volcanogenic nutrient fluxes and plant ecosystems in large igneous provinces an example from the columbia river basalt group research from biological and geological sources has highlighted the role of volcanoes in the outgassing of p and thermal fixation and subsequent atmospheric oxidation of no x in volcanic environments the impact of these nutrient fluxes on biological systems has been demonstrated on present day hawai9i and here we consider the impact on the plant communities within a large igneous province lip the miocene columbia river basalt province the youngest lip on earth contains many sedimentary interbeds between the flows of the major extrusive phase and these interbeds preserve variable but often diverse palynofloras by integration of palynofloral analysis with analysis of macronutrient levels in the interbeds it can be suggested that there may have been significantly elevated levels of p within the lava field proximal to the source vents a distribution mirrored by ca and mg evidence for potential volcanogenic eutrophication is restricted to 10 3 year duration interbeds contemporary with eruptive activity elsewhere in the lip the geochemistry and palynology of other interbeds demonstrate nutrient deficiency with a potential route to nutrient sufficiency available from long term symbiotic n fixation elsewhere within the columbia river basalt province this process is short circuited by the imput of felsic ash from the nearby cascades range of volcanoes

testing the reliability of discrimination diagrams for determining the tectonic depositional environment of ancient sedimentary basins the lower palaeozoic rocks of the west of ireland caledonides provide a novel way to test the reliability of discrimination diagrams in sedimentary rocks a comparison between the geochemical signature of the tuff bands and the signature from the adjacent sedimentary rocks reveals that a combination of multi element diagrams and discrimination diagrams is best suited to identify the tectonic environment of deposition the tuff bands from the ordovician and silurian formations all carry an active continental margin signature only three of the sedimentary discrimination diagrams assigned more than 50 of the samples to this setting the results from the discrimination diagrams indicate an active margin setting for the ordovician mweelrea and rosroe formations and the silurian formations whilst the derrylea and sheeffry sedimentary samples have a passive margin signature overall the results show that in the case where felsic volcanism occurred on a quartz rich source region the active continental margin field on many established discrimination diagrams is not reliable in identifying this setting however the study highlights one of the key strengths of discrimination diagrams in their ability to be used as a lever to reveal significant information about the sediment source as in this case to identify sediment that has been transported from different tectonic settings and deposited in one sedimentary basin

a different perspective on glauconite as a chronometer for geologic time scale studies there are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high temperature minerals others clearly do not and are too young by varying degrees the outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries

stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

geological significance of 40ar 39ar mica dates across a mid crustal continental plate margin connemara grampian orogeny irish caledonides and implications for the evolution of lithospheric collisions1 the connemara region is a world class example of a regional scale high temperature metamorphic terrain its rock record documents formation of a bi vergent orogenic wedge and associated calkalkali

laser probe 40ar 39ar dating studies on sub milligram whole rock basalt samples the age of the steens mountain geomagnetic polarity transition revisited the steens mountain geomagnetic polarity transition is arguably the most comprehensively studied geomagnetic field reversal trapped in igneous rocks over twenty years ago this reversal was dated at 15 5 0 3 ma by the kar method we have now tested the feasibility of carrying out 40ar 39ar dating on 1 mg samples of fine grained whole rock basalt utilizing a laser probe i e on samples 1000 times smaller than have been traditionally analyzed twenty samples taken from lava flows below within and above the reversal n r in steens mountain oregon have been studied by this laser incremental heating technique eighteen of these specimens yield acceptable plateau ages with a weighted mean age of 6 2 0 1 ma standard error on the mean this result is in excellent agreement with the results obtained on two bulk samples of whole rock basalt that average 16 1 0 1 ma an attempt to confirm these ages by fusion of plagioclase laths taken from a few of the individual lava flows was hampered by their low k content 0 2 0 3 our best estimate for the age of six plagioclase laths each 1 mg weight is 16 5 0 5 ma we suggest the techniques utilized herein could be extended to carry out dating of basaltic material of age 1 ma

subsurface settlement profiles above tunnels in clays designers assessing the possible effect of tunnelling on structures relatively close to the tunnel crown need to know how subsurface settlement profiles develop and how these relate to surface settlement profiles the effect on piled foundations or existing tunnels depends on the width of the subsurface settlement profile and on the magnitude of the settlement this note uses available field measurements and centrifuge model test data to determine how the widths of the settlement profiles and the magnitudes of settlement vary with depth above tunnels constructed in clays the results reveal a fairly consistent pattern of behaviour the width of the subsurface settlement troughs at depth is significantly greater than would be predicted by assuming a constant trough width parameter k of 0 5 smaller more realistic subsurface settlements and strains will be predicted if account is taken of the proportionally wider trough widths at depth

a new calibration point for the late miocene section of the geomagnetic polarity time scale 40ar 39ar dating of lava flows from akaroa volcano new zealand recent dating of geomagnetic field reversals over the past 2 ma by both the radiometric and astrochronologic techniques indicates that the k ar derived ages for these reversals are too young we report herein on 40ar 39ar incremental heating study of two geomagnetic field reversals trapped in lavas on akaroa volcano new zealand that demonstrate that the k ar derived ages for field reversals at 10 9 ma are too young by about 7 the new age for the termination of chron 5 a critical tie point on the geomagnetic polarity time scale for late miocene time is 9 67 0 11 ma

basic magmatism in connemara ireland evidence for a volcanic arc it is found by the use of pearce cann diagrams that basic magmatism during dalradian sedimentation in connemara was of a fundamentally different type to that which led to later synorogenic basic intrusions the dalradian magmatism is of a transitional within plate to divergent plate margin type whereas the synorogenic intrusions are of volcanic arc type it is suggested that the synorogenic basic intrusions of connemara tyrone and ne scotland represent the roots of a volcanic arc that developed by arenig times over a northerly directed subduction zone

magnetostratigraphy of the eldorado mountains volcanic complex and the calibration of the early to middle miocene polarity time scale 1 magnetic polarity measurements from a well dated 15 6 13 7 ma volcanic section in southern nevada allow resolution of controversies in the calibration of the middle miocene polarity time scale a sequence of lavas yielding exclusively reversed polarities dated at 15 5 14 9 ma can be confidently identified as c5br and the c5cn ln y reversal is bracketed between 15 67 and 15 48 ma relative to tc 27 92 ma integration with other recent studies shows that polarity chrons c5cr c5abr are now confidently correlated with lava flows dated from 16 9 to 13 4 ma by the 40ar 39ar method

thermal response of a moving lithosphere over a mantle heat source the mid ocean bathymetric swells observed around some volcanic island chains can be modeled using the thermal conduction equations of an oceanic lithosphere moving over a heat source located at the base of the plate some previous studies of this problem have suggested that the heat source must lie well within the lithosphere in order to explain the observed bathymetry and surface heat flow these conclusions are based on an isothermal boundary condition at the base of the lithosphere but this choice requires that the heat source is maintained above this boundary and can actually lead to a flux of heat back into the mantle particularly in the vicinity of the heat source if the lower boundary is insulated insofar as the anomalous temperature field is concerned flux from the heat source back into the mantle is prevented resulting in a more rapid uplift of and in an increased heat flow across the seafloor previous solutions have considered only the steady state solutions whereas the present model also evaluates transient solutions for rapidly moving oceanic plates the transient effects can be neglected provided that the heat source has been active for at least 107 years and that the direction of motion and the intensity of the heat source have remained constant in this interval solutions with the heat source located near the base of the conductive layer of the conduction equations for heat flow bathymetry and geoid heights in the spatial domain reproduce the principal observational characteristics of the hawaiian swell at distances away from the immediate origin of the swell and this part of the swell can be used to estimate the thermal conduction model parameters the model does not wholly explain the observations near the origin and the mismatch may reflect a dynamic component in the support of the swell

ages of the steens and columbia river flood basalts and their relationship to extension related calc alkalic volcanism in eastern oregon discussion hooper et al 2002 1 present stratigraphic chemical and field relationships for tertiary volcanic rocks of eastern oregon and conclude that 1 the imnaha basalt are 15 5 m y old and 2 the steens basalt ca 16 6 ma represents the earliest eruptions of the columbia river basalt flood

analytical approach for estimating ground deformation profile induced by normal faulting in undrained clay although theoretical models have been developed to predict the location of the failure surface in soil induced by bedrock faulting no analytical tool is available to estimate subsurface ground deformation in this paper a newly developed semi empirical approach is introduced and developed for calculating surface and subsurface deformations induced by normal faulting in undrained clay based on observations from centrifuge model tests the ground deformation mechanisms are identified by three regions namely a stationary zone a shearing zone and a rigid body zone by using an error function to represent vertical displacement continuously the ground deformation profile can be described quantitatively it is revealed that the ground deformation profile depends on vertical displacement of the bedrock hanging wall soil thickness dip angle of the bedrock fault plane and a shape parameter that is a function of the undrained shear modulus normalized by the undrained shear strength validation and consist

geomagnetic fluctuations during a polarity transition the extensive roza member of the columbia river basalt group washington state has intermediate paleomagnetic directions bracketed by underlying normal and overlying reverse polarity flows a consistent paleomagnetic direction was measured at 11 widely distributed outcrops the average direction has a declination of 189 and an inclination of 5 with greater variation in the inclination rietman 1966 in this study the roza member was sampled in two pasco basin drillcores where it is a single cooling unit and its thickness exceeds 50 m excellent core recovery allowed uniform and dense sampling of the drillcores during its protracted cooling the roza flow in the drillcores recorded part of a 15 5 ma geomagnetic polarity transition the inclination has symmetric quasicyclic intraflow variation while the declination is nearly constant consistent with the results from the outcrops thermal models of the cooling flow provide the timing for remanence acquisition the inclination is inferred to have progressed from 0 to 15 and back to 3 over a period of 15 to 60 years at rates of 1 6 to 0 5 yr because the geomagnetic intensity was probably weak during the transition these apparently high rates of change are not significantly different from present day secular variation these results agree with the hypothesis that normal secular variation persists through geomagnetic transitions the iow amplitude quasicyclical fluctuations of the field over tens of years recorded by roza suggest that the geomagnetic field reverses in discrete steps and that more than 15 60 years were required to complete this reversal

mesozoic igneous activity in the maranha o province northern brazil 40ar 39ar evidence for separate episodes of basaltic magmatism low and high ti basaltic rocks of the maranha o province in northern brazil appear to be related to the opening of the atlantic ocean 40ar 39ar dating was carried out on powdered whole rock basalts both washed and unwashed in dilute nitric acid to document this relationship acid treatment does not alter the argon retention properties of retentive phases but is effective in removing alteration phases high ca k degassed at low laboratory temperatures our results identify that at least two periods of volcanic activity formed the maranha o province low ti basalts from the western sector are 200 190 ma and are related to the opening of the north atlantic high ti basalts from the eastern sector are 129 124 ma slightly younger than compositionally similar rocks in the serra geral flood basalt province of southern brazil and are related to the opening of the south atlantic other flood basalts such as the columbia river deccan rajmahal ferrar and siberian provinces were each predominantly formed in 1 2 ma the maranha o basalts remnants of separate igneous events spanning 70 ma should not be classed as a single flood basalt province the temporal complexities noted in the maranha o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the atlantic ocean two sets of mesozoic basaltic rocks occur in southern africa india and south america resulting from events leading to the fragmentation and dispersal of sections of gondwanaland in each sub continent the two provinces were formed by events separated by 50 70 ma of these pairs only the maranha o and serra geral show geochemical similarities indicating compositionally distinct upper mantle source regions under sections of brazil separated by 3000 km

caledonian strike slip terrane accretion in w ireland insights from very low grade metamorphism illite chlorite crystallinity and b0 parameter analysis of pelites with detrital white micas in the clew bay galway bay segment of the irish caledonides indicates that b0 data from whole rock and 2 m fractions generally show differences smaller than the errors of the method irrespective of 001 illite crystallinity values probably due to metamorphic recrystallization intermediate pressure metamorphism of the ordovician silurian clew bay group indicates slow subduction allowing partial thermal re equilibration before exhumation in contrast the croagh patrick group laurentian shelf sediments underwent high pressure alteration suggesting rapid subduction exhumation synchronous with strike slip faulting the murrisk group which underwent high intermediate pressure metamorphism in an ordovician back arc forms a separate terrane to the croagh patrick group to the north and also to the ordovician lough nafooey and tourmakeady groups and rosroe formation in the south in which low intermediate pressure alteration occurred these together with the silurian north galway group may have undergone heating due to movement over or deposition on the hot gowlaun detachment as the connemara dalradian was exhumed the south connemara group also underwent a high pressure alteration consistent with its inferred subduction environment evidence of contact alteration due to known or inferred buried late to post caledonian granitoid plutons has been found in the clew bay louisburg clare island croagh patrick murrisk and south connemara groups these show evidence of lower pressure alteration than the surrounding country rocks

low angle extensional faulting and the emplacement of the connemara dalradian ireland a breccia mapped at the contact of the connemara dalradian and the unconformably overlying silurian succession of north galway shows evidence of having formed from strains developed at the brittle ductile deformation transition this breccia was surficially exposed just prior to the onset of an upper llandovery marine transgression the geometry of the breccia outcrop is consistent with its having developed along a regional low angle extensional fault two possible regimes of formation are proposed in the first the breccia represents the remnant of a significant low angle strike slip extensional fault along which the connemara dalradian was laterally emplaced in the second the extensional faulting may be a slightly later event related to the rapid uplift of the connemara massif both models involve rapid footwall uplift an event already inferred from metamorphic criteria the fault may be compared to major extensional detachments such as those in the basin and range province of the united states however this is the first description of an early caledonian fault of this nature and may have important consequences for future tectonic studies in this and other orogen s

late caledonian dolerite sills from sw connacht ireland bimodal silurian igneous activity in sw connacht occurred during basin subsidence and sedimentation and before basin deformation mafic sills averaging 3 m thickness were preferentially intruded into and supported by a thin horizon of fine grained siliceous sediment tonalee fm multiple injection produced the thicker sills a primary igneous lithology of augite plagioclase olivine phyric dolerite has in some sills been obliterated by pervasive hydration and carbonation expressed in a biotite calcite quartz albite mineralogy this represents volatile influx into magma at a sub intrusive level and contrasts with syn intrusive assimilation and post intrusive alteration that introduced silica and potassium into and removed magnesium from marginal dolerite the sw connacht magmatism occurred in an extensional tectonic setting probably in a nne ssw structurally controlled basin and was coeval with bimodal magmatism in other parts of ireland and britain on both sides of the iapetus sutures

water exsolution from quartz implications for the generation of retrograde metamorphic fluids available data from the literature suggest that the solubility of h2o in nominally anhydrous minerals such as quartz should increase with increasing metamorphic grade if this is the case water should precipitate from the host mineral during cooling and uplift of regionally metamorphosed terranes calculations indicate that this mechanism could result in the release of significant quantities of h2o which would then be available to participate in retrograde hydration reactions and the formation of fluid inclusions

geochemical evolution of arc magmatism during arc continent collision south mayo ireland the lough nafooey arc in the western irish caledonides collided with laurentia during the early ordovician this event is recorded in the stratigraphy of the south mayo trough the preserved forearc basin of this system lavas at the base of the oldest lough nafooey group show intraoceanic arc composition end t decreases and light rare earth elements rees become more enriched upsection in the lough nafooey group ca 495 481 ma reflecting early collision with laurentia the subsequent tourmakeady group ca 481 470 ma is rhyolitic light ree enriched and has strongly negative end t these rocks were erupted during the grampian orogeny the rosroe and mweelrea formations 470 ma show wide scatter of la sm and nb zr values suggesting mixed mantle sources this correlates with rapid exhumation of the adjacent connemara metamorphic terrane the chemical evolution of the arc supports models of collision orogeny and tectonic unroofing within 15 m y and shows that genesis of magmas more enriched than continental crust can occur during arc continent collision clarifying the potential role of arc volcanism in continental crust formation

the midcontinent rift in the lake superior region with emphasis on its geodynamic evolution abstract the midcontinent rift is a middle proterozoic continental rift which records about 15 m y of extension subsidence and voluminous volcanism in the period 1109 1094 ma in the central part of north america during that time the crust was nearly totally separated and as much as 25 km of subaerial basalts accumulated in a deep central depression following extension and volcanism a longer period of subsidence resulted in development of a post rift sedimentary basin in which as much a 8 km of fluvial and lacustrine clastic rocks were deposited partial inversion of the central depression occurred about 30 50 m y after extension to produce the current configuration of a central horst composed mostly of thick volcanic accumulations between shallower flanking basins

structural discordance between neogene detachments and frontal sevier thrusts central mormon mountains southern nevada detailed geologic mapping in the mormon mountains of southern nevada provides significant insight into processes of extensional tectonics developed within older compressional orogens a newly discovered wsw directed low angle normal fault the mormon peak detachment juxtaposes the highest levels of the frontal most part of the east vergent mesozoic sevier thrust belt with autochthonous crystalline basement palinspastic analysis suggests that the detachment initially dipped 20 25 to the west and cut discordantly across thrust faults nearly complete lateral removal of the hanging wall from the area has exposed a 5 km thick longitudinal cross section through the thrust belt in the footwall while highly attenuated remnants of the hanging wall nowhere more than a few hundred meters thick structurally veneer the range the present arched configuration of the detachment resulted in part from progressive domino style rotation of a few degrees while it was active but is largely due to rotation on younger structurally lower basement penetrating normal faults that initiated at high angle the geometry and kinematics of normal faulting in the mormon mountains suggest that pre existing thrust planes are not required for the initiation of low angle normal faults and even where closely overlapped by extensional tectonism need not function as a primary control of detachment geometry caution must thus be exercised in interpreting low angle normal faults of uncertain tectonic heritage such as those seen in the cocorp west central utah and birp s moist deep reflection profiles although thrust fault reactivation has reasonably been shown to be the origin of a very few low angle normal faults our results indicate that it may not be as fundamental a component of orogenic architecture as it is now widely perceived to be we conclude that while in many instances thrust fault reactivation may be both a plausible and attractive hypothesis it may never be assumed

the ordovician evolution of the south mayo trough western ireland the south mayo trough a broad synclinorium containing a 10 km sequence of early ordovician turbidites passing up into shallow water sandstones is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north facing arc the arc collided with the laurentian margin in early llanvirn times to deform the dalradian supergroup rifted margin clastic sequence prior to the late llandovery sinistral transcurrent faulting transposed the connemara dalradian terrane to its present position on the outboard ocean ward side of the south mayo trough the south mayo trough was fed principally by arc and ophiolite derived elastics during the arenig with an increasing metamorphic component during the llanvirn the late ordovician transpressional docking of connemara provided the southerly source for high grade metamorphic clasts in the deny veeny formation

fine tuning the radiometrically derived geomagnetic polarity time scale gpts for 0 10 ma new 40ar 39ar step heating ages for geomagnetic field reversals at 3 2 and 4 5 ma along with previous data at 1 2 10 and 16 ma are used to derive a geomagnetic polarity time scale for 0 10 ma these ages for chron boundaries are fitted to the relative widths of the polarity intervals on the ocean floor using a cubic spline technique the interpolated ages of chrons for 0 6 ma are in good agreement with the astronomical time scale and differ considerably from earlier attempts based in part on k ar dates significant differences between the interpolated ages and the astronomical time scale are noted for the onset of the olduvai and reunion sub chrons there is better agreement if 40ar 39ar data obtained from direct dating of these chrons is utilized this time scale is estimated to be accurate to 1 for 0 6 ma for 6 10 ma the lack of a tie point limits the accuracy to about 2

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes by discussion abstract barry et al 2010 presented 40 ar 39 ar ages to argue for a very limited duration 0 4 m y for the eruption of the grande ronde lavas critical examination of their data sets shows 1 a number of their plateau ages are statistically invalid and 2 many of their groundmass matrix samples were altered and did not yield valid estimates of the time of crystallization in general their ages err on the young side reflecting partial 40 ar loss due to alteration and there is no compelling evidence therein to argue for very limited duration for the grande ronde basalt the imnaha grande ronde and wanapum basalt erupted within 0 7 m y starting at 16 4 ma by contrast the ages of barry et al 2010 span the interval 16 6 to 14 7 ma and in some instances are out of stratigraphic sequence

a geological and tectonic cross section of the caledonides of western ireland geological and geophysical criteria allow division of the caledonides of western ireland into four principal zones zone 1 in the north contains grenvillian basement with late proterozoic supra crustals zone 2 contains the westport complex the deer park complex and the south mayo trough connemara and the delany dome formation occupy zone 3 zone 4 contains the south connemara group and mid to late ordovician and silurian sediments and volcanic rocks the various tectonic models proposed for these zones are discussed and a minimum tectonic model is presented which involves early ordovician collision of the northern part of zone 2 with the laurentian margin zone 1 followed by subduction flip northern part of zone 4 the site of early collision between zones 1 and 2 subsequently underwent orogenic collapse and strike slip dismemberment leading to the sinistral transpressive emplacement of zone 3 assemblages in zone 4 were accreted to the now south facing arc northwards stress transfer consequent on suturing deformed the thinned crust and overlying silurian sequences in zones 2 and 3

the cooling history of connemara western ireland from k ar and rb sr age studies k ar age determinations are reported on 26 hornblendes 17 coexisting muscovites and biotites nine muscovites and nine biotites from the dalradian connemara schists metagabbros and gneisses these are supplemented by 19 rb sr whole rock mica ages and four 40 ar 39 ar incremental heating ages on micas a long at least 75 ma and complex cooling history without a simple pattern of ages has been obtained the ages are not correlated with the grain sizes or the chemical compositions of the analysed minerals or the topographic elevation from which the samples came but there is an irregular increase of hornblende k ar ages to 480 10 ma with increase of metamorphic grade southwards towards the metagabbros and gneiss the preservation of the complex cooling pattern is ascribed to the action of syn and post metamorphic slides or faults which moved slices of the massif upwards by varying amounts closely spaced samples give results which rule out simple tilting of the massif rapid cooling c 30 ma 1 at 480 10 ma arenig llanvirn during the f3 fold phase was followed by slower cooling until 455 10 ma caradoc when renewed rapid uplift occurred possibly correlated with the southward thrusting of connemara and with synchronous renewed uplift in the scottish dalradian rocks widespread biotite k ar and rb sr ages of 440 5 ma end ordovician throughout connemara and the dalradian rocks of scotland are ascribed to the major taconic tectonic event which docked the dalradian rocks against the flanking folded ordovician successions of the midland valley and the south mayo trough biotites giving k ar ages of about 415 ma may have been reset during the late silurian folding of the cover rocks the importance of very close sampling integrated with structural studies is emphasized

predicting 2d ground movements around tunnels in undrained clay a new analytical method is introduced for calculating displacements due to tunnelling this is conceived within the framework of the bound theorems of plasticity but allowing for soil strain hardening the ground displacements due to tunnelling are idealised by a simple displacement mechanism of distributed shearing in the plane of the tunnel cross section the tunnel support pressure corresponding to a certain volume loss is calculated from energy balances of the work dissipated in distributed shear the potential energy loss of soil flowing into the tunnel and the work done by this soil against the tunnel support pressure the calculations are carried out in steps of small volume loss accompanying small reduction in support pressure after each of which the tunnel geometry is updated in this way each reduced tunnel support pressure is related to a complete ground displacement field a simplified closed form solution is also provided for the prediction of maximum surface ground settlement for the par

40ar 39ar dating of the brunhes matuyama geomagnetic field reversal magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale gpts to date events in the range 0 to 5 million years ago a critical tie point on the gpts is the potassium argon age of the most recent brunhes matuyama geomagnetic field reversal astronomical values for the forcing frequencies observed in the oxygen isotope record in ocean drilling project site 677 suggest that the age of this last reversal is 780 ka thousand years ago whereas the potassium argon based estimate is 730 ka results from 4039 ar incremental heating studies on a series of lavas from maui that straddle the brunhes matuyama reversal give an age of 783 11 ka in agreement with the astronomically derived value the astronomically based technique appears to be a viable tool for dating young sedimentary sequences

a geomagnetic polarity time scale for the period 0 17 ma based on 40ar 39ar plateau ages for selected field reversals utilizing 40ar 39ar plateau ages for selected field reversals at 1 2 10 16 and 34 ma a geomagnetic polarity time scale gpts is constructed following the technique of cande and kent 1992 this differs markedly from previous gpts especially in middle late miocene time the ages for the chrons in post miocene time are in agreement with values derived by the astrochronological technique refinements in this gpts await acquisition of 40ar 39ar plateau ages for field reversals in the time frames 3 8 ma and 11 15 ma

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

cordilleran metamorphic core complexes in arizona a contribution from geomorphology cordilleran metamorphic core complexes consist of granitic cores surrounded by in an outward direction mylonite chloritic breccia and microbreccia these rocks are separated by a low angle detachment fault from typically unmetamorphosed but normally faulted rocks rocks above the detachment fault have been transported relative to the core rocks in one consistent direction giving a distinct lineation to rocks below the detachment fault dome shaped metamorphic core complex mountains show considerable structural control of landforms mylonitic rocks develop triangular dome facets around the margins and low relief areas in the central parts of the complexes chloritic breccia is much less resistant to erosion and has been almost completely removed from many complexes drainage patterns in metamorphic core complexes are mainly radial but an older set of drainage lines parallel to mylonitic lineation especially channels that drain down the crests of antiformal ridges suggests that drainage developed on a ramp on the detachment fault and the underlying breccia prior to lineation parallel arching

recalibration of the geomagnetic polarity timescale the quest for improved time control to estimate rates of geologic and paleoenvironmental processes is central to the earth sciences until the middle of this century geochronology was based solely on macrofossil evolution providing the largely unmodified definition of geologic stages in the last few decades with impetus from the deep sea drilling project dsdp and the ocean drilling program odp microfossil biostratigraphy has become increasingly important although new radiometric techniques have resulted in improved absolute age precision the direct correlation of absolute ages to biozonations and hence to geologic stage boundaries remains poor this is due to the difficulty of finding materials suitable for radiometric dating which can be directly correlated to biozonations the bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished particularly for cenozoic and late mesozoic time through the geomagnetic polarity timescale gpts the template for late jurassic to quaternary geomagnetic polarity is derived from oceanic magnetic anomalies as demonstrated by heirtzler et al 1968 the gpts can be constructed using oceanic magnetic anomaly record s assuming constant seafloor spreading rate s and interpolating between available radiometric ages in the last quadrennium there have been outstanding developments in radiometric dating and astrochronology which have greatly improved the absolute age control of the gpts

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

concordant sea floor spreading rates obtained from geochronology astrochronology and space geodesy reversals of the earth s magnetic field allied to sea floor spreading have left their imprint in the form of a magnetic tape recorder on the sea floor it is demonstrated that the geomagnetic polarity time scale for the past few million years derived utilizing a radiometric dates of magnetic field reversals trapped in subareal igneous rocks and b as derived from the milankovitch forcing frequencies and their imprint in the sedimentary environment are in good agreement further sea floor spreading rates calculated from these results are in substantial agreement with the rate of plate movement averaged over the last decade as obtained by space geodetic measurements since the principles underlying these dating techniques are entirely different it lends credence to the results obtained for sea floor spreading arguably the most fundamental process in plate tectonics

new tie points for the geomagnetic polarity time scale during the middle miocene from the mogan group on gran canaria and ocean drilling program leg 157 site 953 a thick sequence of volcaniclastic sediments drilled at site 953 during ocean drilling program odp leg 157 northeast of gran canaria canary islands contains an almost complete magneto stratigraphy back to the shield stage of the island 14 8 ma ago onshore a sequence of reversals has been identified and dated in 19 dominantly peralkaline rhyolitic ignimbrites one rhyolitic and one basaltic lava flow of the mogan group 13 35 13 95 ma which overlie basalt flows of the island s shield stage 14 ma the magneto stratigraphy of the ignimbrites onshore has been correlated with the marine magneto stratigraphy at site 953 determined in syn ignimbritic volcaniclastic turbidites which were deposited practically synchronously immediately following the entry of the parent pyroclastic flows into the sea around the circumference of the island the four polarity intervals recorded in the sequence of the mogan group ignimbrites correspond to c5acr c5acn c5adr and c5adn single crystal 40ar 39ar age determinations of the ignimbrites bracketing the polarity changes gave the following ages and uncertainties for the reversals c5ad t 13 95 0 07 ma c5ac o 13 89 0 08 ma and c5ac t 13 47 0 09 ma the newly dated polarity changes fit and refine the miocene age model proposed in the global polarity time scale

comments on deccan flood basalts at the cretaceous tertiary boundary by v courtillot j besse d vandamme r montigny j j jaeger and h cappetta the age of the flood basalts of the deccan area and the length of the interval during which the outpourings took place are not known precisely the authors have tried to date those basalts and estimate how long the volcanic activity lasted on the basis of new paleomagnetic paleontological and geochronological data of the deccan effusives they came to the preliminary conclusion that the volcanic activity may have lasted less than 1 ma and that the majority of the lavas poured out during chron 29r unfortunately the authors were not familiar with our paper on paleomagnetism and radiomettic age determinations of deccan trap lavas 2 in our paper we presented the paleomagnetic data derived from 223 individual lava flows from 16 sections all over the deccan area and gave the k ar age determinations for 13 selected unaltered lava flows we agree that the enormous volume of deccan extrusives poured out in a relatively short interval and that the volcanic activity was probably concerned with the spreading in the central indian ocean in late cretaceous and early tertiary time

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

palaeomagnetism and radiometric dating of the british tertiary igneous province muck and eigg summary the lavas dykes and other intrusive bodies of muck and eigg have been sampled for palaeomagnetic purposes all the bodies are reversely magnetized except for four dykes of muck which are normally magnetized the lavas show variations of direction of magnetism believed to be a record of secular variation the radiometric dates support the geological evidence that the lavas of eigg and muck predate parts of the rhum central igneous complex the lavas of rhum and canna and some of those of skye the combined evidence shows that most of the activity took place during an r n r n sequence between 63 and 58 ma the younger r n sequence is the same as the first part of the r n r sequence found in mull but the sgurr pitchstone of eigg and parts of the eastern redhills of skye belong to later reversed intervals and correspond to chron 26r and chron 26 the igneous activity occurred over a 10 myr period from about 63 ma and can probably be associated with chrons 27r to 23r of the harland et al time scale

a model for continental crust genesis by arc accretion rare earth element evidence from the irish caledonides abstract the formation of continental crust is a complex problem with a paradox at its center continental material is believed to form by arc magmatism a model that does not reconcile the bulk mafic and light rare earth element lree depleted composition of intra oceanic arcs with the andesitic lree enriched composition of continents we present evidence supporting an arc origin for continental crust by demonstrating significant changes in magmatic chemistry during arc continent collision we use as an example the connemara region of the western irish caledonides where metamorphism and orogenesis of dalradian sedimentary sequences record the collision of the oceanic lough nafooey arc with the passive margin of laurentia at 475 ma arenig this arc was mafic and lree depleted during its oceanic activity during collision rhyolites more lree enriched than the surrounding continental crust were erupted while the mid crust was intruded by similarly lree enriched gabbro and diorite crystal fractionation caused lree enrichment to exceed that expected by assimilation of continental crust alone we propose that subsequent loss of the corresponding lower crustal cumulates into the mantle removed this lree depleted material from the net crust added to the continent the combined processes of crystal fractionation and lower crustal loss during arc continent collision drive the average bulk arc composition toward that of average continental crust

comment to distribution and geochronology of the oregon plateau u s a flood basalt volcanism the steens basalt revisited by m e brueseke et al j volcanol geotherm res 161 2007 187 214 abstract the 40 ar 39 ar stepheating results of brueseke et al 2007 are critically evaluated firstly for possible alteration of the samples dated steps passing the alteration index test baksi 2007a can then be used to derive plateau ages which must satisfy rigorous statistical tests of the eleven samples studied by brueseke et al 2007 only four yield plateau values that reflect eruption ages these suggest steens basalt was erupted within a relatively short period of time 0 2 m y considerably less than the 1 m y suggested by the authors the seeming difference between the plateau ages for the steens mountain basalts obtained by these authors and two earlier attempts is resolved by a proper adjustment of ages at various laboratories for interlaboratory calibration differences and by rejecting ages from altered samples the age of the important steens mountain geomagnetic polarity transition is 16 59 0 03 ma

end silurian modifications of ordovician terranes in western ireland the ordovician elements in the caledonides of western ireland may be divided into the clew bay south mayo north connemara and south connemara sub zones evidence suggests the presence of a number of proximal suspect terranes and the remnants of one or possibly two arc complexes palaeontological and sedimentary evidence suggests an allochthonous relationship between the silurian rocks of croagh patrick and north galway these successions may therefore not represent a single sedimentary overlap to the ordovician suspect terranes end silurian deformation was the result of transpression which was responsible in part for the present configuration of the area s geology deep seated shear zones with fuchsite and gold mineralization were foci for lateral translation concomitant with southward directed thrusting northward directed back thrusts and variable degrees of metamorphism being of higher grade in the north the end silurian event is intermediate between the high grade metamorphism in parts of the appalachians and the non metamorphic silurian sequences of scotland

a basin model for the silurian of the midland valley of scotland and ireland identification of terranes is frequently based on the apparent disparity of source area for sediment supply palaeontological evidence and the presence of faults whose sense and amount of displacement is sometimes conjectural as an example of such superficially unrelated stratigraphies the silurian successions of the midland valley of scotland and the northern part of ireland are compared a diachronous late llandovery early middle telychian transgressive episode which is earlier in the midland valley inliers is common to many of the successions the transgression resulted in the development of comparable sedimentary facies and fossil assemblages the clast compositions of llandovery and wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement areas of active volcanism appear to have migrated in a north westerly direction with time a similar sequence is evident in canada it is suggested that the common features of the scottish irish successions may be accommodated in a unified basin model for silurian sedimentation and volcanism such a basin may have formed in an intra arc environment and been controlled by oblique slip fault mechanisms although evidence for the presence of a silurian arc is equivocal

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

estimation of lava extrusion and magma production rates for two flood basalt provinces utilizing the statistical approach of cox and dairymple 1967 it is shown that the geomagnetic polarity time scale of harland et al 1982 may contain errors of 0 85 1 0 m y for middle late miocene times a revised time scale based on the work of mcdougall et al 1984 is correlated with the magnetostratigarphic units of the columbia river basalts united states lava extrusion rates reached a peak of 275 km3 kyr for a period of 0 5 m y around 15 8 ma during the columbia volcanic episode it is suggested that even higher rates of lava extrusion may have occurred in the deccan province central india 65 ma a simple energy budget calculation indicates that the reunion hotspot could provide enough heat to melt the total volume of rock needed to produce the deccan traps during the 5 m y it would have taken the subcontinent to traverse the hotspot

revised age calibration points for the geomagnetic polarity time scale two of the seven cenozoic age calibration points in the widely used cande and kent s geomagnetic polarity time scale have recently been revised the rest of the five calibration points are revised here based on recently available radioisotopic dates and magnetostratigraphic data ages of the magnetic reversals are recalculated using the revised calibration points differences between the present time scale and cande and kent s revised time scale are generally less than 0 3 m y the largest up to 1 3 m y being in the early miocene

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes abstract grande ronde basalt grb lavas represent the most voluminous eruptive pulse of the columbia river snake river yellowstone hotspot volcanism with an estimated eruptive volume of 150 000 km 3 grb lavas form at least 66 of the total volume of the columbia river basalt group new 40 ar 39 ar dates for grb lavas reveal they were emplaced within a maximum period of 0 42 0 18 my a well documented stratigraphy indicates at least 110 grb flow fields or individual eruptions and on this basis suggests an average inter eruption hiatus of less than 4000 years isotopic age dating cannot resolve time gaps between grb eruptions and it is difficult to otherwise form a picture of the durations of eruptions because of non uniform weathering in the top of flow fields and a general paucity of sediments between gr lavas where sediment has formed on top of grb lavas it varies in thickness from zero to 20 30 cm of silty to fine sandy material with occasional diatomaceous sediment individual grb eruptions varied considerably in volume but many were greater than 1000 km 3 in size most probably eruptive events were not equally spaced in time some eruptions may have followed short periods of volcanic repose perhaps 10 2 to 10 3 of years whilst others could have been considerably longer many 1000 s to 10 4 years recent improvements in age dating for other continental flood basalt cfb lava sequences have yielded estimates of total eruptive durations of less than 1 my for high volume pulses of lava production the grb appears to be a similar example where the main pulse occupied a brief period even allowing for moderate to long duration pahoehoe flow field production the amount of time the system spends in active lava producing mode is small less than c 2 6 based on eruption durations of approximately 10 000 years compared to the duration of the entire eruptive pulse of c 420 000 years a review of available 40 ar 39 ar data for the major voluminous phases of the columbia river basalt group suggests that activity of the steens basalt imnaha basalt grb may have at times been simultaneous with obvious implications for climatic effects resolving intervals between successive eruptions during cfb province construction and durations of main eruptive pulses remains vital to determining the environmental impact of these huge eruptions

late ordovician to early silurian amalgamation of the dalradian and adjacent ordovician rocks in the british isles from detailed radiometric and pressure temperature estimates of the metamorphic history uplift and erosion of the dalradian rocks of the british isles we find that 15 25 km of erosion took place mainly in ordovician time none of this vast volume of material is apparently present in the ordovician sedimentary basins that now flank the dalradian block we conclude that the ordovician basins to the south of the dalradian block are exotic terranes and we document their emplacement into their present position during late ordovician to early silurian time i e taconic by using evidence mainly from western ireland

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

evidence for errors in the geomagnetic polarity time scale at 17 15 ma 40ar 39ar dating of basalts from the pacific northwest usa 40ar 39ar incremental heating studies have been carried out on whole rock basalt samples from the columbia river basalt crb and the steens basalt to obtain crystallization ages with a standard error 1 these high precision accurate ages are utilized with the known magnetostratigraphy of the crb to test the accuracy of the early to middle miocene section of the geomagnetic polarity time scales currently in use we conclude that these time scales underestimate the ages of various chrons within this time range by 0 9 m y

lava sediment interaction and drainage system development in a large igneous province columbia river flood basalt province washington state u s a abstract large igneous provinces lips have been implicated in some of the largest environmental perturbations to have affected the earth through geological time yet the impact of lip development on drainage and ecosystem development in the immediate vicinity of these provinces are still poorly constrained to date based on a detailed integrative facies scheme we characterize the interaction between volcanism and fluvial lacustrine and wetland environments in the miocene columbia river flood basalt province crbp lip exposed in central washington state usa the facies scheme proposed here comprises a detailed description and interpretation of siliciclastic bioclastic diatomite volcaniclastic and paleosol facies and subfacies intercalated with lavas of the columbia river basalt group crbg facies and facies associations of individual interbeds are further correlated to reconstruct changes in sedimentary settings and topography of lava fields during deposition field observations and facies analysis help to explain the effects of flood basalt volcanism on drainage system development we propose a generic model of lava drainage interplay and distribution of sedimentary settings in flood basalt provinces which will contribute to our understanding of sedimentological environmental and volcanic processes in the crbp hydrocarbon exploration in volcanic terrains requires detailed information on the distribution and development of sedimentary settings this model will help to better predict the character and distribution of sedimentary bodies and potential hydrocarbon reservoirs in volcanic terrains

volatile fluxes during flood basalt eruptions and potential effects on the global environment a deccan perspective abstract we examine the role that flood basalt eruptions may have played during times of mass extinction through the release of volcanic gases continental flood basalt provinces have formed by numerous eruptions over a short period of geologic time characteristically a few million years within this period a short lived climactic phase that lasts about 1 ma typically emplaces a large proportion of the lava volume this phase consists of a series of huge eruptions each yielding 10 3 10 4 km 3 of magma each eruption lasted on the order of a decade or more and built an immense p hoehoe dominated lava flow field by eruptive activity along fissures tens to hundreds of km long high fire fountains emanating from vents along the fissures at times sustained eruption columns that lofted gas and ash into the upper troposphere and lower stratosphere while the lava flows covered huge areas the combination of large eruption magnitudes maintained high effusion rates during eruptions and the repeated nature of the characteristic large scale eruptive activity occurs in earth history only during periods of flood basalt volcanism based on recent analogs and determination of volatile contents of ancient flood basalt lavas we estimate that individual eruptions were capable of releasing 10 000 tg of so 2 resulting in atmospheric loadings of 1000 tg a 1 during a sustained decade long eruptive event we apply this model of flood basalt volcanism to estimate the potential mass of co 2 and so 2 released during formation of the 65 ma deccan province the deccan lava pile contains the record of hundreds of enormous p hoehoe flow fields erupted within a period of about 1 ma consequently atmospheric perturbations associated with so 2 emissions from just one of these long lasting eruptions were likely to have been severe and constantly augmented over a decade or longer by contrast the amounts of co 2 released would have been small compared with the mass already present in the atmosphere and thus much more limited in effect individual eruptions were followed by hiatuses of hundreds to thousands of years during which the gas contributions to the atmosphere would be recycled it is clear that the nature and potential atmospheric impact of a series of huge volume repeated long term degassing events requires further investigation in conjunction with appropriate climate models

conditions of high grade metamorphism in the dalradian of connemara ireland in e connemara ireland dalradian metasediments were initially metamorphosed to kyanite staurolite grade at pressures above 7kbar the subsequent emplacement of basic to intermediate magmas accompanied by uplift and erosion produce a steepening of the geothermal gradient and led to the formation of migmatitic garnet cordierite sillimanite gneisses in the highest grade regions in the s cation exchange equilibria between coexisting mineral pairs suggest that partial melting took place under high temperatures 750 c and moderate pressures 4 5 6 kbar fluid inclusion studies and andalusite occurrences from migmatite leucosomes suggest that crystallization of the melts occurred at lower pressures

testing the accuracy of the geomagnetic polarity time scale gpts at 2 5 ma utilizing40ar 39ar incremental heating data on whole rock basalts abstract recent40ar 39ar dating and astrochronologic calculations indicate that the k ar derived gpts underestimates the ages of geomagnetic field reversals in the interval 2 0 ma we report tests to check the accuracy of k ar data utilized to date the reunion event 2 1 ma and part of the early pliocene portion 4 6 ma of the gpts 40ar 39ar incremental heating studies on whole rock basalts from the island of reunion yield an age of2 14 0 03 ma for the reunion event somewhat older than the earlier k ar value of2 07 0 02 ma on the same set of lava flows study of a single specimen from the newer volcanics australia indicates that the earlier k ar date was accurate but the new40ar 39ar results yield a plateau age 4 56 ma with a standard error of 0 6 as compared to the k ar value 3 the improved precision and accuracy resulting from such40ar 39ar incremental heating studies will play a key role in calibrating more precisely the gpts for the interval 10 0 ma

rapid tectonic exhumation detachment faulting and orogenic collapse in the caledonides of western ireland collision of the oceanic lough nafooey island arc with the passive margin of laurentia after 480 ma in western ireland resulted in the deformation magmatism and metamorphism of the grampian orogeny analogous to the modern taiwan and miocene new guinea orogens after f470 ma the metamorphosed laurentian margin sediments dalradian supergroup now exposed in connemara and north mayo were cooled rapidly 35 jc m y and exhumed to the surface we propose that this exhumation occurred mainly as a result of an oceanward collapse of the colliding arc southwards probably aided by subduction rollback into the new trench formed after subduction polarity reversal following collision the achill beg fault in particular along the southern edge of the north mayo dalradian terrane separates very low grade sedimentary rocks of the south mayo trough lough nafooey forearc and accreted sedimentary rocks of the clew bay complex from high grade dalradian meta sedimentary rocks suggesting that this was a major detachment structure in northern connemara the unconformity between the dalradian and the silurian cover probably represents an eroded major detachment surface with the renvyle bofin slide as a related but subordinate structure blocks of sheared mafic and ultramafic rocks in the dalradian immediately below this unconformity surface probably represent arc lower crustal and mantle rocks or fragments of a high level ophiolite sheet entrained along the detachment during exhumation orogenic collapse was accompanied in the south mayo trough by coarse clastic sedimentation derived mostly from the exhuming dalradian to the north and to a lesser extent from the lough nafooey arc to the south sediment flow in the south mayo trough was dominantly axial deepening toward the west volcanism associated with orogenic collapse rosroe and

variations in the velocities of the major plates since the late cretaceous abstract we present models for the relative and absolute velocities of the major plates from 80 m y ago to the present as tests of several alternative plate reconstructions and as a guide in the evaluation of the hypothesis that pronounced changes in plate speeds or directions arise directly from changes in the configuration of plate boundary forces digitized plate boundaries were rotated backwards in time using published rotations to determine positions at 5 m y intervals back to the late cretaceous the oceanic plate boundaries were then corrected so as to properly match magnetic anomalies relative plate velocities were obtained by differencing finite rotations absolute velocities were determined by assuming that no net torque is exerted on the lithosphere rms absolute speeds of each plate were calculated as functions of time as a conceptually simple measure of plate velocities and accelerations we have tested several possible reconstructions for the antarctic and pacific plates proposed to explain the failure of simple rigid antarctic and or pacific plate models to match paleomagnetic data hotspot tracks and equatorial crossings defined by pelagic sediment deposition we find that 1 1 plate reconstruction models with either two antarctic or two pacific plates in the early tertiary are distinguishable from models with single antarctic and pacific plates principally on the basis of faster speeds for the kula plate and the northern pacific greater areas for the kula and farallon plates and faster subduction at the aleutians and along the west coast of south america for that time 2 2 oceanic plates as a group have moved faster than those with continental lithosphere only since 50 m y ago active subduction appears to be a sufficient condition for rapid plate motion regardless of the fractional plate area comprised of continental lithosphere 3 3 the absolute motion of india in the late cretaceous was anomalously fast 15 20 cm yr compared to the speeds of the present plates 4 4 the rms velocities of a number of plates and of the lithosphere as a whole are much higher in the late cretaceous than through most of the cenozoic 5 5 the predicted apparent polar wander for north america is in fairly good agreement with paleomagnetic data with the slight discrepancy attributable to a net contribution to the torque balance from plate boundary forces

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

regional correlation of grande ronde basalt flows columbia river basalt group washington oregon and idaho the tholeiitic flood basalts of the columbia river basalt group of middle and late miocene age cover more than 200 000 km2 in washington oregon and idaho the most voluminous formation of the group the grande ronde basalt erupted for 2 m y from north northwest trending fissure systems concentrated in southeast washington and adjacent oregon and idaho four magnetostratigraphic units designated r1 n1 r2 and n2 from oldest to youngest are recognized on the basis of polarity in the grande ronde and provide the broad stratigraphic framework for the formation in this study major element chemistry and relative stratigraphic position within the polarity intervals are used to identify and correlate individual flows and sequences of flows within the grande ronde basalt on a regional scale systematic examination of more than 350 analyses from 47 stratigraphic sections show that most flows fall into one of five major chemical groupings which are distinguished primarily by small but significant variations in mgo tio2 and p2o5 content in addition four minor chemical types local to the eastern part of the province have been identified feeder dikes of each chemical type have also been located flows or packets of flows of each chemical type can be correlated between field sections to define specific chemical stratigraphic subunits these subunits consist of several flows collectively 30 150 m thick subunits of most chemical types are repeated at irregular intervals throughout the formation no progressive chemical trend occurs within the grande ronde many of the chemical stratigraphic subunits extend to the margins of the province although most are confined to the source region in eastern washington although the total number of subunits is less in the west away from the fissure systems the total thicknesses of the n2 and r2 magnetostratigraphic units are each as thick or thicker than the corresponding units in eastern washington the greatest thicknesses occur in the central part of the province within the pasco basin the distribution of basalt relative to the location of vents as well as the relative east west thicknesses suggests that basalt flowed hundreds of kilometres westward during the most voluminous grande ronde eruptions ponding against the irregular margin of the cascade range and being diverted through the ancestral columbia gorge toward the washington oregon coast between these huge sheetflood events smaller eruptions blanketed areas within the source region and ongoing regional subsidence created a shallow westward draining basin in the center of the province

on the kinematics of 2d tunnel collapse in undrained clay a kinematic plastic solution has been developed for ground movements around a shallow unlined tunnel embedded within an undrained clay layer in this solution the pattern of deformation around the tunnel is idealised by a simple plastic deformation mechanism within the boundaries of the deformation mechanism the soil is required to shear compatibly and continuously with no relative sliding at the boundaries the soil is regarded here as a rigid plastic tresca material the plausibility of the proposed mechanism is demonstrated by comparison with limit analysis calculations and centrifuge test results

asynchronous records of brunhes matuyama reversal in marine sediments and chinese loess review and discussion abstract geomagnetic polarity reversals are generally considered to occur synchronously around the world and are commonly used as time markers to correlate events between different sediment archives they also play an indispensable role for understanding the mechanisms which generate the earth s geomagnetic field nevertheless in the case of the most recent reversal the brunhes matuyama b m reversal 780 ka comparison of paleomagnetic studies in chinese loess paleosol sequences versus marine cores reveal a marked discrepancy in timing of this event this timing offset has led to uncertainties in correlating the chinese loess paleosol sequences with other global climate records this paper briefly reviews the history of study on the b m reversal both in marine and chinese loess sediments and reviews several contentious explanations of the aforementioned b m boundary offset finally the most recent progress of paleogeomagnetic field variation studies using cosmogenic 10 be in chinese loess is introduced with the purpose of suggesting a potential way to address this b m boundary timing offset issue

tunnels in sands the effect of size depth and volume loss on greenfield displacements this paper examines the effect that tunnel size depth and volume loss have on greenfield soil displacements above tunnels in sandy ground the results of a series of plane strain centrifuge tests

timing of deformation associated with the syn tectonic dawros currywongaun doughruagh complex nw connemara western ireland new data are presented that demonstrate the syn tectonic intrusion of a suite of mafic and ultramafic magmas into the connemara dalradian magma was intruded during the local d2 episode but immediately prior to the onset of d3 both deformation episodes affect the gabbros forming two distinct groups of fabrics formed at different stages during the history of the intrusion applying thermal considerations to this history suggest that any gap in time between the formation of these fabrics was

terrane amalgamation in the clew bay region west of ireland the caledonides of the west of ireland provide a well exposed and well mapped example of an oblique collision zone the east northeast trending deer park and achill beg fault system is a crustal scale ductile sinistral strike slip duplex of late ordovician age imbricating late precambrian granulite facies lower crustal rocks near eclogite facies supracrustal rocks up to amphibolite facies dalradian metasedimentary rocks and greenschist facies cambro ordovician rocks this fault system is correlated with a pre devonian component of the highland boundary fault system in southern scotland in the clew bay area the high pressure low temperature facies metamorphic rocks in tectonic contact with greenschist facies cambro ordovician rocks are together interpreted as an accretionary prism complex related to northwestward directed subduction both of these are allocthonous terrains with respect to the dalradian terrane to the north north west mayo to the south the cambro ordovician rocks docked with a probable dalradian block containing ultramafic intrusives deer park complex during the late ordovician the deer park complex and south mayo trough linked earlier during the arenig silurian and lower middle devonian redbed successions sit unconformably on the metamorphic rocks deposition and deformation of these cover rocks was controlled by oblique strike slip movements on the leek fault whose strike swings from west northwest to north northeast following earlier basement trends as it is traced eastwards from clew bay the leek fault system may be correlated with the leannan fault of northwest donegal a splay of the great glen fault system of central scotland east of clew bay this sinistral shear generated local dilation on the more northerly trending bend of the leek fault lower and middle old red sandstone redbeds were developed here the west northwest trend of the leek fault in clew bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in silurian rocks post middle old red sandstone pre late tournaisian dextral displacement on the leek fault reversed this pattern with transtension in clew bay allowing intrusion of small carbonated peridotite bodies into silurian rocks and easterly directed thrusting of middle old red sandstone rocks east of the bay on the transpressional north south bend a tectonic model for the region is presented here this model involves a northwestward directed subduction system 150 to 750 km of arenig sinistral strike slip movement and eastwards insertion of the connemara block with formation of the ordovician south mayo trough as a pull apart basin subsequently a further 130 to 650 km eastward displacement of rocks took place south of the deer park fault in later ordovician times the magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original laurentian margin and using the current juxtaposition of terranes a minimum wavelength of 300 km is inferred

paleogene sea levels and climates u s a eastern gulf coastal plain abstract biostratigraphic zonation of the paleogene formations in the eastern gulf coastal plain of the u s a based on calcareous nannoplankton allows a reassessment of the timing and pattern of changing sea levels and climates in the region general similarity is recognizable between the gross pattern of paleogene transgressions regressions in the eastern gulf coastal plain and the recently published vail et al 1977 scheme of eustatic sea level changes local tectonism and sedimentation have however overprinted intra cycle shoreline fluctuations on the eustatic pattern a hiatus spanning at least calcareous nannoplankton zone np 11 exists between the early eocene hatchetigbee and tallahatta formations another significant hiatus representing most or all of np 23 exists between the early oligocene bucatunna clay and the late oligocene chickasawhay limestone nannoplankton assemblages suggest temperate sea surface waters during the early and early late paleocene followed by subtropical sea surface conditions from latest paleocene to earliest oligocene climatic conditions were cooler during the remainder of the early oligocene

critical evaluation of the age of the deccan traps india implications for flood basalt volcanism and faunal extinctions attempts to evaluate the possible link between flood basalt volcanism and the faunal extinctions observed at the k t boundary must rely on precise knowledge of the amount of such volcanism that took place 65 ma the current geochronological data do not permit close resolution of either the timing or the duration of deccan volcanism the age of the bulk of the material is placed in the range 70 60 ma further resolution of this hypothesis awaits acquisition of high quality 40 ar 39 ar plateau ages on key sections of the deccan traps

caledonian transpressional terrane accretion along the laurentian margin in co mayo ireland the highland border fair head clew bay baie verte brompton suture clew bay suture on achill island western ireland is an e w trending zone of vertical ductile dextral transpression bounding the southern edge of the dalradian terrane the achill dalradian a sequence of coarse grained psammite overlain by conglomerate quartzite schist and limestone disposed in recumbent curvilinear folds represents laurentian continental margin deposits which were translated westward during deformation parallel to the suture trace studies of fabric development and albite porphyroblast growth indicate that e w dextral transpression and west directed recumbent folding were synchronous and are interpreted as separate expressions of the same nw directed terrane impingement event this process of terrane docking was marked by a c 4kbar phase of decompression within the suture four separate terrane slivers are recognized the presence of pyrope garnet and crossite amphibole within two imply the clew bay zone experienced sufficient displacement to exhume rocks from deep lithospheric levels in addition the extensive retrogression and destruction suffered by these remnant high pressure mineral assemblages during the albite generating transpressional phase implies that ductile deformation was enhanced by the introduction of fluid into and along the suture zone during impingement

hawaiian xenolith populations magma supply rates and development of magma chambers hawaiian volcanoes pass through a sequence of four eruptive stages characterized by distinct lava types magma supply rates and xenolith populations magma supply rates are low in the earliest and two latest alkalic stages and high in the tholeiitic second stage magma storage reservoirs develop at shallow and intermediate depths as the magma supply rate increases during the earliest stage magma in these reservoirs solidifies as the supply rate declines during the alkalic third stage these magma storage reservoirs function as hydraulic filters and remove dense xenoliths that the ascending magma has entrained during the earliest and latest stages no magma storage zone exists and mantle xenoliths of lherzolite are carried to the surface in primitive alkalic lava during the tholeiitic second stage magma storage reservoirs develop and persist both at the base of the ocean crust and 3 7 km below the caldera only xenoliths of shallow origin are carried to the surface by differentiated lava during the alkalic third stage magma in the shallow subcaldera reservoir solidifies and crustal xenoliths including oceanic crustal rocks are carried to the surface in lava that fractionates in an intermediate depth reservoir worldwide xenolith populations in tholeiitic and alkalic lava may reflect the presence or absence of subvolcanic magma storage reservoirs

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

the origin and significance of the delaney dome formation connemara ireland dalradian meta sediments of the laurentian margin and mafic intrusions thereof in sw connemara ireland tectonically overlie meta rhyolites of the delaney dome formation the two units are separated by the mannin thrust a new u pb age of 474 6 5 5 ma shows that the delaney dome formation is a temporal equivalent of arc volcanic rocks preserved in the adjacent south mayo trough the tourmakeady volcanic group erupted during the collision of an oceanic island arc with the laurentian margin in the grampian orogeny new rare earth and high field strength element data show that the delaney dome formation and tourmakeady volcanic group are chemically similar and arc like in character this suggests that the delaney dome formation is an along strike equivalent of the tourmakeady group strike slip faulted south of the south mayo trough during or after the grampian orogeny further correlation of these units with northern appalachian rhyolites is also possible the delaney dome formation is an extrusive temporal equivalent of intrusions that penetrate the connemara dalradian thus movement along the mannin thrust brought mid crustal plutons and dalradian country rocks tectonically above the extrusive volcanic sequence the mannin thrust is identified as a major imbricating structure within a continental arc but not a terrane boundary

a radiometric age for the cretaceous tertiary boundary based upon k ar rb sr and u pb ages of bentonites from alberta saskatchewan and montana bentonites associated with coal seams at the north american cretaceous tertiary k t boundary in the red deer valley of alberta in the frenchman valley of saskatchewan and near hell creek montana were dated by the k ar rb sr and u pb methods based upon palynological stratigraphic correlation and the presence of a sharp iridium anomaly the three sampled bentonite horizons all occur less than 1 m above the established k t boundary there is evidence of possible systematic errors giving dating results outside nominal statistical error values despite possible constant errors the weighted mean of nine average dates is calculated as 64 3 1 2 ma 2 standard deviation an unweighted averaging produces a grand mean age of 64 4 1 2 ma