equatorial inertia gravity waves observed in topex poseidon sea surface heights we report an evaluation of the ability of the topex poseidon altimeter to observe equatorial inertia gravity waves in the central pacific aliasing problems limit us to examining meridional mode two but this mode is an energetic one in the pacific basin these estimates of the temporal potential energy modulations are compared with estimates from tide gauges in the tropical pacific and we find that the time series obtained at these points show good agreement

transtensional origin of multi order cross folds in a high grade gneiss complex southwestern grenville province formation during post peak gravitational collapse the well mapped western part of the ottawa river gneiss complex orgc new name a large metamorphic core complex hosts a system of gently plunging cross folds of outcrop to regional scale situated in the ductile detachment zone between the lower grade cover and high grade core of the complex the cross folds are buckle structures that deform the attenuated gneissic layering plunge parallel to the regional elongation lineation range from upright to recumbent and exhibit distinctive hinge parallel elongation lineations all features of extension dominated ductile transtension our l s fabric data are consistent both with kinematic modelling that predicts progressive constrictional strain in the hinge zones of transtensional folds and with dynamic modelling that predicts rotation of flattened fold limbs into moderately dipping attitudes on the basis of petrologic data we show that cross folding postdated the peak ottawan metamorphism and took place during retrogression and exhumation of the thrust sheet stack the cross folds form an inclined system that is principally developed in retrograde melt weakened amphibolite facies rocks with the transtensional origin implying that exhumation and retrogression of the high grade core of the complex took place in an oblique extensional setting a transtensional origin for the cross folds removes the need to appeal to orogen parallel regional shortening an implausible requirement of previous interpretations and is compatible with data indicating that much of the visible fabric and structure of amphibolite facies domains of the orgc developed during post peak exhumation retrogression and gravitational collapse of the thrust sheet stack

u pb zircon age of the southwest lobe of the havre saint pierre anorthosite complex grenville province canada u pb zircon dating of the southwest lobe of the havre saint pierre anorthosite intrusion indicates that it is 1062 4 ma old parallelism of magmatic and solid state foliations with the adjacent abbe huard lineament suggest that anorthosite parental magmas rose up this shear zone which was active at that time the age of igneous crystallization is much younger than that of a spatially associated mangerite intrusion but accords with age data from other granitoid intrusions elsewhere in the grenville province evidence points to a widespread 1 09 1 05 ga magmatic event that included massif anorthosite intrusions this magmatic event coincided with late stage convergent tectonics in the southwestern grenville province

pre grenvillian evolution and grenvillian overprinting of the parautochthonous belt in key harbour ontario u pb and field constraints the parautochthonous belt in the region of key harbour ontario is composed of early proterozoic migmatitic para and orthogneiss and mid proterozoic granitoids which were reworked during the grenville orogeny grenvillian deformation is localized into anastomosing arrays of high strain shear zones enclosing elongate bands and lozenges of rock subjected to lower and near coaxial strain crosscutting relationships preserved in the low strain domains document two pre grenvillian plutonic and tectonometamorphic events which are bracketed in age by u pb zircon geochronology a 1694 ma leucogranite intrudes and provides a minimum age for high metamorphic grade gneisses formed during an earlier tectonometamorphic event d1 m1 the leucogranite was intruded by mafic dykes deformed and metamorphosed at uppermost amphibolite facies during d2 m2 before the emplacement of mid proterozoic granitoids at ca 1450 ma following the emplacement of gabbro dykes and pods at ca 1238 ma the area was overprinted by

influence of microscale weak zones on bulk strength shear zones have different rheological properties than the surrounding rocks indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone we demonstrate the value of this concept at the microscale we first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure rather three dimensional weak zones analogous to viscous shear zones can interconnect or bridge weak phases these zones typically form at high stress sites comprise multiple minerals and deform by mechanisms independent of those in the surrounding minerals the presence of weak zones strongly affects the bulk strength of the rock disproportionate to the mode of the weak zones for example the development of 1 mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength instead accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones for this reason we advocate use of the scale independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases more generally the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock this article is protected by copyright all rights reserved

forced oscillations on the equatorial atlantic basin during the seasonal response of the equatorial atlantic program 1983 1984 a subset of data from an array of eight inverted echo sounders and a meteorological station deployed and maintained for 15 months in the equatorial atlantic as part of the seasonal response of the equatorial atlantic program are analyzed to determine the variability of forced oscillations of the basin the analysis is done to study both low and high frequency variability at low frequencies the equatorial basin responds as a whole to the seasonally varying winds the dominant signal is the annual cycle of the equatorial system as a composite of the response to the strong seasonal wind curl that is maximum in the western basin and the part of the zonal transfer to mass associated with the seasonal equatorial upwelling and downwelling system three bands of energy appear as dominant in the combined spectrum of the ocean and the atmosphere at higher frequencies 20 30 days 13 16 days and the inertial gravity wave band each one of the bands is analyzed and discussed instability waves are detected on the echo sounder records the waves have periods centered at 30 2 days and propagate westward with a wavelength of 1000 km and a phase speed of 0 35 m s the most energetic oscillation in the analyzed wind records is at periods centered around 14 days the response of the ocean to this oscillation has a meridional distribution with maximum amplitude at 3 n the most significant oceanic oscillations in the inertial gravity band are centered at 5 2 and 3 5 days from the study of their meridional structure it is concluded that those oscillations correspond to forced inertial gravity waves with meridional structures corresponding to the meridional numbers n 1 and n 3 respectively

a modern structural regime in the paleoarchean 3 64 ga isua greenstone belt southern west greenland abstract the footwall gneisses beneath the western part of the paleoarchean 3 8 ga isua greenstone belt southern west greenland are interpreted here in terms of a 3 64 ga stack of mylonitic crystalline thrust nappes the oldest example known on earth in present coordinates the kinematic history of the thrust nappe stack is couched in terms of initial longitudinal strike parallel thrusting towards the southwest followed by transverse thrusting to the northwest and subsequent extensional collapse of the thickened crust toward the southeast diorite and tonalite that form the western margin of granitoids structurally overlying the western part of the isua greenstone belt and its footwall contain 3 5 ga mafic dykes some of which are deformed and or truncated at fault contacts within the granitoids accordingly a component of the deformation structurally above the isua greenstone belt occurred after 3 5 ga significantly later than the formation of the underlying mylonitic nappes probably during the neoarchean the structural regime of mylonitic thrust nappe stacking is very similar to that in modern mountain belts it would appear that the deformational behaviour rheological constitution and overall strength of paleoarchean and modern continental crust were similar

nature of the elzevir mazinaw domain boundary grenville orogen ontario the mooroton shear zone msz of the metasedimentary belt of the grenville orogen in ontario separates the mazinaw domain to the east from the elzevir domain the shear zone is composed of mafic metavolcanic metaclastic and marble mylonites and is characterized by a subvertical foliation which crosscuts the regional foliation and a near vertical stretching lineation microstructures indicate deformation occurred under crystal plastic conditions and shear sense indicators give movement of east mazinaw side up the geometry of the msz which follows the eastern margin of the elzevir tonalite implies that lithologic anisotropy guided its location garnet biotite and garnet hornblende peak temperatures are juxtaposed across the msz with temperatures ranging from 460 to 530 c in the southeastern elzevir and 490 to 620 c in the southwestern mazinaw conditions in amphibolite facies limiting barometers applied to rocks in both domains show no significant offset petrologic data within the shear zone

thrusting in the lower crust evidence from the oygarden islands kemp land east antarctica layered orthogneisses of the oygarden islands preserve evidence for four high grade deformation events d 1 to d 4 archaean d 1 and d 2 structures are only patchily preserved due to extensive recrystallization during d 3 and d 4 which represent effects of the c 1000 ma rayner structural episode ductile thrusting at middle to lower crustal levels occurred during d 3 which is separated into two mutually cross cutting phases based on structural geometry the two phases represent changes in finite strain that developed during progressive deformation east directed transport during d 3a developed subhorizontal thrusts that contain co axial east trending f 3a folds and l 3a lineations buckling as a consequence of constriction in thrust duplexes developed upright f 3b folds coaxial to f 3a folds and steeply south dipping d 3b shear zones garnet clinopyroxene and garnet orthopyroxene bearing assemblages in mafic lithologies and garnet sillimanite bearing assemblages in pelitic lithologies reflect d 3 conditions of p 9 kbar and t 800 850 c the well exposed d 3 duplex structures indicate that shortening of the lower crust may be accommodated by extensive strain partitioning to develop contemporary kilometre scale thrust stacking and ductile flow

proterozoic reactivation of the southern superior province and its role in the evolution of the midcontinent rift major reverse faults associated with the late compressional phase of the 1 1 ga midcontinent rift in the western lake superior region appear to cut across the rift at the eastern end of the lake and join with reverse faults on the eastern shoreline defined on the basis of geological and potential field data the continuation of the faults across eastern lake superior is inferred on evidence drawn from nearshore shipborne magnetic surveys together with new interpretations of published bathymetric and glimpce aeromagnetic data in the archean superior province about 100 km east of lake superior paleomagnetic and petrographic data from the 2 45 ga matachewan dyke swarm show that the kapuskasing zone a narrow belt of uplifted crust can be extended to within 50 km of the lake superior shoreline and has bounding reverse faults that are almost continuous with two faults of similar dip and sense of displacement that define the inversion of the midcontinent rift in the central and western parts of the lake si

winds at st peter and st paul rocks during the first sequal year wind observations were obtained during the first nine months of the sequal field program from a station maintained at st peter and st paul spp rocks winds are very steady from may to november and very unsteady during april when the intertropical convergence zone itcz crosses the site wind stress components are computed and comparison with 1979 data shows that the onset of the winds occurred one month earlier in 1983 but is of similar amplitude the spectral analysis of the wind shows a significant increase in the variance for periods between 3 5 days and 10 15 days

validation of an equatorial atlantic simulation model using inverted echo sounder data changes in the vertically averaged temperature of the water column observed over a 12 month period at eight locations in the tropical atlantic with inverted echo sounders are described in terms of empirical orthogonal functions these modes are compared with those from a similar analysis for data from a general circulation model of the tropical atlantic the method proves to be useful for the validation of models

precise zircon geochronology in the adirondack lowlands and implications for revising plate tectonic models of the central metasedimentary belt and adirondack mountains grenville province ontario and new york new high precision single grain dating of leucogranites from the adirondack lowlands dated previously by multigrain zircon methods at ca 1416 ma wellesley island and ca 1285 1230 ma hyde sch

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

crustal and lithospheric scale structures of the precambrian superior grenville margin abstract the lithosphere beneath the margin of the archean superior and proterozoic grenville provinces was investigated with a northwest southeast oriented 650 km long profile of 40 magnetotelluric stations dominant geoelectric strike azimuths of n45 e and n85 e were defined for the crust and the lithospheric mantle respectively a 2 d isotropic resistivity model derived using the crustal strike images resistive laurentian margin rocks dipping southeast to the base of the crust bounded to the northwest by the grenville front and to the southeast by the central metasedimentary belt boundary zone the observation is in contrast to conductive mid to lower crust elsewhere in the region a 2 d isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern grenville province the geometry of this body which includes a well defined base and southeast dip suggests that it is superior lithosphere a sub vertical conductor located approximately 50 km along strike from the mesozoic kirkland lake and cobalt kimberlite fields is interpreted to be due to re fertilization of an older mantle scar the resistivity model includes a horizontal conductor at 160 km depth beneath the southern superior province that is possibly the resistivity signature of the lithospheric asthenospheric boundary

basin wide thermocline displacements along the equator of the atlantic in 1983 observations from five inverted echo sounders deployed between 10 w and 38 30 w along the equator during the spring and summer of 1983 are presented the annual development of the eastward acting zonal pressure gradient is described and related to the observed zonal wind stress the minimum and maximum basin wide gradients are observed in mid april and in the latter part of july respectively the upwelling season in the gulf of guinea is identified at the easternmost sounder during may june and july pulses in thermocline displacement first deepening then shallowing propagate eastward across the basin at speeds of 2 8 and 2 1 m s 1 they arrive at 10 w at the onset and just after the cessation of the seasonal upwelling at that location their initialization is related to relaxation periods in the zonal wind for four of the last five years the time of the pulse preceding the upwelling is tentatively identified

a fracture model for quartz ribbons in straight gneisses abstract polycrystalline composite quartz ribbons in straight gneisses of the grenville province are explained as dilatant quartz filled fractures formed parallel to a strong preexisting gneissosity the ribbons are thin 3 mm but long and continuous and have straight and truncating contacts with bordering grains of other minerals they are similar in morphology to polycrystalline ribbons interpreted in the literature as resulting from high strains in contrast little strain is required if the ribbons form as quartz filled fractures a monoclinic symmetry of the shapes of ribbons and inclusions can be explained by fracture interactions as they propagate the symmetry is consistent with a late extensional south side down sense of shearing which has been documented elsewhere in the grenville province

monazite u th pb geochronology of the central metasedimentary belt boundary zone cmbbz grenville province ontario canada the central metasedimentary belt boundary zone cmbbz is a crustal scale shear zone that juxtaposes the central gneiss belt and the central metasedimentary belt of the grenville province geochronological work on the timing of deformation and metamorphism in the cmbbz is ambiguous and the questions that motivate our study are how many episodes of shear zone activity did the cmbbz experience and what is the tectonic significance of each episode we present electron microprobe data from monazite the u th pb chemical method to directly date deformation and metamorphism recorded in five garnet biotite gneiss samples collected from three localities of the cmbbz of ontario west guilford fishtail lake and killaloe all three localities yield youngest monazite dates ca 1045 ma most of the monazite domains that yield these dates are high y rims in comparison with this common late ottawan history the earlier history of the three cmbbz localities is less clearly shared the west guilford samples have m

geology and u pb geochronology of the kipawa syenite complex a thrust related alkaline pluton and adjacent rocks in the grenville province of western quebec the kipawa syenite complex a thin folded sheet of amphibole syenite quartz syenite and minor nepheline syenite lies along a west vergent thrust separating a lower slice comprising the kikwissi granodiorite and biotite tonalite dated at 2717 15 11 ma and unconformably overlying metasedimentary rocks from an overlying slice containing the red pine chute orthogneiss an alkali granite gneiss and the mattawa quartzite the syenite complex dated at 1033 3 ma lies within the lower slice but has metasomatically altered the overlying slice texturally guided u pb spot analyses on partially metasomatised zircons from the alkali granite gneiss yield a cluster of 207pb 206pb ages at 1389 8 ma interpreted as the time of igneous crystallization and four ages overlapping the time of syenite emplacement interpreted as in situ metasomatic growth the highest structural slice comprises garnet amphibolite separated from lower slices by the allochthon boundary thrust metamorphic grade increases upward from

modes of variability of the 1983 thermocline signal acoustic travel time series from an array of nine inverted echo sounders deployed along the equator and in the north equatorial countercurrent necc are decomposed into empirical orthogonal functions eof to analyze the dominant frequencies of oscillations in the thermocline field at low frequencies the first eof mode describes the seasonal signal shows the reversal of the necc and separates the behavior of the gulf of guinea from the rest of the basin the second eof mode shows rapid changes in the depth of the thermocline in short periods of time on the equator at 10 w and at 3 n 38 30 w comparison with a simulation model by philander and pacanowski shows that the theoretical time series behave in a similar way at these two locations at high frequencies the modes show oscillations in the inertia gravity wave band

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

deep crustal shear zones high grade tectonites and associated metasomatic alteration in the limpopo belt south africa implications for deep crustal processes large thrust sheets of complexly folded d 1 migmatitic granulites bounded by a regional system of anastomosing ductile shear zones d 2 characterize the southern marginal zone of the limpopo belt tectonites within these shear zones are subdivided into primary straight gneisses with preserved textures that can unambiguously be referred to the d 2 exhumation stage annealed straight gneisses that recrystallized during post d 2 cooling and classic mylonites that post date the limpopo orogeny primary straight gneisses within these shear zones contain textural and mineralogical features that suggest they developed by a shear related process of grain size reduction and fabric formation due to decreasing pressure and temperature under granulite facies conditions this process is evident from the synkinematic replacement of garnet by cordierite and hypersthene symplectites the resultant asymmetric sigmoidal corona textures enhance the shear fabric in contrast pelitic granulite

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

wall rock structure at the present contact surfaces between repeatedly deformed thrust sheets grenville orogen of central ontario canada in the central gneiss belt of the grenville orogen ontario ca 1020 ma extensional shearing disharmonic buckle folding and seismic faulting at middle to upper crustal levels affected the geological structure of pre 1040 ma ductile thrust sheets because much of the repeated in situ deformation was mechanically discontinuous the present contacts between thrust sheets may not coincide at all localities with the original thrust surfaces we focused special attention on the basal contact of the parry sound domain whose synformal structure may have resulted from gravitational subsidence of its dense rocks immediately after ductile thrusting east of wahwashkesh lake a transverse gradient of total strain is absent on horizontal scales of 100 1000 m in lithologically uniform granite gneiss comprising the uppermost western footwall of the northern parry sound domain this contrasts with the steep transverse strain gradients documented by others on the same scale in the wall rocks of phanerozoic ductil

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

crustal assimilation in the burnt lake metavolcanics grenville province southeastern ontario and its tectonic significance three intimately interbedded suites of volcanic rocks are identified geochemically in the burnt lake area of the belmont domain in the central metasedimentary belt and their petrogenesis is evaluated the burnt lake back arc tholeiitic suite comprises basalts similar in trace element signature to tholeiitic basalts emplaced in back arc basins formed in continental crust the burnt lake continental tholeiitic suite comprises basalts and andesites similar in trace element composition to continental tholeiitic sequences the burnt lake felsic pyroclastic suite comprises rhyolitic pyroclastics having major and trace element compositions that suggest that they were derived from crustal melts rare earth element models suggest that the burnt lake back arc tholeiitic rocks were formed by fractional crystallization of mafic magmas derived by approximately 5 partial melting of an amphibole bearing depleted mantle enriched in light rare earth elements by a subduction component the modelling also suggests that t

geometry of the allochthonous daly bay complex northwest territories a model constrained by geology and a three dimensional gravity interpretation the daly bay complex is one of several metamorphic complexes making up the aqxarneq gneisses north of chesterfield inlet in central district of keewatin granulite facies metamorphism 0 55 gpa 750 c and ductile deformation have affected all of the rocks in the complex a 1 15 km wide inward dipping ductile shear zone forms the outer part of the complex and contains strongly deformed equivalents of rocks in the core mesoscopic structures and metamorphic mineralogy suggest the daly bay complex was emplaced into the surrounding lower grade rocks by northward directed thrusting a three dimensional gravity model constrained by structural observations and 1091 surface density measurements shows that the relatively dense rocks of the complex form a spoon shaped structure with a long axis trending northwest southeast it is approximately 50 km by 120 km in lateral extent and reaches a maximum depth of about 9 km the thin skinned geometry of the daly bay complex supports the notion that the crust in central keewatin between the daly bay complex and baker lake comprises a series of undulating imbricated gneiss sheets of middle and lower crustal material which were juxtaposed by a major tectonic event sometime between 2 5 and 1 9 ga the interpreted basal d6collement is comparable to seismic features in many orogens and a predictable consequence of increased ductility with depth in the crust

structural fabric of the central metasedimentary belt of southern ontario canada from deep seismic profiling abstract the central metasedimentary belt boundary tectonic zone cmbbtz is a 10 20 km wide zone of intense structural deformation within the 1 3 1 0 ga grenville orogen of southeastern canada the crustal structure of the exposed cmbbtz has been well studied but its sub phanerozoic location and geometry beneath the urban development and nuclear stations of the toronto region are not well known a new 75 km lithoprobe reflection profile acquired close to toronto provides a clear image of the cmbbtz as a panel of southeast dipping reflections that extends with moderate dip

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

ductile thrusting at mid crustal level southwestern grenville province the northwestern boundary zone of the central metasedimentary belt grenville province in the haliburton area ontario is a stack of alternating tonalitic and syenitic crystalline thrust sheets transported toward the northwest on out of sequence upper amphibolite facies ductile thrust zones during the grenvillian orogeny at 1060 ma approximately 100 ma after the initiation of thrusting in the underlying central gneiss belt kinematics of the deformation are complex predominant northwestward thrusting was at least partly coeval with subordinate northeastward thrusting late synmetamorphic extensional shears cut both thrusts and thrust sheets minor late thrusting on discrete ductile shear zones postdates the extensional structures belts of mechanically weak pelite appear to have localised the thrust sheets highly mobile marble behaved as a relatively low viscosity fluid during transport able to intrude and erode more competent wall rock

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

near surface current and temperature variability observed in the equatorial atlantic from drifting buoys we examine data from 23 surface drifters which were deployed between june 1983 and july 1984 at 4 w 1 5 4 s the drifters were equipped with a window shade drogue centered at 15 m depth and in most cases a 117 m long thermistor chain drift data indicate a westward south equatorial current with typical speeds of about 20 cm s 1 south of the equator maximum westward flow of 30 40 cm s 1 occurred in june through september of 1983 and 1984 consistent with climatology near zero westward flow which was significantly weaker than was expected from climatology by about 20 cm s 1 occurred from february to april 1984 this probably represents real interannual variability and may be related to the fact that the equatorial atlantic was warmer than usual in the spring and summer of 1984 energetic variations on time scales of o 1 month and less and space scales of o 100 km and less are ubiquitous in the drift and temperature data kinetic energy levels associated with these variations south of the equator are typically 100 cm2 s 2 with slightly higher values in boreal summer energy levels north of the equator are about 500 cm2 s 2 on the basis of data available only during the boreal summer and fall the sources of this energy include inertia gravity waves instabilities of the general circulation and other phenomena for which there is no simple explanation

the queensborough mafic ultramafic complex a fragment of a meso proterozoic ophiolite grenville province canada abstract the queensborough mafic ultramafic complex occurs as a fault bounded block up to 10 km wide and having an area 220 km2 it lies in the grimsthorpe domain of the bancroft elzevir mazinaw sharbot lake terrane in the central metasedimentary belt of the grenville province it has been suggested without adequate supporting data that the complex may represent oceanic crust a fragment of an ophiolite or even a metavolcanic sequence made up of basaltic and komatiitic flows the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a series of ultramafic rocks characterized by metre scale compositional layering represented by several different metamorphic assemblages of talc chlorite carbonate anthophyllite and actinolite tremolite these assemblages indicate that the original rocks were cumulate peridotites and pyroxenites the ultramafic rocks are overlain structurally by a series of mafic rocks predominantly massive to highly sheared gabbros the gabbros are penetrated by a series of mafic dykes and include a few small enclaves of pillowed mafic volcanics major and trace element chemistry shows that the mafic rocks represent a fractionally crystallized sequence of tholeiitic gabbros lavas and mafic dykes and that the ultramafic cumulates are co genetic the regional geological setting and the trace element signatures of the mafic rocks suggest that they were formed in a back arc basin comparison of the queensborough complex with proterozoic and phanerozoic igneous complexes suggests that it represents a partially preserved crustal section of a mesoproterozoic ophiolite in addition the rocks of the queensborough complex are petrographically and geochemically similar to those of the vavilov basin which occurs in the deepest part of the tyrrhenian sea by analogy with this neogene back arc basin we suggest that the central metasedimentary belt which contains the queensborough complex formed by attenuation of the continental crust the complex is preserved along a suture and is interpreted as having formed during the collision of a continental island arc with the continental margin of laurentia the characteristics and preservation of the queensborough mafic ultramafic complex are very similar to those of many phanerozoic ophiolites and suggest that mesoproterozoic thermotectonic processes are also similar to those presently operating

sequential nesting of magmas in marble southwestern grenville province quebec from fracture propagation to diapirism abstract in the southwestern grenville province quebec 1090 1070 my old k rich alkaline equidimensional plutons such as the kensington pluton occur in marble rich country rock but coeval dykes bearing lower crustal xenoliths are found only in a quartzofeldspathic gneiss complex the sequential nesting of magmas the various episodes of skarn formation and the progressively more mafic character of the intruding magmas of the kensington pluton are interpreted in terms of magma ascent through a series of propagating fractures cessation of upward fracturing and pooling of magma in marble rich ductile crust diapirism radial shortening of wall rock and in situ pluton expansion this suite provides evidence that at deep crustal levels the country rock rheology influences the mode of magma ascent and emplacement and that diapirism is a possible mechanism of magma transport through rocks behaving rheologically by a power law model

cordierite gedrite rocks from the central metasedimentary belt boundary thrust zone grenville province ontario mesoproterozoic metavolcanic rocks with affinities to the composite arc belt cordierite gedrite rocks in the southern grenville province occur near the base of the central metasedimentary belt boundary thrust zone interpreted by some as a crustal suture between the 1 29 1 24 ga composite arc belt and 1 4 ga rocks of laurentia major and trace element compositions of these rocks are consistent with volcanic protoliths that range in composition from basalt to dacite these cordierite gedrite rocks have low cao average 1 2 wt and major element and oxygen isotope ratios suggestive of hydrothermal alteration before metamorphism rare earth element ree compositions also indicate igneous protoliths although some ree patterns have been modified by local melt extraction the trace element compositions of cordierite gedrite rocks and neodymium isotope systematics are similar to those of metavolcanic rocks in the composite arc belt and are consistent with the extension of the composite arc belt to the base of the boundary thrust zone

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

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

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 article by j martignole and p pouget november 1994 issue received in january 1995 presents a large body of data on the geothermobarometry of an area that had hitherto received little attention these data will allow the formulation of a more precise geotectonic model for the evolution of a part of the central grenville province however it is regrettable that the authors were remiss in not citing the sources of the information pertaining to the major structural features presented in their figs 2 4 more specifically we are referring to the two major structures previously identified by us the cayamant lineament and the baskatongdcsert lineament which the authors are trying to rebaptize the baskatong shear zone etc since 1990 our studies on the cayamant lineament and the baskatong desert lineament have been well documented published and widely publicized at all available geoscientific events such as ministbre des ressources naturelles du quebec publications and information seminars uvesque et al 1990 sharma et al 1990 1993a uvesque and sharma 1990 sharma 1991 1992a 1992b 1992c sharma and gigubre 1993 madore et al 1993a 1993b 1994 abitibi grenville project lithoprobe workshops sharma and uvesque 1991 sharma et al 1992b the 1992 grenville workshop friends of grenville sharma and uvesque 1992 the 1992 grenville field excursion friends of grenville led by k n m sharma sharma et al 1992a 1993 geological survey of canada current activities forum sharma 1993 toronto 94 annual general meeting and trade exhibition of the cana

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

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

u pb geochronology of the raglan gabbro belt central metasedimentary belt ontario implications for an ensialic marginal basin in the grenville orogen the raglan gabbro belt of the ontario grenville orogen is coincident with the top of the central metasedimentary belt boundary thrust zone a major mid crustal shear zone separating the central gneiss belt in the footwall from the central metasedimentary belt in the hanging wall it has been suggested that the gabbros making up the belt are coeval that they formed in a marginal basin within the central metasedimentary belt and that they formed a horizon of theologically stiff material that controlled the localization of the top of the boundary thrust zone during its initiation as the marginal basin closed at ca 1190 ma u pb zircon dating of plutons within the raglan gabbro belt was undertaken to test the coeval nature of intrusions in the belt magmatic crystallization ages for three of the gabbros fall in the range 1246 1227 ma and a fourth yields a minimum age of ca 1175 ma the results are permissive of a common origin for the gabbros and allow that the raglan gabbro belt may have been related to

nd isotope mapping of the dysart gneiss complex evidence for a rifted block within the central metasedimentary belt of the grenville province abstract more than 65 new nd isotope analyses are presented for the vicinity of the central metasedimentary belt boundary zone cmbbz near haliburton in the grenville province of ontario canada they fall into two well defined groups with distinct ranges of tdm model ages these two groups comprise the cmbbz with an average tdm age 1 46 ga and rocks of the central metasedimentary belt cmb with an average tdm age of 1 28 ga however a distinct block of crust within the cmb comprising the dysart gneiss complex has an older model age signature tdm age 1 40 ga resembling the cmbbz rather than the rift zone the dysart complex is surrounded by marble tectonites interspersed with elongate bodies of orthogneiss that yield young nd model ages typical of the cmb however it is argued that the development of the dysart complex as a distinct crustal block predates ductile deformation during the grenville orogeny instead the dysart block is thought to have rifted away from the central gneiss belt to the northwest during the formation of the cmb as an ensimatic back arc rift zone isotopic mapping of the dysart block along with the previously discovered elzevir block provides evidence for the geometry of the rifting process and also has important implications for the structure of grenville basement in eastern north america

timing and duration of melting in the mid orogenic crust constraints from u pb shrimp data muskoka and shawanaga domains grenville province ontario the central gneiss belt in the grenville province ontario exposes metaplutonic rocks orthogneisses and minor paragneisses that were deformed and metamorphosed at crustal depths of 20 35 km during the mesoproterozoic grenvillian orogeny we present sensitive high resolution ion microprobe shrimp u pb zircon data from eight samples of migmatitic orthogneiss granite and pegmatite from the muskoka and shawanaga domains that constrain the age and duration of partial melting in the mid orogenic crust our results support earlier interpretations that the protoliths to these migmatitic orthogneisses formed at ca 1450 ma emplacement and crystallization of granite and pegmatite in the shawanaga domain took place at ca 1089 ma apparently coevally with deformation and high grade metamorphism leucosomes in the muskoka and shawanaga domains yield ages of 1067 and 1047 ma respectively interpreted as the ages of melt crystallization the geochronological data and field observations suggest that melt was pr

thermocline displacement across the atlantic north equatorial countercurrent during 1983 two time series of thermocline displacement across the north equatorial countercurrent are derived from pairs of inverted echo sounders which straddle the countercurrent along the 28 w and 38 30 w meridians the observations reported are from february to september 1983 from mid may to the end of the record the thermocline deepens to the south and shallows to the north of the countercurrent suggesting a continual strengthening of the countercurrent the rate of increase of the thermocline slope at the western meridian is about twice that at 28 w as expected from historical data an empirical orthogonal functions eof analysis of the four sounders finds the dominant mode accounting for 64 of the variance this is compared to an eof analysis of model results by philander and pacanowski where the forcing is provided by the historical average wind field with the model the dominant mode is almost the entire response 92 of the variance the time dependence of the observed mode tracks that derived from the model during the overlapping seasons

the geochemical nature of the igneous rocks of the sharbot lake domain central metasedimentary belt ontario two bimodal mafic silicic suites of igneous rocks the sharbot lake volcanic rocks and the lavant igneous complex are identified geochemically in the sharbot lake domain of the central metasedimentary belt in ontario and their genesis and thermotectonic environment are evaluated the sharbot lake volcanic rocks comprise a series of basalts characterized by light rare earth element lree depletion and relatively high concentrations of fe2o3 tio2 mno v and y together with rhyolites and silicic pyroclastic rocks they are intruded by rocks of the lavant igneous complex which comprises tholeiitic gabbros characterized by lree enrichment and low concentrations of fe2o3 tio2 mno v and y and granitoid rocks the trace element signatures of the mafic rocks of the sharbot lake volcanic sequences are most like those of back arc tholeiitic basalts and those of the lavant igneous complex are comparable to those of low k tholeiitic basalt suites the trace element signatures of the silicic rocks ass

matrix mosaics brittle deformation and elongate porphyroclasts granulite facies microstructures in the striding athabasca mylonite zone western canada abstract the late archean striding athabasca mylonite zone western canadian shield is a 400 km long linked system of granulite facies mylonite belts developed in the continental crust the granulite facies mylonites 800 1000 c at 0 8 1 1 gpa developed in subsets of the anhydrous mineral assemblage clinopyroxene orthopyroxene garnet plagioclase quartz hornblende comparison of the microstructures of the striding athabasca mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the striding athabasca mylonites in addition to dislocation creep and dynamic recrystallisation extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism the microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain scale and aggregate scale diffusion grain boundary mobility and mass transfer at relatively slow strain rates despite their annealed appearance the granoblastic matrix mosaics developed synkinematically deformation induced dilation may enhance metamorphic reactions where products are more voluminous than reactants in the porphyroclast population highly elongate 20 1 monocrystalline orthopyroxenes appear to be fragments of dismembered kinked parent grains rather than stretched porphyroclasts granulite facies mylonites should not be treated as direct analogues of greenschist facies mylonites in particular it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained high temperature shear zone rocks

kinematical and rheological evolution of a crustal scale ductile thrust zone central metasedimentary belt grenville orogen ontario reply the central metasedimentary belt boundary thrust zone is a 10 km thick 200 km long stack of crystalline thrust sheets enclosed by an anastomosing network of ductile thrust zones formed at mid

time of metamorphism beneath the central metasedimentary belt boundary thrust zone grenville orogen ontario accretion at 1080 ma new u pb zircon and titanite data from the muskoka domain grenville province ontario provide protolith and metamorphic ages for the southwestern central gneiss belt discordant analyses from a migmatitic orthogneiss and its leucosome form a linear array with an upper intercept age of 1457 6 ma and a lower intercept age of 1064 18 ma u pb analyses on metamorphic zircon from an amphibolite yield a set of concordant analyses with an average 207pb 206pb age of 1079 3 ma a weakly migmatitic granitoid rock and a transecting charnockitic vein in the immediate footwall of the central metasedimentary belt boundary thrust zone yielded a discordant array of analyses wth an upper intercept age of 1394 13 ma and a lower intercept age of 1066 8 ma the charnockitic vein yielded concordant zircon ages of 1077 2 ma the upper intercept ages are interpreted in terms of protolith crystallization and the concordant and lower intercept ages as grenviilian high grade metamorphism and associated anatexis we h

upper mantle velocity structure of the lower great lakes region abstract the lithospheric root beneath north america contains a prominent indentation beneath the lower great lakes region that is approximately aligned with the track of the new england seamounts by combining data from the recently installed polaris network in southern ontario canada with data acquired in 1996 during the abitibi grenville teleseismic experiment we have performed a tomographic inversion using 4543 p wave traveltimes from 213 events 5 0 m b 6 6 and 1860 s wave traveltimes from 98 events 5 0 m b 6 6 to obtain high resolution images of the upper mantle beneath the lower great lakes two salient features of the 3 d models are 1 a patchy nnw trending low velocity region and 2 a linear ne striking high velocity anomaly s wave images show that the low velocity anomaly changes from an arcuate feature at 400 km depth to a nw striking linear feature at 100 km depth beneath the neoproterozoic ottawa bonnechere graben the linear high velocity anomaly extends to at least 300 km depth and strikes parallel to surface geological belts and the laurentian continental margin we interpret the high velocity anomaly as a possible relict slab associated with ca 1 35 1 3 ga subduction beneath the composite arc belt whereas the low velocity anomaly is interpreted as a zone of alteration and metasomatism associated with the ascent of magmas that produced the late cretaceous monteregian plutons our data support an interpretation of these plutons as melts generated by the passage of north america across a mantle plume rather than a far field response to opening of the north atlantic

early history of the carthage colton shear zone grenville province northwest adirondacks new york u s a abstract the adirondack mountains expose two distinct tectonic elements of the proterozoic grenville province of northeastern north america the adirondack lowlands and highlands the lowlands are located along the eastern edge of the metasedimentary belt and the highlands form the western portion of the granulite terrane the two are separated by the carthage colton shear zone ccsz u pb titanite and 40ar 39ar hornblende ages in the lowlands are ca 100 m yr older than in the highlands across the ccsz while both the lowlands and the highlands record a history of metamorphism during the elzevirian orogeny ca 1150 ma only the highlands record evidence of a major phase of grenvillian thermotectonic activity ottawan orogeny at ca 1090 1030 ma proposed tectonic models require that the lowlands were either laterally separated from the highlands or were at structurally higher levels during this metamorphism new u pb and 40ar 39ar ages from titanite and hornblende bearing mylonites in the dana hil

structure of the nagavali shear zone eastern ghats mobile belt india correlation in the east gondwana reconstruction abstract interpretation of satellite data in combination with regional field traverses delineating the major structural features such as the nagavali and vamsadhara shear zones and associated fold patterns provides a synoptic picture of the regional tectonic framework of the central part of the eastern ghats mobile belt the complex geology of the study area can broadly be grouped into three distinct deformational events d1 fabrics represented by near flat lying gneissic foliations paralleling the lithological banding are best preserved in low strain domains and are related to middle to late archaean thrusting 3000 2600 ma the second deformational event d2 is characterized by the development of shear zones and associated mylonitic fabrics and magmatism probably during 1450 850 ma the pan african thermal 500 550 ma overprint is restricted to shear zones in the form of reworking regionally the central part of the eastern ghats mobile belt can be divided into five distinct structural domains based on structural geometry of folds foliations and lineations a three dimensional block diagram of the nagavali and vamsadhara shear zones involving fold thrust tectonics associated with westward thrusting is presented here a correlation of pan african shear zones in adjacent continents wrapping around the archaean dharwar craton in the reconstruction of rodinia and east gondwana supercontinent suggests an east west convergence

sw grenville province canada the case against post 1 4 ga accretionary tectonics abstract seven accretionary sutures formed between 1 16 and 1 03 ga have been identified by different authors in the ontario quebec adirondack oqa segment of the mesoproterozoic grenville orogen in canada with one exception the inferred accretionary terrane boundaries lie within or at the margins of the central metasedimentary belt cmb located between the central gneiss belt and the adirondack highlands central granulite terrane however geological geochronological and petrological data suggest that the grenville orogen on both sides of the proposed terrane boundaries sutures preserves a common 1 4 1 03 ga tectonomagmatic history inconsistent with its origin as a post 1 4 ga collage of exotic tectonic blocks features which straddle the proposed 1 16 1 03 ga sutures from the central gneiss belt via the adirondack highlands to the mauricie area include 1 mesoproterozoic continental crust 1 5 1 4 ga forming the host and or basement to younger magmatic and supracrustal suites 2 a 1 35 1 3 ga continental arc remnants of which occur from the cmb boundary zone cmbbz in ontario to the appalachians in the united states built on the 1 5 1 4 ga continental crust 3 intrusions of 1 17 1 13 ga age in the central gneiss belt mafic suite and the adirondack highlands and their quebec extension amcg suite i e anorthosite massifs and related granitoids 4 relics of 1 18 1 14 ga sedimentary basins in the northwestern cmb and the mauricie area we propose that an alternative model can adequately account for the observed geology of this part of the grenville orogen wherein the rocks of the oqa segment were part of an andean type margin between 1 4 and 1 2 ga at 1 35 1 3 ga a continental magmatic arc was built upon the southeastern margin of laurentia represented by the 1 5 1 4 ga mesoproterozoic continental crust the arc split at 1 3 ga forming an ensialic back arc basin relics of which now occur in the northwestern part of the cmb and the back arc basin was flanked to the southeast by an active 1 28 1 25 ga arc collision between the laurentian margin and another continent amazonia occurred at 1 2 ga resulting in closure of the back arc basin and initiation of thrusting along the cmbbz post collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere upper mantle melting and extension of the overlying crust resulting in widespread magmatic activity at 1 17 1 13 ga including emplacement of the amcg massifs crustal extension generated sedimentary basins now represented by the st boniface sediments in the mauricie area 1 18 to between 1 15 and 1 09 ga and the penecontemporaneous flinton group in the northwestern cmb renewed post collisional granulite facies shortening commenced at 1 12 ga manifested as nappes in the central gneiss belt and thrusting in the mauricie area continued post collisional shortening at 1 08 1 05 ga was more localised resulting in reactivation of thrusting in the cmbbz and initiation of the kinematically compatible tawachiche shear zone along the eastern border of the quebec extension of the adirondack highlands the characteristics of the oqa segment of the grenville orogen can all be accounted for in the context of 1 a 1 4 1 2 ga southeast facing andean type margin to a laurentian upper plate associated with northwest dipping subduction 2 continental collision at 1 2 ga and 3 subsequent continued post collisional shortening without invoking accretion of exotic terranes between 1 4 and 1 0 ga

high frequency oscillations and their correlation with the wind forcing during the first sequal year two arrays of inverted echo sounders one along the equator and the other across the north equatorial countercurrent are analysed together with a wind record from st peter and st paul rocks the dominant high frequency feature along the equator is an energetic oscillation with a period of 5 8 days that shows strong coherence with the meridional component of the wind the sounder at 3 n 28 w on the other hand shows a high degree of coherence with the zonal component of the wind at a period of 4 5 days comparison of these measurements in 1983 with measurements made in 1979 suggest considerable non stationarity

observations of inertia gravity waves in the atlantic from inverted echo sounders during fgge abstract the study is based on observations from inverted echo sounders anchored near the equator in the western atlantic ocean it appears to be a good method of detecting inertia gravity waves with large vertical scale the spectral analysis of the records shows variable spectral density levels in the band of 2 5 days three of the four inverted echo sounder ies records show an increase in the variance centered at 3 75 days in addition two sounders deployed during the latter half of the year stronger winds also show a peak in the variance at 2 14 and 2 31 days the most likely interpretation of the spectral peak at 3 75 days is an atmospherically forced vertically propagating inertia gravity wave with a meridional structure given by the n 3 mode the corresponding equivalent depth is h 0 7 m and the vertical wavelength is 730 m the energy density estimated from the variance in the ies records is e 0 7 103 j m 2

internal tidal bore warm fronts and settlement of invertebrates in central chile we studied the occurrence of large high frequency temperature fluctuations and their potential association with settlement of intertidal invertebrates during the spring summer period 1999 2000 at las cruces on the coast of central chile our results showed the existence of internal tidal bores characterized by sharp drops in water temperature at the surface and near the bottom and subsequent temperature increases which occurred with a semidiurnal periodicity measured currents support the hypothesis of alternating onshore and offshore movement of warm water fronts the frequency of the events varied through the summer and their amplitude seems to be modulated by onshore winds the strongest events were observed when strong onshore winds occurred in late afternoon hours and the entire water column showed a semidiurnal temperature signal highest values of chlorophyll concentration in the intertidal zone and daily settlement of bivalves gastropods and crustaceans were observed at times when conditions were favorable for occurrence of internal tidal bores results suggest that internal tidal bore warm fronts could play an important role in the transport of neustonic larval invertebrates and the delivery of phytoplankton to at least some intertidal sites along the coast of chile