reworking of sialic crust as represented in late archean age gneisses northern labrador regionally reworked sialic crust has been recognized in the form of 2 800 m y old nebulitic rocks kiyuktok gneisses with high initial 87sr 86sr ratios 0 7081 and pb isotope ratios that suggest a history of the source back to at least 3 500 m y b p the field geochemical and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the 3 600 m y old uivak gneisses by the introduction of aqueous fluids some 2 800 m y ago contemporary but subordinate tonalitic granodioritic orthogneisses derived from discrete intrusive bodies have contrasting sr isotope compositions some display low initial 87sr 86sr ratios 0 7030 that suggest derivation from a source with mantlelike sr isotope composition however most of the late archean age intrusive rocks display higher initial 87sr 86sr ratios 0 7056 0 7059 recognition of these contrasting isotopic patterns suggests that processes of late archean crust formation and stabilization in the north atlantic craton were more complex than previously realized crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material

inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

coastal processes and the development of shoreline erosion in 1971 the us army corps of engineers inventoried the stability of the nation s coastlines concluding in their final summary a report on the national shoreline study that of the 135 000 km of coastline some 33 000 km were seriously eroding more recently dolan et al 1983 assembled the existing data on shoreline changes for the united states reaffirming the magnitude of this problem although such inventories have not been undertaken for the entire world s shorelines coastal erosion is undoubtedly a global problem beach erosion is for the most part episodic with the major retreats in the shoreline occurring during unusually intense storms on the east coast of the united states beach erosion and property losses are produced by nor easters and hurricanes on the west coast intense storms move down from the gulf of alaska generating waves that attack the coastline this obvious association of beach erosion with the energy level of storm waves creates an oversimplified impression of the actual physical processes involved in the erosion only recently have we begun to understand and appreciatc the diversity of processes that are actually involved in coastal erosion it is now recognized that long period oscillations may dominate water motions in the nearshore sometimes containing more energy than the storm generated waves these oscillations as well as a variety of nearshore currents rearrange the beach sand to cause highly variable rates of shoreline retreat along even short stretches of beach although the long term rise in sea level has been recognized as an important factor in shoreline

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

swash oscillations on a natural beach run up swash oscillations were measured on a gently sloping beach face for a variety of incident wave conditions run up energy spectra at wind wave frequencies show an 3 dependence and energy levels that are independent of incident wave height this suggests saturation in contrast run up energy at surf beat periods increase approximately linearly with increasing incident wave energy thus in the inner surf zone where wave breaking limits the energy at wind wave frequencies the principal manifestation of large incident wind waves is energetic surf beat

the origin of swash cusps on beaches abstract genetically there are two types of beach cusps those formed in the surfzone by the nearshore circulation system and those formed on the beachface by the swash and backwash the latter are here called swash cusps and a simple model relating the physical dimensions of swash cusps to the properties of the incident wave field and the mean beach slope is developed as in some previous models the cusp wavelength is controlled by the longshore wavelength of edge waves but the edge waves are now required only to provide small periodic perturbations on an originally uniform beach further cusp growth occurs because of the interactions between the incident waves and the perturbed topography the maximum cusp height apex to valley is hypothesized to be proportional to the significant vertical excursion of swash motions it follows from the above assumptions that the maximum possible cusp steepness cusp height cusp longshore wavelength is proportional to the mean on offshore beach slope this result agrees with field data and the value of the ratio of maximum cusp height to beach slope differs from that obtained theoretically by guza and bowen 1981 using a complicated non linear model with rather different physics the present model implies that edge waves although necessary for initiating the initial bedform perturbation need not persist for the development of mature cusp topography

numerical model of the shoreline change at oarai beach abstract kraus n c and harikai s 1983 numerical model of the shoreline change at oarai beach coastal eng 7 1 28 a numerical simulation was made of the long term shoreline evolution of the sandy beach adjacent to oarai harbor japan the sand transport and shoreline change at oarai are dominated by wave diffraction at a long breakwater and interruption of the current and sand transport at a large groin the model was first calibrated using detailed wave data for a recent 7 1 2 month period the shoreline change which occurred over three years was then simulated to verify the model s predictive capability the calculation procedure for the breaking wave height and angle along the beach under combined diffraction and refraction was also verified with field measurements the availability of detailed wave and survey data stimulated a number of improvements in the calculation procedures for the breaking waves and shoreline change a comprehensive description of the site is given and relevant shoreline survey and wave data are listed in an appendix to enable interested parties to test and refine their own models

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

ion probe u th pb zircon dating of polymetamorphic orthogneisses from northern labrador canada rounded zircons included within igneous zircons from the uivak i gneisses northern labrador have ages of up to 3863 12 ma 2 showing the former existence of rocks that predate the uivak gneisses by more than 100 ma the igneous zircons themselves have experienced varying amounts of early pb loss but age estimates based on maximum 207pb 206pb from three separate gneiss samples agree to within error at 3732 6 ma 2 which is taken as the age of emplacement of the igneous precursor to the dominant component of the uivak gneisses a new generation of zircons was formed during migmatization at ca 3620 ma the volumetrically less important lister gneiss was emplaced at 3235 8 ma 2 in the late archaean different parts of the gneiss complex were subjected to different grades of metamorphic overprinting the zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events with simultaneous recrystallization of old zircons and growth of new u poor zi

nd and sr systematics in an eclogite xenolith from tanzania evidence for frozen mineral equilibria in the continental lithosphere combined isotopic nd sr and petrographic analyses are reported for an eclogitic xenolith from the vissury mine tanzania an exceedingly large difference in measured nd values in garnet and clinopyroxene 200 units suggest a major mineral equilibration at 1 75 0 014 ga nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in sr and nd isotope compositions is present within the clinopyroxene the variation in measured nd isotope compositions in different generations of garnet corresponds to 16 units exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to sr and nd petrographically different generations of garnet are present one garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron features like this can originate either if a system is not closed e g affected by a migrating fluid or if a remanent isotopic heterogeneity survived the homogenisation at 1 75 ga the subsolidus partitioning of sm and nd between garnet and clinopyroxene results in higher fractionation of the smnd ratio in garnet than in the liquid crystal partitioning since the closure temperature for the sm nd exchange between garnet and clinopyroxene may be around 850 c in a dry system many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria the isotopic data suggest that this eclogite was emplaced at 1 75 ga into the lithosphere mantle at temperatures of about 1040 c and then cooled as a closed system at a rate of 6 50 c my down to 840 c where all the exchange reactions were efficiently frozen in at 1 75 ga ago the coincidence of the emplacement of granites 1 7 1 8 ga into the central part of the tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere

pb isotopic compositions of archean komatiites and sulfides abstract pb isotopic compositions were measured in komatiites and other volcanic rocks from several archean greenstone belts and in magmatic and exhalative sulfides associated with these volcanics thin komatiite flows generally yielded pb pb isochron ages consistent with other geochronological or geological information but thicker basaltic flows gave ages that are too young these results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the pb pb age thicker basaltic flows resisted alteration during eruption but were affected by later events their pb pb ages probably correspond to the time of metamorphism and not to the time of emplacement magmatic and exhalative sulfides from the abitibi belt in canada fall on the same trend in the 207 pb 204 pb 206 pb 204 pb diagram as komatiites basalts sedimentary rocks and k feldspars from granites the calculated 1 value is 7 8 0 1 a similar suite of rocks and minerals from the kambalda area in australia has distinctly higher 207 pb 204 pb and a higher 1 of 8 3 nd isotopic data demonstrate that some kambalda rocks are highly contaminated with older continental crust the differences in initial pb isotopic compositions are attributed to the presence or absence and the age of underlying granitoid basement kambalda greenstones are underlain and contaminated by old 3 0 ga granitoid crust but significantly older granitoid basement appears absent from the abitibi belt the pb isotopic compositions of kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks the abitibi greenstones are not highly influenced by crustal contamination and their pb isotopic compositions may reflect those of archean mantle

variability of shoreline erosion and accretion within a beach compartment of flathead lake montana we measured shoreline reconfiguration and variability in shoreline retreat in flathead lake which is a large glacial lake regulated by a hydroelectric dam at its natural outlet wave induced erosion rates ranged from 0 5 to 2 5 m yr i at the northshore study site where fetch and hence wave energy was maximal shoreline retreat was not solely related to storm intensity or volumetric changes in nearshore bars three erosive processes were involved in the measured variability of shoreline retreat undercutting endstripping and ovcrwash wave scour on an exposed bank caused undercutting and endstripping while overwash resulted when water was forced over the top of the shoreline shoreline morphology and vegetation determined the type of erosion process and the rate of shoreline retreat shoreline retreat was offset by localized and dramatic accretion caused by sediment entrapment by drift logs bordering the shoreline the eroding shoreline and ncarshore bed were sediment sources for the accretion drift logs naturally protcctcd the shoreline from direct wave attack and stimulated sediment accretion providing new rccruitmcnt arca for riparian vegetation each stretch of sea coastline or lake shorcline uniquely responds to the physical processes associated with erosion and accretion change in shoreline position is one measurable response to the complex processes that continually shape the littoral zone shoreline erosion depends on many site specific factors such as ncarshore morphology relative changes in water levels varying wave energies patterns of net littoral drift and sediment composition and grain size warnkc et al 1966 sharaf el din 1974 sakalowsky 1975 morton 1979 therefore when addressing shoreline behavior it is necessary to examine and identify components and processes that determine the observed shoreline response on a local scale beach erosion and shoreline retreat are largely episodic corresponding to periods of intense storms however this general association of erosion with the cncrgy level of waves creates an oversimplified impression of the actual processes causing shoreline retreat

early archaean u pb fractionation and timing of late archaean high grade metamorphism in the saglek hebron segment of the north atlantic craton the archaean gneiss complex of the saglek hebron area in northern labrador is dominated by polyphase similar to 3 7 ga old tonalitic trondhjemitic granodioritic ttg uivak i gneiss that exhibit 3 8 3 9 ga inheritance these gneisses are interleaved with a heterogeneous group of supracrustal rocks known as the nulliak supracrustal assemblage early archaean ultramafic rocks occur either as tectonically emplaced slivers of lithospheric mantle along structural contacts within the uivak i gneiss or as layers of metakomatiite in nulliak supracrustal units the metakomatiites yield a pb pb isochron age of 3845 160 ma reflecting a significant change from an initial archaean mantle like mu 1 value of 7 9 to a range of mu 2 values of 2 0 4 2 members of the tectonically emplaced lithospheric mantle suite have less well correlated pb 206 pb 204 and pb 207 pb 204 compositions both ultramafic suites were collected in an area identified as transitional between granulite and amphibolite facies however model calculations show that the data scatter exhibited by the lithospheric mantle cannot be accommodated by post 3 85 ga u depletion evidence is provided that the lithospheric mantle suite reflects variable degrees of mixing between a depleted mantle and a high mu reservoir suggesting that the two ultramafic rock suites had different evolutionary histories timing of late archaean reworking during medium to high grade metamorphism was determined by mineral whole rock dating of a garnet pyroxenite from an area dominated by kiyuktok gneiss reworked similar to 3 75 ga uivak gneiss sm nd mineral whole rock data for this gneiss yielded an isochron equivalent to an age of 2570 17 ma significantly younger than its pb pb isochron age of 2735 43 ma the negative epsilon nd t 7 9 reflects a significant crustal residence time indicating that the sm nd system of the whole rock has been reset the internal pb mineral isochron is interpreted to reflect the maximum age of peak metamorphism the age discrepancy of ca 165 ma between the sm nd and the pb pb isochron ages suggests slow cooling at a minimum rate of 1 3 degrees c ma 1 this indicates that the saglek hebron segment of the north atlantic craton experienced gradual uplift during late archaean exhumation c 1999 elsevier science b v all rights reserved

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

sand transport by unbroken water waves under sheet flow conditions a model of wave induced onshore sand transport and resulting topographical changes seaward of the break point on a sloping beach is presented for the specific case in which the waves are random normally incident weakly nonlinear and relatively long the bottom boundary layer is fully turbulent and sand transport occurs as sheet flow the model consists of the following components 1 a new empirical expression for sand transport as sheet flow in which the instantaneous transport rate is directly proportional to the instantaneous bottom shear stress 2 an expresssion for the mean bottom shear stress based on a theoretical analysis of the bottom boundary layer 3 a simple representation of the shoaling wave field in which the local properties are described by linear long wave theory the probability density function of the surface displacement is gaussian and the waves conserve energy flux as they shoal and 4 the equation for conservation of sediment mass computations based on the model compare favorably with recently reported field measurements of the onshore motion of a long offshore bar

a paleomagnetic transect of the mid cretaceous peninsular ranges batholith baja california mexico we report structural paleomagnetic and magnetic fabric data for midcretaceous plutons of the peninsular ranges batholith along a transect at 30 n latitude four plutons in the western sector are characterized by characteristic magnetizations residing in magnetite in this sector el milagro aguaje del burro la zarza and san telmo plutons yield a combined paleopole at 82 1 n 169 7 e k 137 6 a 95 7 9 n 4 38 sites which rotated for closure of the gulf of california falls at 79 3 n 179 5 e and it is concordant with the north america reference pole plutons in the transition zone between the eastern and western sectors of the peninsular ranges have magnetizations residing in hematite el potrero and san jose plutons yield highly discordant paleopoles indicating apparent clockwise rotation r and fl attening f of 33 0 5 1 and 27 6 6 1 respectively san jose and 46 1 5 9 and 31 0 7 0 el potrero the discordance is best explained by west down tilt of the crustal block between the main martir thrust and the rosarito fault which are major compressional structures parallel to the trend of the peninsular ranges the san pedro martir pluton a large la posta type pluton on the eastern sector of the transect has magnetizations that reside primarily in hematite the mean

global sea level trend in the past century data derived from tide gauge stations throughout the world indicate that the mean sea level rose by about 12 centimeters in the past century the sea level change has a high correlation with the trend of global surface air temperature a large part of the sea level rise can be accounted for in terms of the thermal expansion of the upper layers of the ocean the results also represent weak indirect evidence for a net melting of the continental ice sheets

formation of beach cusps in a wave tank beach cusps with a longshore spacing of 20 to 150 cm have been built by the continuous action of incident waves on a steep laboratory beach floor covered uniformly with a thin bed of glass beads breaking of incident waves was observed to induce vortices on the bed by interacting with swash motion along the beach face beach cusps formed when the value of a dimensionless parameter hb sgti2 became smaller than 0 042 hb is the breaking height of the incident waves ti their period s the beach slope and g the acceleration due to gravity this critical value occurred at a nearly central part of the generation region 0 003 hb sgti2 0 068 for plunging breakers presented by galvin 1968 breaking wave induced vortices rather than breaker types controlled the movement of bed material in the nearshore zone most of the measured spacings of beach cusps including previous observations were in good agreement with half a wavelength of the zero mode subharmonic edge wave which is generated on the beach by the refraction of incident waves and has twice the period of the waves the role of edge waves at each stage of cusp formation still remains as an important problem to be clarified

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

wave set up on a natural beach wave set up the superelevation of mean water level owing to the presence of breaking incident waves was measured at the shoreline of a natural beach offshore pressure sensors monitored incident wave conditions the set up of the shoreline was found to be about 0 17hs where hs is the significant wave height in deep water

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

chemical and isotopic effects of late archaean high grade metamorphism and granite injection on early archaean gneisses saglek hebron northern labrador

on beach cusp formation a system of shallow water equations and a bed evolution equation are used to examine the evolution of perturbations on an erodible initially plane beach subject to normal wave incidence both a permeable under darcy s law and an impermeable beach are considered it is found that alongshore periodic morphological features reminiscent of swash beach cusps form after a number of incident wave periods on both beaches on the permeable impermeable beach these patterns are accretional erosional in both cases flow is horn divergent spacings of the cusps are consistent with observations and are close to those provided by a standing synchronous linear edge wave an analysis of the processes leading to bed change is presented two physical mechanisms are identified concentration gradient and flow divergence which are dominant in the lower and upper swash respectively and their difference over a wave cycle leads to erosion or deposition on an impermeable beach infiltration enters this balance in the upper swash a bed wave of elevation is shown to advance up the beach at the tip of the uprush with a smaller wave of depression on the backwash it is found that cusp horns can grow by a positive feedback mechanism stemming from decreased increased backwash on positive negative bed perturbations

pitfalls and new approaches in granulite chronometry timing and duration of metamorphism are particularly difficult to decipher in granulites because the metamorphic temperature commonly experienced by such rocks exceeds the closure temperature of all geochronometers other than the u th pb systems in a few minerals age constraints on granulite metamorphism therefore often rely on data from a single geochronometer and consistency tests are rarely possible this case study was conducted on a single hand specimen of an exceptional late archaean granulite from the northern marginal zone of the limpopo belt zimbabwe this rock not only contains zircon monazite and xenotime as accessory phases but is also depleted in pb to such an extent that dating of orthopyroxene was possible in situ 207 206 pb sims zircon monazite and xenotime ages conventional u pb zircon and orthopyroxene ages and pb pb step leaching orthopyroxene ages were obtained the in situ 207 206 pb sims ages of monazite and xenotime inclusions in cordierite are well within an independent age bracket as are ages of an interstitial monazite grain interstitial xenotime however retains a more than 55 ma younger age than the cordierite shielded grain these ages are incompatible with a monazite xenotime u pb closure temperature of 725 25 degrees c but suggest that grains which were shielded from processes leading to additional pb loss are closed to significant pb volume diffusion at temperatures greater than or equal to 800 degrees c zircon ages reflect partial protolith inheritance total resetting during peak temperature conditions and disturbance during minor hydrothermal overprint and are therefore not useful on their own to constrain the timing of granulite facies metamorphism u pb and pb pb step leach systematics of orthopyroxene yielded ages identical to the in situ phosphate ages well within the independent age bracket however the step leach experiment showed that these ages merely date monazite and zircon inclusions rather than the host orthopyroxene itself simple model calculations are used to demonstrate that even minute contamination of low mu minerals with high mu inclusions will have significant effects on the obtained mixed age and may lead to erroneous conclusions regarding the u pb closure temperature of e g pyroxene and garnet these pitfalls in granulite geochronology will usually result in an overall apparent age dispersion and time scales derived from field studies should therefore be viewed as upper limits c published by 1998 elsevier science b v

sr and nd isotope systematics of polymetamorphic archean gneisses from southern west greenland and northern labrador sr and nd isotopic data for middle to late archean polymetamorphic felsic gneisses from localities in the nuuk area west greenland are compared and contrasted with new isotopic results for early archean amitsoq gneisses and with data for isotopically reworked kiyuktok gneisses from the saglek area labrador sr isotopic data for individual suites of felsic gneisses record the time integrated effect of variable rb sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity contrasting petrologic and sr nd isotopic characteristics are the result of differences in level of exposure caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post shearing folding deformation sm nd systematics of felsic gneisses from nordafar ikerasakitsup akornga tinissaq and kangimut sammisoq qasigianguit define a geologically meaningless ca 3280 ma nd isochron which is the result of mixing of samples from unrela

two archaean granulite facies metamorphic events in the nuuk maniitsoq region southern west greenland correlation with the saglek block labrador two dilferent granulite facies events have been identified using shrimp u pb zircon geochronology in the nuuk maniitsoq tract of west greenland previously considered to comprise a continuous contemporaneous granulite facies terrain about 35 km north of nuuk a granulite facies metasedment contains metamorphic zircon overgrowths recording a 207 pb 206 pb age of 2999 4 ma 2 some l30 km further north near maniitsoq an orthopyroxene bearing granite contains metamorphic zircons recording an age of 2738 6 ma previous correlations have liked the early archaean rocks of the saglek block of northern labrador with those in the nuuk region however this causes problems with correlation of crustal blocks on either side of the labrador sea the proposal here of correlating late archaean metamorphic events at 2740 ma provides a better solution to the problem

waves long waves and nearshore morphology abstract recent field measurements on beaches of different slopes have established that wave motion at periods substantially longer than the incident waves dominates the velocity field close to the shore analysis of a number of extensive data sets shows that much of this long wave motion is in the form of progessive edge waves though forced wave motion standing edge waves and free waves propagating away from the shore may also contribute to the energy theoretically the drift velocities in bottom boundary layers due to edge waves show spatial patterns of convergence and divergence which may move sediment to form either regular crescentic or cuspate features when only one edge wave mode dominates or a bewildering array of bars bumps and holes when several phase locked modes exist together convincing field demonstration of the link between nearshore topography and edge waves only exists for the special case of small scale beach cusps on steep beaches formed by edge waves at the subharmonic twice the period of the incident waves at longer periods the link is proving more difficult to establish due to the longer time scales of topographic changes the interaction between pre existing topography and the water motion and the observation of broad banded edge wave motion which is not readily linked to topography with a well defined scale these ideas are however central to the study of nearshore processes as most of the plausible alternate hypotheses do not seem to lead to quantitative predictions clearly further theoretical and observational work is essential

field observations of surf beat 1 progressive edge waves nineteen biaxial electromagnetic current meters have been used to determine the longshore and on offshore structure of currents at surf beat periods 1 4 min the sensors formed two linear arrays a long shore array within the surf zone and an on offshore array stretching from the shoreline to well beyond the breaker line analysis of the longshore current components yields a clear picture of progressive low mode edge waves with frequency wave number dispersion relations which are in remarkably good agreement with predictions some separation of edge wave modes is found with mode zero energy dominating in the frequency band 0 006 and 0 011 hz and mode one between 0 015 and 0 025 hz on offshore currents present a rather different picture which while not inconsistent with the longshore currents suggests that other sources of energy are also important to the on offshore currents these include standing edge waves probably formed by reflections at nearby scripps canyon and motions which are nonresonantly forced by incoming wave groups

timing of late archaean terrane assembly crustal thickening and granite emplacement in the nuuk region southern west greenland abstract a breakthrough in understanding the structural and metamorphic evolution of archaean gneiss complexes occurred with the recognition that the nuuk region of southern west greenland comprised separate terranes assembled in the late archaean from northwest to southeast these are the akia 3220 2970 ma akulleq 3870 3600 and 2820 ma and tasiusarsuaq 2920 2860 ma terranes the minimum time of assembly is recorded by the first event common to all component terranes using shrimp u pb zircon geochronology the oldest events common to all terranes including emplacement of crustally derived granites contemporaneous metamorphism and anatexis have been dated at 2710 2725 ma in the akulleq terrane areas where in situ diatexite formed abundant granitoid sheets were intruded and common growth of metamorphic zircon mostly low th u in most lithologies occurred in the akia and tasiusarsuaq terranes there was only intrusion of a lesser number of 2720 ma granitoid sheets because metamorphic zircons of that age have not been found the 2720 ma event is interpreted as marking or shortly following terrane assembly when the diverse components of the akulleq terrane were tectonically juxtaposed with the other two this new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of archaean gneisses can be due to accretionary tectonics long after the individual components were first formed

very early archean crustal accretion complexes preserved in the north atlantic craton the north atlantic craton contains very early archean supracrustal rocks orthogneisses and massive ultramafic rocks most units of supracrustal rocks are dominated by mafic volcanic rocks layered gabbros and banded iron formations bust some also contain abundant felsic volcanic sedimentary rocks quartzites and marbles some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic sedimentary rocks in these sequences ca 3800 ma and also from older ca 3850 ma sources the presence of the ca 3850 ma detrital zircons suggests that the supracrustal units containing them were deposited on or close to ca 3850 ma sialic crust the massive ultramafic rocks have chemical affinities to upper mantle rocks the voluminous suites of tonalitic gneisses are dominated by 3700 3730 ma bodies that intrude the supracrustal sequences but they also locally contain components with ages between 3820 and 3920 ma the diverse supracrustal units upper mantle rocks and ge 3820 ma components in the gneisses were tectonically interleaved in very early archean convergent plate boundaries giving rise to accretion complexes in the period 3700 3730 ma voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels forming juvenilemore continental crust and leaving behind a refractory lower crustal to upper mantle substrate less

the archean gneiss complex of northern labrador 2 mineral ages secondary isochrons and diffusion of strontium during polymetamorphism of the uivak gneisses strontium isotope systematics of minerals from the ca 3600 ma old uivak gneisses and of compositionally layered uivak i gneisses which define secondary isochrons are the result of sr isotopic homogenization during thermal events ca 2800 ca 2500 and ca 1800 ma ago the strontium isotope data provide information about the thermal history of the archean gneiss complex in northern labrador the scale of sr isotopic equilibration and mechanisms of sr diffusion during polymetamorphism the mineral data indicate that biotite and amphibole were both partially open to sr diffusion during the hudsonian orogeny ca 1800 ma ago however cooling rates and closure to diffusion of sr in these phases varied throughout the terrane potassium feldspars were largely closed to diffusive loss of sr during the 1800 and 2500 ma events and they record evidence of a ca 2800 2900 ma thermal event ages defined by secondary isochrons correlate with variations in the widths of the gneissic layers and correspond with geolog

development of crescentic bars for a periodically perturbed initial bathymetry 1 the development of crescentic bed patterns starting from an undisturbed beach i e one only perturbed so as to simulate background noise and thereby initiate morphological development is compared with that starting from a beach where monochromatic and in some cases bichromatic bed patterns pre exist using a fully non linear model morfo55 a wide range of different lengthscales and amplitudes is investigated the findings suggest that the pre existence of crescentic bed forms can influence the subsequent morphological development of this beach significantly but that this development is related to the undisturbed beach development whether a pre existing bed form remains under certain forcing conditions depends on the position of the pre existing lengthscale along the undisturbed linear growth rate curve if the pre existing mode shows significant linear growth in the undisturbed scenario this pre existing lengthscale remains otherwise the pre existing mode will be replaced by a bed form with a lengthscale closer to the undisturbed fastest growing mode a pre existing lengthscale that is significantly longer than the undisturbed fastest growing mode will be replaced by a higher harmonic of the original lengthscale for shorter pre existing lengthscales this will be the linear fastest growing mode an increased amplitude of the pre existing bed form accelerates the development toward a new stable situation for small initial amplitudes the initial response of the system to pre existing bed forms could theoretically be described using linearized equations alone the migration rates of both the pre existing and newly arising modes correspond to the migration rate of these modes in the undisturbed scenario

wave transformation and swash oscillation on gentle and steep slopes the numerical model developed previously for coastal structures is slightly modified and applied to predict the wave transformation in the surf and swash zones on gentle slopes as well as the wave reflection and swash oscillation on relatively steep beaches the numerical model is one dimensional in the cross shore direction and is based on the finite amplitude shallow water equations including the effect of bottom friction which are solved in the time domain for the incident wave train specified as input at the seaward boundary of the computation located outside the breakpoint the slight modification is related to the effect of the time averaged current on the seaward boundary condition and improves the agreement between the computed and measured mean water levels on gentle slopes the modified numerical model is compared with available small scale test data for monochromatic waves spilling on gentle slopes as well as for monochromatic waves plunging and surging on a relatively steep slope additional comparisons are made with small scale tests conducted using transient monochromatic and grouped waves on a 1 8 smooth slope with and without an idealized nearshore bar at the toe of the 1 8 slope as a whole the numerical model is shown to be capable of predicting both time varying and time averaged hydrodynamic quantities in the surf and swash zones on gentle as well as steep slopes

bars bumps and holes models for the generation of complex beach topography in shallow water any two waves of the same frequency are shown to produce complex patterns of drift velocity above the sea bed if the longshore components hi h of the wave numbers of the two waves are different these steady flow patterns exhibit a longshore periodicity of wave number hi h irrespective of whether the waves propagate in the same direction say hi he positive or in opposite directions he negative the interaction of two edge wave modes is examined in detail the drift velocities are calculated and a simple sediment transport model is used to predict the beach topography that would be in equilibrium with these flow patterns as expected crescentic sand bars are produced by the special case of standing edge waves hi h intriguingly for all other cases a combination of complex transverse bars plus meandering or straight offshore bars result patterns that are surprisingly reminiscent of many published descriptions of complex rhythmic topography the extension of the model to three or more waves produces topography that appears to be very irregular although the pattern should repeat over sufficiently long distances along the beach even with only three waves these distances may be very large compared to the scale of the bars

planar cross bedding associated with rip currents of upper cretaceous formations northeast japan abstract the sedimentary structure produced by rip currents is mainly characterized by seaward dipping planar cross bedding field observations from the upper cretaceous santonian kunitan and taneichi formations in northeast japan show that the occurrence of the cross bedded units extends laterally from the ancient beachlines but is restricted in vertical extent the planar cross set bed of the kunitan formation overlies very coarse grained parallel laminated and trough cross bedded sandstone beds of longshore current origin and underlies a pebbly conglomerated bed that was probably formed by a storm generated bottom current above is a zone of alternated beds of bioturbated and parallel laminated or hummocky cross stratified fine grained sandstones representing either lower shoreface or transitional depositional environments the planar cross bedding of the taneichi formation is intercalated with the underlying trough cross bedding and with the overlying bioturbated or parallel laminated very fine grained sandstone bed which also includes hummocky cross stratification both planar cross set beds of the taneichi and kunitan formations are present in the succession which fines upward suggesting an increasing depth of deposition such field occurrences indicate that bedforms produced within deeply incised rip channels which extend towards the sea can be untouched by ravinement and remain well preserved during transgression