accelerator mass spectrometer ages for the younger dryas event in atlantic canada abstract in atlantic canada bulk sediment dates for the onset of a late glacial cooling widely regarded as the younger dryas event are highly variable ranging from 14 300 270 to 10 800 100 yr b p we present the first accelerator mass spectrometer ams 14c dates from atlantic canada at or close to the boundaries of this event from six sites in new brunswick and nova scotia the mean of five dates places the onset of this cooling at ca 10 770 yr b p the inclusion of a sixth perhaps anomalously old date changes the mean to 10 880 yr b p the termination averages three dates ca 10 000 yr b p these dates place the timing of the younger dryas event in atlantic canada closer in line with the traditional chronozone boundaries of 11 000 and 10 000 yr b p in northwest europe

pre alpine history of the pennine zone in the tauern window austria u pb and rb sr geochronology zircon ages from major lithologies of the zentralgneis suggest that much of the variscan magmatism in the tauern window is older than previously suggested in the southeast tauern window a tonalite has been dated at 314 7 m y and a granodioritic biotite augen gneiss at 313 10 m y two granodiorites from the granatspitzkern yielded zircon data consistent with a similar age these zircon data require re interpretation of some previously published rb sr whole rock ages and raise the possibility that alpine metamorphism caused more widespread disturbance of rb sr whole rocks than commonly supposed

palinspastic reconstruction and topographic evolution of the eastern alps during late tertiary tectonic extrusion this paper presents a new palinspastic restoration of the eastern alps for neogene time and an attempt to reconstruct the neogene palaeogeology palaeotopography and palaeohydrography in connection with the structural evolution the eastern alps underwent radical horizontal displacement during the neogene due to large strike slip systems and formation of structural windows our palinspastic reconstruction considers a the rearrangement of tectonic units dismembered during tectonic extrusion b the tectonic denudation driven by displacement of the crystalline blocks c geochronological arguments and d the sedimentary record of the syn extrusion basins the rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre miocene unstretched pattern of the eastern alps reduced to 65 of its present day e w elongation using this structural frame and considering the sedimentary record a set of palaeogeologic and palaeotopographic sketch maps with the palaeo river systems is presented for three time slices pre syn and post extension situation in late oligocene and early miocene times the western eastern alps were already mountainous whereas the eastern part of the orogen formed lowlands or hilly areas enhanced block movement in the course of the extrusion process around the early middle miocene boundary led to the formation of intramontane sedimentary basins and a fault induced reorientation of the drainage pattern which forms the basis of the modern river system in the area east of the tauern window this region where pre miocene land surfaces are preserved probably became a mountainous area not before late miocene time and never reached the elevations of the areas further west

younger dryas interval and outflow from the laurentide ice sheet a boxmodel of the great lakes is used to estimate meltwater flow into the north atlantic between 8000 and 14 000 calendar years b p controls on the model include the oxygen isotopic composition of meltwaters and lake waters as measured in the shells of ostracodes outflow rates are highest when oxygen isotopic values of the lake waters are most negative denoting a maximum glacial meltwater component flow rates reach maximum values before the onset of the younger dryas and after it ends these maxima appear to be correlative with the major meltwater pulses mwp 1a and 1b although the resumption of north atlantic deep water formation may be tied to the reduction in ice sheet melting neither the onset nor the end of the younger dryas as recorded in the greenland ice sheet project gisp2 records appear tied to maxima in meltwater outflow from the laurentide ice sheet

u pb isotopic dating of fluid infiltration and metasomatism during dalradian regional metamorphism in connemara western ireland a metasomatic diopside rock occurs at the top of the dolomitic connemara marble formation of western ireland and contains titanite and k feldspar in addition to around 90 diopside xmg 0 90 0 97 u pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium rich and isotopically concordant with the result that a precise u pb age of 478 2 5 ma can be determined the age is identical within error to a less precise rb sr age of diopside k feldspar of 483 6 ma petrological evidence indicates that the assemblage crystallized at c 620 c close to or below the closure temperature of titanite the age thus provides a precise estimate of the time of metamorphism this age is 11 3 ma younger than the 490 ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age this difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism the success of this approach is demonstrated by the results from connemara

seismic stratigraphy of lake huron georgian bay and postglacial lake level history five seismic reflectors in the basinal sedimentary section have been identified as representing erosional surfaces in shoaler regions of the lake basins and can be traced throughout most of the study area the reflectors representing these erosional surfaces and their conformable equivalents in the deep lake basins are caused by intervals of coarser grain size in the sediments and are thought to represent lowering of lake levels prior to and following the main algonquin highstand about 10 2 11 0 ka these reflectors are used to define seismic sequences in the postglacial sedimentary fill of the basins two additional erosional surfaces which closely follow the initial fall of the main algonquin lake level are most clearly seen in thicker sections found in the northern part of the basins the two oldest pre main algonquin sequences appear to onlap highs and fill in lows within the basins whereas the younger sequences associated with the main algonquin through mattawa times tend to drape the

the lake superior varve stratigraphy and implications for eastern lake agassiz outflow from 10 700 to 8900 cal ybp 9 5 8 0 14c ka abstract glaciolacustrine rhythmites within sediment cores from lake superior record the regional recession of the laurentide ice sheet lis from 10 700 to 8900 cal ybp ca 9 5 8 0 14 c ka lis retreat from superior opened eastern lake agassiz outlets so that the rhythmites reflect the combined impacts of sediment laden meltwater and lake agassiz discharge multiple rhythmite stratigraphies a time series analysis of the thickness measurements and high resolution inorganic carbonate data demonstrate that this is an annual record varved the varve thickness records primarily document regional ice margin dynamics correlative thick varve sequences at 9100 cal ybp 8 1 14 c ka and 10 400 10 200 cal ybp 9 2 9 0 14 c ka record two periods of enhanced glaciofluvial discharge most likely moraine formation the nakina and nipigon general varve cessation is associated with the circumvention of lake agassiz and glacial meltwater into lake ojibway at 9040 cal ybp 8 1 14 c ka although adjacent to the inlets from lake nipigon rhythmic sedimentation persisted for 200 years positively identifying lake agassiz catastrophic discharge events remains speculative but seems feasible following retreat of marquette ice that had re advanced to fill the basin the initial influx of lake agassiz water is expected at around 10 600 cal ybp 9 4 14 c ka but at this time most of northeastern lake superior was covered by ice three sets of thick thin varves in western lake superior perhaps record influxes of lake agassiz at around 10 630 10 600 and 10 570 cal ybp 9 4 14 c ka varve formation in superior coincides with high lake levels in lake huron suggesting that high lake levels in huron correspond to periods of high agassiz and or meltwater flow into lake superior

el nino like climate teleconnections in new england during the late pleistocene a glacial varve chronology from new england spanning the 4000 year period from 17 500 to 13 500 calendar years before the present was analyzed for evidence of climate variability during the late pleistocene the chronology shows a distinct interannual 3 to 5 years band of enhanced variability suggestive of el nino southern oscillation enso teleconnections into north america during the late pleistocene when the laurentide ice sheet was near its maximum extent and climatic boundary conditions were different than those of today this interannual variability largely disappears by the young end of the 4000 year chronology with only the highest frequency components roughly 3 year period persisting this record provides evidence of enso like climate variability during near peak glacial conditions

correlation of alaskan varve thickness with climatic parameters and use in paleoclimatic reconstruction the thickness of varves in the sediments of skilak lake alaska are correlated with the mean annual temperature r 0 574 inversely correlated with the mean annual cumulative snowfall r 0 794 and not correlated with the mean annual precipitation r 0 202 of the southern alaska climatological division for the years 1907 1934 a d varve thickness in skilak lake is sensitive to annual temperature and snowfall because skilak glacier the dominant source of sediment for skilak lake is sensitive to these climatic parameters trends of varve thickness are well correlated with trends of mean annual cumulative snowfall r 0 902 of the southern alaska climatological division and with trends of mean annual temperature of the southern r 0 831 and northern r 0 786 alaska climatological divisions trends of varve thickness also correlate with trends of annual temperature in seattle and north head washington r 0 632 and 0 850 respectively comparisons of trends of varve thickness with trends of annual temperature in california oregon and washington suggest no widespread regional correlation trends of annual snowfall in the southern alaska climatological division and trends of annual temperature in the southern and northern alaska climatological divisions are reconstructed for the years 1700 1906 a d climatic reconstructions on the basis of varve thickness in skilak lake utilize equations derived from the regression of series of smoothed climatological data on series of smoothed varve thickness reconstruction of trends of mean annual cunulative snowfall in the southern alaska climatological division suggests that snowfall during the 1700s and 1800s was much greater than that during the early and mid 1900s the periods 1770 1790 and 1890 1906 show marked decreases in the mean annual snowfall reconstructed trends of the annual temperature of the northern and southern alaska climatological divisions suggest that annual temperatures during the 1700s and 1800s were lower than those of the early and mid 1900s two periods of relatively high annual temperatures coincide with the periods of low annual snowfall thus determined

el nino southern oscillation events recorded in the stratigraphy of the tropical quelccaya ice cap peru snow accumulation measured during 1982 1983 on the quelccaya ice cap peru was 70 percent of the average from 1975 through 1983 inspection of 19 years 1964 through 1983 of accumulation measured near the summit of quelccaya reveals a substantial decrease 30 percent in association with the last five el nino southern oscillation enso occurrences in the equatorial pacific the enso phenomenon is now recognized as a global event arising from large scale interactions between the ocean and the atmosphere understanding this extreme event with the goal of prediction requires a record of past occurrences the quelccaya ice cap which contains 1500 years of annually accumulated ice layers may provide a long and detailed record of the most extreme enso events

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

timing and duration of north american glacial lake discharges and the younger dryas climate reversal abstract radiocarbon dated sediment cores from the champlain valley northeastern usa contain stratigraphic and micropaleontologic evidence for multiple high magnitude freshwater discharges from north american proglacial lakes to the north atlantic of particular interest are two large closely spaced outflows that entered the north atlantic ocean via the st lawrence estuary about 13 200 12 900 cal yr bp near the beginning of the younger dryas cold event we estimate from varve chronology sedimentation rates and proglacial lake volumes that the duration of the first outflow was less than 1 yr and its discharge was approximately 0 1 sv 1 sverdrup 106 m3 s 1 the second outflow lasted about a century with a sustained discharge sufficient to keep the champlain sea relatively fresh for its duration according to climate models both outflows may have had sufficient discharge duration and timing to affect meridional ocean circulation and climate in this report we compare the proglacial lake discharge record in the champlain and st lawrence valleys to paleoclimate records from greenland ice cores and cariaco basin and discuss the two step nature of the inception of the younger dryas

eastern pacific sea surface temperature since 1600 a d the δ18o record of climate variability in galapagos corals we measured stable oxygen isotope ratios and skeletal growth rates in the massive corals pavona clavus and p gigantea from the west coast of isabela island galapagos to assess interannual to decadal climate variability in the eastern pacific comparisons of instrumental data sets show that sea surface temperatures sst in the galapagos region are representative of a broad portion of the eastern equatorial pacific the site is especially well suited for long term studies of the el nino southern oscillation enso phenomenon as it lies within the eastern pacific center of action for thermal anomalies associated with enso the p gigantea isotope record is nearly monthly in resolution spans the period 1961 1982 and shows strong correlation with a galapagos instrumental sst record r 0 90 for annual averages cross spectral analysis shows that sst can explain greater than 80 of the variance in 18o at both the annual cycle and within the high frequency portion of the enso band 3 5 years the p clavus record is annual in resolution extends from 1587 to 1953 a d and was obtained from a 10 m diameter colony preserved within the urvina bay uplift because seawater 18o variations in the region are very small we interpret the urvina bay coral 18o record in terms of annual average sst the isotopic record appears to be a very good but not perfect indicator of enso events and shows good correspondence with the historical enso reconstruction of quinn et al 1987 a number of low 18o excursions that we observe during the 17th and 18th centuries very likely represent enso events that are missing from the historical tabulations most interannual 18o variations between 1607 and 1953 a d represent annual average temperature excursions of 1 to 2 5 c during the little ice age the annual 18o series correlates well with many north american tree ring records and shows low temperatures during the early 1600s and early 1800s and relatively warmer conditions during the 1700s unlike most northern hemisphere tree ring and instrumental records we see no evidence at this site for warming between 1880 and 1940 but rather observe a slight cooling 28 of the total variance the main enso mode is centered at 4 6 years and accounts for 12 of the total variance additional significant oscillations occur at periods of 3 3 6 8 11 17 22 and 34 years both annual growth rate and 18o show variance at periods equivalent to the solar and solar magnetic periods e g 11 and 22 years respectively in addition the amplitude of the 11 year 18o cycle generally varies with the amplitude of the solar cycle supporting previous suggestions that the solar cycle may modulate interannual to decadal climate variability in the tropics the dominant oscillatory modes both within the enso and interdecadal frequency bands shift to shorter periods from the early to middle 1700s and again from the middle to late 1800s this may reflect major reorganizations within the tropical ocean atmosphere system and suggests that tropical pacific climate variability is linked across timescales ranging from years to decades

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

sediment resuspension and transport in lake michigan during the unstratified period instrumented moorings were deployed during the winter of 1994 95 at three depths 28 58 and 101 m in southern lake michigan storms during the observation period were not unusually severe so the processes observed are typical of those that occur during an average winter time series observations of water temperature beam attenuation coefficient a measure of water transparency and current velocity show that local resuspension of bottom sediment occurred frequently after the breakdown of the thermocline resuspension was most frequent close to the shore but was also observed at the 58 m station local resuspension did not occur at the 101 m station but advection to the site of material resuspended at shallower sites was observed these observations do not support the hypothesis proposed by previous investigators that local resuspension at depths of 100 m or greater occurs during the unstratified period it is more likely that fine grained material resuspended by storm action in intermediate water depths 30 60 m is transported into the deeper parts of the lake by the general lake circulation

lateral extrusion in the eastern alps revisited refining the model by thermochronological sedimentary and seismic data 1 this study comprises a reassessment of the classical model of lateral extrusion in the eastern alps by using recently published geochronological data sedimentary ages from intramontane basins ages and distribution of magmatic rocks and information from seismic profiles extrusion related faulting continuously propagated from the western toward the central eastern part of the eastern alps during oligocene to middle miocene times this is confined by oblique convergence between the adriatic and european plates during middle miocene times extrusion became not only lateral in terms of parallel to the trend of the eastern alps but was characterized by a displacement vector at a high angle to the strike of the orogen this resulted in the exhumation of the schladming and pohorje blocks that were exhumed within extensional bridges at the northern and southern terminations of the pols lavanttal fault system respectively from middle miocene to recent times extrusion was controlled by overall extension between the dinaric and carpathian subduction zones the influence of north directed compression triggered by the northward moving adriatic plate diminished and the influence of the retreating carpathian subduction zone increased this gave rise to miocene volcanism that is exclusively found east of the dinaric subduction zone we therefore consider that lateral extrusion in the eastern alps can be subdivided into distinct tectonic phases with less pronounced eastward extension related displacement between late oligocene and middle miocene times as soon as the eastern alps passed the dinaric subduction zone the entire domain became highly extensive

spatial correlations of interdecadal variation in global surface temperatures we have analyzed spatial correlation patterns of interdecadal global surface temperature variability from an empirical perspective using multi taper coherence estimates from 140 yr records we find that correlations between hemispheres are significant at 95 confidence for non randomness for most of the frequency band 0 06 f 0 24 cyc yr coherence estimates of pairs of 100 yr grid point temperature data series near 5 yr period reveal teleconnection patterns consistent with known patterns of enso variability significant correlated variability is observed near 15 year period with the dominant tele connection pattern largely confined to the northern hemisphere peak to peak t 0 5 with simultaneous warming and cooling of discrete patches on the earth s surface a global average of this pattern would largely cancel

modes of orogen parallel stretching and extensional exhumation in response to microplate indentation and roll back subduction tauern window eastern alps the tauern window exposes a paleogene nappe stack consisting of highly metamorphosed oceanic alpine tethys and continental distal european margin thrust sheets in the eastern part of this window this nappe stack eastern tauern subdome etd is bounded by a neogene system of shear the katschberg shear zone system kszs that accommodated orogen parallel stretching orogen normal shortening and exhumation with respect to the structurally overlying austroalpine units adriatic margin the kszs comprises a 5 km thick belt of retrograde mylonite the central segment of which is a southeast dipping low angle extensional shear zone with a brittle overprint katschberg normal fault knf at the northern and southern ends of this central segment the kszs loses its brittle overprint and swings around both corners of the etd to become subvertical dextral and sinistral strike slip faults the latter represent stretching faults whose displacements decrease westward to near zero the kinematic continuity of top east to top southeast ductile shearing along the central low angle extensional part of the kszs with strike slip shearing along its steep ends combined with maximum tectonic omission of nappes of the etd in the footwall of the knf indicates that north south shortening orogen parallel stretching and normal faulting were coeval stratigraphic and radiometric ages constrain exhumation of the folded nappe complex in the footwall of the kszs to have begun at 23 21 ma leading to rapid cooling between 21 and 16 ma this exhumation involved a combination of tectonic unroofing by extensional shearing upright folding and erosional denudation the contribution of tectonic unroofing is greatest along the central segment of the kszs and decreases westward to the central part of the tauern window the kszs formed in response to the indentation of wedge shaped blocks of semi rigid austroalpine basement located in front of the south alpine indenter that was part of the adriatic microplate northward motion of this indenter along the sinistral giudicarie belt offsets the periadriatic fault and triggered rapid exhumation of orogenic crust within the entire tauern window exhumation involved strike slip and normal faulting that accommodated about 100 km of orogen parallel extension and was contemporaneous with about 30 km of orogen perpendicular north south shortening of the etd extension of the pannonian basin related to roll back subduction in the carpathians began at 20 ma but did not affect the eastern alps before about 17 ma the effect of this extension was to reduce the lateral resistance to eastward crustal flow away from the zone of greatest thickening in the tauern window area therefore we propose that roll back subduction temporarily enhanced rather than triggered exhumation and orogen parallel motion in the eastern alps lateral extrusion and orogen parallel extension in the eastern alps have continued from 12 to 10 ma to the present and are driven by northward push of adria

surface temperature patterns associated with the southern oscillation abstract the typical global and large scale regional temperature patterns associated with the low warm and high cold phases of the southern oscillation so are investigated a total of 12 separate regions were found to have consistent temperature patterns associated with low phase of the so while 11 areas were found to have temperature patterns associated with the high phase of these areas 9 have expected temperature patterns during both phases of the so in the tropics temperature anomalies are of the same sign as the so related sea surface temperature sst anomaly in all land regions except for one area in the west pacific three extratropical responses to the low phase of the so are found over north america and one is found in japan high so temperature patterns were found in the extratropies for japan western europe and northwestern north america the identified temperature responses are more consistent in tropical regions than in the extratropies the so can influence the estimation of

modified cam clay modelling of temperature effects in clays the cam clay model for isothermal mechanical behaviour of clays has been extended to take account of the effects of temperature on stress strain behaviour the assumptions used in constructing the new model are based on published results and on new data presented in the paper the model allows prediction of how heating and cooling af fect volume changes pore water pressures and strengths for both normally consolidated and overconsolidated saturated clays it permits modelling of observed reductions in the overconsolidation ratio with increasing temperature the model provides accessible qualitative explanations for temperature effects that were previously difficult to understand it will also allow easy implementation for quantitative modelling in triaxial stress fields results predicted by the model are compared with data collected by the authors at temperatures up to 100 c the model does not account for changes that occur in clay minerals at higher temperatures for example in bentonites at temperatures higher than about 150 c

u pb and rb sr geochronology of magmatism and metamorphism in the dalradian of connemara western ireland new geochronological data which clarify the timing of syn orogenic magmatism and regional metamorphism in the connemara dalradian are presented u pb zircon data on four intermediate to acid foliated magmatic rocks show important inherited components but the most concordant fractions demonstrate that major magmatism continued until 465 ma whereas the earliest basic magmatism has been dated previously at 490 ma a fine grained fabric cutting granite contains discordant zircons which also appear to be 465 ma old arc magmatism in connemara therefore spanned a period of at least 25 ma recent u pb data on titanite from central connemara which gave a peak metamorphic age of 478 ma are supplemented by u pb data on titanite and monazite from metamorphic veins in the east of connemara which indicate that low p high t regional metamorphism continued there to 465 ma i e at least 10 ma later than in the central region dated previously new rb sr data on muscovites from coarse grained segregations in different structural settings range from 475 to 435 ma in part this range probably also reflects differences in age from west to east with three ages close to 455 ma from the eastern area which is also the site of the lowest pressure metamorphism thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1 2 ma the new dates emphasize the complexity in the spatial and temporal distribution of high level regional metamorphism caused by magmatic activity the relatively simple overall distribution of mineral appearance isograds revealed by regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions primarily in the southern part of connemara the later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism and were localized in the eastern part of the region

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

enso and sunspot cycles in varved eocene oil shales from image analysis abstract on the premise that sequential changes in varve thickness offer a proxy for climatic variations we investigated varve thickness in three core segments from the distal lacustrine oil shales tipton and laney members of the green river formation by means of an image analysis program of two strong bimodal periodicities one at 4 8 5 6 years is interpreted as an el nino type enso phenomenon of atmospheric dynamics while the other at 10 4 14 7 years is interpreted as the sunspot cycle originally recognized in this formation by bradley 1929 1931 weaker periodicities may exist at ca 8 and 33 years the latter also recognized by bradley taken in conjunction with the work of bradley 1929 1931 and of crowley et al 1986 this suggests that some but not all of the oil shale of the green river formation is truly varved and can be used to infer climatic time series automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time but careful selection of facies in their geological framework is necessary

lakes of the huron basin their record of runoff from the laurentide ice sheet the 189 000 km2 hur n basin is central in the catchment area f the present q s r lanrentian great lakes that now drain via the st lawrence river to the north atlantic ocean during deglaciation from 21 7 5 ka bp and owing to the interactions of ice margin positions crustal rebound and regional topography this basin was much more widely connected hydrologi cally draining by various routes to the gulf of mexico and atlantic ocean and receiving over flows from lakes impounded north and west of the great lakes hudson bay drainage divide early ice marginal lakes formed by impoundment between the laurentide ice sheet and the southern margin of the basin during recessions to interstadial positions at 15 5 and 13 2 ka be in i each of these recessions lake drainage was initially southward to the mississippi river and gulf of mexico in the first recession drainage subsequently switched eastward along the ice margin to the north atlantic ocean in the second recession drainage continued southward through the michigan basin and later eastward via the ontario basin and mohawk river valley to the north atlantic ocean during the final retreat of ice in the huron basin from 13 to 10 ka be proglacial lake drainage switched twice from the michigan basin and the mississippi river system to the i north atlantic via the ontario basin and mohawk river valley finally diverting to the champlain sea in the st lawrence river valley at about 11 6 ka be new seismo and litbo stratigraphic information with ostracode data from the offshore lacustrine sediments were integrated with the traditional data of shorelines uplift histories of outlets and radio carbon dated shallow water evidence of transgressions and regressions to reconstruct the water level history and paleolimnological record for the northern huron basin for the 11 7 ka bp period negative excursions in the 8 80 isotopic composition of ostracodes and bivalves in southern lake michigan southwestern lake huron and eastern lake erie indicate an influx of water from ice marginal lake agassiz in central north america about 11 ka be a major decline in water levels of the huron basin after 10 5 ka bp followed the high level main lake algonquin phase as ice receded and drainage was established through the north bay area to ottawa river valley during the subsequent mattawa stanley phase the lake level history was dominated by fluctuations of tens of meters highstands of the earliest oscillations whose origin is not clear might be related to some of the well known post algonquin shorelines after 9 6 ka be it is suggested that large inflows from lake agassiz and hydraulic damming in downstream outlets were the likely cause of the lake mattawa highstands a lowstand at 9 3 9 1 ka bp occurred when these inflows were diverted or impeded by an ice advance in the nipigon basin area while undiluted meltwater continued to enter the huron basin assemblages and isotopic composition of the ostracode fauna indicate very dilute meltwater during the lowstands as late as 7 5 ka be and precipitation runoff with comparatively higher dissolved solids dur ing the highstands we speculate that the water composition of the lake mattawa highstands was dom inated by the agassiz inflows by that time much of lake agassiz was remote from ice marginal envi ronments and the inflows were drawn from surface water of the southern sector of the lake which was largely supplied by runoff and dissolved solids from the exposed land area of western canada major inflows apparently ended about 8 ka be but northern proglacial lakes apparently continued as melt water persisted in the huron basin until about 7 5 ka bp the cessation of major inflows initiated the final lowstand in the huron basin and the present hydrological regime of local runoff

post younger dryas seasonality in the north american midcontinent region as recorded in lake huron varved sediments a sediment core from northwestern lake huron contains a sequence of red and gray layered sediments that are postglacial and early holocene glaciolacustrine varves two hundred and sixty four couple

a thermomechanical model for saturated clays a thermomechanical model for saturated clays is proposed within the framework of recent extensions of the cam clay model the results of some tests found in the literature are analyzed and the main features of the thermomechanical behaviour of clays are identified the effect of the overconsolidation ratio ocr on the volume change of a soil expansion contraction submitted to heating is well established using experimental data obtained for selected soils by various authors however existing models need to be modified to correctly model this feature for this reason a new volumetric thermal plastic mechanism is developed that allows for the prediction of plastic strains at higher ocr values the overconsolidation effect observed when heating a normally consolidated soil is also modelled particular attention is paid to the coupling and hardening phenomena related to the combined effects of stress and temperature a qualitative validation is made by examining the response of the model under a given th

u pb zircon and sm nd geochronology of mafic and ultramafic rocks from the central part of the tauern window eastern alps geochronological and geochemical analyses were carried out in order to identify the pre variscan basement of the tauern window eastern alps maficultramafic rocks from the central part of the tauern window have been studied by ree analysis and u pb and sm nd isotopic analyses on whole rock zircons garnets and sphene u pb and sm nd zircon dating define both magmatic pan african and cambro ordovician events from 650 ma to 486 ma within the alpine fold belt this indicates a time span of 150 ma for magmatic activities in the tauern window of the eastern alps the ages of 657 ma u pb zircon and 644 sm nd zireon obtained from an amphibolite are the oldest dates of the eastern alps they may be related to the pan african orogeny and imply an early cycle of magmatic intrusion before major activity started at around 500 ma sm nd whole rock analyses of the precambrian rocks do not define an isochron reflecting heterogenities within the mantle source the initial end values 1 2 to 4 7 are very low implying an enrichment of the magma source the second main phase of magmatic activity 539 486 ma is characterized by the emplacement of mafic ultramafic rock sequences as no ophiolitic relies are observed in these domains the early paleozoic magmatism was likely associated with extensional tectonics obtained ages of 301 3 and 314 4 3 ma point to a variscan metamorphism the first combined u pb zircon sm nd zircon data for an amphibolite from the basal amphibolite formation baf favoured the sm nd zircon isochron age as a magmatic age whereas the low initial end value point to an enriched magma source as well as to heterogenities within the magma source the obtained ages suggest that parts of the pre variscan basement within the alpine fold belt were formed during the pan africa cycle the detection of pre variscan ages within the alpine basement must reffect a complex history involving significant pre variscan activity

coalification and thermal histories of tertiary basins in relation to late alpidic evolution of the eastern alps the thermal histories of tertiary basins situated around and within the eastern alps are examined using coalification data waples method is used to estimate paleogeothermal conditions the data suggest that the basins thermal histories are intimately related to late alpidic geodynamics of the eastern alps

great lakes paleohydrology complex interplay of glacial meltwater lake levels and sill depths the oxygen isotope record of ostracode and clam shells recovered from great lakes cores of known age allows definition of times when meltwaters from the laurentide ice sheet were important components of lake water in the several lake basins since 12 ka we find that the lowstands in lake huron and georgian bay are characterized by isotopically light waters 18 o values of 20 to 22 relative to smow standard mean ocean water and the highstands by isotopically heavy waters of more local origin these data can be used to determine the degree of hydraulic separation among the early holocene lakes southern lake michigan for instance may mix with northern source waters only during times of rising and high water levels generally it is characterized by waters of local origin

a state parameter based thermomechanical constitutive model for fine grained saturated soils this paper presents a two surface constitutive model for describing thermomechanical behaviour of saturated fine grained soils at both normally consolidated and overconsolidated states a thermal d

seasonal sedimentation in the pliocene villarroya lake n spain inferred from pollen analysis abstract high resolution pollen analysis was performed on a short 8 7 cm thick section with 94 varve couplets made up of dark and light laminae from a lacustrine facies of the pliocene villarroya basin spain the aim of the present study was i to verify the seasonality of the sedimentation and ii to determine if pollen data record the short term climatic fluctuations related to variations in laminae thickness and colour the samples displayed high pollen concentrations and very good taxonomic diversity the composition of the pollen spectra indicated that the vegetation structure was closely linked to relief i a regional open herbaceous vegetation ii a local marshy and riparian environment iii deciduous mixed forest at mid altitudes and iv coniferous mixed forest in the surrounding mountains at higher altitudes in the 48 distinct couplets where the dark and light laminae have been separately sampled the pollen content showed higher concentrations of early flowering trees in the dark ones whereas late flowering plants were more abundant in the light ones total pollen concentrations in the 48 couplets were higher in most of their dark layers these results point to a seasonal layering according to present day flowering times the dark laminae represent sedimentation in winter spring and the light laminae sedimentation in late summer autumn the correlation between the ap arboreal pollen nap non arboreal pollen percentages and lamina thickness linked lamina thickness with the highest values of ap indicating warmer periods variations in the ecological groups enabled us to differentiate eight pollen cycles related to short term climatic fluctuations these alternations seem to be more associated with seasonality in precipitation than with temperature oscillations and could be correlated with the nao enso like and sunspot like cycles

climatic cycles recorded in glacially influenced rhythmites of the gowganda formation huronian supergroup abstract the gowganda formation of the 2 45 2 2 ga huronian supergroup contains glacially induced varve like rhythmites that potentially preserve a detailed record of climatic conditions during the paleoproterozoic era four rhythmic couplet thickness records were measured at two outcrops near wharncliffe ontario for the purpose of time series analysis the couplets which range from 1 to 32 mm thick are composed of alternating layers of siltstone and claystone time series analysis of the couplet thickness records using the mtm toolkit of mann and lees 1996 consistently revealed periodicities in the range of 2 2 2 9 couplets per cycle which is consistent with climatic cycles such as the quasi biennial oscillation qbo and the el nino southern oscillation enso observed in modern times this periodicity suggests that the rhythmic couplets represent annual deposits i e varves evidence for the presence of cycles at 3 0 4 9 couplets 6 6 6 9 couplets 8 8 9 2 couplets 22 8 couplets and 30 1 31 0 couplets were also observed in some couplet thickness records however the presence of these longer term cycles was inconsistent from site to site

stratigraphy and paleolimnologic record of lower holocene sediments in northern lake huron and georgian bay seismic reflection profiling and piston coring identified seismic reflectors in northern lake huron and georgian bay linked with unconformities caused by at least six reductions in lake level in ascending stratigraphic order these lowstands occurred at about 11 200 bp associated with the kirkfield outlet from early lake algonquin 10 200 9900 bp associated with the post algonquin lake level fall 9800 9050 bp the most extreme lowstand associated with the main stanley hough draw down and 7800 7450 bp the concomitant highstands are lake algonquin from about 11 200 10 200 bp early lake mattawa between 9600 and 9350 bp the main mattawa phase 9050 7800 bp and the nipissing highstand at about 4700 bp isotopic and paleoecological data show that all of the lowstands are characterized by cold dilute and isotopically very light 20 waters from the melting laurentian ice cap highstands on the other hand are characterized by higher dissolved solid concentrations and a much smaller

alpine metamorphism in the south east tauern window austria 1 p t variations in space and time the pennine rocks exposed in the south east tauern window austria contain mineral assemblages which crystallized in the mid tertiary late alpine regional metamorphism the pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers the results are a in the garnet chlorite zone 2 5 km structurally above the staurolite biotite isograd t 490 50 c p 7 1 kbar b within 0 5 km of the staurolite biotite isograd t 560 300c p 7 1 kbar c in the staurolite biotite zone c 2 5 km structurally below the staurolite biotite isograd t 610 30 c p 7 6 1 2 kbar d in the staurolite biotite zone 3 4 km structurally below the staurolite biotite isograd t 630 40 c p 6 6 1 2 kbar the pressure estimates imply that the total thickness of overburden above the basement cover interface in the mid tertiary was c 26 4 km this overburden can only be accounted for by the austro alpine units currently exposed in the vicinity of the tauern window if the altkristallin the middle austro alpine nappe was itself buried beneath an upper austro alpine nappe or nappe pile which was 7 4 km thick at that time the occurrence of epidote margarite quartz pseudomorphs after lawsonite in garnet indicates that part of the mesozoic pennine cover sequence in the south east tauern experienced blueschist facies conditions t 450 c p 12 kbar in early alpine times evidence from the central tauern is used to argue that the blueschist facies imprint post dated the main phase of tectonic thickening d1a and was thus a direct consequence of continental collision combined oxygen isotope and fluid inclusion studies on late stage veins thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation suggest that they crystallized from aqueous fluids at 1 1 0 4 kbar and 420 20 c early alpine late alpine and vein formation p t constraints have been used to construct a p t path for the base of the mesozoic cover sequence in the south east tauern window the prograde part of the p t path between early and late alpine metamorphic imprints is unlikely to have been a smooth curve and may well have had a low dp dt overall the decompression presumably due to erosion which occurred immediately before the thermal peak and possibly also earlier in the tertiary was probably partly or completely cancelled by the effects of early to mid tertiary d2a tectonic thickening the thermal peak of metamorphism was followed by a phase of almost isothermal decompression which implies a period of rapid uplift in the middle tertiary the peak metamorphic p t estimates are compared with the solutions of england s 1978 one dimensional conductive thermal model of the eastern alps and are shown to be consistent with the idea that the late alpine metamorphism was caused by tectonic burial of the pennine zone beneath the austro alpine nappes in the absence of extraneous heat sources such as large intrusions at depth

microstructural and textural development of calcite marbles during polyphase deformation of penninic units within the tauern window eastern alps abstract the evolution of calcite microstructures and crystallographic preferred orientations cpos is well understood due to well constrained experimental studies however the interpretation of naturally deformed calcite marbles is more difficult because of less constrained strain paths a multiphase deformation history and variable p t conditions the penninic units within the tauern window eastern alps have been affected by several deformation events and metamorphic overprint generally three major deformational events can be distinguished d 1 is related to underthrusting of penninic units beneath the austroalpine nappe complex and top to the n nappe stacking within the penninic continental units deformation stage d 2 is interpreted as reflecting the subsequent continent collision between the penninic continental units and the european foreland d 3 is related to the formation of the dome structure of the tauern window this polyphase deformation history can be partly reconstructed by the evolution of calcite microfabrics and cpos three types of calcite fabrics are distinguished within the penninic units of the tauern window and the lower austroalpine unit d 1 fabrics are characterized by equilibrated microstructures and lt cpos the cpos are generally strong and symmetric with one well developed cluster near the z axis of the finite strain ellipsoid these fabrics have locally been overprinted by subsequent amphibolite to greenschist facies metamorphism generally the occurrence of lt fabrics coincides with the occurrence of amphibolite facies metamorphic mineral assemblages in the central part of the tauern window while ht fabrics have been observed outside this area fabrics from the central part of the tauern window have likely been strengthened during subsequent thermal equilibration while the fabrics that have been observed at the peripheral parts have been less affected by subsequent metamorphic overprint therefore d 1 fabrics do not reflect d 1 conditions but subsequent thermal equilibration similar observations have been made for the evolution of d 2 fabrics lt fabrics dominate inside the amphibolite facies isograde ht fabrics occur outside greenschist facies metamorphic conditions the fabrics are characterized by high finite strains near the margins of the tauern window the intensity of which decreases towards the central parts where microstructures are characterized by recrystallization and thermal equilibration due to amphibolite facies metamorphic overprint the strong cpos document this influence the ht fabrics and microstructures within peripheral areas indicate that they have less been affected by this thermal event and therefore are more indicative for the deformational conditions during d 2 d 3 is restricted to distinct shear zones along the tectonic boundaries of the tauern window from the central parts to the shear zone boundaries a clear evolution of microfabrics can be observed in internal parts microstructures are characteristic for intracrystalline plasticity with a dominating activity of r glide twinning was less important during the final phases of deformation on approaching the shear zone boundaries the grain size decreases due to dynamic recrystallization and secondary grain size reduction until ultramylonites are formed within these domains grain boundary sliding seems to have been dominant in conclusion calcite cpos from polyphase areas do not only include information on the deformation conditions but also bear information about the thermal overprint subsequent to the main deformational event

geothermobarometry in metasediments of the southern grossvenediger area tauern window austria metasediments in the southern grossvenediger area tauern window austria were studied along a cross section through rocks of increasing metamorphic grade from the margin of the tauern window in the south to the base of the upper schieferhulle including the eclogite zone in the north in the southern part of the cross section there is no evidence for a pre late alpine metamorphic history in the form of high pressure relics or pseudomorphs mineral assemblages are characterized by the stability of tremolite calcite biotite calcite and biotite chlorite calcite in the northern part a more complete alpine metamorphic evolution is preserved primary high pressure assemblages are dolomite quartz tremolite zoisite zoisite dolomite quartz phengite i and probably tremolite dolomite phengite i secondary post kinematic assemblages tremolite calcite talc calcite phengite ii chlorite calcite quartz biotite chlorite calcite biotite zoisite calcite formed as a result of the dominant late alpine metamorphic overprint the occurrence of biotite zoisite calcite is confined to the northernmost area and defines a biotite zoisite calcite isograd p t estimates based on standard thermobarometric techniques and on stability relationships of tremolite calcite dolomite quartz and zoisite give consistent results p t conditions of the main tertiary metamorphic overprint were 525 c p 7 5 1 kbar in the northern part of the cross section the southern part was metamorphosed at lower temperatures of 430 470 c the si content of phengites from this area is almost as high as that of phengites from the eclogite zone simax 3 4 pfu pressures 10 kbar at 420 c are suggested by phengite barometry according to massone schreyer 1987 in the absence of high pressure relics or pseudomorphs these phengites which lack late alpine re equilibration are the only record that rocks of the southern part probably also experienced an early non eclogitic high pressure metamorphism

far field effects of late miocene subduction in the eastern carpathians e w compression and inversion of structures in the alpine carpathian pannonian region kinematic and paleostress data constrain a late miocene e w compressional event that affected the entire alpine carpathian pannonian system after 9 ma and prior to 5 3 ma the maximum horizontal compression directions obtained from 110 stations show a mean 1 orientation of 083 deformation is mainly strike slip e w directed compression followed early to middle miocene upper plate extension in the pannonian basin which was caused by the retreating subduction boundary in the outer carpathians this compressional event terminated early to middle miocene eastward lateral extrusion of the eastern alps and reverted strike slip faults which bounded earlier extruded wedges such as the salzachtal ennstal fault the periadriatic fault the mur murz vienna basin fault system and strike slip faults in the western carpathians e w compression caused the reorientation of the extension direction of the alpine crustal stack east directed tectonic unroofing of the metamorphic domes in the central eastern alps terminated between 9 and 6 ma and early to middle miocene orogen parallel e w extension switched to late miocene n s extension which parallels modern topographic slopes in the pannonian basin the change from synrift extension to late miocene compression during the postrift phase caused the pronounced postrift subsidence by stress induced downward flexure of the loaded lithosphere we relate late miocene e w compression to coeval late stage west directed subduction of the european plate below the eastern carpathians slab pull of the subducted plate caused subduction roll back and upper plate extension of the pannonian area up to the middle miocene subduction slowed down and ceased when up to 60 km thick buoyant continental crust entered the subduction zone during the short period of continued convergence e w directed compressive stress was transmitted across the subduction boundary into the upper plate compressive stress was transferred through the uppermost brittle crust of the earlier extended pannonian region into the eastern alps up to 1400 km behind the subduction zone e w compression terminated during the pliocene when the pliocene to recent nnw sse compressive stress field was established in central europe

global scale modes of surface temperature variability on interannual to century timescales using 100 years of global temperature anomaly data we have performed a singular value decomposition of temperature variations in narrow frequency bands to isolate coherent spatio temporal modes of global climate variability statistical significance is determined from confidence limits obtained by monte carlo simulations secular variance is dominated by a globally coherent trend with nearly all grid points warming in phase at varying amplitude a smaller but significant share of the secular variance corresponds to a pattern dominated by warming and subsequent cooling in the high latitude north atlantic with a roughly centennial timescale spatial patterns associated with significant peaks in variance within a broad period range from 2 8 to 5 7 years exhibit characteristic el nino southern oscillation enso patterns a recent transition to a regime of higher enso frequency is suggested by our analysis an interdecadal mode in the 15 to 18 years period range appears to represent long term enso variability this mode has a sizeable projection onto global average temperature and accounts for much of the anomalous global warmth of the 1980s a quasi biennial mode centered near 2 2 years period and a mode centered at 7 to 8 years period both exhibit predominantly a north atlantic oscillation nao temperature pattern a potentially significant decadal mode centered on 11 to 12 years period also exhibits an nao temperature pattern and may be modulated by the century scale north atlantic variability

mantle lithospheric deformation and crustal metamorphism with some speculations on the thermal and mechanical significance of the tauern event eastern alps changes in the thickness of the mantle lithosphere beneath convergent orogens through processes such as the pervasive thickening of the lithosphere and the convective thinning of the thermal boundary layer dictate the heat flow through the base of the crust and thus may profoundly influence the thermal evolution of crustal metamorphic terrains calculations based on one dimensional thermal energy balances show that when mantle lithosphere is substantially thickened and remains intact through the orogenic cycle then low temperature facies series including blueschist and eclogite facies may be preserved throughout the crust in contrast crust thickened above attenuated mantle lithosphere will develop much higher temperature facies series potential energy arguments suggest that changes in the thickness in the mantle lithosphere induce changes in the elevation and the potential energy stored within the lithosphere therefore the response to mantle lithospheric deformation observable in crustal metamorphic terrains should not only be recorded in the thermal regime but also in changes in the incremental strain history in as much as it reflects the force balance operating within the orogen for example mantle lithospheric thinning beneath thickened crust may cause an increase in crustal temperatures synchronous with the termination of convergent strain or for large reductions in mantle lithospheric thickness with the onset of extensional deformation such histories are recorded in a number of young metamorphic terrains for example the eastern alps which may provide an important thermal and isostatic record of the mantle lithospheric response to convergent deformation here we show how application of these ideas may lead to new insights into some outstanding problems concerning the thermal and mechanical evolution of the eastern alps during the tertiary tauern metamorphic event

alpine metamorphism in the south east tauern window austria 2 rates of heating cooling and uplift existing geochronological data are reviewed and new rb sr k ar and 39ar 40ar ages are presented including a suite of 33 mica ages from a 20 km north south tunnel section these data are discussed in relation to the thermal history from the overthrusting of the autroalpine nappes c 65 myr ago to the present the earliest phase of metamorphism involving lawsonite crystallization is associated with emplacement of these nappes subsequently temperatures in the rocks beneath rose at a mean rate of 3 6 c myr until the climax of metamorphism at high structural levels published data indicate an age 35 myr for the metamorphic climax in contrast a new 39ar 40ar step heating age of 23 8 0 8 myr on amphibole from near the base of peripheral schieferhulle closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels following the climax near isothermal uplift and erosion reduced pressure to c 1 kbar before white mica closure at 19 myr this implies uplift at 3 mm yr along the tunnel section white mica k ar ages vary systematically from 24 myr to 16 5 myr with position relative to a late 4 km amplitude dome whereas biotite rb sr ages are uniform at 16 5 myr across the whole profile doming is thus dated at 16 5 myr with transient uplift rates 5 mm yr at other times uplift rates were 1 mm yr

dating deformation using rb sr in white mica greenschist facies deformation ages from the entrelor shear zone italian alps ages of deformation have been obtained by rb sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation since white micas commonly define deformation fabrics in medium grade metamorphic rocks these ages can be directly related to structural geometries with regional context this direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust it does require that the dated minerals attained isotopic equilibrium with the dominant sr reservoir at temperatures lower than the closure temperature this resetting was apparently achieved during dynamic recrystallization of white micas in greenschist facies metasediments and metagranitoid units in the western alps the results suggest that the sr isotopic composition of the new mica is buffered by the coexisting high sr phases calcite feldspar or epidote via the grain boundary network high strain rocks from the entrelor shear zone system of the western alps have yielded indistinguishable white mica rb sr ages along 30 km of individual and kinematically linked shear zones the age of the back thrusting event is constrained at 34 1ma the age yielded by the younger generation of synkinematically crystallized white micas this event was short lived involving at least 20 km of shortening in 1 m y or less an earlier variably overprinted component dating from 38 to 37 ma has been identified in the mica fabric but its kinematic significance is uncertain this method of dating strain fabrics offers a powerful tool for tectonic studies since isotopic resetting can be directly linked to structural geometries microstructural textures and pt conditions it allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust

argon retentivity and argon excess in amphiboles from the garbenschists of the western tauern window eastern alps 22 hornblende k ar ages and 10 39ar 40ar spectra were obtained for hornblende garbenschists from the western tauern window the post kinematic amphiboles were produced during the late alpine prograde metamorphism 6 10 kb and 500 570 c two nearly potassiumfree cummingtonites rimming hornblende yield k ar ages of 120 ma while the 20 tschermakitic hornblendes scatter between 17 and 37 ma the reason for this scatter is excess ar possibly incorporated into amphiboles during healing of fractures now traceable by trails of fluid inclusions excess ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with k ar isochrons it can be quantified in 4 out of 10 hornblendes by 39ar 40ar stepwise heating experiments ages of 18 20 ma result for corrected hornblendes the retentivity of 40ar after correction for excess shows no correlation with chemistry within the narrow compositional range observed rather it shows intriguing correlations with irregularities in ca k spectra pointing to a microstructurally controlled mechanism for ar loss this observation leads to a critical evaluation of the closure temperature constant which apparently depends on an incompletely known number of mineralogical and environmental parameters in particular those 39ar 40ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure this gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media