evolution of the oceanic lithosphere inferred from po so waves traveling in the philippine sea plate po so waves are characterized by their high frequency content and long duration travel over great distances up to 3000 km through the oceanic lithosphere po so waves are developed by the multiple forward scattering of p and s waves due to small scale stochastic random heterogeneities to study the nature of these heterogeneities po so waves are analyzed in the philippine sea plate which consists of three regions with different lithospheric ages in the philippine sea plate po so waves propagate in the youngest region 15 ma and propagate more effectively in older regions we investigate the mechanism of this propagation efficiency using numerical finite difference method simulations of 2 d seismic wave propagation the results of this study demonstrate that the increase in propagation efficiency of po so waves depends on the age of the oceanic lithosphere and this relationship can be qualitatively explained by thickening of the oceanic lithosphere including small scale heterogeneities and a reduction in the intrinsic attenuation these small scale heterogeneities may form continuously in oceanic lithosphere from the time of its formation at a spreading ridge via the solidification of melts distributed in the asthenosphere

late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

synthetic pn and sn phases and the frequency dependence of q of oceanic lithosphere the oceanic lithosphere is an extremely efficient waveguide for high frequency seismic energy in particular the propagation of the regional to teleseismic oceanic pn and sn phases is largely controlled by properties of the oceanic plates the shallow velocity gradient in the sub moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle the confinement of pn sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed the rapid disappearance of sn and the discontinuous drop in pn sn group velocity beyond a critical distance dependent upon the local thickness of the lithosphere are interpreted as a shadowing effect of the low q asthenosphere wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere the results were compared to data collected during the 1983 ngendei seismic experiment in the southwest pacific the pn sn coda is successfully modeled as a sum of leaky organ pipe modes in the sediment layer and oceanic water column while scattering is present to some degree it is not required to explain the long duration and complicated nature of the pn sn wave trains the presence of extremely high frequencies in pn sn phases and the greater efficiency of sn than pn propagation are interpreted in terms of an absorption band rheology a shorter high frequency relaxation time for p waves than for s waves results in a rheology with the property that q q at low frequency while q q at high frequency consistent with the teleseismic pn sn observations the absorption band model is to viewed as only an approximation to the true frequency dependence of q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed

a regional perspective of the quetico metasedimentary belt superior province canada alternating greenstone granite and metasedimentary gneiss belts are a first order tectonic feature of the southern superior province the tectonic development of the quetico metasedimentary belt is reviewed with regard to depositional structural and metamorphic plutonic history over its 1200 km length the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite polyphase deformation has resulted in a steep easterly striking foliation and regional gently east plunging stretching lineation metamorphic grade varies in a low p facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite intrusive granite zone mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 mpa 2 5 kbar near the canada united states border to 600 mpa 6 kbar in granulites adjacent to the kapuskasi

geodynamic setting crustal architecture and vms metallogeny of ca 2720 ma greenstone belt assemblages of the northern wawa subprovince superior province the greenstone belts along the northern margin of the wawa subprovince of the superior province vermilion shebandowan winston lake manitouwadge formed at ca 2720 ma and have been interpreted to be representative of a rifted arc to back arc tectonic setting despite a common inferred tectonic setting and broad similarities these greenstone belts have a significantly different metallogeny as evidenced by different endowments in volcanogenic massive sulphide vms magmatic sulphide and orogenic gold deposits in this paper we examine differences in geodynamic setting and crustal architecture as they pertain to the metallogeny of each greenstone belt by characterizing the regional scale trace element and isotopic nd and pb geochemistry of each belt the trace element geochemistry of the vermilion greenstone belt vgb shows evidence for a transition from arc like to back arc mafic rocks in the soudan belt to plume driven rifted arcs in the ultramafic bearing newton belt the shebandowan greenstone

the global short period wavefield modelled with a monte carlo seismic phonon method summary at high frequencies 1 hz much of the seismic energy arriving at teleseismic distances is not found in the main phases e g p pp s etc but is contained in the extended coda that follows these arrivals this coda results from scattering off small scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic versus scattering attenuation most analyses of seismic coda to date have concentrated on s wave coda generated from lithospheric scattering for events recorded at local and regional distances here we examine the globally averaged vertical component 1 hz wavefield 10 range for earthquakes recorded in the iris farm archive from 1990 to 1999 we apply an envelopefunction stacking technique to image the average time distance behavior of the wavefield for both shallow 50 km and deep 500 km earthquakes unlike regional records our images are dominated by p and p coda owing to the large effect of attenuation on pp and s at high frequencies modelling our results is complicated by the need to include a variety of ray paths the likely contributions of multiple scattering and the possible importance of p to s and s to p scattering we adopt a stochastic particle based approach in which millions of seismic phonons are randomly sprayed from the source and tracked through the earth each phonon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer discontinuities are modelled by treating the energy normalized reflection and transmission coefficients as probabilities scattering probabilities and scattering angles are computed in a similar fashion assuming random velocity and density perturbations characterized by an exponential autocorrelation function intrinsic attenuation is included by reducing the energy contained in each particle as an appropriate function of traveltime we find that most scattering occurs in the lithosphere and upper mantle as previous results have indicated but that some lower mantle scattering is likely also required a model with 3 to 4 per cent rms velocity heterogeneity at 4 km scale length in the upper mantle and 0 5 per cent rms velocity heterogeneity at 8 km scale length in the lower mantle with intrinsic attenuation of q 450 above 200 km depth and q 2500 below 200 km provides a reasonable fit to both the shallow and deep earthquake observations although many trade offs exist between the scale length depth extent and strength of the heterogeneity

how seismic waves lose energy ocean basins record the life history of a tectonic plate its creation at a mid ocean ridge its thickening over time and its consumption at a subduction zone the movement of tectonic plates is possible because the lithosphere earth s stiff outermost shell slides on top of a weak asthenosphere despite its fundamental role in facilitating plate tectonics the nature of the lithosphere asthenosphere boundary is poorly understood the asthenosphere is on average warmer than the lithosphere but the temperature contrast alone may not provide the necessary viscosity reduction previous work has also proposed a dehydrated lithosphere and damp asthenosphere 1 and a solid lithosphere and partially molten asthenosphere 2 on page 1593 of this issue takeuchi et al 3 present an analysis of aftershocks of the 2011 tohoku earthquake and show how the attenuation of seismic waves has a different frequency response in the lithosphere versus the asthenosphere

folds and folding in the beardmore geraldton fold belt the beardmore geraldton fold belt forms a transitional crustal segment between the quetico and wabigoon subprovinces of the canadian shield bedding surfaces so and a bedding plane schistosity s1

the gutenberg discontinuity melt at the lithosphere asthenosphere boundary the lithosphere asthenosphere boundary lab beneath ocean basins separates the upper thermal boundary layer of rigid conductively cooling plates from the underlying ductile convecting mantle the origin of a seismic discontinuity associated with this interface known as the gutenberg discontinuity g remains enigmatic high frequency ss precursors sampling below the pacific plate intermittently detect the g as a sharp negative velocity contrast at 40 to 75 kilometer depth these observations lie near the depth of the lab in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base i propose that the g reflectivity is regionally enhanced by dynamical processes that produce melt including hot mantle upwellings small scale convection and fluid release during subduction

subprovince accretion tectonics in the south central superior province development of tectonic subprovinces as shear bounded granite greenstone and sediment dominated terranes during the late archaean is reviewed and interpreted from relationships between portions of

upper mantle and crustal seismic structure beneath the northwestern pacific basin using a seafloor borehole broadband seismometer and ocean bottom seismometers abstract in august 2000 a seafloor borehole broadband seismometer wp 2 was installed in the northwestern pacific basin and seismic experiments with ocean bottom seismometers obss a single channel seismic streamer and airguns were performed the wp 2 continued to record data autonomously and a total of 436 days of broadband seismic records were eventually retrieved reflecting the low noise environment many teleseismic events were recorded shallow seismic velocity models just below the obss were derived by the tau p p method for the airgun obs data and the single channel seismic data the crustal structure was estimated from the obss and wp 2 data by forward modeling using a two dimensional ray tracing method the results of the seismic surveys show that the crustal seismic structure around wp 2 is laterally homogeneous and corresponds to typical oceanic crust the uppermost mantle exhibits seismic anisotropy the velocity variations are about 5 for p waves and about 3 5 for s waves and the fast direction appears to be perpendicular to the magnetic lineations travel times of earthquakes recorded by the wp 2 and the previous seismological studies suggest that the lower part of the lithosphere has greater anisotropy than the uppermost mantle to explain late first arrivals from the earthquakes that occurred in the slow direction with epicentral distances between 1600 and 2200 km a low velocity zone below a depth of 30 km and a rapid increase of velocity at a depth of 210 km are inferred receiver function analysis of 16 events with a high signal to noise s n ratio from the wp 2 data was performed and discontinuities were estimated at depths of 416 and 666 km these discontinuity depths are consistent with those of the average beneath the ocean

seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of obs array records at 60 ma seafloor we analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 ma seafloor in the southeastward of tahiti island the south pacific by the tomographic investigation by seafloor array experiment for the society hotspot project we first obtained one dimensional shear wave velocity model beneath the array from average phase velocities of rayleigh waves at a broadband period range of 5 200 s the obtained model shows a large velocity reduction at depths between 40 and 80 km where the lithosphere asthenosphere boundary might exist we then estimated shear wave azimuthal anisotropy at depths of 20 100 km by measuring azimuthal dependence of phase velocities of rayleigh waves the obtained model shows peak to peak intensity of the azimuthal anisotropy of 2 4 with the fastest azimuth of nw se direction both in the lithosphere and asthenosphere this result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid ocean ridge but is roughly parallel to the ancient plate motion at depths of 20 60 km the fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60 100 km additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190 210 km

determination of intrinsic attenuation in the oceanic lithosphere asthenosphere system we recorded p and s waves traveling through the oceanic lithosphere asthenosphere system las using broadband ocean bottom seismometers in the northwest pacific and we quantitatively separated the intrinsic anelastic and extrinsic scattering attenuation effects on seismic wave propagation to directly infer the thermomechanical properties of the oceanic las the strong intrinsic attenuation in the asthenosphere obtained at higher frequency 3 hertz is comparable to that constrained at lower frequency 100 seconds by surface waves and suggests frequency independent anelasticity whereas the intrinsic attenuation in the lithosphere is frequency dependent this difference in frequency dependence indicates that the strong and broad peak dissipation recently observed in the laboratory exists only in the asthenosphere and provides new insight into what distinguishes the asthenosphere from the lithosphere

small scale heterogeneities in the oceanic lithosphere inferred from guided waves 1 we analyze seismic waveforms from deep focus earthquakes occurring in the subducting slab beneath japan recorded by broadband ocean bottom seismometers bbobss installed on the northwestern pacific ocean seafloor the data reveal waveforms with a low frequency direct p onset followed by large amplitude high frequency long duration po and so waves from the analysis of the bbobs records and a numerical finite difference method simulation of seismic wave propagation we elucidate the generation and propagation processes of such guided waves we demonstrate that the low frequency direct p and s waves propagate in the asthenosphere and that the following high frequency long duration po and so waves are developed by multiple forward scattering of p and s waves the scattering occurs due to laterally elongated heterogeneities in both the subducting and horizontal parts of the oceanic lithosphere with the apparent velocities vp 8 1 km s vs 4 6 km s being close to the velocities of oceanic lithosphere

broadening of seismogram envelopes in the randomly inhomogeneous lithosphere based on the parabolic approximation southeastern honshu japan analyzing horizontal component seismograms of small earthquakes with intermediate hypocentral distances in southeastern honshu japan we found that time widths of seismogram envelopes around direct s waves are much longer than source duration times estimated from their magnitudes the time lags of the maximum amplitude and the half maximum after the peak were measured from the onset of the direct s wave arrival from band pass filtered seismograms from 2 to 32 hz even though there is considerable scatter both the time lags are found to increase with increasing hypocentral distance up to 305 km we hypothesize that pulse shape broadens and the maximum amplitude is reduced after propagating through the random structure of the lithosphere the parabolic approximation theoretically predicts that the long wavelength component of velocity inhomogeneities compared with the wavelength of seismic waves produces diffraction fluctuations and makes seismogram envelopes broaden with increasing travel distance in the saturated regime supposing a gaussian autocorrelation function for the randomness and an empirical frequency dependent attenuation we propose a formula for the temporal change in the power spectral density of seismic waves applying this formula to the observed data we statistically evaluated the scale of random inhomogeneities the mean square fractional velocity fluctuation was estimated to be 10 3 times the correlation distance a in kilometers

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

unraveling the geologic history of the hemlo archean gold deposit superior province canada a u pb geochronological study the hemlo deposit is one of canada s largest gold camps gold is both structurally and lithologically controlled but shows unusual relationships relative to many other archean lode deposits field evidence suggests that gold was emplaced within mechanical and chemical traps along a jog in a major sinistral shear zone most likely during early formation of second generation g2 structures which represent the most intense phase of regional deformation mineralization was followed by amphibolite facies regional metamorphism u pb geochronology has been carried out on zircons from rocks associated with the deposit in order to determine the precise chronology of geologic events and their relationship to mineralization zircon populations are commonly affected by both inheritance and hydrothermal metamorphic crystal growth meaningful and precise age information can only be determined by precise single zircon dating on rocks from a well constrained structural and stratigraphic context gold is mostly found near the deformed contact between the moose lake quartz porphyry volcanic complex and stratigraphically underlying metasedimentary rocks these rocks have been folded by a camp scale noncylindrical f2 fold zircon from the moose lake quartz porphyry is dominated by inheritance and gives ages that extend from 2800 to 2690 ma metasedimentary rocks that underlie and overlie the moose lake porphyry show a much narrower range of provenance ages and are dominated by 2 690 to 2 693 million year old zircon two detrital zircons from a metasedimentary rock directly beneath the porphyry define an age of 2685 4 ma this should be an older age limit on volcanism deformation and gold mineralization the four youngest near concordant zircons from the cedar lake pluton define a magmatic age of 2680 1 ma this pluton was emplaced prior to or early during g2 deformation while emplacement of gold was most likely controlled by g2 structures therefore 2680 1 ma probably also represents an older age limit on gold mineralization a feldspar porphyry dike that cuts gold ore gives a younger age bracket of 2677 1 ma the results of zircon dating suggest that a volcanosedimentary complex was constructed in the area over the period of 2693 to 2685 ma this developed on earlier crust ca 2720 ma in age but there is evidence from detritus and xenocrysts for older rocks dating back to over 2800 ma deposition of supracrustal rocks was followed by granodiorite plutonism at 2680 1 ma then major deformation g2 and gold mineralization overprinted by amphibolite facies metamorphism previously dated at 2676 ma from titanite ages late tectonic sanukitoid suite plutons were emplaced at 2677 1 ma thus the primary gold forming event occurred near the beginning of a period of plutonism and crustal reworking that lasted at most a few million years this time association supports the view that granitoid magmas were the source of auriferous fluids plutons intruded into an actively deforming crust and were probably also the heat source for regional metamorphism the most likely environment was a tectonically active timiskaming type sedimentary basin similarly aged basins are found in the timmins area to the east and the shebandowan greenstone belt to the west

emplacement and deformation features of syntectonic leucocratic veins from rainy lake zone western superior province canada abstract networks of leucocratic veins are widespread in the rice bay redgut bay domain of the rainy lake zone at the wabigoon quetico subprovince boundary archean superior province canada vein geometries and their cross cutting relationships with host rocks are indicative of syntectonic emplacement within a regional late archean transpressional deformation event kenoran orogeny the relative folding and boudinage of the veins compared to their host rocks indicate that they were emplaced syntectonically with the d2 deformation semi quantitative strain and vorticity analyses confirm the syn d2 timing of the veins the observed structural features are interpreted as being formed during magma emplacement along dilatant fractures and subsequent solid state deformation under overall ductile deformation conditions the structures related to emplacement are better preserved in domains of low post emplacement strain and veins intruded into competent host rocks whereas the folds and boudins related to post emplacement deformation are well developed in veins intruded into relatively incompetent host rocks in domains of high strain the primary intrusive structures are modified or even locally obliterated due to increasing deformation it is envisaged that regional kinematics and the rheological state of host rocks are the main controls on the style of vein emplacement and deformation the variety of observed structures is considered the result of deformation and lithological heterogeneity rather than several distinct magmatic events

seismic structure of the upper mantle beneath the philippine sea from seafloor and land observation implications for mantle convection and magma genesis in the izu bonin mariana subduction zone abstract we have determined the three dimensional shear wave speed structure of the upper mantle in and around the philippine sea region using seismograms recorded by dense land based and long term broadband ocean bottom seismographic stations we used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of a finite frequency and ray bending the new ocean bottom data provided us with improved spatial resolution 300 km in the philippine sea region in the upper 120 km the shear wave speed structure is well correlated with seafloor age at depths greater than 160 km fast anomalies of the subducting pacific plate are clearly defined we also found slow speed anomalies beneath the kyushu palau ridge at depths greater than 120 km along the izu bonin ogasawara mariana arc we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc each anomaly has a width of 500 km moreover these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct sr nd and pb isotope ratios we suggest that each of the anomalies is a site of large scale flow of upper mantle into the mantle wedge and that each already contains a component from the adjacent subducting slab

seismic attenuation near the east pacific rise and the origin of the low velocity zone abstract low shear wave velocities beneath mid ocean ridges and in the low velocity zone beneath oceanic plates commonly have been attributed to the presence of melt or dissolved water but several recent studies have challenged that interpretation the alternative is that the anelastic effects of increasing temperature may cause the observed drop in velocity along with a predicted increase in attenuation we report the first measurements of surface wave attenuation within regional arrays of seismometers on the seafloor near the east pacific rise there is much less attenuation than is predicted by models in which the velocity is controlled solely by the direct elastic and anelastic effects of changing temperature suggesting that melt and water concentration do play an important role there also is somewhat less attenuation than is found in global studies we speculate that scattering from unresolved velocity heterogeneities contribute to the apparent attenuation in global studies

neoarchean trans subprovince sediment transport in southwestern superior province sedimentological geochemical and geochronological evidence at 2700 ma volcanism spawned by northward directed subduction under wabigoon subprovince canadian shield fed sediment into an adjacent fore arc basin trench system volcanically active island arcs to the south wawa subprovince were transported northward during this interval and at 2690 ma collided with the accretionary complex quetico subprovince metamorphosed sedimentary units intercalated with the ocean floor and island arc volcanic rocks of northern wawa subprovince provided the opportunity to amass data useful in extending the paleogeographic interpretation of the area this study examines the mckellar harbour formation a several kilometre thick assemblage of graded metasandstone beds outcropping to the north of lake superior its sedimentology geochemistry and detrital u pb zircon geochronology were investigated in conjunction with the same attributes of possible correlative units in the region similarities between the mckellar harbour formation and sedimentary rocks composing the fore

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

s type granites in the western superior province a marker of archean collision zones detailed field observations indicate that neoarchean s type granites were emplaced along and or proximal to some terrane tectonic boundary zones in the western superior province southeastern m

spatial distribution of scattering loss and intrinsic absorption of short period s waves in the lithosphere of japan on the basis of the multiple lapse time window analysis of hi net data summary propagation of short period s waves through the crust is strongly controlled by scattering loss and intrinsic absorption scattering loss controls the shape of the seismic envelope and the spatial distribution of the energy while intrinsic absorption produces an exponential decay as a function of time which is independent of the coordinates since both parameters have a different effect it is possible to compute them by means of the multiple lapse time window analysis mltwa in this paper high resolution maps of japan for both seismic s wave attenuation parameters are obtained by using the mltwa with data provided by the hi net seismic network the maps show strong regional variation of the parameters the variations depend mainly on the tectonic setting of each region and volcanism mechanisms in the scattering loss maps in the 1 2 hz map there is a very different behaviour between the northeast stronger scattering loss and the southwest much weaker scattering loss in the northeast of japan areas with strong scattering loss correspond mainly to the location of the volcanic arcs in southwest japan chugoku region and the west side of kii peninsula show strong scattering levels for the 4 8 hz and higher frequency bands in hokkaido strong scattering loss and intrinsic absorption can be observed for the lower frequency bands in the sendai plain we find strong scattering loss for all the frequency bands under all these areas low velocity anomalies have been detected in previous works in tohoku area the east side shows lower intrinsic attenuation than the west side this characteristic seems to extend to the southern part of hokkaido and to central japan in kanto area the values of intrinsic absorption are lower than in the surrounding areas but in the region around the itoigawa shizuoka tectonic line and in its western side strong absorption areas are observed the behaviour of each area for intrinsic absorption is similar to the behaviour observed in the q 1c maps the mltwa is based on the hypothesis of multiple isotropic scattering in a medium with the homogeneous distribution of scattering mean free path and intrinsic absorption although these hypotheses are very simple the results are very informative on the characteristics of each region and show the usefulness of studying the properties of scattered seismic waves in the understanding of the properties of the crust

a seismic reflection image for the base of a tectonic plate a high resolution image for the base of an oceanic plate that is subducting beneath north island new zealand reveals a channel which is interpreted as a sheared zone of ponded partial melts or volatiles this low viscosity channel decouples the plate from mantle flow beneath allowing plate tectonics to work tim stern and co authors use explosion generated seismic waves to form a high resolution image of the base of an oceanic plate that is subducting beneath north island new zealand they find an abrupt seismic wave speed transition less than a kilometre thick at a depth of 100 km dipping at an angle of 15 the presence of a parallel 10 km deeper reflection event shows that the decrease in compressional wave speed is confined to a channel at the base of the plate which the authors interpret as a sheared zone of ponded partial melts or volatiles they conclude that this is independent evidence for a low viscosity channel at the lithosphere asthenosphere boundary that decouples plates from mantle flow beneath and allows plate tectonics to take place plate tectonics successfully describes the surface of earth as a mosaic of moving lithospheric plates but it is not clear what happens at the base of the plates the lithosphere asthenosphere boundary lab the lab has been well imaged with converted teleseismic waves1 2 whose 10 40 kilometre wavelength controls the structural resolution here we use explosion generated seismic waves of about 0 5 kilometre wavelength to form a high resolution image for the base of an oceanic plate that is subducting beneath north island new zealand our 80 kilometre wide image is based on p wave reflections and shows an approximately 15 dipping abrupt seismic wave speed transition less than 1 kilometre thick at a depth of about 100 kilometres the boundary is parallel to the top of the plate and seismic attributes indicate a p wave speed decrease of at least 8 3 per cent across it a parallel reflection event approximately 10 kilometres deeper shows that the decrease in p wave speed is confined to a channel at the base of the plate which we interpret as a sheared zone of ponded partial melts or volatiles this is independent high resolution evidence for a low viscosity channel at the lab that decouples plates from mantle flow beneath and allows plate tectonics to work

constraints on the geodynamic evolution of the southern superior province u pb la icp ms analysis of detrital zircon in successor basins of the archean abitibi and pontiac subprovinces of ontario and quebec canada abstract to investigate the tectonic development of the southern superior province new u pb laser ablation inductively coupled mass spectrometry analysis of detrital zircon was conducted on graywacke samples from successor basins of the abitibi and pontiac subprovinces the three successor basin successions investigated are the 2690 2685 ma porcupine assemblage and the 2679 2669 ma timiskaming assemblage of the abitibi subprovince and the 2682 ma sedimentary rocks of the pontiac subprovince predominately neoarchean zircon grains 80 95 of the total zircon population reflect local sources in the abitibi and pontiac subprovinces the successor basins were deposited within 100 ma of the majority of the detrital zircon grains primary crystallization ages which is consistent with patterns observed at modern convergent margin settings porcupine assemblage samples contain 5 mesoarchean zircon while pontiac subprovince and timiskaming assemblage samples contain 18 and 13 respectively no local mesoarchean sources exist in the abitibi or pontiac subprovinces suggesting input from an adjacent hinterland the higher proportion of mesoarchean zircon in the timiskaming assemblage relative to the porcupine assemblage suggests that detritus from the hinterland was more prevalent during the later stages of collision probably as a result of progressive uplift and denudation of the hinterland the high proportion of mesoarchean zircon in the pontiac subprovince sedimentary rocks may reflect an additional mesoarchean source comparison to previously published geochronology data for the superior province indicates that the hinterland was to the northwest and is in part composed of the winnipeg river marmion and opatica subprovinces patterns of contractional deformation successor basin formation and felsic plutonism progressively young to the south in the southern superior province and are interpreted in terms of accretionary processes detrital transport across subprovince or terrane boundaries progressive hinterland emergence and foreland directed propagation of the deformational front recorded by successor basins is similar to that observed in modern style collisional or accretionary orogens

upper mantle q structure beneath old seafloor in the western pacific the pacific lithosphere anisotropy and thickness experiment ocean bottom seismometer array south of the shatsky rise allows imaging the attenuation structure of the upper mantle in the oldest parts of the pacific ocean the array consisted of eight seismometers with a lateral extent of 200 km by 600 km intermediate and deep focus earthquake sources in the mariana and izu bonin subduction zones provide paths that probe different depths beneath the seafloor acceleration spectra from the vertical component of p waves are inverted for the attenuation operator and corner frequency pn propagation indicates very high qp values in the lithosphere of order 1000 while path average qp values in the upper mantle are on the order of 400 the variety of source epicentral distances and depths enables us to deduce the vertical qp 1 structure of the upper mantle using the amplitude spectra directly in a weighted least squares problem that accounts for the effects of triplications in the traveltime curves we invert for qp 1 as a function of depth we demonstrate that qp is frequency dependent and adopt a power law dependence with coefficient 0 27 we resolve three distinct layers using a reference frequency of 1 hz with qp on the order of 145 at depths 100 250 km qp on the order of 350 at depths 250 410 km and very high qp with attenuation indistinguishable from zero for depths below 410 km the p waves have a component of scattering and thus qp 1 represents the maximum intrinsic attenuation beneath normal undisturbed oceanic lithosphere

metaconglomerate preserves evidence for kimberlite diamondiferous root and medium grade terrane of a pre 2 7ga southern superior protocraton article i nfo we studied heavy minerals extracted from a diamondiferous metaconglomerate that formed 2697 2701 ma in a successor basin within the michipicoten greenstone belt mgb of the wawa abitibi terrane southern superior craton the conglomerate is metamorphosed in the greenschist facies and contains mainly locally derived igneous mafic to felsic detritus but also very minor components of medium grade metamorphic minerals diamonds and paragenetic diamond indicator minerals comparison of the size distribution resorb tion and n aggregation of diamonds in nearby wawa lamprophyres and the metaconglomerate diamonds confirms that the latter were not derived from the proximal lamprophyric source the heavy minerals in the metaconglomerate include diopside olivine corundum chromite almandine pyrope with kelyphitic rims picroilmenite amphibole and anorthite low abundances of the heavy minerals several grains per 4 70 tons of the metaconglomerate are in part explained by their complete or partial replacement by the greenschist mineral assemblage detrital almandine and amphibole are inferred to originate in amphibolite facies rocks cr diopside olivine chromite and anorthite were sourced from mafic ultramafic anorthosite and chromitite bearing layered complexes mapped in the mgb the presence of pyrope with more than 6 wt cr2o3 suggests derivation from a cratonic root picroilmenite has compositions typical of kimberlite and unlike that of ultramafic lamprophyres and other unconventional diamondiferous volcanics the wawa metaconglomerate therefore should be considered analogous to the witwatersrand successor basin conglomerate in recording indirect evidence for archean kimberlites the tight localization of the diamondif erous conglomerate in time and space was controlled by a quick 3 ma erosion of the source kimberlite body the location of the kimberlite bearing n2 7 ga superior protocraton was inferred from the provenance of the metaconglomerate detrital material the clasts could have originated as close as the northern wawa abitibi terrane or as distant as the opatica terrane the pre 2 7 ga diamonondiferous cratonic root below the southern superior was removed in the neoarchean proterozoic the existence of archean kimberlites and deep diamondiferous roots below smaller pre 2 7 ga protocratons emphasizes the similarity of neoarchean and phanerozoic mantle processes

tomography reveals buoyant asthenosphere accumulating beneath the juan de fuca plate the boundary between earth s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth perhaps indicating a thin weak layer that may strongly influence plate motion dynamics the behavior of such a layer at subduction zones remains unexplored we present a tomographic model derived from on and offshore seismic experiments that reveals a strong low velocity feature beneath the subducting juan de fuca slab along the entire cascadia subduction zone through simple geodynamic arguments we propose that this low velocity feature is the accumulation of material from a thin weak buoyant layer present beneath the entire oceanic lithosphere the presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics

trench and slope basin deposits in an archean metasedimentary belt superior province canadian shield the identification of a late archean arc trench assemblage in northwestern ontario provides the opportunity to compare depositional systems developed in a precambrian convergent setting with cenozoic examples two types of sedimentary associations exist in the accretionary complex medium to thick bedded bouma a ab and abc felsic turbidites dominate the belt these are primarily organized into unstructured sequences and reflect deposition in a ramp like environment with multiple feed points supplying sediment from a forearc basin mafic turbidites with possible shallow water reworked intervals form isolated pods within the metasedimentary belt erosion of upthrust blocks of sedimentary strata containing ultramafic masses supplied this sediment to elevated slope basins these types of depositional systems are similar in many respects to those developed in cenozoic and holocene arc trench settings

geological evolution of the manitouwadge greenstone belt and wawa quetico subprovince boundary superior province ontario constrained by u pb zircon dates of supracrustal and plutonic rocks fifty million years of archean evolution is recorded in the manitouwadge greenstone belt and the wawa quetico boundary region from ca 2720 ma volcanism and subvolcanic plutonism associated with m

high resolution seismic constraints on flow dynamics in the oceanic asthenosphere convective flow in the mantle and the motions of tectonic plates produce deformation of earth s interior and the rock fabric produced by this deformation can be discerned using the anisotropy of the seismic wave speed this deformation is commonly inferred close to lithospheric boundaries beneath the ocean in the uppermost mantle including near seafloor spreading centres as new plates are formed via corner flow and within a weak asthenosphere that lubricates large scale plate driven flow and accommodates smaller scale convection seismic models of oceanic upper mantle differ as to the relative importance of these deformation processes seafloor spreading fabric is very strong just beneath the crust mantle boundary the mohorovicic discontinuity or moho at relatively local scales but at the global and ocean basin scales oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere here we use rayleigh waves recorded across an ocean bottom seismograph array in the central pacific ocean the nomelt experiment to provide unique localized constraints on seismic anisotropy within the oceanic lithosphere asthenosphere system in the middle of a plate we find that azimuthal anisotropy is strongest within the high seismic velocity lid with the fast direction coincident with seafloor spreading a minimum in the magnitude of azimuthal anisotropy occurs within the middle of the seismic low velocity zone and then increases with depth below the weakest portion of the asthenosphere at no depth does the fast direction correlate with the apparent plate motion our results suggest that the highest strain deformation in the shallow oceanic mantle occurs during corner flow at the ridge axis and via pressure driven or buoyancy driven flow within the asthenosphere shear associated with motion of the plate over the underlying asthenosphere if present is weak compared to these other processes

the structure degradation of a silty loess induced by long term water seepage abstract loess is a typical silt sized aeolian soil that is widely deposited and possesses a fragile soil structure susceptible to collapse upon wetting the instability of shallow loess slopes is well documented but deeper failures triggered by rainfall and irrigation can also occur in the case of long term seepage clay particles and salts have been observed to leach out of the loess slopes this paper presents a systematic laboratory study of the effects of water conditions on the mechanical behaviour of silty loess with isotropic compression and bender element tests carried out on natural and reconstituted loess recovered at 10 m depth from a site in china the results show that saturation causes reduction in both the compression yield stress and small strain stiffness of the soil and that after being exposed to seepage they decrease further the possible mechanism of structure degradation is investigated by means of microscopic and chemical analyses and it is found that seepage will cause the pore ions concentration to reduce with a loss of clay mineral bonding strength resulting in the detachment of silt particles from aggregates this work highlights the importance of monitoring the water outflow in the field at the loess slope edge since it may causes large deformation in the long term

broadband ocean bottom seismology broadband ocean bottom seismographs bbobss were first developed in the 1980s and have since been used to explore the structure of earth s interior beneath oceanic regions for example mid oceanic ridges subduction zones hot spots and the oceanic lithosphere asthenosphere boundary the best approach for broadband seismic observations in oceanic regions is that of a borehole seismic observatory attached to the ocean floor cable which is realized in several near coast networks considering the high cost of such networks there is still a need to develop autonomous bbobss with a better signal to noise ratio for temporary observations of focused scientific targets far from the coast

turbidites and iron formations beardmore geraldton ontario application of a combined ramp fan model to archaean clastic and chemical sedimentation sandy turbidites grain flows conglomeratic mass flows and oxide facies iron formation are present in the late archaean beardmore geraldton terrain a metasedimentary belt which extends for at least 80 km in an e w direction the marine portion of this basin contains four lithofacies associations la 1 thinbedded iron formation clastic sediment association this association represents a continuum of deposit types containing iron formation subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand silt 2 thin bedded turbidite dominated association these sediments consist mostly of silt sand beds which either show no vertical trends or thin and fine upwards over a few metres 3 medium bedded turbidite dominated association most of these sediments are medium to coarsegrained vertically unstructured sand sequences with occasional structured intervals 4 thick bedded association this is dominated by poorly graded sands up to 7 8 m in thickness sand beds are characterised by a thin basal zone of coarse sand and pebbles a large central interval containing a mixture of medium and coarse sand and a thin upper zone of fine sand silt the overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars we infer a nearshore break in slope locally with large channels la 4 extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans structured portions of la 3 however many deltaic surfaces probably were not tapped by major channels but merged downslope into a submarine ramp sediment was transported across the ramp by slump events and sheet like grain flows unstructured portions of la 3 iron formation and la 2 sediments probably accumulated both in upper mid ramp areas with low sediment delivery rates and distal to fan ramp successions as major streams on the braid plain changed position associated submarine channels and slump fed ramp deposits also would have shifted laterally this produced overlap of different facies associations in both the fan and ramp environments which may explain why observed vertical trends in bedding are limited to several metres we suggest that on narrow active archaean cratonic margins additional non regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity and seismic activity associated with the arc also contributed to the lack of vertically structured sequences turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation

shebandowan greenstone belt western superior province u pb ages tectonic implications and correlations the shebandowan greenstone belt of the western wawa subprovince consists of successions of volcanic and sedimentary rocks impinging onto the metasedimentary quetico subprovince to the north and cored by a batholithic complex to the south u pb geochronology using mainly zircon and titanite demonstrates a relatively rapid accretion of the greenstone belt in the late archean the oldest ages were obtained for 2750 ma tonalitic gneiss and sporadic 2830 2750 ma detrital or xenocrystic zircons a major phase of greenstone belt construction at 2720 ma formed ultramafic to felsic volcanic rocks and peridotitic gabbroic and anorthositic bodies probably in an extensional arc backarc setting these units are laterally correlative with volcanogenic massive sulfide bearing assemblages in the central wawa subprovince and probably with similar successions in northern minnesota the second major stage of felsic volcanism and plutonism at about 2695 ma was associated with d 1 compression causing thrusting imbrication and sedimentation this was succeeded by the deposition of an unconformable sequence of calc alkalic to alkalic volcanic and sedimentary rocks and emplacement of tonalitic to syenitic plutons at about 2690 ma transpressive deformation d 2 constrained between 2685 and 2680 ma caused the development of locally penetrative structures and deposition of clastic sedimentary packages the emplacement of plutons at 2683 2680 ma concluded the evolution of the greenstone belt titanite and rutile yield ages in the same range as zircon showing that the greenstone belt was situated at high crustal levels by 2680 ma and escaped the younger and intense deformational metamorphic events recorded in greenstone belts farther east in the central wawa subprovince as well as in the immediately adjacent migmatitic quetico subprovince to the north

po so synthetics for a variety of oceanic models and their implications for the structure of the oceanic lithosphere summary polso synthetic seismograms were computed for a variety of oceanic structures in order to model data from an t zb 5 7 earthquake recorded during the oss iv seismic experiment satisfactory modelling of polso waveshapes and frequency content was achieved with no lateral heterogeneity but with a small random vertical heterogeneity in the mantle superimposed on a mean velocity structure which is consistent with seismic refraction data the random heterogeneity in the p and s velocities appears to be about 2 per cent with a scale length of 5 km this kind of heterogeneity is easily achieved by varying slightly the major mineral components in either the peridotite or eclogite mantle mineralogy and can be another cause of observed upper mantle anisotropy using a step function source time behaviour our numerical modelling also indicates that in the upper mantle qp is roughly proportional to rising from about 450 at f 1 hz to about 800 at f 15 hz and that q is roughly proportional to rising from about 900 at f 1hz to about 1800 at f 15hz a slight decrease in q from these values in any small zone of the upper mantle reduces so amplitudes drastically and we believe this is why so is sometimes missing from observed seismograms the computed synthetic record sections were also used to examine the relation between intrinsic q and the apparent q inferred from data in earlier studies apparent q was found to be lower than intrinsic q below about 5hz and higher than intrinsic q above 5 hz when these differences are accounted for the mantle q s of this study agree with the mantle q s of butler et al 1987 to within 10 per cent there can no longer be any doubt that oceanic upper mantle q s are very large above 5 hz and that q is about twice as large as qp

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

polycrystal anelasticity at near solidus temperatures elasticity anelasticity and viscosity of polycrystalline aggregates were measured at the near solidus temperatures ranging from below to above the solidus temperature tm the result shows that the mechanical effects of the partial melting are twofold changes just below the solidus temperature in the absence of melt and changes at the solidus temperature due to the onset of partial melting as homologous temperature t tm increases from about 0 92 to 1 high frequency part of the attenuation spectrum significantly grows viscosity of the grain boundary diffusion creep is also reduced in this temperature range these changes are caused by a solid state mechanism and have a large amplitude even for the samples which can generate very small amounts of melt at the solidus temperature at the onset of melting further increases in the elastic anelastic and viscous compliances occur these changes are caused by the direct effects of the melt phase and are very small for the samples with very small melt fractions mechanical properties of a partially molten aggregate are determined by these twofold changes and when melt fraction is small the former changes are dominant we performed a parameterization of the present experimental results and applied the obtained empirical formula to the seismic tomographic data in the upper mantle the present model explains well the steep reduction of the seismic shear wave velocity in the oceanic lithosphere just below the solidus temperature

geochemical discrimination of clastic sedimentary rock sources abstract factors controlling the geochemistry of a clastic sedimentary rock can include 1 composition of source terrain 2 chemical weathering 3 hydraulic sorting 4 diagenesis 5 metamorphism and 6 hydrothermal alteration a linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present the use of graphical analysis of element pairs circumvents the problem chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements this results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner as chemically mobile element plots will produce a scatter of points this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history the constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin a second technique using sio2 plots has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements sio2 immobile element plots result in a linear arrangement of points extending towards either 0 or 100 sio2 the 0 intercept position occurs for elements with major mineral phases concentrated in sand the 100 intercept for those concentrated in clays plotting chemically mobile elements produces different patterns and this can be used to gain information on alteration and sorting history elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material this provides a powerful tool for deducing source terrain from sediment geochemistry techniques outlined above were tested on archaean metasandstones from superior province canada immobile element ratio diagrams for nb al ti and zr al ti indicate that a calc alkaline extrusive intrusive suite lying to the north of the study area was the source and not five other volcanic suites in the region this conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques

the age and provenance of metasedimentary rocks in the quetico subprovince ontario from single zircon analyses implications for archean sedimentation and tectonics in the superior province abstract u pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the quetico subprovince in northwest ontario secondary lead loss was virtually eliminated by choosing crack free grains and removing the outer surface using air abrasion thus the 207 pb 206 pb age can be considered a good approximation to the age of crystallization of each grain zircons in the quetico metasediments show a range of ages from 2698 3 ma to 3009 4 ma deposition and metamorphism of the metasediments is bracketed within a 10 5 ma time span between crystallization of the youngest detrital zircon grain and a late tectonic intrusion the blalock pluton which cuts the sediments and has a zircon age of 2688 4 ma the most easterly metasediment sample contains zircons with ages which cluster around 3007 6 ma which is within error of the age of a batholith dated in the adjacent marmion lake area of the wabigoon subprovince this along with the fact that most detrital zircons were only slightly abraded by sedimentary transport suggests that the source of many zircons may have been proximal from the north there is also a significant cluster of young zircon ages in the range 2698 2709 ma surprisingly only a few ages correspond to the time of most intense arc type felsic volcanism 2710 2745 ma in the main greenstone belt of the western wabigoon subprovince sedimentary deposition occurred at a time of extensive deformation in the adjacent wabigoon subprovince the pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers these data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events

predictions and observations for the oceanic lithosphere from s to p receiver functions and ss precursors the ocean lithosphere is classically described by the thermal half space cooling hsc or the plate models both characterized by a gradual transition to the asthenosphere beneath scattered waves find sharp seismic discontinuities beneath the oceans possibly from the base of the plate active source studies suggest sharp discontinuities from a melt channel we calculate synthetic s to p receiver functions and ss precursors for the hsc and plate models and also for channels we find that the hsc and plate model velocity gradients are too gradual to create interpretable scattered waves from the base of the plate subtle phases are predicted to follow a similar trend as observations flattening at older ages therefore the seismic discontinuities are probably caused by a thermally controlled process that can also explain their amplitude such as melting melt may coalesce in channels although channels 10 km thick should be resolvable by scattered wave imaging

seismic and electrical signatures of the lithosphere asthenosphere system of the normal oceanic mantle although plate tectonics started as a theory of the ocean basins nearly 50 years ago the mechanical details of how it works are still poorly known our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere asthenosphere system las beneath the ocean we review the existing observational constraints on the seismic and electrical properties of the las particularly for normal oceanic regions away from mid oceanic ridges hot spots and subduction zones where plate tectonics is expected to present its simplest form whereas a growing volume of seismic data on land has provided remarkable advances in large scale pictures seafloor observations have been shedding new light on essential details by combing through these observational constraints researchers are unveiling the nature of the enigmatic las future directions for large scale seafloor observations are also discussed

the global attenuation structure of the upper mantle 1 a large data set of fundamental mode rayleigh wave amplitudes is analyzed to derive a new global three dimensional model of shear wave attenuation in the upper mantle the amplitude observations span a range of periods between 50 and 250 s and are derived from earthquakes with mw 6 0 that occurred between 1993 and 2005 four separate factors may influence an amplitude anomaly intrinsic attenuation along the raypath elastic focusing effects along the raypath uncertainties in the strength of excitation and uncertainties in the response at the station in an earlier paper dalton and ekstrom 2006a dependence of the retrieved attenuation structure on these terms was shown to be significant and an approach was developed to invert the amplitudes simultaneously for each term the new three dimensional attenuation model qrfsi12 which is the subject of this paper is derived using this method the model contains large lateral variations in upper mantle attenuation 60 to 100 and exhibits strong agreement with surface tectonic features at depths shallower than 200 km at greater depth qrfsi12 is dominated by high attenuation in the southeastern pacific and eastern africa and low attenuation along many subduction zones in the western pacific resolution tests confirm that the change in pattern of attenuation above and below 200 km depth can be determined with confidence using the fundamental mode data set the new model is highly correlated with global models of shear wave velocity particularly in the uppermost mantle suggesting that the same factors may control both seismic attenuation and velocity in this depth range however forcing the lateral perturbations in attenuation to match those found in global velocity models decreases the data variance reduction which suggests that subtle differences between patterns of attenuation and velocity are robust

subduction zone guided waves and the heterogeneity structure of the subducted plate intensity anomalies in northern japan 1 the subducting pacific plate acts an efficient waveguide for high frequency signals and often produces anomalously large intensity on the eastern seaboard of northern japan during deep earthquakes the waveform records in the region of high intensity show a low frequency f 2 hz later arrivals with a long coda this behavior is not explained by a simple subduction zone model comprising a high velocity plate with low attenuation from the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the pacific subduction zone we demonstrate that the high frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0 5 km in thickness the standard deviation of wave speed fluctuations from the averaged background model is about 2 this new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness but low frequency waves with long wavelengths can easy tunnel through such lamina structure the result is frequency selective propagation characteristics with a faster low frequency phase followed by large and high frequency signals with very long coda a low wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high frequency coda three dimensional simulations using the earth simulator supercomputer for modeling of high frequency seismic wave propagation in the pacific subduction zone including plate heterogeneity clearly demonstrate the scattering waveguide effects for high frequency seismic waves traveling in the plate the region of large intensity for the heterogeneous model migrates away from the hypocenter into northern japan with an elongated zone along the pacific coast almost comparable to the observations from deep events in the pacific plate

high frequency seismic attenuation of oceanic p and s waves in the western pacific analyses of oceanic p and s data from an earthquake in the kuril islands which occurred along the trend of a 1500 km long ocean bottom hydrophone array deployment near wake island have yielded constraints on high frequency seismic attenuation in the western pacific spatial attenuation and q were determined for group velocities between 8 2 and 3 0 km s and for frequencies between 2 5 and 22 5 hz there is greater attenuation of oceanic p than oceanic s at all frequencies suggesting either the presence of significant bulk attenuation in the lithosphere or that oceanic p propagates as a leaky mode the apparent attenuation of oceanic p and s is distance dependent with lower attenuation beyond 30 the apparent q of oceanic p increases with frequency as q 0 7 from about 300 at 2 5 hz to 1500 at 17 5 hz the apparent q of oceanic s increases with frequency as q 1 1 from about 400 at 2 5 hz to 3000 at 22 5 hz

subaqueous sediment gravity flow deposits practical criteria for their field description and classification a new method for the description and classification of subaqueous sediment gravity flow deposits is proposed the classification scheme employs a convenient letter code and divides deposits individual beds into descriptive categories of two hierarchical levels facies and subfacies facies as the higher rank categories are distinguished chiefly on the basis of sediment type i e bed grain size texture a total of 13 facies have been distinguished g gravel gs gravel sand couplet gys gravelly sand s sand sm sand mud couplet ms mud sand couplet tm silt mud couplet mt mud silt couplet m mud mys muddy sand sym sandy mud myg muddy gravel gym gravelly mud subfacies as the lower rank categories are distinguished within the individual facies on the basis of the bed s internal structures the number of subfacies is unlimited and their labelling code includes particular facies symbols see above preceded by lower case letters denoting specific sedimentary structures and their vertical arrangement subfacies thus refer to the bed s intervals or divisions which are labelled as follows m massive unstratified and ungraded g graded unstratified and graded s plane stratified x cross stratified 1 parallel and or cross laminated q liquefied for example subfacies gsg graded to plane stratified gravel are gravel beds that have a lower graded interval and an upper plane stratified interval subfacies xg cross stratified gravel are gravel beds that are cross stratified throughout subfacies sls plane stratified to laminated sand are sand beds that have a lower plane stratified interval and an upper laminated parallel and or cross laminated interval

structure and development of an anastomosing network of ductile shear zones a detailed structural analysis of an anastomosing shear zone network in metagabbros from the archean rainy lake zone canada revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle 130 facing the main shortening direction a typology of shear zone intersections confluences and other features shows that all shears formed during a single deformation event with dextral and sinistral shears being active together or in an alternating fashion in spite of the difficulty of establishing a complete kinematic sequence early and late shears can be distinguished the final angular pattern between dextral and sinistral shears is not an original feature dextral and sinistral shears formed at nearly right angles and the angles progressively opened towards the extension direction as a result of increasing strain the obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge shaped domains of lesser deformed rock bounded by the shears through time there was an increasing prevalence of dextral shears over sinistral ones the studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear dominated dextral transpression to a higher vorticity dextral transpression

effects of partial melting on seismic velocity and attenuation a new insight from experiments the effects of partial melting on seismic velocity and attenuation have long been studied by focusing on the direct effects of melt such as the poroelastic effect the direct effects are generally very small for a very small melt fraction because geochemical studies have shown that the melt fraction during partial melting is very small 0 1 it is difficult to explain upper mantle low velocity regions by the direct effects of melt recent experimental studies by using a rock analog have captured a significant enhancement of polycrystal anelasticity just before partial melting in the absence of melt this newly recognized effect enables us to interpret seismological and geochemical observations consistently the new anelasticity model significantly changes the interpretation of upper mantle seismic structures this review summarizes the recent progress in the understanding of polycrystal anelasticity starting from a basic knowledge of linear anelasticity

evolution of an archean subprovince boundary a sedimentological and structural study of part of the wabigoon quetico boundary in northern ontario a prograded clastic wedge represented by formation to group scale units of metasedimentary strata in the beardmore geraldton terrane developed in a transitional zone at the southern margin of a