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

geologic and kinematic constraints on late cretaceous to mid eocene plate boundaries in the southwest pacific abstract starkly contrasting tectonic reconstructions have been proposed for the late cretaceous to mid eocene 85 45 ma evolution of the southwest pacific reflecting sparse and ambiguous data furthermore uncertainty in the timing of and motion at plate boundaries in the region has led to controversy around how to implement a robust southwest pacific plate circuit it is agreed that the southwest pacific comprised three spreading ridges during this time in the southeast indian ocean tasman sea and amundsen sea however one and possibly two other plate boundaries also accommodated relative plate motions in the west antarctic rift system wars and between the lord howe rise lhr and pacific relevant geologic and kinematic data from the region are reviewed to better constrain its plate motion history during this period and determine the time dependent evolution of the southwest pacific regional plate circuit a model of 1 west dipping subduction and basin opening to the east of the lhr from 85 55 ma and 2 initiation of northeast dipping subduction and basin closure east of new caledonia at 55 ma is supported west dipping subduction and basin opening were not driven by convergence as has previously been proposed our plate circuit analysis suggests that between at least 74 ma and subduction initiation at 55 ma there was little net relative motion between the pacific plate and lhr

contractional tectonics and magma paths in volcanoes abstract this study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings based on field data and physically scaled experiments the data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting in spite of the widely accepted idea that volcanism can develop only in extensional transcurrent tectonic settings the experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes the substrate fault splits into two main shear zones within the volcano a shallow dipping one with reverse motion propagates towards the lower volcano flank and a steeper dipping one with normal motion propagates upwards in plan view the reverse fault zone is arcuate and convex outwards whereas the normal fault zone is rectilinear structural field surveys at volcanoes located in contractional settings show similar features the plio quaternary trohunco and los cardos centinela volcanic complexes argentina lie above plio quaternary reverse faults the late pleistocene holocene el reventador volcano ecuador is also located in a coeval contractional tectonic belt these volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area consistent with the experiments these data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano

the effects of quartz dauphine twinning on strain localization in a mid crustal shear zone abstract we use microstructural electron backscatter diffraction ebsd and crystal vorticity axis cva analyses to evaluate the effects of quartz dauphine twinning on strain localization we compare a granodiorite mylonite and a tonalite mylonite from the grebe shear zone a mid crustal transpressional shear zone in fiordland new zealand ebsd analysis reveals identical quartz grain boundary migration dynamic recrystallization microstructures in both samples with abundant dauphine twinning only in the granodiorite mylonite bulk cva patterns further distinguish the untwinned and dauphine twinned samples indicating a transition from simple shear to pure shear dominated transpression respectively the quartz cva pattern from the dauphine twinned sample indicates that twinned and untwinned grains record a transition from simple shear to pure shear dominated deformation respectively pole figures of the dauphine twinned sample reveal high temperature slip systems in the untwinned and dauphine twinned grains we conclude that dauphine twinning formed early in transpression and rendered the twinned grains less deformable relative to the untwinned grains over time the effectiveness of dauphine twinning in localizing strain is therefore strongly influenced by its timing relative to the evolving shear zone kinematic deformation geometry

metamorphism and deformation of mafic and felsic rocks in a magma transfer zone stewart island new zealand in the mingled mafic felsic halfmoon pluton at the neck stewart island part of the median batholith of new zealand some hornblende gabbros and diorites retain magmatic structures whereas others show evidence of major changes in grain and inclusion shapes and still others are amphibolite facies granofelses with few or no igneous relicts these mafic to intermediate magmas crystallized in felsic magma relatively quickly with the result that most deformation occurred at subsolidus conditions it is suggested that mafic intermediate rocks with predominantly igneous microstructures spent less time in the magmatic system the metamorphism of the mafic rocks appears to be autometamorphic in the sense that elevated temperatures were maintained by magmatic heat during subsolidus cooling elevated temperatures were maintained because of repeated sheet injection and subconcordant dyke injection of hot basaltic and composite mafic felsic magmas into a dominantly transtensional km scale outboard migrating magmatic shear zone that operated semi continuously for between c 140 and c 130 ma complete cooling occurred only when the system evolved to transpressional and the locus of magmatism migrated inboard southward between c 130 and c 120 ma associated with solid state mylonitic deformation intermingled granitic rocks escaped metamorphism because they remained magmatic to lower temperatures and experienced shorter and lower temperature subsolidus cooling intervals however the felsic rocks underwent relatively high temperature solid state deformation as indicated by myrmekite replacing k feldspar and chess board subgrain patterns in quartz locally they developed felsic mylonites the felsic rocks were deformed in the solid state because of their high proportion of relatively weak minerals quartz and biotite whereas the mafic rocks mostly escaped subsolidus deformation except in local high strain zones of hornblende plagioclase schist because of their high proportion of relatively strong minerals hornblende and plagioclase we suggest that such contrasting microstructural features are diagnostic of long lived syntectonic magma transfer zones and contrast with the more typical complex batholith scale magma chambers of magmatic arcs

direct observation of basal sliding and deformation of basal drift at sub freezing temperatures in a tunnel at the base of sub polar urumqi glacier no 1 china three new mechanisms of glacier flow at sub freezing temperatures have been observed taken individually or in combination these modes of flow can account for nearly all 60 80 of the overall glacier motion and yet they act only within the lowermost 1 2 of the effective glacier thickness these mechanisms are 1 enhanced deformation of the frozen and ice laden subglacial drift 2 motion across discrete shear planes or shear bands within the frozen drift or at the ice drift interface and 3 basal sliding at an ice rock interface at a temperature of nearly 5 c the ice laden drift has an effective viscosity of more than one hundred times less than that measured in the overlying ice thus allowing very rapid shear deformation the observed rate or basal sliding at the ice rock interface agrees favorably with that predicted by the recent work of shreve 1984 if proper account is taken of the measured surface roughness and reduced ice viscosity

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

debris transport in a temperate valley glacier haut glacier d arolla valais switzerland hambrey m j goodsell becky glasser n f 2005 debris transport in a temperate valley glacier haut glacier d arolla valais switzerland journal of glaciology 51 172 pp 139 146

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

errata neogene fjordal sedimentation on the western margin of the lambert graben east antarctica the lambert graben is occupied by the world s largest fjord system through which flows the lambert glacier the amery ice shelf and their tributaries along the western margin of the graben in the northern prince charles mountains remnants of uplifted miocene and pliocene strata of the glacigenic fjordal pagodroma group total more than 800 m in thickness these sediments provide evidence for a dynamic east antarctic ice sheet during the neogene period each of the four pagodroma group formations defined from this region rests unconformably on either proterozoic or permo triassic rocks the unconformity surfaces represent parts of the walls and floors of neogene fjords for these surfaces to have been eroded the ice must have been grounded out as far as the continental shelf in prydz bay the pagodroma group was deposited by wet based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern antarctic ice masses the principal lithofacies are massive diamicts and soulder gravels deposited both close to a calving grounded glacier terminus and from icebergs the few stratified diamicts are the product of more distal iceberg sedimentation an ice transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts some lithofacies contain evidence of subaquatic slumping and gravity flowage volumetrically minor lithofacies include laminites with some exposures exhibiting large ice rafted clasts the laminites represent less proximal mainly ice free fjordal sediments resulting either from tidal current sorting of suspended sediment originating from subaquatic glaciofluvial discharge or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment the pagodroma group provides a record of multiple glaciation by dynamic sliding glaciers carrying large amounts of both basal and supraglacial debris the closest modern analogues in terms of the thermal and dynamic characteristics of the neogene lambert glacier appear to be the fast flowing tidewater glaciers of east greenland these glaciers originate from the interior ice sheet and discharge large volumes of icebergs the resulting lithofacies are predominantly diamicts

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

depositional models for moraine formation in east antarctic coastal oases this paper examines the origin of moraine ridges in east antarctic coastal oases and derives depositional models appropriate for the reconstruction of quaternary history on the basis of morphology structure and sedimentology four principal types of ridge may be identified 1 type a moraines form whenthe basal debris zone crops out near an ice margin 2 type b moraines form when large recumbent folds develop in the basal debris zone 3 type c moraines are ice contact screes and fans which form when debris accumulates at streep or cliffed ice margins and 4 type d moraines are thrust block moraines that form when unconsolidated sediment is entrained by freezing shearing and thrusting of sediment blocks at the base of the glacier simple calcumations of the rate of debris accumulation at ice margins suggest that type a b and c moraines take thousands of years to form and record stable ice margins type d moraines are structural features that may form relatively quickly when ice margins override unconsolidated sediment constructing models to explain the origin of the moraines is an important part of reconstructing the quaternary history of antarctic coastal oases because the models provide a basis for reconstructing the position and behaviour of the ice sheet during advance and retreat

biogenic calcite structures forming in lake fryxell antarctica stromatolites are organosedimentary structures produced by microorganisms which have been used extensively in the interpretation of precambrian and to a lesser extent phanerozoic history1 5 modern stromatolites are generally restricted to hot springs or warm hypersaline and or alkaline aquatic environments3 recently however we documented6 the presence of modern stromatolites in several cold perennially ice covered antarctic dry valley lakes here we report a finding in lake fryxell antarctica of biogenic calcite structures not observed in our previous study of other antarctic lakes these structures were laminated indurate unbranched and had either a vertical or horizontal orientation we infer that these calcite structures are biogenic in origin and hence stromatolitic based on the following evidence 1 the presence of an actively metabolizing algal mat on their surface and 2 an internal composition of filamentous blue green algal cell wall fragments diatom frustules and calcite crystals

the influence of basal processes on the dynamic behaviour of cold based glaciers abstract there exists within glaciology a widely held assumption that basal sliding and bed deformation do not operate beneath cold based ice and that their basal velocity is therefore zero irrespective of bed conditions consequently their ability to erode entrain and transport sediment and thereby alter the landscape is assumed to be limited consequently very little research has been focused towards describing and understanding the motion of cold based ice masses and our knowledge of their behaviour remains poor and the assumption of zero basal velocity unchallenged in this review paper it is argued that this assumption is not universally applicable and that in certain circumstances basal processes not only remain active at sub freezing temperatures but can significantly influence glacier motion this is particularly the case in glaciers where sub freezing basal thermal conditions coincide with the presence of fine grained ice rich subglacial sediments due to the lack of work undertaken on contemporary cold based glaciers much of the information used to support this argument is derived from field research in permafrost areas and on quaternary glacial sediments and from the laboratory testing of ice sediment mixtures it is concluded that the assumption of zero basal velocity beneath cold based ice is overly simplistic and that in reality the situation is likely to be much more complex work is therefore urgently required firstly to investigate the circumstances in which basal motion remains active at sub freezing temperatures and to determine their influence on glacier motion and secondly to examine the likely extent of subglacial permafrost in both the contemporary and quaternary glacial environment

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

crustal thickening of the early cretaceous paleo pacific gondwana margin abstract the cause of 7 to 25 km of early cretaceous thickening of parts of the new zealand gondwana margin has been argued to be due to tectonic or magmatic processes new insights into this event have been gained from examination of the grebe mylonite zone an extensive amphibolite facies early cretaceous ductile shear zone that separates volcanic arc rocks of the outboard median batholith from metasedimentary and intrusive rocks of the western province inboard median batholith at lake manapouri in southwest new zealand ar ar and u pb geochronology shows that here exhumation of the outboard median batholith began 128 ma syn kinematic magmatism was taking place at 121 ma and measurable displacement had ceased by 116 ma although the mylonite zone is currently subvertical a combination of thermochronological cooling profiles lineation orientations and back rotation of tilted tertiary unconformities indicate that it most likely formed as an oblique sinistral reverse fault that dipped 60 e and therefore displaced the outboard median batholith over the margin of the western province however the high original angle of this and other compressional shear zones in analogous structural positions the apparent absence of a regional thrust or nappe sheet s and the subtly different ages of thickening for different areas indicate that tectonic loading can have played only one part in the crustal thickening event another factor was the contemporaneous emplacement of 30 000 km 3 of mafic to felsic magma which was likely accommodated by vertical depression of the middle and lower crust tectonic and magmatic processes leading to crustal thickening were therefore not mutually exclusive

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

three stage evolution of lower crustal gneiss domes at breaksea entrance fiordland new zealand 1 new structural analyses reveal the presence of several kilometer scale gneiss domes and many subdomes within an exhumed section of cretaceous lower crust in sw fiordland the domes are defined by complex nearly concentric trajectories of foliations and steep high strain zones in migmatitic granulite and eclogite facies orthogneiss structural patterns combined with evidence of deformation that was synchronous with partial melting and high grade metamorphism steep mineral lineation orientations sense of shear indicators and near isothermal decompression suggest that buoyancy driven diapiric flow of hot partially molten crust aided initial dome formation mafic intermediate magmatism appears to have enhanced this process superimposed on all high grade structures are upper amphibolite facies shear zones that record a change in the style and conditions of lower crustal flow lineation orientations sense of shear evidence of cooling with decompression and other patterns suggest that the retrograde shear zones record bulk horizontal flow and crustal thinning driven by regional tectonic forces a preferred sequence begins with the initial emplacement of mafic intermediate magma into the lower crust at depths up to 70 km d1 diapirism and initial dome formation occurred during a period of waning arc magmatism and episodic partial melting and high grade metamorphism d2 following melt crystallization dominantly horizontal flow of a cooler strain hardened lower crust occurred during late cretaceous continental extension d3 this example highlights the interplay between mechanisms of horizontal and vertical flow at the root of a continental arc and illustrates their roles in the formation of gneiss domes

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

victoria lower glacier and ross sea glaciation dry valleys area south victoria land antarctica observations glaciologiques et etude morphologique et structurale du glacier victoria lower glacier

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

classification of poorly sorted sedimentary rocks abstract the classification and nomenclature of sandstones has evolved over the years into a system which is now generally accepted as standard however no equivalent system exists for the poorly sorted sedimentary rocks which include grain sizes from clay to gravel a descriptive classification is proposed based on an earlier system but using updated nomenclature and lithological fields such a scheme is needed to clearly define lithological terms currently used in the description of sedimentary sequences

the initiation and evolution of the transpressional straight river shear zone central fiordland new zealand abstract structural data and u pb geochronology on zircon from central fiordland new zealand show the role of pre existing structural heterogeneities in the kinematic evolution of a newly discovered zone of transpression the straight river shear zone consists of steep zones of high strain that are superimposed onto older fabrics across a 10 80 km region the older foliation formed during two periods of tectonism contraction and magmatism of mostly carboniferous 312 306 ma age and early cretaceous batholith emplacement ending by 113 4 1 7 ma followed by extension that ceased by 88 4 1 2 ma the primary mechanism for the formation of steep shear zone foliations was the folding of these older fabrics conjugate crenulation cleavages associated with the folding record shortening at high angles to the shear zone boundaries fold axial surfaces and axial planar cleavages strike parallel to the shear zone with increasing strain as they progressively steepened to subvertical in most areas shear sense flips from oblique sinistral east side down component to oblique dextral west side down across zones of intermediate and high strain high strain zones display subvertical mineral lineations steep strike slip faults and shear sense indicators that record strike slip motion across the steep lineations these patterns reflect triclinic transpression characterized by narrow zones of mostly strike slip deformation and wide zones of mostly contraction zones of high strain align with offshore traces of late tertiary strike slip faults suggesting that a previously undocumented component of late tertiary shortening and strike slip motion is accommodated within fiordland

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

ice dynamics and thermal regime of taylor glacier south victoria land antarctica les donnees glaciologiques recueillies sur le taylor glacier ont servi a reconstituer les principaux traits de la dynamique et du regime thermique du glacier les etudes du bilan de masse montrent que le glacier est proche de l equilibre les conditions thermiques de la glace basale sur une grande partie de la zone d ablation ont ete calculees a partir d estimation du flux de chaleur geothermique et de mesures des temperatures de la glace pres de la surface des vitesses et de l epaisseur de la glace on a trouve que jusqu a 50 de la zone d ablation la glace basale pouvait etre fondante

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

development of sediment landform associations at cold glacier margins dry valleys antarctica hambrey m j fitzsimons s j 2010 development of sediment landform associations at cold glacier margins dry valleys antarctica sedimentology 57 3 857 882 michael hambrey is corresponding author sponsorship marsden fund of new zealand university of otago research committee nerc grant no ner a s 2000 01256

testing the model of oblique transpression with oblique extrusion in two natural cases steps and consequences abstract kinematic models of various types of transpression have been used to explain fabric features and strain in many natural deformation studies here a mathematical model that encompasses all monoclinic and triclinic transpressional deformations including triclinic deformation with inclined simple shear and or inclined extrusion orientations can be tested using a step by step approach with available field evidence two cases are presented the first case from the wabigoon quetico boundary in the archean superior province utilizes both fabric orientation and quantified strain data the best fit of the field evidence to the model indicates that deformation likely took place along subvertical shear zones via transpression with subhorizontal simple shear 0 20 and variable inclined extrusion direction extrusion can be either east or west and typically indicates extrusion orientations between 0 and 50 from vertical the second case of the south iberian shear zone has fabric orientation data but no quantifiable strain possibilities the best fit of the field evidence to the model indicates that deformation likely took place along a moderately inclined shear zone via transpression with subhorizontal simple shear 0 20 and variable inclined extrusion direction values between 0 and 80 from the true dip of the shear zone using this protocol in other examples of natural deformation will allow further constraints to be applied to kinematic models

localisation of deformation in the thermal contrast at a granite batholith margin new zealand abstract the wainui shear zone is a 1 2 km wide steeply dipping ductile shear zone that is intimately associated with the western margin of a very extensive granite batholith in central new zealand shear sense indicators and lineation orientations within the central high strain portion of shear zone consistently show a reverse dip slip east side up sense of motion shallower foliation in the margins of the shear zone is interpreted to pre date the central high strain zone however u pb and ar ar data reveal that all ductile deformation occurred between 114 and 109 ma and probably between 114 and 111 ma requiring 1 that deformation kinematics within the shear zone changed over a short time period and 2 the shear fabrics formed immediately after emplacement of the voluminous granite batholith the temporal and spatial link between the deformation and plutonism is hypothesised to be a result of thermal weakening of the cooler paleozoic crust by cretaceous granite intrusion which promoted localisation of deformation onto the intrusive contact deformation was initially distributed over a wide zone but became focused into a central mylonite zone continental scale shortening is recorded as a network of ductile shear zones that formed on thermal contrasts within the upper middle and lower crust along the new zealand gondwana margin

debris entrainment and transfer in polythermal valley glaciers hambrey mj bennett mr dowdeswell ja et al 1999 debris entrainment and transfer in polythermal valley glaciers journal of glaciology 45 149 69 86 published 1999

autochthonous inheritance of zircon through cretaceous partial melting of carboniferous plutons the arthur river complex fiordland new zealand tims and shrimp u pb analyses of zircons from milford orthogneiss metadiorite p 1 1 4 gpa t 750 c of the arthur river complex of northern fiordland reveal a bimodal age pattern zircons are predominantly either paleozoic 357 0 4 2 ma and prismatic with oscillatory zoning or cretaceous 133 9 1 8 ma and ovoid with sector or patchy zoning the younger age component is not observed overgrowing older grains most grains of both ages are overgrown by younger cretaceous 120 ma metamorphic zircon with very low u and th u 0 01 we interpret the bimodal ages as indicating initial igneous emplacement and crystallisation of a dioritic protolith pluton at 357 ma followed by early cretaceous granulite facies metamorphism at 134 ma during which a significant fraction 60 of the zircon grains dissolved and subsequently reprecipitated effectively in situ in partial melt pockets the remaining 40 of original paleozoic grains were apparently not in contact with the partial melt remained intact and show only slight degrees of pb loss sector zoning of the cretaceous grains discounts their origin by solid state recrystallisation of paleozoic grains the alternative explanation that the paleozoic component represents a 40 inherited component in an early cretaceous transgressive dioritic magma is considered less likely given the relatively high solubility of zircon in magma of this composition the absence of 134 ma overgrowths the single discrete age of the older component equivalent time integrated 177hf 176hf compositions of both age groups and the absence of the cambrian proterozoic detrital zircon that dominates regional cambro ordovician metasedimentary populations similar bimodal carboniferous early cretaceous age distributions are characteristic of the wider arthur river complex 8 of 12 previously dated dioritic samples have a paleozoic component averaging 51 furthermore the age and chemical suite affinity of these and several more felsic rocks can be matched with those of the relatively unmetamorphosed carboniferous plutonic terrane along the strike of the mesozoic margin in southern fiordland also supporting the in situ derivation of the carboniferous inherited component

tracking the influence of a continental margin on growth of a magmatic arc fiordland new zealand using thermobarometry thermochronology and zircon u pb and hf isotopes 1 geothermobarometric radiogenic isotopic and thermochronologic data are used to track the influence of an ancient continental margin western province on development of an adjacent carboniferous cretaceous magmatic arc outboard median batholith in fiordland new zealand the data show a record of complicated mesozoic gondwana margin growth paragneiss within the outboard median batholith is of carboniferous to jurassic age and records burial to middle crustal depths in late jurassic early cretaceous during subduction related plutonism and arc thickening in contrast western province metasedimentary rocks in the area of study immediately west of the outboard median batholith are late cambrian early ordovician in age recrystallized at the amphibolite facies in the late devonian early carboniferous and exhibit no evidence for mesozoic textural or isotopic reequilibration a phase of deformation between 128 and 116 ma deformed exhumed and cooled the outboard median batholith to greenschist facies temperatures while large parts of the western province underwent 9 kbar metamorphic conditions zircon grains from mesozoic inboard plutons are isotopically more evolved hf t 2 3 to 4 0 than those in the outboard median batholith hf t 9 4 to 11 1 the contrasting zircon hf isotope ratios absence of s type plutons or proterozoic early paleozoic inherited zircon and the apparent absence of early paleozoic metasedimentary rocks indicates that the outboard median batholith is unlikely to be underlain by the western province continental lithosphere the new data are consistent with the outboard median batholith representing an allochthonous although not necessarily exotic arc that was juxtaposed onto the gondwana continental margin along the intervening grebe mylonite zone

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

experimental constraints on pre eruptive conditions of phonolitic magma from the caldera forming el abrigo eruption tenerife canary islands abstract we have performed phase equilibrium experiments to determine the pre eruptive conditions of the largest caldera forming eruption 20 km 3 of dre that has occurred on tenerife canary islands the abrigo ignimbrite is phonolitic and was erupted during the last caldera forming episode ca 190 ka prior to the construction of the teide pico viejo active central complex the abrigo phonolite contains low amounts of phenocrysts ca 7 vol mainly sanidine biotite clinopyroxene and magnetite with minor amounts of titanite apatite and hauyne we have investigated pressures from 50 to 250 mpa temperatures of 700 900 c water contents in the melt from 1 5 to ca 10 wt and oxygen fugacity f o 2 from 1 log unit above the ni nio solid buffer to quartz magnetite fayalite comparison of the natural and experimental phase proportions and compositions indicates that the phonolite at the roof of the abrigo magma reservoir was at 130 50 mpa corresponding to ca 4 5 km below the surface 825 25 c with 3 1 wt dissolved h 2 o and f o 2 at the ni nio buffer 1 log unit this shows that the magma that produced the largest ignimbrite on tenerife was stored at relatively shallow depths but was water undersaturated and its eruption was probably triggered by input of fresh mafic magma comparison of the abrigo phase equilibrium results with those obtained in other phonolitic magmas e g eifel kerguelen somma vesuvius shows that the major element and probably fluid composition h 2 o cl and s may have a strong effect on the presence or absence of phases and their compositions extrapolating phase equilibria results obtained on a specific phonolite to constrain the pre eruptive conditions of other phonolites should be done with care

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

transformation of two pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust fiordland new zealand granulite facies gabbroic and dioritic gneisses in the pembroke valley milford sound new zealand are cut by vertical and planar garnet reaction zones in rectilinear patterns in gabbroic gneiss narrow dykes of anorthositic leucosome are surrounded by fine grained garnet granulite that replaced the host two pyroxene hornblende granulite at conditions of 750 uc and 14 kbar major and trace element whole rock geochemical data indicate that recrystallization was mostly isochemical the anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss but change in morphology at the contacts from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss to zones with a septum of coarse grained garnet surrounded by anorthositic leucosome in the dioritic gneiss the dioritic gneiss also contains isolated garnet grains enclosed by leucosome and short planar trains of garnet grains linked by leucosome partial melting of the dioritic gneiss mostly controlled by hornblende breakdown at water undersaturated conditions is inferred to have generated the leucosomes the form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation limited retrogression suggests considerable melt escape dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss the migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite

capo granitola sciacca fault zone sicilian channel central mediterranean structure vs magmatism abstract the tectonic framework of the northern sector of the capo granitola sciacca fault zone cgsfz a nne oriented lithospheric strike slip fault zone located in the sicilian channel southern italy has been reconstructed with the aim to clarify the relationships between geometry and kinematics of the structures and the occurrence and distribution of the magmatic manifestations observed in the area this has been achieved by the interpretation of a large dataset composed of 2 d multichannel seismic profiles chirp profiles magnetic data and borehole information in addition to the volcanic edifices known in the graham and terribile banks this study has allowed to recognize several other magmatic manifestations the magmatic occurrences consist of small volcanic cones buried magma ascents and potential igneous sills the cgsfz is bounded by two strike slip fault systems the capo granitola fault system cgfs to the west and the sciacca fault system sfs to the east dominated by positive flower structures generated by tectonic inversion of nne oriented late miocene extensional faults only the southern part of the cgfs shows the presence of a sub vertical n s oriented strike slip master fault the sector between the two fault systems does not show a significant pliocene quaternary tectonic deformation except for its southern part hosting the terribile bank which is dissected by wnw to nw trending normal faults developed during late miocene and later reactivated this set of faults is currently active at the terribile bank whereas is buried by pliocene quaternary deposits in the central and northern sectors of the cgsfz the observed magmatism is driven by a mechanism of non plume origin magmas have used as open paths the faults of the cgfs and sfs which cut the whole lithosphere reaching the asthenosphere and producing partial melting by simple pressure release most of the magmatism develops along the strike slip master fault associated with the cgfs and the normal faults affecting the terribile bank the magmatic feeding of the terribile bank would be related to lateral magma migration coming from the structures of the sfs which would use the open pathways represented by active normal faults in the central northern part of the cgsfz magmas migrate upward along lithospheric faults then move laterally and rise toward the surface through nne and nw trending buried normal faults these late miocene faults do not reach the surface and this may have favoured the emplacement of igneous sills which in turn may explain the observed volcanic centres

high pressure metamorphism caused by magma loading in fiordland new zealand cretaceous granulite facies metamorphism in the fiordland area of new zealand has distinctive mineralogical textural and structural features that set it apart from most other regional metamorphic belts the metamorphism developed over a 30 150 km area and the consequence of a 20 km thick increment to crustal thickness is closely associated in space and time with a large plutonic complex the western fiordland orthogneiss wfo although temperatures and pressures as high as 700 c and 12 kbar were attained the metamorphic overprint on earlier low pressure assemblages is weak and incomplete little strain accompanied the metamorphism the temperature threshold at which metamorphic recrystallization is recorded is over 500 c zoned garnets are preserved at unusually high temperatures indicating duration of metamorphism on the order of 10 times shorter than in most other regional terranes this pattern of features bears close similarity to metamorphism in the coast plutonic complex in north america where a mechanism of magma loading has been invoked in fiordland the high pressure metamorphism can be explained by depression of country rock under a crustal zone that is inflated by intrusion of the wfo regional structure of the wfo as a horizontally sheeted complex suggests that the pluton was emplaced by vertical displacement of country rock and supports the magma loading model

genesis of hummocky moraines by thrusting in glacier ice evidence from svalbard and britain irregular mounds of glacial debris commonly referred to as 9hummocky moraine9 until recently were linked to ice stagnation during rapid climatic amelioration however recent work in scotland has demonstrated that some hummocky moraine dating from the younger dryas event c 10 000 years bp was the product of deposition at active ice margins observations at modern high arctic glacier margins in svalbard 76 80 n indicate that some moraines of this type form under a dynamic glaciological regime mainly by thrusting in active polythermal ice undergoing strong longitudinal compression this situation is particularly common where there is a transition from warm based ice in the interior of a glacier to cold based ice at the margins or snout from their morphological similarity rectilinear slopes facing up glacier and irregular downglacier faces and apparent stacking of slabs of sediment of subglacial derivation it is concluded that some assemblages of british moraines are also the product of thrusting this process appears to be most dominant in polythermal glaciers rather than in temperate glaciers on this basis it is tentatively suggested that the younger dryas in britain was characterized by a climatic regime similar to that in svalbard today mean annual temperature 5 c 400 1000 mm precipitation and that the associated glaciers may have been of polythermal character

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

debris entrainment at the ice bedrock interface in sub freezing temperature conditions terre adelie antarctica the debris rich ice from the bottom 6 m of the 82 m deep caroline coastal antarctic record of last interglacial natural environment ice core reaching bedrock and from five 2 m long surface cores at moraine prudhomme in terre adelie antarctica is described and compared to debris laden ice from the core drilling site d10 isotopic total gas content co2 concentration and sem investigations of embedded particles together with ice textures and fabrics rule our pressure melting regelation around bed obstacles or freezing on as possible mechanisms for the debris entrainment at the ice bedrock interface it is suggested that the debris entrapment by purely mechanical means e g shearing is an efficient process in forming basal ice layers bil at sub freezing temperatures from authors

roles for fluid and or melt advection in forming high p mafic migmatites fiordland new zealand a series of striking migmatitic structures occur in rectilinear networks through western fiordland new zealand involving for the most part narrow anorthositic dykes that cut hornblende bearing orthogneiss adjacent to the dykes host rocks show patchy spatially restricted recrystallization and dehydration on a decimetre scale to garnet granulite although there is general agreement that the migration of silicate melt has formed at least parts of the structures there is disagreement on the role of silicate melt in dehydrating the host rock a variety of causal processes have been inferred including metasomatism due to the ingress of a carbonic mantle derived fluid hornblende breakdown leading to water release and limited partial melting of host rocks and dehydration induced by volatile scavenging by a migrating silicate melt variability in dyke assemblage together with the correlation between dehydration structures and host rock silica content are inconsistent with macroscopic metasomatism and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid

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

reconstructing the transport history of glacigenic sediments a new approach based on the co variance of clast form indices abstract two indices are used to describe the aggregate shape and roundness characteristics of glacially transported clasts the c40 index the percentage of clasts with c a ratios 0 4 and the ra index the percentage of very angular and angular clasts in a sample analysis of the between and within sample co variance of these indices provides a powerful tool for discriminating actively and passively transported clasts in glacigenic deposits use of these indices is illustrated in an analysis of the form characteristics of clasts from lateral and frontal moraines at storbreen jotunheimen these fall between those of actively transported clasts in basal till low c40 and ra values and those of unmodified frost weathered clasts high c40 and ra values both indices decline down moraine within sample analyses of clast shape demonstrates that these aggregate clast form gradients reflect changes in the relative proportions of actively and passively transported debris on the moraines rather than progressive modification of actively transported clasts these analyses also indicate that actively transported clasts form the dominant component of the moraines particularly near the former glacier terminus further applications of the approach developed here are suggested

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