceptionally high precision we have calibrated a reference gas and developed a simple analytical routine involving one peak hop both of which are used to evaluate detector bias correction factors because the 40 ar 39 ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age precise and accurate determination of the ages of these standards and verification of isotopic composition and homogeneity is essential we present results from both total fusion and incremental heating experiments on single crystals of the widely used alder creek acs and fish canyon fcs sanidine standards several recent independent measurements of acs using multi collector mass spectrometers including those presented herein favor an age of 1 1864 0 0003 0 0012 ma 95 confidence analytical full external uncertainty that is 1 younger than the widely used value obtained by nomade et al 2005 using a single collector mass spectrometer incremental heating experiments on single crystals of fcs most commonly do not yield plateau ages instead many crystals appear to be affected by ar loss or excess ar our findings obtained using ion counters independently confirm those recently obtained using a multiple faraday detectors or a combination of faraday and ion counting detectors thus careful consideration must be given to using fcs in future high precision 40 ar 39 ar calibrations of the geologic timescale notwithstanding 40 ar 39 ar dating of sanidine using the 5 collector noblesse can now provide leverage critical to interpreting the large range of u pb zircon dates commonly found in rhyolitic lavas and tuffs

shear wave velocity and attenuation structure for the shallow crust of the southern korean peninsula from short period rayleigh waves abstract we analyzed the short period rayleigh waves from the first crustal scale seismic refraction experiment in the korean peninsula kcrust2002 to determine the shear wave velocity and attenuation structure of the uppermost 1 km of the crust in different tectonic zones of the korean peninsula and to examine if this can be related to the surface geology of the study area the experiment was conducted with two large explosive sources along a 300 km long profile in 2002 the seismic traces recorded on 170 vertical component 2 hz portable seismometers show distinct rayleigh waves in the period range between 0 2 s and 1 2 s which are easily recognizable up to 30 60 km from the sources the seismic profiles which traverse three tectonic regions gyeonggi massif okcheon fold belt and yeongnam massif were divided into five subsections based on tectonic boundaries as well as lithology group and phase velocities for the five subsections obtained by a continuous wavelet transform method and a slant stack method respectively were inverted for the shear wave models we obtained shear wave velocity models up to a depth of 1 0 km overall the shear wave velocity of the okcheon fold belt is lower than that of the gyeonggi and yeongnam massifs by 0 4 km s in the shallowmost 0 2 km and by 0 2 km s at depths below 0 2 km attenuation coefficients determined from the decay of the fundamental mode rayleigh waves were used to obtain the shear wave attenuation structures for three subsections one for each of the three different tectonic regions we obtained an average value of q 1 in the upper 0 5 km for each subsection q 1 for the okcheon fold belt 0 026 is approximately three times larger than q 1 for the massif areas 0 008 the low shear wave velocity in the okcheon fold belt is consistent with the high attenuation in this region

seismic signature of the lake mien impact structure southern sweden the eugeno s seismic profile iv in southern sweden crossed lake mien a confirmed heavily eroded 121 ma impact structure the lake mien structure has a pre erosion rim diameter of about 9 km it is

argon extraction from geological samples by co2 scanning laser step heating lasers are fundamental tools for sampling in geochemical studies and have found wide application in mass spectrometric sample introduction systems here we describe an isotope extraction method for 40ar 39ar geochronology using a new scanning co2 laser system this method can partially un mix radiogenic 40ar from trapped argon components and provides an alternative to furnace step heating methods a key advantage of the laser scanning method developed at the scottish universities environmental research centre suerc is the ability to step heat samples as large as 100 mg to fusion using low raster speeds although care must be taken to avoid self shielding of grains and proper laser targeting the scanning laser extraction system has the potential for lower overall blanks and the ability to run blanks and calibrations between steps of a heating sequence this provides better control on system performance and characterization during sample measurement and can result in improved data quality

40ar 39ar dating of hydrothermal activity biota and gold mineralization in the rhynie hot spring system aberdeenshire scotland this study presents a new high precision 40ar 39ar age for the devonian hot spring system at rhynie hydrothermal k feldspar sampled from two veins that represent feeder conduits and a hydrothermally altered andesite wall rock date the hydrothermal activity the fossilised biota and syn k feldspar gold mineralization at 403 9 2 1 ma 2 oxygen isotope data for the parent fluid 4 to 2 show that the k feldspar was precipitated from a dominantly meteoric fluid which mixed with magmatic fluids from a degassing magma chamber the 40ar 39ar age 403 9 2 1 ma 2 when recalculated 407 1 2 2 ma 2 with respect to the astronomically tuned age for fish canyon sanidine 28 201 0 023 ma 1 also provides a robust marker for the polygonalis emsiensis spore assemblage biozone within the pragian earliest emsian furthermore the age identifies the devonian pull apart volcano sedimentary basins of the british and irish caledonides and their root zones as specific targets for future gold exploration

a metrological approach to measuring 40ar concentrations in k ar and 40ar 39ar mineral standards in geochronology isotopic ages are determined from the ratio of parent and daughter nuclide concentrations in minerals for dating of geological material using the k ar system the simultaneous determination of 40ar and 40k concentrations on the same aliquot is not possible therefore a widely used variant the 40ar 39ar technique involves the production of 39ar from 39k by neutron bombardment and the reliance on indirect natural calibrators of the neutron flux referred to as mineral standards many mineral standards still in use rely on decades old determinations of 40ar concentrations resulting uncertainties both systematic and analytical impede the determination of higher accuracy ages using the k ar decay system we discuss the theoretical approach and technical design of a gas delivery system which emits metrologically traceable amounts of 40ar and will allow for the sensitivity calibration of noble gas mass spectrometers the design of this system is based on a rigorous assessment of the uncertainty budget and detailed tests of a prototype system a number of obstacles and proposed resolutions are discussed along with the selection of components and their integration into a pipette system

multi phase cooling of early cretaceous granites on the jiaodong peninsula east china evidence from 40 ar 39 ar and u th he thermochronology abstract the jiaodong peninsula at the southeastern periphery of the north china craton ncc has undergone complex changes since the mesozoic a striking feature of the ncc is the widespread distribution of late mesozoic magmatic rocks predominantly characterized by granitic intrusions these igneous plutons are economically important due to their polymetallic mineralization and significant as geodynamic tracers of the regional tectonic evolution two representative early cretaceous granites the yashan and yuangezhuang plutons on the jiaodong peninsula were selected for multiple thermochronometer studies to constrain a complete thermal history from their emplacement to near surface conditions we obtained biotite 40ar 39ar ages of two granites on the new generation argus vi noble gas mass spectrometer two biotites registered identical 40ar 39ar plateau ages of 117 ma which are consistent within the error with their zircon u pb ages this suggested that the two granites were emplaced at the same time and cooled rapidly to about 300 c the single crystal zircon u th he zhe dating for the two samples gave weighted mean ages of 93 5 4 5 ma and 93 3 5 0 ma combined with previously published geochronological data and the thermal inversion of the zircon low temperature thermochronological data three phases of rapid cooling were identified for the two studied plutons 1 the early cretaceous 117 110 ma 2 the late cretaceous 100 90 ma and 3 the early tertiary 65 50 ma the early cretaceous phase of cooling occurred in a regional extensional regime on the ncc characterized by various extensional structures the late cretaceous cooling is consistent with the third episode of the yanshan movement which caused a regional basin inversion event on the ncc in comparison to the first two phases of rapid cooling the early tertiary cooling was more gradual this phase corresponded to a widely developed eocene rift on the ncc and the reactivation of major boundary faults such as the giant tan lu fault zone and the taihangshan fault

advanced techniques in site characterization and mining hazard detection for the underground coal industry abstract today s highly productive coal mining systems represent large capital investments careful geotechnical planning and design are essential to protect these systems and to help ensure their overall success too often however the ability of the mine operator to adequately characterize the geotechnical environment and keep up with changing geologic conditions lags the advance rate of the mining equipment seismic tomographic imaging based on the same principles as medical computer aided tomography cat scans offers the ability to identify changing geologic conditions loading patterns and the location of nearby abandoned mine workings although seismic tomography has been used for many years in the oil industry until recently its practicality in the mining industry has been limited because the processes involved interfered with mining productivity and were labor intensive in terms of data collection and interpretation new advancements in signal processing have greatly enhanced the speed resolution and range of applications of tomographic imaging in underground coal mine settings many tomography applications in the mining industry use seismic velocity and or attenuation tomography within a volume enclosed by the seismic source and receiver array although results obtained using this geometry work well in many situations recent developments in reflection tomography show promise in eliminating the requirement that the target volume be surrounded by sources receivers for example in seam seismic reflection tomography may now be used to image structures and or old workings from one general location in the mine e g face areas of mains and panel developments well ahead of planned developments largely eliminating the need to probe hole drill on regular intervals these new developments have greatly expanded the ability of the mine operator to cost effectively characterize previously inaccessible areas of the property this paper gives an overview of the theories and processes involved in seismic tomography applicable to coal mine settings recent examples are presented of tomographic imaging applied to a variety of coal mine ground control problems and old works detection in the united states and europe the applicability of both seismic refraction and seismic reflection tomography is discussed

the argus multicollector noble gas mass spectrometer performance for 40ar 39ar geochronology we describe a new high sensitivity multicollector noble gas mass spectrometer argus that has been specifically designed for simultaneous collection of all ar isotopes and hence is ideally suited for 40ar 39ar geochronology the instrument uses a detector housing that holds five faraday collectors one collector is equipped with a 1011 ohm resistor for 40ar and the remaining four are fitted with 1012 ohm resistors for 36ar to 39ar argus has a mass resolution of 225 250 and a measured sensitivity of 7 10 14 mol v at 200 a trap current during the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40ar 36ar of 300 67 0 07 2 eight data points rejected and mean square weighted deviation 1 18 the ages of three mineral standards taylor creek rhyolite sanidine heidelberg biotite and alder creek sanidine are within error of the accepted ages and uncertainties for two are improvements on previously published data argus decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak jumping single collector mass spectrometers

constraints to the timing of india eurasia collision a re evaluation of evidence from the indus basin sedimentary rocks of the indus tsangpo suture zone ladakh india deposited within the indus tsangpo suture zone the cenozoic indus basin sedimentary rocks have been interpreted to hold evidence that may constrain the timing of india eurasia collision a conclusion challenged by data presented here the eurasian derived 50 8 51 ma chogdo formation was previously considered to overlie indian plate marine sedimentary rocks in sedimentary contact thus constraining the timing of collision as having occurred by this time using isotopic analysis u pb dating on detrital zircons ar ar dating on detrital white mica sm nd analyses on detrital apatite sandstone and conglomerate petrography mudstone geochemistry facies analysis and geological mapping to characterize and correlate the formations of the indus basin sedimentary rocks we review the nature of these contacts and the identification and correlation of the formations our results reveal that previously interpreted stratigraphic contacts identifying chogdo formation unconformably overlying indian plate sedimentary rocks are incorrect rather we suggest that the inaccuracy of previous interpretations is most likely a result of formation misidentification and thus cannot be used to constrain the timing of india asia collision

3d common reflection point based seismic migration velocity analysis three dimensional prestack depth migration and depth residual picking in common image gathers cigs are the most time consuming parts of 3d migration velocity analysis most migration based velocity analysis algorithms need spatial coordinates of reflection points and cig depth residuals at different offsets or angles to provide updated velocity information we propose a new algorithm that can analyze 3d velocity quickly and accurately spatial coordinates and orientations of reflection points are provided by a 3d prestack parsimonious depth migration the migration involves only the time samples picked from the salient reflection events on one 3d common offset volume ray tracing from the reflection points to the surface provides a common reflection point crp gather for each reflection point predicted nonhyperbolic moveouts for local velocity perturbations based on maximizing the stacked amplitude give the estimated velocity updates for each crp gather then the velocity update for each voxel in the velocity model is obtained by averaging over all predicted velocity updates for that voxel prior model constraints may be used to stabilize velocity updating compared with other migration velocity analyses the traveltime picking is limited to only one common offset volume and needs to be done only once there is no need for intensive 3d prestack depth migration hence the computation time is orders of magnitude less than other migration based velocity analyses a 3d synthetic data test shows the algorithm works effectively and efficiently

seismic refraction studies in the basement of the vredefort structure abstract the 30 mgal positive gravity anomaly in the centre of the basement of the vredefort structure has been a feature of discussion for a number of years to supply constraints on the depth and nature of this body refraction seismic measurements were made on a n s line across the basement anomaly the results support a three fold subdivision of the uppermost structure on the seismic profile granitic gneisses in the north dioritic gneisses in the central sector over the gravity anomaly and carboniferous karoo strata in the southern part of the basement throughout the central section of the refraction profile rocks with p wave velocity of 6 40 6 43 km s dominate the profile whilst to the north and south of this central section basement velocities are all less than 6 0 km s computer travel time simulations of the basic model were carried out using 2 d ray tracing

in situ seismic measurements in borehole lb 08a in the bosumtwi impact structure ghana preliminary interpretation in order to assist in the interpretation of previous seismic refraction and reflection surveys a vertical seismic profile was acquired in the lake bosumtwi ghana hard rock core hole lb 08a no seismic reflections are seen in the up going wave field obtained and this observation is consistent with the lack of reflectivity observed in the corresponding 2 d surface seismic profile obtained in earlier studies direct down going p waves were found both in the cased sediment column from a depth of 73 m to 239 m below the lake surface and in the open hole hard rock section from a depth of 239 m to 451 m of lb 08a analysis of the observed travel times indicates a nearly constant p wave velocity of 1520 m s through the soft lacustrine sediments in the hard rock however the p wave velocity rapidly increases by nearly 30 from 2600 m s to 3340 m s these values are in good agreement with the gross velocity structure obtained in the earlier joint inversion of seismic reflection and refraction data these values are low relative to those expected for the metasedimentary protoliths an observation that has been made at other young impact structures of similar size the low velocities together with the fact that they increase so rapidly is suggestive of a decreasing density of fractures and microcracks with depth consequently the seismic velocity trend may provide a proxy measure of damage and hence the decay of the shock pressure from the impact point validation of this requires additional detailed studies of the porosity structure in the core

on folding and shear zone development a mesoscale structural study on the transition between two different tectonic styles abstract an example of coeval development of ductile shear zones and folds in schistose terrains is documented by detailed mapping and structural analysis folds and shear zones were formed under retrogressive conditions after the peak of the regional hercynian metamorphism and both affect a previous regional schistosity a gradual change in structural style on a regional scale is evidenced by the disappearance in higher grade domains of late folds and the preponderance of shear zones geometrical analysis reveals that the bulk structural pattern is compatible with a non coaxial strain history structural variation reflects a gradual change in mechanical behaviour of schists which show a marked variation in crystallinity at the time of deformation

the polygenetic kahouanne seamounts in the northern lesser antilles island arc evidence for large scale volcanic island subsidence abstract remotely operated vehicle rov explorations of the kahouanne seamounts located 25 km se of montserrat in the northern lesser antilles island arc have discovered the occurrence of honeycomb scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level mbsl these features combined with the flat topped morphology of the seamounts the occurrence of shallow water carbonates rhodoliths benthic foraminifera and the presence of oxidized highly vesicular volcanic fragments suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth the subsidence is likely to have been in response to a combination of 1 graben development southeast of the island of montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the lesser antilles volcanic arc and 2 regional subsidence inferred from studies of nearby carbonate platforms 40ar 39ar dating of basalt andesite lavas from the central seamount indicates active volcanism around 5 4 ma shallow water carbonates recovered from an apparent wave cut terrace at 600 mbsl in the same area were deposited approximately 3 ma based on sr isotope stratigraphy indicating significant subsidence of the complex since that time the total subsidence is in line with displacements in the kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of guadeloupe and martinique implying that intra arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc

geophysical aspects of meteorite impact craters in eroded shield environment with special emphasis on electric resistivity abstract the traditional geophysical methods of magnetics and gravity as applied to meteorite impact crater studies are shortly reviewed their optimal resolution power with respect to crater size and pre existing structures is discussed the morphology of some typical impact craters is illustrated to show the character of the post impact deformation a new geophysical method in impact crater studies the in situ measurement of electrical resistivity is introduced and illustrated with case studies from simple and complex craters it is concluded that the resistivity method represents a useful tool for mapping the extent of authigenic brecciation in crater structures and their surroundings the method constitutes in combination with detailed elevation data gravity and magnetic measurements a powerful tool for constraining the classification of impact craters in crystalline shield environments the resistivity measurements have revealed a complex brecciation pattern in large central uplift structures consisting of a central less brecciated core surrounded by a ring of more intense brecciation typical impact brecciated crystalline rocks have resistivities between 2000 and 8000 ohm m the crater structures treated in this paper are bjorko dellen siljan and uppland all located in sweden

argon redistribution during a metamorphic cycle consequences for determining cooling rates 40ar 39ar thermochronology is commonly used to constrain the rates and times of cooling in exhumed metamorphic terranes with ages usually linked to temperature via dodson s closure temperature tc formulation whilst many metamorphic 40ar 39ar data are consistent with the timing of crystallisation or cooling within a chronological framework defined by other higher temperature chronometers other 40ar 39ar data are more difficult to interpret we report white mica and biotite single grain fusion and laser ablation 40ar 39ar ages from felsic gneisses from the western gneiss region norway the rocks record isothermal decompression from peak eclogite facies conditions white mica stable to amphibolite facies conditions biotite stable at c 700 c white mica and biotite yield dispersed single grain fusion dates from 416 to 373 ma and 437 to 360 ma respectively in situ laser ablation analyses provide a similar range with white mica spot ages ranging from 424 to 370 ma and biotite spot ages ranging from 437 to 370 ma the dates span the duration of the metamorphic cycle suggested by previous studies and cannot be reconciled with the results of simple models of ar loss by diffusion during cooling samples that show evidence for different physical processes such as the chemical breakdown of white mica partial melting and fluid ingress generated different age populations to samples that did not experience or record obvious petrological evidence for these processes samples that record significant recrystallization and deformation yielded younger white mica but older biotite single grain fusion ages than more pristine samples amphibolite facies gneisses that preserve evidence for significant partial melting generated younger biotite ages than samples that recorded evidence for significant hydration our data support other reported observations that high temperature metamorphic mica 40ar 39ar dates cannot be assumed to record the timing of cooling through a specific temperature window careful assessment of the petrographic context of the dated minerals and consideration of their post crystallisation history may provide a more robust insight into whether age links to stage in a temporally meaningful way

tristan da cunha constraining eruptive behavior using the 40ar 39ar dating technique the historically active volcanic ocean island of tristan da cunha exhibits a complex and dynamic history with numerous often compositionally distinct parasitic centers punctuating the large edifi ce to date the temporal relationship between differing styles of activity has been unclear we have applied high precision 40ar 39ar dating to 15 carefully selected samples from tristan da cunha to ascertain spatio temporal relationships of recent volcanism explore episodicity and establish if the most recent summit activity post dated eruptions from parasitic centers lower on the fl anks this has yielded a new suite of reliable holocene ages with the youngest dated deposit at 3 1 ka 1 a recent fl ow at the summit was constrained to 5 1 ka 1 confi rming that summit and parasitic activity on the volcano s fl anks overlap in time the oldest dated deposits were 118 4 ka 1 from a parasitic cone in the southern sector and 81 10 ka 1 from one of the lowest sub aerial shield forming lava fl ows in the northern sector large scale sector collapse is bracketed between 34 1 ka and 26 5 ka 1 via dating of the youngest headwall lava fl ow and oldest sub aerial scarp fi lling deposits no systematic relationship between the new temporal framework vent location and eruptive compositions was found although magmatic fl ux has been inferred to be relatively low tristan da cunha is capable of relatively frequent eruptions from a wide variety of vent locations across a broad range of compositions

feldspar 40 ar 39 ar dating of icdp paleovan cores abstract volcaniclastic fall deposits in icdp drilling cores from lake van turkey contain sodium rich sanidine and calcium rich anorthoclase which both comprise a variety of textural zoning and inclusions an age model records the lake s history and is based on climate stratigraphic correlations tephrostratigraphy paleomagnetics and earlier 40 ar 39 ar analyses stockhecke et al 2014b results from total fusion and stepwise heating 40 ar 39 ar analyses presented in this study allow for the comparison of radiometric constraints from texturally diversified feldspar and the multi proxy lacustrine age model and vice versa this study has investigated several grain size fractions of feldspar from 13 volcaniclastic units the feldspars show textural features that are visible in cathodoluminescence cl or back scattered electron bse images and can be subdivided into three dominant zoning types 1 compositional zoning 2 round pseudo oscillatory zoning and 3 resorbed and patchy zoning ginibre et al 2004 round pseudo oscillatory zoning records a sensitive alternation of fe and ca that also reflects resorption processes this is only visible in cl images compositional zoning reflects anticorrelated anorthite and orthoclase contents and is visible in bse eleven inverse isochron ages from total fusion and three from stepwise heating analyses fit the age model four experiments resulted in older inverse isochron ages that do not concur with the model within 2 uncertainties and that deviate from 1 ka to 17 ka minimum c and r type zoning are interpreted as representing growth in magma chamber cupolas as wall mushes or in narrow conduits persistent compositions of po type crystals and abundant surfaces recording dissolution features correspond to formation within a magma chamber c type zoning and r type zoning have revealed an irregular incorporation of melt and fluid inclusions these two types of zoning in feldspar are interpreted as preferentially contributing either heterogeneously distributed excess 40 ar or inherited 40 ar to the deviating 40 ar 39 ar ages that are discussed in this study

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

long term changes in explosive and effusive behaviour at andesitic arc volcanoes chronostratigraphy of the centre hills volcano montserrat volcanism on montserrat lesser antilles arc has migrated southwards since the formation of the silver hills 2 5 ma and has formed three successively active volcanic centres the centre hills volcano was the focus of volcanism from 1 0 4 ma before activity commenced at the currently active soufriere hills volcano the history of activity at these two volcanoes provides an opportunity to investigate the pattern of volcano behaviour on an andesitic arc island over the lifetime of individual volcanoes here we describe the pyroclastic stratigraphy of subaerial exposures around central montserrat identifying 11 thick 1 m pumiceous units derived from sustained explosive eruptions of centre hills from 0 8 0 4 ma over 10 other less well exposed pumiceous units have also been identified the pumice rich units are interbedded with andesite lava breccias derived from effusive dome forming eruptions of centre hills the stratigraphy indicates that large up to magnitude 5 explosive eruptions occurred throughout the history of centre hills alongside effusive activity this behaviour at centre hills contrasts with soufriere hills where deposits from sustained explosive eruptions are much less common and restricted to early stages of activity at the volcano from 175 130 ka subsequent eruptions at soufriere hills have been dominated by andesitic effusive eruptions the bulk composition petrography and mineral chemistry of volcanic rocks from centre hills and soufriere hills are similar throughout the history of both volcanoes except for occasional transient departures to different magma compositions which mark shifts in vent location or dominant eruption style for example the final recorded eruption of centre hills before the initiation of activity at soufriere hills was more silicic than any other identified eruption on montserrat and the basaltic south soufriere hills episode marked the transition to the current stage of predominantly effusive soufriere hills activity the compositional stability observed throughout the history of centre hills and soufriere hills suggests that a predominance towards effusive or explosive eruption styles is not driven by major compositional shifts of magma but may reflect local changes in long term magma storage conditions that characterise individual episodes on 105 year timescales of volcanism on montserrat

the lake bosumtwi impact crater ghana the 1 m y old bosumtwi crater ghana has a nearly circular shape with a rim diameter of 11 km north south and 10 km east west it is surrounded by a circular depression and an outer ridge of diameter 20 km polymict breccias averaging at least 20 m thick with clasts as much as 5 m long occur on the outer ridge and the crater rim shows in situ shattered rock patches of suevite have been found in the circular depression north and south of the crater analogy with better known craters suggests that bosumtwi has a central uplift rising to 200 m beneath the lake floor an aeromagnetic anomaly of amplitude 50 nanotesla nt over the northern half of the lake is interpreted as due to a layer of magnetized fallback breccia beneath the lake sediments the normal polarity of the breccia shows that the crater was formed during the normal jaramillo event of 0 97 to 0 85 m y ago which agrees with the magnetic stratigraphy of the related ivory coast microtektites a regional gravity survey indicates a negative bouguer anomaly over the crater there is some geochemical evidence that the meteorite was an iron and its mass and energy are suggested as about 108 tons and 3 1019 joules or 7 3 103 megatons

shrimp u pb dating of the preeruption growth history of zircons from the 340 ka whakamaru ignimbrite new zealand evidence for 250 k y magma residence times the whakamaru group ignimbrites are products of one of the largest caldera forming eruptions from the central taupo volcanic zone new zealand ca 340 ka imaging of zircons separated from low silica rhyolite pumice from whakamaru group ignimbrite reveals a complex magmatic crystallization history some grains contain resorbed cores cores and rims of individual zircons were dated using the sensitive high resolution ion microprobe due to the high u contents of the zircons 206 pb count rates were sufficiently high to obtain 206 pb 238 u ages with precisions of 5 10 1 s using longer than usual 206 pb count times and this has allowed zircon core and rim ages to be distinguished zircon rim and outer growth zone ages range from within 1 s error of the eruption age to 449 20 ka whereas zircon cores produced ages to 608 20 ka indicating magmatic residence times exceeding 250 k y for some zircons the data are consistent with a prolonged evolution of the magma system over hundreds of thousands of years involving magma replenishment and mixing with residual phases of older silicic magma chambers in addition to crystal fractionation processes prior to eruption the study has shown that 206 pb 238 u dating using high resolution ion microprobe can be used to determine zircon preeruptive growth histories in quaternary lavas and pyroclastic rocks

in situ measurements of shear wave velocity in sediments with higher mode rayleigh waves a seismic survey was carried out on a tidal flat in the sw netherlands in order to determine shear wave velocities in sediments by means of higher mode rayleigh waves the dispersion properties of these rayleigh waves were measured in the 2 d amplitude spectrum or f k spectrum and resulted in phase velocities for six different modes as a function of frequency 5 30 hz these observed phase velocities were inverted for a nine layer model for the shear wave velocity to a depth of 50 m

tomographic velocity estimation with plane wave synthesis in areas with structurally complex geology tomographic velocity analysis is often required to estimate velocities in this paper i describe an algorithm for tomographic velocity estimation that uses plane wave synthesis imaging as a prestack migration the classical iterative two step process measures the traveltime errors with the current velocity model and then update the velocity model is performed as follows the events are picked in the image space after prestack migration with surface oriented plane wave synthesis imaging the traveltime deviations are measured through residual moveout rmo velocity analysis in common surface location csl gathers obtained by reflector oriented plane wave synthesis imaging and the velocity update is calculated by inverting the traveltime deviations through a conjugate gradient the results from synthetic data indicate that the tomographic method successfully estimates interval velocity models that lead to depth migrated images with no residual moveout

the 40ar 39ar and k ar dating of lavas from the hilo 1 km core hole hawaii scientific drilling project mauna kea lava flows cored in the hilo hole range in age from 200 ka to about 400 ka based on 40ar 39ar incremental heating and k ar analyses of 16 groundmass samples and one coexisting plagioclase the lavas all subaerially deposited include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic transitional tholeiitic and tholeiitic basalts the lower section has yielded predominantly complex discordant 40ar 39ar age spectra that result from mobility of 40ar and perhaps k the presence of excess 40ar and redistribution of 39ar by recoil comparison of k ar ages with 40ar 39ar integrated ages indicates that some of these samples have also lost 39ar nevertheless two plateau ages of 391 40 and 400 26 ka from deep in the hole combined with data from the upper section show that the tholeiitic section accumulated at an average rate of about 7 to 8 m kyr and has a mean recurrence interval of 0 5 kyr flow unit samples from the upper section yield relatively precise 40ar 39ar plateau and isotope correlation ages of 326 23 241 5 232 4 and 199 9 ka for depths of 415 7 m to 299 2 m within their uncertainty these ages define a linear relationship with depth with an average accumulation rate of 0 9 m kyr and an average recurrence interval of 4 8 kyr flow unit the top of the mauna kea sequence at 280 m must be older than the plateau age of 132 32 ka obtained for the basal mauna loa flow in the corehole the upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to mauna kea as it rode the pacific plate away from its magma source the hawaiian mantle plume the age depth relation in the core hole may be used to test and refine models that relate the growth of mauna kea to the thermal and compositional structure of the mantle plume

cryptotephrochronology of the eemian and the last interglacial glacial transition in the north east atlantic the eemian interglacial and the onset of the subsequent glacial period serve as the most recent analogue for the natural operation of the climate system during the current interglacial pronounced climatic oscillations occurred during this period but their nature and pattern are poorly understood due to dating limitations and unknown phase relationships between different regions and archives tephrochronology offers considerable potential for precise correlation of disparate palaeoclimatic archives preserving evidence of these rapid climatic transitions through the tracing of common isochronous tephra horizons we outline the identification of three previously unknown cryptotephra horizons within a marine core from the rockall trough north east atlantic this sequence preserves a high resolution record of this interval and shard size geochemical heterogeneity and the co variance of shard concentrations with ice rafted debris data are utilized to demonstrate that primary airfall was the most likely transport and depositional pathway the main geochemical populations of these horizons have similar transitional alkali major and trace element compositions suggesting that they were derived from a common icelandic source potentially the oraefajokull volcanic system these tephra horizons represent additions to the north atlantic event stratigraphy for this period and tentative correlations to icelandic terrestrial deposits are proposed for two horizons

long term low latitude cosmogenic 3he production rate determined from a 126 ka basalt from fogo cape verdes an ankaramite lava from fogo cape verdes has been used to determine the long term cosmogenic 3he 3hecos production rate at low latitude 14 9 n 40ar 39ar incremental heating yield an eruption age of 125 8 7 ka cosmogenic 3he abundances yield local production rates of 75 6 7 0 atoms g year in olivine 1 n 10 and 70 1 2 9 atoms g yr in pyroxene 1 n 11 the lower 3hecos production rate in pyroxene is consistent with compositional control on production when scaled to sea level high latitude using available algorithms the olivine 3hecos production rate ranges from 86 109 atoms g yr these values overlap the long term production rate determined at the canary islands 99 atoms g yr they are however lower than previously reported long term production rates from hawaii and southern south america 129 152 atoms g yr and the range of 20 ka calibration sites 110 130 atoms g yr erosion burial and high atmospheric pressure appear to be of negligible importance the likeliest explanation is that the geomagnetic field strength was higher in the past the agreement between the fogo geomagnetic field strength corrected production rate scaled using various published scaling algorithms suggests that differences in incorporation of the non dipole component of the earth magnetic field in the scaling algorithms are negligible for long term production rate determinations at low elevation and low latitude longitudinal variation in long term cosmogenic nuclide production rates between the central atlantic ocean and the americas hawaii and patagonia may exist this has implications for exposure dating of old surfaces it will require that locally determined production rates should be used where available and that scaling schemes incorporating longitudinal variations should be used if nearby calibration sites are not available furthermore if long term longitudinal production rate variations exist it implies that variation in the geomagnetic field may not average out after around 20 ka but may require significantly longer periods up to 100 ka

palaeomagnetism and the siljan impact structure central sweden summary the siljan impact structure diameter c 52 km located in central sweden is considered the largest meteorite crater in western europe the time of impact has been estimated to be 368 ma from ar ar dating of an impact melt a palaeomagnetic study of rocks from inside and outside the impact structure has been carried out with the aim of identifying natural remanent magnetizations nrms related to the impact event primary magnetizations are partly preserved within the impact area however the directions of these original magnetizations are disturbed due to tectonic movements related to the impact three different directions of magnetizations not recognized elsewhere were identified within the central part of the structure these three magnetizations are defined in order of increasing coercivity hc and blocking temperature tb the origin of a high hc chemical remanent magnetization crm c of late palaeozoic time carried by haematite can be interpreted as due to oxidation caused by circulating meteoric water in cracks and fractures caused by the impact another component b identified in a lower temperature range 40d 580dc is probably carried by magnetite in pseudotachylite this magnetization is considered as a thermal remanent magnetization while in other rocks its pre tectonic character and occurrence also in the low coercivity range implies a shock remanent origin the palaeomagnetic pole for this remanence corresponds to a silurian devonian age which gives an age of impact which is not very diffirent from that estimated by radiometric data a third low hc and tb component a is interpreted to be a crm carried by slowly growing goethite the direction of this crm implies it to be a recent post impact magnetization

40ar 39ar evidence for early deglaciation of the central chilean andes spatial variability of deglaciation and warming since the last glacial maximum is critical for relating hemispheric or regional climate forcing mechanisms to global orbitally induced insolation changes radiocarbon ages of moraines ice cores and deep sea sediments from the southern hemisphere suggest that initial warming and glacial recession began before 23 000 years ago but that in latitudinal belts of concentrated precipitation glaciers readvanced 18 000 and 13 000 years ago 40 ar 39 ar dating of glaciated and unglaciated lava flows 2200 m in elevation at laguna del maule central chilean andes implies that at 36 s the southern andean ice cap retreated between 25 600 1 200 and 23 300 600 years ago and did not expand afterwards these ages are reconciled if increased insolation ablated sea ice cover and mountain glaciers in the southern hemisphere 5 000 years before the northern and if subsequently laguna del maule lay north of the westerly winds delivering moisture to more southerly 39 52 s portions of the ice cap

high resolution 40ar 39ar dating using a mechanical sample transfer system combined with a high temperature cell for step heating experiments and a multicollector argus noble gas mass spectrometer 40ar 39ar dating of young 5 10 16 mol 36ar is better than 0 5 1 0 1 n 4 8 we illustrate the system performance by 40ar 39ar dating of whole rock samples and mineral separates from the oman ophiolite as well as from the siebengebirge heldburg and rhon volcanic provinces in central germany

geology of the cenozoic indus basin sedimentary rocks paleoenvironmental interpretation of sedimentation from the western himalaya during the early phases of india eurasia collision this study reassesses the stratigraphy sedimentology and provenance of the indus basin sedimentary rocks deposited within the indus tsangpo suture zone itsz during the early phases of india eurasia collision using field observations biostratigraphy and petrographic and isotopic analyses we create a paleodepositional reconstruction within the paleotectonic setting of the early phases of india eurasia collision we then re examine existing constraints to the timing of india eurasia collision previously interpreted from the earliest occurrence of mixed indian and eurasian derived detritus in the succession from mid cretaceous to early paleocene times the jurutze and sumda formations were deposited within an arc bounded marine basin between the dras and kohistan ladakh island arcs the 51 ma aged deltaic chogdo formation then filled the basin until deposition of the 50 8 49 4 ma aged nummulitic limestone during a marine incursion before continental facies developed in an evolving intermountain basin with the deposition of the paleogene indus group within these systems sediment was sourced from the eurasian margin to the north and was transported southward into the suture zone in this section we see no unequivocal evidence of indian plate input to the sedimentary succession and thus no evidence of mixed indian eurasian derived detritus indicative of india asia collision until the upper stratigraphic horizons of the indus group when facies are representative of an axial northwesterly flowing river system we suggest that the paleo indus river was initiated within the itsz during late oligocene early miocene times sedimentation of the indus group continued until the late miocene

the temporal record of magmatism at cerro uturuncu bolivian altiplano twenty six new 40ar 39ar plateau ages for 23 lavas and domes from the uturuncu volcano in the altiplano of sw bolivia reveal a protracted eruptive history from 1050 5 to 250 5 ka eruptions have been exclusively effusive producing some 50 km3 of high k dacites and silicic andesites bimodal mineral compositions complex mineral textures the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at uturuncu post 458 ka distinct high and low mgo cr magmas are resolved these magmas erupt during similar times suggesting that eruptions are tapping different parts of the magma system albeit from the same vent system volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath uturuncu and calculated buoyancy forces are sufficient to drive effusive eruptions eruptive activity is episodic with six eruptive periods separated by hiatuses of 50 kyr cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka the episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying altiplano puno magma body

40ar 39ar systematics of melt lithologies and target rocks from the gow lake impact structure canada abstract the age of the gow lake impact structure saskatchewan canada is poorly constrained with previous estimates ranging from 100 to 250 ma using a combination of step heating and uv laser in situ 40ar 39ar analyses we have sought to understand the 40ar 39ar systematics of this small impact crater and obtain a more precise and accurate age this structure is challenging for 40ar 39ar geochronology due to its small size 5 km diameter the silicic composition of the target rock and the large difference in age between the impact event and the target rock 1 2 ga these factors can serve to inhibit argon mobility in impact melts leading to retention of extraneous 40ar and anomalously older measured ages we mitigated the undesirable effects of extraneous 40ar retention by analysing small volume aliquots of impact glass using step heating and even smaller volumes via the uv laser in situ 40ar 39ar technique although primary hydration of impact generated glasses enhanced the diffusivity of 40ar inherited from silica rich melts data still had to be corrected for extraneous 40ar by using isotope correlation plots to define the initial trapped 40ar 36ar components our inverse isochron age of 196 8 9 6 9 9 ma 2 analytical external precision demonstrates that the gow lake event occurred within uncertainty of the triassic jurassic boundary but there is no evidence that it was part of an impact cluster

construction of component maps from aeromagnetic total field anomaly maps1 total field anomalies as defined from normal aeromagnetic surveys give good approximations of the anomalous components along the direction of the main geomagnetic field which is generally much larger than the anomalous field using the relations between vertical and horizontal components of the field the total field anomaly is related to any vertical or horizontal component and the corresponding horizontal and vertical derivatives an example from the siljan impact structure indicates that such directional filters may be applied to extract useful information from magnetic maps

seismic velocities and q factors in the uppermost crust beneath the sveka profile in finland the sveka deep seismic sounding profile in central finland was shot in 1981 to study the crustal structure of the fennoscandian shield in the contact zone between the archaean and svecokarelian provinces apart from p and s waves well developed rayleigh surface waves r waves with periods of 0 5 2 0 s were recorded from all the shot points in the frequency band 0 5 2 5 hz the r waves were clearly observed on the entire 325 km long profile amplitudes of p s and r waves corrected for charge size were used to study the attenuation of waves and q factors from the records of body and r waves the distributions of seismic velocities and quality factors were determined using the dynamic ray tracing method 2 d modelling and the reflectivity method 1 d modelling in the 2 d model of the uppermost crust the p wave velocities vary from 5 8 5 9 km s at the top to 6 2 6 3 km s at a depth of 6 km the velocity ratio solvpvs varies with depth and distance from 1 67 to 1 74 reaching its maximum value of 1 8 2 0 in the uppermost 200 500 m and values of 1 76 1 80 in the uppermost 1 km the quality factors qp and qs vary in the uppermost 1 km from 20 to 120 reaching a higher value of 140 in the marginal zone of the archaean basement in the depth range of 1 2 km q values of 80 400 were found in the depth range 2 6 km q values of 300 800 were found for the svecokarelian province and 200 400 for the archaean the low values of vs and q in the uppermost 1 km are thought to be the result of rock fracturing the crack density was estimated for the uppermost crust along the profile for the sveka profile we found values of 0 35 0 54 in the uppermost 200 m 0 10 0 32 at a depth of 0 2 1 0 km and 0 1 for depths of 1 4 km the comparison of our seismic data with results of theoretical and laboratory investigations implies that the uppermost crustal rocks are saturated the zones of strong attenuation of r waves coincide with faults and schist zones which are well known from surface geology

tracing time in the ocean a brief review of chronological constraints 60 8 kyr on north atlantic marine event based stratigraphies abstract well resolved event based stratigraphies in marine sediments spanning a significant portion of the last glacial period 60 8 kyr provide a unique opportunity for time stratigraphic correlation in the north atlantic region here we review the current methods available to chronologically constrain these event based stratigraphies highlighting in particular the value of tephrochronology as an independent tool to validate correlations between records while the intimate protocols lowe et al 2008 blockley et al 2011 are equally applicable to marine and terrestrial records spatially and temporally variable marine radiocarbon reservoir age effects mres provide a challenge to using marine radiocarbon in the former as an independent chronostratigraphic tool despite the inherent uncertainties associated with tuning we conclude that the mid points of the common abrupt warming transitions associated with the well defined millennial scale climate oscillations the dansgaard oeschger d o cycles observed in the oxygen isotopes of the greenland ice cores and north atlantic sea surface temperature sst records currently provide the most robust correlation tie points from which to derive age control in this invited intimate special issue article we propose a new protocol for establishing marine event based chronostratigraphies in the north atlantic region and focus on areas of chronological potential in palaeoceanographic research

east african climate change and orbital forcing during the last 175 kyr bp abstract variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long term fluctuations in african lake levels the understanding of such relationships is essential for designing climate prediction models for the tropics our assessment of the nature and timing of east african climate change is based on lake level fluctuations of lake naivasha in the central kenya rift 0 55 s 36 20 e inferred from sediment characteristics diatoms authigenic mineral assemblages and 17 single crystal 40ar 39ar age determinations assuming that these fluctuations reflect climate changes the lake naivasha record demonstrates that periods of increased humidity in east africa mainly followed maximum equatorial solar radiation in march or september interestingly the most dramatic change in the naivasha basin occurred as early as 146 kyr bp and the highest lake level was recorded at about 139 133 kyr bp this is consistent with other well dated low latitude climate records but does not correspond to peaks in northern hemisphere summer insolation as the trigger for the ice age cycles the naivasha record therefore provides evidence for low latitude forcing of the ice age climate cycles

explosion generated short period surface wave dispersion and noise studies along linear seismic arrays in southern sweden summary the dispersion of short period fundamental mode rayleigh waves rg was measured along linear arrays on the baltic shield the data used is a selected set of 176 records along three refraction profiles in southern sweden the phase velocity determined in the 0 2 3 5 hz band could be grouped into a number of dispersion regions each regional dispersion showed a high internal consistency and the slowness as function of frequency was almost linear in the 1 3 hz band linear regression was used to separate the effects of the medium from the experimental uncertainties systematic studies of the noise to signal ratio showed that the standard deviation of the dispersion within each region and the above frequency band essentially reflected the lateral heterogeneities from the dispersion shear velocity models were inverted down to about 2 3 km in one case down to 6 km the s wave velocity was weakly constrained to a non decreasing function of depth with decreasing gradient the p wave velocity was constrained using recent results on poisson s constant ranging from 0 28 to 0 25 in the 0 2 km interval the density was constrained using geological knowledge of the area using these constraints the s wave velocity increased rapidly with depth in the first few hundred metres of the crust the models could be naturally grouped into three sets with similar character in close agreement with the large scale surface geology the highest velocities were found in the east in the smiland varmland granitic belt and the smallest in the northwest in the sveconorwegian crust

ultra high precision 40ar 39ar ages for fish canyon tuff and alder creek rhyolite sanidine new dating standards required abstract the 40 ar 39 ar dating technique is a high precision 40 ar 39 ar standards include sanidine from the ca 28 ma fish canyon tuff fct and the ca 1 2 ma alder creek rhyolite acr despite common usage the ages of fct and acr sanidine have proven contentious with reported values varying by 2 well outside the 0 1 aspiration of earthtime www earth time org the current study presents ultra high precision multi collector 40 ar 39 ar results for fct and acr sanidine using a new generation argusvi mass spectrometer these analyses demonstrate the significantly higher by a factor of 10 level of precision achievable using the argusvi system compared to most previous studies significantly 40 ar 39 ar step heating analyses resolve distinct age gradients for both fct and acr sanidine possible explanations for these gradients include combinations of recoil loss of 39 ar instrumental bias isotopic fractionation during step heating thermally induced argon loss and or extraneous argon contamination minor recoil loss of 39 ar and partial retention of inherited argon during extended magma residence are deemed the most plausible causes of the age discordance given this assumption we calculate revised maximum eruption ages of 28 01 0 04 ma 2 for fct and 1 178 0 002 ma 2 for acr relative to the astronomically calibrated age of a1 tephra sanidine crete rivera et al 2011 earth planet sci lett 311 420 426 nonetheless the argusvi data demonstrate that fct and acr sanidine are non ideal as high precision 40 ar 39 ar standards with different heating protocols likely to induce inter laboratory bias the current findings account for at least some of the scatter in inter calibration ages reported previously for various 40 ar 39 ar standards in broader terms the results mandate a re evaluation of the astronomically calibrated ages for these and other sanidine samples while the new generation of multi collector mass spectrometers provides the means to further evaluate 40 ar 39 ar ages used to define new decay constants and standard ages

ar ar muscovite dating as a constraint on sediment provenance and erosion processes in the red and yangtze river systems se asia in this study we applied 40ar 39ar dating methods to muscovite grains in sands from the modern red and yangtze rivers in order to constrain palaeo and modern erosion patterns and sediment transport processes micas in the headwaters of the red river and its southern da river tributary are dominantly cenozoic while the northern lo river tributary shows a typical 160 220 ma population the lower reaches of the red river comprise 53 of such triassic grains indicating that the lo river is the most important net contributor to the modern delta however this contrasts with zircon u pb data indicating that the upper reaches of the red river dominate that mineral group we suggest that the mismatch reflects the rapid transport of mica relative to zircon in the river and the fact that the red river is not in equilibrium we hypothesize that the zircons were eroded 8 ka under a regime of stronger monsoon that enhanced erosion in the northern drainage basin while modern erosion is focused in the south especially in the song chay massif micas from the upper yangtze river are resolvably older than those in the red river 230 250 ma so that this method could be used to test for drainage capture in palaeo delta sediments micas in the modern yangtze delta are generally much younger than those in the upper reaches and indicate that erosion in the longmen shan sichuan and neighbouring regions is more important than the upper yangtze in supplying sediment reflecting the stronger monsoon rains in those areas

the initiation and evolution of the river nile abstract the nile is generally regarded as the longest river in the world knowledge of the timing of the nile s initiation as a major river is important to a number of research questions for example the timing of the river s establishment as a catchment of continental proportions can be used to document surface uplift of its ethiopian upland drainage with implications for constraining rift tectonics furthermore the time of major freshwater input to the mediterranean is considered to be an important factor in the development of sapropels yet the river s initiation as a major drainage is currently constrained no more precisely than eocene to pleistocene within the modern nile catchment voluminous cenozoic continental flood basalts cfbs are unique to the ethiopian highlands thus first detection of their presence in the nile delta record indicates establishment of the river s drainage at continental proportions at that time we present the first detailed multiproxy provenance study of oligocene recent nile delta cone sediments we demonstrate the presence of ethiopian cfb detritus in the nile delta from the start of our studied record c 31 ma by 1 documenting the presence of zircons with u pb ages unique within the nile catchment to the ethiopian cfbs and 2 using sr nd data to construct a mixing model which indicates a contribution from the cfbs we thereby show that the nile river was established as a river of continental proportions by oligocene times we use petrography and heavy mineral data to show that previous petrographic provenance studies which proposed a pleistocene age for first arrival of ethiopian cfbs in the nile delta did not take into account the strong diagenetic influence on the samples we use a range of techniques to show that sediments were derived from phanerozoic sedimentary rocks that blanket north africa arabian nubian shield basement terranes and ethiopian cfb s we see no significant input from archaean cratons supplied directly via the white nile in any of our samples whilst there are subtle differences between our nile delta samples from the oligocene and pliocene compared to those from the miocene and pleistocene the overall stability of our signal throughout the delta record and its similarity to the modern nile signature indicates no major change in the nile s drainage from oligocene to present day

palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the south atlantic we present a dataset of a full vector palaeomagnetic study of late pleistocene lavas from the island tristan da cunha in the south atlantic ocean the current day geomagnetic field intensity in this region is approximately 25 mu t compared to an expected value of similar to 43 mu t this phenomenon is known as the south atlantic geomagnetic anomaly saa geomagnetic field models extending back to the last 10 ka find no evidence for this being a persistent feature of the geomagnetic field albeit all models are constructed from data which is particularly sparse in the southern hemisphere new ar 40 ar 39 incremental heating dating indicates the studied lavas from tristan da cunha extruded between 90 and 46 ka palaeointensity estimations of eight lava flows made using the thellier method yield an average palaeointensity of 18 6 mu t and virtual axial dipole moment vadm of 3 1 1 2 x 10 22 am 2 the lava flows demonstrate four time intervals comparable to the present day saa where the average vadm of the tristan da cunha lavas is weaker than the global vadm average this suggests a persistent or recurring low intensity anomaly to the main geomagnetic field similar to the saa existed in the south atlantic between 46 and 90 ka c 2016 elsevier b v all rights reserved

perspectives on 40ar 39ar dating isotopic dating of geological samples using the k ar method and its variant the ar ar dating technique provides ages that in favourable circumstances are precise and accurate to within 1 limiting factors include the accuracy of the decay constants for k and the age of neutron fluence monitor minerals for rapidly cooled igneous rocks a k ar or ar ar age normally will give a good estimate of the age of eruption but for slowly cooled igneous rocks or metamorphic rocks an age measured on a sample is likely to be a cooling age as ar ar ratios can now be determined often to better than 0 1 the main limitation on accuracy relates to how well the k decay constants are known better determination of the and decays of k the basis for the decay constants is suggested at the present time rather than adoption of new decay constants linked to another decay scheme until improved independently measured decay constants for k become available there may be some circumstances where the newly proposed decay constants need to be used especially when comparing ages on volcanic igneous rocks with those measured in another system

40ar 39ar dating into the historical realm calibration against pliny the younger laser incremental heating of sanidine from the pumice deposited by the plinian eruption of vesuvius in 79 a d yielded a40ar 39ar isochron age of 1925 94 years ago close agreement with the gregorian calendar based age of 1918 years ago demonstrates that the 40ar 39ar method can be reliably extended into the temporal range of recorded history excess 40ar is present in the sanidine in concentrations that would cause significant errors if ignored in dating holocene samples

age and geologic setting of quartz vein hosted gold mineralization at curraghinalt northern ireland implications for genesis and classification acknowledgments we are grateful to dalradian gold ltd for providing the sections for petrographic analysis geochemical data and general support we would also like to thank the following john still alison sandison and jenny johnston of the school of geosciences university of aberdeen for assistance with the sem studies js and with preparing figures as and jj nerc for ongoing funding of the argon isotope facility at suerc jim imlach and ross dymock at suerc for technical assistance and martin lee at the school of geographical and earth sciences at the university of glasgow for use of the sem cl equipment the paper has benefitted significantly from comments by the official reviewers and unofficial reviews by garth earls jamie wilkinson mark cooper and adrian boyce and detailed conversations with ian alsop structural geology of the sperrins and nyree hill and gawen jenkin gold mineralization in the caledonides the authors are entirely responsible for the conclusions expressed

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers