three dimensional lithospheric deformation and gravity anomalies associated with oblique continental collision in south island new zealand summary isostatic considerations exhibit differences between the northern central and southern parts of the pacific australian plate collision in south island new zealand in the northern part mean elevations are moderate and the gravity low is small the central part contains the highest elevations and gravity and elevations correspond to each other relatively well and in the southern part the gravity low is strongest whereas the mean elevations are moderate again these differences indicate changes in the character of the isostatic compensation and are explained by increased thickening and widening of the crustal root from north to south and also by the long wavelength gravity response to a mantle density anomaly that increases towards the south a simple 3 d gravity model is derived that includes the detailed crustal structures from the south island geophysical transect sight experiment as well as a high density anomaly in the mantle inferred from teleseismic data the model indicates that cold and therefore dense upper mantle material penetrates the asthenosphere to a greater extent in the south similar to the behaviour of an apparently highly ductile lower crust as plate reconstruction suggests more lithospheric shortening in the north our model corresponds to lithospheric material escaping laterally to the south almost perpendicular to the compression caused by lithospheric shortening of the mantle therefore in addition to the prevailing mantle shear in new zealand there may also be a component of extrusional mantle creep beneath the southern alps orogen which could have caused some of the observed large seismic anisotropy in this region we may have also found evidence for submerged eocene miocene oceanic lithosphere beneath the southeastern part of south island that has been unaccounted for after plate reconstruction

coarse grained sediment gravity flow facies in a large supraglacial lake near williams lake in the central interior of british columbia the fraser river exposes long sections of late pleistocene glaciolacustrine sediments selectively preserved within a bedrock trough the dominant facies types are thick normally graded gravels and sands that occupy steeply dipping multistorey channels up to 300 m wide and several tens of metres deep channels appear to have been simultaneously cut and filled by high density turbidity currents in a glacial lake floored by stagnant ice fining upward sediment gravity flow sequences up to 50 m thick may be the product of quasi continuous surging turbidity flows triggered by catastrophic meltwater discharges into the trough or retrogressive failure of ice cored sediments large scale post depositional deformation structures such as synclinal folds normal faults sedimentary dyke swarms and dewatering structures record gravitational foundering of sediment and pore water expulsion caused by the melt of underlying glacier ice melting of buried ice masses along the floor of the trough appears to have controlled the flow paths of turbidity currents by producing sub basins within the overlying sediment pile an idealized model of supraglacial lacustrine sedimentation is developed that may be applicable to other glaciated areas with similar bedrock topography

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

modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing during the last glacial maximum tropical glacier snowlines were lower than expected based on estimates of tropical sea surface temperatures sea surface temperature reconstructions suggest the indo pacific warm pool was cooler than previously thought these temperatures and convective mixing processes can explain snowline altitude in this region

last glacial maximum in an andean cloud forest environment eastern cordillera bolivia comment and reply comment whether the climate of tropical south america during the last glacial maximum lgm was colder and drier or colder and wetter than present day has been widely debated it is accepted however that the lgm in tropical south america was 2 9 c colder than today e g betts and ridgway 1992 1

tropical boundary layer equilibrium in the last ice age a radiative convective boundary layer model is used to assess the effect of changing sea surface temperature pressure wind speed and the energy export from the tropics on the boundary layer equilibrium equivalent potential temperature it remains difficult to reconcile the observations that during the last glacial maximum 18 000 yr bp the snowline on the tropical mountains fell 950 m while the tropical sea surface temperatures fell only 1 2 k

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

shock induced resuspension deposits from a pleistocene proglacial lake kleszczow graben central poland abstract several glaciolacustrine successions forming part of the quaternary fill of the kleszczow graben contain massive and widespread fine grained units that are abnormally thick and homogeneous field studies of such a unit indicate that it is the result of rapid settling of particles from a sediment cloud consisting of material that had once settled on the lake bottom but were resuspended the geological setting makes it likely that the trigger mechanism responsible for the sediment cloud in the lake water was a shock and that this shock most probably an earthquake was caused by tectonic activity in the graben

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

tropical climates at the last glacial maximum a new synthesis of terrestrial palaeoclimate data i vegetation lake levels and geochemistry palaeodata in synthesis form are needed as benchmarks for the palaeoclimate modelling intercomparison project pmip advances since the last synthesis of terrestrial palaeodata from the last glacial maximum lgm call for a new evaluation especially of data from the tropics here pollen plant macrofossil lake level noble gas from groundwater and delta 18 o from speleothems data are compiled for 18 2 ka c 14 32 degrees n 33 degrees s the reliability of the data was evaluated using explicit criteria and some types of data were re analysed using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the coldest month mtco mean annual temperature mat plant available moisture pam and runoff p e cold month temperature mat anomalies from plant data range from 1 to 2 k near sea level in indonesia and the s pacific through 6 to 8 k at many high elevation sites to 8 to 15k in s china and the se usa mat anomalies from groundwater or speleothems seem more uniform 4 to 6 k but the data are as yet sparse a clear divergence between mat and cold month estimates from the same region is seen only in the se usa where cold air advection is expected to have enhanced cooling in winter regression of all cold month anomalies against site elevation yielded an estimated average cooling of 2 5 to 3 k at modern sea level increasing to approximate to 6 k by 3000m how ever neotropical sites showed larger than the average sea level cooling 5 to 6 k and a non significant elevation effect whereas w and s pacific sites showed much less sea level cooling 1 k and a stronger elevation effect these findings support the inference that tropical sea surface temperatures ssts were lower than the climap estimates but they limit the plausible average tropical sea surface cooling and they support the existence of climap like geographic patterns in sst anomalies trends of pam and lake levels indicate wet lgm conditions in the w usa and at the highest elevations with generally dry conditions elsewhere these results suggest a colder than present ocean surface producing a weaker hydrological cycle more arid continents and arguably steeper than present terrestrial lapse rates such linkages are supported by recent observations on freezing level height and tropical ssts moreover simulations of greenhouse and lgm climates point to several possible feedback processes by which low level temperature anomalies might be amplified aloft

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

paleometeorology high resolution northern hemisphere wintertime mid latitude dynamics during the last glacial maximum 1 hourly winter weather of the last glacial maximum lgm is simulated using the community climate model version 3 ccm3 on a globally resolved t170 75 km grid results are compared to a longer lgm climatological run with the same boundary conditions and monthly saves hourly scale animations are used to enhance interpretations the purpose of the study is to explore whether additional insights into ice age conditions can be gleaned by going beyond the standard employment of monthly average model statistics to infer ice age weather and climate results for both lgm runs indicate a decrease in north atlantic and increase in north pacific cyclogenesis storm trajectories react to the mechanical forcing of the laurentide ice sheet with pacific storms tracking over middle alaska and northern canada terminating in the labrador sea this result is coincident with other model results in also showing a significant reduction in greenland wintertime precipitation a response supported by ice core evidence higher temporal resolution puts in sharper focus the close tracking of pacific storms along the west coast of north america this response is consistent with increased poleward heat transport in the lgm climatological run and could help explain early glacial warming inferred in this region from proxy climate records additional analyses shows a large increase in central asian surface gustiness that support observational inferences that upper level winds associated with asian pacific storms transported asian dust to greenland during the lgm

snowline depression in the tropics during the last glaciation five primary methods have been used to reconstruct pleistocene snowlines or equilibrium line altitudes elas in the tropics 23 53n 23 53s during the last glaciation but each has inherent errors that limit the accuracy of the results additional potential errorsin determiningela depression involveestimates of modernsnowline altitude datingresolution topographicreconstructionof former glaciers orographic e ects the presence of rockfall debris on glaciers and calculation of regional ela gradients eustatic sea level lowering during the last glaciation is an additional factor in uencing estimates of ela depression ela in cases where modern snowline lies above a mountain summit only a minimum value for ela can be obtained at 12 tropical sites in africa the americas to 103s latitude and paci c islands estimates of average ela range from 440 to 1400m but most fall in the range of 800 1000m mean 1 900 135m in a regional study of ela depression in the southern tropical andes 8 223s an average ela of ca 920 250m has been reported based on the assumption that glacier mass balance was controlled solely by ablation season temperature and assuming a full glacial temperature lapse rate of 63c km depression of mean annual temperature in glaciated alpine areas was ca 5 4 0 83c if adjusted for a sea level fall of 120m at the glacial maximum this value is reduced to 4 7 0 83c the gure is based on the unlikely assumption that accumulation on alpine glaciers has been invariant nevertheless it is similar to values of temperaturedepresson 5 6 43c for the last glaciationobtainedfrom variousterrestrial sites but contrasts with tropical sea surface temperature estimates that are only 1 33c cooler than present 2001 elsevier science ltd all rights reserved

a debris flow deposit with multiple plug flow channels and associated side accretion deposits a grain dominant debris flow deposit of juvenile basaltic volcaniclastic detritus from a late eocene early oligocene marine surtseyan volcanic pile in north otago new zealand contains multiple semi circular to u shaped channel like structures or relics thereof associated with wedges of asymptotic layers of fine and coarse debris the u shaped structures are interpreted as channel like plug flow zones and the associated wedges are interpreted as abandoned parts of the marginal laminar shear flow zone fine layers and adjacent plug zone coarse layers these appear to have been abandoned by frictional freezing as the plug flow channels migrated laterally in an apparently step like fashion the debris flow and its deposit appear to have been tabular in geometry and the development of multiple transport pathways in debris flows is considered in terms of the geometrical ratio of the width of the flow to the dynamic thickness

x ray tomography of tsunami deposits towards a new depositional model of tsunami deposits x ray computed microtomography is used to obtain high resolution imagery of a historical tsunami deposit in andalusia spain 1755 lisbon tsunami the technique allows characterization of grain size distribution structures component analysis and sedimentary fabric of fine grained unconsolidated tsunami deposits at resolutions down to particle scale the results are validated by comparing to data obtained using other techniques such as laser diffraction anisotropy of magnetic susceptibility and x ray microfluorescence on the same deposits specific technical details such as sampling scanning and image processing methods and further improvements are addressed the use of x ray computed microtomography provides new insights into the stratigraphy of the deposits and gives access to significantly more detailed view of key sedimentary features such as mudlines rip up clasts crude laminations convolutions floating outsized clasts and contacts between successive units this analysis of the 1755 tsunami deposits using x ray computed microtomography allows the proposal of new hypotheses for the sedimentary processes forming tsunami deposits deposition by settling is limited and the section analysed here is dominated by a high shear stress leading to the development of traction carpets with laminated mudlines corresponding to the basal frictional region of these carpets the onset of the tsunami backwash is marked by a micro vortex resembling kelvin helmoltz instabilities

jet efflux deposits of a subaqueous ice contact fan glacial lake rinteln northwestern germany abstract the coarse grained ice contact porta subaqueous fan delta complex was deposited in glacial lake rinteln at the margin of the saalian ice sheet that advanced south of the weser chains nw germany the ice proximal depositional system was up to 15 km long and 10 km wide the present study deals with ice proximal subaqueous fan deposits which are interpreted as products of a subcritical plane wall outflow jet that periodically passed into a supercritical jet with hydraulic jump the proximal facies assemblage consists of the coarse clast supported gravelly deposits of a hyperconcentrated high density effluent and of related cohesionless debris flows attributed to the conduit or immediate proximal jet outflow zone of flow establishment the intermediate facies assemblage attributed to the outflow jet proximal zone of flow transition is dominated by normally graded and cross stratified gravels with scour structures at their bases these gravels were deposited by a high density effluent capable of forming mouthbar like features these deposits pass downcurrent into an assemblage of planar parallel stratified and planar and trough cross stratified sands and pebbly sands partially interpreted as antidunes with abundant scour structures and intercalated layers of fine sand silt and silty mud attributed to the jet distal zone of flow transition the distal facies assemblage consists of trough cross stratified sands and pebbly sands and is attributed to the outflow jet proximal zone of established flow the sedimentary succession as a whole has wedge shape geometry with a gentle fan shaped inclination of the bedding from the southeast to the southwest repeated vertical alternations of supercritical and subcritical deposits and muddy interlayers can be attributed to temporary fluctuations in the meltwater outflow whereas the overall upward fining of the succession indicates a net decline of meltwater discharges

an effective elastic thickness map of australia from wavelet transforms of gravity and topography using forsyth s method 1 we have developed a wavelet version of forsyth s coherence method for estimating the effective elastic thickness te of the lithosphere from gravity and topography we have previously shown that the fan wavelet is particularly well suited to making localised coherence estimates and here we progress the method by showing how the coherence can be modelled by using the wavelet transforms of gravity and topography to infer the initial loads that flex the plate the effectiveness of the method is demonstrated on synthetic data from a model having a rectangular area of large te embedded in a uniform low te surrounding and a load ratio that increases with wave number the results for the australia region show a core of high te under north and central australia with much lower values to the east and also unexpectedly under the western parts of the yilgarn and pilbara cratons

evolutionary aspects of lithosphere discontinuity structure in the western u s 1 we have produced common conversion point ccp stacked ps and sp receiver function image volumes of the moho and lithosphere asthenosphere boundary lab beneath the western united states using transportable array data the large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities the moho is a nearly continuous topographic surface whereas the lab is not and the seismic images show a more complex expression the first order change in lab depth in the western u s occurs along the cordilleran hingeline the former laurasian passive margin along the southwestern precambrian north american terranes the lab is about 50 deeper to the east of the hingeline than to the west with most of the increase in lab thickness being in the mantle lithosphere we infer that the moho and the lab are late mesozoic or cenozoic everywhere west of the hingeline modified during farallon subduction and its aftermath between the hingeline and the rocky mountain front the lab and to a lesser extent the moho have been partially reset during the cenozoic by processes that continue today seismicity and recent volcanism in the interior of the western u s are concentrated along gradients in crustal and or lithospheric thickness for example the hingeline and the eastern edge of the coastal volcanic magmatic terranes to us this suggests that lateral gradients in integrated lithospheric strength focus deformation similarly areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness

effective elastic thickness of south america and its implications for intracontinental deformation 1 the flexural rigidity or effective elastic thickness of the lithosphere te primarily depends on its thermal gradient and composition consequently maps of the lateral variability of te in continents reflect their lithospheric structure we present here a new te map of south america generated using a compilation of satellite derived grace and champ missions and terrestrial gravity data including egm96 and sagp and a multitaper bouguer coherence technique our te maps correlate remarkably well with other proxies for lithospheric structure areas with high te have in general high lithospheric mantle shear wave velocity and low heat flow and vice versa in this paper we focus on the te of the stable platform we find that old cratonic nuclei mainly archean and early middle proterozoic have in general high te 70 km while the younger patagonian phanerozoic terrane has much lower te 20 30 km suggesting that te is related to terrane age as has already been noted in europe within cratonic south america te variations are observed at regional scale relatively lower te occurs at sites that have been repeatedly reactivated throughout geological history as major sutures rift zones and sites of hot spot magmatism today these low te areas are surrounded by large cratonic nuclei they concentrate most of the intracontinental seismicity and exhibit relatively high surface heat flow and low seismic velocity at 100 km depth this implies that intracontinental deformation focuses within relatively thin hot and hence weak lithosphere that cratonic interiors are strong enough to inhibit tectonism and that the differences in lithospheric rigidity structure and composition between stable cratons and sites of intracontinental deformation are not transient and may have been maintained in some cases for at least 500 m y

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

on the recovery of effective elastic thickness using spectral methods examples from synthetic data and from the fennoscandian shield there is considerable controversy regarding the long term strength of continents te while some authors obtain both low and high te estimates from the bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength others obtain estimates of only 25 km using the free air admittance and suggest that the mantle is weak at the root of this controversy is how accurately te can be recovered from coherence and admittance we investigate this question by using synthetic topography and gravity anomaly data for which te is known we show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free air admittance we reformulate the admittance method and show that it can recover synthetic te estimates similar to those recovered using coherence in light of these results we estimate te in fennoscandia and obtain similar results using both techniques te is 20 40 km in the caledonides 40 60 km in the swedish svecofennides 40 60 km in the kola peninsula and 70 100 km in southern karelia and svecofennian central finland independent rheological modeling using a xenolith controlled geotherm predicts similar high te in central finland because te exceeds crustal thickness in this area the mantle must contribute significantly to the total strength te in fennoscandia increases with tectonic age seismic lithosphere thickness and decreasing heat flow and low te correlates with frequent seismicity however in proterozoic and archean lithosphere the relationship of te to age is ambiguous suggesting that compositional variations may influence the strength of continents copyright 2004 by the american geophysical union

tropical snowline depression at the last glacial maximum comparison with proxy records using a single cell tropical climate model 1 the substantial lowering of tropical snowlines at the last glacial maximum lgm circa 21 kya is examined using a modified version of the single cell tropical climate model of betts and ridgway 1989 these authors concluded that it was difficult to reconcile the large depression of snowlines at the lgm with the small reduction in mean tropical sea surface temperature sst of the climate long range investigation mapping and prediction climap reconstruction here climatic implications of the snowline depression are compared not with climap but with temperature proxies 18o and sr ca ratios in corals noble gas concentrations in aquifers alkenone and mg ca ratios in deep sea sediments and recent faunal reconstructions whose lgm values have been interpreted as implying lower surface temperatures in the tropics it proves difficult in the framework of this model to reconcile the coldest of the indicated paleotemperatures with the observed snowline depression which by itself is found to be consistent with an sst reduction of 3 k a cooling of this magnitude corresponds most closely to recent warm pool lgm sea surface temperature estimates based on mg ca paleothermometry discordance among the various proxy reconstructions may result at least in part from regional variations in surface temperature change at the lgm

sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice contact fan deposits a pleistocene example ireland a pit located near ballyhorsey 28 km south of dublin eastern ireland displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice contact fan deposits the coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies the lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space mainly controlled by the position of an overlying ice roof during ice bed decoupling these sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments the sorted sediments consist of fine grained laminated facies cross laminated sand and channelized gravels and are interpreted as subglaciofluvial sediments deposited within a subglacial de coupled space the subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds triggered by fluid overpressure and shear stress during episodes of ice bed recoupling clastic dykes and folds the upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake forming a thick sedimentary wedge erosively overlying the subglacial deposits gravel facies and large scale trough bedding sand are observed within this proximal wedge while normally graded sand beds with developed bedforms are observed further downflow the building of the prograding ice contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading normal faults this study facilitates the recognition of subglacial submarginal depositional environments formed in part during localized ice bed coupling episodes in the sedimentary record the sedimentary sequence exposed in ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation the impact on the subglacial drainage system and the consequences on the irish sea ice stream flow mechanisms

temperature control of continental lithosphere elastic thickness te vs vs article i nfo the elastic thickness of the continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes recently it has been questioned whether the lithosphere thickness and strength are dependent on crustal and upper mantle temperatures and compositions as predicted by laboratory data we test this dependence regionally by comparison in northwestern north america of the effective elastic thickness te from topography gravity coherence with upper mantle temperatures mapped by shear wave tomography velocities vs and other temperature indicators the te values are strongly bimodal as found globally less than 20 km for the hot cordillera backarc and over 60 km for the cold stable craton these te correspond to low vs beneath the cordillera and high vs beneath the craton model temperature depth profiles are used to estimate model te for comparison with those observed only limited areas of intermediate thermal regimes i e thermotectonic ages of 300 ma have intermediate te that suggest a weak lower crust over a stronger upper mantle there are large uncertainties in model te associated with composition water content strain rate and decoupling stress threshold however with reasonable parameters model yield stress envelopes correspond to observed te for thermal regimes with 800 900 c at the cordillera moho and 400 500 c for the shield in agreement with temperatures from vs and other estimators our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature and that laboratory based rheology generally provides a good estimate of the deformation behaviour of the crust and upper mantle on geological time scales

hydroclimate changes across the amazon lowlands over the past 45 000 years reconstructing the history of tropical hydroclimates has been difficult particularly for the amazon basin one of earth s major centres of deep atmospheric convection for example whether the amazon basin was substantially drier or remained wet during glacial times has been controversial largely because most study sites have been located on the periphery of the basin and because interpretations can be complicated by sediment preservation uncertainties in chronology and topographical setting here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide our results are based on a decadally resolved uranium thorium dated oxygen isotopic record for much of the past 45 000 years obtained using speleothems from paraiso cave in eastern amazonia we interpret the record as being broadly related to precipitation relative to modern levels precipitation in the region was about 58 during the last glacial maximum around 21 000 years ago and 142 during the mid holocene epoch about 6 000 years ago we find that as compared with cave records from the western edge of the lowlands the amazon was widely drier during the last glacial period with much less recycling of water and probably reduced plant transpiration although the rainforest persisted throughout this time

teleseismic p wave delays and modes of shortening the mantle lithosphere beneath south island new zealand a high speed zone in the mantle directly beneath the southern alps of new zealand is required by the recorded pattern of teleseismic p waves two parallel lines of 80 seismographs spaced at 2 km intervals recorded three earthquakes from the western pacific with epicentral distances of 52 53 and 78 azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines differences between measured delays and those predicted from the crustal structure reach 0 8 s along one line and 1 0 s along the other with the rays for the earliest arriving signals passing the depth of 120 km beneath the center of the island assuming these early arrivals are due to structure within the mantle shallower than 200 km they imply that the core of the high speed zone lies beneath the thickest crust which has been shortened by 100 km of convergence during the past 6 7 myr although the shape and position of the high speed body cannot be fixed uniquely a roughly symmetric body centered about a depth of 120 km 80 100 km wide with a depth extent of 100 km and with a maximum speed advance of 7 satisfies the observations the pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other rather the residuals favor thickening of mantle lithosphere by a more homogenous straining of it as if mantle lithosphere beneath continental crust behaved as a continuum an excess mass in the mantle is also required by the observed gravity anomalies once allowance is made for the seismically determined crustal thickness this high density mantle anomaly provides sufficient force per unit length to maintain the crustal root which is approximately twice as thick as that necessary to support the topography

subduction evolution and mantle dynamics at a continental margin central north island new zealand 1 central north island new zealand provides an unusually complete geological and geophysical record of the onset and evolution of subduction at a continental margin whereas most subduction zones are innately two dimensional north island of new zealand displays a distinct three dimensional character in the back arc regions specifically we observe mariana type subduction in the back arc areas of central north island in the sense of back arc extension high heat flow prolific volcanism geothermal activity and active doming and exhumation of the solid surface evidence for emplacement of a significant percent of new lithosphere beneath the central north island comes from heat flux of 25 mw km of strike of volcanic zone and thinned crust underlain by rocks with a seismic wave speed consistent with underplated new crust seismic attenuation qp 1 is high 240 and rhyolitic and andesitic volcanism are widespread almost complete removal of mantle lithosphere is inferred here in pliocene times on the basis of the rock uplift history and upper mantle seismic velocities as low as 7 4 0 1 km s in contrast southwestern north island exhibits chilean type back arc activity in the sense of compressive tectonics reverse faulting low heat flow thickened lithosphere and strong coupling between the subducted and overriding plates this rapid switch from mariana type to chilean type subduction occurs despite the age of the subducted plate being constant under north island moreover stratigraphic evidence shows that processes that define the extensional back arc area the central volcanic region are advancing southward into the compressional system wanganui basin at about 10 mm yr we link the progression from one system to another to a gradual and viscous removal of thickened mantle lithosphere in the back arc regions thickening occurred during the miocene within the taranaki fault zone the process of thickening and convective removal is time and temperature dependent and has left an imprint in both the geological record and geophysical properties of central north island which we document and describe

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

sandy gravelly mass flow deposits in an ice marginal lake saalian leuvenumsche beek valley veluwe the netherlands with emphasis on plug flow deposits abstract in three pits in the leuvenumsche beek valley near ermelo and elspeet massive and diffusely banded clean gravelly sands found in association with sandy sediments showing some similarities to the bouma sequence were interpreted as originating from sediment gravity flows the flows which came to rest on a ca 6 paleoslope probably originated from slumping of parts of the ice pushed ridges at either side of an ice marginal lake major characteristic features in the three sand pits studied are u shaped channel forms with a maximum depth and width of 10 25 rmm the fills consist of sand and gravels locally containing giant sand clasts the sediments just below the base of these channel forms are commonly distorted and folded especially the massive more sandy fills are surrounded by a finer grained diffusely bedded and inversely graded zone these sedimentary features suggest that rigid sediment plugs sheared downslope generating a finer grained shear zone around the plug scouring a slide scar till its present semi cylindrical form and preserving the steep sides exceeding the angle of repose of the channel form by freezing the u shaped channel forms plugged troughs are locally overlain by shallower gently sided trough structures with mainly diffusely banded infillings very shallow and often smaller troughs are found again on top of these massive and diffusely banded infillings the sediments in these troughs consist of repetitions of two types of microsequences 1 thickly bedded sequences of normally graded massive sands near horizontal stratifications plane beds and cross bedded sets and 2 thinly bedded sequences of relatively thick units t a and thin units t c and t d of the bouma sequence the repetitions of the microsequences are interpreted as flow pulsations upwards thinning of the sequence together with a finer grain size may point to waning flow conditions their association with plugged troughs is suggestive of retrogressive flow slides

new antarctic gravity anomaly grid for enhanced geodetic and geophysical studies in antarctica gravity surveying is challenging in antarctica because of its hostile environment and inaccessibility nevertheless many ground based airborne and shipborne gravity campaigns have been completed by the geophysical and geodetic communities since the 1980s we present the first modern antarctic wide gravity data compilation derived from 13 million data points covering an area of 10 million km2 which corresponds to 73 coverage of the continent the remove compute restore technique was applied for gridding which facilitated levelling of the different gravity datasets with respect to an earth gravity model derived from satellite data alone the resulting free air and bouguer gravity anomaly grids of 10 km resolution are publicly available these grids will enable new high resolution combined earth gravity models to be derived and represent a major step forward towards solving the geodetic polar data gap problem they provide a new tool to investigate continental scale lithospheric structure and geological evolution of antarctica

why is the north america cordillera high hot backarcs thermal isostasy and mountain belts global mountain belts are commonly concluded to be a consequence of crustal thickening resulting from continental collision with high elevations supported by crustal roots however accumulating seismic structure data indicate that many mountain belts have no crustal root most of the north american cordillera has a 30 35 km crust in contrast to 40 45 km for the lower elevation craton and other stable areas it has been shown previously that most such mountain belts are in present or recent backarcs that are uniformly hot from thermal constraints we predict a uniform 1600 m elevation support of the cordillera by thermal expansion compared to stable areas over most of the cordillera relative to stable areas the elevations after correction for variable crustal thickness and density are in excellent agreement when subduction and shallow backarc convection stop the lithosphere may cool and the elevations of mountain belts subside over 300 m y

mapping the mechanical anisotropy of the lithosphere using a 2d wavelet coherence and its application to australia abstract we develop a new method for imaging the spatial variations of the anisotropy of the flexural response of the lithosphere and apply it to recent topographic and gravity data sets over australia the method uses two dimensional morlet wavelet transforms superposed in a strictly controlled geometry to estimate the auto and cross spectra of the two data sets in a number of different directions the resulting wavelet coherence is a function of scale or wavelength as well as orientation and is inverted at each spatial location for the three parameters of an anisotropic thin elastic plate model i e maximum and minimum flexural rigidities and the orientation of the maximum extensive tests of the method on synthetic anisotropic but uniform data sets show that it retrieves the amplitude and orientation of the anisotropy with useful accuracy the results for australia west of 143 e show a strong correlation with the shallower layers 75 175 km of a recent model of seismic sv wave azimuthal anisotropy the weak axes i e of minimum flexural rigidity in most cases are approximately at right angles to the fast axes of the seismic anisotropy implying that for precambrian australia they arise from the same source this is most likely deformation resulting from the most recent episode of orogeny

structure and evolution of the intracratonic congo basin 1 surface wave tomography heat flow and crustal thickness measurements have demonstrated that the thickness of the continental lithosphere varies by at least a factor of 2 since the thermal time constant of the lithosphere depends upon the square of its thickness subsidence records of extensional sedimentary basins offer a potential way of extending these observations into the past here we examine the congo basin a large and iconic intracratonic sedimentary basin in central africa this roughly circular basin covers an area in excess of 1 4 106 km2 with more than 5 km thickness of sedimentary rocks the oldest parts of which are late precambrian in age first we assess the thickness of the lithosphere we have estimated its thickness across africa using maps of shear wave velocity obtained by inversion of fundamental and higher mode surface waveforms the congo basin sits on 220 30 km thick lithosphere and appears to be part of a southern core to the continent encompassing both archean cratons and proterozoic mobile belts this thickness is consistent with published estimates from kimberlites reappraisal of legacy seismic reflection images demonstrates that the sedimentary section is underlain by a late precambrian rift zone and that the basin is still subsiding today subsidence modeling of two deep wells is consistent with uniform extension and cooling of the lithosphere by a factor of 1 2 during latest precambrian and cambrian time we argue that the exceptional 0 55 ga history of the basin is a direct consequence of the lithospheric thermal time constant being a factor of 4 longer than normal today the basin coincides with a long wavelength 30 to 40 mgal gravity anomaly we interpret this gravity anomaly as the surficial manifestation of 400 600 m of recent mantle convective drawdown in response to the onset of upwelling plumes around the flanks of the southern african continent the alternative explanation that it is the static manifestation of locally thick lithosphere is inconsistent with global trends of mantle density depletion our interpretation is consistent with fast seismic velocities observed throughout the sublithospheric upper mantle underneath the basin and recent geodynamic modeling

glacial interglacial dynamics of the eastern equatorial pacific cold tongue intertropical convergence zone system reconstructed from oxygen isotope records record mean late holocene lh last glacial maximum lgm d 18 o amplitudes ranging between 1 0 and 1 3 we estimate that mean sea surface temperatures sst in this region during the lgm were on average 1 5 0 5 c lower than the lh larger d 18 o amplitudes are observed in sites north of the equator indicating a spatial pattern of reduced meridional sst gradient across the equator during the lgm this result is supported by comparison of mg ca sst reconstructions from two sites straddling the equator we interpret the reduction of this gradient during the lgm as evidence for a less intense cold tongue intertropical convergence zone itcz frontal system a more southerly position of the itcz and weaker southeast equatorial trades in the eep indexterms 1620 global change climate dynamics 3309 3339 meteorology and atmospheric dynamics ocean atmosphere interactions 0312 4504 3344 meteorology and atmospheric dynamics paleoclimatology 4231 oceanography general equatorial oceanography 4267 oceanography general paleoceanography keywords cold tongue itcz oxygen isotopes tropical pacific

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

reconstructing climate from glaciers glaciers offer the potential to reconstruct past climate over timescales from decades to millennia they are found on nearly every continent and at the last glacial maximum glaciers were larger in all regions on earth the physics of glacier climate interaction are relatively well understood and glacier models can be used to reconstruct past climate from geological evidence of past glacier extent this can lead to significant insights regarding past present and future climate for example glacier modeling has demonstrated that the near ubiquitous global pattern of glacier retreat during the last few centuries resulted from a global scale climate warming of 1 c consistent with instrumental data and climate proxy records climate reconstructions from glaciers have also demonstrated that the tropics were colder at the last glacial maximum than was originally inferred from sea surface temperature reconstructions future efforts to reconstruct climate from glaciers may provide new constraints on climat

last glacial maximum equilibrium line altitude and paleo temperature reconstructions for the cordillera de merida venezuelan andes abstract the pattern and magnitude of glacier equilibrium line altitude ela lowerings in the tropics during the last glacial maximum lgm are topics of current debate in the northern tropics paleo ela data are particularly limited inhibiting the ability to make regional and large scale paleoclimatic inferences to improve these records nine paleo glaciers in the venezuelan andes were reconstructed based on field observations aerial photographs satellite imagery and high resolution digital topographic data paleo glacier equilibrium line altitudes elas were estimated using the accumulation area ratio aar and the area altitude balance ratio aabr methods during the local lgm in venezuela 22 750 to 19 960 cal yr bp elas were 850 to 1420 m lower than present local lgm temperatures were are at least 8 8 2 c cooler than today based on a combined energy and mass balance equation to account for an ela lowering this is greater than estimates using an atmospheric lapse rate calculation which yields a value of 6 4 1 c cooler the paleo glacial data from the venezuelan andes support other published records that indicate the northern tropics experienced a greater ela lowering and possibly a greater cooling than the southern hemisphere tropics during the lgm

environmental processes of the ice age land oceans glaciers epilog knowledge of the state of the earth at the last glacial maximum lgm an interval around 21 000 years ago is an important benchmark for understanding the sensitivity of global environmental systems to change much progress in understanding climates of the lgm has occurred in the 20 years since the end of the climap project of the 1970s climate long range investigation mapping and prediction here we review this progress based on presentations and discussion at a rst open science meeting of the epilog project environmental processes of the ice age land oceans glaciers held in delmenhorst germany may 1999 we outline key controversies and document protocols for epilog contributions so that a new synthesis of the lgm earth can emerge as an open project of the world s community of scientists 2001 elsevier science ltd all rights reserved

a seismic reflection image for the base of a tectonic plate a high resolution image for the base of an oceanic plate that is subducting beneath north island new zealand reveals a channel which is interpreted as a sheared zone of ponded partial melts or volatiles this low viscosity channel decouples the plate from mantle flow beneath allowing plate tectonics to work tim stern and co authors use explosion generated seismic waves to form a high resolution image of the base of an oceanic plate that is subducting beneath north island new zealand they find an abrupt seismic wave speed transition less than a kilometre thick at a depth of 100 km dipping at an angle of 15 the presence of a parallel 10 km deeper reflection event shows that the decrease in compressional wave speed is confined to a channel at the base of the plate which the authors interpret as a sheared zone of ponded partial melts or volatiles they conclude that this is independent evidence for a low viscosity channel at the lithosphere asthenosphere boundary that decouples plates from mantle flow beneath and allows plate tectonics to take place plate tectonics successfully describes the surface of earth as a mosaic of moving lithospheric plates but it is not clear what happens at the base of the plates the lithosphere asthenosphere boundary lab the lab has been well imaged with converted teleseismic waves1 2 whose 10 40 kilometre wavelength controls the structural resolution here we use explosion generated seismic waves of about 0 5 kilometre wavelength to form a high resolution image for the base of an oceanic plate that is subducting beneath north island new zealand our 80 kilometre wide image is based on p wave reflections and shows an approximately 15 dipping abrupt seismic wave speed transition less than 1 kilometre thick at a depth of about 100 kilometres the boundary is parallel to the top of the plate and seismic attributes indicate a p wave speed decrease of at least 8 3 per cent across it a parallel reflection event approximately 10 kilometres deeper shows that the decrease in p wave speed is confined to a channel at the base of the plate which we interpret as a sheared zone of ponded partial melts or volatiles this is independent high resolution evidence for a low viscosity channel at the lab that decouples plates from mantle flow beneath and allows plate tectonics to work

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

global crust mantle density contrast estimated from egm2008 dtm2008 crust2 0 and ice 5g we compute globally the consolidated crust stripped gravity disturbances anomalies these refined gravity field quantities are obtained from the egm2008 gravity data after applying the topographic and crust density contrasts stripping corrections computed using the global topography bathymetry model dtm2006 0 the global continental ice thickness data ice 5g and the global crustal model crust2 0 all crust components density contrasts are defined relative to the reference crustal density of 2 670 kg m3 we demonstrate that the consolidated crust stripped gravity data have the strongest correlation with the crustal thickness therefore they are the most suitable gravity data type for the recovery of the moho density interface by means of the gravimetric modelling or inversion the consolidated crust stripped gravity data and the crust2 0 crust thickness data are used to estimate the global average value of the crust mantle density contrast this is done by minimising the correlation between these refined gravity and crust thickness data by adding the crust mantle density contrast to the original reference crustal density of 2 670 kg m3 the estimated values of 485 kg m3 for the refined gravity disturbances and 481 kg m3 for the refined gravity anomalies very closely agree with the value of the crust mantle density contrast of 480 kg m3 which is adopted in the definition of the preliminary reference earth model prem this agreement is more likely due to the fact that our results of the gravimetric forward modelling are significantly constrained by the crust2 0 model density structure and crust thickness data derived purely based on methods of seismic refraction

is the tropical atmosphere conditionally unstable abstract we examine the mean and standard deviation of the bouyancy of lifted boundary layer parcels from a large sample of soundings from the western equatorial pacific the inclusion of condensate loading in the definition of buoyancy is emphasized as is the precise originating level of parcels in the subcloud layer we confirm the observation of betts that those parts of the tropical atmosphere experiencing deep convection are nearly neutral to adiabatic parcel ascent from the subcloud layer when adiabatic condensate loading is included in the definition of buoyancy parcels lifted from the top of the subcloud layer are more nearly neutral than those originating near the surface and atmospheres with higher levels of free convection lfcs are closer to neutral than those with low lfcs we explore possible reasons for these observations and discuss their implications for tropical meteorology

flexural isostasy constraints from gravity and topography power spectra we have used spherical harmonic coefficients that describe earth s gravity anomaly and topography fields to quantify the role of isostasy in contributing to crustal and upper mantle structure power spectra reveal that the gravity effect of topography and its flexural compensation contributes significantly to the observed free air gravity anomaly spectra for spherical harmonic degree 33 400 and lower n 33 degrees but topography appears either uncompensated or fully compensated at these degrees irrespective of the actual te all isostatic models underpredict the spectra at 2 n 12 and so we interpret the low order earth s gravity field as caused at least in part by non isostatic processes due to dynamic motions such as those associated with convective upwellings and downwellings in earth s mantle

should quaternary sea level changes be used to correct glacier elas vegetation belt altitudes and sea level temperatures for inferring climate changes abstract changes in the altitudes of glacier snowlines elas and the altitudes of montane vegetation belts vbas measure quaternary climatic change an accepted correction to such changes by deducting the amount of contemporary sea level fall is wrong since the air displaced by the ice sheets approximately fills the space left by the falling sea level and so there is no overall downward movement of the troposphere this also causes a reduced cooling at the lowered sea level relative to that at the former inter glacial sea level about 1 c at the las glacial maximum which reduces the discrepancies previously noted by others between terrestrial and marine estimates of sea level cooling the change in temperature is indicated by the product of the ela or vba lowering and the environmental lapse rate elr prior estimates of ela 900 135 m and elr 6 0 1 c km 1 would indicate a cooling of 5 4 c at interglacial sea level and 4 4 c at glacial sea level although glacial period elrs are not known reliably established ela corrections for local epeirogenic uplift or subsidence are appropriate

deposition in an esker bead and fan complex lanark ontario canada abstract a glaciofluvial landform complex comprising a single esker ridge proximally multiple ridges and beads in the middle section and a complex of ridges and fans distally is mapped and described we present detailed successions in the associated sediment showing sedimentary structure grain size and palaeocurrent estimates sediment in the esker is mainly coarse grained and varies from boulders with a fine sand matrix deposited at maximum discharges to diamicton interpreted as a product of ice pressing under low discharges early stage deposition in the esker at high flow was from suspension and bed load intermediate stage deposition was mainly from bed load and late stage deposition was from hyperconcentrated suspension the esker rests in a bedrock channel which functioned as a nye channel prior to esker formation the channel surface carries a variety of fluvial erosional marks short distributary ridges of the ridges and beads complex contain mainly cobbles and boulders but beads are mainly in sand with rapid down flow fining and change in sedimentary structure faulting and overfolding of bead sediment suggest a subglacial origin deposition of the esker fan complex was highly variable in space and time down fan sedimentary changes with transitions from gravel to cross laminated fine sand in a few tens of metres indicate rapid loss in competence fan location lateral to the esker indicates a broadening of the zone of subglacial meltwater with time deposition from suspension and steep sided scours are interpreted to result from deposition and erosion by hyperconcentrated flows a model of underflows with submerged hydraulic jumps spilling laterally from the esker tunnel into subglacial cavities explains the morphological and sedimentary characteristics of the fans detailed analysis of small scale fining upwards sequences in the fans and beads indicates deposition by a large number of flow events these events are explained by glacier hydraulics whereby periodic separation of the glacier from its bed caused water with a high suspended sediment concentration to spill out from the esker tunnel into adjacent subglacial cavities evolution of the subglacial drainage system indicates a complexity of subglacial sedimentation near to a grounding line which is not readily apparent from observations at modern glaciers terminating in relatively deep water

fan delta sedimentation on the basin margin slope of the cretaceous strike slip izumi basin southwestern japan abstract the late cretaceous campanian maastrichtian izumi basin is an e w trending strike slip basin some 300 km long and 15 km wide filled with turbidites and associated coarse clastic deposits in the northeastern margin of the basin steep face fan deltas were built on a steep slope and prograded directly into deep water forming conglomeratic slope aprons the fan delta deposits comprise debris flow dominated alluvial fan conglomerates delta slope conglomerates originated from various subaqueous sediment gravity flows and prodelta mudstones with intercalations of thin sandstone and channel fill pebbly sandstone the most significant feature of the izumi fan delta system is a contrast between the gravel dominated delta slope and the mud dominated prodelta the development of this fan delta system was due to tectonic activity along the northern margin of the basin sedimentation took place due to vigorous uplift of the adjacent source terrane which caused a high rate of supply of coarse detritus and rapid basin subsidence which resulted in deep water depth

huascaran δ18o as an indicator of tropical climate during the last glacial maximum the interpretation of the isotopic composition of andes tropical mountain glaciers is reconsidered using a simple rayleigh distillation model it is shown that andes snowline depression and huascaran ice core 18o are consistent with a 3 c drop in sea surface temperature during the last glacial maximum accompanied by an increase from 40 at present to 60 during the lgm in the annual mean proportion of atmospheric water lost to continental runoff broecker s interpretation of huascaran 18o as a paleo hygrometer is discussed

steady propagation of delamination events delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation accelerating as the sinking slab of detached lithosphere grows longer this propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings the form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material without coupling the tip of the intrusion cools and stiffens to form a wedge dividing the crust cold mode with coupling the intrusion is forced to convect and remains ductile hot mode the cold mode can propagate at all velocities the hot mode has a lower limiting velocity of 1 2 cm year but offers less resistance at higher speeds resistance to delamination includes a constant term from the buoyant crustal downwarp plus a velocity proportional term representing viscous deformation however the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities matching this resistance to loading lines of 100 to 800 km slabs sinking in a mantle of 1022 p velocities of 0 3 8 0 cm year are obtained changes in viscosity affect this rate but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab the surface expression of delamination is a leading outer rise followed by a submarine trough with a large negative free air anomaly which finally evolves into a 1 km plateau if crustal viscosity and velocity are both low however there is a montonic crustal uplift with no trough thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm year driven by slabs up to 400 km in length

continent scale strike slip on a low angle fault beneath new zealand s southern alps implications for crustal thickening in oblique collision zones new zealand s southern alps lie adjacent to the continent scale dextral strike slip alpine fault on the boundary between the pacific and australian plates we show with a simple 2 d model of crustal balancing that the observed crustal root and erosion expressed as equivalent crustal shortening is up to twice that predicted by the orthogonal plate convergence since 11 ma and even since 23 ma when the alpine fault formed we consider two explanations for this involving a strong component of motion along the length of the plate boundary zone geophysical data indicate that the alpine fault has a listric geometry flattening at mid crustal levels and has accommodated sideways underthrusting of australian plate crust beneath pacific plate crust the geometry of the crustal root together with plate reconstructions requires the underthrust crust to be the hyperextended part of an asymmetric rift system which formed over 500 km farther south during the eocene the narrow remnant part today forms the western margin of the campbell plateau at 10 ma the hyperextended margin underwent shallow subduction in the puysegur subduction zone and then was dragged over 300 km along the length of the southern alps beneath a low angle 20 section of the alpine fault we speculate that prior to 10 ma more distributed lower crustal shortening and thickening occurred beneath the southern alps accommodating southward extrusion of continental crust in the northern part of the plate boundary zone providing a mechanism for clockwise rotation of the hikurangi margin

terrestrial conditions at the last glacial maximum and climap sea surface temperature estimates are they consistent climap 1981 seasonal reconstruction of the earth s surface at the last glacial maximum geological society of america map and chart series mc 36 boundary conditions were used as inputs to the giss general circulation model and the last glacial maximum lgm climate was simulated for six model years the simulation was compared with snow line depression and pollen inferred temperature data at low latitudes specifically for hawaii colombia east africa and new guinea the model does not produced as much cooling at low latitudes as is implied by the terrestrial evidence an alternative experiment in which the climap sea surface temperatures were uniformly lowered by 2 c produces a better fit to the land data although in hawaii model temperatures are still too warm the relatively warm climap tropical sea surface temperatures also provide for only a slight decrease in the hydrologic cycle in the model in contrast to both evidence of lgm tropical aridity and the results of the experiment with colder ocean temperatures with the climap sea surface temperatures the lgm global annual mean surface air temperature is 3 6 c colder than at present if the ocean temperatures were allowed to cool in conformity with the model s radiation balance the lgm simulation would be 5 6 c colder than today and in better agreement with the tropical land evidence

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

geochronology and paleoclimatic implications of the last deglaciation of the mauna kea ice cap hawaii we present new 3he surface exposure ages on moraines and bedrock near the summit of mauna kea hawaii which refine the age of the mauna kea ice cap during the local last glacial maximum llgm and identify a subsequent fluctuation of the ice margin the 3he ages when combined with those reported previously indicate that the local ice cap margin began to retreat from its llgm extent at 20 5 2 5 ka in agreement with the age of deglaciation determined from llgm moraines elsewhere in the tropics the ice cap margin receded to a position at least 3 km upslope for 4 5 5 0 kyr before readvancing nearly to its llgm extent the timing of this readvance at 15 4 ka corresponds to a large reduction of the atlantic meridional overturning circulation amoc following heinrich event 1 subsequent ice margin retreat began at 14 6 1 9 ka corresponding to a rapid resumption of the amoc and onset of the bolling warm interval with the ice cap melting rapidly to complete deglaciation additional 3he ages obtained from a flood deposit date the catastrophic outburst of a moraine dammed lake roughly coeval with the younger dryas cold interval suggesting a more active hydrological cycle on mauna kea at this time a coupled mass balance and ice dynamics model is used to constrain the climate required to generate ice caps of llgm and readvance sizes the depression of the llgm equilibrium line altitude requires atmospheric cooling of 4 5 1 c whereas the mass balance modeling indicates an accompanying increase in precipitation of as much as three times that of present we hypothesize 1 that the llgm temperature depression was associated with global cooling 2 that the temperature depression that contributed to the readvance occurred in response to an atmospheric teleconnection to the north atlantic and 3 that the precipitation enhancement associated with both events occurred in response to a southward shift in the position of the inter tropical convergence zone itcz such a shift in the itcz would have allowed midlatitude cyclones to reach mauna kea more frequently which would have increased precipitation at high elevations and caused additional cooling

hydrospheric transports the oxygen isotope record and tropical sea surface temperatures during the last glacial maximum a model is developed to consider explicitly the effect of glacial to holocene changes in transports of fresh and salt water on the distribution of oxygen isotopes in seawater and therefore on the calibration of the foraminiferal 18o paleothermometer the published foraminiferal isotopic record is shown to be consistent with glacial surface temperatures at low latitudes cooler than today by about 4 c in the atlantic and indian oceans and about 2 c in the pacific given glacial transport patterns supported by the geologic record if in addition net vapor transport from the indian and atlantic oceans to the pacific was enhanced significantly during glacial times relative to today the isotopic record would be consistent with a sea surface cooler by up to 5 c throughout the tropics at the last glacial maximum as expected based on proxies for continental surface temperatures

effective elastic thickness variations along the andean margin and their relationship to subduction geometry 1 we present a new map of the spatial variations in effective elastic thickness te along the andes estimated using bouguer coherence the te variations reflect interactions between subducting slab and preexisting terrane structure in the forearc conductive cooling of the continent by the subducting slab exerts primary control on rigidity resulting in te that is highest 40 km where the oceanic lithosphere is oldest and coldest 20 s in the central andes te is relatively low 20 km along the volcanic chain and the altiplano and puna plateaus we interpret this weakening to reflect a high geothermal gradient maintained by advective magmatic processes a shallow and hot asthenosphere and a very weak lower crust throughout this region east of the plateaus high te delineates underthrusting of the brazilian shield finally north and south of the plateaus flat subduction areas are characterized by high te high shear wave velocity thick thermal lithosphere and low heat flow indicating that continental lithosphere there is thicker colder and stronger on the basis of these relationships we suggest that variations in slab dip along the margin relate to variations in structure of the continental lithosphere in particular we propose that upper plate structure influences the width and viscosity of the asthenospheric wedge which control the suction moment responsible for the subduction angle at depths 70 100 km for example when oceanic lithosphere subducts beneath thin continental lithosphere the low viscosity asthenosphere allows the slab to detach from the continent and sink into the mantle at normal angles however when oceanic lithosphere subducts close or beneath thick and strong continental lithosphere the asthenospheric wedge narrows and corner flow drags high viscosity mantle from the base of the thick 150 km cold continent into the wedge suction forces increase with both narrowing of the wedge and its increasing viscosity we estimate the asthenospheric viscosity underneath thick cold continents to be 1020 pa s sufficient to induce flat subduction later after prolonged hydration and weakening of the continent s base asthenospheric flow into the wedge may resume allowing the slab to sink again into the mantle at normal angles

the long term strength of europe and its implications for plate forming processes field based geological studies show that continental deformation preferentially occurs in young tectonic provinces rather than in old cratons this partitioning of deformation suggests that the cratons are stronger than surrounding younger phanerozoic provinces however although archaean and phanerozoic lithosphere differ in their thickness and composition their relative strength is a matter of much debate one proxy of strength is the effective elastic thickness of the lithosphere te unfortunately spatial variations in te are not well understood as different methods yield different results the differences are most apparent in cratons where the bouguer coherence method yields large te values 60 km whereas the free air admittance method yields low values or 1 5 gyr old is much larger mean te 60 km than that of younger lithosphere mean te 30 km we propose that this strength difference reflects changes in lithospheric plate structure thickness geothermal gradient and composition that result from mantle temperature and volatile content decrease through earth s history

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

sedimentation in a drumlin lee side subglacial wave cavity northwest ireland abstract lee side stratification sequences in a late pleistocene drumlin in northwest ireland are used to assess sedimentation patterns in subglacial wave cavities formed on the lee sides of drumlins three major subglacial cavity sub environments are identified 1 high energy divergent trough like closed conduit characterised by channel fill pebbly sand facies 2 low energy closed conduit characterised by alternating sand silts 3 trough margin dominated by stratified diamicts facies arrangements suggest two major transient states reflecting variations in subglacial meltwater flux the active phase showed meltwater flow directed across the drumlin apex suggesting complete uncoupling from the glacier bed supercavitation as flow velocities waned pebbly sand facies were deposited by in situ freezing of within conduit sediment admixtures the later quiescent phase was characterised by meltwater flow directed transverse to ice flow cavity shrinkage and concomitant deposition of subglacial sheetwash and cohesive sediment flows along cavity margins cavity closure resulted in complete coupling at the ice substrate interface with localised glaciotectonic loading on underlying lee side stratification sequences and concomitant till deposition these late stage sedimentation events at shinnan hill produced a very complex polygene endform which cannot be simply attributed to one formative process more realistically they reflect the influence of a rather complex and very dynamic subglacial system subject to complex spatial and temporal variations in subglacial erosion deposition and meltwater flux at the time of drumlinisation

mantle earthquakes crustal structure and gravitational instability beneath western north island new zealand a cluster of 30 52 km deep earthquakes and a 7 10 km step in the moho beneath western north island new zealand are both interpreted as manifestations of active delamination of the continental lower crust and mantle lithosphere these phenomena occur in the back arc region beneath the east west oriented taranaki ruapehu tr line which strikes at a high angle to the present day plate boundary through new zealand across the line there is an abrupt change in crustal and mantle lid thickness and in upper mantle seismic attenuation q p 1 showing that the mantle lithosphere has been highly thinned on the north side we show from a receiver function profile that nearly all of the deep earthquakes reside in the uppermost mantle on the northern side of the tr line a sum of earthquake moment tensors suggests strike slip motion either parallel or orthogonal to the tr line resulting in northwest southeast oriented horizontal extension active normal faults oriented northwest southeast and north south are seen at the surface on each side of the tr line but the surface is uplifting here at 0 4 mm yr this requires the mantle lithosphere to be thinning at a higher rate than the overlying crust consistent with a delamination process

static and dynamic support of western united states topography article history accepted 7 october 2013 available online 31 october 2013 editor p shearer isostatic and dynamic models of earth s surface topography can provide important insights into the driving processes of tectonic deformation we analyze these two estimates for the tectonically active western united states using refined structural models derived from earthscope usarray for the crust use of recent moho depth measurements and crustal density anomalies inferred from passive source seismology improve isostatic models however seismically determined lithospheric thickness variations from lithosphere asthenosphere boundary lab maps and lithospheric and mantle density anomalies derived from heat flow or uppermost mantle tomography do not improve isostatic models substantially perhaps this is a consequence of compositional heterogeneity a mismatch between thermal and seismological lab and structural complexity caused by smaller scale dynamics the remaining non isostatic dynamic component of topography is large topography anomalies include negative residuals likely due to active subduction of the juan de fuca plate and perhaps remnants of formerly active convergence further south along the margin our finding of broad scale positive residual topography in the basin and range substantiates previous results implying the presence of anomalous buoyancy there which we cannot fully explain the colorado plateau does not appear dynamically anomalous at present except at its edges many of the residual topography features are consistent with predictions from mantle flow computations this suggests a convective origin and important interactions between vigorous upper mantle convection and intraplate deformation

isostasy flexure and dynamic topography abstract a fundamental scientific question is what controls the earth s topography although the theoretical principles of isostasy flexure and dynamic topography are widely discussed the parameters needed to apply these principles are frequently not available isostatic factors controlling lithospheric buoyancy are frequently uncertain and non isostatic factors such as lithospheric bending towards subduction zones and dynamic topography are hard to distinguish the question discussed here is whether a set of simple rules that relate topography to lithospheric structure in various tectonic environments can be deduced in a way that missing parameters can be approximated or does each area behave differently making generalizations problematic we contribute to this issue analyzing the asia africa arabia europe domain following a top down strategy we compile a new crustal thickness map and remove the contribution of the crust from the observed elevation then the challenge is to interpret the residual topography in terms of mantle lithosphere buoyancy and dynamics based on systematic relationships between tectonic environments and factors controlling topography we argue that crustal buoyancy and mantle lithospheric density can be approximated from available geological data and that regions near mantle upwelling or downwelling are easily identified by their extreme residual topography yet even for other areas calculating lithospheric thickness from residual topography is problematic because distinguishing variations in mantle lithosphere thickness from sub lithospheric dynamics is difficult fortunately the area studied here provides an opportunity to examine this issue based on the conjunction between the afar plume and the mid ocean ridge in the nearby gulf of aden and southern red sea we constrain the maximal amplitude of dynamic topography to 1 km this estimate is based on a narrow definition of dynamic topography that only includes sub lithospheric processes and using mid ocean ridges as reference where mantle lithosphere buoyancy is zero