40 r139 r and k ar age constraints on shear zone evolution southern taltson magmatic zone northeastern alberta1 new 40ar 39ar analyses of hornblende muscovite biotite and k feldspar constrain the timing of deformation and cooling of the southern taltson magmatic zone which underwent lower granulite to upper amphibolite grade deformation in part synchronous with voluminous 1 99 1 92 ga magmatism new data are combined with existing k ar dates into a regional cooling framework to provide thermotemporal constraints on the deformational history 40ar 39ar hornblende ages of ca 1900 ma are interpreted to record relatively rapid cooling following ductile thrusting on the andrew lake shear zone and younger anatectic magmatism these data with published k ar and u pb data support relatively rapid cooling of the taltson magmatic zone from monazite closure temperature of 725 c at ca 1930 ma to 525 c at ca 1900 ma cooling rate estimates are about 7 c ma which suggests moderate exhumation rates during the high grade part of the deformational history a muscovite 40 r 39ar plateau age of 1803 f 11 ma is consistent with the mean muscovite k ar age of 1792 ma indicating regional cooling through about 350 c at ca 1800 ma 40ar 39ar ages from magmatic biotite of 1856 and 1799 ma also suggest slow cooling during greenschist grade deformation which can be no older than ca 1860 ma a k feldspar 40ar 39ar age of 1681 ma provides a lower limit for the time of greenschist grade deformation cooling rate estimates during amphibolite to greenschist grade deformation are 1 75 2 25 cima

geochronological constrains on the timing of magmatism deformation and mineralization at the karouni orogenic gold deposit guyana south america abstract the karouni deposit is located in central guyana 35 km to the w of the former producing 5 moz omai mine and is interpreted to be an orogenic gold deposit it is hosted in rhyacian aged 2 2 to 2 1 ga volcano sedimentary and intrusive rocks metamorphosed to greenschist facies detailed structural and crosscutting relationships coupled with robust new u pb geochronology on intermediate to felsic intrusive rocks and hydrothermal minerals from the karouni camp have constrained the structural evolution of the deposit and provided a new age for gold mineralization with important implications for the structural framework and timing of gold mineralization throughout the region magmatism within the 200 km2 of the karouni camp progressed from early synvolcanic intermediate composition dikes at 2147 ma to a major voluminous pulse of felsic magmatism coeval with regional deformation during the trans amazonian orogeny between 2120 and 2088 ma a late pulse of granitic magmatism at 1950 ma may be related to a post orogenic event inherited zircons within the felsic intrusive rocks indicate assimilation of crust as old as 2223 32 ma gold mineralization associated with camp scale d2 deformation has been dated to 2084 15 ma on the basis of u pb geochronology of hydrothermal titanite these results show gold mineralization is coeval with felsic magmatism and contemporaneous with a major pulse of orogenic gold mineralization in the birimian province of west africa

first early permian paleomagnetic pole for the yili block and its implications for late paleozoic postorogenic kinematic evolution of the sw central asian orogenic belt we conducted a paleomagnetic study on the early permian volcanic and sedimentary rocks and the neoproterozoic mafic dikes in the yili block nw china magnetite and hematite were proven to be the principal magnetic remanence carriers demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity the magnetic remanence of only the early permian strata turned out to be primary based on positive fold tests meanwhile the magnetic remanence age of the mafic dikes is ambiguous accordingly the first early permian paleomagnetic pole for the yili block is calculated at 81 5 n 256 5 e n 11 and a95 10 9 comparisons of this new pole with published ones from the yili tarim and south junggar blocks provide new quantitative constraints on late paleozoic kinematic evolution of the sw central asian orogenic belt 1 between the yili and tarim blocks significant relative movement took place along major strike slip faults during the late carboniferous to early permian 580 290 km and the early to late permian 585 340 km and the displacement rate increased from the late carboniferous to early permian 19 3 9 7 mm yr to the early to late permian 29 3 17 0 mm yr 2 a significant relative rotation of 28 3 18 3 in the late permian and a lateral displacement of 630 295 km after the late permian occurred between the yili and south junggar blocks the significant strike slip movements played an important role in the formation of the central asian orogenic belt and should be considered with great attention in tectonic and paleogeographic reconstructions

lake michigan water levels and the development of holocene beach ridge complexes at two rivers wisconsin stratigraphic geomorphic and radiocarbon evidence two beach ridge complexes formed at two rivers wisconsin on the west shore of lake michigan during the past 5700 yr auger drilling closely spaced vibracoring and subsurface geophysical profiling using ground penetrating radar along two east west transects across ridges and swales indicate that several facies of beach sediment are capped with dune sand 14c ages were obtained from basal peat deposits collected from swales between successive beach ridges associated lake levels were interpreted from the elevation of coarse foreshore deposits observed on the east basinward side of the next lakeward beach ridge 14c ages geomorphic relationships and stratigraphic interpretations indicate that progradation and water level drop followed a major transgression lake level fluctuated on a scale of 1 7 2 m during the past 3000 yr

40ar 39ar thermochronology of the southern gawler craton australia implications for mesoproterozoic and neoproterozoic tectonics of east gondwana and rodinia 40ar 39ar thermochronology of the southern gawler craton australia reveals a protracted history of slow cooling with local crustal heating rapid cooling and fault reactivation between 1700 and 700 ma hornblendes from most of the southern part of the craton including the kalinjala mylonite zone yield ages 1580 1610 ma with younger more discordant samples showing evidence of retrogression these results indicate that the region cooled below 500 c 100 m y after the kimban orogeny and coincident with the gawler range hiltaba suite magmatism to the north perhaps associated with postorogenic extension biotite and k feldspar data indicate that regional cooling after this time was more gradual with a few exceptions locally in the southeastern eyre peninsula rapid cooling at 1425 ma is indicated by nearly concordant hornblende and biotite ages and may record a phase of regional extension other exceptions include areas of lower grade archean rocks that cooled to 350 c by 1700 1640 ma greenschist facies reactivation of shear zones occurred at 1100 ma during the musgravian orogeny on the basis of biotite single grain data and retrogressed amphiboles k feldspar age spectra record cooling at 700 800 ma following regional intrusion of the gairdner dike swarm and or denudation during rifting along the pacific margin of east gondwana and breakup of rodinia tectonothermal events revealed by these results previous geochronological and geological studies and available paleomagnetic data are comparable to other cratons and mobile belts in australia east gondwana and laurentia providing evidence that southeastern australia and north america were adjacent during much of proterozoic time

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

magnetic fabric and 3d geometry of the hombreiro sta eulaliapluton implications for the variscan structures of eastern galicia nw spain abstract in this paper the internal structure and 3d geometry of the hombreiro sta eulalia granitic pluton hercynian belt nwspain are described characterization of the internal structure was performed from surface data based on microstructural analysis and anisotropy of magnetic susceptibility studies the geometrical features of the pluton morphology at depth were derived from gravity data the pluton is made up of paramagnetic granites and leucogranites k values ranging between 1 1 and 13 3 10 5 si with well developed magnetic fabrics anisotropy degree p 1 05 for 78 of the sampling localities the results obtained enable us to depict a flat lying magnetic structure with associated n120 e trending magnetic lineations the gravity survey corroborates the subhorizontal laminar geometry of the pluton which has an estimated thickness of 2 km and an almost planar floor the 3d morphology is elongate at depth parallel to the linear internal fabric this would imply that magma emplacement took place along this preferential direction whereas the deepest zones of the pluton trend wnw ese elongate parallel to regional 1 it is almost rootless we present data showing that the hombreiro sta eulalia pluton emplacement was closely related spatially and chronologically along with other granitoid plutons from the lugo dome with two major structures of eastern galicia the basal shear zone of the mondonedo nappe and the extensional vivero fault

how rough sea affects marine seismic data and deghosting procedures most seismic processing algorithms generally consider the sea surface as a flat reflector however acquisition of marine seismic data often takes place in weather conditions where this approximation is inaccurate the distortion in the seismic wavelet introduced by the rough sea may influence for example deghosting results as deghosting operators are typically recursive and sensitive to the changes in the seismic signal in this paper we study the effect of sea surface roughness on conventional 5 160 hz and ultra high resolution 200 3500 hz single component towed streamer data to this end we numerically simulate reflections from a rough sea surface using the kirchhoff approximation our modelling demonstrates that for conventional seismic frequency band sea roughness can distort results of standard one dimensional and two dimensional deterministic deghosting to mitigate this effect we introduce regularisation and optimisation based on the minimum energy criterion and show that this improves the processing output significantly analysis of ultra high resolution field data in conjunction with modelling shows that even relatively calm sea state i e 15 cm wave height introduces significant changes in the seismic signal for ultra high frequency band these changes in amplitude and arrival time may degrade the results of deghosting using the field dataset we show how the minimum energy optimisation of deghosting parameters improves the processing result

40ar 39ar ages for the alkaline volcanism and the basement of gorringe bank north atlantic ocean abstract gorringe bank is situated on the europe africa plate boundary at the eastern end of the azores gibraltar fracture zone it has two summits gettysburg bank to the southwest and ormonde bank to the northeast we applied the 40 ar 39 ar stepwise heating method to date six samples of the alkaline volcanic rocks two gabbros from the ormonde bank and a dolerite from the gettysburg bank the results that the alkaline volcanism lasted probably for less than 6 ma 66 60 ma although the nature of this volcanism precludes any subduction feature during its setting the alkaline volcanism of ormonde is probably linked to upper cretaceous eocene compressive tectonic events the basement rocks of gorringe bank reveal distrubed 40 ar 39 ar age spectra one plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 ma and related to a variation in the direction of the relative movement between europe and africa the more probable age given by a plagioclase of another gabbro and by a dolerite 110 ma corresponds to tilting northeastward of the gorringe massif

p t time characterization of the trans amazonian orogeny in the imataca complex venezuela metaigneous and metasedimentary rocks from the imataca complex of venezuela record high temperature decompression at granulite facies peak metamorphism preservation of convincing reaction assemblages in many samples allows determination of relative timing of development of observed assemblages sillimanite pseudomorphs after andalusite and absence of kyanite requires a high temperature pressure t p prograde geothermal gradient two pyroxene thermometry and garnet orthopyroxene plagioclase quartz assemblages yield peak t and p in the range of 750 800 c and 8 8 5 kbar one sample of garnet bearing meta norite contains orthopyroxene with approximately 6 al2o3 requiring either higher peak pressures or disequilibrium growth of orthopyroxene from garnet breakdown of garnet sillimanite to hercynite quartz and of garnet sillimanite quartz to cordierite require decompression at high temperature mineral compositions in these assemblages indicate p near 6 kbar for t in the range of 500 700 c coexisting orthopyroxene clinopyroxene plagioclase quartz apparently react to form abundant hornblende and strongly zoned plagioclase with an70 cores and an35 margins garnet is also compositionally zoned in some samples finally alkali feldspar and plagioclase coexist in some samples without perthitic intergrowths and these retrograded feldspars yield temperatures near 550 c rhombohedral oxides are extensively and complexly exsolved in some samples exsolution products include ilmenite hercynite and corundum in other samples exsolution of oxides has apparently been complete and only homogeneous discrete grains of rhombohedral and cubic oxides exist these completely exsolved oxides yield apparent retrograde temperatures below 600 c these observations indicate that the imataca complex reached granulite facies metamorphic conditions along a high t p gradient followed closely by tectonic loading subsequent decompression and slow cooling allowed significant progress along several retrograde reactions as well as equilibration of feldspar compositions and extensive exsolution in oxide phases the observed compositional zoning in garnet and plagioclase developed during retrograde reactions a one dimensional heat flow model constrained by rb sr and 40ar 39ar cooling ages suggests the imataca complex was uplifted from 32 to 16 km at a rate of 0 5 1 km ma1 after which isobaric cooling ensued the p t path given by this model is consistent with the retrograde assemblages observed

soil development rates in the transantarctic mountains soil chronosequences were identified at three sites 78 85 s within the transantarctic mountains for determining rates of soil development in cold desert ecosystems more than 175 soils were sampled in xerous and ultraxerous environments on moraines containing sandstone and dolerite and ranging from late holocene ca 6 ka to pre late quaternary ca 250 ka in age based on five linear and nonlinear models highly significant correlations p 0 01 exist between drift age and the following surface boulder weathering features and soil properties percentage of boulders containing desert varnish spalling and pitting depths of staining visible salts ghosts pseudomorphs and coherence in soils electroconductivity in the zone of salt enrichment and total profile salts in the absence of leaching soluble salts readily accumulate in cold desert soils a morphogenetic sequence of soluble salts is presented that relates field observations with laboratory data soil color is highly correlated p 0 01 with dithionite extractable fe within a site the proportion of clasts 15 mm on the surface of the desert pavement increases with time due likely to deflation of fines by strong winds therefore the dominant processes in these cold desert soils are salinization rubification reddening due to oxidation of iron bearing minerals and desert pavement formation

ams radiocarbon dating of lake michigan beach ridge and dune development abstract strandplains of shore parallel beach ridges bordering the great lakes are valuable for reconstructing histories of climate related lake level fluctuations however imprecise radiocarbon dates of ridge formation have frustrated development of dependable chronologies from which information about variation in the frequency of ridge formation and inferred climate fluctuations can be obtained the resolution and precision of radiocarbon chronologies can be improved with ams14c dates of roots and rhizomes of plant species associated with the formation and growth of the sand dune caps of breach ridges these dates reliably estimate the timing of shore progradation when the base of the previously established beach ridge becomes inundated by the water table an ams radiocarbon chronology of beach ridge formation in northern lake michigan shows that information about variation in the frequency of ridge formation is important for paleoclimatic interpretation

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

rough seas and time lapse seismic time lapse seismic surveying has become an accepted tool for reservoir monitoring applications thus placing a high premium on data repeatability one factor affecting data repeatability is the influence of the rough sea surface on the ghost reflection and the resulting seismic wavelets of the sources and receivers during data analysis the sea surface is normally assumed to be stationary and indeed to be flat the non flatness of the sea surface introduces amplitude and phase perturbations to the source and receiver responses and these can affect the time lapse image we simulated the influence of rough sea surfaces on seismic data acquisition for a typical seismic line with a 48 fold stack a 2 m significant wave height sea introduces rms errors of about 5 10 into the stacked data this level of error is probably not important for structural imaging but could be significant for time lapse surveying when the expected difference anomaly is small the errors are distributed differently for sources and receivers because of the different ways they are towed furthermore the source wavelet is determined by the sea shape at the moment the shot is fired whereas the receiver wavelet is time varying because the sea moves significantly during the seismic record

genetic interpretations of elemental and chemical differences in a soil chronosequence california abstract soils developed on fluvial terraces in central california have similar parent materials climatic settings vegetation cover and slopes but range in age from 40 000 to 3 000 000 years the soils have chemical compositions that change systematically with increasing age such chemical differentiation is most likely the result of long term weathering and mineralogical transformations that occurred since deposition of terrace fills and stabilization of the geomorphic surfaces the changes in composition with time closely mimic other studies on mineral weathering in which alkali and alkali earth elements are lost more rapidly than transitional elements the relative rates of element loss were determined by changes in element ratios over time net losses and gains of elements in different size fractions were monitored by their concentrations relative to zr the most stable constituent both sand and finer size fractions have lost considerable amounts of ca mg na and k aluminum appears to have been lost from the sand fraction and gained in the fine fraction over a 3 million year time span although there is no evidence for losses of fe and ti from sands there is a net influx of fe and ti into finer fractions probably gained from undetectable yet significant weathering of sand grains etching of sand grains clay mineralogy and microprobe analyses also indicate that the soils have undergone these chemical transformations during their formation mineralogical analyses also mimic other studies on mineral weathering in which the pyroxenes weather more rapidly than hornblende which weathers more rapidly than sphene or zircon

beach ridge development and lake level variation in southern lake michigan abstract the most accurate source of information on lake level fluctuations in the great lakes is the historical record from lake level gauges although it can be semiquantitatively extended back into the late 1700 s the historical record is too short to recognize long term patterns of lake level behavior to extend the historical record information must be obtained from the great lakes geologic record such information includes the elevation and age of geomorphic features and stratigraphic sequences one of the longest geologic records of late holocene lake level variation is preserved in a beach ridge complex along the southern shore of lake michigan called the toleston beach this strandplain contains over 150 beach ridges that arc across northwestern indiana and fan out into northeastern illinois each ridge was formed during the fall from a high lake level and the elevation of the foreshore deposits in each ridge provides information on the upper physical limit of lake level over the past 4000 years three scales of quasi periodic lake level variation were determined by radiocarbon dating basal peats of wetlands between the ridges and by measuring the elevation of foreshore swash deposits within ridges these three scales are 1 a short term and small scale fluctuation of 25 to 35 years with a range of about 0 5 to 0 6 m 2 an intermediate term and meso scale fluctuation of 140 to 160 years and a range of about 0 8 to 0 9 m and 3 a long term and large scale fluctuation of 500 to 600 years and a range of 1 8 to 3 7 m the short term and intermediate term fluctuations are reflected in the historical record an increase in the rate of shoreline progradation from east to west across indiana s shoreline causes differential preservation of the lake level fluctuations that is groups of four to six ridges in the western part of the strandplain that formed in response to the small scale fluctuations combine eastward into single ridges and groups of ridges representing the meso scale fluctuations the large scale fluctuations produced the most dramatic response in the western part of the toleston beach here following each high stand individual spits prograded southward off of a bedrock headland the successive spit extensions created several small lakes landward of the spits and started the 20 km eastward stream mouth deflection of the grand calumet river across indiana s western lakeshore

thermal and tectonic evolution of the paleoproterozoic transamazonian orogen as deduced from 40ar 39ar and ams along the oyapok river french guyana new 40ar 39ar dating of hornblende and biotite and measurement of anisotropy of magnetic susceptibility ams have been performed on a 150 km section along the oyapok river in order to better constrain the timing of cooling of the transamazonian orogen in the south of french guyana from north to south hornblende cooling ages range from 2052 4 to 1993 7 ma and corresponding biotite cooling ages vary from 1995 4 to 1928 4 ma this suggests a uniform cooling rate 3 5 c ma from 550 to below 250 c the south guyana complex sgc cool down later than the southern guyana belt sgb the sgc displays high temperature metamorphism and higher magnetic anisotropy degree p 1 18 1 44 we interpret the high p values as the result of the development of a ductile fabric consecutively to transcurrent tectonic the regular southward decrease of the cooling ages and the temperature profile determined at 1995 ma along the studied 150 km section indicates that no significant differential vertical movements affected the sgc these results imply that the regional cooling was not associated with a rapid exhumation process as found in modern orogens instead we propose a model of exhumation with a constant speed and later exhumation of the southern oyapok zone belonging to a lower structural level and or retarded by the transcurrent tectonics during the last transamazonian magmatic event at 2100 ma

sea surface shape derivation above the seismic streamer the rough sea surface causes perturbations in the seismic data that can be significant for time lapse studies the perturbations arise because the reflection response of the non flat sea perturbs the seismic wavelet in order to remove these perturbations from the received seismic data special deconvolution methods can be used but these methods require as input the time varying wave elevation above each hydrophone in the streamer in addition the vertical displacement of the streamer itself must also be known at the position of each hydrophone and at all times this information is not available in conventional seismic acquisition however it can be obtained from the hydrophone measurements provided that the hydrophones are recorded individually not grouped that the recording bandwidth is extended down to 0 05 hz and that data are recorded without gaps between the shot records the sea surface elevation and also the wave induced vertical displacement of the streamer can be determined from the time varying pressure that the sea waves cause in the hydrophone measurements when this was done experimentally using a single sensor seismic streamer without a conventional low cut filter the wave induced pressure variations were easily detected the inversion of these experimental data gives results for the sea surface elevation that are consistent with the weather and sea state at the time of acquisition a high tension approximation allows a simplified solution of the equations that does not demand a knowledge of the streamer tension however best results at the tail end of the streamer are obtained using the general equation

holocene soil chronofunctions southern alps new zealand abstract field and laboratory data for two soil chronosequences from the high country of the south island of new zealand suggest rapid soil formation rates the areas studied are the ben ohau range and the mt cook area where mean annual precipitation is about 1 and 4 m respectively most soil ages are estimated from the thicknesses of rock weathering rinds soil chronofunctions are derived for indices that depict field profile and color development and a modified index of profile anisotropy for laboratory data out of a possible 21 chronofunctions 16 are statistically significant at the 0 05 level and 8 are from each area most 13 of the chronofunctions best fit a power model and the remaining ones are split between linear 2 and exponential 1 models although problems exist in the purity of the chronosequences because other soil forming factors are not constant duration of pedogenesis seems to be the most significant factor in explaining the trends despite the large differences in mean annual precipitation between the two areas many of the derived curves for property vs time are quite similar

plant cover effects on hydrology and pedogenesis in a sandy vadose zone abstract the hydrology of three field lysimeters was monitored after 15 years of plant growth to examine pedogenic impacts of plant induced changes to water budgets flow and chemical denudation between 70 and 87 of annual water discharge from all lysimeters sandboxes was generated from short usually 5 days precipitation events and longer usually 25 days snowmelt events these infiltration events occur only 10 23 of the time throughout the year average annual evapotranspiration et from the red pine sandbox was 73 of precipitation and 1 4 2 6 times larger than et from the grass and moss lichen boxes respectively in situ unsaturated hydraulic conductivity estimates based on darcy s law are on the order of 10 8 m s and did not appear to vary significantly with plant cover the upper zone of each sandbox is characterized by rapid large changes in the magnitude of pressure head 700 to 6 cm water content 1 to 9 and magnitude and direction of the hydraulic gradient the vertical extent of this upper zone varies with plant cover it extends to a depth between 30 and 70 cm beneath moss lichen and grass whereas the upper zone beneath red pine extends to at least 120 cm during the growing season synoptic field measurements of water content and pressure head were very different from laboratory measurements of hanging column moisture retention curves the red pine sandbox consistently exhibited the smallest water contents and the most negative pressure heads upper zone soil water samples collected at small and large tension during events had different silica concentrations which supports the existence of a dynamic flow system consisting of water residing in large and small pore spaces respectively however large tension silica concentrations increased systematically with depth in all sandboxes indicating chemical reaction progresses in bulk soil water along flow paths a conservative estimate of the shortest mean soil water residence time in the sandboxes is 9 days which is sufficient time for exchange and weathering reactions to occur silica concentrations at 95 cm beneath red pine were twice as large as those beneath the moss lichen cover however larger water fluxes from the moss lichen sandbox produced an annual silica denudation flux of 910 mol si ha year whereas silica denudation from the pine sandbox was only 470 mol si ha year these results suggest that water and solute uptake by rooting vegetation may actually decrease chemical denudation from young soil building ecosystems

modeling broadband ocean acoustic transmissions with time varying sea surfaces solutions to ocean acoustic scattering problems are often formulated in the frequency domain which implies that the surface is frozen in time this may be reasonable for short duration signals but breaks down if the surface changes appreciably over the transmission time frequency domain solutions are also impractical for source receiver ranges and frequency bands typical for applications such as acoustic communications e g hundreds to thousands of meters 1 50khz band in addition a driving factor in the performance of certain acoustic systems is the doppler spread which is often introduced from sea surface movement the time varying nature of the sea surface adds complexity and often leads to a statistical description for the variations in received signals a purely statistical description likely limits the insight that modeling generally provides in this paper time domain modeling approaches to the sea surface scattering problem are described as a benchmark for comparison the helmholtz inte

broadband seismic over under sources and their designature deghosting abstractthe source depth influences the frequency band of seismic data due to the source ghost effect it is advantageous to deploy sources deep to enhance the low frequency content of seismic data but for a given source volume the bubble period decreases with the source depth thereby degrading the low frequency content at the same time deep sources reduce the seismic bandwidth deploying sources at shallower depths has the opposite effects a shallow source provides improved high frequency content at the cost of degraded low frequency content due to the ghosting effect whereas the bubble period increases with a lesser source depth thereby slightly improving the low frequency content a solution to the challenge of extending the bandwidth on the low and high frequency side is to deploy over under sources in which sources are towed at two depths we have developed a mathematical ghost model for over under point sources fired in sequential and simultaneous modes and we have found an inverse mode

rates of weathering and chemical depletion in soils across a chronosequence of lake michigan sand dunes abstract accumulation of soil organic matter and depletion of base cations and trace metals were measured in surface soils i e o horizon and upper 15 cm of mineral soil across a 4150 year soil chronosequence of dune capped beach ridges bordering northern lake michigan within 400 years of beach ridge formation coniferous forest develops soil c and n accumulate to steady state levels of 40 and 1 3 mg ha 1 respectively and the ph of the upper mineral soil decreases from 8 5 to 4 3 as carbonate minerals dissolve after substantial proportions of ca and mg have been removed an incipient spodic horizon can be observed the upper surface of which progressively deepens with increasing soil age analysis of soil strain of the upper mineral soil showed gradual dilation reaching 121 after several centuries of soil development after accounting for soil dilation quantities of ca mg p zn mn fe and v in the upper 15 cm of mineral soil decreased exponentially to low asymptotic levels representing quantities contained in weathering resistant minerals mass balance modeling indicated weathering and elemental losses in the order of decreasing decay constant as ca mg mn zn p v fe al k na ti this sequence is consistent with weathering of carbonates then ferromagnesian minerals then feldspars p levels are tightly coupled with ca losses in soils older than 200 years suggesting that ca rather than fe or al stabilizes p high concentrations of heavy metals in the organic horizon relative to the upper mineral soil indicate atmospheric inputs of industrial pollutants

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

40ar 39ar thermochronometry of the imataca complex venezuela the imataca complex of venezuela an archean metamorphic belt located on the northern margin of the amazonian craton underwent upper amphibolite to granulite facies metamorphism during the 2 0 ga trans amazonian orogeny 40ar 39ar plateau dates on hornblende separates from the imataca complex vary from 1972 to 1760 ma and record post trans amazonian orogeny cooling biotite and feldspar pairs which were separated from the same samples or from samples collected at the same exposure and should record identical thermal histories yield distinct results the biotite 40ar 39ar spectra are primarily plateau profiles with dates ranging from 2044 to 1254 ma whereas perthitic potassium feldspar yield stair step 40ar 39ar spectra and integrated dates ranging from 1395 to 1166 ma the internally discordant perthite spectra appear to reflect cooling at rates of 0 1 0 2 c ma 1 consistent with the cooling rates estimated from the blocking temperature and age differences between biotite feldspar pairs from the same sample two plagioclase samples yield saddle shaped 40ar 39ar spectra with 1235 and 1221 ma minimum dates these internally discordant spectra are caused by degassing of non atmospheric trapped ar from the plagioclase and of radiogenic ar sited in k rich inclusion phases the 2 0 1 7 ga biotite dates are confined to the margins of the imataca complex and record the waning stages of trans amazonian orogeny uplift and cooling the 1 4 1 2 ga biotite dates which are confined to the center of the imataca complex are correlated with their 200 350 c ar blocking temperatures as estimated from their average grain size and fe fe mg composition the 1 3 1 1 ga feldspar ages are correlated with their 200 300 c ar blocking temperatures as determined from analysis of the 40ar 39ar step heating data the extremely slow cooling rates implied by the biotite feldspar pairs suggest that between 1 7 and 1 1 ga the imataca complex cooled isobarically at intermediate crustal levels all ar mineral systems closed by 1 1 ga perhaps reflecting a renewed phase of uplift associated with the mild tectonism of the nickorie episode

early proterozoic ore deposits and tectonics of the birimian orogenic belt west africa abstract the early proterozoic of the west african craton comprises a series of volcanic troughs and sedimentary basins the birimian with granitic terranes accreted on archean nuclei in the man and reguibat shields studies of the birimian of the man shield indicate a model of polycyclic evolution with a major collision event d 1 2 1 ga thrusting part of the proterozoic terrane over the archean before individualization of numerous volcanic troughs and clastic infill basins the proposed evolution for the birimian orogenic belt comprises 1 deposition of the sedimentary lower birimian b1 with minor tholeiitic volcano sedimentary intercalations containing chert and or mn formations and with most of the detritus being derived from early proterozoic sources apart from contamination near the proterozoic archean contact 2 pre b2 crustal thickening related to d 1 thrusting 3 formation over about 40 ma of the upper birimian b2 with numerous volcanic troughs of different composition tholeiitic and rare komatiitic bimodal tholeiitic to calc alkaline volcano plutonic and tarkwaian clastic infill basins and 4 major transcurrent tectonics with sinistral d 2 and dextral d 3 strike slip faults such a tectonic evolution from a collision d 1 phase to a transcurrent d 2 d 3 phase is typical of collision belts and in the present case the evolution of the birimian orogenic belt can be extended into guyana the metallogenic history of the birimian shows a three phase evolution coinciding with the orogenic evolution and extends over almost 150 ma from the perkoa massive zn ag sulfides 2 12 ga with a clear mantle affinity to the latemesothermal au quartz veins 2 ga with according to lead isotopes a high crustal participation the economic mineralization of belt thus consists of 1 1 pre orogenic pre d 1 deposits related to early extension zones this was diverse with stratiform au tourmalinite type 1 au loulo in mali dorlin in guyana stratiform fe cu faleme in senegal and mn nsuta in ghana tambao in burkina faso and a single massive zn ag sulfide deposit perkoa in burkina faso associated with regional volcanosedimentary variably tholeiitic stratigraphic marker beds 2 2 syn orogenic post d 1 to syn d 2 d 3 deposits with disseminated au sulfides type 2 au yaoure in the ivory coast in extensional zones of the b2 followed by auriferous paleoplacers type 3 au in b2 extensional zones tarkwaian banket conglomerate or syn d 2 transtensional zones debris flow of orapu in guyana 3 3 late orogenic post peak d 2 d 3 deposits with mesothermal au mineralization evolving from a disseminated gold bearing arsenopyrite and au quartz lode type type 4 au ashanti in ghana to a quartz vein type with free gold and cu pb zn ag bi paragenesis most of the gold in west africa comes from this phase finally the metallogeny of the birimian appears as rich in au and mn formations and poor in volcanogenic mineralization and bif it differs from the archean metallogeny of west africa and other regions through the presence of certain deposit types such as au stratiform tourmalinite gold bearing conglomerate with au and fe ti oxides but no uranium or sulfides and au arsenopyrite rich shear zone deposits that are very common in but not exclusive to the proterozoic and paleozoic

rough sea deghosting using a single streamer and a pressure gradient approximation we present a method for receiver ghost correction of towed streamer data that accounts for the rough sea surface the method explicitly uses the fact that the pressure is zero at the free sea surface to estimate the vertical pressure gradient continuous elevation measurements of the wave height directly above the hydrophones are required a measurement which is currently unavailable the new deghosting method is fundamentally limited to frequencies below the first ghost notch the lowest order implementation requires that the streamer is towed no deeper than approximately 6 m and a receiver spatial sampling interval of about 3 m or less using the lowest order and simplest implementation of the new method the rough sea error is reduced from 1 5 2 5 db to about 1 1 5 db in amplitude and from 20 to 10 in phase at 50 hz in a 4 m significant wave height sea higher order terms in the approximation promise to further reduce the error

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

oblique magmatic structures of two epizonal granite plutons hoggar algeria late orogenic emplacement in a transcurrent orogen abstract two granite plutons in the pan african belt of hoggar algeria are studied using the anisotropy of magnetic susceptibility technique ams at low field together with a study of microstructures the tesnou complex is made of coalescent plutons at the margins of which dextral n s oriented shearing imprints solid state textures in the granite microstructures and low magnetic anisotropy magnitudes indicate a magmatic state weak deformation consistent nw se trending sub horizontal magnetic lineations making an overall oblique pattern with respect to bordering shear faults is attributed to low n s trending dextral shear strain magnitudes

soil development on moraines of mendenhall glacier southeast alaska 1 the moraines and soil morphology abstract soils on a sequence of well drained neoglacial moraines up to 240 years old plus an older soil were sampled to investigate soil development through the initial stages of podzolization pebbles and cobbles in the moraines are about 75 to 80 granitic rock computations based on minerals observed optically and on chemical analyses indicate that plagioclase and quartz are the predominant minerals in the fine earth the climate is perhumid leaching of the moraines may have begun before they were free of glacial ice but little organic matter accumulated under the initial vegetative cover of sparse dwarf fireweed 10 year site as alder thicket developed organic matter accumulated rapidly in oi oe and a horizons 38 year site as spruce trees began to replace alder organic matter was lost from the surface mineral soil to form an e horizon and an incipient bs horizon formed below it an oa horizon formed and the a horizon disappeared 70 and 90 year sites organic acid leaching continued to form a spodic bhs horizon in spruce forest 240 year site and a spodic bh horizon in spruce hemlock forest 240 year site hornblende has been etched slightly within 240 years apatite has been lost from e horizons and free fe presumeably in ferrihydrite has accumulated in b horizons the younger moraine soils both with and without incipient bs horizons are typic cryorthents and the 240 year old and older soils with spodic bhs and bh horizons are typic haplocryods they are expected to accumulate more organic matter and become andic humicryods which is the most common subgroup of soils in the area

soil development on moraines of mendenhall glacier southeast alaska 2 chemical transformations and soil micromorphology abstract soils on holocene age moraines of the mendenhall glacier in southeast alaska range from morphologically undeveloped to soils having field identified spodic horizons that are more strongly expressed with increasing soil age a soil chronosequence was sampled to study development of spodic features and to compare the expression of these features with chemical and micromorphological properties organic c accumulates rapidly in these soils with o horizons forming within 38 yr organic c is highly correlated with cation exchange capacity r2 0 92 variable charge r2 0 94 extractable acidity r2 0 83 and 1500 kpa water retention r2 0 97 the ph of the 10 yr old pedon is acidic with a soil ph h20 ranging from 5 0 to 6 4 the ph decreases with soil age with the lowest ph in horizons at the organic mineral contact increases in dithionite citrate sodium pyrophosphate and acid oxalate extractable fe feo are most evident in the 240 yr old soil depth trends for these measured properties within pedons are most distinctive for fe which increases from surface mineral horizons a or e to underlying horizons in all pedons the feo fed ratio increases with soil age in b horizons and in pedons 70 yr this ratio ranges from 0 3 to 0 5 in the e horizons and from 0 4 to 0 83 in b horizons immediately below these data indicate a greater proportion of poorly crystalline fe in illuvial horizons with increasing soil age allophane content is low 240 yr old pedon this chronosequence study indicates that spodic development is achieved within a relatively short period of time in soils of this region

cooling and inferred uplift erosion history of the grenville orogen ontario constraints from 40ar 39ar thermochronology stepwise 40ar 39ar degassing experiments of 57 mineral separates of hornblende muscovite biotite and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the grenville orogen in ontario the 40ar 39ar data support an interpretation of slow nearly uniform cooling 1 4 c m y from temperatures of 500 c to below 150 c in the central gneiss belt cgb hornblendes cooled through ar closure between 930 and 1025 ma whereas in the central metasedimentary belt cmb hornblendes record the following range in 40ar 39ar cooling ages 1104 ma in the frontenac terrane 1007 1067 ma in the sharbot lake terrane 919 1026 ma in the elzevir terrane and 972 ma in the central metasedimentary belt boundary zone regional uplift erosion rates of 0 03 0 14 km m y have been estimated for the grenville orogen in ontario based on the 40ar 39ar data a model retrograde p t path for rocks of the cgb and an upper time constraint provided by flat overlying cambrian and ordovician sediments these erosion rates are consistent with rates estimated for other proterozoic or archean granulite terranes but are an order of magnitude slower than active orogens such as the alps and himalayas a regular variation in hornblende 40ar 39ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the cmb the pattern of 40ar 39ar ages is interpreted to reflect late tectonic extension consistent with field observations in the central metasedimentary belt boundary zone and elsewhere in the cmb up to 13 km of vertical displacement is inferred for some rocks in the cmb between the time they cooled below closure to argon diffusion in hornblende 500 c and their exposure at the surface 25 c

quantitative geochemical approach to pedogenesis importance of parent material reduction volumetric expansion and eolian influx in lateritization abstract using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock through calculation of net mass fluxes through the laterite soils in mali west africa we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density contributes only a very minor fraction of the enrichment of al fe si and au instead we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features at the same sample depths that accumulation reaches extreme values we show that volumetric expansion in excess of 200 has occurred locally we infer that these spatially coincidental zones of mass influx of rock forming metals fe al si and also au with dilational hyperstrains result from a mutually reinforcing mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink swell cycles and root growth and decay we propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded we speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids consequently progressive hyperstrains accumulate above this critical depth by the long term influence of a proposed translocational wedge of chemically resistant minerals against which numerous generations of plant roots have exerted stresses eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted instead the source region is largely the existing regolith column itself which releases local material and in addition is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat

did south america and west africa marry and divorce or was it a long lasting relationship abstract the 2 1 2 0 ga transamazonian orogen in south america can be well correlated with the 2 1 2 0 ga eburnean orogen along the western margin of west africa both the orogens are characterized by early tangential tectonics marked by large scale thrusts and sinistral strike slip faults followed by later transcurrent tectonics marked by dextral shear zones metamorphic evolution of the two orogens involved an initial phase of crustal thrusting and thickening followed by exhumation and final cooling these suggest that south america and west africa may have been jointed along the transamazonian eburnean orogen at 2 1 2 0 ga this is supported by the presence of 2 0 ga fluvio deltaic formations in nearly every craton in south america and west africa the available paleomagnetic data also indicate that between 2 1 1 5 ga coeval rocks in west africa and south america recorded similar polar wander paths if south america and west africa remained coherent from their amalgamation at 2 1 2 0 ga until their incorporation into gondwana at 0 6 0 5 ga the positions of south america and west africa in the present configurations models of rodinia need to be re evaluated since these configurations did not bring the two blocks into close proximity

40ar 39ar and k ar data bearing on the metamorphic and tectonic history of western new england 40ar 39ar ages of coexisting biotite and hornblende from proterozoic y gneisses of the berkshire and green mountain massifs as well as 40ar 39ar and k ar mineral and whole rock ages from paleozoic metamorphic rocks suggest that the thermal peak for the dominant metamorphic recrystallization in western new england occurred 465 5 m y ago taconian although textural data indicate a complex metamorphic tectonic history for paleozoic rocks no evidence in rocks at least as high as kyanite grade dictates an acadian age for the barrovian metamorphism available 40ar 39ar and k ar data suggest that the low grade metamorphism and cleavage formation in taconic allochthons and the higher grade metamorphism and emplacement of the berkshire massif allochthon are taconian 40ar 39ar age data from a poorly defined terrane beginning near the east margin of the green mountain massif and extending along the eastern one third of the berkshire massif as far south as otis massachusetts suggest that the area has been retrograded during a metamorphism that peaked at least 376 5 m y ago acadian available age and petrologic data from western new england indicate the presence of at least three separate metamorphic structural domains of taconian age 1 a small area of relict high pressure and low temperature metamorphism in northern vermont t 1 domain 2 a broad area in vermont and eastern new york of normal barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient t 2 domain and 3 a rather narrow belt of steep gradient barrovian series metamorphic rocks extending from near the cortlandt complex northeastward through dutchess county new york to the berkshire massif in western massachusetts t 3 domain areas of maximum metamorphic intensity within the t 3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting berkshire massif or from recumbent folding manhattan prong of remobilized north american continental crust in the later stages of the taconic orogeny

contrasting mechanism of crustal growth abstract the paleoproterozoic granite greenstone belts of french guiana formed during a period of major crustal growth at the scale of the earth two contrasting mechanisms of crustal growth have been identified by recent structural metamorphic geochronological and geochemical investigations in the cayenne regina region which is part of the northern granite greenstone belt of french guiana following the formation of oceanic crust which is characterized by a tholeiitic volcanic sequence dated at 2174 7 ma the early stage of crustal generation was marked by successive building of calc alkaline plutonic volcanic complexes with the formation of the ile de cayenne complex dated at 2144 6 ma to the north and of the central guiana complex dated at 2115 7 ma to the south this first period of crustal generation by accumulation of mantle derived magmas was followed by a period of crustal recycling and tectonic accretion sediments derived from the granite greenstone belts were deposited in large marginal basins such as the orapu basin separating the plutonic volcanic complexes convergence between the newly formed crustal blocks caused deformation during the transamazonian orogeny of the orapu marginal basin caught in between the ile de cayenne and central guiana complexes oblique convergence is further characterized by the development of en echelon pull apart basins along the north guiana trough sinistral strike slip shear zones affecting the edges of the orapu marginal basin the lithological succession in these basins is typical of foreland basins and the metamorphic and structural evolution of the kaw and regina basins in the cayenne regina region indicate burial to depth up to 20 km in a context of heterogeneous transpression the geochemical signature of small syntectonic granitic plutons dated at 2093 8 ma and 2083 8 ma emplaced along the major strike slip shear zones is consistent with crustal thickening and partial melting of an enriched crustal source these features suggest that the last stage of the transamazonian orogeny was marked by oblique convergence collision and crustal thickening in a similar way to modern orogens

a review of the geochronology of the amazonian craton tectonic implications abstract the available geochronological data for the amazonian craton permit delineation of its main age provinces and their respective tectonic development demarcation of the boundaries of each age province is primarily based on rb sr whole rock isochrons supported by k ar determinations on minerals some u pb results on zircons pb pb whole rock isochrons and sm nd model ages complement these data in key areas at the center of the amazonian craton lies the archean age central amazonian province bounded in its northern part by the maroni itacaiunas mobile belt which developed during early proterozoic times the western and southern margins of the archean core are bordered by three tectonic provinces 1 the rio negro juruena magmatic arc 2 the rondonian mobile belt and 3 the sunsas mobile belt each belt borders the western margin of the preceding belt and each records a successively younger middle proterozoic orogeny within the central amazonian province 2 7 ga age determinations have been obtained from the serra dos carajas area although the maroni itacaiunas belt includes several reworked archean nuclei rock units formed during the 2 2 1 9 ga trans amazonian orogeny predominate the isotopic evidence suggests that a mantle derived component constituted a significant portion of the crust accreted at that time the rio negro juruena province is of mid proterozoic age 1 75 1 50 ga and includes almost entirely mantle derived material juxtaposed with the pre existent continental mass along a series of successive magmatic arcs the rondonian and sunsas provinces both exhibit older sialic basements 1 5 ga the former is characterized by important tectonomagmatic events in the 1 45 1 25 ga interval whereas the latter records tectonic reworking at 1 1 0 9 ga finally the amazonian craton is bordered to the east by the late proterozoic paraguai araguaia or araguaia tocantins fold belt

oblique lineations in orthogneisses and supracrustal rocks vertical partitioning of strain in a hot crust eastern borborema province ne brazil detailed structural work conducted at the eastern area of the neoproterozoic brasiliano zpan african borborema province northeastern brazil has shown two orientations of stretching lineations with ese trend in supracrustal rocks and ne trend in underlying orthogneisses in the metasedimentary sequence numerous kinematic indicators showing a top to the northwest sense of shear denote a welldeveloped non coaxial deformation in the orthogneisses lineations formed dominantly during coaxial deformation although a component of ne directed shear is locally observed the two lineations were produced under similar high temperature metamorphic conditions and are interpreted as the result of a protracted nw se contractional strain regime where i subhorizontal non coaxial shear with superimposed flattening led to an initial phase of nw directed thrusting ii flattening strains mainly accumulated in the orthogneisses with progressive deformation leading to a lineation oblique to the transport direction iii the subhorizontal fabric in basement and cover rocks was refolded by overturned folds and then iv cross cut by conjugate ene striking dextral and nne striking sinistral shear zones contemporaneous with ne trending upright folds it is proposed that vertical partitioning of strain between basement and cover may explain the presence of oblique lineations in this orogenic belt that did not go through a final stage of extensional collapse q 2005 elsevier ltd all rights reserved

lake michigan beach ridge and dune development lake level and variability in regional water balance abstract a sequence of northern lake michigan beach ridges records lake level fluctuations that are probably related to changes in late holocene climate historically episodes of falling and low lake level associated with regional drought led to the formation of dune capped beach ridges the timing of prehistoric ridge formation estimated by radiocarbon dating of plant macrofossils from early successional dune species shows that return periods of inferred drought averaged for time intervals of 100 to 480 yr ranged between 17 and 135 yr per drought during the last 2400 yr in five of ten of these time intervals the average return period ranged between 17 and 22 yr per drought these intervals of frequent ridge formation and drought were associated with the development of parabolic dunes which is indicative of high lake level and moist climate this seeming paradox suggests that unusually moist decades alternated with unusually dry decades during these time intervals regional water balance probably varied less during the time intervals when ridges formed less often and the lake produced no evidence of high level

effects of time varying sea surface in marine seismic data abstracta method of imaging sea surfaces based on marine seismic measurements has recently been developed the imaging technique is based on extrapolating decomposed wavefields obtained from dual sensor streamers to the sea surface where an adequate imaging condition is applied earlier feasibility tests of the method involved only controlled data associated with frozen sea surfaces here the issue of time varying effects will be in focus we introduced a modeling approach based on the kirchhoff helmholtz integral and computed the scattered wavefield from time varying rough sea surfaces e g pierson moskowitz sea surfaces we generated data for a realistic wind speed and verify the robustness of the proposed sea surface imaging technique by taking into account possible effects of moving receivers as well as streamers with variable shape we investigate the feasibility of estimating the surface wave velocity from the spectra of the imaged sea surfaces and finally present a successful application of the

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

petrochemistry and sr pb and nd isotopic geochemistry of early precambrian rocks wutaishan and taihangshan areas china abstract in the wutaishan and taihangshan region china the lowermost high grade metamorphic complex of grey gneiss and amphibolite the fuping complex gives a 2 2 0 2 ga pb pb isochron age with a first stage growth 7 73 and a 2 37 0 07 ga sm nd isochron age with 143nd 144nd 0 0 50963 0 00005 or nd t 1 50 nd depleted mantle model dates tdm of amhibolites 2 48 to 2 60 ga those of gneisses are 2 43 to 2 46 ga the unconformably overlying medium to low grade metavolcanic complex the wutai complex gives a 2 0 0 1 ga rb sr isochron age with 87 86sr 0 0 7025 2 a 2 27 0 02 ga pb pb isochron date with afirst stage growth 7 73 and a 2 26 0 06 ga sm nd isochron first stage 143nd 144nd 0 0 5097 2 or nd t 1 08 individual sample nd depleted mantle model date are 2 5 ga metavolcanic samples from low grade metasedimentary rocks of the hutuo group all lie close to a 2 3 ga reference line on a rb sr isochron plot have nd tdm 2 32 2 34 and 2 62 ga from previous geological work published u pb zircon dating and ourisotopic and geochemical results we infer that the fuping rocks formed 2 6 ga ago in an environment like a modern island arc the wutai complex formed by 2 5 ga ago mostly formed in a tectonic setting similar to that of the fuping complex with the exception that one volcanic cycle formed in an environment like a modern mor and one unit formed in an environment transitional between modern within plate and plate margin settings the hutuo group formed 2 4 ga ago in a within plate tectonic environment there is no evidence for continental crust before 2 6 ga in the wutaishan and taihangshan region this contradicts previous assignment of the fuping complex to 2 8 ga ol older the fuping complex is not an early nucleus of the sino korean craton the new data also confirm that the wutai complex is older than its original assignment as early proterozoic the sm nd systems for metabasaltic rocks in the region either in amphibolite facies or in greenschist facies are all significantly disturbed in contrast with the undisturbed sm nd reported in previous studies of the sino korea craton this study provides further evidence that the archean igneous rocks in china formed at different times from heterogeneous and depleted mantle sources

paleoproterozoic 2155 1970 ma evolution of the guiana shield transamazonian event in the light of new paleomagnetic data from french guiana abstract we present a comprehensive paleomagnetic study on paleoproterozoic 2173 2060 ma plutonic and metamorphic rocks from french guiana representative of the full range of the main transamazonian tectonothermal steps twenty seven groups of directions and poles were obtained from combination of 102 sites 613 samples based on age constraint similar lithology and or geographical proximity paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases this is also reinforced by positive field tests baked contact and reversal tests recent u pb and pb pb on zircon and complementary 40ar 39ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages estimated magnetic ages based on amphibole dating as a proxy enable us to construct a guiana shield apparent polar wander path for the 2155 1970 ma period it is also possible to present paleolatidudinal evolution and continental drift rates related to specific transamazonian tectonic regimes french guiana and probably the guiana shield were located at the equator from ca 2155 to 2130 ma during the meso rhyacian d1 magmatic accretion phase related to subduction of eorhyacian oceanic crust after closure of the eorhyacian ocean and collision of west african and amazonian plates the guiana shield moved the first evolution towards 60 latitude occurs after 2080 ma during the neorhyacian d2a post collisional sinistral transcurrent phase during the late rhyacian d2b phase up to 2050 ma the guiana shield reaches the pole and starts to move to lower latitudes on an opposite meridian by the orosirian d2c phase from ca 2050 to 1970 ma the guiana shield reaches the equator based on the amphibole 40ar 39ar dates we estimate the continental drift between 12 and 16 cm y for the meso to late rhyacian period followed by a lower rate between 9 and 14 cm y up to orosirian time this study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100 200 ma time period our results confirm the possibility of apwp construction on paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi method radiometric dating

imaging the sea surface using a dual sensor towed streamer sea surface profile and reflection coefficient estimates are vital input parameters to various seismic data processing applications the common assumption of a flat sea surface when processing seismic data can lead to misinterpretations and mislocations of events a new method of imaging the sea surface from decomposed wavefields has been developed wavefield separation is applied to the data acquired by a towed dual sensor streamer containing collocated pressure and vertical particle velocity sensors to obtain upgoing and downgoing wavefields of the related sensors time gated upgoing and downgoing wavefields corresponding to a given sensor are then extrapolated to the sea surface where an imaging condition is applied so that the time invariant shape of the sea surface can be recovered by sliding the data time window the temporal changes of the sea surface can be correspondingly estimated ray tracing and finite difference methods were used to generate different controlled data sets used in this feasibility study to demonstrate the imaging principle and to test the image accuracy the method was also tested on a first field data example of a marginal weather line from the north sea

early proterozoic crustal evolution in the birimian of ghana constraints from geochronology and isotope geochemistry abstract representative suites of ghanaian granitoids metavolcanics and metasediments were selected from birimian rock units for rb sr pb pb and sm nd analyses isotopic data on the igneous rocks provide evidence for a major early proterozoic crust forming episode which took place over a maximum time interval from 2 3 to 2 0 ga by a differention from a depleted mantle source there is little evidence for the involvement of significantly older e g archaean crust in the genesis of the igneous rock units the only exception so far found is the winneba granitoid which appears to contain a magmatic contribution from presumably underlying archaean basement sm nd model ages indicates that birimian sediments were derived from adjacent penecontemporaneous volcanic belts the birimian of ghana forms part of a major proterozoic eburnean episode of juvenile crustal accretion which has been recognised in the surrounding areas of mauretania senegal ivory coast burkina faso mali and niger where it has been dated at 2 1 2 2 ga abouchami et al the birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents

variations in weathering processes and rates with time in a chronosequence of soils from glen feshie scotland chemical and mineralogical characteristics have been determined for a chronosequence of six soil profiles ranging in age from 80 13 000 years bp developed on river terraces in the western cairngorms of scotland the c horizons are similar chemically and mineralogically and the soils have similar pedogenetic histories exchangeable ca and mg decrease with time and base saturation decreases exponentially from 24 6 in the ah horizon of the youngest profile to 2 8 in the comparable horizon of the 10 000 year old profile according to the chronofunction y 3 372 22 612 exp 0 0007365t long term weathering rates of base cations calculated from the loss of these cations relative to zr appear to decrease exponentially with time but this may be due to the method of calculation the magnitude of loss of base cations decreases in the sequence na k mg ca but when the relative mobilities of these elements are considered the loss is in the order mg na ca k this reflects the dissolution of chlorite and loss of mg and the more rapid weathering of plagioclase feldspar and loss of na and ca particularly in the coarse sand fraction than k feldspar the clay fractions although 2 of all horizons also show distinct patterns with age in that chlorite and mica are less abundant in older soils and vermiculite is more abundant the latter phase often having hydroxyaluminium polymers in the interlayer region the chemical and mineralogical trends in the soil sequence are closely associated and are induced by pedogenic weathering

modeling scattering effects from time varying sea surface based on acoustic reciprocity in marine seismic processing the sea surface is often considered as a flat mirror hence the effects of different weather conditions during the acquisition are largely ignored however studies have shown that rough sea surface ghosts can severely damage the 4d signal if not handled properly in data processing in order to account for realistic sea surface effects in processing the impact of time varying rough sea surfaces need to be studied in this work we derive a method for modelling source and receiver ghosts from time varying rough sea surface and their interaction with subsurface reflections this method is based on acoustic reciprocity and leads to integral equations of non stationary wavefields these modelling equations can also serve as a basis for investigating source and receiver deghosting methods for time varying rough sea surfaces our developed modelling algorithm is validated against a frequency domain approach for a frozen rough sea surface for a moving simple sea surface the d

nature s diffusion experiment the cooling rate cooling age correlation correlation between the age of orogeny and the cooling rate of metamorphic rocks through 500 180 c as determined through various radioisotopic dating methods suggests that ancient orogens cooled at rates as much as two orders of magnitude slower than those of active orogens i propose that the correlation has arisen because ancient orogens have undergone long postorogenic periods as long as hundreds of millions of years during which the rocks remained effectively isothermal laboratory experiments suggest that argon diffusion in biotite may be sufficiently rapid at temperatures of 220 c to produce the correlation by slow loss of argon during extended isothermal periods one interesting realization is that nature has provided a long term argon diffusion experiment the results of which have been inverted in this paper to give an independent estimate of the diffusivity of argon in biotite under natural conditions this estimate compares favorably with a projection of the results of laboratory hydrothermal experiments down 10 orders of magnitude the analysis confirms the theory that there is a critical thermal window below accepted closure temperatures in which long residence will result in significant partial resetting of thermochronometers