detachment faulting mantle serpentinization and serpentinite mud volcanism beneath the porcupine basin southwest of ireland the porcupine basin southwest of ireland provides an opportunity to study the symmetry of rifting at stretching factors approaching continental breakup profiles across the basin image a bright reflection that appears to represent a detachment fault and may in part be a decollement at the top of partially serpentinized mantle although overall the basin appears symmetric the consistent westward structural dip of the detachment implies that at high stretching factors extension was asymmetric farther south the porcupine median high appears in cross section to be a triangular construction overlying tilted fault blocks and onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

interpretation of the gravity and magnetic anomalies over the mull tertiary intrusive complex nw scotland a 50 mgal local positive gravity anomaly over the mull intrusive complex is interpreted as caused by an underlying dense body of mafic basic and or ultrabasic rocks extending to a depth of between 6 5 and 13 km and occupying a volume of between 2000 and 3600 km 3 depending on density contrast the granites produce smaller superimposed negative gravity anomalies which indicate that the granites of the late loch ba centre extend to a depth of about 2 km and those of the early glen more centre occupy a significantly smaller volume the granites occupy between 5 and 9 of the total volume consistent with a derivative origin the complicated aeromagnetic anomalies have been simplified by applying the pseudogravimetric transformation the most conspicuous positive pseudogravimetric anomaly occurs over the rocks of centres 1 and 2 this is of much smaller areal extent than the positive gravity anomaly and it is attributed to highly magnetic rocks extending to a depth of about 2 km beneath centres 1 and 2 including cone sheets the magnetization of the deeper parts of the complex is much weaker and has not been resolved a partial ring of linear or arcuate negative magnetic anomalies occurs along the se s and w margins of the deep seated intrusive complex and may mark an unexposed early ring dyke with strong reverse magnetization it is estimated that up to a quarter of the volume occupied by the intrusive complex may have been provided by various types of forceful intrusion but that most of the upper crustal space has been made available by ring subsidence and other types of stoping this is feasible because the magma was demonstrably lower in density than the moine and lewisian the mantle derived magma probably rose through the more ductile lower crust by diapirism thereby causing a slight thickening of the underlying crust which may provide isostatic support for the excess mass of the intrusive complex and its associated topography

subsidence analyses from the north sea triple junction when compared with theoretical subsidence curves calculated from the uniform stretching model water loaded subsidence data from the triple junction region of the north sea suggest that the jurassic cretaceous rifting event caused lithospheric thinning by a factor of c 2 0 although somewhat larger than the stretching factors found elsewhere in the north sea this amount of thinning is anticipated from the overall geometry of the three graben system and is consistent with the observed volume and elemental and isotopic composition of the jurassic forties volcanic province apart from in the southern north sea permo triassic extension is thought to have been relatively minor in comparison to the later jurassic cretaceous phase the anomalously small amount of late jurassic syn rift subsidence in those wells where local fault controlled effects are minimal supports the well known idea of localized relative uplift or doming in the triple junction area c 10 4 km 2 prior to and during the early stages of the jurassic cretaceous rift phase a time dependent differential stretching model in which the lithospheric mantle is initially stretched by a greater amount than the crust is stretched might provide an explanation such a model would require the total amount of stretching integrated over space and time to be the same for the lithospheric mantle and for the crust in order to avoid space problems alternatively the same data could be explained by invoking a small transient thermal anomaly in the asthenosphere

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

role of decollement material with different rheological properties in the structure of the aljibe thrust imbricate flysch trough gibraltar arc an analogue modelling approach abstract the miocene aljibe thrust imbricate flysch trough in the external zone of the gibraltar arc shows large changes in structural style towards the foreland rheological variation of the decollement rocks triassic evaporitic rocks and mesozoic limestones appears to be the main factor controlling the contrasts in structural style that characterise adjacent wedge domains in order to test this hypothesis we carried out analogue experiments involving three dimensional variations of decollement material a plate of viscous material was placed in the central part of the models at the base of the sand multilayers different geometries of the viscous plate were used experimental results show a strong link between the rheological properties of the decollement material and the structure of the overlying thrust wedge these results compare well with the internal geometry and vergence distribution in the aljibe thrust imbricate indeed the tectonic style of the wedge can be explained as a consequence of variations of rock type along the basal decollement

earlier stone age archaeology of the vhembe dongola national park south africa and vicinity abstract south of the modern limpopo river near its confluence with the shashe gravels associated with an ancient aggrading terrace of the river attracted earlier stone age esa occupations preserved in the vhembe dongola national park of south africa artefacts from late esa occupations were deflated onto substrates of cemented miocene terrace sediments and sandstone bedrock and today lie buried by fine non alluvial sands of holocene age artefacts are eroding from these sands which cap a 3 km long escarpment forming the northern edge of the remnant terrace close to the escarpment large scale excavation has exposed an artefact accumulation with parallels to the sangoan industry or charaman in one proposed industrial sequence test pitting of the sand cover across 10 km 2 has been conducted along the escarpment and southwards in sediments that overlie both sandstone and calcrete buried artefacts are preserved at many locations but the more informative sites occur at deep sediment traps formed by hilly sandstone outcrops koppies at one such location later and middle stone age artefacts also occur analysis of the cultural phases present on this landscape and osl dating of the deeper deposits is underway to document the pleistocene and holocene periods when this semi arid drought prone environment was habitable for stone age populations

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

lateral structural variation along the kalabagh fault zone nw himalayan foreland fold and thrust belt pakistan abstract the nw himalayan fold and thrust belt in pakistan is of gentler regional slope and wider extent than the other parts of the convergent plate boundary between india and the rest of asia large scale structural re entrants typify the main frontal thrust mft of the nw himalayan fold and thrust belt in pakistan understanding dynamics of the formation of these structural variations has been hampered by the lack of information about the lateral structures bounding the re entrants our mapping of the kalabagh fault zone a lateral ramp linking the salt and the surghar ranges advanced spaceborne thermal emission and reflection radiometer aster data field investigations and the interpreted reprocessed 2d seismic data this integration of surface and subsurface geology provides new insights on the geometry and evolution of the kalabagh fault zone by showing that it forms an oblique ramp to the main frontal thrust and at north a lateral ramp with right lateral strike slip movement our results indicate that the presence and areal extent of the evaporates is the dominant factor controlling lateral structural variation in the nw himalayan fold and thrust belt of pakistan the kalabagh fault zone acts as a zone that accommodates differential shortening and structural variation along the orogenic trend

thrust tectonics and the role of evaporites in the ionian zone of the albanides this study focuses on the alpine fold and thrust belt in the ionian basin of albania where there are giant onshore oil fields the evaporites have acted as a detachment layer but also have been cut by thrusts that carry evaporites to the surface in their hanging walls where the detachment is located at the top of the evaporites they are dragged up in the footwall along thrusts and faults develop at the base and the top of the evaporite simple shear deformation along the thrusts causes greater than 80 thinning of the evaporite so that sheets of only 40 200 m thickness are preserved whereas the original evaporite thickness probably was well in excess of 2000 m where ramps cut through the evaporite and cause repetition of the complete evaporite layer the salt is remobilized into salt filled compressional anticlines or escapes at the surface to produce smeared out surface extrusions the most spectacular example of this is the dumrea structure where the evaporites have developed into a complex shape from an overthrusted allochthonous sheet that has been extruded at the surface the dumrea structure is up to 5 6 km thick proved by drilling and has been displaced laterally by up to 25 km and it has the effective geometry of a giant sheath fold

imaging beneath an opaque basaltic layer using densely sampled wide angle obs data a combined reflection refraction wide angle seismic survey was conducted on the continental shelf north west of britain using a conventional streamer with an air gun source and static ocean bottom seismometers obs to record wide angle energy the shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section the basalts however proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons where an important velocity inversion was anticipated this paper reports on the processing modeling and interpretation of the densely sampled wide angle obs data that were coincident with the reflection profile eleven obs instruments were deployed along a 75 km line and recorded signal from a powerful 149 liter 9100 in sup 3 air gun array fired every 50 m data processing was performed using a standard industrial reflection seismic software package prior to first arrival picking processing steps included geometry definition trace summation and display of the data using various scaling algorithms an initial model was constructed from 1d velocity time profiles digitized every 4 km along the stacked section first arrival travel time modeling rapidly converged to a detailed model of the structure of the top 5 km of the crust modeling revealedmore the existence of a buried low velocity mesozoic sedimentary basin of a prominent basement horst and of a normal fault penetrating to the basement less

three dimensional gravity and magnetic modelling of the irish sector of the ne atlantic margin abstract a new 3d lithospheric model has been constructed using high resolution gravity data from the irish national seabed survey the sedimentary component of the model incorporated density variations due to laterally varying overcompaction associated with cenozoic denudation after optimisation based on gravity inversion regional crustal thickness variations were defined which are in reasonable agreement with the results of wide angle seismic experiments high crustal extension factors 5 characterise the deeper parts of the rockall and porcupine basins and in places the model indicates extreme stretching 10 beneath these basins this could be because of instability in the gravity inversion although other recent investigations have independently suggested similarly high extension factors in contrast the hatton basin is characterised by an apparent extension factor of about 2 the modelling resolves a pattern of ne to nne trending local mesozoic basins on the margins of the rockall trough helping to delineate structures that were previously only sparsely sampled by seismic surveys it appears possible that rifts with similar trends underlie the volcanic rocks which obscure the deeper parts of the hatton basin the linear trends of the basins to the south and east of ireland are interpreted to have been inherited from a basement fabric that was initially established during the late precambrian assembly of this basement and subsequently subjected to caledonian and variscan reactivation magnetic modelling indicates that the variations in the thickness of the crystalline crust predicted by the gravity models can explain the regional magnetic anomaly patterns over the rockall and porcupine basins but that significant additional magnetic material probably igneous rocks of both palaeogene and cretaceous ages is required to explain the anomalies in the hatton basin region the magnetic signature of the rockall basin is distinctly different to that over the basement of similar apparent thickness formed during mid cretaceous c34n opening of the ocean basin to the south this is an impediment to hypotheses that invoke mid cretaceous sea floor spreading rather than intracontinental rifting to explain the development of the basin the exception is in the extreme south of the basin where the volcanism associated with the barra volcanic ridges combined with indications of relatively strong lithosphere could be evidence of incipient ocean opening

triassic evaporites and plio quaternary diapirism in western greece wide triassic evaporite outcrops run parallel to the strike of hellenide thrust faults in western greece they crop out along the leading edges of thrust sheets emplaced as part of the foreland migrating fold and thrust belt the evaporites acted as the plane of decollement for thrust sheet emplacement over much of the external hellenides plio quaternary diapirs occur locally within the broad outcrops of triassic evaporite they are the result of local mobilization of evaporite after its emplacement their rise may have been initially triggered by hellenide thrust sheet loading and subsequently accentuated by plio quaternary subsidence geophysical records and onland exposures show that diapiric rise controlled local sedimentation patterns above the sites of earlier hellenide thrusts the subsequent intrusion of diapirs into these cover sediments caused localized extensional structures to develop this is best seen in se zakinthos where strain hardened conjugate faults developed in friable pliocene sandstones early in the intrusion history principal strains calculated from these sets demonstrate patterns of extension sub orthogonal to intrusion margins they are comparable with patterns known from other field and experimental examples but contrast with some recent studies on diapir related strains

new evidence for the origin of the porcupine median volcanic ridge early cretaceous volcanism in the porcupine basin atlantic margin of ireland 1 two dimensional pre stack depth migrated seismic reflection data gravity and velocity models are used to assess the nature and origin of a prominent buried ridge the porcupine median volcanic ridge pmvr within the porcupine basin offshore ireland the debate on the origin of the pmvr during the past 30 years has followed the evolution of the concept of continental margin genesis in this paper the origin of the ridge is evaluated on the basis of the internal geometry and velocity structure revealed by the seismic data implication of the presence of these type of ridges in hyper extensional rifted margins is discussed and compared with other margins the analysis indicates that the ridge is an extrusive volcanic ridge probably tholeiitic in composition constructed by stacked hyaloclastite deltas and topped by carbonate platforms the results invalidate previously proposed models involving highly rotated fault blocks and the serpentinite mud volcanism the extension magnitude analysis suggests a highly stretched setting where limited mantle serpentinization may have occurred but the architecture and velocity of the pmvr demonstrates that it is made of lower velocity materials than serpentinite during the opening of the north atlantic the pmvr represents the northern time equivalent magmatic event expressed along the newfoundland iberia galicia recorded by the j anomaly that originate from cretaceous volcanic deposits

a review on the driving mechanism of the tyrrhenian apennines system implications for the present seismotectonic setting in the central northern apennines abstract the formation of the tyrrhenian apennines system has been characterized by a complex distribution of major tectonic events such as opening of basins migration and distortion of orogenic belts retreat of trench zones stopping of consuming processes and starting of new subductions the various aspects of each event such as timing location dimension and deformation style impose important constraints on the genetic mechanism in this work we argue that the numerous constraints implied by all events together may be plausibly and coherently explained as effects of the convergence of the confining africa arabia and eurasia plates crucial evidence for discriminating among the various geodynamic hypotheses so far advanced is provided by the structural tectonic information recently provided by crop seismic sections in the study area the proposed evolutionary history is illustrated by several paleogeographic maps which provide a more detailed reconstruction than previous attempts particular attention is paid to describing the tectonic setting developed since the middle pleistocene in the apennines belt and its connection with seismic activity

olivine dolerite intrusionsin the fastnet basin sills of olivine microgabbro and olivine dolerite 0 5 180 m thick intrude lower liassic hettangian sinemurian sediments in the fastnet basin a sw extension of the n celtic sea basin about lat 50 n long 10 w the evidence of 3 wells and 2300 km of seismic reflection profiles indicates 6 major sills and sill complexes and 3 possible igneous centres or plugs a high level sill may intrude lower cretaceous sediments and one plug apparently deforms cretaceous reflectors k ar ages on separated biotites from a thick sill intruding liassic sediments gave 170 4 ma bajocian this sill contains 30 olivine fo 70 73 20 clinopyroxene magnesian salite 45 plagioclase an 60 78 with accessory magnetite ilmenite edenitic hornblende ti phlogopite apatite zircon spinel in olivine and sulphides alteration to serpentine minerals chlorite sericitic mica and leucoxene is sporadic the magmatism in the fastnet province thus appears to have been active from the mid jurassic to the early tertiary related initially to the time of initial rifting between the european and american plates and subsequently to their final separation between greenland and rockall and associated thulean magmatism

stratigraphic evolution and source rock potential of a lower oligocene to lower middle miocene continental slope system hellenic fold and thrust belt ionian sea northwest greece the western flanks of the hellenic fold and thrust belt are similar to the nearby prolific albanian oil and gas provinces where commercial volumes of oil have been produced the lower oligocene to lower middle miocene slope series at this part of the hellenic fold and thrust belt provides a unique opportunity to evaluate the anatomy and source rock potential of such a system from an outcrop perspective slope progradation is manifested as a vertical pattern exhibiting an increasing amount of sediment bypass upwards which is interpreted as reflecting increasing gradient conditions the palaeoflow trend exhibits a western direction during the late oligocene but since the early miocene has shifted to the east the occurrence of reliable index species allowed us to recognize several nannoplankton biozones np23 to nn5 organic geochemical data indicate that the containing organic matter is present in sufficient abundance and with good enough quality to be regarded as potential source rocks the present rock eval ii pyrolytic yields and calculated values of hydrogen and oxygen indexes imply that the recent organic matter type is of type iii kerogen a terrestrial origin is suggested and is attributed to short transportation distance and accumulation at rather low water depth the succession is immature with respect to oil generation and has not experienced high temperature during burial however its eastern down slope equivalent deep sea mudstone facies should be considered as good gas prone source rocks onshore since they may have experienced higher thermal evolution in addition they may have improved organic geochemical parameters because there is no oxidization of the organic matter

how to build an extensional basin in a contractional setting numerical and physical modelling applied to the mejerda basin at the front of the eastern tell of tunisia abstract in tunisia africa eurasia convergence resulted in the tell fold and thrust belt ftb part of the tell rif or maghrebides associated to the south to the atlas intra continental belt the mejerda rift basin is developed over the compressional boundary between the tell and the atlas the main objective of this study is to identify the mechanism responsible for such a basin development the abundance of triassic salt structures in the northern atlas compared to the tell suggests a friction change along the basal decollement formed by the salt level this frictional contrast is expected to produce a heterogeneous deformational response and complex structures on this basis numerical and analogue modelling were used to investigate the impact of a friction change on the deformation style the kinematic approach of the limit analysis theory was applied here via the software optum g2 the onset of faulting in all numerical tests reveals extensional faults appear at the friction boundary beyond the onset of faulting sandbox experiments show extensional features such as collapsing blocks and extensional faults overall the adopted numerical and physical approaches showed evidences of extension in particular the experiment using sandpaper and silicon putty pdms for the decollement transition shows viscous deformation mimicking diapiric movement these observations of numerical and physical model behaviors highlight that compression can produce an extensional basin at the top of a thrust wedge as expression of a strong basal friction change hence the mejerda basin is interpreted as a wedge top basin it developed during the growth of the tell and atlas ftbs the same configuration likely exists in other mountain belts the case of the molasse basin between the jura and the western alps is advocated

development of the porcupine seabight use of magnetic data to show the direct relationship between early oceanic and continental structures the porcupine seabight is a broad composite rectilinear embayment in the margin of the irish continental shelf which seems to be floored at least in the southern part by oceanic crust there is a geometrically straightforward relationship between areas of crustal thinning rifting and the position of probable 9oceanic crust9 it is proposed that an important crustal unit here termed the porcupine segment includes the porcupine bank and the western end of the slyne ridge interpretation of a new magnetic map indicates that opening in the porcupine seabight appears to have taken place along a set of transform faults which 9root9 in an older dextral shear fault set oriented at about 130 three major zones along the longitudinal axis of the seabight relate to the geometry of magnetic stripes in the subjacent basement and can be used in conjunction with opening related to a distant pole of rotation to explain the overall present physical configuration of the basin the porcupine seabight probably opened no earlier than about 170 ma following the establishment of a jurassic volcanic province in the vicinity of the southern seabight elbow

thrust and fold tectonics and the role of evaporites in deformation in the western kuqa foreland of tarim basin northwest china the kuqa foreland between the tarim basin and the tianshan mountains is rich in oil and gas based on field work and seismic profiles the structural styles and their formation mechanisms were determined and the role of evaporites in the deformation was demonstrated the main structural styles in the overburden are detachment folds large scale nappes triangle zones gentle and wide synclines fault propagation folds and pop ups the main structures in the substrate are small scale thrust faults duplexes pop ups and fault bend and fault propagation folds and formed mainly at the end of the pliocene under north south compression the evaporite layer in the lower section of the paleogene is the decollement zone for the disharmonic deformations in the overburden and in the substrate the detachment along the evaporite layer made it possible for compressive stresses to be transmitted farther in the overburden than in the substrate deformation in the overburden is more extensive than in the substrate at the leading edge of deformation at the trailing edge of deformation the structural highs in the overburden closely correspond to those in the substrate which is of significance for petroleum exploration in the western kuqa foreland

influence of depositional setting and sedimentary fabric on mechanical layer evolution in carbonate aquifers abstract carbonate aquifers in fold thrust belt settings often have low matrix porosity and permeability and thus groundwater flow pathways depend on high porosity and permeability fracture and fault zones methods from sedimentology and structural geology are combined to understand the evolution of fracture controlled flow pathways and determine their spatial distribution through this process bed parallel pressure solution surfaces ps1 are identified as a fracture type which influences fragmentation in peritidal and basinal carbonate and upon shearing provides a major flow pathway in fold thrust belt carbonate aquifers through stratigraphic analysis and fracture mapping depositional setting is determined to play a critical role in ps1 localization and spacing where peritidal strata have closer spaced and less laterally continuous ps1 than basinal strata in the peritidal platform facies units with planar lamination have bed parallel pressure solution seams along mudstone laminae in contrast burrowed units of peritidal strata have solution seams with irregular and anastamosing geometries laminated units with closely spaced bed parallel solution seams are more fragmented than bioturbated units with anastamosing solution seams in the deeper water depositional environment pelagic settling and turbidity currents are the dominant sedimentation processes resulting in laterally continuous deposits relative to the peritidal platform environment to quantify the fracture patterns in the basinal environment mechanical layer thickness values were measured from regions of low to high bed dip the results define a trend in which mechanical layer thickness decreases as layer dip increases a conceptual model is presented that emphasizes the link between sedimentary and structural fabric for the peritidal and basinal environments where solution seams localize in mud rich intervals and the resulting pressure solution surface geometry is influenced by sedimentary geometry i e stacked fining upward cycles burrows planar laminations in both facies types laterally continuous ps1 can behave as mechanical layer boundaries as layer parallel slip increases to accommodate shear strain in the fold thrust belt more ps1 behave as mechanical layer boundaries

the evolution of the porcupine and rockall basins offshore ireland the geological template for carbonate mound development the margins of the deep water sedimentary basins west of ireland contain a number of large clusters provinces of spectacular carbonate mounds and build ups these basins have a complex development history involving the interplay of rift tectonics thermal subsidence igneous activity and oceanographic variations the porcupine and rockall basins both rest upon thin continental crust the consequence of major rift episodes in permo triassic late jurassic and early cretaceous times phases of volcanism occurred in the early cretaceous and especially in the early cenozoic fluid flow within the basins is likely to have been controlled by the overall basin geometry and by the distribution and linkage of permeable strata with fault systems stratal surfaces and unconformities a number of regional unconformities controlled by both basin tectonic and regional oceanographic effects can be mapped and correlated throughout the porcupine and rockall basins the youngest of these unconformities c10 early pliocene can be traced throughout much of the nw european atlantic margin it forms the horizon on which virtually all the carbonate mounds in the basins develop suggesting a geologically instantaneous mound nucleation and growth event although the control on their development is uncertain the mound clusters show a spatial association with lithified strata buried contourite and deltaic deposits slope failure features and with the basin margins analysis of these relationships points to a combination of geological and oceanographic processes controlling mound initiation and growth

crustal strain dependent serpentinisation in the porcupine basin offshore ireland abstract mantle hydration serpentinisation at magma poor rifted margins is thought to play a key role in controlling the kinematics of low angle faults and thus hyperextension and crustal breakup however because geophysical data principally provide observations of the final structure of a margin little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension here we present new observational evidence on how crustal strain dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the porcupine basin offshore ireland we present three new p wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the moho across and along the basin axis we use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching c of 2 5 in the north at 52 5 n and 10 in the south at 51 7 n these values suggest that no crustal embrittlement occurred in the northernmost region and that rifting may have progressed to crustal breakup in the southern part of the study area we observed a decrease in mantle velocities across the basin axis from east to west these variations occur in a region where c is within the range at which crustal embrittlement and serpentinisation are possible c 3 4 across the basin axis the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting indicating fault controlled mantle hydration mantle velocities also suggest that the degree of serpentinisation together with the amount of crustal faulting increases southwards along the basin axis seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 15 this observation is consistent with laboratory measurements that show that at this degree of serpentinisation mantle rocks are sufficiently weak to allow low angle normal faulting based on these results we propose two alternative formation models for the porcupine basin the first involves a northward propagation of the hyperextension processes while the second model suggests higher extension rates in the centre of the basin than in the north both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation which eventually controls the development of detachments faults with advanced stretching

predicting fluid pressure in sedimentary basins from seismic tomography 10 pages 9 figures 2 tables geophysical journal international 2019 predicting fluid pressure in sedimentary basins from seismic tomography published by oxford university press on behalf of the royal astronomical society all rights reserved

the influence of preexisting structure and halokinesis on organic matter preservation and thrust system evolution in the ionian basin northwest greece the opening of the ionian basin and its internal differentiation is attested to by lateral facies and thickness variations of the formations deposited during the pliensbachian and tithonian synrift formations the beginning of the synrift sequence is represented by the siniais limestones pliensbachian and their lateral equivalent the louros limestones the geometric characteristics of the extensional basin depend on both extension related to the latest opening of the tethys ocean and halokinesis of the ionian zone evaporitic substratum the orientation of the extensional regime is deduced from the direction of stratigraphic pinch outs of the synrift formations directions of transport of slump features and direction of movement on synsedimentary faults observed in t e base of these formations in the half grabens the postrift period is marked by an unconformity at the base of the vigla limestones which represents the early berriasian breakup during which sedimentation was synchronous in the entire ionian basin the accumulation of organic matter in the lower and upper posidonia beds of the ionian zone during the toarcian and tithonian is directly related to the geometry of the synrift period of the ionian basin restricted subbasins were formed where the geometry of the basin favored stagnation and consequently the locally euxinic conditions of bottom waters anoxic conditions persisted locally to the postrift period in areas where the upper siliceous zone albian cenomanian of the vigla limestones is well developed these areas probably represent subbasins that were preserved by the continuation of halokinetic movements during the postrift period during the early miocene alpine orogeny collision related compressive stresses on the margin induced the reactivation of preexisting fractures which were responsible for the inversion tectonics that affected the mesozoic basin the geometric characteristics of the inverted basin were dependent on lithology evaporites geometry of the extensional structures and direction of the compressional phase the observed geometries do not always correspond to the classical scheme of inversion tectonics the geological evolution in the ionian basin is an example of inversion tectonics of a basin with an evaporitic substratum the opening of the ionian basin and the inversion tectonics influence both the source rocks and the probable hydrocarbon traps of the ionian zone

the development of irish offshore sedimentary basins a variety of ensialic mesozoic and cenozoic sedimentary basins surround ireland three main periods of basin development occurred the first in the permo triassic produced a series of small rift basins which nucleated along reactivated generally caledonian structures the second in the mid and late jurassic represents a period of crustal extension and rifting the third phase reflects thermal subsidence of cretaceous and tertiary age and coincides with seafloor spreading in the north atlantic the basins developed in response to general east west extension simple extensional structures accompanied by crustal thinning dominated in the north south trending porcupine basin the ne sw oriented basins in the celtic sea slyne erris donegal and rockall areas developed in a transtensional setting until early cretaceous times the goban spur basins at the confluence of regional structural trends and adjacent to the continent ocean boundary are dominated by transtensional structures the effects of early tertiary alpine compression are seen especially in the celtic sea basins the basins whilst having some broad geological similarities which reflect eustatic sea level variations differ in their detailed evolution each basin experienced unique facets of local structural control tectonic history and facies distribution

positive fault inversion triggering intrusive diapirism an analogue modelling perspective understanding the formation and the development of salt structures is very important especially because they are of significant economical interest for hydrocarbon trapping and for long term storage of radioactive waste and energy reserves generally the activity of normal faults developed in extensional regimes is considered the most efficient mechanism for salt diapirs the results of analogue models reported in this paper suggest a new triggering mechanism for the rise of salt structures during basin inversion this mechanism relates the localization of ductile diapirs to early normal faults only after their inversion during later shortening in this case diapiric growth is related to the strong dip slip reactivation component along the fault extruding the silicone simulating salt upward some natural cases in which the timing and the mechanism of diapiric growth is not clear can be re interpreted in the light of these analogue model results

sequence stratigraphic response to aptian albian rifting in conjugate margin basins a comparison of the jeanne d arc basin offshore newfoundland and the porcupine basin offshore ireland

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

periodic features of seismic activity in albania by statistica means the temporal variation of the seismic activity in albania is discussed in this study we have investigated the seismic energy release for this century also using the periodogram technique to study the time series of the seismic energy release

coexistence of thin and thick skinned tectonics in zakynthos area western greece insights from seismic sections and regional seismicity abstract the structural style of the fold and thrust belt in the western part of the hellenic foreland in an area offshore the zakynthos island is assessed on the basis of two lithoseismic profiles earthquake data and field data in the broader area the compressional deformation in the area is due to the late phases of subduction and collision between aegean and african plates shortening is affecting a 6 7 km thick sedimentary succession of mesozoic carbonate rocks and evaporites as well as miocene to pleistocene foredeep clastic deposits structures accommodating shortening in the pre apulian and ionian zones comprise a blind imbricate thrust system and fault propagation folds that formed over a 10 11 km deep detachment the thrust system comprises an array of forward verging thrust faults which sole out in a low angle hinterland dipping decollement separating the mesozoic cover from the pre mesozoic basement in this tectonic context the triassic evaporitic layers seem to play an important role in the vertical partitioning of the deformation style acting as either detachment horizons or lining the imbricate thrust surfaces the concentration of recent seismic activity at a depth between 6 and 16 km implies that the pre mesozoic basement is also involved in the deformation in the form of upthrust basement slices suggesting a combination of thick and thin skinned deformation across this domain of the external hellenides fold and thrust belt the frontal part of the belt shows higher seismic activity along a low angle blind thrust as well as also in the areas where the individual thrust planes branch at depth to the basal decollement

ostracod colour change by thermal alteration offshore ireland and western uk abstract ostracod colour changes from white to amber to red brown to black are principally interpreted to be the result of thermal alteration the changes in colour are thought to be caused by progressive carbon fixation of hydrocarbons derived from the surrounding sediments which invaded the ostracod shell exceptions to this scheme include faunas from organically poor limestones which remain white irrespective of the amount of heating and grey brown or yellow brown specimens coloured by the oxidation of diffuse pyrite thirty four exploration wells have been examined from the irish and western uk sectors and these have shown a progressive darkening of colours which is correlated with both burial depth and distance from igneous intrusions the fastnet basin and southern region of the north celtic sea exhibit higher colour change depth ratios to the porcupine basin and northern region of the north celtic sea basin implying greater palaeoheat flow in the former basins

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

geochemistry of upper miocene lower pliocene source rocks in the hellenic fold and thrust belt zakynthos island ionian sea western greece abstract the southwestern margin of the hellenic fold and thrust belt situated along the southern edge of the dinarides albanides hellenides continental convergent zone has been studied in terms of its hydrocarbon generating potential the upper miocene lower pliocene tortonian to zanclean clastic sedimentary succession at zakynthos island provides an excellent opportunity to evaluate the source rock potential of such a system twenty seven mudstone samples were analyzed using organic geochemical techniques such as rock eval pyrolysis bitumen extraction open column chromatography and gas chromatography mass spectrometry the obtained data indicate that the sedimentary sequence contains units with sufficient organic material of good enough quality to be considered as potential source rocks the input of terrestrial organic material and associated gaseous generating potential is indicated by 1 rock eval pyrolytic yields and calculated values of hydrogen and oxygen indices 2 n alkane distribution with dominant long chain components 3 high terrigenous to aquatic organic material ratios 4 odd to even predominance values 5 low sulfur content and 6 occurrence of gammacerane the kerogen is immature as evidenced by 1 t max and production index values 2 odd to even predominance values 3 low diasterane to sterane and pr ph ratios 4 low 20s 20s 20r and sterane ratios and 5 presence of gammacerane

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

the 2014 cephalonia earthquakes finite fault modeling fault segmentation shear and thrusting at the nw aegean arc greece we present refined finite fault models ffm for the 2014 cephalonia keffalinia kefalonia seismic sequence mw 6 0 at the nw edge of the aegean arc ionian sea the area represents the seismically most active part of europe and a continental promontory in which fault modeling is a challenge because of structural complexity and poor coverage by seismological gps and insar data inversion was based on gps data and a new algorithm permitting fusion of slip vectors of individual earthquakes and of their cumulative dislocation and accepting constraints and collocation type analysis of uncertainties computed ffm which correspond to an essentially strike slip fault and a blind shallow oblique slip thrust were assessed by sensitivity analysis and insar data and are consistent with the tectonic fabric of the area they can also explain the observed extreme peak ground accelerations the 2014 faults in combination with ffms of the 2003 and 2015 leucas lefkada lefkas earthquakes farther ne and of the 1983 m7 0 earthquake farther sw constrain a 100 km long immature strike slip fault zone along close to the cephalonia leucas coasts this fault pattern previously regarded as a poorly documented cephalonia transform fault consists of occasionally overlapping oblique slip segments with variable geometric and kinematic characteristics in a shear zone landwards of the plate interface as evidence from seismic profiles reveals this pattern may explain the enigmatic superimposition of shear and compression in the nw edge of the aegean arc

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

internal structure of the erlend tertiary volcanic complex north of shetland revealed by seismic reflection detailed seismic reflection mapping 100 km n of shetland has revealed a large subcrop of palaeocene basalts proved by exploration drilling the geometry of internal reflectors within the buried basalt suggests the existence of two partially eroded volcanoes a vent of about 2 km in diameter surrounded by radially outward dipping basalts is centred over the erlend igneous complex whose existence had been postulated previously on gravity and magnetic evidence alone a new volcano the more deeply eroded west erlend centre is postulated by interpretation of seismic data and by gravity modelling an approximately cylindrical mafic or ultramafic pluton is inferred from the gravity modelling to underlie each of the two centres the diameter of the erlend pluton is about 14 km and that of west erlend about 5 7 km the seismic stratigraphy of volcanic rocks can now be recognized as a valid new field of study with particular relevance to hydrocarbon exploration

the upward transport of inclusions in newtonian and power law salt diapirs abstract this paper studies the ability of salt diapirs to lift large inclusions of dense rocks rafts dense inclusions will be lifted if salt in the diapir rises faster than the inclusions sink the power law rheologies of six different salts and viscosities estimated for newtonian salt are used to calculate the settling velocity of these inclusions as a function of their radii and densiy as well as the temperature of the salt this is done using the known equation describing the velocity of solid spheres settling in unbounded power law fluid two dimensional numerical models are used to study he effect of the ellipticity of inclusions on their sinking velocity as a function of the power law exponentn the calculations of the settling velocities of inclusions neglect several other factors discussed in the paper a the ambient fluid salt is finite bounded b more than one inclusion translates in the salt c further strain rate softening of salt is caused by its diapiric ascent the results suggest that whereas newtonian salt of 1017 1018 pa s and the power law salt of the vacherie dome would have to rise at unreasonably high speeds in order to lift large inclusions most power law salt diapirs would be capable of lifting inclusions of the sizes observed in the iranian domes up to 3 6 km2 if these rise at geologically reasonable velocities and temperatures

the transition between the erris and the rockall basins new evidence from wide angle seismic data abstract the rockall trough is a deep water basin west of britain and ireland the origin of the basin and the nature of the crust beneath it is controversial a series of seismic wide angle experiments carried out during 1988 and 1990 help to clarify the crustal and upper mantle structure in the region results for the crustal structure from a profile which straddles the shelf break between the basin and the irish shelf are discussed here these data together with the available geological information indicate that the basin probably formed in the late palaeozoic to early mesozoic as part of a regionally linked basin assemblage which includes the hatton basin and the shallow sea basins surrounding ireland and britain good data quality has allowed the transition between the relatively unstretched crust of the irish and british mainland defined by previous onshore seismic refraction experiments to be well resolved the erris trough one of the small late palaeozoic to early mesozoic basins which fringe the mainland shelf region north of the porcupine basin is a half graben its major bounding fault is located on its western margin and the basin is divided from the rockall trough by a narrow horst the erris ridge the shelf trajectory along the western flank of the horst deepens smoothly towards the trough centre where the crust thins to 5 km near the trough margin below a sedimentary and water column 8 km thick surprisingly the crustal thickness is slightly greater over a 150 km broad zone at the trough centre i e ca 6 km this change in crustal thickness may be due to lateral strain migration to the warmer basin margins as its centre cooled during the deformation the crustal structure beneath the sediment pile at the trough centre is two layered as opposed to the three layered seismic refraction structure found onshore however the basic character of the lower crust present in onshore ireland in particular the presence of a gradient zone defining the crust mantle transition is still preserved in the trough this similarity in structure precludes the presence of magmatic underplating the crustal structure observed in the rockall trough can be formed by differential stretching of the lithosphere in this model the lower ductile crust and mantle lithosphere are stretched over a wide region by 2 2 3 strain focusing into a much narrower region of brittle upper crust generates severe amounts of crustal thinning 1 8 10 and is responsible for the fusing of the upper and mid crustal seismic refraction layers found beneath onshore ireland and britain mediating detachment surfaces sited at the brittle ductile transition at any time served to relay the strain from the lower lithosphere into the upper crust syntectonic heat loss plays an important role in controlling the deformation pattern

focal mechanism solutions for albanian earthquakes for the years 1964 1988 abstract focal mechanism solutions for 175 albanian earthquakes of albania for the years 1964 1988 were obtained by using an interactive computer technique the results obtained are elaborated upon by applying fisherian statistical analysis to obtain parameters for the mean stress direction in five zones to explain the results a wedging model in the interaction zone between the adria microplate and albanian orogen is suggested

seismicity and seismotectonics in epirus western greece results from a microearthquake survey during a twelve month passive tomography experiment in epirus in northwestern greece a total of 1368 microearthquakes were located the most accu rately located events and focal mechanisms are used here to understand the seismo tectonics of the area the seismicity shows a clear association with the main previously defined deformation zones a total of 434 well defined focal mechanisms were also used for the determination of the stress pattern in the area the computed stress field pattern is quite complex close to the surface and almost homogeneous at depths below 15 km for these depths the stress field is purely compressional in a west southwest direction whereas for shallow depths it is transpressional or even extensional for some smaller areas the abrupt change in the stress pattern which occurs as depth increases suggests the existence of a detachment surface which is provided by the evaporites that have intruded into the upper layers through the thrust zones the presence of the evaporites and their lateral extent is mapped by the seismicity distribution and con firmed by seismic tomography based on the findings we estimate a possible total evaporite thickness of almost 10 km at least for the central part of the study area such a result is important for the oil exploration efforts that have just started in epirus

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

site formation at kudu koppie a late earlier and middle stone age site in northern limpopo province south africa in the mapungubwe national park near the confluence of the shashe and limpopo rivers along south africa s northernmost borders with botswana and zimbabwe the site of kudu koppie is characterized by three lithologically and archaeologically distinct stone age units from bottom to the top these units are 1 the lower kudu koppie unit lkku which includes large tools such as handaxes picks and cleavers characteristic of a late earlier stone age phase 2 the middle kudu koppie unit mkku which contains bifacially retouched points characteristic of a middle stone age msa industry and 3 the upper kudu koppie unit ukku which has sporadic segments and other tools characteristic of the later stone age lsa a refitting and nodule analysis which matches lithic pieces based on microscopic similarities in colour texture and other visible characteristics demonstrates that site formation processes have caused some vertical displacement of material within but not between the lkku and mkku within the national park the kudu koppie sandstone outcrop is unusual in that it has an overhanging structure which undoubtedly contributed to the initial formation and eventual preservation of the archaeological deposits this paper presents several lines of evidence indicating that stratigraphic integrity at kudu koppie has been preserved and that the three horizons are in primary or near primary context

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

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

3d evolution of fold and thrust belts formed by oblique convergence abstract this paper presents the results of a series of scaled analogue models designed to simulate thrust belt development formed by orthogonal and oblique convergence the models were constructed to produce doubly vergent orogenic wedges with a pro wedge and a retro wedge separated by an uplifted orogenic core models were constructed from homogeneous layered sandpacks in a 3 m 1 3 m deformation rig the progressive evolution of the models was recorded using digital photography and the photographs were animated for analysis the experiments investigated convergence obliquities from orthogonal 90 to as low as 15 obliquity orthogonal models produced long linear critically tapered pro wedge fold and thrust belts parallel to the convergence margin together with a narrow uplifted core and a steep retro wedge thrust system the taper angle of the pro wedge was typically 11 12 whereas the retro wedge maintained a steeper taper of 38 42 models where the convergence vector was oblique at angles of 60 and 45 to the margin produced doubly vergent thrust wedges with thrust faults trending parallel to the margin there was little evidence of linked penetrative strike slip faulting but en echelon riedel like shear systems developed in the 45 oblique model subordinate oblique slip motion was observed on some thrust faults in contrast for 30 and 15 oblique convergence models strong deformation partitioning developed with steeply sloping pro wedges that did not have critical tapers through going linked strike slip faulting was developed that cut the uplifted axial zones of these models analysis of the models by animation of the digital photographs shows how the thrust systems initiated and propagated in particular it is clear that at any one time several thrusts were moving simultaneously serial cross sections through the moderately oblique convergence models 60 45 are almost indistinguishable from the orthogonal model cross sections the results of orthogonal convergence experiments compare well to the map patterns of faults and folds in the salt range pakistan 60 and 45 oblique convergence models compare well to the zagros where some strain partitioning on strike slip faults occurs the highly oblique models show very strong strain partitioning with penetrative margin parallel strike slip faulting similar to that found in ne venezuela

effect of spatial distribution of hormuz salt on deformation style in the zagros fold and thrust belt an analogue modelling approach scaled analogue models of thin skinned simultaneous shortening above adjacent viscous and frictional decollements simulate the effect of hormuz salt on the shortening in the zagros fold and thrust belt the models consisted of sand layers that partly overlay a viscous layer of silicone and were shortened from one end spatial distribution of the viscous decollement varied along strike and dip as occurs in part of the zagros fold and thrust belt in this belt phanerozoic sedimentary cover was shortened partly above the hormuz salt lying on the precambrian crystalline basement behaving as a basal viscous decollement model results display how the nature of the decollement affects the evolution of an orogenic belt using model results we explain the development of deflection zones and discuss strain partitioning formation of different topographic wedges and differential sedimentation along the zagros fold and thrust belt model results suggest the formation of a gentle taper consisting of both foreward and backward thrusts above a viscous decollement and a relatively steeper taper consisting only of forward vergent imbricates above a frictional decollement however in our models the steepest wedge with the highest topography formed where the viscous substrate had a limited extent with a transitional boundary pinch out perpendicular to the shortening direction shortening of this boundary led to development of frontal ramps associated with significant uplift of the area behind the deformation front

subparallel thrust and normal faulting in albania and the roles of gravitational potential energy and rheology contrasts in mountain belts the active tectonics of albania and surrounding regions on the eastern margin of the adriatic sea is characterized by subparallel thrust and normal faulting which we suggest is likely to be related to gravitational potential energy contrasts between the low lying adriatic sea and the elevated mountainous areas inland we calculate the magnitude of the force which the mountains and lowlands exert upon each other as a result of this potential energy contrast it is likely that this force is largely supported by shear stresses on faults and if so the average stresses are less than 20 mpa alternatively if the mountains are supported by stresses in the ductile part of the lithosphere the stresses are likely to be 80 240 mpa in magnitude the mountains of albania are significantly lower than other ranges such as the peruvian andes which are thought to be extending in response to potential energy differences and we discuss the relation between albania and these other higher mountain belts from the perspective of differences in lithosphere rheology we suggest that the lowlands of western albania and the adriatic sea may have been weakened through time as a result of the deposition of large thicknesses of sediment which lead to heating of the crystalline basement a reduction in the potential energy contrast that could be supported by the lowlands and so normal faulting in the mountains of eastern albania

geological and geophysical evidence for a mafic igneous origin of the porcupine arch offshore ireland sedimentary basins west of ireland contain a number of deep seismic structures that have been variously interpreted as fault blocks serpentinite extrusions or igneous complexes the porcupine arch pa is a deep level 11 km domal 50 km wide seismic feature associated with a prominent free air gravity anomaly high and high p wave velocities detailed seismic mapping of igneous sill complexes in the porcupine basin suggests a possible connection with the pa the sills form a thick 5 km interconnected network extending from the pa into the flanking post rift cretaceous stratigraphy suggesting that the pa may be the top of a large ultra mafic intrusion that fed the sills an intrusive origin for the pa is in agreement with geophysical modelling gravity and v p the seismic character of the porcupine arch structure and the primitive bulk composition of the porcupine sills and is consistent with documented regional cenozoic uplift in the basin with the development of shallow water and the occurrence of paleocene eocene deltaic depositional systems similar mafic ultramafic intrusive complexes have been documented elsewhere on the northeastern atlantic margin including the rockall trough these findings emphasize that higher heat flow in the early cenozoic may have prevailed over the northern part of the porcupine basin

some buried tertiary dykes of britain and surrounding waters deduced by magnetic modelling and seismic reflection methods dykes of tertiary age have been identified by modelling offshore marine and aeromagnetic data and onshore aeromagnetic data dykes have been found in the north sea off ne england and in the irish sea published aeromagnetic maps have also been used to locate non outcropping tertiary dykes in the scottish borders many of the offshore anomalies are caused by multiple dykes that cannot be separately resolved all dykes retain the dominant nw se trend of the known tertiary dykes although this trend approaches e w near the east coast of england under the north sea the trend swings back and becomes increasingly southerly trending the dykes may be the result of nw se compressional or ne sw tensional stress with deep crustal weaknesses causing changes in trend

magnetic mapping of the butterton dyke an example of detailed geophysical surveying a detailed vertical gradient magnetic survey of part of a small intrusion known as the butterton dyke has been made with the aid of a microcomputer based data gathering system the results of over 16 500 magnetic measurements made by one person in less than a day have given results which reveal a detailed pattern of magnetic anomalies from these it is possible to trace the course of two dykes for a distance of over 300 m and identify places where they change direction and show small offsets the advantage of making vertical gradient rather than total field magnetic measurements include a faster surveying speed and better resolution of near surface anomalies

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

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

evolution of fracture and fault controlled fluid pathways in carbonates of the albanides fold thrust belt the process of fracture and fault formation in carbonates of the albanides fold thrust belt has been systematically documented using hierarchical development of structural elements from hand sample outcrop and geologic map scales the function of fractures and faults in fluid migration was elucidated using calcite cement and bitumen in these structures as a paleoflow indicator two prefolding pressure solution and vein assemblages were identified an overburden assemblage and a remote tectonic stress assemblage sheared layer parallel pressure solution surfaces of the overburden assemblage define mechanical layers shearing of mechanical layers associated with folding resulted in the formation of a series of folding assemblage fractures at different orientations depending on the slip direction of individual mechanical layers prefolding and folding related fracture assemblages together formed fragmentation zones in mechanical layers and are the sites of incipient fault localization further deformation along these sites was accommodated by rotation and translation of fragmented rock which formed breccia and facilitated fault offset across multiple mechanical layers strike slip faults formed by this process are organized in two sets in an apparent conjugate pattern calcite cement and bitumen that accumulated along fractures and faults are evidence of localized fluid flow along fault zones by systematic identification of fractures and faults their evolution and their fluid and bitumen contents along with subsurface core and well log data we identify northeast southwest trending strike slip faults and the associated structures as dominant fluid pathways in the albanides fold thrust belt

structural evolution of salt influenced fold and thrust belts a synthesis and new insights from basins containing isolated salt diapirs abstract lateral shortening is expressed in unique ways in salt basins especially if pre shortening diapirs are present we present an overview of previously published and new physical models and present new 3 d conceptual models that capture the evolution of shortening structures in salt provinces dominated by precursor isolated diapirs termed isolated diapir provinces in such provinces isolated diapirs form only a minor volumetric component of a sedimentary basin however due to the relative weakness of rock salt and their ability to localize strain during shortening they have a disproportionately large influence on structural development we find three key mechanical principles govern the processes and structural styles developed during shortening of isolated diapir provinces first salt diapirs shorten before surrounding sedimentary rocks due to their relative weakness and so form salients in the thrust front during early shortening second diapirs tend to nucleate folds and faults which radiate out from the diapirs third as diapir walls converge the roof must shorten extrusive salt sheets are expelled through thin roofs but thicker roofs resist piercement and so tend to undergo complex folding and faulting as a result of these principles the first order controls on the structural styles expressed across a shortened isolated diapir province are the configuration the diapir array prior to shortening the connectivity of these diapirs prior to shortening total strain magnitude and diapir roof thickness second order controls include the initial cross sectional and map view geometry of diapirs diapir size and diapir orientation with respect to the shortening direction

the geometry and topology of natural sheath folds a new tool for structural analysis abstract curvilinear sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the x y axial surface and the x z medial culmination depression surface which bisects the fold nose however 10 000 geometric analyses of minor folds and fabrics formed during ductile thrusting in the caledonides of northern scotland reveals that major dome and basin sheath folds can display distinct and predictable asymmetries across both axial and medial surfaces the strain is typically heterogeneous so that structural fabrics and younging evidence are preserved within sheath folds at varying stages of development this allows an analysis of the evolution of such structures from tongue folds to more extreme tubular forms geometric relationships between measured orientations of fold hinges axial planes extension lineations and foliations are compared on fabric topology plots ftps which provide an effective tool for monitoring planar and linear fabric rotations with increasing progressive non coaxial deformation they consistently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces and on crossing medial surfaces from short to long hinge line segments axial and medial surfaces effectively therefore divide major sheath folds into quadrants with different amounts senses and combinations of planar and linear fabric rotation within each domain such heterogeneous deformation implies that models of intense non coaxial deformation uniformly affecting pre existing folds may overestimate bulk displacement and shear strain variable fold hinge line rotation about medial surfaces also provides an effective mechanism for the closure of major sheaths which may otherwise project for unfeasible distances in the x direction bedding cleavage intersections are developed at greater angles to the transport direction than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing in cross section asymmetric tear drop eyes indicate the sense of fold hinge line vergence whilst fold limb and hinge line asymmetry may be combined on 3 d vergence ranking and rotation grids which allow location and relative strain states of minor structures to be accurately predicted within the overall sheath framework systematic fabric analysis on ftps may be applied to the investigation of ductile deformation across a broad range of scenarios

fluid flow evolution in the albanide fold thrust belt insights from hydrogen and oxygen isotope ratios of fluid inclusions tectonic forces generated during thrust emplacement along active margins may drive complex fluid flow patterns in fold thrust belts and foreland basins isotope data 2h and 18o of fluid inclusions hosted in calcite vein cements are used to reconstruct regional fluid migration pathways in the albanide fold thrust system the calcite veins used in this study developed in a sequence of naturally fractured cretaceous eocene carbonate rocks as a result of episodic throughput of fluids from the early stages of burial onward the acquired fluid inclusion isotope data demonstrate that fluids circulating in the carbonates were derived from an underlying reservoir that consisted of a mixture of meteoric water and evolved marine fluids probably derived from deep seated evaporites the meteoric fluids infiltrated in the hinterland before being driven outward into the foreland basin and ascended as soon as fracturing induced a sufficient increase in permeability structural and petrographic observations provide time constraints for the various phases of fracture infilling and reveal an increasing dominance of meteoric water in the system through time as migration pathways shortened and marine formation fluids were progressively flushed out similar fluid flow evolutions have previously been recorded in various fold thrust belt settings elsewhere in the world