the present day stress field of australia abstract the unusual present day crustal stress pattern of the australian continent has been the subject of scientific debate for over 25 years the orientation of maximum horizontal present day stress s hmax in continental australia is unlike all other major tectonic plates in that the stress pattern shows regional variability and it is not oriented sub parallel to the direction of absolute plate motion previous studies on the stress pattern of australia revealed that the complex stress pattern of the continent is controlled at a first order by the superposition of plate tectonic forces exerted at the plate boundaries however prior analysis of the contemporary australian crustal stress pattern have been unable to model or explain the stress pattern observed in most of eastern australia and has not extensively addressed the numerous smaller scale variations in stress orientation the recent development of unconventional reservoirs in australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorly constrained areas in eastern australia in addition stress analysis in conventional hydrocarbon mineral and geothermal exploration in all other parts of the continent provides the opportunity to review and update the australian stress map asm this study presents the new release of the asm with a total of 2150 stress data records in australia increased from 594 data records in 2003 the 2016 asm contains 1359 data records determined from the interpretation of drilling related stress indicators 650 from earthquake focal mechanism solutions 139 from shallow engineering measurements and two from geological indicators the results reveal four distinct regional trends for the s hmax orientation in australia including a nne ssw s hmax trend in northern and northwestern australia which rotates to a prevailing e w orientation in most western and south australia the orientation of s hmax in eastern australia is primarily ene wsw and swings to nw se in southeastern australia a comparison between the new asm database and neotectonic features further confirms the role of contemporary stress in recent deformation of the australian crust the 2016 asm reveals significant discrepancies between newly observed s hmax orientations and predictions by published geomechanical numerical models for the australian continent forces generated at the boundary of indo australian plate remain the major control on the regional crustal stress pattern in continental australia however the increased data density particularly in eastern australia reveals numerous local perturbations of the stress field that were not previously clearly captured hence the key findings of the new release are that local intra plate stress sources are more significant than previously recognized particularly in eastern australian basins and cause substantial local scale rotations of the present day crustal stress pattern that have not been factored into existing australian stress models

compressive and tensile failure of inclined well bores and determination of in situ stress and rock strength in this paper we investigate the occurrence of compressive and tensile failures of arbitrarily inclined well bores under a wide variety of stress conditions the principal assumptions in this analysis are that the rock is isotropic and that it deforms elastically to the point of failure as has been shown by previous investigators for a given stress state and well bore orientation it is straightforward to predict the orientation of the failures around the well bore as well as whether failure is likely to occur depending on such parameters as rock strength and borehole fluid pressure however as the stress state is almost never known in situ we demonstrate how observations of compressive and tensile wall failures in inclined holes can be used to constrain in situ stress orientations and magnitudes if there are independent data on the magnitude of the least principal stress from either leak off or microfrac tests and on the formation pore pressure we further demonstrate how once the stress state is determined it is possible to assess both an upper bound on the effective in situ rock strength and the degree to which increasing the borehole fluid pressure or mud weight can reduce the likelihood of borehole failure through application of this methodology to an inclined well bore in an area of complex faulting in the gulf of mexico we illustrate how it is possible to utilize observations of borehole failures to determine the magnitude and orientation of the stress tensor in areas such as offshore sedimentary basins where drilling inclined well bores is quite common

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

rheology of the earth s mantle a historical review abstract historical development of our understanding of rheological properties of the earth s mantle is reviewed rheological properties of the earth s mantle control most of the important geological processes such as the style of mantle convection e g stagnant lid versus plate tectonics and the nature of thermal evolution however inferring the rheological properties of the earth s mantle is challenging because of the presence of multiple mechanisms of deformation that have different dependence on time scale strain rate stress levels and other parameters through the integration of a broad range of observations including the elastic stiffness from tidal deformation and the viscosity from the concept of isostasy a gross picture of rheological stratification of the earth s mantle a strong lithosphere a weak asthenosphere and a strong layer below was proposed in the mid 19th century however not only the physical basis for such a model was weak due to the lack of proper understanding of some materials science issues such as the interplay between elastic and viscous deformation but also the lack of understanding of temperature depth relation associated with convection prevented our understanding of the rheological structure of the earth s interior major progress occurred in the first half of the 20th century in our understanding of the atomistic mechanisms of plastic deformation in solids and much of the theoretical framework on the plastic deformation of solids was established by late 1960s those developments provided a basis for scaling analyses that are critical to the applications of laboratory results to the earth s interior major progress in laboratory studies on rheological properties occurred in the mid 1960s to the early 1970s in griggs lab using a new type of solid medium high pressure deformation apparatus to pressure 2 gpa and temperature 1600 k the basic concepts such as the water weakening non linear rheology and deformation induced lattice preferred orientation were identified by their studies however large uncertainties in the stress measurements with this type of apparatus were recognized in the late 1970s and high resolution experimental studies using synthetic samples were initiated in paterson s lab in the mid 1980s using a high resolution gas medium deformation apparatus however experimental studies with such a gas medium deformation apparatus can be conducted only at low pressures

experimental studies of shear deformation of mantle materials towards structural geology of the mantle a brief outline is given on experimental studies carried out in the minnesota mineral and rock physics laboratory of microstructural evolution and rheology of mantle mineral aggregates or their analogues using a simple shear deformation geometry a simple shear deformation geometry allows us to unambiguously identify controlling factors of microstructural evolution and to obtain large strains at high pressures and temperatures and thus provides a unique opportunity to investigate the structural geology of the mantle we have developed a simple shear deformation technique for use at high pressures and temperatures pressure up to 16 gpa and temperature up to 2000 k in both gas medium and solid medium apparati this technique has been applied to the following mineral systems i olivine aggregates ii olivine basaltic melt iii catio3 perovskite aggregates the results have provided important data with which to understand the dynamics of the earth s mantle including the geometry of mantle convection mechanisms of melt distribution and migration beneath mid ocean ridges and the mechanisms of shear localization limitations of laboratory studies and future directions are also discussed

a multiscale approach to model the anisotropic deformation of lithospheric plates the association of experimental data showing that the plastic deformation of olivine the main constituent of the upper mantle is highly anisotropic and the ubiquitous seismic anisotropy in the upper mantle which indicates that olivine crystals show coherent orientations over scales of tens to hundreds of kilometers implies that the long term deformation in the upper mantle is anisotropic we propose a multiscale approach based on a combination of finite element and homogenization techniques to model the deformation of a lithospheric plate while fully considering the mechanical anisotropy stemming from a strain induced orientation of olivine crystals in the mantle this multiscale model explicitly takes into account the evolution of crystal preferred orientations cpo of olivine and of the mechanical anisotropy during the deformation we performed a series of numerical experiments simulating the uniaxial extension of a homogeneous 100 olivine but anisotropic plate to test the role of the olivine cpo on the plate mechanical behavior and the link between cpo and mechanical anisotropy evolution even for this simple solicitation different orientations and intensity of the initial olivine cpo result in variable plate strengths and deformation regimes a plate with an initial cpo where the olivine 100 and 010 axes are concentrated at 45 degrees to the extension direction has high resolved shear stresses on the easy 010 100 and 001 100 slip systems of olivine this results in low strength and in deformation by transtension plates with an initial cpo where the maximum of 100 axes is parallel or normal to the extension direction show a high initial strength isotropic plates have an intermediate behavior the progressive rotation of olivine 100 axes toward the imposed stretching direction results in hardening in all models except in those characterized by an initial concentration of olivine 100 axes normal to the imposed extension in which softening is followed by hardening

the relationship between closure pressures from fluid injection tests and the minimum principal stress in strong rocks abstract closure pressures measured during injection tests such as mini fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude this paper presents stress strength and image log data from the australian cooper basin which suggests that in reservoirs with high in situ stress high tensile strength and weak geological fabrics interpreted closure pressures may be significantly greater than the minimum principal stress closure pressures interpreted from mini frac injection tests in the cooper basin suggest the minimum principal stress varies from 12 4 27 2 mpa km 0 55 1 2 psi ft to better understand the reasons for this variation in closure pressure image logs and mini frac data from 13 treatment zones and core from seven of these treatment zones were analysed the analysis revealed that treatment zones with high measured closure pressures 18 1 mpa km 0 8 psi ft high treating pressures 31 6 mpa km 1 4 psi ft and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength 7 mpa 1015 psi and geological fabrics planes of weakness including natural fractures conversely treatment zones with lower measured closure stress 19 mpa km 0 84 psi ft and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and or no geological fabrics we suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the cooper basin where they are associated with hydraulic fracture complexity rather they reflect the normal stress incident on pre existing weaknesses that are exploited by hydraulic fluid during the mini frac injection

investigation of increasing hydraulic fracture conductivity within producing ultra deep coal seams using time lapse rate transient analysis a long term pilot experiment in the cooper basin australia abstract ultra low permeability inertinite rich poorly cleated dehydrated overpressured coal seams deeper than 9000 ft 2740 m in the cooper basin of central australia represent a fundamentally different play type to shallow and deep coal seam gas reservoirs proof of concept gas flow was achieved in 2007 four patchawarra formation coal seams were subjected to low proppant concentration slick water hydraulic fracture stimulation within a dedicated vertical wellbore at a depth of 9500 ft 2900 m gas was produced for 81 2 years at a slowly increasing base flow rate averaging 0 1 mmscfd 2 8 mscmd experimental data were gathered for characterising dynamic reservoir behaviour this included multiple extended pressure build up tests the authors have previously investigated the three largest of these using time lapse pressure transient analysis the published results reveal a dominant bilinear flow regime associated with an isolated domain of increasing coal fabric permeability surrounding the hydraulic fracture this paper builds upon the time lapse pressure transient analysis study by specifically investigating conductivity of the hydraulic fracture which the authors postulate to have also increased the hypothesis is tested by applying time lapse rate transient analysis to the two specially designed pressure drawdown tests to atmospheric pressure that immediately follow the first two pressure build up tests of the time lapse pressure transient analysis between the two pressure drawdown tests spaced 586 days apart the wellbore flowed gas continuously for 327 days at an average flowing bottom hole pressure of 580 psig 4 0 mpag hence there was ample opportunity for hydraulic fracture conductivity to change between the tests both pressure drawdown tests were monitored for 24 h using high resolution surface pressure gauges each was initiated from the same surface shut in pressure of 2500 psig 17 2 mpag the flow pressure data are initially used to construct diagnostic plots that clearly identify the hydraulic fracture linear flow regime early in each test immediately after the dissipation of wellbore storage hydraulic fracture properties are then extracted using specialty plots that display rate normalised pseudo pressure difference versus linear superposition time comparing the slopes of the two speciality plot trends indicates that the hydraulic fracture flow property b f k f f increased during the 327 day gas flow period by a factor of 4 this is supported by a 60 increase in the initial gas flow rate from hydraulic fracture storage from 7 5 to 12 0 mmscfd 212 4 to 340 0 mscmd additionally despite a significantly larger volume of hydraulic fracture storage gas being produced to surface during the second test the duration of the hydraulic fracture linear flow regime is less than for the first these observations are consistent with an increasingly more conductive hydraulic fracture over flowback time that allows compressed gas within it to discharge more rapidly

lithospheric deformation beneath the san gabriel mountains in the southern california transverse ranges high resolution tomographic images from los angeles region seismic experiment larse array and southern california seismic network scsn teleseismic data suggest that the entire lithosphere below the san gabriel mountains and san andreas fault in the transverse ranges has thickened in a narrow vertical sheet p wave travel time inversions of the combined data support the presence of the well documented upper mantle high velocity anomaly that extends 200 km into the mantle under the northernmost los angeles basin and transverse ranges and is associated with mantle downwelling due to oblique convergence we find that the high velocity high density upper mantle anomaly comprises a 60 80 km wide sheet of mantle material that lies directly below a substantial crustal root in the san gabriel mountains the velocity perturbations are as large as 3 in the anomaly corresponding to a 2 density increase the tomographic images suggest that deformation in the ductile lower crust and mantle lithosphere may be partially coupled mechanically and thermally if the thickening is occurring together in response to convergence and that it may be a local compressional feature

numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow 1 the development of lattice preferred orientations lpo within olivine aggregates under flow in the upper mantle leads to seismic and rheological or viscoplastic anisotropies we compare predictions from different micromechanical models applying several commonly used theoretical descriptions to an upwelling flow scenario representing a typical oceanic spreading center significant differences are obtained between models in terms of lpo and associated rheology in particular in regions where the flow direction changes rapidly with superior predictions for the recently proposed second order approach this highlights the limitations of ad hoc formulations lpo induced rheological anisotropy may have a large effect on actual flow patterns with 1 2 orders of magnitude variation in directional viscosities compared to the isotropic case citation castelnau o d k blackman and t w becker 2009 numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow geophys res lett 36 l12304

anisotropic rheology of a cubic medium and implications for geological materials summary dislocation creep which is the dominant deformation mechanism in the upper mantle results in a non newtonian anisotropic rheology the implication of non newtonian rheology has been quite extensively studied in geodynamic models but the anisotropic aspect remains poorly in vestigated in this paper we propose to fill this gap by 1 introducing a simple mathematical description of anisotropic viscosity and 2 illustrating the link between plastic crystal defor mation and bulk material rheology the study relies on the highest symmetry of the anisotropic tensor a cubic symmetry for which anisotropy is characterized by one parameter only first order implications of anisotropy are quantitatively explored as a function of the effective rheology of the material is described as a function of the orientation of the crystals and of the imposed stress and the validity of the isotropic approximation is discussed the model applied to ringwoodite a cubic crystal with spinel type structure predicts that the dynamics of the transition zone in the earth s mantle is going to be strongly affected by mechanical anisotropy

using stress deflections to identify slip events in fault systems abstract a new approach for identifying past slip events in strike slip fault systems without geological markers is presented a 2 d distinct element modeling code is used to simulate the stress distribution around a strike slip fault zone after a major slip event mature and immature fault zones are investigated slip on the fault is responsible for stress rotations near the active fault strand series of experiments reveal that the stress deflections are qualitatively the same whatever the fault strike fault friction and far field differential stresses for a left lateral fault stresses are rotated clockwise in the two compressive quadrants near the fault tips and counterclockwise in the two extensive quadrants the stress field signature of a slip event thus highlights the extent of the active fault strand conversely the reconstruction of paleostress fields through inversion of minor striated faults that commonly follow slip on the master fault and the analysis of their deflections allow recognition of the fault strands that slipped in a close or remote past application to the left lateral pontarlier fault zone in the jura fold and thrust belt france late cenozoic in age shows that the fault activity has migrated northward in agreement with the in sequence development of the mountain belt

postcollisional lithospheric evolution of the southeast carpathians comparison of geodynamical models and observations seismic evidence and thermal and topographic transients have led to the interpretation of lithospheric removal beneath the southeast carpathians region a series of numerical geodynamic experiments in the context of the tectonic evolution of the region are conducted to test the surface crustal response to lithosphere delamination and slab break off the results show that a delamination type removal plate like migrating instability causes a characteristic pattern of surface uplift subsidence and crustal extension shortening to occur due to the lithospheric deformation and dynamic thermal forcing of the sublithospheric mantle these features migrate with the progressive removal of the underlying lithosphere model results for delamination are comparable with observables related to the geodynamic evolution of the southeast carpathians since 10 ma the mantle structure inferred by seismic tomography migrating patterns of uplift 1 5 km and subsidence 2 km in the region crustal thinning in the carpathian hinterland and thickening at the focsani depression and regional extension in the carpathian corner e g opening of brasov basin correlating with volcanism e g harghita and persani volcanics in the last 3 myr

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

origins of the european regional stress field abstract we present a study of the origins of the european regional stress field intraplate stresses due to various plate boundary forces and intraplate stress sources were predicted using an elastic finite element analysis the finite element mesh consists of about 1629 nodes in a network of 3117 triangular elements which provide a spatial resolution of about 1 the study builds on the previous modelling studies through consideration of a wide range of plausible forces acting along the collisional eastern and southern plate boundaries and importantly through the inclusion of forces due to other lateral density variations within the lithosphere such as those associated with the continental margins and high topography the modelling was constrained by more than 1800 stress indicators from the world stress map project which provide information about the orientation of the observed maximum horizontal compressive stress sh the torque associated with the ridge push force is well constrained and the principal source of uncertainty relates to the boundary conditions used to represent the collisional resistance acting along the eastern and southern boundaries a gradient in the collisional resistance increasing in magnitude from west to east is assumed to act along the southern plate boundary whereas the eastern boundary is modelled as a pinned margin at 46 e the sh orientation and magnitude of the regional stress field was found to be largely invariant to the boundary conditions used to represent the tectonic forces acting along the eastern and southern margins and is characterized by a nearly uniform nw se compression with a magnitude in the range of 10 20 mpa averaged over a 100 km thick lithosphere lateral density variations within the continental areas of europe were found to reduce the magnitude of the predicted stresses but do not have a significant effect on the orientation of the regional stress field

evidence for low viscosity garnet rich layers in the upper mantle abstract the rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from western norway that garnet rich rocks can have lower solid state viscosities than olivine rich rocks modeling of melt free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet rich and olivine rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine grained garnet deforms by diffusion creep while the coarse grained olivine deforms by dislocation creep and or diffusion creep in general modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine grained garnets in a dry mantle environment or at conditions where the garnet pyroxene layer is partially molten i e close to solidus liquidus conditions in the upper mantle alternatively in cold plate tectonic settings e g in subduction zones some water weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine

high temperature creep of olivine single crystals 2 dislocation structures dislocation structures and densities in experimentally deformed single crystals of san carlos olivine were examined using the oxidation decoration technique the high temperature deformation experiments were conducted at various temperatures t differential stresses and oxygen fugacities o2 the samples were buffered against either orthopyroxene opx or magnesiowustite mw solid state powders and compressed along one of the three 45 orientations in samples that were deformed and subsequently quenched under load seven distinct dislocation structures were observed a for samples compressed parallel to 110 c at 16 50 mpa and o2 10 6atm two different dislocation arrangements were identified for opx buffered samples at 1400 t 1475 c the dislocation structure was composed of curved screw segments that had usually cross slipped and straight edge segments of comparable length at 1300 t 1350 c for opx buffered samples and at 1350 t 1500 c for mw buffered samples the dislocation structure is dominated by arrays of very long edge or mixed dislocations with only a few screw dislocations b for both opx buffered and mw buffered samples compressed parallel to 101 c at 1340 t 1400 c and 30 60 mpa the dislocation configurations are characterized by zigzag near edge dislocations at o2 10 5atm and by straight cross slipped screw dislocations plus pinned edge dislocations at o2 10 9atm c for samples compressed parallel to 011 c at t 1400 c and 70 150mpa the primary dislocations change from long straight edge dislocations for o2 10 4atm and opx buffer to a combination of straight edge dislocations and polygon shaped half loops for o2 10 4atm and mw buffer to gently curved near edge dislocations for o2 10 9atm for both solid state buffers abundant long straight screw dislocations were present in each case for the 011 c samples all of the above variations in dislocation structure are paralleled by changes in the measured power law equation that describes the behavior of creep mechanisms of olivine thus the different dislocation structures are associated with different rate controlling creep mechanisms for all of the dislocation structures the dislocation density increases with differential stress according to the relation 1 4 dislocation structures in samples that were deformed and then statically annealed have also been studied for both 110 c and 011 c samples deformed over a wide range of experimental conditions the dislocation structures on the slip planes generally consist of short curved edge or mixed dislocation segments in 101 1 samples deformed at low o2 50 mpa and 1400 c the dislocation structure was much less altered by annealing than the dislocation structures in 110 c and 011 c samples the density of dislocations decreased much less for these 101 c samples than for the 110 c and 011 c samples due to climb of the edge segments the straight screw segments found in quenched samples became curved and many low angle 100 tilt boundaries connected by straight 100 screw dislocations developed during the annealing process

rayleigh taylor instabilities with anisotropic lithospheric viscosity summary rocks often develop fabric when subject to deformation and this fabric causes anisotropy of physical properties such as viscosity and seismic velocities we employ 2 d analytical solutions and numerical flow models to investigate the effect of anisotropic viscosity av on the development of rayleigh taylor instabilities a process strongly connected to lithospheric instabilities our results demonstrate a dramatic effect of av on the development of instabilities their timing location and most notably their wavelength are strongly affected by the initial fabric specifically we find a significant increase in the wavelength of instability in the presence of av which favours horizontal shear we also find that an interplay between regions with different initial fabric gives rise to striking irregularities in the downwellings our study shows that for investigations of lithospheric instabilities and likely of other mantle processes the approximation of isotropic viscosity may not be adequate and that av should be included

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region

drilling induced tensile wall fractures implications for determination of in situ stress orientation and magnitude abstract detailed investigation of failure of the borehole wall in two scientific drilling projects the german ktb kontinentales tiefbohrprogramm der bundesrepublik deutschland and the european geothermal research project at soultz sous forets france has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress s h a model for the initiation of the fractures is presented which points out the important influences of a the tectonic stress state b increased mud pressures during drilling operation and c thermal stresses induced by circulation of relatively cold drilling mud analysis of drilling induced fractures in the gpk1 borehole at soultz sous forets where the magnitude of s h is known from hydraulic fracturing experiments demonstrates the validity of this model for the initiation of the fractures further a new method is proposed to estimate the magnitude of s h from the occurrence of drilling induced fractures and knowledge of thermally induced stress and pumping pressure during drilling the method is successfully applied to both ktb boreholes an independent method to estimate the magnitude of s h based on the analytical calculation of the stress intensity factor for drilling induced fractures taking into consideration both increased mud pressure and thermal stress is also presented application of this method confirms the results derived with the analysis described above additionally the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress

structural reactivation in plate tectonics controlled by olivine crystal anisotropy structures formed during ancient tectonic events are commonly reactivated during subsequent tectonism numerical models point to mechanical anisotropy arising from the inherited orientation of crystals of the mineral olivine in the lithospheric mantle as the cause of this behaviour

smoothing and extrapolation of crustal stress orientation measurements the theory and mechanics of a statistical smoothing algorithm for estimating stress fields based on observed data are described incorporated into the algorithm are tunable parameters which allow the user to adjust the smoothness and fidelity of the fitted stress field in addition the user has the option of using robustness weights or to weight the predicted stress field by assigning quality ranks to each data point fitted stress fields can be displayed as either a gridded map axes of maximum or minimum horizontal stress are shown as short bars located at each point in an evenly spaced grid or as a stress trajectory map examples of predicted stress fields for southern california and western canada based on borehole elongation data are displayed as stress trajectory maps a predicted paleostress field for the spanish peaks intrusion complex in colorado based on vertical dike orientations is displayed as a gridded map the smoothing algorithm is not limited to the analysis of stress orientation data any data consisting of a set of undirected lines measured at discrete locations are appropriate for input possible candidates include strain data mineral or intersection lineations and lineaments

continued geothermal reservoir stimulation experiments in the cooper basin australia two fluid injection experiments were conducted in the cooper basin australia in 2010 and 2012 following previous stimulations in the habanero reservoir approximately 34 000 m3 of water were injected into the granitic crust at 4160 m depth through the habanero 4 well more than 29 000 induced seismic events were recorded by a local 24 station network event magnitudes are in the range between m l 1 6 and 3 0 hypocenter locations could be determined for 21 720 events the spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified previously fault plane solutions determined for 525 of the strongest events are mostly consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution the fracture zone is interpreted to be of tectonic origin large stress accumulations near a reservoir boundary indicate that the hypocenter distribution is bounded by a structural limit of the fracture zone high pressure stimulation at similar depth in a nearby well caused only minor seismic activity during an eight day stimulation period less than 80 induced seismic events were recorded followed by another 139 events over the next six months the maximum event magnitude was m l 1 6 hypocenters are located in close vicinity of the injection well and align along steeply dipping fractures consistent with their fault mechanisms these fractures are not well oriented for shearing in the regional stress field they were seismically activated at extremely large overpressures and accepted only small amounts of fluid our findings indicate that natural flowing fractures play a key role for hydraulic stimulations in the crystalline crust

viscous anisotropy of textured olivine aggregates part 1 measurement of the magnitude and evolution of anisotropy the development of crystallographic textures in olivine rich rocks leads to a marked anisotropy in viscosity of the upper mantle strongly influencing a variety of large scale geodynamic processes most estimates of the magnitude of viscous anisotropy in the upper mantle are derived from micromechanical models that predict textural and mechanical evolution numerically unfortunately relatively few data exist with which to benchmark these models and therefore their applicability to geodynamic processes remains in question here we present the results from a series of laboratory deformation experiments that yield insight into the magnitude and evolution of the anisotropy of olivine aggregates during deformation along complex loading paths aggregates of fo50 olivine were first deformed in extension in a gas medium apparatus at a temperature of 1473 k confining pressure of 300 mpa and a variety of stresses and strain rates early in the extension experiments samples exhibited viscosities similar to those previously determined for isotropic aggregates extensional deformation was accompanied by formation of crystallographic textures with 100 axes dominantly aligned with the extension axis samples were subsequently deformed in torsion under similar conditions to shear strains of up to 15 5 early in the torsion experiments samples supported stresses a factor of 2 larger than measured at the end of extension experiments demonstrating a marked anisotropy in viscosity textures at the end of torsion experiments exhibited 100 axes dominantly aligned with the shear direction comparable to previous experimental observations evolution of the textures resulting from extension to those resulting from torsion was analyzed through examination of radial sections of torsion samples our results confirm that texture produces viscous anisotropy in olivine aggregates and we provide a simple calibrated parameterization of viscous anisotropy for use in geodynamic models our results also provide an extensive dataset for future calibration of micromechanical models that track the evolution of anisotropy in upper mantle rocks

topographic response to mantle lithosphere removal in the southern sierra nevada region california geological studies of mantle xenoliths entrained in late neogene quaternary lavas from the southern sierra nevada region and regional geophysical studies suggest that the highdensity mantle lithosphere that formed beneath the sierra nevada batholith in conjunction with arc magmatism is being convectively removed as a drip structure this structure as imaged seismically is roughly cylindrical in shape with a diameter of 100 km and extends to 225 km depth centered above this structure is a region 120 km in diameter that is undergoing active subsidence relative to adjacent regions such subsidence is seen in the active fluvial alluvial sediment flooding of mountainous topography of the southwestern sierra and in the development of the adjacent tulare lake basin of the san joaquin valley dynamic modeling of such upper mantle drip structures predicts a phase of overlying surface subsidence during the most vigorous phase of drip formation the southern sierra upper mantle drip and the overlying crust appear to be in this phase of their dynamically coupled evolution

stress induced seismic azimuthal anisotropy in the upper crust across the north west shelf australia seismic azimuthal anisotropy saa is observed in many areas of the earth and fast polarized directions have been mapped using earthquake surface waves and teleseismic s wave splitting over large regional and tectonic scales higher resolution petroleum exploration data including 3 d seismic surveys and dipole shear logs often exhibit azimuthal anisotropy however we are unaware of any published studies mapping saa from exploration scale data to study large scale regional and tectonic saa trends the physical mechanisms causing saa in earthquake data are a subject of great interest comparing regional trends to the higher resolution exploration trends may help understand these mechanisms we present a saa analysis using seismic exploration data across the north west shelf nws of australia we map the fast polarization directions from 34 dipole shear logs and two 3 d seismic surveys and compare them to in situ maximum horizontal stress orientations h our results show that the and h trends correlate across a geographical area spanning almost 2000 km and are similar to published results in the region from earthquake seismology these results suggest that differential horizontal stress is the likely mechanism causing saa at a wide range of spatial scales on the nws of australia we also show that saa is not observed at exploration scales in some areas of the nws and propose a lithology dependent stress sensitivity model in which saa is strongest in clean unconsolidated quartz dominated sandstones and weaker or nonexistent in well consolidated cemented rocks and shale dominated sediments

intraplate stresses and seismicity the role of topography and density heterogeneities abstract the stress induced by the topography and by density heterogeneities in the lithosphere has been computed for two different tectonic regions of the north american plate the lithosphere was assumed to consist of a two dimensional layered elastic slab over an inviscid fluid the calculations show that these stresses are locally comparable in magnitude to the regional tectonic stress 20 50 mpa two areas have been analyzed in greater detail the southeastern united states and the rio grande rift the stress in the southeastern united states is computed along a cross section perpendicular to the strike of the appalachian mountains the computed stress appears largest beneath the blue ridge and valley and ridge provinces where the appalachian seismicity is located the stress in the western united states is computed along a cross section perpendicular to the rio grande rift large stress is shown beneath the rift and the adjacent regions the stress induced by surface and internal loading is extensional in the rio grande rift and compressional in the colorado plateau and in the great plains

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

stress induced by topography and crustal density heterogeneities implication for the seismicity of southeastern canada abstract the stress induced by the topography and density heterogeneities in the lithosphere has been calculated for a large region covering most of southeastern canada the calculations assume that the lithosphere consists of a three dimensional layered elastic slab overlaying an inviscid fluid the calculations show that the deviatoric stress difference induced by density heterogeneities and the topographic load is of the same order as the regional tectonic stress tens of mpa stresses appear largest along the saint lawrence valley where most of the seismicity is located e g the charlevoix and the lower saint lawrence areas the charlevoix seismic zone was analyzed in greater detail and theoretical focal mechanisms were computed this analysis shows that the stress induced by surface and internal loading is compressional in this area and the principal stress orientation is consistent with earthquake focal mechanisms and in situ measurements

investigation of fault mechanisms during geothermal reservoir stimulation experiments in the cooper basin australia abstract subsequent to an initial reservoir stimulation in 2003 a second long term fluid injection experiment was conducted in the cooper basin australia in 2005 to further enhance and enlarge the existent geothermal reservoir approximately 25 000 m 3 of water were injected into the granitic crust at 4250 m depth thereby inducing about 16 000 seismic events which were recorded by a local eight station network event magnitudes calibrated by recordings of the permanent network of geosciences australia are in the range between m l 1 2 and 2 9 with seismic moments between 10 9 and 10 13 n m hypocenter locations could be determined for 8886 events the spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation which is further extended in the course of restimulation early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well the immediate vicinity of the injection well remains seismically quiet during restimulation these effects can be explained by a simple kaiser effect model where the induced seismicity is controlled by the in situ fluid pressure increase compound fault plane solutions based on p phase polarities indicate a similar fault mechanism for nearly all events the dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution a small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly accumulating up to several centimeters of shear displacement

viscous anisotropy of two phase composites and applications to rocks and structures abstract anisotropy of viscosity in rocks has a significant effect on the development of geological structures as demonstrated in several theoretical and analogue model studies this paper develops the results of a self consistent mechanics approach to modelling the bulk viscous properties of two phase composites to reveal the strength of the viscous anisotropy anisotropy factor is defined as the ratio of normal to shear viscosity n s the value of is determined for different types of composites of two viscous phases with varying viscosity ratio m 5 10 100 ranging from bilaminates to those with elliptical shape fabrics given by axial ratio r bilaminates with equal layer thickness are the most highly anisotropic systems where simply depends on m these results are applied to rocks modelled as two phase composites of competent and incompetent lithologies with the aim of quantifying the viscous anisotropy that is associated with natural rock deformation and the formation of geological structures it is deduced that m 5 or 10 is an appropriate maximum viscosity contrast for many metasedimentary rocks e g psammite pelite composites but that m 100 might be effectively reached in limestone marl pairs or in rocks containing surfaces of slip or zones of recrystallisation or grain boundary sliding these m values give rise to maximum values of 3 5 5 and 25 5 respectively an anisotropy factor of 25 is effectively a composite of a rigid and a viscous phase and may only be achieved in solid melt mixtures anisotropic rocks whether layered or containing a strong shape fabric will develop folds more easily than isotropic rocks characteristic structures are similar folds chevron folds kink bands and shear bands geometric features of folds especially fold angularity may provide methods of quantifying the viscous anisotropy of rocks and composites

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

evolving viscous anisotropy in the upper mantle and its geodynamic implications asthenospheric shear causes some minerals particularly olivine to develop anisotropic textures that can be detected seismically in laboratory experiments these textures are also associated with

drip instabilities of continental lithosphere acceleration and entrainment by damage summary geological evidence indicates large volume lithospheric drip instabilities form under regions of continental lithosphere the size of these drips suggests that either the base of the lithosphere is volumetrically increased or the highly viscous upper lithosphere participates in the drip previous theoretical models using simple rheologies have been unable to produce unforced large volume drip instabilities thus large drip instabilities are typically induced by thickening or destabilizing the mobile base of the lithosphere through tectonic forcing such as convergent thickening or dense magmatic emplacement following extension here we propose a theory of lithospheric drip instabilities that may arise naturally from the thermal and rheological structure of earth independent of specific tectonic forcings using damage physics relevant for earth we find a large portion of the lithosphere may be mobilized and entrained into growing drip instabilities for a critical amount of damage the growth is accelerated sufficiently that large volume drip instabilities may form within geologically feasible time frames we therefore suggest large volume lithospheric drip instabilities may arise independently of tectonic settings through damage assisted mobilization and entrainment of the highly viscous lithosphere

viscous anisotropy of textured olivine aggregates 2 micromechanical model the significant viscous anisotropy that results from crystallographic alignment texture of olivine grains in deformed upper mantle rocks strongly influences a large variety of geodynamic processes our ability to explore the effects of anisotropic viscosity in simulations of these processes requires a mechanical model that can predict the magnitude of anisotropy and its evolution unfortunately existing models of olivine textural evolution and viscous anisotropy are calibrated for relatively small deformations and simple strain paths making them less general than desired for many large scale geodynamic scenarios here we develop a new set of micromechanical models to describe the mechanical behavior and textural evolution of olivine through a large range of strains and complex strain histories for the mechanical behavior we explore two extreme scenarios one in which each grain experiences the same stress tensor sachs model and one in which each grain undergoes a strain rate as close as possible to the macroscopic strain rate pseudo taylor model for the textural evolution we develop a new model in which the director method is used to control the rate of grain rotation and the available slip systems in olivine are used to control the axis of rotation only recently has enough laboratory data on the deformation of olivine become available to calibrate these models we use these new data to conduct inversions for the best parameters to characterize both the mechanical and textural evolution models these inversions demonstrate that the calibrated pseudo taylor model best reproduces the mechanical observations additionally the pseudo taylor textural evolution model can reasonably reproduce the observed texture strength shape and orientation after large and complex deformations a quantitative comparison between our calibrated models and previously published models reveals that our new models excel in predicting the magnitude of viscous anisotropy and the details of the textural evolution in addition we demonstrate that the mechanical and textural evolution models can be coupled and used to reproduce mechanical evolution during large strain torsion tests this set of models therefore provides a new geodynamic tool for incorporating viscous anisotropy into large scale numerical simulations

a joint study of experimental deformation and experimentally induced microstructures of pretextured peridotites a series of deformation tests was performed on samples from natural xenoliths and dunites with well defined textures and microstructures the rock samples were deformed perpendicular to the foliation 45 to the foliation and parallel to the foliation at temperatures from 1000 c to 1200 c at a confining pressure of 300 mpa and a constant strain rate of 10 5 s to obtain insight into the rheological behavior of anisotropic lithospheric mantle material the experiments showed strong work hardening for all samples with a large increase of the flow stress at relatively low strain in all samples both brittle and plastic mechanisms were operative we observed nonlocalized dilatant and localized dilatant deformation features nonlocalized and localized plastic deformation created subgrain boundaries deformation lamellae and an increase in dislocation density in the olivine crystals heterogeneous dislocation activities occurred on the 100 0kl system at 1000 c and on the 100 010 and 101 010 systems at 1200 c strong crystallographic fabrics developed in samples with olivines that were initially oriented with low resolved shear stresses on their slip systems the postdeformation fabric intensity was lower in specimens with strong initial petrofabrics interactions between dislocations of different type led to high work hardening rates during high temperature deformation favoring intragranular microcracking fractures opened parallel to the 100 screw dislocations in the 010 plane and in the hk0 planes oblique to the 100 plane the strength of the deformed samples was temperature and orientation dependent rocks deformed perpendicular to the foliation are strongest and those deformed parallel to the foliation have intermediate rock strength similar to those deformed at 45 to the foliation based on the laboratory results we find important implications for the rheology of the lithosphere 1 peridotites develop semibrittle rheology even at high temperature and show characteristic microstructures and fabric changes 2 the orientation of compressive stress and initial petrofabric are important parameters for defining a deformation regime if pressure temperature and strain rate are fixed 3 interaction of slip systems produces strong work hardening and intragranular microcracking

stress orientations from borehole wall fractures with examples from colorado east texas and northern canada azimuthally aligned breakouts in oil wells are explained as shear fractures in the zone of amplified stress difference near the borehole in a stress field having unequal horizontal principal stres

a kinematic model for recrystallization and texture development in olivine polycrystals the interpretation of seismic anisotropy in the mantle requires a knowledge of the relationship between the lattice preferred orientation lpo of crystals and the convective flow field in order better to understand this link we present a model for the evolution of lpo in olivine aggregates that deform by both intracrystalline slip and dynamic recrystallization dynamic recrystallization depends on the dislocation density of the grains which is a function of the applied local stress grains with a large density of dislocations lower their bulk strain energy by nucleating strain free sub grains at a rate proportional to a dimensionless nucleation parameter grains with high energy are then invaded by grains with low energy by grain boundary migration at a rate proportional to a dimensionless grain boundary mobility m the value of is constrained by observed lpo patterns in experimentally deformed olivine aggregates and m is constrained by the temporal evolution of the strength of the lpo for m 125 75 and 3 the model predictions agree well with the experimental results numerical calculations of lpo using our model are significantly faster than those based on viscoplastic self consistent or equilibrium based theories making the model especially suitable for applications for complex convective flows

age geodynamic setting and mantle enrichment processes of a k rich intrusion from the meissen massif northern bohemian massif and implications for related occurrences from the mid european hercynian the plutonic complex of the meissen massif northern margin of the bohemian massif comprises dioritic to mainly monzonitic and granitic rocks the diorite to monzonite intrusions show major and trace element patterns typical for shoshonitic series the chemical signatures of less crustally contaminated diorites are similar to arc related shoshonitic rocks derived from continental lithospheric mantle clm sources previously enriched by subduction of altered oceanic crust laser step heating 40ar 39ar analyses on actinolitic to edenitic amphiboles from geographically different occurrences of the monzonitic intrusion yielded concordant plateau ages as well as total gas ages ranging from 329 1 1 4 to 330 4 1 4 ma and from 330 4 2 1 to 330 6 1 8 ma respectively these cooling ages are indistinguishable from sensitive highresolution ion microprobe shrimp 238u 206pb intrusion ages measured on magmatic zircon rims from the monzonite nasdala et al submitted this shows that the monzonite intrusion is probably not related temporally to active subduction because it postdates eclogites of the adjacent saxonian erzgebirge by approximately 20 ma the shoshonitic magmas intruded during strike slip tectonism along the elbe valley zone the enrichment of their mantle sources may be of upper devonian lower carboniferous age or older intrusions of shoshonitic to ultra potassic k rich rocks during the upper visean namurian are widespread in the moldanubian zone based on similar ages and structural relationships a similar post collisional setting to the meissen shoshonitic rocks can be demonstrated most of these occurrences cut high grade nappe units which were subducted during the upper devonian lower carboniferous in contrast to the meissen massif at least the ultra potassic members of the central and the south bohemian batholiths were derived from clm sources enriched by fluids or melts released from subducted oceanic crust and by greater portions of crustal material despite the similar post collisional geodynamic setting of the k rich intrusions different enrichment processes generated mid european hercynian clm sources with heterogeneous major and trace element and isotopic signatures

some geodynamical effects of anisotropic viscosity summary rheological anisotropy in the mantle may arise from various causes the most important are the preferred orientation of single crystals and the orientation of streaks of eclogitic material in a marble cake mantle in both cases the orientation can be achieved by the flow kinematics and in steady state convection the planes of easy slip will eventually be orientated parallel to the stream lines the dynamic equations for a 2 d incompressible anisotropic viscous fluid are derived and some geodynamic consequences are studied a difference of the order 10 100 between shear and normal viscosity is considered post glacial rebound would be affected by the mechanical properties in deeper parts of the mantle the observed uplift history near the ice margin has been taken as argument against strong viscosity stratification in the mantle it is shown here that with strong viscous anisotropy at least in the upper mantle the uplift history is compatible with a significant increase in viscosity between upper and lower mantle the topographic and geoid signal can show a spatial offset from a generating mass anomaly in the mantle this may provide the opportunity for in situ observation of anisotropic flow properties in the mantle in numerical case studies of steady state convection streamline orientated anisotropy is considered at high rayleigh number for bottom heated convection the formation of velocity boundary layers is observed with no flow in the stagnant core of the convection cell with internal heating or strong pressure and temperature dependence of viscosity this is no longer found but in the latter case anisotropy enhances the channelling of flow into lowviscosity regions e g hot rising plumes a few experiments have been performed for time dependent convection the results suggest that the onset of time dependent boundary layer instabilities may be somewhat retarded but would not be strongly suppressed by anisotropy strong time dependence may destroy any large scale coherence in the orientation of anisotropy and thus render the mantle to appear viscously isotropic

systematic assessment of fault stability in the northern niger delta basin nigeria implication for hydrocarbon prospects and increased seismicities abstract accurate information on fault networks the full stress tensor and pore fluid pressures are required for quantifying the stability of structure bound hydrocarbon prospects carbon dioxide sequestration and drilling prolific and safe wells particularly fluid injections wells such information also provides essential data for a proper understanding of superinduced seismicities associated with areas of intensive hydrocarbon exploration and solid minerals mining activities pressure and stress data constrained from wells and seismic data in the northern niger delta basin nndb nigeria have been analysed in the framework of fault stability indices by varying the maximum horizontal stress direction from 0 to 90 evaluated at depths of 2 km 3 5 km and 4 km we have used fault dips and azimuths interpreted from high resolution 3d seismic data to calculate the predisposition of faults to failures in three faulting regimes normal pseudo strike slip and pseudo thrust the weighty decrease in the fault stability at 3 5 km depth from 1 2 mpa to 0 55 mpa demonstrates a reduction of the fault strength by high magnitude overpressures pore fluid pressures 50 mpa have tendencies to increase the risk of faults to failure in the study area statistical analysis of stability indices si indicates faults dipping 50 60 80 90 and azimuths ranging 100 110 are most favourably oriented for failure to take place and thus likely to favour migrations of fluids given appropriate pressure and stress conditions in the dominant normal faulting regime of the nndb a few of the locally assessed stability of faults show varying results across faulting regimes however the near similarities of some model based results in the faulting regimes explain the stability of subsurface structures are greatly influenced by the maximum horizontal stress shmax direction and magnitude of pore fluid pressures

the present day state of tectonic stress in the darling basin australia implications for exploration and production knowledge of the full present day stress tensor and pore pressure has significant applications in the exploration and production of conventional and unconventional hydrocarbon reservoirs the darling basin of new south wales australia is an old sedimentary basin late cambrian silurian to early carboniferous in which there was limited information about the present day stress field prior to this study in this paper we evaluate the contemporary stress field of the darling basin using a dataset from recent exploration wells and perform a geomechanical risk assessment with respect to borehole stability fracture fault generation and reactivation our interpretations of borehole failures in borehole image logs reveal a prevailing east west orientation of the maximum horizontal stress throughout the darling basin the estimates of the magnitudes for the vertical minimum and maximum horizontal stress in the studied wells indicate a transition between thrust and strike slip faulting stress regime at 600 700 m depths where the magnitude of vertical stress and minimum horizontal stress are close to each other however the presence of borehole breakouts and drilling induced tensile fractures that we observe in the image logs at greater depths 900 2100 m indicate a transition into a strike slip tectonic stress regime below a depth range of approximately 700 900 m these findings are in agreement with overcoring stress measurements east and west of the investigated wells furthermore there are several neogene to recent geological structures in the study area that indicate thrust faulting with an east west oriented maximum horizontal stress orientation around this old sedimentary basin the consistency between the orientation of maximum horizontal stress determined from wellbore data and neotectonic structures is significant and implies that horizontal stress orientations derived from very recent geological features may be valuable inputs to geomechanical models in the absence of wellbore or other data however the recent surface geological structures suggest a thrust faulting stress regime that is in slight contrast to the transition between thrust and strike slip stress regime sh sh similar to sv indicated by petroleum data and highlights a potential pitfall of using neotectonic structures in geomechanical models in particular careful attention and verification should be made when using neotectonic structures for input calibration or confirmation of geomechanical models especially in intraplate tectonic settings such as australia c 2016 elsevier ltd all rights reserved

tectonic drivers of the wrangell block insights on fore arc sliver processes from 3 d geodynamic models of alaska intracontinental shear zones can play a key role in understanding how plate convergence is manifested in the upper plate in regions of oblique subduction however the relative role of the driving forces from the subducting plate and the resisting force from within intracontinental shear zones is not well understood results from high resolution geographically referenced instantaneous 3d geodynamic models of flat slab subduction at the oblique convergent margin of alaska are presented these models investigate how viscosity and length of the denali fault intracontinental shear zone as well as coupling along the plate boundary interface modulate motion of the wrangell block forearc sliver and slip across the denali fault models with a weak denali fault 1017 pa s and strong plate coupling 1021 pa s were found to produce the fastest motions of the wrangell block 10 mm yr the 3d models predict along strike variation in offs et along the denali fault changing from dextral strike slip motion in the eastern segment to oblique convergence towards the fault apex models further show that the flat slab drives oblique motion of the wrangell block and contributes to 20 models with a short fault and 28 models with a long fault of the observed quaternary slip rates along the denali fault the 3d models provide insight into the general processes of forearc sliver mechanics and also offer a 3d framework for interpreting hazards in regions of flat slab subduction

buoyancy driven instabilities of low viscosity zones as models of magma rich zones the low reynolds number dynamics of a thin layer of fluid bounded below by a flat horizontal boundary and above by a fluid of another viscosity and greater density is reported three distinct stages of growth were observed the first stage is a rayleigh taylor instability in which disturbances of one specific wave number grow most rapidly if e is the ratio of the viscosity of the thick layer to the viscosity of the thin layer fastest growth is for wave number e in the second stage distortion of the interface is large and it is found experimentally that the fluid moves out of the thick layer as circular columns surrounded by relatively broad regions of descending material in the third stage fully matured structures are formed if the upwelling material has less viscosity than the surrounding material the structure develops a rim syncline and a pronounced overhang and eventually ascends as a spherical pocket of fluid fed by a conduit two applications to geophysics are given the first application follows from the fact that a melt source must exist in the mantle below mid ocean ridges this source can be approximated as a cylindrical body with lower viscosity and density compared to the overlying mantle if the cylinder develops a gravitational instability it will develop regularly spaced vertical protrusions estimates of the spacing are compared to high resolution segmentation data and some constraints on viscosity of the mantle below spreading centers are made these are that viscosity of the mantle is 1018 1 p and the ratio of this viscosity to the viscosity of the cylinder is less than 100 in the second application the upwelling conduits are measured experimentally and solitary waves are observed a recently found analog with magma rising up through the pores of a viscous crystalline matrix is discussed

subduction initiation at a strike slip plate boundary the cenozoic pacific australian plate boundary south of new zealand 1 we first present a synthesis of the macquarie ridge complex mrc tectonic structures as well as paleo reconstruction models of the kinematic evolution of the pacific australia plate boundary south of new zealand since the eocene we then ascertain the geodynamical conditions that preceded subduction initiation and identify the nature and structures of the crust that first subducted at the puysegur subduction zone this synthesis is used to produce a subduction initiation model for the puysegur region concomitant to inception of the alpine fault ca 23 ma a 150 km wide transpressive relay zone developed along puysegur bank inherited structures enabling localization of compressive deformation right lateral motion at the relay zone has juxtaposed oceanic and continental crusts facilitating inception of subduction and controlling the subduction vergence subsequently the puysegur subduction zone initiated at the transpressive relay zone ca 20 ma upper and lower plate inherited structures guided and facilitated the lengthening of the subduction zone during the neogene the four individual segments of the mrc represent different stages of incipient subduction whose development depends on local geodynamical conditions and lithospheric heterogeneities the example of the mrc demonstrates that subduction can initiate from an oceanic spreading center through progressive changes in plate kinematics within a 10 15 myr time frame index terms 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158 8155 tectonophysics plate motions general 9604 information related to geologic time cenozoic keywords subduction initiation pacific australia plate motion cenezoic macquarie ridge

basement structural architecture and hydrocarbon conduit potential of polygonal faults in the cooper eromanga basin australia abstract a thorough and complete understanding of the structural geology and evolution of the cooper eromanga basin has been hampered by low resolution seismic data that becomes particularly difficult to interpret below the thick permian coal measures as a result researchers are tentative to interpret the basement fault architecture within the basin which is largely undefined to provide a better understanding of the basement fault geometry all available two dimensional seismic lines together with 12 three dimensional seismic surveys were structurally interpreted with assistance from seismic attribute analysis the upper cretaceous cadna owie formation and top permian reflectors were analysed using a common seismic attribute technique incoherency that was used to infer the presence of faults that may have otherwise been overlooked detailed basement fault maps for each seismic survey were constructed and used in conjunction with two dimensional seismic data interpretation to produce a regional basement fault map large north northeast south southwest striking sinistral strike slip faults were identified within the patchawarra trough appearing to splay from the main northeast southwest striking ridge these sinistral north northeast south southwest striking faults together with field scale southeast northwest striking dextral strike slip faults are optimally oriented to have potentially developed as a conjugated fault set under a south southeast north northwest oriented strike slip stress regime geomechanical modelling for a regionally extensive system of cretaceous polygonal faults was performed to calculate the leakage factor and dilation tendency of individual faults faults that extend into lower cretaceous oil rich reservoirs with strikes of between 060 n and 140 n and a high to near vertical dip angle were identified to most likely be acting as conduits for the tertiary migration of hydrocarbons from known lower cretaceous hydrocarbon reservoirs into shallow cretaceous sediments this research provides valuable information on the regional basement fault architecture and a more detailed exploration target for the cooper eromanga basin which were previously not available in literature

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

the role of pre existing mechanical anisotropy on shear zone development within oceanic mantle lithosphere an example from the oman ophiolite structural and fabric analysis of the well exposed hilti mantle section oman ophiolite suggests that shear zone development which may have resulted from oceanic plate fragmentation was influenced by pre existing mantle fabric present at the paleoridge detailed structural mapping in the mantle section revealed a gently undulating structure with an east west flow direction a nw se strike slip shear zone cuts across this horizontal structure the crystal preferred orientation cpo of olivine within the foliation is dominated by 010 axial patterns rather than more commonly observed 010 100 patterns suggesting that the horizontal flow close to the moho involved non coaxial flow olivine cpo within the shear zone formed at low temperature is characterized by 001 100 patterns and a sinistral sense of shear the olivine cpo becomes weaker with progressive mylonitization and accompanying grain size reduction and ultimately develops into an ultra mylonite with a random cpo pattern the olivine 010 axis is consistently sub vertical even where the horizontal foliation has been rotated to a sub vertical orientation within the shear zone these observations suggest that the primary mechanical anisotropy mantle fabric has been readily transformed into a secondary structure shear zone with minimum modification this occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling we propose that primary olivine cpo fabrics may play a significant role in the subsequent structural development of the mantle thus the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center

fracture systems and petrophysical properties of tight sandstone undergoing regional folding a case study of the cretaceous reservoirs in the kuqa foreland thrust belt tarim basin abstract the kuqa foreland thrust belt of the tarim basin hosts a prolific hydrocarbon system the main reservoirs are characterized by tight sandstone with low porosity and ultra low permeability buried presently at more than 5000 m depth natural fracture systems have a significant impact on reservoir performance and it may form paths for fluid flow in tight sandstone reservoirs and therefore plays an essential role in controlling high production rates with the aim of analyzing the distribution of favorable reservoir we studied subsurface fracture patterns and petrophysical properties of the bashijiqike formation with data collected from dabei structure and keshen structure we document the presence of four fracture systems whose distribution is related to far field stress thrusting and folding in the keshen anticlines higher fracture intensities are observed in the limbs with principal east west hinge parallel fracture system direction the hinge parallel fracture system includes a moderate dipping fracture set and a vertical fracture set crosscutting relationships stress field inversion and fracture surface textures suggest that the moderate dipping fracture set consists of shear fractures and can be explained by intra layer shear caused by thrusting the vertical fracture set may be formed due to the local outer arc extension during folding the oblique fracture system shows scattering in strike and is supposed to be related to a shear event of locally left lateral strike slip whereas in the dabei anticlines the major system is hinge perpendicular fracture system nw se showing a trending of increasing fracture intensity from limbs to hinge the hinge perpendicular fractures system in the dabei anticlines consist of shear fractures and tension fractures stress inversion result shows it corresponds to northwest southeast orientation compression stage besides we analyzed petrophysical properties include porosity permeability young s module poisson ratio and compressive strength of the bashijiqike formation concerning their structural position within folds it indicates that the highest porosity and permeability are associated with dissolution and fracturing the compressive strength of different structural positions also restrict the variations of mechanical properties of rocks the best reservoir quality is suggested in the hinge zone and areas close to thrust the findings have given maybe helpful for providing favorable reservoir targets for prospecting in the compressional basin

structural evolution of the permian triassic cooper basin australia relation to hydrocarbon trap styles the structural and depositional history of the cooper basin in eastern central australia has revealed that the basin is a mildly compressional structural depression controlled by northwest trending and northeast trending pre permian basement features pronounced pre permian compressions are indicated by northeast trending major structures the gidgealpa merrimelia innamincka and murteree nappacoongee trends detailed chronostratigraphic facies analysis with closely spaced palynological control of the patchawarra formation revealed that two pronounced phases of uplift occurred during the sakmarian the major intrabasin highs were rejuvenated during these tectonic events as documented by crestal unconformities middle and upper patchawarra unconformities evidence of each event is dominantly tectonic in character with similar depositional patterns over these highs related to each event these events are also recognizable in midflank areas and basin margins with contemporaneous deposition in deeper parts of the basin results from this research show potential for future hydrocarbon discoveries within structural stratigraphic and structural stratigraphic traps in the cooper basin various trap styles are closely associated with faults unconformities and lateral facies changes lowside fault closures onlap plays and unconformity traps are expected to be well copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 22 1995 revised manuscript received august 6 1996 final acceptance november 12 1996 2geological survey of western australia 100 plain street east perth wa australia 6004 e mail n apak dme wa gov au 3national centre for petroleum geology and geophysics ncpgg university of adelaide thebarton campus university of adelaide sa 5005 australia 4santos ltd 101 grenfell street adelaide sa 7000 australia the authors wish to thank the cooper basin consortium group of companies for permission to publish this paper the drafting staffs at santos ltd and geological survey of western australia gswa are sincerely thanked for their efforts in preparing figures this paper is dedicated to the memory of bill stuart

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

anisotropic viscosity changes subduction zone thermal structure abundant observations of seismic anisotropy in subduction zones attest that the material in the mantle wedge has a strong fabric and therefore should be mechanically anisotropic in this paper we examine the effect of anisotropic viscosity on the thermal structure of subduction zone mantle wedges and quantify its importance relative to other thermal and rheological factors using two dimensional finite element kinematic models we find that anisotropic viscosity results in two substantial changes a hotter slab wedge interface and time variability of the melt production rate and excess temperatures although not as significant as the effect of temperature dependent viscosity anisotropy leads to an increase of up to 35 c in the temperature along the slab wedge interface a hotter slab wedge interface can change the depth extent of the seismogenic zone limit the depth to which hydrous minerals can carry water and influence flux melting time variability of the thermal field is a novel result of adding anisotropic viscosity to our models this time variability results from heterogeneity in material alignment and could explain temporal changes in subduction zone magmatism without invoking a change in the wedge geometry slab age or composition

constraining stress magnitudes using petroleum exploration data in the cooper eromanga basins australia abstract the magnitude of the in situ stresses in the cooper eromanga basins have been determined using an extensive petroleum exploration database from over 40 years of drilling the magnitude of the vertical stress sv was calculated based on density and velocity checkshot data in 24 wells upper and lower bound values of the vertical stress magnitude are approximated by sv 14 39 z 1 12 and sv 11 67 z 1 15 functions respectively where z is depth in km and sv is in mpa leak off test data from the two basins constrain the lower bound estimate for the minimum horizontal stress shmin magnitude to 15 5 mpa km closure pressures from a large number of minifrac tests indicate considerable scatter in the minimum horizontal stress magnitude with values approaching the magnitude of the vertical stress in some areas the magnitude of the maximum horizontal stress shmax was constrained by the frictional limits to stress beyond which faulting occurs and by the presence of drilling induced tensile fractures in some wells the maximum horizontal stress magnitude can only be loosely constrained regionally using frictional limits due to the variability of both the minimum horizontal stress and vertical stress estimates however the maximum horizontal stress and thus the full stress tensor can be better constrained at individual well locations as demonstrated in bulyeroo 1 and dullingari north 8 where the necessary data i e image logs minifrac tests and density logs are available the stress magnitudes determined indicate a predominantly strike slip fault stress regime shmax sv shmin at a depth of between 1 and 3 km in the cooper eromanga basins however some areas of the basin are transitional between strike slip and reverse fault stress regimes shmax sv shmin large differential stresses in the cooper eromanga basins indicate a high upper crustal strength for the region consistent with other intraplate regions we propose that the in situ stress field in the cooper eromanga basins is a direct result of the complex interaction of tectonic stresses from the convergent plate boundaries surrounding the indo australian plate that are transmitted into the center of the plate through a high strength upper crust

an analysis of anisotropy of rocks containing shape fabrics of rigid inclusions abstract this paper presents a theoretical basis for estimation of mechanical anisotropy in homogeneous rocks containing shape fabrics of rigid inclusions the analysis is based on two types of viscous models one containing linear fabrics of prolate a b c inclusions cf l tectonite and the other containing planar fabrics of oblate a b c inclusions cf s tectonite models show contrasting bulk viscosities in stretching normal viscosity and shearing shear viscosity parallel to the fabric the axial ratio r a b and the volume concentration v of rigid inclusions appear to be the principal parameters in determining the viscosity contrast in anisotropic models with linear fabrics normal viscosity p increases monotonically with increase in r whereas shear viscosity s increases to a maximum and then drops down to a near stationary value in anisotropic models with planar fabrics the normal viscosity increases little with increasing flatness of inclusions but the variation assumes a steep gradient when the latter is large shear viscosity on the other hand is relatively less sensitive to the shape of inclusions the ratio of normal and shear viscosities conventionally described as anisotropy factor in both the models is always greater than 1 indicating that normal viscosity will be essentially greater than shear viscosity irrespective of the axial ratio of inclusions forming the fabric models with a linear fabric show contrasting normal viscosities in pure shear flow along and across the linear fabric the anisotropy is expressed by the ratio of longitudinal and transverse normal viscosities anisotropic factor it is revealed that the transverse viscosity is essentially less than the longitudinal viscosity as observed in test models

effects of rock anisotropy and heterogeneity on stress distributions at selected sites in north america abstract provinces of regionally consistent horizontal stress orientations persist throughout the upper crust and these may contain local areas where horizontal stress h orientations seem to rotate relative to the regional stress orientations in a consistent manner we suggest that the concept of an inclusion with anisotropy and heterogeneity of mechanical properties may be useful to explain some cases of h rotations h rotations up to 50 and magnitude changes of 40 could reasonably be expected based on simulations patterns correlate well with crustal observations rotation of h orientation reaches 25 to 58 in the different cases studied we also model a strike slip fault zone as a soft inclusion showing reorientations of hmax which agree with observed rotations near the murre fault in the jeanne d arc basin canada results imply that understanding the behaviour of soft and stiff inclusions or expanding and contracting thermal inclusions for that matter can help explain a number of geophysical and geological phenomena related to stress patterns

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

on the robustness of estimates of mechanical anisotropy in the continental lithosphere a north american case study and global reanalysis lithospheric strength variations both influence and are influenced by many tectonic processes including orogenesis and rifting cycles the long complex and highly anisotropic histories of the continental lithosphere might lead to a natural expectation of widespread mechanical anisotropy anisotropy in the coherence between topography and gravity anomalies is indeed often observed but whether it corresponds to an elastic thickness that is anisotropic remains in question if coherence is used to estimate flexural strength of the lithosphere the null hypothesis of elastic isotropy can only be rejected when there is significant anisotropy in both the coherence and the elastic strengths derived from it and if interference from anisotropy in the data themselves can be plausibly excluded we consider coherence estimates made using multitaper and wavelet methods from which estimates of effective elastic thickness are derived we develop a series of statistical and geophysical tests for anisotropy and specifically evaluate the potential for spurious results with synthetically generated data our primary case study the north american continent does not exhibit meaningful anisotropy in its mechanical strength similarly a global reanalysis of continental gravity and topography using multitaper methods produces only scant evidence for lithospheric flexural anisotropy

progressive deformation in anisotropic rocks abstract field observations show that the majority of crustal rocks possess a penetrative foliation defined by either compositional layering preferred orientations of crystals or both ductile deformation involving planar anisotropy of viscosity can be characterized by an anisotropy factor q n r s the ratio of the bulk viscosities in pure and simple shear respectively this ratio of the normal and shear viscosities may be determined analytically if the anisotropy is resolvable in a multilayer sequence with individual laminae of isotropic viscosity in that case the resistance to normal compression will be largely controlled by the competent layers whereas the resistance to shear is controlled by the soft layers more specifically the viscosities of the individual laminae add up like resistances in series and in parallel in the expressions for the normal and shear viscosities respectively the reorientation of the bulk stress within an anisotropic multilayer is systematically investigated for a range of anisotropy factors the mode of progressive deformation is controlled only by the anisotropy factor and the orientation of the principal deviatoric stress the rate of deformation can be scaled if the bulk normal viscosity and the magnitude of the principal deviatoric stress are also known the results are illustrated in a series of nomograms showing the spectrum of strain and rotation histories possible in rock volumes deforming with planar anisotropy it appears that with increasing anisotropy factor the deformation spectrum will narrow on simple shear irrespective of the orientation of the background or bulk deviatoric stress axes plane strain and isochoric conditions are assumed throughout the analysis

maximum horizontal stress orientations in the cooper basin australia implications for plate scale tectonics and local stress sources summary borehole breakouts and drilling induced tensile fractures ditfs were interpreted in 61 wells in the cooper basin indicating an average maximum horizontal stress orientation of 101 n a total of 890 borehole breakouts and 608 ditfs were interpreted in the cooper basin the approximately east west maximum horizontal stress orientation is consistent over much of the basin except in the patchawarra trough where maximum horizontal stress rotates to a northwest southeast orientation this rotation in maximum horizontal stress orientation is consistent with in situ stress data to the northwest of the cooper basin the stress field in the cooper basin appears to mark the apex of a major horseshoe shaped rotation in maximum horizontal stress direction across central eastern australia finite element modelling of the in situ stress field of the indo australian plate iap using a range of plate scale tectonic forces is able to match the regional maximum horizontal stress orientation over most of australia reasonably well including the mean east west maximum horizontal stress orientation in the cooper basin however plate boundary scale modelling does not adequately match the horseshoe shaped stress rotation across central eastern australia the average east west maximum horizontal stress orientation in the cooper basin indicates that stresses from tensional forces acting along the tonga kermadec subduction zone are not transmitted into the interior of the australian plate the majority of the tensional forces associated with the tonga kermadec subduction zone are most likely accommodated along the numerous spreading centres within the lau havre backarc basin a number of more localized stress anomalies have also been identified these cannot be explained by plate scale tectonic forces and are possibly a result of geological structure and or density contrasts locally perturbing the stress field

the australian stress map knowledge of the in situ stress field of the australian continent has increased greatly since compilation of the world stress map in 1992 principally by analysis of borehole breakouts and drilling induced tensile fractures in petroleum wells stress orientations are variable across the australian continent as a whole however within 15 of 16 individual stress provinces defined in the australian continent of one to a few hundred kilometres scale mean stress orientations are statistically significant the stress provinces and stress trajectory mapping reveal that there are systematic continental scale rotations of stress orientation within australia unlike many other continental areas stress orientations do not parallel the direction of absolute plate motion nonetheless the regional pattern of stress orientation is consistent with control by plate boundary forces if the complex nature of the convergent northeastern boundary of the indo australian plate and stress focusing by collisional segments of the boundary is recognized

lithospheric instability in obliquely convergent margins san gabriel mountains southern california 1 oblique convergence across the san andreas fault in southern california has lead to rapid uplift of the san gabriel mountains since 5 ma tomographic models of this region include a sheet like high velocity anomaly extending to depths 200 km one proposed mechanism for the formation of this feature is gravitational instability of the lithosphere resulting in a cold thermal downwelling we present a systematic study of the sensitivity of lithospheric deformation lithospheric downwelling topography deflection of the moho convergent velocity profile to the viscosity structure in 2 d numerical models of lithospheric instability including a pre existing zone of weakness strike parallel strain within a convergent region should create a local zone of weakness due to the non newtonian response of the lithosphere such a weak zone can focus convergent motion increasing and localizing the growth rate of a lithospheric instability we find that the observed characteristics of deformation depend on the width x w and relative viscous weakening factor f w the ratio of crust to mantle lithospheric viscosity eta c eta m and the absolute viscosity of the lithosphere h m for the san gabriel mountains we find that a range of models is capable of reproducing the observed moho deflection and depth extent of the downwelling within the short time since the onset of convergence however only a small subset of models x w 20 km f w 50 100 eta m 7 5 x 10 20 1 5 x 10 21 pa s eta c eta m 100 reproduce both the narrow high topography and horizontal shortening profile

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

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

azimuthal anisotropy in the d layer beneath the caribbean 1 the lowermost mantle beneath central america has anisotropic seismic velocity structure manifested in shear wave splitting of signals from south american earthquakes recorded at north american broadband recording stations prior studies of deep mantle anisotropy in this region have characterized the structure as having vertical transverse isotropy vti which is sufficient to explain a general trend of early tangential sh component arrivals however vti models cannot quantitatively match systematic waveform complexities in the onset of many of the shear waves that graze this region after accounting for splitting effects of upper mantle anisotropy beneath the recording stations we model the corrected waveform data using full wave theory for mantle velocity models with an anisotropic d layer this is the first attempt to quantitatively model a large data set including azimuthal anisotropy in d the models include transverse isotropy with either a vertical or tilted symmetry axis the latter resulting in azimuthal anisotropy for some initial shear wave polarizations tilted transverse isotropy tti produces small reversed polarity arrivals on the sv components at the arrival time of sh consistent with the data geographical variations in the azimuth of the tti symmetry axis are indicated by the data the lack of azimuthal coverage prevents unique resolution of the tti orientation and also precludes distinguishing between tti and other azimuthal anisotropy structures such as that predicted for lattice preferred orientation of minerals nonetheless our modeling demonstrates the need for laterally varying anisotropic structure of more complex form than vti for this region

convective instability of a boundary layer with temperature and strain rate dependent viscosity in terms of available buoyancy summary cold mantle lithosphere is gravitationally unstable with respect to the hotter buoyant asthenosphere beneath it leading to the possibility that the lower part of the mantle lithosphere could sink into the mantle in convective downwelling such instabilities are driven by the negative thermal buoyancy of the cold lithosphere and retarded largely by viscous stress in the lithosphere because of the temperature dependence of viscosity the coldest and therefore densest parts of the lithosphere are unavailable for driving the instability because of their strength by comparing theory and the results of a finite element representation of a cooling lithosphere we show that for a newtonian fluid the rate of exponential growth of an instability should be approximately proportional to the integral over the depth of the lithosphere of the ratio of thermal buoyancy to viscosity both of which are functions of temperature and thus depth we term this quantity available buoyancy because it quantifies the buoyancy of material sufficiently weak to flow and therefore available for driving convective downwelling for non newtonian viscosity with power law exponent n and temperature dependent pre exponential factor b the instabilities grow superexponentially as described by houseman molnar 1997 and the appropriate timescale is given by the integral of the nth power of the ratio of the thermal buoyancy to b the scaling by the available buoyancy thus offers a method of determining the timescale for the growth of perturbations to an arbitrary temperature profile and a given dependence of viscosity on both temperature and strain rate this timescale can be compared to the one relevant for the smoothing of temperature perturbations by the diffusion of heat allowing us to define a parameter similar to a rayleigh number that describes a given temperature profile s tendency toward convective instability like the rayleigh number this parameter depends on the cube of the thickness of a potentially unstable layer therefore mechanical thickening of a layer should substantially increase its degree of convective instability and could cause stable lithosphere to become convectively unstable on short cales we estimate that convective erosion will in 10 myr reduce a layer thickened by a factor of two to a thickness only 20 to 50 per cent greater than its pre thickened value thickening followed by convective instability may lead to a net thinningof a layer if thickening also enhances the amplitude of perturbations to the layer s lateral temperature structure for the mantle lithosphere the resulting influx of hot asthenosphere could result in rapid surface uplift and volcanism

the in situ stress field of the perth basin australia a total of 17 wells in the perth basin were interpreted to have 114 breakouts covering a combined length of 3 2 km and a mean hmax orientation of 108 n the inferred mean orientation of hmax in the perth basin is broadly consistent with that in the adjacent basement yilgarn block the new data confirm that hmax orientation in the region is consistent with other stress indicators different depths and different geological provinces the new data also confirm that the orientation of hmax does not parallel the nne direction of absolute plate velocity anomalous north south hmax orientations identified in a number of wells in the perth basin can be attributed to local structural effects

characterization of stress perturbations near major fault zones insights from 2 d distinct element numerical modelling and field studies jura mountains abstract we consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations based on 2 d distinct element modelling we reconstruct the local stress field around a vertical discontinuity in various geological contexts the resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 we establish simple relationships controlling stress changes around a pre existing fault zone as a function of 1 the remote differential stress magnitude 1 3 2 the friction coefficient on the discontinuity and 3 the strike of the discontinuity relative to the far field stress as a geological example we present the morez fault zone in the internal jura paleostress reconstruction in forty two sites indicates that the trends of the mio pliocene compression are n110 on average near the fault whereas they are n130 in the surrounding areas a comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 counterclockwise deviation is directly related to the reactivation of this large weak zone we thus evaluate the role of mechanical decoupling along pre existing zones of weakness especially with consideration to the accommodation of the alpine deformation in the jura belt

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

topography boundary forces and the indo australian intraplate stress field the relative contribution of topographic e g ridge push continental margins and elevated continental crust and plate boundary e g subduction and collisional forces to the intraplate stress field in the indo australian plate iap is evaluated through a finite element analysis two important aspects of the iap intraplate stress field are highlighted in the present study 1 if substantial focusing of the ridge push torque occurs along the collisional boundaries i e himalaya new guinea and new zealand many of the first order features of the observed stress field can be explained without appealing to either subduction or basal drag forces and 2 it is possible to fit the observed shmax maximum horizontal stress orientation and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes e g tens of megapascals averaged over the thickness of the lithosphere throughout the plate this study therefore presents a plausible alternative to previous studies of the iap intraplate stress field which predicted very large tectonic stress magnitudes hundreds of megapascals in some parts of the plate in addition topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental india and australia and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions a central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin we note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field and this nonuniqueness continues to make modeling the iap stress field problematic while our study is an important step forward in understanding the sources of the iap intraplate stress field a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin

prediction of anisotropy from flow models a comparison of three methods observations of anisotropy in earth are used regularly as constraints for models of deformation using various assumptions about the relationship between deformation and the resulting anisotropic fabric we compare three methods for calculating fabric from velocity fields tracking of finite strain ellipses a kinematic crystallographic code and the evolution of directors we find that the use of the finite strain ellipse provides only limited prediction capabilities as it cannot reproduce experimental observations that involve recrystallization the crystallographic code we tested a variant of the popular code d rex provides a more complete fabric prediction but at a much higher computational cost the directors method provides an intermediate solution while it does not include some of the more complex crystallographic processes that d rex does the results of this method closely resemble those of d rex at a lower computational cost the directors are also capable of tracking anisotropy at much larger strains than d rex we conclude that when computation speed is important for example in self consistent geodynamic models with anisotropic rheology the directors method provides an appropriate approximation

rayleigh taylor instability under a shear stress free top boundary condition and its relevance to removal of mantle lithosphere from beneath the sierra nevada 1 the separation of zones of apparent downwelling flow at the ends of the sierra nevada suggests a relatively large wavelength 500 km of unstable growth but rayleigh taylor instabilities for plausible rheological structures with a fixed top boundary condition require much shorter wavelengths 100 km for maximum growth rates to understand this difference we analyze analytical solutions and perform numerical 2 d plane strain experiments for rayleigh taylor instability of a dense layer overlying a less dense substratum representing the instability between the mantle lithosphere and the underlying asthenosphere focusing on the effects of a shear stress free boundary condition at the top the overall effect of this condition is an enhancement of growth rate factors at long wavelengths which depends greatly on the exponential viscosity variation with depth of the layer with large or little variation across the unstable layer the solutions approximate those with a fixed top boundary condition or for constant viscosity respectively an intermediate zone showing the enhanced growth rates includes ratios of layer thickness to viscosity e folding length h l of 1 8 for newtonian viscosity and 1 4 for nonlinear viscosity the free top condition is likely applicable to geologic situations where the lower crust is weak olivine flow laws and low temperature estimates at 35 km depth 255 355 c place the sierra nevada viscosity scaling ratio h l between 5 and 9 thus longer wavelengths than commonly assumed for rayleigh taylor instabilities seem permissible when viscosity decreases with depth and the top surface of the layer is only weakly constrained

on the role of anisotropic viscosity for plate scale flow 1 we study the role of partial melt or lattice fabric induced viscous anisotropy for lithosphere asthenosphere interactions using mantle flow models mechanical anisotropy has only a moderate influence on plate scale flow and global geoid predictions for the cases considered and anisotropic weakening effects are similar to those due to effective isotropic viscosity reduction while anisotropy modifies details of mantle flow and may be relevant for time dependent scenarios it may be safely ignored for a range of other studies these findings increase our confidence in previous isotropic modeling they also imply that melt rich layers may be a dynamically valid explanation for the lithosphere asthenosphere boundary as far as our anisotropic models are applicable

middle to late paleozoic acadian orogeny in the northern appalachians a laramide style plume modified orogeny the laramide orogeny of the western united states is proposed as a modern analogue for the silurian devonian acadian orogeny and subsequent diachronous voluminous short lived magmatism and basin formation in the northern appalachians shallowing of the benioff zone accounts for several enigmatic features associated with plate convergence in the northern appalachians including 1 wenlockian ludlovian termination of arc related magmatism in the avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 ma and 2 diachronous migration of the acadian deformation front from ca 415 ma in the southeast to ca 370 ma in the northwest extending more than 600 km into the continental interior the flattening of the subduction zone is attributed to overriding of a plume by the convergent margin which may explain 1 the abrupt termination of magmatic quiescence by 380 370 ma and voluminous felsic magmatism and production of plume related lamprophyres in the southeast meguma terrane as the plume thermally eroded the oceanic lithosphere causing melting of the lower crust 2 late devonian regional high temperature low pressure metamorphism in the meguma terrane related to the thermal anomalies above a plume 3 synchronous devonian emplacement of meguma gold deposits and associated siderophile elements possibly derived from fluid circulation above an ascending plume 4 rapid late devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume 5 migration of magmatism to the north avalon terrane cobequid highlands so that plume related carboniferous magmatism occurred in and around the carboniferous permian maritimes basin 6 the high density lens at the base of the crust beneath the maritimes basin the product of plume derived underplated mafic rocks and 7 a subsidence mechanism for formation of the maritimes basin by cooling of a decapitated plume head

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia

geodetic evidence of back arc spreading in the mariana trough 1 repeated gps surveys in the mariana islands show that the mariana block is moving apart from the philippine sea plate the velocities of the islands relative to the philippine sea plate range from about 15 mm yr in the northern islands to about 45 mm yr near guam the data also suggest convergence rates for the mariana forearc with respect to the pacific plate of 35 45 mm yr at 19 n increasing to 55 70 mm yr at 13 5 n in addition the velocity vectors show a slight north south expansion of the arc the estimated location of the euler pole of the mariana forearc with respect to the philippine sea plate is well south of the geographical point where the back arc basin narrows to zero width

overriding plate controls on subduction evolution geologic and geophysical observations indicate that the thickness density and strength of the lithosphere vary on the earth however the role of the overriding plate lithosphere properties on the evolution and morphology of subduction is not well understood this paper presents time dependent numerical models of subduction that vary the overriding plate thickness strength and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology we examine the effect of these parameters on subduction evolution in particular the emergence of a asymmetric versus symmetric subduction b trench retreat versus advance c subduction zone geometry d slab stagnation versus penetration into the lower mantle and e flat slab subduction almost all of the models presented result in sustained asymmetric subduction from initiation trench advance occurs in models with a thick and or strong overriding plate slab dip measured at a depth below the plate boundary interface has a negative correlation with an increase in overriding plate thickness overriding plate thickness exerts a first order control over slab penetration into the lower mantle with penetration most commonly occurring in models with a thick overriding plate periods of flat slab subduction occur with thick strong overriding plates producing strong plate boundary interface coupling the results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction as well as providing an explanation for the variation of slab geometry across subduction zones on earth where similar patterns of evolution are observed