numerical simulation of metamorphic pressure temperature time paths and fluid production in subducting slabs in order to investigate metamorphic reactions and fluid production in subduction zones a 2 dimensional finite difference model has been constructed the model simulates heat transfer in a subduction zone with a 27 dip a spatial resolution of 1 km is used in the region of the subduction thrust in order to carefully monitor metamorphic reactions elsewhere the spatial resolution is 5 km in these calculations the oceanic crust is assumed to contain 2 wt bound volatiles distributed homogeneously within the 7 5 km thick crust fluids are released by continuous and discontinuous end members of three different dehydration models pressure sensitive temperature sensitive and amphibole negative dp dt dehydration subduction zone pressure temperature time p t t paths predicted by the model intersect the wet basaltic solidus only at the initiation of subduction in mature subduction zones and in the absence of significant frictional heating and induced mantle convection p t t paths encounter subsolidus conditions at depths of 100 150 km beneath magmatic arcs suggesting that slab dehydration reactions play an important role in arc magma genesis metamorphic dehydration reactions that consume 50 kj kg result in p t t paths that are only 5 k cooler than models that neglect such reactions in both the temperature and amphibole dehydration models the fluid production region moves to greater depths over time as the subduction zone cools within the oceanic crust fluid production starts at the top of the crust and migrates downward in the temperature and amphibole dehydration models this phenomenon coupled with the weakening effect of devolatilization reactions may result in the accretion of oceanic crust to the hanging wall of the subduction zone for a convergence rate of 3 cm yr average fluid fluxes out of the subducting slab range from 0 1 kg fluid m yr for the continuous reaction models to 1 kg fluid m yr for the discontinuous pressure and amphibole dehydration models vertical fluid fluxes on the order of 1 kg fluid m yr can substantially perturb the thermal structure of overlying rocks and can cause large scale hydration and metasomatism of the overlying mantle wedge in contrast the thermal effect of fluid flow parallel to the subduction thrust zone is insignificant compared to the thermal effect of subducting oceanic lithosphere

cleavage fold relationships in the lower cambrian tubize group southeast anglo brabant fold belt lembeek belgium little is known about the cleavage fold relationships in the lower palaeozoic brabant massif primarily because of the scarcity of suitable exposures moreover to date folds have only been described in the ordovician and silurian sequences along the southern extremity of the basement however excavation works for the construction of the tgv railway track south of brussels created an opportunity to study cleavage fold relationships in the lower cambrian terrigenous series tubize group in a more central part of the brabant massif the structural features observed seemed inconsistent with the suspected regional trends primarily a divergent cleavage fan was observed in a region thought to have a regular cleavage attitude the symmetrical but divergent disposition of the cleavage with regard to the fold hinges is explained by flexural folding of a pre existing bedding parallel compaction fabric cleavage development and folding are considered synchronous the divergent cleavage fan reflects local strain variations also the steeply plunging hinge lines of the open subangular folds are seemingly inconsistent with the regional trend characterized by subhorizontal fold hinges taking into account the structural position of the fold assemblage in the subvertical limb of a large scale upright isoclinal fold structure the fold assemblage is interpreted as an incongruous parasitic feature the steeply plunging hinge lines are considered to be caused by fold hinge rotation during progressive coaxial deformation although this incongruous fold assemblage with its divergent cleavage fan is localized it provides important information on both local and regional deformation circumstances in this part of the anglo brabant fold belt

quantitative estimates of interplate coupling inferred from outer rise earthquakes interplate coupling plays an important role in the seismogenesis of great interplate earthquakes at subduction zones the spatial and temporal variations of such coupling control the patterns of subduction zone seismicity we calculate stresses in the outer rise based on a model of oceanic plate bending and coupling at the interplate contact to quantitatively estimate the degree of interplate coupling for the tonga new hebrides kurile kamchatka and marianas subduction zones depths and focal mechanisms of outer rise earthquakes are used to constrain the stress models we perform waveform modeling of body waves from the gdsn network to obtain reliable focal depth estimates for 24 outer rise earthquakes a propagator matrix technique is used to calculate outer rise stresses in a bending 2 d elastic plate floating on a weak mantle the modeling of normal and tangential loads simulates the total vertical and shear forces acting on the subducting plate we estimate the interplate coupling by searching for an optimal tangential load at the plate interface that causes the corresponding stress regime within the plate to best fit the earthquake mechanisms in depth and location

the olivine spinel transformation and the rheology of subducting lithosphere during subduction relatively hot lithospheric slabs are expected to deform in a superplastic flow regime below a depth of 300 km if olivine reacts to fine grained spinel this process reduces the flow strength of such slabs below the phase boundary so that they become aseismic at about this depth in colder slabs temperatures are too low for superplastic deformation and a high flow strength and seismic activity are maintained to a depth of 600 700 km

3 d anatomy of an active fault propagation fold a multidisciplinary case study from tsaishi western caucasus georgia abstract ongoing deformation processes combining fault propagation and folding are the cause of diffuse seismicity in many areas of the world a detailed understanding of the structural evolution of tectonically active folds is crucial for the evaluation of seismic hazard this paper proposes an integrated analysis of an active fold consisting in the development of a 3d model by combination of geomorphological observations field geological structural data and seismic reflection sections our case study is the tsaishi anticline located at the southwestern tip of the rioni basin uplifted area at the foothill of greater caucasus western georgia we recognized that the fold started to form since the beginning of the middle miocene although preliminary data suggest the possibility of initial local uplift in the oligocene folding process continues up to nowadays giving rise to a south vergent anticline as shown by upwarped late quaternary river deposits the fold backlimb is affected by three main back thrusts whereas at the foot of the forelimb a main north dipping thrust comes very close to the surface based on seismic sections here field data show the presence of a 13 km long fault scarp or fold scarp along the scarp is located the epicenter of the strongest earthquake to date the ms 6 0 tsaishi earthquake that struck the area in 1614 ce based on our results we propose that the overall structure can be classified as an active fault propagation fold the recognition of its very recent growing associated with a major underlying active fault represents also a major contribution to the seismic hazard assessment of this populated area

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

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1

seismicity and structural investigations of the romanian vrancea region evidence for azimuthal variations of p wave velocity and poisson s ratio abstract a study of the shallow and intermediate depth seismicity of the romanian vrancea region in the period 1964 1981 has been performed the seismic events have been relocated by a standard location procedure using a regional velocity model from the temporal and spatial distribution of the seismic activity aspects of the seismicity before the large march 4 1977 earthquake are treated in particular the seismic gap in space and time prior to this event found by m rze 1979 which is critically discussed and revised the concept of the precursor time magnitude relationships of different authors is applied and its validity to the vrancea region assessed the hypocentral distribution shows that the intermediate depth seismic activity is confined to a small volume with dimensions of only some tens of kilometers the results are interpreted in terms of the tectonics of the region from an analysis of the travel time residuals at different local stations evidence for lateral velocity heterogeneities beneath the region is obtained e g a high velocity zone southeastwards of the carpathian chain finally mean p s ratios i e poisson s ratios for various stations are calculated from p and s wave travel times they show azimuthal variations of up to 6 for stations within the area where the intermediate seismic activity occurs in comparison with the station focsani situated eastwards in the carpathian foredeeps all these results are compatible with the plate tectonic concept for the vrancea region that is the subduction of an oceanic lithospheric slab under the carpathian mountain arc giving rise to such a highly active seismic zone

slab temperature and thickness from seismic tomography 1 method and application to tonga delay times of compressional body wave phases from both teleseismic and local events are used to invert for a high resolution p wave velocity model in the tonga subduction zone the images obtained show a high velocity subducting slab with velocity deviations of the order of 3 4 assuming to first order that the positive velocity anomalies within the slab are caused by a temperature effect a theoretical slab temperature model based on the diffusion equation is used to explain velocity anomalies within the tomographic slab temperature differences between the interior of the slab and the ambient mantle are converted to velocity perturbations using the scaling parameter dvp dt 4 8 10 4 km s 1 c 1 for lithosphere material the optimal values for the parameters in the temperature model are found using a nonlinear optimization that compares the integrated velocity anomalies in the tomographic slab region to integral of high velocities in a synthetic slab derived from a temperature model the parameters for slab thickness and mantle potential temperature are not uniquely determined therefore a fixed value for the mantle potential temperature based on laboratory values for the temperature of the spinel to perovskite transition at 660 km is used using 1180 c as the potential temperature the theoretical temperature model gives an optimal slab thickness of 82 km for a region near 29 s in tonga the uncertainty in the thickness is dominated by the uncertainty in the mantle temperature and would be 8 km for an uncertainty of 100 in mantle temperature but nonsystematic errors are less in order to enhance the tomographic result the velocity model is biased towards the theoretical slab model however a posteriori changes made to the tomogram will most likely violate the fit to the delay time data to prevent this the difference between the tomogram and the predicted slab model is projected onto the null space of the inversion to remove components which do not satisfy the seismic data using only null space components to modify the minimum norm solution an enhanced model is obtained which has been biased toward the theoretical solution but has the same data misfit as the minimum norm solution the final image shows a very narrow and continuous slab with maximum velocity anomalies of the order of 6 7 many of the gaps within the slab as well as artifacts around the slab which were present in the minimum norm solution are absent in the biased image

variations in slab dip along the subducting nazca plate as related to stress patterns and moment release of intermediate depth seismicity and to surface volcanism 1 the subducting nazca plate shows a high degree of along strike heterogeneity in terms of intermediate depth seismicity 70 300 km orientations of slab stress and volcanism we compile the intermediate depth earthquakes of south america from the harvard centroid moment tensor cmt catalogue to determine along strike dip variations and we explore the variable level of correlation between these observations primary results are three fold 1 consistency among orientation of tension axes highest level seismicity and occurrence of volcanism is associated with steeply dipping regions 20 30 2 volcanism and intermediate depth seismicity exhibit no direct correlation suggesting that earthquake faulting is not a necessary condition for transport of dehydrated fluids out to the mantle wedge for initiation of melting 3 comparison of slab thermal structures between regions with and without volcanism suggests that for certain wedge widths the maximum temperature in the mantle wedge is higher for the former than for the latter

40ar 39ar isotope constraints on the age of deformation in charnwood forest uk 40 ar 39 ar dating by a combination of spot analysis and step heating has been carried out on mica fabrics from slaty cleavage and a shear zone in the neoproterozoic to cambrian rocks of charnwood forest leicestershire the results indicate that crust adjacent to the eastern margin of the midlands microcraton was affected by localized epizonal metamorphism and penetrative deformation between 425 and 416 ma silurian to earliest devonian somewhat earlier than the acadian phase of the caledonian orogeny recorded elsewhere in southern britain the charnwood cleavage geometry suggests that the deformation arose within a dextral transpressional regime along the eastern margin of the midlands microcraton this tectonism may have overlapped with pre acadian sinistral transtensional movements documented for the welsh basin and lake district areas to the west and north of the midlands microcraton

viscosity of the earth s mantle inference from dynamic support by flow stress abstract in this study the viscosity is estimated from a new and different point of view the plates with various dip angles and penetration depths are descending into the mantle at many locations around the world since these descending slabs are affected by viscous flow negative buoyancy forces in the slabs and other effects some information about the viscosity can be obtained through the balance of torques exerted on the slabs a model is constructed on the basis of the corner flow treatment for a given viscosity model the stress exerted normal to the slab by the flow is calculated from the balance of torques by this stress the negative buoyancy force and the bending moment the dip angles of the descending slabs are calculated by the comparison between these calculated angles and the observed angles better viscosity models are selected the viscosity distribution obtained here consists of a low viscosity layer with a thickness of about 100 km and a viscosity of about 10 20 p poises an intermediate viscosity layer with about 10 22 p viscosity and either a highly viscous 10 24 p or more layer around 700 km depth or the highly viscous lower mantle

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

a deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes a distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1 2 here we report the first extensive deep earthquake aftershock sequence to be observed it was recorded by an array of eight broadband seismographs following the 9 march 1994 deep tonga earthquake the aftershocks show a power law decay with time following the main shock as is typical of shallow events most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main shock mechanism and the mechanisms of three large aftershocks assuming these aftershocks denote the main shock rupture area they define a 50 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region the width of the aftershock zone is wider than the expected width of the metastable olivine wedge demonstrating that either the width of the metastable olivine material exceeds previous estimates or the aftershocks are not confined in such a wedge

stress and flow beneath island arcs summary the flow pattern stress distribution topography and gravity anomalies were computed from numerical models having density and viscosity distributions resemblant to the aleutian arc the results were compatible with the hypothesis that the excess density of the slab drives its descent and that hydrodynamic forces are responsible for topographic and gravity highs over the outer rise seaward of the trench and the frontal arc and lows over the trench in models with simple distributions of rheological parameters the force from the slab was transmitted directly upward producing a negative gravity anomaly over the arc material with low resistance to flow was needed along the fault plane above the slab or within the crust of the frontal arc and within the wedge of asthenosphere above the slab to reduce that force and to allow the horizontal lithosphere to move with the slab models with the resistance to flow thus reduced had outer rises deep trenches horizontal tension seaward of the trench horizontal compression under the trench and downdip tension in the slab free air gravity anomalies which are the sum of between deflections of the free surface due to hydrodynamic forces and direct attractions from the masses driving the flow were not fit excellently by any of the models in part because the coarse grid used precluded accurate representation of the fault zone above the slab and the frontal arc an alternate to the hypothesis that about 5 kb of stress on the fault plane is needed to produce an outer rise is offered by these models shear stress between the slab and the island arc was always below 700 bars in the more successful models if the density distribution was scaled to match the topography of the trench this is much less than the 2000 bars stresses needed if frictional heating causes island arc volcanism

using shear wave amplitude patterns to detect metastable olivine in subducted slabs 1 olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to phase this may produce a tapering low seismic velocity wedge within the slab in the transition zone transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic motivated by previous two dimensional numerical calculations for upgoing and downgoing seismic waves a three dimensional gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions five slab velocity structures are considered a thermal model with horizontal phase transitions at the same depths as in ambient mantle two models incorporating equilibrium transitions within the slab and two models with tongues of metastable olivine a depth and temperature dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to phase are included in the thermal modeling three dimensional dynamic ray tracing and gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models for sources near 400 km depth the presence of low velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models comparison with observed amplitudes for four events in the kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models this indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs

a model of post seismic recovery induced by a deep focus earthquake abstract post seismic behavior induced by a deep focus earthquake in subduction zone is closely investigated numerically on realistic upper mantle models taking into account the pressure and temperature conditions for this purpose the post seismic stress changes expected from the deep event are calculated by a two dimensional finite element method 2d fem assuming newtonian and power law creep behaviors as justified by high p t experiments several cases are investigated in detail for various fault depths and thermal structures two cases are considered for the location of the fault one when an earthquake occurs within oceanic crust overlying a subducting plate and the other when an earthquake is located within the plate we also examine the case when the temperature distribution within the plate is lower by about 200 c on average than that in the upper mantle at the same depth in addition the post seismic behavior due to a deep focus event in a petrological thermal model of a mantle wedge is investigated the results show that the post seismic behavior associated with the deep event occurring within the oceanic crust indicates a pattern of relaxation of the coseismic stress changes lower temperatures inside the plate would suppress the post seismic displacements and stress changes effectively the petrological model also has some influence on the post seismic flow pattern in the mantle wedge we also study a possible correlation between deep focus earthquakes and a shallow large event which has been detected in the northwestern pacific region however effective mechanisms to explain the phenomenon could not be identified from the viewpoint of post seismic behavior caused by power law creep

pre and synorogenic burial metamorphism in the ardenne and neighbouring areas rhenohercynian zone central european variscides abstract the rhenohercynian zone of the central european variscides contains in the central ardenne eifel brabant campine region of belgium france germany and luxemburg an omnipresent thermal overprint which reaches in places uppermost epizonal conditions because of its distribution and structural relationships this metamorphism cannot be explained by a singular tectonometamorphic event field and microstructural observations new illite crystallinity and other data relating to this metamorphism are presented together with a review of existing petrological illite crystallinity vitrinite reflectance conodont alteration index and fluid inclusion data and their relationship to small and regional scale geological structures three distinct geotectonic situations can be described which overlap and alternate in space and time 1 the oldest metamorphism which reached epizonal conditions can be localized in the pre devonian basement rocks of the brabant massif and with minor evidence of the stavelot massif further south it is clearly related to a pre variscan caledonian deformation event and could be of burial origin 2 the main regional metamorphism affected all devonian and carboniferous rocks and the lower palaeozoic basement highs of the ardenne eifel area it does not exceed diagenetic conditions in some areas but reaches maximal values of 500 c and 400 mpa in the serpont and rocroi massif areas theye and fransolet 1993 robion et al 1995 it is pre to synkinematic to the main variscan contractional penetrative deformation and correlates well with the age and thickness of the sedimentary cover rocks the metamorphism is associated with the pre carboniferous preorogenic rifting stage of the rhenohercynian basin and documents the peak of subsidence and sediment accumulation we interpret it as diastathermal metamorphism it reaches its maximum in the low grade metamorphic zones of the ardenne area which are inverted synsedimentary structures 3 the diastathermal metamorphism is overprinted in the central part of the ardenne eifel area middle devonian to carboniferous rocks of the dinant synclinorium by an anchizonal beginning epizonal metamorphism in the foreland basins north of the midi fault zone it reaches diagenetic to lowermost anchizonal conditions it is the expression of the burial by thick synorogenic mid to upper carboniferous cover rocks which were deposited in piggyback and foreland basins this clastic wedge relates to the carboniferous contraction of the rhenohercynian basin whose metamorphosed rocks were themselves affected by the deformation of the advancing orogenic front

distinguishing syn cleavage folds from pre cleavage folds to which cleavage is virtually axial planar examples from the cambrian core of the lower palaeozoic anglo brabant deformation belt belgium abstract within the cambrian jodoigne formation in the easternmost part of the anglo brabant deformation belt sub horizontal to gently plunging folds occur within the limbs of steeply plunging folds the latter folds are cogenetic with cleavage and are attributed to the brabantian deformation event in contrast although cleavage is also 1 virtually axial planar to the sub horizontal to gently plunging higher order folds shows 2 a well developed divergent fanning across these folds 3 an opposing sense of cleavage refraction on opposite fold limbs and 4 only very small cleavage transection angles an analysis of the cleavage bedding intersection lineation suggests that these higher order folds have a pre cleavage origin on the basis of a comparison of structural and sedimentological features these higher order folds are interpreted as slump folds the seemingly normal cleavage fold relationship across the slump folds within the limbs of the large steeply plunging folds is due to the very small angle between cleavage and bedding as such a normal cleavage fold relationship is no guarantee for a syn cleavage fold origin it is not unlikely that also within undeformed recumbent slump folds a well developed compaction fabric formed parallel to the axial surface of the slump folds may show fanning and contrasting senses of cleavage refraction on opposite fold limbs

seismic discontinuities and subduction zones abstract the seismic discontinuities at 410 and 660 km depth in the mantle are generally believed to be due to phase transformations in its major component olivine an increasing observational base of short period regional seismic network data supports this view here we analyse cross sections of discontinuity topography in the northwest pacific subduction zones to infer the thermal and chemical state of the mantle in a subduction environment most of the data is from the southern part of the izu bonin subduction zone penetration of the lower mantle by the steeply dipping subducting slab is clearly indicated by depression of the 660 km discontinuity elevation of the 410 km discontinuity in this region by up to 60 km implies a thermal anomaly of approximately 900 1000 c relative to the ambient mantle observation of the elevated 410 in the seismically active part of the slab indicates that the olivine wadsleyite phase transformation occurs under essentially equilibrium conditions there which argues against the transformational faulting hypothesis as the mechanism for deep earthquakes upgoing p wave reflections from the 410 km discontinuity indicate that it is sharp in the mantle far from subducting slabs reflection coefficient variability in and near slabs indicates either a chemical effect or the consequences of topographic focusing

on melting of the subducted oceanic crust effects of subduction induced mantle flow abstract based on petrological and geochemical arguments it is possible that arc magma is derived from subducted oceanic crust in this paper regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth i e at depths of about 100 km by a dynamic mantle calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab this current continuously feeds hot mantle material into the corner and onto the slab surface a high temperature thermal environment can be maintained in the vicinity of the wedge corner immediately beneath the over riding plate our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner additional geological processes such as reasonable amounts of shear heating and minor dehydration which will lower the local melting temperature will further increase the probability of melting a cold subducted oceanic crust at shallow depth

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

resolution experiments for nw pacific subduction zone tomography we investigate the resolving power of isc neic p travel time data in tomographic inversions for the geometry of the subduction zones in the nw pacific from thermal models for the kurile japan izu bonin mariana and ryukyu slabs we generate three dimensional synthetic velocity anomalies for subducting slabs which are then projected onto a cell model for the uppermost 1400 km of the mantle using these synthetic models we compute synthetic delay times for ray paths corresponding to the source and receiver locations used for the actual data add gaussian noise invert the synthetic data and compare the resulting velocity structure to the initial synthetic models this comparison is illustrated for sections through the kuriles and the mariana arcs a variety of resolution artifacts are observed which in many cases resemble features visible in the tomographic results obtained from inverting the actual isc neic data

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

a hypothesis for the seismogenesis of a double seismic zone summary the seismogenesis of a double seismic zone in particular the lower layer of a double seismic zone has not been adequately explained in the literature on the basis of seismic data and geothermal structures along three well studied cross sections in the kuril kamchatka and japan subduction zones we investigate the temperature pressure conditions associated with seismogenic structures of the double seismic zones the corresponding tip loci seem to suggest that earthquakes observed in the lower layer and in the lower part below approximately 130 20 km of the top layer of a double seismic zone were caused by metastable phase transition a mechanism similar to that responsible for deep focus earthquakes only at lower temperature pressure conditions under this hypothesis the wedge shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition according to theoretical experimental studies and the constraints imposed by our observations a likely candidate for such a phase transition is the metastable al rich enstatite decomposing into the assemblage of al poor enstatite plus garnet earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and or facies change from basalt to eclogite that the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 f 20 km because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition it is inferred that single seismic zones are also caused by the same mechanisms except that the implicit layer of a supposed double seismic zone is missing due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

density profiles of oceanic slabs and surrounding mantle integrated thermodynamic and thermal modeling and implications for the fate of slabs at the 660 km discontinuity abstract we have calculated the mineralogical properties of the earth s mantle and the lithological units constituting the subducting oceanic slabs within a wide range of p t conditions within the cao feo mgo al2o3 sio2 system except for the basalt top layer of a slab for which the system is extended to include na2o the mineralogical data are then converted using the appropriate p v t relations to bulk densities the calculated adiabatic density vs depth profile of the mantle between 200 and 725 km depths is in good agreement with geophysical and experimental data the density data of the different compositional units are combined with calculated thermal structures for a variety of slab mantle systems to construct equilibrium density profiles as a function of depth the mean equilibrium densities of the slabs within the transition zone 400 660 km depth are found to be 0 04 0 05 g cm3 greater than those of the ambient mantle within the same depth interval for the entire upper mantle density differences between slabs and ambient mantle are slightly less but the slabs still remain denser than the latter at 670 km depth slabs have lower density than the ambient lower mantle because of the commencement of perovskite forming reactions within the mantle and displacements of these reaction boundaries to higher pressures within the slabs as a consequence of their negative p t slopes if perovskite forming reactions within slabs are hindered for kinetic sluggishness then neutral buoyancy would be achieved when the slabs have penetrated 100 km into the lower mantle however using the available data on the kinetics of spinel to perovskite plus periclase reaction we conclude that the reaction would go to completion in a peru type young slab 41 myr and very likely also in a tonga type old slab 110 myr before these penetrated 100 km into the lower mantle thus slabs should always remain negatively buoyant and therefore continue to subduct through the lower mantle once it penetrates through the 660 km discontinuity despite a negative buoyancy force a slab could deflect at the top of the lower mantle 660 km because of factors resisting subduction namely viscosity jump low dip angle slab roll back and metastable persistence of olivine in cold slabs if published scale model experiments represent realistic approximations of the factors affecting plate subduction then according to our density data any slab with a dip angle of 40 50 would bend at the 660 km discontinuity if there is a viscosity jump of at least by a factor of 10 and trench migration the basalt top layer of a slab is denser than other slab components and the ambient mantle at all depths to 660 km and therefore should continue to sink into the lower mantle especially if a slab directly penetrates the 660 km barrier instead of peeling off in the transition zone to form a perched eclogite or piclogite layer as previously proposed the harzburgite layer which is sandwiched between denser basalt and lherzolite layers faces greater resistance to subduction especially in a young slab and thus could significantly contribute to the deformation of a slab near the 660 km discontinuity

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

properties of the lithosphere in collisional belts in the mediterranean a review abstract a brief review of the lithosphere asthenosphere properties is given for the mediterranean area the strongest lateral heterogeneities are found in the proximity of tectonically active areas such as collisional belts high s wave velocities below about 70 km denote the presence of deep seated almost aseismic lithospheric roots under the western alps the north central apennines and the cordillera betica an alternative explanation of the data related to mechanisms of intermediate and deep focus earthquakes in the tyrrhenian sea and aegean sea is discussed in the frame of the dynamic subduction model proposed by freund et al 1980 which postulates that the slab is disrupted in blocks and pulled down gravitationally

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

rheological control of wadati benioff zone seismicity intermediate and deep focus earthquakes in wadati benioff zones are thought to occur in the cold interiors of downgoing slabs which are significantly stronger than the warmer mantle given that earthquakes in oceanic lithosphere appear restricted by an isotherm and hence a given value of lithospheric strength we investigate whether a similar formulation is useful for subducting plates strength in downgoing slabs should be affected by both pressure and temperature an effect previously treated using a depth dependent limiting temperature for seismicity wortel 1982 we find this limiting temperature implies that a possible limiting strength increases strongly with depth unless either the temperatures were too low or the activation volume too large comparison of the analytic model used by wortel with numerical thermal models appears to exclude the first possibility we explore the second possibility by using the numerical thermal model to compute strength contours for flow law constants reported from laboratory experiments and find that the expected pressure strengthening is large enough that the slab should have considerable strength well below the deepest seismicity we conclude that if laboratory results are applicable to these conditions either a strongly depth dependent limiting strength exists or factors in addition to strength control the distribution of subduction zone earthquakes

simultaneous travel time inversion for earthquake location and subduction zone structure in the central aleutian islands a combined location and velocity inversion technique is applied to travel time data from 151 well recorded central aleutian earthquakes the data include p and s wave arrivals at stations of a local network and p pp and sp wave arrivals at teleseismic stations after correction for crustal structure at the reflection points the depth phases pp and sp provide important constraints on subduction zone structure not ordinarily resolved by other types of data the structure is assumed to vary only across the arc and in depth it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section to avoid ray tracing it is assumed that the velocity part of the problem is linear preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab a slab thickness of 80 100 km and a downdip length of about 400 km well below the deepest seismic activity are indicated the slab is characterized by seismic velocities as much as 11 higher than the surrounding mantle in its upper portions and 4 6 higher at depth a sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab the slab anomaly appears to spread out and fall off very slowly with depth these are probably not real effects but consequences of the omission of three dimensional ray tracing

earthquake generating stresses in a descending slab abstract a numerical study of stress within descending slabs beneath island arcs provides a general interpretation for the occurrence of deep and intermediate focus earthquakes the computation of stress due to several factors which have been proposed for the interpretation of the double planed seismicity of intermediate depth events shows that thermal expansion and olivine spinel phase change play an important role in earthquake generation the stress field due to the combination of these two causes gives the following results in the depth range 50 300 km the compressional and tensional stress predominates in the upper and central part of the slab respectively maximum shear stress amounting to 1 6 gpa in compressional and 0 4 gpa in tensional fields occurs at a depth of 200 km the stress is generally very small at around 300 350 km below 400 km compressional stress exists at the central part of the slab and tensional stress appears near the upper and lower surfaces of the slab maximum shear stress is 2 8 gpa in compression and 1 6 gpa in tension at a depth of 500 km the principal axes in these high stress fields align almost parallel to the dip direction of the slab these stress features explain well such observed seismological facts beneath island arcs as the seismicity depth relationship planer configuration of seismic zone existence or non existence of double planed seismicity and focal mechanisms of intermediate and deep focus earthquakes

the anisotropy of magnetic susceptibility ams in low grade cleaved pelitic rocks influence of cleavage bedding angle and type and relative orientation of magnetic carriers

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

structural analysis of the adjacent acadian and variscan fold belts in belgium and northern france from geophysical and geological evidence abstract two palaeozoic fold belts are juxtaposed in belgium and northern france the northern belt of the brabant massif traditionally considered a part of the european caledonides was folded during the acadian orogeny in the early devonian although it is largely concealed gravity and magnetic maps give a general picture of the existing structures a curved gravity trend is interpreted as an arc shaped succession of granitic batholiths structures north and south of this arc are completely different the northern subdomain containing magnetic lower cambrian metasediments shows multiple thrust faults towards the south over the granites whereas the southern subdomain without magnetic rocks is moulded around the granitic core the southern variscan belt is characterized in general by thrusting with a north nw and ne vergence during the variscan orogeny the brabant massif acted as an obstacle to the general transport of the ardennes massif to the n nw the oblique convergence with the brabant massif caused differential displacement towards the northwest the western areas having travelled farther as seen from a pattern of wnw oriented strike slip faults

the p velocity within the tonga benioff zone determined from traced rays and observations p waves with travel time residuals between 0 s and 12 s are observed at regional stations in samoa afi and raoul island rao for 39 earthquakes in tonga with focal depths between 70 km and 300 km these anomalously large residuals apparently are produced because seismic phases travel along the strike of the tonga benioff zone within the high velocity subducted lithosphere for up to 1200 km before arriving at afi and rao to eliminate erroneous residuals caused by poor event locations we selected 11 stations and reread the available p times at these stations for the 39 events these arrivals and pp p intervals were used to relocate the events using a variant of the joint hypocenter determination method then the pattern of residuals at afi and rao not used in the relocation was compared to the pattern of residuals expected for various models of the subducted lithosphere as determined by ray tracing the observed pattern of residuals at afi is consistent with the ray traced models if some of the first arrivals are produced by rays traveling directly along the strike of the subducting lithosphere and if others are produced by rays which reflect once off the upper surface of the subducting lithosphere before arriving at afi the observed residuals can be explained by a model where the p velocity in the subducted lithosphere is 8 higher than the velocity in the herrin model the residuals are fit even better by a layered slab model in which the seismic velocity is about 6 higher than the herrin velocity at the upper surface of the subducted lithosphere and about 9 higher at the bottom of the slab these velocity contrasts could be produced if the temperature in the slab was 700 c cooler than the surrounding mantle and if there were no partially melted material within the slab the existence of these anomalously large residuals suggests that the high velocity region in the upper 300 km of the mantle beneath tonga must be fairly continuous over distances of 1000 km and more

relationship of deep seismicity to the thermal structure of subducted lithosphere recent experimental work on silicate olivine polymorphs1 has confirmed earlier observations of mechanical failure in analogous compounds2 3 as a result of phase transformations when non hydrostatic stresses were applied to a metastable phase this transformational faulting 4 mechanism is considered to be a leading candidate5 among others involving olivine transformation6 7 for the cause of deep earthquakes evidence consistent with this mechanism comes from the observation2 4 that worldwide earthquakes become more numerous with increasing depth after a seismicity minimum at about 350 km depth8 this depth lies within the range anticipated for mineralogical transformations in the subducted lithosphere9 but individual subduction zones differ in their thermal structures so if transformational faulting indeed contributes to deep seismicity this should be reflected in the location of the seismicity minimum for each zone the depth at which the minimum occurs should depend on temperature in the same way as do the polymorphic transformations of olivine and should always lie deeper than the depth at which the equilibrium transformation takes place here we test these predictions for eight subduction zones worldwide we find that with one exception the north japan zone the depth of the seismicity minimum decreases linearly with increasing thermal age of the slab a measure of its temperature profile these results support the proposal that deep earthquakes are a consequence of phase transformations in olivine

subduction zone calibration and teleseismic relocation of thrust zone events in the central aleutian islands abstract teleseismically determined epicenters for thrust zone earthquakes in the central aleutian islands are systematically mislocated by about 40 km to the north relative to local network solutions we attribute the mislocation to travel time anomalies produced by the higher p wave velocity in the subducting slab these travel time anomalies can be estimated by seismic ray tracing through a thermally modeled velocity structure for the slab observed teleseismic station residuals and slab travel time anomalies computed from local network hypocenters plotted as a function of event distance to the center of curvature of the island arc show good agreement this distance versus residual relationship can be used to validate the local network hypocenters and to constrain the length of subducting lithosphere computed slab source corrections when incorporated in a standard location program reduce teleseismic mislocations to 20 km the addition of empirical station corrections reduces the mislocation to about 10 to 15 km depth and origin time estimates can be further improved through use of better station corrections a local station within 2 and depth phases

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

fluid flow alteration and polysulphide mineralisation associated with a low angle reverse shear zone in the lower palaeozoic of the anglo brabant fold belt belgium abstract in the lower palaeozoic rocks of the brabant massif belgium a recently discovered polysulphide mineralisation is related to a low angle reverse shear zone this shear zone has been attributed to the main early devonian deformation event data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification in addition chloritisation is associated with this alteration the alteration zone may reach a thickness of 250 m ore mineralisation occurred synkinematically and is spatially related to the shear zone the mineralisation consists of pyrite marcasite arsenopyrite pyrrhotite chalcopyrite sphalerite galena stibnite and smaller amounts of tetrahedrite and other sulphosalts it is concentrated in quartz sulphide veins or occurs diffusely in the host rock the mineralising fluids have a low salinity h 2 o co 2 ch 4 nacl kcl composition and a minimum temperature of 250 320 c the 18 o values of quartz vary between 12 3 and 14 5 smow and d compositions of the fluid inclusions in the quartz crystals range from 65 to 35 v smow the d and the calculated 18 o values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids the 34 s values between 4 7 and 10 6 cdt fall outside the interval typical for i type magmas important migration of likely metamorphic fluids causing a widespread alteration and a polysulphide mineralisation along a low angle shear zone has thus been identified for the first time in the caledonian anglo brabant fold belt

arcuate fold and cleavage patterns in the southeastern part of the anglo brabant fold belt belgium tectonic implications abstract the brabant massif belgium is situated at the southeastern extremity of the anglo brabant fold belt the predominantly concealed eastern branch of the british belgian caledonides in contrast to the other parts of this caledonian slate belt the deformation history in the brabant massif has to date never been constrained properly in this paper a synthesis is presented of the structural observations in the limited outcrop area of the brabant massif in particular with respect to cleavage and fold trajectories and mutual cleavage fold relationships arcuate fold and cleavage patterns as well as the presence of cleavage transected folds are demonstrated similar to the other parts of the belt moreover the arcuate trajectories are considered to reflect the trace of the caledonian deformation front once obscured by the variscan ardenne allochthon combined with interpretations of the aeromagnetic and bouguer anomaly maps an overall kinematic model is proposed for the acadian deformation event in the brabant massif with the proposed model calling upon processes of indentation by promontories and lateral escape of lithostructural domains as well as on the control exerted by rigid granitoid basement blocks the strong similarities with the other parts of the british belgian caledonides become apparent by defining an overall shortening direction and proposing a trace for the caledonian deformation front in this southeastern extremity of the anglo brabant fold belt an integrated tectonic image is obtained for the entire british belgian caledonides

seismic constraints on the morphology of deep slabs residual sphere images from deep earthquakes not only detect the presence of slab associated velocity anomalies but also lend insight into the flow and deformation of lithosphere subducted into the lower mantle we have compared travel times from deep events in the kuril and mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth we assume that the details of the deformation whether the slab buckles imbricates fragments etc are averaged out along the ray paths and hence our models constrain the scale not the mode of slab thickening the deep event travel times are best fit by undeformed models but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects although the mariana times are consistent with advective thickening factors of 5 or more factors larger than 3 are ruled out by the kuril data for all models examined the data require that slab material extends to depths of 900 1000 km global tomographic models and regional studies which delineate high velocity anomalies in the lower mantle beneath zones of cenozoic subduction are consistent with our results as is recent work on pulse distortion by slab gradients comparison of observed and predicted rates of seismic moment release suggests that if substantial advective thickening does occur it is largely aseismic

three dimensional numerical modeling of earthquake migration along a northwestern pacific subduction slab three dimensional numerical modeling has been performed to account for possible earthquake migration phenomena along a northwestern pacific subduction slab on the assumption that the lithosphere is a linear visco elastic body but the asthenosphere and the mesosphere respond to the applied stress as non linear newtonian materials obeying the power law of creep on the basis of a three dimensional thermal structure of the northwestern pacific region the preliminarily computed stress regimes show that 1 to induce a stick slip type trench event prefailures at a depth of about 40 km in the front of the locked zone between two lithospheric plates are required 2 the compositional density contrast due to phase change at typical depths is prerequisite of stress concentration to cause a dislocation type deep focus earthquake and 3 the co seismic change in gravitational potential is a controlling factor for earthquake migration downward along the subduction slab

gravity anomaly map of north america the gravity anomaly map of north america is the product of a 12 year international effort to compile critically edit and merge gravity anomaly data on a continental and global scale this color pixel map printed on four quadrant sheets at a scale of 1 5 000 000 and including a fifth sheet showing a color index map with data references is the first such map at this large scale to include several hundreds of thousands of precise surface data of the united states canada mexico and central america as well as other high quality surface data from neighboring continental and oceanic areas the map which shows bouguer gravity anomalies on land and free air gravity anomalies over oceans is remarkable for its detail sixty six colors or shadings have been used in a carefully conceived nonlinear scheme to show anomalies at a 5 or 10 mgal interval over a dynamic range from about 300 mgal to 130 mgal

phase transition buoyancy contributions to stresses in subducting lithosphere the sequence of phase transitions undergone by minerals with increasing depth in earth s mantle is perturbed within subducting lithospheric slabs by their thermal structure such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab while other factors contribute to overall slab stresses thermal and phase transition effects largely control the structure of the stress field within the slab the resulting maximum in down dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity furthermore metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity these observations are independent of the variety of failure mechanisms proposed for deep seismogenesis

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

sedimentological thickness variations within silurian mudstone dominated turbidite deposits and the effects on cleavage fanning anglo brabant deformation belt belgium the effect of vertical changes in lithology on cleavage refraction and cleavage fanning is relatively well understood in contrast the control that lateral changes in bed thickness and related multilayer characteristics have on cleavage fanning has not been widely documented mudstone dominated wenlock age turbidites of the anglo brabant deformation belt belgium exhibit pronounced lateral thickness changes which we attribute to intraslope flow ponding during foreland basin development the mudstone dominated nature of the turbidites is considered to reflect a particularly fine grained source area rather than a distal origin formation boundaries and lateral changes in lithofacies unit thickness are reflected in the amount of cleavage fanning the degree of convergent cleavage fanning increases with an increase in thickness of the less competent units and with a decrease in thickness and number of the more competent units as such a detailed analysis of changes in cleavage bedding angle aids in the distinction of different lithostratigraphic units of similar appearance the location of their boundaries and identification of subtle lateral sedimentological changes the utilization of cleavage and bedding data to recognize and describe vertical and lateral changes in a semi quantitative way allows the linkage of subtle changes in multilayer rheology to finite strain trajectories supplementary materials details of position of lithological profiles are available at www geolsoc org uk sup18708

model calculations of regional network locations for earthquakes in subduction zones algorithms for locating earthquakes recorded by local networks commonly assume a flat layered velocity structure even in island arc regions where the presence of a dipping lithospheric slab introduces large lateral velocity variations moreover because seismic stations occur along the narrow island arc the resulting linear network geometry degrades earthquake location to model this situation we trace rays in a spherical earth from hypothetical earthquakes through appropriate velocity models to networks of various geometries to isolate the effect of the slab most of the velocity models consist simply of a planar dipping constant velocity region embedded in a sphere with a constant lower seismic velocity we then use the calculated p and s travel times to relocate the hypothetical earthquakes with a flat layered velocity structure we perform calculations of this kind in a variety of models varying the dip of the slab the velocity contrast between the slab and the mantle the station geometry and the distance between the earthquakes and the slab mantle boundary we also perform calculations in more realistic models with geometry closely resembling two seismic networks in alaska these model calculations emphasize the great care that seismologists should exercise when interpreting local network locations in island arc regions in the present study all the relocations incorporate p and s arrivals at all stations and include no random or picking errors nevertheless the relocated hypocenters differ from the actual model hypocenters by up to 100 km or more and produce significant distortions in the wadati benioff zone the most prominent spurious feature is a clear increase in the apparent dip of the wadati benioff zone beneath a certain depth furthermore the relocation reduces the width or apparent thickness of the wadati benioff zone e g they produce an apparent thickness of 5 km or smaller for a zone of earthquakes with an actual thickness of 20 km however if different combinations of stations report different events it is possible to determine an apparent thickness which exceeds the actual thickness for local networks near subduction zones it is useful to estimate the mislocation and rms residual expected for each event recorded and located this can be accomplished by performing model calculations in a simple model with a constant velocity dipping slab embedded in a half space for such a simple model there exists only one ray path from each earthquake within the slab to each station and the travel time for this path can be calculated as a part of the routine network location process a relocation using the routine network location algorithm and these calculated travel times provides an indication of the expected mislocation and rms residual for the actual recorded event

observations on earthquake stress axes and seismic morphology of deep slabs summary focal mechanism solutions from centroid moment tensor cmt and other studies in the nw pacific and tonga kermadec regions are analysed in association with velocity images from tomographic inversions deep compression axes and fast velocity slab anomalies are usually in consistent alignment even when the slab is contorted or flattened the down dip direction for each event is given based on slab geometry interpreted from velocity anomalies and seismicity the well known pattern of tension to compression down dip stress by isacks molnar is seen and fitted with a tangent curve which gives the location of the neutral down dip stress zone ndsz an abnormal pattern of ndsz exists beneath major junctions of the arcs the ndsz is generally in the depth range of 100 250 km in the central parts of the arcs while it is deeper in the mariana region the depth seems to depend on the dip and topology of the slab but the ndsz probably starts at more or less the same subducted length along the slab

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

differences between magnetic and mineral fabrics in low grade cleaved siliciclastic pelites a case study from the anglo brabant deformation belt belgium abstract theoretically within a given pelitic rock in which the main paramagnetic carriers are white mica and chlorite and in which judging from the ams parameters ams is controlled by the paramagnetic carriers the results of x ray pole figure goniometry should show a qualitative relationship with ams this idea is tested on single phase deformed low grade pelites of the lower palaeozoic brabant massif belgium representing the southeastern part of the anglo brabant deformation belt for the vast majority of the samples investigated the different lf ams parameters suggest a paramagnetic control on lf ams different parameters of lf ams measured at 77 k low t lf ams and a comparison of these with the lf ams parameters indicate that lf ams is controlled virtually entirely by paramagnetic carriers high field torque magnetometry hf ams indicates further that ams is controlled entirely by paramagnetic carriers for the vast majority of the samples two apparent discrepancies are observed however these are 1 a frequent mismatch between the lf ams fabric on the one hand and the paramagnetic fabric indicated by low t lf ams and hf ams on the other hand and 2 a rather common mismatch between the preferred orientation of chlorite and white mica suggested by x ray pole figure goniometry on the one hand and the lf ams fabric low t lf ams fabric and hf ams paramagnetic fabric on the other hand these apparent discrepancies may partly be attributed to the composite nature of both the lf ams fabric and the paramagnetic susceptibility fabric the presence of different populations of paramagnetic carriers possibly in combination with minor traces of ferromagnetic s l carriers which are not picked up by the methods applied may explain some of the mismatches observed and are compatible with the basin evolution history of the brabant massif in addition it is possible that the low temperatures low t lf ams and high fields hf ams are capable of enhancing one of the paramagnetic phases thus leading to discrepancies between methods that define the mineral and the magnetic fabrics the results of this study show that even when the lf ams data suggest that the lf ams of a given pelitic rock in which phyllosilicates form the main paramagnetic carriers is controlled entirely by paramagnetic carriers alternative methods such as phyllosilicate x ray pole figure goniometry will not necessarily reflect the paramagnetic carrier orientation although a number of cases have successfully correlated lf ams with mineral fabric and occasionally even strain this study serves as an example for how methods to define mineral and magnetic fabrics can be sensitive in detecting different paramagnetic phases

the waveguide effect of metastable olivine in slabs we use a finite difference algorithm to compute p sv synthetic seismograms for deep earthquakes recorded at regional seismic stations located above a subducting slab we calculate synthetics for 2 d slab models in which the 410 km discontinuity is uplifted owing to the exothermic nature of the transition the equilibrium model and models in which the 410 km discontinuity is depressed owing to inhibited kinetics of the olivine transition the metastable model in the latter case the existence of the low velocity wedge of metastable olivine has a profound influence on the waveforms of deep focus earthquakes the wedge acts as a geometrical waveguide with the energy focusing effect apparent even after the wavefront has traveled the several hundred kilometers from the wedge boundary to the earth s surface the guided energy is most pronounced for receivers located near the surface projection of the deep slab but is observable over a range of 300 km and so may provide a valuable diagnostic for discriminating between equilibrium and metastable models of subduction a recent search for such guided energy using regionally recorded p and s waves from deep events in tonga yielded negative results

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

mapping the tonga slab we applied residual sphere analysis to 25 intermediate and deep focus earthquakes in the tonga subduction zone and mapped variations in slab strike and dip depth extent and deep lateral advection at all latitudes along the arc as in the northwest pacific travel time data from intermediate and deep focus events are well modeled by simple thermal slab structures and a thermal coefficient of velocity p t of 0 5 0 2 m s k i e peak velocity perturbations of about 3 7 in all regions where the seismicity extends to depths of 600 km or more the shortest thermal slab models that cannot be rejected at the 90 confidence level reach depths of 750 km northern tonga to 850 km southern tonga these models however require a value of p t 1 5 m s k 3 times the value suggested by laboratory studies if the magnitude of p t is constrained to be less than 1 0 m s k the minimum penetration depth varies from 800 km to 900 km the travel time data are equally consistent with slab structures that deform at the base of the upper mantle thickening by a factor of 3 and models that contain no advective thickening inferred slab structure varies considerably throughout the subduction zone and the nearly ubiquitous match between the observed seismicity and detailed undulations in slab strike and dip is evidence that residual spheres do reflect near source heterogeneity travel times in southern tonga 21 5 s 27 s are consistent with and in some cases require slab models that bend to a steeper dip at depth this phenomenon is also observed in parts of the northwest pacific but is not required in central 19 s 21 5 s and northern 13 s 19 s tonga the residual spheres of both intermediate and deep focus events suggest a sharp bend in slab strike at 25 s this feature coincides with a kink in strike observed at all depths in the seismicity and occurs at the intersection of the trench and the louisville ridge northern tonga is the locus of considerable complexity in deep slab structure seismicity strain rate and travel time data are consistent with although they do not uniquely require a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes to the north at shallower depths and in the lower mantle the contorted segment would have begun subducting roughly 10 m y ago during a major plate reorganization in northernmost tonga a decrease in the maximum depth of seismicity and inferred slab structure provides evidence for southward shear of deep material relative to the shallow slab compared to data from the northwest pacific where travel time observations also suggest slab penetration below the deepest earthquakes tonga seismicity and residual spheres show more complexity at depth and may indicate more deformation internal to the slab these observations argue against convective models which are rigorously stratified at 650 km depth but they are consistent with a discontinuity in mantle properties such as an increase in viscosity or a small percent variation in chemistry if it merely resists and deforms the descending slabs without acting as a complete barrier to flow