uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

distribution and mechanism of neogene to present day vertical axis rotations pacific australian plate boundary zone south island new zealand remarkably little knowledge exists about mechanisms of vertical axis rotation in continental crust steeply dipping basement rocks in south island new zealand provide an opportunity to map the distribution of rotations across the pacific australian plate boundary zone and to delineate boundaries of rotated blocks in unusual detail we synthesize new structural data with new and existing paleomagnetic data with geodetic data and with patterns of neogene quaternary faulting in the strike slip marlborough fault system for the past 20 m y vertical axis rotations have been hinged about two crustal scale boundaries near the east coast the ne hinge accommodated 50 of early middle miocene clockwise rotation which caused deformation of the eastern ends of the alpine wairau and clarence strike slip faults the sw hinge has accommodated a further 30 50 of finite clockwise rotation since 4 ma and deflects active fault traces the locus of rotation has shifted southwestward astride a subduction margin that is lengthening in that direction rotating rocks are pinned to the south against a locked collision zone where the continental chatham rise impinges against the margin slip on inland strike slip faults is transformed seaward across a zone of fault termination into rigid body rotation of a large continental block that has been thrust eastward over the downgoing subducted slab of the pacific plate the rotation mechanism is a migrating hinge which resembles a flexed telephone book strike slip faults are translated through a brecciated hinge region that does not coincide with a fixed material line in the rock

late cretaceous cenozoic tectonic development of the southwest pacific region abstract a new model of the plate tectonic development of the southwest pacific integrates the continental geology of new zealand with the age structure of the surrounding oceanic crust revealed previously from magnetic anomaly lineations the model differs from previous ones in that the onland geology of new zealand is used to constrain the tectonic development in two important ways 1 the modern australia pacific plate boundary did not transect the new zealand sector until 23 million years ago 2 there has been a total of only c 500 km of dextral displacement on the plate boundary through new zealand the model is described with reference to a series of paleotectonic maps drawn to represent the setting at the times of anomalies 32 24 7 and 5 novel interpretations based on these reconstructions include the following during the late cretaceous the campbell fault was a continental transform fault between the tasman sea spreading centre and the bounty rift its 330 km of dextral displacement accompanying 25 of counterclockwise rotation of a campbell plateau block when the eastward propagating southeast indian ridge broke into the south tasman sea 57 million years ago it continued to propagate directly into southern south island and generated late eocene oligocene continental rifting through western new zealand at the same time back arc spreading in the norfolk basin caused continental rifting along a similar trend in western north island the australia pacific plate boundary originated as a transform fault between the southeast indian ridge and the pre existing kermadec trench it developed in response to the sudden cessation of spreading on the segment of the southeast indian ridge east of a fracture zone here named fracture zone z two implications arise from this model regarding the integrity of antarctica 1 1 during the cretaceous marie byrd land was probably no more than 200 km northwest of its present position 2 2 the proposition made in some earlier plate tectonic reconstructions that if the alpine fault did not form until after the late eocene there must be a late cretaceous early tertiary plate boundary within antarctica is most probably valid

tectonic rotations about vertical axes during the last 4 ma in part of the new zealand plate boundary zone the declinations of the primary magnetization of sedimentary rocks in the northern part of the new zealand plate boundary zone after thermal or alternating field cleaning have been used to determine tectonic rotations about vertical axes of rigid crustal blocks the pattern of rotations during the last 4 ma combined with structural style and continuity defines seven structural domains each ca 100 100 200 km across which contain crustal blocks up to 100 km across and 20 km thick large crustal blocks tens of km across in two of these domains have rotated clockwise more than 20 relative to one of the margins of the plate boundary zone in the last 4 ma their behaviour appears to be controlled by the nature of the plate boundaries such as the presence of an underlying subducted slab and the strength of the crust at the back of the overlying crustal wedge small crustal blocks 10 km across may have rotated clockwise through angles greater than 20 during the last 4 ma floating on an underlying zone of more distributed deformation the tectonic rotations of the large crustal blocks and the nature of the deformation at their boundaries combined with an interpolation of the relative plate positions can be used to reconstruct the plate boundary zone at ca 4 ma

paleogene seafloor spreading in the southeast tasman sea magnetic anomalies 11 18 have been identified in the southeast tasman sea the area of ocean crust southwest of fiordland new zealand west of the puysegur trench the present plate boundary and southeast of the older 80 58 ma ocean crust of the tasman sea basin structures associated with two changes in spreading direction are preserved in the area spreading between the australian and pacific plates began in the eocene about 40 ma in this area orthogonal to cretaceous paleocene spreading between them in the tasman sea the boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the resolution ridge system rifting appears to have propagated northeast along a fracture zone the northern blocks of the resolution ridge system may be isolated fragments of continental crust of the campbell plateau between anomalies 18 and 11 40 30 ma the spreading rate was about 1 5 cm yr swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11 about 30 ma and led to a change from tension to transcurrent motion along the plate boundary the timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data

surface deformation of westerly granite during creep holographic interferometry has been used to observe the surface deformation of westerly granite during creep experiments at 50 mpa 500 bars confining pressure interference fringes on the holograms produced by this technique constitute contour maps of the sample surface shape change between exposures several distinctive characteristics of the surface shape change have been observed in general surface deformation during primary and secondary creep while highly inhomogeneous is distributed fairly uniformly no large scale pattern is seen in the surface shape changes at approximately the onset of tertiary creep surface displacements become more locallized eventually approx 85 90 of time to failure a distinct ridge is formed which exactly coincides with the location of the final fracture

three dimensional numerical experiments of strain partitioning at oblique plate boundaries implications for contrasting tectonic styles in the southern coast ranges california and central south island new zealand a three dimensional numerical model is used to investigate the nature and distribution of crustal scale deformation and strain partitioning across obliquely convergent transpressive plate boundaries a uniform elastic plastic crustal layer obeying murrell s extension to three dimensions of the griffith failure criterion was deformed under basal kinematic boundary conditions that correspond to the oblique convergence between two mantle lithospheres one of which detaches and subducts beneath the other the results show deformation concentrated in relatively narrow shear zones the character planar versus curved and dip of these shear zones or crustal scale faults depend on the relative amount of convergence versus transcurrent motion imposed across the plate boundary in models dominated by transcurrent boundary conditions two separate sets of coexisting structures develop 1 a pair of planar relatively shallow dipping thrust zones arranged in a v structure that accommodate convergent motion and 2 a set of curved near vertical strike slip zones arranged in a flower structure that accommodates the transcurrent motion in models dominated by convergence both contractional and transcurrent deformations are accommodated by oblique slip across a single pair of structures that have the same geometry as the thrust shear zones there is a gradational behavior between these end member partitioned and nonpartitioned modes for intermediate boundary conditions in all cases the dip of the shear zones predicted by the numerical model is in good agreement with the dip of ideal faults predicted by mohr s incipient faulting hypothesis the two basic modes are interpreted to underlie the contrasting tectonic styles of the southern coast ranges california and central south island new zealand factors other than obliquity of convergence which play a role in strain partitioning are discussed

stress concentrations and failure modes in horizontal wells accounting for elastic anisotropy of shale formations abstract this study presents a systematic analysis of stress changes near the horizontal well sections drilled in shale and clay formations using a new approach most of the prior analyses of stress concentrations and wellbore stability in anisotropic shale formations use the traditional lekhnitskii amadei solutions we present a faster set of analytical stress functions based on largely overlooked solutions by green and taylor first the full theoretical range of possible compliances for transverse isotropic materials is evaluated using data for 5 shale clay samples and 12 additional test materials contour plots show the principal stress trajectory patterns and variation in the spatial stress concentration due to deviation from isotropic elasticity next we show how uni axial far field stress solutions can be mutually superposed and combined with stress function solutions for local stress concentration due to internal pressure on the wellbore the reappraised set of green and taylor equations is then applied to horizontal wells in three andersonian stress states for compressional strike slip and extensional basins the stress concentrations in the horizontal wellbore are evaluated for well depths between 0 and 10 000 ft using the elastic stiffnesses from five shale and clay samples to show the stress deviations from the isotropic reference case finally a full wellbore stability evaluation model combines the stress concentrations around the wellbore with failure criteria to determine the safe drilling window at each well depth for each andersonian stress state practical graphs and supporting equations are provided to assess wellbore stability in transverse isotropic shale

the oligocene miocene pacific australia plate boundary south of new zealand evolution from oceanic spreading to strike slip faulting abstract since the eocene the pacific australia plate boundary south of new zealand has evolved from a spreading system into a transform boundary swath data acquired in the southeast tasman oceanic crust between the macquarie ridge complex and the resolution ridge system show that the spreading fabric changes orientation southwards along the puysegur trench striking successively n60 e n85 e and n120 e this reflects the reorganisation of the plate boundary in response to changes in relative plate motion a comparison of these orientations with the positions of the pacific australia relative poles of rotation enables us to estimate the age of stoc where there are no identified magnetic anomalies the youngest age of the oceanic crust is ca 12 ma at the south end of the puysegur trench this age is consistent with spreading rates and the amount of crust generated since 31 ma curved fracture zones on either side of the macquarie ridge complex suggest a continuous reorientation of transform faults between 31 ma and ca 15 ma small scale seafloor morphology shows a 13 change of orientation in the l atalante fracture zone that indicates incremental rather than continuous changes in azimuth of the transform faults patterns of fanning ridges indicate that periods of asymmetric spreading accompanied the spreading segment reorientations using swath data and plate reconstruction models we infer that between 31 and 12 ma the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cumulative length of spreading segments along the whole plate boundary this indicates that since 14 15 ma the plate boundary has become progressively predominantly transcurrent allowing strike slip motion to develop along a line of merging transform faults that connected to the intracontinental alpine fault

an analytical model of coal wellbore stability based on block limit equilibrium considering irregular distribution of cleats abstract an analytical model of coal wellbore stability is established based on the block limit equilibrium method taking the irregular distribution of cleats into account because coal seam is cut into discontinuous blocks with different shapes and sizes block slip can be treated as the symbol of coal wellbore instability according to the geometric characteristic of blocks blocks surrounding the wellbore can be divided into 3 categorizes type i type ii and type iii to evaluate the block slip the stress distribution near the wellbore is determined based on the continuous medium mechanics and the fracture mechanics using mohr coulomb criterion and limit equilibrium method the critical conditions of block slip are derived by defining the failure coefficient including type i type ii and type iii the proposed model is verified by comparison of the obtained results from the proposed analytical method with the discrete element method as well as the actual drilling it can be proved that the proposed model can predict accurately block slip additionally the effect of drilling fluid pressures on the wellbore stability is specially investigated the results show that with the increasing of drilling fluid pressure the degree of wellbore collapse may be more severe meanwhile the analysis method of reasonable drilling fluid pressure for coal drilling is proposed based on the interval theory combining the slip area of blocks finally the parametric study is focused on investigating the effects of the internal friction angle of face cleats the cohesion of face cleats and the tensile strength of butt cleats on the failure coefficients of blocks

geodetically determined strain across the southern end of the tonga kermadec hikurangi subduction zone summary a method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used but the angles of the old survey have not necessarily been reobserved this method is applied to triangulation in the northern south island at the southern end of the tonga kermadec hikurangi subduction zone from a profile of shear strain across the indian pacific plate boundary the displacement of the indian plate relative to the pacific is calculated to be 54 9 mm yr at an azimuth of 84 f lo in remarkable agreement with the motion predicted by global plate tectonic models most of this motion occurs within a 15okm wide zone bounded on the east by the hikurangi trough within this zone the motion is partitioned near the hikurangi trough no slip is occurring at the upper surface of the subducting pacific plate the subduction thrust and motion is predominantly thrusting normal to the trough axis to the west is a region of predominantly dextral strike slip faulting this pattern is consistent with fitch s model of oblique subduction to the south of the profile a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab indicating the end of the partitioned motion

borehole breakouts and compaction bands in two high porosity sandstones we compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high porosity sandstones subjected to unequal far field principal stresses tablerock sandstone has a porosity of 28 and is composed of 55 quartz and 37 weaker feldspar grains grain cementation is substantial through microcrystalline quartz critical far field stresses induce failure around boreholes in the form of v shaped dog eared breakouts the result of dilatant intra and trans granular microcracking subparallel to both the maximum horizontal far field stress and to the borehole wall on the other hand boreholes in mansfield sandstone which has similar porosity 26 but contains mainly quartz grains 90 held together primarily by spot sutured contacts fail by developing fracture like breakouts we show evidence suggesting that mansfield sandstone breakouts are preceded by the development of a narrow several grain diameters localized compaction zone along the direction of the least horizontal stress which is where the maximum compressive tangential stress concentration occurs failure mechanism here is simply the removal by the circulating drilling fluid of mainly intact grains loosened during the formation of the compaction band the type of cementation mineral homogeneity grain strength and sphericity appear to be major factors in the formation of compaction bands some breakout dimensions in both rocks are correlatable to the far field principal stresses and can be potentially used in conjunction with other information as indicators of their magnitudes

investigation of present day in situ stresses and pore pressure in the south cambay basin western india implications for drilling reservoir development and fault reactivation abstract this work presents the assessment of present day stresses and pore pressures in a prolific onshore hydrocarbon bearing field located in the southern cambay basin western india through the development of a comprehensive geomechanical model a c quality wellbore breakouts inferred from the available four arm caliper logs and anisotropy analyses of dipole shear sonic logs indicate a ne n40 e to n50 e azimuth for maximum horizontal stress shmax pore pressure characterisation reveals mild overpressure in the lower eocene shale caused by disequilibrium compaction resulting from high sedimentation rates vertical stress sv was derived from density logs and a poroelastic strain model was employed to quantify the minimum shmin and maximum horizontal stress components shmax being later validated with leak off tests based on the relative magnitudes of three principal in situ stress components post rift strata upper eocene and younger reveal a normal faulting regime sv shmax shmin whereas a normal fault to strike slip faulting shmax sv shmin transition is observed in the syn rift unit lower eocene and older the mohr coulomb rock failure criterion was utilised to depict the shear failure gradient to prevent wellbore instability and results were validated with caliper logs we have also analysed the fault reactivation potential at reservoir level using frictional fault theory and stress polygons the results indicate that a minimum of 31 increase in pore pressure by fluid injection and hydraulic fracturing treatment can cause shear slippage of critically oriented faults resulting in induced seismicity our study has helped to understand the interplay between stress states and pore fluid pressure and their impact on reservoir development drilling and completion design and fault stability analysis in any given oil and gas field

crystal seismic observations across the convergent plate boundary north island new zealand abstract seismic reflection profiles recorded across southern north island new zealand define the structural details of a convergent plate boundary the eastern profile delineates the thrust wedges and back tilted basins of the accretionary prism which overlies a detachment zone marking the top of the subducted pacific plate most of the younger part of this wedge is isolated from the landward part of the sequence by an oceanward dipping detachment zone the western profile defines a broad crustal downwarp in the back arc region of the plate boundary overlying the 20 50 km deep subduction zone the crustal section farther to the west comprises a generally transparent middle crustal layer and a reflective lower crust dipping reflective zones in the middle crust may delineate low angle normal faulting associated with a cretaceous extensional episode detailed seismological data demonstrate the close association of the top of a zone of high seismicity inferred to mark the subducted pacific plate with the base of the reflective sequence identified as the base of the overriding australian plate

borehole stability analysis in a thermoporoelastic dual porosity medium the problem of diffusion and mass transfer in dual porous media is considered in a threephase framework the solid phase is assumed to contain two distinct cavities filled with fluid the porous mixture is composed of two overlapping media the porous blocks and the fissure network the fluid can transfer between the cavities due to fluid pressure difference in addition hydraulic and thermal diffusions take place through the mixture a global understanding of mass transfer diffusion and deformation is provided the governing equations associated with these phenomena are presented for a mixture in thermal equilibrium the finite element approximation of the governing equations is formulated and applied to the stability analysis of a vertical borehole a parametric analysis is carried out to evaluate the influence of mass transfer on the pressure profiles of the fluids around the borehole permeable and a semipermeable boundary conditions are compared to predict the potential for failure of the wellbore under drained and partially undrained conditions

large tectonic rotations in a wide zone of neogene distributed dextral shear northeastern south island new zealand abstract the new zealand plate boundary zone in the northern part of south island new zealand comprises a series of active dextral strike slip faults referred to as the marlborough fault zone mfz which link oblique continental convergence farther south along the alpine fault to subduction of the pacific plate along the hikurangi margin new paleomagnetic data are presented which together with previous studies suggest coherent neogene clockwise rotation up to 130 about a vertical axis of crustal blocks in the mfz starting at 20 ma there is a striking correlation between the amount of observed rotation and the trend of mesozoic basement structures creating this part of the new zealand orocline in a zone of distributed dextral shear about 100 km wide which has persisted for about 20 ma in the southern part of the mfz crustal blocks appear to be on a scale of 1 10 km with boundaries that are subparallel to the basement strike this way deformation is accommodated by a combination of fault slip and rotation in a zone of more pervasive dextral shear resulting in coherent bending of the basement structure in the central part of the mfz this rotation appears to be that of elongate crustal blocks 50 km 10 km in size rotating 80 clockwise at an average rotation rate 4 ma in detail the rotation rate has decreased towards the present from 6 ma at 20 ma to

isrm suggested methods for rock stress estimation part 3 hydraulic fracturing hf and or hydraulic testing of pre existing fractures htpf contents1 introduction 10112 summaryoftestmethods 10123 assumptionsandlimitations 10123 1 hydraulicfracturing hf 10123 2 hydraulictestsonpre existingfractures htpf 10124 apparatus 10134 1 fractureorientationdetectiondevices 10145 personnelquali cation 10146 equipmentperformance 10147 hydraulicfracturingtestprocedure 10148 hydraulic fracturedelineation 10168 1 impressionpackertechnique 10168 2 geophysicalimagingtechnique 10179 obtainingcriticalpressureparametersfromthepressure timerecords 101710 stresscalculations 101810 1 hydraulicfracturing hf 101810 2 hydraulictestsonpre existingfractures 101911 reportingofresults 1019references 1020

multiscale insights into borehole instabilities in high porosity sandstones this paper presents a multiscale analysis of the classical borehole instability problem in high porosity sandstones using a hierarchical multiscale approach a rigorous two way message passing coupling of finite element method and discrete element method is employed where the finite element method is employed to solve a boundary value problem and the constitutive material responses required at each of its integration points are derived by the discrete element method solution to an embedded representative volume element instead of by using an assumed phenomenological model we employ this multiscale approach to examine the successive failure of a borehole subjected to gradually decreased support stresses at the inner borehole wall or increased far field stresses to reproduce the material behavior of high porosity sandstone representative volume elements with a high porosity structure and interparticle bonds are generated it is found that stress concentration triggers the initial failure at the borehole wall and the subsequent failure in form of deformation bands is driven by further stress concentration ahead of the band tips the failure pattern around the borehole varies with initial stress state and global loading path the comparison of the local stress paths of the initial failure points in various cases reveals that the change of failure pattern from shear failure to mixed mode failure and to compaction failure is dominated by the increased mean stress cross scale parametric studies show that the failure mode changes from clear compaction bands to multiple arrays of shear bands by changing the high porosity specimen to a low porosity one the increase in cohesion strength expands the yield locus and enlarges the critical mean stress between different failure patterns hence the failure mode may change from compaction failure to mixed mode failure or from mixed mode failure to shear failure due purely to the increased cohesion strength the qualitative comparisons indicate greater length larger area and more severe damage of the diametrically opposite failure with the increase in minor principal stress 0 and principal stress ratio the diametrically opposite failure mode is found a good indicator for 0 direction but it may also occur under hydrostatic far field stress due to material anisotropy

impacts of catastrophic volcanic collapse on the erosion and morphology of a distal fluvial landscape hautapu river mount ruapehu new zealand debris avalanches caused by the collapse of volcanic flanks pose a great risk to inhabited areas and may permanently change the surrounding landscape and its drainage systems in this research we explored the interplay between a debris avalanche and a tectonically uplifting surrounding landscape providing insights into the long term consequences of volcanic edifice failures exposures of coarse volcaniclastic sediments along the hautapu river 50 km southeast of mount ruapehu new zealand show evidence of the largest known collapse event of the stratovolcano which was followed by a vigorous regrowth phase that produced numerous pyroclastic eruptions and pumice rich lahars similar diamicton deposits are exposed within the river catchment adjacent to the west cover bed stratigraphy and geochemical correlation of andesitic lava blocks within the debris avalanche deposit with dated lavas exposed on the cone indicate that deposition occurred between 125 and 150 ka the collapse took place during the shift from a glacial to an interglacial climate when glaciers on the cone were in retreat and high pore water pressures combined with deep hydrothermal alteration weakened the cone in addition collapse may have been accompanied by magmatic unrest the 2 3 km3 debris avalanche inundated an area of 260 km2 and entered the proto hautapu catchment where it was channelized within the deeply entrenched valley mass wasting events associated with postcollapse volcanism continued to be channeled into the proto hautapu river for another 10 k y producing long runout lahars subsequently the river catchment was isolated from the volcano by incision of the intervening whangaehu river into the proximal volcaniclastic sediments accompanied by regional faulting and graben deepening around ruapehu at present the volcaniclastic deposits form a distinctive plateau on the highest topographic elevation within the hautapu valley forming a reversed topography caused by preferred incision of the hautapu river into softer late tertiary sediments concurrent with constant uplift

closed form solution of stress state and stability analysis of wellbore in anisotropic permeable rocks abstract this work aims to develop a closed form solution of stress state around wellbore drilled in the anisotropic permeable rocks taking into account the transient effect of fluid flow for this purpose a simplified method was adopted to solve the anisotropic hydraulic diffusion equation knowing the explicit transient evolution of pore pressure the hydraulic and hydro mechanical potentials were proposed through which stress state around wellbore are determined thanks to the use of the well known complex potential approach the validity of this closed form solution was verified by comparing with the result obtained from the numerical simulation by combining the analytical solution of stresses on well wall with a chosen anisotropic failure criterion of rock mass the stability analysis of wellbore was carried out through the parametric study we elucidated the influence of different factors on the safe mud pressure window notably the anisotropic poro elastic properties the anisotropic strength of the rock masses as well as the anisotropic initial stress state and the inclined bedding plane of formation

abrupt strike slip fault to subduction transition the alpine fault puysegur trench connection new zealand swath bathymetry and other geophysical data collected over the fiordland margin southwest of new zealand are used to investigate the mechanism of transform subduction transition between the alpine fault and the puysegur trench two segments of the pacific australian plate boundary in this region the cenozoic southeast tasman basin which obliquely underthrusts fiordland at the puysegur trench is separated from the cretaceous tasman basin by the resolution ridge system a major lithospheric discontinuity of the downgoing plate interpretation of seafloor morphology shows that the alpine fault extends offshore along the fiordland coast and splits into west and east branches the west branch cuts obliquely across the margin and connects sharply to the puysegur subduction front at the northeastern tip of the resolution ridge system earthquake and seismic reflection data indicate that the west branch is genetically controlled by downgoing plate structures associated with the resolution ridge system hence the west branch is interpreted as the surface trace of the plate boundary segment extending between the alpine fault and the puysegur trench we conclude that the development of the strike slip segment of the plate boundary and its sharp transition to the puysegur subduction are controlled by inherited structures of the australian plate furthermore according to geophysical data presented here a tearing of the downgoing plate can be interpreted beneath the west branch a review of geophysical data along the region of the alpine fault hikurangi trough northeast new zealand shows a progressive transform subduction transition that is accommodated by motion partitioning between the subduction interface and strike slip faults this transition is accounted for by an interplate coupling that progressively increases toward the alpine fault in relation with a gradual thickening of the downgoing crust the comparison between the fiordland and the hikurangi strike slip subduction transitions show that presence of inherited downgoing plate crustal faults properly oriented with respect to the plate motion facilitates a sharp strike slip subduction transition

thermochronological analysis of the dynamics of the southern alps new zealand the isotopic ages of material exposed close to the alpine fault in the southern alps new zealand vary in a consistent fashion along the orogen for the k ar system muscovite and biotite ages display two distinct spatial trends that meet in the area of fox glacier where a strong relationship is observed between k ar age and altitude excess argon is present in samples between mount kinnaird and haast pass in the southwest of the southern alps giving unrealistically old inconsistent k ar ages across this region incorporation of excess argon appears to be highly variable however and minimization of its influence on dynamical interpretations of thermochronology is possible through considering only the youngest age from a given area such treatment indicates that excess argon contamination does not play an important role in the systematic k ar age variation identified along the southern alps the observed age distribution is consistent with estimates of the modern rapid uplift and exhumation of the southern alps beginning ca 5 ma k ar mica ages older than this south of the copland valley indicate that substantial late cenozoic exhumation occurred along the southern alps prior to the development of the modern tectonic regime but at rates lower than at present total exhumation during the current phase of tectonism in the southern alps decreases from the whataroa river southward north of fox glacier the southern alps have attained a dynamical steady state such that the observed distribution of exhumation will not change significantly with further deformation and denudation under current conditions

effects of water on rock strength in a brittle regime abstract failure strengths of granite and andesite have been measured under various conditions of strain rate and confining pressure both in the dry and wet states constant stress creep tests for granitic samples have been also conducted all specimens show that the failure strength decreased linearly as the logarithm of the strain rate decreased the strain rate dependence of the failure strength is increased at higher confining pressures the strain rate effect is more apparent on the failure strengths of wet samples than dry samples in lower confining pressure ranges in the constant stress creep experiments the creep failure strength decreased as the logarithm of the time to failure increased time dependent failure strength of all rocks observed in the regime of the present study may be interpreted by subcritical crack growth assisted by the stress corrosion mechanism

large tectonic rotation of part of new zealand in the last 5 ma abstract detailed paleomagnetic data from the wairoa syncline a middle miocene to the present forearc basin on the east coast of the north island new zealand show that the rate of clockwise rotation for the last 5 ma has been 7 8 ma of which less than 1 5 ma can be explained by apparent polar wander due to motion of the australian or pacific plates this rotation is similar to a present day rate of 7 ma determined from geodetic data between 5 and 20 ma ago the rate of tectonic rotation is poorly determined and may be between 0 and 2 ma the change in the rate of rotation of the wairoa syncline around 5 ma is probably related to a markedly different tectonic style in the new zealand region within the last 5 ma associated with a change in position of the euler poles of rotation for the pacific australian plates

the structure of the south fiji basin abstract new magnetic seismic and bathymetric data show that the south fiji basin is a structurally complex marginal basin a gap in the identifiable magnetic anomaly lineations exist over the central part of the basin and prevents the unequivocal linking of the anomaly lineations anomalies 7a to 12 associated with the ridgc ridge ridge triple junction in the north with an apparent single spreading centre of the same age in the south this gap which makes a detailed synthesis of the historical development of the basin difficult may arise from post spreading intraplate tectonics if symmetric spreading is assumed part of the oceanic lithosphere formed during the oligocene episode of seafloor spreading has subsequently been consumed presumably by subduction westward under three kings rise

influence of cumulative convergence on lithospheric thrust fault development and topography along the australian pacific plate boundary south of new zealand the development of faulting and topography resulting from initial convergence within oceanic lithosphere is largely unknown we explore relationships among convergence structural development and topography along 1500 km of the submarine australian pacific plate boundary south of new zealand the macquarie ridge complex mrc due to the variable orientation of the boundary and close proximity of the australian pacific poles of rotation individual segments of the plate boundary have experienced different convergence histories since 10 95 ma chron 5o because interaction along the oceanic extent of the boundary involves oceanic lithosphere of broadly the same age and therefore thermal structure structural and morphologic differences can be attributed primarily to variations in angles and rates of convergence with respect to the plate boundary orientation since 10 95 ma we relate plate boundary normal convergence determined from stage pole rotations to structural development focusing on transitions from purely strike slip faulting to partitioned thrust and strike slip faulting between 47 s and 60 s since 10 95 ma our results indicate that boundary normal convergence of 100 km marks the transition from strike slip dominated faulting to partitioned underthrusting and strike slip faulting incipient subduction establishment of subduction at the puysegur trench and incipient subduction at the hjort trench corresponds to convergence at rates between 2 and 4 cm yr angles 30 and with durations of at least 10 m y resulting in 100 km of boundary normal convergence anomalous topographic volumes resulting from tectonic deformation are quantified from swath bathymetry and compared to convergence history results from analysis of the central mrc suggest that prior to lithospheric scale thrust faulting 100 km of boundary normal convergence can be accommodated by crustal deformation 6 km relief from ridge crest to adjacent trough and strike slip faulting our research supports induced forced intraoceanic initiation of subduction

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

crustal heterogeneity and subduction processes 3 d vp vp vs and q in the southern north island new zealand summary along the southern hikurangi subduction zone in the southern north island new zealand the subduction zone changes from volcanic with moderately weak interseismic coupling to non volcanic with strong coupling in wellington to investigate how material heterogeneity relates to subduction processes we compute 3 d seismic properties using 342 earthquakes and 2 shots recorded by a 165 station seismic array the time base for some telemetered temporary stations was not stable which required revising the inversion code to allow an additional origin time related to the telemetered stations progressive inversions from 2 d to fine 3 d were carried out for vp vp vs and qp combined interpretation of these three seismic properties highlights features of the subduction zone the subducted slab is imaged as a high velocity high q feature in the overlying crust from southeast to northwest there is low velocity in the accretionary wedge high velocity in the axial ranges and low velocity in the wanganui basin high vp vs and low qp characterize the crust east of the north island dextral fault belt high vp vs is imaged above the subducted slab to 40 km depth in the northern half of the study area low qp is also prominent above the slab particularly trenchwards of the volcanic region in marked contrast to the high vp vs the low qp above the slab extends as a continuous feature up to the low qp in the shallow crust underlying the volcanic region the low qp and high vp vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release vp vs is particularly sensitive to overpressured fluid in the compressional forearc whereas qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region the rheology of the lower crust of the overlying plate is a factor in large scale subduction coupling as evidenced by the terrane boundaries and haast schist in the section near wellington the changing rheology of the lower crust of the overlying plate in the southern north island also provides a possible explanation for the southward cessation of volcanism

morphostructure of an incipient subduction zone along a transform plate boundary puysegur ridge and trench multibeam bathymetric and geophysical data reveal a major strike slip fault that extends along the summit of the puysegur ridge east of the puysegur trench the northward structural development of this ridge trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m y at the southern end of the trench where subduction has not yet started the puysegur ridge has a narrow 50 km steep sided cross section and the axial strike slip fault separates a shallow 125 625 m flat topped eastern crest from a deeper 400 1600 m western crest these characteristics indicate differential uplift during the initial stage of shortening on the lower plate an incipient 5 2 km deep trench developed in conjunction with normal and reverse faults suggesting strong interplate coupling across the trench northward the ridge broadens linearly to 80 km wide its western flank has locally collapsed and the ridge summit has subsided possibly by 1 5 km suggesting that the interplate coupling decreases and that a benioff zone is being formed concomitant to the northward ridge evolution the trench deepens to 6 2 km and normal fault throws increase along its outer wall indicating greater flexure of the downgoing plate

lithology dependent minimum horizontal stress and in situ stress estimate abstract based on the generalized hooke s law with coupling stresses and pore pressure the minimum horizontal stress is solved with assumption that the vertical minimum and maximum horizontal stresses are in equilibrium in the subsurface formations from this derivation we find that the uniaxial strain method is the minimum value or lower bound of the minimum stress using anderson s faulting theory and this lower bound of the minimum horizontal stress the coefficient of friction of the fault is derived it shows that the coefficient of friction may have a much smaller value than what it is commonly assumed e g f 0 6 0 7 for in situ stress estimate using the derived coefficient of friction an improved stress polygon is drawn which can reduce the uncertainty of in situ stress calculation by narrowing the area of the conventional stress polygon it also shows that the coefficient of friction of the fault is dependent on lithology for example if the formation in the fault is composed of weak shales then the coefficient of friction of the fault may be small as low as f 0 2 this implies that this fault is weaker and more likely to have shear failures than the fault composed of sandstones to avoid the weak fault from shear sliding it needs to have a higher minimum stress and a lower shear stress that is the critically stressed weak fault maintains a higher minimum stress which explains why a low shear stress appears in the frictionally weak fault

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis underground borehole drilling usually causes instability in the surrounding coal due to in situ stress redistribution including stress concentration and stress release however the mechanisms of unloading induced coal strength reduction are still poorly understood the primary objective of this study is to investigate the effect of confining pressure unloading on soft coal strength reduction for borehole stability analysis a series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experimental stress paths including conventional uniaxial triaxial compression and triaxial compression with confining pressure unloading the unloading stress path was obtained by analyzing the stress redistribution around a borehole to capture a more accurate coal mechanical response according to our experimental results plastic deformation generated before failure under the unloading stress path is smaller than that generated under the conventional loading stress path furthermore the cohesion of the traditionally and newly reconstituted samples diminishes approximately by 44 77 and 29 66 respectively with confining pressure unloading indicating that there is a significant reduction in coal strength due to confining pressure unloading the mechanism for unloading induced coal strength reduction comes from confining pressure unloading induced increase in shear stress on the fracture surface and a decrease in shear strength this effect increases the shear slipping potential whose driving force generates tension fractures at both ends of the preexisting fractures

a reassessment of the stress and natural fracture orientations from analysis of image logs in the chinese continental scientific drilling program borehole at donghai county jiangsu province china abstract the chinese continental scientific drilling ccsd project has drilled a 5100 m deep research borehole in the sulu ultra high pressure uhp metamorphic belt eastern china the uhp metamorphic belt is thought to be a product of continent continent collision and has consequently experienced very intensive structural deformation based on a more complete well log data set of ccsd borehole we can have a more detailed and reliable study on the structure features of borehole wall and other rock physical properties than a previous study published in 2009 abundant data related to borehole breakouts bos drilling induced tensile fractures ditfs and natural fractures were collected from the image logs the bo and ditf data indicate that the average direction of the maximum horizontal principal stress sh of the ccsd borehole site is about 79 2 which is consistent with the convergent direction e w of the pacific sea plate with respect to the eurasian plate analysis of ditfs indicated that in the case of the ccsd borehole axial drilling induced tensile fractures aditfs occur occasionally in the upper section 0 2300 m of the borehole with low dipping angle 0 10 while transverse drilling induced tensile fractures tditfs occur significantly in the lower section 3800 5000 m with high dipping angle 10 30 the natural fracture distribution at depth in the metamorphic rocks of the ccsd borehole indicates that 1 the failure strength of rocks and borehole depth are two factors that affects natural fracture frequency 2 most of the dip azimuth of natural fractures is consistent with the dip azimuth of foliations observed in the core 3 the development of most of the natural fractures probably was dominated by the development of foliations and both the natural fractures and foliations developed in response to the subduction and exhumation of the sulu terrene

observed stress state for the iodp site c0002 and implication to the stress field of the nankai trough subduction zone abstract conducting a stress state analysis is one of major challenges for evaluating earth crust and formation conditions stress magnitude is especially essential to comprehend stress heterogeneity significant structural anisotropy and or pre existing fracture systems stress state is influenced by rock strength structural properties and near field principal stresses which impact borehole integrity during drilling factors that we can measure through logging while drilling lwd and measure while drilling mwd to better understand borehole conditions include mud weight overburden weight and velocity the lwd resistivity images in holes c0002a and c0002f of the nankai trough seismogenic zone experiment nantroseize indicate that most of the drillings in stable environments however the bottom hole assembly became stuck around 3 km depth in hole c0002p and did not reach the mega thrust fault we evaluated the stress state for the site c0002 by the geomechanical model with the breakout width and rock strength estimation we constrain the stress profiles in the drilled depth our analysis inferred that the instability of hole c0002p had been caused by an overpressure drilling fluid state and low horizontal principal stress furthermore we developed an optimally oriented plane oop model to exam the stress state in the site c0002 from nankai accretionary prism to the nankai trough subduction zone the normal faulting stress regime modeled from oop is consistent with the geomechanical model in site c0002 the horizontal principal stresses magnitude appears to less than the overburden weight above this subduction zone

sedimentary architecture of a plio pleistocene proto back arc basin wanganui basin new zealand the sedimentary architecture of active margin basins including back arc basins is known only from a few end members that barely illustrate the natural diversity of such basins documenting more of these basins types is the key to refining our understanding of the tectonic evolution of continental margins this paper documents the sedimentary architecture of an incipient back arc basin 200 km behind the active hikurangi subduction margin north island new zealand the wanganui basin wb is a rapidly subsiding plio pleistocene sedimentary basin located at the southern termination of the extensional back arc basin of the active central volcanic region tvz the wb is asymmetric with a steep thrust faulted outer arc ward margin and a gentle inner craton ward margin it contains a 4 km thick succession of plio pleistocene sediments mostly lying offshore composed of shelf platform sediments it lacks the late molasse like deposits derived from erosion of a subaerial volcanic arc and basement observed in classical back arc basins detailed seismic stratigraphic interpretations from an extensive offshore seismic reflection data grid show that the sediment fill comprises two basin scale mega sequences 1 a pliocene 3 8 to 1 35 ma sub parallel regressive bpre growthq sequence that overtops the uplifted cratonward margin above the reverse taranaki fault and 2 a pleistocene 1 35 ma to present divergent transgressive bsyn growthq sequence that onlaps i the craton ward high to the west and ii uplifted basement blocks associated with the high angle reverse faults of the arc ward margin to the east along strike the sediments offlap first progressively southward mega sequence 1 and then southeastward mega sequence 2 with sediment transport funnelled between the craton and arc ward highs towards the hikurangi trough through the cook strait the change in offlap direction corresponds to the onset of arc ward thrust faulting and the rise of the axial ranges at ca 1 75 ma resulting in 5100 5700 m of differential subsidence across the fault system sedimentation has propagated south to southeast ward over the last 4 myrs at the tip of successive back arc graben volcanic arcs and the associated thermally uplifted parts of the north island following the southward migration of the hikurangi subduction margin subsidence occurred by mantle flow driven flexure the result of active down drag of the lithosphere by locking of the hikurangi subduction interface and sediment loading the wb is considered to be a proto back arc basin that represents the intermediate stage of evolution of an epicratonic shelf platform impacted by active margin processes

bonded particle discrete element modeling of mechanical behavior of transversely isotropic rock abstract this paper presents bonded particle discrete element modeling with embedded smooth joints for simulating the mechanical behavior of transversely isotropic rock with systematic verifications and extensions to laboratory and upscaled problems verification was conducted against the analytical solutions in terms of anisotropic elastic and strength behavior the developed model was compared with laboratory observations of three rock types gneiss shale and schist leading to good agreements bonded particle dem modeling successfully captured the failure patterns observed in anisotropic rock in which weak planes play a significant role the model was upscaled to large scale foundation problems subjected to surface line load the study achieved reasonable agreements both for isotropic and anisotropic solutions the results demonstrate that the bonded particle dem with embedded smooth joints can model the equivalent anisotropic medium and it has potential to be applied to problems of larger engineering problems in which anisotropy is important

mode of crustal shortening adjacent to the alpine fault new zealand the late cenozoic episode of crustal shortening in the south island new zealand which has caused present day uplift rates of greater than 10 mm yr and has formed the southern alps may have also caused clockwise rotation of the alpine fault by up to 20 in contrast to the northern end of the alpine fault which appears to have remained fixed relative to the australian plate increasing westward movement of the fault trace has occurred southward along the fault this westward movement may exceed 100 km where the fault crosses the coastline in northern fiordland most shortening adjacent to the southern end of the alpine fault has occurred within australian plate continental crust which has been thrust beneath pacific plate continental crust this zone of continental underthrusting merges with the fiordland subduction zone further south where australian plate oceanic crust is being subducted gravity modelling of the continental collision zone beneath the southern alps indicates that the leading edge of underthrust australian plate crust may be close to a relatively sharp 10 km change in moho depth inward dipping thrust faults on both plates mark the outer limits of the collision zone which is over 300 km wide near the southern end of the alpine fault the probable increase in crustal thickness due to the late cenozoic shortening suggests that the amount of crustal thickening is similar in size to the amount of eroded crust

subduction zone retreat and recent tectonics of the south island of new zealand the central south island of new zealand is a zone of recent oblique continent continent convergence between the australian and pacific plates convergence is localized on the alpine fault and its broader expression has created the southern alps we propose that many of the strike variations in orogenic style of this zone e g physical extent and style of crustal shortening argon release patterns and fission track observations can be explained by differential amounts of subduction zone retreat beneath the orogen the observed patterns suggest greater eastward retreat of the subducting pacific plate in the south than in the north to support this hypothesis we compare geodynamic models having varying amounts of subduction zone retreat with geological and geophysical observations incorporating subduction zone retreat not only creates tectonic variations similar to those observed but also provides a geodynamically consistent explanation for the size shape and location of the broad negative bouguer gravity anomaly associated with the orogen in addition these geodynamic models suggest that subduction zone retreat may result in local topographic lows which may serve as preferential locations for the formation of intermontane basins and associated lakes possibly explaining the chain of south island lakes our preferred interpretation of the geologic history of the central south island convergence has an initial phase of pacific plate lower crust and mantle subduction underthrusting with no retreat followed by a second phase of subduction zone retreat in the southern part of the zone this retreat may be a consequence of oblique convergence which creates interference beneath fiordland between the south east dipping australian plate and the underthrust west dipping pacific plate to the north and forces retreat

bending within the axial tectonic belt of new zealand in the last 9 myr from paleomagnetic data abstract tongaporutuan stage late miocene 11 6 myr sedimentary rocks at two localities within the axial tectonic belt of new zealand have a direction of magnetisation in which the declination is 020 11 and 030 8 d 95 the directions of magnetisation in rocks of similar age outside the belt have a declination like that of the earth s present axial dipole 360 the clockwise deviation of the declination is inferred to indicate bending of the rocks within the zone of deformation between the pacific and indian plates due to the oblique convergence of these plates

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

comparison between analytical and 3d finite element solutions for borehole stresses in anisotropic elastic rock we present a rigorous validation of the analytical amadei solution for the stress concentration around an arbitrarily orientated borehole in general anisotropic elastic media first we revisit the theoretical framework of the amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry contrary to previous understanding provided that the reduced strain coefficients b11 and b55 are not equal it is shown from theoretical considerations and published experimental data that the b11 and b55 are not equal for realistic rocks second we develop a 3d finite element elastic model within a hybrid analytical numerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation third we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations vertical deviated and horizontal and for several cases involving isotropic transverse isotropic and orthorhombic symmetries it is concluded that the analytical amadei solution is valid with no restriction on the borehole orientation or the symmetry of the elastic anisotropy

reconstructions of the new zealand region for the neogene abstract the major structural elements of the present day plate boundary zone through new zealand are a subduction zone under the northern half of the country linked by a wide transform zone to the southward extension of the plate boundary through fiordland and the macquarie ridge using finite rotations of the pacific australian plate pair a sequence of reconstructions of the relative positions of the plates is presented reconstructions of the plate boundary zone based on paleomagnetic data involve essentially the same structural elements as today but with large clockwise changes in their trend with time

the tectonic evolution of the southern alps new zealand insights from fully thermally coupled dynamical modelling summary we present here the results of numerical models of the southern alps in which the thermal and dynamical developments of the orogen are fully coupled to one another this interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models in particular the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the alpine fault zone in these models this thermal weakening of the crust also produces a zone of high strain antithetic to the alpine fault in the upper crust that is comparable to the main divide fault zone within the southern alps the introduction of orographic rainfall and erosional processes into the model leads to the development of high asymmetric topography comparable to that of the southern alps this topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side due to the time required to accumulate suycient topography for this rain capture evect to become significant the establishment of this high asymmetric topography lags the initiation of surface uplift by several million years comparison of the observed thermal history of the southern alps with that seen in models based on divering hypotheses of the tectonic evolution of the south island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 ma with relative stability of the tectonic regime since that time the variation in isotopic ages along the southern alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the australian and pacific plates

dual porosity poroelastic analyses of wellbore stability this paper presents a numerical solution of dual porosity poroelastic formulations that couple solid deformations with fluid flow in both matrix and fracture systems of naturally fractured reservoirs the finite element numerical method was applied to solve the inclined wellbore problem in which mud weight was considered for both permeable and impermeable boundary conditions failure criteria relevant to tensile collapse and shear failure modes were introduced into the numerical model and resulting failure areas around boreholes were subsequently defined several related applications including the stability of inclined and horizontal wellbores under various in situ stress regimes were evaluated upper and lower mud weight bounds and the most stable borehole orientations were determined the best trajectory selections for horizontal boreholes were also investigated

age and origin of the new zealand orocline in relation to alpine fault movement the alpine fault and the recurved arc structures orocline in the late palaeozoic mesozoic rangitata orogen are two of the first order structures of the new zealand subcontinent a commonly held view is that the orocline originated as a gigantic drag fold with alpine fault movement in view of the post oligocene age of the alpine fault however this concept involving a single recurved arc cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the rangitata orogen and its cover rocks these data indicate at least 62 of dextral rotation during each of the mesozoic rangitata orogeny and the late cenozoic kaikoura orogeny a new model presented here postulates that there are actually two dextral recurved arcs a western arc which lies mainly outside the australia pacific plate boundary zone has a multiple origin having formed chiefly during the late triassic phase of the rangitata orogeny by the subduction accretion of the exotic rakaia terrane an eastern arc coincides with the area of the present plate boundary zone it formed as a mega brittle ductile shear zone with the late cenozoic development of the modern australia pacific plate boundary which includes the alpine fault this solves the problem of the nature and location north of the alpine fault of the relative plate movement evident as 480 km of displacement on the alpine fault a major implication of this model for plate tectonic reconstruc tions of the sw pacific is that only approximately 500 km of dextral displacement has occurred through new zealand during the late cretaceous cenozoic compared with the commonly adopted figure of 1000 km

new insight into the dynamic development of the southern alps new zealand from detailed thermochronological investigation of the mataketake range pegmatites

up dip partitioning of displacement components on the oblique slip clarence fault new zealand abstract active strike slip faults in new zealand occur within an obliquely convergent plate boundary zone although the traces of these faults commonly delineate the base of mountain ranges they do not always accommodate significant shortening at the free surface along the active trace of clarence fault in northeastern south island new zealand displaced landforms and slickenside striations indicate predominantly horizontal displacements at the ground surface and a right lateral slip rate of ca 3 5 5 mm year during the holocene the inland kaikoura mountain range occupies the hanging wall of the fault and rises steeply from the active trace to altitudes of ca 3 km the geomorphology of the range indicates active uplift and mountain building which is interpreted to result in part from a vertical component of fault slip at depth these data are consistent with the fault accommodating oblique slip at depth aligned parallel to the plate motion vector and compatible with regional geodetic data and earthquake focal mechanisms oblique slip on the clarence fault at depth is partitioned at the free surface into 1 right lateral displacement on the fault and 2 hanging wall uplift produced by distributed displacement on small scale faults parallel to the main fault decoupling of slip components reflects an up dip transfer of fault throw to an off fault zone of distributed uplift such zones are common in the hanging walls of thrusts and reverse faults and support the idea that the dip of the oblique slip clarence fault steepens towards the free surface

discussion on integrating borehole breakout dimensions strength criteria and leak off test results to constrain the state of stress across the chelungpu fault taiwan abstract the in situ stress computations presented by haimson et al 2010 overestimated the maximum horizontal stress in the vicinity of the chelungpu fault taiwan our analyses from extended leak off tests drilling induced tensile fractures and anderson s faulting theory show considerable lower values for the maximum horizontal stress our results suggest that closer to the ruptured fault more s h is released implying a strong earth crest in the shallower section which is far away from the ruptured fault however it would require more detailed and intensive work in order to achieve an accurate interpretation of the in situ stress in this region

temporal variations in latest quaternary slip across the australian pacific plate boundary northeastern south island new zealand rates of latest quaternary slip from stream terraces and moraines displaced by faults of the alpine shear system in ne south island new zealand vary in space and time detailed histories of fault slip are obtained from combining displacement data with estimates of the age of the surface from weathering characteristics precision in surface ages is 5 20 using rock weathering rinds and 15 50 using soil properties the oldest surfaces examined are 15 20 ka and have right lateral fault offsets up to 400 600 m the main faults of the in the ne south island the wairau awatere clarence hope kekerengu and porters pass faults have average latest quaternary right lateral slip rates of 3 10 5 10 7 10 25 40 5 7 and 3 4 mm yr respectively every fault has undergone a substantial decrease in lateral slip rate in the last 3 5 kyr summed across the plate boundary the average latest quaternary slip rates are comparable to long term rates of horizontal slip across the australian pacific plate boundary around 40 mm yr parallel to the boundary and rates of geodetic strain and seismic moment release over the last 50 100 years approximately the same however sums of lateral fault slip rates over the interval from 15 to 5 ka exceed the plate motions whereas late holocene lateral fault slip rates are less than half the long term average the best explanation of these variations is slip across the plate boundary is episodic varying over perhaps 5 kyr intervals this implies that 15 20 kyr is the time interval necessary to average out shorter 5 kyr episodic variations in plate boundary motions

seawater redox variations during the deposition of the kimmeridge clay formation united kingdom upper jurassic evidence from molybdenum isotopes and trace metal ratios 1 the kimmeridge clay formation kcf and its equivalents worldwide represent one of the most prolonged periods of organic carbon accumulation of the mesozoic in this study we use the molybdenum mo stable isotope system in conjunction with a range of trace metal paleoredox proxies to assess how seawater redox varied both locally and globally during the deposition of the kcf facies with lower organic carbon contents toc 1 7 wt were deposited under mildly reducing suboxic conditions while organic rich facies toc 7 wt accumulated under more strongly reducing anoxic or euxinic local conditions trace metal abundances are closely linked to toc content suggesting that the intensity of reducing conditions varied repeatedly during the deposition of the kcf and may have been related to orbitally controlled climate changes long term variations in 98 95mo are associated with the formation of organic rich intervals and are related to third order fluctuations in relative sea level differences in the mean 98 95mo composition of the organic rich intervals suggest that the global distribution of reducing conditions was more extensive during the deposition of the pectinatites wheatleyensis and lower pectinatites hudlestoni zones than during the deposition of the upper pectinatites hudlestoni and pectinatites pectinatus zones the global extent of reducing conditions during the kimmerigidan was greater than today but was less widespread than during the toarcian early jurassic oceanic anoxic event this study also demonstrates that the mo isotope system in jurassic seawater responded to changes in redox conditions in a manner consistent with its behavior in present day sedimentary environments

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

pliocene exhumation and landscape evolution of central north island new zealand the role of the upper mantle 1 mudstone porosity analyses show that over 2 km of exhumation has occurred in central north island new zealand since 5 ma this combined with a coeval surface uplift of 500 m indicates a maximum 2 5 km of rock uplift in central north island the magnitude and wavelength 400 km of the rock uplift is similar to some topographic doming observed in continental areas associated with mantle upwelling yet central north island is recognized as a region of back arc extension marine terraces 100 150 km from the center of maximum exhumation show uplift rates of 0 2 0 3 mm yr 1 consistent with about 1 km of exhumation if averaged over 4 5 myr which compares well with mudstone porosity analyses seismic attenuation and slow pn velocities indicate that a thermal anomaly in the upper mantle is the source of this rock uplift the rock uplift may be explained by the replacement of a 100 km thick mantle lid by asthenosphere with a density 66 kg m 3 less than the lid about half this density contrast can be explained by thermal expansion and the rest is ascribed to partial melts in the upper mantle southward migration for the locus of maximum rock uplift is consistent with the zone of maximum incision erosion being 50 km south of the southernmost volcano such a migration is in keeping with other geological markers of volcanic and thermal phenomena and further strengthens the link of landscape evolution at active continental margins with processes in the mantle

in situ stress pore pressure and stress dependent permeability in the southern qinshui basin this study focuses on the in situ stress pore pressure and permeability in the southern qinshui basin one of largest coalbed methane basins in china well tests show that permeability in this basin is higher than other coalbed methane reservoirs this is because it is located in an extensional basin where the normal faulting stress regime is dominated this in situ stress regime is advantageous to keep coal cleats open hydraulic fracturing tests indicate that the fracture gradient or minimum horizontal stress is much lower than the shales in the gulf of mexico and other oil basins the minimum horizontal stress model is proposed with consideration of the stress coefficient based on the uniaxial strain method this model provides a fairly good prediction on the minimum stress permeability data show that the effective stress dependent permeability is pronounced in the coalbed methane reservoir this is significant for the dual porosity and dual permeability coal reservoirs which consist of coal porous matrices and cleats the reason is that a rapid increase in effective stress can induce the closure of cleats which may cause a permanent loss of permeability in the cleats this reduces the connectivity between the cleats and coal matrices hence the coal matrices cannot deliver gas pressure to the cleats for supporting the cleat space therefore slowing down the effective stress change during production e g slowing reservoir drawdown can decelerate the permeability reduction this is particularly important for the reservoir in which the pore pressure is not significantly overpressured such that in the southern qinshui basin

coal strength and young s modulus related to coal rank compressional velocity and maceral composition abstract this paper presents an experimental investigation of coal rank and maceral composition influences on the coal mechanical behaviors the complete stress strain behavior uniaxial compressive strength young s modulus and acoustic compressional velocity were measured and correlated to coal ranks and microstructures the test results show that coal is an elasto brittle geo material and its uniaxial compressive strength and young s modulus increase as coal rank increases this occurs because as vitrinite reflectance or coal rank increases coal has less microporous structure and thus higher uniaxial compressive strength therefore using vitrinite reflectance value instead of vitrinite content is advantageous for correlating coal strength the experimental results also demonstrate that compressive strength and young s modulus have positive exponential correlation even for different types of coal therefore the compressive strength of coal is highly related to its young s modulus the uniaxial compressive strength and acoustic compressional velocity of coal are also correlated but a single correlation does not exist for different coal ranks instead different relationships occur for different types of coal

a model of the cenozoic evolution of northern new zealand and adjacent areas of the southwest pacific abstract new information from the southwest pacific indicates that earlier attempts at formulating the evolution of the area assuming a single upper cenozoic magmatic arc are untenable it now appears that there were two arcs during the miocene and pliocene a western northland three kings rise arc and an eastern tonga lau kermadec colville arc both appear to have developed above west dipping subduction zones it is suggested that the norfolk and reinga basins formed as back arc basins to the western arc and that eastern north island lay adjacent to northland and formed the accretionary prism to that arc upper cenozoic evolution of the region involved the simultaneous opening of the norfolk reinga basins consumption of the western portion of the oligocene south fiji basin by subduction beneath the western arc and eastwards movement of new zealand three kings rise towards the tonga kermadec arc when the kermadec and hikurangi trenches came into line late in the pliocene the tonga kermadec arc was able to propagate rapidly southwards into north island simultaneously the western arc became extinct and the tectonic tempo and strike slip faulting accelerated markedly throughout new zealand eastern north island was moved dextrally an uncertain distance relative to the western north island and rotated 25 30 clockwise this accounts for the paradox of a 22 m y old accretionary margin lying adjacent to a 2 m y old arc taupo volcanic zone at the present day

poroelastic response of a borehole in a non hydrostatic stress field abstract this paper deals with the solution of the stress displacement and pore pressure field induced by the drilling and or the pressurization of a vertical borehole the rock which is permeated by fluid is assumed to behave as a poroelastic material with compressible constituents following the biot theory the analytical solution is derived in the laplace transform space and is transformed to the time domain using an approximate numerical inversion technique the solution reveals a full range of coupled poroelastic effects which provide potential mechanisms for delayed borehole instability and shear fracture initiation inside the rock

characterization of pore pressure fracture pressure shear failure and its implications for drilling wellbore stability and completion design a case study from the takouazet field illizi basin algeria abstract this work presents the assessment of overburden stress pore pressure and fracture pressure distribution in the paleozoic stratigraphic units in the takouazet field eastern illizi basin algeria we studied a vertical exploration well where significant hydrocarbon discoveries have been established in devonian and ordovician sandstone reservoirs overburden stress interpreted from density log has an average 1 07 psi ft 24204 04 pa m gradient pore pressure derived from sonic log reveals that hydrostatic conditions prevail in devonian and ordovician units whereas abnormal pore pressure with a maximum 0 66 psi ft 14929 60 pa m gradient was recorded against the marine silurian shale fracture pressure was modeled using effective stress ratio models as well as a poisson s ratio based approach rock property based fracture pressure estimate is more sensitive to lithological variations and hence this estimate was used for determining safe drilling mud window and casing placements the estimated pore pressure and fracture pressure were calibrated with direct downhole formation pressure measurements and leakoff test lot measurements respectively shear failure using mohr coulomb rock failure criterion detected the minimum allowable mud pressure to avoid wellbore instability issues and the result was validated with caliper log observations it indicated that solid production from the reservoir is very likely during initial production from the reservoir and a solid control mechanism has been suggested also widening of breakouts and deteriorating wellbore interpreted from time lapse formation imaging logs strongly suggest against open hole completion based on interpreted pressure gradients and image log analyses optimum drilling mud and casing design and completion policies were recommended for safe and successful operational planning

the kinematics of the plate boundary zone through new zealand a comparison of short and long term deformations summary surveys of shear strain rates derived from repeated triangulation cover about one quarter of the area of the plate boundary zone through new zealand and are well distributed within it using a method developed by haines relating shear strains to the total strain and velocity fields the dilatational and rotational strain rates and the velocities within the deformed zone are derived the plate boundary zone is shown to be two dimensional for the most part in that gradients of the velocity along the length of the zone are small or zero the shear strain rates are not uniform however they reach a maximum along a central axis and decrease gradually to zero toward the sides nor are the two independent components of strain one involving shortening or extension perpendicular and the other shear parallel to the zone equally developed everywhere there is a clear geographic separation with shortening developed over the subduction thrust and shear developed behind in a zone of wrench faulting time variations in the strain field in part of the region above the subduction interface are related to changes in the degree of coupling across the interface a velocity field is computed the relative velocity of the two plates from this data agree very well with that derived from seafloor spreading data these strain and velocity fields derived from geodetic studies give the short term kinematics of the plate boundary zone the long term kinematics of the zone are estimated from 1 uplift and subsidence data which are related to changes in crustal thickness and hence to dilatational strain and 2 variation in the declination of primary magnetization in rocks of known age related to the rotational strain the rate of rotational strain has increased in time from about 2 to 3 myr 1 10 myr ago to about 9 myr 1 today the increase can be understood as a secondary effect of a developing compressional component increasing the coupling between the plates so that more of the motion parallel to the zone is accommodated by distributed shear and less by aseismic slip the rates of strain averaged over the last 1 myr or so and at a scale length of a few tens of kilometres are closely similar in distribution and magnitude to the strain rates derived from short term kinematics while therefore at scale lengths of a few hundred metres or so faulting is very important and the deformation is discontinuous with unstrained blocks faulted with large but uncertain slip against adjacent blocks the deformation at these longer scale lengths is continuous and regular the two consequences of this study are that 1 strain from geodetic data can be directly related to the average long term deformation and therefore can be an important tool in the study of deformation in regions of current tectonic activity and 2 at scale lengths of a few tens of kilometres the deformation of a region can be described as the deformation of a continuum and we can ignore the effects of faulting

ridge reorientation mechanisms macquarie ridge complex australia pacific plate boundary the macquarie ridge complex portion of the australia pacific plate boundary south of new zealand provides a unique complete record of a 60 90 change in spreading direction since 40 ma that resulted in the transition from a spreading center to a transform plate boundary marine geophysical data show that during reorientation most ridge segments completely disappeared and all shortened additionally modification of newly created crust caused differences in widths of correlative spreading segment corridors on the two plates we propose two models for ridge reorientation that explain the observed spreading fabric and arcuate fracture zone relationships nonrigid plate deformation was accommodated by failing and propagating spreading ridge segments rifts and transfer of crust between plates during the gradual reorientation of the spreading axes

evolution of stress induced borehole breakout in inherently anisotropic rock insights from discrete element modeling the aim of this study is to better understand the mechanisms controlling the initiation propagation and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings a two dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs both isotropic and anisotropic hollow square shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale the isotropic model is validated by comparing the stress distribution around borehole wall and along x axis direction with analytical solutions effects of different factors including the particle size distribution borehole diameter far field stress anisotropy and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress far field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings results from this study can provide fundamental insights on the underlying particle scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock

a comparative study of the anisotropic dynamic and static elastic moduli of unconventional reservoir shales implication for geomechanical investigations abstractwe obtained the complete set of dynamic elastic stiffnesses for a suite of shales representative of unconventional reservoirs from simultaneously measured p and s wave speeds on single prisms specially machined from cores static linear compressibilities were concurrently obtained using strain gauges attached to the prism regardless of being from static or dynamic measurements the pressure sensitivity varies strongly with the direction of measurement furthermore the static and dynamic linear compressibilities measured parallel to the bedding are nearly the same whereas those perpendicular to the bedding can differ by as much as 100 compliant cracklike porosity seen in scanning electron microscope images controls the elastic properties measured perpendicular to the rock s bedding plane and results in highly nonlinear pressure sensitivity in contrast those properties measured parallel to the bedding are nearly insensitive to stress this anisotropy to the pressure dependency of the stra

the last 25 ma of rotational deformation in part of the new zealand plate boundary zone abstract the marlborough fault system is a zone of dextral transpression in continental crust and marks the southern end of the hikurangi subduction system between the obliquely convergent australian and pacific plates detailed mapping over an area of 100 km2 has revealed a complex history of deformation during the last 25 ma the development of a thrust belt in the miocene referred to as d1 deformation was synchronous with the deposition of a thick submarine fan complex the beginning of d1 deformation may mark the initiation of the subduction zone in this part of the plate boundary zone subsequently in the plio pleistocene the region formed a zone of dextral shear marked by major strike slip faults and zones of distributed deformation referred to as d2 deformation crustal blocks less than 5 km across may have rotated ca 60 clockwise about vertical axes relative to the pacific plate in the last 4 ma the pattern of deformation during the last 10 ka to 100 years is consistent with the long term d2 deformation and suggests that large crustal blocks resting on the subducted pacific plate have rotated clockwise about vertical axes relative to the pacific plate accommodated by dextral shear further to the sw in the marlborough fault system the overall effect of the deformation has been the rotation of the edge of the southern part of the hikurangi margin from a w to nw trend in the early miocene to a ne trend today

neogene evolution of lower trench slope basins and wedge development in the central hikurangi subduction margin new zealand detailed analysis of the stratigraphic architecture and deformation of lower trench slope sedimentary basins permits the tectonic evolution of subduction margins to be constrained this study utilises offshore seismic reflection profiles and onshore outcrop data to examine the entire lower trench slope of the hikurangi subduction margin in the eastern north island new zealand our results constrain the main spatial and temporal changes of facies and sedimentary units since about 25 ma we demonstrate that the geometries and locations of miocene to quaternary sedimentary basins are controlled by tectonic activity and reflect stages of subduction wedge development four types of sedimentary basins have been recognized 1 flysch basins with local olistostromes at the front of seaward propagating thrust sheets 2 5 10 km wide turbidite rich trench slope basins between uplifting structural ridges i e anticlines associated with shortening within 100 km of the subduction front at the seafloor 3 30 40 km wide trench slope basins associated with an upslope increase in thrust and ridge spacing and 4 mixed siliciclastic carbonate shelves formed in association with margin uplift after filling of the wider 30 40 km trench slope basins the lateral and vertical successions of basin geometries and sedimentary infill are consistent with the overall progressive uplift of the subduction wedge formation of some of the wide trench slope basins may be accompanied by significant local subsidence and normal faulting synchronous with active shortening at the subduction front margin wide normal faulting during the middle late miocene may have formed due to upslope collapse related to tectonic erosion all of the basins studied contain major unconformities at their base and top with basin strata deposited over about 2 8 myr the short life span of these lower trench slope sedimentary basins is consistent with a succession of short paroxysmal tectonic episodes rather than continuous deformation for the duration of subduction stratigraphic discontinuities within basins e g facies changes and reversal of paleo currents also record short term tectonic events c 1 2 myr on the basin bounding structures and attest to the episodic nature of upper plate deformation in response to continuous subduction beneath the active margin

experimental and theoretical study of the anisotropic properties of shale abstract the effect of bedding plane orientations on shear strength and failure mechanisms of shale is investigated with an emphasis on the shear parameters of the bedding planes this study is based on direct shear tests conducted on directional shale samples obtained from formations which exhibit well developed stratification the tested cylindrical specimens were prepared by coring at 30 intervals from the direction of the bedding planes the results indicate that the anisotropy of the shear strength of shale can be readily observed due to the anisotropic effects of bedding plane orientation and normal stress the shear parameters such as strength cohesion and internal friction angle reach their maximum and the minimum values at 60 and 0 respectively where is the angle between the bedding planes and the coring orientation which is also the direction of normal stress during the laboratory tests it is demonstrated that the bedding planes are weak not only with regards to cementing strength but also with regards to friction three different shear failure modes were built up depending on the shearing angle 1 sliding failure across the bedding planes 2 sliding failure along the bedding planes and 3 sliding failure across the bedding planes combined with tensile splitting along the bedding planes an expression of the shear stress concentration factor of the direct shear model is derived to evaluate the anisotropy of shear strength

sub critical fracture and associated retrogression of garnet during mylonitic deformation garnets from the alpine fault mylonites are fractured most fractures have a retrogressive fracture fill a small number have garnet fill a detailed sem study has been carried out to elucidate the mechanisms of fracture and associated retrogression wide fracture zones contain free floating islands of garnet in a retrogressive matrix electron channelling patterns show that segments of garnet across such fracture zones rarely show any relative rotations where rotations are observed they are small and systematic some large fractures show clearly that fracture and retrogression occurs by a replacement process fracture surface morphologies are indicative of surface dissolution a model is established where crack tips advance by a process of dissolution at a high stress crack tip whilst precipitation occurs as the tip and its associated stress field advance precipitation of retrogressive fracture fill occurs if fracture occurs out of the field of stability of garnet