seismic images of the grenville orogen in ontario in 1990 lithoprobe acquired 240 km of seismic reflection data across parts of the central gneiss belt cgb and the central metasedimentary belt cmb within the western grenville province of southern ontario interpretation of these data in conjunction with geological constraints provided by bedrock mapping supports a model of northwest directed thrusting and crustal shortening for the grenville orogen within the cgb the parry sound shear zone is imaged as a 3 km wide zone of reflections dipping southeastward at 20 25 and soling at depths 7 km in the north and 3 km in the south beneath parry sound domain parry sound domain and the immediately adjacent domains are underlain by a gently southeast dipping reflective zone at 4 5 12 0 km depth interpreted as a detachment surface likely associated with the central britt shear zone this zone may have accommodated northwesterly transport of parry sound southern britt and northwestern rosseau domains over britt domain during grenvillian thrusting wit

seismic reflection profiles of the central part of the clarendon linden fault system of western new york in relation to regional seismicity abstract geological and geophysical research in upstate new york with few exceptions has not definitively associated seismicity with specific proterozoic basement or paleozoic bedrock structures the central part of the clarendon linden fault system clfs between batavia and dale ny is one of those exceptions where seismicity has been studied and has been spatially associated with structure the clfs is either a complex system of long faults with associated shorter branches and parallel segments or a region of many short faults aligned north south from the lake ontario shore southward to allegany county ny interpretation of 38 km of vibroseis and approximately 56 km of conventional seismic reflection data along 13 lines suggests that the clfs is a broad zone of small faults with small displacements in the lower paleozoic bedrock section that is at least 77 km long and 7 17 km wide and spatially coincident with a north trending geophysical combined aeromagnetic and gravity lineament within the basement the relative offset across the faults of the system is more than 91 m near attica ny the clfs is the expression of tectonic crustal adjustments within the paleozoic rock above the boundary of two basement megablocks of differing petrologic provinces and differing earthquake characteristics that forms the eastern side of the elzevir frontenac boundary zone deep seismic reflection profiles display concave eastward listric faults that probably merge at depth near the mid crustal boundary layer an interpretive vertical section provides the setting for refined definitions of the clfs its extensions at depth and its relation to seismicity most modern seismicity in western new york and the niagara peninsula of ontario occurs in apparent patterns of randomly dispersed activity the sole exception is a line of seven epicenters of small earthquakes that trend east from attica ny into the rochester basement megablock earthquakes may be triggered at the intersections of north and east trending brittle faults within the niagara basement megablock current interpretations of the mechanisms for earthquake generation in western new york and the niagara peninsula of ontario require conservative estimates of seismic hazards that assume that an earthquake the size of the 1929 attica ny event m b 5 2 or larger could occur anywhere in the eastern great lakes basin eglb the broad zone of small displacement faults that marks the clfs in the lower paleozoic sedimentary section and the uppermost basement may not provide the structural environment for generation of earthquakes in western new york if this interpretation is correct most seismicity is generated within the niagara basement megablock beneath or west of the clfs consequently we may have to look to the deeper tectonic regime of basement megablocks to understand the distribution of modern seismicity in the eglb

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

late orogenic plastic to brittle extension along the robertson lake shear zone implications for the style of deep crustal extension in the grenville orogen canada rocks regionally metamorphosed and deformed at middle to lower crustal levels in the contraction dominated mesoproterozoic grenville orogen are exposed in southern ontario canada investigation of the robertson lake shear zone rlsz indicates that extension is a significant component in the late tectonic evolution of this deeply eroded orogen the rlsz is a discrete zone 0 5 1 0 km thick that can be traced for nearly 100 km cross cutting regional structures and lithologic units the zone is composed of a thick mylonite zone and a narrow overlying brecciated zone mylonite foliation shallowly dips east southeast and contains a down dip lineation mylonites are characterized by microstructures indicative of crystal plastic deformation and contain a variety of shear sense indicators including s c and c c composite fabrics sigma porphyroclasts mica fish and other shape fabrics oblique to shear planes consistently indicating a normal down to the east sense of shear a zone of cataclastic structures overlies the mylonite zone and brittle deformation overprints the mylonitic fabrics slickensides in the zone generally strike parallel to foliation in the mylonites but have steeper dips sense of slip indicators and brittle fault orientations also indicate down to the east displacement during east west extension juxtaposition of brittle and crystal plastic structures is resolved with a model of displacement during exhumation whereby localization of cataclasis occurred along a previously active mylonite zone metamorphic facies are upper greenschist in the hanging wall and upper amphibolite in the footwall and metamorphic grade increases from west to east in both the hanging wall and footwall the regional variations in metamorphic grade and the low angle shear zone geometry are a result of isostatic flexural rotations that accompanied extension combined with the crystal plastic to cataclastic nature of the rlsz new 40ar 39ar isotope data from biotite constrain the timing of shear zone displacement until at least 901 1 ma late in the evolution of the grenville orogen

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

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

listric normal faulting during postorogenic extension revealed by 40ar 39ar thermochronology near the robertson lake shear zone grenville orogen canada the robertson lake shear zone is a major plastic to brittle extensional shear zone in the grenville orogen that bounds the mazinaw and sharbot lake domains and provides information on the style of late extension and the unroofing history of the orogen argon isotope data were collected from hornblende and micas to determine 40ar 39ar ages constrain the temperature time histories of these two domains and infer the unroofing history of the region hornblende cooling ages across the mazinaw domain footwall show little variation indicating uniform unroofing of the footwall since 950 ma phlogopite muscovite and biotite cooling ages of the footwall are 924 to 890 ma the cooling history of the mazinaw domain is characterized by slow cooling after peak metamorphism circa 1000 ma accelerated cooling 4 5 c m y from 950 ma to 890 ma and an average cooling rate of 1 c m y to circa 590 ma when these rocks were at or near the surface the cooling history of sharbot lake hanging wall domain is drastically different than that of the mazinaw domain hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 ma respectively indicating a cooling rate of 5 c m y after slow cooling from metamorphic temperatures biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve indicating that the cooling history varied across the domain cooling rates in the hanging wall adjacent to the shear zone are low 2 c m y a biotite cooling age 1029 ma and preservation of an amphibole growth age 1205 ma in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 ma these data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone the varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement

a revised model for the crustal structure of the sw grenville province ontario canada the grenville province forms the exhumed remnants of a 1 1 ga collisional orogeny that telescoped an older continental margin terranes with distinct crustal formation ages can be mapped using nd isotopes revealing a ramp flat thrust structure the ramp is identified by the presence of retrogressed eclogites and its trajectory is refined using nd model ages the main allochthon is locally overlain by the parry sound klippe but is also underlain by a tectonic duplex northwest directed nappes represent remnants of a corrugated thrust sheet but a ring shaped remnant was also preserved where the thrust sheet was down buckled under the dense rocks of parry sound domain

the grenville orogen explained applications and limitations of integrating numerical models with geological and geophysical data numerical models offer powerful insights into tectonic processes especially when their validity can be tested against geological and geophysical observations from natural orogenic belts here we explain some of the criteria for success in integrating orogenic models with data using examples from the grenville orogen model designs must be simplified by comparison with nature to illuminate the first order processes that control orogenic evolution which limits the extent to which model results can reproduce geological observations for the western grenville orogen observed variations in geological properties are represented by lower crustal blocks with strength decreasing from the exterior to the interior of the model go series models with this design reproduce the first order crustal architecture of the georgian bay and montreal val d or lithoprobe transects both constant convergence and stop convergence models produce similar geometries but only stop convergence models produce normal sense shear

interpretation of seismic and potential field data from western new york state and lake ontario abstract lithoprobe and industry seismic profiles have furnished evidence of major zones of easterly dipping grenville deformed crust extending southwest from exposed grenville rocks north of lake ontario additional constraints on subsurface structure limited to the postulated clarendon linden fault system south of lake ontario are provided by five east west reflection lines recorded in 1976 spatial correlations between seismic structure and magnetic anomalies are described from both lake ontario and the newly reprocessed new york lines in the paleozoic to precambrian upper crust the new york seismic sections show 1 an easterly thickening wedge of subhorizontal paleozoic strata unconformably overlying a precambrian basement whose surface has an apparent regional easterly dip of 1 2 minor apparent normal offsets possibly on the order of tens of meters occur within the paleozoic section the generally poorly reflective unconformity may be locally characterized by topographic relief on the order of 100 m 2 apparent local displacement on the order of 90 m at the level of the black river group diminishes upward to little or no apparent offset of queenston shale 3 within the limited seismic sections there appears to be no evidence that the complete upper crustal section is vertically or subvertically offset 4 dipping structure in the paleozoic strata 15 to 35 resembles some underlying precambrian basement elements 5 the surface continuity of inferred faults constituting the clarendon linden system is not strongly supported by the seismic data beneath the paleozoic strata the seismic sections show both linear and arcuate reflector geometry with easterly apparent dips of 15 to 35 similar to the deep structures imaged on seismic lines from nearby lake ontario and on lithoprobe lines to the north the similarity supports an extension of easterly dipping central metasedimentary belt structures of the grenville orogen from southern ontario to beneath western new york state from a comparison of the magnetic and gravity fields with the new york seismic sections we suggest 1 the largely nonmagnetic paleozoic strata appear to contribute negligibly to magnetic anomalies seismically imaged fractures in the new york paleozoic strata appear to lie mainly west of a positive gravity anomaly the relationship between magnetic and gravity anomalies and the changes in the geometry of interpreted precambrian structures remains enigmatic 2 north to northeast trending curvilinear magnetic and gravity anomalies parallel but are not restricted to the principal trend of the postulated clarendon linden fault system paleozoic fractures of the clarendon linden system may partly overlie a southward extension of the composite arc belt boundary zone

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

suturing and extensional reactivation in the grenville orogen canada sutures are zones of weakness within orogenic belts that have the potential to become reactivated during orogenic evolution the robertson lake shear zone marks a major tectonic boundary in the southeastern grenville orogen of canada that has been intermittently active for at least 130 m y the shear zone played a major role in the compressional stage of the orogenic cycle as well as during postorogenic collapse the zone separates the elzevir terrane to the west and the frontenac terrane to the east sphene ages u pb indicate that these two terranes have distinct tectonothermal histories and that the shear zone represents a cryptic suture in its current state the shear zone is a low angle 30 ese dip plastic to brittle extensional shear zone that separates the mazinaw footwall and sharbot lake hanging wall domains integration of structural metamorphic and chronologic data leads to a model that describes the complete evolution of this fundamental tectonic boundary that evolved from an early compressional zone ca 1030 ma to a late extensional zone until at least 900 ma

an asperity model to simulate rupture along heterogeneous fault surfaces a model has been developed to simulate the statistical and mechanical nature of rupture on a heterogeneous strike slip fault the model is based on the progressive failure of circular asperities of varying sizes and strengths along a fault plane subjected to a constant far field shear displacement rate the basis of the model is a deformation and stress intensity factory solution for a single circular asperity under a unidirectional shear stress the individual asperities are unified through the fault stiffness and the far field stress and displacement during fault deformation asperities can fail and reheal resulting in changes in the local stresses in the asperities stress drops and changes in the stiffness of the fault depending on how the stress is redistributed following asperity failure and on the strenghts of the neighboring asperities an earthquake event can be the failure of one or more asperities following an earthquake event seismic source parameters such as the stress drop energy change and moment magnitude are calculated results from the model show a very realistic pattern of earthquake rupture with reasonable source parameters the proper magnitude frequency behavior and the development of characteristic earthquakes also the progression ofb values in the model gives some insight into the phenomenon of self organized criticality

crustal velocity structure of the superior and grenville provinces of the southeastern canadian shield the 1992 lithoprobe abitibi grenville seismic refraction experiment was conducted using four profiles across the grenville and superior provinces of the southeastern canadian shield delay time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another with the largest velocity values occurring underneath the central gneiss and the central metasedimentary belts south of the grenville front the grenville front tectonic zone is imaged as a southeast dipping region of anomalous velocity gradients extending to the moho the velocity anomaly maps suggest an archean crust may extend horizontally 140 km beneath the northern grenville province near surface velocity anomalies correlate well with the known geology the most prominent of these is the sudbury structure which is well mapped as a low velocity basinal structure the tomography images also suggest underthrusting of the pontiac and quetico subprovinces

a regional paleomagnetic study of lithotectonic domains in the central gneiss belt grenville province ontario abstract we have made a regional paleomagnetic study of lithotectonic domains in the central gneiss belt of the grenville province in ontario along ten n s and e w traverses up to 200 km in length although originally intended to clarify the tectonic mechanism by which these exotic terranes were assembled and welded to the archean superior craton during the 1150 ma grenvillian orogeny we actually learned much more about the timing of post orogenic uplift of the various domains around 1000 900 ma as they stabilized and became part of the rodinia supercontinent the normal n and reverse r natural remanent magnetizations nrms of all domains as well as those of reactivated regions flanking the grenville front gf the superior grenville boundary to the north and south have paleomagnetic poles falling on the 980 920 ma 40ar 39ar calibrated portion of the grenville apparent polar wander track for laurentia there is a general tendency for paleopoles to young with increasing distance of domains from the gf implying that more southerly domains were uplifted and magnetized later but two of the domains do not fit this pattern previously reported younging trends away from the gf based on k ar thermochron maps and paleomagnetic zone poles are untrustworthy because of hydrothermal alteration which causes chemical remagnetization and anomalously old k ar ages near the gf another trend in our data is a regular increase in the r n ratio with increasing distance south of the gf in the reactivated zones flanking the gf nrms are overwhelmingly of n polarity whereas well away from the gf r n is close to 50 50 also nrm intensities and susceptibility values increase 100 fold away from the gf peaking 10 km south of the front with a pulse like pattern similar to that documented in anomalously high 40ar 39ar dates in the same region both the magnetic and ar ar results are likely due to a wave of hydrothermal alteration and remagnetization during which fluids were driven away from the gf since the remagnetized nrm near the gf has approximately the same direction as nrms in domains away from the gf hydrothermal activity along fault systems south of the gf must have continued until 1000 ma long after the collisional orogeny itself

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

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

nd isotope mapping of the allochthon boundary thrust on the shores of georgian bay ontario significance for grenvillian crustal structure and evolution nearly 50 new nd isotope analyses are presented for the shawanaga region of georgian bay ontario to study crustal evolution in the grenvillian central gneiss belt depleted mantle t dm nd model ages are used to map a major grenvillian tectonic boundary the allochthon boundary thrust abt which in the shawanaga area separates gneisses with t dm ages above and below 1 65 ga this is lower than the 1 8 ga age cut off observed further north and is attributed to a southward increase in mesoproterozoic magmatic reworking of an original palaeoproterozoic continental margin causing a progressive southward decrease in nd model ages between shawanaga island and franklin island nd isotope mapping yields an abt trajectory that closely matches published geological mapping and passes within 100 m of four retrogressed eclogite bodies typically associated with the thrust boundary this validation of the method gives confidence in the mapped trajectory south of snake island where sparse outcrop inhibits lithological mapping the new results suggest that published 1 7 1 9 ga t dm ages in the lower go home domain of the central gneiss belt further south are also indicative of parautochthonous crust hence we propose that the main ramp of the abt is located in the immediate hangingwall of the go home domain much further south than generally recognized this has important implications for the large scale crustal structure of the sw grenville province suggesting that the abt ramp has a similar curved trajectory to the grenville front and the central metasedimentary belt boundary thrust

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

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

structure and chronology of the oval peak batholith and adjacent rocks implications for the ross lake fault zone north cascades washington the 200 km long ross lake fault zone rlfz forms the northeastern border of the crystalline core of the north cascades this late cretaceous and paleogene structure is an 15 km wide system of faults and fault zones several of which deform the paleocene 65 ma u pb zircon oval peak batholith the southern segment of the gabriel peak tectonic belt of the rlfz is defined by strongly deformed rocks of the foliated southwestern margin of the oval peak batholith structurally underlying pre late cretaceous twisp valley schist and structurally highest part of the subjacent skagit gneiss which here consists of late cretaceous 87 ma and paleocene 60 ma orthogneiss kinematic indicators in moderately steeply northeast dipping mylonitic orthogneiss record reverse slip the strong solid state flattening fabric in the foliated margin of the oval peak batholith formed during or shortly after emplacement and overprinted a magmatic fabric observed in the core of the batholith foliation and associated stretching lineation within the gabriel peak tectonic belt are oblique to the regional structural grain but concordant with the contacts of the batholith these patterns in part record emplacement of the batholith as an expanding diapir into the tectonic belt reverse slip in the tectonic belt occurred before during and shortly after emplacement but ended by 55 58 ma the core of the oval peak batholith intruded a large tongue of twisp valley schist on the north porphyroblast microstructures and ar ar and k ar geochronology indicate that amphibolite facies dynamothermal metamorphism of the schist occurred during intrusion the foggy dew and twisp river fault zones separate the batholith and its wall rocks on the northeast from weakly metamorphosed strata of the methow basin the foggy dew fault zone contains mylonitic gneiss derived from the oval peak batholith skagit gneiss and tonalitic rock that displays u pb zircon systematics suggestive of an eocene age sillimanite schist and amphibolite are also present mylonites record dextral strike slip with a component of normal slip down to the northeast movement in the fault zone continued subsequent to deformation in the gabriel peak tectonic belt and ended by 48 ma mylonites in the twisp river fault zone also reveal dextral strike slip but apparently with less normal slip the rlfz experienced a diachronous history from at least 65 to 45 ma this history was dominated by dextral strike slip but there were significant components of normal slip and reverse slip on some structures during the latter part of this interval 57 45 ma the rlfz probably recorded a transtensional tectonic regime transitional between extension to the east in omineca metamorphic core complexes and dextral strike slip to the west related to oblique convergence along the continental margin early cenozoic plutonism metamorphism and ductile deformation have strongly overprinted any of the major cretaceous deformation postulated in previous tectonic models for the rlfz

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

the effect of small random crustal reflectors on the complexity of pg and pmp coda abstract in the conventional method of incorporating heterogeneity into crustal models of the earth smoothed random small scale velocity fluctuations are added to a uniform seismic velocity gradient field numerical experiments on such models show that the addition of the velocity fluctuations will tend to break up a smooth travel time curve into segments with erratic amplitudes giving one the illusion that the earth is layered with significant intra crustal boundaries many experiments which employ finite difference methods produce synthetic seismic sections that closely match the nature of the observed seismic sections in appearance and many of the features that we have observed owe their origin to velocity fluctuations rather than to deterministic effects in this paper we investigated an alternative method of adding heterogeneity to an earth model smoothed randomly oriented small scale seismic reflectors are embedded in a uniform seismic velocity gradient field the effect of the velocity gradient is to make the reflective field for downward waves much greater than upward waves the on off switching light bulb effect characteristics of the reflection coefficients of the intra crustal reflectors as a function of angle of incidence plays a major role in generating the pg coda in seismic wide angle reflection experiments numerical experiments show that the complexity of the coda or the presence absence of the pmp travel time branch is related to random orientations and velocity contrasts of the small scale reflectors the shingling effect of pg first arrivals is predicted by these experiments the complexity of the record sections which is predicted from the above analysis was compared to actual data obtained from recent canadian lithoprobe seismic coincident near vertical and wide angle reflection experiments

nd isotope mapping of a tectonic window near chalk river ontario implications for large scale grenvillian tectonics abstract over forty new nd isotope determinations are presented for granitoid orthogneisses from the chalk river area on the border between ontario and quebec in the sw grenville province tdm model ages provide evidence for a tectonic window through the allochthonous belt into the grenvillian parautochthon in the chalk river area this tectonic window constrains the location of the main ramp of the allochthon boundary thrust southeast of the chalk river area however the structure of the tectonic window has been complicated by down faulting on the ontario side of the ottawa river due to reactivation of late precambrian normal faulting associated with the ottawa bonnechere graben thus on the ontario side of the border the surface expression of the tectonic window is represented by a strongly sheared tectonic duplex that underlies the main allochthon however the lower section of a 1 km deep borehole drilled by canadian nuclear laboratories has tdm model ages indicating penetration into the underlying parautochthon the new data combined with previous nd isotope mapping and lithoprobe seismic reflection results point to a relatively constant thickness for the exhumed muskoka allochthon by showing that this exhumation occurred immediately below the central metasedimentary belt boundary this evidence strongly supports a ramp flat thrusting model for the sw grenville province this model greatly reduces the likelihood of major crustal exhumation to the nw of parry sound domain thus removing the theoretical basis for pervasive southeast directed orogenic collapse across the ontario gneiss belt

softening the lower crust modes of syn transport transposition around and adjacent to a deep crustal granulite nappe parry sound domain grenville province ontario canada 1 the parry sound domain is a granulite nappe stack transported cratonward during reactivation of the ductile lower and middle crust in the late convergence of the mesoproterozoic grenville orogeny field observations suggest the following with respect to the ductile sheath 1 formation of a carapace of transposed amphibolite facies gneiss derived from and enveloping the western extremity of the parry sound domain and separating it from high strain gneiss of adjacent allochthons this ductile sheath formed dynamically around the moving granulite nappe through the development of systems of progressively linked shear zones 2 transposition initiated by hydration amphibolization of granulite facies gneiss by introduction of fluid along cracks accompanying pegmatite emplacement shear zones nucleated along pegmatite margins and subsequently linked and rotated the source of the pegmatites was most likely subjacent migmatitic and pegmatite rich units or units over which parry sound domain was transported comparison of gneisses of the ductile sheath with high strain layered gneiss of adjacent allochthons show the mode of transposition of penetratively layered gneiss depended on whether or not the gneiss protoliths were amphibolite or granulite facies tectonites before initiation of transposition resulting in e g folding before shearing no folding before shearing respectively meter scale truncation along high strain gradients at the margins of both types of transposition related shear zones observed within and marginal to parry sound domain mimic features at kilometer scales implying that apparent truncation by transposition originating in a manner similar to the ductile sheath may be a common feature of deep crustal ductile reworking

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

joint inversion of gravity and magnetic anomalies of eastern canada the power spectra and degree correlation of the surface topography and free air gravity anomalies of eastern canada show that the gravity anomalies are subdivided into three parts the short wavelength components 30 170 km shorter than 30 km are not well resolved largely arise from density perturbations in the crust and to a lesser extent from the surface topography and moho undulation whereas the contribution of intracrustal sources to the intermediate wavelength components 170 385 km is comparable with that of the topography the long wavelength components 385 1536 km are overcompensated at the moho we present a crustal model for the intermediate and long wavelength components which takes into account the surface topography density perturbations in the crust and moho undulation with a certain degree of isostatic compensation the general characteristics of this model resemble the crustal structure revealed from seismic measurements the reduced to pole magnetic anomalies of eastern canada sh

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

three dimensional visualization of top down superimposed thrust sheets in the sw grenville province ontario to properly understand the tectonic history of the grenville province it is necessary to have a reliable scientifically based understanding of the present day three dimensional 3d structure of the orogen based on detailed nd isotope mapping of surface boundaries and lithoprobe seismic sections this study provides the first detailed visualization of the 3d structure of the grenville gneiss belt in ontario using the sketchup software package the 3d visualization supports a model in which thrust geometry was imposed from the top downwards controlled by the nw boundary of the central metasedimentary belt that originated as a failed back arc rift zone the central metasedimentary belt boundary controlled the trajectory of the allochthon boundary thrust its underlying tectonic duplex and ultimately the grenville front this process of superimposed thrusting explains the large scale change in the trajectory of the grenville front north of georgian bay that has been called the big bend to assist in visualizing the 3d model a fly through animation is provided in the supplementary material

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

models of large hot orogens containing a collage of reworked and accreted terranes we describe a classification scheme for orogens using temperature magnitude t m diagrams and use this framework for modelling large hot orogens that evolve in continents comprising cratonic nuclei bordered by a series of juvenile accreted reworked and metamorphosed terranes modelling the complete evolution of an orogen is difficult particularly large orogens with multiple orogenic phases early phases during which a continent is assembled produce a tectonic and metamorphic fabric that needs to be taken into account when modelling the main collisional orogeny this inherited fabric is represented in a simple way in models described here by a series of lower crustal blocks that are arranged to be systematically stronger toward the cratonic continental interiors we investigate how this fabric influences the development of the model orogen during the main collisional phase using upper mantle scale ums and crustal scale cs finite element models the models exhibit a diachronous three phase evolutio