breakup of the newfoundland iberia rift abstract the newfoundland iberia rift is considered to be a type example of a non volcanic rift key features of the conjugate margins are transition zones tzs that lie between clearly continental crust and presumed normal penrose type oceanic crust that appears up to 150 180 km farther seaward basement ridges drilled in the iberia tz consist of exhumed serpentinized peridotite of continental affinity consistent with seismic refraction studies although the boundaries between continental crust and the tzs can be defined with relative confidence there are major questions about the position and nature of the change from rifting to normal sea floor spreading at the seaward edges of the tzs notably drilling of presumed oceanic crust in the young m series anomalies to investigate and potentially resolve these conflicts we analysed the tectonic history and deep pre cenomanian stratigraphy of the rift using seismic reflection profiles and drilling results rifting occurred in two main phases late triassic earliest jurassic and late jurassic early cretaceous the first phase formed continental rift basins without significant thinning of continental crust the second phase led to continental breakup with extension concentrated in three episodes that culminated near the end of berriasian hauterivian and aptian time the first two episodes appear to correlate with separation of continental crust in the southern and northern parts of the rift respectively suggesting that the rift opened from south to north in a two step process the third episode persisted through barremian and aptian time we suggest that during this period there was continued exhumation of subcontinental mantle lithosphere at the plate boundary and that elevated in plane tensile stress throughout the rift caused intraplate extension primarily within the exhumed mantle this rifting may have been interrupted for a time during the barremian when melt was introduced from the southern edge of the rift by plume magmatism that formed the southeast newfoundland ridge and j anomaly ridge and the conjugate madeira tore rise we propose that the rising asthenosphere breached the subcontinental mantle lithosphere in latest aptian earliest albian time initiating sea floor spreading this resulted in relaxation of in plane tensile stress i e a pulse of relative compression that caused internal plate deformation and enhanced mass wasting this aptian event produced a strong rift wide reflection that is unconformably onlapped by post rift sediments that were deposited as a stable sea floor spreading regime was established although previously considered to be a breakup unconformity associated with separation of continental crust the event instead marks the final separation of the subcontinental mantle lithosphere our analysis indicates that interpretation of tectonic events in a non volcanic rift must consider the rheology of the full thickness of the continental lithosphere in addition to spatial and temporal changes in extension that may occur from segment to segment along the rift

seismic diffraction from the north pyrenean fault a depth migrated line drawing of the ecors profile abstract we present here a complete depth migrated line drawing of the ecors pyrenees profile the main feature of the migrated section is an important diffraction zone generated by the north pyrenean fault we image several inhomogeneities in both the iberian and the european upper crust some of them can be linked to proterozoic materials that have been affected by hercynian tectonics deeper around 25 s two way time twt several dipping reflectors initially identified below the aquitaine basin migrate to the south they form the northern limit of the iberian lower crust dipping beneath a wedge consisting of european crust and upper mantle a previously described but now better imaged feature of the lithosphere beneath the pyrenees

structural setting of the bay of naples italy seismic reflection data implications for campanian volcanism abstract this paper focuses on the recent tectonic evolution of the bay of naples with the aim of exploring the connection between local tectonics and volcanism we reprocessed the seismic reflection dataset acquired in the area in the late 1973 the new processing was highly successful in obtaining a decisive strong reduction of random noise removal of coherent noise and reduction of spatial aliasing classical interpretative schemes and complex attributes of seismic traces were used to reconstruct fault kinematics and reflector patterns the results show that the faults affecting the bay of naples exhibit prevailing ne structural strikes with the exception of the pozzuoli caldera where nw patterns are also common many faults are subvertical and show seismic evidence of volcanic activity along them a main alignment of conjugate ne sw faults named here as magnaghi sebeto line intersects several submarine volcanic banks and separates the bay into two sectors characterized by important geological geophysical and petrochemical differences the structural configuration of the bay may reflect the occurrence of either oblique extension or a transfer zone of the nw se fault system along which in the campanian lucanian apennine chain great vertical displacements occur

volcanism and faulting of the campania margin eastern tyrrhenian sea italy a three dimensional visualization of a new volcanic field off campi flegrei by means of information technology 3d digital geologic models are increasingly the best methods of constraining interpretations of geology at depth visualization in three dimensions has allowed us to develop solutions that explain the stratigraphic and structural complexity of the campania margin of the eastern tyrrhenian sea especially the lateral relationships among volcanoes and underlying normal faults using seismic and sequence stratigraphy and structural geology approaches in a dedicated gis environment we interpreted a seismic grid and investigated a subsurface volcanic field in gaeta bay off campi flegrei located within the campania margin the gaeta bay basin fill is characterized by several volcanic units that were analyzed using 2d structure contour maps and isochron maps these volcanoes are comparable in size to the edifices of campi flegrei vesuvius and the submerged volcanoes of the bay of naples their age spans from the lower pleistocene for the isolated v0 volcano sited in central gaeta bay to 0 4 0 1 ma for the volcanic field of southern gaeta bay the study of the interplay between volcanic units and faults allowed us to distinguish 1 the oldest fault buried by the lower pleistocene v0 volcano in central gaeta bay 2 a normal fault swarm ne sw that in the middle pleistocene gave rise to a half graben filled by the v1 v5 volcanic field in southern gaeta bay and 3 post 0 4 ma normal faults that downthrow the v1 and v3 volcanoes we correlated the 1 500 m thick late quaternary volcanoes of southern gaeta bay to the buried lava volcanoes interlayered with volcanoclastic deposits drilled at campi flegrei geothermal wells

shear in the lower crust during extension not so pure and simple abstract as lower crustal reflectivity appears to be related to extension it is sensible to consider the role of the lower crust in the extension of the lithosphere and in particular the way the lower crust deforms the occurrence of transparent zones wrapped around by reflective bands can be interpreted as low strain regions enclosed within anastomosing shear zones suggesting that deformation in the lower crust is by the relative movements of low strain lozenges along acoustically reflective high strain zones although this deformation can easily accommodate bulk pure shear it can also develop during bulk simple shear a component of subhorizontal simple shear within the lower crust is necessary if deformation in the mantle is localised into shear zones with the geometry and distribution imaged on birps profiles the lower crust may be the critical link in the lithosphere transferring deformation between the relatively strong upper crust and upper mantle and being strongly sheared in the process the form of this shearing is a direct consequence of the way the entire lithosphere extends

why fast polarization directions of sks seismic waves are parallel to mountain belts abstract the splitting of sks waves shows that the fast velocity is commonly parallel to the trend of mountain belts this has been related to the anisotropy induced by frozen mantle flow at lithospheric depth when the flow direction is parallel to mountain belts this flow reflects a transcurrent motion parallel to the boundaries of the plates whose collision is responsible for mountain building considering the width of major shear zones transcurrent flow should not reorient the mantle fabric on a transverse width larger than 20 40 km unless the flow is partitioned between several major transcurrent shear zones therefore such a scale seems to be too small to explain the sks results another contribution to the observed mantle anisotropy is suggested to be the fabric induced by mantle flow during a prior event of rifting at the colliding plate boundaries which are derived from passive margins

a balanced section across the pyrenean orogenic belt evolutionary models that have been proposed for the pyrenean orogenic belt involve gravitational gliding of major thrust sheets to both the north and south of an uplifted central axial zone it has been suggested by previous workers that uplift is a result of alpine compression or long lived strike slip movement on the north pyrenean fault we critically assess these models and propose a new model based on the generation of a thin skinned linked thrust system due to alpine collision of iberia and europe pyrenean alpine tectonics is dominated by southward thrusting on a major sole fault dipping north at approximately 6 within hercynian basement northerly directed thrusts of the north pyrenean zone and northern folded foreland are backthrusts from this sole fault a total orogenic shortening of 106 km in the central pyrenees was achieved between the palaeocene and oligocene at a time averaged rate of 0 62 cm yr 1 a thin skinned model requires that the north pyrenean fault is truncated by both northward and southward moving thrusts and occurs in the footwall of the sole fault some 60km north of its present surface trace

fault reactivation during mesozoic extension in eastern offshore canada abstract new seismic data are presented that document reactivation of a thrust fault as a normal fault four deep seismic reflection profiles now straddle the hibernia oil field region of the grand banks off eastern canada the profiles intersect each other near the western edge of the 22 km deep jeanne d arc basin thereby giving a three dimensional picture of basement structures the basin bounding fault which marks the eastern edge of the bonavista platform appears to follow a pre existing fault system more precisely the fault that bounds the jeanne d arc basin just north of hibernia can be traced on the seismic data along strike to the south beneath the bonavista platform where it has not been reactivated the seismic data show that this feature was probably a palaeozoic thrust fault a laterally extensive palaeozoic basin occupies most of the bonavista platform the deeper part of the crust is quite reflective and is characterized by numerous dipping reflective packages this detailed study of the hibernia region suggests that the narrow elongated basins of the grand banks may have been formed within the reactivated hanging walls of older thrust sheets similar observations have been made for the mesozoic extensional basins of the u k continental shelf the pre mesozoic compressional structures documemed on the central grand banks lie along the appalachian orogen and may be of variscan age

the emplacement of intermediate volume ignimbrites a case study from roccamonfina volcano southern italy a model is presented for the emplacement of intermediate volume ignimbrites based on a study of two 6 km3 volume ignimbrites on roccamonfina volcano italy the model considers that the flows were slow moving and quickly deflated from turbulent to non turbulent conditions yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows in proximal locations on the caldera rim heterogeneous exposures including discontinuous lithic breccias stratified and cross stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows in medial locations thick massive ignimbrite occurs associated with three types of co ignimbrite lithic breccia which we interpret as being emplaced by non turbulent flows multiple grading of different breccia lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower moving front part this overriding of different parts of non turbulent pyroclastic flows could be caused by at least two different mechanisms 1 changes in flow regime such as hydraulic jumps that may occur at breaks in slope and 2 periods of increased discharge rate possibly associated with caldera collapse producing fresh pulses of lithic rich material that sheared onto the slower moving part of the flow in front

a tomographic study of the lithosphere beneath the pyrenees from local and teleseismic data a tomographic image of the pyrenean lithosphere from the surface down to 200 km is deduced from the propagation of the seismic waves recorded at the regional pyrenean seismograph network crustal and subcrustal models are derived independently from the inversion of two different data sets p and s velocity models for the crust are obtained from the propagation of local earthquakes the upper crust appears very heterogeneous and exhibits sharp velocity contrasts in the uppermost 7 km an east west trending zone with velocities up to 6 higher than the surrounding material is observed all along the pyrenees this zone coincides in most places with the exposure of the paleozoic rocks of the axial zone low velocities are observed on both sides of this high velocity zone at the surface they coincide with well known sedimentary basins but dip down to about 11 km depth toward the axis of the range in the central and eastern pyrenees two large high velocity bodies stand out in the north pyrenean zone and extend down to the base of the upper crust they coincide with large positive gravity anomalies and are interpreted as lower crust or upper mantle blocks uplifted into the upper crust the lower crust is poorly resolved but appears to be less heterogeneous than the upper crust moreover the lower crustal structure is almost uncorrelated with the upper crustal structure the major feature of the lower crust is a low velocity anomaly which stretches along the north pyrenean fault in the western pyrenees and beneath the axial zone in the central pyrenees its location nearly coincides with the distribution of the seismicity in the upper mantle p models are derived from the travel times of the remote events recorded at the pyrenean stations the most important feature is the presence in the eastern and central pyrenees of a vertical low velocity heterogeneity south of the north pyrenean fault down to 80 100 km this structure exhibits a 2 velocity contrast with respect to the surrounding structures and is interpreted as lower crustal material subducted during the pyrenean convergence this heterogeneity is not observed in the western pyrenees suggesting that the crustal shortening has been completely accommodated by several thrusts in the thinned aquitaine crust

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

geology of proximal small volume trachyte trachyandesite pyroclastic flows and associated surge deposits roccamonfina volcano italy abstract this paper describes the 232 ka b p mttt trachyte trachyandesite pyroclastic succession of roccamonfina volcano this small volume proximal sequence crops out along mulino di sotto paratone and pisciariello ravines in the southwest sector of the central caldera and covers a minimum extent of 3 5 km2 area it is made up of seven pyroclastic flows and pyroclastic surge units consisting of trachytic ash matrix containing juvenile trachyandesitic scoria and dense lava fragments pumice clasts of uncertain trachyandesite and a foreign trachyandesitic lithic facies two stratigraphic markers allow correlation of the units no paleosoils and plinian fallout have been observed at the base and within the succession some lateral grading of scoria and lithic clasts suggests that mttt derived from three distinct source vents the sequence consists of a basal ash flow passing laterally to laminated surge deposits unit a this is overlain by a reversely graded scoria and pumice lapilli flow unit b which is in turn overlain by a thinly cross stratified scoria lapilli surge unit c unit c is capped by a prominent ash and scoria flow unit d a ground layer marker mk1 divides unit d from a massive ignimbrite which grades upcurrent to sand wave surge deposits unit e another ground layer marker mk2 separates unit e from unit f this unit consists of a basal ignimbrite passing laterally to bedded surge deposits with convolute structures subunit fl and grading upcurrent to a subhorizontally plane laminated ash cloud subunit f2 containing near the top a layer of millimetric lithic clasts embedded in fine ash the succession is closed by the pyroclastic flow unit g surge unit c can be interpreted in terms of vertical gradients in turbulence particle concentration and velocity during flowage whereas the bedded surge parts present in the massive deposits of units a and e f1 can be related to abrupt changes of velocity down the steep slopes of ravines reverse grading in unit b is probably due to grain dispersive pressures the convolute structures within fl are related to zones of diagenetic cementation associated with groundwater finally the laminated fine grained nature of subunit f2 is interpreted as due to ash clouds elutriated from the basal part of unit f stratigraphic markers mk1 mk2 are ground layer breccias formed by settling of lithic and scoria clasts from overlying units e and f respectively vesiculation and morphologies of glass shards of the mttt succession suggest that eruptions were essentially driven by magmatic explosions which had an appreciable hydromagmatic component

electrical resistivity model of the crust and upper mantle from a magnetotelluric survey through the central pyrenees summary an mt profile which roughly coincides with the south side of the ecors pyrenees deep reflection seismic profile was made from the north pyrenean zone through the central pyrenees and ebro basin the periods recorded ranged from 0 0128s to 2000 s analysis of mt data shows a 2 d conductive structure the strike direction of the structure is e w parallel to the pyrenean chain interpretation of the data includes geomagnetic transfer functions and is constrained by some relevant geometric crustal features observed in seismic data accordingly a 2 d resistivity cross section has been obtained the most striking results are 1 detection of subvertical conductors in the upper crust coinciding with major fault zones 2 detection of a deep high conductivity zohe at the contact between the iberian and the european plates 3 the fact that the european plate is thicker and more resistive than the iberian one interpretation of the deep high conductivities suggests the existence of partial melting in the pyrenees related to the subduction of the iberian plate beneath europe

the role of transfer structures on volcanic activity at campi flegrei southern italy abstract the tyrrhenian margin of central italy underwent extension during pliocene and quaternary extension occurred mainly through nw se normal faults bordering a sequence of plio quaternary basins these basins are offset by coeval ne sw faults which show strike slip and normal motions and have been interpreted as transfer faults plio quaternary volcanic activity along the margin occurred along a nw se belt systematically in correspondence with ne sw transverse systems the campi flegrei volcanic district cfvd on the southern tyrrhenian margin consists of an active ne sw volcanic ridge developed along ne sw fractures we performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at campi flegrei field analysis at campi flegrei recognized ne sw and to a lesser extent coeval nw se active fractures analogue experiments have simulated the development of transfer fault systems in brittle extensional domains the experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 60 the mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses it shows how pre existing subvertical fractures require the smallest magmatic pressures to be penetrated for a given magmatic pressure subvertical fractures might be penetrated more deeply tapping more easily primitive magmas these results suggest that the cfvd is located along a ne sw transfer zone connecting nw se regional normal faults volcanic activity along such ne sw trend would be induced by the subvertical dip of the transfer faults the subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along ne sw systems at campi flegrei these considerations find a wider application on the remaining volcanic districts of the margin located in the same overall structural setting

the pyrenean architecture as revealed by teleseismic p to s converted waves recorded along two dense transects between 2011 and 2013 two dense transects were deployed across the central and western pyrenees to get better constraints on the deep lithospheric architecture and discriminate the competing models of the structure and formation of the pyrenees each transect recorded the regional and global seismicity during a period of approximately 1 yr here we exploit the records of teleseismic compressional waves and of their conversions to shear waves on internal discontinuities in order to map lithospheric interfaces beneath the two transects the migrated sections obtained by performing common conversion point stacks are in remarkable agreement with the results of the ecors pyrenees and ecors arzacq deep seismic surveys however the migrations of converted waves reveal new details of the deep lithospheric architecture that could not be seen with the active source experiments the new images provide clear and definite evidence for the subduction of a thinned iberian crust down to at least 70 km depth a result that has important implications for the formation of the pyrenees the subduction of the iberian lithosphere leads to reconsider the amount of convergence between iberia and europe during the cenozoic a recent regional p wave tomography relying on the data of the pyrope and iberarray temporary experiments revealed the segmentation of lithospheric structures by inherited hercynian ne sw transfer faults that were reactivated during the albian rifting our migration images are consistent with this model and give further support to the idea that the pyrenees were produced by the tectonic inversion of a segmented hyperextended rift that was buried by subduction beneath the european plate

crustal structure of the goban spur continental margin northeast atlantic from deep seismic reflection profiling deep reflection data across the uk continental margin are used to test theories concerning the origin of a pervasive reflective fabric in the lower continental crust seen on many deep reflection profiles worldwide the origin of the reflections is at present little understood but clearly represents an important crustal process the western approaches margin wam line was designed to determine the behaviour of the lower crustal reflections in crust which has undergone significant extension the wam profile across the continental shelf is characterized by bright continuous lower crustal reflections some of which reach 30 km in length and have minimum reflection coefficients of 0 1 deep water multiples in the area of the continental slope partially obscure reflections from the lower crust but detailed analyses show that reflectors are present in the lower crust on the continental slope adjacent to the continent ocean boundary the lower crust on the wam profile is well imaged and free of noise and multiples this very thin crust exhibits the same reflectivity pattern as is observed on the continental shelf i e the upper half of the crust is blank and the lower half contains sub horizontal reflections the lower crustal reflections adjacent to the continent ocean boundary are less continuous than the reflections seen on the shelf and are interpreted as being the remnants after extension of the lower crustal reflectors observed on the continental shelf the lower crustal reflectors are thus interpreted to pre date the extensional episode that led to the opening of the atlantic the preservation of thinned sections of both transparent upper crust and reflective lower crust adjacent to the continent ocean boundary suggests that extension by pure shear was the mechanism by which the crust was thinned in the goban spur area

40ar 39ar and k ar dating of k rich rocks from the roccamonfina volcano roman comagmatic region italy abstract roccamonfina is the northernmost volcano of the campanian area of the k rich roman comagmatic region of italy it erupted a huge amount of pyroclastics and lavas belonging to both the leucite basanite and leucitite series lbls and the shoshonite series ss spread over an area of about 300 km2 the above series correspond to the high k series hks and low k series lks of appleton 1972 respectively 40ar 39ar and k ar dating of samples from both series gave ages ranging from 0 656 to 0 096 ma for the ss and from 1 03 to 0 053 ma for the lbls these results indicate that the products of the two series were outpoured together at least between 0 7 and 0 1 ma ago i e during both the so called pre caldera phase and the post caldera phase of activity the latest products of the volcanism at roccamonfina were erupted just before the deposition of the grey campanian ignimbrite which erupted from vents located about 50 km to the south in the phlegrean fields near naples and has an age of about 33 000 years taking into account all the available radiometric data we conclude that roccamonfina was active between 1 5 and 0 05 ma ago in excellent agreement with the stratigraphie evidence in this same time span is concentrated the activity of all the centers of the roman region north of naples

origin of the calderas and evolution of roccamonfina volcano roman region italy abstract the roccamonfina volcano consists of a volcanic fluvial succession dated 549 ka bp paleoauruncus overlain by a 546 ka bp stratacone main cone formed by leucite bearing lava and tephra the main cone was dissected by four nested calderas a ring fracture concentric with and outside of the central caldera is also present detailed mapping of the volcano inspection of landsat satellite images and radiometric dating establish that the gli stagli and northern calderas subsided between respectively 546 474 and 403 374 ka bp and that the summit part collapsed between 546 and 446 ka bp to form the central caldera a chronologically intermediate state dated in the range 376 323 ka bp was identified between the 549 374 ka bp leucite bearing high k series hks stage i and the 317 96 ka bp leucite free low k series lks stage ii this stage is made up of concurrent hks lks rocks stage ics the central gli stagli and northern calderas are older than blt and wtt as supported by the strong discrepancies between calculated volumes of these pyroclastic rocks and the volume of the caldera also caldera collapses due to sector sliding are improbable as tested by the absence of rock slide avalanche deposits the collapses occurred incrementally through repetitive draining of the magma chamber s resulting from a succession of moderate sized volcanic events gli stagli caldera was infilled by a lake early after collapse so that sediments alternated with leucite tephrite lava flows and tephra within the lake the sequence was overlain by a thick succession of young pyroclastic rocks downfaulting downsagging cut the flat lying gli stagli filling sequence resulting in the old valle caldera successive pyroclastic and debris flows derived by updoming of the central caldera deposits infilled the last structure and downfaulted part of the valle fill producing the young valle caldera

analogue models of the effect of long term basement fault movement on volcanic edifices long term fault movement under volcanoes can control the edifice structure and can generate collapse events to study faulting effects we explore a wide range of fault geometries and motions from normal through vertical to reverse and dip slip to strike slip using simple analogue models we explore the effect of cumulative sub volcanic fault motions and find that there is a strong influence on the structural evolution and potential instability of volcanoes the variety of fault types and geometries are tested with realistically scaled displacements demonstrating a general tendency to produce regions of instability parallel to fault strike whatever the fault motion where there is oblique slip faulting the instability is always on the downthrown side and usually in the volcano flank sector facing the strike slip sense of motion different positions of the fault beneath the volcano change the location type and magnitude of the instability produced for example the further the fault is from the central axis the larger the destabilised sector also with greater fault offset from the central axis larger unstable volumes are generated such failures are normal to fault strike using simple geometric dimensionless numbers such as the fault dip degree of oblique motion angle of obliquity and the fault position we graphically display the geometry of structures produced the models are applied to volcanoes with known underlying faults and we demonstrate the importance of these faults in determining volcanic structures and slope instability using the knowledge of fault patterns gained from these experiments geological mapping on volcanoes can locate fault influence and unstable zones and hence monitoring of unstable flanks could be carried out to determine the actual response to faulting in specific cases

seismic images and evolution of the iberian crust in the pyrenees abstract crustal images of the pyrenean range based on coincident wide angle and vertical deep seismic profiles are presented in this study wide angle experiments in the thickened iberian crust along strike lines have provided no evidence for reflections from interfaces between layers of different velocities in the middle or lower crust in contrast reversed sounding on a line across the pyrenean range shows wide angle reflectivity its unusual reflectivity pattern points to widespread impedance contrasts of localized continuity embedded in the crust this observation is consistent with a new image of vertical incidence reflectivity obtained by coherency weighted migration of the coincident ecors traverse this profile shows numerous discontinuous and dipping reflections from the middle to the base of the crust the reflections dip towards the axis of the range the upper limit and the attitude of the reflective zones are so strongly variable that the crust can hardly be envisaged as the simple continuous three layered model previously used for geodynamical reconstructions the reflective zones cannot be brought back to the horizontal image of a layered lower crust of proposed extensional origin by plate unbending to the pre alpine stage the dipping reflections may result from compressional deformation as the south pyrenean deep crustal domain has undergone both hercynian and alpine orogenic deformation if part of the present reflectivity pattern was due to alpine remobilization the corresponding lower crustal deformation would have spread out thereby accomodating convergence by thickening even outside the axial zone where the upper crustal deformation appears to be concentrated

structure across the northeastern margin of flemish cap offshore newfoundland from erable multichannel seismic reflection profiles evidence for a transtensional rifting environment we present the results from processing and interpreting nine multichannel seismic reflection lines collected during the 1992 erable experiment over the northeastern margin of flemish cap offshore newfoundland these lines combined into five cross sections provide increased seismic coverage over this lightly probed section of the margin and reveal tectonically significant along strike variations in the degree and compartmentalization of crustal thinning similar to the southeastern margins of flemish cap and the grand banks a transitional zone of exhumed serpentinized mantle is interpreted between thinned continental and oceanic crust the 25 km wide transitional zone bears similarities to the 120 km wide transitional zone interpreted as exhumed serpentinized mantle on the conjugate irish atlantic margin but the significant width difference is suggestive of an asymmetric conjugate pair a 40 50 km wide zone of inferred strike slip shearing is interpreted and observed to extend along most of the northeastern margin of flemish cap individual shear zones szs may represent extensions of szs and normal faults within the orphan basin providing further evidence for the rotation and displacement of flemish cap out of orphan basin the asymmetry between the flemish cap and irish conjugate pairs is likely due in large part to the rotation and displacement of flemish cap which resulted in the flemish cap margin displaying features of both a strike slip margin and an extensional margin

new geophysical constraints on the deep structure of the pyrenees a 2d modeling of geoid quasigeoid data across the pyrenees is consistent with electromagnetic images of the pyrenean lithospheric structure showing a subducted iberian lower crust using new magnetotelluric data acquired through the orogen a 3d electrical conductivity model of the pyrenean lithospheric structure is obtained the most outstanding feature is the presence of a high conductivity zone at lower crustal depths along the orogen in the contact zone between the iberian and european plates the integration of various geophysical parameters suggests that the simplest and most reasonable mechanism to explain the observed data is partial melting of the subducted iberian lower crust

gravity constraints on the deep structure of the pyrenean belt along the ecors profile abstract in order to furnish further constraints to modelling the ecors pyrenees deep seismic profile gravity data were collected along a stripe 20 km wide and 220 km long containing the seismic line a total of 4020 stations have been used to derive a bouguer anomaly map of the area a large and roughly symmetrical negative anomaly 100 mgal is centered on the axial zone a local positive anomaly over the north pyrenean zone disturbs the negative regional trend somewhat detailed densities were assigned to the sedimentary cover on the basis of borehole samples refraction data and a velocity density relationship were used to constrain the densities adopted for the deeper layers the results of the two dimensional gravity modelling confirm the asymmetrical shape of the crust mantle boundary beneath the pyrenean belt the downward deflection of the iberian moho from the ebro basin leads to a maximum depth of 65 km under the paleozoic axial zone whereas a depth of 35 km is found for the european moho under the north pyrenean zone two extreme models are proposed for the northern limit of the iberian crust a vertical contact in the downward continuity of the north pyrenean fault or a 70 northward sloping interface the positive anomalies beneath the north pyrenean zone may be explained by dense intracrustal mantle slices or lower crustal sheets the outcrop of granulitic basic gneiss within the labourd massif suggests that the anomaly results from lower crustal sheets

a 3d gravimetric crustal model for the northeastern region of the iberian peninsula we combine different methodologies normally used separately in exploration geophysics to obtain the topographies of the moho discontinuity and the boundary between the upper and lower crusts by separating their effects in the bouguer anomaly map the sfm method was used to define the average depth of the layers causing the anomalies the resulting three dimensional model which reproduces the observed gravity anomaly was estimated by inverting the corresponding domains of the power spectrum a good correlation exists between the calculated depths and the upper and lower boundaries of the layered lower crust observed along the reflection seismic lines in the area

normal faulting along the western side of the matese mountains implications for active tectonics in the central apennines italy abstract we provide new field data from geologic mapping and bedrock structural geology along the western side of the matese mts in central italy a region of high seismicity strain rates among the highest of the entire apennines 4 5 mm yr gps determined extension and poorly constrained active faults the existing knowledge on the aquae iuliae normal fault aif was implemented with geometric and kinematic data that better constrain its total length 16 5 km the minimum long term throw rate 0 3 0 4 mm yr post late glacial maximum lgm and the segmentation for the first time we provide evidence of post 350 ka and possibly late quaternary activity of the ailano piedimonte matese normal fault apmf the apmf is 18 km long it is composed of a main 11 km long segment striking nw se and progressively bending to the e w in its southern part and a 7 km long segment striking e w to ene wsw with very poor evidence of recent activity the available data suggest a possible post lgm throw rate of the main segment of 0 15 mm yr there is no evidence of active linkage in the step over zone between the aif and apmf prata sannita step over an original tectonic model is proposed by comparing structural and geodetic data the aif and apmf belong to two major nearly parallel fault systems one system runs at the core of the matese mts and is formed by the aif and the faults of the gallo letino matese lake system the other system runs along the western side of the matese mts and is formed by the apmf linked to the se with the piedimonte matese gioia sannitica fault the finite extension of the apmf might be transferred to the nw towards the san pietro infine fault the nearly 2 3 mm yr gps determined extension rate is probably partitioned between the two systems with a ratio that is difficult to establish due to poor gps coverage the proposed model though incomplete several faults transfer zones need further investigations aids in the seismotectonic interpretation of poorly known earthquakes e g 346 355 ad earthquake on the ailano piedimonte matese gioia sannitica fault system and stimulates and further orients seismotectonic investigations aimed at constraining the segmentation pattern and seismogenic potential of the area

seismic image of the cantabrian mountains in the western extension of the pyrenees from integrated escin reflection and refraction data abstract integrated analysis of normal incidence and large aperture seismic reflection data collected in 1992 and 1993 within the spanish escin and complementary projects provide a first complete n s crustal transect across the northern iberian peninsula and continental margin images of the crustal structure of the cantabrian mountains and their transition to the duero basin and to the cantabrian margin are obtained from a a 65 km long vertical reflection profile escin 2 on land b a 200 km long reversed refraction profile and c wide angle recordings of the marine escin 4 profile consistent results between reflectivity pattern and velocity depth distribution reveal important lateral variations in the deep structure the reflective crust imaged in the escin 2 profile changes its attitude from sub horizontal beneath the duero basin to north dipping beyond the mountain front basement thrusts are observed in the upper crust merging into a detachment at 6 s twt and may have triggered the alpine uplift of the range the moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile about 35 km inland modelling of the refraction data laterally extends the seismic image and provides evidence for variscan crustal features beneath the duero basin northwards the velocity in the lower crust decreases and the moho constrained by the wide angle data from profile escin 2 deepens to about 60 km ending abruptly at the shoreline the velocity depth model is constrained along the asturian platform up to the continental slope where the crust mantle boundary is located at 24 km depth this margin moho shows a progressive deepening southwards and extends to the coast where it is found at 30 km depth the present seismic data support an important alpine reworking and thickening of the crust under the cantabrian mountains the onshore offshore transition is marked by an imbrication of two crusts of very different thicknesses this signature offers a strong parallelism with the one previously observed further east across the pyrenees in the ecors seismic profile

lithoprobe east results from reflection profiling of the continental margin grand banks region summary the 1985 lithoprobe east profiles resolve deep crustal structure of the grand banks off eastern canada basins are 7 to 20 km deep and bounded by major faults traceable to moho depth the lower crust is reflective along much of the survey and the top of this reflective layer has a variable depth prominent and often surprisingly flat moho reflections are observed puzzling rotated fault blocks are imaged at the continent ocean transition

a deep seismic reflection profile across the northern north sea the viking graben is a major mesozoic rift basin in the northern north sea1 seismic refraction data2 gravity surveys3 and theoretical modelling4 have led to the general acceptance of an extensional tectonic origin for the viking graben involving an early mesozoic phase of extension and fault controlled subsidence followed by a more regional thermal contraction subsidence the early success of the birps deep seismic reflection profiling5 and the results from cocorp6 led to the planning and acquisition during 1983 of a deep seismic line recorded to 15 s twt two way time across the north viking graben the aims of the present study were to provide constraints on the geometry of the viking graben and its extensional faults to provide data on the geometry of crustal thinning beneath the graben and to provide an insight into the relation between the shallow extensional features of the sedimentary basin and the deep crustal thinning here we present a specific interpretation of this seismic profile further details will be given elsewhere7

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

upper crustal investigation of the gulf of saint lawrence region eastern canada using ambient noise tomography we studied the 3 d shear wave velocity vs structure in the gulf of st lawrence gsl and adjacent onshore areas to 20 km depth by inverting rayleigh wave dispersion extracted from the vertical components of continuous ambient seismic noise waveforms the region is divided into three broad zones based on their vs characteristics in the northwest the grenville province i e the exposed edge of predominantly middle proterozoic laurentian crust is dominated by high vs except for well known anorthosite sites which are characterized by relatively lower vs in contrast the central segment of the gsl region corresponds to a belt with generally low vs at upper crustal levels in the southeastern part of the gsl prominent low vs in the uppermost crust are found to coincide with locations of subsidiary basins of the canadian maritime basin while higher vs characterize the accreted appalachian terranes where they are exposed on land the grenville province is wedged out at depth by the red indian line which is the suture between composite laurentia and peri godwanan ganderia in the canadian appalachians the geometry and vs characteristics of the south easternmost peri gondwanan terranes of avalonia and meguma suggest that they may be fully or partially structurally overlying a basement with distinct seismic characteristics which could be a vestige of the west african craton that was under thrust beneath composite laurentia during the terminal alleghenian continent continent collision in the middle of the gsl the 3 d geometry of the canadian maritimes sedimentary basins overlying the appalachian terranes shows that the depth to the top of basement is in excess of 8 km

groundwater mixing in a heterogeneous multilayer aquifer driven by geogenic co2 fluxes evidence from chemical and isotopic composition of ferrarelle waters riardo plain southern italy abstract the successful management of carbon in the earth s crust is critical for mitigating the increase of anthropogenic co2 in the atmosphere carbon capture and storage ccs requires an understanding of the behavior of carbon in the crust and the development of robust monitoring techniques to constrain the movement mechanisms and pathways for any potential co2 leakage here we examine an aquifer from the riardo plain campania region southern italy which serves as a suitable natural analogue for co2 migration to the critical zone i e shallow crust and aquifers and as a case study to evaluate the geochemical processes that occur when co2 saturated fluids mix with freshwater in shallow aquifers we investigate the behavior of various geochemical constituents major and trace elements 18o h2o 13c dic and rn content water from this area has a high degree of mineralization ec 2500 3000 s cm high hco3 2 5 g l is saturated with respect to caco3 and is enriched in alkali ions e g na k the high degree of mineralization occurs in groundwater that discharges from the basal aquifer of the roccamonfina volcanic edifice 6 km nw with vast co2 inputs that promote host rock leaching superficial volcanic aquifers are recharged by fresh meteoric precipitation when groundwater flows from carbonates at the edge of the plain to aquifers hosted in the southeastern slope of the roccamonfina volcano the presence of normal faults in this area permits natural upwelling of co2 rich groundwater which locally mixes with shallow freshwater present within the upper volcanic succession significant r 0 8 linear correlations between conservative elements suggest that groundwater geochemistry is dominated by a mixture of two main endmembers i deep mineralized waters and ii shallow diluted waters the intrusion of freshwater to volcanic aquifers induces oxidation leading to adsorption of select elements e g as and ba onto fe oxyhydroxide precipitates within these aquifers geochemical modeling suggests that co2 saturation approaches 3 g l which agrees with direct measurements of co2 flux we conclude that our conceptual geochemical model helps to constrain mixing of co2 with freshwater and to diagnose the secondary geochemical processes that influence aqueous geochemistry within co2 influenced groundwater

a deep seismic crustal transect from the ne iberian peninsula to the western mediterranean the crustal structure of the transition from the ne iberian peninsula to the western mediterranean is characterized by large scale variations related to its successive compressional and extensional cenozoic tectonics the spanish esci spanish acronym for seismic studies of the iberian crust program recorded 450 km of deep vertical reflection profiles on land and at sea to complete a first crustal transect from the pyrenees to the south balearic basin highly energetic north dipping reflections imaged by combining the etude continentale et oceanique par reflexion et refraction sismique ecors and esci land profiles mark a crustal thickening toward the pyrenees the reflective band becomes horizontal beneath the catalan coastal ranges and the moho is found at 11s no evidence of crustal thinning affecting the iberian mainland is observed the transition to the mediterranean basin is constrained by complementary onshore offshore recordings forward modeling combined with multichannel processing of the wide angle data set shows that a strong thinning of the continental crust by a factor of 2 over a distance of 50 km affects both the catalan and balearic flanks of the valencia trough this thinning affects mainly the deep crust characterized by low velocities of 6 4 6 5 km s suggesting a removal of part of the lower crust during the rifting processes velocities of 7 8 7 9 km s are interpreted in the uppermost mantle with no evidence of underplating the moho appears as a marked discontinuity throughout the transect the nw part of the balearic promontory is characterized by a thick and reflective crust from 12 to 30 km depth an intensely fracturated evaporitic sequence is seen for 100 km along the south balearic basin the seismic images of the continent ocean transition are disturbed by these prominent sediments however the deep reflectivity suggests a crustal thinning moho at 6 7 s to be related to the middle miocene present extensional tectonics

north grand banks basin offshore newfoundland a sedimentary epeiric basin west of the murre mercury detachment fault abstract the eastern newfoundland offshore area is geologically the best known region of the northwestern atlantic margin extensive geophysical coverage and about 140 exploratory and delineation wells have allowed structural and stratigraphic mapping of a mesozoic rift basin network that includes the south whale whale horseshoe south jeanne d arc carson bonnition jeanne d arc orphan and flemish pass basins a less explored basin not yet targeted by the oil industry and accordingly less explored has recently been discussed in the marine geoscience literature located on the bonavista platform west of the jeanne d arc basin bounding fault and defined here as the north grand banks basin this area has been described from refraction work correlated with deep seismic profiling lithoprobe east data as a possible mesozoic rift basin with hydrocarbon potential or as a palaeozoic basin detected first by gravity measurements but unconfirmed as a rift basin by reflection data this basin is only 2 km deep and contains palaeozoic and probably undeformed mesozoic sedimentary infill its hydrocarbon potential is low the mesozoic sedimentary rocks if present are probably platform carbonates and clastics formed during thermal subsidence episodes following the first two rifting stages that affected the grand banks of newfoundland the placement of the pre rift unconformity within the sedimentary successions of the north grand banks basin and the precise nature of the rocks situated above the avalon basement remain unresolved a geodynamic model involving simple shear extension in the upper crust is used to explain the evolution of newfoundland offshore sedimentary basins including the north grand banks according to this model the basin is located on the upthrown side of the murre mercury fault system that represents the upper crustal expression of a crustal penetrative detachment

temporal evolution of the roccamonfina volcanic complex pleistocene central italy abstract the roccamonfina volcanic complex rvc in southern italy is an early to middle pleistocene stratovolcano sharing temporal and morphological characteristics with the somma vesuvius and the alban hills both being associated with high volcanic hazard for the cities of naples and rome respectively the rvc is important for the understanding of volcanic evolution in the roman and campanian volcanic provinces we report a comprehensive study of its evolution based on morphological geochemical and k ar geochronological data the rvc was active from c a 550 ka to 150 ka its evolution is divided into five stages defining a volcanic pulse recurrence time of c a 90 100 kyr the two initial stages consisted in the construction of two successive stratovolcanoes of the tephrite phonolite namely high k series the first stage was terminated by a major plinian eruption emplacing the trachytic rio rava pumices at 439 9 ka at the end of the second stage the last high k series stratovolcano was destroyed by a large sector collapse and the emplacement of the brown leucitic tuff blt at 353 5 ka the central caldera of the rvc is the result of the overlapping of the rio rava and of the blt explosions the plinian eruption of the blt is related to the emptying of a stratified deep seated hks magma chamber during the upwelling of k series ks magma marking a major geochemical transition and plumbing system re organization the following stage was responsible for the emplacement of the lower white trachytic tuff at 331 2 ka and of basaltic trachytic effusive products erupted through the main vent the subsequent activity was mainly restricted to the emplacement of basaltic shoshonitic parasitic cones and lava flows and of minor subplinian deposits of the upper white trachytic tuff between 275 and 230 ka the northern crater is most probably a maar that formed by the phreatomagmatic explosion of the yellow trachytic tuff at 230 ka the latest stage of activity featured the edification of the central shoshonitic domes at c a 150 ka

the newfoundland basin ocean continent boundary and mesozoic seafloor spreading history the ocean continent boundary in the newfoundland basin is defined as the seaward limit of a continental margin magnetic smooth zone east of the grand banks this boundary is marked by a prominent nne trending magnetic anomaly that is correlated with the j anomaly 115 m y south of flemish cap the smooth zone boundary strikes approximately 060 and is approximately 15 m y younger magnetic anomaly trends suggest two directions of motion during separation of iberia and north america the first phase of motion commencing at j anomaly time with a spreading center strike of 015 produced a rifted margin along the grand banks south of the newfoundland seamounts no spreading occurred north of the seamounts during this phase implying a counter clockwise rotation of iberia and no grand banks galicia bank separation the second phase began at about 102 m y with a shift of the pole of rotation to a location near paris producing a ridge orientation of approximately 060 this spreading center extended north and east into the northern newfoundland basin and bay of biscay producing a rifted margin south of flemish cap and opening of biscay this ridge geometry produced a component of extension across the newfoundland fracture zone and the southeastward migration of the resultant leaky transform fault between 102 m y and the next pole shift produced the volcanic edifice of the southeast newfoundland ridge fracture zone trends during this phase also exerted strong control on volcanism within the newfoundland seamount province this activity ceased at about 97 m y the date at which the second phase ended is not well defined by presently available data a rrr triple junction existed in the northeastern newfoundland basin western biscay region for a short time prior to anomaly33 34 80 m y which marks the inception of a continuous mid atlantic ridge spreading center between the newfoundland and charlie gibbs fracture zones

reply to comment by yanni gunnell and marc calvet on origin of the highly elevated pyrenean peneplain gunnell and calvet 2006 hereinafter referred to as gc challenge the recent model that we proposed for the origin of the highly elevated pyrenean peneplain by contest ing our morphometric analysis of this chain and the relation we made between the morphological evolution and the piedmont sedimentation their reasoning is as follows 1 according to calvet 1996 on which their comment is largely based the high elevation low relief surfaces in the eastern pyrenees are remnants of a peneplain that devel oped before the pliocene from applanation near to sea level and which was later uplifted by 2000 m during the plio quaternary in other words gc belong to the applanation school whereas we woul d belong to the altiplanation school 2 high elevation low relief surfaces do not exist in the central pyrenees 3 therefore the relationships we made between the morphology of the central pyrenees and the pattern of the detrital sedimentation in the adjacent ebro foreland basin is meaningless 4 contrary to the initial interpretation of calvet 1996 gc recognize that crustal thickening did not develop since the pliocene in the eastern pyrenees so they appeal to another geodynamical process such as extension or lithosphere delamination to explain the supposed uplift

crustal structure of the north iberian continental margin from seismic refraction wide angle reflection profiles abstract the structure and geodynamics of the southern margin of the bay of biscay have been investigated from a set of 11 multichannel seismic reflection profiles recorded also at wide angle offsets in an onshore offshore network of 24 obs obh and 46 land sites this contribution focuses on the analysis of the wide angle reflection refraction data along representative profiles the results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the iberian peninsula offshore the crust has a typical continental structure in the eastern tip of the bay which disappears smoothly towards the nw to reach crustal thickness close to 10 km at the edge of the studied area 45 n 6 w the analysis of the velocity depth profiles altogether with additional information provided by the multichannel seismic data and magnetic surveys led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic typical oceanic crust has not been imaged in the investigated area onshore the new results are in good agreement with previous results and document the indentation of the bay of biscay crust into the iberian crust forcing its subduction to the north the interpreted profiles show that the extent of the southward indentation is not uniform with an alpine root less developed in the central and western sector of the basque cantabrian basin n s to ne sw transfer structures seem to control those variations in the indentation degree

geophysical constraints on the deep structure along the ecors pyrenees line the structure of the central part of the pyrenean range traversed by the ecors vertical reflection seismic line is further constrained by a joint interpretation of complementary and reprocessed data wide angle low frequency profiling of moho topography indicates an abrupt termination of the northern european moho at the north pyrenean fault and a range of possible geometries of a deeper iberian moho which does not seem to extend significantly to the north of this fault when reprocessing vertical reflection data to 25 s two way travel time strong clear straight reflections are discovered in the part of the section under the northern foreland long after the moho reflection their strong northward dip indicates that they originate in fact at reflectors at depth beneath the north pyrenean fault which may correspond to the connection between the two levels of moho the reflections then being from the northward inclined limit between the wedge of iberian crust underlying the southern edge of the european mantle a range of possible shapes is allowed by the conjunction of these seismic data which is compatible with the results of a detailed gravity profile this would indicate that the imbrication of european into iberian lithosphere has been of limited extent with respect to the surface trace of the north pyrenean fault provided no strong density contrast in the mantle part of the section contributes significantly to the gravity anomaly

a three dimensional model of the pyrenean deep structure based on gravity modelling seismic images and petrological constraints summary the present study aims at building a 3 d density model of a whole collisional mountain belt at the lithospheric scale the geometry of internal structures is constrained by seismic studies while density values are imposed by petrological considerations the model must fit both the bouguer and geoid anomalies this approach is applied to the pyrenees for which numerous geophysical data are available the crustal structure is constrained from the results of seismic profiles the most important crustal feature is the large moho jump which coincides with the surface expression of the north pyrenean fault considered as the suture between the iberian and eurasian plates the eurasian crust is about 30 km thick whereas the iberian crust is 50 km thick in the central part of the range thick sedimentary basins at the piedmont and two large heavy intracrustal bodies located in the western and central parts of the range are also important features of the crustal model at the lithospheric scale a low velocity strip under the north pyrenean fault down to 80 100 km depth interpreted as the subduction of the iberian lower crust has been revealed by tomographic models the density of this structure has been constrained using petrological arguments which assume that the crustal material is transformed into eclogite at about 12 15 kbar 40 50 km depth an eclogite type density value much higher than the mantle density is thus imposed from 50 to 100 km depth theoretical bouguer and geoid anomalies are computed for this 3 d density model using a summation of density block anomalies a good agreement is found with observations giving strong support to the proposed model the crustal structure as seen by seismic profiles explains most of the observed long wavelength negative bouguer anomaly as well the local anomalies related to the intracrustal bodies a possible mantle origin of these bodies is ruled out on the other hand the transformation of the subducted lower crust into dense eclogite provides a successful fit to the long wavelength positive geoid anomalies in addition the water released during eclogite formation may help to explain the high conductivity zone detected down to 80 km south of the north pyrenean fault the 3 d model proposed here thus reconciles petrological considerations and geophysical data

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

seismic and tectono metamorphic characters of the lower continental crust in phanerozoic areas a consequence of post thickening extension deep seismic profiles of phanerozoic continental crust commonly show a highly reflective lower crust rheological considerations suggest that the seismic fabric of the lower crust can be attributed to the tectonic transposition of various petrological heterogeneities in the main flow plane the variscan provinces of western europe have been affected during phanerozoic times by several extensional and compressional events the geometrical relationships between seismic and geological structures indicate that the layering of the lower crust was acquired during the late carboniferous to permian when the thickened variscan crust was affected by gravitational collapse petrological and geochronological analyses of deep crustal rocks xenoliths and exposed sections indicate that the lower crust has recorded a major high t medium p granulite facies metamorphism during the late variscan extension whereas on the surface upper carboniferous to permian basins were being deposited a similar scenario characterizes other phanerozoic orogenic belts in the caledonian provinces of the british islands the lower crust is seismically reflective it has undergone medium p granulitic metamorphism during the deposition of devonian sedimentary basins at the end of the caledonian orogeny in the same way collapse of the mesozoic belt in the western part of north america is responsible during the cenozoic for pervasive crustal extension whose consequence is a seismic layering of the lower crust accompanied by a low p granulite grade metamorphic event while in the mid and upper crusts low angle ductile normal faults give rise to the basin and range province therefore it is proposed that there is a genetic relationship between 1 post thickening crustal extension 2 low to medium p granulite facies metamorphism of the lower crust and 3 seismic layering of the lower crust

high resolution imaging of the pyrenees and massif central from the data of the pyrope and iberarray portable array deployments the lithospheric structures beneath the pyrenees which holds the key to settle long standing controversies regarding the opening of the bay of biscay and the formation of the pyrenees are still poorly known the temporary pyrope and iberarray experiments have recently filled a strong deficit of seismological stations in this part of western europe offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution here we report the results of the first tomographic study of the pyrenees relying on this rich data set the important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections beneath the massif central the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the pyrenees in contrast they reveal the segmentation of lithospheric structures mainly by two major lithospheric faults the toulouse fault in the central pyrenees and the pamplona fault in the western pyrenees these inherited hercynian faults were reactivated during the cretaceous rifting of the aquitaine and iberian margins and during the cenozoic alpine convergence therefore the pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the european plate

a new tomographic image of the pyrenean lithosphere from teleseismic data abstract a new tomographic model of the pyrenean lithosphere is determined down to 200 km depth from teleseismic p and pkp travel times with a lateral resolution of 0 25 compared to previous models two important improvements are 1 a larger number of stations with a more even distribution in particular to the west of the range and 2 the introduction before inversion of crustal corrections inferred from previous refraction and reflection experiments this last point is crucial because a strong moho jump up to 20 km is present at the north pyrenean fault the former boundary between eurasian and iberian plates the comparison of the models obtained with and without crustal corrections reveals the strong contamination of the models by the crust down to 100 km depth in the uncorrected model a large strip with negative p velocity anomalies previously interpreted as subduction of lower crust is observed it disappears in the corrected model moreover the introduction of crustal corrections allows us to reveal short wavelength heterogeneities which were hidden by the crustal signal an attempt is made to relate the heterogeneities revealed by the tomographic model with the tectonic history of the pyrenees in particular with the alpine orogeny the alpine phase includes an extensive episode with generation of the thin continental crust and possibly the opening of an oceanic sea floor and then a compressive stage in our model no signature of an oceanic subducted slab could be detected all along the range a result which rules out the opening of a large oceanic floor before the compressive stage a subduction of continental crust is possible but due to the transformation of lower crust into eclogite at depth it can not be detected by seismological methods whereas it was observed from electrical and gravity data to the east of the range large heterogeneities with low velocities are ascribable to the neogene extension related to the rotation of the corso sardo block and the opening of the gulf of lion a prominent high velocity anomaly extending down to 200 km in eastern central pyrenees could possibly be interpreted as a detached piece of the tethys slab in north of iberia outside the range deep down to 200 km low velocity structures oriented n130 e are probably related to hercynian orogeny

implications of the seismic structure for the orogenic evolution of the pyrenean range abstract the dynamic evolution of the pyrenees is discussed in the light of geophysical data recent deep seismic sounding have revealed the crustal structure of the pyrenees which is used to test the different evolutionary models proposed until now the crustal thickness of the paleozoic axial zone paz and the north pyrenean zone npz differ by more than 10 km ranging from about 30 km in the npz to 40 50 km in the paz the transition from paz to the npz identified at the surface as the north pyrenean fault npf is sharp at depth and marked by a vertical step at least in the eastern half of the range the npz is characterized by additional throws and dips of the moho in the east whereas in the west a heterogeneous middle to lower crust is encountered with high velocity anomalies the seismic results suggest that the paz and the npz belong to different plates the npf being the plate boundary these results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following nne ssw directions they rather suggest that after a period of extension two main orogenic events took place a phase involving shearing and thinning which affected mainly the present day npz and a later compressive phase which explains the building up of the chain the thickening of the crust and the enhancement of a pre existing difference in crustal thickness between the european and iberian plates

slant stacks of refraction data from maine effects of lateral variations in velocity structure abstract we discuss some of the advantages and pitfalls associated with p analysis of wide angle profiles recorded over terranes characterized by significant lateral heterogeneity we construct composite slant stacks for a pair of reversed shots recorded along the axis of the merrimack synclinorium in southwest maine and invert picks for bounds on averaged one dimensional slowness depth structure extremal bounds on total crustal thickness derived from shot point 12 are 40 to 43 km average crustal velocities for individual slowness models corresponding to the extremal depths range from 6 5 to 6 6 km sec predicted normal incidence two way travel times for the moho lie between 12 2 and 13 2 sec the utility of one dimensional inversion of p data is largely in estimating the maximum resolving power of the data set this is information that forward modeling approaches do not provide comparisons of slant stacks of reversed shot gathers with intercept times predicted for a two dimensional model derived by ray tracing highlight some of the pitfalls associated with interpretation of p data in terms of one dimensional structure in some cases the principal arrival may become multi valued resulting in misidentification of phases and compounding biases in estimates of average crustal parameters slant stacks can be highly useful however in refining two dimensional models derived from travel times because of the sensitivity of ray parameters to dip and lateral velocity gradients we use slant stacks in conjunction with the original shot gathers to refine our estimate of the configuration of the moho beneath southwest maine our final model shows a step in the moho that is consistent with variations in normal two way travel times inferred from analysis of postcritical fan shots this abrupt change in crustal thickness may mark the northeast extent of significant underthrusting of avalon crust beneath medial new england basement during the acadian orogeny the preservation of offsets in moho that were formed during paleozoic compression is consistent with the limited amount of rifting experienced by this portion of the orogen during the opening of the atlantic

a three dimensional model of the pyrenees and their foreland basins from geological and gravimetric data abstract we construct a three dimensional geological model of the pyrenees and their foreland basins with the geomodeller this model which accounts for different sources of geological and geophysical informations covers the whole pyrenees from the atlantic ocean to the mediterranean sea and from the iberian range to the massif central down to 70 km depth we model the geological structure with a stratigraphic column composed of a superposition of layers representing the mantle lower middle and upper crusts the sedimentary basins are described by two layers which allow us to make the distinction between mesozoic and cenozoic sediments which are characterized by markedly different densities and seismic velocities since the pyrenees result from the convergence between the iberian and european plates we ascribe to each plate its own stratigraphic column in order to be able to model the imbrication of iberian and european crusts along this fossile plate boundary we also introduce two additional units which describe the orogenic prism and the water column in the bay of biscay and in the mediterranean sea the last ingredient is a unit that represents bodies of shallow exhumed and partly serpentinized lithospheric mantle which are assumed to produce the positive bouguer gravity anomalies in the north pyrenean zone a first 3d model is built using only the geological information coming from geological maps drill holes and seismic sections we use the potential field method implemented in geomodeller to interpolate these geological data this model is then refined in order to better explain the observed bouguer anomalies by adding new constraints on the main crustal interfaces the final model explains the observed bouguer anomalies with a standard deviation less than 3 4 mgal and reveals anomalous deep structures beneath the eastern pyrenees

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

deep seismic reflection profile across the northern appalachians a marine seismic reflection line across the northeast extremity of the canadian appalachians in newfoundland indicates a collisional suture in the lower crust beneath the central ophiolitic dunnage terrane the thrust and fold belt miogeocline above the grenville basement and the dunnage and gander tectonostratigraphic terranes all appear to be allochthonous with respect to lower crustal basement the gander avalon terrane boundary to the east is a near vertical feature that penetrates the crust the data also suggest that the ancient passive margin of north america extends eastward under the dunnage terrane for about 70 km the newfoundland deep seismic experiment indicates major tectonic differences compared to results for the southern appalachians

mapping the indentation between the iberian and eurasian plates beneath the western pyrenees eastern cantabrian mountains from receiver function analysis abstract in the last decades active seismic profiling in the northern part of the iberian peninsula has evidenced that the alpine collision between the iberian and eurasian plates resulted in a complex crustal structure with the iberian crust underthrusting the eurasian crust and reaching depths of at least 45 50 km beneath the pyrenean chain and the cantabrian mountains in the transition between these two zones the situation is particularly complex as evidenced in previous wide angle and passive seismic studies this contribution focuses in getting new clues on the crustal structure of this transitional zone through receiver function rf analysis of teleseismic data recorded at permanent and temporary stations located in both the spanish and french sides of the western pyrenees different techniques h stacking pseudo migration synthetic 2d modeling have been considered in the analysis passive seismic data from previous temporary deployments in the zone have been reworked and added to the discussion a first order result is that passive seismic data are broadly consistent with the indentation of the iberian and eurasian crusts inferred from active seismic profiling thus providing a completely independent confirmation of this feature for the first time an iberian moho underlying the eurasian crust is documented from rf beneath the stations located at the northern side of the pyrenean range moreover clear indications of dipping interfaces are observed at some stations the new rf results suggest that in the crustal indentation beneath the basque massifs area the eurasian crust extends farther south with respect to the image inferred from active seismic data this new geometry implies that the pamplona transfer zone has played a major role in the regional geodynamic history

the marconi reflection seismic data a view into the eastern part of the bay of biscay abstract seismic data acquisition for the marconi project took place in 2003 in the eastern half of the bay of biscay the overall goal of the multichannel experiment was to collect seismic data to elucidate the structure and evolution of the southern margin of the bay where an along the margin transition between oceanic subduction in the west and continental convergence in the east occurs and its relationship to the pyrenean realm we describe the multichannel experiment and present an interpretation of the abyssal plain sector of the profiles altogether eleven regional seismic reflection profiles were collected with a total length of 1800 km between the longitudes 6 and 2 w and latitudes 43 5 and 46 n the seismic profiles show first images of the thick sedimentary pile that covers the eastern abyssal plain of the bay the interpretation includes three sedimentary sequences separated by unconformities that correspond to the pre syn and post alpine sequences the post orogenic deposits lay unconformably over the previous compressional structures in the eastern part of the bay floored by transitional crust these compressional features are evident in the presence of a young accretionary prism sealed at its first stages of development the overall structure and shape is similar to those of active subduction prisms worldwide but evidencing differences in width and sediment wedge thickness from west to east along the margin buoyancy of the transitional crust and resistance to subduction led to stronger plate coupling towards the east where oceanic crust is absent and may be the main cause for these morphological changes along the east west traverse of the wedge the marconi multichannel deep seismic dataset provides the first estimation of the thickness of the sediment load on the 5000 m deep abyssal plain of bay of biscay where no direct data were available

crustal structure of the flemish cap continental margin eastern canada an analysis of a seismic refraction profile summary the crustal structure of the ne flemish cap margin was determined along a 460 km long refraction wide angle reflection seismic transect flame line to define the thickness structure and composition of the crust and uppermost mantle along the line a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers a coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to moho the model displays a sediment cover of up to 3 6 km thick subdivided into three layers with velocities of 1 8 1 9 km s 1 2 8 3 1 km s 1 and 4 7 4 8 km s 1 for the western part of the flame line over flemish cap the p wave velocity model displays an up to 32 km thick three layer continental crust the continental crust has velocities of 5 8 6 1 km s 1 6 3 6 45 km s 1 and 6 65 6 85 km s 1 and thicknesses of about 5 km 7 km and 20 km in the upper middle and lower layers respectively the thick continental crust thins to a two layer 6 km thick crust upper layer is 5 55 6 0 km s 1 and the layer below is 6 65 6 8 km s 1 over a distance of 45 km s wave velocities are determined in the upper layer of the thick continental crust over flemish cap and the transition zone by assigning poisson s ratios in the p wave velocity model comparison of calculated to observed arrival times gives a poisson s ratio of 0 27 in the upper layer and 0 28 in the layer below which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone the thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7 5 7 9 km s 1 we interpret this layer as partially serpentinized mantle which is consistent with observationsfromthenewfoundlandmargintothesouth theserpentinizedmantleterminates 30 km seaward of the thick continental crust at the seaward most end of the thin continental crust a prominent ridge feature is observed the seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust the reflection seismic data show a high relief basement from the ridge feature and seaward the flame line crosses magnetic anomaly 34 and extends another 50 km seaward well into oceanic crust the ridge is flanked seaward by a two layer oceanic crust the upper layer layer 2 has velocities of 4 8 5 0 km s 1 for the landward most 35 km of oceanic crust and 4 8 6 2 km s 1 for the seaward 60 km the average thickness of layer 2 is 2 km the lower layer layer 3 has velocities of 6 7 7 2 km s 1 and a thickness of 3 5 km the velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature

crustal thickening and variations in architecture from the qaidam basin to the qang tang north central tibetan plateau from wide angle reflection seismology abstract wide angle reflection refraction profiles at crustal scale have been obtained along a 700 km transect of northeastern tibet from six shotpoints a combination of sampling by broadside fan recordings and in line profiling allows us to image increases in moho depth towards the centre of the plateau these occur with different styles across the two block boundaries between north kun lun bayan har and qang tang as shown also by the changes resolved in the internal architecture and layer velocities in the crust clear s waves are observed together with p waves they bring rare constraints on the composition of crustal units which reveals an abnormally felsic average composition for the whole crust and its lower half along the whole length of the transect across the north kun lun qaidam block variations of crustal thickness are detected with moho depth between 55 and 65 km and an inverse correlation with basement topography in the north of the bayan har block a strong heterogeneity in the middle of the 70 km thick crust is shown as suggested from synthetic seismograms modelling it may result from thin layer inclusion marking the northward tectonic superposition of the bayan har crust on the north kun lun crust as imaged by a fan profile the corresponding upper part could have been transported northwards to thicken also the upper crust of the present north kun lun block in the north of qang tang a model is obtained with a 75 km thick crust and a lower half of the crust with average velocities characteristic of felsic composition from bayan har to qang tang variations of deep architecture are documented across the jinsha suture and xianshuihe fault from recording in line and broadside to several shotpoints together with the felsic composition of the lower part of the crust a moho offset suggests the imbrication of the bayan har crust in the qang tang lithosphere between a south dipping moho above and a north dipping moho below a model of indentation of the qang tang by an originally thinner bayan har crust and lithosphere with part of the qang tang brought to greater depth would also be consistent also with the moho topography across the transition from the qang tang to bayan har as imaged by fan profile

a hybrid method to compute short period synthetic seismograms of teleseismic body waves in a 3 d regional model we present a hybrid method to simulate the propagation of short period teleseismic body waves through 3 d regional models the incident wavefield is computed in an spherically symmetric reference earth model based on the direct solution method the global and regional wavefields are matched at the boundaries of the regional mesh in the regional domain we implement a spectral element method with absorbing boundaries to cancel the outgoing scattered wavefield the hybrid method is successfully benchmarked against the direct solution method in the reference earth model iasp91 the potential of the method is illustrated by computing short period p wave synthetic seismograms in a 3 d model with a 20 km moho offset and or topography on the free surface focusing on the actual example of the pyrenees

thinned and subsided continental crust on the rifted margin of eastern canada crustal structure thermal evolution and subsidence history summary seismic refraction measurements were carried out using ocean bottom seismometers over foundered continental crust in orphan basin and flemish pass on the continental margin north east of newfoundland tau p travel time inversion synthetic seismogram analysis and conventional layered model calculations were applied to the data the results show that these outer regions of the margin are underlain by thinned continental crust with a total depth to the m discontinuity of about 22 km there are two main crustal layers with p velocities of about 6 1 and 7 0 km s 1 which appear to be homogenous and separated by sharp interfaces these are overlain by a layer in which p velocities are about 5 5 km s 1 this is interpreted to be precambrian or palaeozoic basement mesozoic and cenozoic sediments cover the basement rocks and are over 4 km thick the results imply that crustal thinning to about 50 per cent of the original crustal thickness occurred the gravity anomaly data show that the thin crust has a maximum horizontal extent of about 450 km from the ocean continent boundary near orphan knoll landward to the outer continental shelf the subsidence history and thermal evolution of the region was computed assuming that the observed thinning is produced by horizontal extension of the lithosphere it is suggested that extension can only satisfy the observed crustal structure and elevation of the margin during the rift phase if more extension took place in the lower lithosphere than in the upper lithosphere the computed subsidence is compared to the observed subsidence and the total amounts of subsidence are similar however the shape of the observed subsidence curves measured in deep exploratory wells differs significantly from the predicted subsidence assuming cooling began when final continental breakup occurred in the late cretaceous the temperature distribution within the lithosphere due to extension may be related to the flexural rigidity of the plate as a function of both time and position across the margin therefore it is suggested that the response of the lithosphere to sediment loading and the large amplitude of the gravity shelf edge anomalies are directly related to the thermal history of the region

traveltime tomography of a dense wide angle profile across orphan basin abstractwe used first arrivals and moho reflections from the 500 km long orphan basin wide angle velocity experiment obwave profile with 3 5 km instrument spacing to construct a traveltime tomography section and to delineate the moho discontinuity across the orphan basin the orphan basin is a failed rift located offshore newfoundland canada showing thinned continental crust over an unusually wide region we observed 1 a zone of extreme crustal thinning 7 km thick crust with no evidence for mantle serpentinization 2 basement morphology exhibiting tilted blocks linked to the crustal thinning and 3 a thicker central crustal segment that is probably related to prerift structural inheritance comparison with the adjacent jeanne d arc basin to the southeast suggested the presence of a decoupling zone between the two basins accommodating the difference in extension rates there was a good correlation between the tomographic velocities and the reflection structure derived from a coincident seismic

constraints on crustal structure and complex moho topography beneath pinon flat california from teleseismic receiver functions abstract we use teleseismic p waves recorded at pinon flat observatory pfo to constrain the three dimensional crustal and upper mantle velocity structure beneath the station by forward modeling radial receiver function waveforms we construct a one dimensional crustal model that includes a significant shear velocity inversion at 9 km in depth arrivals on the tangential components indicate dip of at least the uppermost discontinuity complicated moho topography deepening to the northwest of pfo is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal p wave reverberations although fine details cannot be resolved each of those sets of observations plus mislocation vectors provide strong indications of abrupt moho topography possibly including step offsets of several kilometers this is not only consistent with gravity data in implying airy isostasy with compensation at moho depth but extends that model to a much finer length scale than had been resolved

comparison of lithosphere structure across the orphan basin flemish cap and irish atlantic conjugate continental margins from constrained 3d gravity inversions regionally constrained 3d gravity inversion results on the orphan basin flemish cap and the irish atlantic conjugate continental margins are compared to investigate crustal structure early rifting history and geological evolution of this part of the north atlantic the full crustal density anomaly distributions provide some of the first depth images of how rifted structures compare along and across these conjugate margins broad similarities in crustal structure are identified with some noticeable differences linked to rifting and crustal stretching processes extreme crustal thinning stretching factors 3 5 is indicated beneath much of the southern porcupine basin the western half of west orphan basin the eastern half of jeanne d arc basin the southeastern half of east orphan basin and in pockets beneath rockall basin this appears to have resulted in the serpentinization and possible exhumation of mantle lithosphere on the irish atlantic and flemish cap margins but not beneath orphan basin a simple evolution model is proposed for the early stages of rifting between the margins it is suggested that ancient orogenic sutures played an important role in controlling the northward migration of rifting and the rotation and displacement of flemish cap out of orphan basin supplementary material enlarged maps from this paper are available at www geolsoc org uk sup18527

a synthesis of the geophysical characteristics of terranes in eastern canada abstract geological identification of terranes in the canadian appalachians has led in general to the proposing of terranes which have recognizably distinct geophysical signatures indicative of fundamental differences in crustal structure between the terranes the rapidly expanding geophysical data base has enabled an integrated approach to geophysical terrane analysis the geophysical data enable the humber with its diagnostic gravity low and variable magnetic anomalies to be recognized throughout the canadian appalachians deep seismic data suggest that it is found beneath the western part of the dunnage terrane the latter is typified by dominantly positive gravity anomalies in newfoundland these together with the magnetic data enable the zone to be subdivided into the notre dame and exploits subzones the seismic character clearly identifies the presence of the dunnage northeast of newfoundland and suggests that it may terminate south of newfoundland the gander terrane has typically negative gravity and magnetic anomalies which may be confidently traced from northeast of newfoundland south through cape breton a belt of similar anomalies occurs in new brunswick the deep seismic evidence suggests that the gander may represent the outcrop of a deep crustal block underlying the eastern part of the dunnage terrane the avalon terrane is identified throughout the canadian appalachians by a diagnostic set of arcuate alternating gravity and magnetic anomalies the boundary between the avalon and gander is clearly recognized on deep seismic sections as a near vertical feature cutting through the crust northeast of newfoundland the meguma avalon boundary is clearly delineated by a gravity and magnetic gradient throughout the system there is limited and ambiguous deep seismic data regarding the nature of it and the meguma terrane in general a subtle set of nw trending lineations perpendicular to the dominant regional trend has recently been identified geological evidence that suggests these are post carboniferous in age and may be related to the development of a tensional regime associated with the mesozoic rifting of the atlantic

the eastern end of the pyrenees seismic features at the transition to the nw mediterranean the deep crustal features at the transition from the pyrenean range to the nw mediterranean basin have been investigated by onshore offshore recording of ligurian sardinian lisa seismic profiles the predominance in this area of the neogene extensional tectonics over the paleogene compressional one is revealed from the present day deep structure sampled by the new wide angle data the existence of a crustal root beneath the pyrenean axial zone and the outstanding differences in moho depths between the iberian and european domains well established from previous seismic studies on central and western pyrenees are not observed anymore at the easternmost part of the range this area is marked by a continued crustal thinning seawards the moho rises about 10 km along distances of 60 km that affects already the mainland further evidences on lateral variations and topography of the crust mantle boundary are indicative of the extent of the different tectonic episodes

the eruption pyroclastic flow behaviour and caldera in filling processes of the extremely large volume 1290 km3 intra to extra caldera permian ora ignimbrite formation southern alps italy abstract the permian ora formation 277 274 ma preserves the products of the ora caldera super eruption northern italy the stratigraphic architecture of the exceptionally well preserved intra caldera succession provides evidence for caldera collapse at the onset of the eruption a multiple discharge point fissure eruption style and progressive incremental caldera in filling by numerous pyroclastic flow pulses within the caldera the ignimbrites of the ora formation are voluminous 1290 km3 crystal rich 25 to 55 and ubiquitously welded the ora formation has been divided into four members a d which also define the principal eruption phases the eruption proceeded in four main stages 1 early caldera collapse and vent opening producing locally distributed basal co ignimbrite lithic breccia member a 2 vent clearing which produced the eutaxitic lithic rich ignimbrite and minor thin ground and ash cloud surge deposits member b 3 waxing and steady eruption which produced the dominant eutaxitic coarse crystal rich ignimbrite with local lithic rich and fine crystal rich ignimbrite and minor surge deposits member c and 4 waning eruption recorded by the eutaxitic fine crystal rich ignimbrite with local lithic rich ignimbrite deposits member d the incremental filling and late stage outpouring of pyroclastic material from the caldera is recorded by vertical and lateral lithofacies deposit variation and some correlation between stratigraphic sections these findings reveal a structure to the outwardly monotonous 1300 m thick intra caldera fill and thinner

the seismogenic zone of the continental crust in northwest iberia and its relation to crustal structure the distribution of seismicity at the western end of the cantabrian mountain range nw iberia reflecting current crustal dynamics is investigated integrating seismically active structures long term structures and the topographic features at the surface the thickness of the seismogenic zone within the continental crust is established in 20 km two crustal domains can be distinguished in the study area in terms of the seismicity pattern the style of alpine structures and the relief the presence of crustal fluids arises as a very likely contributing factor to the excessive thickness of the seismogenic zone in the study area more importantly a switch in tectonic style at the transition between crustal domains in coincidence with the lateral termination of orogenic frontal thrusts is envisaged as involving sufficient stress heterogeneity and amplification to explain the current concentration and characteristic pattern of historical seismicity in the region overall the distribution of seismicity in the crust is sensitive to the type and style of crustal structures

seismic signature of variscan and alpine tectonics in nw iberia crustal structure of the cantabrian mountains and duero basin the seismic structure of the crust of the nw iberian peninsula along the cantabrian mountains and their southern foreland the duero basin is investigated from the first data set available from refraction and wide angle reflection profiles the velocity depth distributions obtained along e w and n s transects evidence lateral variations in crustal structure that can be associated to variscan and alpine tectonic episodes the western part of the range toward the variscan hinterland zones as well as the duero basin below the sediments show a crustal structure similar to that of the variscan belt elsewere in europe the total crustal thickness is 30 32 km and three main layers upper middle and lower crust with average velocities 6 0 6 25 and 6 8 km s are resolved some paleozoic structures such as the basal detachment of the cantabrian zone are expressed in the seismic models the crust appears to vary eastward in the external zones the most outstanding feature is a crustal root beneath the highest cantabrian summits where the moho is found at 47 km depth a prominent alpine reworking of the crust in the eastern part of the cantabrian zone is thus revealed and the n s seismic transects delineate a geometry similar to the one found across the central pyrenees in the etude continentale et oceanique par reflexion et refraction sismique ecors profile revealing the importance of the alpine deformations in the northern part of the iberian plate

the ecors deep reflection seismic survey across the pyrenees the pyrenees are an alpine age mountain belt located on the boundary between the iberian and european plates using the same geological and geophysical data earth scientists studying the structure of the pyrenees during the past ten years have not reached a consensus here we report the first results from a nearly continuous 255 km long reflection seismic survey across the pyrenean chain this first deep reflection profile of an entire erogenic belt will help to constrain possible models for the structure and formation of the pyrenean chain

pleistocene magnetochronology of early hominin sites at ceprano and fontana ranuccio italy article i nfo paleomagnetic analyses were conducted on two cores drilled at ceprano in central italy where an incomplete hominin cranium was discovered in 1994 as well as on two additional cores from the nearby site of fontana ranuccio that yielded hominin remains associated with an acheulean industry no evidence for the 0 78 ma brunhes matuyama boundary was found at ceprano down to 45 m below the level that yielded the hominin cranium the ceprano lithostratigraphy and the paleomagnetic age constraints are broadly consistent with the stratigraphy of the liri lacustrine sequence of the latina valley constrained by published k ar ages between 0 6 and 0 35 ma and according to an age model with magnetic susceptibility supported by pollen facies data suggest that the level that yielded the hominin cranium has an age of 0 45 0 05 0 10 ma evidence for the brunhes matuyama boundary was found at fontana ranuccio about 40 m below the hominin level consistent with a k ar age of 0 46 ma reported for this level hence the ceprano and fontana ranuccio hominin occurrences may be of very similar mid brunhes age

structural setting of a typical alkali potassic volcano roccamonfina southern italy abstract the structural setting of roccamonfina volcano is reconstructed using gravity and magnetotelluric plus time domain electromagnetic data a 887 m deep bore hole drilled within the summit caldera was utilized to constrain the inversion models roccamonfina volcano appears located at the intersection of two east northeast and north trending narrow grabens the location of historical seismic activity indicates that at least one of them the north trending is still active mt tdem apparent resistivity curves show the existence of a ubiquitous conductive layer which includes hydrothermally altered volcanic rocks and flysch formations the average thickness of the volcanic sequence is about 800 1000 m no evidence was found for the presence of the intracrustal magmatic reservoir which is required to justify the chemistry of magmas erupted during the second and final stage of activity of the volcano

thermomechanical evolution of a thinned continental lithosphere under compression implications for the pyrenees in order to demonstrate the interactions between plate boundary forces and preexisting zones of weakness and how those control the localization of continental deformation inside the lithosphere the thermomechanical evolution of an initially thinned and thermally perturbed continental lithosphere in compression was analyzed within the framework of the pyrenees while a concensus exists as to the shallow structures the extent and geometry of deformation at depth have been the subject of great controversy the thermomechanical implications of recent geological hypotheses on the initial state prior to the compression recently derived from balanced cross section reconstructions are here quantified and compared to the recent ecors deep seismic reflection profile across the pyrenees a thermomechanical model based on finite elastoplastic viscoplastic deformations has therefore been formulated using phenomenological constitutive equations derived from experimental studies the thermally coupled problem is solved by an appropriate vertical two dimensional plane strain finite element approximation in those models most of the shortening is accommodated inside a local zone of intense plastic shearing that creates a sharp wedge shaped root progressively sinking deep into the mantle the main thickening takes place inside the lower crust where it can reach a factor 2 this indentation and crustal imbrication process is controlled by a buckling instability occuring inside the uppermost part of the mantle the evolution of the topography can be related to these phenomena lateral conductive heat transfer overtakes the advective transfer in the vicinity of the inflow of colder crustal material inside a warmer mantle the deformation pattern can be explained by a three layer mechanical lithosphere with a soft ductile internal layer of potential decoupling between two more competent layers characterized by a significant elastic stored energy the overall response of this composite plate is not much influenced by smaller scale variations of crustal rheology but is very much dependent on the contrast in rates of viscous dissipation between the lower crust and the upper mantle and on the possibility of singling out a layer of significant elastic stored energy by the balance between rate of loading and rate of dissipation inside the uppermost mantle predicted deformation pattern is found to agree with the seismic reflection and gravity data even though extrapolation to other rates of shortening must be verified by appropriate numerical modelling these physical mechanisms may be regarded as a paradigm for other crustal imbrication phenomena found in the himalayas tibet or the alps

the receiver structure beneath the ebro basin iberian peninsula abstract a model of the lithospheric structure of the eastern sector of the ebro basin has been obtained by means of an inversion of the receiver functions from teleseismic events recorded by one of the institut d9estudis catalans stations located at the northeastern iberian peninsula the technique consists of the isolation and modeling of the amplitude and travel time of the direct p teleseismic wave reverberations in a local structure and has been used for events recorded at poblet station pob placed in the southeast margin of the basin from 1994 to 1997 the analysis of pob records shows the influence of large scale heterogeneities in the wave pulses coming from azimuths crossing the catalan coastal ranges whereas a simple medium response is obtained when teleseismic waves only sample the basin the inversion of such simple receiver functions in terms of a flat layered media 2 to 3 km layer thicknesses along with a comparison of the results with previous and independent works enables us to obtain a crustal and upper mantle velocity structure for the ebro basin the preferred solution models show a strong near surface velocity gradient a nearly constant p velocity upper crust of about 5 9 km sec and a sharp velocity contrast at the crust mantle boundary located at 32 km depth overlayed by a lower crust consisting of a significant positive velocity gradient the upper mantle shows an average p velocity of about 8 0 km sec