offshore granulites from the bay of biscay margins fission tracks constrain a proterozoic to tertiary thermal history over short distances partly as pebbles from albian conglomerates we chose the granulite samples for our fission track study anticipating that they have recorded most of the history of the continental crust in the area and because the origin and emplacement of these rocks is still not clear the results have important implications and show that fission track studies on offshore basement have great potential for furthering our under standing of continental margin pro cesses abstract zircon and apatite fission track ages were determined on granulites dredged along the bay of biscay margins a sample from ortegal spur iberia margin yielded 725 67 ma zircon a sample from le danois bank iberia margin yielded 284 58 ma zircon indicating post variscan cooling apatite from this sample gave 52 2 ma interpreted as final cooling after pyrenean thrust imbrication two other samples from le danois bank have early cretaceous apatite ages 138 7 and 120 8 ma interpreted to result from exhumation during rifting finally a granulite from goban spur armorican margin gave 212 10 ma apatite coinciding with a precursory rifting phase together with published radiometric results these data indicate a precambrian high grade terrane at the site of the current margins the distribution of the granulites on the seafloor reflects tectonic and erosional processes related to a mesozoic rifting and b early tertiary incipient subduction of the bay of biscay beneath iberia

persistent basement wrenching as controlling mechanism of variscan thin skinned thrusting and sedimentation cantabrian mountains spain abstract in this classical area of thin skinned tectonics current models of the complex fold and thrust belt seem to be approaching a successful synthesis without the necessity for extreme regional bending to account for the characteristic horse shoe form of the orogen fundamental wrench fault zones e g the leon and sabero gordon lineaments whose influence is recorded throughout the palaeozoic stratigraphic history have also played an important role in the sedimentary and structural events of the variscan cycle strike slip motion not only interfered with thrusting far into stephanian times but also effectively controlled molasse fanglomerate sedimentation rendered traceable by multiple clast sources depocentre migration fan skewing and progressive unconformities from the surficial pattern of sedimentation and structure a left stepping pull apart basement structure is deduced it is held responsible for block tilting opposite to the thrusting modifying the backfolding of the thrust sheets and generating the concurrent surficial collapse of its fanglomerate cover it is considered that the persistence of strike slip motion throughout the palaeozoic justifies the conclusion that deeper crustal events of this type may form the source of the thrusting and induce the variable stress orientations in the upper crust to implement the complex near surface deformation the proposed basement configuration fits well into the plate tectonic concept of a palaeo africo iberian promontory to account for the initiation of the ibero armorican arc

early cretaceous syn rotational extension in the organya basin new constraints on the palinspastic position of iberia during its rotation article i nfo in this study we interpret the paleo stress pattern in the organya basin southern pyrenees northern spain as inferred from the anisotropy of magnetic susceptibility ams of 39 sites distributed over the entire basin combined with information from other cretaceous iberian basins such analysis adds to constrain kinematic reconstructions of the cretaceous rotation of iberia allied with the opening of the bay of biscay and the northward propagation of the north atlantic the organya basin is an inverted cretaceous basin in the hanging wall of the boixols thrust the lithologies are mainly weakly deformed pelagic and hemi pelagic limestones and marls which recorded the aptian 35 counterclockwise rotation of iberia three types of ams fabrics could be distinguished all representing typical intermediate and tectonic fabrics ew magnetic lineations dominate in the eastern part of the basin and are related to crustal shortening during the pyrenean orogeny this interpretation is consistent with structural cross sections across the basin showing more intense shortening in the east in the central part of the basin approximately ns oriented magnetic lineations are observed interpreted as the original extensional direction during basin foundering so in line with results from previous studies ams can still unveil the original extensional direction in an inverted sedimentary basin something which may be difficult to reconstruct from geological data alone the original extension direction in the organya basin is perpendicular to the boixols thrust bounding the basin to the south correction for the aptian rotation of iberia leads us to infer a ne sw oriented extension direction at the onset of the rotation of iberia this extension direction is inconsistent with current plate kinematic reconstructions of the cretaceous rotation and motion of iberia we therefore suggest that during the opening of the bay of biscay and related iberian rotation iberia was in a much more westerly position than assumed in current models and that the rotation of iberia was followed by dominantly eastward translation with respect to southern france prior to n s convergence and shortening in the pyrenees since the latest cretaceous

finite element simulation of fluid flow in fractured rock media fluid dynamics models are used by the petroleum industry to model single and or multi phase flow within fractured rock formations in order to facilitate extraction of fluids such as oil and natural gas and in other areas of engineering to study groundwater flow as well as to estimate contaminant seepage and transport in this paper the numerical modelling software comsol is used to simulate air and water flow through a specimen of granite with a single vertical fracture subjected to triaxial loading conditions the intent of the model is to simulate triaxial test findings on a rock specimen with a natural fracture fluid flow is simulated at various confining and inlet pressures using the cubic law model results were in good agreement with laboratory findings pressure distribution along the fracture and across the specimen are as expected with a near linear pressure distribution along the length of the fracture a drawdown effect on pressure distribution across the specimen in the vicinity of the fracture is also observed pressure gradient was largely uniform however some localised zones of high gradient along the fracture are observed

paleomagnetic evidence for dextral strike slip motion in the pyrenees during alpine convergence mauleon basin france abstract the paleomagnetic study of the aptian albian black marls from the inverted early cretaceous mauleon basin in the northern pyrenees reveals two components of magnetization that both yield negative fold tests the carriers of each component show different unblocking temperatures indicating that ferrimagnetic iron sulphides are the main carriers of the intermediate temperature component itc unblocks around 360 400 c and magnetite is the main carrier of the high temperature component htc unblocks around 460 500 c both components show a clockwise deviation in declination with respect to the reference field directions itc di and htc di both in in situ coordinates the reference direction for stable europe for the cenozoic is d i 001 50 d 5 i 3 however the reversal test for both of the magnetization components in situ is positive the structural evolution of the basin involves transtensional deformation during the main stage of basin formation and inversion during compression with migration of hydrocarbons and development of thrusts and folds with axial planar cleavage secondary magnetizations were acquired at the latest stages of the evolution of the inverted basin post folding as the result of a chemical mechanism and they were blocked during two different and likely close spaced in time polarity chrons the blocking of the two different magnetizations with different carriers implies chemical changes in the environment from anoxic sulphate reduction to oxic inferred clockwise rotation of the paleomagnetic vectors can be interpreted in terms of dextral shear across the north pyrenean zone during the tertiary probably eocene according to the plate tectonic scenario this distributed dextral shear was probably favored by discrete shear along cleavage planes as shown by geologic markers of consistent dextral strike slip motion in non competent units with strong cleavage development there is no evidence of such dextral movement in the south pyrenean zone thus indicating a strong partitioning of deformation during the pyrenean orogeny

simulation of groundwater flow in fractured rocks using a coupled model based on the method of domain decomposition groundwater flow in fractured rocks is modeled using a coupled model based on the domain decomposition method in the model the fractured porous medium is divided into two non overlapping sub domains one is the rock matrix in which the medium is described using a continuum model the other consists of deep fractures and fissure zones where the medium is described using a discrete fracture network dfn model the two models are coupled through the continuity of the hydraulic heads and fluxes on the common boundaries the coupled model is used to simulate groundwater flow in a hydropower station the results show that the model simulates groundwater levels that are in agreement with the measured groundwater levels furthermore the model s parameters relating to deep fractures and fissure zones are verified by comparing three different models the continuum model coupled model and dfn model the results show that the coupled model can capture and duplicate the hydrogeological conditions in the study domain whereas the continuum model overestimates and the dfn model underestimates the measured hydraulic heads a sensitivity analysis shows that fracture aperture has a considerable effect on the groundwater level so when the fracture aperture is large the coupled model or dfn model is more appropriate than the continuum model in the fracture domain

structure and origin of the j anomaly ridge western north atlantic ocean the j anomaly ridge is a structural ridge or step in oceanic basement that extends southwest from the eastern end of the grand banks it lies beneath the j magnetic anomaly at the young end m 4 to m 0 of the m series magnetic anomalies its structural counterpart beneath the j anomaly in the eastern atlantic is the madeira tore rise but this feature has been overprinted by post middle cretaceous deformation and volcanism in order to study the origin and evolution of the j anomaly ridge madeira tore rise system we obtained seismic refraction and multichannel reflection profiles across the j anomaly ridge near 39 n latitude the western ridge flank consists of a series of crustal blocks downdropped along west dipping normal faults but the eastern slope to younger crust is gentle and relatively unfaulted the western flank also is subparallel to seafloor isochrons becoming younger to the south anomalously smooth basement caps the ridge crest and it locally exhibits internal eastward dipping reflectors similar in configuration to those within subaerially emplaced basalt flows on iceland when isostatically corrected for sediment load the northern part of the j anomaly ridge has basement depths about 1400 m shallower than in our study area and deep sea drilling has shown that the northern ridge was subaerially exposed during the middle cretaceous we suggest that most of the system originated under subaerial conditions at the time of late stage rifting between the adjacent grand banks and iberia the excess magma required to form the ridge may have been vented from a mantle plume beneath the grand banks iberia rift zone and channelled southward beneath the rift axis of the abutting mid atlantic ridge resulting edifice building volcanism constructed the ridge system between anomalies m 4 and m 0 moving southward along the ridge axis at about 50 mm yr about m 0 time when true drift began between iberia and the grand banks this southward venting rapidly declined the results were rapid return of the spreading axis to normal elevations division of the ridge system into the separate j anomaly ridge and madeira tore rise and unusually fast subsidence of at least parts of these ridges to depths that presently are near normal this proposed origin and evolutionary sequence for the j anomaly ridge madeira tore rise system closely matches events of uplift and unconformity development on the adjacent grand banks

new constraints on the thermal history of north east greenland from apatite fission track analysis apatite fission track analyses from the area north of jameson land east greenland indicate that the region has undergone at least three phases of cooling during the mesozoic and cenozoic two major periods of cooling during the tertiary have been identified a middle tertiary episode in which cooling began between 40 and 30 ma and a late tertiary episode in which cooling began between 10 and 5 ma the middle tertiary event is synchronous with emplacement of major intrusive bodies associated with continental rifting and may be due either to uplift and erosion or hydrothermal effects the late tertiary event appears to be related to erosion associated with uplift resulting from changes in the north atlantic spreading direction and associated events no paleothermal effects have been identified related to the onset of rifting in the early tertiary results from samples farthest from the continental margin reveal an earlier event in which cooling began between 225 and 165 ma the origin of this event is not clear but it may reflect uplift and erosion associated with recognized unconformities within the jurassic section

seismic modelling of outcrop carbonate systems an application to the cretaceous platform to basin system of the maiella mountain central apennines italy

the early cretaceous of north east greenland a crossroads of belemnite migration abstract recent field work in lower cretaceous successions of traill o and wollaston forland north east greenland have resulted in c 2350 belemnite guards collected bed by bed from the upper ryazanian hauterivian the most common belemnite genera observed acroteuthis pachyteuthis and cylindroteuthis are of boreal arctic affinities and closely related to nw european and siberian faunas other taxa including hibolithes common pseudobelus relatively common and duvalia rare show faunal links to both nw european and mediterranean faunas this paper describes and discusses these findings in their taxonomic biostratigraphic palaeobiogeographic palaeoecologic and palaeoceanographic context in particular the occurrence of pseudobelus which is common in the circum mediterranean area is remarkable since it is the first observation of this tethyan genus in the entire boreal realm the palaeoecological interpretation of these observations result in the recognition of four different palaeobiogeographic belemnite assemblages for the boreal realm 1 north east greenland 2 spitsbergen 3 nw europe and 4 siberia in contrast to the other assemblages the belemnite faunas of north east greenland consist of a boreal arctic elements b boreal european taxa c endemic belemnites of tethyan ancestry and d tethyan species these findings make north east greenland part of an immigration route from the tethyan realm via the north atlantic to the high boreal this allowed tethyan species which are otherwise unknown from the boreal realm to reach north east greenland the occurrence of the tethyan genus pseudobelus in north east greenland also supports the interpretation of this taxon as a hemipelagic dweller capable of crossing major distances the belemnite patterns further suggest the existence of a proto gulf stream documenting a south to north flow of warm surface waters as far north as greenland already in the earliest cretaceous valanginian this has substantial implications for the interpretation of early cretaceous climate and oceanic current systems as well as for the palaeobiology of belemnites

towards a new concept of the tectogenesis of the pyrenees abstract crustal structures of the central and eastern pyrenees are compared with those of the alps and carpathians by analogy northward subduction underthrusting in the central and eastern pyrenees is proposed southward subduction underthrusting is found along the north iberian margin and western pyrenees hence a flip a change in the direction of subduction underthrusting is postulated between the central and western pyrenees

rift propagation and eustacy as controlling factors during jurassic inshore and shelf sedimentation in northern east greenland abstract mesozoic sedimentation in northern east greenland was initiated in the middle jurassic with transgression of the peneplaned caledonian basement approximately 1 km of inshore and shelf sediments were deposited in progressively deeper water in middle and late jurassic times the sedimentary sequence is interpreted as resulting from the interplay of fault controlled subsidence borderland uplift and major eustatic sea level changes facies analysis shows that sedimentation in the rift type basin was influenced by fluvial tidal wave and storm related processes

early tertiary magmatism in ne greenland abstract between 73 and 74 n in ne greenland a lower series of quartz tholeiite lavas is overlain by an upper series which varies from quartz tholeiitic to basanitic the lower series basalts are attributed to large melt fractions produced east of the present outcrop where lithospheric thinning permitted the iceland plume to ascend to shallow levels relocation of eruption sites up to 150 km further west preceeded upper series activity in this small degree melts generated principally from garnet lherzolite facies sources were followed by products of more extensive shallower melting which gave rise successively to the upper series lavas a post upper series basaltic dyke swarm and its associated caldera complex palaeomagnetic observations and 40 ar 39 ar dating suggest that the lower series and part of the upper series erupted in c24r at 58 57 ma rejuvenescent activity at c 32 ma marking the termination of volcanism may relate to decompressive mantle melting associated with late regional uplift

paleomagnetism of the lower cretaceous redbeds from northern spain evidence for a multistage acquisition of magnetization abstract an extensive study of the wealden series from the cantabric chain and the sierra de los cameros northern spain reveals that the n r m contains four types of components tentatively classified in decreasing ages the older components group 1 are considered as the primary magnetization they yield a paleomagnetic pole located at lat 41 n long 237 e n 6 k 186 95 5 this pole position implies the existence of a large upper jurassic lower cretaceous swing in the iberian polar wander path the comparison with other polar paths north america africa supports the hypothesis of an initial lower cretaceous rotation of iberia prior to the main mid cretaceous shift the remainder of the components represent remagnetizations the major phase groups 2 and 3 extended from the upper cretaceous to the eocene at least a period closely correlated with the period of convergence between the european and iberian plates the directions of group 3 are characterized by low inclination this is a common behaviour for eocene directions in the mediterranean area previous data obtained from the wealden redbeds largely agrees with the directions of one or other of the groups 2 and 3 as there is strong evidence of an acquisition subsequent to the main mid cretaceous movement of iberia the geodynamical inferences of the paleomagnetic data have to be re examined the last type of components group 4 was built up during recent remagnetization and is of little interest

crestal unconformities on an exposed jurassic tilted fault block wollaston forland east greenland as an analogue for buried hydrocarbon traps abstract the stratigraphy of successions exposed in footwall crests of tilted fault blocks is commonly highly complex crestal stratigraphy and structure are particularly difficult to unravel in the subsurface due to poor seismic resolution across fault zones footwall collapse and coalescing syn and post rift unconformities crestal ridges are important elements in basin evolution as they form drainage divides and sediment sources for aprons along footwall scarps and hangingwall deltas a middle jurassic lowermost cretaceous footwall crest is exceptionally well exposed in the mountain stratumbjerg in wollaston forland east greenland rifting and block tilting was initiated in the bajocian intensified in the oxfordian kimmeridgian culminated in latest jurassic volgian time and faded out in the earliest cretaceous the main border faults of the westward tilted blocks trend roughly n s the first early syn rift block was formed in the middle late jurassic and was 40 km wide during rift climax in the latest jurassic it was fragmented into three blocks each 10 15 km wide the early syn rift succession rests on thin upper permian evaporites and carbonates or directly on peneplaned crystalline basement it is composed of the stepwise backstepping marine pelion jakobsstigen and bernbjerg formations deposited in progressively deeper water reflecting the combined effects of increased rifting and long term eustatic rise the rift climax and late syn rift succession was deposited along the main western basin margin fault scarp up to several kilometres thick it consists of coalesced mainly conglomeratic deep marine slope apron fans over the block crest this succession unconformably overlies early syn rift strata whereas in the deeper parts of the halfgraben the base of the succession is conformable a post rift unconformity was formed in the late hauterivian probably during early post rift emergence and has an irregular stratigraphically and structurally controlled erosional topography the eroded early syn rift rift climax and late syn rift successions were subsequently draped by deep marine barremian and younger post rift strata the outcrop example highlights the interplay between large scale block faulting minor synthetic intra block faulting crestal degradation and development of rift climax and post rift unconformities with pronounced erosional topography it provides an excellent structural sedimentological field analogue to deeply buried marine halfgraben settings which are key elements in many hydrocarbon reservoirs its tectonic and stratigraphic development is thus highly similar to a number of large oil fields in the north sea and the norwegian shelf

fault development and stress evolution of the post hercynian asturian basin asturias and cantabria northwestern spain abstract the asturian basin an emerged part of the north iberian continental margin has been investigated in terms of fault development and stress evolution the mesozoic history of this domain is mainly linked to the extensional faulting processes which preceded the opening of the bay of biscay preserved jurassic faulted blocks and associated half graben structures bounded by wnw ese to nw se faults are related to a late jurassic kimmeridgian rifting stage this fault system seems to have played a major role during the mesozoic structuring of the basin suggesting an extension oriented approximately ne sw the cenozoic evolution is related to the iberia eurasia convergence and collision leading to compressional and subsequent extensional structures the direction of compression documented in various sites was nnw sse during an initial early middle eocene stage reoriented probably in the oligocene to ne sw this succession of stress regimes was applied to the same three major fault systems ne sw nw se and e w which were inherited from the variscan orogeny some of them having already been involved in the latest stephanian early permian rifting episode during the tertiary compressional phase the nw se trend which is believed to have controlled the late jurassic tectonic development was rejuvenated into dextral strike slip faults whereas the ne sw system moved in a sinistral sense the e w system was reactivated as reverse faults giving rise to a partial inversion of the basin

paleomagnetism of permo triassic redbeds from the asturias and cantabric chain northern spain evidence for strong lower tertiary remagnetizations the paleomagnetic analysis of the permo triassic redbeds outcropping in the western part of the cantabric chain and the small mesozoic basin from the asturias shows that these formations have a history of complex magnetization only a few sites did not experience the remagnetization processes and retained original directions the most reliable results yield a paleomagnetic pole located at lat 49 n long 217 e n 11 95 3 7 which is suggested as reliable permo triassic data for the iberian plate two remagnetization phases are recognized a moderate phase predating the folding gave rise to a first overprinting it is connected with the distension which occurred in the pyreneo cantabrian region during the upper jurassic lower cretaceous the main remagnetization phase which occurred after the folding is dated from the lower tertiary and can be related to the compression induced on the northern boundary of iberia from upper cretaceous onwards in some cases this phase led to a complete replacement of the primary magnetization previously published data which were at the time interpreted as being european like in direction are attributed to this phase hence our results do not support the hypothesis of a micro plate called le danois block which was suggested in order to explain these results we believe that there is no paleomagnetic evidence supporting the existence of a complicated boundary between europe and iberia during the mid cretaceous opening of the bay of biscay

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

comparison between the galicia and aquitaine margins abstract there is a great similarity between the galicia margin and the aquitaine margin i e the aquitaine basin and the north pyrenean zone in particular the timing of the rifting as well as the resulting structures exposure of perodotite tilted fault blocks normal and transverse faults half grabens are practically the same this comparison leads to the interpretation that the northern side of the pyrenean fold belt is inherited from the mesozoic deep passive margin of europe the cenozoic thrust sheets and reverse faults of the northern pyrenees zone are tentatively interpreted as former rift structures tilted blocks and listric faults which were removed during the convergence of the iberian and european plates during the paleocene and eocene

extended finite element method for analysis of multi scale flow in fractured shale gas reservoirs economic exploration for shale gas is highly dependent on the complexity of the fracture network caused by hydraulic fracturing technology so it is necessary to accurately assess the effect of the fracture network on gas flow behavior and productivity having multiple scales is a remarkable characteristic of gas flow in fractured shale gas reservoirs which involves multiple flow regimes including the gas desorption from shale matrix knudsen flow in nanoscale pores darcy flow in general porous media darcy flow in the fracture networks and fluid exchange between the matrix system and the fracture system the extended finite element method xfem has been integrated with the dual permeability method dpm to investigate the multi scale problem some previous studies have adopted the same solution scheme but usually considered gas flow in the natural fracture network and in the hydraulic fractures as belonging to the same scale and in addition the coverage area of the stimulated reservoir volume srv was underestimated based on the xfem dpm this paper subsumes flow in the micro mecro fractures and macro hydraulic fractures under two scales because of their different effects on shale gas flow and then presents a new multi scale extended finite element model to study the multi scale flow problem in shale gas reservoirs moreover the lagrangian multiplier method is integrated to introduce the internal well boundary conditions into the xfem so the arbitrariness and the asymmetry of the complex fracture network can be taken into account easily to reflect the real flow mechanisms in fractured shale case studies indicate that the improved extended finite element model constructed in this paper is effective especially for complicated asymmetrical physical condition problems

implications of fault scarp degradation for brent group prospectivity ninian field northern north sea although the majority of the oil reserves contained in the ninian field are being produced from tilted middle jurassic sandstones of the brent group significant volumes of oil have also been discovered within a poorly documented subtle structural trap that lies in the immediate footwall to the field s controlling eastern boundary fault integration of well data and the interpretation of seismic surveys allow the definition of an area characterized by major footwall degradation the data demonstrate that the 1 25 mi 2 km wide area affected lies to the east of the zone of brent group truncation along the whole 15 mi 25 km length of the field a combination of sedimentary and stratigraphic information shows that the degradation and products obtain a thickness that locally exceeds 450 ft 126 m the degradation process appears to have been triggered by accelerated extension during the callovian early oxfordian deposition of the heather formation the development of pronounced fault terraces due to footwall collapse copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 26 1996 revised manuscript received august 12 1996 final acceptance december 24 1996 2department of geology and geophysics the university of edinburgh kings buildings west mains road edinburgh eh9 3jw scotland e mail jru glg ed ac uk 3chevron uk ltd ninian house crawpeel road aberdeen ab12 3lg scotland 4department of earth sciences university of manchester oxford road manchester m13 9pl england we would like to thank the ninian partners chevron uk ltd murphy petroleum ltd oryx uk energy company sands petroleum formerly neste north sea ltd sun oil britain ltd and ranger oil for permission to publish this article however the views expressed here are ours and are not necessarily those of chevron or the ninian partnership help was received from rick davis robert heming bryan man bill higgs andrew harding and john connor of chevron gerry white is thanked for drafting the diagrams interpretations carried out using geoquest ies and iesx software mounted on edinburgh s seismic workstation facility were supported by the petroleum science and technology institute psti norsk hydro shell expro and esso exploration and production uk ltd the study forms part of a wider research program studying the occurrence and effects of fault scarp degradation and subtle stratigraphic trap formation in the brent province currently being supported at the university of edinburgh by a natural environmental research council nerc realising our potential award ropa grant gr3 r9521 finally we would like to acknowledge aapg reviewers alan roberts and j brint for helpful and constructive comments

structural and sedimentological evolution of part of the west central spanish pyrenees in the late tertiary during the late oligocene and early miocene the frontal thrust complex of one of the south pyrenean thrust sheets the guarga or gavarnie sheet became emergent to form part of the northern margin of the ebro basin the principal structures in the frontal thrust complex show a progressive development from a simple recumbent fold in the west to an emergent thrust in the east southward movement at the thrust front decreased from east to west and in the west the thrust tip line is buried these structures influenced miocene continental sedimentation on two scales the emergent thrust front formed an orographic barrier to rivers draining the higher pyrenees these rivers passed around the barrier rather than across it and the positions of the apices of fluvial distributary systems in the ebro basin were thus controlled by the low points in the structure of the thrust front on a smaller scale sedimentary sequences and syn depositional structures in small alluvial fan bodies at the basin margin were controlled by late thrust movements in the adjacent frontal thrust complex syn depositional faults folds and unconformities in the alluvial fan conglomerate sequences can be related to movements on individual faults within the thrust complex

microscopic modelling of the hydraulic fracturing process a microscopic perspective is introduced in this study which offers a detailed insight at the inter particle level to the geo mechanical responses caused by fluid injection and the resulting pressure build up this was achieved by employing the discrete element method dem to model the pressure development and the subsequent fracturing and or cavity propagation this technique represents the formation material as an assembly of discrete particles linked to each other through contacts numerical experiments were carried out on two sample materials for the first instance tests were carried out on a bulk material representative of a generic intact rock with the breakage of inter particle bonds indicating the formation of cracks the second series of tests was carried out on granular type materials such as sand where particle separation signified cavity initiation and separation it was observed from the dem modelling results that the intact rock material showed a predominance of mode ii fracturing at high fluid velocities however when the fluid velocity is reduced considerably the fracturing behaviour tended towards more of mode i also records of the pressure development were taken from the numerical results and were used to monitor the fracturing events the outcome of this study highlights important aspects of the hydraulic fracturing process especially at the particle particle scale and thus provides a strong basis for more exhaustive studies involving larger scale reservoir modelling and more complex fracturing scenarios

influence of rock failure behaviour on predictions in sand production problems the mechanical strength of rock in terms of shear or compressive failure has been previously adopted as a criterion for sand production and when used solely has been proven to overestimate the process on the other hand ignoring the mechanical strength behaviour of the material increases the tendency for inaccurate estimations of the erosion process in this work an equally proportionated inclusion of the mechanical strength and erosion based criteria in sanding predictions is proposed and assessed by numerical models several rock failure models and their influences on the sanding process have been analysed including models such as the drucker prager dp the drucker prager hardening dp hardening the mohr coulomb mc and the mohr coulomb softening mc softening modelling outcomes show distinct differences in rock response to operating and boundary conditions e g flow rate and drawdown and predictions of sand production it was confirmed by modelling results that despite the low magnitude of stresses and strains developed at the well face and perforation regions post yield hardening behaviour increases the estimation of the amount and intensity of sand production also incorporating a post yield softening behaviour increases the magnitude of stresses and strains however this effect is observed to have a negligible impact on sand production the role of void ratio has been recognised as a dominant factor as its evolution significantly determines the pattern and intensity of sand production a more cautious selection and rigorous coupling of rock strength models in sand production modelling is therefore essential if accuracy of predictions is to be improved

palaeomagnetism and isotopic age data from upper cretaceous igneous rocks of w portugal geological correlation and plate tectonic aspects summary palaeomagnetic results and k ar age data for the sintra and sines intrusive complexes w portugal and further details on the palaeomagnetic structure of the lisbon volcanics are reported the sintra complex consists of two main intrusive phases having been emplaced in the upper cretaceous at around 90 ma and 75 ma respectively the radiometric results show that the sines complex formed concurrently with the second sintra magmatism the early main sintra pluton has a characteristic magnetization of d 358 i 27 95 3 3 this remanence direction is defined by gabbros and diorites as the granitic rocks constituting the bulk of the complex are shown to possess stable secondary magnetization imposed during quaternary weathering the characteristic magnetization of the younger intrusive event as defined by the sines rocks has a mean direction of d 041 i 41 95 3 3 these two upper cretaceous palaeomagnetic directions are significantly different at the 95 per cent probability level the lisbon volcanics show the presence of the 75 ma magnetization suggesting that also this volcanic complex dates from the upper cretaceous in addition the palaeomagnetic results from the lisbon complex have given further substance to a previously reported magnetization component with shallow inclination and north northwest declination now defined by d 333 i 14 95 7 4 it is inferred that this magnetization which has a dual polarity structure formed through low temperature oxidation in late cretaceous to lower tertiary time the declination difference between the 90 ma and 75 ma magnetizations may be interpreted in terms of a two phase rotation of iberia in the upper cretaceous prior to the assumed late cretaceous early tertiary secondary magnetic imprint implying a counterclockwise rotation of minimum 40 followed by a clockwise rotation of similar magnitude however if the relevant palaeomagnetic reference frame for iberia before and after its independent tectonic movements in the late cretaceous is the african plate then the clockwise rotation is about 20 larger than the counterclockwise one apparently accounting for the compressive tectonism in the bay of biscay

implications of the seismic structure for the orogenic evolution of the pyrenean range abstract the dynamic evolution of the pyrenees is discussed in the light of geophysical data recent deep seismic sounding have revealed the crustal structure of the pyrenees which is used to test the different evolutionary models proposed until now the crustal thickness of the paleozoic axial zone paz and the north pyrenean zone npz differ by more than 10 km ranging from about 30 km in the npz to 40 50 km in the paz the transition from paz to the npz identified at the surface as the north pyrenean fault npf is sharp at depth and marked by a vertical step at least in the eastern half of the range the npz is characterized by additional throws and dips of the moho in the east whereas in the west a heterogeneous middle to lower crust is encountered with high velocity anomalies the seismic results suggest that the paz and the npz belong to different plates the npf being the plate boundary these results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following nne ssw directions they rather suggest that after a period of extension two main orogenic events took place a phase involving shearing and thinning which affected mainly the present day npz and a later compressive phase which explains the building up of the chain the thickening of the crust and the enhancement of a pre existing difference in crustal thickness between the european and iberian plates

analysis of flow path around the sealing section hg a experiment in the mont terri rock laboratory as part of the hg a experiment in the mont terri rock laboratory large scale in situ water gas injection experiments was conducted in a microtunnel this research work focuses on the numerical analysis of the experimental data and the in situ observations concerning a temporary change of the hydromechanical properties of opalinus clay during experimental operations three phases were numerically interpreted these included the generation of excavation damaged zone during tunnel excavation in which highly permeable flow paths around the tunnel have been formed the self sealing effect during water tests and the pressure evolution during a long term gas injection test a coupled two phase flow and mechanics model taking into account the strong anisotropic properties of opalinus clay was developed to interpret the measured data the hydraulic anisotropy was described by a transversely isotropic permeability tensor an elasto plastic model was established to consider both stiffness anisotropy and strength anisotropy anisotropic plasticity was studied using the microstructure tensor method

a new kinematic model for iberia further palaeomagnetic and isotopic age evidence abstract palaeomagnetic results and k ar age data from the algarve igneous province have uncovered three ancient relative field components it is inferred that the principal magnetization of the monchique complex with a mean direction of d 42 i 181 95 4 5 was acquired at around the time of the pluton s emplacement some 72 2 ma ago a second magmatic pulse affected the region at 68 2 ma leaving behind a relative field component of d 42 i 341 95 1 7 furthermore an overprinted magnetization characterized by shallow inclination and somewhat smeared northwesterly southeasterly declination most likely originates from the lower tertiary the new palaeomagnetic data are seen in conjunction with previously published upper cretaceous results from the sintra and sines intrusive complexes the resulting database gives entirely new perspectives for evaluating the plate tectonic history of the peninsula in upper cretaceous lower tertiary time a two phase rotation of the iberian micro plate an anticlockwise one of 40 followed by a clockwise one of 70 is clearly in evidence the net clockwise rotation provided a major impetus for the compressive deformation in the pyrenees and the bay of biscay and led to extensive remagnetization of older rock formations in the northern regions of iberia previous palaeomagnetic conclusions in favour of a single anticlockwise rotation of 35 supposed to be directly connected with the formation of the bay of biscay appear to be artefacts which stem from an incorrectly adopted palaeomagnetic reference frame

organic petrography and geochemistry of inertinite rich mudstones jakobsstigen formation upper jurassic northeast greenland indications of forest fires and variations in relative sea level abstract the lower middle oxfordian jakobsstigen formation wollaston forland northeast greenland consists mainly of stacked coarsening upward successions of offshore to shoreface heteroliths sandstone and rare foreshore sandstones the units are separated by thin laterally extensive sheets of terrigenous carbonaceous mudstones which have been subjected to organic petrographic and geochemical studies the mudstones are thermally immature with maturities corresponding to r 0 in the range 0 35 0 50 the mudstones contain very high proportions of allochthonous inertinite subordinate huminite char and negligible proportions of liptinite inertinite reflectance distributions are markedly bimodal with maxima at approximately 1 73 and 4 91 r m both pyrolysis yields and solvent extract yields are low the distributions of n alkanes are markedly light end skewed and show a pronounced predominance of even numbered compounds in the lower carbon number range biomarker distributions feature a dominance of c 29 steranes slight enhancement of extended hopanes and steranes low proportion of tricyclic triterpanes and very low hopane sterane ratios sedimentological organic petrographical and geochemical evidence suggests that the regular alternation between marine and terrestrial depositional environments during deposition of the jakobsstigen formation was related to low amplitude high frequency changes in relative sea level and local climate the mudstones were deposited during early rise of relative sea level in shallow flat bottomed lakes or lagoons on a broad coastal plain the lakes acted as traps for fine elastic sediment and for predominantly windborne inertinite generated by wildfires in the hinterland high rates of evaporation rendered the lakes mildly saline hampering their colonization by vegetation other than cyanobacteria and halophilic microorganisms similarly saline porewaters excluded higher plant vegetation from emergent areas upon continued rise of the relative sea level the lakes were gradually flooded and their deposits became covered by sandy shallow marine sediments the larger areas covered by shallow marine waters during periods of high relative sea level led to a more humid local climate and to lower frequency of wildfires during falling relative sea level the marine deposits were eroded and partially removed and the cycle subsequently repeated upon renewed rise in relative sea level hence minor changes in relative sea level gave rise to the regular alternation of two vastly different depositional environments as well as to marked variations in local climate

sequence stratigraphy in the upper cretaceous of the basco cantabrian basin northern spain the sedimentary cycles of the cenomanian to maastrichtian were investigated in the basco cantabrian basin bcb in northern spain provinces of alava vizcaya and burgos the depositional area was a distally steepened carbonate ramp which extended from catalonia northwestwards to the basque country the investigated sediments range from calciturbidites and pelagic marls to marl limestone alternations deposited on a distal carbonate ramp shallow marine limestones marls and intertidal clastics and carbonates were deposited on the proximal part of the carbonate ramp the establishment of a regional sequence analysis is based on the investigation of seismic profiles well logs and outcrop sections examples of outcrop sections are interpreted in terms of sequence stratigraphy unconformities of third and second order cycles depositional geometries systems tracts the sequence stratigraphic interpretation of outcrop sections is based on facies analysis interpretation of observed depositional geometries and correlation of unconformities and marine flooding surfaces through the basin a biostratigraphic framework is established based on ammonites inoceramids planktonic and benthic foraminifera as a result a regional sequence stratigraphic cycle chart is presented and compared with published global cycle charts the correlation of the regional cycle chart with published cycle charts is good in the cenomanian and turonian several sequence boundaries in the bcb are shifted by up to one biozone compared with the global chart some type 1 boundaries of the standard chart are only type 2 in the bcb important type 1 boundaries in the bcb are top geslinianum zone with a 100 m lowstand wedge at the basis of the sequence sequence boundary 92 2 base petrocoriense zone with a 250 m shallowing upwards lowstand wedge at the basis sequence boundary 89 2 and within the syrtale zone sequence boundary 85 0

study on three dimensional fracture network connectivity path of rock mass and seepage characteristics based on equivalent pipe network nuclear waste repositories have extremely stringent requirements for geological environment however natural fractures in rock mass can be potential channels for nuclide migration therefore the influence of fractures on the permeability of rock mass must be assessed in this paper a well research was conducted on well exposed granite outcrops in the xinchang site the chinese high level radioactive waste repository the high precision three dimensional model of a typical outcrop is built to obtain fracture information combined with field measurement and then the three dimensional fracture network model is generated using the relevant parameters by monte carlo method to obtain more comprehensive fracture connectivity while avoiding the traditional method of searching the connectivity path in the complicated 3d fracture model taking up a lot of storage space and costing a lot of time this paper presents an approach using matlab cell array instead of traditional adjacency matrix to search and store fracture network connectivity paths in dfn model the fracture disc with certain thickness is equivalent to three dimensional pipe network model epnm with variable diameter and the equivalent path permeability coefficient epc is proposed to objectively study the permeability characteristics of the seepage path in fractured rock mass based on that especially noteworthy is that some fractures in a certain strike range belong to open type while those in another range belong to cemented closed fractures when fresh fractures were exposed by cutting off the surface rock to a certain depth the calculation of epc under different conditions shows that the order of magnitude of epc mean value is 1e 7m s and 1e 3m s respectively when fractures are cemented and not partly on this basis the size of the representative elementary volume rev of the fractured rock mass in the study area is determined to be about 25 m by rotating the matrix in model the spatial permeability tensor of the region including permeability principal value and main direction is obtained which is within the range of borehole data the predicted results may provide some reference for the related projects in the future

paleoconstraints during rifting of the northeast atlantic passive margins the northeast atlantic passive margins formed during the early cretaceous phase of rifting from goban spur to galicia bank these margins are characterized by a tilted fault block geometry well defined by numerous single channel and multichannel seismic data stress directions cannot be deduced from these structural data but recently withjack and jamison 1986 have described the strain states and fault patterns produced by oblique rifting using experimental and analytical techniques we have applied these techniques to the northeast atlantic continental margins all types of extensional margins are represented there with pure extensional rifting goban spur and west galicia margins oblique rifting north spanish and celtic margins and close to pure wrenching armorican margin the rifting directions are parallel to the adjacent oceanic fracture zones or flow lines defined in the northeast atlantic and in the bay of biscay except for the west galicia margin where a 20 difference exists this difference could be within the error range of the method or could be due to a slight relative motion between galicia bank and iberia during rifting globally the coherence between rifting and spreading directions is an argument in favor of the validity of the method on the basis of kinematic arguments the motion of iberia with respect to eurasia has an eastward component related to the west to east motion of africa with respect to eurasia while iberia was a small plate whose motion was roughly coupled to africa this is confirmed by our analysis we conclude that during both the rifting and spreading episodes in the bay of biscay the motion of iberia with respect to eurasia was in the southeastward direction recently boillot 1984 suggested that iberia moved about 150 km southwestward with respect to eurasia during the rifting phase and then moved in the southeastward direction during the spreading phase we suggest that the boillot hypothesis could be wrong

the influence of the evolution of the pyrenees on adjacent basins abstract important thicknesses of syn and post orogenic sediments are observed in the southern part of the aquitaine basin and in the north of the ebro basin their deposition cannot be explained only as a result of the extensional history of the early cretaceous thus the influence of the compression associated with the pyrenees formation upon the subsidence of these basins is studied europe and iberia are considered to behave as two semi infinite elastic plates the evolution through time of the sedimentary thicknesses is explained by the deflections of these two plates induced not only by the topographic load of the pyrenees but also by additional forces 10 12 n m that may have their origin in the formation of a cold lithospheric root during the shortening a series of calculations is presented for an approximately north south profile across the aquitaine basin the pyrenees and the ebro basin a good fit for the shape of the basins and the gravity anomalies was obtained with flexural rigidities of 1 5 10 23 nm for the northern plate and the outer part of the southern plate and 1 4 10 21 nm for the inner part of the southern plate

characterization of two phase flow in a single rock joint abstract an experimental study using a triaxial apparatus was used to analyze the two phase flow patterns in jointed rock specimens rock specimens having a single natural fracture were tested for two phase flow of water and air triaxial tests were conducted to characterize the two phase flow through fractured granite specimens at low confining pressures it was found that for a relatively smooth joint jrc 0 25 mpa gas injection pressures the gas occupied the main part of the fracture and the liquid was able to flow as an unstable film forming an annular flow along the joint when the annular flow developed the mixture flow pattern was independent of the air flow velocity this was due to the fact that once the injected air velocity reached a critical value i e 20 m s water velocity inside the joint was negligible for a given confining pressure and injected water pressure further increase in inlet air pressures developed a single phase air flow with no water flow

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization