seismic reflection study of the precambrian crust of central minnesota a cocorp consortium for continental reflection profiling seismic reflection survey across the boundary zone between the late archean superior province granite greenstone terrane and older archean gneiss and granulite terrane of the minnesota river valley shows numerous north to northwest dipping reflection events throughout the crust a particularly prominent and continuous series of such events projects to the surface near the trace of a fault zone that had been previously identified as the boundary between these two terranes within the great lakes tectonic zone the moderate dips of these events about 30 their correlation with presumed faults near the surface their continuity over tens of kilometres and the persistence of these and other reflection zones throughout the crust suggest that these events correspond to fault zones probably thrusts they may have originated during late archean collision between the superior province crust and older continental crust to the south reflections are sparser and more concordant in the gneiss and granulite terrane than in the superior province there are several prominent reflection groups with gentle northerly dips in the middle and upper crust of the granulite and gneiss terrane their distribution appears to correlate with the sources of aeromagnetic and gravity anomalies that are part of a belt that extends west to central south dakota and that apparently reveal the position of the postulated archean suture the axes of deposition and deformation of proterozoic metasedimentary rocks are approximately parallel to and apparently coincide with the boundary between two archean terranes farther east in minnesota wisconsin and michigan but proterozoic penokean deformation has not been distinguished either in the study area or in the seismic sections the minnesota cocorp survey has demonstrated the capability of this seismic reflection method to reveal important but previously unsuspected structures in complexly deformed archean terranes this has provided structural evidence that the separate subprovinces of the north american archean crust might have been assembled by subduction related collision

seismic reflection profiling in the proterozoic arunta block central australia processing for testing models of tectonic evolution abstract the region extending from the northern amadeus basin to the northern arunta province is characterised by a large change in bouguer gravity 1400 s 2 abrupt changes in teleseismic travel time residuals and large thrust structures in which rocks from the lower crust are now exposed at the surface deep seismic reflection data other geophysical data and geological mapping within this region have contributed to the development of a crustal model that would result from thick skinned tectonic processes in which deformation occurred along moderately dipping thrusts or shear zones that extended from the surface down to at least the crust mantle boundary two important features of this model have emerged with the help of unconventional stacking techniques the first is a series of reflections from the redbank deformed zone that can be traced from surface outcrop to depths of more than 30 km the second is evidence of a fault extending through the crust and displacing the moho

the labradorian orogeny in the grenville province eastern labrador canada two minor intrusions within the hawke river terrane in the grenville province eastern labrador have upper intercept u pb ages as follows i for a megacrystic intermediate dyke and ii 1622 3 ma for a pegmatite dyke the upper intercepts date time of emplacement and the lower intercepts 508 and 320 ma respectively record the timing of early paleozoic events in the region a third sample a sillimanite bearing pelitic gneiss from the paradise metasedimentary gneiss belt in the hawke river terrane yielded a range of ages between 1647 and 1627 ma from seven single zircon analyses the zircons in the metasedimentary gneiss are interpreted as detrital because of their variation in morphology the range of ages obtained and their extreme variation in u content a detrital origin implies that the sedimentary protolith must have been deposited after 1627 ma and is therefore not the same age as morphologically similar metasedimentary gneiss that occurs as enclaves within pre 1670 ma migmatized quartz d

seismic images of the grenville orogen in ontario in 1990 lithoprobe acquired 240 km of seismic reflection data across parts of the central gneiss belt cgb and the central metasedimentary belt cmb within the western grenville province of southern ontario interpretation of these data in conjunction with geological constraints provided by bedrock mapping supports a model of northwest directed thrusting and crustal shortening for the grenville orogen within the cgb the parry sound shear zone is imaged as a 3 km wide zone of reflections dipping southeastward at 20 25 and soling at depths 7 km in the north and 3 km in the south beneath parry sound domain parry sound domain and the immediately adjacent domains are underlain by a gently southeast dipping reflective zone at 4 5 12 0 km depth interpreted as a detachment surface likely associated with the central britt shear zone this zone may have accommodated northwesterly transport of parry sound southern britt and northwestern rosseau domains over britt domain during grenvillian thrusting wit

pre 2 8 ga crust in the opatica gneiss belt a potential source of detrital zircons in the abitibi and pontiac subprovinces superior province canada ages of detrital zircons in post 2 7 ga metasedimentary rocks of the southern abitibi and northern pontiac subprovinces require that pre 2 8 ga crust was exposed during or following deformation of the 2 70 and 2 68 ga and the belt forms a high grade crystalline core to a south vergent mountain belt along the northern margin of the abitibi subprovince erosion of the opatica and northern abitibi during thrusting would shed detritus southward in advance of a south vergent thrust system the lac rodayer complex may be the enigmatic source of 2 8 ga detrital zircons within turbidites of the southern abitibi and northern pontiac subprovinces

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

deep seismic profiling in the nosop basin botswana cratons mobile belts and sedimentary basins abstract deep seismic reflection data from the kalahari of western botswana has been interpreted in terms of the tectonics of southern africa the 600 km of 12 fold data exhibit good quality reflections in the range 5 15 s twt the data outline the nosop basin a depositional centre with greater than 10 km of palaeozoic and younger sediments five prominent reflecting horizons are traced in the basement underlying the sediments on the basis of the seismic data the nosop is now interpreted as a single basin the previous multi basin interpretation of the aeromagnetic data esults from features within the magnetic basement the uppermost two basement units are interpreted as proterozoic sediments that rest on archean crust the seismic data show that the crust appears to thicken to the southeast as the kaapvaal craton is approached at the same time the sedimentary cover progressively thins suggesting that the western edge of the kaapvaal is an extended margin interpreted deep crustal shear zones indicate a major terrane boundary separating the proterozoic from the archean depths to basement inferred from aeromagnetic data are consistent with the depth to the seismic boundary associated with the top of the archean some of the aeromagnetic expression may be due to karoo volcanic intrusives in the basement a thrust belt in the northwest of the area now buried in the mid crust is correlated with the nearby ghanzi chobe belt and allows the mid crustal layering to be dated as late proterozoic

roots of the labradorian orogen in the grenville province in southeast labrador evidence from marine deep seismic reflection data marine deep seismic reflection data obtained as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe project from offshore southeast labrador are evaluated using geodynamic and gravity models and a tectonic interpretation is developed the geodynamic model explains the seismic reflection data in terms of doubly vergent tectonism resulting from southward directed underthrusting of mantle and lowermost crust middle and upper crustal seismic reflectors are considered to be proshears and retroshears and correlated through potential field data with major mylonite zones at terrane boundaries mapped onshore as there is little evidence of subsequent severe structural modification apart from strike parallel grenvillian dextral transpression which does not alter the crustal geometry along the line of the seismic transect the seismic reflectors are considered to represent labradorian structures inferred from onshore geochronological data to be mostly related to a 1665 ma labradorian accretionary event the southward directed underthrusting of the mantle and lowermost crust is consistent with existing models for southward labradorian subduction coast parallel gravity and magnetic anomalies are interpreted as an expression of a neoproterozoic to early phanerozoic extensional basin the existence of which had little effect on the underlying crystalline basement other than to downfault it relative to that exposed onshore

labradorian and grenvillian crustal evolution of the goose bay region labrador new u pb geochronological constraints major initial crust formation in the northeastern grenville province of labrador occurred during the labradorian orogeny 1710 1620 ma prior to re deformation in the grenvillian orogeny between 10

the wathaman batholith an early proterozoic continental arc in the trans hudson orogenic belt canada the wathaman batholith a major element of the trans hudson orogenic belt in northern saskatchewan and manitoba has an exposed strike length of 900 km this immense body separates dominantly archean rae hearne provinces to the northwest from early proterozoic arc related terranes of the orogen to the southeast major and trace element data for 42 samples from the saskatchewan part of the batholith are similar to those of phanerozoic arc batboliths for example sierra nevada and andean batholiths data plotted on trace element discrimination diagrams show that the batholith has continental arc affinities u pb zircon geochronology of five main phase granite samples suggests that most of the batholith formed between 1865 and 1850 ma and is coeval with smaller plutons of the la rouge arc the main deformation of the batholith is limited by the 1830 ma age of a late granite sheet that crosscuts folds and foliation of main phase granites the 15 to 25 m y crystallization span of the batholith is similar to the histories of many more recent continental magmatic arcs

eclogite facies shear zones deep crustal reflectors strongly foliated eclogite facies rocks in 30 150 m thick shear zones of caledonian age occur within a grenvillian garnet granulite facies gabbro anorthosite terrain in the bergen arcs of norway the predominant eclogite facies mineral assemblages in the shear zones are omphacite garnet zoisite kyanite in gabbroic anorthosite and omphacite garnet in gabbro eclogite facies rocks in shear zones are generally fine grained alternating omphacite garnet and kyanite clinozoisite rich layers define gneissic layering a strong shape preferred orientation of omphacite kyanite and white mica phengitic muscovite and or paragonite define the foliation the anorthositic eclogites show omphacite b axis maxima approximately normal to the foliation and c axis girdles within the foliation plane p wave velocities v determined at confining pressures to 600 mpa for samples from eciogite facies shear zones range from 8 3 to 8 5 km s l and anisotropy ranges from 1 to 7 the few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation the fast propagation direction lies within the c axis girdles parallel to foliation and the slow propagation direction is parallel to the b axis concentration normal to foliation in samples for which omphacite crystallographic preferred orientation was determined v for the granulite facies protoliths average about 7 5 km s high calculated reflection coefficients for these shear zones 0 04 0 14 indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high pressure conditions but escaped thermal reactivation

crustal history of the rae and hearne provinces southwestern canadian shield saskatchewan constraints from geochronologic and isotopic data abstract the cree lake zone hearne province and western granulite domain rae province in northern saskatchewan are contrasting parts of the variably reworked upper plate western hinterland of the paleoproterozoic trans hudson orogen their junction is defined by the virgin river shear zone a part of the more extensive snowbird tectonic zone subsurface extension of the 2 0 ga thelon orogen lies west of the western granulite domain we present u pb zircon ages sm nd and rb sr isotopic data from the cree lake zone and western granulite domain and from the junction granite which intrudes the virgin river shear zone data from the western granulite domain indicate that prevalent granulite facies metamorphism occurred between 2 3 and 2 0 ga but is imposed on archean protoliths 2 8 3 0 ga old low amphibolite facies retrogression which increases toward the virgin river shear zone probably postdates 2 1 ga data from the cree lake zone also yield archean protolith ages as old as 3 0 ga and some rocks yield interpreted high grade metamorphic overprint ages of 2 3 2 0 ga however two samples yield data indicating hudsonian high grade metamorphic overprint 1 87 to 1 81 ga the junction granite has a u pb zircon age of 1 82 0 30 ga but relict zircons and sm nd data show that it was derived by melting of archean crust high grade mid crustal thermal resetting in the western granulite domain may be coeval with the 1 9 2 0 ga thelon orogen although the data do not preclude an earlier event or events similar 2 3 2 1 ga reworking extends into the cree lake zone but high grade hudsonian 1 87 1 84 ga reworking is also evident in parts of this zone well to the west of the wollaston domain and may extend entirely across the zone to the virgin river shear zone relative importance of the thelon and hudsonian overprints in cree lake zone is unknown data from the junction granite and western granulite domain suggest that all recorded high strain events in the virgin river shear zone postdate 2 3 ga and are thus unrelated to deep crustal archean granulite facies mylonites in the tantato black lake segment of the snowbird tectonic zone north of the athabasca basin

seismic images of eclogites crustal scale extension and moho relief in the eastern grenville province quebec seismic images from a 250 km lithoprobe reflection profile in the interior of the eastern grenville province provide important new constraints on the crustal architecture of this part of the orogen prominent upper crustal reflections can be correlated with exposures of high pressure metamorphic rocks in the manicouagan shear belt providing the first direct evidence for eclogite reflectivity in the grenville province the eclogites are cut by major late grenvillian normal faults that penetrate the deep crust and preserve evidence of extensional collapse of the overthickened orogen north to south crustal thinning indicated by a change in moho reflection time from 16 to 13 s correlates well with regional bouguer gravity trends and is accompanied by a dramatic increase in the reflectivity of the lower crust these features underscore the significance of recently recognized along strike variations in tectonic style within the grenville province and point to the internal complexity that characterizes the root zones of collisional orogens

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

the deep structure of the grenville front a new perspective from western quebec the grenville front is a major tectonic boundary exposed on the canadian shield the front is defined as the northwestern limit of grenvillian deformation on the basis of geochronological and metamorphic data this boundary is also evident in some geophysical data sets the lithoprobe abitibi grenville transect crosses the grenville front near lac temiscamingue in western quebec a new 114 km long deep seismic reflection line shows a crustal structure quite different from that seen on previous surveys across the grenville front the archean foreland pontiac subprovince has a pattern of reflectivity similar to that seen in most of the superior province this pattern continues for some 30 km south of the surface exposure of the grenville front there is no evidence for a band of dipping reflectors truncating the horizontal pontiac reflectors in fact the leading edge of the grenville province is difficult to identify on the seismic section the moho is well defined and reveals that the crust thins under

contrasting tectonic styles in the northern grenville province implications for the dynamics of orogenic fronts in western labrador tectonic elements of the parautochthonous belt of the northern grenville orogen include a regional mid crustal thin skinned metamorphic fold thrust belt structurally overlain by a thick skinned thrust sheet with a high pressure metamorphic signature in central ontario the parautochthonous belt is composed of a major reverse sense shear zone along which deep crustal rocks have been uplifted and exhumed tectonic models for both areas involve crustal shortening by delamination in the lower crust and tectonically driven exhumation up a crustal scale ramp the fold thrust belt northwest of the ramp in labrador is a result of the development of critical topography in the thrust wedge at the top of the ramp and the accretion of thrust slices to the base of the wedge during its propagation onto the foreland the fold thrust belt is absent in ontario because syntectonic erosion of the thrust wedge prevented the development of critical topography

eclogitized gabbros from the eastern grenville province textures metamorphic context and implications undeformed lenses of olivine orthopyroxene gabbro belonging to the 1429 ma shabogamo intrusive suite parautochthonous belt of the eastern grenville province host the first occurrence of eclogite mineral assemblages reported from the grenville province these rocks preserve igneous textures overprinted by eclogite mineralogy and by textures related to subsequent unloading high pressure signatures include omphacite pseudomorphs after augite plagioclase replacement by garnet kyanite corundum and orthopyroxene omphacite garnet coronas between olivine and plagioclase transformations related to decompression include replacement of garnet kyanite corundum by spinel plagioclase growth of plagioclase collars between clinopyroxene and garnet and de jadeitization of omphacite clinopyroxenes with numerous plagioclase inclusions and garnets surrounded by clinopyroxene plagioclase symplectites in granoblastic mafic rocks from the same area are also evidence of decompression from high pressures these

lithoprobe crustal reflection cross section of the southern canadian cordillera 1 foreland thrust and fold belt to fraser river fault seismic reflection data from the south central canadian cordillera covering the interval from the easternmost metamorphic core complexes near arrow lakes to the fraser river fault system along the fraser river reveal a highly reflective and complex crust the base of the crustal reflectivity interpreted as the reflection moho is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12 0 s in the east to about 10 5 s in the west this virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust even though part of this area was subjected to large amounts of eocene extension the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the u s cordillera three structural culminations the monashee complex the vernon antiform and the central nicola horst are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during jurassic to eocene compression and then to have been modified and exposed during early and middle eocene extension an example of a compressional structure observed on the profiles is the monashee decollement which can be traced from the surface westward into the lower crust extension is manifested along a variety of normal faults including the regionally extensive low angle okanagan valley eagle river fault system moderately dipping faults such as the columbia river and slocan lake faults and high angle faults such as the quilchena creek and coldwater faults both jurassic to eocene compressional shear zones and early to middle eocene extensional shear zones are listric into the lower crust or moho under the intermontane belt

deep structure of an archaean greenstone terrane more than half of the continental crust was formed before about 2 500 million years ago1 yet there is still considerable uncertainty regarding the structure and evolution of the ancient archaean cratons2 7 modern analogues proposed for the setting of archaean greenstone belts include aborted intra cratonic rifts continental margins and various oceanic settings2 7 alternatively archaean tectonics may have been quite different from the plate tectonics we see today2 4 6 the abitibi belt in canada is the largest archaean greenstone ternane3 and has been a touchstone for many models because its surface geology is relatively well known8 10 it contains significant gold and base metal deposits10 and it is less deformed and has suffered less erosion than most archaean ter ranes here we present deep seismic reflection data from part of the abitibi belt which reveal extensive subhorizontal and gently dipping reflection zones truncated by steeply inclined discontinuities which project to major fault zones at the surface our preliminary model of the abitibi belt comprises a mostly juvenile magmatic crust similar to that exposed in the nearby kapuskasing uplift11 12 dissected and imbricated by deep penetrating discontinuities associated with thrust and strike slipe faulting in view of the similarity of greenstone belts worldwide2 7 13 our seismic sections provide new constraints on the general processes that have affected archaean metavolcanic terranes

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

geologic evolution of the late archean abitibi greenstone belt of canada the southern volcanic zone svz of the late archean abitibi belt of the superior province of canada is dominated by komatiitic to tholeiitic volcanic plateaus and large bimodal mafic felsic volcanic centers these volcanic rocks were erupted between about 2717 and 2700 ma in a series of rift basins that formed as a result of wrench fault tectonics they overlie and juxtapose a volcano plutonic assemblage characterized in the northern abitibi belt the age of the assemblage is about 2720 ma or older and it comprises basaltic to andesitic and dacitic subaqueous massive volcanics cored by comagmatic sills and layered anorthositic complexes and overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma a tectonic model is proposed in which the svz formed in a series of rift basins that dissected an earlier formed volcanic arc comparisons are made with rift environments that have been postulated for phanerozoic areas such as the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs

gravity flexure and the deep structure of the grenville front eastern quebec and labrador the grenville front in eastern quebec and southern labrador is characterized by a large negative bouguer gravity anomaly with flanking highs forward flexural modelling of this anomaly provides constraints on the deep structure of the eastern grenville the grenville front is assumed to mark the surface trace of a boundary between less dense crust to the northwest and more dense crust to the southeast with constant dip all the way to the moho an end load is applied to the northwestern plate to simulate the effect of orogenic and or subsurface loads but is then removed underneath a continuous plate to simulate the transient nature of the load this simple model yields good fits to the gravity anomaly provided the effective elastic thickness of the lower plate is less than about 30 km and the crustal boundary dips at about 20 the deep structure of the eastern grenville front may therefore be dominated by a major shallow dipping straight thrust the applied end loads for the best fitting models are o

results of a seismic reflection survey across the fault zone between the thompson nickel belt and the churchill tectonic province northern manitoba approximately 11 km of four fold common reflection point data have been recorded across a region that spans the contact fault zone between the thompson nickel belt and the churchill tectonic province from these data it is shown that the upper crust in this region and to a lesser extent the lower crust are characterized by numerous scattered events that originate from relatively small scale features within the thompson nickel belt two extensive and particularly high amplitude reflection zones at two way travel times of t 5 0 5 5 s and t 6 0 6 5 s are recorded with apparent northwesterly dips of 0 20 c these reflection zones which have a laminated character are truncated close to the faulted contact with the churchill province both the contact fault zone and the churchill province in this region have crustal sections that are relatively devoid of significant reflectors the evidence presented here confirms that the crustal section of the thompson nickel belt is fundamentally different from th

moho in the archean minnesota gneiss terrane fossil alteration or layered intrusion a seismic survey in the archean minnesota gneiss terrane 3 6 3 8 ga imaged a seismically variable discontinuous moho at both vertical incidence and wide angles the wide angle data show the gneiss terrane crust to be thick 49 km with a high average velocity 6 8 km s the vertical incidence data show a sharp 10 km long moho depth fresnel zone radius 2 8 km reflection that disappears laterally and data analysis suggests a 1 km moho zone with 100 m thick internal layering changes in the signal noise ratio across the survey or energy losses from reflectors within the crust do not affect the variability in the reflection moho therefore the reflection discontinuity is a geologic feature and lateral structural variation of the moho occurs over short tens of kilometres distances possible interpretations of the layered structure are 1 a layered intrusion 2 localized alteration to eclogite by intrusion of fluids along shear zones or zones of weakness or 3 fossil granulite facies structures preserved within an eclogitized lower crust a thin gradient or thin layered zone with small impedance contrasts or an abrupt velocity change are possible models for the regional moho the moho shows considerable lateral structural variation within an early archean high grade terrane

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

deep crustal structure of the precambrian basement beneath northern lake michigan midcontinent north america a deep seismic reflection profile in northern lake michigan midcontinent north america provides a cross section of the crust across the 1850 ma penokean orogen in which an early proterozoic island arc complex was deformed between two converging archean continental masses the island arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base above which it is variably reflective to transparent the archean terranes have thicker crust as much as 50 km the lower 20 25 km of which is strongly reflective abrupt offsets of moho near terrane boundaries may have been preserved since accretion during the penokean orogeny

assessing source rock maturity in frontier basins importance of time temperature and tectonics rate constants are presently not established well enough to allow accurate theoretical quantification of organic matter evolution instead a rapid empirical method for determining source rock maturity in relatively unexplored basins as a function of sediment age and thermal history is developed from a compilation of published worldwide data the oil threshold temperature may be empirically described in a non arrhenius relationship as a function of sediment age t 164 4 19 39 ln t where t is threshold temperature in c and t is time in 106 years the depth to the oil ceiling in a sedimentary basin is then doc 100 t ts dt dz where doc is depth in meters t is derived from the preceding relationship ts is the average surface temperature and dt dz is the geothermal gradient in c 100 m similarly the depth in meters to the oil floor dof may be calculated from dof 100 150 ts dt dz sedimentary geothermal gradients can vary in time and space these nonsteady state and or tectonically perturbed basin thermal histories are more complicated and must be considered in order to apply maturation models correctly for sedimentary basins floored by oceanic crust less than 120 m y old the geothermal gradient may be modeled as dt dz 113 k radicalt where k is the thermal conductivity basins in convergent tectonic settings have characteristic geothermal gradients that vary as a function of tectonic setting

seismic images of a grenvillian terrane boundary a series of gently dipping reflection zones extending to mid crustal depths is recorded by seismic data from lakes ontario and erie these prominent reflection zones define a broad complex of southeast dipping ductile thrust faults in the interior of the grenville orogen one major reflection zone provides the first image of a proposed grenvillian suture the listric boundary zone between allochthonous terranes of the central gneiss and central metasedimentary belts curvilinear bands of reflections that may represent ramp folds and ramp anticlines that originally formed in a deep crustal scale duplex abut several faults vertical stacking of some curvilinear features suggests coeval or later out of sequence faulting of imbricated and folded thrust sheets grenvillian structure reflections are overlain by a thin wedge shaped package of shallow dipping reflections that probably originates from sediments deposited in a local half graben developed during a period of post grenville extension this is the first seismic evidence for such extension in this region which could have occurred during terminal collapse of the grenville orogen or could have marked the beginning of pre appalachian continental rifting

three dimensional collisional structure of the trans hudson orogen canada abstract the three dimensional structure of the eastern trans hudson orogen tho part of a paleoproterozoic continent continent collision zone in central north america is revealed through a network of lithoprobe seismic reflection profiles the seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust e w profiles show strong e dipping reflections extending throughout the crust while n s profiles record events outlining antiformal and synformal structures this allows the identification of decollements that may have localized along pre existing structures e g possible basin bounding extensional faults and at major rheological boundaries e g basement cover contact upper middle crust transition the present topographic surface displays oblique crustal sections with 10 15 km of structural relief generated during post collisional intracontinental transpression of tho as a result of crustal scale folding and faulting

orogen parallel and transverse shearing in the opatica belt quebec implications for the structure of the abitibi subprovince the late archean opatica granitoid gneiss belt is situated within the northern abitibi subprovince along the northern margin of the abitibi greenstone belt approximately 200 km of structural section was mapped along three traverses within the previously unstudied opatica belt the earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing d1 late d1 deformation was concentrated into kilometre scale ductile fault zones typically with l s tectonite fabrics two families of lineations are associated with d1 indicating shearing both parallel and transverse to the east northeast trend of the belt lineations trending east northeast or northwest southeast tend to be dominant within domains separated by major fault zones in light of the abundant evidence for early north south compression documented throughout southern superior province including the abitibi greenstone belt d1 is interpreted in terms of mid crustal thrusting prob

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

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

paleo proterozoic thick skinned tectonics lithoprobe seismic reflection results from the eastern trans hudson orogen new seismic reflection data collected by lithoprobe across the trans hudson orogen manitoba and saskatchewan provide striking images of juvenile paleo proterozoic arc rocks flin flon and kisseynew belts juxtaposed against the deformed northwestern margin of the archean superior craton crustal imbrication on a scale imaged in few other orogens is observed within the flin flon belt where a package of shallowly east dipping reflections extends from the surface to 14 s these reflections are attributed to middle to lower crustal arc rocks that appear to have been stacked below a major detachment that underlies the upper crustal rocks of the flin flon belt surprisingly the seismic images show the juvenile arc rocks dipping moderately eastward beneath the craton in apparent contradiction to existing tectonic models geological and geochronological evidence suggest that the observed crustal imbrication probably reflects late collisional or postcollisional convergence rather than earlier oceanic subduction

lithoprobe new perspectives on crustal evolution lithoprobe is canada s national collaborative multidisciplinary earth science research program directed toward an enhanced understanding of how the north american continent evolved research in i

relative amplitude preserving processing for crustal seismic reflection data an example from western canada abstract most crustal scale seismic reflection profiles are processed using methods that preserve reflection geometries but not true relative amplitudes and therefore do not provide direct constraints on the magnitude of impedance contrasts at major crustal boundaries the effects of attenuation in the crust or the maximum time of signal penetration here we describe a new relative amplitude preserving rap processing scheme tailored for high fold land based acquisition systems that can be used to provide some of these constraints our processing philosophy employs a statistical treatment of amplitudes and is based on the following assumptions 1 temporal decay of recorded amplitudes is proportional to the product of zero offset geometrical spreading in a layered earth and attenuation based on a constant q model and 2 spatial variations in recorded amplitudes are surface consistent we apply this methodology to a profile from western canada where well logs from oil and gas exploration are available to calibrate observed seismic reflections using a spectral ratio method tailored for noisy data we obtain an estimate of 450 for apparent q in the sub phanerozoic crust signal penetration is frequency dependent and is estimated to decrease from 20 to 13 s over the frequency range of 10 to 40 hz application of our processing methodology leads to a stack section in which lower crustal reflectivity is much more prominent than reflectivity in the middle crust compared with conventional processing of the same data which yields a stack section characterized by conspicuous mid crustal reflections the p wave reflection coefficient for the moho is estimated to be about 0 1

a three dimensional perspective on the evolution of archaean crust lithoprobe seismic reflection images in the southwestern superior province in 1990 1991 the lithoprobe project completed 450 km of seismic reflection profiles across the late archaean crust of the southwestern superior province the results define a broad three fold division of crust upper crust in the abitibi greenstone belt is non reflective and is a 6 8 km veneer of volcanic and plutonic supracrustal rocks whereas in the sediment gneiss dominated pontiac subprovince upper crust comprises shallow northwest dipping turbidite sequences mid crust in both the abitibi and the pontiac subprovinces is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic intermediate gneisses the seismic signature of the northern abitibi greenstone belt may be represented in an exposed 25 km crustal section in the kapuskasing stuctural zone preliminary tectonic models based on the seismic data are consistent with a plate tectonic scenario involving oblique subduction and imbrication of sedimentary plutonic and volcanic sequences the northern abitibi supracrustal sequences either represent an allochthon or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the pontiac or quetico seismic velocities have yet to be defined however crustal thicknesses are relatively constant at 35 40 km the thinnest crust is adjacent to the grenville front where moho is very well defined

deep reflection seismics in the precambrian of sweden summary a total of 161 km of deep seismic profiles have been shot in the region one profile crosses the protogine zone in sw sweden over most of the profile short weak reflectors are seen the only area with a concentration of reflectors is in the upper two seconds between the two tectonic zones a nearly transparent area east of the protogine zone is interperted as a deep granite intrustion in the siljan impact structure where four profiles were shot the ne part of the structure is dominated by upper crustal high amplitude reflectors possible causes are discussed

proterozoic orogens of the northeastern canadian shield new information from the lithoprobe ecsoot crustal reflection seismic survey as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe acquired 1250 km of deep seismic reflection data along the coast of labrador and across ungava bay to image evidence of proterozoic crustal accretion to the archean nuclei of the nain and superior provinces of the canadian shield the relatively pristine archean crust of the nain province has low reflectivity and generally lacks systematic reflector orientations reworking of archean crust on the margins of the makkovik province has little effect on this weak signature in contrast the archean crust in the eastern churchill rae province appears to have been overprinted by a strongly developed whole crustal easterly dipping reflection fabric interpreted to result from proterozoic collision of the nain and superior provinces in the paired new quebec and torngat orogens juvenile proterozoic crust in the makkovik and grenville provinces also shows strong whole crustal dipping reflection fabrics interpretable as outw

review of recent results from continental deep seismic profiling in australia abstract the australian geological survey organisation regularly collects 450 500 km of onshore deep seismic reflection data and up to 4500 km offshore each year in australia these recordings are made in a wide range of tectonic provinces including in the last few years late palaeozoic mesozoic intracontinental and palaeozoic mesozoic cenozoic continental margin extensional basins moderately deformed palaeozoic transtensional basins and compressional fold belts and archaean greenstone terranes several of these provinces are major petroleum exploration provinces whereas others contain significant mineral deposits the primary purpose of the deep seismic profiling program is to resolve the tectonic history of the australian continent and thereby to encourage exploration for hydrocarbons and mineral resources in australia on the northwest australian continental margin major basin systems including the bonaparte basin formed as a result of complex interactions since the carboniferous involving episodes of extension followed by strike slip movements and inversion which reactivated both the primary extensional and ancient basement structures off southeastern australia basins such as the gippsland basin formed as part of a linked transtensional system related to movement on a common mid crustal detachment complex on continental australia the bowen basin in the northeast was deformed by thrust faults that root in a major e dipping detachment that flattens in the middle crust the cobar basin in the southeast is a case where the seismic data support a detachment model in which the upper plate displacement vector can be calculated by plate reconstructions linking the geometry of the detachment surface with that of the basin the greenstone terranes within the eastern goldfields region of western australia show crustal scale fault systems that are planar and steep dipping more in keeping with those interpreted in data from other precambrian provinces rather than those of the palaeozoic provinces

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere abstract crustal scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere the plane strain finite element model results provide indications of the styles of deformation to be expected in small compressional orogens in particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other the crust is modelled using coulomb plastic frictional and thermally activated power law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary surface denudation partial and total subduction of the crust and compositional layering are examined the results show the development of inclined step up shear zones which are a consequence of conjugate thrusting in regions of coulomb controlled rheology these zones link to sub horizontal shear zones which occur where composition and temperature render the crust viscously weak the model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation finally we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens

buried proterozoic foredeep under the western canada sedimentary basin electromagnetic studies of the precambrian basement beneath the western canada sedimentary basin in alberta indicate a narrow linear conductivity anomaly spatially correlated with a strong positive magnetic feature the red deer high the conductor is located below sedimentary cover near the top of the crystalline basement and has limited depth extent we propose that this anomalous feature represents graphitic metasedimentary rocks in the euxinic flysch facies of a proterozoic foredeep sequence the strong magneticanomalyresultsfromanassociatedironformationdepositedontheouterrampofthe foredeep this model explains the geophysical anomalies has analogues on the exposed shield and is consistent with the timing deformation history and known geology of the precambrian basement

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

paleoproterozoic collisional orogen beneath the western canada sedimentary basin imaged by lithoprobe crustal seismic reflection data exceptionally clear images of crustal structure of the canadian shield that underlies the western canada sedimentary basin beneath 3 5 2 2 km of phanerozoic sedimentary strata have been obtained on a seismic reflection profile acquired by lithoprobe the profile crosses tectonic domains of central alberta and delineates a major buried orogenic belt of paleoproterozoic 1 8 ga age associated with crustal scale thrust imbrication and deflections in the crust mantle boundary available geochronologic data suggest that crustal imbrication observed in the alberta basement was coeval with that documented in the trans hudson orogen to the east 1 80 1 83 ga and implies that a large region of continental crust extending 1000 km from the western superior province to the snowbird tectonic zone underwent considerable shortening during assembly of this part of the canadian shield

seismic imaging of extended crust with emphasis on the western united states understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust this is particularly true in areas where the last tectonic event was extensional such as the basin and range province of the western united states and much of western europe in these regions a characteristic reflective pattern has emerged whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective in the metamorphic core complex belt in the western united states where extension can be as much as an order of magnitude greater than in the more classic continental rift zones the lower crustal reflectivity thickens and rises yielding a picture of a crust that is reflective throughout synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across characterized by inter layered high and low velocities two geologic factors are interpreted as contributing to this layered character ductile strain responding to stress in the thermally weakened middle and lower crust and intrusive layering corresponding to injection of subhorizontal sheets of mantle derived magmas these two processes yield a variety of geologic structures including transposed compositional layering mylonitic ductile shear zones and intrusive mafic sheets all of which occur at the proper scales to cause the prominent reflectivity observed if metamorphic core complexes are representative of extended continental crust world wide then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world

seismic reflection images of high angle faults and linked detachments in the trans hudson orogen postcollisional 1 8 1 7 ga intracontinental deformation in the trans hudson orogen canada produced a series of orogen parallel high angle faults and folds in seismic reflection profiles the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4 8 s 12 24 km the folded and faulted upper part of the crust is underlain by laterally coherent shallow dipping reflections that are locally bounded by discrete highly reflective zones these zones are interpreted as detachments shear zones and can be traced from the upper to lower crust where some of them appear to pass into laterally continuous reflections that define the moho two distinct regimes of postcollisional crustal deformation are inferred from the seismic images high angle faulting and lateral block extrusion in the upper crust and low angle ductile shearing in the mid lower crust the surface geology indicates that the faults resulted in southwest orogen parallel extrusion of the orogen s internal zone relative to the bounding archean hearne and superior cratons faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction the seismic images suggest that the high angle fold fault structures are kinematically linked to low angle detachments represented by laterally coherent highly reflective zones the detachment shear zones are inferred to have a top to the southwest sense of shear associated with a subhorizontal northeast southwest extension direction parallel to those observed for 1 83 1 80 ga collisional shear zones exposed in major postcollisional fold culminations long lived orogen parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the superior and hearne margins

structure of the high grade opatica belt and adjacent low grade abitibi subprovince canada an archaean mountain front plutonic gneisses in the archaean opatica belt in canada record crustal scale wsw vergent i e longitudinal high temperature d1 ductile shearing similar structures occur in adjacent greenstone belts but only in narrow amphibolite facies zones near the base metavolcanics in the greenschist facies portions do not record penetrative d1 structures the greenstone belts are interpreted as allochthonous on the gneisses they form the uppermost d1 thrust sheets sse vergent i e approximately transverse d2 thrusting imbricated the opatica belt and formed a large antiform either a culmination or an antiformal stack in the southern opatica the imbricated opatica belt wedged under the northern part of the abitibi subprovince and created a tight syncline with nnw vergent d2 thrusts interpreted as backthrusts on its northern limb the opatica belt abitibi subprovince contact therefore resembles some phanerozic mountain fronts further southwards propagating imbrication of the crust beneath the abitibi subprovince created a s vergent foreland fold and thrust belt in the overlying greenstones high grade opatica gneisses and adjacent low grade abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an archaean mountain belt similar in many respects to phanerozoic examples

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

nontypical birps on the margin of the northern north sea the shet survey summary striking similarities in the reflectivity of the crust and upper mantle on birps profiles has led to the development of the typical birp a model seismic section for the british continental lithosphere the shet survey collected in the region of the shetland islands and the northern north sea fits the general pattern to a certain extent caledonian structures and devonian or younger basins are imaged in the otherwise acoustically transparent upper crust an unexpected and exciting feature imaged on shet is a short wavelength structure on the moho or abrupt moho offset beneath the strike slip walls boundary fault shet differs markedly from the swat typical birp however by showing a poorly reflective lower crust only a narrow zone 1 s at the base of the crust contains high amplitude reflections the shet survey therefore highlights the wide variation in lower crustal reflectivity within the total birps data set rather than the similarities

deep structure beneath lake ontario crustal scale grenville subdivisions lake ontario marine seismic data reveal major grenville crustal subdivisions beneath central and southern lake ontario separated by interpreted shear zones that extend to the lower crust a shear zone bounded transition between the elzevir and frontenac terranes exposed north of lake ontario is linked to a seismically defined shear zone beneath central lake ontario by prominent aeromagnetic and gravity anomalies easterly dipping wide angle reflections and fractures in paleozoic strata we suggest the central lake ontario zone represents crustal scale deformation along an elzevir frontenac boundary zone that extends from outcrop to the south shore of lake ontario seismic images from lake ontario and the exposed western central metasedimentary belt are dominated by crustal scale shear zones and reflection geometries featuring arcuate reflections truncated at their bases by apparent east dipping linear reflections the images show that zones analogous to the interpreted grenville front tectonic zone are al