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

strain patterns in models of spreading gliding nappes three experiments have been carried out in order to study the progressive and finite strain in nappes where gravitational spreading and gliding occur together these experimental models were made from analogue materials able to collapse under their own weight on an inclined plane throughout most of the models the stretch trajectories in a vertical plane parallel to the flow show a low amplitude sigmoidal pattern consistent with a previous theoretical model other stretch trajectories exist at the front and back ends of each model the kinematic significance of all these trajectories is discussed in detail highest strain intensities are always found next to the base of the model strain paths calculated at different stages of flow indicate clearly that nowhere in the model is the motion simple there is a complex combination in time and space of simple and pure shear except at the base of the model where the motion approximates a simple shearing near the two parallel lateral sides of the model a lateral boundary effect can be observed from the strong curvature of the transverse markers surface grid and vertical layers strain paths have also been estimated in this complex zone where wrench shearing is superimposed upon vertical shortening and thrust shearing the geological implications of these experimental models are discussed in relation to recent theoretical studies and field work

understanding progressive arc and strike slip related rotations in curve shaped orogenic belts the case of the olevano antrodoco sibillini thrust northern apennines italy 1 we report on a paleomagnetic study of the southern sector of the olevano antrodoco sibillini oas thrust front which corresponds to the southern limb of the northern apennines italy orogenic salient a lively debate has developed regarding the oroclinal progressive arc versus non rotational nature of the oas which has been alternatively interpreted as a dextral strike slip fault dextral transpressive fault or frontal to oblique ramp that reactivated pre existing jurassic normal faults here we document the paleomagnetism integrated with biostratigraphic and structural data of 52 new sites from both the oas hanging wall and footwall on the basis of 39 retained sites we find a peculiar pattern of tectonic rotations along the oas thrust that evidences four rotational domains the thrust footwall is characterized by a southern domain that undergoes an approximately 30 counterclockwise rotation with respect to the stable foreland and an approximately non rotated domain the data from the hanging wall indicate the occurrence of a dextral strike slip component along the southern sector of the oas thrust supported by a strong clockwise rotation close to the ne sw lateral ramp which rapidly fades 1 km from the thrust front a slight but significant cw rotation observed in the remaining sites from the hanging wall confirms the progressive nature of the oas and its structural position as the southern limb of the northern apennines salient our detailed paleomagnetic study is crucial in discriminating between progressive arc and strike slip related components in the main curved orogenic front of the northern apennines

volume loss model for trace element enrichments in mylonites mylonites from three widely separated localities along the fries ductile deformation zone in the blue ridge province show substantial enrichments in trace and minor elements tio sub 2 p sub 2 o sub 5 zr y and v and depletions in rb and sr relative to the protolith two end member hypotheses can explain the enrichments one involves partitioning to and from an infiltrating fluid and assumes that all the elements were mobile the second hypothesis assumes the high field strength cations were largely immobile and that their enrichment is due to large volume losses in the mylonites modeling indicates that ti is immobile for fluid rock ratios as high as 10 sup 4 and petrographic and modal data support a volume loss mechanism volume loss was accommodated by loss of sio sub 2 and alkalies during feldspar dissolution calculated fluid rock volume ratios required to produce the observed sio sub 2 losses assuming ti mobility range from 70 to 600 variable enrichments in p zr y and v are attributed to selective mobility of these elements during fluid infiltration

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

a modern structural regime in the paleoarchean 3 64 ga isua greenstone belt southern west greenland abstract the footwall gneisses beneath the western part of the paleoarchean 3 8 ga isua greenstone belt southern west greenland are interpreted here in terms of a 3 64 ga stack of mylonitic crystalline thrust nappes the oldest example known on earth in present coordinates the kinematic history of the thrust nappe stack is couched in terms of initial longitudinal strike parallel thrusting towards the southwest followed by transverse thrusting to the northwest and subsequent extensional collapse of the thickened crust toward the southeast diorite and tonalite that form the western margin of granitoids structurally overlying the western part of the isua greenstone belt and its footwall contain 3 5 ga mafic dykes some of which are deformed and or truncated at fault contacts within the granitoids accordingly a component of the deformation structurally above the isua greenstone belt occurred after 3 5 ga significantly later than the formation of the underlying mylonitic nappes probably during the neoarchean the structural regime of mylonitic thrust nappe stacking is very similar to that in modern mountain belts it would appear that the deformational behaviour rheological constitution and overall strength of paleoarchean and modern continental crust were similar

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

rolling structures at large shear strain abstract at large shear strain shear criteria are often obliterated or become ambiguous from examination of both natural examples and experimental models we describe a new criterion called rolling structure widely represented in sheared rocks a typical rolling structure is composed of a rigid or competent object e g porphyroclast boudin fossil etc with two tails asymmetrically disposed around it in most cases tails are of the same material as that of the rotating object and result from strain softening and grain size reduction at the object periphery forming a mantle z and s asymmetries of rolling structures represent dextral and sinistral senses of shearing respectively tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense besides the determination of the sense of shear the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks since rolling structure length is proportional to shear strain

the structure and timing of lateral escape during the scandian orogeny a combined strain and geochronological investigation in finnmark arctic norwegian caledonides abstract determining the timing duration and mechanism of tectonic events within an orogenic cycle such as ocean subduction continent continent collision or gravitational collapse is challenging especially in ancient orogenic belts variations in the tectonic transport direction however can be used as a guide to these stages of orogeny while thrust sheets within the caledonian allochthon in north norway were emplaced broadly eastwards perpendicular to the trend of the orogen many features indicate material transport in other orientations one dominant feature of the mageroy nappe sitting above and infolded with the kalak nappe complex is the development of a strong n s lineation orthogonal to the main transport direction strain measurements in part determined by a new method are used in the context of the regional structural data to identify the critical stage in orogeny when compressional forces are balanced by orogen parallel lateral escape quantitative 3 d strain estimation in the mageroy nappe indicates prolate deformation with c 50 horizontal shortening parallel to the thrusting direction e w and c 200 extension along the orogenic strike n s with c 30 vertical shortening temporal constraint on this fabric is provided by ar ar isotopic analysis of undeformed white mica in cross cutting granitic pegmatites these data show that prolate deformation occurred before the white mica cooling age of 416 4 ma while the previously determined depositional age of the hellefjord schist indicates that it occurred after 438 4 ma a granitic pegmatite that intruded the hellefjord schist after an initial deformation phase but during or prior to a later deformation has been dated at 431 2 ma by u pb zircon a previous lower age constraint on this deformation of 428 5 ma is given by metamorphic zircon overgrowths on fractured grains these results constrain the continental collision between baltica and laurentia in finnmark to the interval c 431 428 ma placed in a regional context these results indicate that lateral escape was orthogonal to the thrusting direction and occurred during the continent continent collision stage in the scandian orogeny before gravitationally driven collapse

stratigraphic evolution in the ligurian alps between variscan heritages and the alpine tethys opening a review abstract the ligurian alps are the southernmost segment of the alpine orogenic belt their formation is the result of a multistage process that comprises two orogenic cycles the variscan and the alpine one since the late paleozoic onwards superposed sedimentary successions deposited in this area as a response to the changing tectonic setting in the frame of the western mediterranean evolution the transition between the two orogenies was characterized by the formation of a net of sedimentary basins controlled by diffused extension which is not correlated to a clear geodynamic context yet as the fault network is severely affected by alpine reworking the multiple pre alpine tectonic stages experienced by the ligurian sector can be unravelled only by the analysis of the well exposed stratigraphic succession which offers very distinctive feature for each tectonic phase from the present day situation of a nappe pile orogen backward to the originary basinal setting the aim of this work is to reconstruct the paleogeographic evolution of the ligurian alps between permian and jurassic through a detailed stratigraphic analysis several sedimentary packages belonging to different tectonic units are analysed grouped into specific domains and correlated five major steps in the evolution of the ligurian alps have been pointed out they have been contextualized within the latest paleogeographic reconstructions of the alpine sector contributing to detail the role of the study area in the plate tectonic dynamics the sedimentary record is referred to successive geodynamic stages from the pangea break up and variscan belt dismantling through a triassic diffused extension to the alpine tethys rifting and finally to the spreading that generated the piedmont ligurian oceanic branch the stratigraphic reconstruction of the ligurian sector also indicates the lacking of an ocean interposed between the european continent and the alpine collisional wedge thus representing the southward termination of the valais basin on the whole the paleogeographic reconstructions provided in this work highlight that the ligurian alps were a domain in which for over a hundred million years orogenesis rifting and oceanisation strongly affected the integrity of the upper crust the heterogeneity of the crust that suffered multiple mantle uplifts partial melting and extensive faulting over a long period is testified by field evidences and several effusive events the last and most important rifting and the following alpine tethys oceanisation developed around a narrow elongated area of crustal weakness generated by multiple geodynamic events

a new structural and kinematic framework for the alboran domain betic rif arc western mediterranean orogenic system the processes that drastically thinned and rapidly exhumed the alboran domain in southern spain are poorly understood geological maps cross sections and a synthesis of previous work in the alpujarride complex provide a new structural framework for the alboran domain with major revision of the previously defined lithotectonic units alboran continental lithosphere was buried and thickened in the paleogene d 1 and subsequently thinned during late orogenic extension in the early miocene regional patterns of stretching lineations and late recumbent folds record important spatiotemporal changes in the kinematics of extension east directed shear d 2 is preserved in deep orogenic levels and thinned the crustal sequence during rapid exhumation and high temperature metamorphism north directed shear d 3 d 4 is preserved in upper orogenic levels and reworked earlier structures during the later stages of exhumation and cooling this large scale switch in the direction of finite stretching resulted from the interaction between components of orogen parallel d 2 and orogen perpendicular d 3 d 4 extension and associated vertical and lateral gradients in strain we provide a novel kinematic model for gravity driven extensional collapse of the alboran domain compatible with both westward motion of the alboran domain relative to the iberian margin and subsequent northward extensional flow and emplacement of alboran lithosphere onto the iberian margin supplementary material a list of sample names coordinates and descriptions plus a google earth kmz file of sample locations is available at https doi org 10 6084 m9 figshare 5508643 a geological sketch map of the sierra de las estancias indicating the trend of stretching lineations in the alpujarride complex and location of major geological boundaries is available at https doi org 10 6084 m9 figshare 5513398

initiation and development of the pennine basal thrust swiss alps a structural and geochronological study of an exhumed megathrust abstract the pennine basal thrust pbt is an exhumed megathrust developed during continental collision from late eocene to miocene to trace its evolution five samples with indications for up to three microstructurally diachronous white mica generations were investigated by laser in situ and step heating 40 ar 39 ar dating three deformation related crystallization ages can be distinguished 1 d 1 characterized in the pbt hanging wall by an s 1 foliation defined by white mica chloritoid began at or before 38 0 ma 2 d 2 formed a pervasive s 2 cleavage and synchronous white mica rich veins dated at 27 ma 3 d 3 produced an s 3 crenulation cleavage and chlorite white mica veins dated at 23 ma older ages of 96 ma footwall and 115 ma hanging wall are interpreted as minimum ages for the detrital component finally discrete faulting produced fault gouge with an illite k ar age of 19 ma a simplified back restored reconstruction provides a tectonic context for the dated structures in this framework d 1 occurred during middle to late eocene tectonic accretion after late eocene initiation of continental collision d 2 reflects oligocene top to nw shearing with both in and out of sequence thrusting d 3 then developed from 23 to 19 ma during the progressive deactivation of the pbt

polyorogenic deformation history recognized at very shallow structural levels the case of the antola unit northern apennine italy abstract the antola unit is a ligurian unit occurring at the top of the nappe pile of the northern apennines italy structural analysis indicates that the antola unit has been involved in a complex polyphase deformation history developed at a shallow structural level the successive deformation phases belong to both the alpine and apenninic orogenic cycles therefore the antola unit is a good example of a polyorogenic tectonic unit deformed at a very shallow structural level the older part of the structural evolution is assigned to the meso alpine tectonic stage and consists of two deformation phases d1 and d2 characterized by opposite tectonic transport directions the d1 and d2 structures were sealed by the middle eocene miocene deposits of the tertiary piedmont basin the younger part of the deformation history instead can be referred to the northern apennines tectonics and consists of two phases d3 and d4 both involving the tertiary piedmont basin succession the structural data indicate that the d1 deformations developed in mostly unlithified rocks and that these structures can be related to the westward emplacement of the antola unit in the alpine orogenic wedge the d2 phase is characterized by folds and low angle extensional faults and is referable to a gravity driven deformation both the d3 and d4 phases characterized by folds and thrusts are due to the compressive regime related to the development of the northern apennines

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

experiments on folding in spreading gliding nappes abstract a small scale model of spreading gliding nappe was used to study the development of folds which are controlled by the geometry of external boundaries throughout in the model the deformation combines pure shear and simple shear recumbent folds are produced above irregularities of the base the cylindricity of folds is directly dependant on the cylindricity of basal irregularities models with homogeneous rheological properties produce only recumbent folds whereas models of contrasting rheological properties produce recumbent folds and en echelon upright folds in the zones of lateral shear the experiments demonstrate that 1 a large scale recumbent folds in nappes can result from passive amplification processes in unsteady shear flow 2 b upright folds and coaxial refolding result from mechanical instability due to contrasting rheological properties between layers and develop during a unique progressive deformation these laws can be directly applied to a spreading gliding nappe but also on a larger scale to all nappes in which the shear component dominates the deformation

the theoretical effect of metasomatism on sm nd isotopic systems abstract if sm and nd are mobilized during fluid rock interaction fractionation of the ratio between sm and nd in geological units may take place the fractionation can be characterized by a fractionation factor f which can attain any value between d sm d nd and 1 d sm d nd depending on the fluid rock ratio during ree transfer if such fractionations are not recognized this will induce errors in sm nd isotopic interpretations of age and provenance of the affected geological units the apparent ages and nd values are functions of the true values f and the time lapse between petrogenesis and secondary disturbance the functional relationships can be closely approximated by direct proportionalities

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

microfabrics of calcite ultramylonites as records of coaxial and non coaxial deformation kinematics examples from the rocher de l yret shear zone western alps microfabrics were analysed in calcite mylonites from the rim of the pelvoux massif western alps france wnw directed emplacement of the internal penninic units onto the dauphinois domain produced intense deformation of an eocene age nummulitic limestone under lower anchizone metamorphic conditions slightly below 300 c two types of microfabrics developed primarily by dislocation creep accompanied by dynamic recrystallisation in the absence of twinning coaxial kinematics are inferred for samples exhibiting grain shape fabrics and textures with orthorhombic symmetry their texture crystallographic preferred orientation cpo is characterised by two c axis maxima symmetrically oriented at 15 from the normal to the macroscopic foliation non coaxial deformation is evident in samples with monoclinic shape fabrics and textures characterised by a single oblique c axis maximum tilted with the sense of shear by about 15 from the analysis of suitably oriented slip systems for the main texture components under given kinematics it is inferred that the orthorhombic textures which developed in coaxial kinematics favour activity of b10 11n and b02 21n slip along the f and r planes respectively with minor contributions of basalban slip in contrast the monoclinic textures which developed during simple shear are most suited for duplex ban slip along the basal plane the transition between the dominating slip systems for the orthorhombic and monoclinic microfabrics is interpreted to be due to the effects of dynamic recrystallisation upon texture development since oblique c axis maxima documented in the literature are most often rotated not with but against the shear sense calcite textures alone should not be used as unequivocal shear sense indicators but need to be complemented by microstructural criteria such as shape preferred orientations grain size estimates and amount of twinning

vertical and fold axis parallel extension within a slate belt in a transpressive setting northern appalachians abstract an incremental and finite strain study of a mid paleozoic slate belt in a transpressive setting in gaspe canadian appalachians has been undertaken using syntectonic fibers from pyrite type pressure shadows regionally strain estimates are quite consistent and reflect only slightly higher strain in the central part of the anticlinorial structure defining the slate belt extensions of up to 160 vertically and 110 parallel to the fold axis were recorded in the slate belt combined with an estimated overall regional shortening of 80 the fibers first rotated toward the final position of the cleavage within a vertical section normal to the cleavage and then rotated from a vertical to a horizontal orientation this change in fiber orientation observed in cleavage parallel sections indicates an abrupt change in the incremental stretching direction from sub vertical to sub horizontal fold axis parallel the vertical extension is related to folding and cleavage development during coaxial flattening the horizontal extension reflects the strain imposed on the rocks during regional simple shear which produced strike slip faulting and further tightening of the folds folding cleavage development fold axis parallel extension and faulting are all related to oblique convergence during the middle devonian acadian orogeny in this part of the canadian appalachians and clearly indicate an overall transition from a pure shortening deformation to a simple shear dominant deformation in an overall transpressive setting

geological setting of the southern termination of western alps a revision of the stratigraphic and tectonic setting of the southern termination of the western alps at the junction of the maritime alps with the westernmost ligurian alps is proposed in response to the alpine kinematic evolution a number of tectonic units formed on the deformed palaeo european continental margin and were arranged in a nw se striking anastomosed pattern along the north eastern boundary of the argentera massif because these tectonic units often cut across the palaeogeographic subdivision of the alpine literature and show only partial affinity with their distinctive stratigraphic features new attributions are proposed the subbrianconnais domain is here intended as a deformation zone and its tectonic units have been attributed to dauphinois and provencal domains furthermore the eocene alpine foreland basin succession has been interpreted based on the affinity of its lithologic characters and age as a single feature resting above all the successions of the different mesozoic domains the cretaceous tectono sedimentary evolution of the studied domains was characterized by intense tectonic controls on sedimentation inducing lateral variations of stratigraphic features and major hydrothermal phenomena since the early oligocene transpressional tectonics induced a ne sw shortening together with significant left lateral movements followed by late oligocene middle miocene right lateral movements along e w to se nw striking shear zones this induced the juxtaposition and or stacking of brianconnais dauphinois and ligurian tectonic units characterized by different metamorphic histories from anchizonal to lower greenschist facies this evolution resulted in the arrangement of the tectonostratigraphic units in a wide transfer zone accommodating the oligocene wnw ward movement of portions of the palaeo european margin placed at the south western termination of western alps and the miocene dextral shearing along se striking faults that bound the argentera massif on its ne side

arcuate thrust systems in sandbox experiments a comparison to the external arcs of the western alps in the external western alps two regional structural arcs were generated during tertiary northwest directed collision between the apulian indenter and the european passive margin these arcs distinguished by their geographic position and their age are examined by using a new compilation of structural data and comparing these to the results of sandbox analogue experiments the principal western alpine arc comprises two orthogonal synchronous thrust systems in the late eocene early miocene major shortening 105 km was toward the northwest to west northwest and minor shortening 11 km was toward the southwest shortening in each branch decreased toward the core of the arc during the late miocene and pliocene the jura arc accommodated 35 km of northwest directed shortening while 10 5 km of southwest to south southwest directed shortening was accommodated on the digne thrust system sandbox experiments were used to investigate the role of the motion vector of a rigid rectangular indenter orthogonal diagonal curved or rotational paths and the mechanical stratigraphy of the foreland in the evolution of upper crustal arcuate systems e g presence of a basal easy slip silicone horizon comparison of experimental results with the external alpine arc suggests that the indenter followed a slightly diagonal path with respect to the european margin from the eocene to the early miocene and curved counterclockwise by 10 15 in the middle miocene mechanical stratigraphy experiments support the hypothesis that thick triassic evaporites played a primary role in the evolution of the jura arc the influence of mechanical stratigraphy was most prominent during weak deformation at the external boundaries of the alpine orogen jura fold and thrust belt digne thrust

nd and sr isotope study of hydrothermally altered granite at san manuel arizona implications for element migration paths during the formation of porphyry copper ore deposits a study of the nd and sr isotope composition of the late cretaceous monzogranite stock at san manuel arizona and its mineralized alteration halo was undertaken in order to assess the transport of nd and sr that accompanied the hydrothermal alteration sampling was along a 1 km transect perpendicular to the alteration zones extending from the potassically altered core to the propylitically altered fringe of the system large isotopic differences between the stock and the enclosing 1 4 b y old granitic wall rock coupled with the original homogeneity of the latter provide optimal conditions for detecting evidence of chemical transport the mineralizing porphyry stock has a lower 87 sr 86 sr ratios 0 708 and a higher epsilon nd 6 than the precambrian granite wall rock 0 782 11 the 87 sr 86 sr ratios of the altered wall rock decrease regularly from 0 782 to 0 715 from the propylitic alteration zone to the stock contact indicating that material flow was primarily from the stock into the wall rock consistent with an orthomagmatic model for porphyry copper ore genesis the sr in the ore zone is 75 percent stock derived and evidence of stock derived sr is found up to 700 m into the wall rock the sr and rb concentration variations are mainly controlled by the alteration mineralogy and do not correlate with the isotopic variations the epsilon nd values and the sm and nd concentrations of the altered wall rock show subtle changes that are mostly confined to the potassically altered zone only one wall rock sample located 0 5 m from the stock contact is strongly disturbed the results suggest that sr isotopes may provide a useful tool for studying or locating fossil hydrothermal systems whereas the relatively immobile nd isotopes are reliable indicators of the magmatic provenance of altered intrusions

patterns of stretch trajectories and strain rates within spreading gliding nappes abstract stretch trajectories in vertical flow planes through superficial nappes are thought to be specific for some classical mechanical models so that their recognition in the field is of particular interest in order to find a pattern of stretch trajectories related to a gravitational process a strain factorization is attempted from experimental scale models of nappes where spreading and gliding are combined the strain within this type of nappes is schematically considered as the simultaneous combination of two components a simple shearing component and a pure shearing component spatial and time distribution of these two components is computed from both time and spatial evolution of the strain pattern within the scale models a better understanding of stretch trajectories in spreading gliding nappes is consequently provided strain rates are also computed from scale models and the sudden increase of strain rate from top to bottom is explained finally a mechanical model of spreading gliding nappe is briefly discussed

overthrust shear deformation of a foreland basin structural studies south east of the pelvoux massif se france abstract the late tertiary champsaur prapic fold zone lies to the southeast of the pelvoux basement massif in the footwall of the frontal pennine thrust of the western alps the 0 8 1 3 km thick late eocene oligocene foreland basin succession lies directly on crystalline basement and was deformed by wsw directed shear below the overthrusting internal alpine units on the frontal pennine thrust this deformation decouples from the basement in a top to wsw sw basal shear zone above the turbidite sequence gres du champsaur is folded in chevron like folds a 12 km long continuous profile across the champsaur fold zone documents variations in chevron fold shape axial plane dips and zonal distribution of folding which indicate progressively higher shear strains towards the frontal pennine thrust in the east chevron fold axes trend generally nnwsse except close to the pelvoux massif where fold axes curve to n s possibly due to lateral drag in the western part of the investigated area sw directed thrusting on the champsaur thrust almost doubles the tertiary stratigraphy the champsaur thrust and chevron folding in its footwall deform the overlying internal thrust sheets the champsaur prapic fold zone is a particular example of overthrust shear deformation in the footwall of internal alpine nappes in which deformation is concentrated in diffuse ramp zones to the north late alpine deformation changes to wnw directed partly post dating the wsw sw directed deformation further south therefore in the core of the external western alpine are two sub perpendicular transport directions which barely overlap can be distinguished

strain partitioning in the axial nw alps since the oligocene 1 this work analyzes the postmetamorphic evolution of the northwestern alps thanks to a multiscale field geology study integrated by fission track analysis it illustrates how the fission track method constraining throws along major faults and age of mesoscale deformation provides interpretation keys to detangle complex structural data sets results are discussed within the framework of the alps apennines knot and give new insights on strain partitioning within transpressional orogenic belts the axial nw alps acted since the late oligocene as an orogen scale shear zone accommodating in the aosta valley dextral strike slip coeval with forward propagation of external thrusts within this transcurrent belt right lateral deformation was partitioned between the fast exhuming western block and the already unroofed eastern block these blocks show contrasting structural characters the former is cut by first order longitudinal structures bounding major tectonostratigraphic units the latter is fragmented by complex patterns of lower order faults interpreted as subsidiary shears the onset of mesoscale extension widespread in the eastern block is diachronous across the belt and migrated through time toward the european foreland transition from syncollisional shortening to orogen scale transcurrence observed in the nw alps corresponds to a regional strain field variation documented in the mid oligocene all around the alps apennines knot which is possibly related to a change of the adria trajectory relative to europe since the late oligocene deformation associated with oblique plate convergence was partitioned between strike slip in the axial belt and thrusting in external areas most of the shortening was accommodated in the helvetic chains 102 km deformation was limited elsewhere to an order of 101 km

the magnetic fabric of weakly deformed late jurassic shales from the southern subalpines chains french alps evidence for sw directed tectonic transport direction abstract extensive magnetic fabric investigations have been conducted on the late jurassic sedimentary black shales 37 sites 362 samples the so called terres noires formation from the southern subalpine chains western alps this study completes a previous study done in the northern subalpine chains which demonstrated that the terres noires formation preserves a subtle magnetic lineation related to tectonic transport direction in the southern subalpine chains diois devoluy embrunais digne nappe similar magnetic fabrics have been found the shape of the magnetic fabric is oblate and locally mirrors the gradient of deformation in the embrunais footwall the magnetic foliation is parallel to the bedding or to the slaty cleavage about 50 of the magnetic lineations have down dip magnetic directions while the other magnetic lineations are sub horizontal or parallel to the intersection bedding cleavage the tectonic magnetic fabrics recorded in the embrunais confirm the ability of the terres noires to record tectonic transport direction the magnetic fabric geometry suggests sw directed tectonic transport in the southern subalpine chains the diois appears as a tip zone where transport directions are abruptly clockwise rotated from nw directed in the northern subsalpine chains to sw directed the magnetic fabric from the digne nappe exhibits a fan pattern of transport directions in close agreement with the arcuate shape of the thrust

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

a routine high precision method for lu hf isotope geochemistry and chronology a method for chemical separation of lu and hf from rock meteorite and mineral samples is described together with a much improved mass spectrometric running technique for hf this allows i geo and cosmochronology using the176lu 176hf decay scheme and ii geochemical studies of planetary processes in the earth and moon chemical yields for the three stage ion exchange column procedure average 90 for hf chemical blanks are 0 2 ng for lu and hf from 1 g of hf a total ion current of 0 5 10 11 ampere can be maintained for 3 5 h yielding 0 01 0 03 precision on the ratio176hf 177hf normalisation to179hf 177hf 0 7325 is used extensive results for the johnson matthey hf standard jmc 475 are presented and this sample is urged as an international mass spectrometric standard suitable aliquots prepared from a single batch of jmc 475 are available from denver lu hf analyses of the standard rocks bcr 1 and jb 1 are given the potential of the lu hf method in isotope geochemistry is assessed

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

behavior of isotopic systematics during deformation and metamorphism a hf nd and sr isotopic study of mylonitized granite a combined compositional and hf nd and sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the harquahala mountains western arizona to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks the 1 4 ga harquahala granite was deformed during mesozoic thrusting along the harquahala thrust granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite in most of these mylonites the protolith is not megascopically recognizable and can be discerned only by the progressive transition to undeformed granite isotopic analyses of hf nd and sr from the shear zone document the immobility of the hf and nd isotopic systems relative to that of the sr isotopic system during deformation the rb sr isotopic data show considerable scatter on an isochron plot exhibiting both gains and losses of rb and sr from the whole rock systems in contrast the sm nd and lu hf isotopic systematics are mostly well behaved on isochron diagrams plotting mostly in tight clusters or along 1 4 ga isochrons these results show that while the sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance both hf and nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism

detrital zircon u pb ages and hf isotopic constraints on the terrigenous sediments of the western alps and their paleogeographic implications detrital zircons from cretaceous micaschist late eocene earliest oligocene sandstone and early oligocene siltstone of the western alps fall into three main separable age clusters at 610 540 ma 490 430 ma and 340 280 ma that correspond to the cadomian neoproterozoic ordovician and variscan carboniferous events widespread in western and central europe hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust a distinguishable group of triassic zircons around 250 200 ma which is considered to derive from the southern alps has been detected in the early oligocene schistes a blocs formation and the brianconnais flysch noir in contrast this age group is absent in late eocene earliest oligocene sandstones in agreement with sedimentological studies our results show that the main source areas of the eocene sandstone were probably located in the european continent the arrival of detritus from the internal zone occurred in early oligocene coeval with the tectonic rotation from northwestward to westward in the propagation of allochthonous units based on previous studies and our new data we argue that the the brianconnais zone was likely a paleorelief since the middle eocene that accounts for the lack of detritus from the adriatic units contemporary sediments were accumulated in the foredeep of the adriatic plate from oligocene time onwards the blockage was cut through after a regional uplifting and thus the internal zone started to provide detritus into the western flexural basins

structural pattern and emplacement mechanism of the neka valley nappe complex eastern alborz iran the neka valley nappe complex is exposed in the south of gorgan county in the eastern alborz fold and thrust belt we use the results of a regional survey of the structural data and their patterns to interpret the mechanisms that emplaced the unmetamorphosed nappes in the foreland fold and thrust belt of the alborz mountains most of the strain magnitudes are low in the study area but increase slightly towards what are probably their proximal ends strain ellipsoid is dominantly oblate with xy aligned along and across the belt or the nappe complex the average kinematic vorticity number w k 0 6 which indicates most of the strain partitioning resulted in a general shear most of flinn s k values and the stretch along the shear plane values are lower than 1 structural indicators such as orthogonal extensional joints pinch and swell structures anastomosing cleavages and listric normal and growth faults developed by push from the rear large scale thrust complexes with opposed dips such as triangle zones as well as k and values 1 are compatible with the shear flow diverging distally and streamlines expected of the rear compression emplacement mechanism together with a later minor brittle deformation these major ductile strains appears to provide a general model suitable for the emplacement of the nappes studied in a thin skinned fold and thrust belt where the sedimentary cover strata shortened and imbricated in the upper crust

testing models of orogen exhumation using zircon u th he thermochronology insight from the ligurian alps northern italy testing models of orogen exhumation requires precise constraint of the time temperature paths of the exhumed rocks the zircon u th he zhe thermochronometer has a closure temperature of 140 210 c and can provide temporal constraints on the exhumation history of rocks through a thermal range which crucially corresponds the onset of brittle behaviour of an exhuming orogen we performed zhe analyses to test the existing contradictory models for the exhumation of the ligurian alps the zhe ages indicate a very rapid 1 3 6 8 mm yr and southward migrating exhumation of the orogen between 32 and 25 ma these high exhumation rates are unique within the alpine belt and cannot be reconciled with existing geodynamic models of the evolution of the ligurian alps we propose a model of tectonic denudation via detachment accomplished in the shallowest crust 5 km as a result of the last orogenic phase of extension

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

strain models within spreading nappes abstract internal strain within nappes and thrust sheets is especially important in the understanding of emplacement mechanisms in this paper strain patterns within spreading nappes are investigated theoretically and experimentally three types of spreading nappe are distinguished using various boundary conditions spreading gliding spreading and extruding spreading nappes each type is placed in its proper natural environment strain components and strain gradients are summarized from the use of scale models performed with analogue materials mathematical computations from experimental modelling allow the calculation of 1 1 the schistosity trajectories in vertical flow planes 2 2 the relationship between stretch and transport direction and 3 3 the spatial distribution of different types of strain ellipsoid these provide some tests which make it possible to demonstrate in the field whether nappes are or are not gravity induced finally some field examples from the central alps and the pyrenees are briefly discussed they provide good support to the strain models presented in the present paper

thrust zone kinematics in a basement cover imbricate stack eastern pelvoux massif french alps abstract thrust imbrication is now widely accepted as a major process in controlling basement cover relationships in the external parts of the western alps complications in the simple geometries expected from thrusting alone arise from alpine deformation affecting pre existing extensional faults inherited from the old mesozoic basins this contribution describes the structural geology of part of the eastern pelvoux massif centred on the rocher de l yret where alpine deformation has masked any pre existing structures a series of major thrusts carry basement onto cover these lie west of and in the footwall to the pennine front a major zone of thrusting which carries the internal alps onto the external zones stretching lineations in calc mylonites derived from eocene limestones deposited on the basement indicate a tectonic transport axis of wnw ese plunging ca n100 e this direction is supported by the orientation of striae and corrugations on faults which cut basement rocks shear criteria in the calc mylonites imply top to the wnw shearing despite this simple kinematic picture basement cover relationships cannot be explained by thrusting alone a series of faults cut down through the thrust stack towards the wnw and show net extensional offsets with respect to the tectonic pile these faults and associated shears in the calc mylonites cannot be traced to depth rather they appear to branch onto earlier thrust surfaces this geometry is interpreted as due to distributed shearing in the hanging wall to the local floor thrust perhaps reflecting late reactivation of the pennine front

neogene remagnetization of normal polarity in the late jurassic black shales from the southern subalpine chains french alps evidence for late anticlockwise rotations abstract this study presents paleomagnetic investigations of 23 sites from the late jurassic black shales the so called terres noires that outcrop in the southern subalpine chains ssc six sites are located in the diois six sites in the trieves and devoluy and eleven sites in the digne nappe the magnetizations are weak and are carried by titanomagnetites we found generally one component per site within the medium range of temperature 200 to 500 c all magnetizations are characterized by the lack of reverse polarity the directions of magnetization are generally better grouped in geographic coordinates in the trieves the devoluy and the digne nappe fold testing at one site from the trieves demonstrated a late synfolding acquisition of magnetization in contrast pre tilting magnetizations are suggested in the diois and locally in the digne nappe given an average inclination close to late tertiary expected inclinations in geographic coordinates we propose that these magnetizations are neogene overprints our results point to two major results 1 the lack of reverse polarity which is puzzling during the neogene where 50 of the magnetic field is reverse 2 the discovery of quite systematic anticlockwise rotations these anticlockwise rotations do not occur in the diois and are larger in the southern part of the digne nappe we presume that these rotations were active during the late stage of alpine folds emplacement and are still active today

subduction related petrogenesis of late archean calc alkaline lamprophyres in the yilgarn craton western australia abstract we present a comprehensive petrographic mineralogical and geochemical study of calc alkaline lamprophyres cal from the archean yilgarn craton western australia previous studies have shown that the emplacement age of cal from the eastern goldfields superterrane of the yilgarn craton is 2684 to 2640 ma a new rb sr mica age for a cal sample in the western yilgarn is 2070 ma both archean and proterozoic cal analysed in this study display porphyritic textures and contain phenocrysts of amphibole minor clinopyroxene and biotite in a fine grained groundmass dominated by feldspar high mgo ni and cr abundances up to 11 9 wt 373 and 993 ppm respectively are consistent with derivation of primitive magmas from a mantle source enrichment in h2o reflected in the abundance of magmatic amphibole and mica combined with high whole rock lile th yb ratios and negative nb ta anomalies in trace element patterns are consistent with a source that was metasomatised by hydrous fluids analogous to those generated by phanerozoic subduction related processes chondritic nd and hf signatures and archean mantle like sr isotope signatures of the late archean cal indicate that the fluid metasomatism required to explain their volatile and trace element enriched composition shortly preceded partial melting i e there was insufficient time to develop enriched radiogenic isotopic signatures the concurrence of apparently juvenile radiogenic isotopes and fluid related trace element compositions requires a geodynamic scenario whereby dehydration of a subducted slab triggered metasomatism of the overlying mantle wedge our findings therefore support a subduction setting at 2 6 2 7 ga along the eastern margin of the yilgarn craton the cal from the western yilgarn have similar compositions but enriched sr nd hf isotopes compared to those in the eastern goldfields superterrane this signature is consistent with melting of lithospheric mantle domains previously enriched by subduction related metasomatism hence our study suggests the presence of a subduction setting in the western yilgarn during the archean which is consistent with previous geodynamic reconstructions however the geodynamic trigger for the early proterozoic event that generated cal magmatism in the western yilgarn is currently unclear

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

reconstructing the kinematic evolution of curved mountain belts a paleomagnetic study of triassic red beds from the wyoming salient sevier thrust belt u s a determining the kinematic history and mechanics of curved fold and thrust belts is fundamental to understanding the tectonic evolution of mountain systems to better understand the development of a classic curved fold and thrust belt we completed an integrated paleomagnetic and strain study of the wyoming salient paleomagnetic data are reported here from 154 sites collected from red beds of the triassic ankareh formation in the salient and nine sites collected from the relatively stable foreland red beds display three components with distinctly different magnetic behaviors 1 a near primary triassic magnetization carried by hematite that is stable up to 680 c tr component 91 sites 2 a cretaceous chemical remagnetization carried partly by magnetite k component 32 sites and 3 a recent viscous magnetization that is mostly removed by 350 c site mean vectors for the tr and k components show a high degree of scatter from expected triassic and cretaceous reference directions suggesting significant tilt and rotation subsequent to magnetization acquisition restoration of tilt and folding for individual site means results in well clustered shallow and moderate inclinations for the tr and k components respectively and in variable declinations related to systematic vertical axis rotations statistical analysis of declinations for both components indicates that 75 of present day salient curvature resulted from secondary rotation and 25 of primary curvature was likely related to sedimentary basin architecture analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets 80 compared to the frontal thrust sheets 65 suggesting that rotations were concentrated near the leading edge of the propagating fold and thrust wedge with only minor additional rotation of internal sheets transfer zones oblique ramps and more deformed overturned fold limbs display locally more complex patterns which can be understood through careful structural analysis when combined with internal strain data and regional structural relations paleomagnetic data support a kinematic model of a progressive arc with curved thrust slip paths and differential shortening that rotated early layer parallel shortening fabrics and produced minor strike parallel extension this kinematic history likely reflects a combination of processes including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient presence of a weak basal detachment and fault zone weakening that favored lower taper and buttressing by laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient

element mobility associated with syn metamorphic shear zones near scotchport nw mayo ireland three major blastomylonitic synmetamorphic epidote amphibolite to mid amphibolite facies shear zones are seen on the nw coast of the mullet peninsula in nw mayo these shear zones occur at the contacts of major structural units and in an imbricated slice where rocks of the erris complex are deformed and chemically modified chemical changes associated with individual shear zones have been deduced by comparing the compositions of various gneisses both within and adjacent to the shear zones compositional changes are different in the constituent rock types within each unit and many elements normally considered immobile have been selectively mobilized within the shear zones little evidence of wholesale metasomatic introduction of components into these shear zones was found to accompany the selective mobilization