basin development examples from the united kingdom and comments on hydrocarbon prospectivity abstract analysis of on and offshore basins in the ukcs using a variety of structural modelling techniques including and now confirmed by deep seismic reflection profiles has shown that basins develop on linked fault systems these fault systems comprise both steep and shallow dipping faults which can have either dip or strike slip displacements while linked fault systems are common to all basins they are developed to different degrees and hence produce a variety of structural expression within and between the basins this allows basins to be classified either in terms of their dominant structural elements or seen as a mechanical and geological continuum as the basin grows changes in crustal thickness in the upper brittle crust can take place either on linked staircase arrays of low angle listric faults or on steeper planar domino faults or commonly on a combination of both linked by crustally conservative strike slip systems the development of the controlling basement elements effects the distribution and pattern of internal deformation in the evolving sedimentary fill resulting in a characteristic basin architecture this architecture has profound implications for exploration in both extensional and strike slip basins providing possibilities for the prediction of trap geometry and distribution as well as opportunities for new plays

triassic pebbles derived fossils and the ordovician to devonian palaeogeography of europe papers published by salter 1864 and davidson 1870 on the faunas from pebbles in a triassic conglomerate at budleigh salterton devon are reviewed after modern reassessment these pebbles although of apparently similar quartzites have been found to be of four different ages two ordovician mid arenig and late llandeilo and two devonian lochkovian pragian and frasnian by comparing these four faunas with those contemporary in adjacent palaeocontinents it can be shown that apart from the earliest one they have affinities closer to those of the armorican peninsula of brittany and normandy than to the rest of britain and that these armorican faunas are in clasts which were transported northward by triassic rivers consideration of all the various faunas in the whole of northwest europe reflect the earliest ordovician of southern britain as part of the vast gondwanan continent from which it became detached by the mid ordovician with a widening rheic ocean between the two palaeocontinents and the subsequent merging of avalonia with baltica and laurentia to form laurussia by mid devonian times new palaeogeographical maps depicting phases from the ordovician to the devonian are presented

the structure and geomorphology of the dashgil mud volcano azerbaijan the large scale and small scale 10 m structure and geomorphology of the active dashgil mud volcano in central azerbaijan have been studied through historical records it is known that this mud volcano has major eruptions every 6 32 years the activity observed during two field trips in 1995 consisted of low energy venting and debouching of mud water oil and gas from three mud pools salses and about 20 individual cones gryphons located on the summit plateau of the mud volcano besides the bubbling mud pools ranging from 30 to 75 m across and the cones ranging from 0 8 to 3 m height a 200 m long string of about 10 sinter mounds was found on the summit plateau the internal structure of the mud volcano and the processes responsible for the observed dashgil geomorphology are discussed assuming the dashgil mud volcano to be continuously active with an average methane production similar to that observed in 1995 between the rare major eruptions we have conservatively estimated the dashgil mud volcano to produce an annual amount of at least 800 sm3 standard cubic metres of gas mainly methane which is emitted directly into the atmosphere this estimate indicates that the natural contribution of atmospheric greenhouse radiatively active gases from the world s terrestrial and submarine mud volcanoes is highly significant

triggering and dynamic evolution of the lusi mud volcano indonesia abstract mud volcanoes are geologically important manifestations of vertical fluid flow and mud eruption in sedimentary basins worldwide their formation is predominantly ascribed to release of overpressure from clay and organic rich sediments leading to impressive build up of mud mountains in submarine and subaerial settings here we report on a newly born mud volcano appearing close to an active magmatic complex in a backarc sedimentary basin in indonesia the location of the mud volcano close to magmatic volcanoes results in a high background temperature gradient that triggers mineralogical transformations and geochemical reactions at shallow depth the eruption of 100 c mud and gas that started the 29th of may 2006 flooded a large area within the sidoarjo village in northeast java thousands of people have so far been evacuated due to the mud flood hazards from the eruption since the initial eruption the flow rate escalated from 5000 to 120 000 m 3 d during the first eleven weeks then the erupted volume started to pulsate between almost zero and 120 000 m 3 d in the period august 14 to september 10 whereas it increased dramatically following swarms of earthquakes in september before reaching almost 180 000 m 3 d in december 2006 sampling and observations were completed during two fieldwork campaigns on the site the eruption of boiling water is accompanied by mud aqueous vapour co 2 and ch 4 based on geochemical and field results we propose a mechanism where the eruptions started following the 27th of may earthquake due to fracturing and accompanied depressurization of 100 c pore fluids from 1700 m depth this resulted in the formation of a quasi hydrothermal system with a geyser like surface expression and with an activity influenced by the regional seismicity

geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone heubeck ch and mann p 1991 geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone tectonophysics 191 l 26 four published plate kinematic models for the present day north american caribbean plate boundary predict significantly different fault behavior along this dominantly strike slip plate boundary zone pbz using a computer graphics system we compare the deformational pattern predicted by the four kinematic models with geologic and seismic observations along the entire length of the pbz from central america to the northern lesser antilles our results indicate that none of the kinematic models predict all the geologic and seismic observations across the entire pbz however all four models predict mapped fault character and observed earthquake focal mechanisms along certain segments of the pbz we conclude that the present day deformation within the north american caribbean plate boundary zone cannot be described by rotation about a single pole instead the caribbean plate appears to be segmented into at least three major sub plates or blocks each of which requires an individual pole of rotation to describe its motion relative to the north american plate sub plate boundaries correspond to two poorly studied zones of diffuse deformation within the caribbean plate honduras depression of central america and beata ridge of the caribbean sea

the nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems in accretionary wedges often morphologically similar sedimentary intrusions when observed by remote geophysical means may have one of two quite different driving mechanisms and a highly variable significance for the regional hydrogeologic picture for example mud diapirs are driven by buoyancy forces that arise from bulk density contrasts in them mud and pore fluid upwell en masse and fluid migration is a related sometimes important but generally subsidiary process in contrast diatremes contain sediments fluidized during rapid fluid advection and are forcibly and directly driven by the hydrogeologic system the nature of fluid input from local and exotic source regions can therefore strongly affect sedimentary intrusive processes and vice versa this complicates the process of defining the main features of the hydrogeological systems operating in accretionary wedges focused vertical advection through steep sided piercement mud diapirs requires conduit systems otherwise flow will be diffuse and directed more horizontally out of the low permeability mud mass however where the permeability of the overburden is less than that of the diapir the whole diapir may act as a conduit apart from this special case conduits will be associated with highly anisotropic scaly fabrics that can sometimes develop in the marginal shear zone of diapirs scaly fabrics form during deformation and compaction of a mud matrix under conditions of constant or increasing effective stress however the effective stress path can be complex as it is both controlled by the relative rates of upward intrusion and burial by sedimentation and or structural thickening and the hydrogeologic system due to this it appears likely that even in a geographically related group of diapirs effective stress histories will vary widely between intrusions so that some can form advective pathways for fluids and some cannot mixed systems of behavior may also be present with local diatremes developing within diapirs above the terminations of conduit systems and rapidly expanding methane gas pockets the potentially heterogeneous near surface behavior may be why the surface manifestations of sedimentary intrusions are so variable when observed in the field diatremes can also form separately as large primary features above any structural or stratigraphic conduit that rapidly expels water or gas into the base of an unlithified sediment column when active large diatremes require enormous quantities of fluid water or gas to drive them particularly if they are long lived features and hence are a direct indication of at least an episodically vigorous hydrogeologic system

geophysical evidence concerning the southern boundary of the london platform beneath the hog s back surrey the results of a gravity survey have been interpreted with seismic reflection and borehole data to show that a major change in the structure of the upper crust underlies the hog s back w of guildford surrey the pre mesozoic basement beneath the thin mesozoic succession n of the hog s back is of a lower density than that underlying the thick mesozoic succession to the s this lower density basement was probably the principal cause of the relative stability of the london platform during the mesozoic era the sourthern boundary of the london platform may have been initiated by variscan thrusting with subsequent reversal of displacement or may be inherited from an older crustal lineament

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

the structural development of the wessex basin the mesozoic and cenozoic structures of southern england form part of a system of disturbances which extends across the channel into northern france they are reviewed in the light of published data from south dorset basin development started with permo triassic rifting and is believed to have continued through the jurassic and early cretaceous with intermittent growth on deep seated listric normal faults probably associated with roll over anticlines this regime had ceased by the aptian and the basin passed through a period of relative stability in the late cretaceous followed by tectonic inversion and in the miocene by northwards compressive movement along the former normal faults this partially restored the earlier separative displacement at depth and probably accentuated pre existing roll over anticlines it was expressed in the surficial upper cretaceous palaeogene sequence as monoclinal flexuring the early rifting and the change from overall n s crustal stretching to compression may reflect plate tectonic events in the north atlantic region

aspects of basin inversion in southern britain the wessex basin of southern england a permian to cretaceous extensional basin can be structurally divided into a set of constituent asymmetrical graben bounded by major e w trending zones of en echelon syn depositional normal faults the graben were inverted in late cretaceous and tertiary times by compressive stresses oriented roughly north south inversion structures fall into two related categories regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben fill superimposed upon and geographically delimiting the regional upwarps are roughly east west trending linear zones of en echelon inversion structures these coincide with the earlier graben bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault the linear reverse fault monocline inversion structures were a relatively inefficient method of basin shortening because upper crustal faults in the region steepen upward markedly reversal of these faults under compression resulted in a shortening discrepancy at shallow depths locally particularly in south dorset this was overcome by bucklefolding and low angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures elsewhere however the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary fill this occurred preferentially in those graben containing young poorly lithified and therefore weak sediments thus the early cretaceous depocentres of the weald and channel basins were strongly upwarped with axial uplifts of over 1000 m conversely graben having older more lithified sequences suffered little regional upwarp shortening primarily by fault reversal along the linear inversion structures the amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension

in situ cone penetration tests at the active dashgil mud volcano azerbaijan evidence for excess fluid pressure updoming and possible future violent eruption active mud volcanism is a global phenomenon that represents a natural hazard by self igniting eruptions and the continuous emission of methane gas in both marine and continental settings mud domes are often found in compressional tectonic settings such as the caucasus orogenic wedge dashgil mud volcano the most prominent of 200 features in azerbaijan has erupted vigorously in historic times for several years we have observed variations in the activity of dashgil dome including transients in methane flux build up of extrusive mud cones on the main feature and flexural polygonal cracks adjacent to the main crater lake and new mud cones in spring 2007 we carried out in situ cptu cone penetration testing with pore pressure measurement experiments in the crestal area of dashgil our data suggest that the central portion of the crater lake which hosts the conduit for gas and possible mud ascent shows both low sediment shear strength 5 20 kpa and excess pore fluid pressures between 15 and 30 kpa supra hydrostatic at 1 m sub bottom depth in situ cone resistance as a measure for undrained shear strength is as low as 150 kpa in the conduit whereas the mud is found rather stiff in all other testing locations 300 700 kpa probably a result of deeply buried shales of the maikop formation parts of which now liquefy and ascend pore pressure is low in the centre of the conduit probably because of rapidly migrating gas it increases to 30 kpa at the lake bottom and deep flank then decreases upslope on the lake flank and reaches hydrostatic values at the crater rim from the overpressured region beneath the fluid filled crest of dashgil dome combined with the other observations we suspect to currently witness an ongoing period of updoming the presence of sintered mudstones from explosive eruptions in 1908 and 1928 and most likely before suggests that a similar violent activity may occur in the near future

the structural evolution of the english channel area the structural evolution of the english channel area is controlled by structure and particularly by the pre existing cadomian and variscan crustal discontinuities which have been reactivated repeatedly in post variscan times they controlled the crustal subsidence that produced basin development in the mesozoic prior to the sea floor spreading in the north atlantic region they were then reactivated during the cenozoic compression and basin inversion the english channel development is ascribed to mid tertiary differential uplift oligocene to miocene during late tertiary to quaternary times the channel displays characteristics of a tectonically controlled fluvial basin periodically invaded by the sea at the lithospheric scale the channel can be considered as an active intraplate area influenced by the nw se alpine push the nw se atlantic ridge push and glacial rebound stresses copyright 2003 john wiley sons ltd

crustal structure beneath southern england from deep seismic reflection profiles seismic reflection events from recently acquired profiles in southern england are interpreted in terms of stratigraphic and structural interfaces between rock units in particular evidence is brought forward that a series of reflection events dipping between 22 and 27 with azimuth averaging 160 represents 2 major thrust zones occurring in the variscan floor beneath mesozoic cover a geological model based upon borehole information and regional geological considerations is presented the tentative recognition of crystalline basement overlain and overstepped by late proterozoic rocks of variable thickness suggests that basement in this area was unstable from late precambrian times the mesozoic structural evolution can be related to tectonic re activation of major concealed thrust zones in the palaeozoic 9basement9

when mud volcanoes sleep insight from seep geochemistry at the dashgil mud volcano azerbaijan the worlds 1500 mud volcanoes are normally in a dormant stage due to the short duration of eruptions their dormant stage activity is often characterized by vigorous seepage of water gas and petroleum however the source of the fluids and the fluid rock interactions within the mud volcano conduit remain poorly understood in order to investigate this type of activity we have combined satellite images with fieldwork and extensive sampling of water and gas at seeping gryphons pools and salsa lakes at the dashgil mud volcano in azerbaijan we find that caldera collapse faults and e w oriented faults determine the location of the seeps the seeping gas is dominated by methane 94 9 99 6 with a 13c v pdb in the 43 9 to 40 4 range consistent throughout the 12 analysed seeps ethane and carbon dioxide occur in minor amounts seventeen samples of seeping water show a wide range in solute content and isotopic composition pools and salsa lakes have the highest salinities up to 101 043 ppm cl and the lowest 18o v smow values 1 4 the mud rich gryphons have low salinities 18 000 ppm cl and are enriched in 18o 18o 4 6 the gas geochemistry suggests that the gases migrate to the surface from continuously leaking deep seated reservoirs underneath the mud volcano with minimal oxidation during migration however variations in gas wetness can be ascribed to molecular fractionation during the gas rise in contrast the water shows seasonal variations in isotopic composition and surface evaporation is proposed as a mechanism to explain high water salinities in salsa lakes by contrast gryphons have geochemical signals suggesting a deep seated water source these results demonstrate that the plumbing system of dormant mud volcanoes is continuously recharged from deeper sedimentary reservoirs and that a branched system of conduits exists in the shallow subsurface while the gas composition is consistently similar throughout the crater the large assortment of water present reflects the type of seep i e gryphons versus pools and salsa lakes and their location within the volcano our data highlight the importance of a carefully planned sampling strategy when the target is water geochemistry whereas the methane content and isotopic composition is relatively independent of the particular seep morphology

3d lithosphere scale density model of the central european basin system and adjacent areas abstract to analyze the first order structural features characterizing the crust and the lithospheric mantle below the central european basin system and adjacent areas a new lithosphere scale 3d structural density model has been derived that integrates published knowledge with 3d gravity modeling with this study we aim to integrate previous results on sub domains of the study area and regional results of limited resolution in conjunction with 3d gravity analysis to derive the configuration of the lower crust and lithospheric mantle that is consistent with the known density distribution of the sediment fill and with observed deep seismic data as well as with observed gravity the derived 3d density model resolves the internal configuration of the crystalline crust below the central european basin system and adjacent areas at the regional scale and shows that the crystalline crust is characterized by a layered structure the upper to middle crystalline crust is characterized by relatively low seismic p wave velocities and densities it shows strong variations in thickness across the study area that partly correlate spatially with the inherited segmentation in response to crustal amalgamation this upper to middle crystalline crust is underlain by a continuous high density high velocity lower crustal layer locally more than 30 km thick beneath the east european craton and generally decreasing in thickness from the older to the younger tectonic domains though the thickness of this high density lower crust is generally small below the permo cenozoic central european basin system it can reach locally more than 18 km below the northeast german and the norwegian danish basins the derived depth to the lithosphere asthenosphere boundary confirms previously proposed geometries and is up to 230 km deep beneath the precambrian domain but less than 100 km deep beneath the phanerozoic domains implying that the average thermal gradient is higher in the phanerozoic lithosphere than in the precambrian part

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

a possible basaltic deep source of the south central england magnetic anomaly the group of large positive magnetic anomalies in south central england is shown to originate from two sources at different depths the shallow source originates from silesian basic igneous rocks the anomaly of the deeper source has been isolated by upward continuation and inverted via pseudofield transformation the magnetic model is interpreted as a large body of basaltic rocks intruding the middle crust it is proposed that the body was emplaced by buoyant ascent and may be the cause of the stability of the london platform during mesozoic times it is speculated that the basalt was intruded in a back arc environment during the period preceding the variscan orogeny

integrated geophysical geological modelling of the caledonian and precambrian basement of southern britian quantitative modelling of potential field data has been used to test and extend geological sections constructed for the new british geological survey tectonic map of britain and ireland three of the profiles cross part of the anglo brabant massif and provide new information on the nature of the pre mesozoic basement a profile across southern england passing just to the west of london suggests that a significant contribution to observed gravity variations in the region results from changes in the thickness of relatively low density lower palaeozoic rocks it also identifies a major deep seated body with relatively high magnetic susceptibility and low density which is interpreted as a precambrian cratonic core underlying the southeastern part of the midlands microcraton profiles across central and eastern england reveal major boundaries within the precambrian basement some of which coincide with structures mapped at surface a number of intrusions of probable caledonian age have been recognized including bodies beneath the widmerpool gulf and the wash those beneath the wash appear to lie in a discrete basement region which separates belts of more magnetic basement lying to the northwest and southeast

modified dynamic cptu penetrometer for fluid mud detection one particularly complex phenomenon is the episodic tidally driven variation of navigable depth level as a result of fluid mud settlement this paper presents results from dynamic cone penetration testing with pore pressure measurement cptu as a nonacoustical direct device to support surveying and management of these areas the new technique is modular and uses a disk configuration for fluid mud detection both disk resistance and pore pressure measurements accurately identify suspended matter concentrations of 90 g l or more and the transition from fluid mud to consolidating mud once concentrations exceed 150 g l hence the procedure attests the potential for rapid reliable assessment of a fluid mud layer and concurrent characterization of the underlying consolidated sediment by monitoring the pore pressure and strength changes during penetration

field measurements of shear strength of an underconsolidated marine clay abstract this paper presents the observations of cone penetration testing cpt in situ vane shear testing and undrained triaxial testing of underconsolidated marine clay in the craney island dredged material management area cidmma norfolk va comparison of vane shear measurements taken during 1981 1983 to cpt measurements taken during 1993 indicates a change in undrained shear strength both the vane shear and cone penetration resistances are lowest at the mid heights of the clay layers and the excess pore water pressures are highest at the mid height of the clay layers indicating that the clay layer is underconsolidated

fold and thrust propagation in the western himalaya based on a balanced cross section of the surghar range and kohat plateau pakistan layer parallel slip is dominant in a wedge tapering about 2 degrees across the 70 km wide kohat foreland fold and thrust belt between the main boundary thrust and the surghar range of northern pakistan balanced cross sections show over 50 line length shortening by blind or partially emergent folding and thrusting of which only about half is documented cover sequence deformation folds with 1 to 5 km wavelengths thrust faults and backthrusts occur in the siwalik clastic wedge over buried tip lines and a zone of decoupling decollement thrusting advanced across the kohat plateau to the southernmost frontal thrust ramp of the surghar range about 2 ma before thrusting 12 20 km to the north of the surghar range in the northern kalabagh fault zone this out of sequence th usting is interpreted as a taper building response to an increased decollement dip above the basement the low critical taper of the kohat surghar wedge is explained by the nearly flat basement and low basal shear traction along eocene and eocambrian evaporite layers

the caucasus an actual example of the initial stages of continental collision abstract a field study of the neotectonics of the georgian caucasus is combined with geophysical geodetic and geological information in order to obtain a mechanical model of actual deformation the analysis of satellite photos together with topographical and geological maps shows that the great caucasus is bisected by a nne trending sinistral strike slip fault zone the western great caucasus is thrusted to the south over the rioni basin while on the other hand its northern flank is a monocline that descends gently into the russian platform active volcanism is present from elbruz to kazbeg the eastern great caucasus is limited by thrusts to the north and to the south and intermediate structures are disposed in a fan like shape no recent volcanism is observed the eastern limit of the chain is a nnw trending dextral strike slip fault zone seismicity from a local network illustrates the activity of the thrusts and gives evidence for the strike slip faults microtectonic data confirm the recent activity and the sinistral character of the borjomi kazbeg strike slip fault the strike slip faults bordering the eastern caucasus may be seen as the limits of a plastic wedge in front of the arabian plate at an early stage of continental collision

an experimental setup for fluid venting in unconsolidated sediments new insights to fluid mechanics and structures abstract focused outflow of water at the seafloor termed seepage or fluid venting is a widely recognized global phenomenon in an effort to better understand the mechanisms by which fluid flow related structures form we performed laboratory seepage experiments using samples representing a broad range of grain sizes and hydraulic conductivities 10e 3 to 10e 7 m s detailed observations led to the recognition of a typical often repeated progression of fluid flow structures that form during processes of increasing and decreasing hydraulic gradients the development of fluid flow structures like macro pores conduits and pipes is strongly dependant upon cohesion but is unrelated to grain size variations from sample to sample partial liquefaction leads to permanent changes in the grain fabric and to an increase in hydraulic conductivity our observations explain the longevity of seep sites and systematic relationships between hydraulic gradients grain fabric appearance and hydraulic conductivity that could be used to estimate flow rates based on structural findings

an investigation of the reading newbury magnetic anomaly the complex magnetic anomaly situated in the region of reading and newbury has been investigated using several techniques analysis of the power spectrum of the anomaly indicates that it results from two groups of sources a shallow group at a maximum depth of some 1 1 km and a more extensive deeper group at a depth in excess of 2 5 km the anomalies due to these two sources have been separated by wavenumber filtering and continuation methods the deeper group of sources appears to have a significant remanent magnetization and could be modelled by an approximately prismoidal body with a side length of some 20 km the shallow group of sources appears to correlate with igneous rocks encountered in local boreholes the magnetic properties of these rocks have been determined

structural analysis of the adjacent acadian and variscan fold belts in belgium and northern france from geophysical and geological evidence abstract two palaeozoic fold belts are juxtaposed in belgium and northern france the northern belt of the brabant massif traditionally considered a part of the european caledonides was folded during the acadian orogeny in the early devonian although it is largely concealed gravity and magnetic maps give a general picture of the existing structures a curved gravity trend is interpreted as an arc shaped succession of granitic batholiths structures north and south of this arc are completely different the northern subdomain containing magnetic lower cambrian metasediments shows multiple thrust faults towards the south over the granites whereas the southern subdomain without magnetic rocks is moulded around the granitic core the southern variscan belt is characterized in general by thrusting with a north nw and ne vergence during the variscan orogeny the brabant massif acted as an obstacle to the general transport of the ardennes massif to the n nw the oblique convergence with the brabant massif caused differential displacement towards the northwest the western areas having travelled farther as seen from a pattern of wnw oriented strike slip faults

tertiary structures and hydrocarbon entrapment in the weald basin of southern england abstract the weald basin of southeast england was formed by rapid subsidence associated with thermal relaxation following early mesozoic extensional block faulting the basin appears initially to have taken the form of an easterly extension of the wessex basin but became the major depocentre during the upper jurassic and lower cretaceous with associated active faulting these movements appear to have ceased prior to albian times and a full upper cretaceous cover is believed to have been deposited in a gentle downwarp which extended far beyond the confines of the weald and wessex basins major inversion of the weald basin took place in the tertiary with both gentle regional uplift which in the eastern part of the basin is estimated to have exceeded 5000 feet 1525 metres and intense local uplift along pre existing zones of weakness which led to the formation of compressional features such as tight folds and reverse faults zones of tertiary deformation appear to have been strongly influenced by underlying particularly hercynian structural trends lower jurassic source rocks reached maturity in the early cretaceous and initial migration occurred at this time often over long distances into traps closed by pre aptian faults tertiary tilting and uplift led to the breaching of many of these pre existing traps and the formation of large folded closures a second phase of hydrocarbon migration particularly of gas took place at this time with significant vertical migration along fault zones major reservoirs located to date occur in middle jurassic carbonates and upper jurassic sandstones but deep burial in the basin has caused considerable destruction of primary reservoir characteristics changes in the temperature and pressure regimes and the mobilization of fluids within the basin resulting from the tertiary uplift caused further diagenetic changes particularly in the carbonate reservoirs exploration of the weald basin remains at a very early stage with a low drilling density to date the more recent drilling has focussed on earlier structures but traps formed or modified during the tertiary movements represent important exploration objectives although general deterioration in reservoir quality towards the centre and east of the basin makes large fold closures in these areas less attractive

correlation of the rhenohercynian variscides recent structural and stratigraphical data from the rhenohercynian variscides of belgium germany and south cornwall indicate tectonic continuity within this zone prior to disruption along a late carboniferous dextral transcurrent fault this extends from the bristol channel to the vosges parallel to similar lineaments of variscan age in europe and north america consequently a tectonic model for south cornwall is extrapolated to europe and a plate tectonic setting for the evolution of the north european variscides by the closure of an oceanic basin is discussed

gravity gradients and upper crustal tectonic fabrics ireland gravity gradients derived from an updated bouguer anomaly map of ireland reveal large scale lineaments many of which can be related to geological structures and tectonic development first horizontal derivatives of the anomaly calculated at various azimuths accentuate linear trends in different directions while the second derivative helps resolve near surface structure the dominant ne sw caledonian trending lineaments are interpreted as due to fault boundaries and folds in pre carboniferous basement and caledonian fault reactivation during the variscan orogeny subsidiary nne ssw trending lineaments are present as well as weaker nw se trending lineaments the largest negative anomalies are produced by late caledonian granites and widespread buried granitic rock in the middle to upper crust is proposed both exposed and inferred granites appear to be spatially related to intersections of ne sw with nne ssw and nw se trending lineaments the orientation of the dominant gravity trend changes from ne sw towards ene wsw going westwards across ireland and forms part of a more regional change in the caledonian gravity fabric recognised throughout the northeast atlantic region variscan e w gravity trends in the south of ireland overlay the caledonian trends relationships between gravity lineaments and anomalies support a thin skinned model for the variscan cover

interpretation of the eastern sulaiman fold and thrust belt pakistan a passive roof duplex abstract surface geology seismic reflection and well data from the eastern sulaiman fold and thrust belt sftb were integrated to analyze its structural geometry deformation style foreland structure development and tectonic shortening the n s oriented fold belt s deformation front has a monoclinal expression over 350 km along the eastern edge of the sftb the cross section shows a 9 km thickness of platform sequences that are detached from the basement by an inferred decollement in paleozoic strata the base of cretaceous strata is 6 km deep in the foredeep syncline rising 7 km in the foreland in the absence of a major thrust and is interpreted as a thin skinned passive roof duplex style of deformation the duplex sequence of jurassic and older strata are separated from the cretaceous and younger roof sequence which is intact over foreland structures for 60 km a balanced structural cross section of this area restores to an original length of 172 km implying 112 km 65 of shortening since 6 2 ma with rates of 18 mm yr this is greater than the 50 shortening along a 359 km long n s balanced cross section across the sftb but is likely to occur along stacked duplexes with high relief along the eastern sftb

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

cambrian and ordovician stratigraphy related to structure and seismic profiles in the western part of the english midlands in england the cambrian successions include relatively continuous dominantly argillaceous sequences nuneaton area 900 m thick and arenaceous successions punctuated by hiatuses malvern shropshire the tremadoc lower ordovician succeeds paraconformably it is widespread dominantly argillaceous and locally very thick 2000 m deposition of the lower tremadoc is thought to have been rapid uplift of regional extent occurred in early ordovician post tremadoc times and inversion of the basins such as the tremadoc worcester graben occurred during the variscan orogeny the disposition of the cambrian tremadoc beneath the upper llandovery unconformity is thought to reflect the arrangement of basins and highs at that time evidence from seismic profiles indicates deposition in half grabens in the welsh borderland evidence from outcrops and boreholes suggests that the st david s series middle cambrian is locally thickened against syn depositional faults the thinning and local absence of the st david s series across such faults is attributed to the influence of tilt block highs rifting and tilting allowed thick tremadoc to accumulate e g in a precursor worcester graben but evidence for the direction of thickening is masked by lack of seismic markers and by the effect of pre landovery erosion

the seismic reflection moho beneath the united kingdom and adjacent areas abstract onshore and offshore deep seismic reflection data have been integrated to produce two way travel time and depth maps of the seismic reflection moho beneath the uk moho travel times are mostly in the range 9 5 11 5 s showing only weak correlation with specific upper crustal structural features but decreasing generally northwestward towards the atlantic margin moho depths show a much stronger correlation with upper crustal structure than do travel times there is pronounced crustal thinning moho depths 32 km are found generally beneath basement massifs particularly onshore short wavelength topographic features on the moho are quite rare and are generally restricted to ancient zones of continental collision and or subduction some of these ancient collision and or subduction zones are spatially associated with a highly reflective moho this is particularly evident where low angle shear zones in the lower crust or uppermost mantle converge with or intersect the moho it is suggested that development of a moho detachment surface can tectonically smooth the moho and thereby markedly increase its reflectivity a tendency in certain areas for the moho to remain at roughly constant two way travel time though not constant depth beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium conversely travel time perturbations beneath smaller features 10 km across indicate that these are at least partially uncompensated the uk crust is therefore weak but not sufficiently weak to attain local airy isostasy basement massifs west of scotland show systematically smaller moho two way travel times than similar structures farther south within the constraints of regional isostasy this can be interpreted in two main ways greater rigidity and strength of lewisian rocks within the west scottish massifs manifest as higher seismic velocity relative to density could be a cause of the observed travel time differences alternatively low density material beneath the crust could buoy up the western scottish massifs if this buoyancy were the consequence of igneous underplating then any underplated material would have to lie beneath the seismic reflection moho

the nature of the variscan basement in southeast england evidence from integrated potential field modelling the variscides of southeast england are buried beneath post carboniferous cover interpretations of the basement are based mainly on deep boreholes geophysical signatures from the basement are contained within the regional gravity and magnetic data a gravity stripping exercise has been undertaken to remove the gravitational effect of the post variscan cover to generate a residual gravity map this map is interpreted along with integrated potential field modelling along four long interconnected profiles and compared with a revised pre permian subcrop map the magnetic evidence suggests that precambrian magnetic basement of the midlands microcraton has been buried southwards by north vergent variscan thusting over the foreland north of the variscan front short wavelength anomalies superimposed upon this deep precambrian source are due to shallower silurian and carboniferous volcanic rocks many residual gravity lows within the rhenohercynian zone may be related to thick low density devonian basins in the english channel a change in geophysical signature occurs north of the portland wight fault coinciding with phyllites in the basement models are presented in which the english channel magnetic anomalies originate within the pre permian basement comparisons with anomalies in the southwestern approaches suggest that the portland wight thrust is a terrane boundary possibly a subduction related suture implying southerly directed variscan subduction

the newer granite problem a geotectonic view the siluro devonian suite of granitic plutons in the british caledonides known as the newer granites together with their associated extrusive rocks represent one of the most extensively researched examples of calc alkaline magmatism apparently related to orogeny although recent chemical studies have credibly interpreted some of the scottish intrusions and volcanic rocks as part of a continental margin magmatic arc generated by the subduction of lapetus oceanic lithosphere beneath laurentia insurmountable problems of distribution and timing arise when attempts are made to relate the magmatic activity as a whole to a traditional two plate collision model for the orogeny newer granite magmatism is here discussed in the context of more mobilistic models for the post grampian evolution of the british caledonides which involve e w closure between laurentia and baltica terminated by collision in the silurian followed by the northward accretion of gondwana derived terranes in the early devonian the former produced the main caledonian tectonometamorphism of the northern highlands of scotland the latter the late caledonian deformation of the slate belts in the paratectonic caledonides these models imply much more complex convergence geometries which can in principle account for the whole newer granite suite as a series of subduction generated magmatic arcs overlapping in space and time the model proposed involves three late caledonian magmatic arcs in addition to the ordovician borrowdale arc which is not considered in this paper one is related to laurentia baltica convergence with westward subduction beneath the scottish sector of the laurentian margin in the ordovician and early silurian which generated the early members of the newer granite suite in the highlands a second is related to northward silurian early devonian subduction at the solway line which generated the younger newer granites and volcanic rocks north of the highland border and a third related to northward accretion of the armorican terrane in early devonian time produced intrusive and extrusive magmatism as far south as southeast ireland and the english midlands

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

a geochemical approach to earthquake reconnaissance at the baratang mud volcano andaman and nicobar islands abstract variations in the relative composition of constituent gases coming out of the mud volcano near jarwa creek in baratang island middle andaman are discussed on the basis of regional tectonic settings and seismo geochemical features the most abundant gas is ch4 55 7 vol followed by n2 32 2 o2 ar 9 9 co2 2 0 and he 0 2 concentration of the radioactive gas 222rn is relatively high 7500 bq m3 this paper discusses the experimental methods adopted to continuously monitor he ch4 co2 and 222rn with a view to identify pre seismic gas concentration anomalies associated with regional seismic occurrences results related to this gas geochemical monitoring are presented with a purpose to develop ability in ascribing a distinct geochemical anomaly to a particular seismic activity in the region

structural trends in central britain from images of gravity and aeromagnetic fields digital image processing techniques have been used to analyse the regional gravity and aeromagnetic datasets in central britain colour and shaded relief images have been generated which convey information on both anomaly amplitude as colour and anomaly gradient as relief and highlight structural trends lineaments and textural contrasts not easily discernible on standard contour maps we have identified some of the more important features and have tried to relate these to the evolution of the crust in each region on a broad scale the images emphasize caledonoid ene trends to the north of the solway line and both tornquist se and caledonoid ne to ene trends to the south the pattern of magnetic anomalies over central england seems to define the extent of the shallow late precambrian early palaeozoic basement of the midlands microcraton and delineates crustal elements and characteristic trends within it magnetic lineaments and the grain of gravity anomalies trending in northeasterly and northwesterly directions on either side of the microcraton seem to relate to structures which originated during the evolution of the welsh and eastern english caledonides respectively where lower palaeozoic and precambrian rocks form the concealed basement as in most of england the gravity lineaments tend to reflect the influence of basement fractures on the subsequent pattern of sedimentation whereas many of the features on the magnetic images are more directly related to structures within the basement itself