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

resurgent strike slip duplex development along the hitra snasa and verran faults more trondelag fault zone central norway abstract lineament analysis of the ene wsw verran and hitra snasa faults of the long lived more trondelag fault zone mtfz has revealed the presence of a complex array of anastomosing faults and fractures this has the geometric configuration of a major dextral strike slip fault zone and compares well with characteristic patterns of braided wrench faulting e g the san andreas fault system of california the fault array recognized in conjunction with the verran fault is clearly post caledonian and is considered to define a dextral strike slip duplex system associated with the parallel hitra snasa fault are riedel like structures which tend to point to an earlier component of sinistral movement rock products present along the hitra snasa fault and its secondary faults comprise mylonites hydrothermally altered rocks and small scale recrystallized breccias along the main verran fault evidence of late polyphasal deformation is seen in several discrete episodes of brecciation hydrothermal alteration and locally pervasive prehnite and stilbite veining the fault structures occurring along the hitra snasa and verran faults are thought to have originated as a sinistral fault system of late devonian age especially for the hitra snasa lineament subsequently strike slip movement reversed in sense and shifted locus towards the verran fault system during the late jurassic or early cretaceous during this strike slip reversal some earlier fractures related to the sinistral system were rejuvenated within the stress field of the evolving dextral duplex system strike slip displacements of a similar age are known from fault complexes on the norwegian continental shelf and in the northern north sea the regional picture indicates that the mtfz was almost certainly linked to the fault systems of northern scotland prior to the opening of the viking graben

tectonostratigraphic development of the trondheim region caledonides central norway abstract the metamorphic allochthon of the central norwegian caledonides comprises a complex of discrete nappes of metasediments and igneous rocks ranging in age from probable svecofennian through vendian to silurian this southeastward translated allochthon overlies a thin cover of autochthonous vendian to cambrian sediments deposited upon a crystalline precambrian basement and is superseded by late orogenic intermontanebasinal sediments of latest silurian to middle devonian age stratigraphical sequences in higher allochthonous units are floored by oceanic tholeiitic basalts with rare subjacent sheeted dyke and gabbro units considered as fragments of an ophiolite assemblage which suffered initial eastward transport in pre middle arenig times an important orogenic event which is well represented in northern and southwestern norway the overlying ordovician silurian sequences disturbed by episodic parorogenic events embrace a variety of sedimentary facies from shallow water carbonates to deep marine terrigenous turbidites and include both island arc and marginal basin lavas and intrusives polyphase middle silurian metamorphism and deformation resulted in a complex telescoping and dissection of the lower palaeozoic rocks and their precambrian substrate with nappe translation in the order of several hundred kilometres folding and thrusting of old red sandstone molasse sediments attests to continuing tectonism well into devonian times

development of basins in the inner moray firth and the north sea by crustal extension and dextral displacement of the great glen fault abstract reflection seismic data provide evidence that mesozoic dextral movements along the great glen fault line have had an important influence on the development of the inner moray firth basin geophysical evidence further indicates that deep structure beneath the inner basin is dissimilar to that beneath the outer part and viking and central grabens in the north sea tectonic development of the inner basin can nevertheless be fitted into a pattern of north sea extensional movements which led to the formation of the graben system with which the major north sea hydrocarbon resources are associated

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

kinematic stratification in the hinterland of the central scandinavian caledonides abstract a transect through west central norway illustrates the changing geometry and kinematics of collision in the hinterland of the central scandinavian caledonides a depth section through the crust is exposed on fosen peninsula comprising three tectonic units separated by two shear zones the lowest unit exposed in the roan window is a modestly deformed caledonian granulite complex framed by a subhorizontal decollement with nw se oriented lineations and kinematic indicators showing top to the northwest transport the middle unit the vestranden gneiss complex contains relict granulites but was penetratively deformed at amphibolite facies to produce an orogen parallel family of structures during translation on the decollement shallow plunging lineations on steep schistosities are subparallel to fold axes of the dominant upright non cylindrical folds a small component of sinistral strike slip is also recorded in contrast southernmost fosen peninsula contains an abundance of cover rocks infolded with proterozoic basement in a fold nappe with shallow e dipping schistosities down dip lineations and orogen oblique top to the west shear sense indicators a ne striking sinistral shear zone separates the gneisses from southern fosen deformation in the scandian hinterland was partitioned both in space and time with orogen parallel extension and shear at middle structural levels and orogen oblique transport at shallower levels

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

scandinavia s north atlantic passive margin 1 the tectonics of the eastern passive margin of the north atlantic are reexamined the scandinavian north atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the scandinavian caledonides combined information from structural geology potential field data regional geology basin development and geomorphology made it possible to propose a new definition of the passive margin the rift shoulder is formed by linked faults defining the present rift flank these faults are the extensional structures located furthest to the east onshore norway sweden that can be linked to the rifting that led to the formation of the north atlantic this innermost boundary fault system ibf is formed by a set of normal west dipping crustal faults it extends over a distance exceeding 2000 km from the north sea across the caledonian mountain belt to the barents sea the passive margin width between the continent ocean boundary and the ibf ranges from 550 km in the south to over 700 km in mid norway to 165 km north of lofoten rifting and faulting on the ibf started in permo carboniferous and a succession of rift phases eventually culminated in continental breakup and the formation of the north atlantic basin development between permian and cretaceous was toward the future breakaway fault but faulting was also active on land from mesozoic to present onshore offshore crustal scale cross sections show the geometry of the passive margin the dip orientation of the major faults and the changes in crustal thickness

bridge structures as sense of displacement criteria in brittle fault zones abstract fault zones often occur as arrays of en echelon fractures there are two main types of en echelon patterns involving right or left stepping fractures in such arrays first generation fractures define compressive or tensile bridges the structural features of such natural bridges at any scale can be used as criteria to determine the sense of displacement small compressive bridges provide criteria based on the increase in linear density of solution seams related to the degree of asymmetry of the fault trace and to the sequence of formation of the different types of en echelon fractures the criteria deduced from large compressive bridges involve upward displacement of the material by folds and hill or mountain ranges asymmetry of the fault profile and relative chronology of the segments tensile bridges yield criteria based upon vein fillings asymmetry and relative chronolog in large scale structures some pull apart basins are shown to be bounded by normal faults parallel to but outside the strike slip segments from a comparison of analytical models with well known large scale fault zones a new criterion can be proposed in fault zones formed through coalescence of en echelon fractures the bridging segments are wide if facing leeward of the bulk movement sense and narrow if forward facing

origin damping and pattern of development of faults in granite observations of brittle fractures in the granite of the massif de la borne as well as in other rocks allow us to discuss the different hypotheses regarding the origin of faults it seems that all faults originate by shear along a preexisting plane which can be an important fracture joint former fault or a microflaw pore grain boundary the shear on the plane gives rise to an array of en echelon cracks that makes the rock less resistant and permits the lengthening of the fault we also found an arrangement of horsetail fractures damping the movements on the faults we then propose a pattern for the development of the faults they originate on preexisting plane and propagate by initiating en echelon cracks the movements are damped by horsetails which permit by faults connection in relays the creation of larger faults

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

dating fault reactivation by ar ar laserprobe an alternative view of apparently cogenetic mylonite pseudotachylite assemblages apparently cogenetic mylonite pseudotachylite assemblages are commonly reported in major fault zones they represent two very incompatible modes of deformation and complex formation mechanisms have been proposed to explain this paradox we report here one such assemblage from central norway in which apparently synchronous mylonite and pseudotachylite formation is separated by 100 ma mean muscovite ar ar laserprobe spot ages from mylonite and pseudotachylite matrix are 406 11 ma and 290 10 ma respectively in preference to a complex cogenetic model we invoke fault reactivation as a viable alternative and suggest that assumed contemporaneity in such assemblages may be invalid in many cases

stress transfer in the storegga area offshore mid norway abstract the recently discovered large gas field ormen lange within the storegga submarine slide area offshore mid norway poses important questions as to the origin of these large submarine slides and their possible implications in terms of the safety of the development and the production in this respect the present study was initiated with the main objective of investigating the possibility of stress transfer as triggering mechanism for multiple earthquake occurrence and hence for the storegga submarine slide ca 8200 years before present bp stress changes following different scenario earthquakes along the selected faults in the area are modeled and computed for a number of cases using the coulomb failure criterion we demonstrate the possibility of large earthquakes triggering multiple ruptures along neighbouring fault segments the combined effect of the co seismic slip and strong ground shaking due to a large earthquake m 7 accompanied by the stress transfer on the optimally oriented normal fault planes which are analogous to the slide scar may have played an important role for the triggering of the storegga submarine slide

fault zone reactivation kinematics and mechanisms the kinematics and mechanisms of fault zone reactivation are reviewed reactivation is dependent upon fault zone orientation and the existence of weak mylonites along these zones the recognition of reactivation within mylonite zones and the softening processes that first concentrate deformation into these and secondly provide a weak medium for reactivation are discussed attention is given to the darling mobile zone western australia and the redbank zone see obee white this symposium as examples of reactivated zones

palaeomagnetic argument for a stationary spitsbergen relative to the british isles western europe since late devonian and its bearing on north atlantic reconstruction abstract palaeomagnetic results from the devonian of central spitsbergen mimer valley formation billefjorden suggest an average palaeofield direction for the late devonian early carboniferous with declination 227 5 inclination 30 6 corresponding to a relative pole position at lat 24 s long 325 e this magnetization which is post tectonic is regarded as acquired chemically linked with the upper devonian svalbardian phase of deformation this result together with some recent data from central spitsbergen defines a devonian polar wander segment that accords extremely well with corresponding british data we interpret this in terms of no major displacement of central spitsbergen as compared with western europe since devonian time svalbard and the barents sea region have probably formed a fairly stable platform since the early devonian on the balance of available geological and palaeomagnetic evidence from spitsbergen the british isles and newfoundland the north atlantic region does not appear to have been subjected to strike slip motions in the order of thousands of kilometers in or since the late devonian

numerical modeling of cenozoic stress patterns in the mid norwegian margin and the northern north sea numerical modeling of cenozoic stress patterns in the northern north sea and the mid norwegian margin is presented and the sense of potential slip along major fault planes belonging to the two areas is restored we assume that the main regional source of stresses is the atlantic ridge push as demonstrated by previous studies furthermore we also assume a nearly consistent nw se strike for the far field stress from continental breakup between greenland and norway earliest eocene to present day first we applied the commercial two dimensional distinct element method udec to simulate cenozoic stress and displacement patterns in the study area variations in rheology and major fault zones were introduced into the model the more trondelag fault complex and its inferred continuation into the shetland platform forms the major mechanical discontinuity in the model second we used the sortan method developed at the university of paris vi to predict the sense of potential slip along major fault planes the input for the sortan model was constrained by the geometry of the selected fault planes and local principal stress directions extracted from the udec modeling our results show that the more trondelag fault complex and its inferred continuation into the shetland platform act as a weak fault zone this fault zone divides the study area into two different stress provinces the continental margin and the northern north sea this result agrees well with the observed differences in cenozoic structural evolution of the two areas compressive structures are observed along the continental margin whereas relative tectonic quiescence characterizes the northern north sea during the tertiary the restored stress patterns in the northern north sea and the mid norwegian margin also agree well with the observed present day stress configuration our analysis demonstrates a method to reconstruct the sense of slip on major fault planes by combining two complementary numerical tools udec and sortan as a result it is demonstrated that oblique slip motions are mainly expected in particular strike slip and reverse dip slip faulting are simulated

cretaceous tectonics off mid norway implications for the rockall and faeroe shetland troughs between 62 and 68 n on the norwegian continental margin the hypothesis of a cretaceous or permian ocean extending northwards from the rockall trough can be tested with data from commercial reflection seismic profiles two major mesozoic basin systems occur the helgeland basin is a late triassic and early jurassic depocentre with a thin upper jurassic and cretaceous cover except in two late cretaceous sub basins the more lnner voring basin system is a middle to late cretaceous depocentre in which the pre cretaceous section is deeply buried and rarely seen on seismic profiles the two basin systems are separated by a complex zone of faulted 9marginal highs9 active in jurassic times and reactivated during cretaceous times pre cenomanian lavas or sills are interpreted within the more basin and a pre cretaceous block faulted sequence can be identified across the inner voring basin except in a narrow axial zone these features suggest the presence of an extremely narrow zone of oceanic crust of cretaceous age fault patterns confirm a major tectonic episode of mid cretaceous age the geology off mid norway supports a mid cretaceous age for sea floor spreading in the rockall trough but suggests that the generated oceanic crust narrowed northwards and that plate motion was partially accommodated by dextral transcurrent faulting reconstruction of jurassic tectonic trends in nw europe must allow for subsequent cretaceous distortion

reconstructing morphological and depositional characteristics in subsurface sedimentary systems an example from the maastrichtian danian ormen lange system more basin norwegian sea understanding large scale sediment distribution patterns and morphological characteristics in subsurface sedimentary systems is highly challenging and generally requires regional seismic and well coverage here we test a method that aims to predict first order morphological characteristics and type of sedimentary transport system in ancient source to sink systems based on trends observed in submodern pliocene holocene depositional environments an example from the paleocene ormen lange system more basin norwegian sea demonstrates the application of the method and several descriptive parameters are estimated for this ancient subsurface system in the ormen lange system basin floor fan and distal slope parameters are well constrained from seismic and well control however knowledge of the morphology and relationships between upper slope shelf and catchment characteristics and their relationships to deep water systems is poor and these are the parameters that are discussed in this study estimated parameters of catchment size derived from this technique are in good agreement with preserved remnants of fluvial valleys located onshore predicted sediment transport characteristics are also comparable to the depositional mechanisms interpreted from cores and well logs suggesting a small tectonically active system with high fluvial discharge and low sediment storage potential in the catchment and shelf subenvironments the discussed method is thus capable of predicting first order segment characteristics in subsurface sedimentary systems with an uncertainty of one to two orders of magnitude this information can be used to increase the understanding of unexplored basins or to add data and uncertainty ranges to well known petroleum systems

the role of fault reactivation and growth in the uplift of western fennoscandia new structural data published aft data and topographic data suggest that the latest cretaceous cenozoic uplift of western norway was associated with normal reactivation of the more trondelag fault complex reactivation focused along the base of today9s topographic rise with maximum displacements in the order of 2 3 km structural and aft data indicate that reactivation occurred along a displacement gradient with least normal displacement in the ne and increasing displacements towards the sw reactivation commenced in connection with the triassic to earliest cretaceous phases of rifting on the norwegian margin and probably continued through most of the tertiary the late cretaceous to cenozoic topographic development of the scandes mountains followed structurally induced templates such as those described from other areas of active normal faulting a fault model for the cenozoic uplift of the norwegian mainland provides a framework for provenance erosion and transport of sediments eventually deposited in the offshore post rift basins in the latest cretaceous palaeogene and neogene the asymmetric topographic profile of fennoscandia is reflected in the aft data and suggests that lithospheric flexure places a first order control on the shape of the scandes mountains and the central fennoscandian craton

a regional scale geochemical survey of soil o and c horizon samples in nord trondelag central norway geology and mineral potential abstract a highly efficient low density sampling strategy was employed to study the geochemical expression of geological bodies and the mineral potential on the county scale in central norway soil o and c horizon samples n 752 were collected in nord trondelag and parts of sor trondelag and analysed for 53 chemical elements ag al as au b ba be bi ca cd ce co cr cs cu fe ga ge hf hg in k la li mg mn mo na nb ni p pb pd pt rb re s sb sc se sn sr ta te th ti tl u v w y zn and zr and pb isotopes in an aqua regia extraction at the sample density of one site 36 km2 the four metal deposits which have been mined in the area within the last 50 years were all detected as geochemical anomalies in addition a number of new anomalies that may warrant follow up surveys were found in terms of geology the grong olden culmination is marked by a distinct 206pb 207pb isotope anomaly geochemical differences distinguish the most important belts of mafic metavolcanic lithologies in the area though the fosdalen iron ore deposit is only marked in the soil o horizon the c horizon outlines the more prominent anomalies of possibly economic interest climatic factors like the input of marine aerosols along the coast are clearly visible in the soil o horizon low density geochemical mapping of two sample materials provides important complementing information for the interpretation of the geochemical variation in nord trondelag county

devonian orogen parallel opposed extension in the central norwegian caledonides late orogenic early to middle devonian extension in the scandinavian caledonides was unidirectional in western norway new data from two detachment zones hoybakken and kollstraumen north of the more trondelag fault complex show that bidirectional opposed orogen parallel extension dominated in this region at this time the fault complex acted as a transfer zone for the hoybakken detachment extension and uplift in central norway triggered significant magmatic activity in contrast to the lack of granite intrusions during exhumation of western norway

accretion and lateral extension in an orogenic wedge evidence from a segment of the seve ko li terrane boundary central scandinavian caledonides abstract the seve ko li boundary in the central scandinavian caledonides represents the transition from baltica continental margin related rocks seve to outboard westerly derived terranes ko li in this study we present and discuss structural and kinematic data recording the syn and post accretionary evolution of this boundary in west central sweden amphibolite facies thrusting mylonitization imbrication and folding occurred before and during the establishment of the seve ko li boundary subsequent asymmetric lateral extension considerably modified earlier structures and the boundary at lower grade contractional ramps and extensional synforms developed and translation continued during progressive deformation kinematic analysis indicates dominant movement towards the east or southeast local movement in the opposite direction may be due to stretching and rotation in the hanging wall above extensional shear zones a pattern of minor shear zones and shear bands demonstrate stretching in an approximately e w direction the presence of extensional structures developed during top to the east movement suggests alternative mechanisms for the formation of some characteristic caledonian structures e g late synforms and thin lenticular tectonic units these features may be due to asymmetric extension during scandian top to the east motion of the nappe pile contemporaneously basement antiforms may have been stacked at lower tectonic levels

pb concentrations and isotope ratios of soil o and c horizons in nord trondelag central norway anthropogenic or natural sources abstract soil o and c horizon samples n 752 were collected at a sample density of 1 site 36 km 2 in nord trondelag and parts of sor trondelag c 25 000 km 2 and analysed for pb and three of the four naturally occurring pb isotopes 206 pb 207 pb and 208 pb in a hno 3 hcl extraction soil o and c horizons are decoupled in terms of both pb concentrations and pb isotope ratios in the soil c horizon the grong olden culmination a continuous exposure of the precambrian crystalline basement across the general grain of the caledonian orogen is marked by a distinct 206 pb 207 pb isotope ratio anomaly no clear regional or even local patterns are detected when mapping the pb isotope ratios in the soil o horizon samples variation in the isotope ratios declines significantly from the soil c to the o horizon on average pb concentrations in the o horizon are four times higher and the 206 pb 207 pb isotope ratio is shifted towards a median of 1 15 in comparison to 1 27 in the c horizon it is demonstrated that natural processes like weathering in combination with plant uptake need to be taken into account in order to distinguish anthropogenic input from natural influences on pb concentration and the 206 pb 207 pb isotope ratio in the soil o horizon

patterns of basement structure and reactivation along the ne atlantic margin a lineament pattern on the ne atlantic margin is discussed illustrated by gravity and magnetic images in the norwegian sea and reviewed in the context of onshore field evidence while most possible fault trends exist three major sets predominate a ne sw left stepping lineament set defines the gross geometry of the margin while interposing northerly trends impose a rhomboidal geometry at a variety of scales the margin is segmented by nw se transfer zones sometimes involving significant offsets the principal trends are primarily a function of mesozoic cenozoic plate wide extensional stress fields certain proterozoic and caledonian lineaments were however opportunistically reactivated according to the extension direction caledonian ne sw orogen oblique shears typified by the more trondelag fault zone were reactivated via jurassic strike slip or oblique slip and were further exploited during cretaceous early cenozoic extensional episodes leading to continental break up jurassic e w extension may also have reactivated n s faults existing in the basement or generated in duplex systems between the ne sw shears precambrian and caledonian basement lineaments striking at a low angle to the extension direction probably predisposed the formation of major transfer zones

late jurassic half graben control on the siting and structure of hydrocarbon accumulations uk norwegian central graben abstract the late jurassic structure of the central graben is discussed both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures the regional kinematics of the north sea rift suggest that during the late jurassic the central graben opened approximately orthogonally to the main nnw sse fault trend within the basin some evidence that triassic extensional faults were reactivated at this time is seen towards the basin margins and a significant triassic extensional history is inferred the late jurassic basin comprises the axial c 100 km of the central graben s l the basin axis is offset progressively to the se by both transfer zones and discrete transfer faults footwall uplift of the basin margins during extension provided the source for the upper jurassic reservoir sands encountered by exploration drilling these sands were probably preferentially fed into the basin at transfer zones in the margins salt structures are ubiquitous within the central graben and their siting is everywhere controlled by the underlying fault block topography salt structures form preferentially towards the crests of half graben or buttressed against their boundary faults inversion structures in the norwegian sector reactivated with a reverse sense of displacement late jurassic extensional faults a model is proposed which shows the two main inversion structures feda graben norway tail end graben denmark to be detached on antithetic reverse faults riding in the salt these faults terminate at tip folds within major salt walls the geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late jurassic half graben

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

the middle to late devonian hoybakken detachment central norway 40ar 39ar evidence for prolonged late post scandian extension and uplift the regionally significant 0 5 2 km thick hoybakken detachment in central norway bounds the southern margin of the central norway basement window and exhibits a well developed top to the wsw fabric characteristic of late scandian devonian ductile extension 40 ar 39 ar data obtained from hornblende mica and k feldspar mineral separates of rocks collected in a transect through the hoybakken detachment yield well defined plateau and isochron mineral ages early devonian exhumation and cooling of the hoybakken detachment footwall is recorded by hornblende ages of 400 ma and mica ages of 390 ma the mylonitic fabric overlying the footwall records younger middle devonian mica crystallization ages of 384 381 ma that are among the youngest extensional ductile fabrics dated in the caledonides and suggest prolonged extensional activity on the hoybakken detachment after inferred cessation of ductile extension at 381 ma the rate of uplift and cooling was reduced and the footwall records late devonian to early carboniferous k feldspar ages of 371 356 ma prolonged extensional activity at hoybakken is compatible with recent u pb ages of deformed titanite crystals and established rb sr ages of white mica in shear related pegmatites both from the southwestern part of the fosen peninsula and 40 ar 39 ar ages of syn tectonic mica overgrowth from the adjacent hitra snasa fault together these ages suggest the onset of ductile extension soon after 401 ma and with the middle devonian crystallization ages determined here suggest that ductile extension on the hoybakken detachment had a duration of 11 20 ma the youngest age of 320 ma was obtained from a k feldspar in a cataclastic granite of the hoybakken detachment s hangingwall and is considered to date a phase of post scandian brittle deformation that overprinted the mylonitic shear fabric

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

kinematics of the hoybakken detachment zone and the more trondelag fault complex central norway the tectonic disintegration of the caledonian orogen through combined extension contraction and strike slip was characterized by spatial and temporal strain partitioning through a period of at least 30 ma early to mid devonian exhumation of the central norway basement window was associated with retrograde top to the sw extensional shearing in the hoybakken detachment zone sinistral shearing along the more trondelag fault complex and formation of extension parallel folds progressive exhumation led to increasing strain localization and to the transition from ductile to brittle deformation in the interval between c 370 and 320 ma the hoybakken detachment fault cut previously folded detachment mylonites capturing mylonites in its hanging wall 40 ar 39 ar mica and k feldspar ages indicate a late devonian or younger age for the uppermost parts of the adjacent old red basin gentle folding of this stratigraphic level attests to the continuation of shortening and orogen oblique extension into late devonian carboniferous time shortening was intensified along strands of the more trondelag fault complex as shown by mutually cross cutting reverse and strike slip faults flower structures may be particularly common in constrictional strain systems where strike slip faults develop parallel to the principal elongation trend but normal to the principal axis of shortening

kinematics of post caledonian polyphase brittle faulting in the sunnfjord region western norway abstract structural studies in the sunnfjord region of western norway focused on faulting episodes that post date the major period of early devonian extension clarify the style and prolonged complex evolution of brittle faulting major brittle structures in the region can generally be divided into 1 e w faults and 2 n s fault joint zones with subordinate ne sw and nw se structures in close association with these large scale features four distinct groups populations of mesoscopic faults can be distinguished with basis in common characteristics spatial distribution orientation sense of slip and cross cutting relationships two reference areas the eastern parts of the kvamshesten and hornelen devonian basins feature clear cross cutting relationships of major faults and folds such relations and kinematic patterns of the distinct populations of mesoscopic faults and their relationship to dated fault rocks and dykes form the basis for a four stage history of post early devonian brittle faulting stage 1 is characterised by n s shortening expressed by thrusting and folding related to either late devonian carboniferous transpression or compression stage 2 is characterised by n s shortening and e w extension on conjugate strike slip faults and bisecting normal faults and is probably related to the regional e w folding during progressive shortening stage 3 is characterised by approximate e w extension and is manifested as low angle silicified fault breccias of possible permian age that mainly bound the eastern margin of the devonian basins stage 4 features n s extension on steep e w striking normal faults that possibly relates to a regional late jurassic early cretaceous extensional phase significant throw 500 1000 m on some of the faults emphasise the importance of late devonian and younger faulting indicating that these deformation episodes had a regional extent and therefore also occurred in the neighbouring offshore areas the established four stage fault evolution is examined in the light of known episodes of rejuvenation of faults on the nearby shelf and along the more trondelag fault zone emphasising the importance of movement on master faults with respect to local strain conditions in the sunnfjord region