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

how is strain localized in a meta granitoid mid crustal basement section spatial distribution of deformation in the central aar massif switzerland abstract this study investigates strain distribution in granitoid rocks formerly in the middle crust in the central aar massif switzerland and places the deformation behavior in the tectonic framework of the alpine orogeny strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones sz 80 sz km with sz thicknesses the majority of the steep shear zones preferentially accommodated upward motion by the southern block leading to an increase in peak metamorphic conditions from 250 c in the north to 450 c in the south of the aar massif the shear zones initiated at about 18 20 km depths during a stage of crustal thickening handegg phase subsequent deformation reactivated some shear zones with a gradual transition from reverse dip slip over oblique slip to strike slip shear zones under local transpressional conditions oberaar phase

shear zone patterns and strain distribution at the scale of a penninic nappe the suretta nappe eastern swiss alps abstract shear zone patterns within the suretta nappe of the swiss alps are studied in order to understand finite strain and bulk tectonic displacements the suretta nappe is chosen for two main reasons the availability of late variscan granites deformed during alpine tectonics only and because of continuous outcrop from bottom to top of the nappe this alpine nappe recorded two major ductile deformation phases which developed under low grade metamorphic conditions the analysis of shear zone patterns in the roffna granite reveals that the first deformation is well developed in the entire nappe the second deformation however is more localized in the lower and upper parts the bulk asymmetry of the shear zone patterns confirms the kinematics recorded by other penninic nappes in this area first a stacking of the nappe towards the nw followed by the top to the e shearing during post thickening extensional tectonics the shear zone patterns and strain distribution allow us to deduce kinematics at the nappe scale

microstructures mineral chemistry and geochronology of white micas along a retrograde evolution an example from the aar massif central alps switzerland abstract this study investigates gneissic tectonites of different mechanical origins and aims at unravelling the link between microstructures white mica chemistry and k ar data the highest temperature mylonitic deformation which is investigated is dominated by dislocation creep in monomineralic quartz and viscous granular flow in polymineralic domains these mylonites show homogenous mineral chemistry resulting in robust and consistent k ar age data the lowest temperature deformation investigated is that of a retrograde brittle deformation producing cohesionless fault gouges within the cohesionless fault gouges frictional granular flow is considered as the main deformation mechanism the k bearing phases inside the fault gouges record no chemical resetting during this brittle overprint but contain mechanically fragmented k bearing micas and significant amounts of newly produced smectite this yields k ar ages inside of a gouge that are in the range of adjacent mylonite ages and may not represent the timing of gouge formation in an effort to understand these uncertainties in k ar ages detailed observations of the microstructural context of k bearing white mica were made microstructural criteria in combination with quantitative element mapping allowed for the discrimination of five important mica related processes 1 fracturing recrystallization of white mica 2 re precipitation of new fine grained white mica inside the deforming rock matrix 3 pseudomorphic replacement of precursor minerals by white mica and chlorite 4 pseudomorphic replacement of pre existing white mica without grain size reduction and 5 remnant grains the combination of these different processes led to a high variability in mineral chemistry and isotope data for tectonites which relates directly to variations in the size of the equilibrium volumes despite the samples being highly deformed in the presence of fluids inherited sheet silicates may survive if inherited sheet silicates are present they contribute to an increase in both the chemical heterogeneity and the apparent k ar ages of a sample

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

control of the geometry of precursor brittle structures on the type of ductile shear zone in the adamello tonalites southern alps italy abstract amphibolite facies ductile shear zones developed during the immediate post intrusive cooling history of the adamello tonalites southern alps italy shear zones include i thin a few mm s thick fault like shear zones that accommodate shear strain values up to several 100 s the dominant type ii mylonitic horizons dm s thick in sharp contact with the undeformed wall rock iii continuous shear zones with sigmoidal shaped s or composite s c foliations a transition between the different types occurs along strike over short distances discontinuous shear zones are also localized on pegmatite dykes and quartz veins and both i and ii are discontinuously outlined by quartz layers detailed surface mapping reveals that most fault like shear zones are arranged en echelon mainly forming contractional steps markers crosscut by shear zones allow the displacement to be measured at several positions along shear zones and this reveals very steep displacement gradients close to the shear zone tips differential displacement is mainly accommodated at contractional steps by the development of foliated domains geochemical analyses of major and trace elements show that there is no compositional change along strain gradients the overall features are consistent with nucleation of shear zones on former sets of en echelon joints in many cases intruded by pegmatite dykes or filled with quartz reactivation of joints produced strongly localized shear zones whereas broader foliated zones evolved from the contractional jogs between adjacent stepped joint terminations during progressive shearing these jogs were progressively involved in the accommodation of shear displacement and overall shear zone development

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

contribution of aeolian sand to backbarrier marsh sedimentation abstract the frequency of island overwash is commonly assumed to be the main factor regulating sand flux to backbarrier marshes however aeolian transport of sand across a barrier has received little attention in saltmarsh research the contribution of aeolian sand to backbarrier marshes is examined here with marsh cores and measurements of wind blown sediment transport across transgressive and regressive parts of a barrier island backbarrier marshes fronted by a wide washover fan or a high elevation dune ridge were supplied with aeolian sand that significantly contributed to vertical accretion especially near the marsh barrier island edge the percent volume of the marsh originating from aeolian sand based on the upper 1 cm of marsh sediment decreased by an order of magnitude only 20 m from the dune marsh boundary sandy beds sampled in every marsh core at depth are composed of aeolian sand and were likely emplaced over time after storms deposited a sandy washover fan near the marsh and or reduced vegetation cover across the island sand beds preserved within backbarrier marsh deposits are commonly thought to have been emplaced rapidly during a storm however post storm aeolian transport should also be considered as an important mechanism for forming sand beds over a longer period of time within saltmarsh strata the contribution of aeolian sand to backbarrier marsh accretion should increase as sea level continues to rise causing many barrier islands to narrow thereby reducing transport distances between the dune and marsh along developed coastlines however anthropogenic factors associated with increased development largely decrease the connectivity between the dunes and backbarrier marsh

control of the geometric arrangement of material heterogeneities on strain localization at the brittle to ductile transition in experimentally deformed carbonate rocks abstract triaxial high temperature 900 c deformation experiments were conducted at constant strain rate in a paterson type deformation apparatus on cylinders of carrara marble with two right or left stepping non overlapping weak inclusions of solnhofen limestone oriented at 45 to the cylinders longitudinal axes applying different values of confinement 30 50 100 and 300 mpa we induced various amounts of brittle deformation in the marble matrix and investigated the effect of brittle precursors on the initiation and development of heterogeneity induced high temperature shear zones viscosity contrast between the matrix and the inclusions induces local stress concentration at the tips of these latter the initial arrangement of the inclusions results in either an overpressured contractional or underpressured extensional domain in the step over region of the sample at low confinement 30 and 50 mpa abundant brittle deformation is observed but the spatial distribution of microfractures is dependent on the kinematics of the step over region microcracks occur either along the shearing plane between inclusions in extensional bridge samples or broadly distributed outside the step over region contractional bridge samples accordingly ductile deformation localizes along the inclusions plane in the extensional bridge samples as opposed to distributing over large areas of the matrix in the contractional bridge samples if microcracking is suppressed high confinement strain is accommodated by viscous creep and strain progressively de localizes in extensional bridge samples our experiments demonstrate that brittle precursors enhance the degree of localization in the ductile deformation regime but only if the interaction of pre existing heterogeneities induces an extensional mean stress regime in between

effective beam pattern of the blainville s beaked whale mesoplodon densirostris and implications for passive acoustic monitoring the presence of beaked whales in mass strandings coincident with navy maneuvers has prompted the development of methods to detect these cryptic animals blainville s beaked whales mesoplodon densirostris produce distinctive echolocation clicks during long foraging dives making passive acoustic detection a possibility however performance of passive acoustic monitoring depends upon the source level beam pattern and clicking behavior of the whales in this study clicks recorded from digital acoustic tags dtags attached to four m densirostris were linked to simultaneous recordings from an 82 hydrophone bottom mounted array to derive the source level and beam pattern of the clicks as steps towards estimating their detectability the mean estimated on axis apparent source level for the four whales was 201 dbrms97 the mean 3 db beamwidth and directivity index estimated from sequences of clicks directed towards the far field hydrophones were 13 and 23 db respectively while searching for prey blainville s beaked whales scan their heads horizontally at a mean rate of 3 6 s over an angular range of some 10 thus while the di indicates a narrow beam the area of ensonification over a complete foraging dive is large given the combined effects of body and head movements associated with foraging

tectonics of the central swiss molasse basin post miocene transition to incipient thick skinned tectonics in this paper we investigate the tectonics of the central swiss molasse basin smb situated between the central european alps and the jura fold and thrust belt based on re interpreted 2d reflection seismic data and geological 3d modeling we present new insights into the structural configuration and evolution of the central smb during the thin skinned main deformation phase of the jura fold and thrust belt 12 4 ma the central smb was riding passively above a major detachment horizon within triassic evaporites which roots beneath the external crystalline massifs of the central alps in the smb low amplitude detachment folds and detachment rooted wsw ene striking thrust and reverse faults are related to this event younger major nne ssw and nnw sse trending strike slip fault zones cross the basin over a length of several kilometers affecting the cenozoic and mesozoic cover units and extending into the pre mesozoic crystalline basement in cases the fault zones apparently reactivate inherited permo carboniferous trough pct bounding paleozoic normal faults seismic data further reveals mild inversion of wsw ene striking pcts these structures clearly cut the basement cover contact thus postdating the main deformation phase of the jura fold and thrust belt present day earthquake focal mechanisms and epicenter distribution show strike slip faulting and basement seated seismicity respectively thus corroborating our observations from seismic interpretation our data suggests a change from late miocene thin skinned to pliocene to present day incipient thick skinned tectonics this major change reflects the establishment of a mid crustal detachment and may be related to changes in slab dynamics of the subducting european lithosphere

the geometry and kinematics of flow perturbation folds abstract minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow using structures associated with wnw directed caledonian thrusting in n scotland we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction these define a series of larger scale fold culminations reflecting surging flow or depressions reflecting slackening flow that are bisected by transport parallel culmination and depression surfaces the fold patterns suggest a dominance of layer normal differential shearing due to gradients in shear strain normal to transport culmination surfaces are marked by along strike reversals in the polarity of structural facing and vergence of minor folds which contrary to classic fold patterns define reverse asymmetric relationships culmination surfaces separate folding in to clockwise z folds and anticlockwise s folds domains relative to the transport lineation the dip of fold axial planes systematically increases as their strike becomes sub parallel to transport resulting in a 3d statistical fanning arrangement centred about the transport direction thus mean s and z fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved culminations display convergent fold patterns with fold hinges becoming sub parallel to transport towards the culmination surface and underlying detachment whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface thus around culminations and depressions there are ordered scale independent relationships between transport direction shear sense fold facing vergence and hinge axial plane orientations the techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow

late stages of continent continent collision timing kinematic evolution and exhumation of the northern rim aar massif of the alps abstract the alpine front s external crystalline massifs represent exhumation of mid crustal basement during late stage continent continent collision we unravel the geodynamic evolution of the northalpine front aar massif by combining structural and low t thermochronometric data a crustal scale tectonic restoration neogene present and estimation of associated changes in exhumation rates become possible the european continental margin experienced multiple switches from horizontal to vertical and again to horizontal dominated tectonics early thrust related inversion of rift basins is followed by a switch to rapid vertical exhumation with deformation along steep reverse normal faults this vertical tectonics is related to a positive buoyancy force of the european crust induced by lower crustal delamination of the aar massif during an orogenic rollback process of the european lithosphere 22 20 ma differential rock uplift rates of up to 1 3 1 6 km myr resulted which decreased northwards to 0 1 km myr reducing buoyancy forces isostatic compensation led to a horizontal tectonic style again persisting compressional forces induced an orogen scale strain partitioning with dextral transpressive faults in the south and simultaneous crustal scale northwest directed thrusting in the north en bloc exhumation of the entire aar massif in the thrust s hanging wall occurred between 13 5 ma and 5 0 ma at rates of 0 5 0 9 km myr and 0 6 0 8 km myr respectively it is this late stage thrusting in combination with surface erosion which shaped today s prominent northalpine front switches between horizontal and vertical tectonics might generally be relevant for late continent continent collisional stages here buoyancy forces of non thinned continental crust entering the subduction zone cause a strong vertical tectonic deformation style instead of classical thrust tectonics

sedimentological and geomorphological imprints of holocene tsunamis in southwestern spain an approach to establish the recurrence period this paper reviews the sedimentological and geomorphological imprints of prehistoric and historical tsunamis in the four main estuaries of sw spain these imprints include beach erosion filling of intertidal channels deposition of bioclastic layers washover fans and reworked aeolian sheets and the breaching of spits and tombolos most of these imprints were caused by the 218 209 bc and ad 1755 tsunamis although evidence of other tsunamis has been identified in these two events effects on human populations were severe and diverse such as human loss of life changes in coastal settlements and international borders damage to port infrastructure or flooding of marsh and inhabited areas new radiocarbon reservoir data were included in order to obtain an approach to the recurrence period of these high energy events 700 1000 years in this area 2013 elsevier b v

early stage weathering systematics of pb and nd isotopes derived from a high alpine holocene lake sediment record highlights deglacial chemical weathering monitored by adsorbed isotopic and elemental signals resilience of authigenic geochemical signals towards variable lake redox conditions pronounced early deglacial weathering of labile accessory minerals remarkably congruent uranogenic and reduced thorogenic pb isotope release anthropogenic atmospherically deposited pb pollution since over 2000 years abstract radiogenic pb and nd isotopes are well established tools in paleoceanographic science tracing ambient climate and continental runoff to the oceans down to sub millennial timescales particularly in case of pb isotopes a clear climate dependency of continental isotopic runoff on glacial interglacial transitions has been observed pb isotopes were reported to be released incongruently during initial chemical weathering this incongruent release implies that pb isotopic runoff compositions differ from the bulk catchment pb isotopic signal yet only little is known about the processes leading to the incongruent release and the timescales of weathering on the continents in this study we targeted the adsorbed trace metal signature in sediments from a swiss high alpine lake that have accumulated since the retreat of the large alpine ice domes during the last deglaciation to investigate initial pb and nd isotope weathering processes in a granitic environment additionally selected adsorbed element concentrations and ratios were analysed to complement the isotopic physico chemical weathering information the integrity of the presented isotope records is supported by further investigation into the lake environment e g oxic anoxic conditions and its potential influence on the isotopic record the pb isotope records during the early lake phase witnessed high amplitude isotopic fluctuations linked to the initial chemical weathering of fine glacial substrate this finding is also supported by the lithology of the core and rapidly decreasing adsorbed th and u concentrations following this early lake phase the majority of the holocene traced congruent release of 207pb 204pb and 206pb 204pb and a significant depletion of 208pb 204pb in the adsorbed phase these findings corroborate earlier suggestions of more effective weathering of uranogenic minerals yet also call for the presence of more weathering resistant thorogenic minerals in the lake catchment the latest 2 2 ka of the record are significantly overprinted by anthropogenic pb deposition coinciding with the rise and fall of the roman empire finally our data suggest that nd isotopes are equally affected by incongruent weathering during the initial deglacial weathering processes albeit at smaller magnitude than seen for pb isotopes

deformation at the frictional viscous transition evidence for cycles of fluid assisted embrittlement and ductile deformation in the granitoid crust abstract mid crustal deformation is classically characterized by the transition from ductile to brittle deformation defining the frictional to viscous transition fvt here we investigate an exhumed continental mid crustal basement section in order to envisage the relationship between ductile and brittle deformation at the fvt our detailed study from km to micro scale shows that under greenschist metamorphic conditions deformation is accommodated by a dense network of highly localized ductile shear zones in the investigated case it is not quartz which defines the overall ductile deformation behavior but the viscous granular deformation in shear zones with an ultrafine grained polymineralic matrix consisting of quartz feldspar sheet silicates and epidote during viscous granular flow mass transfer processes under the presence of fluids promote a chemo mechanical mixing resulting in grain size reduction and reaction softening coeval with this ductile deformation fluid assisted embrittlement occurs as indicated by biotite coated fractures cataclasites and injection of non cohesive polymineralic gouge material into secondary fractures inside the host rock the embrittlement during predominant ductile deformation occurs in cycles i e prolonged periods of slow viscous granular flow are interrupted by rapid brittle deformation we interpret this fluid assisted cyclic embrittlement evidenced by injection of the fluidized material into off fault fractures as an alternative equivalent to pseudotachylites and as a microstructural indicator for paleo seismic activity with exhumation and associated cooling localized deformation persists in the ultrafine grained polymineralic shear zones but progressively transitions to cataclastic flow and finally to pressure dependent frictional flow always showing cycles of slow interseismic flow and fast seismic injection events overall in the granitic crust of the aar massif brittle and ductile deformation coexist up to deformation temperatures of minimum 450 c indicating that the fvt has to be placed in a rather wide range from 8 km up to 18 20 km in the granitoid crust

nucleation of ductile shear zones on dilatant fractures small shear zones in granitic rocks of the sierra nevada california and near roses rosas northeast spain display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation many zones have sharp nearly planar boundaries between their highly deformed interiors and the undeformed wall rock the roses shear zones narrow continuously to hairline fractures at their ends mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones we suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes thereby promoting chemical alteration and or hydrolytic weakening

characterization and imaging of a hydrothermally active near vertical fault zone in crystalline rocks based on hydrophone vsp data abstract a shallow hydrothermally active fault zone embedded in fractured crystalline rocks of the central swiss alps has been drilled and geophysically explored in view of its potential analogies to planned petrothermal reservoirs in the alpine foreland vertical seismic profiling vsp experiments have been performed using a 24 channel borehole hydrophone array with a sensor spacing of 1 m in order to image the fault core and to detect hydraulically open fractures the dense sensor spacing allows for an accurate sampling of the wavefields this in turn enables their separation through standard seismic processing methodologies thus allowing for the generation of seismic body wave vsp profiles and a tube wave corridor stack which maps brittle deformation in the surrounding rock mass multiple offset hydrophone vsp data were acquired with a crooked line survey geometry and processed with 3d methods hereby a laterally changing velocity cube representative of the vertical structural variations associated with the fault zone is generated and utilized for kirchhoff pre stack depth migration psdm imaging the resulting seismic image delineates various vertical structures notably the target fault zone the interpretation of the vsp data is complemented by and corroborated through a suite of geophysical borehole logs which characterize the fine scale petrophysical variations in response to brittle deformation in the fault core and the surrounding damage zones this study exemplifies typical targets of hardrock seismic exploration where the structures are frequently near vertical discontinuous and where acoustic impedance contrasts are commonly associated with deformation as such it illustrates the benefits of using borehole seismic methods in such environments and the use of borehole hydrophone arrays for targeting fractures and shear zones

pre orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central pyrenees pre kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts but rarely dated and accounted for in orogenic cycle reconstructions studying pre kinematic alteration is challenging because of its usual obliteration by subsequent syn kinematic metamorphism often occurring at conditions typical of the brittle ductile transition it is however to be expected that pre kinematic alteration has major implications for the rheology of the upper crust in the 305 ma old variscan basement of the bielsa massif located in the axial zone of the pyrenees successive fluid rock interaction events are recorded in granites below 350 c combined microstructural and petrographic analysis low t thermobarometry and in situ u th pb dating of anatase titanite and monazite show extensive pre orogenic pre alpine and pre kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270 350 c at 230 300 ma this event is followed by a second fluid rock interaction stage marked by new crystallization of phyllosilicates at 200 280 c and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes u pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40 70 ma consistent with independent regional geology constraints the variscan basement was therefore softened at late to post variscan time at least 150 200 ma before the main alpine shortening while alpine age compression c 35 50 ma leads to the formation of a dense net of mylonites the associated deformation both distributed at the scale of the bielsa massif and localized at decametric scale in mylonitic corridors precedes the strain localization along the major thrusts of the axial zone the bielsa massif is a good example where inherited pre orogenic upper crustal softening controls the deformation patterns in granitic basement units through low grade metamorphic reactions

brittle precursors of plastic deformation in a granite an example from the mont blanc massif helvetic western alps abstract in the mont blanc helvetic massif granites record mesoscale alpine structures which include joints veins cataclastic to mylonitic shear zones and foliated granites a detailed structural analysis indicates that brittle deformation predates plastic strain joints never pass through and veins are offset by cataclastic shear zones and mylonites the mylonites progressively develop by plastic reactivation of cataclastic shear zones during greenschist facies metamorphic conditions plastic deformation is first localized in the brittle discontinuities and the fine grained matrix of cataclasites then it involves the granite within brittle shear zones and this is initially accomplished mainly by flow of reaction softened aggregates of sericite widely replacing the strain supporting magmatic plagioclase the brittle to plastic evolution has resulted in highly localized discontinuous plastic shear zones with high lateral continuity and these characteristics are derived from reactivation of and focusing along pre existing brittle discontinuities in addition mylonites may inherit high angles of intersection and may contain granite porphyroclasts these features may allow the inference of a precursor brittle deformation where the plastic overprint has completely erased the initial brittle fabrics

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

spatial and temporal controls on overwash occurrence on a great lakes barrier spit field measurements from 1985 to 1990 and sequential aerial photography since 1945 show that overwash plays an extremely important role in the dynamics of long point a large barrier spit on the north shore of lake erie overwash occurs primarily in the transgressive proximal and central zones of the spit which together account for some 65 of the total shoreline length of 41 km during periods of high lake level over 50 of the shoreline in these zones may be overwashed washover morphology ranges from continuous washover terraces in areas of low foredunes to isolated washovers with narrow throats and distinct fans where breaching has occurred through high dunes individual overwash events commonly produce deposits on the fan surface 0 25 0 75 m or more in thickness washover fan sediments are dominated by nearly horizontal planar bedding with deposits near the fan margins often having foreset bedding reflecting deposition in standing water of the bay or of interdune ponds the frequency of overwash oc

late holocene evolution of the southwestern donana national park guadalquivir estuary sw spain a multivariate approach four phases are distinguished in the late holocene evolution of the southwestern donana national park sw spain based on a multidisciplinary analysis of the sediments present in drill cores in the oldest phase 2400 2500 cal yr bp a coastal lagoon the roman lacus ligustinus was recognized in the central part of this area partly closed by the donana spit and limited by fluvial levees the following phase 2400 2200 cal yr bp is characterized by high energy events which caused the breakthrough of the donana spit and the creation of new littoral strands in the inner areas in the third phase 2200 2050 cal yr bp this new outlet was closed coinciding with the progradation of the donana spit the last phase 2050 cal yr bp recent comprises three periods a an unstable period 2050 1950 cal yr bp with the deposit of cheniers over the previous levees b an infilling period with a diminution of the marine influence and c the appearance of temporary ponds between the emerged levees and cheniers

shear failure mechanism in granite inferred from multi scale brittle structures the brittle structures of a hercynian granite la borne french massif central observed at several scales from regional to microscopic are presented and interpreted on mechanical terms emphasis is placed on strike slip faults joints and cracks related to incipient shear fracturing during horizontal compression three compressive tectonic phases have been identified according to their brittle structures and characterised by the burial depth at the time of their generation the two first phases h1 h2 are hercynian and occurred while the granite was deeply buried 5 to 3 km depth whereas the last phase p is pyrenean and occurred at very low depth the geometric organizations of cracks joints and faults are clearly similar at various scales from about 10 m to several m these field observations are strong arguments for the occurrence of shear structures at microscopic to macroscopic scales during tectonic events following this inference it is proposed that at least in the case of hercynian phases cracks joints and macroscopic strike slip faults have been generated by the same mechanical process under high overburden weight therefore incipient fractures seem to be generated as shearing structures and to evolve by coalescence with neighbouring ones with the same direction

constraining deformation phases in the aar massif and the gotthard nappe switzerland using th pb crystallization ages of fissure monazite ce abstract fissure monazite ce hereafter called monazite commonly crystallizes during deformation at low metamorphic grade and offers the possibility to date protracted deformation over several millions of years and to identify distinct deformation phases we performed th pb geochronology at the microscale on 10 samples of fissure monazite from two alpine crystalline massifs aar massif and gotthard nappe ion microprobe ages show that the earliest stage of crystallization recorded by fissure monazite domain ages occurred around 15 9 ma in the gotthard nappe and about 1 my later in the aar massif with the latest crystallization event recorded at 6 ma protracted monazite crystallization in fissures indicates that deformation datable with fissure monazite lasted about 10 ma comparison of th pb crystallization ages of fissure monazite to existing thermochronological data shows that early monazite crystallization coincide with zircon fission track ages whereas the youngest monazite crystallization overlaps with apatite fission track ages monazite also grows contemporaneous with muscovite illite crystallization k ar ages in fault gouges as monazite can grow during dissolution precipitation cycles induced by tectonic activity their chronology allows to further constrain in the aar massif the final handegg phase at 12 11 5 ma and the coeval activity of the pfaffenchopf 11 5 9 ma and the oberaar and rhone simplon phases 11 5 and 6 ma in the gotthard nappe monazite crystallization constrains a major portion of the chiera backfolding phase at 14 13 ma and confirms that the south western termination of the nappe was affected by the rhone simplon phase

storm flood impacts along the shores of micro tidal inland seas a morphological and sedimentological study of the vesterlyng beach the belt sea denmark abstract the morphological and sedimentological impacts of an extreme storm event bodil in december 2013 were investigated at a mixed sand and gravel beach vesterlyng in a micro tidal environment along the belt sea denmark the storm response was characteristic for mixed sand and gravel beaches gravelly storm berms with sandy to gravelly washover fans were formed at the upper beach where swash processes dominated during extreme water levels in combination with overtopping and overwash in addition storm scarps and gravel flats were formed the elevation of the storm berm crest was identical to a summation of the still water level 1 60 m in the belt sea during the storm flood and the wave run up 0 75 m at the shore the crest of 2 35 m is therefore a precise measure of the storm flood level this storm flood was classified as a large scale extreme event in a holocene perspective it is suggested that storm berm beach ridge heights above contemporaneous sea level can be used as a proxy of past flood events along micro tidal shores

lateral displacement during neogene convergence in the western and central alps although major deformation in the western alps is clearly the result of north northwest directed thrust tectonics there is evidence suggesting that orogen parallel deformation may have been important in the late tectonic history of the western alps the authors propose that this northeast trending deformation includes 1 southwest directed normal fault movement along the simplon line 2 a diffuse zone of northeast striking dextral strike slip deformation along the rhone valley in switzerland between the mont blanc and aiguilles rouges massifs and through the belledonne massif and 3 southwest directed thrusting in the embrunais ubaye and digne nappe systems of southeastern france the correlation of these three broad regions of deformation is based on similar amounts of minimum displacement consistent kinematics and timing of deformation

networking of shear zones at the brittle to viscous transition cap de creus ne spain a crustal scale shear zone network at the fossil brittle to viscous transition exposed at cap de creus ne spain evolved by coeval fracturing and viscous mylonitic overprinting of an existing foliation initial fracturing led to mylonitic shearing as rock softened in ductilely deformed zones surrounding the fractures mylonitic shear zones widened by lateral branching of fractures from these shear zones and by synthetic rotation of the existing foliation between the fractures and shear zones shear zones lengthened by a combination of fracturing and mylonitic shearing in front of the shear zone tips shear zones interconnected along and across their shearing planes separating rhomb shaped lozenges of less deformed rock lozenges were subsequently incorporated into the mylonitic shear zones by widening in the manner described above in this way deformation became homogeneous on the scale of initial fracturing metre to decametre scale in contrast the shear zone network represents localisation of strain on a decametre length scale the strength of the continental crust at the time of coeval fracturing and viscous shearing is inferred to have decreased with time and strain as fracturing evolved to mylonitic shearing and as the shear zones coalesced to form a through going network subparallel to the shearing plane crustal strength must therefore be considered as strain and scale dependent

role of fluid in strain softening within the main central thrust in sikkim the origin of quartz rich mylonites abstract the presence of fluids promotes strain softening and profoundly affects the evolution of shear zones the main central thrust mct is a major shear zone that accommodated at least 90 km of shortening and played a significant role in cenozoic evolution of the himalaya surprisingly no information exists on the role of fluid in evolution of the mct this study integrates mineralogical and geochemical analyses with mass balance calculations to explore the role of fluids in evolution of the mct shear zone from four exposures which span 40 km distance along the transport direction in sikkim eastern india our analyses reveal significant fluid induced strain softening in the investigated exposures attested by retrogression of biotite to chlorite and feldspar to muscovite and quartz the retrogressed feldspar grains are surrounded by recrystallized quartz and the muscovite and chlorite produced due to retrogression define the mylonite foliations which indicate fluid activity was syn tectonic we propose a three stage conceptual model to analyze the role of fluid in the evolution of the mct in sikkim first the shear zone nucleated along pre existing structurally weak and compositionally distinct zones following which the proto shear zone developed in closed system conditions and deformation localized by dynamic recrystallization and dislocation creep resulting in formation of 200 1000 m thick protomylonites then fluids infiltrated the shear zone and induced retrogression and element mobility in the hinterland most exposures significant fluid induced retrogression transformed the calc silicate and paragneiss protoliths to mylonites resembling micaceous quartzites in the frontal most and structurally intermediate exposures the muscovite and chlorite grains underwent further dissolution leaving behind a quartz rich residue thus transforming tonalite orthogneiss and paragneiss protoliths to mylonites resembling monomineralic quartzites the deformation front narrowed through time and became progressively localized within the 120 410 m thick strain softened mylonites at the shear zone core whereas deformation ceased or continued slowly in the 100 880 m thick protomylonites at the margins the study improves our understanding of the strain softening mechanisms operative within the mct and the origin of quartz rich mylonites

strain pattern in the aar granite central alps orthogneiss developed by bulk inhomogeneous flattening the central part of the aar granite aar valley shows lens shaped domains of low strain separated by anastomosing domains of high strain a comparable pattern is found on all scales of observation deformation gradients are outlined by a gradual change from isotropic granite to orthogneiss and locally ultramylonites ultramylonites are concentrated within conjugate ductile shear zones where moderate the strain is concentrated in very narrow small scale faults which affect relatively undeformed granite within foliated domains finite strain measurements on deformed xenoliths indicate that i strain intensity increases gradually with grain size reduction and ii finite strain ellipsoids are of flattening type irrespective of the degree of mylonitization the pattern of small scale faulting the trends of the regional foliation and the orientation of ductile shear zones are consistent with a bulk strain field of flattening type throughout progressive deformation

nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions abstract in the neves area the pre alpine intrusive protolith of the zentralgneise unit tauern window eastern alps is well preserved in a kilometric scale low strain domain without pervasive alpine deformation it is compositionally heterogeneous consisting predominantly of granodiorites with lesser leucocratic granites and different generations of lamprophyres and aplites the intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes incipient alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities resulting in the development of several different types of shear zones fractures without alteration haloes initially accommodate slip entirely on the fracture itself with increasing deformation a foliation is progressively developed in the adjacent host rock eventually producing a single heterogeneous ductile shear zone with the typical sigmoidal foliation pattern strong layers aplite dykes and bleached alteration haloes developed to either side of precursor fractures localize shear on their boundaries to produce characteristic paired shear zones shearing is more evenly distributed within weak layers lamprophyres and quartz veins with a marked discontinuity in shear strain against the adjacent little deformed granodiorite shear zone development was accompanied by the formation of new fractures and quartz rich veins in the host rock which in turn also localized shear magmatic contacts fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations the kinematics of the overprinting strike slip shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis 1 here oriented ca 345 discontinuities of almost all orientations show shear reactivation even in the case of very low resolved shear stress indicating an overall viscous response of the system without a specific yield stress the geometry and kinematics of the shear zone network suggest that the overall deformation in low strain domains was close to coaxial during deformation along the shear zone network compatibility was maintained by fracturing with the development of new quartz veins and by distributed ductile deformation of the host rock especially within contractional domains of the shear network and at shear zone intersections deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities shear zone development in intact rock is always preceded by fracturing which localizes subsequent shear reactivation

rheological implications of heterogeneous deformation at multiple scales in the late cretaceous sierra nevada california late cretaceous deformation in the east central sierra nevada arc of california was heterogeneous at multiple scales we quantify this heterogeneous deformation at centimeter meter and kilometer scales in the vicinity of the gem lake shear zone and infer variations in effective viscosity from our data at the centimeter scale variations in strain of different clast types in conglomerate suggest that effective viscosities varied by less than an order of magnitude lithology controlled the magnitude and nature of deformation recorded by clasts at the meter scale cleavage refraction between stratigraphic layers records variations in finite strain comparison of our observations with cleavage refraction models suggests a maximum effective viscosity contrast of 10 between layers bulk composition controls variations in deformation of the different layers at the kilometer scale variation in finite strain magnitude and orientation in similar rock types both within and outside the shear zone demonstrates that deformation inside the zone was relatively intense comparing these results to numerical models of heterogeneous regional deformation we estimate that the angle of oblique convergence inside the zone was 15 10 while outside it was greater than 60 these kilometer scale results imply regional deformation was moderately strike slip partitioned during the late cretaceous and suggest regional effective viscosity varied by a factor between 6 and 17 at each scale of observation the apparent range of effective viscosity varies by an order of magnitude or less consequently we infer that relatively modest strength variations produced the structures observed at hand sample to tectonic scales

the evolution of the polymetamorphic basement in the central alps unravelled by precise u pb zircon dating the u pb age determinations of zircon and rutile from the aar massif reveal a complex evolution of the central alpine basement the oldest components are found in zircons of metasediments which bear cores of archean age the u pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 ma an age that is typical for pan african metamorphism the zircons are interpreted as pan african detritus with archean inheritance the provenance region of the pan african zircons is assumed to be a terrane of gondwana affinity i e the w african craton or the pentevrian microplate the caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the aar massif it is dated at 456 2 and 445 ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome respectively both occurring in the northernmost zones of the massif hercynian metamorphism never exceeded greenschist facies conditions and is recorded by zircon in a garnet amphibolite and by rutile in a meta psammite that yield an age of 330 ma both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of hercynian metamorphism the gastern granite at the western end of the aar massif is a contaminated granite that intruded at 303 4 ma contemporaneously with the wide spread late hercynian post collisional i type magmatism the study demonstrates the potential of isotope dilution u pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains

local shear zone pattern and bulk deformation in the gran paradiso metagranite nw italian alps the gran paradiso nappe of the northwestern alps mostly consists of augen gneisses derived from the alpine deformation of permian granitoids the regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top to west sense of shear the granitoid protolith is preserved in the kilometre scale low strain domain of the piantonetto valley and mainly consists of a porphyritic metagranite including joints leucocratic dykes and biotite rich schlieren in this low strain domain the alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite the shear zones mostly dip towards s se in a shallow shear zones1 to steep inclination shear zones2 the shear zones show typical features that can be explained by reactivation of pre existing joints and planar compositional heterogeneities palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening the kinematics of individual shear zones depends on the orientation of the original heterogeneities acting as nucleation planes and by partitioning of strain components at the kilometre scale with concentration of the flattening component to the piantonetto low strain domain the strain geometry and the kinematics of individual shear zones within piantonetto are not directly connected to the top to west sense of tectonic transport observed elsewhere in the gran paradiso nappe however the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the gran paradiso massif we conclude that the shear zone network of the piantonetto valley is representative of the incipient stages of ductile deformation of a granite nappe even if its architecture is determined by the arrangement of pre existing structural and compositional heterogeneities aspects of the large scale bulk strain can be derived from this local shear zone pattern

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

micromechanisms of shear zone propagation at the brittle viscous transition our investigation of progressively strained rock samples from the margins of greenschist facies shear zones utilizes a space for time approach to reveal how the mylonitic overprint of metapsammitic and pelitic host rocks at the cap de creus involved brittle fracturing we present a set of microscale observations indicating that microfractures formed immediately prior to or coevally with a fine grained mylonite microfracturing dominated early stages of strain localization on the scale of the shear zones on the microscale centimeter long fractures facilitated strain softening by allowing enhanced fluid access thereby accelerating the dynamic recrystallization of quartz and a metamorphic reaction of biotite as these two processes produce a polyphase matrix of small dislocation poor grains that eventually form an interconnected rheologically weak phase fractures become inactive this represents a strain dependent brittle viscous transition we outline this transition in a conceptual model for the rheological evolution of mid crustal shear zones

contrasting deformation of s and i type granitoids in the lachlan fold belt eastern australia abstract several examples of contrasting deformation under greenschist facies conditions of closely associated silurian devonian s type peraluminous and i type mainly metaluminous granitoid plutons of very similar age occur in the lachlan fold belt of eastern new south wales australia they occur in and or near the wologorong batholith the wyangala batholith and the kosciusko batholith the s type granitoids generally are strongly foliated and locally mylonitic whereas most of the i type granitoids typically show alteration to hydrous minerals chlorite epidote and actinolite but are unfoliated or only locally weakly foliated commonly fractured and have spaced ductile shear zones the more obvious and penetrative deformation in the s type granitoids probably is due mainly to their larger contents of relatively weak and or ductile minerals especially quartz and biotite and to the common occurrence of quartz in relatively large patches these features providing more continuous strained volumes and enabling quartz and mica rich folia to develop between resistant grains of feldspar in most of the i type granitoids the smaller amounts of quartz and biotite together with the common occurrence of quartz in smaller more scattered patches produce more localized strained volumes and may give rise to local stress concentrations that lead to fracturing the latter provides access to fluids responsible for hydrothermal alteration however some i type granitoids have high enough proportions of less ductile minerals to develop foliations although typically this occurs only adjacent to major faults some variation in the deformation of s types also is to be expected thus the main factor controlling the development of foliations appears to be the ratio of strong to weak or brittle to ductile minerals rather than the s i subdivision with which it generally correlates although other possible factors need further investigation such contrasting styles of deformation could lead to the erroneous interpretation that the massive i types are post deformation plutons and that the s types which commonly are slightly older are foliated by syn emplacement deformation whereas in fact both are pre deformation granitoids

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

discrimination of annealed and dynamic fabrics consequences for strain localization and deformation episodes of large scale shear zones abstract large scale shear zones are often characterized by long lasting deformation with complex strain localization behavior based on a drill core across polymineralic carbonate mylonites of the doldenhorn nappe helvetic alps switzerland the present study uses spatially resolved microstructural variations to discriminate unambiguously deformation microstructures from annealed ones to infer the deformation mechanisms and to reconstruct the strain localization history these goals require quantitative microstructural and textural analyses of sample suites in sections across the shear zone particularly the occurrence of two distinct zener trends i e the relationship between grain size and volume fraction of matrix and second phase grains allows for the identification of annealed from dynamic microstructures the data presented here indicate that continued shearing after peak metamorphic conditions and during exhumation induced cooling resulted in progressive strain localization at the base of the shear zone while the inactive parts of the formerly high temperature shear zone were progressively overprinted by annealing

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

finite element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting the development of shear zones initiating on random weaker initial perturbations is modelled numerically for low deborah number viscoelastic materials considering the influence of effective viscosity contrast power law rheology strain softening and different imposed bulk deformation geometries ranging from pure to simple shear conjugate shear zones initiate at 90 to one another and rotate with increasing bulk deformation the basic pattern not being markedly influenced by the vorticity of imposed deformation the rate of propagation of individual conjugate shear zones is little affected by increased effective viscosity contrast between matrix and inclusion but is promoted by power law rheology however the most marked effect is observed for strain softening behaviour where rapid propagation produces straighter and narrower shear zones the localisation of strain is reflected in a correspondingly heterogeneous stress distribution in particular mean stress or pressure is higher in the extending near planar weaker zones of localised shear melting of gneissic or pelitic compositions is pressure dependent with free water present increased pressure promotes melting whereas the opposite is true for water absent melting for water present conditions a positive feedback could develop between localised shearing increased pressure and partial melting this is potentially more effective in concentrating melt in shear zones than shear heating where melt related softening has a negative feedback effect

cheniers and chenier plains a general introduction abstract cheniers and chenier plains have been studied in many coastal regions throughout the world an inventory of the investigations currently being carried out is presented in this and accompanying papers cheniers and chenier plains are azonal features their distribution ranges from tropical to the subarctic regions and they show a variety of features progradational shorelines where mud accretion is temporarily interrupted favour the development of these features in addition to the concept of otvos and price who define cheniers and chenier plains as being formed exclusively in relation to littoral progradational environments it is suggested here that chenier formation in deltaic progradational environments should also be included genetically two types of cheniers can be distinguished depending on the origin of the related sediments cheniers developing due to wave winnowing of the muddy sediments and the subsequent concentration of the coarser particles to form a longshore bar or ridge which is driven shoreward and cheniers built by littoral drift in both cases they can be recognised by their sedimentary structures when cheniers arrive onshore they will migrate further landward due to washover processes under extreme hydrodynamical conditions e g spring tides storm surges etc to summarise the information concerning the episodic development of chenier plains in order to produce a or a few universal model s appears to be very difficult owing to the numerous factors that influence their formation these include river mouth shifting sealevel changes climatic changes storm frequency mortality of shellfish and bay geometry owing to this sensitivity to environmental changes during their formation cheniers are considered to be very important geomorphological features they provide a relatively accurate record of sealevel and environmental changes which can be dated with high resolution

small scale ductile shear zones neither extending nor thickening nor narrowing abstract the length thickness and strain gradients of small scale 10 3 10 1 m thick ductile shear zones are pre determined by the presence of surface precursors e g fractures or compositional layers and associated fluid rock interaction fractures with their surrounding host rock damage zone and concurrent tectonic under pressure during dilation provide efficient fluid pathways and networks with diffusive fluid rock interaction at their margins the fluid induced gradational to zoned compositional haloes that symmetrically surround a fracture control the strain gradients of developing shear zones and result in a diversity of geometric types including single homogeneous to heterogeneous shear zones and paired shear zones as a consequence geochemical differences between shear zone and host rock may reflect fluid rock interaction during the precursor brittle history rather than during slip especially considering that shear zones with even a small component of stretch may be over pressured and therefore unable to drain fluids from the surrounding rocks support from field observation for this interpretation is given by 1 the lack of correlation between shear zone thickness and accumulated displacement and 2 the similar thickness of shear zones and locally unexploited alteration haloes surrounding fracture precursors most small scale shear zones in massive rocks granitoids are initially neither thickening nor narrowing with increasing strain this concept may also apply to more foliated rocks but does not necessarily hold for larger scale shear zones

boudinage as a material instability of elasto visco plastic rocks abstract pinch and swell structures are commonly interpreted to evolve out of viscosity contrasts which are induced by geometric interactions and material imperfections from materials science an additional localization phenomenon is well established where localization emerges out of steady state for critical material parameters and or loading rates here we investigate the conditions under which this second type of instabilities prevails and whether geological materials necessarily require a trigger by imperfections in order to generate instability we focus on imperfections in terms of grain size variations embedded in a deformation environment controlled by thermo mechanical feedbacks we introduce a random distribution of grain sizes over two orders of magnitude in a central layer embedded in a matrix with a diffusion creep rheology the rheology of the layer evolves with dislocation and diffusion creep as end member deformation mechanisms applying pure shear extension the 3 layer model is subjected to natural deformation conditions the central layer quickly establishes a viscous steady state as a natural response of the system due to relaxation and energy optimization upon continued loading localization then intriguingly arises out of a homogeneous state we present an analysis which confirms that this type of instability is indeed physically admissible using vibration analysis we verify the robustness of the numerical solution by first identifying the natural mode shapes and frequencies of the simulated structure and material parameters including geometric imperfections in a second step the eigenmodes are perturbed and superposed to the initial conditions we conclude that this pattern of perturbations guides the onset of strain localization boudinage can therefore be seen both as a geometric problem and or a material bifurcation which evolves out of homogeneous state the latter class offers the great possibility of extracting fundamental material parameters out of localized structures directly from field observations

probabilistic tsunami hazard assessment at seaside oregon for near and far field seismic sources 1 the first probabilistic tsunami flooding maps have been developed the methodology called probabilistic tsunami hazard assessment ptha integrates tsunami inundation modeling with methods of probabilistic seismic hazard assessment psha application of the methodology to seaside oregon has yielded estimates of the spatial distribution of 100 and 500 year maximum tsunami amplitudes i e amplitudes with 1 and 0 2 annual probability of exceedance the 100 year tsunami is generated most frequently by far field sources in the alaska aleutian subduction zone and is characterized by maximum amplitudes that do not exceed 4 m with an inland extent of less than 500 m in contrast the 500 year tsunami is dominated by local sources in the cascadia subduction zone and is characterized by maximum amplitudes in excess of 10 m and an inland extent of more than 1 km the primary sources of uncertainty in these results include those associated with interevent time estimates modeling of background sea level and accounting for temporal changes in bathymetry and topography nonetheless ptha represents an important contribution to tsunami hazard assessment techniques viewed in the broader context of risk analysis ptha provides a method for quantifying estimates of the likelihood and severity of the tsunami hazard which can then be combined with vulnerability and exposure to yield estimates of tsunami risk

apatite and monazite an effective duo to unravel superimposed fluid flow and deformation events in reactivated shear zones abstract mylonitic shear zones crosscutting homogenous granitoids can retain evidence of fluid driven metasomatic retrogression and reactivation however the relationships between fluid rock interaction retrogression deformation and mylonitisation and the timing thereof are often cryptically recorded this study focuses on the granulite facies boothby orthogneiss from the reynolds range central australia which contains a large scale mylonitic shear zone with an apparent record of structural inheritance fluid infiltration and reactivation the chosen site provides an ideal natural laboratory in which to investigate the timing of deformation associated fluid flow and mass transport u pb isotope analyses of monazite indicate an average pb recrystallization age of c 1560 ma demonstrating that the orthogneiss fabric developed during the mesoproterozoic chewings orogeny 1590 1550 ma structural mapping suggests that this shear zone represents a riedel branch of larger structures that were subsequently reactivated during the paleozoic alice springs orogeny 450 300 ma the timing of reactivation and fluid flow is constrained by u pb dating of apatite which is present as a stable u bearing mineral in both orthogneiss and mylonite modelling of apatite radiogenic pb retention ages accounting for a wide potential range in common pb compositions demonstrates at least some growth and or recrystallization at c 1500 ma and c 400 ma confirming apatite precipitation during alice springs shearing and the reactivation of chewings age structures in addition alice springs aged apatite is found along pre existing fabrics in the orthogneiss in the vicinity of the shear zone indicating pre kinematic fluid flow across the shear zone boundary and into country rock that was otherwise largely unaffected the combined datasets demonstrates that integrated apatite and monazite u pb geochronology is an effective method to unravel the record of superimposed fluid flow and deformation events this includes the detection of an inverse younging relationship where younger ages are preferentially recorded in the wall rock as rather than in the reactivated shear zone such effects are potentially common where deformation is driven by pre kinematic fluid rock interaction with subsequent deformation enhancing the removal of replacement assemblages in more deformed rocks and favouring their preservation in less deformed rocks

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

facies analysis foraminiferal record and chronostratigraphy of holocene sequences from saltes island tinto odiel estuary sw spain the origin of high energy deposits abstract a multidisciplinary analysis of holocene sediments from la cascajera tinto odiel estuary sw spain has allowed to differentiate five sedimentary facies sandy tidal flat clayey tidal flat washover fan salt marsh edaphic horizon according to their granulometric characteristics internal structure mineralogical composition clay mineral assemblage and paleontological record these data coupled to 14c ams dating suggest an evolution from a sandy tidal plain to a chenier before 2100 cal yr bp this initial chenier was partially eroded later by regional storms between 2100 cal yr bp and 1900 cal yr bp leading to the deposition of two washover fans these facies have been eroded in the last centuries by the action of an ebb tide channel

microfabric memory of vein quartz for strain localization in detachment faults a case study on the simplon fault zone abstract this manuscript deals with the adaptation of quartz microfabrics to changing physical deformation conditions and discusses their preservation potential during subsequent retrograde deformation using microstructural analysis a sequence of recrystallization processes in quartz ranging from grain boundary migration recrystallization gbm over subgrain rotation recrystallization sgr to bulging nucleation blg is detected for the simplon fault zone sfz from the low strain rim towards the internal high strain part of the large scale shear zone based on i the retrograde cooling path ii estimates of deformation temperatures and iii spatial variation of dynamic recrystallization processes and different microstructural characteristics continuous strain localization with decreasing temperature is inferred in contrast to the recrystallization microstructures crystallographic preferred orientations cpo have a longer memory cpo patterns indicative of prism and rhomb glide systems in mylonitic quartz veins overprinted at low temperatures 400 c suggest inheritance of a high temperature deformation in this way microstructural textural and geochemical analyses provide information for several million years of the deformation history the reasons for such incomplete resetting of the rock texture is that strain localization is caused by change in effective viscosity contrasts related to temporal large and small scale temperature changes during the evolution of such a long lived shear zone the spatially resolved quantitative investigation of quartz microfabrics and associated recrystallization processes therefore provide great potential for an improved understanding of the geodynamics of large scale shear zones

scales of deformation partitioning during exhumation in a continental subduction channel a petrofabric study of eclogites and gneisses from nw spain malpica tui allochthonous complex

post magmatic resetting of rb sr whole rock ages a study in the central aar granite central alps switzerland this study investigates the post magmatic reequilibration of rb sr whole rock isochrons in the central aar granite aar massif central alps a post magmatic prealpine hydrothermal alteration caused rehomogenization of sr isotopes in local subsystems of the granites by subsolidus reactions mainly involving k feldspar the size of these rehomogenized subsystems is dependent on rock mineralogy and chemistry interference between sample size size of the rehomogenized subsystem and undisturbed magmatic system size yields a series of rb sr wholerock ages intermediate between an upper carboniferous intrusion age and the age of a hypothetical triassic liassic alteration event the microscopic reaction textures as well as the rb sr age systematics of this hydrothermal event can be distinguished from alpine metamorphic effects

structure and development of an anastomosing network of ductile shear zones a detailed structural analysis of an anastomosing shear zone network in metagabbros from the archean rainy lake zone canada revealed the existence of prevalent dextral and minor sinistral conjugate shear zones with the obtuse angle 130 facing the main shortening direction a typology of shear zone intersections confluences and other features shows that all shears formed during a single deformation event with dextral and sinistral shears being active together or in an alternating fashion in spite of the difficulty of establishing a complete kinematic sequence early and late shears can be distinguished the final angular pattern between dextral and sinistral shears is not an original feature dextral and sinistral shears formed at nearly right angles and the angles progressively opened towards the extension direction as a result of increasing strain the obtuse angles were achieved by the combined effects of continued shearing on newly forming shears and internal deformation of the lozenge shaped domains of lesser deformed rock bounded by the shears through time there was an increasing prevalence of dextral shears over sinistral ones the studied pattern and sequential analysis indicate that the bulk deformation was noncoaxial with a deformation regime evolving from a pure shear dominated dextral transpression to a higher vorticity dextral transpression

40ar 39ar dating of synkinematic white mica insights from fluid rock reaction in low grade shear zones mont blanc massif and constraints on timing of deformation in the nw external alps this paper highlights the use of synkinematic white mica biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low grade metamorphic conditions the mont blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow resulting in substantial differences in mineralogy and whole rock geochemistry on the basis of their synkinematic alteration assemblages and geographic distribution within the mont blanc massif three main metamorphic zones are distinguished within the network of shear zones these are i epidote white mica bearing assemblages ii chlorite phlogopite bearing assemblages and iii white mica biotite calcite actinolite epidote bearing assemblages 40ar 39ar age spectra of biotite and phlogopite are complex and reflect significant variations in chemical composition in biotite this is partly due to inheritance from precursor variscan magmatic biotite in contrast new white mica grew at the expense of feldspar during alpine deformation and its ar spectra do not show any excess 40ar on the se side of mont blanc ages of shear zone phengites have a narrow range of 15 8 16 0 0 2 ma which is in the same age range as 40ar 39ar ages of minerals from kinematically related veins the top to se sense of shear is consistent with initiation of a mont blanc flower structure within a dextral transpressional system by 16 ma on the nw side mini plateaux ages of 14 5 0 3 and 23 4 0 4 ma are preserved in the same sample suggesting the possibility of two phases of deformation this is also supported by partly preserved ages of 18 36 6 ma in biotites and phlogopites ages between 36 and 18 ma might reflect ongoing top to nw thrusting following penninic front activation in a context of nappe stacking and crustal thickening nw directed thrusting on the nw side of mont blanc continued after 18 ma synchronous with se directed thrusting on the se side of the massif these divergent movements produced the overall pop up geometry of the mont blanc massif which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending sw from the rhone valley into the mont blanc area

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

from static alteration to mylonitization a nano to micrometric study of chloritization in granitoids with implications for equilibrium and percolation length scales strain accommodation in upper crustal rocks is often accompanied by fluid mediated crystallization of phyllosilicates which influence rock strength and shear zone formation the composition of these phyllosilicates is frequently used for pressure temperature time constraints of deformation events although it is often highly heterogeneous even in mylonites this study investigates the reactions producing a phyllosilicate chlorite in and below grenschist facies conditions and the variations in chlorite composition along a strain gradient in the variscan bielsa granitoid axial zone pyrenees compositional maps of chlorite including iron speciation are compared to nanostructures observed by transmission electron microscopy in increasingly strained samples and related to mechanisms of fluid percolation in the bielsa granitoid altered at the late variscan alpine age shear zones are found with high strain gradients the undeformed granitoid exhibits local equilibria pseudomorphic replacement and high compositional heterogeneities in chlorite this is attributed to variable reaction mechanisms at nanoscale and element supply little interconnected intra and inter grain nanoporosity and isolation of fluid evolving in local reservoirs in samples with discrete and mm sized fractures channelized fluid triggered the precipitation of homogeneous alpine chlorite in fractures preserving late variscan chlorite within the matrix in low grade mylonites where brittle ductile deformation is observed micro and nanocracks and defects allows the fluid percolating into the matrix at the scale of hundreds of m this results in a more pervasive but incomplete replacement of late variscan chlorite by alpine chlorite despite the high strain in studied granitoids deformed under greenschist facies conditions local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according reaction mechanisms and element mobility controlled by i matrix fracture porosity contrasts at nanoscale and ii the location and interconnection of nanoporosity between crystallites of phyllosilicates this preservation influences our ability to reconstruct the pre and syn kinematic metamorphic history of granitic rocks in low grade units of orogens