extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

gravitational collapse of orogenic crust a preliminary three dimensional finite element study although gravitational collapse has been widely accepted as a viable explanation for both synorogenic and postorogenic extension its mechanisms have not been well understood previous models of continental deformation that assumed a viscous or viscoplastic rheology are not well suited for simulating stress states and brittle deformation within the upper crust where most extensional collapse occurs here we present preliminary results of a three dimensional 3 d finite element modeling of stress and faulting patterns within the orogenic crust using a viscoelastic rheology major parameters controlling orogenic extension including topographic loading tectonic compression basal shear thermal structure and 3 d tectonic boundary conditions are systematically explored in the model for typical tectonic compression 60 100 mpa and 2 3 km elevation the model predicts synorogenic extension in high plateaus and concomitant thrusting in the lowlands near the foothills viscous relaxation within the ductile crust may amplify deviatoric stresses within the brittle crust by a few times and stress amplification is greater when the crust is hotter and therefore the brittle crust is thinner most synorogenic extension occurs in the direction orthogonal to regional compression while postorogenic extension is more likely to occur normal to the trend of mountain belts we apply the model results to the tibetan plateau and suggest that the age of the north trending grabens is not a reliable proxy for the time when the plateau had reached it present mean high elevation conversely development of the east trending south tibetan detachment system in the miocene by gravitational collapse was more difficult and may require a higher elevation of the himalayas than present or much of the tibetan plateau to have remained low till late miocene

vertical averages of rheology of the continental lithosphere relation to thin sheet parameters abstract we calculate vertically averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional compressional and strike slip regimes we assume that brittle deformation occurs by byerlee s law and that ductile deformation occurs by steady state creep mechanisms under these conditions the vertically averaged rheology may be approximated by a single power law over the range of strain rates from 10 13 to 10 17 s 1 this is true not only when either brittle or ductile behavior dominates the strength of the lithospere but also when the contributions of brittle and ductile behavior are comparable the value of the power law exponent n and the strength of the lithosphere depend on the depth and stress difference at the brittle ductile transition the temperature at the moho and the geothermal gradient in the range of vertically integrated stress between 1012 and 1013 n m 1n reaches a minimum of between 5 and 10 under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior the strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere

a relative sea level history for arviat nunavut and implications for laurentide ice sheet thickness west of hudson bay abstract thirty six new and previously published radiocarbon dates constrain the relative sea level history of arviat on the west coast of hudson bay as a result of glacial isostatic adjustment gia following deglaciation sea level fell rapidly from a high stand of nearly 170 m elevation just after 8000 cal yr bp to 60 m elevation by the mid holocene 5200 cal yr bp the rate of sea level fall decreased in the mid and late holocene with sea level falling 30 m since 3000 cal yr bp several late holocene sea level measurements are interpreted to originate from the upper end of the tidal range and place tight constraints on sea level a preliminary measurement of present day vertical land motion obtained by repeat global positioning system gps occupations indicates ongoing crustal uplift at arviat of 9 3 1 5 mm yr in close agreement with the crustal uplift rate inferred from the inferred sea level curve predictions of numerical gia models indicate that the new sea level curve is best fit by a laurentide ice sheet reconstruction with a last glacial maximum peak thickness of 3 4 km this is a 30 35 thickness reduction of the ice 5g ice sheet history west of hudson bay

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

indentation of a continent with a built in thickness change experiment and nature orogens oblique to the direction of plate convergence are currently attributed to obliquity between the margins of one or both of the sutured continents to their direction convergence we use a single analogue experiment and natural examples to illustrate a potential additional factor variations in strength of the indented continent at a high angle to the convergence direction the wavelengths of structures in laterally shortened lithosphere depend on the strength of the most competent layers lateral variations in crustal thickness must therefore lead to structures oblique to any applied lateral compression an analogue experiment was performed to explore this phenomenon a two layer indented continent was modelled by a brittle upper crust of sand above a lower crust of high viscosity polymer floating on a single layer of low viscosity syrup representing the mantle the well known strike slip structures allowing lateral escape to distant weak boundaries were hindered by lateral boundaries in front of the indenter this allowed us to focus on the effects of a thickness change built into the indented continent along a zone parallel to the direction in which a vertical rigid wall advancing at a steady rate represented the indenter vertical escape led to an orogenic belt oblique to the advancing wall this obliquity influences subsequent lateral escape model scaling and interpretations are based on extended thin sheet approximation etsa and standard theories of faulting four sectors of the alpine himalayan orogen iran tunisia the eastern alps and the himalaya are oblique to the continental convergence direction and we point to thickness changes at high angles to the suture that may account for this geometry as crustal thicknesses north of oblique sectors of the himalayas are not yet known we speculate on them we infer from the main difference between our experiment and all our examples chosen from nature that vertical orogenic escape was oblique to our model suture but can be parallel to natural sutures

topography the geoid and compensation mechanisms for the southern rocky mountains the southern rockies of colorado are anomalously high elevations greater than 2800 m topographically rough implying active uplift and underlain by significant low velocity anomalies in the upper mantle that suggest an intimate relationship between mantle geodynamic processes and the surface topography the region is in isostatic equilibrium i e near zero free air gravity anomaly however the poor correlation between the high topography and crustal thickness makes the application of simple compensation models e g pure heiskanen or pratt hayford problematic knowledge of how the current topography of the rockies is isostatically compensated could provide constraints on the relative role of sublithospheric buoyancy versus lithospheric support here we evaluate the geoid and its relationship to the topography using the geoid to elevation ratio gtr in the spatial domain and the admittance in the frequency domain to constrain the mechanism of compensation we separate the upper mantle geoid anomalies from those with deeper sources through the use of spherical harmonic filtering of the egm2008 geoid we exploit the fact that at wavelengths greater than the flexural wavelength where features are isostatically compensated the geoid topography ratio can be used to estimate the depth of compensation and the elastic thickness of the lithosphere the results presented below indicate that the main tectonic provinces of the western united states have moderate geoid topography ratios between 3 5 and 5 5 m km 3 9 for the southern rockies 4 25 for the colorado plateau and 5 2 for the northern basin and range suggesting shallow levels of isostatic compensation in terms of the elastic thickness of the lithosphere our results indicate an elastic thickness of less than 20 km these value support the notion that a major portion of the buoyancy that has driven uplift resides at depths less than 100 km and that upper mantle processes such as small scale convection may play a significant role in the buoyant uplift of the southern rockies as well as other actively uplifting areas of the western united states further support for this hypothesis is provided by high coherence for the geoid topography relationship for nearly all wavelengths between 50 and 1000 km

paleolithospheric structure revealed by continental geoid anomalies abstract lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere estimates of local isostatic equilibration and potential energy in tandem with lithosphere related geoid anomalies can be used to estimate paleolithospheric thickness providing a clearer understanding of how and why continental topography is developed we employ several simplifying assumptions about the crustal and mantle lithosphere density and structure and readily acknowledge that our results are therefore first order approximations to predict the pre orogenic structure of the lithosphere at the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere in this way we are able to use independent measurement of lithospheric geoid anomalies current post orogenic elevation and lithospheric structure and paleoelevation information to estimate topographic development and structural support over time application of this technique to the southwestern united states indicates that the uplift of the colorado plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre orogenic southern basin and range was thinned relative to the northern basin and range and colorado plateau although we use the southwestern u s as an example this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood

seismic response to slab rupture and variation in lithospheric structure beneath the savu sea indonesia article i nfo variations in seismic moment release and stress state across the transition from subduction of oceanic crust to arc continent collision in the banda arc are constrained by focal mechanism solutions from the cmt earthquake catalogue in particular the slab under the western savu sea is unusual in that intermediate depth 70 300 km events indicate that at this depth range the slab is largely in down dip compression this contrasts with the intermediate depth down dip tension that typifies the sunda slab to the west and the far eastern banda slab to the east down dip compression beneath the savu sea reflects subduction of transitional crust of the scott plateau more buoyant than the indian ocean crust subducting further west in this region enhanced magma flux is indicated by unusually narrow volcano spacing in the overlying arc and suggests that down dip compression reflects not only more buoyant transitional crust but also a reduction in slab wedge coupling induced by enhanced magma flux east of the savu sea the near complete absence of intermediate depth seismicity is attributed to a slab window that has opened where australian continental crust has collided with the arc differences in seismic moment release around this slab window indicate asymmetric rupture propagating to the east at a much faster rate than to the west

stress modeling in the andes constraints on the south american intraplate stress magnitudes regions of high topography which exhibit both compressional and extensional deformation provide a natural laboratory to place constraints on horizontal intraplate stress magnitudes an elastic finite element analysis of the lithospheric stress field in the cordillera bianca region of peru has been used to evaluate the lithospheric stress state and constrain the south american intraplate stress magnitudes the orthogonal orientation of the andean high topography to the east west regional stress field in conjunction with the well documented change in deformation style from compression to extension across the cordillera provides a unique setting for the stress modeling a two dimensional finite element grid consisting of an assembly of isotropic elastic quadrilateral elements in a state of plane strain was used to represent the lithosphere an isostatically balanced crustal density model was used to calculate the stresses associated with the lateral density variations between the andean topography and the brazilian shield in some models horizontal stress was concentrated at midcrustal depths by including vertical variation in young s modulus the modeling results indicate bounds of 10 and 75 mpa for the magnitude of the average horizontal stress averaged over a 100 km thick lithosphere a preferred value of 25 mpa is obtained on the basis of fitting all of the data for the rheological models considered and predicting a transition from extension to compression near the 3000 m elevation contour such a far field average stress is consistent with a ridge push origin because the modeling performed in this study assumes that the topographic load is entirely compensated by the buoyancy of the deflected moho these estimates represent upper bounds the effect of incorporating thermal support for the topographic load of the cordillera bianca would be to further reduce our estimate of the intraplate stress magnitudes analysis of the distribution of calculated stress with depth indicates that previous assumptions about the vertical plane shear stress within the lithosphere being negligible are acceptable for the geometry of the andes

mesoscale oceanographic features associated with the central bering sea ice edge february march 1981 midwinter 1981 observations from the central bering sea shelf showed a hydrographic structure which was two layered in temperature salinity and density this structure was confined to a 100 km wide band which coincided approximately with the ice edge the colder lower salinity upper layer was continuous in its t s properties with homogeneous water to the north beneath the ice the warmer more saline lower layer was similarly continuous with water to the south near the shelf break a northwestward baroclinic flow of the surface relative to 75 dbar was associated with the layered structure the layered structure and associated flow appear to be regular winter features associated with the ice edge in this region input of low salinity water from melting of ice along the edge is adequate to maintain the layered structure and its associated baroclinic circulation against the effects of tidal and wind mixing the resulting observed oceanographic features were similar to those which have been found in association with midshelf and shelf break fronts in other regions and appear to define a dynamically controlled boundary between the cold low salinity water on the northern shelf and the warmer more saline water near the shelf break

problems in the extrapolation of laboratory rheological data abstract the many types of variables and deformation regimes that need to be taken into account in extrapolating rheological behaviour from the laboratory to the earth are reviewed the problems of extrapolation are then illustrated with two particular cases in the case of divine rich rocks recent experimental work indicates that within present uncertainties of extrapolation the flow in the upper mantle could be either grain size dependent and near newtonian or grain size independent and distinctly non newtonian both types of behaviour would be influenced by the present of trace amounts of water in the case of quartz rich rocks the uncertainties are even greater and it is still premature to attempt any extrapolation to geological conditions except as an upper bound the fugacity and the scale of dispersion of the water are probably two important variables but the quantitative laws governing their influence are not yet clear

variation of upper layer dynamics during breakup of the seasonal ice cover in hudson bay the present study describes circulation and stratification changes associated with the melt and breakup of the seasonal ice cover in the coastal waters of southeast hudson day field work was carried out at a station located 25 km north of the great whale river buoyancy fluxes and dissipation rates were calculated as well as changes in potential energy surface velocity data were partitioned into frequency bands and complex demodulated throughout the sampling period most of the current energy was found to be in the semi diurnal tidal band after ice breakup however low frequency and inertial motions became relatively more important in response to direct wind forcing at the sea surface changes in amplitudes and phases of the major tidal constituents occurred and are related to the presence of the sea ice cover between early april and mid june semi diurnal current amplitude doubled while its phase shifted by 45 to 60 degrees in early june the ice cover was sufficiently dispersed to allow the surface turbulence to overcome the buoyancy flux and mix the upper water column

mechanisms of crustal deformation thirty sixthwilliam smith lecture the energy source for crustal deformation is isotopic heating and secular cooling of the mantle in a true solid heat would be lost to the surface by conduction however solid state creep processes allow the earth s solid mantle to exhibit a fluid behaviour thus thermal convection can convert heating into directed motion variations in temperature lead to variations in densitythrough thermal expansion and contraction although the resultant body forces are vertical horizontal variations in temperature lead to horizontal body forces at an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge the result is a substantial horizontal ridge push force this force along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics lithospheric thinning must occur beneath continental rifts and plateau uplifts i suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust mantle boundary this penetration may also cause lithospheric stoping and plateau uplifts lithospheric thinning leads to large tensional stresses above the zone of thinning these forces are generally much larger than the compressional forces generated by crustal thinning the forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts in island arcs and andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces probably the most complex zone of crustal deformation is the continental collision variations in types of continental collisions are discussed

some isostatic and thermal consequences of the vertical strain geometry in convergent orogens abstract the thermal and isostatic consequences of continental deformation particularly surface elevation horizontal buoyancy forces and metamorphism in the crust depend to a considerable extent on the way in which strain is distributed between the crust and the mantle lithosphere 1 since these vertical strains may be strongly decoupled by processes such as convective thinning or detachment of the lower thermal boundary layer 2 with the extent of this decoupling varying in space and time in an orogen 3 it is useful to parameterise the vertical strain on the scale of the crust c and lithosphere 1 respectively we illustrate this parameterisation by considering the thermal and isostatic consequences of a number of c 1 paths which may develop during the convergent deformation of a thermally stabilised lithosphere three qualitatively distinct c 1 paths in orogens are distinguished on the extent of decoupling between vertical strains in the crust and mantle lithosphere type 1 paths involve no decoupling so the vertical strain in the lithosphere is homogeneous for a given c type 1 paths result in relatively low surface elevation and low intermediate metamorphic temperatures appropriate for the development of glaucophane schist and barrovian metamorphic assemblages type 2 paths allow decoupling due to processes such as convective thinning of the mantle lithosphere on the orogenic timescale but only after considerable initial lithospheric thickening type 2 paths lead to relatively high surface elevations for a given c and intermediate to high metamorphic temperatures only after the mantle lithosphere is thinned the increase in potential energy associated with mantle lithospheric thinning resulting in an increase in buoyancy forces by as much as 10 13 nm 1 will quickly terminate convergent deformation with the result that metamorphic heating will postdate the thickening deformation and may induce extensional collapse of the orogen type 3 paths involve efficient mantle lithosphere thinning as the crust thickens with a consequence being particularly the possibility of relatively high metamorphic temperatures during active crustal thickening for a given driving force convergent deformation along a type 3 path will be terminated at relatively small c providing an appropriate setting for the development of high t low p metamorphic assemblages

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

thermal control on post orogenic extension in collision belts abstract intracontinental extension develops in two different ways in the rhine graben the baikal or the east african rifts it is localized in a long and narrow zone along a few normal faults which often delineate a single graben in contrast in the basin and range province or the tibetan plateau it is distributed over a much broader area up to more than 1000 km across this difference arises from differences in rheology and applied forces widespread extension probably reflects the thinning of a previously thickened crust geological observations indicate that only wide orogenic belts undergo late or post orogenic thinning a related fact is that anatectic granites seem to be most abundant in wide orogenic belts the general correlation between belt width and occurrence of both extension and anatectic granites suggest that it reflects in part the characteristics of horizontal heat transfer between thickened crust and surroundings in wide orogenic plateaux the large amount of heat produced in the crust by radioactive decay and lack of erosion leads to high temperatures in narrow mountain belts horizontal heat transfer and high erosion rates concur to prevent temperatures from rising to values at which the crust may yield in extension we have tested this hypothesis using transient two dimensional numerical models of orogenic belts focussing on the effects of the shortening rate the final width of the belt and the rate of crustal radiogenic heat production in the range of geological values the shortening rate has little influence on the results crustal temperatures increase as a function of belt width we find that the sensitivity of crustal temperatures to belt width is most pronounced in the 100 300 km width range and for large values of crustal heat generation the pressure temperature paths followed by geological formations depend on the belt width which is consistent with available data the integrated shear strength of the lithosphere decreases with time as the belt is being built and then eroded this evolution is sensitive to belt width if extension occurs when the integrated shear strength drops below a critical value our results yield a framework for comparing different orogenic belts we predict that extension starts late in the history of wide belts and never occurs in narrow belts a final implication of this study is that the average rate of crustal heat production is probably close to 1 1 10 6 w m 3 which is in the upper range of possible values we present a simple and convenient diagram summarizing the thermal characteristics of orogenic belts as a function of width and shortening rate

a rapid method to map the crustal and lithospheric thickness using elevation geoid anomaly and thermal analysis application to the gibraltar arc system atlas mountains and adjacent zones we present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness the main assumptions are local isostasy and a four layered model composed of crust lithospheric mantle sea water and the asthenosphere we consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle we perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of rms error of elevation and geoid anomaly databases the application of this method to the gibraltar arc system atlas mountains and adjacent zones reveals the presence of a lithospheric thinning zone sw ne oriented this zone affects the high and middle atlas and extends from the canary islands to the eastern alboran basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western betics eastern rif rharb basin and gulf of cadiz a number of different even mutually opposite geodynamic models have been proposed to explain the origin and evolution of the study area our results suggest that a plausible slab retreating model should incorporate tear and asymmetric roll back of the subducting slab to fit the present day observed lithosphere geometry in this context the lithospheric thinning would be caused by lateral asthenospheric flow an alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the canary island and extending as far as central europe

seasonal current variations observed in western hudson bay yearlong current meter data from western hudson bay show that the annual ice cover modifies the currents currents observed at three depths are dominated by barotropic semidiurnal tidal currents which reach amplitudes of 28 cm s 1 during the ice covered season the tidal currents and tidal heights observed inshore decrease and their arrival times are advanced monthly mean currents set predominantly to the south and east in accordance with the cyclonic circulation pattern of the bay mean current speeds are less than 4 cm s 1 and are made up of wind driven and density driven components their onshore offshore components vary in phase with the ice buildup and ice breakup seasons of the annual ice cover storm generated inertial currents are as strong as the tidal currents but rotate clockwise opposite to the tidal current direction they decrease with depth and are absent during the ice covered season inertial currents below the pycnocline lead those in the surface mixed layer by more than 180 simulating an upward phase velocity passing weather systems also generate 5 to 6 day periodic currents which slightly decrease but are coherent with depth they reach magnitudes of up to 25 cm s 1 and occur throughout the year regardless of the ice condition

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

a conceptual model of an arctic sea 1 we propose a conceptual model for an arctic sea that is driven by river runoff atmospheric fluxes sea ice melt growth and winds the model domain is divided into two areas the interior and boundary regions that are coupled through ekman and eddy fluxes of buoyancy the model is applied to hudson and james bays hjb a large inland basin in northeastern canada for the period 1979 2007 several yearlong records from instruments moored within hjb show that the model results are consistent with the real system the model notably reproduces the seasonal migration of the halocline the baroclinic boundary current spatial variability of freshwater content and the fall maximum in freshwater export the simulations clarify the important differences in the freshwater balance of the western and eastern sides of hjb the significant role played by the boundary current in the freshwater budget of the system and its sensitivity to the wind forcing are also highlighted by the simulations and new data analyses we conclude that the model proposed is useful for the interpretation of observed data from arctic seas and model outputs from more complex coupled climate models

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

thick skinned versus thin skinned thrusting rheology controlled thrust propagation in the variscan collisional belt the southeastern bohemian massif czech republic austria the variscan nappe assembly within the southeastern bohemian massif includes 1 crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels 2 nappe assembly of footwall units under very low to low grade metamorphic conditions and 3 frontal thin skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate thick skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin skinned tectonic character of foreland units 40ar 39ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during late variscan thrusting between 340 ma and 325 ma the 40ar 39ar mineral ages together with previously published 40ar 39ar and sm nd data record systematically decreasing variscan mineral ages structurally downward into footwall units this is explained by rapid exhumation and cooling synchronously with thrusting along crustal scale ramps during foreland directed nappe propagation the location of decollement zones within specific structural levels was apparently controlled by crustal scale rheological con trasts late variscan cooling ages contrast with cadomian 40ar 39ar hornblende and muscovite ages recorded within the structural basement immediately below the major variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during variscan thrusting these record a sequence of cadomian magmatic and metamorphic events in foreland units including 1 pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions 610 ma 2 greisen formation during a cadomian collisional event 600 ma and 3 intrusion of calcalkaline granitoid bodies with i type affinities 590 ma

intraplate stresses and plate driving forces in the philippine sea plate we use a spherical shell elastic finite element analysis to characterize intraplate stresses and plate driving forces in the philippine sea plate our finite element mesh is comprised of 489 nodes and 914 elements and provide a spatial resolution of about 1 the philippine sea plate is unusual in that it does not have large numbers of seismic or tectonic stress indicators and there are no midplate borehole stress data which can be used as constraints in this finite element study however it does possess a unique boundary configuration which potentially can lend more insight to plate forces and stresses than can similar studies on plates with more seismically active interiors because the plate s boundary consists primarily of subduction zones with roughly equal proportions of boundary on the overlying and downgoing sides of trench the resultant plate stress field is very sensitive to subduction related forces as a result borehole stress measurements in certain parts of the plate would be useful in further constraining these forces we find that trench pull forces can explain a large component of the velocity of the philippine sea plate however trench pull has somewhat less overall effect on the stresses across the plate than do the lateral density and topographic variations within the plate i e gravitational or potential energy force and trench suction forces are probably about an order of magnitude smaller than the net trench pull forces collisional forces may dominate the stress fields near taiwan and the izu peninsula but they are of only secondary importance in determining the motion and deformation of the philippine sea plate the forces that generate stress orientations most closely matching the observed stress indicators produce a lithospheric stress field across the plate that averages about 15 mpa over a 70 km thick lithosphere scaling inversely with lithosphere thickness values up to four times as great are produced locally

dynamical evaluation of the surface circulation using remote sensing of drifting ice floes an analysis of ice drift from consecutive landsat multispectral scanner images of the same area taken at 24 h interval allowed the calculation of the surface circulation using a dynamical approach comparison between calculated and measured values shows that this method leads to a good approximation of the surface circulation in a region where ice floes are free drifting on the other hand the sole use of the floe drift over time to evaluate the currents leads to results that are far from the measured values tests made using various air and ice drag coefficients and wind speeds and directions for different drift distances show that the error associated with these parameters is small although the values obtained using the numerical data involved considerably more effort and expense the results were not significantly better than those found using photographic products a good approximation of the surface circulation can thus be made using a simple digitization of the floe positions on photographic products application of the methodology to a set of four image pairs led to the recognition of two modes of circulation in southeastern hudson bay in spring 1985 this method can thus become a useful tool for rapid evaluations of surface currents in ice covered areas that can hardly be sampled otherwise

modeling of dynamic uplift denudation rates and thermomechanical consequences of erosion in isostatically compensated mountain belts in this paper we present a simple one dimensional model to examine the interplay between dynamic uplift and denudation in response to simultaneous surface erosion and lithospheric deformation under the assumption that the surface elevation of a mountain belt is isostatically compensated and the lithosphere undergoes vertically homogeneous deformation we also discuss the dynamics of isostatically compensated mountain belts in relation to surface erosion based on a brace goetze lithospheric model consisting of a quartz dominated crust and an olivine dominated upper mantle the mechanical strength of such a lithosphere is governed by a combination of brittle failure and power law or dorn law creep assuming a simple linear dependence of erosion rate on elevation we find that contrary to previous suggestions the effect of high denudation rates on the mechanical strength of the lithosphere does not involve significant weakening of the lithosphere

contribution of deviatoric stresses to metamorphic p t paths an example appropriate to low p high t metamorphism p t paths for a simple situation appropriate to many low p high t lpht terranes in which metamorphism and deformation are localized by advection of heat in magmas has been modelled assuming a medium with a power law rheology with an inverse exponential dependence of stress and temperature and capable of sustaining deviatoric stress in the order of 100 mpa at 400d c and strain rates of up to 10 13s 1 numerical simulations and analytical approximations for p t histories appropriate for simple convergent deformation histories show that the destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic temperature peak moreover for a specified strain rate and temperature evolution p t paths may vary from clockwise to anticlockwise merely as a function of vertical distance from the heat source inasmuch as mounting independent evidence suggests that the crust can support deviatoric stresses of up to about 100 mpa at temperatures of 400 500d c and that the shear strength of the crust is strongly temperature dependent in the range 400 800d c these results suggest that caution should be taken in the tectonic interpretation of p t paths involving decompression of the order of 100 mpa or less in lpht terranes the results illustrate a plausible mechanism for the close spatial association of both clockwise and anticlockwise p t paths documented in some lpht terranes

on uplift and exhumation during convergence a simple kinematic model is used to illustrate the vertical motions of rocks during convergent deformation and erosion at the surface it is shown that rocks may move upward or downward in the crust depending on the relative rates of erosion and thickening and depending on their initial depth in the crust exhumation during thickening can only occur if rapid denudation accompanies the thickening process during homogeneous thickening with erosion that is elevation dependent the initial depth from which rocks can be exhumed is only determined by the density distribution in the column and is independent of erosion or thickening rates the maximum initial depth from which rocks can be exhumed is of the order of 30 km or less there is therefore no conflict in the observation of synchronous exhumation and convergent deformation if the peak pressure of rocks is equivalent to no more than 30 km of burial it is also shown that uplift defined as vertical motion of the surface with respect to a reference level for example the geoid and exhumation defined as vertical motion of rocks with respect to the surface may follow different patterns in time and that the difference between the two evolutions may be a useful indicator of the exhumation process the model serves to emphasize the important differences between uplift and exhumation which are often not distinguished in the literature

some remarks on high temperature low pressure metamorphism in convergent orogens many high temperature low pressure high t low p metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating cooling cycle such terranes are not readily explained as the conductive response to crustal thickening since the resulting moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric scale strains must greatly outlast the deformation consequently high t low p metamorphism may be generated during crustal thickening only when significant heat is advected within the crust as for example may occur during the segregation of granitic melts we show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially for example by removing the mantle part of the lithosphere a simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust c and the lithosphere 1 respectively allows the potential energy of the evolving orogen to be readily evaluated using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high t low p metamorphism during crustal thickening with c1 is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations fc f1 for the same initial lithospheric configuration

from lithospheric thickening and divergent collapse to active continental rifting 1 abstract 32 introduction 33 modelling approach main assumptions and simpli cations 53 1 tectonic thickening 63 2 erosion sedimentation 63 3 thermal relaxation 73 4 gravity driven ow 74 physical model 74 1 temperature pro le 84 2 density pro le and surface elevation 84 3 gravitational force 94 4 strength pro le 104 5 lithospheric geometry through time 105 presentation of the results 115 1 stable thermal boundary layer during convergence 115 2 post convergence removal of the thermal boundary layer 126 discussion and conclusion 137 acknowledgements 158 references 162

origins of the european regional stress field abstract we present a study of the origins of the european regional stress field intraplate stresses due to various plate boundary forces and intraplate stress sources were predicted using an elastic finite element analysis the finite element mesh consists of about 1629 nodes in a network of 3117 triangular elements which provide a spatial resolution of about 1 the study builds on the previous modelling studies through consideration of a wide range of plausible forces acting along the collisional eastern and southern plate boundaries and importantly through the inclusion of forces due to other lateral density variations within the lithosphere such as those associated with the continental margins and high topography the modelling was constrained by more than 1800 stress indicators from the world stress map project which provide information about the orientation of the observed maximum horizontal compressive stress sh the torque associated with the ridge push force is well constrained and the principal source of uncertainty relates to the boundary conditions used to represent the collisional resistance acting along the eastern and southern boundaries a gradient in the collisional resistance increasing in magnitude from west to east is assumed to act along the southern plate boundary whereas the eastern boundary is modelled as a pinned margin at 46 e the sh orientation and magnitude of the regional stress field was found to be largely invariant to the boundary conditions used to represent the tectonic forces acting along the eastern and southern margins and is characterized by a nearly uniform nw se compression with a magnitude in the range of 10 20 mpa averaged over a 100 km thick lithosphere lateral density variations within the continental areas of europe were found to reduce the magnitude of the predicted stresses but do not have a significant effect on the orientation of the regional stress field

heat sources of cretaceous metamorphism in the eastern alps a discussion abstract the alpine orogenic cycle is characterised by a series of early metamorphic events that occurred in the cretaceous the eoalpine events a collection of data on peak metamorphic conditions pt paths and on relationships between structure and metamorphism for these events in the eastern alps shows that 1 eoalpine metamorphism occurred largely simultaneously with pervasive deformation 2 the eoalpine thermal peak was reached near the pressure peak 3 lower grade structurally higher rocks reached their metamorphic peak earlier than higher grade structurally lower rocks and there is a wide spread of metamorphic ages 4 low grade metamorphic conditions are probably more perturbed than high grade conditions and 5 there is a conspicuous regional distribution of metamorphic grade with amphibolite and eclogite facies metamorphism occurring predominantly in two regions located symmetrically west and east of the tauern window possible heating mechanisms are discussed that can satisfy these observations and a review of previous thermal models is provided it is shown that models which rely on conductive heat transfer cannot account for all of these observations in order to assess other heat sources the effects of shear heating are evaluated here with simple model calculations and a straightforward comparison of length and time scales of dissipative heat production the results suggest that shear heat episodically released during the stacking of the austroalpine nappe pile may be a significant contributor to the observed thermal perturbation

sedimentary thickness variations and deformation intensity during basin inversion in the flinders ranges south australia abstract the central and northern parts of the adelaide fold belt in the flinders ranges south australia consist of a sequence of neo proterozoic cambrian sediments overlying a meso proterozoic basement complex both of which were mildly deformed in an intracratonic setting during the 500 ma delamerian orogeny the fold belt lies within a prominent heat flow anomaly average heat flows of 90 mwm 2 reflecting extraordinary enrichments in heat producing elements in the meso proterozoic basement suggesting that anomalous thermal regimes may have been significant in localising delamerian deformation however spatial variations in deformation intensity correlate more closely with variations in the thickness of the sedimentary sequence than with observed variations in heat flow suggesting that the thickness of the sedimentary blanket plays a crucial role in localising delamerian deformation during basin inversion we use simple numerical models of lithospheric strength to investigate the potential role of sedimentary thickness variations on the distribution and style of deformation focussing on the impact of a variable thickness sediment pile deposited above a radioactive basement we show that for thermal parameters appropriate to the flinders ranges moho temperatures may vary by 25 30 c for every additional kilometre of sediment for a brace goetze lithospheric rheology controlled by a combination of temperature dependent creep processes and frictional sliding the observed variations in thickness of the sedimentary pile are sufficient to cause dramatic reductions in the vertically integrated strength of the lithosphere by many orders of magnitude thereby providing a plausible explanation for observed correlation between sediment thickness and deformation intensity during basin inversion

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

the role of gravitational potential energy in active deformation in the southwestern united states in orogenic belts intrinsic buoyancy and extrinsic boundary forces act on the continental lithosphere determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation in the western united states estimates of gravitational potential energy show that given available constraints on rheology buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation including absence of deformation this underscores the importance of gravitational potential energy for deformation in orogenic belts

current variability under landfast sea ice in lutzow holm bay antarctica current measurements were made under landfast sea ice in lutzow holm bay antarctica in 1991 in spite of no direct wind forcing and negligible thermohaline forcing currents with a magnitude of 0 1 0 3 m s 1 were observed it is suggested that current variability strongly depends on the extent of the landfast ice on the eastward continental shelf before the fast ice edge extends to the shelf break the current at ongul strait located near the continental coast is highly coherent with the wind with a time lag of 1 1 5 days and somewhat coherent with the adjusted sea level with no time lag at periods of 6 8 days after the fast ice edge is stably located along the shelf break the current is not at all coherent with the wind or sea level we infer that the variability at periods of 6 8 days is mainly due to the first mode shelf wave excited by the wind stress over the eastward continental shelf the direction of the prevailing current at ongul strait changed drastically from southward the same direction of the wind to northward in may although the prevailing wind direction did not change this change in timing coincides with the rapid development period of the fast ice extent

isopycnal deepening of an under ice river plume in coastal waters field observations and modeling 1 the great whale river located on the southeast coast of hudson bay in canada forms a large river plume under complete landfast ice during early spring short term fluctuations of plume depth have motivated the present numerical study of an under ice river plume subject to tidal motion and friction we introduce a simple two layer model for predicting the vertical penetration of the under ice river plume as it propagates over a deepening topography the topography is idealized but representative friction on the bottom surface of the ice cover on the seabed and at the plume interface is parameterized using the quadratic friction law the extent of the vertical penetration is controlled by dimensionless parameters related to tidal motion and river outflow model predictions are shown to compare favorably with under ice plume measurements from the river mouth this study illustrates that isopycnal deepening occurs when the ice cover vertical motion creates a reduced flow cross section during the ebbing tide this results in supercritical flow and triggers the downward plume penetration in the offshore for a given river discharge the freshwater source over a tidal cycle is unsteady in terms of discharge velocity because of the variation in the effective cross sectional area at the river mouth through which freshwater flows

on the relationship between surface uplift and gravitational extension this paper highlights the relationship between isostatically supported surface elevation and the lateral buoyancy forces of an orogen the former is a linear function the latter is a quadratic function of the thickness of the lithosphere thus surface elevation and lateral buoyancy forces may have differing evolutions in the development of an orogen indeed during the evolution of collisional orogens the qualitative relationship between these two parameters may reverse so that only in the early orogenic evolution topographically high regions exert a net lateral buoyancy force on topographically low regions later in the evolution this may change to a counterintuitive scenario in which there is a net lateral buoyancy force from the topographically lower regions toward the topographically higher regions to illustrate this fact we use a simple one dimensional kinematic model for an orogen which undergoes simultaneous thickening due to convergence and erosion at the surface it can be shown that for reasonable erosion and thickening rates surface elevation increases through time until a steady state is reached while the lateral buoyancy force changes from positive to negative giving rise to the inward collapse of an orogen in the field this may be recognized as the occurrence of two successive coaxial convergent deformation events present day examples may be the formation of back arc basins where extension in the low lying hinterland of an andean type mountain belt is caused by a gravitational compressive force exerted by the hinterland onto a belt of significantly higher surface elevation but possibly of lower potential energy

on the stability of isostatically compensated mountain belts the formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence the evolution of potential energy stored in an orogen is dependent on 1 the density structure 2 the thermal evolution and 3 the way convergent deformation is partitioned between crust and mantle lithosphere it is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension or collapse may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened e g england 1987 in this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the brace goetze model for the rheology of the lithosphere the reference lithosphere defined to be in potential energy and isostatic balance with the mid ocean ridges changes with different thermal parameters of the lithosphere the instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts measured relative to the reference state with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range it is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates greater than a few times 10 16 s 1 and leading to significant finite extension providing the moho temperatures exceed about 650 700 c a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust consequently processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse the calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening i e fl fc 2 where fl and fc are the lithospheric and crustal thickening factors

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

reply by the author to t r lafehr and k c chan discussion on the normal vertical gradient of gravity geophysics 48 1011 1031 july 1983 discussion geophysics 51 1505 1508 july 1986 frequently geophysical methods can be useful to those without detailed knowledge of physics buried conducting objects can be found by a beachcomber using only a crude metal detector conducting ore bodies can be located with an em instrument distinct basement units can be mapped on an aeromagnetic map as magnetic terrains and salt domes are apparent on gravity maps of the gulf coast in all of these cases anomalous values of some measurement stand out distinctly against their background values however this is not the case with ground gravity surveys in areas of high topographic relief where elevation corrections can be far greater than the target anomalies

mantle lithospheric deformation and crustal metamorphism with some speculations on the thermal and mechanical significance of the tauern event eastern alps changes in the thickness of the mantle lithosphere beneath convergent orogens through processes such as the pervasive thickening of the lithosphere and the convective thinning of the thermal boundary layer dictate the heat flow through the base of the crust and thus may profoundly influence the thermal evolution of crustal metamorphic terrains calculations based on one dimensional thermal energy balances show that when mantle lithosphere is substantially thickened and remains intact through the orogenic cycle then low temperature facies series including blueschist and eclogite facies may be preserved throughout the crust in contrast crust thickened above attenuated mantle lithosphere will develop much higher temperature facies series potential energy arguments suggest that changes in the thickness in the mantle lithosphere induce changes in the elevation and the potential energy stored within the lithosphere therefore the response to mantle lithospheric deformation observable in crustal metamorphic terrains should not only be recorded in the thermal regime but also in changes in the incremental strain history in as much as it reflects the force balance operating within the orogen for example mantle lithospheric thinning beneath thickened crust may cause an increase in crustal temperatures synchronous with the termination of convergent strain or for large reductions in mantle lithospheric thickness with the onset of extensional deformation such histories are recorded in a number of young metamorphic terrains for example the eastern alps which may provide an important thermal and isostatic record of the mantle lithospheric response to convergent deformation here we show how application of these ideas may lead to new insights into some outstanding problems concerning the thermal and mechanical evolution of the eastern alps during the tertiary tauern metamorphic event

finite element modelling of stress patterns along the mid norwegian continental margin 62 to 68 n the inferred nature of the regional intraplate stress field off the coast of norway is based primarily on borehole breakout information and earthquake focal mechanisms the stress indicators reveal a slightly clockwise rotation of the orientation of the principal horizontal stress sh to the northwest with respect to the dominant e w orientation observed in the northern north sea the stress regime is predominantly strike slip and thrust fault although two normal faulting events have occurred on the shelf and shelf edge within the mid norwegian margin segment 62 to 68 n the tr0ndelag platform exhibits large variations of observed sn orientations this may be due to an almost isotropic horizontal stress state sh sh resulting from the superposition of local and regional stress sources this paper presents the predicted tectonic stresses along the mid norwegian margin based on a 3 d finite element analysis the two principal modelling objectives are to elaborate the effects of 1 changes in the regional stress magnitude sh and 2 variations in rock properties i e young s modulus on the horizontal stress anisotropy hsa of the v0ring basin and the tr0ndelag platform the model incorporates simplified mainland topography the tr0ndelag platform the voring basin the v0ring plateau and the lofoten basin our modelling results indicate that horizontal stress anisotropy hsa and the regional sh magnitude within the various structural units is dependent on their strike with respect to the orientation of the present day regional stress field specifically we find 1 a near isotropic horizontal stress magnitude configuration for the southern part of the v0ring basin where the gradient of topography across the mid norwegian margin is oriented oblique to the regional trend of sh 2 a maximum horizontal stress anisotropy hsa in the southern part of the tr0ndelag platform this local hsa maximum bordering the hsa minimum of the voting basin 3 a narrow band of high hsa at the eastern edge of the tr0ndelag platform imposed by compressive gravitational stresses related to the elevated topography of mainland norway 4 a low hsa at the western margin of the tr0ndelag platform related to spreading stresses associated with the topographic lowering towards the v0ring basin and 5 in case of a far field stress of sh 1 1sh the modelling results predict exchanged principal horizontal stresses sh and sh at the western margin of the tr0ndelag platform

on the modification of tides in a seasonally ice covered sea 1 new observations from eight moorings located in foxe basin hudson strait and hudson bay are used to study the seasonal variability of the m 2 tide significant seasonal variations of the m 2 surface elevation are found in all these regions and at all seasons the largest variations occur during winter while both elevation increase hudson strait and decrease hudson bay foxe basin are observed these variations are found recurrent at the stations where multiyear observations are available observations from a velocity profiler are consistent with a seasonal damping of the tides because of friction under ice numerical simulations with a sea ice ocean coupled model and realistic forcing qualitatively reproduce most of the features of the observed variability the simulations show that the winter m 2 variations are essentially caused by the under ice friction albeit with strong regional differences under ice friction mostly occurs in a limited region foxe basin and can account for both increased and decreased m 2 elevations during winter

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia

plate scale potential energy distributions and the fragmentation of ageing plates abstract geoid anomalies associated with mid ocean ridge systems and a number of continental margins imply that on the scale of individual plates the old ocean lithosphere represents a gravitational potential energy sink since lateral variations in potential energy contribute to deviatoric stresses in the lithosphere the changing potential energy distributions in individual plates associated with the growth and ageing of the oceanic lithosphere may be expected to result in changes in the intraplate stress field analytical models for simple plate geometries using lithospheric density models consistent with small positive 6 m geoid anomalies across continental margins show that the growth of oceanic lithosphere over a period of 200 ma may contribute to a decline in the mean plate potential energy u p of about 1 10 12 n m 1 and thus contribute a mean extensional stress difference zz xx in continental lithosphere of up to about 8 mpa averaged over a 125 km thick lithosphere these estimates are sensitive to the assumed mean continental potential energy u c about which there is some uncertainty for higher u c approaching that of the mid ocean ridges u mor the net decline in u p may be as much as 1 7 10 12 n m 1 whereas for significantly lower u c approaching that of old ocean lithosphere plate growth may increase u p transiently by up to 2 7 10 12 n m 1 leading to compression in the continents in the african and antarctic plates the ageing of the ocean lithosphere since the late jurassic is estimated to have contributed to a decline in u p of about 0 6 10 12 and 0 95 10 12 n m 1 respectively contributing a mean stress difference of about 5 mpa and 7 5 mpa in the respective continents whereas the predicted stress changes associated with ageing of the oceanic lithosphere are significantly smaller than most laboratory based estimates of the extensional strength of the lithosphere they may provide an important contribution to the stress fields that eventually lead to the fragmentation of ageing plates

the combined inversion of seismological and goce gravity data new insights into the current state of the pacific lithosphere and upper mantle abstract in this study we combine seismological and goce satellite gravity information by using a bayesian like technique with the aim of inferring the density structure of the pacific 90 n 90 s 121 e 60 w lithosphere and upper mantle we recover a 1 1 3 d density model down to 300 km depth which explains gravity observations with a variance reduction of 67 41 the model with an associated a posteriori standard deviation provides a significant contribution to understanding the evolution of the pacific lithosphere and answers to some debated geodynamic questions our methodology enables us to combine the recovery of density parameters with the optimum density v sv scalings the latter account for both seismological and gravity observations in order to identify the regions characterized by chemically induced density heterogeneities which add to the thermally induced anomalies chemically modified structures are found west of the east pacific rise epr and are of relevant amplitude both below the north western side of the pacific plate at the base of the lithosphere and up to 100 km depth beneath the hawaiian and super swell regions thus explaining the anomalous shallow regions without invoking the thermal buoyancy as the sole justification coherently with the chemically modified structures our results a support a lighter and more buoyant lithosphere than that predicted by the cooling models and b are in favor of the hypothesized crustal underplating beneath the hawaiian chain and beneath the volcanic units in the southern branch of the super swell region the comparison between calculated mantle gravity residuals and residual topography a suggests a lateral viscosity growth associated with the increasing thickness and density of the plate and b correlates well with sub lithospheric mantle flow from the epr towards west up to the kermadec and tonga trench in the south and the kuril kamchatka trench in the north

pulsating continents on venus an explanation for crustal plateaus and tessera terrains we propose that tessera terrains on venus represent continental crust that does not participate in the periodic recycling of the lithosphere through global subduction events we have studied the force balance on the boundary of a continental area that survives a global subduction event using an analytical model in the proposed model the ratio between the crustal and lithospheric mantle thicknesses controls the force balance if the crust thickness is less than 2 5 of the lithospheric mantle thickness the continental area will be compressed but if the crustal thickness is higher than 2 5 of the lithospheric mantle thickness the continental area will spread out and collapse consequently if the lithospheric mantle beneath a continental region is delaminated during a global subduction event the continent will collapse generating tessera inliers dominated by extensional tectonics but if a significant portion of lithospheric mantle remains then the continental area will be compressed generating a plateau by crustal shortening the observed plateau heights can be explained by this model a 2 km height plateau can be generated by a lithospheric mantle thickness of 40 km while a 4 km height plateau can be generated by a 90 km thick lithospheric mantle we have modelled this crustal thickening of a continental area by tectonic contraction using a thin viscous sheet approach with a newtonian viscosity for the crust the force from a hot mantle elevated during a global subduction event is enough to build up a plateau by compression in 50 ma using a viscosity for the continental crust of 1021 pa s and 200 ma for 5 1021 pa s during this compressional stage concentric fold and thrust belts are generated in the plateau continent erasing any impact craters that were present the subsequent stabilization of a new crust and lithosphere in the surrounding mantle changes the force balance allowing a moderate gravitational collapse of the plateau continent accommodated by radial grabens the pulsating continent model links for the first time the generation of crustal plateaus and the origin of the volcanic plains predicting the observed equivalent effective crater density for both terrains