the variation of temperature with time and hydrocarbon maturation in sedimentary basins formed by extension abstract when a sedimentary basin is formed by extension both the subsidence history and the temperature gradient can be calculated hence the time temperature history of any stratum can be obtained three types of behaviour occur the temperature of material which never reaches depths greater than about 4 km increases at all times the time temperature history of material which finally sinks to greater depths reaches a maximum during the subsidence history the time at which this maximum occurs decreases as the depth increases at greater depths the maximum temperature occurs at the time the basin is formed the time temperature history is then used to calculate lopatin s maturation index for each stratum as a function of time and the index plotted as a function of depth and time for different amounts of extension the maximum temperature at a given depth and hence lopatin s index is almost independent of the amount of extension when the age of the basin is greater than about 150 ma similar calculations taking account of sediment compaction show the same behaviour though the maturity of the material at a given depth is increased because both lopatin s index and its calibration are empirical it is important to develop new maturation indices based on individual molecular reactions whose activation energies can be determined in the laboratory

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

japan sea a pull apart basin abstract recent field work in the hokkaido central belt and marine geology studies along the eastern margin of japan sea in addition to previously published data lead us to propose a new model of opening of the japan sea the synthesis of both on land and offshore structural data gives new constraints about the structural evolution of the system the rhombohedral shape of the japan basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early eo oligocene rifting of a pull apart basin accommodated along two large right lateral shear zones east of korea and west of northeast japan and sakhalin it is followed during upper oligocene lower miocene by the main opening of the japan basin as a mega pull apart then a back arc spreading probably related to the subduction process induced the creation of the yamato and tsushima basins at the end of lower miocene and in middle miocene clockwise rotation of southwest japan larger than 20 or major bending of honshu mainland deduced from paleomagnetic studies is unlikely at this time since 1 or 2 my b p to present compression prevails along the eastern margin of the japan sea the generation of marginal basins as pull apart basins along intracontinental strike slip faults is a mechanism which has been proposed by other authors concerning the south china sea the question then is whether the fragmentation of the asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one

calculating velocity fields across plate boundaries from observed shear rates summary a general horizontal velocity field can be expressed uniquely in terms of the velocities at points along a line and an integral function of the 2 d shear rate consequently the relative motion across a plate boundary can be calculated from observed shear rates by taking the velocities along a line within one of the plates to be zero

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

the dynamics of extensional sedimentary basins constraints from subsidence inversion on the continents sedimentary basins are generally formed by small amounts of lithospheric extension extension might cease because the lithospheric mantle has cooled and strengthened or because the driving force is removed in the former case a relationship should exist between the maximum strain rate during extension and the final stretching factor attained we explore how this relationship depends on the rheology of the lithospheric mantle and of the crust as well as on sedimentation and prior rifting events we also present strain rate information obtained by inverting subsidence data from a variety of sedimentary basins there is evidence of a systematic relationship between strain rate and stretching factor in each basin implying that lithospheric rheology is consistent with the rheology of olivine measured in laboratory experiments our data are difficult to explain if removal of the driving force governs the magnitude of stretching factors

pangaean divergent margins historical perspective abstract pangaean divergent margins caused by the breakup by rifting of that supercontinent display three distinct evolutionary stages in their youth or rift stage divergent margins display a synsediment fault style that has considerable tectonic instability during the mature or drift stage seafloor spreading continental margins are relatively stable and are dominated by thermal subsidence these two stages rifting and drifting are separated in many areas by an unconformity coeval with the oldest associated oceanic crust the breakup unconformity in some areas the onset of seafloor spreading is marked by intense magmatic activity sediment facies the rifting stage range from continental clastics landward of the continental basement hinge and along the flanks of the basin to saline deposits and more open water carbonates in the axial zone of the rift lithologies emplaced during the rift or mature stage of margin development range from shallow clastics carbonates to deep water carbonate siliceous oozes turbidites and contourites some of which were deposited in an anoxic environment deposition then is controlled by changes in configuration of basins changes in sea level and plate migration through several climatic belts during the final or old age of margin development divergence gives way to convergence this stage which generally terminates with continental collision is characterized by tectonism magmatic activity and crustal shortening mesozoic divergent margins along the east west trending former tethys ocean have reached this stage in their evolution

vesiculation in magmas from stromboli and implications for normal strombolian activity and paroxysmal explosions in basaltic systems 1 we performed a series of x ray tomographic experiments and lattice boltzmann permeability simulations on pyroclastic products from explosive activity at stromboli between december 2004 and may 2006 we reconstructed the 3 d textures of vesicles to investigate the relationship between the nature of vesiculation in the erupted products and the dynamics of gas transport in the shallow conduit in order to derive implications for the eruptive behavior of basaltic volcanoes scoriae from normal strombolian explosions display remarkably consistent vesicle volume distributions fit by power laws with an exponent of 1 0 2 we ascribe the origin of such distributions to the combined effect of coalescence and continuous nucleation events in the steady state shallow magma system that supplies normal strombolian activity volume distributions and textures of vesicles in pumice clasts from the 5 april 2003 and 15 march 2007 paroxysmal activity are markedly different from those in the scoriae besides a power law function with a higher exponent portions of these distributions can be also fit by an exponential function suggesting the attempt of the system to reach near equilibrium conditions the investigated pumice clasts also lack the large connecting vesicles responsible for the development of degassing pathways in the stromboli magma that erupts the scoriae this testifies to a decreased degassing efficiency of the magma associated with paroxysmal explosions and potential overpressure buildup at depth by comparison with degassing experiments on basaltic melts we derive a time constraint on the order of minutes to hours for the incubation of paroxysms at stromboli

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

three fragmentation mechanisms for highly viscous magma under rapid decompression abstract analysis of the behavior of viscous magma under short term tensile stresses demonstrates that the strain rates anticipated in the eruptive stages of collapse related events may be high enough under certain circumstances to provide fragmentation of the melt by brittle failure mechanisms of fragmentation of a highly viscous vesicular magma by the propagation of an unloading wave by layer by layer vesicle bursting fragmentation wave propagation and by rapid filtration gas flow are outlined fragmentation scenarios involving possible combinations of the three fragmentation mechanisms are evaluated for two models of magma with isolated and with connected pores experimental results for magma fragmentation under high decompression rates appear to support such mechanisms these mechanisms of highly viscous magma fragmentation by rapid decompression probably have their greatest application in cases of explosive disintegration of lava domes and cryptodomes caused by collapse events

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

rift tectonics on the passive continental margin off galicia spain abstract new migrated multichannel seismic profiles on the starved passive continental margin off galicia spain illustrate the tectonic style of the early cretaceous rifting event which preceded the separation of iberia and north america the main episode of rifting was preceded by other tensional episodes probably of earliest cretaceous and late jurassic age listric faulting associated with rifting was accompanied by large scale mass movements from the crests of the tilted blocks two subaerial erosional surfaces are visible one of late jurassic or earliest cretaceous age was affected by the final faulting episode the other was synchronous with the main phase of rifting and is present only on the upper part of the margin because the central trough was under deep water the significance of a ridge near the ocean continent boundary where altered lherzolite has been dredged and cored is discussed below the tilted blocks which include the basement rocks the seismic horizon s is visible as on the armorican margin its significance is also discussed

the ilopango tierra blanca joven tbj eruption el salvador volcano stratigraphy and physical characterization of the major holocene event of central america abstract the ilopango caldera is the source of the large tierra blanca joven tbj eruption that occurred about 1 5 ka years ago between ca ad270 and ad535 the eruption dispersed volcanic ash over much of the present territory of el salvador and pyroclastic density currents pdcs extended 40 km from the volcano in this study we document the physical characteristics of the deposits from all over el salvador to further constrain the eruption processes and the intensity and magnitude of the different phases of the eruption the succession of deposits generated by the tbj eruption is made of 8 units the eruption started with pdcs of hydromagmatic origin unit a0 followed by fallout deposits units a and b that are

physical properties of the 1980 mount st helens cryptodome magma physical properties of cryptodome and remelted samples of the mount st helens grey dacite have been measured in the laboratory the viscosity of cryptodome dacite measured by parallel plate viscometry ranges from 10 82 to 9 94 log10 pa s t 900 982 c and shrinkage effects were dilatometrically observed at t 900 c the viscosity of remelted dacite samples measured by the micropenetration method is 10 60 9 25 log10 pa s t 736 802 c and viscosities measured by rotational viscometry are 3 22 1 66 log10 pa s t 1298 1594 c comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water bearing melt demonstrates significant deviations between measured and calculated values this difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal bearing vesicular melt viscosities at low temperature assuming that the cryptodome magma of the 18 may 1980 mount st helens eruption was residing at 900 c with a phenocryst content of 30 vol a vesicularity of 36 vol and a bulk water content of 0 6 wt we estimate the magma viscosity to be 1010 8 pa s

tectonic implications of canyon directions over the northeast atlantic continental margin the basis of this study is a new bathymetric map of the northeast atlantic compiled from previously published maps made from conventional echosounder data plus all sea beam data acquired on board the r v jean charcot since 1977 as most of the sea beam data have been obtained on the continental margin from porcupine seabight to the south of the iberian peninsula a precise picture of the continental slope is given a statistical analysis of the canyons based on 750 measurements reveals that many of the canyons present sharp changes in their direction indicating a structural control mainly linked to the late hercynian trends especially around the iberian peninsula nevertheless the paths of canyons may merely reflect recent gravity processes as in the porcupine seabight canyons locally follow the directions of listric and associated transecting faults permian to triassic and upper jurassic to lower cretaceous as on the celtic margin and every type of tectonic lineament for example the north pyrenean paleogene thrust front which fringes the gouf of cap breton a comparison of diagrams for the northern and southern bay of biscay margin especially trends predating the opening is compatible with a 25 rotation of iberia with respect to europe

seismic image of undercrusted serpentinite beneath a rifted margin during the continental rifting the upper mantle was unroofed and the mantle rocks were transformed into serpentinite at the ocean continent transition of the west galicia margin spain the serpentinite layer several km thick extends probably eastwards beneath the highly thinned continental crust of the margin the serpentinite layer was recently imaged by seismic reflection it is discontinuously and deeply layered as serpentinized peridotite can have densities and seismic velocities comparable to those of the lower continental crust we suggest that undercrusting by serpentinite can play a part in building the lower seismic crust in highly stretched continental rifted areas

integrating the holocene tephrostratigraphy for east asia using a high resolution cryptotephra study from lake suigetsu sg14 core central japan abstract tephra volcanic ash layers have the potential to synchronise disparate palaeoenvironmental archives on regional to hemispheric scales highly productive arc regions like those in east asia offer a considerable number of widespread isochrons but before records can be confidently correlated using these layers a refined and integrated framework of these eruptive events is required here we present the first high resolution holocene cryptotephra study in east asia using the lake suigetsu sedimentary archive in central japan the holocene tephrostratigraphy has been extended from four to twenty ash layers using cryptotephra extraction techniques which integrates the deposits from explosive eruptions from north korea china south korea and along the japanese arc this lake suigetsu tephrostratigraphy is now the most comprehensive record of east asian volcanism and the linchpin site for correlating sequences across this region major element glass geochemical compositions are presented for the tephra layers in the sequence which have been compared to proximal datasets to correlate them to their volcanic source and specific eruptions this study has significantly extended the ash dispersal of many key holocene marker layers and has identified the first distal occurrence of isochrons from ulleungdo and changbaishan volcanoes utilising the high precision lake suigetsu chronology we are able to provide constrained eruption ages for the tephra layers which can be transferred into other site specific age models containing these markers this new framework indicates that several isochrons stratigraphically bracket abrupt climate intervals in japan and could be used to precisely assess the regional and hemispheric synchronicity of these events

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

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

extensive ash falls in and around the sea of japan from large late quaternary eruptions abstract five widespread ashes of late quaternary age have been recognized on land and also in deep sea sediments in and around the sea of japan all are favorable for determining the magnitude and type of the great eruptions and for establishing a quaternary chronology around japan accurate determination of refractive indices of volcanic glass shards and phenocrysts together with chemical data have enabled successful characterization of the tephra layers and correlation between land and sea of the five tephras two are alkalic and are the products of major eruptions of two korean volcanoes the baegdusan tomakomai ash b tm and the ulreung oki ash u oki the b tm ash is recognized at the top of most cores in the northern part of the sea of japan and also on hokkaido and the northern part of honshu where a reliable archeological age is given around the 11th century the u oki ash occurs between the kikai akahoya and aira tn ashes in cores from adjacent to the ulreung do to central honshu where three radiocarbon ages of around 9300 yr b p were obtained the other three tephras are subalkalic and are correlated with large scale eruptions producing voluminous pyroclastic flows and calderas in kyushu the kikai akahoya k ah aira tn at and aso 4 ashes the former two have been well described already but their fallout areas are partly revised in this manuscript the at ash is the most prominent marker recognized not only on land of japan but also in cores from the whole area of the sea of japan and part of the pacific ocean the airfall aso 4 ash simultaneous with the aso 4 pyroclastic flow eruption is also described it mantles very extensive areas and serves as an important time marker about 70 000 years ago

contemporary horizontal velocity and strain rate fields of the pacific australian plate boundary zone through new zealand we have inverted velocity solutions from nine geodetic networks distributed across new zealand to derive present day continuous horizontal velocity and strain rate fields at the earth s surface throughout the country the nine networks contain a total of 362 global positioning system gps stations that have been observed at least twice and at least a year apart between 1991 and 1998 the model velocity field is expanded as bicubic spline interpolation functions defined within a curvilinear grid that covers the country and extends into the assumed rigid australian and pacific plates to west and east the inversion jointly minimizes the magnitudes of fitted strain rates and the misfit to the observed velocity data the spline technique allows high spatial resolution of strain rate variations especially in regions with spatially dense gps data expansion of the model velocity field on the surface of a sphere allows arbitrarily large areas to be studied previously known aspects of new zealand plate boundary deformation are highlighted with improved resolution including back arc extension in the taupo volcanic zone rotation of the hikurangi forearc away from this zone in the north island and a band of high shear strain rate under the southern alps to the southeast of the alpine fault our results reveal several new features including a region of enhanced shear straining apparently associated with strike slip faults in the southern north island and a band of contractional straining subparallel and well east of the alpine fault that is similar to features found in numerical and sandbox models of continental collision

late cretaceous and cenozoic intra plate compressional deformations in the alpine foreland a geodynamic model abstract the mesozoic grabens and wrench induced troughs of western and central europe developed in response to triassic early cretaceous intra plate tensional stresses which affected the arctic north atlantic and the tethys borderlands during the rifting phase preceding the opening of the respective oceans this resulted in the fragmentation of the continental crust of the alpine foreland during the late cretaceous and tertiary alpine orogeny collision related compressive stresses exerted on the foreland induced the reactivation of these fracture systems causing the inversion of mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present alpine deformation front the structural style of these inverted basins and basement uplifts is indicative of compressional and transpressional deformations the total amount of crustal shortening associated with these deformations is unlikely to exceed a few tens of kilometres these displacements require a coupling between the foreland and the orogen at the alpine a subduction zones and within the foreland a decoupling at intra crustal levels between the crust and mantle and or at deeper lithospheric levels the example of the alpine foreland indicates that compressive stresses inducing major deformations can be transmitted from a collisional plate boundary through continental lithosphere over distances of up to 1300 km circumstantial evidence suggests that collision related compressive stresses not large enough to give rise to significant deformations can be transmitted through continental lithosphere over even greater distances

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

the plinian phase of the campanian ignimbrite eruption phlegrean fields italy evidence from density measurements and textural characterization of pumice textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content size shape vesicle wall thickness and the degree of interconnectivity the plinian fallout deposit directly underlying the campanian ignimbrite ci eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater the deposit consists of a lower lfu and upper ufu pumice lapilli bed generated by the sub steady eruption of trachytic magma with 0 90 with vesicle number density ranging from 107 108 cm 3 the degree of vesicle coalescence is high for all pumice types with interconnected vesicles generally representing more than 90 of the bulk vesicle population the results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase the different pumice types and also within each single pumice type fragment the origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior the conclusions of this study are that vesicle deformation degree of coalescence and intense shear at the conduit walls play a major role on the degassing process hence affecting the entire conduit dynamics

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

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

geological prior information and its applications to geoscientific problems geological information can be used to solve many practical and theoretical problems both within and outside of the discipline of geology these include analysis of ground stability predicting sub surface water or hydrocarbon reserves and assessment of risk due to natural hazards in many cases geological information is provided as an a priori component of the solution that is information that existed before the solution was formed and which is incorporated into the solution such information is termed geological prior information the key to the successful solution of such problems is to use only reliable geological information in turn this requires that a multiple geological experts are consulted and any conflicting views reconciled b all prior information includes measures of confidence or uncertainty without which their reliability and worth is unknown and c as much information as possible is quantitative and qualitative information or assumptions are clearly defined so that uncertainty or risk in the final result can be evaluated this paper discusses each of these components and proposes a probabilistic framework for the use and understanding of prior information we demonstrate the methodology implicit within this framework with an example this shows how prior information about typical sequence stacking patterns allows aspects of 2 d platform architecture to be derived from 1 d geological data alone such as that obtained from an outcrop section or vertical well this example establishes how the extraction of quantitative multi dimensional geological interpretations is possible using lower dimensional data the final probabilistic description of the multi dimensional architecture could then be used as prior information sequentially for a subsequent problem using exactly the same method

a two dimensional inverse model for extensional sedimentary basins 2 application 1 we apply the two dimensional strain rate inversion algorithm described in the companion paper to five sedimentary basins located world wide this general algorithm makes no assumptions about the number duration or intensity of rifting episodes within a basin instead the distribution of strain rate is allowed to vary smoothly through space and time until the misfit between observed and predicted stratigraphy is minimized potentially important two dimensional effects such as lateral heat flow and elastic thickness have been incorporated in each case observed subsidence profiles can be automatically and accurately fitted the calculated distributions of strain rate are corroborated by independent information about the number and duration of rifting episodes peak strain rate estimates are comparable to present day measurements in actively extending basins such as the aegean sea and the basin and range province temporal and spatial strain rate patterns from a large number of sedimentary basins in diverse settings will help to constrain the dynamical evolution of thinning continental lithosphere in all five sedimentary basins the smallest acceptable misfit is achieved when the elastic thickness is less than 2 4 km this surprising result indicates that the lithosphere beneath sedimentary basins remain weak during and after rifting it also underscores the importance of inverting data sets so that the existence uniqueness and resolution of solutions can be analyzed our two dimensional approach is one small step toward a generalized three dimensional inverse model of sedimentary basins

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

post 19 ka b p eruptive history of ulleung island korea inferred from an intra caldera pyroclastic sequence ulleung island is a quaternary volcanic island located in the mid western part of the east sea sea of japan back arc basin which has erupted from the pliocene until the late holocene this study focuses on reconstructing the latest eruptive history of the island by describing the sedimentological and stratigraphic characteristics of the most recent trachytic phonolitic pyroclastic sequence named the nari tephra formation this formation is preserved as a succession of unwelded pyroclastic and epiclastic deposits within an embayed margin of the nari caldera the embayment acted as a topographic trap for proximal pyroclastic deposits and contains a complete record of the past 19 000 years of eruption history the formation includes evidence for five separate eruptive episodes member n 1 to n 5 with intervening weathered and or soil horizons indicating hundreds to thousands of years of repose between each eruption eruption styles and depositional mechanisms varied between and during individual episodes reflecting changing dynamics of the magma plumbing system magmatic gas coupling and a variable role of external water extra caldera sequences show that only a few of these eruptions generated sustained eruption columns or pyroclastic density currents pdcs large enough to overtop the caldera wall thus tephra sequences outside the caldera provide an underestimate of eruption frequency and care needs to be taken in the interpretation and correlation to distal tephra sequences recognized in marine and terrestrial records in addition topographic effects of caldera structures should be considered for the assessment of pdc related hazards in such moderately sized pyroclastic eruptions

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

structure and development of porcupine seabight sedimentary basin offshore southwest ireland the porcupine basin is a deep sedimentary basin on the continental margin west of ireland all available geologic and geophysical data have been used to interpret the history of the basin it contains up to 8 km of cretaceous and younger sediments underlain by an unknown thickness of jurassic and triassic rock gravity and magnetic anomaly models have been used to investigate the nature of the crust beneath the porcupine basin these models suggest a thinned continental crust rather than the oceanic crust previously proposed by some authors although the nature of a narrow zone of crust 10 km thick in the central southern basin cannot be determined conclusively a high amplitude elongate gravity high associated with a negative magnetic anomaly in the axis of the n rthern porcupine seabight correlates with a volcanic ridge probably of early cretaceous age seen on seismic reflection profiles regional geologic and geophysical studies show that the porcupine basin was originally structured in the early mesozoic as part of a complex northeast to north northeast trending intracontinental rift system extending from the grand banks of newfoundland to the present northwest europe continental shelf the basin can be divided into a sequence of segments bounded by north northeast trending normal faults and northwest trending strike slip faults the degree of extension within the basin decreases northward across each strike slip fault accounting for the overall northward narrowing of the basin the early mesozoic rift fabric of the porcupine basin was reactivated during the early cretaceous when the basin became caught up in the northwest trending rift that later evolved into the n rth atlantic this second phase of extension led to subsidence and the accumulation of extreme thicknesses of cretaceous and younger sediments

america eurasia plate boundary in eastern asia and the opening of marginal basins abstract the opening of the arctic ocean during the past 55 ma resulted in relative rotation of america with respect to eurasia about a pole located in eastern siberia near the plate boundary the extensional plate boundary enters deep inside the eurasian continent up to the rotation pole on the opposite side of the pole on the pacific side of the plate boundary compressive tectonics are recorded along sakhalin and hokkaido from the oligocene to middle miocene the relative movement was accommodated by strike slip motion along sakhalin and hokkaido although the rotation pole was not located at a significatively different position from now in this paper we explain this by independent motion of the southernmost tip of the american plate towards the pacific margin which behaves as a free boundary this oceanward motion resulted in an extension of the american plate giving rise to the wedge structure of the okhotsk sea the japan sea opened as a pull apart basin along the strike slip boundary finally the increasing extension in the okhotsk sea led to the opening of the oceanic kuril basin

a model for the mechanism of the may 18 1980 mount st helens blast abstract a model is suggested for the mechanism of the blast that accompanied the may 18 1980 mount st helens eruption the model explains the large duration of the blast process and accounts for several explosive episodes recognized during the blast the mechanism is based on the idea of destruction of the near solidus porous cryptodome by a fragmentation wave termed the f wave originating from cryptodome decompression induced by a landslide the f wave propagates from a free surface to the interior of the cryptodome at velocity n the products of cryptodome fragmentation a gas pyroclast mixture are ejected into the atmosphere at velocity u calculations show that for pressure of gas in the cryptodome pores of 20 mpa and porosity 0 4 0 7 the f wave travels at 5 m s and velocity of pyroclast ejection is 100 250 m s for a cryptodome with diameter of 700 m the duration of blast fragmentation is estimated at 140 s energy released during the blast is estimated as 4 8 7 2 1015 j blast power is 3 4 5 1 1013 wt gas pyroclastic mixture mass flux is 5 0 7 5 103kg m2 s the maximum discharge is 1 9 2 9 sx 109 kg s and the weight fraction of water vapor in the cryptodome is 0 006 0 04 several explosive episodes during the blast are accounted for by fragmentation wave stoppage due to blockage of the exploding cryptodome by landslide material

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

the horizontal velocity field in the deforming aegean sea region determined from the moment tensors of earthquakes we use the spatial distribution of seismic moment tensors of earthquakes in the aegean region over the time interval 1909 1983 to recover a continuous horizontal velocity field that describes the overall deformation of the lithosphere at large length scales the calculated velocity field is dominated by two effects 1 an e w right lateral shear of the eastern aegean related to motion on the north anatolian fault becoming distributed as it enters the aegean and 2 a n s extension probably related to the sinking of the slab in the hellenic trench the southern part of the central aegean is found to be moving in a sw direction relative to europe at a rate of about 30 mm yr probably a lower bound with an error of around 10 mm yr and rotating clockwise in the seismogenic upper crust this velocity field is accommodated by right lateral strike slip faults in the eastern aegean and by normal faults that rotate clockwise in central greece a comparison of paleomagnetic declination data with the expected rotation of rigid elongate inclusions in the velocity field shows in most places an agreement in sense and approximate agreement in rate of rotation expected rotation rates of line elements are sometimes too low probably because our derived velocity field is smoothed and unable to match locally high strain rates there is only one part of the region where line elements are predicted to rotate in either clockwise or counterclockwise directions depending on their orientation this is in western turkey which coincident ally is the only place where paleomagnetic rotations in both directions have been observed this coincidence in particular suggests to us that the analogy of rigid elongate inclusions in the velocity field responding to forces on their bases may be useful in predicting the senses and approximate rates of rotation of crustal blocks in deforming continental regions the velocity field we obtain preserves the strike directions of the major faults as directions of zero length change in spite of considerable smoothing we use this observation to speculate that the interaction between the upper crust and the rest of the lithosphere beneath it may involve an interplay of effects on one hand the variation of strength with direction in the crust may control the strike directions of faults that form or become reactivated and may also limit the velocity fields that are allowable on the other hand the fault bounded blocks may rotate in the velocity field in response to forces on their bases which would require the velocity field to change with time if the directions of zero length change are fixed to the blocks

holocene tephrostratigraphic framework and monsoon evolution of east asia key tephra beds for synchronising palaeoclimate records abstract in east asia our understanding of holocene climate change forcing mechanisms and propagation require the precise chronological control of palaeoclimate records to allow robust integration of data sets the existing chronologies predominantly based on 14c method however are not sufficient to constrain key questions about abrupt climate shifts that occur within a century in the transitions between states widely dispersed tephra layers allow precise dating and synchronisation of sedimentary archives providing a chronological framework for integrating records especially where the visible tephra record is complemented by the addition of cryptotephra non visible ash despite significant tephra studies in this region however a comprehensive holocene tephra framework is not available to address this issue we carry out a thorough review on holocene tephra investigations undertaken in japan using widespread tephra beds we present an integrated tephra framework and suggest the way forward for establishing this as a wider approach for east asia the framework is based on twenty two ash layers that are mainly from japan and to a lesser extent china n korea s korea and russia each tephra is assessed in terms of chronology geochemistry and distribution the framework is compared with high resolution palaeoclimate records from east asia using this we demonstrate regional variations in monsoon evolution and more importantly the potential of tephra isochrons in constraining such variations given the scarce identification of tephra layers in those well resolved palaeoclimate records we advocate a more systematic employment of the cryptotephra method which would potentially lead to a significant advance in east asian tephrochronology and the correlation of palaeoclimate archives in the region

caldera development during the minoan eruption thira cyclades greece the well known caldera of thira santorini greece was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a ne trending line of vents before the minoan eruption of 1400 b c thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half andesitic lavas formed the overlapping shields of the north and were contemporaneous with and in many places interbedded with the southern tuff deposits although there appears to be little difference between the composition of magmas erupted differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial producing lava flows whereas in the south eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs many of these tuffs are plastered onto the walls of what appears to have been an older caldera most probably associated with an eruption of rhyodiacitic tephra 100 000 years ago the minoan eruption of about 1400 b c had four distinct phases each reflecting a different vent geometry and eruption mechanism the minoan activity was preceded by minormore eruptions of fine ash 1 the eruption began with a plinian phase from subaerial vent s located on the easternmost of the lave shields 2 vent s grew toward the sw into the flooded depression subsequent activity deposited large scale base surge deposits during vent widening by phreatomagnetic activity 3 the third eruptive phase was also phreatomagmatic and produced 60 of the volume of the minoan tuff this activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding less

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

on traction carpet sedimentation traction carpets are highly concentrated bedload layers that are developed beneath and driven by turbulent overlying flows they have a convex up velocity profile and comprise a lower frictional and an upper collisional region the frictional region having a particle concentration more than 80 of the packed bed is characterized by nearly continuous grain contacts low strain rate and hampered grain segregation on the other hand the collisional region has a particle concentration between 15 and 80 of the packed bed and is characterized by active grain collisions and higher strain rate deposition from the traction carpets occurs via progressive aggradation of the bed rather than via en masse freezing while the downward grain flux from the overlying flow maintains them the thickness of a traction carpet stratum is therefore determined by the cumulative amount of sediment settled during the lifespan of a traction carpet and can be much larger than the thickness of a moving traction carpet inverse grading can be produced in the collisional region by the vertical gradient of dispersive pressure which is related to nonuniform distribution of particles when a thick frictional region develops however the inverse size distribution in the collisional region is poorly recorded in the deposits depositional features of traction carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collisional to frictional regions the thickness ratio is further controlled by the applied shear stress sediment fallout rate and grain size generally a collisional region is better developed beneath a highly competent and coarse grained gravelly flow whereas a thick frictional region is developed beneath a heavily sediment laden fine grained sandy flow this explains the more common occurrence of inverse grading in coarse grained deposits

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

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

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

seismic reflection study of acoustic basement in the south korea plateau the ulleung interplain gap and the northern ulleung basin volcano tectonic implications for tertiary back arc evolution in the southern east sea abstract in order to understand the volcano tectonic evolution history of the east sea back arc basin this study focuses on the seismic characteristics of acoustic basement based on the multi channel seismic reflection data acquired from the south korea plateau the northern ulleung basin and the ulleung interplain gap according to the seismic reflection characteristics the acoustic basement in the study area can be classified into three types type a acoustic basement is a remnant of rifted continental crust comprising relatively shallow seated continental margin platforms elongated ridges and equidimensional blocks in the south korea plateau and its vicinities type b acoustic basement is volcanic sills flows sediments complexes infilling the rift controlled basement lows in the south korea plateau the ulleung interplain gap and the northern ulleung basin type c acoustic basement is a cluster or chain of volcanic edifices which shows internal facies variation from main body to apron slope facies volcanic activities identified in the types b and c acoustic basements can be classified into three stages based on the volcano stratigraphic correlation with the existing stratigraphic framework stage 1 volcanism is characterized by a fissure type eruption represented by scattering volcanic cones and mounds type c and volcanic sills and lava flows interlayering with syn rift sedimentary units type b early stage continental rifting prior to the middle miocene most likely controlled this volcanism stage 2 volcanism is characterized by ene wsw trending chain of volcanic edifices type c along the northern margin of the ulleung basin and ulleung interplain gap during the middle miocene the weakening of back arc extension might have resulted in limited volcanic eruption which formed elongate volcanic fields across the ulleung and yamato basins stage 3 volcanism is represented by highly peaked volcanic islands and seamounts type c showing vertically stacked apron slopes in seismic cross sections this vertical stacking pattern is indicative of multiple eruption events from a single eruption center at least from the late miocene to the quaternary expanded volcanic age control based on volcano stratigraphic and isotopic age dating reveals that the volcanic activities in the southwestern and southeastern east sea show similar spatio temporal variations this implies that the entire southern east sea has experienced almost same volcano tectonic evolution during the cenozoic back arc evolution especially the stage 2 volcanic chain along the northern ulleung basin and the yamato seamount chain in the yamato basin matches in their eruption timing ca 12 ma and geomorphic properties i e direction of the volcanic chains and distances from subduction zone which strongly suggests the existence of regional scale chain volcanism across the southern east sea considering the chemical properties of volcanic rock samples from the volcanic chain in association with the suggested tectonic frameworks during the tertiary the remnant magma source derived from the waning mantle convection most likely fed the volcanic chain in the terminal stage of back arc opening

an inverse method for determining lithospheric strain rate variation on geological timescales abstract inverse theory is used to calculate the strain rate variation as a function of time needed to fit observed subsidence patterns within extensional sedimentary basins no a priori information concerning either the duration of rifting or the total strain is required the method is one dimensional but can be generalised for use with two and three dimensional data conventional kinematic modelling of such basins normally assumes that lithospheric stretching is instantaneous or that the strain rate during a rifting period of known duration is constant the scheme is tested on a variety of synthetic data and applied to subsidence data from the dolomites northern italy linear error analysis combined with random and systematic noise tests show that variations in strain rate are significant and well resolved in the dolomites inverse calculations produce good fits to the data yielding total strains and durations of lithospheric stretching that are consistent with geological constraints calculated strain rates are not constant but vary between 10 18 10 15 s 1 information concerning spatial and temporal strain rate variation will provide important constraints for dynamic models of lithospheric deformation

the mesozoic tertiary evolution of the aquitaine basin the aquitaine basin situated in southwest france with an area of about 60 000 km 2 has the form of a triangle which opens towards the atlantic bay of biscay and is limited to the north by the hercynian basement of brittany and the massif central and to the south by the pyrenean tertiary orogenic belt beneath the tertiary sequence 2 km thick and which outcrops over much of the basin a mesozoic series up to 10 km thick rests generally on a tectonized hercynian basement but locally it covers narrow nw se trending post orogenic trenches of stephano permian age the mesozoic history can be subdivided into four major structural sedimentary episodes 1 during a triassic taphrogenic phase a continental evaporitic complex developed with associated basic magmatism 2 throughout the jurassic a vast lagoonal platform developed initially lower lias as a thick evaporitic sequence followed by a uniform shale carbonate unit indicating a relative structural stability 3 the end of the jurassic and the lower cretaceous saw a fragmentation of this platform due to an interplay between the iberian and european tectonic plates resulting in an ensemble of strongly subsident sub basins 4 during the upper cretaceous and until the end of the neogene the evolution of the aquitaine basin was influenced by the pyrenean orogenic phase with the development towards the south of a trench infilled by flysch which from the upper eocene is succeeded by a thick post orogenic molasse complex the main hydrocarbon objectives in the basin are situated in the jurassic platform e g the lacq giant gas field and the cretaceous sub basins e g the cazaux and parentis oil fields to date production has been about 4 times 10 7 m 3 of oil and about 15 times 10 10 m 3 of gas since the first gas discovery st marcet in 1939

geophysical constraints on the deep structure of a limited ocean continent subduction zone at the north iberian margin 1 late cretaceous to cenozoic convergence between iberia and europe led to the partial closure of the bay of biscay with limited southward subduction of oceanic crust below the north iberian margin inclined sub moho reflections and diffractions observed in deep seismic reflection profiles shot across the margin are especially well represented in two reflection profiles escin 3 2 and escin 3 3 these two profiles have been chosen to test if the sub moho reflections correspond to true primary deep events and provided that they are reflecting off the subduction zone to investigate its deep structure spectral analysis together with travel time estimation and migration allow us to characterize a number of these sub moho events as deep source low frequency 19 hz reflections and diffractions synthetic seismograms were generated by three dimensional seismic modeling of a sub moho southward dipping surface interpreted to correspond to the top of subducted oceanic crust comparison between the real and synthetic data show that inclined low frequency sub moho reflections in both escin 3 2 and escin 3 3 profiles may correspond to reflections from southward subducted bay of biscay oceanic crust geoid free air gravity and absolute topography modeling provides additional constraints on the lithospheric scale structure of this limited ocean continent subduction zone beneath the north iberian margin

formation of non volcanic passive margins a composite model applies to the conjugate galicia and southeastern flemish cap margins one of the most controversial issues in geology today is the way the lithosphere extends the galicia margin is a particularly good place to adress this problem as it is fairly sediment starved has clear upper crustal structure well imaged by a variety of seismic reflection profiles and has well control to calibrate seismic data observed subsidence is compared with extension of the uppermost crust of the entire margin as computed from the geometry of tilted fault blocks imaged on a series of parallel seismic lines the pure shear or stretching model clearly accounts for the extension of the upper continental margin but not for the lower margin where the s reflector zone a prominent zone of intra crustal reflectors is present i suggest that the s reflector zone is a detachment zone accomodating decreasing displacement towards the continent and that this detachment zone active since the beginning of rifting outcrops on the canadian side and corresponds to the steep southeastern flemish cap continental slope pure shear is the mechanism which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is locally simple shear where the detachment zone is active

occurrence of pockmarks on the ortegal spur continental margin northwestern iberian peninsula a total of 445 pockmarks were observed on the upper continental slope of the northwest corner of the iberian peninsula the ortegal spur area by swath bathymetric and ultrahigh resolution seismic data the pockmarks are u v and w shaped and have terraces or indentations in cross section and are dish shaped circular to oval in plan view they occur on the surface of the seabed and buried within the plio quaternary and neogene sediments four types of pockmarks were identified and mapped on the basis of their plan view and cross section morphology regular irregular asymmetric and composite the concentration of pockmarks is attributed to seepage of fluids migrating up dip from deeper parts of the sedimentary basin a linear high density concentration with a nnw to n ne and ese trend of pockmarks is observed above inferred basement faults that do not affect the quaternary succession these pockmarks are thus caused by seepage of thermogenic gas and or other pore fluids from deeper late cretaceous units and their distribution may help to improve our understanding of the fluid system and migration regime in this part of the galicia continental margin

dynamics of the pacific north american plate boundary in the western united states the vertically averaged deviatoric stress tensor field within the western united states was determined with topographic data geoid data recent global positioning system observations and strain rate magnitudes and styles from quaternary faults gravitational potential energy differences control the large fault normal compression on the california coast deformation in the basin and range is driven in part by gravitational potential energy differences but extension directions there are modified by plate interaction stresses the california shear zone has relatively low vertically averaged viscosity of about 1021 pascal seconds whereas the basin and range has a higher vertically averaged viscosity of 1022pascal seconds

wave planation surfaces in the mid western east sea sea of japan indicators of subsidence history and paleogeographic evolution of back arc basin abstract this study focuses on revealing the origin of deep marine wave planation surfaces and its geomorphic and tectonic implications for the back arc evolution based on multi channel seismic reflection profiles acquired from the mid western east sea sea of japan on seismic reflection profiles the wave planation surfaces are recognized as continuous and high amplitude single reflectors outlining the flattened top of submarine terraces or isolated topographic highs the wave planation surfaces are classified into four types depending on the geomorphic characteristics and origins of substratal landmasses i e continental margin terrace type 1 rifted continental fragment type 2 volcanic edifices type 3 and uplifted sedimentary successions type 4 which have common origin of wave erosion near the sea level but different subsidence history the mean subsidence rate of wave planated substratum is calculated by dividing the maximum depth of a wave planation surface below modified datum sea level d e by the time elapsed since the onset of subsidence t information of local subsidence history enables reconstruction of paleogeographic evolution of mid western east sea during the neogene quaternary time 1 incipient continental rifting and onset of marine incursion ca 23 18 ma 2 progressive continental rifting and onset of wave planation ca 18 11 ma 3 final eruption of post rift volcanoes ca 11 ma 4 uplifting of shelf margin banks and onset of the pliocene holocene volcanic activities ca 11 2 5 ma 5 change in the locus of volcanic eruption and cessation of contractile uplifting in the basin margin ca 2 5 0 ma

a two dimensional inverse model for extensional sedimentary basins 1 theory 1 a simple two dimensional inverse model for extracting the spatial and temporal variation of strain rate from extensional sedimentary basins is described our starting point is a forward model which allows basin stratigraphy to be calculated from any given strain rate distribution this forward model includes potentially important effects such as flexural rigidity and the two dimensional conduction advection of heat conversely inverse modeling determines strain rate variation by minimizing the misfit between predicted and observed patterns of basin subsidence no a priori assumptions about the number duration or intensity of rifting episodes are necessary instead strain rate is allowed to vary smoothly throughout time and space an important restriction is that vertical lines remain vertical but this restriction can be relaxed if deemed necessary we have successfully inverted different synthetic sedimentary basin geometries which were generated by forward modeling sensitivity analysis and noise tests demonstrate that strain rate patterns can be recovered with confidence the relationship between stratigraphic misfit and elastic thickness shows that usually an upper limit but not a lower limit for elastic thickness can be retrieved our general approach is applicable to any extensional sedimentary basin including highly extended passive margins and it should be regarded as the stepping stone to more sophisticated two dimensional and three dimensional inverse modeling in the companion paper we apply our algorithm to a set of phanerozoic extensional sedimentary basins located worldwide

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

longitudinal evolution of the suez rift structure egypt abstract a three dimensional study of the structure of the suez rift has been carried out using field and subsurface data in an attempt to determine the role of transverse faults and the longitudinal evolution of the rift as in most intracontinental rifts the structure of the gulf of suez area is governed by normal faults and tilted blocks whose crests constitute the main target of exploratory wells the fault pattern consists of two major sets of trends 1 1 longitudinal faults parallel to the rift axis and created in an extensional regime where 3 was trending ene wsw and 2 2 transverse faults with a n s to nne ssw dominant trend the transverse faults are inherited passive discontinuities while most of the longitudinal faults were created during neogene times in a purely extensional regime both sets were simultaneously active producing a zigzag pattern and rhombic shaped blocks the transverse faults can show horizontal strike slip components and act as relays between major normal faults although the suez rift appears as a simple narrow elongated trough dominated by two almost symmetrical shoulders its internal structure is asymmetrical cross profiles show that all the major blocks are tilted in the same direction however the tilt direction changes twice along the rift to the north and to the south of the rift the blocks are tilted eastward while in the central part they are tilted westward to the north the change of dip is accompan ed by a graben type twist zone without transverse faulting at least in the neogene series to the south the change of dip is accompanied by a more complex structure involving both a major transverse fault and a horst type twist zone in this latter case the transverse fault does not cut through the entire rift balanced cross sections established from subsurface data show that the tilt angle and the amount of extension increase from north to south while the width of the blocks decreases indicating a pole of opening close to the northern end of the gulf minimum values for the amount of opening range from 5 km in the north to about 20 km in the south

holocene explosive eruptions of witori and dakataua caldera volcanoes in west new britain papua new guinea abstract witori and dakataua caldera volcanoes have been very active in the middle to late holocene using tephrochronology this paper establishes the chronostratigraphy of these eruptions and their magnitude and the frequency of explosive volcanism at witori and dakataua after a long dormancy witori started explosive activity at ca 5600 conventional radiocarbon years bp producing in the next 4500 years five major tephra layers w k1 to w k4 w g with veis of 5 to 6 after the w g eruption at around 1200 bp the activity decreased in magnitude but increased in frequency with some eruptions forming central cones the major eruption of dakataua began with alternating ejections of phreatomagmatic ashfalls and plinian deposits followed by the cataclysmic eruption resulting in lithic rich pyroclastic flows ca 1100 1200 bp the major tephra layers cover extensive areas in west new britain due to their large volumes and the prevailing easterly winds providing valuable time markers for establishing holocene chronology the largest eruption the w k2 event of ca 3300 bp shaped much of the present landscape with an extensive area significantly devastated by tephra falls and pyroclastic flows obsidian and other artefacts buried by the tephras indicate that the area was repeatedly occupied the major tephra events formed new coastal plains favourable for human occupation

understanding the thermal evolution of deep water continental margins areas of exploration for new hydrocarbons are changing as the hydrocarbon industry seeks new resources for economic and political reasons attention has turned from easily accessible onshore regions such as the middle east to offshore continental shelves over the past ten years there has been a marked shift towards deep water continental margins 500 2 500 m below sea level in these more hostile regions the risk and cost of exploration is higher but the prize is potentially enormous the key to these endeavours is a quantitative understanding of the structure and evolution of the thinned crust and lithosphere that underlie these margins

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

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

revised stratigraphy of holocene tephras on ulleung island south korea and possible correlatives for the u oki tephra abstract detailed field surveys and measurements of major oxide compositions of volcanic glass shards using electron microprobe analyses revealed the stratigraphy of holocene tephras u 4 u 3 and u 2 on ulleung island south korea the bubble wall type glass shards of kikai akahoya k ah tephra detected in soil intercalated with tephra layers indicate that the depositional horizon of the tephra is situated between the u 3 and u 2 tephras this study has established a revised stratigraphy in ascending order of u 4 u 3 k ah and u 2 tephras comparisons of the major chemical compositions and eruption ages of the u 4 and u 3 tephras on ulleung island and those of ulleung tephras detected in and around the japanese islands suggest that these tephras are distributed widely and potentially can serve as useful and important time markers of the early to middle holocene radiocarbon dates and stratigraphic relationships between the k ah and ulleung tephras on the island suggest that the ulleung oki tephra u oki which has been used as an essential time marker of the early holocene is most likely correlated with the u 4 tephra therefore detailed analyses of chemical compositions of glass shards with dating of these tephras are necessary to identify u oki in and around japanese islands

causes and consequences of pressurisation in lava dome eruptions abstract high total and fluid pressures develop in the interior of high viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing two effects are recognised and are modelled quantitatively first large increases in magma viscosity result from degassing during magma ascent strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent second microlites grow in the melt phase as a consequence of undercooling caused by gas loss rapid microlite growth can cause large excess fluid pressures to develop at shallow levels theoretically closed system microlite crystallization can increase local pressure by a few tens of mpa although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects including spurts of growth cycles of dome growth and subsidence sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows the characteristic shallow level long period and hybrid seismicity characteristic of dome eruptions is attributed to the excess fluid pressures which are maintained close to the fracture strength of the dome and wallrock resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit

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

crustal transition between continental and oceanic domains along the north iberian margin from wide angle seismic and gravity data deep crustal features of the transition between the north iberian mainland and the bay of biscay are constrained from new wide angle seismic and gravity data velocity depth models are derived from far offset recordings onshore of two n s marine profiles complemented with new refraction data inland and tested against models that fit the gravity anomalies important lateral variations are inferred along the north iberian margin in the western transect a variscan crust is documented from the mainland across the continental shelf and a rapid crustal thinning beneath the slope marks the transition to the oceanic domain the eastern transect shows an outstanding crustal thickening beneath the coastline moho depths around 50 km and a gradual thinning across the shelf differences are related to the v shape of the bay of biscay that causes a w e transition along the margin from a short lived oceanic subduction to continental collision during the tertiary oblique convergence between europe and iberia

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

identification of the late quaternary tephra layers in the ulleung basin of the east sea using geochemical and statistical methods abstract the late quaternary tephra layers discovered in piston cores from the ulleung basin of the east sea sea of japan were studied using x ray fluorescence spectrometry xrf inductively coupled plasma atomic emission icp aes and inductively coupled plasma mass spectrometer icp ms in order to identify and differentiate the tephra layers and to characterize their major and trace element compositions the morphology and geochemistry of glass shards combined with 14c dating and statistical methods confirm the ulleung and aira tanzawa eruptions as the origin of the tephra layers respectively the so called ulleung oki u oki 10 1 cal ka and ulleung yamato u ym 30 9 cal ka tephra layers consist predominantly of pumice type glass shards in contrast the aira tanzawa at 23 cal ka tephra layer consists mainly of plane type and or bubble wall glass shards the u oki and u ym tephra layers contain k rich glasses r k2o sio2 0 09 ranging from phonolitic to trachytic alkali compositions whereas the at tephra layer which contains k poor glasses r ca 0 04 is rhyolitic according to trace element content the relative amounts of the elements ti zr y nb and p from the tephra layers evidently indicate the different tectonic zones between the ulleung back arc and kyushu island arc islands especially the relatively depleted concentrations of feo v co and ni in the u oki tephra layer most likely reflect more fractional crystallization of olivine pyroxene hornblende and magnetite than those in the u ym tephra layer

the taupo eruption new zealand i general aspects the ca a d 186 taupo eruption was the latest eruption at the taupo volcanic centre occurring from a vent at horomatangi reefs now submerged beneath lake taupo in the central north island of new zealand minor initial phreatomagmatic activity was followed by the dry vent 6 km 3 hatepe plinian outburst large amounts of water then entered the vent during the 2 5 km 3 hatepe phreatoplinian ash phase eventually stopping the eruption though large amounts of water continued to be ejected from the vent area causing gullying of the ash deposits after a break of several hours to weeks phreatoplinian activity resumed generating the 1 3 km 3 rotongaio ash notable for its fine grainsize and for containing significant quantities of non or poorly vesicular juvenile material the vent area then became dry again and eruption rates and power markedly increased into the 23 km 3 taupo ultraplinian phase which is the most powerful plinian outburst yet documented synchronous with this ultraplinian activity lesser volumes of non to partially welded ignimbrite were generated by diversion of ejecta from or partial collapse of the eruption column the rapid rate of magma withdraw al during this phase removed support from the vent area to trigger local vent collapse and initiate the catastrophic eruption of the ca 30 km 3 taupo ignimbrite finally after some years lava was extruded on to the floor of the reformed lake taupo and floating fragments derived from the lava carapace were driven ashore the known eruption volume is more than 65 km 3 while additional volumes are represented by primary material now beneath lake taupo and layer 3 to the ignim brite phases a total volume of more than 105 km 3 is likely equivalent to more than 35 km 3 of magma plus more than 3 km 3 of lithic debris airfall deposits more than 10 cm thick blanketed 30000 km 2 of land east of the vent while ignimbrite covers a near circular area of 20000 km 2 widespread and locally severe ground shaking occurred during but mostly after the eruption associated with subsidence in the lake taupo basin secondary deposits are abundant above and extending beyond the taupo ignimbrite consisting of the products of surface water interacting with the still hot ignimbrite and subsequent water reworking of the light pumiceous materials the complexity and size of this eruption preclude accurate forecasting of the size nature and return period of the inevitable next eruption from the taupo volcanic centre

new constraints on the formation of the non volcanic continental galicia flemish cap conjugate margins numerous multichannel seismic data acquired on the starved continental margin off galicia show that the prominent intra crustal s reflector is observed only below the oceanward portion of the continental margin close to the ocean continent boundary superficial extension has been computed from the geometry of the well imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose the pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the s reflector is present the s reflector is a detachment zone which presents a decreasing offset in the direction of the continent and this detachment zone has been active since the beginning of rifting recent kinematic reconstructions show that the conjugate margin to galicia is the southeastern flemish cap margin from the few seismic profiles available on this canadian margin a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the galicia margin as the s reflector the proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is either pure shear for the continentward portion of the galicia margin or simple shear for the oceanward portion of the galicia and the whole southeastern flemish cap margins where the detachment zone is active i consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust

seismic structure of the northern continental margin of spain from escin deep seismic profiles abstract by the end of the carboniferous the crust of the continental shelf in northwestern spain was made up of deeply rooted structures related to the variscan collision from permian to triassic times the tectonic setting had changed to mainly extensional and the northern iberian continental margin underwent rifting during late jurassic early cretaceous times along with sea floor spreading and the opening of the bay of biscay until the late cretaceous subsequently the northern iberian margin was active during the north south convergence of eurasia and iberia in the tertiary a multichannel seismic experiment consisting of two profiles one north south escin 4 crossing the platform margin offshore asturias and another escin 3 crossing the platform margin to the northwest of galicia was designed to study the structure of the northern iberian margin the escin 4 stacked section reveals inverted structures in the upper crust within the le danois basin north of the steep continental slope escin 4 shows a thick sedimentary package from 6 to 9 5 s two way travel time twt within this latter package a 40 km long north tapering wedge of inclined mainly south dipping reflections is thought to represent a buried alpine age accretionary prism in the north western part of the escin 3 escin 3 1 stacked section horizontal reflections from 6 5 to 8 5 s correspond to an undisturbed package of sediments lying above oceanic type basement in this part of the line a few kilometres long strong horizontal reflection at 11 2 s within the basement may represent an oceanic moho reflection also a band of reflections dips gently towards the southeast from the base of the gently dipping continental slope the part of escin 3 line that runs parallel to the nw galicia coast escin 3 2 is characterized by bright continuous lower crustal reflections from 8 to 10 s beneath the lower crustal reflectivity a band of strong reflections dips gently toward the southwest from 10 to 13 5 s the part of escin 3 that parallels the northern margin escin 3 3 shows good reflectivity in all levels upper crustal reflections image the sedimentary fill of probable mesozoic to recent basins mid crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional variscan features the lower crustal level shows layered reflections between 8 and 12 s dipping reflections are found below the continental moho

mechanism of caldera collapse and resurgence observations from the northern part of the kumano acidic rocks kii peninsula southwest japan abstract the northern part of the kumano acidic rocks on the kii peninsula southwest japan formed during miocene felsic igneous activity that involved caldera forming eruptions and subsequent intrusion of a massive sheetlike body of granite porphyry into intracaldera deposits detailed mapping has revealed that the northern part of the kumano acidic rocks has two separate collapse structures that show the structural margin of a caldera one is an arcuate fault zone characterized by several normal faults with gentle dips pyroclastic deposits characterized by the dominance of crystal rich welded ash vary in thickness across the fault zone the ash beds contain numerous boulders on the inner side of the collapse structure representing caldera collapse breccias sourced from the fault scarp during the caldera forming eruption the other is a 1 3 km wide ring dike of granite porphyry that encloses the prevolcanic sedimentary rocks and the pyroclastic deposits and is interpreted as a magma conduit along a subsurface circular fracture the caldera collapse area enclosed by these two structures is elliptical and about 20 30 km in size our observations suggest a coherent piston type collapse for the northern part of the caldera while asymmetric subsidence along the fault zone resulted in trapdoor or piecemeal subsidence in the southern part the ring dike is connected to the sheetlike body of granite porphyry we suggest that this sheetlike intrusion is a laccolith and may have caused resurgent doming of the caldera floor