three dimensional necking during viscous slab detachment we study the three dimensional 3 d deformation during detachment of a lithospheric slab with simple numerical models using the finite element method an initially vertical layer of power law viscous fluid mimics the slab and is surrounded by a linear or power law viscous fluid representing asthenospheric mantle we quantify the impact of slab size and shape symmetric asymmetric on slab detachment and identify two processes that control the lateral i e along trench slab deformation 1 the horizontal deflection of the lateral vertical slab sides 100 km with velocities up to 16 mm yr and 2 the propagation of localized thinning necking inside the slab with velocities 9 cm yr the lateral propagation velocity is approximately constant during slab detachment larger slabs here wider than approximately 300 km detach with rates similar to those predicted by 2 d models whereas smaller slabs detach slower implications for geodynamic processes and interpretations of seismic tomography are discussed

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

multiple episodes of continental subduction during india asia convergence insight from seismic tomography and tectonic reconstruction abstract high wavespeed tomographic anomalies shallower than 1100 km beneath the india asia collision zone are interpreted as continental slabs subducted during collision combining anomaly positions with paleogeographic reconstructions of india we constrain the spatio temporal evolution of multiple episodes of continental subduction likely related to these anomalies this study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone the evolution of the western part is characterized by two episodes of steep subduction of the northern margin of india the first episode involving an area with a lateral extent as large as 1500 km started at about 40 30 and ended by a slab break off process at 15 ma the second episode consists on subduction beneath the hindu kush mountains since 8 ma to the east of the collision zone no anomaly related to steep subduction along the northern edge of india is found we interpret two tomographic anomalies beneath the eastern border of the indian plate beneath burma and beneath the andaman sea as the result of two successive episodes of southeastward extrusion followed by subduction we suggest that both extruded portions were initially located along the northern margin of india and that they slid around the eastern syntaxis then southward along the eastern boundary of indian plate both portions subducted along the eastern border of india south of the eastern syntaxis we provide a rough estimate of the amount of indian lithosphere consumed during these subduction and extrusion episodes by comparing this amount with the total amount of indian lithosphere at the onset of collision we conclude that these processes accommodated most of the india asia convergence during collision

slab portal beneath the western aleutians tomographic images of the distribution of shear wave speed beneath the northwestern pacific delineate the configuration of the subducted oceanic lithosphere beneath the western aleutian arc at 100 km depth a fast shear wave speed anomaly is beneath the aleutian arc everywhere east of 1738e between 1648e and 1738e however seismic velocities at this depth are slow relative to the surrounding mantle the lateral termination of the fast shear wave speed anomaly at depth coincides with a gap in deep seismicity beneath the aleutians the absence of these two distinctive traits of subducting slabs leads us to conclude that this segment of the aleutian arc overlies a very large window in the otherwise continuous lithospheric slab that we term a slab portal this portal is likely to facilitate the production of distinctive volcanic rocks adakites or high mg andesites by partially melting the adjacent edges of the slab the miocene age of most adakites in the westernmost aleutians where no slab is present may indicate that the portal formed relatively recently the chemistry of western aleutian adakites approximates that of typical continental crust so their genesis and subsequent lateral transport toward kamchatka is a plausible mechanism for new continent formation

slab break off abrupt cut or gradual detachment new insights from the vrancea region se carpathians romania slab break off is a plate tectonic process which does not only return lithospheric material into the deeper mantle but also has severe effects on surface movements and seismic hazard slab pull induced seismicity is reduced when the subducted slab decouples from the overlying crust in the vrancea region se carpathians strong earthquakes frequently occur at intermediate depths 70 180 km in a laterally small volume while the crust shows low but distributed seismicity the stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust both features indicate a decoupling between slab and overlying crust either by slab break off or delamination however the strong vertical elongation of the slab requires that the upper end of the slab is fixed vertically thus a soft coupling is assumed that is strong enough to enable elongation of the slab but weak enough to inhibit quasi static stress transfer to the overlying crust

continental collision and slab break off a comparison of 3 d numerical models with observations abstract conditions and dynamics of subduction collision and subsequent 3 d slab break off and slab tear propagation are quantified for the first time using fully dynamic numerical models model results indicate that collision after the subduction of old strong subducting oceanic slab leads to slab break off at 20 25 myr after the onset of continental collision and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100 150 mm yr in contrast young weak oceanic slabs show the first break off already 10 myr after continental collision and can experience tear migration rates up to 800 mm yr slab strength plays a more important role in the timing of slab break off and the speed of a propagating slab tear than negative slab buoyancy does slab break off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle the density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break off a 40 km thick continental crust can be buried to depths greater than 200 km although this maximum depth is significantly less for younger or very weak slabs or thicker continental crust slab break off typically starts at a depth of 300 km mostly independent of mantle rheology but like continental crustal burial can be shallower for young or buoyant plates our 3 d models illustrate how due to the difference in necking in 2 d and 3 d break off has an intrinsic small preference to start as a slab window within the slab s interior rather than as a slab tear at the slab edge however any significant asymmetry in the collision setting e g earlier collision at one end of the subduction zone would override this and leads to slab tearing starting near one edge of the slab these results put important new constraints on the dynamics of the collision and subsequent slab break off for modern collision belts for a proposed timing of the initial arabia eurasia collision at 35 ma break off of the 200 myr old neo tethys slab is unlikely to have occurred before 15 10 ma furthermore our results illustrate that shallow early break off of weak slabs provides a viable explanation for the absence of blueschists and ultra high pressure metamorphism in the precambrian geological record

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

towards absolute plate motions constrained by lower mantle slab remnants the movement of tectonic plates before the cretaceous period is poorly understood a global mantle tomography model suggests that the longitude of oceanic subduction zones up to 300 million years ago was offset by up to 18 compared with tectonic reconstructions for the same period

slab breakoff insights from 3d thermo mechanical analogue modelling experiments abstract the detachment or breakoff of subducted lithosphere is investigated using scaled three dimensional thermo mechanical analogue experiments in which forces are measured and deformation is monitored using high speed particle imaging velocimetry piv the experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs slow subduction experiments with scaled convergence rates 1 cm yr 1 have lower peclet numbers and are characterized by lower tensile strength subducted lithosphere causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low therefore when continental collision is preceded by slow oceanic subduction the hot and weak subducted lithosphere need not be very long or extremely negatively buoyant to cause detachment under such conditions detachment may occur sooner after the onset of continental subduction than previously predicted in contrast if collision is preceded by rapid subduction 10 cm yr 1 breakoff will be delayed and occur only when the convergence rate has slowed sufficiently to thermally weaken the slab and cause its eventual failure the analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary stereoscopic piv reveals a characteristic strain pattern that accompanies the detachment horizontal contraction and subsidence with scaled values up to 1200 m in the trench and forearc area precedes the passage of the detachment and is followed by horizontal extension and uplift up to 900 m high frequency monitoring captures rapid propagation of the detachment along the plate boundary at scaled rates of up to 100 cm yr 1 however this rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether

investigating the base of the mantle using differential travel times abstract several techniques using differential seismic travel times to map lateral structure in the lowermost mantle are discussed results are shown for recent studies involving the established techniques of core reflected phases scs s and pcp p and diffracted phase profiles sdiff and new techniques involving the differential times of both core transmitted and core diffracted phases pkp pdiff and sdiff sks skks are described the recent databases of digital seismograms have allowed for a study of d velocities in the eastern hemisphere using scs s and sscs ss differential times from the many western pacific earthquakes the result is an image at a resolution of a few hundred kilometers of a slow velocity anomaly of 2500 km width beneath micronesia 2 relative to the preliminary reference earth model prem that is surrounded on three sides by fast d rock that is 3 faster than prem a study using the differential arrivals of core diffracted s waves sdiff from digital records is providing information about long wavelength variations in d shear velocities though the rigorous earthquake station geometry requirements limit the study to particular regions of the globe another study is using over 40 000 pcp p differential travel times as reported to the international seismological centre to map global p velocities at the base of the mantle and it shows that global coverage of the core mantle boundary cmb is very poor though there are some regions northern asia northern pacific central america with enough data sampling to allow a quantification of average d p velocities with a total robust range of 4 lateral variation they cover only a small portion of the total cmb as a means of increasing our understanding of the long wavelength variations of seismic velocities a description is given of two techniques that will take advantage of totally different sets of earthquake station geometries from the core reflected phase studies in the distance range from 120 to beyond 165 the differential times of pkp and pdiff can be used to map long wavelength d p velocities these two phases are very different in shape and frequency content so the differential times are found by a waveform cross correlation with reflectivity synthetic counterparts in the distance range from 105 to beyond 135 the differential times of sdiff sks and skks can be examined simultaneously to test models of velocity structure above and below the cmb also through comparison with synthetic counterparts

a simple analytical solution for slab detachment abstract an analytical solution is presented for the nonlinear dynamics of high amplitude necking in a free layer of power law fluid extended in layer parallel direction due to buoyancy stress the solution is one dimensional 1 d and contains three dimensionless parameters the thinning factor i e ratio of current to initial layer thickness the power law stress exponent n and the ratio of time to the characteristic deformation time of a viscous layer under buoyancy stress t tc tc is the ratio of the layer s effective viscosity to the applied buoyancy stress the value of tc n specifies the time for detachment i e the time it takes until the layer thickness has thinned to zero the first order accuracy of the 1 d solution is confirmed with 2 d finite element simulations of buoyancy driven necking in a layer of power law fluid embedded in a linear or power law viscous medium the analytical solution is accurate within a factor about 2 if the effective viscosity ratio between the layer and the medium is larger than about 100 and if the medium is a power law fluid the analytical solution is applied to slab detachment using dislocation creep laws for dry and wet olivine results show that one of the most important parameters controlling the dynamics of slab detachment is the strength of the slab which strongly depends on temperature and rheological parameters the fundamental conclusions concerning slab detachment resulting from both the analytical solution and from earlier published thermo mechanical numerical simulations agree well indicating the usefulness of the highly simplified analytical solution for better understanding slab detachment slab detachment resulting from viscous necking is a combination of inhomogeneous thinning due to varying buoyancy stress within the slab and a necking instability due to the power law viscous rheology n 1 application of the analytical solution to the hindu kush slab provides no order of magnitude argument against slab detachment and therefore supports existing studies suggesting a currently ongoing slab detachment in the hindu kush slab

slab detachment in laterally varying subduction zones 3 d numerical modeling understanding the three dimensional 3 d dynamics of subduction collision systems is a longstanding challenge in geodynamics we investigate the impact of slab detachment in collision systems that are subjected to along trench variations high resolution thermomechanical numerical models encompassing experimentally derived flow laws and a pseudo free surface are employed to unravel lithospheric and topographic evolutions first we consider coeval subduction of adjacent continental and oceanic lithospheres sco this configuration yields to two stage slab detachment during collision topographic buildup and extrusion variable along trench convergence rates and associated trench deformation the second setting considers a convergent margin which is laterally limited by a transform boundary stb such collisional system is affected by a single slab detachment little trench deformation and moderately confined upper plate topography the effect of initial thermal slab age on sco and stb models are explored similarities with natural analogs along the arabia eurasia collision are discussed

evolution of pinch and swell structures in a power law layer analytical and finite element fem solutions for the necking of a single power law layer up to large finite amplitude are obtained continuous necking of the layer produces pinch and swell structures the layer is either a free plate or embedded in a homogeneous medium an analytical solution for finite amplitude necking based on the assumption that plane sections remain plane psrp agrees well with the fem result for a layer power law stress exponent n 5 and for a ratio of layer to medium effective viscosities m 100 fem simulations for embedded layers verify that psrp for m 20 the presented numerical experiments generate localized necking and pinch and swell structures similar to natural ones for n 5 and m 20 additional weakening mechanisms such as strain softening although likely to be operative in nature are not required to generate natural pinch and swell structures fem experiments with random perturbation of the layer interfaces show that even with strong necking instability the layer is thinned at the swell as well as at the necks affecting strain estimation from pinch and swell geometry 2008 elsevier ltd all rights reserved

three dimensionality of slab detachment due to ridge trench collision laterally simultaneous boudinage versus tear propagation the detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide in particular the propagation of slab detachment tearing of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift however there is little understanding of the causes and dynamics associated with three dimensional 3 d slab tearing especially in the case of ridge trench collision here we show using fully dynamic 3 d numerical models that the process of detachment due to ridge trench collision depends on the geometry of the ridge segments approaching the trench for a finite laterally symmetric slab the 3 d detachment process occurs nearly simultaneously along strike by way of boudinage type necking and opening of holes central to the slab for a case involving the approach of two offset ridge segments to the trench slab tearing occurs in the form of 1 a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and 2 horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments however lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone whether detachment occurs simultaneously along strike or propagates laterally the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

delamination or slab detachment beneath vrancea new arguments from local earthquake tomography 1 vrancea located at the southeastern carpathians arc bend is one of the areas in the alpine himalayan belt that features strong earthquakes occurring at intermediate depths 60 200 km in this study we investigated the crustal and lithospheric structure beneath the vrancea seismic area using a local earthquake tomography approach we used an updated and revised catalog spanning from 1982 to 2006 that uses data from both permanent and temporary networks in the target area simultaneous tomographic inversion for the vp and vs anomalies and the vp vs ratio and source locations was done using the lotos code the reliability and robustness of the results were rigorously checked using various tests e g by studying the role of different parameters on the results of the inversion performing the inversion using random data subsets and synthetic modeling the tomography results clearly indicate the presence of a high velocity material beneath vrancea at a depth interval of about 60 200 km that coincides with the distribution of intermediate depth seismicity this result agrees generally with previous tomographic studies we compare two scenarios leading to this structure 1 subduction and slab detachment and 2 drop forming or delamination the latter mechanism presumes that the thickening of the crust due to continent continent collision causes transformation of the mafic lower crust into denser eclogite this material accumulates until it reaches a critical mass at which point it forms a large drop that begins to fall down we propose that the high velocity anomaly we observe in our tomogram might represent the descending eclogitic lower crust material enveloped by the entrained lithosphere it is possible that a similar delamination process can be observed in other parts of the alpine himalayan belt such as in the pamir hindu kush area

collision of continental corner from 3 d numerical modeling abstract continental collision has been extensively investigated with 2 d numerical models assuming infinitely wide plates or insignificant along strike deformation in the third dimension however the corners of natural collision zones normally have structural characteristics that differ from linear parts of mountain belt we conducted 3 d high resolution numerical simulations to study the dynamics of a continental corner lateral continental oceanic transition zone during subduction collision the results demonstrate different modes between the oceanic subduction side continuous subduction and retreating trench and the continental collision side slab break off and topography uplift slab break off occurs at a depth 100 km to 300 km that depends on the convergence velocity the numerical models produce lateral extrusion of the overriding crust from the collisional side to the subduction side which is also a phenomenon recognized around natural collision of continental corners for instance around the western corner of the arabia asia collision zone and around the eastern corner of the india asia collision zone modeling results also indicate that extrusion tectonics may be driven both from above by the topography and gravitational potentials and from below by the trench retreat and asthenospheric mantle return flow which supports the link between deep mantle dynamics and shallower crustal deformation

structure and evolution of lithospheric slab beneath the sunda arc indonesia tomographic imaging reveals seismic anomalies beneath the sunda island arc indonesia that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers the sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the mesozoic tethys ocean in accord with previous studies the lithospheric slab was imaged as a continuous feature from the surface to the lower mantle below java with a local deflection where the slab continues into the lower mantle the deep slab seems to be detached from the upper mantle slab beneath sumatra this complex slab structure is related to the tertiary evolution of southeastern asia and the indian ocean region

estimating hinterland exhumation from late orogenic basin volume nw borneo a new approach for restoring sediment volumes onto the sediment source area to estimate uplift timing and magnitude is discussed and used in the context of late tertiary basin and topographic development in nw borneo sediment volumes for the baram deltaic province were estimated for four time periods latest early middle and late miocene and pliocene recent using 2d and 3d seismic horizons wells and outcrops volume restoration onto the palaeo sediment source area determined exhumation amount at the drainage divide c 5 km from 17 ma to recent and provided a reasonable match with other denudation estimation methods cooling ages and an older regional sediment restoration study using 1979 vintage sediment isopach maps restoration took into account 1 changing sediment densities during the erosion deposition cycle 2 changing area of the sediment source province with time e g changing shoreline location as a result of eustacy uplift and delta progradation tectonic shortening 3 uplift and partial erosion of baram deltaic province sediment denudation at the drainage divide for the middle miocene late miocene and pliocene recent has proceeded at a similar rate for each period initial uplift of central borneo has been attributed to buoyancy of thinned continental crust that jammed the subduction zone under nw borneo in the early miocene however the absence of decay in the erosion rates with time from the middle miocene to recent suggests operation of an additional uplift mechanism that may be related to delamination of mantle lithosphere slab detachment is the favoured explanation

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

new analogue materials for nonlinear lithosphere rheology with an application to slab break off abstract stress dependent nonlinear upper mantle rheology has a firm base in rock mechanical tests where this nonlinearity results from dislocation creep of minerals in the last few decades there has been some attention to nonlinear power law materials for application in scaled analogue experiments for tectonic processes however studies describing the rheology of analogue materials with the same nonlinear dependency on stress as observed for lithospheric mantle materials at relevant stress levels are still lacking in this study we have developed and rheologically tested materials based on combinations of silicone polymers and plasticine with the aim of obtaining a material that can serve as a laboratory analogue to the power law rheology of olivine aggregates at lithospheric mantle conditions from our steady state creep tests we find that it is possible to obtain such a power law material with effective viscosities over relevant model stress ranges 5 4000 pa that allow for nonlinear deformation at laboratory time scales we apply the developed material to a process where localized deformation of the lithosphere can be expected slab break off we study this process using analogue models where we apply the new nonlinear material to the lithospheric mantle domains while we use newtonian glucose to represent the low viscous asthenosphere now that we properly manage power law behavior in our analogue lithosphere materials we are able to model localized lithospheric tearing

slabs do not go gently in geology textbooks the fate of the oceanic crust seems straightforward the ocean floor is created by upwelling of lighter magma at spreading ridges the magma cools as it moves away from the ridge forming a stiff layer or plate called the oceanic lithosphere having increased in density it then descends back into the mantle in trench regions precise seismic tomography studies have revealed that many descending slabs have a more complex evolution and have developed tears detached from the surface plate or even broken up into fragments on page 1173 1 of this issue obayashi et al 1 not only show clear tomographic evidence for the development of a vertical tear under southwest japan but have also found evidence for ongoing plate rupturing the authors correlated the images directly with measurements of stress revealed by active seismic sources 1 lookup doi 10 1126 science 1172496

unfolding the subducting plate in the central new hebrides island arc geometrical argument for detachment of part of the downgoing slab in the central part of the new hebrides island arc a sizable gap exists in the seismic activity seismic observations in the zone of the gap have been previously interpreted as either related to a detachment of part of the downgoing slab or associated with a continuous slab we give a geometrical argument which added the seismic observations from these previous studies adds one step in favor of the detachment hypothesis after unfolding the downgoing plate and restoring it to its initial state using a 3d balance technique we show that the gap can be filled with three pieces two of these pieces are seismic zones located beneath the seismic gap and the third one is a deep 550 650 km seismic zone located about 200 km north east of the arc these three pieces could be detached parts of the subducting plate

melting features along the western ryukyu slab edge northeast taiwan tomographic evidence 1 behind the sedimentary ryukyu arc lies the okinawa trough whose termination is located at the tip of the ilan plain northern taiwan just above the ryukyu slab edge the present day active volcanic front is located 80 100 km above the ryukyu slab and extends from japan to kueishantao island an islet situated 10 km offshore the ilan plain between december 1990 and may 1999 3370 earthquakes recorded in northern taiwan by 65 seismic land stations were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a low vs but high vp vs sausage like body 30 km in diameter lies within the eurasian mantle wedge on top of the western ryukyu slab extremity at depths ranging between 20 and 100 km we suggest that the h2o rich component formed by dehydration processes from subducting sediments oceanic crust and serpentinized mantle above the ryukyu slab and along the vertical portion of the slab edge might explain the presence of the sausage like body a low vs but high vp vs channel rises obliquely from the sausage like body at a depth of 40 km in direction of the andesitic kueishantao island we propose that the h2o rich component and or melt rise up from the sausage like body and interfere with the okinawa trough back arc basin magmas formed in the upper mantle lower crust then magmas propagate upward within the upper brittle crust through veins and or narrow conduits

subduction and slab breakoff controls on asian indentation tectonics and himalayan western syntaxis formation 1 we test the link between large scale asian continent deformations and indian slab subduction and breakoff during convergence by means of three dimensional numerical models of the subducting upper plates system we find that the subduction of the buoyant continent results in the reduction of convergence velocity comparable to that observed in the indian motions yet the upper plate deformation remains accommodated in a narrow belt along a straight margin comparable rates are measured when the subducting slab breakoff is modeled although the convergent margin deforms and curves markedly with large underplating contiguous to ongoing subduction similar to what observed along the himalayan range the models support the interpretation of the himalayan western syntaxis evolution the progressive curvature of the indian margin and the underthrusting as a consequence of the indian slab breakoff the modeled slab detachment is followed by short lived large stresses in the upper plate interiors propagating at large distance from the margin with a trend similar to several major asian lithospheric faults such localized stress has likely provided the conditions for the formation of the central asian intracontinental faulting the bangong red river and the altyn tagh faults that followed successive indian slab breakoff episodes continent subduction and breakoff during india asia convergence offer an explanation for the different deformation mechanisms as the long lived understhrusting and the episodic lithospheric faulting in the asian continent and their link to deep processes

how different cross sectional shapes influence the separation zone of an open channel confluence abstracta key feature of a schematized open channel confluence is the separation zone that is present when the tributary flow detaches from the downstream corner of the confluence this zone of rec

topography of the calabria subduction zone southern italy clues for the origin of mt etna 1 calabria represents an ideal site to analyze the topography of a subduction zone as it is located on top of a narrow active wadati benioff zone and shows evidence of rapid uplift we analyzed a pattern of surface deformation using elevation data with different filters and showed the existence of a long wavelength 100 km relatively positive topographic signal at the slab edges the elevation of mis 5 5 stage marine terraces supports this pattern although the record is incomplete and partly masked by the variable denudation rate we performed structural analyses along the major active or recently reactivated normal faults showing that the extensional direction varies along the calabrian arc and laterally switches from arc normal within the active portion of the slab to arc oblique or even arc parallel along the northern and southern slab edges this surface deformation pattern was compared with a recent high resolution p wave tomographic model showing that the high seismic velocity anomaly is continuous only within the active wadati benioff zone whereas the northern and southwestern sides are marked by low velocity anomalies suggesting that large scale topographic bulges volcanism and uplift could have been produced by mantle upwelling we present numerical simulations to visualize the three dimensional mantle circulation around a narrow retreating slab ideally similar to the one presently subducting beneath calabria we emphasize that mantle upwelling and surface deformation are expected at the edges of the slab where return flows may eventually drive decompression melting and the mount etna volcanism

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

upper mantle structure beneath the alpine orogen from high resolution teleseismic tomography 1 to understand the evolution of the alpine orogen knowledge of the actual structure of the lithosphere asthenosphere system is important we perform high resolution teleseismic tomography with manually picked p wave arrival times from seismograms recorded in the greater alpine region the resulting data set consists of 4199 relative p wave arrivals and 499 absolute p wave arrivals from 76 teleseismic events corrected for the contribution of the alpine crust to the travel times the three dimensional 3 d crustal model established from controlled source seismology data for that purpose represents the large scale alpine crustal structure absolute p wave arrival times are used to compute an initial reference model for the inversion tests with synthetic data document that the combination of nonlinear inversion high quality teleseismic data and usage of an a priori 3 d crustal model allows a reliable resolution of cells at 50 km 50 km 30 km hence structures as small as two cells can be resolved in the upper mantle our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km along strike of the alps the inversion reveals a high velocity structure that dips toward the se beneath the adriatic microplate in the western and central alps in the eastern alps we observe a northeastward dipping feature subducting beneath the european plate we interpret this feature in the western and central alps as subducted mainly continental european lower lithosphere for the east we propose that parts of the vardar oceanic basin were subducted toward the ne forcing continental adriatic lower lithosphere to subduct northeastward beneath the european plate

a tomographic image of indian lithosphere break off beneath the pamir hindukush region summary the p and s seismic anomalies in the upper mantle beneath the pamir hindukush collision zone and surrounding areas are investigated using more than 4 10 5 traveltimes from 5775 events located in the study region recorded by the stations of the worldwide seismic network and reported in the isc catalogue all available epicentral distance ranges of seismic rays are considered all sources are relocated using an algorithm which includes the double difference method in addition source parameters are corrected in the main inversion step simultaneously with the velocity models the high reliability of the resulting p velocity images is confirmed by various tests the s model provides much smoother patterns which are however generally consistent with p anomalies our model shows a complex image of lithospheric plate under the study region under sw hindukush the northwards subducting indian plate is imaged throughout the upper mantle and transition zone further to the ne within a 300 km wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at 250 km depth the tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern tien shan down to at least 250 km depth to the north of tarim we observe a northward dipping of the tarim plate down to 500 km depth at the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults which indicate complex crustal structure in this region

dynamics of slab detachment 1 we investigate the dynamics of slab detachment around the detachment zone and evaluate the amount of time necessary for slabs to detach the study combines results of two dimensional 2d state of the art thermomechanical numerical simulations and a 1d analytical solution for viscous necking under gravity we show that the dominant deformation mechanism during slab detachment is viscous necking independent of the depth of slab detachment when the slab dip is moderate 35 70 slab detachment is partly affected by localized simple shearing in the colder parts of the slab brittle fracturing breaking plays a minor role during slab detachment our 2d thermomechanical models indicate that the duration of slab detachment quantified from the onset of slab thinning until the actual detachment i e vanishing of slab pull force is relatively short 250 km can occur within a short time interval 1 ma which has implications for geodynamic interpretations using slab detachment as explanation for processes such as melting exhumation or surface uplift the thinning of the slab during detachment observed in 2d simulations agrees well with predictions from a 1d analytical solution indicating that the 1d solution captures the first order features of the detachment process we also evaluate the impact of shear heating on the duration of slab detachment the predictions of a simple semi analytical solution based on dimensionless parameters agree well with our and previously published results

quantifying the forces needed for the rapid change of pacific plate motion at 6 ma studies have documented several rapid changes along the pacific north american pacific antarctic and pacific australian plate boundaries in latest miocene to earliest pliocene time consistent with a sudden clockwise rotation of pacific plate velocity relative to hotspots during this period we test the hypothesis that this change in plate motion was initiated by cessation of subduction along the northern melanesian arc due to the collision between the arc and the ontong java plateau this hypothesis has long been formulated but never tested quantitatively we use a geomechanical model of the lithosphere to determine the changes in plate boundary forces that are necessary to obtain the observed change in the pacific plate motion our model results show that the change in motion can be explained by a clockwise rotation of the slab related basal strength component of plate driving force the change of slab related force from a post 6 ma to a pre 6 ma setting is perpendicular to the arc and points towards the australian plate the force per unit length is in the range of currently accepted values for subduction zones since there have been no other relevant changes at subduction zones along the pacific plate boundary during the latest miocene we relate this change in slab related force to the former southward dipping pacific plate slab along the northern melanesian arc system which is now detached our model results suggest that rapid changes in plate motion can be triggered by slab detachment with consequences for plate boundary processes even at great distances from the event

a 3 d lagrangian finite element algorithm with remeshing for simulating large strain hydrodynamic instabilities in power law viscoelastic fluids we present a three dimensional 3 d numerical algorithm pink 3d that is based on the finite element method the algorithm is designed to simulate hydrodynamic instabilities in power law viscoelastic fluids under gravity these instabilities are caused by large and sharp contrasts in mechanical strength and or density between different materials e g folding necking or rayleigh taylor diapirism the instabilities are controlled by the geometry of the material interfaces and the related intralayer stress distribution when amplitudes of the material interfaces are still low the presented algorithm combines a deformable lagrangian mesh with remeshing in order to accurately simulate the low amplitude stages of the emerging instabilities and also to simulate the large strain evolution of the structures emerging from these instabilities the remeshing is based on material interfaces that accurately track the boundaries between materials with strongly varying material properties e g effective viscosity or power law stress exponent we describe here the main technical details of the 3 d algorithm the accuracy of the 3 d algorithm is demonstrated with comparisons between the numerical results and 2 d and 3 d analytical solutions for folding necking rayleigh taylor diapirism and circular inclusions in viscous medium we also benchmark the 3 d algorithm with results of a different 2 d finite element algorithm to test the accuracy of the large strain results with remeshing furthermore two tests are presented that show the accuracy of the viscoelasticity implementation pink 3d is also used to study 3 d necking applied to lithospheric slab detachment and 2 d and 3 d folding applied to fold nappe formation in particular we apply the 3 d code to quantify and visualize the evolution of the 3 d finite strain ellipsoid for the developing 3 d structures

3 d numerical models of viscous flow applied to fold nappes and the rawil depression in the helvetic nappe system western switzerland abstract the helvetic nappe system exhibits three dimensional 3 d features such as the lateral variation in geometry between the morcles and doldenhorn fold nappes or the rawil depression we perform 3 d finite element simulations of linear and power law viscous flow to investigate fold nappe formation during shortening of a half graben with laterally varying thickness 3 d ellipsoids and corresponding 2 d intersection ellipses are used to quantify finite strain fold nappes which formed above a thicker graben have i larger amplitudes ii a less sheared and thinned overturned limb and iii a larger thickness than fold nappes formed above a thinner graben these results agree with observations for the morcles and doldenhorn nappes we also perform 3 d simulations for a tectonic scenario suggested for the evolution of the rawil depression the basement is shortened and extended laterally and includes a graben which is oblique to the shortening direction and acts as mechanical weak zone the graben causes laterally varying basement uplift generating a depression whose amplitude depends on the graben orientation and the stress exponent of basement and sediments the axial plunge of the depression is smaller approximately 10 than the observed plunge approximately 30 indicating that additional processes are required to explain the geometry of the rawil depression

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle