stratigraphy and strontium geochemistry of messinian evaporite bearing successions of the southern apennines foredeep italy implications for the mediterranean salinity crisis and regional palaeogeography an integrated study comprising detailed mapping stratigraphy evaporite facies analysis strontium content and 87sr 86sr ratio data is presented for two messinian evaporite bearing units cropping out in irpinia daunia mountains southern apennines italy these units record the tectonic and palaeogeographic evolution of the messinian southern apennine foreland basin system during the abrupt salinity crisis which occurred in the mediterranean region the daunia unit comprises the monte castello evaporites formed by euxinic diatomitic marls evaporitic limestones and shallow water gypsum having a 87sr 86sr average value of 0 70898 0 00005 close to the sr isotopic value of the coeval seawater these marginal evaporites are capped by an erosional unconformity and continental clastic deposits the vallone del toro unit is made up of thin bedded marls claystones calcilutites and diatomite layers with small gypsum crystals laminated gypsum and gypsarenite testifying deposition in relatively deep water settings gypsum has variable sr isotopic data with an average value of 0 70899 0 00005 for the argilliti policrome del torrente calaggio formation and an average value of 0 70863 0 00014 for the argilliti di mezzana di forte formation reflecting major riverine freshwater input the different patterns of lithofacies and sr isotopic composition of gypsum discriminate two distinctive evaporite units reflecting the tectonic and palaeogeographic controls within the apennines mountain belt and the mediterranean data for evaporites of the southern apennines testify the presence of basins in the central and eastern mediterranean sea which never desiccated during the entire salinity crisis and evolved to widespread lago mare conditions in the latest messinian

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

latent heat effects of the major mantle phase transitions on low angle subduction abstract very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate we have investigated the effects of latent heat for this situation where mantle material is pushed through the major mantle phase transitions during shallow low angle subduction below the overriding plate the significance of the buoyancy forces arising from the latent heat effects on the dynamics of the shallowly subducting slab is examined by numerical modeling when a 32 ma old slab is overridden with 2 5 cm yr by a continent flat subduction occurs with a 4 5 cm yr convergence rate when latent heat is included in the model forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces under these circumstances the flat slab segment subducts horizontally about 350 km further and for about 11 ma longer than in the case without latent heat before it breaks through the 400 km phase transition the style of subduction strongly depends on the mantle rheology increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow similarly an overriding velocity of less than 1 cm yr leads to steep subduction which gradually changes to flat subduction when increasing the overriding velocity however these model parameters do not change the aforementioned effect of the latent heat provided that low angle subduction occurs in all models latent heat resulted in a substantial increase of the flat slab length by 300 400 km varying the olivine spinel transition clapeyron slope from 1 to 6 mpa k reveals a roughly linear relation between and the horizontal length of the slab based on these results we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low angle subduction below an overriding plate

three dimensional instantaneous mantle flow induced by subduction 1 we conduct three dimensional subduction experiments by a finite element approach to study flow around slabs which are prescribed based on a transient stage of upper mantle subduction from a laboratory model instantaneous velocity field solutions are examined focusing on the toroidal vs poloidal components as a function of boundary conditions plate width and viscosity contrast between slab and mantle we show how the slab to mantle viscosity ratio determines the strength of toroidal flow and find that the toroidal flow component peaks for slab mantle viscosity ratios 100 independent of slab width or geometry

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

the effect of rheological approximations in 3 d numerical simulations of subduction and collision abstract subduction and collision zones evolve differently from one another due to different rheological properties different amounts of regional isostatic compensation and the different mechanisms by which forces are applied to the convergent plates the rheology of mantle and lithosphere is known to have the largest influence on the dynamics of subduction and continental collision however previous 3 d geodynamic models of subduction collision processes have used various rheological approximations making their results difficult to compare since there is no clear understanding on the extent of these approximations on the dynamics here we test the effect of rheological approximations in a geometrically simplified 3 d model in which the dynamics of subduction and collision is entirely driven by slab pull i e free subduction the results exhibit a wide range of behaviors depending on the rheological law employed from linear viscous lv to temperature dependent visco elasto plastic vep rheology that takes into account both diffusion and dislocation creep rheological approximations exert a strong effect on slab morphology trench migration rates and topography in the upper plate for example we find that slab dynamics varies drastically between end member models in lv approximations slabs do not break off but instead buckle at the transition zone the dynamics is dominated by low strain rates and diffuse stresses and topography is controlled by lateral extrusion of material which homogenizes amplitudes in the upper plate in simulations with non linear vep rheology slab break off occurs at the corners of the indenting continent opening of a back arc basin is fast and topography has a laterally heterogenous distribution of amplitudes opening of a back arc basin is observed only in simulations with non linear vep rheology when the rheology is approximated with multiple discrete layers the dynamics also resembles more the non linear vep case

the northern apennines palynological record as a contribute for the reconstruction of the messinian palaeoenvironments abstract the messinian stage has long been associated with an overall warm and dry climate whereas recent researches indicate either a warm and humid or a cool and dry climate the integrated stratigraphic record of vegetation and climatic changes from northern apennines sites provides the solution to this apparent contradiction its integration with the updated geological and sedimentological studies provides additional data for the reconstruction of the depositional palaeoenvironments in both marginal and deeper sub basins of the apennines foredeep the onset of the mediterranean salinity crisis msc is recorded in the gessoso solfifera of the vena del gesso marginal sub basin cyclical humid conditions corresponding to precession minima developed during the deposition of the shales interbedded with the gypsum 5 9 to 5 6 ma some cooler events took also place under the effects of global glacial stadials and regional factors apennines uplift at present no major changes from moist to dry conditions are attested to just before the salinity crisis as well as in sicily so climate did not play a major role in the onset of the msc despite the favourable context provided by inferred thermo xeric conditions in southern italy a drier episode indicated by the expansion of the open vegetation including the northward migration of lygeum postdates the onset of the salinity crisis of about 400 kyr in the lower post evaporitic deposits of maccarone deeper sub basin it falls within a period of global warming whereas at a regional scale it could correlate p p to the evaporite deposition in deeper basins and to hiatuses in the marginal basins of sicily and of the western sector of northern apennines its sudden end about 100 kyr later in coincidence with a significant increase of pinaceae indicates a turnover in the terrestrial setting not linked to major climate changes but possibly to a complex interaction between other palaeoenvironmental factors e g tectonics and eustatism in contrast organic walled dinoflagellate cysts exclude significant modifications in aquatic settings insaturation of either open marine or brackish conditions in the latter a later change is marked by the arrival of impagidinium sp 1 a species here referred instead to caspidinium rugosum about 7 m below the first colombaccio this occurrence together with the spread of pediastrum indicates a freshwater dilution i e the lago mare event during wetter climatic conditions on the adjacent landmass increase of tsuga and cedrus the successive arrival and or dominance of other paratethyan taxa such as i sp 2 i sp 3 and galeacysta etrusca indicate highly variable water environments marine vs continental water inputs during the deposition of the uppermost post evaporitic deposits the lago mare is stratigraphically sandwiched between an ash layer 130 m below dated at 5 5 ma and the beginning of the pliocene where a peak of impagidinium patulum marks the onset of open marine conditions the dominant humid subtropical to warm temperate climate indicates differences in both temperature and moisture values with respect to the coeval southern sections revealing climatic gradients within the mediterranean at least from the messinian no dramatic vegetation and climate changes have been recorded during the msc major changes occurred later as indicated by the palynological record from 2 6 ma this palynostratigraphic record is a good reference for more recent models of the development of the msc and for establishing time relationships between the apennine and sicilian successions

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

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

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

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

the pliocene pleistocene stratigraphic and tectonic evolution of the central sector of the western periadriatic basin of italy abstract the central portion of the western periadriatic basin is a foreland basins systems associated to the central apennine outer orogenic wedge that stretches along the marche and abruzzi regions of italy an area that has been studied since long time and where a plenty of surface and subsurface data are available the present study is aimed to review and synthetize these data add new surface and subsurface data in order to define a firm stratigraphic and tectonic framework that is base for the reconstruction of the pliocene pleistocene tectonic stratigraphic evolution of the central portion of the western periadriatic basin the study integrates field mapping published geological maps seismic lines well logs and an updated chronostratigraphic scheme six major nne ssw thrust stacks and eight unconformity bounded stratigraphic units have been defined then a ne sw oriented zone separates a northern from a southern sector which have differences in structural styles amount of shortening basins widths and thicknesses of stratigraphic units despite these differences the deformation timing the depositional history and the migration of basins system are common in both sectors and eight tectonic stratigraphic evolutionary stages have been reconstructed with sequential restoration of two representative cross sections the results highlight that the pliocene pleistocene stratigraphic and tectonic evolution of the central western periadriatic basin preserves the records of supra regional and global controls which drive thrust propagation orogen growth basins migration and basins sedimentary infill in particular the stratigraphy preserves global third order eustatic climatic changes that have a strict feed back and concomitance with tectonic pulses the latter are regulated 1 by cyclic growth of an orogenic wedge undergoing coeval erosion and sedimentation and 2 by eastward retreat of a westward subducting and flexing continental lithosphere

subduction mode selection during slab and mantle transition zone interaction numerical modeling global seismic tomography of the subduction zones shows that the subducting slabs could either stagnate around the 660 km discontinuity or penetrate into the lower mantle the stagnating slabs also have various morphologies these are directly related to the interaction between the subducting slabs and the mantle transition zone mtz the dynamics of which are still debated using a 2 d thermo mechanical model we systematically investigated the modes of subduction in the mantle transition zone and explored the key constraints of various subduction styles four basic subduction modes are obtained in the numerical experiments including one with slab penetrating through the 660 km discontinuity and three other modes with slab stagnating in the mtz i e folding lying and rolling back the numerical models indicate that the age of subducting oceanic plate the thickness of overriding continental lithosphere and the convergence velocity play crucial roles in the dynamics of subducting slab and mtz interaction in general the young subducting slab favors the penetration or folding mode whereas the old subducting slab tends to result in lying or rolling back mode although other parameters can also affect our models also show a strong correlation between the subduction mode selection and dip angle of the slab tip when reaching the 660 km phase boundary

easternmost mediterranean evidence of the zanclean flooding event and subsequent surface uplift adana basin southern turkey according to the literature the adana basin at the easternmost part of the mediterranean basin in southern turkey records the pliocene stage with shallow marine to fluvial deposits our micropalaeontological analysis of samples from the adana basin reveal late lago mare biofacies with paratethyan ostracod assemblages pertaining to the loxocorniculina djafarovi zone grey clays rich in planktonic foraminifera lie above the lago mare deposits within the grey clays the continuous occurrence of the calcareous nannofossil reticulofenestra zancleana and the base of the reticulofenestra pseudoumbilicus paracme points to an early zanclean age 5 332 5 199 ma both ostracod and benthic foraminifera indicate epibathyal and bathyal environments 87 sr 86 sr measurements on planktonic and benthic foraminifera fall below the mean global ocean value for the early zanclean indicating potentially insufficient mixing of low 87 sr 86 sr mediterranean brackish lago mare water with the global ocean in the earliest pliocene we utilize the ages and palaeodepths of the marine sediments together with their modern elevations to determine uplift rates of the adana basin of 0 06 to 0 13 mm a 1 since 5 2 5 3 ma total uplift of 350 650 m from surface data and 0 02 0 13 mm a 1 since c 1 8 ma total uplift of 30 230 m from subsurface data

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

folding of viscous plumes impinging on a density or viscosity interface summary the possibility that slabs of subducted oceanic lithosphere may buckle and fold under longitudinal compressive stresses when they encounter the major seismic discontinuity near a depth of 670 km in the mantle has led us to explore experimentally the behaviour of very viscous plumes falling onto a density interface or a viscosity step we begin with the case of axisymmetric and planar streams falling through air onto the free surface of a viscous fluid which has the same or smaller density and observe the conditions under which they coil or fold in order to remove surface tension and to introduce effects of a viscous environment we study plumes falling onto a density interface between two generally very viscous liquid layers stability to buckling is found to depend largely on a geometrical criterion based on the length to thickness ratio of plumes but also the tendency to buckle is greater for large density differences across the interface and larger viscosity contrasts between plume and upper layer when plumes are unstable coils or folds are laid down with a horizontal scale in a fixed proportion to the thickness of the plume and the frequency of folding is determined by the velocity and thickness of the slab as it enters the region of compressive stress entrainment of upper layer fluid can have a marked effect on the fate of the folded plume material which may spread at the interface or continue to descend into the lower layer similar behaviour is observed at a viscosity interface application of results to the mantle depends on the nature of the 670 km discontinuity and is therefore inconclusive however cool slabs are likely to be unstable if mantle circulation involves two layers separated by a density interface or if whole mantle circulation involves a large viscosity increase with depth near 700 km

magnetic and structural constraints for the noncylindrical evolution of a continental forebulge hyblea italy 1 the hyblean plateau in southeastern sicily italy consists of an isolated and elevated forebulge structure formed since the early miocene time by bending the foreland lithosphere below the advancing maghrebian thrust fold belt the hyblean forebulge is presently located in front of an orogenic recess and partly surrounded by two orogenic salients we analyzed magnetic anisotropy of magnetic susceptibility and structural joints faults slickenside lineations and bed attitudes fabric data from neogene carbonate rocks exposed atop the hyblean forebulge results showed unidirectional fabrics developed in response to a nw se extension at the end the of early miocene time and beginning of middle miocene time and duple to multiple fabrics developed in response to major nw se and ne sw extensions from the langhian time onward we interpreted these results as the evidence for the growth of a doubly plunging forebulge due to the presence of foreland crustal heterogeneities which enhanced differential retreating processes of the foreland along the subduction zone and the subsequent formation of orogenic salients and recesses a semianalytical model shows that the observed brittle deformations potentially involved a significant thickness of the carbonate sedimentary cover thereby reducing the lithosphere flexural rigidity and enhancing its bending aptitude

on the origin of the strait of gibraltar most interpretations of the early pliocene opening of the strait of gibraltar involve a tectonic process however no tectonic structure of this age has been unequivocally documented that could account for such a hypothesis on the other hand the sea level drop of the mediterranean during the messinian salinity crisis has dramatically enhanced continental erosion and in particular regressive fluvial erosion we show that such erosional process inevitably developed in the gibraltar area we finally propose that regressive fluvial erosion was at the origin of the opening of the strait of gibraltar

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

dynamics of free subduction from 3 d boundary element modeling 1 in order better to understand the physical mechanisms underlying free subduction we perform three dimensional boundary element numerical simulations of a dense fluid sheet with thickness h and viscosity 2 sinking in an ambient mantle with viscosity 1 the mantle layer is bounded above by a traction free surface and is either 1 infinitely deep or 2 underlain by a rigid boundary at a finite depth h d similar to the typical geometry used in laboratory experiments instantaneous solutions in configuration 1 show that the sheet s dimensionless stiffness s determines whether the slab s sinking speed is controlled by the viscosity of the ambient mantle s 10 time dependent solutions with tracers in configuration 2 demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides the extra edge drag exerted by the flow around the sides causes transverse deformation of the slab and makes the sinking speed of a 3 d slab up to 40 less than that of a 2 d slab a systematic investigation of the slab s interaction with the bottom boundary as a function of 2 1 and h h delineates a rich regime diagram of different subduction modes trench retreating slab folding trench advancing and reveals a new advancing folding mode in which slab folding is preceded by advancing trench motion the solutions demonstrate that mode selection is controlled by the dip of the leading edge of the slab at the time when it first encounters the bottom boundary

messinian paleogeography and erosional surfaces in italy an overview abstract the messinian deposits of italy display an extreme variability both lateral and vertical including strong changes in salinity and in sediment accumulation rates this variability strongly contrasts with the much more uniform nature of both the older i e tortonian and younger i e zanclean strata changes in salinity range from fresh water to hypersaline conditions leading to evaporite precipitation salinity crisis the chloride facies it recorded in calabria facing the deep ionian sea in the central sicilian basin and in a single satellite basin of southern tuscany fed by the ligurian sea erosional surfaces are widespread at the southern edge of the alps and extend to the subsurface of the po basin rapidly subsiding and outward moving foredeeps deep seated brine pools uplifted in post messinian times post tectonic extensional basins provide a different stratigraphic record of the same paleogeographic event a dramatic drop of sea level which lasted overall less than one million years

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

effect of plastic viscous layering and strain softening on mode selection during lithospheric extension 1 factors controlling the selection of deformation modes during continental extension are investigated using analytical and numerical methods we view the lithosphere as a laminate and examine a simple system with a uniform plastic layer overlying a uniform linear viscous layer the rate of energy dissipation is analyzed for pure shear ps symmetric plug sp and asymmetric plug ap extension modes and the analysis reveals that the primary control is the relative rate of dissipation in the two layers a basic difference is that the plastic layer yield strength is independent of the strain rate whereas the viscous stress depends on strain rate therefore dissipation scales linearly and quadratically with extension velocity for these respective layers when other parameters e g extension velocity and properties are held constant minimum dissipation predicts that the modes ap sp and ps will be selected in this order with increasing viscosity of the lower layer transition viscosities between modes t1 and t2 are 4 1021 pa s and 8 1022 pa s respectively for our parameters values numerical models confirm the analysis results inferred mode controls and order of mode selection when strain softening of the plastic layer occurs during extension implications for lithosphere that acts as a bonded plastic viscous laminate include the following 1 asymmetric extension ap mode is preferred when the extension rate and or effective viscosity is low or the viscous region temperature is high 2 symmetric sp mode extension is preferred for intermediate combinations of parameters and 3 overall pure shear ps mode may occur for opposite end member parameter combinations

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

climatically versus tectonically forced erosion in the alps thermochronometric constraints from the adamello complex southern alps italy abstract we apply u th sm he dating and fission track thermochronometry of apatites to the adamello complex located in the southern european alps our goal is to constrain exhumation rates since the miocene in particular to determine whether the increase in sedimentation documented in the foreland basins since 5 ma is recorded in cooling rates of the southern alps thermochronometry provides a cooling rate for rocks which can be converted to an exhumation rate through thermal modeling thereby quantifying the amount of rock exhumed at different time scales apatite fission track and u th sm he analyses all record miocene ages we find no evidence for an increase in exhumation rate during the plio pleistocene the results of our exhumation rate estimates and tectono geomorphic modeling suggest that unroofing during thrust propagation of the southern alps in the late miocene drove most of the exhumation while changes in relief mainly associated with plio pleistocene climate change likely did not affect the adamello complex the findings from this study combined with other studies highlight that there appears to be a gradient across the alps with more intensive plio pleistocene erosion in the western and central alps compared to the eastern and southern alps

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

dynamic models of downgoing plate buoyancy driven subduction subduction motions and energy dissipation abstract it is much debated whether the forces associated with the downgoing plate the overriding plate passive or active mantle flow are dominant in controlling the paths of plates into the mantle we investigate the dynamics and energetics for a free subduction system driven solely by downgoing plate buoyancy using a finite element model of a viscoelastic plate with a free surface sinking into a passive unbounded mantle represented by drag forces parameters are varied to study effects of an asthenosphere ridge push and a passive overriding plate for a range of subducting plate viscosities and densities such a single free plate achieves subduction mainly through trench retreat most of the energy dissipation occurs in driving the passive mantle response as a result the slab s sinking velocity is its stokes velocity determined by lithospheric buoyancy and mantle viscosity the total subduction velocity and dip adjust to minimize bending dissipation in the lithosphere and are affected by slab rheology as well as buoyancy a low viscosity asthenosphere and ridge push facilitate plate advance increasing plate dips and lowering subduction velocity while suction and buoyancy of a work free passive overriding plate decreases plate dips thus increasing subduction and rollback velocities however the geometrical relation between the different parameters is the same in all model cases because the slabs sink according to their stokes velocity the free subduction models thus provide a reference that can be used to distinguish the signature of downgoing plate buoyancy from that of other driving forces in global compilations of subduction parameters

dynamics of outer rise faulting in oceanic continental subduction systems 1 during subduction bending of downgoing oceanic lithosphere gives rise to normal faulting due to the extensional stress state generated in the upper plate as deformation patterns inherently reflect a material s state of stress and rheology extensive global observations of outer rise faulting patterns and subduction dynamics provide a unique opportunity to examine the factors controlling outer rise deformation despite a wide range of observed oceanic plate ages convergence rates and slab pull magnitudes across modern subduction systems however measured outer rise faulting patterns show effectively no correlation to variations in these parameters this lack of correlation may reflect that outer rise faulting patterns are strongly sensitive to all of these parameters are dependent on additional parameters such as downgoing overriding plate coupling or that existing faulting measurements require additional analysis in order to provide a basis for future analysis of outer rise faulting patterns we build on previous thermo mechanical numerical models of outer rise deformation and explore the relationship between outer rise faulting patterns subduction dynamics and brittle rheology in an oceanic continental subduction system analysis of time averaged outer rise faulting patterns indicates that downgoing plate age and velocity downgoing overriding plate coupling and slab pull all significantly affect faulting patterns while variations in brittle rheology have a significantly smaller impact these relationships reflect that the sensitivity of outer rise faulting patterns to the frictional properties of the oceanic crust and mantle is small compared to variations in the overall stress state and deformation rate of subduction systems in order to gain additional insight into the origin outer rise faulting patterns future numerical studies should focus on specific regions in order to place constraints on the structure of the downgoing plate and dynamics of the subduction system

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

predicting trench and plate motion from the dynamics of a strong slab the motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle these models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation here using a recent compilation of a global data set we found that the motion of trenches either advancing or retreating with respect to upper plates scales with their corresponding subducting plates motion based on simple experimental tests we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships we deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle

the early geodynamic evolution of mars type planets abstract it is not clear whether mars once possessed active tectonics yet the question is critical for understanding the thermal evolution of mars and the origin and longevity of its early dynamo to address these issues we have coupled mantle flow simulations together with parameterized core evolution models to simulate the early evolution of mars like planets and constrain the influence of early mobile lid tectonics on core evolution we have explored a wide parameter suite encapsulating a range of uncertainties in initial conditions rheological parameters and surface strength we present successful models that experience early mobile lid behaviour with a later transition into a stagnant lid mode which reproduce core dynamo histories similar to the magnetic history of early mars

the bending mechanics in a dynamic subduction system constraints from numerical modelling and global compilation analysis abstract lithospheric plates bend at subduction zones where the vertical motions of the slabs are converted to surface plate motions to understand the mechanics of plate bending we derive scaling laws for the deflection at the margin i e radius and dip from numerical models of a subducting viscoelastic plate in such dynamic system we find that the buoyancy and the stiffness of the plates control the radius and the dip as well as the plate motions toward the trench this mechanical model successfully predicts the curvature of published three dimensional laboratory and numerical models for a thorough comparison with the observable we have also implemented forces additional to the slab pull such as the suction force and far field stresses by increasing or resisting the torque applied at the trench by the slab these forces can largely rearrange the dip and the radius of slabs and the inherent plate motions although they do not alter the observed anticorrelation between radius and dip similar inverse correlation relationship and dip radius ranges are shown by most of the subduction zones analysed from a global compilation radii in the range of 100 350 km and dips of 30 70 for slabs that extends to the bottom of the upper mantle are compatible with the models and allow estimating an average lithospheric viscosity contrast of 200 in the bending with respect to the ambient mantle radii and dips outside of this range are in good agreement with the trends and the magnitudes of models that include suction and far field forces in all these subduction zones the correlation between dip radius and plate velocity is found to be compatible with that of the models showing how relevant bending is for the dynamics of earth

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

mantle rheology and the scaling of bending dissipation in plate tectonics 1 plate tectonics on earth involves the bending deformation of plates at subduction zones and because plates are generally considered to be stiff owning to the rheology of mantle minerals the role of energy dissipation by plate bending in the global energy balance has been frequently debated in the recent literature here we consider how bending dissipation should scale with slab parameters such as dip angle plate age the radius of curvature and plate velocity by systematically exploring the parameter space with instantaneous stokes flow calculations we derive the scaling of bending dissipation for a range of mantle viscosity functions including pseudoplastic rheology with olivine flow laws our results indicate that as we move away from the isoviscous case the scaling gradually deviates from what has commonly been assumed in previous studies most notably for the radius exponent which exhibits more than threefold reduction and even a sign reversal in some cases these modifications in scaling exponents originate in the complication of the deformation field caused by viscosity variations within the bending plate approximating the lithospheric rheology by a single effective viscosity in the dynamical models of subduction has been a common practice but we suggest that such approximation may limit the geological relevance of modeling studies in particular when estimating the significance of bending dissipation

modelling the plate driving mechanism this paper demonstrates how sub lithospheric loading gives rise to plate interior stresses and plate boundary forces the stressing of the lithosphere resulting from plume heads in the upper mantle hot spots from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis it is first demonstrated how a sub lithospheric load exemplified by a sub continental plume head produces local deviatoric stress in the strong upper lithosphere within a plate interior region it is shown that a 500 km wide region of anomalous density averaging 8 kg m 3 between 100 km and 400 km depths gives rise to a deviatoric tension of 75 mpa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load the large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load when such a stress system is cut across by a zone or plane of weakness then plate boundary forces and distant plate interior stresses are produced a weak zone beneath the crest is incorporated in a model of a normal ocean ridge and this yields a ridge push force of 2 5 10 12 nm 1 referenced to old ocean floor and deviatoric compression of about 40 mpa in old ocean floor a model of a ridge underlain by an anomalously low density upper mantle plume head yields a much larger ridge push force and large deviatoric compressions of nearly 100 mpa extend into the bordering continents subduction pull slab pull and trench suction of about 4 5 10 12 nm 1 is modelled for a 300 km deep slab separated from the overriding plate by a weak fault and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges

the record of the messinian salinity crisis in mobile belts insights from the molise allochthonous units southern apennines italy abstract mediterranean marginal basins recorded the messinian salinity crisis msc in various structural settings including syn rift thrust top foredeep and foreland basins during the msc the apennines were one of the mobile belts of the peri mediterranean chain starting from more hinterland areas allochthonous units of both the northern and southern apennines migrated toward the adriatic foreland developing allochthonous top sedimentary basins one of these basins on top of the molise allochthonous units southern apennines recorded almost all of the main steps of the msc in a gypsum quarry district the occurrence of conduits yielding flow mobilized sediments which cross cut the lower evaporites testifies to a fluid migration event responsible for the formation of a brecciated mudgrade limestone buildup this event can be connected with the mediterranean drawdown responsible for the messinian erosional surface mes the post evaporitic marly succession that unconformably overlies the lower evaporites and the brecciated limestones is characterized by the presence of paratethyan molluscs and ostracods loxoconcha muelleri and loxocorniculina djafarovi zones and small mammals among which is recorded the occurrence of stephanomys debruijni a disconformity within the post evaporitic succession divides it into lower p ev1 and upper p ev2 lago mare deposits a further erosional surface separates the messinian lago mare sediments from fully marine lower zanclean mpl2 zone deposits whilst the unconformity separating the lower evaporites and the lago mare deposits although enhanced by a tectonic event could be related to a late messinian base level drop mes1 as well as the disconformity between p ev1 and p ev2 mes2 the younger angular unconformities affecting both the lago mare deposits p ev2 and the fully marine pliocene sediments might be related to two different phases of orogenic transport those events affected the molise allochthonous units during their late miocene early pliocene forelandward migration

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

palaeoenvironments of the mediterranean basin at the messinian hypersaline hyposaline transition evidence from natural radioactivity and microfacies of post evaporitic successions of the adriatic sub basin at the end of the messinian salinity crisis changes in the palaeoceanography and palaeoclimate induced a transition from hypersaline to hyposaline conditions in the mediterranean water body detailed investigation of natural radioactivity and microfacies analyses of two early post evaporitic messinian sections show that the transition occurred in poorly oxygenated and well stratified water masses these transitional deposits which mainly consist of marls and caco3 rich horizons are generally well laminated and totally barren in benthic and planktonic fauna the presence of barite small 5 8 lm authigenic framboidal pyrite and high values of u indicate persistent anoxic conditions during the early postevaporitic messinian stage in the adriatic sub basin of the mediterranean sea the presence of post evaporitic welllaminated barren deposits from both mediterranean borderland sections and odp sites suggests that these anoxic conditions extended to other sub basins of the mediterranean region

the mediterranean messinian salinity crisis an apennine foredeep perspective abstract owing to its expanded stratigraphic sections the apennine thrust belt offers the opportunity to better understand the evaporitic and post evaporitic messinian events a physical stratigraphic framework of messinian deposits based on facies analysis and basin wide correlation of key surfaces and sedimentary cycles is presented it is shown that the messinian apennine foredeep had marginal basins with shallow water primary evaporites and deeper basins where resedimented evaporites accumulated under relatively deep water conditions like many other mediterranean examples primary shallow water evaporites of apenninic marginal basins show evidence for subaerial exposure and erosion however the development of such an erosional surface does not correspond to the deposition of primary evaporites in the deepest part of the basin s here the unconformity can be traced towards the base of resedimented evaporites or to a level within them implying that the deeper basins of the apennine foredeep never underwent desiccation during the messinian salinity crisis but rather received the eroded marginal evaporites this fact usually overlooked raises important questions about the deep desiccation model of the mediterranean

a low viscosity wedge in subduction zones geochemical petrologic and seismological observations indicate that there may be high concentrations of water in the region above a subducting slab the mantle wedge which could decrease the viscosity of the mantle locally by several orders of magnitude using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography gravity and geoid a regional dynamic model of the tonga kermadec subduction zone shows that the viscosity of the wedge is at least a factor of 10 smaller than surrounding mantle lithosphere and asthenosphere consistent with estimates from seismic dissipation and deformation experiments

the dynamics of subduction and trench migration for viscosity stratification summary although subduction plays a dominant role in plate tectonics related processes such as trench migration are not well understood we investigate the influence of viscosity stratification and different flow boundary conditions on the trench migration dynamics of a subducting lithosphere in a 2 d model using a finite difference code subduction and mantle flow are driven by a compositionally prescribed density contrast between the high viscosity slab and the ambient mantle no overriding plate is assumed the model rheology is viscoplastic at shallow and viscous at greater depth the mantle is stratified with a higher viscosity in the lower part of the computational domain the plastic rheology allows the plate to decouple from the top of the box and is described by a simplified friction law to mimic the brittle deformation of the shallow lithosphere the flow patterns and slab morphologies differ as a function of the velocity boundary conditions bcs for reflective bcs we model either a plate which is fixed on the box side or a plate that can freely move a slab with periodic bcs is always laterally free we find that when the slabs interact with the highly viscous lower mantle the slab with reflective bcs is more folded than the periodic one due to the confinement of mantle flow around the slab in general the trench is observed to roll back toward the oceanic plate for free slabs with reflective bcs the trench retreat velocity deceases after the interaction with the lower mantle in contrast for the free slab with periodic bcs it is nearly constant or even increases with time if described in a reference frame fixed to the lower mantle the free slab with periodic bcs exerts a force on the lower mantle which causes a net differential flow between the upper and lower mantle we also carried out additional experiments with different mechanical slab properties and thicknesses stiffer free slabs subduct more steeply and are able to penetrate straight into the lower mantle after penetration into the lower mantle the trench moves toward the continent thinner slabs deform more easily and are folded for all boundary conditions we can generalize the rollback systematics for free slabs by normalizing trench velocities by the plate speed free slabs being more representative of the west pacific subduction zones show large temporal variations of normalized trench velocities ranging between a retreat value of 1 0 to an advancing value of 0 3

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

the impact of the south american plate motion and the nazca ridge subduction on the flat subduction below south peru 1 flat subduction near peru occurs only where the thickened crust of the nazca ridge subducts furthermore the south america continent shows a westward absolute plate motion both the overriding motion of south america and the subduction of the nazca ridge have been proposed to explain the flat slab segment below south peru we have conducted a series of numerical model experiments to investigate the relative importance of both mechanisms results suggest that the average upper mantle viscosity should be about 3 5 1020 pa s or less and basaltic crust should be able to survive 600 to 800 c ambient temperature before transforming into eclogite to explain the slab geometry below peru the effect of the overriding plate is estimated to be as large or twice as large as that of the plateau subduction

three dimensional acoustic monitoring and modeling of a deep sea co2 droplet cloud 1 we show that release of 5 liters of liquid co2 at 1000 m depth can be readily detected acoustically and tracked for over 30 minutes and 150 m of ascent with both surface ship 38 khz and rov 675 khz sonars the released liquid broke up into droplets covered with a hydrate film the remarkably sensitive acoustic response of the droplets may be attributed to the high sound speed contrast between co2 300 m sec and sea water 1500 m sec the near spherical shape of the droplets created by the hydrate shell and the high compressibility of the liquid the observed cloud conformed closely to models of co2 disposal allowing for reasonable predictions of larger scale processes this offers a remarkably sensitive technique for examination in real time of engineered releases of co2 volcanic sea floor liquid co2 plumes or leakage from geologic co2 storage

spherical dynamic models of top down tectonics 1 we use the multipole boundary element method mp bem to simulate regional and global geodynamics in a spherical 3 d setting we first simulate an isolated subducting rectangular plate with length llitho and width wlitho varying between 0 5 and 2 times the radius of the earth rearth and with viscosity litho varying between 100 and 500 times the upper mantle um sinking in a layered mantle characterized by lower upper mantle viscosity ratio lm um varying between 1 and 80 in a mantle with small upper lower viscosity contrast 1 trench and plate motions are weakly dependent on wlitho plate motion is controlled by slab pull if llitho rearth while for longer plates plate speed strongly decreases because of the plate basal friction and flow reorganization an increasing viscosity ratio gradually breaks this pattern and for 10 combined with wlitho rearth and greater trench advance and retreat are simultaneously observed these results offer a first order explanation of the origin of the size llitho wlitho rearth of the largest plates observed over the past 150 myr finally two global plate tectonic simulations are performed from reconstructed plates and slabs at 25 ma before present and before 100 ma respectively it is shown that mp bem predicts present plate kinematics if plate mantle decoupling is adopted for the longest plates llitho rearth models for 100 ma show that the slab slab interaction between india and izanagi plates at 100 ma can explain the propagation of the plate reorganization from the indian to the pacific plate

outer slope faulting associated with the western kuril and japan trenches summary elongated fault escarpments on the outer slopes of the western kuril and japan trenches have been investigated through detailed swath bathymetric mapping numerous horsts and grabens formed by these escarpments were identified distinct n70 e linear alignment of the escarpments parallel to the magnetic anomaly lineations was revealed on the outer slope of the western kuril trench in the japan trench north of 39 00aen most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations a zig zag pattern of faulting exists south of 39 00aen each topographic profile was decomposed by computer analysis into two curves representing 1 the smoothed long wavelength slope of the subducting ocean crust surface and 2 the short wavelength 10 km roughness of plateaus and valleys edged by outwardand inward facing fault escarpments throughout the surveyed areas escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall but with no distinct increase below that depth no significant diverence is recognized in fault throws towards and away from the trench it can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis which is caused by downward bending of the subducting lithosphere the relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the east pacific rise crest the maximum length and height of escarpments are small in the kuril trench compared with those in the japan trench implying a diverence in mechanical strength depending on the fault orientation the crust is weakest along the inherited spreading fabric second weakest probably along the non transform ovset direction and strongest in directions very oblique to these orientations seamounts appear to be more rigid than normal ocean crust with no particular weak orientations resulting in fewer but larger faults along the axis of plate bending as most clearly represented in the subducting daiichi kashima seamount

subduction zone earthquakes and stress in slabs the pattern of seismicity as a function of depth in the world and the orientation of stress axes of deep and intermediate earthquakes are explained using viscous fluid models of subducting slabs with a barrier in the mantle at 670 km 670 km is the depth of a seismic discontinuity and also the depth below which earthquakes do not occur the barrier in the models can be a viscosity increase of an order of magnitude or more or a chemical discontinuity where vertical velocity is zero longn versus depth wheren is the number of earthquakes shows 1 a linear decrease to about 250 300 km depth 2 a minimum near that depth and 3 an increase thereafter stress magnitude in a subducting slab versus depth for a wide variety of models shows the same pattern since there is some experimental evidence thatn is proportional toe where is a constant and is the stress magnitude the agreement is encouraging in addition the models predict down dip compression in the slab at depths below 400 km this has been observed in earlier studies of earthquake stress axes and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion at intermediate depths the models predict an approximate but not precise state of down dip tension when the slab is dipping the observations do not show an unambiguous state of down dip tension at intermediate depths but in the majority of regions the state of stress is decidedly closer to down dip tension than it is to down dip compression chemical discontinuities above 670 km or phase transitions with an elevation of the boundary in the slab predict when incorporated into the models stress peaks which are not mirrored in the profile of seismicity versus depth models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double benioff zones but also yield stress magnitude profiles consistent with observed seismicity models where a nonlinear rheology is used are qualitatively consistent with the linear models

origin of localized fast mantle flow velocity in numerical models of subduction 1 the origin of localized fast mantle flow near subduction zones found in regional jadamec and billen 2010 2012 and global stadler et al 2010 alisic et al 2010 numerical models using non newtonian rheology is explored using 3d models with simple geometry two suites of models using both a free slab and a slab attached to a lithospheric layer are presented to determine 1 the origin of high magnitudes of mantle flow 50 cm yr and 2 focusing of flow within the mantle wedge with mantle wedge velocities up to two times higher than slab sinking velocities the rheology in the models is either newtonian with a prescribed low viscosity wedge lvw or non newtonian wherein low viscosity regions form in response to high stress surrounding the sinking slab the results show that there are two aspects of the models that lead to localized fast velocities reduction in the mantle viscosity surrounding the sinking slab leads to faster overall flow rates induced by the negative buoyancy of the sinking slab and geometric effects that are caused by lateral variations in viscosity including retrograde pivoting motion of the slab

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

development of texture and seismic anisotropy during the onset of subduction 1 how reliable are shear wave splitting measurements as a means of determining mantle flow direction this remains a topic of debate especially in the context of subduction the answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation here we present an integrated model to simulate strain history dependent texture development and estimate resulting shear wave splitting in subduction environments we do this for a mantle flow model that in its geometry approximates the double sided molucca sea subduction system in eastern indonesia we test a single sided and a double sided subduction case results are compared to recent splitting measurements of this region by di leo et al 2012a the setting lends itself as a case study because it is fairly young and therefore early textures from the slab s descent from the near surface to the bottom of the mantle transition zone which we simulate in our models have not yet been overprinted by subsequent continuous steady state flow second it allows us to test the significance of the double sided geometry i e the need for a rear barrier to achieve trench parallel subslab mantle flow we demonstrate that although a barrier amplifies trench parallel subslab anisotropy due to mantle flow it is not necessary to produce trench parallel fast directions per se in a simple model of a type olivine lattice preferred orientation and one sided subduction trench parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle

subduction triggered magmatic pulses a new class of plumes article i nfo a variety of atypical plume like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed and seismological imaging has shown ubiquitous small scale convection in the uppermost mantle in regions such as the mediterranean region the western us and around the western pacific we argue that the three dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics the testable surface expressions of this process are the outside arc alkaline volcanism topographic swell and low velocity seismic anomalies associated with partial melt using three dimensional simplified numerical subduction models we show that focused upwellings can be generated both ahead of the slab in the back arc region though five times further inward from the trench than arc volcanism and around the lateral edges of the slab in the order of 100 km away from slab edges vertical mass transport and by inference the associated decompression melting in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities the upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation we discuss representative case histories from the pacific and the mediterranean where we find possible evidence for such slab related volcanism

deformation of a weak subducted slab and variation of seismicity with depth oceanic lithosphere is assumed to possess platelike properties whether it is lying at the earth s surface or descending deep into the mantle yet the geometries of subducting slabs indicate significant deformation as they descend through the mantle1 4 suggesting that lithospheric plates might be so weakened during subduction as to act not as a rigid solid but as a viscous fluid numerical and laboratory experiments have shown that fluid slabs can indeed take on realistic profiles5 7 however it has not been clear whether a fluid or weak model of slab dynamics can account for deep earthquakes which are usually ascribed to the deformation of a rigid or strong slab here we show using numerical simulations of slab evolution that a weak slab model is in fact consistent with seismic observations assuming that earthquakes occur at a rate proportional to deformation rate we reproduce the observed variation with depth of seismicity rate and focal mechanisms and the cessation of seismicity at 670 km depth provided that sinking material encounters resistance at depth here modelled as a viscosity jump at 670 km the pattern of seismicity can be explained by any mechanism for deep earthquakes in which the rate of seismicity is proportional to strain rate in the slab

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

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

slab stiffness control of trench motion insights from numerical models 1 subduction zones are not static features but trenches retreat roll back or advance here we investigate the dominant dynamic controls on trench migration by means of two and three dimensional numerical modeling of subduction this investigation has been carried out by systematically varying the geometrical and rheological model parameters our viscoplastic models illustrate that advancing style subduction is promoted by a thick plate a large viscosity ratio between plate and mantle and a small density contrast between plate and mantle or an intermediate width w 1300 km advancing slabs dissipate 45 to 50 of the energy in the system thin plates with relatively low viscosity or relatively high density or wide slabs w 2300 km on the other hand promote subduction in the retreating style i e slab roll back the energy dissipated by a retreating slab is 35 to 40 of the total dissipated energy most of the energy dissipation occurs in the mantle to accommodate the slab motion whereas the lithosphere dissipates the remaining part to bend and unbend with a simple scaling law we illustrate that this complex combination of model parameters influencing trench migration can be reduced to a single one plate stiffness stiffer slabs cause the trench to advance whereas more flexible slabs lead to trench retreat the reason for this is that all slabs will bend into the subduction zone because of their low plastic strength near the surface but stiff slabs have more difficulty unbending at depth when arriving at the 660 km discontinuity those bent slabs tend to cause the trench to advance in a similar way variation of the viscoplasticity parameters in the plate may change the style of subduction a low value of friction coefficient weakens the plate and results in a retreating style while higher values strengthen the plate and promote the advancing subduction style given the fact that also on earth the oldest and therefore probably stiffest plates have the fastest advancing trenches we hypothesize that the ability of slabs to unbend after subduction forms the dominant control on trench migration

curvature of subducted lithosphere from earthquake locations in the wadati benioff zone estimates of slab curvature from 133 profiles across 12 major subduction zones are obtained by fitting splines through earthquakes locations several geometric quantities are computed from the spline fits including the minimum radius of curvature we also introduce a quantity called the equivalent radius of curvature to precisely define the bending dissipation in either a viscous or a plastic slab the minimum and equivalent radius of curvature are approximately equal in slabs with simple geometry i e a single bend but deviate substantially when the geometry is more complicated a compilation of estimates shows that the equivalent radius of curvature is insensitive to the age of the plate at the trench mean values are 175 km or 191 km depending on the rheology of the slab when bending occurs by viscous deformation the resulting dissipation can exceed the power available from subduction with reasonable choices for viscosity a more consistent description of the energetics is obtained by assuming a plastic rheology

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

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

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

geochemistry and habitat of natural gases in po basin northern italy three types of natural gases can be distinguished in the po basin 1 bacterial gases dgr13cch4 60 ppt dgrdch4 180 to 200 ppt c2 0 2 which are found predominantly in pliocene pleistocene reservoirs 2 mixed gases dgr13cch4 60 to 50 ppt dgrdch4 180 to 200 ppt c2 0 2 to 5 which are predominantly in reservoirs of messinian upper miocene age but also in a few places in older and younger reservoirs 3 thermogenic gases dgr13c 50 ppt dgrd 200 to 150 ppt c2 5 which commonly are associated with oil and are produced predominantly from miocene and mesozoic reservoirs a quantitative assessment of gas in place for these three gas types is 1 80 2 10 and 3 10 respectively the areal distribution and amount of the three gas types are controlled strictly by geologic factors synsedimentary tectonics and turbidite sedimentation are the best conditions for the trapping of biogenic gas in the southeastern part of the area where 70 of the bacterial gas in place is produced here bacterially formed methane is produced from depths of up to 4 500 m 15 000 ft and is the deepest bacterial gas so far reported in the literature in the pedealpine homocline lower sedimentation rates and a weak deformation of pliocene pleistocene sediments prevented appreciable accumulations only 15 of the bacterial gas in the basin is produced in this area thermogenic gases are found in the vicinity of the apennines where strong tectonic movements have favored its migration from deeper mesozoic sources into tertiary reservoirs or into deep mesozoic carbonate reservoirs of the pedealpine homocline

did the mediterranean marine reflooding precede the mio pliocene boundary paleontological and geochemical evidence from upper messinian sequences of tuscany italy abstract the events related to the messinian salinity crisis are among the most deeply investigated of earth s history according to the current hypothesis of neogene paleogeographic evolution approximately 5 5 ma ago after evaporitic sedimentation and before the mio pliocene boundary the mediterranean was characterized by the widespread development of non marine environments inhabited by molluscs and ostracods of brackish affinity the messinian post evaporitic deposits that testify such a dramatic environmental change are commonly referred to as lago mare and have been reported from several outcrops and boreholes throughout the entire mediterranean basin the origin of lago mare conditions is commonly interpreted as the result of the synergistic effect of both the humid climatic conditions and change of the drainage patterns at the mediterranean scale with the capture of paratethyan brackish waters a few recent studies however suggest that such a scenario probably represents an oversimplification of the original context from both a paleogeographical and paleoenvironmental point of view unfortunately the results of these studies have never been commonly accepted and the proposed evidences have been considered questionable in this paper we describe the fish assemblages from the lago mare deposits of two localities cava serredi and podere torricella located in the neogene hinterland basins of tuscany central italy these assemblages consists of a mixture of marine euryhaline and stenohaline taxa suggesting that the depositional environments were characterized by permanently open connections with a marine basin filled with normal marine waters in order to better define the paleontological significance of these upper messinian fish assemblages the oxygen carbon and strontium isotopic composition of fish otoliths and other fossils has been measured these isotopic compositions are strongly indicative of the presence of normal marine conditions close to the depositional environments testified by the sedimentary sequences thereby implying that the interpretation of the geochemical results are consistent with those derived from the paleoecological analyses of the fish assemblages based on the integrated paleoichthyological geochemical approach discussed in this paper it is possible to unambiguously demonstrate that normal marine rather than fresh or brackish waters were present in the mediterranean at least during the upper part of the lago mare event providing an unquestionable evidence that the marine refilling of the basin preceded the mio pliocene boundary

role of lateral mantle flow in the evolution of subduction systems insights from laboratory experiments summary we present 3 d laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions in particular we present a set of experiments characterized by different conditions imposed at the trailing edge of the subducting plate that is plate fixed in the far field plate detached in the far field imposed plate motion experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle for each set we show differences between models that do or do not include the possibility of out of plane lateral flow in the mantle by varying the lateral boundary conditions our results illustrate how a subducting slab vertically confined over a 660 km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways on the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

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

mountain building and mantle dynamics 1 mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen a general dynamic model for this process is still lacking here we propose that mountain belts can be classified between two end members first those of slab pull type where subduction is mainly confined to the upper mantle and rollback trench motion lead to moderately thick crustal stacks such as in the mediterranean second those of slab suction type where whole mantle convection cells conveyor belts lead to the more extreme expressions of orogeny such as the largely thickened crust and high plateaus of present day tibet and the altiplano for the slab suction type deep mantle convection produces the unique conditions to drag plates toward each other irrespective of their nature and other boundary conditions we support this hypothesis by analyzing the orogenic volcanic and convective history associated with the tertiary formation of the andes after 40 ma and himalayas after collision at 55 ma based on mantle circulation modeling and tectonic reconstructions we surmise that the forces necessary to sustain slab suction mountain building in those orogens derive after transient slab ponding from the mantle drag induced upon slab penetration into the lower mantle and from an associated surge of mantle upwelling beneath africa this process started at 65 55 ma for tibet himalaya when the tethyan slab penetrated into the lower mantle and 10 myr later in the andes when the nazca slab did this surge of mantle convection drags plates against each other generating the necessary compressional forces to create and sustain these two orogenic belts if our model is correct the available geological records of orogeny can be used to decipher time dependent mantle convection with implications for the supercontinental cycle

dynamics of subduction and plate motion in laboratory experiments insights into the plate tectonics behavior of the earth 1 three dimensional laboratory experiments have been designed to investigate the way slab bearing plates move during subduction inside the mantle in our experiments a viscous plate of silicone lithosphere subducts under its negative buoyancy in a viscous layer of pure honey mantle varying thickness width viscosity and density of the plate and mantle three characteristic modes of subduction are observed a retreating trench mode mode i a retreating trench mode following a transient period of advancing trench mode ii and an advancing trench mode mode iii these modes are characterized by different partitioning of the amount of subduction into plate and trench motion our experiments show that the velocity of subduction can be modeled by the dynamic interaction between acting and resisting forces where lithospheric bending represents 75 95 of the total resisting forces however our experimental results also show the impossibility to predict a priori the plate velocity only from the velocity of subduction without considering trench migrations we find experimentally that the lithospheric radius of curvature which depends upon plate characteristics stiffness and thickness and the mantle thickness exerts a primary control on the trench behavior our results suggest that the complexity of the style of subduction could be controlled by geometrical rules of a plate bending inside a stratified mantle the earth system is in the crucial range for the interplay between the rigidity of the plate and the mantle stratification this setting may be the responsible for the complexity of the past and present tectonic styles

back arc rifting initiated with a hot and wet continental lithosphere abstract how back arc rifting is initiated is studied by inferring the tectonic force and strain rate regimes from published subsidence data for nine back arc basins in the european tethyan belt and in the western pacific the results show that rifting only begins after some particular magnitude of tectonic force is reached this implies that subduction has already progressed to a point such that the total negative buoyancy of the slab becomes so large that the relevant tectonic force exceeds its bending strength once back arc rifting has been initiated the force required to maintain it gradually decays this indicates that the processes governing back arc extension are fast enough that lithosphere weakening due to an increase in geothermal gradient exceeds strengthening due to the thermal relaxation and crustal thinning this is achieved given currently accepted estimates for the maximum force 7 tn m if the thickness of the thermal lithosphere is significantly less than 125 km and if the lithosphere has a wet rheology meaning that a weak continental lithosphere is required for the initiation of back arc rifting the presence of significant strength in the uppermost mantle is not precluded but it is not required

deep water clastic evaporites deposition in the messinian adriatic foredeep northern apennines italy did the mediterranean ever dry out a new genetic facies model for deep water clastic evaporites is presented based on work carried out on the messinian gessoso solfifera formation of the northern apennines during the last 15 years this model is derived from the most recent siliciclastic turbidite models and describes the downcurrent transformations of a parent flow mainly composed of gypsum clasts the model allows clearer comprehension of processes controlling the production and deposition of clastic evaporites representing the most common evaporite facies of the northern apennines and the definition of the genetic and stratigraphic relationship with primary shallow water evaporites formed and preserved in marginal settings due to the severe recrystallization processes usually affecting these deposits petrographic and geochemical analyses are needed for a more accurate interpretation of the large spectrum of recognized gravity driven deposits ranging from debrisflow to low density turbidites almost all the laminar balatino gypsum previously considered a deep water primary deposit is here reinterpreted as the fine grained product of high to low density gravity flows facies associations permit the framing of the distribution of clastic evaporites into the complex tectonically controlled depositional settings of the apennine foredeep basin the messinian salinity crisis occurred during an intense phase of geodynamic reorganization of the mediterranean area that also produced the fragmentation of the former miocene apennine foredeep basin in this area primary shallow water evaporites equivalent to the mediterranean lower evaporites apparently only formed in semi closed thrust top basins like the vena del gesso basin the subsequent uplift and subaerial exposure of such basins ended the evaporite precipitation and promoted a widespread phase of collapse leading to the resedimentation of the evaporites into deeper basins vertical facies sequences of clastic evaporites can be interpreted in terms of the complex interplay between the messinian tectonic evolution of the apennine thrust belt and related exhumation erosional processes the facies model here proposed could be helpful also for better comprehension of other different depositional and geodynamic contexts the importance of clastic evaporites deposits has been overlooked in the study of other mediterranean areas based on the apennine basins experience it is suggested here that evaporites diffused into the deeper portions of the mediterranean basin may consist mainly of deep water resedimented deposits rather than shallow water to supratidal primary evaporites indicative of a complete basin desiccation

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