neoarchaean volcanic rocks central hearne supracrustal belt western churchill province canada geochemical and isotopic evidence supporting intra oceanic supra subduction zone extension abstract the kaminak segment of the central hearne supracrustal belt chsb western churchill province canada comprises a diverse sequence of neoarchaean volcanic and less abundant metasedimentary rocks that were emplaced as two assemblages between 2695 2711 ma and 2681 2686 ma respectively these are intruded by uncommon pre tectonic diorites and tonalites ca 2691 ma voluminous syn tectonic tonalites and granodiorites ca 2679 2686 ma and rare post tectonic potassic monzogranite and syenite ca 2659 2666 ma metasedimentary rocks include turbidites epiclastic tuffs minor iron formation oxide and rare volcanic conglomerates volcanic rocks of assemblage i include abundant pillowed and massive basalts to andesites with less common silicic lavas tuffs and volcaniclastic debris flow deposits assemblage ii contains voluminous silicic tuff and volcaniclastic debris flow deposits but fewer basaltic to andesitic flows the critical diagnostic feature of the chsb is the stratigraphic intercalation of compositionally diverse basaltic andesitic and felsic volcanic rocks throughout both assemblages mapping u pb geochronology and lithogeochemistry suggest that an initial morb like basaltic plain containing widespread intercalations of dacite to rhyolite was replaced at ca 2688 ma by a relatively short lived dacite to rhyolite dominated magmatic environment characterized by localized felsic volcanic centres and a bloom of 2686 2679 ma tonalitic to granodioritic plutons basaltic to andesitic rocks are dominated by iron rich tholeiites although the proportion of calc alkaline rocks increases with silica content felsic volcanic rocks all exhibit calc alkaline affinities the wide range in chemistry of the basaltic to andesitic rocks of both volcanic assemblages implies diverse mantle sources capable of generating voluminous morb with less common arc neb oib and rare babb like rocks similarly the variable composition of the felsic volcanic rocks indicates both anatexis of eclogitic to garnetiferous mafic crust and also extensive fractionation of mafic precursors in crustal magma chambers two geochemically distinct arc like mafic suites were generated through contamination of primary mantle derived magmas by juvenile ca 2700 ma silicic crust either in their mantle source or through assimilation upon ascent ndt isotopic data are comparable to contemporaneous depleted mantle with only local evidence for incorporation of older 2700 ma crust the chsb may have formed via tectono magmatic processes comparable to those of the eocene infant arcs of the sw pacific whereby the formation of a thick sequence of coeval intercalated compositionally diverse mantle and crustal derived rocks are generated in an extensional supra subduction setting the cessation of supra subduction zone extension at ca 2688 ma was followed by the short lived development of felsic volcanic edifices incipient arc the extrusion of mafic to felsic magmas and the concomitant intrusion of voluminous syn kinematic tonalitic plutons this accompanied a major change in the tectono magmatic setting accompanying and presumably following the termination of extensional supra subduction zone processes

off line acquisition of crustal reflection and refraction data a piggy back seismic reflection survey was carried out in 1984 as part of the dekorp 2 s profile the signal from explosive sources was digitally recorded on 120 channels the objectives were a to record crustal reflections at a wide range of incidence angles 5 85 km offset b to provide 3 d control of deep crustal reflectors seen on the main profile an important consideration throughout the fieldwork was to avoid interference with the main reflection survey this experiment showed that explosive charges as small as 5 90 kg may provide sufficient energy for observations from the steep angle to the wide angle range the off line position of the receivers allowed the reconstruction of steep angle reflection profiles parallel and perpendicular to the main reflection line a strong diffraction cluster on the main line could be correlated laterally and indications about the diffractors shape could be obtained

synthetic seismograms for a complex crustal model the algorithm of the original reflectivity method has been vectorized and implemented on a cdc cyber 205 computer calculation times are shortened by a factor of 20 to 30 compared with a general purpose computer with a capacity of several million floating point operations per second mflop the rapid calculation of synthetic seismograms for complex models high frequency sources and all offset ranges is a provision for modeling not only particular phases but the whole observed wavefield as an example we model refraction data of the black forest southwest germany and are able to derive rather tight constraints on the physical properties of the lower crust

paleoproterozoic basement cover infolding and thick skinned thrusting in hearne domain nunavut canada intracratonic response to trans hudson orogen enveloped by orogenic belts during the assembly of laurentia 2 0 1 7 ga the rae and hearne domains of the western churchill province experienced a complex sequence of tectonic events historically referred to as the hudsonian orogeny in hearne domain diverse thick skinned intraplate structures developed in response to accretionary processes in trans hudson orogen these structures include infolds of archean basement and paleoproterozoic cover and broken foreland thrusts in central hearne domain an intracratonic basin succession of continental to shallow marine strata hurwitz group 2 45 1 91 ga is exposed in outliers that constitute the keels of doubly plunging basement cover synclinoria defining a fold train with wavelengths of 35 45 km with the exception of local out of syncline shear zones basement and hurwitz group are welded formed under greenschist facies conditions the basement cover infolds are open concentric and commonly display complex structures in their interiors in the poorfish windy outlier southwestern hearne domain continental rift deposits of the kiyuk group unconformably overlie the hurwitz group in contrast to basement cover infolding to the north and east archean basement the hurwitz group and the kiyuk group are imbricated by thick skinned northwest vergent thrusts and folds thrusting in the poorfish windy belt was completed by 1815 10 ma as indicated by new u pb data from hydrothermal metamorphic titanite recovered from recrystallized cements at the top of the kiyuk group we interpret that shortening from shallow level basement cover infolding is balanced at depth by intrabasement faulting and distributed ductile strain and that the geometric link between central hearne basement cover infolding and poorfish windy thrusting is via intrabasement faults that cut to the surface the main controls on thick skinned deformation were the high competency of the lower hurwitz group previous thermal weakening basement heterogeneities and possibly shallow subduction preceding terminal collision in trans hudson orogen 2002 elsevier science b v all rights reserved

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

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

the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia correlation of three high p and low to intermediate t belts from nw and sw iberia allows recognition of a single terrane affected by late devonian continental subduction during the variscan orogeny this terrane is located in the lower part of the allochthonous complexes of nw iberia and in a lower position in most of the ossa morena zone that therefore emerges as another allochthonous complex of iberia the ossa morena complex together with the allochthonous complexes of nw iberia form a huge rootless nappe that was transported towards the variscan foreland during final assembly of pangea

the spi lake formation of the central hearne domain western churchill province canada an axial intracratonic continental tholeiite trough above the cogenetic kaminak dyke swarm the spi group is a package of plagioclase porphyritic amygdaloidal basalt flows and sandstone spi lake formation conformably overlain by coarse conglomerate and sandstone old boot formation these unconformably overlie the kaminak lake segment of the central hearne supracrustal belt western churchill province the spi group developed in a small isolated basin 8 km2 and is older than the paleoproterozoic hurwitz group our data demonstrate that the basalts of the spi lake formation are genetically related to the ca 2450 ma kaminak dyke swarm a speculation previously promoted by some workers based on field characteristics alone they are continental tholeiites with large ion lithophile and light rare earth element enriched multielement profiles with prominent high field strength element troughs the 143nd 144nd compositions overlap the contemporaneous chondritic bulk earth data all of these are features we infer to have resulted from mixing of melts from dominantly depleted mid ocean ridge basal

pg shingles preliminary results from the onshore glimpce refraction experiment abstract in 1986 a series of near vertical seismic reflection lines and coincident seismic refraction lines were recorded across the great lakes in the glimpce experiment from a refraction point of view this experiment was unique as this was the first time in north america that long range refraction lines up to 350 km were recorded with a trace spacing of less than 100 metres the source of energy was a large air gun some preliminary results of the nature of the onshore recordings made by the university of western ontario are presented in this paper observations of many of the in line profiles show that the character of the pg crustal phase is not simple but has a shingle like pattern which may be caused by wide angle reflection effects from numerous long and short reflectors within the crust theoretical studies indicate that the amplitudes of the wide angle reflected signals from thin high velocity layers are in general much larger than those from thin low velocity layers supercritical reflected waves are not possible from the top of a low velocity layer the closely spaced traces show in minute detail the manner in which one travel time branch fades and the next one arrives the results seem to indicate that the pg phase in our data set has little direct wave component but instead is made up of a whole series of shingled or fish scaled wide angle reflection components each with successively larger apparent velocities these results are in agreement with the findings of the near vertical reflection component of the experiment

large scale anisotropy within the crust in the basin and range province seismic reflection records of the 1986 nevada passcal seismic experiment in the roughly east west direction dip line show a highly reflective crust with a broad diffuse moho which is characterized at normal incidence by strong subparallel laterally discontinuous multicyclic events lacking observable diffractions recordings in the roughly north south direction strike line show a transparent crust with short scattered events and a strong continuous moho elastic finite difference modeling indicates that 1 effects due to the topography of the reflecting interfaces and or the existence of heterogeneities increase the travel times thus lowering the velocity estimates and 2 the seismic response of a layered structure can be disrupted from a strong laterally continuous event to a series of short multicyclic events by increasing the dimensions of the overlying heterogeneities finite difference modeling demonstrates that large scale anisotropy achieved by roughly east west alignment of strongly elongated lenses boudins located at mid and lower crustal depths can explain the observed differences in the seismic response along the perpendicular lines

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths

neoarchean arc magmatism and subsequent collisional orogenesis along the eastern rae domain western churchill province implications for the early growth of laurentia abstract the athabasca granulite terrane on the eastern margin of the rae domain of the western churchill province contains a well exposed record of neoarchean magmatism deformation and metamorphism circa 2 63 2 60 ga plutonic rocks range from 58 01 to 73 71 sio2 and vary from ferroan to magnesian they form shallow linear trends on modified alkali lime index diagram plotting largely in the calc alkaline field on tectonic discrimination diagrams all samples plot in the volcanic arc field samples are lree enriched and contain relatively steep negative slopes and flat depleted hree profiles when normalized to chondrites the entire neoarchean plutonic complex independent of lithology contains strong enrichments in the liles and pb and depletions in hfses monazite u th total pb results from a migmatitic paragneiss suggest that partial melting occurred at ca 2 57 ga within the chipman subdomain monazite results from the adjacent northwestern subdomain suggest that deformation occurred at ca 2 57 ga in sims and icp ms zircon u pb geochronology corroborates these conclusions although the region is dominated by paleoproterozoic localized shear zones internal to each subdomain is evidence of a widespread tectonothermal event during the neoarchean that involved arc plutonism crustal thickening lower crustal flow at pressures exceeding 1 0 gpa and extensive partial melting of the lower continental crust we interpret this sequence of magmatic deformational and anatectic processes to reflect the transition from a continental arc to a collisional tectonic setting the recognized series of events presented herein is likely correlative to the 2 60 2 50 ga macquoid orogen which has been recognized further to the northeast and likely parallels the eastern margin of the rae domain of the western churchill province these data support an important component of modern style plate tectonics during continental magmatism and accretion in the neoarchean

dual sources of ensimatic magmas hearne domain western churchill province nunavut canada neorchean infant arc processes abstract in the henik segment of the central hearne supracrustal belt neoarchean supracrustal rocks accumulated in deep water depositional systems that were well removed from a continental influence repeated in time and space these systems consisted of mafic lava plain slope and apron subenvironments with local emergent to near emergent felsic volcanic centres volcanic and plutonic rocks are subalkaline and range in composition from basalt to rhyolite consistent with field relationships the volcanic rocks are juvenile and with predominantly positive values that are independent of compositional variation display little evidence of contamination by continental crust however despite primary interlayering on scales of 10s of metres mafic and felsic rocks display markedly different rare earth element ree characteristics the mafic volcanic rocks are morb like and display a tholeiitic magma evolution trend flat ree patterns and positive values suggesting that they were derived from partial melting of depleted mantle in marked contrast the felsic rocks display steeply sloping ree patterns with significantly lower heavy ree abundances negative nb anomalies and positive zr and hf anomalies suggesting derivation from partial melting of slightly older basaltic crust this duality of magma sources supports a generalized model for the central hearne supracrustal belt that draws analogies from eocene intraoceanic infant arc processes described from the southwestern pacific ocean rather than from those normally associated with fully developed phanerozoic subduction

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

elastic wave velocities in rocks from a lower crustal section in southern calabria italy compressional and shear wave velocities vp and vs have been measured on 14 rock samples recovered from a high grade metamorphic terrane in the southern part of the calabrian massif which is considered to represent a continuous section of a former deep continental crust the experiments have been carried out in a cubic anvil apparatus at constant confining pressures up to 600 mpa and temperatures ranging from 20 to 700 c using the ultrasonic transmission technique at 600 mpa the compressional and shear wave velocities range from 6 33 km s 1 to 7 74 km s 1 and from 3 54 km s 1 to 4 30 km s 1 respectively variations in velocity are strongly related to variations in mineralogy both vp and vs increase with increasing amphibole pyroxene garnet and sillimanite content the range of velocities measured in the granulite facies rocks under conditions of lower crustal depth is similar to the range of seismic velocities for the crust beneath calabria importantly the metapelite rocks with intermediate silica and high alumina contents have in situ compressional and shear wave velocities which are equivalent to the mafic rocks or even significantly higher the detected high velocity layer at a depth between 15 and 20 km can be explained by a sequence of garnet cordierite sillimanite gneisses for which actual velocities between 6 6 and 7 6 km s 1 have been determined experimentally

the ferguson lake deposit an example of ni cu co pge mineralization emplaced in a back arc basin setting the world s largest ni cu platinum group element pge deposits are dominantly hosted by ultramafic rocks within continental extensional settings e g raglan voisey s bay resulting in a focus

temporal evolution of the neoarchean central hearne supracrustal belt rapid generation of juvenile crust in a suprasubduction zone setting abstract new u pb zircon ages are reported for 17 volcanic plutonic and sedimentary rocks from the neoarchean central hearne supracrustal belt in the western churchill province of the canadian shield the central hearne supracrustal belt historically referred to as the rankin ennadai greenstone belt is a major granite greenstone terrain with a strike length of over 700 km this study focused on the central and eastern segments of the belt in the kaminak lake area represents the first detailed chronological study of this extensive greenstone belt the geochronological data indicate rapid formation of crust between 2711 and 2667 ma rocks of the oldest assemblage i formed between 2711 and 2691 ma and consist of mixed tholeiitic and calc alkaline mafic and felsic submarine to subaerial volcanic rocks minor plutonic rocks group 1 are associated with this interval a younger assemblage ii of volcanic rock with significant calc alkaline plutonic rocks group 2 developed between 2686 and 2679 ma a period of deformation d1 locally separates the two assemblages major regional penetrative deformation and metamorphism developed during the latter stages of the plutonic event at ca 2680 ma an extensive belt of metasedimentary rocks and associated iron formation was deposited after 2681 ma based on ages of detrital zircon the youngest events include intrusion of 2666 4 1 1 ma post deformational granite group 3 2659 ma carbonatite can j earth sci 29 1992 896 and deposition of post 2660 ma conglomerates and arenites of possible timiskiming type there is no indication of contributions from mesoarchean crust either in inherited grains in igneous rocks or as detrital grains in sedimentary rocks although the evolutionary sequence has parallels to greenstone belts such as the abitibi belt in terms of the rock types and relative timing of events there are some differences we envision development of the belt in an extensional oceanic suprasubduction environment

relationship between the montgomery lake and hurwitz groups and stratigraphic revision of the lower hurwitz group district of keewatin recent detailed mapping has resulted in a possible solution to a long standing stratigraphic controversy as to whether the contact between the montgomery lake group age uncertain and the hurwitz group paleoproterozoic is an unconformity in its type area the montgomery lake group consists of mainly immature litharenite and lithic wacke and is separated from underlying archean greenstones and the overlying hurwitz group by angular unconformities in the padlei belt a unit previously referred to as montgomery lake sedimentary rocks montgomery lake group or map unit x consists of mainly submature to mature subarkose to quartz arenite with interbeds of pyritic auriferous and nonpyritic locally red quartz pebble conglomerate these rocks differ from the montgomery lake group in its type area but bear strong lithologic and stratigraphic resemblence to rocks in the lower hurwitz group furthermore interfingering between map unit x and hurwitz group lithotypes suggests that map unit x is an

the conrad discontinuity and the top of the reflective lower crust do they coincide abstract in order to investigate an often quoted coincidence of an intracrustal velocity jump the conrad discontinuity and lower crustal reflections found in tectonothermally young areas 54 reflection seismic profiles have been analysed and combined with 50 refraction seismic profiles adjacent to the reflection lines such a relationship is however contradicted by the observation that in some places a reflective lower crust is found while a velocity discontinuity is missing most of the compared data sets clearly demonstrate that the velocity jump if present is found more than 2 km below the uppermost reflections from the lower crust it is thus concluded that the two phenomena are not related occasionally both a reflective lower crust rlc and a conrad discontinuity can be observed in this case the differences in depth between the two can be explained by the structure of the velocity increase as in the example of the kapuskasing uplift in canada due to the overall depth dependent increase of seismic velocity a genetic correlation of high velocity and reflectivity is negligible this conclusion is supported by the observed highly reflective lower crust of the urach low velocity body by explaining the rlc with thermorheological arguments the upper limit of reflections simply appears as a thermal boundary depending only on temperature material and fluid content the theological models presented are explaining a rlc with and without the presence of a velocity discontinuity

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

estimation of interval velocities within the earth s crust abstract experiments to assess reliable interval velocities within the earth s crust have been carried out 1 1 using an enlarged spread length e g 23 km for the urach u1 profile 2 2 moving source and fixed geophone arrays up to 80 km in length e g dekorp 2 s and 3 3 using expanded spread configurations up to 100 km in length e g dekorp 4 the choice as to which of these methods will give the best and most reliable velocity depth data depends on the differentiation and the tectonic style of the crust which determine whether continuous near vertical and or quasi continuous wide angle reflectors from the intracrustal boundaries develop method 1 has been applied in an area of young tectonothermal activity miocene volcanism by means of the excellent data quality with phase correlations to depths of 20 km in the lower crust it has provided the most detailed velocity structure on the other hand with increasing tectonothermal age of the crust a general decrease in reflector length is observed and method 3 seems to provide the most reliable velocity depth information in this situation although only in a limited area

initiation of 2 45 2 1 ga intracratonic basin sedimentation of the hurwitz group keewatin hinterland northwest territories canada continental to shallow marine rocks of the hurwitz group exposed as outliers across the hearne province in northern canada were deposited between 2 45 and 2 11 ga on a 3 3 2 6 ga granite greenstone basement initiation of the hurwitz basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging during stage 1 polymictic conglomerate lower noomut formation was deposited in palaeovalleys cut into archaean basement and subarkose to quartz arenite quartz pebble conglomerate upper noomut formation was deposited on a low relief sand plain abundant first cycle quartz rich sandstones are attributed to intense humid climate weathering in low relief source areas and at sites of temporary storage as well as to repeated fluvial and eolian reworking interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen stage 2 padlei formation was marked by a climatic shift toward cold glacial conditions as indicated by dropstones greywacke till pellets and possible ikaite pseudomorphs in lacustrine rhythmites stage 3 kinga formation represents a return to wet warm conditions with fluvial eolian deposition of subarkose and quartz arenite on a low relief sand plain maguse member and deposition of supermature quartz arenite in a vast 100 000 km2 minimum shallow water lake or series of lakes whiterock member local chert beds at the top of the kinga formation hawk hill member formed by spring discharge due to regional gravity driven convective groundwater flow an intracratonic setting is implied by 1 a symmetric basin centred fill 2 absence of features indicating a shelf slope break 3 absence of rift rocks 4 a prolonged slow subsidence history 5 likely correlation with other continental to shallow marine deposits scattered across the keewatin hinterland and 6 positioning 1000 2000 km away from closest coeval passive margin sequences we speculate that hurwitz basin formed due to lithospheric stretching related to a large scale mantle upwelling during the protracted breakup of an earliest palaeoproterozoic supercontinent kenorland

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

petrogenesis of neoarchaean volcanic rocks of the macquoid supracrustal belt a back arc setting for the northwestern hearne subdomain western churchill province canada abstract rocks exposed in the macquoid gibson lakes region northwest hearne subdomain western churchill province canada comprise three major lithotectonic assemblages the principal volcanic belt the metasedimentary macquoid homocline and the cross bay plutonic complex neoarchaean supracrustal rocks of the belt range in age from 2700 ma crust likely did not occur ndt 2680 ma values for three specimens one from each of the arc like suites and one babb like basalt are slightly lower than the remainder suggesting very minor incorporation of slightly older crust these features imply that the processes that generated the macquoid supracrustal belt required simultaneous tapping of geochemically distinct mantle reservoirs with concomitant anatexis of sialic crust garnet stability field and fractionation of felsic magmas in upper crustal magma chambers shallow water deposition of abundant volcaniclastic rocks and semipelite along with minor conglomerate and quartzite was broadly contemporaneous with this magmatism we envisage a geodynamic setting characterized by tectonomagmatic processes similar to those of modern supra subduction zone back arc marginal basins such as the sea of japan therein an extensional back arc setting likely proximal to continental crust provides an explanation for a broad swath of diverse mantle derived rocks intercalated with less common felsic rocks as well as an abundance of immature clastic metasedimentary rocks

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

neoarchean convergent margin ni cu mineralization axis lake and nickel king ni cu deposits in the south rae craton of the canadian shield abstract the southern portion of the rae craton in the canadian shield is host to multiple ca 2 63 ga ni cu rich metamorphosed mafic intrusions with noritic noritic anorthositic and pyroxenitic compositions including those hosting the undeveloped axis lake and nickel king deposits found in northern saskatchewan and southeast northwest territories in order to understand the setting of these intrusions and their concomitant mineralization we investigated their geochemical and isotopic characteristics mineralized intrusions at axis lake and nickel king are metamorphosed up to granulite facies with local partial melting the intrusions preserve whole rock major element geochemistry consistent with a sub alkaline basaltic composition e g sio2 46 54 wt tio2 the nd isotope values of the axis lake intrusion ndt2630 0 3 to 2 1 are more juvenile and closer to the depleted mantle value relative to those of the nickel king deposit ndt2630 2 4 to 0 4 this agrees with their also distinctive th yb and nb yb distributions suggesting oceanic and continental arc like settings respectively the axis lake and nickel king nd isotope compositions are indistinguishable from other arc like 2 61 to 2 58 ga mafic to felsic intrusions and volcanic rocks recognized in the rae craton and the neighboring chesterfield block the age spatial distribution and geochemistry of the studied intrusions coincide with the initial stages of a recently defined 2 63 2 60 ga continental arc the roots of which are found in the south rae craton our data indicate that neoarchean intrusions with an arc like geochemistry may be a significant exploration target in the rae craton this broadens possible host suites beyond the established paleoproterozoic rift related rocks most commonly considered as the prime target for ni cu pge mineralization

lower crustal reflectivity patterns in wide angle seismic recordings many features of wide angle reflection data have been attributed to finely layered structure in the lower crust reflections with similar character however can be produced by small amplitude 2 d random velocity variations finite difference seismograms for a lower crustal model with isotropic random velocity variations match field data recorded at the black forest germany at source receiver offsets beyond about twice the target depth waves scattered from a random zone produce the appearance of horizontal layering in wide angle data lower crustal reflectivity patterns can thus be explained by models that have lateral and vertical random fluctutations rather than fine scale layering

isotopic evidence for neoarchean continuity across the snowbird tectonic zone western churchill province canada abstract the snowbird tectonic zone stz the ca 2800 km long boundary between rae and hearne domains of the western canadian shield remains of uncertain tectonic significance but is currently interpreted as a ca 1 9 ga suture the eastern rae domain is well studied and underlain by a 20 000 km2 region of high pressure granulites containing ca 2 60 ga arc rocks and granulite to amphibolite grade structures which formed at ca 2 57 ga d1 m1 and 1 90 ga d2 m2 the 1 85 ga legs lake shear zone juxtaposed the high p granulites of the eastern rae domain structurally above lower amphibolite to greenschist grade supracrustal gneisses of the hearne domain zircon u pb and hf isotopic analyses were acquired from igneous and metasedimentary rocks across the stz and rae hearne boundary to assess the validity of the suture model with specific focus given to the 5 km wide legs lake shear zone results indicate that both 2 60 2 55 ga magmatism and deformation continue unimpeded across the stz and rae hearne boundary furthermore hf isotopic data from igneous zircon from 2 60 ga plutonic rocks across the stz display little variation in ehf indicating that no major crustal break corresponds to the rae hearne boundary or any of the shear zones that parallel the stz on the ground older plutonic rocks show a greater variability in ehf which may indicate a disparate origin grade intensity and timing of paleoproterozoic features vary along the transect and display little continuity across the rae hearne domain boundary in conjunction with existing field and isotopic data we interpret that the rae and hearne domains were together by 2 60 ga and that the stz was host to repeated paleoproterozoic reactivation driven by the latter stages of taltson thelon orogenesis final indentation of the slave craton into the rae domain and accretionary processes on the southern periphery of the hearne domain from 1 93 to 1 84 ga furthermore assessing the viability of major shear zones within high grade terranes as potential sutures is complicated by the lateral and vertical heterogeneity of the crust juxtaposed by them

the scattering of seismic waves through a crust and upper mantle with random lateral and vertical inhomogeneities abstract the propagation of seismic waves through earth models with slightly random lateral and vertical inhomogeneities superimposed on one composed of layers with vertical velocity gradients was investigated the maximum deviation of velocity from a mean value at a given depth and a correlation distance derived from a two dimensional smoothing filter were two parameters used to vary the amplitude and size of the velocity anomalies the resulting models show short discontinuous reflectors scattered about at various depths throughout the model and are thus in agreement with many deep seismic reflection experiments on the other hand numerical experiments using ray tracing techniques showed that the effect of the lateral and vertical velocity anomalies is to scatter the energy and break up the continuous travel time lines from vertical gradient models into travel time segments with different slopes similar to those observed in many long range seismic refraction experiments and to those resulting from layering effects in the media many of the numerical experiments which modelled the random crust produced a pg segment and a p segment with an apparent conrad discontinuity at a depth of 10 20 km this apparent depth being related to the correlation distance when a seismic wave propagates through a heterogeneous earth the amount of its energy which is converted into scattered energy will be a function of the inhomogeneous characteristics of the medium through which it has passed if a ray passes through a homogeneous earth the energy arriving at an array station should be relatively coherent whereas if the ray encounters lateral and vertical inhomogeneities its energy will be incoherent and much more complex a series of coherency measurements done on array recordings of earthquakes at various distances showed that large lateral and vertical variations in complexity exist for different ray paths through the earth with the region below the 650 km discontinuity in the mantle tending to be much simpler than the region just below the lithosphere

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

a stochastic view of lower crustal fabric based on evidence from the ivrea zone despite its complicated history the ivrea zone is considered to be a representative surface exposure of extended continental crust we have digitized two standard 1 25 000 geological maps from this area and evaluated their structural statistics because of the subvertical orientation of the ivrea zone these maps can be considered as small scale cross sections through the lower continental crust the autocorrelation functions of the digitized lithologies measured from these maps show a clear self similar or fractal rather than a gaussian or deterministic trend we found that an anisotropic von karman correlation function with an aspect ratio around 4 and a hurst number of 0 3 corresponding to a fractal dimension of 2 7 matches the observed data our results represent an explicit confirmation of previous indirect evidence for the fractal nature of lithospheric heterogeneities and provide the means to construct realistic crustal scale seismic models of ivrea type lower continental crust

a comparison of the moho interpreted from gravity data and from deep seismic reflection data in the northern north sea summary we present the results of gravity modelling of the nsdp 84 deep seismic reflection profiles in the northern north sea the moho depths inferred from forward modelling of marine bouguer gravity anomalies from independent inversion of free air gravity anomalies derived from satellite altimetry and from deep seismic reflection profiling are in good mutual agreement this agreement suggests that the moho discontinuity interpreted from seismic reflection data corresponds to a major jump in density and therefore is likely to represent not only the geophysical but also the petrological boundary between the continental crust and the upper mantle in spite of the significant lateral heterogeneity of the crystalline basement below the northern north sea area as documented by seismic reflection profiling the average density contrast between the basement and the upper mantle is interpreted to show rather little variation this is not true for the deepest part of the viking graben where the nature of the lower crust and upper mantle may have been changed by magmatic processes accompanying the graben formation a relationship between the degree of crustal extension and the volume of magmatic intrusion into the lower crust is tentatively supported by our results