are we studying deformed rocks in the right sections best practices in the kinematic analysis of 3d deformation zones abstract a standard procedure in kinematic analyses of deformed rocks is studying samples only on the foliation perpendicular lineation parallel section based on the assumption that the rotational component of deformation is restricted to that plane the theoretical background for this procedure is that deformation is plane strain which is often useful but fails to account for all relevant features observed in many natural cases more general 3d kinematic models reproduce several of these features and help to better understand 3d deformation likely predominant in nature so they are a more appropriate starting point for deformation zone analysis theoretical and natural 3d deformation examples suggest that the section with highest degree of asymmetry the vorticity normal section vns and the corresponding vorticity vector are independent from the finite strain fabric and thus may present varied orientations with respect to the tectonic foliation and lineation the vns orientation depends on the kinematics resulting from boundary conditions whereas the strain fabric orientation also depends on accumulated strain the orientation of the vns is itself an essential element of shear zone kinematics hence it cannot be pre assumed but must be determined independently

lithospheric structure across oblique continental collision in new zealand from wide angle p wave modeling 1 oblique convergence between the australian and pacific plates in south island new zealand has resulted in crustal thickening and distributed deformation within both plates we measure this thickening and image the deformation with seismic wide angle data along a 600 km long transect that spans the plate boundary p wave arrival times from 34 000 rays are used to construct a two dimensional model for seismic velocity with depth crustal velocities average 6 1 and 6 2 0 23 km s for the australian and pacific sides of the boundary respectively upper mantle velocities average 8 1 0 25 km s distributed deformation is indicated by the following observations a reduction in upper crustal and midcrustal velocities by 4 west of the plate boundary ascribed to flexural bending stresses a velocity reduction of 8 immediately east of the alpine fault linked to high pore fluid pressures 17 km of crustal thickening to form a 44 1 4 km deep crustal root that is offset 10 20 km se of the highest mountains a velocity reversal below the midcrust in the eastern part of the crustal root caused either by heating or an artifact from seismic anisotropy strong thickening of a 7 0 0 4 km s fast lower crust within the crustal root and a low velocity zone in the australian upper mantle due to sampling the slow orientation of upper mantle anisotropy

variations in exhumation level and uplift rate along the obliqu slip alpine fault central southern alps new zealand we use geophysical and geological data from the southern alps to explore the relationship between plate motions and crustal structure on the geomorphology exhumation state and deformation style of rocks uplifted along amajor oblique slip fault a 50 km long segment of the southern alps has a higher uplift rate more relief deeper exhumation and a narrower width than surrounding regions there the delaminated east tilted crust of the pacific plate yields the youngest late cenozoic thermochronometric ages contours for fission track ar ar and k ar ages on several different minerals define an asymmetrically nested pattern of ages that increase away from the western side of the central southern alps eleven new 4 0 ar 3 9 ar samples of hornblende from the hanging wall of the alpine fault indicate that lower crustal rocks exhumed from 500 c in the late cenozoic are confined to a 20 km long culmination at the southern end of the central southern alps ages as low as 3 5 ma imply time integrated vertical exhumation rates as high as 6 9 mm yr this is the only part of this 5 8 ma range that may have achieved exhumational steady state remnant plugs of the original crustal hanging wall ramp are apparently preserved outside the central alps implying 70 km of fault convergence there 4 0 ar 3 9 ar age trends for hornblende near the alpine fault suggest that horizontal surfaces in the lower crust in the pacific plate have been overturned by reverse slip ductile shearing across a zone of distributed deformation that extends 2 km beyond the 1 km thick basal mylonite zone at the broadest orogen scale higher uplift rates throughout the central southern alps may be related to a rheologically controlled increase in the convergent velocity of points to the east of the alpine fault associated with a strengthening of the pacific crust at a more local scale maximum rates of uplift are inferred to occur near franz josef and fox glaciers because the alpine fault steepens at depth structural data suggests that its footwall ramp is curved and that the fault s dip steepens by 15 20 relative to its attitude farther to the south this 10 20 km long restraining bend may enhance local rates of rock uplift near franz josef and fox glaciers contemporary normal stress and shear resistance may also be increased on this part of the alpine fault helping to explain the central region s quiet historic seismicity and apparently strongly locked nature

lateral uniformity of india plate strength over central and eastern nepal the current understanding of the himalayan lithosphere stems mostly from cross sections through the range at the longitude of the kathmandu basin in this paper we laterally extend the analyses of structures and rheology along the nepal himalayas between the pokhara valley and the arun river we take advantage of available information and a new data set including gravity measurements and a receiver function profile it appears that the geometry of the moho inferred from seismological profiles and long wavelength gravity anomalies does not exhibit major east west variations within the 350 km wide study area using thermomechanical modelling we show that the northward deepening of the moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong india plate this result suggests a gradual mechanical decoupling between the crust and the mantle leading to a northward decrease of the effective elastic thickness of the indian lithosphere from 75 km to 25 km beneath the ganga basin and the tibetan plateau respectively our results also confirm partially eclogitized lower indian crust beneath southern tibet at shorter wavelengths the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east west variations of the thickness of subsurface geological structures such as the ganga basin and the tethyan sedimentary sequence

very high strains recorded in mylonites along the alpine fault new zealand implications for the deep structure of plate boundary faults abstract oblique displacement on the alpine fault which forms the principal structure along the australian pacific plate boundary in south island new zealand has resulted in exhumation of a kilometre wide mylonite zone in the hanging wall adjacent to the current brittle fault trace the mylonites formed under amphibolite facies conditions at depths of ca 25 km and have been uplifted during the past 5 ma a suite of 65 70 ma pegmatite veins in the hanging wall alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca 100 km measurements of the thickness distribution of the pegmatite veins within the non mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites the thicknesses approximate a log normal distribution with a mean value that is progressively reduced through the protomylonites mylonites and ultramylonites by assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions shear strains calculated range from 12 to 22 for the protomylonites 120 to 200 for the mylonites and 180 to 300 for the ultramylonites corresponding to pure shear values of 1 3 in each case these values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 ma were accommodated within a 1 2 km wide mylonite zone through the middle and lower crusts the results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth

quartz fabrics in the alpine fault mylonites influence of pre existing preferred orientations on fabric development during progressive uplift strong quartz crystallographic preferred orientations cpos were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse alpine fault zone new zealand the cpos have a consistent asymmetry indicating a high ratio of simple to pure shear strain with a shear sense of dextral up to the nw consistent with the mesoscopic shear sense indicators and with slip on the active fault there is a transition from y maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace to cross girdle fabrics in the protomylonites further from the fault the strong y maxima or single girdle cpos are ascribed to high ductile shear strains under amphibolite facies conditions while the cross girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions however the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation the highly oriented y maximum fabrics formed at high temperature contain very few grains suitably oriented for basal slip so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism or rhomb further from the fault where shear strains under high temperature conditions were lower weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns this interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high temperature fabrics to become sufficiently intense to be preserved this in turn implies that deep seated localised shear was taking place early during the evolution of the current oblique slip plate boundary fault the results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect future work should consider the effects of deformation history total strain and the presence of other mineral phases

ductile fabrics in the zone of active oblique convergence near the alpine fault new zealand identifying the neotectonic overprint the mid crustal alpine schist in central southern alps new zealand has been exhumed during the past similar to3 m y on the hanging wall of the oblique slip alpine fault these rocks underwent ductile deformation during their passage through the similar to 150 km wide pacific australia plate boundary zone likely to be cretaceous in age peak metamorphism predates the largely pliocene and younger oblique convergence that continues to uplift the southern alps today late cenozoic ductile deformation constructively reinforced a pre existing fabric that was well oriented to accommodate a dextral transpressive overprint quartz microstructures below a recently exhumed brittle ductile transition zone reflect a late cenozoic increment of ductile strain that was distributed across deeper levels of the pacific plate deformation was transpressive including a dextral normal shear component that bends and rotates a delaminated panel of pacific plate crust onto the oblique footwall ramp of the alpine fault progressive ductile shear in mylonites at the base of the pacific plate overprints earlier fabrics in a dextral reverse sense a deformation that accompanies translation of the schists up the alpine fault ductile shear along that structure affects not only the 12 km thick section of alpine mylonites but is distributed across several kilometres of overlying nonmylonitic rocks c 2001 elsevier science ltd all rights reserved

controls on lineation development in low to medium grade shear zones a study from the cap de creus peninsula ne spain lineations composed of similarly oriented elongate mineral aggregates or grains are a common feature in deformed rocks but it is unclear which factors control the development of such lineations field observations and microstructural analysis of samples which were taken from discrete greenschist to lower amphibolite facies shear zones of the easternmost variscan pyrenees show that strain is only one of several factors that control the strength and type of a lineation dynamic recrystallization metamorphic reactions and rigid body rotation are also important controlling factors for the development of lineations the most important of these is dynamic recrystallization the way in which dynamic recrystallization influences lineation development is largely a function of the initial fabric in a specific rock type different lithologies with different initial fabrics produce distinctly different types and strengths of lineations even if deformed to the same finite strain in the same shear zone an initial fine grain size and a monomineralic composition of the parent rock type commonly hinder the development of lineations in contrast in initially coarse grained polymineralic rocks well developed lineations commonly develop therefore the ratio of initial to dynamically recrystallized grain size determines to a large extend the development of such lineations

fission track analysis of the late cenozoic vertical kinematics of continental pacific crust south island new zealand during the late cenozoic the pacific plate has been converging obliquely with the australia plate in south island new zealand a result of this convergence has been the growth of a major mountain range the southern alps at the leading edge of the pacific plate the results of fission track analysis of 140 samples from 13 transects across the alps reported here establish the late cenozoic vertical kinematics amount age and rate of rock uplift of the pacific crust underlying the alps the late cenozoic rock uplift of the pacific crust is asymmetrical with respect to the alpine fault being a maximum 19 km immediately east of the central part of the fault with lesser values at the eastern 3 km northern 10 km and southern 8 km extremities of the alps the age of the start of rock uplift varies spatially across the southern alps the earliest indications from fission track analysis being at 8 ma at the southern end of the alps decreasing to 5 ma at the northern end and 3 ma along the southeastern margin this age variation reflects the longer time over which the southern parts of the alps have been in collision the rate of propagation of rock uplift southeastward into the pacific plate has been 30 mm yr nearly 4 times the late cenozoic average rate of convergence normal to the plate boundary late cenozoic mean rock uplift rates range from a maximum of 2 8 mm yr at the alpine fault to a minimum of 1 0 mm yr in the east and have been sustained for periods of 3 8 m y accompanying denudation has exhumed amphibolite grade rocks immediately east of the alpine fault the rock uplift has been controlled by oblique slip displacement on the alpine fault a continental crustal section at least 19 km thick has been uplifted on the alpine fault comparison of the late cenozoic mean rock uplift rates with uplift rates derived from reset zircon data 2 10 mm yr near the alpine fault shows that uplift has accelerated over time but only significantly since 1 3 0 3 ma the amount of mesozoic uplift ranged from minimal amounts north of arthur s pass to 3 km near mount cook to 10 km in the south at lake wanaka

ore mineralogy of pgm placers in siberia and the russian far east abstract platinum group minerals pgm placers of siberia and the russian far east are subdivided into three associations on the basis of mineralogical and geochemical features the first is a ruthenium iridium osmium ophiolitic association from zolotaya suyenga and balyksa river placers a type restricted to ultramafic rocks of the ophiolitic belts os ir ru alloys are dominant over pt fe nuggets two magmatic parageneses of source rocks are typical in a type association early osmium iridium and isoferroplatinum ruthenium which forms after the os ir ru system is saturated with ru in early paragenesis the distribution coefficient of ru between osmium and iridium kdruhex cube is 1 35 1 97 with the composition plots of os ir ru alloys on an os ru ir pt diagram forming a ruthenium trend the second association is ruthenium iridium osmium b type the source of which has not been located although b type materials are found along the ko tayenza rivers in this assemblage os ir ru alloys are dominant over other platinum group element pge minerals and their compositions vary from os to ir on the os ru ir pt diagram with no ruthenium trend os ir ru alloys are characterized by gradual and contrast zoning in composition from osmium in the centre to ruthenium or iridium in the rim the kdruhex cube is less than 1 with an average 0 4 isoferroplatinum occurs as discrete grains and inclusions in these os ir ru alloys a third association is that of isoferroplatinum and is restricted to the alaskan type inagli gal moenan philippa massifs and occurs in the kuznetsk alatau along the kaura kaurchak rivers placers pt fe solid solution mostly pt3fe is dominant in these placers two parageneses are typical in source rocks an early isoferroplatinum osmium parageneses and later isoferroplatinum iridium parageneses the second one is the result of exsolution structure development from primary pt ir ru os solid solutions which formed as the ore forming system saturated with ir plots of osmium composition form an osmium trend on the os ru ir pt diagram in the all associations products of alteration and replacement of primary magmatic minerals caused by late magmatic s and as enriched residual fluids or by late serpentinization are observed the main secondary minerals comprise pge sulphoarsenides and laurite in the os it ru alloys and cooperite and sperrylite in the pt fe alloys

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

deformation path partitioning within a transpressive shear zone marble cove newfoundland detailed study of the structures in marble cove newfoundland indicates that they formed during a single transpressional event during which deformation was partitioned between dominantly strike slip and dominantly dip slip domains marble cove exhibits steeply dipping foliations steeply to shallowly plunging stretching lineations greenschist facies mineral assemblages and abundant quartz veins within the broad domain where stretching lineations plunge steeply sense of shear indicators and asymmetry of quartz c axis distributions in mylonitized quartz veins indicate dominantly reverse motion with a component of dextral shear adjacent greenschists exhibit orthorhombic symmetry and apparently accommodated pure shear the abrupt transition to a narrow high strain domain of shallowly plunging stretching lineations is accommodated by a rheologically distinct serpentinite body shear bands in this domain indicate dextral strike slip movement with a reverse component smaller conjugate shear bands are evident in thin section indicating a component of pure shear sense of shear indicators and asymmetry of c axis distributions in mylonitized quartz veins also record dextral strike slip and reverse shear with a component of pure shear crenulations and asymmetric folds in foliation overprint all other fabrics in marble cove and record a final phase of reverse shear with a dextral component with partitioning of this kind structures recording strike slip movement may be limited in areal extent and therefore less likely to be exposed than structures recording dip slip motion such partitioning within transpressional shear zones may therefore impede recognition of strike parallel motions within orogenic belts

cenozoic plate boundary evolution in the south island of new zealand new thermochronological constraints 1 thermochronological investigations of samples collected west of the alpine fault zone provide new insight into the early development of the australian pacific aus pac plate boundary through new zealand that is not preserved elsewhere in the modern orogenic system of the south island the 40ar 39ar fission track and u th he ages for these samples span the cenozoic and provide direct constraints on the timing and character of two discrete episodes in the evolution of this tectonic boundary the initial propagation of the aus pac boundary through the south island in the early miocene is expressed as short lived geographically localized cooling which we infer to be due to exhumation beginning at 23 25 ma this is consistent with development of this boundary as an initially distributed zone of deformation progressively localizing onto discrete fault structures developing primacy of strain accommodation over time regionally synchronous development of rapid cooling at 8 ma corroborates the association of the present obliquely convergent character of the aus pac boundary through the south island with a marked change in pacific plate motion at that time subsequent increases in cooling rate and a regionally prominent 5 ma thermochronologic signal illuminate the progressive adaptation of the plate boundary to this new tectonic system over a period of 3 myr culminating in development of the extant tectonic regime

quaternary slip rate and geomorphology of the alpine fault implications for kinematics and seismic hazard in southwest new zealand glacial landforms at 12 localities in 9 river valleys are offset by the southern end of the onshore alpine fault offsets cluster at 435 1240 and 1850 m consistent with evidence for glacial retreat at 18 58 and 79 calendar ka the peak of an offset fluvial aggradation surface is correlated with the last glacial maximum at 22 ka displacement rates derived from features aged 18 22 58 and 79 cal ka are 24 2 2 2 23 2 4 9 21 4 2 6 and 23 5 2 7 mm yr respectively with uncertainties at the 95 confidence level the joint probability weighted mean and arithmetic mean of all observations pooled by rank are 23 1 1 5 23 2 1 4 and 23 1 1 7 mm yr respectively we conclude that the mean surface displacement rate for this section of the alpine fault is 23 1 mm yr with standard error in the range of 0 7 0 9 mm yr the reduction in estimated long term slip rate from 26 6 mm yr to 23 2 mm yr results in an increase in estimated hazard associated with faulting distributed across the rest of the plate boundary model dependent probabilities of alpine fault rupture within the next 50 yr are in the range 14 29 the 36 3 mm yr of total plate motion nuvel 1a is partitioned into 23 2 mm yr of alpine fault dextral strike slip 12 4 mm yr of horizontal motion by clockwise block rotations and oblique dextral reverse faulting up to 80 km southeast of the alpine fault and 5 3 mm yr of heave on reverse faults at the peripheries of the plate boundary

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

reprint of extensional shear band development on the outer margin of the alpine mylonite zone tatare stream southern alps new zealand abstract schistose mylonitic rocks in the central part of the alpine fault af at tatare stream new zealand are cut by pervasive extensional c shear bands in a well understood and young natural ductile shear zone the c shears cross cut the pre existing mesozoic aged foliation displacing it ductilely synthetic to late cenozoic motion on the af using a transect approach we evaluated changes in geometrical properties of the mm cm spaced c shear bands across a conspicuous finite strain gradient that intensifies towards the af precise c attitudes c foliation dihedral angles and c s intersections were calculated from multiple sectional observations at both outcrop and thin section scales based on these data the direction of ductile shearing in the alpine mylonite zone during shear band activity is inferred to have trended 20 clockwise down dip of the coeval pacific australia plate motion indicating some partitioning of oblique slip motion to yield an excess of dip slip relative to plate motion azimuth or some up dip ductile extrusion of the shear zone as a result of transpression or both constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary szb across all examined parts of the shear zone the mean dihedral angle between the c shears and the mylonitic foliation s remains a constant 30 1 1 the aggregated slip accommodated on the c shear bands contributed only a small bulk shear strain across the shear zone 0 6 0 8 uniformity of per shear slip on c shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain hardening rheology such that they locked up at a finite shear strain inside c bands of 12 15 shear band boudins and foliation boudins both record extension parallel to the szb as do the occurrence of extensional shear band sets that have conjugate senses of slip we infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with w k

influence of exhumation on the structural evolution of transpressional plate boundaries an example from the southern alps new zealand concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength three dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate bounding structure is stable if asymmetric erosion patterns such as those observed in orographic mountain belts pertain and if earth9s crust has a strong on weak rheology in early stages of oblique convergence of an initially laterally homogeneous material lateral boundary parallel strain is accommodated along a near vertical structure and convergent boundary normal strain is concentrated on structures dipping at moderate angles into the orogen exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure when the upper crust beneath the orogen is significantly weakened by exhumation lateral strain abandons the vertical structure and shifts to the dipping structure combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust as is the case along the alpine fault new zealand the process of thermal thinning is controlled by advection and occurs on time frames of 1 2 m y the two components of strain remain separate in the lower crust during active convergence exhumation of lower crustal material occurs only along those structures accommodating convergent strain consequently material exhumed from lower regions of ductile deformation as is the case along the alpine fault contains lineations that indicate a greater component of convergence than predicted from the total plate motion postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones one dominantly convergent and one dominantly strike slip widely reported orogen parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector but the exhumation of the lateral transport zone

platinum mineralization of the svetly bor and nizhny tagil intrusions ural platinum belt abstract this paper presents the results of a detailed mineralogical and micro analytical study of two ural alaskan type intrusions in the ural platinum belt 1 the dunite hosted mineralization of the svetly bor intrusion and 2 the chromitite mineralization of the nizhny tagil intrusion two generations of platinum minerals are typical of both intrusions magmatic pt fe ni alloys and post magmatic pt fe ni cu alloys a trend from ferroan platinum to isoferroplatinum pt fe pt 3 fe is shown for magmatic alloys of the svetly bor intrusion magmatic alloys of the nizhny tagil intrusion are represented by ferroan platinum with ni only varying in fe the magmatic pt fe alloys of both intrusions were depleted in fe during the evolution of ore forming systems and crystallized during the entire magmatic process generally as fine cubic crystals and anhedral grains hosted by dunite at the magmatic stage during and after the crystallization of dunite platinum dunite type of the svetly bor intrusion the evolution of mineral paragenesis was accompanied by a temperature drop as well as increases in f o 2 and f s 2 most of the platinum was concentrated in the residual melts together with chromium and crystallized in the final stage of the magmatic process platinum chromite ore of the nizhny tagil intrusion post magmatic pt fe cu ni alloys formed during the serpentinization of dunite evolved according to a general compositional trend from tetraferroplatinum and ferronickelplatinum to tulameenite and later to pt cu alloy and pt oxide ptfe pt fe cu ni cu 3 pt pt o platinum rich mineralization of both intrusions was formed in the late magmatic stage from a melt rich in volatiles regardless of their location in dunite or chromitites most probably the rock structure fracture systems was the determining factor in the migration of pge rich residual melts and ore accumulation

microstructural evolution and rheology of quartz in a mid crustal shear zone abstract we present microstructural and crystallographic preferred orientation cpo data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization deformation processes and cpo evolution the sample investigated here is a moderately deformed quartz rich mylonite from the blue ridge in virginia we have created high resolution crystallographic orientation maps using electron backscatter diffraction ebsd of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100 recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast we document a systematic decrease in fabric intensity with recrystallization suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre existing cpo relationships between recrystallization fraction and shear strain suggest that complete microstructural re equilibration requires strains in excess of 5 variation in the degree of recrystallization implies that strain was accumulated heterogeneously and that a steady state microstructure and rheology were not achieved

colloform high purity platinum from the placer deposit of koura river gornaya shoriya russia abstract a microinclusion of colloform high purity platinum in a grain of platinum group minerals pgm from the alluvial gold bearing placer deposit in the south of western siberia russia was detected and characterized for the first time it is different in composition texture and conditions of formation from high purity platinum of other regions described in the literature the main characteristics of investigated high purity platinum are colloform layered texture admixture of fe 0 37 0 78 wt and paragenesis of cu rich isoferroplatinum hongshiite and rhodarsenide the pgm grain with high purity platinum is multiphase and heterogeneous in texture it is a product of intensive metasomatic transformation of cu rich isoferroplatinum pt 3 fe 0 6 cu 0 4 the transformation was carried out in two stages 1 copper stage including three substages cu rich isoferroplatinum copper platinum and hongshiite and 2 arsenic rhodarsenide the formation of high purity platinum was separated in time from the formation of isoferroplatinum and was carried out by precipitation from postmagmatic solutions

lattice rotation in polycrystalline aggregates and single crystals with one slip system a numerical and experimental approach abstract in this study lattice rotations in polycrystals and single crystals with one slip system have been analysed for pure shear and simple shear by re visiting previously published data and by conducting new numerical models that are compared with results from experiments on polycrystalline ice the numerical models are based on the finite difference method and on the assumption that dislocation glide on one slip system is the dominant crystalline deformation mechanism and is controlled by the critical resolved shear stress law such a deformation scheme corresponds to the operation of glide on 0001 in polycrystalline ice used in the physical deformation experiments the results show that lattice rotation is primarily controlled by the bulk deformation kinematics in both the polycrystalline aggregates and the single crystals in the single crystals the lattice rotation is entirely consistent with the vorticity of the bulk deformation kinematics whereas in the polycrystalline aggregates extensive grain interactions significantly modify the local lattice rotations and may even lead to the lattice planes of individual grains rotating in an opposite sense to that of the bulk deformation these results can reasonably explain the development of crystallographic preferred orientations widely reported in the literature

deformation path in high strain zones with reference to slip partitioning in transpressional plate boundary regions abstract the current status of the kinematics and strain geometry of high strain zone studies is briefly summarized a general high strain zone has a triclinic deformation path and monoclinic shear zones are special end member cases fabrics observed in natural shear zones and theoretical considerations based on continuum mechanics are compatible with this conclusion non steady deformation paths remain difficult to deal with and may ultimately rely on a realistic mechanical treatment of high strain zones which may be possible when our knowledge of the mechanical behavior of rocks under natural deformation conditions is improved an examination of the phenomenon of slip partitioning in transpressional plate boundary regions shows that the bulk deformation path in the forearc area trench parallel high strain zone is generally triclinic the alpine fault in the south island of new zealand provides an example of a currently active triclinic shear zone the southern knee lake shear zone of manitoba canada provides an archean example of a triclinic shear zone

monoclinic model shear zones abstract although many ductile shear zones are supposed to have developed by approximately simple shear flow some must have formed under different conditions a few types of such non simple shear zones have been proposed in the literature such as transpression transtension and stretching shear zones this paper presents a full three dimensional kinematic model of shear zones with monoclinic flow geometry monoclinic shear zone types can be classified according to flow parameters and according to the geometry and orientation of accumulating finite strain modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop in monoclinic shear zones these include stretching lineations normal to the displacement direction foliations normal to the shear zone boundary switching of the position of principal finite strain axes x y and z during progressive deformation and strain gradients along shear zones that may give rise to obliqueness of fabric elements of the same age in adjacent rock volumes

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

seismic anisotropy and its impact on imaging the shallow alpine fault an experimental and modeling perspective the transpressional alpine fault in new zealand has created a thick shear zone with associated highly anisotropic rocks low seismic velocity zones lvz and high seismic reflectivity are recorded in the alpine fault zone but no study has explored the underlying physical rock parameters of the shallow crust that control these observations protomylonites are the volumetrically dominant lithology of the fault zone here we combine experimental measurements of p wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged numerical models that account for the porosity free real samples fabric elastic tensors from ebsd are calculated by mtex and a finite element model fem while microfractures are modeled with differential effective media dem theory at effective pressures representative of the alpine fault brittle zone experimental wave speeds are lower than those predicted by mtex fem a possible dem model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures 70 mpa such micro porosity in the form of foliation and mica basal plane parallel microfractures and grain boundaries is validated with synchrotron x ray microtomography and tem images finally by modeling anisotropy of seismic reflection coefficients with angle of incidence we demonstrate that the high reflectivity and lvz observed at the alpine fault can only be explained if this microporosity is accounted for throughout the brittle fault zone even at depths of 7 10 km

relationships between kinematic indicators and strain during syn deformational exhumation of an oblique slip transpressive plate boundary shear zone the alpine fault new zealand abstract in order to assess how different elements of plate boundary scale shear zone tectonite fabrics reflect the boundary kinematics and strain history we have examined a range of such structures within the active alpine fault mylonite zone new zealand in this oblique slip zone we find that quartz cpo fabric symmetry and individual mica grain lineations reflect incremental stretches but develop stable orientations parallel to the finite stretching direction s 1 this s 1 trends oblique to the plate motion vector because the shear zone experiences some extrusion toward the free surface conversely asymmetric structures such as shear bands and mica fish reflect simple shear dominated deformation late in the shear zone history parallel to the imposed relative plate motion vector finally linear fabrics are a poor record of the mylonitic deformation in this zone because 1 they are dominantly inherited and have been only partially re oriented during mylonitisation and 2 macroscopically they become less visible at higher strains due to fabric homogenisation generally these results indicate that particularly in heterogeneous polyphase shear zones a full understanding of the movement history can only be obtained by considering kinematic information from as many different sources as possible

crustal deformation during 1994 1998 due to oblique continental collision in the central southern alps new zealand and implications for seismic potential of the alpine fault the positions of 115 ground marks in a 150 100 km area of oblique continental collision in the central southern alps new zealand have been measured by global positioning system gps two to four times between 1994 and 1998 contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and alpine fault across strike more than 60 of the strain occurs within a band from 5 km nw to 20 km se of the alpine fault but significant strain continues at least a further 60 km se to near the edge of the southern alps foothills projections of the fault parallel and fault normal components of velocity onto an alpine faultnormal profile show that about 85 of the nuvel 1a model relative plate motion is observed within the gps network the surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50 70 of the relative plate rate in a region deeper than about 5 8 km on the down dip extension of the se dipping alpine fault material shallower than this is behaving elastically and thus storing elastic strain in the region of the alpine fault the longer wavelength displacements can be modeled either as distributed deformation beneath the southern alps or by localization of elastic strain around the upper end of a discrete nw dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the se boundary of the mountains if extrapolated to the surface strain determined from a small scale survey network crossing the alpine fault indicates no significant near surface aseismic fault slip on the central alpine fault over the past 25 years our results are consistent with independent geological evidence that the central section of the alpine fault is capable of producing large to great earthquakes

continental transforms a view from the alpine fault abstract continental transform faults are dominantly highly localized strike slip shear zones hundreds of kilometers long that accumulate tens to hundreds of kilometers of displacement from work on the alpine fault we pose the questions what is the deep structure of a continental transform and how does the displacement become localized we review research on the alpine fault and propose a model in which the fault partitions at depth into a steep zone extending into the mantle with largely fault parallel motion and a flat ductile decollement in the lower crust the fault localizes around two thirds of the plate motion within a 100 km wide zone of distributed deformation a review of other active continental fault systems suggests that variation between them may reflect their tectonic origins the nature of the crust in which they develop the presence of a significant oblique component of motion and the displacement rate all however have evidence for the development of a single principal fault zone that carries 50 of the total displacement and extends as a localized zone of shear into the upper mantle we review mechanisms of strain weakening and suggest that localization of a principal fault may be initiated in the seismogenic crust and through a series of positive feedbacks eventually extend through the lower crust into the upper mantle

continental crust under compression a seismic refraction study of south island geophysical transect i south island new zealand 1 the 1996 south island geophysical transect sight active source seismic survey was designed to show the style of lithospheric thickening due to late cenozoic oblique convergence across the australian pacific plate boundary in new zealand as part of this study two seismic refraction lines were shot across central south island and offshore extensions of the continental crust in the tasman sea and pacific ocean we present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects a tomographic inversion of 62 563 travel times from crustal and upper mantle refractions and wide angle reflections resulted in a model with a two layer crust upper crustal velocities were between 5 9 and 6 3 km s and lower crustal velocities were between 6 5 and 7 0 km s continental compression has locally reduced the seismic velocities in the pacific plate crust by 0 2 0 3 km s a possible effect of high strain and fluids in the crust the thickening of the crust from 28 km at the east coast of south island to 37 km beneath the southern alps can account for about 25 of the 80 110 km shortening of pacific plate crust while the rest must be accounted for by rapid erosion of mesozoic sedimentary rocks on the west side of the orogen in our model the lower crust forms a continuous 2 6 km thick layer beneath central south island the asymmetric topography of the southern alps is reflected in the crustal root which has a steeper flank at the west coast this observation is consistent with westward underthrusting of pacific lower lithosphere beneath south island that has been suggested in earlier studies

brittle and ductile deformation in a zone of rapid uplift central southern alps new zealand schists exposed in the central southern alps new zealand 1 5 km east of the alpine fault zone have been rapidly uplifted during the late cenozoic a regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle ductile transitional behaviour to ductile deformational features kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction indicating their relation to late cenozoic uplift of the southern alps analysis of deformed veins suggests a minimum shortening strain of about 50 perpendicular to the foliation the veins crosscut upright mesoscopic to macroscopic folds which commonly develop high strain zones on their limbs within such zones a strong stretching lineation plunges gently sw approximately perpendicular to the stretching lineation in the mylonites along the alpine fault the folds and high strain zones do not appear to be related to late cenozoic uplift but may have originated during an earlier phase of dominantly strike slip motion fluid inclusion studies give depth temperature estimates of around 3 km and 285 c for the development of the brittle structures and 6 8 km at 310 350 c for the brittle ductile transition the deformed veins show evidence of extensive fluid infiltration during the later phases data from fluid inclusions and metamorphic assemblages give an estimate of 15 20 km and 400 450 c for their deformation these data combined with uplift rates determined by other studies allow the construction of a depth temperature time path which indicates nearly isothermal decompression associated with rapid uplift a shallow brittle ductile transition 6 8 km is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area because of the small temperature change over a large amount of decompression pressure may be an important factor in controlling the depth of the brittle ductile transition the marked temperature drop above the brittle ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism

late quaternary slip rates and slip partitioning on the alpine fault new zealand abstract published geological data on late quaternary offsets on the alpine fault new zealand have been assembled into a common format and analysed with respect to uncertainties uncertainties arise mainly from measurement of offset features relating apparent offsets to actual fault slip and dating the offset features despite the considerable uncertainties the data form a coherent set consistent with a relatively constant rate of strike slip of 27 5 mm year between milford sound and hokitika this rate represents 70 75 of the fault parallel interplate motion north of the confluence with the hope fault the rate drops substantially dip slip rates on the other hand show considerable variation along strike rising to a maximum of more than 10 mm year in the central section and decreasing to zero at the southern end partitioning of c 25 of the interplate slip on to structures east of the alpine fault occurs in the central section consistent with predictions from critical wedge models the partitioning of all the fault normal component of displacement on to other structures in the south may be related in part to a doubling in width of the deforming wedge to the east most probably however the fault normal displacement is mainly accommodated by underthrusting of the australian plate offshore due to a change in the nature of the crust from continental to oceanic

silicate inclusions in isoferroplatinum constraints on the origin of platinum mineralization in podiform chromitites abstract the origin of large platinum group mineral pgm nuggets inside chromitite schlieren in some ural alaskan type ultramafic complexes has been assigned to orthomagmatic processes a promising phenomenon which may provide new insights into the processes responsible for pge mineralization formation or at least place constraints on existing models are the multiphase inclusions hosted in pgm which have been previously found in platinum nuggets from alluvial deposits for the first time this study examines multiphase inclusions hosted in isoferroplatinum nuggets from lode chromitite schlieren in almost unaltered dunite from the matysken ural alaskan type complex koryak highlands far east russia these multiphase inclusions are comprised of diopside hydrous silicates apatite plagioclase k feldspar silica and other minerals that are distinctly different from the host dunite mineralogy taken together with similar mineral assemblages in cr spinel hosted inclusions these inclusions cannot be crystallization products of a mafic ultramafic melt but instead require an alternative explanation we critically evaluate different genetic models and conclude that a range of fluid assisted metamorphic processes should be involved in producing massive pgm within chromitite schlieren in dunites

transpressive ductile flow and oblique ramping of lower crust in a two sided orogen insight from quartz grain shape fabrics near the alpine fault new zealand 1 the three dimensional 3 d grain shape fabric gsf of deformed quartz veins was measured in 30 samples from the hanging wall of the oblique slip alpine fault in the central southern alps new zealand these record aspects of the neotectonic strain experienced by lower crustal rocks in the pacific plate as they were transported through a two sided orogen deformation was nonsteady state transpression was followed by near vertical shearing as the rocks negotiated the oblique footwall ramp of the alpine fault at the brittle ductile transition ramping up was accommodated by an escalator like back shearing process this thinned the transpressed crust and boosted dip slip rate on the alpine fault mechanically this suggests a sharp step in the footwall ramp adjacent to which differential stresses were transiently high margin parallel motion was partitioned onto these shears fabric development is simulated using a 3 d kinematic model into which we input constraints on plate motion fault slip rates crustal structure fabrics and geodetic strain in the southern alps this model can explain many aspects of the southern alps fabrics including their nonvertical foliation and lineation similarities between the model predictions and the observed fabrics suggest that lower crustal flow in the pacific plate mimicked the geodetic velocity pattern at the surface today with obliquely convergent deformation being distributed across a wide region to the east of the alpine fault the model shows that the timescale for an exhumational steady state with respect to deformational fabrics is slow 107 years a condition which the 6 m y old southern alps have probably not yet achieved

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

zoned cretaceous and cenozoic garnet and the timing of high grade metamorphism southern alps new zealand new garnet sm nd and lu hf and existing rb sr 40ar 39ar u pb and sm nd ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the alpine schist belt and southern alps orogen new zealand are diachronous and partly cenozoic in age the dominant metamorphic isograds in the alpine schist formed during crustal thickening at a previously unsuspected time at c 86 ma immediately prior to the opening of the tasman sea at c 84 82 ma obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism and fabric elements can be highly diachronous a detailed timing history for the growth of a single garnet is recorded by a sm nd garnet whole rock age of 97 8 8 1 ma for the inmost garnet core zone 1 lu hf ages of 86 2 0 2 ma and 86 3 0 2 ma for overgrowth zones 2 and 3 a step leach sm nd age of 12 37 ma for zone 4 and growth of the garnet rim zone 5 over the alpine fault mylonite foliation during the modern phase of oblique collision that began at c 5 6 ma plate convergence along the new zealand portion of the gondwana margin continued after c 105 ma almost certainly culminating in the oblique collision of a large oceanic plateau hikurangi plateau the metamorphism of the alpine schist at c 86 ma is evidence of that hit the mid to late cretaceous extension that is widespread elsewhere in the new zealand region is attributed to upper plate extension and slab roll back the effects of the collision with the hikurangi plateau may have contributed to the changing plate motions in the region leading up to the opening of the tasman sea at c 82 ma

quartz c axis fabric development and large scale post nappe folding wandfluhhorn fold penninic nappes quartz c axis fabrics have been investigated within a suite of quartz veins and monomineralic layers around a major post nappe fold hinge the wandfluhhorn fold in the bosco area swiss italian border within the lower penninic nappes two kinematic domains which are separated by the axial plane trace of the wandfluhhorn fold are recognized on the lower limb the measured quartz c axis fabric asymmetry indicates a sense of shear in which the overlying layers move to the southwest i e top to sw whereas on the upper limb the shear sense is reversed with the top moving to northeast the shear direction n60 e n80 e however is constant in both areas and oblique to an older stretching lineation as well as to the d3 fold hinge such a distribution of asymmetric quartz c axis fabrics and the constant orientation of their interpreted shear direction which is apparent only from the fabric data and not from field evidence indicates fabric development pre or early syn wandfluhhorn folding with subsequent folding and modification of the existing textures and possibly rotation of the initial fold axis an overall westward directed shear has been suggested for the whole of the lepontine alps however this study demonstrates that this simple general pattern has been modified locally by later folding it also demonstrates that the dominant lineation may be a finite stretching lineation due to more than one phase of deformation and is not necessarily related to any particular transport direction

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

unique cu rich sulphide ores of the southern 2 orebody in the talnakh intrusion noril sk area russia geochemistry mineralogy and conditions of crystallization abstract we present new geochemical and mineralogical data from the southern 2 orebody located in the south western branch of the talnakh intrusion russia and composed of massive and disseminated sulphides the southern 2 orebody consists dominantly of chalcopyrite along with lesser pentlandite and subordinate cubanite bornite and pyrrhotite and differs from cu rich ores of the oktyabr sky deposit due to the absence of moikhukite talnakhite assemblages ores from the southern 2 orebody are highly enriched in copper nickel and platinum group elements pges cu up to 28 12 and 10 58 wt ni up to 6 29 and 2 45 wt and pt pd up to 220 and 173 ppm in the massive and disseminated ores respectively the ratios of cu ni 3 85 7 34 in massive and 2 64 6 58 in disseminated ores the ratios of the platinum to iridium group elements ppge ipge reaches more than 40000 these features indicate an extremely different degree of fractionation of the sulphide melt during the formation of ores the sequence of crystallization of the pge minerals pgms on the basis of textural information was as follows pt pd sn rustenburgite atokite pd cu sn stannopalladinite pt fe cu ni tetraferroplatinum pd ni as majakite pd as palladoarsenide pd pb bi zvyagintsevite polarite and pd5pb3 unnamed phase au cu pd tetra auricupride and auricupride au ag alloys this is consistent with experimental data on pge minerals the pgm assemblage was caused by the evolution of a significantly fractionated sulphide melt that was enriched in cu ni pt pd pb and au and as a result produced minerals with peculiar compositions e g cu rich stannopalladinite pb rich polarite ni rich palladoarsenide which do not occur in other ores furthermore pt fe cu ni alloys are abundant and sperrylite is absent which is a very common phase in all ores of the noril sk area the composition of pentlandite with ni ni fe ratios ranging from 0 48 to 0 68 indicates that it formed between 560 c ni poor to 500 c ni rich and 10 5 to 7 lgfs2 respectively the pgms and au minerals were formed at temperatures ranging from 550 to 225 c the specificity of sulphide with pgms association is due to the evolution of the cu rich and ni rich melt under elevated sulphur fugacity conditions the mechanism of filter pressing which precipitated sulphide drops at the bottom of the chamber and penetration of sulphide melts into the country rocks is supported by the same geochemical composition of disseminated and massive ores

kinematic analysis of the southern iberian shear zone and tectonic evolution of the acebuches metabasites sw variscan iberian massif 1 the acebuches metabasites constitute a slice of oceanic crust located at the variscan suture between the ossa morena and the south portuguese zones iberian massif the emplacement of the metabasites onto the accretionary prism was associated with the activity of the southern iberian shear zone careful measurements of the fabrics and structures related to this shear zone show consistent kinematic criteria pointing to the presence of a simple shear component with an oblique predominantly left lateral sense of displacement the data are consistent with existing models of triclinic or oblique shear zones considering coeval simple shear and pure shear components these models provide a satisfactory explanation of the observed spatial variations in the foliation lineation and fold hinge data the kinematics of other shear deformations older and younger than the southern iberian shear zone have also been established these results offer new insights into the tectonic evolution of this suture of the variscan belt and show how complex transpressional strains can arise during oblique crustal deformation associated with the motions of tectonic plates

uplifted marine terraces along the alpine fault new zealand three types of evidence indicate that marine terraces are widespread in the southern alps of new zealand i remnants of shore platforms occur as distinct levels of notched ridge crests and flat summits degraded sea cliffs are common ii scattered quartz beach pebbles occur on 16 of 18 levels of exhumed shore platforms in the fox franz josef type area to altitudes as high as 1700 meters iii altitudinal spacings of new zealand terrace flights allow correlation with 18 dated global marine terraces at new guinea which were formed during glacio eustatic highstands of sea level within the last 336 x 10 3 years inferred uplift rates at fox franz josef increased from 3 2 to 7 8 meters per 10 3 years since about 135 x 10 3 to 140 x 10 3 years ago presumably because of increased convergence between the pacific and australian plates

kinematics of oblique collision and ramping inferred from microstructures and strain in middle crustal rocks central southern alps new zealand the hanging wall of the alpine fault near franz josef glacier has been exhumed during the past similar to2 3 m y providing a sample of the ductilely deformed middle crust of a modem obliquely convergent orogen presently exposed rocks of the pacific plate are inferred to have undergone several phases of ductile deformation as they moved westward above a mid crustal detachment initially they were transpressed across the outboard part of the orogen resulting in oblate fabrics with a down dip stretch later they encountered the alpine fault experiencing an oblique slip backshearing on vertical planes this escalator like deformation tilted and thinned the incoming crust onto that crustal scale oblique ramp this style of hanging wall deformation may affect only the most rapidly uplifting central part of the southern alps because of the low flexural rigidity of the crust in that region and its displacement over a relatively sharp ramp angle at depth a 3d transpressive flow affected mylonites locally near the fault but their shear direction remained parallel to plate motion ruling out ductile extrusion as an important process in this orogen outside the mylonite zone late cenozoic shortening is inferred to be modest 30 40 as measured from deformation of younger biotite grains oblique collision is dominated by translation on the alpine fault and rocks migrate rapidly through the deforming zone preventing the accumulation of large finite strains transpression may play a minor role in oblique collision c 2001 elsevier science ltd all rights reserved

structural analysis of mylonitic rocks in the cougar creek complex oregon idaho using the porphyroclast hyperbolic distribution method and potential use of sc type extensional shear bands as quantitative vorticity indicators abstract mylonitic rocks of the cougar creek complex of northeastern oregon and west central idaho provide an opportunity to document the deformational structures produced during general non coaxial shear within quartz feldspar mylonites and to explore the potential role of sc type extensional shear bands in vorticity analysis well developed feldspar porphyroclasts within six mylonite zones were utilized to estimate bulk kinematic vorticity w k using the porphyroclast hyperbolic distribution phd method w k values for the cougar creek mylonites range from w k 0 26 to w k 0 37 synthetic and antithetic shear band inclinations were measured relative to observed shear zone boundaries within five mylonite zones with estimated w k values and compared to the non coaxial flow field geometries and eigenvector orientations in each mylonite zone synthetic sc type shear band populations exhibit a range of inclination with maximum inclination lying approximately parallel to the acute bisector ab of the eigenvectors similarly antithetic shear band populations show a range of inclination near the obtuse bisector ob of the eigenvectors we infer that sc type extensional shear bands form initially parallel to ab and ob and rotate towards the flow plane with progressive deformation decreasing their inclination relative to the shear zone boundary ab and ob have significance in the strain field in that they represent orientations of maximum angular strain rate thus planes perpendicular to ab and ob are mechanically favorable for small zones of localized simple shear shear bands within the heterogeneous bulk strain of the mylonite orientation analysis of populations of sc type shear bands may provide a direct quantitative means of estimating w k

erosional control on the structural evolution of a transpressional thrust complex on the alpine fault new zealand abstract the waikukupa thrust is a 4 km long oblique thrust segment of the new zealand alpine fault that has developed over the last 65 000 years emplacing mylonite and cataclasite over fluvioglacial gravel during the last 20 000 years or so a 2 km long out of sequence imbricate the hare mare thrust has formed on the east side of the waikukupa river valley the average rate of slip on the two thrusts is estimated at 22 30 mm year analysis of the fault structure in terms of critical wedge theory is consistent with imbrication resulting from a reduction in wedge taper below the critical value due to rapid river erosion the internal structure of the waikukupa thrust sheet consists of reverse faults and angular folds forming duplex like systems associated with minor strike slip faults within the limits of uncertainty the structures are consistent with strain accumulating during internal transpressional shear of the thrust sheet while the taper of the latter was subcritical prior to the transfer of fault displacement to the hare mare imbricate the thrust complex forms part of a serrated range front fault system consisting of oblique thrust sections linked by strike slip faults we introduce the term serial partitioning for this type of partitioned transpressional fault system its development is linked to the erosional processes at the range front an alternative system consisting of parallel thrust and strike slip faults we here term parallel partitioning while angle of obliquity is an important parameter in determining which partitioning model develops we suggest that erosion rate is also an influential factor

rotation of long tectonic clasts in transpressional shear zones abstract the two dimensional analysis of rotation of rigid ellipsoidal inclusions is not applicable to situations in which either the bulk deformation deviates from plane strain or one of the principal axes of the inclusion is at an angle to the vorticity vector both these situations may occur in certain types of transpressional ductile shear zones the model presented here shows how long cylindrical or ellipsoidal tectonic clasts with the longest axis parallel to the walls of the shear zone but at an angle to the vorticity vector rotate in transpressional shear zones long clasts initially at a low angle to the vorticity vector and at a large angle to the stretching lineation will rotate and tend to become subparallel to the stretching lineation the rotation of the principal axes of the elliptical cross section around the cylinder axis cannot be unlimited for any inclusion including the inclusion with circular cross section r 1 if the deformation is sufficiently large there is always a stable position of orientation the model explains simultaneous occurrence of monoclinic rolling structures in sections parallel and perpendicular to the vorticity vector depending upon the initial orientation the long axes of different clasts in the foliation plane may rotate clockwise and counterclockwise consequently opposite senses of asymmetry of rolling structures may appear in sections parallel to the vorticity vector

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

testing models for obliquely plunging lineations in transpression a natural example and theoretical discussion theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal many field descriptions of transpressional zones however indicate a range of lineation orientations between these extremes several theoretical models have been developed to explain such departures from expected lineation orientation and we discuss these in the context of a field example from the archean superior province in the north american craton existing models are insufficient to explain obliquely plunging lineations in this example because 1 obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no slip condition 2 foliations and lineations vary independently 3 the vorticity normal section is subhorizontal limiting possibilities for inclined simple shear 4 high vorticity is needed for finite strains and lineations to match previously proposed triclinic models but vorticity is relatively low and 5 juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models because existing theoretical models are not applicable to our field example we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression

mica controlled anisotropy within mid to upper crustal mylonites an ebsd study of mica fabrics in the alpine fault zone new zealand

thermal regime in the central alpine fault zone new zealand constraints from microstructures biotite chemistry and fluid inclusion data abstract the thermal state within plate boundary fault zones is likely to influence the dimensions of large co seismic fault ruptures and strain localisation in underlying mylonitic shear zones vein microstructures biotite chemistry and fluid inclusion data offer new insights into the thermal structure of the mylonitic shear zone down dip of the alpine fault the main pacific australian plate boundary structure through southern new zealand quartz veins within the fault rocks were formed and deformed at temperatures ranging from 500 c to immediately above that of the brittle viscous transition 325 c illustrating that fluids are present throughout the fault zone microthermometric data from primary mixed co 2 h 2 o fluid inclusions in boudin neck quartz carbonate veins indicate that these inclusions were trapped at temperatures of 325 15 c and pressures of c 40 mpa vein microstructures indicate that fluid temperatures were equal to rock temperatures assuming hydrostatic fluid pressure the veins were trapped at depths of 2 o co 2 fluids in the shallower crust of 500 kg m 3 biotite grains in the mylonites are a mixture of porphyroclastic grains with high r ti mg inherited from the parent alpine schist and smaller low r grains that recrystallised during mylonitisation particularly those that are found within syn mylonitic shear bands assuming a simple step function model approximating diffusive equilibration the changes in biotite chemistry indicate t during mylonitic deformation of 340 c according to the ti in biotite thermometer of henry et al 2005 a best fit uplift p t path to existing data and these new results indicate that the uplift path p t gradient of fault zone rocks above the brittle viscous transition is of the order of 40 c km 1 below the brittle viscous transition the uplift path p t gradient is 10 c km 1 the p t profile within the fault rocks is comparable to that in the immediate hanging wall alpine schist indicating there is no peak in the crustal isotherms in the fault hanging wall as was indicated by previous numerical models and analyses that did not consider data from the fault rocks themselves

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

how do lineations reflect the strain history of transpressive shear zones the example of the active alpine fault zone new zealand abstract lineations within mylonites exhumed in the hanging wall of new zealand s active alpine fault zone have a complicated relationship to contemporary plate kinematics the shear zone is triclinic and macroscopic object lineations are not usually parallel to the simple shear direction despite high total simple shear strains 150 this is mostly because the lineations are inherited from pre mylonitic fabrics and have not been rotated into parallelism with the mylonitic stretching direction which pitches c 44 in the fault plane furthermore some lineations have been variably rotated depending on whether they are present in shear bands or microlithons which accommodated bulk strains with different vorticities total strains required to obtain parallelism between the finite maximum principal stretching direction calculated from transpression models and these mylonitic lineations are pure shear stretch s 1 3 5 simple shear 11 7

platinum group element oxides from the pirogues ophiolitic mineralization new caledonia origin and significance a number of platinum group element pge oxides and hydroxides have been discovered in a pge mineralization in the new caledonia ophiolite complex they include pt fe oxides containing variable amounts of oxygen 5 50 at ir fe rh oxides metal oxygen approximately 3 fe rh pt oxides and pt ir fe rh oxides metal oxygen 1 and an ru mn fe phase with a very low metal oxygen ratio interpreted as hydroxide these minerals have been found either as inclusions in chromite crystals from chromitite or as discrete grains from alluvial concentrates pt fe oxides are characterized by unusual textures including zoning and symplectite optical and scanning electron microscope descriptions of all the pge oxides are given together with electron microprobe analyses including the quantitative determination of oxygen the pge oxides may result from the alteration of magmatic pgm alloys and from primary crystallization of oxides in lateritic conditions their presence indicates the existence of mechanisms of transport and crystallization of pge as oxide in surface conditions mechanisms contributing to the redistribution and enrichment of pge in laterite