clastic intrusions in deep sea fan deposits of the rosroe formation lower ordovician western ireland abstract the rosroe formation contains discordant elastic intrusions and essentially stratiform clastic intrusions up to 40 cm thick that display little evidence of injection features of the elastic intrusions such as upper surface irregularities and tapered lateral margins also occur in wedged clastic layers up to 30 cm thick in the rosroe formation and suggest they may be injected sheets a few of the mostly granule grade injected sheets in the rosroe formation contain banding that is remarkably similar to the banding in the formation s stratified pebbly sandstone together with the existence of essentially stratiform injected sheets the banding suggests that some elastic sills in deep sea fan deposits may be indistinguishable from normally deposited beds contactual features sharp upper contacts upper surface irregularities truncation of superjacent strata upper surface inclusions nesting and blunt steep and tapered lateral margins and internal characters lenticular breccia and trains of aligned laminite clasts of the elastic intrusions in the rosroe formation provide tentative criteria for the identification of clastic sills that simulate normally dep sited beds in deep sea fan deposits

a condensed lower to middle ordovician graptolite succession at matane quebec canada the highly condensed lower to middle ordovician deep water succession from the cap des rosiers group at matane bears three conspicuous graptolite faunas of late tremadoc to mid darriwilian age the tremadoc fauna belongs to the aorograptus victoriae zone in the castlemainian isograptus victoriae maximodivergens zone fauna the earliest pseudisograptus species pseudisograptus initialis n sp appears together with the ancestral arienigraptus gracilis a darriwilian fauna with holmograptus spinosus and bergstroemograptus crawfordi represents the youngest pre nemagraptus gracilis zone fauna described from the quebec appalachians

large scale crustal heterogeneities and lithospheric strength in cratons abstract the rheology and thermal structure of the continental lithosphere are intimately linked in old cratons the effective elastic thickness of the lithosphere has been estimated by various spectral inverse methods based on the correlation between topography and gravity anomalies estimates vary within a very large range from 40 km to 120 km depending on the method used in this paper we use forward models to account for lateral variations in mechanical properties and their effect on the equivalent elastic thickness eet of the lithosphere from these models which allow brittle elastic ductile rheologies and mechanical discontinuities faults we have calculated the strain stress distributions and displacement fields vertical integration of the stress permits a local determination of the effective elastic thickness the computed displacements were used to calculate related bouguer and free air gravity anomalies and compare them with the observations the analysis is applied to the 2000 ma kapuskasing uplift in the superior province of the canadian shield where the presence of a high density block in the upper crust is due to the upthrusting of midcrustal rocks along a major thrust fault the study shows that the stability of this structure on geologic time scales requires a strong lower crustal rheology a cold geotherm and the fault to be healed this study also shows that because of stress dependence of the non linear rheology crustal heterogeneities may cause significant 40 local reductions of the lithospheric strength away from the kapuskasing structure the average strength of the lithosphere remains high eet 100 km conventional methods for estimating the elastic thickness would not resolve such local strength reductions in cratons but would predict depending on the method used highly overestimated or instead underestimated eet

franciscan complex olistostrome at crescent city northern california an olistostrome and bounding turbidites are exposed within the late mesozoic franciscan complex along the crescent city california coastline facies grade in character from mutti ricci lucchi 1978 mixed facies b c and d to f the olistostrome to mixed a b and e progressing upwards within the franciscan stratigraphic section the facies f unit outcrop is up to 600 m thick and extends 12 km along strike it consists of oblate to tabular blocks up to 200 m in maximum dimension of greenstone tonalite radiolarian chert limestone phyllite and greywacke dispersed in a scaly argillite matrix the olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites by the presence of abundant recycled sedimentary clasts within the unit and by the presence of sedimentary breccias and associated dismembered slump folded turbidites both within the olistostrome and among subjacent turbidites sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected late mesozoic sierran klamath magmatic are franciscan rocks record an early pervasive layer parallel flattening strain in such features as extensional faults necking and pinch and swell structures several scales of extensional faulting account for the juxtaposition of turbidites of different facies and or with varying degrees of stratal disruption the formation of sandstone lozenges and the formation of scaly foliation in the olistostrome matrix the latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay these were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome the presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix and of sandstone dykes that intrude fractures and associated drag folded turbidite beds indicate that franciscan sediments were not lithified during their early deformation these sediments were deposited in either a trench or trench slope basin and were first deformed either by gravity collapse of the trench slope cover or less likely by vertical loading beneath the toe of the accretionary wedge they later were folded during internal shortening of the growing franciscan accretionary prism

mechanisms of melange formation examples from sw japan and southern scotland abstract the formation of synthetic riedel shears either as discrete faults or ductile shear zones is important in the fragmentation of layers to form melanges the magnitude of longitudinal and shear strain necessary for this fragmentation has been assessed using a simple rigid domino model for layer parallel shear the strain required for layer fragmentation is a function of block aspect ratio and initial fracture dip relative to layering layer stretching can occur during simple shear if folds are developed but a component of extension parallel to the shear zones reduces the strain required for fragmentation simple models of this type break down when there is a component of dilation across the melange block bounding fractures allowing shale matrix infill or mineral vein growth some melanges show evidence in the form of symmetrical structures of a more coaxial deformation history and this may in part reflect deformation partitioning between sandstone and shale layers other mechanisms of melange formation include the development of out of sequence thrusts cutting through already dipping beds the causes of strike parallel extension are tentatively related to large scale evidence of arc parallel forearc stretching

evidence for a weak and overpressured decollement beneath sediment dominated accretionary prisms we present structural data from several underthrust sequences in southwest alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical 1 and regionally extensive layer parallel shearing these results contrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where 1 plunges gently seaward or toward the deformation front on the basis of mohr circle diagrams we propose that these different stress directions are consistent only with a model in which the two regimes are separated by a very weak decollement and calculate coefficient of friction values that range from 0 25 to 0 08 these values are significantly lower than coefficient of friction values calculated for the basal decollement in the taiwan fold and thrust belt e g 0 2 barr and dahlen 1989a and they are also lower than laboratory friction measurements for most rocks e g 0 6 0 8 byerlee 1978 consequently these low values suggest the presence of rocks with unusual material properties e g montmorillonite logan and rauenzahn 1987 or anomalous fluid pressures along the decollement on the basis of the occurrence of shale diapirs the distribution of early veins and the absence of montmorillonite in the underthrust sequences we propose that there was an anomalously high fluid pressure along the decollement fault instead of there being such a low coefficient of friction these results emphasize the importance of dewatering and fluid flow in controlling the position of the decollement which in turn determines how the accretionary prism grows in essence it is the development of a weak overpressured decollement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes

experimental studies of the fluidization of layered sediments and the formation of fluid escape structures experiments demonstrate that fluid escape structures can be produced as a result of unstable fluidization behaviour where a lower base layer of granular material is inhibited from fluidizing by the presence of an overlying non fluidizing top layer before the base layer can fluidize the weight of the overlying material must be balanced and this is accomplished by base layer material pressing against the bottom surface of the confining top layer forming a static layer this static layer allows the top layer to lift away from the base layer which is then free to fluidize a water filled crack forms below the static layer and as this grows instability causes the static layer and top layer to bend and conical voids to form below the antiformal sections rupture occurs at the apex of the water void allowing the underlying water and fluidizing material to burst out through the top layer the fluidized base layer material then flows through the rupture until all of this material except that in the static layer is deposited above the previously overlying layer and a stable fluidization system results the top layer material is bent upwards around the rupture and the resulting pillar type escape structure is preserved if flow then ceases the vigour of the burst out is greatest when the base layer material has a grain size 15 of the top layer material if the base layer grain size is less than 8 of the top layer then base layer material will pass through the top layer pore spaces without forming an escape structure if cohesive material is present escape structures form when a layer of fine grained cohesive material overlies a layer of cohesionless material at low flow rates small pipes with scattered angular bends pierce the top layer and base layer material passes through them the base layer material is ejected on to the top layer and builds up around the mouth of each pipe to form constructional structures sand volcanoes this is in contrast to the cohesionless experiments where the weight of material being deposited on the top layer caused an ejecta filled depression to form around the rupture if flow then ceases both the pipes and the sand volcanoes are preserved at high flow rates where the base layer fluidizes the top cohesive layer becomes fragmented small fragments circulate within the fluidizing base layer and are preserved as floating clasts large fragments sink to the bottom of the fluidizing base layer erosion of the bottom surface of these larger fragments causes this surface to become convex downward the experimentally derived structures are similar to pillar type structures observed in the field and the processes described can be used to investigate the development of these structures fluidization experiments also demonstrate the genesis of dish structures and the cohesive behaviour can be applied to the deformation of these structures after initial formation

integration and spatial analysis of high resolution geophysical and geological data eastern gaspe peninsula in the vicinity of the town of gaspe the relationships between the silurian devonian sedimentary succes sion of the gaspe belt and the humber and the dunnage zones are complex to unravel these relationships we used high resolution aeromagnetic data and regional gravimetric data coupled with field tectonostratigraphic information the magnetic vertical derivative located several magnetic anomalies associated with near surface features in the silurian devonian cover sequence in particular a conglomerate with magnetic fragments that overlies the late silurian salinic unconformity is clearly recognizable large ovoid anomalies of significant intensity located in the silurian devonian sedimentary cover area cannot be associated with any known geological feature the interpretation of the high pass and low pass filtered aeromagnetic survey indicates that the ovoid anomalies originate below the silurian devonian cover sequence the most significant of the ovoid anomalies is associated with a gravimetric anomaly it is proposed that these geophysical anomalies are probably associated with ultramafic and or volcanic rocks correlative in the subsurface with outcrops of the cambrian ordovician lithologies of the lady step complex and or the shickshock group resume dans la region de gaspe les relations entre la couverture sedimentaire siluro devonienne de la ceinture de gaspe et les zones de humber et de dunnage sont complexes pour mieux comprendre ces relations nous avons utilise des donnees aeromagnetiques de haute resolution et des donnees gravimetriques regionales appariees aux donnees tecto nostratigraphiques de terrain le gradient magnetique vertical a mis en evidence plusieurs signatures magnetiques peu profondes dans la couverture sedimentaire siluro devonienne en particulier un conglomerat a fragments magnetiques surmontant la discordance salinique du silurien tardif est aisement identifiable de grandes anomalies ovoides d intensite importante localisees dans la couverture siluro devonienne n ont pu etre associees a aucun element geologique connu le passage du leve magnetique dans des filtres passe haut et passe bas a permis de mieux cerner cer taines signatures magnetiques et de determiner que les anomalies ovoides prennent source sous la couverture siluro devonienne la plus grande des anomalies ovoides est associee a une anomalie gravimetrique il est propose que ces anomalies geophysiques soient vraisemblablement associees a des roches ultramafiques et ou volcaniques correlatives en profondeur a des lithologies affleurantes des unites cambro ordoviciennes du complexe de lady step et ou du groupe de shickshock

seismic properties of rock samples from the pikwitonei granulite belt god s lake domain crustal cross section manitoba laboratory measurements of compressional and shear wave velocity to confining pressures of 600 mpa for a suite of representative samples collected from the pikwitonei granulite belt and god s lake domain an archean crustal cross section in the northwestern superior province provide the basis of comparison of these terranes with the seismic characteristics of archean lower crust we found that felsic rocks in the pikwitonei granulite belt and god s lake domain which make up the bulk of these terranes have a similar average compressional wave velocity of 6 5 km s at 600 mpa indicating that felsic rocks show little velocity change across the amphibolite granulite facies transition compressional wave velocities for mafic rocks from each terrane are between 7 1 and 7 3 km s apparent poisson s ratio ranges from 0 24 to 0 26 and 0 26 to 0 28 for felsic and mafic rocks respectively these velocity data compare favorably with data for similar lithologies from the kapuskasing uplift using the relative abun

generation and obduction of ophiolites constraints from the bay of islands complex western newfoundland constraints provided by the bay of islands ophiolite complex boic western newfoundland suggest that its generation as well as its obduction is related to collision of the dunnage zone island arc system with the irregular continental margin of eastern north america the constraints include the supra subduction zone setting of the boic its extensional environment of generation the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the north american continental margin and the restriction of the boic as well as other appalachian ophiolites and associated taconian allochthons to reentrants in the north american margin during the early phases of collision of the dunnage arc with the north american margin subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation the first develops through trench rollback leading to ophiolite generation in the overriding arc plate outboard of the reentrant whereas the second develops in a transtensional environment with spreading centers offset by strike slip faults which propagate through the overriding plate away from the indenter promontory the configuration of the boic spreading ridge normal to the continental margin and the direction of initial obduction parallel to the margin favor generation of the ophiolite in an overall strike slip regime with complete subduction of the oceanic remnant extension in the overriding plate ceases and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the dunnage zone and for termination of arc volcanism along the north american margin

tectonic sediment thickening fluid expulsion and the thermal regime of subduction zone accretionary prisms the cascadia margin off vancouver island tectonic sediment thickening fluid expulsion and the resulting thermal regime of the northern cascadia subduction zone accretionary prism have been described by numerical models both the sediment thickening and the fluid expulsion are found to have important thermal effects constraints on the models are provided by 1 detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom simulating reflector bsr and 2 porosity changes across the accretion zone inferred from the landward variation in the p wave velocity depth profiles the heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20 below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes the porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea cascadia basin the model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically generating a high porosity underconsolidated section reconsolidation and fluid expulsion in response to the tectonic thickening is slow occurring mainly 20 30 km landward of the deformation front the maximum predicted fluid expulsion rate is about 4 10 11 m s 1 1 3 mm a 1 the pore pressure distribution across the deformation front region has been estimated from the velocity data using the simplifying assumption that velocity is only a function of effective pressure pore pressures are high reaching 80 of lithostatic 15 to 20 km landward of the deformation front an estimate of the bulk permeability in the seaward tens of kilometers of the prism ranging from 10 16 m2 near the top to about 10 18 m2 near the base has been obtained through matching the pore pressure distribution in the numerical model to that obtained from the velocity data the prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism

stratigraphie framework for the cambrian ordovician rift and passive margin successions from southern quebec to western newfoundland the taconian humber zone stretches from western newfoundland to southern quebec the early cambrian slope succession in newfoundland is found in the curling group whereas in quebec various units were deposited during that first time slice biostratigraphic data allow correlation of the curling group with the labrador group in newfoundland and with the newly time constrained slope succession in quebec the end of the rift drift transition is marked by a sea level lowstand at the end of the early cambrian the middle cambrian to latest early ordovician passive margin history recorded five cyclic sea level fluctuations three of these cycles are recorded in the shallow marine middle to late cambrian platform port au port group and slope sediments preserved in the cow head and northern head groups in newfoundland the biostratigraphic information assists correlation with cambrian passive margin units in quebec major sea level lowstands are recognized along the continental margin in early middle late camb

a 40ar 39ar study of oceanic and continental deformation processes during an oblique collision taconian orogeny in the quebec reentrant of the canadian appalachians received 7 december 2006 revised 9 january 2008 accepted 10 march 2008 published 1 july 2008 1 two phases of penetrative deformation are documented in the taconian hinterland of the appalachian orogen in the gaspepeninsula d1 is associated with the obduction of the mont albert ophiolite onto the paleozoic laurentian margin whereas d2 corresponds to later transport of allochthons across the margin in the metamorphic sole s1 is a se dipping mylonitic fabric with a downdip lineation in underlying metabasalts d1 is characterized by nw overturned and recumbent folds and a subhorizontal s1 schistosity with an ene trending orogen parallel lineation d2 is characterized by a s2 steeply dipping penetrative axial planar crenulation cleavage and ne trending f2 folds the intraoceanic thrusting of ophiolite is dated at 465 ma early d1 whereas emplacement of ophiolite andsubsequentdeformationofthemarginwasrecorded by isotopic signatures between 459 and 456 ma late d1 d2 is dated at 448 ma throughout the hinterland taconian transpressive deformation is related to an oblique collision within the quebec reentrant of the canadian appalachians during the ordovician citation malo m g ruffet a pincivy and a tremblay 2008 a 40 ar 39 ar study of oceanic and continental deformation processes during an oblique collision taconian orogeny in the quebec reentrant of the canadian appalachians tectonics 27 tc4001 doi 10 1029 2006tc002094

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

diverse melanges of an ancient subduction complex abstract three lithologically and structurally diverse melanges occur within an early paleozoic early middle ordovician subduction complex in the central appalachian orogen type i melange characterized by horizon of variabls y deformed sandstone and scaly mudstone that alternate with coherent sandstone rich sequences is interpreted to reflect accretion related deformation of water saturated trench deposits type ii melange composed of exotic radiolaria bearing mudstone clasts in a scaly mudstone matrix can be explained by remo biliza tion and mixing of inner trench slope sediments type iii melange is a poorly sorted polymict assemblage of native lithology clasts in a scaly mudstone matrix evidence o f forceful injection of matrix mud into clasts and inferred discordant contacts between melange and surrounding bedded deposits suggest that the type iii melange formed from mud diapirism the close association o f these melanges points out the diversity of tectonic and sedimentary processes previously documented from modern convergent margins that may be reflected in older sub duction complexes introduction

structural history of continental margin sediments beneath the bay of islands ophiolite newfoundland deformed continental margin sediments of the curling group underlie the bay of islands ophiolite in the humber arm allochthon of western newfoundland within the allochthon tectonic slices of sediments are separated by zones of melange the earliest structures in the slices are synsedimentary features produced by soft sediment deformation on the continental slope or rise later west facing asymmetrical f 1 folds without penetrative axial plane cleavage were probably produced during the emplacement of the allochthon in the middle ordovician taconic orogeny associated extensional structures include shear fracture and extension fracture boudins pervasive cataclastic shearing of shale and boudinage of competent beds produced melange between slices shear surfaces and the original bed parallel fissility of the shale both contribute to the anastomosing fabric of the melange matrix a subsequent folding event of probable acadian age produced upright to moderately inclined f 2 folds with axial plane cleavage increasing in intensity eastwards across the allochthon this event refolded the folds thrust slices and melange zones produced during emplacement later gentle cross folds associated with sporadic development of crenulation cleavage produced culminations and depressions on the f 2 fold hinges

retrograde eclogite from mont albert gaspe quebec retrograde eclogite is found along the northern perimeter of ultramafic rocks in the mont albert complex gaspe quebec garnet clinopyroxene geothermometry gives 840 c as the peak temperature of metamorphism kyanite in associated metapelites requires that at this temperature pressure be a minimum of 12 kbar based on symplectic textures in the eclogitic and associated rocks and clinozoisite white mica quartz plagioclase equilibria a counterwise pressure temperature trajectory that terminates near 500 c and 8 kbar can be traced for the mont albert complex

deformational styles in a sequence of olistostromal melanges pacific rim complex western vancouver island canada the pacific rim complex is exposed in a fault bounded slice along the west coast of vancouver island it is divided into two parts 1 the ucluth formation a basement unit composed of lower mesozoic arc volcanic rocks and 2 a suprajacent sedimentary sequence more than 2 km thick composed of lower cretaceous olistostromal melanges an unconformity is present at least locally at the ucluth melange contact and indicates that the ucluth formation is stratigraphic basement to the melanges melange style deformation is restricted to the sedimentary sequence and has affected virtually all parts of that sequence the melanges formed without the development of a cleavage or pervasive cataclasis indicating that deformation occurred prior to consolidation of the sediments three types of melanges have been recognized the first consists of a matrix of inter bedded mudstone chert sandstone and green tuff which encloses numerous exotic blocks of igneous rocks and limestone beds within the matrix show variable amounts of layer parallel fragmentation which is attributed to in situ liquefaction and lateral flowage within individual beds the blocks in this melange most of which were derived from the underlying ucluth formation are considered to be submarine slide blocks produced by rock falls from nearby basement scarps the matrix sediments near the blocks commonly contain wispy lenses of poorly sorted plutonic and volcanic detritus green tuff interpreted as scree from the rock slides the second and third types of melange are distinguished by the predominance of either mudstone or sandstone but are otherwise quite similar these melanges lack exotic blocks and consist of a contorted assemblage of mudstone turbidite sandstone conglomerate pebbly mudstone and rare chert all of which are depositionally interrelated sedimentary structures and stratal coherence are generally well preserved the internal structure of these melanges consists of a series of dismembered depositional sequences individual sequences are commonly more than 75 m thick some are upright and others are overturned paleoecological evidence indicates that some of these sequences were originally deposited in an infra slope basin thus these melanges are interpreted to have formed mainly by the accumulation of large slump sheets at the base of a submarine slope previous interpretations have considered the pacific rim complex to be a late mesozoic subduction complex constructed along the western margin of the wrangellia terrane vancouver island several factors argue against a subduction melange interpretation 1 the melanges were deposited on an older arc related basement not oceanic crust 2 exotic blocks were introduced as rock slides and not by subsurface faulting and 3 at least some of the strata originated in an infra slope basin the heterogeneous deformational style is more compatible with an origin by near surface mass movement processes including submarine slides rock falls debris flows and in situ liquefaction the amount of mass wasting recorded in the pacific rim melanges probably requires a seismically active setting where frequent ground shaking produced large transients in pore fluid pressure similarities in stratigraphy and metamorphism suggest that during late cretaceous time the pacific rim complex was part of the san juan cascades thrust system located 220 km to the southeast in northwestern washington state if this is correct the pacific rim melanges would have formed on the inboard side of wrangellia about 30 m y prior to the collision of wrangellia with the american continent the present location of the pacific rim outboard of wrangellia is attributed to younger latest cretaceous or early tertiary transcurrent displacement

40ar 39ar ages for minerals from the amphibolite dynamothermal aureole mont albert gaspe quebec new 40ar 39ar ages are presented for two hornblendes and one biotite from the dynamothermal aureole that is situated at the base of the mont albeit ophiolite analysis by the incremental release technique reveals a small component of excess 40ar in one hornblende though both exhibit plateau type spectra the biotite was analyzed by the total fusion technique all apparent ages are concordant and average 456 3 ma dynamothermal aureoles form during the obduction process which juxtaposes ophiolites and amphibolite protoliths during a compressional tectonic event at a plate margin in this instance thrusting of the mont albert ophiolite over rocks of the shickshock group the concordance of biotite and hornblende ages indicates rapid postmetamorphic cooling and firmly dates obduction at 456 3 ma ago an age younger than any of the other northern appalachian ophiolites the high pressure metamorphism of the amphibolite is inconsistent with the present high structural level of the composite mont albert

tempo of earliest ordovician graptolite faunal succession conodont based correlations from the tremadocian of quebec successive tremadocian planktic dendroid graptolite assemblages from continental slope sequences in quebec can be correlated with north american platform biozonations on the basis of conodonts anisograptid bearing assemblage 2 middle tremadocian matane faunas are associated with early ordovician rossodus manitouensis zone new designation conodonts younger middle tremadocian faunas with adelograptids assemblage 3 are no younger than the rossodus manitouensis zone key dendroid evolutionary immigration events take place within the lower conodont fauna b interval rooted dendroids near cap des rosiers quebec and in eastern new york state occur with lower fauna b conodonts and the trilobites pareuloma and borthaspidella however the earliest tremadocian and earliest ordovician dendroid immigration event represented by the local lowest occurrence of faunas with dictyonema flabelliforme s l at localities in western newfoundland eastern new york state norway and eastern china also lies with

quartz arenites of the cambro ordovician kamouraska formation quebec appalachians canada i deep water depositional processes in a continental slope environment the kamouraska formation is an uppermost cambrian lowermost ordovician quartz arenite dominated unit of controversial origin deposited on the southeastern slope of laurentia bordering the iapetus ocean it is exposed in the quebec appalachians on the south shore of the st lawrence estuary the formation consists of basal polymictic conglomerate and overlying massive sheet like quartz arenite the conglomerate beds are reversely and reversely to normally graded the quartz arenite beds are generally massive although they may show coarse tail grading beds containing full or partial bouma sequences are rare paleoflow directions from ripple cross lamination ripple marks on bed surfaces and sole marks point towards southeast south and southwest the clastic sediments of the kamouraska were transported into the deep sea by sediment gravity flows that evolved from hyperconcentrated to concentrated density flows and then to turbidity currents the depositional environment is interpreted to have been a

ramp flat geometry within the central kapuskasing uplift evidence from potential field modeling results potential field data constrained by seismic reflection seismic refraction geologic and physical properties data have been used to study the deep crustal structure of the central kapuskasing uplift the val rita block is interpreted as a thrust sheet of granulites detached at mid crustal depths and uplifted along a ramp producing an arched double granulite layer a thin thrust sheet of southward thickening granulites best describes the groundhog river block whereas simple thrusting along a ramp characterizes the chapleau block from this interpretation a sequential evolution of the kapuskasing uplift is proposed at about 2450 ma northwest southeast compression 1 resulted in variable thrusting along the ivanhoe lake fault zone this was followed by a period of normal faulting resulting in the juxtaposition of high and low pressure rocks across the lepage and saganash lake faults with possible truncation of the val rita block arch further rotation of 1 to an east west direction resulted in the

age and composition of the ruisseau isabelle melange along the shickshock sud fault zone constraints on the timing of melanges formation in the gaspe appalachians melanges are common in the humber and dunnage zones of the quebec appalachians humber zone melanges are spatially associated with taconian nappes and believed to be of tectonic origin dunnage zone melanges are interpreted as relicts of an ordovician accretionary prism the ruisseau isabelle melange rim in the gaspe ap palachians was first interpreted as a dunnage zone melange because of its tectonic position along the baie verte brompton line and lithological components such as dark mudstone blocks of ultramafic rocks and greenish chromite bearing sandstone the rim comprises sedimentary rock assemblages composite shale black shale and chromite bearing sandstone assemblages an exotic block a pebbly mudstone assemblage and slivers of serpentinized peridotite and metamorphic tectonite most melanges of the dunnage zone in the quebec appalachians contain partly similar rocks but their age is not well constrained recent work on the rim and cap chat melange of the humber zone and new fossil finds in the rim rock assemblages allow us to conclude that previous distinction between melanges of the dunnage and humber zones on the basis of rock type associations is not valid according to the age of the rim rock assemblages melanges in the northern gaspe appalachians continued forming after the mid caradocian later than the dunnage zone melanges of southern gaspe appalachians the juxtaposition and structural features of the rim rock assemblages result from repeated faulting along the shickshock sud fault from late ordovi cian to middle devonian

arenig biostratigraphy of the pointe de levy slice quebec appalachians canada the pointe de levy slice includes a nearly complete arenig graptolite succession a conspicuous gap exists in the yapeenian ya to lower darriwilian da part ya 1 da 1 the lithological column is compiled from several sections at levis quebec that yield only parts of the complete succession the graptolite zonation can be compared closely with that from the cow head group of western newfoundland and the deep kill formation of eastern new york the shumardia limestone is differentiated into four graptolite zones and subzones of which the youngest zone clearly belongs to the llanvirn the araneograptus murrayi and holmograptus lentus zones are documented for the first time from the levis area the undulograptus dentatus zone of the latest arenig is introduced

crystalline lithology across the kapuskasing uplift determined using in situ poisson s ratio from seismic tomography seismic refraction tomography has been used to image lateral velocity variations both vp and vs within the shallow crystalline basement using travel times from a high resolution reflection profile across the kapuskasing uplift structure in northern ontario comparison of these in situ velocities and the outcrop geology indicates a correlation of velocity with the major lithologic units anorthosite vp 6150 6350 m s and vs 2700 3000 m s mafic gneiss vp 6000 6250 m s and vs 2775 3000 m s tonalite vp 5850 6000 m s and vs 2950 3025 m s vp measurements on core samples from the main lithologic units confirm that tonalite can be be clearly distinguished from mafic gneiss and anorthosite based on vp the observed vp values are attributed to mineralogy based on modal analyses of core samples from the same region poisson s ratio calculated from in situ vp and vs helps to discriminate between velocity variations caused by porosity effects from those due to lithology values of poisson s ratio obtained for the three lithologic units within the upper 100 m of the basement are anorthosite 0 35 0 39 mafic gneiss 0 34 0 38 and tonalite 0 32 0 34 these relatively high values of poisson s ratio indicate that the shallow basement is porous and water saturated poisson s ratio is a particularly effective discriminator in this instance because of its extremely low value in quartz 0 08 relative to other silicate minerals 0 21 0 32 and the greater quartz content of tonalite 27 36 relative to anorthosite and mafic gneiss 0 23 reflection coefficients calculated for the contrasting lithologies range up to 0 065 for tonalite mafic gneiss contacts and up to 0 044 for tonalite anorthosite contacts confirming that an interlayered sequence of tonalitic anorthositic and mafic rocks mapped at the surface may be the cause of the shallow reflections observed in the coincident seismic reflection data

surface and subsurface signatures of gas seepage in the st lawrence estuary canada significance to hydrocarbon exploration abstract the seabed of the st lawrence estuary is characterized by many fluid releasing features on multibeam bathymetric images these features correspond to crater like depression pockmarks predominantly found on the northwestern shoulder of the 300 m deep laurentian channel as well as on the channel floor aligned pockmarks which define segments up to 12 km long are frequent in the laurentian channel whereas they are preferentially associated with submarine landslides on the northwestern shoulder of the channel on high resolution seismic profiles pockmarks found in the laurentian channel are characterized by seismic chimneys that may be traced down to the autochthonous paleozoic rocks st lawrence platform suggesting a thermogenic origin for the gas on the northwest shoulder of the laurentian channel the seismic signature of pockmarks does not extend downward to the reflector that corresponds to bedrock st lawrence platform and or grenvillian province suggesting together with high sedimentation rate a biogenic origin for the gas these results are discussed in the light of the data collected in the onshore parts of the st lawrence platform and suggest the presence of a mature hydrocarbon source the conclusions are supportive arguments for the hydrocarbon prospectivity of both the onshore and offshore segments of the paleozoic autochthonous domain st lawrence platform

petrogenetic significance of solid carbonate inclusions in apatite of the sukulu carbonatite uganda abstract many solid inclusions occur in apatite of the sukulu carbonatite uganda of which the most abundant are carbonate which can be classified into clear mg calcite and pitted calcite inclusions based on their morphology texture and chemical composition although such solid inclusions are ubiquitous in carbonatite apatite and have been described by many workers this study provides new insight into their genesis and petrogenetic significance the pitted inclusions commonly have elongate or spherical shapes and are spatially related to microfractures in the apatite host they probably developed from early primary aqueous or mg calcite solid inclusions by infiltration of post magmatic fluids through the microfractures the clear inclusions generally have spheroidal shapes and are thought to represent an early magmatic phase and to be typically magmatic in origin electron microprobe analysis indicates that the clear inclusions contain 1 6 wt mgo and the pitted ones 1100 c prior to apatite formation in contrast other clear inclusions became dark or brownish and remarkably homogeneous on heating at relatively moderate temperatures 740 912 c indicating that they may represent true melt inclusions trapped as melts during apatite growth the present findings clearly illustrate the importance of both magmatic and post magmatic processes in the genesis of the carbonate of the sukulu carbonatite complex they also suggest that extensive post magmatic processes are likely to have been responsible for development of the low mg calcite and associated dolomite which dominate the sovites of this complex

growth processes and melange formation in the southern apennines accretionary wedge abstract the southern apennines owe their present geometry to the subduction of an apulian continental lithospheric slab and are thus best interpreted as an accretionary wedge southward the apenninic thrust front branches on to the still active ionian wedge which is related to the present calabro sicilian subduction the interpretation of conventional seismic reflection profiles and exploration wells combined with geologic field studies biostratigraphy and structural geology provide constraints for a geodynamic model for the post messinian growth of the apenninic prism our model for the apennines is a good analogy for submerged oceanic accretionary prisms in both instances features such as out of sequence thrusts underplating and duplexes are viewed as processes to explain the thickening of accretionary wedges syntectonic terrigenous deposits have accumulated in the synclines developed above the flats of pliocene thrust faults these depocenters represent piggy back basins younger accretion at the eastern border of the accretionary wedge has led to a progressive uplifting and tilting of the already emplaced thrust sheets including a continous westward migration of the depocenters in the piggy back basins during pliocene and early quaternary times a contrary west to east migration of the depocenters locally occurred where preexisting flats of pliocene thrust faults have been refolded rather than only being rotated dewatering processes and effects of overpressures on undercompacted sediment deformations can also be studied in the southern apennines melanges and broken formations form an important component of the trench apparently restricted to the base of tectonic nappes listric normal faults are a major structural element which preserves in combination with the erosion of topographic highs and resedimentation in the piggy back basins a uniform geometric shape critical taper for the prism the age and location of these distensive or gravitational effects are also discussed

criteria helpful in recognizing shear zone and diapiric melanges examples from the hoh accretionary complex olympic peninsula washington a study of the middle miocene hoh melanges exposed on the western olympic peninsula washington provides criteria for differentiating between shear zone and diapiric melanges the shear zone melange is characterized by a consistent foliation orientation throughout the outcrop intense pervasive scaly foliation uniform fold vergence directions exotic clasts predominantly phacoidal clasts and consistent clast long axis lineations the diapiric melange is characterized by radial foliation in map view well developed scaly foliation at the contacts and poorly developed scaly foliation in the center of the melange opposite fold vergence directions across the melange and rare exotic clasts additionally clasts vary in shape from phacoidal at the contacts to angular in the center in the diapiric melange large clasts are found only in the center of the melange and clast long axes vary from tightly clustered at the contacts to near random orientations in the center the systematic variation in clast size in the diapir may be due to increased shear at the margins and or clast migration to the center of the diapir caused by a velocity viscosity contrast interpretation of field data indicates that some clasts rotate continuously in the matrix yet clast long axes have a definite preferred orientation this contradiction may be resolved by theoretical models indicating that clasts rotate fastest at high angles to the shear plane thus although clasts above a critical aspect ratio asymptotically approach stability parallel to the shear plane the long axes of rotating clasts spend a proportionally greater amount of time close to the shear plane parasitic folds in the matrix indicate that clast long axes align parallel to transport direction small scale folds in the matrix clast long axis lineations and the mean foliation indicate that the hogsback section of the hoh melange was emplaced by a high angle oblique thrust directed to n10 e

introduction to special section on the role of fluids in sediment accretion deformation diagenesis and metamorphism in subduction zones accretionary prisms which develop from materials scraped from the subducting oceanic lithosphere form within one of the most dynamic structural environments on earth and constitute critical elements of mountain belts in the marine setting these large accumulations of sediments grow in a water saturated environment consequently their tectonic chemical and thermal evolution is strongly influenced both by the seawater that is incorporated with the sediment as it is accreted and by water that is rendered from the clays and rocks of the prism as they are subjected to increasing pressure and temperatures figure 1 although accretionary prisms are mechanically similar to subaerial fold and thrust belts the latter may develop in consolidated partially saturated sediments with less pronounced hydrogeologic effects

melanges intrusive and extrusive sediments and hydraulic arcs water carrying sediment may intrude with or without associated extrusion wherever the fluid pressure exceeds the intrusion strength of the surrounding materials it has recently been suggested that thrust sheets moving toward continental interiors act as giant squeegees driving fluids through their substrates ahead of them active ice sheets and sedimentary prisms can also squeeze fluids through rocks such fluids potentially inject hydraulic melanges along their plumbing systems but these are difficult to distinguish from other melanges and from broken formations or olistostromes because of surficial slumping or extrusion miniature examples of melanges pumped through a marine slope by the tidal grounding of an ice sheet of permian carboniferous dwyka age in south africa are used to illustrate that there are even more categories of melange than previously suspected instead of being clarified by recent work the problem of deducing the origin of individual melanges becomes more difficult this is because melanges are formed by the mixing of thick hydrous slurries in any confined conditions and are not diagnostic of subduction nonetheless the results of hydraulic activity within a subglacial marine slope suggest that the sedimentary volcanoes developed on many accretionary prisms overlie relatively cold hydraulic arcs oceanward of the hot magmatic arcs behind subduction zones

holmograptus spinosus and the middle ordovician darriwilian graptolite biostratigraphy at les mechins quebec canada the les mechins region in quebec provides a unique opportunity to understand the darriwilian graptolite suc cession in eastern north america the graptolite faunas of the upper member of the anse au crapaud formation include the arienigraptus gracilis oncograptus upsilon undulograptus austrodentatus and holmograptus spinosus biozone fau nas and thus complement the well known darriwilian faunas of the cow head table head and goose tickle groups of western newfoundland in particular the holmograptus spinosus biozone interval of calcarenitic limestones bears a rare combination of benthic shelly and planktic graptolitic faunas with conodonts brachiopods ostracods sponge spicules and algal remains associated with common graptolites that allow an inter continental correlation the holmograptus spino sus biozone is introduced for the interval based on a diverse and characteristic graptolite fauna the fauna includes among others atopograptus woodwardi recognized for the first time in north america the holmograptus spinosus biozone fauna can be found worldwide and is easily correlated into the conodont biofacies using species of the genus histiodella ties between graptolite and conodont biozonations are discussed for the darriwilian silicified ostracods are illustrated for the first time from the interval hustedograptus quebecensis is described as new resume au quebec la region de les mechins fournit une occasion unique de comprendre la succession de graptolites du darriwilien dans l est de l amerique du nord les faunes graptolites du membre superieur de la formation de l anse au crapaud comprennent les faunes des biozones aarienigraptus gracilis aoncograptus upsilon aundulograptus austro dentatus et aholmograptus spinosus ces fossiles s ajoutent ainsi aux faunes bien connues des groupes darriwilien de cow head de table head et de goose tickle de l ouest de terre neuve l intervalle de la biozone aholmograptus spi nosus des calcaires calcarenitiques comporte notamment une association rare de faunes benthiques acoquilles et de planctons agraptolites avec des conodontes des brachiopodes des ostracodes des spicules d eponges et des restes d algues associes aux graptolites communs qui permettent une correlation intercontinentale la biozone aholmograptus spinosus est introduite pour l intervalle en se basant sur une faune de graptolites diversifiee et caracteristique la faune comprend entre autres atopograptus woodwardi reconnu pour la premiere fois en amerique du nord la faune de la bio zone aholmograptus spinosus peut etre trouvee partout atravers le monde et il est assez facile de la faire correspondre au biofacies de conodontes en utilisant des especes du genre histiodella les liens entre les biozones agraptolites et acono dontes sont discutes pour le darriwilien des ostracodes silicifies sont illustres pour la premiere fois apartir de l intervalle hustedograptus quebecensis est decrit en tant que nouveau fossile traduit par la redaction

stratal disruption and development of melange western newfoundland effect of high fluid pressure in an accretionary terrain during ophiolite emplacement abstract the bay of islands ophiolite was emplaced onto the continental margin of north america during the mid ordovician taconic orogeny when tectonic slices of continental margin sediments were accreted to the moving allochthon tectonic slices grade into and are surrounded by melange early fracture in sandstones formed without grain breakage and allowed penetration of liquid petroleum along fracture planes other fractures involved cataclastic flow and were sometimes re activated during formation of later pressure solution cleavage shear fracture and extension fracture boudinage affect competent strata extensional veins cut cement in limestone beds and are filled by shale quartz calcite and bitumen folds also formed at a time when siltstone and sandstone were at least partially lithified melange matrix shows abundant shear and extension fractures in a variety of orientations coaxial extension responsible for disruption of bedding can be explained by a brittle accretionary wedge model in which high fluid pressures resulted from tectonic dewatering of shales surface slope decreased as fluid pressure rose beneath the ophiolite causing horizontal extension of the wedge after escape of excess water the surface slope steepened again as renewed stacking occurred

regional metamorphism of the appalachian humber zone of gaspe peninsula 40ar 39ar evidence for crustal thickening during the taconian orogeny geochronological studies on the timing of deformation and metamorphism along the laurentian margin have shown that the ages of metamorphic events change along strike within the newfoundland southern quebec segment of the canadian appalachians the gaspe peninsula is located at mid point of the two extremities of this segment new single grain laser 40ar 39ar plateau ages solely reflect latest middle late ordovician metamorphism samples taken within the internal humber zone in the shickshock group rocks yield 40ar 39ar muscovite and hornblende ages ranging from 457 to 454 ma samples from the amphibolite du diable the metamorphic sole of the mont albert ophiolite yield 40ar 39ar muscovite and hornblende ages ranging from 459 to 457 ma ordovician ages of the internal humber zone are consistent with 40ar 39ar ages from southern quebec and are interpreted as the result of the emplacement onto the margin of both the ophiolite and its metamorphic sole

a geophysical model for the kapuskasing uplift from seismic and gravity studies the kapuskasing uplift is an oblique cross section of archean crust exposed by a major thrusting event in early proterozoic times previous results from the traveltime and amplitude analysis of compressional wave p wave arrivals from a seismic refraction experiment have been used to constrain the modelling of shear wave s wave arrivals and gravity anomalies along the seismic profiles s wave and p wave velocity information have been combined to obtain the variations of poisson s ratio within the crust high and low poisson s ratio values have been linked to the mafic and felsic content respectively of the shield rocks density variations along the profiles constrained by the p wave velocity structures and the observed gravity anomalies again have been linked to the lithological variations as observed in the exposed cross section geological models constrained by the geophysical observations and the cross sectional exposure have been constructed for profiles across the northern and southern porti

raman microspectrometry of fluid inclusions abstract for many kinds of fluid inclusions the coupling of microthermometry and raman microspectrometry is still the only viable option to obtain compositions of single fluid inclusions a review is given on the basis of 16 years of experience and helped with about 120 references of the instrumentation analytical conditions and methodology of the application of raman microspectrometry to gaseous aqueous and hydrocarbon inclusions and their daughter minerals

ophiolitic source rocks for taconic age flysch trace element evidence flysch generated along the appalachian orogen during the ordovician taconic orogeny ranges in age from late arenig to early caradoc allochthonous flysch and from late llanvirn to ashgill autochthonous flysch sandstones in these units are dominantly lithic arenites with a heavy mineral suite containing common to abundant mg rich chromite north of the canada united states border the only reasonable sources for the chromite were the lower ultramafic units of obducted ophiolites trace element analyses cr ni v cu ti zr y of flysch sandstones indicate a striking increase in cr and ni concentrations from the united states appalachians to the canadian appalachians in the united states units cr is in the range of 10 to 50 ppm canadian flysch units average an order of magnitude more cr these results suggest that ophiolite obduction either did not occur or was of minor importance during the taconic orogeny in the united states appalachians the difference in ophiolite abundance between the canadian and united states appalachians suggests a fundamental difference in tectonic style along the orogen a feature that must be accommodated by plate tectonic models new models will probably need to consider both obliquity of subduction and geological complexity of the ordovician collision zone in order to explain the spatial distribution of obducted ophiolite sheets

clastic dike swarms and the structural evolution of the foreland fold and thrust belt of the southern andes clastic dike swarms are intruded into a cenozoic foreland fold and thrust belt in southern chile the dike material consists of sandstone and conglomerate with clasts as much as 25 cm in diameter brecciated wall rocks are also found in the clastic matrix the dikes often exceed 50 cm in width and hundred of metres in length in outcrop and are oriented overwhelmingly transverse to fold axes and parallel to cross joints the dike swarms are concentrated near the toe of major thrusts where they are found injected into the hanging wall a regional axial planar spaced cleavage is developed in the clastic dikes and sills clearly postdating injection of clastic material dike injection postdates at least the initiation of extension fractures the cross joint set within the swarms the dike intrusions represent as much as 20 extension parallel to fold axes in addition to several percent extension represented regionally by the cross joints the structural features of the dike swarms and their geometric relationship to other structures suggest rapid single phase injection the arrival of the front edge of a thrust on an already highly overpressured sedimentary sequence would cause an instantaneous increase in the overburden pressure and therefore could result in injection of a dense slurry of wet coarse sediments along preexisting planes of weakness field relationships of mesoscopic structures suggest the following evolution cross joint development incipient cleavage development initiation of thrusting and buckling clastic dike injection regional spaced cleavage development and associated late stage tightening of folds

paleogeography of the matapedia basin in the gaspe appalachians initiation of the gaspe belt successor basin the matapedia basin consists of the uppermost ordovician lowermost silurian deep water fine grained carbonate siliciclastic rocks of the honorat garin formation and matapedia groups pabos and white head formations the lower rock assemblage of the gaspe belt in the gaspe appalachians paleogeographic maps of eight time slices from the caradocian to the llandoverian are presented to better understand the tectonosedimentary evolution of the matapedia basin deposition evolved from siliciclastic garin fm to argillaceous limestones pabos fm to limestones white head fm the overall change from terrigenous garin fm to limestone facies white head fm reflects a change in the source area paleocurrent directions and composition of sandstones indicate an orogenic source area to the south for the garin formation which is believed to be the inliers of the humber and dunnage zones in the southern gaspe and new brunswick appalachians lime muds deposited by turbidity currents coming from the nor

the saint daniel melange evolution of an accretionary complex in the dunnage terrane of the quebec appalachians the saint daniel melange is part of a series of melanges located along the baie verte brompton line in the northern appalachians this line marks the suture between rocks of oceanic affinities and those of the ancient passive margin of north america with which they collided during the taconian middle to late ordovician orogeny the saint daniel melange contains a wide variety of lithologies including well bedded to dismembered sedimentary sequences pebbly mudstone olistostromes and slivers of igneous and metamorphic rocks black shales with interbeds of green shale calcareous siltstone or sandstone are the dominant units they exhibit various stages of melange formation such as those present in shallow parts of an accretionary complex units of oceanic origin include sediments derived from the forearc basin and slivers of an ophiolite and of a magmatic arc units derived from sediments of the passive margin of north america are also present the ratio between these various lithologies changes greatly within the melange on a kilometric scale along strike the saint daniel melange is a structural complex in which the various units were assigned a sequential order mimicking a stratigraphic order the saint daniel melange is interpreted as the relict of an accretionary complex because of its actual structural position within the northern appalachians and because all its lithologies and their structural fabric can be found in modern accretionary complexes

development of inverted metamorphic gradient in the internal domain of the taconian belt gaspe peninsula the mont logan nappe is part of the taconian internal domain of the quebec appalachians and is entirely made up of synrift to passive margin elastics and volcanics of the shickshock group rocks o

underplated units in an accretionary complex melange of the shimanto belt of eastern shikoku southwest japan imbricated thrust stacks composed of melange and oceanic slabs occur in the shimanto belt eastern shikoku sw japan the melanges are divided into two types melange i consists of lenticular sandstone blocks surrounded by scaly shale matrix and melange ii is composed of blocks of oceanic material such as basalts cherts and red shale in a scaly matrix of shale melange ii is observed only near the boundaries between melange i and oceanic slabs the fabric of melange i indicates progressive deformation during underthrusting of originally coherent trench fill turbidites layer parallel extension characterizes early deformation and is divided into two stages an early stage formed during normal faulting and a subseguent stage formed perpendicular to the early extension the early extension is interpreted in terms of riedel shear associated with layer parallel shearing and tectonically induced loading normal to the layering the second extension is due to sticking thrust movement late deformation of melange i is represented by folding in association with layer parallel shearing this appears to be related to compression just before or during underplating of melange i beneath the accretionary prism layer parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments subsequently the shear zone penetrates pelagic sediments and finally oceanic basement at this time melange ii is formed by the shearing an imbricated stack of melanges i ii and oceanic slabs forms duplexes during the underplating process the weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench fill is thin

experimental soft sediment deformation structures formed by the liquefaction of unconsolidated sands and some ancient examples the effects of liquefaction in saturated sand bodies under a variety of driving forces are described from shaking table experiments and structures from the geological record are presented which are analogous to the experimental structures the collapse of sloping heaps of cross bedded sand under a gravitational body force generates low angle essentially uncontorted stratification a basal zone of shearing may be present with steepened and folded foresets stretching of foresets may be accommodated on normal faults and bottomsets may be contorted into inclined folds in natural systems the substrate may also liquefy causing deformation driven by an unevenly distributed confining load stratification in the surface bedform is flattened and stratification in the substratum contorted experiments failed to produce relative displacement at the interface between stacked sand bodies liquefaction of gravitationally unstable systems in sands generates load structures comparable to those from sand mud systems recumbent folded deformed cross bedding is formed by current shear over a liquefied bed as has been inferred from field and theoretical analyses shear of nonliquefied sand forms angular folds other deformation mechanisms such as fluidization or seepage may generate structures similar to all of these local water escape structures driven by fluidization occur in the upper parts of some liquefied sand bodies they include cusps sand volcanoes and clastic dykes transient cavities formed in some experiments and seemed to be preserved as breached cusps although the experiments tried to isolate individual driving forces driving forces may operate together and there may be a continuum between deformation driven by water escape and deformation driven by loading different structures from those described here may form where liquefaction develops in a buried layer as opposed to at the sediment surface

a reevaluation of the position of the baie verte brompton line in the quebec appalachians the influence of middle devonian strike slip faulting in gaspe peninsula the baie verte brompton line bbl the surface expression of the taconian suture in the canadian appalachian orogen extends from southern quebec to the northeast end of newfoundland in the que

large scale liquefaction and fluidization in the cap chat melange quebec appalachians the cap chat melange crops out discontinuously for 200 km along the south shore of the st lawrence river in the gaspe peninsula it is located just south of logan s line the northern limit of the humber zone with the taconian foreland basin this melange is composed of dismembered rocks of the adjacent formations in particular the lower ordovician riviere ouelle and middle ordovician tourelle formations with lesser contributions by the middle ordovician des landes and the cambrian orignal formations blocks in the melange vary in size from a few centimetres to several kilometres with well preserved internal stratigraphy in the larger blocks the distribution of blocks is not uniform and the composition of the surrounding matrix changes with corresponding changes in block composition tectonic processes mostly extensional and compressional faulting are responsible for some of the chaotic aspects of the melange however the main mechanism was as follows i large scale liquefaction of the mudstone rich riviere ouelle formation ii sinking with consequent dismembering of the tourelle formation into this underlying weakened riviere ouelle formation and iii fluidization of the lowermost sand beds of the tourelle formation resulting in abundant sandstone sills and dikes in the riviere ouelle formation

hinterland directed transtensional faulting at an orogen structural front the example of the cap chat melange quebec appalachians the cap chat melange is a regional scale dismembered unit that occurs close to the quebec appalachian front the melange records early d 1 taconian and late d 3 acadian regional deformations the greater degree of disruption of the melange compared with surrounding units is due to the superimposition of d 2 structures that are rare outside the melange d 2 structures include both layer extensional features such as block disruption along shear zones and faults and layer contractional features such as folds that record the transtensional collapse of the taconian orogenic wedge along a rheologically weak deformation zone the cap chat melange is interpreted as the westernmost remnant of a zone of mechanical decoupling in the upper crust which allows normal or transtensional faulting in the inner parts of the orogen this paper presents evidence that beyond the general complexity of melange units geological observations may still contribute to unraveling their tectonic history and that transtensional reactivation of original thrust faults is a viable mechanism in the frontal parts of orogens

collision along an irregular margin a regional plate tectonic interpretation of the canadian appalachians an idealized plate tectonic model for the pre carboniferous development of the canadian appalachians explains the 400 km dextral offset of tectonostratigraphic zones from quebec and northern new brunswick to newfoundland and the up to 600 km offset of oppositely verging belts of acadian deformation from the gaspe peninsula to eastern newfoundland it is proposed that these offsets which occur at the st lawrence promontory result from the collision of an irregular north american passive continental margin with island arc and continental crust to the east along an east dipping subduction zone the line of subduction is assumed to have been linear and the subducting slab to have maintained its mechanical integrity during collision a jigsaw fit of the opposite sides of the iapetus ocean is made unnecessary by invoking lithospheric delamination and tectonic wedging during the acadian orogeny in newfoundland the model is consistent with surface geology and recent deep seismic reflection observations fro

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

yapeenian early ordovician graptolites in the quebec appalachians a newly discovered oncograptus upsilon fauna occurs in a sequence of turbiditic siltstones within the lower ordovician ste anne river nappe in the gaspe area of quebec this is the first discovery

stratigraphy of the aroostook perce anticlinorium in the gaspe peninsula quebec in quebec the aroostook perce anticlinorium exposes two stratigraphic groups the honorat and the matapedia the terrigenous units of the honorat group form the core of the anticlinorium and are overlain by the carbonate sequence of the matapedia group each group comprises two formations the honorat is herein divided into two new formations the arsenault and the garin which are defined for the first time the matapedia group includes the pabos and white head formations the pabos formation a transitional unit between the noncalcareous strata of the honorat group and the carbonate facies of the white head formation is here redefined the white head formation comprises four members these two groups represent continuous sedimentation from the late ordovician caradocian to the early silurian llandoverian the lithostratigraphy of the type areas of the honorat group honorat township region and pabos formation chandler region are fully described whereas the lithostratigraphy of the type area of

magmatic fluids in the fen carbonatite complex s e norway three different types of carbonatite magma may be recognized in the cambrian fen complex s e norway 1 peralkaline calcite carbonatite magma derived from ijolitic magma 2 alkaline magnesian calcite carbonatite magma which yielded biotite amphibole sovite and dolomite carbonatite and 3 ferrocarbonatite liquids related to 2 and or to alkaline lamprophyre magma damjernite apatite formed during the pre emplacement evolution of 2 contains inclusions of calcite and dolomite devitrified mafic silicate glass and aqueous fluid all of these inclusions have a magmatic origin and were trapped during a mid crustal fractionation event p 4 kbars t 625 c where apatite and carbonates precipitated from a carbonatite magma which coexisted with a mafic silicate melt the fluid inclusions contain water dissolved ionic species mainly nacl with minor polyvalent metal salts and in some cases co2 two main groups of fluid inclusions are recognized type a co2 bearing inclusions of approximate molar composition h2o 88 90 co 27 5 nacl 5 d 0 85 0 87 g cm3 type b co2 free aqueous inclusions with salinities from 1 to 24 wt nacleq and densities betwen 0 7 and 1 0 g cm3 more strongly saline type b inclusions salinity ca 35wt d 1 0 to 1 1 g cm3 contain solid halite at room temperature and occur in overgrowths on apatite type a inclusions probably contain the most primitive fluid from which type b fluids have evolved during fractionation of the magmatic system type b inclusions define a continuous trend from low towards higher salinities and densities and formed as a result of cooling and partitioning of alkali chloride components in the carbonatite system into the fluid phase available petrological data on the carbonatites show that the fluid evolution in the fen complex leads from a regime dominated by juvenile co2 h2o fluids during the magmatic stage to groundwater derived aqueous fluids during post magmatic reequilibration

lower paleozoic configuration of the quebec reentrant based on improved along strike paleogeography an improved paleogeography of the quebec reentrant has been achieved by examining the nature and distribution of major sediment buildups within this reentrant the quebec reentrant has one side produced by rift faults and one side by a major transform fault the oak hill group was deposited near a triple rift zone clastic sediments of this group formed a large delta that was fed by a major river flowing in the rift valley of the ottawa graben the delta must have prograded on a gentle slope platform no other thick deltaic deposits are known towards the northeast instead facies analyses favor the existence of a promontory near quebec still farther to the northeast the saguenay graben was not the product of a triple rift junction however it served to funnel sediments to a deep sea fan and the adjacent platform was narrow with a steep slope the lateral distribution of sediments from the ordovician taconic orogeny can be explained by this improved paleogeographic reconstruction the configuration of

crustal structure of the kapuskasing uplift from lithoprobe near vertical wide angle seismic reflection data reprocessing of one lithoprobe kapuskasing deep seismic reflection profile discloses significant new information on the structure of the kapuskasing uplift ku the shallow structure of the ivanhoe lake fault zone ilfz along which the high grade granulites of the ku were thrust to the surface is conspicuously imaged on a seismic section as a series of prominent northwest dipping reflections with listric geometry the new images show that the ilfz is a steep fault 50 near the surface which quickly flattens out at shallow depths these reflections coincide with bright positive aeromagnetic anomalies over the fault zone direct correlation with geological observations indicates that the high reflectivity and high magnetism associated with the fault zone likely originate from mylonites the reprocessing also reveals a pronounced midcrustal reflector within the abitibi greenstone belt agb dipping northwest and plunging under the ku the existence of such a reflector is independently confirmed by wide angle reflection data acquired from a cross profile this reflector is apparently also detected on two other reflection profiles crossing the ilfz about 80 km to the southwest its concave down shape and broad lateral extent suggest that it represents underthrusting of the agb beneath the ku with these new results a more complete structural cross section can be constructed it shows that on the northwest side the ku was overthrust along the northwest dipping ilfz and on the southeast side the agb was underthrust northwesterly it seems evident that the ku is a product of intraplate collision during the early proterozoic and the underthrusting of the agb is primarily responsible for the emplacement of the ku

age and correlation of the honorat group southern gaspe peninsula the honorat group of southern gaspe consists of two formations the arsenault and the garin the arsenault formation heretofore considered barren has yielded a graptolite faunule indicative of a llanvirn llandeilo age early middle ordovician suggesting a correlation with the lower mictaw group of gaspe as revised by de broucker a hiatus of indeterminate magnitude corresponding to the taconic unconformity probably separates the arsenault formation from the overlying garin formation the garin has yielded graptolites ranging from the upper climacograptus spiniferus zone to the paraclimatograptus manitoulinensis zone late middle to early late ordovician the c spiniferus zone graptolites are identical to those of the upper sequence of the cloridorme formation of northern gaspe and the blind brook formation of the munsungun anticlinorium of maine but differ somewhat from those from the upper tetagouche group of new brunswick which are closer to those from the summerford and exploits groups of no

melanges and disrupted rocks at the leading edge of the humber arm allochthon w newfoundland appalachians deformation under high fluid pressure abstract the humber arm allochthon was structurally emplaced onto the laurentian margin in western newfoundland during taconian ordovician and acadian devonian deformation on port au port peninsula disrupted allochthonous rocks previously mapped as melange and scaly shale include three mappable variably disrupted stratigraphic units in addition mixed rocks constitute melange with much smaller area than previously mapped at outcrop scale a qualitative assessment of disruption distinguishes broken but coherent stratigraphy from a more disrupted and mixed melange unit within coherent regions three generations of folds are probably related to taconian acadian and carboniferous deformation events more disrupted regions show an average of 24 blocks to 76 matrix with block sizes 0 5 158 cm a new sampling technique allowed recovery of oriented melange samples for thin section multiple orientations of extensional fractures suggest approximately coaxial extension abundant carbonate and less common bitumen filled veins suggest that high fluid pressure played a role in the emplacement of the allochthon high fluid pressure was probably also responsible for dewatering structures sandstone dykes and partially brecciated carbonate beds map relationships outcrop and thin section scale observations lead to a reinterpreted structural history for western newfoundland in which an early taconian west bay thrust sheet was rapidly emplaced onto the laurentian margin during emplacement debris flows initially contributed igneous blocks to the allochthon but the majority of fragmentation took place in an environment of horizontal tectonic extension promoted by high fluid pressure that encouraged brittle fracture the west bay thrust sheet was subsequently overridden by the out of sequence lourdes thrust parts of the allochthon were probably re imbricated in later events but because of previous disruption an organized imbricated thrust belt was not developed at the tip of an advancing thrust wedge a clear distinction between tectonic and surficial processes of melange formation may not be possible

the riviere des plante ophiolitic melange tectonic setting and melange formation in the quebec appalachians the riviere des plante ophiolitic melange rpom is the largest and best exposed of the three known ophiolitic melanges that contain blocks of chain lakes massif clm all three lie along the baie verte brompton line which marks the suture between the continental rocks of the humber zone and the oceanic rocks of the dunnage zone the ophiolitic melange is composed of serpentinized ultramafic rocks some of which are sheared and or carbonatized blocks of amphibolitized gabbro basalt volcanogenic breccia and conglomerates it also contains continental k rich granitoid rocks and high grade metamorphic upper amphibolite facies rocks the rpom is part of the saint daniel melange an accretionary prism onto which the rpom has been technically emplaced the clm was part of a terrane accreted to the laurentian margin during the taconian orogeny blocks of the clm along the baie verte brompton line are interpreted as fragments of this terrane caught within the suture zone it is proposed that the clm co

diagenetic and low grade metamorphic terranes of gaspe peninsula related to the geological structure of the taconian and acadian orogenic belts quebec appalachians illite crystallinity ic measurements determination of the proportion of 2m mica polytypes and organic matter reflectance measurements establish regional diagenetic low grade metamorphic trends for the taconian and acadian belts of gaspe peninsula ic varies as a function of many factors besides maximum burial temperature and heating time correlation between ic and 2m illite polytypes for the fortin group and temiscouata formation suggests i that the amount of high grade detrital mica in the samples is low and ii that ic can be used with some confidence as an estimator of regional thermal maturation levels correlation of these parameters with available organic reflectance values further supports this assumption the illites of the temiscouata and western fortin groups are mostly phengitic in composition whereas in the eastern outcrop belt they are more mg and fe rich celadonitic but generally also of lower grade and lower 2m content the d 060 values for illites measured on the unorientated 2 m fraction of samples fall between 1 502 and 1 503 a range 1 500 1 504 a indicating relatively low octahedral occupancy by mg and fe between one fifth and one third of the available spaces pyrophyllite and paragonite were not detected chlorites are fe rich and ripidolitic the ic map for the acadian belt of the peninsula displays general congruence between ic contours 2200 sample points and structural trends for the 27 000 km2 area the highest grades anchimetamorphic are associated with the oldest rocks honorat and matapedia groups exposed in the cores of major anticlines anchimetamorphic grades associated with the western outcrop belt of the lower devonian fortin group require 7 8 km of subsidence to accommodate sufficient thickness of overlying younger rocks on top of 4 5 km of fortin group deep water clastics to explain the grades in terms of burial metamorphism assuming a geothermal gradient of 30 c km 1 the lowest grade diagenetic rocks occupy a large area in the northeastern part of the peninsula smaller areas in the northwestern part of the acadian belt in the centre of chaleurs bay synclinorium and in the ordovician mictaw group the contact between the taconian and acadian belt is marked by a distinct maturation discontinuity the grand pabos fault juxtaposes rocks of contrasting maturation levels matapedia group against fortin group in the west but shows no maturation offset further east in the honorat group the fault zone limiting the fortin group in the north is also associated with a major ic jump

pillow beds a new type of seismites an example from an oligocene turbidite fan complex alicante spain a new type of soft sediment deformed beds termed pillow beds originally a packet of calcarenitic turbidites show sand in sand loading with plane lower and upper surfaces the pillow beds have some resemblance to ball and pillow structures and they are considered to have formed as a result of dewatering the pillow beds are oligocene in age and are part of turbidite lobes in a submarine fan system which was fed through a canyon along the southern border of the ancient iberian continent three hypotheses may explain their occurrence overloading sliding or seismic shock a seismic origin is preferrred on the basis of the flat undeformed lower surface of the pillow beds excluding dewatering of and loading into the previously deposited beds furthermore the pillow bed structures are related to grain size and may show repetitions thus excluding sliding the scale and nature of the pillow beds suggest they could be the result of an earthquake with a magnitude of 6 7 on the richter scale

using 3d kinematic models in subduction channels the case of the chanaral tectonic melange coastal cordillera northern chile abstract the chanaral tectonic melange northern chile is a local unit within the late paleozoic accretionary complex formed at the southwestern margin of gondwana the structural characteristics of the studied melange were mostly developed during a first deformation phase d1 and include a block in matrix fabric lineations l1 and foliations s1 tight to intrafoliar folds s c and s c c composite planar fabrics and a conspicuous spatial separation of domains with predominantly linear and linear planar fabrics folding during a second stage d2 modified the orientation of the previous fabrics and structures the eastern boundary of the chanaral melange is n s to nnw sse oriented moderately dipping to the east its western boundary is not exposed the plane showing the maximum structural asymmetry the vorticity normal section is ene wsw directed and sub vertical kinematic criteria consistently reveal top to the wsw displacement a kinematic model of triclinic transpression with inclined extrusion has been applied to evaluate the d1 structural features of the chanaral melange the pitch of the simple shear direction on the deformation plane ranged from 60 n to 90 the pitch of the estimated extrusion direction was of 30 40 s the coaxial component was clearly constrictional logarithmic k value of 2 to 5 the vorticity number has not been constrained by the model but its spatial variation can explain the domainal distribution of the fabrics in the melange the simulated particle paths show the predominance of material displacement parallel to the margin with low to moderate down dip displacements which is in accordance with the low pressure metamorphic assemblages found in the melange the convergence direction between the blocks separated by the melange unit was n50 60 e kinematic blocking of the chanaral melange probably related to the accretion of an oceanic volcanic domain allowed the d2 folding of the previous structures a process that at least initially proceeded without a change in the convergence direction

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

taconian and acadian transpression between the internal humber zone and the gaspe belt in the gaspe peninsula tectonic history of the shickshock sud fault zone the shickshock sud fault has a history of ordovician taconian silurian salinic and devonian acadian movements taconian deformation involving ductile dextral oblique slip faulting is recorded in cambrian rocks in the footwall of the shickshock sud fault metabasalt and metaarkose at amphibolite grade are converted into phyllonite and mylonitic schist shear bands asymmetric garnet porphyroclasts c s fabrics and mica fish textures indicate dextral shearing the regional sense of shear is top to west and southwest on generally southeast dipping shear zones hornblende of metabasalt yielded an 40ar 39ar age of 455 9 2 ma and muscovite from the mylonitic schist yielded an 40ar 39ar age of 454 3 0 9 ma which indicate metamorphism and deformation during the taconian orogeny evidence for silurian activity is indicated by the salinic unconformity to the south related to normal block faulting deformation features in the ordovician and silurian devonian rocks in the hanging wall were predominantl

the casanova complex of the northern apennines a melange formed on a distal passive continental margin abstract the cretaceous palaeocene casanova complex occurs in two thrust sheets of the eugeosynclinal ligurids of the northern apennines it is a sedimentary melange with ophiolitic and quartzose turbidites or limestone shale olistostrome submarine debris flows as matrix exotic blocks of ophiolite and granite serpentinite breccias and lenticular ophiolitic breccias and olistostromes contribute to the melange character of the complex deformational structures include soft sediment slump folds indicating a sw dipping palaeoslope and boudins a gradational slumped top to the melange small scale faults in chert blocks and deformation associated with the emplacement of the exotic slide blocks the blocks were shed as rotational slides from submarine fault scarps and are surrounded by haloes of debris created by submarine weathering the stacking pattern of the blocks with the originally stratigraphically highest ophiolite lithologies lowest in the pile of blocks is explained by a diverticulation model with progressively deeper erosion mechanical analysis shows that the blocks were stable when partly exposed resting on a soft sediment substratum criteria which distinguish the casanova complex from a tectonic melange and which may be of value in other melanges are discussed previous interpretations of the complex as a precursor olistostrome to northeastward nappe emplacement the bracco ridge model are rejected the melange is believed to have formed on ocean crust as a result of turbidite and debris flow sedimentation soft sediment deformation block faulting gravity sliding and submarine erosion at the distal edge of a uniformly sw dipping continental margin

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle

ophiolite obduction in the quebec appalachians canada 40 ar 39 ar age constraints and evidence for syn tectonic erosion and sedimentation 1 2 3 detailed field work conducted in the dunnage zone of the quebec appalachians is herein combined with 40 ar 39 ar dating on a series of ophiolitic massifs crosscutting granites and associated metamorphic rocks occurring along the baie verte brompton line the taconian suture between laurentia and lower paleozoic peri laurentian oceanic terranes studied massifs are the lac brompton ophiolite and the riviere des plante ultramafic complex in southern quebec and the nadeau ophiolitic melange in the gaspe peninsula our work suggests that these massifs form remnants of eroded ophiolitic nappes which are unconformably overlain by the saint daniel and riviere port daniel melanges and correlate with the thetford mines and mont albert ophiolitic complexes our 40 ar 39 ar data and compiled regional age constraints indicate that ophiolite obduction was diachronous along the strike of the orogen the timing of obduction and melange formation varies according to the irregular geometry of the early paleozoic laurentian margin with earlier collision occurring along or at the margins of promontories obduction was initiated with the formation of infraophio litic metamorphic soles between ca 479 and 472 ma in southern quebec and the nadeau ophiolitic melange and pos sibly as late as ca 470 466 ma for the mont albert complex these sole rocks were later exhumed and translated onto the laurentian margin with the overlying ophiolites between 475 and 460 457 ma the uplifting and erosion of the orogenic wedge during the waning stages of obduction has resulted in the sedimentation of olistostromal melanges and onlapping flysch units above the ophiolitic nappes as well as foredeep flysch successions during the latest arenig to earliest caradoc