crustal structure and surface zonation of the canadian appalachians implications of deep seismic reflection data in 1986 1181 km of marine seismic reflection data was collected to 18 20 s of two way traveltime in the gulf of st lawrence area the seismic profiles sample all major surface tectono stratigraphic zones of the canadian appalachians they complement the 1984 deep reflection survey northeast of newfoundland together the seismic profiles reveal the regional three dimensional geometry of the orogen three lower crustal blocks are distinguished on the seismic data they are referred to as the grenville central and avalon blocks from west to east the grenville block is wedge shaped in section and its subsurface edge follows the form of the appalachian structural front the grenville block abuts the central block at mid crustal to mantle depths the avalon block meets the central block at a steep junction that penetrates the entire crust consistent differences in the seismic character of the moho help identify boundaries of the deep crustal blocks the moho signature varies from uniform over extended di

stable isotope and radiocarbon balances of two tibetan lakes sumxi co longmu co from 13 000 bp hydrologic and climatic changes in western tibet since 13 000 bp are inferred from mineralogical and isotopic analyses of lacustrine sediments from the sumxi longmu co basin the interpretation is based on isotope data from the modern system carbon isotope chemistry is complex because of the presence of old limestones on the watershed sediments contain detrital and authigenic calcites the proportions of which are quantitatively estimated by x ray diffractometry stable isotope contents of the authigenic calcite are reconstructed from mass balance equations the high 13c contents up to 6 are attributed to an enhancement of calcite precipitation from water surface during freezing periods the 18o content is related to residence time and evaporation the validity of 14c dates obtained from different types of material is discussed our record in good agreement with paleobiological data from the same material shows the sudden establishment of wet warm conditions at 10 000 bp which culminated at 7500 6000 bp after a brief reversal event at 7900 bp maximum aridity for the whole holocene is recorded at 4300 bp

the complete synthesis of seismic crustal phases at regional distances using a new method to evaluate green s functions for elastic layered media we compute complete synthetic seismograms at regional distances for an earthquake in the earth crust we choose as source parameters the ones inferred for an earthquake in central france and compare the computed seismograms to the ones recorded at several short period stations of the french seismic network the major features of the records including complexity and duration are well reproduced by the synthetics lg waves which are the prominant phase are shown to be guided waves made up of sh and sv waves incident on the moho at angles of incidence more grazing than the critical angle and multiply reflected within the crust the length of the computed lg wave coda extends between group velocities of 3 5 and 2 8 km s and is in good agreement with the data

an investigation of the regional variations and frequency dependence of anelastic attenuation in the mantle under the united states in the 0 5 4 hz band summary studies of teleseismic p and s wave amplitudes and spectra in the 0 5 4 hz band show large variations in the attenuative properties of the upper mantle under the united states the data indicate that attenuation is greatest under the south western united states including but not confined to the basin and range province the lowest attenuation prevails under the north central shield regions the north eastern part of the country consisting of new england and possibly including a larger area along the eastern seaboard is characterized by moderate attenuation in the mantle the level of the high frequency energy in short period seismic waves and the differences between q values derived from short and long period data indicate that q is frequency dependent the form of frequency dependence of t compatible with the data in the 0 5 4 hz range does not allow a rapid decrease of t with increasing frequency rather it supports a gradual decrease covering the broader 0 1 4 hz range the curves of t versus frequency for shield to shield and mixed shield to western united states type paths are parallel with an average difference of 0 2 s in t in the short period band but may diverge towards the long period band for both curves t p is below 1 s for shield to shield paths t p must be below 0 5 s at 1 hz

drastic desalination of small lakes in east siberia russia in the early twentieth century inferred from sedimentological geochemical and palynological composition of small lakes reconstruction of temporal and spatial climate development on a seasonal basis during the last few centuries may help us better understand modern day interplay between natural and anthropogenic climate variability the objective of this paper is to reconstruct hydrology and landscape changes of east siberia during the termination of the little ice age and the subsequent recent warming we analysed sediment samples from the saltwater sulfatnoe lake bolshoye alginskoe and freshwater shuchie lake using high resolution x ray fluorescence spectroscopy at 1 mm scan resolution fourier transform infrared techniques and pollen analyses the depth age models of the cores were constructed by 210pb activity using the constant rate of supply model the lake sediment cover of these lakes began to form from ca 1870 three significant periods 1870 1895 1895 1925 and from 1925 to the present were defined in hydrology and chemical regime of these lakes for the past 140 years lake levels were extremely low and high saturated with salts during the final period of the little ice age lake levels began to slowly rise from 1870 to 1895 and vegetation was poor at that period intensive desalination of the lakes occurred in 1895 1925 and environment conditions were temperate and favourable for the majority of the taxa of the regional vegetation regional precipitation significantly increased and water saturation of the catchments was high from 1925 to the present the chemical precipitation of carbonate stopped completely in lake shichie and reduced considerably in lake sulfatnoe and b alginskoe strong increasing trend of weathering of the lake catchments began in 1970 and still continues

holocene palaeoenvironmental records from the high altitude wular lake western himalayas we present a comprehensive record of holocene 11 590 628 cal yr bp climate and hydrographic changes around the wular lake located in kashmir valley india based on the multi proxy investigation

regional variation and frequency dependence of qβ in the crust of the united states approximate lg attenuation coefficient values are determined for paths in eastern north america at periods between 2 and 4 sec where no data were previously available these new data together with reported values at a period of 1 sec are consistent with values predicted by a frequency dependentent q of the crust in which q varies as 0 2 the new data are inconsistent with values which would be predicted by models having prominent maxima or minima in q 1 within the period range 1 to 5 sec lg attenuation coefficients were computed for the frequency independent crustal q models of cheng and mitchell 1981 for the basin and range province and colorado plateau of the western united states the predicted values at 1 sec for the colorado plateau and basin and range province are about twice as large and three times as large respectively as those predicted for the eastern united states recently reported values of q for 1 sec lg in those regions are consistent with the attenuation coefficients and q values of lg predicted by the colorado plateau and basin and range province models a model with a small degree of frequency dependence of q would be consistent but is not required by the data

attenuation of high frequency earthquake waves in south america earlier attenuation studies for the south american continent indicate that for sn and lg waves there is low attenuation in the shield region east of the andes whereas in the west of south america with some exceptions there is high attenuation those studies however were nonquantitative in this study theq0 1 hz values for lg waves for south america are presented based on a scattering model ofaki 1969 as extended byherrmann 1980 for the coda waves of shallow local and near regional earthquakes the results of the codaq method are compared with those obtained bynuttli s method 1973 theseq0 values are in good agreement with the apparentq0 of lg waves obtained by the latter method the data were obtained from over 100 local and regional earthquakes recorded by 12 wwssn stations throughout continental south america they provided a range of frequencies from 0 4 to 1 4 hz for which the frequency dependence ofq was investigated by assumingq q0 f f0 the observed data indicate that the tectonic region of western south america is characterized by lowq0 and a large frequency dependent factor the values ranging from 150 to 350 and 0 4 to 0 7 respectively theq0 values increase in the shield region east of the andes but frequency dependence decreases the average crustalq0 for north and central argentina ranges from 420 to 580 and ranges from 0 2 to 0 3 theq0 is larger in the brazil region ranging from 580 to 980 and varies from 0 0 to 0 2 in the lower attenuation region of eastern south america higher values of attenuation correlate with greater thickness of the sedimentary layers

the seismic velocity structure of northern appalachian crust around western newfoundland the crustal velocity structure beneath western newfoundland is constrained by a reanalysis of older regional refraction profiles together with an analysis of one new profile initial interpretation of the older data gave inconsistent and discontinuous structures that are difficult to reconcile with more recent deep reflection profiles we also show that traveltimes predicted by the earlier models often do not yield acceptable fits to the original observations our reinterpretation reveals a simpler pattern in which the crust is characterized by a persistent high velocity lower crustal hvlc layer this layer has velocities of 7 0 7 9 kmls and thicknesses of 5 23 km it is thickest beneath the grenville crustal block east of the appalachian structural front and thins or is possibly absent within the central block analysis of the new much higher resolution profile off western newfoundland confirms the existence of the hvlc layer with a velocity of 7 2 kmls and thicknesses of 11 19 km increasing to

wide angle seismic reflections from two dimensional random target zones we have analyzed the shot gather seismic response of a buried zone of isotropic random velocity fluctuations scattering analysis predicts that events at offsets greater than the depth of the target zone will appear more continuous and coherent than events at zero and small offsets due to an effective dip filter inherent to the source target receiver geometry the scattering theory is confirmed with finite difference synthetic seismograms calculated for an idealized crustal model the closely spaced synthetic seismograms show bright events having the appearance of refraction arrivals which suggest a horizontally layered structure in the target zone these effects appear to be independent of the magnitude of the velocity fluctuations provided that the magnitude of the fluctuations is small 3 0 of the mean velocity we have made a second finite difference calculation of a crustal scale model to reinterpret previously published wide angle data from the black forest germany sandmeier and wenzel 1986 the response of the lower crust seen in the conventionally recorded field data can be explained by a lower crustal model of isotropic small amplitude random velocity fluctuations we present this in contrast to a laterally homogeneous model of a finely layered lower crust of alternating high and low velocities proposed by sandmeier and wenzel 1986 the synthetic studies and the scattering analysis demonstrate that the geometry of the wide angle experiment produces signals which appear to be from layered target zones even where no layering exists they also underscore the importance of conducting seismic surveys which make use of closely spaced receivers and which integrate vertical incidence and wide angle recording

small scale heterogeneity and large scale velocity structure of the continental crust since conrad 1925 first identified a midcrustal refraction event in central europe seismologists have developed layered models of the continental crust with velocity steps at various levels modern analysis of crustal refraction data including automated inversion relies on ray theoretical techniques or wave theory for plane layered media which generally parameterize the earth as a series of grossly horizontal layers the bias toward layered models is in part based on physical and geologic intuition but it is to a large measure a result of available interpretation methods for example layer based reflectivity and ray tracing methods in contrast reflection seismology has produced a picture of a highly heterogeneous crust with wavelength scale variations in velocity existing at different levels of the crust in different tectonic regimes finite difference modeling shows that zones of small amplitude random velocity fluctuations not only produce short discontinuous reflection segments as observed on deep seismic reflection data but also produce strong wide angle reflections some of the events identified as crustal wide angle reflections in field data may in fact be the result of scattering from wavelength scale heterogeneities and need not result from reflection from a first or second order velocity discontinuity significant changes in the large scale velocity structure associated with zones of random velocity fluctuations produce relatively small changes in the dynamic properties of the backscattered wave field this observation is in disagreement with the common credo that the seismic refraction method is primarily sensitive to changes in the large scale velocity structure as a consequence the ray theoretical interpretation of such wide angle reflections as first order discontinuities leads to crustal velocity structures that are nonunique and may be erroneous in detail while correctly predicting average crustal velocity and thickness we are not suggesting that crustal velocity does not change with depth on the large scale rather we are proposing that crustal seismic events observed at intermediate and large offsets may be the result of scattering from wavelength scale velocity fabric rather than specular reflection or refraction from first order discontinuities

apparent q for upper crustal rocks of the central rio grande rift spectra from 130 digitally recorded microearthquakes in the magnitude range 0 9 to 0 3 were used to compute apparent seismic quality factors q for upper crustal rocks within 45 km of socorro new mexico in this study apparent q was defined as a measure of seismic wave attenuation due to intrinsic absorption and scattering and was computed over the frequency range 0 50 hz most of the recorded energy was in the 3 30 hz band it was found that apparent q increases with event distance for the eight stations used in this study this increase was successfully modeled with a varying thickness low q low velocity layer lying above a relatively high q high velocity half space it was found that 1 near surface low q regions lying directly beneath the recording sites vary from 0 3 to 2 0 km in thickness and exhibit qp and qs values less than 50 2 the average qs for the upper crust in the central rio grande rift excluding the near surface low q region is 535 95 confidence interval 374 938 3 the observed qp qs ratio for events recorded at eight stations varied from 0 34 to 1 39 for earthquakes recorded at three stations the qp qs ratio decreased with increasing event distance more for two other stations no systematic increase or decrease in qp qs with distance was evident and for the remaining three stations events were too closely clustered to draw any conclusions these changes in the qp qs ratio with increasing event distance may reflect varying degrees of fluid saturation in upper crustal rocks 4 at least three anomalously low qs regions exist within 20 km southwest of socorro these abnormally low q regions may reflect small magmatic intrusions or high fracture densities at depth and may lie in close proximity to microearthquake swarms at depths of 7 2 10 3 km 63 references 10 figures 4 tables less

holocene environmental changes in bangong co basin western tibet part 4 discussion and conclusions abstract a 12 4 m core taken in lake bangong provides a continuous holocene climatic record we summarize information on changes in stable isotope and radiocarbon balances in the lake hydrobiology and vegetal cover in the catchment deduced from detailed analytical results given in the three preceding papers the bangong record is then compared with the environmental history of the neighbouring lake sumxi also constructed from multidisciplinary analyses the two records show a major environmental change at 10 9 5 ka b p attributed to a rapid strengthening of the summer monsoonal circulation which led to wet warm conditions this event was followed by a long term trend toward aridity which culminated around 4 3 ka b p in western tibet maximum monsoon rainfall seems to have occurred from 9 5 to 8 7 ka b p and from 7 2 to 6 3 ka b p as two wet pulses separated by a reversal event centered on 8 0 7 7 ka b p our results broadly agree with the records from lake seling in central tibet and lake qinghai at the plateau s northeastern margin and with palaeoclimatic studies in western china which document conditions wetter and warmer than those of today during the early middle holocene the environmental fluctuations recorded in western tibet appear in phase with climatic changes recognized in tropical north africa suggesting that the 8 0 7 7 ka b p and the 4 0 3 0 ka b p dry events were caused by abrupt disequilibrium in the climatic system

coincident seismic wave velocity and reflectivity properties of the lower crust beneath the appalachian front west of newfoundland we have determined the seismic velocity structure of the crust in the vicinity of the appalachian deformation front off western newfoundland and the adjacent gulf of st lawrence these measurements

a deep seismic reflection profile across the northern north sea the viking graben is a major mesozoic rift basin in the northern north sea1 seismic refraction data2 gravity surveys3 and theoretical modelling4 have led to the general acceptance of an extensional tectonic origin for the viking graben involving an early mesozoic phase of extension and fault controlled subsidence followed by a more regional thermal contraction subsidence the early success of the birps deep seismic reflection profiling5 and the results from cocorp6 led to the planning and acquisition during 1983 of a deep seismic line recorded to 15 s twt two way time across the north viking graben the aims of the present study were to provide constraints on the geometry of the viking graben and its extensional faults to provide data on the geometry of crustal thinning beneath the graben and to provide an insight into the relation between the shallow extensional features of the sedimentary basin and the deep crustal thinning here we present a specific interpretation of this seismic profile further details will be given elsewhere7

biogeochemistry and paleoclimate variability during the holocene a record from mansar lake lesser himalaya investigation of biogeochemistry and amino acids on a 30 m long core from mansar lake has thrown light on palaeoclimate variability during the holocene period the c n ratio between 6 and 10 with some deviations and 13c mostly between 20 and 22 in the shallow core as well as a c n ratio greater than 13 reaching from 19 to 20 and correspondingly lower 13c of 28 in the deeper core suggest an aquatic source of carbon in the former and a cellulose rich land plant source in the latter this is supported by the abrupt increase in organic carbon content in the deeper core compared to the shallow core which indicates a hot and wet climatic regime during the early holocene and dry and cold during the late holocene period the amino acid data aa c c and aa n n are higher in shallow sediments compared to deeper sediments indicating an aquatic plant source in the shallow core and greater supply of land plant sources in the deeper core the lower percentage of the non protein amino acids alanine ala and amino butyric acid in the shallow core compared to the deeper core indicates different sources of organic matter in the lake basin the higher amino acid ratio asp ala 16 99 av and glu aba 18 18 av in the shallow core and lower ratios 10 32 and 12 41 av in the deeper core and asp glu 1 52 av and ala aba 1 61 av ratios in the former which are potential indicators of the nature of the organic matter are higher in the shallow core relative to the deeper core 1 33 and 1 23 av indicating relative biodegradation of organic matter in deeper sediments it has been observed that the organic matter associated with the dry season is relatively less biodegraded as evidenced from their higher ratios and is more biodegraded in the wet season as their ratios are lower in the river sediments in the absence of a bacterial contribution of organic matter from the soil source in this lake since mansar lake is a non drainage type it is envisaged that the climatic variation may be responsible for biodegradation in the deeper core sediments therefore the c n ratio and 13c values supported by amino acid data the latter being significant in revealing primary productivity and a terrestrial source of organic matter suggest a hot and wet climatic regime during the early holocene ca 7580 bp and a dry and cold in the late holocene period ca 4050 bp

lithoprobe onshore seismic reflection transects across the newfoundland appalachians vibroseis seismic reflection data have been recorded to 18 s two way traveltime along three transects across the island of newfoundland the upper crust has both steep and subhorizontal reflectors consistent with a ramp flat style of deformation whereas the middle and lower crust are largely free of regional flats reflectors descend through ca 20 km of vertical section in the middle and lower crust to flatten into the moho or perhaps cut through it in places the moho is interpreted to be no younger than the dipping reflectors reflection fabrics interpreted to be indicators of dominantly mid ordovician to mid silurian strain show consistent orientations among the transects and divide the crust into two blocks a northwestern block is characterized by upper and middle crustal reflectors dipping mostly southeast at variable angles this block is underlain to the southeast by supposedly younger and dominantly northwesterly dipping reflectors that define a northwest tapering wedge shaped block floored

a tectonic window in central newfoundland geological evidence that the appalachian dunnage zone may be allochthonous in the through hill area of central newfoundland mafic to ultramafic complexes which preserve varying amounts of ophiolite stratigraphy mark the trace of a major fault zone that outcrops in a roughly elliptical pattern the major northeast trending axis is about 60 km and the shorter axis is about 30 km in length the most complete ophiolite stratigraphy is preserved in the coy pond and pipestone pond complexes which have steep dips and face east and west respectively outwards from the centre of the ellipse the ophiolitic rocks are bounded on the outside by rocks typical of the dunnage zone of central newfoundland principally lower to middle ordovician volcanic and volcaniclastic rocks interpreted to be of island arc affinity davidsville victoria lake and baie d espoir groups cold spring pond formation and upper ordovician to silurian clastic sediments botwood group the volcano sedimentary sequences are interpreted to have been deposited on the ophiolite conformably but the contacts are n

lithoprobe east onshore offshore seismic refraction survey constraints on interpretation of reflection data in the newfoundland appalachians abstract combined onshore offshore seismic refraction wide angle reflection data have been acquired across newfoundland eastern canada to investigate the structural architecture of the northern appalachians particularly of distinct crustal zones recognized from earlier lithoprobe vertical incidence studies a western crustal unit correlated with the grenville province of the laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7 2 km s in central newfoundland beneath the central mobile belt the crust thins to 35 km or less and is marked by average continental velocities not exceeding 7 0 km s in the lower crust further east in a crustal unit underlying the avalon zone and associated with the gondwanan plate margin the crust is 40 km thick and has velocities of 6 8 km s in the lower crust explanations for the thin crust beneath the central mobile belt include 1 post orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin 2 mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination and 3 transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle except for a single profile in western newfoundland velocities in the crust are of typical continental affinity with lower crustal velocities less than 7 0 km s this indicates that there was no significant magmatic underplating under the newfoundland appalachians during mesozoic rifting of the atlantic ocean as proposed elsewhere for the new england appalachians a mid crustal velocity discontinuity observed in the newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles however we suggest that localized velocity heterogeneities at mid crustal depths correspond to organized vertical incidence reflections

influence of bottom water anoxia on nitrogen isotopic ratios and amino acid contributions of recent sediments from small eutrophic lonar lake central india lonar lake is a eutrophic saline soda lake with permanently anoxic deep water the high ph and deoxygenation result in very elevated d15n of suspended particulate matter spm and sediments due to denitrification and ph related loss of gaseous ammonium spm and sinking particles are predominantly aquatic in origin whereas surface sediments are of mixed terrestrial plant and planktonic source an indicator of degradation intensity was derived from a principal component analysis of the spectral distribution of amino acids and named lonar degradation index li a ratio of individual amino acids ox anox ratio was additionally used to determine the relative degree of aerobic vs anaerobic degradation these two biogeochemical indicators can be used to detect changes in degradation intensity and redox conditions in the geological history and thus the paleoclimatic interpretation of lonar sediments surface sediments can be divided into three zones 1 a nearshore oxic zone of predominantly aquatic organic matter in which oxidation leads to a strong diagenetic increase of d15n 2 an alluvial zone with a predominance of isotopically depleted land plant and soil organic

frequency dependence of crustalqβ in stable and tectonically active regions fundamental mode rayleigh wave attenuation data for stable and tectonically active regions of north america south america and india are inverted to obtain several frequency independent and frequency dependentq models because of trade offs between the effect of depth distribution and frequency dependence ofq on surface wave attenuation there are many diverse models which will satisfy the fundamental mode data higher mode data such as 1 hz lg can however constrain the range of possible models at least in the upper crust by using synthetic lg seismograms to compute expected lg attenuation coefficients for various models we obtained frequency dependentq models for three stable and three tectonically active regions after making assumptions concerning the nature of the variation ofq with frequency

the seismic velocity structure of the newfoundland appalachian orogen the deep structure of the newfoundland appalachian orogen is investigated by analyses of three intersecting seismic refraction wide angle reflection profiles which traverse the gander and dunnage zones or central mobile belt of newfoundland a simultaneous travel time inversion for velocity and interface was applied to the in line seismic refraction wide angle reflection data and constrained by synthetic amplitude models the results of the modeling procedure show a subhorizontally layered crust with upper crustal velocities ranging from 5 4 to 6 2 km s a midcrustal velocity of 6 25 6 35 km s and a lower crustal velocity of 6 7 0 2 km s the top of the lower crust is marked by a series of prominent reflections between 18 and 23 km depth which suggest a complex layered velocity interface strong laterally coherent moho reflections indicate a sharp crust mantle transition at 35 3 km the uppermost mantle has a velocity of 8 0 0 2 km s and a reflecting horizon at 55 km depth suggests an increase to velocities approaching 8 5 km s normal moveout corrections applied to fan profiles provide constraining evidence for the reflecting horizon at the top of the lower crust and laterally continuous moho reflections at 11 12 s two way travel time comparisons with a coincident deep seismic reflection profile show that the refraction and reflection mohos match to better than 2 3 km bulk poisson s ratios of 0 23 0 24 for the whole crust calculated from pmp sms travel times suggest a crust dominated by quartzofeldspathic lithologies and a notable absence of voluminous mafic additions to the lower crust the absence of a deep crustal root coupled with the bulk intermediate composition inferred for the lower crust from the seismic refraction wide angle data implies that the crust beneath central newfoundland has undergone multiple periods of reactivation and equilibration following successive orogenic episodes

crustal structure of the southeastern grenville province northern new york state and eastern ontario the grenville province exposes an oblique cross section through mid lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the grenvillian orogeny 1 1 ga the southeastern grenville province is divided into two subterranes by the carthage colton mylonite zone a 110 km long lineament characterized by intense ductile shear and igneous intrusion which separates the amphibolite facies metasediments of the central metasedimentary belt to the west from the granulite facies metaplutonics of the central granulite terrane to the east the recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the grenville province raises questions concerning the deep structure of these subterranes and in particular the means by which the mid lower crustal rocks exposed in the grenville province were emplaced seismic refraction wide angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern grenville province a travel time inversion for velocity and interface depth was applied to the seismic data together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern grenville province in the central metasedimentary belt the upper crust is characterized by velocities in the range 6 3 6 4 km s and a poisson s ratio of 0 26 0 01 which are attributed to quartzofeldspathic rocks farther east in the central granulite terrane upper crustal velocities of 6 55 km s and a poisson s ratio of 0 28 0 01 are associated with the marcy anorthosite the seismic homogeneity of the upper crust in the region of the carthage colton mylonite zone suggests that this boundary is a shallow feature limited to the upper 2 3 km of the crust the deep crustal structure of the southeastern grenville province is characterized by two discrete and laterally discontinuous seismic interfaces in the central metasedimentary belt the top of the lower crust is delineated by an eastward dipping interface at 24 28 km depth in the central granulite terrane prominent en echelon reflections referred to as the tahawus complex form a gently arched dome at 17 22 km depth interpretation of the tahawus complex as a zone of layered mafic cumulates is supported by its high velocity 7 1 km s and poisson s ratio 0 27 0 02 the lower crust is characterized by a velocity of 7 0 7 2 km s and an anomalously high poisson s ratio of 0 30 0 02 which are representative of pyroxene garnet granulites in contrast velocities of 6 8 7 0 km s are modeled beneath the central granulite terrane and appear to signify a lateral change in composition the moho lies at 44 45 km depth and is characterized by pronounced en echelon reflection segments suggesting compositional interlayering around the crust mantle boundary the velocity of the upper mantle is 8 0 8 2 km s an anomalous upper mantle layer with a reversed velocity of 8 6 km s dips eastward from 50 to 60 km depth beneath the southeastern grenville province our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the grenvillian orogeny are preserved in the mid lower crust as a layered cumulate body tahawus complex and in the upper mantle as an eclogitic lens possibly delaminated from the overthickened crust during uplift of the southeastern grenville province

frequency dependence of qlg and its relation to crustal anelasticity in the basin and range province the high observed frequency dependence of q for 1 hz lg waves f0 4 0 6 in the basin and range province can be explained by a frequency independent layered model of intrinsic q for shear waves q which has very low values in the upper crust and rapidly increasing values at greater depths by contrast earlier work in the eastern united states indicated q values in the upper crust which are high and which vary with frequency the q model which best explains reported values of qlg and its frequency dependence in the basin and range consists of an 8 km thick low q layer 60 or less overlying q values as high as 1000 or more at mid crustal depths this q distribution is best explained by an upper crust which contains fluids in interconnected cracks overlying a lower crust where higher lithostatic pressure has closed those cracks these results indicate that whereas the frequency dependence of q is an intrinsic property of crustal material the frequency dependence of qlg arises from both the intrinsic frequency dependence of crustal material and its layered structure a notation is suggested which distinguishes between the frequency dependence of qlg and q

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

environmental evolution and hydrodynamic process of dongping lake in shandong province china over the past 150 years sporopollen when recorded in the sedimentary record is a characterization factor of vegetation over geological time of an area the grain size characteristics are direct reflection of the hydrodynamic conditions based on analyses of high time resolution sporopollen and grain size of a 55 cm long sediment core of dongping lake it is shown that the sedimentary environment and hydrodynamic processes of dongping lake have seen significant changes over the past 150 years from 1855 to 1865 the lake is in a hydrostatic depositional environment where submerged plants were in great abundance emergent and floating leaved aquatic plants were comparatively fewer and reworked sporopollen fossils were not observed these indicate a weak and stable hydrodynamic condition during this stage however fluctuation of mz amounts of clay and silt were present which may be due to high volumes of sediments around dongping lake basin transported into dongping lake via the yellow river during the early lake forming stages from 1865 to 1922 the lake is in a backswamp depositional environment where aquatic plants are nearly nonexistent appearing occasionally in some layers with relatively low abundance fern spores show a twofold increase in occurrence and reworked sporopollen fossils are frequently identified all of these indicate a strong hydrodynamic condition and frequent environmental change grain size characteristics also support this interpretation from 1922 to 1961 aquatic plants appeared at the upper layers of this zone at relatively low amounts and reworked sporopollen fossils were frequently identified grain size characteristics show an obvious change in grain size distribution indicating a strong hydrodynamic condition and unstable depositional environment these relationships were attributed to a backswamp depositional environment from 1961 to 2007 dongping lake is in a hydrostatic depositional environment where the hydrodynamic condition is weak and stable evidence for this is shown by a remarkable increase in submerged plants a rapid decrease in fern spores and occasional appearances of reworked sporopollen fossils in the lower part of the zone however grain size characteristics reveal that fluctuation of mz amounts of clay silt and sand are noticeable and frequent which may be mainly related to impacts of ever enhancing human activities in recent years

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

deep seismic reflection profile across the northern appalachians a marine seismic reflection line across the northeast extremity of the canadian appalachians in newfoundland indicates a collisional suture in the lower crust beneath the central ophiolitic dunnage terrane the thrust and fold belt miogeocline above the grenville basement and the dunnage and gander tectonostratigraphic terranes all appear to be allochthonous with respect to lower crustal basement the gander avalon terrane boundary to the east is a near vertical feature that penetrates the crust the data also suggest that the ancient passive margin of north america extends eastward under the dunnage terrane for about 70 km the newfoundland deep seismic experiment indicates major tectonic differences compared to results for the southern appalachians

diagenetic changes of lignin compounds in a more than 0 6 million year old lacustrine sediment lake biwa japan a vertical profile of lignin in the upper 700 m layer of a 1400 m sediment core of lake biwa an oligotrophic freshwater lake in japan was determined using a cuo oxidative degradation method the results indicated that lignin is found throughout the core demonstrating lignin to be very stable for over 0 6 million years moreover the upper 250 m approximately 0 6 million years old segment of the sediment core was investigated to determine the apparent long term degradation rate of lignin a downward lignin concentration decrease is observed over the upper 250 m of the core which corresponds to a calculated half life of at least approximately 40 104 years assuming that lignin decrease is due to its in situ degradation diagenesis

absorption band q model for the earth body wave surface wave and normal mode data are used to place constraints on the frequency dependence of q in the mantle with a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range the quality factor qs is proportional to in the band and to and 1 at higher and lower frequencies respectively as appropriate for a relaxation mechanism with a spectrum of relaxation times the parameters of the band are q min 80 0 15 and width 5 decades the center of the band varies from 101 seconds in the upper mantle to 1 6 103 seconds in the lower mantle the shift of the band with depth is consistent with the expected effects of temperature pressure and stress high qs regions of the mantle are attributed to a shift of the absorption band to longer periods to satisfy the gravest fundamental spheroidal modes and the scs data the absorption band must shift back into the short period seismic band at the base of the mantle this may be due to a high temperature gradient or high shear stresses a preliminary attempt is also made to specify bulk dissipation in the mantle and core specific features of the absorption band model are low q in the body wave band at both the top and the base of the mantle low q for long period body waves in the outer core an inner core qs that increases with period and low qp qs at short periods in the middle mantle the short period qs increases rapidly at 400 km and is relatively constant from this depth to 2400 km the deformational q of the earth at a period of 14 months is predicted to be 463

sharp decrease in summer monsoon strength 4000 3500 cal yr b p in the central higher himalaya of india based on pollen evidence from alpine peat age constrained pollen data and magnetic susceptibility of an alpine peat profile from the garhwal higher himalaya display a continuous record of climate and monsoon trends for the past 7800 yr about 7800 cal yr b p dominance of evergreen oak quercus semecarpifolia alder alnus and grasses in the pollen record reflect a cold wet climate with moderate monsoon precipitation from 7800 to 5000 cal yr b p vegetation was progressively dominated by conifers indicating ameliorated climate with a stronger monsoon a warm humid climate with highest monsoon intensity from 6000 4500 cal yr b p represents the mid holocene climatic optimum between 4000 and 3500 cal yr b p the abundance of conifers sharply decreased with the greatest increase in evergreen oak this trend suggests progressive cooling with a decrease in the monsoon to its minimum about 3500 cal yr b p two relatively minor cold dry events at ca 3000 and 2000 cal yr b p marked step wise strengthening of the monsoon until ca 1000 cal yr b p after a cold dry episode that culminated ca 800 cal yr b p the monsoon again strengthened and continued until today a sharp decrease in temperature and rainfall at 4000 3500 cal yr b p represents the weakest monsoon event of the holocene record this cold dry event correlates with proxy data from other localities of the indian subcontinent arabian sea and western tibet

vegetation climate and sea level in the past 55 000 years hanjiang delta southeastern china abstract pollen in quaternary deposits from the subtropical hanjiang delta records three major phases in the local vegetation and climate history during the last 55 000 yr 1 a prevalent cool to temperate and humid climate at ca 24 000 14c yr b p is indicated by abundant pollen of temperate trees including conifers 2 between 20 000 and 15 000 14c yr b p a cold dry environment was associated with low sea level during the last glaciation leading to subaerial exposure weathering and interruption of sedimentation as well as departure from the region of dacrydium and sonneratia 3 a short term expansion of grassland at ca 10 300 14c yr b p reduced the predominant lauraceae fagaceae evergreen forest possibly corresponding to the younger dryas cooling the combined data indicate a maximum sea level rise in the mid holocene 7500 4000 14c yr b p and a marine influence in the late pleistocene at 45 000 20 000 14c yr b p the holocene warming however did not bring back moisture sensitive taxa indicating high seasonal aridity probably caused by renewed monsoon conditions

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

a synthesis of the geophysical characteristics of terranes in eastern canada abstract geological identification of terranes in the canadian appalachians has led in general to the proposing of terranes which have recognizably distinct geophysical signatures indicative of fundamental differences in crustal structure between the terranes the rapidly expanding geophysical data base has enabled an integrated approach to geophysical terrane analysis the geophysical data enable the humber with its diagnostic gravity low and variable magnetic anomalies to be recognized throughout the canadian appalachians deep seismic data suggest that it is found beneath the western part of the dunnage terrane the latter is typified by dominantly positive gravity anomalies in newfoundland these together with the magnetic data enable the zone to be subdivided into the notre dame and exploits subzones the seismic character clearly identifies the presence of the dunnage northeast of newfoundland and suggests that it may terminate south of newfoundland the gander terrane has typically negative gravity and magnetic anomalies which may be confidently traced from northeast of newfoundland south through cape breton a belt of similar anomalies occurs in new brunswick the deep seismic evidence suggests that the gander may represent the outcrop of a deep crustal block underlying the eastern part of the dunnage terrane the avalon terrane is identified throughout the canadian appalachians by a diagnostic set of arcuate alternating gravity and magnetic anomalies the boundary between the avalon and gander is clearly recognized on deep seismic sections as a near vertical feature cutting through the crust northeast of newfoundland the meguma avalon boundary is clearly delineated by a gravity and magnetic gradient throughout the system there is limited and ambiguous deep seismic data regarding the nature of it and the meguma terrane in general a subtle set of nw trending lineations perpendicular to the dominant regional trend has recently been identified geological evidence that suggests these are post carboniferous in age and may be related to the development of a tensional regime associated with the mesozoic rifting of the atlantic

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

a statistical model for ground motion produced by earthquakes at local and regional distances to adapt random process theory techniques for statistical estimation of peak ground motion to more realistic earth models we constrain the parameters of duration geometrical spreading and spectral shape by modeling the main ground motion as being the result of major contributions by the direct s wave and supercritically reflected s waves the results of our modeling are constrained to be consistent with values from full wave synthetics for the test models the combination of estimation theory and theoretical amplitude spectrum of the main ground motion within the ergodic window successfully predicts the mean peak vertical ground displacements velocities and accelerations of the 1982 miramichi earthquakes in new brunswick canada in addition upon considering the effects of source depth and crustal structure for the november 25 1988 saguenay earthquake m 5 8 in quebec canada the predicted mean peak horizontal ground accelerations match the observed data very well the effects of source depth and crustal structure on the peak ground motion are complicated for different source sizes and at different epicentral distance ranges

physical properties and structure of the lower crust revealed by one and two dimensional modelling summary data from a refraction and a reflection seismic survey in the black forest southwest germany are used for extensive one and two dimensional modelling the data are available along approximately the same line and therefore the same piece of crust is probed by two seismic methods we utilize this favorable circumstance for detailed model calculations concerning both data sets lower crustal properties vary on the scale of a wavelength and thus full solutions of the elastic equations are required the reflectivity method for the evaluation of refraction seismograms and numerical solutions of the acoustic wave equation for the reflection response details of the geometry and physical properties of the lamination are derived vertical layering on a scale of 100 m is found horizontal extent of reflecting elements is in the range of a few hundreds of meters rocks with velocities between 5 6 and 7 2 kds constitute the lower crust