a seismic discontinuity in the upper mantle between the eastern alps and the western carpathians constraints from wide angle reflections and geological implications abstract seismic investigation of the lithosphere by means of active source experiments is mostly confined to the crust and the moho structures in the upper mantle are more likely to be discovered by analyses of teleseismic data although these methods are restricted in their resolution capabilities the relatively rare evidence for upper mantle refractors or reflectors in active source data enables challenging and interesting studies of the lower and not so well known part of the lithosphere we present such an example from the tectonically complex region between the eastern alps and the western carpathians this area was covered by several extensive 3d wide angle reflection refraction experiments within the last decade and their layout was designed to illuminate the crustal structure and in particular the moho discontinuity in some areas reflections from below the moho are also recorded these reflections occur at recording offsets between 200 and 500 km and they are particularly strong in cross line recordings we derive a set of travel times from the data and perform a tomographic inversion for the depth and shape of the reflecting interface the inversion makes use of an existing 3d crustal model which also includes the moho topography since the upper mantle velocities are poorly constrained and the azimuthal distribution of the rays is biassed several tests are applied to investigate the reliability of possible solutions the results from the tomographic inversion indicate an overall horizontal and radially dipping reflector the average depth of the reflector is 55 km which is about 25 km below the crust mantle transition and amplitude modelling suggests that the reflecting interface represents a velocity increase the investigated area is further characterised by deep sedimentary basins high heat flow high velocities in the lower crust diffuse moho signature and a strong positive bouguer anomaly nearby xenolith outcrops exhibit a pronounced change in anisotropy and indicate the presence of two distinct layers in the lithospheric mantle whereas the deeper layer is thought to present more juvenile lithosphere derived from thermal relaxation in the post extension phase most likely the upper mantle reflector also represents this change in anisotropy though other scenarios are also possible we conclude that the entire lithosphere is significantly shaped by extensional processes which affect the area since the late oligocene early miocene

general features of the uppermost mantle stratification from long range seismic profiles abstract long range seismic studies made by russian institutions during the last two decades reached depths of 700 800 km on the continent peace nuclear explosions were used as a source and 100 km in the ocean ocean bottom stations recording 500 kg explosions in water comparison of the wave fields of these and other seismic profiles in russia western europe and the south atlantic shows that the classical lithosphere asthenosphere model is too simple to explain the observed data a stratified medium with alternating weak and rigid layers is a more realistic model of the upper mantle one of the layers is globally observed at a depth of 80 to 100 km it is located in the thermal lithosphere beneath old platforms at the bottom of the lithosphere under active tectonic areas and inside the asthenosphere below mid oceanic ridges relatively high velocities 8 4 to 8 5 km s are typical for this boundary both in the old platforms and in the high heat flow oceanic areas a change of structural pattern where the block structure of the uppermost mantle is transformed into a horizontally homogeneous one is typical at that boundary level such features suggest that the boundary separates the brittle and more ductile upper mantle layers and may be considered as the bottom of the global mechanical lithosphere

sulfide composition and microthermometry of fluid inclusions in the leg 111 sheeted dike section of ocean drilling program hole 504b costa rica rift hole 504b of the ocean drilling program is dedicated to the study of crustal structure and hydrothermal processes in 5 9 m y old oceanic basement continuing the work of previous legs hole 504b was extended 212 3 m to a total depth of 1562 3 m below seafloor bsf during leg 111 in 1986 quartz sulfide veins occur at a depth of 1369 1388 m bsf in basalts of the sheeted dike complex the ore minerals are predominantly pyrite less chalcopyrite rare corich cu fe s phases and a thiospinel linnaeite carrollite microprobe analyses yield a high co content in zoned vein pyrites 8 wt as well as in the cu fe s phases 5 wt up to 35 8 wt co was detected in the thiospinel a co ni ratio of 100 distinguishes the vein pyrite from pyrite in the basaltic wall rock and from pyrite formed as an alteration product of olivine co ni 5 the co ni ratios correlate positively with cu and negatively with as co rich nonstoichiometric cu fe s sulfides in chalcopyrite are interpreted as metastable phases which have been quenched at a high temperature and prohibited from exsolution of the stable products chalcopyrite and pyrite fluid inclusions in quartz from the quartz sulfide veins are two phase and vary from liquid to vapor dominated their salinities range from 4 2 to 7 2 wt equivalent nacl and average 5 5 wt pressure 360 bars corrected average filling temperatures vary from 271 to 408 c with a maximum of 486 c this is consistent with calculated quartz formation temperatures for a single quartz separate 4 2 18o using oxygen isotope thermometry the 18o value of the hydrothermal fluid was determined to be 1 7 the temperature data indicate fluid alteration of the sheeted dikes at about 350 to 500 c the maximum homogenization temperatures intersect the liquid vapor two phase boundary above the critical point of seawater thus phase separation could have occurred before or during the formation of the mineralized veins and the alteration of the sheeted dike sequence

the identification of mantle reflections below hungary from deep seismic profiling abstract in the 1970 s the eotvos lorand geophysical institute performed seismic reflection deep sounding experiments with multiple coverage in several parts of the pannonian basin posgay 1975 one of the most interesting results was that arrivals were received in the time range corresponding to the depth of the asthenosphere they were interpreted as reflections posgay et al 1981 conspicuous arrivals from the time range of the asthenosphere 15 20 s were also received along the pannonian geotraverse in 1987 in the majority of sections published in other countries the upper mantle seems to be transparent the present paper discusses investigations and preliminary results which lead to the conclusion that this apparent transparency is due to the frequency range below 10 hz being neglected in both signal generation and recording in most deep continental reflection surveys the seismogeological conditions of the pannonian basin are certainly favourable for reflection studies of the upper mantle the correctness of the assumption that arrivals from the time range of the asthenosphere are primary reflections is subjected to further investigation and conclusions are made concerning the structure of the asthenosphere

imaging the lithospheric mantle in northwestern canada with seismic wide angle reflections normal moveout corrected and stacked sections of wide angle reflection data collected in the archean slave province and paleoproterozoic wopmay orogen in northwestern canada image the crust mantle boundary as a subhorizontal discontinuity at about 32 36 km depth along a 450 km lateral extent and for the first time from such data layering within the lithospheric mantle at depths between 75 and 90 km over a lateral distance of 750 km these images correspond to moho and mantle reflections observed on a co located multichannel seismic mcs reflection profile structural features and or the basalt eclogite phase change associated with subduction of oceanic and or delaminated lower crust are possible sources of the deep reflectivity one mantle reflection group lies west of the mcs data and east of the cordilleran deformation front it could have similar sources or result from the mesoproterozoic racklan orogen or meso neoproterozoic rifting of the ancient margin

asthenospheric structure beneath a neogene basin in southeast hungary abstract beneath one of the subbasins of the pannonian basin formed between the alps carpathians and dinarides the bekes basin in southeast hungary a typical structure of the lithosphere and an upwelling of the asthenosphere can be inferred from seismic reflection geothermal geomagnetic gravity and magnetotelluric investigations in the bekes basin the thickness of neogene sediments reaches 6 7 km i e the basement of the neogene basin sinks 3 4 km over the basin area in the reflection section of the pannonian geotraverse that ends in the bekes basin the crust mantle boundary rises by 5 6 km up to a depth of 22 km in the course of performing and processing seismic reflection measurements particular care was given to the preservation of low frequency 2 10 hz information this explains that data were successfully received even from the upper mantle amplitude distribution of reflection arrivals and the dipping of reflecting surfaces permit to infer an uplift by 15 20 km in the lithosphere asthenosphere boundary which thus is assumed to be at a depth of 40 45 km beneath the bekes basin in geothermal investigations the model was established with seismic results being accounted for assuming an upwelling in the lithosphere asthenosphere boundary the superficial heat flow density corrected for the cooling effect of sediments was in good agreement with the values of model calculations the latest magnetotelluric results support the upwelling position of the lithosphere asthenosphere boundary the geomagnetic anomaly determined over the area of the bekes basin can be explained by the uplifted position of the lower crust the gravity anomaly can be attributed to the combined effect of this and the upwelling in the crust mantle boundary it is hoped that the integrated interpretation of results obtained by various geophysical methods confirms the assumption that said methods permit to investigate the structure of the whole lithosphere and thus the disclosed results may contribute to the completion and precision of basin models studied so far

depth of crustal conducting layer and asthenosphere in the pannonian basin determined by magnetotellurics abstract on the basis of numerical modelling and statistical investigations of the horizontal electrical inhomogeneities narrow fractures change of conductance of the surface sediments min magnetotelluric mt sounding curves were found to be less distorted and therefore they can be used for the determination of deep conducting formations in the crust and upper mantle carefully selected mt curves indicate the crustal conductive layer at depths of 7 13 km and the conductive asthenosphere between 50 and 80 km the relatively shallow position of the asthenosphere corresponds to the high heat flow q 80 mw m 1 in the pannonian basin according to the empirical relation derived by adam 1978

crustal structure of the central sector of the betic cordillera se spain deep seismic reflection gravimetric and magnetometric data allow the main features of the deep structure of the central sector of the betic cordillera to be established the moho is horizontal or dips slightly toward the s below the mountain chain and its morphology has no direct relationship with the region s topography the deep reflectors are not deformed by the neogene kilometric scale folds that produced the main topographic features probably due to the existence of detachment levels in the crust the crust is slightly thickened in the betic cordillera almost 35 km and has an abrupt transition to the thin crust of the alboran sea 15 km in the central alboran sea along an e w oriented narrow band subparallel to the coast line where the moho dips more than 60 n the neogene evolution of the mountain chain caused compressive deformation in the external zones in the internal zones pre tortonian low angle normal faults developed after the alpine metamorphism and later high and low angle normal faults and strike slip faults occurred up to the quaternary geophysical and field geology data can be used to establish a geological model for the recent evolution of the crustal deformation in this model the recent evolution of the region is probably determined by thrusting of the thin continental crust of the alboran sea toward the nw over the southern margin of the iberian massif this deformation could be a consequence of the oblique convergent movement between the african and eurasian plates which may have uplifted the entire region causing regressions in the sedimentary basins and the formation of folds and extensional structures in the upper part of the thickened crust

tertiary palaeomagnetic data for the carpatho pannonian region in the light of miocene rotation kinematics abstract the tectonic interpretation of the palaeomagnetic data for the neogene history of the carpatho pannonian region has usually been based on the palaeomagnetic data for the pre cenozoic complexes here a review of the tertiary palaeomagnetic data is given for the sites with two or more sets of data new site mean directions are calculated for most areas new area mean directions are given with a uniform criterion for the limitation of the normally distributed sub populations based on radiometric data some neogene volcanic sequences are subdivided into two complexes with separate area mean directions the possible influence of the areal tilts on the area mean palaeomagnetic directions is analysed and conclusions on the horizontal rotations are drawn the palaeomagnetic rotations are interpreted in the light of the author s kinematic model for the neogene history of the carpatho pannonian region the rotation of both the southeastern unit by about 100 clockwise and the northwestern unit by about 35 anti clockwise are supported by palaeomagnetic data in addition some rotations predicted by the model are documented by palaeomagnetic data as follows 1 the anti clockwise rotation of the flanks of the darno line gemer bukk area with sinistral displacements in the early miocene 2 the anti clockwise rotations along the southeastern margin of the northwestern unit with sinistral displacements before 10 ma 3 the anti clockwise rotation of the ukrainian carpathians before 12 ma some palaeomagnetic rotations have not been predicted by the kinematic model but occur in the regions where deformations are quite probable these unexpected palaeomagnetic rotations throw a new light on some details of the kinematics 1 the advance of the outer west carpathian nappes later than 14 ma is documented by the increased palaeomagnetic inclination of the pieniny andesites 2 the movement of the central and inner west carpathians later than 12 ma is recorded in the palaeomagnetic rotations of the central and east slovakian volcanites this movement seems to reflect a bay of thin crust within the underthrust european plate margin or more precisely the reaction of the eastern contour of the bay on the drag of the northwestern unit towards the northeast by the still rotating southeastern unit the rotation and deformation of the northwestern unit in the miocene have to be taken into account in the reconstructions of the pre neogene situations since they intensely influenced the arrangement of all older palaeotectonic and palaeogeographic zones the southeastern unit also suffered intense rotation and deformation in the miocene unfortunately there are almost no palaeomagnetic data available for its tertiary complexes

super deep reflection profiling exploring the continental mantle lid abstract reflections from the continental mantle lithosphere have now been reported from common mid point cmp profiles at numerous sites around the world these reflections include 1 distinct dipping events that appear to originate in the lower crust and continue some distance into the upper mantle 2 kilometer scale discontinuous diffusely distributed or isolated reflections at varying depths 3 continuous sub horizontal reflections tens of kilometers in extent often interpreted as the base of the lithosphere in spite of a wide variation in depth and 4 very deep reflections in the mantle lithosphere 125 km the first category of mantle reflections is most commonly observed in proximity to crustal sutures lending credence to their interpretation as fossil subduction zones isolated discontinuous mantle reflections are by far the most common and though observed in both ancient and modern collisional environments are difficult to interpret due to both their limited extent and a corresponding lack of three dimensional control sub horizontal mantle reflections appear in disparate tectonic environments in at least nine locations worldwide of special note are the various reports of specific reflections at 80 100 km 22 24 s twt in the north sea eastern europe and the urals which suggest a more ubiquitous mantle horizon the deepest events imaged thus far in a controlled source cmp survey are the super deep mantle reflections of the urseis project that have been identified at 135 165 km 35 43 s and 225 km 56 s of the many reported mantle reflections these are the most likely to be associated with the base of the lithosphere although such an interpretation averts the question of why an essentially thermo mechanical transition should be reflective at high frequencies 10 20 hz

the timan pechora basin northeastern european russia tectonic subsidence analysis and a model of formation mechanism abstract we study the tectonic subsidence of the timan pechora basin by using a number of exploration wells and lithologic stratigraphic sections along deep seismic sounding profiles the tectonic analysis reveals a rapid phase of subsidence in the late devonian followed by a slower one we suggest a two phase model of persistent subsidence in the basin 1 in the ordovician 505 438 ma b p and 2 in the silurian 438 408 ma b p the thinning of the lithosphere beneath the basin due to these rift phases led to the uplift of the asthenosphere and to partial melting we explain the vertical motions of the crust by a magmatism eclogitization model the formation mechanism includes accumulation of magmatic melt below the lithosphere asthenosphere boundary in the early middle devonian a phase transformation of magmatic material to eclogites during the cooling and a flow in the uppermost mantle induced by the evolved heavy bodies since the late devonian we construct a two dimensional numerical model of the flow in the asthenosphere compute this flow and describe the resulting changes in surface topography the model is in keeping with observed gravity seismic and subsidence data using the results of tectonic subsidence analysis and the model we interpret the geodynamic evolution of the timan pechora basin and discuss the effect of eclogitization in the uppermost mantle on the basin development

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

a refined apparent polar wander curve for the transdanubian central mountains and its bearing on the mediterranean tectonic history abstract recently obtained palaeomagnetic data have given a fairly detailed apparent polar wander a p w curve for the mesozoic of the transdanubian central mountains which has been compared with other a p w curves from the mediterranean area the similarity of the a p w curves enabled a unified a p w curve to be constructed for the central mediterranean region it has been shown that the new mediterranean a p w curve is in full agreement even in details with the updated a p w curve for africa this confirms the existence during the entire mesozoic of a single central mediterranean megatectonic unit rigidly attached to africa its positions with respect to the pole can now be traced back in more detail than before

deep structure of the continental lithosphere in an unextended orogen an explosive source seismic reflection profile in the urals urals seismic experiment and integrated studies urseis 1995 explosive source near vertical seismic reflection data from the urals seismic experiment and integrated studies urseis profile displays an image of orogenic lithosphere to depths of 225 km the reflective crustal section is characterized by dipping crustal scale shear zones and a pronounced crustal root that show no evidence of overprinting by extension and are preserved since late paleozoic collision toward the east the uralian crustal root is composed of a partially subducted volcanic arc while to the west it has affinity with the east european platform the reflection character of the moho varies across tectonic strike and implies that the moho 1 acted as a structural detachment beneath the trans uralian zone 2 forms a tectonic boundary between terranes beneath the east uralian zone 3 consists of a transitional zone from eclogitized lower crust to peridotitic upper mantle in the crustal root 4 is a 3 9 km thick reflective zone beneath the fold and thrust belt and 5 is a boundary 200 m thick in the west mantle reflections at 80 100 km depth 22 24 s may represent a continuation of the main uralian fault into the subcrustal lithosphere alternatively these reflectors may signify paleozoic or younger mantle shear zones or the top of a zone of partial melting in the upper mantle deeper mantle reflections at 140 160 km 35 45 s and 225 km 55 s may image mafic intrusions at the base of the lithosphere or along localized shear zones in the upper mantle

moho reflectivity and seismic signal penetration abstract analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by lithoprobe and the consortium for continental reflection profiling cocorp changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth results from montana show pronounced increases in average frequency and slight drops in amplitude associated with a die out of reflections at the moho results from quebec show amplitude and frequency change occurring through the moho on lines with and without moho reflections these contrasting observations confirm both adequate signal penetration and that the differences in moho reflectivity are representative of geologic differences

discussion of origins of hinterland basins to the rif betic cordillera and carpathians abstract the hinterland basins of the rif betic cordillera western alboran sea and the carpathians pannonian basin are in part composed of a thick in excess of 3 4 km thermal sag basin sequence the underlying crust has been thinned to 25 km in parts of the pannonian basin and to 15 km in parts of the alboran sea modern tectonic models for these areas envision tectonic collision and thickening of the crust followed by crustal thinning either by gravitational collapse back arc extension or a combination of the two contemporaneous with thrusting basin and range type extension has been envisioned for parts of these extensional collapse systems extensional features are common and they interact in a complex way with strike slip systems flexural cantilever modelling agrees with earlier mckenzie type pure shear modelling of the extensional systems in suggesting that extension alone cannot generate sufficient thermal decay to explain the great thickness of the sag basins associated with the alboran sea and pannonian basin consequently it is suggested that uplift of the asthenosphere accompanying subduction roll back is an important factor in the evolution of these basins roll back of the mantle lithosphere permits hot asthenospheric mantle to upwell and lie close to the base of the crust initially uplift and erosion of the crust overlying the asthenosphere occurs as the thermal perturbation cools the crust thermally subsides to produce a large sag basin

sub annual moraine formation at an active temperate icelandic glacier this paper presents detailed geomorphological and sedimentological investigations of small recessional moraines at fjallsjokull an active temperate outlet of oraefajokull southeast iceland the moraines are characterised by striking sawtooth or hairpin planforms which are locally superimposed giving rise to a complex spatial pattern we recognise two distinct populations of moraines namely a group of relatively prominent moraine ridges mean height 1 2 m and a group of comparatively low relief moraines mean height 0 4 m these two groups often occur in sets systems comprising one pronounced outer ridge and several inset smaller moraines using a representative subsample of the moraines we establish that they form by either i submarginal deformation and squeezing of subglacial till or ii pushing of extruded tills locally proglacial glaciofluvial sediments are also incorporated within the moraines during pushing for the first time to our knowledge we demonstrate categorically that these moraines formed sub annually using repeat uncrewed aerial vehicle uav imagery we present a conceptual model for sub annual moraine formation at fjallsjokull that proposes the sawtooth moraine sequence comprises i sets of small squeeze moraines formed during melt driven squeeze events and ii larger push moraines formed during winter re advances we suggest the development of this process form regime is linked to a combination of elevated temperatures high surface meltwater fluxes to the bed and emerging basal topography a depositional overdeepening these factors result in highly saturated subglacial sediments and high porewater pressures which induces submarginal deformation and ice marginal squeezing during the melt season strong glacier recession during the summer driven by elevated temperatures allows several squeeze moraines to be emplaced this process form regime may be characteristic of active temperate glaciers receding into overdeepenings during phases of elevated temperatures especially where their englacial drainage systems allow efficient transfer of surface meltwater to the glacier bed near the snout margin

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

the crustal and upper mantle geophysical signature of narrow continental rifts in the pannonian basin summary the pannonian basin provides a very good opportunity to study extensional tectonics the image of narrow continental rifts which represent one of the modes of continental extensional tectonics was studied in detail in the bekes basin in order to constrain the lithosphere structure beneath the narrow rifts in the pannonian basin density models and interpretation of magnetotelluric measurements along the pannonian geotraverse are presented the results obtained agree with the most recent deep seismic data most of the narrow rifts in the pannonian basin are characterized not only by thinner crust but also by thinner lithosphere a typical phenomenon of the gravity field over the central part of the narrow rifts in the pannonian basin is the existence of a relative local gravity high these gravity highs are probably due to intrusions of high density masses into the lower crust and lower part of the upper crust beneath the narrow rifts which are related to the extension of the basins subbasins

tectonic rotations south of the bohemian massif from palaeomagnetic directions of permian red beds in hungary abstract palaeomagnetic studies were carried out in permian red beds of the balaton highlands the mecsek mountains and the bukk mountains of hungary statistically well defined directions were obtained from six localities in the balaton highlands and two localities in the mecsek mountains no meaningful results were obtained from the bukk mountains three magnetic components were identified from red beds of the balaton highlands 1 in haematite with a very high unblocking temperature 700 c interpreted as a permian magnetization d c 79 i c 11 k 24 95 13 6 in six samples from three beds in a single locality 2 a secondary but ancient component residing mainly inmaghemite d 314 i 49 k 48 95 10 0 in 84 samples from six localities with a within locality scatter increasing on unfolding and 3 a direction parallel to the present field d 7 i 62 k 46 95 7 7 in nine samples from a single locality for the balaton highlands the component 1 direction agrees with directions obtained from permian red beds and volcanics in the eastern part of the southern and eastern alps and the inner west carpathians all show large apparent rotations relative to stable europe since the permian component 2 is of post folding post aptian age its direction agrees with known late cretaceous directions from the transdanubian central mountains which also show significant counterclockwise rotation relative to stable europe the characteristic magnetization for the mecsek mountains resides in haematite and may be primary the directions indicate only a slight net counterclockwise rotation of the mecsek mountains with respect to stable europe since the permian

observations of seismic reflectors in the lower lithosphere beneath the skagerrak the lithosphere the rigid outer shell of the earth includes both the crust and the upper mantle in the past decade the complex continental crust and underlying shallower mantle lithosphere have been extensively studied using reflection seismology but the lower lithosphere has not been successfully imaged here we report the observation of continuous and virtually horizontal reflectors in the lower continental lithosphere down to depths of 100 110 km below the skagerrak scandinavia these are apparently not structurally associated with the crust whereas previously reported mantle reflectors such as those observed by the birps group around the british isles1 2 are often located near large faults in the overlying crust like the crust the mantle lithosphere shows a seismically transparent upper and a reflective lower part thus the processes that form seismic reflectors seem to have an affinity for ductile rheological environments in the mantle as well as in the crust our observations are consistent with a model consisting of a mechanically strong upper lithosphere underlain by a thermal boundary layer which separates it from the convecting asthenosphere

gravity constraints on the crustal structure and slab evolution along a transcarpathian transect abstract a new unified bouguer anomaly map of the carpathian arc and the pannonian basin has been compiled from previously prepared and recently published gravity maps in order to constrain the crustal structure and the tectonic evolution of the area a two dimensional gravity model is presented along a western carpathians pannonian basin southern carpathians transect the model is based on deep seismic lines where available and on detailed geological sections the results confirm that the extensional crustal structure of the pannonian basin revealed by deep seismic surveys agrees with the gravity data furthermore they suggest that the western and southern carpathians area at different stages of their evolution the subducted oceanic slab under the western carpathians has already been assimilated to the asthenosphere while a crustal slab is still present under the southern carpathians these findings are compatible with the observation that the last major phase of crustal shortening terminated at the early middle miocene in the western carpathians but continued throughout the pliocene in the southern carpathians

death valley bright spot a midcrustal magma body in the southern great basin california a previously unrecognized midcrustal magma body may have been detected by cocorp deep seismic reflection profiles in the death valley region of the southern great basin high amplitude relatively broad band reflections at 6 s 15 km are attributed to partially molten material within a subhorizontal intrusion this bright spot extends laterally at least 15 km beneath central death valley a moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000 yr old basaltic cinder cone the fault zone is inferred to have been a magma conduit during the formation of the cinder cone vertical variations in crustal reflection character suggest that the death valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and or intruded lower crust the death valley bright spot is similar to reflections recorded by cocorp in 1977 in the rio grande rift where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20 km depth

seismic image of the deep crust at the eastern margin of the alps austria indications for crustal extension in a convergent orogen abstract a 39 km long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the eastern alps austria the ese wnw trending low fold seismic line crosses austroalpine basement units and extends approximately from 20 km west of the penninic window group of rechnitz to 60 km sse of the alpine thrust front the explosive source seismic data reveals a transparent shallow crust down to 5 km depth a complexly reflective upper crust and a highly reflective lowermost crust the upper crust is dominated by three prominent west dipping packages of high amplitude subparallel reflections the upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low angle normal shear zones related to the miocene exhumation of the rechnitz metamorphic core complex in the western portion of the upper crust east dipping and less significant reflections prevail the lowermost package of these reflections is suggested to represent the overall top of the european crystalline basement along the western portion of the line the lower crust is characterised by a 6 8 km thick band of high amplitude reflection lamellae typically observed in extensional provinces the moho can be clearly defined at the base of this band at approximately 32 5 km depth due to insufficient signal penetration outstanding reflections are missing in the central and eastern portion of the lower crust we speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower moho in the east preferable at about 30 5 km depth the high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust the latter interpreted to represent broad extensional mylonite zones emphasises the latest extensional processes in accordance with eastward extrusion

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

on the nature of mantle heterogeneities and discontinuities evidence from a very dense wide angle shot record abstract a seismic survey with a receiver spacing of 50 m provided one of the most densely sampled wide angle seismic reflection images of the lithosphere this unique data set recorded by an 18 km long spread reveals that at wide angles the shallow subcrustal mantle features high amplitude reflectivity which contrasts with a lack of reflectivity at latter travel times this change in the seismic signature is located at approximately 120 150 km depth which correlates with the depth estimates of the lithosphere asthenosphere boundary lab of previous dss studies this seismic signature can be simulated by two layer mantle model both layers with similar average velocities differ in their degree of heterogeneity the shallow heterogeneous layer and the deeper and more homogeneous one correlate with the lithosphere and the asthenosphere respectively studies involving surface outcrops of ultramafic massifs and mantle xenoliths infer that the upper mantle is a heterogeneous mixture of ultramafic rocks lherzolites harzburgites pyroxenites peridotites dunites and small amounts of eclogites laboratory measurements of physical properties of these mantle rocks indicate that compositional variations alone can account for the wide angle reflectivity a temperature increase would homogenize the mixture decreasing the seismic reflection properties due to melting processes it is proposed that this would take place below 120 150 km 1200 c the lab