topographic expressions of mantle dynamics in the mediterranean abstract the surface of the earth is the ever changing expression of the dynamic processes occurring at depth and at or above its surface however our ability to read landscapes in terms of their underlying tectonic or climatic forcing remains rudimentary during the last decade particular attention has been drawn to the deep dynamic contributions to topography related to the stresses that are produced at the base of the lithosphere by mantle convection and their relevance compared to the iso static component of topography here we use examples from the mediterranean and estimate residual and dynamic topography we then compare those with surface uplift from geology for specific regions to disentangle the dynamic from the static components considering the different topographic signatures of tectonic processes e g actual residual and dynamic topography as well as uplift rates jointly provides a powerful way to distinguish between the contributions of mantle crustal and surface processes such an approach might bring us closer to reading topographic expressions in terms of their geological cause

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

a mass failure model for the initial degradation of fault scarps with application to the 1959 scarps at hebgen lake montana abstract calculation of earthquake scarp ages from scarp morphology usually assumes that scarp materials reach their angle of repose immediately after a rupture however observations of the 1959 hebgen lake montana earthquake scarp and similar features worldwide confirm that scarps require a finite period of mass failure to reach the initial conditions for hillslope diffusion so the age of features less than 1000 yr old cannot be accurately estimated with methods based only on the linear diffusion equation we apply a numerical model of this interval of mass failure degradation to vertical initial angle scarps from the 1959 rupture at hebgen lake montana the mass failure rate coefficient r m ranges from 1 0 10 2 to 1 2 10 1 m yr 1 for young scarps at hebgen lake and nine other locations and has little or no dependence on climate conditions such as annual temperature range or average rainfall including an interval of mass failure gives more accurate age estimates where a scarp age is of the same order as the characteristic mass failure relaxation time of 10 1000 yr

a mantle flow mechanism for the late paleozoic subsidence of the parana basin the carboniferous permian stage in the evolution of the parana basin is characterized by the long wavelength deposition of up to 2 5 km of sediment in the continental interior the tectonics of the region during this time were dominated by convergence and active subduction along the nearby panthalassan margin of gondwana while subsidence in the basin is clearly related to the major tectonic events at the plate margin there is a distinct delay between the onset of subduction and the initiation of basin deposition this and the nature of basin stratigraphy suggests that the large scale subsidence may have been the dynamic response of the lithosphere to mantle flow associated with the penetration of accumulated slab material through the phase change at 660 km depth we present numerical experiments of mantle convection which show that subducting slabs can stagnate at the 660 km depth phase change and intermittently penetrate through the associated negative dynamic topography remains at relatively low amplitudes as the subducting material pools and then increases rapidly to a maximum during the phase of penetrative flow we find that the predicted delay time between initial descent of the slab and the maximum surface topography corresponds with evidence from the geological record several models of sedimentation based on the evolving dynamic topography are presented and a simulation generated using infill rates independently estimated for the basin provides a close fit to the amplitude and horizontal length scale of material accumulation in the parana basin

the influences of lower mantle viscosity stratification on 3d spherical shell mantle convection we have studied the dynamical effects of a stratified viscosity profile on time dependent mantle circulations by using a 3d boussinesq spherical shell convection model this particular viscosity profile taken from the inversion of geoid data has the distinct feature of having a large viscosity maximum in the middle lower mantle surface rayleigh numbers ranging from 5 104 to 8 106 have been considered there are several distinct differences in both the morphologies of the plumes and the sinking currents from constant viscosity models in a viscously stratified lower mantle plumes are formed invariably only after the cold downwelling has reached the bottom the mean mantle temperature is reduced and this allows for the sinking of very cold material into the deep mantle the number of upwelling plumes is reduced and the overall circulation is stabilized with well organized triple junctions of cold sheets developed the presence of internal heating increases the number of upwelling plumes and breaks up the network of downwelling sheets these plumes become larger in size and have a hotter interior because of the nearly stationary character

evidence for a sharp 670 km discontinuity as inferred from p to s converted waves the fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short period and broadband seismograms using a stacking technique to analyze the p wave coda of teleseismic records evidence is found for coherent near receiver p to s converted phases generated by the 400 and 670 km discontinuity beneath a number of the stations used in this study variations in travel time slowness and amplitude of these phases as observed among the stacks for stations of the regional seismic test network in the united states and of the network of autonomously recording seismographs in western europe are very likely the expression of upper mantle heterogeneity observations of coherent converted phases from the 400 km discontinuity are fewer in number than of phases converted at the 670 km discontinuity suggesting that the latter is more pronounced some of the seismograms especially of station rscp show extremely high amplitude p to s converted phases from the 670 km discontinuity these seismograms allow a detailed waveform comparison of the converted phase with the direct p phase and present evidence for a sharp 670 km discontinuity

arc lavas on both sides of a trench slab window effects at the solomon islands triple junction sw pacific abstract the woodlark spreading center wsc is subducted at the san cristobal trench forming a triple junction at the new georgia group ngg arc in the solomon islands wsc lavas are n morb at 100 km from the trench but with decreasing distance they have increasingly arc like sr nd pb isotopic ratios enrichments in rb k pb sr and depletions in hfse and y within 50 km of the trench on the simbo and ghizo ridges many recovered samples are island arc tholeiites to medium k calc alkaline andesites and dacites and many have the same or similar major and trace element and isotopic characteristics as true arc lavas in the ngg on the other side of the trench previous investigations have concluded that these wsc lavas are the result of relic back arc mantle enrichments resulting from subduction of the pacific plate prior to the late miocene at the north solomon trench 200 km to the north however the high silica wsc lavas are more arc like than those recovered from other distal back arcs and are more voluminous forming large submarine ridges and stratovolcanoes we suggest that true arc mantle migrates across the plate boundary from the adjacent ngg arc through slab windows created by the subduction of the wsc this leads to variable mixing between ngg arc and wsc n morb end members forming the transitional lavas recovered from the wsc lavas with similar arc like characteristics have previously been recovered on the chile rise near where it is subducted at the chile trench raising the possibility that such mantle transfer is a common phenomenon where active spreading centers are subducted the presence of slab windows may also be responsible for the unusual forearc volcanism in the ngg and melting of slab window margins may account for the presence of high silica adakite like lavas on the wsc

lower mantle seismic discontinuities and the thermal morphology of subducted slabs abstract the nature of the lowermost few 100 km of the mantle holds insights into the scale of convection of the earth s mantle here we investigate the fine structure of this region using seismic migration of shear waves reflecting from velocity discontinuities near the core mantle boundary cmb recorded by a dense network of european seismic stations two main features are found one structure exhibits large scale topography and can be explained by a sharp increase in seismic velocity 206 316 km above the cmb the second structure lies 55 85 km above the cmb and marks a sharp reduction in velocity to explain the origin of these discontinuities we appeal to numerical simulations of three dimensional 3d mantle convection incorporating rigidly moving plates the plates organize the convective flow to allow for the development of large convection cells and cool sheet like downwellings at a convergent plate boundary the results suggest that in places observed seismic complexity in d discontinuity structure may be due to the thermal gradients resulting from the accumulation of cool subducted plate material at the cmb

the role of the heating mode of the mantle in intermittent reorganization of the plate velocity field summary the geological record indicates that stages of relatively steady plate motion have been punctuated by comparatively brief periods in which plate velocities have reorganized the distribution of buoyancy sources in the mantle has generally been regarded as evolving too slowly to explain these rapid transitions in plate velocity we investigate the feedback between mantle convection and plate velocity using 2 d and 3 d mantle convection models that incorporate mobile dynamic plates we focus on the influence of internal heating in the mantle and consider the effect of mantle viscosity stratification and different plate geometries on the plate velocity time dependence as either the rayleigh number or the internal heating rate is increased to magnitudes approaching mantle values the record of the plate motion from our calculations becomes characterized by intermittent changes in direction this behaviour is a result of the influence of plates on heat loss from the inherently unsteady internally heated convecting system plate motion instills a pattern of organization on the underlying convection that reflects the plate geometry and results in the formation of sheet like downwelling structures at convergent plate boundaries in both 2 d and 3 d calculations in contrast upwellings in 3 d models are not sheet like the role of the sheet like downwellings is critical in the observed episodic reorganization of the plate velocities warm material below the plates is entrained by plate motion into regions enveloping the downwelling sheets during periods of fairly steady plate motion buoyancy associated with the build up of heat around the downwelling sheets leads to the creation of an unstable convection pattern this build up of heat is dramatic in calculations with mantle like internal heating rates and resists continued long term plate motion towards mature downwellings when there are limitations on the degree of freedom of the direction of plate movement such as in 2 d models these effects become even more pronounced accordingly the effect of plates on mean global thermal quantities is more dramatic in 2 d calculations than it is in 3 d calculations nevertheless 3 d calculations incorporating plates of different sizes do exhibit rapid reorganizations in their convection patterns as the pull of young slablike features supersedes the pull of mature downwelling sheets we compare the timing and frequency of the reorganization events in our calculations with the general characteristics of plate motions determined from plate reconstruction studies

temperature measurements and heat transfer in near surface snow at the south pole to study near surface heat flow on the antarctic ice sheet snow temperatures were measured at south pole station to a depth of 3 m at 15 min intervals during most of 1992 solar heating and water vapor transport were negligible during the 6 month winter as was inter grain net thermal radiation leaving conduction as the dominant heat transport mechanism the rate of temperature change at depth over 15 min intervals was smaller than that at the surface by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth a finite difference model with conduction as the only heat transfer mechanism and measured temperatures as the upper and lower boundary conditions was applied to foursets of three thermistors each the thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor the thermal conductivity obtained at shallow depths above 40 cm was lower than that given by existing parameterizations based on density probably because the snow grains were freshly deposited cold and poorly bonded a model using only vertical conduction explains on average 87 ofthe observed 15 min temperature changes at less than 60 cm depth and 92 below 60 cm the difference between modeled andmeasured temperature changes decreased with depth the discrepancies between model and observation correlated more strongly with the air snow temperature difference than with the product of that difference with the square of the wind speed suggesting that the residual errors are due more to non vertical conduction and to sub grid scale variabilis of the conductivity than to windpumping the residual heating rate not explained by the model of vertical conduction exceeds 0 2 w m 3 only in the top 60 cm of the near surface snow

root mean square velocities of the continents with respect to the hot spots since the early jurassic the root mean square velocities of the major continents with respect to the hot spots were estimated for the past 180 m y by combining previously published plate reconstructions these velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data from 169 m y b p to l44 m y b p the hot spot data indicate that laurasia moved rapidly 70 mm yr but the reconstructions permit a velocity as low as 50 mm yr we also estimate that from 63 m y b p to 48 m y b p india moved rapidly 140 20 30 mm yr faster than the present velocity of any plate and more than twice as fast as the present velocities of several subducting oceanic plates those attached to downgoing slabs along a substantial fraction of their margin the data require no other continent to have exceeded a velocity of 45 mm yr during the last 180 m y in agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no nettorque analyses we conclude that the continents have moved faster occasionally much faster than at present from this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere instead oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents on the basis of the very rapid early tertiary motion of india we conclude that the velocities of subducting plates can vary by a factor of 2 therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid

episodic tectonic plate reorganizations driven by mantle convection abstract periods of relatively uniform plate motion were interrupted several times throughout the cenozoic and mesozoic by rapid plate reorganization events r hey geol soc am bull 88 1977 1404 1420 p a rona e s richardson earth planet sci lett 40 1978 1 11 d c engebretson a cox r g gordon geol soc am spec pap 206 1985 r g gordon d m jurdy j geophys res 91 1986 12389 12406 d a clague g b dalrymple us geol surv prof pap 1350 1987 5 54 j m stock p molnar nature 325 1987 495 499 c lithgow bertelloni m a richards geophys res lett 22 1995 1317 1320 m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 it has been proposed that changes in plate boundary forces are responsible for these events m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 we present an alternative hypothesis convection driven plate motions are intrinsically unstable due to a buoyant instability that develops as a result of the influence of plates on an internally heated mantle this instability which has not been described before is responsible for episodic reorganizations of plate motion numerical mantle convection experiments demonstrate that high rayleigh number convection with internal heating and surface plates is sufficient to induce plate reorganization events changes in plate boundary forces or plate geometry are not required

platform subsidence behind an active subduction zone numerical models of viscous mantle flow indicate that a broad subsidence should occur above subduction zones1 4 this platform subsidence is predicted to cause an asymmetrical depression 500 1 000 km wide with a maximum depth of 2 km if filled with water direct evidence linking platform subsidence to mantle flow has remained elusive however here we offer evidence from western new zealand for a complete link in both space and time between subduction initiation and platform subsidence because of the broad wavelength 200 km of the observed 1 500 m subsidence it is unlikely that the subsidence is due to a flexural foredeep the observed subsidence does however match in both wavelength and amplitude that predicted by a model of subduction induced mantle flow the occurrence of platform subsidence has implications not only for understanding mantle convection but also for explaining geoid anomalies over active margins5 and patterns of continental submergence6 and sedimentation7 that could otherwise be interpreted as a eustatic sea level change8

late miocene adakites and nb enriched basalts from vizcaino peninsula mexico indicators of east pacific rise subduction below southern baja california a typical slab melt association was emplaced from 11 to 8 ma in the santa clara volcanic field vizcaino peninsula baja california sur it includes adakitic domes and associated pyroclastic flow deposits together with lava flows of niobium enriched basalts the trace element and isotopic sr nd pb signatures of adakites are consistent with melting of altered mid ocean ridge basalts and the sources of the nb enriched basalts contain an enriched mantle wedge component such associations commonly form at depths of 70 80 km during low dip subduction of very young oceanic crust however the santa clara field is relatively close 100 km to the paleotrench which suggests that the genesis of its adakites and nb enriched basalts occurred in a very high thermal regime linked to the subduction of the then active guadalupe spreading center of the east pacific rise our data suggest that the asthenospheric window documented below northern baja california also developed beneath the south of the peninsula during the neogene this hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region

far field tilting of laurentia during the ordovician and constraints on the evolution of a slab under an ancient continent during a brief period of 10 to 15 million years in the middle ordovician the michigan basin departed from its bull s eye subsidence pattern and tilted toward the east opening to the appalachian basin this tilting is observed in maps of tectonic subsidence estimated for the black river and trenton formations and extends over 300 km across the michigan basin and into eastern wisconsin contours of constant tectonic subsidence rate are approximately parallel to the inferred position of the laurentian iapetus convergent margin the distance between the inferred position of the subduction zone to the limit of tilting is approximately 1000 km three alternative models for the tilting are tested two relying on the rigidity of the continental lithosphere and a third on the viscous flow generated by a subducted slab in the first elastic model we assume the edge of the elastic plate is simply loaded from above by a fold and thrust sheet volcanic pile etc this model however cannot simultaneously satisfy the space of tectonic subsidence and subsidence rate even for lithospheres which have a rigidity of 10 28 nm in the second elastic model the laurentian continental margin descends into a trench of an eastward dipping slab that is a slab descending under the iapetus ocean this process cannot generate any significant far field displacements even for extremely rigid plates and must be rejected for the third model we use finite element solutions of a negatively buoyant slab in a viscous medium with a faulted lithosphere such slabs can easily generate not only realistic trenches on the under thrusting plate but also significantly tilt the lithosphere as much as 1000 km from the plate margin the magnitude and distribution of far field displacements depend on the age length and dip angle of the slab in contrast to the elastic models penetration of a west dipping slab beneath the continent can reproduce both the extent magnitude and rate of tectonic subsidence observed in the trenton and black river formations the observed data are best fit by an old slab 140 ma which initially descended at a moderate dip 20 30 for 10 15 m y which then steepened as the slab penetrated deeper into the mantle at the position of the previous ordovician plate margin there are narrow block faulted basins which underwent rapid subsidence 10 m y before the michigan basin tilted toward the east we propose that these earlier subsidence events were caused by the initial descent of the slab under the preexisting cambro ordovician passive margin the time lag of 10 m y may be due to the time it takes the slab to penetrate the upper mantle this result is important for understanding the time evolution of mantle convection and mechanisms for the initiation of subduction

thermal evolution of the mantle following continental aggregation in 3d convection models the claim that supercontinents insulate the mantle is largely based on recognition that seismically slow mantle below the atlantic african geoid high coincides with the former location of pangea we investigate the viability of continental insulation by varying both plate geometries and mantle properties in six three dimensional 3d mantle convection models the efficiency of continental insulation is quantified by calculating the rate of change of the average temperature in the subcontinental mantle our findings generally agree with two dimensional 2d modeling results however we conclude that 2d convection models may exaggerate subcontinental heating particularly in largely bottom heated cases and that subcontinental heating results from an absence of subduction in the subcontinental mantle rather than the insulation of active upwellings correspondingly we find that when large degrees of internal heating are present in the mantle hotter than average regions evolve below large oceanic plates

tearing and breaking off of subducted slabs as the result of collision of the panama arc indenter with northwestern south america we present two regional lithospheric cross sections that illustrate eastward and southeastward dipping subducted slabs to depths of 315 km beneath the surface of colombia in northwestern south america these cross sectional interpretations are based on relocated earthquake hypocentral solutions models supported on gravity and magnetic regional data and coda q q c tomography the method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and thermal structure of the lithospheric system mapping of earthquake defined benioff zones combined with tomographic imaging reveals the presence of an 240 km long east west striking slab tear named here the caldas tear the proposed caldas tear separates a zone of shallow 20 30 dipping southeastward subduction in the area of colombia adjacent to panama and the caribbean sea which is not associated with subduction related volcanism from an area of steeper 30 40 dipping slab adjacent to the eastern pacific ocean that is associated with an active north south chain of active arc volcanoes we propose that the caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the panama indenter an extinct island arc that began subducting beneath northwestern south america about 12 ma the area south of the panama indenter is miocene oceanic crust of the nazca plate which subducts eastward beneath northwestern south america at normal angles and melts to form a north south trending active volcanic arc in addition to the formation of the caldas tear we propose that impedance of the thicker crustal area of the panama arc indenter over the past 12 ma may have led to down dip break off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate depth earthquakes beneath east central colombia

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

thermal structure of the panama basin by analysis of seismic attenuation abstract using recordings of earthquakes on oceanic bottom seismographs and onshore stations on the coastal margins of colombia panama and ecuador we estimate attenuation parameters in the upper lithosphere of the panama basin the tomographic images of the derived coda q values are correlated with estimates of curie point depth and measured and theoretical heat flow our study reveals three tectonic domains where magmatic hydrothermal activity or lateral variations of the lithologic composition in the upper lithosphere can account for the modeled thermal structure and the anelasticity we find that the costa rica ridge and the panama fracture zone are significant tectonic features probably related to thermal anomalies detected in the study area we interpret a large and deep intrinsic attenuation anomaly as related to the heat source at the costa rica ridge and show how interactions with regional fault systems cause contrasting attenuation anomalies

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

morphodynamics of a high discharge tropical delta san juan river pacific coast of colombia abstract the san juan river has one of the most extensive and best developed deltas on the pacific coast of south america measuring 800 km 2 the river drainage basin measures 16 465 km 2 and is located in one of the areas with the highest precipitation in the western hemisphere the annual rainfall varies from 7000 to 11 000 mm and as a result the san juan river has the highest water discharge 2550 m 3 s 1 sediment load 16 10 6 t yr 1 and basin wide sediment yield 1150 t km 2 yr 1 on the west coast of south america the san juan delta growth began approximately 5000 years bp the structure of the delta is determined by the interactions between fluvial deposition and the effect of 1 7 m significant swells mostly from the sw and strong tidal currents analysis of delta progradation indicates that during 1848 1992 the morphology of the delta was characterized by beach ridge accretion spit growth narrowing of inlets and a general advance of the delta shoreline during the past decade processes such as rapid erosion of the delta shore narrowing of barrier islands and breaching of a new inlet are the result of a long term relative sea level rise of 2 6 mm yr 1 due to tectonically induced subsidence coupled with a eustatic rise of sea level the delta also experiences strong oceanographic manifestations associated with the el nino la nina cycle causing regional sea level elevation of 20 30 cm during el nino years recent coastal subsidence in the delta is evidenced by 1 increased occurrence of non storm washover events 2 increased erosion of barrier islands with average loss of 11 m yr 1 during 1993 1997 and 3 a relative sea level rise of 3 4 mm yr 1 during 1991 1999 the morphology and recent evolution of the san juan delta are unique when compared to other deltas of south america because of the singular combination of extreme climatic geologic and oceanographic conditions under which the delta has formed and the absence of human induced impact in the drainage basin

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

origins of topography in the western u s mapping crustal and upper mantle density variations using a uniform seismic velocity model to investigate the physical basis for support of topography in the western u s we construct a subcontinent scale 3 d density model using 1000 estimated crustal thicknesses and s velocity profiles to 150 km depth at each of 947 seismic stations crustal temperature and composition are considered but we assume that mantle velocity variations are thermal in origin from these densities we calculate crustal and mantle topographic contributions typical 2 uncertainty of topography is 500 m and elevations in 84 of the region are reproduced within error remaining deviations from observed elevations are attributed to melt variations in crustal quartz content and dynamic topography compositional variations in the mantle while plausible are not necessary to reproduce topography support for western u s topography is heterogeneous with each province having a unique combination of mechanisms topography due to mantle buoyancy is nearly constant within 250 m across the cordillera relief there 2 km results from variations in crustal chemistry and thickness cold mantle provides 1 5 km of ballast to the thick crust of the great plains and wyoming craton crustal temperature variations and dynamic pressures have smaller magnitude and or more localized impacts positive gravitational potential energy gpe anomalies 2 1012n m calculated from our model promote extension in the northern basin and range and near the sierra nevada negative gpe anomalies 3 1012n m along the western north american margin and yakima fold and thrust belt add compressive stresses stresses derived from lithospheric density variations may strongly modulate tectonic stresses in the western u s continental interior

bulge migration and pinnacle reef development devonian appalachian foreland basin abstract detailed stratigraphic analyses of late emsian and early eifelian lower to middle devonian carbonate dominated strata in the northern appalachian basin indicate anomalous locally varying relative sea level changes and inversions of topography the distribution of a major basal bounding unconformity basinal pinnacle reefs local absence of parasequences and eastward migration of shallow marine carbonate lithofacies and related biofacies in the onondaga limestone and underlying strata mark the retrograde migration of an elongate northeast southwest trending area of positive relief bordered on its cratonward side by a similarly migrating basin of intermediate depth these features are thought to represent the forebulge and back bulge basin of the appalachian foreland basin system as it developed during a time of relative quiescence within the acadian orogeny however the relatively small size of the bulgelike feature ca 80 100 km wide 20 50 m positive relief its great distance from the

tectonic segmentation of the north andean margin impact of the carnegie ridge collision the north andean convergent margin is a region of intense crustal deformation with six great subduction earthquakes mw 7 8 this century the regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the andes segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction this segmentation is related to heterogeneity on the subducting nazca plate in particular the influence of the carnegie ridge collision is investigated four distinct seismotectonic regions can be distinguished region 1 from 6on to 2 5on with steep ese dipping subduction and a narrow volcanic arc region 2 from 2 5on to 1os showing an intermediate depth seismic gap and a broad volcanic arc region 3 from 1os to 2os with steep ne dipping subduction and a narrow volcanic arc region 4 south of 2os with flat subduction and no modern volcanic arc the carnegie ridge has been colliding with the margin since at least 2 ma based on examination of the basement uplift signal along trench parallel transects the subducted prolongation of carnegie ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed broad volcanic arc these findings conflict with previous tectonic models suggesting that the carnegie ridge entered the trench at 1 ma furthermore the anomalous geochemical adakitic signature of the volcanoes in the broad ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant shallowly subducting carnegie ridge from segments of steep subduction in regions 1 and 3 it is further suggested that carnegie ridge supports a local flat slab segment similar to that observed in peru the impact of the carnegie ridge collision on the upper plate causes transpressional deformation extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the san andreas fault system the pattern of instrumental and historical seismicity indicates 1 great earthquakes on the northern and southern flanks of the colliding ridge 2 a slight reduction in observed seismicity at the trench ridge intersection 3 increased stress far into the continent and 4 a nne displacement of the n andes block to be further effects of the collision 1999 elsevier science b v all rights reserved

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

excitation of true polar wander by subduction true polar wander the global motion of the mantle relative to the earth s rotation axis is known from the analysis of palaeomagnetic data hotspot tracks and plate motions to have occurred at velocities of up to 0 5 myr 1 since the late cretaceous period1 4 we address here the longstanding question of how fast episodes of true polar wander tpw can be excited5 by analysing the impact of the distribution and activity of subduction zones on polar motion using nonlinear liouville equations which allow us to treat large excursions of the polar axis we show that unrealistically fast tpw is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle this need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on post glacial rebound and geoid anomalies theoretical creep laws and some preliminary results on tpw induced by density anomalies embedded in the mantle6 10 the lower viscosity in the upper mantle means that upper mantle density anomalies are most effective in exciting tpw changes in the pattern of subduction through time may be responsible for both episodes of fast tpw and times of quiescence in polar motion

mantle avalanches and the dynamic topography of continents abstract the dynamic topography of continents associated with mantle avalanche events are discussed our flow simulations involve two dimensional axisymmetric multi phase mantle models which incorporate compressibility and a realistic factor of 40 increase in viscosity with depth the surface kinematic boundary conditions involve a mix of free and rigid i e no slip constraints with the latter intended to mimic the influence of a spherical continental cap we find that avalanche events support regional vertical deflections of up to 2 5 km the horizontal length scale of the dynamic topography is 2500 km and non negligible deflections can persist on the order of 100 200 ma we also find that convergence rates in regions surrounding the continents are variable but can reach rates of the order of 3 cm a finally we speculate that an avalanche event may have been responsible for the development of the large scale intracontinental basin system in the western interior of north america from the early devonian to late carboniferous 1

effects of mantle heat source distribution on supercontinent stability a two dimensional cartesian geometry numerical model is used to study the influence of internal heating on the ability of mantle convection to rift and disperse an assembled supercontinent our wide aspect ratio models incorporate migrating plate boundaries and finite thickness rigid mobile oceanic and continental plates 12 000 km across we examine models characterized by both whole mantle and upper mantle convection parameters specifying different degrees of internal heating in the mantle we compare the patterns of convection that develop below stationary continental plates that form as a result of the aggregation of smaller mobile continents following continental collisions we find that a reorganization of mantle flow may result in the development of deviatoric stresses at the base of the continental lithosphere which are capable of initiating continental breakup our results suggest that continental basal stresses are largely controlled by the wavelength of the subcontinental convection pattern and that the inclusion of internal heating in the mantle generally increases this wavelength for models with 80 internal heating in which active upwellings are intrinsically absent basal stresses arise primarily from subduction at the continental margins and result in a geologically reasonable duration for our model supercontinent assemblages 200 myr for models with 40 or less internal heating continental breakup is controlled by mantle upwellings and our supercontinents remain intact much longer 600 700 myr

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

seismic imaging of the upper mantle beneath the rocky mountain foreland southwestern alberta the vibroseis augmented listen time vault experiment is a special seismic reflection survey designed to image the fine scale structure of the continental upper mantle of the rocky mountain foreland in southwestern alberta to depths of 200 km or more two mutually perpendicular profiles were acquired across and within the vulcan structure a roughly east west trending tectonic belt in the crystalline basement beneath the western canada sedimentary basin that separates the medicine hat block from the loverna block relative amplitude preserving processing procedures were developed to estimate the seismic signal penetration limit which varies between 100 and 220 km depth amplitude decay analysis and q estimation show that a seismically unreflective zone within the vulcan structure is not caused by inadequate signal penetration this blank zone is interpreted as part of an intrusive complex that has overprinted the preexisting structural fabric unlike most other parts of alberta the reflection moho is i

development of the gulf of guayaquil ecuador during the quaternary as an effect of the north andean block tectonic escape 1 interpretation of industrial multichannel seismic profiles and well data are used to identify the main tectonic features of the gulf of guayaquil gg area these include two e w trending major detachments the posorja and the jambeli detachment systems which represent half grabens with oppositely dipping detachments to the south and to the north respectively the ne sw trending puna santa clara fault system developed as a transfer fault system between the posorja and the jambeli detachments the esperanza and the jambeli basins exhibit 3 4 km of sediment that accumulated during the past 1 6 1 8 myr the puna santa clara fault system bounds the esperanza and the jambeli basins evidencing that the evolution of these basins is tightly controlled by the two detachments at depth to the west the n s trending domito fault system bounding the posorja detachment system and the esperanza basin to the west acted as a transfer zone between the shelf area and the continental slope the pliocene series show no significant variations in thickness throughout the gulf of guayaquil area suggesting that no important tectonic deformation occurred from 5 2 to 1 8 1 6 ma the major period of tectonic deformation in the gulf of guayaquil area occurred during the pleistocene times three main tectonic steps are identified from early pleistocene to 180 ka major subsidence occurred along the esperanza and jambeli basins from 180 to 140 ka most of the gulf of guayaquil area was above sea level during the isotope substage 6 low stand from 140 ka to present tectonic activity is restricted along the normal faults bounding the esperanza basin the tenguel fault and the puna santa clara and domito fault systems a n s trending tensional stress regime characterizes the pleistocene times throughout the northward drifting of the north andean block is proposed to control the tectonic evolution and associated subsidence of the gulf of guayaquil area it is also accepted that the collision of the carnegie ridge with the trench axis has to play a major role in controlling the north andean block northward drift because the carnegie ridge subduction is possibly an ongoing process which began prior to the pliocene we postulate the along strike morphology of the ridge at the origin of interplate coupling variations the subduction of an along strike positive relief of the ridge is proposed at the origin of the major tectonic reorganization of the gg area occurring at 1 8 1 6 ma

curie point depth in the sw caribbean using the radially averaged spectra of magnetic anomalies abstract we have estimated the curie point depth cpd using the average radial power spectrum in a tectonically complex area located in the sw caribbean basin data analyzed came from the world digital magnetic anomaly map and three methods have been used to compare results and evaluate uncertainties centroid spectral peak and forward modeling results show a match along the three methods suggesting that the cpd values in the area ranging between 6 km and 50 km the results share the following characteristics a high values 30 km are in continental regions b there is a trend of maximum cpd values along the sw ne direction starting from the central cordillera in colombia to the maracaibo lake in venezuela c there is a maximum cpd at the sierra nevada de santa marta colombia as well as between costa rica nicaragua and nicaragua honduras borders the lowest cpd values

the opening of sirte basin result of slab avalanching abstract north africa s sirte basin opening is an enigmatic feature in the complex meso cenozoic rearrangement of mediterranean tectonics new borehole data inversion constrains its deformation history showing a stretching event starting 70 ma and terminating in a further abrupt increase at 50 ma rapidly fading afterwards the timing of this event hardly reconciles with the mesozoic major plates reorganization following the spreading of the atlantic and the neogene central mediterranean tectonics active at different times here we propose that sirte rifting could have been driven by the pull exerted by the hellenic subduction reconstructions of hellenic convergence and slab deep subduction as constrained by plate kinematics and tomography show that large slab mass accumulates in the upper mantle by late cretaceous paleogene eventually forcing further sinking in the lower mantle coeval to the growing strain evolution recorded in sirte furthermore the 20 m y phase of large pull here recorded the consequent rapid growth over 10 m y and following decay are compatible with the dynamics of slab avalanche as revealed by numerical models showing that the sirte basin opened in response to the large pull force developed during the mass flush and transferred from deep slab to surface

the contribution of discontinuous rock mass failure to glacier erosion geomechanical rock mass properties control the response of bedrock to applied stresses and can be summarized in a linear mohr coulomb equation which defines the principal parameters determining failure nevertheless in studying the erosion of bedrock by glacier ice little attention has been paid to failure criteria though a coincidence of erosion all and forms with fracture systems at regional and local scales has been demonstrated few studies have analysed the precise nature of the fracture geometry or proposed its mechanical impact in association with glacier ice this investigation proposes that since almost all bedrock possesses identifiable fracture systems the properties of discontinuous rock mass drm be regarded as defining primary conditions of stress and stability which are subsequently modified directly and indirectly by glacier ice consequent rock mass failure modes are prescribed by discontinuity geometry and applied stresses and evidence from north wales confirms the validity of the theoretical treatment of rock mass properties and explains the accordance of landforms with structure

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

catastrophic overturn of the earth s mantle driven by multiple phase changes and internal heat generation the effects of phase changes and strong internal heat generation may combine to bring about brief but extremely intense episodes of rapid thermal convection in the earth s mantle numerical calculations using realistic thermodynamic properties for the exothermic olivine spinel and endothermic spinel perovskite magnesiowustite phase transitions suggest the transition region of the earth s mantle may act as a capacitor for subducting slabs slab material accumulates in the transition region until a threshold level of thermal buoyancy is reached and is then rapidly discharged into the lower mantle in my calculations this occurs as a catastrophic burst of convection lasting 10 myr with elevated heat transfer rates lasting 100 myr such episodes may be analogs to superplume activity which has been hypothesized to give rise to an intense episode of intra plate volcanism and stabilize the geodynamo against reversals the topography on the two phase change boundaries is found to be negatively correlated with the correlation becoming more negative during periods of rapid convection furthermore the results of this study suggest the transition region of the earth s mantle could be 250 k cooler on average and consequently more viscous than the surrounding mantle

extreme firn metamorphism impact of decades of vapor transport on near surface firn at a low accumulation glazed site on the east antarctic plateau snow and firn properties control the transport of vapor gases and water between the atmosphere and the underlying strata an understanding of this transport and the properties that control it is important for predicting air snow transfer of chemical species and for interpreting ice cores remote sensing images of east antarctica show large areas of alternating light and dark bands these low amplitude long wavelength features have glazed downwind faces and rough upwind faces and are called megadunes the first linked measurements of the permeability and the associated microstructure for a glazed area within a well defined megadune area are reported in this paper permeability and density were measured along with grain scale properties derived from digital image processing of preserved thick sections at this cold low accumulation glazed site a clear layering pattern exists in the top meter the firn density ranges from 0 24 to 0 50 g cm 3 permeability measurements range from 50 x 10 10 to 200 x 10 10 2 several times greater than corresponding profiles from warmer higher accumulation sites like siple dome antarctica it is shown that buoyancy driven natural convection may be important in post depositional processes in very cold low accumulation sites like this

fan beheading and drainage diversion as evidence of a 3200 2800 bp earthquake event in the esmeraldas tumaco seismic zone a case study for the effects of great subduction earthquakes abstract the san lorenzo area belongs to the esmeraldas tumaco seismic zone where some of the strongest earthquakes of south america occurred during the 20th century this paper provides evidence for a succession of geomorphic changes characterized by the disruption of the quaternary drainage network and the reshaping of the cayapas santiago estuary the rise of the la boca uplift bordered by the la boca and san lorenzo faults is responsible for the southward diversion of the palabi tululbi bogota and carolina rivers toward the santiago and cayapas rivers the increase of the discharge directed to the cayapas river generated the change of the channel pattern downstream from the confluence and the avulsion of a new estuary through the coastal plain according to the dating of beach ridges the avulsion occurred in the period 3200 2800 bp this period corresponds also to a faster accretion of the beach ridge margin interpreted as a response to a small uplift of the shore the coherency of the three morphologic evidences diversion of drainage network avulsion and increase of coastal accretion suggest a unique morphotectonic event in relation with the activity of the esmaraldas tumaco seismic zone the opening of a direct communication through the mangrove margin may have brought favorable conditions for the development of the la tolita archaeological site after 3000 bp

potential tsunamigenic submarine landslides in active margins abstract a review of modern historical and submarine landslides from the geological record shows that landslides in active continental margins can generate tsunamis the tsunamis may damage coastal and seabed infrastructure and so represent an important element of marine geohazards research due to their potentially significant impacts on society the primary trigger mechanism of tsunamis in this type of setting was thought to be earthquake activity however there are also a number of alternative hypotheses regarding the likely initiation mechanism including the generation of submarine landslides in this paper we briefly review the geological features and trigger mechanisms of tsunamigenic submarine landslides on active margins large tsunamigenic submarine landslides appear to occur mostly on margins characterized by non accretion these observations have implications for tsunami warning systems as the japanese system does not consider the scenario of tsunami excitation by submarine landslides

new constraints on the properties of the yellowstone mantle plume from p and s wave attenuation tomography 1 we estimate attenuation t for teleseismic p and s arrivals to seismometers in the yellowstone intermountain seismic array tomographically invert these data for upper mantle qp 1 and qs 1 structure and with the aid of the upper mantle velocity model of waite et al 2006 interpret the results for mantle temperature partial melt and water content because attenuation analysis is susceptible to contamination by noise we employ time and frequency domain analyses and a careful assessment of the uncertainty associated with each estimate by using the misfit between actual and predicted traces or spectra the greatest noise source is signal generated noise which affects s wave results more than p wave results s waves also appear more susceptible to the effects of plume focusing we find the upper mantle to be highly attenuative but that above 200 250 km the low velocity plume is progressively less attenuative than the adjacent mantle with decreasing depth we conclude that water dissolved in mantle minerals causes the upper mantle to be highly attenuative that the yellowstone plume is only 50 c warmer than the surrounding mantle below the north america lithosphere and that melting within this plume begins at depths of 200 250 km we attribute the lower attenuation in the partially molten plume to the dehydration of the solid matrix as water partitions into the melt the source of the upper mantle hydration is attributed to subduction including water flux from a hydrated transition zone and laramide age shallow subduction and direct hydration of north america lithosphere

mantle stratification and long term polar wander there is accumulating evidence1 2 that the average location of the earth s magnetic pole has moved relative to the deep mantle over the past 200 myr this phenomenon of true polar wander may be caused by the advance and retreat of ice sheets3 or by mass redistribution in the earth s interior due to changes in the pattern of mantle convection4 5 new analyses1 2 6 of polar wander data show a significant shift of the pole in the late cretaceous but this period is thought to have been too warm for glaciation to have occurred thus most of the true polar wander must be due to mass movements in the mantle here we show by analysing the appropriate equations for polar wander that both viscosity and chemical stratification in the mantle are important in determining the rate of polar wander the dynamical effects of chemical stratification and high viscosity 1023 poise in the lower mantle are to decrease polar wander speed to a level consistent with the averaged velocities inferred from palaeomagnetic data whole mantle convection models without non adiabatic density jumps at 670 km depth and with viscosities less than 1023 p would produce wandering rates in excess of 5 per myr

creep in pelagic sediments and potential for morphologic dating of marine fault scarps cores from the flanks of the galapagos spreading centre show that the top 25 cm of pelagic sediments is highly bioturbated kenyan and turcotte 1985 have proposed that bioturbation may cause down slope creep of the sediment surface and bioturbation may therefore smooth sediment topography in a similar way to hillslope creep on land assuming this produces movement consistent with the diffusion transport model sediment topography across scarp crests digitised from deep tow profiler records is modelled with a simple analytic solution to the diffusion equation and with a mean diffusion constant k of 0 007 m yr this small value suggests creep rates are modest for example 2 cm yr for a 30 slope and is an upper bound since sediment avalanching and bottom currents probably also remove sediment from scarp crests although requiring further work these results suggest that morphologic dating methods may be applicable to marine fault scarps and could be useful in relatively sediment free areas such as mid ocean ridges where it is commonly not possible to use datable offset seismic horizons

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

creation of the cocos and nazca plates by fission of the farallon plate abstract throughout the early tertiary the area of the farallon oceanic plate was episodically diminished by detachment of large and small northern regions which became independently moving plates and microplates the nature and history of farallon plate fragmentation has been inferred mainly from structural patterns on the western pacific plate flank of the east pacific rise because the fragmented eastern flank has been subducted the final episode of plate fragmentation occurred at the beginning of the miocene when the cocos plate was split off leaving the much reduced farallon plate to be renamed the nazca plate and initiating cocos nazca spreading some oligocene farallon plate with rifted margins that are a direct record of this plate splitting event has survived in the eastern tropical pacific most extensively off northern peru and ecuador small remnants of the conjugate northern rifted margin are exposed off costa rica and perhaps south of panama marine geophysical profiles bathymetric magnetic and seismic reflection and multibeam sonar swaths across these rifted oceanic margins combined with surveys of 30 20 ma crust on the western rise flank indicate that i localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred km wide fissural volcanism along some of these fractures built volcanic ridges e g alvarado and sarmiento ridges that are 1 2 km high and parallel to absolute farallon plate motion they closely resemble fissural ridges described from the young western flank of the present pacific nazca rise ii for 1 2 m y prior to final rupture of the farallon plate perhaps coinciding with the period of lithospheric stretching the entire plate changed direction to a more easterly nazca like course after the split the northern cocos part reverted to a northeasterly absolute motion iii the plate splitting fracture that became the site of initial cocos nazca spreading was a linear feature that at least through the 680 km of ruptured oligocene lithosphere known to have avoided subduction did not follow any pre existing feature on the farallon plate e g a fracture zone trail of a transform fault iv the margins of surviving parts of the plate splitting fracture have narrow shoulders raised by uplift of unloaded footwalls and partially buried by fissural volcanism v cocos nazca spreading began at 23 ma reports of older cocos nazca crust in the eastern panama basin were based on misidentified magnetic anomalies there is increased evidence that the driving force for the 23 ma fission of the farallon plate was the divergence of slab pull stresses at the middle america and south america subduction zones the timing and location of the split may have been influenced by i the increasingly divergent northeast slab pull at the middle america subduction zone which lengthened and reoriented because of motion between the north america and caribbean plates ii the slightly earlier detachment of a northern part of the plate that had been entering the california subduction zone contributing a less divergent plate driving stress and iii weakening of older parts of the plate by the galapagos hotspot which had come to underlie the equatorial region midway between the risecrest and the two subduction zones by the late oligocene

influence of convergent plate boundaries on upper mantle flow and implications for seismic anisotropy shear wave splitting observations in the region of the upper mantle enveloping subduction zones have been interpreted as showing extensive regions of trench parallel flow despite the difficulty of reconciling such behavior with a sound model based on the forces that drive mantle motion to gain insight into the observations we systematically investigate flow patterns around the cold downwelling sheets associated with consumed plate material in a three dimensional numerical mantle convection model first we compare results from calculations employing prescribed plate geometries and kinematic plate velocities where the convergent plate boundary morphology is varied while keeping the plate velocity and convective parameters fixed subsequently we examine the flow around sheet like downwellings in a number of convection calculations featuring dynamically evolving plate velocities all of the calculations include thick viscous plates and a stratified mantle viscosity in all of the models examined we find that at mid upper mantle depths flow directions no longer align with plate motion and the influence of buoyancy driven downwellings clearly dominates flow solutions in the first models analyzed a pair of plates are included in the calculations and the large scale flow is generally roll like in the final model we investigate the interaction of four plates and a plate geometry characterized by triple junctions we examine a sequence from this calculation that features a triple junction of convergent boundaries in this model large scale flow characterized by convection rolls is superseded by a complex flow solution where flow in the mid upper mantle neither aligns uniformly with the plate motion nor necessarily follows the forcing associated with local buoyancy sources in this setting upper mantle flow in the vicinity of the sheet like downwellings featured in the solution moves orthogonal obliquely and even parallel to different sections of the convergent plate boundaries in the latter case our calculations of the deformation of a fixed volume parcel of upper mantle material suggest that an olivine lattice preferred orientation should develop that would result in a fast polarizing direction for seismic shear waves parallel to the slab our findings have implications for the interpretation of flow in the upper mantle based on seismic anisotropy

mountain building and mantle dynamics 1 mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen a general dynamic model for this process is still lacking here we propose that mountain belts can be classified between two end members first those of slab pull type where subduction is mainly confined to the upper mantle and rollback trench motion lead to moderately thick crustal stacks such as in the mediterranean second those of slab suction type where whole mantle convection cells conveyor belts lead to the more extreme expressions of orogeny such as the largely thickened crust and high plateaus of present day tibet and the altiplano for the slab suction type deep mantle convection produces the unique conditions to drag plates toward each other irrespective of their nature and other boundary conditions we support this hypothesis by analyzing the orogenic volcanic and convective history associated with the tertiary formation of the andes after 40 ma and himalayas after collision at 55 ma based on mantle circulation modeling and tectonic reconstructions we surmise that the forces necessary to sustain slab suction mountain building in those orogens derive after transient slab ponding from the mantle drag induced upon slab penetration into the lower mantle and from an associated surge of mantle upwelling beneath africa this process started at 65 55 ma for tibet himalaya when the tethyan slab penetrated into the lower mantle and 10 myr later in the andes when the nazca slab did this surge of mantle convection drags plates against each other generating the necessary compressional forces to create and sustain these two orogenic belts if our model is correct the available geological records of orogeny can be used to decipher time dependent mantle convection with implications for the supercontinental cycle

the influence of tectonic plates on mantle convection patterns temperature and heat flow summary the dynamic coupling between plate motion and mantle convection is investigated in a suite of cartesian models by systematically varying aspect ratios and plate geometries the aim of the study presented here is to determine to what extent plates affect mantle flow patterns temperature and surface heat flux to this end we compare numerical convection models with free slip boundary conditions to models that incorporate between one and six plates where the geometries of the plates remain fixed while the plate velocities evolve dynamically with the flow we also vary the widths of the plates and the computational domain in order to determine what constraint these parameters place on the mean temperature heat flux and plate velocity of mantle convection models we have investigated the influence of plates for three whole mantle convection cases that differ in their heating modes internally heated and basally heated and rheologies isoviscous and depth dependent viscosity we present a systematic investigation of over 30 models that exhibit increasingly complex behaviour in order to understand highly time dependent systems using the insight gained from simpler models in models with aspect ratios from 0 5 to 12 we find that for the same heating mode variations in temperature can be as much as 40 per cent when comparing calculations with unit width plates to models incorporating plates with widths equal to five times the model depth mean surface heat flux may decrease by 60 per cent over the same range of plate widths we also find that internally heated mantle convection models incorporating plates exhibit novel behaviour that we believe has not been described previously in mantle convection studies specifically in internally heated models plate motion is characterized by episodic reversals in direction driven by changes in the mantle circulation from clockwise to counterclockwise and vice versa these flow reversals occur in internally heated convection and are caused by a build up of heat in the interiors of wide convection cells close to mantle downwellings we find that flow reversals occur rapidly and are present in both single plate and multiple plate models that include internal heating this behaviour offers a possible explanation for why the pacific plate suddenly changed its direction some 43 ma

plate tectonics and aspherical earth structure the importance of poloidal toroidal coupling we derive poloidal flow internal loading green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities from the green function we obtain kernels connecting the surface divergence the geoid and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques and with these kernels we argue that both the surface divergence and geoid fields consisting of harmonic degrees 2 5 may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km we point out however that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of 2 0 0 5 1021 pa s we comment on the implication of the difference between this number and the somewhat lower value 1 1021 pa s which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle