structural mineralogical and petrophysical effects on deep crustal rocks of fluid limited polymetamorphism western gneiss region norway the proterozoic banded granulite facies complex of bardsholmen western gneiss region norway t 815 845 c was locally transformed to eclogite t 455 510 c p 12 kbar and amphibolite facies rocks t 460 c during the caledonian continental collision the granulite complex consists of mafic two pyroxene granulite and leucocratic orthopyroxene garnet bearing layers alternating on a scale from 1 cm to 10 m the granulite facies rocks change to eclogite facies rocks over centimetre scale distances along well defined fluid infiltration fronts the mafic granulite was transformed to omphacite garnet rich eclogites and the leucocratic rocks were converted to quartz phengite rich assemblages with minor garnet and local omphacite melange like lithologies consisting of mafic lenses of eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust to eclogite facies during amphibolitization this melange like lithology evolves to a rock where amphibolite lenses and layers are surrounded by granitoid gneiss a lithology typical of the western gneiss region the bardsholmen locality illustrates the profound control exerted by fluids on the timing of metamorphism the structural make up and petrophysical properties such as density and rheology of crustal root zones fluid induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyancy of the subducted crust we suggest that orogens may develop differently depending on the fluid budget

interhemispheric air temperature phase relationships in the nonlinear dansgaard oeschger oscillation it has previously been suggested that the southern ocean might act as a thermal reservoir in mediating the relationship between the northern and southern hemisphere air temperature signals during a dansgaard oeschger cycle on the basis of a climate model based analysis we demonstrate on the contrary that it is equatorial and subtropical north atlantic thermocline waters that act so as to integrate and damp the northern hemisphere signal to fix the amplitude and phase of the southern hemisphere counterpart to the more intense north atlantic component of the oscillation the dynamics of the southern ocean component of the atlantic meridional overturning circulation are critical in explaining the evolution of the interhemispheric temperature phase relationships european project for ice coring in antarctica dronning maud land inferred air temperature variations are in phase with sea surface temperatures north of the antarctic sea ice edge suggesting that it is via a fast atmospheric pathway that this temperature signal is transmitted into the antarctic ice core record

the modification of bottom boundary layer turbulence and mixing by internal waves shoaling on a barrier reef abstractresults are presented from an observational study of stratified turbulent flow in the bottom boundary layer on the outer southeast florida shelf measurements of momentum and heat fluxes were made using an array of acoustic doppler velocimeters and fast response temperature sensors in the bottom 3 m over a rough reef slope direct estimates of flux richardson number rf confirm previous laboratory numerical and observational work which find mixing efficiency not to be a constant but rather to vary with frt reb and rig these results depart from previous observations in that the highest levels of mixing efficiency occur for frt 1 suggesting that efficient mixing can also happen in regions of buoyancy controlled turbulence generally the authors find that turbulence in the reef bottom boundary layer is highly variable in time and modified by near bed flow shear and stratification driven by shoaling internal waves

estimating the mean diapycnal mixing using a finescale strain parameterization abstractfinescale methods are currently being applied to estimate the mean turbulent dissipation rate and diffusivity on regional and global scales this study evaluates finescale estimates derived from isopycnal strain by comparing them with average microstructure profiles from six diverse environments including the equator above ridges near seamounts and in strong currents the finescale strain estimates are derived from at least 10 nearby argo profiles generally 60 km distant with no temporal restrictions including measurements separated by seasons or decades the absence of temporal limits is reasonable in these cases since the authors find the dissipation rate is steady over seasonal time scales at the latitudes being considered 0 30 and 40 50 in contrast a seasonal cycle of a factor of 2 5 in the upper 1000 m is found under storm tracks 30 40 in both hemispheres agreement between the mean dissipation rate calculated using argo profiles and mean from microstructure profiles is wi

turbulence and diapycnal mixing in drake passage abstractdirect measurements of turbulence levels in the drake passage region of the southern ocean show a marked enhancement over the phoenix ridge at this site the antarctic circumpolar current acc is constricted in its flow between the southern tip of south america and the northern tip of the antarctic peninsula observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the acc frontal zones where strong flow reaches the bottom in these areas turbulent dissipation levels reach 10 8 w kg 1 at abyssal and middepths the mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 10 4 m2 s 1 this level is an order of magnitude larger than the mixing levels observed upstream in the acc above smoother bathymetry outside of the frontal regions dissipation rates are o 10 10 w kg 1 comparable to the background levels of turbulence found throughout most mid and

global energy conversion rate from geostrophic flows into internal lee waves in the deep ocean a global estimate of the energy conversion rate from geostrophic flows into internal lee waves in the ocean is presented the estimate is based on a linear theory applied to bottom topography at o 1 10 km scales obtained from single beam echo soundings to bottom stratification estimated from climatology and to bottom velocity obtained from a global ocean model the total energy flux into internal lee waves is estimated to be 0 2 tw which is 20 of the global wind power input into the ocean the geographical distribution of the energy flux is largest in the southern ocean which accounts for half of the total energy flux the results suggest that the generation of internal lee waves at rough topography is a significant energy sink for the geostrophic flows as well as an important energy source for internal waves and the associated turbulent mixing in the deep ocean

extremely long kelvin helmholtz billow trains in the romanche fracture zone in the atlantic ocean the densest water mass antarctic bottom water aabw can only cross the mid atlantic ridge from its southwestern to northeastern basins in limited because deep conduits at the southwestern entrance of one of these the equatorial romanche fracture zone aabw crosses a sill at 4550 m depth in a 7 km narrow channel before plunging into the deep at the sill slope the rapidly flowing aabw causes shear induced turbulent mixing with the overlying water masses we present an excerpt of 1 hz sampled half yearlong moored observations from 99 high resolution temperature sensors that demonstrate and quantify the turbulence details on top of quasi steady shear flow an internal tide modulates the mixing together they constitute a means for an extremely long train of 250 consecutive kelvin helmholtz billows in a day that vary between 5 and 100 m in vertical scale

continental eclogites and the wilson cycle the wilson cycle the repeated opening and closing of oceans commonly along roughly the same lines is here attributed to the presence of eclogite facies roots of partially collapsed orogens we present a finite element thermal model that suggests that such roots will weaken the orogenic lithosphere relative to that of the adjacent foreland for hundreds of millions of years and make it a preferred site for later rifting

crustal structure of the middle urals based on reprocessing of russian seismic reflection data summary reprocessed russian seismic reflection data combined with existing russian geological maps and seismic interpretations provide the basis for a revised tectonic interpretation of the upper crust in the middle urals the main uralian fault muf the principal suture of the urals separating the east european craton to the west from accreted terranes to the east is found to penetrate to depths of at least 18 km and perhaps much deeper with a uniform eastward dip of 35 after migration as the muf appears to truncate reflections in both the foreland and hinterland of the urals it is arguably one of the youngest structural features of the orogen basement rocks of the east european platform underthrust the suture and comprise the deep crust a minimum of 30 km east of the surface expression of the muf the tagil volcanic arc complex greater than 1000 km in length along strike comprises at least the upper 10 km of the crust east of the muf this terrane shallows eastwards where it appears to be truncated by the west dipping serov mauk fault based on correlation with surface geology the hinterland of the urals east of the muf is characterized by a previously unrecognized zone of discontinuous mid crustal reflectivity at depths of 14 16 km that is discordant with upper crustal reflections this mid crustal zone of reflectivity is interpreted to represent either a major subhorizontal structural detachment or a zone of magmatic intrusion

distal occurrence of mid holocene whakatane tephra on the chatham islands new zealand and potential for cryptotephra studies abstract the whakatane tephra a rhyolitic tephra erupted ca 5500 cal bp from okataina volcanic centre central north island has been identified on the chatham islands which lie 900 km east of christchurch new zealand the visible tephra layer 5 mm in thickness and preserved within peat on pitt island was identified using both radiocarbon dating and analysis of glass shards by electron microprobe whakatane tephra is the first holocene tephra to be identified on the chatham islands and it is the most distal holocene tephra yet recorded in the new zealand region being 850 km from source the pitt island occurrence extends the tephra s dispersal area markedly by an order of magnitude possibly to 300 000 km 2 an estimated dispersal index d of approximately 10 5 km 2 indicates that the eruption generated a very high plinian column possibly exceeding 30 km in height with strong winds blowing the ash plume southeastwards this new discovery of distal whakatane tephra as a thin but visible layer strongly implies that cryptotephras are likely to be preserved on the chatham islands and within adjacent ocean floor sediments therefore the potential exists to develop enhanced cryptotephrostratigraphic records from these distal areas which in turn would help facilitate precise correlation via tephrochronology of palaeoenvironmental records such as nz intimate from mainland new zealand the southwest pacific ocean and the chatham islands

rapid cross density ocean mixing at mid depths in the drake passage measured by tracer release diapycnal mixing across density surfaces is an important process in the global ocean overturning circulation1 2 3 mixing in the interior of most of the ocean however is thought to have a magnitude just one tenth of that required to close the global circulation by the downward mixing of less dense waters4 some of this deficit is made up by intense near bottom mixing occurring in restricted hot spots associated with rough ocean floor topography5 6 but it is not clear whether the waters at mid depth 1 000 to 3 000 metres are returned to the surface by cross density mixing or by along density flows7 here we show that diapycnal mixing of mid depth 1 500 metres waters undergoes a sustained 20 fold increase as the antarctic circumpolar current flows through the drake passage between the southern tip of south america and antarctica our results are based on an open ocean tracer release of trifluoromethyl sulphur pentafluoride we ascribe the increased mixing to turbulence generated by the deep reaching antarctic circumpolar current as it flows over rough bottom topography in the drake passage scaled to the entire circumpolar current the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups 1 sv 106 m3 s 1 upwell in the southern ocean with cross density mixing contributing a significant fraction 20 to 30 per cent of this total and the remainder upwelling along constant density surfaces the great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean floor topography

interaction between fluid flow fracturing and mineral growth during eclogitization an example from the sunnfjord area western gneiss region norway in sunnfjord western gneiss region of south norway a proterozoic layered gabbro complex displays various degrees of transformation to eclogite in the unreacted parts layering in the gabbro is defined by modal variations of plagioclase olivine pyroxenes and minor fe ti oxide and spinel coronitic and foliated eclogite formed from the gabbro by hydration reactions at t 510 620 c causing a volume decrease of c 13 in the coronitic eclogite the mafic magmatic phases are replaced by aggregates of omphacite barroisite tremolite talc and rutile whereas the plagioclase domains are pseudomorphed by omphacite barroisite clinozoisite kyanite paragonite and garnet the felsic and mafic domains are separated by a garnet rim up to 5 mm thick garnet was also formed along dilational veins connecting and radiating out of coronas including the same eclogite facies minerals in addition microfractures filled by amphibole and omphacite cut through the corona and vein garnet oriented perpendicular to the garnet layering the transformation of dry gabbro to eclogite with hydrous minerals requires supply of water the timing of metamorphic reactions is therefore dependent on the timing of fluid introduction the inclusion pattern of garnet in the holt tyssedalsvatnet metagabbro complex indicates that transformation started under eclogite facies conditions brittle deformation in form of fractures allowing infiltration of fluids and mobilisation of elements is shown to be the most important process initiating transformation brittle deformation is thereby active in deep crustal levels corresponding to eclogite facies conditions fracturing is interpreted as having been caused by a combination of high fluid pressure volume changes during mineral transformations and external stresses ductile deformation started after the initial metamorphic transformation garnet chemistry and zoning pattern are controlled by the chemistry of the growth place the fluid influx and element supply abrupt chemical variations across corona garnet may have been formed by an interaction between rapid growth and transport along microfractures between plagioclase and mafic domain the metamorphic transition proceeded rapidly the transformation of the deep crust to eclogite can be viewed as a dynamic process where the volume changes result in fracturing and these fractures funnelling fluid and element enhancing mineral reaction the densification causes further fracturing indicating that the process is self driven provided that fluid is available

the combined effect of tidally and eddy driven diapycnal mixing on the large scale ocean circulation several recent studies have shown that ocean western boundaries are the primary regions of eddy energy dissipation globally theeddyenergysinkshavebeenestimatedtointegratetoabout0 2tw thisisasizable fraction of the tidal energy dissipation in the deep oceanic interior estimated at about 1 0 tw and contributing to diapycnal mixing the authors conduct sensitivity experiments with an ocean general circulation model assuming that the eddy energy is scattered into high wavenumber vertical modes resulting in energy dissipation and locally enhanced diapycnal mixing when only the tidal energy dissipation maintains diapycnal mixing the overturning circulation and stratification in the deep ocean are too weak with the addition of the eddy dissipation the deep ocean thermal structure becomes closer to that observed and the overturning circulation and stratification in the abyss become stronger furthermore the mixing associated with the eddy dissipation can on its own drive a relatively strong overturning the stratification and overturning in the deep ocean are sensitive to the vertical structure of diapycnal mixing when most of this energy dissipates within 300 m above the bottom the abyssal overturning and stratification are too weak allowing forthedissipationtopenetratehigherinthewatercolumn suchassuggestedbyrecentobservations resultsin stronger stratification and meridional circulation zonal circulation is also affected in particular the drake passage transport becomes closer to its observational estimates with the increase in the vertical scale for turbulence above topography consistent with some theoretical models the drake passage transport increases with the increase in the mean upper ocean diffusivity

a new characterization of the turbulent diapycnal diffusivities of mass and momentum in the ocean the diapycnal diffusivity of mass supported by turbulent events in the ocean interior plays a fundamental role in controlling the global overturning circulation the conventional representation of this diffusivity due to osborn 1980 assumes a constant mixing efficiency we replace this methodology by a generalized osborn formula which involves a mixing efficiency that varies nonmonotonically with at least two nondimensional variables using these two variables we propose dynamic parameterizations for mixing efficiency and turbulent prandtl number the latter quantifies the ratio of momentum to mass diapycnal diffusivities based on the first synthesis of an extensive direct numerical simulation of inhomogeneously stratified shear induced turbulence data from argo floats are employed to demonstrate the extent of the spatial and statistical variability to be expected in both the diapycnal diffusivities of mass and momentum we therefore suggest that previous estimates of these important characteristics of the global ocean require reconsideration

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

the quest for an unambiguous parameterization of mixing efficiency in stably stratified geophysical flows estimates of turbulent mixing in geophysical settings typically depend on the efficiency at which shear driven turbulence mixes density across isopycnals to date however no unifying parameterization of diapycnal mixing efficiency exists due to the variability of natural flows and also due to certain ambiguities that arise from descriptions based on a single parameter here we highlight important ambiguities of some common single parameter schemes in the context of a multiparameter framework that considers the independent effects of shear buoyancy and viscosity parameterizations based on the gradient richardson number ri the turbulent froude number frt and the buoyancy reynolds number reb are considered the diagnostic ability of these parameters is examined using published data from both direct numerical simulations and field observations

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

spatial and temporal variability of global ocean mixing inferred from argo profiles 1 the influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water mass transformation stratification maintenance and the overturning circulation however the distribution of ocean mixing is not well described by sparse ship based observations since this mixing is both spatially patchy and temporally intermittent we use strain information from argo float profiles in the upper 2 000 m of the ocean to generate over 400 000 estimates of the energy dissipation rate indicative of ocean mixing these estimates rely on numerous assumptions and do not take the place of direct measurement methods temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans they corroborate previous observations linking elevated dissipation rates to regions of rough topography we also observe heightened estimated dissipation rates in areas of high eddy kinetic energy as well as heightened diffusivity in high latitudes where stratification is weak the seasonal dependence of mixing is observed in the northwest pacific suggesting a wind forced response in the upper ocean

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material