the mid norwegian margin a discussion of crustal lineaments mafic intrusions and remnants of the caledonian root by 3d density modelling and structural interpretation the high density lower crustal body lcb on the mid norwegian margin is almost universally interpreted to represent magmatically underplated material added to the crust during early tertiary opening of the ne atlantic the thickness of the lcb is uneven and its distribution along the margin is sharply limited by margin perpendicular lineaments three dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments bivrost jan mayen gleipne etc in the crustal architecture with and without the lcb the bivrost lineament separating the voring and lofoten margin segments is clearly expressed and so is arguably a lineament in the sw voring margin notably most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from late caledonian orogenic collapse an alternative interpretation of the lcb is thus that it represents high grade metamorphic rocks remnant from the caledonian root determining the nature of the lcb has profound effects for the volume estimate of magmatic rocks in the north atlantic igneous province and consequently also for the degree of crustal thinning and heat flow

remote sensing of desert dune forms by polarimetric synthetic aperture radar sar abstract this report describes remote sensing experiments using highly capable polarimetric synthetic aperture radar sar in preparation for the launch of the sir c x sar mission nasa s jpl airborne polarimetric sar airsar was deployed over many locations worldwide the report describes sar observations of desert dune fields in north america bolivia and australia acquired by airsar and in namibia acquired by sir c x sar the use of polarimetric and multiwavelength sar is reviewed and comments on the advantages of the various combinations are noted sar data are very useful in mapping dunes because of the ability to control illumination parameters such as look angle wavelength and polarization furthermore the unique responses of sar to surface roughness are found very useful in mapping dune forms for most sites the longer wavelengths p and l band at 68 cm and 24 cm are advantageous over the shorter c band at 5 6 cm providing better contrast overall copolarized channels hh and vv are found best for mapping the dunes and dune types when trying to discriminate active from inactive dunes cross polarized channels provide essential information on vegetation

moho signature from wide angle reflections preliminary results of the 1993 trans hudson orogen refraction experiment abstract in 1993 a combined crustal seismic refraction and wide angle reflection experiment was conducted over the northern saskatchewan and manitoba portion of the trans hudson orogen by lithoprobe the program included three seismic lines extending over 1900 km in length the exceptionally high quality data collected during the field experiment included seismic reflections and refraction events from the moho and upper lithospheric mantle the relatively high seismic fold and signal to noise ratio of the field observations made it possible to process the wide angle portion of the data set with standard reflection signal enhancement techniques the moho is revealed as a complex interface with significant structural relief 40 52 km that correlates in first order fashion with crustal geology and structure the moho topography is interpreted as a result of variable ductile reworking and eclogitization of the lower crust

ground penetrating radar simulation in engineering and archaeology forward modeling of ground penetration radar is developed using exact ray tracing techniques structural boundaries for a ground model are incorporated via a discrete grid with interfaces described by splines polynomials and in the case of special structures such as circular objects the boundaries are given in terms of their functional formula in the synthetic radargram method the waveform contributions of many different wave types are computed using a finely digitized antenna directional response function the radar crosssection of buried targets and the effective area of the receiving antenna can be statistically modeled attenuation along the raypaths is also monitored the forward models are used 1 as a learning tool to avoid pitfalls in radargram interpretation 2 to understand radar signatures measured across various engineering structures and 3 to predict the response of cultural structures buried beneath important archaeological sites in japan

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

basement structure and its influence on the structural configuration of the northern north sea rift the northern north sea rift basin developed on a heterogeneous crust comprising structures inherited from the caledonian orogeny and devonian post orogenic extension integrating two dimensional regional seismic reflection data and information from basement wells we investigate the pre rift structural configuration in the northern north sea rift three seismic facies have been defined below the base rift surface 1 relatively low amplitude and low frequency reflections interpreted as pre caledonian metasediments caledonian nappes and or devonian clastic sediments 2 packages of high amplitude dipping reflections 500 ms thick interpreted as basement shear zones and 3 medium amplitude and high frequency reflections interpreted as less sheared crystalline basement of proterozoic and paleozoic caledonian origin some zones of seismic facies 2 can be linked to onshore devonian shear zones whereas others are restricted to the offshore rift area interpreted offshore shear zones dip s ese and wnw in contrast to w to nw dipping shear zones onshore w norway our results indicate that devonian strain and ductile deformation was distributed throughout the caledonian orogenic belt from central south norway to the shetland platform most of the devonian basins related to this extension are however removed by erosion during subsequent exhumation basement shear zones reactivated during the rifting and locally control the location and geometry of rift depocenters e g in the stord and east shetland basins pre rift structures with present day dips 15 were reactivated although some of the basement shear zones are displaced by rift faults regardless of their orientation relative to rift extension direction

the use of multifrequency and polarimetric sir c x sar data in geologic studies of bir safsaf egypt abstract bir safsaf within the hyperarid core of the sahara in the western desert of egypt was recognized following the sir a and sir b missions in the 1980s as one of the key localities in northeast africa where penetration of dry sand by radar signals delineates previously unknown sand buried paleodrainage valleys radar rivers of middle tertiary to quaternary age the bir safsaf area was targeted as a focal point for further research in sand penetration and geologic mapping using the multifrequency and polarimetric sir c x sar sensors analysis of the sir c x sar data from bir safsaf provides important new information on the roles of multiple sar frequency and polarimetry in portraying specific types of geologic units materials and structures mostly hidden from view on the ground and on landsat tm images by a relatively thin but extensive blanket of blow sand basement rock units granitoids and gneisses and the fractures associated with them at bir safsaf are shown here for the first time to be clearly delineated using c and l band sar images the detectability of most geologic features is dependent primarily on radar frequency as shown for wind erosion patterns in bedrock at x band 3 cm wavelength and for geologic units and sand and clay filled fractures in weathered crystalline basement rocks at c band 6 cm and l band 24 cm by contrast quaternary paleodrainage channels are detectable at all three radar frequencies owing among other things to an usually thin cover of blow sand the sir c x sar data investigated to date enable us to make specific recommendations about the utility of certain radar sensor configurations for geologic and paleoenvironmental reconnaissance in desert regions

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

paleozoic mesozoic tectono sedimentary evolution and magmatism of the egersund basin area norwegian central north sea abstract this study focuses on the tectonic evolution of the greater egersund basin in the norwegian central north sea with special emphasis on late paleozoic extensional tectonics following caledonian collapse and the variscan orogeny and the impact of the basement structural grain on this evolution the caledonian collapse likely resulted in development rejuvenation of the deepest e w trending structures depocenters by late devonian time thus a late devonian early carboniferous age can be assigned to the initial extension which was associated with the development of an e w striking basin system to be overprinted by n s extensional structures of similar age a phase of regional magmatism at the carboniferous permian transition 300 ma may be associated with a large igneous province centred on the skagerrak area faulting during late carboniferous early permian was minor within the study area as reflected by uniform sedimentary thicknesses of the uppermost carboniferous and lower permian sequences major normal faults mainly trending n s were active during a late permian early triassic rift phase affecting large parts of the central and northern north sea area a later phase of extension was initiated in late middle jurassic time and the egersund basin proper formed during the late jurassic early cretaceous the depocenters that developed during this phase was influenced by the deep late paleozoic sub salt structural grain including strike slip movements along the sorgenfrei tornquist zone later events include mild inversion along the northern flank of the egersund basin possibly as a late cretaceous response to far field alpine compression and cenozoic regional tilting

reply to the comment by a g jones et al on deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and related quaternary volcanism 1 scientific discussion and different points of view are a basis of the advancement of knowledge we acknowledge the comments of jones et al 2012 as an opportunity to publicly discuss the structure and origin of the atlas mountains moreover we welcome the opportunity to compare our results with those recently published by the group responsible for the comment ledo et al 2011 although it is not pertinent to comment in detail on a paper published in another journal we also wish to remark that the paper of ledo et al 2011 was reviewed and published during the revision period of our contribution anahnah et al 2011 therefore they are two different approaches and data sets measured in different sites and by different instruments for the same region lending readers the chance to compare different interpretations the main differences on the data sets are the profile of anahnah et al 2011 compared with the profile of ledo et al 2011 is 170 km longer vertical magnetic data were obtained and lower frequencies were recorded 2 we regret the style and way used by jones et al 2012 we shall answer only those comments of jones et al 2012 related to objective issues 3 one of the final conclusions of jones et al 2012 might serve as the starting point of our reply

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

seismic images of continental lithosphere seismic reflection profiles in usa and west germany over thirty years ago showed strong reflections from the lower crust but technical advances through the 1970s were needed for the method to image the continental lithosphere the british institutions reflection profiling syndicate birps found advantage in working offshore and has obtained over 12 000 km of profiles around britain during the past decade giving the densest regional coverage anywhere reflections are observed from strata in sedimentary basins from low angle faults that can be followed from surface to mid crust from a laterally variable reflective zone in the lower crust from the moho and from fault like features in the upper mantle there is evidence around britain that faults bounding sedimentary basins are older structures which have been reactivated it is argued that lower crustal reflectivity around britain is mainly due to anastomosing ductile shears which accommodate extension in the lower crust commensurate with extension in the upper crust by faulting and block rotation there is also evidence of reflections preserved from older orogenic features so that reflections interpreted as extensional ductile shears in the lower crust may also derive from older features that have been reactivated most reflections in the lower crust arise from rocks that have been highly strained evidence of lateral variability in the reflective character of the moho suggests that the moho has been actively involved in the structural evolution of the lithosphere upper mantle reflections are interpreted as shear zones which transfer crustal deformation through the lithosphere

shuttle imaging radar experiment the shuttle imaging radar sir a acquired images of a variety of the earth s geologic areas covering about 10 million square kilometers structural and geomorphic features such as faults folds outcrops and dunes are clearly visible in both tropical and arid regions the combination of sir a and seasat images provides additional information about the surface physical properties topography and roughness ocean features were also observed including large internal waves in the andaman sea

uplift of deep crust during orogenic extensional collapse a model based on field studies in the sogn sunnfjord region of western norway the western gneiss region wgr in norway experienced high pressure metamorphism during silurian devonian continent continent collision the eclogite bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen formation of the devonian basins is related to the extension the footwall of the detachment zones comprises three structural and metamorphic zones the upper zone zone 1 is characterized by penetrative homogeneous down to the west simple shear developed under retrograde greenschist facies metamorphism zone 2 suffered inhomogeneous simple shear of the same polarity petrography and mineral chemistry data from the lower zone zone 3 show a record of initial eclogite facies metamorphism at 600 c and 16 kbar which was decompressed almost isothermally to amphibolite facies conditions at 550 c and 10 12 kbar both the eclogite and amphibolite facies metamorphism developed in a regime of pure shear with vertical shortening the rapid decompression records an approach of approximately 20 km to the surface related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere the pure shear regime which developed initially in the lower crust was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear the shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust a considerable amount of tectonic stripping of the orogenic welt predates deposition of the devonian sedimentary basins

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

faults and folds fact and fiction abstract after reviewing the microscopic and macroscopic texture of fault zones the localisation of rupture and creep is described and the reduced strength of fault zones is investigated simple strength and viscosity models for the whole lithosphere play a major role for the geometry of fault zones compressional faults thrusts show ramp and flat structures often soling in a weak detachment zone extensional faults prefer a listric shape often turning subhorizontal and being invisible in the seismic lamellae of the ductile lower crust which according to the models is also a broad zone of detachment and decoupling faults in the lower crust are rare and restricted to strain hardened fault rocks or intrinsically rigid rocks like oceanic mafic ultramafic rocks of suture zones transient faults in the lower crust are created by rupture processes of large earthquakes some deep faults moderately dipping or flat are again observed in the rather rigid uppermost mantle the observation of folding inside the crust seems to be connected with an intermediate viscosity range and with a broad vertical transition zone where rigid reactions and ductile processes are mixed various methods of detecting faults by seismic studies are critically reviewed and some sequence of reliability is suggested impedance contrasts polarity dip thickness and multiple fault strings have to be deciphered by wavefront modelling origin and survival of faults are considered to be a consequence of tectonic and thermal evolution under specific stress systems

integrated seismic studies of the baltic shield using data in the gulf of bothnia region in the autumn of 1989 a co operative experiment involving 12 research institutions in northwestern europe collected 2268 km of deep seismic reflection profiles in the gulf of bothnia and the baltic

early carboniferous unroofing in western norway a tale of alkali feldspar thermochronology abstract 40ar 39ar thermochronologic data from an extensional detachment zone in western norway document punctuated late paleozoic through mesozoic tectonothermal activity diffusion and thermal modeling of alkali feldspars sampled in a profile through the detachment replicate laboratory argon release patterns when feldspars comprise three or four diffusion domains with differing activation energies that represent high intermediate and low closure temperatures these domains correspond to distinct changes in late paleozoic cooling rates 1 slow cooling at 0 4 2 2 c m yr from ca 380 360 ma 2 rapid cooling at 15 c m yr in late devonian early carboniferous time 360 340 ma 3 slow cooling at 0 4 1 7 c m yr after ca 340 ma with partial argon loss in permian and late jurassic early cretaceous times the early carboniferous rapid cooling event recorded by the feldspars links the western norway margin to contemporaneous unconformities regional extension basin inversion and igneous activit

eclogite facies shear zones deep crustal reflectors strongly foliated eclogite facies rocks in 30 150 m thick shear zones of caledonian age occur within a grenvillian garnet granulite facies gabbro anorthosite terrain in the bergen arcs of norway the predominant eclogite facies mineral assemblages in the shear zones are omphacite garnet zoisite kyanite in gabbroic anorthosite and omphacite garnet in gabbro eclogite facies rocks in shear zones are generally fine grained alternating omphacite garnet and kyanite clinozoisite rich layers define gneissic layering a strong shape preferred orientation of omphacite kyanite and white mica phengitic muscovite and or paragonite define the foliation the anorthositic eclogites show omphacite b axis maxima approximately normal to the foliation and c axis girdles within the foliation plane p wave velocities v determined at confining pressures to 600 mpa for samples from eciogite facies shear zones range from 8 3 to 8 5 km s l and anisotropy ranges from 1 to 7 the few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation the fast propagation direction lies within the c axis girdles parallel to foliation and the slow propagation direction is parallel to the b axis concentration normal to foliation in samples for which omphacite crystallographic preferred orientation was determined v for the granulite facies protoliths average about 7 5 km s high calculated reflection coefficients for these shear zones 0 04 0 14 indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high pressure conditions but escaped thermal reactivation

oblique rifting and sequential faulting in the jurassic development of the northern north sea abstract jurassic structural development of the northern north sea has characteristics of oblique rifting this is interpreted to result from an obliquity between jurassic nw se to wnw ese directed extension and a preexisting n s trending zone of deformation related to permo triassic rifting and thinning of the crust the zone of maximum jurassic extension i e the viking and sogn grabens defines a narrow 25 40 km depression which trends diagonally nne ssw across a broader 130 150 km permo triassic basin the jurassic rift zone consists of a system of ene chelon rift segments with changing symmetry along strike bounded by faults oblique to the overall graben envelope sequential jurassic fault development as well as the interference between n s and ne sw striking faults are explained within the context of oblique rifting the early late bajocian oxfordian rift stage was dominated by reactivation of n s striking major faults whereas a number of coeval but smaller faults have a ne sw orientation ne sw striking faults show increasing importance during the jurassic stretching phase and major faults defining this orientation accommodated most of the kimmeridgian volgian extension faults striking nesw are observed to cross cut earlier n s trending faults they interfere with n s trending faults to define faultblocks with rhombohedral shape and they are associated with partial collapse of crestal areas of n s oriented fault blocks to form structural terraces

timing and kinematics of caledonian thrusting and extensional collapse southern norway evidence from 40ar 39ar thermochronology 40ar 39ar dating methods have been applied to rocks across the caledonian orogen in southwestern norway in the eastern nappe area k feldspar thermochronological modeling microfabric characteristics and conodont color alteration together indicate that temperatures were not above the closure temperature of muscovite for any significant period of time two groups of muscovite and biotite ages from this area 415 408 for most samples with top to the se fabrics and 402 394 for samples with top to the nw fabrics are therefore interpreted as ages of contractional thrusting and extensional hinterland directed nappe translation deformation respectively in the west hinterland peak caledonian temperatures were higher and 40ar 39ar plateau ages are generally interpreted as cooling ages the western basement and lower nappes cooled rapidly through 500 c basement at 404 ma and 350 c basement and lower nappes shortly after i e during extensional top to the nw transport of the orogenic wedge in addition tectonostratigraphically higher nappes in the hinterland show evidence of earlier cooling probably following ordovician orogenic activity prior to the main collisional stage the new 40ar 39ar data conform to kinematic observations that contraction and extension in the caledonian nappe region were sequential and that the change from contraction convergence to extension divergence was quick between 408 and 402 ma

a seismic geotraverse across the iberian variscides orogenic shortening collisional magmatism and orocline development 1 a crustal geotraverse through the iberian variscides is presented by integrating the available geological and seismic profiling data different modes of orogenic shortening are identified with varying degrees of coupling between upper and lower crust in northern and southern regions of the geotraverse a decoupling in the middle crust permits the lower crust to subduct underthrust thus compensating for strong upper crustal shortening this behavior does not result in great crustal thickening except in sectors of crustal underthrusting in southern central iberia moderate upper crustal shortening is due to a mechanically strong lower crust impeded to subduct underthrust in this region shortening is partitioned between upper and lower crust deformation being distributed in the upper crust while localized at major fault zones in the lower crust finally the central region of the geotraverse northern central iberia shows a coupled crustal deformation having given way to the largest orogenic thickening in the iberian variscides the thermal maturity of this much thickened crust originated voluminous crustal melting and concomitant normal fault detachments developed while shortening dominated in other regions theoretical models suggest that compressive stresses may prevail in the lower crust beneath the extending upper crust thus explaining the efficient syncollisional exhumation in this part of the orogen a particular feature of the iberian variscan geotraverse is the great importance of out of section mass movements mainly left lateral shear zones concentrated in two suture boundaries which displaced to the nw present coordinates central and northern iberia with respect to southern iberia

microtopographic evolution of lava flows at cima volcanic field mojave desert california microtopographic profiles were measured and power spectra calculated for dated lava flow surfaces at cima volcanic field in the eastern mojave desert of california in order to quantify changes in centimeter to meter scale roughness as a function of age for lava flows younger than about 0 8 m y roughness over all spatial scales decreases with age with meter scale roughness decreasing slightly more than centimeter scales flows older than about 0 8 m y show a reversal of this trend becoming as rough as young flows at these scales modeling indicates that eolian deposition can explain most of the change observed in the offset or roughness amplitude of power spectra of flow surface profiles up to 0 8 m y other processes such as rubbling and stone pavement development appear to have a minor effect in this age range changes in power spectra of surfaces older than about 0 8 m y are consistent with roughening due to fluvial dissection these results agree qualitatively with a process response model that attributes systematic changes in flow surface morphology to cyclic changes in the rates of eolian soil formation and fluvial processes identification of active surficial processes and estimation of the extent of their effects or stage of surficial evolution through measurement of surface roughness will help put the correlation of surficial units on a quantitative basis this may form the basis for the use of radar remote sensing data to help in regional correlations of surficial units

subsurface valleys and geoarcheology of the eastern sahara revealed by shuttle radar the shuttle imaging radar sir a carried on the space shuttle columbia in november 1981 penetrated the extremely dry selima sand sheet dunes and drift sand of the eastern sahara revealing previously unknown buried valleys geologic structures and possible stone age occupation sites radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys some nearly as wide as the nile valley and perhaps as old as middle tertiary the now vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat hyperarid region underfit and incised dry wadis many superimposed on the large valleys represent erosion by intermittent running water probably during quaternary pluvials stone age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation the presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present day oases which have been centers of episodic human habitation radar penetration of dry sand and soils varies with the wavelength of the incident signals 24 centimeters for the sir a system incidence angle and the electrical properties of the materials which are largely determined by moisture content the calculated depth of radar penetration of dry sand and granules based on laboratory measurements of the electrical properties of samples from the selima sand sheet is at least 5 meters recent september 1982 field studies in egypt verified sir a signal penetration depths of at least 1 meter in the selima sand sheet and in drift sand and 2 or more meters in sand dunes

permian and mesozoic extensional faulting within the caledonides of central south norway palaeomagnetic data from fault rocks along major faults in the laerdal gjende fault system cutting the caledonian structure in the jotunheimen area of central south norway reveals a multi component remanence pattern sample and site mean directions from the fault rocks obtained by thermal cleaning demonstrate a simple pattern of normal polarity low blocking components and reverse polarity high blocking directions the magnetic signature of the laerdal gjende fault system fault rocks is identical to that observed on breccias on late faults along the west coast of norway based on available palaeomagnetic reference data we assign ages of mid late permian and late jurassic early cretaceous for important phases of faulting and breccia formation along the laerdal gjende fault system in central south norway structural windows partly exposing basement along the axis of the caledonides in southern norway were exhumed by footwall uplift on major faults in the laerdal gjende fault system the consanguinity of fault rock data from the laerdal gjende fault system and fault rocks in western norway and comparison with displacement on the offshore continuation of the laerdal gjende fault system along the hardangerfjorden shear zone indicate that the main tectonic events responsible for the development of the north sea basin also significantly affected the geology of central south norway

model inspired interpretation of seismic structures in the central alps crustal wedging and buckling at mature stage of collision analogue models of continental convergence inspire a new interpretation of the deep seismic profile across the central alps the classical collision belt of western europe we conclude that active mountain building processes involve wedging and crustal buckling and that seismic information does not reflect subduction related orogeny but documents recent crustal shortening we also infer that present day subduction of the delaminated european mantle lithosphere takes place below the northern side of the mountain belt

detection of sand covered geologic features in the arabian peninsula using sir c x sar data eolian sand covers a large part of the arabian peninsula the flow of eolian sand is ongoing and sand covers bedrock as well as fluvial features of the peninsula consequently there is a gap in the understanding of the geological processes shaping the region the extensive coverage of shuttle imaging radar sir c x sar data over the arabian peninsula has provided a new means of mapping sand obscured features the preliminary interpretation of the data has identified several sites where l band reveals sand covered geological structures and drainage channels that are not shown on published maps and are not traceable with more conventional methods the indications are that these radar data may lead to improving the geological and fluvial geomorphic understanding of the region

microwave remote sensing of physically buried objects in the negev desert implications for environmental research we report remote detections of physically buried specularly reflecting objects using microwave radar at two sites ashalim and tseelim in the northern region of the negev desert israel these detections provide confirmation that microwave subsurface remote sensing is a genuine phenomenon at ashalim a scatterometer operating in the p band 441 mhz 68 cm was mounted on a cherry picker truck at a height of 8 m and used to detect two triangular aluminum mesh reflectors forming a 1 m square area reflector buried down to a depth of 8 cm in dry sand at tseelim the same scatterometer was mounted on an airplane flying at an altitude of 70 m and used to detect 1 m square aluminum reflectors each one submerged at a different location along the airplane flight path buried down to a depth of 20 cm the experimental results compare favorably with a theoretical model that incorporates radar absorption effects arising in the sandy subsurface layer and radar interference effects arising from phase differences between reflections from the surface and buried reflector the theoretical modeling also predicts the detection of a subsurface reflector down to a depth of about 4 4 m this experiment and the associated modeling approach is the first of a series of planned experiments which we outline for the detection and the theoretical evaluation of buried reflectors using remote microwave and vhf radar we identify potential subject areas for environmental research

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

crustal reflectivity underneath the central scandinavian caledonides abstract we have studied data from a 160 km long explosive vibroseis reflection seismic profile part of the central caledonian transect acquired in the central scandinavian caledonides between 1988 and 1992 extended correlation has been used to increase the record length to 20 s for the vibroseis component the reflectivity interpreted as primarily due to dolerities is strong at depths

comment on deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and related quaternary volcanism citation jones a g d kiyan j fullea j ledo p queralt a marcuello a siniscalchi and g romano 2012 commenton deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and relatedquaternary volcanism tectonics 31 tc5011 doi 10 1029 2011tc003051

crustal evolution of the middle urals based on seismic reflection and refraction data abstract integration of seismic refraction and deep reflection data from the middle urals of central russia provides important new constraints on the structure of the uralian crust re analysis of the granit refraction profile and comparison with coincident reflection data from the esru profile shows a high velocity 7 6 7 8 km s root zone from c 45 to 55 km with low reflectivity beneath the urals we interpret this interval as crustal material consistent with previous russian interpretations of this velocity anomaly above this crustal root one of the principle features imaged on the esru profile is a thick zone 3 4 s twt of relatively strong reflectivity which may characterize the lower crust of the east european platform but is considerably shallower to the east at 8 12 s twt 25 37 km than in the west at 10 14 s twt 31 43 km reprocessing of 20 s records from the r 17 profile in the west siberian basin 315 km northeast of the esru profile reveals a similar pronounced lower crustal reflectivity between 11 14 s twt 34 43 km in the hinterland of the middle urals this lower crustal reflection fabric may represent a feature developed during collisional orogenesis or a younger property imparted through post orogenic extension future deep reflection profiling will be critical to address the continuity and accordingly the tectonic significance of this lower crustal reflectivity in the urals

shuttle imaging radar physical controls on signal penetration and subsurface scattenng in the eastern sahara sir a signal penetration and subsurface backscatter within the upper meter or so of the sediment blanket in the eastern sahara of southern egypt and northern sudan are enhanced both by radar sensor parameters and by the physical and chemical characteristics of eolian and alluvial materials interpretation of sir a images by mccauley et al 1 2 dramatically changed previous concepts of the role that fluvial processes have played over the past 10000 to 30000000 years in shaping this now extremely flat featureless and hyperarid landscape in the present paper we summarize the near surface stratigraphy the electrical properties of materials and the types of radar interfaces found to be responsible for different classes of sir a tonal response the dominant factors related to efficient microwave signal penetration into the sediment blanket include a favorable distribution of particle sizes b extremely low moisture content and c reduced geometric scattering at the sir a frequency 1 3 ghz the depth of signal penetration that results in a recorded backscatter here called radar imaging depth was documented in the field to be a maximum of 1 5 m or 0 25 of the calculated skin depth for the sediment blanket radar imaging depth is estimated to be between 2 and 3 m for active sand dune materials diverse permittivity interfaces and volume scatterers within the shallow subsurface are responsible for most of the observed backscatter not directly attributable to grazing outcrops

mapping exposed and buried drainage systems using remote sensing in the negev desert israel abstract this paper examines the use of optical visible and infrared ir data synthetic aperture radar data sar and ground penetrating radar gpr for detection identification and mapping of exposed and buried channels under sands in the western negev israel and northern sinai egypt field observations and shallow geophysical methods corroborated the observations the halutza agur and shunra sand dunes are the continuation of the northern sinai dunes playa type sediments from the late quaternary are found in upper reaches of dry river channels between the dunes of the northwest negev and represent a deterioration of the drainage system caused by shifting sand dunes a dual polarimetric radar sir c image from srl 1 having two frequencies and two polarizations exhibits meaningful differences in reflectance between the buried and abandoned channels and their surroundings the buried continuation of wadi mobra and the larger nahal nizzana channels under the western shunra sand dunes can be observed in l band radar data these channels are not clearly observed either in the landsat thematic mapper tm visible nor in the near infrared nir images the radar wavelengths and polarizations that improve the contrast between the buried river beds and the surrounding sand mantle are in descending order l hv l hh and c hh however in places where the sand mantle is very thick or where channels cut through bedrock the visible and near infrared data from landsat tm are easier to interpret and tend to be more useful for mapping fluvial patterns

development of rift basins illustrated by the structural evolution of the oseberg feature block 30 6 offshore norway the mesozoic tilted fault block in norwegian north sea block 30 6 containing the giant oseberg oil and gas field is used to illustrate basin development during the jurassic and cretaceous there is seismic evidence for an early rifting phase in the northern viking graben on which the jurassic cretaceous rifting phase early bathonian to berriasian is overprinted the oseberg structure is representative of other fault blocks bordering the northern viking graben axis planar normal faults and listric faults which were restricted to the rifting phase dominate both fault types exhibit syn fault sedimentation listric faulting is recognized on seismic sections by the differential tilting reverse drag antithetic faulting and a wedge shaped syn rift reflector package the initiation of rifting is dated by baselap onto pre rift reflectors and cessation is dated by an unconformity beneath the generally onlapping post rift sediments steeply dipping planar normal faults often active along previously established fault zones played a significant part in the post rift development of the basin the last movement on planar normal faults in the oseberg fault scarp becomes younger away from the rift axis and this produced the complex fault scarp visible on many seismic lines the topography defined by the post rift unconformity is a composite surface which formed in response to subsidence processes it is not the result of orogenic events such as the kimmerian 9orogeny9 footwall uplift associated with planar normal faults caused local unconformities in complex fault scarps and on fault block crests

the reflectivity of magmatic underplating using the layered mafic intrusion analog abstract the reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates one dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0 18 a seismic profile across the bjerkreim intrusion in southern norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0 05 produces detectable reflections two dimensional models demonstrate that the subhorizontal cumulate layering in typical mid to upper crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image reflection coefficients predicted for granulite facies mineral assemblages are similar to those predicted for the igneous assemblages reflection coefficients predicted for cumulates intruded or re equilibrated at eclogite facies conditions may be either decreased or enhanced with respect to the igneous assemblages depending on the abundance of plagioclase in the cumulate layers and the pressure results suggest that magmatic underplating should not be invoked as an explanation for non reflective crust except in cases where eclogitization has occurred

crustal inhomogeneities in the northern north sea from potential field modeling inherited structure and serpentinites abstract a new crustal model for the northern north sea was developed by gravity and magnetic modeling along the deep seismic line nsdp84 1 utilizing vertical gradients allowed distinguishing between shallow and deep crustal sources the upper crust is characterized by low magnetic susceptibilities and low densities which is typical for felsic rocks a new finding was that the deep crust below the western viking graben and the east shetland basin is the source of high magnetic anomalies combined with low gravity anomalies which was interpreted to represent rocks with very high magnetic susceptibilities and low to intermediate densities such rock parameters may indicate serpentinites but intermediate intrusives or a combination of both is also possible honoring the string of three near equidistant magnetic maxima which follow the trend of the nne ssw striking east shetland basin in the map plane it is suggested that this area is part of an island arc of the iapetus ocean which has been assembled during the collision between laurentia and baltica in late silurian times partly serpentinized peridotites and intermediate intrusives will relate in such a model to slab dehydration of the subducting oceanic plate below the island arc these inherited or synorogenic serpentinites are expected to persist in the geothermal regime of the caledonian orogeny to a depth of at least 50 km increased heat flow by later rift phases will have caused metamorphism of the remaining serpentinites to meta peridotites at depth below the present day moho fluid release related to dehydration of the serpentinites may have triggered further serpentinization of the inherited partly serpentinized rocks at shallower depth an alternative origin for the suggested serpentinites valid only for the area under the western part of the viking graben may be synrift serpentinization due to the heavy faulting during the jurassic rift phase

shortening of analogue models of the continental lithosphere new hypothesis for the formation of the tibetan plateau initial stages of compression produce periodic buckling of models analogous to a four layer continental lithosphere with further shortening amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle the rescaled results suggest that a continental lithosphere under compression will buckle with a first order wavelength of about 200 km and a second order wavelength of 20 30 km continued shortening results in the amplification of undulations troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle thus giving rise to an irregular topography of the moho fifty percent shortening results in a 60 km thick continental crust we argue that this model history may explain the elevation and deep crustal structure of the tibetan plateau

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

origin and significance of intra basement seismic reflections offshore western norway we use 3d seismic data to image a series of enigmatic sw dipping reflection packets within pre mesozoic crystalline basement offshore western norway based on their low angle dip and complex reflection wave train our preferred interpretation is that the reflection packets are the seismic expression of mylonitic zones generated by nappe emplacement during the caledonian orogeny late jurassic faults truncate and offset these reflection packets by several hundred metres suggesting that these faults did not exploit pre existing basement weaknesses our observations suggest that older basement fabrics may not always play a significant role in determining the geometry of later fault systems supplementary materials details of the time depth relationships from wells that have been used to depth convert interpretations s1 a supplementary figure showing an uninterpreted version of the seismic profile presented in fig 2 s2 and a throw v distance plot for fault f1 at the structural levels of the basement reflection and intra basement reflection 1 are available at www geolsoc org uk sup18683

the deep seismic image of the crustal structure of the tornquist zone beneath the skagerrak sea northwestern europe abstract in the sw skagerrak sea a set of deep seismic profiles cross the tornquist zone a complex fault zone extending from the black sea to the north sea these sections show that the scandinavian segment of the tornquist zone represents the boundary between the thick crystalline crust of the fennoscandian shield to the northeast and the thinner basin crystalline crust to the southwest it is associated with a complex area of moho offsets and sub moho reflectors flexural subsidence of the shield edge appears to have occurred during the mesozoic without substantial thinning of the crust on the basin side the thinned crust together with the uppermost mantle appears to have been rigid enough to support segmentation and rotation of lithosphere blocks the deep and narrow fjerritslev trough is located at a bend of the tornquist zone and is observed to contain at least 15 km 8 5 s twt of sediments it is interpreted as a pull apart basin created by transtensional movements along the tornquist zone the presence of at least 6 km 3 s twt of palaeozoic sediments in the fjerritslev trough indicates that this segment of the tornquist zone was active also during this period

crustal stretching in the scandinavian caledonides as revealed by deep seismic data the normalization of overthickened orogenic crust after a continent continent collision typically involves several different processes including erosion plate divergence gravity driven collapse of the orogenic wedge and viscous flow of the lower crust as a contribution to this discussion we here utilize reprocessed deep seismic data to image the scandinavian part of the major caledonian continent continent collision zone where we identify a double set of extensional shear zones one in the upper to middle crust hardangerfjord shear zone and a major oppositely dipping moho offsetting shear zone in the lower crust and upper mantle the latter shear zone may have a true displacement close to 50 km and appears to offset the moho vertically by 10 km extending downward beyond the 50 km depth limit of the seismic data while the upper structure offsets the basal caledonian thrust the moho shear zone is more difficult to date however they are kinematically consistent and located at the transition zone between thick and thin skinned post collisional tectonics and we suggest that they may represent a crustal pinch or zipper like structure that separated viscously flowing caledonian lower crust from cooler rigid basement representative for the rest of the baltic shield later extension created the north sea rift but the general rift axis and the associated riftrelated thinning developed in a narrower zone that runs oblique to the devonian trend hence the two generations of extension and thinning can be distinguished at the crustal scale

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

mega scale moho relief and the structure of the lithosphere on the eastern flank of the viking graben offshore southwestern norway the international lithosphere project deep reflection seismic survey in the norwegian sector of the north sea has been reprocessed particularly focusing on the deep crust the reflection moho and the upper mantle the data display shifting reflection patterns of the crust and the upper mantle parallel to the eastern margin of the viking graben in the upper crust which is mainly seismically transparent by the processing techniques utilized here large scale structural features like detachment shear zones and master faults can be identified several of the major onshore faults and shear zones match seismic features in the seismic lines many of these structures acted as extensional shear zones in the devonian the middle crust is of variable reflectivity whereas the lower crust is generally strongly reflective and is particularly so in the southern domain the reflection moho is identified throughout the study area but is of variable character the presence of a s e dipping structure hardanger moho offset that displaces the moho by approximately 10 km extends deep into the mantle below the 50 km line depth is positioned where the shallower hardangerfjord shear zone which flattens on the level of the middle crust is situated the hardangerfjord shear zone hardanger moho offset system coincides with change of the crustal thickness depth to moho a change that also coincides with the transition from thin to thick skinned caledonian deformation intramantle reflections are common in the study area some of which are interpreted as shear zones whereas others most likely represent magmatic intrusions

tectonic setting of post orogenic granites within sw fennoscandia based on deep seismic and gravity data within the svecononvegian province of sw scandinavia granite intrusions are abundant prominent ones are the bohus iddefjord and the fla granites the so called bohus fla granite belt the age of these granites consistent at 900 ma coincides with the late extensional stage of the svecononvegian grenvillian orogen gravity observations and deep seismic profiling lines are presented that also cover the skagerrak sea the geophysical data suggest that the bohus granite continues seaward for at least 80 km and its thickness offshore is estimated to be 2 4 km where the seismic profiling lines intersect the inferred seaward extension of the bohus granite a distinct reflection pattern is observed at 1 6 s twt this coincides with a gravity modelled thickness of 4 km the seismic profiles also show a large moho offset beneath the modelled granite it is proposed that this offset is related to svecononvegian crustal underthrusting and that the granite melt could have formed by anatexis of mid crustal rocks downthrusted to greater depths in the vicinity of the seismically observed moho offset

seismic velocity heterogeneity and the composition of the lower crust abstract many wide angle seismic experiments attempt to constrain lower crustal seismic velocity and thus composition deep near normal incidence seismic data recorded during the past decade suggest a widely varying and highly complex pattern of lower crustal heterogeneity synthetic seismic modelling indicates that velocities measured from wide angle experiments may depend significantly upon the geometrical arrangement and scale of the heterogeneities heterogeneities with a correlation length considerably large than the seismic wavelength will tend to produce observed velocities biased towards high values heterogeneities much smaller than the seismic wavelength can lead to a velocity bias of either sense depending upon their arrangement seismic velocities greater than 7 3 km s measured near the base of the crust in underplated and intruded passive continental margins suggest compositions that are markedly more mafic than mantle melting calculations predict our seismic modelling suggests that the mismatch between modelled and observed velocities at passive margins may be attributable to large scale correlation length greater than seismic wavelength heterogeneities

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

the hardangerfjord shear zone in sw norway and the north sea a large scale low angle shear zone in the caledonian crust the hardangerfjord shear zone is a more than 600 km long low angle extensional structure that affects the south norway and north sea caledonides the ductile shear zone which shows total maximum onshore displacement of the order of 10 15 km is primarily a basement structure with an associated passive monoclinal fold structure of the overlying caledonian nappes deep seismic data indicate that the shear zone continues down to the lower crust 20 25 km at a dip of 22 23 where it appears to flatten and merge with the general lower crustal deformation fabric onshore the hardangerfjord shear zone consists of a system of hard linked ductile shear zone segments brittle faults the l rdal gjende fault system occur in the folded caledonian allochthons in the ne part of the hardangerfjord shear zone and reappear in the north sea these may represent a high level brittle response to the devonian development of the hardangerfjord shear zone but were reactivated during permo triassic and late jurassic extensional events a c 5 km thick package of seismic reflectors along the hardangerfjord shear zone is presumed to represent a mylonite zone which is too thick to be formed entirely by 10 15 km of devonian displacement hence the hardangerfjord shear zone is likely to be a proterozoic shear zone reactivated during devonian extension

migration of wide angle seismic reflection data from the grenville front in lake huron under the auspices of the great lakes international multidisciplinary program on crustal evolution glimpce coincident steep and wide angle seismic reflection data sets were acquired in lake huron the wide angle data were recorded at stationary onshore receivers during the shooting and recording of the conventional marine steep angle survey to extract information from the wide angle data a variety of processing techniques including automatic trace editing amplitude balancing f k filtering static corrections and predictive deconvolution were applied to the wide aperture common receiver gather before migration the application of a slowness weighted depth migration algorithm which was first tested on synthetic data and which included a slowness filter resulted in an image of the grenville front tectonic zone gftz remarkably similar to that observed in the multichannel steep angle section the migrated wide angle data suggest that prominent bands of east dipping seismic reflections observed across the gftz extend unattenuated to depths of at least 45 km

crustal thinning and nature of extension in the northern north sea from deep seismic reflection profiling a regional network of deep seismic reflection profiles recorded in the northern north sea has been used to map mesozoic and cenozoic basin thickness and crustal thickness in the viking graben and adjacent platform areas depth to the reflection moho varies from about 20 km in parts of the viking graben to about 32 km beneath the shetlands and the norwegian margin the shallowing of the reflection moho beneath the viking graben implies crustal stretching factors for mesozoic extension greater than 2 in the center of the north sea basin the axis of crustal thinning is located directly beneath the viking graben the axis of the sedimentary basin steeply dipping basin bounding faults are imaged in the upper crust and several dipping reflectors are observed in the upper mantle but no continuous reflective feature extending from the sedimentary basin to beneath the moho is observed on any of the deep profiles thus these data do not support the existence of lithosphere penetrating low angle detachments zones of simple shear as the cause of extension in the northern north sea some of the mantle reflectors dip to the west and some to the east suggesting that simple shear if it occurs in the upper mantle is not of uniform sense rather these data suggest a complex depth dependent pattern of brittle extensional deformation in the upper crust pervasive ductile extension bulk pure shear in the lower crust which is decoupled from deformation in the mantle and extension accommodated by discrete dipping shear zones in the lithospheric mantle

seismic image of an early proterozoic rift basin abstract three high resolution vibroseis reflection profiles were acquired across the paleoproterozoic huronian supergroup 2 5 2 2 ga of the canadian shield in the vicinity of the temagami lake magnetic anomaly one of the largest positive magnetic anomalies in north america the supergroup formed as a passive margin sequence and composed of volcanic and sedimentary rocks unconformably overlies archean rocks of the superior province quality seismic images obtained through innovative pseudo 3d seismic processing and constrained by lithological data from deep boreholes reveal a 20 km wide early proterozoic rift basin beneath a thick cover of metasediments one of the oldest well preserved rift basins on earth

the proximal domain of the mid norwegian rifted margin the trondelag platform revisited abstract located along the coast of norway the trondelag platform area corresponds to the proximal domain of the mid norwegian voring rifted margin we investigated its architecture and structural evolution based on interpretation of a dense network of high resolution seismic reflection profiles supplemented by comparison with local onshore analogues the entire area is floored by ductile crustal shearing and regional scale core complexes that form a corrugated surface with basement culminations that are flanked and sometimes cut by detachment faults the detachments are segmented suggesting long lived and multiphase tectonic activity the observed geometries are directly comparable to the ones mapped onshore in and around devonian old red sandstone basins in western and south western norway and in the adjacent north sea the combination of intensively sheared basement core complexes flanking and cutting detachment faults is interpreted to represent the offshore record of the transition from orogenic collapse to the first phases of rifting the observations demand a revision of our conceptual approach of margins proximal domains geometries for rifted margins that develop on collapsed orogens

deep crustal structure of the northern north sea viking graben results from deep reflection seismic and gravity data abstract the combination of deep reflection seismic interpretation nsdp 84 data set and 3 d modelling of free air gravity data resulted in a regional crustal model for the viking graben area the interpretation focuses on the structure of the crystalline crust which is displayed by depth sections a map of the moho a map of the thickness of the crystalline crust and a map of the extension factor the thickness of the crystalline crust varies from 10 km near the centre of the graben to 30 km under the adjacent platforms the thickness of the sedimentary cover varies from 11 km to 1 km respectively the topography of the moho reflects in a highly smoothed way the top of the crystalline basement the seismic moho coincides with the gravity moho although some discrepancies are observed in the most extensively stretched areas high values are observed but no indications of the presence of oceanic type crust associated with a crustal rupture have been found the results are thought to favour a stretching model of graben development a strong inclined reflection and associated reflection patterns in the east shetland platform are tentatively interpreted as a fragment of the iapetus suture the asymmetrical structure of the graben is assumed to be associated with initial inhomogeneities in the crust

the influence of structural inheritance and multiphase extension on rift development the northern north sea the northern north sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement the geometry and evolution of syn rift depocentres during this multiphase evolution and the mechanisms and extent to which they were influenced by pre existing structural heterogeneities remain elusive particularly at the regional scale using an extensive database of borehole constrained 2d seismic reflection data we examine how the physiography of the northern north sea rift evolved throughout late permian early triassic rp1 and late jurassic early cretaceous rp2 rift phases and assess the influence of basement structures related to the caledonian orogeny and subsequent devonian extension during rp1 the location of major depocentres the stord and east shetland basins was controlled by favorably oriented devonian shear zones rp2 shows a diminished influence from structural heterogeneities activity localises along the viking sogn graben system and the east shetland basin with negligible activity in the stord basin and horda platform the utsira high and the devonian lomre shear zone form the eastern barrier to rift activity during rp2 towards the end of rp2 rift activity migrated northwards as extension related to opening of the proto north atlantic becomes the dominant regional stress as rift activity in the northern north sea decreases through documenting the evolving syn rift depocentres of the northern north sea rift we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics controlling the segmentation of depocentres as well as the locations styles and magnitude of fault activity and reactivation during subsequent events

absolute dating of brittle fault movements late permian and late jurassic extensional fault breccias in western norway 40ar 39ar geochronological and palaeomagnetic dating methods applied to fault breccias in western norway have isolated two brittle reactivation episodes of the syn post caledonian extensional nordfjord sogn detachment these events of latest permian and latest jurassic early cretaceous ages demonstrate temporal relationships between development of chemical remanent magnetism and partial resetting of ar isotopic systems during distinct breccia forming episodes a third event of carboniferous age was also identified in the breccias with the 40ar 39ar technique and is a relict unroofing signature inherited from the fault wall rocks these brittle faults are significant time markers and become relevant to interpretations of offshore seismic data which attempt to place ages on faults that have undergone multiple reactivation episodes

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

spatio temporal evolution of strain accumulation derived from multi scale observations of late jurassic rifting in the northern north sea a critical test of models for lithospheric extension abstract we integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern north sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire viking graben and its flanking platform areas these data show how syn rift accommodation within the basin varied in space and time with sub kilometer scale spatial resolution and a temporal resolution of 2 3 myr regional interpretations of 2d seismic reflection refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust these image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio temporal evolution of strain accumulation throughout the lithosphere during the 40 myr 170 130 ma period of late jurassic extension across this basin for the first 25 30 myr strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined as strain rates across the whole basin were consistently very low 16 s l thermally induced strength loss cannot explain this phenomenon the strain localisation is manifest in the near surface by a systematic migration of fault activity the pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control especially when estimates of flexural rigidity for this area are considered the best explanation for what is observed in this time period is a coupling between near surface strain localisation driven by brittle or plastic failure and the evolving thermal structure of the lithosphere we demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate dependence of frictional strength observed in laboratory studies during the final 10 myr of basin formation strain accumulation was focused within the axis and strain rates declined rapidly replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

latest caledonian to present tectonomorphological development of southern norway abstract the regional resultant stress field of the northeastern north atlantic has shifted significantly throughout the phanerozoic in fennoscandian parts of the caledonian orogen mountain building which was characterized by nw se contraction reference to present north was followed by a collapse with transport both parallel and transverse to the mountain chain the late palaeozoic mesozoic saw several stages of e w to nw se extension varying in time and position local episodes of inversion are traceable in some cases particularly in connection with deep seated and long lived zones of weakness the cenozoic has to a larger degree been affected by compression including folding and basin inversion again some of the more pronounced effects of local inversion are related to pre existing fault systems neogene uplift of the western mountainous area in scandinavia can be unravelled by potential field study aft data and reflection seismic sections assuming that the region is close to isostatic equilibrium the uplifted areas must be supported at depth by substantial volumes of low density material within the crust or the mantle close to the crust mantle interface or close to the lithosphere asthenosphere interfaces a series of nw se oriented cross sections representing successive stages of development from the late caledonian to present were prepared and used to analyse the tectonomorphological development as related to the shifting stress configurations three stages are inferred stage 1 devonian late permian denudation of the caledonian mountain chain during the latest stage of closing of the iapetus ocean in early to mid devonian times the caledonian mountain chain reached its maximum elevation of perhaps as much as 8 9 km including the foreland basins to the east the average topographic gradient of the eastern flank may have been in the order of 1 3 the denudation of the caledonian mountains continued into the permian and by the end of the permian the mountain system was more or less obliterated with a gradient of the eastern flank probably continued to fall to less than 0 1 stage 2 triassic late cretaceous tectono thermal uplift of central south norway uplift of central south norway was rejuvenated in triassic times and continued throughout the jurassic and early cretaceous the gradient of the eastern flank increased to a maximum of approximately 0 15 and master structural elements like the laerdal gjende olestol fault complex again played important roles in the development stage 3 tertiary present renewed tectono thermal activity and post glacial rebound a second phase of post caledonian tectono thermal uplift of south central norway started in oligocene time again increasing the gradient of the eastern flank of south norway around 0 2

depth and configuration of the crystalline basement in the viking graben area northern north sea the depth of the crystalline basement in the viking graben area has been mapped the dominant structural feature in the study area is a n s trending zone approximately 150 km wide containing a variety of structures graben basins and structural highs and named the northern north sea rift zone the viking graben is only one structural element though a major one in this zone the following depth ranges appear to apply a on platforms basement shelves and major structural highs 1 3 km b in basins 7 9 km maximum depth c in graben 8 10 km maximum depth

erosion rate and previous extent of interior layered deposits on mars revealed by obstructed landslides we describe interior layered deposits on mars that have obstructed landslides before undergoing retreat by as much as 2 km these landslides differ from typical martian examples in that their toe height increases by as much as 500 m in a distinctive frontal scarp that mimics the shape of the layered deposits by using crater statistics to constrain the formation ages of the individual landslides to between ca 200 and 400 ma we conclude that the retreat of the interior layered deposits was rapid requiring erosion rates of between 1200 and 2300 nm yr 1 we suggest that the interior layered deposits are either extremely friable if eroded strictly by wind or composed of a material whose degradation has been enhanced by ice sublimation these erosion rates also confirm that the interior layered deposits have been in a state of net degradation over the past 400 m y suggesting that the process that caused net deposition in the past has ceased or slowed substantially on mars relative to erosion our results imply that interior layered deposits with a similar morphology across mars including the mound in gale crater have probably undergone similar rapid erosion and retreat suggesting that their total modern volume underrepresents the depositional record and thus sedimentary history of mars

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

interaction of permo triassic and jurassic extensional fault blocks during the development of the northern north sea the main structural elements resulting from late permian early triassic extension have been resolved across the total width of the northern north sea the northern segment of the basin north of 61 n bounded to the west by the hutton fault and to the east by the norwegian mainland was occupied by four westerly tilted half grabens 25 35 km in width prior to jurassic extension the central segment of the basin 60 20 61 n exhibits a full graben beneath the jurassic horda platform flanked by half grabens tilted towards the basin margins the change in permo triassic fault block configuration defining structural domains is largely constrained by a basement discontinuity which runs diagonally ne sw across the basin maximum jurassic extension is represented by several centres of subsidence aligned nne ssw across a n s oriented and broader permo triassic basin jurassic extension was concentrated within the viking graben proper south of 60 n which in this area is represented by single westerly tilted en echelon half graben segments 20 25 km wide the area affected by jurassic extension becomes progressively wider to the north again governed by the basement discontinuity eventually encompassing the total width of the basin c 200 km north of 61 n the central segment of the northern north sea displays the greatest contrast in permo triassic and jurassic structural styles and basin geometry jurassic stretching and faulting was negligible to the east horda platform precisely the area which had been the site of maximum permo triassic stretching and fault activity it has generally been assumed that jurassic fault configuration along the eastern margin of the central segment of the viking graben to a large extent reflects the reactivation of major permo triassic faults this study demonstrates the interference of permo triassic and jurassic basin geometries in this area due to major faults of the two generations dipping in opposite directions as a consequence the e tilted oseberg fault block of jurassic origin and c 20km in width is underlain by a wider c 35 km and w tilted permo triassic fault block

deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and related quaternary volcanism 1 the atlas mountains are characterized by high elevations and quaternary volcanism long period magnetotelluric data acquired along a nnw sse transect reveal the presence of a conductive anomalous mantle below the high atlas data dimensionality analyses show a preferent n80 e strike of the deep resistivity structure in agreement with the induction vector alignment at long periods accordingly a 2d inversion of the data set was carried out large resistive bodies at the crustal basement most likely correspond to batholiths emplaced in more conductive metapelites they are covered by outcropping conductive sedimentary detritic and carbonate rocks lithospheric thinning producing anomalous mantle and basin development in the atlas probably started during triassic jurassic rifting inversion tectonics since the oligocene produced low shortening on previous lithospheric weak zones with thrusting of the atlas above the stable african plate melting at the top of the anomalous mantle is connected with quaternary basaltic volcanism in the middle atlas

softening the lower crust modes of syn transport transposition around and adjacent to a deep crustal granulite nappe parry sound domain grenville province ontario canada 1 the parry sound domain is a granulite nappe stack transported cratonward during reactivation of the ductile lower and middle crust in the late convergence of the mesoproterozoic grenville orogeny field observations suggest the following with respect to the ductile sheath 1 formation of a carapace of transposed amphibolite facies gneiss derived from and enveloping the western extremity of the parry sound domain and separating it from high strain gneiss of adjacent allochthons this ductile sheath formed dynamically around the moving granulite nappe through the development of systems of progressively linked shear zones 2 transposition initiated by hydration amphibolization of granulite facies gneiss by introduction of fluid along cracks accompanying pegmatite emplacement shear zones nucleated along pegmatite margins and subsequently linked and rotated the source of the pegmatites was most likely subjacent migmatitic and pegmatite rich units or units over which parry sound domain was transported comparison of gneisses of the ductile sheath with high strain layered gneiss of adjacent allochthons show the mode of transposition of penetratively layered gneiss depended on whether or not the gneiss protoliths were amphibolite or granulite facies tectonites before initiation of transposition resulting in e g folding before shearing no folding before shearing respectively meter scale truncation along high strain gradients at the margins of both types of transposition related shear zones observed within and marginal to parry sound domain mimic features at kilometer scales implying that apparent truncation by transposition originating in a manner similar to the ductile sheath may be a common feature of deep crustal ductile reworking

short lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids collision tectonics and the associated transformation of continental crust to high pressure rocks eclogites are generally well understood processes but important contradictions remain between tectonothermal models and petrological isotopic data obtained from such rocks here we use 40ar 39ar data coupled with a thermal model to constrain the time integrated duration of an orogenic cycle the burial and exhumation of a particular segment of the crust to be less than 13 myr we also determine the total duration of associated metamorphic events to be 20 kyr and of individual heat pulses experienced by the rocks to be as short as 10 years such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events earthquakes such events triggered transient heat advection by hot fluid along deformation shear zones which cut relatively cool and dry subducted crust in contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust our non steady state cold crust model satisfactorily explains several otherwise contradictory geological observations the duration of the geological processes that shape the earth generates vigorous debate mountain building where crust is consumed continuously by burial and recycled as buried remnants rise to the surface is one process where new dating methods can resolve the issue high precision dating of rock remnants left by continental collision in the caledonides of southern norway shows that the entire cycle can happen quickly in about 13 million years in addition the rapid transport of hot fluid zones through the cold crust may explain many puzzling geological observations

biodiversity patterns of early middle ordovician marine microphytoplankton in south china based on new materials from six sections and all available literature data new diversity curves are presented for the phytoplankton acritarchs from south china covering the early middle ordovician interval when the great ordovician biodiversification event took place the total diversity curve and the origination data imply that a major radiation of the phytoplankton occurred during the analysed interval a peak of the total acritarch diversity curve appears in the a suecicus graptolite biozone the diversity changes vary in the different parts of the investigated area most probably depending on the position of the analysed sections on the carbonate shelf or the slope reflecting diversity differences due to the position on an inshore offshore transect the early middle ordovician diversity pattern of the phytoplankton is compared with those of several marine invertebrate groups compared with the diversity curve peak of the acritarchs the conodonts and brachiopods reached their highest diversities before the acritarchs while the highest diversity of the chitinozoans appears slightly later the graptolites show two peaks during the early middle ordovician while the trilobites diversity curve shows a peak only in the sandbian the different fossil groups such as chitinozoans conodonts graptolites brachiopods and trilobites show therefore different evolutionary patterns to that of the acritarchs that are not yet fully understood and correlations are so far difficult the acritarch diversity changes can partly be compared to the local sea level changes from four sections in south china at a larger scale the acritarch radiation coincides with a general transgression at a regional or local scale correlations are not straightforward pointing out that more detailed data based on both acritarch studies and more precise sea level investigations are necessary

fault distribution in the precambrian basement of south norway abstract mapping of the structural pattern by remote sensing methods landsat spot aerial photography potential field data and field study of selected structural elements shows that the cratonic basement of south norway is strongly affected by a regular lineament pattern that encompasses fault swarms of different orientation age style attitude and frequency albeit counting numerous fault and fracture populations the faults are not evenly distributed and n s to nne ssw nnw sse and ne se ene wsw systems are spatially dominant n s to nnw sse structures can be traced underneath the caledonian nappes to the western gneiss region in western and central south norway emphasizing their ancient roots dyke swarms of different ages are found within most of these zones also the ostfold oslo trondheim and the mandal molde lineament zones coincide with trends of sveconorwegian post collision granites we conclude that the n s trend includes the most ancient structural elements and that the trend can be traced back to the proterozoic svecofennian and sveconorwegian orogenic events some of the faults may have been active in neoproterozoic times as marginal faults of rift basins at the western margin of baltica remnants of such fault activity have survived in the cores of many of the faults belonging to this system the ancient systems of lineaments were passively overridden by the caledonian fold and thrust system and remained mostly but note entirely inactive throughout the sub cambrian peneplanation and the caledonian orogenic collapse in the silurian devonian the system was reactivated in extension from carboniferous times particularly in the permian with the formation of the oslo rift and parts of it remain active to the present albeit by decreasing extension and fault activity

regional sedimentology and petroleum geology of marine late bathonian valanginian sandstone in the north sea abstract a large number of marine late bathonian valanginian sandstone units have been identified in the north sea north of the mid north sea high this paper discusses their complex areal distribution and outlines sedimentological models the sandstone formations are interpreted as shallow marine transgressive and regressive units interbedded with beach lagoonal and coastal plain deposits coarse grained scarp fed fans occur along fault controlled rift margins the total proven recoverable reserves in these sandstone reservoirs are 2 3 x 10 9 ton oil equivalent of which approximately one third is oil the play types are intimately linked with the mesozoic rift system of the viking witch ground and central grabens along this rift system the sand units provide good trapping potential and they are generally interbedded with or lie immediately below the kimmeridgian ryazanian source rocks so that migration paths are relatively simple and migration efficiency is high

nontypical birps on the margin of the northern north sea the shet survey summary striking similarities in the reflectivity of the crust and upper mantle on birps profiles has led to the development of the typical birp a model seismic section for the british continental lithosphere the shet survey collected in the region of the shetland islands and the northern north sea fits the general pattern to a certain extent caledonian structures and devonian or younger basins are imaged in the otherwise acoustically transparent upper crust an unexpected and exciting feature imaged on shet is a short wavelength structure on the moho or abrupt moho offset beneath the strike slip walls boundary fault shet differs markedly from the swat typical birp however by showing a poorly reflective lower crust only a narrow zone 1 s at the base of the crust contains high amplitude reflections the shet survey therefore highlights the wide variation in lower crustal reflectivity within the total birps data set rather than the similarities

extension parallel folding in the scandinavian caledonides implications for late orogenic processes abstract structural analyses of the devonian basins and basement rocks in the scandinavian caledonides show the existence of large scale e w oriented folds which affect the para autochthonous western gneiss region of the baltic craton these folds mostly occur in areas of widespread and intense late orogenic extension chronology based on structural relationships both within the devonian sediments and caledonian rocks shows that folding has been active from the first stages of the ductile extension and ended after devonian basin sedimentation folding evolved through time towards crustal scale transcurrent left lateral faulting in the zones of maximum deformation such an evolution is explained by progressive unloading of the western gneiss region due to extension which involved a permutation of the maximum stress from a vertical position at the onset of extension to a horizontal position in the subsequent stages folding parallel to late orogenic extension as well as left lateral translation on east to northeast trending crustal scale faults suggest the occurrence of a regional direction of compression perpendicular to the direction of extension this in turn argues for an external force triggering the late orogenic extensional process in the scandinavian caledonides

a comparison of the moho interpreted from gravity data and from deep seismic reflection data in the northern north sea summary we present the results of gravity modelling of the nsdp 84 deep seismic reflection profiles in the northern north sea the moho depths inferred from forward modelling of marine bouguer gravity anomalies from independent inversion of free air gravity anomalies derived from satellite altimetry and from deep seismic reflection profiling are in good mutual agreement this agreement suggests that the moho discontinuity interpreted from seismic reflection data corresponds to a major jump in density and therefore is likely to represent not only the geophysical but also the petrological boundary between the continental crust and the upper mantle in spite of the significant lateral heterogeneity of the crystalline basement below the northern north sea area as documented by seismic reflection profiling the average density contrast between the basement and the upper mantle is interpreted to show rather little variation this is not true for the deepest part of the viking graben where the nature of the lower crust and upper mantle may have been changed by magmatic processes accompanying the graben formation a relationship between the degree of crustal extension and the volume of magmatic intrusion into the lower crust is tentatively supported by our results