acceleration source spectra anticipated for large earthquakes in northeastern north america a set of 16 large intraplate earthquakes are considered on the basis of their tectonic settings compressive focal mechanisms and shallow focal depths as possible analogs for large earthquakes that may be anticipated to occur in northeastern north america the earthquakes range in seismic moment from 5 1023 to 3 1026 dyne cm and include the mainshocks and largest aftershocks of the 1982 miramichi canada the 1985 to 1988 nahanni canada and the 1988 tennant creek australia earthquake sequences the teleseismic recordings of these earthquakes are spectrally analyzed by correcting the logarithmically averaged acceleration spectra for the frequency dependent effects of attenuation and interference of the depth and direct phases the resulting acceleration source spectra are generally flat at high frequencies the mainshock spectra exhibit intermediate spectral falloffs that is u where 1 over frequencies that characteristically extend from 0 1 to 0 5 hz in contrast the largest aftershocks of the three sequences named above exhibit 2 spectral shapes the high frequency source spectral levels increase with seismic moment approximately as ru m 1 3 the 1988 saguenay canada earthquake which occurred significantly deeper than the other earthquakes exhibits an anomalous spectral shape combining the teleseismic data with strong motion recordings indicates that the intermediate spectral falloff for this earthquake extends from 0 2 to 3 hz the spectral amplitude at 3 hz is a factor of 3 higher than the amplitude anticipated for an earthquake of this size from the other 15 earthquakes

regional mapping of pre metamorphic spilitization and associated chemical mobility in greenschist facies metabasalts of the sw scottish highlands abstract both spilitic and non spilitic metabasaltic sills are hosted by greenschist facies metasediments in the sw scottish highlands spilitization is mainly characterized by enrichment in na 2 o elevated modal plagioclase and epidote segregations mapping of the spatial distribution of spilitic metabasalts reveals an ancient sub sea floor fluid cell centred on the extrusive tayvallich volcanics fluid circulation was most extensive at shallow levels where most sills were spilitized we attribute this to pervasive flow of saline fluid which was thermally driven by the cooling suite of lava flows and sills spilitization below this lithostratigraphic depth was restricted to only a few sills their spilitization is largely unrelated to specific properties of these sills e g width chemistry or host lithology we conclude that fluid channelling was an intrinsic property of sub sea floor fluid flow either at deeper levels or earlier during fluid circulation by profiling of the size distributions of relic phenocrysts in a partly spilitized sill we conclude that spilitization proceeds with the symmetric propagation of a spilitization front from the sill margins towards the sill interior based on chemical profiling across the margin of an epidote segregation we conclude that spilitization is associated with chemical transport on scales ranging from 0 1 to 10 m

ground motion relations for eastern north america predictive relations are developed for ground motions from eastern north american earthquakes of 4 0 m 7 25 at distances of 10 r 500 km the predicted parameters are response spectra at frequencies of 0 5 to 20 hz and peak ground acceleration and velocity the predictions are derived from an empirically based stochastic ground motion model the relations differ from previous work in the improved empirical definition of input parameters and em pirical validation of results the relations are in demonstrable agreement with ground motions from earthquakes of m 4 to 5 there are insufficient data to adequately judge the relations at larger magnitudes although they are consistent with data from the saguenay m 5 8 and nahanni m 6 8 earthquakes the underlying model parameters are constrained by empirical data for events as large as m 6 8

glacial outburst floods and loess sedimentation documented during oxygen isotope stage 4 on the columbia plateau washington state abstract stratigraphy and age control of late pleistocene loess associated glacial outburst flood deposits and flood cut unconformities in the channeled scabland washington state united states indicate a significant cordilleran ice sheet advance during marine oxygen isotope stage 4 glacial outburst flood deposits from stage 2 classic bretz flood deposits ca 21 to 13 ka and related features in the channeled scabland overlie a widespread layer of loess that contains buried soils and the mount st helens set c tephra ca 46 ka this loess in turn overlies deposits of the penultimate episode of giant outburst floods and an unconformity cut by those floods regional trends in the thickness texture and overall composition of the older loess are strikingly similar to those from the youngest loess known to be derived from stage 2 flood deposits we conclude that the older loess also is derived from fine grained flood deposits luminescence ages tephrochronology and soil development rates indicate that the bulk of deposition of the older loess occurred during stage 3 following glacial outburst flooding marked by a regional flood cut unconformity the apparent cyclical pattern of cold climate buried soils flood deposits and thick loess accumulations demonstrate that sediment supply renewed by flood episodes is a major control on accumulation of loess on glacial timescales

conflicting sm nd whole rock and u pb zircon ages for archean lavas from newton township abitibi belt ontario abstract archean komatiitic and tholeiitic lavas from newton township ontario have a sufficient range of sm nd ratios to define a well constrained line on the normal 143 nd 144 nd vs 147 sm 144 nd isochron plot the data give an isochron age of 2826 64ma and an initial e nd of 2 65 0 26 however u pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite tholeiite suite give an age of 2697 1 1ma there is strong evident that the zircon age is the eruption age suggesting that the older sm nd age is incorrect and probably results from mixing between isotopically distinct mantle sources at the time of eruption the sources had e nd values from about 4 2 to 1 6 indicating that the archean mantle in this area was markedly heterogeneous and not uniformly depleted

modeling of source rupture characteristics for the saguenay earthquake of november 1988 the magnitude mb 6 0 saguenay earthquake of 25 november 1988 in quebec canada was one of the largest to have occurred in eastern north america during the past half century recorded high frequency ground motions exceeded anticipated values for an event of its size by a factor of 10 on both the regional network and strong motion instruments two proposed explanations for the discrepancy are 1 that the source was a rare high stress drop event and 2 that it was an asymmetrical fractional stress drop rupture involving only normal effective stresses in this article detailed fault slip models are derived to fit characteristics of strong motion displacement velocity and acceleration data the results establish that the effective rupture stress was normal less than 100 bars that the fault rupture was highly asymmetrical with respect to the point of rupture initiation and that the average slip time for points within the rupture area approx 0 2 sec was considerably less than that associated with the standard brune 1970 source spectral model the rupture area developed in a number of episodes each widening or lengthening the previously ruptured area which may explain the short average slip time the results indicate that the widely used assumption in hazard analyses that earthquake spectra are adequately represented by the standard brune 1970 complete stress drop model may be seriously unreliable for prediction of strong ground motion in eastern north america

regional propagation characteristics and source parameters of earthquakes in northeastern north america abstract the vertical components of the s wave trains recorded on the eastern canadian telemetered network ectn from 1980 through 1990 have been spectrally analyzed for source site and propagation characteristics the data set comprises some 1033 recordings of 97 earthquakes whose magnitudes range from m 3 to 6 the epicentral distances range from 15 to 1000 km with most of the data set recorded at distances from 200 to 800 km the recorded s wave trains contain the phases s s m s s n and l g and are sampled using windows that increase with distance the acceleration spectra were analyzed from 1 0 to 10 hz to separate the source site and propagation characteristics an inversion for the earthquake corner frequencies low frequency levels and average attenuation parameters is alternated with a regression of residuals onto the set of stations and a grid of 14 distances ranging from 25 to 1000 km the iteration between these two parts of the inversion converges in about 60 steps the average attenuation parameters obtained from the inversion were q 1997 10 and 0 998 0 003 the most pronounced variation from this average attenuation is a marked deamplification of more than a factor of 2 at 63 km and 2 hz which shallows with increasing frequency and increasing distance out to 200 km the site response spectra obtained for the ectn stations are generally flat the source spectral shape assumed in this inversion provides an adequate spectral model for the smaller events m o 21 dyne cm in the data set whose brune stress drops range from 5 to 150 bars for the five events in the data set with m o 10 23 dyne cm however the source spectra obtained by regressing the residuals suggest that an 2 spectrum is an inadequate model for the spectral shape in particular the corner frequencies for most of these large events appear to be split so that the spectra exhibit an intermediate behavior where u is roughly proportional to

use of empirical green s functions spectral ratios and kinematic source models for simulating strong ground motion abstract ground motions for large and moderately large earthquakes at short and moderate distances are particularly important for seismic hazard estimation in eastern north america ena very few direct observations of such ground motions have been obtained however because of the sparsity of recording sites and the relatively low rates of occurrence of large earthquakes inside the region estimation of strong ground motion must therefore rely heavily on theoretical models to extend empirical results obtained from small earthquakes and from the few larger ones for which reliable data are available because of the generally large distances between recording stations the main source of useful data comes from lg wave trains observed at relatively large distances for the two largest earthquakes to have occurred near populated regions of southeastern canada during the past decade spectral ratios of the lg wave trains of the mainshocks with respect to those of their aftershocks are found to depend almost entirely upon the source radiation characteristics of the sources alone this result is utilized to derive elastodynamically based kinematic rupture models that are consistent with the empirical spectral ratio data such models provide a firm physical basis from which to infer the most probable spectral characteristics for future large earthquakes in the region in converse application it is shown that spectral ratios obtained from such models along with empirical seismograms from small earthquakes can be used to accurately simulate strong ground motions at short and moderate as well as large distances as such small event seismograms are relatively plentiful the problem of reliable strong ground motion estimation is therefore reduced to that of obtaining reliable representative source spectral models the solution of this latter problem must continue to depend upon whatever empirical data are available and upon appropriately detailed theoretical modeling

number and size of last glacial missoula floods in the columbia river valley between the pasco basin washington and portland oregon field evidence and radiocarbon age dating combined with hydraulic flow modeling provide new information on the magnitude frequency and chronology of late pleistocene missoula floods in the columbia river valley between the pasco basin washington and portland oregon more than 25 floods had discharges of 1 0 106 m3 s at least 15 floods had discharges of 3 0 106 m3 s at least six or seven had peak discharges of 6 5 106 m3 s and at least one flood had a peak discharge of 10 106 m3 s a value consistent with earlier results from near wallula gap but better defined because of the strong hydraulic controls imposed by critical flow at constrictions near crown and mitchell points in the columbia river gorge stratigraphy and geomorphic position combined with 25 radiocarbon ages and the widespread occurrence of the ca 13 ka radiocarbon years mount st helens set s tephra show that most if not all the missoula flood deposits exposed in the study area were emplaced after 19 ka radiocarbon years and many were emplaced after 15 ka more than 13 floods perhaps postdate ca 13 ka including at least two with discharges of 6 106 m3 s from discharge and stratigraphic relationships upstream we hypothesize that the largest flood in the study reach resulted from a missoula flood that predated blockage of the columbia river valley by the cordilleran ice sheet multiple later floods probably including the majority of floods recorded by fine and coarse grained deposits in the study area resulted from multiple releases of glacial lake missoula that spilled into a blocked and inundated columbia river valley upstream of the okanogan lobe and were shunted south across the channeled scabland

rare earth element mobility in the roffna gneiss switzerland the roffna gneiss a deformed hercynian granite porphyry within the penninic nappes of eastern switzerland underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies this resulted in the selective mobilization of major and trace elements over distances of 10 s to 100 s of meters and the resetting of the rb sr whole rock isotopic systems some 100 my ago the component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric the mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline albite and biotite and the formation of phengite and quartz the fluids introduced mg and h2o promoting the development of phengite and removed the na being released by the breakdown of albite the fluids were most probably derived from the surrounding triassic carbonates and quartzites these relatively high fo2 and carbonate rich fluids also introduced rare earth elements ree into the gneiss the gneiss was progressively enriched in eu up to 60 y up to 40 and yb up to 100 these enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite rich sample while the ree were mobile uranium and thorium were essentially immobile the formation of xenotime was suggested to explain the observed heavy ree enrichment when large differences in the ree contents were found for replicate analyses using hf and then lithium metaborate for dissolution these differences arose because xenotime like monazite can be difficult if not impossible to dissolve in hydrofluoric acid due to the possibility of incomplete sample dissolution we now recommend fusion with lithium metaborate for all ree lu hf or sm nd studies

major and trace element characterization expanded distribution and a new chronology for the latest pleistocene glacier peak tephras in western north america keuhn s c froese d g carrara p e foit jr f f pearce n j g rotheisler p 2009 major and trace element characterization expanded distribution and a new chronology for the latest pleistocene glacier peak tephras in western north america quaternary research 71 2 201 216 sponsorship nserc discovery grant alberta ingenuity new faculty award to duane froese

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

age and petrogenesis of na rich felsic rocks in western iran evidence for closure of the southern branch of the neo tethys in the late cretaceous abstract intermediate to felsic volcanic and granitic rocks with high na2o concentrations 5 2 9 1 wt are widely distributed in the harsin area along the zagros thrust zone in western iran most of these rocks are classified as low potassium tholeiite display affinity with oceanic plagiogranite and contain somewhat high na content and low concentrations of k2o al2o3 rb sr and ca thus we prefer to apply the term na rich felsic rocks to this complex u pb dating yielded ages of 94 6 2 7 ma 2 from baddeleyite and 95 0 2 4 ma 2 from zircon grains indicating that the complex crystallized in the late cretaceous based on the mineral compositions the crystallization occurred at low pressures mostly

case for periodic colossal jokulhlaups from pleistocene glacial lake missoula two classes of field evidence firmly establish that late wisconsin glacial lake missoula drained periodically as scores of colossal jokulhlaups glacier outburst floods 1 more than 40 successive flood laid sand to silt graded rhythmites accumulated in back flooded valleys in southern washington hiatuses are indicated between flood laid rhythmites by loess and volcanic ash beds disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest slopes gentle and time between floods short 2 in several newly analyzed deposits of pleistocene glacial lakes in northern idaho and washington lake beds comprising 20 to 55 varves average 30 40 overlie each successive bed of missoula flood sediment these and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood they dispel the notion that the prodigious floods numbered only a few the only outlet of the 2 500 km 3 glacial lake missoula was through its great ice dam and so the dam became incipiently buoyant before the lake could rise enough to spill over or around it like grimsvotn iceland lake missoula remained sealed as long as any segment of the glacial dam remained grounded when the lake rose to a critical level 600 m in depth the glacier bed at the seal became buoyant initiating underflow from the lake subglacial tunnels then grew exponentially leading to catastrophic discharge calculations of the water budget for the lake basin including input from the cordilleran ice sheet suggest that the lakes filled every three to seven decades the hydrostatic prerequisites for a jokulhlaup were thus re established scores of times during the 2 000 to 2 500 yr episode of last glacial damming j harlen bretz9s spokane flood outraged geologists six decades ago partly because it seemed to flaunt catastrophism the concept that lake missoula discharged regularly as jokulhlaups now accords bretz9s catastrophe with uniformitarian principles

calibration of time history simulation methods abstract ground motion time histories for use in engineering analyses of structures in eastern north america are often simulated from seismological models owing to the paucity of real recordings in the magnitude and distance ranges of interest two simulation methods have been widely used in recent years the stochastic method and the ray theory method in the stochastic method as im plemented in this study ground motion is treated as filtered gaussian noise whose underlying spectrum is determined from an empirical region specific seismological model of the source and propagation processes in the ray theory method as implemented in this study the ground motions are simulated by convolving an empirical source function with theoretical green s functions for a specified crustal structure model this article compares results of the two simulation methods for four well recorded calibration events and assesses the applicability of the methods the assessment is based on comparisons of ground motion parameters from the simulated data with those of the actual recordings ground motion parameters in the frequency range from 1 to 10 hz are sat isfactorily predicted by both methods averaged over the four events studied the stochastic method underpredicts 1 hz response spectra by 20 to 40 but accurately predicts response spectra for frequencies of greater than 2 hz it also accurately predicts peak ground acceleration and velocity the wave propaga tion method underpredicts 1 hz response spectra by 10 to 40 but accurately predicts response spectra for higher frequencies it overpredicts peak ground acceleration and velocity by 10 to 40 both methods are imprecise the stan dard error of an estimate is a factor of about 2 2 the bias and standard error of an estimate for the wave propagation method are generally slightly lower than for the stochastic method if the focal depth of the event can be specified i e as for a past earthquake if the focal depth of the event is not known i e as for a future earthquake then the accuracy and precision of thetwo methods are about the same the chief advantage of the wave propagation method is its predictive power since its attenuation func tion is derived from the focal depth and crustal structure it does not require knowledge of the empirical attenuation function the chief advantage of the stochastic model is its economy and simplicity introduction seismic hazard studies are often used to identify design earthquakes in terms of magnitude and dis tance from a site of interest ground motion time his tories for the desired magnitude s and distance s are then sought to be used as input for engineering analyses of a proposed structure for regions such as california where strong ground motion records are relatively abun dant a suite of recordings from past earthquakes is usu ally available in eastern north america by contrast relevant records for the desired magnitudes and distances are seldom available it is therefore necessary to simu late the required records based on seismological models of earthquake ground motion generation processes two simulation methods have been widely used in recent years the stochastic method and the ray theory method in the stochastic method ground motion is treated as bandlimited finite duration gaussian noise whose underlying spectrum is determined from a seismological model of source and propagation processes in the ray theory method the ground motions are simulated by convolving an empirical source function usually derived from a past event with theoretical green s functions for 400

active faulting on the wallula fault zone within the olympic wallowa lineament washington state usa the wallula fault zone is an integral feature of the olympic wallowa lineament an 500 km long topographic lineament oblique to the cascadia plate boundary extending from vancouver island british columbia to walla walla washington the structure and past earthquake activity of the wallula fault zone are important because of nearby infrastructure and also because the fault zone defines part of the olympic wallowa lineament in south central washington and suggests that the olympic wallowa lineament may have a structural origin we used aeromagnetic and ground magnetic data to locate the trace of the wallula fault zone in the subsurface and map a quarry exposure of the wallula fault zone near finley washington to investigate past earthquakes along the fault we mapped three main packages of rocks and unconsolidated sediments in an 10 m high quarry exposure our mapping suggests at least three late pleistocene earthquakes with surface rupture and an episode of liquefaction in the holocene along the wallula fault zone faint striae on the master fault surface are subhorizontal and suggest reverse dextral oblique motion for these earthquakes consistent with dextral offset on the wallula fault zone inferred from offset aeromagnetic anomalies associated with ca 8 5 ma basalt dikes magnetic surveys show that the wallula fault actually lies 350 m to the southwest of the trace shown on published maps passes directly through deformed late pleistocene or younger deposits exposed at finley quarry and extends uninterrupted over 120 km

resetting of smnd systematics during metamorphism of 3 7 ga rocks implications for isotopic models of early earth differentiation abstract new sm nd isotopic results for sixteen samples from the garbenschiefer amphibolite unit of the 3700 ma isua supracrustal belt of west greenland are reported the data illustrate the difficulty in identifying primary nd t values in poly metamorphic and poly deformed rock sequences the investigated samples are interpreted as derived from a mg and al rich igneous protolith which can be argued to have been essentially isotopically homogeneous at least on the outcrop scale at the time of its emplacement however the data do not give uniform nd t values even from single outcrops the wide variation of nd t which we obtain 1 0 to 5 0 demonstrates that the sm nd systematics of the samples were disturbed by one or more metamorphic event s which took place some considerable time after the original crystallization of the isua belt a sm nd isochron age of 2849 116 ma has been obtained on plagioclase and amphibole separates which represent the oldest metamorphic assemblage now preserved at isua this result indicates that the earliest amphbolite facies metamorphism of the isua belt for which pressure and temperature conditions can be obtained p 5 kbar t 500 600 c is late archaean and not early archaean as had been assumed earlier by some authors 2850 ma is also thought to represent the main time of disturbance of the sm nd system in the garbenschiefer amphibolite unit a consideration of available isotopic data suggests that many of the rocks from isua underwent some nd isotopic re equilibration and or fractionation of their 147 sm 144 nd ratio during the 2850 ma metamorphic episode in these rocks the computed nd t are apparent values only and need not represent either the primary isotopic composition of their protoliths or early archaean mantle values only nd rich rocks 20 ppm such as some felsic schists and the grey gneisses adjacent to the belt are arguably sufficiently robust to retain nd t values which are close to their original composition these average 2 0 which may represent a close approximation of the nd value of the earth s mantle at 3800 ma the results presented in this paper show that great care should be exercised in the use of sm nd data from very early archaean rocks as constraints for isotopic models of early earth s evolution particularly in terranes which have been affected by several metamorphic episodes

new stratigraphic markers in the late pleistocene palouse loess novel fossil gastropods absolute age constraints and non aeolian facies four stratigraphic sections in the southern part of the columbia basin preserve a sequence of aeolian and non aeolian sediments ranging in age from 9ae43 to 47ae0 14 c ka based on accelerator mass spectrometry radiocarbon dating of fossil molluscs geochemistry of cascade mountain sourced tephra and association with formally recognized pedostratigraphic units the washtucna and old maid coulee soils study sections are interpreted as representing concurrent deposition of loess and distal missoula flood rhythmites in valleys tributary to main drainages backflooded during the missoula floods and formation of carbonate and iron rich soils sediments belong to the formally recognized l 1 and l 2 loess units established for the palouse loess which were deposited in the columbia basin subsequent to events of glacial outburst flooding sediments associated with the mount saint helens set s and set c tephras in the study sections preserve a fauna of five species of gastropod mollusc which have not been reported previously from sediments of late pleistocene age in the palouse region the fossils comprise two distinct faunules stratigraphically separated by the mount saint helens so tephra accelerator mass spectrometry radiocarbon dating of the fossils collected above the tephra in two of the sections yielded ages of 12ae48 0ae06 and 9ae43 0ae05 14 c kyr these ages suggest that independent determinations of the 13ae35 14 c kyr age of the so tephra in other areas where missoula flood sediments are preserved are probably accurate and help to refine the age of the latest events in the most recent sequence of catastrophic glacial outburst flooding

record of pre late wisconsin giant floods in the channeled scabland interpreted from loess deposits loess deposits adjacent to flood coulees on the columbia plateau in eastern washington state record multiple episodes of pre late wisconsin cataclysmic floods evidence in the loess consists of flood cut unconformities upward fining in individual loess strata and flood deposits that consist of basaltic gravel fragments of calcified paleosols and slackwater sediment at least six episodes of floods during the pleistocene are indicated by paleomagnetic measurements tephra correlations and the stratigraphic position of flood indicators one episode of the six occurred prior to 790 ka the most recent episode before the last glacial floods occurred at 36 ka at the latest

upb zircon geochronology of late archaean metamorphic rocks in the taihangshan wutaishan area north china abstract a geochronological study has been made in the taihangshan wutaishan area shanxi province on a sequence of precambrian supracrustal rocks and stratigraphically related intrusives these constitute part of the north china basement block the oldest rocks of the fuping group consist predominantly of mafic granulites paragneisses quartzites and amphibolites this group was always considered archaean and early k ar measurements yielded some minimum ages as old as 2300 ma and others as young as 1500 1900 ma the unconformably overlying wutai group is considered as the base of the proterozoic succession and consists of amphibolite grade paragneisses carbonates and amphibolites in its lower part and greenschist grade clastic sediments and volcanics in its upper part it has given apparent k ar mineral ages between 1500 and 2000 ma present investigations have employed the u pb zircon approach on selected parts of the sequences to provide direct depositional ages or age constraints for the fuping and wutai groups and to find the age s of subsequent metamorphic processes a minimum age for the fuping group and maximum age for the wutai group has been derived from zircon age measurements on the lanzhishan granite 2560 9 ma which intrudes the fuping group and is unconformably overlain by the wutai group this late archaean result can be interpreted as the age of an igneous crystallization event and therefore a maximum for the depositional age of the wutai group the above result is well substantiated by zircon dating of the wutai group itself zircons from a low grade metamorphosed keratophyric lava in the upper part of the group give a volcanic crystallization age of 2520 17 ma this provides close constraints for the age of deposition of the wutai group at between 2560 and 2520 ma ago if the wutai group is indeed basal proterozoic the archaean proterozoic boundary in this part of china is also dated at between 2560 and 2520 ma further evidence as to the antiquity of the wutai group comes from a u pb zircon age of 2520 30 ma for the ekou granite which intrudes the wutai group sequence the age of the underlying late archaean rocks in the tainhangshan wutaishan area the fuping group could not be directly determined because of the paucity of volcanic markers and the higher grade of metamorphism its minimum age is 2560 ma see above but only an approximate maximum age can be gleaned from the zircon systematics in granulite and amphibolite grade paragneiss samples these data indicate a degree of pb inheritance which combined with metamorphic disturbance and more recent pb loss inhibit any further interpretation other than that they indicate an approximate maximum age of around 2800 ma several of the paragneisses and one amphibolitic schist contain zircon systems which are interpreted as having completely responded to a well defined amphibolite facies metamorphic event at 2500 ma ago a younger but less well defined metamorphism caused complete pb loss from zircon and recrystallization in the amphibolite and granulite facies terrains at between 2000 and 1800 ma ago rocks which show evidence of this younger metamorphic event were not recognized from the field or structural data base but from the distribution of the few such samples analyzed it seems possible that this younger terrain may now be interpretable as a cross cutting proterozoic mobile belt this proterozoic metamorphic province is also reflected by k ar mica ages between 1600 and 2000 ma and some preliminary rb sr total rock ages at 1700 ma on nearby paragneisses whose u pb zircon age is 2500 ma

paleoproterozoic crustal growth in the western block of the north china craton evidence from detrital zircon hf and whole rock sr nd isotopic compositions of the khondalites from the jining complex detrital zircons from the khondalites in the western block of the north china craton were mainly derived from a 1 9 to 2 1 ga paleoproterozoic provenance most of the more than 200 detrital zircons analyzed in this study have positive initial ehf values and a wide range of hf depleted mantle model ages tdmc that range from 2 08 ga to 2 74 ga with a peak at 2 24 to 2 40 ga the whole rock nd isotopic analyses for the khondalite samples give quite uniform 143nd 144nd ratios from 0 511188 to 0 511858 corresponding to initial end 2 10ga values ranging from 1 6 to 0 6 in contrast sr isotopic compositions of these samples are more varied with initial 87sr 86sr ratios at 2 10 ga ranging from 0 7017 to 0 7076 in the end versus 87sr 86sr correlation diagram most samples plot near the mixing hyperbola of the depeleted mantle and crust with the contribution of depleted mantle about 65 to 75 percent these results suggest that a large amount of juvenile materials from the depleted mantle was accreted to the western block at 2 1 ga a period of magmatic quiescence in many cratons of the world

determining precambrian crustal evolution in china a case study from wutaishan shanxi province demonstrating the application of precise shrimp u pb geochronology

spectral estimates of seismic shear waves the scaling of seismic sources according to brune 1970 1971 with a stress drop of 100 bars allows accurate prediction of average fourier spectra of seismic shear waves in the far field for the frequency band 1 to 10 hz random vibration theory under the assumption of stationary response accurately predicts average linear elastic response spectrum amplitudes in this same frequency band these conclusions are reached through comparison of theoretical estimates with strong motion observations during a wide range of california earthquakes local magnitudes 4 0 to 7 2 distances 9 to 130 km thus these models are an accurate physically based method of estimating high frequency strong ground motion for a range of earthquake magnitudes and distances and offer the advantage over purely empirical methods of a physical basis for extrapolating to predict ground motions for magnitudes and distances which are poorly documented with data

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

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

influence of exhumation on the structural evolution of transpressional plate boundaries an example from the southern alps new zealand concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength three dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate bounding structure is stable if asymmetric erosion patterns such as those observed in orographic mountain belts pertain and if earth9s crust has a strong on weak rheology in early stages of oblique convergence of an initially laterally homogeneous material lateral boundary parallel strain is accommodated along a near vertical structure and convergent boundary normal strain is concentrated on structures dipping at moderate angles into the orogen exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure when the upper crust beneath the orogen is significantly weakened by exhumation lateral strain abandons the vertical structure and shifts to the dipping structure combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust as is the case along the alpine fault new zealand the process of thermal thinning is controlled by advection and occurs on time frames of 1 2 m y the two components of strain remain separate in the lower crust during active convergence exhumation of lower crustal material occurs only along those structures accommodating convergent strain consequently material exhumed from lower regions of ductile deformation as is the case along the alpine fault contains lineations that indicate a greater component of convergence than predicted from the total plate motion postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones one dominantly convergent and one dominantly strike slip widely reported orogen parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector but the exhumation of the lateral transport zone

rb sr and sm nd isotopic dating of an early precambrian spilite keratophyre sequence in the wutaishan area north china preliminary evidence for nd isotopic homogenization in the mafic and felsic lavas during low grade metamorphism the spilite keratophyre sequence in the wutaishan area north china has been dated by zircon from the quartzkeratophyre and a volcanic crystallization age of 2522 ma liu dunyi et al 1985 was obtained this volcanic rock sequence experienced a low grade metamorphism of greenschist facies during 1800 2000 ma however combined sm nd whole rock data from the volcanic rock sequence if interpreted in terms of an isochron gives a younger age of 2250 182 ma if separately calculated sm nd isochrons for quartz keratophyre and spilite yield two consistently younger ages of 1977 385 ma and 1981 178 ma these sm nd isochron ages are consistent with a rb sr age of 1871 98 ma and with the time of regional greenschist facies metamorphism consequently we interpret the 2250 ma alignment on the sm nd diagram as an errorchron the two sm nd metamorphic ages suggest that partial homogenization of nd isotopic composition in water rich spilite keratophyric meta volcanics can be achieved during low grade metamorphism as a result of ree mobility using the nd values at the time of metamorphism and the average 147sm 144nd ratios of the spilite and the quartzkeratophyre initial nd values of 2 6 3 5 and 3 0 6 1 respectively are estimated at the time of volcanic eruption 2522 ma similar positive initial nd values for the spilites and quartz keratophyres suggest that they might have been derived from the same depleted mantle source or that the quartz keratophyres are derived from a short lived mafic source

ichnological evidence of jokulhlaup deposit recolonization from the touchet beds mabton wa usa abstract the late wisconsinan touchet beds section at mabton washington reveals at least seven stacked jokulhlaup deposits five showing evidence of post flood recolonization by vertebrates tracemakers are attributed to voles or pocket mice 1 3 cm diameter burrows and pocket gophers or ground squirrels 3 6 cm diameter burrows the mount st helens s tephra deposited between flood beds contains the invertebrate generated burrows naktodemasis and macanopsis estimates of times between floods are based on natal dispersal distances of the likely vertebrate tracemakers 30 50 m median distances 127 525 m maximum distances from upland areas containing surviving populations to the mabton area a distance of about 7 9 km tetrapods would have required at least two to three decades to recolonize these flood beds based on maximum dispersal distances invertebrate recolonization was limited by secondary succession and estimated at only a few years to a decade these ichnological data support multiple floods from failure of the ice dam at glacial lake missoula separated by hiatal surfaces on the order of decades in duration ichnological recolonization times are consistent with published estimates of refill times for glacial lake missoula and complement the other field evidence that points to repeated autogenically induced flood discharge

metamorphic overprinting of smnd isotopic systems in volcanic rocks the telemark supergroup southern norway abstract the telemark supergroup of southern norway consists of a series of metamorphosed mesoproterozoic volcanics and sediments the volcanic pile records three metamorphic events a pervasive regional greenschist facies event ca 1050 ma starmer 1993 a more localised early pre regional metamorphism burial type event and a late thermal metamorphism related to granite intrusion ca 900 ma priem et al 1973 in the present study major and trace element data are employed to illustrate the influence of these metamorphic events on apparent initial nd isotopic compositions in metamorphosed volcanics in the case of the basaltic volcanics burial and thermal metamorphism caused substantial fractionation of the ree and alteration of nd isotopic compositions by interaction with open system fluids such that spurious ages might result from age calculations using the nd isotopic data during the regional metamorphism element mobility was limited to the lile rb sr k and ba the ree were not fractionated and the original nd isotopic signatures were preserved the regional metamorphic effects on the acid volcanics are related to the degree of matrix recrystallisation since this matrix contains a large fraction of the ree where groundmass recrystallisation was limited the original nd isotopic signatures were retained but where the groundmass has been extensively recrystallised the ree have been fractionated resulting in a sm nd isochron age menuge 1985 ca 300 ma younger than the u pb zircon age dahlgren et al 1990 the results of this study emphasise the importance of distinguishing the nature of metamorphic fluid rock reactions the potential for ree mobility during low grade metamorphism and the need for careful selection and geochemical screening of samples for nd isotopic studies in metamorphosed volcanic terrains

th u pb monazite geochronology of the luliang and wutai complexes constraints on the tectonothermal evolution of the trans north china orogen abstract recent lithological metamorphic and geochronological studies show that the basement of the north china craton ncc formed by collision of two discrete archean to paleoproterozoic blocks eastern and western blocks along a paleoproterozoic orogen named the trans north china orogen however the timing of the collision is controversial and post collisional exhumation has not been precisely dated this study applies the electron microprobe th u pb monazite dating technique to determine the ages of the peak metamorphic event and subsequent exhumation of the low to medium grade luliang and wutai complexes in the trans north china orogen electron microprobe th u pb monazite data for the luliang complex reveal five tho 2 pbo age ranges 1 1940 1938 ma 2 1880 1847 ma 3 1795 1755 ma 4 1720 1703 ma and 5 1648 ma of these age ranges the first four have also been recorded in the wutai complex which yields tho 2 pbo ages of 1 1930 ma 2 1838 1822 ma 3 1793 ma and 4 1719 ma the oldest age range of 1940 1930 ma is interpreted to record the widespread emplacement of mafic dikes in rocks of the luliang and wutai complexes the age range of 1882 1822 ma is in accord with the shrimp u pb ages of metamorphic zircons and mineral sm nd and 40 ar 39 ar ages obtained for other complexes in the orogen and is interpreted as the age of the major metamorphic event caused by amalgamation between the eastern and the western blocks the age range of 1795 1755 ma is consistent with emplacement of large scale unmetamorphosed mafic swarms at 1800 1765 ma interpreted as the time of post orogenic extension the 1720 1703 ma and 1648 ma ages are considered to date later multiple stages of hydrothermal alteration since monazites with such young ages only occur along fractures in older monazite grains and at the rims these new monazite tho 2 pbo ages support the tectonic model for the evolution of the north china craton that envisages discrete eastern and western blocks colliding along the trans north china orogen at 1 85 ga and then undergoing post collisional extension in the period 1795 1755 ma

earthquake source spectra in eastern north america abstract a new model for average earthquake s wave source spectra in eastern north america ena is derived the model is consistent with fundamental elasto dynamic principles as well as the familiar spectral scaling laws appropriate to sources having geometrical and dynamical similarity observed characteristics of s wave spectra in ena are fully explained as simple consequences of directivity effects entailed by the classical crack rupture model kostrov 1964 madariaga 1976 with normal effective stresses of the order of 100 bars and fractional stress drop brune 1970 abnormally high stress drops are not required recognition of the fundamental role of directivity explains most features of observed data that are currently interpreted as anomalous among other things neglect of such effects is shown to be responsible for the conclusion by boatwright and choy 1992 and boore and atkinson 1992 that high frequency spectral levels for the magnitude 6 saguenay earthquake of 1988 are anomalously large other important implications of the new model are that some previously inferred systematic variations in brune stress drop with magnitude are simply artifacts of the invalid use of the standard brune 1970 1971 model assumption and that current estimates of q in ena which depend on this assumption are likely to be significantly too large for high frequencies associated uncertainties allow the possibility that effective stress does not vary significantly with magnitude for earthquakes having magnitudes in the range of 4 to 7 the new model suggests that some currently favored source spectra may under estimate spectral amplitudes of future larger earthquakes in ena by as much as a factor of 6 for some frequencies the proposed new spectra are practically identical with those of the brune standard model for an effective stress of 100 bars for frequencies up to about twice the brune corner frequency but then increase to values 3 times larger over a range of about 4 in frequency as such the high frequency spectral levels are equivalent to standard brune model values for earthquakes of one full magnitude unit larger with consequent important implications for seismic hazard estimates in the region

u pb and hf isotopic study of detrital zircons from the luliang khondalite north china craton and their tectonic implications two types of metasedimentary rocks occur in the trans north china orogen of the north china craton one type consists of highly metamorphosed supracrustal rocks with protoliths of mature cratonic shale called khondalites as found in the luliang complex rocks of the other type are also highly metamorphosed but less mature as represented by the wanzi supracrustal assemblage in the fuping complex u pb isotopic data for detrital zircons from khondalites show a provenance dominated by 1 9 2 1 ga palaeoproterozoic rocks these detrital zircons display a wide range of e hf values from 16 0 to 9 2 and give hf isotopic model ages mostly around 2 3 ga the high positive e hf values approach those for the depleted mantle at 2 1 ga highlighting a juvenile crustal growth event in palaeoproterozoic times hf isotopic data also imply that c 2 6 ga old crustal material was involved in the palaeoproterozoic magmatic event these data are similar to those for the khondalitic rocks from the interior of the western block of the north china craton suggesting a common provenance in contrast other metasedimentary rocks in the trans north china orogen such as the wanzi supracrustal assemblage in the fuping complex have a source region with both palaeoproterozoic and archaean rocks their detrital zircon hf isotopic data indicate reworking of old crustal material and a lack of significant juvenile palaeoproterozoic magmatic input these rocks are similar to the coevally deposited meta sedimentary rocks in the interior of the eastern block we propose that the luliang khondalites were deposited on the eastern margin of the western block in a passive continental margin environment and were thrust eastward later during collision with the eastern block other metasedimentary rocks in the trans north china orogen were deposited on the western margin of the eastern block in a continental arc environment our data support the eastward subduction model for the palaeoproterozoic tectonic evolution of the north china craton

stochastic prediction of ground motion and spectral response parameters at hard rock sites in eastern north america empirical predictions of ground motions from large eastern north american earthquakes are hampered by a lack of data for such events for this reason most prediction techniques have been based at least in part on data from the seismically active and well instrumented western north america concentrating on the prediction of response spectra on hard rock sites we have used a relatively new theoretical technique that does not require western data to make ground motion predictions for eastern north america this method often referred to as the stochastic model has its origins in the work of hanks and mcguire who treat high frequency motions as filtered random gaussian noise for which the filter parameters are determined by a seismological model of both the source and the wave propagation the model has been successfully applied to the predictions of ground motions in the western united states and to short period magnitudes from large to great earthquakes worldwide for our application the essential parameters of the model are estimated by using existing data from small to moderate eastern north american earthquakes a crucial part of the model is the relation between seismic moment and corner frequency the relation proposed in 1983 by nuttli for mid plate earthquakes leads to predictions of ground motions that are lower than available data by a factor of about 4 on the other hand a constant stress parameter of 100 bars gives model predictions in good accord with the data to aid in applications the ground motion predictions are given in the form of regression equations for earthquakes of magnitude 4 5 to 7 5 at distances within 100 km of the source the explanatory variables are hypocentral distance and moment magnitude m because predictions are often required in terms of m g rather than m we have used the theoretical model to establish a relation between the two magnitudes the predicted relation agrees with the sparse data available although the large uncertainties in the observed magnitudes for the larger events as well as the sensitivity of the theoretical magnitude to the attenuation model make it difficult to discriminate between various source scaling models

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

the age of glacier peak tephra in west central montana at sheep mountain bog near missoula montana a late glacial tephra that probably fell in late summer is preserved as an 8 mm thick graded bed overlain by another 8 mm of redeposited ash mixed with lake deposits sediment surrounding the ash was 14c dated to about 11 200 yr b p electron microprobe analyses of the volcanic glass and hornblende phenocrysts from this ash layer indicate that they are similar in major element chemistry to those of glacier peak layer g previously considered to be about 12 000 yr old or older

nd isotopic systematics and chemistry of central australian sapphirine granulites an example of rare earth element mobility abstract lower proterozoic sapphirine bearing and associated granulites from central australia exhibit the greatest range of present day 143 nd 144 nd ratios nd o 26 5 to 112 3 yet reported for rocks believed to be cogenetic the nd isotopic data and ree abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform cu pb zn sulfide deposits with which they are closely associated field relationships petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism calculations employing ree abundances of the sapphirine granulites and associated rocks combined with bulk solid fluid distribution coefficient data yield high fluid rock ratios consistent with a pre metamorphic hydrothermal origin for the unusual ree patterns the sm nd data for these rocks define an age of 1760 75ma which is significantly younger than the crust formation age of the terrain 2070 125ma but indistinguishable from the rb sr whole rock age for granulite facies metamorphism 1790 35ma these data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly perhaps tens of millions of years before granulite facies metamorphism followed by redistribution of nd isotopes or small fractionations of the sm nd ratio during the granulite facies event and possibly also during intense retrogression which reset rb sr whole rock and u pb zircon and monazite systematics at about 1700 ma

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

archean geodynamics in the central zone north china craton constraints from geochemistry of two contrasting series of granitoids in the fuping and wutai complexes abstract the archean to paleoproterozoic central zone of the north china craton is situated between the eastern and western archean continental blocks and contains two contrasting series of neoarchean granitoids the 2523 2486 ma tonalite trondhjemite granodiorite ttg gneisses in the fuping complex and the 2555 2525 ma calc alkaline granitoids tonalite granodiorite granite and monzogranite in the wutai complex the fuping ttg gneisses most likely formed from partial melting of 2 7 ga basalts at 50 km with an involvement of 3 0 ga crustal material the wutai granitoids have higher k 2 o lile and rb sr but lower sr y and la n yb n than the fuping ttg gneisses are characterized by nd t dm from 2 5 to 2 8 ga and e nd t from 0 49 to 3 34 and are derived from partial melting of a juvenile source at the geochemistry of these two contrasting series of neoarchean granitoids provides further evidence that the wutai complex originated and evolved separately from the fuping complex the wutai complex most likely formed as an oceanic island arc with volcanism and synvolcanic granitoid intrusions at 2555 2525 ma the wutai complex was subsequently accreted onto the eastern archean continental block and was probably responsible for crustal thickening and ttg magmatism at 2523 2486 ma in the fuping complex as part of the taihangshan hengshan block at the western margin of the eastern archean continental block

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

origin of spilitic rocks from the southern slope of the greater caucasus a suite of 39 spilitic basalts and andesites of bajocian age was analyzed for the major elements and li rb sr ba sc v cr co ni and seven ree hf ta and th were analyzed in 20 samples sr and o isotope ratios have been determined for selected whole rock samples the occurrence of biotite and actinolite indicates maximum temperatures of hydrothermal reactions slightly above 400 c prehnite pumpellyite and epidote formed at lower temperatures chlorite and sodic plagioclase are abundant products of hydrothermal seawater alteration a comparison between weakly and intensively altered basalts demonstrates that the largest proportion of al v cr co ni and the ree was immobile during spilitization enrichment in 18o is correlated with the gain of h2o co2 na2o and li which exceeds pre spilitic levels initial 87sr86sr ratios for 10 whole rock samples range from 0 7040 to 0 7049 relics of fresh clinopyroxene phenocrysts from the spilitic basalts and andesites have initial 87sr86sr ratios between 0 7037 and 0 7044 and 18o values between 4 5 and 5 6 smow whole rock samples have higher 87sr86srinit ratios and higher 18o values when compared to their clinopyroxene phenocrysts which is explained as a result of alteration by seawater average 18o values of 11 smow along with the enrichment of li suggest that the major alteration occurred at temperatures below 200 c the spilitic basalts have higher average sc v cr co and ni concentrations relative to the spilitic andesites the abundances of these elements in the spilitic andesites are quite similar to those in typical orogenic andesites the correlation of cr and ni in both the spilitic basalts and the spilitic andesites is explained by various degrees of fractionation of olivine and clinopyroxene although ta and th may have been mobilized to minor extents the majority of the analyzed samples has hf ta th relations which are typical of subduction related volcanic rocks assuming an olivine tholeiite as the parental melt from which the basaltic and andesitic magmas were derived the average la and ce concentrations in these rocks require a metasomatically enriched peridotite source with 5 times chondritic la and ce abundances the coexistence of andesites and basalts at the southern slope of the greater caucasus along with the occurrence of ophiolites in the lesser caucasus suggests that both could be related to a middle jurassic subduction zone in this region

pre late wisconsinan pleistocene biota from southeastern michigan u s a abstract fossiliferous sediments underlie the late wisconsinan mill creek and fisher road tills at the mill creek site northwest of port huron michigan fragmentary remains of 5 fish and 11 mammal taxa occur with 39 taxa of molluscs 6 of ostracodes and a pine spruce pollen assemblage while the fish are wide ranging the mammals include distinctly northern elements such as dicrostonyx sp lemmus sp mictomys borealis and microtus xanthognathus the molluscs include a mixture of cold vertigo alpestris oughtoni and v modesta and warm lioplax sulculosa taxa none of the vertebrates have been found at michigan sub till sites before and lioplax is new to michigan fossil or living vertigo hannai is known only as a pleistocene fossil at present the age of the biota is indeterminate the stratigraphy indicates a middle wisconsinan or greater age a finite 14c age of 48 3 0 8 ka ql 1215 was later supplemented by a tl date of 57 9 ka but perhaps as old as 300 ka two clusters of amino acid allo isoleucine isoleucine values obtained from six genera of molluscs are interpreted to represent the co occurrence of early sangamonian aile ile values ranging from approximately 0 15 to 0 25 and illinoian aile ile values ranging from approximately 0 25 to 0 35 mollusc fossils at the mill creek site these results highlight the need for more reliable quaternary dating methods

smnd geochronology of greenstone belts in the yilgarn block western australia abstract sm nd geochronology has been used to date 3 widely spaced metavolcanic greenstone belts in the yilgarn block the measured isochron ages are 2 78 0 03 ga initial nd 2 5 0 3 for greenstones at kanowna in the eastern goldfields province 3 05 0 10 ga initial nd 0 9 0 7 for diemals marda in the southern cross province and 2 98 0 12 ga initial nd 0 7 1 2 for the warriedar fold belt in the murchison province these data show that substantial crustal components of age 3 0 ga exist in the southern cross and murchison provinces and support published evidence that the eastern goldfields province contains no crustal material 2 8 ga these ages are all significantly younger than ages commonly observed in the western gneiss terrain and so support the suggestion of a progressive crustal age trend across the yilgarn block the measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at 2 8 ga the age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas 0 2 ga so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences leaching experiments on altered warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system probably prior to the 2 6 ga regional metamorphic event the initial nd values do not conform to previously suggested mantle depletion curves when considered with published data for greenstone belts they suggest complex mantle heterogeneity with end diverging over time to both positive and negative values

the wutaishan orogenic belt within the shanxi province northern china a record of late archaean collision tectonics abstract the archaean crust of northeastern shanxi province northern china is dominated by three distinct terranes the high grade gneiss terranes hengshan in the north and fuping in the southeast and the intervening low grade terrane of the wutaishan belt the 2 5 ga beitai arc complex which constitutes the major part of the wutaishan belt is interpreted as a pre collision magmatic arc there was a marginal sea basin between the beitai arc and the fuping terrane at the fore arc collision stage the passive margin deposits which occurred along the western margin of the fuping terrane became a foreland fold thrust belt after the closure of the marginal sea basin the remnants of sea floor occur as a melange zone along the suture zone between the beitai arc and fuping terrane nearly at the same time at 2 5 ga the back arc oceanic lithosphere started to subduct beneath the hengshan terrane then the hengshan terrane came into collision with the arc and the fuping terrane some dismembered sea floor rocks are preserved as a back arc melange zone between the hengshan terrane and the beitai arc the term wutaishan orogeny should be reserved for the processes which lasted from 2600 to 2450 ma

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

evidence for isotopic equilibration of smnd whole rock systems in early archaean crust of enderby land antarctica smnd isotopic data indicate that differential ree mobility occurred on a whole rock scale during transitional amphibolite to granulite facies regional metamorphism 700 c 7 kbar in early archaean rocks 3930 ma of the napier complex of enderby land antarctica the degree of mobility is independent of metamorphic grade but correlates directly with development of tectonic fabric whole rock samples with d3 m3 internal fabrics lie along an array corresponding to an age of 2410 100ma whereas samples preserving only earlier fabrics preserve an older albeit imprecisely defined isochron age in contrast to a widely held belief such changes did not require the presence of a large hydrous fluid flux if the mechanism responsible for smnd resetting at this locality where tchur ages range from 1990 ma to 6090 ma is more widespread than is currently recognised isolated smnd model ages particularly in complex terrains should be treated with caution

rare earth element mobility in vesicular lava during low grade metamorphism a geochemical comparison of basaltic relicts and spilitic domains from two burial metamorphosed flows in central chile of similar original composition and rich and poor in amygdules respectively demonstrates a relationship between initial vesicularity and rare earth element ree mobility during spilitization the ree were partly leached from permeable parts of the flows and precipitated in voids now amygdules and veinlets the ree excluding eu moved coherently in the highly amygdaloidal flow spilitic domains and amygdules inherited the basaltic ree pattern besides being characterized by a positive eu anomaly epidotes separated from amygdules have a ree distribution which mimics that of the basalt the absolute contents range widely suggesting local and or temporal ree variations in the metamorphic fluids pumpellyite differs by being strongly enriched in heavy ree similar ratios of th hf and ta in samples as contrasting as relict basalt and a geode are consistent with coherent leaching coherent mobility when established for a rock system can be used to elucidate for example whether minerals in cross cutting veins were formed by local redistribution or from introduced material

smnd isotopic systematics of enderby land granulites and evidence for the redistribution of sm and nd during metamorphism measurements of143nd 144nd and147sm 144nd are reported for whole rocks and mineral separates from granulites of the napier complex at fyfe hills charnockites leuconorites and gabbros yield a whole rock smnd isochron age of3060 160m y and an initial143nd 144nd ratio of0 50776 10 nd 3060m y 2 0 1 8 the negative nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of sm and nd and partial re equilibration of143nd 144nd ratios this probably occurred during the upper granulite facies metamorphism which has also been dated at 3100m y by rbsr and upb zircon studies 1 coexisting clinopyroxene apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 300 m y this array indicates that redistribution of sm and nd and re equilibration of143nd 144nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at 2450m y due to the resetting of the smnd system on both whole rock and mineral scales the primary crystallization age of the igneous protolith is not well constrained by the present data although it is clearly3100m y if it is assumed that the complex was derived initially from a depleted mantle reservoir nd t 2 evolution of the negative nd value of 2 0 with the observed sm nd ratios requires a prehistory of 380m y this implies a primary age of 3480m y however substantially older primary ages can be inferred if the source reservoirs had nd t 2 and or substantial reductions in the sm nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m y such an inferred reduction in the sm nd ratio may have been the result of preferential loss of sm relative to nd or introduction of a low sm nd fluid with nd 0 during granulite facies metamorphism

neoarchean paleoproterozoic magmatic arc evolution in the wutai hengshan fuping area north china craton new perspectives from zircon u pb ages and hf isotopic data abstract the wutai granite greenstone belt which formed in the neoarchean paleoproterozoic with the adjoining high grade metamorphic hengshan fuping region record important transitions in crustal and tectonic evolution although these evolutionary processes have still been in dispute in this study new u pb and lu hf la icp ms zircon analyses were obtained from eight metamorphosed magmatic rocks in the wutai hengshan fuping area in north china craton ncc the results reveal that most of wutai granitoids yield emplacement ages of 2 56 2 52 ga with corresponding ehf t2 of 4 7 to 8 3 the tonalitic gneiss from dongshandi intrusion however gives a new age of 2482 ma and ehf t2 of 2 5 to 4 6 which are similar with yixingzhai pluton marked by ages of 2501 2499 ma and ehf t2 of 3 2 to 4 4 also 2 7 ga and 2 5 ga continental crust generation are further revealed but 2 5 ga crustal growth is more complicated due to the involvement of old crustal components which can be identified by variable ehf t2 values ranging from 1 8 to 7 5 thus based on the distinctions of hf isotopes compositional and spatial features two consecutive groups of 2 56 2 52 ga and 2 52 2 48 ga are recognized and a two step tectonic evolution model for the wutai hengshan fuping area is proposed during 2 56 2 52 ga intra oceanic subduction led to the formation of wutai oceanic island arc accompanied mainly by the development of wutai granitoids ehf t2 of 4 7 to 8 3 and coeval volcanics predominant arc tholeiitic chemical affinity corresponding to the juvenile crustal growth between 2 52 ga and 2 48 ga with intervening oceanic crust between intra oceanic arc and continent consumed the wutai intra oceanic arc then collided and was obducted onto the western edge of the hengshan fuping continental block large amounts of ttg gneisses produced in both the wutai and hengshan fuping ehf t of 1 8 to 7 5 along with calc alkaline volcanism in wutai possibly in response to the thickening through arc accretion and commencing the continental arc this proposed two step tectonic evolution model combined with a late neoarchean 2 6 2 5 ga intra oceanic arc system along the northwestern of the eastern block eb probably provides a better explanation for the fundamental geodynamic mechanism for continental arc development at the proterozoic dawn

sm nd isotope study of early archean rocks qianan hebei province china a sm nd study of mafic amphibolite which occurs as lensoid masses in migmatitic quartz dioriticto granitic gneiss was undertaken in order to determine if it has an early archean age six amphibolite samples define an isochron yielding an age of 3 50 0 08 b y with an initial nd value of 3 3 0 3 three samples of quartz dioritic gneiss lie close to this isochron and have sm nd model ages of ca 3 5 b y referenced to a epleted mantle source a sample of k feldspar rich granitic gneiss gives a younger model age of about 2 9 b y the non zero initial nd value of the mafic amphibolites can be compared with chondritic meteorite samples measured in this laboratory and is therefore precisely determined if the initial nd value can be regarded as representative of the original igneous rocks then it indicates that the mantle magma source was severely depleted with respect to magmaphile elements this suggests that the geochemistry of the early archean mantle may have been similar to that of the modern upper mantle and may require that a large pre archean continental crust have been present on the earth

the isotope systematics of a juvenile intraplate volcano pb nd and sr isotope ratios of basalts from loihi seamount hawaii abstract sr nd and pb isotope ratios for a representative suite of 15 basanites alkali basalts transitional basalts and tholeiites from loihi seamount hawaii display unusually large variations for a single volcano but lie within known ranges for hawaiian basalts nd isotope ratios in alkali basalts show the largest relative variation 0 51291 0 51305 and include the nearly constant tholeiite value 0 51297 pb isotope ratios show similarly large ranges for tholeiites and alkali basalts and continue tatsumoto s 31 loa trend towards higher 206pb 204pb ratios resulting in a substantial overlap with the kea trend 206pb 204pb ratios for loihi and other volcanoes along the loa and kea trends 31 are observed to correlate with the age of the underlying lithosphere suggesting lithosphere involvement in the formation of hawaiian tholeiites loihi lavas display no correlation of nd sr or pb isotope ratios with major element compositions or eruptive age in contrast with observations of some other hawaiian volcanoes 38 isotope data for loihi as well as average values for hawaiian volcanoes are not adequately explained by previously proposed two end member models new models for the origin and the development of hawaiian volcanoes must include mixing of at least three geochemically distinct source regions and allow for the involvement of heterogeneous oceanic lithosphere

sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt ellensburg formation late miocene central washington late miocene volcaniclastics of the ellensburg formation washington northwestern usa are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a cascade range source area two facies associations are recognized the first is composed of laterally restricted well sorted polymictic conglomerates representing a gravel bedload regime during inter eruption periods the second comprises laterally extensive sheets bounded by deeply developed paleosols composed of monomictic pebbly pumiceous dacitic sandstones with intercalated debris flow and hyperconcentrated flood flow deposits these sheets aggraded in response to eruptive events that are sometimes recorded by air fall tephras at the bases of sequences debris flow and hyperconcentrated flood flow deposits occur as far as 120 km from source but are uncommon beyond 50 km hyperconcentrated flood flow deposits initially increase in abundance away from source as debris flow deposits diminish suggesting formation of hyperconcentrated flood flows by dilution of debris flows sandy facies form broad sheets dominated by scour fill bedding in proximal and medial settings and grade to narrower trough cross bedded sheets in distal settings suggesting moderation of flood discharges with distance base level changes associated with episodic sediment influx caused incision aggradation cycles in mainstream settings and episodic impoundment of tributaries to form lakes or rapidly aggrading poorly drained floodplains although volcanism was the primary control on depositional style concurrent development of the yakima fold belt produced a structure consequent drainage pattern that determined sediment dispersal and basin subsidence permitted preservation of both syn eruption and inter eruption facies detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics but dominates deposits of small tributary streams flowing toward rather than from the cascades

correlation of distal tephra layers in loess in the channeled scabland and palouse of washington state abstract thirty two distal tephra layers that are interbedded in quaternary loess at 13 sites in the channeled scabland and palouse were sampled as part of a regional study of the stratigraphy and chronology of dominantly windblown sediments on the columbia plateau an electron microprobe was used to determine the elemental composition of volcanic glass in all of the samples and also to determine the composition of ilmenite in 14 of them two of the distal tephra layers correlate with glacier peak eruptions 11 200 yr b p five with mount st helens tephra set s 13 000 yr b p and nine with mount st helens tephra set c ca 36 000 yr b p based on analysis of glass and ilmenite in reference pumices from glacier peak mount st helens mount mazama mount rainier and mount jefferson on the calculation of similarity coefficients for comparisons of both glass and ilmenite reference compositions with those of distal tephras and on considerations of stratigraphic position the composition of glass and ilmenite and the stratigraphic position of one distal tephra layer in the loess suggests that it is from an eruption of mount st helens at least several thousand years older than the set c eruptions glass composition and stratigraphic position of a distal tephra at another site in loess suggested a possible correlation with some layers of the pumice castle eruptive sequence at mount mazama ca 70 000 yr b p but similarity coefficients on ilmenite of only 45 and 48 fail to support the correlation and show why multiple correlation methods should be used similarity coefficients higher than 96 for both glass and ilmenite establish a correlation with mount st helens layer cw for distal layers in two widely separated sites these layers are in sedimentary successions that are closely associated with giant floods in the channeled scabland the 36 000 yr b p radiocarbon age of the mount st helens set c establishes a minimum limiting date for an episode of flooding that predates the widespread late wisconsin floods a correlation of distal tephra layers at two other sites in the scabland and palouse establishes a chronostratigraphic link to a still older episode of flooding within the brunhes normal polarity chron six distal tephra layers in pre late quaternary loess that are not correlated with known or dated eruptions have compositions and distinctive stratigraphic positions relative to magnetic reversal boundaries that make them key markers for future work

late quaternary stratigraphy of loess in the channeled scabland and palouse regions of washington state abstract loess deposits of late quaternary age in the channeled scabland and western palouse regions of washington state contain distinctive sequences of calcic buried soils loess layers tephra layers and stratigraphic unconformities caused by cataclysmic flooding four new soil stratigraphic units the sand hills coulee soil washtucna soil old maid coulee soil and the devils canyon soil are recognized across a 100 km transect the pedostratigraphic correlations are strengthened by two tephra layers found at several sites that are correlative with the mount st helens set s radiocarbon ages about 13 000 yr b p mount st helens set c radiocarbon ages about 36 000 yr b p and by a third one that may be from an earlier pre set c mount st helens eruption the set s tephra layer separates the sand hills coulee and washtucna soils the set c tephra layer separates the old maid coulee and devils canyon soils and the pre set c tephra layer underlies the devils canyon soil the well known late wisconsin episode of scabland flooding and two older episodes of flooding are marked by regional stratigraphic unconformities that lie between pedostratigraphic units one unconformity that underlies the washtucna and old maid coulee soils was caused by a major episode of scabland flooding that occurred before 35 000 yr b p probably during the early wisconsin 80 000 to 65 000 yr b p a possible second and older episode of flooding recognized by an unconformity that underlies the devils canyon soil may be illinoian ca 130 000 to 200 000 yr b p in age

summary of pre 1980 tephra fall deposits erupted from mount st helens washington state usa mount st helens has been a prolific source of tephra fall deposits for about 40 000 years these tephra deposits 1 record numerous explosive eruptions 2 form important regional time stratigraphic marker beds and 3 record repeated changes in composition within and between eruptive periods

thermoluminescence dating of late pleistocene loess and tephra from eastern washington and southern oregon and implications for the eruptive history of mount st helens the first numeric dating of late pleistocene loess from the palouse region of eastern washington is reported this loess is interbedded with thin distal tephra layers some identified as mount st helens products both the interbedded tephra layers and thick paleosols that have developed in the loess have previously been correlated across the region a thicker correlative of one of these tephra layers mount st helens set c was collected from summer lake oregon for direct dating by thermoluminescence tl polymineral fine grain tl ages for loess from three chosen sections in washington range from 17 to 83 ka these imply ages for some tephra beds that are significantly older than implied by earlier radiocarbon 14c dating for example tl ages of 17 21 ka for loess bracketing a tephra layer correlated to mount st helens set s suggests a somewhat older age for this ash than the previously accepted 14c age of 15 5 0 5 ka cal yr corrected to calendar years tl ages of 46 6 1 ka and 57 7 ka for loess above and of 53 7 ka and 75 6 ka below a tephra bed correlated to a mount st helens set cw layer along with a tl age of 46 5 ka for fine silt sized glass from a mount st helens set cy bed in oregon revise upward from 38 ka cal yr 14c age is 36 ka b p to 50 ka the age of this regionally important marker ash set several tl ages taken together imply a much longer eruptive history for mount st helens than previously thought a tl age of 83 8 ka just below a previously undated tephra layer from mount st helens emsh ash suggests that the eruptive history of the mountain extends back to at least 80 ka furthermore age depth extrapolation through our sections to an ash wa 5c probably correlated to mount st helens suggests an eruptive history of at least 120 ka

crustal evolution of the gyeonggi massif south korea nd isotopic evidence and implications for continental growths of east asia abstract we investigated the sm nd isotopic characteristics of the gyeonggi massif korea in order to delineate its crustal growth history nd isotopic depleted mantle model ages t dm obtained from 32 new measurements of basement and supracrustal samples and from the literature suggest that the gyeonggi massif is as old as middle archean 3 0 ga and was primarily extracted from the mantle during the late archean 2 9 2 5 ga in addition minor crustal formation occurred during the early proterozoic ca 1 9 1 8 ga late archean early proterozoic crustal growth of the gyeonggi massif is comparable with that of the sino korean or yangtze craton the t dm distribution pattern alone is not sufficient to determine the crustal affinity of the gyeonggi massif with either of the chinese cratons however late proterozoic crustal evolution of the gyeonggi massif appears to favor its tectonic correlation with the yangtze craton

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

granitoid evolution in the late archean wutai complex north china craton abstract two temporally discrete episodes of granite magmatism have been identified in the wutai complex of the north china craton based on shrimp u pb zircon dating of all major granitoid bodies in the area the older period is latest archean and ranges in age from 2560 to 2515 ma whereas the younger episode is paleoproterozoic with magmatism at 2170 and 2120 ma archean magmatism can be further separated into two pulses based on geochemistry and the association of the younger plutons with extensive coeval volcanic activity in the wutai complex the early phase of archean activity is represented by the lanzhishan ekou and portions of the chechang betai granites with ages ranging from 2560 to 2540 ma the lanzhishan granite is the only one at wutaishan that contains inherited zircon with a number of cores and individual grains giving ages in excess of 2700 ma these are relatively evolved granitoids interpreted to be derived from older basement of the eastern block of the north china craton the shifo guangmingshi the main grey phase of the wangjiahui granite and the major components of the chechang betai granite have ages of 2540 2515 ma and are coeval with felsic volcanism in the wutai complex which is a consequence of subduction and related arc magmatism the dawaliang granite and pink phase of the wangjiahui granite are considerably younger with paleoproterozoic ages of 2170 and 2120 ma respectively these are also evolved granitoids most likely derived by partial melting of the older basement although we have found no evidence of inheritance the petrogenesis of the granitoids reflects the precambrian amalgamation history of the north china craton indicating derivation along the western edge of the eastern block during development and amalgamation of the wutai arc and prior to collision with the western block of the craton 1 8 ga ago

long history of pre wisconsin ice age cataclysmic floods evidence from southeastern washington state abstract cataclysmic ice age floods in the pacific northwest began as early as 1 5 2 5 ma on the basis of an evaluation of surface exposures and recent borehole studies within southeastern washington field evidence suggests at least two episodes of pre wisconsin i e 130 ka glacial outburst flooding a middle pleistocene flood is identified by normal magnetic polarity calcrete capped deposits that yield maximum th u age dates from 200 to 400 ka the deposits with reversed polarity are correlated to early pleistocene 780 ka floods while exposures of pre wisconsin deposits are limited because of erosion and or burial the record of earlier pleistocene flooding is preserved within giant flood bars these bars show incremental growth representing a composite from cataclysmic floods deposited intermittently through the pleistocene in one giant flood bar up to 100 m thick deposits interpreted as matuyama age indicate that the bar had grown to half its present height by 780 ka furthermore matuy

paleomagnetic and tephra evidence for tens of missoula floods in southern washington paleomagnetic secular variation and a hiatus defined by two tephra layers confirm that tens of floods from glacial lake missoula montana entered washington s yakima and walla walla valleys during the last glaciation in these valleys the field evidence for hiatuses between floods is commonly subtle however paleomagnetic remanence directions from waterlaid silt beds in three sections of rhythmically bedded flood deposits at zillah touchet and burlingame canyon display consistent secular variation that correlates serially both within and between sections the secular variation may further correlate with paleomagnetic data from fish lake oregon and mono lake california for the interval 12 000 17 000 14 c yr b p deposits of two successive floods are separated by two tephras derived from mount st helens washington the tephras differ in age by decades indicating that a period at least this long separated two successive floods the beds produced by these two floods are similar to all of the 40 beds in the slack water sediment sequence suggesting that the sequence is a product of tens of floods spanning a period of perhaps a few thousand years

a subdivision of the precambrian of china abstract precambrian rocks are widely distributed in china the precambrian is divided into two time units i e the archaean and proterozoic eon each of these is separated into three chronological intervals also with the status of eras with the prefixes early middle or late the time boundary between the archaean and proterozoic eon is placed at 2500 ma according to the present isotopic data the proposed subdivision for the archaean of china is two fold the age of the fuping group is younger than 2800 2900 ma and that of the qianxi group and the corresponding stratigraphic units of eastern liaoning are older than 2800 ma so that 2800 ma is selected as the boundary between the early middle and late archaean based on the representative stratigraphic units the wutai and huto groups and an intervening major unconformity formed by the wutaiian orogeny at 2200 2300 ma the early proterozoic is further divided into two periods with a time demarcation at 2200 ma a major episode of orogeny known as the luliangian movement occurred at the end of the early proterozoic at 1900 ma this disturbance was very extensive and is in a way responsible for the difference in geological conditions between the lower and middle upper proterozoic in china the boundary 1900 ma that relates to the luliangian movement is more important than the boundary corresponding to the age of 1600 ma which is recommended as the time boundary between proterozoic i and ii so we propose to use 1900 ma as the boundary between the early and middle proterozoic in china the time boundary between the middle proterozoic including the changcheng system and the jixian system and the late proterozoic which is composed of the qingbaikou and sinian systems is 1000 ma the age for the boundary between cambrian and precambrian based upon the recent isochron data is inferred to be 610 ma

a loess record of pre late wisconsin glacial outburst flooding pleistocene paleoenvironment and irvingtonian fauna from the rulo site southeastern washington usa abstract worldwide loess paleosol sequences are archives of terrestrial quaternary paleoenvironment data the rulo site is a newly described site in the palouse loess of southeastern washington state usa the site reveals a 19 m thick deposit of loess and flood sediment preserving a total of 30 m of reconstructed stratigraphic section five unconformity bound sequences of sediment are exposed at the site the lowest two of which have yielded fossil remains including the first reported occurrence of flat headed peccary platygonus compressus in northwestern north america the unconformities two diamicts bearing extrabasinal clasts and a number of clastic dikes provide evidence for pre wisconsin glacial outburst floods in southeastern washington more than a dozen paleosols are preserved at the site these soils are relatively unweathered and therefore primarily record periods of slow sedimentation accumulation in the palouse rather than changes in weathering rates a paleosol near the base of the sequence is unusually weathered for palouse paleosols and records a period of increased weathering during the middle pleistocene all units above the sand are normal paleomagnetic polarity additional age control is provided by a tephra in the upper third of the sequence that matches a ca 220 ka tephra from newberry volcano and by stratigraphic correlation of later units to other sites in the palouse the rulo site adds to the record of irvingtonian fauna in north america and supports accumulating evidence for pre late wisconsin glacial outburst floods in the pacific northwest

three dimensional finite difference simulation of fault dynamics rectangular faults with fixed rupture velocity we analyze three dimensional finite difference solutions for a simple shear crack model of faulting to determine the effects of fault length and width on the earthquake slip function the fault model is dynamic with only rupture velocity fault dimensions and dynamic stress drop prescribed the numerical solutions are accurate for frequencies up to 5 hz and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies near the hypocenter the slip velocity exhibits a square root singularity whose intensity increases with hypocentral distance at distances greater than the fault width w growth of the velocity intensity ceases and the slip function becomes nearly invariant with distance along the fault length closed form expressions are developed for the dependence of static slip s slip rise time tr and slip velocity intensity v on fault geometry along the center line of a long narrow fault at hypocentral distances exceeding w these expressions reduce to s w tr 0 5 w vr and v w 2 vr where is the dynamic stress drop the shear modulus and vr the rupture velocity the numerical results imply that uniform dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high frequency ground motion relative to low frequency ground motion a further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width but will be independent of rupture length