a moderate melting model for the voring margin norway based on structural observations and a thermo kinematical modelling implication for the meaning of the lower crustal bodies high p wave velocities 7 1 7 8 km s lower crustal bodies lcbs imaged along volcanic margins are commonly interpreted as plume and breakup related thick mafic underplating this interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer voring basin norway an exceptional strong amplitude reflection the t reflection is particularly well defined below the north gjallar ridge ngr between 7and 8 s twt the t reflection is located near the volcanic lava flows emplaced during the ne atlantic breakup 55 54 ma ago and coincides with the top of the lcb forming a mid crustal dome based on structural and temporal relationships we show that the dome clearly influences the structural development of the ngr and predates the continental breakup at least by 10 15 ma using a thermo kinematical model we tried also to investigate and quantify the relationships between the extension lcb and the magmatic production modelling suggests that significant paleocene early eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the breakup initiation the conclusion of 2d thermo kinematical parametric analysis is that the magmatic model predicts either little extension 5 with significant melting along the outer voring basin we suggest that the continental part of the lcb could not necessarily be breakup related and so magmatic as has often been stated previously it is concluded here that the continental part of the lcb observed beneath the outer voring basin may be partly or fully attributed to inherited high pressure granulite eclogite lower crustal rocks the real amount of mafic material emplaced along the outer voring basin could be 20 40 less than thought

the kinematics of movements along the insubric line and the emplacement of the ivrea zone schmid sm zingg a and handy m 1987 the kinematics of movements along the insubric line and the emplacement of the ivrea zone in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tecta a hys 135 47 66 the insubric line west of locamo is characterized by a 1 km thick greenschist facies mylonite belt east of locamo these mylonites are overprinted by a discrete brittle fault the mylonites are derived from basement units of the central alps sesia zone and the southern alps ivrea zone as well as from the permo mesozoic cover of the southern alps canavese sense of shear criteria indicate that the mylonites accomm ated backt s g followed by dextral strike slip motion mylonitization during backtbrusting was synchronous with backfolding of the central alpine nappes under higher metamorphic conditions a horizontal temperature gradient resulted from the rapid juxtaposition of the warm central alpine block against the cold southern alpine block mylonites formed during the later dextral strike slip event are related to large transcurrent displacements in the central alps deduced from regional kinematic considerations thus both mylo t ation events are contempor eous with deformation to the north and south of the insubric line insubric phase extensively modifying the pre insubric crustal configuration of the alps the insubric phase post dates the bergell intrusion 30 m y the emplacement of the geophysical ivrea body is a combined effect of vertical uplift due to e w directed crustal thinning during the early jurassic and underplating by continental crust associated with late cretaceous compression a deep crustal normal fault pogallo line subsequently rotated during tertiary alpine orogenesis separates deeper parts of the southern alpine crust ivrea zone from intermediate crustal levels strona ceneri zone the rigid ivrea body localir ed large strains within the insubric mylonite belt and is responsible for the present curvature of the insubric line in uetion

shear wave anisotropy of laminated lower crust beneath urach sw germany a comparison with xenoliths and with exposed lower crustal sections in order to investigate the laminated lower crust beneath urach sw germany we analysed 1 the anisotropy and reflectivity of seismic shear waves and 2 the composition of xenoliths samples we conclude that the directional dependence of shear waves of the lower crust is effectively transversely isotropic with a maximum velocity difference of 3 to 6 both seismic signature and petrological information are compared with reference profiles deduced from the exposed lower crustal sections of ivrea and calabria italy the comparison based on synthetic seismograms shows that the observed combination of shear wave anisotropy and reflectivity pattern corresponds to alternating layers containing a high amount of metapelites such as found in the calabria profile and in the val strona subsection of ivrea the main portion of the whole rock anisotropy is caused by the lattice preferred orientation of anisotropic minerals so called intrinsic anisotropy the effect of alternating rock layering so called layering anisotropy contributes only a minor portion to the whole rock anisotropy layered mafic intrusions such as found in the val sesia subsection of ivrea are nearly isotropic both seismic anisotropy and xenoliths indicate that mafic layers do not play a major role for the laminated lower crust at urach they may be restricted to a thin layer at the crust mantle boundary

dynamic link between the level of ductile crustal flow and style of normal faulting of brittle crust abstract in a rheologically layered crust compositional layers have an upper elasto plastic part and a lower viscous one when broken the upper elastic part undergoes flexure which is upward for the foot wall and downward for the hanging wall as a consequence of bending stresses will develop locally that can overcome the strength of the plate and therefore impose the migration of active fault in the lower viscous part of each compositional layer rocks can potentially flow numerical modelling of the behaviour of a crust made up of two compositional layers during and following extension shows that flow can take place not only in the lower crust but also and more importantly in the lower part of the upper crust the ability of crustal rocks to flow influences the style and kinematics of rifted regions when no flow occurs subsidence will affect the extending areas both hanging wall and foot wall will subside with respect to an absolute reference frame such as sea level and there will be a strict proportionality between extension and thinning in addition the downward movement of the fault blocks will decrease the local stresses created in the foot wall and increase those of the hanging wall thereby imposing a migration of fault towards the hanging wall this is the behaviour of extensional settings developed on stabilised crust and which evolved in a passive margin when flow does take place middle crustal rocks will move towards the rifting zone causing isostatically driven upward movements that will be superimposed on movements associated with crustal and lithospheric thinning consequently fault blocks will move upwards and the crust will show more extension than thinning the upward movements will decrease the stresses developed in the hanging walls and increase those of the foot wall faults will then migrate towards the foot wall such a mode of deformation is expected in regions with thickened crust and has its most apparent expression in core complexes

contrasting seismic reflectivity of the lower crust and uppermost mantle between ne japan and sw japan as illustrated by petrophysical analyses of mafic and ultramafic xenoliths 1 petrophysical analyses of mafic and ultramafic xenoliths reveal large contrasts in acoustic impedance between the lower crustal rocks and uppermost mantle rocks both beneath ichinomegata crater in ne japan and beneath oki dogo island in sw japan in contrast to minor contrasts in acoustic impedance both among the lower crustal rocks and among the uppermost mantle rocks beneath ichinomegata crater relatively large contrasts in acoustic impedance are present beneath oki dogo island between granulite and olivine gabbro in the lower crustal rocks as well as between plagioclase bearing wehrlite and other cumulus peridotites in the uppermost mantle rocks based on the existing petrologic models of the lower crust and uppermost mantle beneath ichinomegata crater and oki dogo island we then constructed model structures of the lower crust and uppermost mantle beneath these two areas composed of units with randomly arranged 100 m thick foliation parallel horizontal layers of xenolith samples and used these model structures to examine the seismic reflectivity beneath these two areas the results suggest the presence of strong seismic reflectors between the lower crust and the uppermost mantle in both areas although no other recognizable seismic reflector is expected beneath ichinomegata crater many seismic reflectors due to the above contrasts in acoustic impedance are expected both in the lower crust and in the uppermost mantle beneath oki dogo island thus a remarkable contrast in seismic reflectivity in the lower crust and uppermost mantle is likely present between these two areas as well as between ne japan and sw japan

some geophysical implications of the deformation and metamorphism of the ivrea zone northern italy abstract the ivrea zone is commonly cited as an upthrust and steepened section through the lower continental crust being contiguous via the geophysical ivrea body with the present day lower crust beneath the po basin the rocks presently exposed were buried to lower crustal depths through the variscan orogeny and were gradually exhumed to mid crustal depths through permian and early mesozoic times as a result of crustal extension they were emplaced into their present position during the alpine orogeny an important time marker in the deep crustal evolution of the rocks was the development of a coarse grained equigranular texture following the attainment of the amphibolite and granulite facies metamorphic peak this effectively separates earlier pervasive synmetamorphic deformation from later highly localized plastic deformation associated with crustal extension and progressive cooling the geophysical properties of the section would have been markedly different during each of these two stages of development in the later stage the granulite facies rocks remained relatively dry and when buried would have been characterized by the high seismic velocities that they display under pressure in the laboratory during the prograde orogenic part of the history seismic velocities would have been substantially depressed through the effects of elevated pore water pressure particularly at high temperatures

the evolution of the serie dei laghi strona ceneri and scisti dei laghi the upper component of the ivrea verbano crustal section southern alps north italy and ticino switzerland abstract the serie dei laghi is the upper intermediate component of the exposed crustal section of massiccio dei laghi in the southern alps the latter including also the ivrea verbano zone its oldest event is a sedimentary cycle at least of early paleozoic age it starts with a pelitic sequence scisti dei laghi that later suffered a tectono metamorphic event of rather high pressure this event is recorded by rare mineral and textural relics detected in the scisti dei laghi of the m gambarogno area later an arenaceous sequence strona ceneri was deposited it is now represented at the base by basaltic metatuffites resulting from the reworking of an ophiolite belt then followed by paragneisses with detrital phenoclasts ceneri gneiss and by fine grained gneisses gneiss minuti during the ordovician conspicuous tonalitic to granitic masses were emplaced the induced thermal disturbance and the circulation of pegmatitic stage magmatic residuals caused contact metamorphism and granitization in the sedimentary country rocks during the hercynian a main prograde event generated the regional foliation with isoclinal geometry and transposed character a syn kinematic stage produced medium grade parageneses of relatively high pressure static recrystallization and sillimanite growth then followed during the uplift due to the combined action of hydration and decompression margarite bearing coronitic textures grew around kyanite a retrograde metamorphism was then recorded by the appearance of greenschist facies parageneses at particular structural sites the formation of the cossato mergozzo brissago cmb and pogallo fault system against the ivrea verbano zone as well as the schlingen folds in the northeast is of late hercynian age along the cmb fault calc alkaline dykes and stocks appinites were intruded probably towards the end of the upheaval but deep enough to induce partial melting in their country rocks between val d ossola and valsesia the hercynian evolution is concluded by the early permian magmatic cycle that gave rise to both extrusive rhyolitic sheets and to granitic epiplutonic bodies accompanied by mafic dykes in mesozoic times the serie dei laghi was dissected by many extensional faults these partly reactivated the hercynian lineaments such as the cmb and pogallo lines during the alpine orogenic cycle the serie dei laghi was affected by s verging cataclastic overthrusts

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

mechanisms of shear localization in the continental lithosphere inference from the deformation microstructures of peridotites from the ivrea zone northwestern italy the ultramafic massif of balmuccia northwestern italy shows a variety of deformation fabrics including some localized shear zones that resulted in nearly complete melting pseudotachylyte a series of peridotite specimens were collected near one of the pseudotachylyte fault veins to investigate the mechanisms of shear localization the microstructural analyses show at least three deformation stages the first stage i is nearly homogeneous deformation at low stress 3 mpa and high temperature 1300 1500 k followed by localized deformation stage ii at a higher stress 60 mpa and moderate temperature 1000 1150 k and finally semi brittle deformation stage iii at a very high stress 400 mpa that resulted in the formation of pseudotachylyte the stage ii deformation resulted in relatively small strains in most areas but significant localized deformation leading to dynamic recrystallization occurred the degree of which increases systematically toward the fault zone based on this observation together with the observation that the brittle deformation post dated dynamic recrystallization we conclude that the shear localization in this locality occurred as a result of ductile deformation rather than brittle deformation various mechanisms of shear localization in the ductile regime are examined and we conclude that the grain size reduction due to dynamic recrystallization at relatively high stresses and moderate temperatures is a probable mechanism of shear localization the intermediate regime between dislocation and diffusion creep where rheology is grain size sensitive yet continuous recrystallization occurs due to dislocation creep is suggested to play an important role in shear localization in the upper mantle the conditions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization it is shown that shear localization occurs at relatively low temperatures and or high stresses localized deformation in the continental lithosphere will occur well below the brittle ductile transition 10 km depth down to 40 80 km depth depending upon the geothermal gradient and water fugacity

thrusting in the lower crust evidence from the oygarden islands kemp land east antarctica layered orthogneisses of the oygarden islands preserve evidence for four high grade deformation events d 1 to d 4 archaean d 1 and d 2 structures are only patchily preserved due to extensive recrystallization during d 3 and d 4 which represent effects of the c 1000 ma rayner structural episode ductile thrusting at middle to lower crustal levels occurred during d 3 which is separated into two mutually cross cutting phases based on structural geometry the two phases represent changes in finite strain that developed during progressive deformation east directed transport during d 3a developed subhorizontal thrusts that contain co axial east trending f 3a folds and l 3a lineations buckling as a consequence of constriction in thrust duplexes developed upright f 3b folds coaxial to f 3a folds and steeply south dipping d 3b shear zones garnet clinopyroxene and garnet orthopyroxene bearing assemblages in mafic lithologies and garnet sillimanite bearing assemblages in pelitic lithologies reflect d 3 conditions of p 9 kbar and t 800 850 c the well exposed d 3 duplex structures indicate that shortening of the lower crust may be accommodated by extensive strain partitioning to develop contemporary kilometre scale thrust stacking and ductile flow

the tectonic and rheological evolution of an attenuated cross section of the continental crust ivrea crustal section southern alps northwestern italy and southern switzerland the tectonic and rheological evolution of the southern alpine continental crust is reconstructed from structural petrological and radiometric studies in the ivrea and strona ceneri basement units the deep crust of the southern alps acquired its present compositional and metamorphic zonation during paleozoic magmatism and amphibolite to granulite facies regional metamorphism inferred strength contrasts between lower crustal and upper mantle rocks in the ivrea zone are low at the high temperatures of regional metamorphism late paleozoic transtension and basic to intermediate magmatism in all crustal levels preceded extensional faulting associated with the formation of a passive continental margin during early mesozoic time extensional uplift and cooling of the basement section is coupled with crustal scale trends of increasing rheological stratification grain size reduction and strain localization mylonitic shear zones in the lower crust and a broad zone of noncoaxial shear at the base of the intermediate crust in the ivrea zone are inferred to have controlled the high strain rheology of the attenuating southern alpine crust the deep crust in the thinned ivrea crustal cross section transferred noncoaxial strain from the upper mantle to the intermediate and upper crust final uplift and exposure of the deep southern alpine crust occurred during late cretaceous to tertiary alpine thrusting

seismic anisotropy and shear wave splitting in lower crustal and upper mantle rocks from the ivrea zone experimental and calculated data to quantify the relationships between anisotropy s wave splitting and tectonics we determined the seismic properties of lower crustal and upper mantle rocks outcropping in the ivrea zone northern italy we obtained p and s wave seismic velocities by laboratory direct velocity measurements and or by calculations based on the modal compositions of the rocks the lattice preferred orientations lpos and the single crystal stiffness coefficients measured p and s wave velocities 6 0 7 5 km s 1 and 3 6 4 2 km s 1 are typical of the lower crust the p wave anisotropy is in the range 0 10 shear wave birefringence is in the range 0 0 0 6 km s 1 with typical values between 0 0 and 0 2 km s 1 in many cases the birefringence is clearly related to fabric elements foliation lineation mafic rocks such as anorthosite or pyroxene bearing gabbros exhibit low p wave anisotropies 5 and low shear wave birefringences less than 0 1 km s 1 in contrast the seismic properties of felsic rocks such as biotite bearing gneisses and mafic rocks such as amphibolites display high vp anisotropy 10 and strong birefringence 0 3 km s 1 biotite and amphibole preferred orientations clearly control seismic anisotropy and particularly shear wave splitting in these rocks maximum splitting is observed in directions parallel to the foliation with the fast split shear wave polarized parallel to the foliation plane to have an overview of the seismic properties of this lower crustal section at a broader scale we calculated from our data the anisotropic seismic properties of several hypothetical samples that are perhaps more representative of the regional anisotropy than each sample individually for instance the average lower crustal sample displays an anisotropy of 5 5 for p waves and a birefringence around 0 14 km s 1 for s waves propagating parallel to the foliation we observe little splitting for waves propagating at high angle to the foliation

polyphase seismic faulting in the ivrea zone italian alps revealed by 40ar 39ar dating of pseudotachylytes terra nova 23 162 170 2011 abstract most fault related pseudotachylytes in the val sesia area ivrea zone nw italian alps occur in two areas near the canavese line in the kinzigite formation paragneisses and in an elongate belt in the mafic complex gabbros away from the canavese line 40ar 39ar dating indicates that pseudotachylytes from the kinzigite formation were formed in the late eocene likely during the early stages of the collision between the nw corner of the adriatic indenter and the european margin pseudotachylytes from the mafic complex were formed under a transpressive strain regime as suggested by kinematic indicators and yield early cretaceous albian 40ar 39ar ages their formation is possibly related to shortening during the inception of the tectonic inversion of the adriatic passive margin

large scale folding in the upper part of the ivrea verbano zone nw italy new geological mapping has led to a new interpretation of the large scale superimposed folding in the upper part of the ivrea verbano zone italian alps the region is widely held to represent an upended section through lower continental crust of northern italy the dominant fold structure extending some 40 km along strike is the massone tight to isoclinal antiform with a hinge line strongly curved through 115 this folds pre existing large scale folds that formed during regional migmatization probably during the hercynian orogeny to form a type 2 interference geometry the region then suffered post orogenic mafic magmatic underplating and other magmatism accompanied by crustal stretching with contact metamorphism and migmatization causing the imposition of the final pattern of metamorphic isograds the ivrea verbano zone was brought into contact with the overlying metamorphic rocks of the serie dei laghi on a major shear zone sub solidus stretching continued though displacements on low angle high temperature shear zones most of the ivrea verbano zone was finally tilted to the vertical and emplaced into its present position after the mesozoic era and probably during alpine orogenesis forming the vertical limb of a crustal scale double kink

lithology and evolution of the crust mantle boundary region in the southwestern basin and range province mantle and crustal xenoliths from volcanic rocks in the southwestern basin and range province and colorado plateau transition zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region seismic transects in this area show a strongly reflective moho of generally low relief which in the area of modern transects consists of a thin zone 2 km thick of short reflectors the upper mantle is transparent and has a pn of 7 8 8 0 km s similar to much of the western united states a lower crustal zone 2 13 km thick has variable internal reflectivity and a relatively low velocity of 6 6 6 8 km s upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths whereas melt infiltration preceded and followed ductile deformation brittle deformation represented by closely spaced joint systems and faults followed ductile deformation and is related to the youngest magmatic episodes these structural characteristics and high uppermost mantle temperature 1000 c may combine to explain the relatively low pn alternating layers of ductily deformed and undeformed peridotites with or without igneous intrusions may contribute to the reflectivity of the moho lower crustal xenoliths are dominantly igneous textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths the crustal xenoliths also commonly are jointed and in addition many show partial melting and have abundant cavities that probably were filled with co2 rich fluids these rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records the mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems partial melts and high temperatures garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late miocene extension felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust the preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes the earliest magmatic events preceded extension and later events accompanied and followed extension

seismic velocities and anisotropy of the lower continental crust a review seismic anisotropy is often neglected in seismic studies of the earth s crust since anisotropy is a common property of many typically deep crustal rocks its potential contribution to solving questions of the deep crust is evaluated the anisotropic seismic velocities obtained from laboratory measurements can be verified by computations based on the elastic constants and on numerical data pertaining to the texture of rock forming minerals for typical lower crustal rocks the influence of layering is significantly less important than the influence of rock texture surprisingly most natural lower crustal rocks show a hexagonal type of anisotropy maximum anisotropy is observed for rocks with a high content of aligned mica it seems possible to distinguish between layered intrusives and metasediments on the basis of in situ measurements of anisotropy which can thus be used to validate different scenarios of crustal evolution

geodynamic evolution of the continental allochthonous terrane cat of the braganca nappe complex ne portugal the braganca massif in particular its continental allochthonous terrane cat is a keystone in the comprehension of the pre variscan and variscan geodynamic evolution of the eurasian plate in particular of the high grade metamorphic evolution the braganca cat comprises three types of granulites recording quite distinct metamorphic histories and paragneisses with enclosed eclogite boudins it has undergone five characterizable main deformation phases during two orogenic cycles separated by the extensive intrusion of mafic ultramafic layered bodies types 2 and 3 granulites into a pervasively deformed granulitic lower continental crust type 1 granulites geochronological data obtained on high grade metamorphic rocks from braganca and other similar rocks in the variscan fold belt have been typically interpreted in two different ways polycyclical evolution precambrian and paleozoic variscan or monocyclical evolution variscan this second hypothesis is in clear contradiction with field evidence with tectonometamorphic data and even with some geochronological data as shown in this article thus we suggest that the tectonometamorphic evolution of the braganca cat and by comparison of other similar cat in the variscan fold belt took place during two orogenic cycles one precambrian grenvillian pan african cadomian and another one paleozoic variscan

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

tectonic subsidence history and thermal evolution of the orange basin the orange basin offshore southwest africa appears to represent a classical example of continental rifting and break up associated with large scale transient volcanism the presence of lower crustal bodies of high seismic velocities indicates that large volumes of igneous crust formed as a consequence of lithospheric extension we present results of a combined approach using subsidence analysis and basin history inversion models our results show that a classical uniform stretching model does not account for the observed tectonic subsidence moreover we find that the thermal and subsidence implications of underplating need to be considered another departure from the uniform stretching model is renewed sub crustal thinning and linked to that uplift in the cenozoic that is necessary to reproduce the observed phases of erosion and the present day depth of the basin the dimension of these events has been examined and quantified in terms of tectonic uplift and sub crustal thinning based on these forward models we predict the heat flow evolution not only for the available real wells but also for virtual wells over the entire study area finally the hydrocarbon potential and the temperature evolution is presented and shown in combination with inferred maturation of the sediments for depth intervals which comprise potential source rocks 2009 elsevier ltd all rights reserved

isotopic evidence from the ivrea zone for a hybrid lower crust formed by magmatic underplating the large mafic complex overlying mantle peridotites in the ivrea zone of northern italy originated by underplating of mantle derived magma onto the base of continental crust and may be a unique outcrop of typical mafic lower crust sampled elsewhere only by mafic xenoliths neodymium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material the resulting hybrid magmas have the same isotopic signature as and may represent the lower crustal roots of upper crustal hercynian granitoid intrusions

the development of orientated symplectites during deformation orientated symplectites have been observed in deformed granulite facies metabasic rocks from the ivrea verbano zone in northern italy the area underwent lower crustal extension accommodated by movement on localized high t shear zones in areas of relatively low strain such as at the margins of shear zones symplectites of orthopyroxene plagioclase and spinel have formed the symplectites are vermiform and orientated parallel to the main foliation and in the regional stretching direction the reaction was synkinematic with the deformation and only developed in potentially dilatant grain boundaries in the rock it was presumably inhibited in grain boundaries subjected to higher normal stress due to the relatively large volume increase involved in the reaction the observations support the interpretation that the deformation was related to regional extension under high t granulite facies conditions the symplectites forming as a result of decreasing pressure

mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust the sondalo magmatic system n italy

investigation of the dynamic sea ice component of a coupled atmosphere sea ice general circulation model this paper discusses the development of a coupled atmospheric ice model with a comprehensive ice dynamics cavitating fluid rheology and an open water parameterization the results center on the ice thickness and concentration distribution in the arctic basin where the inclusion of ice rheology has maximum impact the influence of the wind stresses calculated by an atmospheric model with stability dependent surface drag coefficients is discussed along with the role of the ocean currents and ocean ice drag coefficient under winter wind stress conditions the model had a tendency to buildup ice on the siberian coast the introduction of a partial slip condition at land ice boundaries in the model designed to prevent this buildup is discussed and the best choice for the amplitude of the slip at the land boundaries in this model is found to be 0 4 the ice model results are compared with the available observational data sets on ice thickness concentration and ice velocity fields to test the validity of the model simulations

structural geometry lower crustal magmatic underplating and lithospheric stretching in the ivrea verbano zone northern italy abstract the ivrea verbano zone is believed to represent an up ended cross section through the lower continental crust as it existed at the end of the hercynian orogeny in the southern alps structurally oriented geological mapping has been carried out in the central and northern parts of the ivrea verbano zone a region some 30 km along strike the geology of the southern half of the zone is dominated by a large basic ultrabasic complex the mafic formation which is in contact with a strongly banded metasedimentary metabasic sequence to the north particular attention was paid to a the structural relationship between the rocks of the mafic formation and their envelope of high grade metasedimentary and metabasic rocks b the geometrical configuration throughout the ivrea verbano zone of high temperature shear zones which accommodated post hercynian crustal extension and c the geometry of late low temperature faulting the effects of which have been removed in order to produce a restoration of the structure as it existed during the post hercynian extensional phase the intrusion of large volumes of basic magma ca 50 of the outcrop area to form the rocks of the mafic formation appears to be coeval with the onset of extension ca 280 ma the main basic body has a laccolithic form which was originally more than 10 km thick overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot immediately subsolidus or partially molten body incorporating its envelope of hot metasediments into a large originally recumbent fold a geometrical association with a high temperature low angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion the ivrea verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating which may aid in the interpretation of present day deep seismic profiles it also demonstrates the geometry of a network of conjugate high temperature low angle shear zones in a well layered lower crustal section

subsidence and sedimentation on jurassic passive continental margin southern alps italy the southern alps of italy preserve a tectonically intact array of jurassic facies that record the evolution of a part of the margin of the apulian plate from its ancestral beginnings in a complex of permian and triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs then as the new ligurian oceanic basin began to open farther west and as subsidence gradually slowed accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility carbonate dissolutio levels and the strength of bottom currents estimates of the history of seafloor depths based on a simple subsidence law of the form subsidence k age 12 provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the jurassic in this region the rapid 1 km deepening of the compensation depth for calcite during the late jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling fertile silica rich carbonate dissolving to downwelling less fertile silica poor carbonate preserving

petrogenesis and tectonic significance of amphibolites interlayered with metasedimentary gneisses in the ivrea zone southern alps northwest italy abstract amphibolites occurring as elongate lenses and boudins within the metasedimentary kinzigite formation of the ivrea zone are all of igneous parentage a detailed study of their trace element characteristics shows that they can be divided into two groups type 1 amphibolites occur as layers up to 100 m thick and have trace element distributions showing depletion in incompatible elements typical of normal n type mid ocean ridge basalts type 2 amphibolites occur as thinner layers and boudins and have higher levels of the more incompatible elements similar to transitional or enriched e type ocean basalts although metamorphism appears to have resulted in loss of sio 2 and variable enrichment in na k rb ba sr and locally ca the less mobile elements including the rare earth elements have preserved the primary basaltic geochemical characteristics the tectonic model which best explains the assemblage of slivers of morb like amphibolites within extremely highly deformed semi pelitic metasediments is that of an accretionary sedimentary wedge formed at a palaeozoic subduction zone

variation in amphibole and plagioclase composition with deformation investigation of the mineral chemistry of a shear zone in an amphibolite facies metagabbro has shown that the composition of the amphibole and plagioclase varies with deformation chemical analyses of the rocks indicate that the shear zone approximates an isochemical system the amphibole varies progressively from an initial magnesio hornblende to a ferroan pargasitic hornblende in the shear zone alkalies ti and fe2 fe2 mg increasing while the plagioclase becomes more anorthitic varying from labradorite to bytownite these changes in composition of amphibole and plagioclase from metabasic rocks are normally associated with increasing temperature in prograde regional metamorphic terrains such chemical variations observed in localized shear zones would suggest that shearing stress in regional metamorphism may play an important role in producing the documented discrepancies in amphibole chemistry between different metamorphic belts

pogallo line south alps northern italy an intermediate crystal level low angle normal fault the pogallo line is a major crustal shear zone in the crystalline south alps of northern italy which separates middle amphibolite granulite facies rocks of the ivrea zone from lower middle amphibolite facies rocks of the strona ceneri zone u pb rb sr and k ar whole rock and mineral ages for samples from this region indicate that the two zones pursued different time temperature paths after caledonian peak metamorphism cooling curves for the two terrains finally converge in the late triassic early jurassic a time of large scale crustal extension in the south alps that anticipated the opening of the western tethys we suggest that the pogallo line may represent a low angle normal fault that developed at intermediate crustal levels during this period of extension and attained its present subvertical orientation during alpine deformation

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

pseudotachylytes in the balmuccia peridotite ivrea zone as markers of the exhumation of the southern alpine continental crust terra nova 22 70 77 2010 abstract two types of pseudotachylytes are observed in the balmuccia peridotite of the ivrea zone southern alps italy a type pseudotachylytes correspond to previously studied occurrences and were formed under temperatures comprising between 550 and 900 c and pressures comprising between 0 6 and 1 2 gpa these conditions were met in the ivrea crust between 350 and 270 ma suggesting that a type pseudotachylytes were formed during variscan tectonics or permian transtensional tectonics b type pseudotachylytes post date a type pseudotachylytes textural characteristics of b type veins suggest a formation in the upper continental crust at depths of about 5 10 km or less petrological constraints indicate that the exhumation of the ivrea crust at such shallow depths was achieved later than c 70 ma thus providing a maximum age of 70 ma for b type veins pseudotachylytes appear as markers of the poly orogenic evolution of the alpine belt

texture development of recrystallised quartz polycrystals unravelled by orientation and misorientation characteristics abstract the development of microstructures and textures i e crystallographic preferred orientations during recrystallisation of naturally deformed quartz polycrystals has been studied via electron diffraction techniques in the scanning electron microscope in the investigated sample series of quartz rich rocks originating from different deformation regimes the microstructural and textural changes in quartz have been significantly influenced by dynamic recrystallisation based on microstructural observations paired with orientation and misorientation analyses down to the scale of grains and subgrains criteria could be established which characterise the dominant recrystallisation process and its influence on texture development it is shown that the texture development during dynamic recrystallisation is controlled by a differential activation of slip systems in grains of soft and hard orientations the analyses provide further evidence that specific grain orientations are preferred during crystal plastic deformation recrystallisation and grain growth the influence of twinning after the dauphine law was also investigated observations of a progressive reduction in the population of dauphine twin boundaries during recrystallisation and a penetrative deformation in both hosts and twins indicate a generation prior to deformation and recrystallisation a mechanical origin for twinning and possible influence on texture development was therefore discarded

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

stress gradients around porphyroclasts palaeopiezometric estimates and numerical modelling abstract based on strain gradients commonly found around porphyroclasts the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite facies shear zone of the ivrea verbano zone italy was investigated differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast matrix interface of about 1 7 2 0 with respect to the average in the matrix the palaeopiezometric stress data are compared to the results of a finite element model composed of two rheologically different phases in this model the local distribution and concentration of stresses around porphyroclasts mostly depend on clast matrix coupling and the effective viscosity contrast of the two phases the bulk strength and work hardening of the composite system increases with interface coupling both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities and promote strain localization and a change in the dominant deformation mechanism

late palaeozoic deformation of post variscan lower crust shear zone widening due to strain localization during retrograde shearing the santa lucia shear zone slsz corsica is a granulite facies permian shear zone that developed after the emplacement of a deep seated gabbroic intrusion new structural data shows that the slsz results from the juxtaposition of three spatially distinct mylonite belts which are the product of the interaction between magmatism metamorphism and shearing over a temperature range from 800 to 400 c during the earlier high grade deformation stage which was accompanied by decompression from 7 to 5 kb at 800 c the slsz has accommodated high finite strain on a shear zone 1 km wide strain became increasingly localized as temperature decreased but rather than reactivating pre existing shear zones as commonly expected younger mylonites expanded into previously unsheared rock extending the total width of the shear zone the zonation of different fabrics across the slsz suggests that pre existing compositional and grain size heterogeneities in the starting material played a key role in governing superposed generations of shear zones

the role of tectonic grain size reduction in the rheological stratification of the lithosphere with the aid of published constitutive flow laws for common crustal and upper mantle rock types it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere assuming a particular crustal stratification of rock types and a particular geothermal gradient such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces such as the moho discontinuity which are therefore likely to be associated with a tendency for major sub horizontal detachments to form constitutive flow laws used for such purposes are usually assumed to represent steady state flow at constant rock microstructure

on the structure of the ivrea verbano zone northern italy and its implications for present day lower continental crust geometry the ivrea verbano zone in northern italy represents a section through the lower continental crust which has been tilted and emplaced into its present position during the alpine orogeny recent and on going structurally oriented geological mapping in this region is providing new information about the geometry of the complex the central part of the zone is dominated by a large basic complex the mafic formation which is intrusive into the surrounding gneisses the foliation within the envelope of gneisses is deflected around the intrusive complex as if by ballooning but in the region south west of monte capio both units are folded together into a tight to isoclinal steeply plunging fold with an amplitude of c 10 km this fold locally inverts the stratigraphy of the layered basic group of the complex and is thought to be the result of gravitational collapse following intrusion and inflation of a large magma body into the lower crust several high temperature shear zones have now been traced within the country rock for distances up to 20 km the geometry of these and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body close parallels can be drawn between the observed structure in the ivrea verbano zone after removing the effects of late low temperature faulting and folding related to emplacement of the rocks into their present position and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the us basin and range province

crustal modelling of the ivrea verbano zone in northern italy re examined coseismic cataclasis versus extensional shear zones and sideways rotation abstract new field and microstructural observations in the ivrea verbano zone ivz northern italy reveal extensive post hercynian cataclasis indentation structures microtectonic escape structures microcataclastic injectites and occasional true pseudotachylytes the deformation occurred in situ and was governed by pure shear these observations are incompatible with prevailing crustal models of the ivz most of these models assume the presence of lower crustal originally flat lying extensional shear zones sideways block rotation and exposure of the petrological moho although the original structural observations from the central ivz suggested that the post hercynian deformation was cataclastic rather than shear dominated other already published information also contradicts the sideways rotation of a crustal block comprising the ivz and the adjacent serie dei laghi sdl after the early permian as assumed in the models the ivz and sdl have different detrital zircon populations and are separated by a steep shear zone and a transcurrent fault not rotated listric faults as assumed in the models rotation is also prevented by a flat lying zone of miarolitic cavities in the early permian baveno granite and by contemporaneous vertical basic dykes the widespread in situ cataclasis is interpreted as due to coseismic deformation by high frequency seismic waves generated by earthquakes along the insubric fault and exerting oscillatory compression dilation and torsion in a 10 km wide zone along the footwall of the insubric line the in situ post hercynian cataclasis absence of lower crustal extensional shear zones and lack of sideways rotation of the ivz and sdl call for a new and simpler crustal model of the region

the cossato mergozzo brissago line and the pogallo line southern alps northern italy and their relationships with the late hercynian magmatic and metamorphic events abstract the hercynian basement of the southern alps west of lago di lugano consists of two units the ivrea verbano which is well known for its lower crust and upper mantle rocks and as the seat of a marked gravimetric positive anomaly and the serie dei laghi an intermediate to upper crustal metasedimentary and metagranitic complex the contact between them is a subvertical tectonic discontinuity the cossato mergozzo brissago line cmb and pogallo line recently the subject of considerable interest as the possible analogue of a low angle normal fault later tilted to the present upright position such as those increasingly revealed by the deep reflection seismics in thinned continental crust late hercynian igneous activity mainly took place along this lineament the contact is however very complex the cossato lower valsesia segment is poorly exposed and has been reactivated by a younger fault cremosina line in the valsesia val d ossola toce river segment the cmb line is expressed as a strong structural and lithological contrast as well as by a narrow belt of mylonites in the val cannobina lago maggiore sector the mylonites are distributed over a considerable thickness mostly within the serie dei laghi they are connected with large scale ductile folding on nearly vertical axes schlingenbau the cmb line is dissected by several orthogonal and oblique subvertical faults grottaccio quarna val lessa val pogallo faults the presence of appinitic calcalkaline dykes and stocks of about 285 ma intruded along particular segments of the cmb isolated by those faults suggests that the attitude of the main tectonic boundary was upright at the time of intrusion the intrusion apparently occurred in a transtensional environment and induced a temperature increase in the country rocks causing a low pressure metamorphic overprint in the val cannobina lago maggiore segment and dehydration melting in a belt 2 3 km wide between valsesia and val d ossola the pogallo line cuts across the cmb line with a sinistral horizontal slip of about 11 km and is also characterized by amphibolite facies mylonites the absence of mylonitic appinites and consistent geologic evidence point to formation of the fault before 285 ma a significant mesozoic displacement along both faults is therefore excluded even though post permian reactivations are demonstrated by the presence of rb sr and k ar mica mixed ages along the cmb line a new evolutionary model can be proposed to account for most of the geological petrological and geophysical data early paleozoic continental accretion subduction and magmatic arc followed by a more external probably oblique hercynian reactivation inducing important transcurrent movements along cmb and pogallo before the early permian magmatic phase

drilling predation and taphonomy in modern mollusk death assemblages san salvador island bahamas abstract studies of modern mollusk death assemblages provide an important framework for evaluating the fossil record an analysis of 1951 shells from two beaches sandy point and haitian boat beach on the isolated carbonate platform of san salvador island bahamas has yielded significantly different frequencies of predation in bivalve dominated assemblages at sandy point beach bivalves and gastropods had a drilling frequency of 0 11 and 0 07 respectively at haitian boat beach bivalves and gastropods showed a higher drilling frequency of 0 37 and 0 22 respectively the prevalence of beveled drill holes suggests that the dominant predator was a naticid gastropod at both sites some ecological and environmental differences between the beaches could account for these disparate signals of predation at haitian boat beach there is a higher abundance of the naticid polinices sp and it is a lower energy setting than sandy point beach furthermore radiocarbon dating of 17 shells at both beaches yielded a maximum age of 6000 years before present suggesting that these assemblages record a complex and long taphonomic history these results imply that future large scale coupling of predation studies with age dating of assemblages may provide important insights into the temporal structure of predation as well as the role of taphonomic loss of drilled shells

high velocity breakup related sills in the voring basin off norway multichannel seismic reflection profiles in the hel graben v0ring basin reveal a sill complex at approximately 5 km depth it is associated with exceptionally high 7 4 km s 1 seismic wide angle velocities the existence of observable wide angle arrivals shows that the sills act as efficient waveguides seismic reflection data and amplitude modeling constrain the thickness of individual sills to approximately 100 m sonic logs from sills of similar thickness on the nearby utgard high show an average velocity of 7 0 km s 1 such high velocities require an olivine gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes a possible reason for such an emplacement environment is the hei graben s role as an intrusion center during breakup volcanism this would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex affecting depth conversion and gravity modeling furthermore the results question the concept of lower crustal bodies as large scale homogeneous accumulations of mafic melt