generation of continental rifts basins and swells by lithosphere instabilities 1 continents may be affected simultaneously by rifting uplift volcanic activity and basin formation in several different locations suggesting a common driving mechanism that is intrinsic to continents we describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent we carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above dynamical behavior depends on three dimensionless numbers a rayleigh number for the unstable block a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block within the block instability develops in two different ways in an outer annulus and in an interior region in the outer annulus upwellings and downwellings take the form of periodically spaced radial spokes the interior region hosts the more familiar convective pattern of polygonal cells in geological conditions such instabilities should manifest themselves as linear rifts striking at a right angle to the continent ocean boundary and an array of domal uplifts volcanic swells and basins in the continental interior simple scaling laws for the dimensions and spacings of the convective structures are derived for the subcontinental lithospheric mantle these dimensions take values in the 500 1000 km range close to geological examples the large intrinsic buoyancy of archean lithospheric roots prevents this type of instability which explains why the widespread volcanic activity that currently affects western africa is confined to post archean domains

evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

the depth of trenches as a function of the subduction rate and age of the lithosphere abstract the trench is one of the most characteristic features of the subduction zones for a given arc or for all the subduction zones put together relationships between their depth variations the variations of the subduction rate and the age of the sinking lithosphere can be established certain other parameters have been examined with those mentioned above but without success it will be demonstrated that the observed relationships can be interpreted in terms of an elastic model and of a viscous model

evolution of 3 d subduction induced mantle flow around lateral slab edges in analogue models of free subduction analysed by stereoscopic particle image velocimetry technique abstract we present analogue models of free subduction in which we investigate the three dimensional 3 d subduction induced mantle flow focusing around the slab edges we use a stereoscopic particle image velocimetry spiv technique to map the 3 d mantle flow on 4 vertical cross sections for one experiment and on 3 horizontal depth sections for another experiment on each section the in plane components are mapped as well as the out of plane component for several experimental times the results indicate that four types of maximum upwelling are produced by the subduction induced mantle flow the first two are associated with the poloidal circulation occurring in the mantle wedge and in the sub slab domain a third type is produced by horizontal motion and deformation of the frontal part of the slab lying on the 660 km discontinuity the fourth type results from quasi toroidal return flow around the lateral slab edges which produces a maximum upwelling located slightly laterally away from the sub slab domain and can have another maximum upwelling located laterally away from the mantle wedge these upwellings occur during the whole subduction process in contrast the poloidal circulation in the mantle wedge produces a zone of upwelling that is vigorous during the free falling phase of the slab sinking but that decreases in intensity when reaching the steady state phase the position of the maximum upward component and horizontal components of the mantle flow velocity field has been tracked through time their time evolving magnitude is well correlated to the trench retreat rate the maximum upwelling velocity located laterally away from the subducting plate is 18 24 of the trench retreat rate during the steady state subduction phase it is observed in the mid upper mantle but upwellings are produced throughout the whole upper mantle thickness potentially promoting decompression melting it could thereby provide a source for intraplate volcanism such as mount etna in the mediterranean the chiveluch group of volcanoes in kamchatka and the samoan hotspot near tonga

numerical modeling of stress and topography coupling during subduction inferences on global vs regional observables interpretation abstract the mechanical strength of the oceanic lithospheres exerts a fundamental control on the dynamics and tectonics of convergent margins here we use numerical models of coupled subducting upper plates and mantle to investigate the impact of the subducting lithosphere properties on the parameters commonly measured at subduction zones the models predict a correlation among parameters such as subducting and convergence velocities slab dip and curvature and stress coupling with the upper plates such signature emerges in a dynamic system through the slab buoyancy similar trends are found when assessing the impact of laboratory constrained non linear rheologies uncertainties on the mechanical strength and mantle flow although some features are more affected when the role of upper plate thickness is tested we find a positive correlation of upper plate subsidence and fore arc topography with the buoyancy of the down going plate however plasticity in the crust limits the development of topography becoming increasingly relevant with thinner upper plates so that no correlation can be established similar control is found on the width of the long wavelength dynamic subsidence due to flexure which increases non linearly with the upper plate thickness the models constrain relations among parameters within the dynamic system driven that allow inferences on the observable parameters the comparisons supports the idea that the subduction dynamics emerges as a global phenomenon in plate motions bending and dynamic subsidence yet andean type margin tectonics is best addressed in a regional context

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

the effect of hot spots on the oceanic age depth relation hot spots located both midplate and on midocean ridges can bias the empirical age depth relation ocean floor bathymetry for seafloor greater than 80 m y b p in age has been previously used to determine if the oceanic lithosphere behaves as a cooling half space or as a cooling plate however large areas of the ocean basins do not fit a simple age depth relation anomalously shallow swells approximately 1000 km wide and 1 km high surround hot spots such as iceland the azores and the cape verde islands by sorting seafloor depth data by distance to the nearest hot spot track we show that the along isochron variation in depth in the north atlantic correlates with distance from a hot spot track due to the absence in the north atlantic of seafloor both old and far from hot spots we compare depth versus age at constant distance and do not observe flattening of the age depth relation in any distance range the age depth relation depth meters 2700 295 t m y 1 2 derived for seafloor 1200 to 1800 km from a hot spot track represents our best estimate of the age depth relation appropriate to seafloor unaffected by a hot spot track in the north atlantic the previously inferred flattening of the age depth relation appears to be caused by the inclusion of hot spot swells this implies that hot spots may be the primary means of heat transfer from the deeper mantle to the base of the lithosphere

subduction hinge migration along the izu bonin mariana arc abstract hinge migration or rollback is thought to have a significant influence on the tectonics of trench arc backarc systems where the backarc region is actively opening the hinge is usually either stationary or retreating in a mantle hotspot frame of reference the izu bonin mariana system represents a highly anomalous case in which the hinge is reportedly advancing behind the retreating eastern edge of the overriding philippine sea plate we have re evaluated the motion of the subduction hinge using the best available estimates of the motion of the philippine sea plate and estimated maximum rates of extension within the izu bonin arc if the plate motions have not changed appreciably the subduction hinge of the izu bonin trench has advanced over the last 5 ma the approximate average rate of hinge advance is 0 6 mm a 1 at the north end of the izu bonin arc increasing to 24 30 mm a 1 at 27 n the rate of advance of the mariana hinge at 18 n is 10 20 mm a 1 the dip of the subducted slab is strongly correlated with the motion of the philippine sea plate suggesting that the rapid retreat of the overriding plate affects mantle flow we have found a tentative hinge advance mechanism in a simple newtonian viscous corner flow model though the model has severe geometrical and rheological limitations it suggests that the retreat of the overriding plate gives rise to excess uplift pressures acting on the top surface of the subducted slab if the hinge is not assumed to be fixed the excess pressure will tend to rotate the slab arcward steepening the slab and advancing the subduction hinge thus the westward motion of the philippine sea plate may explain both the advance of the subduction hinge and the observed relationship between the motion of the philippine sea plate and the dip of the subducted slab along the izu bonin mariana trench

on the role of slab pull in the cenozoic motion of the pacific plate we analyze the role of slab pull acting on the pacific plate during its early tertiary change in motion slab pull forces are estimated by integrating the negative buoyancy of a 700 km long slab along a revised subduction boundary model adopting the muller et al 2008 seafloor age reconstructions our results indicate that torques predicted from a simple slab pull model match the pacific plate euler vectors during the tertiary fairly well the change of the pacific motion at similar to 50 40 ma appears to be driven by the onset of the izu bonin mariana system and soon afterwards by the tonga kermadec subduction zones

antarctic peninsula continental magnesian andesites indicators of ridge trench interaction during gondwana break up subduction of young hot oceanic lithosphere and a major episode of lithospheric extension have been considered as key tectonomagmatic factors in the genesis of setouchi type continental magnesian andesites we present data on a new discovery of such andesites from the northern part of the antarctic peninsula and propose that subduction of newly created oceanic lithosphere occurred along the proto pacific margin of antarctica during the middle jurassic continental magnesian andesite magmatism was contemporaneous with the early stages of gondwana break up and we suggest that subduction of young hot oceanic lithosphere and ridge trench interaction may have contributed to the disintegration of gondwana by thermally weakening the lithosphere and developing a broad linear extensional zone along the proto pacific margin

on the origin of the cocos nazca spreading center to investigate the mechanism underlying the break up of the farallon plate into the cocos plate and nazca plate we analyze the state of stress in the farallon plate at about 30 m y b p just prior to the fragmentation to this purpose we use finite element methods and a reconstruction of regional plate boundaries appropriate for 30 m y b p a key role in the model is played by the dependence of two important plate tectonics forces slab pull and ridge push on the age of oceanic lithosphere the results show a highly tensional stress field with maximum principal stresses of 5 to 6 kbar north south tension in the vicinity of present day panama is proposed to have been the cause for the fragmentation of the farallon plate and the inception of spreading along the new cocos nazca plate boundary because the kind of plate interaction that gave rise to the high level of tensional stresses in the farallon plate is not restricted to the area of this study the proposed mechanism seems to shed light on the problem of fragmentation of oceanic plates in general

cenozoic tectonics of western north america controlled by evolving width of farallon slab the basin and range province of western north america exemplified by the alternating mountain ridge and valley landscapes across nearly the entire u s state of nevada started to form 50 million years ago through a series of extensions of the continental crust prior to that massive mountain building collisions at the boundary between north america and the pacific ocean formed a subduction zone and compressed the continent by combining observations of global subduction zone velocities with numerical modeling schellart et al p 316 1 demonstrate that the thinning of the subducting portion of the oceanic plate controlled how and when the transition from compression to extension occurred even today where the much smaller remnant oceanic plate continues to slowly subduct below north america in the pacific northwest the width of the slab and not its age controls the velocity of subduction indeed this relation may explain the dynamics of other modern subduction zones from south america to japan 1 lookup doi 10 1126 science 1190366

breakup of young oceanic lithosphere in the upper part of a subduction zone implications for the emplacement of ophiolites it is investigated by means of numerical modelling whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate first a thermal model of the upper part of a subduction zone the region between the trench and the volcanic arc is presented temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases culminating in the arrival of a spreading ridge at the trench temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology next it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere recently created at a spreading center in the vicinity of a subduction zone can lead to its breakup our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench if breakup occurs some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate the detached sheet of thin oceanic lithosphere will not be subducted to mantle depths instead subduction of the trailing plate at the opposite side of the ridge can lead to the incorporation of this sheet into the forearc region our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc trench region and eventually upon closure of the ocean basin into an orogenic belt it is shown that a large subset of ophiolites those with a harzburgite dominated mantle section has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase

the orthogonality of subduction an empirical rule abstract measurement of the angle of plate convergence obliquity of subduction along twenty one trench and transform boundaries reveals that the direction of plate convergence is not randomly distributed but rather is strongly bimodal plate convergence is most commonly taken up along a system of strike slip faults and trench segments in which the direction of plate convergence is approximately normal to the trench axis orthogonal subduction slip vectors measured along these trench segments confirm the observation that in general oceanic lithosphere is subducted nearly perpendicular to the trench axis plate boundaries evolve often by forming new plate boundaries in ways that tend to maintain this pattern of orthogonal subduction

on the stress dependence of the earthquake b value laboratory experiments have shown that the b value in the size distribution of acoustic emission events decreases linearly with differential stress there have been a number of observations that indicate that this relation may also hold for earthquakes here using a simple frictional strength model for stresses in the continental lithosphere combined with earthquake b values measured as a function of depth in a wide variety of tectonic regions we verify and calibrate that relation finding b 1 23 0 06 0 0012 0 0003 1 3 where the stress difference 1 3 is in megapascal for subduction zones we find that b value correlates linearly with the slab pull force and with the net reduction of plate interface normal force both of which also indicate a negative linear relation between b value and differential stress

age of oceanic plates at subduction and volatile recycling the age of the subducting plate as it enters the trench controls the maximum depth of volatile transport by the downgoing plate as the slab descends and heats up decarbonation and dehydration reactions cause alteration minerals and sediments to release volatiles our calculations show that subducting oceanic plates 11 m y old in oceanic arcs and 34 m y old in continental arcs heat up so rapidly that no h2o or co2 can return to the asthenosphere instead these volatiles rise into the over riding lithospheric plate co2 and h2o are released differently during subduction a thickly sedimented plate subducting beneath an oceanic arc will return h2o to the asthenosphere only if the subducting plate is older than 11 m y and co2 only if it is older than 25 m y if archaean oceanic lithosphere had a maximum age of 30 50 m y and an average age of 10 18 m y then the amount of volatile recycling to the asthenosphere could have been much lower than at present despite a greater total consumption rate

on modeling the thermal evolution of the oceanic upper mantle an assessment of the cooling plate model the plate cooling model for the thermal evolution of the oceanic upper mantle has been widely accepted to explain observed variations of depth to oceanic basement and conductive heat flow with the age of the seafloor several estimates of best fitting plate model parameters derived from depth heat flow and age data have been proposed but the viability of the plate model itself has not been rigorously evaluated we have used published mean depths and depths to basement at deep sea drilling project ocean drilling program dsdp odp drilling sites to test the plate cooling model based on two criteria first viable plate models must have coefficients that are consistent with the slope of the corresponding root t line because the half space or root t subsidence of young seafloor is implicit in the plate model i e the slope of the root t line can be calculated directly from the coefficients of the plate subsidence model second any viable physical model must fit the data with an acceptable degree of systematic misfit large systematic misfits indicate that the model cannot explain the observations fits of half space root t models to depth versus age data for young 80 ma seafloor indicate basal temperatures in the range 1300 to 1370 c based on the age at which the depths deviate from the root t line the minimum plate thickness that is compatible with the best fitting half space models for young seafloor is 120 km in contrast all best fitting plate models yield systematically higher temperatures 1450 to 1470 c and thinner plates 102 118 km the plate model can explain the depth to basement at dsdp odp drill sites with a satisfactory degree of systematic misfit but we find that there is no plate cooling model that can explain the variation of the mean depths derived from the dbdb5 database with age over the entire range of ages 0 to 165 ma models that best fit the entire data set have unacceptably large systematic misfits over the entire range of seafloor age whereas models that minimize the systematic misfit for young seafloor 0 to 81 ma fail for older seafloor the plate model clearly fails to explain the observations the cooling history of old seafloor is not simply an extension of the cooling history of young seafloor according to a simple plate cooling model previous studies have suggested as an alternative to the plate model that observed variations of basement depth with age are best explained by the combined effects of a normal half space cooling process and the dynamic and or thermal effects of hot spots or mantle plumes our results are entirely consistent with the half space cooling model and we find that the best reference model for normal subsidence is d t 2600 20 m 345 3 m m y t we also find that the heat flow predicted by this half space model is consistent with the most reliable average heat flow values from the pacific

the driving mechanism of plate tectonics relation to age of the lithosphere at trenches sea floor age depth and age trench depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 ma if so slab pull fs and ridge push fr forces can be calculated from the age of the sea floor at trenches t and if the shear stress acting on the base of the plate increases with absolute velocity a the approximate force balance equation is dva2 svat32 rt where d s and r are constants related to drag slab pull and ridge push respectively the relation between velocity and age is va2 s vat12 r we have tested this model by linear regression using rate and age data for 15 trenches va mm a 2 9 20 32 vat12 26 250 the strength of this correlation r 0 98 is strong evidence in favor of the validity of this simple model these results suggest that ridge push is just sufficient to overcome drag at the base of the plate and does not contribute significantly to the motions of oceanic plates though the value of r is consistent with the motions of plates not attached to subducted slabs

control of seafloor aging on the migration of the izu bonin mariana trench article i nfo recent global kinematic studies reveal that most of the trenches roll back but a significant number of them advance toward the upper plate those advancing trenches are mostly located in the western pacific and correspond to the subduction of very old mesozoic oceanic lithosphere while retreating trenches are commonly explained by the slab pull action of the descending lithosphere the origin of advancing trenches is still debated since this relationship is dependent upon the adopted reference frame we select region where geological studies show the variability of trench migration style with time the izu bonin mariana ibm region represents a key example the detailed reconstruction of the trench migration of the ibm subducting system reveals that after a long episode of asymmetric rollback the ibm trench recently started advancing we propose that this change from retreating to advancing trench mode results from the subduction of progressively older and stiffer lithospheric material we test this hypothesis by means of two dimensional 2 d numerical models reproducing the effects of the lithospheric aging during subduction the result of our numerical tests shows that the entrance of old and stiff lithosphere forces the trench to advance because the increasing stiffness of the slab prevents the slab to unbend once it has subducted we adapt this physical result to the ibm evolution showing that the switch from trench retreat to trench advance can reconcile the shape of ibm slab as previously suggested finally we discuss the possibility that the subduction of the cretaceous lithosphere might have triggered the change of the motion of the pacific plate around 5 ma

plate moving mechanisms constraints and controversies the nature of the driving forces for plate tectonics has been a subject for intense often heated debate for more than a quarter of a century on 15 april 1992 peter ziegler delivered the annual william smith lecture to the geological society on the subject of plate moving mechanisms followed by a series of lectures on related themes this thematic set of papers resulting from the meeting covers some of the major issues discussed for more than a quarter of a century earth scientists have debated the nature of the forces which drive the lithospheric plates one of the major problems has been whether mantle convection including the activity of mantle plumes dominates or whether boundary and plate forces such as slab pull and ridge push exert the most important controls on plate motions clearly the relative importance of the different forces involved may vary both in space and time and they may either reinforce or counterbalance each other depending upon the tectonic setting on 15 april 1992 peter ziegler delivered the annual william smith lecture to the geological society on the subject of plate moving mechanisms this was followed by a further eight presentations on related themes by an international group geoscientists from europe and north america of the nine papers presented at the meeting six are included in this thematic set ziegler bott wilson searle et al doglioni pavoni the other speakers were randall m richardson university of arizona tuscon usa john f dewey oxford and sierd cloetingh free university

change in tectonic force inferred from basin subsidence implications for the dynamical aspects of back arc rifting in the western mediterranean abstract a method has been developed that allows temporal changes in tectonic force during rift basin formation to be inferred from observed tectonic subsidence curves and has been applied to the gulf of lions the provencal basin and the valencia trough in order to gain some understanding of the dynamical aspects of back arc basin rifting in the western mediterranean sea two distinct tectonic force regimes active at different times during the evolution of each of these back arc basins are identified the first which can be seen in both basins is characterized by tensional forces that gradually abate with time to vanish some 20 my after the onset of rifting the magnitude of tectonic force required to initiate the rifting process is significantly greater in the valencia trough than in the provencal basin subsequently the dynamic development of these back arc basins differs in the provencal basin there is a renewal of force with extensional deformation concentrated in the central part of the rift whereas in the valencia trough the second tectonic force regime is inferred to be one that causes compression that subsequently relaxes such temporal patterns of tectonic force are interpreted to be related to the causative driving processes allowing constraints to be placed on the transient interaction between the overriding and subducting plates in a back arc setting the models also allow inferences to be made about the rheological structure of the lithosphere a significant variation of initial crustal thickness is inferred for the provencal basin but not for the valencia trough in both basins a wet rheology is required in order to initiate rifting given currently accepted bounds on tectonic force magnitudes adoption of a dry rheology leads to insufficiently high strain rates for significant lithosphere extension in both cases

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

plates and their motions the last reviews which covered research in plate motions were those of herron and pitman 1975 on marine magnetic anomalies and dewey 1975 on plate tectonics both reporting on research for the period 1971 1974 in the last 12 years there has been considerable refinement of relative motion models as well as further development of global models also major effort has gone into defining reference frames for the study of plate motions and discriminating between them more recently the role of errors in individual plate rotations and the associated uncertainties in reconstructions have received attention in this report i concentrate on work published in the last 4 years 1983 1986 citing a minimum of publications from previous years however some papers in press for early 1987 have been included this report first reviews work on relative plate motions for the present and past and reconstructions and then models for these motions global ones for driving forces and local ones determining subduction history and back arc spreading the problem of reference frames both hotspot and paleomagnetic is discussed next i end with a review of work on the errors in plate motions

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

retracted signatures of downgoing plate buoyancy driven subduction in motions and seismic coupling at major subduction zones this article has been retracted at the request of the editor in chief and authors please see elsevier policy on article withdrawal http www elsevier com locate withdrawalpolicy reason after publication errors were discovered in the plate motion database that it was based on this dataset was an updated version of the dataset presented in sdrolias and muller 2006 provided to us by the first author the errors in this version were in the away from trench towards trench assignment for subduction zones with back arcs and also due to the fact that the next generation plate model had only partially been completed these errors affect the conclusions about seismic coupling they also change some of the points in most of the other plots and although this does not invalidate the other conclusions the discussion to reach them would be altered

the himalayas a modern analogue for archaean crustal evolution abstract present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals if the oceanic crust was appreciably thicker in the archaean as geothermal models would indicate then oceanic crustal collision on the scale of the present day himalayan continental collision zone may have been a frequent occurrence in the archaean resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum

new insights on the offshore extension of the great sumatran fault nw sumatra from marine geophysical studies 1 over the last 20 years the great sumatran fault gsf has been studied on land but we have very little information about its offshore extension nw of sumatra and its link with the west andaman fault to the north the problem is further complicated by its vicinity to the volcanic arc here we present detailed analyses of the offshore extension of the gsf based on recently acquired high resolution bathymetry multichannel seismic reflection data and some old single channel seismic reflection data our findings demonstrate that the branches of the gsf near banda aceh proceed further northwestward producing two 15 20 km wide adjacent basins the southwestern transpressional breueh basin is 1 2 km deep and has a flower structure with a push up ridge in the center suggesting the presence of an active strike slip fault the presence of strike slip earthquakes beneath this basin further suggests that one active branch of the gsf passes through this basin the northeastern transtensional weh basin is up to 3 4 km deep and the absence of recent sediments on the basin floor suggests that the basin is very young the presence of a chain of volcanoes in the center of the basin suggests that the sumatran volcanic arc passes through this basin the anomalous depth of the weh basin might be a site of early back arc spreading or may have resulted from pull apart extension we examine all these new observations in the light of plate motion local deformation and possible seismic risk

the destruction of lenses and generation of wodons abstract since midocean eddies migrate westward they eventually reach the western boundaries it is therefore of interest to find out what happens after the eddies collide with the walls an isopycnic two layer primitive equation model on a plane and a simple analytical model on an f plane are constructed to investigate the meridional migration of an oceanic eddy along a western wall on a plane three factors determine the eddy s migration along a western meridional wall first the image effect pushes an anticyclonic cyclonic eddy northward southward second the force resulting from the larger coriolis force on the northern side of the eddy pulls an anticyclonic cyclonic eddy southward northward third after an anticyclonic cyclonic eddy collides with the wall parts of the anticyclonic eddy s interior fluid leak out southward northward along the wall forming a thin jet in an analogy to a rocket this jet pushes the eddy northward southward our aim is to investigate in whi

variations of slab dip and overriding plate tectonics during subduction insights from analogue modelling we present small scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions the oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate in this configuration the long term process of subduction is not steady state slab interaction with the upper mantle lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes followed by periods of slab steepening the overriding plate tectonic regime is influenced by the dynamics of subduction slab anchoring favouring trench perpendicular shortening when the slab is anchored slab flattening further favours shortening while slab steepening favours extension or smaller shortening rates non steady state long term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present day subduction zones experiments suggest that despite boundary conditions applied on the converging plates do not change tectonics pulses within the overriding plate may be caused by this non steady state behaviour

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

the variation with frequency of the long period tides the oceanic response to long period tidal forcing in closed basins approaches equilibrium for periods much longer than a month for moderate values of dissipation at a given frequency the response is closest to equilibrium near the eastern wall of the basin global calculations of the mf and mm tides made using a 2 2 grid are presented changes in the length of day have also been computed for these tides

evolution of the slab bending radius and the bending dissipation in three dimensional subduction models with a variable slab to upper mantle viscosity ratio abstract three dimensional laboratory subduction models are presented investigating the influence of the slab upper mantle viscosity ratio sp um on the slab bending radius r b with sp um 66 1375 here r b is non dimensionalized by dividing it by the upper mantle thickness t um the results show that r b t um varies with time reaching a maximum when the subduction velocity is maximum furthermore r b t um increases approximately linearly with increasing sp um for the investigated viscosity range the model results show that the slab bending force f be and the energy dissipation during bending be are small compared to the negative buoyancy force of the slab f bu and the potential energy release during sinking bu maxima in be bu f be f bu are reached in the early stage of subduction when r b t um is minimum and the slab tip is at 220 440 km depth maximum be bu increases with increasing sp um with be bu max 0 06 0 11 0 18 and 0 22 for sp um 66 217 709 and 1375 respectively for subduction depths 220 440 km be bu 0 02 0 11 for all viscosity ratios assuming that in nature sp um r b t um and be bu to natural subduction zones implies that in nature sp um 1 7 10 2 and um 0 8 2 7 10 20 pa s finally the laboratory models which use glucose syrup and silicone oil as modelling materials highlight the importance of accurate control on temperature during an experiment new material investigations show that the viscosity of these two materials decreases exponentially with temperature in the range 3 33 c their density decreases approximately linearly with temperature and their coefficient of thermal volumetric expansion is 3 8 4 2 10 4 c 1 glucose syrup and 9 2 10 4 c 1 silicone oil

boundary forces and plate velocities the notion that plate motions result from forces associated with plate boundaries implies that average plate velocities in a mantle hot spot frame of reference are systematically related to the nature and geometry of plate boundaries and to the proportion of continental lithosphere included in the plates several simple linear models relating plate velocity v to net ridge push rp slab pull sp trench suction ts and continental drag cd have been evaluated by multiple linear regression ridge push slab pull and continental drag are found to be vitally important trench suction may be a real force but because effective trench lengths are small and only three of the twelve plates considered are affected it is not a statistically significant contributor to plate motions the preferred relation v cm yr 2 7 4 5 2 1 9 rp 13 2 1 6 sp 4 7 6 cd is a remarkably good predictor of average plate velocities with an rms error of 0 50 cm yr and a correlation coefficient of 0 97 the implied balance between these net forces is generally consistent with the results of force balance models the constant term includes all forces not explicitly considered and suggests that the combined effects of such forces are about equal for all plates finally because the parameters used for any particular plate are independent of all other plates the success of this simple model strongly suggests that the forces which drive plate motions are intimately related to the plates themselves