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

collapse basin a new type of extensional sedimentary basin from the devonian of norway pure shear and simple shear models provide interpretations of sedimentary basins formed by extension of crust and lithosphere of normal thickness devonian basins of western norway developed during collapse of the thickened caledonian crust and return to a crust and lithosphere of normal thickness because extension did not thin the lithosphere to less than its normal thickness no thermal subsidence phase followed the initial subsidence

microstructurally constrained versus bulk fault gouge k ar dating abstract structural mineralogical and geochronological data were collected from two brittle fault zones fault i and ii deforming the ordovician rolvsnes granodiorite in sw norway microstructurally constrained k ar data from cohesive damage zone samples are compared to bulk k ar data from fault gouges from the adjacent fault cores in fault i fluid ingress along the fault core caused pervasive alteration of the host granodiorite plagioclase and biotite are almost completely altered to kaolinite quartz illite and smectite in fault ii a complex deformation history is recorded by multiple fault rock microdomains including an ultracataclasite with 50 authigenic illite and an illite jarosite bearing pseudotachylyte microdomains hosting authigenic illite jarosite from both studied faults were separated characterized mineralogically and dated by k ar results confirm that each domain contains only one generation of authigenic illite jarosite and provide robust time constraints on fault initiation in the carboniferous 339 8 ma and reactivation in the permian 289 6 ma k ar geochronology of the adjacent fault gouges yields inclined age versus grain size relationships the ages of the coarsest grain size fractions are similar to the age range of the damage zone data whereas the ages of the finer fractions are significantly younger our study provides new and important insights into the dynamics of long lived fault zones isotopic signatures and structural features representative of fault initiation are preserved in the damage zones while they are progressively overprinted due to cumulative strain localization in the fault cores finally our study further clarifies the significance of age plateaus in age versus grain size plots which allows for a robust and geologically meaningful interpretation of k ar data

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

palaeomagnetism magnetic fabrics and the structural style of the hornelen old red sandstone western norway the old red sandstone of the hornelen district western norway forms part of a major fold terrane and the principal magnetization is associated with the late devonian to early carboniferous svalbardian solundian orogeny the present fault margins of the hornelen massif claimed by various authors to represent subordinate devonian splay faults associated with megashearing in the north atlantic or lateral ramps in a regional extensional tectonic setting are shown to be post devonian structures and the present outline of the hornelen massif is a mesozoic jurassic graben

the evolution of the hornelen basin detachment system western norway implications for the style of late orogenic extension in the southern scandinavian caledonides abstract the geology of the devonian hornelen basin and the surrounding western gneiss region wgr western norway has been examined in detail structural clast provenance and geothermobarometric data have been combined to reconstruct the motion history of part of a major late orogenic extensional shear zone in the southern scandinavian caledonides the nordfjord sogn detachment nsd the nsd in the hornelen basin area is a zone of mylonites 4 6 km thick tectonostratigraphically constrained within the lower and middle allochthons lma and the uppermost part of the wgr it separates eclogite bearing gneisses of the wgr from greenschist and amphibolite facies rocks of the middle and upper allochthons marking a profound metamorphic break and indicating major excision of crustal section kinematic indicators give a consistent top to the west shear sense indicating a reversal of the movement sense of the scandian nappes with the ductile strain being distributed through the entire lma section the wgr was exhumed from depths of up to 100 km in the footwall of the shear zone partly aided by pure shear vertical flattening below the nsd and extension and erosion of the hanging wall the majority of the movements on the nsd in the hornelen basin area occurred under amphibolite facies conditions prior to 410 ma subsequent shear caused limited greenschist retrogression along extensional crenulation cleavages depending on interpretation of mica 40 ar 39 ar ages this lower grade shearing may be due to a continuation of movements or renewal of motion associated with opening of the hornelen basin at about 385 ma the lack of clear evidence of penetrative greenschist facies mylonites in the hornelen area may be due to the faulted omission of tectonites of this grade following scandian a subduction and eclogite facies metamorphism at 425 ma the p t t path of the wgr shows very rapid decompression associated with early slip on the extensional detachment horizons unroofing was initially near isothermal followed by a phase of rapid near isobaric cooling after 410 ma decompression and cooling slowed as slip on the nsd slowed or even ceased by the middle devonian the wgr lay at about 10 15 km depth and the crust had almost returned to isostatic equilibrium at this time a brittle fault splaying from the nsd ramped upwards through the upper allochthon opening a scoop shaped west facing half graben the hornelen basin a further 50 km of heave on the nsd occurred during alluvial infilling of the basin subsequently the basin fill was folded by late devonian n s compression and the basin margins were modified by mesozoic faulting these features are consistent with an orogenic collapse model for southern scandinavia that involves large scale backsliding of the scandinavian caledonides within tectonostratigraphic units by this mechanism the baltic shield could have been efficiently unloaded with tectonic translations producing significant metamorphic breaks within a seemingly preserved tectonostratigraphic sequence the model favours orogenic collapse driven by an external pull rather than internal body forces

strain migration during multiphase extension observations from the northern north sea many rifts develop through multiphase extension it can be difficult however to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried using 2 d and 3 d seismic reflection and borehole data from the northern north sea we examine the style magnitude and timing of reactivation of a preexisting permian triassic rift phase 1 fault array during a subsequent period of middle jurassic to early cretaceous rift phase 2 extension we show that rift phase 2 led to the formation of new n s striking faults close to the north viking graben but did not initially reactivate preexisting rift phase 1 structures on the horda platform we suggest that at the beginning of rift phase 2 strain was focused in a zone of thermally weakened lithosphere associated with the middle jurassic north sea thermal dome rather than reactivating extant faults diachronous reactivation of the permian triassic fault network eventually occurred with those faults located closer to the middle jurassic to early cretaceous rift axis reactivating earlier than those toward the eastern margin this diachroneity may have been related to flexural down bending as strain became focused within the north viking graben and or the shifting of the locus of rifting from the north sea to the proto north atlantic our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field

a long hard road reviewing the evidence for environmental change and population history in the eastern adriatic and western balkans during the late pleistocene and early holocene the eastern adriatic and western balkans are key areas for assessing the environmental and population history of europe during the late pleistocene and early holocene it has been argued that the balkan region served as a late glacial refugium for humans animals and plants much like iberia and the italian peninsula and in contrast to the harsh conditions of eastern and central europe as post glacial amelioration occurred and sea level rose these regions to the north and west of the balkan mountains became forested and were populated by mesolithic forager fishers meanwhile to the south the domestication of plants and animals in the near east began to cause large scale environmental as well as lifestyle changes even as the balkan peninsula was a likely crossroads on the route for the spread of agriculture and herding from southwest asia into europe issues such as pre neolithic settlement the discussion of human environment interactions and the role of climate events such as the 11 4 9 3 and 8 2 ka cal bp in this critical landscape are often overlooked efforts to counter this challenge have been hampered by an apparent lack of data so that the region hardly occurs in distribution maps in part this is due to patchy research and a complicated political history which have contributed to a fragmented archaeological and paleoecological record yet as we show here there is in fact plenty of evidence available for review we present a survey of different proxies for environment and settlement throughout the late pleistocene and into the early holocene combining radiocarbon data with zooarchaeological lithic and palynological records by mapping this evidence we are able to discuss the impact of climate change during the pleistocene holocene transition and consider the role of environment and landscape on human population distribution at this crossroads in place and time

the solund stavfjord ophiolite complex and associated rocks west norwegian caledonides geology geochemistry and tectonic environment

the age and tectonic significance of dolerite dykes in western norway coast parallel dykes in sw norway primarily of permo triassic age have been linked regionally to the early tectonic evolution of the norwegian continental shelf we demonstrate from palaeomagnetic data mean declination 206 1 inclination 30 1 a 95 11 8 that dolerite dykes in the coastal sunnfjord region of western norway and immediately west of the devonian basins are also of permian c 250 270 ma age and not lower or pre devonian as previously advocated the sunnfjord dykes appear to be contemporaneous with dykes from sw norway at sotra 262 6 ma and the oldest dykes from sunnhordland 260 280 ma and geochemical data attest to a transition from sub alkaline to alkaline magmatism at the dawn of the mesozoic the sunnfjord dykes are not simple records of e w extension and magma intrusion but instead represent significant mid late permian time markers within a complex zone of fault activation and rejuvenation late mesozoic cenozoic magnetic overprinting mean declination 348 6 inclination 68 9 a 95 12 and metamorphic alteration documented by these dykes are directly dependent upon proximity to major e w brittle faults south of the hornelen devonian basin hence some motion and related fluid activity do post date dyke intrusion the e w high angle normal or oblique slip faults can be regionally traced offshore to the oygarden fault zone onshore these faults truncate the hornelen low angle detachment which in turn cuts folded d evonian strata these observations along with evidence for permian and late jurassic cretaceous extension from the nearby dalsfjord region demonstrate important reactivation of a late to post caledonian detachment and high angle fault system in western norway

complete remagnetization of some permian dykes from western norway induced during burial uplift abstract stable paleomagnetic directions in four basaltic dykes and in some associated caledonian metamorphic rocks define high latitude mesozoic paleomagnetic pole positions which are not compatible with the k ar age of 250 my determined for the dykes a monotonic increase of 40 occurs in the potassium content of samples taken across a dyke 32 cm wide this is accompanied by only a 1 2 variation in the k ar age of the samples suggesting the absence of any significant level of initial argon titanomagnetite grains having bulk compositions around x 0 6 have suffered extensive low temperature alteration forming assemblages of ferri rutile granules in a matrix of pure magnetite the complete remagnetisation of both the dykes and the associated country rocks is probably an expression of a vrm acquired at elevated temperatures 150 500 c at the emplacement depth of the dykes a stable remanent magnetisation was locked in during uplift of the area probably related to the kimmerian basin development in the adjacent north sea and the epeirogenic uplift of western fennoscandia

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

palaeomagnetism of coast parallel alkaline dykes from western norway ages of magmatism and evidence for crustal uplift and collapse summary palaeomagnetic investigation of 18 coast parallel alkaline dykes from the sunnhordland region western norway reveal the presence of multi component magnetization in half of the dykes the distribution of reversely magnetized directional end points corresponds to a kiaman permian geomagnetic field direction but the intersection of great circle trends coincide with the well grouped and stable normal magnetization present in the remaining nine dykes the stable remanence deviates significantly from any known palaeomagnetic field direction for western europe and although a non dipole field spot reading origin cannot be ruled out it is tentatively suggested that the magnetically stable dykes were intruded when the sunnhordland area was affected by a temporary eastward tilt associated with a phase of crustal uplift over the adjacent north sea area a tilt connection of 15 will bring the stable remanence directions in close agreement with the triassic jurassic pole position for western europe implying a kimmerian age of these intrusions the range of reported k ar ages of some of the dykes is not in accordance with the palaeomagnetic results and is tentatively attributed to a partial argon loss in the reversely magnetized dykes kiaman caused by reheating during the intrusion of the assumed younger sequence of the magnetically stable and normally magnetized dykes b excess argon in the assumed younger dykes inherited from xenoliths present a kiaman intrusive age of all the dykes cannot be excluded however in which case a complete remagnetization of some of the dykes occurring possibly during the proposed event of crustal uplift was associated with partial loss of argon

the sunnfjord melange evidence of silurian ophiolite accretion in the west norwegian caledonides a major composite terrane the sunnfjord melange has been identified in the west norwegian caledonides the rocks of the melange provide a terrane link between the allochthonous continental rocks of the dalsfjord suite with its cover of continental margin deposits and the oceanic terrane of the solund stavfjord ophiolite complex the melange was formed as the ophiolite was emplaced on the the fossiliferous lower to middle silurian continental margin deposits of the herland group this group unconformably overlies older metasedimentary rocks of the hoyvik group and the crystalline basement of the dalsfjord suite a structural style characteristic of thin skinned thrust foldbelts is locally preserved within the herland group on atloy and the thrust foldbelt was developed in the foreland of the continental margin during the ophiolite accretion a u pb zircon age of 443 3 ma from the ophiolite the silurian fossils in the herland group and the identification of the sunnfjord melange as an obduction melange provide the basis for a well constrained model of ophiolite accretion in the scandinavian caledonides

rock magnetic and magnetic fabric variations across three thin alkaline dykes from sunnhordland western norway influence of initial mineralogy and secondary chemical alterations summary profiles of natural remanent magnetization nrm rock magnetic properties and anisotropy of magnetic susceptibility ams across three thin 26 to 109 cm alkaline dykes from sunnhordland western norway are presented the magnetic remanence is carried by pseudo single domain psd lowtemperature oxidized titanomagnetite with circa 6 per cent of cations substituted by aluminium systematic variations in magnetic properties across the dykes are caused by differences in titanomagnetite concentration as well as grain size lowtemperature alteration and laboratory oxidation by heating in air to a maximum temperature of 600 c do not homogenize the magnetic properties across the dykes fine grained regions near dyke margins carry the most stable remanent magnetization but are also most amenable to secondary chemical remanent magnetization crm resetting between dyke variations in magnetic properties are larger than within dyke variations a stable primary remanence is found in two of the dykes while the third is totally reset by a soft viscous remanent magnetization vrm despite similar optical grain sizes magnetic mineralogy and oxidation state the difference is attributed to a lower titanium content in the latter dyke preventing effective partitioning of magnetic grains during low temperature alteration laboratory heating affects anisotropy variations across the dykes resulting in more consistent axis orientations within each dyke and symmetric edge to edge variations in the degree of anisotropy ams directions are different from those in the country rock and vary systematically across the thinnest dyke before heating

brecciation related argon redistribution in alkali feldspars an in naturo crushing study abstract alkali feldspar thermochronologic modeling with the 40ar 39ar method has generated marked advances in knowledge of the mechanisms for argon diffusion in feldspars while the goal in many cases has been to extrapolate the observed and modeled argon behavior in feldspars to natural geological settings scientific debate surrounding the true feasibility of such extrapolations and indeed the validity of thermochronologic modeling in itself have provided much impetus to improve laboratory techniques to test and increase basic understanding of argon diffusion two cornerstones for the debate over the feasibility of alkali feldspar thermochronology for modeling natural geologic processes have been 1 is volume diffusion the main mechanism for argon movement in feldspars and 2 if volume diffusion is a viable mechanism does argon then reside in discrete domains within the feldspar lattice we describe a study of alkali feldspars from a profile through a well controlled brittle fault zone in western norway the feldspars document argon loss during deformation and strongly suggest the existence of argon domains within the feldspars at least during laboratory step heating the progressive change in the character of argon diffusion is recognizable in the logr ro diffusion data from the feldspars and is mimicked by physical changes observed optically in the feldspars through progressive degrees of brittle deformation modeling results indicate a reduction in size of the biggest domains and the appearance of smaller domains during the strongest stages of deformation whether or not this reveals the existence and the transformation of the domain structure in naturo is difficult to prove from our data alone but interestingly this behaviour corresponds directly to the physical optical appearance of more intense crack networks and subgrains in progressively more brecciated feldspars because the thermochronologic histories derived from modeling the feldspar data conform very well to the known tectonic history of the area the feldspars appear to have successfully retained physical optical and isotopic records of episodic tectonic processes operating from ductile through low temperature brittle regimes in rocks with a caledonian history overprinted by several later younger geologic events however because the cold brecciation is the last tectonothermal event recorded by these rocks it is impossible to truly test for the existence of diffusion domains in naturo argon loss appears to have been effective only in the most highly brecciated deformed samples where the combination of the connected crack network increased fluid flow and higher temperatures enhanced diffusion via fast diffusion pathways and thus volume diffusion from the lattice only minor argon loss occurred in zones of lower brittle strain although some development of cracks and brittle features is evident independent of the existence of diffusion domains this study highlights the possible pitfalls when cooling histories are deduced from brecciated feldspars in which age and diffusion charateristics have been decoupled while the geochronological memory has survived and is identical to that of nonbrecciated feldspars suggesting no loss and minor effects of deformation the diffusion characteristics have been completely transposed by brecciation and the appearance of new domains modeling feldspars with these latter characteristics would effectively utilise a new feldspar diffusion structure with an old relict age memory

a paleomagnetic pole position of late jurassic early cretaceous dolerites from hinlopenstretet svalbard and its tectonic implications abstract paleomagnetic examination of late jurassic early cretaceous dolerites hinlopenstretet reveal a mean paleofield direction d 181 i 71 95 4 2 k 117 9 17 sites 96 samples which is argued to represent a deuteric magnetization formed at the time of cooling the resultant paleopole 66 n 200 e dp 7 dm 8 accords with paleomagnetic poles of similar age for north america after taking into account post jurassic continental separation the result from the present study is interpreted to represent a late jurassic early cretaceous paleopole for eurasia the similarity of the paleopoles from cratonic north america and eurasia in its pre drift configuration at about 145 ma makes it reasonable to conclude that relative displacement due to extension between eurasia and north america prior to the onset of sea floor spreading must have taken place after 145 ma and before the start of sea floor creation in the cretaceous

caledonian compressional and late orogenic extensional deformation in the staveneset area sunnfjord western norway abstract the studied area in western norway constitutes part of the hangingwall of the extensional kvamshesten detachment zone kdz the kdz separates eclogite bearing lower crust from a hangingwall consisting of a precambrian basement cover pair silurian continental margin sediments a silurian obduction melange a late ordovician ophiolite and the devonian kvamshesten basin deposited unconformably on the pre devonian rocks contractional deformation related to ophiolite obduction and to the caledonian scandian orogeny produced a suite of se verging structures developed under greenschist facies conditions as orogenic collapse commenced extensional shear zones were formed in the upper and middle crust re activating contractional shear zones and fabrics the direction of transport on the inherited fabrics was reversed from top to southeast to top to northwest and structures related to the regional extension were superimposed on the contractional structures back folding of the main contractional foliation by asymmetric w vergent folds together with nw and w directed shearing along weak lithologies and semi ductile faulting in the melange are the main structural expressions of early stages of the orogenic collapse in the staveneset area there is apparently no metamorphic break between the contractional fabrics and the earliest extensional structures the extensional structures developed however under progressively more brittle conditions the devonian sediments were deposited upon a sequence of rocks in the upper plate of the kvamshesten detachment that had already undergone significant extensional deformation and tectonic exhumation in the late silurian early devonian the extensional deformation in the upper plate of the western norwegian extensional detachments has up to recently been largely ignored in the discussion of the structural control of the formation of the devonian basins

thermochimie database developments in the framework of cement clay interactions we present here the global results of an eight year project where we attempted to complete the thermochimie database for cement minerals zeolites and clay minerals this work reports the methodology adopted to ensure the consistency between the three groups of minerals our first step concerns the relations between the three groups of minerals from field observations and experimental results gathered from the literature a summary of the selection process is then given for clay and cement minerals and a specific focus is proposed for zeolites with examples of analcime and mordenite the global consistency of the selection is investigated with respect to the main chemical tendencies found for cement clay interactions especially considering the alkalinity of the minerals consistency is checked by establishing predominance diagrams in the chemical sub systems of interest for the three groups of minerals then comparing the phase relations with respect to experimental results or field observations the methodology is illustrated using the case of gismondine and zeolite p ca

a late carboniferous palaeomagnetic pole recorded from a syenite still stabben central norway high blocking remanence components d 192 i 12 recorded in a syenite sill from the island of stabben central norway are considered to be of primary deuteric origin and to correspond to the radiometric age 297 ma the upper part of the sill and the contact gneiss however are strongly contaminated by a younger magnetic overprint d 8 i 59 the primary component may represent a reliable upper carboniferous pole determination from western scandinavia whereas the secondary component most probably relates to cretaceous or younger faulting

the tectonic significance of pre scandian 40ar 39ar phengite cooling ages in the caledonides of western norway pre silurian continental margin deposits in western norway non conformably overlying allochthonous continental orthogneisses retain ordovician 40 ar 39 ar cooling ages for phengites implying either rapid cooling immediately after a late ordovician orogenic event or less likely a slow cooling following an early ordovician or older orogeny the dalsfjord suite hoyvik group basement cover pair are probably a lateral equivalent to late proterozoic sandstones sparagmites covering the jotun nappe gneisses of the middle allochthon in central south norway the hoyvik group underwent polyphase deformation greenschist facies metamorphism t max 40 ar loss probably occurred during subsequent scandian deformation and late to post orogenic extension the hoyvik group rocks were unroofed before the wenlock time 423 428 ma and cooled through the temperature for argon retention in phengite at c 447 4 ma indicating a maximum cooling rate between 14 and 22c ma 1 through ashgill and llandovery times before being subjected to low grade metamorphism during the scandian orogeny rapid pre scandian cooling combined with peak metamorphic conditions of 450c or less may indicate that the dalsfjord hoyvik basement cover pair were avected by an orogenic event during the late ordovician caradoc time the data also suggest that the caledonian margin of baltica may have experienced a more protracted tectonism during the caledonian cycle than previously models focusing on early caledonian and tremadoc or older ophiolite obduction and the scandian continental collision between baltica and laurentia

palaeomagnetic dating of fault rocks evidence for permian and mesozoic movements and brittle deformation along the extensional dalsfjord fault western norway summary palaeomagnetic and structural studies of the dalsfjord nappe western norway show that the basal ow angle detachment dalsfjord fault is a long lived fault zone and that the most important phase of faulting was of devonian extension probably nucleated on an earlier silurian scandian thrust fault rocks produced during subsequent movements indicate that the dalsfjord fault underwent periods of brittle low angle extensional reactivation during the permian 250 260 ma and upper jurassic lower cretaceous c 150 ma corresponding with stages of major extensional movements on the continental shelf palaeomagnetic studies may be of great importance for dating faults and major movement stages in long lived fault systems the particular importance of the results is that they show that low angle normal faults can operate in a brittle upper crustal regime