late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

the intrinsic behaviour of a decomposed granite soil the intrinsic mechanics of a decomposed granite soil were investigated by triaxial tests at standard pressures and at high pressures up to 35 mpa to compare its behaviour in its saturated partially saturated and dry states in each case it was found that the behaviour could be described within a critical state framework with particle breakage as the principal means of plastic volumetric compression the behaviour of partially saturated samples compacted at around the optimum water content was found not to differ appreciably from that of saturated samples in contrast tests on dry samples revealed a state boundary surface for the dry soil which exists outside that of the saturated soil sieve analyses showed this to result from the fact that the soil in its dry state was subject to a smaller amount of particle breakage although the critical state friction angle is not affected by the presence of water the location of the state boundary surface at higher volumes stresses for the dry soils results in the

helium distribution in a mantle shear zone from the josephine peridotite a previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high temperature ductile deformation in the mantle in order to test this result this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small 6 m width ductile mantle shear zone from the josephine peridotite oregon usa all measurements were made by coupled in vacuo crushing and melting demonstrating that most of the helium 80 resides within the solid phases rather than fluid or melt inclusions the present study confirms the influence of deformation on helium contents but only at the highest shear strain 20 are helium contents significantly higher the highest helium concentration by roughly a factor of two is found in the center most sample which also has grain size reduction by a factor of 4 dislocations and sub grain boundaries are present in all samples and do not correlate with helium concentrations mineralogy also appears to have a negligible influence in this shear zone as modal mineralogy is relatively homogeneous with all samples being harzburgites these observations suggest that the increase in helium concentration is related to grain size reduction with grain boundaries proposed as an additional storage site for helium in the mantle the present data also characterize the isotopic composition of the josephine peridotite 3he 4he 6 7 0 2 ra n 33 between 6 3 and 7 1 ra the presence of cosmogenic 3he in the matrix is indicated by the helium isotopic composition released by melting 3he 4he 8 5 0 3 ra n 10 from 7 9 to 10 9 this corresponds to an exposure age of 10 kyr which is approximately concordant with the end of the last glacial maximum very little radiogenic helium is present in the samples suggesting extremely low uranium and thorium contents u 0 3 ppb helium isotope measurements in four samples outside the shear zone suggest that mineralogy and melt infiltration are also important factors for understanding helium storage in the mantle

instability and liquefaction of granular soils under undrained and partially drained states an experimental investigation of the initiation of instability liquefaction in saturated sand under partially drained conditions is presented the domain of stress space in which this instability develops is identified under various degrees of drainage and its relationship to the zone of instability observed under undrained shear is explored it is shown that partially drained conditions may render sand unstable that would otherwise be stable in a completely undrained state extremely small void ratio increases that cannot be regarded as physical loosening of sand if sand is partially drained contribute to instability implications of the findings are discussed in practical problems of liquefaction key words sand liquefaction undrained partially drained instability triaxial test

influence of fines on the collapse of loose sands a study has been carried out to investigate the influence of fines and gradation on the skeletal collapse behaviour of loosely prepared sand samples loose sand samples formed by moist tamping and consolidated to the same effective stress level were prepared with varying percentages of both plastic and non plastic fines 74 m samples were isotropically consolidated and subjected to monotonic undrained triaxial compression results indicate that increased percentages of fines 0 5 this change is represented by a decreased strain softening behaviour and a shifting of the steady state line variation of the gradation of the host sand appears to have little effect on the monotonic undrained behaviour a supplemental study involving use of the scanning electron microscope was carried out to evaluate the findings of the test program in terms of sand fabric key words colla

effects of fluid rock interaction on 40ar 39ar geochronology in high pressure rocks sesia lanzo zone western alps abstract in situ uv laser spot 40ar 39ar analyses of distinct phengite types in eclogite facies rocks from the sesia lanzo zone western alps italy were combined with sims boron isotope analyses as well as boron b and lithium li concentration data to link geochronological information with constraints on fluid rock interaction in weakly deformed samples apparent 40ar 39ar ages of phengite cores span a range of 20 ma but inverse isochrons define two distinct main high pressure hp phengite core crystallization periods of 88 82 and 77 74 ma respectively the younger cores have on average lower b contents 36 g g than the older ones 43 48 g g suggesting that loss of b and resetting of the ar isotopic system were related phengite cores have variable 11b values 18 to 10 indicating the lack of km scale b homogenization during hp crystallization overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40ar 39ar ages than the respective cores they also show variable effects of heterogeneous excess 40ar incorporation and ar loss one acceptable inverse isochron age of 77 1 1 1 ma for rims surrounding older cores 82 6 0 6 ma overlaps with the second period of core crystallization compared to the phengite cores all rims have lower b and li abundances but similar 11b values 15 to 9 reflecting internal redistribution of b and li and internal fluid buffering of the b isotopic composition during rim growth the combined observation of younger 40ar 39ar ages and boron loss yielding comparable values of both parameters only in cores and rims of different samples is best explained by a selective metasomatic overprint in low permeability samples this overprint caused recrystallization of phengite rims whereas higher permeability in other samples led to complete recrystallization of phengite grains strongly deformed samples from a several km long blueschist facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent 40ar 39ar ages 64 7 68 8 ma yielding an inverse isochron age of 65 0 3 0 ma almost complete b and li removal in mylonitic phengite is due to leaching into a fluid the b isotopic composition is significantly heavier than in phengites from the weakly deformed samples indicating an external control by a high 11b fluid 11b 7 4 we interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone this event also caused partial resetting of the ar isotope system and further b loss in more permeable rocks of the adjacent unit we conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40ar 39ar data in partially metasomatized rocks

laser probe 40 ar 39 ar dating of strain fringes mid cretaceous synconvergent orogen parallel extension in the interior of the sevier orogen 1 uv and co2 laser probe 40ar 39ar in situ analyses of phlogopite and muscovite in fibrous strain fringes from greenschist facies metamorphic rocks document mica growth ages at temperatures lower than their closure temperatures and therefore directly date deformation the new dates resolve the age of the earliest ductile fabric recorded in the raft river albion grouse creek metamorphic core complex of utah and idaho phlogopite was dated in quartz calcite phlogopite strain fringes around pyrite in pennsylvanian permian rocks from the grouse creek mountains utah using both the uv and co2 laser probe muscovite was dated in quartz muscovite strain fringes around pyrite in deformed jurassic sills from the black pine mountains idaho using the co2 laser probe phlogopite 40ar 39ar ages for individual strain fringes grouse creek mountains range from 92 ma to 110 ma with the most reliable ages ranging from 101 ma to 110 ma mean age 105 0 5 8 ma muscovite 40ar 39ar ages for individual strain fringes black pine mountains range from 97 ma to 112 ma mean age 104 7 5 8 ma strain fringes are associated with a subhorizontal foliation and a generally n trending elongation lineation exhibiting components of top to the north simple shear and coaxial strain accommodating n s extension and subvertical shortening midcrustal northward flow at 105 6 ma within the interior of the sevier orogen coeval with east directed shortening in the foreland and with plate convergence records orogen parallel synconvergent extension we favor gravitational relaxation of structural culminations resulting from focused crustal shortening as a driving mechanism for orogen parallel flow

constraining deformation stages in brittle ductile shear zones from combined field mapping and 40ar 39ar dating the structural evolution of the grimsel pass area aar massif swiss alps abstract 40 ar 39 ar dating of syn kinematic white mica phengite and biotite is used to constrain the age of shear zones within the granodiorites of the grimsel pass area aar massif central alps three sets of steeply dipping shear zones have been mapped stage 1 shear zones are very broad 200 m wide zones of biotite bearing rocks with a progressive increase in intensity of ductile deformation from rim to core and have dip slip kinematics subsequent stage 2 shear zones are narrower contain phengite rich mylonites and show a gradual change from dextral strike slip to dip slip kinematics across strike strain localisation during the formation of stage 2 shear zones is interpreted to have been aided by reaction weakening during extensive fluid rock interaction brittle precursors of stage 2 shear zones are marked by biotite rich cataclasites preserved along the northern contact of the overprinting main stage 2 shear zone biotite from a stage 1 shear zone yields an age of 21 1 0 2 ma phengites in stage 2 mylonites in the grimsel pass area have a very narrow age range 13 8 12 2 ma which is interpreted to bracket the duration of ductile deformation during stage 2 deformation at mid crustal levels however the biotite rich cataclasites at the margin of a stage 2 shear zone may have localised fluid flow up to 2 ma prior to the onset of main stage 2 ductile deformation stage 3 shear zones are brittle strike slip faults containing cataclasites breccias and clay fault gouges earlier studies have established that these late faults formed at

new insights from 40ar 39ar laserprobe dating of white mica fabrics from the pelion massif pelagonian zone internal hellenides greece implications for the timing of metamorphic episodes and tectonic events in the aegean region

steady state concepts and static liquefaction of silty sands the results from an experimental study on silty sands are presented and evaluated in view of the framework of critical state or steady state soil mechanics drained and undrained compression tests were performed on nevada sand containing nonplastic silt the drained and undrained steady state lines diverged this divergence was caused by the tendency for static liquefaction at low pressures results from undrained tests with different initial void ratios produced different steady state lines unique steady state lines may therefore not always exist for silty sands denser specimens liquefied at low confining pressures while looser specimens at higher confining pressures showed stable behavior thus a unique relation between void ratio and confining pressure namely the steady state line may not represent the behavior of loose silty sands this occurs because silty sands exhibit reverse behavior characteristics more dilative behavior is observed with increasing confining pressure under undrained shearing load and deformation control testing was performed to evaluate the effect of strain rate compressibility was examined as an alternative measure of liquefaction potential

effects of stress paths on the small strain stiffness of completely decomposed granite research on the small strain 0 001 1 characteristics of sedimentary soils and sands has advanced to the stage where it has been utilized in engineering analysis and design for some time despite the progress the stiffness characteristics of weathered materials such as completely decomposed granite cdg at small strains have still attracted relatively little research attention this paper describes a systematic laboratory investigation of the small strain characteristics of intact cdg subjected to various triaxial stress paths including drained compression and extension tests the small strain stiffness was measured using bender elements and internal local transducers measurements from bender elements illustrate that the elastic shear modulus of cdg increases as the mean effective stress increases and the void ratio decreases significant nonlinear shear stiffness shear strain and bulk modulus volumetric strain relationships were observed at 0 01 shear strain the measured average shear stiff

the strangways impact structure northern territory australia geological setting and laser probe 40ar 39ar geochronology abstract the strangways impact structure occurs within the mcarthur sedimentary basin northern territory australia the target rocks are primarily quartzites and subordinate shale siltstone sequences of the mesoproterozoic roper group these are primarily underlain by granitoid gneiss which is exposed in a 9 11 km diameter central uplift exposure is relatively limited and the upper parts of the impact structure have been removed by erosion based on the extent of the disturbance of the regional geology the diameter of strangways is estimated to be 24 26 km it is thus a complex crater and probably a central peak basin stratigraphic constraints place the time of impact as post roper group 145 ma in order to better define the time of impact infrared laser spot fusion 40 ar 39 ar data have been obtained from two impact melt lithologies and a highly shocked breccia clast these yield a combined age of 646 42 ma biotite dated from the granitoid gneiss central uplift yields 1686 1237 ma with the younger age reflecting the effects of shock induced resetting and or hydrothermal alteration due to hot melt sheet water interaction following impact the new age of 646 42 ma for strangways places it firmly in the neoproterozoic rather than being ordovician or younger as previously proposed

monotonic and cyclic liquefaction of very loose sands with high silt content the results from an experimental study on sands with high nonplastic silt content are presented drained and undrained triaxial compression tests undrained cyclic triaxial tests and drained undrained instability tests were performed on specimens of loose nevada sand with 40 silt content the behavior was observed to be somewhat different from previously published tests with sands at lower silt content the greater silt content appears to provide a more volumetrically contractive response throughout the entire stress strain curve however some aspects of the response were similar to sands with lower silt content monotonic undrained tests indicated reverse behavior i e static liquefaction occurred at low confining pressures and increasing dilatant volume change tendency was observed with increasing confining pressure analyzing the results using the concepts of steady state resulted in a unique steady state line only when undrained tests were sheared from the same isotropic compression line when

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

rb sr isotopic and compositional retentivity of muscovite during deformation abstract permian metapegmatite muscovite from the upper austroalpine matsch unit in southern tyrol italy was investigated regarding its rb sr and compositional retentivity during cretaceous upper greenschist facies deformation the data imply that microstructurally relic permian magmatic muscovite largely maintained its major and trace element compositions during deformation whereas the rb sr geochronometer is strongly affected by a net loss of sr lower sr concentrations of muscovite correlate with higher 87rb 86sr and 87sr 86sr ratios in most samples the muscovite grain size and magnetic fractions with the lowest 87rb 86sr and 87sr 86sr ratios preserve a permo triassic muscovite whole rock rb sr apparent age interpreted as to reflect formation during or cooling after pegmatite emplacement contrastingly muscovite fractions with higher 87rb 86sr and 87sr 86sr ratios are arranged along a roughly linear array with a positive correlation of the 87rb 86sr and 87sr 86sr ratios in the 87rb 86sr vs 87sr 86sr space they yield successively lower muscovite whole rock rb sr apparent ages we explain the variations in the rb sr isotopic character of microstructurally relic muscovite by a presumably deformation related loss of sr during the cretaceous event contemporaneously only very limited amounts of isotopically different sr from the matrix reservoir might possibly have entered the muscovite consequently the rb sr of the relic muscovite is affected by a net loss of sr the results imply that at temperatures of

reassessment of proterozoic granitoid ages in ghana on the basis of u pb zircon and monazite dating abstract the west african craton in ghana is of early proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the birimian supergroup as well as younger conglomerates and quartzites of the tarkwaian group and extensive granitoid plutons no consensus exists on absolute and relative ages of plutons within belts and basins u pb geochronology of four different granitoid complexes indicates that contrary to long held views belt granitoid plutons ashanti belt 2172 2 ma sefwi belt 2179 2 ma are 60 to 90 ma older than basin plutons kumasi basin 2116 1 ma belt granitoid are approximately coeval with volcanic rocks of the birimian supergroup while basin plutons are late kinematic and postdate tarkwaian sedimentation the first high precision geochronological data for rocks from ghana i e the eastern part of the birimian domain on the west african craton presented here show that magmatism within this region occurred over a time span of at least 90 ma and provide further evidence that the period between 2 1 and 2 2 ga was one of important magmatism in west africa

the liquefaction of sands a collapse surface approach recent large scale slides occurring during the hydraulic placement of an artificial island berm in the beaufort sea resulted from the liquefaction of the berm sand subsequent laboratory tests and

excess argon evolution in hp lt rocks a uvlamp study of phengite and k free minerals nw turkey excess argon is a common problem in high pressure metamorphic rocks here we present 40ar 39ar data from metabasite metachert and metapelite lithologies and a range of minerals the results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high pressure metamorphism samples were derived from blueschist and low temperature eclogite facies rocks from the tavsanli zone of nw turkey samples were analysed using the 40ar 39ar ultra violet laser ablation microprobe uvlamp and yield ages ranging from 72 3 to 154 7 ma detailed intra grain profiling of phengites in situ and analyses of sodic amphibole lawsonite quartz and garnet present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite whole rock xrf data indicates a scattered relationship between bulk k2o content of the rock and the concentration of excess argon in phengite we conclude that excess argon was derived in situ in a relatively closed fluid absent system rather than introduced by fluids

experimental investigation of both the disturbed and undisturbed granitic saprolite soil granitic saprolites are abundant in many regions around the world and they are widely employed in various engineering projects behavior of such kinds of soils is not sufficiently understood and our knowledge about the fundamental behavior of saprolites is to a far less extent than the sedimentary soils owing to the limited knowledge many problems occurred in the engineering applications that used saprolites as a construction material this study presents results of a set of conventional drained and undrained triaxial compression shearing tests on the granitic saprolites in both the disturbed and undisturbed conditions this study showed that the soil fabric was the paramount factor governing on the soil stress strain response rather than the soil bonds under the drained shearing condition the undisturbed soil revealed a strain softening behavior with a dilative response at the low consolidation stress level while a contractive response was present at the high consolidation stress levels however the disturbed soil showed a strain hardening behavior with a contractive response regardless of the consolidation stress levels during the drained shearing the undrained response was complicated as the soil showed both the contractive and dilative responses with phase transformation points regardless of the soil condition i e disturbed or undisturbed and the consolidation stress levels the bond effect of the intact soil persisted during the consolidation stage however its effect did not extend to the large strain controlled shearing stage where the tangential stiffness suddenly decreased at the axial strain of 1 5

influence of silt size and content on liquefaction behavior of sands this study investigates the fines content influence on liquefaction potential of a single base sand mixed with three different essentially nonplastic silts through strain controlled monotonic undra

the early stages of the alpine collision an image derived from the upper eocene lower oligocene record in the alps apennines junction area the upper eocene lower oligocene sediments deposited in the eastern part of the tertiary piedmont basin in northern italy provide a complete record of the unroofing of the alpine orogenic prism during the early stages of exhumation in the ligurian sector from late priabonian till late rupelian time the sediments in the study area were derived from two different sources one characterised by white micas with sib6 5 pfu and permian 40 ar 39 ar ages 270 ma and the other characterised by white micas with sin7 pfu and eocene oligocene 40 ar 39 ar ages 32 50 ma the first source is considered to be indicative of low pressure metamorphic rocks that covered the hp rocks of the ligurian alps and were completely eroded by chattian time from this time on the study area started to record the first input from western alpine sources characterised by a larger span of ages with a more frequent eoalpine signal thus sediments deposited in the eastern part of the tertiary piedmont basin contain the only available evidence of rocks belonging to high crustal levels in the alpine orogenic prism that were not affected by the alpine overprint these data also provide time constraints to the poorly dated first conglomerates deposited in this area 40 ar 39 ar geochronology reveals a minimum age of 33f1 4 ma for the pianfolco conglomerates in the type locality and of 31 4f3 5 ma for the borbera conglomerates d 2004 elsevier b v all rights reserved

failure instability and the second work increment in loose silty sand triaxial tests have been performed to demonstrate the conditions for stability and instability in loose silty sand drucker 1951 and hill 1958 stability conditions in terms of the sign of the second work increment were employed in the design of the stress paths used in the triaxial compression and extension tests performed with quasi constant shear stress while the mean normal stress was reduced until failure occurred it is shown that the sand is completely stable under drained conditions for any stress path and irrespective of the sign of the second work increment this is demonstrated by completely stopping the change in stresses and observing the stable behavior in the range of stresses where the sand contracts and where it dilates once the effective stress failure surface is passed the sand becomes unstable and the sign of the second work increment is always negative run away instability can occur inside the failure surface for loose silty sand under un drained conditions for which the sand tends to contract pore pressures continue to develop and the second work increment is negative liquefaction may follow if the loose silty sand is sufficiently loose resume des essais triaxiaux ont ete realises afin de demontrer les conditions de stabilite et d instabilite dans des sols sil teux l ches les conditions de stabilite de drucker 1951 et hill 1958 en termes du signe du deuxieme increment de tra vail sont utilisees dans la conception des cheminements des contraintes pour les essais triaxiaux en compression et en extension realises avec une contrainte de cisaillement pratiquement constante tandis que la contrainte normale moyenne etait reduite jusqu a la rupture il a ete observe que le sable est completement stable dans des conditions drainees quelque soit le cheminement des contraintes et quelque soit le signe du deuxieme increment de travail ceci est demontre en arretant com pletement le changement de contraintes et en observant le comportement stable dans la variation des contraintes parmi les quelles le sable se contracte et se dilate une fois la surface de rupture de la contrainte effective passee le sable devient instable et le signe du deuxieme increment de travail est toujours negatif l emballement de l instabilite peut se produire a l interieur de la surface de rupture pour du sable silteux l che dans des conditions non drainees durant lesquelles le sable a tendance a se contracter les pressions interstitielles continuent a se developper et le deuxieme increment de travail est nega tif la liquefaction peut se produire par la suite si le sable silteux l che est suffisamment l che

landslide hazard assessment by smoothed particle hydrodynamics with spatially variable soil properties and statistical rainfall distribution rainfall induced landslide has caused significant damage to structures and casualties in the past decades and it is of great importance to assess the post failure behavior of slopes this study pr

behavior of a loosely compacted unsaturated volcanic soil in this paper the stress strain relationsihp and volumetric behavior of a loosely compacted unsaturated decomposed volcanic soil fill were studied by conducting three series of triaxial stress path tests 1 consolidated undrained on the saturated fill 2 constant water content and 3 a reducing suction under constant deviator stress on the unsaturated fill the last two series of tests were designed to simulate the effects of undrained response and rainfall infiltration in initially unsaturated slopes respectively it was found that the saturated loose volcanic soil behaves like clay under isotropic compression but it resembles sand behavior when it was subjected to undrained shear for isotropically consolidated unsaturated specimens sheared under a constant water content a hardening stress strain and a nonlinear shear strength suction relationship are observed at relatively high suctions both angle of friction and apparent cohesion appear to be independent of suction volumetric contraction during shear is observed in this series of tests on the other hand anisotropically consolidated loose unsaturated specimens subjected to a reducing suction change from contractive to dilative behavior as the net mean stress increases this observed volumetric behavior unlike the shear strength is stress path dependent and cannot be explained by using the existing elastoplastic critical state theoretical framework extended for unsaturated soils

neotethys closure history of anatolia insights from 40ar 39ar geochronology and p t estimation in high pressure metasedimentary rocks the multiple high pressure hp low temperature lt metamorphic units of western and central anatolia offer a great opportunity to investigate the subduction and continental accretion related evolution of the eastern limb of the long lived aegean subduction system recent reports of the hp lt index mineral fe mg carpholite in three metasedimentary units of the gondwana derived anatolide tauride continental block namely the afyon zone the oren unit and the southern menderes massif suggest a more complicated scenario than the single continental accretion model generally put forward in previous studies this study presents the first isotopic dates white mica 40ar 39ar geochronology and where possible are combined with p t estimates chlorite thermometry phengite barometry multi equilibrium thermobarometry on carpholite bearing rocks from these three hp lt metasedimentary units it is shown that in the afyon zone carpholite bearing assemblages were retrogressed through greenschist facies conditions at c 67 62 ma early retrograde stages in the oren unit are dated to 63 59 ma in the kurudere nebiler unit hp mesozoic cover of the southern menderes massif hp retrograde stages are dated to c 45 ma and post collisional cooling to c 26 ma these new results support that the oren unit represents the westernmost continuation of the afyon zone whereas the kurudere nebiler unit correlates with the cycladic blueschist unit of the aegean domain in western anatolia three successive hp lt metamorphic belts thus formed the northernmost tavsanli zone c 88 82 ma the oren afyon zone between 70 and 65 ma and the kurudere nebiler unit c 52 45 ma the southward younging trend of the hp lt metamorphism from the upper and internal to the deeper and more external structural units as in the aegean domain points to the persistence of subduction in western anatolia between 93 90 and c 35 ma after the accretion of the menderes tauride terrane in eocene times subduction stopped leading to continental collision and associated barrovian type metamorphism because by contrast the aegean subduction did remain active due to slab roll back and trench migration the eastern limb below southwestern anatolia of the hellenic slab was dramatically curved and consequently teared it therefore is suggested that the possibility for subduction to continue after the accretion of buoyant e g continental terranes probably depends much on palaeogeography

rapid crystallization of precious metal mineralized layers in mafic magmatic systems the solidified remnants of mafic magmatic systems host the greatest concentrations of platinum group metals in the earth s crust our understanding of precious metal mineralization in these intrusive bodies is underpinned by a traditional view of magma chamber processes and crystal mush solidification however considerable uncertainty remains regarding the physical and temporal controls on concentrating these critical metals despite their importance to modern society we present high precision 87sr 86sr analyses of plagioclase and clinopyroxene from within centimetre thick precious metal enriched layers in the palaeogene open system rum layered intrusion northwest scotland isotopic heterogeneity is present between plagioclase crystals between clinopyroxene and plagioclase and within plagioclase crystals throughout the studied section on the basis of these observations we demonstrate that platinum group element mineralization formed by repeated small volume reactive melt percolation events the preservation of strontium isotope heterogeneities at 10 100 m length scales implies cooling of the melts that formed the precious metal rich layers occurred at rates greater than 1 c per year and cooling to diffusive closure within tens to hundreds of years our data highlight the importance of cyclic dissolution recrystallization events within the crystal mush and raise the prospect that precious metal bearing mafic intrusions may form by repeated self intrusion during cooling and solidification mineralization of platinum group elements in mafic intrusions occurs due to repeated self intrusion of magma according to strontium isotope heterogeneities preserved in the rum layered intrusion scotland

theoretical investigation of the undrained stability of shallow submerged slopes the problem of the criteria of stability of the equilibrium state of an infinitely long slope with constant inclination is tackled and the conditions of spontaneous collapse of the slope are proposed an experimental programme is described to define the stability locus for loose hostun rf sand the validity of the instability line concept introduced by lade for isotropically consolidated virgin specimens is corroborated it is further shown that this instability line can still be defined when specimens are anisotropically consolidated although then the slope of the line is greater it is shown that an instability line cannot be defined for specimens pre loaded in drained conditions and subsequently reloaded undrained as the level at which peak occurs in undrained conditions is dependent on the pre loading history the results are examined in the light of elastoplastic strain hardening theory by using a model to allow for mixed isotropic kinematic hardening all the experimental phenomena observed can

the thermal response of a metamorphic belt to extension constraints from laser ar data on metamorphic micas abstract metamorphic rocks may be exhumed to the surface either by isostatic rebound uplift and erosion or tectonically via thrusting or extension the latter mechanism is particularly important in the himalayas where a large extensional detachment system appears to be responsible for very fast unroofing at 20 ma here we present the results of a laser argon study of micas and quartz aimed at investigating the thermal response of an orogenic belt to such extension the data when combined with existing constraints on the prograde evolution lead to a uniquely complete thermal history for a metamorphic belt laser argon data are reported for quartz and micas from semi pelitic metamorphic rocks from the zanskar himalayas nw india the samples derive from three traverses extending from low grade rocks near the detachment to high grade rocks in the core of the belt most samples demonstrably contain no excess component and the ages of muscovite and biotite show a correlation with distance from the extensional detachment the ages themselves as well as the difference between the muscovite and biotite ages both increase as the detachment is approached muscovite and biotite ages from within the shear zone date deformation associated with extension at 20 25 ma the remaining data are most readily interpreted in terms of early slow cooling of the lower grade rocks near the detachment in response to erosional denudation before the detachment became active rocks in the core of the orogen were still heating at this time later fast cooling of the high grade rocks in the core of the belt ensued when the extensional regime became active simple heat transfer calculations based on the distance of the high grade rocks from the fault and the argon ages of their micas provide further constraints on the timing of extension and suggest that the fault first moved 23 25 ma ago and that it abruptly terminated metamorphism in the core of the metamorphic belt our analysis of the thermal history suggests that cooling and exhumation rates determined from metamorphic belts beneath extensional detachments will often be minima

collapse of dry sand loose cohesionless saturated materials have proved responsible for a number of serious or catastrophic flow slides liquefaction failures with no obvious triggering mechanism have also been recorded this phenomenon of sudden liquefaction without a presence of cyclic shear stresses is often referred to as spontaneous or static liquefaction results from previously published studies suggest that liquefaction is triggered not by the undrained loading and generation of pore pressures but by the collapse of the metastable sand structure which in turn generates the driving pore pressures in a saturated material hence the collapse is a characteristic response of a material to certain stress states rather than a result of some enforced undrained loading this theory is evaluated on very loose dry ottawa sand it is shown that the very loose dry sand when subjected to a constant deviatoric stress path significantly changes its behavior at a certain discreet stress state increases compressibility and becomes

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

influence of stress ratio and stress path on behavior of loose decomposed granite this paper presents results from four series of triaxial compression tests of loosely compacted decomposed granite dg or silty sand on both isotropically and anisotropically consolidated specimen

a 40ar 39ar laser probe study of micas from the sesia zone italian alps implications for metamorphic and deformation histories two samples from the eclogite micaschist complex emc and the seconda zona diorito kinzigitica iidk of the sesia zone have been studied using a high spatial resolution laser probe 40ar 39ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories data from a single large phengite grain from the emc show a range of ages from mid jurassic to upper cretaceous these apparent age variations are spatially related to both location within the grain and to intragrain microstructure modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain parallel to the mica cleavage profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places the temperatures of the initiation of deformation and the possible time scales for the deformation can be calculated as a function of cooling rate all estimates suggest deformation at greenschist facies in accord with the observed retrograde mineral assemblage absolute temperature estimates for deformation vary by less than 22 c for different cooling rates of 10 and 30 c ma 1 but vary by 80 c with different estimates of diffusion parameters the duration of deformation was for at least 2 ma at 10 c ma 1 or 0 7 ma at 30 c ma 1 biotites from the iidk sample record a permian to upper cretaceous age range that correlates with grain size the smallest grain sizes yielding the youngest ages this relationship is best explained by a partial resetting of biotites during an alpine thermal event initiated not more than 70 ma ago modelling of these data suggest that the sample never exceeded 300 c during the alpine the profoundly different thermal histories of the two units the emc recrystallized at 550 c whilst the iidk remained below 300 c suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism

instability caused by a seepage impediment in layered fill slopes the underlying cause of loose fill slope failures in hong kong has been attributed to static liquefaction during heavy rainfall a series of centrifuge model tests and numerical analyses were condu

rainfall induced slope failure considering variability of soil properties both hydraulic properties and shear strength properties of unsaturated soils can affect the stability of a soil slope during a rainstorm the uncertainties of hydraulic property parameters for unsaturated soils and their effects on the reliability of slopes under rainfall condition have been studied recently chong et al 2000 tung chan 2003 for example chong et al 2000 studied the influences of the uncertainty of each parameter in the soil water characteristic curve swcc and the permeability function on the safety factor of unsaturated soil slopes using monte carlo simulations tung and chan 2003 performed probabilistic analyses of rainfall infiltration and slope stability considering uncertainties in swccs by the latin hypercube sampling technique in both studies the deterministic model used is an uncoupled seepage analysis program seep w geo slope ltd 2001a and a companion slope stability analysis program slope w geo slope ltd 2001b seep w is formulated for conditions of constant net normal stress and non deformable soil media however under real situations the seepage and flow processes in a deformable soil are influenced by soil deformations a coupled hydromechanical model is therefore preferred to analyse the behaviour and stability of a deformable soil slope subjected to rainfall the objectives of this paper are to develop a coupled hydromechanical model and a finite element based slope stability program to study the behaviour and stability of deformable unsaturated soil slopes and to illustrate their application in probabilistic study by an example of a hypothetical unsaturated soil slope considering the variability of soil properties a

timing and nature of fluid flow and alteration during mesoproterozoic shear zone formation olary domain south australia the development of shear zones at mid crustal levels in the proterozoic willyama supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration monazite dating of shear zone fabrics reveal that they formed at 1582 22 ma at the end of the olarian d3 deformational event and immediately prior to the emplacement of regional s type granites two stages of fluid flow are identified in the area first an albitizing event which involved the addition of na and loss of si k and fe and a second phase of calcsilicate alteration with additions of ca fe mg and si and removal of na fluid fluxes calculated for albitization and calcsilicate alteration were 5 56 109 to 1 02 1010 mol m 2 and 2 57 108 5 20 109 mol m 2 respectively these fluxes are consistent with estimates for fluid flow through mid crustal shear zones in other terranes the fluids associated with shearing and alteration are calculated to have 18o and d values ranging between 8 and 11 and 33 and 42 respectively and nd values between 2 24 and 8 11 our results indicate that fluids were derived from metamorphic dehydration of the willyama supergroup metasediments fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent retrograde fluid flow event thus the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times prior to regional granite formation during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing

magma flow regimes in sills deduced from ar isotope systematics of host rocks the best place to seek evidence of the style of past magma flow through a conduit is in the country rock heat flow has been studied in country rock adjacent to two tertiary dolerite sills intruding the caledonian schists and quartzites on the isle of mull scotland radiogenic 40ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the ultra violet laser ablation micro probe to discriminate between a history of prolonged magma flow a history of conductive cooling following laminar flow and instantaneous emplacement of the intrusions the 40ar 39ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3 5 months resulted in extensive argon loss from the micas country rock melting and mineral breakdown adjacent to a 6 m sill these features were absent from the wall rocks of a smaller 2 7 m thick sill which exhibited even less argon loss than might have been predicted for an instantaneous intrusion if the heat loss from the 6 m sill observed in one locality had been repeated along its length it would have formed an important magma conduit to the mull volcano but dolerite is not a common flow composition on mull if on the other hand the heat loss from the sill varies along strike it constitutes strong evidence for channelling and heterogeneous flow within the sill

dating deformation using rb sr in white mica greenschist facies deformation ages from the entrelor shear zone italian alps ages of deformation have been obtained by rb sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation since white micas commonly define deformation fabrics in medium grade metamorphic rocks these ages can be directly related to structural geometries with regional context this direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust it does require that the dated minerals attained isotopic equilibrium with the dominant sr reservoir at temperatures lower than the closure temperature this resetting was apparently achieved during dynamic recrystallization of white micas in greenschist facies metasediments and metagranitoid units in the western alps the results suggest that the sr isotopic composition of the new mica is buffered by the coexisting high sr phases calcite feldspar or epidote via the grain boundary network high strain rocks from the entrelor shear zone system of the western alps have yielded indistinguishable white mica rb sr ages along 30 km of individual and kinematically linked shear zones the age of the back thrusting event is constrained at 34 1ma the age yielded by the younger generation of synkinematically crystallized white micas this event was short lived involving at least 20 km of shortening in 1 m y or less an earlier variably overprinted component dating from 38 to 37 ma has been identified in the mica fabric but its kinematic significance is uncertain this method of dating strain fabrics offers a powerful tool for tectonic studies since isotopic resetting can be directly linked to structural geometries microstructural textures and pt conditions it allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust

multiple deformations along the honam shear zone in southwestern korea constrained by rb sr dating of synkinematic fabrics implications for the mesozoic tectonic evolution of northeastern asia abstract the honam shear zone dextrally running along the southern margin of the ogcheon belt korea has drawn much attention in reconstructing the mesozoic tectonic framework of northeastern asia here we present rb sr data for the middle to southwestern part of the honam shear zone in an attempt to directly determine ages of deformation in the middle part the younger limit of deformation is constrained to be 162 11 ma by feldspar biotite rb sr dating of the deformed sunchang granodiorite on the other hand an early cretaceous deformation age is estimated for synkinematic fibers in ultramylonites from the southwestern part by two reproducible rb sr errorchrons at 138 4 and 139 23 ma this deformational time scale is consistent with earlier interpretations based on field evidence magnetic susceptibility data and age data for jurassic plutons within the shear zone our data demonstrate that the honam shear zone is not a permian to triassic tectonic feature related to the chinese continental collision

laser ablation rb sr dating by online chemical separation of rb and sr in an oxygen filled reaction cell abstract the rb sr beta decay dating system is one of the most attractive tools in geochronology as rb is sufficiently abundant in common k bearing minerals like biotite muscovite and k feldspar this allows dating of a wide variety of rocks e g volcanic magmatic metamorphic sedimentary and hydrothermal environments without the need of preconcentration as is often required for u pb dating of zircon however this advantage was to date negatively counteracted by the lack of a suitable in situ technique as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope a reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer icp ms allows exactly this the online chemical separation of two different elements coupled to a laser ablation la system in situ rb sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates sr from rb we present here a simple procedure in which rb sr ages can be obtained from a suite of individual phases in regular thin sections using the most established reaction gas oxygen o 2 it is possible to react part of the ablated sr to sro while no rbo is formed measurements of feldspars and micas with a spot size of 80 m were calibrated against glass standards nist srm 610 bcr 2g and biotite from la posta california 91 6 ma results are presented for a variety of magmatic rocks with well established thermal records a sample each from the klokken syenodiorite greenland 1161 ma the ulvo alkaligabbro sweden 1256 ma and a pegmatite from the bohus granite sweden 920 ma obtained in situ rb sr isochron ages are accurate 87 sr 86 sr ratios are accurate the methodology outlined in this study has significant implications for rb sr studies due to the high spatial resolution 1 quality of measurements can be better controlled by avoiding alterations and inclusions 2 large sample numbers can be investigated simply by using regular thin sections 3 several mineral generations can be targeted that would not be distinguishable in mineral separates and 4 isotope zonation within single crystals can be revealed this will open the rb sr dating system to new fields of study for example thermochronologic studies can use 87 sr 86 sr and 87 rb 86 sr zonation within crystals to better work out cooling and or reheating paths further applications are in the field of provenance studies and temporal evolution of shear zones

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