a deep crustal shear zone exposed in western fiordland new zealand an approximately 180 km long deep crustal shear zone in western fiordland new zealand has been shown to have formed during continental extension between 116 and 100 ma the western fiordland shear zone forms a major tectonic boundary between two sets of gneisses with unrelated deformation histories a detailed study of the northern part of the shear zone has revealed that the shear zone formed at depths of 40 km approximately 12 kbar and 680 c and was originally very gently dipping or subhorizontal prior to subsequent upright folding the current thickness postfolding of the shear zone is 3 4 km abundant asymmetric structures in the shear zone indicate noncoaxial deformation and a top to the ne sense of shear unlike ductile shear zones in metamorphic core complexes from elsewhere in the world there is no evidence that decompression of the shear zone rocks or foot wall rocks occurred during the active life of the shear zone suggesting that displacement along the shear zone by noncoaxial deformation must have been largely horizontal this shear zone provides important evidence for the nature of deep crustal deformation occurring during continental extension

evidence of early cretaceous collisional style orogenesis in northern fiordland new zealand and its effects on the evolution of the lower crust structural metamorphic and kinematic data from a well exposed section of lower crustal rocks in northern fiordland new zealand reveal a history of intense contractional deformation and high p metamorphism at the roots of a convergent orogen high p 14 kbars granulite facies garnet clinopyroxene bearing reaction zones occur adjacent to anorthositic veins within gabbroic and dioritic gneiss these veins and reaction zones were variably deformed by two phases of high p granulite facies deformation quantitative kinematic analyses conducted using systems of rotated veins and reaction zones indicate that the first phase produced steeply dipping shear zones within a sinistral pure shear dominated flow regime wk 0 69 this deformation occurred at conditions of p 14 0 1 3 kbars and t 676 34 c and resulted in subhorizontal arc parallel ne sw stretching and up to 60 subhorizontal shortening in high strain zones of the lower crust at depths 45 km the second phase of deformation occurred at p 14 1 1 2 kbars and t 674 36 c and produced vertically stacked gently dipping ductile thrust faults that accommodated arc normal nw directed displacement these features reflect major tectonic thickening of the crust oblique convergence and high p metamorphism during the collision of the roots of a convergent orogen represented by plutons of the median tectonic zone in eastern fiordland with the paleo pacific margin of gondwana represented by western fiordland distinctive kinematic styles suggest that this collision resulted in a partitioning of the arc parallel ne sw and arc normal nw se components of oblique convergence onto sinistral strike slip and ductile thrust faults respectively at lower crustal levels

thermomechanical evolution of the crust during convergence and deep crustal pluton emplacement in the western province of fiordland new zealand 1 fiordland new zealand contains exposures of high pressure p 12 14 kbar granulite facies rocks that form one of earth s largest exposed lower crustal roots of an early cretaceous magmatic arc these exposures allowed us to examine the mechanisms and processes that controlled crustal thickening and large vertical displacements at the deepest levels of a deforming arc system we present structural and metamorphic data that show how the root of this arc was tectonically thickened by imbricate granulite facies thrust zones during and after the emplacement of sheeted plutons into the middle and lower crust the imbricate thrust zones form part of a well exposed 12 km wide two sided fold thrust belt that preferentially developed in crust that was thermally softened by magmatism changes in metamorphic mineral assemblages and microstructural data from the contact aureole of a composite arc batholith called the western fiordland orthogneiss record thrust and pluton emplacement conditions of p 7 9 kbar paleodepths of 25 30 km these data show that thrust imbrication and tectonic loading at 30 km depth is a viable mechanism of large vertical displacements and crustal thickening at the deepest levels of magmatic arcs this mechanism produced a characteristic up pressure metamorphic history that is similar to that observed in many other large magmatic belts worldwide

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

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

processes controlling vertical coupling and decoupling between the upper and lower crust of orogens results from fiordland new zealand the pre cenozoic configuration of western new zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a cretaceous orogen from upper to lower crustal levels 10 50 km beginning at 126 ma a composite batholith dominated by diorite was emplaced into the lower crust during emplacement deformation was partitioned into zones weakened by magma and heat leading to the development of two layer parallel shear zones at the upper and lower contacts of the batholith transient vertical decoupling of the crust above and below the batholith occurred from 126 ma until 120 ma as magma was emplaced into and moved through a weak thick lower crust by 116 ma however much of the batholith had crystallized and the lowermost crust had cooled from 750 c t 850 c to t 650 700 c cooling was aided by the juxtaposition of pre existing crust against hot new crust and by the efficient extraction of partial melts out of the lower crust cooling together with dehydration of the lower crust and mafic compositions led to the development of a strong dry lower crustal root by 116 ma a strong lower crust resulted in high degrees of vertical coupling between the upper and lower crust during contraction from 116 to 105 ma even as magma continued to be emplaced into the mid upper crust a narrow focused orogenic style in the upper crust at this time reflected a highly viscous lower crust through which compressional stresses were transferred vertically the results imply that changes in plate boundary dynamics rather than the thermal weakening of thick lower crust during convergence controlled the onset of regional extension at 108 105 ma

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

geologic consequences of plate reorganization an example from the eocene southern alaska fore arc observation of relative timing of deformation metamorphism and plutonism in a high temperature low pressure metamorphic belt in the eastern chugach mountains of alaska leads to a model of ridge subduction followed by plate reorganization to account for the abnormally high geothermal gradients in the fore arc between 56 and 53 ma a change in the direction of the kula farallon spreading halted the previous southward migration of the kula farallon north american triple junction this forced the triple junction to migrate back north along the plate margin enlarging the slab window beneath the accretionary margin the expanded slab window produced a large scale thermal manifestation now recognized as the chugach metamorphic complex the accretionary complex responded to plate reorganization by orogen parallel extension associated with oblique subduction of the kula plate contraction began again following passage of the triple junction and subduction of the farallon plate

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

high temperature arc parallel normal faulting and transtension at the roots of an obliquely convergent orogen structural and kinematic data from northern coastal british columbia 54 5 n document intra arc deformation patterns at mid crustal levels during and after emplacement of the coast mountains batholith major arc parallel displacements occurred along high temperature 700 c ductile normal faults and steep sinistral transtensional shear zones within an obliquely convergent margin strain patterns and pluton emplacement were controlled by 1 structural anisotropies in host rocks that predate batholith emplacement and 2 kinematic compatibility requirements created by simultaneous motion on curved pluton bounding shear zones these controls superseded a partitioning of the arc parallel and arc normal components of oblique plate convergence onto strike slip and thrust faults respectively

deep crustal embrittlement and fluid flow during granulite metamorphism in the limpopo belt south africa deep crustal rocks generally deform in a ductile manner and have a very low permeability however in the southern marginal zone of the limpopo belt solutions fluids have moved through high grade rocks and reacted with these on a regional scale along a major shear zone strike slip environments there have more widespread alteration than thrust fault environments under high grade metamorphic conditions brittle failure and hydraulic fracture processes in particular have occurred in these deep crustal rocks the latter phenomenon provides a mechanism for developing permeability and plumbing systems for the migration of fluids and or liquids through the deep crust modeling of the physical conditions of brittle failure under these unusual conditions would suggest that a mobile phase of low viscosity and water activity participated in the fracture process

structure and kinematics of oblique continental convergence in northern fiordland new zealand abstract southeast of the alpine fault in fiordland new zealand late tertiary faults envelope exposures of early cretaceous high p 12 16 kbar granulite facies orthogneisses these exposures allowed us to examine how pre cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence fault slip data and kinematic modelling show that oblique slip and reverse displacements preferentially were partitioned east of the alpine fault onto curved ne striking surfaces and moderately dipping 30 52 n and nw striking surfaces respectively approximately 3 0 5 5 km of exhumation and 6 5 7 0 km of oblique dextral displacement occurred on faults that display palm tree style geometries and subhorizontal detachments in profile this heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the alpine fault north of milford sound the principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7 9 ma the effects of these pre existing structures and a focusing of contractional deformation along the northeast margin of fiordland resulted in an unusually high degree of strike slip partitioning across the southernmost onshore segment of the alpine fault

ion microprobe upb zircon geochronology of granitic magmatism in the western province of the south island new zealand abstract zircon u pb isotopic systems are described for three mid palaeozoic and four late mesozoic granites from the western province of the south island new zealand the granites chosen are representative of the major plutons in this region two distinct episodes of palaeozoic magmatism have been identified middle devonian 380 ma and early carboniferous 330 ma the mesozoic granites are all mid cretaceous in age and range from 120 to 110 ma the o sullivans granite forming a large part of the karamea batholith has a crystallisation age of 377 7 4 1 ma and the windy point granite from the paparoa batholith has an age of 328 6 4 4 ma further west the granite at cape foulwind has a similar age of 327 3 6 2 ma from east to west the cretaceous granites appear to decrease in age the separation point batholith is dated at 118 ma the oldest phase at separation point gave an age of 116 6 1 9 ma and the youngest phase the pearse granodiorite gave the same age within error at 119 4 2 3 ma the mount olympus pluton in nw nelson gave an age of 111 4 2 0 ma and the same age is found for the buckland granite which forms most of the paparoa batholith at 109 6 1 7 ma inherited zircon components as old as 1700 ma have been identified in many of the granites and distinct components are present at 1000 and 500 600 ma this pattern of zircon ages has also been observed in granites from the lachlan fold belt and in ordovician sediments from se australia and new zealand however it is not clear whether the old zircons are derived from the lower crust from the incorporation of upper crustal material or both the palaeozoic granites can be broadly correlated with rocks of similar age in antarctica se australia and tasmania that formed part of the gondwana supercontinent the cretaceous granites are probably part of an extensive mesozoic magmatic arc that can be traced from south america through west antarctica and on into new zealand

the growth of orogenic belts and the role of crustal heterogeneities in decollement tectonics a series of finite element models of contractional deformation are presented that examine the role preexisting mechanical heterogeneities within the lithosphere play in controlling the partitioning of stress and strain during orogenesis the models focus particularly on orogens that develop near previously rifted continental margins although many of the observations are relevant to other fold and thrust belts the models indicate that orogen evolution is strongly influenced by crustal architecture during the first 150 to 200 km of shortening major decollements develop at midcrustal and lower crustal levels partitioning strain into upper crustal lower crustal and mantle strain domains the magnitude and spatial distribution of strain within each domain are nearly uniform but differ significantly between domains the decollements extend throughout the entire width of the orogen providing a successively increasing degree of decoupling between deformation in the shallow crust and deeper levels the midcrustal decollement terminates beneath the leading edge of the thrust front but the deep crustal decollement extends more than 500 km beyond the foreland this allows up to 5 10 strain to develop well in front of the orogenic belt in the interior of the orogen deformation extends through all levels of the crust beginning during the earliest stages of contraction deformation propagates continentward with increasing shortening with the foreland regions undergoing the most intense shortening in the shallow crust the loci of maximum total shortening within the upper and lower crust are laterally offset with the thickest crust lying continentward of the greatest elevations and the site of most intense surface deformation this pattern of crustal thickness variation is a primary feature of orogenesis in the models and is not related to postorogenic collapse after 150 to 200 km of shortening sufficient topography develops to cause the balance between rock strength gravitational forces and shear stress on the basal detachment to dominate the style of deformation preexisting mechanical heterogeneities thereafter exert little influence on orogenic processes

stochastic model of heterogeneity in earthquake slip spatial distributions summary finite fault source inversions reveal the spatial complexity of earthquake slip or pre stress distribution over the fault surface the basic assumption of this study is that a stochastic model can reproduce the variability in amplitude and the long range correlation of the spatial slip distribution in this paper we compute the stochastic model for the source models of four earthquakes the 1979 imperial valley the 1989 loma prieta the 1994 northridge and 1995 hyogo ken nanbu kobe for each earthquake except imperial valley we consider both the dip and strike slip distributions in each case we use a 1 d stochastic model for the four earthquakes we show that the average power spectra of the raw that is non interpolated data follow a power law behaviour with scaling exponents that range from 0 78 to 1 71 for the four earthquakes we have found that a non gaussian probability law that is the levy law is better suited to reproduce the main features of the spatial variability embedded in the slip amplitude distribution including the presence and frequency of large fluctuations since asperities are usually defined as regions with large slip values on the fault the stochastic model will allow predicting and modelling the spatial distribution of the asperities over the fault surface the values of the levy parameters differ from one earthquake to the other assuming an isotropic spatial distribution of heterogeneity for the dip and the strike slip of he northridge earthquake we also compute a 2 d stochastic model the main conclusions reached in the 1 d analysis remain appropriate for the 2 d model the results obtained for the four earthquakes suggest that some features of the slip spatial complexity are universal and can be modelled accordingly if this is proven correct this will imply that the spatial variability and the long range correlation of the slip or pre stress spatial distribution can be described with the help of five parameters a scaling exponent controlling the spatial correlation and the four parameters of the levy distribution constraining the spatial variability

granulite formation during continental extension in fiordland new zealand the formation of the high pressure high temperature rocks known as granulites has traditionally been attributed to metamorphism in the deeper parts of continental collision zones and magmatic arcs1 3 more recently greater consideration has been given to the possibility of granulite formation during continental rifting4 6 and in particular4 to whether granulites are forming today beneath the north american basin and range province here we report thermobarometric data and single zircon u pb ages for an analogous but much more deeply exhumed extensional terrain in fiord land new zealand7 8 where early cretaceous granulites underwent renewed granulite facies metamorphism at significantly shallower crustal levels following the extensional collapse of magmatically thickened continental crust our data show that granulite facies metamorphism can take place during continental extension and demonstrate that in western new zealand it was accompanied at higher structural levels by the emplacement of one or more metamorphic core complexes7 9 in keeping with the model postulated4 for the basin and range province

the interdependence of deformational and thermal processes in mountain belts crustal temperatures within collisional orogens are anomalously high compared with temperatures at comparable depths in stable continents which is evidence of thermal processes that are fundamental to orogenesis these temperatures can be explained by the redistribution of crust enriched in heat producing elements through the accretion of crust from the down going plate to the upper plate and surface erosion with the use of geologically reasonable rates the model results predict high temperatures over 600 c and inverted upper plate geotherms about 100 c over 20 kilometers at shallow depths 20 to 40 kilometers by 25 to 35 million years after collision this study emphasizes the interdependence of deformational surficial and thermal processes

successive hydration and dehydration of high p mafic granofels involving clinopyroxene kyanite symplectites mt daniel fiordland new zealand three texturally distinct symplectites occur in mafic granofels of the arthur river complex at mt daniel fiordland new zealand these include symplectic intergrowths of clinopyroxene and kyanite described here for the first time pods of mafic granofels occur within the contact aureole of the early cretaceous western fiordland orthogneiss batholith the pods have cores formed entirely of garnet and clinopyroxene and rims of pseudomorphous coarse grained symplectic intergrowths of hornblende and clinozoisite that reflect hydration at moderate to high p these hornfelsic rocks are enveloped by a hornblende clinozoisite gneissic foliation s1 narrow garnet reaction zones in which hornblende and clinozoisite are replaced by garnet clinopyroxene assemblages developed adjacent to fractures and veins that cut s1 fine grained symplectic intergrowths of 1 clinopyroxene and kyanite and 2 clinozoisite quartz kyanite and plagioclase form part of the garnet reaction zones and partially replace coarse grained s1 hornblende and clinozoisite the development of the garnet reaction zones and symplectites was promoted by dehydration most probably following cooling of the contact aureole maps of oxide weight percent and cation proportions calculated by performing matrix corrections on maps of x ray intensities are used to study the microstructure of the symplectites

shrimp monazite and zircon geochronology of high grade metamorphism in new zealand ion microprobe dating of zircon and monazite from high grade gneisses has been used to 1 determine the timing of metamorphism in the western province of new zealand and 2 constrain the age of the protoliths from which the metamorphic rocks were derived the western province comprises westland where mainly upper crustal rocks are exposed and fiordland where middle to lower crustal levels crop out in westland the oldest recognisable metamorphic event occurred at 360 370 ma penecontemporane ously with intrusion of the mid palaeozoic karamea batholith c 375 ma metamorphism took place under low pressure high temperature conditions resulting in upper amphibolite sillimanite grade meta morphism of lower palaeozoic pelites greenland group orthogneisses of younger cretaceous age formed during emplacement of the rahu suite granite intrusives c 110 ma and were derived from protoliths including cretaceous separation point suite and devonian karamea suite granites in fiordland high grade paragneisses with greenland group zircon age patterns were metamorphosed m1 to sillimanite grade at 360 ma concomitant with crustal thickening and further granite emplace ment m1 mineral assemblages were overprinted by higher pressure kyanite grade metamorphism m2 at 330 ma it remains unclear whether the m2 event in fiordland was primarily due to tectonic burial as suggested by regional recumbent isoclinal folding or whether it was due to magmatic loading in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous westland region metamorphism in fiordland accompanied and outlasted emplacement of the western fiordland orthogneiss wfo at 110 125 ma the wfo equilibrated under granulite facies conditions whereas cover rocks underwent more limited recrystallization except for high strain shear zones where conditions of lower to middle amphibolite facies were met the juxtaposition of palaeozoic kyanite grade rocks against cretaceous wfo granulites resulted from late mesozoic extensional defor mation and development of metamorphic core complexes in the western province

crustal scale rheological transitions during late orogenic collapse abstract orogeny involves crustal thickening followed by thermal relaxation and radiogenic heat production in the thickened crust culminating in crustal melting and magma intrusion which decrease the crustal viscosity by several orders of magnitude and cause late orogenic collapse collapse of the canadian cordillera is expressed in the early tertiary shuswap metamorphic core complex british columbia which displays a three layer crustal section separated by two fundamental rheological discontinuities 1 the brittle ductile transition across which high angle normal faults in the upper crust control basin formation merge into a low angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic to low temperature conditions and 2 the metatexite diatexite transition across which the rocks lose their solid framework and behave like a viscous magma this transition has the potential to mechanically decouple the upper crust from the rest of the lithosphere during a late orogenic gravitational collapse

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

kinematic vorticity and tectonic significance of superposed mylonites in a major lower crustal shear zone northern fiordland new zealand abstract new structural metamorphic finite strain and kinematic vorticity data from a 4 km wide subvertical shear zone in fiordland new zealand reveal a history of deformation reflecting different tectonic regimes an analysis of ductile fabrics within the shear zone and its wall rocks shows two distinctive stages of amphibolite facies mylonitic deformation dasz2 and dasz3 that are superimposed on older paleozoic or early mesozoic structures variations in strain intensity and well defined shear zone boundaries have allowed us to examine the progressive development of lasz2 sasz2 and lasz3 sasz3 fabrics and compare the types kinematics and conditions of deformation that produced them mineral assemblages defining lasz2 sasz2 provided calculated peak conditions of 11 9 1 1 kbar and 581 34 c indicative of lower crustal depths mineral assemblages defining lasz3 sasz3 provided calculated peak conditions of 8 7 1 2 kbar and 587 42 c finite strain and kinematic vorticity studies show that dasz2 involved ductile normal faulting and crustal thinning leading to decompression and exhumation of lower crustal rocks at some time during the cretaceous rifting of ancestral new zealand from gondwana dasz3 represents an episode of mid crustal dextral transpression that may have resulted from latest mesozoic or cenozoic oblique convergence reactivation of the subvertical sasz3 foliation by cataclastic shear zones and brittle faults dasz4 was accompanied by limited recrystallization at greenschist facies conditions dasz4 shear zones record upper crustal dextral strike slip faulting that resembles late tertiary deformation patterns associated with the australian pacific transform plate boundary

seismic excitation and directivity of short period body waves from a stochastic fault model abstract a theoretical study is made to investigate the excitation of short period p and s waves based on a stochastic fault model the source model describes an earthquake as a finitely propagating unilateral rupture on a fault plane where fault heterogeneities fault patches are randomly distributed the fundamental parameters are 1 average stress drop 2 fault dimension 3 variance of stress drop 4 patch corner frequency relevant to the fault patch sizes and 5 fault patch rupture directions it is shown that the source effectively radiates short period s waves this would reveal the reason why the strong ground motion of large earthquakes is mainly composed of s waves the short wavelength description of an earthquake is obtained by an energy additive superposition of the stochastic fracturing of random fault patches this gives rise to a seismic directivity effect peculiar to the incoherent short waves this effect is studied in the frequency and time domains especially for root mean square and maximum accelerations the excitation of strong motion accelerations is formulated by the stochastic part of earthquake faulting in terms of the variance of stress drop fault dimension and cut off frequency in addition to the short period seismic directivity

role of melt during deformation in the deep crust deformation in the deep crust is strongly influenced by the presence of melt injected melt or magma weakens the crust because strain will tend to localize where melt is present the amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate for plutons that intrude into rocks which are near the solidus temperature of the melt crystallization times can be quite long 1myr partial melting of deep crustal rocks can lead to melt enhanced embrittlement this occurs because the volume change for most melting reactions is positive therefore when the rate of melt production outpaces the rate at which melt can leave the system the melt pressure increases eventually the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the central gneiss belt canada this suggests that melt enhanced embrittlement occurred in these rocks an expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived assuming that migmatite has a small tensile strength differential stresses are 20 mpa in migmatitic rocks at the time melt enhanced embrittlement occurs the occurrence of melt enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting furthermore repeated episodes of melt enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains

cretaceous high p granulites at milford sound new zealand metamorphic history and emplacement in a convergent margin setting granulite facies orthogneiss of the arthur river complex arc at milford sound western fiordland records a complex early cretaceous magmatic and orogenic history for the pacific gondwana margin that culminated in the emplacement and burial of a dioritic batholith the western fiordland orthogneiss wfo enstatite bearing mafic to intermediate protoliths of the arc and wfo intruded the middle to upper crust the early deformation history of the arc is preserved in the pembroke granulite where two pyroxene s1 assemblages that reflect p 750 c were only patchily recrystallized during later deformation s1 is cut by garnet bearing leucogabbroic to dioritic veins which are cut by distinctive d2 fractures involving anorthositic veins and garnet diopside plagioclase bearing reaction zones these zones are widespread in the arc and wfo and record conditions of p 14 kbar and t 750 c garnet clinopyroxene bearing corona reaction textures that mantle enstatite in both the arc and wfo reflect early cretaceous burial by approximately 25 km of continental crust most of the arc is formed from the milford and harrison gneisses which contain steeply dipping s4 assemblages that envelop the pembroke granulite and involve garnet hornblende diopside clinozoisite rutile and plagio clase with or without kyanite the p t history of rocks in western fiordland reflects pronounced early cretaceous convergence related tectonism and burial possibly related to the collision and accretion of island arc material onto the pacific gondwana margin

western fiordland orthogneiss early cretaceous arc magmatism and granulite facies metamorphism new zealand u pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks here termed the western fiordland orthogneiss wfo in fiordland southwest new zealand indicate synkinematic magmatic emplacement between 120 and 130 ma ago these rocks were previously interpreted as possibly being of precambrian age initial pb and sr ratios are consistent with arc subduction related magmagenesis with little or no involvement of ancient continental crust subsequent high pressure 12 kb metamorphism of the wfo may reflect a major collision event involving crustal thickening by overthrusting of a 15 km thick sequence metamorphism ceased 116 ma ago based on206pb 238u ages of zircon from a retrogressed granulite u pb isotopic analysis of apatite along with previously published rb sr mineral ages indicate that final uplift and cooling to 300 400 c was largely completed by 90 ma the average uplift rate during this period is inferred to have been in excess of 1 mm yr unmetamorphosed gabbronorites of the darran complex in eastern fiordland inferred by some investigators to be the granulite protolith yield concordant u pb zircon ages of 137 1 ma u pb ages of apatite and previously published k ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years the high grade wfo and unmetamorphosed darran complex are now separated by a profound structural break however the ages and similarities in initial pb and sr isotopic ratios suggest that both suites are products of the same early cretaceous cycle of subduction related magmatism the timing of early cretaceous magmatism and metamorphism collision and resultant crustal thickening and subsequent great uplift and erosion in fiordland has important implications for terrane accretion and the evolution of relative plate motions along the new zealand segment of the gondwana margin

early palaeozoic terranes in new zealand and their relationship to the lachlan fold belt abstract two tectonostratigraphic terranes of palaeozoic age are recognised in the western province of new zealand separated by the north trending anatoki thrust the buller western terrane is composed largely of a relatively uniform suite of quartz rich elastics mainly turbiditic sandstone siltstone and black shale and ranges from basal ordovician to late ordovician in age it is an equivalent of the ballarat bendigo belt of victoria the takaka eastern terrane contains a varied assemblage of middle and late cambrian volcanics volcaniclastics carbonates and turbidites ordovician carbonate shale and sandstone and silurian quartzite the closest equivalent in australia is early palaeozoic rocks of west tasmania the main deformational events in the latest ordovician or silurian buller terrane latest silurian takaka terrane and middle devonian both terranes can be equated with the benambran bowning and tabberabberan orogenies of eastern australia respectively palaeozoic granites are restricted to buller terrane and are spread over a belt 300 450 km wide age range on sparse isotopic data is 370 310 ma late devonian carboniferous with the bulk in the early carboniferous s type granite predominates with several a type granites and a single i type the similarity in the nature and timing of the main events in the sedimentary tectonic and volcanic history of new zealand clearly link it with the lachlan fold belt on the other hand the age and chemistry of the s type new zealand palaeozoic granites make it unlikely that they are the direct equivalents of granite terranes of the lachlan fold belt or antarctica

plate boundary deformation in south island new zealand is related to inherited lithospheric structure abstract the alpine fault is the main active structure in the oblique continental collision zone of south island new zealand it is continuous at the surface for 800 km and accommodates 70 of current plate motion a 460 km offset of basement rocks suggests it has accommodated 50 of plate displacement since 45 ma geophysical geological and contemporary kinematic data are successfully modeled by slip on a narrow alpine fault zone extending into the lower crust the data are consistent with plate boundary deformation occurring on either northwest or southeast dipping shear zones in the upper mantle or with widely distributed strain in the upper mantle we propose that the position of the alpine fault and any associated lithospheric discontinuity is controlled by an eocene passive margin that separated palaeozoic continental lithosphere of the challenger plateau from much younger oceanic lithosphere strike slip motion since 25 ma has translated the passive margin into the continental collision zone resulting in subduction of oceanic lithosphere beneath south island and progressive localisation of shear strain near the alpine fault the eocene rift boundary formed by exploiting cretaceous oceanic transform faults and an older discontinuity within the new zealand continent we propose that a precursor to the alpine fault could be as old as palaeozoic inherited structure has clearly controlled the first order deformation pattern through new zealand since at least 100 ma and demonstrates that ancient faults can play an important role in determining lithospheric scale patterns of deformation at continental plate boundaries

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

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

kyanite paragonite bearing assemblages northern fiordland new zealand rapid cooling of the lower crustal root to a cretaceous magmatic arc fiordland new zealand exposes the lower crustal root of an early cretaceous magmatic arc that now forms one of earth s most extensive high p granulite facies belts the arthur river complex a dioritic to gabbroic suite in northern fiordland is part of the root of the arc and records an early cretaceous history of emplacement tectonic burial and high p granulite facies metamorphism that accompanied partial melting of the crust late random intergrowths of kyanite quartz and plagioclase partially pseudomorph minerals in the earlier high t assemblages of the arthur river complex indicating high p cooling of an over thickened crustal root by c 200 c the kyanite intergrowths are themselves partially pseudomorphed by paragonite commonly in the presence of phengitic white mica biotite plagioclase intergrowths that partially pseudomorph phengitic white mica and diopside plagioclase intergrowths that partially pseudomorph jadeitic diopside combined with published thermochronology results are consistent with later rapid decompression a short duration anticlockwise p t path may be explained by the high p juxtaposition of comparatively cool upper crustal rocks following their tectonic burial and under thrusting during the waning stages of early cretaceous orogenesis this was then followed by the decompression giving the rapid exhumation within 20 myr of peak metamorphism as suggested by the isotopic data

new zealand s darran complex and mackay intrusives rb sr whole rock isochrons in the median tectonic zone the cretaceous opening of the tasman sea followed by uplift east of the alpine fault has given access to the roots of a mesozoic convergent gondwana margin in western new zealand this long term margin contains the complex median tectonic zone mtz of recent authors and is pefectly exposed between milford sound and the darran range in the western part of this profile in the milford gneiss complex earlier mtz history is obscured by cretaceous metamorphism and structural transposition however the pembroke granulite is well preserved and the metamorphic curtain gradually lifts toward the east rb sr whole rock isochron dates and initial 87 sr 86 sr i o values are presented the darran complex yields a whole rock isochron of 133 or 17 ma concordant with published zircon u pb and hornblende k ar results the mackay intrusives mistake diorite and hut suite yield a whole rock isochron of 224 or 4 ma mswd 1 21 in agreement with published u pb dates of mistake diorite rafts of christina gneiss in mackay intrusives were subject to partial melting at the time of the mackay intrusive activity however the darran complex also appears spectacularly intruded by the christina crosscut irruptives of mackay granitoids which plot precisely on the 224 ma mackay rb sr whole rock isochron obvious ways of resolving this paradox are 1 that the darran complex has undergone thermal resetting after its intrusion by mackay magma and 2 the christina crosscut irruptives are late back intrusive darran differentiates containing assimilated mackay material these solutions fail on the grounds of heat transfer mackay isochron was not re set or of heat balance surmised irruptives excessively contaminated for the available heat a way past the constraints is the passive tectonic emplacement of the mt crosscut irruptive as a dike like roof pendant the mt christina irruptive is dated less reliably is tectonically less modified and may represent a late darran liquid a much older age and lower i o of the darran correlative rotoroa complex may be inherited from a common rotoroa darran protolith i o increases systematically from rotoroa to western darran milford pembroke to darran rocks and finally to western fiordland orthogneiss wfo which is chemically distinct from the pembroke granulite and darran complex

influence of lithological heterogeneity mechanical anisotropy and magmatism on the rheology of an arc north cascades washington abstract many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere several of the controlling factors for rheology are difficult to study experimentally particularly lithological heterogeneity mechanical anisotropy and magmatism and we focus on these in a study of the deformation patterns in a thick crustal section 5 to 40 km paleodepth through the cretaceous cascades core of the nw cordillera this magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section and the remainder of the section is characterized by heterogeneous fold dominated ductile deformation early tight to isoclinal recumbent folds and associated axial planar fabrics are refolded by one or more cycles of nearly coaxial open to isoclinal upright to overturned folds layering played a mechanically active role during folding at all levels as indicated by cleavage refraction boudinaged layers and kinematic indicators that record fold related shear ductile deformation intensifies in the narrow structural aureoles of plutons and sw directed reverse shear was partitioned into some of the aureoles the poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus as commonly predicted all of the plutons have magmatic foliations that at least in part reflect regional strains and these foliations are strong in the deeper plutons the thinner sheets acted as competent bodies during folding and boudinage after they reached the solidus but generally did not cause marked strain gradients in their hosts a relative strength profile constructed for the cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles however in more detail relative strengths are markedly variable units were able to accumulate large ductile strains but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid to deep crustal part of the profile this profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale and should be applicable to many other magmatic arcs

sm nd and rb sr isotopic and geochemical systematics in phanerozoic granulites from fiordland southwest new zealand sm nd and rb sr isotopic analyses are reported for granulite facies orthogneisses from fiordland southwest new zealand whole rock samples define a rb sr isochron age of 120 15 ma and an initial 87sr 86sr ratio of 0 70391 4 nd values at 120 ma show a relatively wide range of from 0 4 to 2 7 indicating decoupling of sr nd isotope systems associated ultramafic rocks have initial 87sr 86sr ratios of from 0 70380 to 0 70430 and nd values of from 0 1 to 3 0 the different initial ratios suggest that the various intrusions although contemporaneous were not derived through fractionation of a single parent magma a metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a proterozoic isotopic signature with a measured nd 0 value of 10 2 87sr 86sr ratio of 0 73679 and a tnd provenance age of 1490 ma the rb sr whole rock age of the granulites is the same as obtained from recent u pb zircon dating mattinson et al 1986 and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism rb sr and sm nd analyses of mineral systems indicate that the terrain had cooled below 300 c by 100 ma providing further evidence that high grade metamorphism was of exceptionally short duration

the deep crust of the southern rhine graben reflectivity and seismicity as images of dynamic processes abstract the rhine graben part of the european cenozoic rift system deserves special attention because of its location in the foreland of the alpine orogen the phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the variscan orogeny late orogenic crustal re equilibration to the interference of rifting and alpine orogeny in tertiary times the rhine graben is one of the most detailedly studied continental grabens prospecting for hydrocarbons in its sedimentary fill and intensive pre site studies for the continental deep drilling program ktb on its eastern crystalline flank black forest provided a comprehensive set of geoscience data seismic investigations ranging from a deep seismic network of near vertical reflection to concurrent wide angle refraction experiments were accompanied by seismological and geological surveys therefore a direct observational comparison between the two depth definitions of crustal subdivisions elastic vs strength rheological was possible in large parts of western europe deep seismic sounding dss investigation distinguishes between a crystalline upper crust of comparatively low p wave velocities 5 9 6 0 km s with little and discrete reflectivity and a strongly reflective lower crust with higher p wave velocities about 6 4 6 8 km s both are separated by the conrad discontinuity in contrast strength and rheological considerations define a brittle upper crust and a ductile lower crust the boundary between these two realms is usually referred to as the brittle ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres the present brittle ductile transition in the rhine graben region as defined by the deepest hypocentres does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies this suggests that the laminated lower crust developed during the crustal re equilibration phase at the end of the variscan orogeny the second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the variscan lower crust a remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern rhine graben and is possibly induced by the young rifting process we suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the alpine orogen this allows us to propose the distinction between the influence of compression extension continental subduction and intra crustal detachment during graben formation the dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small scale collisional escape tectonics associated with an intra continental transform fault at the southern end of the rhine graben the rhine sao ne transform zone

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

geochronology and geochemistry of high pressure granulites of the arthur river complex fiordland new zealand cretaceous magmatism and metamorphism on the palaeo pacific margin the arthur river complex is a suite of gabbroic to dioritic orthogneisses in northern fiordland new zealand the arthur river complex separates rocks of the median tectonic zone a mesozoic island arc complex from palaeozoic rocks of the palaeo pacific gondwana margin and is itself intruded by the western fiordland orthogneiss new shrimp u pb single zircon data are presented for magmatic metamorphic and deformation events in the arthur river complex and adjacent rocks from northern fiordland the arthur river complex orthogneisses and dykes are dominated by magmatic zircon dated at 136 129 ma a dioritic orthogneiss that occurs along the eastern margin of the complex is dated at 154 4 3 6 ma and predates adjacent plutons of the median tectonic zone rims on zircon cores from this sample record a thermal event at c 120 ma attributed to the emplacement of the western fiordland orthogneiss migmatitic palaeozoic orthogneiss from the arthur river complex 346 6 ma is interpreted as deformed wall rock very fine rims 5 20 lm also indicate a metamorphic age of c 120 110 ma a post tectonic pegmatite 81 8 1 8 ma may be related to phases of crustal extension associated with the opening of the tasman sea the arthur river complex is interpreted as a batholith emplaced at mid crustal levels and then buried to deep crustal levels due to convergence of the median tectonic zone arc and the continental margin