bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonic, eustatic and environmental controls on mid‐Cretaceous carbonate platform deposition, south‐central Pyrenees, Spain
Authors: Drzewiecki Peter A. , Simo J. A. (toni)
Published: March 2003

Cenomanian–Turonian strata of the south‐central Pyrenees in northern Spain contain three prograding carbonate sequences that record interactions among tectonics, sea level, environment and sediment fabric in controlling sequence development. Sequence UK‐1 (Lower to Upper Cenomanian) contains distinct lagoonal, back‐margin, margin, slope and basin facies, and was deposited on a broad, flat shelf adjacent to a deep basin. The lack of reef‐constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. Sequence UK‐2 (Upper Cenomanian) contains similar shallow‐water facies belts, but syndepositional tectonic modification of the margin resulted in a steep slope and deposition of carbonate megabreccias. Sequence UK‐3 (Lower to Middle Turonian) records a shift from benthic to pelagic deposition, as the shallow platform was drowned in response to a eustatic sea‐level rise, coupled with increased organic productivity. Sequences UK‐1 to UK‐3 are subdivided into lowstand, transgressive and highstand systems tracts based on stratal geometries and facies distribution patterns. The same lithologies (e.g. megabreccias) commonly occur in more than one systems tract, indicating that: (1) the depositional system responded to more than just sea‐level fluctuations; and (2) similar processes occurred during different times throughout sequence development. These sequences illustrate the complexity of carbonate platform dynamics that influence sequence architecture. Rift tectonics and flexural subsidence played a major role in controlling the location of the platform margin, maintaining a steep slope gradient through syndepositional faulting, enhancing slope instability and erosion, and influencing depositional processes, stratal relationships and lithofacies distribution on the slope. Sea‐level variations (eustatic and relative) strongly influenced the timing of sequence and parasequence boundary formation, controlled changes in accommodation and promoted platform drowning (in conjunction with other factors). Physico‐chemical and climatic conditions were responsible for reducing carbonate production rates and inducing platform drowning. Finally, a mud‐rich sediment fabric affected platform morphology, growth geometries (aggradation vs. progradation) and facies distribution patterns.
A field study of turbulence and sediment dynamics over subaqueous dunes with flow separation
Authors: Kostaschuk Ray
Published: March 2003

This study examines flow, turbulence and sand suspension over large dunes in Canoe Pass, a distributary channel of the Fraser River delta, Canada. Dune morphology is characterized by a symmetrical shape and steep leeside slopes over 30°. Velocity was measured with an electromagnetic current meter and suspended sand concentration with four optical backscatter (OBS) probes. The general patterns of time‐averaged velocity and sand suspension are consistent with previous studies, including an increase in mean velocity and decrease in turbulence intensity and sand concentration with height above the bed, reversed flow with high turbulence intensity and high sand concentrations in the leeside flow separation zone and an increase in near‐bed velocity and sand concentration along the stoss side of the dune. Frequency spectra of near‐bed velocity and OBS records from leeside separation zones are composed of two distinct frequencies, providing field confirmation of scale relations based on flume experiments. The low‐frequency spectral signal probably results from wake flapping and the high‐frequency signal from vortex shedding. The wake‐flapping frequency predominates outside the separation zone and is linked to turbulent structures that suspend sand. Predictions from a depth‐scale Strouhal Law show good agreement with measured wake‐flapping frequencies. Cross‐correlations of OBS records reveal that turbulent sand suspension structures advect downstream at 23–25° above the horizontal. These advection angles are similar to coherent flow structures measured in flumes and to sand suspension structures visualized over large dunes in the field.
Sedimentary processes in the Selvage sediment‐wave field, NE Atlantic: new insights into the formation of sediment waves by turbidity currents
Authors: Wynn Russell B. , Weaver Philip P. E. , Ercilla Gemma , Stow Dorrik A. V. , Masson Douglas G.
Published: March 2003

An integrated geophysical and sedimentological investigation of the Selvage sediment‐wave field has revealed that the sediment waves are formed beneath unconfined turbidity currents. The sediment waves occur on the lower continental rise and display wavelengths of up to 1 km and wave heights of up to 6 m. Wave sediments consist of interbedded turbidites and pelagic/hemipelagic marls and oozes. Nannofossil‐based dating of the sediments indicates a bulk sedimentation rate of 2·4 cm 1000 years–1, and the waves are migrating upslope at a rate of 0·28 m 1000 years–1. Sediment provenance studies reveal that the turbidity currents maintaining the waves are largely sourced from volcanic islands to the south. Investigation of existing models for sediment‐wave formation leads to the conclusion that the Selvage sediment waves form as giant antidunes. Simple numerical modelling reveals that turbidity currents crossing the wave field have internal Froude numbers of 0·5–1·9, which is very close to the antidune existence limits. Depositional flow velocities range from <6 to 125 cm–1. There is a rapid increase in wavelength and flow thickness in the upper 10 km of the wave field, which is unexpected, as the slope angle remains relatively constant. This anomaly is possibly linked to a topographic obstacle just upslope of the sediment waves. Flows passing over the obstacle may undergo a hydraulic jump at its boundary, leading to an increase in flow thickness. In the lower 15 km of the wave field, flow thickness decreases downslope by 60%, which is comparable with results obtained for other unconfined turbidity currents undergoing flow expansion.
Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis?
Authors: Hillgärtner Heiko , Dupraz Christophe , Hug Wolfgang
Published: March 2003

Characteristic fabrics such as micrite envelopes, calcified filaments and micritic grain‐to‐grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm‐ and hardgrounds of Lower Cretaceous and Upper Jurassic platform carbonates (Swiss and French Jura Mountains). Their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands. ‘Meniscus‐type cements’ (to distinguish them from vadose meniscus cements), which clearly formed in subtidal environments, are related to filament calcification, trapping of percolating micrite and microbially induced carbonate formation. Such meniscus‐type cements are commonly micritic, but meniscus‐shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed. Therefore, an interpretation of vadose early diagenesis should not be based on meniscus cements alone. Similarly, subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure.
Coarse carbonate breccias as a result of water‐wave cyclic loading (uppermost Jurassic – South‐East Basin, France)
Authors: Bouchette Frédéric , Séguret Michel , Moussine‐Pouchkine Alexis
Published: March 2003

In the uppermost Jurassic of the central part of the South‐East Basin of France, an association of lime mudstone beds, calcarenite beds and coarse carbonate breccia bodies form an informal stratigraphical unit called the ‘Barre Tithonique’. In the ‘Barre Tithonique’, gradual transitions from lime mudstone or calcarenite to breccia show different stages of deformation leading to progressive brecciation of the original lithologies. The study of the breccia facies, and the observed gradual transitions as a whole, document a new early diagenetic process in carbonate environments, resulting from water‐wave and seabed interaction. Water‐wave induced brecciation and its abundance in the ‘Barre Tithonique’ indicate that sea–seabed interaction was significant. Comparison with modern studies of the mechanics of wave–seabed interaction suggests that water depth was less than 200 m. It is demonstrated that sedimentary features such as channel‐like structures, previously interpreted as being the result of erosion and deposition of mud‐flows, were in fact produced by wave‐induced, in situ reworking of lime mud, without any significant unidirectional flow or gravity induced displacement.
Submarine fan environment inferred from turbidite thickness distributions
Authors: Carlson J. , Grotzinger J. P.
Published: March 2003

Recent studies of turbidite bed thickness distributions have demonstrated power‐law as well as log‐normal statistical distributions. The different distributions may reflect different fan processes and environments and, therefore, could be used as a quantitative method to help identify those environments, including those devoid of sequential patterns. The cumulative distributions of well‐known turbidite deposits spanning a range of interpreted fan subenvironments are used to illustrate the potential correlation between cumulative distribution and environments. Assuming that power‐law distributions may, for some systems, be the primary input signal, one‐dimensional modelling allows semi‐quantitative characterization of the effects of different fan processes such as erosion and bed amalgamation. Environments indicative of different fan processes may be characterized based on the degree to which processes have acted as a ‘filter’ to modify the assumed power‐law distribution systematically. This model of the effect of fan processes on the power‐law distribution is used to help to account for bed thickness distributions observed in several field sites.
Siliciclastic and volcaniclastic influences on equatorial carbonates: insights from the Neogene of Indonesia
Authors: Wilson Moyra E. J. , Lokier Stephen W.
Published: March 2003

ABSTRACT In active tectonic areas of humid equatorial regions, nearshore shallow‐water environments are commonly sites of near‐continuous siliciclastic influx and/or punctuated volcaniclastic input. Despite significant clastic influence, Neogene carbonates developed in SE Asia adjacent to major deltas or volcanic arcs, and are comparable with modern mixed carbonate–clastic deposits in the region. Research into delta‐front patch reefs from Borneo and fore‐arc carbonate platform development from Java is described and used to evaluate the effects of siliciclastic and volcaniclastic influx on regional carbonate sedimentation, local changes in carbonate‐producing biota and sequence development. Regional carbonate development in areas of high siliciclastic or volcaniclastic input was influenced by the presence of antecedent highs, changes in the amounts or rates of clastic input, delta lobe switching or variations in volcanic activity, energy regimes and relative sea‐level change. A variety of carbonate‐producing organisms, including larger benthic foraminifera, some corals, coralline algae, echinoderms and molluscs could tolerate near‐continuous siliciclastic or volcaniclastic influx approximately equal to their own production rates. These organisms adopted various ‘strategies’ for coping with clastic input, including a degree of mobility, morphologies adapted to unstable substrate inhabitation or shedding sediment, and shapes adapted to low light levels. Local carbonate production was also affected by energy regime, clastic grain sizes and associated nutrient input. Clastic input influenced the inhabitable depth range for photoautotrophs, the zonation of light‐dependent assemblages and the morphology and sequence development of mixed carbonate–clastic successions. This study provides data on the dynamic interactions between carbonate and non‐carbonate clastic sediments and, when combined with information from comparable modern environments, allows a better understanding of the effects of siliciclastic and volcaniclastic influx on carbonate production.
Syntectonic hydrothermal calcite in a faulted carbonate platform margin (Albian of Jorrios, northern Spain)
Authors: Aranburu A. , Fernández‐Mendiola P. A. , López‐Horgue M. A. , García‐Mondéjar J.
Published: March 2003

ABSTRACT A calcite mass more than 1·5 km long and 20 m wide crops outs along the faulted margin of the Albian carbonate platform of Jorrios in northern Spain. The mass contains abundant dissolution cavities up to 7 m long and 1 m high, filled with cross‐stratified quartz sandstone and alternating sandstone–calcite laminae. Similar cavities are also present in a 50‐m‐wide zone of platform limestones adjacent to the calcite mass that are filled with limestone breccias and sandstone. The calcite mass has mean δ18O values of 19·6‰ (SMOW), whereas platform limestones have mean δ18O values of 24·4‰ (SMOW). Synsedimentary faulting of the carbonate margin and circulation of heated fault‐related waters resulted in replacement of a band of limestone by calcite. Soon after this replacement, dissolution by undersaturated fluids affected both the calcite mass and the adjacent limestones. Percolating marine quartz sand filled all dissolution cavities, sometimes alternating with precipitating calcite. The resulting cavities and fills, which recall products of meteoric diagenesis, are attributed to a hydrothermal origin based on their geometry, occurrence along the profile and synsedimentary tectonic relationships. The early faulting and diagenesis are related to local extensional tectonism in a large‐scale strike‐slip setting. Movements occurred during the early dispar/appenninica zone of the Late Albian.
Repeated cycles of submarine channel incision, infill and transition to sheet sandstone development in the Alpine Foreland Basin, SE France
Authors: McCaffrey William D. , Gupta Sanjeev , Brunt Rufus
Published: March 2003

The Grès de Champsaur turbidite system, deposited in a distal setting in the Alpine Foreland Basin of south‐eastern France, exhibits a repeated upsection alternation in sand body geometry between incised channels and sheet sands. The channels form symmetric lenticular erosional features, of width 900–1000 m (measured between the lateral limits of incision) and depth 65–115 m, and can be traced axially for up to 5 km. In each case, the channel fill is capped by a laterally persistent sandy sheet‐form interval, which lies upon a fine‐grained substrate beyond the channel margins. No intrachannel elements have been traced into the substrate sequence, suggesting that, before infill, the channels acted as open sea‐floor conduits of essentially the same dimensions as the preserved channel deposits. The channels are vertically stacked, although axial erosion juxtaposes younger channel axis deposits against the fill of older channels and their channel‐capping sheet sandstones to produce an apparently well‐connected composite sandstone body geometry. The predominant channel‐fill facies comprises coarse‐grained, amalgamated sandstones, which are commonly parallel‐ or cross‐stratified. Subsidiary facies of finer grained sandstone–mudstone couplets and clast‐bearing muddy debrites are commonly preserved as erosional remnants, suggesting a complex channel history of aggradation and erosion. The repeated cycles of channel incision, infill and transition to sheet sandstone development indicate repetitive incision and healing of the palaeo‐sea floor. A model is proposed that links incision to the development of relatively steep axial gradients (parallel to the mean dispersal direction) and the return to sheet‐form deposition to the re‐establishment of lower axial gradients, with the repetitive switch between incisional channels and sheet sandstones driven by changes in sediment input rate against a background of ongoing sea‐floor tilting.
Facies analysis of the Trypali carbonate unit (Upper Triassic) in central‐western Crete (Greece): an evaporite formation transformed into solution‐collapse breccias
Authors: Pomoni‐Papaioannou F. , Karakitsios V.
Published: March 2003

The Trypali carbonate unit (Upper Triassic), which crops out mainly in central‐western Crete, occurs between the parautochthonous series (Plattenkalk or Talea Ori‐Ida series, e.g. metamorphic Ionian series) and the Tripolis nappe (comprising the Tripolis carbonate series and including a basal Phyllite–Quartzite unit). It consists of interbedded dolomitic layers, represented principally by algally laminated peloidal mudstones, foraminiferal, peloidal and ooidal grainstones, as well as by fine‐grained detrital carbonate layers, in which coarse baroque dolomite crystals and dolomite nodules are dispersed. Baroque dolomite is present as pseudomorphs after evaporite crystals (nodules and rosettes), which grew penecontemporaneously by displacement and/or replacement of the host sediments (sabkha diagenesis). However, portions of the evaporites show evidence of resedimentation. Pre‐existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal‐shaped hoppers, as well as of anhydrite nodules and rosettes (salt crusts). All microfacies are characteristic of peritidal depositional environments, such as sabkhas, tidal flats, shallow hypersaline lagoons, tidal bars and/or tidal channels. Along most horizons, the Trypali unit is strongly brecciated. These breccias are of solution‐collapse origin, forming after the removal of evaporite beds. Evaporite‐related diagenetic fabrics show that there was extensive dissolution and replacement of pre‐existing evaporites, which resulted in solution‐collapse of the carbonate beds. Evaporite replacement fabrics, including calcitized and silicified evaporite crystals, are present in cements in the carbonate breccias. Brecciation was a multistage process; it started in the Triassic, but was most active in the Tertiary, in association with uplift and ground‐water flow (telogenetic alteration). During late diagenesis, in zones of intense evaporite leaching and brecciation, solution‐collapse breccias were transformed to rauhwackes. The Trypali carbonate breccias (Trypali unit) are lithologically and texturally similar to the Triassic solution‐collapse breccias of the Ionian zone (continental Greece). The evaporites probably represent a major diapiric injection along the base of the parautochthonous series (metamorphic Ionian series) and also along the overthrust surface separating the parautochthonous series from the Tripolis nappe (Phyllite–Quartzite and Tripolis series). The injected evaporites were subsequently transformed into solution‐collapse breccias.
Anatomy of shelf deltas at the edge of a prograding Eocene shelf margin, Spitsbergen
Authors: Mellere Donatella , Plink‐Björklund Piret , Steel Ronald
Published: March 2003

Abstract Although shelf‐edge deltas are well‐imaged seismic features of Holocene and Pleistocene shelf margins, documented outcrop analogues of these important sand‐prone reservoirs are rare. The facies and stratigraphic architecture of an outcropping shelf‐edge delta system in the Eocene Battfjellet Formation, Spitsbergen, is presented here, as well as the implications of this delta system for the generation of sand‐prone, shelf‐margin clinoforms. The shelf‐edge deltas of the Battfjellet Formation on Litledalsfjellet and Høgsnyta produced a 3–5 × 15 km, shelf edge‐attached, slope apron (70 m of sandstones proximally, tapering to zero on the lower slope). The slope apron consists of distributary channel and mouth‐bar deposits in its shelf‐edge reaches, passing downslope to slope channels/chutes that fed turbiditic lobes and spillover sheets. In the transgressive phase of the slope apron, estuaries developed at the shelf edge, and these also produced minor lobes on the slope. The short‐headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf, and the shelf‐edge position of the discharging deltas, made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin. The abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion. Evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane‐parallel laminae within turbidite beds in the slope channels, and thick spillover lobes with repetitive alternations of massive and flat‐laminated intervals. The examined shelf‐edge to slope system, now preserved mainly below the shelf break and dominated by sediment gravity‐flow deposits, has a threefold stratigraphic architecture: a lower, progradational part, in which the clinoforms have a slight downward‐directed trajectory; a thin aggradational zone; and an upper part in which clinoforms backstep up onto the shelf edge. A greatly increased density of erosional channels and chutes marks the regressive‐to‐transgressive turnaround within the slope apron, and this zone becomes an angular unconformity up near the shelf edge. This unconformity, with both subaerial and subaqueous components, is interpreted as a sequence boundary and developed by vigorous sand delivery and bypass across the shelf edge during the time interval of falling relative sea level. The studied shelf‐margin clinoforms accreted mostly during falling stage (sea level below the shelf edge), but the outer shelf later became estuarine as sea level became re‐established above the shelf edge.
Hydrodynamic behaviour of Taupo 1800a pumice: implications for the sedimentology of remobilized pyroclasts
Authors: Manville V. , Segschneider B. , White J. D. L.
Published: March 2003

Abstract Large‐scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments. On land, erosion, remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run‐off. Conversely, in the submarine realm, both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium. Despite this, and the number of studies devoted to volcaniclastic sedimentation, relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves, which ought to underpin any assessment of transport or depositional process. This is crucial, as many volcanic particles exhibit variable density: according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and/or gaseous phases during transport and deposition. Investigation of the physical and hydrodynamic properties of Taupo 1800a pumice, with reference to sedimentary facies developed during the eruption aftermath, shows that, although buoyant when dry, when sufficiently waterlogged, cool pumice clasts will sink and behave more like quartzo‐feldspathic material. Saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air–water exchange in smaller pores. Low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent‐sized quartzo‐feldspathic or crystal/lithic particles. Fine‐grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles. These unusual properties of temporary buoyancy, variable saturation, low density and size‐dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies. These findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments, but also for primary depositional processes during subaqueous explosive volcanism.
Numerical modelling of depositional sequences in half‐graben rift basins
Authors: Gawthorpe Rob L. , Hardy Stuart , Ritchie Bryan
Published: March 2003

ABSTRACT A three‐dimensional numerical model of sediment transport and deposition in coarse‐grained deltas is used to investigate the controls on depositional sequence variability in marine half‐graben extensional basins subject to eustatic sea‐level change. Using rates of sea‐level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall‐sourced deltas in high‐subsidence locations near the centre of a fault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope. Differences in the three‐dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall‐sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault‐tip and hangingwall dip‐slope deltas mean that they are subject to relative sea‐level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault‐tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet‐like delta lobes characterize the hangingwall dip‐slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. Furthermore, the results highlight pitfalls in sequence‐stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.
Style of active intraplate deformation from gravity and seismicity data: the Baikal Rift, Asia
Authors: Petit
Published: March 2003

To better understand how active deformation localizes within a continental plate in response to extensional and transtensional tectonics, a combined analysis of high‐quality gravity (Bouguer anomaly) and seismicity data is presented consisting of about 35000 earthquakes recorded in the Baikal Rift Zone. This approach allows imaging of deformation patterns from the surface down to the Moho. A comparison is made with heat flow variations in order to assess the importance of lithospheric rheology in the style of extensional deformation. Three different rift sectors can be identified. The southwestern rift sector is characterized by strong gravity and topography contrasts marked by two major crustal faults and diffuse seismicity. Heat flow shows locally elevated values, correlated with recent volcanism and negative seismic P‐velocity anomalies. Based on earthquake fault plane solutions and on previous stress field inversions, it is proposed that strain decoupling may occur in this area in response to wrench‐compressional stress regime imposed by the India–Asia collision. The central sector is characterized by two major seismic belts; the southernmost one corresponds to a single, steeply dipping fault accommodating oblique extension; in the centre of lake Baikal, a second seismic belt is associated with several dip‐slip faults and subcrustal thinning at the rift axis in response to orthogonal extension. The northern rift sector is characterized by a wide, low Bouguer anomaly which corresponds to a broad, high topographic dome and seismic belts and swarms. This topography can be explained by lithospheric buoyancy forces possibly linked to anomalous upper mantle. At a more detailed scale, no clear correlation appears between the surficial fault pattern and the gravity signal. As in other continental rifts, it appears that the lithospheric rheology influences extensional basins morphology. However, in the Baikal rift, the inherited structural fabric combined with stress field variations results in oblique rifting tectonics which seem to control the geometry of southern and northeastern rift basins.
Continental breakup by detachment faulting: field evidence and geochronological constraints (Tasna nappe, Switzerland)
Authors: Froitzheim N. , Rubatto D.
Published: March 2003

The Lower Tasna Detachment (LTD) is a low‐angle fault contact between serpentinized peridotite below and continental basement above. It was formed during Jurassic to Early Cretaceous rifting of a Tethyan continental margin and later captured in a thrust nappe during Tertiary plate convergence. Foliated gabbro, gabbro mylonite, and granitoid mylonite occurring along the LTD record shearing under decreasing temperatures. U–Pb dating of zircon from the gabbro mylonite yielded a Permian age, interpreted as the age of gabbro intrusion, whereas the breakup of the passive margin occurred as late as Early Cretaceous. This suggests that the gabbro belongs to a prerift, lower to middle crustal intrusion ‘smeared out’ along the detachment by extensional faulting. The juxtaposition of mantle and upper crust along the Lower Tasna detachment may serve as a model for several seismic reflectors observed in distal passive continental margins (e.g. S reflector of the Galicia margin).
Insights into the collection and emplacement of granitic magma based on 3D seismic images of normal fault‐related salt structures
Authors: Quirk  , D’Lemos  , Mulligan  , Rabti
Published: March 2003

ABSTRACT

3D seismic data from salt structures in the Southern North Sea show how viscous fluids tend to migrate towards and flow upwards along the footwalls of active normal faults where zones of low stress are inherent. This process provides one potential solution for the question of how enough space is created to accommodate large granitic bodies in the crust. During episodes of regional extension, granitic melt may collect in the lower crust below normal detachments in domed areas from which magma may be sourced. Subsequently, in the upper crust, emplacement can occur in fault‐bounded laccoliths at rates commensurate with other geological processes.
Climate records and changes in deep outflow from the Norwegian Sea ∼150–55 ka
Authors: Rasmussen T.L. , Balbon E. , Thomsen E. , Labeyrie L. , Van Weering T.C.E.
Published: March 2003

In order to contribute to our understanding of the linkage between climate and ocean circulation we have studied benthic foraminifera from near the northern end of the Faeroe–Shetland Channel covering isotope stages 6 to lower stage 3 (∼150–55 ka). Our records demonstrate shifts between recurring assemblages, which on millennial timescales monitor the outflow history of Norwegian Sea Deep Water. The records show that the outflow is closely linked to the climate of the region as documented in the Greenland ice cores. Outflow was relatively strong during all major warmer interstadials whereas there was no outflow during the colder stadials. During isotope substage 5e outflow was stable for ∼10–12 kyr with significant changes at the beginning and end only.
Conditions favouring catastrophic landslides on Tenerife (Canary Islands)
Authors: Hürlimann M. , Ledesma A. , Martí J.
Published: March 2003

Catastrophic failures of volcano flanks represent one of the most hazardous geological phenomena. These immense mass movements originate either by increasing the destabilizing forces (driving forces) or by reducing the strength of the materials involved, or both. The study of large volcanic landslides on Tenerife suggests that the presence of weak residual soils (palaeosols) in combination with the pre‐existence of deep, narrow canyons created by fluvial erosion, have played a fundamental role in the initiation of large‐scale sector collapses of the volcanic edifice, regardless of the triggering mechanism considered. Residual soils strongly reduce the material strength during undrained loading, while pre‐existing canyons control the lateral limits of the landslide. The existence of a wet climate in some sectors of the island favours these circumstances.
Sand‐box modelling of basement‐controlled transfer zones in extensional domains
Authors: Acocella  , Faccenna  , Funiciello  , Rossetti
Published: March 2003

The interaction modalities of transfer zones connecting rift segments may be influenced by several factors. Amongst these, the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies. Sand‐box models were made to investigate how the orientation, geometry and kinematics of transfer zones depend upon pre‐existing basement anisotropies. Analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults. Strike‐slip faults are present inside the transfer zone, provided that angle α (between the extension direction and the axis of the transfer zone) is less than 50°. Narrower transfer zones, striking oblique to the extension direction, occur for angle φ (between the direction perpendicular to the extension direction and the direction of the basement anisotropy) less than 90°; wider transfer zones, subparallel to the extensional direction, occur for φ > 90°. Increasing the overstep induces narrower transfer zones striking subparallel to the extension direction. Similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts. The comparison between experimental and natural data shows how the geometry, kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies: transfer zones along inherited basement structures set at lower angles to the extension direction, and display lower aspect ratios, than transfer zones where inherited anisotropies are absent.
Microbial–serpulid build‐ups in the Norian–Rhaetian of the Western Mediterranean area: ecological response of shelf margin communities to stressed environments
Authors: Cirilli  , Iannace  , Jadoul  , Zamparelli
Published: March 2003

Microbial–serpulid communities are the main reef‐builders of most Norian–Rhaetian carbonate platforms of Italy. They represent minor, yet significant communities in the shallow western Tethys, in contrast with the highly diversified sponge‐ and coral‐dominated Dachstein reefs widespread from Eastern Alps to Australia. The microbial–serpulid build‐ups are systematically associated with narrow intraplatform troughs and developed on the outer margin–upper slope area under marine conditions unsuitable for the development of coralgal reefs. The development of these settings can be related to transtensional tectonics affecting an elongate belt which was roughly parallel to the Piedmont–Ligurian Ocean spreading axis during the Jurassic. Dysoxic and mesosaline waters are the main cause of the success of the Norian microbial–serpulid build‐ups; whereas, from the late Norian to the Rhaetian, eutrophication and climate change control their distribution. These environments acted during Norian–Rhaetian times as refuges, where opportunitistic organisms could survive and build frameworks during a period otherwise dominated by coralgal reefs.
Discovery of Gondwana plant fossils and palynomorphs of Late Asselian (Early Permian) age in the Karakoram Block
Authors: Upadhyay R. , Chandra R. , Sinha A.K. , Kar R.K. , Chandra S. , Jha N. , Rai H.
Published: March 2003

We report the discovery of Early Permian (late Asselian, ∼280–275 Ma) plant fossils and associated palynomorphs from a marine sedimentary sequence of the eastern Karakoram plate, in northern India. These specimens show affinities with those found in marine Lower Gondwana sediments of the Indian subcontinent. This supports the contention that during the Early Permian Period, the Karakoram plate was Peri‐Gondwanan. It is suggested to have had an intermediate position between the Indian plate and the Qiangtang–Lhasa microcontinents, at a latitude of about 35° south.
Tectonothermal history of the Hercynian continental crust of the Serre (southern Calabria, Italy) monitored by Rb–Sr biotite resetting
Authors: Del Moro A. , Fornelli A. , Piccarreta G.
Published: March 2003

In the Serre mountains of Calabria, Italy, an exposed section of the continental crust, as left by the Hercynian orogeny, consists of intermediate‐lower to upper crustal units. Huge masses of granitoids separate the lower from the upper crustal units. Many mica ages have been obtained from metamorphic and plutonic rocks, which have been interpreted as reflecting continuous cooling or discrete Mesozoic events. A reappraisal of previously determined isotopic data integrated with new Rb–Sr biotite ages is presented and assessed at regional scale to better constrain the post‐Hercynian geological evolution of the continental crust of the Serre. The ages cover a wide span of time and form clusters which fit a model involving magmatic, hydrothermal and tectonic events preceding and accompanying the opening and closure of the Tethyan ocean.
High U/Th partitioning by clinopyroxene from alkali silicate and carbonatite metasomatism: an origin for Th/U disequilibrium in mantle melts?
Authors: Foley S. F. , Petibon C. M. , Jenner G. A. , Kjarsgaard B. A.
Published: March 2003

Clinopyroxene/melt pairs in strongly potassic silicate and carbonatite melts exhibit unusually high U/Th partitioning ratios of ˜ 3 and ˜ 2, respectively. These values are much higher than those found for aluminous clinopyroxenes in peridotite, and have the potential to cause significant (230Th)/(238U) isotope enrichment in volcanics. The potassic silicate (lamproite) and carbonatite melts correspond closely to the main agents of mantle metasomatism, indicating that clinopyroxene in metasomatized regions of the mantle may greatly affect U/Th disequilibria. Recycling of alkali pyroxenite veins in the oceanic lithosphere formed by solidification of melt in the extremities of the MORB melting region presents an alternative to eclogite recycling in MORB and OIB genesis.
Magma homogenization during anatexis, ascent and/or emplacement? Constraints from the Variscan Weinsberg Granites
Authors: Gerdes A.
Published: March 2003

Recent studies have shown that melts and residues may not equilibrate during anatexis, and uncertainty exists about the scale on which magmas can be homogenized. This study of elemental and isotopic homogeneity of the South Bohemian Weinsberg granites (˜ 5000 km2) identifies three voluminous, relatively homogeneous magma batches. Each batch has different 87Sr/86Srinit (0.7080, 0.7093 and 0.7106), but all equilibrated at ˜ 327–329 Ma, very similar to the time of monazite crystallization. The data cannot entirely prove melt/residue equilibration during anatexis. However, elemental and isotopic compositions imply magma generation by partial melting of heterogeneous South Bohemian crust and chemical differentiation subsequent to Sr‐isotope equilibration. Assuming relatively rapid ascent and solidification rates, magma homogenization must have occurred mostly just after partial melting, during melt segregation and accumulation in the deeper crust with slow prograde heating. Models of rapid crustal heating and instantaneous melt extraction are incompatible with the data.
Permo‐Silesian movements between Baltica and Western Europe: tectonics and ‘basin families’
Authors: Mattern Frank
Published: March 2003

During the late Carboniferous, NW‐trending faults of Central Europe’s Bohemian Massif acted as a dextral shear zone between Baltica and Western Europe. During the early Permian, the shear sense was reversed, and a group of strike‐slip basins formed along major NW‐trending faults. This reversal is attributed to early Permian left‐lateral motion between Baltica and Western Europe recorded by faults and basins of the Bohemian Massif. The Uralian orogeny is identified as the driving force for the sinistral motion between Baltica and Western Europe, shifting Baltica to the north‐west along the Tornquist–Teisseyre zone.
Too‐low magnetic inclinations in central Asia: an indication of a long‐term Tertiary non‐dipole field?
Authors: Si Jingwei , Voo Rob Van der
Published: March 2003

Observed Tertiary palaeolatitudes in central Asia are more southerly (by about 1600 km on average) than those predicted from the Eurasian reference palaeopoles. Subsequent northward displacements of the central Asian terranes are unlikely to have been this large. In this study we analyse to what extent non‐dipole fields, especially octupole fields, can explain this phenomenon. A global (zonal) octupole field manifests itself in two ways. (1) Because the reference APWP is based mostly on results from the UK and North America, its palaeopoles will be far‐sided as seen from the North Atlantic, but near‐sided as seen from eastern Asia, giving predicted palaeolatitudes that are too high. (2) An octupole field contribution produces observed palaeolatitudes, as calculated with the dipole formula for central Asia, that are too low. Both effects therefore increase the palaeolatitude anomalies in Asia. We find that an octupole/dipole field ratio (G3) of 0.06 or greater will reduce the palaeolatitude discrepancies significantly and is of the same magnitude as the G3 estimate of a recent analysis of Early Tertiary European and North American data.
The Cretaceous/Palaeogene (K/P) boundary at Aïn Settara, Tunisia: restructuring of benthic foraminiferal assemblages
Authors: Peryt Danuta , Alegret Laia , Molina Eustoquio
Published: March 2003

Benthic foraminiferal assemblages, in contrast to planktic foraminifera, generally did not suffer mass extinctions at the Cretaceous/Palaeogene (K/P) boundary; extinctions were fewer in deeper water. However, the outer shelf, upper bathyal section at Aïn Settara, Tunisia, records a dramatic change in the structure of benthic foraminiferal assemblages across the K/P boundary. At the level of extinction of planktic assemblages and enrichment in Ir and other geochemical anomalies, highly diversified, low‐dominance Upper Maastrichtian assemblages with infaunal and epifaunal morphogroups were suddenly replaced by taxonomically impoverished assemblages, strongly dominated by epifaunal morphogroups. This extinction or temporary emigration of most infaunal morphogroups is interpreted to be the result of a sudden breakdown in food supply. This, in turn, is the consequence of a sudden collapse in primary productivity, probably resulting from the impact of the K/P asteroid.
Crustal‐scale boudinage and migmatization of gneiss during their exhumation in the UHP Province of Western Norway
Authors: Labrousse L. , Jolivet L. , Agard P. , Hébert R. , Andersen T. B.
Published: March 2003

ABSTRACT A field study in the coesite province, the deepest unit of the Norwegian Caledonides, gives new constraints on the rheological behaviour of the continental crust during exhumation. Lithological heterogeneities and differential retrogression led to crustal‐scale boudinage during the late‐orogenic intense E–W stretching event in the footwall of the Nordfjord‐Sogn Detachment. The main gneissic lithologies display a modest but widespread syn‐exhumation migmatization. Textural criteria allow estimation of a 30% fusion rate. Partial melting mostly post‐dates eclogitization and is synchronous with ductile stretching and top‐to‐west shearing. Presented observations suggest that the melt reactions and migmatization resulted in a soft rheology. During subduction to ~ 100 km depth and subsequent exhumation, crustal viscosity can be reduced by up to four orders of magnitude. Models are discussed that consider a transition from a small internal strain of the crust to viscous flow during exhumation.
Evidence for high‐temperature diffusional creep preserved by rapid cooling of lower crust (North Bohemian shear zone, Czech Republic)
Authors: Zulauf G. , Dörr W. , Fiala J. , Kotková J. , Maluski H. , Valverde‐Vaquero P.
Published: March 2003

ABSTRACT Volume diffusion and dislocation creep at T ∼ 800 °C led to high finite strain in granulite and orthogneiss of the Ohře crystalline complex (North Bohemian shear zone). Intragranular creep by volume diffusion is indicated by (i) lobate phase boundaries between feldspar and quartz, and (ii) removal of perthite lamellae and precipitation of tiny, aluminium‐rich needles at the margins of K‐feldspar. The striking diffusional‐creep structures imply high interfacial free energy that has been preserved from equilibration as a result of rapid cooling. U–Pb dating of monazite (342 ± 1 Ma) and 40Ar–39Ar dating of muscovite (341 ± 4 Ma) of Kadaň orthogneiss result in a cooling rate of 50 + 25/−17 °C Myr−1. This high value is attributed to collapse‐related ‘elevator‐style’ movements along the North Bohemian shear zone, resulting in the juxtaposition of upper crustal rocks of the Tepla–Barrandian unit against lower crustal rocks of the Erzgebirge crystalline complex.
Expression of antisense nodulin‐35 RNA in Vigna aconitifolia transgenic root nodules retards peroxisome development and affects nitrogen availability to the plant
Authors: Lee Na‐Gyong , Stein Barry , Suzuki Hideki , Verma Desh Pal S.
Published: March 2003

A nodulin‐35 (N‐35) cDNA encoding nodule‐specific uricase (EC 1.7.3.3.) was isolated from a Vigna aconitifolia (mothbean) root nodule cDNA library. Sequence analysis of Vigna uricase (VN‐35) cDNA revealed 90% homology to that of soybean. The VN‐35 cDNA was inserted in the antisense orientation downstream of the CaMV—35S promoter, and transgenic hairy roots were formed on Vigna plants using Agrobacterium rhizogenes. Infection with Bradyrhizobium (cowpea) gave rise to root nodules on transgenic hairy roots supported by the wild‐type shoot. Expression of antisense VN‐35 RNA was detected in transgenic nodules on individual roots using polymerase chain reaction (PCR). The nodules expressing antisense VN‐35 RNA were smaller in size and showed lower uricase activity than nodules formed on the hairy roots transformed with a binary vector containing β‐glucuronidase (GUS) gene (used as control), and the plants exhibited nitrogen deficiency symptoms. Ultrastructural analysis and immunogold labeling with antibody against soybean N‐35 revealed that the growth of peroxisomes was retarded in transgenic nodules expressing antisense VN‐35 RNA. These data suggest that a reduction in ureide biosynthesis limits the availability of symbiotically reduced nitrogen to the plant. The nodules of tropical legumes appear to be specialized in nitrogen assimilation and are developmentally controlled to produce and transport ureides.