the dynamics of extrusion tectonics insights from numerical modeling to elucidate the dynamics of extrusion tectonics i investigate the relation between subduction processes and intraplate deformation three dimensional numerical models of subducting upper plate systems show that subduction of a heterogeneous buoyancy slab that is hosting continental and oceanic lithosphere causes the retreat of the oceanic trench and the localization of deformation in the back arc similar trench evolution is found when the subducting slab partially detaches from the impinging continent however deformation in the upper plate interiors widens with indentation plastic yielding and rotation of a large block extruding laterally toward the retreating margin indentation stresses are consequence of the partial slab breakoff at depth and vanish when the detached slablet reaches the mantle transition zone resulting in transient extrusion tectonics the distance to which indentation deformation propagates depends on the ability of subducting lithosphere to propagate stress laterally and is linear with the lithospheric strength while in the stretching back arc the plate deformation and the underlying mantle flow are coupled they are uncoupled in the extrusion tectonics where mostly stresses propagated through the margin drive rotations the potential role of these diverse subduction processes in the eastern mediterranean is discussed suggesting that different mechanisms likely drive the tectonics of this area

high temperature overprint in u hpm rocks exhumed from subduction zones a product of isothermal decompression or a consequence of slab break off slab rollback abstract this paper presents and discusses petrological observations from high to ultrahigh pressure u hp metamorphic terrains in relation to existing geophysical and numerical models for subduction and exhumation processes in orogenic belts the interpretations are mostly based on observations from gneiss terrains bearing abundant bodies mafic meta eclogite and ultramafic garnet peridotite and pyroxenite exposed in collisional orogens the inclusions and compositional zoning of minerals are considered to be first order information that is needed to constrain pt paths of hp uhp rocks and reconstruct the related geodynamic models for subduction and exhumation of crustal and mantle rocks the bohemian massif of the european variscides is used as the basis for a model example to explain these processes but u hp rocks from various other terrains are taken into consideration to discuss available pt paths in relation to proposed subduction and exhumation rates of u hp rocks based on geophysical and geochronological data primarily information used in this respect include textural relations and preserved prograde zoning in minerals from many u hp rocks which reveal that a relatively cool geothermal gradient typical of subduction zones tended to prevail during the prograde and peak pressure segments of pt paths prior to initiation of exhumation and may have continued even with cooling if exhumation rates were rapid the commonly applied interpretation of isothermal decompression during exhumation is critically appraised considering whether a simple thermal relaxation and radiogenic heating during exhumation is responsible for formation of post peak pressure retrograde mineral assemblages and textures observed in u hp rocks we go on to consider whether this can satisfactorily explain the often pervasive medium pressure high temperature metamorphic re equilibration of u hp rocks or whether an additional external source of heat is a better explanation we conclude that the commonly observed high temperature metamorphic overprint exhibited by u hp rocks occurs mostly after rocks have been exhumed from the subduction channel and have reached normal crustal positions when mantle upwelling resulting from slab breakoff delamination or slab rollback takes place at the onset of continent continent collision we also explore contrasting pt trajectories for mantle rocks that have been entrained into crustal material during their subduction or exhumation pt paths of mantle and subducted crustal rocks tend to converge as mantle rocks impinge upon the cooler subduction zone and once entrained share a common evolution that depends on the exhumation mechanism and rate considering all of the data presented in this work we conclude that the diverse polyphase metamorphic evolution exhibited by u hp terrains embodied in the pt paths of hp and uhp rocks has important consequences for reconstructing their changing thermal regimes and provides important constraints for geodynamic models involving subduction and the transition to collision

trench migration and upper plate strain over a convecting mantle trench motion and upper plate deformation ultimately respond to mantle flow herein i build upon the mantle flow model results of conrad and behn 2010 and compute the drag forces underneath all plates and show that they control the dynamics of plates and plate boundaries the small misfit angle between between the traction azimuths of mantle traction and absolute plate motion corroborates the idea that convective mantle drag is a prominent driver of plate tectonics less intuitive is the fact that the interplay between the drag forces from the upper and lower plates that amounts to 5 to 8 5 1012nm 1 per unit trench length dictates both trench migration rates and upper plate deformation at odds with the classic view that assigns the prime role to the idiosyncrasies of subduction zones slab age interplate friction water content etc i find that the intrinsic properties of subduction zones in fact only modulate this behavior more specifically the mean value of the integrated trenchward mantle drag force from the lower and upper plates from 2 to 6 5 1012nm 1 controls upper plate deformation conversely it is the difference between the lower and upper plates mantle drag forces from 3 to 10 1012nm 1 that controls trench migration rates in addition i find that a minimum trenchward force of 2 5 1012nm 1 must be supplied by mantle drag before trenches can actually advance and before upper plates undergo compression this force results from the default tendency of slabs to rollback when solely excited by their own buoyancy and is thus the effective tensional force that slab pull exerts on the plate interface

influence of lateral slab edge distance on plate velocity trench velocity and subduction partitioning 1 subduction of oceanic lithosphere occurs through both trenchward subducting plate motion and trench retreat we investigate how subducting plate velocity trench velocity and the partitioning of these two velocity components vary for individual subduction zone segments as a function of proximity to the closest lateral slab edge dse we present a global compilation for 207 trench segments from 17 active subduction zones on earth and three dimensional numerical models of progressive free subduction of a single oceanic plate that subducts into a stratified mantle the results show that the subducting plate velocity is always high 5 1 cm yr models and 4 2 cm yr nature and trench velocity is always low 2 5 cm yr models and 1 7 cm yr nature in the center of wide subduction zones dse 2200 km only in regions close to lateral slab edges dse 1000 km be it for narrow or wide subduction zones can the trench velocity exceed 4 cm yr models and 6 cm yr nature and can the subducting plate velocity go below 4 cm yr models and 2 cm yr nature in general plate velocities trench velocities and subduction partitioning are much more variable near slab edges than in the center of wide subduction zones owing to other parameters that affect subduction kinematics we conclude that subduction kinematics can vary considerably along individual subduction zones and that the upper bound values for trench velocity and lower bound values for subducting plate velocity and subduction partitioning at individual subduction zone segments depend critically on dse

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

opposite subduction polarity in adjacent plate segments the work of m p m f and j v is part of the projects alpimed pie csic 201530e082 and mite cgl2014 59516 p and partly financed by the generalitat de catalunya 2014 sgr 1471 and 2017 sgr 847 the work of s z is funded by the spanish ministry of economy and competitiveness through grant cicyt dpi2017 85139 c2 2 r and generalitat de catalunya 2017 sgr 1278

3d slab breakoff in laboratory experiments abstract we study slab breakoff of subducting plates with variable orientations of passive margins ocean continent transition oct using three dimensional laboratory experiments our results show that the initial obliquity of the oct with the trench determines the depth and timing of slab breakoff the reference model involves a straight oct parallel to the trench slab breakoff first starts in the central section of the subduction zone at a depth equivalent to 250 km and then propagates rapidly towards the edges of the subduction zone where it develops at slightly higher depths 280 km in the models where passive margin arrives first at the edges of the subduction zone slab breakoff starts at that location and propagates laterally towards the center of the subduction zone it results in higher slab breakoff depths at the edges of the subduction zone than at the center maximum difference of 170 km in models where the passive margin arrives first in the center of the subduction zone slab breakoff starts at that location and propagates towards the edges of the subduction zone over a period that can be as large as 31 ma and with a large range of depths maximum difference of 170 km the rate of lateral propagation of slab breakoff decreases with the increasing initial obliquity of the oct with the trench the model with the maximum obliquity shows a correlation between the surface topography and the slab breakoff at depth the plate surface is uplifted after the oceanic and continental slab segments are mechanically decoupled and in the final stage of the experiment when the slab is educted subsidence develops as the isostatic and dynamic support diminish

non steady state subduction and trench parallel flow induced by overriding plate structure abstract the direction of plate tectonic motion and the direction of mantle flow as inferred from observations of seismic anisotropy measurements show a good global correlation far from subduction zones however this correlation is poor near subduction zones where below the slab seismic anisotropy is aligned parallel to the trench and above the slab has a complex pattern which has not been fully explained here we present time dependent three dimensional 3d fully dynamic simulations of subduction to study the effect of overriding plate structure on the evolution of slab geometry and induced mantle flow we find that along strike variation in thermal thickness of the overriding plate causes increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate the variation in slab geometry drives strong trench parallel flow beneath the slab and a complex flow pattern above the slab this new mechanism for driving trench parallel flow provides a good explanation for seismic anisotropy observations from the middle and south america subduction zones where both slab dip and overriding plate thermal state are strongly variable and correlated and thus may be an important mechanism in other subduction zones the location and strength of trench parallel flow vary with the time dependent evolution of the slab suggesting that the global variability in seismic anisotropy observations in subduction zones is in part due to the non steady state behavior of these systems

dependency of slab geometry on absolute velocities and conditions for cyclicity insights from numerical modelling summary the aim of this study is to quantify the relationship between the kinematics of subduction deformation in the overriding plate and the evolution of slab geometry a 2 d finite element numerical code is used and a first objective consists in benchmarking previously published analogue models far field plate velocities are applied and once the subducting plate reaches the 660 km discontinuity modelled as a rigid base we obtain two different forms or styles of subductionthatdependontheoverridingplatevelocity vop ifvop 0 theslabliesforwardson the 660km discontinuity style 1 and if vop 0 the slab lies backwards on the discontinuity style 2 we also obtain a cyclic pattern with the slab folding on itself repeatedly when vsp 0 and 2vop vsp 0 where vsp is the subducting plate velocity these conditions result from the analysis of several simulations in which the subduction velocities and plate viscosities are varied when the slab periodically folds on the 660km discontinuity periods of shallow slab dip and compression in the overriding plate are followed by periods of slab steepening and relative extension in the overriding plate folding periodicity is controlled by the slab viscosity and subduction velocity when a low viscosity zone is included in the overriding plate the trench motion is effectively decoupled from the overriding plate velocity therefore allowing it to be directly controlled by the deep dynamics of the slab for the cyclic style 2 corresponding to forward folding of the slab the low viscosity region in the overriding plate increases the stress amplitudes oscillations the trench motion and the folding periodicity with time therefore the strength of the entire overriding plate is shown to directly control the dynamics of subduction using the nazca and south american plate velocities we produce models of cyclic folding with a period of ca 22ma and a minimal dip angle of ca 10 episodic folding of the slab on the 660km discontinuity would produce the necessary changes in slab dip and overriding plate deformation that explain episodes of volcanic quiescence alternating with greater rates of shortening along the andes

effects of plate bending and fault strength at subduction zones on plate dynamics for subduction to occur plates must bend and slide past overriding plates along fault zones because the lithosphere is strong significant energy is required for this deformation to occur energy that could otherwise be spent deforming the mantle we have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology by increasing the effective viscosity of either the plate or the fault zone we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy we have developed a simple physical theory that predicts this slowing by estimating a convecting cell s total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone the energy required to bend the slab is proportional to the slab s viscosity and to the cube of the ratio of its thickness to its radius of curvature the distribution of dissipation among the mantle lithosphere and fault zone causes the speed of a plate to depend on its horizontal length scale using the observation that earth s plate velocities are not correlated to plate size we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity with higher values required if the fault zone can support shear tractions 50 mpa over 300 km these subduction zone strengths imply that the mantle fault zone and lithosphere dissipate about 30 10 and 60 of a descending slab s potential energy release if the slab is 100 km thick the lithospheric component is highly dependent on slab thickness it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km ubduction zone strength should be more stable than mantle viscosity to changes in mantle temperature so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the earth cools because the details of convergent plate boundaries are so important to the dynamics of plate motion numerical models of mantle flow should treat subduction zones in a realistic way

3d numerical modeling of mantle flow crustal dynamics and magma genesis associated with slab roll back and tearing the eastern mediterranean case interactions between subduction dynamics and magma genesis have been intensely investigated resulting in several conceptual models derived from geological geochemical and geophysical data to provide physico chemical constraints on these conceptual models self consistent numerical simulations containing testable thermo mechanical parameters are required especially considering the three dimensional 3d natural complexity of subduction systems here we use a 3d high resolution petrological and thermo mechanical numerical model to quantify the relative contribution of oceanic and continental subduction collision slab roll back and tearing to magma genesis and transport processes our modeling results suggest that the space and time distribution and composition of magmas in the overriding plate is controlled by the 3d slab dynamics and related asthenospheric flow moreover the decrease of the bulk lithospheric strength induced by mantle and crust derived magmas promotes the propagation of strike slip and extensional fault zones through the overriding crust as response to slab roll back and continental collision reduction of the lithosphere asthenosphere rheological contrast by lithospheric weakening also favors the transmission of velocities from the flowing mantle to the crust similarities between our modeling results and the late cenozoic tectonic and magmatic evolution across the eastern mediterranean region suggest an efficient control of mantle flow on the magmatic activity in this region which in turn promotes lithospheric deformation by mantle drag via melt induced weakening effects

trench migration net rotation and slab mantle coupling article i nfo laboratory models have been conducted to improve our understanding of the role that the resistance of the slab to bending and its coupling to the ambient mantle play in subduction dynamics over geological time scales our models are set up with a viscous plate of silicone lithosphere subducting under negative buoyancy in a viscous layer of glucose syrup mantle for our study the lithosphere upper mantle viscosity contrast has been systematically varied from 10 to 10 5 in order to explore the parameter space between weak and strong slab dynamics we found that subduction is characterized by a retreating mode for viscosity ratios n10 4 by the coexistence of a retreating mode and an advancing mode for viscosity ratios between 10 4

flat subduction dynamics and deformation of the south american plate insights from analog modeling received 14 june 2007 revised 13 january 2008 accepted 12 march 2008 published 21 june 2008 1 we present lithospheric scale analog models investigating how the absolute plates motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction possibly resulting in the appearance of flat slab segments and how it changes the overriding plate tectonic regime experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab slab flattening is a long term process 10 ma which requires the subduction of hundreds of kilometers of buoyant plateau the overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate the increase of the overriding plate topography close to the trench results from 1 the buoyancy of the plate subducting at trench and 2 the overriding plate shortening experiments are compared to the south american active margin where two major horizontal slab segments had formed since the pliocene along the south american subduction zone flat slab segments below peru and central chile nw argentina appeared at 7 ma following the beginning of buoyant slab segments subduction in northern ecuador and northern chile the process of slab flattening resulting from the carnegie and iquique ridges subductions respectively seems to be active but not completed the formation of flat slab segments below south america from the pliocene may explain the deceleration of the nazca plate trenchward velocity citation espurt n f funiciello j martinod b guillaume v regard c faccenna and s brusset 2008 flat subduction dynamics and deformation of the south american plate insights

influence of overriding plate geometry and rheology on subduction 1 subduction dynamics is strongly dependent on the geometry and rheology of the subducting slab and adjacent plates as well as on the induced mantle flow driven by the evolution of tectonic configurations along subduction zones however these processes and the associated plate tectonic driving forces are difficult to study using time dependent 3 dimensional computer simulations due to limitations in computing resources we investigate these phenomena with a novel numerical approach using bem earth a stokes flow solver based on the boundary element method bem with a fast multipole fm implementation the initial bem earth model configurations self consistently determine the evolution of the entire lithosphere mantle system without imposing additional constraints in a whole earth spherical setting we find that models without an overriding plate overestimate trench retreat by 65 in a 20 m y model run also higher viscosity overriding plates are associated with higher velocity subducting slabs analogue to faster oceanic plates subducting beneath more rigid continental lithosphere in our models poloidal flows dominate the coupling between the down going and overriding plates with trench orthogonal length variations in overriding plates inducing flows at least 2 stronger than trench parallel width variations however deformation in the overriding plate is related to its length and width with narrower and longer plates extending more than wider and shorter plates

influence of surrounding plates on 3d subduction dynamics 1 our 3d modelling study shows that the presence of lithospheric plates around a subducting plate has a significant influence on subduction dynamics in particular on trench retreat rate slab dip and lateral shortening of the subducting plate neighbouring plates prevent unrealistic plate behaviour with no need for complex rheologies because at the earth s surface plates form a continuous shell they should not be neglected

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

episodic slab rollback fosters exhumation of hp uhp rocks summary the burial exhumation cycle of crustal material in subduction zones can either be driven by the buoyancy of the material by the surrounding flow or by both high pressure and ultrahigh pressure rocks are chiefly exhumed where subduction zones display transient behaviours which lead to contrasted flow regimes in the subduction mantle wedge subduction zones with stationary trenches mode i favour the burial of rock units whereas slab rollback mode ii moderately induces an upward flow that contributes to the exhumation a regime that is reinforced when slab dip decreases mode iii episodic regimes of subduction that involve different lithospheric units successively activate all three modes and thus greatly favour the exhumation of rock units from mantle depth to the surface without need for fast and sustained erosion

overriding plate thickness control on subducting plate curvature subducting plate sp curvature exerts a key control on the amount of bending dissipation associated with subduction and the magnitude of the subduction resisting bending force however the factors controlling the development of sp curvature are not well understood we use numerical models to quantify the role of sp rheology on the minimum radius of curvature rmin we find that rmin depends strongly on the sp thickness when the rheology is viscous this dependence is substantially reduced when the sp behaves plastically in line with the lack of correlation between rmin and sp thickness on earth in contrast plasticity leads to a strong positive correlation between rmin and the overriding plate op thickness using an analysis of rmin versus op thickness we show that such a positive correlation exists on earth this suggests that op structure in conjunction with sp plasticity is crucial in generating slab curvature systematics on earth

stress and deformation mechanisms at a subduction zone insights from 2 d thermomechanical numerical modelling numerous processes such as metamorphic reactions fluid and melt transfer and earthquakes occur at a subducting zone but are still incompletely understood these processes are affected or even controlled by the magnitude and distribution of stress and deformation mechanism to eventually understand subduction zone processes we quantify here stresses and deformation mechanisms in and around a subducting lithosphere surrounded by asthenosphere and overlain by an overriding plate we use 2 d thermomechanical numerical simulations based on the finite difference and marker in cell method and consider a 3200 km wide and 660 km deep numerical domain with a resolution of 1 km by 1 km we apply a combined visco elasto plastic deformation behaviour using a linear combination of diffusion creep dislocation creep and peierls creep for the viscous deformation we consider two end member subduction scenarios forced and free subduction in the forced scenario horizontal velocities are applied to the lateral boundaries of the plates during the entire simulation in the free scenario we set the horizontal boundary velocities to zero once the subducted slab is long enough to generate a slab pull force large enough to maintain subduction without horizontal boundary velocities a slab pull of at least 1 8 tn m 1 is required to continue subduction in the free scenario we also quantify along profile variations of gravitational potential energy gpe we evaluate the contributions of topography and density variations to gpe variations across a subduction system the gpe variations indicate large scale horizontal compressive forces around the trench region and extension forces on both sides of the trench region corresponding vertically averaged differential stresses are between 120 and 170 mpa furthermore we calculate the distribution of the dominant deformation mechanisms elastoplastic deformation is the dominant mechanism in the upper region of the lithosphere and subducting slab from ca 5 to 60 km depth from the top of the slab viscous deformation dominates in the lower region of the lithosphere and in the asthenosphere considering elasticity in the calculations has an important impact on the magnitude and distribution of deviatoric stress hence simulations with increased shear modulus in order to reduce elasticity exhibit considerably different stress fields limiting absolute stress magnitudes by decreasing the internal friction angle causes slab detachment so that slab pull cannot be transmitted anymore to the horizontal lithosphere applying different boundary conditions shows that forced subduction simulations are stronger affected by the applied boundary conditions than free subduction simulations we also compare our modelled topography and gravity anomaly with natural data of seafloor bathymetry and free air gravity anomalies across the mariana trench elasticity and deviatoric stress magnitudes of several hundreds of mpa are required to best fit the natural data this agreement suggests that the modelled flexural behaviour and density field are compatible with natural data moreover we discuss potential applications of our results to the depth of faulting in a subducting plate and to the generation of petit spot volcanoes

upper plate deformation and trench retreat modulated by subduction driven shallow asthenospheric flows upper plate deformation within a subduction zone depends on the complex relationship between surface plate motions trench motion slab pull and asthenospheric flow previous modelling studies suggest that trench motion rates should be related to slab buoyancy but this relationship is neither clear nor verified by observations of natural subduction systems trench motion is also thought to induce upper plate deformation however no clear correlation has been identified between the direction of trench motion and the mode of upper plate deformation in this study we construct 2 d thermo mechanical models to explore the relationship between slab pull trench retreat and upper plate deformation focusing on subduction systems with retreating trenches we start with quasi steady state subduction and introduce a positive density anomaly into the slab to transiently increase slab pull we vary both the value of the density anomaly and the properties of the upper plate to isolate key controls on trench retreat and upper plate deformation our models indicate that asthenospheric flow responds to changes in slab pull and influences both trench retreat and upper plate deformation we propose that trench retreat depends on the competition between shallow and opposite asthenospheric flows below the subducting and upper plates and that a fast sub slab flow can hamper trench retreat even when slab buoyancy is high after a transient slab pull increase the mode of upper plate deformation partly depends on the upper plate s ability to translate horizontally an upper plate with a ridge at its trailing edge deforms by shortening while a fixed upper plate may deform by extension finally in some cases upper plate deformation seems to allow trench retreat if the upper plate is weak enough to be deformed by the basal shear from underlying asthenospheric flow our results provide insights into retreating subduction systems with contrasting upper plate deformation modes such as the compressive andes and the extensional aegean

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

slab rollback rate and trench curvature controlled by arc deformation subduction systems are inherently three dimensional with significant along strike variations in the timing and style of deformation and magmatism geodynamic models used to explain such variations and associated trench curvature typically invoke mantle flow or along strike variations in the properties of the subducting lithosphere ignoring the role of the overriding plate here we use analogue experiments to investigate the dynamics of diachronous backarc basin opening in the models horizontal tension increases in the upper plate due to progressive subduction of negatively buoyant oceanic lithosphere the magmatic arc lithosphere eventually fails and backarc opening occurs via rollback of the forearc block arc failure initiates near the model edge and propagates diachronously along strike producing an arcuate plate boundary the experiments demonstrate that the trench rollback rate is limited by the propagation rate of arc failure conversely slab rollback generates additional horizontal tension in the adjacent magmatic arc lithosphere which drives along strike propagation of arc failure feedback between the rates of trench rollback and arc failure propagation eventually leads to a steady state which dictates the geometry of the backarc basin in map view in the experiment reported here the force and rate balance leads to a backarc basin with an ellipticity of 1 7 which matches the shape of the west mariana basin pacific ocean

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

episodicity in back arc tectonic regimes abstract the evolution of back arc basins is tied to the development of the dynamics of the subduction system they are a part of we present a study of back arc basins and model their development by implementing 3d time dependant computer models of subduction including an overriding plate we define three types of episodicity pseudo quasi and hyper episodicity and find evidence of these in nature observations of back arc basin ages histories of spreading quiescence and compression in the overriding plate give us an understanding of the time development of these subduction zones and back arc basins across the globe today a number of trenches are advancing the izu bonin trench the mariana trench the japan trench the java sunda trench and the central portion of the peru chile trench the andes subduction zone the izu bonin mariana and japan all have established back arc basins while the others have documented episodes of spreading quiescence compression or a combination of these the combination of advancing and retreating trench motion places these subduction zones in the category of hyper episodicity quasi episodicity in which the back arc shifts between phases of rifting spreading and quiescence is the dominant form of episodic back arc development in the present we find this type of episodicity in models for which the system is dynamically consistent that we have allowed the subducting plate s velocity to be determined by the sinking slabs buoyancy quasi and hyper episodicity are only found in subduction zones with relatively high subducting plate velocities between 6 and 9 cm year finally those subduction zones for which the subducting plate is moving slowly such as in the mediterranean or the scotia sea experience only pseudo episodicity where the spreading moves linearly towards the trench but often does so in discrete ridge jump events

dynamic topography above retreating subduction zones dynamic topography provides a measure of stresses within the earth s interior dense slabs induce an upper mantle flow that deflects the surface of the earth downward above them by combining a simple theoretical stokeslet model of subduction gravity modeling and seismic tomography i show that a significant fraction as much as 2000 m of the topographic variations observed above the scotia mariana and hellenic subduction systems appears to be dynamically induced by stresses related to the underlying subduction

subducting slabs jellyfishes in the earth s mantle the constantly improving resolution of geophysical data seismic tomography and seismicity in particular shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle this observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle the distribution and character of seismicity reveal however that slabs undergo vertical extension in the upper mantle and compression near the transition zone in this paper we demonstrate that during the subduction process the shape of low viscosity slabs 1 to 100 times more viscous than the surrounding mantle evolves toward an inverted plume shape that we coin jellyfish results of a 3d numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head the head is linked to the body of the subducting slab by a thin tail a complete parametric study reveals that subducting slabs may achieve a variety of shapes in good agreement with the diversity of natural slab shapes evidenced by seismic tomography our work also suggests that the slab to mantle viscosity ratio in the earth is most likely to be lower than 100 however the sensitivity of slab shapes to upper and lower mantle viscosities and densities which remain poorly constrained by independent evidence precludes any systematic deciphering of the observations

deformation history inferred from magnetic fabric in the southwestern okcheon metamorphic belt korea abstract magnetic fabric and rock magnetic studies have been carried out for the non fossiliferous low to medium grade metasedimentary rocks in the southwestern part of the okcheon metamorphic belt omb two major metamorphic events in the study area were previously recognized regional metamorphism m 1 in the late carboniferous to early permian and contact metamorphism m 2 due to the intrusion of granite in the middle jurassic the metamorphic grade of the study area increases from the biotite zone in southeast through the garnet zone to the staurolite zone towards the northwest magnetic fabrics of the study area are generally well defined and can be characterized according to the metamorphic zones magnetic foliation is the dominant magnetic fabric in the biotite zone while magnetic lineation prevails in both garnet and staurolite zones we interpret the metamorphism related deformation history of the study area based on magnetic fabrics magnetic mineralogy and previously reported metamorphic evolution as follows penetrative nw dipping cleavage represented by magnetic foliation was formed in the study area by prevailing nw se shortening event during the m 1 regional metamorphism in the late carboniferous early permian this shortening event is interpreted to be associated with the collisional event between the north and south china blocks cleavages dipping steeply to the southeast in the staurolite zone are attributed to the pressure exerted from the intrusion of jurassic granite in the northwestern area

intraoceanic subduction of heterogeneous oceanic lithosphere in narrow basins 2d numerical modeling abstract we provide 2d thermomechanical numerical models to constrain the subduction process of narrow 600 km wide oceanic basins resulting in formation and exhumation of serpentinite bearing high pressure hp and ultrahigh pressure uhp complexes presently exposed as dismembered massifs in orogenic belts we simulated subduction of heterogeneous i e non layered oceanic lithosphere testing different serpentinite rheologies and varying other major parameters initial slab dip convergence rate age and structure of oceanic lithosphere distance subduction zone continental margin weak zone geometry we show that serpentinite rheology is the most important parameter affecting both the geometry and behavior of serpentinite channels that form in the mantle wedge as the result of upward migration of slab fluids a strong serpentinite power law rheology is responsible for the continuous accretion and underthrusting of slab slices at the base of mantle wedge to form planar or wedge shape serpentinitic channels the great part of sediments is buried only during continental crust subduction but is never exhumed moreover slab slices do not mix with mantle wedge rocks exhumation of mafic ultramafic rocks occurs from pressure not exceeding 15 kbar on the other hand a weak newtonian behavior of serpentine best describes subduction and exhumation dynamics this rheology produces channels that usually widen to depth slab and sediment slices are easily incorporated in mantle wedge serpentinite during ongoing convergence in this case slab and mantle wedge serpentinite are mixed together and can be closely associated in the channel the models running a newtonian rheology highlight that slices of the oceanic overriding plate can be eroded and dragged in the channel till great depth a weak power law serpentinite rheology produces a transitional behavior between that of strong power law and newtonian rheology the serpentinitic melange produced using a newtonian rheology is finally exhumed from pressure 20 kbar during the final stages of continental crust subduction and collision the rise to shallow depth of the hp melange is driven by concurrent serpentinite buoyancy slab roll back and asthenospheric mantle upwelling moreover we propose that the subduction exhumation dynamics observed in the model acted during closure of the narrow alpine tethys ocean in particular we suggest that some slices presently outcropping in hp ophiolitic massifs of the western alps voltri and monviso massifs were tectonically coupled at slab mantle interface and exhumed during collision in a soft low viscosity serpentinite channel we finally investigated intensity and dynamics of magmatic processes during subduction our numerical experiments suggest that even subduction of narrow oceanic basins may indeed allow the development of a magmatic arc in such relatively small short living arcs steeper faster and younger slabs produce earlier onset of the volcanism after the beginning of subduction

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

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

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

the role of boundary conditions in numerical models of subduction zone dynamics abstract numerical studies of subduction zone dynamics on a regional scale usually use a limited spatial extent for their models and therefore need to define boundary conditions on all model edges these boundary conditions imply a choice for the mechanical and thermal state of the surrounding regions which may influence the evolution of the model system we investigate the role of the surface and lateral boundary conditions for simple mechanical subduction models using a standard finite element method we find that subduction is only possible if the slab can decouple from the surface this decoupling can be achieved by a true free surface a free slip surface with a weak crust for the subducting plate or a free slip surface with a layer of low viscosity low density material sticky air between the model surface and the crust models of slab dynamics that employ a free slip surface reproduce trench migration slab sinking velocities and slab geometry of models with a free surface as long as they use either a weak crust which can be viscous viscoelastic and or brittle or a sticky air layer the vertical topography will however not be reproduced for free slip models without a sticky air layer for ocean ocean convergent models we find that the application of inflow boundary conditions at the edges of the subducting or overriding lithosphere controls trench motion and the geometry of the subducting slab inflow on the overriding side causes trench retreat and a slab resting on the lower mantle whereas inflow restricted to the subducting side can cause trench advance and a slab which folds on the lower mantle

parameters controlling dynamically self consistent plate tectonics and single sided subduction in global models of mantle convection recent advances in numerical modeling allow global models of mantle convection to more realistically reproduce the behavior at convergent plate boundaries in particular the inclusion of a free surface at the outer boundary has been shown to facilitate self consistent development of single sided subduction this allows for a more extensive study of subduction in the context of global mantle convection as opposed to commonly used regional models our first study already indicated important differences between mantle convection with single sided subduction and mantle convection with double sided subduction here we further investigate the effect of various physical parameters and complexities on inducing earth like plate tectonics and its evolution in time results reinforce the previous finding that using a free surface instead of a free slip outer boundary dramatically changes subduction style with free surface cases displaying many episodes of single sided subduction which leads to more realistic slab dip stress state trench retreat rate and slab induced mantle flow longevity of single sided subduction is promoted by a layer of hydrated crust with a low yield strength to lubricate the subduction channel a low viscosity asthenosphere and a high strength of the slab determined by a combination of high diffusion creep viscosity and intermediate friction coefficient although its effective viscosity is in the observationally constrained range in the bending region the time evolution displays interesting events including subduction polarity reversals subduction shut off and slab break off

effects of mantle and subduction interface rheologies on slab stagnation and trench rollback abstract trench rollback has been a widely discussed phenomenon in recent years and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the upper mantle here we concentrate on the effects of rheological description yield stress lower mantle viscosity viscosity of crust in controlling the rollback and associated stagnation of slabs in the transition zone 410 660 km depth we perform numerical simulations of slab evolution in a 2d cartesian model with strongly nonlinear rheology combining diffusion creep dislocation creep and a power law stress limiter we demonstrate that trench retreat develops in most models considered regardless of the subducting plate age or prescribed strength rollback then mostly produces slabs that are horizontally deflected at the 660 km phase boundary and remain subhorizontal at the bottom of the transition zone slab morphologies are in agreement with stagnant horizontally deflected structures reported in the transition zone by seismic tomography furthermore if the strength of the slab is limited to less than 0 5 gpa the slab experiences a significant amount of horizontal buckling the amplitude of the rollback velocity is sensitive to several model parameters as one might expect it increases with the age of the subducting plate thus reflecting its increasingly negative buoyancy on the other hand rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates high friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation also reducing the additional negative buoyancy imparted by the 410 km exothermic phase transition suppresses trench rollback interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models where for example buoyancy effects may be counteracted by associated yield stress weakening

quantifying the net slab pull force as a driving mechanism for plate tectonics 1 it has remained unclear how much of the negative buoyancy force of the slab fb is used to pull the trailing plate at the surface into the mantle here i present three dimensional laboratory experiments to quantify the net slab pull force fnsp with respect to fb during subduction results show that fnsp increases with increasing slab length and dip up to 8 12 of fb making fnsp up to twice as large as the ridge push force the remainder of fb is primarily used to drive rollback induced mantle flow 70 to bend the subducting plate at the trench 15 30 and to overcome shear resistance between slab and mantle 0 8

trench motion slab geometry and viscous stresses in subduction systems summary a semi analytic 3 d model for subduction within a newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress trench motion and slab geometry in subduction systems although negative slab buoyancy provides the basic driving force for subduction slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow 100 150 km mantle and undersupported by viscous stresses at greater depth in the upper mantle these deeper parts of the subduction system act as an engine for subduction while shallower parts act as a brake on trench motion trench migration rates and slab geometry reflect a competition between these two effects during steady state subduction trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of 2 10 20 pa s above 300 km and 5 10 20 pa s below steady state slab dip increases as slab density decreases especially for very low density slabs which dip significantly more steeply than high density slabs the horizontal velocity at the top of the lower mantle measured relative to the foreland has a very large effect on trench migration rates rivalling or even exceeding that of slab buoyancy slab width parallel to the trench also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab the stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere very stiff slabs with elastic plate thicknesses more that 40 km or viscosities in the range of 10 25 10 26 pa s subduct significantly more slowly than weak slabs with trench migration rates in the range of half to a third that of weak slabs large unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate indicating that local geology can exert important constraints on slab dynamics during non steady state subduction rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere in the absence of other driving forces for convergence trench migration rates can change by a factor of two or more in as little as 2 3 myr for example from 35 to 70 mm yr 1 when an oceanic piece of slab follows a continental one into the subduction system subduction of variable buoyancy lithosphere is accompanied by changes in slab dip with depth and through time

effect of margin curvature on plate deformation in a 3 d numerical model of subduction zones summary the large amount of data acquired along most subduction zones underlines the importance of three dimensions in the subduction mechanism in this work we use a fully 3 d mechanical numerical model to analyse the impact of the plate boundary geometry on the deformation of the upper plate the model consists of two initially horizontal independent plates overlying an inviscid fluid and continuously pushed towards each other the initial geometry of the contact zone between the two plates controls the stress regime and the strain pattern in the upper plate the case with a linear plate boundary and orthogonal convergence is first considered and used as a reference model with this simple model the effects of some important parameters i e the interplate friction and the lithosphere asthenosphere density contrast are investigated finally cases with curved plate boundaries are considered and the results are used to shed light on the importance of the dip direction of the interplate plane for the upper plate strain pattern unnecessary oceanward convexity causes an accumulation of the subducted material beneath the upper plate and induces an important uplift of the convex area on the contrary the material escapes from an oceanward concavity and provokes subsidence in the forearc zone such a behaviour induces preferential zones of weakeness in the overriding plate and may allow for explaining some local stress regime variations along convergent margins

unraveling topography around subduction zones from laboratory models the relief around subduction zones results from the interplay of dynamic processes that may locally exceed the iso static contributions the viscous dissipation of the energy in and around subduction zones is capable of generating kilometer scale vertical ground movements in order to evaluate dynamic topography in a selfconsistent subduction system we carried out a set of laboratory experiments wherein the lithosphere and mantle are simulated by means of newtonian viscous materials namely silicone putty and glucose syrup models are kept in their most simple form and are made of negative buoyancy plates of variable width and thickness freely plunging into the syrup the surface of the model and the top of the slab are scanned in three dimensions a forebulge systematically emerges from the bending of the viscous plate adjacent to the trench with a large wavelength dynamic pressure offsets the foreside and backside of the slab by 500 m on average the suction that accompanies the vertical descent of the slab depresses the surface on both sides at a distance equal to the half width of the slab the topographic depression amounts to 500 m on average and becomes negligible at a distance that equals the width of the slab in order to explore the impact of slab rollback on the topography the trailing edge of the plates is alternatively fixed to fixed mode and freed from free mode the end wall of the tank both the pressure and suction components of the topography are 30 lower in the free mode indicating that slab rollback fosters the dynamic subsidence of upper plates our models are compatible with first order observations of the topography around the east scotia tonga kermadec and banda subduction zones which exhibit anomalous depths of nearly 1 km as compared to adjacent sea floor of comparable age

interaction of subducted slabs with the mantle transition zone a regime diagram from 2 d thermo mechanical models with a mobile trench and an overriding plate transition zone slab deformation influences earth s thermal chemical and tectonic evolution however the mechanisms responsible for the wide range of imaged slab morphologies remain debated here we use 2 d thermo mechanical models with a mobile trench an overriding plate a temperature and stress dependent rheology and a 10 30 or 100 fold increase in lower mantle viscosity to investigate the effect of initial subducting and overriding plate ages on slab transition zone interaction four subduction styles emerge i a vertical folding mode with a quasi stationary trench near vertical subduction and buckling folding at depth vf ii slabs that induce mild trench retreat which are flattened horizontally deflected and stagnate at the upper lower mantle interface hd iii inclined slabs which result from rapid sinking and strong trench retreat isr iv a two stage mode displaying backward bent and subsequently inclined slabs with late trench retreat bir transitions from regime i to iii occur with increasing subducting plate age i e buoyancy and strength regime iv develops for old strong subducting and overriding plates we find that the interplay between trench motion and slab deformation at depth dictate the subduction style both being controlled by slab strength which is consistent with predictions from previous compositional subduction models however due to feedbacks between deformation sinking rate temperature and slab strength the subducting plate buoyancy overriding plate strength and upper lower mantle viscosity jump are also important controls in thermo mechanical subduction for intermediate upper lower mantle viscosity jumps 30 our regimes reproduce the diverse range of seismically imaged slab morphologies

no exploits back arc basin in the iapetus suture zone of ireland a controversial aspect of the closure history of the iapetus ocean concerns the existence of the exploits basin in ireland and britain the exploits tetagouche back arc basin of the canadian appalachians opened during the middle ordovician as the popelogan victoria volcanic arc migrated northwards from the ganderian margin the bellewstown terrane within the iapetus suture zone of ireland lies between the ganderian avalonian leinster terrane and the laurentian grangegeeth terrane an early ordovician celtic shelly fauna is hosted in volcanogenic breccia that we demonstrate is stratigraphically overlain by shales with no volcanic horizons the lower 24 m of shale are unfossiliferous but shale above this stratigraphic level has yielded a new graptolite fauna consistent with the upper part of the artus biozone darriwilian c 464 ma u pb zircon dating of the volcanic horizon yields an age of c 474 ma the shale therefore appears to represent c 10 myr of deposition and a significant gap in volcanism this together with a sandbian anglo welsh fauna suggests that the bellewstown terrane remained in a peri gondwanan position throughout the ordovician hence exploits back arc basin opening and ganderian arc migration did not occur in the irish sector of iapetus supplementary materials the table of u pb zircon data cl images of zircon grains concordia plot of all u pb zircon analyses and additional drawings and notes on the graptolite fauna are available at www geolsoc org uk sup18858

dynamics of the ryukyu izu bonin marianas double subduction system trench motions represent the surface expression of the interaction between subducting plates and the underlying mantle but the inherent dynamics are not fully understood one interesting case is the migration of the izu bonin marianas trench ibm that accommodates the subduction of the pacific beneath the philippine sea plate psp which is in turn subducting beneath the eurasian plate along the ryukyu trench the history of the ibm trench is dominated by fast episodic retreat from 40 to 15 ma however around 10 5 ma the ibm trench reversed its motion from retreat to advance the switch in trench motion occurred soon after the breakoff of the psp slab along the ryukyu trench and the onset of new subduction and represents a fundamental change in the dynamics of the western pacific subduction zones here following the modelling study of ci kova and bina 2015 which suggested a link between ibm trench advance and ryukyu subduction we run 2 d numerical experiments to test the influence of a newly formed ryukyu slab on the established ibm subduction zone we run two dimensional numerical experiments to test the influence of this newly formed ryukyu slab on the ibm subduction zone the results from our geodynamic model compare favourably with the reconstructed trend indicating that the switch in trench motion along the ibm trench may indeed be related to the onset of a new subduction zone along the ryukyu trench our analysis substantiates the idea that advancing trench motions in the western pacific are due to the establishment of a double subduction system further analysis of such dynamics provides insights for the mechanisms controlling subducting plate and trench motions and mantle force transmission

does subduction polarity changes below the alps inferences from analogue modelling the surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere scale physical models key parameters investigated were the degree of lateral coupling between adjacent domains of opposing subduction polarity the width of the zone separating the domains and the lithosphere geometry and rheology the model results illustrate an asymmetric lithospheric structure induced by deformation of the downgoing plates which have been separated by a narrow transition zone a wide and symmetric orogenic wedge overlying a region of thickened mantle lithosphere and hampered subduction characterizes this transition zone in addition interaction between the neighboring subduction domains caused downbending of the upper plates and resulted in the lateral termination of crustal structures and lowering of surface topography the lateral extent of interaction between the domains strongly depends on the degree of coupling between the domains the rheology of the mantle lithosphere and the amount of bulk shortening the modelling results have major implications on the interpretation of seismic and tomographic data from the european alps in terms of the crust and lithosphere geometries it appears that an observed lateral change of subduction polarity at mantle depth can explain the variations of wedge build up between the western central and eastern alps

mantle flow influence on subduction evolution abstract the impact of remotely forced mantle flow on regional subduction evolution is largely unexplored here we investigate this by means of 3d thermo mechanical numerical modeling using a regional modeling domain we start with simplified models consisting of a 600 km or 1400 km wide subducting plate surrounded by other plates mantle inflow of 3 cm yr is prescribed during 25 myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench perpendicular mantle inflow from either the back or front of the slab leading to 10 50 km changes in slab morphology and trench position while no strong slab dip changes were observed as compared to a reference model with no imposed mantle inflow in experiments with trench oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100 200 km lastly we investigate how subduction in the western mediterranean region is influenced by remotely excited mantle flow that is computed by back advection of a temperature and density model scaled from a global seismic tomography model after 35 myr of subduction evolution we find 10 50 km changes in slab position and slab morphology and a slight change in overall slab tilt our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion still these secondary effects occur on scales 10 50 km typical for the large scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution

geodynamics of trench advance insights from a philippine sea style geometry abstract for terrestrial parameter sets trench retreat is found to be nearly ubiquitous and trench advance quite rare largely due to rheological and ridge push effects recently updated analyses of global plate motions indicate that significant trench advance is also rare on earth being largely restricted to the marianas izu bonin arc thus we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the philippine sea region in this subduction system a slab pull force from distal subduction is transmitted to the overriding plate at the pacific trench our 2d modeling demonstrates that trench advance can occur for terrestrial rheologies in such special geometries we observe persistent trench advance punctuated by two episodes of back arc extension characteristic features of the model such as time interval between extensional episodes high back arc heat flow and stress state of philippine plate correspond to processes recorded in the region

the effect of slab gaps on subduction dynamics and mantle upwelling abstract gaps within a subducting plate can alter the surrounding mantle flow field and the overall subduction zone dynamics by allowing hot sub slab mantle to flow through the gaps and into the mantle wedge this through slab flow can produce melting of the slab gap edges as well as significant upwelling that can lead to anomalous alkaline volcanism and or dynamic uplift in the overriding plate while the altered mantle flow patterns affect the trench evolution numerous geodynamic models have investigated the processes that form slab gaps but few studies have examined the dynamics of slab gap altered mantle flow its effects on trench morphology and kinematics or the controlling parameters on these processes here laboratory subduction models with a pre cut gap in a subducting silicone plate are used to explore how slab gap size and slab gap depth influence the surrounding mantle flow field and trench dynamics results suggest that both the vertical extent and the depth of the top trailing edge of the slab gap are crucial parameters for modulating overall subduction dynamics they show that a slab gap which occurs near the surface and initially comprises 30 of the subducting plate width can extend enough vertically in the slab to produce significant vertical flow through the gap changes to the trench geometry and kinematics are also evident in the models such that double and triple arc geometries are formed during subduction of a shallow slab gap all of these results are consistent with observations of slab gaps and their induced surface expressions or the lack thereof in eastern anatolia east java italy and argentina

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

laboratory experiments of slab break off and slab dip reversal insight into the alpine oligocene reorganization we present laboratory experiments to study the consequence of the rupture of a subducting slab on the deep geometry of the subduction zone in our experiments slab break off occurs after the entrance of buoyant material at trench causing a slowing down of the subduction and producing an increase of the shortening rate and a transient episode of subduction dip reversal we discuss the potential application of these processes to the recent collisional evolution of the alps proposing that the rupture of the slab produced a re organisation of the belt with a transition from one sided to a doubly vergent orogen

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102

the relationship between plate velocity and trench viscosity in newtonian and power law subduction calculations convection with a newtonian temperature dependent rheology leads to little or no surface velocity unless zones of weakness are introduced plate like features are observed in calculations both with newtonian rheology employing imposed weak zones and with power law non newtonian rheology where high stresses at the trench reduce the effective viscosity since deformation at subduction zones involves faulting both of these parameterizations should be treated with some skepticism it is important to understand how the parameterizations affect the model results we study the relationship between trench viscosity and plate velocity using a newtonian rheology by varying the viscosity at the trench the plate velocity is a function of the trench viscosity for fixed rayleigh number and plate slab viscosity slab velocities for non newtonian rheology calculations are significantly different from slab velocities from newtonian rheology calculations at the same effective rayleigh number both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate non newtonian rheology eliminates the need for imposed weak zones and provides a self consistent fluid dynamical mechanism for subduction in numerical convection models