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

integrated stratigraphy of the early miocene lacustrine deposits of pag island sw croatia palaeovegetation and environmental changes in the dinaride lake system article i nfo an integrated stratigraphic study of a neogene lacustrine succession on the pag island croatia combining quantitative pollen analysis magnetostratigraphy cyclostratigraphy biostratigraphy and gamma ray measurements provides new insights into orbitally controlled variations in palaeo vegetation and depositional patterns in the dinaride lake system the quantitative palynological record shows a cyclical pattern of vegetation changes that closely corresponds to sedimentological patterns the intervals with a high abundance of thermophilous and xeric indicators suggesting a warm and dry climate generally coincide with intervals of frequent lignite deposition and shallow lake facies this suggests that both records are dominantly controlled by variations in past climatic conditions and lake level our data show two large scale warming and shallowing upward cycles which are interpreted to be forced by the 100 kyr eccentricity cycle of the earth s orbit magnetostratigraphic data of the examined section reveal a long 113 m reversed polarity interval followed by a 7 m thick interval of normal polarity at the top the inferred depositional rate of 0 3 mm yr combined with biostratigraphic constraints by mollusks suggests that the most logical correlation of the reversed interval is to chron c5cr this indicates that the pag succession was deposited between 17 1 and 16 7 ma and that it corresponds to the burdigalian stage of the early miocene and the regional karpatian stage of the central paratethys the high relative percentage of thermophilous pollen taxa engelhardia and taxodium type being the most prominent generally indicates a subtropical humid climate for the sw croatian part of the dinaride lake system the observed warming trend is possibly related to the onset of the miocene climatic optimum

numerical simulations of bedrock valley evolution by meandering rivers with variable bank material bedrock river valleys are fundamental components of many landscapes and their morphologies from slot canyons with incised meanders to wide valleys with strath terraces may record environmental history several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing but the uniqueness of valley evolution pathways and the long term stability of valley morphology under constant forcing are unknown and are not predicted in existing numerical models for vertically incising rivers because rivers often migrate more rapidly through alluvium than through bedrock we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology simulations using a numerical model of river meandering with vector based bank material tracking indicate that channel lateral erosion rate in sediment and bedrock vertical erosion rate and initial alluvial belt width explain first order differences in bedrock valley type that bedrock bound channels can evolve under steady forcing from alluvial states and that weak bedrock and low vertical incision rates favor wide shallow valleys while resistant bedrock and high vertical incision rates favor narrow deep valleys during vertical incision sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill the inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features including strath terraces and underfit valleys commonly ascribed to external drivers

la mc icpms and shrimp u pb dating of complex zircons from quaternary tephras from the french massif central magma residence time and geochemical implications abstract analyses of zircon grains from the queureuilh quaternary tephras pumice provide new information about their pre eruptive history u pb dating was performed in situ using two methods shrimp and la mc icpms equipped with a multi ion counting system both methods provided reliable 207pb 206pb and 206pb 238u ratios as well as u and th abundances required for u pb concordia intercept age determination after initial 230th disequilibrium correction the new la mc icpms method was validated by dating a reference zircon 61 308b and zircons from a phonolitic lava dated independently with the two techniques a time resolution of about 20 kyr for 1 ma zircon crystals was achieved for both methods the clear euhedral zircon population from queureuilh tephras is quite complex from several points of view 1 some grains are reddish or yellowish while others are colorless 2 the u and th composition changes by more than an order of magnitude and th u is generally high 1 2 3 there are three discrete ages recorded at 2 35 0 04 1 017 0 008 and 0 640 0 010 ma from the previously determined 40ar 39ar age at 0 571 0 060 ma duffell h 1999 contribution geochronologique a la stratigraphie volcanique du massif des monts dore par la methode 40ar 39ar d e a univ clermont ferrand 56 p the discontinuous zircon age populations the color of the grains and their composition we favor the following model as explanation the oldest less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent a primitive magma chamber perhaps deep in crustal level was formed at 1 0 ma the related magma previously characterized by high th u ratio 2 2 1 1 underwent rejuvenation during ascent to a new chamber at shallow depth and or during injection of more mafic magmas during this stage at 0 64 ma the colorless zircon grains of lower th u ratio 1 3 0 5 crystallized this last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40ar 39ar method however if the primitive magma is considered the magma residence time as a whole from this first stage reached 446 kyr in the light of the complex history of such magmas which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions the use of zr concentration in geochemical modeling of whole rock compositional data can be problematic

a sequence stratigraphic investigation of a miocene formation supported by coal seam quality parameters central paratethys n hungary abstract a stratigraphic investigation of the coal bearing miocene siliciclastic succession in structurally dissected basins of northern hungary was performed based on log correlations and is supported by coal petrology and sulphur data the succession represents the karpatian lower order global eustatic cycle of the paratethys and was dissected into six higher order sedimentary sequences sq 1 sq 6 the coal bearing strata in the nograd basin sq 1 sq3 represent a lowstand systems tract lst prior to the karpatian relative sea level rise the transgressive systems tract tst starts with the oncophora beds sq 4 accumulated above a regional unconformity and containing coal beds in the borsod basin the tst is superimposed by tectonically forced higher order lowstand systems tracts sq 5 sq 6 containing coal beds only in the relatively elevated east borsod basin to support the sequence stratigraphic model the average maceral and sulphur content of coal seams was also interpreted the low sulphur 0 47 content in sq 1 reflects the lowstand environment determined by continental derived freshwater input transitional sulphur contents 0 66 1 02 in the late lowstand sq 2 and sq 3 indicate occasional saline invasions related to relative sea level rises high 2 3 2 6 sulphur contents in the tst sq 4 were caused by regular seawater inundations into the lagoon while decreasing sulphur contents in sq 5 and sq 6 2 28 2 09 reflect the increasing rate of freshwater income in association with the stepwise basinward shifts of the shoreline and the lagoonal facies within the lower to middle miocene salgotarjan lignite formation the analysis of coal seams contributed to the interpretation of relative sea level changes and the sequence stratigraphic context facilitated both the interpretation of sulphur content in coals and the reliable correlation of coal seams

neogene mammalian faunal change in southern asia correlations with climatic tectonic and eustatic events the fluvial neogene siwalik formations of northern pakistan span long time intervals with only minor hiatuses and being highly fossiliferous are uniquely suited for studies of change in mammalian faunas magnetostratigraphic correlations of a critical stratigraphic section give dates for 45 middle and late miocene biostratigraphic events these mark either first appearances or extinctions in the mammal fauna and show that in the siwaliks there were major fauna turnovers at between 20 and 16 ma and at 9 5 and 7 4 ma two minor faunal events are dated at 13 2 and about 12 ma many species making their first appearance were immigrants from europe or africa and indicate when connections to those regions existed immigration and extinction were the dominant modes of faunal change in situ evolution was much less important the siwalik biostratigraphic record correlates closely to climatic oceanographic and tectonic events which probably controlled immigration into southern asia abiotic events were therefore important factors affecting evolution of the mammal communities

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

changing seas in the early middle miocene of central europe a mediterranean approach to paratethyan stratigraphy the miocene palaeogeographic evolution of the paratethys sea is still poorly constrained here we use modern mediterranean biochronology to provide an up to date overview of changing seas in central europe instead of a paratethys sea that waxed and waned with fluctuating global sea level we show that the evolution of different seas was mainly controlled by tectonic phases the early miocene ottnangian sea 18 ma was connected to the mediterranean via the rhone valley while the karpatian sea 16 5 ma was initiated by a tectonically induced marine transgression through the trans tethyan gateway in most central european basins the establishment of the badenian sea 15 2 ma triggered by subduction related processes in the pannonian and carpathian domain is significantly younger by 1 myr than usually estimated the updated palaeogeographic reconstructions provide a better understanding of the concepts of basin dynamics land sea distribution and palaeoenvironmental change in the miocene of central europe this article is protected by copyright all rights reserved

variability of rock erodibility in bedrock floored stream channels based on abrasion mill experiments we quantify variations in rock erodibility kr within channel cross sections using laboratory abrasion mill experiments on bedrock surfaces extracted from streams with sandstone bedrock in utah and basaltic bedrock in the hawaiian islands samples were taken from the thalweg and channel margins the latter at a height that is inundated annually for each sample a sequence of abrasion mill experiments was completed to quantify variations in erosion rate with erosion depth erosion rate data from these experiments shows two things first the erosion rate from channel margin samples is greater than for thalweg samples with the greatest difference observed for the rock surface that was exposed in the stream channel second erosion rate decreases with depth beneath the original rock surface by an order of magnitude in most cases the erosion rate becomes steady at depths of 1 3 mm for channel margin samples and 0 1 0 4 mm for thalweg samples because only rock properties and microtopography vary throughout the sequence of mill experiments these results suggest that kr of the bedrock surface exposed in stream channels is higher at the margins than near the channel center and that kr decreases over depths of 1 mm the simplest explanation for these patterns is that kr is enhanced at the bedrock surface and along the channel margins due to the effects of weathering on rock strength and surface roughness we hypothesize that a balance exists between weathering enhanced erodibility and episodic incision to allow channel margins to lower at rates similar to the thalweg

magneto biostratigraphy and paleoenvironments of the miocene freshwater sediments of the sarajevo zenica basin abstract the sarajevo zenica basin of bosnia herzegovina was part of the dinaride lake system a large network of miocene long lived freshwater basins in southeastern europe the basin contains a thick sedimentary succession of carbonates coals and mixed siliciclastic deposits that reflects the paleoclimatic and tectonic evolution of the region in this study we present novel integrated magneto bio stratigraphic and sedimentological data and reconstruct the paleoenvironmental evolution of the sarajevo zenica basin during its two main evolutionary phases thrusting and extension the basal oligo miocene freshwater paleoenvironments are characterized by alternating palustrine shallow lacustrine and distal fluvial phases the base level fluctuations are largely controlled by syn sedimentary pulses of tectonic loading during the final phase of thrusting in the internal dinarides the majority of this succession is considered early miocene in age which contrasts with previous oligocene age estimates the subsequent extensional phase initiated not later than 18 4 ma this coarsening upward sequence of lacustrine carbonates silts sands and conglomerates is correlated between 17 2 and 15 ma c5cr c5br by means of integrated bio magnetostratigraphy during this upper extensional phase subsidence and sediment influx was generally controlled by activity along the basin bounding normal fault overruling smaller scale climatic influences we conclude that the existence of the long lived sarajevo lake is coeval with other dinaric and southern pannonian lakes and overlaps in time with the miocene climatic optimum sedimentation in the sarajevo zenica basin terminated at 15 14 ma which concurs with both the end of the climatic optimum as well as the cessation of extension in the dinarides these results will help to better quantify the paleoclimatic changes in the dinaride lake system as well as the regional tectono sedimentary events such as potential migrations of deformation across the dinarides

u pb and 40ar 39ar dating of the miocene fossil track site at ipolytarnoc hungary and its implications abstract abundant early miocene mammal and bird tracks and a rich plant assemblage is preserved by the emplacement of an ignimbrite sheet of the gyulakeszi rhyolite tuff formation grtf near ipolytarnoc in northern hungary the tuff that overlies the track bearing sandstone yielded a single crystal zircon u pb total isochron age of 17 42 0 04 ma and a single crystal laser fusion plagioclase 40 ar 39 ar age of 17 02 0 14 ma uncertainties are quoted at the 2 level an additional 40 ar 39 ar age of 16 99 0 16 ma was obtained from the equivalent rhyolite tuff near nemti where the underlying terrestrial clay yielded early proboscidean remains assigned to the mn4 mammal zone the new high precision dates allow revision of the numeric age and correlation of the ipolytarnoc fossil site and the grtf previously based on an average k ar age of 19 6 1 4 ma the difference of 0 40 0 15 ma between the u pb and 40 ar 39 ar ages support the growing evidence for a systematic bias between the two isotopic systems due to the inaccurately known 40 k decay constant but likely also includes an undetermined pre eruptive residence time of zircon published biostratigraphic data from under and overlying marine strata establish correlation with the nn3 nannoplankton zone and together with the new radioisotopic ages suggest assignment of the fossils and the tuff to the ottnangian regional stage of the central paratethys a global correlation lends support to the recently suggested astronomical calibration of the early miocene time scale that revised the previous scales towards younger ages the 40 ar 39 ar age from nemti provides a reliable correlation of the mn4 mammal zone in central europe with the numeric time scale and places a minimum constraint on the age of the regional proboscidean datum the migration event of proboscideans from africa to europe through the emerging gomphotherium landbridge contrary to suggestions for a significantly earlier european proboscidean datum it appears that the originally suggested age of c 17 5 ma is realistic but it is significantly younger than the south asian proboscidean datum

the link between tectonics and sedimentation in back arc basins new genetic constraints from the analysis of the pannonian basin the architecture of sedimentary basins reflects the relationship between accommodation space and sediment supply their rates and localization being variable during basin evolution the mechanisms driving the interplay between tectonics and sedimentation in extensional back arc basins overlying rheological weak zones inherited from an earlier orogenic evolution are less understood a typical example is the pannonian back arc basin of central europe it is floored by continental lithosphere and was affected by large amounts of extension driven by the subduction rollback that took place in the carpathians and or dinarides a novel kinematic and seismic sequence stratigraphic interpretation calibrated by wells allows the quantification of the link between the formation of half grabens and coeval sedimentation in the great hungarian plain part of the basin while the lower order tectonic induced cycles characterize the main phases of extension in various subbasins the higher order cyclicity and associated unconformities define individual moments of fault re activation our novel interpretation of a temporal and spatial migration of extension during miocene times explains the contrasting present day strike of various subbasins as a result of their gradual clockwise rotation incorporating the observed asymmetry in particular the associated footwall exhumation infers that the amount of extension is much larger than previously thought the quantitative link between tectonics and sedimentation has allowed the definition of a novel model of sedimentation in asymmetric basins that can be ported to other natural scenarios of similarly hyperextended back arc basins observed elsewhere

a calibrated mammal scale for the neogene of western europe state of the art abstract a magnetobiostratigraphically calibrated mammal scale for the neogene of western europe is presented in this paper the mammal neogene mn units originally proposed by mein report on activity rcmns working groups 1975 have been re defined here on the basis of first appearances of selected small and large mammal taxa the chronology of the lower boundaries of each unit had been established mostly after the significant magnetobiostratigraphic framework developed in the last decade in a number of spanish basins ebro calatayud daroca valles penedes teruel fortuna cabriel and guadix baza in the case of the early and middle miocene particularly mn 1 mn 2 and mn 3 the authors have also taken into account the magnetobiostratigraphic framework developed in the north alpine foreland basin some alternative correlations of the magnetostratigraphic data from this last basin are proposed in order to achieve a higher degree of consistence with the data from the iberian basins a quite well established magnetostratigraphic calibration of the mn boundaries can be proposed for most of the neogene from middle miocene to late pliocene on the other hand the chronological boundaries of the early miocene mn units are still poorly constrained due to 1 scarcity of well studied continuous thick magnetostratigraphic sections 2 the difficulty in defining the boundaries of the mn zones for this time span due to the relative homogeneity and persistence of the fossil rodent faunas and the absence of significant large mammal dispersal events some of the troubles which arise with the application of the mn units strengthen the need to take into account the palaeobiogeographical meaning of these units and their real suitability to date and correlate through extensive geographic areas

the mid langhian flooding in the eastern central paratethys integrated stratigraphic data from the transylvanian basin and se carpathian foredeep the mid langhian badenian flooding fully reconnected the semi isolated central paratethys realm with the mediterranean and thereby drastically changed the middle miocene paleogeography of central europe due to the scattered stratigraphic record and scarcity of independent age constraints in some areas the precise age and underlying mechanism are still debated we present integrated chronostratigraphic data from five sections in the eastern part of the system to reconstruct the flooding event distal from the strait to the mediterranean by applying modern mediterranean biochronology planktonic foraminifera and calcareous nannofossils supplemented by an 40ar 39ar age on a tuff we demonstrate that the widespread open marine settings in the nw transylvanian basin were definitely installed after 14 9 ma mmi4d biozone and in most areas before 14 4 ma in the marginal study area in the se carpathian foredeep fully marine conditions likely set in slightly later 14 6 14 4 ma there short lived marine incursions into the brackish environment occurred since the latest burdigalian pre flooding phase the new ages overlap with the flooding in the majority of the central paratethys 14 9 14 4 ma and with marine overflow into the black sea 14 85 ma we suggest that the transgression was driven by subsidence of the pannonian basin by creating accommodation space and diminishing barriers between sub basins but was likely enhanced by a global sea level rise finally we speculate that the scarcity of all calcareous material in the se carpathian foredeep before the mid langhian flooding might be related to pulses of nutrient rich brackish and low ph water from the neighboring black sea basin

natural variations in calcite dissolution rates in streams controls implications and open questions models of bedrock channel evolution typically assume that chemical erosion is negligible in comparison to mechanical erosion while this assumption is reasonable for channels in silicate rocks it is questionable within highly soluble strata such as carbonates the magnitude and variability of calcite dissolution rates in streams has remained as a critical unknown for models of bedrock incision and karst conduit formation here we use u s geological survey data to estimate calcite dissolution rates from 77 different streams located in a wide range of settings the calculated rates are commonly on the order of 1 mmyr 1 which is 1 to 2 orders of magnitude larger than previous estimates we also find that pco2 is the strongest control on at a site variability though some sites also display dilution controlled variability typically dissolution rates vary within a relatively narrow range which has important implications for the relative importance of chemical and mechanical erosion

eocene to miocene magnetostratigraphy biostratigraphy and chemostratigraphy at odp site 1090 sub antarctic south atlantic at ocean drilling program odp site 1090 lat 42854 89s long 8854 09e located in a water depth of 3702 m on the agulhas ridge in the sub antarctic south atlantic 300 m of middle eocene to middle miocene sediments were recovered with the advanced piston corer apc and the extended core barrel xcb u channel samples from the 70 230 meters composite depth mcd interval provide a magnetic polarity stratigraphy that is extended to 380 mcd by shipboard whole core and discrete sample data the magnetostratigraphy can be interpreted by the fit of the polarity zone pattern to the geomagnetic polarity time scale gpts augmented by isotope data and bioevents with documented correlation to the gpts three normal polarity subchrons c5dr 1n c7ar 1n and c13r 1n not included in the standard gpts are recorded at site 1090 the base of the sampled section is correlated to c19n middle eocene although the interpretation is unclear beyond c17r the top of the sampled section is correlated to c5cn late early miocene although in the uppermost 10 m

tertiary evolution of the intra carpathian area a model abstract the outer carpathian flysch nappes encircle an intra carpathian domain which can be divided into two megatectonic units north pannonian and tisza mostly on the basis of contrasting mesozoic and palaeogene facies development we see two major kinematic problems to be solved 1 1 the present distribution of the mesozoic and palaeogene facies is mosaic like and some belts form exotic bodies within realms of austroalpine affinity 2 2 late eocene palinspastic reconstruction of the outer carpathian flysch nappes suggest that the entire intra carpathian area must have been located several hundreds of kilometres to the south and to the west of its present position neogene extension can account for shortening in the external carpathian nappes but is unable to explain mesozoic facies anomalies and offsets of palaeogene formations we suggest that evolution of the intra carpathian area involved first late palaeogene early miocene juxtaposition of the north pannonian and tisza megatectonic units accompanied by the closure of the external carpathian flysch troughs thereafter extension of this amalgamated unit occurred which was compensated by thrusting of flysch nappes onto the european foreland and formation of molasse foredeeps eastward escape of the north pannonian unit from the alpine collisional belt involved left lateral shear along the pieniny klippen belt and right lateral shear along the mid hungarian zone parts of the late palaeogene basin and an early miocene volcanic edifice were dissected offset and elongated by several 100 kms the driving mechanism of the eastward escape of the intra carpathian area can be related to the collision of apulia and europe and the subduction of the external carpathian crust under the pannonian units the escape ceased gradually in the early miocene when oblique collision between the north pannonian unit and european continent occurred neogene extension of the pannonian region was an areal deformation the extension at locally variable rate resulted in the break up of the heterogenous floor of the neogene basin the driving mechanism of basin extension and contemporaneous compressional deformation of the external carpathians is thought to be related to ongoing subduction involving the marginal part of the attenuated european continental crust tectonic activity in the carpathians and basin subsidence and volcanism shifted in time and in unison from the west toward the east southeast

astronomical calibration age for the oligocene miocene boundary the stratotype section for the base of the miocene is at a reversed below to normal above magnetic transition that is claimed to represent magnetic chron c6cn 2n o deep sea drilling project dsdp site 522 is the only location we are aware of that unambiguously records the three normal events of c6cn we have quantitatively determined the range of the short lived nannofossil sphenolithus delphix and the lower limit of s disbelemnos in dsdp holes 522 and 522a in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site astronomical tuning of ocean drilling program odp sites 926 928 and 929 shows that s disbelemnos appears at 22 67 ma and that the entire range of s delphix is from about 22 98 ma to 23 24 ma using these ages linear interpolation in dsdp site 522 suggests that the age of c6cn 2n o and of the oligocene miocene boundary is 22 92 0 04 ma our value conservatively expressed as 22 9 0 1 ma is 0 9 m y younger than the currently accepted age of the oligocene miocene boundary and of c6cn 2n o which was assigned an age of 23 8 ma based on an estimate of 23 8 1 ma for the oligocene miocene boundary the bulk sediment carbon isotope data from dsdp site 522 is correlated to the record from benthic foraminifera at odp site 929 to refine the calibration of magnetic reversals from c6cn 1n o to c7n 2n o at dsdp site 522 on the astronomical time scale

early miocene basin evolution in northern hungary tectonics and eustasy abstract it is presumed that the oligocene early miocene basin evolution in northern hungary was primarily driven by compressional tectonics producing a major second order transgressive regressive facies cycle the early miocene basin evolution is best understood in terms of molasse sedimentation in an overfilled flexural basin during this time the gradual cessation of thrusting in the adjacent west carpathian thrust fold belt resulted in its uplift and subaerial exposure significant amounts of sediment were delivered to the flexural basin filling it up to sea level during the late stage uplift of the flexural basin a shallow marine depositional environment developed as a result of isostatic rebound and signals of third order eustatic sea level changes can be revealed sedimentological studies of outcrops of the lower miocene succession proved marked changes in facies along the gently dipping distal southeastern flank of the basin a sudden inception of shallow marine coarse elastics on top of siltstones deposited in significantly deeper water may have been the result of a third order eustatic sea level fall shortly before the burdigalian at the boundary of the nn1 nn2 nannofossil zones during the resulting lowstand various tide dominated facies aggraded which were subsequently flooded by the burdigalian sea level rise the highstand is represented by upwards shallowing progradational units due to accelerating sedimentation and or tectonic uplift the typically elongated and narrow flexural basin characterized by small water depth had dimensions which were particularly suitable for the amplification of tidal motions this resulted in the deposition of tide influenced sandy sediments in contrast to examples where the evolution of a strong tidal influence is related to transgressions here such conditions developed following a drop of sea level

paleogeographic evolution of the southern pannonian basin 40ar 39ar age constraints on the miocene continental series of northern croatia the pannonian basin originating during the early miocene is a large extensional basin incorporated between alpine carpathian and dinaride fold thrust belts back arc extensional tectonics triggered deposition of up to 500 m thick continental fluvio lacustrine deposits distributed in numerous sub basins of the southern pannonian basin extensive andesitic and dacitic volcanism accompanied the syn rift deposition and caused a number of pyroclastic intercalations here we analyze two volcanic ash layers located at the base and top of the continental series the lowermost ash from mt kalnik yielded an 40ar 39ar age of 18 07 0 07 ma this indicates that the marine continental transition in the slovenia zagorje basin coinciding with the onset of rifting tectonics in the southern pannonian basin occurs roughly at the eggenburgian ottnangian boundary of the regional paratethys time scale this age proves the synchronicity of initial rifting in the southern pannonian basin with the beginning of sedimentation in the dinaride lake system beside geodynamic evolution the two regions also share a biotic evolutionary history both belong to the same ecoregion which we designate here as the illyrian bioprovince the youngest volcanic ash level is sampled at the glina and karlovac sub depressions and both sites yield the same 40ar 39ar age of 15 91 0 06 and 16 03 0 06 ma respectively this indicates that lacustrine sedimentation in the southern pannonian basin continued at least until the earliest badenian the present results provide not only important bench marks on duration of initial synrift in the pannonian basin system but also deliver substantial backbone data for paleogeographic reconstructions in central and southeastern europe around the early middle miocene transition

paleogene retroarc flexural basin beneath the neogene pannonian basin a geodynamic model abstract the neogene middle miocene recent pannonian basin is superimposed on an earlier paleogene middle eocene early miocene basin complex as a result of back arc extension although the stratigraphy of the latter is very well known due to raw material exploration its formation and structural evolution is much less understood a transtensional origin was generally proposed for the paleogene basin of hungary assuming an origin analogous to that of the overlying neogene pannonian basin we suggest an alternative geodynamic scenario for the development of the paleogene basin complex as we think that the observations are better understood in terms of a retroarc foredeep basin model according to this a flexural basin developed south of the backthrust inner west carpathian units the thrust load resulted in a generally deep underfilled flysch basin during middle eocene early oligocene times in the late oligocene early miocene predominantly shallow marine and continental deposition occurred indicating the gradual cessation of thrusting in the adjacent thrust fold belt and the beginning of molasse sedimentation throughout the whole paleogene a shift can be observed of the bathyal dominantly siliciclastic in the axis of the foredeep and neritic partly carbonate in the foreland flank of the foredeep facies belts this migration of the middle eocene early miocene depocenters of the hpb hungarian paleogene basin to the east northeast in present day coordinates is the result of two processes 1 1 the advancement of backthrusting to the southeast 2 2 along strike shift of thrusting to the northeast while a thin skinned thrust front can be reconstructed for the late eocene buda anticline the diosjeno hurbanovo line is interpreted as the result of a major thick skinned thrusting event at the base of the np24 zone 31 ma the subsidence pattern the distinct depositional environments and the structural observations can be more easily integrated into a flexural basin model than into a wrench basin model based on the flexural basin model of this work the early tertiary western carpathians formed a compressional arc while this arc probably developed above a b subduction zone during the late eocene with a corresponding trench magura a forearc basin central carpathian or podhale and a retroarc flexural basin hungarian paleogene from the early oligocene to the early miocene it was characterized by a subduction with a corresponding peripheral foredeep dukla and silesian and a retroarc flexural basin hungarian paleogene

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

magnetic polarity zonation within the el teniente copper molybdenum porphyry deposit central chile el teniente porphyry copper deposit the world s greatest intrusion related cu mo ore body is hosted within basaltic andesitic volcanic and gabbroic rocks mafic complex this ore body is strongly affected by multiple events of alteration mineralization with pervasive potassic and chloritic alteration and coetaneous with associated copper mineralization we present paleomagnetic results obtained from oriented samples at four locations within the mine and from two drill cores 200 and 400 m long respectively samples are representative of all the main hydrothermally altered rock units with emphasis on the mafic host rock and dacitic teniente dacite porphyry and dioritic porphyry intrusions magnetic experiments hysteresis loop isothermal remanent magnetization irm k t curves thermal and alternating field demagnetization show the presence of prevailing magnetite microscope and sem observations show two families of magnetite a large multidomain magnetite grains associated with biotite and chlorite of various different hydrothermal alteration events and b abundant small to medium grain size magnetite 0 01 and up to 0 2 si with nrm in the range 0 1 2 am 1 most mafic complex rock samples have univectorial magnetizations during alternating field or thermal demagnetization within the mine the magnetic polarity is spatially distributed in the northern part of the deposit the teniente dacite porphyry the associated hydrothermal breccias and the hosting mafic complex record a reverse polarity magnetization also observed in the el teniente sub 6 mine sector immediately to the east and southeast in the eastern part of the deposit a normal polarity is observed for samples of the mafic complex from the two long drill cores there is no evidence for superimposed magnetizations of opposite polarities in samples of the mafic complex anhysteretic remanent magnetization arm in a dc field of 40 t and nrm have similar magnitude and comparable behavior upon alternating field demagnetization the well defined strong remanent magnetizations associated with high unblocking temperatures 500 c indicate an acquisition of remanent magnetization during mineralization by circulating high temperature fluids related with ore deposition paleomagnetic results and the recorded polarity zonation suggest multiple mineralization events occurred at el teniente each one with its own evolution stages superimposed within the district these results indicate that a simplified broad four stage model for el teniente as presented and overly employed by many authors divided in 1 late magmatic 2 main hydrothermal 3 late hydrothermal and 4 posthumous stage does not recognize various short lived single mineralization events some superimposed and some distinctly separated in time and space there is no paleomagnetic evidence for post mineralization deformation

constraints on miocene oceanography and climate in the western and central paratethys o sr and nd isotope compositions of marine fish and mammal remains abstract the paratethys evolved as a marginal sea during the alpine himalayan orogeny in the oligo miocene sediments from the northern alpine molasse basin the vienna and the pannonian basins located in the western and central part of the paratethys thus provide unique information on regional changes in climate and oceanography during a period of active alpine uplift oxygen isotope compositions of well preserved phosphatic fossils recovered from the sediments support deposition under sub tropical to warm temperate climate with water temperatures of 14 to 28 c for the miocene 18o values of fossil shark teeth are similar to those reported for other miocene marine sections and using the best available estimates of their biostratigraphic age show a variation until the end of the badenian similar to that reported for composite global record the 87sr 86sr isotope ratios of the fossils follow the global miocene seawater trend albeit with a much larger scatter the deviations of 87sr 86sr in the samples from the well constrained seawater curve are interpreted as due to local input of terrestrially derived sr contribution of local sources is also reflected in the end values consistent with input from ancient crystalline rocks e g bohemian massif and or mesozoic sediments with end 7 excluding samples showing local influence on the water column an average end value of 7 9 0 5 may be inferred for the miocene paratethys this value is indistinguishable from the end value of the contemporaneous indian ocean supporting a dominant role of this ocean in the western and central paratethys

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

in situ soil specific nonlinear properties back calculated from vertical array records during 1995 kobe earthquake in situ nonlinear soil properties are investigated based on vertical array records obtained during the 1995 hyogoken nambu earthquake kobe earthquake at soft soil sites near the earthquake fault zone spectrum ratios between ground surface and deeper levels are calculated for the main shock and associated small shocks and s wave velocities and damping ratios in surface soil layers to best reproduce them are back calculated by means of an inversion analysis extended bayesian method assuming one dimensional vertical horizontal shear sh wave propagation obvious differences in s wave velocities and damping ratios are found between the main shock and the small shocks clear strain dependent modulus degradations which can be differentiated for different soil types are recognized the degradations are essentially consistent with some of laboratory test results to date for each soil type at least for g g0 0 5 or larger though for gravelly soils back calculated values tend to show milder degradations tha

chronology and integrated stratigraphy of the miocene sinj basin dinaride lake system croatia article i nfo ar 39 ar dating mollusk evolution conohyus in the miocene the intra montane basins of the dinaric mountain chain harbored a series of long lived lakes constituting the so called dinaride lake system the thick lacustrine sedimentary records of these lakes provide an excellent opportunity to study evolution and radiation of mollusks in an isolated environment the 500 m thick infill that accumulated in the sinj basin is one of the key records because of its excellent mollusk preservation recent studies on the depositional history pollen assemblages and large mammals have enhanced the understanding not only of lake sinj but also of the regional climatic developments and faunal migratory patterns a reliable chronology of the development of lake sinj which is crucial for global correlation of its endemic realm was still lacking in this paper we present a detailed time frame for the miocene sinj basin based on palaeomagnetic and 40 ar 39 ar data we conclude that deposition took place between 18 0 to 15 0 ma a time span that correlates with the upper burdigalian and lower langhian mediterranean stages and ottnangian karpatian and lowermost badenian paratethys stages furthermore we determined the timing of several events in mollusk evolution important for correlation across the dinarides an age of 15 0 ma is attributed to the large mammals conohyus and gomphotherium preserved in the upper part of the basin stratigraphy 2010 elsevier b v all rights reserved

paleomagnetic and geochronologic constraints on the geodynamic evolution of the central dinarides the geodynamic evolution of the dinaride mountains of southeastern europe is relatively poorly understood especially in comparison with the neighboring alps and carpathians here we construct a new chronostratigraphy for the post orogenic intra montane basins of the central dinarides based on paleomagnetic and 40ar 39ar age data a first phase of basin formation occurred in the late oligocene a second phase of basin formation took place between 18 and 13 ma concurrent with profound extension in the neighboring pannonian basin our paleomagnetic results further indicate that the dinarides have not experienced any significant tectonic rotation since the late oligocene this implies that the dinarides were decoupled from the adjacent adria and the tisza dacia mega units that both underwent major rotation during the miocene the dinaride orogen must consequently have accommodated significant shortening this is corroborated by our ams data that indicate post middle miocene shortening in the frontal zone wrenching in the central part of the orogen and compression in the hinterland a review of paleomagnetic data from the adria plate which plays a major role in the evolution of the dinarides as well as the alps constrains rotation since the early cretaceous to 48 10 counterclockwise and indicates 20 of this rotation took place since the miocene it also shows that adria behaved as an independent plate from the late jurassic to the eocene from the eocene onwards coupling between adria and africa was stronger than between adria and europe adria continued to behave as an independent plate the amount of rotation within the adria dinarides collision zone increases with age and proximity of the sampled sediments to undeformed adria these results significantly improve our insight in the post orogenic evolution of the dinarides and resolve an apparent controversy between structural geological and paleomagnetic rotation estimates for the dinarides as well as adria

comparison of electronic and mechanical schmidt hammers in the context of exposure age dating are q and r values interconvertible application of schmidt hammer exposure age dating shd to landforms has substantially increased in recent years the original mechanical schmidt hammer records r rebound values although the newly introduced electronic schmidt hammer silverschmidt facilitates greatly improved data processing it measures surface hardness differently recording q velocity values that are not a priori interconvertible with r values this study is the first to compare the performance of both instruments in the context of field based exposure age dating with a particular focus on the interconvertibility of r values and q values the study was conducted on glacially polished pyroxene granulite gneiss jotunheimen southern norway results indicate that mean q values are consistently 8 10 units higher than mean r values over the range of values normally encountered in the application of shd to glacial and periglacial landforms a convenient conversion factor of 10 units may therefore be appropriate for all but the softest rock types close to the technical resolution of the instruments the electronic schmidt hammer should therefore be regarded as a useful complement and potential replacement for the mechanical schmidt hammer conversion of published r values data to q values requires however careful control and documentation of instrument calibration copyright 2014 john wiley sons ltd