tectono metamorphic evolution of a nappe stack a case study of the swiss alps fold and thrust belts are prominent structures that occur at the front of compressional orogens to unravel the tectonic and metamorphic evolution of such complexes kinematic investigations quantitative microstructural analysis and geothermometry calcite graphite calcite dolomite were performed on carbonate mylonites from thrust faults of the helvetic nappe stack in central switzerland paleo isotherms of peak temperature conditions and cooling stages fission track of the nappe pile were reconstructed in a vertical section and linked with the microstructural and kinematic evolution mylonitic microstructures suggest that under metamorphic conditions close to peak temperature strain was highly localized within thrust faults where deformation temperatures spatially continuously increased in both directions from n to s within each nappe and from top down in the nappe stack covering a temperature range of 180 380 c due to the higher metamorphic conditions thrusting of the lowermost nappe the doldenhorn nappe was accompanied by a much more pronounced nappe internal ductile deformation of carbonaceous rock types than was the case for the overlying wildhorn and gellihorn nappes ongoing thrusting brought the doldenhorn nappe closer to the surface the associated cooling resulted in a freezing in of the paleo isotherms of peak metamorphic conditions contemporaneous shearing localized in the basal thrust initially still in the ductile deformation regime and finally as brittle faulting and cataclasis inducing ultimately an inverse metamorphic zonation with ongoing exhumation and the formation of the helvetic antiformal nappe stack a bending of large scale tectonic structures thrusts folds peak temperature isotherms and cooling isotherms occurred while this local bending can directly be attributed to active deformation underneath the section investigated up to times of 2 3 ma a more homogeneous uplift of the entire region is suggested for the very late and still active exhumation stage

on the structure of grain boundaries in tectonites abstract grain boundaries in several tectonites have been examined using high voltage and scanning transmission electron microscopy on a gross scale grain boundaries are observed to differ from the host crystal in exhibiting faster rates of preferential etching during ion beam thinning and faster rates of ionization damage during exposure to the electron beam grain boundary features can be grouped according to decreasing size as follows grain junction voids that can form linked networks of tubules arrays of isolated voids and inclusions along the grain boundary surface and a narrow grain boundary film of distorted crystal structure the presence of inclusions in voids and the propensity for electron damage along the boundary suggests that the former were initially fluid filled and that the latter may be hydrated initial microchemical analyses using stem indicate that no compositional difference exists between the grain boundaries and host crystals the overall complex grain boundary structure in tectonites suggests that the effective grain boundary width for intercrystalline diffusion is much greater than that proposed based on metals and that diffusion path lengths along grain boundaries are much less than the theoretical maximum the net effect is to greatly enhance deformation mechanisms dependent on intercrystalline mass transport

cooling induced strain localization in carbonate mylonites within a large scale shear zone glarus thrust switzerland abstract the glarus thrust switzerland offers a great field example of strain localization under retrograde conditions along the thrust mylonitic microfabrics are characterized by a temperature stress controlled balance of grain size reducing mechanisms and grain growth consequently mean grain sizes decrease along the thrust with decreasing metamorphic conditions and towards the thrust contact in an opposite manner calcite twin densities increase towards the contact cpos are strongest between 0 5 and 15 m away from the thrust but become generally weaker in the last centimeters where also cataclasites occur the cpo weakening and grain size reduction towards the shear zone point to a change from predominant dislocation creep to granular flow finally ending in cataclastic deformation the microfabric changes correlate with a decrease in 13c and 18o towards the thrust contact indicating the presence of fluids during cycles of brittle and plastic deformation based on the microfabric changes variations in stable isotopes and cross cut relationships a subdivision into a low and high temperature shear zone can be made the microfabric modifications resulted from changes in deformation conditions due to ongoing thrusting and exhumation induced cooling promoting further strain localization of an existing high strain shear zone

brittle ductile deformation in the glarus thrust lochseiten lk calc mylonite the glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1 m layer of lochseiten lk calc mylonite this layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture lattice and shape preferred orientations are observed within the fine grained recrystallized matrix these features indicate the alternate activity of brittle and ductile deformation mechanisms in contrast to the classical view that grain boundary sliding superplasticity is the dominant deformation mechanism it is advocated that fluids derived from the footwall and expelled along the thrust are responsible for hydrofracturing and cataclastic deformation in periods between fracture events deformation was ductile in this new interpretation a substantial amount of the total thrust displacement was accommodated by numerous short lived and strongly localized fracture events at the base of the verrucano thrust sheet rather than a permanently weak decollement lithology

an sem study of porosity and grain boundary microstructure in quartz mylonites simplon fault zone central alps a backscattered and secondary electron sem study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde simplon fault zone established three characteristic components 1 fine isolated pores 1 m diameter are scattered across two grain interfaces preferentially concentrated on surfaces in extension pores are uncommon on three grain junctions and there is no evidence for fluid interconnectivity along three and four grain junctions the fine porosity may develop by accumulation of original mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding dynamic cavitation implies that the ductile mylonitic deformation is at least locally dilatant and therefore pressure sensitive 2 large vug like pores up to mm scale extend along multi grain boundaries observed in all samples they are most common in the higher initial temperature coarse grained samples with a microstructure dominated by grain boundary migration recrystallization grains bordering this connected porosity develop perfect crystal faces undecorated by fine pores or pits the irregular lobate optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces the coarse porosity is probably due to accumulation during dynamic recrystallization of co2 rich fluid with a high wetting angle against quartz 3 in one sample interconnected sinuous ridges 0 2 m high are observed to follow three and four grain junctions and disjoint into more isolated worms and spheroidal globules on two grain interfaces these are transitional to more branching vein like or convoluted brain like forms the brain like and globular forms have been observed with varying frequency through the range of samples with the globules attaining sizes of up to 60 m vein structures have also been observed on intragranular fractures these topologies do not match across adjoining surfaces and must have developed into free space the ridge vein brain spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established they could represent unstable meniscus necking of a thin grain boundary phase of low viscosity developed due to quasi adiabatic shear and or local stress induced dilatancy during microcracking

low temperature deformation of limestone isola palmaria northern apennine italy the role of primary textures precursory veins and intracrystalline deformation in localization abstract fine grained limestones of the tuscan nappe from isola palmaria northern apennine italy contain sporadically developed bedding parallel shear zones some of which exhibit well developed calcite mylonites the presence of shear zones that formed at temperatures less than 250 c allows the study of limestone deformation mechanisms active under low temperature upper crustal conditions two materially distinct types of shear zones developed during the syn nappe stage of deformation and have been characterized by integrated field strain microstructural optical microscopy cathodoluminescence transmission electron microscopy and ebsd fabric studies type a shear zones occur within decametre thick packages accommodating heterogeneously distributed strain in very fine grained limestone micrite type b shear zones are localized within calcite veins that develop into recrystallized calc mylonites both types form at the contact with dolomite although this does not play an important role in strain localization in both cases low temperature ductility occurs by extensive dislocation glide sustained by athermal network accommodated recovery although the end state microstructures of both shear zone types cannot be differentiated optically differences do exist in the crystallographic preferred orientations cpo and the grain boundary structure of deformed micrite and vein calcite deformed micrite grain boundaries are rich in voids consistent with extensive fluid flux that can enhance material transport and independent grain displacements while suppressing cpo development the crystallographically defined grain boundaries in recrystallized vein calcite suggest that lower grain boundary diffusivity enables more typical dislocation mediated cpo formation

fission track reconstruction of the front of the northern apennine thrust wedge and overlying ligurian unit in the northern apennines foreland the marnoso arenacea formation ma fm records post depositional burial temperatures overlapping with those of the zone of partial annealing of apatite fission tracks because the stratigraphy sedimentology petrography and structural evolution of this turbidite succession has been intensively studied over the last 40 years the ma fm provides an ideal case to apply the apatite fission track method the data show a general decrease of the maximum paleotemperature undergone by the ma samples toward the foreland area the maximum burial calculated using a geothermal gradient of 20 c km spans from more than 5 km to less than 2 5 km and indicates that the reconstructed total thickness of the ma succession is not enough to justify the determined burial values stratigraphic data indicates that the missing section consisted of a lower component foredeep successions and an upper allocthonous component composed of disrupted and chaotic oceanic sediments and minor ophiolites ligurian unit the ligurian unit overrode the foredeep succession during deposition its advancement toward the foreland area was associated with subsidence and only locally contrasted by thrust growth the reconstructed wedge shape of the ligurian unit in its final and pre erosional configuration at time of the maximum bunal of the ma succession shows a flat upper surface which corresponds to a paleosurface of early pliocene age this surface marks at 4 to 5 ma the onset of the exhumation phase which occurred at a mean rate of 1 2 mm yr

cosmogenic 10be production rate calibrated against 3he in the high tropical andes 3800 4900 m 20 22 s abstract many geomorphologic applications notably glacier chronologies require improvements in both the precision and the accuracy of the cosmogenic dating tool of particular importance is the need to better constrain the spatial variability of the cosmogenic nuclides production rates at high elevation and low latitudes one strategy that can be adopted for this is to couple absolute calibrations from independently dated surfaces with cross calibration studies performed by measuring several cosmogenic nuclides in the same rock in the present study we report the highest elevation 4800 m cross calibration published to date comprising measurements of cosmogenic 3 he and 10 be in cogenetic pyroxene and quartz the samples were collected from six dacitic moraine boulders exposed from 32 to 65 ka at 4820 m on the flanks of the uturuncu volcano 22 s 67 w southern lipez bolivia the samples yield a remarkably tight cluster of 3 he 10 be production ratios with a weighted mean of 33 3 0 9 1 this production ratio is undistinguishable within uncertainties from the 3 he 10 be production ratio of 32 3 0 9 determined in the same mineral pair at low elevation 1333 m by amidon et al 2009 these results agree at the 1 level and suggest that any hypothetical increase of the 3 he 10 be production ratio in pyroxene and quartz is likely to be lower than 5 over this elevation range 1000 5000 m moreover the production ratio is almost insensitive to the li content of the pyroxene 20 to 50 ppm li suggesting that the cosmogenic thermal neutron production of 3 he is very low in this setting the high elevation 3 he 10 be production ratio is used in combination with a local determination of the 3 he production rate in the high central altiplano 3800 m blard et al 2013 to establish a local 10 be production rate of 30 0 1 4 at g 1 yr 1 at 3800 m and 20 s after scaling to sea level high latitude with the time dependent lal stone model this yields a 10 be production rate in quartz of 3 63 0 17 at g 1 yr 1 importantly this rate can be used for high precision geomorphologic dating for example for determining glacial chronologies 1 4 through 10 be dating of moraines in the high tropical andes any inaccuracy attached to the scaling model is canceled out when the calibration site is located close to the dated object the same experiment was also undertaken in pyroxene and plagioclase from two andesitic boulders from the tunupa volcano moraines exposed at 3800 m and 4200 m a 3 he 10 be production ratio of 35 9 1 3 1 is obtained for this mineral pair indicating that the 10 be production rate in plagioclase is about 8 lower than in quartz

experimental study of the influence of stress temperature and strain on the dynamic recrystallization of carrara marble carrara marble has been deformed experimentally at temperatures ranging between 500 and 1000 c at confining pressures of 200 and 300 mpa and up to very large strains in extension and compression in order to study the microstructural and mechanical property changes associated with dynamic recrystallization microstructural studies were made by optical microscopy on ultrathin sections and automatic grain size measurement techniques were used when the temperature is sufficiently high 600 c and the stresses are high enough for deformation twinning to occur twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size at stress levels too low to activate twinning recrystallization occurs in two stages the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume the recrystallization process requires larger prestrains at the lower temperatures the size to which migration recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms within the range of experimental observations dynamically recrystallized grain size does not depend on strain rate or temperature separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry no unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found however extrapolation of the experimental data to low natural strain rates suggests that weakening following recrystallization may occur in nature

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

microstructures of cataclasites in a limestone on shale thrust fault implications for low temperature recrystallization of calcite abstract the hunter valley thrust hvt a low temperature foreland thrust fault in the valley and ridge province southern appalachians produced two distinct cataclasites a limestone cataclasite derived predominantly from the hanging wall limestone and a shale cataclasite derived predominantly from the footwall shale the limestone cataclasite consists of fragments of calcite limestone pre existing limestone cataclasite quartz and quartz aggregates in a fine grained

grain coarsening maps a new tool to predict microfabric evolution of polymineralic rocks polymineralic rocks undergo grain coarsening with increasing temperature in both static and deformational environments as long as no mineral reactions occur the grain coarsening in such rocks is complex because the different phases influence each other and it is this interaction that controls the rate of grain coarsening of the entire aggregate we present a mathematical approach to investigate coupled grain coarsening using a set of microstructural parameters including grain size and volume fraction of both second phases and matrix mineral in combination with temperature information based on samples from polymineralic carbonate mylonites that were deformed at different temperatures we demonstrate how the mathematical relation can be calibrated for this natural system using such data sets for other lithologies grain coarsening maps can be generated which allow the prediction of microstructural evolution in polymineralic rocks such predictions are crucial for all subdisciplines in the earth sciences that require fundamental knowledge about microstructural changes and rheology of an orogen at different depths such as structural geology geophysics geodynamics and metamorphic petrology

on the relationship between the formation of shear zones and the form of the flow law for rocks undergoing dynamic recrystallization abstract the formation and maintenance of shear zones in rocks at high temperatures when viscous flow takes place at constant volume requires strain weakening several processes can lead to a strain weakening characteristic in the flow of such rocks to high strains but conventional experimental rock mechanics techniques cannot normally provide high quality mechanical data to high strains recent experimental data for calcite marble have demonstrated strain weakening accompanied by dynamic recrystallization a power law with strain exponent r can be used to describe the dependence of the flow stress on strain together with the conventional description of the dependence of strain rate on stress described by a stress exponent n the stable localization of strain into shear zones is favoured by more negative values of r e g r 0 8 and more positive values of n e g n 5 to 7 a further important factor is the critical strain e0 for the onset of recrystallization and hence strain weakening shear zone formation is favoured by smaller values for e0 shear zone formation weakens the crustal section in which they occur but the amount of weakening may not be great perhaps no more than 50 of the previous steady state flow stress certain rock types such as mafic igneous rocks may through their mechanical properties be more susceptible to shear zone formation than others such as mica schists it is emphasized that the analysis presented is based upon a strain dependent flow law that describes single phase dynamic recrystallization by grain boundary migration that leads to enhanced recovery without any change in deformation mechanism no consideration is given to possibly important effects arising from the activation of grain size sensitive flow or to potential weakening effects arising from syntectonic metamorphic reactions

microfabrics in carbonate mylonites along a large scale shear zone helvetic alps abstract carbonate mylonites with varying proportions of second phase minerals were collected at positions of increasing metamorphic grade along the basal thrust of the morcles nappe helvetic nappes switzerland variations of temperature stress and strain rate changes in chemistry of solid and fluid phases and differing degrees of strain localization and annealing were tracked by measuring the shapes mean sizes and size distributions of both matrix and second phase grains as well as crystal preferred orientation cpo of the matrix field structures suggest that strain rate was constant along the fault the mean and distribution of the calcite grain sizes were affected most profoundly by temperature increased temperature presumably accompanied by decreased stress correlated with larger mean sizes and wider size distributions at a given location the matrix grains in mylonites with more second phase particles are on average smaller have narrower size distributions and have more elongate shapes for example mylonites with 50 vol of second phases have matrix grain sizes half that of pure mylonites changes in calcite chemistry and the presence of synkinematic fluids seemed to influence microfabric only weakly temporal variations in conditions such as exhumation induced cooling apparently provoke changes in temperature stress and strain rate along the nappe these changes result in further strain localization during retrograde conditions and cause the grain size to be reduced by an additional 50 the matrix cpo strengthens with increasing temperature or strain but weakens and rotates with increasing second phase content these fabric changes suggest differing rates of grain growth grain size reduction and development of cpo owing to variations in the deformation conditions and perhaps mechanisms to interpret natural mylonite structures or to extrapolate mechanical data to natural situations requires careful characterization of the microfabric and in particular second phase minerals

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

pressure fluctuation during uplift of the northern apennines italy a fluid inclusions study abstract p t conditions existing after the main syn collision tectonic phase in the western part of the northern apenninic chain italy e g in the tuscan nappe outcropping in the la spezia area were estimated on the basis of a detailed microstructural structural petrographic and fluid inclusion study of quartz of syn tectonic d2 veins developed in the tertiary flysch at the top of the tuscan nappe macigno formation three main fluid events have been distinguished as follows during retrograde metamorphism d1 to d2 phase fluids in equilibrium with turbidites from the tuscan nappe were h2o ch4 mixtures issued from water organic matter interactions in temperatures conditions that may have reached at least 260 c or more 280 c depending on the considered depth estimates and maximum pressures around 210 250 mpa evidence of strong fluid pressure fluctuation between lithostatic and hydrostatic within the metamorphic formations up to 100 150 mpa possibly linked to fault valve activity at the beginning of the uplift triggered phase separation of the water methane fluids and production of methane rich and water rich fluids fluctuations in pressure during these events played a crucial role in quartz crystallization especially in extensional fissures formed perpendicular to the d2 folds axial foliation changes in the fluid regime and sources with time are evidenced by the input of brines which mix to distinct degrees and are trapped in healing microfissures during retrograde fluid evolution such mixing processes are an indication of the connection between separate fluid reservoirs with different temperature conditions consequently mass and heat transfer have to be taken into account as the downward percolation of cooler fluids probably accelerated the rate of cooling of the exhumed formations these processes are certainly common to most orogenic terrains and can be quantitatively studied through p v t x reconstruction using fluid inclusion data on the drainage zones where the mixing processes occurred this work confirms that fluid inclusion studies can provide accurate quantitative estimates of fluid pressure variations during the evolution of orogenic terrains and could therefore constitute a useful tool in tectonic interpretations at the light of the models developed for pressure variations in the upper crust

the role of veining and dissolution in the evolution of fine grained mylonites the mcconnell thrust alberta microstructural observations of the mcconnell thrust fault rocks demonstrate the importance of vein formation and calcite dissolution in the evolution of the mcconnell thrust limestone mylonites the protolith to the mylonite is a micritic limestone the mylonite contains numerous discontinuous bedding parallel calcite veins that range from relatively undeformed except for twinning and undulose extinction to highly sheared the highly sheared veins show evidence for dynamic recrystallization and grain size reduction along grain and twin boundaries that can lead to complete recrystallization to a grain size of approximately 1 3 m the bulk of the calcite matrix within the mcconnell thrust mylonite is interpreted to have formed by dynamical recrystallization of veins the emplacement of relatively coarse grained veins promoted the activation of dislocation creep within the fault rocks of the thrust in the absence of the increase in grain size afforded by vein emplacement it is anticipated that the fine grained protolith would have deformed predominantly by solution transfer with grain boundary sliding and localized brittle shearing the layer parallel calcite veins are found in close proximity to sheared clay seams which are interpreted to represent the insoluble residue left behind after dissolution of calcite these clay seams may have contributed to the build up of high pore fluid pressures which if intermittently reaching lithostatic conditions during displacement would result in transient brittle failure and thrust surface parallel vein emplacement in an otherwise ductilely deforming rock

thermochronologic evidence for the exhumational history of the alpi apuane metamorphic core complex northern apennines italy 1 the apennine range is a young convergent orogen that formed over a retreating subduction zone the alpi apuane massif in the northern apennines exposes synorogenic metamorphic rocks and provides information about exhumation processes associated with accretion and retreat u th he and fission track ages on zircon and apatite are used to resolve exhumational histories for the apuane metamorphic rocks and the structurally overlying very low grade macigno formation stratigraphic metamorphic and thermochronologic data indicate that the apuane rocks were structurally buried to 15 30 km and 400 c at about 20 ma exhumation to 240 c and 9 km depth below sea level occurred at 10 13 ma by 5 ma the apuane rocks were exhumed to 70 c and 2 km the macigno and associated tuscan nappe were also structurally buried and the macigno reached its maximum depth of 7 km at 15 to 20 ma stratigraphic evidence indicates that the apennine wedge was submarine at this time thus we infer that initial exhumation of the apuane was coeval with tectonic thickening higher in the wedge as indicated by synchronous structural burial of the tuscan nappe from 6 to 4 ma thinning at shallow depth is indicated by continued differential exhumation between the apuane and the tuscan nappe at high rates after 4 ma differential exhumation ceased and the apuane and the tuscan nappe were exhumed at similar rates 0 8 km ma which we attribute to erosion of the apennines following their emergence above sea level