structural evolution of the laurentian margin revisited southern quebec appalachians implications for the salinian orogeny and successor basins the laurentian margin of the appalachians is divided into external and internal zones on the basis of metamorphic and structural contrasts in the southern quebec internal zone silurian to early devonian southeast verging structures are superimposed on northwest verging structures whereas most of the external zone lacks such overprints regional backthrust faults define a major upper plate lower plate boundary the external zone rocks are in the hanging wall and internal zone rocks are in the footwall metamorphic rocks with silurian early devonian 40 ar 39 ar ages 430 410 ma characterize the lower plate to the east the saint joseph fault and the baie verte brompton line are southeastdipping normal faults that crosscut the upper plate lower plate boundary metamorphic rocks with middle ordovician 40 ar 39 ar ages 469 461 ma and rocks of the external zone both occur in the downthrown side of the saint joseph fault and the baie verte brompton line u pb and 40 ar 39 ar ages suggest that the northwest verging structures are related to ophiolite obduction and crustal thickening during the taconian orogeny ca 480 445 ma whereas the southeast verging structures formed during silurian early devonian backthrusting and normal faulting the revised structural interpretation has implications for the salinian orogeny and involves 1 southeast directed transport of the taconian crustal wedge of the upper plate followed by normal faulting and juxtaposition with the lower plate along the saint joseph fault and the baie verte brompton line and 2 the forma

a new stratigraphic framework for the gaspe belt in southern quebec implications for the pre acadian appalachians of eastern canada the post taconian units in the quebec and northern new brunswick appalachians constitute the gaspe belt and geological studies have mostly focussed on its eastern quebec segment biostratigraphic data indicate that the succession in southern quebec is no older than late silurian and extends into the early devonian two distinct stratigraphic assemblages are present the first assemblage saint luc cranbourne and lac aylmer formations and glenbrooke group unconformably overlies the humber and dunnage zones the units show a basal alluvial conglomerate that passes progressively to deeper marine facies upsection which have recorded a post late silurian transgressive event the second assemblage saint francis group and frontenac formation is faulted against either dunnage units or autochthonous post taconian units it locally unconformably overlies units of the dunnage zone the succession shows progressively deeper marine conditions upsection and also has recorded a post late silurian transgressive ev

detrital zircon u pb geochronology of the magog group southern quebec stratigraphic and tectonic implications for the quebec appalachians

forward potential field modeling of the appalachian orogen in the gaspe peninsula quebec canada implications for the extent of rift magmatism and the geometry of the taconian orogenic wedge the nature and extent of potential source bodies associated with prominent regional scale gravity and magnetic anomalies in the northern quebec appalachians are investigated through forward modeling potential field anomalies are interpreted to be the geophysical expression of basalt flows extruded during the neoproterozoic to cambrian rifting of laurentia prior to opening of iapetus geophysi cal modeling provides substantial new constraints on seismic and structural cross sections and the map distribution of volcanism this study indicates that the volcanism was significantly more widespread than previously recognized basaltic rocks form a sheet up to 4 kilometers thick transported on the main taconic decollement most of this sheet is deeply buried and overlain by a thick pile of paleozoic sediments later tectonic modifications by backthrusting and hinterland directed normal faulting as well as by acadian strike slip faulting substantially changed the geometry of the transported volcanic rocks and the structural style both across and along the strike of the orogen the extension of the gravity anomaly under the st lawrence river also suggests that the ordovician deformation front is 20 to 30 kilometers further to the north than previously interpreted

the supra ophiolitic sedimentary cover of the asbestos ophiolite quebec canada first geochemical evidence of transition from oceanic to continental sediment flux abstract the 473 5 3 myr asbestos ophiolite complex of the quebec appalachians was formed in a forearc basin and obducted on a margin of laurentia 460 myr ago the complex together with its sedimentary cover is well exposed at burbank hill 130 km sw of quebec city where eight distinct lithologies have been identified 1 pyroxenites and wehrlites with minor dunitic layers 2 fractured gabbros 3 breccia containing gabbro and diabase fragments 4 polygenic conglomerates 5 red mudstone chert sandstone succession 6 tuffs with intercalated grey chert 7 greenish grey mudstones and 8 slates and sandstones of the saint daniel melange the gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites tio2 0 8 0 85 wt and boninites tio2 45 la sm n 1 9 3 3 these rock types also occur as mafic fragments in the polygenic conglomerate the mafic fragments crystals and rocks within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex the conglomerate was probably formed after the forearc crust was fragmented by pre obduction normal faults these fault scarps would have promoted the erosion of the oceanic crust leading to the accumulation of diverse detritus in grabens the sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent derived material la sm n 5 and la yb 20 and a decrease in ophiolitic material moving upward in the stratigraphic sequence the sedimentary rocks at the bottom of the sequence group 3 are mostly composed of ophiolite material with only 20 of crustal material they also have a high content of cr and mgo with values of 350 480 ppm and 5 7 8 respectively the rocks of the uppermost part of the pile group 1 which contain up to 80 crustal material have low abundances of cr and mgo 30 100 ppm and 2 7 4 5 wt continental detritus on the top of the asbestos ophiolite suggests the complex formed near the laurentian continental margin and that the eroded continental material had access to the basin where the ophiolite was formed this is consistent with the obduction of the asbestos and thetford mines ophiolites on the laurentian margin shortly after their formation the proximity of thick laurentian continental crust near the trench could explain why the subduction zone blocked up soon after the ophiolites formation

the contribution of loess magnetism in china to the retrieval of past global changes some problems abstract in the loess plateau of china the stratigraphical position of the brunhes matuyama boundary was not found in loess unit l8 at luochuan and jixian as is commonly accepted the interbedded palaeosols may be differentiated from the parent loess not only by their high magnetic susceptibility but also by their enhanced remanence properties these contrasts mainly result from concentration changes of the inherited magnetic minerals grain size changes and authigenic formation of magnetic minerals which are all linked to climate controlled soil formation processes

timing of tectonometamorphism across the green mountain anticlinorium northern vermont appalachians 40ar 39ar data and correlations with southern quebec in the pre silurian lithotectonic units of the northern vermont appalachians the timing of orogenesis and tectonometamorphism has traditionally been ascribed to the combined effects of the middle ordovician taconian orogeny and middle to late devonian acadian orogeny however numerous geochronological studies throughout the northern appalachians including neighboring southern quebec have obtained silurian and early devonian age data that document more or less continuous tectonometamorphic activity throughout the ordovician devonian the structural and metamorphic evolution of northern vermont can be separated into three regional phases which are characterized by distinct structures fabrics and metamorphic parageneses the first phase d 1 associated with westward emplacement of various thrust slices leading to crustal thickening and regional metamorphism and the second phase d 2 characterized by bivergent structures and metamorphic overprint have both been considered to be taconian the third phase the structure and fabric of which are also observed in the silurian devonian rocks to the east is considered to be acadian we present new step heating and spot fusion 40 ar 39 ar geochronological data on amphibole and fabric forming muscovite from samples taken across the green mountain anticlinorium which coupled with published data provide improved age constraints on tectonometamorphism of d 1 latest cambrian to middle ordovician d 2 silurian early devonian and d 3 middle devonian events by comparing structural and metamorphic characteristics and now timing these phases are interpreted to be correlative to the tripartite tectonometamorphic evolution documented in southern quebec and they further exemplify the along strike diachronism of tectonism induced by the inherited irregular geometry of the laurentian margin

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

thrusting and back thrusting in the taconian internal zone southern quebec appalachians cambro ordovician continental margin rocks of the humber zone of the quebec appalachians were mainly deformed during the taconian orogeny middle ordovician to early silurian two taconian deformational events are recorded west of the sutton notre dame mountains anticlinorium axis they are characterized respectively by northwest directed faulting and synmetamorphic folding d1 2 and by southeastward back thrusting motion d3 the latter deformation has previously been poorly documented in the quebec appalachians this duality of structural vergence is probably induced by the progressive tectonic wedging of basement rocks during a nearly constant northwest southeast taconian shortening in correlative higher grade metamorphic rocks of new england back thrusting structures d3 have not been described and are most probably absent because their root zone is located well above the present day erosional surface of that part of the appalachian belt

taconian orogenesis sedimentation and magmatism in the southern quebec northern vermont appalachians stratigraphic and detrital mineral record of iapetan suturing cambrian ordovician units of the southern quebec appalachians oceanic domain include obducted ophiolites and an overlying syncollisional sedimentary basin represented by the saint daniel melange and flysch units of the magog group these terrains were thrust onto the laurentian continental margin during the taconian orogeny and are unconformably overlain by silurian devonian successor basins this contribution presents and discusses new stratigraphic data acquired in the oceanic domain as well as u pb and 40ar 39ar geochronological data on detrital zircons and muscovites sampled at various stratigraphic levels from the base of the sedimentary units of the oceanic domain and the silurian devonian units of the gaspe belt this work combined with the compilation of existing data for southern quebec and western new england allows a better definition of the tectono stratigraphic evolution of the area and are used to propose correlations with different rock units of the northern new england appalachians which represents a collage of laurentian margin and oceanic units it is shown that cambrian ordovician rocks of southern quebec and northern vermont have undergone a similar tectonic evolution the ophiolites were obducted onto the laurentian margin as a more or less coherent slab of oceanic lithosphere both the ophiolites and continental margin rocks were uplifted eroded and unconformably overlain by middle to late ordovician units of the saint daniel melange and magog group in a forearc setting the presence of mafic and felsic volcanic flows that are interstratified within the saint daniel melange can be correlated with an episode of delamination and slab breakoff during the final stages of obduction and exhumation of the orogenic wedge the magog group was deposited during the emplacement of the taconian nappes as the outboard volcanic arc was still active but progressively shutting down the provenances suggested by the age of the dated detrital minerals can be entirely reconciled with the peri laurentian context of the studied rock units and testify of the tectonic evolution and sedimentary recycling of the taconian orogen from ophiolite obduction onto the laurentian margin to the collapse of the orogenic belt and the formation of successor basins

tectonometamorphic evolution of the southern quebec appalachians 40ar 39ar evidence for middle ordovician crustal thickening and silurian early devonian exhumation of the internal humber zone published age data from the humber zone of the southern quebec appalachians indicate that deformation and metamorphism occurred during middle to late ordovician time this paper presents the results of a regional scale geochronological study of 70 amphibole and mica single grains by the 40 ar 39 ar laser step heating method which brings forward a more complete middle ordovician early devonian tectonometamorphic evolution of the quebec appalachian humber zone biotites have plateau ages varying between 420 and 411 ma amphiboles are often compositionally rimmed and or contain biotite inclusions their age spectra are generally disturbed but may suggest the occurrence of two distinct events silurian 429 424 ma and middle ordovician 462 460 ma two main groups of muscovite ages have been obtained middle ordovician 469 461 ma and silurian early devonian 431 411 ma but middle to late devonian 387 376 ma ages are also locally present middle ordovician ages are interpreted to record crustal thickening by nappe emplacement during regional taconian metamorphism silurian early devonian ages are attributed to consequences of backthrusting and normal faulting a statistical analysis of silurian early devonian muscovite age spectra reveals an irregular decrease in weighted apparent age maxima across the internal humber zone this age decreasing trend is interpreted to result from the temporal ca 431 411 ma and possibly spatial northwest to southeast progression of deformation recrystallization and fluid circulation during tectonic exhumation of the internal humber zone in southern quebec tectonic models for the northern appalachian internal humber zone should take into account the strong body of evidence for silurian early devonian hinterland directed and extensional deformation as documented in southern quebec

dating polyphase deformation across low grade metamorphic belts an example based on 40ar 39ar muscovite age constraints from the southern quebec appalachians canada geochronologic 40ar 39ar data of fabric forming metamorphic minerals in conjunction with structural and metamorphic studies are being increasingly used to constrain the deformation and tectonometamorphic evolution of polyphase low grade orogens careful data interpretation is needed to extract meaningful age constraints from neo and recrystallized minerals affected by isotopic disturbances in the southern quebec appalachians the sutton mountains anticlinorium exposes the metamorphic core of the early paleozoic continental margin of laurentia orogenesis in this part of the appalachians was the result of tectonic events that have been classically attributed to the combined effects of the middle to late ordovician taconian and the middle devonian acadian orogenies however evidence of separate and distinct silurian early devonian tectonism has also been recently documented laser step heating 40ar 39ar data on single grains of muscovite from polydeformed greenschist facies samples of the sutton mountains anticlinorium indicate that these tectonometamorphic events are heterogeneously preserved as a prograde taconian event at ca 456 ma and an acadian overprint at ca 390 ma the integration of 40ar 39ar age spectra analyses with structural relationships provides precise age constraints on the duration propagation and evolution of silurian early devonian hinterland directed deformation as it migrated across the anticlinorium including back thrusting from ca 433 ma to ca 420 ma and extensional faulting from ca 417 ma to ca 405 ma along the laurentian margin of the northern appalachians such silurian early devonian tectonism is currently attributed either to the collapse of the taconian orogen triggered by the delamination of the subducted slab or to the outboard accretion of peri gondwanan terranes during the salinic orogeny

k ar dating constraints on the tectonothermal evolution of the external humber zone southern quebec appalachians in the external humber zone the emplacement episodes of allochthonous nappes and related low grade metamorphism are mainly based on paleontological ages from the st lawrence lowlands in fact late ordovician flysches are affected by thrusting and it is not clear if the nappe imbrication is the result of the taconian or the acadian deformation along the chaudiere river transect fourteen shales and phyllites of cambrian to ordovician age were sampled for determination of the k ar values of their 2 m size fractions to interpret the k ar data mineral composition and clay mineral crystallinity of the 2 m size fractions and maturation of the organic matter were determined in addition the intensity of deformation was characterized by describing the rock texture at macroscopic and microscopic scale the combined approach used in this study emphasized that nearly all k ar dates are mixed or partly reset values however it can be stated that in the quebec promontory nappe and part of the chaudiere

tectonic evolution and significance of silurian early devonian hinterland directed deformation in the internal humber zone of the southern quebec appalachians in the southern quebec appalachians the early tectonic history of the laurentian margin humber zone comprises foreland propagating northwest directed thrust faulting nappe emplacement and regional prograde metamorphism in response to the obduction of large ophiolitic nappes during the taconian orogeny in the internal humber zone this event is dated at 462 3 ma late middle ordovician which is interpreted to represent the timing of near peak taconian metamorphism superimposed hinterland directed structures are accompanied by retrograde metamorphism and consist of back thrusts and normal faults which respectively delimit the northwestern and southeastern limbs of the sutton and notre dame mountains anticlinoria both salient structures of the internal humber zone of southern quebec geochronologic data on the timing of hinterland directed deformation vary from 431 to 411 ma two tectonic models are presented and discussed which may account for the silurian early devonian evolution of the la

tracking low temperature tectonism of the st lawrence platform and humber zone southern quebec appalachians through apatite and zircon u th he thermochronology the st lawrence platform slp and humber zone hz of the southern quebec appalachians have been the subject of extensive studies to resolve the degree of thermal maturation yet the timing of the thermal maximum is not well understood we have employed apatite ahe and zircon zhe u th he thermochronometry across a network of late cambrian to late ordovician siliciclastic and grenvillian basement outcrops to resolve the thermal history below 200 c single crystal dates from individual samples exhibit age dispersion by as much as 300 million years with a strong positive to negative correlation with increasing effective uranium eu concentration the data in the southwestern portion of the basin allow for thermal maxima of up to 200 c during the late ordovician or early devonian regional burial trends combined with local estimates of paleogeothermal gradients indicate that if sedimentation continued after the late ordovician there was no significant increase in burial in southwestern portions

late neoproterozoic to permian tectonic evolution of the quebec appalachians canada abstract due to the unique location of the quebec appalachians in a re entrant of the paleo north american margin and to the fact that the area largely escaped late paleozoic deformation this segment of the orogen preserves evidence of the early orogenic stages that are partly obscured elsewhere despite diachronicity both along and across strike of the orogen five tectonic events of regional significance are recognized 1 the earliest event is mainly recorded by neoproterozoic to early ordovician normal faults associated with extension of the laurentian craton and the establishment of a passive continental margin 2 the second event the ordovician taconian orogeny is marked by foreland directed deformation and corresponds to the principal deformational and metamorphic events recorded by the laurentian margin 3 the third event early salinic is early silurian it is restricted to the southern part of the quebec appalachians and consists of hinterland se directed thrusts folds and retrograde metamorphism 4 the fourth event late salinic is recorded by regional scale late silurian to early devonian normal or transtensional faults and is associated with the formation of the connecticut valley gaspe trough and 5 the last event the devonian acadian orogeny affects with variable intensity all the pre carboniferous lithologies this tectonic synthesis implies that the subdivision of the peri laurentian oceanic tracts proposed in other segments of the appalachians should be applied with caution in quebec and that the structural style the number of oceanic domains and possibly the vergence of subduction change along the strike of the orogen in particular the similarities of the thetford mines and boil mountain ophiolitic complexes and sedimentary covers and the lack of evidence for a major intervening suture are consistent with the maximum allochtony hypothesis i e the assumption that the southern quebec ophiolites are rooted east of the chain lake massif a geodynamic model involving the taconian obduction of a large ophiolitic nappe s the docking of a peri gondwanan gander medial new england terrane in early salinic the formation of large sedimentary basins connecticut valley gaspe and merrimack troughs due to trench rollback and or delamination during the late salinic event and collision at the outer margin of composite laurentia during the acadian orogeny can account for the protracted structural and metamorphic history of the quebec appalachians

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

exploring laser ablation u pb dating of regional metamorphic garnet the straits schist connecticut usa abstract garnet is a prominent recorder of metamorphic processes and plays a pivotal role in deciphering the pressure temperature evolution of metamorphic rocks however the potential of regional metamorphic garnet for laser ablation u pb dating remains unexplored as variable and low u we applied la icp ms u pb dating to garnet and zircon monazite and xenotime from the straits schist western connecticut usa to constrain the timing of garnet growth within the overall acadian age metamorphic history recorded by this unit we assessed the effects of u rich inclusions on garnet u pb data using high resolution microbeam sampling grain size analysis and calculations assessing how inclusion size and abundance affect u concentrations measured in garnet when analysing garnet our results demonstrate that by scrutinizing the time resolved u signal u rich inclusions can be identified and effectively removed during data evaluation if u rich inclusions are unaccounted for garnet analyses have elevated u contents compared to pure garnet and in a tera wasserburg diagram data points are shifted to the left right or along the garnet regression line for inclusions older younger or of the same age as the garnet respectively garnet from two metapelitic samples yielded lower intercept ages of 409 10 ma and 406 15 ma and a combined u pb age of 406 1 8 3 ma metamorphic zircon yielded age clusters at 446 2 4 2 ma 413 3 3 0 ma and 382 9 2 8 ma monazite ages range between 390 and 355 ma and xenotime yielded 370 1 5 6 ma the 446 ma age peak is attributed to detrital zircon formed during the waning stages of the taconian orogeny early acadian metamorphism is recorded by garnet and 413 ma zircon whereas monazite and zircon ages of 390 to 383 ma coincide with peak metamorphic conditions early carboniferous monazite and xenotime ages could reflect the incursion of fluids into the straits schist during neo acadian metamorphism coeval plutonism in new england or breakdown of garnet during regional exhumation

fossil brines preserved in the st lawrence lowlands quebec canada as revealed by their chemistry and noble gas isotopes abstract brines in cambrian sandstones and ordovician dolostones of the st lawrence lowlands at becancour quebec canada were sampled for analysis of all stable noble gases in order to trace their origin and migration path in addition to quantifying their residence time major ion chemistry indicates that the brines are of na ca cl type possibly derived from halite dissolution 87 sr 86 sr ratios and ca excess indicate prolonged interactions with silicate rocks of the proterozoic grenville basement or the cambrian potsdam sandstone the brines constrain a 2 3 contribution of mantle 3 he and large amounts of nucleogenic 21 ne and 38 ar and radiogenic 4 he and 40 ar 4 he 40 ar and 21 ne 40 ar ratios corrected for mass fractionation during incomplete brine degassing are identical to their production ratios in rocks the source of salinity halite dissolution plus the occurrence of large amounts of 40 ar in brines constrain the residence time of becancour brines as being older than the cretaceous evaporites in the st lawrence lowlands likely existed only during devonian silurian time brines might result from infiltration of devonian water leaching halite penetrating into or below the deeper cambrian ordovician aquifers during the devonian the basin reached temperatures higher than 250 c allowing for thermal maturation of local gas prone source rocks utica shales and possibly facilitating the release of radiogenic 40 ar into the brines the last thermal event that could have facilitated the liberation of 40 ar into fluids and contributed to mantle 3 he is the cretaceous monteregian hills magmatic episode for residence times younger than the cretaceous it is difficult to find an appropriate source of salinity and of nucleogenic radiogenic gases to the becancour brines

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

oceanic core complexes and ancient oceanic lithosphere insights from iapetan and tethyan ophiolites canada and albania abstract oceanic ridges are extensional environments characterized by brittle to ductile normal faulting and episodic magma supply studies in modern oceanic settings suggest that the interplay of low angle detachments and high angle normal faults locally lead to the exhumation of lower crust and upper mantle peridotites to the seafloor as oceanic core complexes along the mid atlantic ridge core complexes typically occur in inside corners of intersections between the ridge axis and major transform faults the core complexes were exhumed by movement along long lived detachment faults that extend for 10s of km and that are crosscut by high angle normal faults oriented parallel to the rift axis ridge segments with abundant detachments would be characterized by lithosphere having little or no upper crust similar features in ophiolites may also reflect the dominance of amagmatic extension this contribution presents two examples of fossilized oceanic core complexes preserved in the canadian appalachian and albanian dinaride ophiolite belts in the canadian appalachians the southern quebec ophiolites experienced two paleozoic orogenies after their obduction onto the laurentian margin although locally obscured by tectonic fabrics and structures the original relationships between the ophiolitic mantle the overlying plutonic section and onlapping ordovician siliciclastic rocks are well preserved in the thetford mines ophiolite which also shows similarities with lithological and structural features of the mirdita ophiolite where conformable contacts between the sedimentary cover sequence debris flows and flysch deposits and the mantle and crustal sections are superbly exposed the jurassic mirdita ophiolite in albania underlies ca 4000 km2 and was only slightly affected by the tertiary alpine orogeny of eastern europe well exposed sections of the ophiolite suggest that the oceanic mantle was exhumed prior to obduction and locally records the formation of an oceanic core complex mafic cumulates of the mirdita ophiolite have been totally excised in places and exhumation related shear zones are preserved in the krabbi puka massif the shear zones are manifested both within the mantle and at the contact with overlying basalts the ophiolitic sedimentary sequence was deposited on basalts sheeted dykes or gabbros and typically consists of debris flows intercalated with and overlain by slumped oceanic sedimentary rocks the lithological and structural characteristics of both the thetford mines and the mirdita ophiolites are consistent with extensional tectonism crustal excision and lower crust upper mantle exhumation during formation both ophiolites are believed to have originated during asymmetrical oceanic extension in a suprasubduction zone setting with preservation of the upper plate lower plate segments during obduction

extension versus shortening models for hinterland directed motions in the southern quebec appalachians abstract in the taconian internal zone of southern quebec structures related to hinterland directed motion are found on both limbs of the sutton notre dame mountains anticlinorium d3west and d3east structures and along the st joseph fault these ductile to brittle ductile structures were formed during upper to mid greenschist grade metamorphic events thus suggesting that the area occupied a mid crustal position during hinterland motion layer extensional fabrics along the eastern limb of the sutton notre dame mountains anticlinorium can be interpreted either as structures contemporaneous to backthrusting deformation rooted on the western limb of the anticlinorium or as structures formed during normal faulting along the st joseph fault the study of layer extensional structures found in the taconian internal zone of the southern quebec appalachians shows that it is not obvious to infer wether normal sense fabrics that occur in orogenic hinterlands are related to regional crustal extension or to local syn collisional extensional faulting

distribution and characteristics of taconian and acadian deformation southern quebec appalachians the regional structure of the southern quebec appalachians is redefined in terms of superposed taconian and acadian deformation events taconian structures and metamorphism dominate in cambro ordovician rocks west of the baie verte brompton line bbl the taconian orogen is subdivided into a northwestern external zone composed of fault imbricated continental rocks displaying single phase foreland structures and a south eastern internal zone of polydeformed and metamorphosed continental and oceanic rocks the structural relationships of cambro ordovician rocks astride the bbl demonstrate that the continent ocean contact was deformed by both southeast verging late taconian and upright acadian folds east of the bbl syn and post taconian rocks were mainly affected by acadian deformation the acadian external zone consists of cambro ordovician and post ordovician rock units characterized by a single phase of folding the zone is bounded to the southeast by the riviere victoria fault the acadian internal zone displays polyphase deformation in the easternmost part of the quebec appalachians the acadian deformation shows a significant overlap with taconian structures and metamorphism and structural windows of the taconian orogen are found within both the external and internal zones of the acadian orogen

u pb geochronology and origin of granitoid rocks in the thetford mines ophiolite canadian appalachians we integrate new and existing age data from ophiolite and subophiolite rocks in the thetford mines region quebec appalachians to determine the timing of 1 oceanic crust formation 2 oceanic decoupling 3 ophiolite emplacement onto the continent and 4 postemplacement taconic orogenesis few ophiolites possess high precision temporal data for all these events this allows us to develop models to explain the origin of emplacement related granitoids that have intruded the ophiolite u pb zircon ages have been determined for four samples from the thetford mines ophiolite oceanic plagiogranite from the lac de l9est region of the thetford massif yields an age of 480 2 ma this age is identical to that of the adjacent mont ham massif thrust sheet 478 3 2 ma suggesting a close temporal and spatial association of these two units in the source oceanic domain two peridotite hosted granitoid samples yield crystallization ages of 470 5 3 ma and 469 4 ma a precambrian inherited zircon component is also present in one sample these granitoids are strongly peraluminous contain abundant inherited zircon exhibit variable but high 87 sr 86 sr initial ratios and low 208 pb 206 pb ratios in igneous zircon which suggest that they were derived by partial melting of laurentian margin sedimentary rocks the ages indicate that the ophiolite was thrust over the laurentian margin within 1 17 m y of oceanic crustal crystallization and that these obduction related granites were generated during a period of shear heating calculations indicate that shearing can generate the peridotite hosted granitoid melts and their observed contact zone mineralogies under specific conditions although margin sedimentary rock melting in response to west dipping subduction related arc magmatism cannot be excluded

ordovician and silurian metamorphic cooling ages along the laurentian margin of the quebec appalachians bridging the gap between new england and newfoundland geochronological studies on the timing of deformation and metamorphism along the laurentian margin have shown that the ages of metamorphism events change along the strike of the newfoundland new england segment of the appalachian orogen the quebec appalachians represent a critical transition zone where new single grain laser 40 ar 39 ar plateau ages show that both middle ordovician and silurian deformation and metamorphism are recorded parts of the analyzed samples give 40 ar 39 ar muscovite ages ranging from 468 to 461 ma these samples were taken within structural windows of the eastern internal humber zone that are overlain by ophiolitic and melange rocks of the dunnage zone other analyzed samples yielded silurian 40 ar 39 ar muscovite ages clustering from 425 to 421 ma these samples were taken across the notre dame mountains anticlinorium which represents a structural and metamorphic dome within the internal humber zone the middle ordovician ages of southern quebec are coeval with data from western new england and are interpreted as representing cooling shortly after the peak taconian regional metamorphism the silurian ages which have never been previously documented in the quebec appalachians are comparable to those described in newfoundland however they are not attributed to tectonism that shows characteristics of an orogen scale continent continent collision salinian orogeny but rather those of late taconian backthrusting and normal faulting or a period of crustal extension that followed the main taconian crustal thickening

diachronous supracrustal extension in an intraplate setting and the origin of the connecticut valley gaspe and merrimack troughs northern appalachians in the appalachians of mainland canada and new england silurian early devonian rocks are preserved in the connecticut valley gaspe and merrimack troughs and rest unconformably or in fault contact with older rocks belonging to laurentia and to gander avalon respectively the silurian early devonian rocks consist of marine clastic deposits with subordinate carbonates lava flows and terrestrial deposits the origin of these sedimentary basins is still poorly understood metamorphic ages and structures in the laurentian margin major unconformities and syn sedimentary normal faulting in both troughs argue for significant crustal extension during deposition the connecticut valley gaspe and merrimack troughs are separated by inliers of pre ordovician to ordovician rocks which we interpret as silurian basement highs that would have been buried in devonian times to form a composite sedimentary basin volcanic rocks are widely distributed in time and space in both basins and are mostly subalkaline within plate tholeiites which is consistent with a tectonic setting involving crustal extension rather than with subduction granitic plutons are abundant in the merrimack trough and attest to high temperatures at mid crustal levels it is suggested that crustal extension was responsible for the formation of both basins and that heating of the lower crust in the merrimack trough during the silurian was the result of synorogenic collapse likely triggered by delamination at the laurentia medial new england boundary delamination of the subducted slab and the upwelling of the asthenosphere would have caused 1 isostatic uplift and formation of basement highs 2 magmatism in the lower crust and regional scale contact metamorphism in the upper crust and 3 collapse of metamorphic terranes and the formation of subsiding sedimentary basins

stratigraphy and geochemistry of the lac brompton ophiolite canada evidence for extensive forearc magmatism and mantle exhumation in the southern quebec ophiolite belt three ophiolites are currently recognized in the southern quebec appalachians from south to north the mont orford ophiolite dominated by tholeiitic to transitional alkaline basalt and diabase a

acadian metamorphism in the dunnage zone of southern quebec northern appalachians 40ar 39ar evidence for collision diachronism in new england the acadian orogen is divided into western and eastern metamorphic belts on the basis of contrasting pressure temperature p t paths peak metamorphism conditions and ages along strike correlations of structures indicate that rock units affected by devonian metamorphism and deformation in southern quebec belong to the western acadian belt and share the same tectonic evolution as in new england muscovite 40 ar 39 ar ages obtained from greenschist grade ordovician volcanic and plutonic rocks of the ascot complex in southern quebec indicate that acadian metamorphism and deformation peaked ca 380 375 ma ordovician muscovite ages of ca 462 ma are locally well preserved in the granitic intrusion of the ascot complex and provide constraints on the timing of island arc plutonism in the dunnage zone of southern quebec ordovician 40 ar 39 ar age spectra do not show important losses of radiogenic argon indicating that acadian metamorphism has been low grade with temperatures below the ar isotopic closure temperature proposed for muscovite acadian peak metamorphism is significantly younger and p t conditions of regional deformation have been lower in southern quebec than in amphibolite grade rocks of the western acadian belt of new england in the northern appalachians acadian metamorphism and deformation are attributed to devonian age crustal overthickening related to plate convergence the comparison of devonian tectonic fabrics in new england and southern quebec suggests that progression in the timing and p t conditions of acadian peak metamorphism in both areas are the result of the irregular geometry of the collision zone and of the northward migration of metamorphic and structural fronts along the orogen

the late ordovician middle devonian gaspe belt in eastern canada recent advances the appalachian forelands and platform natmap national geoscience mapping program project in eastern canada is a multi discipline and multi organization research endeavour aimed at the understanding of the evolution and architecture of the ancient continental margin of laurentia this canadian journal of earth sciences special issue presents some recent research progress for our knowledge of the silurian devonian gaspe belt of that ancient margin

syncollisional basin development in the appalachian orogen the saint daniel melange southern quebec canada the saint daniel melange is an orogen scale sedimentary basin of the quebec appalachians that is commonly interpreted as the remnant of a subduction complex formed during the taconian orogeny however geochronological structural and stratigraphical data from laurentian continental margin rocks and adjacent ophiolitic rocks in southern quebec indicate that the saint daniel melange is an olistostromal syncollisional piggyback basin that represents the base of the magog group forearc basin the regional stratigraphic framework of the saint daniel melange and its relationships with underlying and overlying rock units have been established on the basis of six stratigraphic sections from the thetford mines asbestos and mont orford ophiolitic complexes our results imply that 1 the saint daniel melange is a sedimentary sequence that unconformably overlies different structural and pseudostratigraphic levels of the southern quebec ophiolites 2 it is made up of four distinct and laterally discontinuous units that record the obduction of ophiolites onto laurentia and exhumation of basement rocks followed by subsidence and deposition of the overlying saint victor formation of the magog group 3 the sedimentary rocks of the saint daniel record a transition from ophiolite dominated to continental sources indicating progressive exhumation of both the ophiolite and the continental margin upon which it was obducted 4 0 ar 3 9 ar analysis of muscovite from metamorphic rock fragments in debris flows of the melange yield an age of 467 2 ma which is within the range of ar ages measured in metamorphic rocks that underlie the ophiolites and indicates the uplift of metamorphosed continental deposits during or shortly after obduction 4 the saint daniel melange is stratigraphically overlain by the magog group and represents the base of a syncollisional basin developed in a forearc setting during the taconian orogeny

lg attenuation in the central and eastern united states as revealed by the earthscope transportable array abstract lg waveforms recorded by earthscope s transportable array ta are used to estimate lg q 0 in the central and eastern united states ceus attenuation is calculated based on lg spectral amplitudes filtered at a narrow band from 0 5 to 1 5 hz two station and reverse two station techniques are used to calculate q 0 values 39 events occurring from 2010 to 2012 and ranging from magnitude 3 to magnitude 6 are used in this study 39 258 and 4 627 interstation q 0 values are determined using the two station and reverse two station techniques respectively the results show northeast trending high q 0 regions low attenuation ranging from eastern new mexico to wisconsin regions of low q 0 high attenuation are seen along the minnesota wisconsin border the gulf coastal plain gcp along the south dakota nebraska border and along the oklahoma texas border a positive correlation between heat flow sediment thickness recent tectonic activity fluids and low q 0 was observed areas with low heat flow thin sediment cover no recent tectonic activity and a lack of fluids were observed to have consistently high q 0 these new models use a greater amount of data attain more coverage than previous studies and better constrain attenuation this increase in detail can improve high frequency ground motion predictions of future large earthquakes for more accurate far field hazard assessment and improve the overall understanding of the structure and assemblage of the ceus

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

structural evolution of the thetford mines ophiolite complex canada implications for the southern quebec ophiolitic belt 1 the thetford mines ophiolite complex tmoc preserves a complete ophiolitic sequence and occupies the hanging wall of a major se dipping normal fault the saint joseph fault preobduction synobduction and postobduction structures can be recognized in the tmoc ns trending preobduction paleonormal faults are parallel to ultramafic minor intrusions and to sheeted dykes recording extension related to seafloor spreading in a pericontinental suprasubduction zone basin wnw trending synobduction synmetamorphic fabrics are found toward the base of the tmoc and in the underlying continental margin rocks but are absent in the upper part of the tmoc and overlying sedimentary rocks these ordovician taconian structures record the development of a dynamothermal aureole immediately below the mantle margin contact and emplacement of the young ophiolite onto the continental margin postobduction structures include late silurian early devonian se verging backthrusts and back folds that inverted the tmoc and middle devonian acadian nw verging folds and reverse faults the tectonic history established for the tmoc is consistent with that of the adjacent laurentian margin and can be applied to the southern quebec ophiolitic belt as a whole the structural synthesis of the ophiolitic belt complemented with new observations and our compilation of stratigraphical geochemical geochronological and petrological data suggests that the southern quebec ophiolites may represent the remnants of the obduction of a single large slab of suprasubduction oceanic lithosphere extending for over a 100 km of strike length

interpretation of ages of arc magmatism metamorphism and collisional tectonics in the taconian orogen of western new england

obduction of tethyan type ophiolites a case study from the thetford mines ophiolitic complex quebec appalachians canada abstract oceanic plagiogranites and peridotite hosted granites phg of the thetford mines ophiolite tmo yield mean u pb zircon ages of 479 2 1 6 ma and 469 5 2 8 ma and are interpreted to be the products of fractionation of basaltic magma and melting of the laurentian margin respectively the existing 40ar 39ar age for the ophiolitic metamorphic sole of the tmo is a hornblende isochron age of 477 5 ma for amphibolite facies rocks sites previously sampled for u pb and 39ar 40ar analyses were resampled for more accurate 39ar 40ar dating in order to better constrain the thermo tectonic evolution of the ophiolite and underlying laurentian margin rocks on which they were overthrust and quantify the time required for obduction of tethyan type ophiolites in the canadian appalachians amphiboles from tmo gabbro and plagiogranite yield 39ar 40ar plateau ages of 477 475 ma in contrast muscovites from phg yield cooling ages of 466 465 ma and show evidence for thermal resetting at ca 460 ma for both a granitic mylonite and a crosscutting pegmatite facies the amphibolitic metamorphic sole yields 39ar 40ar ages of ca 471 ma in amphibole and high temperature ages of ca 466 ma in muscovite with evidence for recrystallization to ages as young as 457 ma muscovites from the sub ophiolitic laurentian metasedimentary basement and micaschist clasts in supraophiolitic debris flows of the tmo yield similar pseudo plateau and high temperature ages of ca 463 464 ma the u pb and 39ar 40ar data suggest that the obduction of peri laurentian oceanic crust lasted approximately 5 to 10 m y since both facies formed approximately m y after the ophiolitic crust this supports models whereby the still hot ophiolitic mantle provided the heat needed to generate the phg and the metamorphic sole between ca 475 and 470 ma muscovite ages of phg record cooling below ca 450 500 c by ca 465 ma but 39ar 40ar ages from both a mylonitized phg facies and micaschist clasts suggest that the emplacement of the oceanic lithosphere on the laurentia margin had nearly ended by ca 460 ma likely due to foreland directed downward thrust propagation and related exhumation of the collisional wedge the new age data imply that the southern quebec ophiolites were thrust 100s of km over the laurentian margin which has major implications for the location of the subduction suture and the early stages of the taconic orogeny in the northern appalachians

the northward tectonic transport in the southern apennines examples from the capri island and western sorrento peninsula italy we analyzed a thrust fault system located in the western sorrento peninsula and capri island southern italy where several mesoscale structures related to the main thrusts such as riedel shear planes overturned folds minor thrust and back thrust faults suggest a dominant northward tectonic transport major and minor thrust faults generally characterized by a ramp flat geometry involved the mesozoic apennine carbonates the middle miocene foredeep and the unconformable thrust top basin deposits the biostratigraphic analysis of calcareous nannoplankton assemblages on the thrust top basin sediments indicates an age not older than late tortonian we propose that this out of sequence thrusting stage was related to a regional tectonic event widespread in the entire southern apennines probably occurred in the pliocene time simultaneously with the activity of deep seated thrust faults that involved the buried carbonates of the apulian platform these out of sequence thrust faults here referred to as envelopment thrusts were enucleated in a lower structural level with respect to the allochthonous wedge representing the w e segments of large regional arcuate structures

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