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

seismic evidence for late proterozoic orogenic structures below the phanerozoic sedimentary cover in the kattegat area sw scandinavia 1 commercial seismic reflection profiles from sw scandinavia image sveconorwegian orogenic structures in the late precambrian basement beneath the thin phanerozoic cover we interpret 1 pronounced west to wnw dipping reflections as sveconorwegian localized ductile shear which overprint 2 weaker east dipping reflections interpreted as gothian or early sveconorwegian compressional structures folded and laminar seismic fabrics may represent f and c type crystalline thrust sheets emplaced during the gothian and sveconorwegian events some of the dipping reflections are observed to sole into 3 an interval of subhorizontal reflections at 2 5 4 5 s two way travel time which we interpret as a fossil ductile to brittle transition that was unroofed during the gothian orogeny 4 nw se striking thrusts cutting the sveconorwegian structures may indicate a shift in the late sveconorwegian stress field they may represent an early stage in the subsequent phanerozoic basin evolution in the danish area

applications of seismic pattern recognition and gravity inversion techniques to obtain enhanced subsurface images of the earth s crust under the central metasedimentary belt grenville province ontario summary project lithoprobe s abitibi grenville transect seismic reflection lines 32 and 33 traverse the exposed central metasedimentary belt cmb located in the grenville province of the precambrian shield of canada in southern ontario these seismic lines image a zone with a protracted deformational history spanning more than 300 myr detailed examination of the commercially processed stacked sections reveals a number of significant deficiencies in some important areas the image quality in these zones of reduced coherency needs to be enhanced to examine specific features and their relation to the surface geology examination of near vertical seismic data from lines 32 and 33 revealed that the signal to noise ratio was not improved by stacking due to misalignment of signals even after static normal moveout corrections and residual static corrections the presumed reason is that reflected seismic energy following long ray paths in heterogeneous media suffers from relative advances and delays in its propagation and hence arrives at slightly different times at the receivers tending to be poorly aligned relative to its theoretical traveltime curves a pattern recognition pr method for signal enhancement followed by energy stacking in moving time windows was used in this study to improve the images in spite of misalignments reprocessing has refined the geometry of the reflection profiles the objective of this paper is to use enhanced images of the seismic reflection data obtained by using a pr approach together with gravity data using 2 5 d forward and 3 d inversion routines to give an improved model of subsurface structure in the vicinity of lines 32 and 33 line 32 is dominated by southeast dipping reflectors soling into the lower crust the listric geometry of the strong reflection packages of the cmb boundary thrust zone is interpreted to represent a crustal scale ramp flat geometry that accommodated northwest directed tectonic transport of the cmb this interpretation is also supported by the gravity data

petrology and age of the a type mulock granite batholith northern grenville province ontario abstract the mulock granite batholith is exposed over 530 km2 in the northern part of the grenville province 25 km north of north bay ontario the batholith has a u pb zircon emplacement age of 1244 4 3 ma two regional tectonic events affected the batholith 1 moderate grade regional metamorphism and 2 shearing and mylonitization related to the grenville front tectonic zone which slices across the northern third of the batholith a suite of fine to coarse grained pink gneissic granites containing accessory fluorite and with colour indices between 1 and 27 dominate the batholith and are cut by abundant aplite dikes anorthosite dikes cut the granites and aplite in the central part of the batholith the granites were originally alkali feldspar granite and syenogranite and the coarser grained varieties were originally hypersolvus the suite is mainly metaluminous to marginally peraluminous peralkaline varieties are rare most of the suite is subalkalic but subordinate alkalic granites enriched in biotite and ferrohastingsite are early phases the suite shows all the major and trace element compositions and elemental ratios diagnostic of a type granite suites metamorphism altered the primary distribution of some of the trace elements particulary rb and f the granites crystallized under relatively low pressures

the origin of fold abutments in the map pattern of the westernmost grenville province central ontario abstract the westernmost part of the proterozoic grenville orogen widely interpreted as an aggregate of structural domains and subdomains has been subjected to superimposed disharmonic buckling on the scale of kilometres the domain boundaries are characterized by abutments of upright to overturned folds attributed by others to thrust induced truncation of inherited structures the present study shows that abutments on geological maps are fold termination lines which mark narrow zones of adjustment flow caused by buckle strain heterogeneity on the kilometre scale such adjustment zones are ductile equivalents of fold decollement surfaces in the brittle upper crust and are normally characterized by high strain gradients prominent adjustment zones may be located within strain softened rocks at domain boundaries coincident with major ductile thrusts

a cocorp deep reflection profile across the buried reelfoot rift south central united states abstract a cocorp deep reflection survey across the northern mississippi embayment reveals essential features of the late precambrian early paleozoic reelfoot rift stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness the base ranging from 2 0 to a maximum of 3 5 s twt 4 to 8 km depth in contrast the base of phanerozoic the sedimentary section immediately east of the rift is at 1 4 s 2 8 km depth correlation with a basement penetrating well indicates that the base of the upper cretaceous holocene mississippi embayment supergroup base of the cambrian ordovician knox group and the lower upper cambrian bonneterre formation are basin wide seismic stratigraphic markers clastic sedimentary strata of the lamotte formation which immediately overlie precambrian basement show clear angular discordance with the overlying subhorizontal bonneterre formation indicating that the base of the bonneterre marks the transition from active extension to passive subsidence within the reelfoot rift palinspastic restoration of the basement suggests relatively modest extension associated with rift formation 17 the blytheville arch an axial antiformal feature as well as a number of lesser structures indicative of multiple episodes of fault reactivation are also evident on the cocorp profile the crystalline basement beneath the reelfoot rift is relatively devoid of reflections down to 5 0 s 12 15 km depth beyond which it passes gradationally into a complexly reflective diffractive middle and lower crust the lower crust however does not exhibit the distinctly laminated character commonly observed in mesozoic cenozoic extensional provinces the crust mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12 13 s 40 km depth followed by an unreflective zone which in turn is followed by a distinct horizontal reflection at 14 5 s the latter is not observed on existing profiles outside the rift at present it is not possible to determine strictly whether the deep horizon represents moho or alternatively a discrete feature sill within the uppermost mantle however its spatial association with the reelfoot rift suggests that the two are genetically related regional relations together with the existence of dipping fabric in the deep crust beneath the rift suggest the possibility that the reelfoot rift formed along the southern extension of the circa 1 ga grenville front

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

deep crustal structure beneath the central and inner west carpathians abstract the 150 km long deep seismic transect 2t crossing the central part of the slovakian west carpathians has revealed an important whole crustal seismic suture zone dipping to the south this zone is dominated by six large and several minor reflection events because of our good knowledge of the surface geology and a large number of comparative studies of the eastern alps it is clear that the upper reflections are due to overthrust sheets that originated during compression of the apulian passive margin tectonically deeper reflections are interpreted as being due to ligurian oceanic igneous and sedimentary rocks sheared in ductile domain during continental collision with the brianconnais passive margin and continental masses north of the central apulian west carpathians cwc behind a significant strike slip fault are the cenozoic structures of the pieniny klippen belt and the magura rhenodanubian flysch south of the cwc the inner tethyan west carpathians iwc are thrust over the cwc the iwc originated during the late jurassic early cretaceous closure of the tethyan ocean and the seismic image of the iwc is different from the highly reflective cwc suture these three pre neogene nappe systems amalgamated as one overthrust system during the neogene subduction of the moldavian sea as the moldavian accretionary wedge the vepor gemer block of the west carpathians where the cretaceous suture is observed has been untouched by any major cenozoic deformation and has remained relatively rigid this unique structure therefore enables us to observe seismically the cretaceous suture zone which elsewhere in the european alpine belt has been almost completely destroyed more general data about the beginning of subduction and subsequent collision processes are presented significant compression and partial subduction of the apulian upper plate passive margin lasted from the hauterivian to the beginning of the turonian for about 25 30 ma subduction of the ligurian ocean occurred from turonian to campanian times after the early campanian the collision of the apulian and brianconnais plates took place

dating the source of lower crystal reflectivity using birps deep seismic profiles across the lapetus suture abstract six birps deep seismic reflection profiles around the british isles that cross the trend of the lapetus suture allow a geophysical definition of the suture zone for 900 km along its strike and demonstrate that specific reflective structures have survived since the mid palaeozoic caledonian orogen in an area northeast of england spanning 70 km along caledonian strike and 200 km across strike three profiles show very considerable similarities in the reflectivity patterns of the lower crust when these similarities are mapped the boundaries separating regions of lower crust with the same reflective character run parallel to caledonian strike this strongly suggests that the distribution of reflectivity in the lower crust in this area is directly controlled by caledonian structures geologic models for the area imply that many of the reflectors themselves may also be of caledonian age however though an age for the imposition of patterns of reflectivity can be inferred the age of an individual reflector cannot be determined unless it can be traced to outcrop or unless there is independent evidence as to the physical nature of the reflector

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

crustal structure of the central graben of the midcontinent rift beneath lake superior abstract travel times and amplitudes of refracted and reflected arrivals observed during glimpce great lakes international multidisciplinary program on crustal evolution from the midcontinent rift in lake superior have been modeled using two dimensional ray tracing techniques forward modeling was used to iteratively refine an initial velocity model that was constructed from results of tau sum analysis of the travel time data at each station combined with information from a coincident common midpoint cmp reflection profile when converted to time the resulting model agrees quite well with the cmp reflection profile within the central graben of the midcontinent rift approximately 2 km of sedimentary rock 2 8 4 6 km s overlie an 8 km thick sequence of volcanics and interflow sediments 5 0 6 5 km s within the central rift graben observed on the reflection data beneath this sequence is a 6 8 km thick sequence of 6 6 7 0 km s material interpreted to represent metamorphosed volcanics the velocity of the material at the base of the rift graben is not well constrained approx 7 0 7 2 km s but probably comprises an additional 10 12 km of metavolcanic rocks and intrusions that extend to the base of the graben as imaged on the coincident seismic reflection profile the central graben is ranked by reverse faults the isle royal and keweenawan faults that probably represent normal faults associated with graben formation that were reactivated during the late precambrian a 50 100 ms time advance anomaly associated with the isle royal fault is observed at every station this anomaly has been modeled as shallow high velocity blocks located directly beneath the fault the blocks correlate well with the walls of a steep sided bathymetric trough and are believed to represent highly indurated upper keweenawan sediments resulting from hydrothermal alteration a marked decrease in the apparent velocity and amplitude of the first arrivals is observed on reversed sections at offsets exceeding 100 km and has been modeled as pre rift continental crustal rocks with a velocity of about 6 5 km s adjacent to the 7 0 km s material in the graben calculation of the gravity response of the seismic model demonstrates that the gravity high centered over the rift can be entirely attributed to high density rocks occupying the central half graben imaged on the cmp profile wide angle reflections from about 15 30 km depth beneath the flanks of the graben indicate the presence of velocity discontinuities that may represent rift related detachment surfaces and or pre rift structures within the pre rift crust additional wide angle reflections define the top and bottom of high velocity lower crustal layer the high velocity lower crustal layer is thickest beneath the rift graben and is interpreted to represent magmatic material underplated to the crust during rifting as discussed by trehu et al 1991 total crustal thickness beneath the lake varies from about 55 km beneath the graben to about 35 km beneath the southern shore and 40 km beneath the northern shore

geoscience impact a synthesis of studies of the sudbury structure geophysical probing results are synthesized into a three dimensional framework necessary for understanding the genesis of the sudbury structure based primarily on seismic reflection results centred on the sudbury igneous complex remnants of crustal melting from a catastrophic meteorite impact are superimposed on the juxtaposition of mid crustal rocks exhumed during the archean against deformed paleoproterozoic sedimentary rocks sedimentation metamorphism deformation and metasomatic overprints are all part of the post impact history of sudbury and tend to dominate the geophysical response of the structure pre impact deformation although certainly preserved in some aspects of sudbury geology is not clearly expressed in the geophysical data nor are any elements of impact induced deformation geophysical views of the sudbury igneous complex are thus somewhat biased in representing mostly the post impact but pre grenvillian history of the region with the exception of igneous events establishing th

a neodymium isotope study of plutons near the grenville front in ontario canada abstract sm nd analysis of meta plutonic rocks near the grenville front in ontario allows their division into two groups granitoid plutons with model ages of 2 ga are intruded into a crustal section formed largely of archean or re worked archean basement plutons with model ages of plutons south of the proposed suture line can be explained by anatexis of penokean age crust alone while plutons north of this line contain mixtures of archean and proterozoic nd these mixed signatures suggest that the nw grenville province may be analogous to northern wisconsin and michigan in this region a very early proterozoic miogeocline supplied by the superior province is overlain by a formation of mixed superior penokean provenance which heralded the collision of a penokean arc with the archean craton despite very strong sm nd evidence for the presence of old crustal components in the plutons u pb analysis did not reveal the presence of inherited zircon hence the absence of such zircons should not be taken in isolation as evidence against crustal re working

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

an interlaboratory study of tex86 and bit analysis of sediments extracts and standard mixtures two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers gdgts are the tex86 tetraether index of 86 carbon atoms paleothermometer for sea surface temperature reconstructions and the bit branched isoprenoid tetraether index for reconstructing soil organic matter input to the ocean an initial round robin study of two sediment extracts in which 15 laboratories participated showed relatively consistent tex86 values reproducibility 3 4 degrees c when translated to temperature but a large spread in bit measurements reproducibility 0 41 on a scale of 0 1 here we report results of a second round robin study with 35 laboratories in which three sediments one sediment extract and two mixtures of pure isolated gdgts were analyzed the results for tex86 and bit index showed improvement compared to the previous round robin study the reproducibility indicating interlaboratory variation of tex86 values ranged from 1 3 to 3 0 degrees c when translated to temperature these results are similar to those of other temperature proxies used in paleoceanography comparison of the results obtained from one of the three sediments showed that tex86 and bit indices are not significantly affected by interlaboratory differences in sediment extraction techniques bit values of the sediments and extracts were at the extremes of the index with values close to 0 or 1 and showed good reproducibility ranging from 0 013 to 0 042 however the measured bit values for the two gdgt mixtures with known molar ratios of crenarchaeol and branched gdgts had intermediate bit values and showed poor reproducibility and a large overestimation of the true i e molar based bit index the latter is likely due to among other factors the higher mass spectrometric response of branched gdgts compared to crenarchaeol which also varies among mass spectrometers correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of bit index measurements among laboratories as well as a substantially improved estimation of molar based bit values this suggests that standard mixtures should be used in order to obtain consistent and molar based bit values

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

three dimensional visualization of top down superimposed thrust sheets in the sw grenville province ontario to properly understand the tectonic history of the grenville province it is necessary to have a reliable scientifically based understanding of the present day three dimensional 3d structure of the orogen based on detailed nd isotope mapping of surface boundaries and lithoprobe seismic sections this study provides the first detailed visualization of the 3d structure of the grenville gneiss belt in ontario using the sketchup software package the 3d visualization supports a model in which thrust geometry was imposed from the top downwards controlled by the nw boundary of the central metasedimentary belt that originated as a failed back arc rift zone the central metasedimentary belt boundary controlled the trajectory of the allochthon boundary thrust its underlying tectonic duplex and ultimately the grenville front this process of superimposed thrusting explains the large scale change in the trajectory of the grenville front north of georgian bay that has been called the big bend to assist in visualizing the 3d model a fly through animation is provided in the supplementary material

deep seismic structure and thermo mechanical modelling of continental collision zones abstract a series of 2d thermo mechanical models are presented which quantify crustal thickening and the resulting temperature perturbations associated with thrusting in an attempt to constrain the existence of temperatures responsible for the generation of anatectic granites in a compressional environment deformation is assumed to occur in the crust by a simple thrusting mechanism the moho acting as a detachment surface as has been observed on deep seismic reflection data while the more ductile sub crustal lithosphere is assumed to behave elastically and deform in a pure shear manner investigated are the relative effects of the rate of crustal shortening the distribution of radioactive elements within the crust the reduced heat flow from the mantle isostasy and erosion our results show that previous oversimplified 1d models significantly overestimate the elevated temperatures in lower crustal regions caused by radiogenic heating and that this mechanism will only have a significant effect in regions of relatively thin lithosphere or where there are unusually high rates of radiogenic heating in the crust generation of synthetic p t t data for locations deep down in the lower crust enables us to quantify the temperatures produced following a period of compressional deformation providing insight into the time scales involved for the generation of crustal melt granites within an orogenic belt

precise u pb ages for grenvillian and pre grenvillian thrusting of proterozoic and archean metamorphic assemblages in the grenville front tectonic zone canada while the position of the grenville front is placed at a structural feature associated with the onset of high grade rocks when approached from the north the time of first metamorphism in these rocks is in general pre grenvillian and highly variable at killarney amphibolite to granulite grade rocks were metamorphosed lineated and foliated to their present front parallel configuration mainly by 1453 6 ma north of north bay amphibolite grade migmatites have been subjected to a pre 2650 ma metamorphism granulite grade gneisses south of the grenville front near val d or record a 2640 ma metamorphic event south of chibougamau mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 ma in central labrador a 1650 ma foliated granite is present south of the grenville front and at least some of the banding is this old at the atlantic coast granulite facies metamorphism in front parallel gneisses predates a 1790 1662 ma deformation in contrast each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies nw directed thrusting or crustal loading near killarney on georgian bay titanite records an episode of 977 2 ma partial resetting the degree of which increases with distance into the grenville province here grenvillian deformation where present is manifest as feldspar porphyroclastic fabric with strong se plunging lineations near north bay archean pegmatites in the grenville front zone were similarly deformed at 988 10 ma and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 2 ma a 991 4 ma cooling age for titanite in this melt is analytically identical to the zircon growth age hence rapid cooling occurred south of chibougamau new zircons grew in high pressure coronite in response to tectonic loading at 995 6 ma and in central labrador melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 2 ma titanite in the granitic host rock has an analytically identical age of 992 4 ma again indicating rapid cooling this second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 5 ma to 988 5 ma two age mixing line grenville resetting at about 1037 19 ma in the grenville front zone on the labrador coast is indicated but major thrusting is pre 1662 ma u pb isotopic results indicate that the grenville front zone is characterized along its length by partially reset u pb systems high pressure assemblages local melting and rapid cooling crustal loading and shearing with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling are required

a laser 40ar 39ar study of minerals across the grenville front investigation of reproducible excess ar patterns 40ar 39ar ages have been determined on single grains of biotite and amphibole from rocks on a 40 km traverse across the grenville front gf in the temagami area ontario minerals in the vicinity of the gf contain excess ar in varying amounts characterized by both anomalously high integrated and plateau ages the excess ar in biotite varies with distance from the gf such that the overall age distribution along the gf forms an asymmetric wave like pattern an argonami with two age maxima one on either side of the gf the integrated age maxima are 2 34 ga for biotite and 3 21 ga for amphibole located 6 and 0 2 km north of the gf respectively whereas maxima are 1 33 ga for biotite and 1 50 ga for amphibole 7 km south of the gf this wave like pattern of apparent ages shows a remarkable similarity with that seen in an earlier k ar study across the gf at chibougamau quebec and also with a small scale pattern of excess ar associated with the emplacement of a dyke variations in the magnitude of excess a

heat flow gravity and structure of the abitibi belt superior province canada implications for mantle heat flow the results of new heat flow measurements are presented for eleven sites located in the abitibi greenstone belt of the archean superior province these new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the abitibi subprovince and to determine the mantle heat flow beneath the canadian shield in order to analyze the long wavelength variations in heat flow and bouguer gravity anomaly the crust was divided into three layers with distinctive composition and physical properties average density heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data the dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density the models require mantle heat flow to range between 10 and 14 mw m 2 the westward increase in heat flow requires the tonalitic layer to thicken from the grenville front toward the kapuskasing uplift at the expense of the greenstone supracrustals long wavelength gravity data are more affected by changes in crustal thickness than by composition and the low bouguer anomaly near the eastern grenville front is caused by 5 6 km of crustal thickening in greenstone terranes the long wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

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

seismic imaging of mid crustal structure beneath central and eastern north america possibly the elusive grenville deformation iris passcal instrument center at the new mexico institute of mining and technology socorro new mexico usa u s national science foundation nsf national science foundation nsf ear 1261681 u s department of energy national nuclear security administrationnational nuclear security administrationunited states department of energy doe nsfnational science foundation nsf ear 1251515 ear 1251329 ear 1250988

a regional paleomagnetic study of lithotectonic domains in the central gneiss belt grenville province ontario abstract we have made a regional paleomagnetic study of lithotectonic domains in the central gneiss belt of the grenville province in ontario along ten n s and e w traverses up to 200 km in length although originally intended to clarify the tectonic mechanism by which these exotic terranes were assembled and welded to the archean superior craton during the 1150 ma grenvillian orogeny we actually learned much more about the timing of post orogenic uplift of the various domains around 1000 900 ma as they stabilized and became part of the rodinia supercontinent the normal n and reverse r natural remanent magnetizations nrms of all domains as well as those of reactivated regions flanking the grenville front gf the superior grenville boundary to the north and south have paleomagnetic poles falling on the 980 920 ma 40ar 39ar calibrated portion of the grenville apparent polar wander track for laurentia there is a general tendency for paleopoles to young with increasing distance of domains from the gf implying that more southerly domains were uplifted and magnetized later but two of the domains do not fit this pattern previously reported younging trends away from the gf based on k ar thermochron maps and paleomagnetic zone poles are untrustworthy because of hydrothermal alteration which causes chemical remagnetization and anomalously old k ar ages near the gf another trend in our data is a regular increase in the r n ratio with increasing distance south of the gf in the reactivated zones flanking the gf nrms are overwhelmingly of n polarity whereas well away from the gf r n is close to 50 50 also nrm intensities and susceptibility values increase 100 fold away from the gf peaking 10 km south of the front with a pulse like pattern similar to that documented in anomalously high 40ar 39ar dates in the same region both the magnetic and ar ar results are likely due to a wave of hydrothermal alteration and remagnetization during which fluids were driven away from the gf since the remagnetized nrm near the gf has approximately the same direction as nrms in domains away from the gf hydrothermal activity along fault systems south of the gf must have continued until 1000 ma long after the collisional orogeny itself

tectonic and metamorphic events in the westernmost grenville province central ontario new results from high precision u pb zircon geochronology the u pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation samples from the central gneiss belt of the grenville province in the southern georgian bay region provide a means of linking areas formed during common episodes of ductile flow three zircon fractions from one attenuated dyke in the moon river domain gave identical concordant ages of 1062 1065 and 1064 2 ma while those from another gave identical concordant ages of 1052 1051 and 1050 2 ma for the time of metamorphism zircons from a folded dyke from the go home domain gave identical concordant ages of 1049 1047 and 1046 2 ma while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 2 ma as if folding metamorphic zircon growth and postfolding pegmatite are part of the same proc

the geometric and kinematic development of the emma lake thrust stack grenville front southwestern labrador the emma lake thrust stack is a duplex structure consisting of basement cored fold nappes with thrust through overturned limbs developed in greenschist facies metamorphic rocks geometric relationships between fold nappes and thrust surfaces indicate both regular sequence and out of sequence thrusting occurred during development of the thrust stack microstructural analysis shows that regular sequence thrusts developed under ductile conditions whereas out of sequence thrusts developed in a brittle ductile field the structural geometry and metamorphic grade of the emma lake thrust stack indicate that it developed under ductile conditions at upper midcrustal levels the geometry of the emma lake thrust stack suggests that structural styles common to higher level fold and thrust belts may be traced into deeper levels of the crust

exhumed lower crust in nw ireland and a model for crustal conductivity granulite facies gneisses within the ox mountains inlier nw ireland are interpreted as an exhumed segment of sub dalradian lower crust formed at 605 37 ma and later wedged upwards by crustal imbrication along the n dipping highland boundary fault clew bay terrane boundary a lower crustal provenance for the gneisses is suggested by evidence for eclogite decompression and for the later replacement of sillimanite by kyanite in response to slow cooling at depth granulite facies rocks are believed to be present throughout much of the lower continental crust granulites are electrically resistant in contrast to the lower crust which is usually electrically conducting this paradox may be resolved by inferring that the granulite facies rocks form dry kilometre scale lenses mega augen wrapped around by anastomosing retrograded brine soaked electrically conducting shear zones retrogressive hydration is believed a to seal the margins of the dry mega augen with expanded reaction products and b to concentrate the residual brine amphibolite facies rocks associated with the lower crustal shear zones above about 350 c would equilibrate with strong unsaturated brine and become electrically conducting in the upper crust similar rocks would concentrate the brine to the point of saturation whereupon halite would precipitate all water be consumed and the rock would become an insulator

structure and tectonic evolution of the transitional region between the central appalachian foreland and interior cratonic basins abstract the transitional region between the central appalachian foreland basin and michigan and illinois interior cratonic basins is characterized by first and second order structures related to plate convergent processes at the laurentian plate margin the first order structures are the waverly and cincinnati arches believed to be forebulges associated with the taconic and alleghanian orogenies respectively the second order structures are basement faults that exhibit a protracted history of recurrent displacements first and second order structures are linked in that the basement faults have had a major effect in controlling the location and uplift history of the arches the waverly and cincinnati arches share many similarities that suggest a common process of arch development the waverly arch developed over the low density rocks in the central metasedimentary belt of the grenville province near its structural contact with the central gneiss belt uplift on the arch commenced during passive margin development in the cambrian but reached its maximum development at the end of the middle ordovician taconic orogeny reactivation of faults along the arch continued as late as the pennsylvanian the cincinnati arch as expressed in the precambrian basement surface is coincident with the underlying grenville front on a regional scale the front is a major crustal suture which served to localize and control the history of the cincinnati arch local deviations between the location of the arch and front are explained by the presence of proterozoic rift basins filled with low density sedimentary rocks by interactions between the developing appalachian michigan and illinois basins and by a northward decrease in alleghanian tectonic loading in a pattern similar to the waverly arch the cincinnati arch began its history with a period of regional uplift as early as the middle to late ordovician followed by maximum arch development in the pennsylvanian in response to the alleghanian orogeny neither arch showed significant migration following initial development as predicted by general models of forebulge development we attribute this to the effect of preexisting basement anisotropies such as lithotectonic boundaries which served to control the initial location of the forebulge and affect its subsequent history a third major arch the kankakee arch occurs in the western part of the transitional region and is oriented at right angles to the other arches unlike the waverly and cincinnati arches this arch owes its development to subsidence in the adjacent michigan and illinois basins rather than orogenic activity in the appalachians

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

crystallization sequence and magma chamber processes in the ferrobasaltic sept iles layered intrusion canada the sept iles layered intrusion quebec canada 564 ma is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro and an upper border series with anorthosite capped by cupolas of a type granite chilled margin compositions suggest a ferrobasaltic parental magma close to that of the skaergaard intrusion but much richer in iron and titanium samples from drill cores and surface sampling of the 4 7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 fe ti oxide layers cm to m thick and many anorthositic autolithic blocks the sequence of crystallization in the layered series is plagioclase an72 34 and olivine fo75 21 followed by magnetite and ilmenite then ca rich pyroxene and finally apatite an olivine gap is observed between fo66 and fo59 the saturation of fe ti oxides before ca rich pyroxene is interpreted to be the result of the high feot and tio2 contents and the low cao content of the parental magma contamination by old continental crust has occurred during crystallization of the layered series as indicated by sr isotopic compositions 87sr 86sr564 0 70360 0 70497 the differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases cr content of magnetite and lower sr isotope ratios these reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma the author 2010 published by oxford university press all rights reserved for permissions please e mail journals permissions oxfordjournals org

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

migration of wide angle seismic reflection data from the grenville front in lake huron under the auspices of the great lakes international multidisciplinary program on crustal evolution glimpce coincident steep and wide angle seismic reflection data sets were acquired in lake huron the wide angle data were recorded at stationary onshore receivers during the shooting and recording of the conventional marine steep angle survey to extract information from the wide angle data a variety of processing techniques including automatic trace editing amplitude balancing f k filtering static corrections and predictive deconvolution were applied to the wide aperture common receiver gather before migration the application of a slowness weighted depth migration algorithm which was first tested on synthetic data and which included a slowness filter resulted in an image of the grenville front tectonic zone gftz remarkably similar to that observed in the multichannel steep angle section the migrated wide angle data suggest that prominent bands of east dipping seismic reflections observed across the gftz extend unattenuated to depths of at least 45 km

precambrian crust beneath the mesozoic northern canadian cordillera discovered by lithoprobe seismic reflection profiling 1 the cordillera in northern canada is underlain by westward tapering layers that can be followed from outcrops of proterozoic strata in the foreland belt to the lowermost crust of the orogenic interior a distance of as much as 500 km across strike they are interpreted as stratified proterozoic rocks including 1 8 0 7 ga supracrustal rocks and their basement the layering was discovered on two new deep seismic reflection profiles in the yukon line 3 650 km and northern british columbia line 2 1245 km in two segments that were acquired as part of the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in the mackenzie mountains of the eastern yukon the layering in line 3 is visible between 5 0 and 12 0 s 15 to 36 km depth it is followed southwestward for nearly 650 km 500 km across strike and thins to less than 1 0 s 3 0 3 5 km thickness near the moho at the yukon alaska international boundary in the northern rocky mountains of british columbia the upper part of the layering on line 2 correlates with outcrops of proterozoic 1 76 1 0 ga strata in the muskwa anticlinorium at this location the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for 700 km 300 km across strike it disappears as a thin taper at the base of the crust 150 km east of the coast of the alaskan panhandle the only significant disruption in the layering occurs at the tintina fault zone a late to postorogenic strike slip fault with up to 800 km of displacement which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles 300 km apart the base of the layered reflection zone coincides with the moho which exhibits variable character and undulates in a series of broad arches with widths of 150 km in general the mantle appears to have few reflections however at the southwest end of line 3 near the alaska british columbia border a reflection dips eastward from 14 0 s to 21 0 s 45 to 73 km depth beneath exposed eocene magmatic rocks it is interpreted as a relict subduction surface of the kula plate our interpretation of proterozoic layered rocks beneath most of the northern cordillera suggests a much different crustal structure than previously considered 1 ancient north american crust comprising up to 25 km of metamorphosed proterozoic to paleozoic sediments plus 5 10 km of pre 1 8 ga crystalline basement projects westward beneath most of the northern canadian cordillera 2 the lateral 500 km by at least 1000 km and vertical up to 25 km extent of the proterozoic layers and their internal deformation are consistent with a long lived margin for northwestern north america with alternating episodes of extension and contraction 3 the detachments that carry deformed rocks of the mackenzie mountains and northern rocky mountains are largely confined to the upper crustal region above the layering 4 accreted terranes include thin klippen that were thrust over north american pericratonic strata e g yukon tanana and terranes such as nisling and stikinia that thicken westward as the underlying proterozoic layers taper and disappear 5 the ages of exposed rocks are not necessarily indicative of the ages of underlying crust a frequent observation in lithoprobe interpretations so that estimates of crustal growth based on surface geology may not be representative

seismic images of a tectonic subdivision of the grenville orogen beneath lakes ontario and erie new seismic data from marine air gun and vibroseis profiles in lake ontario and lake erie provide images of subhorizontal phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust these reflections are interpreted as structural features of crustal scale subdivisions within the grenville orogen broadly deformed imbricated and overlapping thrust sheets within the western central metasedimentary belt are succeeded to the west by a complex zone of easterly dipping apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the central metasedimentary belt and the central gneiss belt the interpreted central metasedimentary belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes ontario and erie to structures exposed 150 km to the north less reflective west dipping events are interpreted as structures within the

variations in the reflectivity of the moho transition zone beneath the midcontinent rift system of north america results from true amplitude analysis of glimpce data true amplitude processing of the great lakes international multidisciplinary program on crustal evolution seismic reflection data from the midcontinent rift system of north america shows large differences in the reflectivity of the moho transition zone beneath the axial rift beneath the rift flanks and outside of the rift the moho reflection from the axial rift has a discontinuous diffractive character marginally stronger several decibels than an otherwise transparent lower crust and upper mantle beneath the axial rift moho is interpreted to be a synrift igneous feature beneath the rift flanks the reflectivity of the moho transition is generally well developed with two identifiable boundaries although in places it is weakly reflective to nonreflective similar to moho outside the rift the two boundaries are interpreted as the base of essentially intact although stretched prerift archean crust upper boundary and new synrift moho 1 2 s 6 7 km deeper lower boundary beneath the rift flanks the layered reflection moho transition results from the preexisting crustal composition and fabric modified by synrift igneous processes and extensional tectonic metamorphic processes the geologic evidence for extensive basaltic magmatism in the rift is the basis for interpreting the moho signature as a keweenawan structure that has been preserved for 1 1 b y extension and magmatism appear to enhance reflectivity in the lower crust and moho transition zone only where stretching factors are moderate rift flanks and not where they are extreme axial rift this leads to the prediction that the reflectivity across analogous volcanic passive continental margins should be greatest beneath the moderately stretched continental shelves and should decrease towards the ocean continent boundary

rupture creep lamellae and crocodiles happenings in the continental crust from the observation of certain seismic reflectivity patterns in phanerozoic continental crusts a relationship is suggested between 1 rupture and seismicity generally restricted to the upper crust 2 crocodile tectonics a kind of vertical indentation observed by diverging plane reflectors preferably in the middle crust 3 creep and the formation of reflecting lamellae in the lower crust based on these observations and on viscosity depth estimates a conceptual model for the above mentioned structures is presented which is also compatible with observations in older crusts boundary conditions are given for rupture as well as for lamellae forming creep

widespread buried precambrian layered sequences in the u s mid continent evidence for large proterozoic depositional basins large regions of the north american mid continent are underlain by precambrian layered rocks buried beneath phanerozoic sedimentary strata south of the wichita mountains published seismic reflection profiles show a precambrian layered assemblage extending for at least 40 km in both the north south and east west directions and industry data show that it may continue 150 km to the southeast seismic reflection data in the illinois region show a precambrian layered assemblage extending 320 km in an east west direction and 200 km in a north south direction in both cases the layered rocks are as much as 12 km thick apparent sequence boundaries onlap downlap within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strat due to faulting the layered sequences correlate with regions of relatively long wavelength and low amplitude magnetic anomalies the extent of this magnetic signature suggests that about 200 000 km 2 of illinois indiana and western ohio about 50 000 km 2 of southernmost oklahoma and north central texas and about 32 000 km 2 of southern missouri and northern arkansas may be underlain by similar precambrian strata drill holes indicate that the top of the mid continent precambrian basement is composed largely of silicic igneous rocks such material may comprise a large part of the layered sequences alternatively these igneous rocks could be intermixed with or underlain by nonvolcanic meta sedimentary strata the strong reflectivity of some layers suggest that minor mafic flows and or sills may also be present analysis of u pb and nd sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1 7 2 0 ga and predate or are contemporaneous with the 1 3 1 5 ga crystallization ages of the granites and rhyolites though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed the possibility tha they contain precambrian hydrocarbons that escaped heating is as yet untested

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

deep seismic profile of the amazonian craton northern brazil deep seismic profiling of the amazonian craton northern brazil reveals previously unknown deep structural features buried beneath a thin relatively flat phanerozoic sedimentary cover amazon basin the seismic signature of the precambrian crust is characterized by a transparent shallow basement down to 8 9 km followed by a highly reflective middle and lower crust middle and lower crustal events are dominated by abundant dipping asymmetric arcuate reflections and diffractions the shallowest of which occur beneath a structural arch jutai arch although wide angle refraction data are unavailable a decrease in the number of coherent reflections at about 15 s two way travel time 45 km and small drops in amplitude and increases in average frequency are interpreted to mark the moho the deep amazonian seismic line is located near the westernmost boundary of the amazonian craton imaging the rondonian province one of the youngest geochronological provinces of the craton middle proterozoic to the south the rondonian basement crops out revealing schists gneisses granulites and important volumes of anorogenic granites the prominent laterally extensive arcuate reflections observed within the middle and lower crust may be related to this anorogenic magmatic activity perhaps corresponding to layered intrusive complexes on the other hand intrusion of mantle derived material during an early paleozoic rifting as well as the mesozoic episodes of mafic intrusion may also have contributed to this seismic signature similar seismic signatures found in eastern australia and on cocorp profiles of the central u s craton suggest that such cratonic terranes may have undergone similar tectonic evolution in the upper crust probable fault plane reflections associated with ene wsw reverse faults of jurassic early cretaceous age may sole into the top of the reflective zone suggesting that the lower limit of the transparent upper crust may represent a rheological boundary

faults and folds fact and fiction abstract after reviewing the microscopic and macroscopic texture of fault zones the localisation of rupture and creep is described and the reduced strength of fault zones is investigated simple strength and viscosity models for the whole lithosphere play a major role for the geometry of fault zones compressional faults thrusts show ramp and flat structures often soling in a weak detachment zone extensional faults prefer a listric shape often turning subhorizontal and being invisible in the seismic lamellae of the ductile lower crust which according to the models is also a broad zone of detachment and decoupling faults in the lower crust are rare and restricted to strain hardened fault rocks or intrinsically rigid rocks like oceanic mafic ultramafic rocks of suture zones transient faults in the lower crust are created by rupture processes of large earthquakes some deep faults moderately dipping or flat are again observed in the rather rigid uppermost mantle the observation of folding inside the crust seems to be connected with an intermediate viscosity range and with a broad vertical transition zone where rigid reactions and ductile processes are mixed various methods of detecting faults by seismic studies are critically reviewed and some sequence of reliability is suggested impedance contrasts polarity dip thickness and multiple fault strings have to be deciphered by wavefront modelling origin and survival of faults are considered to be a consequence of tectonic and thermal evolution under specific stress systems

geophysical investigations and crustal structure of the north american midcontinent rift system abstract geophysical investigations have had a prominent role in studying the 1100 ma midcontinent rift system because of limited surface exposures and deep drilling of the rift rocks gravity and magnetic anomaly data delineate the rift system its central volcanic filled graben and overlying and adjacent post rift sedimentary basins and the major faults of the system over its 2000 km length in central north america crustal seismic studies indicate a high velocity crust that is thickened to as much as 55 km along the rift deeply penetrating seismic reflection data obtained across the rift show that the crust was thinned to less than one half of its original thickness during rifting and the present crust has been thickened by addition of up to a total of 30 km of mantle derived volcanic rocks within grabens and clastic sediments deposited in overlying sag basins as well as by intrusions into the crust and crustal underplating these data also confirm the occurrence of a post rift regional compressional event that locally caused broad folding of the rift rocks and reactivation of the graben normal faults as high angle reverse faults with vertical throws of several kilometers geophysical data indicate that the rifting event had a profound effect on the entire vertical range of the crust and that structures and thus the tectonic evolution are similar along the entire length of the rift

crustal modeling from spectrally correlated free air and terrain gravity data a case study of ohio we investigate the use of spectral correlation theory to analyze terrain gravity effects and free air gravity anomalies of ohio for possible constraints on the thickness variations and neotectonics of the crust terrain gravity effects are computed from the topography by gauss legendre quadrature integration and are compared against independent free air gravity anomaly observations for their wavenumber correlation spectrum spectral correlation filters are designed accordingly to extract terrain correlated free air gravity anomalies that are subtracted from the terrain gravity effects for estimates of the compensated terrain gravity effects these effects are used to model the moho by inversion assuming they predominantly reflect crustal thickness variations our results characterize the middle third of ohio as a broad zone of thickened precambrian crust which also may include rifted regions where the thickness of the prerift crust has been reduced greatly furthermore we find that about 83 of the ins

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

a new map of crustal terranes in the united states from cocorp deep seismic reflection profiling a new map has been constructed to summarize major systematic variations in seismic reflection character observed by over 10000 km of cocorp profiling in the us among the seismic characteristics represented on this map are crustal scale detachments which can be traced as relatively low angle reflectors over large areas deeply penetrating dipping reflection zones some of which mark major shear zones perhaps even crustal sutures moho reflectivity which appears to vary from craton to phanerozoic periphery seismic transparency which can sometimes be interpreted as geologic homogeneity seismic layering prominent in the lower crust in many areas and now known to dominate the upper basement in other regions and unusually strong reflections or bright spots at mid crustal levels that may mark deep crustal fluids

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

mega scale moho relief and the structure of the lithosphere on the eastern flank of the viking graben offshore southwestern norway the international lithosphere project deep reflection seismic survey in the norwegian sector of the north sea has been reprocessed particularly focusing on the deep crust the reflection moho and the upper mantle the data display shifting reflection patterns of the crust and the upper mantle parallel to the eastern margin of the viking graben in the upper crust which is mainly seismically transparent by the processing techniques utilized here large scale structural features like detachment shear zones and master faults can be identified several of the major onshore faults and shear zones match seismic features in the seismic lines many of these structures acted as extensional shear zones in the devonian the middle crust is of variable reflectivity whereas the lower crust is generally strongly reflective and is particularly so in the southern domain the reflection moho is identified throughout the study area but is of variable character the presence of a s e dipping structure hardanger moho offset that displaces the moho by approximately 10 km extends deep into the mantle below the 50 km line depth is positioned where the shallower hardangerfjord shear zone which flattens on the level of the middle crust is situated the hardangerfjord shear zone hardanger moho offset system coincides with change of the crustal thickness depth to moho a change that also coincides with the transition from thin to thick skinned caledonian deformation intramantle reflections are common in the study area some of which are interpreted as shear zones whereas others most likely represent magmatic intrusions

proterozoic extensional tectonics of the central fennoscandian shield results from the baltic and bothnian echoes from the lithosphere experiment a three dimensional crustal structure of the bothnian sea is deduced from the near vertical reflection data of the crosscutting lines 1 6 7 and c of the babel baltic and bothnian echoes from the lithosphere experiment for this study the common depth point stacked sections have been migrated with a velocity 6 5 km s and displayed as stacked envelope sections to show their enhanced large scale crustal structures the seismic sections show listric shear zones which dip toward se and flatten at major detachment zones 35 40 km and 48 km in depth the detachment zones are associated with changes in p wave velocity from 6 8 km s to 7 0 km s and from 7 4 km s to 8 2 km s and coincide with the middle and lower crustal boundary and the moho boundary respectively the geometry and crosscutting relationship of the detachment zone are well developed on the e w directed line 7 from where the moho detachment zone can be interpreted as to be younger different shear zone systems operated in the parallel n s directed lines 1 and 6 and therefore it is not easy to correlate crustal structures between lines 1 and 6 in the thinned southern part of study area the lower crust has a highly reflective lamellar structure and a strongly reflective subhorizontal moho increased reflectivity of the lower crust is attributed to stretching and mafic underplating of the crust associated with the subjotnian diabase dykes and rapakivi magmatism in the upper crust the rapakivi granite and diabase association is characterized by a rather transparent area within which strong reflections mimic graben and horst structures the distribution of the bimodal association seems to be controlled by the low angle listric shear zones the spatial distribution of outcropping lithologies combined with deeply penetrating shear zones suggests a simple shear extensional deformation the first stage of deformation which concentrated in the upper and middle crust records the collapse of the overthickened svecofennian crust the second stage involving the entire crust may record either the continuation of the postcollisional collapse or a separate anorogenic extensional period the seismic structures predict major post svecofennian deformation faulting and tilting in the central fennoscandian shield the extensional tectonics operating in the fennoscandian shield in the proterozoic is comparable to modern extensional tectonics

north west propagation of the grenville orogen grenvillian structure and metamorphism near key harbour georgian bay ontario canada abstract we report structural and metamorphic data from a c 25 km transect across the eastern grenville front tectonic zone gftz to the britt domain at the northern end of georgian bay near key harbour ontario constrasting grenvillian structural and fabric elements characterize the eastern gftz northern britt domain and a narrow transition zone between them moderately to steeply dipping foliations with strong down dip lineations in all three divisions appear to be associated with nw directed thrusting in the transition zone and northern britt domain early s l fabrics with steep lineations are overprinted by younger structures s l with shallow se sse plunging lineations in which sparse dominantly but not exclusively normal sense kinematic indicators are recognized pressure and temperature estimates from grenvillian metamorphic assemblages in metadi abase indicate that conditions of p 12 kbar and t c 800 c were achieved before or during the thrust related deformation with p t t paths that indicate near isothermal decompression to p c 4 kbar and t c 700 c correlation of fabric elements with points on the p t t paths suggests that exhumation occurred during two stages the first associated with thrusting 1035 ma and the second with extension and thrusting pre 1003 ma the gftz contains steeply to moderately dipping thrust related fabrics and lacks shallow extensional structures the latest episode of thrusting in the gftz is inferred to have taken place at 990 980 ma the data are interpreted in terms of a tectonic model involving two stages of propagation of the grenville orogen towards its foreland 1035 ma and 980 ma with an intervening period of extension although the tectonic regime probably remained compressional on the scale of the orogen

multi genetic origin of the continental moho insights from lithoprobe this paper reviews four hypotheses for the origin of the continental crust mantle boundary and discusses seismic parameters with which these hypotheses might be tested the relict moho hypothesis posits that the oceanic moho is preserved during continental assembly the magmatic underplating hypothesis posits formation of a new moho by episodic emplacement of sill like intrusive bodies the metamorphic or metasomatic front hypothesis posits that the moho is overprinted by a phase transformation and the regional decollement hypothesis posits that the moho behaves as a structural detachment these hypotheses are not mutually exclusive and examples from the canadian lithoprobe program suggest that all four may be applicable in different regions of north america comparison of seismic images from a fossil subduction zone with modern subduction at cascadia suggests that serpentinization of the forearc mantle a previously unrecognized mechanism for overprinting and erasing the reflection moho may have occurred in the proterozoic

a structural analysis of a metamorphic fold thrust belt northeast gagnon terrane grenville province northeast gagnon terrane is located within the parautochthonous belt of the grenville orogen near the projected intersection of the front zones of the grenville and new quebec orogens the area consists principally of supracrustal units of the early proterozoic knob lake group and a newly recognized unit the equus lake formation both are intruded by the middle proterozoic shabogamo gabbro structural elements in the rocks record evidence of a polyorogenic history that is attributed to both the ca 1800 ma hudsonian and the ca 1000 ma grenvillian orogenies this paper is concerned with the latter grenvillian deformation resulted in the formation of a relatively deep level fold thrust belt three thrust sheets can be defined on the basis of basal thrusts variations in morphology and orientation of structural elements and internal thrust sheet geometry the polydeformational style of the area rotation of fold axes into subparallelism with the tectonic transport direction and internal imbrication lea

crustal structure beneath the sub himalayan fold thrust belt kangra recess northwest india from seismic reflection profiling implications for late paleoproterozoic orogenesis and modern earthquake hazard abstract most compressional orogens include salients and recesses along their strike the age and origin of which can be hard to ascertain in the kangra recess in the trace of the main boundary thrust mbt the largest such recess within the active himalayan orogen the sub himalayan sedimentary fold thrust belt increases in width to as much as 90 km the kangra basin but narrows to as little as 10 km in the adjoining nahan salient of the mbt the subathu basin to the southeast new seismic reflection profiling places the himalayan decollement at 6 8 km depth above thin but reflective meso to neoproterozoic vindhyan lesser himalayan series equivalent strata these data show that the vindhyan sedimentary rocks are thinner in the kangra recess than further southeast allowing the hypothesis that the width of the lesser himalayan thrust belt and the existence of the kangra recess could be related to the pre deformation basin thickness this hypothesis obviates the need for control of the kangra recess by a lateral ramp in the main himalayan thrust so making it more likely that the kangra segment could rupture as part of an earthquake far larger than the devastating 1905 m 7 8 kangra earthquake below the proterozoic sedimentary rocks our reflection data show a west southwest dipping reflective fabric spanning a 30 km crustal thickness which we infer corresponds to a widespread ulleri wangtu orogenic event at c 1850 ma affecting a pre tethyan indian continental margin thickening the basement by c 20 the deepest 10 km of this 50 km thick crust shows a more horizontal arguably younger reflectivity though the moho is not clearly marked by strong reflectors

a magnetotelluric survey on manitoulin island and bruce peninsula along glimpce seismic line j black shales mask the grenville front the results of a magnetotelluric survey are presented a cover of phanerozoic sediments including a highly conductive layer of black shales of limited extent precludes the exact definition of electrical features at depth thus a comparison with the seismic reflections observed on glimpce line j is impossible however the interpretation of the electromagnetic data proves to be an excellent illustration of electrical distortion and attenuation in 3 d environments we show in particular that features originally interpreted as due to a thin conductor at mid crustal depth are actually representative of edge effects related to the sedimentary cover

seismic imaging of upper crustal basement faults in the skagerrak sea abstract the continental upper crystalline crust appears almost non reflective on many deep seismic reflection profiling sections typically only the major faults and lithological boundaries are imaged whereas smaller scale faults are usually not identified a deep seismic survey 0 16 s twt covering the skagerrak sea was undertaken in 1987 although the northeastern part of the skagerrak has only thin or no sediment cover the data in the depth range 0 6 s twt were reprocessed to commercial exploration standards close inspection of these high quality sections provides evidence of abundant moderately steep dipping 20 50 faults that can be traced to depths of 10 13 km fault geometry which can be inferred at profile intersections is in accordance with the general tectonic fabric observed on land and beneath the skagerrak sea these areas were deformed during the proterozoic sveconorwegian orogeny and during the formation of the permian oslo rift due to post permian erosion of the skagerrak sea proper the relative age and movements of the basement faults could not be determined this study shows that under favorable conditions faults in the upper crystalline crust are mappable by seismic means such measurements have the potential for resolving variations in block geometry with depth and also recent seismotectonic movements

crustal reflections underneath lake vanern sweden summary in conjunction with airgun profiles on lake vanern 48 channel seismic recordings were made along a 7 km long profile on the northern peninsula varmlandsnas this peninsula lies in the mylonite zone which runs through sw sweden four airgun profiles were recorded two on the eastern side of the mylonite zone and two on the western side each profile being on the order of 20 30 km long these profiles have been processed using the method of common data point cdp stacking which resulted in a total of approximately 50 km of sub surface coverage we have interpreted the data in the context of the regional geology and results from other geophysical studies in the area important results are the imaging of lower crustal reflectors and moho at near vertical incidence angles the dips and depths of these reflectors are in fair agreement with other studies the upper crust on the eastern side of the mylonite zone appears to be considerably more reflective this may be due to the presence of mafic intrusions or to the reworking of the crust during the sveconorwegian orogeny a brief comparison with results from the glimpce profiles over the grenville province is also made

deep seismic reflectivity patterns in specific tectonic units of western and central europe a systematic comparative study of the reflectivity of the continental crust is undertaken made possible by the expanded data basis of deep seismic reflection profiles in europe the reflectivity is not at all the same but can change dramatically from one seismic line to the next different seismic reflectivity patterns independent of applied techniques are observed correlations between these seismic patterns and specific tectonic units have been found

seismic images of continental lithosphere seismic reflection profiles in usa and west germany over thirty years ago showed strong reflections from the lower crust but technical advances through the 1970s were needed for the method to image the continental lithosphere the british institutions reflection profiling syndicate birps found advantage in working offshore and has obtained over 12 000 km of profiles around britain during the past decade giving the densest regional coverage anywhere reflections are observed from strata in sedimentary basins from low angle faults that can be followed from surface to mid crust from a laterally variable reflective zone in the lower crust from the moho and from fault like features in the upper mantle there is evidence around britain that faults bounding sedimentary basins are older structures which have been reactivated it is argued that lower crustal reflectivity around britain is mainly due to anastomosing ductile shears which accommodate extension in the lower crust commensurate with extension in the upper crust by faulting and block rotation there is also evidence of reflections preserved from older orogenic features so that reflections interpreted as extensional ductile shears in the lower crust may also derive from older features that have been reactivated most reflections in the lower crust arise from rocks that have been highly strained evidence of lateral variability in the reflective character of the moho suggests that the moho has been actively involved in the structural evolution of the lithosphere upper mantle reflections are interpreted as shear zones which transfer crustal deformation through the lithosphere

crustal thickness and vp vs variations in the grenville orogen ontario canada from analysis of teleseismic receiver functions abstract we have developed a simple semblance weighted stacking technique to estimate crustal thickness and average vp vs ratio using teleseismic receiver functions we have applied our method to data from 32 broadband seismograph stations that cover a 700 400 km2 region of the grenville orogen a 1 2 0 98 ga himalayan scale collisional belt in eastern north america our seismograph network partly overlaps with l ithoprobe and other crustal refraction surveys in 8 out of 9 cases where a crustal refraction profile passes within 30 km of a seismograph station the two independent crustal thickness estimates agree to within 7 our regional crustal thickness model constructed using both teleseismic and refraction observations ranges between 34 0 and 52 4 km crustal thickness trends show a strong correlation with geological belts but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium the thickest crust 52 4 1 7 km was found at a station located within the 1 1 ga mid continent failed rift the central gneiss belt which contains rocks exhumed from deep levels of the crust is characterized by vp vs ranging from 1 78 to 1 85 in other parts of the grenville orogen vp vs is found to be generally less than 1 80 the thinnest crust 34 5 37 0 km occurs northeast of the 0 7 ga ottawa bonnechere graben and correlates with areas of high intraplate seismicity

the possible role of fluids for the structuring of the continental crust abstract some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes the role of fluids in enhancing or destroying such structures is studied seismicity is strongly dependent on the different pore pressure of fluids in intraplate settings in strike slip or extensional areas hydrostatic pore pressure seems to be approached in accordance with standard rheological models and standard seismicity depth distributions in suture zones or zones of developing loads possibly also along zones of near horizontal normal faults extremely high pore pressure develops which approaches lithostatic values they are accompanied by a number of weakening processes with enhanced shear and reduced viscosity these values correlate with low seismic velocities v p in accordance with an empirical v p relationship

origin of deep crystal reflections seismic profiling across high grade metamorphic terranes in canada abstract in an attempt to better understand the origin of deep crustal reflections lithoprobe has sponsored or co sponsored seismic reflection surveys across tracts of high grade metamorphic rock in the archean superior craton the proterozoic grenville orogen and the phanerozoic cordilleran orogen common to these three diverse terranes are near surface zones of prominent seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology at some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks stratified assemblages sills etc whereas in other regions it was generated by extreme attenuation stretching and ductile flow of weakly layered or irregularly organized rocks it seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust with reflections originating at lithological boundaries and zones of mylonite

reviewing the configuration and extent of the midcontinent rift system abstract uncertainty exists in the configuration and extent of the midcontinent rift system mrs because of deficiencies in geophysical data and limited information from outcrops and basement drill holes additional ambiguity is caused by misunderstanding the definition of continental rifts six principal problematic regions in mapping the mrs are described gravity and magnetic data supported by drill hole and seismic reflection data show that the eastern lake superior rift segment of the mrs continues south from lake superior and connects to a much narrower rift in northern lake michigan the eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data but is interpreted to occur near the u s canada border the rift segment in southeastern michigan intersects the grenville front and likely continues eastwards in modified form to near the boundary with canada cross cutting gravity and magnetic signatures may reflect grenvillian overthrusts near the terminus of the mrs in michigan the proposed southerly extensions of both branches of the rift system into oklahoma and ohio are based primarily on positive gravity anomalies but neither postulated extension appears to be associated with rifted troughs rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by grenville orogenesis recent paleomagnetic investigations in conjunction with high resolution radiometric dating imply that the mrs developed during the rapid southward movement of laurentia during a quiescent period along its eastern continental margin massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin the heated crust was made more ductile fostering rifting due to extensional stresses the nipigon embayment remains as a possible candidate for an early third branch of the mrs but current evidence is insufficient to include the fort wayne rift as part of the mrs future studies of the mrs would be well served by new age dating and high resolution seismic studies of the lithosphere

crustal velocity structure of the superior and grenville provinces of the southeastern canadian shield the 1992 lithoprobe abitibi grenville seismic refraction experiment was conducted using four profiles across the grenville and superior provinces of the southeastern canadian shield delay time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another with the largest velocity values occurring underneath the central gneiss and the central metasedimentary belts south of the grenville front the grenville front tectonic zone is imaged as a southeast dipping region of anomalous velocity gradients extending to the moho the velocity anomaly maps suggest an archean crust may extend horizontally 140 km beneath the northern grenville province near surface velocity anomalies correlate well with the known geology the most prominent of these is the sudbury structure which is well mapped as a low velocity basinal structure the tomography images also suggest underthrusting of the pontiac and quetico subprovinces

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

limitations of quantitative analysis of deep crustal seismic reflection data examples from glimpce amplitude preservation in seismic reflection data can be obtained by a relative true amplitude rta processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally after compensating for amplitude distortion by near surface effects and propagation effects quantitative analysis of relative true amplitudes of the great lakes international multidisciplinary program on crustal evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression processing techniques such as deconvolution f k filtering and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless the relative strength of individual events however is preserved on curves generated at different stages in the processing we suggest that qualitative comparisons of relative strength if used carefully provide a meaningful measure of variations in reflectivity simple theoretical models indicate that peg leg multiples rather than water bottom multiples are the most severe source of noise contamination these multiples are extremely difficult to remove when the water bottom reflection coefficient is large 0 6 a condition that exists beneath parts of lake superior and most of lake huron

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

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

seismic images of the continental moho of the indian shield abstract deep seismic reflection profiling has imaged different patterns of the moho across the indian shield with a variety of tectonic environments from archean to recent the character of the moho varies from a discrete strong event the base of strong coherent lower crustal sub horizontal reflections the base of dipping lower crustal reflections into mantle to no clear reflection boundary the seismic reflection data suggest a laminated lower crust in several places and offsets in the moho at others kinematic and dynamic modeling of wide angle reflection data across the mesoproterozoic south delhi fold belt and the central indian tectonic zone suggests a laminated lower crust for these regions in general the lower crust of the indian shield is heterogeneous a clear moho is identified in some of the precambrian orogenic belts and sedimentary basins along with reflective lower crust whereas the cratonic areas exhibit a diffused moho post collisional extensional process such as orogenic collapse delamination magmatic intrusions low viscosity ordering and underplating might have played a role in the generation of lower crustal laminated zone and formation of a younger moho the termination of lower crustal reflectivity at the moho with a transparent upper mantle need not necessarily indicate homogeneous upper mantle

the grenville front tectonic zone results from the 1986 great lakes onshore seismic wide angle reflection and refraction experiment the grenville front which marks the orogenic boundary between the archean superior structural province and the much younger grenville province to the southeast is one of the major tectonic features of the canadian shield within canada it is approximately 1900 km in length extending from the north shore of lake huron across ontario and quebec to labrador in 1986 a major coincident onship near vertical reflection and onshore wide angle reflection refraction experiment glimpce great lakes international multidisciplinary program on crustal evolution was conducted along a series of lines across the great lakes one of the lines line j ran across georgian bay and lake huron for a distance of 350 km and crossed the grenville front tectonic zone gftz the seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50 62 5 m the gftz had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections data recorded by the stations on the east end of the line indicate that the crustal p phases are very complex and form a shinglelike pattern of reflected waves data recorded by stations at the center and at the western end of the line show that the pg phases are normal and lack the shinglelike appearance this character of arrivals was also observed on the corresponding s wave sections a combined p and s wave forward modeling analysis shows that the gftz is composed of bands of reflectors dipping at angles of 20 35 extending to the lower crust these reflectors were also well imaged on the coincident near vertical reflection data reflectors under the britt domain to the east of the gftz have a shallower dip than those along the zone the structure of the crust under the manitoulin terrane to the west of the gftz is laterally homogeneous with a major intracrustal reflector at a depth of 17 20 km below the surface poisson s ratio is slightly higher to the east of the grenville front compared to the region to the west the travel times of the pmp signals indicate that the moho may be deeper under the gftz than under the surrounding regions our results give added support to tectonic theories that the grenville front owes its origin to a continental collision process

crustal scale shortening and extension across the grenville province of western quebec a deep seismic reflection survey shot in 1993 crosses almost the full width of the grenville province in western quebec the seismic transect provides a very clear image of the crust mantle boundary and the most precise definition to date of the various grenvillian terranes the crust is around 44 km thick beneath the grenville front but thins rapidly to 36 km some 60 km to the southeast it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect far from the inferred location of the main grenvillian collision the grenville front zone in which nw directed thrusting at about 1 ga was followed by se directed extension is defined by discontinuous se dipping reflections which extend down to the moho to the southeast the overlying migmatitic archean parautochthon almost half of the transect is characterised instead by nw dipping reflectors extending into the lower crust these reflectors are in turn truncated to the southeast by a 12 km thick zone of intense se dipping reflections the baskatong crustal ramp the base of the allochthonous terranes allochthon boundary thrust is likely located at this ramp which flattens out at around 30 km depth into the base of the relatively transparent intermediate crust a highly reflective upper crustal deck corresponding to rocks of the mont laurier terrane was thrust over the baskatong crustal ramp and is represented further to the northwest by the klippelike cabonga allochthon the synformal transparent morin anorthosite charnockite complex belongs to the same upper crustal level ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the nw directed propagation of tectonic transport during the grenvillian orogeny involving deformation and displacement along the baskatong ramp with a relay into the grenville front zone postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the grenville front and affecting the crust up to 350 km away from the front

the effect of small random crustal reflectors on the complexity of pg and pmp coda abstract in the conventional method of incorporating heterogeneity into crustal models of the earth smoothed random small scale velocity fluctuations are added to a uniform seismic velocity gradient field numerical experiments on such models show that the addition of the velocity fluctuations will tend to break up a smooth travel time curve into segments with erratic amplitudes giving one the illusion that the earth is layered with significant intra crustal boundaries many experiments which employ finite difference methods produce synthetic seismic sections that closely match the nature of the observed seismic sections in appearance and many of the features that we have observed owe their origin to velocity fluctuations rather than to deterministic effects in this paper we investigated an alternative method of adding heterogeneity to an earth model smoothed randomly oriented small scale seismic reflectors are embedded in a uniform seismic velocity gradient field the effect of the velocity gradient is to make the reflective field for downward waves much greater than upward waves the on off switching light bulb effect characteristics of the reflection coefficients of the intra crustal reflectors as a function of angle of incidence plays a major role in generating the pg coda in seismic wide angle reflection experiments numerical experiments show that the complexity of the coda or the presence absence of the pmp travel time branch is related to random orientations and velocity contrasts of the small scale reflectors the shingling effect of pg first arrivals is predicted by these experiments the complexity of the record sections which is predicted from the above analysis was compared to actual data obtained from recent canadian lithoprobe seismic coincident near vertical and wide angle reflection experiments

a global analysis of heat flow from precambrian terrains implications for the thermal structure of archean and proterozoic lithosphere previous studies of heat flow from precambrian terrains have yielded two different relationships a global temporal relationship between heat flow and tectonic age and a regional spatial relationship between heat flow and the proximity of archean cratons we analyze heat flow from tectonically stable precambrian terrains worldwide to address questions associated with these two heat flow patterns 1 is the spatial relationship a global pattern 2 do the two heat flow relationships have a common underpinning in answer to the first question our data analysis reveals a widespread spatial pattern between heat flow and the proximity of archean cratons which is characterized by low heat flow in archean cratons and proterozoic terrains adjacent to cratonic margins and higher heat flow in proterozoic terrains that are more than a few hundred kilometers from a craton to address the second question we examine three previous interpretative models of precambrian heat flow 1 simple cooling of a thermal boundary layer 2 thicker lithosphere in archean terrains than in proterozoic and 3 greater heat production in proterozoic crust than in archean model 1 predicts essentially no change in heat flow in terrains older than 1 5 ga and therefore does not likely provide a common underpinning for the precambrian heat flow patterns models 2 and 3 when combined with the special structural configuration of sutures can independently yield both the spatial and temporal heat flow distributions and thus alone or together may be considered candidates to explain the precambrian heat flow patterns however thermal models of the lithosphere in which the heat flow patterns are explained entirely by variations in crustal heat production cannot satisfy constraints on upper mantle temperatures beneath archean cratons derived from xenolith thermobarometry the thermobarometry constraints can be satisfied by models in which differences in lithospheric thickness are the principal factor controlling the surface heat flow distributions if the heat flow patterns result primarily from variations in lithospheric thickness then a different temporal heat flow relationship for precambrian terrains can be proposed one in which heat flow varies inversely with the age of stabilization of the lithosphere

nd isotope mapping of a tectonic window near chalk river ontario implications for large scale grenvillian tectonics abstract over forty new nd isotope determinations are presented for granitoid orthogneisses from the chalk river area on the border between ontario and quebec in the sw grenville province tdm model ages provide evidence for a tectonic window through the allochthonous belt into the grenvillian parautochthon in the chalk river area this tectonic window constrains the location of the main ramp of the allochthon boundary thrust southeast of the chalk river area however the structure of the tectonic window has been complicated by down faulting on the ontario side of the ottawa river due to reactivation of late precambrian normal faulting associated with the ottawa bonnechere graben thus on the ontario side of the border the surface expression of the tectonic window is represented by a strongly sheared tectonic duplex that underlies the main allochthon however the lower section of a 1 km deep borehole drilled by canadian nuclear laboratories has tdm model ages indicating penetration into the underlying parautochthon the new data combined with previous nd isotope mapping and lithoprobe seismic reflection results point to a relatively constant thickness for the exhumed muskoka allochthon by showing that this exhumation occurred immediately below the central metasedimentary belt boundary this evidence strongly supports a ramp flat thrusting model for the sw grenville province this model greatly reduces the likelihood of major crustal exhumation to the nw of parry sound domain thus removing the theoretical basis for pervasive southeast directed orogenic collapse across the ontario gneiss belt

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

early proterozoic collisional orogeny along the northern thelon tectonic zone northwest territories canada evidence from the foreland the subsidence history of kilohigok basin 1 9 ga n w t canada reflects the opening and closing of an ocean basin along the present thelon tectonic zone the basal sedimentary sequence represents a passive margin shelf that faced se toward the orogen following passive margin subsidence tectonic loading occurred resulting in rapid subsidence of the outer shelf margin and simultaneous uplift and erosion of the platform interior a flexural moat formed subparallel to the thelon tectonic zone the shelf was subsequently drowned and successively buried by deep water flysch shallow marine shelf sediments and fluvial molasse forming a foreland basin tapered to the nw direct structural evidence for the tectonic load is provided by the leading edge of a thin skinned thrust fold belt exposed in oblique cross section within the se extremity of the basin upright folds at high structural levels can be traced downward to recumbent tight folds and thrusts near the basement cover contact documenting increasing shear strain with depth overall northwest vergence and about 50 shortening is indicated by fold thrust geometry and finite stretching lineations above a brittle ductile decollement that roots within the thelon tectonic zone

proterozoic sutures and terranes in the southeastern baltic shield interpreted from babel deep seismic data abstract a hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal incidence and wide angle seismic data along the babel profile in the baltic sea this intermediate terrane is situated between a northern terrane of svecofennian age and a southwestern terrane of gothian age it is delimited upwards by two low angle and oppositely dipping sutures and occupies mainly middle and lower crustal levels with a width of 300 km at moho level the 1 86 ga suture against the northern terrane is imaged by a prominent almost continuous ne dipping crustal reflection from 3 5 to 14 s twt over 175 km where it downlaps on the moho sub moho velocities change from 8 2 to 7 8 km s 0 2 over less than 25 km a relatively strong ne dipping normal incidence and wide angle reflection at 19 23 s twt indicates that the suture extends into the upper mantle the pervasive ne dipping reflection fabric of the intermediate terrane is interpreted as shear zones that developed during collision and possibly were reactivated by later events high poisson s ratios suggest a mafic composition or high fluid content the 1 86 ga collision was probably succeeded by continental break up and removal of an unknown continent except for the intermediate terrane subsequent formation of an east dipping subduction zone further to the west led to the emplacement of 1 81 1 77 ga old granitoids in the southern part of the transscandinavian igneous belt the 1 65 1 60 ga suture against the southwestern terrane is defined by a semi continuous band of strong sw dipping reflections between 3 and 8 s twt over 65 km which are interpreted as a low angle thrust zone along which gothian crust overrode the intermediate terrane the identification of three individual seismic terranes in the southeastern part of the baltic shield provides new evidence for palaeoproterozoic plate tectonic processes

deformation within part of a major crustal shear zone parry sound ontario structure and kinematics deformation within part of the parry sound shear zone pssz in the southwestern grenville structural province has been examined at the transition from mylonitized gabbroic anorthosite and orthogneiss within the body of the pssz to retrograde granulites in the immediate hanging wall detailed structural analysis shows a framework of internally deformed imbricate domains within the hanging wall granulites that are truncated by a major dislocation within the pssz mylonites deformation geometries within discrete hanging wall domains and the pssz exhibit strong similarities ascribable to a common deformation cycle that typically preserves three generations of overprinting structures this cycle comprises i formation of shear induced recumbent isoclinal folds and stretching lineations with concomitant transposition of compositional layering ii rotation of fold axes during noncoaxial shear into parallelism with the stretching lineation and formation of sheath folds and iii refolding about axes nearly