the saamian of the belomorian mobile belt new geochronological constraints abstract most gneisses and amphibolites of the belomorian belt in the white sea region have previously been considered to represent the oldest saamian crustal component in the baltic shield which has been referred to the early archaean new u pb zircon ages of some of the most typical saamian rocks from the northern segment of the belomorian belt are reported these are 1 2 65 2 60 ga for the main rebolian event of folding high p metamorphism and migmatization 2 2 8 ga for the protolith of an orthogneiss in a granulite facies relic area within the region of rebolian folding on the eastern shore of lake kovdozero 3 2 74 ga for tonalitic orthogneisses that dominate the belomorian gneiss terrain around kandalaksha bay of the white sea 4 2 4 ga for a gabbro granite intrusion on the tolstik peninsula of the white sea this intrusion has previously been considered to be one of the type rock units of the early archaean these and other age determinations previously only reported in russian constrain the age of the assumed saamian in the belomorian belt in conjunction with new field observations they place most of the dated rock units within the same age group as the late archaean lopian rocks as a result of the age determinations the concept of a single array of late archaean geodynamic events in the baltic shield is advanced the belomorian belt is interpreted as an edge of the late archaean granite greenstone terrain in karelia which had been involved in strong structural reworking during an event of tectonic stacking and thrusting 2 65 2 60 ga ago part of the belomorian belt was subsequently also affected by palaeoproterozoic deformation the new results throw doubt on the previously assumed late archaean lopian age of some metavolcanic rock units occurring within the belomorian belt but overlying the presently dated orthogneisses these metavolcanics which comprise for instance the so called kolvitsa belt and its continuation towards the northwest are assigned tentatively to the palaeoproterozoic they may be age equivalents of lapponian sumian 2 5 2 4 ga old rocks underlying the lapland granulite belt

magnetic vs structural fabrics whipple detachment structure whipple wash california abstract orientations of ellipsoids of anisotropy of magnetic susceptibility ams are compared with those of mesoscopic structural features for the breccia and upper mylonite zones for the whipple detachment zone in whipple wash whipple mountains eastern california the anisotropy of the breccia which ranges up to 5 is much less than that of the mylonitic gneiss which can exceed 100 slight differences in trend are evident between the ams ellipsoid elongations and those of mineral lineations maximum axes 1 of the ams ellipsoids for both breccia and mylonite trend generally ssw nne with shallow plunge whereas mineral stretching lineations in the mylonite gneiss have trends near ne sw with shallow plunge measured fold axes are close to the 1 axes ssw nne 1 axes in the breccia show more variation e w and ssw nne at different structural levels mesoscopic mineral lineations in the breccia were not discernible for the planar elements ams ellipsoid flattening is generally more closely parallel to foliation surfaces in the mylonite than in the breccia this is evident from overlapping minimum susceptibility axes 3 and foliation pole clusters 3 axes are more scattered than are poles of foliation the scatter in 3 is possibly the superimposed susceptibility effect of several structural surfaces of different ages joints small faults transposed metamorphic foliations and mylonitic foliation especially interesting are the discovery of good 1 definitions in the breccia the apparent lack of correlation of 1 axes with the very prominent stretching lineations in the mylonite and the similarity of the 1 axial trends with those of the fold fracture pole patterns

results from a detailed aeromagnetic survey across the northeast newfoundland margin part i spreading anomalies and relationship between magnetic anomalies and the ocean continent boundary abstract a detailed aeromagnetic survey carried out across the northeast newfoundland margin clearly shows the presence of sea floor spreading anomalies 25 to 34 correlation of these anomalies with synthetic profiles shows an increase in the rate of spreading soon after anomaly 27 time three fracture zones can be identified by dislocations in the magnetic anomalies their positions are confirmed on the depth to basement map of this region an eastward extension of the southernmost fracture zone at latitude 49 n matches well with the faraday fracture zone across the mid atlantic ridge and with a basement ridge known as pastouret ridge mapped off goban spur by combining the present survey data with the previously collected shipborne measurements we have also traced the westward continuation of the charlie gibbs fracture zone under the newfoundland shelf a large amplitude magnetic anomaly lies along the margin and separates two zones with different magnetic characteristics long wavelength small amplitude anomalies on the landward side and quasi lineated anomalies on the seaward side seismic data compilations show that this large anomaly coincides with the ocean continent boundary at most places north of flemish cap modelling of the magnetic anomalies indicate that the large amplitude anomaly is caused by the juxtaposition of highly magnetized oceanic crust against weakly magnetized continental crust this situation is similar to that observed across the goban spur margin which is a conjugate of the flemish cap margin the presence of highly magnetized oceanic crust landward of anomaly 34 and within the cretaceous magnetic quiet zone is attested to by the presence of similar large amplitude anomalies south of the flemish cap and goban spur regions but these do not mark the ocean continent transition

crustal structure of the goban spur continental margin northeast atlantic from deep seismic reflection profiling deep reflection data across the uk continental margin are used to test theories concerning the origin of a pervasive reflective fabric in the lower continental crust seen on many deep reflection profiles worldwide the origin of the reflections is at present little understood but clearly represents an important crustal process the western approaches margin wam line was designed to determine the behaviour of the lower crustal reflections in crust which has undergone significant extension the wam profile across the continental shelf is characterized by bright continuous lower crustal reflections some of which reach 30 km in length and have minimum reflection coefficients of 0 1 deep water multiples in the area of the continental slope partially obscure reflections from the lower crust but detailed analyses show that reflectors are present in the lower crust on the continental slope adjacent to the continent ocean boundary the lower crust on the wam profile is well imaged and free of noise and multiples this very thin crust exhibits the same reflectivity pattern as is observed on the continental shelf i e the upper half of the crust is blank and the lower half contains sub horizontal reflections the lower crustal reflections adjacent to the continent ocean boundary are less continuous than the reflections seen on the shelf and are interpreted as being the remnants after extension of the lower crustal reflectors observed on the continental shelf the lower crustal reflectors are thus interpreted to pre date the extensional episode that led to the opening of the atlantic the preservation of thinned sections of both transparent upper crust and reflective lower crust adjacent to the continent ocean boundary suggests that extension by pure shear was the mechanism by which the crust was thinned in the goban spur area

results from a detailed aeromagnetic survey across the northeast newfoundland margin part ii early opening of the north atlantic between the british isles and newfoundland abstract poles of rotation for the north atlantic have been derived from the results of a new aeromagnetic survey northeast of newfoundland reconstruction of the north atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the atlantic from flemish cap and goban spur to the azores gibraltar fracture zone a group of similar anomalies has also been identified in the bay of biscay north of goban spur and flemish cap these anomalies follow the ocean continent boundary poles of rotation derived for this anomaly show that it forms an isochron 100 110 m y during the long cretaceous normal polarity interval the cause of this anomaly is not definite but it may represent an increase in the magnetization of the crust during a limited time within the cretaceous magnetic quiet zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by raymond and labrecque the north atlantic has also been reconstructed at the time of the initial opening in the region between flemish cap and the charlie gibbs fracture zone using inferred ocean continent boundaries on the west and east sides it has been shown that the entire region could not have saparated at one time but that spreading between the british isles and newfoundland had to progress from south to north consequently when active sea floor spreading was taking place between goban spur and flemish cap about 110 m y the region to the north was still being stretched the calculated amount of stretching as derived from the reconstructions about 25 agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region and with the results of deep seismic studies active spreading in the north started about 100 m y ago

the role of fracture zones during early red sea rifting structural analysis using space shuttle radar and landsat imagery the structural pattern of the red sea region an area where continental lithosphere is cut by a young oceanic rift was studied by means of satellite and space shuttle radar imagery the space shuttle mission 2 made two passes over the red sea bracketing a region transitional between the southern red sea where true seafloor spreading started about 5 ma bp and the northern red sea where crust is probably continental though thinned and stretched structures on these passes define bands of shearing in the african and arabian margins parallel to the dead sea fracture zone 000 020 combinations of these data with landsat information reveal four shear bands along the margins of the red sea and one oblique to the gulf of suez we have named the northern three red sea shear zones detected on the sir a data the brothers dead sea fracture zone the zabargad fracture zone and the shagara fracture zone two sets of normal faults were also detected trending 330 340 parallel to the northern red sea coastline and 310 320 parallel to the axial deeps in the red sea a map of the 310 320 faults bordering the red sea resembles a v shaped wedge narrowing to the north near the gulf of suez these faults may have formed as a result of the most recent push of seafloor spreading if this is so then there may be a propagating tectonic wedge moving from south to north which thins the continental crust up to distances of 300 km from the axis of spreading following continental deformation oceanic crust may be emplaced in 9hot points9 along the rift axis these 9punctiform9 oceanic spreading centres should propagate in the same direction as continental rifting during this episode palaeo shear zones probably act as barriers 9locked zones9 to the axial propagation one such case is at the zabargad fracture zone where the oblique impact of the propagating rift against the palaeo shear may have placed the red sea9s western side under compression and uplift while the eastern side may have been subject to oblique extension with the formation of pull apart basins the space shuttle imagery allows integration of some of the northern red sea morphotectonic features with structures on the continent

structural framework and tectono stratigraphic evolution of the porcupine seabight basin offshore western ireland abstract the porcupine seabight basin is an intra cratonic rift basin which evolved through recurrent episodes of lithospheric stretching structure contour and isopach maps presented here have been constructed following the interpretation of approx 10 000 km of seismic data together with all the available well data this integrated database has allowed the tectono stratigraphic history of the basin to be traced in detail from its earliest inception the basin was initially formed during the carboniferous when up to 3 7 km of autuno carboniferous sediments were deposited on a caledonian and hercynian structured basement late permian and early mesozoic sediments at least 1 km thick are preserved in contiguous grabens and their presence is also inferred within localized sub basins late liassic crustal doming heralded the onset of the final rifting episode and in the north promoted severe erosion and a rapid discharge of clastic sediments rapid subsidence block faulting and magmatism accompanied the deposition of up to 3 5 km of middle upper jurassic sediments in asymmetrical sub basins the transition from stretching to passive subsidence across the jurassic cretaceous boundary in combination with localized inversion abrupt eustatic and environmental changes generated a basin wide unconformity the rift drift transition in the adjacent proto atlantic rift system is recorded in the porcupine seabight basin by voluminous volcanism and a late lower cretaceous unconformity cretaceous sediments are strongly transgressive and up to 6 km thick these were superseded during the early palaeogene by several regressive transgressive cycles before the arrival of an upper slope fauna in the mid middle eocene cenozoic sediments are up to 4 km thick and record the interplay between 1 the location of the basin pn the pyrenean alpine foreland superimposed on the periphery of the icelandic plume and 2 its development as a marginal atlantic shelf region the former is thought to be responsible for eo oligocene faulting subsidence and igneous activity in the north east part of the basin and the latter for unconformities and a change in the style of sedimentation principally due to oceanic circulation changes

the structure and subsidence of rockall trough from two ship seismic experiments coincident multichannel seismic reflection and refraction profiles acquired northwest of britain during a two ship seismic experiment in the southern rockall trough provide evidence for a thin 6 km syn rift crust below rockall trough the variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the north atlantic or with thinned continental crust heavily intruded by syn rift igneous rocks the syn rift crust beneath rockall trough is now buried by nearly 5 km of sediments that are intruded by tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally the basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which they interpret as sediments intercalated with submarine lava flow generated during the opening of the trough estimates of stretching from crustal thinning and from subsidence indicate that rockall trough has undergone extensive rifting beta 6 upwelling asthenosphere beneath the thinned lithosphere generated at least 1 3 km thickness of melt as it decompressed the molten rock rose upward from the mantle until it was in part extruded as lava flows and in partmore intruded into the crust less

major cretaceous volcanic province in southern rockall trough abstract a group of three large curvilinear ridges called the barra volcanic ridge system has been mapped in the acoustic basement of southern rockall trough typically each ridge is about 2 km high and 20 km wide at its base a crudely layered acoustic character moderate density but high strength of magnetisation point to a volcanic sedimentary ash composition for the ridges seismic continuity with the acoustic basement of the rest of rockall trough suggests that the trough basement is of similar composition an age for the ridges of lower cretaceous is indicated by well ties and consideration of regional geology volumetrically the ridges are on the scale of hot spot features such as the wyville thomson ridge

pre mesozoic geology of the western and north western irish continental shelf exploration wells drilled offshore west and north west of ireland penetrated a c 3 7 m cumulative thickness of permo carboniferous sediments overlying dalradian metasediments intruded by caledonian granitic plutons in the north late tournaisian to early visean clastics were deposited coeval with the rheic transgression upon an emergent craton in a continental to marginal marine environment they record an increasing marine influence upwards with the establishment of a carbonate shelf in the asbian clastic carbonate sedimentation which became cyclical in late visean to early namurian temporally correlates with the yoredale cyclothems younger namurian sediments are not preserved sedimentation in early westphalian progressed from prodelta to coal enriched delta top and abandonment facies and is conformably succeeded by brackish to freshwater sediments that contain evidence for local marine incursions of stephanian age an autuno silesian section that is preserved offshore may also have been deposited onshore prior to erosion seismic data enable partial delineation of n s and ne sw early sub basins marginal to the main porcupine basin these sub basins which contain silesian sediments may have styled successive extensional episodes in the basin and augment support for rift models of carboniferous basin development in the north atlantic nw european domain birps data provide supporting evidence for the offshore prolonga tion of pre mesozoic basement fault zones

gravity and magnetic studies of crustal structure across the porcupine basin west of ireland abstract gravity magnetic and seismic data from a profile across the porcupine basin are used to suggest a model for the crustal structure in the region the 280 km long profile bears wsw off the southwest coast of ireland and overlaps partially with the coole 3a and 3b lines of makris et al 13 the gravity data are processed to produce an isostatic residual anomaly which is then modelled by two and a half dimensional methods using the seismic data to provide geometrical constraints similar modelling techniques are used for the magnetic data the final model shows crystalline crust which thins from 28 km at the eastern end of the profile to less than 8 km beneath the central part of the basin the thinned crust is intruded by dense magnetic bodies whilst the eastern margin is underlain by a large low density body which is assumed to be a granite these new findings have parallels in other basins on thinned and rifted crust

anomalous magnetic properties in rocks containing the mineral siderite paleomagnetic implications sampling and magnetic measurement of a 1 m bed in a new road cut of the upper cretaceous austin chalk northeastern texas has yielded anomalous magnetic results initial measurement of the anisotropy of magnetic susceptibility ams indicated unusually high anisotropies and low bulk susceptibilities characteristic of a magnetocrystalline anisotropy which might be expected for siderite feco3 natural remanent moments nrm for these samples were low 1 10 4 a m2 and directions were typical for samples which had acquired a normal geomagnetic field overprint at the site periodic remeasurement of the nrm yielded an increase in moment for some samples and periodic reversals of rm in a direction parallel or antiparallel to the ambient field in the laboratory isothermal remanent moments of these samples saturates at low induction values 200 mt ams remeasurement over a period of weeks to months revealed a general decrease in the anisotropy magnitudes an increase in susceptibility and a change in principal axis orientations these data were compared with heating oxidation experiments on austin chalk siderite ore and clastic sediments with siderite cements all of these results are consistent with the x ray diffraction and mossbauer data which indicate the presence of siderite in the austin chalk samples the mossbauer spectra of the samples obtained at room temperature and 78 k indicate the presence of approximately 80 pyrite 10 siderite and 10 of a clay component possibly chlorite when only the iron bearing mineral components are considered after sampling exposure to the air and subsequent oxidation in the laboratory the siderite in austin chalk samples appears to have altered to fe2o3 maghemite or fe3o4 magnetite increasing the magnetic moment and changing the nrm and ams directions in the samples the continuing changes appear to reflect a conversion from the less stable fe2o3 to fe2o3 hematite or oxidation of fe3o4 to fe2o3 we infer from our study that oxidation of siderite bearing limestone e g during the formation of unconformities or due to low magnitude tectonic events may produce a very stable secondary magnetic moment in the rock such changes may be very important when interpreting paleomagnetic data from limestones

from continental hyperextension to seafloor spreading new insights on the porcupine basin from wide angle seismic data key points new analysis of wide angle seismic data from the southern porcupine basin evidence for presence of oceanic crust in the southern porcupine basin jurassic rifting propagated from south to north resulting in non uniform strain when rifting stopped the deep structure and sedimentary record of rift basins provide an important insight into understanding the geological processes involved in lithospheric extension we investigate the crustal structure and large scale sedimentary architecture of the southern porcupine basin offshore ireland along three wide angle seismic profiles supplemented by thirteen selected seismic reflection profiles the seismic velocity and crustal geometry models obtained by joint refraction and reflection travel time inversion clearly image the deep structure of the basin our results suggest the presence of three distinct crustal domains along the rifting axis a continental crust becoming progressively hyperextended from north to south through the basin b a transitional zone of uncertain nature and c a 7 8 km thick zone of oceanic crust the latter is overlain by a 8 km compacted upper paleozoic mesozoic succession and 2 km of cenozoic strata due to the lack of clear magnetic anomalies and in the absence of well control the precise age of interpreted oceanic crust is unknown however we can determine an age range of late jurassic to late cretaceous from the regional context we propose a northward propagating rifting process in the porcupine basin resulting in variations in strain along the rift axis

determining continuous basins across conjugate margins the east orphan porcupine and galicia interior basins of the southern north atlantic ocean abstract the east orphan porcupine and galicia interior basins are several of the most promising basins for hydrocarbon exploration along the rifted continental margins of the southern north atlantic ocean despite having formed at similar geological times the basins exhibit fundamental differences in symmetry crustal thickness sedimentary cover thickness and amount of extension interpretation of seismic reflection and well data was integrated with published 3d grids of depth to basement and moho proxy depth to interpret and restore select seismic reflection lines publicly available kinematic evolution models were also integrated to evaluate and compare the restored seismic reflection lines in a more global regional context interpretation of five seismo stratigraphic units and three tectonostratigraphic megasequences along the east orphan porcupine and galicia interior basins reveals similar seismic character for each unit and comparable tectonic history the structural restoration of the selected lines indicates that evolution sedimentary cover thickness faulting style crustal structure and kinematic evolution of the east orphan and porcupine basins differ significantly a variable and asymmetrical crustal structure is found in the east orphan basin contrasting with the elongated and symmetric porcupine and galicia interior basins rift domain maps of the three basins reveal that they are each underlain by hyperextended crust with possible exhumed mantle in the centre of the porcupine basin based on a holistic analysis of the results obtained the linkage between the east orphan and the porcupine basins seems implausible but rather a contemporaneous relationship is interpreted moreover a potential connection between the galicia interior basin and the porcupine basin during the early to late jurassic is proposed such scenarios imply an oblique and synchronous rifting evolution around the bay of biscay triple junction of the southern north atlantic

comparison of lithosphere structure across the orphan basin flemish cap and irish atlantic conjugate continental margins from constrained 3d gravity inversions regionally constrained 3d gravity inversion results on the orphan basin flemish cap and the irish atlantic conjugate continental margins are compared to investigate crustal structure early rifting history and geological evolution of this part of the north atlantic the full crustal density anomaly distributions provide some of the first depth images of how rifted structures compare along and across these conjugate margins broad similarities in crustal structure are identified with some noticeable differences linked to rifting and crustal stretching processes extreme crustal thinning stretching factors 3 5 is indicated beneath much of the southern porcupine basin the western half of west orphan basin the eastern half of jeanne d arc basin the southeastern half of east orphan basin and in pockets beneath rockall basin this appears to have resulted in the serpentinization and possible exhumation of mantle lithosphere on the irish atlantic and flemish cap margins but not beneath orphan basin a simple evolution model is proposed for the early stages of rifting between the margins it is suggested that ancient orogenic sutures played an important role in controlling the northward migration of rifting and the rotation and displacement of flemish cap out of orphan basin supplementary material enlarged maps from this paper are available at www geolsoc org uk sup18527

acoustic stratigraphy of the sedimentary succession west of porcupine bank ne atlantic ocena a preliminary account abstract acoustic basement lies at an average of between 6 0 and 6 5 sec two way time below sea level in the southern rockall trough and northern porcupine abyssal plain the overlying sedimentary succession reaches maximum thicknesses of at least 4 0 sec and can be divided by 3 regionally developed seismic reflecting horizons which are used as a framework to establish an acoustic stratigraphy for the area by selecting three type seismic sections these reflectors are named in ascending order shackleton charcot and challenger the area is crossed by e w basement high structures the clare lineament which may be an easterly extension of the charlie gibbs fracture zone that separates the porcupine abyssal plain from the eastern part of southern rockall trough under the latter the post shackleton acoustic sequence is thickened as if dammed to the north of the clare lineament whilst a further thickening above reflector charcot occurs along a ne sw line somewhat farther north into the southern rockall trough this can also be related to shallow lying acoustic basement features pre shackleton sediments overlie a very irregular basement topography the acoustic characters of the various sediment packages are described and it is speculated that major changes in the sedimentary environments took place across reflectors shackleton and challenger the latter probably establishing the modern bottom current circulation patterns no ages can be unequivocally assigned to the main reflectors but previously published data suggest a late eocene oligocene age for challenger possible lavas or sills are identified in the succession between reflectors shackleton and charcot

fault patterns in the cretaceous and tertiary end syn rift thermal subsidence succession of the porcupine basin offshore ireland abstract fault lengths distributions and orientations were determined at six post jurassic horizons in the porcupine basin offshore ireland three rifting periods occurred in the porcupine basin with fault orientations and lengths measured for the second and third of these dominant fault directions changed through time while remaining broadly parallel to the basin margins particularly in the southern part of the basin and especially for the larger faults observed changes in activity suggest that strain was evenly distributed throughout the basin during the periods of extension length population curves for the faults throughout the basin show an approximate straight line log log relationship between cumulative number and fault length indicating that the fracture length populations are self similar over a range of scales the fault patterns reflect the response to the transition from a syn rift dominant phase of basin evolution to one where the development was primarily influenced by thermal subsidence with subsidiary rifting

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

late permian to early mesozoic rifting and sedimentation offshore nw ireland abstract a late permian triassic and early lias rift succession up to 1 km thick is revealed from offshore nw ireland sediments are preserved in deep narrow grabens from the donegal basin in the north southwestwards through the erris and slyne troughs to a series of newly defined basins collectively forming the north porcupine basin with a further structure the galway graben on the porcupine bank this line of basins superimposed on a caledonoid fractured basement underwent a complex structural evolution but since the late permian they developed primarily as a wrench modified rift system during the latest lias crustal doming west of britain may have produced a regional angular unconformity and created a source for thick middle jurassic alluvial clastics deposited in the main porcupine basin to the south early tertiary transpression of the north porcupine area caused late stage fragmentation of the basin the rift succession consists of continental red beds bounded by two dateable transgressive intervals these intervals provide valuable stratigraphic control for constraining the timing and scale of seaways propagating across the fracturing pangean supercontinent from the late permian to the early jurassic evidence is also presented for an episode of syn rift volcanism temporally unique to western britain and ireland