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

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

rate and mechanism in prograde metamorphism for a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature disequilibrium depends on the slowest of three sequential steps a surface detachment of reactant minerals b transport of material to the site of mineral growth and c nucleation and growth of the product mineral we have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism the dissolution and growth steps are linear functions of the driving force g of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself

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

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

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

reaction weakening and emplacement of crystalline thrusts diffusion control on reaction rate and strain rate abstract in the southern appalachians the blue ridge piedmont crystalline thrust sheet was emplaced onto low grade late precambrian and paleozoic sedimentary rocks in the footwall along a basal detachment consisting of phyllosilicate rich mylonites phyllonites the phyllonites developed first by mechanical breakdown of feldspar followed by chemical breakdown to white mica in the presence of a pore fluid diffusion of solute in the pore fluid is the rate limiting step in controlling reaction rate and also the strain rate assuming solute diffusion follows the stokes einstein equation the shear strain rate is given by t 2 n k t 5 r x 2 for shear stress 20 mpa where n is a constant is a geometric factor k is boltzmann s constant t is absolute temperature is water viscosity r is the atomic radius of the diffusing species and x is the diffusion distance a bulk diffusion coefficient in the range of 10 10 to 10 12 m 2 s over distances of 10 100 m results in strain rates of 10 14 to 10 13 s 1 in the temperature range 200 400 c it is concluded that greenschist grade crystalline thrust sheets develop on pre existing basement faults that become weak during reaction softening and localize into high strain phyllonite zones in which pore fluid diffusion controls reaction rate and strain rate

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

strain and displacement measurements for the june 9 1980 victoria mexico earthquake a microgeodetic network 22 km south east of est guadalupe victoria baja california norte installed in late may 1980 has been resurveyed in an experiment that started 12 hours after the june 9 1980 victoria earthquake which had an epicenter at 10 km depth about 12 km from the network the resurvey was complete by june 13 both the initial observations and the resurvey were done with hp3800 distance meter equipment some angular control was provided with a wild t3 theodolite the network underwent a compressive strain of 7 3 micro strain essentially parallel the cerro prieto fault about the time of the earthquake strains of this size are associated with simple dislocation models of earthquakes of this magnitude its direction appears to be anomalous however this may indicate compression related to soil liquefaction processes or strain near the end of the slip plane

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

phyllonitization and development of kilometer size extension gashes in a continental scale strike slip shear zone north goias central brazil abstract several km scale vertical extension gashes occur in a low metamorphic grade strike slip shear zone in central brazil these mega gashes show many of the characteristics commonly found in en echelon extension gashes of cm and outcrop scale reflecting a wide range of scale invariance for this phenomenon the gashes are filled with quartz veins which commonly host gold mineralization microstructures show a progressive deformation of the original cavity infilling vein structures towards the gash margins logarithmic plots of length vs thickness for gashes from thin section outcrop and air photograph scales define a power law l 11 4 t 0 96 logarithmic plots of cumulative frequency define curves with power law segments whose slopes indicate fractal dimensions d around 1 4 1 5 for both length and thickness the phyllonite zones adjacent to the mega gashes are interpreted to exert a crucial role in their development calculations show that the amount of quartz depleted in the phyllonite zones correspond to the amount of quartz precipitated in the mega gashes 7 10 6 m 3 volumes of fluid in the order of 10 10 m 3 must have been channelled through the opened fractures to precipitate such an amount of quartz we conclude that these mega gashes have developed from continuous propagation and opening of tension fractures in zones relatively preserved from phyllonitization protomylonites it is suggested that the development of kilometric extension gashes in the non phyllonitic domains produces a volume gain in response to the volume loss produced in the phyllonite zones the whole shear zone is envisaged therefore as an isovolumetric system with alternating lateral zones of volume loss phyllonites and volume gain extension gashes

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

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

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

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

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

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

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

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

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault

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