taconic orogeny in pennsylvania a 15 20 m y apennine style ordovician event viewed from its martic hinterland abstract in pennsylvania the taconic orogeny lasted from 461 to 443 ma as cambro ordovician slope deposits were deformed into mountains edging the laurentian craton at the same time that materials from an adjacent deep water basin were being transported 50 70 km across a carbonate platform into foreland basins this paper focuses on shelf edge hinterland features mostly the martic zone as a folded stack of imbricate thrust sheets of slope materials that corresponds to vermont s taconic mountains and southern quebec s zone of taconic allochthons work of the last century is summarized corrected and combined with a new 450 ma radiometric date and fluid inclusion data from the pequea mine within the martic zone these and abundant new graptolite and conodont dates in the foreland paint a revised pennsylvania picture differing from the northern taconic areas differences are 1 transport of very large allochthonous masses of deep water material the dauphin formation far across the carbonate platform and 2 deformation migrating progressively across that platform during a 15 20 m y period incorporating it and its foreland cover into alpine scale recumbent folds and thrusts the scenario has many analogies to italy s modern apennine mountains minus the latian volcanics

deep air convection in the firn at a zero accumulation site central antarctica abstract ice cores provide unique archives of past atmospheres and climate but interpretation of trapped gas records and their climatic significance has been hampered by a poor knowledge of the prevalence of air convection in the firn layer on top of polar ice sheets in particular the phasing of greenhouse gases and climate from ice cores has been obscured by a discrepancy between empirical and model based estimates of the age difference between trapped gases and enclosing ice which may be due to air convection here we show that deep air convection 23 m occurs at a windy near zero accumulation rate site in central antarctica known informally as the megadunes site 80 77914 s 124 48796 e deep convection is evident in depth profiles of air withdrawn from the firn layer in the observed pattern of the nitrogen isotope ratio 15 n 14 n the argon isotope ratio 40 ar 36 ar and in the mixing ratios of the anthropogenic halocarbons methyl chloroform ch 3 ccl 3 and hfc 134a ch 2 fcf 3 transport parameters diffusivities were inferred and air was dated using measured carbon dioxide co 2 and methane ch 4 mixing ratios by comparing with the law dome atmospheric record which shows that these are the oldest firn air samples ever recovered co 2 mean age 1863 ad the low accumulation rate and the consequent intense metamorphism of the firn due to prolonged exposure to seasonal temperature cycling likely contribute to deep air convection via large grain size and vertical cracks that act as conduits for vigorous air motion the megadunes site provides a possible modern analog for the glacial conditions in the vostok dome fuji and dome c ice core records and a possible explanation for lower than expected 15 n 14 n ratios in trapped air bubbles at these times a general conclusion is that very low accumulation rate causes deep air convection via its effect on firn structural characteristics

isotopic age constraints and metamorphic history of the talladega belt new evidence for timing of arc magmatism and terrane emplacement along the southern laurentian margin abstract u pb and 40ar 39ar dates from the talladega belt southernmost appalachians provide insight into the timing and nature of pre alleghanian tectonism in this region low grade metasedimentary rocks of the talladega belt represent the outermost preserved portions of the southern laurentian margin thus recording the earliest orogenic events that affected the margin in addition to later overprinting events these rocks are structurally overlain by the hillabee greenstone metavolcanic sequence from which metadacite yields an age of 470 4 ma ion microprobe u pb zircon age hillabee geochemistry indicates formation in an arc or back arc setting we suggest correlation of the hillabee greenstone with 460 470 ma arc related rocks in the dahlonega gold belt which extends along strike with the hillabee through georgia and into north carolina and interpret the hillabee as the southernmost volcanic expression of an early middle ordovician arc that formed outboard of laurentia rocks of the talladega be

ordovician k bentonites issues in interpreting and correlating ancient tephras abstract prominent upper ordovician k bentonites include the deicke and millbrig beds in the late ordovician of eastern north america and the kinnekulle k bentonite in the uppermost sandbian of baltoscandia these beds are thought to represent some of the largest ash fall deposits known in the phanerozoic record this report presents a comprehensive study of biotite compositions from the three beds covering a far more extensive geographic range than has previously been described these data show that the kinnekulle and millbrig are multiple event ash beds some parts of which are indistinguishable from one another tectonomagmatic discrimination diagrams combined with mg number data indicate that the deicke millbrig kinnekulle sequence represents the progressive transformation from calc alkaline to peraluminous magmatic sources consistent with a model of progressively evolving magmatism during the closure of the iapetus ocean it is concluded that the millbrig and kinnekulle beds are coeval and represent primarily simultaneous episodes of explosive volcanism although it cannot be excluded that some portions of both beds were produced by the same event

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

crustal evolution in the southern appalachian orogen evidence from hf isotopes in detrital zircons abstract paired u pb and lu hf isotopic compositions of individual detrital zircons reveal that mesoproterozoic lithosphere in the southernmost appalachian orogen evolved almost continuously from 1200 to 900 ma with a noticeable concentration only at 1050 ma the evolution of the lu hf system in this crust is characterized by a systematic linear evolution pattern of 176hf 177hf versus age over this interval which suggests a limited range of mixing ratios between juvenile and older crustal components to create a new mesoproterozoic lithospheric reservoir extrapolation of the lu hf systematics of this reservoir from the mesoproterozoic to the paleozoic using typical crustal values shows general coincidence with the lower part of the hf isotopic array defined by paleozoic detrital zircons because the paleozoic array extends to both much higher and much lower ehf values than are likely to develop from extrapolations of the mesoproterozoic array the paleozoic array is most compatible with an orogen fo

gas age ice age differences and the chronology of the vostok ice core 0 100 ka 1 gas is trapped in polar ice at depths of 50 120 m and is therefore significantly younger than the ice in which it is embedded the age difference is not well constrained for slowly accumulating ice on the east antarctic plateau introducing a significant uncertainty into chronologies of the oldest deep ice cores vostok dome fuji and dome c we recorrelate the gas records of vostok and greenland ice sheet project 2 gisp2 cores in part on the basis of new ch4 data and use these records to construct six vostok chronologies that use different assumptions to calculate gas age ice age differences we then evaluate these chronologies by comparing times of climate events at vostok with correlative events in the well dated byrd ice core west antarctica from this evaluation we identify two leading chronologies for the vostok core that are based on recent models of firn temperature firn densification and thinning of upstream ice one chronology involves calculating gas age ice age differences from these models the second new approach involves calculating ice depths in the core that are contemporaneous with depths in the same ice core whose gas ages are well constrained this latter approach circumvents problems associated with highly uncertain accumulation rates in the vostok core the uncertainty in vostok chronologies derived by correlating into the gisp2 gas record remains about 1 kyr and high precision correlations continue to be difficult

late palaeogene quaternary geology of halmahera eastern indonesia initiation of a volcanic island arc the late palaeogene quaternary stratigraphy of halmahera is described and new formation names are proposed based on recent field investigations of the ne and central part of the island this stratigraphic information provides new insights into the neogene history of halmahera and the development of the present island arc the late palaeogene and younger rocks rest unconformably on an ophiolitic basement complex which formed part of a late cretaceous early tertiary fore arc after volcanic arc activity ceased in the eocene the former fore arc terrane was uplifted and deeply eroded in the late palaeogene some of the late palaeogene early miocene river valleys are currently being re excavated by the present rivers slow subsidence began in the mid late oligocene and by the end of the miocene all eastern halmahera was the site of shallow water carbonate deposition there is no evidence for arc volcanism in central halmahera at this time and the reported oligo miocene volcanism in nearby regions is interpreted as volcanism related to the sorong fault system the miocene shallow water region subsided rapidly in the early pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites with increasing amounts of calc alkaline volcanic debris from a pliocene volcanic arc built on the western arms of halmahera probably on the eroded early tertiary arc this phase of rapid subsidence in the pliocene back arc region resulted from the initiation of subduction of the molucca sea lithosphere eastwards beneath halmahera differential subsidence on nw se and ne sw sets of faults in the region immediately behind the active arc led to the formation of deep sediment filled basins adjacent to the eastern arms a major deformation event in the pleistocene resulted in folding and local thrusting at the junction between eastern and western halmahera and volcanism ceased in the pliocene arc the third halmahera arc the quaternary arc currently active in the northern part of the islands began activity within the last 1 ma and is built upon the deformed and partly eroded pliocene arc the pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward dipping molucca sea plate with adjacent plates either with a fragment of the australian continent in the sorong fault zone and or with the philippine sea plate beneath northern halmahera

δ15n of n2 in air trapped in polar ice a tracer of gas transport in the firn and a possible constraint on ice age gas age differences with respect to gas transport the fim near the surface of an ice sheet can be divided into three zones the uppermost is a convective zone located just below the surface of the ice sheet in which the air is rapidly flushed by convective exchange with the overlying atmosphere below the convective zone there is a diffusive air column in which diffusion is rapid but there is no convection between the bottom of the diffusive air column and the bubble close off region there may be a nondiffusive zone in which diffusion is so slow that negligible gas transport occurs the diffusive air column is characterized by progressive enrichment with depth of 15n in n2 and heavy isotopes of gases in general as predicted using the barometric equation in this paper we present data on the 15n of n2 in recently trapped air samples from 12 ice cores along with numerous downcore samples from byrd vostok and dome c bubble close off depths for these cores calculated from a densification model ranged from 51 to 114 meters below the surface mbs we used these data and the barometric equation to calculate the thickness of the diffusive air column and found that it comprised 46 to 93 of the total firn thickness at our study sites paleo close off depths calculated from the densification model for glacial sections of byrd vostok and dome c are 15 25 m deeper than close off depths today diffusive column heights calculated from 15n varied in a more complex manner the diffusive column height at byrd appears to have decreased from 74 m during the last glacial period to 50 m during the holocene at vostok and dome c the diffusive column height calculated from 15n increase from about 65 m during the last glacial period to about 80 m in the holocene we use records of surface temperature and co2 at byrd and vostok along with their respective chronologies to constrain the ice age gas age difference age throughout the section of the vostok ice core corresponding to the last glacial termination in principle age values calculated from these data can be used to discriminate whether gas in the fim mixes to the bubble close off depth or to a depth equivalent to the diffusive column height in practice however uncertainties in the chronology of byrd and vostok are too great to allow us to distinguish between these two possibilities one can only say that at the time of the last termination age for vostok was between 3 and 10 5 kyr previous estimates fall within this range

terrane amalgamation in the philippine sea margin abstract the philippine sea plate includes plateaus of thickened crust interpreted as imbricated ophiolite and arc related terranes of late mesozoic early tertiary age separated by thinner oceanic crust the arrival of plateaus at the subducting southwest margin of the philippine sea plate has caused the philippine trench to propagate southward in increments and caused transfer of terranes to the philippine margin new data from the halmahera region indicate that the position nature and evolution of plate boundaries have been strongly influenced by the heterogeneous character of the philippine sea plate at present the philippine trench terminates at an oceanic plateau which is structurally continuous with an old forearc and ophiolite terrane on halmahera the position of this terrane has caused philippine sea plate eurasia convergence to be transferred from subduction at the philippine trench to the molucca sea collision zone through a broad ne sw dextral transpressional zone across halmahera this plate boundary configuration is unstable and requires the future development of a new subduction zone to the east of halmahera which will result in amalgamation of the halmahera ophiolite terrane to the philippine margin in the halmahera region amalgamation of terranes to the evolving philippine microcontinent is currently in process

taconian orogeny in the new england appalachians collision between laurentia and the shelburne falls arc tectonic models of the ordovician taconian orogeny in western new england usually invoke a collision between the laurentian margin and a magmatic arc identified as the bronson hill arc however in central massachusetts and southern new hampshire rocks in the bronson hill arc are 454 to 442 ma and therefore younger than much of the taconian deformation and metamorphism in western new england and eastern new york which began by 470 ma u pb and single grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc the shelburne falls arc that formed west of the bronson hill arc at 485 to 470 ma the shelburne falls arc formed above an east dipping subduction zone by the early ordovician the taconian orogeny was the result of the collision between laurentia and the shelburne falls arc beginning ca 475 to 470 ma the younger bronson hill arc formed above a west dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the taconian orogeny the taconian orogeny ended when plate convergence between laurentia and iapetus was accommodated by the newly developed west dipping subduction zone instead of by crustal shortening in the taconian thrust belt the tectonic history of the new england appalachians is inconsistent with a middle ordovician collision between laurentia and the proto andean margin of gondwana

bridging the gap between the foreland and hinterland i geochronology and plate tectonic geometry of ordovician magmatism and terrane accretion on the laurentian margin of new england

snow and firn permeability at siple dome antarctica the nature of air snow exchange processes depends upon both the physical characteristics of the snow and forcing from the atmosphere an understanding of snow air transport processes and models of these processes are important for interpreting ice cores and for predicting remotely sensed snow surface characteristics accurate modeling requires knowledge of the physical characteristics of the snow and firn in this paper measurements of snow and firn permeability from the surface down to 12 m depth at siple dome antarctica are presented the permeability varies greatly as a function of layer and depth generally increasing to approximately 3 m and generally decreasing below that the maximum permeability 280 10 10 1 0 m2 occurs at approximately 3 m depth the lowest permeability in the top 12 m of snow and firn 10 10 10w m2 was measured in the surface wind pack the measurements show that layering and microstructure have large effects on permeability a mathematical expression for the overall variation of permeability with depth is presented it is shown that snow density is a poor indicator of permeability quantitative microscopy may be helpful in relating observed snow characteristics to permeability and useful in modeling firnification and chemical species transport

south pole paleowind from automated synthesis of ice core records 1 we develop a fully automated reconstruction of south pole surface roughness as a measure of past wind intensity using dynamic warping feature recognition and internal consistency checks to reduce subjectivity and analysis time we synchronized millimeter resolution optical profiles of deep south pole glacial dust together with ice core data between the ages 26 and 90 thousand years b p the images were captured using a laser dust logger in six boreholes of the icecube neutrino detector array a massive construction project in the ice at south pole and an unusual opportunity for glaciology and paleoclimate research south pole surface roughness anticorrelated with curves of antarctic pco2 and temperature which we propose were connected through secular migrations of the southern hemisphere westerlies this new paleoclimate signal may be direct evidence of atmospheric reorganization during the glacial helping to deconvolve the thermodynamics and biogeochemistry of climate change we also found that local intensity of atmospheric circulation roughly tracked the quantity of wind borne particulates deposited in antarctic ice during the glacial period although the largest dust increases were likely furnished by the source regions

volcanic arc emplacement onto the southernmost appalachian laurentian shelf characteristics and constraints in the southernmost appalachians the hillabee greenstone an ordovician volcanic arc fragment lies directly atop the outermost laurentian devonian earliest mississip pian shelf sequence at the structural top of the greenschist facies talladega belt the frontal metamorphic allochthon along this orogenic segment the hillabee greenstone was emplaced between latest devonian and middle mississippian time it and the upper most laurentian section were later repeated together within a series of map scale imbri cate slices of a postmetamorphic dextral transpressional alleghanian thrust duplex system that placed the high grade eastern blue ridge allochthon atop the talladega belt geochemical and geochronologic u pb zircon studies indicate that the hillabee greenstone s interstratifi ed tholeiitic meta basalt and calc alkaline metadacite rhyolite formed within an extensional setting on con tinental crust ca 460 470 ma palinspastic reconstructions of the southern appalachian ordovician margin place the hillabee green stone outboard of the present position of the pine mountain terrane and suggest links to ordovician plutonism in the overlying east ern blue ridge and possibly to widespread k bentonite deposits within ordovician plat form units the tectonic evolution of the hil labee greenstone exhibits many unusual and intriguing features including 1 premeta morphic emplacement along a basal cryptic thrust which is remarkably concordant to both hanging wall and footwall sequences across its entire extent 230 km 2 forma tion transport and emplacement of the arc fragment accompanied by minimal deforma tion of the hillabee greenstone and under lying outer margin shelf rocks 3 emplace ment temporally coincident with the adjacent collision of the younger tectonically indepen dent ouachita volcanic arc with southeastern laurentia these features highlight strong contrasts in the ordovician taconian evolu tion of the southern and northern parts of the appalachian orogen

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

convective mixing of air in firn at four polar sites air withdrawn from the firn at four polar sites dome fuji h72 and ym85 antarctica and north grip greenland was measured for 15 no f n2 and 18 oo f o2 to test for the presence of convective air mixing in the top part of the firn known as the convective zone understanding the convective zone and its possible relationship to surface conditions is important for constructing accurate ice core greenhouse gas chronologies and their phasing with respect to climate change the thickness of the convective zone was inferred from a regression line with barometric slope of the data in the deep firn it is less than a few meters at h72 and ngrip whereas a substantial convective zone is found at dome fuji 8 6 2 6 m and ym85 14 0 1 8 m by matching the outputs of a diffusion model to the data effective eddy diffusivities required to mix the firn air are found at the surface of dome fuji and ym85 these are found to be several times greater than the molecular diffusivity in free air the crossover from dominance of convection to molecular diffusion takes place at 7 2 11 2 and 0 5 0 5 m at dome fuji ym85 and ngrip respectively these depths can be used as an alternative definition of the convective zone thickness the firn permeability at dome fuji is expected to be high because of intense firn metamorphism due to the low accumulation rate and large seasonal air temperature variation at the site the firn layers in the top several meters are exposed to strong temperature gradients for several decades leading to large firn grains and depth hoar that enhance permeability the thick convective zone at ym85 is unexpected because the temperature accumulation rate and near surface density are comparable to ngrip the strong katabatic wind at ym85 is probably responsible for creating the deep convection the largest convective zone found in this study is still only half of the current inconsistency implied from the deep ice core gas isotopes and firn densification models 2006 elsevier b v all rights reserved

lithospheric sources of north florida usa tholeiites and implications for the origin of the suwannee terrane abstract sr nd and pb isotopic data for mesozoic tholeiites from six drillholes in northern florida suwannee terrane indicate derivation of the magmas from a source that was slightly incompatible element enriched with respect to bulk earth probably continental lithospheric mantle variably contaminated by more enriched crust initial isotopic compositions of the florida tholeiites show a considerable range 87 sr 86 sr 0 7059 to 0 7097 end 4 6 to 0 1 206 pb 204 pb 18 30 to 18 65 207 pb 204 pb 15 57 to 15 66 and 208 pb 204 pb 38 22 to 38 35 these compositions are distinctly different from those of mesozoic alkali basalts from the subsurface of southwestern florida heatherington a l mueller p a 1991 geochemical evidence for triassic rifting in southwestern florida tectonophysics 188 291 302 it is suggested that the north florida tholeiites were derived from within the continental lithosphere in contrast with mesozoic volcanic rocks from southwestern florida which have been previously suggested to be related to a mantle plume sm nd isotope systematics suggest derivation of the tholeiites from a 1 ga source a similar age to that proposed by pegram pegram w j 1990 development of continental lithospheric mantle as reflected in the chemistry of the mesozoic appalachian tholeiites usa earth and planetary science letters 97 316 331 for the source of tholeiitic basalts within the carolina terrane this supports proterozoic tectonic models which suggest that both the suwannee and carolina terranes had affinities with south american portions of gondwana containing 1 ga crust specifically the sunsas rondonian province nance murphy 1996 mueller p a kozuch m heatherington a l wooden j l offield t w koeppen r p klein t l nutman a p 1996 evidence for mesoproterozoic basement in the carolina terrane and speculations on its origin in nance r d thompson m d eds avalonian and related peri gondwanan terranes of the circum north atlantic gsa special paper 304 pp 207 218

metamorphism of stratified firn at dome fuji antarctica a mechanism for local insolation modulation of gas transport conditions during bubble close off 1 the evolution of the structure of a 112 59 m long firn core recovered at dome fuji east antarctica was investigated in order to improve understanding of firn densification and bubble formation processes which are important for interpreting local insolation proxies used for astronomical dating of deep ice cores using selected samples we measured physical properties including 1 the relative dielectric permittivities in both the vertical and horizontal planes 2 the bulk density at a resolution of millimeters 3 the three dimensional geometric structure of pore space and 4 crystal orientation fabrics we found that the firn at dome fuji contains horizontal strata with thicknesses of several centimeters near the surface of the ice sheet these strata are characterized by contrasting bulk density earlier field studies suggest that summer insolation causes densification of surface firn down to 30 m density maxima exhibited a clear positive correlation with the strength of structural anisotropy and c axis clustering around the vertical in contrast the correlation is negative in deeper firn confirming previous findings that initially less dense firn became denser than initially dense firn in addition numerous examples of textures indicating that deformation preferentially occurred in weaker layers were found moreover the initially dense firn layers were more permeable for air near the bottom of firn we propose a model linking firn properties with conditions for the gas transport processes near the bottom of firn the model explains how stronger insolation can lead to bulk ice with a lower o2 n2 ratio and smaller total gas content

pinwarian to grenvillian magmatic evolution in the central grenville province new constraints from id tims u pb ages and coupled lu hf s mc icp ms data understanding the magmatic evolution of the rocks once comprising the hinterland of the grenville orogen through their mesoproterozoic formation is a key to understanding the grenvillian orogeny as a whole in this contribution we present high precision isotope dilution thermal ionization mass spectrometry id tims u pb and coupled solution multicollector inductively coupled plasma mass spectrometry s mc icp ms lu hf zircon data from magmatic rocks occurring in the allochthonous belt of the grenville orogen in the central part of the grenville province we document the presence of a large tract of pinwarian crust represented by a 1497 5 ma granitic gneiss as well as large late geon 14 to early geon 13 1434 7 11 1413 12 1393 8 1383 1 ma magmatic complexes one grenvillian plutonic suite of 1015 2 ma that cross cuts the host rock metamorphic fabric has also been dated this age provides a minimum age of ottawan metamorphism in the region the hf isotopic data show that the magmatic roc

40ar 39ar thermochronology and alleghanian development of the southernmost appalachian piedmont alabama and southwest georgia 40ar 39ar age spectra of hornblende muscovite and microcline and total fusion ages of biotite from metamorphic rocks of the inner piedmont pine mountain and uchee belts are reported mineral cooling ages from the eastern part of the inner piedmont are as follows hornblende 320 ma muscovite 296 ma biotite 293 ma and microcline diffusional release patterns tmax 267 ma tmin 234 ma a 347 ma hornblende spectrum from the highest inner piedmont structural level sampled is the oldest date determined and implies earlier passage of this level through the 500 c isotherm most release spectra from pine mountain belt units are discordant with little or no apparent geologic meaning modified saddle shaped release patterns for hornblende indicate extraneous argon with a maximum age of 358 ma muscovite from the pine mountain belt cover sequence is 286 ma plateau age and one from the underlying grenville basement is 277 ma correlation age indicating cooling below the 350 c isotherm plateau ages on uchee belt rocks are as follows hornblende from 297 to 288 ma muscovite 285 ma biotite 276 ma and microcline tmax 261 ma tmin 230 ma muscovite fish from a bartletts ferry fault zone phyllonite have a plateau age of 283 ma the 40ar 39ar results combined with other geologic data indicate that 1 a large part of the southern and inner piedmonts of alabama and southwest georgia experienced a late paleozoic amphibolite facies thermal and deformational event contemporaneous with the alleghanian orogeny observed in the foreland 2 the tectonic development of this event characterized by initial crustal thickening followed by right slip and normal slip movements is grossly similar to that described for the amphibolite facies alleghanian belt in the eastern piedmont of south carolina and georgia and 3 extensional movements along the flanks of the pine mountain window occurred between ca 277 ma and the late triassic early jurassic and thus may reflect latest alleghanian extensional collapse or mesozoic rifting

the southern termination of the philippine trench abstract recent studies of the southern end of the philippine trench using gloria sidescan sonar and single channel seismic data and geological studies on the island of halmahera suggest that the trench is in the process of propagating south and that some of the ese wnw convergence is transferred via a broad ne sw zone of dextral strike slip across northern halmahera into the molucca sea collision zone ne sw ridges and lineaments on the seafloor can be traced into major faults and structural trends on land the bathymetric expression of the philippine trench is lost where it meets an oceanic plateau on the philippine sea plate this elevated and probably thickened plateau appears to have inhibited any further propagation of the trench southwards to the north of the plateau there is a well developed accretionary prism but to the south deformation of sediments on the seafloor is less intense a prominent ridge with sediments passively banked up against it marks the southern limit of deformation associated with the philippine trench to the south lies the eastern halmahera waigeo ophiolite terrane an area of shallow water and islands underlain by ophiolitic basement between halmahera and the sorong fault zone there are no bathymetric or structural features to indicate any form of active link between the philippine trench and the sorong fault through or along the northern side of this ophiolite terrane there is no earthquake evidence for any form of fault linking the philippine trench to either the palau trench or the sorong fault but there are numerous earthquake epicentres reported between the southern end of the trench and the molucca sea the island of halmahera is considered to lie in a diffuse boundary zone at the margin of the philippine sea plate continued collision of the opposing arcs in the molucca sea will ultimately prevent further e w convergence between halmahera and sulawesi it is predicted that either the philippine trench will propagate south along its present line to meet the sorong fault or that another trench will develop further to the east in either case the arc and ophiolite terrane of halmahera might then be accreted onto the eurasian margin

the age of the air in the firn and the ice at summit greenland in the framework of the eurocore drilling project 1989 at summit central greenland air samples were collected from a drill hole in the firn at several depth levels the samples have been analyzed for 85kr co2 ch4 cfc s f 11 and f 12 contents and for the isotopic composition of nitrogen and oxygen the measured data are compared with the results of a diffusion model co2 for example takes 12 years with a standard deviation of 7 5 years to diffuse from the surface to the firn ice transition under the present climatic conditions at summit the difference between the age of the ice and the mean age of the occluded air is approximately 210 years an enrichment of the heavier isotopes due to separation by gravity has been observed molecular diffusion alone suffices to explain the observed concentration trends in the firn column we can however not exclude the existence of a convective zone below the surface we have also measured the density and closed porosity along the firn core that has been retrieved when drilling the hole the porosity data reveal that the air is occluded into bubbles mainly between 65 and 80 m corresponding to densities of 790 and 830 kg m3 respectively

the importance of along margin terrane transport in northern gondwana insights from detrital zircon parentage in neoproterozoic rocks from iberia and brittany abstract detrital zircons from late neoproterozoic rocks of the peri gondwanan cadomian belt of sw iberia and north armorican domain of brittany record neoproterozoic ca 860 550 ma palaeoproterozoic ca 2300 1800 and archaean ca 3300 2600 ma u pb ages the absence of mesoproterozoic zircons suggests that these terranes evolved in a peri w african realm this is in contrast to other western european terranes that preserve mesoproterozoic zircons and are likely to have evolved in a peri amazonian realm such a contrast in detrital zircon populations coupled with the presence of mesoproterozoic zircons in the ordovician armorican quartzite deposited in a peri african platform is interpreted to record along margin terrane transport the change in provenance suggests that subduction was replaced by transform faults that juxtaposed amazonia derived terranes against w africa derived terranes to form the avalonia and armorica microcontinents subsequent extension along the margin resulted in the birth of the rheic ocean and the outboard drift of avalonia

temperatures heating rates and vapour pressures in near surface snow at the south pole a finite volume model is used to simulate 9 years 1995 2003 of snow temperatures at the south pole the upper boundary condition is skin surface temperature derived from routine upwelling longwave radiation measurements while the lower boundary condition is set to the seasonal temperature gradient at 6 5 m depth taken from prior measurements at the south pole we focus on statistics of temperature heat fluxes heating rates and vapour pressures in the top metre of snow but present results from the full depth of the model 6 5 m the monthly mean net heat flux into the snow agrees with results from previous studies performed at the south pole on shorter timescales the heating rates and vapour pressures show large variability the net heat flux into the snow which is between 5wm 2 in the monthly mean can be greater than 20 w m 2 on hourly timescales on sub daily timescales heating rates exceed 40 k d 1 in the top 10 cm of the snow subsurface temperatures and therefore heating rates are more variable during winter april september due to increased synoptic activity and the presence of a strong surface based atmospheric temperature inversion the largest vapour pressures 60 70 pa and vertical gradients of vapour pressure are found in the top metre of snow during the short summer december january in contrast during the long winter the low temperatures result in very small vapour pressures and insignificant vapour pressure gradients the high summertime vapour pressure gradients may be important in altering the isotopic composition of snow and ice on the antarctic plateau

terrane accretion production and continental growth a perspective based on the origin and tectonic fate of the aleutian bering sea region orogenesis in the aleutian bering sea region would create an expansive new area of pacific rim mountain belts the region itself formed about 55 ma as a consequence of the suturing of a single exotic fragment of oceanic crust aleutia to the pacific s alaskan siberian margin a massive overlap assemblage of the igneous crust of the aleutian arc and the thick sedimentary masses of the aleutian basin have since accumulated above the captured basement terrane of aleutia future closure of the aleutian bering sea region either northward toward the continent or southward toward the aleutian arc would structurally mold new continental crust to the north american plate the resulting beringian orogen would be constructed of a collage of suspect terranes although some terranes would include exotic crustal rocks formed as far as 5000 km away most terranes would be kindred or cotetonic blocks composed of the overlap assemblage and of relatively local 100 1000 km derivation the aleutian bering sea perspective bolsters the common supposition that although disrupted and smeared by transcurrent faulting examples of kindred assemblages should exist and perhaps commonly in ocean rim mountain belts

relationship between the ellisville pluton and chopawamsic fault establishment of significant late ordovician faulting in the appalachian piedmont of virginia

constraints on provenance stratigraphic correlation and structural context of the volta basin ghana from detrital zircon geochronology an amazonian connection abstract we have dated 1060 detrital zircon grains from the neoproterozoic to lower palaeozoic volta basin ghana and from sandstones in the adjoining thrust sheets belonging to the pan african dahomeyide belt all dated zircons in the lower voltaian bombouaka group are older than 1000 ma while samples from the middle voltaian oti group and the upper voltaian obosum group contain numerous zircons of 600 1000 ma the samples we have studied from the dahomeyide thrust sheets buem and togo structural units have zircon age spectra similar to those from the bombouaka group confirming a correlation of the investigated sandstones with lower voltaian strata the bombouaka group was deposited between 1000 and 600 ma perhaps shortly after 1000 ma as suggested by earlier rb sr data on clay minerals deposition of the oti and obosum groups took place shortly after 600 ma perhaps continuing into the lower palaeozoic most samples contain palaeoproterozoic zircons with ages of 2000 2200 ma that probably have been derived from the surrounding crystalline birimian basement archaean zircons present in smaller proportions may have come from archaean rocks in the west african craton most zircons of 1000 1900 ma were probably derived from sources outside the west african craton the amazonian craton being a plausible source region zircons with ages around 1200 ma are believed to have been derived from grenvillian orogenic belts perhaps those that fringe the amazonian craton if this is so no major seaways could have been present between these belts and the west african craton during the early neoproterozoic zircons younger than 1000 ma were probably derived from an eastern continental block that collided with the west african craton during the pan african orogeny 600 ma ago

mesoproterozoic trans laurentian magmatism a synthesis of continent wide age distributions new sims u pb ages zircon saturation temperatures and hf and nd isotopic compositions abstract the origin and tectonic setting of the mesoproterozoic 1 50 1 34 ga granite rhyolite provinces of the midcontinent region of the united states and the coeval a type granite plutons of the southwestern united states have been an enigma for decades in this paper we review the history of this problem examine the significance of a wealth of previously published u pb zircon and whole rock sm nd data and present new u pb zircon age and lu hf isotopic data from selected zircon samples our data show that the initiation of this magmatism spans the interval 1 50 1 40 ga across the continent but there is a general younging westward younger 1 39 1 34 ga magmatism also occurred across the continent but forms a major magmatic event only in the south central mid continent there is generally a good correlation between sm nd and lu hf isotopic compositions and inferred mantle derivation model ages except possibly in southeastern oklahoma the absence of older cores in zircons is compatible with calculated zircon saturation temperatures of ca 850 c and supports mantle driven basaltic underplating and resulting crustal partial melting as a petrogenetic model for the a type magmatism our new data compilation refines the existence of the nd line that extends from southwestern texas into southeastern michigan and the western grenville province in canada it marks an isotopic boundary between granites and rhyolites of cratonal origin to the northwest 1 8 1 6 ga model ages and those of more juvenile origin to the southeast 1 5 1 4 ga model ages our suggested tectonic model for the mesoproterozoic a type magmatism across eastern and southern laurentia involves the development of an active laurentian margin extending from eastern canada across what is now the midcontinent region of the united states and at least into the current southwestern united states magmatism along this margin was likely related to asthenosphere lithosphere interactions associated with the accretion of outboard arcs and possible exotic terranes that caused complex deformational and magmatic responses in the continental interior this activity evidently spread from northeast to southwest over a period of ca 70 million years the occurrence of the younger ca 1 39 1 34 ga event indicates that similar processes occurred some 130 million years later suggesting a long lived convergent plate margin and recurring basaltic underplating in some locations perhaps due to inboard lithospheric delamination and or mantle upwelling

4 1 5 c abrupt warming 11 270 yr ago identified from trapped air in greenland ice nitrogen and argon isotopes in air trapped in a greenland ice core gisp2 show two prominent peaks in the interval 11 800 10 800 b p which indicate two large abrupt warming events the first abrupt warming 10 4 c is the widely documented event at the end of the younger dryas here we report on the second abrupt warming 4 1 5 c which occurred at the end of a short lived cooler interval known as the preboreal oscillation 11 270 30 b p a rapid snow accumulation increase suggests that the climatic transition may have occurred within a few years the character of the preboreal oscillation and the subsequent abrupt warming is similar to the dansgaard oeschger d o events in the last glacial period suggestive of a common mechanism but different from another large climate change at 8 200 b p in which cooling was abrupt but subsequent warming was gradual the large abrupt warming at 11 270 b p may be considered to be the final d o event prior to the arrival of the present stable and warm epoch

observing and modeling the influence of layering on bubble trapping in polar firn interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn much work has been done to understand the processes affecting air transport in the open pores of the firn but a paucity of data from air trapped in bubbles in the firn ice transition region has limited the ability to constrain the effect of bubble closure processes here we present high resolution measurements of firn density methane concentrations nitrogen isotopes and total air content that show layering in the firn ice transition region at the west antarctic ice sheet wais divide ice core site using the notion that bubble trapping is a stochastic process we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach we include the process of bubble trapping into an open porosity firn air transport model and obtain a good fit to the firn core data we find that layering broadens the depth range over which bubbles are trapped widens the modeled gas age distribution of air in closed bubbles reduces the mean gas age of air in closed bubbles and introduces stratigraphic irregularities in the gas age scale that have a peak to peak variability of 10 years at wais divide for a more complete understanding of gas occlusion and its impact on ice core records we suggest that this experiment be repeated at sites climatically different from wais divide for example on the east antarctic plateau

synchronising edml and northgrip ice cores using δ18o of atmospheric oxygen δ18oatm and ch4 measurements over mis5 80 123 kyr abstract water isotope records from the epica dronning maud land edml and the northgrip ice cores have revealed a one to one coupling between antarctic isotope maxima aim and greenland dansgaard oeschger do events back to 50 kyr in order to explore if this north south coupling is persistent over marine isotopic stage 5 mis 5 a common timescale must first be constructed here we present new records of 18o of o2 18oatm and methane ch4 measured in the air trapped in ice from the edml 68 147 kyr and northgrip 70 123 kyr ice cores we demonstrate that through the period of interest ch4 records alone are not sufficient to construct a common gas timescale between the two cores millennial scale variations of 18oatm are evidenced over mis 5 both on the antarctic and greenland ice cores and are coupled to ch4 profiles to synchronise the northgrip and edml records they are shown to be a precious tool for ice core synchronisation with this new dating strategy we produce the first continuous and accurate sequence of the north south climatic dynamics on a common ice timescale for the last glacial inception and the first do events of mis 5 reducing relative dating uncertainties to an accuracy of a few centuries at the onset of do events 24 to 20 this edml northgrip synchronisation provides new firm evidence that the bipolar seesaw is a pervasive pattern from the beginning of the glacial period the relationship between antarctic warming amplitudes and their concurrent greenland stadial duration highlights the particularity of do event 21 and its antarctic counterpart our results suggest a smaller southern ocean warming rate for this long do event compared to do events of mis 3

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

basin formation and neogene sedimentation in a backarc setting halmahera eastern indonesia abstract it has been proposed that basins in backarc settings form in association with subduction by thinning of continental crust by backarc spreading in oceanic crust by compression retro arc basins or by trapping of pieces of oceanic plate behind an arc the halmahera basin in eastern indonesia developed in a backarc setting but does not fall into any of these categories it formed by subsidence of thickened crust made up of imbricated mesozoic palaeogene arc and ophiolite rocks halmahera lies at the western edge of the philippine sea plate in a complex zone of convergence between the eurasian margin the oceanic plates of the west pacific and the australian indian plate to the south the basement is an imbricated complex of mesozoic to palaeogene ophiolite arc and arc related rocks during the miocene this basement complex formed an area of thickened crust on which carbonate reef and reef associated sediments were deposited we interpret this shallow marine region of thickened crust to be similar to many of the oceanic plateaux and ridges found within the philippine sea plate to day in the late miocene convergence between the philippine sea plate and the eurasian margin resulted in the formation of the halmahera trench to the west of this region of thickened crust subduction of the molucca sea plate at the trench caused the development of a volcanic island arc subsidence in the backarc area produced a broad sedimentary basin filled by clastics eroded from the arc and from uplifted basement and cover rocks the basin was asymmetric with the thickest sedimentary fill on the western side against the volcanic arc the halmahera basin was modified in the plio pleistocene by east west compression as the molucca sea plate was eliminated by subduction

homogeneous climate variability across east antarctica over the past three glacial cycles recent ice core studies have raised the disturbing possibility that glacial interglacial climate changes may be non uniform across antarctica1 2 these findings have been confined to records from the ross sea sector of the continent but significant deviations in other areas would call into question the widely assumed validity of the climate record obtained from vostok east antarctica on large spatial scales3 here we present an isotopic profile from a core drilled at dome fuji4 5 situated 1 500 km from vostok in a different sector of east antarctica the two records show remarkable similarities over the past three glacial cycles the extent of the dome fuji record in both large amplitude changes such as terminations interglacials and interstadials and more subtle glacial events even when the origin of precipitation is accounted for our results indicate that antarctic climate is essentially homogeneous at the scale of the east antarctic plateau possibly as a consequence of the symmetry of the plateau and the adjacent ocean

firn air δ15n in modern polar sites and glacial interglacial ice a model data mismatch during glacial periods in antarctica the phase lag between atmospheric composition air bubbles and temperature water isotopes can be quantified from ice cores provided that the age difference between entrapped air and the surrounding air can be correctly estimated this difference depends on the lock in depth lid when air no longer mixes with the atmosphere the lid can be estimated from firnification models or from the air isotopic composition 15n and 40ar both methods give consistent results for greenland and one coastal site in antarctica byrd new firn measurements in greenland northgrip and antarctica berkner island bas depot dome c confirm that firnification models correctly reproduce the present lid over a large range of surface conditions however a systematic mismatch is observed for the last glacial maximum lgm in east antarctic sites vostok dome c dome f questioning the model s validity here we use new 15n measurements from two coastal antarctic sites kohnen station and law dome providing depth estimates again distinct from firnification model calculations we show that this discrepancy can be resolved by revising the estimate of past accumulation rates 15n measurements can therefore help to constrain past accumulation rate and improve ice core dating

ordovician volcanic arc terrane in the central appalachian piedmont of maryland and virginia shrimp u pb geochronology field relations and tectonic significance

formation of high pressure metabasites in the southern appalachian blue ridge via taconic continental subduction beneath the laurentian margin 1 tectonic models that account for the occurrence of eclogite retrograde eclogite peridotite and migmatitic basement gneisses in the southern blue ridge invoke closure of a small ocean basin between the eastern blue ridge ebr and western blue ridge wbr in this obducted ophiolite model franciscan type eastward subduction of oceanic crust beneath the piedmont arc occurred prior to obduction of the arc onto the laurentian margin the discovery of kilometer scale eclogite bodies near the ebr wbr boundary are cited as evidence for obducted ophiolite accreted island arc models however thermobarometric estimates based on mineral chemistry from hp metabasites of the western central and eastern blue ridge indicate that eclogites and granulites are medium temperature type 550 900 c inconsistent with the obducted ophiolite model mid ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550 c thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the blue ridge allow for a model of westward subduction of continental material beneath laurentia during the taconic orogeny that produced a megamelange now preserved in the central and eastern blue ridge tectonic provinces p t paths determined from blue ridge hp metabasites are similar to p t paths determined previously from caledonian and variscan eclogites thermobarometric similarities imply a common tectonic history of continental collision subduction of continental crust to depths of 50 65 km followed by rapid eduction

modeling the densification of polar firn including heat diffusion application to close off characteristics and gas isotopic fractionation for antarctica and greenland sites 1 modeling the densification of polar firn is essential to estimate variations of close off characteristics density close off depth delta age in relation with past climate parameters temperature and accumulation rates furthermore the air composition in the firn is modified by gravitational and thermal fractionation and stable isotope measurements of permanent gases like nitrogen or argon can provide information on the amplitude of these fractionations in this work we propose a new model coupling firn densification and heat diffusion in addition to the determination of the firn thickness and gas ice age differences the model allows a reconstruction of the time evolution of the temperature for different sites in antarctica vostok and greenland gisp2 and therefore the evolution of gravitational and thermal isotopic fractionations in firn under present day conditions the modeled profiles are in good agreement with the available temperature measurements in firn for sites with low accumulation rates such as vostok the results show the existence of temperature gradients in the firn column even when no rapid climatic changes occur the comparison of the modeled 15n results to measurements allows to better constrain the 18o temperature relationship used to infer the surface temperature history and for gisp2 the model validates the long term borehole based temperature

a method for precise measurement of argon 40 36 and krypton argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in greenland and at siple dome antarctica we describe a method for measuring the 40ar 36ar ratio and the 84kr 36ar ratio in air from bubbles trapped in ice cores these ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured 15n 14n both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on earth s climate the overall precision of the 40ar 36ar method 1 standard error of the mean is 0 012 for a sample analyzed in duplicate corresponding to 0 6 m in reconstructed firn thickness we use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation about 100 g of ice is used for a duplicate pair of analyses an example of the technique applied to the gisp2 ice core yields an estimate of 11 3k of abrupt warming at the end of the last glacial period 15 000 years ago the krypton argon ratio can provide a diagnostic of argon leakage out of the bubbles which may happen naturally during bubble close off or artifactually if samples are warmed near the freezing point during core retrieval or storage argon leakage may fractionate the remaining 40ar 36ar ratio by 0 007 per change in 84kr 36ar introducing a possible bias in reconstructed firn thickness of about 2 m if thermal diffusion is not accounted for or 6 m if thermal diffusion effects are quantified with measured 15n 14n reproducibility of 84kr 36ar measured in air is about 0 2 1 standard error of the mean but is about 1 for ice core samples ice core samples are systematically enriched in 84kr 36ar relative to atmosphere by 5 probably reflecting preferential size dependent exclusion of the smaller argon atom during bubble entrapment recent results from the siple dome ice core reveal two climate events during the last deglaciation including an 18 m reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr bp and a partial or total removal of the firn during an ablation event at 15 3 kyr bp

greenland temperature response to climate forcing during the last deglaciation greenland ice core water isotopic composition 18 o provides detailed evidence for abrupt climate changes but is by itself insufficient for quantitative reconstruction of past temperatures and their spatial patterns we investigate greenland temperature evolution during the last deglaciation using independent reconstructions from three ice cores and simulations with a coupled ocean atmosphere climate model contrary to the traditional 18 o interpretation the younger dryas period was 4 5 2 c warmer than the oldest dryas due to increased carbon dioxide forcing and summer insolation the magnitude of abrupt temperature changes is larger in central greenland 9 to 14 c than in the northwest 5 to 9 c fingerprinting a north atlantic origin simulated changes in temperature seasonality closely track changes in the atlantic overturning strength and support the hypothesis that abrupt climate change is mostly a winter phenomenon

inferences on late stage evolution of the russell lake allochthon and the soapstone ridge complex in georgia southern appalachians based on chlorite geothermometry

thermal fractionation of air in polar firn by seasonal temperature gradients 1 air withdrawn from the top 5 15 m of the polar snowpack firn shows anomalous enrichment of heavy gases during summer including inert gases following earlier work we ascribe this to thermal diffusion the tendency of a gas mixture to separate in a temperature gradient with heavier molecules migrating toward colder regions summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion here we explore and quantify this process further in order to 1 correct for bias caused by thermal diffusion in firn air and ice core air isotope records 2 help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion severinghaus et al 1998 and 3 address whether air in polar snow convects during winter and if so whether it creates a rectification of seasonality that could bias the ice core record we sampled air at 2 m depth intervals from the top 15 m of the firn at two antarctic sites siple dome and south pole including a winter sampling at the pole we analyzed 15n 14n 40ar 36ar 40ar 38ar 18o 16o of o2 o2 n2 84kr 36ar and 132xe 36ar the results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30 although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision relative values of different gas pairs may at siple dome 40ar 4 is 66 2 as sensitive to thermal diffusion as 15n in agreement with laboratory calibration 18o 2 is 83 3 and 84kr 48 is 33 3 as sensitive as 15n the corresponding figures for summer south pole are 64 2 81 3 and 34 3 accounting for atmospheric change the figure for o2 n2 4 is 90 3 at siple dome winter south pole shows a strong depletion of heavy gases as expected however the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities except for kr constant at 34 4 suggesting the action of some other process that is not currently understood no evidence for wintertime convection or a rectifier effect is seen

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

plate boundary evolution in the halmahera region indonesia abstract halmahera is situated in eastern indonesia at the southwest corner of the philippine sea plate active arc arc collision is in process in the molucca sea to the west of halmahera new stratigraphic observations from halmahera link this island and the east philippines and record the history of subduction of the molucca sea lithosphere the halmahera basement complex and the basement of east mindanao were part of an arc and forearc of late cretaceous early tertiary age and have formed part of a single plate since the late eocene early oligocene there is no evidence that halmahera formed part of an oligo miocene arc but arc volcanism associated with eastwards subduction of the molucca sea beneath halmahera began in the pliocene and the pliocene arc is built on a basement of the early tertiary arc arc volcanism ceased briefly during the pleistocene and the arc shifted westwards after an episode of deformation the present active arc is built upon deformed rocks of the pliocene arc the combination of new stratigraphic information from the halmahera islands and models of the present day tectonic structure of the region deduced from seismic and other geophysical studies is used to constrain the tectonic evolution of the region since the miocene diachronous collision at the western edge of the philippine sea plate which began in mindanao in the late miocene impeded the movement of the philippine sea plate and further motion has been achieved by a combination of strike slip motion along the philippine fault subduction at the philippine trench and subduction of the molucca sea lithosphere beneath halmahera

three trench endings in eastern indonesia abstract underway surveys using long range sidescan sonar as well as conventional marine geophysical techniques have been made of three trench endings in eastern indonesia in each case the termination is associated with the presence of a block of thickened crust the transition between the sunda trench and the troughs which encircle the banda arc is a consequence of collision with the northern margin of the australian continent whereas the southern termination of the philippine trench is defined by the presence of an oceanic plateau which impedes further southwards propagation both these trenches are currently active and are associated with clearly defined wadati benioff zones the new guinea trench terminates in the west at a n trending seafloor rise known as the mapia ridge the relationship between the two features is not well understood there are no clear indications of present day subduction along the northern margin of new guinea and insufficient data from the flanks of the mapia ridge to show whether this ever had a subduction history the new data obtained within the trench immediately east of the ridge suggests that subduction has ceased in the westernmost part of the new guinea trench and that the oceanic crust of the ayu basin west of the mapia ridge post dates the period of active subduction

a plate tectonic scenario for the iapetus and rheic oceans abstract the tectonics dynamics and biogeographic landscape of the early paleozoic were dominated by the opening and expansion of one large ocean the rheic and the diminution to terminal closure of another iapetus an understanding of the evolution of these oceans is thus central to an understanding of the early paleozoic but their chronicle also presents a rich temporal profile of the wilson cycle illustrating continental scale rifting microcontinent formation ocean basin development arc accretion and continent continent collision nevertheless contemporary paleogeographic models of the iapetus and rheic oceans remain mostly schematic or spatiotemporally disjointed which limits their utility and hinders their testing moreover many of the important kinematic and dynamic aspects of the evolution of these oceans are impossible to unambiguously resolve from a conceptual perspective and the existing models unsurprisingly present a host of contradictory scenarios with the specific aim to resolve some of the uncertainties in the evolution of this early paleozoic domain and a broader aim to instigate the application of quantitative kinematic models to the early paleozoic i present a new plate tectonic model for the iapetus and rheic oceans the model has realistic tectonic plates which include oceanic lithosphere that are defined by explicit and rigorously managed plate boundaries the nature and kinematics of which are derived from geological evidence and plate tectonic principles accompanying the presentation and discussion of the plate model an extensive review of the underlying geological and paleogeographic data is also presented

u pb geochronologic nd isotopic and geochemical evidence for the correlation of the chopawamsic and milton terranes piedmont zone southern appalachian orogen abstract we report u pb crystallization ages from four metavolcanic rocks and two granitic gneiss samples as well as whole rock chemical analyses and sm nd isotopic ratios from 25 metaigneous and metasedimentary rocks from the chopawamsic and milton terranes southern appalachian orogen a metarhyolite sample from the chopawamsic formation and a metabasalt sample from the ta river formation in the chopawamsic terrane have indistinguishable u pb crystallization ages of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 471 4 pm 1 3

firn processes and δ15n potential for a gas phase climate proxy abstract in order to quantify the sequence of events between changes in atmospheric composition and climate changes recorded in ice cores we must accurately account for the age difference between ice and gas at a given depth this gas age ice age difference depends on the age of the ice at the bottom of the firn layer where the bubbles are closed off firn densification models are used to calculate how this age difference varied in the past but have an uncertainty on the order of 1000 years for central antarctic sites here we explore the possibility that 15 n of n 2 is a gas phase proxy of climate which can be used to synchronize gas and ice records we present the 15 n record from the epica dome c edc ice core covering the last three glacial terminations and five glacial interglacial cycles between 300 and 800 ka previous studies have shown that gravitational settling enriches 15 n as a function of the diffusive column height in the firn if densification models prediction of deeper firn close off under glacial conditions is correct then we would expect heavier 15 n during glacial periods and a negative correlation with temperature instead edc 15 n is positively correlated with the ice deuterium content a proxy for temperature as previously reported at vostok dome fuji and epica dronning maud land we propose a mechanism that links accumulation rate firn permeability and convective mixing in the top meters of the firn to explain this correlation between 15 n and ice deuterium content the tightest correlation is observed over glacial terminations supporting the idea that 15 n is a property in the gas phase that records changes in surface conditions linked to deglacial warming

a newly identified gondwanan terrane in the northern appalachian mountains implications for the taconic orogeny and closure of the iapetus ocean the taconic and salinic orogenies in the northern appalachian mountains record the closure of the iapetus ocean which separated peri laurentian and peri gondwanan terranes in the early paleozoic the taconic orogeny in new england is commonly depicted as an ordovician collision between the peri laurentian shelburne falls arc and the laurentian margin followed by silurian accretion of peri gondwanan terranes during the salinic orogeny new u pb zircon geochronology demonstrates that the shelburne falls arc was instead constructed on a gondwanan derived terrane preserved in the moretown formation which we refer to here as the moretown terrane metasedimentary rocks of the moretown formation were deposited after 514 ma and contain abundant ca 535 650 ma detrital zircon that suggest a gondwanan source the moretown formation is bound to the west by the peri laurentian rowe belt which contains detrital zircon in early paleozoic metasedimentary rocks that is indistinguishable in age from zircon in laurentian margin rift drift successions these data reveal that the principal iapetan suture in new england is between the rowe belt and moretown terrane more than 50 km farther west than previously suspected the moretown terrane is structurally below and west of volcanic and metasedimentary rocks of the hawley formation which contains laurentian derived detrital zircon providing a link between peri laurentian and peri gondwanan terranes the moretown terrane and hawley formation were intruded by 475 ma plutons during peak activity in the shelburne falls arc we propose that the peri laurentian rowe belt was subducted under the moretown terrane just prior to 475 ma when the trench gap was narrow enough to deliver laurentian detritus to the hawley formation interaction between peri laurentian and peri gondwanan terranes by 475 ma is 20 m y earlier than documented elsewhere and accounts for structural relationships early ordovician metamorphism and deformation and the subsequent closure of the peri laurentian taconic seaway in this scenario a rifted arc system on the gondwanan margin resulted in the formation of multiple terranes including the moretown that independently crossed and closed the iapetus ocean in piecemeal fashion

magnitude and temporal evolution of dansgaard oeschger event 8 abrupt temperature change inferred from nitrogen and argon isotopes in gisp2 ice using a new least squares inversion abstract polar temperature is often inferred from water isotopes in ice cores however non temperature effects on 18 o are important during the abrupt events of the last glacial period such as changes in the seasonality of precipitation the northward movement of the storm track and the increase in accumulation these effects complicate the interpretation of 18 o as a temperature proxy here we present an independent surface temperature reconstruction which allows us to test the relationship between 18 o ice and temperature during dansgaard oeschger event 8 38 2 thousand yrs ago using new 15 n and 40 ar data from the gisp2 ice core in greenland this temperature reconstruction relies on a new inversion of inert gas isotope data using generalized least squares and includes a robust uncertainty estimation we find that both temperature and 18 o increased in two steps of 20 and 140 yrs with an overall amplitude of 11 80 1 8 c between the stadial and interstadial centennial mean temperature the coefficient d o 18 d t changes with each time segment which shows that non temperature sources of fractionation have a significant contribution to the 18 o signal when measured on century averaged values we find that d o 18 d t 0 32 0 06 c which is similar to the glacial holocene value of 0 328 c

geology and geochemistry of the mafic and ultramafic intrusive rocks dadeville belt alabama the majority of mafic complexes within the dadeville belt of alabama9s inner piedmont contain two plutonic suites the doss mountain suite consisting of metanorite dmtn and metaorthopyroxenite dmto and the slaughters suite which is composed of meta gabbro sg all plutonic rocks are pre regional metamorphism in age but in some places are predominantly free of macroscopic metamorphic fabrics in the doss mountain area the dmtn and dmto show progressive mineralogical and textural changes into a poorly foliated and nonlayered amphibolite and an actinolite schist respectively such changes are essentially isochemical the dmtn dmto amphibolite actinolite schist complex is interpreted to be a layered norite orthopyroxenite intrusion that has suffered variable degrees of metamorphism in other areas the dmtn and dmto intrude well lineated and foliated amphibolites of contrasting composition that belong to the ropes creek formation the sg has well developed olivine plagioclase reaction coronas as the result of metamorphism in all areas examined the sg appears to intrude the amphibolites the sg has high alumina and incompatible element ba sr ti nb zr y pb th hf ree concentrations and high la yb ratios in contrast the dmtn and its metamorphosed equivalent has low alkalies and other incompatible element contents has ree patterns that parallel chondritic abundances and has low la yb ratios on the basis of several geochemical parameters the dmtn is tholeiitic whereas the sg is calc alkaline with some alkalic tendencies discriminant diagrams together with the extremely low ti concentration in the dmtn and the calc alkaline nature of the sg strongly indicate emplacement in an arc environment high al 2 o 3 tio 2 and cao tio 2 ratios as well as low incompatible element concentrations in the dmtn suggest that its origin was by partial melting of a depleted source the dmto represents a cumulate of the norite slaughters gabbro magmatism resulted from partial melting of a less depleted source

basement rocks of the halmahera region eastern indonesia a late cretaceous early tertiary arc and fore arc halmahera is a k shaped island located at the junction of several major arc trench systems of the western pacific eastern indonesia region western halmahera is an active volcanic arc above a zone of intense seismicity which characterizes the north molucca sea eastern halmahera has a basement of dismembered ophiolitic rocks with slices of mesozoic and eocene sediments overlain unconformably by middle oligocene and younger sedimentary and volcanic rocks the mesozoic and eocene sediments reveal notable stratigraphical and petrological similarities to the marianas fore arc and the eastern halmahera basement complex is interpreted as a pre oligocene fore arc lacking an accretionary complex there is some evidence that the pre oligocene volcanic arc behind this fore arc now forms part of the basement of western halmahera the mesozoic and tertiary sediments were imbricated together with igneous and metamorphic rocks representing the deeper parts of the fore arc during the late eocene plate reorganization event recognized throughout the western pacific margins the eastern halmahera basement complex can be traced into eastern mindanao probably further northwards in the eastern philippines and may be related to similar terranes within and around the present philippine sea plate in contrast the southern part of the island of bacan at the southwestern end of the halmahera group has a basement of continental metamorphic rocks associated with a deformed ophiolitic complex quite different to the basement of eastern halmahera the metamorphic rocks are interpreted to be part of the north australian continental margin basement which is separated from the halmahera basement complex by a splay of the sorong fault system and the deformed ophiolite complex of bacan is suggested to represent magmatism in the fault zone

origin and motion history of the philippine sea plate abstract the philippine sea plate is the one major plate whose tertiary motion is poorly constrained and whose origin is problematical its southern boundary is the sorong fault system which is part of a major left lateral fault system at the northern margin of the australian plate the southern part of the plate in eastern indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the daito ridge province of the northern philippine sea by the west philippine central basin the east indonesian islands of the halmahera waigeo region contain a good mesozoic and tertiary stratigraphic record indicating a long arc history for the southern part of the plate new palaeomagnetic data from these islands define two sub areas an area forming part of the philippine sea plate north of the sorong fault and an area within the sorong fault system the area north of the fault records a long term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge rocks of philippine sea plate origin within both areas record similar latitudinal shifts the rotation of the area north of the sorong fault is considered to represent the motion of the southern part of the philippine sea plate the new data indicate large tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested for the southern part of the plate there was 40 rotation with northward translation between 0 and 25 ma no significant rotation between 25 and 40 ma and there was 50 rotation with southward translation between 40 and 50 ma we show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate the translation history of the southern part of the plate in eastern indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate 15 n 160 e for the interval 5 25 ma and 10 n 150 e for the interval 40 50 ma reconstructions based on these poles predict that at 45 ma the palau kyushu ridge had a wnw ese orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the izu bonin marians forearc at this time the long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the west philippine basin by spreading in a backarc basin

what was the surface temperature in central antarctica during the last glacial maximum abstract the temperature increase at vostok antarctica from the last glacial maximum to the present warm period is about 8 c based on the deuterium isotope profile the bore hole temperature temperature profile in the ice sheet indicates that the temperature difference may have been much larger about 15 c the temperature dependent gas occlusion process is the key to evaluate the two scenarios atmospheric air penetrates the porous firn layer of the ice sheet and gets trapped at the firn ice boundary consequently the air is younger than the surrounding ice when it gets enclosed in bubbles this age difference age between ice and enclosed gas is temperature and accumulation rate dependent therefore it is possible to estimate paleotemperatures from a known age we use the linkage between chronologies of ch 4 and water isotopes from byrd station and vostok to obtain an experimental age for vostok this experimental age is then compared to modeled age for the two temperature scenarios our results indicate that the temperature reconstruction deduced from the water isotopic composition is the more probable one

attenuation of wind induced pressure perturbations in alpine snow windpumping has been identified as a process that could potentially enhance sublimation of surface snow at high forcing frequency and spawn air movement deeper in firn at lower frequencies we performed an experiment to examine the relationship between high frequency wind and pressure measurements within the top meter of an alpine snowpack and compared experimental results with two theoretical predictions we find that both theoretical predictions underestimate high frequency perturbation pressure attenuation with depth in the near surface snowpack and the discrepancy between theory and measurement increases with perturbation pressure frequency the impact of this result for near surface snow is that potential enhanced sublimation will occur over a shallower snow depth than these two theories predict correspondingly interstitial air mixing at depth in firn will be driven by lower frequencies than these two theories predict while direct measurement of these energy rich lower frequencies is beyond the scope of this paper stationary pressure measurements validate the presence of a pressure field that could drive near surface circulation

lithospheric deformation within the molucca sea arc arc collision evidence from shallow and intermediate earthquake activity this paper presents the results of a local earthquake survey in the molucca sea arc arc collision zone of eastern indonesia although intermediate depth earthquakes define zones dipping to the west beneath the sangihe arc and to the east beneath the halmahera volcanic arc only shallow activity less than 60 km depth occurred beneath the central part of the collision zone where the majority of activity is observed the concentration of earthquake foci in the 10 to 50 km depth range in a limited region beneath the talaud mayu ridge suggests that convergence between the arcs proceeds by shortening within basement of the intervening molucca sea plate rather than by slip along shallow dipping planes between the arcs and the subducted slabs published focal mechanism solutions suggest that high angle reverse faulting beneath the talaud mayu ridge generates almost all of the seismic energy release within the collision zone the predominance of shallow reverse and strike slip faulting at the axis of the bilaterally subducting molucca sea lithosphere suggests that stresses are not due to bending alone and have a large horizontal regional component perpendicular to the island arcs the geometry and state of stress within the molucca sea lithosphere requires an external driving force for the convergence between the two arcs and a strong degree of coupling between the island arcs and the subducting molucca sea plate at least at the present stage of collision

influence of pre existing plate margin structures on foredeep filling insights from the taconian blountian clastic wedge southeastern usa break up of continents gives rise to conjugate margins with irregular shapes and with thermal histories and sedimentary records that differ along strike these elements need to be considered in the integrated study of a foreland basin orogenic belt pair that formed during subsequent contractional events this paper relates the promontory and embayment configuration of southeastern laurentia north american craton to 1 along strike variations in width of the initial depositional profile of the middle to upper ordovician taconian blountian foredeep 2 northeastward toward the embayment along strike thickening and migration of siliciclastic depocenters in the foredeep that favored the resumption of carbonate ramp deposition on the promontory during the late ordovician and 3 variation in intensity of deformation of intrabasinal structures as recorded by reactivation of across strike structures located on the transition zone between the promontory and embayment geometry and composition of the blountian foreland strata indicate that the foredeep slope was narrower and steeper on the alabama promontory than on the southwestern flank of the tennessee embayment differential slope geometries were accommodated by reactivation of transverse basement faults which controlled local conglomerate deposition in deep water settings on the transition zone between the promontory and embayment in contrast conglomerate beds at the top of coarsening upward successions on the southwestern flank of the tennessee embayment record the cratonward progradation of deltaic depositional systems the results presented here may have relevance to other peripheral foreland basins formed over rifted continental margins or retro arc foreland basins formed adjacent to inverted intracratonic rifts

analogous tectonic evolution of the ordovician foredeeps southern and central appalachians new stratigraphic data suggest that the diachronous evolution of the ordovician foredeeps in the southern and central appalachians was remarkably similar stratigraphic features that characterize the middle ordovician sevier basin in tennessee and the middle and late ordovician martinsbuirg basin in pennsylvania are in identical ascending order 1 disconformity on the knox group beekmantown group 2 shelf carbonates 3 slope deposits 4 submarine fan turbidites and 5 contourites and muddy turbidites we propose that diachronous attempted subduction of the north american craton beneath southeastern microplates and or volcanic arcs resulted in uplift and erosion of the western shelf followed by its rapid subsidence basinward migration of eastern and northeastern terrigenous source areas and associated submarine fan turbidites resulted from continued convergence and filled the basins finally tectonic stabilization and lowering of the source area is recorded by contourites and muddy turbidites the evolutionary model proposed for the sevier and martinsburg basins closely resembles present day tectonics of the timor foredeep and the adjoining sahul shelf north of australia similar comparisons have also been made for ordovician basins in the northern appalachians analogous tectonic mechanisms therefore appear to have operated diachronously along the eastern margin of north america from tennessee to new england and possibly to newfoundland during ordovician time

argon and nitrogen isotopes of trapped air in the gisp2 ice core during the holocene epoch 0 11 500 b p methodology and implications for gas loss processes argon and nitrogen isotopes of air in polar ice cores provide constraints on past temperature and firn thickness with relevance to past climate we developed a method to simultaneously measure nitrogen and argon isotopes in trapped air from the same sample of polar ice this method reduces the time required for analysis allowing large numbers of measurements we applied this method to the entire holocene sequence of the gisp2 ice core 82 37 1692 22 m with a 10 20 year sampling interval 670 depths d 40 ar and d 15 n show elevated values in the oldest part of the dataset consistent with a thicker firn layer and increased temperature gradient in the firn due to the legacy of the abrupt warming at the end of the younger dryas interval and the gradual warming during the preboreal interval 11 5 10 0 ka the preboreal oscillation and the 8 2k event are clearly recorded the data show remarkable stability after the 8 2k event available data suggests that post coring gas loss involves two distinct types of fractionation first smaller molecules with less than a certain threshold size leak through the ice lattice with little isotopic fractionation second gas composition changes via gas loss through microcracks which induces isotopic fractionation these two gas loss processes can explain most trends in our data and in other ice core records 2008 elsevier ltd all rights reserved