The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km² and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?     

Chemical record of the projectile in the graded fall-back sedimentary unit from the Ries Crater, Germany

The so-called “Graded Unit” comprises 17.2 m of the core from the research borehole Nördlingen 1973 and is thought to represent fall-back material, which has been sorted similar to tephra. Samples from six levels of these air-borne impact debris sediments in the Ries Crater have been analyzed for Ir, Ni, Co, Cr and Fe. The concentrations of these elements are negatively correlated with average grain size and accordingly the depth of the samples indicating the presence of a surface correlated component and are hence consistent with vapour deposition of meteoritic material on small dust grains. The largest Ir concentration found is 230 pg/g, a factor of 28 higher than the indigenous concentration. Enrichment factors in the uppermost layer of the “Graded Unit” are about 28 for Ir, approximately 2 for Fe, Co, Ni and 1.5 for Cr. If the enrichment of all elements determined were of meteoritic origin, then the “net meteoritic composition” is most closely matched by aubrites as suggested previously. However, it seems more likely that the observed concentration trends of Cr, Fe, Co and Ni are due to mineral sorting. Therefore the only as yet certain indication of meteoritic material is the presence of Ir, which is not sufficient to further constrain the nature of the projectile.     

Deccan flood basalts at the Cretaceous/Tertiary boundary?

Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.     

The structure of the guinean continental margin: Implications for the connection between the central and the south atlantic oceans

The results of a recent geological-geophysical survey, conducted off Guinea, are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the West African continental margin. Three sectors are distinguished:

A northwestern portion of the margin comprises a wide and deeply submerged plateau — the Guinean Marginal Plateau underlain by a thick sedimentary sequence and facing westward toward the Gambia Abyssal Plain. Scismic stratigraphy and structures show clear analogies to the Jurassic margins of the central Atlantic. Including the presence of a Cretaceous paleoslope covered by Cenozoic deposits.

A southern area of the margin comprises a series of aligned (W-E trending), acoustic basement features extending along the slope and bounding the Guinean Plateau to the south. These features, basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data. The bordering deep Sierra Leone abyssal plain, also dissected by E-W-trending oceanic fracture zones, contains a sedimentary cover apparently not older than middle Cretaceous.

Between both sectors and between two NW-SE trending scarps lies an intermediate area. The seismic profiles show that here, the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the S-E and covered by untectonized (but partly eroded) Upper Cretaceous to Cenozoic sediments.

The overall structure of the Guinean Margin is interpreted as the result of two major events. During a first phase the margin was created at the southern extremity of the central Jurassic Atlantic and developed like other comparable margins. During a s econd phase (beginning in Early Cretaceous times) the margin was progressively submitted to the opening of the equatorial South Atlantic. This process gave rise to the margin of the southern Guinean plateau (locally injected by volcanics) and generated the tectonic features of the intermediate zone. This protion may thus represent a part of the rifted Jurassic margin discordantly dissected by the oblique opening of the south Atlantic in the area. The oceanic crust of the central and south Atlantic were definitively connected only during Late Albian times as indicated by the end of the tectonic activity and the early Upper Cretaceous unconformity.     

Dynamic stress wave reflections/attenuation: Earthquake simulation in centrifuge soil models

In this investigation, the feasibility of earthquake simulation in centrifuge soil experiments is studied. The strong detrimental effect of standing waves for such an endeavour is clearly shown. A modest degree of success toward producing a model earthquake is reported via two devices—a certain kind of physically tuned internal excitor and an effective absorbent material at the walls.     

Les minéralisations ferrifères du Sahara Algerien: le gisement de fer oolithique de Mecheri Abdelaziz (bassin de Tindouf)

Résumé Le gisement de fer oolithique de Mecheri Abdelaziz (Famennien terminal du bassin de Tindouf), présente des caractères morphologiques et sédimentologiques qui permettent de penser que la minéralisation s'est mise en place dans des édifices deltaïques progradants.La minéralisation est complexe et comprend trois paragenèses principales; maghémite-magnétite, bavalite-hématite, bavalite-magnétite et quatre faciès types: faciès microconglomératique (FMC), faciès à oolithes dispersées dans une matrice détritique (FOD), faciès à oolithes dispersées dans une matrice non détritique (FOND) et faciès à oolithes jointives, cimentées (FOC).Les caractères des oolithes et de la mésostase montrent que les corps minéralisés résultent de l'accumulation d'oolithes, développées essentiellement par concrétionnement intrasédimentaire et ayant subi, avant le dépôt définitif, des phases alternantes d'oolithisation et de remise en suspension.Sur l'ensemble du gisement, les teneurs moyennes sont les suivantes: Fe total: 43,49%; SiO₂=13,66%; P₂O₅=2,31%; CaO=5,35%.En ce qui concerne la source du fer, il semble que la mise en place du minerai soit liée aux importantes réserves de quartzites ferrugineux situées dans le Précambrien du socle Réguibat, dont le stock métal initial a subi des remobilisations successives.

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The Mecheri Abdelaziz oolitic iron deposit (late Famennian of Tindouf Basin) shows morphological and sedimentological features of prograding deltaic sequences.The mineralization is complex and allows the recognition of 3 types of paragenesis: maghemite-magnetite, bavalite-hematite, bavalite-magnetite; and 4 types of facies: microconglomerate facies, oolites dispersed in a matrix with detritic elements, oolites dispersed in a matrix without detritic elements and oolites with cement.The comparison of oolites and matrix suggests that the orebodies are the result of oolites deposition, developed by intrasedimentary processes and before the final deposition, by alternating phases of oolitization and suspension.For the whole ore deposit, the main composition is: total Fe=43.49%; SiO₂=13.66%; P₂O₅=2.31%; CaO=5.35%.The major portion of iron was probably provided by continental leaching of the very important iron formations of the Precambrien Reguibat shield.

     

Sedimentation cycle of a freshwater tidal flat in the St. Lawrence estuary

Cap-Tourmente tidal flat is located on the north shore of the St. Lawrence middle estuary, 50 km downstream from Quebec city, Canada. Seasonal variations in sedient accretion were studied during the period without ice from May to December, using horizontal plates in conjunction with vertical stakes. The sedimentological regime which is characterized by very rapid changes, can be divided into four periods. There are two periods of intense erosion in May and October–November; during these months, erosion is initiated by walking and beak probing of very large flocks of snow geese. During the summer, very rapid accretion occurs for approximately 100 days; during this period, the sedimentation rate is 2 mm per day in the lower section of the tidal marsh which is covered with vegetation. Among the numerous factors influencing the sedimentation cycle, the very gradual slope of the upper part of the flat (0.25%) seems to be one of the most important because it results in a very large area (550 m wide) which is protected from strong tidal currents. This latter area is covered with dense and high vegetation which provides an effective protection against erosion unitl the snow geese arrive in October and raze it in a few days. The presence of vegetation reduces the velocities of tidal currents, particularly during ebb and modifies their direction for both flood and ebb, orienting them perpendicularly to the main tidal flow.     

Volatile C1C8 organic compounds in sediments from the Peru upwelling region

Volatile C₁C₈ organic compounds were analyzed in seven cores recovered from a transect across the Peru Shelf Upwelling Region. Compounds detected in ng/g dry weight quantities included methane, ethane, propane and other C₄C₇ alkanes as well as functionalized compounds including alkenes, furans and aldehydes. Strong correlations were observed between some groups of compounds with similar structures. Maxima in two groups of compounds were observed: (1) C₁, C₂, C₃, furans, and butanals (plus sporadic alkanes) as nitrate levels decreased below about 2–4 μM in the subsurface, and (2) C₄C₆ alkenes together with predominantly branched and cycloalkanes at the sediment-water interface in cores recovered from oxygenated bottom waters. The levels of C₁C₇ alkanes in these sediments are comparable to levels found in DSDP cores buried to depths of less than 600 m (or not exposed to geothermal temperatures exceeding about 30°C).     

Composition and origin of fluids associated with lode gold deposits in a Mesoarchean greenstone belt (Warrawoona Syncline, Pilbara Craton, Western Australia) using synchrotron radiation X-ray fluorescence

Microthermometry and Raman spectroscopy techniques are routinely use to constrain ore-fluids δ¹⁸O and molar proportions of anhydrous gas species (CO₂, CH₄, N₂). However, these methods remain imprecise concerning the ore-fluids composition and source. Synchrotron radiation X-ray fluorescence allows access to major and trace element concentrations (Cl, Br and K, Ca, Fe, Cu, Zn, As, Rb, Sr) of single fluid inclusion. In this paper, we present the results of the combination of these routine and newly developed techniques in order to document the fluids composition and source associated with a Mesoarchaean lode gold deposit (Warrawoona Syncline, Western Australia). Fluid inclusion analyses show that quartz veins preserved records of three fluid inclusion populations. Early fluids inclusions, related to quartz veins precipitation, are characterized by a moderate to high Br/Cl ratio relative to modern seawater, CO₂ ± CH₄ ± N₂, low to moderate salinities and significant base metal (Fe, Cu, Zn) and metalloid (As) concentrations. Late fluid inclusions trapped in secondary aqueous fluid inclusions are divided into two populations with distinct compositions. The first population consists of moderately saline aqueous brines, with a Br/Cl ratio close to modern seawater and a low concentration of base metals and metalloids. The second population is a fluid of low to moderate salinity, with a low Br/Cl ratio relative to modern seawater and significant enrichment in Fe, Zn, Sr and Rb. These three fluid inclusion populations point to three contrasting sources: (1) a carbonic fluid of mixed metamorphic and magmatic origin associated with the gold-bearing quartz precipitation; (2) a secondary aqueous fluid with seawater affinity; and (3) a surface-derived secondary aqueous fluid modified through interaction with felsic lithologies, before being flushed into the syncline. Primary carbonic fluids present similar characteristics than those ascribed to Mesoarchaean lode gold deposits. This suggests similar mineralization processes for mid- and Mesoarchaean lode gold deposits despite contrasting fluid–rock interaction histories. However, in regard to the protracted history documented in the Warrawoona Syncline, we question the robustness of the epigenetic crustal continuum model, as ore-fluid characteristics equally support an epigenetic or a polyphased mineralization process.