factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

a finite amplitude necking model of rifting in brittle lithosphere we formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic plastic layer overlying a fluid substrate a perfectly plastic rheology is a continuum description of faulting in rift zones two important aspects of rift evolution are illustrated by this model the evolution of the rift width as extension proceeds and the finite strain that occurs a region at yield initially develops with a width determined by the thickness of the brittle layer and the internal deformation within this yield zone is proportional to the topographic slope as extension proceeds the surface within the rift subsides and the width of the subsiding yield zone decreases at any stage of rifting material in regions just outside the yield zone is deformed but no longer deforming the width of these deformed regions increases with increasing extension vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone creating flanking uplifts the external force required to maintain active rifting increases with the amount of lithospheric stretching indicating that rifting is a quasi static stable process because the yield zone will revert to elastic behavior if the external force causing extension is removed the model predicts that the rift depression and flanking uplifts will be preserved after extension stops our simple mechanical model demonstrates the inherent relationship among graben formation lithospheric thinning and rift shoulder uplift in rift zones

a two phase model for compaction and damage 3 applications to shear localization and plate boundary formation a new two phase theory employing a nonequilibrium relation between interfacial surface energy pressure and viscous deformation bercovici et al this issue provides a model for damage void generation and microcracking and thus a continuum description of weakening failure and shear localization here we demonstrate applications of the theory to shear localization with simple shear flow calculations in which one phase the matrix representing for example silicate is much stronger more viscous than the other phase the fluid this calculation is motivated as a simple model of plate boundary formation in a shear zone even without shear the two phases eventually separate due to gradients in surface tension however the influence of shear on phase separation is manifest in several ways as shear velocity increases the separation rate of the phases increases demonstrating a basic feedback mechanism accumulation of the fluid phase causes focused weak zones on which shear concentrates causing more damage and void generation and thus greater accumulation of fluid beyond a critical shear velocity phase separation undergoes intense acceleration and focusing leading to a tear localization in which the porosity becomes nearly singular in space and grows rapidly like a tear or crack at an even higher value of shear velocity phase separation is inhibited such that shear localization gives way to defocusing of weak zones suggestive of uniform microcracking and failure throughout the layer our two phase damage theory thus predicts a wide variety of shear localization and failure behavior with a continuum model applications of the theory to various fields such as granular dynamics metallurgy and tectonic plate boundary formation are numerous

extent of azores plume influence on the mid atlantic ridge north of the hotspot a complete bathymetry and geophysical survey of the recent domain of the mid atlantic ridge north of the azores between 40 30 n and 45 n was achieved during the 1998 triatnord cruise of r v l9atalante interpretation of along track data and geochemical analyses of rock samples show that the surveyed section represents a peculiar section of the mid atlantic ridge this new data set allows us to tentatively place the northern limit of the influence of the azores hotspot on the accretion processes between lat 43 n and 44 n this limit is marked by a 1 wide transition zone this fits well with the asymmetry of the upper mantle thermal structure as previously inferred from seismic tomography and geoid models thus enhancing the links between shallow and deeper accretion processes in a hotspot ridge interaction context observations of along ridge variations of geochemical parameters at various scales shed light on the relations between mantle geochemical heterogeneities and the accretion processes along the ridge axis confirming the asymmetry in the along ridge horizontal extent of the azores plume

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

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

effects of ridge geometry on mantle dynamics in an oceanic triple junction region implications for the azores plateau abstract plate boundary geometry can affect the nature of magmatism along a mid ocean ridge the azores plateau is located in a complex geological setting that includes a triple junction tj an oblique and recently formed ultra slow spreading ridge a zone of diffuse seafloor deformation a major fracture zone and a postulated hotspot the precise character of the hotspot is somewhat debated as some lines of evidence indicate it may not be a classic deep seated plume however seismic and gravity data suggest plateau crustal thicknesses of 8 km or more implying some mechanism for excess melting to assess the role of ridge geometry in creating the azores plateau this study uses a finite element numerical model to isolate the effects of selected aspects of plate boundary configuration on mantle flow and melt production in a tj kinematically similar to the azores tj the model focuses on the slowest spreading ridge in the tj analogous to the terceira rift the effect of the varying ridge obliquity observed along the terceira rift is also assessed using an independent 1 d melting model in general relatively little melt production is predicted along the terceira rift analogue except for regions closest to the tj where the proximity of a faster spreading ridge increases temperatures within the melting zone in the 1 d melting model with mantle temperatures of 1350 c melt thicknesses of 2 km are calculated for the least oblique segments while more oblique segments produce little to no melt the presence of a long discontinuity simulating the gloria fz has little effect on mantle dynamics for axial distances

reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores the morpho structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass wasting processes we present here a multi disciplinary study of the successive stages of development of faial azores during the last 1 myr using high resolution digital elevation model dem and new k ar tectonic and magnetic data we reconstruct the rapidly evolving topography at successive stages in response to complex interactions between volcanic construction and mass wasting including the development of a graben we show that 1 sub aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca 0 85 ma followed by its partial destruction over half a million years 2 beginning about 360 ka a new small edifice grew on the ne of the island and was subsequently cut by normal faults responsible for initiation of the graben 3 after an apparent pause of ca 250 kyr the large central volcano cv developed on the western side of the island at ca 120 ka accumulating a thick pile of lava flows in less than 20 kyr which were partly channelized within the graben 4 the period between 120 ka and 40 ka is marked by widespread deformation at the island scale including westward propagation of faulting and associated erosion of the graben walls which produced sedimentary deposits subsequent growth of the cv at 40 ka was then constrained within the graben with lava flowing onto the sediments up to the eastern shore 5 the island evolution during the holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano tectonic depression we conclude that the whole evolution of faial island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation each pulse has been separated by considerable periods of volcanic inactivity during which the faial graben gradually developed we propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement as observed historically when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the capelinhos eruptive crises 1957 58

p t paths derived from garnet growth zoning in an extensional setting an example from the tormes gneiss dome iberian massif spain the tormes gneiss dome nw iberian massif variscan belt of spain comprises a metamorphic core complex lower unit bounded by a major extensional detachment despite metamorphic temperatures in the upper amphibolite facies 700 740 c metapelites from the highest levels of the lower unit contain garnet with preserved growth zoning these rocks were used for reconstruction of quantitative p t paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning the results are consistent with a two stage tectonothermal evolution under high grade conditions 1 an early compressional phase of deformation that led to upper amphibolite facies barrovian type metamorphism and to p increase and t rise to approximately 9 kbar and 700 725 c 2 a subsequent major extensional phase of deformation that led to quasi isothermal decompression from 8 9 to 3 kbar at t conditions between 700 and 740 c several lines of structural textural and petrological evidence suggest that up to 15 20 km of overburden was removed from the lower unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions a final stage of quasi isobaric cooling to greenschist facies conditions is locally recorded in late low grade detachments

rb sr isotopic systematics of alkali rich fragments in the yamato 74442 ll chondritic breccia abstract we have undertaken mineralogical petrographical and rb sr isotopic studies on alkali rich igneous rock fragments in the yamato y 74442 ll chondritic breccia the fragments are a few mm in size and are composed mainly of porphyritic olivine and dendritic pyroxene set in alkali rich groundmass glass minor phases include chromite troilite and metallic nickel iron bulk chemical compositions of the fragments are almost identical to the host chondrite except for a depletion of sodium and an enrichment of potassium isotopic analyses of nine fragments from y 74442 yield a rb sr age of 4429 54 ma 2 for 87 rb 0 01402 ga 1 with an initial ratio of 87 sr 86 sr 0 7144 0 0094 2 assuming precursors of the fragments formed 4568 ma with 87 sr 86 sr 0 69889 when the solar system formed a time averaged rb sr weight ratio of the source material for the fragments is calculated to be 2 58 0 91 0 93 the extremely high rb sr value of this source is difficult to interpret by any igneous fractionation or liquid immiscibility but can be explained by mixing of a chondritic component with an alkali rich component formed in the early solar nebula in our preferred model the alkali component with rb sr 30 would have condensed from the residual nebular gas after removal of refractory strontium and must have been isolated for a long time in a region where the temperature was sufficiently low to prevent reaction with other silicates oxides a mixture of the alkali component early nebular condensates and the ferromagnesian component could reflect flash heating induced by impact on an ll chondritic parent body at least 4429 ma ago and further enrichments of rubidium and potassium relative to strontium could have occurred during this event the resulting impact melt rocks could have been fragmented by later impact event s and finally incorporated into the y 74442 parent body thus a remarkable signature of alkali enrichments both in the early solar nebula and later on the ll chondrite parent body is preserved as a minor component of some chondritic breccias such as y 74442

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

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

the role of lithospheric processes on the development of linear volcanic ridges in the azores abstract linear volcanic ridges lvrs are widespread along the azores plateau and are often used as a tectonic marker of the surface stress field nevertheless the mechanisms that drive the emplacement and development of these structures are not well established and they have been attributed to the plateau diffuse deformation off rift extension or the result of the interaction between a hotspot and the brittle lithosphere this study hypothesizes that linear volcanic ridges are the result of magma emplacement into pre existing damaged lithosphere using a 3d finite element representation of the brittle lithosphere and underlying ductile mantle and assuming that the deformation is driven by plate boundary forces applied at the edges as describe by global plate kinematic models the brittle layer is described by an elastoplastic rheology with progressive damage where fractures are assumed to be analogous to localized shear bands the ductile mantle underneath is modeled as a viscoelastic layer that exerts a shear drag at the base of the brittle layer the modeling shows that lithospheric processes alone can justify the spatial distribution of linear volcanic ridges and even the development of the faial ridge the factors controlling the fracturing pattern are the plate geometry and velocity boundary conditions the shearing introduced at the east azores fracture zone gloria fault limit and the interaction between the viscous mantle and the spatially varying brittle plate thickness along the terceira rift the predicted fractures match the orientation of the lvrs in the second n135 n140 and third n150 to n s sectors and provide an explanation for the arcuate shape of the rift itself the brittle plate thickness variations are crucial for the development of the more recent lvrs which are predicted to occur along the faial ridge in the best fit model the top mantle viscosity is 1 10 22 pa s at 5 15 km depth and the present day fracture network takes 3 ma to develop

the location of the allochthon boundary thrust and the archean proterozoic suture in the mattawa area of the grenville province nd isotope evidence abstract over 100 nd isotope analyses are used to map the crustal formation ages of grenville orthogneisses in a 10 000 km 2 area near mattawa on the ontario quebec border lithotectonic terranes with mapped outcrops of eclogite or coronitic metagabbro have nd model ages less than 1 8 ga and are interpreted as fragments of the allochthonous polycyclic belt however large areas of orthogneiss to the north of mattawa have model ages of 1 9 2 9 ga interpreted as archean crust that was extensively plutonised in paleo and mesoproterozoic magmatic events based on this evidence a revised location is proposed for the allochthon boundary thrust abt which passes to the south of the bonfield batholith and mattawa but then trends to the north and is correlated with the lac watson shear zone some allochthonous klippen are identified northwest of the main trace of the abt representing the dissected remains of what was originally a large allochthonous thrust sheet within the parautochthon a major step in depleted mantle model ages is observed in the vicinity of the mattawa fault separating archean and paleoproterozoic crustal blocks this boundary is interpreted as an ancient crustal suture reactivated by faulting associated with the ottawa bonnechere graben it is concluded that nd isotope analysis is a useful technique for detailed mapping of major tectonic boundaries in highly metamorphosed orogenic belts such as the grenville province

volcano spacing and plate rigidity in plane stresses which accompany the flexural deformation of the lithosphere under the load adjacent volcanoes may govern the spacing of volcanoes in hotspot provinces specifically compressive stresses in the vicinity of a volcano prevent new upwelling in this area forcing a new volcano to develop at a minimum distance that is equal to the distance in which the radial stresses change from compressional to tensile the inflection point if a volcano is modeled as a point load on a thin elastic plate then the distance to the inflection point is proportional to the thickness of the plate to the power of 3 4 compilation of volcano spacing in seven volcanic groups in east africa and seven volcanic groups of oceanic hotspots shows significant correlation with the elastic thickness of the plate and matches the calculated distance to the inflection point in contrast volcano spacing in island arcs and over subduction zones is fairly uniform and is much larger than predicted by the distance to the inflection point reflecting differences in the geometry of the source and the upwelling areas

a morphometric analysis of the submarine volcanic ridge south east of pico island azores a region of crustal extension the azores plateau contains excellent examples of submarine volcanic edifices constructed over a wide range of ocean depths along the pico ridge using bathymetric data and towed ocean bottom instrument tobi side scan sonar imagery we measured the dimensions diameter height slopes shape and texture of these volcanic edifices to further understanding of the geometric development of a submarine ridge our analysis and interpretation of the measurement and texture data suggest the following 1 the various edifice types do not correlate with depth ranges suggesting that ambient water pressure is not a controlling factor in configuration of the edifice formed 2 the cones have a mean diameter of 948 m height of 152 m and slope of 15 6 and are peaked rather than displaying flat topped summits and 3 while hummocky textured cones also occur smooth textured cones predominate the cones seem to develop preferentially outwards then upwards with only a weak correlation between diameter and height suggesting that the cone population does not evolve self similarly although smooth and hummocky cone populations are not statistically different in mean slope angle and eruption depth the hummocky cones have a significantly greater mean diameter than the smooth cones we suggest that hummocky textured cones probably involving eruption of pillow lavas are formed after voluminous smooth textured flows nearest neighbour analysis suggests that submarine cones are distributed randomly whereas subaerial cones are not we interpret this finding to suggest subaerial cones being masked by over covering flows which tend to flow further than submarine lava flows furthermore the spatial distribution of cones away from the ridge centre is not easily explained by a magma supply fed via lava tubes from central eruptions because of a lack of consistent pathway down gradient fissure vents seem to be important in determining construction of the ridge and result in linear arrangements of edifices and cone elongation consistent with the regional tectonic trend of this part of the azores plateau cones form as each feeding dyke intrusion cools and the eruption becomes localized along point source vents along the fissure the ridge seems to be predominantly formed from fissure eruptions along the ridge axis with subordinate transport of lava down its flanks

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

polymetamorphism in the archean hemlo heron bay greenstone belt superior province p t variations and implications for tectonic evolution the tectono metamorphic history of the late archean 2800 2600 ma hemlo heron bay greenstone belt in the superior province has been delineated from textural relationships mineral chemistry and p t paths in metapelites cordierite orthoamphibole rocks and metabasites from the white river exploration property hemlo area ontario an early low temperature medium pressure metamorphism about 500 c and 6 6 5 kbar 1 kbar 100 mpa is indicated by the occurrence of relict kyanite and staurolite porphyroblasts and zoned garnet porphyroblasts in metapelites and the presence of zoned calcic amphiboles in metabasites this early metamorphism appears to have been coeval with the previously documented d1 deformation that is associated with for example low angle thrusts a second regional metamorphism predominates in the hemlo heron bay greenstone belt and is generally of relatively low grade at about 510 530 c and 3 2 3 5 kbar over most of the study area and increases to medium grade 550 650 c and

tectonothermal analysis of high temperature mylonitization in the coimbra cordoba shear zone sw iberian massif ouguela tectonic unit portugal evidence of intra continental transcurrent transport during the amalgamation of pangea abstract mylonites in pelitic and quartzofeldsphatic gneisses from the ouguela tectonic unit coimbra cordoba shear zone sw iberian massif have been studied as an example of high temperature ductile deformation associated with transcurrent tectonics detailed microstructural and p t analysis indicates that ductile deformation evolved from a metamorphic peak at approximately 650 750 c and 7 5 9 5 kbar quartzofeldsphatic gneisses and 730 790 c and 7 5 9 5 kbar pelitic gneisses to retrograde conditions at 500 575 c and 4 5 5 5 6 5 7 5 kbar quartzofeldsphatic gneisses and 525 600 c and 3 5 4 5 5 5 7 5 kbar pelitic gneisses following the metamorphic peak exhumation was very fast the p t trajectory which does not reach the curve for granite melting is distinct that of isothermal decompression instead the progressive and contemporaneous decrease in pressure and temperature was a direct response to strong heat dissipation along the contacts between the ascending slice and the adjacent blocks the horizontal component of exhumation path calculated for middle and shallower crustal levels sum to ca 57 km to 94 km for the pressure peak assuming this offset acted in the visean during a time interval of ca 9 ma the estimated exhumation horizontal slip rate is in the order of 6 3 to 10 4 mm yr which corresponds to an exhumation oblique slip exhumation rate of 6 6 to 10 7 mm yr for ductile deformation these values indicate that the transcurrent tectonic displacements accommodated by these mylonitic are similar to those of modern intra continental shear zones such as the still active karakoram fault 8 3 mm yr in the himalayas the coimbra cordoba shear zone is therefore a typical intra continental transcurrent zone with ten to one hundred kilometre along strike mass movement of material that aided the exhumation of deep crustal rocks study of this large scale structure in the sw iberian massif is therefore central to models of orogenic deformation during the amalgamation of pangea

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

crustal architecture and tectonic assembly of the central gneiss belt southwestern grenville province canada a new interpretation the central gneiss belt southwestern grenville province is characterized by parautochthonous crust in the north and allochthonous lithotectonic domains in the south despite nearly two decades of study the basal decollement to allochthonous domains transported from the southeast known as the allochthon boundary thrust has not been precisely located throughout much of the belt between lake nipissing and georgian bay where its surface trace is known it separates 1 24 ga sudbury metadiabase in the footwall from eclogite remnants and 1 17 1 15 ga coronitic olivine metagabbro confined to its hanging wall on the premise that this relationship can be used to trace the allochthon boundary thrust elsewhere in the central gneiss belt we have sought to extend the known distribution of these mafic rock types making use of field petrographic and geochemical criteria to identify them new occurrences of all three mafic types are identified in a region extending from south of lake nipissing to western quebec

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

volcanic geology of furnas volcano sao miguel azores abstract furnas is the easternmost of the three active central volcanoes on the island of sao miguel in the azores unlike the other two central volcanoes sete cidades and fogo furnas does not have a well developed edifice but consists of a steep sided caldera complex 8 5 km across it is built on the outer flanks of the povoacao nordeste lava complex that forms the eastern end of sao miguel constructive flanks to the volcano exist on the southern side where they form the coastal cliffs and to the west the caldera margins tend to reflect the regional local tectonic pattern which has also controlled the distribution of vents within the caldera and areas of thermal springs activity at furnas has been essentially explosive erupting materials of trachytic composition products associated with the volcano include plinian and sub plinian pumice deposits ignimbrites and surge deposits phreatomagmatic ashes block and ash deposits and dome materials most of the activity has occurred from vents within the caldera or on the caldera margin although strombolian eruptions with aa flows of ankaramite and hawaiite have occurred outside the caldera the eruptive history consists of at least two major caldera collapses followed by caldera infilling based on 14 c dates it appears that the youngest major collapse occurred about 12 000 10 000 years bp new 14 c dates for a densely welded ignimbrite suggest that a potential caldera forming eruption occurred at about 30 000 years bp recent eruptions

distributed damage faulting and friction we present a formulation for mechanical modeling of geological processes in the seismogenic crust using damage rheology the seismogenic layer is treated as an elastic medium where distributed damage modifying the elastic stiffness evolves as a function of the deformation history the model damage rheology is based on thermodynamic principles and fundamental observations of rock deformation the theoretical analysis leads to a kinetic equation for damage evolution having two principal coefficients the first is a criterion for the transition between strength degradation and recovering healing and is related to friction the second is a rate coefficient of damage evolution which can have different values or functional forms for positive degradation and negative healing evolution we constrain these coefficients by fitting model predictions to laboratory data including coefficient of friction in sawcut setting intact strength in fracture experiments first yielding in faulting experiments under three dimensional strain onset and evolution of acoustic emission and dynamic instability the model damage rheology accounts for many realistic features of three dimensional deformation fields associated with an earthquake cycle these include aseismic deformation gradual strength degradation development of process zones and branching faults around high damage areas strain localization brittle failure and state dependent friction some properties of the model damage rheology e g cyclic stick slip behavior with possible accompanying creep are illustrated with simplified analytical results the developments of the paper provide an internally consistent framework for simulating long histories of crustal deformation and studying the coupled evolution of regional earthquakes and faults this is done in a follow up work

a test of detailed nd isotope mapping in the grenville province delineating a duplex thrust sheet in the kipawa mattawa region over sixty new neodymium model ages were determined on orthogneisses from the kipawa mattawa region of the grenville province to refine previous nd isotope mapping work in this area the combined nd data sets support a tectonic model involving three major thrust sheets in the kipawa area separated by major shear zones the uppermost sheet is correlated with the allochthonous polycyclic belt represented in the study area by the lac watson nappe along with two allochthonous klippen these have nd model ages 2 6 ga whereas the overlying sheet consists of magmatically reworked archean parautochthon with model

the southern limit of archean crust and significance of rocks with paleoproterozoic model ages nd model age mapping in the grenville province of western quebec abstract detailed nd model age mapping in the grenville province of western quebec has identified biotite gneisses with crustal formation age intervals of 2 5 2 9 ga and 1 8 2 0 ga although the presence of archean crust in the grenville parautochthonous belt has been known for some time the results of nd model age mapping combined with geochemical and aeromagnetic data indicate that the archean parautochthon extends more than 20 km to the south of the renzy shear belt this belt which was previously defined as the southern limit of archean crust corresponds to an isolated allochthon with nd model ages ranging from 2 0 to 1 4 ga and does not demarcate the allochthon boundary thrust rocks with model ages of 1 8 2 0 ga exposed to the south of the archean crust geochemically possess an arc affinity and are interpreted as a paleoproterozoic arc a model age boundary between archean crust and this arc has been defined by this study the age boundary probably represents a suture along which the southern margin of laurentia and the arc converged

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

stratification of the earth beneath the azores from p and s receiver functions abstract seismic recordings of iris ida gsn station cmla and of several temporary stations in the azores archipelago are processed with p and s receiver function prf and srf techniques contrary to regional seismic tomography these methods provide estimates of the absolute velocities and of the vp vs ratio up to a depth of 300 km joint inversion of prfs and srfs for a few data sets consistently reveals a division of the subsurface medium into four zones with a distinctly different vp vs ratio the crust 20 km thick with a ratio of 1 9 in the lower crust the high vs mantle lid with a strongly reduced vp vs velocity ratio relative to the standard 1 8 the low velocity zone lvz with a velocity ratio of 2 0 and the underlying upper mantle layer with a standard velocity ratio our estimates of crustal thickness greatly exceed previous estimates 10 km the base of the high vs lid the gutenberg discontinuity is at a depth of 80 km the lvz with a reduction of s velocity of 15 relative to the standard iasp91 model is terminated at a depth of 200 km the average thickness of the mantle transition zone tz is evaluated from the time difference between the s410p and sks660p seismic phases that are robustly detected in the s and sks receiver functions this thickness is practically similar to the standard iasp91 value of 250 km and is characteristic of a large region of the north atlantic outside the azores plateau our data are indicative of a reduction of the s wave velocity of several percent relative to the standard velocity in a depth interval from 460 to 500 km this reduction is found in the nearest vicinities of the azores in the region sampled by the prfs but as evidenced by srfs it is missing at a distance of a few hundred kilometers from the islands we speculate that this anomaly may correspond to the source of a plume which generated the azores hotspot previously a low s velocity in this depth range was found with srf techniques beneath a few other hotspots

patterns of stress at midocean ridges and their offsets due to seafloor subsidence the effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two dimensional 2d horizontal stresses arising from isostatically compensated vertical loads the topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge parallel tensile stresses these may predominate over the ridge perpendicular stresses and explain observations at midocean ridge offsets such as 1 oblique normal faulting at ridge transform intersections trending up to 60 relative to the ridge axis and 2 nontransform offsets consisting of structures oriented at 45 relative to the ridge trend at midocean ridge overlaps rotation of the ridge parallel tensile stresses favours rift propagation at more than 45 relative to the ridge trend it is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets and as a result large overlaps may eventually start to rotate and evolve into a microplate

the azores triple junction evolution since 10 ma from an aeromagnetic survey of the mid atlantic ridge abstract in the past two decades several models have been presented to describe the evolution and the present structure of the azores triple junction these models were mainly based on morphological analysis of sea bottom topography sparse magnetic profiling sidescan sonar surveying over the plateaus and global plate kinematic considerations for the north atlantic in this paper we follow a different approach from a detailed aeromagnetic survey covering both sides of the mid atlantic ridge between 37 n and 40 30 n the magnetic anomalies up to anomaly 5 are accurately identified allowing careful modelling of the kinematics of this region for the past 10 ma and thus establishing a coherent framework for the design of geophysical models for the azores triple junction the analysis of magnetic anomalies and the use of fourier domain inversion techniques show that the ridge is made up of six segments each one varying in length from 50 to 60 km the more continuous section of the ridge can be defined from the first four northern segments although the north azores fracture zone right offsets the ridge at 39 30 n 29 40 w the fifth and sixth segments are respectively right offset by the acor fracture zone at 38 23 n 30 15 w and by the princess alice fracture zone at 38 00 n 30 50 w anomaly identifications using a two dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters the results thus obtained reveal that at least between anomalies 5 and 3 10 3 85 ma the azores displayed an independent motion relative to the neighbouring plates and after anomaly 2a 2 45 ma the azores moved attached to the eurasian plate the triple junction azores north america africa or eurasia north america africa moved northward from 38 00 n 30 50 w to 38 20 n 30 15 w between 4 and 3a and probably to 38 50 n 30 00 w at anomaly 2a time a detailed reconstruction model is presented

three dimensional passive flow and temperature structure beneath oceanic ridge ridge ridge triple junctions abstract this study uses numerical modeling to investigate generalized characteristics of mantle flow and thermal structure in the vicinity of ridge ridge ridge rrr triple junctions oceanic triple junctions present a unique opportunity to study three dimensional 3d mantle dynamics in a tectonic setting considerably different than where only two plates diverge in many prominent oceanic triple junctions including rodrigues azores and galapagos the slowest spreading ridge branch intersects the near collinear faster spreading branches quasi orthogonally this study focuses on triple junctions free of influence from nearby hotspots similar to the geometry of the rodrigues triple junction a finite element model was used to calculate the steady state 3d velocity flow field and temperature patterns resulting from advective and conductive heat transfer for the slowest spreading branch model results predict a strong component of along axis velocity directed away from the triple junction both upwelling velocity and temperature are calculated to increase along the slowest spreading ridge toward the triple junction approaching the upwelling rate and temperature of the fastest spreading branch within 200 km of the triple junction upwelling velocity is predicted to increase more than threefold along the slowest spreading ridge in contrast the calculated upwelling velocity and temperature for the fastest spreading branch are not significantly different from the case of a single spreading ridge for triple junctions where the three ridges spread faster than the rodrigues triple junction system such as the galapagos triple junction the contrast in axial upwelling velocity and temperature between the slowest and fastest spreading ridges is predicted to be less significant however for triple junctions with overall slower spreading velocities such as the azores triple junction this contrast is more pronounced

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

control on seafloor spreading geometries by stress and strain induced lithospheric weakening abstract seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults few regions exhibit different patterns such as the east pacific rise epr which additionally displays overlapping spreading centres oscs and microplates we introduce a dynamical model using two independent scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins the elastic modulus that is affected by the damage determines the type of localized deformation damage reducing the bulk modulus tends to result in tensile fractures while a reduction in shear modulus leads to shear fractures the damage source determines the fracture orientation material weakening in tension results in fractures perpendicular to the most tensile principal stress while shear weakening results in two conjugate fractures at 45 relative to the applied stress strain or energy dependent damage results in propagating localized fractures stress dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones starting from small perturbations with reduced elastic moduli as nucleation points all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress the orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones oscs form when the shear damage leading to transform faults is suppressed while microplate formation requires additional reduction of the shear modulus by tensile energy oblique spreading at 45 recently discovered along ultraslow spreading ridges occurs when both moduli are weakened by shear energy a parameter regime exists in which ridge transform patterns develop at low applied tension while microplates form at higher stresses these results indicate that at least three different micromechanical processes operate with different evolution rates oscs and oblique spreading require different material properties

the geochemistry of mafic gneisses from the renzy terrane western grenville province quebec implications for the geodynamic setting of the early mesoproterozoic laurentian margin abstract this study presents new geochemical data on mafic gneisses from the renzy terrane rt located in the grenvillian parautochthonous belt of western quebec this region is of particular interest since it contains one of the rare ni cu co massive sulfide deposits ever mined in the grenville province rt mafic gneisses have been divided into 3 groups based mainly on major element compositions trace element profiles normalized to the primitive mantle and incompatible trace element ratios the behaviour of high field strength elements hfse ta zr hf ti th and large ion lithophile elements lile ba rb are particularly useful for discriminating rt mafic gneisses major and trace element compositions of group 1 mafic ultramafic gneisses are very similar to those of the rt ultramafic sheets the mafic gneisses are therefore interpreted as melagabbroic sills cogenetic with these sheets the trace element composition of group 1 mafic gneisses is typical of magmas formed from a previously metasomatized depleted mantle source these rocks have high th ta ratios and show strong to moderate negative anomalies in ti zr and hf on multi element plots normalized to the primitive mantle mafic gneisses of group 2 show geochemical features transitional between extensional and compressional settings these rocks generally have low concentrations of hfse and high concentrations of lile typical features of mafic rocks formed in a compressional environment however some of them are fe ti rich with minor or no negative anomalies in hfse and low to moderate concentrations of lile on multi element diagrams normalized to primitive mantle features more typical of an extensional environment mafic gneisses of group 3 have a geochemical signature typical of extensional environments or of ocean island basalt oib these gneisses have no negative anomalies in hfse and have low concentrations of lile rt mafic gneisses are similar in composition to mafic granulites in the bondy gneiss complex in the central metasedimentary belt and to shawanaga domain amphibolites from the central gneiss belt two paleoarc back arc environments in the grenville province based on the contrasting geochemical signatures found in the rt mafic gneisses and the similarities with known arc back arc settings in the grenville province we propose that the rt was formed by arc back arc magmatic activity most likely associated with a period of arc magmatism along the proterozoic laurentian margin oib like mafic gneisses found in the rt are similar to those from the kipawa region formed by intraplate continental magmatism in the parautochthonous belt of the grenville province

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

erratum the high pressure belt in the grenville province architecture timing and exhumation we propose that the grenvillian allochthonous terranes may be grouped into high pressure hp and low pressure lp belts and examine the hp belt in detail in the western and central grenville province the hp belt is developed in paleo and mesoproterozoic rocks of the pre grenvillian laurentian margin and characterized by grenvillian eclogite and co facial hp granulite in mafic rocks pressure temperature p t estimates for eclogite facies conditions in well preserved assemblages are about 1800 mpa and 850 c in the central grenville province hp rocks formed at 1060 1040 ma and underwent a single stage of unroofing with transport into the upper crust by 1020 ma whereas farther west they underwent two stages of unroofing separated by penetrative mid crustal recrystallization before transport to the upper crust at 1020 ma unroofing processes were comparable in the two areas involving both thrusting and extensional faulting in an orogen propagating into its foreland by understacking in detail thrusting episodes preceded extension in the western grenville province whereas in the central grenville province they were coeval resulting in unroofing by tectonic extrusion in the central grenville province the footwall ramp is well preserved but any former ramp in the western grenville province was obliterated by later lower crustal extensional flow continuation of the hp belt into the eastern grenville province is not established but likely on geological grounds however the pattern of deep crustal seismic reflection in the lithoprobe eastern canadian shield onshore offshore transect ecsoot line contrasts with that father west suggesting that if present the hp rocks were exhumed by a different mechanism

the terceira rift as hyper slow hotspot dominated oblique spreading axis a comparison with other slow spreading plate boundaries abstract we suggest the 550 km long terceira rift tr azores plateau is the world s slowest spreading hyper slow 4 mm a plate separation 2 3 3 8 mm a perpendicular to oblique axial segments organized accreting plate boundary in its slightly sinuous ca 300 km radius of curvature axial trace its oblique spreading angles ca 40 65 and in frequency and first motions of earthquakes the tr resembles better known ultra or super slow spreading ridges e g gakkel and southwest indian ridges interpreted simply as volcanically unfilled rift valley segments the inter island basins e g the 3200 m deep hirondelle basin are slightly wider 30 60 km but not significantly deeper 1000 2200 m than the mid atlantic ridge mar median valley 20 28 mm a 10 n 53 n however along axis segmentation wavelengths ca 100 km are double those along the central mar but make tr comparable to the ultra slow 15 16 mm a southwest indian and gakkel 7 13 mm a ridges if this segmentation wavelength reflects rayleigh taylor instabilities the viscosity contrast between the overlying axial lithosphere and the partial melt zones is about an order of magnitude greater at ca 4 16 mm a than at 20 30 mm a the tr differs dramatically from ultra slow ridges only in the large amplitude of along strike topography 2000 4000 m 4200 m total variation owing perhaps to a copious melt flux from the azores hotspot combined with a spreading rate determined greater axial flexural strength and plate thickness and slower export of volcanics from the rift axis the probable tr youth ca 1 ma requiring less than 4 km new oceanic crust suggests lack of steady state spreading conditions which may explain the published gravity evidence against tr spreading absolute plate motions support the creation of the azores plateau by successive ne jumps of the rift axis to maintain its position over a fixed hotspot

the mont laurier quebec earthquake of 19 october 1990 and its seismotectonic environment on 19 october 1990 at 07 01 ut a magnitude mblg 5 0 earthquake occurred approximately 9 km southwest of mont laurier quebec canada latitude 46 47 n and longitude 75 59 w this earthquake is the largest to have occurred in the western quebec seismic zone since the 1944 magnitude 5 6 cornwall massena earthquake the mont laurier earthquake was widely felt up to distances of 500 km and caused some minor damage in the epicentral region intensity modified mercalli mm v during the week following the earthquake a field survey recorded 23 aftershocks magnitudes were all less than ml 1 5 analysis of the mainshock and its aftershocks indicates focal depths in the range 10 to 12 km of the two east west oriented nodal planes given by the predominantly thrust mechanism of the mainshock the steep northerly dipping one probably represents the fault plane as indicated by the trend of aftershock hypocenters the last 10 yr of recording have confirmed that most western quebec earthquakes including the mont laurier earthquake occur in a northwest southeast trending zone inside the central metasedimentary cm belt with most focal depths varying between 7 and 25 km although some northwest trending structural features are known correlating these with the epicentral trend is premature the mid crustal hypocentral depths of many earthquakes the east west trend of the fault plane of the mont laurier earthquake and variations in regional focal mechanisms all suggest reactivation of deep structural features which may not have a surface expression inside the central metasedimentary belt most earthquakes occur along the eastern side where the activity seems to correlate with the labelle shear zone and the morin anorthosite body which may act as a stress concentrator the western side of the activity does not end with the central metasedimentary belt a fact that implies that even though the factors controlling seismicity lie predominantly within the central metasedimentary belt the adjacent geologic domains are also seismogenic

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

reevaluation of magnetic chrons in the north atlantic between 35 n and 47 n implications for the formation of the azores triple junction and associated plateau 1 in this paper we present a new magnetic compilation for an area of the north atlantic located between 35 n and 47 n and up to anomaly 33r we also present a strategy to pick magnetic isochrones and compute finite rotation poles this technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector a cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite eulerian rotations for the chrons 5 6 6c 11 12 13 18 20 21 22 23 24 25 26 28 29 30 32 33 and 33r this set of chrons and poles is used to discuss the evolution of the north atlantic close to iberia and in particular the onset and early development of the azores triple junction area we show that the relative motion between the eurasian and the african plates can be coherently described in terms of rigid plate kinematics respecting both the anomalies shapes and the precise location of the main structural elements of the area the pico fracture zone the east azores fracture zone and the gloria fault we distinguish from the magnetic point of view two different areas of the azores plateau the south azores domain where almost undisturbed nnw magnetic lineations can be found and the azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations probably matuyama and brunhes we present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of iberia to eurasia was younger than previously thought lower miocene triggering the formation of the azores domain in which stretching took place essentially in the last 20 ma at an average rate of 3 8 mm a and progressively attaching the south azores domain to the african plate by a northward progression of the triple junction