late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

origin and metamorphic history of an early cretaceous polybaric granulite terrain fiordland southwest new zealand regionally extensive two pyroxene granulites in fiordland southwest new zealand are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes the granulite protolith igneous charnockitic rock synkinematically intruded metasediment and other orthogneiss in an early cretaceous subduction related magmatic arc and during cooling experienced deformation induced recrystallization to form granoblastic gneiss the granulites occur side by side with coeval rocks of amphibolite facies mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization 1 an early high temperature comparatively low pressure event accompanying magmatic intrusion andalusite sillimanite facies series recorded locally in the country rock followed by 2 high pressure metamorphism under conditions of 650 700 c at 12 13 kbar garnet granulite locally overprinted earlier formed two pyroxene granulite during the latter event the pressure increase 6 kbar between the two events is attributed to crustal thickening by overthrusting and is equivalent to unloading of a 20 km thick slab over rocks already buried at mid crustal depths both events occurred over a 20 m y interval between the time of magmatic emplacement of the granulite protolith and uplift controlled final cooling of the terrain the phanerozoic granulites in fiordland share some petrologic similarities with precambrian granulite terrains suggesting that at least some aspects of the former may serve as a useful model for development of the latter

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

intra arc transpression in the lower crust and its relationship to magmatism in a mesozoic magmatic arc abstract structural observations and u pb geochronology from fiordland new zealand support a model of partitioned transpression within the lower crust of an early mesozoic magmatic arc called the median batholith we use this lower crustal section to test whether transpression was an efficient mechanism for transporting magma through the deep lithosphere a continentward migration of magmatic activity occurred within the margin of gondwana after 140 ma followed by a period of concentrated magmatism in a vertical 12 15 km wide lower crustal shear zone after 119 ma the shear zone named the indecision creek shear zone contains variably oriented dioritic intrusions and displays systematic variations in the three dimensional orientation of ductile structures from the margins to the center of the shear zone the pitch of stretching lineations on foliation surfaces changes from 10 35 to 55 82 with increasing finite strain this increase in pitch is accompanied by a steepening and counter clockwise rotation of foliation planes these and other structural patterns indicate that arc parallel sinistral oblique slip and strike slip displacements occurred at the shear zone margins and that deformation in its center was dominated by horizontal arc normal shortening and near vertical extrusion this style of partitioned transpression reflects the effects of rheological contrasts created by a heterogeneous pattern of magmatism within the arc field relationships and u pb dates on zircon suggest that the shear zone formed along the boundary between outboard older and inboard younger parts of the batholith and facilitated the transfer of small volumes of magma vertically through the lower crust until at least 111 ma when convergence and arc magmatism waned

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

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

vertical coupling and decoupling in the lithosphere recent advancements in the understanding of mountain belts have focused on vertical coupling of the lithospheric layers this volume describes the extent to which observed or inferred sub horizontal coupling or attachment zones provide vertical kinematic linkage between rheologically distinct layers in the continental lithosphere a common theme is whether the deformation which partitioned differently in each layer is linked kinematically across attachment zones and driven by flow in the deeper crust and mantle lithosphere the papers are divided into six sections the first analyses the extent to which mantle flow controls deformation of the overlying layers the vertical axis block rotations section uses geological and palaeomagnetic data to constrain the role and magnitude of basal shear across mid crustal attachment zones the lower crustal flow and topography section addresses the time dependent development of orogenic plateaux and their role in the orogenic cycle multiple examples of the spatial and temporal development of lithospheric coupling are given in both the orogenic examples and subduction examples sections finally rheological constraints of vertical coupling in the lithosphere are investigated

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

natural deformation and recrystallization of some intermediate plagioclase feldspars abstract intermediate an25 an48 plagioclase feldspars from the nordre stromfjord shear zone central west greenland have undergone extensive ductile deformation by intracrystalline slip and by twinning the plagioclases were deformed under low granulite facies conditions optical substructures include undulatory extinction deformation bands subgrains and deformational twins mainly on the albite law dynamic recrystallization occurred primarily by subgrain rotation with bulge nucleation migration of a high angle boundary as a secondary mechanism in some cases recrystallization is accompanied by a small decrease in the an content which is thought to indicate prograde metamorphic conditions the plagioclases exhibit a good crystallographic preferred orientation which is compatible with 010 001 being the principal slip system quartz has in the same rocks undergone more plastic deformation and more extensive recovery than plagioclase this is at least in part due to the fact that most dislocations in plagioclase are dissociated

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

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

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

tempo of burial and exhumation within the deep roots of a magmatic arc fiordland new zealand the u pb thermochronology of titanite apatite and rutile from a crustal profile through a mesozoic magmatic arc in fiordland new zealand is used to constrain the timing and duration of significant vertical movements during arc construction and evolution titanite data from deep crustal 12 13 kbar basement and cover rocks of central fiordland imply that contractional arc thickening 25 km occurred by 111 1 113 4 ma within a few million years of a major phase of mid to deep crustal magmatism this finding suggests that this cycle of magmatism arc thickening and high grade metamorphism occurred in 6 2 m y in contrast to rapid burial significant unroofing of the central fiordland gran ulites was more protracted requiring an additional 40 45 m y these new data are con sistent with continued residence of the granulites in thickened arc crust for 15 20 m y with subsequent major unroofing recorded by rutile cooling to 450 8c by ca 70 ma such temporal constraints are essential for comprehensive models of the growth modifi cation and unroofing of magmatic arc systems

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

the anatomy and evolution of a transpressional imbricate zone southern uplands scotland this paper describes in detail a spectacularly exposed transpressional imbricate zone from the southern uplands of se scotland a highly heterogeneous assemblage of contemporaneous structures is preserved and is interpreted to have formed during bulk inclined triclinic sinistral transpression in an obliquely convergent thrust wedge the resulting structures include a piggy back imbricate system of closely related strike slip detachments highly curvilinear folds oblique thrust faults and a clockwise transecting cleavage a series of sub domains are recognised which are interpreted to result from imperfect kinematic partitioning of the bulk transpression into either contraction or wrench dominated strains in this case the partitioning appears to be controlled by pre existing lithological anisotropy and how this was subsequently modified by processes such as folding once deformation began our findings demonstrate that transpressional deformation zones can develop structural patterns geometrically very similar to those found in dip slip fold and thrust belts such geometries are likely to occur in many other obliquely convergent wedges because deformation zone boundaries are very often inclined and lie at low angles or subparallel to regional bedding imbricate systems inevitably form because the sedimentary anisotropy is a key mechanical control in their development our observations illustrate therefore the fundamental importance of measuring fault movement directions in the field using fault slickenline lineations in imbricate systems it is possible that a number of imbricate zones identified in other thrust wedges may turn out to be transpressional features q 2004 elsevier ltd all rights reserved

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

origin of small scale segmentation and transpressional thrusting along the alpine fault new zealand the alpine fault is the major structural feature of the australian pacific plate boundary in the south island of new zealand geologic evidence suggests that half to three quarters of the plate boundary displacement is accommodated by movement on the fault detailed investigation of the central section of the alpine fault has revealed that it consists of oblique thrust sections striking 020 050 that are linked by subvertical right lateral faults striking between 065 and 090 the segmentation is on a scale of 1 10 km similarly oriented right lateral faults are abundant southeast of the alpine fault and are consistent with stresses induced in an elastic layer by an oblique slip ductile fault zone below propagation of the fault to the surface is predicted to result in an en echelon arrangement of strike slip and thrust segments it is suggested that the spatial distribution of segments is affected by the existence of deeply incised valleys in the hanging wall that disturb the stress field to depths of 1 4 km the segmentation is near surface and does not appear to act as a barrier to the propagation of large earthquakes

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

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

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

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

physical experiments of vertical transpression with localized nonvertical extrusion abstract previously proposed models of vertical walled transpression implicitly assume that material extrudes upwards during deformation this assumption is not necessarily valid at all scales given that a in areas of diverse lithologies local zones of relatively rigid materials may cause extruding material to deflect around those zones and b ductile strain often forms anastomosing geometries of shear zones therefore it is possible that a local extension direction in otherwise classic transpression may be nonvertical for rocks deforming at depth using an analogue experiment we have modeled a transpression zone with localized nonvertical extrusion this extrusion is accomplished by the addition of a side leak that allows sideways extrusion in addition to vertical extrusion the net extension direction depends on the material s position within the deforming zone resulting in a significant range of lineation orientations with deformation the strain patterns produced by transpression with localized nonvertical extrusion may explain the wide array of lineation orientations found in some natural ductile transpression zones

evidence of high sea level during isotope stage 5c in queensland australia abstract thirty nine species of scleractinian corals have been recovered from under a high dune on the western mainland side of north stradbroke island eastern australia the corals are associated with thin intertidal sediments and their good condition implies burial in situ and preservation in a saturated zone most likely this occurred as the coast prograded and a large dune advanced into the littoral zone burying intertidal sediments and coral the species assemblage indicates a sheltered environment but one open to the ocean without wide fluctuations in salinity three species yielded a mean 230 th 234 u age of 105 000 yr b p which is significantly younger than the nearest pleistocene corals at evans head new south wales the corals provide evidence of a sea stand near present sea level during isotope stage 5c which is considerably higher than previously suggested for this period their good condition implies that the overlying parabolic dune is of comparable age and formed during that high stand of sea level also the isotope age provides a maximum period for the development of giant podzols in the podzol chronosequences on coastal dunes in southern queensland

rheologically induced structural anomalies in transpressive regimes abstract local structural anomalies are sometimes found in transpressive regimes they can be related to strain partitioning processes but in some cases other types of heterogeneities could affect the deformation style this paper shows that rheological heterogeneities can radically influence the geometry of faults in transpression zones and cause them to be markedly non planar in the marao region of northern portugal a zone in which variscan structures are sw facing is found within the regional ne facing geometry this anomaly is adjacent to a complex shear zone mina ribeira das cestas where the fault plane is curved however the surrounding material does not show any signs of a related folding event detailed field studies mainly concerning the rheological contrasts between the deformed lithostratigraphic units show a close relationship between the structural anomaly and the movement along rheological anisotropies these observations complemented by experimental deformation of multilayers composed of analogue materials allow the construction of a generic model strain partitioning during the initial stage of the sinistral transpressive variscan deformation produced the juxtaposition of lithostratigraphic units with strongly different rheological contrasts subsequent flattening deformation induced heterogeneous indentation phenomena producing the non planar geometry of the complex shear zone the structural heterogeneities caused could have an important role in the evolution of a transpressive domain

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

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

isotopic ages from the nelson region of south island new zealand crustal structure and definition of the median tectonic zone abstract plutonic rocks in the rotoroa complex and drumduan terrane of south island new zealand yield zircon u pb dates of 156 and 142 ma respectively that are interpreted as crystallization ages hornblende and biotite 40 ar 39 ar dates of 140 130 ma from the rotoroa represent either emplacement ages cooling ages or a metamorphic resetting event these two units crop out between the brook street terrane and the separation point batholith and lack any clear affinity with tectonostratigraphic terranes of the new zealand western or eastern provinces the rotoroa complex and drumduan terrane are interpreted as part of a series of dismembered mesozoic volcanic plutonic arc complexes that are sandwiched between terranes of the western and eastern provinces occupying a structural position here referred to as the median tectonic zone mtz correlative units in fiordland on the opposite side of the alpine fault include the mackay intrusives darran complex largs terrane lochburn formation and the halfway peak gabbro farther south on stewart island the anglern complex and paterson group are part of the same structural belt the mtz is an extension of the original concept of the median tectonic line put forth by landis and coombs 1967 dismemberment and juxtaposition of arc magmatic assemblages in the mtz with western and eastern province terranes is related to large scale transcurrent faulting in the early cretaceous its essential features as a regional tectonostratigraphic terrane were established by 117 ma as indicated by stitching of the rotoroa complex to the takaka terrane western province by the separation point batholith 117 114 ma the echinus granite yields a 310 ma u pb zircon crystallization age that suggests the granite and associated gneisses are part of the western province which may constrain the position of the western margin of the mtz near nelson city

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

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

seismo and neotectonics in finnmark kola peninsula and the southern barents sea part 1 geological and neotectonic framework abstract as a background for assessing the seismo and neotectonics of the finnmark kola peninsula region emphasis has been directed towards the lineament framework involving a careful examination of digital landsat tm imagery integrated with a variety of data sets ranging from geophysics and digital elevation models to microtectonics based on a detailed field structural control of many of the multiphase lineaments in this 70 000 km2 area of the northeastern part of the fennoscandian shield as well as microscopic investigation of the fault mylonites and breccias five principal lineament sets were distinguished of the nw se polyphasal megafractures the murmansk shear zone carries geological and isotopic evidence of an archaean tectonothermal history and demarcates an important terrane boundary lineaments irrespective of their trends were classified according to their significance in the regional geological framework some lineaments are clearly associated with gravity and aeromagnetic anomalies and probably extend to deep crustal levels quite a few of the prominent faults within lineament zones were reactivated in post glacial times in one area in finnmark core drilling through a se dipping post glacial reverse fault has confirmed its listric shape at depth earthquake focal mechanisms for this same fault are presented in the second part of this dual contribution

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