structural and metamorphic history of a segment of the sesia lanzo zone and its bearing on the kinematics of alpine deformation in the western alps

two phase orogenic convergence in the external and internal sw alps the nw se trending sector of the sw alps includes the dora maira massif where tertiary eclogites record ultrahigh pressures and rapid exhumation along a ne sw crustal cross section italy france compiled pressure temperature time data in internal zones are correlated with tertiary stratigraphy in external zones to reconstruct orogen evolution revealing a coherent two phase convergence history during the first subduction accretion phase eocene 55 34 ma rapid north south plate convergence caused the subduction and exhumation of high pressure and ultrahigh pressure uhp rocks in a steady state subduction channel this coincided with the north to nnw migration of an underfilled flexural basin across the european foreland nappe stacking within the subduction channel did not create significant relief implying that primarily subduction forces generated this flexural basin from 34 ma onward the second collisional phase was characterized by slower nw se plate convergence the internal units of the sw alps underwent considerable anticlockwise rotation as they became involved in a nw se oriented sinistral transpression zone between the european and adriatic plates to the north of the orogen the north alpine foreland basin became overfilled as a result of high sediment supply from increasing orogen relief in contrast in se france active flexure of the european plate appears to have ceased and sedimentation became limited to small thrust sheet top basins created by continuing gentle ne sw shortening internal units were exhumed slowly from depths of c 20 km principally by erosion in the sw alps the transition between these two phases was marked by the rapid subduction and exhumation of the dora maira uhp unit assuming lithostatic pressure this unit would have been exhumed from 100 km depth requiring a rate that exceeds that generated by plate convergence therefore either exhumation was accelerated by additional stresses locally generated by transpression slab breakoff or high density contrasts or more controversially the ultrahigh pressure occurred at a considerably shallower depth as a result of local overpressure

thrust structures in the eastern dauphinois zone french alps north of the pelvoux massif abstract an examination of thrust structures in the eastern part of the dauphinois zone of the external french alps referred to in the literature as the ultradauphinois zone shows that major basement thrusts climb up section to produce cover basement synclines these thrusts also climb laterally and are continuous with thrust in the cover rocks the external basement massifs are recognized as thrust sheets with variably deformed and thrust cover sequences the distinction made in the previous literature between the dauphinois and ultradauphinois zones is no longer tenable cover thrusting proceeded by both smooth slip and rough slip the latter producing a duplex of cover thrust slices restoration of this duplex indicates that a shortening of 70 km in the cover occured during its formation possible errors in this estimate include uncertainties in the original stratigraphic thickness and in the overall shape of the duplex another duplex is thought to have formed at a basement ramp created by the presence of an early basement normal fault partial footwall collapse of this basement ramp gave rise to a basement horse at the bottom of the duplex the overall relation between cover and basement thrusting is indicated using a hanging wall sequence diagram recent geophysical studies suggest that the basement thrusts developed from a mid crustal decollement which passes down dip to offset the moho model studies of thin skinned tectonics may not be appropriate to such thrust geometries

extrusion and radial spreading beyond a closing channel abstract the development of experimental models of an extruding spreading zone beyond a closing channel is studied and a possible natural example described the strain pattern folding and radial displacements within the extruding spreading zone are analysed folds developed in the closing channel are either passively rotated or refolded when carried into the spreading zone a simple theoretical model of the experiments demonstrates that the stretching lineation is radial and parallel to the displacement direction towards the base and the front of the extruding spreading zone but perpendicular towards the top an intermediate domain with no stretching lineation separates the radial and concentric domains and corresponds to a transition through a uniaxial oblate ellipsoid k 0

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

geothermobarometry in albite garnet orthogneisses a case study from the gran paradiso nappe western alps abstract the aim of this paper is to estimate syntectonic p t conditions within albite and garnet bearing orthogneisses these rocks are generally characterized by the assemblage quartz albite biotite phengite cafe garnet epidote titanite garnet contains up to 55 mole per cent of grossular k feldspar is a relict magmatic phase p t conditions are estimated using several independent methods first it is shown that exchange reactions based on the fe mg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks due to the high grossular content of garnet second maximum and minimum pressures are constrained respectively by the occurrence of albite instead of jadeite quartz and by the assemblage phengite biotite quartz third phase equilibria in albite and garnet bearing metagranites are modelled in the system k 2 o cao feo al 2 o 3 sio 2 h 2 o equilibrium curves are calculated for the observed phase compositions uncertainties in p t estimates mainly result from the choice of appropriate non ideal solution models for the garnet an application is developed for granites from the gran paradiso nappe western alps these granites show an heterogeneous deformation of alpine age expressed by mylonitic shear zones cutting across weakly deformed domains estimated p t conditions for the synkinematic assemblages are 10 16 kbar at 550 50 c

a kinematic analysis incorporating incremental strain data for the frontal pennine zones of the western french alps abstract an incremental and finite strain study of the zones adjacent to the frontal pennine thrust fpt has been undertaken using syntectonic fibres taken from veins pyrite type pressure shadows and similar fringes from around pebbles in conglomeratic rocks the resultant strain data together with detailed mapping of local deformational geometries and regional trends has shown that the pennine zones show a more complex structural history than previously envisaged the structural analysis divides the pennine zones into two regions north and south of creve tete the southern termination of the valais basin the zones to the south of creve tete mont fut valbuche nielard and aiguille d arves show an essentially piggy back thrust sequence directed towards the foreland in which the elongation of the strain ellipse initially parallels the transport direction wnw but finally rotates to become sub parallel to the tectonic strike nne ssw the structures of the valais zone to the north of creve tete are more complex with both n and wnw verging folds thrusts and strike slip faults here the long axis of the strain ellipse appears initially to parallel the tectonic strike ne sw this is followed by a rotation into the late transport direction wnw this localised alternation between strike and transport parallelism is problematic specific examples of strike parallel extension may be explained in terms of radial transport buttressing against earlier extensional structures strike slip motion and or stretching over lateral culmination walls the major division in fibre pattern north and south of creve tete however is best explained as being a function of compartmentalization of deformation styles between adjacent areas models which invoke a dominant deformation mechanism e g thrust tectonics along the entirety of the alpine chain thus need to be rethought similarly the recognition of widespread strike parallel extension suggests that two dimensional balanced section techniques are simplistic giving only minimum shortening estimates in a single direction and thus failing to reflect the true situation

the geology and structural evolution of a caledonian fold and ductile thrust zone kyle of tongue region sutherland northern scotland the middle to upper proterozoic metasedimentary rocks of the northern highland moine in sutherland are disposed in a series of major caledonian c 470 430 ma metamorphic thrust nappes produced by nw wnw directed shortening and reworking of parts of a pre existing precambrian c 1000 ma orogen lewisian inliers occur throughout sutherland and represent tectonically interleaved units of the original basement gneisses upon which the moine cover sequence was deposited in the western moine nappe of the kyle of tongue region the rocks are deformed by two main episodes d1 and d2 which are thought to represent the effects of precambrian and caledonian orogenesis respectively little basement cover interleaving or large scale folding occurred during the d1 event in the moine nappe but the associated metamorphism to ut leust garnet grade produced peak metamorphic mineral assemblages subsequent caledonian reworking d2 produced a foreland propagating system of wnw directed ductile thrusts and related major f2 sheath folds both of which resulted in much of the observed moine lewisian interleaving metamorphism associated with d2 decreases in grade westwards occurring at similar or slightly lower grades than the earlier d1 event this retrogressive overprint is most obvious at the w margin of the moine nappe the d2 folds and ductile thrusts are geometrically and kinematically similar to the caledonian structures recognized within the moine thrust zone to the west the very low 5 ramp angles suggesting that the thrusts must have very shallow dipping trajectories through the crust f2 fold formation is attributed to an early phase of wnw directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued wnw directed thrusting the occurrence of internal extension in the orogenic wedge during active thrust related foreshortening implies that the shear strength of the material close to the basal detatchment s was continuously reduced i e wedge motion became supercritical in this context strain softening processes within the ductile thrust mylonites were probably of major significance

structural history of high pressure metamorphic rocks in the southern vanoise massif french alps and their relation to alpine tectonic events abstract a nappe of amphibolite facies metamorphic rocks of pre permian age in the southern vanoise massif the arpont schist has been affected by an alpine hp lt metamorphism the first mesoscopically recognizable deformation d 1 post dated the high pressure peak jadeitic pyroxene quartz glaucophane lawsonite and was associated with glaucophane epidote d 1 produced a flat lying schistosity and a nw trending glaucophane lineation and was probably associated with nappe displacement involving nw directed subhorizontal shear d 2 formed small scale folds and a foliation associated with chlorite albite the changing parageneses during the period pre d 1 to d 1 to d 2 suggest decreasing pressure so that the deformation appears to have been related to the uplift history rather than to the process of tectonic burial d 2 was followed by a static metamorphism green biotite chlorite albite possibly of lepontine age se directed backthrusting and folding d 3 and later differential uplift along steep faults took place under low grade conditions

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

deformation and motion in the western alpine arc we attempt to reconstruct the kinematics deformation and motion of the western alpine arc between the mercantour and aar massifs first we compile a map showing principal extension x direction within three major structural units in the external zone x directions are broadly radial to the alpine arc in the internal zone they are uniform or smoothly varying in some domains whereas in the intermediate zone they often show much scatter next we use geochronological data and p t conditions estimated from metamorphic mineral reactions to time the deformation on each of a series of interval maps we show x directions for strain accumulated during a chosen time interval we argue that these incremental x directions are approximately equivalent to segments of particle paths showing displacements relative to stable europe for 120 100 ma the displacement direction was about n150 and coeval with eclogite facies metamorphism it is interpreted as a direction of overthrusting by the african plate around 40 ma the displacement in the south part of the arc became westerly finally between 25 and 15 ma displacement directions were generally radial to the arc

subtle stretching lineation revealed by magnetic fabric of callovian oxfordian black shales french alps abstract the anisotropy of magnetic susceptibility ams of callovian oxfordian black shales from the subalpine massifs of the french alps has been measured the magnetic susceptibility of these formations originates from ferromagnetic titanomagnetite and paramagnetic phyllosilicates illite and chlorite although the magnetic fabric is complicated by the interaction of planar and linear structural elements and the presence of variable amounts of minerals two types of lineation are preserved a stretching lineation which exhibits counterclockwise rotation as a result of deformation from south to north and an intersection lineation parallel to the fold axis this set of new structural data together with semiquantitative analysis of ams is in agreement with the geodynamic evolution of the western alps

source tracing of detrital serpentinite in the oligocene molasse deposits from the western alps barreme basin implications for relief formation in the internal zone we present the first contribution of tracing the source area of ophiolitic detritus in the alpine molasse deposits by raman spectroscopy the lower oligocene molasse deposits preserved in the barreme basin in the sw foreland of the western alpine arc are known for the sudden arrival of the first exotic detritus coming from the internal alpine zones among them the pebbles of serpentinized peridotites have so far not been studied we show that they only consist of antigorite serpentinite implying that they originate from erosion of high temperature blueschists in contrast the upper oligocene lower miocene molasse shows mixed clasts of serpentine including antigorite and lizardite without any evidence of chrysotile this suggests that they were derived from a less metamorphosed unit such as the low temperature blueschist unit taking into account the sediment transport direction in the basin and the varied metamorphic characteristics of the other ocean derived detritus we constrain the lithologic nature of the source zones and the location of the relief zones identified as the internal alps se of the pelvoux external crystalline massif available structural data and in situ thermochronological data allow the reconstruction of the oligocene to early miocene collisional geometry of the palaeogene subduction wedge this phase corresponds to two major phases of uplift evolving from a single relief zone located above the ivrea body during early oligocene times and persisting up to early miocene times then during late oligocene early miocene times a second relief zone developed above the brianconnais zone at that time the internal western alps acquired its double vergency

present day vertical isostatic readjustment of the western alps revealed by numerical modelling and geodetic and seismotectonic data the active tectonics of the western alps reveals contrasting regimes ongoing extension at the heart of the chain and transpression compression at its external sectors the active processes currently affecting this region are still a matter of debate the classical models proposed in the literature invoke eurasia adria plate collision counterclockwise motion of the adria micro plate slab retreat of the subducted continental lithosphere and slab detachment more recently several authors prefer the hypothesis of tectonics driven by isostasy buoyancy forces to better understand the influence of these processes on the velocity strain and stress fields at the surface and in the crust we developed 2d viscoelastic numerical models along a vertical cross section perpendicular to the western alps we run our models with different driving forces in order to investigate one by one the geodynamic processes proposed in the literature results are compared with available geodetic geological and seismotectonic data in order to bring into coincidence model predictions and observations an important vertical isostatic readjustment must be included in the modelling together with a slight horizontal compression 0 5 mm year 1 probably due to africa eurasia convergence we show that the subduction process in this alpine region is likely to be dead and that buoyancy forces may be dominating the present day tectonics the geological society of london 2010

the geometrical relationship between the stretching lineation and the movement direction of shear zones abstract the geometrical relationship between foliations and lineations in shear zones is considered quantitatively assuming that the internal structures of shear zones have monoclinic symmetry the common practice of directly correlating the pitch or plunge of the stretching lineation with the obliquity of the shear direction is challenged it is the orthogonal projection of the stretching lineation on the shear zone boundary that is parallel to the shear direction a shear zone is parallel to a plane containing both the pole to the symmetry plane and the strike of the shear zone and the shear direction is parallel to the intersection of the symmetry plane and the shear zone boundary the symmetry plane is determined by the stretching lineation and the poles to the s foliation the implications of this study for the interpretation of shear zones are discussed with reference to an actual example

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

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

change of direction of overthrust shear in the helvetic nappes of western switzerland abstract the lowest of the helvetic nappes in western switzerland the morcles and the doldenhorn nappes are large scale recumbent folds in equivalent tectonic positions in the alpine chain a comparison of the orientations of stretching lineations in both nappes shows that the lineations in the two stratigraphically normal sequences trend n s whereas those in the two stratigraphically inverted sequences trend nw se a similar geometrical relationship has been found from an analysis of the calcite c axis directions of these limestones the two normal sequences show nw se directed c axes whereas the two inverted sequences show w e directed c axes these observations lead to the following conclusions 1 regionally consistent shear movements occurred over a strike distance of more than 60 km and these gave rise to similar deformation patterns in both nappes 2 a progressive rotation of these shear movements from a northerly direction in the earlier formed limbs to northwesterly in the later formed inverted limbs can be deduced the finite strain stretching lineations lag behind the crystallographic fabric axes formed in the later part of the deformation history

incremental strain analysis using two generations of syntectonic coaxial fibres an example from the monte marguareis brianconnais cover nappe ligurian alps italy abstract composite calcite quartz fibres developed in strain fringes on pyrites in the monte marguareis cover nappe ligurian brianconnais northern italy are utilised for an incremental strain analysis classical methods require non coaxial fibres to reconstruct incremental strain histories as an innovation this contribution shows that two generations of coaxial fibres grew independently on different pyrite sites during the two major deformation phases d1 and d2 which affected the monte marguareis structural unit rare curved fibres are shown by meso and microstructural analyses to have grown during d1 and been deformed by d2 folds this peculiarity provides a way to estimate quantitatively the strain associated with these deformation phases without the uncertainties related to classical methods d1 strain shows a marked strain partitioning among different lithologies and within the same lithology the measured elongations ranging between 1 84 and 8 45 measurements of d2 incremental elongation are available only for one lithology and indicate that d2 stretching is approximately half that of d1 and its contribution to the finite strain ellipsoid cannot be neglected this finding is consistent with qualitative inferences on strain partitioning between d1 and d2 phases measured d2 elongations display higher homogeneity throughout the area both l1 fibre orientation generally ne sw and d1 fold asymmetry suggest d1 deformation occurred in a ductile shear zone possibly induced by the south westward motion of nappes consistently with regional tectonics the l2 lineation orientation approximately ne sw and the d2 folds asymmetry suggest that the d2 phase is likely to be the product of a regionally recognised backthrusting event

subduction polarity reversal at the junction between the western alps and the northern apennines italy abstract the controversial relationship between the orogenic segments of the western alps and the northern apennines is here explored integrating recently published 3d tomographic models of subduction with new and re interpreted geological observations from the eclogitic domain of the voltri massif ligurian alps italy where the two belts joint each other the voltri massif is here described as an extensional domain accommodating the opposing outward migration of the alpine and apennine thrust fronts since about 30 35 ma using tomographic images of the upper mantle and paleotectonic reconstructions we propose that this extensional setting represents the surface manifestation of an along strike change in polarity of the subducted oceanic slab whose polarity changed laterally in space and in time our tectonic model suggests that the westward shift of the alpine thrust front from the oligocene onward was the consequence of the toroidal asthenospheric flow induced by the retreat of the apenninic slab

kinematics of the alpine plate margin structural styles strain and motion along the penninic austroalpine boundary in the swiss austrian alps a structural and kinematic analysis of parts of the alpine suture zone i e the arosa zone and the edges of the overlying austroalpine and underlying penninic units is presented the arosa zone is interpreted as part of an accretionary wedge formed by the overriding of the austroalpine units the northern part of the adriatic plate over the penninic units structures strain and stretching trajectories indicate that two main deformation events followed each other discontinuously the earlier event was a wsw to nw directed thrusting under noncoaxial deformation and led to melange type structures the second event less penetrative in the arosa zone and with a downward increase in strain and noncoaxial flow formed n to ne verging folds and imbrication zones west directed displacement commenced in the early cretaceous and the motion changed to a prevailing northerly direction during the eocene as revealed by sediments involved in thrusting and by radiometric dating of the mylonites the kinematic history deduced from structural analysis agrees with the history obtained by recent work on plate margin sediments we correlate the anticlockwise rotation of the adriatic plate relative to europe between about 130 and 60 ma with the west directed motion in the arosa zone a change to relative n s motion during the lower tertiary led to the n to ne directed displacements a direct relationship thus exists between the motion of the adriatic plate and the resulting deformation along its northern margin during the early history of the eastern alps partitioning in brittle and ductile deformation and structural slicing under noncoaxial deformation is responsible for the melange type structures

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

present day geodynamics in the bend of the western and central alps as constrained by earthquake analysis summary the contrasted tectonics of the western central alps is examined using a synthesis of 389 reliable focal mechanisms the present day strain regime is mapped and interpolated for the entire alpine belt based on a newly developed method of regionalization the most striking feature is a continuous area of extension which closely follows the large scale topographic crest line of the alpine arc thrusting is observed locally limited to areas near the border of the alpine chain a majority of earthquakes within the alps and its forelands are in strike slip mode stress inversion methods have been applied to homogenous subsets of focal plane mechanisms in order to map regional variations in stress orientation the stress state is confirmed to be orogen perpendicular both for 3 in the inner extensional zones and 1 in the outer transcurrent transpressional zones extensional areas are well correlated with the part of the belt which presents the thickest crust as shown by the comparison with the bouguer anomaly and the average topography of the belt in the northwestern swiss alps extension is also correlated with currently uplifting zones these observations and our strain stress analyses support a geodynamic model for the western alps in which the current activity is mostly a result of gravitational body forces earthquakes do not provide any direct evidence for ongoing convergence in the alpine system but a relationship with ongoing activity of complex block rotations of the apulian microplate cannot be ruled out

kinematic model for the curvature of the northern subalpine chain france abstract analyses of cross sectional shortening mapped faults and calcite twin strain are used in conjunction with geometric kinematic modeling to interpret the mechanism of curvature formation in the northern subalpine chain of france there is a 45 change in strike around the chain that is convex towards the foreland northwest five models for fold thrust belt curvature are applied to the northern subalpine chain they can reproduce the geometry and the kinematic history of a curved fold thrust belt and can be used to predict the magnitude and orientations of strains and the three dimensional geometry of the curvature the models are pure bending radial thrusting curve parallel simple shear uniform displacement uniform shortening and transport parallel simple shear two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain extension or contraction parallel to strike and cross sectional shortening the transport parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains this is the pattern observed in the northern subalpine chain therefore the curvature of the northern subalpine chain is attributed to a transport parallel simple shear mechanism

the curved translation path of the parpaillon nappe french alps the parpaillon nappe is one of the two helminthoid flysch nappes emplaced on the external dauphinois zone of the western alps a structural analysis of the nappe is presented two superposed deformations d1 and d2 are described that are mainly characterized by large scale recumbent folds whose axes are quasi orthogonal ne sw for d1 and nw se for d2 their vergence is northwestward for d1 and southwestward for d2 during the d2 deformation the nappe was separated into two units one of these being thrusted over the other an analysis of incremental strain using quartz and calcite fibre growth indicates that d2 follows d1 without discontinuity therefore the superposition of d1 and d2 structures is interpreted as a progressive deformation instead of two distinct phases of deformation the emplacement of the nappe is discussed under two aspects the relations between displacement and strain and the role of gravity it is concluded that the translation has been twofold first towards the nw and then towards the sw and that the displacement result essentially from gravity forces kinematic implications for the alpine collision are suggested

thrust tectonics and crustal shortening in the external french alps based on a seismic cross section abstract duplex geometry has an important bearing on understanding the development of orogenic zones based on a published seismic section through the french alps the external massifs are considered to form a duplex structure as a result of basement thrusting the duplex is restored to a uniform basement thickness giving a model estimate of shortening of 45 km the interpretation of this figure in terms of early alpine crustal thinning is discussed

hydrogen isotope fractionation in land based serpentinization systems comment on origin of methane in serpentinite hosted hydrothermal systems the ch4 h2 h2o hydrogen isotope systematics of the hakuba happo hot spring by suda et al earth planet sci lett 386 2014 112 125

finite strain patterns in some alpine nappes abstract the patterns of finite strain in some alpine nappes and thrust sheets are reviewed together with their tectonic evolution glarus nappe the finite strain pattern approximately that of heterogeneous simple shear was acquired during upper oligocene times when the helvetic thrust block was deformed together with its infrastructure and basement the glarus thrust was formed during miocene times and modified by pliocene to recent differential uplift morcles nappe this large recumbent fold nappe overrode the aiguilles rouges massif and its cover towards the northwest the finite strain pattern resulted from successive diagenetic compaction multilayer buckling and heterogeneous simple shear high strains occur along the inverted limb of the nappe and fold hinges are oriented subperpendicular to the nw stretching lineation final emplacement of the nappe during the miocene was perhaps a later event chartreuse massif 33 e w shortening was developed across this fold and thrust belt during post helvetian times by decollement along the oxfordian shales in response to thrusting and uplift in the belledonne basement the oxfordian shales had earlier accommodated a pre helvetian probably oligocene deformation in the basement and its trias bajocian cover when n030 trending subhorizontal folds cleavage and a stretching lineation 18 n030 stretching were developed digne nappe detached along the keuper evaporites the cover was displaced westwards in two stages 1 late oligocene with folding cleavage development formation of the frontal slices and an overall e w shortening of 20 and 2 late miocene pliocene after emplacement of the embrunais nappes in the east producing the digne thrust it is concluded that for these alpine nappes geological criteria must be used to establish their evolution if the internal deformation can then be assigned to part of this evolution the pattern of finite strain can be used to quantify and elucidate that part a maximum of information is gained if the incremental deformation history can also be obtained from study of syntectonic fibres

progressive evolution from uplift to orogen parallel transport in a late acadian upper amphibolite to granulite facies shear zone south central massachusetts fabrics within a 1 km wide shear zone along the eastern margin of the bronson hill anticlinorium in central massachusetts provide important information on the regional tectonic development during the devonian acadian orogeny two distinct mineral lineations observed within the zone are each defined by alignment of coarse sillimanite needles and elongated quartz and feldspar grains and aggregates both lineations are associated with shearing and development of mylonitic fabrics the character of the two lineations and related fabrics differs mainly in orientation and in the relationship of each lineation to the local orientation of quartz c axis fabrics the interpreted earlier lineation has a steep west plunge and the later lineation has a shallow plunge and north south to southwest trend parallel to the regional trend of the orogen extensive dynamic recrystallization of feldspar the possible activity of prism slip in quartz and the mineral assemblages in apparent equilibrium during deformation suggest that both lineations probably formed under upper amphibolite facies conditions just after the thermal peak of acadian metamorphism a progressive change between the two deformation fabrics is suggested by evidence of overlap between the two by their striking similarity in style and metamorphic conditions of formation and by the similarity in orientation of their bulk strain axes with a change only in the transport direction between them kinematic indicators associated with each phase of lineation development are consistent throughout the shear zone the earlier phase has a west side up sense of shear and the later phase indicates west side north movement these phases are correlated with the regional backfold and dome stages of acadian deformation the kinematics and relationships between them suggest a revised tectonic model for the later part of the acadian orogeny and for development of orogen parallel lineations in this region in this model uplift and thickening of basement gneisses in the bronson hill anticlinorium during the backfold stage was impeded by gravitational instability in a thermally weakened crust this led to a change from subvertical to subhorizontal extension east west acadian shortening was still in progress constraining this extension to a north south direction parallel to the trend of the orogen

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

the arc of the western alps in the light of geophysical data on deep crustal structure recently two international deep seismic campaigns in the western and central alps etude continentale et oceanique par reflexion et refraction sismique progetto strategico crosta profonda ecors crop roure et al 1996b and national research program 20 nrp 20 pfiffner et al 1997 have been completed here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the alps collected during the past 40 years including the two above mentioned geophysical projects the tectonic reinterpretation of the ecors crop high resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology combined with previously published interpretations regarding the central alps schmid et al 1996 this study reveals substantial differences in geometry and kinematics between transects across the western and central alps respectively at depth the transition between the central and western alps is marked by the western limits of an adriatic lower crustal wedge shaped structure and the northeastern limits of a similar structure made of european lower crustal material at the surface it coincides with a corridor of dextral strike slip along the tonale and simplon lines in addition the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the penninic realm of the western alps to less than 20 km in the central alps the formation of the western alpine arc was initiated during convergence and collision before 35 myr ago when the adriatic micro plate moving northward with respect to the european foreland caused sinistral transpression in the western alps while the central and eastern alps underwent head on convergence and collision during the post collisional stage i e after 35 myr ago the arcuate shape of the western alps was accentuated by wnw directed movement and anticlockwise rotation of the adriatic microplate decoupled from the central and eastern alps along the tonale simplon dextral shear zone this led to wedging of lower crustal slices both in the western alps and in the central alps the new tectonic interpretation of the ecors crop transect allows a kinematic model to be established for crustal shortening in the western alps during the past 35 myr the ivrea mantle plays the role of a backstop in our tectonic model we distinguish three episodes of post collisional crustal shortening along the ecors crop transect from 35 to 30 ma the brianconnais basement was back thrusted over the gran paradiso units leading to 30 km of crustal shortening first episode in the early miocene movements concentrated mainly along the penninic frontal thrust and resulted in about 60 km shortening second episode post 12 ma shortening within the external massifs is associated with folding in the jura mountains when the crust was shortened by an additional 30 km third episode

development of a transverse to orogen parallel extension lineation in a complex collisional setting trans hudson orogen manitoba canada abstract in the southeastern trans hudson orogen tho the wide dispersion in orientation of the l3 extension lineation from orogen parallel to transverse is considered primary as it cannot be accounted for by simple overprinting of a single precursor orientation interpretation of the origin of this extension lineation is complicated by the fact that its gently northeast plunging orientation at the north end of the study area is inconsistent with the evidence for sinistral east side up movement of the steep oblique slip d3 faults the hypothesis presented here to explain the complicated lineation pattern is based on evidence that the preceding period d2 southwest directed transport overlapped temporally and spatially with d3 northwest southeast transpressional shortening d2 and d3 are attributed to successive but overlapping collisional events d3 transpressional shortening is interpreted to have propagated westward through the tho resulting in a prolonged and complex transition between d2 and d3 deformation regimes the dispersion of the l3 extension lineation is interpreted to reflect the competing effects of both deformation regimes which appear to have been partitioned in the crust with d2 effects best developed in higher metamorphic grade rocks and d3 effects best developed in lower metamorphic grade rocks

heterozoan carbonate deposition on a steep basement escarpment late miocene almeria south east spain heterozoan temperate water carbonates mixed with varying amounts of terrigenous grains and muddy matrix azagador limestone accumulated on and at the toe of an inherited escarpment during the late tortonian early messinian late miocene at the western margin of the almeria nijar basin in south east spain the escarpment was the eastern end of an uplifting antiform created by compressive folding of triassic rocks of the betic basement channelized coralline algal bryozoan rudstone to coarse grained packstone together with matrix supported conglomerate are the dominant lithofacies in the higher outcrops comprising the deposits on the slope these sediments mainly fill small canyon shaped half graben depressions formed by normal faults active before during and after carbonate sedimentation roughly bedded and roughly laminated coralline algal bryozoan rudstone to coarse grained packstone are the main lithofacies forming an apron of four small kilometre scale lobes at the toe of the south eastern side of the escarpment almeria area channelized and roughly bedded coralline algal bryozoan rudstone to coarse grained packstone conglomerates packstone and sandy silt accumulated in a small channel lobe system at the toe of the north eastern side of the escarpment las balsas area carbonate particles and terrigenous grains were sourced from shallow water settings and displaced downslope by sediment density flows that preferentially followed the canyon shaped depressions roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope whereas the rest of the carbonate lithofacies were deposited by high density turbidite currents the steep escarpment and related break in slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse grained low transport efficiency skeletal dominated sediment in the apron lobes accelerated uplift of the basement caused a relative sea level fall resulting in the formation of outer ramp carbonates on the apron lobes which were in turn overlain by lower messinian coral reefs this is the first known example of a base of slope apron within temperate water carbonate systems this article is protected by copyright all rights reserved

calcite fabrics in a natural shear environment the helvetic nappes of western switzerland the aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the upper jurassic limestones of the helvetic nappes between the mont blanc and aar massifs over a distance along strike of about 180 km the c axis preferred orientation pattern which could be inferred is regionally very constant the c axis point maxima are everywhere subperpendicular to the regional strike of the chain there is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains the c axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain shape fabric this fabric geometry indicates either continuous shear deformation or alternatively a two stage deformation history the obliquity of the grain shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process or it may represent a later finite deformation which has led to the formation of a regionally developed second cleavage there is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern this suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction the observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms a combination of slip on r and twinning on e however the e twinning observed in the specimens investigated is younger than the development of the crystallographic fabric a new method for stress determination from e twinning is proposed

kinematics of the northern part of the simplon line central alps abstract structural studies were carried out along the northern part of the simplon line in order to constrain the kinematic history of the line during tertiary times the ductile strain accumulated within both basement units and mesozoic cover reveals the upper units sheared towards the west southwest the simplon line can be described as a major shear zone along which the main movement occurred perpendicular to the line this shearing developed during retrograde metamorphic conditions and postdates earlier deformations associated with the high temperature lepontine episode finally it was followed by the formation of the berisal syncline back folding episode which generated some brittle movements towards the south localized at the contact between the lower and upper pennine units i e on the simplon line

retrogressive fabric development during exhumation of the voltri massif ligurian alps italy arguments for an extensional origin and implications for the alps apennines linkage geological mapping coupled with structural investigations carried out in the voltri massif eastern ligurian alps italy provide new data for the interpretation of the tectonic context controlling main fabric development during exhumation of its high pressure core the voltri massif is here interpreted as a c 30 km long eclogite bearing asymmetric dome formed by the progressive verticalisation of the regional second phase mylonitic foliation developed during retrogressive greenschist metamorphic conditions in this light the exhumation history is driven by a ductile to brittle extensional process operating through low angle top to the w multiple detachment systems a late eocene early oligocene age for this extensional episode is proposed on the basis of structural correlations stratigraphic and radiometric constraints in this scenario the voltri massif is interpreted as an extensional domain developed to accommodate the late eocene early oligocene arching of the western alps northern apennines orogenic system

from mantle to crust stretching the mediterranean abstract the origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated this work is based on a compilation of sks fast splitting directions that give an image of flow lines in the mantle around the mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts we find that 1 the two sets of directions are parallel in the three main backarc regions namely the alboran sea the tyrrhenian sea and the aegean sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle suggesting that 2 crustal deformation is mainly driven from below by slab retreat and 3 the lithospheric fabric is reset within a few millions of years in backarc environments

kinematics of ductile shearing from outcrop to crustal scale in the monte rosa nappe western alps significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the mesozoic cover quartzites and marbles of the monte rosa nappe in the inner part of the western alps we summarize here the results of a detailed microstructural study at several scales of observation in the northern part of the nappe and discuss the kinematic significance and compatibility of such results most of the observed deformations can be accounted for by progressive wnw vergent shearing during and after the late eocene within a ductile shear zone of crustal scale the displacement of the hanging wall may have been in excess of several tens of kilometers however some ese vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the monte rosa nappe

widespread post nappe normal faulting in the internal western alps a new constraint on arc dynamics abstract field data collected in the brianconnais and piemont zones at the centre of the western alpine arc demonstrate a high density of normal faults normal displacements are pervasive from the millimetric to the kilometric scales they occur both parallel and at high angles to the arc overprinting all the alpine folds and thrusts and the associated schistosities the orientation of fault striae implies that the pile of metamorphic nappes se of the pelvoux massif has undergone a general extension extension tends to be multidirectional in the brianconnais zone orogen parallel extension prevails in most of the piemont schistes lustres to the west in brianconnais zone brittle extension is associated with a reversal of movement along the brianconnais frontal thrust that started before the end of the oligocene in the piemont schistes lustres the onset of normal faulting occurred early in the miocene and continued the ductile extension that accompanied the exhumation of hp lt metamorphic thrust sheets during the palaeogene in both zones extension remains currently active resulting in the regional seismicity since more than 20 ma ago this normal faulting in the internal arc coexists with an outward propagation of folds and thrusts in the external arc we discuss its possible significance in the context to the dynamic evolution of the alps

alpine tectonics and rotation pole evolution of iberia abstract the geological evolution of the betic cordilleras and pyrenees reflects the cretaceous and tertiary rotation pole and kinematic evolution of the iberian and african plates new constraints on the alpine tectonic evolution of the iberian plate are provided by p t t data and regionally consistent stretching lineations from the metamorphic parts of the betic cordilleras high pressure low temperature metamorphism in the betic cordilleras resulted from continent continent collision which caused subduction to a maximum depth of 37 km a preliminary 116 10 ma radiometric age for this event corresponds to the initiation of seafloor spreading to the west of iberia which lasted until about 80 ma intracontinental thrusting in the betics between 99 ma and 83 ma took place after subduction ended e w to ese wnw trending stretching lineations indicate the direction of thrusting which resulted in extensional strains of 200 600 the timing of thrusting in the betics coincides with a 95 80 ma tectonic phase in northern africa during which e w stretching lineations were formed the stretching lineations are coincident with the 110 80 ma motion vector of africa iberia with respect to eurasia thrusting in the betics and deformation in northern africa was driven by convergence of africa iberia and eurasia cretaceous deformation is further recorded by terrigeneous sedimentation in the mauritanian flysch and by the tectosedimentary evolution of the malaguide complex crustal thinning magmatism and metamorphism in the pyrenees during the 110 85 ma period is governed by a left lateral strike slip of africa iberia with respect to eurasia around the same rotation pole as thrusting in the betics during the 80 54 ma period the rotation pole was situated west of gibraltar near the previous active collision zone this inhibited large scale overthrusting and related penetrative deformation in northern africa and the betic cordilleras deformation was instead transferred to the northern boundary of iberia now acting as an african promontory from the campanian on wards oblique convergence took place around the combined gibraltar rotation pole deformation culminated in the late eocene corresponding to spreading in the norwegian greenland sea at 55 ma which induced an additional compression in western eurasia during the pyrenean collision high pressure metamorphic rocks in the betic cordilleras were exhumed and they cooled substantially the cooling trend was disturbed by oligocene extensional deformation and introduction of a transient heat source which correlates with the mantle being uplifted during extension heating culminated at the oligocene miocene boundary in the betic cordilleras and in northern africa this evolution agrees with the development of a plate boundary between iberia and africa at 30 ma after completion of the pyrenean collision the new plate boundary was connected to the western european rift system renewal of compression and overthrusting in the betic zone took place after 20 ma overthrusting is succeeded by two phases of wrenching juxtaposing crustal segments with different moho depths inherited from the late oligocene to early miocene extension

kinematics of the alpine arc and the motion history of adria the alps have been interpreted traditionally in terms of north or northwest directed convergence between africa and europe1 with adria the continental underpinnings of the adriatic sea and northern italy acting as a promontory of the african continent2 this has been supported by recent analyses of africa europe relative plate motions3 5 which indicate dominantly north south directed convergence during the tertiary phase of intracontinental shortening we show here however that kinematic indicators from alpine tectonites are difficult to reconcile with this motion history figs 1 and 2 and are explained best by roughly west northwest directed adria europe motion since about 70 million years ago combined with an arc normal pattern of motion driven by local body forces this implies that adria has moved independently of both africa and europe since at least the late cretaceous

western alpine back thrusting as subsidence mechanism in the tertiary piedmont basin western po plain nw italy abstract basin formation dynamics of the tertiary piedmont basin tpb are here investigated by means of cross section numerical modelling previous works hypothesised that basin subsidence occurred due first to extension oligocene and then to subsequent loading due to back thrusting miocene however structural evidence shows that the tpb was mainly under contraction from oligocene until post pliocene time while extension played a minor role furthermore thermal indicators strongly call for a cold flexure induced mechanism but are strictly inconsistent with a hot thermally induced mechanism our new modelling shows that the tpb stratigraphic features can be reproduced by flexure of a visco elastic plate loaded by back thrusts active in the western alps in oligo miocene times far field compression contributed to the tpb subsidence and controlled the basin infill geometry by enhancing basin tilting forebulge uplift and erosion of the southern margin of the basin these results suggest that the tpb subsidence is the result of a combination of mechanisms including thrust loading and far field compressional stresses