dietary reconstruction of an early to middle holocene human population from the central california coast insights from advanced stable isotope mixing models the inherent sampling and preservational biases of the archaeological record make it difficult to quantify prehistoric human diets especially in coastal settings where populations had access to a wide range of marine and terrestrial food sources in certain cases geochemical proxies such as stable isotope ratios may be utilized to provide robust estimates on the relative proportions of various food resources consumed by prehistoric populations the harkins slough archaeological site scr 60 130 is an early to middle holocene coastal assemblage located on monterey bay in central ca the carbon d 13 c and nitrogen d 15 n isotope composition of human burials and associated archaeofauna excavated from the site were measured for input into a concentration dependent isotope mixing model that is able to statistically discriminate among multiple o3 food sources the human burials segregate into two distinct groups an early w7000 bp and middle w4500 bp holocene population with significant dietary differences stable isotope analyses indicate a 70e84 marine food source contribution for the early holocene group ehg but only a 48e58 marine dietary contribution for the middle holocene group mhg results also suggest that pinnipeds were an important marine food source for both groups modeling results are in agreement with archaeological evidence from southern ca suggesting that early holocene coastal populations were highly dependent on marine resources with the ability to procure both littoral and pelagic species further the use of terrestrial plant sources likely increased over time with the feasible contribution range doubling from the ehg 4e30 mean 19 to mhg 10e52 mean 38 this trend is supported by regional archaeological evidence for the advent of technological innovations linked to intensive terrestrial plant processing beginning in the middle holocene methodologically this work demonstrated the utility of a concentration dependent stable isotope mixing model in an iterative framework to provide feasible ranges of dietary contribution when the number of food sources is too high to allow a unique solution 2004 published by elsevier ltd

a model for fibrous illite nucleation and growth in sandstones we have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in situ k feldspar grain dissolution or imported into the model reference frame illite fiber nucleation and growth are modeled using arrhenius expressions that consider saturation state in addition to temperature and time nucleation occurs on pore walls and muscovite and detrital illite may be defined as energetically favorable substrates the model is integrated with other touchstone models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations we evaluated the illite model performance on two data sets 1 jurassic quartzose samples from offshore mid norway with maximum temperatures ranging from 108 to 173c 226 to 343f and 2 miocene lithic samples from offshore southeast asia that have maximum temperatures ranging from 157 to 182c 315 to 360f the model matches measured abundances of illite kaolinite and k feldspar in both data sets using identical kinetic parameters predicted k ar ages are consistent with available data given uncertainties associated with detrital contaminants although no illite particle size data are available from the analyzed samples modeled crystallite thicknesses from the norway data set are comparable to published measurements of 0 004 to 0 012 m from north sea samples with similar temperature histories

holocene lake level trends in the rocky mountains u s a abstract the availability of water shapes life in the western united states and much of the water in the region originates in the rocky mountains few studies however have explicitly examined the history of water levels in the rocky mountains during the holocene here we examine the past levels of three lakes near the continental divide in montana and colorado to reconstruct holocene moisture trends using transects of sediment cores and sub surface geophysical profiles from each lake we find that mid holocene shorelines in the small lakes 4 110 ha were as much as 10 m below the modern lake surfaces our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000 2000 years ago we also discuss evidence for millennial scale moisture variation including an abruptly initiated and terminated wet period in colorado from 4400 to 3700 cal yr bp and find only limited evidence for low lake stands during the past millennium the extent of low water levels during the mid holocene which were most severe and widespread ca 7000 4500 cal yr bp is consistent with the extent of insolation induced aridity in previously published regional climate model simulations like the simulations the lake data provide no evidence for enhanced zonal flow during the mid holocene which has been invoked to explain enhanced mid continent aridity at the time the data including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium confirm the ability of large boundary condition changes to push western water supplies beyond the range of recent natural variability

mountain environment and early human adaptation in nw himalaya india a case study of siwalik hill range and leh valley abstract prehistoric sites are reported from the entire length and breadth of india most of these are surface sites and invite less attention and significance compared to the few sites that are buried the systematic excavation of prehistoric sites has contributed to the understanding of typo technological evolution in the backdrop of time and space however the early artifact bearing sites in himalaya beyond the indo gangetic plains although open air sites hold significance in terms of understanding the extent of human colonization in himalaya in relation to climate the sporadic evidence of early humans in the siwalik ranges of jammu and along the indus river in the leh valley undoubtedly confirm the advent of early humans to this geographical realm when most of the area had tectonically stabilized and the more or less present day landscape and ecology had developed in the region with ameliorated climate for early human settlement

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

a modern analogue of western united states summer palaeoclimate at 6000 years before present a modern summer climate analogue of western united states 6 ka palaeoclimate based on broad scale patterns from proxy data and general circulation model simulations provides details concerning its sum mer synoptic climatology august 1955 serves as a close analogue featuring an amplified subtropical ridge aloft and anticyclonic flow near the surface over the central united states and stronger westerly flow along the canadian northwest united states border the august 1955 summer temperature pattern similar to those from general circulation model simulations exhibits positive anomalies throughout most of the continental interior the august 1955 precipitation pattern corresponds very closely with patterns of effective moisture from proxy data showing widespread aridity and some stronger monsoonal activity in the southwest the analogue suggests that the amplified subtropical ridge is a very important feature of 6 ka palaeoclimate with the circulation pattern differing from that in the early holocene implying interactions between both milankov itch and sub milankovitch forcing factors

the evolution of illite to muscovite an example of the behaviour of isotopes in low grade metamorphic terrains abstract a review of the possibilities and limitations of the use of radiogenic isotopes in low grade terrains i e in the temperature range below the blocking temperature of potassic white mica is discussed using a background of a data base of over 500 isotope analyses on more than 250 illitic muscovitic samples published in various papers special stress is laid on the main problems 1 distinction between detrital and neoformed minerals 2 the slow evolution of minerals without pronounced breaks and 3 retention problems of radiogenic daughter products a brief description of low grade metamorphism is given and criteria enabling a subdivision into low to very low grade metamorphism are discussed comprising illite crystallinity mineralogy fluid inclusions and coal rank measurements the useful type of lithologies for this kind of work ranges from shales slates through volcaniclastic tuffs with high primary glass contents and limestone dolomites marbles to mylonites it can be concluded that carefully selected illitic muscovites yield geologically meaningful ages provided that more than one isotopic method is applied

the development of polygonal fault systems by syneresis of colloidal sediments abstract polygonal fault systems occur in numerous sedimentary basins worldwide are generally located on passive margins in onlap fill units and tend to comprise the finest grained sediments in this geological setting these fault systems have been most thoroughly described in the central north sea basin and the detailed structure shows a significant correlation with lithological variations both vertically and laterally extension measured in stacked decoupled tiers of polygonal faults correlates positively with both clay fraction and smectite content lateral facies variations are also observed and indicate that time equivalent sequences upslope from the smectite rich polygonally faulted sediments are coarse grained clay poor and undeformed this leads us to believe that the structure and geometry of the fault system are controlled by the colloidal nature of the sediments and that the volumetric contraction measured on seismic sections can be accounted for by syneresis of colloidal smectitic gels during early compaction syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of the constituent pore fluid this process occurs due to the domination of interparticle attractive forces in marine clays dependent on environment and is governed by the change of gel permeability and viscosity with progressive compaction the process of syneresis can account for a number of structural features observed within the fault systems such as tiers of faults the location of maximum fault throw and growth components at upper fault tips as such this paper represents the first attempt to correlate microscale properties of clay rich sediments to their macroscale seismic character

a microphysical model for fault gouge friction applied to subduction megathrusts a microphysical model is developed for the steady state frictional behavior of illite quartz fault gouge and applied to subduction megathrust p t conditions the model assumes a foliated phyllosilicate supported microstructure which shears by rate independent frictional slip on the aligned phyllosilicates plus thermally activated deformation of the intervening quartz clasts at low slip rates or high temperatures the deformation of the clasts is easy accommodating slip on the foliation without dilatation with increasing velocity or decreasing temperature the shear of the clasts becomes more difficult increasing bulk shear strength until slip is activated on inclined portions of the phyllosilicate foliation where it anastomoses around the clasts slip at these sites leads to dilation involving clast matrix debonding balanced at steady state by compaction through thermally activated clast deformation model predictions taking pressure solution as the thermally activated mechanism show three regimes of velocity dependent frictional behavior at temperatures in the range of 200 500 c with velocity weakening occurring at 300 400 c in broad agreement with previous experiments on illite quartz gouge effects of slip rate normal stress and quartz fraction predicted by the model also resemble those seen experimentally extrapolation of the model to earthquake nucleation slip rates successfully predicts the onset of velocity weakening behavior at the updip seismogenic limit on subduction megathrusts the model further implies that the onset of seismogenesis is controlled by the thermally activated initiation of fault rock compaction through pressure solution of quartz which counteracts dilatation due to slip on the fault rock foliation

chemical and k ar systematics of a mylonitic mica after natural and experimental interactions with varied fluids abstract flow through and batch leaching experiments combining mineralogical chemical and k ar isotopic analyses of mica separates from a mylonitic sample of the tyndrum faulting system in scotland u k were conducted to evaluate and simulate the natural interaction with h 2 so 4 loaded river water at phs of 3 4 the flow through experiments with h 2 so 4 loaded and pure deionized water completed by a batch leaching experiment with 1 m hcl at room temperature had varied effects on soluble mineral phases such as fe sulfates and ca mg carbonates that were present in the varied mica size fractions but none caused the mineralogical and k ar characteristics of the mica to differ from separates of the natural environment despite the limited number of k ar ages size fractionation of the mylonitic mica identified two generations of mica with different ree patterns the coarser 2 8 m fraction yielded a high la yb ratio of 4 3 and crystallized at 359 6 ma at a temperature of 250 300 c probably during a major tectonic thermal activity the smaller

clay mineralogy and isotope geochemistry of the alteration halo at the cigar lake uranium deposit the clay size fraction of the alteration halo associated with the cigar lake uranium deposit consists of illite with subordinate kaolinite sudoite al chlorite fe chlorite hematite and traces of an illite chlorite mixed layer mineral the kubler index of illite crystallinity ranges from 2 0 to 6 7 the most altered samples average 4 0 these data indicate that high grade diagenetic to anchizonal conditions have been reached within the alteration halo intensity ratios of the 002 and 001 illite reflections i 002 i 001 are generally greater than 0 4 the phengite muscovite boundary reflecting aluminum rich illites illite composition approaches hydromuscovite whereas 1m and 2m polytypes prevail regionally the presence of 3t and 2m polytypes in the alteration halo suggests that locally higher temperatures have been attained than the regional estimates of 180 220 c k ar dates for illite rich samples from 1255 to 1148 ma could reflect variable resetting of the k ar system during hydrothermal f

holocene vegetation and fire reconstructions from the aquarius plateau utah usa micropaleontological assemblages from two lakes located on the aquarius plateau in southern utah were investigated to evaluate changes in vegetation fire disturbance and paleoclimate over the last 8600 years this paper presents reconstructions of holocene landscape conditions and wildfire dynamics in the subalpine zone of the colorado plateau during the early holocene forests on the aquarius plateau were dominated by spruce and pine at upper elevations and mixed conifer forests at mid elevation sites fire return intervals were the most frequent of the record averaging 250 years between burning episodes proxy based reconstructions of the middle holocene suggest warmer climate conditions relative to present with variable moisture availability during the middle holocene the forest zone on the aquarius plateau contracted where both lower and upper timberline retreated upslope and downslope respectively fire frequency was similar to early holocene though overall charcoal accumulations were reduced in the late holocene forests in the subalpine zone became dominated by spruce fir and aspen while at mid elevation forests dominated by douglas fir spruce and pine established the most striking result indicates that evidence of fire was essentially absent from the aquarius plateau during the late holocene until the arrival of european settlers to the region c 1850

friction on subduction megathrust faults beyond the illite muscovite transition abstract previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges dominated by smectite illite and quartz here we aim to determine the effect of the transition to muscovite rich fault rock at depths beyond the illite dominated region of the seismogenic zone to achieve this rotary friction experiments were performed on simulated muscovite quartz 65 35 gouge at 100 600 c an effective normal stress of 170 mpa pore pressures of 100 and 200 mpa and sliding velocities of 1 100 m s we isolated the effects of muscovite by shearing pure muscovite gouge at 200 600 c under otherwise identical conditions at low shear strains 1 muscovite quartz gouge showed a friction coefficient of 0 4 0 5 while pure muscovite gouge exhibited 0 3 both gouges showed an increase in with and temperature the muscovite quartz gouge was characterised by velocity strengthening or neutral behaviour at 100 350 c velocity weakening at 350 500 c and velocity strengthening at 500 600 c in contrast pure muscovite gouge showed predominantly velocity strengthening or neutral behaviour demonstrating a key role of quartz in producing velocity weakening the velocity weakening regime in the mixed gouge extends that of 250 400 c recently reported for illite quartz gouge up to 500 c taking into account the observed effect of sliding velocity on a b the data for illite quartz and muscovite quartz together help explain the depth extent of seismogenesis on subduction megathrusts interestingly comparison with previous microphysical models suggests that the mechanism causing velocity weakening in phyllosilicate quartz gouges is one of granular flow involving competition between shear induced dilation and compaction through thermally activated quartz deformation

neocrystallization or cooling 40ar 39ar ages of white micas from low grade mylonites abstract 40 ar 39 ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or alternatively ages of deformation induced neocrystallization to address these interpretations three types of information are needed 1 the effective diffusion length scale controlling volume diffusion of argon in white micas 2 an estimate of the diffusion parameters for volume diffusion of argon and 3 the thermal history completely recrystallized quartzites exhibit smooth grain size distributions for white micas with good exponential or power law fits suggesting that the grains grew under similar conditions the microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history measured grain size distributions provide a maximum limit for the diffusion length scale thermal histories are estimated by 40 ar 39 ar diffusion experiments on k feldspars using the multi diffusion domain method of lovera lovera o m richter f m harrison t m 1989 the 40 ar 39 ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes j geophys res 94 17917 17935 these results are consistent with the expected temperature time path for the thrusting environment and predict maximum temperatures of 250 350 c during the time span recorded by the white micas experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages forward modeling of the passage of the white micas through the closure window for argon diffusion using the thermal histories derived from the k feldspars indicates that the white micas record neocrystallization rather than cooling and supports the idea that the white micas record the timing of when ductile deformation ceased

on the structural evolution of the central trough in the norwegian and danish sectors of the north sea abstract the central trough of the north sea is not a simple rift graben it is an elongated area of regional subsidence which was initiated in mid cretaceous times and continued to subside through to the late tertiary its form is not representative of pre mid cretaceous tectonics in late permian times the north sea was divided into a northern and southern zechstein basin by the e w trending mid north sea ringkobing fyn high the latter was dissected by a narrow graben trending nnw through the tail end graben and the sogne basin the feda graben was a minor basin on the northern flank of the mid north sea high at this time this structural configuration persisted until end middle jurassic times when a new wnw trend separated the tail end graben from the sogne basin right lateral wrench movement on this new trend caused excessive subsudence in the tail end and feda grabens while the sogne basin became inactive upper jurassic subsidence trends continued during the early cretaceous causing the deposition of large thicknesses of sediments in local areas along the trend from mid cretaceous times the regional subsidence of the central trough was dominant but significant structural inversions occurred in those areas of maximum early cretaceous and late jurassic subsidence

hydrothermal alteration of the ediacaran doushantuo formation in the yangtze gorges area south china abstract the geochemical and fossil record preserved in the ediacaran age 635 551 ma doushantuo formation of south china has been extensively examined to explore the impact of changing climate and the oxidation state of the oceans on the development and distribution of early multicellular life in the yangtze gorges area this formation shows many of the geochemical trends and features thought to typify global ocean chemistry in the ediacaran period but there are indications that post sedimentary processes modified these signals this study of clay minerals and organic matter builds a more detailed picture of the type and degree of post sedimentary alteration at different stratigraphic levels of the formation and focuses on how this alteration influenced stable carbon and oxygen isotope records in the cratonward jiulongwan and huajipo sections of the doushantuo formation its lower part members 1 and 2 consists largely of dolomitic shale rich in authigenic saponite that crystallized in an alkaline sedimentary basin saponite has been altered to chlorite via corrensite across tens of meters of strata in lower member 2 with increased alteration downward toward the cap dolostone the greater chloritization is accompanied by lower 18 o and higher d values of trioctahedral clays this pattern of alteration of trioctahedral clays is likely due to hydrothermal fluid activity in the underlying relatively permeable nantuo formation and cap dolostone a concomitant increase of solid bitumen reflectance toward the base of the formation supports this idea in the uppermost part of the formation in the yangtze gorges area member 4 a typical open water marine dolomitic shale rich in illite and organic matter increases in the methylphenanthrenes ratio index and solid bitumen reflectance correlate with decrease of the bulk rock k al ratio upward providing evidence for hot fluid migration above the nearly impermeable shale clay from the upper part of the formation is enriched in 18 o but not in d relative to clay from the lower parts indicating progressive 18 o enrichment of hydrothermal fluids that percolated upward and laterally through permeable 18 o rich carbonates a maximum hydrothermal alteration temperature of 200 c is estimated from a calibration curve for illitization during burial diagenesis but given that the hydrothermal activity probably occurred in short pulses the temperature could have been much higher k ar ages are consistent across different size fractions of fine illite from member 4 shale 430 ma and from a k bentonite bed near the base of member 2 in the jiuqunao section 325 ma 25 km from jiulongwan and huajipo these age values show that the diagenetic illite of the doushantuo formation is a product of either deep burial diagenesis overprinted by spatially limited hydrothermal activity or of two localized hydrothermal events patterns of carbonate 13 c and 18 o depletion in the basal doushantuo formation are similar to chloritization trends and 18 o variation in diagenetic clay minerals given independent evidence for 13 c depletion of hydrothermal fluids these trends indicate carbonate fluid isotope exchange commensurate with the degree of post sedimentary alteration supporting a model of lithologically controlled differential diagenesis induced by hydrothermal fluids as the main control on c and o isotope variability in this stratigraphic interval this model could potentially explain other notable 13 c excursions higher up in member 3

thermochronometry across the austroalpine pennine boundary central alps switzerland orogen perpendicular normal fault slip on a major overthrust and its implications for orogenesis fifty one new and 309 published thermochronometric ages nine systems with closure temperatures ranging from 450 to 70 c from the graubunden region of the central alps demonstrate that a pronounced thermal mismatch between the austroalpine allochthon alpine orogenic lid and the pennine zone persisted until at least 29 ma and allowably until circa 18 ma the observed mismatch supports previous suggestions that the famous overthrust between the austroalpine allochthon and the pennine zone historically regarded as primarily an eocene top north thrust fault is in fact primarily an oligocene miocene normal fault that has a minimum of 60 km of displacement with top south or top southeast sense of shear two hallmarks of alpine geology deposition of the foredeep molasse and emplacement of the helvetic nappes appear to be coeval peripheral manifestations of crustal thickening via the interposition of the pennine zone as a northward intruding wedge between the austroalpine lid and the european cratonic margin with the helvetic system european margin acting as the floor of the wedge we presume the penninic wedge is driven by the buoyant rise of subducted crust no longer able to remain attached to the descending slab if so emplacement of the pennine wedge could have occurred mainly after adria was juxtaposed against cratonic europe

pleistocene water cycle and eastern boundary current processes along the california continental margin 1 coastal marine sediments contain mixtures of terrestrial and marine paleoclimate proxies that record how the coastal water cycle has behaved over long time frames we explore a 600 kyr marine record from odp site 1018 located due west of santa cruz california to identify coastal wet and dry periods and to associate them with oceanographic processes wet periods in central california identified by increased tree pollen relative to pollen from grasslands and scrublands are found on every major deglaciation in the last 600 kyr sea surface temperature sst data were collected for the last two deglaciations wet periods are associated with a rapid rise in sst off central california sst gradients along the california margin and changes in biogenic deposition show that wet periods in central california are associated with a weakening of the california current and weakened coastal upwelling high carbonate production suggests that there was significant curl of wind stress upwelling offshore we propose that wet periods in central california are associated with a meteorological connection to the tropical pacific and weakened southward flow in the california current that shunted temperate pacific water northward into the alaska gyre we do not observe evidence for a south shifted westerly storm track at the last glacial maximum but find that wet periods are diachronous along the california margin the wettest period around the santa barbara basin peaked at 16 ka preceding the wet peak in central and northern california by 4 kyr

regional aridity in north america during the middle holocene increased aridity throughout the great plains and rocky mountain region during the middle holo cene has been documented from pollen records aeolian proxy variables in lake cores and active sand dune migration varve calibration provided by a continuously varved record of the holocene from a core from elk lake northwestern minnesota shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3 8 ka with peak aeolian activity at about 6 ka if aeolian influx to elk lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in minnesota dune fields then the varve calibration in elk lake provides precise time calibration of periods of peak aeolian activity in minnesota palaeowind studies from the minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest the same as the dominant wind direction in dune fields throughout the great plains and rocky mountains if the mid holocene aeolian act

mineralogical kar stable and sr isotope systematics of k white micas during very low grade metamorphism of limestones helvetic nappes western switzerland abstract mineralogical k ar rb sr and stable isotope analyses have been carried out on k white micas from helvetic malm limestones in order to examine their evolution during very low to low grade alpine metamorphism associated with intense ductile deformation metamorphic temperatures were estimated at 300 c from stable isotopes quartz calcite thermometry occurrence of chloritoid and epizonal illite crystallinity index k white micas consist of variable mixtures of 2m 1 phengite and muscovite as revealed by detailed x ray diffraction analyses using peak decomposition of the 060 331 spectra k ar apparent ages display a strong grain size dependence in which mainly fine grained size fractions m record alpine ages 37 15 ma however these ages provide a relative rather than an absolute chronology of the diachronous alpine metamorphic evolution of the helvetic nappes the resetting of the k ar isotopic system of k white micas to alpine metamorphic conditions reflects an apparent combination of crystallization recrystallization and radiogenic 40 ar diffusion loss the oxygen isotope compositions of micas 15 to 22 are intermediate between detrital and 18 o enriched values expected for micas neoformed within an abundant marine carbonate matrix no isotopic equilibrium has been reached between calcite and micas the variable depletion of hydrogen isotope compositions 126 to 82 is influenced by the interaction with organic matter under closed system conditions organic matter if not removed may also represent a serious source of error in k ar age determination by introducing radiogenic 40 ar contamination 87 sr 86 sr isotope ratios of micas range from 0 70879 to 0 70902 with one outlier at 0 71794 the low values reflect sr exchange with calcite occurring during crystallization recrystallization of micas under closed system conditions

radioactive production and diffusional loss of radiogenic 40ar in clays in relation to its flux to the atmosphere abstract this paper addresses the release mechanisms of radiogenic 40 ar from clays for the purpose of estimating the 40 ar flux from the sedimentary crust to the atmosphere clays from sedimentary sequences of cambrian to neogene age show discrepancies between their depositional stratigraphic ages and the k ar apparent ages or dates of the different size fractions the possible geochemical mechanisms that may be responsible for the k ar dates being either higher or lower than the stratigraphic age are discussed in general the known observation of a decrease in the k ar apparent age of clays with decreasing particle size is interpreted as due to a faster escape of radiogenic 40 ar from the smaller particles in the younger neogene clays the mass fractions of 40 ar lost are in a range from 13 to 34 in the fine size particles that are smaller by a factor of 10 in their linear dimension than the bigger particles in the older formation samples cambrian to triassic the 40 ar loss is smaller from 7 to 19 for the same particle size range this smaller decrease in the 40 ar 40 k ratio in the stratigraphically older samples may be at least in part accounted for by the closure of the mineral system after the process of 40 ar escape went on for some time in all sediments a lowering of the k ar apparent age of the different size fractions mainly in the smaller is possibly due to authigenesis of mixed layered illite smectite and illite as a new generation of particles that result from and are therefore better adapted to the new diagenetic environment in a neogene basin sediments of depositional age as an overall model we present a mechanism that explains the lowering of the 40 ar 40 k ratio and k ar apparent age in fine clay size fractions based on the production of 40 ar in a closed system for some period of time followed subsequently by the diffusional escape of 40 ar with its continuing production from 40 k in the particles this analysis gives 40 ar diffusion coefficients consistently in a range from 10 28 to 10 27 cm 2 s from the results in this paper the flux of radiogenic 40 ar from the sedimentary crust to the atmosphere is estimated as 5 to 13 10 6 mol year which is about 15 to 40 of the flux from the whole continental crust as reported by other investigators

a multi proxy paleolimnological reconstruction of holocene climate conditions in the great basin united states article i nfo article history received 11 february 2009 available online 14 august 2009 a sediment core spanning 7000 cal yr bp recovered from stella lake a small sub alpine lake located in great basin national park nevada was analyzed for subfossil chironomids non biting midges diatoms and organic content estimated by loss on ignition loi subfossil chironomid analysis indicates that stella lake was characterized by a warm middle holocene followed by a cool neoglacial period with the last two millennia characterized by a return to warmer conditions throughout the majority of the core the stella lake diatom community composition is dominated by small periphytic taxa which are suggestive of shallow cool alkaline oligotrophic waters with extensive seasonal ice cover a reconstruction of mean july air temperature mjat was developed by applying a midge based inference model for mjat two component wa pls consisting of 79 lakes and 54 midge taxa rjack 2 0 55 rmsep 0 9 c comparison of the chironomid inferred temperature record to existing regional paleoclimate reconstructions suggests that the midge inferred temperatures correspond well to regional patterns this multi proxy record provides valuable insight into regional holocene climate and environmental conditions by providing a quantitative reconstruction of peak holocene warmth and aquatic ecosystem response to these changes in the great basin a region projected to experience increased aridity and higher temperatures

hydrothermal and unexpected diagenetic alteration in permian shales of the lodeve epigenetic u deposit of southern france traced by k ar illite and k feldspar dating abstract mineralogical and chemical analyses combined with k ar dating of illite and k feldspars were carried out on bulk samples and different fractions from permian shales around the lodeve u deposit to identify diagenetic and hydrothermal alteration episodes and their timing authigenic 2m1 and 1m illite with k ar ages between 270 and 245 ma are the predominant mineral types in the shales they are of early diagenetic origin relative to deposition and formed apparently directly from pyroclastic materials in a lagoonal to deltaic environment under semi arid conditions albite and a first generation of k feldspar developed by a replacement of analcime the k feldspar growing only in the former lagoonal environment that favored also early diagenetic dolomitization at the expense of calcite the hydrothermally altered shales next to the u deposit contain authigenic quartz fe chlorite neoformed illite and a second generation k feldspar with k ar ages beginning at about 220 ma triassic although jurassic and cretaceous ages were also obtained this younger age spectrum jurassic cretaceous is close to previous u pb isotope data from the u deposit probably reflecting several episodes of u mobilization therefore the hydrothermal activity extended episodically from triassic to cretaceous time within the u deposit the hydrothermal temperatures attained locally 220 c as the k ar system of illite from early permian diagenesis is partially reset triassic hot fluids ascended along faults into the northern area of the lodeve basin during an initial rifting of the nearby western tethys ocean further fluid pulses were recorded at lodeve during the opening of the north atlantic and separation of europe from africa jurassic cretaceous which also caused the peak hydrothermal activities in central and western europe as the lodeve area is located between two independently developing oceans it could have been supplied with hot fluids for a long period of time

timing of alpine fault gouges k ar ages from clay rich fault gouges in the european alps are consistent internally with established field constraints and with fission track ages demonstrating the applicability of this method for direct dating of brittle deformation illite grown by retrograde hydration of granitic and or high grade metamorphic protoliths is the major k bearing mineral in the fractions that have been separated 0 1 to 6 10 m the ages obtained are bracketed by apatite and zircon fission ages from adjacent localities this together with the interpreted cooling curve for one well constrained sample from the simplon fault indicates that illite grew or the k ar isotopic system in illite closed at temperatures in the range 120 150 c consistent with the stability of illite alone rather than illite smectite average ages obtained for fractions finer than 2 m were 1 5 2 2 6 ma 2 for 11 analyses from the detachment and footwall of the simplon fault 2 8 8 0 6 ma for four analyses from the centovalli fault at trontano 3 19 9 2 6 ma for two analyses from a major riedel fault related to the periadriatic fault offsetting the tertiary bergell tonalite at gesero 4 21 5 5 5 ma for two analyses from a zone parallel to the periadriatic fault in the adjacent southern alps in val morobbia and 5 14 5 3 9 ma for two analyses from a gouge zone on the northern rim of the tertiary mauls tonalite in the eastern alps the ages from the simplon centovalli and periadriatic faults are not identical and do not lend support to models linking these three faults as a single coeval structure

chemostratigraphy k ar ages and illitization of silurian k bentonites from the central belt of the southern uplands down longford terrane british isles the central belt of the southern uplands terrane in both scotland and ireland is a faulted and imbricated sequence of caradoc to llandovery shales and llandovery turbidites with numerous interbedded k bentonites the bentonites are composed dominantly of r3 ordered mixedlayer illite smectite i s containing 90 95 illite and represent the product of illitization during low grade metamorphism k ar age determination on the m size fraction gave a range from 379 10 to 406 10 ma the fixed k is thought to have originated in the precursor ash and was remobilized during the transformation of smectite to i s k ar ages record a retrograde thermal event that post dates wenlock prehnite pumpellyite facies metamorphism and is contemporaneous with cooling and uplift during the end silurian early devonian collision of laurentia with the east avalonian terrane differences in rb and other trace elements between the k bentonite beds are due in part to differences in original ash composition and can be used to group the beds within biostratigraphically defined boundaries the chemical identification of groups of k bentonite beds offers additional criteria for their stratigraphical correlation on a regional scale

mesozoic hydrothermal impact on rotliegende and bunter immature sandstones of the high rhine trough and its adjacent eastern area southern black forest germany abstract the mesozoic hydrothermal and geodynamic evolution of a complex region of the european crust is studied here on immature and highly reactive rotliegende and bunter sandstones of the intramontane high rhine trough southern black forest and its adjacent eastern area bounded by the upper rhine graben and the alpine foreland microscopic observations outline varied degrees of illitization of detrital feldspar grains muscovite flakes and illite particles xrd analysis shows that the clay fractions are dominated by illite with minor illite smectite mixed layered minerals of r3 and r1 ordering the chemical composition of these minerals is controlled by precursor minerals since the illitic material inherited its chemical components from detrital muscovite and illite towards the centre of the trough the sandstones were more strongly affected by the hydrothermal impact showing extensive replacement of feldspar grains by al rich illite k ar dating of the authigenic illite type material points to multiple hydrothermal alteration episodes a late jurassic event at 147 ma seems to have affected extensively the study area especially towards the centre of the trough at the periphery of the trough relics of an early jurassic activity are likely preserved an additional early cretaceous hydrothermal event seems to have occurred locally near schopfheim and next to the rhine graben boundary fault long before its main rifting phase the 147 ma event was obtained also nearby in the previously investigated breisgau schramberg offenburg teinach and baden baden troughs and is characteristic for the black forest and some other areas outside this region it concludes obviously a period of regional hydrothermal activities between 160 and 145 ma whereas the early jurassic and cretaceous signatures are also known throughout central and western europe suggesting repeated fluid pulses the hydrothermal flows were controlled by reactivated basement faults due to periods of extensive reorientation of the tectonic stress during the break up of pangaea illitization was triggered by hot and low saline solutions of about 180 c and eq whereas a later fluid pulse of about 80 c and 20 wt nacl eq caused quartz overgrowth fluid flows delivered by the long lasting orogeny of the nearby alps were probably limited towards the black forest becoming part of the european margin of the tethys in the mesozoic the pre alp area evidently developed a separate fluid flow system

small scale faulting as an indicator of deformation mechanism in the tertiary sediments of the northern danish central trough abstract the tertiary sediments in the northern part of the danish central trough are characterized by a large number of small faults 25 50 m offset models for the accommodation of the deformation of the sediments are presented and it is shown quantitatively that a bending model best explains the observed strain distribution from this we infer that the internal deformation of the tertiary sediments is due to differential subsidence along older fault trends and not caused by applied external stresses on the tertiary sediments the effect of rock rheology and excess fluid pressure is discussed in order to explain the size of the throw and the smaller number of reverse faults compared to normal faults although the rocks are brittle they may in regions of shortening suffer more grainscale creep deformation than in regions of extension the strain distribution is determined on seismic sections and the main limitations of the method is thus the vertical and horizontal resolution of the seismic surveys used

paleoclimate simulations for north america over the past 21 000 years features of the simulated climate and comparisons with paleoenvironmental data maps of upper level and surface winds and of surface temperature and precipitation illustrate the results of a sequence of global paleoclimatic simulations spanning the past 21 000 years for north america we review a the large scale features of circulation temperature and precipitation that appear in the simulations from the ncar community climate model version 1 ccm 1 b the implications of the simulated climate for the past continental scale distributions of three plant taxa picea spp pseudotsuga menziesii and artemisia tridentata which are broadly representative of the vegetation across the continent and c the potential explanations in terms of atmospheric circulation or surface energy and water balance processes for mismatches between the simulations and observations most of the broad scale features of previous paleoclimatic simulations with the ncar ccm 0 for north america are present in the current simulations many of the elements of a conceptual model based on previous climate simulations that describes the controls of paleoclimatic variations across north america during the past 21 000 years are found in simulations reviewed here these include 1 displacement of the jet stream by the laurentide ice sheet to the south of its present position in both winter and summer 2 generation of a glacial anticyclone over the ice sheet at the lgm and the consequent induction of large scale sinking motions induced over eastern north america 3 changes in the strength of surface atmospheric circulation features through time including weakening of the aleutian low in winter and strengthening of the eastern pacific and bermuda high pressure systems in summer as the ice sheet decreased in size 4 development of a heat low at the surface and a strengthened ridge in the upper atmosphere over the continent at the time of the maximum summer insolation anomaly 5 increases in summer temperature earlier in regions remote from the ice sheet these increases appear earlier in the present ccm 1 simulations than in the previous ccm 0 ones however and 6 continuation of negative winter temperature anomalies into the middle holocene in general simulated surface conditions that are discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns e g those that suppress precipitation or advect warm air into a region with these mismatches amplified in beringia and the southeastern united states by surface energy and water balance processes

in situ stress from breakouts in the danish sector of the north sea abstract borehole breakouts from 26 wells situated in the danish central graben and one well situated in the northwestern section of the ringkobing fyn high have been studied the data provide information on the orientation of the contemporary principal horizontal stresses down to depths of 4 7 km statistics were used to obtain a mean azimuth of breakouts for each well and each individual breakout was weighted by its length data quality of breakouts is assigned according to the world stress map quality ranking scheme the breakout data reveal that s h orientations in the danish central graben are extremely scattered no regional trend in mean s h orientation can be identified as a result of the large standard deviation 77 similar scatterings of s h orientations have been reported from adjacent regions of the north sea the results indicate that the state of stress in the sedimentary cover of the danish central graben is influenced by a local stress generating mechanism rather than platewide stress transmissions mechanisms three units with different states of stress have been identified and found to correspond somewhat with the lithostratigraphic divisions of the post chalk group chalk group and pre chalk group

cretaceous early tertiary inversion tectonism in the danish central trough abstract the inversion of the danish central trough was heralded by a mild precursor during the late hauterivian before rifting had ended the main inversion took place during the late cretaceous it can be subdivided into several sub phases of which those occurring during the turonian santonian mid maastrichtian and post danian early tertiary are the most pronounced minor inversion tectonism occurred more or less continuously between these major events the turonian santonian event had the greatest impact in the northern part of the danish central trough whereas the effects of later events are most pronounced in the south throughout the late cretaceous danian a gradual change from more brittle deformation with faulting to more plastic deformation characterised by folding and flexuring can be seen

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

high frequency sequence stratigraphy of upper cenozoic deposits in the central and southeastern north sea areas abstract during late cenozoic times southwestward prograding deposits filled in the central and southern part of the epicontinental north sea basin the ringkobing fyn high acted as a topographical high partly separating the central and southeastern north sea into two embayments which were connected towards the west by a deeper seaway located in the central graben area the stratal geometry deduced from seismic sections indicates different basin physiography subsidence rates and sediment influx between the two embayments eustatic sea level cycles of several frequencies interacted with low subsidence rates in the southern embayment to generate composite relative sea level cycles each composed of relative sea level cycles of higher frequencies forty five upper cenozoic sequences are identified in the southern embayment whereas only 21 sequences are recognized in the northern embayment major hiati and intervals of starved sedimentation may therefore be present in the northern embayment the sequences stack into nine composite sequences each reflecting a depositional cycle representing lowstand succeeded by transgressive highstand conditions in general a composite sequence is characterized by a period of increasing sediment accumulation rates

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

radiometric age constraints on mineral growth metamorphism and tectonism of the gummfluh klippe brianconnais domain of the prealpes switzerland a combined 40ar 39ar k ar rb sr and stable isotope study has been made of white micas from the gummfluh klippe brianconnais domain of the prealpes switzerland the klippe consists mainly of mesozoic to early tertiary carbonate rocks metamorphosed from anchizonal to epizonal conditions at the base of the klippe is a 10 50 m thick ductilely deformed marble mylonite containing deformed authigenic quartz segregations stable isotope measurements of the coexisting calcite 18osmow 24 5 and quartz 18osmow 28 4 from the mylonite indicate relatively low temperatures 300 c during mylonitization analyses of white mica separates of varying size fractions from the mylonitic rocks by k ar and rb sr techniques yield ages between 57 and 103 ma this variation is correlated with two parameters the size of the mineral fraction and the proportion of 2m1 more phengitic to 1m more muscovitic polytype in the sample the k ar and rb sr ages are generally younger in the smaller size fractions which also containless 2m1 phengite high precision 40ar 39ar age spectra from different size fractions of these micas record three distinct components a small hercynian component ca 200 300 ma a significant eoalpine component 64 80 ma forming 40ar 39ar age plateaus and a very minor tertiary component ca 20 40 ma characterization of the samples by sem indicates the presence of two white mica populations a coarser grained deformed detrital mica that probably corresponds to the 2m1 phengite and a finer grained neoformed 1m mica collectively these observations suggest that the gummfluh samples contain a mixture of detrital phengites of hercynian age together with neocrystallized muscovites grown during the late eoalpine metamorphic event followed by minor argon loss during the tertiary the main geologic episode recorded in the 40ar 39ar age spectra of white micas in the mylonite is of late cretaceous early tertiary age 64 80 ma representing the first reliable eoalpine ages ever to be reported from the prealpes contrary to tectonic models the marble mylonite at the base of the gummfluh klippe appears to be a cretaceous thrust plane and not the thrust surface formed during transport of the klippe into its present position from the penninic alps during the tertiary the late cretaceous thrust developed during marine sedimentation at a depth of 800 m below the seafloor at temperatures of 280 c facilitated by warm fluids along the tectonic discontinuity

holocene fire and vegetation along environmental gradients in the northern rocky mountains abstract holocene records of fire vegetation and climate were reconstructed from four sites in the bitterroot range region of the northern rocky mountains in order to examine the vegetation and fire histories and evaluate the hypothesis proposed by whitlock and bartlein 1993 regarding the effects of increased summer insolation on precipitation patterns vegetation history in the series of sites was broadly similar in the late glacial period the pollen data suggest open parkland dominated by picea or alpine meadow which reflect conditions cooler and drier than present these open forests were replaced in the early to middle holocene by forests composed mainly of pinus and pseudotsuga which suggest conditions warmer than present modern forest compositions were in place by ca 3000 cal yr bp and small variations in the timing of the vegetation shifts reflect local differences among sites the long term trends in fire occurrence support the hypothesis proposed by whitlock and bartlein 1993 that precipitation regimes were sharpened during the early holocene summer insolation maximum but their location has remained unchanged as a result of topographic constraints sites located in areas currently summer dry were drier than present during the early holocene and fires were more frequent conversely sites located in the areas that are summer wet at present were wetter than present in the early holocene and fires were less frequent on millennial time scales it appears that the climate boundary is controlled by topography and does not shift

a 2000 yr reconstruction of air temperature in the great basin of the united states with specific reference to the medieval climatic anomaly abstract a sediment core representing the past two millennia was recovered from stella lake in the snake range of the central great basin in nevada the core was analyzed for sub fossil chironomids and sediment organic content a quantitative reconstruction of mean july air temperature mjat was developed using a regional training set and a chironomid based wa pls inference model r 2 jack 0 55 rmsep 0 9 c the chironomid based mjat reconstruction suggests that the interval between ad 900 and ad 1300 corresponding to the medieval climate anomaly mca was characterized by mjat elevated 1 0 c above the subsequent little ice age lia but likely not as warm as recent conditions comparison of the stella lake temperature reconstruction to previously published paleoclimate records from this region indicates that the temperature fluctuations inferred to have occurred at stella lake between ad 900 and ad 1300 correspond to regional records documenting hydroclimate variability during the mca interval the stella lake record provides evidence that elevated summer temperature contributed to the increased aridity that characterized the western united states during the mca

simulated changes in extreme temperature and precipitation events at 6 ka abstract we have employed a high resolution regional climate model rcm to test the sensitivity of extreme climate events to 6 ka orbital forcing using western north america as a case study calculated differences 6 ka control in annual extreme precipitation event frequency were mostly positive throughout the rcm domain as were differences in extreme event contribution to total annual precipitation these annual mean differences were driven by changes in the seasonal distribution of extreme precipitation events at 6 ka which was in turn linked to changes in the daily variability of upper level zonal wind speeds over the northeast pacific positive differences in the frequency of extreme maximum daily temperature values occurred inland in the rcm domain with peak differences of 28 days year over the great basin likewise days in which the maximum daily temperature exceeded 32 c were 24 more frequent in the 6 ka integration than in the control integration and heat waves lasted up to 12 days longer mean first and last freeze dates were generally earlier in the 6 ka integration along the pacific coast and over the southern great basin and later over the northern great basin and interior of the pacific northwest we propose that by exerting controls on paleoclimate archives unique from changes in the mean climate state such changes in the frequency and duration of extreme climate events could have played a substantial role in shaping the mid holocene paleoclimate record of the region and that extreme climate events may have been similarly important in other regions and during other time periods

a 7600 yr vegetation and fire history from anthony lake northeastern oregon usa with linkages to modern synoptic climate patterns we used pollen and high resolution charcoal analysis of lake sediment to reconstruct a 7600 yr vegetation and fire history from anthony lake located in the blue mountains of northeastern oregon from 7300 to 6300 cal yr bp the forest was composed primarily of populus and fire was common indicating warm dry conditions from 6300 to 3000 cal yr bp populus declined as pinus and picea increased in abundance and fire became less frequent suggesting a shift to cooler wetter conditions from 3000 cal yr bp to present modern day forests composed of pinus and abies developed and from 1650 cal yr bp to present fires increased we utilized the modern climate analogue approach to explain the potential synoptic climatological processes associated with regional fire the results indicate that years with high fire occurrence experience positive 500 mb height anomalies centered over the great basin with anomalous southerly component of flow delivering dry air into the region and with associated sinking motions to further suppress precipitation it is possible that such conditions became more common over the last 1650 cal yr bp supporting an increase in fire despite the shift to more mesic conditions

three dimensional consolidation of fine grained sediments current theories of one to three dimensional consolidation are based on the concept of gravitational loading with concomitant expulsion of pore fluid as a result of mechanical rearrangements of particles and porosity reduction however the recent discovery of extensive networks of layer bound fractures forming polygonal patterns in extremely fine grained smectitic clays and chalks suggests that current theory may not adequately explain the consolidation process in such sediments these faults develop during shallow burial and often appear to have been active at the sediment water interface radially isotropic strain suggests that the process of formation was nontectonic measured values of strain suggest that deformation resulted from volumetric contraction of the sediments we argue that the formation mechanism for these fault systems is related to the colloidal properties of the constituent sediments and that as such stresses due to gravity may not be the only factor controlling pore fluid expulsion

orbital suppression of wind driven upwelling in the california current at 6 ka 1 marine records clearly show activity in the california current cc system to have been altered during the mid holocene however the specific nature of cc activity during this period remains ambiguous as proxies that suggest cooler than present sea surface temperatures co occur in mid holocene marine sediments with multiple proxies that suggest decreased coastal upwelling to test the sensitivity of wind driven upwelling in the cc to mid holocene orbital forcing we have employed a high resolution regional climate model rcm the seasonality of modern cc wind driven coastal upwelling simulated by the rcm is in strong agreement with observational data further we show that changes in the seasonality of solar insolation induced by mid holocene milankovitch forcing decrease early and peak season coastal upwelling in the cc along with increasing late season coastal upwelling these results are in agreement with several marine and terrestrial proxies suggesting that relative to present cc activity during the mid holocene was characterized by a longer and less vigorous upwelling season with decreased seasonal contrast

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

investigating the sensitivity of the atmospheric general circulation model echam 3 to paleoclimatic boundary conditions we present atmospheric simulations of three different time slices of the late quaternary using the echam 3 general circulation model in t42 resolution in this work we describe the results of model runs for the time slices 6000 years bp last climate optimum 21 000 bp last glacial maximum and 115 000 years bp glacial inception although the solar insolation is known for all time slices a complete data set of the other boundary conditions which are necessary for running the atmospheric model exists only for the last glacial maximum in the form of the climap reconstruction for the other two time slices which are interglacial states like the modern climate sea surface temperatures land albedo and ice sheet topography are kept at modern values and only the solar insolation is changed appropriately the response of the model to solar insolation changes is quite reasonable the modelled anomalies are small and roughly opposite in sign for 6000 bp and 115 000 bp respectively in the case of last glacial maximum the glacial ice sheet topography and ice albedo produce a much larger climate anomaly in the model however to enable a real test of model performance under glacial boundary conditions the climap sea surface temperatures which are now known to be partly inaccurate should be replaced by an updated state of the art reconstruction

response of the north american monsoon to regional changes in ocean surface temperature 1 the north american monsoon nam an onshore wind shift occurring between july and september has evolved in character during the holocene largely due to changes in northern hemisphere insolation published paleoproxy and modeling studies suggest that prior to 8000 cal years bp the nam affected a broader region than today extending westward into the mojave desert of california holocene proxy sst records from the gulf of california goc and the adjacent pacific provide constraints for this changing nam climatology prior to 8000 cal years bp lower goc ssts would not have fueled northward surges of tropical moisture up the goc which presently contribute most of the monsoon precipitation to the western nam region during the early holocene the north pacific high was further north and ssts in the california current off baja california were warmer allowing monsoonal moisture flow from the subtropical pacific to take a more direct northwesterly trajectory into an expanded area of the southwestern u s west of 114 w a new upwelling record off southwest baja california reveals that enhanced upwelling in the california current beginning at 7500 cal year bp may have triggered a change in nam climatology focusing the geographic expression of nam in the southwest usa into its modern core region east of 114 w in arizona and new mexico holocene proxy precipitation records from the southwestern u s and northwestern mexico including lakes vegetation pollen and caves are reviewed and found to be largely supportive of this hypothesis of changing holocene nam climatology

kar systematics of clay to mica minerals in a multi stage low grade metamorphic evolution the behavior of the ar isotopic system of clay to mica fractions was studied in cambrian greywacke samples along a transect of northwestern morocco these rocks were affected by a progressively increasing metamorphic overprint spanning from diagenetic to epizonal metamorphic conditions k ar isotopic data of 0 2 10 m size fractions recovered from seven samples allowed to recognized three episodes of illite crystallization 1 during an upper ordovician thermal event at 445 ma which is characterized by an illite chlorite mineral paragenesis in the northernmost part of the transect 2 during a carboniferous low grade overprint at 305 ma which is characterized by a similar mineral paragenesis and 3 during a triassic to liassic hydrothermal activity at 205 ma which generated a smectite dominated paragenesis the ages were examined in combining x ray mineralogic and k ar isotopic data in terms of linear arrays in both isochron and harper diagrams to differentiate true isochrons from mixing lines it could also be shown that mixtures of two generations of micaceous minerals can induce different trends in the k ar dates depending whether the first generation of minerals was either or not affected by the event inducing the later generation

spatial variations of holocene climatic change in the yellowstone region abstract the paleoecologic record of the northern rocky mountains shows considerable spatial variability which has been difficult to explain as a unified response to large scale changes in effective moisture or temperature through time comparison of past and present climatic patterns suggests that the spatial heterogeneity in the fossil record arises from a local response to the effects of greater summer solar radiation during the early holocene solar radiation directly determines evapotranspiration and indirectly controls the strength of the summer monsoon and the eastern pacific subtropical high pressure system the yellowstone region provides an example of an area where the indirect effects of solar radiation are manifest by the juxtaposition of two contrasting climatic regimes today and where these regimes were amplified during the early holocene fossil pollen data suggest that central and southern yellowstone the summer dry area became drier in the early holocene while northern yellowstone the summer wet area became wetter the boundary between regimes however is controlled by topography and has not changed significantly in the holocene the spatial variations of holocene vegetation and climatic changes in the northern rocky mountains may therefore be explained as a trade off between strengthened monsoonal circulation and a stronger subtropical high in the early holocene and the relative importance of these precipitation regimes on a topographically complex landscape

thermal 40ar 39ar separation of diagenetic from detrital illitic clays in gulf coast shales the 40ar 39ar age spectra for illite rich mixed layered illite smectite i s from texas gulf coast bentonites contain only a diagenetic component and are characteristically staircase shaped this spectrum shape contrasts with patterns seen for anchizonal and epizonal grade bentonites that are flatter this may be due to the structural and compositional heterogeneity mixed layering typical of low grade clays especially with respect to k on the other hand shales from depths exceeding those of the smectite to illite transformation have both detrital and diagenetic components although diagenetic illite rich i s dominates shale fine fraction detrital mica is observed even in the 0 2 m size fraction age spectra of shales consist of two portions 1 a staircase shaped low temperature section corresponding to degassing of the diagenetic illite rich i s component and 2 a peak in the high temperature section corresponding to degassing of well crystallized detrital mica the data obtained from 40ar 39ar stepwise analysis of these mixtures of diagenetic and detrital components permit at least partial resolution of the diagenetic from the detrital ages we have developed techniques for quantifying the separate diagenetic and detrital components based on subtraction of the detrital contribution from the age spectrum of mixture giving rise to an age corrected for the detrital contamination the resulting corrected 40ar 39ar ages are closer to the diagenetic ages this is in sharp contrast to the old k ar ages of the same samples although still in the developmental state these techniques appear to represent a solution to the problem of contamination of diagenetic illitic clays by detrital illitic phyllosilicates

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

climatic influences on holocene variations in soil erosion rates on a small hill in the mojave desert a detailed study of a small hill in ne mojave desert in eastern california was conducted to elucidate the effect of climate on the variations in soil erosion rates through holocene field surveys and sampling were carried out to obtain information on topography geomorphology soil and vegetation conditions seismic refraction sediment deposition and hillslope processes integration of this information allowed reconstruction of the hill topography at the end of the pleistocene deduction of the evolution of the hill from the end of the pleistocene to the present and estimation of total soil losses resulting from various hillslope processes the estimates are consistent with the premise that early holocene climate change resulted in vegetation change soil destabilization and topographic roughening current very slow hillslope transport rates e g 5 mm ky 1 by rodent burrowing a presently important transport form appear inconsistent with the inferred total soil loss rate 31 mm ky 1 packrat midden studies imply that the ne mojave desert experienced enhanced monsoonal precipitation in the early holocene presumably accentuating soil loss water erosion on one slope of the hill was simulated using water erosion prediction project wepp a process based erosion model using 4 and 6 ky of precipitation input compatible with an appropriate monsoonal climate and the present climate respectively the wepp predicted soil losses for the chosen slope were compatible with inferred soil losses identification of two time periods within the holocene with distinct erosion characteristics may provide new insight into the current state of mojave desert landform evolution

provenance and variscan low grade regional metamorphism recorded in slates from the basement of the sw hungary a metapelitic sequence with silurian protolith from the horvathertelend unit tisza mega unit hungary has a k white mica chlorite quartz albite anatase tourmaline mineral assemblage moderately developed disjunctive and well developed continuous foliations are present geochemical results reflect a dominantly felsic source of the protoliths suggesting an intermediate to acidic volcanic arc in the provenance area metasandstone pebbles in the metaconglomerate indicate a recycled sedimentary source raman spectroscopy of carbonaceous material indicates a 350 370 c peak metamorphic temperature the kubler index kibasel values of the phengitic k white mica indicate epizonal metamorphism 0 22 0 04 2 the chlorite crystallinity index chccis suggests metamorphic alteration near to the anchizone epizone boundary 0 31 0 06 2 k ar ages of k white mica are interpreted as a result of variscan metamorphism c 310 ma and post variscan uplift c 290 ma the predominance of lydite and slate of llandoverian age and the overlying coarse grained metagreywacke and metaconglomerate beds of the szalatnak slate formation show strong lithological similarities with the proximal silurian sequences in the malopolska massif kielce region holy cross mountains poland the original position of the horvathertelend unit is presumably to the northeast from the bohemian massif next to the upper silesian block moravo silesian zone and the malopolska terrane

using fire regimes to delineate zones in a high resolution lake sediment record from the western united states article i nfo article history received 20 february 2012 available online 10 november 2012 paleoenvironmental reconstructions are important for understanding the influence of long term climate variabil ity on ecosystems and landscape disturbance dynamics in this paper we explore the linkages among past climate vegetation and fire regimes using a high resolution pollen and charcoal reconstruction from morris pond located on the markagunt plateau in southwestern utah usa a regime shift detection algorithm was applied to back ground charcoal accumulation to define where statistically significant shifts in fire regimes occurred the early holocene was characterized by greater amounts of summer precipitation and less winter precipitation than mod ern ampleforest fuel and warm summer temperatures allowed for large firestooccur themiddleholocenewasa transitional period between vegetation conditions and fire disturbance the late holocene climate is characterized as cool and wet reflecting an increase in snow cover which reduced opportunities for fire despite increased avail ability of fuels similarities between modern forest fuel availability and those of the early holocene suggest that warmer summers projected for the 21st century may yield substantial increases in the recurrence and ecological impacts of fire when compared to the fire regime of the last millennium

late quaternary vegetation and climate history near little lake central coast range oregon pollen and plant macrofossils recovered from little lake oregon provide the first record of late quaternary climate and vegetation changes in oregon9s central coast range the pollen data suggest that the period from ca 42 000 to 24 770 yr b p featured an open forest of western white pine western hemlock and fir the inferred climate was cooler and wetter than today the full glacial period was characterized by a parkland of spruce lodgepole pine and mountain hemlock which suggests that the climate was colder and possibly drier than today the oregon coast range was not a glacial refugium for temperate conifers as has been proposed by 16 000 yr b p the pollen and plant macrofossil data suggest the development of a montane forest composed of pine fir western hemlock and mountain hemlock temperate taxa were present in the vegetation at 13 500 yr b p during a period of climatic amelioration a reversal in this warming trend may have occurred between 11 000 and 10 500 yr b p when pine western and mountain hemlock and spruce were slightly more abundant early holocene forests featured douglas fir red alder and bracken fern implying more severe summer drought and more frequent fires after 5600 yr b p douglas fir western hemlock and western red cedar dominated the record marking the introduction of the present day cool moist climate however during the past 2800 yr increased douglas fir and decreased cedar suggest the possibility of reduced effective moisture at little lake

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

anatomy of great plains protracted heat waves especially the 1980 u s summer drought abstract the protracted heat wave and drought of the great plains during summer 1980 was a manifestation of an abnormal form of the general circulation an upper level continental high developed rapidly over the southern plains in late may and persisted with only small changes throughout the summer the associated subsidence dry soil lack of cloud and high insolation led to unrelenting heat descriptions of several interlocking features are given followed by an attempt to ascribe the development and maintenance of the drought producing cell to several physical factors strong low latitude westerlies expanded circumpolar vortex over the north pacific north america and north atlantic during spring 1980 led to latitudinally depressed storm tracks relative to normal combined with normal seasonal forcing this anomalous pattern was stable in spring but unstable in summer after the climatologically dependable northward march of the westerlies moreover normal spring to summer mid tropospheric height chang

les deformations ordoviciennes dans la zone des sehoul maroc septentrional une orogenese caledonienne en afrique du nord the sehoul zone of northern morocco differs from the remainder of the hercynides in having the structural signature of an earlier event of deformation folds and thrusts overturned to the south aff

late paleozoic mesozoic tectonic evolution of the trans altai and south gobi zones in southern mongolia based on structural and geochronological data abstract the trans altai zone in southern mongolia is characterized by thrusting of greenschist facies silurian oceanic rocks over devonian and lower carboniferous volcano sedimentary sequences by e w directed folding affecting the early carboniferous volcanic rocks and by the development of n s trending magmatic fabrics in the devonian carboniferous arc plutons this structural pattern is interpreted as the result of early carboniferous thick skinned e w directed nappe stacking of oceanic crust associated with syn compressional emplacement of a magmatic arc the southernmost south gobi zone represents a proterozoic continental domain affected by shallow crustal greenschist facies detachments of ordovician and devonian cover sequences from the proterozoic substratum whereas supracrustal carboniferous volcanic rocks and permian sediments were folded into n s upright folds this structural pattern implies e w directed thin skinned tectonics operating from the late carboniferous to the permian as demonstrated by k ar ages ranging from 320 ma to 257 ma for clay fractions separated from a variety of rock types moreover the geographical distribution of granitoids combined with their geochemistry and shrimp u pb zircon ages form distinct groups of carboniferous and permian age that record typical processes of magma generation and increase in crustal thickness the field observations combined with clay ages the geochemical characteristics of the granitoids and their ages imply that the e w trending zone affected by tectonism migrated southwards leaving the trans altai zone inactive during the late carboniferous and permian suggesting that the two units were tectonically amalgamated along a major e w trending strike slip fault zone this event was related to late carboniferous subduction that was responsible for the vast volume of granitoid magma emplaced at 300 305 ma in the south gobi and at 307 308 ma in the trans altai zones the formation and growth of the crust was initially due only to subduction and accretion processes during the post collisional period from 305 to 290 ma the addition of heat to the crust led to the generation of per alkaline melts once amalgamated these two different crustal domains were affected by n s compression during the triassic and early jurassic 185 173 ma resulting in e w refolding of early thrusts and folds and major shortening of both tectonic zones

use and abuse of seismic data in reservoir characterisation all seismic data contain a mixture of signal and noise in detailed reservoir characterisation it is commonly difficult to distinguish between real features and seismic artefacts this is especially a problem when interpreting seismic attribute maps such maps are widely used tools during reservoir description but serious pitfalls exist which may lead to erroneous interpretations and fatal development plans for oil fields a recent interpretation of seismic attribute maps from a seismic survey collected across gullveig an oil field located in the northern north sea has been used to illustrate how small faults can be recognised and mapped from such maps we applied available well data including core data and two seismic surveys from the same area and present convincing evidence that the vast majority of linear features seen on the seismic attribute maps are in fact seismic artefacts and not faults the data from gullveig are furthermore supplemented by observations from the nearby gullfaks field and discussions on the topic of seismic noise we use these observations and discussions to stress the importance of using all available data to guide and quality control the structural interpretation of attribute map features before utilising such interpretations as input to reservoir modelling or well planning

40ar 39ar muscovite dating of thrust activity a case study from the axial zone of the pyrenees abstract the pyrenees belt features well exposed deformation structures that make it ideally suited to study thrust faults in this paper we present the first 40ar 39ar age of the pic de port vieux thrust ppvt located in the south western part of the pyrenean axial zone the ppvt is a secondary thrust related to the major gavarnie thrust gt with a footwall comprising upper cretaceous limestones and a hanging wall made of lower triassic pelites and sandstones a vertical transect in the fault hanging wall was investigated to characterize the deformation mechanism and to date the thrust activity by using 40ar 39ar step heating technique on muscovite petrographic observations confirm that pressure solution and recrystallization dissolution processes in presence of fluids are the main mechanisms that controlled the chemical textural changes in the fault core zone three groups of muscovite are identified in the fault zone i e detrital diagenetic and newly formed muscovites the detrital and diagenetic muscovites are mainly present in the damage zone while the newly formed muscovite is abundant in the core zone and is synkinematic to the fault activity the fractions 50 16 m 16 2 m 2 0 5 m

cenozoic differential subsidence of north sea the structural evolution of the central north sea has been dominated by subsidence throughout the last 65 m y which occurred at different rates in various segments of the basin the bulk of the tertiary sediments accumulated in a relatively narrow elongate basin in the central north sea the base of the tertiary today is deeper than 3 250 m in the axis of the central graben the overall shape of the basin is well defined by contours decreasing in depth northward to the norwegian sea and landward in all other directions different rates of subsidence of the various subbasins are evident by comparison of isopach maps during the paleocene and eocene the main depocenter was located in the northwest part of the north sea the viking graben whereas the oligocene and miocene depocenters are observed to have been in the southern part of the central graben in addition two ancillary depocenters have been identified for other intervals of the tertiary the moray firth and the central jutland portion of the danish subbasin

the relation between grain size and 40ar 39ar date for alpine white mica from the siviez mischabel nappe switzerland abstract variations in alpine white mica 40 ar 39 ar dates from the cover units of the siviez mischabel nappe relate to regional variations in the thermal history of the nappe we focus on three regions within the nappe the central siviez mischabel csm the southern siviez mischabel ssm and the eastern siviez mischabel esm our approach weaves together observations of quartz and mica textures in thin section the variation of 40 ar 39 ar date with grain size considerations of the effective diffusion dimension edd of argon in white mica and a comparison of dates with diffusion model results in the csm pressure solution of quartz and dislocation glide in mica accommodated alpine deformation dates record mica growth during nappe emplacement from 40 to 36 ma and do not vary with grain size in the ssm and esm both mica and quartz show textures associated with dynamic recrystallization and dates decrease with grain size in the ssm dates also agree with the timing of nappe emplacement but in the esm dates significantly post date the timing of nappe emplacement a comparison of dates with diffusion model results supports inferences from rock fabrics that the ssm experienced higher peak temperatures than the csm even though dates from both units approximate the timing of mica growth dates obtained from the esm however do not compare well with simple models and the thermal evolution of this region of the nappe in the neighborhood of the simplon fault zone is not well understood

disequilibrium compaction as the cause for cretaceous paleogene overpressures in the danish north sea cretaceous paleogene overpressure distribution in the danish central graben shows a remarkable coincidence with the thickness of the rapidly deposited middle miocene to holocene succession slow deposition of smectite dominated clays in a deep marine environment occurred from the late paleocene until the middle miocene and the resultant mudstone succession constitutes the main barrier that delays pressure dissipation between the late miocene and the holocene the upper cretaceous paleogene succession became overpressured probably because of accelerated depositional rates quantification of this disequilibrium compaction mechanism relies mainly on a determination of permeability and effective compressibility of the paleogene shales this article shows that realistic permeabilities can be assumed provided that compressibilities describing the plastic process of compaction are used in the pressure equation instead of the elastic compressibilites that for instance can be derived from log data one dimensional 1 d modeling is applied in two cases a well from the dan chalk field where accelerated deposition since the tortonian 11 2 ma produced a present day overpressure of 7 97 mpa 1156 psi and a well from the south arne chalk field where accelerated deposition since the early serravallian 14 6 ma produced a present day overpressure of 13 9 mpa 2016 psi this is based on an identical set of parameters and compares with the observed 7 7 and 14 8 mpa 1117 and 2147 psi overpressure at the two locations the modeled development of the pressure profiles shows that an effective stress minimum occurred in the upper part of the paleogene succession this is consistent with the observed ubiquitous intraformational faulting at that level about 80 of the added neogene load is estimated to have been converted to overpressure

episodic basin wide fluid expulsion from geopressured shale sequences in the north sea basin a proposed general model for the episodic dewatering of thick shale successions is based on the recognition of a pervasive polygonal extensional fault network developed in the dominantly fine grained lower tertiary of the north sea basin seismic data show that the faults are arranged in stratigraphically bound structural units tiers that are delimited vertically by almost undeformed condensed sections and are restricted in distribution to the lowest permeability slope and basin floor facies i propose an episodic three stage mechanism to explain the fault genesis involving 1 the development of basin wide overpressured compartments 2 a density inversion between the overpres sured units and the overlying seal and 3 natural hydraulic fracturing pressure bleed off and resealing of the pressure compartment leading to a repeat of the cycle

episodic basin wide hydrofracturing of overpressured early cenozoic mudrock sequences in the north sea basin a deformational fabric associated with the early cenozoic mudrock dominated sequences of the north sea is described seismic data show that the mudrock sequences are pervasively faulted over a large area of the basin axis in the central and northern north sea mapping the faults using regional two and three dimensional data shows that the faults are minor extensional structures arranged in stratigraphically bound tiers in the deep water sequences and bounded by regional condensed sequences seals the geometry of the faulting is complex faults are organized into cellular networks comprising polygonal prismatic and pyramidal forms a gravitational mechanism of downslope collapse faulting is rejected on the grounds that the distribution of the deformed sequences extends over large areas of formerly horizontal basin plain a mechanism involving the repeated development of basin wide overpressured compartments is proposed in which the cellular structure is related to deformation patterns associated with a density inversion between the overpressured units and the overlying seal hydrofracturing is invoked to account for the almost random strike distribution of the faulting the region affected by the pervasive fluid expulsion faulting encompasses almost half of the north sea basin basin wide collapse of the overpressured compartments could have resulted in a geologically rapid flux of enormous volumes of pore fluids this mechanism has implications for concepts of steady state compaction heat flow in basins hydrocarbon transport and the genesis of lead zinc ore deposits

spatial variability of late quaternary paleoclimates in the western united states abstract paleoclimatic interpretation of proxy data is complicated sometimes by the appearance of heterogeneous patterns of climatic responses across networks of sites modern climate analogues for the western united states similar to those patterns of atmospheric circulation of 18 000 and 9000 yr b p were examined in order to explain such patterns of spatial heterogeneity modern analogues were defined by comparing modern atmospheric circulation patterns with those simulated by general circulation models maps of temperature and precipitation anomalies of the modern analogues reveal patterns of spatial heterogeneity which resemble the patterns of effective moisture compiled from paleoclimatic data january 1957 was found to be a reasonable 18 000 yr b p analogue and it features isolated areas of increased wetness in the northern great basin and increased dryness in the northwest interior analogues for 9000 yr b p from a composite of 11 augusts display patterns of spatial heterogeneity of effective moisture over most of the mountainous areas the analogues suggest that spatial heterogeneity of climate is the rule rather than the exception over much of the western united states with the climatic anomalies at any particular time representing the outcome of the mediation of large scale atmospheric circulation controls by smaller scale topographic features

geochemical and k ar isotopic behaviour of alpine sheet silicates during polyphased deformation abstract several deformational events with several generations of phengites occur in the schistes lustres from the northern cottic alps the phengites belong mainly to 1 1 old forms from a s 1 schistosity 2 2 deformed forms from s 2 cleavage tightening zones 3 3 neoformed forms from second generation stretching zones the si tetrahedral substitutions of these minerals which lie between 3 28 and 3 46 depend on their microstructural site this chemical variation cannot only be related to p t conditions it is also induced by deformation related solution deposition processes the k ar apparent ages of the phengites are widely scattered between 38 and 65 ma the data when plotted in an isochron diagram suggest a two step response at 58 54 ma and 50 49 ma of the phengites to deformation this could happened during the same event which produced the second generation of microstructures