the outer hebrides fault a major proterozoic structure in nw britain mylonites in the outer hebrides fault zone on scalpay hams are cut by metabasite sheets which are probably of scourie dyke age 2400 2200 ma and by pegmatites of late laxfordian age c 1750 ma the mylonites contain relicts of a gneissose banding which probably formed in the archaean c 2700 ma ago these age constraints suggest that the fault formed in the inverian event c 2500 ma ago not in the caledonian silurian devonian as widely reported the fault is a long lived structure which was reactivated as an extensional fualt in the laxfordian late proterozoic torr idonian and the carboniferous early mesozoic and as a thrust in the caledonian in a regional context this interpretation has important implications for the structural evolution of the lewisian complex for the nature of the caledonian event in the laurentian craton caledonian foreland and for the age and geometry of the sedimentary basins in the hanging wall of the outer hebrides fault

inversion of aeromagnetic data using digital terrain models although draped magnetic surveys contain more information about the magnetization of the rocks near the surface of the earth than surveys at constant elevation allowance for the effects of the terrain is critical for their correct interpretation a new method for calculating the magnetic effect of the topography from a digital terrain model by integrating analytically in the vertical direction and then numerically in the horizontal plane is presented this method lends itself to the calculation of anomalies when the magnetization of the rocks varies with position and thus is well suited to the inversion of draped aeromagnetic surveys to obtain the apparent magnetization of the surficial rocks this inversion is achieved by repeated use of an approximate inverse function in the form of a two dimensional 2 d filter that is applied to gridded data an example using draped magnetic data collected over white island an active volcanic island of high relief shows that although the anomaly pattern is domina

deformation of the caledonide lithosphere of northwest scotland abstract an interpretation of the moist seismic reflection profile along the north coast of scotland has shown the pattern of deep crustal structure across the margin of the caledonian orogen with the hebridean cratonic foreland to the northwest it is proposed that the lower crust of the orogen is characterised by a suite of easterly dipping thrusts that divide it into flakes that were imbricated during lateral compression of the lithosphere the effects of concomitant and subsequent uplift and erosion led to the removal of the major part of the upper crust within the orogen so that the present crust and moho are largely the relics of the caledonian lower crust as time progressed the thrust front encroached further into the foreland and cut down to a basal decollement at successively deeper levels this behaviour is explained as a consequence of the response of the brittle ductile transition to changing the temperature regime rock composition and strain rate as the orogeny proceeded palinspastic reconstructions of the whole lithosphere illustrate the process following the compressional phase of the orogeny uplift led to initiation of extension and the reactivation of the thrusts as normal listric faults rotation of basement blocks gave rise to wedge shaped sedimentary basins

proterozoic mafic dykes as monitors of hp granulite facies metamorphism in the grenville front tectonic zone western quebec metamorphosed mafic dykes and pods in the grenville front tectonic zone gftz are correlated on the bases of map patterns and geochemical trends with paleo and mesoproterozoic dykes that cross cut archean rocks of the foreland nw of the grenville front the gftz consists of a se dipping 30 km thick slab of quartzo feldspathic granulites and biotite garnet opx gneisses overriding along the mclaurin thrust a 4 km thick zone of muscovite sillimanite micaschists that abuts on the grenville front the granulite slab in turn dips under amphibolite grade migmatites of the reservoir dozois terrane along the dorval detachment according to published u pb and new u th pb ages on monazite the granulites the gneisses and the schists of the gftz were metamorphosed at ca 2 7 2 8 ga and reworked at ca 1 0 ga grenvillian proterozoic mafic dykes and pods cross cutting the gftz are thus monocyclic i e they went through the grenvillian metamorphic event only several of these metamorphosed mafic dykes and pods have preserved evidence of emplacement in a brittle regime such as intersection of the gneissic foliation chilled margins magmatic layering this in spite of a strong metamorphic overprint attested by coronitic fabrics in particular metamorphosed mafic dykes of the granulite slab contain large pyroxenes rimmed by garnet coronas with minute quartz inclusions suggesting the reaction pl opx grt qtz these garnet coronas in turn are locally replaced by symplectites of opx hbl pl and qtz probably formed through back reactions grt cpx qtz pl opx and grt cpx hbl qtz thermobarometric calculations using the above assemblages indicate peak equilibration at about 1 2 1 5 gpa at temperatures of about 800 c followed by a quasi isothermal decompression down to 0 9 gpa at temperatures around 700 c proterozoic mafic dykes thus record a grenvillian hp granulite metamorphism resulting from burial of upper crustal levels down to sub moho depths with concomitant heating this event was followed by rapid exhumation of the gftz zone probably controlled by normal displacement along the dorval detachment

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

deep seismic reflection profiling of the lewisian foreland sum m a r y a number of marine deep seismic reflection profiles cross the hebridean shelf the lewisian foreland to the caledonian orogen they include the birps moist winch and drum profiles several recent commercial deep lines and many old and new conventional commercial exploration profiles basement comprising the upper crust is largely devoid of coherent seismic reflections in contrast the mid crust contains many reflectors which may be relics of early palaeozoic caledonian or earlier grenvillian eastward dipping thrust zones which pass into an acoustically strongly layered lower crust the outer isles thrust is mapped from the surface to the mid crust and tied into its land outcrop on north lewis reactivation of this thrust as a normal fault caused the formation of the sea of the hebrides minch and north lewis basins the moho is defined by a strong band of reflections at a rather uniform 27 km depth the eastward dipping flannan thrust can be mapped into the upper mantle from about is to 45 km depth the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the east on moist and is either the western most of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or a more easterly thrust which structurally overlies these easterly dipping reflectors in neither case are the easterlydipping reflectors themselves likely to be simply caledonised lewisian foreland

contrasting zoning profiles in high grade garnets evidence for the allochthonous nature of a grenville province terrane in the central part of the grenville province proterozoic metasediments of the reservoir cabonga terrane arc underlain by gently dipping archean migmatites a major mylonite zone located within the migmatites is the base of a large thrust sheet made up of the reservoir cabonga terrane and a migmatitic sole garnets from meta aluminous assemblages in proterozoic metasediments mylonites and migmatites display contrasting types of ca zoning patterns garnets with ca rich cores are found along the western edge of the reservoir cabonga terrane whereas garnets with flat ca profiles are found in the remaining proterozoic metapelites in the mylonites and in the archean migmatites considering samples in which an infinite reservoir of ca ions is warranted by abundant plagioclase in the matrix pressures well in excess of 1200 mpa at temperatures between 700 and 800 c are indicated for a metamorphic event recorded in the cores of garnets from the western edge of the reservoir cabonga terrane the inner rim of garnet records pressures of around 1000 mpa for the same temperature range in homogeneous garnets a single p t set around 800 mpa and 750 c is recorded in the migmatites some of the mylonites initiate their recrystallization under relatively high pressures which is continued during decompression and record the lowest p t values these metamorphic signatures suggest that the proterozoic metapelites were already metamorphosed under relatively high pressure when emplaced on to archean migmatites and that the entire area was subsequently subjected to metamorphism at a lower pressure

the erris ridge a major geological feature in the nw irish offshore basins the erris ridge is a narrow structural high that separates the erris and rockall troughs it has a composite internal character and its orientation is thought to be controlled by a pre caledonian structural fabric which was frequently reactivated during caledonian to cenozoic time in the north it is fault bounded and probably contains rocks of palaeozoic and older age it formed a positive topographic feature which was progressively onlapped during permo triassic to cretaceous times fault reactivation occurred along its western boundary in tertiary times and it was submerged by approximately latest miocene times further south the ridge is more complex the palaeozoic and older core is flanked to the se by interpreted permo triassic to lower jurassic strata which constitute a fault controlled uplifted remnant of a half graben basin this composite structure was probably inverted during late jurassic times and remained as a topographic feature until cretaceous times it was progressively onlapped during cretaceous and tertiary times with a minor phase of fault reactivation during the early tertiary the ridge acted as a rigid block which facilitated the focusing of several phases of crustal extension in the erris trough and in the rockall trough

the influence of depth on reactivation in normal faulting abstract the study examines the influence of depth on the development of steep normal faults above a gently dipping fault which is reactivated in its deeper part and abandoned in its upper part the isotropic and anisotropic mohr coulomb anderson theory is used to determine the ability of any part of a fault to be reactivated by calculating the difference between the stress deviator necessary to induce reactivation and that required to induce fracturing when linear depth dependent properties are assumed it appears that a new normal fault can branch off a gentle pre existing fault only if the friction coefficient of the latter is much less than that used by byerlee and if the ratio cohesion of fault cohesion of intact rock is non negligible for a 30 dip fault the friction coefficient has to be less than 0 35 friction angle less than 20 neither pore fluid pressure nor constant excess fluid pressure affects the above conditions when the cohesion of the surrounding rocks increases with depth steep normal faults can branch off byerlee faults with dips as gentle as 30 a low shear strength of the lowest part of the preexisting fault also increases the chances of reactivation of the deeper part and abandonment of the upper part

docking of the central metasedimentary belt to laurentia in geon 12 evidence from the 1 17 1 16 ga chevreuil intrusive suite and host gneisses quebec the chevreuil intrusive suite 1 17 1 16 ga represents a chronological field marker of regional extent that intruded the central metasedimentary belt in the western grenville province of quebec after peak metamorphism style and site of magma emplacement and extent of deformation of chevreuil plutons and dykes permit unravelling of the early grenvillian evolution of the belt with respect to cratonal north america the suite comprises a series of vertically layered gabbro stocks and monzonite diorite gabbro sheet intrusions and a swarm of microdiorite dykes that cut across gneisses the dykes display systematic variations in extent of deformation across the belt we targeted u pb geochronology on gneisses within the identified strain windows they preserve the record of a ca 1 20 ga high pressure temperature p t metamorphic event the sheet intrusions define magmatic corridors all along and concordant with the western northern and eastern tectonic boundaries of the belt the concordant and elonga

gravity terrain corrections calculated using digital elevation models corrections for terrain effects are required for virtually all gravity measurements acquired in mountainous areas as well as for high precision surveys even in areas of low relief terrain corrections are normally divided into two parts one part being the correction for terrain relatively close to the gravity station the inner zone correction and the other part being the correction for more distant say 2 km terrain the latter correction is normally calculated using a machine procedure that accesses a digital terrain data set the corrections for terrain very close to the gravity station are done manually using hammer s 1939 procedures or a similar method are guessed in the field or simply are neglected occasionally special correction procedures are used for the inner zone terrain corrections e g lafehr et al 1988 but such instances are uncommon

structural discordance between neogene detachments and frontal sevier thrusts central mormon mountains southern nevada detailed geologic mapping in the mormon mountains of southern nevada provides significant insight into processes of extensional tectonics developed within older compressional orogens a newly discovered wsw directed low angle normal fault the mormon peak detachment juxtaposes the highest levels of the frontal most part of the east vergent mesozoic sevier thrust belt with autochthonous crystalline basement palinspastic analysis suggests that the detachment initially dipped 20 25 to the west and cut discordantly across thrust faults nearly complete lateral removal of the hanging wall from the area has exposed a 5 km thick longitudinal cross section through the thrust belt in the footwall while highly attenuated remnants of the hanging wall nowhere more than a few hundred meters thick structurally veneer the range the present arched configuration of the detachment resulted in part from progressive domino style rotation of a few degrees while it was active but is largely due to rotation on younger structurally lower basement penetrating normal faults that initiated at high angle the geometry and kinematics of normal faulting in the mormon mountains suggest that pre existing thrust planes are not required for the initiation of low angle normal faults and even where closely overlapped by extensional tectonism need not function as a primary control of detachment geometry caution must thus be exercised in interpreting low angle normal faults of uncertain tectonic heritage such as those seen in the cocorp west central utah and birp s moist deep reflection profiles although thrust fault reactivation has reasonably been shown to be the origin of a very few low angle normal faults our results indicate that it may not be as fundamental a component of orogenic architecture as it is now widely perceived to be we conclude that while in many instances thrust fault reactivation may be both a plausible and attractive hypothesis it may never be assumed

gravity modelling along a lithoprobe seismic traverse northern grenville province western quebec high precision gravity data were collected along lithoprobe seismic reflection lines in the northern part of the grenville province in western quebec an interpretation is presented for line 52 which starts some 60 km southeast of the grenville front traverses the parautochthonous reservoir dozois terrane including the allochthonous slice of the reservoir cabonga terrane and ends near the town of mont laurier in the allochthonous mont laurier terrane on the regional scale the bouguer gravity anomaly is consistent with the interpretation of the seismic reflection data it supports crustal thinning southward of the grenville front under the cabonga allochthon this thinning may be related to postorogenic extension the gravity modelling shows dramatic thinning of the lower crust and suggests that extension was accommodated by extrusion of the lower crust the gravity modelling also requires a steep boundary between the reservoir cabonga and the reservoir dozois terranes extending to 15 km the geo

quaternary evolution of glaciated gneiss terrains pre glacial weathering vs glacial erosion vast areas previously covered by pleistocene ice sheets consist of rugged bedrock dominated terrain of innumerable knolls and lake filled rock basins the cnoc and lochan landscape or landscape of areal scour these landscapes typically form on gneissose or granitic lithologies and are interpreted 1 either to be the result of strong and widespread glacial erosion over numerous glacial cycles or 2 formed by stripping of a saprolitic weathering mantle from an older deeply weathered landscape we analyse bedrock structure erosional landforms and weathering remnants and within the cnoc and lochan gneiss terrain of a rough peneplain in nw scotland and compare this with a geomorphologically similar gneiss terrain in a non glacial arid setting namaqualand south africa we find that the topography of the gneiss landscapes in nw scotland and namaqualand closely follows the old bedrock saprolite contact weathering front the roughness of the weathering front is caused by deep fracture zones providing a highly irregular surface area for weathering to proceed the weathering front represents a significant change in bedrock physical properties glacial erosion and aeolian erosion in namaqualand is an efficient way of stripping saprolite but is far less effective in eroding hard unweathered bedrock significant glacial erosion of hard gneiss probably only occurs beneath palaeo ice streams we conclude that the rough topography of glaciated cnoc and lochan gneiss terrains is formed by a multistage process 1 long term pre glacial chemical weathering forming deep saprolite with an irregular weathering front 2 stripping of weak saprolite by glacial erosion during the first glaciation s resulting in a rough land surface broadly conforming to the pre existing weathering front etch surface 3 further modification of exposed hard bedrock by glacial erosion in most areas glacial erosion is limited but can be significant beneath palaeo ice stream the roughness of glaciated gneiss terrains is crucial for modelling of the glacial dynamics of present day ice sheets this roughness is shown here to depend on the intensity of pre glacial weathering as well as glacial erosion during successive glaciations

restored transect across the exhumed grenville orogen of laurentia and amazonia with implications for crustal architecture new 40 ar 39 ar analyses from a transect across the major tectonic units of the southwest amazon craton document the heterogeneous effects of the late mesoproterozoic collision with the grenville margin of north america basement rocks of the amazon and adjacent paragua cratons mostly preserve pre grenvillian ages older than 1 3 ga localized iso topic age resetting at 1 18 1 12 ga is caused by grenvillian activation of widespread sinistral strike slip shear zones in the amazon basement in the nova brasilandia belt between these two cratons new 40 ar 39 ar data record cooling through 920 ma after the granulite facies deformation of this suture zone regional cooling rates calculated from compiled u pb 40 ar 39 ar and rb sr thermochronologic data are used to establish post grenvillian exhumation patterns for the southwest amazon and the north american belt paleodepths calculated for 1 0 ga along a transect of the restored 1300 km wide belt vary from uniformly deep levels 15 30 km exposed in north america to shallower levels 5 15 km observed in the southwest amazon we interpret this difference as reflective of a change in tectonic architecture i e thrust dominated deformation in laurentia versus strike slip dominated deformation in the amazon with a commensurate variation in crust al thickness this interpretation explains the widespread preservation of both pre grenvillian ages and collisional ages from the amazon craton in contrast with the more homogeneous array of cooling ages from the north american grenville province marking the postorogenic extensional collapse of an overthickened crust the asymmetrical oro genic architecture from the reconstructed grenville belt mirrors cross sections proposed for modern orogenic belts where deep crustal rocks are not yet exposed

transcrystalline shear fracturing and pseudotachylite generation in a meta anorthosite harris scotland abstract the meta anorthosite is locally deformed by brittle shear fracturing which progressively increases from isolated fractures with little cataclasite to many generations of closely spaced fractures the intervening rock being highly deformed in both a plastic and brittle way in most cases an e w compression on gently dipping to steep reverse shear planes occurs which we relate to a caledonian thrust zone in places the highly deformed rock is cut by pseudotachylite veins which locally form networks the pseudotachylite is generally intrusive but does not appear to be related to movement on major slip surfaces very locally it may have formed in situ pseudotachylite only occurs in highly deformed rock is only very occasionally deformed itself and thus generally represents at each locality the last stage of a complex deformation history as if its presence welded the rock and prevented further deformation these striking differences from the country rock gneisses in which pseudotachylite occurs on well developed fractures in very slightly deformed rock are considered to be due to the low anisotropy of the meta anorthosite to its lower shear strength and to the easy propagation and branching of the shear fractures in plagioclase the source of the heat necessary to generate the pseudotachylite melt is not clear it may come from crack propagation as well as frictional sliding

crustal evolution along a seismic section across the grenville province western quebec results of deep seismic reflection survey along a 375 km long transect of the grenville province in western quebec are combined with a review of geological observations and published isotopic ages the seismic profile offers a remarkably clear image of the crust mantle boundary and a good definition of the various crustal blocks crust about 44 km thick beneath the grenville front zone thins abruptly to ca 36 km southeastward perhaps the result of extension on southeast dipping surfaces extending to the moho other zones of relatively thin crust although less pronounced occur where proterozoic crust overlies archean crust and beneath the morin anorthosite complex the thickest crust is found at the extreme southeast of the transect east of the morin anorthosite from northwest to southeast three main crustal subdivisions are 1 deformed archean rocks with southeast dipping reflectors in the grenville front zone 2 an archean parautochthon with northwest dipping reflectors extending to the lower

metamorphic constraints on the tectonic evolution of the allochthonous monocyclic belt of the grenville province western quebec the tectono metamorphic evolution of grenvillian monocyclic terranes in western quebec has been constrained along a northwest southeast transect from reservoir cabonga to the morin anorthosite by t

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

the grenville front south of val d or quebec abstract the tectonometamorphic history of rock assemblages is unravelled throughout a 50 30 km strip across the grenville front the boundary between the superior and the grenville provinces in the canadian shield this was accomplished by applying geothermobarometric techniques to zoned garnets systematic analysis of garnetiferous assemblages revealed that 1 1 primary garnet grti cores grew during a high t low p metamorphic episode and 2 2 secondary garnet grtii cores as well as most of the grti rims grew during a low low t high p metamorphic episode the two sets of p t conditions can by comparison with available geochronological data be assigned to the kenoran late archaean and grenvillian mid proterozoic orogenies respectively a prominent ductile shear zone traverses the 50 30 km strip and divides the area southeast of the grenville front into two regions with contrasted tectonometamorphic behaviour the eastern region is composed of migmatitic and charnockitic gneisses representing intermediate to lower levels of the archaean crust the western region is composed of migmatitic metasediments which show no evidence of significant crustal thickening during the grenvillian orogeny along a transect across the grenville front eastern region significant offsets of grenvillian paleopressure levels indicate that the thickened crust is a stack of imbricated slices with vertical components of displacement in the order of several kilometres after compensating for the grenvillian movement along the appropriate shear zones offsets remain between some of the archaean paleopressure levels suggesting that delamination of the archaean crust was initiated prior to the peak of grenvillian metamorphism the thrust slices thus identified in the eastern region have thicknesses of a few kilometres but they could be much thinner in the western region here the grenville front is ill defined and lies in a zone of gradual transition between the pontiac group metasediments of the superior province and the lac lambert migmatites of the grenville province the overall deformations of the two regions southeast of the grenville front are incompatible and cause the development of an intervening shear zone although the two regions experienced different tectonometamorphic histories they both belong to the parautochthonous belt of the grenville orogen

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

does draping aeromagnetic data reduce terrain induced effects contrary to intuition draped aeromagnetic surveys when compared to typical level surveys amplify rather than reduce the problem of magnetic terrain anomalies calculations of the total magnetic field of various simple magnetic topographies on level and draped surfaces support this conclusion in cases where draped surfaces are lower than level surfaces the draped profiles exhibit steeper gradients and deeper polarity lows over topography than do the level profiles on the other hand where draped surfaces are higher than level surfaces all anomalies are attenuated so that magnetic terrain effects might be reduced relative to subsurface sources depending upon the magnetization of each the difference in magnetic behavior between level and draped data can be explained by a contribution of a vertical derivative component in the draped case that is absent in the level case the contribution is most significant near topographic features because both the observation surface and the topographic surface

rheological controls on grenvillian intrusive suites implications for tectonic analysis abstract studies of plutonic suites in western grenville along with the recent advances in understanding magma ascent and emplacement and interactions between magma and their country rock provide insights on the use of intrusive rocks to infer the accretionary history of deeply exposed precambrian orogens in the grenvillian central metasedimentary belt of quebec terrane juxtaposition was inferred to be late based on the distribution of 1 08 ga potassic alkaline plutons this view was questioned however upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting rheologically softer units for example grenvillian marble in this context preferential ponding of magmas in marble rather than tectonics can control regional distribution of plutons the spatial association of plutons with marble and their contact relationships are at the local scale obscured by transformation of marble to skarn and by mechanical excision of marble from the paragneiss wall rock sequence in contrast plutons and dykes of an older 1 17 ga less alkaline magma association are shown by field and remote sensing studies to be evenly distributed across the various lithotectonic domains of the belt their sheet like emplacement along the belt boundary constrains grenvillian tectonic assembly in the region to be early contrasting loci type and degree of deformation of intrusive bodies illustrate that host rock rheological differences influence the final characteristics of an intrusive suite consequently the nature of the host is an important variable in inferring timing relationships and tectonic scenarios from intrusive suites in high grade terranes

erratum metamorphic constraints on the evolution of the gneisses from the parautochthonous and autochthonous polycyclic belts grenville province western quebec the tectono metamorphic history of polycyclic grey gneisses located in the central grenville province of western quebec has been constrained along a transect perpendicular to the length of the gr

new constraints on the geometry of the lac bouchette gabbro anorthosite from magnetotellurics and magnetics a cross section of the resistivity structure through the lac bouchette gabbro anorthosite provides a new image of the thin skinned geometry of an allochthonous terrane in the western grenville prov

a two stage emplacement for the cabonga allochthon central part of the grenville province evidence for orthogonal and oblique collision during the grenville orogeny the cabonga allochthon central part of the grenville province is a proterozoic unit of high grade metasedimentary and metaigneous rocks resting on top of reworked parautochthonous archean migmatites of the reservoir dozois terrane the geometry and final assembly of terranes in this area result from the following succession of strains and displacements 1 thrusting towards the northwest of a proterozoic terrane 2 northwest southeast shortening of the transported terrane as indicated by large scale northeast trending buckle folds and by coeval development of a decollement within the upper layers of the archean footwall 3 reactivating of this decollement in the form of an east dipping thrust the cabonga thrust piggy back sequence the detached basement and the proterozoic allochthon forming the cabonga thrust sheet 4 northeast trending sinistral wrenching in the parautochthon responsible for the sinuous contour of the overlying cabonga thrust and probably also for the westward thrusting i

geochronological constraints on the last stages of terrane assembly in the central part of the grenville province abstract new u pb zircon and monazite ages for syn and post kinematic pegmatites that cut major shear zones within the western quebec portion of the grenville province constrain the timing of deformation and final terrane assembly within this region from the grenville front se along the present section the grenville province consists of a reworked archean promontory partly covered by proterozoic allochthons and two proterozoic metasedimentary and metaigneous terranes which are probably allochthonous the age of the grenville front is still poorly constrained in this area as pegmatites intersecting high grade rocks exhumed some 30 km south of the front have upper intercept u pb ages of 2642 6 5 ma field and geochronological data in this paper however lend support to the possibility of late archean exhumation of high grade terranes along the front south of the front two types of ductile shear zone are responsible for the geometry of terranes in this part of the grenville province the first type corresponds to the boundary of allochthons transported onto the archean parautochthon the corresponding structures are not directly dated but reheating of the parautochthon at about 1020 ma u pb ages on monazite sets a minimum age for thrusting in this area late movement along an east dipping shear zone located within the parautochthon the cabonga thrust ct 998 9 5 ma is rather unique in that it involves rocks of the parautochthon and is characterized by a westerly directed transport the shear zones of the second type are subvertical ne trending generally sinistral and intersect the parautochthon the allochthon or both from the front towards the se these shear zones are the cadgecrib shear zone 998 16 6 ma intersecting the parautochthon the renzy shear zone 1003 4 5 ma involving allochthonous metasediments the baskatong shear zone 1020 2 1 ma which marks the nw limit of proterozoic rocks of the mont laurier terrane the labelle shear zone 1078 6 ma separating the mont laurier and the morin terrane and the taureau shear zone 1074 4 ma bounding the morin terrane to the east all the above timing constraints are from u pb ages on syn to post kinematic pegmatites emplaced within the respective shear zones this age distribution suggests a propagation of transcurrent deformation towards the nw which is responsible for the final configuration of terranes

geometry and kinematics of a major crustal shear zone segment in the appalachians of southern new brunswick in southern new brunswick the kennebecasis fault follows the northern boundary of a crystalline portion of the late precambrian cambrian avalon terrane this low grade crystalline complex forms the basement to a series of carboniferous through triassic basins this complex also contains a major shear zone relic that is largely flat lying but upturned adjacent to the fault and that with it defines the pocologan kennebecasis fault zone the orientation of the main composite foliation s1 and the included mineral stretching lineation l1 indicate that this geometry is a primary feature of the shear zone representing a linked pair of horizontal and vertical detachments bounding an allochthonous unit kinematic indicators show that this allochthon moved parallel to the strike of the north appalachian orogen with top towards the west or west southwest the bounding shear zone is not uniform but consists of a mylonite phyllonite adjacent to the detachment the upturned segment of the shear zone has bee

age of the cabonga nepheline syenite grenville province western quebec this study presents isotope dilution thermal ionization mass spectrometry id tims u pb data for megacrystic zircon from the cabonga nepheline syenite complex of the reservoir cabonga terrane grenville province this terrane is an allochthonous slab metamorphosed under granulite grade conditions 1 18 1 14 ga and transported onto migmatites of the grenvillian parautochthon at about 1 02 ga the very low uranium and lead concentrations of zircon from the cabonga nepheline syenite cns did not permit determination of a statistically significant crystallization age using laser ablation microbeam techniques consequently extensive microsampling 15 zircon chips guided by x ray and electronic imaging followed by id tims analyses were carried out on a single megacrystic zircon a regression of 13 out of 15 u pb isotopic analyses results in a crystallization age of 117 1 3m a for the cns criteria based on zircon morphologies zoning patterns varying th u ratios 0 3 0 9 and a highly fractionated zr hf ratio 68 suggest an igneous derivation for the cns the cabonga alkaline rocks intruded under high grade metamorphic conditions pre ceded the onset of the widespread and ubiquitous 1 15 0 01 ga anorthosite mangerite charnockite granite magmatism in the southern part of the grenville province resume cette etude presente des donnees de geochronologie u pb par dilution isotopique spectrometrie de masse par ionisation thermique id tims sur des zircons a megacristaux provenant du complexe de syenite a nepheline de cabonga du terrane du reservoir cabonga province de grenville ce terrane est une dalle allochtone qui a ete meta morphisee au facies des granulites vers 1 18 1 14 ga et transporte par dessus des migmatites du parautochtone gren villien vers 1 02 ga les tres faibles concentrations d uranium et de plomb dans le zircon de la syenite a nepheline de cabonga n ont pas permis de determiner un ge de cristallisation statistiquement significatif au moyen de techniques d ablation par laser a microfaisceaux de nombreux micro echantillonnages 15 eclats de zircon guides par imagerie aux rayons x et electronique et suivis d analyses par dilution isotopique et spectrometrie de masse par ionisation thermique id tims ont ete effectues sur un seul megacristal de zircon une regression de 13 des 15 analyses u pb isotopiques a donne un ge de cristallisation de 117 1 3m apour la syenite a nepheline de cabonga selon des criteres bases sur la morphologie des zircons les patrons de zonalite les rapports th u variables 0 3 a 0 9 et un rapport zr hf hautement fractionne 68 la syenite a nepheline de cabonga proviendrait de roches ignees les roches alcalines de cabonga mises en place sous des conditions de metamorphisme eleve precedent le debut du magmatisme 1 15 0 01 ga anorthosite mangerite charnokite granite generalise et tres etendu dans la partie sud de la province de grenville

a new variable magnetization terrain correction method for aeromagnetic data terrain effects in aeromagnetic data are produced by rugged magnetic topography these effects mimic the shape of topography and can often be so large that they obscure anomalies of interest thus it is desirable to remove terrain effects from aeromagnetic data in order to isolate the anomalies to be investigated however removal of aeromagnetic terrain effects has been a longstanding problem previously developed methods have succeeded only in certain specific geologic situations i present a new aeromagnetic terrain correction method that is superior to the previously developed methods for the general case this method takes into account the highly variable magnetic properties of rocks and can remove terrain effects whether the sources of interest are shallow or deep the new method is based on the assumption that magnetic sources of interest are often geometrically unrelated to terrain it finds the magnetization that gives a magnetic field residual with minimum correlation to terrain effects for a

nd isotope mapping of allochthonous grenvillian klippen evidence for widespread ramp flat thrust geometry in the sw grenville province abstract over sixty new nd isotope analyses are used to constrain the extent of several small grenvillian lithotectonic domains that form erosional outliers of the grenvillian allochthonous polycyclic belt the identification of these tectonic klippen when combined with previous mapping of large nappes shows that the allochthonous polycyclic belt forms a mid upper crustal sub horizontal thrust sheet that was transported to the nw over the archean proterozoic parautochthon this thrust sheet was corrugated by folding and dissected by erosion leaving the klippen as synformal remnants nd isotope mapping of these remnants when coupled with seismic reflection data from the lithoprobe programme suggests that these thrust sheets were exhumed on relatively steep crustal scale ramps before transportation to the nw on sub horizontal shear zones this type of thrust geometry previously observed in the central eastern grenville province was termed ramp flat geometry by rivers et al 2012 re examination of lithoprobe seismic data for the georgian bay transect shows that reflectors under the parry sound klippe have a synformal character in the mid crust suggesting that this is also part of an undulose sub horizontal thrust sheet exhumed on a steep crustal scale ramp rather than the previous model of a very long ramp with low inclination hence we argue that the georgian bay section has a geometry resembling the ramp flat trajectory of the allochthon boundary thrust previously discerned elsewhere in the grenville province

tectono thermal evolution of deep crust in a mesoproterozoic continental collision setting the manicouagan example high pressure metamorphic rocks in continental collision belts provide important information on the orogenic evolution of deep crust in the grenville province a prime example is the manicouagan imbricate zone miz a mesoproterozoic crustal section that was metamorphosed up to 1800 mpa and 850 950 c at 1050 ma high tempera tures and the presence of synmetamorphic within plate tholeiitic gabbro intrusion in the miz attest to thermal perturba tion during burial and emplacement of mantle derived melts consistent with thinning of lithospheric mantle subsequently the miz was extruded by a combination of northwest directed thrusting over a crustal scale ramp and ex tension at higher crustal levels in the middle miz extension was coeval with thrusting but perpendicular to the thrust direction and was apparently controlled by ductility contrasts between slices in contrast the highest levels show evi dence of southeast directed extension i e of opposite sense to the thrust direction that postdated thrusting within the miz this episode was followed by renewed northwest directed thrusting of medium pressure mesoproterozoic units over the miz and then by final southeast directed extension overall configuration of the miz was achieved between 1040 990 ma and is consistent with outward propagation of the orogen alternating periods of thrusting and exten sion displacement are a likely result of adjustments permitting maintenance of dynamic equilibrium within the advanc ing stack the tectonothermal characteristics of the miz allow comparison between the grenville province and younger thermally perturbed orogens i e variscan belt and may be the final product of a tibetan plateau style evo lution

the central metasedimentary belt grenville province as a failed back arc rift zone nd isotope evidence abstract nd isotope data are presented for granitoid orthogneisses from the central metasedimentary belt cmb of the grenville province in order to map the extent of juvenile grenvillian age crust within this orogenic belt that is composed mostly of older crustal terranes the data reveal a 150 km wide belt of juvenile crust in ontario but this belt contains a block of pre grenvillian crust containing the elzevir pluton which yields an estimated crustal formation age of 1 5 ga the recognition of an older block within the cmb has profound implications for its structure and tectonic evolution because it implies that juvenile grenvillian crust apparently forming a wide ne sw belt is in fact distributed in two narrower segments with approximately n s strike we suggest that the cmb comprises an en echelon series of ensimatic rift segments created by back arc spreading behind a continental margin arc these rift segments extend southwards in the subsurface into the northeastern unites states the rift segments contain abundant marble outcrops consistent with marine incursion into the rift zone and these deposits also continue northwards into a marble domain of the cmb in quebec however crustal formation ages in the latter domain are largely pre grenvillian implying that the quebec rift segment was ensialic hence we interpret the cmb in ontario and quebec as the northern termination of a failed back arc rift zone

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

three dimensional mapping of seismic reflections from the crust and upper mantle northwest of scotland abstract a survey area 190 km by 110 km to the northwest of scotland on the uk continental shelf has been the site of intense exploration using the deep seismic reflection technique a closely spaced grid of deep reflection data totalling 1600 line km has been acquired by the birps group between 1981 and 1987 the majority of the data were recorded to 15 s two way travel time 50 km depth profiles with recording times up to 60 s 230 km depth were also acquired the survey includes a two ship synthetic aperture profile the slave line which synthesises a 0 16 km offset cdp profile the main features revealed and consistently imaged across the survey area by this combined dataset are an upper crust containing half graben style basins bounded to the west by planar faults a reflective lower crust and dipping and sub horizontal high amplitude reflections within the mantle three dimensional mapping of the dipping reflectors shows a complex package of reflections which updomes into the lower crust in the central area of the survey disrupting the moho and continuing into the uppermost mantle where it thins to a couplet of reflections in the north and south of the survey area this structure is observed only in the mantle to a depth of 80 km this dataset is unique in deep seismic reflection profiling in terms of the quality of the image returned the great depth to which reflections are imaged and the intensity of surveying of the lower crust and upper mantle the continuity and strength of the reflectors seen on these records show that even at the great depths at which these structures exist the seismic reflection technique is able to image them as high amplitude coherent and highly continuous reflections the reflections are interpreted as shear zones or faults within the lower crust and upper mantle this indicates that in this part of the world at least both the lower crust and upper mantle are able to sustain discrete zones of deformation

a model for the moist seismic reflection profile n scotland an interpretation of the moine and outer isles seismic traverse reflection profile along the north coast of scotland is developed here by the construction of a model crustal cross section from which by ray tracing a synthetic seismic section has been prepared for comparison with the observed record section faults are modelled by narrow low velocity zones and the moho by narrow high and low velocity zones the geometry of the faults shown in the model is believed to imply that during the caledonian orogeny crustal thickening was achieved by imbrication of thrust flakes from the se in which the thrust front propagated northwestward into the foreland and at the same time cut down so that the basal decollement deepened relative to the moho eventually into the mantle concomitant erosion and uplift initiated extension and the development of sedimentary basins in half grabens

deep structure of the foreland to the caledonian orogen nw scotland results of the birps winch profile the winch marine deep seismic reflection profile crosses the hebridean shelf the proterozoic foreland to the caledonian orogen west of scotland the data quality is very good the upper crust is largely devoid of coherent seismic reflections although this may in part be due to acquisition techniques being inappropriate for this problem in contrast the middle and lower crust 10 25 km depth exhibits good reflections the mid crust contains reflectors which may be relics of early palaeozoic caledonian or earlier grenvillian eastward dipping thrust zones which pass into an acoustically strongly layered lower crust the outer isles thrust is mapped from the surface to the mid crust and tied into its land outcrop on north lewis reactivation of this thrust offshore in a normal sense in mesozoic times caused the formation of the minch and north lewis basins the mono is defined by a strong band of reflections at about 27 km depth which correlates well with results from the hmse explosion seismic survey moho depth is apparently rather uniform in the area of the foreland crossed by winch winch and hmse results together suggest a mean crustal velocity for the hebridean shelf of 6 4 0 1 km s 1 the eastward dipping flannan thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth well into the upper mantle neither the flannan thrust nor the outer isles thrust appear to pass straight through the reflective lower crust suggesting that the lower crust is a region of high strain the outer hebrides is a positive block probably formed as an isostatic response to mesozoic normal faulting which reactivated the outer isles thrust

magnetic models of crystalline terrane accounting for the effect of topography igneous rocks commonly have large magnetic susceptibilities so that high topographic relief in crystalline terrane can produce significant anomalies in aeromagnetic surveys topographic anomalies are particularly significant in relatively undeformed volcanic terrane because young volcanic rocks generally have large natural remanent magnetizations as well as large susceptibilities these anomalies commonly appear in aeromagnetic surveys as a complex pattern of high amplitude short wavelength magnetic features that tend to obscure anomalies caused by deeper geologic sources we have facilitated geologic interpretation of an aeromagnetic survey of the oregon cascade range by calculating the magnetic field caused by a three dimensional 3 d topographic model maps of the calculated field are compared with observed aeromagnetic data both visually and with a numerical technique that produces a contour map of correlation coefficients for the model these comparisons allow quick recognition of anomalies caused

high energy flood events recorded in the mesoproterozoic meall dearg formation nw scotland their recognition and implications for the study of pre vegetation alluvium the mesoproterozoic neoproterozoic torridonian sandstone of nw scotland has renewed sedimentological significance following recent advances in the understanding of pre vegetation alluvium as it is one of the most extensive and easily accessible successions of such strata worldwide this paper presents the first modern sedimentological analysis of the unit9s constituent meall dearg formation late mesoproterozoic recognizing a dominance of alluvial facies with subordinate aeolian facies alluvial strata within the meall dearg formation contain direct evidence for event deposition by high energy floods including the following 1 widespread upper and transitional upper flow regime elements 2 frequent stacking of successively lower flow regime elements 3 common subcritical subaqueous dune fields with superimposed ripple marks 4 occasional thin desiccated mudstones 5 evidence that microbial mats colonized substrates during intervals of sedimentary stasis together these strands of primary sedimentary geological evidence indicate that the alluvial deposition of the meall dearg formation was typified by supercritical flows during high energy floods punctuated by prolonged intervals of sedimentary stasis the preservation potential of all of the features was boosted by highly aggradational sedimentary conditions these primary observations have implications for the norm in pre vegetation alluvium and confirm that despite increasing recognition of diversity within precambrian fluvial systems classical pre vegetation motifs of high energy alluvial flood deposits preserved as sheet braided alluvium are still an archetypal sedimentary signature in some instances using supportive evidence from the meall dearg formation we recommend that the term sheet braided be used in inverted commas in future studies this emphasizes the polygenetic depositional nature of the 20 1 width thickness fluvial style while maintaining the value of the term in the isolation of a key characteristic of pre vegetation sedimentary architecture

magnetic field changes on white island new zealand and the value of magnetic changes for eruption forecasting for about 15 years after their commencement in 1968 magnetic measurements in the crater of white island gave a good indication of the state of activity and the likelihood of an eruption episode commencing during this period magnetic thermal ground level and chemical measurements were all generally consistent with a model in which these surface changes were caused by increasing temperatures under the donald mound area in the central sub crater in more recent times there has not been such a relationship between the changes around the remnants of donald mound and the volcanic activity which has become localised in the west sub crater it appears that changes under the crater especially lowering of the ground surface and the water table at the west end of the crater have meant that eruptions are no longer preceded by heating under donald mound the west sub crater is not easily accessible for repeat surface surveys one way in which we might be able to monitor future volcanic activity at white island would be by repeated aeromagnetic surveys as the magnetic changes are so large that changes of hundreds of nanoteslas are likely to be observed at practical flying heights

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault

basin structure and tertiary magmatism on skye nw scotland the emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre existing basin structures these controls are investigated using examples from the tertiary igneous complexes of skye part of the sea of hebrides basin of nw scotland the basin consists of an array of half graben related to se dipping normal faults these pre volcanic mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields the unconformity represents a period of cretaceous uplift and denudation of the basin and its flanks entirely pre dating the tertiary volcanism of nw scotland this unconformity seals stratigraphically the major basin faults preserving field relationships that permit the tracing of these faults in the country rocks to the tertiary intrusions the major camasunary fault is separated from the raasay fault via a series of minor graben linked by a series of steep nw se trending faults that transfered mesozoic displacements between the principal fault strands a broad range of igneous material of various compositions was intruded into part of the mesozoic sea of hebrides basins and their flanks during palaeocene times different emplacement styles and different structural controls are found the major gabbroic centres do not appear to be controlled by upper crustal structures having been emplaced into the footwalls of major faults however minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions thereby facilitating emplacement in contrast the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile reactivating segments of the basin fault network doming occurred from an array of sills the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions the sill levels and their transgressive forms are strongly related to inferred mesozoic basin structures the major fold structures of tertiary age in southern skye are interpreted as accommodating granitic emplacement rather than crustal shortening the nw se mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes these interpretations are illustrated using field relationships mapped in the vicinity of the coire uaigneich granophyre it is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions the deeper levels of the mesozoic faults show no evidence of substantial reactivation

40ar 39ar thermochronology along a western quebec transect of the grenville province canada ar ar thermochronology on 24 hornblendes 3 biotites 2 muscovites and 2 k feldspars collected along a 400 km long nw se geotraverse through the grenville province in western quebec is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the grenville orogen in the grenville front zone the c 1000 ma time of exhumation previously established from thermobarometric and isotopic studies is supported by the hornblende age data presented here from 60 km to 160 km se of the front reworked archaean migmatites of the parautochthonous reservoir dozois terrane rdt 1004 ma old metamorphic monazites contain hornblendes with 972 950 ma cooling ages assuming metamorphic geotherms between 25 and 30 c km 1 calculated cooling and unroofing rates are about 6 c ma 1 and 0 33 km ma 1 in the p t range 725 c 800 mpa and 450 c 400 mpa hornblendes from monocyclic rocks of the mont laurier and morin terranes mlt and mt monazite ages c 1165 ma give ages of about 1040 and 1010 ma respectively calculation of cooling unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated grenville collision which took place approximately between 1060 and 1020 ma cooling ages of c 900 ma for muscovite and biotite and 860 810 ma for k feldspar show that cooling rates decreased to around 1 5 c ma 1 under retrograde greenschist facies conditions in the mlt on a time vs distance diagram the hornblende data define several distinct age ranges suggesting that each terrane had a characteristic thermal history thus cooling was diachronous and probably non homogeneous throughout this segment of the grenville orogen the time lag between the cooling history of the parautochthon 972 950 ma and the allochthons 1040 1010 ma is compatible with an earlier pre 1040 ma peak of metamorphism in the allochthons the reservoir cabonga allochthon was transported toward the nnw from its probable root zone in the mlt during the 1060 1020 ma grenvillian collision as a partially cooled slab the remobilization of the archaean parautochthon is attributed to this collision in the grenville front zone slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid tectonic exhumation shortly after collision the minor delay 20 30 ma in unroofing of the mt compared to the adjacent mlt is most likely related to post 1040 ma extensional displacement along the labelle shear zone in terranes like those described above where metamorphism is diachronous determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure however identification of contrasting cooling histories contributes to unravelling the independent movement of terranes