a comparison of eastern north american seismic strain rates to glacial rebound strain rates glacial rebound strain rates computed using a simple laurentide glacial loading model are of the order of 10 9 per year within the region of glaciation and extending several hundred kilometers beyond the horizontal strain rates receive approximately equal contributions from horizontal and vertical velocities a consequence of the spherical geometry adopted for the earth model in the eastern united states and southeastern canada the computed strain rates are 1 3 orders of magnitude greater than an estimate of the average seismic strain rate anderson 1986 and 1 order of magnitude greater than predicted erosional strain rates the predicted glacial rebound strain rates are not in general oriented in such a way as to augment the observed state of deviatoric stress possibly explaining why the seismic strain rates are much smaller than the glacial rebound strain rates an exception to this may be seismically active regions in the st lawrence valley

new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

validation of the ice 3g model of wurm wisconsin deglaciation using a global data base of relative sea level histories a global data base consisting of radiocarbon controlled relative sea level rsl histories from 392 different geographic locations has been compiled for use in analyses of the glacial isostatic adjustment process these data may be employed to constrain both the spatiotemporal characteristics of the last deglaciation event of the current ice age and the rheological structure of the planetary interior in a previous paper in this series tushingham and peltier 1991 we employed the rsl data in this set from sites that were once ice covered to refine the apriori model of deglaciation that had been delivered by past research in the area of pleistocene geomorphology peltier and andrews 1976 wu and peltier 1983 to produce a new global model of this event that we have called ice 3g in the present paper following a description of the full data base and of the methods used to compile it we employ data from the sites that were not ice covered 200 time series to verify the plausibility of this refined model of deglaciation and to test the extent to which the totality of the observations constrain the radial viscoelastic structure of the planet the analysis of solutions to the forward problem presented here reinforce previous inferences of a preference for a thick lithosphere by rsl data from sites along the u s east coast and a preference by data from most near field sites for an increase in viscosity across the 670 km discontinuity in the mantle by a factor of approximately 2 when the viscosity of the upper mantle is fixed to the value of 1021 pa s originally deduced by haskel 1935 on the basis of his analysis of the data from fennoscandia

fossil strain from the 1811 1812 new madrid earthquakes observations of postseismic deformation generally suggest that the effects of postseismic viscoelastic relaxation may persist for many decades after an earthquake in particular strain effects from the three great earthquakes that occurred in the new madrid seismic zone in 1811 1812 may influence present day measurements of ground deformation in this active seismic region of the central united states forward calculations using an earth rheology that may be appropriate for a continental intraplate region and a simple fault model for the 1811 1812 sequence suggest that postseismic relaxation may be an important factor in driving the unexpectedly high rate of shear deformation recently observed in the southern new madrid seismic zone

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

the proterozoic of eastern bolivia and its relationship to the andean mobile belt abstract the previously unmapped bolivian sector of the central brazil shield 220 000 km in area has been studied during an anglo bolivian technical co operation programme the metamorphic basement comprises the lomas maneches granulitic complex the chiquitania gneissic complex and the san ignacio schist supergroup which crops out as metasedimentary belts rb sr dates on some of the granulites suggest a protolith age of 2000 ma but the outcrop pattern of the metamorphic rocks as a whole is due to the 1300 ma san ignacio orogeny which was accompanied by widespread granite emplacement and metamorphism up to granulite facies the sunsas cycle began with the deposition of the supracrustal sunsas vibosi group and closed with the development of the sunsas and aguapei mobile belts at 1000 ma the upper proterozoic brasiliano cycle is represented by a bifurcating sedimentary basin in the se of the area with a closing tectonic event of 520 ma correlation is suggested with units in the central brazil shield the bolivian belts are extrapolated east into brazil and their limits indicated to the west the belts show trends parallel to the phanerozoic andean chain and they reflect the major deflection in the chain the arica elbow furthermore when the belts are traced towards the andes their ages broadly correspond to those of the sub andean precambrian basement this leads to the proposal of an enlarged andean orogenic belt system 1000 km wide comprising a 1300 ma old proto andean orogen which has controlled the trends of subsequent mobile belts

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

the 1983 goodnow earthquake in the central adirondacks new york rupture of a simple circular crack on 7 october 1983 a magnitude 5 1 mb earthquake occurred in the central adirondack mountains near the town of goodnow new york the earthquake was well recorded both by regional stations and by a large number of digital and analog seismographs in north and south america and europe these regional and teleseismic data are complemented by accurate locations of aftershocks recorded by a portable network from 17 hours after the main shock providing an unusually complete dataset to study the source process of an intraplate earthquake the results of an analysis involving the formal inversion of the vertical component rayleigh waves and of short period p waves show that the earthquake was due to reverse faulting with a centroidal depth of 7 5 km striking north south and dipping at 60 to the west the scalar seismic moment is 1 9 1023 dyn cm for p waves for paths from northeastern united states plausible values of t attenuation range from 0 4 to 0 7 sec and for these values the estimates of source duration range from 0 60 to 0 35 sec assuming a circular crack model with a rupture velocity of 3 0 km sec the bounds on source duration give upper and lower limits for the fault radius of 0 9 and 0 5 km and for the stress drop of 670 and 115 bars the preferred value of t 0 6 sec yields a source duration of 0 45 sec a radius of 0 7 km and a stress drop of 265 bars this fault radius agrees with that inferred from the spatial distribution of aftershocks the inferred stress drop of the goodnow earthquake is higher than generally observed for interplate events if the goodnow earthquake is representative of events in eastern north america the high stress drop may partially explain the large felt areas produced by these relatively small intracontinental earthquakes

postglacial rebound and the focal mechanisms of eastern canadian earthquakes a detailed numerical model of postglacial rebound for the eastern canadian arctic is described with special attention being paid to baffin island this numerical model is able to account for the observed uplift patterns as recorded by the relative sea level histories of discrete sites distributed throughout the study area these uplift patterns show regional trends so the same model can be used to interpolate between data sites and to estimate the uplift history at any arbitrary site within the study area by treating the lithosphere as a thin elastic plate spatial variations in the uplift pattern can be translated into estimates of lithospheric stress the model predicts the variations in uplift as a function of both space and time since deglaciation and can therefore be used to estimate the temporal evolution of lithospheric stress following deglaciation the stress so calculated is treated as a perturbation to an ambient stress field having its origin in other processes postglacial rebound is shown to

changes in orientation of near surface stress field as constraints to mantle viscosity and horizontal stress differences in eastern canada observations indicate that the orientation of the stress field in eastern canada has changed significantly during the last 9 000 years if this temporal rotation of stress orientation is due to changes of the dominant stress component from rebound stress 9 000 years ago to tectonic stress at the present then this rotation in stress orientation can be used to constrain mantle viscosity and the difference between the horizontal tectonic principal stresses rebound stress in a maxwell earth due to the application of a realistic ice load is calculated with the finite element method and is superposed on tectonic stress to give the total stress it is demonstrated that large tectonic stress differences 10 mpa or high viscosity 1022 pa s in the lower mantle cannot explain this temporal stress rotation because in the former case stress orientation is determined by the static tectonic stress while for the latter case rebound stress continues to dominate the current stress orientation

activity along the osning thrust in central europe during the lateglacial ice sheet and lithosphere interactions abstract the osning thrust is one of the major fault systems in central europe with a length of approximately 115 km it underwent a polyphase tectonic evolution in the mesozoic that ranged from extensional movements in the jurassic to reverse faulting and thrusting during inversion in the late cretaceous a series of complex metre scale faults and related fold structures are developed within the upper pleistocene alluvial aeolian complex of the upper senne 1 km away from the thrust the faults dip towards the north and show offsets in a range of several decimetres growth strata indicate a two fold evolution of the structures the faults began as normal faults and were later transformed into reverse faults which resulted in the formation of small scale inversion structures with a typical harpoon shape glaciotectonics can be ruled out as the driving mechanism for these structures because new optically stimulated luminescence osl ages imply that the sedimentary succession was deposited during the late pleistocene between 29 3 2 9 ka and 13 1 1 5 ka at which time the ice margin was much further to the northeast we believe the inversion structures were caused by movement on the osning thrust due to generation of a forebulge of the late pleistocene weichselian ice sheet this led to normal faulting as a consequence of extension in the forebulge area during glaciation the osl ages for the normal fault related growth strata are in a range of 16 13 ka later reverse movements occurred during deglaciation due to the n s directed compressional stress field in northern germany numerical simulations of the deglaciation seismicity point to seismic events with a thrust mechanism in the study area between 15 5 and 12 3 ka bp although normal faulting is also possible in this time period numerous soft sediment deformation structures imply that movement on the osning thrust caused earthquakes with a significant magnitude in the autumn of 1612 an earthquake took place in this area it is very likely that this event is related to background seismicity on this fault although movement due to the influence of the ongoing glacial rebound in fennoscandia is also possible

modelling of stresses in the fennoscandian lithosphere induced by pleistocene glaciations the seismicity observed in fennoscandia is usually explained by tectonic stresses the question of the contribution of stresses induced by glacial isostatic adjustment to the seismicity has not finally been settled yet in order to quantify the isostatic stress field generated by the fennoscandian glacial load we consider layered maxwell viscoelastic half spaces loaded by axisymmetric discs in particular we calculate the maximum shear stress distribution induced by the load model and the associated stress regimes in the lithosphere emphasis is placed on a discussion of the sensitivity of the calculated isostatic stresses to variations in load cross section lithosphere thickness and asthenosphere viscosity we also examine the effects produced by the viscous relaxation of the lithosphere whose viscosity is parameterized according to the stratification expected for temperature activated diffusion creep our results show that the isostatic shear stresses typically reach 2 mpa near the surface at the present time but that their distribution may be modified strongly by the details of the earth and load models investigated the viscous relaxation of the lithosphere becomes particularly important if more than one glaciation is considered since the last ice age in fennoscandia consisted of several glaciations the model of a perfectly elastic lithosphere conventionally employed in models of glacial isostatic adjustment therefore may not be justified when calculating the isostatic stresses

evidence of strong earthquake shaking in the lower wabash valley from prehistoric liquefaction features earthquake induced liquefaction features in holocene sediments provide evidence of strong prehistoric shaking magnitude m b 6 2 to 6 7 in the wabash valley bordering indiana and illinois the source of the one or more earthquakes responsible was almost certainly in or near the wabash valley the largest event is interpreted to have occurred between 7500 and 1500 years ago on the basis of archeological pedological and stratigraphic relations

viscoelastic responses of a hard transition zone effects on postglacial uplifts and rotational signatures abstract recent studies from laboratory s karato 1989 and monte carlo inversion y ricard et al 1989 have argued that the transition zone between 400 and 670 km depth may be stiffer than the lower mantle and the upper mantle above it using a five layer viscoelastic spherical model we have calculated the transient displacements of postglacial rebound the induced polar motions and the temporal variations of the geopotential up to degree eight of the zonal coefficients we have compared two models a one with two viscoelastic layers separated at 670 km and b the other with three layers in which a hard garnet layer lies between the upper and lower mantle forward modelling shows that it may be possible to discern the presence of a hard garnet layer with a viscosity of at least ten times greater than the upper mantle on the basis of uplift data near the center of the former laurentide ice sheet and from polar wander and j 2 data temporal variations of higher gravity harmonics such as j 6 and j 8 can potentially place even tighter constraints on the rheological properties of the hard transition zone a lower mantle viscosity between 2 and 4 10 22 pa s is generally preferred in models with a garnet layer which may be as large as 50 times more viscous than the upper mantle

glacial isostatic adjustment on 3 d earth models a finite volume formulation summary we describe and present results from a finite volume fv parallel computer code for forward modelling the maxwell viscoelastic response of a 3 d self gravitating elastically compressible earth to an arbitrary surface load we implement a conservative control volume discretization of the governing equations using a tetrahedral grid in cartesian geometry and a low order linear interpolation the basic starting grid honours all major radial discontinuities in the preliminary reference earth model prem and the models are permitted arbitrary spatial variations in viscosity and elastic parameters these variations may be either continuous or discontinuous at a set of grid nodes forming a 3 d surface within the regional or global modelling domain in the second part of the paper we adopt the fv methodology and a spherically symmetric earth model to generate a suite of predictions sampling a broad class of glacial isostatic adjustment gia data types 3 d crustal motions long wavelength gravity anomalies these calculations based on either a simple disc load history or a global late pleistocene ice load reconstruction ice 3g are benchmarked against predictions generated using the traditional normal mode approach to gia the detailed comparison provides a guide for future analyses e g what grid resolution is required to obtain a specific accuracy and it indicates that discrepancies in predictions of 3 d crustal velocities less than 0 1 mm yr 1 are generally obtainable for global grids with 3 106 nodes however grids of higher resolution are required to predict large amplitude 1 cm yr 1 radial velocities in zones of peak post glacial uplift e g james bay to the same level of absolute accuracy we conclude the paper with a first application of the new formulation to a 3 d problem specifically we consider the impact of mantle viscosity heterogeneity on predictions of present day 3 d crustal motions in north america in these tests the lateral viscosity variation is constructed with suitable scaling from tomographic images of seismic s wave heterogeneity and it is characterized by approximately 2 orders of magnitude peak to peak lateral variations within the lower mantle and 1 order of magnitude variations in the bulk of the upper mantle below the asthenosphere we find that the introduction of 3 d viscosity structure has a profound impact on horizontal velocities indeed the magnitude of the perturbation of order 1 mm yr 1 is as large as the prediction generated from the underlying 1 d radial reference model and it far exceeds observational uncertainties currently being obtained from space geodetic surveying the relative impact of lateral viscosity variations on predicted radial motions is significantly smaller

deformation of internal boundaries in a viscoelastic earth and topographic coupling between the mantle and core summary in a two layer self gravitating viscoelastic maxwell earth is solved analytically and the behaviour of the internal interface which is non monotonic in time is shown to be due to the buoyancy effect across the interface deformation inside realistic earth models is calculated using the new normal mode method some modes with long relaxation times which do not contribute significantly to surface deformations are found to be significant for deformations inside the mantle flow patterns inside the mantle and the topograph at the core mantle boundary cmb due to surface glacial loads have been calculated glacial induced topography at the cmb unless suitably located is shown to be too small to significantly alter the total coupling mechanism and thus flow in the fluid core if there is any causal relationship between climatic changes and geomagnetic reversals then it is probably due to the transfer of rotational energy from the mantle to the core a revised estimate of 100 m drop in sea level is required to cause changes in the flow pattern in the core

a three dimensional p wave velocity model for the charlevoix seismic zone quebec canada 1 a three dimensional p wave velocity model has been developed for the charlevoix seismic zone csz the csz is located along the st lawrence river 100 km northeast of quebec city canada and is one of the most active seismic zones in eastern north america five earthquakes with magnitudes equal to or exceeding 6 0 have occurred in the csz in historic time and around 200 earthquakes occur annually hypocenters are located in precambrian basement rocks basement rocks have been affected by numerous tectonic events including grenvillian collision iapetan rifting and meteor impact we performed a sequential tomographic inversion for p wave velocity structure based upon 3093 p wave arrivals from 489 earthquakes recorded by 12 stations high velocity is associated with the center of the impact crater the region of high velocity is surrounded by low velocities interpreted to be highly disrupted rocks an elongated high velocity region is present at midcrustal depths that trends parallel to the st lawrence river earthquakes avoid the high velocity body and separate into two bands one on either side of the feature larger earthquakes magnitude 4 have occurred along the northern edges of the high velocity region

lower mantle viscosity constrained by seismicity around deglaciated regions knowledge of the viscosity structure of the earth s mantle is important for constraining models of mantle convection and isostatic rebound here we show that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the lower mantle in addition to those previously proposed1 2 calculations using a spherical viscoelastic earth model show that the present day magnitude of the stress fields induced in the lithosphere beneath the now disappeared laurentide and fennoscandian ice sheets is very sensitive to the value of the lower mantle viscosity stresses of 100 bar sufficient to cause seismicity can still remain in the lithosphere for lower mantle viscosities greater than 1022 pa s for lower mantle viscosities of 1021 pa s only a few tens of bars of stress persist in the lithosphere today this influence of lower mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes and hence for earthquake recurrence times

lagrangian particle transport in the indian ocean a model study lagrangian transport of passive particles in the indian ocean is diagnosed using the global modular ocean model 2 of the geophysical fluid dynamics laboratory first the model is forced by time varying monthly mean climatological wind stresses and a particle cloud released in the indonesian throughflow itf region is tracked for one year in the next experiment two separate clouds one released in the itf region and the other in the arabian sea region are followed for three years by forcing the model with weekly varying nmc9195 wind stress data in both the experiments short term liapounov exponents indicate that particle trajectories are chaotic with typical e folding stretching time about 125 days the zonal dispersion of the itf cloud is faster than the shear flow dominated transport process in the first experiment as well as for the first two years in the second experiment however at later times the zonal dispersion rate is subquadratic in the latter experiment for the arabian sea cloud zonal dispersion is diffusive when the whole time range of three years is considered however for the initial six months it is shear flow dominated and thereafter becomes even subdiffusive the meridional dispersion rate is roughly superdiffusive in both experiments when the whole time range is taken into account our estimates of correlation dimension a lower bound on the fractal dimension suggest that none of the particle clouds reach the area filling limit in the time range of the model integration we also compare the present results with those from earlier studies of chaotic stirring in the oceans

implications of a ductile crustal layer for the deformation caused by the fennoscandian ice sheet summary previous studies of glacial isostatic adjustment have usually considered earth models with a purely elastic lithosphere a possibly significant deviation from this assumption is the presence of a ductile crustal layer embedded in an otherwise elastic lithosphere such a layer has been suggested for various continental regions on the basis of seismic and tectonic evidence the present study investigates the implications of a ductile crustal layer for the interpretation of glacial isostatic adjustment using a layered incompressible maxwell viscoelastic earth model and a simplified representation of the fennoscandian glaciation the relaxation time and amplitude spectra show that besides the conventional buoyancy mode m0 also present in earth models with a purely elastic lithosphere the ductile layer supports a slowly decaying mode mc which is capable of modifying the deformation markedly thus measures of the absolute deformation such as the stress clearly reflect the presence of the ductile layer in contrast to this are measures of the relative deformation such as the vertical displacement rate and the rate of gravity change to a good approximation these rates are inversely proportional to the relaxation time of the more quickly decaying mode m0 and are therefore less affected by the ductile layer taken together the present study suggests that provided a ductile crustal layer exists its presence should be accounted for in interpretations of the glacial isostatic adjustment following the fennoscandian or smaller glaciations

the southern border of the guapore shield in western brazil and bolivia an interpretation of its geologic evolution abstract the geology of the southeastern and southwestern margins of the guapore shield is described the shield rocks comprise high grade granulite and low grade greenschist sequences these rocks are poorly known oldest recorded ages include transamazonian datings 2 ma the margins of the shield show the development of a mobile belt formed by rocks of upper proterozoic to cambrian age within this mobile belt the cuiaba group of uncertain but possibly upper proterozoic age outcrops to the east and shows tectonic contacts with younger rocks that span the vendian upper proterozoic to cambrian age the rocks of vendian to cambrian age form three isolated elongated basins which show evidence of fault control the axes of these basins are equally separated by angles of 120 and meet at a triple junction near corumba the stratigraphy of the vendian to cambrian basins is correlated from bolivia to brazil based on tectonic continuity sedimentological similarity and fossil content the central location at corumba shows instability since the middle proterozoic when important igneous activity is recorded at rincon del tigre bolivia and in brazil this igneous activity is interpreted as being related to hot spot activity in an intercratonic setting from which the younger upper proterozoic basins originated

postglacial rebound and fault instability in fennoscandia summary the best available rebound model is used to investigate the role that postglacial rebound plays in triggering seismicity in fennoscandia the salient features of the model include tectonic stress due to spreading at the north atlantic ridge overburden pressure gravitationally self consistent ocean loading and the realistic deglaciation history and compressible earth model which best fits the sea level and ice data in fennoscandia the model predicts the spatio temporal evolution of the state of stress the magnitude of fault instability the timing of the onset of this instability and the mode of failure of lateglacial and postglacial seismicity the consistency of the predictions with the observations suggests that postglacial rebound is probably the cause of the large postglacial thrust faults observed in fennoscandia the model also predicts a uniform stress field and instability in central fennoscandia for the present with thrust faulting as the predicted mode of failure however the lack of spatial correlation of the present seismicity with the region of uplift and the existence of strike slip and normal modes of current seismicity are inconsistent with this model further unmodelled factors such as the presence of high angle faults in the central region of uplift along the baltic coast would be required in order to explain the pattern of seismicity today in terms of postglacial rebound stress the sensitivity of the model predictions to the evects of compressibility tectonic stress viscosity and ice model is also investigated for sites outside the ice margin it is found that the mode of failure is sensitive to the presence of tectonic stress and that the onset timing is also dependent on compressibility for sites within the ice margin the evect of earth rheology is shown to be small however ice load history is shown to have larger evects on the onset time of earthquakes and the magnitude of fault instability

on postglacial geoid subsidence over the equatorial oceans we develop two new spectral formalisms for the gravitationally self consistent solution of the sea level equation which governs the redistribution of glacial meltwater on a visco elastic earth the first is a purely spectral technique based on an extension of theory outlined by dahlen 1976 for determining the equilibrium oceanic tide on an elastic planet using this technique with a feasible spherical harmonic truncation level l 30 we can obtain convergence in the computed sea level variations within the degree range l 10 we also develop a second pseudospectral technique however which permits the construction of gravitationally self consistent solutions to much higher degree and order l 100 the pseudospectral formalism is employed here to develop a comprehensive physical explanation for the global pattern of present day sea level variations due to ongoing glacial isostatic adjustment in particular we focus upon a mechanism which we term equatorial ocean syphoning that acts to draw water toward the oceanic portion of the collapsing peripheral bulge that encircles previously glaciated regions the collapse of the forebulge induces a flow of water which is required to maintain hydrostatic equilibrium the syphoning mechanism dominates the relative sea level rsl variation in oceans in the far field of the ice sheets that is beyond the peripheral bulge during periods such as the interglacial of the past 4000 years during which the volume of the ice sheets has not apparently changed appreciably while at the same time isostatic adjustment persists the rsl change in the near field is in contrast dominated by the local vertical displacement of the solid surface finally comparison of gravitationally self consistent predictions of rsl change in the far field with the observational data at small pacific island sites has provided an upper bound of 1 to 2 m for the eustatic sea level rise produced by any recent last 3000 years melting of the antarctic ice sheet

glacio seismotectonics ice sheets crustal deformation and seismicity abstract the last decade has witnessed a significant growth in our understanding of the past and continuing effects of ice sheets and glaciers on contemporary crustal deformation and seismicity this growth has been driven largely by the emergence of postglacial rebound models pgm constrained by new field observations that incorporate increasingly realistic rheological mechanical and glacial parameters in this paper we highlight some of these recent field based investigations and new pgms and examine their implications for understanding crustal deformation and seismicity during glaciation and following deglaciation the emerging glacial rebound models outlined in the paper support the view that both tectonic stresses and glacial rebound stresses are needed to explain the distribution and style of contemporary earthquake activity in former glaciated shields of eastern canada and fennoscandia however many of these models neglect important parameters such as topography lateral variations in lithospheric strength and tectonic strain built up during glaciation in glaciated mountainous terrains glacial erosion may directly modulate tectonic deformation by resetting the orogenic topography and thereby providing an additional compensatory uplift mechanism such effects are likely to be important both in tectonically active orogens and in the mountainous regions of glaciated shields in general the short term response to ice fluctuations is similar to the earth s response to fluctuations in water reservoirs with the subsequent increase or decrease in seismicity which depends on the pre existing stress state when the ice fluctuations occur on the spatial scale and magnitude of the late pleistocene glaciation and deglaciation however the viscoelastic response of the earth especially the mantle causes significant changes in crustal deformation and earthquake activity that is spatially extensive and temporally complex the regions of greatest ice thickness and the regions marginal to the late pleistocene ice sheets indicate the most dramatic evidence of earthquake faulting the mantle response to these large ice mass fluctuations and the change in mass between the oceans and land caused and continues to cause measureable crustal deformation at hundreds of kilometres from the ice margins for both tectonically active and cratonic regions palaeoseismic investigations of late pleistocene and holocene faults are an important tool in evaluating earthquake hazard the predicted response of the earth over this time however is very dependent on the model assumed for most regions far more carefully designed field observations are needed to constrain the existing rheological and ice models

evidence of late and postglacial seismic activity in the temiscouata madawaska valley quebec new brunswick canada three types of geological phenomena independently suggest that the temiscouata madawaska valley was affected by one or more seismic events following its deglaciation 1 subbottom acoustic profiling of lac temiscouata and grand lac squatec revealed disturbance of bottom sediments by mass transport processes in both lakes erosional truncation of preexisting acoustically laminated sediments and accumulation of hummocky chaotic deposits over older hummocky surfaces or laminated sequences both result from mass transport processes unidirectional mass flows from several points in these symmetrical basins in situ disruption of laminated sediment beneath flat bottoms and the large area of the lake floors affected suggest strong similarities in sedimentation style with lakes that have been disturbed during strong earthquakes 2 southeast of lac temiscouata in saint jacques new brunswick two separate mass flow deposits made up largely of coarse 0 5 m angular boulders of local bedrock occur on oppos

stress and fault parameters affecting fault slip magnitude and activation time during a glacial cycle stress and fault parameters affecting fault slip magnitude and activation time during a glacial cycle

glacial isostatic adjustment and earth rotation refined constraints on the viscosity of the deepest mantle we explore the extent to which two properties of earth s present day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle the analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660 km seismic discontinuity appears to be allowed by these data such observations do not constrain the viscosity of the mantle below about 1400 km depth however and here we estimate the average viscosity in this deepest region by invoking observations of the present day rate and direction of polar wander and the present day magnitude of the nontidal acceleration of the rate of planetary rotation since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation this response is expected to provide the most specific sampling possible of deep mantle properties our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial interglacial transition namely the ice 4g model this analysis shows that when earth rotation observations are combined with relative sea level constraints the viscosity of the deepest mantle is required to be 1 order of magnitude higher than the viscosity of the upper part of the lower mantle such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid the analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations

andean trending mobile belts in the brazilian shield much of the phanerozoic andean mountain chain is floored by precambrian basement1 3 in places as old as 2 000 myr4 5 but relations between these rocks and those of the brazilian shield have long been obscure mainly because of the intervening cenozoic sedimentary cover we describe here recent work over the bolivian sector of the brazilian shield fig 1 which shows a pattern of proterozoic mobile belts approximately parallel with the andean chain to the west when the trends of these bolivian belts are projected towards the andes under the cenozoic cover their ages broadly accord with those of the andean basement our interpretation is that the two domains are linked in the form of a chain of five andean trending mobile belts with a combined width of 1 000 km becoming progressively younger to the west with ages ranging from 1 300 myr to the present

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

can deglaciation trigger earthquakes in n america a spherical self gravitating viscoelastic earth model is used to calculate the spatial and temporal evolution of the glacially induced lithospheric stress and fault stability in north america the predicted onset time of a pulse of earthquake activities the mode of failure and the magnitude of instability are investigated for three sites with different distance from the former ice margin it is found that glacial unloading is able to trigger paleo earthquakes within the ice margin near charlevoix 47 5 n 70 1 w and in wabash valley 38 5 n 87 w outside the ice margin however rebound stress decays away from the former ice margin thus glacial unloading is unlikely to have triggered the large m8 earthquakes in new madrid 36 6 n 89 5 w

the proterozoic history of eastern bolivia abstract about 100 000 km2 of the previously unmapped bolivian sector of the central brazil shield has been studied by proyecto precambrico an anglo bolivian technical cooperation programme the lower proterozoic is represented by the lomas maneches granulite group and the bulk of the chiquitania paragneiss complex which were formed during the trans amazonic orogenic cycle 2000 ma the middle proterozoic spans the orogenic cycles of san ignacio 2000 1300 ma and sunsas unmetamorphosed upper proterozoic and possibly cambrian sediments on the southern and eastern flanks of the shield represent marine transgressions related to the intra continental braziliano orogenic cycle east trending dolerite dykes were probably intruded during this period within the shield

deglaciation seismotectonics a principal influence on intraplate seismogenesis at high latitudes abstract in many high latitude stable continental regions deglaciation dominates the current crustal strain field interference between polarised tectonic and radial deglaciation strain fields affects the deviatoric stress state and fault stability margins and in consequence both the levels and properties of seismicity throughout glacial loading and unloading the most notable present day affects of this strain interference are predicted to include alternating quadrants of enhanced seismicity and aseismicity around rebound domes and former peripheral forebulges such strain interference can explain much of the geography and timing of holocene earthquake occurrence across northern europe and eastern north america

mantle viscosity and ice age ice sheet topography ice age paleotopography and mantle viscosity can both be inferred from observations of earth s response to the most recent deglaciation event of the current ice age this procedure requires iterative application of a theoretical model of the global process of glacial isostatic adjustment results demonstrate that the iterative inversion procedure converges to a paleotopography that is extremely close to that from the ice 4g model the accompanying mantle viscosity profile is furthermore shown to reconcile the requirements of aspherical geoid anomalies related to the mantle convection process thus resolving a fundamental issue concerning mantle rheology the combined model also explains postglacial sea level histories for the east coast of the united states

viscoelastic perturbations of the earth significance of the incremental gravitational force in models of glacial isostasy summary the incremental gravitational force igf arises from perturbations of the earth s gravitational potential in glacial isostasy its sources are the surjicial and internal mass redistributions associated with the growth and decay of the continental ice sheets we examine the errors caused by the neglect of the igf using closed form solutions of the equations governing surface load induced perturbations of two types of incompressible spherical earth models 1 maxwellian viscoelastic mantle enclosed by elastic lithosphere and 2 maxwellian viscoelustic mantle enclosing inviscid core calculations in the legendre domain of the radial surface displacement for these models show that neglecting the igf causes enhancement of the elastic response and acceleration of the viscous relaxation in the space domain these changes entail corresponding modifications of the calculated land adjustment the magnitude of the error caused by the neglect of the igf strongly depends on the deglaciation history and load radius adopted assuming a typical deglaciation history the error reaches a maximum of less than 20 m at the end of the deglaciation phase for loads comparable in size to the canadian or the fennoscandian ice sheets we also compare sphericul earth models with igf and plane earth models without igf calculations of the radial surface displacement show that the errors due to the neglect of sphericity and the igf partially compensate each other taking the uncertainties of the observational data into account we conclude that the majority of the canadian and fennoscandian glacial isostatic adjustment data can be modelled with sufficient accuracy using a plane earth model without igf

proterozoic crustal evolution at the southern margin of the amazonian craton in the state of mato grosso brazil evidence from rbsr and k ar data abstract geologic mapping on a 1 100 000 scale in the sao jose dos quatro marcos sjqm area southwest of the state of mato grosso central western brazil showed a remarkable variety of precambrian lithologic units predominant are grey tonalitic gneisses with a rb sr isochron age of 1971 770 ma an initial 87 sr 86 sr ratio of 0 7071 and a k ar biotite cooling age of 1500 ma the gneisses gneisses show interlayered first generation amphibolite lenses and bands k ar amphibole age of 1500 ma and some rare relicts of grey gneissic granulites together with associated pink augen gneisses rare outcrops of deformed banded calc silicate rocks are recrystallized by contact metamorphism these units are intruded by a granitoid suite constituted by early usually somewhat deformed tonalites and granodiorites and late massive pink granodiorites and monzo and syegogranites rb sr isochron age 1472 19 ma i r 0 7037 they show enclaves of grey gneisses and second generation amphibolites a large amphibolite body to the west is also considered part of this second generation basic sequence scattered rhyolite outcrops to the east represent the southernmost remnants of the volcanic rio branco group the grey gneisses here formally identified as the sao jose dos quatro marcos gneiss and probably derived from lower proterozoic tonalites were complexly deformed and recrystallized at 2 0 ga into predominantly amphibolite facies rocks the first and second generation amphibolites document two distinct commingling episodes separated by at least 0 5 ga each directly associated with the corresponding host rock granitoid magmatism the porphyritic granites were converted into pink augen gneisses before 1500 ma and are thus not contemporaneous to the 1380 ma syn kinematic san ignacio granitoids from eastern bolivia calc silicate rocks are interpreted as remnants of supracrustal sequences perhaps related to the quatro meninas group the 1500 ma granitoids formally named the jaboti intrusive suite invaded under a syn to late kinematic regime determining a unique thermal magmatic event in this region

induced stresses and fault potential in eastern canada due to a realistic load a preliminary analysis summary in order to understand the causal relation between postglacial rebound and earthquakes a simple disc load model is used to 1 calculate stresses induced in the lithosphere and mantle by glacial loading melting and postglacial rebound and 2 evaluate the effect of glacial loading rebound on the failure potential for earthquakes in the upper crust the dependence of the failure potential and the actual mode of failure on the coefficient of friction the ambient tectonic stress magnitude direction the stress due to the overlying rocks and lithospheric thickness are investigated prominent features of this paper are the inclusion of 1 a viscoelastic mantle and thus the migration of stress and 2 the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field it is assumed that throughout the earth there are optimally oriented pre existing virtual faults that are initially close to but not at failure thus a time dependent quantity called dfsm related to the coulomb mohr failure criterion can be defined such that a negative value of dfsm would advocate faulting or earthquake activities whereas a positive value of dfsm would promote stability the results indicate that under all combinations of tectonic stress magnitude and overburden stress crustal loading promotes fault stability directly underneath the load upon the removal of the load thrust faulting is predicted within the ice margin if the horizontal stress s induced by the overburden is greater than or equal to the vertical component s of the overburden stress 121 where sjs under this condition theory predicts that faulting or earthquake activity should have reached a maximum immediately after deglaciation if the horizontal stress induced by the overburden is less than the vertical component of the overburden stress 1 l then theory predicts fault stability within the ice margin the theory predicts fault instability both north and south of the ice margin the mode of failure however is completely determined by the value of i the trade off between the tectonic stress magnitude and the overburden stress parameter 1 is also investigated it is shown that a larger tectonic stress magnitude can be used to compensate a smaller value of 1 the results of this analysis show that variations in the coefficient of friction lithospheric thickness and a ductile zone below the upper crust do not significantly affect the above conclusions

will earthquake activity in eastern canada increase in the next few thousand years knowledge of whether earthquake activity will increase in the next few thousand years is important for the planning of nuclear waste repositories assuming that fault instability portends earthquak

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

stress strain changes and triggered seismicity following the mw 7 3 landers california earthquake calculations of dynamic stresses and strains constrained by broadband seismograms are used to investigate their role in generating the remotely triggered seismicity that followed the june 28 1992 mw7 3 landers california earthquake i compare straingrams and dynamic coulomb failure functions calculated for the landers earthquake at sites that did experience triggered seismicity with those at sites that did not bounds on triggering thresholds are obtained from analysis of dynamic strain spectra calculated for the landers and mw6 1 joshua tree california earthquakes at various sites combined with results of static strain investigations by others i interpret three principal results of this study with those of a companion study by gomberg and davis this issue first the dynamic elastic stress changes themselves cannot explain the spatial distribution of triggered seismicity particularly the lack of triggered activity along the san andreas fault system in addition to the requirement to exceed a coulomb failure stress level this result implies the need to invoke and satisfy the requirements of appropriate slip instability theory second results of this study are consistent with the existence of frequency or rate dependent stress strain triggering thresholds inferred from the companion study and interpreted in terms of earthquake initiation involving a competition of processes one promoting failure and the other inhibiting it such competition is also part of relevant instability theories third the triggering threshold must vary from site to site suggesting that the potential for triggering strongly depends on site characteristics and response the lack of triggering along the san andreas fault system may be correlated with the advanced maturity of its fault gouge zone the strains from the landers earthquake were either insufficient to exceed its larger critical slip distance or some other critical failure parameter or the faults failed stably as aseismic creep events variations in the triggering threshold at sites of triggered seismicity may be attributed to variations in gouge zone development and properties finally these interpretations provide ready explanations for the time delays between the landers earthquake and the triggered events

material versus isobaric internal boundaries in the earth and their influence on postglacial rebound summary most previous earth models used to calculate viscoelastic relaxation after the removal of the late pleistocene ice loads implicitly assume that there is no exchange of mass across the mantle density discontinuities on periods of tens of thousands of years the material boundary formulation in the present study simple incompressible models are used to determine the earth s behaviour in the case where the density discontinuity remains at a constant pressure rather than deforming with the material the isobaric boundary formulation the calculation of the movement of the boundary is more rigorous than in earlier studies and uses the local incremental pressure calculated at the depth of the boundary and allows for the vertical deformation caused by the change in volume as material changes phase it is shown that the buoyancy modes associated with the density discontinuities decrease in strength and increase in relaxation time analogous to what results when the density contrast is reduced also two viscoelastic modes arise from an isobaric boundary which is also predicted when there is a contrast in rigidity or viscosity across a material boundary the difference in predicted radial deformation between the isobaric boundary model and the material boundary model is largest for long wavelength loads for which the material incremental pressure at depth is largest if the isobaric boundary model is appropriate for the treatment of the mineral phase changes in the mantle on glacial rebound timescales then previous inferences of the deep mantle to shallow mantle viscosity ratio based on large scale deformation spherical harmonic degree lo of the earth and including data from the early part of the glacio isostatic uplift are too small

a review of the geochronology of the amazonian craton tectonic implications abstract the available geochronological data for the amazonian craton permit delineation of its main age provinces and their respective tectonic development demarcation of the boundaries of each age province is primarily based on rb sr whole rock isochrons supported by k ar determinations on minerals some u pb results on zircons pb pb whole rock isochrons and sm nd model ages complement these data in key areas at the center of the amazonian craton lies the archean age central amazonian province bounded in its northern part by the maroni itacaiunas mobile belt which developed during early proterozoic times the western and southern margins of the archean core are bordered by three tectonic provinces 1 the rio negro juruena magmatic arc 2 the rondonian mobile belt and 3 the sunsas mobile belt each belt borders the western margin of the preceding belt and each records a successively younger middle proterozoic orogeny within the central amazonian province 2 7 ga age determinations have been obtained from the serra dos carajas area although the maroni itacaiunas belt includes several reworked archean nuclei rock units formed during the 2 2 1 9 ga trans amazonian orogeny predominate the isotopic evidence suggests that a mantle derived component constituted a significant portion of the crust accreted at that time the rio negro juruena province is of mid proterozoic age 1 75 1 50 ga and includes almost entirely mantle derived material juxtaposed with the pre existent continental mass along a series of successive magmatic arcs the rondonian and sunsas provinces both exhibit older sialic basements 1 5 ga the former is characterized by important tectonomagmatic events in the 1 45 1 25 ga interval whereas the latter records tectonic reworking at 1 1 0 9 ga finally the amazonian craton is bordered to the east by the late proterozoic paraguai araguaia or araguaia tocantins fold belt

moment tensors state of stress and their relation to post glacial rebound in northeastern canada summary in the stable craton of northeastern canada moderate seismicity as well as glacial isostatic adjustment gia has been observed we investigate five earthquakes with moment magnitudes between 3 6 and 4 1 that have occurred in northern hudson bay since 2007 which may be triggered by gia focal mechanisms of the earthquakes are determined using a waveform fitting procedure for surface waves in which the best double couple mechanism is obtained through a grid search over strike dip and rake all events exhibit a thrust fault mechanism in general agreement with mechanisms of previously analysed earthquakes in that area stress inversion results incorporating our new mechanisms as well as previously published results exhibit significant differences from previously calculated stresses from gia models for the first time maximum horizontal stress direction shmax is compared to the modelled stress values for northeastern canada we find that the maximum horizontal stress direction strikes roughly nnw sse in contrast to previously inferred ne sw orientation of shmax our results indicate that the local stress field in northern hudson bay deviates from regional stresses resulting from the existence of a roughly e w oriented zone of faulting which is not included in existing gia models

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

mantle viscosity from the simultaneous inversion of multiple data sets pertaining to postglacial rebound we employ the formalism of bayesian inference to invert three distinct data sets related to the phenomenon of glacial isostatic adjustment these data consist respectively of the wavenumber spectrum of relaxation times for fennoscandian rebound inferred by mcconnell 1968 the characteristic relaxation times for a large collection of distinct 14c dated relative sea level curves from both canada and fennoscandia and two data related to the rotational response of the earth to the glaciation deglaciation cycle of the late pleistocene ice age our analyses show that the spherically symmetric viscosity structure objectively inferred on the basis of these data from the once glaciated regions of the northern hemisphere allows a fully satisfactory reconciliation of both us east coast relative sea level observations as well as providing a close fit to the requirements of aspherical geoid anomalies

viscous gravitational relaxation summary this paper is concerned with a detailed examination of the response of maxwell models of the planet to surface mass loads particular attention is devoted to an examination of the factors which determine the isostatic response since the understanding of this response is crucial in a number of different geodynamic problems one particular example which we discuss in detail is concerned with the prediction of free air gravity anomalies produced by large scale deglaciation events using the methods developed here we are able to provide the first direct assessment of the importance of initial isostatic disequilibrium on the observed relative sea level variations and free air gravity anomalies forced by the melting of the laurentide ice sheet we are therefore able to estimate the extent to which such initial disequilibrium might influence the inference of mantle viscosity from isostatic adjustment data our calculations establish that free air gravity data although they are sensitive to the degree of initial disequilibrium provide an extremely high quality constraint upon the viscosity of the lower mantle

on postglacial sea level summary an exact method is presented for calculating the changes in sea level that occur when ice and water masses are rearranged on the surface of elastic and viscoelastic non rotating earth models the method is used to calculate the instantaneous elastic and delayed vi scoelastic sea level changes following the partial melting of late quaternary ice sheets we find that there can be large errors in the usual assumption that changes in sea level are uniform over the ocean basins if a quantity of ice equivalent to a uniform 100 m rise in sea level melts from the laurentide and fennoscandian ice sheets then in the south pacific the instantaneous rise in sea level can be as large as 120m in the north atlantic the instantaneous rise is always less than 100 m there is a zone in the north atlantic with almost no sea level change and near greenland and norway the sea level falls rather than rises by over 100 m one thousand years after the melting a forebulge migrating towards the ice loads causes water to flow from the south pacific into the north pacific suggesting that raised beaches should occur in the south pacific the gravitational attraction of an ice mass upon a nearby ocean tends to hold sea level high in the vicinity of the ice this extra load near the ice may have a significant influence on postglacial isostatic adjustment

postglacial faulting in eastern canada nature origin and seismic hazard implications abstract at more than 70 locations in southeastern canada and the adjacent united states tiny bedrock faults displace glacial striations that are about 10 000 years old these reverse faults mostly of small displacement occur in a broad arc from western ontario to newfoundland the faults lie near the margin of the laurentian ice sheet strike generally tangential to it and imply compression nearly normal to the ene directed contemporary regional stress these observations and some age relationships suggest that the faults formed very soon after deglaciation perhaps in the first few thousand years after ice melting in each place and probably represent the release of stresses caused by flexural deformation of the upper crust although the individual offsets are small the faults are believed to be pervasive and individual outcrops reflect amounts of stress relief consistent with stresses implied by the postglacial tilt of nearby lake shorelines

proterozoic geologic evolution of the sw part of the amazonian craton in mato grosso state brazil abstract this paper presents new geochronologic isotopic and geochemical data bearing on the evolution of proterozoic crust in sw mato grosso state brazil which is at the southern end of the 1 6 1 8 ga rio negro juruena orogenic belt of the amazonian craton amazonia our data define three major crustal events i the alto jauru terrane occurs in the eastern part of the region and is comprised of island arc related rocks with u pb ages from 1 79 to 1 74 ga these rocks have e nd t values range from 2 8 to 2 0 with crustal residence ages t dm from 1 93 to 1 78 ga indicating a mainly juvenile signature at 1 8 ga ii the cachoeirinha suite occurs in the central part of the region and consists of calc alkaline plutons emplaced into alto jauru terrane host rocks these plutons have u pb ages from 1 56 to 1 54 ga and e nd t values ranging from 1 0 to 0 8 with crustal residence ages t dm from 1 88 to 1 75 ga we interpret this orogen as the roots of a continental margin arc built upon basement comprised of the alto jauru terrane iii the santa helena batholith occurs in the western part of the region and is a large elongate body of calc alkaline rocks ranging from granodiorite to highly evolved granite these units yield u pb ages from 1 45 ma to 1 42 ga with e nd t values ranging from 4 1 to 2 6 and crustal residence ages t dm from 1 70 to 1 50 ga the rio alegre domain occurs west of the santa helena batholith and includes juvenile 1 52 to 1 47 ga volcanic and mafic plutonic rocks regional geologic relationships suggest that these rocks are part of the crust into which the santa helena batholith was emplaced we interpret this batholith as the magmatic core of a juvenile arc accreted to the edge of the alto jauru terrane with incorporation of some older crust alto jauru terrane in the east and derivation from mainly juvenile crust rio alegre domain in the west the rio branco suite occurs to the east of exposed alto jauru terrane rocks as large hills protruding through younger aquapei group sedimentary rocks it consists of gabbro and granophyric rocks with u pb ages of 1 47 ga gabbro and 1 43 ga granophyre and inherited older nd suggesting hinterland derivation from alto jauru terrane basement during development of the santa helena batholith several undeformed 1 5 1 4 ga granitic plutons occur within the alto jauru terrane these are also regarded as inboard manifestations of subduction related magmatism associated with accretion of the rio alegre domain and formation of the santa helena batholith the siliciclastic aguapei group was deposited sometime between 1 0 and 1 4 ga it overlies jauru terrane basement rocks of the santa helena batholith and the rio alegre domain in the east it is flat lying and undeformed but in the west it is deformed and metamorphosed in the nnw trending aguapei thrust belt farther west the aguapei group is horizontal undeformed and overlies the paleoproterozoic paragua block in bolivia where it has been correlated with the sunsas group the aguapei thrust belt has k ar cooling ages of about 930 ma and is apparently a foreland fold and thrust belt formed by reactivation of an older rift basin during the 1 0 ga sunsas orogeny which occurs west of the paragua block in bolivia

effect of viscosity structure on fault potential and stress orientations in eastern canada summary previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern canada have focused on the mode of failure and the observed timing of the pulse of earthquake faulting activity following deglaciation in this study the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times it is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 1021 pa s mantle the effects of viscosity structure on the above predictions are also examined it is shown that since most of the above observations are found within the ice margin they are not very sensitive to lithospheric thickness also the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust high viscosity 1022 pa s in the lower mantle is rejected by the stress orientation and rotation observations a low viscosity 6 times 1020pa s upper mantle with 1 6 times 1021 pa s in the upper part of the lower mantle and 3 times 1021 pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations

holocene glacial rebound and sea level change in nw europe summary observations of late pleistocene and holocene sea level change relative to the crust exhibit very considerable variations across nw europe in consequence of the response of the earth s crust to the deglaciation of fennoscandia and of the water added to the oceans from the melting of all late pleistocene ice sheets inversion of sea level observations from a site near the centre of the fennoscandian ice sheet and from three sites located beyond the margin of the ice sheet at the time of maximum glaciation yield a range of plausible models for the earth s response and for the ice models further constraints on this range of models is placed by a comparison of observed sea levels with predicted values at other sites near the former ice sheet margins the resulting mantle parameters are upper mantle viscosity 3 5 x 10 pa s lower mantle viscosity 2 7 x loz1 pa s lithospheric thickness 100150 km these values represent effective parameters that describe the response of the earth to surface loading of short to intermediate wavelengths on a time scale of 104yr the lower mantle viscosity is poorly constrained but the marked increase from upper to lower mantle is a characteristic of all plausible solutions the inversion places a constraint on the total volume of ice in the fennoscandian ice sheet such that the equivalent sea level rise from this contribution is about 13 14 m a less well determined constraint of about 10 m equivalent sea level rise is suggested for the barents kara ice sheet the inversion also indicates that a small amount of melt water from ice sheets far away from europe continued to be added into the oceans during late holocene time so as to raise the equivalent sea level by about 3 m during the past 6000 yr consistent with similar inversions of data from sites in the australian and pacific regions

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

crustal anisotropy beneath hudson bay from ambient noise tomography evidence for post orogenic lower crustal flow the crust underlying hudson bay canada records a long and complex tectonic history in this study we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle hudson bay the ambient noise data were processed to obtain group velocity dispersion measurements from 10 35 s period which were inverted using an algorithm that incorporates the effects of anisotropy this work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy the inversion method uses smoothing and damping to regularize the solution due to the significantly increased number of model parameters relative to the isotropic case we performed a careful analysis for parameter selection to determine whether leakage occurs between isotropic and anisotropic model parameters we observe a robust pattern of anisotropic fast directions in the mid crust that are consistent with large scale tectonic trends based on magnetic anomaly patterns in particular a distinctive double indentor shape for the superior craton is clearly expressed in both data sets this pattern breaks down deeper in the crust suggesting that some degree of lithospheric decoupling in the lower crust such as channel flow occurred during orogenesis given regional evidence for vertically coherent deformation in the crust and underlying mantle we interpret this pattern in the lower crust as a tectonic overprint that post dates the main phase of trans hudson deformation at most levels in the crust we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath hudson bay