the dynamics of motion of the south american plate observations of the intraplate stress field as compiled and evaluated in the context of the world stress map project can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates in this paper we investigate the driving and resistive plate tectonic forces acting on the south american plate in order to establish a detailed basis for future stress modelling our basic assumption is that the plate is in mechanical equilibrium that is the net torque of the forces acting on the plate vanishes the complexity of the forces exerted along the margins of the south american plate requires the dynamical analysis to be performed step wise incorporating the forces which are expected to be of major importance a first order model is defined the effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first order model with the first order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived the ridge push force driving the motion of the south american plate can be quantified independently and is included in the calculations as a known quantity solutions are obtained using four different determinations of the euler vector of absolute motion of the south american plate which is taken to define the orientation of the basal shear stress the results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis further with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere estimates of the maximum amount of continental basal shear resistance are derived evaluation of an alternative model for the south american plate excluding the northernmost pan of the andes demonstrates the significance of the resistive force exerted on the south american plate due to convergence with the caribbean plate it is to be expected that comparison of theoretical distributions of intraplate stress computed on the basis of the present dynamical analysis with observations of the actual state of stress will further constrain the range of feasible force models

tectonic erosion and accretion in steady state trenches abstract the geometry of subduction in a steady state trench is analysed to derive simple equations linking the rate of plate convergence across the trench with the outer slope and trough axial sedimentation rates the dips of the outer slope and the slope trough sequence boundary and the angle of decollement below the inner slope it is concluded that tectonic accretion will generally occur where the decollement is steeper than the outer trench slope and conversely that tectonic erosion will occur if the decollement is shallower than the dip of the outer slope the analysis is applied to the andean trench to explain tectonic erosion in the arica bight and to the timor trough to demonstrate that subduction has ceased

plate tectonics at an awkward junction rules for the evolution of sovanco ridge area ne pacific summary the elegant geometrical rules of plate tectonics do not allow for a gradual shift in plate motion directions or the gradual as opposed to sudden cessation of subduction at the scale of the small plates in the ne pacific imperfections in boundary processes have a large effect on the net torque on the plates and heavily influence the evolution of the geometry in this area the rotation of the spreading directions and the diminution of true subduction along the southern canadian coast has not occurred by the sudden switching of plate motions from one stable condition to another instead it appears as if the dominant factor for the evolution is the resistance of the ocean floor to formation of new smoothly slipping transform faults compressive deformation of even young lithosphere is not only mechanically unlikely but is not helpful to the particular configurations found in this area instead a migrating shear zone and an episode of highly en echelon spreading along a new axis nearly perpendicular to the present juan de fuca ridge have resulted the present sovanco ridge was never a transform fault neither is the nootka fault a shear zone but the locus of stretching between plates whose motions are congruent at the juan de fuca ridge but diverge toward the continental margin

forces controlling the present day state of stress in the andes the present day state of stress in the andes is expected to be controlled primarily by two different types of forces 1 the resistive force exerted on the western overriding margin of the south american plate and 2 forces that arise from the thickened crust of the andes i e the effect of topography and its compensating crustal root we have studied these forces on the basis of a model for the dynamics of the entire south american plate in which the andes are embedded in this model a given set of forces is constrained by the criterion that the net torque on the plate should vanish a thin elastic shell representation is used to calculate the intra plate stress field associated with the various force distributions we define a reference model for the present study that incorporates a uniform magnitude for the resistance associated with convergence along the western plate margin fpcr plate contact resistance and does not include the effects of topography subsequently we investigate the effect of lateral variation in the magnitude of fpcr and add the topography related forces the main results are 1 a uniform magnitude of fpcr leads to a better match with the observations than a magnitude that is a function of the dip of the lithosphere subducting below the western plate margin 2 the amount of horizontal compression across the andes found in the case that ridge push is considered to be the only force driving the south american plate is small compared to the value required to sustain the andes

three dimensional numerical simulation of the subduction dynamics in the sunda arc region southeast asia abstract the sunda arc has been modelled as a test subduction zone with geometrical complexities and varying dynamics we simulated spatially the structural heterogeneities of the subduction zone and carried out our computations by a three dimensional finite element approach the aim is to estimate the lateral variation in the effective horizontal compressive stress in the interplate region which can produce the present day observed seismogenic stress field in our simplified approach the stress field has been modelled by varying the traction ridge push while keeping the age dependent gravitational body force slab pull constant our result suggests that there is a considerable amount of stress dissipation in the decoupled segment of java at sumatra the required traction is much greater than that expected from the age dependency relations in our modelling the transition from the dominance of the horizontal compressive stress to that of the gravitational pull in the regional stress field occurs around a depth range of 150 200 km lateral variation in the subducted slab volume emerges crucial to the seismogenic stress field and the derived pattern of displacement subduction of the lighter oceanic rise material should enhance the stress level the effect of abrupt lateral variation in structure e g sudden changes in the slab dip or slab depth may also be significant on the regional stress field viscoelastic modelling attempted for estimating the temporal variation in subduction dynamics suggests that the thrust level will increase southward of the present day thrust zone in java while strike slip fault type stress field will be developed south of sumatra

outer rise stress changes related to the subduction of the juan fernandez ridge central chile 1 although outer rise seismicity is less common than interplate seismicity in subduction zones a significant level of seismicity has occurred between the nazca trench and juan fernandez ridge in central chile during the past 20 years we first study the 9 april 2001 mw 7 0 event and determine its focal mechanism depth and source time function by body waveform inversion from teleseismic broadband data the results indicate tensional faulting in the upper part of the mechanical lithosphere its strike 41 is similar to those observed in events down dip of the slab at about 100 km depth which could indicate that these earthquakes occur in preexisting structures formed at the trench compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 mw 7 8 valparaiso earthquake to understand their relation with the state of stress of the lithosphere we construct yield stress envelopes of the oceanic lithosphere including static and dynamic stresses dynamic stresses are due either to slab pull ridge push resistive and drag forces we explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the valparaiso interplate event the yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after

on the dynamics of the juan de fuca plate the juan de fuca plate is currently fragmenting along the nootka fault zone in the north while the gorda region in the south shows no evidence of fragmentation this difference is surprising as both the northern and southern regions are young relative to the central juan de fuca plate we develop stress models for the juan de fuca plate to understand this pattern of breakup another objective of our study follows from our hypothesis that small plates are partially driven by larger neighbor plates the transform push force has been proposed to be such a dynamic interaction between the small juan de fuca plate and the pacific plate we aim to establish the relative importance of transform push for juan de fuca dynamics balancing torques from plate tectonic forces like slab pull ridge push and various resistive forces we first derive two groups of force models models which require transform push across the mendocino transform fault and models which do not intraplate stress orientations computed on the basis of the force models are all close to identical orientations of predicted stresses are in agreement with observations stress magnitudes are sensitive to the force model we use but as we have no stress magnitude observations we have no means of discriminating between force models model stresses in the north and the south of the juan de fuca plate are similar breakup along the nootka fault zone and not in the gorda region can be explained if the local strength of a lithosphere column depends on lithospheric age stresses are invariant to whether or not transform push is acting on the mendocino transform fault we therefore conclude that the introduction of this extra plate driving force is not required by observations for the juan de fuca plate fl 2001 elsevier science b v all rights reserved

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

why does near ridge extensional seismicity occur primarily in the indian ocean abstract the possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest this effect is suggested by three key observations a decrease in seismicity with lithospheric age the fact that focal mechanisms show extension perpendicular to the spreading direction and a depth stratification of mechanism types a difficulty with this idea is that although thermoelastic stresses should be comparable in different regions the intraplate seismicity seems to occur in local concentrations in particular the ridge parallel extensional seismicity occurs preferentially in the central indian ocean region we explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects in particular the dramatic concentration of extensional seismicity in the central indian ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the himalayan collision and the various subduction zones the ridge parallel extensional stresses show a decrease with age similar to that of the seismicity as earthquakes in this area provide a major portion of the data for both ridge parallel extension and depth stratification these effects may be due more to the regional stress we thus propose that thermoelastic stresses provide a low level background in all plates but that the dominant effect is that of individual plate geometry and local boundary effects

subduction induced fragmentation of the nazca plate off peru mendana fracture zone and trujillo trough revisited the mendana fracture zone and the trujillo trough are two major intra oceanic features on the nazca plate off peru during the seaperc cruise of the r v jean charcot in 1986 sea beam bathymetric data single channel seismic records geomagnetic and gravity measurements and heat flow data have been obtained over these two areas in this paper we confirm that the mendana fracture zone is actively opening perpendicular to its trend resulting in the formation of new oceanic crust since about 3 5 ma we calculate that this new rift is propagating westward along the fracture zone at a velocity of about 10 cm yr 1 or 2 cm yr 1 with respect to the trench this extensional feature appears to be genetically linked with the trujillo trough located 200 km to the north this trough is bounded to the east by a left lateral transpressive fault which became active probably at the time when the mendana rift started we then discuss a possible mechanism to explain the origin of these two areas of intraoceanic deformation

kinematics of convergence deformation and stress distribution in the taiwan collision area 2 d finite element numerical modelling abstract using a 2 d plane stress finite element model with elastic and elasto plastic rheologies appropriate for deformation within the brittle upper crust we analyse the relationship between kinematics of convergence deformation and stress distribution in the present taiwan collision occurring between the ryukyu and luzon subduction zones the distribution of stress trends calculated in our models is compared with a synthetic map of actual stress trajectories based on the most recent data available in the collision zone these data combine present day sources from borehole breakouts and earthquake focal mechanism with the reconstruction of quaternary palaeostress from fault slip data analyses resulting in a complete map of compressional stress trajectories which is used to constrain our models we show that the distribution of stress trajectories in the active taiwan collision is principally controlled by 1 the geometric configuration of the boundary between eurasia and the philippine sea plate 2 the shape and rheological properties of major structural units 3 the direction of convergence of the philippine sea plate relative to eurasia and 4 the influence of the opening of the okinawa trough the study of a two dimensional elastic and elasto plastic finite element modelling of the subduction collision in and around taiwan allows us to estimate the influences of these different parameters in the stress pattern taking into account both the simplifying assumptions of the numerical modelling and the angular uncertainties of field determinations the fit between the calculated stress pattern of the finite element model and that determined based on the geometrical synthesis of field analyses is rather good in general indicating that our model is valid to first approximation misfits remain minor and can be explained by data uncertainties and simplifying modelling assumptions for instance the shape of the corner of the collision zone is critical but is not accurately known also limited decoupling in the longitudinal valley collision zone was not considered in our models although it certainly plays a role some interesting features of our model are 1 the greater influence of the shape of the collision zone in comparison with that of the direction of convergence 2 the requirement for a trench retreat related to suction force in the ryukyu arc and 3 the crucial role of the interaction between okinawa trough opening and collision at the sharp northwestern corner of the philippine sea plate including its influence on the geological evolution of northeastern taiwan

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

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

perspectives on the roles of melanges in subduction accretionary complexes a review abstract melanges play three principal overlapping roles in the architecture of subduction accretionary complexes sacs during and after sac formation first tectonic melanges serve as zones of concentrated deformation within and below the accreted rocks that are assembled during the subduction accretion process these melanges facilitate preservation of inter melange less deformed accretionary units aus beneath the trench side of the sac the initial deformation zone along the decollement at the top of the down going plate is marked by non melange tectonically dismembered formations or thinner units of scaly rock and breccia that separate accreted rocks above from the subducting rocks below olistostromal rocks may be incorporated into the decollement here in the mid arc to inner arc areas of the sac exotic block bearing melanges develop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy in mud diapirs and along out of sequence faults some of which facilitate uplift of high pressure rocks and serpentinite matrix melanges second after accretion the sedimentary tectonic diapiric and polygenetic melange units serve as single block or sheet architectural aus or subunits within larger accreted aus of the sac diapiric melanges and sedimentary olistostromal melanges formed in and above sac fault blocks respectively may become incorporated into the sac during ongoing deformation and become architectural units as well third melanges serve as post subduction stress guides that focus shear strain during continuing and post accretion deformation of sacs allowing ongoing modification of the sac during progressive deformation of the orogen examples of each type of role reveal the importance of all three processes in the current architecture of outer orogenic belts

finite element modelling of tertiary paleostress fields in the eastern part of the tajo basin central spain three subsequent tertiary paleostress fields that are deduced from fault slip data for the eastern part of the tajo basin are analyzed by finite element studies the modelling results show that maximum horizontal stresses shmax are mainly controlled by the geometry of the model limits and the boundary conditions applied the models are used to test two hypotheses on the origin of the altomira range a local stress field responsible for its formation altomira can be modelled successfully by superposition in time and place of two major paleostress fields iberian and guadarrama stress trajectories have been modelled with respect to a homogeneous cover and heterogeneous basement to investigate the role of rheological contrasts between different basement blocks on the orientation of the stress field results of this kind of modelling suggest a mechanical decoupling between the cover and the basement especially for the altomira paleostress field

use of the elastic recoil detection analysis erda microbeam technique for the quantitative determination of hydrogen in materials and hydrogen partitioning between olivine and melt at high pressures abstract a nuclear microbeam technique called elastic recoil detection analysis erda or forward recoil spectroscopy which is capable of yielding bulk h in silicates at ppm sensitivities is described this technique is nondestructive and uses a 4he beam which may be routinely focused to dimensions less than 5 5 m the technique is suitable for the analysis of both materials of appreciable h content e g amphiboles and materials with trace h content the technique is calibrated in the range 0 2 wt h2o using a set of mineral standards of known h2o concentration with sensitivities of 0 04 wt h2o achieved there is a good correlation between h2o contents derived by spectra simulations and concentrations derived by empirical calibration although the former yield data 10 20 lower when compared to known values the equilibration of olivine with a potassic silicate melt at high pressures 1 5 to 10 gpa in experiments shows more h is accommodated in the mineral with increasing pressure the olivine melt kh2o expressing h as wt h2o concentration in mineral concentration in melt at 1 5gpa 1400 c was ca 0 04 0 015 at 5 8 6 gpa 1740 c olivine melt kh2o increased to 0 13 0 03 a single experiment at 10 gpa ca 2000 c yielded a minimum kh2o of 0 12 the amount of h which minerals accommodate is also highly correlated with bulk system composition which controlled melt composition in alkali absent bulk systems the kh2o for olivine equilibrated with a mg si melt was 0 30 at 1 gpa 1400 c an order of magnitude increase over the alkali bearing system at this pressure this reflects the reduced facility of a wholly mg si melt to accommodate h2o relative to an alkali bearing melt the increase in kh2o with pressure for olivine melt combined with data for kh2o of natural olivine and orthopyroxene in basaltic glass at p

spatial and temporal variations in the andean subduction zone a comprehensive global study of the process of subduction of oceanic lithosphere has made it evident that a major role is played by the age of the subducted lithosphere the age dependence of subduction is of special interest in an understanding of the late cenozoic history of the andean subduction zone many of the complexities of this zone result from the peculiar circumstance that the zone is in transition from an early stage of consumption of old lithosphere 70 ma producing deep earthquakes towards the latest stage of consumption of young lithosphere without deep earthquakes this transition has induced an evolutionary pattern with significant temporal variations notably changes in dip and length of the subducted slab moreover the various segments of the south american subduction zone are in different stages of this evolution in comparison with the other segments the northern chile segment is in the earliest stage and therefore shows a much closer resemblance to the classic deep subduction zones in the western pacific than do other parts of the south american zone a roughly n s symmetry in development is found around northern chile with the ecuadorian and southern chilean near 40 45 s segments representing the latest stage to a large extent the temporal and spatial variations in lithospheric age at the trench originate from the reorganization in plate configuration which started with the break up of the farallon plate into the cocos plate and the nazca plate at about 25 ma

accretion and lateral variations in tectonic structure along the peru chile trench abstract extensive marine geophysical data sets on the deformational characteristics of the oceanic crust and of sediments in and near the peru chile trench notably those presented by schweller kulm and co workers have established the presence of significant north south variations along the trench in this paper we investigate the physical basis for these variations by studying the stress field in the nazca plate with emphasis on the region near the trench finite element methods were employed to calculate the stress distribution the resulting regional stress field near the trench is characterized by compression normal to the trench at latitudes between 1 s and approximately 16 s tension in the range 16 s 27 s and again compression from 27 s to 45 s many conspicuous and hitherto unexplained variations along the trench system can be understood by taking into account these lateral changes a close correlation appears to exist between the latitude ranges with compression and tension normal to the trench and the accreting and non accreting provinces of the peru chile trench respectively the basis for this correlation must be sought in the role which grabens in the seaward trench slope play in the subduction or accretion of sediments deep grabens can trap the sediments and carry them down into the subduction zone leaving little or no material to be accreted onto the margin the regional stress field is superimposed on the stresses generated in bending the nazca plate prior to subduction tension in the direction normal to the trench as inferred for the nazca plate off southern peru and northern chile 16 s 27 s lowers the neutral plane in the bending plate and increases the tensional stresses near the surface this promotes the formation of grabens and consequently the subduction of sediments similarly compression normal to the trench counteracts the formation of deep grabens in the latitude ranges 1 s 16 s and 27 s 45 s thus favouring accretion of sediments from this study we infer that the regional stress field may have a significant effect on the topography of the seaward trench slope taking into account the role of regional stress in graben formation not only explains first order latitudinal variations in accretion and subduction of sediments along the peru chile trench but it is also expected to contribute significantly to understanding similar variations observed in other subduction zones

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

origin and tectonic evolution of the caroline ridge and the sorol trough western tropical pacific from admittance and a tectonic modeling analysis abstract an independent caroline sea plate surrounded by prominent seismotectonic features is said to exist in the western tropical pacific the caroline sea plate is a transient feature existing only when the caroline philippine sea plate boundary is jumping to the caroline pacific boundary the origin and the tectonic evolution of the caroline ridge sorol trough region which forms the northern part of the caroline pacific plate boundary are examined using a standard admittance analysis a tectonic modeling study and additional direct gravity and magnetic analyses the observed admittance function is fitted well by shallow airy model in the 250 125 km 0 025 k 1 wavelength region whose average crustal thickness t c is 20 km and also by thin plate flexure model of isostasy within the diagnostic waveband of 250 80 km 0 025 k 1 with an effective elastic thickness t e of 5 7 km a simple interpretation of the observed admittance function is that the local airy model of isostasy is the main support mechanism in the study region for the longer wavelength more fundamental topography corresponding to that of the caroline ridge the caroline ridge is inferred to have formed in a near ridge tectonic setting on top of oceanic lithosphere which was young thin and weak at the time of emplacement the tectonic model study after an explanation for rifting along the mendana fracture zone in the nazca plate suggests that the sorol trough formed in the 17 7 ma interval splitting the caroline ridge into two longitudinal halves the rifting along the sorol trough which took place in the manner described in weissel and karner 1989 flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension j geophys res 94 13 919 13 950 necking of unbroken lithosphere post dated the formation of the caroline ridge by 10 15 m y during which time t e in the region increased to 5 10 km the rifting necking along the sorol trough represents a second loading event in the region the combined gravity effects of the rifting as well as the increase in the t e in the region from essentially nil to 5 10 km is reflected in the agreement between the observed admittance and the regional model with a 5 7 km t e in the wavelength region 80 km these results are consistent with the findings and the suggestions of the previous investigators in the region in that 1 the caroline ridge was constructed in the late oligocene by hot spot volcanism on top of oligocene seafloor of the caroline sea back arc basin 2 the eauripik rise genetically related to the caroline ridge formed on young and weak caroline basin lithosphere and is supported by airy manner crustal thickening and that 3 the lithosphere of the caroline ridge was young at the time of rifting necking along the sorol trough on the basis of the current results and the findings of the previous workers a tentative hypothetical rheological model for the crustal density structure of the caroline ridge sorol trough is also given the caroline sea plate appears to be bounded in the north and south by e w trending boundaries that deform diffusely while more normal narrow n s trending boundaries exhibiting more localized albeit slow deformation bound the plate in the east and west

stress magnitude estimates from earthquakes in oceanic plate interiors we propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms using a depth dependent rheology we show it is possible to estimate the differential stress 1 3 averaged over some reference lithospheric thickness the resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis we apply the method to the central indian ocean where intraplate seismicity is high from well studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals this result is consistent with the high level of stress that was found from numerical model calculations by cloetingh and wortel 1985 1986 from the few intraplate events in the pacific plate we also estimate differential stresses in this area

the relationship between plate curvature and elastic plate thickness a study of the peru chile trench the age of the nazca plate where it enters the peru and northern chile trenches varies from 30 ma in the north to 45 ma in the south as its dip beneath the south american continent steepens from 13 to 30 if the elastic thickness te of oceanic lithosphere depends only on its age and therefore thermal state we would expect that te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth geoid and gravity anomalies would increase from the peru trench in the north to the northern chile trench further south we find that the opposite is true the lithosphere appears suffer outboard of the peru trench than it does offshore chile to explain deflections of the lithosphere seaward of peru the isotherm controlling the elastic ductile transition must be near 800 c providing the thermal structure of the plate is that predicted by the standard thermal plate model because the decrease in plate stiffness to the south is correlated with an increase of plate curvature over the outer rise and outer trench wall we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains the magnitude of the reduction in strength however suggests that the standard oceanic yield strength envelope underestimates the amount of failure for lithosphere bent to high strains unless the stress field off northern chile is characterized by 100 mpa level tension the more highly bent segment of subducting lithosphere at this trench also dips at a steeper angle at greater depth beneath the continent but detailed analysis of plate geometry does not support a relationship between slab dip as determined by earthquake hypocenters and elastic behavior over the outer rise