crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

seismic structure of the central metasedimentary belt southern grenville province crust and upper mantle structure interpreted from wide angle seismic data along a 260 km profile across the central metasedimentary belt of the southern grenville province in ontario and new york state shows i relatively high average crustal and uppermost mantle velocities of 6 8 and 8 3 km s respectively ii east dipping reflectors extending to 24 km depth in the central metasedimentary belt iii weak lateral velocity variations beneath 5 km iv a mid crustal boundary at 27 km depth and v a depth to moho of 43 46 km the wide angle model is generally consistent with the vertical incidence reflectivity of an intersecting lithoprobe reflection line the mid crustal boundary correlates with a crustal detachment zone in the lithoprobe data and the depth extent of east dipping wide angle reflectors regional structure and aeromagnetic anomaly trends support the southwest continuity of grenville terranes and their boundaries from the wide angle profile to two reflection lines in lake ontario a zon

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

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

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

a k ar and 40 ar 39 ar study on white micas from the brixen quartzphyllite southern alps the brixen quartzphyllite basement of the southern alps italy consists of metasediments which had suffered progressive deformation and low grade metamorphism pmax 4 kbar tmax 375 25 c during the palaeozoic it has been excavated by pre permian erosion buried again beneath a pile of permo mesozoic to cainozoic sediments estimated tmax 150 c and is now exposed anew due to late alpine uplift and erosion the behavior of the k ar system of white micas is investigated taking advantage of the narrow constraints on their thermal history imposed by the geological stratigraphic reference systems

influence of the atlantic inflow and mediterranean outflow currents on late quaternary sedimentary facies of the gulf of cadiz continental margin abstract the late quaternary pattern of sedimentary facies on the spanish gulf of cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the atlantic inflow currents flowing southeastward across the cadiz shelf toward the strait of gibraltar an inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10 20 m that extends deeper and obliquely down paleo valleys as a result of southward down valley flow a mid shelf holocene mud facies progrades offshore from river mouth sources but atlantic inflow currents cause extensive progradation along shelf toward the southeast increased inflow current speeds near the strait of gibraltar and the strong mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf at the outer shelf edge and underlying the mid shelf mud and inner shelf sand facies is a late pleistocene to holocene transgressive sand sheet formed by the eustatic shoreline advance the late quaternary pattern of contourite deposits on the spanish gulf of cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges coincident with the northwestward decrease in outflow current speeds from the strait there is the following northwestward gradation of contourite sediment facies 1 upper slope sand to silt bed facies 2 sand dune facies on the upstream mid slope terrace 3 large mud wave facies on the lower slope 4 sediment drift facies banked against the diapiric ridges and 5 valley facies between the ridges the southeastern sediment drift facies closest to gibraltar contains medium fine sand beds interbedded with mud the adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margins by comparison the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand because of the complex pattern of contour parallel and valley perpendicular flow paths of the mediterranean outflow current the larger scale bedforms and coarser grained sediment of valley facies trend perpendicular to the smaller scale bedforms and finer grained contourite deposits of adjacent sediment drift facies radiocarbon ages verify that the inner shelf shoreface sand facies sedimentation rate 7 1 cm kyr mid shelf mud facies maximum rate 234 cm kyr and surface sandy contourite layer of 0 2 1 2 m thickness on the cadiz slope 1 12 cm kyr have deposited during holocene time when high sea level results in maximum water depth over the gibraltar sill and full development of the atlantic inflow and mediterranean outflow currents the transgressive sand sheet of the shelf and the mud layer underlying the surface contourite sand sheet of the slope correlate respectively with the late pleistocene sea level lowstand and apparent weak mediterranean outflow current

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

a rupture model for the great earthquake of 1897 northeast india we assume that the unusually deep extensive and long lasting floods of 1897 along the section of the brahmaputra river north of the western shillong plateau were due to local ground subsidence associated with the great earthquake which occurred on june 12 of that year in the western part of northeast india numerical simulations of ground level changes due to slip on a buried low angle thrust fault dipping due north then show that the northern limit of the rupture zone of this earthquake should have been along the e w flowing brahmaputra river about 40 km north of the northern edge of the shillong plateau and about 70 km south of the himalayan mountain front a similar interpretation of a ground tilt observation suggests that the western limit of the 1897 rupture zone was along the western margin of the shillong plateau the e w and n s dimensions of the rupture zone are estimated to be 170 km and 100 km respectively so that it enclosed the western half of the shillong plateau and areas north of it up to the brahmaputra river the rupture depth could not be estimated from the available data on ground level changes and was constrained at 15 km beneath the southern margin of the shillong plateau on other evidence the above thrust fault should be of the nature of a detachment at midcrustal depth which arose because the continental crust associated with the indian shield terrains of the shillong plateau and mikir hills immediately to the east could not subduct under the continental crust of the eurasian plate to the north and east it is tentatively suggested that although this detachment may extend under the himalaya it may not be the detachment on which the great earthquakes of 1905 1934 and 1950 have occurred in the northwestern central and eastern himalaya respectively it is also suggested that a distinction should be made between the seismicity of the himalaya and the seismicity of the himalayan convergent plate margin hcpm an earthquake of the himalayan seismic belt is also an earthquake of the hcpm but the converse need not hold true since the inferred northern limit of the 1897 rupture zone is about 70 km south of the himalayan mountain front it is suggested that this earthquake belongs to the hcpm but not to the himalayan seismic belt thus conservatively a seismic gap of about 700 km may exist along the himalayan seismic belt between the eastern and western limits of the ruptures zones of the great 1934 and 1950 earthquakes respectively

deep geoelectric structure over the lower brahmaputra valley and shillong plateau ne india using magnetotellurics summary magnetotelluric studies over the shillong plateau and lower brahmaputra sediments have delineated the dauki fault as a ne sw striking thrust zone with a dip angle of about 30 along which the low resistivity layer of bengal sediments and the underlying oceanic crust subduct to the northwest at present about 50 km length of these sequences has subducted beneath the shillong plateau and is traced up to depth of about 40 km another thrust zone sub parallel to the dauki thrust is observed in the lower brahmaputra valley corresponding to the brahmaputra fault this is interpreted to be an intracratonic thrust within the indian plate these results suggest that a large fraction of the seismicity over the shillong plateau is associated with the ne sw striking dauki thrust contrary to the earlier belief that this fault zone is relatively aseismic the present studies also suggest that the shillong plateau and the adjoining sedimentary layers act as a supracrustal block not directly participating in the subduction process however in response to the compressive tectonic forces generated by the himalayan and indo burman subduction processes the shillong plateau together with the brahmaputra sediments overlying the indian crust drift eastwards relative to the bengal sediments along the surface expression of the dauki fault leading to a dextral strike slip movement we thus propose that the ne indian crust responds to the compressive forces differently at different depths governed by the rheological considerations at deeper levels the crustal readjustments take place through the subduction along the dauki and brahmaputra thrusts where as at the shallow levels the relative deformability of the supracrustal blocks have a strong influence on the tectonics leading to the strike slip mechanism along the surface expression of the dauki fault

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

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

extension in the central metasedimentary belt of the ontario grenville timing and tectonic significance a laterally extensive zone of southeast dipping marble mylonites in the central metasedimentary belt cmb of the southern ontario grenville the bancroft shear zone records southeast directed extension and a displacement of about 10 12 km calcite graphite carbon isotope thermometry indicates that the maximum temperature at which shearing occurred was ca 450 c in contrast carbon isotope fractionations 13 c cc gr within the marble protolith consistently give temperatures of 650 700 c the bancroft shear zone postdates 1060 ma and older high grade shear zones in the central gneiss belt and the cmb that are associated with northwest directed thrusting comparison of the temperature data from the retrograde marble mylonites with cooling curves for the cmb gives a late grenvillian age range of 935 1010 ma for the extensional event the geometric relation between thrusting and normal faulting in the cmb is comparable to that in parts of the himalayas and southern tibet and similarly we interpret late normal faulting in the cmb to be a result of gravitational collapse of a thrust thickened crust in contrast to the himalaya tibet region however accretion in the grenville becomes older in the direction of regional thrusting

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

an 40ar 39ar geochronological study of the elzevir batholith and its bearing on the tectonothermal history of the southwestern grenville province canada results of 40ar 39ar step heating for muscovite potassium feldspar and plagioclase from the ca 1240 ma elzevir trondhjemite in the central metasedimentary belt of the canadian grenville province have been combined with earlier data and used to deduce a thermal history for the eastern elzevir terrane following the ottawan orogeny 1050 950 ma muscovite yielded precise plateau dates of ca 900 ma whereas the potassium feldspar displayed disturbed age spectra with ca 730 760 ma dates for broad plateau regions this age difference could be explained by slow cooling or by thermal overprinting at 760 800 ma that has updated the microcline the plagioclase spectra provide evidence for a low temperature hydrothermal event ca 380 ma ago that has generated sericite in saussurite and has caused partial argon loss from the microcline this result agrees with the other plagioclase thermochronometry and paleomagnetism on the cordova gabbro 25 km to the west but provides a lower estimate for the upper age brac

high density crustal intrusive bodies beneath shillong plateau and indo burmese range of northeast india revealed by gravity modeling and earthquake data abstract modeling and analysis of gravity and earthquake data of a critically dynamic northeastern region of india assumes a special significance for understanding the seismicity and geodynamics in this paper the relation between lateral heterogeneity of gravity interfaces with subsurface structures and seismic characteristics of northeast india are studied in detail to understand the seismicity and dynamics of the region using bouguer gravity earthquake and magnetotelluric data the modern scaled power spectral analysis of gravity data has brought out crustal lateral heterogeneity at a depth of 8 33 km 19 73 km and 37 14 km that correlate well with 10 km upper crust 20 km lower crust and a moho at 28 42 km revealed by 1d seismic crustal velocity and tomography studies the inferred crustal depths support thinner crust over uplifted shillong plateau and gradual increase of depths over subduction interface of indo burmese range 2d gravity modeling constrained by seismic velocity model and 3d euler deconvolution technique along two specific e w and three n s regional gravity transects delineated three significant high density anomalous heterogeneous intrusive bodies presumably due to underplating beneath shillong plateau mikir massif and indo burmese range based on the seismicity and stress pattern analysis through seismic b value variation we infer that the study region exhibits compressive stress concentrations in the form of brittle deformation at shallow depths and weakening of lower crust by high density intrusive mafic ultramafic bodies of ductile nature which may cause intense seismicity besides the ongoing collision of north drifting indian plate with eurasia to the north and the eastern subduction interface of indo burmese range we also infer that the ne to nw trending v shaped areas outlined north of shillong plateau south of mikir massif and ne sw trending thrust belt of schuppen exhibit aseismic nature and may be the future impounding seismic sources in this region

thermochronology of economic mineral deposits dating the stages of mineralization at panasqueira portugal by high precision 40 39 ar age spectrum techniques on muscovite 40 ar 39 ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history thermochronology of the panasqueira portugal tin tungsten deposit a deposit spatially associated with a belt of hercynian plutons muscovite samples with an age difference as small as 2 2 m y 0 7 of the age are statistically distinct statistics are even better for comparison of multiple samples from separate events that is a difference of 0 9 m y 0 3 can be resolved in this approximately 300 m y old deposit the major tin and tungsten ore forming stages which are the oxide silicate stage the main sulfide stage and greisenization occurred between 296 3 or 0 8 1 sigma and 291 6 or 0 8 m y 1 sigma the first substage of the oxide silicate stage was a short lived thermal pulse at 296 3 or 0 6 m y the fluids responsible may have emanated from the known granite cupola the main sulfide stage was active at 294 5 or 0 9 m y as a slightly longer lived pulse with oldest evidence for this stage 295 8 or 0 6 m y coming from areas farthest away from the known cupola and youngest evidence 293 5 or 0 8 m y closest to the cupola a second substage of the oxide silicate stage occurred as a short lived thermal pulse at 292 9 or 0 7 m y synchronous with greisenization of the cupola and alteration of the silica cap at 292 1 or 0 4 m y the duration of activity of the oxide silicate stage the main sulfide stage greisenization and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4 2 or 0 5 m y 1 sigma minor argon loss from all dated muscovites occurred during later reheating probably during the longer lived pyrrhotite alteration stage a single center the known cupola had a prolonged role and was the source for main sulfide stage oxide silicate stage ii greisenization and alteration of the silica cap and possibly oxide silicate stage i and the pyrrhotite alteration stage however a separate source for these latter two stages cannot be ruled out this study is an example of a new and powerful application of 40 ar 39 ar age spectrum dating of muscovite because of the high precision demonstrated in this study it is now possible to establish time constraints necessary for solving some of the long standing problems in economic geology beyond this the unique geologic situation of panasqueira has allowed us to quantify the thermal characteristics of muscovite published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees c during rapid cooling or short reheating and a temperature of approximately 270 degrees c during slow cooling or extended reheating argon loss patterns displayed by all dated muscovites resulted from reheating after original closure the mechanism for this argon loss appears to have been argon transport by volume diffusion thus 40 ar 39 ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age duration and number of episodes of mineralization

age of the grenville dyke swarm ontario quebec implications for the timing of lapetan rifting u pb baddeleyite and zircon ages for three diabase dykes from widely spaced localities within the grenville dyke swarm indicate a single age of emplacement at ma the 700 km long grenville dyke swarm located in the southeastern part of the canadian shield was emplaced syntectonically with the development of the ottawa graben this graben may represent a plume generated lapetan failed arm that developed at the onset of the breakup of laurentia other precisely dated lapetan rift related units such as the callander alkaline complex and the tibbit hill formation volcanic rocks indicate a protracted 36 ma period of rifting and magmatism prior to volcanism along this segment of the lapetan margin the age of the grenville dykes is the youngest in a progression of precisely dated mafic magmatic events from the 723 ma franklin dykes and sills to the 615 ma long range dykes along the northern and northeastern margins of laurentia respectively thus the age for these dykes represents a key time marker for

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

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

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

exsolution in hornblende and its consequences for 40ar39ar age spectra and closure temperature abstract step heating studies of hornblende and biotite which coexist in an amphibolite near north walpole vermont reveal dramatically different 40 ar 39 ar age spectra the biotite mg 59 is characterized by a plateau over 98 percent 39ar release at 343 5 ma in strong contrast to the amphibole which exhibits both a complex age spectrum and a k ar age significantly younger than the biotite the amphibole release pattern contains three complete age cycles which range between extremes of 199 ma and 408 ma interestingly the k ca ratio calculated from reactor produced isotopes also varies with ar release but follows a trend which is antipathetic to the 40 ar 39 ar age pattern detailed microstructural examination including transmission electron microscopy tem reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40ar 40ar retentivity in the amphibole this result underscores the variability of closure temperature tc for 40ar in metamorphic amphiboles and should warn against accepting a single universal value of tc

age of deformation within the central metasedimentary belt boundary thrust zone southwest grenville orogen constraints on the collision of the mid proterozoic elzevir terrane u pb zircon and titanite geochronology on syntectonic intrusions from the ontario central metasedimentary belt boundary thrust zone has constrained the timing of two major periods of northwest directed ductile thrusting on this crustal scale shear zone initiation of deep seated uppermost amphibolite facies deformation is constrained to a period before 1190 ma over 100 ma earlier than previous estimates major reimbrication of the shear zone took place ca 1080 1060 ma and was more pronounced in the southwestern segment the initiation of deformation within the boundary thrust zone coincides with the emplacement of the hanging wall elzevir terrane of the central metasedimentary belt onto the mid proterozoic laurentian margin the structural nature and temporal and kinematic consistency of the reimbrication phase along the 200 km exposed length of the shear zone is thought to be due to within plate reactivation of the existing rheologically weak structure a result of continent continent collision ca

crustal structure beneath northeast india inferred from receiver function modeling abstract we estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast india by using h vp vs stacking method and a non linear direct search approach neighbourhood algorithm na technique followed by joint inversion of rayleigh wave group velocity and receiver function calculated from teleseismic earthquakes data results show significant variations of thickness shear velocities vs and vp vs ratio in the crust of the study region the inverted shear wave velocity models show crustal thickness variations of 32 36 km in shillong plateau north 36 40 in assam valley and 44 km in lesser himalaya south average vp vs ratio in shillong plateau is less 1 73 1 77 compared to assam valley and lesser himalaya 1 80 average crustal shear velocity beneath the study region varies from 3 4 to 3 5 km s sediment structure beneath shillong plateau and assam valley shows 1 2 km thick sediment layer with low vs 2 5 2 9 km s and high vp vs ratio 1 8 2 1 while it is observed to be of greater thickness 4 km with similar vs and high vp vs 2 5 in rup lesser himalaya both shillong plateau and assam valley show thick upper and middle crust 10 20 km and thin 4 9 km lower crust average vp vs ratio in assam valley and shillong plateau suggest that the crust is felsic to intermediate and intermediate to mafic beneath shillong plateau and assam valley respectively results show that lower crust rocks beneath the shillong plateau and assam valley lies between mafic granulite and mafic garnet granulite

deep electrical structure over the igneous arc of the indo burman orogen in sagaing province myanmar from magnetotelluric studies abstract magnetotelluric studies over the igneous arc of the indo burman range in the sagaing province of myanmar have delineated the high resistivity indian plate subducting westwards beneath the burmese block to depths of 30 km and beyond the thick moderately resistive 20 100 m layer overlying the subducting indian plate may be due to the low resistivity sediments the entire region is covered with prominent sedimentary layer with a conductance varying between 20 and 3000 s showing a general increase from the east to west suggesting that their thickness increases toward the west the large unsystematic variations in the conductance are indicative of the widely varying depositional environments and also possible vertical block movements during the course of their deposition a west dipping low resistivity zone to the east of burmese block seems to demarcate its eastern limit suggesting the possibility of a hitherto unknown deep seated fault which is also supported by the several earthquake foci located over this zone the nature of the crustal movements over this fault is not immediately apparent possibility exists that the sagaing fault is an en echelon fault and the present feature observed here is a part of this en echelon fault the possibility of channel flows of the weakened rocks in the deep crust observed in the vicinity of the eastern himalayan syntaxis may also cause such low resistivity zones

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

age and magnitude of dip slip faulting deduced from differential cooling histories an example from the hope fault northwest montana determination of the age of fault motion poses a challenge in tectonics yet rarely produces satisfactory results we describe a new method in which the age and magnitude of dip slip faulting are estimated from contrasting cooling histories of footwall and hanging wall rocks adjacent to the hope fault northwest montana the hope fault has been interpreted in the past as a mostly right slip fault new kinematic data 40 ar 39 ar thermochronometry and geobarometry indicate that cooling of footwall rocks at 40 ma resulted from dip slip movement this movement caused vertical separation of about 3 to 5 km between footwall and hanging wall rocks suggesting that a minimum dip slip component of 4 km developed during the late eocene these results indicate that the hope fault experienced substantial normal slip in the late eocene making it coeval with other normal and detachment style faults in the northern u s cordillera the western lewis and clark line which in part may share a common tectonic his

interpreting the style of faulting and paleoseismicity associated with the 1897 shillong northeast india earthquake implications for regional tectonism 1 the 1897 shillong assam northeast india earthquake is considered to be one of the largest in the modern history although oldham s 1899 classic memoir on this event opened new vistas in observational seismology many questions on its style of faulting remain unresolved most previous studies considered this as a detachment earthquake that occurred on a gently north dipping fault extending from the himalayan front a recent model proposed an alternate geometry governed by high angle faults to the north and south of the plateau and it suggested that the 1897 earthquake occurred on a south dipping reverse fault coinciding with the northern plateau margin in this paper we explore the available database together with the coseismic observations from the region to further understand the nature of faulting the geophysical and geological data examined in this paper conform to a south dipping structure but its location is inferred to be in the brahmaputra basin further north of the present plateau front our analyses of paleoseismic data suggest a 1200 year interval between the 1897 event and its predecessor and we identify the northern boundary fault as a major seismic source the shillong plateau bounded by major faults behaves as an independent tectonic entity with its own style of faulting seismic productivity and hazard potential distinct from the himalayan thrust front a point that provides fresh insight into the regional geodynamics

crustal shear wave velocity structure beneath northeast india from teleseismic receiver function analysis abstract we investigated the seismic shear wave velocity structure of the crust beneath nine broadband seismological stations of the shillong mikir plateau and its adjoining region using teleseismic p wave receiver function analysis the inverted shear wave velocity models show 34 38 km thick crust beneath the shillong plateau which increases to 37 38 km beneath the brahmaputra valley and 46 48 km beneath the himalayan foredeep region the gradual increase of crustal thickness from the shillong plateau to himalayan foredeep region is consistent with the underthrusting of indian plate beyond the surface collision boundary a strong azimuthal variation is observed beneath shl station the modeling of receiver functions of teleseismic earthquakes arriving the shl station from ne backazimuth baz shows a high velocity zone within depth range 2 8 km along with a low velocity zone within 8 13 km in contrast inversion of receiver functions from se baz shows high velocity zone in the upper crust within depth range 10 18 km and low velocity zone within 18 36 km the critical examination of ray piercing points at the depth of moho shows that the rays from se baz pierce mostly the southeast part of the plateau near dauki fault zone this observation suggests the effect of underthrusting bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the ew ne striking dauki fault dipping 300 toward northwest

microseismicity and tectonics in northeast india abstract three microearthquake surveys were carried out in shillong plateau mikir hills and assam valley areas during 1984 to 1986 some 422 events are relocated by the homogeneous station method the microseismicity map reveals intense crustal 10 to 40 km activity beneath the tura area of shillong plateau the areas to the west of shillong and the area around mikir hills also show high activity the microseismicity map further shows major tectonic lineaments that are compatible with the landsat imagery lineaments and the major faults composite focal mechanisms of the microearthquakes show spatial variation of the tectonic stresses in the region an ene wsw horizontal compressive stress is dominant in the tura area whereas a se nw horizontal compressive stress is dominant in the shillong and mikir hills areas

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

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

a re examination of the influence of composition on argon retentivity in metamorphic calcic amphiboles abstract the effect of composition on argon retention in metamorphic hornblende has been re evaluated by the 40 ar 39 ar step heating method utilizing the same samples from a k ar study in which a relationship between hornblende composition and argon retention was inferred incremental heating of the hornblendes yields 40 ar 39 ar age plateaus or near plateaus between 1081 3 and 1112 3 ma these data support a simple cooling history through argon closure following high grade metamorphism consistent with recent geochronological studies of the bamble sector of norway no clear evidence of extraneous argon is preserved in the 40 ar 39 ar data nor is there any sign of a thermal disturbance following argon closure contrary to the k ar results the 40 ar 39 ar data show no relationship between hornblende cooling age and composition the results of this investigation together with those from other studies suggest that the composition of metamorphic hornblende is not an important factor in controlling its argon closure temperature published studies inferring a compositional dependence on the argon closure temperature in metamorphic hornblende are commonly based on analytical data from impure retrogressed altered recrystallized or microstructurally complex hornblendes which are not representative of the diffusive properties of a pure hornblende such mineralogical and structural complexities may form both above and below the closure temperature of hornblende which may give rise to variable hornblende ages from rocks that have experienced the same geologic history such complexities forming at high temperature may contribute to a modified or variable characteristic grain size s of diffusion resulting in a modified complex hornblende closure temperature these complexities may contribute to low apparent hornblende ages if formed below its true closure temperature in addition variable hornblende ages may be obtained if fine grained mineral intergrowths are present and undergo recoil related phenomena during irradiation geologically meaningful hornblende ages are possible only when proper care is taken in sample selection and characterization

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

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

magnetotelluric study to characterize kachchh mainland fault kmf and katrol hill fault khf in the western part of kachchh region of gujarat india abstract the kachchh mainland fault kmf is a major e w trending fault in the kachchh region of gujarat extending 150 km from lakhpat village in the west to the bhachau town in the east the katrol hill fault khf is an e w trending intrabasinal fault located in the central region of kachchh basin and the south of kmf the western parts of both of the faults are characterized and the sediment thickness has been estimated in the region using a magnetotelluric mt survey at 17 sites along a 55 km long north south profile with a site spacing of 2 3 km the analysis reveals that the maximum sediment thickness is 2 3 km quaternary tertiary and mesozoic in the region out of which the mesozoic sediments feature a maximum thickness of 2 km the estimated sediment thickness is found consistent with the thickness suggested by a deep borehole depth approx 2 5 km drilled by oil and natural gas corporation ongc at nirona northern part of the study area from 2 d inversion of the mt data three conductive zones are identified from north to south the first conductive zone is dipping nearly vertical down to 7 8 km depth it becomes north dipping below 8 km depth and is inferred as kmf the second conductive zone is found steeply dipping into the southern limbs near manjal village 28 km south of nirona which is inferred as the khf a vertical dipping down to 20 km depth conductive zone has also been observed near ulat village located 16 km north of manjal village and 12 km south of nirona village this conductive zone becomes listric north dipping beyond 20 km depth it is reported first time by a geophysical survey in the region

source parameters of earthquakes and intraplate deformation beneath the shillong plateau and the northern indoburman ranges we determined the fault plane solutions and focal depths of 17 earthquakes beneath the shillong plateau and the northern indoburman ranges by combining results from the inversion of long period p and sh waveforms and amplitudes from polarities of first motions and from the identification of pp and sp phases on short period seismograms fault plane solutions of 15 earthquakes show mixtures of thrust and strike slip faulting but the p axes for these events are nearly horizontal and consistently oriented north northeast south southwest all of these earthquakes occurred at depths greater than 29 km beneath the shillong plateau one event occurred at a depth of 52 km the relatively large depths for earthquakes in an intraplate setting suggest that the indian lithosphere in this area is especially cold earthquakes beneath the northern indoburman ranges define a gently east southeast dipping zone from 30 to 45 km beneath the bengal basin to 40 to 90 km beneath the ranges this zone seems to steepen and connect with the zone of intermediate depth seismicity that dips eastward beneath burma these earthquakes cannot have occurred along the interface between a subducting indian plate and the overriding indoburman lithosphere because the p axes not the nodal planes are parallel to the north south trending seismic zone although a couple of the earthquakes might have occurred within the indoburman lithosphere most of this seismicity seems to have occurred within the indian plate recently and currently being subducted eastward beneath the indoburman ranges the consistent north northeast trend of the p axes implies that the orientation of maximum compressional strain in the indian plate throughout its northeastern pan is nearly perpendicular to that responsible for roughly north south trending folds of the indoburman ranges thus either recently in geologic time since 1 ma the orientation of maximum compression changed dramatically or more likely the deformation in the indoburman ranges is decoupled from that in the underlying indian plate meanwhile the seemingly identical northward displacement of india and the indoburman ranges with respect to south china must be accommodated farther east along the sagaing and other faults

paleomagnetic and 40 ar 39 ar studies of late kibaran intrusives in burundi east africa implications for late proterozoic supercontinents the kibaran tectonothermal event in east africa 1400 1200 ma peaked at 1300 ma and was followed by the emplacement of a linear belt of mafic ultramafic and felsic plutons between 1275 1220 ma a series of the mafic ultramafic plutons in burundi from the congo craton carry a stable paleomagnetic remanence with a mean direction of d 108 circ i 16 circ n 10 sites k 37 alpha 95 8 circ with a corresponding paleomagnetic pole at 17 s 113 e biotite samples from the mukanda buhoro massif yield well defined 40 ar 39 ar plateau ages averaging 1223 pm 4 ma and slightly younger integrated ages of 1211 pm 2 ma the 40 ar 39 ar ages combined with previously published u pb and rb sr ages suggest that the magnetic directions were acquired during cooling of the massif between 1260 1210 ma 1236 pm 24 ma one pluton at nyabikere yields an anomalous direction of d 46 circ i 29 circ and a corresponding virtual geomagnetic pole at 43 n

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