marine geology of sohm basin canadian atlantic margin multichannel seismic reflection seismic refraction and magnetic anomaly data are used to interpret the mesozoic sea floor spreading history and subsequent sedimentary evolution of the sohm basin southeast of nova scotia observed m series mesozoic magnetic anomalies show that oceanic crust in the sohm basin was generated at very slow rates of sea floor spreading 5 15 mm yr half rates the slow spreading created very rough crustal topography but the crustal thickness and seismic velocity structure are typical of normal oceanic basement basement depths corrected for sediment loading fall below the predicted age depth curve in the anomaly m25 m4 interval these depth anomalies and an associated regional negative geoid anomaly suggest that the sohm basin is underlai by asthenosphere denser and possibly colder than normal correlation of seismic reflection profiles in the basin to regional seismic stratigraphy together with age constraints from reflection pinch outs on dated oceanic crust provides a chronology for interpreting the seismic sequences the jurassic to lower cretaceous sequences are similar in thickness and acoustic character to sequences observed along the united states atlantic margin suggesting that the new england seamount chain had little effect on sediment distribution patterns within the early western north atlantic basin upper cretaceous sediments in the sohm basin are thin probably because seaward transport of sediment was restricted by eustatic sea level rise and by a sedimentary ridge uplifted because of salt tectonics along the central and upper continental rise progradati nal sedimentary wedges and erosional unconformities characterize the tertiary and quaternary sedimentary record along the lower continental rise the prograding sequences generally correlate to sea level lowstands repeated episodes of erosion by abyssal currents are marked by seismic unconformities horizon au lower oligocene lm lower miocene mm middle miocene and l upper pliocene the deep currents also controlled local patterns of deposition and modified cross slope sediment transport and accumulation patterns thick supra horizon l sediments cover the sohm basin and reflect rapid sediment influx caused by glacio eustatic fluctuations in sea level during the quaternary

magnetochronologie des formations rocheuses du mont megantic et des environs quebec meridional a paleomagnetic study was made to elucidate the ambiguities of the radiometric ages of mont megantic intrusions in relation to field observations and to determine the magnetochronology of the intruded rocks some 179 samples 550 specimens were collected over 58 sites and their magnetization was cleaned by thermal and or alternating field treatment the paleopoles obtained for the compton formation metasediments indicate an early to middle devonian age and for the overlying frontenac formation metavolcanics indicate an age definitely different from that for the intrusive rocks the baked contact test on the hornfels forming the contact metamorphic aureole is positive and the magnetization component was acquired in the early to late cretaceous interval syenite contains two components one with normal polarity the other with reversed polarity their ages are juro cretaceous the gabbro contains only one magnetization component reversed which was acquired in the early cretaceous whereas the granit

the adjustment of mantle plumes to changes in plate motion the relative motion of hotspots and lithospheric plates implies a velocity shear in the underlying mantle causing horizontal advection of mantle plumes as they rise toward the lithosphere consequent tilting of plumes parallel to the direction of plate motion indicates that plumes must undergo a period of readjustment after the velocity vector for plate motion is altered thus the shape of bends in the surface tracks of hotspots resulting from changes in plate motion will reflect the plume adjustment laboratory experiments as well as computations using a simple theory developed in richards griffiths 1988 for the dynamics of continuous plume conduits demonstrate that the bend in the surface track has a radius of curvature approximately equal to the maximum horizontal deflection of the conduit thus the sharpness of the bend at an age of 43ma in the hawaiian emperor volcanic chain implies that the deflection of the underlying plume in that case was small 200 km this small deflection is expected for plumes carrying large buoyancy fluxes and it indicates that tilting of the conduit is unlikely to be sufficient to cause diapiric instability

emplacement of the monteregian hills of quebec geophysical evidence abstract constraints are placed on the shape of intrusions of the monteregian hills and a model of their emplacement is proposed integrated geophysical data magnetic gravimetric and seismic reflection and drainage pattern information indicate that the monteregian hills are shallow intrusives having the form of saucers nourished by thin planar feeders these feeders are apparently tectonically controlled by steeply dipping normal faults in the crystalline basement and intermediate to low angle thrust faults in the overlying lower paleozoic sequence field geological evidence geochemical and geophysical data indicate a slow ascending motion of the cooling magma and a sub vertical rather than sub horizontal injection mode in view of the relatively cold state of the intruded magma and paleomagnetic data partial and progressive melting of an olivine basalt magma is a more acceptable origin for the monteregian hills than a continental hot spot theory

subduction initiation at passive margins the scotian basin eastern canada as a potential example thermal evolution modeling based on the geometry of the scotian basin eastern canada and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins previous studies have demonstrated the difficulty of subduction initiation at mature passive margins however unique features along a passive margin may facilitate subduction initiation subduction may initiate along a part of the margin 1 by extensional or strike slip deformation rather than shortening 2 by deformation of the weaker continental lithosphere rather than oceanic lithosphere 3 by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere and 4 by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin the values of shear resistance under these conditions are still higher than those calculated for ridge push indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms however pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins

geochemistry of sediments from barkol lake in the westerly influenced northeast xinjiang implications for catchment weathering intensity during the holocene abstract though considerable studies concerning past climate changes in the xinjiang northwest china have been performed holocene climate changes of this area are still poorly documented and understood in this study we emphasize the potential of geochemical elements in sediments for indicating the chemical weathering intensity cwi of bakol lake catchment during the holocene the results indicate that the cwi represented by ratios of ca mg na k al ca mg ca al mg al si al mn al and ca al has the potential to reflect past climatic conditions three weakened cwi periods i e from 9400 7600 cal a bp from 6000 5000 cal a bp and since 2000 cal a bp afterwards and two strengthened cwi periods i e from 7600 6000 cal a bp and from 5000 4000 cal a bp are revealed in combination of multi proxy records including carbonate 18o and 13c total carbonate content as well as total organic carbon we find the thermal conditions possibly played an important role in the cwi of lake catchment with a warmer climate favoring stronger cwi and vice versa in contrast the moisture conditions appear less influence on the cwi this new result is expected to be helpful for strengthening our understanding of the spatial and temporal patterns of holocene climate changes in the westerly influenced region

reconstruction of lake balkhash levels and precipitation evaporation changes during the last 2000 years from fossil diatom assemblages environmental changes in arid central asia are prominently manifest through changes in the water balance lake balkhash the largest lake in kazakhstan is a shallow lake situated in a closed basin we obtained a 600 cm long core from the western part of the lake in 2007 and carried out analyses of fossil diatoms and ostracods geochemistry and grain size of the sediments the age control of the core is based on radiocarbon ages of fossil ostracods and in the uppermost part the ad 1963 137cs activity peak and the first appearance of 137cs in ad 1952 we recognized two main diatom assemblages by detrended correspondence analysis one characterized by the dominance of freshwater planktonic species and the other dominated by brackish planktonic brackish and marine benthic and freshwater benthic species the first assemblage suggests higher lake levels and the second suggests lower lake levels the observed assemblage changes are consistent with lake level observations recorded during the last 120 years the diatom assemblage data from the core indicate that there were seven periods when the lake level was low during the last 2000 years moreover the ph of the lake changed together with the lake level in addition during periods when the lake level was low from ad 1260 2000 the abundance of fossil ostracods and the ca and toc contents were low in comparison with the periods before ad 1260 the causes of low water levels were essentially the same between lake balkhash and the aral sea during the studied period one likely cause of the low water levels especially those during the last 1200 years is changes in solar activity furthermore some environmental changes during the past 40 years can be explained by human activities

seasat altimetry the north atlantic geoid and evaluation by shipborne subsatellite profiles a geological evaluation of geoid anomalies over the north atlantic is presented with emphasis on short 40 300 km wavelength undulations mapped by seasat radar altimetry long 3000 km and intermediate 300 3000 km wavelength undulations constrain subcrustal processes and are briefly reviewed portions of three seasat revolutions revs totaling 9300 km were followed by research ship which measured gravity magnetics topography and for a 800 km segment also seismic reflection data to form a ground truth data set for comparison to the altimetry the intermediate wavelength geoid the residual upon removal of a 14 14 long wavelength surface correlates with plate age slope 10 cm m y over cenozoic crust and major fracture zones appear as geoid steps a 3 5 to 5 m high over the bermuda rise may reflect thermal rejuvenation created within the plate 40 50 ma short wavelength geoid anomalies typically 10 50 cm in the open ocean were analyzed by high pass 200 or 400 km cutoff along track filtering and smoothing the vertical deflection computed from the seasat geoid compares well with one based on seismic reflection shipborne free air gravity except for a higher frequency cutoff correlates closely with geoid anomalies except near seamounts the principal contribution to geoid anomalies is fracture zone topography all major and many minor fracture zones fz s appear as geoid lineations particularly between the 15 20 and kane fz from the ridge axis to anomaly 34 the wsw trend of fz s between anomaly 21 and 34 is clearly expressed in the altimetry as is the smooth geoid over mid cretaceous smooth basement formed by fast spreading crustal ribbons 50 150 km wide of higher or lower geoid parallel fz trends some geoid lineations are oblique to both isochron and fz trends east of the puerto rico trench such anomalies may reflect compressional deformation within atlantic lithosphere

minimum continental velocities with respect to the pole since the archean abstract paleomagnetic apparent polar wander paths provide a method for estimating minimum plate velocities which can be extended much further into the geologic past than can be estimated based on sea floor magnetic anomalies minimum velocities can be determined from the rate of change of latitude as derived relative to a fixed pole such latitudinal velocities have been determined for the center of mass of the north american baltic siberian and african shields since the archean the results indicate that plate velocities in the past have at times easily exceeded those for present day continents and that they often were equivalent to present day oceanic velocities although there are peaks and troughs through time north american velocities are significantly greater prior to one billion years ago than those of siberia or baltica

the mont saint hilaire plutonic complex occurrence of excess 40ar and short intrusion history mont saint hilaire is an approximately 10 km2 alkaline plutonic complex in the monteregian petrographic province of quebec the complex consists of an older western half of alkali gabbros and an eastern portion of nepheline syenites and magmatic breccias the intrusives were emplaced at depths of no more than a few kilometres into paleozoic sedimentary rocks through the underlying grenville basement 40ar 39ar analyses of amphibole yield complex age spectra in which the apparent ages decrease with increasing heating temperature conventional k ar analyses of amphibole pyroxene and feldspar yield anomalously old dates these data indicate the presence of small yet significant amounts up to 8 10 11 mol g of excess 40ar the quantities of excess argon are variable between rocks and minerals from the same rock in contrast biotites yield concordant plateaus and consistent ages ranging only from 124 1 to 124 6 ma these data define the age of the complex 124 4 1 2 ma and imply a short time span of

anomalous mantle structure at the proterozoic paleozoic boundary in northeastern us using teleseismic p wave tomography we detect very large 5 variation in the compressional velocity of the upper mantle beneath the proterozoic paleozoic boundary in the northeastern us in this region the upper mantle is heterogeneous on the 100 km scale to a depth of 300 km below this depth the mantle appears to be laterally homogenous at least on the scale of 200 250 km given the tectonic history of the region the large amplitude short wavelength heterogeneities can not be caused by temperature variations but may be due to chemical heterogeneity or variations in the amount and orientation of flow induced seismic anisotropy

time dependence of mantle plumes some simple theory dynamical calculations indicate that plate tectonics are likely to have a major effect on the initiation of plumes from the basal boundary layer in the mantle and their subsequent evolution plume start because the boundary layer is hotter than the overlying mantle and hence unstable to perturbations the growth rate of perturbations is too weakly dependent on wavelength to control the geographic distribution of plumes instead the distribution of plumes is strongly influenced by the shape of large perturbations associated with slabs and surface plates plumes evolve because downward return flow is needed to replace the material tapped from the boundary layer this flow downwarps the top of the boundary layer several hundred kilometers from the plume and thus restricts further flow of hot material toward an active plume the geometry of this downwarp is strongly influenced by the initial relief of the top of the boundary layer and hence by plate processes the actual history of plates and slabs is thus needed in numerical models for comparison with real data on plume fluxes

residual geoid anomalies of the north atlantic ocean and their tectonic implications residual geoid undulations of the north atlantic ocean are obtained and interpreted in terms of upper mantle processes starting from the 1 1 averaged seasat altimeter derived geoid the effects of long wavelength features were first removed by subtracting the degree and order 10 geoid of gem 10c standard earth model the empirical geoid age relationship of the north atlantic after removing the long wavelength features shows closer affinity to the geoid age relationship of the plate model than to that of the half space model for the older part greater than 100 m y of the ocean floor the geoid effects associated with the thermal evolution of the oceanic lithosphere mask the geoid signal associated with deeper processes we thus removed these effects and compared the resulting geoid with residual depth in order to investigate upper mantle processes a principal conclusion here is that the axis of the azores geoid high associated with the azores triple junction shifts east to azores islands after we account for the thermal effects this may indicate that the main axis of smaller scale upper 600 km convective upwellings does not lie beneath the present day spreading center but beneath the azores islands further areas of extensive volcanism such as the bermuda rise and cape verde rise in the older part of the ocean floor ages greater than 100 m y exhibit relative residual geoid highs which correlate very well with the residual depth anomalies

correlation of the late pleistocene paleogeographical events of the caspian sea and russian plain abstract the present work shows the author s concept of paleogeographic development of the caspian sea in the late pleistocene based on the comprehensive analysis of the caspian deposits and their relation and synchronization with glacial interglacial cycles on the russian plain based on published materials global climate changes were fundamental for both the transgressive regressive state of the caspian basins as well as for glacier formation and retreat on the russian plain at the same time glaciers influenced the caspian basin development on a regional level the long term low caspian sea level complicated by the double stage minor late khazar transgression corresponded to the mikulino eemian interglacial with two endothermals this warm stage in the caspian history encompassed the interval corresponding to marine isotope stage mis 5 this is in agreement with the point of view of researchers who correlate the mikulino interglacial with the entire mis 5 the cold vast khvalynian transgression and preceding to it the atel regression developed during the valday glaciation mis 4 2 the atel regression corresponds to cold mis 2 and the beginning of mis 3 its lower boundary in the lower volga region is marked with ice wedges a transgressive tendency began to show in the second half of the interstadial second half of mis 3 and continued into early stages of the late valdai cooling but it was interrupted during the dry and cold glacial maximum peak of mis 2 by the regressive stage further transgression development occurred during the degradation of the glaciation its course was impacted by transgressive regressive events of lower magnitude linked to oscillations of climatic parameters i e cold dry conditions of the middle dryas by the enotaevka regression and the continentalization of the boreal period of the holocene by the mangyshlak regression

the effect of hot spots on the oceanic age depth relation hot spots located both midplate and on midocean ridges can bias the empirical age depth relation ocean floor bathymetry for seafloor greater than 80 m y b p in age has been previously used to determine if the oceanic lithosphere behaves as a cooling half space or as a cooling plate however large areas of the ocean basins do not fit a simple age depth relation anomalously shallow swells approximately 1000 km wide and 1 km high surround hot spots such as iceland the azores and the cape verde islands by sorting seafloor depth data by distance to the nearest hot spot track we show that the along isochron variation in depth in the north atlantic correlates with distance from a hot spot track due to the absence in the north atlantic of seafloor both old and far from hot spots we compare depth versus age at constant distance and do not observe flattening of the age depth relation in any distance range the age depth relation depth meters 2700 295 t m y 1 2 derived for seafloor 1200 to 1800 km from a hot spot track represents our best estimate of the age depth relation appropriate to seafloor unaffected by a hot spot track in the north atlantic the previously inferred flattening of the age depth relation appears to be caused by the inclusion of hot spot swells this implies that hot spots may be the primary means of heat transfer from the deeper mantle to the base of the lithosphere

accuracy of post bomb 137cs and 14c in dating fluvial deposits abstract the accuracy and precision of 137cs and 14c for dating post 1950 alluvial deposits were evaluated for deposits from known floods on two rivers in arizona the presence of 137cs reliably indicates that deposition occurred after intensive above ground nuclear testing was initiated around 1950 there was a positive correlation between the measured level of 137cs activity and the clay content of the sediments although 137cs was detected even in sandy flood sediments with low clay content 137cs is a valuable dating tool in arid environments where organic materials for 14c or tree ring dating are scarce and observational records are limited the 14c activity measured in different types of fine organic detritus yielded dates within 1 to 8 yr of a 1980 flood deposit and the accuracy was species dependent however undifferentiated mixtures of fine organic materials from several post bomb deposits of various ages repeatedly yielded dates between 1958 and 1962 and detrital charcoal yielded a date range of 1676 1939 in semiarid environments the residence time of most types of organic debris precludes accurate annual resolution of post bomb 14c dates

shear wave structure in the grenville province beneath the lower great lakes region from rayleigh wave tomography 1 we have constructed a high resolution 3 d shear wave velocity model of the upper mantle beneath the grenville province in the lower great lakes region a two plane wave tomography method was applied to fundamental mode rayleigh wave data recorded at 35 broadband seismic stations rayleigh wave phase velocities were obtained at 15 periods from 20 s to 143 s and then inverted for shear wave velocity variation a high velocity lid to 150 km depth is imaged beneath most of the studied grenville province one low velocity anomaly of 4 appears at a failed rift in the northeast above 100 km depth coinciding with the great meteor hot spot track and the high seismicity band which can be interpreted as high temperature and high iron and or volatile content related to the hot spot activity low velocity anomaly of 2 at the depths of 100 200 km are found beneath the east of lake ontario and lake huron where the lithosphere is relatively thin these anomalies follow the nw se trend of the slow indentation imaged on the continental scale models we conclude that the location of low velocity anomalies is not only related to the hot spot track but also controlled by the pre existing lithosphere morphology and strength a high velocity column appears between southern lake ontario and the appalachian front centered at 100 km depth it is likely caused by high mg content at the formation of the lithosphere or the presence of ecologite or garnet phases generated during the opening and closure of the iapetus ocean

tree ring recorded hydroclimatic change in tienshan mountains during the past 500 years abstract in central asia tree rings provide one of the best sources of paleoclimatological information however dendroclimatology has not been widely applied in the coniferous forests of central asia tree cores of picea schrenkiana from four sites in the hutubi river basin were developed into a 606 year tree ring width chronology the analyses showed that the tree ring width indices highly correlate with mean april may pdsi in the hutubi river basin mean april may pdsi of the hutubi river was reconstructed using the tree ring data with 41 5 of the variance explained the reconstructed series contains both high and low frequency climate signals our new pdsi reconstruction agrees reasonably well with the dry and wet periods previously estimated from tree rings in western tien shan the results reveal common climatic extremes over much of central asia spatial analysis shows that the pdsi reconstructions have strong common signals for the tien shan the first principal component of pdsi reconstructions in tien shan is significantly correlated with sea surface temperature in the eastern equatorial pacific ocean and atlantic ocean the linkages to the atlantic and pacific oceans suggest the connection of regional moisture variations to the asian monsoon and the westerlies

root mean square velocities of the continents with respect to the hot spots since the early jurassic the root mean square velocities of the major continents with respect to the hot spots were estimated for the past 180 m y by combining previously published plate reconstructions these velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data from 169 m y b p to l44 m y b p the hot spot data indicate that laurasia moved rapidly 70 mm yr but the reconstructions permit a velocity as low as 50 mm yr we also estimate that from 63 m y b p to 48 m y b p india moved rapidly 140 20 30 mm yr faster than the present velocity of any plate and more than twice as fast as the present velocities of several subducting oceanic plates those attached to downgoing slabs along a substantial fraction of their margin the data require no other continent to have exceeded a velocity of 45 mm yr during the last 180 m y in agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no nettorque analyses we conclude that the continents have moved faster occasionally much faster than at present from this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere instead oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents on the basis of the very rapid early tertiary motion of india we conclude that the velocities of subducting plates can vary by a factor of 2 therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid

mantle fabric at multiple scales across an archean proterozoic boundary grenville front canada abstract in eastern ontario and southwest quebec canada the proterozoic grenville province abuts against the archean superior province the complex tectonic history of the region is reflected in the pattern of electrical and seismic anisotropy within the lithosphere while asthenospheric anisotropy is expected to reflect current patterns of mantle flow magnetotelluric and teleseismic data from the polaris and fednor experiments and the lithoprobe abitibi grenville transect are examined for sks splitting and geoelectric strike and receiver functions are generated at selected stations in order to characterise both vertical and horizontal variations in anisotropy in the eastern ontario upper mantle the average shear wave split direction coincides with the direction of plate motion split times are found to be strongest in the southern part of the grenville province where asthenospheric flow is enhanced by the presence of a lithospheric divot the ottawa bonnechere graben and the immediate vicinity of the grenville front are regions of altered split direction indicative of a lithospheric component of anisotropy north of the grenville front there is a gradual reorientation of the split direction from ese to ene which is not easily attributable to crustal tectonics and may represent the northern limit of lithospheric deformation produced by the grenville orogen electrical anisotropy is pervasive in the study area the pattern of magnetotelluric strikes is more complex than the sks pattern though after correction for local distortion the geoelectric strikes correlate fairly well with sks measurements at nearby stations obliquity between sks and magnetotelluric results shows no consistent orientation across the study area receiver function analysis at three selected stations is indicative of a subcrustal anisotropic layer with a consistent sse fast direction underlain at station gac by a sequence of anisotropic layers with varying directions this sequence is not observed at station sado combining these results we interpret the strength and direction of anisotropic fabric in the grenville to vary strongly with depth the upper part of the lithosphere contains thin anisotropic layers perhaps related to eclogitization and relict slabs but which are insufficient to explain the observed sks splits the lower lithosphere is likely to be more ductile and uniformly anisotropic and may be an important control on magnetotelluric strike directions as well as a significant contributor to sks splitting the largest contributor to sks splitting in this region is interpreted to be asthenospheric anisotropy related to absolute plate motion

crustal and lithospheric scale structures of the precambrian superior grenville margin abstract the lithosphere beneath the margin of the archean superior and proterozoic grenville provinces was investigated with a northwest southeast oriented 650 km long profile of 40 magnetotelluric stations dominant geoelectric strike azimuths of n45 e and n85 e were defined for the crust and the lithospheric mantle respectively a 2 d isotropic resistivity model derived using the crustal strike images resistive laurentian margin rocks dipping southeast to the base of the crust bounded to the northwest by the grenville front and to the southeast by the central metasedimentary belt boundary zone the observation is in contrast to conductive mid to lower crust elsewhere in the region a 2 d isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern grenville province the geometry of this body which includes a well defined base and southeast dip suggests that it is superior lithosphere a sub vertical conductor located approximately 50 km along strike from the mesozoic kirkland lake and cobalt kimberlite fields is interpreted to be due to re fertilization of an older mantle scar the resistivity model includes a horizontal conductor at 160 km depth beneath the southern superior province that is possibly the resistivity signature of the lithospheric asthenospheric boundary

137cs dating of lacustrine sediments and human impacts on dalian lake qinghai province china cs 137 and grain size analyses indicate the total amounts of cs 137 deposited by wind and water in the dalian lake of gonghe basin qinghai province the vertical profile of cs 137 in the lacustrine sediments shows three cs 137 activity peaks and one trough the main peak corresponds to the global cs 137 fallout in 1963 the two secondary peaks correspond to the leakage of the chernobyl reactor in 1986 and complete desiccation of the lake in 1994 the trough corresponds to aeolian deposition caused by extensive reclamation of farmland in late 1980s and early 1990s the cs 137 dating was used to evaluate sedimentation rates in dalian lake since 1963 they are divided into three periods two with slow deposition 1963 1986 and 1994 1998 and one with rapid deposition 19861994 cs 137 seems to have the same potential for indicating environmental changes and human interference in the and and semiarid regions as in humid regions c 2002 elsevier science b v all rights reserved

teleseismic studies of the lithosphere below the abitibi grenville lithoprobe transect in the past decade the abitibi grenville lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern superior and adjoin ing grenville provinces among the different geophysical methods that have been employed earthquake seismology provides the widest range of information on the deep structures of the upper mantle this paper presents a review of studies both complete and ongoing involving teleseismic datasets that were collected in 1994 and 1996 along the transect a complete shear wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies results suggest a correlation between the two anisotropies supported by xenolith data and favour a lithospheric origin for the seismic anisotropy the two anisotropies are believed to represent the fossilized remnants of archean strain fields in the lithospheric roots of the canadian shield preliminary splitting results for the 1996 experiment suggest that the s wave azimuthal anisotropy may be depth dependent and laterally varying ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area preliminary receiver function results reveal the presence of an s velocity increase at 90 100 km depth which appears to be laterally continuous over 200 km traveltime inversion models indicate the presence of an elongate low velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface e g grenville front preliminary interpretation relates this anomaly to the same process e g fixed mantle plume continental rifting responsible for the emplacement of the monteregian hills igneous province

lithospheric heating by mantle plumes summary a broad topographic swell is associated with the hawaiian hotspot a kinematic model is developed assuming that downstream of the hotspot this swell is produced by lithospheric heating by a mantle plume material from the model plume flows radially in the asthenosphere and eventually heats the lower lithosphere upstream from the hotspot the model swell owes its existence mainly to hot material at asthenospheric depths the model predicts extensive lithospheric heating downstream from the hotspot interpreted data to tell if the lower lithosphere behaves in this way are lacking

tracking low temperature tectonism of the st lawrence platform and humber zone southern quebec appalachians through apatite and zircon u th he thermochronology the st lawrence platform slp and humber zone hz of the southern quebec appalachians have been the subject of extensive studies to resolve the degree of thermal maturation yet the timing of the thermal maximum is not well understood we have employed apatite ahe and zircon zhe u th he thermochronometry across a network of late cambrian to late ordovician siliciclastic and grenvillian basement outcrops to resolve the thermal history below 200 c single crystal dates from individual samples exhibit age dispersion by as much as 300 million years with a strong positive to negative correlation with increasing effective uranium eu concentration the data in the southwestern portion of the basin allow for thermal maxima of up to 200 c during the late ordovician or early devonian regional burial trends combined with local estimates of paleogeothermal gradients indicate that if sedimentation continued after the late ordovician there was no significant increase in burial in southwestern portions

low temperature thermal history and landscape development of the eastern adirondack mountains new york constraints from apatite fission track thermochronology and apatite u th he dating the adirondack mountains in northern new york state form an elongate domal exposure of mainly high grade metamor phic tectonites in a mountainous setting with topographic relief of 1 km the origin of the adirondack mountains and this relief has long been enigmatic since the adirondacks presently lie within an intracratonic setting inboard of the north american passive mar gin and far from any active plate boundaries through the application of apatite fi ssion track aft thermochronology and apatite u th he ahe dating within the eastern adirondack mountains this study provides constraints on both the thermal and ero sional effects of mesozoic passage near a hot spot and the timing of relief development aft thermochronology and ahe dating record relatively stable thermal conditions within the eastern adirondacks from the middle jurassic into the early cretaceous during the early cretaceous ca 130 120 ma the region underwent heating as sociated with progressive movement near the great meteor hotspot resulting in the temporary establishment of an elevated geo thermal gradient following regional heat ing cooling rates increased considerably ca 105 95 ma likely due to both thermal doming producing an increase in erosion rate and the relaxation of isotherms after passage near the hotspot the regional distribution of aft ages across the eastern adirondacks reveals no systematic age gradient from core to periph ery which would be expected under con ditions of persistent high relief during the decay of a crustal root over many tens to hun dreds of millions of years instead thermo chronological data suggest that the present relief developed during the late cretaceous cenozoic through plateau dissection during periodic base level changes

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

comments on deccan flood basalts at the cretaceous tertiary boundary by v courtillot j besse d vandamme r montigny j j jaeger and h cappetta the age of the flood basalts of the deccan area and the length of the interval during which the outpourings took place are not known precisely the authors have tried to date those basalts and estimate how long the volcanic activity lasted on the basis of new paleomagnetic paleontological and geochronological data of the deccan effusives they came to the preliminary conclusion that the volcanic activity may have lasted less than 1 ma and that the majority of the lavas poured out during chron 29r unfortunately the authors were not familiar with our paper on paleomagnetism and radiomettic age determinations of deccan trap lavas 2 in our paper we presented the paleomagnetic data derived from 223 individual lava flows from 16 sections all over the deccan area and gave the k ar age determinations for 13 selected unaltered lava flows we agree that the enormous volume of deccan extrusives poured out in a relatively short interval and that the volcanic activity was probably concerned with the spreading in the central indian ocean in late cretaceous and early tertiary time

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

deflection of plumes by mantle shear flow experimental results and a simple theory summary linear hotspot tracks indicate relative motion between some hotspot sources and the overlying lithosphere in order to better understand the effects of plate motions and mantle convection on mantle plumes and associated hotspots we consider the path of a continuous plume of buoyant material in a shear flow laboratory experiments show that steady plumes bent over by a linear velocity profile follow parabolic trajectories plume trajectories are also shown to evolve quadratically in time toward a new steady shape after a change in the ambient flow a remarkably simple description of plume dynamics provides good agreement with the experimental data the plume trajectory is determined by the combination of horizontal advection by mantle flow and vertical rise according to a modified stokes law these results are applied to some idealised examples which demonstrate effects of depth dependent mantle viscosity and return flow at least some mantle plumes are likely to be sufficiently robust that they remain close to vertical and therefore stable

the origin of objects of invertebrate descent from the khvalynsk eneolithic cemeteries northern caspian region abstract the summary of the study of invertebrates used for decorative items from khvalynsk eneolithic cemeteries is presented ams radiocarbon dating confirmed that freshwater unio mother of pearl shells and nacre discs manufactured from them are coeval with the site and most likely local in origin tubes of the sea worms and marine shells are fossil and some may originate from local deposits other are probably related to the ancient paratethys basin sediments and can have diverse origin spanning from caucasus and caspian shores to carpathian and mediterranean area glycymeris l clams are most likely originated in the latter area the discussion of the obtained ams 14 c results agrees with a possibility of a noticeable ingress of younger carbon irremovable by standard methods and affecting the determinations probably related to a bacterial activity in deposits

210pb chronology and trace metal geochemistry at los tuxtlas mexico as evidenced by a sedimentary record from the lago verde crater lake lago verde is a fresh water maar found on the lower slopes of san martin volcano at the sierra de los tuxtlas mexico currently the northernmost remnant of the tropical rain forest in america 210 pb and 137 cs analyzed in a sediment core were used to reconstruct the historical fluxes of ag cd cu pb hg and zn to the site during the last 150 yr the 210 pbxs derived sediment accumulation rates the magnetic susceptibility c n ratios and 13 c data evidenced background conditions at the lake until 1960s when enhanced erosion related to the clearing of large forested areas at los tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material recent sediments from lago verde were found enriched by pb 26 fold natural concentration level ncls and moderately enriched by cd cu zn and hg 6 5 4 and 4 fold corresponding ncls respectively the fluxes of cu hg pb and zn have significantly increased since 1940s with peak ratios of total modern to pre industrial fluxes of 11 11 19 and 49 respectively the lake occupies a relatively pristine non industrialized basin and therefore the increased metal fluxes might be related to long distance aeolian transport of trace metals

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

mesozoic hotspot epeirogeny in eastern north america geologic data especially conodont paleotemperatures imply that southeastern canada and new england have been elevated at least 4 km relative to the central appalachian region apatite fission track ages suggest that the uplift and consequent erosion occurred in cretaceous early tertiary time consistent with the formation of a broad swell as north america moved northwestward over the great meteor hotspot

lacustrine sediment chronology defined by 137cs 210pb and 14c and the hydrological evolution of lake ailike during 1901 2013 northern xinjiang china abstract dating lake sediments in arid and semi arid zones by using 137cs and excessive 210pb 210pbex methods is challenging because of the irregular climatic variations and anthropogenic effects here we try to establish the reliable geochronology of the lacustrine core drilled from lake ailike northern xinjiang of china by using 137cs and 210pbex dating methods and improve our understanding of the principal factors that affect the dating results the results indicate that it is important to normalize the 137cs activities by particle size and organic matter in order to diminish their enrichment effects on 137cs activities and thus help us to obtain the accurate 137cs ages the performance of the sediment isotope tomography sit model is better than the other 210pbex derived models the erosion process results in the failure of the commonly used constant rate of supply crs model as well as the low values of the sedimentation rate the 137cs inventory and the post 1963 mean annual flux of 210pbex in ailike core furthermore based on the sit model chronology we further clarify the lake evolution history of the past century and assess the anthropogenic impacts on lake ailike the hydrology of lake ailike was mainly controlled by natural climate before 1960 ce whereas it was completely controlled by intensive human activities after 1960 ce this study supports that the 210pbex based sit model appears to be an adequate method for deriving ages in the lacustrine systems with strong hydrological variability

fossil brines preserved in the st lawrence lowlands quebec canada as revealed by their chemistry and noble gas isotopes abstract brines in cambrian sandstones and ordovician dolostones of the st lawrence lowlands at becancour quebec canada were sampled for analysis of all stable noble gases in order to trace their origin and migration path in addition to quantifying their residence time major ion chemistry indicates that the brines are of na ca cl type possibly derived from halite dissolution 87 sr 86 sr ratios and ca excess indicate prolonged interactions with silicate rocks of the proterozoic grenville basement or the cambrian potsdam sandstone the brines constrain a 2 3 contribution of mantle 3 he and large amounts of nucleogenic 21 ne and 38 ar and radiogenic 4 he and 40 ar 4 he 40 ar and 21 ne 40 ar ratios corrected for mass fractionation during incomplete brine degassing are identical to their production ratios in rocks the source of salinity halite dissolution plus the occurrence of large amounts of 40 ar in brines constrain the residence time of becancour brines as being older than the cretaceous evaporites in the st lawrence lowlands likely existed only during devonian silurian time brines might result from infiltration of devonian water leaching halite penetrating into or below the deeper cambrian ordovician aquifers during the devonian the basin reached temperatures higher than 250 c allowing for thermal maturation of local gas prone source rocks utica shales and possibly facilitating the release of radiogenic 40 ar into the brines the last thermal event that could have facilitated the liberation of 40 ar into fluids and contributed to mantle 3 he is the cretaceous monteregian hills magmatic episode for residence times younger than the cretaceous it is difficult to find an appropriate source of salinity and of nucleogenic radiogenic gases to the becancour brines

seismic stratigraphic correlation across the new england seamounts western north atlantic ocean correlation in the western north atlantic basin of deep water seismic stratigraphy across a basement high within the new england seamounts shows that the seismic sequence of the nova scotian rise is broadly similar to that off the united states time of seamount volcanism inferred from stratigraphic relations around the basement high is consistent with a hot spot origin but nearby stratigraphic data indicate that earlier and later eruptions occurred suggesting lithospheric weakness that predisposed this area to volcanism patterns of tertiary abyssal current erosion and sediment deposition were controlled by sea floor morphology around the seamounts and interaction of abyssal currents with the gulf stream

the newfoundland basin ocean continent boundary and mesozoic seafloor spreading history the ocean continent boundary in the newfoundland basin is defined as the seaward limit of a continental margin magnetic smooth zone east of the grand banks this boundary is marked by a prominent nne trending magnetic anomaly that is correlated with the j anomaly 115 m y south of flemish cap the smooth zone boundary strikes approximately 060 and is approximately 15 m y younger magnetic anomaly trends suggest two directions of motion during separation of iberia and north america the first phase of motion commencing at j anomaly time with a spreading center strike of 015 produced a rifted margin along the grand banks south of the newfoundland seamounts no spreading occurred north of the seamounts during this phase implying a counter clockwise rotation of iberia and no grand banks galicia bank separation the second phase began at about 102 m y with a shift of the pole of rotation to a location near paris producing a ridge orientation of approximately 060 this spreading center extended north and east into the northern newfoundland basin and bay of biscay producing a rifted margin south of flemish cap and opening of biscay this ridge geometry produced a component of extension across the newfoundland fracture zone and the southeastward migration of the resultant leaky transform fault between 102 m y and the next pole shift produced the volcanic edifice of the southeast newfoundland ridge fracture zone trends during this phase also exerted strong control on volcanism within the newfoundland seamount province this activity ceased at about 97 m y the date at which the second phase ended is not well defined by presently available data a rrr triple junction existed in the northeastern newfoundland basin western biscay region for a short time prior to anomaly33 34 80 m y which marks the inception of a continuous mid atlantic ridge spreading center between the newfoundland and charlie gibbs fracture zones

western quebec seismic zone canada clustered midcrustal seismicity along a mesozoic hot spot track 1 the western quebec seismic zone wqsz is a 160 km wide band of intraplate seismicity extending 500 km from the adirondack highlands united states to the laurentian uplands canada previous authors have proposed that the wqsz is localized over the mesozoic track of the great meteor hot spot here we explore this hypothesis further by investigating regional seismicity characteristics focal mechanisms for wqsz earthquakes including a new mechanism for a moderate mn 4 5 earthquake reveal a pattern of reverse sense faulting with sw trending p axes changing to e w in the southern part of the zone we introduce a simple box counting method to delineate spatial clusters based on exceedance of random seismicity density combining this approach with focal depths from regional depth phase analysis we find that seismicity with shallow focus 0 7 km is characterized by a random spatial distribution whereas earthquakes with an intermediate focal depth 8 18 km are strongly clustered along a diffuse linear band trending n50 w earthquakes deeper than 18 km are confined to a few distinct clusters these clusters are characterized by differing b values and for at least one cluster repeating events projection of hypocenters onto a deep seismic profile and comparison with preexisting crustal structures suggest that local reactivation of precambrian structural features may have occurred however the great meteor hot spot track remains the only compelling explanation for the overall distribution of earthquakes proximity of seismicity clusters to historic and prehistoric earthquakes lends support to the hypothesis that modern seismicity may represent exceptionally long lived aftershocks of large past events

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

determination of geochronology and sedimentation rates of shallow lakes in the middle yangtze reaches using 210pb 137cs and spheroidal carbonaceous particles abstract accurate chronologies for recent sediments of shallow lakes in the yangtze floodplain are critical to calibrate proxy records for reconstructing environmental changes during the past century this study presents the results of detailed 210pb analysis from eight lake sediment cores collected from the middle yangtze reaches southeast china unsupported 210pb activities generally declined exponentially with mass depth in the eight cores the chronologies and sedimentation rates for the sediment cores were calculated using different 210pb based mathematical models the 137cs chronomarker i e the 1963 fallout peak and the spheroidal carbonaceous particle scp chronomarker i e the start of the rapid increase in 1970 ce were selected to validate the 210pb dating sedimentation rates derived from different models were validated using historical data including lake area arable land area sediment discharge and reservoir volume in the yangtze basin the scp corrected crs constant rate of supply model performs better than other models based on validation using historical documents in the yangtze basin the 137cs chronomarker might be erroneous due to catchment driven 137cs inputs from soil erosion and post depositional diffusion both scps and 137cs are susceptible to inputs from catchment soil erosion but scps show no apparent degradation and post depositional changes in lake sediments the scp profile provides a relatively reliable chronomarker which can be used for validating 210pb chronologies in these floodplain lakes generally sedimentation rates in the eight lakes were

conductivity structure of the lithosphere asthenosphere boundary beneath the eastern north american margin tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere asthenosphere boundary lab yet this boundary remains enigmatic regarding its properties and geometry the processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies here we employ marine magnetotelluric mt data acquired along a 135 km long profile offshore martha s vineyard new england usa to image the electrical conductivity structure beneath the new england continental margin for the first time we invert the data using two different mt 2 d inversion algorithms and present a series of models that are obtained using three different parameterizations fully unconstrained unconstrained with an imposed lab discontinuity and a priori constrained lithosphere resistivity this suite of models infers variability in the depth of the lab with an average depth of 115 km at the eastern north america passive margin models robustly detect a 350 m lithospheric anomalous conductivity zone lacz that extends vertically through the entire lithosphere our preferred conductivity model is consistent with regional p to s receiver function data shear wave velocity gravity anomalies and prominent geological features we propose that the lacz is indicative of paleo lithospheric thinning either resulting from kimberlite intrusions associated with rifting and the new england great meteor hotspot track or from shear driven localized deformation related to rifting

monteregian hotspot track a long lived mantle plume the monteregian hills the younger white mountains and the new england seamounts constitute a well defined hotspot track the evolution of the plume through time is obtained from the marine track the buoyancy flux of the plume when it made the new england seamounts was about 2 mg s 1 which would make it the seventh or eighth most active plume today the hotspot declined in activity after it made nashville seamount the buoyancy flux when it made corner seamounts and today is less than 0 5 mg s 1 the changes in the flux of the marine hotspot limit how far the marine flux can be extrapolated to the west onto the continental track and what can be learned in general about vulnerability of the continental lithosphere to hotspots the lack of an obvious track west of montreal may be due either to failure of the plume to penetrate the canadian shield to the lack of recognizable intrusions or to strengthening of the plume when it approached montreal

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

no regional anisotropic domains in the northeastern u s appalachians a fabric frozen within the continental lithosphere should lead to regional anisotropic domains with close correlation between tectonics seismic anisotropy and shear wave travel time delays alternatively seismic anisotropy could be maintained by active deformation in the sub lithospheric mantle this scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting surface tectonics and travel time delays a coherent pattern of shear wave splitting is observed in the northeastern united states covering large parts of new york and new england spanning the appalachian orogen and areas underlain by the grenvillian basement the direction of fast shear wave propagation as measured from core refracted sks skks and pks phases varies systematically with event back azimuth at all sites examined and is fit well by two layers of anisotropy with subhorizontal symmetry axes relative travel time delays of shear waves that sample the region vary over 100 km length scale and correlate with surface geology tomographic images suggest lateral shear velocity variations 3 including a slow anomaly beneath the grenvillian basement exposed in the adirondack mountains there is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting we therefore conclude that in the northeastern us the concept of regional anisotropic domains i e distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation does not apply

viscous heating a potential mechanism for the formation of the ultralow velocity zone abstract the detection of several ultralow velocity zones ulvz on the core mantle boundary cmb together with the recent estimates of the lower mantle solidus would suggest that some sort of melting may occur inside the upwellings in the deep mantle we have studied the possibilities of viscous heating as a mechanism for producing favorable conditions leading to melting inside deep mantle upwellings a two dimensional model in an aspect ratio 3 box with temperature and pressure dependent rheology and two major phase transitions has been employed both viscous and adiabatic heating have been included and various amounts of internal heating have been applied viscous heating can heat up the interior of the deep mantle plumes to around 100 degrees above the cmb temperature this amount of temperature rise is sufficient to cause melting inside upwellings in the deep lower mantle we propose here that viscous heating plays an important role in producing conditions leading to the formation of the ulvz massive melting within rising plumes might have been more prevalent in the past

3d lithosphere density structure of southern indian shield from joint inversion of gravity geoid and topography data abstract we present the 3d crustal and lithospheric structure and crustal average density distribution of southern indian shield south of 18 n sri lanka and adjoining oceans the model is based on the assumption of local isostatic equilibrium and is derived from joint inversion of free air gravity and geoid anomalies and topography data the derived crustal thickness of 10 25 km in the oceanic region increases to 34 35 km along the coast a crustal thickness of 34 38 km is obtained beneath the eastern dharwar craton and 36 45 km beneath the western dharwar craton and the southern granulite terrain sri lanka has a thinner crust of 30 35 km the lithosphere asthenosphere boundary is located at depths of 70 120 km under oceanic regions and 150 180 km below the dharwar craton and the northern block of southern granulite terrain a notably thinned lithosphere of 130 km near bangalore in the eastern dharwar craton 140 km beneath the southern block of southern granulite terrain and 130 km in sri lanka is observed the thickness of the lithosphere 130 km near bangalore is inferred as the frozen in signature of a small fossil mantle plume and or tectono compositional effect of a rifted margin and a suture considerable stretching and or convective removal of pristine lithosphere in the southern block of southern granulite terrain and adjoining sri lanka before disappearing completely in the archaean northern block of southern granulite terrain and dharwar craton is suggested

new insights into the lithosphere beneath the superior province from rayleigh wave dispersion and receiver function analysis we study the azimuthally anisotropic upper mantle structure of the superior craton and grenville province in ontario canada using rayleigh wave phase velocity data in the period range 40 160 s 152 two station dispersion measurements are combined in a tomographic inversion that solves simultaneously for isotropic and anisotropic terms using a least squares technique we perform a series of tests to derive optimal regularization smoothing and damping and to assess the resolution of and trade offs between isotropic and anisotropic anomalies the tomographic inversion is able to resolve isotropic phase velocity anomalies on a scale of 200 300 km and to distinguish between different anisotropic regimes on a 500 km scale across the study region isotropic phase velocity anomalies in the tomographic model span a range of up to 2 per cent around a regional average which is similar to the canadian shield dispersion curve of brune dorman 1963 with phase velocities up to 3 per cent above global reference models the amplitude of azimuthal phase velocity anisotropy reaches a maximum of 1 2 per cent a clear east west division of the study area based on both isotropic phase velocity anomalies and azimuthal anisotropy is apparent in the western superior isotropic phase velocities are generally higher than the regional average anisotropy is observed at all periods with ene wsw to ne sw fast propagation directions at periods 120 s the anisotropy likely results from frozen lithospheric fabric aligned with tectonic boundaries whereas the anisotropy at longer periods is interpreted to arise from present day sublithospheric flow the fast directions from published sks measurements are close to the fast rayleigh wave propagation directions throughout the period range sampled and the large sks splitting times may be accounted for by this near coincidence of fast propagation directions across most of eastern ontario phase velocities are lower than the regional average fast propagation directions rotate from nw se at 40 130 s period to wnw ese at periods 140 160 s the results suggest a difference in fast propagation directions between the anisotropic fabric frozen into the lithosphere and the fabric due to current and recent sublithospheric flow the superior craton and grenville province are characterized by large scale structural variations that reflect the complex tectonic history of the region this study highlights differences between the characteristics of eastern and western ontario and indicates the occurrence of multiple layers of anisotropy in the subcratonic upper mantle

the eight classical monteregian hills at depth and the mechanism of their intrusion the eight classical monteregian hills are monadnocks on the st lawrence lowlands and adjacent appalachian foothills in a swath that sweeps 80 km eastward from montreal gravity anomalies suggest t

distribution of seamounts in the north atlantic the paucity of multi narrow beam bathymetric data in the ocean basins has meant that previous studies of seamount distribution and size have relied on wide beam echo sounder data and small1 8 amounts of seabeam multi narrow beam data since 1967 however the us naval oceanographic office has been collecting multi narrow beam bathymetric data9 14 here we use an extensive collection of such data to locate over 800 seamounts in the north atlantic we find that ridge crest generated seamounts are formed only between iceland and the hayes fracture zone where there are contributions of plume material to the mid ocean ridge plumbing system it appears that in the slow spreading north atlantic an injection of plume material into the ridge plumbing system is necessary for the formation of seamounts at the ridge crest

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

mantle plumes continental magmatism and asymmetry in the south atlantic melt production rates are estimated along the ocean tracks rio grande rise walvis ridge and the onshore parana etendeka flood basalt province for the mantle plume currently associated with tristan da cunha in the south atlantic assuming that the volcanic edifice is locally compensated with a thickened crust the maximum oceanic rate 0 3 km3 a 1 is similar to the value for the continental flood basalt calculated using the surface volume of lava and a duration of 5 m y however this latter estimate is conservative and the appropriate rate is probably at least a factor of 5 higher the maximum mantle potential temperature for the ocean plume tracks is estimated to have been 1450 1540 c assuming a dry peridotite source using a similar model temperatures in excess of 1600 c are required to explain the observed magma volumes onshore this is because there is no significant extension in the region where the oldest lavas occur it is argued that the observed volumes of melt can be generated with small amounts of extension and with plume temperatures of 1500 c if melting in the lithosphere is strongly influenced by the relative enrichment of volatiles moreover the model predicts the inferred change to an asthenosphere dominated source when significant extension occurs recent geochonological data have shown that the continental volcanics are 138 ma in the northwest of the parana province and become progressively younger to the southeast oceanic melt production rates decrease by a factor of 3 5 towards the mid ocean ridge tristan da cunha intriguingly melt production rates on the rio grande rise vary to more than twice that of the walvis ridge for oceanic crust of similar age this asymmetry is also seen in the continental regions most obviously in the preserved volume of lava in the parana and etendeka but also in the topography of the underplated continental margin and the orientation of dykes and oblique extension structures these observations reflect the combined roles of the plume temperature lithospheric thickness crustal tectonics and magma source region composition during the evolution of the south atlantic over the last 150 ma

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

phanerozoic vertical motions of hudson bay we analyze the sedimentary record of the hudson bay basin an intracratonic basin which is widely believed to have been formed by convective downwelling within the mantle this belief largely stems from the coincidence of a long wavelength negative gravity anomaly and an approximately circular basin which is filled with about 2 km of predominantly paleozoic sedimentary rocks our starting point is a set of wells and seismic reflection profiles which demonstrate that the basin subsided rapidly from ordovician to devonian times one and two dimensional subsidence modeling show that water loaded subsidence occurred in two stages and is too rapid to be accounted for by thermal contraction alone instead we suggest that basin subsidence is consistent with minor phases of lithospheric thinning which occurred during ordovician silurian and early middle devonian times stretching factors for these minor extensional episodes vary from 1 05 to 1 2 with vertical strain rates of 2 10 ga 1 our interpretation of the

anomalous seaward dip of the lithosphere asthenosphere boundary beneath northeastern usa detected using differential array measurements of rayleigh waves summary rayleigh wave phase velocities and azimuth anomalies in the period range of 30 100 s are measured for a set of four triangular arrays of broad band seismometers in coastal northeastern usa this is a region in which a strong upper mantle slow shear velocity anomaly a new england anomaly crosses the continental margin earthquakes from a wide range of directions are used to detect the variation of parameters with azimuth of propagation no lateral heterogeneity in phase velocity is detected at these periods between stations at the centre and the edge of the anomaly however large 10 20 per cent azimuthal variations occur and have a cos 1 dependence which is indicative of a dipping structure in the upper mantle corresponding azimuth variations with a magnitude of 5 are also detected this behaviour is consistent with a southeasterly n150 e dip of the lithosphere asthenosphere boundary beneath new england this dip is associated with the shoaling of the new england anomaly beneath the adirondack mountains west of the array it is opposite to the dip associated with lithospheric thickening toward the interior of the craton

three decades of dating recent sediments by fallout radionuclides a review this paper reviews the 210pb dating programme carried out at the university of liverpool over a period of three decades since its inception by frank oldfield in the mid 1970s the work at liverpool

the limitations of using 137cs for estimating soil redistribution in semi arid environments the 137cs technique for estimating net ca 30 years soil flux has been used successfully in many environments its widespread use is probably because the 137cs technique overcomes many of the problems of monitoring soil erosion and deposition flux over the medium term 5 to 50 years and at the hillslope scale in this respect the technique probably offers the greatest potential for measuring net soil flux in semi arid environments where soil flux monitoring difficulties are compounded by considerable spatial and temporal variability of the controlling factors however there remain uncertainties in the underlying assumptions of the technique and difficulties in satisfying these assumptions especially in semi arid areas several key assumptions of the 137cs technique for measuring net soil flux in semi arid areas were investigated using data from southwest niger samples were obtained along a toposequence typical of the region and at nested grid nodes stratified using geomorphological information for geostatistical estimation at unsampled locations providing complete coverage of the study area the pervasive occurrence of dust made it difficult to identify an undisturbed site a first approximation of the reference 137cs inventory 2066 125 bq m 2 was provided by modelling the 137cs profile at an unvegetated site despite little evidence in the literature on problems of identifying a reference inventory especially those suffering from wind erosion it is likely that similar problems occur in other semi arid areas problems with the preferential transport of 137cs were identified by expressing the 137cs concentration as a proportion of the weight in each grain size fraction however it was partially accounted for in the calibration relationship by reducing by a factor of 10 the 137cs concentration of soil samples from the plateau it is likely that workers in other semi arid regions have similar problems of preferential transport of 137cs and the method discussed here appears to be a valuable tool to indicate the potential for preferential 137cs movement a large disparity was found between the undisturbed model and the structure of a 137cs profile from a small alluvial fan this was due to the removal of 137cs rich soil and its replacement with soil largely devoid of 137cs which probably originated from gully walls two separate models that related 137cs movement to soil redistribution were used to calculate net soil flux additional modelling would be required to account for 137cs dilution in order to measure net soil flux in gullied and badly eroded rangeland areas net soil flux was calculated at sites along the toposequence at the nested grid sites and at unsampled locations across the study using geostatistics the toposequence samples considerably underestimated the net soil loss relative to the nested grid samples and the geostatistical estimates of net soil flux toposequence sampling does not account for the spatial variation in net soil flux unless very well defined geomorphological units that control the soil redistribution processes are evident this form of sampling is of limited value for investigating the variation of 137cs depth distribution at all but the smallest scale largest areas since it cannot easily be used to determine local variation in soil redistribution processes a small improvement in net soil flux accuracy was provided by the geostatistical estimates relative to the nested grid samples of net soil flux this was thought to be due to the efficiency of the nested grid samples suggesting that geomorphological information can be used to limit the number of samples necessary to encompass several scales of net soil flux variation

evidence from geoid data of a hotspot origin for the southern mascarene plateau and mascarene islands indian ocean a high resolution geoid map of the madagascar and mascarene basins is constructed from geos 3 and seasat satellite altimetric data with an accuracy of 0 7 m large positive anomalies are due to topographic highs and their regional compensation the deflection of the lithosphere under volcanic loads and its effect on the geoid are modeled for the southern mascarene plateau mauritius reunion and rodriguez islands assuming a purely elastic lithospheric model the effective flexural rigidity is estimated for the four zones rigidity varies from 1 10 21 to 2 10 23 n m increasing from north to south the highest rigidity occurs under reunion island and suggests a recent origin for the volcanism 3 5 ma the southern mascarene plateau shows the lowest rigidity suggesting a near ridge origin rodriguez island shows the same trend but in another tectonic framework the topographic and geoid swell is studied as an effect of a thermal mantle plume using a conductive model of lithospheric heating we confirm the assumption of hotspot migration for the origin of the southern mascarene plateau and the mascarene islands

upper mantle velocity structure of the lower great lakes region abstract the lithospheric root beneath north america contains a prominent indentation beneath the lower great lakes region that is approximately aligned with the track of the new england seamounts by combining data from the recently installed polaris network in southern ontario canada with data acquired in 1996 during the abitibi grenville teleseismic experiment we have performed a tomographic inversion using 4543 p wave traveltimes from 213 events 5 0 m b 6 6 and 1860 s wave traveltimes from 98 events 5 0 m b 6 6 to obtain high resolution images of the upper mantle beneath the lower great lakes two salient features of the 3 d models are 1 a patchy nnw trending low velocity region and 2 a linear ne striking high velocity anomaly s wave images show that the low velocity anomaly changes from an arcuate feature at 400 km depth to a nw striking linear feature at 100 km depth beneath the neoproterozoic ottawa bonnechere graben the linear high velocity anomaly extends to at least 300 km depth and strikes parallel to surface geological belts and the laurentian continental margin we interpret the high velocity anomaly as a possible relict slab associated with ca 1 35 1 3 ga subduction beneath the composite arc belt whereas the low velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the late cretaceous monteregian plutons our data support an interpretation of these plutons as melts generated by the passage of north america across a mantle plume rather than a far field response to opening of the north atlantic

middle late pleistocene marine molluscs from izmit bay area eastern marmara sea turkey and the nature of marmara black sea corridors abstract in this study marine pleistocene mollusc faunas from deposits along the southern shores of the izmit bay area marmara sea turkey are reviewed ten samples from five localities spanning middle and late pleistocene intervals are assessed and compared to four samples of a holocene age for the pleistocene fauna a total of 59 mollusc species 33 bivalve and 26 gastropod species are recorded some of which are reported for the first time for this region the middle pleistocene fauna bears large resemblance to the late pleistocene faunas even though marine conditions were lacking during sea level drops in the marmara sea basin in intervening times the middle pleistocene mollusc fauna is dominated by species that typify the so called uzunlarian faunas while the late pleistocene mollusc fauna is indistinguishable from karangatian faunas of the black sea basin several common species including bittium reticulatum rissoa spp and lucinella divaricata show that the both uzunlarian and karangatian assemblages of yalova fauna mainly represent seagrass palaeoenvironments palaeosalinities estimates for the yalova area are uppermost mesohaline to polyhaline typically around or above 20 psu which is almost similar to the optimum salinity estimates for the middle pleistocene uzunlarian however it is slightly lower then the upper pleistocene karangatian representing the location of the yalova area in an embayment with freshwater input at that time the yalova sections are located in the southern flank of the north anatolian fault system responsible for a strong local uplift of the study area the sections contain evidence for three marine high stands separated by terrestrial intervals when regional uplift rates approximately 0 2 mm y and oceanic sea levels are considered it is likely that the three highstand intervals correspond to relative sea level high stands of mis7 mis5e and possibly mis5d but further study is required to confirm such a suggestion

origin and geochemical evolution of the madeira tore rise eastern north atlantic the madeira tore rise located 700 km off the nw african coast forms a prominent ridge in the east atlantic the age and origin of the rise are controversial this study presents major and trace element sr nd pb hf isotope and 40ar 39ar age determinations from volcanic rocks dredged from different sites along the rise in addition isotopic compositions of rock samples from great meteor seamount in the central atlantic are presented the new radiometric and paleontologically constrained ages identify two major episodes of volcanism the first is the base of the rise circa 80 to 95 ma and the second is seamounts on the rise 0 5 16 ma it is proposed that interaction of the canary hot spot with the mid atlantic spreading center formed the deep basement of the madeira tore rise and the j anomaly ridge west of the atlantic spreading center in the mid cretaceous age and geochemical data and plate tectonic reconstructions suggest however that the recovered late cretaceous volcanic rocks represent late stage volcanism from the time when the madeira tore rise was still close to the canary hot spot long after moving away from the influence of the canary hot spot the madeira tore rise was overprinted by late cenozoic volcanism miocene to pleistocene volcanism at the northern end of the rise can be best explained by decompression mantle melting beneath extensional sectors of the azores gibraltar fracture zone african eurasian plate boundary the geochemical compositions of these volcanic rocks suggest that the magmas were variably contaminated by enriched material within or derived by melting of enriched material underplated at the base of the lithosphere possibly originating from the cretaceous canary plume alternatively these late cenozoic volcanic rocks may have derived from decompression melting of enriched pyroxenitic eclogitic material in the upper mantle isotopically more depleted pliocene to pleistocene volcanism at the southern end of the madeira tore rise may be related to the nearby madeira hot spot

reexamination of magnetotelluric responses and electrical anisotropy of the lithospheric mantle in the grenville province canada magnetotelluric mt responses at the proterozoic grenville front in canada have been interpreted as being caused by lithospheric electrical anisotropy and the area is often noted as a classic example of lithospheric anisotropy this study reevaluates evidence for the electrical anisotropy using 56 mt stations the spatially uniform mt responses noted at the grenville front extend to 200 km southeast and for at least 400 km along strike and are associated with rocks at less than 150 km depth examination of induction arrows at longer periods shows arrows at high angle to the mt conductive direction consistent with the presence of macroscopic resistivity structures new 2 d anisotropic inversions show that electrical anisotropy is not required to fit the mt data the results indicate that in the resistive mantle lithosphere beneath the grenville front and in conductive lithosphere in adjacent areas the maximum horizontal resistivity anisotropy is 10 much less than the factor of 15 determined in earlier 1 d studies the results suggest that the upper lithospheric mantle in the area is devoid of significant electrical anisotropy and that the observed mt response directionality is due to large scale resistivity structure we interpret the spatially consistent mt responses observed at the grenville front as being associated with the resistive archean lithosphere extending southeast beneath the grenville front the obliquity between seismic and mt responses arises because the superior fabric is oblique to the seismic fast direction if dextral shearing occurred it appears to have not caused any significant shape preferred electrical anisotropy

manzala lagoon nile delta egypt modern sediment accumulation based on radioactive tracers this study was undertaken to determine whether recent anthropogenic changes in the nile basin have affected the modern rate of sediment accumulation in the nile delta excess 210pb 137cs and 239 240pu were used to develop a sediment chronology for a core from central manzala lagoon the delta sector which has had the highest average rate of sediment accumulation during the holocene to about 0 7 cm year 1 excess 210pb was detected in the top 32 cm of the core yielding an accumulation rate of 1 2 cm year 1 higher than the mean rate for the holocene a high 137cs 239 240pu ratio requires a reactor source possibly chernobyl for these nuclides low concentrations of excess 210pb and weapons fallout nuclides precluded recognition of changes in sediment accumulation rate in manzala lagoon during this century and may limit the use of tracer radionuclides for modern sediment chronology in the nile delta

age progressive volcanism in the new england seamounts and the opening of the central atlantic ocean radiometric ages k ar and oar 39ar methods have been determined on dredged volcanic rocks from seven of the new england seamounts a prominent northwest southeast trend ing volcanic lineament in the northwestern atlantic ocean the oar 39ar total fusion and increnental heating ages show an increase in seamount construction age from southeast to northwest that is consistent with northwestward motion of the north american plate over a new england hot spot between 103 and 82 ma a linear volcano migration rate of 4 7 cm yr fits the seamount age distribution these ages fall within a longer age progression from the corner seamounts 70 to 75 ma at the eastern end of the new england seamounts to the youngest phase of volcanism in the white mountain igneous province new england 100 to 124 ma the new england hot spot estimated from the new radio metric ages and motion of north america in the hot spot reference frame to be near 28on 33ow probably generated a short line of mid to late tertiary age seamounts on the african plate but appears to be presently inactive the hot spot reference frame is used to calculate the motion of the north american plate away from the african plate from early cretaceous time to the present rominent magnetic anomalies recorded in the central atlantic seafloor give the positions of the spreading ridge for a bange of known ages hot spots that now underlie the african plate in the eastern central atlantic new england canary madeira cape verde azores could have produced cretaceous seamount and island chains on the north american plate during the early opening of the central atlantic each of these hot spots has been overridden by spreading ridges at pre dictable times some of these hot spot crossings are expressed as geochemical anomalies in the oceanic crust now far removed from spreading ridges

residual geoid anomalies in atlantic ocean basin relationship to mantle plumes the relations of geoid data from radar altimeter measurements geos 3 to water depth and geochemical variations along the mid atlantic ridge are investigated eight geophysical lines across the ridge in the atlantic ocean were constructed approximately normal to the local ridge trend slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography which has better symmetry between the two flanks subtracting a regional field based on previous work produces residual geoid anomalies whose source must be in the earth s outer 600 km the residual geoid anomalies of the north atlantic show striking correlation with the axis of the mid atlantic ridge but those in the south atlantic do not instead major positive residual geoid anomalies appear over the rio grande and walvis ridges determination of slope s residual geoid per million years crustal age reveals that s increases with greater residual geoid anomaly and with higher shallower crestal ridge topography the slope of 22 cm m y west of the azores high near 38 n suggests that the anomalously high crestal residual geoid and depth anomalies may not be entirely of thermal origin which may normally produce slopes only of about 8 cm m y as found near 28 n away from shallow ridge topography to account for this difference in slopes simply by a variation in mantle temperature would require a difference of about 940 c between the two sites which appears excessive also the decrease in topographic height with crustal age away from the azores high is much greater than predicted by theoretical cooling curves correlation of residual geoid anomalies with depth geochemical variations along the length of the mid atlantic ridge and the chemistry of the rio grande rise and walvis ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal effects

origin of the louisville ridge and its relationship to the eltanin fracture zone system we have combined shipboard and seasat altimeter derived data in an intergrated geological and geophysical study of the louisville ridge a 3500 km long seamount chain extending from the tonga trench to the eltanin fracture zone a break in the smooth trend of the ridge at latitude 37 5 s has been recognized in both bathymetric and altimetric data the 40ar 39ar dating of rocks dredged either side of the break suggest that it is analogous to the bend in the hawaiian emperor seamount chain although the general trend of the ridge can be fit by small circles about pacific absolute motion poles determined from other seamount chains the new bathymetric and age data allow us to refine pacific absolute motion poles the continuity in smooth trend of the ridge and the eltanin fracture zone suggests some relationship between them however a major offset developed on this transform between 60 and 80 ma prior to the oldest dated rocks from the ridge although magmatism was more or less continuous on the ridge during 28 60 ma it probably occurred on crust with little or no offset thus magmatism appears to have been little influenced by the developing fracture zone by 28 ma the distance between the magmatic source and the fracture zone had decreased sufficiently for a portion of the ridge to have been emplaced on crust with an offset after about 12 ma however volcanic activity on the louisville ridge apparently waned despite a possible influence on the magmatism of the fracture zone

the mont laurier quebec earthquake of 19 october 1990 and its seismotectonic environment on 19 october 1990 at 07 01 ut a magnitude mblg 5 0 earthquake occurred approximately 9 km southwest of mont laurier quebec canada latitude 46 47 n and longitude 75 59 w this earthquake is the largest to have occurred in the western quebec seismic zone since the 1944 magnitude 5 6 cornwall massena earthquake the mont laurier earthquake was widely felt up to distances of 500 km and caused some minor damage in the epicentral region intensity modified mercalli mm v during the week following the earthquake a field survey recorded 23 aftershocks magnitudes were all less than ml 1 5 analysis of the mainshock and its aftershocks indicates focal depths in the range 10 to 12 km of the two east west oriented nodal planes given by the predominantly thrust mechanism of the mainshock the steep northerly dipping one probably represents the fault plane as indicated by the trend of aftershock hypocenters the last 10 yr of recording have confirmed that most western quebec earthquakes including the mont laurier earthquake occur in a northwest southeast trending zone inside the central metasedimentary cm belt with most focal depths varying between 7 and 25 km although some northwest trending structural features are known correlating these with the epicentral trend is premature the mid crustal hypocentral depths of many earthquakes the east west trend of the fault plane of the mont laurier earthquake and variations in regional focal mechanisms all suggest reactivation of deep structural features which may not have a surface expression inside the central metasedimentary belt most earthquakes occur along the eastern side where the activity seems to correlate with the labelle shear zone and the morin anorthosite body which may act as a stress concentrator the western side of the activity does not end with the central metasedimentary belt a fact that implies that even though the factors controlling seismicity lie predominantly within the central metasedimentary belt the adjacent geologic domains are also seismogenic

210pb sediment radiochronology an integrated formulation and classification of dating models abstract environmental archives such as sediments banded corals and tree rings are widely used to reconstruct past environmental conditions of ecosystems in particular 210 pb has been used during more than 4 decades through dating of undisturbed sediment cores to study ecosystem changes during the last 100 yr since its inception diverse dating models have been proposed developed used and validated in numerous environments but their nomenclature and formulation is not homogenous in this work we emphasize that conceptual models used to date undisturbed sediment cores can be deduced from a single fundamental equation which relates excess 210 pb concentration in sediment its flux to the sediment surface and mass accumulation rate from this equation models were consistently named based on the main hypothesis used and using a unified and logical nomenclature dating equations were derived in detail we applied in detail most of the described models to a sediment core from mexico and uncertainty expressions were also included finally some recommendations to use the models are proposed we believe that this work can facilitate the use of 210 pb as a tool to date sediment cores in a more homogeneous way

onset of mantle plumes in the presence of preexisting convection the onset of plumes where broad scale convection already exists is investigated along with the thermal history of the earth geological evidence relating to the onset time of mantle plumes is not definitive no well defined hotspot tracks older than mesozoic have been identified but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes since only continental crust of this age exists using the hotspot track distribution for the pacific the absence of pre mesozoic hotspot tracks on the canadian shield cannot easily be attributed to chance however young continental hotspot tracks are controversial and it is not clear whether tracks such as the monteregian hills white mountain magma series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well defined seamount chains our simple models yield potentially testable predictions the instability of plume formation in a system where broad scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source geographically plumes are most likely to form where the boundary layer is thickest and where broad scale ascending flow already exists parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad scale convection to effective boundary layer viscosity in terms of material properties this ratio is the ratio of viscosity across the boundary layer to about the 0 45 power for the case where the broad scale convection is heated from below the viscosity contrast across the boundary layer depends on the temperature contrast across the layer a late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time that is if the core has cooled more slowly than the interior of the mantel in one scenario before about 3 b y b p the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity during this time the mantle was hotter than present and its internal viscosity lower hie temperature contrast at the base of the mantel was thus small after 3 b y internal radioactivity no longer balanced surface heat flow and the mantle cooled about a few hundred degrees celsius as indicated by petrological observations the core cooled slower than the mantle and the temperature contrast across the bottom boundary layer increased with time leading to a late onset of plumes phenomenological inferences are obtained as venus may have cooled more rapidly than the earth its mantle may be too cool for rapid plate tectonics and the gravity field and the convective pattern are dominated by hot upwelling regions

the northern appalachian anomaly a modern asthenospheric upwelling the northern appalachian anomaly naa is an intense laterally localized 400km diameter low velocity anomaly centered in the asthenosphere beneath southern new england its maximum shear velocity contrast at 200km depth is about 10 and its compressional to shear velocity perturbation ratio is about unity values compatible with it being a modern thermal anomaly although centered close to the track of the great meteor hot spot it is not elongated parallel to it and does not crosscut the cratonic margin in contrast to previous explanations we argue that the naa s spatial association with the hot spot track is coincidental and that it is caused by small scale upwelling associated with an eddy in the asthenospheric flow field at the continental margin that the naa is just one of several low velocity features along the eastern margin of north america suggests that this process may be globally ubiquitous

morphometric analysis and amino acid geochronology of uplifted shorelines in a tectonic region near reggio calambria south italy the tectonic coastline of the calabrian peninsula provides an opportunity to study the morphology and history of elevated marine shorelines near capo dell armi eleven shorelines are preserved between sea level and 256 m with some correlation difficulties posed by the dissection of the area by intermittent streams we are able to correlate several of the shorelines across the entire study area with the support of correlations provided by amino acid geochronology we have identified four shorelines dating from the last interglacial period isotope stage 5 the oldest two both contain the strombus bubonius fauna and are of indistinguishable early last interglacial ages stage 5e aminozone e ca 125 ka the highest terrace vii is associated with a shoreline at 157 m and the lower with a shoreline around 125 105 m a s l of the two younger stage 5 without strombus bubonius the higher v lies at about 90 100 m and produces amino acid ratios slightly younger than but statistically indistinguishable from those of aminozone e however the youngest last interglacial shoreline deposits iv have produced amino acid ratios correlated to late stage 5 5a elsewhere in the mediterranean basin amino acid and geomorphic data have shown that uplift varies the study area from 0 85 to 1 03 m ka and suggeststhat constant uplift along any one traverse is unlikely