latent heat effects of the major mantle phase transitions on low angle subduction abstract very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate we have investigated the effects of latent heat for this situation where mantle material is pushed through the major mantle phase transitions during shallow low angle subduction below the overriding plate the significance of the buoyancy forces arising from the latent heat effects on the dynamics of the shallowly subducting slab is examined by numerical modeling when a 32 ma old slab is overridden with 2 5 cm yr by a continent flat subduction occurs with a 4 5 cm yr convergence rate when latent heat is included in the model forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces under these circumstances the flat slab segment subducts horizontally about 350 km further and for about 11 ma longer than in the case without latent heat before it breaks through the 400 km phase transition the style of subduction strongly depends on the mantle rheology increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow similarly an overriding velocity of less than 1 cm yr leads to steep subduction which gradually changes to flat subduction when increasing the overriding velocity however these model parameters do not change the aforementioned effect of the latent heat provided that low angle subduction occurs in all models latent heat resulted in a substantial increase of the flat slab length by 300 400 km varying the olivine spinel transition clapeyron slope from 1 to 6 mpa k reveals a roughly linear relation between and the horizontal length of the slab based on these results we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low angle subduction below an overriding plate

the coupling of indian subduction and asian continental tectonics abstract in order to understand the potential controls on asian tectonics during the subduction of the tethys and indian lithospheres we reconstruct the coupled subduction continent deformation history using tomographic imaging kinematics constraints and numerical modeling the global p waves tomographic images of the mantle below the india asia collision zone provide constraints on the deep structure of continents and subduction history linking the slab positions in the mantle to the asian tectonics reconstructions and the indian plate kinematics we reconstruct the timing and location of successive subduction and breakoff events showing one major breakoff occurred between india and the tethys ocean 45 ma in the western syntax a vertical slab continuous to the continent is shown to override the deeper detached tethys slab in the central region similar structure is found with a detached slab yet closer to the tethys slab in the eastern syntax no slab is imaged it is inferred that after tethys slab had broke off subduction only resumed in the center of the margin while underthrusting took place at both extremities of the convergent margin during following convergence a second breakoff event detached the central indian slab from the margin 15 ma ago which renewed indian lithosphere underthrusting below asia this most probably occurred when the tibetan plateau was already uplifted implying that uplift is not a direct result of underthrusting numerical models of breakoff during subduction illustrate the controls of slab detachment on the complexities of the indian margin in these models the subduction of continental lithosphere resumes after breakoff only where this is entrained by the mantle flow associated with the long lasting oceanic slab sinking that is in the center of the margin while converging continent edges underthrusts the upper plate furthermore the breakoff during subduction has profound implications on the asian intra plate tectonics in the models the breakoff is rapidly followed by large stresses in the upper plate interiors propagating at large distance from the margin along a belt oriented at 45 from the trench the long term evolution of the asian continental tectonics shows drastic changes in the fault pattern with successive strike slip faulting across the asian continent which are in agreement with the mechanisms illustrated by the models transient large coupling at the trench caused by the breakoff events during india asia convergence offers an explanation for episodic nucleation of lithospheric faults within the asian continent and their link to deep processes

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

an evaluation of proposed mechanisms of slab flattening in central mexico central mexico is the site of an enigmatic zone of flat subduction the general geometry of the subducting slab has been known for some time and is characterized by a horizontal zone bounded on either side by two moderately dipping sections we systematically evaluate proposed hypotheses for shallow subduction in mexico based on the spatial and temporal evidence and we find no simple or obvious explanation for the shallow subduction in mexico we are unable to locate an oceanic lithosphere impactor or the conjugate of an impactor that is most often called upon to explain shallow subduction zones as in south america japan and laramide deformation in the us the only bathymetric feature that is of the right age and in the correct position on the conjugate plate is a set of unnamed seamounts that are too small to have a significant effect on the buoyancy of the slab the only candidate that we cannot dismiss is a change in the dynamics of subduction through a change in wedge viscosity possibly caused by water brought in by the slab

intracrustal extension of an archean orogen revealed using single grain u pb zircon geochronology the metaplutonic wawa gneiss domain in the southern superior province is part of an uplifted block that was exhumed during the proterozoic exposing the lower crustal 30 km levels of the late archean 2720 to 2670 ma abitibi wawa orogen previous structural and geophysical studies of the midcrustal wawa gneiss domain reveal subhorizontal shear zones that formed during orogen parallel extension and correspond to gently dipping seismic reflectors present at depth elsewhere in the orogen in order to bracket the age of this extensional deformation the ages of metre scale igneous bodies that crystallized before and after extension have been determined in four outcrops using single grain u pb zircon dating techniques the results constrain the time of extension to the interval between 2660 and 2645 ma and show that deformation was coeval with pegmatite emplacement partial melting of tonalite gneisses as old as 2925 35 ma and regional circa 2660 ma granulite facies metamorphism of adjacent more deeply eroded crust this extension and associated high grade metamorphism in the wawa gneiss domain occurred 10 to 25 m y after the cessation of regional compression major transcurrent fault movement and sediment deposition in the upper structural levels of the orogen the middle and lower crust was therefore mechanically decoupled from the upper crust during this period of orogen parallel extension this intracrustal style of deformation is interpreted to be the consequence of gravitational forces working on a thermally softened middle crust the detachment of lithosphere from the orogen base is the favored mechanism for generating these forces and for introducing the heat that caused synextension metamorphism cyclic construction and delamination of orogen lithosphere could account for even younger metamorphic strain events evident in nearby exposures of lower crust

shear heating induced lithospheric scale localization does it result in subduction abstract even though it is a well established fact that the earth is currently in a plate tectonics mode the question on how to break lithospheric plates and initiate subduction remains a matter of debate here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates it is shown that shear heating under some conditions i facilitates the formation of a lithospheric scale shear zone and ii is capable of stabilizing a lithospheric scale shear zone thus creating the necessary condition for subduction initiation to occur furthermore we demonstrate that not only the localization process is of importance but also the post localization stage where rapidly growing convective instabilities might prevent an incipient subduction zone from developing we develop scaling laws for both the localization and the post localization stages in the case of lithospheric localization the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere thus of the order of the competent layer thickness this shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller scale heterogeneities we compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that for earth like conditions a dry olivine rheology is more likely to result in subduction initiation a large plate age does not always increase the potential for subduction initiation as it increases the potential for convective instabilities to occur instead an optimal plate age exists for shear heating induced subduction initiation which is around 40 ma for a wet olivine rheology

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

dynamics of free subduction from 3 d boundary element modeling 1 in order better to understand the physical mechanisms underlying free subduction we perform three dimensional boundary element numerical simulations of a dense fluid sheet with thickness h and viscosity 2 sinking in an ambient mantle with viscosity 1 the mantle layer is bounded above by a traction free surface and is either 1 infinitely deep or 2 underlain by a rigid boundary at a finite depth h d similar to the typical geometry used in laboratory experiments instantaneous solutions in configuration 1 show that the sheet s dimensionless stiffness s determines whether the slab s sinking speed is controlled by the viscosity of the ambient mantle s 10 time dependent solutions with tracers in configuration 2 demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides the extra edge drag exerted by the flow around the sides causes transverse deformation of the slab and makes the sinking speed of a 3 d slab up to 40 less than that of a 2 d slab a systematic investigation of the slab s interaction with the bottom boundary as a function of 2 1 and h h delineates a rich regime diagram of different subduction modes trench retreating slab folding trench advancing and reveals a new advancing folding mode in which slab folding is preceded by advancing trench motion the solutions demonstrate that mode selection is controlled by the dip of the leading edge of the slab at the time when it first encounters the bottom boundary

numerical modeling of thermal structure circulation of h2o and magmatism metamorphism in subduction zones implications for evolution of arcs abstract numerical models on thermal structure convective flow of solid generation and transportation of h2o rich fluid in subduction zones are consolidated to have a comprehensive view of the subduction zone processes heat balance circulation of h2o magmatism metamorphism growth of arcs and continental margins a large scale convection model with steady subduction of a cold old slab 130 myr old predicts rapid 100 myr cooling of subduction zones resulting in cessation of magmatism the model also predicts that the mantle temperature beneath arcs and continental margins is greatly affected by the effective temperature of the subducting slab i e the age of the subducting slab if subduction of a young hot slab including ridge subduction occurs every 60 to 120 myr as is suggested for eastern asia the average temperature beneath arcs is increased by about 300 c which may explain the long lasting magmatism in eastern asia associated with subduction of young slabs and ridges thermal structure and circulation of h2o are greatly modified to cause a transition from 1 normal arc magmatism 2 forearc mantle melting to 3 slab melting to produce a significant amount 100 km3 of granitic melts associated with both high p t and low p t type metamorphism the last stage of 3 can result in formation of a granitic batholith belt and a paired metamorphic belts synthesis of the numerical models and observations suggest that episodic subduction of young slabs and ridges can explain heat source for generating a large amount of granitic magmas of batholiths synchronous formation of batholith and regional metamorphic belts and p t conditions of the paired metamorphism even the high p t metamorphism requires an elevated geothermal structure in the forearc region associated with ridge subduction although the emplacement of the batholiths and the regional metamorphic belts and the mass balance in subduction zones are not well constrained at present the episodic event associated with ridge subduction is thought to be essential for net growth of arcs and continental margins as well as for the long term heat balance in subduction zones

influence of tectonic overpressure onp tpaths of hp uhp rocks in continental collision zones thermomechanical modelling the principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial exhumation depth from pressure given by geobarometry however pressure deviation from lithostatic i e tectonic overpressure underpressure due to deviatoric stress and deformation is an intrinsic property of flow and fracture in all materials including rocks under geological conditions in order to investigate the influences of tectonic overpressure on metamorphic p t paths 2d numerical simulations of continental subduction collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle the experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab in the subduction channel and within the overriding plate during continental collision the main overpressure region that may influence the p t paths of hp uhp rocks is located in the bottom corner of the wedge like confined channel with the characteristic magnitude of pressure deviation on the order of similar to 0 3 gpa and 10 20 from the lithostatic values the degree of confinement of the subduction channel is the key factor controlling this magnitude our models also suggest that subducted crustal rocks which may not necessarily be exhumed can be classified into three different groups i uhp rocks subjected to significant 0 3 gpa overpressure at intermediate subduction depth 50 70 km p 1 5 2 5 gpa then underpressured at depth 100 km p 3 gpa ii hp rocks subjected to 0 3 gpa overpressure at peak p t conditions reached at 50 70 km depth in the bottom corner of the wedge like confined subduction channel p 1 5 2 5 gpa iii lower pressure rocks formed at shallower depths 40 km depth p 1 gpa which are not subjected to significant overpressure and or underpressure

thermo mechanical controls of flat subduction insights from numerical modeling this study was supported by national basic research program of china 2014cb440901 the strategic priority research program b of cas xdb18020104 national science foundation of china 41190073 41372198 and 41304071 and nerc grant ne j021822 1

subduction induced mantle flow finite strain and seismic anisotropy numerical modeling surface measurements of shear wave splitting patterns are widely used to infer the mantle circulation around subducting slabs however the relation between mantle flow and seismic anisotropy is still elusive finite strain is a direct measurement of time dependent deformation and has been proposed as a proxy for the crystal preferred orientation cpo of mantle minerals we have conducted a series of numerical models to systematically investigate the mantle flow finite strain olivine cpo and sks wave splitting in oceanic subduction zones with variable slab width they demonstrate that the preferred orientations of olivine a axes generally agree with the long extensional axes of the finite strain ellipsoid fse even in these very complex mantle flow fields however neither the a axis nor the fse axes necessarily aligns with the instantaneous mantle velocity vector we identify two domains with distinct deformation mechanisms in the central subplate mantle where simple shear induced by plate advance dominates at shallow depths and produces trench normal fast splitting while pure shear induced by slab rollback dominates the deeper mantle and results in trench parallel fast splitting the sks splitting patterns are thus dependent on the competing effects of these two mechanisms and also on the subduction partition ratio xp xt trench parallel when 1 in addition different mantle deformation mechanisms and sks splitting patterns are observed in the mantle wedge and around the slab edges which may aid in the general interpretation of seismic anisotropy observations in natural subduction zones

excitation mechanism of large amplitude volcanic tremor associated with the 1989 ito oki submarine eruption central japan abstract unusually large amplitude volcanic tremor was observed in association with the 1989 ito oki submarine eruption off the east coast of izu peninsula central japan the largest amplitude tremor occurred on july 13 1989 and was studied by using digital seismic data national research institute for earth science and disaster prevention in the distance range of 6 250 km spectral analysis shows that the seismograms have two kinds of predominant spectral peaks low frequency peaks around 1 hz and middle frequency peaks between 2 and 7 hz the relative amplitude ratios of these peaks show distinctive spatial variation depending primarily on the distances from the crater low frequency waves are predominant at close stations distances less than 50 km from the crater similar to typical low frequency volcanic tremor while at more distant stations the middle frequency waves are more prominent than low frequency waves propagation velocity analysis for narrow frequency bands on the basis of correlation of envelopes of seismograms reveals that the middlefrequency waves are compressional body waves and the excitation of shear wave was ineffective suggesting an explosive or implosive source as the simplest source model in contrast to the middle frequency waves our analysis showed no evidence for the low frequency waves to have propagated with any body wave velocity consideration of the spatial pattern of both waves suggests that surface waves were trapped in sedimentary layers from the shallow source and that no lowfrequency oscillations dominated at the source

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

testing the influence of far field topographic forcing on subduction initiation at a passive margin abstract despite favourable gravitational instability and ridge push elastic and frictional forces prevent subduction initiation from arising spontaneously at passive margins here we argue that forces arising from large continental topographic gradients are required to initiate subduction at passive margins in order to test this hypothesis we use 2d numerical models to assess the influence of the andean plateau on stress magnitudes and deformation patterns at the brazilian passive margin the numerical results indicate that plateau push in this region is a necessary additional force to initiate subduction as the se brazilian margin currently shows no signs of self sustained subduction we examined geological and geophysical data to determine if the margin is in the preliminary stages of subduction initiation the compiled data indicate that the margin is presently undergoing tectonic inversion which we infer as part of the continental oceanic overthrusting stage of subduction initiation we refer to this early subduction stage as the brazilian stage which is characterized by 10 km deep reverse fault seismicity at the margin recent topographic uplift on the continental side thick continental crust at the margin and bulging on the oceanic side due to loading by the overthrusting continent the combined results of the numerical simulations and passive margin analysis indicate that the se brazilian margin is a prototype candidate for subduction initiation

geometry of subduction and depth of the seismogenic zone in the guerrero gap mexico the western coast of guerrero in southern mexico has been identified as a seismic gap on the middle american trench in which no large earthquakes have occurred at least since 1908 it has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake a permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region the seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast the resulting geometry of the subduction zone shows that the cocos plate dips at a shallow angle beneath the north american plate to a depth of 40 km from there the subducted slab is bent upward following a subhorizontal trajectory extending inland at a depth of 50 km this plate geometry is reminiscent of that found in peru and central argentina two other regions where a young oceanic plate is being subducted in mexico however the slab underplates an overriding plate which is only half as thick as that observed in south america a possible oceanic origin of the allochtonous terranes comprising southern mexico may explain the presence of the anomalously thin lithosphere in this region

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

bending mechanics and mode selection in free subduction a thin sheet analysis summary to elucidate the dynamics of free buoyancy driven subduction of oceanic lithosphere i study a model in which a 2 d sheet of viscous fluid with thickness h and viscosity 1 subducts in an infinitely deep ambient fluid with viscosity 1 numerical solutions for the sheet s evolution are obtained using the boundary element method bem starting from an initial configuration comprising a short protoslab attached to a longer horizontal plate that is free to move laterally beneath an impermeable traction free surface interpretation of the solutions using thin viscous sheet theory shows that the fundamental length scale controlling the subduction is the bending length lb defined at each instant as the length of the portion of the sheet s midsurface where the rate of change of curvature is significant geophysically speaking lb is the sum of the lengths of the slab and of the region seaward of the trench where flexural bulging occurs the bending length in turn enters into the definition of the sheet s dimensionless stiffness s h lb 3 which controls whether the sinking speed of the slab is determined by the viscosity of the sheet itself s 1 or by that of the ambient fluid s 1 motivated by laboratory observations of different modes of subduction retreating versus advancing trench folding versus no folding etc in fluid layers with finite depth i calculate numerically the dip d of the slab s leading end as a function of and the normalized depth d h to which it has penetrated the contours of the function d d h strongly resemble the intermode boundaries in the laboratory based regime diagram of schellart supporting the hypothesis that the mode of subduction observed at long times in experiments is controlled by the dip of the slab s leading end when it reaches the bottom of the layer in particular the bem solutions explain why trenches advance in the laboratory only when lies in an intermediate range and why they retreat when is either smaller or larger than this application of the bem model to wu et al s compilation of the minimum curvature radii of subducted slabs suggests 140 510 for the earth this is too small to permit the laboratory type trench advancing mode in agreement with the lack of tomographic evidence for slabs that are bent over backwards

the impact of the south american plate motion and the nazca ridge subduction on the flat subduction below south peru 1 flat subduction near peru occurs only where the thickened crust of the nazca ridge subducts furthermore the south america continent shows a westward absolute plate motion both the overriding motion of south america and the subduction of the nazca ridge have been proposed to explain the flat slab segment below south peru we have conducted a series of numerical model experiments to investigate the relative importance of both mechanisms results suggest that the average upper mantle viscosity should be about 3 5 1020 pa s or less and basaltic crust should be able to survive 600 to 800 c ambient temperature before transforming into eclogite to explain the slab geometry below peru the effect of the overriding plate is estimated to be as large or twice as large as that of the plateau subduction

thermal anomalies and magmatism due to lithospheric doubling and shifting abstract we present some thermal and magmatic consequences of the processes of lithospheric doubling and lithospheric shifting lithospheric doubling concerns the obduction of a cold continental or old oceanic lithospheric plate over a young and hot oceanic lithosphere upper mantle system including an oceanic ridge lithospheric shifting concerns the translation and rotation of a lithospheric plate relative to the upper mantle in both cases the resulting thermal state of the upper mantle below the obducting or shifting lithosphere may be perturbed relative to a normal continental or oceanic geothermal situation the perturbed geothermal state gives rise to a density inversion at depth and thus induces a vertical gravitational instability which favours magmatism we speculate about the magmatic consequences of this situation and infer that in the case of lithospheric doubling our model may account for the petrology and geochemistry of the resulting magma the original layering and composition of the overridden young oceanic lithosphere may strongly influence magmatic processes we dwell shortly on the genesis of kimberlites within the framework of our lithospheric doubling model and on magmatism in general lithospheric recycling is inherent to the mechanism of lithospheric doubling

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

subduction initiation at relic arcs although plate tectonics is well established how a new subduction zone initiates remains controversial based on plate reconstruction and recent ocean drilling within the izu bonin mariana we advance a new geodynamic model of subduction initiation si we argue that the close juxtaposition of the nascent plate boundary with relic oceanic arcs is a key factor localizing initiation of this new subduction zone the combination of thermal and compositional density contrasts between the overriding relic arc and the adjacent old pacific oceanic plate promoted spontaneous si we suggest that thermal rejuvenation of the overriding plate just before 50 ma caused a reduction in overriding plate strength and an increase in the age contrast hence buoyancy between the two plates leading to si the computational models map out a framework in which rejuvenated relic arcs are a favorable tectonic environment for promoting subduction initiation while transform faults and passive margins are not

polyphase formation and exhumation of high to ultrahigh pressure rocks in continental subduction zone numerical modeling and application to the sulu ultrahigh pressure terrane in eastern china 1 high to ultrahigh pressure hp uhp metamorphic rocks commonly form and exhume during the early continental collision and many questions related to their origin still remain unresolved we focus our study on explaining the poly metamorphic origins of many hp uhp terranes composed of tectonic units having strongly variable ages peak metamorphic conditions and p t paths these features are especially well characterized for the sulu uhp terrane in eastern china which we have chosen therefore as the reference case we conducted 2 d thermomechanical numerical modeling of continental subduction associated with formation and exhumation of hp uhp rocks our experiments suggest existence of several consequent episodes of u hp rocks exhumation related to the inherently cyclic origin of continental crust subduction detachment exhumation processes three major phases of these processes are identified in our reference model for the sulu uhp terrane 1 first and 2 second exhumation episodes of hp rocks originated in the subduction channel at lithospheric depths and 3 exhumation of uhp rocks originated at asthenospheric depths numerical models also suggest that subducted uhp rocks which are positively buoyant compared to the mantle may detach from the slab forming a flattened plume underplating the overriding lithosphere this sublithospheric plume may exist for several million years being heated to 800 900 c by the surrounding hot mantle at the later stage upward extrusion of hot partially molten rocks from the plume may exhume high temperature ht uhp complexes toward the surface

geodynamic models of mature continental collision evolution of an orogen from lithospheric subduction to continental retreat delamination 1 the behavior of deep lithospheric processes in continental collision are frequently speculated on but poorly understood in this study we build on previous work to explore the styles of continental lithosphere deformation during mature collision where 1800 km of convergence has been accommodated by horizontal shortening we conducted a suite of numerical geodynamic experiments that test the sensitivity of mature continent collision to varying mantle lithosphere density mantle lithosphere yield stress lower crustal strength and to the presence of phase change related density changes in the lower crust i e eclogitization of mafic lower crust the models suggest that the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate following this initial stage of subduction the subducting lower crust and mantle lithosphere can retreat from the collision zone permitting the sub lithospheric mantle to upwell and intrude the overriding plate as a result the lower crust and mantle lithosphere of the overriding plate delaminate from the overlying crust with local isostatic adjustment subduction and delamination driven crustal processes plateau like uplift occurs the sub crustal evolution also causes bands of syn convergent crustal extension to develop in models with a rheologically weaker lower crust surface crustal response to the deep lithosphere dynamics becomes more diffuse as an example the numerical experiments satisfy a number of surface observables of the himalayan tibetan orogen namely 1 the current general mantle lithosphere architecture as defined by seismic analyses 2 the long wavelength plateau uplift of the western tibetan plateau 3 the surface heat flow measurements in india the tethyan himalaya qiangtang and in qaidam basin and 4 the anomalous syn convergent extension in the southern portion of the orogen

geodynamic regimes of intra oceanic subduction implications for arc extension vs shortening processes abstract 40 of the subduction margins of the earth are intra oceanic they show significant variability in terms of extension and shortening we investigated numerically the physical controls of these processes using a 2d petrological thermo mechanical intra oceanic subduction model with spontaneous volcanic arc growth and deformation we varied the fluid and melt related weakening the ages of both the subduction slab and the overriding plate the subducting plate velocities and the cohesive strength of rocks three main geodynamic regimes were identified retreating subduction with opening of a backarc basin stable subduction and advancing compressive subduction the main difference between these regimes is the degree of rheological coupling between plates which is governed by the intensity of rheological weakening induced by fluids and melts retreating subduction regimes require plate decoupling which results from strong weakening due to both fluids and melts spreading centers nucleate either in forearc or in intraarc regions episodic trench migration is often due to variations of plate coupling with time which is caused by fore arc deformation stable subduction regime with little variation in the trench position forms at an intermediate plate coupling and shows a transient behavior from the retreating to advancing modes the advancing subduction regime results from strong plate coupling at the mature stage this subduction mode is associated with both partial fragmentation and subduction of the previously serpentinized forearc region forearc subduction is typically associated with a magmatic pulse which is caused by dehydration of subducted serpentinized forearc fragments our models demonstrate distinct differences in thermal and lithological structure of subduction zones formed in these different geodynamic regimes results compare well with variations observed in natural intra oceanic arcs

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

constraints on early franciscan subduction rates from 2 d thermal modeling abstract mesozoic subduction along the western margin of north america in california jumped west in mid jurassic time to form the franciscan accretionary wedge outboard of the coast range ophiolite and the great valley forearc basin based on mesozoic plate motion analysis for the pacific region overall rates of subduction are likely to have been on the order of 50 km m y new and previously published lu hf garnet growth ages on high grade blocks in the franciscan however suggest that early franciscan metamorphism took place in a slowly cooling environment characteristic of slow subduction rates in order to assess rates of early franciscan subduction 2 d thermal modeling was carried out based on the following assumptions 1 subduction was initiated at 170 ma based on the age of the oldest tectonic block and on the age of the coast range ophiolite believed to have formed in a forearc setting behind the newly initiated subduction zone 2 at the start of subduction the hanging wall mantle wedge was at asthenospheric temperatures consistent with initiation of subduction at the ophiolite spreading center 3 the earliest subducted lithosphere was 10 m y old at the start of subduction 4 tectonic blocks were metamorphosed at an average pressure of 1 4 gpa corresponding to a depth of 43 km beneath mantle rocks assuming 3 3 g cm 3 subduction rates were investigated between 1 and 300 km m y and the best fit between the ages and estimated metamorphic temperatures of the blocks was found to be 10 5 km m y this is five times slower than the 50 km m y determined from plate motion analysis for that time and suggests that some reappraisal of the configuration and velocities of the oceanic plates may be needed

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

contrasting styles of phanerozoic and precambrian continental collision abstract there are differences in the style of collisional orogens between the phanerozoic and the precambrian most notably the appearance of blueschists and ultrahigh pressure metamorphic uhpm rocks in the geological record since the late neoproterozoic whereas these rocks are absent from older orogens understanding collisional orogenesis in the context of present day values for ambient upper mantle temperature and radiogenic heat production provides a reference from which to extrapolate back to conditions in the precambrian to evaluate differences in the way phanerozoic and precambrian collisional orogens develop a series of experiments was run using a 2 d petrological thermomechanical numerical model in which the collision of spontaneously moving continental plates was simulated for values of ambient upper mantle temperature and radiogenic heat production increasing from those appropriate to the present day thus models of modern collisional orogens involving different modes of exhumation of uhpm rocks were extrapolated back to conditions appropriate for the precambrian based on these experiments an increase of the ambient upper mantle temperature to 80 100 k above the present day value leads to two distinct modes of collision that are different from the modern collision regime and for which the terms truncated hot collision regime strong mafic lower continental crust and two sided hot collision regime weak felsic lower continental crust are proposed some proterozoic orogens record post extension thickening to generate counter clockwise metamorphic p t paths followed by slow close to isobaric retrograde cooling such as occurred in the paleoproterozoic khondalite belt in the north china craton and the late mesoproterozoic early neoproterozoic eastern ghats province part of the eastern ghats belt of peninsular india these orogens have similarities with the truncated hot collision regime in the numerical models assuming subsequent shortening and thickening of the resulting hot lithosphere other proterozoic orogens are characterized by clockwise looping metamorphic p t paths and extensive granite magmatism derived from diverse crustal and subcontinental lithospheric mantle sources these orogens have similarities with the two sided hot collision regime in the numerical models both regimes are associated with shallow slab breakoff that precludes the formation of uhpm rocks the temperature of the ambient upper mantle where this transition in geodynamic regimes occurs corresponds broadly to the neoproterozoic era

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

dynamics of subduction and plate motion in laboratory experiments insights into the plate tectonics behavior of the earth 1 three dimensional laboratory experiments have been designed to investigate the way slab bearing plates move during subduction inside the mantle in our experiments a viscous plate of silicone lithosphere subducts under its negative buoyancy in a viscous layer of pure honey mantle varying thickness width viscosity and density of the plate and mantle three characteristic modes of subduction are observed a retreating trench mode mode i a retreating trench mode following a transient period of advancing trench mode ii and an advancing trench mode mode iii these modes are characterized by different partitioning of the amount of subduction into plate and trench motion our experiments show that the velocity of subduction can be modeled by the dynamic interaction between acting and resisting forces where lithospheric bending represents 75 95 of the total resisting forces however our experimental results also show the impossibility to predict a priori the plate velocity only from the velocity of subduction without considering trench migrations we find experimentally that the lithospheric radius of curvature which depends upon plate characteristics stiffness and thickness and the mantle thickness exerts a primary control on the trench behavior our results suggest that the complexity of the style of subduction could be controlled by geometrical rules of a plate bending inside a stratified mantle the earth system is in the crucial range for the interplay between the rigidity of the plate and the mantle stratification this setting may be the responsible for the complexity of the past and present tectonic styles

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces