diurnal seismicity cycle linked to subsurface melting on an ice shelf seismograms acquired on the mcmurdo ice shelf antarctica during an austral summer melt season november 2016 january 2017 reveal a diurnal cycle of seismicity consisting of hundreds of thousands of small ice quakes limited to a 6 12 hour period during the evening in an area where there is substantial subsurface melting this cycle is explained by thermally induced bending and fracture of a frozen surface superimposed on a subsurface slush water layer that is supported by solar radiation penetration and absorption a simple one dimensional model of heat transfer driven by observed surface air temperature and shortwave absorption reproduces the presence and absence as daily weather dictated of the observed diurnal seismicity cycle seismic event statistics comparing event occurrence with amplitude suggest that the events are generated in a fractured medium featuring relatively low stresses as is consistent with a frozen surface superimposed on subsurface slush waveforms of the icequakes are consistent with hydroacoustic phases at frequency bf gt bf 75 bf hz and flexural gravity waves at frequency bf bf lt 25 bf hz our results suggest that seismic observation may prove useful in monitoring subsurface melting in a manner that complements other ground based methods as well as remote sensing

glaciology ice shelf stability questioned surface lakes and streams are forming on antarctica s ice shelves making them susceptible to instability and possible collapse but rivers could mitigate this effect by efficiently exporting meltwater to the ocean see letters p 344 p 349 the collapse of ice shelves can be hastened by surface meltwater that can seep into crevasses and refreeze causing hydrofracturing such a mechanism was observed in the collapse of the larsen b ice shelf on the antarctic peninsula robin bell et al show that in the nansen ice shelf antarctica a surface river has been in operation for over a century the river terminates in a waterfall that can drain all of the surface meltwater produced annually by the ice shelf in one week thus for the nansen it seems that this surface drainage mechanism might be preventing meltwater from destabilizing the ice shelf it isn t clear at present why meltwater is retained in some systems and drained in others but the finding suggests that meltwater may not be a universally destructive force elsewhere in this issue jonathan kingslake et al report on the extent of meltwater across antarctica and find that surface drainage systems have existed for decades as far as 85 s melt ponds and streams form across antarctica but their full extent and the duration of their existence have remained unclear now jonathan kingslake et al show that surface drainage systems including interconnected ponds and streams have existed for decades as far as 85 s and up to 1 300 metres elevation they show that surface water can be transported more than 100 kilometres creating pond networks almost 80 kilometres long although the implications of the findings for ice sheet dynamics are not explored the study demonstrates that surface water drainage is more extensive than previously thought and is probably a persistent feature of the antarctic ice sheet elsewhere in this issue robin bell et al report on the influence of surface meltwater on antarctic ice shelf stability and find that it might not be a universally destructive force as was previously thought

late tectonic and metamorphic evolution of the piedmont accretionary wedge queyras schistes lustres western alps evidences for tilting during alpine collision this paper addresses the contribution of extension in the upper crust during syncollisional exhumation of subducted material we present structural data based on the analysis of ductile brittle ductile and brittle deformation features in metapelitic schists and meta basalts from the schistes lustres nappe stack in the piedmont zone of the western alps to link the structures with synkinematic pressure and temperature estimates geothermobarometry was performed using fluid inclusion investigation in quartz together with temperature estimations based on metamorphic chlorite composition samples used for geothermobarometry were collected taking into account the structural framework within veins associated with extensional structures observed at the regional scale the early generation of veins associated with boudinage occurred at 415 345 c and 6 2 5 kbar and subsequent incipient normal faulting occurred at temperatures of 315 235 c and pressures 1 7 kbar the transition from ductile to brittle extension occurred between ca 27 ma and ca 10 ma these results indicate a continuous extension in the upper crust during exhumation and continuous deformation associated with regional scale tilting of the exhumed schistes lustres tectonic pile the extensional tectonic regime in greenschistfacies conditions remains active today in most of the internal alpine zones as the alpine collision continues

transport processes induced by metastable boiling water under martian surface conditions liquid water on the martian surface is expected to be metastable owing to low atmospheric pressure experiments at martian conditions reveal that water and briny flows induce grain saltation and slope destabilization with geomorphic consequences

unraveling the exhumation history of high pressure ophiolites using magnetite u th sm he thermochronometry abstract magnetite is a ubiquitous oxide in ultramafic and mafic rocks which is present in a large range of geological and tectonic settings in the case of high pressure ultramafic rocks exhumation timing is commonly constrained by geochronometers in nearby lithologies the development of the magnetite u th sm he method mghe has opened new perspectives to refine the exhumation history of such rocks however no thermal history has been deduced using mghe data so far here we applied mghe dating to magnetite from an hp lt alpine ophiolite body rocher blanc from the schistes lustres units western alps france where part the p t t history is constrained to further refine the thermal history and demonstrate validity of the method textural and geochemical characterizations of the magnetite grains reveal two crystallization environments leading to distinct grain morphologies euhedral and pseudo euhedral with different amounts of mineral inclusions titanite chlorite and augite aegirine both magnetite types crystallized during the same retrograde path at t 250 c under hydrothermal conditions shown by si and v oscillatory zoning and low ti content mghe ages derived from euhedral magnetite crystals range from 14 8 2 1 to 20 9 3 0 ma between zircon and apatite fission track zft and aft ages deduced from the literature pseudo euhedral grains display older ages interpreted as resulting from the implantation of he from u rich neighboring minerals such as titanite for the first time mghe data are inverted along with aft and zft data to refine the thermal history of an ophiolite related to their exhumation the mghe data suggest that the rocher blanc sample has undergone an exhumation in ductile conditions related to subduction dynamics and underplating processes followed by an exhumation reflecting denudation by erosion under brittle conditions this exhumation regime change is found to take place at 20 ma as opposed to a less constrained 26 ma cooling as would be inferred from the zft and aft ages alone considering published thermochronometric data along the western alps we propose that the exhumation is controlled by the upward indentation of the apulian mantle producing regional tilting and associated erosion with the present schistes lustres ophiolite example we demonstrate that the mghe method can be used as a low temperature thermochronometer closure temperature in the 200 250 c range that aids in unraveling the cooling history associated with exhumation of high pressure ultramafic and mafic rocks at the brittle to ductile transition

formation of recurring slope lineae on mars by rarefied gas triggered granular flows active dark flows known as recurring slope lineae have been observed on the warmest slopes of equatorial mars the morphology composition and seasonality of the lineae suggest a role of liquid water in their formation however internal and atmospheric sources of water appear to be insufficient to sustain the observed slope activity experimental evidence suggests that under the low atmospheric pressure at the surface of mars gas can flow upwards through porous martian soil due to thermal creep under surface regions heated by the sun and disturb small particles here we present numerical simulations to demonstrate that such a dry process involving the pumping of rarefied gas in the martian soil due to temperature contrasts can explain the formation of the recurring slope lineae in our simulations solar irradiation followed by shadow significantly reduces the angle of repose due to the resulting temporary temperature gradients over shaded terrain and leads to flow at intermediate slope angles the simulated flow locations are consistent with observed recurring slope lineae that initiate in rough and bouldered terrains with local shadows over the soil we suggest that this dry avalanche process can explain the formation of the recurring slope lineae on mars without requiring liquid water or co2 frost activity transient streaks on martian slopes have been attributed to liquid water simulations show that a dry avalanche process involving the flow of gas in the martian soil due to temperature contrasts can instead explain these recurring features

the tectonic implications of high pressure metamorphism in the western alps five mineralogical stages are recognized in the metamorphism of alpine ophiolites from one area in the zermatt saas zone constraints on p t values indicate an abrupt change at the metamorphic peak 550 600 c 17 20 kbar from a cold positive slope of 25 c kbar or less to cooling initially without discernible change of pressure correspondence of the prograde path to the p t trend of t in c 150 23 p in kbar shown by a variety of alpine units suggests that they all have p t histories for which an england richardson 1977 relaxation model also england thompson 1984 is inappropriate major tectonism at the peak pressure is indicated by association between minerals kyanite chloritoid lawsonite in eclogite vein growths fluid action and thrust surfaces the proposed metamorphic environment involves thrust repetition subcreting ophiolite to the over riding slab in a thermally mature subduction zone at a depth of approximately 70 km preservation is favoured when ocean closure leads to subduction of sialic crust so providing a combination of buoyant uplift with the continued refrigeration by underthrusting of cold surface materials

rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

recurring slope lineae in equatorial regions of mars the presence of liquid water is a requirement of habitability on a planet possible indicators of liquid surface water on mars include intermittent flow like features observed on sloping terrains these recurring slope lineae are narrow dark markings on steep slopes that appear and incrementally lengthen during warm seasons on low albedo surfaces the lineae fade in cooler seasons and recur over multiple mars years recurring slope lineae were initially reported to appear and lengthen at mid latitudes in the late southern spring and summer and are more common on equator facing slopes where and when the peak surface temperatures are higher here we report extensive activity of recurring slope lineae in equatorial regions of mars particularly in the deep canyons of valles marineris from analysis of data acquired by the mars reconnaissance orbiter we observe the lineae to be most active in seasons when the slopes often face the sun expected peak temperatures suggest that activity may not depend solely on temperature although the origin of the recurring slope lineae remains an open question our observations are consistent with intermittent flow of briny water such an origin suggests surprisingly abundant liquid water in some near surface equatorial regions of mars dark streaks that appear on the surface of mars during warm seasons have been observed at the mid latitudes and tentatively attributed to the flow of briny water imagery from the mars reconnaissance orbiter over multiple mars years suggests that these seasonally active features are also present in equatorial regions where liquid surface water is not expected

near surface environmentally forced changes in the ross ice shelf observed with ambient seismic noise author s chaput j aster r c mcgrath d baker m anthony r e gerstoft p bromirski p nyblade a stephen r a wiens d a das s b stevens l a

slope elevation and thermal inertia trends of martian recurring slope lineae initiation and termination points multiple possible processes occurring on coarse sandy slopes abstract recurring slope lineae rsl are dark linear features on the surface of mars that advance incrementally downslope fading and re growing annually numerous hypotheses have been proposed to explain rsl formation including wet models that involve liquid water or brines and dry mechanisms involving liquid free mass wasting or solid vapor phase changes that trigger granular flow in part hypotheses to explain rsl formation rely on observations of the physical characteristics of the slopes on which rsl are present to determine if rsl exhibit physical characteristics typical of one particular formation mechanism we examined the initiation and termination points of over 10 000 rsl across 16 confirmed rsl sites to determine their slope elevation orientation and thermal inertia rsl typically form within an envelope of high elevations at each mapping site are oriented equatorward and west and have hillslope thermal inertia values consistent with sand and gravel notably the rsl begin and end at slopes above within and below the angle of repose required to trigger and sustain dry granular flows sensitivity testing indicates that these slope values are not a result of pixel scale noise discontinuous rsl are observed at several study sites the slope and thermal inertia values of the rsl as well as these additional geomorphic observations of discontinuous dark features are not consistent with the characteristics of dry flows in which continuous dust avalanches begin within the dynamic angle of repose and terminate within the static angle of repose although rsl are found to be common on coarse sandy and rocky slopes not all sandy slopes in rsl regions that are close to the angle of repose contain rsl we conclude that mechanisms other than dry mass wasting are needed to explain all of the observed geomorphic characteristics of rsl source and termination points

hirise observations of recurring slope lineae rsl during southern summer on mars abstract recurring slope lineae rsl are active features on mars that might require flowing water most examples observed through 2011 formed on steep equator facing slopes in the southern mid latitudes they form and grow during warm seasons and fade and often completely disappear during colder seasons but recur over multiple mars years they are recognizable by their incremental growth relatively low albedo and downhill orientation we examined all images acquired by hirise during l s 250 10 slightly longer than southern summer l s 270 360 of mars years 30 31 03 2011 10 2011 and supplemented our results with data from previous studies to better understand the geologic context and characteristics of rsl we also confirmed candidate and likely sites from previous studies and discovered new rsl sites we report 13 confirmed rsl sites including the 7 in mcewen et al mcewen et al 2011 science 333 6043 740 743 the observed seasonality latitudinal and slope orientation preferences and themis brightness temperatures indicate that rsl require warm temperatures to form we conclude that rsl are a unique phenomenon on mars clearly distinct from other slope processes that occur at high latitudes associated with seasonal co 2 frost and episodic mass wasting on equatorial slopes however only 41 82 out of 200 of the sites that present apparently suitable conditions for rsl formation steep equator facing rocky slopes with bedrock exposure in the southern mid latitudes 28 60 s contain any candidate rsl with confirmed rsl present only in 7 13 sites of those locations significant variability in abundance size and exact location of rsl is also observed at most sites indicating additional controls such as availability of water or salts that might be playing a crucial role

incomplete blueschist re crytallization in high grade metamorphics from the sesia lanzo unit vasario sparone subunit western alps a case history of metastability abstract the vasario sparone vs subunit is part of the sesia lanzo unit western alps and represents a thin and discontinuous slice of pre alpine deep continental crust it is tectonically interposed between the eclogitic mica schists and the gneiss minuti subunits metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the vs subunit the vs rocks record a pre alpine high grade of metamorphism t 780 50 c p 6 kbar which is comparable to the metamorphism dated around 500 ma in the ivrea zone during the alpine evolution the vs rocks sustained an early blueschist stage t 300 400 c p 8 10 kbar of probable cretaceous age and a later low t low p stage of tertiary age the blueschist transformations are governed by slow reaction kinetics by the heterogeneous distribution of pervasive deformations and by syn metamorphic fluid phase in particular transient kyanite formed in the early stages of the blueschist re crystallization in response to a process controlled by fluid deficient conditions and by internal buffering of a h 2 o at low values

martian gullies in the southern mid latitudes of mars evidence for climate controlled formation of young fluvial features based upon local and global topography a new survey of mars orbiter camera moc narrow angle images of gullies in the 30 45 s latitude band includes their distribution morphology local topographic setting orientation elevation and slopes these new data show that gully formation is favored over a specific range of conditions elevation 5000 to 3000 m slope 10 and orientation 83 8 on pole facing slopes these data and the frequent occurrence of gullies on isolated topographic highs lead us to support the conclusion that climatic related processes of volatile accumulation and melting driven by orbital variations are the most likely candidate for processes responsible for the geologically recent formation of martian gullies

contrasted metamorphic evolutions in thrusted cover units of the brianconnais zone french alps a model for the conservation of hp lt metamorphic mineral assemblages the evolution of organic matter silicate and fluid phases in cover units of the three structural zones of the vanoise area allows one to distinguish differentp t cooling paths these paths depend on whether the cover has been transported western and northern vanoise or not southern vanoise all units first underwent a common high pressure low temperature hp lt metamorphic stage 300 c 6 kbar of blueschist type fe mg carpholite chloritoid facies the cover units transported on the extermal colder zone coal measure series metamorphosed in albite chlorite facies preserved their hp mineralogy fe mg carpholite lawsonite and organic matter content oils wet gases and kerogen while the unit which remained in contact with its more thermally conductive basement polymetamorphic now shows extensive greenschist facies overprinting breakdown of fe mg carpholite and lawsonite appearance of chlorite pyrophyllite chloritoid and clinozoisite absence of oils and wet gases these two tectonic evolutions might serve as a more general model for the preservation of hp lt mineral assemblages

yo yo subduction recorded by accessory minerals in the italian western alps high pressure metamorphic rocks form during subduction of earth s crust to mantle depths at convergent plate margins their exposure at the surface of earth today provides a record of the subduction zone process in general such metamorphic rocks record only a single cycle of subduction and exhumation yet tectonic models suggest that individual rock units should undergo multiple subduction exhumation cycles here we investigate the microstructure and chemical composition of metamorphic minerals in high pressure rocks exposed in the sesia zone in the italian western alps we find that the minerals white mica garnet allanite and zircon each exhibit multiple generations of mineral overgrowths in particular two generations of white mica with high silicon content indicative of formation at high pressure are separated by an overgrowth with low silicon content that formed during exhumation at low pressures furthermore the trace element signatures of distinct zones within zircon and allanite also reveal two episodes of high pressure metamorphism separated by a period of rapid exhumation we use uranium lead dating of zircon and allanite overgrowths to constrain the timing of this subduction exhumation subduction cycle to 79 65 myr ago we conclude that slices of the sesia zone crust experienced two cycles of burial to mantle depths in less than 20 million years the yo yo subduction occurred during oblique convergence between the african and european plates which involved a continental margin

high resolution thermal inertia derived from the thermal emission imaging system themis thermal model and applications 1 thermal inertia values at 100 m per pixel are determined using nighttime temperature data from the thermal emission imaging system themis on the mars odyssey spacecraft producing the highest resolution thermal inertia data set to date themis thermal inertia values have an overall accuracy of 20 a precision of 10 15 and are consistent with both thermal emission spectrometer orbital and miniature thermal emission spectrometer surface thermal inertia values this data set enables the improved quantification of fine scale surface details observed in high resolution visible images in the tharsis region surface textures and crater rims observed in visible images have no corresponding variation in the themis thermal inertia images indicating that the dust mantle is pervasive at themis scales and is a minimum of a few centimeters and up to 1 2 m thick the thermal inertia of bed form material indicates particle diameters expected for aeolian sediments and these materials are likely currently saltating variations in the thermal inertia within interior layered deposits in hebes chasma can be distinguished and the thermal inertia is too low to be consistent with bedrock or a lava flow thus a secondary emplacement of volcanic material or a volcanic ash deposit is a more likely method of formation higher resolution themis thermal inertia enables the identification of exposed bedrock on the martian surface in nili patera and ares vallis bedrock material corresponds to distinct compositional and morphologic surfaces indicating that a specific unit is exposed and is likely currently being kept free of unconsolidated material by aeolian processes

laboratory studies of perchlorate phase transitions support for metastable aqueous perchlorate solutions on mars abstract perchlorate salts recently discovered on mars are known to readily absorb water vapor from the atmosphere and deliquesce into the aqueous phase at room temperature here we study the deliquescence crystalline solid to liquid transition and efflorescence liquid to crystalline solid transition of perchlorate salts at low temperatures relevant to mars a raman microscope and environmental cell were used to determine the deliquescence relative humidity drh and efflorescence relative humidity erh of naclo 4 and mg clo 4 2 as a function of temperature and hydration state we find that the deliquescence of anhydrous naclo 4 is only slightly dependent on temperature and occurs at 38 rh the drh of naclo 4 h 2 o increases with decreasing temperature from 51 at 273 k to 64 at 228 k the drh of mg clo 4 2 6h 2 o also increases with decreasing temperature from 42 at 273 k to 64 at 223 k the efflorescence of both naclo 4 and mg clo 4 2 salt solutions occurs at a lower rh than deliquescence due to the kinetic inhibition of crystallization for all temperatures studied the erh values of naclo 4 and mg clo 4 2 are 13 and 19 respectively these results indicate perchlorate salts can exist as metastable supersaturated solutions over a wide range of rh and temperature conditions summer diurnal temperature and relative humidity cycles at low latitudes on mars could allow the surface salts to be aqueous for several hours per day

context camera investigation on board the mars reconnaissance orbiter 1 the context camera ctx on the mars reconnaissance orbiter mro is a facility instrument i e government furnished equipment operated by a science team not responsible for design and fabrication designed built and operated by malin space science systems and the mro mars color imager team marci ctx will 1 provide context images for data acquired by other mro instruments 2 observe features of interest to nasa s mars exploration program e g candidate landing sites and 3 conduct a scientific investigation led by the marci team of geologic geomorphic and meteorological processes on mars ctx consists of a digital electronics assembly a 350 mm f 3 25 schmidt type telescope of catadioptric optical design with a 5 7 field of view providing a 30 km wide swath from 290 km altitude and a 5000 element ccd with a band pass of 500 700 nm and 7 m pixels giving 6 m pixel spatial resolution from mro s nearly circular nearly polar mapping orbit raw data are transferred to the mro spacecraft flight computer for processing e g data compression before transmission to earth the ground data system and operations are based on 9 years of mars global surveyor mars orbiter camera on orbit experience ctx has been allocated 12 of the total mro data return or about 3 terabits for the nominal mission this data volume would cover 9 of mars at 6 m pixel but overlapping images for stereo mosaics and observation of changes and meteorological events will reduce this area ctx acquired its first instrument checkout images of mars on 24 march 2006

relative geobarometry from high pressure rocks of quartzofeldspathic composition from the sesia zone western alps italy in the sesia zone of the western alps italy early alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high pressure assemblage of quartz na pyroxene na amphibole paragonite phengite zoisite garnet magnetite sphene and fe sulphide the qfs assemblage over an area 150 km2 relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions mineralogical discontinuities do not occur in the qfs assemblage of this region increases in the jadeite content of the pyroxenes xjd 0 48 to xjd 0 93 and in the glaucophane content of the amphiboles xgl 0 89 to xgl 0 96 occur from the southwest to the northeast of the region studied analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet zoned pyroxenes and garnets together with the regional trends in mineral chemistries suggest that the evolution of the qfs assemblage with increasing pressure may be modelled by pressure sensitive continuous reactions in which amphibole zoisite and the more jadeitic pyroxene constitute the high pressure assemblage chemographic constraints permit the positioning in pressure temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages the low dp dt slope 20 bars c of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz in addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the naalca 1mg 1 exchange or the mgca 1 exchange are calculated comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied with these barometric observations it is possible to show that the p t path which the sesia body travelled towards the final recorded state was one of increasing pressure other blueschist and eclogite occurrences from syros and sifnos which contain rocks of quartzofeldspathic composition are also examined

morphometry of small recent impact craters on mars size and terrain dependence short term modification most recent studies of crater morphometry on mars have addressed large craters d 5 km using elevation models derived from laser altimetry in the present work we examine a global population of small 25 m d 5 km relatively well preserved simple impact craters using hirise stereo derived elevation models we find that scaling laws from prior studies of large simple craters generally overestimate the depth and volume at small diameters we show that crater rim curvature exhibits a strong diameter dependence that is well described by scaling laws for d 1 km above this diameter upper rim slopes begin to exceed typical repose angles and crater rims sharpen significantly this transition is likely the result of gravity driven collapse of the upper cavity walls during crater formation or short term modification in addition we identify a tendency for small craters d 500m to be more conical than large craters and we show that the average cavity cross section is well described by a power law with exponent 1 75 neither conical nor paraboloidal we also conduct a statistical comparison of crater subpopulations to illuminate trends with increasing modification and target strength these results have important implications for describing the initial condition of simple crater shape as a function of diameter and geological setting and for understanding how impact craters are modified on the martian surface over time

thickening history of the western alps a model of evolution for the western segment of the alpine chain is presented available data concerning the structural metamorphic and geochronological evolution in the majorunits involved in the thickening processes are takeninto account io constrain the model the successive stages of the collisional history are correlated with the evolution of thermal structure the african paleomargin was deformed at the onset of collision just after the abduction of the oceanic crust after these initial stages progressive stacking affected more and more external domains of the european plate lastly the deformation of the entire pile is linked io the thermal reequilibration of the considered thickened crust the alpine chain is a good example of a collision zone where the interaction of thermal and mechanical effects can be treated quantitatively

gravity modelling along the ecors crop vertical seismic reflection profile through the western alps abstract new gravity data from 500 stations and results from all previous surveys on the western alps are used to compile a detailed profile along the ecors crop vertical seismic profile the bouguer anomaly consists of two approximately two dimensional regional features namely a wide negative anomaly occurring on the central part of the belt and the pronounced positive anomaly of the geophysical ivrea zone at the border between the alps and the po plain the results of a wide angle seismic campaign and the preliminary line drawing from the vertical seismic have been the main constraints for the modelling of the anomaly two models are proposed their common feature is a piling up of the crust upper mantle units in the penninic zone of maximum crustal thickness as well as in the ivrea zone the contribution of the apulian lower crust to the positive anomaly is greater than formerly suggested several hypotheses are put forward for the zones where reasonable seismic information is available as an illustration of the uncertainties surrounding their geological interpretation the density models are consistent with the hypothesis of lithospheric accretionary processes for the collision between the european and apulian plates

contact zone interaction of metabasites with metapelites amphibolite facies mineral assemblages chemical profiles and their origin the puolankajarvi formation finland abstract the amphibolite facies iron rich metapelites garnet staurolite mica schists of the early proterozoic puolankajarvi formation are intercalated with metamorphosed basic rocks which vary from iron rich amphibolites to magnesium rich layered sills and serpentinites garnet cummingtonite hornblende assemblages are abundant at the contacts of the metapelites with amphibolites while garnet chlorite garnet gedrite chlorite garnet gedrite cummingtonite chlorite and chlorite cummingtonite hornblende assemblages occur at their contacts with the layered magnesium rich sill the bulk composition profiles of these contacts normalised with respect to chromium and or aluminium show conspicuous enrichment in fe s ga and co and depletion in si k ba and rb some elements e g mg mn ni p zr and hf are depleted or enriched in only small amounts while others e g ca na sr and the rare earth elements may show both enrichment and depletion in the course of a profile although early infiltration during the hydration of metabasites and tectonic reworking may have produced some interaction of the basites with the pelites the major metasomatic event is considered to have bile elements the composition of the metapelites for example approached that of magnesium rich ultramafites through potassium depletion while the ultramafites became markedly enriched in iron to conserve the volume silica was depleted from the contact which underwent iron enrichment

evaluation of wet and dry recurring slope lineae rsl formation mechanisms based on quantitative mapping of rsl in garni crater valles marineris mars abstract recurring slope lineae rsl are narrow 0 5 5 m low albedo features that incrementally lengthen down steep slopes during warm seasons fade in colder seasons and recur each mars year to reduce the effort involved in manually mapping and analyzing each rsl in garni crater we developed mapping and automated analysis of rsl maarsl to analyze a set of orthorectified high resolution imaging science experiment hirise images and a digital elevation map maarsl along with manual mapping allowed us to detect rsl compute descriptive statistics and characterize changes over time we mapped 2910 rsl in 22 orthoimages from mars year my 31 solar longitude ls 133 0 to my32 ls 323 7 the maarsl results confirmed that rsl lengthening and fading occur concurrently on slopes with the same orientation and many times within some individual rsl slope angles of rsl and a slope slump show that some rsl start stop and have mean slope angles that are below the angle of repose our analysis shows that rsl are actively lengthening on at least one slope facing direction in all hirise observations of the crater we also found that ne n and nw facing rsl in garni crater lengthened during times of increasing shortwave insolation while s and sw facing rsl lengthened during increasing and decreasing shortwave insolation a non orthorectified hirise image acquired shortly after the my34 dust storm and shows rsl on every slope facing direction which is anomalous with respect to observations prior to the dust storm we find that dust removal and deposition could explain the darkening and fading respectively of rsl and could also explain the apparent lack of material being transported observations of rsl lengthening and fading occur concurrently on slopes could suggest overprinting of dry granular flows dry flows could also explain the significant lengthening activity of every slope facing direction after a fresh layer of dust was deposited via the my34 dust storm alternatively briny shallow subsurface flows are consistent with observations of rsl on slopes below the angle of repose and those that exhibit concurrent lengthening and fading however the most significant problem with briny rsl flows is accessing a source of briny water and removing excess salt from the regolith

high pressure metamorphism in the nappes of the valle dell orco traverse western alps collisional belt a detailed field and petrological study of rocks from nappes cut by the valle dell orco italian western alps in particular the sesia lanzo composite unit has revealed geological and metamorphic histories which started in pre alpine times and lasted up to the alpine subduction collisional processes during these processes the nappes sustained an early high p low t stage and a later low p greenschist facies stage but followed partly distinctive p t time trajectories this paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction underthrust to the later exhumation stage the metamorphic crystallization is often governed by incomplete and or local equilibrium and the pervasive syn metamorphic deformation and the composition of the syn metamorphic fluid phase if present have exerted an effective local control on reaction kinetics

the importance of serpentinite mylonites for subduction and exhumation of oceanic crust abstract in the ultramafic erro tobbio unit voltri massif western alps a set of overprinting structures in serpentinite mylonites is related to alpine subduction to about 80 km depth and to subsequent exhumation antigorite mylonites are cut by en echelon olivine veins which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite the transition from olivine free to olivine bearing structures indicates recrystallization during prograde metamorphism all structures display the same top to the nw kinematics providing evidence for a continuous non coaxial deformation the serpentinite mylonites surround km scale bodies of pre alpine peridotite which show only minor alpine overprint this indicates that during subduction related deformation recrystallization and fluid flow were strongly localized within serpentinite mylonites olivine bearing discontinuous shear planes with top to the se sense of movement crosscut the prograde structures the inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab i e from the subducted slab to the upper plate during accretion thus the shear sense inversion marks the change from burial to exhumation of the serpentinites the low density of antigorite serpentinites 2 75 g cm3 causes strong buoyancy thus providing a mechanism for the exhumation of deeply subducted rocks it is suggested that serpentinites may act as carriers for the uprise of eclogite bodies which have higher densities than the peridotitic upper mantle

the water content of recurring slope lineae on mars observations of recurring slope lineae rsl from the high resolution imaging science experiment have been interpreted as present day seasonally variable liquid water flows however orbital spectroscopy has not confirmed the presence of liquid h2o only hydrated salts thermal emission imaging system themis temperature data and a numerical heat transfer model definitively constrain the amount of water associated with rsl surface temperature differences between rsl bearing and dry rsl free terrains are consistent with no water associated with rsl and based on measurement uncertainties limit the water content of rsl to at most 0 5 3 wt in addition distinct high thermal inertia regolith signatures expected with crust forming evaporitic salt deposits from cyclical briny water flows are not observed indicating low water salinity if any and or low enough volumes to prevent their formation alternatively observed salts may be preexisting in soils at low abundances i e near or below detection limits and largely immobile these rsl rich surfaces experience 100 k diurnal temperature oscillations possible freeze thaw cycles and or complete evaporation on time scales that challenge their habitability potential the unique surface temperature measurements provided by themis are consistent with a dry rsl hypothesis or at least significantly limit the water content of martian rsl

firn air depletion as a precursor of antarctic ice shelf collapse since the 1970s the sudden rapid collapse of 20 of ice shelves on the antarctic peninsula has led to large scale thinning and acceleration of its tributary glaciers the leading hypothesis for the collapse of most of these ice shelves is the process of hydrofracturing whereby a water filled crevasse is opened by the hydrostatic pressure acting at the crevasse tip this process has been linked to observed atmospheric warming through the increased supply of meltwater importantly the low density firn layer near the ice shelf surface providing a porous medium in which meltwater can percolate and refreeze has to be filled in with refrozen meltwater first before hydrofracturing can occur at all here we build upon this notion of firn air depletion as a precursor of ice shelf collapse by using a firn model to show that pore space was depleted in the firn layer on former ice shelves which enabled their collapse due to hydrofracturing two climate scenario runs with the same model indicate that during the 21st century most antarctic peninsula ice shelves and some minor ice shelves elsewhere are more likely to become susceptible to collapse following firn air depletion if warming continues into the 22nd century similar depletion will become widespread on ice shelves around east antarctica our model further suggests that a projected increase in snowfall will protect the ross and filchner ronne ice shelves from hydrofracturing in the coming two centuries

a tem study of disequilibrium plagioclase breakdown at high pressure the role of infiltrating fluid high pressure metamorphism 600 c 20 kbar of the allalin gabbro western alps resulted in the breakdown of plagioclase an63 to fine grained zoisite jadeite kyanite and quartz in rare cases this reaction failed to reach completion the resulting textures of partial reaction have been studied by transmission and analytical electron microscopy in localised regions of a plagioclase crystal where the extent of reaction is 10 only zoisite developed and the orientation relationship left 100 right text zo left 101 right text p1 1 and left 012 right text zo left 010 right text p1 1 is frequently present in regions where 10 50 of plagioclase has transformed the reaction plagioclase h2o zoisite kyanite quartz nasica 1al 1 pl has occurred the systematic orientation relationship between plagioclase and zoisite is absent at 50 transformation complete breakdown of plagioclase occurred in localized micron scale domains by the reaction plagioclase h2o zoisite jadeite kyanite quartz and the reaction products are variably orientated with respect to each other incomplete reaction together with the concentration of reaction products around cracks in original plagioclase grains suggests that extent of reaction was controlled primarily by the availability of h2o the textural observations are interpreted in terms of two possible disequilibrium reaction models 1 plagioclase persists metastably with its original igneous composition to a pressure 17 kbar at 600 c reaction to the equilibrium assemblage then develops adjacent to cracks in response to the presence of aqueous fluid at intracrystalline sites only partial reaction occurs because jadeite and sometimes kyanite and quartz fail to nucleate for kinetic reasons 2 localized regions of a plagioclase crystal partially equilibrate at several stages during the increase of pressure 9 17 kbar at 600 c possibly due to discrete episodes of fluid infiltration in both these models the extent of reaction may be limited by nasi caal interdiffusion in plagioclase

chemical microstructural changes in eclogite facies shear zones monviso western alps north italy as indicators of strain history and the mechanism and scale of mass transfer abstract variations of eclogite facies mineral chemistry are related to the textural characteristics of shear zones in fe ti metagabbros from the monviso meta ophiolitic complex in the italian western alps three different kinds of microstructural domains are described 1 small lenses of low strain rocks with preserved igneous textures 2 larger high strain domains where deformation caused pervasive mylonitic foliation s1 and a stretching mineral lineation l1 and 3 two types of eclogite facies veins the latter can be subdivided into veins composed of only na pyroxenes filling small tension gashes and dilatation fractures and veins composed of a na pyroxene garnet rutile apatite assemblage occupying large fractures that randomly cross cut the mylonites in low strain domains and mylonites the na pyroxenes are omphacites and the garnets are almandine rich in some mylonite layers containing phengite and blue amphibole crosscutting s1 chloromelanite iron rich omphacites exsolved from omphacite in type 1 veins the pyroxene is either omphacite or concentric and sector zoned omphacite chloromelanite in type 2 veins pyroxenes are omphacite and or concentric and sector zoned omphacite chloromelanite and or zoned kosmochlor chromium rich omphacite omphacites associated garnets are richer in pyrope component than those from low and high strain domains possible origins of both sector and concentric zoning and exsolution are discussed it is emphasized that a miscibility gap between the omphacite and acmite end members partly controlled the omphacite chloromelanite distribution we define a chemical vector coat constrains the strain history in eclogite facies rocks and give indications of the scale and mechanism of mass transfer involved during foliation development and fracturing p t conditions are calculated for each microstructural domain the garnet pyroxene thermometer yields scattered temperature values these values relate to the dependence of temperature estimates on the amount of fe3 in the pyroxene and possibly on non ideal substitution in omphacite acmite solid solution

multiple metamorphic stages within an eclogite facies terrane sesia zone western alps revealed by th u pb petrochronology convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel studies of high pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to midcrustal levels but the details of such dynamics are controversial to understand the dynamics of a subduction channel we report the results of a petrochronological study from the central sesia zone a key element of the internalwestern alps this comprises two polymetamorphic basement complexes eclogitic micaschist complex and gneiss minuti complex and a thin dismembered cover sequence scalaro unit associated with pre alpine metagabbros and metasediments bonze unit structurally controlled samples from three of these units eclogitic micaschist complex and scalaro bonze units yield unequivocal petrological and geochronological evidence of two distinct high pressure stages ages u th pb of growth zones in accessory allanite and zircon combined with inclusion and textural relationships can be tied to the multi stage evolution of single samples two independent tectono metamorphic slices showing a coherent metamorphic evolution during a given time interval have been recognized the fondo slice which includes scalaro and bonze rocks and the druer slice belonging to the eclogitic micaschist complex the new data indicate separate stages of deformation at eclogite facies conditions for each recognized independent kilometer sized tectono metamorphic slice between 85 and 60 ma with evidence of intermittent decompression p 0 5 gpa within only the fondo slice the evolution path of the druer slice indicates a different p t time evolution with prolonged eclogite facies metamorphism between 85 and 75ma our approach combining structural petrological and geochronological techniques yields field based constraints on the duration and rates of dynamics within a subduction channel

crustal reworking and hydration insights from element zoning and oxygen isotopes of garnet in high pressure rocks sesia zone western alps subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere a better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial in this study major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the sesia zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin garnet shows a variety of textures requiring dissolution precipitation processes in presence of fluids in polycyclic metasediments garnet preserves a partly resorbed core related to pre alpine high temperature low pressure metamorphism and one or multiple rim generations associated with alpine subduction metamorphism in eclogites garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim in metasediments pre alpine garnet relics show 18o values up to 5 3 higher than the alpine rims while no significant variation is observed among different alpine garnet generations within each sample this suggests that an extensive re equilibration with an externally derived fluid of distinct lower 18o occurred before or in correspondence to the first alpine garnet growth while subsequent influxes of fluid had 18o close to equilibrium the observed shift in garnet 18o is attributed to a possible combination of 1 interaction with sea water derived fluids during pre alpine crustal extension and 2 fluids from dehydration reactions occurring during subduction of previously hydrated rocks such as the serpentinised lithospheric mantle or hydrated portions of the basement

tectonic implications of lawsonite formation in the sesia zone western alps lawsonite of early alpine age formed in different tectonic sub units of the sesia zone western italian alps in the southern part of the eclogitic micaschists subunit em lawsonite grew with quartz jadeite during the early alpine retrograde path which just post dates the eclogite thermal peak the early retrograde path of the southern em occurred at decreasing temperature and at p t conditions comparable to those of the metamorphic peak in some blueschist facies thrust sheets which separate the em from the southern alps such a post eclogitic cooling path may have been induced by conductive cooling linked to migration of the underthrust surfaces and to stacking with colder nappes like the other sesia subunits or the piemonte metaophiolites stacking and thermal interaction between the different units probably occurred at depth during the early stages of exhumation which followed subduction underthrusting of early mid cretaceous age during the paleogene continental collision between the european and african plates determined further underthrusts of more westerly units these units underwent a high pressure very low temperature recrystallization meso alpine event characterized by the growth of lawsonite at the peak metamorphic conditions consequently propagation of the underthrust surfaces may represent an effective mechanism to prevent the rise of the isotherms and to favor post peak cooling paths in some units of the western alps this effect is more obvious in the structurally upper units which were first brought to deep crustal levels in the early alpine imbricated subduction zone

geologic context of recurring slope lineae in melas and coprates chasmata mars one of the major mars discoveries of recent years is the existence of recurring slope lineae rsl which suggests that liquid water occurs on or near the surface of mars today these dark and narrow features emerge from steep rocky exposures and incrementally grow fade and reform on a seasonal basis and are detected in images from the high resolution imaging science experiment camera rsl are known to occur at scattered midlatitude and equatorial sites with little spatial connection to one another one major exception is the steep low albedo slopes of melas and coprates chasmata in valles marineris where rsl are detected among diverse geologic surfaces e g bedrock and talus and landforms e g inselbergs and landslides new images show topographic changes including sediment deposition on active rsl slopes midwall locations in coprates and melas appear to have more areally extensively abundant rsl and related fans as compared with other rsl sites found on mars water budget estimates for regional rsl are on the order of 105 to 106 m3 of fluid for depths of 10 to 100 mm and suggest that a significant amount of near surface water might be present many rsl are concentrated near local topographic highs such as ridge crests or peaks which is challenging to explain via groundwater or ice without a recharge mechanism collectively results provide additional support for the notion that significant amounts of near surface water can be found on mars today and suggest that a widespread mechanism possibly related to the atmosphere is recharging rsl sources

a deep groundwater origin for recurring slope lineae on mars the recurring slope lineae on mars have been hypothesized to originate from snow melting deliquescence dry flow or shallow groundwater except for the dry flow origin these hypotheses imply the presence of surficial or near surface volatiles placing the exploration and characterization of potential habitable environments within the reach of existing technology here we present observations from the high resolution imaging science experiment heat flow modelling and terrestrial analogues which indicate that the source of recurring slope lineae could be natural discharge along geological structures from briny aquifers within the cryosphere at depths of approximately 750 m spatial correlation between recurring slope lineae source regions and multi scale fractures such as joints and faults in the southern mid latitudes and in valles marineris suggests that recurring slope lineae preferably emanate from tectonic and impact related fractures we suggest that deep groundwater occasionally surfaces on mars in present day conditions observations and heat flow modelling suggest that briny groundwater surfacing from fractures forms recurring slope lineae on mars

an aeolian grainflow model for martian recurring slope lineae abstract recurring slope lineae rsl on mars have been enigmatic since their discovery their behavior resembles a seeping liquid but sources of water remain puzzling this work demonstrates that the properties of rsl are consistent with observed behaviors of martian and terrestrial aeolian processes specifically rsl are well explained as flows of sand that remove a thin coating of dust observed rsl properties are supportive of or consistent with this model which requires no liquid water or other exotic processes but rather indicates seasonal aeolian behavior these settings and behaviors resemble features observed by rovers and also explain the occurrence of many slope lineae on mars that do not meet the strict definition of rsl this indicates that rsl can be explained simply as aeolian features other processes may add complexities just as they could modify the behavior of any sand dune

fluid migration above a subducted slab constraints on amount pathways and major element mobility from partially overprinted eclogite facies rocks sesia zone western alps

numerical simulations of an ocean continent convergent system influence of subduction geometry and mantle wedge hydration on crustal recycling the effects of the hydration mechanism on continental crust recycling are analyzed through a 2 d finite element thermomechanical model oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method hydration is accomplished by lawsonite and serpentine breakdown topography is treated as a free surface subduction rates of 1 3 5 7 5 and 10 cm yr slab angles of 30 45 and 60 and a mantle rheology represented by dry dunite and dry olivine flow laws have been taken into account during successive numerical experiments model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material the larger the viscosity contrast between hydrated and dry mantle the larger the percentage of recycled material into the mantle wedge slab dip variation has a moderate impact on the recycling metamorphic evolution of recycled material is influenced by subduction style tpmax generally representative of eclogite facies conditions is sensitive to changes in slab dip a direct relationship between subduction rate and exhumation rate results for different slab dips this relationship does not depend on the used mantle flow law thermal regimes predicted by different numerical models are compared to pt paths followed by continental crustal slices involved in ancient and recent subduction zones making ablative subduction a suitable precollisional mechanism for burial and exhumation of continental crust

new observations of martian southern mid latitude recurring slope lineae rsl imply formation by freshwater subsurface flows abstract southern mid latitude sml recurring slope lineae rsl are narrow 0 5 5 m dark albedo features that emanate from bedrock and incrementally lengthen down steep slopes that preferentially face the equator we observe that sml rsl begin lengthening prior to southern summer at a solar longitude ls of 245 11 when mars global surveyor thermal emission spectrometer tes derived near maximum surface temperatures are 296 5 k and mars reconnaissance orbiter mars climate sounder mcs and mars odyssey thermal emission imaging system themis derived mid afternoon surface temperatures are 273 k sml rsl continue to lengthen for 104 38 sols with an average near maximum surface temperature of 298 5 k the sml rsl then stop lengthening at ls 314 12 when mid afternoon surface temperatures drop below 273 k they remain dark for another 116 41 sols until ls 16 14 as surface temperatures continue to fall although the rsl recharge mechanism remains unknown our observation that the vast majority of rsl lengthen only when mid afternoon surface temperatures are 273 k supports the hypothesis that they are formed by shallow subsurface liquid water flows without significant freezing point depression the number and length of rsl at multiple sites increased dramatically following the mars year 28 globe encircling dust storm we interpret this increase to be due to warmer subsurface temperatures created by a dust laden greenhouse effect that may be unique to the southern mid latitudes near ls 270 therefore sml rsl flow is quite sensitive to ground temperature and may only occur under favorable orbital parameters when mean insolation during the rsl lengthening season is above that of the current southern mid latitude mean insolation value this value is currently at a peak that has not been attained for the last 100 ka meanwhile the rsl poor northern mid latitude mean insolation is near a minimum and has a value 27 lower than the southern value if sml rsl are indeed formed by shallow subsurface freshwater flows then they may be some of the best locations on mars to explore for extant life

hydrological characteristics of recurrent slope lineae on mars evidence for liquid flow through regolith and comparisons with antarctic terrestrial analogs abstract on the basis of their morphology and seasonal thermal characteristics recurrent slope lineae rsl on mars have been inferred to be a possible result of the flow of a liquid likely a saline brine through the upper portions of the martian regolith in this note we analyze repeat hirise imaging of rsl to show that the downslope growth rate of recurrent slope lineae is well fit by an elementary groundwater flow model that also describes the downslope propagation of antarctic water tracks terrestrial analogs to rsl and that the apparent permeability of rsl bearing slopes is consistent with the observed sandy regolith substrate

a thermal tectonic model for high pressure metamorphism and deformation in the sesia zone western alps the sesia zone western alps is a slice of austroalpine continental crust that was once metamorphosed at t 500 560 c and p 14 16 kbar at 130 100 ma b p pervasive blueschist eclogite facies assemblages record this event and a later greenschist facies overprint is only well developed in the nw part of the zone on the basis of thermal calculations it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift erosion an alternative model is proposed in which the sesia zone was emplaced by underthrusting of the austroalpine continental margin prior to subduction of oceanic lithosphere of the piemonte basin subduction of relatively cool oceanic lithosphere below the underthrust sesia zone then continued for a period of time e g 20 ma thus suppressing thermal recovery of this latter unit exhumation of sesia zone rocks began during this subduction episode by uplift on the insubric line erosion and by underplating of oceanic

ice shelf structure derived from dispersion curve analysis of ambient seismic noise ross ice shelf antarctica author posting the author s 2016 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 205 2016 785 795 doi 10 1093 gji ggw036

present day impact cratering rate and contemporary gully activity on mars the mars global surveyor mars orbiter camera has acquired data that establish the present day impact cratering rate and document new deposits formed by downslope movement of material in mid latitude gullies on mars twenty impacts created craters 2 to 150 meters in diameter within an area of 21 5 106 square kilometers between may 1999 and march 2006 the values predicted by models that scale the lunar cratering rate to mars are close to the observed rate implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating the new gully deposits formed since august 1999 are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water relatively long extended digitate distal and marginal branches diversion around obstacles and low relief the observations suggest that liquid water flowed on the surface of mars during the past decade

the formation and evolution of youthful gullies on mars gullies as the late stage phase of mars most recent ice age gullies are extremely young erosional depositional systems on mars that have been carved by an agent that was likely to have been comprised in part by liquid water malin m c edgett k s 2000 evidence for recent groundwater seepage and surface runoff on mars science 288 2330 2335 mcewen a s et al 2007 a closer look at water related geologic activity on mars science 317 1706 1709 the strong latitude and orientation dependencies that have been documented for gullies require 1 a volatile near the surface and 2 that insolation is an important factor for forming gullies these constraints have led to two categories of interpretations for the source of the volatiles 1 liquid water at depth beneath the melting isotherm that erupts suddenly groundwater and 2 ice at the surface or within the uppermost layer of soil that melts during optimal insolation conditions surface near surface melting in this contribution we synthesize global hemispheric regional and local studies of gullies across mars and outline the criteria that must be met by any successful explanation for the formation of gullies we further document trends in both hemispheres that emphasize the importance of top down melting of recent ice rich deposits and the cold trapping of atmospherically derived h 2o frost snow as important components in the formation of gullies this provides context for the incorporation of high resolution multi spectral and hyper spectral data from the mars reconnaissance orbiter that show that 1 cold trapping of seasonal h 2o frost occurs at the alcove channel level on contemporary mars 2 gullies are episodically active systems 3 gullies preferentially form in the presence of deposits plausibly interpreted as remnants of the late amazonian emplacement of ice rich material and 4 gully channels frequently emanate from the crest of alcoves instead of the base showing that alcove generation is not necessarily a product of undermining and collapse at these locations a prediction of the groundwater model we interpret these various lines of evidence to mean that the majority of gullies on mars are explained by the episodic melting of atmospherically emplaced snow ice under spin axis orbital conditions characteristic of the last several myr

formation of recurring slope lineae by liquid brines on present day mars 1 recurring slope lineae rsl are small scale seasonal flow features identified by mars reconnaissance orbiter that present several interesting characteristics such as an albedo contrast seasonal dependence and a strong preference for equator facing slopes all of these characteristics strongly suggest a thermally driven mechanism such as a liquid triggered or dominated flow here we investigate the possibility that these features are formed by melting of brines of various compositions via a combination of thermodynamic and kinetic numerical models results suggest that a solution with a freezing temperature of 223 k can best reproduce the observed seasonality relatively high surface evaporation rates at the rsl locations make the flows quickly disappear over a single season our model reproduces well the seasonality of rsl and can explain the preference for equator facing slopes suggesting that brine flows and therefore liquids are possible on a small time and space scale today on mars however if the rsl are indeed formed by brines it may indicate that a recharge mechanism is active in order to maintain a source of brine over even short geological timescales which would have important implications for the martian water cycle

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

uplift and exhumation of high pressure metamorphic terrains the example of the cycladic blueschist belt aegean sea the cycladic blueschists emerged from a depth of more than 50 km during the tertiary these rocks are now covered only by a thin fraction of the synmetamorphic overburden upper plate remnants of the upper plate include low p crystalline rocks and sediments which have been juxtaposed onto the lower plate blueschists in several stages since the early miocene the low angle contacts operated as normal faults which removed the overburden from above the cycladic blueschists detailed investigations on tinos island indicate that the fractured blocks of the upper plate lying above a low angle normal fault did not rotate this shows that the faults initiated as rather gently dipping fractures similar structures were also reported from other blueschist terrains kinematic analysis of such flat lying faults implies that the synmetamorphic overburden should be mostly located several hundred kilometers away from the present exposure of the blueschists considering that the cycladic blueschist belt exposes successively shallower units in the west and that rocks of the upper plate have an anatolian nature we suggest that these blueschists were dragged from beneath turkey we recognize that the blueschist unit has undergone further thrusting in the final stage of the exhumation as have its counterparts elsewhere in the hellenides the emergence of the blueschists in the tertiary was concomitant with continental collision and thrusting in the hellenides the operation of low angle normal faults with large extensional displacements occurred within this context of collision tectonics this extension occurred parallel to the strike of the convergent orogen

detrital high pressure low temperature minerals in a late turonian flysch sequence of the eastern alps western austria implications for early alpine tectonics along the northern margin of the eastern alps structurally intercalated between the main austroalpine thrust and the pennine rhenodanubic flysch nappe a thin tectonically complex zone occurs that comprises a series of tectonic melanges and imbricates of oceanic south pennine and distal continental margin austroalpine origin this zone is the lateral continuation of the ophiolite bearing arosa zone of the swiss alps we refer to it as the walsertal zone in a broken flysch formation of the walsertal zone turbiditic sandstones yield detrital grains of the high p low t metamorphic minerals lawsonite and glaucophane crossite ferroglaucophane the sandstones are dated as late turonian to earliest coniacian about 87 to 89 ma this age falls into the time span of radiometric ages of subduction related high p low t metamorphism along the south pennine austroalpine convergent margin and of coeval cooling ages of medium p medium t metamorphism in the middle austroalpine nappes if the high p low t minerals are related to mid cretaceous high p low t metamorphism as we think the high p metamorphic rocks must have been brought up from about 20 km depth within about 10 m y or even less as isostatic uplift alone would have been too slow alternative tectonic mechanisms such as corner flow must be responsible for rapid uplift and erosion in a fore arc environment

topographic measurements of slope streaks on mars abstract slope streaks are enigmatic actively forming albedo features occurring on slopes in high albedo low thermal inertia dust rich equatorial regions on mars they are a specifically martian phenomenon with no direct analogs on the earth their morphology suggests that the streaks are initiated at their upslope tips and propagate down to their termini however the physical mechanism of their formation is uncertain we performed a large series of measurements of slopes associated with slope streaks using stereo pairs of high resolution orbital images obtained by hirise camera and generated several digital elevation models for selected streaks we found that 1 slopes at the upslope streak tips range widely however there is a strong indication that streaks can be initiated only on slopes steeper than 20 2 slopes of the streak termini show an even wider range with some streaks terminating at slopes as steep as their tips while others propagate all the way down to horizontal surfaces 3 the streaks can propagate stably for long many hundreds of meters distances and can turn following the topographic gradient on 10 15 slopes 4 no uphill propagation of streaks is detected over baselines of tens of meters 5 the slope streaks often propagate over 1 2 m high obstacles and can climb 1 2 m uphill over short meters distances we used these findings to assess the viability of two classes of hypotheses about slope streak formation mechanisms proposed earlier 1 dry some kind of run away avalanche like dry granular flow and 2 wet some kind of run away propagation of a front of percolating brines in the shallow subsurface no specific observation unambiguously proves or rejects either of the two mechanisms several of our findings are readily explained by the dry mechanism and cannot be easily explained with any kind of wet mechanism while other findings are closely consistent with the wet mechanism and are difficult to reconcile with the dry mechanism this situation might be explained by equifinality meaning that the streaks despite their similarity are formed by different physical processes alternatively some modifications to either the dry or wet mechanisms may be able to provide a coherent explanation it is also possible that new ideas are needed to understand the processes involved

present day formation and seasonal evolution of linear dune gullies on mars linear dune gullies are a sub type of martian gullies as their name suggests they only occur on sandy substrates and comprise very long compared to their width straight or sinuous channels with relatively small source areas and almost non existent visible deposits linear dune gullies have never been observed on terrestrial dunes and their formation process on mars is unclear here we present the results of the first systematic survey of these features in mars southern hemisphere and an in depth study of six dunefields where repeat imaging allows us to monitor the changes in these gullies over time this study was undertaken with hirise images at 25 30 cm pix and 1 m pix elevation data derived from hirise stereo images we find the latitudinal distribution and orientation of linear dune gullies broadly consistent with the general population of martian gullies they occur predominantly between 36 3 s and 54 3 s and occasionally between 64 6 s and 70 4 s they are generally oriented toward ssw at bearings between 150 and 260 we find that these gullies are extremely active over the most recent 5 martian years of images activity comprises 1 appearance of new channels 2 lengthening of existing channels 3 complete or partial reactivation and 4 disappearance of gullies we find that gully channels lengthen by 100 m per year the intense activity and the progressive disappearance of linear dune gullies argues against the hypothesis that these are remnant morphologies left over from previous periods of high obliquity millions of years ago the activity of linear dune gullies reoccurs every year between the end of winter and the beginning of spring ls 167 4 216 6 and coincides with the final stages of the sublimation of annual co ice deposit this activity often coincides spatially and temporally with the appearance of recurrent diffusing flows rdfs digitate shaped dark patches with low relative albedo up to 48 lower than the adjacent dune that encompass the active site south and ssw facing dune slopes are those which preferentially host co2 frost deposits however it is only those with angles of 20 just below the crest which possess linear dune gullies suggesting a slope limited formation process these observations provide a wealth of temporal and morphometric data that can be used to undertake numerical modeling to direct future image monitoring and guide laboratory experiments that can be used to better constrain the formation process of these features

physical models and predictions for recurring slope lineae formed by wet and dry processes abstract the observation of low albedo elongated and transient features lengthening downhill on mars has attracted wide interest because of the possible role of aqueous fluids in their formation these recurring gravity driven processes called recurring slope lineae rsl remain mysterious in that although the influence of local climate conditions has been established their nature dry versus wet and the mechanisms that govern their growth and fading are debated we present three different physical models for the growth of rsl the first two are wet based models with different aqueous fluid evaporation models and the last is based on dry granular theory we discuss the prediction of each model with regards to the growth and fading of rsl and their morphology we finally discuss the strengths and weaknesses of these models in light of what we currently observe on mars we find that both wet and dry rsl scenario face challenges mostly regarding aspects of mass balance however water based scenarios provide a consistent framework to reconcile the different sets of morphological observations made on rsl

syn convergent exhumation implied by progressive deformation and metamorphism in the valle dell orco transect nw italian alps structural metamorphic and geochronological data from the valle del9orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the alpine orogeny deformation generally migrated westwards in this section and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain the data suggest a dynamic crustal stacking system lasting from 70 to 35 ma rather than continental collision following long lived b type subduction and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear bounded slices within the orogenic wedge the easternmost internal unit of the sesia lanzo zone the eclogitic micaschist complex was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late cretaceous during the development of the major preserved structures the eastern eclogitic micaschist complex was undergoing general non coaxial shear with an overall top down to the east vorticity suggesting a potential mechanism of exhuming an eclogitic 9pip9 slices of pre alpine basement the vasario sparone subunit were incorporated into the lower western parts of this unit during the latest part of this phase but do not show penetrative effects of the early alpine deformation they were passively folded during the early decompressional part of the p t history prior to the unit being juxtaposed against the underlying gneiss minuti complex under blueschist greenschist facies conditions the gneiss minuti complex itself was then recrystallized at high strain in the greenschist facies in the latest eocene fabrics developed at this stage indicate top down to east extension which may have accommodated the exhumation of eclogitized piemont oceanic material and the european continental material of the gran paradiso massif evidence of northwest directed thrusting within the gran paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse sense shear zone that presumably links to the more external thrusts now exposed to the west high pressure mineral lineations within the piemont zone cannot be unequivocally assigned to either the burial or exhumation paths but are consistent with the generally east west transport directions observed in the structurally higher units

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

ultrahigh resolution topographic mapping of mars with mro hirise stereo images meter scale slopes of candidate phoenix landing sites 1 the objectives of this paper are twofold first to report our estimates of the meter to decameter scale topography and slopes of candidate landing sites for the phoenix mission based on analysis of mars global surveyor mgs mars orbiter camera moc images with a typical pixel scale of 3 m and mars reconnaissance orbiter mro high resolution imaging science experiment hirise images at 0 3 m pixel 1 and second to document in detail the geometric calibration software and procedures on which the photogrammetric analysis of hirise data is based a combination of optical design modeling laboratory observations star images and mars images form the basis for software in the u s geological survey integrated software for imagers and spectrometers isis 3 system that corrects the images for a variety of distortions with single pixel or subpixel accuracy corrected images are analyzed in the commercial photogrammetric software socet set bae systems yielding digital topographic models dtms with a grid spacing of 1 m 3 4 pixels that require minimal interactive editing photoclinometry yields dtms with single pixel grid spacing slopes from moc and hirise are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully only the mars exploration rover mer b site in meridiani planum is smoother moc results at multiple locations have root mean square rms bidirectional slopes of 0 8 4 5 at baselines of 3 10 m hirise stereopairs one per final candidate site and one in the former site yield 1 8 2 8 slopes at 1 m baseline slopes at 1 m from photoclinometry are also in the range 2 3 after correction for image blur slopes exceeding the 16 phoenix safety limit are extremely rare

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

phase petrology and p t conditions of mafic blueschists from the meliata unit west carpathians slovakia blueschists occurring as layers in calcite marbles of the meliata unit occur along the so called roznava tectonic line situated in the southern part of the gemericum slovakia mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated the most common minerals in the blueschists are blue amphibole epidote and albite some fe2 and al rich rocks also contain garnet and chloritoid respectively na pyroxene with a maximum 50 jadeite component was also found the blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples almandine and spessartine rich garnet has very low mgo content 3 wt the si content in phengite ranges between 3 3 and 3 5 pfu calculated on the basis of 11 oxygens the zoning patterns of blue amphibole garnet and chloritoid suggest their formation during a prograde stage of metamorphism the p t conditions of metamorphism are estimated to be about 380 460 c and 10 13 kbar pressures of 7 5 8 5 kbar and temperatures of 350 370 c were obtained for some actinolite and aegirine rich rocks apart from chlorite other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite

3d reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the alpine subduction system the example of mt mucrone sesia lanzo zone western alps geological mapping multiscale structural analysis and estimations of the degree of fabric evolution and of reaction progress allow the construction of a 3d quantitative model of structural and metamorphic gradients in a portion of continental crust deeply involved in the alpine subduction system and mainly structured under eclogite facies conditions before the continental collision the investigated and modelled rocks outcrop in the surroundings of mt mucrone in the central sesia lanzo zone the geomodeller software allowed a quantitative 3d estimation of domains characterized by homogeneous fabric evolution and metamorphic reaction progress dfe and drp respectively for two of the seven structural and metamorphic imprints detected in this area the d2 eclogitic and d5 greenschist stages which are the most pervasive at km scale such a 3d modelling clarifies mutual relationships between fabric and metamorphic gradients and indicates that i dfe and drp are closely related regardless of the rock type ii the syn deformational thermal regime can influence the degree of metamorphic transformation if dfe remains below the 60 threshold iii the phase transitions can not be properly implemented in quantitative geodynamic modelling without considering the heterogeneity of reaction progress and fabric evolution

new and recent gully activity on mars as seen by hirise 1 fresh appearing gully deposits are found at tens of sites in the southern hemisphere of mars these deposits have latitudinal and azimuthal dependences similar to the overall preferences of southern hemisphere gullies suggesting that most gullies can undergo such events definite changes are seen at ten sites including two previously reported these include visible modification of gully channels and aprons those formation intervals constrained to better than one mars year tend to include winter and exclude summer suggesting seasonal activity this seasonal activity is consistent with proposed models for gully formation driven by co2 frost although at least some of the new deposits are probably due to dry granular flow with no volatile involvement as these deposits are capable of effecting distinct topographic changes to gully aprons and channels they represent a significant component of recent gully evolution

are different martian gully morphologies due to different processes on the kaiser dune field we describe and compare the morphology and activity of two types of gullies with different orientations collocated on the kaiser dune field in the southern hemisphere of mars large apron gullies and linear dune gullies the activity of large apron gullies follows an annual cycle i material collapse into the alcove mid autumn late winter as co2 condenses ii remobilization by mass flows late winter and iii continuous appearance of hundreds of digitate flows on the fan autumn winter we find that large apron gullies could form in hundreds of martian years in contrast linear dune gullies are active briefly in late winter when the co2 frost disappears their activity is characterized by the extension of channels the creation of pits and the darkening of the surface linear dune gullies are likely to form within one to tens of martian years we infer that insolation which influences the depth to ground ice and the amount of volatile deposited may be the factor differentiating large apron gullies and linear dune gullies sediment transport by co2 sublimation is a good candidate for the activity observed in all of these features however linear gullies could also be formed by brine release when the temperature rises abruptly after the removal of the co2 ice

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

structural evolution of the contact between two penninic nappes zermatt saas zone and combin zone western alps and implications for the exhumation mechanism and palaeogeography the boundary zone between two penninic nappes the eclogite facies to ultrahigh pressure zermatt saas zone in the footwall and the blueschist facies combin zone in the hanging wall has been interpreted previously as a major normal fault reflecting synorogenic crustal extension quartz textures of mylonites from this fault were measured using neutron diffraction together with structural field observations the data allow a refined reconstruction of the kinematic evolution of the pennine nappes the main results are 1 the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast directed normal faulting 2 exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening 3 the sesia dent blanche nappe system originated from a continental fragment cervinia in the alpine tethys ocean and the combin zone ophiolites from the ocean basin southeast of cervinia 4 out of sequence thrusting played a major role in the tectonic evolution of the penninic nappes

palaeozoic continental type gabbros in the gran paradiso nappe western alps italy early alpine eclogitization and geochemistry abstract palaeozoic gabbros intruded by late variscan granitoids occur in the southern part of the gran paradiso nappe western alps they are the first gabbroic rocks described in the variscan basement of the internal pennine nappes both gabbros and granitoids sustained the early alpine high pressure metamorphism at t around 500 c and p between 12 and 20 kbar and garnet ca na amphiboles omphacite talc paragonite assemblages developed in the gabbros in little deformed metagabbros the complete eclogitic recrystallization and the relatively advanced compositional homogenization of some eclogitic minerals were probably favoured by fluid infiltration however the microstructural re organization failed and the pre alpine magmatic domains survived in response to the lack of pervasive deformation the little deformed metagabbros are bounded by glaucophane bearing ductile shear zones formed during the early post eclogitic path major and trace element abundances and ree patterns of the gabbros indicate derivation from a basaltic sub alkaline melt with a continental affinity although a hiatus exists between the gabbros and the associated gran paradiso granitoids the ratios between some hygromagmatophile elements and the normalized patterns for some elements suggest a few affinities between the two lithologies these rocks probably formed during the same magmatic cycle and the granitoids originated either from anatexis of continental crust during gabbro intrusion or more likely by fractionation and interaction between the early basaltic melt and the sialic crust

geochemical and k ar isotopic behaviour of alpine sheet silicates during polyphased deformation abstract several deformational events with several generations of phengites occur in the schistes lustres from the northern cottic alps the phengites belong mainly to 1 1 old forms from a s 1 schistosity 2 2 deformed forms from s 2 cleavage tightening zones 3 3 neoformed forms from second generation stretching zones the si tetrahedral substitutions of these minerals which lie between 3 28 and 3 46 depend on their microstructural site this chemical variation cannot only be related to p t conditions it is also induced by deformation related solution deposition processes the k ar apparent ages of the phengites are widely scattered between 38 and 65 ma the data when plotted in an isochron diagram suggest a two step response at 58 54 ma and 50 49 ma of the phengites to deformation this could happened during the same event which produced the second generation of microstructures