isotopic constraints on the thermal history of the wind river range wyoming implications for archean metamorphism precise u pb monazite and 40ar 39ar hornblende ages have been obtained from three locations in the high grade archean core of the wind river range wyoming monazites from metapelites in the paradise basin medina mountain and crescent lake have u pb ages of 2718 1 2633 5 and 2657 2 ma respectively hornblendes from amphibolites and granulites from the same locations yield plateau 40ar 39ar isotope ages of 2652 11 2572 9 and 2527 8 ma respectively and are interpreted as cooling ages from the last thermal event the three localities experienced similar peak pressure temperature conditions the timing of high grade metamorphism in the paradise basin is older than the emplacement of large subjacent batholiths at 2 63 2 67 ga calculated cooling rates based on monazite hornblende pairs of 3 4 1 0 c ma for paradise basin 3 8 1 2 c ma for medina mountain and 1 7 0 4 c ma for crescent lake cannot be used to rule out reheating during subsequent pluton emplacement the markedly slowe

determining the significance of high grade shear zones by using temperature time paths with examples from the grenville orogen ductile shear zones preserve essential information on processes that are active in orogenic roots but the significance of these zones is often difficult to interpret structural petrologic and geochronologic data from shear zones yield elements of the history that are not necessarily synchronous however by combining these data with temperature time t t paths insights are obtained into the nature of shear zones the relation between bounding blocks and orogenic evolution of the deep crust this procedure is illustrated with two examples from the mid proterozoic grenville orogen t t paths from 1160 to 900 ma are based on u pb dating of metamorphic minerals including garnet closure temperature t c 800 c monazite t c 725 c sphene t c 600 c and rutile t c 400 c and 40 ar 39 ar hornblende ages t c 480 c comparison of t t paths from adjacent blocks allows predictions about the significance kinematics and timing of displacement of shear zones in the grenville orogen t t paths can distinguish between major terrane boundaries e g the carthage colton shear zone and within terrane shear zones e g the bancroft shear zone thus these data can also be used to identify individual tectonic terranes in the deep crust this integrated approach to analysis of shear zones provides constraints needed to determine the nature and rate of deep orogenic processes in areas that are complicated by high metamorphic grades

evidence for multiple metamorphic events in the adirondack mountains n y field evidence consisting of 1 rotated foliated xenoliths 2 country rock foliation truncated by isoclinally folded igneous intrusions bearing granulite facies assemblages document one or more early dynamothermal event s of regional scale and high grade early metamorphism resulted in pronounced linear and planar fabric throughout the adirondacks and preceded the emplacement of the anorthosite mangerite charnockite granite alaskite amca suite which contains xenoliths of the metamorphosed rocks olivine metagabbros believed to be approximately contemporaneous with the amca suite also crosscut and contain xenoliths of strongly foliated metasediments these intrusive rocks caused contact metamorphism in the metasediments which locally exhibit both anatectite and restite assemblages subsequently this already complex framework underwent three phases of folding including an early recumbent isoclinical event and was metamorphosed to granulite facies p t conditions the age of the early metamor

deep crustal ductile extension following thrusting in the southwestern grenville province ontario two lithotectonic assemblages in southern britt domain have different histories of plutonism metamorphism structural development and mafic dyke emplacement these differences are accounted for by postulating that a cryptic grenvillian thrust separates the assemblages amphibolite facies extensional shear along the central britt shear zone cbsz overprinted the thrust obscuring kinematic evidence for its existence the structurally lower bayfield nadeau island assemblage contains orthogneiss suites of disparate age lesser amounts of supracrustal rocks deposited before intrusion of the youngest orthogneiss pre grenvillian and grenvillian metamorphic assemblages and at least three mafic dyke suites the overlying ojibway sand bay assemblage contains only younger orthogneiss with grenvillian metamorphic assemblages volumetrically important supracrustal rocks that are younger than the youngest orthogneiss and lacks cross cutting mafic dykes comparable tectono stratigraphic changes are present acros

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

u pb geochronology of the mazinaw terrane an imbricate segment of the central metasedimentary belt grenville province ontario the mazinaw terrane in the central metasedimentary belt of the grenville province comprises volcanic sedimentary and plutonic rocks that were intensely folded and faulted and metamorphosed to as high as upper amphibolite facies u pb geochronology establishes an early period of magmatism and sedimentation at about 1280 1240 ma probably in a marginal basin setting and a multistage rhetamorphic evolution in the period between 1100 and 980 ma which was probably related to crustal thickening by imbrication during compression and wedging of the terrane some of the earliest magmatism formed calc alkalic volcanic rocks of kashwakamak formation at 1276 2 ma an associated sedimentary assemblage was intruded by the helena trondhjemite stock at 1267 5 ma a younger succession formed between 1250 and 1240 ma includes the bimodal volcanic mazinaw lake formation alaskitic granites of the norway lake and abinger plutons and a granodioritic gneiss phase of the cross lake pluton these units were covered

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

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

coeval thrusting and extension during lower crustal ductile flow implications for exhumation of high grade metamorphic rocks numerical models of tectonic processes in large hot orogens offer insight into the styles controls and consequences of ductile flow of middle and lower orogenic crust we present results from crustal scale thermal mechanical models illustrating contrasting styles of syn and post convergent ductile flow crustal viscosity in the models is controlled by temperature dependent flow laws scaled to represent lithological strength variations an additional viscosity decrease at high temperature can be imposed to represent weakening during incipient partial melting two end member tectonic styles in which lower crustal flow is driven either by gravity or tectonics develop in response to the initial properties and evolving strength of the model crust homogeneous channel flow driven by the gravitational potential energy difference between a plateau and its foreland is characteristic of himalayan tibetan ht series models with relatively weak laterally homogeneous crust and moderately high heat production in contrast grenville orogen go series models with relatively strong laterally heterogeneous crust do not reach the low effective viscosity eff 1019 pa s required for gravity driven channel flow instead tectonically driven lateral flow and explusion of lower crust is triggered by underthrusting of a strong indentor the dominant structures are ductile fold nappes that may be transported hundreds of kilometres towards the foreland when tectonic convergence is turned off the model orogens undergo gravitational spreading in which ductile middle and lower crust flow laterally from the plateau towards the foreland driving thrusting on the orogenic flanks and ductile thinning in the orogenic core the resulting geometry can resemble that produced by syn convergent ductile flow except for the development of regional scale normal sense shear zones in the post convergent stage model feasibility is tested against data from the western grenville orogen in ontario the comparison suggests that high grade metamorphic rocks in the orogenic core record the effects of protracted syn and post convergent ductile flow whereas those on the flanks record mainly post convergent thrusting and extension the models suggest a number of ways to distinguish among different styles of syn and post convergent ductile flow metamorphic p t t paths alone are not diagnostic of tectonic process without additional information on crustal scale structure and timing in combination with erosion both types of flow may contribute to exhumation of regionally extensive high grade gneiss terranes

proterozoic multistage ca 1 1 and 0 8 ga extension recorded in the grand canyon supergroup and establishment of northwest and north trending tectonic grains in the southwestern united states the grand canyon supergroup records at least two distinct periods of intracratonic extension and sedimentation in the late mesoproterozoic and neoproterozoic new 40 ar 39 ar age determinations indicate that the mesoproterozoic unkar group was deposited between ca 1 2 and 1 1 ga basins in which the unkar group was deposited and the related northwest striking faults were created by northeast southwest extension which was contemporaneous with regional northwest southeast grenville contraction new u pb data indicate that the neoproterozoic chuar group was deposited between 800 and 742 ma sedimentary and tectonic studies show that chuar deposition took place during east west extension and resulting normal slip across the butte fault this event is interpreted to be an intracratonic response to the breakup of rodinia and initiation of the cordilleran rift margin laramide monoclines of the grand canyon region have north and northwest trends reactivate faults that originated at the time of unkar and chuar deposition and can be traced for great distances hundreds of kilometers from the grand canyon we use the distribution of monoclines in the southwest to infer the extent of proterozoic extensional fault systems the 1 1 ga northwest trending structures and ca 800 700 ma north trending extensional structures created regional fault networks that were tectonically inverted during formation of the ancestral rocky mountains and laramide contraction and reactivated during tertiary extension

timing constraints of orogeny to cratonization thermochronology of the paleoproterozoic trans hudson orogen manitoba and saskatchewan canada abstract a comprehensive thermochronometric investigation across a 600 km transect of the paleoproterozoic trans hudson orogen tho in canada has resolved two first order thermal events 1 widespread peak metamorphism at 1 82 1 79 ga associated with terrane amalgamation and 2 a late collisional metamorphic episode at ca 1 77 ga that is characterized by significant strike slip deformation and rapid cooling geophysical observations of the lithosphere beneath the tho reveal crust that is markedly different on either side of the crustal scale tabbernor sturgeon weir tsw strike slip fault system which offsets lower crustal reflectors the tho east of the fault system records u pb titanite and total pb monazite domain ages that are relatively indistinguishable from 40ar 39ar hornblende and mica cooling ages indicating rapid cooling and perhaps exhumation following 1 77 ga metamorphism seismic imaging indicates the eastern orogen is an irregular package of stacked crustal blocks and our thermochronometric data demonstrate it behaved as a single coherent element shortly 20 30 m y following cessation of convergence in contrast rocks located west of the tsw fault system cooled distinctly later preserving 40ar 39ar muscovite and biotite ages between 1 74 and 1 72 ga this crustal architecture delineated from seismic surveys illustrates a more homogeneous slightly 5 km thinner crust in the case of this orogen apparent stabilization and cratonization occurred ca 100 m y after peak terrane amalgamation and notably the orogen was likely cratonized as two blocks east and west of the tsw fault system

the thermal history of the new york basement determined from 40ar 39ar k feldspar studies constraints on the thermochronologic evolution of the new york crystalline basement are mainly restricted to the high temperature early cooling history following 1 1 ga grenville metamorphism u pb methods and the late cooling history fission track methods the thermal history for the one billion years between these intervals is herein assessed using 40ar 39ar hornblende muscovite biotite and k feldspar thermochronology hornblende preferred ages suggest cooling below 450 500 c between ca 900 and 950 ma total gas ages of biotites range from 746 1060 ma and their age spectra complexity is probably related to younger thermal events and or excess argon a single muscovite analysis yields a preferred age of 854 ma argon isotope analyses of k feldspar provide a quantitative evaluation of the thermal history between 175 and 350 c multi diffusion domain thermochronology on highly variable k feldspar results yield internally consistent thermal histories and along with geologic constraints suggest thermal maxima of 275 350 c at ca 700 470 450 and 300 ma combining all of the k feldspar analyses provides an internally consistent thermal history for the region and allows for the following inferences reheating at 700 ma associated with late precambrian rifting during the formation of the iapetus ocean apparently affected the entire adirondack region with the highest temperatures occurring in the eastern part of new york local reheating in the ordovician is inferred to have affected the eastern adirondack mountains and probably resulted from the combined effects of burial during the early paleozoic emplacement of taconic thrust sheets and migration of hot fluids along normal faults during the taconic orogeny high paleotemperatures for the devonian section of eastern new york are related to maximum burial of the basement during the carboniferous at ca 300 ma

the 40ar 39ar laser analysis of k feldspar constraints on the uplift history of the grenville province in ontario and new york 1 a comprehensive geochronologic database describes a period of late extension across the metasedimentary belt mb of the grenville province in northeastern north america because extension continued through much of the young unroofing history of the region these data do not bracket the timing of final extension across all shear zones and do not constrain the timing of final juxtaposition of terranes analysis of k feldspar in the mb by the 40ar 39ar method can give information on the very latest perturbations in the area since k feldspars have multiple low closure temperatures ranging from about 150 to 300 c these multiple diffusion domains require temperature time modeling to fully interpret argon release spectra this modeling requires precise knowledge of the temperature of degassing of each step which usually requires the use of a resistance furnace for sample analysis we establish a first order relationship between laser power and temperature which can be used for multidiffusion domain modeling of k feldspars comparison of samples analyzed by both methods reveals virtually no difference between the systems supporting the validity of k feldspar analyses by laser step heating our data suggest that using the laser for k feldspars can give results that are geologically reasonable precise and easier to collect in this case these data are then applied to the cooling history of the north american grenville province our k feldspar analyses show that the latest extensional motion along shear zones in the mb is after 900 ma and the region is uplifting as a uniform block by 780 ma

geochemistry and tectonic significance of the tangihua ophiolite complex new zealand abstract the ophiolitic igneous complexes of the tangihua complex of northland new zealand consist of massive and pillowed basaltic lava sequences with intercalated sediments and lesser gabbro microgabbro and basaltic intrusions the complex also includes minor felsic derivatives younger alkalic intrusions which exhibit within plate characteristics and rare ultramafic rocks the lavas are relatively homogeneous and are dominantly tholeiitic basalts with minor calc alkaline and alkaline affinities the complex nature of the chemistry in particular those lavas with transitional arc signatures and the presence of non arc lavas is clear evidence that these volcanics formed in a suprasubduction zone environment the combined geochemical and tectonic constraints suggest that the tangihua complex formed in either a transitional zone between an arc and a back arc setting or in a zone of migration from arc to back arc volcanism as such the field age and geochemical data support the model in which these rocks formed suprasubduction zone crust and are fragments recording some or all stages of arc initiation arc rifting and back arc basin formation the orientation of the arc and back arc volcanic rocks in relation to the over tectonic regime suggests subduction towards the west recent work in the sw pacific shows that late cretaceous ophiolites are present in papua new guinea new caledonia and new zealand the formation of these ophiolitic systems supports the presence of a large scale tectonic regime convergence and consequent westward subduction stretching from papua new guinea to new zealand during the late cretaceous to early palaeocene

differential unroofing within the central metasedimentary belt of the grenville orogen constraints from 40ar 39ar thermochronology an 40ar 39ar thermochronological investigation of upper greenschist to granulite facies gneiss amphibolite and marble was conducted in the central metasedimentary belt cmb ontario to constrain its cooling history incremental 40ar 39ar release spectra indicate that substantial differential unroofing occurred in the cmb between 1000 and 600 ma a consistent pattern of significantly older hornblende and phlogopite 40ar 3ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation variations in hornblende 40ar 39ar age plateaus exceeding 200 ma occur over distances less than 50 km with major age discontinuities occurring across the robertson lake shear zone and the sharbot lake mylonite zone which separate the sharbot lake terrane from the elzevir and frontenac terranes extensional displacements of up to 14 km are inferred between the frontenac and elzevir terranes of the cmb no evidence for significant post argon closure vertical displacement is indicated in the vicinity of the perth road mylonite within the frontenac terrane variations of nearly 100 ma in phlogopite 40ar 39ar plateau ages occur in undeformed marble on either side of the bancroft shear zone phlogopites from sheared and mylonitized marble within the shear zone yield 40ar 39ar diffusional loss profiles but have older geologically meaningless ages thought to reflect incorporation of excess argon by 900 ma southeast directed extension was occurring throughout the cmb possibly initiated along previous zones of compressional shearing an easterly migration of active zones of extension is inferred possibly related to an earlier overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone the duration of extensional shearing is not well constrained but must have ceased before 600 ma as required by the deposition of overlying undeformed cambrian and or ordovician sedimentary rocks

late proterozoic ca 930 ma extension in eastern laurentia the northwest dipping carthage colton shear zone located near the eastern margin of the grenville province in northern new york separates the adirondack lowlands of the metasedimentary belt from the adirondack highlands of the granulite terrane published u pb sphene ages are 1156 1103 ma in the lowlands and 1050 982 ma in the highlands indicating an 100 m y offset in metamorphic ages across the boundary our reported hornblende 40 ar39 ar ages are 1060 ma in the lowlands and 950 ma in the highlands this confirms the offsets in sphene ages across the carthage colton shear zone and indicates an average cooling rate in both the lowlands and the highlands of 18 28 m y over 100 m y biotite 40

exhumation of a collisional orogen a perspective from the north american grenville province combined structural and geochronologic research in the southernmost portion of the contiguous grenville province of north america ontario and new york state show protracted periods of extension after the last episode of contraction the grenville province in this area is characterized by synorogenic extension at ca 1040 ma supported by u pb data on titanites and 40 ar 39 ar data on hornblendes followed by regional extension occurring along crustal scale shear zones between 945 and 780 ma as recorded by 40 ar 39 ar analysis of hornblende biotite and k feldspar by ca 780 ma the southern portion of the grenville province from ontario to the adirondack highlands underwent uplift as a uniform block tectonic hypotheses have invoked various driving mechanisms to explain the transition from compression to extension however such explanations are thus far geodynamically unconstrained numerical models indicate that mechanisms such as gravitational collapse and mantle delamination act over timescales that cannot explain a protracted 300 m y extensional history that is contemporaneous with ongoing uplift of the grenville province rather the presence of a plume upwelling underneath the laurentian margin combined with changes in regional stress directions permitted the observed uplift and extension in the grenville province during this time the uplift history while on a slightly different timescale from those of most plume models is similar to that seen in models of uplift and extension caused by the interaction of a plume with the base of the lithosphere some of the protracted extension likely reflects the contribution of farfield effects possibly caused by tectonic activity in other cratons within the rodinian supercontinent effectively changing the stress distributions in the grenville province of northeastern north america

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

the carthage colton mylonite zone adirondack mountains new york the site of a cryptic suture in the grenville orogen u pb ages were determined on metamorphic sphenes and monazites from the late proterozoic adirondack highlands and lowlands in the vicinity of the carthage colton mylonite zone monazites were extracted from metapelites and sphenes were separated from marbles calc silicate gneisses and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone monazite ages from the lowlands range from 1171 1137 ma sphene ages in the lowlands range from 1156 1103 ma those from the highlands immediately to the east of the mylonite zone range from 1050 982 ma the ages indicate that the last high grade metamorphism in the highlands is ca 100 m y younger than in the lowlands and that both terranes had separate cooling histories at least until ca 1000 ma sphenes from within the carthage colton mylonite zone yield ages of about 1098 ma which are distinct from sphene ages on either side of the shear zone the mineral

current controversies in the caledonides this set of papers arose from a discussion meeting held in the department of geology applied geology university of glasgow on 9 10 september 1997 the aim of the meeting was to review the progress which had been made in resolving some of the more controversial aspects of the pre silurian caledonide geology of the british isles and scandinavia it was also held to mark the retirements of two well known caledonide geologists professors bernard leake and donald ramsay there were 34 talks and poster contributions and the meeting was attended by over 100 people we would like to thank our colleague tim dempster for helping to convene and organize the meeting it was fitting that the meeting should be held in the gregory building which was named after the glasgow professor who not only pioneered work on rift tectonics and regional uplift in africa but also wrote a perceptive book on dalradian geology gregory 1931 in this work gregory pointed out that his observations were made over a period of 25 years being made on recreational weekends and mainly in winter when the ground is less obscured by vegetation and the highland mountains most attractive an approach motivated by a spirit of enquiry and very productive but one which would be out of step with current research strategies we would like to dedicate this set of papers to mark piasecki who died on 13 june 1999 mark was the first person to draw attention to major problems concerning the relationship

thermal regime and post orogenic extension in collision belts in order to assess the conditions leading to post orogenic extension the temperature and strength of the lithosphere during continental collision are calculated the calculations assume that heat is transported by conduction and either that the lithosphere is homogeneously thickened or that only the crust is thickened during the collision despite increased crustal thickness and heat production the temperature does not increase if the rate of shortening is moderate 5 10 16s 1 leading to 100 increase in crustal thickness in 40 myears the temperature distribution is used to determine a rheological profile that is compared with the stress induced by compensated topography the calculations show that for homogeneous lithospheric thickening the strength of the lithosphere increases and is higher than the tensile stress except in the shallow crust the total strength of stable continental lithosphere at the end of shortening is on the order of 1013 n m 1 it is larger by one order than the force induced by compensated topography 1012 n m 1 it appears that homogeneous lithospheric thickening would not lead to post orogenic extension unless the initial conditions are very special with the lithosphere hotter than normal for crustal thickening only the strength of the lithosphere decreases slightly and is on the same order as the tensile stress extension does not necessarily follow from thickening of the crust only but it could take place for a relatively wide range of initial conditions with initial surface heat flow 60 mw m 2 or higher alternatively an event such as rapid removal of the mantle lithosphere by delamination or small scale convection would increase the tensile stress heat rapidly the lithosphere reduce its strength and always trigger extension

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

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

evolution of deep crustal normal faults constraints from thermobarometry in the grenville orogen ontario canada abstract the robertson lake shear zone rlsz is a low angle 30 dip plastic to brittle extensional shear zone located in the southeastern grenville orogen ontario canada thermobarometric data have been collected to evaluate the effects of extension on regional metamorphic field gradients upper greenschist facies metamorphic conditions are preserved in the footwall and upper amphibolite facies metamorphic conditions in the hanging wall however steep gradients in metamorphic pressure are present adjacent to the robertson lake shear zone in the footwall pressures increase from 550 mpa to 800 mpa within 8 km of the rlsz and in the hanging wall pressures decrease from 900 mpa to 500 mpa within 12 km of the rlsz the pressure gradients are interpreted to be a product of isostatic flexural rotations in response to extension along the zone the degree of rotation indicated by the baric gradients in the hanging wall and footwall are compatible and require an initial shear zone orientation of 60 90 which suggests reactivation of an initially high angle terrane boundary previous studies provide constraints of a depth extent of 40 km for the rlsz and crustal thickness of 60 km during extension thus extensional rotations were accommodated by decoupling and regional scale flow in the lower third of the crust

age of xenolith bearing olivine metagabbro eastern adirondack mountains new york a u pb zircon age of 1143 6 pm 7 ma has been obtained on a xenolith bearing olivine metagabbro in the eastern adirondack highlands thus fixing a minimum age for metamorphism represented by truncated foliation in the xenolith and the igneous crystallization age of the rock the igneous age predates commonly accepted metamorphic ages of 1000 1100 ma for the adirondacks and documents that more than one high grade metamorphic event affected the region

sharbot lake terrane and its relationships to frontenac terrane central metasedimentary belt grenville province new insights from u pb geochronology the sharbot lake terrane of the central metasedimentary belt in ontario comprises marbles minor tholeiitic basalts and siliciclastic rocks metamorphosed at low to moderate degrees and is intruded by several suites of mafic to felsic plutons gabbroic and associated monzogranitic phases of the lavant complex in northwestern parts of the terrane yield ages of 1224 2 ma for zircon and monazite and 1221 1211 ma for titanite the wolf grove structure a gneissic domain in the northeastern part of the area was migmatized and intruded by granites at 1168 ma prior to its tectonic juxtaposition on amphibolites and marbles of the sharbot lake terrane correlative events recorded by gneiss quartzite and marble from carleton place farther east stress the strong affinity of the wolf grove structure with rocks of the frontenac terrane the 1156 2 ma posttectonic maberly stock intrudes rocks near the boundary zone between the frontenac and sharbot lake terranes suggesting that the two terranes were linked by

cooling and inferred uplift erosion history of the grenville orogen ontario constraints from 40ar 39ar thermochronology stepwise 40ar 39ar degassing experiments of 57 mineral separates of hornblende muscovite biotite and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the grenville orogen in ontario the 40ar 39ar data support an interpretation of slow nearly uniform cooling 1 4 c m y from temperatures of 500 c to below 150 c in the central gneiss belt cgb hornblendes cooled through ar closure between 930 and 1025 ma whereas in the central metasedimentary belt cmb hornblendes record the following range in 40ar 39ar cooling ages 1104 ma in the frontenac terrane 1007 1067 ma in the sharbot lake terrane 919 1026 ma in the elzevir terrane and 972 ma in the central metasedimentary belt boundary zone regional uplift erosion rates of 0 03 0 14 km m y have been estimated for the grenville orogen in ontario based on the 40ar 39ar data a model retrograde p t path for rocks of the cgb and an upper time constraint provided by flat overlying cambrian and ordovician sediments these erosion rates are consistent with rates estimated for other proterozoic or archean granulite terranes but are an order of magnitude slower than active orogens such as the alps and himalayas a regular variation in hornblende 40ar 39ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the cmb the pattern of 40ar 39ar ages is interpreted to reflect late tectonic extension consistent with field observations in the central metasedimentary belt boundary zone and elsewhere in the cmb up to 13 km of vertical displacement is inferred for some rocks in the cmb between the time they cooled below closure to argon diffusion in hornblende 500 c and their exposure at the surface 25 c

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

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

delamination in collisional orogens during continental collision the weight of the downgoing slab may lead to extensional shearing in the descending plate permitting separation of the downdip dense oceanic lithosphere from the adjacent lighter continental lithospher separation may occur by means of a lithosphere penetrating normal sense shear zone by means of pure shear necking of the lithosphere or by some combination of these mechanisms during separation the continental crust attached to the lower plate may undergo an episode of extensional tectonics the proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism if the continents continue to converge following delamination a compressive tectonic regime will be reestablished in the crust the subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat moho the likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime as well as underplating associated with syntectonic plutonism may account for the seismic complexities observed at moho depths beneath some mountain belts

constraints on the duration of tectonic processes protracted extension and deep crustal rotation in the grenville orogen geochronological data on metamorphic minerals within and outside the bancroft shearzone alarge scaleproterozoicshearzone provideevidenceforextensionaldisplacement over a period of 150 m y this is the longest time interval yet demonstrated for any faultrespondingtooneorogenicphase extensionaldisplacementratesalongthebancroft shear zone averaged over 150 m y from 1040 to 893 ma are 0 06 to 0 13 km m y during extension unroofingofthefootwallwasaccompaniedbyrotationas5to15kmofoverlying crustwasunloaded similarlower platerotationhasbeeninferredformanyofthetertiary metamorphic core complexes exposed in the western united states the protracted displacement and rotation history of the proterozoic bancroft shear zone contrasts with the muchfasterextensionalunroofingoftheyoungermetamorphiccorecomplexes suggesting inherent differences in the tectonic processes operating in these two geologic settings similarities however indicate that both settings may share common aspects of tectonic deformation these findings underscore the geologic importance of large deep crustal faults and indicate that present day zones of active extension such as in the himalayas could remain active many millions of years into the future

synorogenic collapse a perspective from the middle crust the proterozoic grenville orogen structural petrological and geochronological studies of the middle to late proterozoic grenville orogen in ontario canada indicate that a major extensional fault developed synchronously with late thrusting this fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers the temporal and spatial relations among faulting metamorphism and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology comparison of grenville with recent himalayan orogenic activity suggests that during proterozoic times physiographic height crustal thickness and crustal strength were similar to modern conditions in orogenic belts

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

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

circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from laurentia abstract the paleogeography of the precambrian cambrian transition is still poorly known but is fundamental for understanding late neoproterozoic climate extremes and the spatial associations of the rapidly evolving ediacaran and cambrian fauna this period has also been proposed to host unusual geodynamic events such as rapid plate motions and or the bulk tumbling of the earth with respect to its spin axis called true polar wander tpw new paleomagnetic and 40ar 39ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western quebec the mont rigaud and the chatham grenville stocks are related to a failed rift arm of the iapetus ocean both intrusions bear hornblendes which provide overlapping 40ar 39ar plateau ages of 533 2 1 1 ma mont rigaud and 531 4 3 4 ma chatham grenville corresponding to early cambrian nemakit daldynian time characteristic paleomagnetic reversed directions from both intrusions are easterly and shallow d 89 3 i 30 1 95 6 2 k 38 8 n 15 sites and the mont rigaud intrusion also carries antipodal normal polarity directions the combined result yields a north paleopole at location 11 9 s 184 6 e dp 3 8 dm 6 9 placing the montreal region at 16 4 s paleolatitude in the early cambrian in good agreement with late ediacaran and middle to late cambrian paleomagnetic results from laurentia laurentia likely resided at low southerly paleolatitudes throughout the cambrian ruling out a major inertial interchange tpw event for early to middle cambrian time a comparison of late ediacaran to middle cambrian paleomagnetic results from laurentia baltica siberia and elements of west gondwana implies that by early cambrian time a large paleolatitudinal gap existed between laurentia and its presumed conjugate rift margins thus placing a wide iapetus ocean between them the position of baltica within the circum iapetus paleogeography is contentious due to differing interpretations of the few paleomagnetic results during ediacaran time laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 ma to the south polar region at 590 575 ma and then rapidly back to low latitude by 565 550 ma the large and perhaps rapid apparent polar wander of laurentia and other continents in the ediacaran remains enigmatic and controversial

estimating the compensation depth of the hawaiian swell with linear filters broad areas of anomalously shallow seafloor surround regions of active or recently active hot spot volcanism the cause of these midplate swells has been modeled both as elevated temperatures in a convecting layer and as thermal expansion within the conducting portion of the lithosphere according to the former explanation we would expect the low density material compensating the swell to lie beneath the conducting lid whereas the latter model allows compensation at shallower depths we present a technique based on the application of linear filters for determining the compensation depth of midplate swells given observations of topography and geoid or gravity anomalies our method does not require an a priori estimate of swell height or width nor does it assume complete spatial or spectral separation between the swell topography and the signal from the hot spot volcanoes which are compensated by flexure of the oceanic moho rather we exploit the difference in the predicted sign and amplitude of the moho deflection beneath elevated topography produced by loading above versus loading below the elastic plate in order to separate the two effects an application of these linear filters to residual topography and geoid data from the region surrounding the hawaiian hot spot yields a compensation depth of 70 10 km within the lower lithosphere

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

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

onset of seafloor spreading in the iapetus ocean at 608 ma precise age of the sarek dyke swarm northern swedish caledonides abstract the sarek dyke swarm sds crops out in the sarektjakka nappe sn of the seve kalak superterrane in the northern swedish caledonides the sn has two main components 1 a 4 5 km thick succession of rift related sedimentary rocks which is intruded by 2 a suite of tholeiitic dykes the sds constituting 70 80 of the nappe the nappe was deformed during caledonian thrusting but dykes and sedimentary rocks in the interior of the eastern parts of the sn are preserved in a pristine state the tholeiitic dykes of the sds commonly occur in sheeted dyke complexes and up to 11 successive generations can be identified from crosscutting relations the sn represents the fossil continent ocean transition between the baltic craton and the iapetus ocean marking the initiation of seafloor spreading bubble shaped pods and veinlets of diorite are present in the sds sheeted dyke complexes the pods are absent in the oldest dykes but the younger a dyke the more frequent the pods the diorite pods are the equivalent of gabbro pegmatites and both cogenetic and coeval with the dykes the rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons six zircon fractions from the diorite pods including two single grains were analysed geochronologically by the u pb thermal ionization mass spectrometry method the data yield a linear array of points that are 0 4 0 8 normally discordant indicating a crystallization age of 608 1 ma 207pb 206pb 607 9 0 7 ma mswd 0 33 this age is inferred to date the onset of seafloor spreading in the iapetus ocean along the baltoscandian margin

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane