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

role of fluid mixing and wallrock sulfidation in gold mineralization at the semna mine area central eastern desert of egypt evidence from hydrothermal alteration fluid inclusions and stable isotope data abstract the semna gold deposit is one of several vein type gold occurrences in the central eastern desert of egypt where gold bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks the semna gold deposit is related to a series of sub parallel quartz veins along steeply dipping wnw trending shear zones which cut through tectonized metagabbro and granodiorite rocks the orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre existing fault segments textural structural and mineralogical evidence including open space structures quartz stockwork and alteration assemblages constrain on vein development during an active fault system the ore mineral assemblage includes pyrite chalcopyrite subordinate arsenopyrite galena sphalerite and gold hydrothermal chlorite carbonate pyrite chalcopyrite and kaolinite are dominant in the altered metaggabro whereas quartz sericite pyrite kaolinite and alunite characterize the granodiorite rocks in the alteration zones mixtures of alunite vuggy silica and disseminated sulfides occupy the interstitial open spaces common at fracture intersections partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins petrographic and microthermometric studies aided recognition of co 2 rich h 2 o rich and mixed h 2 o co 2 fluid inclusions in the gold bearing quartz veins the h 2 o co 2 inclusions are dominant over the other two types and are characterized by variable vapor liquid ratios these inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids the generally light 34 s of pyrite and chalcopyrite may suggest a magmatic source of sulfur spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226 267 c and 350 1100 bar under conditions transitional between mesothermal and epithermal systems the semna gold deposit can be attributed to interplay of protracted volcanic activity dokhan volcanics fluid mixing wallrock sulfidation and a structural setting favoring gold deposition gold was transported as au bisulfide complexes under weak acid conditions concomitant with quartz sericite pyrite alteration and precipitated through a decrease in gold solubility due to fluid cooling mixing with meteoric waters and variations in ph and f o 2

strain analysis of wrench faults and collision tectonics of the arabian nubian shield a conjugate set of regional scale wrench faults produced by collision of converging plate boundaries about 500 700 m y ago extended across the arabian shield five major wrench fault zones distributed across the precambrian continent were subjected to finite strain analysis all were found to have undergone ductile strains in addition to brittle fault displacements four of the zones together enabled a composite strain profile to be constructed east west across the exposed precambrian continental crust that yielded an estimate of a total sinistral displacement of 280 120 km across the najd fault system the strain fields of all the zones deviated from simple shear strain with systematic heterogeneous variations in extensional strain across the zones finite shortening strains increased in intensity eastwards across the continent and were accompanied by a change in deformation mechanism which indicated that the zones in the east formed at higher crustal levels than the zones in the west the conjugate

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

the succession of ice flow patterns in north central quebec labrador canada abstract quebec labrador canada is one of the postulated dispersal centres of the late wisconsinan laurentide ice sheet and also contains an unusually complex and rich glacial geological record a reliable reconstruction of glaciation history represented by this record would improve our comprehension of the dynamics of the eastern sector of the laurentide ice sheet such as the evolution of the basal thermal regime and the timing of enhanced ice flow into ungava bay we mapped glacial landforms over a 200 000 km2 area using aerial photographs and conducted a detailed field based survey of the central part of the area to establish the relative age of crosscutting glacial geological features the position of the last ice remnant was south of ungava bay well north of what has commonly been perceived as the location for this dispersal centre southward radial flow from a northward migrating and predominantly cold based dispersal centre was the last glacial event in north central quebec labrador a widespread occurrence of older ice flow traces towards the southwest was detected in some areas large drumlins formed during this ice flow event occur in alignment with landforms formed during the last deglaciation this older southwestward flow is however overprinted by and therefore predates traces of northward convergent flow into ungava bay ungava bay flow traces of a northeastward flow event and in some areas traces of the deglacial flow event flutes and glacial striae indicating ice flow towards northeast are superimposed on the ungava bay flow the northeastward ice flow system has been recognised across the mapped area and because of its direction and regional significance we infer formation approximately during the last glacial maximum

constriction in a transpressive regime an example in the iberian branch of the ibero armorican arc abstract detailed strain analyses in the armorican quartzites have been performed in various places within the centro iberian zone of western europe the observed strain pattern shows that in the inner ibero armorican arc as well as near the hinge zone sinistral transpression produces constrictive ellipsoids in the outer arc away from the hinge zone the deformation regime produces plane strain to oblate strain ellipsoids some of these data clearly contradict existent kinematical models which associate constriction with transtension and consider transpression zones to generate oblate strain ellipsoids however newly developed models show that it is possible to obtain a wide range of finite strain ellipsoids in transpression zones if one takes account of lateral escape axial depression and volume change the geodynamic implications of the strain analyses suggest generation of constriction in a transpressive regime caused by lateral expulsion in front of the cantabrian indenter that produced the ibero armorican arc this mechanism was responsible for the variscan wrench component in western europe which was mainly sinistral in iberia and dextral in britanny in iberia the displacement by lateral escape was easily accommodated by subduction of the south portuguese terrane under the ossa morena zone of the iberian terrane

erosion of sognefjord norway sognefjord is formed by a combination of exploitation of rock structure and subaerial and subglacial processes the fjord system follows zones of rock structural weakness above sea level the landforms are due mainly to subaerial processes and somewhat surprisingly not to glacial activity removal of erosive products and deepening of the fjord below sea level is due to glacial erosion

paleomagnetism of the istria peninsula yugoslavia 280 core samples were collected from upper jurassic cretaceous and eocene sediments outcropping in the istria peninsula yugoslavia due to the very low intensities of the initial natural remanent magnetizations more than 50 of the collection consisting mainly of rock samples of jurassic and eocene sediments was not suitable for paleomagnetic studies the cretaceous samples yield a mean paleomagnetic pole lat 53 long 275 and 95 4 8 which is significantly different from the african and european paleomagnetic poles of the same age the position of the istria peninsula on the autochthonous adriatic platform allows the result to be interpreted as applicable to all the autochthonous periadriatic region this new paleomagnetic result indicates that the autochthonous adriatic platform rotated counterclockwise over an angle of about 30 with respect to africa in post mesozoic times

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

quaternary erosion in the sognefjord drainage basin western norway abstract the preglacial paleic landscape configuration in the sognefjord drainage basin of western norway is reconstructed and the total quaternary erosion is quantitatively determined to 7610 km 3 by subtracting the present topography from the reconstructed preglacial surface with an average erosion of 610 m during the suggested time span of 600 000 yr when scandinavian ice sheets reached the continental shelf off western norway mean quaternary erosion rates are calculated to ca 1 02 mm yr using the average relief of 2000 m along the fjord however the rate of glacial erosion was 3 3 mm yr taking into account the selective nature of glacial erosion by ice streams along the fjord an estimate of 2 0 5 mm yr is likely limited to the east by the main watershed a rough estimate of the total quaternary glacial erosion in the fjord region of western norway is calculated to about 35 000 km 3 of rock with a soft sediment porosity of 30 50 this corresponds to about 45 500 52 500 km 3 of continental shelf deep sea sediments

miocene pliocene tectonic evolution of the slovenian periadriatic fault implications for alpine carpathian extrusion models the periadriatic line pal is a remarkable several hundred kilometer long fault system of the alpine orogen its dextral character was documented by several authors using diverse criteria but detailed kinematics and timing of movements had not been investigated along its whole length structural and paleomagnetic measurements mapping and stratigraphic and sedimentological studies have helped to unravel the miocene pliocene evolution of the slovenian segment of the pal brittle deformation was characterized by nw se to n s compression and perpendicular tension deformation has resulted in dextral strike slip faulting folding and tilting of beds the first transpressional event corresponds to the first phase of lateral extrusion of the east alpine western carpathian northern pannonian block in the early miocene 24 17 5 ma after a short period of transtension during the karpatian 17 5 16 5 ma dextral transpression reoccurred during the middle miocene to pliocene and lasted up to the quaternary middle miocene dextral slip can be connected to the second phase of extrusion the highly deformed rocks within the dextral shear zones show variable clockwise sometimes counterclockwise rotations the mechanism of rotation seems to be complex ranging from regional rotation to local folding due to pure or simple shear domino type rotation

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

evolution of the stress fields in the carpatho pannonian area during the neogene abstract observations of microtectonic faults in and around the pannonian basin suggest that the tertiary evolution of the area was controlled by large scale strike slip fault activity statistical analysis of the fault pattern for different localities makes it possible to separate several conjugate sets of strike slip faults and related dilatational and compressive structures these stress fields have a distinct rotation pattern with time inversion of fault data suggests a characteristic rotation of the principal stress axes with time some rigid body rotations are indicated by paleomagnetic data there are basically three contrasting mechanisms to explain this observation 1 1 appropriate rotation of the regional e g european stress field 2 2 rotation of larger tectonic units e g microcontinents under a stable regional stress field and 3 3 coherent rotation of smaller detached continental blocks under a stable regional stress field our model for the neogene tectonic evolution of the pannonian area combines these mechanisms moderate microcontinent rotation may have occurred during the paleogene early miocene in the early middle miocene the microcontinents were dissected by a system of strike slip and normal faults created by a n s compressive e w extensive stress field gravity spreading of the pannonian units and block rotation within bounding strike slip faults are suggested to explain rotated magnetic directions and fault patterns in the late neogene clockwise rotation of the general stress field by at least 40 is thought to be responsible for the reactivations of older strike slip faults and for the creation of new sets this change in the orientation of the stress field is related to the eastward and then southward shift of the carpathian subduction activity

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

structural applications and discussion of a paleomagnetic post paleozoic data base for the central mediterranean abstract a post paleozoic paleomagnetic data base has been compiled for the central mediterranean with the intention of listing directions at the highest possible level of data combination the value of the entries depends on the number and quality of data available the most reliable data are those based on regionally distributed localities and sites where the age is known to a stage and where tectonic correction has been made these represent a whole tectonic unit in a given period of time the lowest quality directions are those based on few samples and isolated in space and time in the tectonic interpretation of the listed data this variation in quality of data has been taken into consideration indications for cenozoic movements having a fundamental influence on the now observable distribution of the clockwise and counter clockwise rotated paleomagnetic directions were treated first and those for older movements afterwards microplate block and nappe movements were considered as possible mechanisms for the rotation of these the last two mechanisms explained satisfactorily the occasional local deviations from a general pattern other differences however such as those which exist between the northern calcareous alps or the tisza unit and the counter clockwise rotated superunit of the central mediterranean which are large and persisted through a longer period of time are best explained by independent movement of the whole unit

review of late proterozoic sutures ophiolitic melanges and tectonics of eastern egypt and north east sudan the distribution structural relations and ages of ophiolites and ophiolitic melanges in the eastern deserts of egypt and north east sudan and their relation to arc and post arc magmatism are used to infer the probable positions of sutures and the directions of obduction and subduction the ages of the successive tectonic stages in the terranes decrease northwards by about 100 ma in the region discussed obduction was north westwards the obducted sheets of ophiolite and ophiolitic melange extending about 500 km north from the onib sol flamed suture may all represent a single back arc basin the direction of subduction is still not clearly proved more geochemical evidence of arc magmatic polarity is needed the late tectonic north westward thrusting the nw se stretching lineation the transpressional north south hamisama shear zone and the nw se sinistral najd faults are all attributed to north westward movement present coordinates of terranes towards the older craton west of the nile the najd faults are interpreted as intracontinental transform faults

late paleozoic rotations of corsica and sardinia new evidence from paleomagnetic and k ar studies abstract a detailed paleomagnetic investigation of so called permian volcanics was undertaken in southeastern and northwestern sardinia k ar dating on separate minerals representing 30 sites on corsica and sardinia showed that the first calc alkaline cycle had taken place in westphalo stephanian times and the second alkaline one at the end of the stephanian 246 paleomagnetic specimens from 28 sites of southeastern sardinia and 2 sites from the northwest were demagnetized by alternating field and thermal processes the directions of characteristic magnetizations are d 85 i 6 95 5 for the volcanics of southeastern sardinia and d 126 i 1 for the nurra samples these mean directions differ from those of northwestern corsica and northern sardinia gallura the paleomagnetic results display good grouping for each volcanic complex but exhibit a geographical dispersion of the mean directions this scattering suggests significant motions in late hercynian times

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

new constraints on red sea rifting from correlations of arabian and nubian neoproterozoic outcrops new constraints on the mechanics of red sea opening were obtained by correlating neoproterozoic outcrops of the arabian and nubian shields along two thirds of the red sea coastlines using a mosaic of 23 landsat thematic mapper scenes 5 105 km together with field geochemical and geochronological data we identified and mapped lithologic units mobile belts and terranes within the arabian and nubian shields features best align if arabia is rotated by 6 7 around a pole at latitude 34 6 n longitude 18 1 e implications of our reconstruction include 1 the amount of continental crust underlying the red sea is small because the restored red sea coasts are typically juxtaposed 2 only a single pole is needed implying that the arabian and nubian shields were rigid plates during red sea rifting 3 coastlines reorient to align with preexisting structures suggesting the rift propagated in part along pre existing zones of weakness 4 large sinistral displacements of up to 350 km along the red sea are not supported 5 the pole is inconsistent with the pliocene pleistocene motion along the dead sea transform pole 32 8 n 22 6 e 0 5 joffe and garfunkel 1987 indicating that more than one phase of motion is required to account for the red sea opening however our pole is similar to that for the total motion along the dead sea transform pole 32 7 n 19 8 e 2 joffe and garfunkel 1987 suggesting that the motion between arabia and nubia was parallel to the total motion along the dead sea transform

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 last glaciation of west central ellesmere island arctic archipelago canada locally abundant ice marginal landforms lie in a 500 km long zone with a distal margin 10 60 km west of the margins of modern ice caps on central ellesmere island much of this drift belt at the heads of the fiords was deposited by the oscillating margin of a coalesced predecessor of the modern ice caps between 9000 and 7000 bp the ice continued to retreat east of the present margin and readvanced to its modern limit in a middle and late holocene cooler climate unweathered but undated till and striations at the base of the drift suggest that the belt does not mark the western limit of central ellesmere island ice in the last glaciation the limit lies an unknown distance downfiord glaciers in the fiords may have floated no reliable evidence was found for a complete ice cover of western ellesmere island and eureka sound in the last glaciation nevertheless much of central and southern ellesmere island and devon island may have been glaciated by a regime that left few erosional or depositional landfo

glaciomarine sedimentation in disraeli fiord high arctic canada abstract disraeli fiord represents a glaciomarine environment unique to the north coast of ellesmere island featuring floating glacier tongues multi year landfast sea ice and a strongly stratified water body which results from enclosure of the fiord by ward hunt ice shelf it is a very low energy depositional environment even at sites proximal to glaciers sediment enters the fiord by interflow and is deposited through suspension settling ice rafting is precluded by the year round sea ice cover sedimentation rates in the inner fiord since about 7000 yrs b p are estimated to be 5 10 cm ka a reflection of the limited sediment influx from the cold based glaciers which contact the fiord changes in sediment structure through the holocene reflect changes in the rate and process of sediment input as well as changes in fiord circulation all of which relate to regional climate during deglaciation early holocene sedimentation rates were orders of magnitude greater than at present subglacial meltwater was the dominant source of inflow with sediment transported in turbid plumes as over or interflow and deposited through suspension settling this is supportive of other studies indicating that a pronounced climatic amelioration led to warm based glaciers at this time

geomorphological evidence of cold based glacier activity in south victoria land antarctica cold based glaciers have long been recognized as capable of covering and protecting landscapes however recent studies of modern cold based glaciers in antarctica show that in some situations erosion deformation and deposition can occur recognizing the dual ability of cold based glaciers to protect and preserve surfaces on the one hand and erode and modify on the other is important for correctly interpreting the often subtle imprint of cold based glaciers on landscapes a range of geomorphological features related to cold based glacier activity has now been documented along with an improved understanding of cold based glacier structure processes and interaction with various substrates collectively this provides an enhanced ability to understand the impact of cold based glaciers on landscapes and reappraise the geomorphological record such insight allows recognition of previously unknown glacial events and better interpretations of the landscape exposure record this is particularly important at the margins of the antarctic ice sheets where past fluctuations in ice sheet volume and its contributions to post glacial sea level rise are poorly constrained this paper reviews the known geomorphological evidence associated with cold based glaciers in the south victoria land region of the transantarctic mountains it aims to provide progress towards a set of criteria for recognizing cold based glacier activity in other regions and to highlight the implications of cold based glacial activity for surface exposure studies and interpreting glacial history cold based glaciers are below pressure melting point throughout and are therefore frozen to the substrate and move primarily by internal deformation drewry 1986 this led to the long standing view that cold based glaciers cannot slide and are thus incapable of erosion and deposition and therefore protect the underlying surface while numerous studies have shown this to be true in many situations recent investigations of modern cold based glaciers have revealed that in some circumstances complex processes operate at subfreezing temperatures in the basal zone and that cold based glaciers can generate a variety of geomorphological features these studies have forced a reappraisal of the geomorphic effectiveness of cold based glaciers and have confirmed that they can both protect and modify landscapes e g fitzsimons 1996 atkins et al 2002 fitzsimons 2003 waller et al 2012 thus the debate has moved on and the challenge now is to understand the full range of processes and products associated with cold based glaciers and the implications for landscape development hambrey fitzsimons 2010 waller et al 2012 recognizing the often subtle features indicative of cold based glacial activity offers the potential to delineate and date previously unknown or poorly constrained glacial events this is particularly relevant for interpreting glacial history and modelling ice sheet reconstructions for example numerous surface exposure dating studies have been carried out on ice marginal landforms with the aim of constraining past variations in the ice sheet volume however many of these studies have produced inconsistent contradictory results including evidence of complex exposure histories suggesting episodes of burial and reworking by cold based ice e g strasky et al 2008 di nicola et al 2009 compared with warm based glaciers there are still relatively few studies of cold based glaciers and significant gaps in our understanding of the processes and products remains in addition lingering misconceptions regarding the geomorphic effectiveness of cold based glaciers and historic use of ambiguous terminology to describe cold based sediments and landforms mean that the evidence and implications of past cold based glacial events have yet to be fully appreciated by the wider geomorphological community this review provides a summary of known geomorphological features associated with cold based glaciers in the south victoria land region of antarctica it aims to provide a basis for recognizing similar features elsewhere and attempts to draw attention to the implications of cold based glacial geomorphology for the correct application of surface exposure dating studies and interpretation of cold based glacial history

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

crustal scale extension in the central pannonian basin abstract within the hungarian segment of the panonian basin a 100 km long seismic reflection profile images the sedimentary fill of two major depressions and their associated crustal signatures these deep 7000 m subbasins are asymmetric half grabens bounded along their western margins by low angle crust penetrating subparallel listric faults the hanging walls are intensely fractured and comprise pre tertiary basement and upper crustal rocks the footwalls are formed by uplifted and ductilely deformed middle to lower crust the crust is relatively thin 25 27 km and decoupled by well defined detachment faults the moho has a gentle northeasterly dip and a spatially variable seismic signature the seismic images imply that the present crustal setting of the area was developed contemporaneously through the northeastward extension of the pannonian basin

oligo miocene rotation of sardinia kar ages and paleomagnetic data of tertiary volcanics abstract conventional k ar dating was carried out on mineral separates mostly plagioclases from middle tertiary volcanics of sardinia along with paleomagnetic studies the close agreement of dates from the two methods establishes the rotation age of sardinia relative to europe within a rather short time span of 1 5 ma between 20 5 and 19 ma from correlation of our results with the geomagnetic time scale it is suggested that the rotation took place shortly after the magnetic anomaly 6

metamorphism and 40ar 39ar dating of the halaban ophiolite and associated units evidence for two stage orogenesis in the eastern arabian shield the arabian shield has long been recognized as a region where plate tectonic processes have been in action during most of the late proterozoic resulting in the amalgamation of the shield s five constituent terranes along four major suture zones studies thus far carried out on these collisional belts have concentrated on the origin and dating of intermediate to acidic igneous rocks and very little emphasis has been placed on the understanding of metamorphic processes operative during plate convergence the al amar suture separates the afif microplate from the ar rayn block in the easternmost section of the shield among several ophiolite occurrences in this suture the halaban belt is by far the largest and preserves within and around it a record of a number of thermal structural events related to the opening and eventual closure of a supra subduction zone back arc basin that existed in the period 695 680 ma the bimodal nature of 40 ar 39 ar dates from the halaban ophiolite and associated units is indicative of a two stage orogenesis climaxing at 680 and 600 ma instead of the previously proposed model of a single orogenic episode between 670 and 630 ma the first stage 680 ma is believed to be related to basin closure and ophiolite emplacement as the ar rayn island arc collided with the afif microcontinent the second episode 600 ma was the outcome of a major collisional event between the arabian craton and a large continental mass east of the ar rayn block

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

structural interpretation and tectonic evolution of a part of the najd shear zone saudi arabia using landsat thematic mapper data abstract digital landsat thematic mapper data were used to extrapolate structural features associated with the ajjaj shear zone from well characterized to less well known areas the satellite images were enhanced using a linear mixing model with the spectra of known rock types measured from the images as endmember components the approach uses information for all six spectral bands the effects of desert varnish coating in modulating the spectral response of rock exposures are specifically accounted by using of an empirically derived coat component comparison between the lithologic component images produced from the unmixing procedure and published geologic maps demonstrate that these images can be effectively used to map brittle faults ductile shear zones and certain spectrally distinct lithologies by combining interpretation of the enhanced images with available geologic data it has been shown in this study that the ajjaj shear zone is dominantly a ductile structure characterized by the continuity of rotated layers across the shear zone with a minimum sinistral displacement of 25 to 30 km the northern boundary of this shear zone is dominantly ductile whereas the southern boundary consists of a continuous brittle displacement surface which juxtaposes contrasting lithologies several brittle faults within and crosscutting the ductile shear zone had previously been mapped as najd faults these faults are not strike slip in character and clearly postdate najd deformation in the middle of the area studied the ajjaj shear zone changes orientation from wnw to nnw this change corresponds spatially to the intersection of the shear zone with a n s oriented structure here named the hanabiq shear zone the hanabiq shear zone is one of a set of broadly n s trending dextral shear zones which crosscut the arabian shield proponents of the rigid indentor model for the origin of the najd fault system have suggested that these faults form a conjugate set to the nw trending sinistral najd faults our study presents clear evidence that the ajjaj shear zone postdates the hanabiq shear zone this supports recent criticisms of the rigid indentor model as applied to the najd the tectonic significance of the najd fault system remains as intriguing problem

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

the paleomagnetism of the upper cretaceous magmatic rocks in the banat area of south carpathians tectonic implications abstract forty four sites were sampled in upper cretaceous magmatic rocks in the western part of the southern carpathians banat area for a paleomagnetic study after detailed af demagnetization only 10 sites yielded primary remanent magnetizations the directions of which are similar to those identified in senonian magmatic rocks from the apuseni mountains and the ha eg and rusca montan basins the mean paleomagnetic pole for these three areas is compared with those from coeval formations of stable europe and stable africa this comparison shows a large clockwise rotation both with respect to africa and europe and a northward transport to reach their present day position during the tertiary the relations with adjacent zones of the tethys mobile belt are also discussed

mesozoic palaeomagnetism of the transdanubian central mountains and its tectonic implications abstract the mesozoic apparent polar wandering apw of the transdanubian central mountains determined from thermally isolated natural remanences at 13 localities shows a remarkable similarity to the mesozoic apw of africa in that they both exhibit the same loop like movement moreover the difference between the two apw s can practically be eliminated by a 35 clockwise rotation of the palaeodeclinations it is concluded therefore that the region of the transdanubian central mountains was part of the african adriatic plate up to some time in the cenozoic when it moved to its present position resulting in a 35 anticlockwise rotation relative to africa

palaeomagnetic evidence of tectonic rotations in the southern margin of the inner west carpathians abstract palaeomagnetic samples were collected at 33 triassic localities from three nappes and a pseudo autochthonous unit in the aggtelek rudabanya mountains in order to establish the rotation pattern of the area as a whole with respect to the enclosing principal continents and to check whether relative movements in the past between the various units that make up the region are resolvable by the palaeomagnetic data after detailed thermal demagnetization and careful data selection it has emerged that the entire study area was rotated anticlockwise with respect to both europe and africa i e in a similar manner and to a similar degree as other units in the central mediterranean region e g the transdanubian central mountains on the other hand small scale relative movements such as those between the aggtelek and rudabanya areas are not apparent in the present data

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

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

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

the solid body tilt of deformed paleohorizontal planes application to an archean transpression zone southern canadian shield abstract the solid body tilt sbt of deformed paleohorizontal planes is equal to the angle between their unstrained top direction and an upward pointing vertical axis kinematic models of important tectonic processes such as transpression and ductile thrusting have specific sbt requirements these models can be tested in ancient and modern orogens by determining the actual sbt of strata early magmatic sills and late horizontal intrusions with relict primary layering classical transpression models predict that except for sheath structures all folds in bedding be upright and horizontal i e that sbt values be close to zero within the hinge zones moreover all passive folding is accomplished by amplification of asperities and caused by the pure shear component of the transpression tensor nine sbt analyses were made in the archean crust of northwest ontario to test the transpression hypothesis north of the quetico wabigoon boundary western superior province central canada the nine sbt values do not discredit the hypothesis that the quetico fault marks an archean transpression zone which included the wabigoon rocks adjacent to the quetico metasediments

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

implications of a combined biostratigraphic and palaeomagnetic study of the umbrian maiolica formation abstract the study of a 275 5 m thick section of white pelagic limestones occupying the valley of the fonte del giordano river on the southern slope of mt montagnola has yielded a biostratigraphically controlled clear magnetic reversal pattern after thermal cleaning the magnetic stratigraphy of the lower 131 m of the section calpionellid zones is correlatable with the m sequence of oceanic magnetic anomalies between m 19 and m 14 the reversal stratigraphy of the upper 81 5 m of the section radiolaria zone has also been tied to the oceanic polarity time scale by making linear interpolation for a missing 63 m thickness underneath it besides the fonte del giordano section two berriasian outcrops each with a different bedding attitude were studied at gubbio and near cagli for tectonic tilt test giving positive results the mean palaeomagnetic pole position for the late jurassic early cretaceous after bedding correction is 19 1 288 2 k 148 7 95 10 2 n 3 confirming the presence of a large swing in the polar path a common behaviour of apparent polar wandering for the peri adriatic region during this time

subglacial preservation of valley morphology at amundsenisen western dronning maud land antarctica on the high altitude polar plateau of amundsenisen western dronning maud land east antarctica a subglacial valley with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta was studied by radio echo sounding in addition a small probably glacial valley was mapped within the same subglacial massif basal ice temperatures were calculated using field data on precipitation air temperature and ice sheet thickness discoveries of old landforms which have been preserved more or less intact beneath the former fennoscandian and laurentide ice sheets have received increasing attention during the last decade the aim of this study is to investigate whether preservation of landforms occurs under the east antarctic ice sheet and to discuss under that climatological and glaciological circumstances preservation may take place the results show that the ice sheet covering the investigated localities is frozen to bed and therefore has an insignificant erosional capability the observations suggest that a large scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold based ice sheet marginal zone for a long time period the process of glacial preservation recognized for bedrock features and tentatively observed for sediment accumulations should act on similar large scale landforms under any cold based ice sheet present or past on the basis of existing studies of the age and stability of the east antarctic ice sheet a middle pliocene age is suggested for the preserved landforms the presence of the presumed sediment filled valley further indicates that no prolonged periods of basal melting have occurred at the amundsenisen study area during the ice sheet history which includes the quaternary glaciation periods finally calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation 1997 by john wiley sons ltd

upper mantle reflectors modelling of seismic wavefield characteristics and tectonic implications summary deep seismic experiments on continental lithosphere generally reveal marked reflectivity from structures in the crust and a significant decrease in reflectivity from the upper mantle however reflected and refracted energy from mantle lithosphere are observed in both near normal incidence and wide angle data the origin of the reflective structures is a matter of debate hypotheses include remnant subduction zones shear zones fluids and seismic anisotropy through analytical and numerical modelling studies including full wavefield modelling we investigate seismic characteristic signatures generated from a variety of geologically plausible models we have found that both upper mantle shear zones of reduced velocity and density and remnant subduction slabs containing high density eclogites may contain sufficient seismic impedance contrasts to normal mantle peridotites to generate near normal incidence reflectivity wide angle energy originates from subduction slabs containing either high or lowvelocity eclogites whereas intermediate velocity eclogites are unlikely to produce significant wide angle phases in general energy of seismic phases originating from upper mantle zones of anomalous seismic velocities and densities is significantly increased if homogeneous zones are replaced by zones of inhomogeneous petrophysical properties resulting from constructive interference maximum wavefield anomalies are generated from sub moho dipping slabs of incomplete transformation of low velocity low density crustal material to high velocity highdensity eclogites localized shear zones generated in mantle peridotite generally do not produce significant wide angle energy only if highly inhomogeneous structures containing material of marked ca 10 per cent velocity and density reduction are present may shear zones be observed in wide angle data analyses of two specific deep seismic data sets mona lisa data from the north sea and babel data from the baltic sea show good agreement between observations and modelling results for dipping remnant subduction slabs containing small scale inhomogeneities associated with incomplete transformation our modelling results improve our possibilities of distinguishing between two often contrasting tectonic interpretations for dipping upper mantle seismic reflectors the remnant subduction and shear zone models

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

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

direct shear tests of materials from a cold glacier implications for landform development abstract the mechanical properties of materials at the ice bed interface exert a major control on glacier behaviour and define the nature and rate of subglacial erosion this paper presents new data on the strength and behaviour of basal ice and substrate of the suess glacier south victoria land antarctica which is a small alpine dry based glacier that has a basal temperature of 17 c a tunnel excavated in the glacier revealed a substrate composed of frozen sand and gravel and a basal zone that was 3 8 m thick from bottom to top the basal zone was composed of 1 8 m of stratified complexly deformed layers of ice and debris laden ice that was overlain by a 0 9 m thick layer of frozen sediment and a 0 8 m thick layer of discoloured ice that lies immediately beneath clean glacier ice direct shear tests were performed in the field on 36 samples using a strain rate of 0 85 mm h and on replicate samples in a laboratory at strain rates of 0 08 and 0 01 mm h the experiments performed in the field show that the average peak shear strength of substrate samples was 2 53 mpa which is almost twice as strong as the average value for basal ice 1 28 mpa and the glacier ice samples 1 39 mpa the laboratory experiments show that the behaviour and peak strength of samples sheared at the higher strain rates are considerably greater than the peak and residual strengths measured during the lower strain rate tests the direct shear tests suggest that the glacier substrate is unlikely to deform at the current temperature

paleomagnetic aspects of plate tectonics in the carpatho pannonian region paleomagnetic data available from the carpatho pannonian region are unevenly distributed at some places observations are sporadic at others the concentration is high in the latter case it is possible to decide whether the movements indicated are related to plate microplate displacements or are of local significance the paleomagnetically best known parts of the carpatho pannonian region belong to the transdanubian central range to the cserhat matra bukk area and to the mecsek villany zone comparison among paleomagnetic data sets characterizing each of them reveals fundamental differences i e points to their independent displacements during the mesozoic and cenozoic most of the isolated observations can be tied to one of the above mentioned paleomagnetic units via coeval mostly late cretaceous cenozoic data thus further subdivision of the carpatho pannonian area at least in the cenozoic does not seem required by available paleomagnetic observations

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

characteristics of romanian lithosphere from deep seismic reflection profiling abstract the tectonic units of romania contain both alpine orogenic regions and platforms the investigations of crustal structure by near vertical reflection seismic profiling are based on less than 100 lines unequally distributed and of limited lengths 5 30 km the seismic features studied are the crustal seismic reflectivity and transparency dipping events and diffractions and the moho and subcrustal reflectivity the major common features are a reflective sedimentary cover of mostly neogene layers a weak signal from the sedimentary basement boundary a quasi transparent upper crust a mid crust with short and subhorizontal events a transparent or layered lower crust and a moho that is either poorly visible or evidenced by a zone with reflection bands and some subcrustal reflections the moesian platform together with the adjacent carpathian foredeep show a mid crust with short and subhorizontal events a crust mantle transition zone in the west an unclear or prominent moho and some subcrustal reflectivity the northern part of the carpathian foredeep shows strong seismic layering of the lower crust the central part of the transylvanian depression is characterized by short and subhorizontal events at mid crustal level a slightly reflective lower crust with diffractions and a weak moho signal the southern sector of the pannonian depression is generally marked by a reflective lower crust the tectonic significance of these seismic signatures is interpreted as follows a brittle upper crust a decoupling mid crustal zone a lower crust with brittle to ductile behaviour a possible extensional or isostatic rebalanced moho and laminated zones with reduced viscosities at the lower crust and upper mantle levels a series of crustal models based on the reflection seismic sections are presented for several areas of romania

paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we first re examined the apparent polar wander curves for stable eurasia and africa since the triassic these curves were then combined together with curves of north and south america according to the kinematics of the atlantic ocean and a synthetic polar wander curve was given then most of the paleomagnetic results from the tethys mobile belt from the atlantic to the pamirs were analysed several groups of plates microplates and blocks can be seen first relatively stable regions like maghreb and sicily which have not moved much then we have a group formed by iberia sardinia italy and to a lesser extent corsica and the western and central alps for these blocks movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of africa but they are sometimes stronger to the east a major change takes place the north of the aegean sea and the ionian zone are clockwise rotated and these rotations are recent oligocene miocene for the first part pliocene to the present for the second part a major problem arises in turkey caucasus and iran paleomagnetic results indicate a position far to the south of eurasia and at the same time geological evidence is in favour of a position close to eurasia we discuss these discrepancies

gneiss cored interference dome associated with two phases of late pan african thrusting in the central eastern desert egypt abstract structural analysis of the wadi um had area in the central eastern desert ced of egypt reveals the presence of another nw trending elliptical gneissic and migmatitic domal structure west of gabal meatiq outlined by a thick schistose and mylonitized carapace the sheared carapace formed during a nw nnw ward thrusting event which emplaced thrust slices of ophiolitic melange dokhan volcanics and hammamat group sediments over the higher grade gneisses the gneisses are subdivided by retrograde shear zones of this event the sheared carapace is interpreted as the roof thrust of an imbricate stacked antiformal duplex a second thrusting and folding event associated with ne sw shortening deformed the earlier structures producing the dome and re thrusting low grade units against it as sw and ne dipping thrusts the features of this gneiss dome are inconsistent with it being a metamorphic core complex caution must be exercised in the recognition of transcurrent shears in the eastern desert since folded low angle thrust related mylonite zones may be steepened by folding producing steep shear zones with low pitching slip lineations extension bi directional thrusting and najd faulting in the ced may be explained in an escape tectonic deformation regime involving extrusion of material away from the pivotal collision zone in the mozambique belt to the south

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

the najd fault system saudi arabia and egypt a late precambrian rift related transform system the najd fault system is a complex set of left lateral strike slip faults and ductile shear zones that strike nw se across the precambrian of arabia and egypt this system was developed during the interval 540 620 ma it is up to 400 km wide with an exposed length of 1100 km inferred buried extensions of the najd give it a total length of 2000 km it is the best exposed and may be the largest pre mesozoic zone of transcurrent faulting on earth previous models for the najd fault system suggest it formed as a result of a major late precambrian continent continent collision this model is not preferred here because 1 the lack of evidence for a pre late precambrian continent to the east of the najd fault system 2 the difference between the orientation of the najd fault system and that predicted by slip line theory 3 the younger age of najd movements compared with that of collisional sutures in the arabian shield and 4 lack of evidence for wide spread crustal uplift that would be expected to accompany collision a new model for the origin of the najd fault system accounts for each of these objections the najd fault system formed in response to a broad zone of nw se directed crustal extension that accompanied juvenile continental crustal formation in northernmost afro arabia this model also accounts for the following observations 1 strands of the najd parallel the direction of extension in the north eastern desert of egypt and sinai 2 the timing of the principal rifting movements ca 575 600 ma overlap with those of the najd ca 560 620 ma 3 in spite of observation 2 the najd fault system is not recognized in northernmost afro arabia instead the najd deformation becomes increasingly ductile and these zones are more commonly intruded by sheared and foliated granites as the principal zone of extension is approached the najd fault system thus represents a set of continental transforms developed in response to a major episode of late precambrian extensional continental crust formation in northernmost afro arabia

proterozoic deformation of the east saharan craton in southeast libya south egypt and north sudan abstract the basement areas in southeast libya south egypt and north sudan west of the nile between gebel uweinat and the bayuda desert are part of an approximately 1000 km wide complexly folded polymetamorphic zone with a regional n nne ne ene trend of foliation and fold axis since this belt extends southwestward into the area of zalingei in the southern darfur block west sudan it is named the northern zalingei fold zone sr and nd isotopic studies suggest that this zone is older than pan african and further indicate that apart from archean rocks in the gebel uweinat area this belt is of early middle proterozoic age an early middle proterozoic three stage deformational and anatectic event established the present day fold and fault geometry in the western parts of this zone in the gebel uweinat gebel kamil area the pan african tectono thermal episode was most effective in the eastern part of the belt near the boundary with the nubian shield volcano sedimentary ophiolite granitoid assemblages it caused migmatization granite emplacement mylonitization and large scale wrench faulting which was related to late proterozoic accretionary and collisional events of the arabian nubian shield with the margin of the east saharan craton

the arabian infracambrian extensional system abstract during the late precambrian and early cambrian about 600 540 ma extensive left lateral faulting along the complex nw se trending najd fault system cut across the arabian shield this tectonic episode was accompanied by nw se directed extension in northern egypt and the sinai peninsula based on these relationships stern 1985 interpreted the najd fault system as a set of transform faults emanating from a rift basin in ne egypt contemporaneous to this system in the western part of the arabian plate and northern egypt extensive evaporitic basins developed in northern pakistan southern oman and the arabian gulf and zagros mountains these basins are generally bounded by nw trending faults such as the najd hawasina and zagros faults and ne trending faults such as the ghudun khasfah and dibba faults these evaporitic basins are interpreted as rift basins such that stern s 1985 extensional model in northern egypt may be expanded regionally to include the eastern part of the arabian plate and the indian subcontinent this broader interpretation implies that after the e w directed collisional tectonics of the late precambrian in western arabia and africa which terminated around 640 600 ma the arabian plate experienced nw se directed crustal extension and continental break up

extensional fault related folding northwestern red sea egypt the duwi and hamadat areas on the northwestern margin of the red sea rift system exhibit superb outcrop examples of kilometric scale extensional fault related folds in this area the nakheil and hamadat faults of the rift border fault system consist of a series of wnw and nw trending segments that link through breached relay ramps the fault hanging walls are characterised by a series of offset synclines with non linear axial traces sub parallel to the faults the synclines are doubly plunging and in the immediate hanging wall to the normal faults the strata dip sub parallel to the fault these folds result from along strike displacement variations on the individual fault segments together with extension related fault propagation folding as the faults propagated upwards through a highly anisotropic pre rift sedimentary section in the duwi area the axial traces of the synclines are offset and bend in the regions of relay ramps in the hamadat area the synclines associated with individual fault segments are separated by transverse hanging wall anticlines that formed where segment linkage occurred numerical simulations of extension related fault propagation folding using a trishear model produce geometries that closely match those of the duwi and hamadat folds the dimensions of these hanging wall synclines and their relationships to fault displacement variations indicate that they are formed by extensional fault propagation folding rather than by frictional drag along the fault surface

strain localisation in an orthogneiss laccolith the pinet massif aveyron southern france abstract sheared granitoids of the pinet laccolith southern france display strain gradients from undeformed protolith to s c mylonites and ultramylonites that formed at amphibolite facies of metamorphism we study shear strain distribution at a regional scale and use image processing techniques to measure strain across continuous shear gradients local perturbations of strain intensity seem to represent early stages of shear strain localisation parallel to the main mylonitic foliation our study suggests that localisation of shear strain shows a periodicity at regional and outcrop scales

ice sheet growth under dirty conditions implications of debris cover for early glaciation advances abstract debris covered glaciers respond differently to any given set of climatic conditions than clean glaciers this difference stems from change in ablation rate caused by a debris cover approaching zero ablation with sufficiently thick protective cover a mechanism not yet considered in the context of ice sheet growth critical to applying the mechanism to ice sheets is supply of debris we postulate that periods after major interglacials offer the best conditions for dirty advances this is because the volume of debris previously deposited and weathered in interglacial time the latter a function of exposure length should be at a maximum advances of dirty ice sheets generate landforms and in positions deviating from clean ice advances under similar climatic conditions hence inferences of both ice sheet properties inferred from such dirty ice advances and the climate conditions related to such advances must be cautious furthermore modeling of past ice sheets must consider the effect of debris cover whenever indications exist for such a cover

strain analysis of a northern apennine shear zone using deformed marble breccias abstract the alpi apuane region of the northern apennines appears to have been deformed within a large scale low angle shear zone with an overthrust sense of movement the presence of mineral stretching lineations folds progressively rotated into the x strain direction and schistosities which intersect the nappe boundaries at small angles suggest that a component of shear strain occurred during the deformation the strain ratios and orientations on two dimensional sections have been determined from deformed marble breccias reduction spots and oncalites data from three or more non perpendicular non principal sections have been combined to determine the finite strain ellipsoids at 33 sites within the shear zone the finite strains have been separated into components of simple shear longitudinal strain and volume change algebraic expressions have been derived and graphs constructed which enable components of and and and to be determined directly from a knowledge of strain ratio r within the shear zone and the angle between the principal strain direction and the shear zone boundary the alpi apuane data indicate that neither simple shear alone nor simple shear with volume change can satisfactorily explain the observed strains consideration of simple shear plus longitudinal strain leads to a general relationship in which the value of shear increases and the values of longitudinal strain change along a sw ne profile across the zone integration of the resulting shear strain distance curves gives a minimum displacement of 4 km within the shear zone combination of the finite strains with the total time of deformation known from k ar studies leads to average strain rates from 1 4 to 9 6 10 15 sec 1 a characteristic flat ramp flat geometry initially formed the boundaries of what was later to develop into the overthrust shear zone and deformation of the underlying crystalline basement is believed to have occurred by ductile shearing estimates of 21 crustal shortening for the region suggest that the crustal thickness prior to deformation was approximately 20 km in this part of the northern apennines

preservation of landforms under ice sheets and ice caps abstract this article addresses the question of whether or not distinct glacial and non glacial landforms can survive beneath ice sheets and ice caps with little or no morphological alteration a review of recent work documents the existence of pre last stadial landforms and landscapes in areas covered by the fennoscandian and laurentide ice sheets a substantial number of independent works indicate that landforms such as eskers drainage channels and boulder fields have escaped destruction despite complete ice overriding during several tens of millenia full preservation of former ground surfaces or delicate landforms probably is linked to areas where the ice sheet base was continuously frozen to its bed larger robust landforms such as large drumlins appear to have been preserved even under wet based conditions in glaciated areas patches preserved under dry cold based conditions provide important windows towards the past showing landscapes that were destroyed in surrounding areas affected by wet based and eroding ice some consequences for the research fields of non glacial geomorphology archaeology and botany include the possibility of subglacial museums and refugia a time space model describes geomorphological access to information from older events in glaciated areas

palaeotectonic reconstruction of the central alpine mediterranean belt for the neogene abstract the structural patterns of the central alpine mediterranean belt ranging from the eastern balearic sea to the western aegean sea have been reconstructed for 10 and 20 ma b p they have been obtained in the frame of the africa europe convergence through a step by step rigid plastic reconstruction using available geological and palaeomagnetic data all domains outlined in the present situation are shown in the palaeotectonic reconstructions while the structural pattern at 10 ma b p is similar to that of the present except for the tyrrhenian region that at 20 ma b p differs from it in most areas the well known adriatic promontory of the african plate is recognizable in both pictures and at 20 ma its original boundaries are revealed as strongly deformed flysch belts the adriatic promontory was broken up in the early or middle miocene into two microplates of comparable sizes the norther one is the adriatic microplate s s which suffered counter clockwise rotation by 20 relative to africa around the alpine apenninic whirl centre the southern one is the apulian microplate first outlined in the present work including the south apenninic and the apulian s s south of the gargano peninsula platforms which suffered no rotations relative to africa but which moved towards the northeast of it the primary reason for the break up of the adriatic promontory was most probably the opening of the balearic ligurian sea since 10 ma the adriatic and apulian microplates were again integral with africa or perhaps they moved towards the west relative to africa pushed by the turkish microplate the orogeny in the area discussed is mostly linked with collisional processes on the boundaries of the adriatic apulian unit

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

extension of the najd shear system from saudi arabia to the central eastern desert of egypt based on integrated field and landsat observations the najd shear system in saudi arabia extends over 1200 km in a nw se direction and has a width of approximately 300 km a digital color mosaic compiled from seven landsat thematic mapper scenes was used to delineate characteristic structural features of the najd system in the midyan region of saudi arabia and to search for similar features in the egyptian eastern desert the digital mosaic was generated using ratios of landsat thematic mapper bands bands 5 4 3 4 5 1 5 7 that are sensitive to the rock content of fe bearing aluminosilicates spectrally opaque phases and hydroxyl bearing or carbonate minerals respectively the mosaic covers approximately 130 000 km of late proterozoic exposures of the arabian nubian shield and has the eastern desert and the midyan region placed in their approximate pre red sea locations the ajjaj shear zone ajz marks the termination of the najd system against the eastern margin of the red sea in the midyan region the ajz aligns with the central eastern desert based on analysis of pre red sea locations analyses of landsat data and field observations show that the ajjaj shear zone and the central eastern desert exhibit the following features in common 1 outcrops that are generally elongate in a nw se direction as a result of folding with fine scale lithologic heterogeneity at the outcrop scale related to deformation associated with faulting 2 nw trending left lateral faults and ductile shear zones 3 subhorizontal nw trending mineral lineations and variably dipping nw trending foliations with local changes in attitude around large competent e g granitic bodies and 4 lithologic contacts that are generally tectonic in nature and related to faulting these features are less common to the north and south of both the ajjaj shear zone and the central eastern desert results are consistent with the najd shear system extending into the eastern desert and dominating the structural patterns within the central part of the eastern desert

isostatic adjustments in a full glacial sea during the last glaciation an ice free corridor existed between the northeast ellesmere island and northwest greenland ice sheets this corridor constituted a peripheral depression in which the marine limit marks the uppermost extent of a full glacial sea the full glacial sea is characterized by 1 14c dates on in situ marine shells that predate initial emergence unloading followed by 2 synchronous emergence from the marine limit throughout the peripheral depression relative sea level curves from the full glacial sea confirm previous morphostratigraphic and glacioisostatic evidence for limited ice extent during the last glaciation these curves also document the history of glacial unloading and the form of the relative sea level curve that one would theoretically expect in the peripheral depression the form of the curves presented here is unlike any other published emergence curves from arctic canada or from fennoscandia the relative sea level curves for northeast ellesmere island show three segme

geochronology of the late precambrian hamisana shear zone red sea hills sudan and egypt the hamisana shear zone hsz is a broad zone of deformation approximately 50 km wide and at least 300 km long making it one of the largest basement structures in ne africa it has been interpreted as a precambrian suture as a zone of strike slip displacement or as a zone of crustal shortening the results of new rb sr and u pb zircon geochronological studies indicate that the northern hsz was thermally active during the pan african event until c 550 mg ago initiation of the structure may have begun 40 110 ma earlier all units have low initial 87 sr 86 sr ratios indicating juvenile derivation the timing of activity in the hsz is 50 150 ma younger than collisional suturing and terrane assembly in the arabian nubian shield but is similar to the 655 540 ma najd fault system of egypt and arabia deformation and metamorphism along the hsz clearly post date terrane accretion and probably are closely related to the najd tectonic cycle the most important deformation of the hsz is unrelated to suturing and at least one late precambrian suture must extend west from arabia into the interior of n africa