the mid norwegian margin a discussion of crustal lineaments mafic intrusions and remnants of the caledonian root by 3d density modelling and structural interpretation the high density lower crustal body lcb on the mid norwegian margin is almost universally interpreted to represent magmatically underplated material added to the crust during early tertiary opening of the ne atlantic the thickness of the lcb is uneven and its distribution along the margin is sharply limited by margin perpendicular lineaments three dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments bivrost jan mayen gleipne etc in the crustal architecture with and without the lcb the bivrost lineament separating the voring and lofoten margin segments is clearly expressed and so is arguably a lineament in the sw voring margin notably most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from late caledonian orogenic collapse an alternative interpretation of the lcb is thus that it represents high grade metamorphic rocks remnant from the caledonian root determining the nature of the lcb has profound effects for the volume estimate of magmatic rocks in the north atlantic igneous province and consequently also for the degree of crustal thinning and heat flow

imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

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

record of plate boundary metamorphism during gondwana breakup from lu hf garnet geochronology of the alpine schist new zealand

the development of subgrain misorientations with strain in dry synthetic nacl measured using ebsd abstract the development of subgrain boundary misorientations with strain in dry synthetic nacl polycrystals deformed at elevated temperature has been investigated using electron backscattered diffraction ebsd at low natural strains up to 0 5 average misorientations of subgrain boundaries increase with strain and a power law relationship exists between strain and average misorientations the average misorientations are strongly influenced by grain orientation suggesting that the misorientation strain relationship may also be texture dependent in materials with high plastic anisotropy like nacl a slight grain size dependency of the average misorientations was observed the results indicate that with suitable calibration average subgrain boundary misorientations may offer a method for estimating the strain accommodated by dislocation creep in nacl and thus perhaps in other geological materials although current theories for polycrystalline plasticity imply that misorientations may also depend on stress in some situations

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

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

origin of a thrust related fold geometric vs kinematic tests abstract geometric tests to determine the origin of fault related folds are common but as is typical in structural geology more than one fold origin may yield the final natural geometry thus the results of geometric tests are usually non unique in contrast kinematic tests of origin which employ both geometry and data about deformation commonly yield more constrained if not unique results unfortunately the necessary data collection requires much more work than for a geometric test in this study the thrust related barclay anticline is analyzed both geometrically and kinematically to determine which test is more effective geometric tests using angular relationships indicate three possible origins fault bend fault arrest and break thrust for the kinematic test predicted deformations for interlayer and flexural slip flexural flow simple shear and bending strains are compared to micro and mesostructural distributions solution strain and finite strain from the anticline strain measurements indicate that microscale deformation is uniformly distributed through the structure and is lithification dominated the microscale deformation does not match kinematic predictions and did not accommodate fold formation fold growth was achieved primarily through layer parallel slip restricted mostly to the forelimb and absent in the hinge which eliminates fault bend and fault arrest origins that require material transport from forelimb to backlimb the barclay anticline is therefore interpreted to be a break thrust structure interestingly a suite of contraction faults in the forelimb and hinge indicates material transport from forelimb to backlimb such transport has been discounted for break thrust folds the most important result of this study is that a kinematic test was a necessary step for distinguishing fold origin geometric testing alone was insufficient given that the barclay anticline has geometric characteristics typical of many thrust related folds kinematic testing appears necessary to determine their structural origin

hyperextension serpentinization and weakening a new paradigm for rifted margin compressional deformation the early eocene magma rich northeast atlantic rifted margins contain a large number of pre breakup and post breakup compressional structures located in abandoned early cretaceous hyperextended basins with crustal stretching factors of 3 4 or more the deformation both predates and postdates the magma rich breakup the hyperextended basins are often underlain by high velocity lower crustal bodies which we argue represent partially serpentinized upper mantle long lived lithospheric weakening and proneness to deformation is proposed to relate to crustal hyperextension probably enhanced by mantle hydration

the structural geometry and evolution of foreland thrust systems northern virginia seismic reflection data reveal that the structural geometry of the central appalachians of northern virginia consists of three distinct thrust systems each thrust system is characterized by a unique internal geometry the blue ridge thrust sheet is a composite thrust sheet composed primarily of imbricated precambrian crystalline rocks it over rode cambrian ordovician carbonates and formed a sheared basement cored fault bend fold thrusts within the sheet may be taconic and earliest alleghanian whereas final thrusting and emplacement of the sheet were probably slightly younger but still early alleghanian the north mountain thrust sheet is characterized by imbricated cambrian ordovician carbonates that are deformed into large amplitude mode ii fault bend folds and fault propagation folds rocks of this sheet were transported more than 60 km across a similar section of carbonates the leading edge of the north mountain thrust sheet was deformed into a fold with a mode ii fault bend fold geometry and was juxtaposed against middle paleozoic rocks the middle paleozoic rocks occur in a ramp across which displacement along the north mountain thrust was transferred to a higher detachment more than 60 km of cover rocks displaced during the emplacement of the north mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western valley and ridge province or were backthrust above the sheet the timing for imbrication and emplacement of the north mountain thrust sheet is probably main phase alleghanian the lower carbonate duplex extends from beneath the blue ridge and north mountain thrust sheets westward across the western valley and ridge province the lower carbonate duplex is characterized by imbricated cambrian ordovician carbonates that form low amplitude fault bend folds this thrust system also probably formed during main phase alleghanian deformation

variation in quartz arenite deformation mechanisms between a roof sequence and duplexes abstract microstructural abundances and histories in quartz arenite of the lower silurian tuscarora sandstone were used to determine the nature and role of microscale deformation in a cover sequence and underlying thrust system and to assess the degree to which the cover sequence accommodated emplacement of the thrust system sandstone samples are located across the transition from the central to southern appalachian foreland thrust system where the thrusts change from blind to emergent southwards and where southern deformation of early alleghanian age has been previously shown to be overprinted by central deformation of alleghanian age microstructures observed with transmitted light and cathodoluminescence microscopy indicate that grain scale deformation occurred by dislocation flow pressure solution and microfracturing with the last being generally the most important the sequence of deformation mechanisms is the same for the cover sequence and the thrust system pressure solution during sedimentary compaction dislocation flow during layer parallel shortening and localized microfracturing with limited pressure solution near major thrust ramps and in steep fold limbs a greater abundance of dislocation flow microstructures in the cover sequence from layer parallel shortening indicates grain scale accommodation in the tuscarora sandstone of some shortening associated with emplacement of the thrust system the transition zone between the central and southern appalachians contains the greatest occurrence of every microstructure which is consistent with the area having been affected by diachronous central and southern alleghanian deformations

the contribution of salt fingers to vertical mixing in the north atlantic tracer release experiment abstract the north atlantic tracer release experiment natre was performed in an area moderately favorable to salt fingers however the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent this is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems despite the lack of a staircase optical shadowgraph profiles revealed that small scale tilted laminae previously observed in a salt finger staircase were abundant at the natre site using microstructure observations the strength of salt finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy the dissipation ratio by examining the dissipation ratio in a parameter space of density ratio r and richardson number ri the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred while the model for turbulence descr

the pounamu terrane a new cretaceous exotic terrane within the alpine schist new zealand tectonically emplaced deformed and metamorphosed during collision of the lip hikurangi plateau with zealandia abstract the petrologically renowned haast schist previously acknowledged as having formed by jurassic collision and amalgamation of the exotic rakaia and indigenous caples terranes during late mesozoic orogenesis contains a component of the alpine schist whose protolith was deposited in the mid cretaceous an air fall tuff deposited on the ophiolitic pounamu ultramafic belt pub has an eruption age of 108 ma the pub is in tectonic contact to the east with rakaia terrane derived alpine schist of triassic age and is overlain by turbiditic metasediments that have cretaceous 109 140 ma detrital zircon populations the dominant zircon component of these sediments is permo triassic compatible with autocannibalistic reworking from the torlesse composite terrane reconnaissance analysis of the alpine schist indicates that cretaceous protoliths extend at east 200 km along strike while possible correlatives elsewhere in new zealand suggest that the original basin may have been thousands of kilometres in extent models for the emplacement of these exotic cretaceous sediments the proposed pounamu terrane are explored our favoured interpretation is that the cretaceous alpine schist protoliths were deposited in an ensimatic sedimentary basin north of earlier mesozoic subduction zones multi phase deformation and metamorphism occurred during collision of the lip hikurangi plateau with zealandia resulting in the emplacement of an ophiolite pub soled allochthonous thrust sheet of the pounamu terrane onto the previously metamorphosed rakaia otago schist caples massif based on new and regional evidence this collision and the climactic metamorphism of the pounamu terrane occurred at 72 ma

origin of the torlesse terrane and coeval rocks south island new zealand the carboniferous to lower cretaceous torlesse terrane and haast schist derivatives constitute the major part of the complexly deformed facies of the eastern province of new zealand strata consist mainly of quartzofeldspathic graywacke and mudstone intercalated with minor but widely distributed conglomerate and volcanics with associated chert and limestone clastic rocks were deposited largely by sediment gravity flow mechanisms in a deep marine environment also present are a few highly fossiliferous shallow marine and terrestrial deposits of limited areal extent which rest unconformably on or in fault contact with torlesse flysch several periods of deformation are recognized and melange is present on both local and regional scales metamorphism ranges from zeolite to greenschist facies the bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages permian atomodesma middle triassic early late triassic torlessia late triassic monotis and late jurassic early cretaceous contacts between major fossil zones are mainly tectonic petrographic analysis permits subdivision of torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones the sandstone petrofacies mainly arkosic together with the composition of conglomerate clasts mainly indurated torlesse rocks indicate that the source terrane was a continental volcano plutonic arc probably part of gondwanaland coupled with autocannibalistic reworking of older uplifted torlesse rocks in contrast to the quartzofeldspathic nature of the torlesse coeval sedimentary rocks of the eastern province are volcanogenic they are thought to represent related forearc basin maitai murihiku terranes and trench complex caples terrane deposits derived from a volcanic island arc brook street terrane three petrofacies are established for maitai murihiku and caples sandstones the petrofacies indicate a common evolving immature to submature volcanic island arc source for these terranes a reconstruction of new zealand s eastern and western provinces is proposed in permian and triassic times the torlesse was deposited in trench slope or borderland basins along a trench transform margin fronting a continental volcano plutonic arc source western province gondwanaland deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of torlesse rock younging outward from the gondwanaland margin at the same time the brook street terrane volcanic arc and associated terranes were forming to the west of the torlesse site separated from gondwanaland by a marginal sea in latest triassic or early jurassic times the torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the gondwana margin the collision event resulted in tectonic thickening of torlesse and caples rocks at the plate interface and metamorphism to haast schist the source was then dominated by older partly metamorphosed torlesse terrane newly uplifted along the collision front closing of the marginal sea behind the brook street terrane in late jurassic early cretaceous times resulted in juxtapositioning with the western province gondwanaland along the median tectonic line

strain analysis using quartz deformation bands abstract quartz deformation bands are kink bands in quartz crystals a deformation band develops as a region of localized crystal plastic deformation with boundaries perpendicular to the slip plane and slip direction which usually is along an axis in the basal plane under cross polarized light the difference in crystallographic orientation between a deformation band and its host is indicated by a difference in extinction positions the displacement between the c axis in a deformation band and the c axis in the host represents the angular shear of the deformation band in the direction of the c axis in the host grain assuming the deformation is homogeneous at the grain scale the angular shear of the grain the gauge is calculated by multiplying the angular shear of the deformation band by the ratio of the sheared part to the whole grain using the strain gauge method for three dimensional infinitesimal strain analysis a minimum number of five grains measured on universal stage is needed to solve for the deviatoric strain components of the aggregate if the strain is homogeneous in the aggregate data from more than five grains are used to find the best fit strain components by a least squares method the principal strains and their orientations are found from these strain components by calculating the eigenvalues and eigenvectors a 3 d strain ellipsoid also is obtained from strain ellipses in three perpendicular planes determined from the two dimensional flat stage measurements by the wellman method both the strain gauge method and the wellman method are tested by using synthetic data sets and applied to a naturally deformed sample both methods give similar results the established wellman method thus confirms the strain gauge calculation

what determines the thickness of layers in a thermohaline staircase the article of record as published may be found at http dx doi org 10 1017 s0022112004002290

extension discrepancy at north atlantic nonvolcanic rifted margins depth dependent stretching or unrecognized faulting rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide angle data or subsidence this extension discrepancy has been interpreted in terms of depth dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere here i show that at those nonvolcanic margins where the velocity structure is well known the upper and lower crust appear to thin equally toward the margin implying that the discrepancy is not due to depth dependent stretching but more likely due to unrecognized polyphase and top basement faulting

observational and laboratory insights into salt finger convection improving observational capabilities in the ocean are revealing a substantial body of evidence that the double diffusive instability known as salt fingers is widely present and plays a significant role in oceanic mixing this evidence includes unique fine and microstructure features in particular regions and distinct patterns in the temperature and salinity variations over large areas laboratory experiments have also advanced our understanding of salt fingers the insights gleaned from both laboratory and ocean are reviewed and related to large scale ocean structure and climate modeling

tracking repeated subduction of the hikurangi plateau beneath new zealand abstract the hikurangi plateau has been subducted beneath new zealand twice firstly at ca 100 ma during n s convergence with gondwana and currently during e w convergence between the pacific and australian plates here we use a nationwide 3 d seismic velocity model derived from seismic tomography studies and seismicity relocated with this model to determine the location of the subducted western and southern edges of the plateau in the hikurangi subduction zone in the north island and northern south island we identify the western edge in the 37 140 km depth range based on the distribution of seismic velocities in the top of the subducted pacific plate changes in the thickness of the dipping seismic zone and steepening of the subducted plate downdip of the buoyant plateau in the central south island where the hikurangi plateau is subducting for a second time the plateau dips less steeply its southern edge is well defined by the collision of the delaminated lower crust of the plateau with the northern fiordland subduction zone where it bends the younger subducted australian plate to vertical the hikurangi plateau was originally rifted from the ontong java plateau and the subducted portions of the hikurangi plateau identified here fill a gap in reconstructions of this large igneous province

microfractures and their role in deformation of a quartz arenite from the central appalachian foreland abstract microfractures are pervasive in the massanutten sandstone a quartz arenite exposed in a doubly plunging synclinorium in the central appalachian foreland they are present as fluid inclusion planes fips and microveins parallel the fips fips occur in three orthogonal sets horizontal vertical and normal to the fold axis and vertical and parallel to the fold axis the geometry distribution and age of fips relative to other microstructures shows that microfractures developed during folding microfracturing followed layer parallel shortening by dislocation flow and was coeval with the peak of pressure solution that marked the final stages of folding microfracture densities are extremely high 160 mm 1 in many samples rapid closure of microfractures suppressed brecciation and prevented microfractures from propagating unstably to form joints microfracture opening accounts for significant finite strain with extensional strains of up to 11 1 and associated volume increases of over 16 0 the general lack of shortening microstructures indicates that a net volume increase is likely and that the massanutten sandstone may have served as a sink for dissolved phases from adjacent units undergoing volume loss deformation

mechanics of merging events for a series of layers in a stratified turbulent fluid this study attempts to explain the evolutionary pattern of a series of well mixed layers separated by thin high gradient interfaces frequently observed in stratified fluids such layered structures form as a result of the instability of the equilibrium with uniform stratification and their subsequent evolution is characterized by a sequence of merging events which systematically increase the average layer thickness the coarsening of layers can take one of two forms depending on the realized vertical buoyancy flux law layers merge either when the high gradient interfaces drift and collide or when some interfaces gradually erode without moving vertically the selection of a preferred pattern of coarsening is rationalized by the analytical theory the merging theorem which is based on linear stability analysis for a series of identical layers and strongly stratified interfaces the merging theorem suggests that the merger by erosion of weak interfaces occurs when the vertical buoyancy flux decreases with the buoyancy variation across the step if the buoyancy flux increases with step height then coarsening of a staircase may result from drift and collision of the adjacent interfaces our model also quantifies the time scale of merging events and makes it possible to predict whether the layer merging continues indefinitely or whether the coarsening is ultimately arrested the merging theorem is applied to extant one dimensional models of turbulent mixing and successfully tested against the corresponding fully nonlinear numerical simulations it is hypothesized that the upscale cascade of buoyancy variance associated with merging events may be one of the significant sources of the fine scale 10 m variability in the ocean

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

origin of thermohaline staircases hypotheses concerning the origin of thermohaline staircases in salt fingering regions are reviewed and assessed one such hypothesis that staircases arise from thermohaline intrusions is developed into a quantitative theory it is shown that growing intrusions evolve toward staircases when the background density ratio lies below a threshold value and nonlinear computations confirm that staircases are viable intrusion equilibria staircase properties such as step heights lateral density ratios and layer slopes lie closest to observed values when salt fingers are assumed not to contribute to shear stress and when turbulent mixing rates are smaller than usual thermocline values

the formation and growth of pressure solution seams in clastic rocks a field and analytical study abstract pressure solution seams psss are closing mode structures of localised dissolution resulting in a volume reduction we present new observations constraining the processes by which psss initiate and grow in low porosity clastic rocks from southern ireland psss initiate as inter granular psss at quartz grain to grain contacts as quartz dissolves clay remains as a residue along the contacts as well as filling the adjacent pores to form incipient psss involving multiple grain boundaries further lengthening and thickening occurs by lateral linkage and transverse coalescence of neighbouring segments of incipient psss multiple pss segments are observed to concentrate in thin tabular zones that appear as single macroscopic psss visible in hand samples published numerical modelling results show the ability of both lateral and transverse growth of psss due to the presence of asperities on their flanks which confirm our interpretations based on our laboratory and field observations

the problem of strain marker centers and the fry method abstract the fry method and its variants will underestimate finite strain when applied to strain markers deformed heterogeneously at marker scale despite homogeneous behavior at a population scale this difference results from the post deformation centers not coinciding with the original pre deformation centers the amount of error may rapidly approach 50 for small strains in ideal cases but is likely to be in the range of 20 30

crustal structure of the ne rockall trough from wide angle seismic data modeling two wide angle seismic lines located in the northern rockall trough were acquired in may 2000 one line line e crosses the trough from the continental shelf off lewis to normal oceanic crust west of lousy bank in nw se direction the other line line d intersects with line e crosses the wyville thomson ridge in a sw ne direction and ends in the faeroe shetland basin sonobuoy data and expanding spread profiles acquired in the same area have been remodeled analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1 5 km thick basaltic layer which is present in most of the trough further conclusions of this study are that the rockall trough is underlain by highly stretched continental crust of 13 km thickness the crust thickens to 24 km beneath lousy bank which is interpreted to be of continental nature beneath the hebrides continental shelf a three layer continental crust of 26 km is modeled an up to 12 km thick high velocity layer is observed underneath the ocean continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting no evidence for an underplate layer could be distinguished beneath the trough area modeling of the structure of the wyville thomson ridge revealed no existing igneous core of the ridge confirming existing theories that it is a compressional structure

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

deep structure of the armorican basin bay of biscay a review of norgasis seismic reflection and refraction data the bay of biscay is bounded to the north by the north biscay margin which comprises the western approaches and armorican segments in the 1970s and 1980s most researchers considered this margin typical of a non volcanic passive margin it is characterized by a striking succession of tilted blocks beneath which occurs the s reflector and the continent ocean boundary is abrupt this paper examines the armorican segment and is based on a study of all early seismic profiles together with new multichannel reflection and refraction seismic data norgasis cruise an important result is the discovery of a 80 km wide ocean continent transition zone that coincides with the armorican basin a deep sedimentary basin it is characterized by a high velocity lower crustal layer 7 4 7 5 km s 1 overlain by sediments the other results are 1 the main crustal thinning occurs exclusively under the narrow continental slope 2 the tilted blocks and the s reflector are observed only at the base of the continental slope in the narrow domain called the neck area 3 the north biscay ridge is a large oceanic plateau present only off the nw armorican margin rather than a long ridge elongated off the whole north biscay margin

crustal structure and evolution of the southern voring basin and voring transform margin ne atlantic abstract the dominantly passive volcanic voring and more margins ne atlantic are separated by the 200 km long voring transform margin vtm the southern voring basin and the vtm have been studied by use of four regional ocean bottom seismograph obs profiles combined by gravity modelling the models demonstrate a complex pattern of magmatism along the transform margin the distribution of magmatism seems to be related to the existence and trend of a lower crustal 8 km s body interpreted as eclogitized rocks present in the southern voring basin early tertiary breakup related magmatic leakage across the continent ocean transition cot appears to be facilitated where this layer is absent these results support earlier workers who have concluded that the jan mayen fracture zone originated from a caledonian zone of weakness we propose that partly eclogitized rocks were uplifted into the lower crust close to this zone during the caledonian orogeny and that this body acted as a barrier to magma emplacement during the late cretaceous early eocene phase of rifting breakup the eclogitized terrain also appears to have caused northeastward channeling of the late cretaceous early tertiary intrusions within the voring basin an up to 10 km thick pre cretaceous sedimentary basin in the southern voring basin may be genetically related to the ns trending late paleozoic and mesozoic rift basins in north east greenland

evidence from zircon u pb age spectra for crustal structure and felsic magma genesis at taupo volcano new zealand u pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at taupo volcano as well as unexpected crustal structure beneath the taupo volcanic zone ages from the xenolith in a 28 ka rhyolite dome match those from early cretaceous pahau terrane graywackes that crop out 75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process boundaries between graywacke terranes in the north island are thus more complex than implied from map views in contrast ages from zircons in the 20 ka omega dacite match the nearby surficial graywacke of the jurassic kaweka terrane 55 of grains quaternary plutonic sources 39 and pahau terrane graywacke 6 five of 72 grains analyzed contain graywacke cores and pleistocene rims survival of euhedral xenocrystic zircons through the generation processes for omega dacite magma reflects wholesale melting of crustal protoliths including varieties not previously considered for petrogenetic modeling open system behavior and crustal melts being generated and erupted within 1 10 yr

crustal structure of the central part of the voring basin mid norway margin from ocean bottom seismographs abstract regional ocean bottom seismograph obs data acquired in the central and northern part of the voring basin mid norway margin have been modelled by use of two dimensional 2 d ray tracing the regional dataset comprises thirty obss deployed along seven 100 170 km long profiles the deeper part of the voring basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels 2 5 km below sea floor but the modelling of the obs data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea floor to the upper mantle the shallow and intermediate sediments to 5 km below sea floor are characterized by a vertical increase in velocity due to increased confining pressure there is also considerable lateral variation in the velocities within sedimentary layers at all levels the obs data confirm that intrusions of sills are important at intermediate and deep sedimentary levels 2 10 km below sea floor in most parts of the area the sills seem to vary in lateral extent from about 20 km to more than 100 km and their thickness is generally inferred to be about 200 m the low velocity in the upper crystalline crust 6 2 km s confirms that the crust in the voring basin is of continental origin in most parts of the area the velocity of the lower crust is as high as 7 3 7 6 km s this high velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness the base of the 7 3 km s layer is interpreted as the moho and the upper mantle velocity is estimated to 8 2 km s

archean uplift of a subprovince boundary in the canadian shield revealed by magnetic fabrics abstract a high strain zone reported elsewhere as a subprovince or tectonic terrane boundary shows evidence of uplift during a penetrative d 1 event some outcrops reveal this by conventional l s petrofabric observations however all outcrops permit measurements of the anisotropy of low field magnetic susceptibility ams which here records the preferred crystallographic orientation of chlorite biotite and amphibole furthermore anisotropy of anhysteretic magnetic remanence aarm reflects the preferred orientation of ferromagnetic grains magnetite and pyrrhotite all three types of fabric show that the rocks were shortened in a north south direction perpendicular to the terrane boundary and stretched upwards in a nearly coaxial strain history a few anomalous magnetic fabrics may indicate some late d 1 subhorizontal e w extension compatible with transpression along the boundary we conclude that this boundary is a zone of high strain associated with the uplift of the high grade plutonic rocks of the winnipeg river subprovince with only slight evidence of minor components of late transpression during the penetrative d 1 fabrics

strain variations and three dimensional strain factorization at the transition from the southern to the central appalachians abstract the transition between the southern and the central appalachian foreland thrust system is a region subjected to at least two non coaxial deformations where typical two dimensional analyses such as cross section balancing or plane strain retro deformation cannot be applied three dimensional factorization can simulate the non coaxial strain history simulation results for compaction and layer parallel shortening lps were compared to finite strains of 16 quartz arenite samples from the tuscarora sandstone successful simulations indicate in the southern appalachians 30 35 total compaction by volume loss including a 4 5 9 e w horizontal shortening during lithification and 5 lps along 150 330 by plane strain or axial symmetric flattening in the central appalachians 30 35 total compaction by volume loss including a 3 7 n s horizontal shortening during lithification and 10 lps along 120 300 by plane strain or axial symmetric flattening in the transition zone 30 compaction by volume loss including a 3 e w horizontal shortening during lithification 5 southern lps along 150 330 by axial symmetric flattening and 5 10 central shortening along 130 or 140 by plane strain or axial symmetric flattening at 10 to the normal central trend from the simulations horizontal shortening in the southern appalachians during lithification in the mississippian was greater than the later alleghanian deformation this shortening probably represents a very early lps developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation also the oblique and weaker central tectonic lps in the transition zone probably represents behavior at the limits of central appalachian deformation finally the lps simulations indicate that axial symmetric flattening oblate strain is more successful than plane strain as the representative behavior so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands

quantitative assessment of low temperature deformation mechanisms in a folded quartz arenite valley and ridge province west virginia abstract analysis of the grain scale deformation mechanisms in folded tuscarora sandstone from the western margin of the appalachian foreland near keyser wv reveals that deformation was accomplished by a combination of pressure solution microfracturing and crystal plastic mechanisms the finite strain was assessed using the normalized fry method and showed a predominant apparent flattening strain regime resulting from diagenetic compaction partitioning of the strain between operative deformation mechanisms revealed that layer normal shortening 25 and layer parallel shortening 10 by pressure solution were the greatest contributors to the finite strain followed by microfracturing and crystal plastic strains respectively although the crystal plastic contribution to the finite strain is small 1 0 layer normal shortening strain partitioning allowed detection of layer parallel extension consistent with the structural position of the samples on the outer arc of a fold the variation in inferred stress directions throughout the progressive deformation indicates that local stratigraphic and structural control on the stress field is important in the development of certain microstructures only the earliest stages of deformation record far field alleghanian compression whereas later stages reflect local stress fields controlled by bedding orientation and position on developing folds similarities in the microstructure evolution between the western and eastern margins of the appalachian foreland indicate that the entire belt underwent a similar deformation history

enhanced diapycnal mixing by salt fingers in the thermocline of the tropical atlantic diapycnal mixing plays a significant role in the ocean s circulation and uptake of heat and carbon dioxide but has not been quantified in salt finger driven thermohaline staircases we recently performed a tracer release experiment in the western tropical atlantic staircase at approximately 400 m depth the observed dispersion implies an effective diapycnal diffusivity for tracer and salt of 0 8 to 0 9 x 10 4 m2 s temperature microstructure data interpreted in terms of a vertical production dissipation balance yields a smaller effective diffusivity for heat of 0 45 0 2 x 10 4 m2 s consistent with salt fingers and well above the mixing ascribable to mechanical turbulence

the mid cretaceous transition from basement to cover within sedimentary rocks in eastern new zealand evidence from detrital zircon age patterns detrital zircon u pb ages for 30 late jurassic and cretaceous sandstones from the eastern province of eastern new zealand combined with previously published geochronological and palaeontological data constrain the time of deposition in the pahau and waioeka terranes of the cretaceous accretionary margin of zealandia and their adjacent cover strata the zircon age patterns also constrain possible sediment source areas and mid cretaceous geodynamic models of the transition from basement accretionary wedge to passive margin cover successions pahau terrane deposition was mainly barremian to aptian but continued locally through to late albian time with major source areas in the adjacent kaweka and waipapa terranes and minor inputs from the inboard median batholith waioeka terrane deposition was mainly albian with distinctive and exclusive sediment sources principally from the median batholith but with minor inputs from the western province alternative tectonic models to deliver such exclusive median batholith and western province derived sediment to the mid cretaceous zealandia continental margin are 1 the creation of a rift depression across zealandia or 2 sinistral displacement of south zealandia with respect to north zealandia to expose western province rocks directly at the zealandia margin detrital zircon age patterns of cretaceous cover successions of the eastern province of eastern new zealand demonstrate purely local sources in the adjacent kaweka and waipapa terranes cretaceous zircon components show a decline in successions of late early cretaceous age and disappear by late late cretaceous time suggesting the abandonment or loss of access to both the median batholith and western province as sediment sources

experimental models of hangingwall deformation in ramp flat listric extensional fault systems abstract the results of three experimental models of ramp flat extensional fault systems are presented the experiments were constructed from sand and mica layers to form an anisotropic sequence that would respond to the imposed deformation by both faulting and folding the geometry of the underlying detachment controls the deformation in the hangingwall sequence ramp flat detachment geometries produce an upper roll over with a crestal collapse graben system a ramp syncline and fold reverse fault zone and a lower roll over with a crestal collapse graben system short ramp geometries produce only minor perturbations in a larger roll over structure as extension progresses the crestal collapse systems migrate and superimpose earlier systems the models of ramp flat extensional structures produce templates for the interpretation of natural ramp flat listric fault systems

continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins abstract depth uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup it cannot explain the observed depth dependent lithosphere stretching and mantle exhumation at rifted continental margins depth dependent lithosphere thinning in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust has been observed at many non volcanic and volcanic rifted continental margins including conjugate margin pairs passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea floor spreading initiation a new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere and that this deformation evolves into sea floor spreading the new model successfully predicts depth dependent stretching of continental margin lithosphere for both non volcanic and volcanic margins and mantle exhumation at non volcanic margins both of which are observed but are not explained by existing depth uniform continental lithosphere stretching models the new model provides a balance of extensional strain supplies an explanation for the paucity of synrift brittle deformation and offers a simple transition from prebreakup lithosphere thinning to sea floor spreading the observed diversity of rifted continental margin structure and width of the ocean continent transition can be explained by variability in the form of the upwelling divergent flow field the new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea floor spreading initiation in the woodlark basin in the western pacific and the observed bathymetry and free air gravity anomaly for the newfoundland and iberian margins

ice tethered profiler observations of the double diffusive staircase in the canada basin thermocline vertical heat fluxes through the staircase are in the range 0 05 0 3 w m 2 only about one tenth of the estimated mean surface mixed layer heat flux to the sea ice it is thus concluded that the vertical transport of heat from the atlantic water in the central basin is unlikely to have a significant impact to the canada basin ocean surface heat budget icebreaker conductivity temperature depth data from the beaufort gyre freshwater experiment show that the staircase is absent at the basin periphery turbulent mixing that presumably disrupts the staircase might drive greater flux from the atlantic water at the basin boundaries and possibly dominate the regionally averaged heat flux

crustal structure of the northern part of the voring basin mid norway margin from wide angle seismic and gravity data abstract three regional ocean bottom seismograph obs profiles with a total of fifteen recovered obss were acquired in the northern part of the voring basin mid norway margin in 1996 the data have been modelled by inversion and forward modelling of the obs vertical components p waves the velocity is found to increase downwards within the sedimentary layers due to increasing depth of burial confining pressure within the two deepest sedimentary layers there seems to be an increase of 0 5 1 km s in velocity northwestwards this increase is most likely caused by high velocity sill intrusions in the sedimentary rocks emplaced during the rift episode leading to early eocene opening of the ne atlantic the presence of significant amounts of sills seems to terminate close to the nyk high along profile 5 dip profile and close to a dome northwest of the shelf edge along profile 6 dip profile the velocity in the top of this dome is anomalously high 3 4 km s at only 1 km depth beneath the seafloor suggesting that the high consists of volcanic rocks or heavily intruded sedimentary rocks the relatively low velocities derived from within the upper crystalline crust 5 9 6 2 km s confirms that the crust is of continental origin within the lower crust there seems to be a clear increase in velocity northwestwards from about 7 0 km s to about 7 4 km s suggesting that the amount of high velocity intrusions underplating in the lower crust decreases landwards the modelling of the strike profile profile 7 suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile the model for profile 7 ties closely southwestwards to the models derived from obs data acquired in 1992 and northeastwards to lofoten obs data acquired in 1988 the models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey and that the transition zone is related to the surt lineament the distribution of magmatic rocks thus seems to correlate strongly with pre breakup structures in addition to the p wave modelling gravity modelling has been performed along all profiles the gravity modelling confirmed all main aspects of the obs models and provided constraints on the crustal structures towards the ends of the profiles

dynamics of fingering convection part 1 small scale fluxes and large scale instabilities received and in revised form double diffusive instabilities are often invoked to explain enhanced transport in stablystratified fluids the most studied natural manifestation of this process fingering convection commonly occurs in the ocean s thermocline and typically increases diapycnal mixing by two orders of magnitude over molecular diffusion fingering convection is also often associated with structures on much larger scales such as thermohaline intrusions gravity waves and thermohaline staircases in this paper we present an exhaustive study of the phenomenon from small to large scales we perform the first three dimensional simulations of the process at realistic values of the heat and salt diffusivities and provide accurate estimates of the induced turbulent transport our results are consistent with oceanic field measurements of diapycnal mixing in fingering regions we then develop a generalized mean field theory to study the stability of fingering systems to large scale perturbations using our calculated turbulent fluxes to parameterize small scale transport the theory recovers the intrusive instability the collective instability and the instability as limiting cases we find that the fastest growing large scale mode depends sensitively on the ratio of the background gradients of temperature and salinity the density ratio while only intrusive modes exist at high density ratios the collective and instabilities dominate the system at the low density ratios where staircases are typically observed we conclude by discussing our findings in the context of staircase formation theory

thermochronology of the torlesse accretionary complex wellington region new zealand the torlesse complex comprises several mesozoic accretionary prism complexes together forming continental basement over large parts of new zealand this study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern north island that crosses the structural grain of the torlesse complex including its older and younger parts zircon fission track ft ages for the late triassic rakaia terrane which is the most inboard of the accretionary complexes are partially annealed some possibly reset and may indicate early cretaceous 134 10 ma cooling from maximum temperatures tmax probably related to imbrication of younger complexes of the pahau terrane numerical modeling of the zircon ft ages and published 40ara 39ar muscovite and biotite ages for the rakaia terrane suggest tmax values of 265 310 c and exhumation from depths of 10 12 km the rocks underlying the aorangi range and involving the youngest accretionary complex have experienced much lower tmax values of 210 and 110 c bracketed by reset apatite ft ages and detrital zircon ft ages the occurrence of a circa 100 ma component of zircon ft ages in both the weakly and highly indurated rocks beneath the aorangi range as well as in remnants of an overlying albian accretionary slope basin whatarangi formation imply multistorey accretion and incorporation of sediment into the youngest prism this circa 100 ma zircon ft age component also places a maximum age on the termination of mesozoic subduction beneath the new zealand region the occurrence of reset apatite ft ages across the whole of the wellington transect indicates that at least 4 km of exhumation occurred during the late miocene

heat salt and momentum transport in a laboratory thermohaline staircase flow characteristics and fluxes in thermohaline staircases are measured in two tanks differing in aspect ratio a where a is the ratio of tank width to fluid depth in one tank the 1 x 1 tank which is 30 cm deep and 30 cm wide a staircase of one salt finger layer and one convecting layer develops for a certain setting of the control parameters the convecting layer has a 2 shadowgraphs show convecting plumes that appear disorganized and a large scale flow never develops instead the finger layer grows in height overtakes the convecting layer and within a few days becomes one finger layer the second tank the 1 x 5 tank is also 30 cm deep but is 150 cm wide for the same control parameter setting a similar staircase with a finger layer 20 cm deep and a convecting layer 10 cm deep develop the convecting layer with a 15 has quite a different character a large scale flow develops so that the convecting layer has one cell 10 cm deep and 150 cm wide in this flow are large plumes which are transient and tilted particle image velocimetry measurements of reynolds stresses show they help to maintain the large scale flow against viscous dissipation shadowgraphs show all the finger tips swept in the direction of the large scale flow adjacent to the finger layer measurements show that the large scale flow collects the salt delivered by the many fingers so that the accumulated negative buoyancy leads to deep convection this is a more stable arrangement with the configuration lasting to the order of 10 2 days

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

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

mooring based observations of double diffusive staircases over the laptev sea slope abstracta yearlong time series from mooring based high resolution profiles of water temperature and salinity from the laptev sea slope 2003 04 2686 m depth 78 26 n 125 37 e shows six remarkably persistent staircase layers in the depth range of 140 350 m encompassing the upper atlantic water aw and lower halocline despite frequent displacement of isopycnal surfaces by internal waves and eddies and two strong aw warming pulses that passed through the mooring location in february and late august 2004 the layers preserved their properties using laboratory derived flux laws for diffusive convection the authors estimate the time averaged diapycnal heat fluxes across the four shallower layers overlying the aw core to be 8 w m 2 temporal variability of these fluxes is strong with standard deviations of 3 7 w m 2 these fluxes provide a means for effective transfer of aw heat upward over more than a 100 m depth range toward the upper halocline these findings suggest that double diffusion is an imp

structure of the more basin mid norway continental margin abstract in the more basin offshore mid norway the basis of the cretaceous sequence reaches depths of more than 5000 m the basin is limited on its eastern margin by a system of extensional faults which probably were initiated in the triassic and which were active periodically throughout the aptian the internal basin floor is divided into subbasins by structural highs which are rotated fault blocks presumably rooted in the basement the main structuring of the basin started in late triassic and was followed by accelerating subsidence in the early cretaceous subsidence was interrupted by a mild phase of inversion in the latest cretaceous and a more pronounced inversional phase in the eocene miocene the latter developed folds with wavelengths in the order of kilometers and local low angle reverse faults and the inversion of depocenters

40ar 39ar thermochronologic constraints on deformation metamorphism and cooling exhumation of a mesozoic accretionary wedge otago schist new zealand abstract structural thickening of the torlesse accretionary wedge via juxtaposition of arc derived greywackes caples terrane and quartzo feldspathic greywackes torlesse terrane at 120 ma formed a belt of schist otago schist with distinct mica fabrics defining i schistosity ii transposition layering and iii crenulation cleavage thirty five 40ar 39ar step heating experiments on these micas and whole rock micaceous fabrics from the otago schist have shown that the main metamorphism and deformation occurred between 160 and 140 ma recorded in the low grade flanks through 120 ma shear zone deformation this was followed either by very gradual cooling or no cooling until about 110 ma with some form of extensional tectonic exhumation and cooling of the high grade metamorphic core between 109 and 100 ma major shear zones separating the low grade and high grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo tectonic histories apparent ages on the low grade north flank hanging wall to the hyde macraes and rise and shine shear zones range from 145 to 159 ma n 8 whereas on the low grade south flank hanging wall to the remarkables shear zone or caples terrane range from 144 to 156 ma n 5 most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica several of the hanging wall samples with ages of 144 147 ma show no evidence for detrital contamination in thin section or in the form of the age spectra apparent ages from the high grade metamorphic core garnet biotite albite zone range from 131 to 106 ma n 13 with a strong grouping 113 109 ma n 7 in the immediate footwall to the major remarkables shear zone most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica 430 330 c samples with apparent ages of about 110 109 ma tend to give concordant plateaux suggesting more rapid cooling the youngest and most disturbed age spectra come from within the alpine chlorite overprint zone where samples with strong development of crenulation cleavage gave ages 85 107 and 101 ma due to partial resetting during retrogression the bounding remarkables shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127 122 ma whereas overprinting shear zones within the core of the schist show apparent ages of 112 109 and 106 ma these data when linked with extensional exhumation of high grade rocks in other parts of new zealand indicate that the east gondwana margin underwent significant extension in the 110 90 ma period

styles of extension offshore mid norway and implications for mechanisms of crustal thinning at passive margins 1 eocene magmatic breakup along the mid norway rifted margin was preceded by extreme jurassic cretaceous crustal thinning in a magma poor environment along the se borders of the rift top basement detachment faults with heaves on the order of 15 40 km evolved in at least two stages to become the boundaries between moderately thinned 20 30 km thick crust and a 100 200 km wide highly extended area with crustal thicknesses generally between 2 and 12 km under the present day more and voring basins in the footwalls of the basin flank detachments lower and middle crust was exhumed in extensional domes that became incised by a younger set of normal faults under the most highly thinned areas a more distal set of deep seated basin floor detachments incised and extended remnant crust and probably the upper mantle leaving as little as 5 km of continental crust to be preserved under thick synrift and postrift deposits we suggest that basin flank detachments such as the ones described above hold the potential to reduce the crustal thickness down to the critical value required for embrittlement of continental crust thus they prepare the ground for incision by the basin floor detachments which may become responsible for exhumation of deep crust or continental mantle if extension is allowed to proceed

magnetic susceptibility of mesozoic and cenozoic sediments off mid norway and the role of siderite implications for interpretation of high resolution aeromagnetic anomalies abstract the study was designed to examine lithological and stratigraphic variations in magnetic susceptibility of the mesozoic and parts of the cenozoic sedimentary succession in petroleum exploration areas offshore norway and investigate the source of the susceptibility the study includes measurements of more than 6 km core from exploration wells and shallow drill sites with main focus on the northern north sea norwegian sea area considerable variation in susceptibility is detected and includes 1 average higher susceptibility in the finer grained lithologies and lowest susceptibility in mineralogically mature sandstones 2 relatively high susceptibility in upper jurassic cretaceous shaly sequences 3 provenance controls on susceptibility in triassic jurassic coarse clastic sediments 4 in all exploration wells positive correlation of high susceptibility to diagenetic siderite cementation the siderite cemented beds and nodules are of particular importance in controlling the susceptibility distribution in the dark shale units susceptibility log patterns from different sediment environments are also exemplified and can tentatively be divided into four types of patterns reflecting combinations of dispersed and concentrated mineral sources of susceptibility magnetic laboratory measurements verify presence of both paramagnetic and ferrimagnetic components in samples with high susceptibility values and with a dominance of the induced magnetisation

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

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

the diffusive regime of double diffusive convection the diffusive regime of double diffusive convection is discussed with a particular focus on unresolved issues that are holding up the development of large scale parameterizations some of these issues such as interfacial transports and layer interface interactions may be studied in isolation laboratory work should help with these however we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory these lie beneath some fundamental questions about how double diffusive structures are formed in the ocean and how they evolve in the competitive ocean environment

thermohaline staircases in a caribbean eddy and mechanisms for staircase formation the structure of a cyclonic eddy in the eastern caribbean was observed by caribbean vorticity experiment cruise cavortex i in august 2003 prominent thermohaline staircases are evident in profiles of temperature and salinity at locations that appear to correlate with the presence of lateral property gradients and shear within the eddy although such a connection is less than definitive due to the coarseness of horizontal sampling these observations are examined in light of predictions from two theories for staircase formation aspects of the data are found to be consistent with each raising the possibility that both mechanisms are operating

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

petrofabric test of viscous folding theory abstract compression and extension axes are deduced from quartz deformation lamellae in a quartzite and a graywacke folded into an asymetrical syncline deformation lamellae fabrics in the two sandstones are distinctly different in the graywacke regardless of bedding orientation or position on the fold compression axes are normal or nearly normal to the axial planar rough cleavage extension axes generally lie in the cleavage plane parallel to dip in most quartzite samples compression axes are parallel or subparallel to bedding at high angles to the fold axis and extension axes are normal to bedding two samples from the very base of the formation indicate compression parallel to the fold axis with extension parallel to bedding at high angles to the fold axis one of these two shows both patterns the lamellae fabric geometry in these two samples suggests the presence of a neutral surface in the quartzite the lamellae derived compression and extension axes are in good agreement with the buckling behavior of a viscous layer quartzite embedded in a less viscous medium graywacke and shale below and shale and carbonate above

double diffusive recipes part ii layer merging events this study explores the dynamics of thermohaline staircases well defined stepped structures in temperature andsalinityprofiles commonlyobservedin regionsofactive doublediffusion theevolutionofstaircases in time is frequently characterized by spontaneous layer merging events these phenomena the authors argue are essentialin regulating theequilibriumlayerthicknessin fullydevelopedstaircases thepatternand mechanics of merging events are explained using a combination of analytical considerations direct numerical simulations and data analysis the theoretical merger model is based on the stability analysis for a series of identical steps and pertains to both forms of double diffusion diffusive convection and salt fingering the conceptual significance of the proposed model lies in its ability to describe merging events without assuming fromtheoutsetspecificpowerlawsfortheverticaltransportofheatandsalt theapproachadoptedbyearlier merging models the analysis of direct numerical simulations indicates that merging models based on the four thirds flux laws offer adequate qualitative description of the evolutionary patterns but are less accurate than models that do not rely on such laws specific examples considered in this paper include the evolution of layers in the diffusive staircase in the beaufort gyre of the arctic ocean

quantifying internal strain and deformation temperature in the eastern himalaya bhutan implications for the evolution of strain in thrust sheets abstract quartz microstructural analysis together with quantification of 3 d crystal plastic strain of quartz clasts normalized fry and rf methods constrain deformation temperature ranges and internal strain magnitude orientation within himalayan thrust sheets in bhutan lesser himalayan lh thrust sheets display an inverted deformation temperature gradient from 250 to 310 c at the main boundary thrust to 500 670 c at the main central thrust mct attributed primarily to stacking of lh thrust sheets deformed at progressively higher temperatures toward the hinterland the hot iron effect of the mct hanging wall only affects the upper few hundred meters of hinterland lh thrust sheets frontal thrust sheets exhibit 7 layer parallel shortening lps strain all other thrust sheets exhibit layer normal flattening lnf strain with 1 9 1 8 1 0 lh rocks and 2 1 1 8 1 0 greater and tethyan himalayan rocks median ellipsoids x parallel to lineation we propose that lps strain developed foreland ward of the thrust deformation front and that lnf strain resulted from later tectonic loading the lps to lnf transition occurs at minimum temperatures for quartz plasticity ca 250 270 c lnf ellipsoids are folded along with strata within thrust sheets indicating that internal strain preceded thrust imbrication and translation at the scale of 10 s of meters structural distance strain magnitude of hanging wall rocks does not increase near thrusts which supports the existence of discrete faults with large translations as generally depicted in balanced cross sections strain magnitudes in low grade greater and tethyan himalayan rocks in central bhutan indicate that the top to the north component of channel flow extrusion is most likely less than 19 km

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

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

determination of pressure solution shortening in sandstones abstract a new method for the determination of pressure solution shortening in sandstones uses the geometry of grain to grain interpenetrations and grains truncated against solution surfaces these features are used to construct plots from which the magnitude and direction of the pressure solution shortening can be determined using simulated pressure solution deformation of artificial and natural grain populations the new method is shown to correctly assess a variety of coaxial and non coaxial shortenings although primarily intended to determine shortening the method can also quantify extension related to growth of beards or overgrowths during pressure solution application of the method to naturally deformed quartz arenite samples shows that pressure solution shortening of up to 26 occurred during compaction and 22 during layer parallel shortening

provenance analysis using conglomerate clast lithologies a case study from the pahau terrane of new zealand abstract the field of provenance analysis has undergone a revolution with the development of single crystal isotope dating techniques using mainly silt to sand sized single minerals this study focuses on coarser grained rocks notably conglomerates which involve much shorter transport distances and therefore may be used to trace proximal sources the torlesse terranes part of the new zealand eastern province are accretionary complexes that comprise an enormous volume of quartzofeldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages two terranes are recognised in the south island the permian to late triassic rakaia terrane and the late jurassic to early cretaceous pahau terrane geochronology geochemistry and sr nd isotopes of igneous clasts from pahau terrane conglomerates identify the median tectonic zone s l as a major contributor of detritus to the pahau depositional basin shrimp u pb zircon ages of igneous clasts range from 128 to 123 ma and from 147 to 135 ma calc alkaline clasts of the latter range are indistinguishable in age chemical composition and petrogenesis from the calc alkaline granitoids of the darran suite whereas alkaline clasts correlate with the electric granite both parts of the median tectonic zone an early jurassic calc alkaline rhyolite clast from kekerengu 188 3 ma correlates with the bounty island granite campbell plateau the petrography and geochemistry of pahau terrane sandstone clasts indicate the recycling of permian to early late triassic older rakaia terrane rocks this is further supported by the detrital zircon age data igneous conglomerate clasts are capable of placing exceptionally tight constraints on pahau provenance and on its mesozoic tectonic setting within the southwest pacific margin of gondwana they provide the best evidence yet that the pahau terrane is locally derived

cretaceous termination of subduction at the zealandia margin of gondwana the view from the paleo trench abstract during the permian to cretaceous zealandia occupied a position on the proto pacific facing convergent margin of gondwana subduction on this margin ceased somewhere between 105 ma and perhaps 70 ma but the timing of this tectonic transition remains controversial resolution of this uncertainty is important for tectonic reconstructions of the southwest pacific and for global plate tectonic models here we revisit the problem by reference to new stratigraphic and geochemical data from the east coast basin of new zealand which occupied a position adjacent to and possibly superimposed on the relict cretaceous subduction trench at the time subduction ceased this basin is expected to preserve direct structural and stratigraphic evidence for or against late cretaceous subduction in western parts of the east coast basin the western sub belt a conspicuous unconformity separates undoubted accretionary prism rocks of the torlesse composite terrane from younger cretaceous cover units strata beneath and overlying this unconformity vary in age from place to place but abundant paleontological data and published ages of detrital zircons some reinterpreted herein indicate that exposed torlesse rocks are nowhere younger than 100 ma overlying zealandia megasequence cover strata are mostly younger than 110 ma between 110 and 85 ma the entire length of the western sub belt reveals complex stratigraphic architecture of relatively small scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion there is widespread evidence for compression on the western sub belt over this period although the overall amount of shortening is likely to be relatively modest in contrast the eastern sub belt preserves a record of near continuous and apparently simple deposition over the same interval of time we see little and somewhat equivocal evidence for extension in either sub belt through the late cretaceous superimposed on this general pattern we observe discrete widespread or basin wide tectonic events at 95 ma and within the intervals 86 83 ma and 83 81 ma indicated by the presence of unconformities and sedimentary facies changes importantly all these events can be correlated with coeval events recorded elsewhere in zealandia suggesting that the east coast basin was subject to the same tectonic regime as the rest of zealandia and shared a common history during the mid to late cretaceous integrating these findings with data from elsewhere in zealandia we argue that there is diverse evidence to indicate that subduction finished along the new zealand segment of the gondwana margin by 100 ma that said the causes of on going late cretaceous compression in the east coast basin and in other parts of zealandia are not well constrained although several possible explanations are plausible there are few analogue abandoned subduction margins described in the literature however the situation in the south shetland trench shows strong similarities with patterns and complexities observed in the east coast basin

strain partitioning and factorization in a quartz arenite abstract most rocks deform by multiple grain scale deformation mechanisms in order to assess completely the contributions of each mechanism the strain associated with that mechanism must be known and compared to the finite strain this study describes the partitioning of strain between three mechanisms pressure solution dislocation creep and microfracturing in a quartz arenite deformed at low temperatures pressure solution which occurred primarily during compaction dominates the finite strain with an average of 24 shortening normal to bedding dislocation creep which occurred during layer parallel shortening accounts for about 2 shortening parallel to bedding dip microfractures which occur in three orthogonal sets resulted in 2 4 extension normal to bedding parallel to bedding strike and parallel to bedding dip the validity of the mechanism strains was tested using strain factorization models were constructed using the mechanism strains in an order determined by their relative ages because porous well sorted quartz arenites such as the one studied are likely to undergo some mechanical compaction in the early stages of diagenesis variable amounts of mechanical compaction were included in factorization models models using the measured deformation mechanism strains with an initial 5 10 mechanical compaction yield finite strains in close agreement with the measured finite strains this suggests that the mechanisms identified the strain associated with each mechanism and the deformation sequence are plausible

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

comparison of fry strain ellipse and ams ellipsoid trends to tectonic fabric trends in very low strain sandstone of the appalachian fold thrust belt abstract in moderately to highly strained sandstones both the long axis of the bedding parallel finite strain ellipse as calculated by the normalized fry method and the projection of the long axis of the ams ellipsoid on the plane of bedding align well with local structural grain trends of cleavage folds and faults this relationship implies that results of both 2d fry and ams analyses represent the local layer parallel tectonic strain component do both methods provide comparable results for very low strain sandstone e g

assessing brittle volume gain and pressure solution volume loss processes in quartz arenite abstract quartz arenite of the tuscarora sandstone has been deformed by dislocation flow pressure solution and microfracturing of which the last two are most important pressure solution involved shortening with no extension most samples have approximately 20 shortening normal to bedding interpreted to be compaction and a few have up to 20 layer parallel shortening of tectonic origin because there is no extension these shortenings resulted in an equivalent amount of bulk volume loss multiple sets of microfractures in the form of fluid inclusion planes and microveins account for up to 10 extension and bulk volume increases up to 17 when both microfracture volume gains and pressure solution volume losses are considered together all samples show a net bulk volume loss ranging from 14 to 35 changes in material volume are different than bulk volume because of the presence of porosity at the time of pressure solution whereas all samples show bulk volume losses most samples show net material volume gains of up to 16 with only those having experienced both compactional and tectonic pressure solution showing material volume losses pressure solution during compaction can account for most but not all cement compactional pressure solution surfaces do not extend into pore filling cement suggesting that much of the cement was externally derived after compaction therefore the flux of material may have been considerably greater than the material volume changes would indicate

extensional geometry of the mid norwegian margin before early tertiary continental breakup abstract interpretation of reflection seismic data integrated with existing crustal models provide an interpretation of the deeply buried mesozoic extensional geometry of the voring and more margins in the norwegian sea whereas late paleozoic to triassic basins are limited by high angle normal faults with nw and se dips the late jurassic early cretaceous voring and more basins are characterised by low angle normal faults that dip towards the nw between the voring and more basins at the se end of the jan mayen fracture zone a synthetic transfer zone is interpreted geometrical models based on basin and fault geometries show that the rift system detached near the upper crust lower crust transition in the study area this rift system accommodated between 110 and 140 km of post early jurassic extension according to the geometry of the present day pre cretaceous upper crust a simple geometrical forward model suggests that a substantial part of this stretching occurred during the late cretaceous paleocene thinning the central and nw segments of the late jurassic early cretaceous rift

metamorphism and uplift of alpine schist in the franz josef fox glacier area of the southern alps new zealand geothermometry and geobarometry of 10 garnet oligoclase zone schists in the franz josef fox glacier area southern alps new zealand give temperatures ranging from 415 to 625 c and pressures from 5 2 to 9 2 kbar indicating a t p array of about 50 c kbar and inferred peak temperature conditions over a c 15 km thick section at depths between c 20 and 34 km the present day distribution of the schist samples implies that only about one third of the original crustal section is now exposed the garnet oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late palaeozoic early mesozoic subduction along the gondwana continental margin partial uplift c 0 2 m ma and erosion of the complex during jurassic cretaceous times rangitata uplift was synchronous with d2 deformation and recrystallization as recorded by the p t array cenozoic kaikoura uplift and exhumation of the schist since c 30 ma to form the southern alps was associated with oblique slip movement on the alpine fault the present day position and steep eastward dip of isograds and d2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two thirds of the original schist sequence largely due to simple shear along schistosity planes as the schist generally shows only incipient greenschist facies retrograde recrystallization an apparently complete although contracted prograde mineral sequence has been preserved by rapid uplift 5 km ma of hot rock and the effects of limited shear heating near the alpine fault

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

double diffusive recipes part i large scale dynamics of thermohaline staircases abstractthree dimensional dynamics of thermohaline staircases are investigated using a series of basin scale staircase resolving numerical simulations the computational domain and forcing fields are chosen to reflect the size and structure of the north atlantic subtropical thermocline salt finger transport is parameterized using the flux gradient formulation based on a suite of recent direct numerical simulations analysis of the spontaneous generation of thermohaline staircases suggests that thermohaline layering is a product of the gamma instability associated with the variation of the flux ratio with the density ratio after their formation numerical staircases undergo a series of merging events which systematically increase the size of layers ultimately the system evolves into a steady equilibrium state with pronounced layers 20 50 m thick the size of the region occupied by thermohaline staircases is controlled by the competition between turbulent mixing and double diffusion assuming in acc

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

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

thermal evolution of the sisters shear zone southern new zealand formation of the great south basin and onset of pacific antarctic spreading 1 the separation of zealandia from west antarctica was the final stage in the cretaceous breakup of the gondwana pacific margin continental extension resulting in formation of the great south basin and thinning of the campbell plateau leading to development of the pacific antarctic spreading ridge was partially accommodated along the sisters shear zone this east northeast striking brittle ductile structure exposed along the southeast coast of stewart island new zealand is a greenschist facies extensional shear zone that separates a hanging wall of chloritic brecciated granites and undeformed conglomerate from a footwall of mylonitic carboniferous and early cretaceous granites this complex structure exhibits bivergent kinematics and can be subdivided into a northern and southern segment the 40 ar 39 ar thermochronology indicates that cooling of the shear zone footwall began at 94 ma with accelerated cooling over the interval 89 82 ma structural and thermochronological data indicate a spatial and temporal link between the sisters shear zone initial sedimentation within the offshore great south basin extension of the campbell plateau and initiation of the pacific antarctic spreading ridge citation kula j a j tulloch t l spell m l wells and k a zanetti 2009 thermal evolution of the sisters shear zone southern new zealand formation of the great south basin and onset of pacific antarctic spreading tectonics 28 tc5015 doi 10 1029 2008tc002368