short paper seismic reflection evidence for the location of the iapetus suture west of ireland a new deep seismic profile recorded west of ireland by the british institutions reflection profiling syndicate birps images prominent north dipping reflectors west of the shannon estuary qualitatively similar north dipping reflectors have been imaged on previous birps profiles recorded across the solway line both east and west of britain and have there been interpreted as structures related to the iapetus suture by analogy with these other north dipping reflectors and because the newly discovered reflectors lie close to previously hypothesized locations of the iapetus suture the structures seen on the new birps profile are here interpreted as representing part of the iapetus suture zone

regional neogene exhumation of britain and the western north sea tertiary exhumation of britain and the western north sea is interpreted to have taken place in two episodes each with amplitudes of about 1 km a palaeogene phase that principally affected the present onshore britain west of the present extent of the late cretaceous danian chalk group and a neogene phase that affected both onshore areas and the western north sea consequently maximum burial of mesozoic and older rocks in the present onshore areas generally occurred in the palaeocene c 60 ma in the western north sea the preservation of the chalk group today suggests relatively stable conditions prevailed immediately after its deposition rather than substantial palaeocene exhumation in this area maximum burial is interpreted to have occurred in the neogene the proposal is consistent with published exhumation estimates based on studies of sediment compaction offshore and of fission tracks onshore it also does away with unlikely high burial rates which are implied in the latest cretaceous earliest tertiary if removal of overburden from the chalk is assumed to have occurred in the palaeocene

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

the north wales earthquake of 19 july 1984 the intra plate earthquake on the lleyn peninsula north wales afforded a rare opportunity to obtain accurate focal parameters for the main shock and the aftershock sequence the focal mechanism does not correlate with movement on any of the large faults showing surface expressions in the area but does indicate a nw se oriented maximum compressional stress direction in agreement with measurements made in other parts of britain aftershock activity was concentrated around the focus of the main shock at depths of between 21 and 24 km the depth indicates an unusually deep transition between brittle and ductile zones that is possibly a consequence of low heat flow in the region

valuable lessons from acquiring 3d seismic for coal seam gas coalbed methane cbm or coal seam gas csg is being developed on a basinwide scale primarily as feedstock for three new liquefied natural gas lng facilities on the east coast of australia a recent 3d seismic survey acquired specifically for csg in the surat and bowen basins shows that although structural detail of the reservoir is a valuable tool deriving other information for reservoir characterization from thinly bedded coals requires a thorough investigation of the effects of high reflectivity thin beds although work continues to progress the results so far suggest that 3d seismic surveys can provide information for improved reservoir characterization

mesozoic cover over northern england interpretation of apatite fission track data whether a significant thickness of mesozoic cover rocks once rested upon the palaeozoic rocks which form the structural highs of the english lake district and the northern pennines is a subject of long debate which has been revived recently by apatite fission track analysis afta studies evidence from adjacent basins suggests that 1200 1750 m of mesozoic cover was probably formerly present on the highs prior to cenozoic erosion higher estimates c 3000 m of cover thickness based on such fission track derived palaeotemperatures result from underestimation of early cenozoic surface temperatures and from not allowing for the relatively low thermal conductivity of the eroded cover rocks for similar reasons apatite fission track estimates of eroded cover rocks elsewhere in britain may also prove to be overestimates

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

menai strait fault system an early caledonian terrane boundary in north wales a steep fault system runs parallel to the menai strait which separates anglesey from the mainland of north wales the menai strait fault system is interpreted as a major terrane boundary along which terrane docking took place during the late precambrian or earliest cambrian the evidence is based on 1 marked geologic dissimilarities exhibited by the pre late early cambrian basement to these terranes and 2 the existence of early major ductile shear zones within the fault system rocks as varied as blueschists low grade melange and shallow level granitic complexes were juxtaposed along these steep shear zones prior to the eruption of ash flow tuffs and deposition of upper lower cambrian marine sediments in the welsh basin since the amalgamation of the anglesey area with the avalonian terrane then a part of gondwana frequent phanerozoic reactivation of the resulting composite terrane boundary has been restricted to mostly dip slip fault movements

progress in birps deep seismic reflection profiling around the british isles abstract the results of 4000 km of new birps profiles are briefly outlined as an introduction to other birps papers in this volume highlights of these profiles include the tracing for 900 km of a reflection pattern in the lower crust associated with the lapetus suture and the observation of more than 20 separate reflections from the mantle while progress in solving the problem of deep water multiples has enabled the joining of the continental and oceanic moho reflections off sw england developments in old interpretational problems are reviewed and the new problem of dating reflections is mentioned

a latest cretaceous hotspot and the southeasterly tilt of britain the southeasterly regional dip of england and wales is usually ascribed to miocene movements loss of up to 2 km of cover from northern england shown by apatite fission track analysis has been related to uplift and erosion induced by a mantle hotspot but its effects were far more widespread than considered hitherto outcrop patterns throughout much of england wales and eastern ireland can be related to a hotspot beneath the irish sea uplift occurred within the late maastrichtian and erosion continued into the late palaeogene after post oligocene down faulting of the irish sea the modern drainage was initiated on the eroded surface unconformities provide the best means of dating orogenic movements in some localities the time interval between the youngest rocks preserved beneath the unconformity and the oldest preserved above may be small enough to enable the dating of fold movements to be determined with some precision thus in southeastern britain the major e w structures of the hampshire basin the weald and the london basin can be dated as miocene as rocks of oligocene age are involved in the folding in the hampshire basin and pliocene sands rest in erosional pockets of the chalk of se england because fold movements tend to affect large areas of the crust at discrete intervals of time the southeasterly tilt of britain evident from the fact that a nw se transect from anglesey to london crosses outcrops of rocks getting progressively younger from the precambrian of anglesey to the tertiary of the london basin has

cenozoic weathering covers in buchan scotland and their significance isolated occurrences of chemically weathered rock or saprolite are common in many formerly glaciated areas around the north atlantic1 6 these pockets of saprolite are widely regarded as remnants of cenozoic deep weathering covers which existed before the first extensive northern hemisphere glaciations at 2 4 myr ref 7 and their apparent survival has long been used as evidence for the local ineffectiveness of glacial erosion2 4 8 9 however the scarcity of correlative deposits means that the age and significance of these fragmented saprolites are uncertain i report here the results of a detailed survey which shows that in buchan scotland a combination of cenozoic tectonic stability and extremely limited pleistocene glacial erosion has allowed the preservation of deep weathering covers of probable miocene to early pleistocene age on a scale not previously reported from any other formerly glaciated area

crustal evolutionary model for the variscides of ireland and wales from swat seismic data the swat seismic reflection profiles recently acquired across the celtic sea demonstrate that the crust which underlies the munster and pembroke variscan foldbelts contains extensive south dipping reflections interpreted as hercynian foreland directed ramp thrusts largely involving the pre devonian basement these large scale crustal ramps developed without any intervening flat lying decollement levels and are thought to have originated in a deep infra crustal decoupling zone not imaged on the swat profiles because it lies within the present day layered lower crust at approximately 25 km depth the pre existing extensional basinal pattern is assumed to have highly influenced the geometry of these deeply rooted thrusts and in particular that of the frontal ramp which emerges along a discontinuous n100 trending thrust zone located above the faulted extensional margin of the munster pembroke and mendips devonian basins shortening of the irish and welsh external zones is hence assigned to dominantly crustal stacking achieved along steep ramps dying out upwards and giving way to mainly undisrupted folds in the thick overlying devonian infill in addition a shallow thin skinned fold thrust system developed above an intra namurian master decollement still preserved in the footwall of the frontal ramp where it either stays blind ireland or emerges pembroke above a dominantly wrench faulted caledonian basement compiling swat seismic data and structural geology leads to a crustal evolutionary model for the irish and south welsh external variscides from a devonian basin template to a final deformation pattern

the sticklepath lustleigh fault zone tertiary sinistral reactivation of a variscan dextral strike slip fault dextral strike slip movement on the sticklepath lustleigh fault zone slfz is indicated by displacements of permian and older rocks previous authors have inferred that the main dextral movement which caused these displacements was post permian and noting the presence of small tertiary pull apart basins along the fault zone probably of tertiary age however the geometry of these early tertiary pull apart basins indicates sinistral rather than dextral strike slip movement we present an alternative model for the history of the sticklepath lustleigh fault zone summarized below 1 late variscan strike slip movement with a total displacement of up to 10 km produced the slfz and offset dextrally an earlier variscan thrust 2 extensional reactivation of this thrust led to rapid permo triassic subsidence in the crediton trough and a dextrally offset neighbour the hatherleigh outlier 3 approximately 6 km of early tertiary sinistral movement on the slfz produced small pull apart sedimentary basins and reduced the net dextral offset across the fault 4 in mid tertiary times minor dextral movements on the slfz may have produced reverse faulting on the margins of the tertiary basins

palaeogene dyke swarm nw wales evidence for cenozoic sinistral fault movement the berw and aber dinlle faults represent the brittle manifestation of a major lineament the menai strait fault system which is known to have been active intermittently since late precambrian times documentation of historical seismicity in this area provides information on the most recent activity along this structure although there is a paucity of field evidence to support conclusively cenozoic or later movements this work documents data from three separate aeromagnetic datasets combined with field relationships which reveal a well defined cenozoic dyke swarm traversing anglesey and snowdonia showing clear sinistral offset up to 1 5 km along the berw fault the most northwesterly component of the menai strait fault system these data provide conclusive evidence of cenozoic

the deep structure of northern england and the iapetus suture zone from birps deep seismic reflection profiles the nec deep seismic reflection profile images structures in the lower crust which represent the iapetus suture zone beneath southern scotland and northern england on the basis of reflectivity characteristics of the lower crust and reflection moho we recognize four different crustal zones which correspond to four crustal terrane types midland valley zone a a sub continental subduction complex zone b lake district zone c and midland platform zone d the junctions between each of these zones are interpreted as being tectonic a and b originate from the northern continental margin of the iapetus ocean and are separated from c and d which are derived from a southern continent by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust the structure responsible for these reflections does not displace the reflection moho north of the ab cd junction the base of zone b and base of the reflective crust is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km these reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust at their northerly limit the deepest of the two parallel reflectors transects the moho to a depth of 6 7 km into the uppermost mantle we suggest that their termination delimits the northerly extent of a decollement or shear zone at the present continental moho the shear zone was active during the late stages of caledonian collision when mantle originally from beneath the southern continent underthrust old and newly created crust of the collision zone

constraints from deep crustal structure on long term landform development of the british isles and eastern united states abstract it is assumed that some major landforms not easily explained by surface geology could be controlled by deep crustal properties the concept was first applied in the british isles and then to the u s atlantic provinces both regions were affected by similar orogenies in late precambrian paleozoic times and presently belong to the passive margins of the north atlantic ocean in both areas the pattern of rifting was greatly influenced by preexisting tectonic fabric of the continental crust in the british isles pre mesozoic structures are cut obliquely by the atlantic margin while in the u s border the present margin is nearly parallel to the paleozoic sutures the british isles were affected by differential block tectonics during the tertiary examples are given of regional scale landforms linked with crustal hinges recent data confirm the great heterogeneity of the crust beneath the british isles each crustal unit has its own isostatic response according to the state of stress and its own dynamic equilibrium between rock resistance erosion processes and uplift rate in the atlantic provinces of the united states the regional physiographic framework is simpler but allochthonous terranes thrust westward over the grenville basement and shelf sediments have buried some major discontinuities in the deep crust reverse faults associated with the nw se compressional stressfield are numerous but cannot explain regional scale landforms like the blue ridge escarpment above the piedmont rock control is not the direct cause of the escarpment the blue ridge from reaches its highest relief where discontinuities in the deep crust interfere with surficial geological features ocean to continent creep processes are suggested to explain the sustained uplift of the appalachian highlands and the relatively stable position of the mountainous front since middle mesozoic times

dating the source of lower crystal reflectivity using birps deep seismic profiles across the lapetus suture abstract six birps deep seismic reflection profiles around the british isles that cross the trend of the lapetus suture allow a geophysical definition of the suture zone for 900 km along its strike and demonstrate that specific reflective structures have survived since the mid palaeozoic caledonian orogen in an area northeast of england spanning 70 km along caledonian strike and 200 km across strike three profiles show very considerable similarities in the reflectivity patterns of the lower crust when these similarities are mapped the boundaries separating regions of lower crust with the same reflective character run parallel to caledonian strike this strongly suggests that the distribution of reflectivity in the lower crust in this area is directly controlled by caledonian structures geologic models for the area imply that many of the reflectors themselves may also be of caledonian age however though an age for the imposition of patterns of reflectivity can be inferred the age of an individual reflector cannot be determined unless it can be traced to outcrop or unless there is independent evidence as to the physical nature of the reflector

the pre glacial evolution of wales the classical explanation of the development of the welsh landscape by the effects of a pulsed eustatic uplift during the cenozoic times is challenged by new data on the geology and geomorphology of wales and the adjacent regions the post caledonian surface developed over most of wales in the upper westphalian plays a major role in landform development since its emergence before mid cretaceous times the welsh massif has suffered a long subaerial evolution which favoured a powerful chemical weathering in anglesey and st david s land residual hills were formed by a process of differential weathering and downwearing in saprolites the millstone grit quartzites have been deeply weathered and dolines or flat bottomed basins have been developed from cretaceous to miocene times a morphotectonic equilibrium has existed between uplift and denudation so that in the major part of wales the original planation surface was constantly maintained by lowering on itself and in some places it is possible to prove that no more than a few dozen metres of rock thickness disappeared during that period several main escarpments were produced by the warping of the planation surface along sharp flexures during neogene times the cenozoic vertical movements were controlled by the caledonian or pre caledonian tectonic pattern which has divided the continental crust into major structural units welsh tectonic development is related to the opening of the north atlantic associated stresses reactivating ancient lines of weakness in the heterogeneous lower crust or upper mantle

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

magnesian basalts from the hebrides scotland chemical composition and relationship to the iceland plume abstract the hebridean tertiary igneous province lay 700 900 km south of a presumed plume centre and 500 km landward of a rifted continental margin prior to opening of the northeast atlantic the province includes a small number of magnesian basalts containing olivine phenocrysts with core compositions equal to or in excess of fo 89 olivine liquid equilibrium calculations suggest that these phenocrysts formed from liquids with mgo contents of 14 15 wt and in rare instances 18 20 wt the liquidus temperatures of these magmas imply mantle potential temperatures of 1350 1460 c at distances up to 900 km from the axis of the iceland plume their emplacement at c 60 ma implies either a long established hotspot or active channeling of plume material along the base of the plate towards a region undergoing considerable lower crustal extension

terrane boundaries in ireland inferred from the irish magnetotelluric profile and other geophysical data this paper presents explanations for some important geophysical features associated with terrane boundaries in the iapetus suture zone in ireland the acquisition of data along the irish magnetotelluric profile imp provided the basis for a re assessment of magnetic gravity and seismic refraction results and further resolved sub surface structures associated with terrane development during the caledonian orogeny the magnetotelluric data located two upper crustal conductors associated with the fair head clew bay and virginia magnetic lineaments one of the conductors probably extends along strike to scotland and its top surface may be associated with the t reflector seen on the north east coast nec deep seismic reflection profile the other conductor has a southeastern edge directly beneath a major throw in seismic basement and defines the northern boundary of the midland valley terrane in ireland the conductors and magnetic lineaments are probably due to black shales in sedimentary basins which have been thrust under the avalonian and laurentian continents during the closure of the iapetus ocean the magnetotelluric data also suggest that the southern uplands magnetic lineament represents a crustal break but the navan tipperary magnetic lineament is probably the consequence of a shallow volcanic block sedimentary trough structure

iapetus suture located beneath the north sea by birps deep seismic reflection profiling the iapetus suture one of the more fundamental geologic boundaries of the british isles has been located beneath the north sea by deep seismic reflection profiling carried out by the british institutions reflection profiling syndicate birps reflection profiling and magnetotelluric sounding have previously been used to infer the position of the iapetus suture beneath the irish sea and the southern uplands but the present study provides the first direct evidence for the location of the iapetus suture east of britain a comparison of the seismic reflection profiles over the iapetus suture east and west of britain shows variations in the dip of the suture which steepens to the northeast beneath the north sea the comparison also reveals unexpected changes in lower crustal reflectivity along the strike of the suture and suggests that there may be no straightforward relation in this area between the crustal reflectivity and the crustal age

unraveling multiple phases of sulfur cycling during the alteration of ancient ultramafic oceanic lithosphere abstract ultramafic hosted hydrothermal systems characterized by ongoing serpentinization reactions exert an important influence on the global sulfur cycle extensive water rock interaction causes elemental exchange between seawater and the oceanic lithosphere effectively removing sulfate from seawater through both abiogenic and biogenic processes here we use bulk rock multiple sulfur isotope signatures 32s 33s 34s and in situ sulfide analyses together with petrographic observations to track the sulfur cycling processes and the hydrothermal evolution of ancient peridotite hosted hydrothermal systems we investigate serpentinized peridotites from the northern apennine ophiolite in italy and the santa elena ophiolite in costa rica and compare those with the iberian margin ocean drilling program odp leg 149 and 173 and the 15 20 n fracture zone along the mid atlantic ridge odp leg 209 in situ measurements of sulfides in the northern apennine serpentinites preserve a large range in 34ssulfide of 33 8 to 13 3 with significant heterogeneities within single sulfide grains and depending on mineralogy detailed mineralogical investigation and comparison with bulk rock 33ssulfide and in situ 34ssulfide data implies a thermal evolution of the system from high temperatures 350 c that allowed thermochemical sulfate reduction and input of hydrothermal sulfide to lower temperatures

reflection seismic evidence for a moho offset beneath the walls boundary strike slip fault birps has acquired two deep seismic reflection profiles across the walls boundary fault a major late caledonian strike slip fault that is probably the northern continuation of the great glen fault of scotland where crossed by the shet deep seismic survey north of the shetlands the walls boundary fault appears to be a narrow vertical structure which penetrates the entire crustal thickness and juxtaposes crusts of different thicknesses the seismic data show a moho offset in travel time of 1 0 1 5 seconds and two high amplitude diffractions originating at moho travel times these diffractions are as yet unique among seismic data across strike slip faults and can be used to argue that the moho is offset in depth by 2 3 km over a lateral distance of less than 6 km seismic modelling is used to constrain this interpretation the preservation of this moho topography suggests that the structure and rheology of the lower crust of the shetland platform was not significantly modified during post devonian extension of the north sea and atlantic margin

geophysical images of the deep crust the iapetus suture the iapetus suture arguably the most fundamental lineament of british and irish structure has been previously identified on the birps winch deep seismic reflection profiles offshore as a nw dipping feature new depth migrated interpretations of these and other reflection data show good correlation of structure for 100 km along strike in the northern irish sea towards magnetotelluric stations onshore in the north of england and southern uplands of scotland three different methods of inverting data from these sites all reveal a thin nw dipping slab of high conductivity a contour map of the suture trace dipping at 15 25 nw can be constructed down to and through the moho at 28 km depth this map suggests crustal stretching below the solway basin the mylonitic suture zone is characterized by good seismic reflection layering high conductivity presumably fluid produced and a steep increase of p wave velocity over the few kilometres of its width crustal refraction experiments lispb and cssp provide useful velocities but locally misleading structure so that the combination of the seismic reflection and geoelectric methods may be the most powerful means of determining deep crustal structure with a good degree of both vertical and lateral resolution

the iapetus suture zone in england scotland and eastern ireland a reconciliation of geological and deep seismic data deep seismic profiles in the irish and north seas have successfully imaged crustal structure across the iapetus suture zone a problem with current interpretations is that the reflectors chosen as the suture project to the top of the basement well to the south of the solway line the palaeontologically defined trace of the suture we present a revised interpretation in which a reflectivity boundary common to all the profiles is identified as the leading edge of imbricated avalonian crust in effect the footwall of the suture zone this projects to the surface close to the solway line obviating the need to invoke northward backthrusting of the suture

the role of basement reactivation in continental deformation the papers in this thematic set arise from a discussion meeting sponsored by the tectonic studies group and held at burlington house on 6 7 march 1996 about 170 people heard 19 formal oral presentations and studied 17 posters of which 15 were briefly introduced with five minute talks sixteen of these contributions are published in this thematic set pre existing heterogeneities in the continental lithosphere are thought to influence markedly its response to subsequent deformation a major reason for this thinking is the long held view e g hills 1956 that fault zones are inherently weak virtually all regions of continental lithosphere are riddled with faults and shear zones thus any subsequent deformation might be expected to utilize these pre existing weak zones as reviewed by white et al 1986 this type of reactivation where faulting activity is localized onto pre existing faults can be contrasted with more general cases of deformation reworking broader volumes of lithosphere there need not be complete reactivation of individual faults and shear zones the distinc tion is nicely illustrated by the concepts of basin inversion a topic that has been well aired in recent years e g cooper williams 1989 coward 1994 in many sedimentary basins there is evidence of contractional reactivation of segments of originally extensional faults e g letouzey 1990 yet for the contrasting tectonic setting of collisional mountain belts recent work has emphasized that in general thrusts do not reactivate pre existing extensional faults e g gillchrist et al 1987 however the sites of almost all the shortened continental

deep structure of the foreland to the caledonian orogen nw scotland results of the birps winch profile the winch marine deep seismic reflection profile crosses the hebridean shelf the proterozoic foreland to the caledonian orogen west of scotland the data quality is very good the upper crust is largely devoid of coherent seismic reflections although this may in part be due to acquisition techniques being inappropriate for this problem in contrast the middle and lower crust 10 25 km depth exhibits good reflections the mid crust contains reflectors which may be relics of early palaeozoic caledonian or earlier grenvillian eastward dipping thrust zones which pass into an acoustically strongly layered lower crust the outer isles thrust is mapped from the surface to the mid crust and tied into its land outcrop on north lewis reactivation of this thrust offshore in a normal sense in mesozoic times caused the formation of the minch and north lewis basins the mono is defined by a strong band of reflections at about 27 km depth which correlates well with results from the hmse explosion seismic survey moho depth is apparently rather uniform in the area of the foreland crossed by winch winch and hmse results together suggest a mean crustal velocity for the hebridean shelf of 6 4 0 1 km s 1 the eastward dipping flannan thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth well into the upper mantle neither the flannan thrust nor the outer isles thrust appear to pass straight through the reflective lower crust suggesting that the lower crust is a region of high strain the outer hebrides is a positive block probably formed as an isostatic response to mesozoic normal faulting which reactivated the outer isles thrust

thermal and tectonic history of the east midlands shelf onshore uk and surrounding regions assessed by apatite fission track analysis apatite fission track analysis has been used to study the thermal and tectonic history of the east midlands shelf and surrounding areas of onshore uk apatites from outcrops of palaeozoic crystalline basement give fission track ages between 300 29 and 420 31 ma which are the oldest yet reported in the uk region and reflect the relative stability of this cratonic region mean track lengths in these samples are between 12 and 13 m namurian and triassic sandstones from outcrop give apatite fission track ages between c 100 and c 390 ma with mean track lengths generally between 10 and 12 5 m distributions of single grain ages and track lengths suggest that all samples have been annealed implying exposure to higher temperatures since deposition with maximum palaeotemperatures reached some time in the 50 80 ma range prior to the onset of cooling samples from five exploration wells show consistent patterns in which apatite fission track ages decrease sharply from around or just less than the stratigraphic age at near surface to values in the 50 80 ma range at depths of c 1 2 km present temperatures of c 40 70 c showing that the samples studied have all been exposed to higher temperatures single grain ages and track length data suggest that maximum palaeotemperatures were attained at or before c 60 ma subsequent cooling was protracted and the data are compatible with two phases in the early and mid tertiary palaeotemperature analysis suggests that palaeo geothermal gradients were similar to present values and that elevated palaeotemperatures were due to greater depth of burial with cooling due to subsequent uplift and erosion estimates of uplift and erosion vary from c 1 1 5 km on the east coast around the humber estuary to c 2 2 km in the east midlands and southern pennines compilation of palaeotemperature estimates from all samples analysed from the uk onshore region shows that mesozoic burial and tertiary uplift has affected a wide area and that uplift was due to broad scale regional warping with recognized inversion axes being areas where uplift and erosion were particularly pronounced or where younger sediments and or inverted basinal structures allow the identification of uplift and erosion the recognition of late cretaceous early tertiary unconformities across wide areas of northwest europe both onshore and offshore suggests that the regional uplift and erosion may have been far more widespread than previously recognized this has important implications for hydrocarbon exploration in northwest europe

lower crustal heterogeneity beneath britain from deep seismic reflection data 450 km of deep seismic reflection data have been acquired over the uk landmass a qualitative assessment of lower crustal reflectivity based on the amplitude and continuity of deep crustal events has been made in the cratonic area of central england the lower crust is generally poorly reflective while in the foldbelt terrains of southern eastern and northern england it is more strongly reflective beneath the variscides of southern england an important distinction can be made between poorly reflective foreland type lower crust lying beneath the variscan front thrust and strongly reflective orogenic lower crust farther south a brief review is given of hypotheses for the origin of lower crustal reflectivity and the relationship of the latter to crustal tectonic regime both in the uk and elsewhere it is concluded that the types of lower crustal heterogeneity required to produce the observed reflections are more easily generated in orogenic provinces fold thrust metamorphic belts than in areas which have undergone only low to moderate crustal extension beneath typical sedimentary basins the generation of granites by partial melting within the lower crust and processes of metamorphic differentiation and recrystallization which accompany orogenic activity are likely to play a significant role in the development of such heterogeneities in addition post orogenic processes of magmatic underplating and low viscosity ordering may also have played a part

crustal structure south of the iapetus suture beneath northern england the crustal structure just south of the iapetus suture1 along a line extending from the mid north sea high across northern england to the solway basin has been investigated as part of the caledonian suture seismic experiment of 1982 this unique sea to land wide angle explosion seismology project has produced data of unprecedented quantity and quality the main innovation was the use of both closely spaced shots mainly at sea and closely spaced stations mainly on land yielding a data set which can eventually be arranged in a variety of new ways for phase correlation and modelling such as constant offset and common mid point the line was extended by irish and german groups across ireland to the shannon estuary having a total length of over 600 km but here we present only preliminary results for northern england and adjacent marine regions the results for this region differ radically from those of the earlier lispb experiment2 in that we have recognized a well defined 6 15 km s 1 basement interpreted as pre caledonian metamorphic basement beneath most of the line at relatively shallow depth and there is a well defined mid crustal refractor starting at approximately 16 km depth beneath the north sea and northern england which is not recognized beneath the irish sea

some observations on basin evolution and dynamics sedimentary basin evolution is intimately bound in with continental lithosphere dynamics deep seismic reflection profiling has revealed images of the crust and upper mantle that suggest linkages between the shape and structure of sedimentary basins as they evolve with the rheological responses to tectonic stress of upper and lower crust and upper mantle components of the lithosphere this is well illustrated by the european geotraverse lithosphere profile from north cape norway to tunisia the alpine mountain belt has a lithospheric root as well as a crustal root by virtue of its asymmetric disposition this may be the cause of compressive and tensile forces in close association in the upper crust which have created mountain uplift and basin subsidence extension and thrusting all in juxtaposition evidence for an elastic upper layer of crust above a plastic substratum can be used as a basis for modelling basin geometry structure and stratigraphy built upon the flexural nature of the upper crust such models go a long way towards simulating the evolution of basins such as the north sea viking graben in more detail analogue structural models can reveal the geometry of basin fill in relationship to crustal movement and the relationships between structure and sedimentation patterns however sedimentary basin modelling needs to begin with the quantitative simulation of depositional environments in relation to such factors as tectonics sea level variations sediment supply and erosion in order to match sedimentary architecture only then can the range of deposits be placed in space and time within a basin environment as building blocks within a dynamically evolving structural framework to create predictive models for comparison with geological reality