folding within seconds hypervelocity impacts of cosmic projectiles larger than 200 m diameter are capable of forming complex craters on earth at these craters shock loading shock damage and excavation flow are followed by a gravity driven collapse of the deep transient cavity such impact structures are characterized by a central uplift a flat crater floor and a terraced crater rim collapse induced deformation features like folds and brittle fault zones have many similarities to tectonic structures typical deformation patterns of complex terrestrial impact craters of 5 15 km diameter are compiled and analyzed with respect to their kinematic development unlike their tectonic counterparts deformation structures are always the result of non plane strain deformation and are formed in a single event that takes place in seconds to minutes to understand the high strain rate processes the microstructure of an impact induced fold of the crooked creek impact crater 7 km diameter missouri united states is investigated in detail a period of 20 30 s at the most is determined for the collapse phase of this crater the gross plastic deformation behavior of the fold is achieved by localized brittle deformation along millimeter to centimeter spaced fault zones forming a network of veins shock damage has fractured 40 of grain boundaries the onset of collapse and associated deformation started in rocks with a reduced cohesion and is friction controlled

flow and fabric development during the diapiric rise of magma scaled isothermal experiments of the internal circulation induced by viscous drag at a diapir country rock interface are compared with published theoretical studies of flow induced finite deformation inside a rising sphere and new numerical results are presented for the incremental strain pattern developed in spherical diapirs the results predict a time dependent deformation history within rising diapirs that produces finite strains characterized by strong oblate flattening strains at the margin and strong vertical prolate strains in the center with an intervening zone of low plane strain finite strains are discussed in terms of the different sub fabrics expected to form in granitoids after considering the history of rheology and temperature distribution in rising magma strong rheological and temperature gradients are predicted for the margins of internally circulating diapirs that can lead to the formation of strong penetrative foliations at the margin and magmatic or even isotropic fabrics in the i

unroofing of active metamorphic core complexes in the d entrecasteaux islands papua new guinea metamorphic core complexes in the d9entrecasteaux islands are forming as a result of active extension at the western end of the woodlark basin spreading center high grade metamorphic rocks exist in the cores of gneissic domes including eclogites formed at minimum depths of 45 75 km i e 13 21 kbar high temperatures were maintained during uplift of basement rocks from 13 21 kbar t 730 900 c through 7 11 kbar t 570 730 c to 5 6 kbar t 730 c subsequent rapid cooling 100 c m y of the basement occurred between 1 and 2 ma on the basis of 40 ar 39 ar geochronology the core zone rocks were transected by kilometre scale amphibolite facies ductile shear zones active at depths of 25 35 km extensive intrusion of granodiorite plutons occurred and the age of granodiorite intrusions constrains late stage shear zone activity to 2 ma and younger unroofing was accomplished by faults and shear zones bounding the gneiss domes juxtaposing them against ultramafic rocks and sedimentary deposits surface uplift continues today and has led to topographic elevations of up to 2 5 km

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

two dimensional strain determination by the inverse surfor wheel abstract a simple method of two dimensional strain determination from the preferred orientation of deformed lines is presented it is based on the change of probability of intersecting lines on a given traverse as a function of traverse orientation the method can be applied to real lines such as grain boundary outlines or to virtual lines such as the tie lines between centre points of neighbouring grains in this manner estimates for the strain of surfaces i e of the particles enclosed by the surfaces and of bulk strain can be obtained

fracturing and diking in incompletely crystallized granitic plutons abstract dikes in granitic plutons commonly have mineralogic and textural characteristics indicating a genetic relation to their host the contact between the dike and the host is marked signifcantly by interlocking unfractured crystals rather than by truncations and overgrowths although megascopically the contacts between dike and host pluton may be exceedingly sharp and planar an explanation of these relationships by fracturing an incompletely crystallized host rather than by entirely post magmatic fracturing with subsequent diking is postulated to be consistent with experimental date and certain petrologic observations published experimental research indicates that fracturing of crystal melt systems may occur if there is at least 70 crystals in the system at which point the system is capable of transmitting compressive and shear stresses via a crystalline framework dilatancy pumping of melt into fracture zones and subsequent hydraulic fracturing is thought to be one possible way late stage fluids of granitic systems evolve into dikes in granitic plutons such as leucogranites aplites and pegmatites interlocking textures between dike and host may also develop if magma from some distant source is interjected along the plane of fracturing in crystal melt systems

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

numerical simulations of impact crater formation with dilatancy impact induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater this paper describes a simple model of dilatancy the creation of porosity in a shearing geological material and its implementation in the isale shock physics code the model is used to investigate impact induced dilatancy during simple and complex crater formation on earth simulations of simple crater formation produce porosity distributions consistent with observations dilatancy model parameters appropriate for low quality rock masses give the best agreement with observation more strongly dilatant behavior would require substantial postimpact porosity reduction the tendency for rock to dilate less when shearing under high pressure is an important property of the model pressure suppresses impact induced dilatancy in the shock wave at depth beneath the crater floor and in the convergent subcrater flow that forms the central uplift consequently subsurface porosity distribution is a strong function of crater size which is reflected in the inferred gravity anomaly the bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius larger craters exhibit a central gravity high within a suppressed gravity low lower crustal pressures on the moon relative to earth imply that impact induced dilatancy is more effective on the moon than earth for the same size impact in an initially nonporous target this difference may be mitigated by the presence of porosity in the lunar crust

dynamic modeling suggests terrace zone asymmetry in the chicxulub crater is caused by target heterogeneity we investigate the cause of terrace zone asymmetry in the chicxulub impact crater using dynamic models of crater formation marine seismic data acquired across the crater show that the geometry of the crater s terrace zone a series of sedimentary megablocks that slumped into the crater from the crater rim varies significantly around the offshore half of the crater the seismic data also reveal that at the time of impact both the water depth and sediment thickness varied with azimuth around the impact site to test whether the observed heterogeneity in the pre impact target might have affected terrace zone geometry we constructed two end member models of upper target structure at chicxulub based on the seismic data at different azimuths one model representing the northwest sector had no water layer and a 3 km thick sediment layer the other model representing the northeast sector had a 2 km water layer above a 4 km sediment layer numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at chicxulub moreover the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the chicxulub crater we conclude that asymmetry in the pre impact target rocks at chicxulub is likely to be the primary cause of asymmetry in the terrace zone

deformation of incompletely crystallized magma systems granitic gneisses and their tectonic implications certain textures in two feldspar calc alkaline granitic gneisses have been identified as indicators of the presence of intercrystalline melt during deformation of crystal melt mushes fluid relocation or fracting is to be expected in an incompletely crystallized magmatic system as deviatoric stresses are applied to it relocation of late stage fluids capable of crystallizing k feldspar quartz microaplite and myrmekite to low pressure zones such as the pressure shadow region around earlier crystals occurs within some developing mylonitic gneiss fabrics and results in a distinctive assemblage of textures strain in these rocks is expressed by various brittle and ductile features and by fluid relocation textures the petrogenesis of these particular gneisses is therefore both magmatic and metamorphic contrasting with gneiss forming processes that had no participating magmatic component migmatitic complexes evolving by partial anatexis of non magmatic rocks in a tectonically active environment are not

the ascent of felsic magmas in dykes abstract the results of a thermal and fluid dynamical analysis are presented that allow geologists to estimate the critical dyke or fracture width w c necessary for the transport of felsic magma through the crust without control by solidification the principal geological variables required to estimate w c are the magmatic viscosity the density and temperature contrasts and t between magma and country rock and the height of ascent h the results show that for typical values of and and for transport distances of 20 to 30 km the critical dyke width is constrained to lie between about 2 and 20 m

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

how the development of magma chambers affects collapse calderas insights from an overview here we review the studies of pluton emplacement and caldera collapse proposing a model linking the two processes the shallow rise of magma occurs by dyking and its emplacement occurs along major anisotropies many plutons are emplaced at subhorizontal discontinuities forming sills these will eventually grow forming laccoliths the most common mechanism to store shallow magma calderas are a surface expression of these reservoirs and can be approximated by a piston cylinder which sinks due to 1 underpressure in the reservoir producing outward dipping reverse faults 2 overpressure within a silllike chamber undergoing doming and 3 overpressure within a laccolith generating apical tensile stresses in the latter two cases inward dipping normal faults form we suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions tabular intrusions generating ring faults mainly at their tips form calderas with a similar width to the intrusion laccoliths may generate ring faults along the intrusion roof forming calderas narrower than the intrusion the outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

heterogeneous deformation and quartz crystallographic fabric transitions natural examples from the moine thrust zone at the stack of glencoul northern assynt quartz crystallographic fabric transitions in well exposed mylonites immediately beneath the moine thrust at the stack of glencoul nw scotland have been investigated by optical microscopy x ray texture goniometry and orientation distribution function analysis a progressive change is observed from asymmetrical kinked single girdle c axis fabrics at 0 5 cm beneath the moine thrust through asymmetrical type i cross girdle fabrics to symmetrical type i cross girdle fabrics at 30 cm beneath the thrust this c axis fabric transition is accompanied by a transition from asymmetrical single a axis maximum fabrics 0 5 cm beneath the thrust through asymmetrical two maxima fabrics to essentially symmetrical two maxima a axis fabrics odf analysis of these s l and l s tectonites indicates that c axis positions on the leading edge of the fabric skeleton are related by a common a direction oriented within the xz plane at a moderate angle to the lineation x in contrast c axis positions on the peripheral trailing edge are related by a positive r rhomb pole oriented close to z a directions lying within this common rhomb plane progressively change through 180 in orientation traced around the c axis fabric skeleton such contrasting single crystal rhomb a preferred orientations on the leading and trailing edges of the fabric skeleton are interpreted as indicating localized grain scale plane strain and flattening deformation respectively they result in tectonites with essentially symmetrical c and a axis fabrics which display strongly asymmetrical positive r and negative z rhomb pole figures the observed transition in quartz c and a axis fabrics is interpreted as indicating an increasing importance of non coaxial plane strain deformation as the moine thrust is approached even immediately 1 cm beneath the thrust however flow has still significantly departed from bulk simple shear and involved an important heterogeneous component of contemporaneous flattening deformation

rock fluidization during peak ring formation of large impact structures large meteorite impact structures on the terrestrial bodies of the solar system contain pronounced topographic rings which emerged from uplifted target crustal rocks within minutes of impact to flow rapidly over large distances these target rocks must have weakened drastically but they subsequently regained sufficient strength to build and sustain topographic rings the mechanisms of rock deformation that accomplish such extreme change in mechanical behaviour during cratering are largely unknown and have been debated for decades recent drilling of the approximately 200 km diameter chicxulub impact structure in mexico has produced a record of brittle and viscous deformation within its peak ring rocks here we show how catastrophic rock weakening upon impact is followed by an increase in rock strength that culminated in the formation of the peak ring during cratering the observations point to quasi continuous rock flow and hence acoustic fluidization as the dominant physical process controlling initial cratering followed by increasingly localized faulting

structure of late variscan millevaches leucogranite massif in the french massif central ams and gravity modelling results in the limousin area variscan leucogranitic plutons are spatially associated with normal faults and major strike slip shear zones that are a continuation of the south armorican shear zone our study focuses on the large n s trending millevaches granitic massif massif central france and intends to highlight through gravity modelling structural and anisotropy of magnetic susceptibility ams the massif structure at depth and to discuss the mode of emplacement of granites within a strike slip tectonic context the mica subfabric suggests that the magnetic foliations display a general nw se sub horizontal pattern on both sides of the n s pradines dextral wrench fault zone that deforms the core of the massif on 5 km width the magnetic lineation trend exhibits a sigmoidal pattern n s in the pradines fault zone and nw se on both sides of it which are consistent with a dextral wrench component the horizontal magnetic foliations and lineations are consistent with the thin granite laccolith model there is no significant imprint of the extensional variscan belt collapse on the internal fabric of millevaches granites than the tectonic dextral transcurrent movement prevailing in this area

shear zone hosted polymetallic sulfides in the south limousin area massif central france remobilized sulfide deposits related to variscan collisional tectonics and amphibolite facies metamorphism this paper investigates the relationship between deformation and mineralization developed in amphibolite facies shear zones in the south limousin area using structural microstructural and textural analyses the investigated shear zone is a ductile thrust fault that stacked low grade metamorphic rocks onto a highly metamorphosed basement with both being folded during a late orogenic tectonic phase the shear zone hosts sulfide bearing mylonitic quartzites composed of dynamically recrystallized quartz syntectonic pyrite pyrrhotite sphalerite chalcopyrite and late covellite mylonites display northwest southeast trending lineation and asymmetric microstructures including sigmoidal quartz aggregates and shear bands that indicate a northwestward movement along the shear zone sulfides occur mainly along shear bands and exhibit primary undeformed textures attesting to their syntectonic emplacement based on petrographic analyses of the metamorphic host rocks mineralization formed at p 5 0 to 5 5 kbars and t 300 to 600 c consistent with p t conditions for recrystallization of metapelites and metabasites involved in the shear zone all data indicate that ductile deformation and emplacement of mineralization were coeval the occurrence of similar pretectonic sulfide deposits in the hanging wall of the mineralized shear zone suggests that the sulfides in the thrust fault were remobilized from preexisting mineralization by fluid that migrated toward the thrust this is the first evidence for thrust zone hosted mineralization within the massif central and attests to the role of crustal scale thrust faults as occasional pathways for mineralized fluids during the upper paleozoic continental collision these results suggest that ductile thrust faults may be important sites of ore deposition in the variscan belt of western europe and in similar collisional belts elsewhere

polyphase wrench tectonics in the southern french massif central kinematic inferences from pre and syntectonic granitoids in the variscan french massif central the south limousin area consists of low to medium grade metamorphic rocks intruded by two granitic bodies the structural and textural analyses of these plutons undertaken in parallel with the structural analysis of their host rocks allow us to characterize and to date different stages in the tectonic evolution of this area this study shows that the south limousin area experienced successivelly two strike slip events along two geographically distinct shear zones from north to south the left lateral estivaux and the right lateral south limousin strike slip faults respectively these ductile faults subdivide the south limousin into three structural units from north to south they are the upper gneiss unit thiviers payzac unit and genis unit the two granitic bodies intrude the thiviers payzac unit only the younger estivaux granite is a syntectonic pluton which emplaced during left lateral wrenching 40ar 39ar dates from biotites indicate an early carboniferous age 346 3 ma the older granite is a pretectonic body it is the ordovician saut du saumon augen orthogneiss in which detailed structural analyses show the polyphase nature of the solid state deformation our microtectonic data indicate that the right lateral motions overprint the left lateral ones and produce apparently symmetrical fabrics

granite magma migration and emplacement along thrusts this paper investigates the influence exerted by brittle tectonic structures in the emplacement of granite plutons in contractional settings we address both cases where contractional tectonics and magma intrusion are 1 coeval to study how active contractional tectonics controls the transport of magma and 2 diachronous to study the role of pre existing structures on the transport of magma in light of new experimental models we show that magma can rise along thrusts ramps and flats this phenomenon occurs for both low viscosity magma basalts to andesite and high viscosity magma dry granite the experimental results also allow the evaluation of the role played by magma viscosity in determining pluton geometries in addition a review of literature demonstrates a spatial and causal relationship between granites and thrusts and highlights the geometric control of magma pathways in the pluton final shape the abundance of subhorizontal and tabular granitic intrusions indicates that the location of inflating granitic sills along thrust flats can be common we argue that active and pre existing flats and ramps thrusts provide a preferential continuous planar anisotropy susceptible to become a granitic magma migration pathway

crustal thickening and attenuation as revealed by regional fold interference patterns ciudad rodrigo basement area salamanca spain abstract the structure of the ciudad rodrigo area iberian massif central iberian zone has been revisited in order to integrate new geological data with recent models of the evolution of the iberian massif detailed mapping of fold structures along with a compilation of field data have been used to constrain the geometry and relative timing of ductile deformation events in this section of the hinterland of the variscan belt the structural evolution shows in the first place the development of a regional train of overturned folds with associated axial planar foliation d 1 towards the lower structural levels the deflection of the fold limbs and a subhorizontal crenulation cleavage depict the upper structural boundary of a superimposed low angle shear zone d 2 which extends at least to the deepest parts of the basement exposed in the study area the amplification and rotation of d 1 folds about a horizontal axis also occurred within this shear zone the flat lying character of the d 2 structures accounts for the attenuation of the previously thickened crust which developed following gravity gradients during thermal re equilibration subsequent deformation led to the formation of two orthogonal sets of upright folds d 3 representing a new shift between crustal thinning and crustal thickening in the region

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

questions about myrmekite in deformed rocks abstract several questions about the development of myrmekite in deformed felsic and metapelitic rocks remain unanswered although a general relationship between deformation and myrmekite appears to be well established the fact that myrmekite grows into relatively non deformed k feldspar while being simultaneously deformed and recrystallized at the rear of the growing lobes suggests that strain energy may not be a major contributor to the advance of the growth interface of the myrmekite colony furthermore because myrmekite typically nucleates on existing plagioclase rather than in k feldspar strain energy may not contribute directly to nucleation of myrmekite either at least in many instances however strain energy probably contributes indirectly by facilitating access of fluids to growth sites thereby altering the local chemical environment and so promoting development of myrmekite because myrmekite recrystallizes readily in deformed felsic rocks it is one of the main contributors to the development of folia

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

episodic exhumation of hp rocks inferred from structural data and p t paths from the southwestern massif central variscan belt france abstract in the southwestern massif central france hp rocks are found in lower allochthonous units i e hp units sandwiched between upper allochthonous units i e lp units and the parautochthonous unit both devoid of hp rocks hp rocks were formed at minimum conditions of 1 7 gpa 700 c in response to the northward subduction of gondwana beneath armorica during the silurian 430 390 ma their exhumation results from an 80 ma long sequence of tectonic processes effective at various scales and active specifically at each main stage of the evolution of the gondwana lithosphere hp rocks were first exhumed by upward southwestward extrusion during the early devonian 390 380 ma in relation to late stage of subduction in the northern area exhumation is achieved by rifting during the upper devonian 365 ma the most obvious event corresponds to the northwestward thrusting of lp units above hp units during early stages of continental collision 355 ma hp rocks were exhumed for a second time during orogen parallel transpression in response to late stage of continental collision 350 345 ma exhumation in internal zones is achieved by crustal scale normal faulting and erosion during the middle carboniferous 335 315 ma synorogenic extension while compression was still active in the southern external zones exhumation of the whole region is completed by regional uplift and erosional denudation during the upper carboniferous 310 290 ma postorogenic extension subduction and collision have played a key role for driving hp rocks to the surface but their efficiency and the resulting strain pattern depend on the angle of convergence upper devonian rifting and the subsequent early carboniferous crustal thickening prevents to consider exhumation of variscan hp rocks as a single and continuous process and open new questions on the related paleozoic lithosphere history

superimposed tectonic and hydrothermal events during the late orogenic extension in the western french massif central a structural and 40ar 39ar study in the western french massif central the argentat fault is a major structure through which as au fluids percolated in the late carboniferous along brittle fractures new petrostructural investigations show that an early ductile normal dextral faulting coeval to leucogranite emplacement took place during the late visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose 40ar 39ar ages cluster around 335 ma this early fluid channelling is associated with brittle deformation only in the hangingwall of the argentat fault whereas ductile deformation is restricted to the footwall these results provide new evidence for the upper crust implication during the syncollisional extension in the french massif central this study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes especially in basement areas where the tectonic plutonic and metamorphic evolution is polyphased

characteristics and mode of emplacement of gneiss domes and plutonic domes in central eastern pyrenees abstract gneiss domes and plutonic granitoid domes make up almost 50 of the pre hercynian terrains in the central and eastern pyrenees from a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism the study also shows that the internal structure the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes this appears to be in good agreement with h ramberg s 1967 gravity deformation and the earth s crust academic press london 1970 model studies in relation to intrusion of plutonic bodies in mechanisms of igneous intrusion edited by newall g rast n geol j spec issue 2 261 286 model studies showing that dome or mushroom like structures similar to those observed develop when there is a small viscosity ratio between the rising body and its enclosing medium this implies a high crystal content for the granitoid magma this crystal content has been estimated by i calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension using the model and program of j p carron et al 1978 bull soc geol fr 20 739 744 ii using the recent results of experimental deformation of partially melted granites of i van der molen m s paterson 1979 contr miner petrol 70 299 318 and ii comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes the minimum crystal content for the development of a dome like structure has been thus estimated to about 70 i e a value very close to that estimated by van der molen paterson 1979 to be the critical value separating the granular framework flow from suspension like behaviour the effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments these small variations appear to exert a strong control on the shape and rate of rise of the domes they are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes finally more complex experiments with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data are reported their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed

importance of pre impact crustal structure for the asymmetry of the chicxulub impact crater the final geometry of an impact crater can be significantly influenced by the geological and geomorphological structure of the impact site therefore it may not on its own provide accurate information regarding the direction and angle of impact

a 43 kyr record of protist communities and their response to oxygen minimum zone variability in the northeastern arabian sea abstract an extensive oxygen minimum zone omz occurs in the northeastern ne arabian sea where sedimentary records show evidence of alternating strong and weak omzs that correlate with north atlantic climate variability during the last glacial interglacial cycle omzs are expanding world wide but information on long term omz ecosystem interactions is mainly limited to fossilized species notably foraminifera here we provide a first comprehensive ancient sedimentary dna record of both fossilizing and non fossilizing protists and their response to omz variability in the ne arabian sea over the last 43 ka protist communities changed significantly during strong vs weak omz conditions coincident with interstadials and stadials respectively dinoflagellates were identified as significant indicator taxa for strong omzs during glacial as well as interglacial interstadials whereas diatoms were significant indicators for strong omzs only during glacial interstadials the chlorophyte chlorella was found to be the main phototrophic protist in nutrient depleted surface waters during glacial stadials notably strong omz conditions shaped past protist communities by creating isolated habitats for those capable of sustaining oxygen depletion either by adapting a parasitic life cycle e g apicomplexans or by establishing mutualistic connections with others e g radiolarians and mixotrophic dinoflagellates or by forming cysts e g colpodeans notably a long term increase in eutrophication and a decrease in the diatom dinoflagellate ratio was observed during the late holocene favoring the pelagic component of the marine food web a similar scenario could be expected in the context of predicted worldwide expansion of coastal omzs associated with global climate change

full waveform tomographic images of the peak ring at the chicxulub impact crater peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation the precise lithology and kinematics of peak ring formation however remains unclear previous work has revealed a suite of bright inward dipping reflectors beneath the peak ring at the chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity new two dimensional full waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin 100 200 m thick layer of low velocity 3000 3200 m s rocks this low velocity layer is most likely composed of highly porous allogenic impact breccias our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors across the peak ring velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation where the rocks that form the peak ring originate from an uplifted basement that has been subjected to high shock pressures 10 50 gpa and lie above downthrown sedimentary rocks that have been subjected to shock pressures of 5 gpa these observations suggest that low velocities within the peak ring may be related to shock effects and that the dipping reflectors underneath the peak ring might represent the boundary between highly shocked basement and weakly shocked sediments

extensional orogenic collapse captured by strike slip tectonics constraints from structural geology and upb geochronology of the pinhel shear zone variscan orogen iberian massif abstract the late paleozoic collision between gondwana and laurussia resulted in the polyphase deformation and magmatism that characterizes the iberian massif of the variscan orogen in the central iberian zone initial continental thickening d1 folding and thrusting was followed by extensional orogenic collapse d2 responsible for the exhumation of high grade rocks coeval to the emplacement of granitoids this study presents a tectonometamorphic analysis of the trancoso pinhel region central iberian zone to explain the processes in place during the transition from an extension dominated state d2 to a compression dominated one d3 we reveal the existence of low dipping d2 extensional structures later affected by several pulses of subhorizontal shortening each of them typified by upright folds and strike slip shearing d3 d4 and d5 as identified by superimposition of structures the d2 pinhel extensional shear zone separates a low grade domain from an underlying high grade domain and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust crustal thinning decompression melting and magma intrusion progressive lessening of the gravitational disequilibrium carried out by this d2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the pinhel shear zone by strike slip tectonics during renewed crustal shortening high grade domains of the pinhel shear zone were folded together with low grade domains to define the current upright folded structure of the trancoso pinhel region the d3 tamames marofa satao synform new dating of syn orogenic granitoids shrimp u pb zircon dating intruding the pinhel shear zone together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca 331 311 ma with maximum tectonomagmatic activity at ca 321 317 ma the capture and apparent cessation of movement of the pinhel shear zone occurred at ca 317 311 ma

folding and granite emplacement inferred from structural strain tem and gravimetric analyses the case study of the tulle antiform sw french massif central abstract in the south limousin french massif central the tulle anticline is a kilometre scale upright open fold which re folds an already foliated variscan stack of nappes three main granodioritic plutons associated with numerous folded dykes and boudinaged sills crop out in the core of the tulle anticline microstructural study including quartz c axis petrofabrics and transmission electron microscope tem analysis allows us to recognize the timing of deformation during pluton emplacement and cooling the earliest fabric is a pre full crystallization deformation the second stage corresponds to a high temperature ca 650 c solid state deformation the last deformation is responsible for the c axis preferred orientation at low temperature several methods of strain analysis applied in various mineral phases show that pluton emplacement is coeval to a constrictional strain this strain is characterized by a nw se maximum stretching axis x parallel to the tulle anticline axis and an intermediate ne sw trending shortening axis y also responsible for large open folding of the host rocks gravimetric modelling allows us to determine a laccolithic shape for the plutons hot stokes diapirism is ruled out and the hypothesis of granitic magma channelling through dykes is preferred a laccolith emplacement model based on a syn folding decollement level is proposed

on granitoid emplacement and related structures a review granitoid magmas are emplaced at the upper crust under a complex interaction of gravitational processes and horizontal tectonics natural intrusive bodies show a wide variety of structural patterns that must be strongly related with their dynamic history of ascent and emplacement comparisons of natural structural patterns with experimental and numerical models are necessary to interpret in a coherent fashion the emplacement mechanism involved from natural experimental numerical and theoretical considerations one can reach important conclusions on the problem of ascent and final emplacement of granitoid magmas in the earth s crust the present paper reviews the principal emplacement mechanisms referred to in the literature that is doming diapirism ballooning stoping cauldron subsidence and dike propagation dike propagation is the most effektive process in magma transport from deep zones in the crust and upper mantle extensional fractures for dike propagation and magma ascent can be developed at depth in the crust according to the modern dike propagation theory in orogenic domains magmas reach the upper crust through narrow channels or dikes and are accumulated in a final reservoir in the form of a discordant supracrustal pluton or a ballooning pluton if during emplacement any regional deformation acts simultaneously

using pluton ages to date regional deformations problems with commonly used criteria studies of pretectonic plutons in the foothills terrane central sierra nevada california along with a review of studies on syntectonic and post tectonic plutons indicate that no single criterion can establish the relative timing of pluton emplacement and regional deformation criteria most often used are whether a pluton is deformed cuts regional structures or is associated with porphyroblasts that postdate regional structures however all three types of plutons may 1 appear deformed or nondeformed 2 have contacts that appear to cut structures in the wall rocks and 3 have porphyroblasts in their aureoles that display varied timing relations the ambiguity of these criteria emphasize the need for careful studies of the structures present throughout the pluton and surrounding wall rock with particular attention paid to regions where the pluton contact is at high angles to regional structures porphyroblast cleavage relations are an important tool in this regard but also must be used with caution

a 3 d kinematic model of fabric development in polycrystalline aggregates comparisons with experimental and natural examples abstract a 3 d generalization of the approach previously developed by etchecopar for the simulation of fabrics in polycrystalline aggregates is presented the model is based on a geometric minimization of gaps overlaps and boundary displacements between deformed neighbouring cells each cell may deform using a small number

pluton accommodation at high strain rates in the upper continental crust the example of the central extremadura batholith spain abstract emplacement in the tensional bridge of a stepped dextral shear zone system is proposed for the central extremadura batholith spain the country rocks show a pervasive anisotropy that conditioned the style of the structures developed as a consequence of the transference of displacement from the stepped shear zones to the releasing area the kinematic evolution of the resulting megakink fold provided the volume increase necessary for the granite emplacement thermal and kinematic models suggest that the growth of individual plutons took place in periods of no more than several hundred to a few thousand years fast strain rates 10 10 10 11 s 1 must concentrate in local structures e g initiation of kink folds even in zones deforming as a whole under typical strain rates 10 14 1 s 1 granite plutons might be used as strain rate gauges for syn plutonic structures

quartz fabric transition in a cooling syntectonic granite hermitage massif france abstract the hermitage granite situated in the northwest massif central france is a syntectonic hercynian leucogranite emplaced along an active transcurrent shear zone during emplacement and cooling the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands c s mylonites increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline a slip and rotation recrystallization migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction we discuss relevant deformation mechanisms and rheological implications for syntectonic plutons in particular we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid state flow

spreading of salt structures in the gulf of mexico abstract past and present overhangs on diapirs ductile intrusions of salt are potential hydrocarbon traps and increasing numbers of larger overhangs are being recognised as seismic acquisition and processing improves salt overhangs develop by three process drag by ductile surroundings sinking around salt diapirs thinning of diapiric stems or the topic considered here gravity spreading gravity spreads salt where it is easier for salt to flow sideways than it is to float or sink the potential level to which individual salt diapirs rise depends on the pressure applied to their source by the overburden load and any lateral forces the potential level to which salt diapirs rise can be independent of both the top free surface and the level of neutral buoyancy of the salt even the most vigorous diapirs cannot rise indefinitely they gravitationally spread at barriers they cannot penetrate below at or above their level of neutral buoyancy it has been suggested that salt diapirs spread below their level of neutral buoyancy in weak layers beneath stiff barriers however no case of deep subsurface salt spreading appears to have been documented instead shales rise with salt along the us gulf coast some salt diapirs in the gulf of mexico may simulate ductile ice flows and spread both upwards and downwards to a subsurface level of neutral buoyancy in surroundings of similar strain rate however most diapirs in the gulf are driven above their level of neutral buoyancy so that they spread downwards back towards it these spread over denser or stifter layers in less dense and weaker barriers of air water or unconsolidated sediments this work focuses on the geological implications of the shapes of the tops of small scale secondary salt bodies that spread superficially in weaker and less dense barriers under the northern gulf of mexico analytical material and natural models are used to show that the shape of the top free surface of spreading salt contains information about both the properties of the materials involved and their kinematics many salt diapirs in the gulf spread downslope from viscous salt fountains in muds over sand salt flowing downslope towards a lower level of neutral buoyancy exhibits dynamic extrusion the temporary dynamic nature of salt sheets is emphasised by restating standard arguments for overburden strength in terms of low strain rates contrary to recent views that salt spreading occurs in weak horizons during fast sedimentation it is argued that most salt spreads superficially during periods of slow sedimentation small salt laccoliths advance downslope at long term rates between 1 and 5 mm a after burial spreading and gliding of their overburdens can detach such laccoliths from their source and smear them to salt sills that advance downslope at 8 mm a as development wells are likely to be complicated by such active salt flow we should begin now direct measurements of the rates at which salt spreads at superficial levels beneath the northern gulf of mexico

architecture of late orogenic plutons in the aracuai ribeira fold belt southeast brazil abstract post collisional to late orogenic magmatism 580 to 480 ma in the aracuai ribeira fold belt se brazil is characterized by the predominance of high k metaluminous allanite titanite bearing granitoids small lenses of coronitic gabbro anorthosite pyroxenite and phlogopite peridotite are also common in deeper exposed areas of this fold belt in the region of southern espirito santo state the deep erosional level associated with a steep topography reveals the internal architecture of the intrusions a tendency to funnel shaped bodies with sub vertical hemi ellipsoidal conical roots changing upwards to shallow dipping tops associated stocks sills and dykes of basic and acid rocks generally intrude the enclosing gneisses along the foliation planes local ductile shear zones and parallel to fold hinges the contact between intrusions and the enclosing rocks is sharp in deeply eroded plutons e g santa angelica venda nova mimoso do sul and varzea alegre but at shallow levels e g castelo pedra azul and conceicao de muqui agmatic stoping zones occur along the borders a magmatic foliation within the granitoids is usually well marked but the schistosity in the surrounding gneisses wraps around the plutons the intrusions have a bimodal chemical distribution and generally are reversely zoned with mafic cores gabbro diorite to tonalite surrounded by a mingled marble cake zone where basic and acid rocks are interfingered and an external zone of syenomonzonite and granite widespread evidence of mingling and mixing between contrasting magmas of gabbroic and granitic and or syenomonzonitic compositions is characteristic for all intrusive complexes we suggest that replacement of lithospheric mantle by hot asthenospheric mantle induced partial melting of the crust the mantle exchange was due to lithospheric mantle delamination and slab breakoff following collisional orogenesis the bimodal plutons result from interaction among the contrasting magmas ascent and emplacement followed older regional structures such as regional fold hinges and ductile shear zones

magnetic overprints in granitoids and metamorphic rocks from limousin france evidence for late variscan rotations crustal folding and tilting abstract generally the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics nevertheless this work demonstrates that successive remagnetizations recorded in early carboniferous metamorphic and plutonic units without clear bedding criteria can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation followed by a folding phase a tilting phase and a second regional clockwise rotation metamorphic ultrabasic tonalitic and granitic rocks from different parts of limousin western french massif central 45 5 n 1 25 e which underwent metamorphism during devonian early carboniferous or were intruded in the early middle carboniferous were sampled in order a to identify the magnetic overprinting phases and the related tectono magmatic events and b to constrain the regional and plate tectonic evolution of limousin paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90 of the magnetization isolated in rocks older than 330 ma are overprints in agreement with results from adjacent areas of the variscan belt the major overprinting phases occurred a in the last stages of the major exhumation phase 332 328 ma mean virtual geomagnetic pole vgp cp 37 n 70 5 e b during the post collisional syn orogenic extension 325 315 ma vgp b 11 n 114 e c in the latest carboniferous and early permian vgp a1 27 n 149 e and d in the late permian vgp a 48 n 146 e the middle late carboniferous overprints cp and b are contemporaneous with emplacement of leucogranitic crustal derived plutons and probably result from the hydro thermal activity related to the magmatism the drift from cp directions to b directions implies that after 330 ma limousin underwent a clockwise rotation by 65 together with the central europe variscides the bt components the vgps of which deviate from the mean apparent polar wander path apwp of the belt are interpreted as b overprints tilted during late variscan tectonics that is in the time range 325 315 ma the first and most important generation of bt overprints was tilted during nw se folding associated with ne sw shortening updoming and emplacement of leucogranitic plutons the second generation reveals southeastward tilting due to ne striking normal faulting the drift from b to a1 directions implies that limousin has participated to the second clockwise rotation by 40 of the whole belt in westphalian times

extensional tectonics within a subduction type orogen the case study of the wugongshan dome jiangxi province southeastern china abstract in southeastern china the wugongshan massif consists of metamorphic rocks and granites generally considered as basement rocks formed during an early paleozoic orogeny our structural and radiometric reappraisal of this massif shows that it results from a mesozoic doming in the metamorphic rocks the foliation defines an e w elliptical shape and bears a n s stretching lineation the axial zone of the dome is occupied by oriented plutons the outer part of the dome consists of devonian quartzite carboniferous permian limestone and triassic jurassic sandstone ductile deformation observed in the permian carbonates and older units develops contrasting kinematics the northern and southern flanks present top to the north and top to the south motions respectively the same divergent pattern of motion is recorded from brittle structures up to the jurassic sandstone the late cretaceous red beds are undeformed in the axial zone coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates along the n and s flanks of the dome post folial recumbent folds are symmetrically overturned to the n and s respectively but at the two extremities of the dome the fold axes are parallel to the stretching lineation 40 ar 39 ar ages of biotite and muscovite from mylonite and granite suggest a late triassic age for the ductile deformation and an early cretaceous age for the final doming the wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime a model which emphasizes plutonism is discussed from permian to early cretaceous southeastern china was a subduction type orogen the crust of which was thickened by accretion of mantle derived magmas the emplacement of a large amount of mesozoic granitoids in southeastern china promoted thermal softening and gravitational instability that allowed the crust to extend the formation of the wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology

magnetic fabrics in the holum granite vest agder southernmost norway implications for the late evolution of the sveconorwegian grenvillian orogen of sw scandinavia abstract the holum granite vest agder southernmost norway has been considered to be the oldest 980 ma undeformed post tectonic pluton of a late sveconorwegian granitoid series that form two belts in southern norway and sw sweden an integrated structural study of this granite pluton based on a survey of its anisotropy of low field magnetic susceptibility ams complemented by microstructural observations optical microscopy and rock magnetism investigations including measurements of partial anhysteretic remanent magnetization and its anisotropy paarm is presented here the magnetic susceptibility and the ams of the holum granite are carried by large magnetite grains coaxiality between the magnetite controlled ams fabric and the rock fabric is verified which allows the use of the former as a proxy for the latter microstructural evidence of a deformation continuum in the granite from magmatic to solid state and homogeneity and continuity of the ams fabrics across the pluton demonstrate that the holum granite is not post tectonic but that it was emplaced in a tectonic strain field a model of emplacement and deformation during the last sveconorwegian regional folding phase evidenced in the country rocks of the pluton is proposed

amphibolitization of metagabbros in the scottish highlands compositions of actinolite hornblende and cummingtonite together with pyroxene and plagioclase are studied in basic intrusions in the dalradian of north east scotland and the glen scaddle complex in the west moine amphibolitization is due to influx of water from the country rocks pyroxene compositions are found to have adjusted to the regional metamorphic environment owing to the difficulty of diffusion of al and si calcic amphiboles are zoned and commonly contain quartz blebs discontinuities in zoning give rise to actinolite hornblende pairs compared with north east scotland disequilibrium is less strong in the glen scaddle area in the latter plagioclase compositions have been greatly changed na partition between hornblende and plagioclase is close to equilibrium the maximum al content of hornblende is lower and zoning patterns are more consistent the fe mg ratio in calcic amphiboles varies with al content while approaching equilibrium partition with other minerals both zoning patterns and fe mg partition with cummingtonite suggest that fe mg of the calcic amphiboles increases more strongly with increasing alvi fe3 than can be explained simply by substitution of al fe3 for mg on m2 model reactions for amphibole formation are constructed cummingtonite formed at lower chemical potential of cao than actinolite ca was exchanged for mg fe between orthopyroxene derived and clinopyroxene derived local systems both cummingtonite and actinolite were formed because of kinetic constraints as intermediate reaction products actinolite hornblende pairs represent disequilibrium this work suggests that many occurrences of actinolite with hornblende where the minerals are zoned may also be due to diffusion kinetics

strain patterns within the late variscan granitic dome of velay french massif central the velay granites are intruded as a late variscan dome in the french massif central strain patterns within the velay granites reflect the geometry and kinematics of late carboniferous deformation at the site of final emplacement the internal deformation developed during granitic magmatism is not homogeneously distributed from inner to outer zones increasing deformation is marked by changes from high temperature foliations to s c orthogneisses strain concentration at the margins reflects a strength contrast due to cooling and subsequent magma hardening according to the internal strain field two zones can be distinguished in the velay dome i a central and southern zone where concentric patterns indicate a southward expansion of granites and ii a northern zone where the lateral expansion is limited and combined with top to the north extensional shearing along the mont pilat detachment zone and high temperature wrenching these asymmetric strain patterns relate to the expansion and deformation of granites emplaced below a detachment fault during the uplift of a thickened area post thickening combination of extensional and wrench tectonics is inferred

thermal and barometric constraints on the intrusive and unroofing history of the black mountains implications for timing initial dip and kinematics of detachment faulting in the death valley region california unroofing of the black mountains death valley california has resulted in the exposure of 1 7 ga crystalline basement late precambrian amphibolite facies metasedimentary rocks and a tertiary magmatic complex the 40ar 39ar cooling ages obtained from samples collected across the entire length of the range 55 km combined with geobarometric results from synextensional intrusions provide time depth constraints on the miocene intrusive history and extensional unroofing of the black mountains data from the southeastern black mountains and adjacent greenwater range suggest unroofing from shallow depths between 9 and 10 ma to the northwest in the crystalline core of the range biotite plateau ages from 13 to 6 8 ma from rocks making up the death valley turtlebacks indicate a midcrustal residence with temperatures 300 c prior to extensional unroofing biotite 40ar 39ar ages from both precambrian basement and tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest subparallel to the regional extension direction diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest the cooling age pattern and geobarometric constraints in crystalline rocks of the black mountains suggest denudation of 10 15 km along a northwest directed detachment system consistent with regional reconstructions of tertiary extension and with unroofing of a northwest deepening crustal section mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment the well known amargosa chaos and perhaps the badwater turtleback are examples of this splaying process considering the current distance of the structurally deepest samples away from moderately to steeply east tilted tertiary strata in the southeastern black mountains these data indicate an average initial dip of the detachment system of the order of 20 similar to that determined for detachment faults in west central arizona and southeastern california beginning with an initially listric geometry a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward this pattern may be explained by a model where migration of footwall flexures occur below a scoop shaped hanging wall block one consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading this process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

submagmatic microfractures in granites microstructures in hercynian granite plutons of the pyrenees indicate fracturing of plagioclase crystals in the presence of melt this is represented mainly by quartz and or feldspar that fills fractures electron microprobe analyses demonstrate that in compositional agreement with the rims of the zoned plagioclase the feldspar infillings are more sodic with decreasing melt fraction fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic i e when the melt was below the critical fraction for magmatic flow material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures because during cooling the volume fraction of melt remains low for a large temperature range a strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas

the biards sb au bearing shear zone massif central france an indicator of crustal scale transcurrent tectonics guiding late variscan collapse structural textural mineralogic microthermometric and isotopic data obtained on the biards sb au district located in the south limousin area massif central france provide new evidence for the tectonic setting of stibnite and gold deposition during the late orogenic variscan collapse in western europe a variety of geologic and geophysical data define a major crustal scale left lateral strike slip shear zone that acted as a trap for sb au mineralization during these movements fault fill veins and then extensional veins of limited extent were created along rheologic heterogeneities that focused brittle deformation geometry and orientation of both vein sets indicate two main episodes of emplacement of ore related to a progressive wrench movement in response to an counterclockwise rotation of the regional stress axes both veins sets are filled by quartz and sulfides in three stages an early arsenopyrite pyrite stage with fine grained quartz a sphalerite pb sb sulfosalt native gold stage with comb quartz and a late stibnite berthierite stage with comb quartz combined fluid inclusion and stable isotope studies on quartz muscovite and sulfides reveal that the three stages result from the continuous evolution of a deep seated hydrothermal system in which the latest stages mixed with meteoric water these data argue for cooling and a pressure drop during ore deposition and indicate a shallow level of emplacement for sb au veins a comparison of the biards deposits with other upper paleozoic gold and antimony deposits in the massif central the western variscides and elsewhere in the world illustrates their common association with crustal scale strike slip shear zones the upper carboniferous age 305 300 ma of the sb au deposits indicates that they formed during the early stages of the postthickening extension of the variscan lithosphere sb au bearing strike slip shear zones resulted from the late variscan crustal collapse guided by the very large scale dextral shearing between central europe northern africa and the northern appalachians and associated with the opening of the paleo tethys and the closing of the theic ocean during the late paleozoic gondwana laurasia translation

extensional deformation assisted by mineralised fluids within the brittle ductile transition insights from the southwestern massif central france abstract extensional deformation assisted by mineralized fluids within the brittle ductile transition is inferred using structural microstructural and textural investigations applied to the sarlande shear zone of the southwestern massif central france the shear zone is a low angle normal fault that bounds the meuzac antiform to the south these structures were formed during the nw se trending synorogenic extension of middle carboniferous age pre existing lithologies involved in the shear zone led to partitioning of behaviour and hydrothermal alteration in response to crustal extension and channelling of mineralising fluids intense fluid rock interactions produce softening and weakening reactions induced by alteration of k feldspar remobilisation of graphite and rutile and growth of white micas chlorite and polymetallic sulphides extensional shear deformation and mineralization progressively and concomitantly developed within the brittle ductile transitional crust 9 13 km depth the derived crustal scale model emphasises the importance of collision which led to the production of large quantities of granite derived fluids and the creation of a crustal permeability that drove these fluids near to the brittle ductile transition late orogenic extension associated with fluid rock interactions led to growth of mineralizing low angle shear zones and footwall domes the role of mineralising fluids fluid pressure appears to be essential in promoting deformation within the brittle ductile transition of the extending crust

tabular intrusion and folding of the late archaean murehwa granite zimbabwe during regional shortening the murehwa granite belongs to the late archaean chilimanzi suite of granites that occupy more than 50 of the zimbabwe craton the granite has compositional layers on metre to tens of metre scales with generally shallow dips and dip directions towards the centre of the granite near its margins the layering is systematically folded about sub horizontal east west axes microcline phenocrysts are generally strongly aligned parallel to the layering but locally crosscut the layering because they are axial planar to the folds crystals are not significantly deformed fabrics are thus magmatic the granite was intruded in a tabular sheet that may have been only about 6 km thick and fed by dykes contrary to previous concepts of diapirism ballooning for some archaean granites of the zimbabwe craton the systematic orientation of magmatic folds and axial planar fabrics demonstrate that the granite was intruded during regional north south shortening this shortening is consistent with the orientation of cratonic scale fractures that were intruded by the great dyke within 25 ma of the intrusion of the murehwa granite the granite may have intruded in a relatively short interval of less than 1 26 ma within the longer duration of the entire chilimanzi granite event

modeling damage and deformation in impact simulations numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters particularly in cases where the final pristine crater morphology is ambiguous or unknown in this paper we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results we demonstrate this with an example simulation of an impact into a terrestrial granite target that produces a 10 km diameter transient crater the results of the simulation are shown in terms of damage a scalar quantity that reflects the totality of fragmentation and plastic strain both total plastic strain the accumulated amount of permanent shear deformation regardless of the sense of shear and net plastic strain the amount of permanent shear deformation where the sense of shear is accounted for damage and plastic strain are both greatest close to the impact site and decline with radial distance however the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains plastic strain in brittle rocks is very heterogeneous however continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock large compared to the spacing between individual zones of sliding this paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones furthermore we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity

folding of granite and cretaceous exhumation associated with regional scale flexural slip folding and ridge subduction kitakami zone northeast japan abstract the early cretaceous granitic plutons intrude the kitakami zone northeast japan whose southern and northern regions consist of forearc basin and accretionary complex rocks respectively all country rock of the kitakami zone exhibit prominent pressure solution cleavage and associated folds formed during shortening with a small component of sinistral shear whereas most plutons show only igneous textures the kesengawa granite and some other plutons however have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks we document a tectonic fold with axial planar foliation in part of the kesengawa granite the metamorphic minerals associated with the contact aureole and country rocks of the kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled the kesengawa pluton and country rocks of the southern kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern kitakami zone are overturned and verge to the east the kitakami basement is not bounded by normal or reverse faults so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions nor is the exhumation associated with extensional doming instead the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines the regional shortening of accretionary prism forearc basin and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation the adakitic magmatism and forearc shortening resulted from subduction of the buoyant izanagi kula ridge a regional event known as the oshima orogeny

late orogenic carboniferous extensions in the variscan french massif central the variscan french massif central experienced two successive stages of extension from middle carboniferous to early permian in the northern massif central the first stage began in the late visean immediately after nappe stacking and is well recorded by namurian westphalian synkinematic plutonism the middle carboniferous leucogranites widespread in the nw massif central limousin and sioule area were emplaced within a crust extending along a ne sw direction at the same time the hanging wall or gueret extensional allochton moved toward the se several examples of the synextensional plutonism are also recognized in central limousin saint mathieu dome la porcherie and cornil leucogranites these examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by nw se stretching and ne sw shortening in the central and southern massif central cevennes ch taigneraie and margeride areas plutonism is dominantly granodioritic and exhibits the same structural features nw se maximum stretching and overturning to the se middle carboniferous namurian westphalian extension was parallel to the variscan belt both in the massif central and southern armorican area this extensional regime was active from the late visean in the north while compression dominated in the southernmost domains montagne noire and pyrenees the second extensional stage occurred from late carboniferous to early permian this event was responsible for the opening of intramontane coal basins brittle deformation in the upper crust and ductile normal faulting localized on the margin of cordierite granite migmatite domes data from the coal basins show that the half graben is the dominant structural style except for basins located along submeridianal left lateral faults which have pull apart geometries late carboniferous extension occurred along the ne sw direction the ne sw maximum stretching direction can be found in the whole massif central but is more developed in the eastern part the extensional direction is transverse to the general trend of the belt and top to the ne shearing is dominant correlations of these two extension directions with neighboring variscan massifs are discussed

peak ring formation in large impact craters geophysical constraints from chicxulub a seismic reflection and three dimensional wide angle tomographic study of the buried v200 km diameter chicxulub impact crater in mexico reveals the kinematics of central structural uplift and peak ring formation during large crater collapse the seismic data show downward and inward radial collapse of the transient cavity in the outer crater and upward and outward collapse within the central structurally uplifted region peak rings are formed by the interference between these two flow regimes and involve significant radial transport of material hydrocode modeling replicates the observed collapse features impact generated melt rocks lie mostly inside the peak ring the melt appears to be clast rich and undifferentiated with a maximum thickness of 3 5 km in the center fl 2000 elsevier science b v all rights reserved

space accommodation by roof lifting during pluton emplacement at amiata italy the amiata area is characterized by an uplift attributed to pluton emplacement during the pliocene epoch the shape of the buried pluton is constrained by gravity data fieldwork was carried out to identify possible deformations within the overburden in order to evaluate the mechanism of emplacement the attitudes of lower pliocene marine sediments deposited immediately before the uplift were measured also the overall pattern of the deposits defines a dome approximately 2 5 km high and 35 50 km wide consistency between these field data and the existing geophysical data suggests that the dome results from the emplacement of a laccolith like intrusion which was responsible for lifting and flexuring the overburden the amiata case shows that roof lifting is a viable process by which space in the upper crust to a depth of 7 km can be accommodated during pluton emplacement