investigation of a lower mantle shear wave triplication using a broadband array sh signals recorded by an array of four broadband benioff 1 90 instruments run by caltech are inspected for evidence of a lower mantle shear wave velocity discontinuity previously detected using wwssn data deep focus earthquakes beneath argentina span the distance range 78 to 82 from the array where the triplication in the travel time curve produced by the discontinuity is expected to result in an arrival between the direct shear wave s and the reflection off of the core mantle boundary scs the sh data for these events do show evidence of the arrival however the broadband signals also show significant receiver structure complexity empirical receiver functions are determined for each station by stacking observations for events in south america closer than 75 from the array incorporating these receiver functions into the synthetic modeling demonstrates that lower mantle shear velocity models with a discontinuity fit the observations better than models without one

the computation of body wave synthetic seismograms in laterally homogeneous media synthetic seismograms computed for realistic horizontally stratified media are now routinely used as an aid to seismic interpretation this paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms reflectivity and wkbj seismograms for a variety of earth models the transformed wave equations are developed from the equations for a spherical gravitating medium in a symmetric form suitable for body wave calculations four methods of solving these equations in general inhomogeneous layers are described the wkbj and langer asymptotic expansions and the wkbj and langer iterative solutions together with the earth flattening transformation and the ray expansion transformed solutions for body waves can then be obtained for realistic layered media four methods of inverting the frequency and wave number transformations are also described the real and complex spectral and slowness methods although realistic seismic models are normally sufficiently complicated that numerical calculations are essential before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms these analytic results for direct rays partial and total reflections turning rays on forward and reversed branches head waves interface waves airy caustics and fresnel and interface shadows are useful to anticipate and understand numerical problems and results finally a comparison of green s functions for crustal mantle and whole earth models calculated using the wkbj and reflectivity algorithms is included

stratigraphy and sequence stratigraphy of the moscovian in the donets basin new results are presented on the stratigraphy based on fusulinids and palynomorphs and the sequence stratigraphy of the moscovian in the donets basin they have been developed by comparison of our observations on cross sections near artemovsk and the published coal mine logs of artemovsk geological survey the moscovian of the donets basin is equivalent to the uppermost westphalian b and the westphalian c and d of western europe in the donets basin the moscovian forms a second order sequence as in the moscow basin that can be subdivided into four third order sequences subdivided into fourth order sequences the upper bashkirian shows two fourth order sequences the vereian exhibits two fourth order sequences the kashirian shows four fourth order sequences the podolskian offers eight fourth order sequences the myatchkovian presents four fourth order sequences these sequences begin with an erosive base and show a succession of elementary sequences with fluvial sandstone coal seam limestone and deltaic facies the fluvial sandstone corresponds with an aggradation period the coal and the limestone with a transgressive period and the deltaic facies with a progradation period a decrease of the sandstone thickness and an increase of the marine deposits that are transgressive and regressive upwards are observed from the base to the top of sequences these fourth order sequences present a higher spatial amplitude than the third order sequence and obscure these sequences the second order sequence equivalent to the moscovian shows a lowstand system tract with more fluvial sandstone in the lower part of the vereian a transgressive system tract in the upper part of the vereian tsinskaya and kashirian a maximum flooding period during the podolskian and lower part of the myatchkovian with more marine facies and a highstand system tract in the upper part of the myatchkovian with more lagoonal and swampy palaeoenvironments the sequences are controlled by the regional tectonic subsidence in the graben and the uplift of the horsts and or by eustasy with either a plate tectonic or a glacial origin other investigations particularly radiometric dating will be useful for solving this problem

emplacement of the cuera and picos de europa imbricate system at the core of the iberian armorican arc cantabrian zone north spain new precisions concerning the timing of arc closure the final stages of the variscan orogeny mostly carboniferous in the western european variscan belt involved the development of the iberian armorican arc which is cored by the cantabrian zone nw iberia the cantabrian zone is the foreland of the western european variscan belt and it is interpreted to record the waning stages of the closure of the rheic ocean the distalmost tectonic units within the cantabrian zone the cuera unit and the picos de europa province were the last tectonic units emplaced at the core of the western european variscan belt orocline together they form an imbricate system and associated wedge top basins that are key to understanding the development of the orocline the emplacement of the cuera unit and the picos de europa province occurred in the latest pennsylvanian between moscovian and gzhelian times new detailed mapping together with stratigraphic sedimentologic and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the cantabrian variscan foreland fold and thrust belt most probably during the oroclinal bending of the sw european variscan belt that formed the iberian armorican arc the n s directed imbricate system was characterized by a shallow dip of the topographic surface 150 15 km and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying gondwana lithosphere and may reflect the closure of the iberian armorican arc

cyclothem digital correlation and biostratigraphy across the global moscovian kasimovian gzhelian stage boundary interval middle upper pennsylvanian in north america and eastern europe the long standing difficulty of correlating pennsylvanian strata among provincial faunal regions is resolved by effecting digital correlation of major glacial eustatic cyclothems that represent high stands when certain species achieved more global distribution than usual in the late moscovian early gzhelian late desmoinesian early virgilian succession in the midcontinent united states several major cyclothems are correlated by both conodont species in common and cyclothem scale with cyclothems in russia moscow basin and ukraine donets basin and the remaining cyclothems fit into the framework by position and scale in this way the suggested event marker for the global kasimovian gzhelian stage boundary first appearance of idiognathodus simulator is supported while possible event markers for the moscovian kasimovian boundary await further evaluation

multiple phase analysis of the shear velocity structure in the d region beneath alaska a large data set more than 990 total waveforms comprised of north american station recordings of a variety of long period s phases s scs sdiff ss sscs sks skks from 36 earthquakes in western pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the d region beneath alaska three hypotheses that have been proposed to explain observed scs precursors and s waveform distortions for this region are appraised with the multiple phase data set and the radial and lateral variations of the shear velocity structure in the 1500 km broad portion of d sampled are also assessed the observed travel time and waveform variations may result from diffraction by near source slab structure scattering by low velocity heterogeneity in the lower mantle or interaction with complex velocity gradients in d associated with a deep mantle velocity discontinuity we utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques and find that the discontinuity model is most consistent with the data subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior which is consistent with extensive local stratification of d the data show sufficient coherence over the 1500 km broad region that we find it useful to present a local average radial model the preferred model sylo has a 2 75 velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the d layer analysis of sks s sks scs and sks skks differential times also indicates that the outermost 100 km of the core below the fast d region underlying alaska must be slower than the prem structure the observations exhibit significant scatter most of which can be attributed to unmodeled heterogeneity in the midmantle and near source regions as well as lateral variations in the d region the complex structure of d that we prefer may be associated with large scale downwelling in the lower mantle but it is not certain whether it is a purely thermal or a combined chemical and thermal effect

evidence of a laterally variable lower mantle structure from p and s waves summary examples of grafenberg array data showing anomalous p waves which typically arrive 3 5s after the direct p wave and which have a slowness 0 7 0 8 sdeg smaller than direct p are presented this additional phase is most frequently observed for events located in the ne portion of the southern kurile island subduction zone 73 80 from grafenberg but systematically disappears for events in the sw portion of this zone because of the magnitude of the slowness difference these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab likewise they may not be accounted for by near receiver structure because these phases are not seen for all kurile events if present they appear at all stations of the array but they follow direct p too closely to be a multiple from the moho therefore we conclude they are very likely caused by lower mantle velocity structure the most likely explanation is the presence of a p velocity jump of about 3 per cent approximately 290 km above the core mantle boundary since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional p phase the distribution of bounce points on this reflector for the kurile events indicates a lateral extension of this velocity anomaly under northern siberia of about 150 km by at least 200 km the best fitting s wave model has a reflector in the same depth but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the p and s velocity in d few events from other regions in this distance range are suitable for a definitive analysis of this kind from among this group some observations indicate a lower mantle anomaly under the lomonosow ridge and under northern greenland but since the lower mantle under western siberia northern novaya zemlya the azores islands region and the ussr afghanistan border region does not produce an additional phase in the grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature

quantitative radiometric and biostratigraphic calibration of the pennsylvanian early permian cisuralian time scale and pan euramerican chronostratigraphic correlation a quantitative biostratigraphic and radiometric calibration is presented for the pennsylvanian through early permian global time scale based upon high precision isotope dilution thermal ionization mass spectrometer id tims u pb zircon ages for interstratified ash beds in the parastratotype sections of the southern urals of russia twenty four ash bed ages in three outer ramp and basinal sections of the pre uralian foredeep bracket the biotic definitions of global stages and regional substages from the base of the upper pennsylvanian kasimovian stage to the base of the lower permian artinskian stage four additional ash bed ages in two sections of the eastern slope of the urals constrain the global bashkirian and serpukhovian stages quantitative stratigraphic methods conop9 are applied to a compilation of over 2000 bioevents in 22 stratigraphic sections supplemented by our dated volcanic horizons to refine the pennsylvanian early permian global time scale significant shifts in the duration of several stages are demonstrated ranging from one to six million years compared with prior estimates the unprecedented density of radiometric calibration points for the pennsylvanian permian transition provides a high resolution 0 1 ma global chronostratigraphic standard for testing and improving biostratigraphic correlations across euramerica we integrate radiometric ages biostratigraphic correlation and cyclostratigraphic tuning of major cyclothems to the long period 404 ka eccentricity cycle to elucidate the tempo magnitude and forcing of eustatic changes and cyclothemic deposition associated with the waxing and waning of gondwanan ice sheets and establish a pan euramerican chronostratigraphic framework for most of pennsylvanian and early permian time

deconvolution of teleseismic recordings for mantle structure summary delineation of detailed mantle structure frequently requires the separation of source signature and structural response from seismograms recorded at teleseismic distances this deconvolution problem can be posed in a log spectral domain where the operation of time domain convolution is reduced to an additive form the introduction of multiple events recorded at many stations leads to a system of consistency equations that must be honoured by both the source time functions and the impulse responses associated with propagation paths between sources and receivers the system is inherently singular and stabilization is accomplished through the supply of an initial estimate of the source time function although alternative choices exist an effective estimate is derived from the eigenimage associated with the largest eigenvalue in a singular value decomposition of the suite of aligned seismograms corresponding to a given event the relation of the deconvolution scheme to simultaneous least squares deconvolution is examined application of the methodology to broadband teleseismic p waveforms recorded on the canadian national seismograph network demonstrates the retrieval of effective green s functions including secondary phases associated with upper mantle structure

mass transport deposits in a variscan wedge top foreland basin pisuerga area cantabrian zone nw spain abstract large scale mass transport deposits mtds from two wedge top depocentres in a variscan foreland basin system cantabrian zone iberian massif northern spain have been studied to gain a better understanding of the processes involved in comparable deposits found in modern submarine environments the well exposed outcrops allow documenting the composition internal organization and lateral changes of the mtd units and additionally to know their frequency and significance in a tectonically active depositional setting the synorogenic 5000 m thick succession that contains these resedimented units comprises seven major tectonically controlled sequences whose bounding unconformities were recognised by seismic scale stratal patterns such as fault propagation and growth fold geometries wedging truncation and onlap these major sequences range between 4 and 0 4 myr in duration and between 1300 and 200 m in thickness and display an overall shallowing upward trend from a basal mtd and deep water sediments prodeltaic mudstone wedges turbidites to shallow marine carbonates and deltaic deposits each major sequence consists of one or more transgressive regressive high frequency sequences up to several hundred metres in thickness showing rapid lateral changes of facies associations across the wedge top depocentres the mtd units are composed of megaclasts up to 2 km long and 360 m thick embedded in mudstones matrix and clast supported breccias and conglomerates with variable internal organization and subordinate turbidite sandstones deposited from slides and gravity flows from cohesive debris flows to high density turbidites these en masse deposits originated from the failure of shelf to slope reaches and some of them constitute multiepisodic units resulting from several large scale collapse events these collapses could have been triggered by several factors mainly related to the growth of fault propagation anticlines during thrust emplacement the ensuing tectonic overload would lead to strong flexural subsidence and to the modification of the basin profile both the major sequences and the mtd units are a useful tool for correlating the highly deformed and faulted successions across the three fault bounded synclines and establishing a coherent stratigraphic framework

chronostratigraphy and paleoclimatology of the lodeve basin france evidence for a pan tropical aridification event across the carboniferous permian boundary abstract paleosols preserved within the carboniferous permian succession of the lodeve basin massif central france change stratigraphically from histosols to calcic vertisols and calcisols to gypsic vertisols and ultimately back to calcic vertisols and calcisols new high precision u pb zircon ages ca idtims for tuff beds within the lodeve and adjacent graissessac basins significantly revise the chronostratigraphy of these and correlated permian terrestrial basins of eastern euramerica under the newly revised chronostratigraphy presented here these stratigraphic changes in morphology indicate a substantial drying of paleoenvironments across the carboniferous permian boundary with a trend toward progressively more arid and seasonal climates through most of the early permian this newly realized chronology provides a paleoenvironmental and paleoclimatic timeline for eastern tropical pangea that is contemporaneous with similar records in western pangea and suggests pan tropical progressive climate change toward aridity and seasonality occurred from the late carboniferous through early permian

seismic stratigraphy of upper quaternary shallow water contourite drifts in the gulf of taranto ionian sea southern italy abstract the occurrence of articulated seafloor morphology over continental shelf upper slope environments may result in a significant change in the patterns and intensity of basin scale thermohaline circulation during eustatic sea level fluctuations these changes may cause in turn erosion deposition and or transport of sediments at the seafloor to form shallow water contourite drifts here we investigate this process in the nw sector of the gulf of taranto ionian sea during and following the last glacial maximum lgm by integrating multibeam bathymetric data ultra high resolution seismic reflection data and gravity core data sea level fall caused subaerial exposure of the summit of the amendolara bank forming a short lived island off the eastern coast of calabria and also creating a narrow passageway between the island and the northern calabria mainland integrated seismic stratigraphic data show that upper quaternary shallow water contourite drifts and associated erosional features locally formed both around the flanks of the amendolara bank ambk and the continental shelf and upper slope off the amendolara village contourite drifts are bounded at the bottom and at the top by two major unconformities indicating that the formation of the sediments drifts occurred between the onset of the lgm and the gs 1 younger dryas event the stratal architecture suggests the occurrence of various types of contourite deposits mostly represented by a axial and lateral channel patch drifts and channel related drifts along the incision to the ne of the ambk b sheeted drifts along the northeastern slope of the ambk c elongated drifts along the continental shelf and upper slope off the coast of amendolara village erosional features also developed on the south eastern flank of the ambk where the levantine intermediate water flows from the central ionian sea towards the gulf of taranto until the present day both processes and timing responsible for erosion of the seafloor and the formation of sediment drifts in the gulf of taranto may be similar to that occurred in the tyrrhenian margins during the late quaternary

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

ediacaran epifaunal tiering epifaunal tiering the subdivision of vertical space within a community is a fundamental attribute of phanerozoic suspension feeding communities this paper documents tiering including the presence of meter tall organisms in neoproterozoic ediacaran communities ediacaran tiering was studied from three exceptionally preserved deep water communities at mistaken point newfoundland which contain in situ census populations of hundreds to thousands of organisms tiering consists of overlapping populations of dominant organisms and is characterized by gradational rather than abrupt tier boundaries at least three tiers are apparent a lower level 0 8 cm above the seafloor an intermediate level between 8 22 cm above the seafloor and an upper level that extends as high as 120 cm tier boundaries are relatively consistent between communities but the constituent organisms in each level are variable suggesting that some ediacaran taxa could fill different tiers interchangeably development of a tiered epifaunal structure is consistent with suspension feeding or absorbing dissolved nutrients directly from seawater despite the common occurrence of tall organisms all communities share a similar population structure in which biomass is concentrated in the basal 10 cm above the seafloor comparison with shallow water ediacaran localities suggests that the observed tiering structure is typical of ediacaran communities ediacaran tierers also show the fundamental subdivision between organisms and or colonies that fed along their entire length and those that developed a specialized feeding apparatus implying that the features of phanerozoic tiered skeletal ecosystems were first initiated in soft bodied communities in the late neoproterozoic

crustal sources in lower palaeozoic rocks from nw iberia insights from laser ablation u pb ages of detrital zircons detrital zircons from greywackes belonging to an ordovician volcanosedimentary igneous complex in nw spain were dated using the laser ablation inductively coupled plasma mass spectrometry technique u pb results indicate the existence of three pre cambrian crustal sources in nw iberia the age grouping dated at c 590 620 ma confirms the presence of a cadomian avalonian basement in this zone the second age group c 1 1 1 2 ga is the first evidence for a grenvillean age crustal component in nw iberia the age grouping dated at c 1 9 2 0 ga corresponds to the age of the icartian igneous basement exposed in other areas of the european variscides

a shear velocity discontinuity in the lower mantle a lower mantle s wave triplication is detected using short and long period sh seismograms in the distance range 70 sup 0 to 95 sup 0 modeling of the observations with synthetic seismograms indicates that the triplication is produced by a 2 75 0 25 shear velocity increase about 280 km above the core mantle boundary the sh data from intermediate and deep focus events for three distinct source region receiver array combinations show generally consistent travel time and relative amplitude behavior of the triplication there are small systematic travel time shifts between the source regions which suggest regional variations in the depth of the discontinuity of up to 40 km

photographic evidence of variable bottom current activity in the suruga and sagami bays central japan abstract complex patterns of bottom current behaviour were clarified by studies of sedimentary features and orientations of benthic animals in the deep suruga and sagami bays on the pacific side of central honshu japan both the suruga and sagami bay measure about 60 km in length as well as in width at their mouths and are about 1500 m deep in their central portions the size of each bay is comparable to that of ancient forearc basins the bottom sediments are characterized by turbidites and slump deposits at 32 stations on the bottom of the bays deep sea photographs were taken most of which clearly indicate bottom current activity current induced bottom features are current lineations moat like scours around resistant objects crag and tail structures behind obstacles ripple marks sand ridges and deformed biogenic structures such as burrows mounds tracks and faeces these features are produced by bottom currents with rather high velocities other important current indicators are some benthic organisms which in general show a sensitive response to currents and adopt particular orientations typical examples of megabenthos identified in the bottom photographs as effective current indicators are the small deimatid holothurian peniagone japonica the benthic shrimp glyphocrangon hastacauda sea anemones and sea pens among them the orientation of peniagone japonica shows abrupt changes of current direction with time for example from n 0 to sw 240 during 15 min and from n to s and back to n a complete rotation during 40 min the results of these observations indicate that the bottom currents in deep bays tend to fluctuate rapidly in velocity and direction probably owing to strong internal tidal waves in the very steep embayments upslope currents appear to be present at the mouths of submarine canyons thus it should be borne in mind that palaeocurrent analysis of ancient bottom current deposits or contourites is limited in application

la icp ms u pb dating of carboniferous ash layers in the cantabrian zone n spain stratigraphic implications seven centimetre thick volcanic ash fall layers interbedded within the thick carboniferous successions of the cantabrian zone in northern spain were dated by u pb zircon laser ablation inductively coupled plasma mass spectrometry across an interval ranging from visean to kasimovian thus covering most of the carboniferous period all these ash layers occur in fossiliferous successions allowing us to insert the radiometric data within a well constrained biostratigraphic framework considering the analytical uncertainty the obtained ages match the ages inferred from the conodont biostratigraphy established in the mississippian succession which hosts the oldest two ash layers visean in age and the fusuline and mega and microflora data from the strata hosting the moscovian and kasimovian westphalian stephanian tonsteins the age of a langsettian tonstein along with data provided by several papers stating that in the cantabrian zone langsettian floras were contemporaneous with lowermost moscovian fusulines suggest that langsettian floras could have been younger in spain than in other areas our absolute ages provide new constraints not only for the correlation of the carboniferous successions of the cantabrian zone with time equivalent reference successions in other parts of the world but also for calibrating the carboniferous global chronostratigraphic units based on marine fossils with the west european regional units supplementary material stratigraphic position and biostratigraphic information on the studied ash fall layers analytical methods and geochronological results are available at https doi org 10 6084 m9 figshare c 3768701

lateral variations near the core mantle boundary differential travel times between s and sks ts sks and skks and sks tskks sks along with amplitude behavior have been used to study lateral variations at the core mantle boundary the data were recorded by the world wide seismographic station network wwssn and the canadian seismograph network csn from source regions in south america and the fiji tonga area a core mantle boundary region traversed by sks and s beneath the mid pacific from the fiji tonga area to north american stations shows the most anomalous behavior in ts sks sks and s are anomalously separated by up to 8 seconds in addition to ts sks anomalies sks and skks are anomalously separated by 2 to 3 seconds a model fitting ts sks for this anomalous region requires one or more of the following 1 up to 5 increase in the top 300 km of outer core velocity 2 a large scale high vs region 2 increase in the lower mantle ne of tonga 3 lateral variations in d shear wave velocities of up to 3 however 1 above predicts a decrease in tskks sks contradicting the observations this suggests that 2 and 3 are more likely explanations in modeling this data set

the pyroclastic tonsteins of the sabero coalfield north western spain and their relationship to the stratigraphy and structural geology abstract the sabero coalfield comprises a succession of strata of over 2000 m of late barruelian to early stephanian b sensu carmaux age proposals have earlier been made to base the boundary stratotype for the top of the barruelian on a section in the lower half of the sabero succession ten different tonstein bands have been located in the principal coal bearing lithostratigraphic units raposa formation sucesiva formation herrera beds and the unica beds material from over 50 sample locations from both surface outcrops and from underground coal workings has been studied by means of microscope thin sections supported by x ray diffraction spectrometry xrd analysis and whole rock chemical analysis on a small number of selected samples all of the 10 bands occur as kaolinite coal tonstein in which the main constituent is kaolinite both as matrix and as abundant fragmentary crystals and phenocrysts the latter predominantly comprising pseudomorphs after biotite and feldspar graupen and irregular kaolinite masses occur in some cases conserving relict structures of volcanic glass feldspars have been identified in the range anorthite albite potassic feldspars are in general absent quartz occurs as acicular splinters and as form and paraform with magmatic re absorption embayments and glass inclusions tridymite has been identified in two bands other accessory primary constituents include idiomorphic apatite and zircon the composition and petrographical features are entirely consistent with a pyroclastic origin locally some apparently secondary alteration of kaolinite to illite is observed and one band shows a lateral progression from kaolinite tonstein through kaolinite illite tonstein to illite tonstein which is apparently coincident with increased intensity of tectonisation a number of the bands have served as the means of resolving structural and stratigraphic problems in the sabero coalfield in particular the identification of level 31 has permitted the reinterpretation of paquete central one of the traditionally principal areas of coal working previously considered to comprise some five or six seams and now recognised as a series of three or more structural repetitions of two workable seams only the overall thickness interpreted for the herrera beds is correspondingly reduced the relatively common presence of ca plagioclase phenocrysts in some bands suggests a rather more dacitic magmatic origin than for the otherwise closely comparable tonsteins of the massif central of france the sabero tonstein bands are closely similar in character and composition with those known in the cinera matallana coalfield and a similar area of origin is surmised although further stratigraphical and petrological work is required before firm correlations can be achieved

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

dynamical influences from thermal chemical instabilities at the core mantle boundary two dimensional time dependent calculations show that the thermal and flow structures can be influenced by the lower mantle thermal chemical instabilities the style of the core mantle boundary cmb deformation in thermal chemical convection is different from that obtained in thermal convection the amplitude of cmb hills is reduced greatly from the depth dependence of thermal expansivity found in recent high pressure experiments

iberian late variscan granitoids some considerations on crustal sources and the significance of mantle extraction ages a suite of post tectonic granitoids mostly peraluminous broadly i type granodiorites and monzogranites and mafic rocks from nwiberia with crystallization ages between ca 309 and 290 ma has been investigated for sm nd isotopes and inherited zircon content in order to constrain the nature of their source rocks end values at 300 ma vary from 0 2 to 5 9 and tdm values range from 1 01 to 1 58 ga inherited xenocrystic zircons yielded ages ranging from 458 to 676 ma with 90 of data between 490 and 646 ma corresponding to neoproterozoic mostly ediacaran cambrian andordovician ages only three highlydiscordant analyses yielded ages older than 650 ma based on the data reported herein and relevant data fromthe literaturewe contend that post tectonic granitoids of the iberian variscan belt with exception of the scarce anatectic s type granitoids were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca 0 9 and 1 1 ga i type granitoids and mantle derived mafic rocks both underwent varying degrees of contamination by ametasedimentary lower crust depleted in pre 650 ma zircon through previousmelting episodes with a time integrated sm nd evolution different to that of the metaigneous lower crust participation of this metasedimentary crust in the genesis of these granitoids may account for nd isotopic variability and nd model ages well in excess of 1 2 ga

a review of the middle late pennsylvanian west european regional substages and floral biozones and their correlation to the geological time scale based on new u pb ages abstract lower moscovian duckmantian to lower asselian ash beds in the continental basins of the central and western czech republic have yielded 15 new high precision u pb single crystal zircon ca id tims ages these are used to improve the calibration of the west european regional terrestrial stages and their correlation to the geological time scale they also allow an improved correlation between the macrofloral biozones that are used for biostratigraphy in the european pennsylvanian coalfields the marine biozones on which the global geological time scale was partly established and the fresh water and terrestrial faunal biozones most of the floral biozones have proved to be essentially isochronous although the ranges of some individual but stratigraphically important taxa may differ between basins

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

rayleigh wave phase velocity maps of tibet and the surrounding regions from ambient seismic noise tomography ambient noise tomography is applied to the significant data resources now available across tibet and surrounding regions to produce rayleigh wave phase speed maps at periods between 6 and 50 s data resources include the permanent federation of digital seismographic networks five temporary u s program for array seismic studies of the continental lithosphere passcal experiments in and around tibet and chinese provincial networks surrounding tibet from 2003 to 2009 totaling 600 stations and 150 000 interstation paths with such a heterogeneous data set data quality control is of utmost importance we apply conservative data quality control criteria to accept between 5000 and 45 000 measurements as a function of period which produce a lateral resolution between 100 and 200 km across most of the tibetan plateau and adjacent regions to the east misfits to the accepted measurements among passcal stations and among chinese stations are similar with a standard deviation of 1 7 s which indicates that the final dispersion measurements from chinese and passcal stations are of similar quality phase velocities across the tibetan plateau are lower on average than those in the surrounding nonbasin regions phase velocities in northern tibet are lower than those in southern tibet perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained at short periods 20 s very high velocities are imaged in the tarim basin the ordos block and the sichuan basin these phase velocity dispersion maps provide information needed to construct a 3 d shear velocity model of the crust across the tibetan plateau and surrounding regions

high precision u pb zircon ca id tims dates from western european late visean bentonites three new u pb zircon chemical abrasion isotope dilution thermal ionization mass spectrometry dates obtained from late visean belgian bentonites are reported and are used to estimate the periodicity of early warnantian shallowing upwards carbonate parasequences that are interbedded with the dated bentonites early warnantian parasequences exhibit mean cycle periodicity values that are consistent with the c 100 ka milankovitch cycle which is the dominant milankovitch frequency recognized from recent pleistocene glacial records and thus strengthen the arguments for 1 these sedimentary cycles being of glacio eustatic origin and 2 the initiation of the main phase of late palaeozoic glaciation before the start of or during earliest warnantian times the new dates also provide additional high precision age constraints for the improved calibration of the mississippian time scale using the new dates the stratigraphical age of the clyde plateau volcanic formation midland valley of scotland is revised from holkerian to early asbian supplementary material u pb zircon ca id tims data tables and the method of the mean cycle periodicity calculations are available at www geolsoc org uk sup18747

the devonian and carboniferous of the moscow syneclise russian platform stratigraphy and sea level changes abstract the moscow syneclise is a vast sedimentary basin located in the centre of the east european platform the palaeozoic sequence of this syneclise and especially its devonian carboniferous part is most suitable for sea level fluctuation analysis the devonian and carboniferous sediments consist mainly of shallow marine carbonates containing several minor terrigenous intervals they attain total thicknesses of up to 1 5 km the modern stratigraphic scale includes 33 regional stages for the devonian and 32 for the carboniferous correlation of this scale with global scales is supported by conodont miospore and foraminifera zonations the new calibrated devonian and carboniferous relative sea level curves for the moscow syneclise reflect late eifelian mid frasnian early tournaisian late visean early serpukhovian and moscovian gzhelian transgressions several of these sea level changes which are particularly evident on the margins of the moscow syneclise originated in response to differential tectonic movements of the voronezh anteclise and the tokmovo uplift as well as to vertical oscillations of the entire east european craton

age and timing of the permian mass extinctions u pb dating of closed system zircons the age and timing of the permian triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts zircons from nine ash beds within the shangsi and meishan sections china pretreated by annealing followed by partial attack with hydrofluoric acid result in suites of consistent and concordant uranium lead u pb ages eliminating the effects of lead loss the u pb age of the main pulse of the extinction is 252 6 0 2 million years synchronous with the siberian flood volcanism and it occurred within the quoted uncertainty

short wavelength geoid bathymetry and the convective pattern beneath the pacific ocean summary the geoid and bathymetry data in the pacific ocean are used to shed new light on the structure of upper mantle convection at short wavelengths 200 km the geoid pattern is characterized by parallel lineations lineations however appear less clearly at larger wavelengths 400 or 600 km where the signal due to convective thermal anomalies might be expected the most prominent features among the short wavelength lineations are associated with the well known chains of seamounts and islands other lineations observed in the geoid coincide with computed tracks of other volcanic centres although these tracks are not continuous this suggests that long lived plumes are at the origin of the apparently short chains of seamounts apart from the lineations corresponding to the tracks of classical hotspots there are numerous similar lineations visible on the filtered geoid maps which have the same directions as hotspot tracks these preferential directions and the gravity to bathymetry ratios seem to indicate that the lineations are due to magmas implaced just above or within the lithosphere convective plumes fixed with respect to each other and much more numerous than the classical hotspots could be the source of these magmas a geometrical test of this hypothesis is performed by comparing the lineation pattern with the computed tracks for a set of tentative new hotspots in this way numerous plumes long lived and fixed with respect to each other are found to provide an explanation of the observed small wavelength geoid features

high precision u pb zircon age calibration of the global carboniferous time scale and milankovitch band cyclicity in the donets basin eastern ukraine 1 high precision id tims u pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic cyclostratigraphic and biostratigraphic framework of the carboniferous donets basin of eastern europe chemical abrasion of zircons use of the internationally calibrated earthtime mixed u pb isotope dilution tracer and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of 0 05 or 100 ka for these carboniferous volcanics this precision allows the resolution of time in the milankovitch band and confirms the long standing hypothesis that individual high frequency pennsylvanian cyclothems and bundles of cyclothems into fourth order sequences are the eustatic response to orbital eccentricity 100 and 400 ka forcing tuning of the fourth order sequences in the donets basin to the long period eccentricity cycle results in a continuous age model for the middle to late pennsylvanian moscovian kasimovian ghzelian strata of the basin and their record of biological and climatic changes through the latter portion of the late paleozoic ice age detailed fusulinid and conodont zonations allow the export of this age model to sections throughout euramerica additional ages for mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations

evidence for a p wave velocity anomaly in d a number of strong p coda phases have been observed on european wwssn station short period recordings of intermediate magnitude kurile arc earthquakes which occurred at several different depths of focus these phases have been observed for several different events and cannot be explained by known phases such as pcp or source multiplicity a lower mantle model containing a first order discontinuity with a 2 75 p wave velocity contrast and located about 344 km above the core mantle boundary produces synthetic seismograms which approximately reproduce the observed amplitudes and arrival times of the coda phases however the fact that these phases are not always observed and that their amplitudes vary significantly for different events and stations suggests that the discontinuity at the top of d may be a transition zone and may be laterally variable in thickness and contrast

new high precision u pb dates from western european carboniferous tuffs implications for time scale calibration the periodicity of late carboniferous cycles and stratigraphical correlation we present three new high precision u pb zircon chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims dates from western european carboniferous diagenetically altered volcanic ash layers bentonites and tonsteins that occur interbedded with cyclic siliciclastic sedimentary rocks the dates constrain the average periodicity of western european late carboniferous arnsbergian to early langsettian cyclic sedimentation to between 89 and 124 ka per cycle this is consistent with the 100 ka short eccentricity milankovitch cycle and supports previous suggestions that the cycles are the product of glacio eustasy cyclostratigraphic tuning of namurian cycles to the 100 ka eccentricity cycle revises the timing of the global mississippian pennsylvanian boundary to c 323 9 ma ca id tims dates also facilitate correlation between different facies belts where biostratigraphical correlations are difficult correlation between eastern and western european sequences confirms that the base of the moscovian stage is located within the duckmantian westphalian b western european regional substage supplementary materials zircon separation and concentration procedures u pb zircon ca id tims methods and data table average periodicity calculations for western and central european namurian and westphalian sedimentary cycles and procedures for recalibrating legacy 40 ar 39 ar dates are available at www geolsoc org uk sup18547

climate and eustasy driven cyclicity in pennsylvanian fusulinid assemblages donets basin ukraine abstract a model of cyclic recurrence 0 6 1 2 myr of three fusulinid assemblages in the middle pennsylvanian siliciclastic carbonate succession of the donets basin is proposed each cycle records progressive turnover of assemblages in shallow marine environments in response to sea level and regional climate change a hemifusulina assemblage a adapted to cooler and reduced salinity seawater records the onset of sea level rise accompanied by humid climatic conditions sea level high stand is captured by the beedeina neostaffella ozawainella taitzehoella or beedeina dominated assemblage b characteristic of relatively deeper water environments the b assemblage is successively replaced by the most diverse population of the warm water fusulinella dominated assemblage c this assemblage which occurs in the upper limestones of each fusulinid cycle records the onset of sea level fall accompanied by a shift to drier conditions and likely increased seawater salinity the proposed model permits robust interbasinal correlation of the pennsylvanian successions of the tethyan realm fusulinids of the a and c 2 assemblages are the most provincial and therefore the most useful for paleogeographic reconstructions specifically they delineate originally contiguous regions that subsequently were dispersed hundreds to thousands of kilometers whereas fusulinids of the b assemblage hold the highest potential for global correlation extinction at the moscovian kasimovian transition of fusulinid genera of the a and b assemblages which inhabited predominately cooler and normal salinity perhaps hyposaline waters can be explained by the onset of global warming in the earliest late pennsylvanian fusulinid assemblages define various types of distribution patterns that differ by tectonic setting of the studied basins suggesting that fusulinid assemblage patterns hold potential for reconstructing the paleogeography and tectonic evolution of pennsylvanian basins of eastern laurasia

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

evidence against a discontinuity at the top of d summary we investigate a recent suggestion of lay helmberger that there is a world wide discontinuity in 5 wave velocity about 250 300 km above the core mantle boundary with an increase of 2 5 3 per cent from above to below their model predicts pronounced pulse form changes of long period sh body waves in the epicentral distance range 95 120 a range which has not been studied by them similar pulse form changes are expected in p wave seismograms under the reasonable assumption of a similar increase in p velocity nine long period seismogram sections of sh and p waves from deepfocus earthquakes below fiji west brazil the sea of japan the sea of okhotsk and the banda sea are presented with practically no indication of the complications expected also the decay constants of pdiff as predicted by the new model differ strongly from observed values thus there is clear evidence against this model the pulse form peculiarities between mantle s and scs in transverse component seismograms at distances from 70 to 80 which lay helmberger interpreted as reflections from the discontinuity of their model must have a different cause

constraints of magnetostratigraphic and mineralogical data on the provenance of sediments in the parece vela basin of the western pacific abstract the sediment provenance and sedimentary processes of the parece vela basin in the western pacific during the last ca 1 myr were studied based on the analysis of magnetic properties mineral features and paleomagnetic ages of core c p19 the results show that the sedimentary sequence has a lower limit age younger than 1 072 ma and the sedimentation process can be divided into two stages with sedimentation rates of 4 0 mm kyr during ca 1 0 78 ma and 3 6 mm kyr since 0 78 ma these sedimentation rates are higher than those in the surrounding area where the sedimentation rate is generally less than 2 mm kyr taken together with the mineral features and magnetic parameters these findings show that volcanic sediments eroded from the nearby ridge were transported by currents to the sampling site these volcanic sediments represent the main source of sediment in the area with eolian inputs contributing little and nonbiogenic authigenic magnetite accounting for part of the magnetic fraction the sedimentary environment and sediment provenance have varied during the last approximately 1 million years during 997 880 ka volcanic activity increased and the input of volcanic clasts was high whereas during 880 443 ka the sedimentary environment tended to be stable and the sediment supply decreased with the increase in volcanic activity in the western pacific the sediment input increased again during ca 443 ka present

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

characteristics of deep currents along trenches in the northwest pacific in the deep sea area of the japan trench located about 150 km east from northern honshu one observes southward flow on the landward slope and the northward flow on the oceanward slope in 1994 and 1995 moorings were deployed on the japan trench landward slope around 40 n one mooring was deployed for 275 days at about 5800 m and another mooring was deployed for 20 days at 4200 m depth results showed the following 1 a deep western boundary current flows south southwest on the landward slope of the japan trench with mean speed of 3 7 cm sec and maximum speed of 15 20 cm s 2 current changes depend mainly on the m2 tide 3 previous measurements as well as inferences from observed benthic fauna imply oppositely directed flow along the oceanward slope of the trench forming a trench countercurrent

preliminary study of tonsteins of the pastora formation stephanian b of the cinera matallana coalfield northwestern spain abstract the correlation of seams of the pastora formation in the cinera matallana basin is affected by facies changes seam discontinuities such as seam splitting and joining and tectonic features exploratory drillings in the matallana syncline and in an underground exposure at the northern flank of the llombera syncline prove for the first time the existence of six tonsteins the investigation provided three kaolin tonsteins one kaolinite illite transition tonstein and two illite tonsteins which are described in detail the determination of coal rank gives evidence that there is no strict co incidence with the degree of diagenesis or metamorphosis of the original volcanic ashes thus further reasons for the alteration have to be considered such as varying compositions of the original volcanic ashes biochemical and chemical processes after deposition local hydrothermal metasomatism caused by the intrusion of basic magma or tectonic stress besides known faunal and facies marker horizons these newly detected tonsteins represent first class markers within the pastora formation the possibility of using these layers for the correlation of strata and for palaeomorphological studies depends on the successful continuation of the search for additional tonsteins as well as on the compilation and evaluation of subsequent investigations

crustal melting and recycling geochronology and sources of variscan syn kinematic anatectic granitoids of the tormes dome central iberian zone a u pb la icp ms study in this study we report u pb laser ablation icp ms zircon and id tims monazite ages for peraluminous granitoid plutons biotite muscovite cordierite sillimanite in the tormes dome one of the gneiss cored domes located in the central iberian zone of the variscan belt of northern spain textural domains in zircon interpreted to represent the magmatic crystallization of the granitoids and one monazite fraction in the ledesma pluton yielded ages around 320 ma in agreement with other geochronological studies in the region this age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the variscan chain of western europe in addition there are several populations of inherited xenocrystic zircon 1 carboniferous zircon crystals ca 345 ma as well as one of the monazite fractions in the coarse grained facies of the ledesma pluton that also yielded an age of ca 343 ma 2 devonian silurian zircon xenocrysts with scattered ages between ca 390 and 432 ma 3 middle cambrian ordovician ca 450 511 ma 4 ediacaran cryogenian zircon ages ca 540 840 ma 5 mesoproterozoic to archaean zircon 900 2700 ma the abundance of carboniferous inherited zircon shows that crustal recycling cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence in our case study it seems plausible that a crustal layer of ca 340 ma granitoids migmatites was recycled partially or totally only 15 20 my after its emplacement

a search for anomalies in the lowermost mantle using seismic bulletins abstract for deep events and epicentral distances around 75 the bulletins of the international seismological centre bisc contain many secondary arrivals between p and pcp the reflection from the core mantle boundary cmb which cannot be explained by current spherical symmetric earth models assuming that these arrivals in the p coda are reflections from p velocity anomalies in the lower mantle we calculate the associated reflection points geographical coordinates and distance from the cmb and map both strength relative density of reflection points in bins in the lower mantle and distance of the reflectors from the cmb on a global scale the regions with these anomalous bins cover 3 of the 50 of the lowermost mantle that can be analysed with this data set the most significant regions with anomalous bins values in parentheses give a preliminary estimate of the distance of the reflector from the cmb are under the north atlantic ridge 250 km northern central siberia 230 km japan 180 km the central mid atlantic ridge 310 and 260 km and off the coast of ecuador 340 km the presence of reflectors in one of these regions has already been confirmed by detailed seismic studies using waveform data from arrays and networks a slight 2 3 p velocity contrast is sufficient to produce measurable reflection amplitudes in the distance range of 70 80 ten percent of the lower mantle sampled by this study is covered by bins with no or very few secondary arrivals in the p coda the most significant regions with such a normal lower mantle are under central america the caroline and marshall islands in the eastern pacific the area of hawaii and the easter islands and the northern atlantic identification of the regions with anomalous bins serves to locate the areas of the lowermost mantle which should be analysed further by using the full information content of seismograms the study of the travel time the amplitude and the form of the wavefield recorded preferably at seismic arrays dense networks will allow the determination of the anomalous velocity structure of the core mantle transition zone in more detail the most promising areas for such a study are given here but further anomalies are likely to exist

better mathematical constraints on ages of carboniferous stage boundaries using radiometric tuff dates and cyclostratigraphy using more geologically reasonable constraints on radiometric dates from tuffs in a cyclic mixed carbonate siliciclastic succession in the donets basin ukraine newer and more geologically accurate ages of stage boundaries are calculated for the carboniferous period these calculations assume astronomically forced milankovitch cyclicity and use radiometric dates of ash layers in the stratigraphic record unlike previous work measurement uncertainties are propagated through calculations to provide mathematically constrained uncertainty in the ages of the boundaries this work uses more realistic errors in the known dates of zircons in the ash layers and does not assume an a priori value for the milankovitch long eccentricity period these calculations are compatible with other previous methods of determining the boundaries and represent improvement over existing age constraints

provenance analysis of the paleozoic sequences of the northern gondwana margin in nw iberia passive margin to variscan collision and orocline development abstract the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges from neoproterozoic to early permian in age its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana or ii an active continental margin or iii a drifting ribbon continent in this paper we present detrital zircon u pb laser ablation age data from 13 samples taken in detrital rocks from the cantabrian zone sequence ranging from early silurian to early permian in depositional age the obtained results together with previously published detrital zircon ages from ediacaran ordovician strata allow a comprehensive analysis of changing provenance through time collectively these data indicate that this portion of iberia was part of the passive margin of gondwana at least from ordovician to late devonian times zircon populations in all samples show strong similarities with the sahara craton and with zircons found in libya suggesting that nw iberia occupied a paleoposition close to those regions of present day northern africa during this time interval changes in provenance in the late devonian are attributed to the onset of the collision between gondwana and laurussia additionally the middle carboniferous to permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the variscan orogeny late devonian to permian samples yield zircon populations that reflect topographic changes produced during the variscan orogeny and development of the lithospheric scale oroclinal buckling

further evidence for the lay discontinuity beneath northern siberia and the north atlantic from short period p waves recorded in france abstract we analyze data from the french laboratoire de detection geophysique ldg network of digital short period seismic stations the p waves from several earthquakes in the kuril okhotsk region display clear secondary arrivals in the 75 85 range which are not explained by smooth lower mantle models the move out of the arrivals across the network indicates a deep mantle origin we interpret them as waves reflected off or transmitted at the lay discontinuity about 300 km above the core mantle boundary the bounce points at this discontinuity are located beneath northern siberia slightly to the northwest of the region for which a similar structure has been reported we propose tools for investigating the lateral variations of the d structure on the scale 100 1000 km the method we propose is based on the deconvolution of the data or convolution of synthetics by a source time function derived from the data we show that the method helps identification of the waves that interact with the lay discontinuity and enables a more objective mapping of its lateral variations evidence for bumps a few kilometers high on the lateral scale of a few tens of kilometers is proposed we also probe a spot of the lower mantle beneath the north atlantic a detailed analysis of the waveforms from a chiapas earthquake suggests that the lay discontinuity is also present in that part of the world

off scraping accretionary process under the subduction of young oceanic crust the shimanto belt of okinawa island ryukyu arc the eocene kayo formation exposed at the southwestern end of the shimanto belt in the ryukyu arc records the progressive development of off scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint the formation consists almost wholly of coherent trench turbidites supplied by axial currents detailed structural analysis has revealed that two successive deformation stages are recognized in the formation the early deformation stage 1 occurred by horizontal shortening related to the initial stage of off scraping accretion in this stage sands were water rich and nearly unlithified and the competence contrast between sandstone and shale was low the late deformation stage 2 is marked by the formation of seaward verging imbricate fold and thrust system within the accretionary wedge compared with stage 1 sands in stage 2 were more dewatered and lithified and the competence contrast between sandstone and shale increased in accordance with this deformation history structures caused by high pore fluid pressure indicate increased cohesion of sands during off scraping accretion studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the formation became buried and suffered thermal imprint pressure solution cleavage developed as an axial planar to the stage 2 folds thermal imprint on the formation is characterized by high paleotemperatures at approximately 250 c 300 c in a region of the prehnite pumpellyite facies metamorphism this thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate

fundamental thermodynamic relations and silicate melting with implications for the constitution of d we describe fundamental thermodynamic relations helmholtz free energy as a function of volume and temperature for solids and liquids simple physically based expressions which contain all thermodynamic information about a system the solid fundamental relation consists of debye and birch murnaghan finite strain theory combined in the mie gruneisen framework the liquid fundamental relation is derived by taking the high temperature limit of the solid expression we derive the liquid equation of state which contains only four parameters from the liquid fundamental relation and show that it successfully describes measurements of liquid alkali metals water and liquid diopside over a wide range of pressure and temperature we find optimal fundamental relation parameters for diopside enstatite ilmenite and perovskite and find the solid relation to be in excellent agreement with data including heat capacities thermal expansion and mgsio3 phase equilibria we then combine the liquid and solid fundamental relations to calculate the melting curves of diopside enstatite and perovskite which are found to be in excellent agreement with experiment all predicted melting curves have dt dp slopes which decrease steadily with pressure eventually becoming negative because of liquid crystal density inversion our predicted melting temperature of perovskite in the d region 3750 k at the base of the mantle is thousands of degrees lower than previous estimates yet it is consistent with experimental data the predicted melting curve although consistent with the lack of widespread melting in the lower mantle is much lower than recently proposed geotherms in the d layer at the base of the mantle by combining our results with seismic observations of the deep mantle we propose that the d layer consists of magnesiowustite and silica in the form of stishovite or its recently discovered high pressure modification

radiometric ages of the fire clay tonstein pennsylvanian upper carboniferous westphalian duckmantian a comparison of u pb zircon single crystal ages and 40ar 39ar sanidine single crystal plateau ages abstract the fire clay tonstein pennsylvanian upper carboniferous westphalian series duckmantian stage a kaolinized volcanic ash deposit occurring in kentucky west virginia tennessee and virginia is the most widespread bed in the middle pennsylvanian of the central appalachian basin usa a concordant single crystal u pb zircon datum for this tonstein gives a 206pb 238u age of 314 6 0 9 ma 2 this age is in approximate agreement with a mean sanidine plateau age of 311 5 1 3 ma 1 n 11 for the fire clay tonstein the difference between the two ages may be due to bias between the 40k and 238u decay constants and other factors the age of the fire clay tonstein has important implications for duckmantian stage westphalian series sedimentation rates correlations with the westphalian series of europe middle pennsylvanian volcanic events and the late paleozoic time scale

modelling d structure beneath central america with broadband seismic data deep south american events recorded at distances between 70 and 96 are used to constrain the lower mantle structure beneath central america employing data from the terrascope bdsn iris and cnsn canadian national seismic network broadband arrays we obtain a 1 d shear model with a 200 km thick d layer with a 3 velocity jump at the top and a negative gradient within the layer from broadband waveform modelling of sh polarized s scs and scd the shear structure above d is similar to the preliminary reference earth model prem dziewonski and anderson 1981 preliminary reference earth model physics of earth planetary interiors 25 297 356 in terms of dt d and s sks differential times sv synthetics for this model assuming a prem core produce a satisfactory fit to the data except at the larger ranges to the north where the waveforms display rapid variation this feature is indicative of lateral variation of the type suggested by grand 1994 where the discontinuity becomes less well defined synthetics computed for a simplified 2 d cross section through this structure produce the scd phase and simple s at larger distances while scd is easily observed in the shear wave data there is little evidence of its counterpart in the p data by testing several models with different p velocity jumps at the same depth as the d jump in shear structure we conclude that the velocity jump must be less than 1 in fact prem provides an excellent fit both in timing and waveform shapes and therefore we conclude that lower mantle structure beneath central america shows clear evidence for a discontinuity in s velocity but not in p

bottom current controlled sedimentation a synthesis of the contourite problem abstract an overview of the main items concerning deep bottom current controlled deposits is presented these include the definition of contourites their processes of deposition and facies characteristics contourite drift types and seismic patterns the interplay of turbidity and bottom current processes and the correlation between bottom current processes and global changes in climate sea level from this discussion several of the more significant problems are highlighted which we believe should be priority targets for future research 1 identification of contourites especially in ancient series where they are interbedded with turbidites and other facies 2 refinement of the numerous parameters used as tracers of palaeocirculation patterns 3 understanding the interaction between the different types of bottom currents and also between bottom current and other deep water processes 4 distinction in the sedimentary record between the effects of short time scale bottom current variations due to regional causes like benthic storms and geological scale current fluctuations related to climatic or astronomic control 5 understanding the control of glacial interglacial climatic cycles on the nature and rate of contourite deposition