space geodetic observations of nazca south america convergence across the central andes space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic nazca and continental south american plates in peru and bolivia roughly half of the overall convergence about 30 to 40 millimeters per year accumulated on the locked plate interface and can be released in future earthquakes about 10 to 15 millimeters per year of crustal shortening occurred inland at the sub andean foreland fold and thrust belt indicating that the andes are continuing to build little 5 to 10 millimeters per year along trench motion of coastal forearc slivers was observed despite the oblique convergence

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

rollback orogeny model for the evolution of the swiss alps the construction of the european alps and the himalayas has been related to the convergence and subsequent collision of two continental plates nearly all models of orogeny build on this concept and all of them relate the stacking of nappes and the build up of topography to compressional forces at work in response to the collision between two continental plates for the central european alps however these models fail to explain the first order observations of a mountain belt which particularly includes the striking isostatic imbalance between the low surface topography and the thick crust beneath the alps here we review and synthesize data on the geologic architecture of the central alps the chronology and pattern of crustal deformation and information about the deep crustal structure derived from seismic tomography furthermore we discuss the intrinsic and explicit assumptions in the kinematic models of alpine evolution in the context of plate tectonic considerations we combine these views with progress in understanding that has been gained through subduction and collision isostatic mass and force balancing and with information that has been collected on the modern seismic regime we conclude that a rollback orogeny model for the european plate offers the most suitable concept to explain the ensemble of surface and deep lithosphere observations in this model gravity forces drive the evolution of the orogen and the construction of surface topography is accomplished without the requirement of a hard collision between two continents

linking collision slab break off and subduction polarity reversal in the evolution of the central indian tectonic zonetectonic evolution of the central indian tectonic zonet deshmukh n prabhakar the central indian tectonic zone demarcates the zone of amalgamation between the north indian craton and the south indian craton presently the major controversies in the existing tectonic models of the central indian tectonic zone revolve around the direction of subduction and the precise timing of accretion between the north indian craton and the south indian craton a new model for the tectonic evolution of the central indian tectonic zone is postulated in this contribution based on recent geological and geophysical evidence combined with previously documented tectonic configurations the present study employs the slab break off hypothesis and subsequent polarity reversal to explain the tectonic processes involved in the evolution of the central indian tectonic zone we propose that the subduction initiated c 2 5 ga in a s directed system producing island arc sequences on the south indian craton the southward subduction regime culminated with slab break off underneath the south indian craton between c 1 65 ga and 1 55 ga which subsequently induced subduction polarity reversal and set the course for n directed subduction

a quantitative approach to spring hydrograph decomposition summary a combined analytical numerical study for the characterization of spring hydrographs is presented two dimensional analytical solutions for diffusive flux from rectangular blocks of arbitrary shape facilitate a quantitative characterization of exponential hydrograph components together with analytical solutions for block discharge a systematic analysis of numerically simulated spring hydrographs of synthetic karst systems provides an insight into karst hydrodynamics different hydrograph components do not represent different classes of rock permeability hydrographs of individual homogeneous blocks can be decomposed into several exponential components discharge hydrographs of symmetric rectangular blocks can be reconstructed by the sum of only three exponential components increasing block asymmetry results in an increasing number of exponential components contributing significantly to total discharge spring hydrographs represent a sum of individual block discharges originating from diffuse infiltration and conduit discharge originating from concentrated recharge beyond the inflection of the recession limb a spring hydrograph can be decomposed in a similar manner to that of individual homogeneous blocks the presented hydrograph analytical method facilitates the estimation of hydraulic and geometric parameters of karst hydrogeological systems

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

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

overriding plate shortening and extension above subduction zones a parametric study to explain formation of the andes mountains mountain building above subduction zones such as observed in the andes is enigmatic and the key parameter controlling the underlying dynamics remains a matter of considerable debate a global survey of subduction zones is presented here illustrating the correlation between overriding plate deformation rate and twelve physical parameters overriding plate velocity subducting plate velocity trench velocity convergence velocity subduction velocity subduction zone accretion rate subducting plate age subduction polarity shallow slab dip deep slab dip lateral slab edge proximity and subducting ridge proximity all correlation coefficients are low r 0 39 irrespective of the global reference frame relative plate motion model or overriding plate deformation model except for the trench velocity 0 33 0 68 exact value depends on adopted global reference frame and subduction velocity which shows an anticorrelation 0 55 0 57 this implies that no individual parameter can explain overriding plate deformation except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined orogenesis occurs in overriding plates bordering central regions of wide subduction zones 4000 km when the overriding plate is moving trenchward at 0 2 cm yr e g andes japan because the center of a wide slab offers large resistance to lateral migration the overriding plate effectively collides with the subduction hinge forcing the slab to attain a shallow dip angle e g nazca and japan slabs overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone but in the latter scenario maximum extension velocities are much lower than in the former scenario for subduction settings close to lateral slab edges overriding plate motion plays no significant role in overriding plate deformation thus for rapid overriding plate extension the key ingredient is rapid trench retreat which only occurs close to lateral slab edges while for overriding plate shortening the key ingredients are 1 the resistance to rapid trench and hinge retreat which occurs far from lateral slab edges and 2 trenchward overriding plate motion

kinematic setting and structural control of arc volcanism abstract the evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono magmatic processes along convergent plate boundaries however our knowledge is fragmented and the available data highlight different behaviours this study analyzes the kinematic setting the structural control and the volcanic productivity of 16 arcs these arcs are characterized by predominant extensional compressional strike slip or oblique motions there is an overall coupling between the normal vs parallel motions along the arc and those of the underlying slab therefore the higher the trench normal or parallel component of the subduction rate the higher is the amount of arc normal or parallel motion this relation confirms that strain partitioning is in general feasible at many convergent settings involving also the volcanic arc portion the arc normal motion may be characterized by extension or compression as a function of the increase in the trench normal convergence or subduction velocity the lack of an evident relation between the subduction rate or convergence rate and the volcanic productivity of the arc is partly inconsistent with previous studies which highlight a relation between the subduction rate and melt production below arcs this discrepancy mainly suggests that different processes control the generation rise and eruption of magma at different depths varying the intrusive extrusive ratio along arcs the structure of the arc does not control the distribution of the volcanoes however it does control the volcanic output through different processes even though regional or local extension associated with strike slip or compressive structures is the ultimate requisite in any setting in general the higher the amount of extension the higher is the output rate along the arc

trench migration and subduction zone geometry seismic tomography results show a large variety in upper mantle structure along convergent plate boundaries we numerically investigate the effect of trench migration on the evolution of a slab with temperature dependent viscosity encountering a viscosity interface we find that subduction zone geometry is sensitive to even small rates 1 cm yr of retrograde motion increase in trench migration rate decreases the dip angle of the slab and its ability to penetrate the lower mantle upon including a background mantle flow it turns out that trench migration relative to the upper mantle flow is more decisive than the absolute plate velocities subduction zone geometry appears to be strongly time dependent we conclude that individual tectonic setting and time dependent slab behaviour can account for many different types of observable subduction zone anomalies our model temperature can adequately account for magnitudes and patterns of seismic anomalies as obtained from seismic tomography

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

trench curvature and deformation of the subducting lithosphere summary the subduction of oceanic lithosphere is generally accompanied by downdip and lateral deformation the downdip component of strain is associated with external forces that are applied to the slab during its sinking namely the gravitational force and the mantle resistance to penetration here we present theoretical arguments showing that a tectonic plate is also subject to a predictable amount of lateral deformation as a consequence of its bending along an arcuate trench zone independently from the long term physical processes that have determined the actual curvature of the subduction zone in particular we show that the state of lateral strain and the lateral strain rate of a subducting slab depend from geometric and kinematic parameters such as trench curvature dip function and subduction velocity we also demonstrate that the relationship between the state of lateral strain in a subducting slab and the geometry of bending at the corresponding active margin implies a small component of lateral shortening at shallow depths and may include large extensional lateral deformation at intermediate depths whereas a state of lateral mechanical equilibrium can only represent a localized exception our formulation overcomes the flaws of the classic ping pong ball model for the bending of the lithosphere at subduction zones which lead to severe discrepancies with the observed geometry and style of deformation of the modern subducting slabs a study of the geometry and seismicity of eight modern subduction zones is performed to assess the validity of the theoretical relationship between trench curvature slab dip function and lateral strain rate the strain pattern within the eight present day slabs which is reconstructed through an analysis of harvard cmt solutions shows that tectonic plates cannot be considered as flexible inextensible spherical caps whereas the lateral intraslab deformation which is accommodated through seismic slip can be explained in terms of deviations from the mechanical equilibrium

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

extensional tectonics during late cretaceous evolution of the southern central andes evidence from the chilean main range at 35 s abstract the study of late cretaceous earliest paleogene series in the chilean andes at 35 s provides new constraints on the coeval evolution of the area and the southern central andes these correspond to the continental series of brcu and the overlying plan de los yeuques formation pyf zircon age determinations reveal that these represent an almost continuous record of deposition respectively from the cenomanian early campanian to the campanian danian according to paleogeographic reconstructions the brcu series corresponds to the coeval foreland basin deposits in a near arc position which was westerly bounded by the earliest late cretaceous orogen detrital zircon age spectra for this reveal only a cretaceous component thus indicating a depocenter disconnected to that represented by coeval synorogenic deposits in argentina the volcanic rocks of the pyf record an eastern migration of the magmatic loci into the area at least since 80 ma its outcrops display a series of syn sedimentary normal faults indicating extensional conditions throughout its deposition its volcanic rocks conform an apparent bimodal suite characterized by arc like affinities low pressure conditions for the corresponding magmas and juvenile isotopic signatures for all the compositional spectra comparison and modeling of such compositional characteristics indicate a derivation from parental magmas similar to those currently sourcing the southern segment of the andean southern volcanic zone 37 42 s altogether these characteristics suggest an extensional intra arc basin setting for the deposition of this unit probably framed by a normal to thin crust of 30 35 km the extensional conditions observed for this unit are also recorded in numerous late cretaceous arc deposits along the andean margin thus indicating that this corresponds to a regional scale feature this evidence records a change from late cretaceous from compressional to extensional conditions in the arc area which agrees well with major plate tectonic configuration proposed for this time span in the andean margin

overriding plate deformation and variability of fore arc deformation during subduction insight from geodynamic models and application to the calabria subduction zone in nature subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates such large plates can impose far field boundary conditions that influence the style of subduction and overriding plate deformation here we present dynamic laboratory models of progressive subduction in three dimensional space in which the far field boundary conditions at the trailing edges of the subducting plate sp and overriding plate op are varied four configurations are presented free both plates free sp fixed op fixed and sp op fixed we investigate their impact on the kinematics and dynamics of subduction particularly focusing on overriding plate deformation the results indicate that the variation in far field boundary conditions has an influence on the slab geometry subduction partitioning and trench migration partitioning our models also indicate that in natural narrow subduction zones assuming a homogeneous overriding plate the formation of back arc basins e g tyrrhenian sea aegean sea and scotia sea is generally expected to occur at a comparable location 250 700 km from the trench irrespective of the boundary condition in addition our models indicate that the style of fore arc deformation shortening or extension is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface trench suction our geodynamic model that uses the sp op fixed setup is comparable to the calabria subduction zone with respect to subduction kinematics slab geometry trench curvature and accretionary configuration furthermore the model can explain back arc and fore arc extension at the calabria subduction zone since the latest middle miocene as a consequence of subduction of the narrow calabrian slab and the immobility of the subducting african plate and overriding eurasian plate this setting induced strong trench suction driving fore arc extension and forced subduction to be accommodated almost entirely by slab rollback not trenchward subducting plate motion while trench retreat was accommodated almost entirely by back arc and fore arc extension not trenchward overriding plate motion comparable to our sp op fixed model

late cretaceous to recent plate motions in western south america revisited abstract the andean cordillera has evolved since the late cretaceous in the context of subduction of oceanic lithosphere beneath continental lithosphere making the kinematics between south america and its adjacent oceanic plates in the pacific basin valuable to analyze the development of the andean orogen the latest cretaceous cenozoic convergence history in western south america may be divided into three stages the youngest stage 1 25 0 ma is characterized by ene directed convergence of the nazca plate toward most of south america and by e w subduction of the antarctic plate beneath southern patagonia the nazca south america convergence rate in stage 1 shows a continuous decrease from the highest values in the cenozoic 15 cm yr to the present day values from gps measurements 7 cm yr stage 2 47 28 ma is characterized by ne directed subduction of farallon with the convergence rate remaining almost constant during the entire interval in those times obliquity was dextral in chile sinistral in southern peru while almost head on convergence occurred in central and northern peru during latest cretaceous to early eocene times stage 3 the farallon plate was subducted beneath peru and the phoenix plate was subducted farther south where a triple junction migrated southward along the chilean margin the subduction of the farallon plate was rather slow with variable direction imposed by the position of the triple junction whereas subduction of the phoenix plate was rapid 10 cm yr and ese directed we present a working hypothesis suggesting no major changes in the age of subducted lithosphere in the chile trench from middle eocene to late oligocene followed by subduction of progressively older oceanic lithosphere in the early neogene and progressively younger lithosphere during the late neogene and the quaternary in addition it is shown that south american motion as predicted by available hotspot models has insufficient resolution to be applied to the analysis of cenozoic andean deformation

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

geomorphic evidence for plio quaternary compression in the andean foothills of the southern neuquen basin argentina the southern central andes foothills are made of imbricate thick and thin skinned fold and thrust belts they were formed during cretaceous and miocene compressive periods by successive eastward propagation episodes of the orogenic front we analyze remnant terraces and alluvial deposits of the neuqun river and its tributaries to study the plio quaternary tectonic regime in the southern neuqun basin topographic profiles of remnant terraces show a crustal scale tens of kilometers anomaly above the chihuido south anticline this anomaly is accompanied by sedimentary aggradations downstream and upstream of the anticline another anomaly in the terrace profiles correlated with a knickpoint in the current neuqun river is observed farther east near aelo we interpret these anomalies to be the consequence of a plio quaternary uplift of the chihuidos and aelo anticlines folding and surface faulting of plio quaternary alluvial deposits show that this uplift is concomitant with the reactivation of the miocene orogenic front and of the predominantly thin skinned agrio fold and thrust belt from these data we infer that the southern neuqun basin is still shortening under a compressive stress regime and that the andean orogenic front is migrating farther to the east extensional features in the study area correspond mainly to thin skinned gravitational gliding resulting from crustal uplift thus we argue that the tectonic compressive regime is preserved in the southern neuqun basin throughout the plio quaternary though the slab may have steepened during this period

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

analogue modeling of arc and backarc deformation in the new hebrides arc and north fiji basin in most backarc basins extension is perpendicular to the arc thus individual spreading ridges extend approximately parallel to the arc in the north fiji basin however several ancient and active spreading ridges strike 70 90 to the new hebrides arc these high angle spreading ridges relocated southward during the asymmetric opening of the north fiji basin we have simulated the structural development of the north fiji basin and the new hebrides arc with scaled analogue models and the results have inspired us to come to several tentative conclusions we interpret the orientation of the high angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge where they form close to the hinge relocation of these spreading ridges is most likely related to subduction of the west torres plateau along the new hebrides trench this resulted in localized collision retarded rollback of the subducting slab along the northwest corner of the trench and reduced extension and shearing in the northwest corner of the north fiji basin backarc extension continued in the rest of the north fiji basin owing to continued rollback of the southern part of the subducting slab here active extension was separated from the slightly or nonextending northwest corner by a zone striking at high angle to the new hebrides arc i e the hazel holme extensional zone moreover impingement of the d entrecasteaux ridge into the overriding plate led to local deformation and fragmentation of the arc

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

global correlations between maximum magnitudes of subduction zone interface thrust earthquakes and physical parameters of subduction zones abstract the maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on earth with some subduction zone segments producing giant subduction zone thrust earthquakes e g chile alaska sumatra andaman japan and others producing relatively small earthquakes e g mariana scotia here we show how such variability might depend on various subduction zone parameters we present 24 physical parameters that characterize these subduction zones in terms of their geometry kinematics geology and dynamics we have investigated correlations between these parameters and the maximum recorded moment magnitude m w for subduction zone segments in the period 1900 june 2012 the investigations were done for one dataset using a geological subduction zone segmentation 44 segments and for two datasets rupture zone dataset and epicenter dataset using a 200 km segmentation 241 segments all linear correlations for the rupture zone dataset and the epicenter dataset r 0 00 0 30 and for the geological dataset r 0 02 0 51 are negligible low indicating that even for the highest correlation the best fit regression line can only explain 26 of the variance a comparative investigation of the observed ranges of the physical parameters for subduction segments with m w 8 5 and the observed ranges for all subduction segments gives more useful insight into the spatial distribution of giant subduction thrust earthquakes for segments with m w 8 5 distinct narrow ranges are observed for several parameters most notably the trench normal overriding plate deformation rate v opd i e the relative velocity between forearc and stable far field backarc trench normal absolute trench rollback velocity v t subduction partitioning ratio v sp v s the fraction of the subduction velocity that is accommodated by subducting plate motion subduction thrust dip angle st subduction thrust curvature c st and trench curvature angle t the results indicate that m w 8 5 subduction earthquakes occur for rapidly shortening to slowly extending overriding plates 3 0 v opd 2 3 cm yr slow trench velocities 2 9 v t 2 8 cm yr moderate to high subduction partitioning ratios v sp v s 0 3 1 4 low subduction thrust dip angles st 30 low subduction thrust curvature c st 2 0 10 13 m 2 and low trench curvature angles 6 3 t 9 8 epicenters of giant earthquakes with m w 8 5 only occur at trench segments bordering overriding plates that experience shortening or are neutral v opd 0 suggesting that such earthquakes initiate at mechanically highly coupled segments of the subduction zone interface that have a relatively high normal stress deviatoric compression on the interface i e a normal stress asperity notably for the three largest recorded earthquakes chile 1960 alaska 1964 sumatra andaman 2004 the earthquake rupture propagated from a zone of compressive deviatoric normal stress on the subduction zone interface to a region of lower normal stress neutral or deviatoric tension stress asperities should be seen separately from frictional asperities that result from a variation in friction coefficient along the subduction zone interface we have developed a global map in which individual subduction zone segments have been ranked in terms of their predicted capability of generating a giant subduction zone earthquake m w 8 5 using the six most indicative subduction zone parameters v opd v t v sp v s st c st and t we identify a number of subduction zones and segments that rank highly which implies a capability to generate m w 8 5 earthquakes these include sunda north sulawesi hikurangi nankai northern ryukyu kamchatka kuril japan aleutians alaska cascadia mexico central america south america lesser antilles western hellenic and makran several subduction segments have a low score most notably scotia new hebrides and mariana

evolution of shallow and deep structures along the maipo tunuyan transect 33 40 s from the pacific coast to the andean foreland fil giambiagi laura beatriz consejo nacional de investigaciones cientificas y tecnicas cientifico tecnologico mendoza instituto argentino de nivologia glaciologia y ciencias ambientales argentina

cordillera formation mantle dynamics and the wilson cycle we propose that the andes cordillera formed by a collision between the trenchward moving south american plate and the nazca slab and subslab mantle resistance to trenchward motion in the form of horizontal compressive normal stress deforms south america9s leading edge the advancing continent also perturbs the subslab mantle flow field as mantle is displaced this flow field detected via shear wave splitting includes trench parallel flow away from a central stagnation point and corner flow around the northern and southern ends of the continent the corresponding spatial variation in applied stress accounts for the large scale characteristics of andean deformation buckling at the stagnation point maximum normal stress forming the central andes and bolivian orocline and corner flow associated with the eastward motion of the caribbean and scotia plates the high stresses required to form and maintain the andes imply that south america9s trenchward motion is primarily driven by deep mantle flow coupled to its base no other driving force of sufficient magnitude is available similarities between north and south american plate motion history and large scale cordillera structure indicate that the western north american laramide rockies formed in a similar manner and that the north american plate is also driven westward by deep mantle flow we infer that atlantic spreading itself is similarly driven by contrast slab rollback in a closing ocean basin opens marginal basins at convergent margins as in the western pacific we suggest that there is a causal link between the opening and closing of ocean basins and the formation of cordilleras and marginal basins cordillera formation occurs at the leading continental margin during the spreading phase of the wilson cycle whereas marginal basins form during the closing phase

relationships among climate erosion topography and delamination in the andes a numerical modeling investigation cordilleran orogenic systems such as the andes are controlled by shortening rates climatically controlled erosion rates and in some cases eclogite production and delamination all of these processes are coupled however making it diffi cult to uniquely determine the relative importance of each process and the feedbacks among them in this paper we develop a massbalanced numerical model that couples an actively shortening orogen and crustal root with eclogite production delamination and climatically controlled erosion the model provides a fi rst order quantifi cation of the sources shortening and sinks erosion and eclogite production and delamination of crustal volume during the cenozoic in the andes as a function of latitude and time given reasonable estimates for the rates of eclogite production and the threshold size of the eclogitic root required for delamination the model suggests that in the central andes between 5 s and 32 s the orogen has grown to a suffi cient height to produce and maintain eclogite which in turn has promoted delamination in the lower crust and mantle in this region climatically controlled erosion rates infl uence the size of the orogen through two separate mechanisms by exporting mass via surface processes and by controlling the lithostatic pressure in the lower crust which modulates the rates of eclogite production and or delamination to the north and south of the central andes relatively low shortening rates and high precipitation and erosion rates have slowed eclogite production such that delamination likely has not occurred during the cenozoic

a laboratory investigation of effects of trench migration on the descent of subducted slabs a laboratory investigation of viscous slabs subducted from a migrating trench reveals a range of possible behaviours the slab dip in a uniform mantle is found to be steady or oscillatory depending on the rates of descent and trench migration in addition density and viscosity interfaces are used to model the increased resistance to sinking of stiff slabs through the seismic discontinuity at a depth of about 660 km in earth s mantle if the slab is denser than the lower layer and its dip in the upper layer is steady it can continue to descend through the lower layer in a tabular form and in either a steady or oscillatory manner or it can be laid horizontally on the interface for a distance before descending in a chain of diapirs resulting from gravitational instability at the interface if the slab dip in the upper layer is unstable the slab sinks into the lower layer in a chain of large diapirs at a spacing determined by the frequency of oscillations set by instability of the slab within the upper layer the style of penetration depends on the trench migration speed and the ratio of sinking velocities in the two layers estimates for mantle slabs indicate that they may range across the major regime transitions the experimental system provides a gross simplification of mantle conditions but the results are a testimony to the possibility of a range of complex behaviour of subducted lithosphere they also indicate that the relationship between the structural features in the lower mantle revealed by seismic imaging and present day tectonic processes at the surface may not be obvious tomographic images of western pacific subduction zones and data for the migration rates of associated trenches suggest a dependence of slab behaviour on migration rate similar to that seen in the experiments

control of forearc density structure on megathrust shear strength along the chilean subduction zone abstract unveiling how the interaction of friction pore pressure p and normal stress n controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains an existing density model of the chilean margin 18 45 s allows the computation of the so called vertical stress anomaly vsa that accounts for the component of normal stress due to the weight of the overlying crustal column a spatial correlation between vsa surface geology and several earthquake parameters suggests a significant control exerted by the geologically dominated density structure of the crust on megathrust shear strength the very seismogenic forearc in front of the high central andes 18 34 s has positive vsa values up to 60 mpa and a mafic dominated crust which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column in addition to a likely dry subduction channel characterized by high and low p in contrast the southern andean forearc 34 45 s shows neutral to negative vsa down to 30 mpa correlated with a felsic dominated crust which added to a presumably wet subduction channel low and high p configures a rather weak megathrust below a light forearc earthquakes here are concentrated around the arauco peninsula a first order morpho structural anomaly characterized by comparatively higher values of vsa than the rest of this segment which is the nest of the giant mw 9 5 valdivia 1960 earthquake and the southern rupture termination of the recent mw 8 8 maule earthquake the ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative vsa values the correlation between vsa and the along strike segmentation of the andean orogen indicates that the geologically inherited density structure of the forearc also rules long term mechanical coupling between converging plates and prescribes the possible structure of the orogen because central and southern andes represent two extremes in terms of orogenic structure and seismogenic behavior results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins

climate controlled shifts in sediment provenance inferred from detrital zircon ages western peruvian andes provenance analysis of pleistocene terrace deposits together with modern sediments from the same streams from four catchments draining the western margin of the andes in peru is used to infer changes in erosion patterns between the past and the present period by matching detrital zircon ages with crystallization ages of source rocks age populations suggest major changes in sediment provenance through the past 100 k y at present sediment sources are mainly located along the steep middle reaches of the rivers whereas during the pleistocene sources were additionally located in the low relief headwaters of these catchments these shifts in the loci of erosion are interpreted to reflect changes in precipitation patterns where periods of stronger precipitation on the altiplano allowed the entrainment of material from the low relief plateau in the past in contrast modern precipitation patterns result in negligible erosion rates on the altiplano and the site of material entrainment shifts to the knickzone reaches where steeper slopes and higher stream power promote erosion in that sense this work illustrates that terrace aggradation is associated with major shifts in provenance sources

plate kinematics slab shape and back arc stress a comparison between laboratory models and current subduction zones a combination of statistical studies on present day subduction zones and three dimensional 3d laboratory models is performed with the aim to clarify the way that plate kinematics control the geometry of the slab and the overriding plate deformation in subduction zones in 3d laboratory models the analogue of a two layer linearly viscous lithosphere upper mantle system is achieved by means of silicon putty glucose syrup tank experiment the subducting and overriding plate velocities are systematically changed by exploring the variability field of natural plate kinematics both statistical and modelling approaches recognize the importance of overriding plate motion on subduction process behavior 1 trenches migrate at a rate close to the overriding plate motion but always move slower than the overriding plates the mechanism at work is a direct consequence of slab anchoring opposed by both lithosphere and mantle viscous resistance and is responsible for overriding plate deformation and slab geometry variability 2 an overriding plate shortens when the overriding plate moves toward the trench and conditions that are favourable for overriding plate extension are created when the overriding plate moves away from the trench 3 shallow and steep dips are found if the overriding plate moves toward and away from the trench respectively

an integrated global model of present day plate motions and plate boundary deformation summary in this paper we present a global model gsrm 1 of both horizontal velocities on the earth s surface and horizontal strain rates for almost all deforming plate boundary zones a model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field in our model we perform a least squares fit between model velocities and observed geodetic velocities as well as between model strain rates and observed geological strain rates model velocities and strain rates are interpolated over a spherical earth using bi cubic bessel splines we include 3000 geodetic velocities from 50 different mostly published studies geological strain rates are obtained for central asia only and they are inferred from quaternary fault slip rates for all areas where no geological information is included a priori constraints are placed on the style and direction but not magnitude of the model strain rate field these constraints are taken from a seismic strain rate field inferred from normalized focal mechanisms of shallow earthquakes we present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones generally the strain rate tensor field is consistent with geological and seismological data with the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs we find that in general there is a good agreement between the present day plate motions we obtain and longterm plate motions but a few significant differences exist the rotation rates for the indian arabian and nubian plates relative to eurasia are 30 13 and 50 per cent slower than the nuvel1a estimate respectively and the rotation rate for the nazca plate relative to south america is 17 per cent slower on the other hand caribbean north america motion is 76 per cent faster than the nuvel 1a estimate while crustal blocks in the india eurasia collision zone move significantly and self consistently with respect to bounding plates only a very small motion is predicted between the nubian and somalian plates by integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in for instance redefining a no net rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation

tectonic shortening and crustal thickness in the central andes how good is the correlation exceptional thickening of continental crust beneath the central andes is believed to be mainly a result of tectonic shortening of the south american plate in neogene time this shortening has been estimated to have contributed as much as 70 80 of the present crustal volume a compilation of published shortening values and our own estimates based on balanced cross sections from the central andes between lat 3 s and 40 s suggest that 70 80 is a maximum rather than an average value for this part of the andes tectonic shortening and the crustal cross section area are only loosely correlated variations in tectonic shortening are more abrupt than those of crustal areas particularly near the northern and southern ends of the altiplano puna high plateau where thick crust is associated with relatively small amounts of shortening shortening there may account for no more than about 30 of the present crustal cross section area the processes that created the remaining crustal area are not clear but are likely to involve poorly constrained pre neogene tectonic shortening moderate magmatic additions to the crust tectonic underplating of material derived from the forearc and possible flow of ductile lower crust along strike

post 8 ma reconstruction of papua new guinea and solomon islands microplate tectonics in a convergent plate boundary setting papua new guinea and the solomon islands are located in a complex tectonic setting between the converging ontong java plateau on the pacific plate and the australian continent here we present a late neogene to quaternary plate tectonic reconstruction for this region the reconstruction was performed using gplates software and is based on data derived from multiple geological datasets including seafloor magnetic isochrons 3d models of subducted slabs paleomagnetic data deformation patterns and the spatio temporal distribution of arc magmatism the reconstruction shows that since ca 6 ma the crustal elements that comprise papua new guinea and the solomon islands began interacting in advance of the impending collision between the ontong java plateau and australian continent leading to the inception of regional microplate tectonics and escalation in tectonic complexity the bismarck sea initially formed as a back arc basin behind the new britain arc but was later modified during arc continent collision following collision the west bismarck sea developed into a transpressional zone associated with subduction of the caroline plate and north bismarck microplate at the new guinea trench farther east seafloor spreading occurred in response to the clockwise rotation of the south bismarck microplate relative to the australian plate the solomon sea plate was subjected to anticlockwise rotation relative to papua new guinea primarily due to the west dipping subduction at the new britain trench which resulted in initiation of subduction at the trobriand trough and extension in the woodlark basin from ca 5 ma crustal shortening in the upper plate which now forms the solomon islands resulted from the resistance of the relatively hot and buoyant woodlark basin lithosphere to subduction at the san cristobal our reconstruction shows that tectonism at convergent plate boundaries could involve an intricate relationship between microplate rotations seafloor spreading and subduction segmentation over timescales considerably 10 ma by unraveling the tectonic evolution of this complex region we provide insights into the development of microplate tectonics at convergent margins in general and the possible diagnostic geological records that might be preserved in ancient orogenic systems

spherical shell tectonics effects of sphericity and inextensibility on the geometry of the descending lithosphere the shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere the lithosphere before subduction is a spherical shell and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone upon bending a spherical shell often deforms in a very different way from a simple plate we examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell which shows little surface deformation under moderate stress this examination is made region by region for most of the subduction zones in the world by means of an analogue method the lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin the wadati benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the international seismological centre isc bulletins the spherical shell is forced to fit the miniature of the wadati benioff zone fitting is first attempted only by bending if a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region the spherical shell is torn along this discontinuity or gap and the goodness of fit is reexamined the results of the analysis are summarized as follows 1 the shape of the wadati benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation 2 in almost all of the regions of poor fit with bending a good fit can be achieved by tearing the spherical shell along the trace of low seismicity the sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape this means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it further several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

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

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

the potential influence of subduction zone polarity on overriding plate deformation trench migration and slab dip angle abstract a geodynamic model exists the westward lithospheric drift model in which the variety of overriding plate deformation trench migration and slab dip angles is explained by the polarity of subduction zones the model predicts overriding plate extension a fixed trench and a steep slab dip for westward dipping subduction zones e g mariana and predicts overriding plate shortening oceanward trench retreat and a gentle slab dip for east to northeastward dipping subduction zones e g chile this paper investigates these predictions quantitatively with a global subduction zone analysis the results show overriding plate extension for all dip directions azimuth 180 to 180 and overriding plate shortening for dip directions with 90 to 110 the wide scatter in data negate any obvious trend and only local mean values in overriding plate deformation rate indicate that overriding plate extension is somewhat more prevalent for west dipping slabs west dipping subduction zones are never fixed irrespective of the choice of reference frame while east to northeast dipping subduction zones are both retreating and advancing in five out of seven global reference frames in addition westward dipping subduction zones have a range in trench migration velocities that is twice the magnitude of that for east to northeastward dipping slabs finally there is no recognizable correlation between slab dip direction and slab dip angle east to northeast dipping slabs 30 to 120 have shallow 0 125 km slab dip angles in the range 10 60 and deep 125 670 km slab dip angles in the range 40 82 while west dipping slabs 60 to 120 have shallow slab dip angles in the range 19 50 and deep slab dip angles in the range 25 86 local mean deep slab dip angles are nearly identical for east and west dipping slabs while local mean shallow slab dip angles are lower by only 4 7 8 1 for east to northeast dipping slabs it is thus concluded that overall there is no observational basis to support the three predictions made by the westward drift model and for some sub predictions the observational basis is very weak at most alternative models which incorporate and underline the importance of slab buoyancy driven trench migration slab width and overriding plate motion are better candidates to explain the complexity of subduction zones including the variety in trench migration velocities overriding plate deformation and slab dip angles

oblique collision in the northeastern caribbean from gps measurements and geological observations 1 previous caribbean gps studies have shown that the rigid interior of the caribbean plate is moving east northeastward 070 at a rate of 18 20 3 mm yr relative to north america this direction implies maximum oblique convergence between the island of hispaniola on the caribbean plate and the 22 27 km thick crust of the bahama carbonate platform on the adjacent north america plate we present a tectonic interpretation of a 15 site gps network which spans the hispaniola bahama oblique collision zone and includes stable plate interior sites on both the north america and caribbean plates measurements span the time period of 1994 1999 in a north america reference frame gps velocities in puerto rico st croix and the lesser antilles indicate that these areas move as a single block in an east northeast direction 070 at a rate of 19 20 mm yr consistent with the movement of the rigid interior of the caribbean plate gps velocities at six sites in central and eastern hispaniola dominican republic show drastically different behavior with more eastwardly strikes 080 and much slower rates 4 17 mm yr than areas of the stable caribbean plate the boundary between the relatively slower moving hispaniola collisional zone and the relatively faster moving uncollided puerto rico virgin islands area is the mona passage where late neogene rifting occurs in a broad zone elastic modeling favors strain partitioning with oblique slip on the outer low angle submarine thrust faults north hispaniola muertos and strike slip on the inner subvertical subaerial strike slip faults septentrional enriquillo

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

three dimensional dynamic models of subducting plate overriding plate upper mantle interaction 1 we present fully dynamic generic three dimensional laboratory models of progressive subduction with an overriding plate and a weak subduction zone interface overriding plate thickness top is varied systematically in the range 0 2 5 cm scaling to 0 125 km to investigate its effect on subduction kinematics and overriding plate deformation the general pattern of subduction is the same for all models with slab draping on the 670 km discontinuity comparable slab dip angles trench retreat trenchward subducting plate motion and a concave trench curvature the narrow slab models only show overriding plate extension subduction partitioning vsp vsp vt increases with increasing top where trenchward subducting plate motion vsp increases at the expense of trench retreat vt this results from an increase in trench suction force with increasing top which retards trench retreat an increase in top also corresponds to a decrease in overriding plate extension and curvature because a thicker overriding plate provides more resistance to deform overriding plate extension is maximum at a scaled distance of 200 400 km from the trench not at the trench suggesting that basal shear tractions resulting from mantle flow below the overriding plate primarily drive extension rather than deviatoric tensional normal stresses at the subduction zone interface the force that drives overriding plate extension is 5 11 of the slab negative buoyancy force the models show a positive correlation between vt and overriding plate extension rate in agreement with observations the results suggest that slab rollback and associated toroidal mantle flow drive overriding plate extension and backarc basin formation

no net rotation model of current plate velocities incorporating plate motion model nuvel 1 we present nnr nuvel1 a model of plate velocities relative to the unique reference frame defined by requiring no net rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model nuvel 1 demets et al 1990 differences between nnr nuvel1 and no net rotation plate motion model am0 2 minster and jordan 1978 are as large as 15 mm yr in nnr nuvel1 the pacific plate rotates in a right handed sense relative to the no net rotation reference frame at 0 67 m y about 63 s 107 e this rotation nearly parallels but is 32 slower than the pacific hotspot euler vector of hs2 nuvel1 which is a global model of plate hotspot velocities constrained to consistency with nuvel 1 gripp and gordon 1990 at hawaii the pacific plate moves relative to the no net rotation reference frame at 70 mm yr which is 25 mm yr slower than the pacific plate moves relative to the hotspots differences between nnr nuvel1 and hs2 nuvel1 are described completely by a right handed rotation of 0 33 m y about 49 s 65 e the 99 confidence ellipsoids for plate hotspot motion in model hs2 nuvel1 exclude the corresponding euler vectors from model nnr nuvell thus the no net rotation reference frame differs significantly from the hotspot reference frame if the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame then hotspots beneath the pacific plate move with coherent motion towards the east southeast alternatively the difference between reference frames may show that the uniform drag no net torque reference frame which is kinematically equivalent to the no net rotation reference frame is based on a dynamically incorrect premise possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans

far field contractional polarity changes in models and nature abstract a change in contractional polarity occurs when the direction of tectonic transport switches along strike this switch is conditioned by lateral variations in rheology or inherited asymmetries such as contrasts in structure or changes in the polarity of subduction zones the parameters controlling contractional polarity changes are less understood in situations when the strain is transferred at large distances from indenters analysing this type of strain transfer is critical for understanding the mechanics of thrusting in fore or back arc settings of orogenic areas comparison of crustal scale analogue modelling with the inversion of the black sea back arc and the formation of the new guinea new britain fore arc suggest that far field changes in contractional polarity are related to rheological contrasts across inherited normal faults the initial extension creates rheological weak zones that localize the subsequent far field contractional deformation along groups of thrusts with opposite vergence along the strike of the system the largest amount of far field contractional deformation is recorded in the transfer zone located between the two indenters moving in opposite directions and is particularly high when inverting oblique extensional systems

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

cenozoic climate change as a possible cause for the rise of the andes causal links between the rise of a large mountain range and climate have often been considered to work in one direction with significant uplift provoking climate change here we propose a mechanism by which cenozoic climate change could have caused the rise of the andes based on considerations of the force balance in the south american lithosphere we suggest that the height of and tectonics in the andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands and shear stresses in the subduction zone depend on the amount of subducted sediments we propose that the dynamics of subduction and mountain building in this region are controlled by the processes of erosion and sediment deposition and ultimately climate in central south america climate controlled sediment starvation would then cause high shear stress focusing the plate boundary stresses that support the high andes

kinematics of subduction and subduction induced flow in the upper mantle 1 results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle the experiments model a dense high viscosity plate subducting lithosphere overlying a less dense low viscosity layer upper mantle the overriding lithosphere is not incorporated several important features of slab behavior were investigated including the temporal variability of hinge line migration the kinematic behavior of the slab and the subduction induced upper mantle flow both fixed and free trailing edge boundary conditions of the subducting plate were investigated results show that hinge line retreat is a natural consequence of subduction of a negatively buoyant slab the migration rate increases until the slab approaches the upper lower mantle discontinuity resulting in a decrease in migration rate followed by a renewed increase and finally approaching a steady state slab retreat results in mantle flow with material initially located underneath the slab flowing around the lateral slab edges toward the mantle wedge experimental results indicate that all rollback induced flow occurs around the lateral slab edges forcing the hinge line to attain a convex shape toward the direction of retreat no signs for poloidal flow underneath the slab tip have been detected only a small component of toroidal type flow was observed underneath slanting slab tips for a fixed trailing edge the slab does not sink vertically downward but sinks at an angle in a regressive manner for a free trailing edge slab sinking is oriented more vertically while the surface part of the subducting plate is pulled into the subduction zone

fracture zones in the north atlantic from combined geosat and seasat data we present a new tectonic fabric map of the northern and central north atlantic ocean floor from 10 s to 66 n based on combined geosat and seasat altimetry data short wavelength vertical gravity along track was derived by hilbert transforming and high pass filtering geosat and seasat deflection of the vertical profiles the along track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and v shaped structures we traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible we also mapped a number of unnamed fracture zones and fracture zone segments many of which were previously uncharted we have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the north central north and south atlantic most fracture zones in this area correlate quite well with synthetic flow lines supporting the proposition that geosat seasat altimetry data serve as a useful tool to map fracture zones where insufficient bathymetric data are available many fracture zones including some small offset fracture zones in the central north atlantic have been stable and follow tectonic flow lines for tens of millions of years however many other fracture zones do not reflect a railroad track pattern of plate motions the geometry of many small and medium offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge causing off axis traces that deviate from tectonic flow lines velocity space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low angle v shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame other low angle v shaped structures reflect slow migrations oscillations of small offset ridge axis discontinuities that are distinct from both tectonic flow lines and v shapes reflecting absolute plate motions high angle v shaped structures are the result of fast migrating ridge axis discontinuities propagating rifts

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

feedback between mountain belt growth and plate convergence while it is generally assumed that global plate motions are driven by the pattern of convection in the earth s mantle the details of that link remain obscure bouyancy forces associated with subduction of cool dense lithosphere at zones of plate convergence are thought to provide significant driving force but the relative magnitudes of other driving and resisting forces are less clear as are the main factors controlling long term changes in plate motion the ability to consider past as well as present plate motions provides significant additional constraints because changes in plate motion are necessarily driven by changes in one or more driving or resisting forces which may be inferred from independent data here we present for the first time a model that explicitly links global mantle convection and lithosphere models to infer plate motion changes as far back as miocene time by accurately predicting observed convergence rates over the past 10 m y we demonstrate that surface topography generated at convergent margins is a key factor controlling the long term evolution of plate motion specifically the topographic load of large mountain belts and plateaus consumes a significant amount of the driving force available for plate tectonics by increasing frictional forces between downgoing and overriding plates

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

two phase contractional deformation of the jurassic daebo orogeny chungnam basin korea and its correlation with the early yanshanian movement of china 1 the daebo orogeny is a major orogenic event in korea that affected the early to middle jurassic daedong supergroup our investigation of the nampo group part of the supergroup in the chungnam basin reveals the occurrence of two phases of shortening the majority of the nampo group and a basement slab were folded and displaced toward the wnw along the ocheon thrust during the first phase middle jurassic early late jurassic field relationships suggest a ramp flat geometry for the ocheon thrust mechanical stratigraphy within the slice is consistent with a detachment fold model geometric modeling suggests 5 5 km and 3 3 km as the minimum amount of shortening accommodated by the ocheon thrust and its slice for the ramp slopes of 15 and 30 respectively the second phase late late jurassic was a n s shortening which produced roughly e w trending reverse faults and folds to form a dome and basin structure these shortening events occurred over the entire chungnam basin and coincide in orientation and timing with those reported from the ogcheon belt and the gyeonggi massif the first and second phases of the daebo orogeny are respectively correlated with the shortening in eastern china and the sinistral transpression along the tan lu fault both belonging to the early yanshanian movement subsequently in the early cretaceous a regional extension commenced over korea and eastern china these remarkable correlations suggest that two stages of crustal thickening occurred throughout the eastern asia during the middle to late jurassic followed by regional thinning in the early cretaceous

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

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

oligocene to present shallow subduction beneath the southern puna plateau abstract the southern puna plateau is a conspicuous example of a high elevation orogenic plateau in a non collisional setting this orogenic sector is currently located above an anomalously shallow subduction segment in which timing and relation to upper plate tectonics have been widely overlooked this subduction segment here referred to as the southern puna shallow subduction spss is characterized by a 200 km wide shallow area located at 300 km from the trench at a depth of 100 120 km and dipping 10 12 to the east to determine the onset of the spss and its link to the tectonic and magmatic activity in this region we analyzed the tectonomagmatic record of the southern puna plateau from preexisting datasets also we present a new approach based on global subduction data that provides a straightforward methodology to extract potential paleo slab angles from the bedrock arc record this analysis reveals that a pronounced eastward arc front migration and magmatic broadening took place at 26 ma and was preceded by 4 ma of reduced magmatic activity which we link to the inception of the spss as expected in shallow subduction settings a change to basement cored distributed deformation south of 25 s in the southern puna plateau coincides with the beginning of shallow subduction also the spss is coincident with the enigmatic post eocene intraplate deformation of the otumpa hills located at 950 km from the trench we suggest that this succession of events is not fortuitous and that the development of the spss impacted directly the overriding plate since the oligocene contributing to the building of one of the largest topographies 3 km and thickest orogenic crusts 70 60 km on earth the shallow subduction would have acted jointly with cenozoic changes in plate kinematics and climate enhancing andean orogenesis at studied latitudes

the dynamic history of 220 million years of subduction below mexico a correlation between slab geometry and overriding plate deformation based on geology paleomagnetism and seismic tomography global tectonic reconstructions of pre cenozoic plate motions rely primarily on paleomagnetic and geological data from the continents and uncertainties increase significantly with deepening geological time in attempting to improve such deep time plate kinematic reconstructions restoring lost oceanic plates through the use of geological and seismic tomographical evidence for past subduction is key the north american cordillera holds a record of subduction of oceanic plates that composed the northeastern panthalassa ocean the large oceanic realm surrounding pangea in mesozoic times here we present new paleomagnetic data from subduction related rock assemblages of the vizcaino peninsula of baja california mexico which yield a paleolatitudinal plate motion history equal to that of the north american continent since late triassic time this indicates that the basement rocks of the vizcaino peninsula formed in the forearc of the north american plate adjacent to long lived eastward dipping subduction at the southern part of the western north american continental margin tomographic images confirm long lived uninterrupted eastward subduction we correlate episodes of overriding plate shortening and extension to flat and steep segments of the imaged slab by integrating paleomagnetic geological and tomographic evidence we provide a first order model that reconciles absolute north american plate motion and the deformation history of mexico since late triassic time with modern slab structure