models for midcontinent tectonism an update during the past decade significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical geologic data base which permits an integration of seismicity data crustal structures and current stress directions critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns the direction of the maximum horizontal compressive stress is generally directed east northeast which supports an origin associated with ridge push forces related to movement of the north american plate crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data these data have been used to relate current seismicity and associated tectonism to one of two models increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field the zone of weakness model as the dominant control on contemporary tectonism and the local basement inhomogeneity model as a mechanism for minor low energy release earthquake activity

stress field in the subducting lithosphere and comparison with deep earthquakes in tonga 1 we present a numerical model of the subducting lithosphere that provides an alternative explanation for stresses causing deep earthquakes our model lithosphere is composed of olivine spinel spinel and perovskite magnesiowustite the heat conduction equation is solved to determine temperature conditions in the slab and to locate the equilibrium phase transformations in pressure temperature space volumetric strains in the subducting lithosphere are calculated from the density of individual phases and from the heat released or consumed in the phase changes these strains are used as sources of stress in the subducting lithosphere dislocation creep and peierls stress creep laws are included in the viscoelastic rheology volumetric reductions due to equilibrium phase transformations cause high shear stress in the transition zone because of the variable viscosity inside the subducting slab aspects of the model shear stresses are in agreement with observations of high seismic activity in the tonga wadati benioff zone compression is oriented along the dip of the slab and extension is oriented in the plane perpendicular to the compression axis since our model stresses agree with the seismic observations and because the model stresses are larger than those caused by buoyancy forces our model provides a possible explanation of stresses causing deep earthquakes also our model does not need metastability of olivine to explain the occurrence of high shear stress in the transition zone

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

estimation of ground motion for bhuj 26 january 2001 mw 7 6 and for future earthquakes in india only five moderate and large earthquakes mw 5 7 in india three in the indian shield region and two in the himalayan arc region have given rise to multiple strong ground motion recordings near source data are available for only two of these events the bhuj earthquake mw 7 6 which occurred in the shield region gave rise to useful recordings at distances exceeding 550 km because of the scarcity of the data we use the stochastic method to estimate ground motions we assume that 1 s waves dominate at r 100 km and lg waves at r 100 km 2 q 508f 0 48 is valid for the indian shield as well as the himalayan arc region 3 the effective duration is given by fc 1 0 05r where fc is the corner frequency and r is the hypocentral distance in kilometer and 4 the acceleration spectra are sharply cut off beyond 35 hz we use two finite source stochastic models one is an approximate model that reduces to the x 2 source model at distances greater that about twice the source dimension this model has the advantage that the ground motion is controlled by the familiar stress parameter dr in the other finite source model which is more reliable for near source ground motion estimation the high frequency radiation is controlled by the strength factor sfact a quantity that is phys ically related to the maximum slip rate on the fault we estimate dr needed to fit the observed amax and vmax data of each earthquake which are mostly in the far field the corresponding sfact is obtained by requiring that the predicted curves from the two models match each other in the far field up to a distance of about 500 km the results show 1 the dr that explains amax data for shield events may be a function of depth increasing from 50 bars at 10 km to 400 bars at 36 km the corresponding sfact values range from 1 0 2 0 the dr values for the two himalayan arc events are 75 and 150 bars sfact 1 0 and 1 4 2 the dr required to explain vmax data is roughly half the corresponding value for amax while the same sfact explains both sets of data 3 the available far field amax and vmax data for the bhuj mainshock are well explained by dr 200 and 100 bars respec tively or equivalently by sfact 1 4 the predicted amax and vmax in the epi central region of this earthquake are 0 80 to 0 95 g and 40 to 55 cm sec respectively

effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

passive seismological imaging of the narmada paleo rift central india abstract the narmada son lineament nsl is a prominent nearly e w trending tectonic feature dividing india into the peninsular and extra peninsular regions it marks the possible suture between two contrasting geological settings termed proto continents lying to the north and south of it various views prevail about the nature of the rift zone the present study delineates the seismic structure of the crust using p wave receiver functions at 10 broadband seismic stations installed along a profile across the nsl the results from common conversion point ccp stacking reveal a near continuous image of the moho boundary with the amplitudes of the ps conversions from the moho being weaker at stations within the rift modelling the receiver functions employing nearest neighborhood algorithm approach brings out a distinct 15 km thickening of the crust and a complex nature of the moho within the rifted zone we find that the backazimuthal variations of the sv and sh receiver functions at the permanent station jabalpur can be explained better by introducing azimuthal anisotropy in the middle and lower crust with the fast axes perpendicular to the rift axis also we find substantial evidence of post rifting magmatic underplating in the lower crust which could be reminiscence of deccan volcanism and or reunion plume path

deep lower crustal earthquakes in central india inferences from analysis of regional broadband data of the 1997 may 21 jabalpur earthquake summary the only instance of a confirmed deep lower crustal earthquake occurrence in the indian shield region has been that of the 1938 satpura earthquake m 6 3 of central india reportedly at a depth of about 40 km moment tensor inversion of regional broadband waveform data of the 1997 may 21 jabalpur earthquake mw 5 7 confirms yet another such earthquake at about 35 km depth in the central part of the narmada son lineament nsl zone the study is based on a refined velocity model obtained using a traveltime grid search method a reverse fault mechanism is obtained which for a palaeo rift valley zone indicates the possibility of reactivation of a pre existing fault under the influence of the ambient stress field due to the india eurasia plate collision forces the occurrence of earthquakes at lower crustal depths quite unusual for the indian shield region indicates a possible causative mechanism related to crust mantle interaction based on the close proximity of the two deep earthquakes and their disposition with respect to the local trend of the central part of the nsl we suggest a model of stress accumulation due to horizontally elongated or elliptical possibly serpentinized mafic intrusives in the lower crust to explain the occurrence of deep earthquakes in the heart of the indian shield

stress distribution within subducting slabs and their deformation in the transition zone abstract the mechanical behavior of slabs in the mantle transition zone is a poorly understood phenomenon to obtain a better understanding of the stress and deformation pattern inside subducting slabs we performed 2d numerical simulations of the subduction process in a purely viscous non linear rheological model including diffusion creep dislocation creep and a stress limiting mechanism the model calculations are used to investigate the effects of yield stress plate age and surface boundary condition free slip versus given plate velocity on the stress development in and around a subducting slab and on the deformation of the subducting plate in the transition zone the deformation and morphology of subducted slabs may differ between free slip and kinematic boundary condition models in some cases especially for young and relatively weak slabs even though kinematically driven subduction morphology may look realistic in most cases our results show that stress distributions can differ significantly if the discrepancy between free slip and kinematic subduction velocity is large and in some model cases even influence the ability of the plate to penetrate into the lower mantle stress distributions such as the ones presented in this study can be a valuable tool in understanding the complex and enigmatic interaction between subducting slabs and the transition zone

a comparison of seismic and structural measurements of scaling exponents during tensile subcritical crack growth abstract the observed fractal nature of both fault length distributions and earthquake magnitude frequency distributions suggests that there may be a relationship between the structure of active fault systems and the resulting seismicity in previous theoretical work a positive correlation between the exponent d from the fracture length distribution and the seismic or acoustic emission ae b value has been inferred from a simple dislocation model of the seismic source here we present the first experimental evidence for a correlation between d and b from a series of tensile fracture mechanics tests on crystalline rock carried out in different environmental conditions both air dry and water saturated and at ambient temperature and pressure the microseismic acoustic emissions were monitored during subcritical crack growth under controlled conditions of constant stress intensity k i and quantitative analyses of the resulting fracture patterns were carried out on the same specimens it is found that ae b values ranging from 1 0 to 2 3 correlate negatively with the normalized stress intensity k i k ic where k ic is the fracture toughness of the specimen the microcrack length distribution exponent d ranges from 1 0 to 1 7 fluid presence has a first order influence on both the ae and structure produced in these experiments for experiments at low stress intensity or high fluid content the activation of the stress corrosion mechanism for k i k ic leads to a greater relative proportion both of small cracks and of low amplitude acoustic emissions reflected in higher values of d and b the exponent d is found to correlate positively with the ae b value

crustal and upper mantle structure of peninsular india and source parameters of the 21 may 1997 jabalpur earthquake mw 5 8 results from a new regional broadband network abstract in the past two years a permanent network of 10 broadband seismographs has become operational in the indian peninsular shield region in this article we analyze the data recorded by this network with two goals 1 to lay ground for a quick estimation of source parameters of future earthquakes in the region and 2 to estimate the source parameters of the destructive jabalpur earthquake of 21 may 1997 mw 5 8 which occurred within the network toward these goals we measure rayleigh and love wave group velocity dispersion curves in the period range of 10 to 75 sec and invert these curves to estimate the crustal and the upper mantle structure below the indian shield region our best model consists of a two layer crust the upper layer is 14 km thick with a shear wave velocity v s of 3 55 km sec the corresponding values for the lower layer are 25 km and 3 85 km sec v s for the upper mantle is 4 65 km sec based on this structure we perform a moment tensor mt inversion of the bandpassed 0 05 0 02 hz seismograms of the jabalpur earthquake the best fit is obtained for a source located at a depth of 42 km with a seismic moment m 0 of 7 7 10 24 dyne cm and a focal mechanism with strike 61 dip 64 and rake 74 these source parameters are similar to those previously reported in the literature demonstrating that a routine mt inversion of moderate and large indian penisular earthquakes is now possible a careful examination of the seismograms and modeling of the two closest records reveal that the jabalpur earthquake nucleated in the lower crust at a depth of 36 km it consisted of two subevents separated in time by 0 65 sec with the second subevent located 2 km above the first one the total source duration was 1 4 sec

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

practicalities of extrapolating one dimensional fault and fracture size frequency distributions to higher dimensional samples previously published theory which extrapolates fault and fracture population statistics observed in a one dimensional sample to two and three dimensional populations is found to be of limited value in practical applications we demonstrate how significant the discrepancies may be and how they arise there are two main sources for the discrepancies 1 deviations from ideal spatial uniformity spatial poisson process of a fault or fracture pattern and 2 non power law scaling of the size frequency distributions of the population we show that even small fluctuations in spatial density combined with variance in the estimator of population statistics can lead to considerable deviations from the theoretical predictions ambiguity about power law scaling or otherwise of the underlying population is a typical characteristic of natural data sets and we demonstrate how this can affect the extrapolation of one dimensional data to higher dimensions in addition we present new theoretical approaches to the problem of extrapolation when clustering of faults and fractures is explicitly considered clustering is commonly observed in the field as en echelon arrays of fault or fracture segments and we show how this property of natural patterns can be quantified and included in the theory these results are relevant to building more realistic three dimensional models of the physical properties of fractured rocks such as fracture permeability and seismic anisotropy

crustal density structure across the central indian shear zone from gravity data abstract the central indian shear zone is a distinct tectonic feature in the central part of india that separates the central indian tectonic zone from the bastar craton the complete bouguer anomaly map of the region encompassing central indian shear zone is characterised by a broad relative gravity high over central indian tectonic zone as compared to the bastar craton the paired gravity anomaly across the two crustal domains signifies that the central indian shear zone is a locus of density discontinuity along which the two crustal domains accreted horizontal gravity gradient analysis further demonstrates the northward subduction of the bastar craton beneath the bundelkhand craton 2 d gravity modelling along mungwani rajnandgaon profile constrained by seoni kalimati seismic section delineates a thick crust beneath the bastar craton as compared to the central indian tectonic zone northward dipping contact of the two crustal domains when projected on the surface coincides with the central indian shear zone with a well defined moho offset and crustal density discontinuity central indian shear zone represents a suture zone that separates the bastar craton from the central indian tectonic zone a low velocity 6 4 km s density 2 90 g cm 3 layer at the base of the crust and relatively lower density 3 21 g cm 3 subcrustal mantle may be the imprint of thermal remobilization beneath the central indian tectonic zone

correlation analysis of fracture arrangement in space abstract we present new techniques that overcome limitations of standard approaches to documenting spatial arrangement the new techniques directly quantify spatial arrangement by normalizing to expected values for randomly arranged fractures the techniques differ in terms of computational intensity robustness of results ability to detect anti correlation and use of fracture size data variation of spatial arrangement across a broad range of length scales facilitates distinguishing clustered and periodic arrangements opposite forms of organization from random arrangements moreover self organized arrangements can be distinguished from arrangements due to extrinsic organization traditional techniques for analysis of fracture spacing are hamstrung because they account neither for the sequence of fracture spacings nor for possible coordination between fracture size and position attributes accounted for by our methods all of the new techniques reveal fractal clustering in a test case of veins or cement filled opening mode fractures in pennsylvanian marble falls limestone the observed arrangement is readily distinguishable from random and periodic arrangements comparison of results that account for fracture size with results that ignore fracture size demonstrates that spatial arrangement is dominated by the sequence of fracture spacings rather than coordination of fracture size with position fracture size and position are not completely independent in this example however because large fractures are more clustered than small fractures both spatial and size organization of veins here probably emerged from fracture interaction during growth the new approaches described here along with freely available software to implement the techniques can be applied with effect to a wide range of structures or indeed many other phenomena such as drilling response where spatial heterogeneity is an issue

deep crustal structure of the indian shield from joint inversion of p wave receiver functions and rayleigh wave group velocities implications for precambrian crustal evolution 1 the s wave velocity structure of the crust and uppermost mantle of the indian shield has been investigated by jointly inverting p wave receiver functions and rayleigh wave group velocities at 38 broadband stations in the subcontinent the indian shield is an amalgamation of several terranes of archean and proterozoic age that were partly flooded by deccan trap volcanism during cenozoic times and that make up a natural laboratory for assessing models of precambrian crustal evolution our results reveal significant variations in crustal thickness and deep crustal velocities 45 50 km thick under the archean west dharwar craton and southern granulite terrane with lower crustal velocities around 4 1 km s 32 35 km thick beneath the archean east dharwar and bundelkhand cratons with lower crustal velocities around 3 8 3 9 km s 50 65 km thick under the proterozoic bhandara craton with lower crustal velocities around 4 2 4 3 km s and 55 km thick under the proterozoic aravalli delhi belt with lower crustal velocities around 4 2 km s s velocities in the 4 1 4 3 km s range in the deep crust can be attributed to mafic lithologies suggesting there has been no secular change in the precambrian evolution of the south indian shield moreover pervasive mafic dike swarming throughout the indian shield suggests that the layer of mafic cumulates is 2 5 1 6 ga old and that it delaminated from some archean terranes our interpretation is that mafic underplating of the terranes making up the indian shield occurred in proterozoic times and that a refractory root developed under the archean terranes after the proterozoic event

experimental evidence for different strain regimes of crack populations in a clay model we report results from clay extension experiments used as a model for the evolution of fault populations due to stress interactions at yielding cracks begin to appear and the brittle strain due to them quickly reaches a rate matching the applied stretching rate the crack density number of cracks per unit area initially increases apace then reaches a maximum at a critical strain decreasing thereafter at low strains where the crack population is dilute a power law length distribution is observed which at high strain gradually transitions to an exponential this agrees with fault populations data observed in low and high strain settings these results indicate that fault populations ranging from power law to exponential size frequency distributions reflect the population evolution with increased strain

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

seismic evidence for significant crustal thickening beneath jabalpur earthquake 21 may 1997 source region in narmada son lineament central india 1 we investigate crustal properties beneath the deep crustal 35 km jabalpur earthquake of 21 may 1997 narmada son lineament nsl central india in search of a possible cause of stress accumulation in the region teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests moho downwarp to 52 km across the width of the lineament in contrast with an average 40 km depth elsewhere in addition the crust beneath the nsl has higher vp vs of 1 84 compared to 1 73 in the surrounding suggestive of a high density mafic mass at depth that compensates the crustal root also supported by small topographic variation 200 m across the lineament presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre existing narmada son fault leading to the earthquake in deep crust

mantle circulation models with sequential data assimilation inferring present day mantle structure from plate motion histories data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow such an approach is essential in mantle circulation models where we seek to constrain an unknown initial condition some time in the past and thus cannot hope to use first principles convection calculations to infer the flow history of the mantle one of the most important observables for mantle flow history comes from models of mesozoic and cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle buoyancy forces due to subducted oceanic slabs here we present five mantle circulation models with an assimilated plate motion history spanning the past 120 myr a time period for which reliable plate motion reconstructions are available all models agree well with upper and mid mantle heterogeneity imaged by seismic tomography a simple standard model of whole mantle convection including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation reveals downwellings related to farallon and tethys subduction adding 35 bottom heating from the core has the predictable effect of producing prominent high temperature anomalies and a strong thermal boundary layer at the base of the mantle significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep factor 100 viscosity rise from the upper to the lower mantle results in substantial transition zone heterogeneity enhanced by the effects of trench migration implicit in the assimilated plate motion history an expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km which is influenced by jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the cretaceous this result implies that sequential assimilation of past plate motion models is ineffective in studying the temporal evolution of core mantle boundary heterogeneity and that a method for extrapolating present day information backwards in time is required for short time periods of the order of perhaps a few tens of myr such a method exists in the form of crude backward convection calculations for longer time periods of the order of a mantle overturn a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model

the influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity results of high resolution seismic tomography showing subducting slabs deflected in the transition zone and thickened in the lower mantle seem to call for slab material weaker than inferred from mineral physics deformation mechanisms a possible mechanism suggested by several authors could be the weakening due to grain size reduction which should occur in the cold portion of fast slabs after an exothermic phase transition at a depth of 400 km since the amount of weakening as well as the rate of subsequent strengthening due to the grain growth are not precisely known we present here a parametric study of slab behavior in the transition zone and upper part of the lower mantle we simulate a subducting slab in a two dimensional 2 d cartesian box in the numerical model with composite rheology including diffusion creep dislocation creep and a general stress limiting rheology approximating peierl s creep we concentrate on two rheologic effects the dynamic effect of slab weakening due to grain size reduction at the phase boundary and the effect of yield stress of stress limiting rheology the effect of trench migration on slab deformation is also included in our study results show that the slab ability to penetrate into the lower mantle is not significantly affected by a trench retreat in the absence of grain size weakening however in case of a 4 cm yr trench retreat grain size weakening provides a viable mechanism to deflect the slab in the transition zone provided that stress limiting deformation mechanism would limit the effective viscosity outside the areas of grain size weakening to about 10 24 pa s 2002 elsevier science b v all rights reserved

a review of crust and upper mantle structure beneath the indian subcontinent this study has been supported by a grant from the ministry of earth sciences moes iitkgp ckh

magnetotelluric studies in the central india tectonic zone implications for intraplate stress regimes and generation of shallow earthquakes abstract the central india tectonic zone citz dissects the indian peninsula into the northern and southern crustal blocks the citz has been a seismically active region since the precambrian whereas the relatively deep crustal earthquakes near the narmada faults in the eastern part of the citz have been well investigated the mechanisms for the shallow earthquakes in the western part remain unknown here we present results from a new magnetotelluric study to derive the crustal structure and to understand its implications our data show a thick and highly resistive 500 ohm m crust in the south of tapti river as against a less resistive one in the north these results in conjunction with heat flow values indicate that the crust below the southern part has stable continental cratonic signatures on the northern side of the tapti river we infer the ascent of basaltic magmas from the mantle into the shallow crust and crystallization into layered intrusions these mafic ultramafic bodies could be a potential cause for the shallow earthquakes in the western part of the citz the mafic ultramafic bodies below the crust of citz would locally modify the intraplate stresses which in turn would facilitate the occurrence of earthquakes due to reactivation of pre existing faults thus the large accumulation of strain energy in the deep crust beneath the region can be attributed to the presence of high stress bodies emplaced at depth during the deccan volcanic activity

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

slope of the geoid spectrum and constraints on mantle viscosity stratification spectral analysis of a recently obtained high resolution tomographic model describing the top 1200 km of the mantle shows a power law dependence on the degree for degrees greater than around 10 the spectrum of recent geoid models is also found to decay in a linear fashion with degree on a log log plot we have employed the logarithmic slope of the geoid between degrees 10 and 25 as a constraint on the viscosity structure of the top 1200 km of the mantle the constraint of fitting the geoid slope represents a new and independent approach to the determination of the upper mantle viscosity structure from conducting over one million runs in a monte carlo inversion we have found that there are basically three families of viscosity which can fit the geoid slope they are 1 with a viscosity hill between 660 and 1000 km 2 with a weak viscosity increase at 660 km and 3 with a significant viscosity increase at depths between 820 and 1000 km below 1000 km the viscosity of the lower mantle for all 3 families is larger than that in the upper mantle these results corroborate the complexity of the mantle viscosity profile between 660 and 1200 km which would have important ramifications on flows between the upper and lower mantle

scaling of fault displacements and implications for the estimation of sub seismic strain fault displacement populations have been shown to follow a power law scaling relationship characterized by an exponent d this relationship can be used to make predictions of the sub seismic fault population from data derived from seismic surveys although fault populations exist in three dimensions the use of section data is recommended d values derived from sections can be applied directly to several problems and are also related to the d value for the fault set in higher dimensions accurate determination of d requires proper consideration of the scale range and sample size limitations of available data the most common technique of using a cumulative frequency graph often leads to an upwards bias an iterative correction procedure is proposed discrete frequency methods avoid this bias but as a standard linear interval graph has other associated problems a log interval graph method is preferred simulations of these methods applied to random computer generated samples from power law distributions have been made to examine the accuracy of d values derived from typical data equations to estimate the confidence intervals for these d values have been derived from a synthesis of the results the application of the techniques is shown using fault data measured on seismic sections from the southern north sea and the inner moray firth where local differences in d are shown to be significant there is usually a marked change in structural style fault data are used to make improved estimates of crustal extension b by extrapolating the derived powerlaw relationship a value of 13 1 20 is calculated for the inner moray firth applications predicting the intersection of horizontal wells with large sub seismic faults and quality control of fault interpretation on seismic sections are also described

two dimensional crustal velocity structure along hirapur mandla profile from seismic refraction and wide angle reflection data the seismic velocity structure of the upper and lower crusts along hirapur mandla pro fi le was obtained by 2 d forward modeling of the refraction and wide angle reflection data digitized record sections of the analog data were utilized for delineation of the crustal velocity model across the vindhyan basin and the narmada son lineament by travel time modeling and qualitatively matching the amplitudes of synthetic seismograms with seismic record sections an interface at a depth of 8 km has been traced from wide angle reflection data of shot points 160 and 235 between jabalpur and mandla the wide angle reflection phase from crust mantle boundary is very prominent in all the seismic record sections of shot points 0 40 80 120 and 235 the moho horizon along the entire study area was continuously traced depicting an upwarp between katangi and jabalpur below the narmada son lineament the three faults inferred near narsinghgarh katangi and jabalpur divide the crust into four blocks the number of faults and displacement patterns along them depicted along the profile indicates that block tectonics appear to be active in this region throughout the geological history

amount of extension on small faults an example from the viking graben extension estimates based on palinspastic restoration of only the largest faults in a deformed terrane may be significantly deficient due to neglect of small faults i e unsampled faults most of which have smaller displacements than do sampled faults small faults individually account for little extension but there may be large numbers of them the cumulative extension on small faults can be calculated from the fractal size distributions of fault displacements independent extension estimates support the inference from fault population statistics that small faults account for significant amounts of extension for example restorations of mesozoic faults interpreted from seismic reflection profiles across the viking graben in the north sea yield extension estimates that are only 40 75 of independent estimates the fractal size distribution of fault displacements in the viking graben is consistent with the hypothesis that the remaining 25 60 of extension resulted from displacements on small faults

three dimensional modeling of intraplate stresses in the epicentral zone of the 21 may 1997 jabalpur earthquake of mw 5 8 central india abstract three dimensional intraplate stresses induced by topography and crustal density heterogeneities have been modeled for a region in central india latitude 21 26 n longitude 77 82 e that was affected by the 1927 son valley m w 6 4 and the 1997 jabalpur m w 5 8 earthquakes the modeled stresses show that these two events occurred in the zones of significant increase in gradients of the principal stress difference 1 3 contours the highest value of these differential stresses 90 mpa is obtained at 5 km depth beneath the epicentral zone of 1927 son valley earthquake whereas a value of 35 mpa is obtained at the focal depth 35 km of the 1997 jabalpur mainshock further depth sections of estimated differential stresses delineate a south dipping high stress body penetrating the whole crust probably a mafic intrusive intruded during the deccan volcanism episode beneath a zone bounded by narmada north fault in the north and narmada south fault in the south within which all aftershocks including mainshocks of the 1997 jabalpur and the 1927 son valley have occurred the modeled direction of principal stresses as well as inferred mode of faulting at 5 20 35 and 40 km depths correlates strongly with the long period 20 50 s centroid moment tensor solutions for the broadband recordings of the m w 5 8 jabalpur mainshock results suggest that estimated large intraplate stresses associated with an inferred mafic intrusive in the presence of high pore pressure resulted from the dehydration of the serpentinite minerals and deep brittle ductile transition may contribute to the failure of rock along the pre existing weak narmada south fault leading to the earthquakes in the deep crust it is proposed that the causal association of large intraplate stresses and the pre existing deep crustal faults would control the future locales of earthquakes in the narmada son lineament

fractal relations for the diameter and trace length of disc shaped fractures analytic relations are developed between the fractal parameters of a random isotropic population of disc shaped fractures and the parameters of the corresponding population of fracture traces expressed in outcrop these relations indicate that a fractal distribution of fracture diameters translates to a fractal distribution of trace lengths and that the parameters for diameter may be uniquely determined from the parameters for trace length probabilistic results demonstrate the accuracy of the analytic relations and identify nonideal behaviors at the limits of the computed trace length data it is expected that these relations will be useful in the three dimensional characterization of fracture systems in rock from in situ trace length data

changes in fault displacement populations correlated to linkage between faults in carboniferous strata exposed at the western margin of the southern appalachian u s a fold thrust belt mesoscopic faults accommodated strains as the cumberland plateau thrust sheet moved 1 km wnw on a sub horizontal detachment these faults are confined to a layer 75 m thick above the detachment in the upper part of this 75 m thick layer a laterally persistent coal seam separates two stratigraphic units subjected to different amounts of sub horizontal elongation distinct sets of mesoscopic faults that cut bedding at high angles and have normal offsets accommodate the strains in each unit below the coal seam a sample line 500 m long intersects 504 faults a plot of log n size rank vs log d dip separation in mm for these faults is linear suggesting that a scaling law n ad c holds for this population least squares regression gives c 0 8 and log10 a 3 1 above the coal seam a sample line 250 m long intersects 745 faults a plot of log n vs log d for these faults has two distinct linear trends faults with separations 1 25 m yield a scaling law n ad c with c 0 5 and log10 a 2 9 seventeen faults with separations d 1 25 m define a second linear segment n a d c with c 1 8 and log10 a 6 9 the break in slope at d 1 25 m corresponds to faults with heights of 30 m roughly equal to the thickness of the faulted strata faults here with d 1 25 m and d 1 25 m which exhibit different scaling characteristics are respectively analogous to faults within and those that extend across the brittle crust in regional studies comparing the two displacement populations described here small offset faults make a proportionally smaller contribution to the total strain in the larger strain setting this pattern which is observed elsewhere suggests that fault systems within individual layers exhibit different scaling characteristics at different stages in their history this contribution argues that fault linkage is the key to such changes in fault scaling behavior it is a mesoscopic to macroscopic structural change that alters the subsequent development of faults fault linkage like fault size or sampling dimension should be considered in examinations of fault scaling laws

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

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

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

compressible convection in the earth s mantle a comparison of different approaches numerical models of mantle convection using the boussinesq extended bousinessq and anelastic liquid approximation are compared for steady state solutions there is good quantitative agreement between the results if they are scaled in a proper way time dependent extended boussinesq and anelastic liquid flows show only qualitative agreement the main difference being a distortion of timescale compressibility induces an asymmetry in the structure of upper and lower boundary layers that cannot be observed in boussinesq fluids

aggregate properties of fracture populations abstract empirical studies indicate that the individual attributes of both faults and extension fractures follow power law scaling aggregate properties of fracture populations are important in a variety of problems and can be specified in terms of the scaling parameters of individual fracture attributes development of an expression for an aggregate property requires consideration of a number of independent factors including the topologic dimension of the aggregate property the topologic dimension of sampling and the possibility of scaling changes for fractures that span some mechanically significant layer the riemann zeta function provides an alternative to integration for the analytical and numerical solution of aggregate problems previous work regarding aggregate properties of fracture populations has focused on the strain due to faulting new expressions are developed here for other aggregate properties of interest fracture surface area fracture porosity fracture permeability and shear wave anisotropy a general characteristic of these aggregate properties is that for most values of scaling exponents the aggregate properties are dependent on the size of the sampling domain this implies that the aggregate properties are scale dependent additionally it appears that fracture surface area is concentrated in the smallest fractures of many populations fracture porosity is concentrated in the largest fractures of most populations but not as strongly as fracture permeability which probably derives almost entirely from the largest fractures in populations

crustal velocity inhomogeneities along the hirapur mandla profile central india and its tectonic implications abstract wide angle seismic and gravity data across the narmada son lineament nsl in central india are analyzed to determine crustal structure velocity inhomogeneities and hence constrain the tectonics of the lineament we present the 2 d crustal velocity structure from deep wide angle reflection data by using a ray trace inverse approach the main result of the study is the delineation of fault bounded horst raised to a subsurface depth 1 5 km and the moho upwarp beneath the nsl the crust below the basement consists of three layers with velocities of 6 45 6 7 6 2 6 5 and 6 7 6 95 km s and interface depths of about 5 5 8 7 14 17 and 18 23 km along the profile the low velocity 6 2 6 5 km s layer goes up to a depth of 5 km and becomes the thickest part 13 km while the overlying high velocity 6 45 6 7 km s layer becomes the thinnest 3 km and upper boundary lies at a depth of 1 5 km beneath the nsl the overall uncertainties of various velocity and boundary nodes are of the order of 0 12 km s and 1 40 km respectively the up lifted crustal block and the up warping moho beneath the nsl indicate that the north and south faults bounding the nsl are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap the lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the narmada south fault during the 1997 jabalpur earthquake

effects of depth variations in creep laws on the formation of plates in mantle dynamics we have studied the influences of depth varying rheologies on the structure of mantle convection and the near surface flow fields upwelling flows crossing the upper lower mantle boundary become considerably thinner and faster streamlines are concentrated near the surface and other plate like characteristics are enhanced by this type of rheological stratification in which the averaged effective viscosity of the upper mantle is about two orders of magnitude lower than that of the lower mantle the presence of a lithosphere with a higher non linear dependence in the flow law further promotes plate like behavior plate like character of the top boundary layer is encouraged by a decreasing depth dependent viscosity in the non newtonian upper mantle and by greater convective vigor

thickness composition and evolution of the indian precambrian crust inferred from broadband seismological measurements abstract we present seismological constraints to model the evolution of indian precambrian crust through receiver function analysis of teleseismic waveforms recorded on 22 broadband seismographs along a n s corridor from southern granulite terrain to delhi on the aravalli craton archean terrains eastern dharwar bastar bundelkhand and aravalli cratons are underlain by crust with felsic intermediate composition vp vs 1 65 1 75 in contrast to mafic composition vp vs 1 78 of crust beneath vindhyan basin godavari basin and the narmada son lineament nsl of proterozoic age the moho depth varies between 33 and 43 km with no definite relationship with age of the crust duplex moho with highly mafic underplated lower crust is characteristic feature of the nsl signifying its paleo rift character present day felsic character of the indian archean crust could be a consequence of post formation modification of the initial mafic lower crust through the process of lithospheric delamination this major tectonic event also created layered lithosphere beneath parts of india imprinted as positive velocity contrast at a depth of 80 km usually referred to as hales discontinuity the upper part of the lithosphere has shear velocity vs 4 52 km s and the lower one 4 77 km s we speculate that such a layered lithosphere could be due to a low density depleted spinel peridotite with archean affinity lying over higher density fertile garnet peridotite of proterozoic age

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

depth to the bottom of magnetic sources dbms from aeromagnetic data of central india using modified centroid method for fractal distribution of sources abstract the depth to the bottom of the magnetic sources dbms has been estimated from the aeromagnetic data of central india the conventional centroid method of dbms estimation assumes random uniform uncorrelated distribution of sources and to overcome this limitation a modified centroid method based on scaling distribution has been proposed shallower values of the dbms are found for the south western region the dbms values are found as low as 22 km in the south west deccan trap covered regions and as deep as 43 km in the chhattisgarh basin in most of the places dbms are much shallower than the moho depth earlier found from the seismic study and may be representing the thermal compositional petrological boundaries the large variation in the dbms indicates the complex nature of the indian crust

quantitative fault studies on the east pacific rise a comparison of sonar imaging techniques a synthesis is presented of published statistics on the lengths scarp heights and spacing of normal faults on the flanks of the east pacific rise epr between 19 s and 21 n nine data sets are included which were collected at different times employing one of four different sonar mapping systems the scripps deep tow instrument seamarc i seamarc ii and gloria with widely ranging data collection capabilities the epr in this region is morphologically quite similar and in some cases the data sets were obtained from the same area of the seafloor it is assumed therefore that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes we try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made we discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters we also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution the theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution we also discuss the effects that surveying geometry can have on the interpretation of fault lengths the data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3 5 s and 12 n on the epr the correlation which we find to be linear is used to estimate 1 the minimum operational fault target size for each of the four mapping systems and 2 the topography due to faulting in an area where only fault lengths are known

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

the evolution of the vindhyan basin vis a vis the narmada son lineament central india from deep seismic soundings abstract the evolution of the vindhyan basin has been studied from deep seismic soundings block tectonics appear to be active throughout the geological history in this region the crust is divided into four major blocks by deep faults at narsinghgarh katangi and jabalpur the crustal block between narsinghgarh and katangi forms a graben in the crystalline basement giving rise to the great vindhyan basin in which precambrian vindhyan sediments of about 5 5 km thickness are deposited the crustal block between katangi and jabalpur is a horst block which represents the narmada son lineament a crustal depth section obtained from wide angle reflections shows a large number of reflector segments up to the mono boundary indicating crustal heterogeneity the boundary faults at the northern and the southern ends of the vindhyan basin extend up to the moho the crustal thickness varies from 39 5 to 45 km

when do fractured media become seismically anisotropic some implications on quantifying fracture properties fractures are pervasive features within the earth s crust and they have a significant influence on the multi physical response of the subsurface the presence of coherent fracture sets often leads to observable seismic anisotropy enabling seismic techniques to remotely locate and characterise fracture systems in this study we confirm the general scale dependence of seismic anisotropy and provide new results specific to shear wave splitting sws we find that sws develops under conditions when the ratio of wavelength to fracture size s d is greater than 3 where rayleigh scattering from coherent fractures leads to an effective anisotropy such that effective medium model emm theory is qualitatively valid when 1 s d 3 there is a transition from rayleigh to mie scattering where no effective anisotropy develops and hence the sws measurements are unstable when s d 1 we observe geometric scattering and begin to see behaviour similar to transverse isotropy we find that seismic anisotropy is more sensitive to fracture density than fracture compliance ratio more importantly we observe that the transition from scattering to an effective anisotropic regime occurs over a propagation distance between 1 and 2 wavelengths depending on the fracture density and compliance ratio the existence of a transition zone means that inversion of seismic anisotropy parameters based on emm will be fundamentally biased more importantly we observe that linear slip emm commonly used in inverting fracture properties is inconsistent with our results and leads to errors of approximately 400 in fracture spacing equivalent to fracture density and 60 in fracture compliance although emm representations can yield reliable estimates of fracture orientation and spatial location our results show that emm representations will systematically fail in providing quantitatively accurate estimates of other physical fracture properties such as fracture density and compliance thus more robust and accurate quantitative estimates of in situ fracture properties will require improvements to effective medium models as well as the incorporation of full waveform inversion techniques

double seismic zones kinematics the kinematics of a double seismic zone of subducting lithosphere at an intermediate depth is studied a simple geometrical argument shows that the strain rate due to unbending of subducting lithosphere can be as large as 10 15 s 1 and large enough to account for the seismic strain rate in this depth range the geometrical unbending of subducting lithosphere in the mantle and the presence of a double seismic zone there can be therefore considered essentially equivalent phenomenon we model a double seismic zone required from this unbending in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere the thickness of a double seismic zone is controlled by the thermal structure and the rheological properties of the subducting slab a preliminary result for the double seismic zone beneath tohoku japan indicates a possibility that the currently available flow law of dry olivine does not properly describe the double seismic zone or at least presents some inconsistency in our knowledge of the mechanical properties e g thermal structure rheology of oceanic lithospheres

the effects of temperature dependent viscosity on mantle convection with the two major phase transitions abstract we have studied the consequences of including temperature dependent viscosity in mantle convection with both of the major phase transitions included we have employed a two dimensional cartesian model in a box of aspect ratio four the time dependent equations have been cast within an extended boussinesq framework in which each of the phase changes has been simulated by an enhanced thermal expansivity both latent heat release and viscous dissipation have been considered two different forms of temperature dependent viscosity the arrhenius and the frank kamenetzky forms have been employed we have considered both models with a constant temperature equilibrium boundary condition at the core mantle boundary and models with a time varying non equilibrium temperature boundary condition from cooling of the core the effects of temperature dependent viscosity are to enhance the tendency of the convective system to be layered because of the influences of the latent heat release and the mechanical heating on creating a sharp viscosity drop across the transition zone we have found that there is a non linear coupling between latent heat release from the interaction of the hot plume with the endothermic phase transition and the temperature dependent viscosity this positive feedback process can give rise to the production of very high temperatures in the transition zone which may cause melting in the region between the two major phase transitions flushing events produced by the non equilibrium boundary condition are much more dramatic than those generated by the equilibrium boundary condition the non linear coupling between latent heat release and temperature dependent viscosity is also stronger in the non equilibrium models comparison with models without the presence of latent heat release and viscous dissipation shows that the models without the heating terms produce a greater degree of mass flux across the endothermic phase boundary because of the lack of the viscosity stratification in the interior of the convective flow results of this moded can be applied to other types of endothermic phase transition found in the lithosphere or to the spinel perovskite transition in the deep portion of the martian mantle

stress fields associated with metastable phase transitions in descending slabs and deep focus earthquakes inside subducting slabs the interaction of different metastable phase transformations olivine spinel spinel spinel spinel perovskite magnesiowustite associated with the latent heat release and absorption for these transitions can result in a complex thermal slab structure thermokinetic coupling processes cause thermal anomalies and buoyancy contrasts between slab and mantle that strongly influence the stress field within subducting plates we have described the thermal field within the downgoing slab in a lagrangian framework by a self consistent high resolution two dimensional thermokinetic coupling model which also incorporates the effects of nucleation site saturation using an adaptive method the threshold for the nucleation site saturation is found to be less than 0 1 transformation degree in the cold slab interior this can significantly shift the metastable wedge of olivine deeper as suggested from previous thermokinetic slab models however this effect is compensated by the latent heat release during the olivine spinel and spinel spinel phase transitions latent heat release produce very sharp phase boundaries in the slab for fast slabs the phase boundaries reveal significant metastable perturbations from the equilibrium state in the cold interior however the strong thermal interaction between both ongoing phase transitions results in sharp overlapping phase boundaries with dramatic consequences for the slab stress field we have investigated the detailed physical properties and the state of stress in the deeper portion of a subducting plate by using up to date temperature field and physical properties determined from recent high p t experiments we find that the slab is denser up to 100 kg m 3 than the surrounding mantle remarkable denser portions up to 260 kg m 3 are located just below the uplifted and equilibrium phase transitions while lighter portions up to 210 kg m 3 can be found just above the depressed spinel to perovskite magnesiowustite transition and for the metastable wedge of olivine these density differences effectively act as a body force and produce a significant stress field which we calculated with a two dimensional finite element code the results show that the buoyancy induced forces produce maximum shear stress up to 23 mpa along the metastable wedge and deeper portion just above the depressed last phase transition involving spinel for the latter the dominant state of stress is down dip compression the calculated p t dependent state of stress is very similar to the depth distribution of deep focus

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

the central india tectonic zone a geophysical perspective on continental amalgamation along a mesoproterozoic suture abstract the central india tectonic zone citz is a prominent divide and a major suture zone between the north indian and south indian crustal blocks the resistive upper crust as modeled in the magnetotelluric data from citz suggests a dominant tonalite trdondhjemite granodiorite composition associated with an accretionary complex characterized by mainly felsic rock components the highly conductive bodies in this zone might represent mafic ultramafic layered intrusives derived from a deeper reservoir of underplated basaltic magma related to the formation of the cretaceous deccan flood basalts the uniformly thick mafic lower crust below the cratons on both sides of the suture is interpreted as the accreted remnants of archaean and paleoproterozoic subducted slabs we redefine the nature of deep faults traversing the citz which were described as steep and penetrating the moho by previous workers and classify them as listric faults with gentle dips at depth seismic reflection data from the eastern side of the suture suggest a northwestward subduction of the bhandara craton reflection data from the central part of the citz show northerly dip in the southern part suggesting northward subduction of the dharwar craton however an opposite trend is observed in the northern part of the suture with a southward dip of the bundelkhand craton based on these features and in conjunction with existing magnetotelluric models we propose a double sided subduction history along the citz this would be similar to the ongoing subduction accretion process in the western pacific region which possibly led to the development of paired collision type and pacific type orogens one important feature is the domal structure along the central part of the suture with a thick felsic crust occurring between mafic and intermediate crust the high resistivity felsic domain suggests underplated sediments felsic crust that would have caused the doming our model also accounts for the extrusion of regional metamorphic belts at the orogenic core and the occurrence of high pressure ultrahigh temperature paired metamorphic belts within the suture

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

stereological analysis of fracture network structure in geological formations relationships between three dimensional fracture networks consisting of poly disperse disks and the corresponding two dimensional trace maps are systematically analyzed bulk densities of disks and disk intersections are related to surface densities of traces and trace intersections this results in a new relation for the average length of disk intersections the probability densities of trace lengths and disk intersections are studied for several disk diameter distributions the inverse problem of deriving the disk distribution from the trace distribution is then solved assuming only that the disks have uniformly random locations and orientations these results are applied to a variety of synthetically generated data as well as to several sets of field data analysis of the field data suggests that fracture diameter distributions follow a power law in agreement with previous conclusions based on trace length histograms with exponents varying from 1 3 to 2 1

stresses along the metastable wedge of olivine in a subducting slab possible explanation for the tonga double seismic layer abstract a numerical calculation of the stresses associated with changes in volume during phase transitions of olivine in a descending slab results in a double layer of high shear stress along the metastable olivine wedge in the depth range 350 460 km stress in the upper layer is in plane tensional and stress in the lower layer is down dip compressional the modeled stress field agrees with observations of stress in the tonga double seismic zone high shear stress also exists in the slab at depths below the metastable wedge this stress distribution involves down dip compression and trench parallel tension which agrees with about half of the focal mechanisms in the tonga slab at depths of 460 690 km the model supports the idea that at least two possible stress release mechanisms for deep earthquakes may act in the tonga subducting slab one transformational faulting is restricted to the metastable wedge while the other one acts below the metastable wedge

stress perturbation associated with the amazonas and other ancient continental rifts the state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts specifically a dense rift pillow in the lower crust may modify substantially the regional stress field both shallow 2 0 2 6 km depth breakout data and deep 20 45 km depth crustal earthquake focal mechanisms indicate a n to nne maximum horizontal compression in the vicinity of the paleozoic amazonas rift in central brazil this compressive stress direction is nearly perpendicular to the rift structure and represents a 75 rotation relative to a regional e w compressive stress direction in the south american plate elastic two dimensional finite element models of the density structure associated with the amazonas rift as inferred from independent gravity modeling indicate that elastic support of this dense feature would generate horizontal rift normal compressional stresses between 60 and 120 mpa with values of 80 100 mpa probably most representative of the overall structure the observed 75 stress rotation constrains the ratio of the regional horizontal stress difference to the rift normal compressive stress to be between 0 25 and 1 0 suggesting that this rift normal stress may be from 1 to 4 times larger than the regional horizontal stress difference a general expression for the modification of the normalized local horizontal shear stress relative to the regional horizontal shear stress shows that the same ratio of the rift normal compression relative to the regional horizontal stress difference which controls the amount of stress rotation also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress the potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike slip stress regimes in the amazonas rift case because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift normal compression to be between 0 25 and 1 0 our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced for ratios 0 5 or enhanced for ratios 0 5 additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust faulting stress regime in the vicinity of the amazonas rift a rift normal stress associated with the seismically active new madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike slip faults parallel to the axis of the rift although this results in a 20 40 reduction in the local horizontal shear stress within the seismic zone sparse stress data in the vicinity of the seismically quiescent midcontinent rift of the central united states suggest a stress state similar to that of new madrid with the local horizontal shear stress potentially reduced by as much as 60 thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows the modeling and analysis here demonstrate that rift normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

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

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

cenozoic epeirogeny of the indian peninsula peninsular india is a cratonic region with asymmetric relief manifest by eastward tilting from the 1 5 km high western ghats escarpment toward the floodplains of eastward draining rivers oceanic residual depth measurements on either side of india show that this west east asymmetry is broader scale occurring over distances of 2 000 km admittance analysis of free air gravity and topography shows that the elastic thickness is 10 3 km suggesting that regional uplift is not solely caused by flexural loading to investigate how indian physiography is generated we have jointly inverted 530 river profiles to determine rock uplift rate as a function of space and time key erosional parameters are calibrated using independent geologic constraints e g emergent marine deposits elevated paleosurfaces uplifted lignite deposits our results suggest that regional tilt grew at rates of up to 0 1 mm a 1 between 25 ma and the present day neogene uplift initiated in the south and propagated northward along the western margin this calculated history is corroborated by low temperature thermochronologic observations by sedimentary flux of clastic deposits into the krishna godavari delta and by sequence stratigraphic architecture along adjacent rifted margins onset of regional uplift predates intensification of the indian monsoon at 8 ma suggesting that rock uplift rather than climatic change is responsible for modern day relief a positive correlation between residual depth measurements and shear wave velocities beneath the lithosphere suggests that regional uplift is generated and maintained by temperature anomalies of 100 c within a 200 25 km thick asthenospheric channel this article is protected by copyright all rights reserved

how valid are dynamic models of subduction and convection when plate motions are prescribed a detailed comparison between fully dynamic and kinematic plate formulations has been made in models of mantle convection plate velocity is computed self consistently from fully dynamic plate models with temperature and stress dependent viscosity and preexisting mobile faults in fully dynamic models the flow is driven solely by internal buoyancy while in kinematic models the flow is driven by a combination of the prescribed surface velocity and internal buoyancy only a temperature dependent viscosity close to the effective viscosity determined from the fully dynamic models is used in the kinematic models the two types of models give very similar temperature structures and slab evolutionary histories when the effective viscosity and surface velocity are nearly identical in kinematic plate models the additional work introduced by the prescribed velocity boundary condition is apparently dissipated within the lithosphere and has little influence on the convection under the lithosphere in models with periodic lateral boundary conditions slabs sink into the lower mantle at an oblique angle and this contrasts with the vertical sinking which occurs with reflecting boundary conditions models show that we can simulate fully dynamic models with kinematic models under either periodic boundary conditions or reflecting boundary conditions

nature of continental crust along the narmada son lineament inferred from gravity and deep seismic sounding data abstract the narmada son lineament nsl is a very prominent feature on the tectonic map of india and divides the indian peninsula into two parts the northern part characterised by lower elevations and more positive bouguer gravity anomalies as compared to the southern part the lineament is characterised by a gravity high practically all along its length four published deep seismic sounding dss profiles across the nsl give vital information concerning its origin an analysis of the bouguer gravity field along these profiles has been carried out using all available geological and velocity density information the analysis reveals that the areas immediately south of the nsl are characterised by higher density crust in comparison to the areas north of it this increase in density could have taken place during mesozoic to tertiary times when major tectonic volcanic activity took place over peninsular india the presence of deep seated faults sediments and anomalous crust shows that the lineament has some characteristics of a rift valley

distributed deformation in the lower crust and upper mantle beneath a continental strike slip fault zone marlborough fault system south island new zealand converted phases from teleseisms recorded by a seismic array spanning the northern half of the marlborough fault system south island new zealand show a continuous unbroken moho underlying a seismically anisotropic lower crust beneath the two north ernmost faults of the fault system these observations suggest that distributed deforma tion not slip on a narrow vertical fault accommodates displacement in the lower crust below the 120 480 km of right lateral slip across the wairau fault one splay of the marl borough fault system and the northward continuation of the alpine fault beneath the wairau fault the moho dips 258 308 southeast from a depth of 26 km northwest of the fault to a depth of 34 km southeast of the fault farther to the southeast ps conversions from the moho continue under the awatere fault 34 6 10 km of slip with a constant amplitude and depth of 34 km across the two faults converted energy from 16 20 km depth varies with back azimuth in a manner suggesting the presence of anisotropy in the lower crust these observations imply that one of the tenets of plate tectonics that faults defining plate boundaries pass through both crust and upper mantle does not apply to new zealand or to continents in general

prediction of macrofracture properties using microfracture information mesaverde group sandstones san juan basin new mexico abstract a better understanding of relationships between microfractures and macrofractures is prerequisite to the extrapolation of observed microfracture properties as a basis for inferring properties of associated macrofractures the hypothesis that micro and macrofractures represent different size fractions of the same fracture sets and consequently have linked attributes was tested on sandstones of the mesaverde group san juan basin new mexico the use of a cathodoluminescence detector attached to a scanning electron microscope sem allowed observation of microfractures not readily visible using conventional microscopic techniques the orientations of microfractures reproduce the orientations recorded at the macroscopic scale from cores and outcrops fracture size distributions are best modeled using power laws and extrapolation of microfracture frequencies to the scale of macrofractures successfully predicts the macrofracture frequencies fracture size distributions change at the length scale of the thickness of the mechanical layer typically by increasing the exponent of their power law distribution this change is independent of censoring biases and reflects changes in sampling topology and or fracture growth mechanisms

ground surface temperature history in central canada inferred from 10 selected borehole temperature profiles among the 57 temperature depth profiles recently measured in manitoba and saskatchewan canada only 10 are suitable for inferring recent changes in ground surface temperature many of the rejected temperature profiles show an apparent climatic signal but are affected by topography changes in vegetation or the proximity of lakes at the northernmost site near lynn lake manitoba shallow horizontal variations in temperature have been identified that are related to intermittent permafrost such variations can induce an apparent climatic perturbation in areas where lateral thermal conductivity contrasts have been measured heat refraction effects can also incorrectly be interpreted as ground surface temperature histories the analysis suggests that forest fires that occurred at some of the sites have had little influence on the temperature profiles the 10 selected temperature profiles cover a wide area in northern manitoba and saskatchewan they have been independently and jointly inverted by a singular value decomposition method the ground surface temperature history shows two main episodes a cold period tentatively identified as little ice age with a minimum around 1820 a d was followed by marked warming after 1920 these trends are similar to those recognized in eastern canada

effects of mantle and subduction interface rheologies on slab stagnation and trench rollback abstract trench rollback has been a widely discussed phenomenon in recent years and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the upper mantle here we concentrate on the effects of rheological description yield stress lower mantle viscosity viscosity of crust in controlling the rollback and associated stagnation of slabs in the transition zone 410 660 km depth we perform numerical simulations of slab evolution in a 2d cartesian model with strongly nonlinear rheology combining diffusion creep dislocation creep and a power law stress limiter we demonstrate that trench retreat develops in most models considered regardless of the subducting plate age or prescribed strength rollback then mostly produces slabs that are horizontally deflected at the 660 km phase boundary and remain subhorizontal at the bottom of the transition zone slab morphologies are in agreement with stagnant horizontally deflected structures reported in the transition zone by seismic tomography furthermore if the strength of the slab is limited to less than 0 5 gpa the slab experiences a significant amount of horizontal buckling the amplitude of the rollback velocity is sensitive to several model parameters as one might expect it increases with the age of the subducting plate thus reflecting its increasingly negative buoyancy on the other hand rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates high friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation also reducing the additional negative buoyancy imparted by the 410 km exothermic phase transition suppresses trench rollback interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models where for example buoyancy effects may be counteracted by associated yield stress weakening

deformation rates from faulting at the tempe terra extensional province mars 1 rates of deformation are estimated from exposed normal faults within the tempe terra extensional province mars through an analysis that incorporates fault segment linkage and utilizes regional observations of displacement length relationships measured from mars orbiter laser altimeter topographic data moment rates from 1014 1016 n m yr strain rates 10 12 10 11 yr 1 and rifting velocities 0 003 mm yr are comparable to rates of deformation on stable plate interiors of earth the calculated low rates of rifting may result from the poorly constrained timing of deformation on mars cumulative moment release estimates decrease linearly with time from noachian 1 1025 n m to early hesperian 6 1024 n m and late hesperian early amazonian 1 1024 these results indicate that a portion of tharsis related deformation remained localized within the tempe rift throughout much of martian history and provide important constraints for models of martian mantle convection

deep structure of the baikal rift zone revealed by joint inversion of gravity and seismology 1 the question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas particularly in the baikal rift zone asia which is colder than other continental rifts as previous gravity and teleseismic studies support the dominance of opposing mechanisms in the baikal rift we reconsidered both data sets and jointly inverted them this more effective approach brings insight into location of the perturbing bodies related to the extension in this region our new joint inversion method allows for inverting the velocity density relationship with independent model parametrization we obtain velocity and density models that consistently show 1 crustal heterogeneities that coincide with the main tectonic features at the surface 2 a faster and denser cratonic mantle nw of lake baikal that we relate to the thermal contrast between old and depleted archean siberian platform and paleozoic orogenic belt sayan baikal belt 3 three dimensional topographic variations of the crust mantle boundary with well located upwarpings and 4 the lithosphere asthenosphere boundary uplift up to 70 km depth with a nw dip our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the baikal rift zone index terms 1234 geodesy and gravity regional and global gravity anomalies and earth structure 7218 seismology lithosphere and upper mantle 8122 citation tiberi c m diament j deverchere c petit mariani v mikhailov s tikhotsky and u achauer deep structure of the baikal rift zone revealed by joint inversion of gravity and seismology