Description and chemical composition of manganese nodules during R/V "Tyro" cruise APNAP 1 to the Madeira Abyssal Plain in 1986

The porewater and sediment composition of two boxcores and of a small gravity core, taken on a manganese-nodule-covered hill and in the Madeira Abyssal Plain proper respectively, are compared. The pore-water study of the two boxcores indicates that oxic conditions prevail in both cores. In addition, it indicates that no detectable fluxes of Mn or Fe occur from the porewater to the ocean bottom water. Variations in the geochemical composition of the sediments can be explained by fluctuations in the amount of carbonate, which acts as a diluting agent. A clear carbonate minimum is observed at 20-22 cm depth in the two cores. This minimum is likely to be associated with the last glacial period (10-20 kyr B.P.). This association is supported by the sediment accumulation rate of 15 mm/kyr as found by extrapolation from the rate for pelagic sediments in the Madeira Abyssal Plain. The bulk composition of the manganese nodules recovered from the submarine hill is chemically almost identical to the average composition of Atlantic nodules. The trace metal and Rare Earth Elements composition indicate a hydrogenous origin for the manganese nodules of this study. On the basis of the chemical composition, and that of nodules relative to that of the adjacent sediments, an average nodule accretian rate of 2.8-3.3 mm/myr has been calculated. Although the analyses of the entire ferromanganese nodules that have been studied seem to indicate a homogenous composition, internal structures of the nodules reveal great inhomogeneity, both visually and chemically. These fluctuations may be related to variations in the fluxes of Mn and Fe, which in turn could be climate-related.

Slope stability analyses based on sediment cores of the Ligurian Margin, Southern France

Submarine slope failures of various types and sizes are common along the tectonic and seismically active Ligurian margin, northwestern Mediterranean Sea, primarily because of seismicity up to ~M6, rapid sediment deposition in the Var fluvial system, and steepness of the continental slope (average 11°). We present geophysical, sedimentological and geotechnical results of two distinct slides in water depth >1,500 m: one located on the flank of the Upper Var Valley called Western Slide (WS), another located at the base of continental slope called Eastern Slide (ES). WS is a superficial slide characterized by a slope angle of ~4.6° and shallow scar (~30 m) whereas ES is a deep-seated slide with a lower slope angle (~3°) and deep scar (~100 m). Both areas mainly comprise clayey silt with intermediate plasticity, low water content (30-75 %) and underconsolidation to strong overconsolidation. Upslope undeformed sediments have low undrained shear strength (0-20 kPa) increasing gradually with depth, whereas an abrupt increase in strength up to 200 kPa occurs at a depth of ~3.6 m in the headwall of WS and ~1.0 m in the headwall of ES. These boundaries are interpreted as earlier failure planes that have been covered by hemipelagite or talus from upslope after landslide emplacement. Infinite slope stability analyses indicate both sites are stable under static conditions; however, slope failure may occur in undrained earthquake condition. Peak earthquake acceleration from 0.09 g on WS and 0.12 g on ES, i.e. M5-5.3 earthquakes on the spot, would be required to induce slope instability. Different failure styles include rapid sedimentation on steep canyon flanks with undercutting causing superficial slides in the west and an earthquake on the adjacent Marcel fault to trigger a deep-seated slide in the east.

(Table 1) 40Ar/39Ar plateau and isochron ages for basalts from the Réunion hotspot track

Concordant plateau and isochron ages were calculated from 40Ar/39Ar incremental heating experiments on volcanic rocks recovered by drilling at four Leg 115 sites and two industry wells along the volcanic lineament connecting Reunion Island to the Deccan flood basalts, western Indian Ocean. The new ages provide unequivocal evidence that volcanic activity migrated southward along this sequence of linear ridges. The geometry and age distribution of volcanism are most compatible with origin above a stationary hotspot centered beneath Reunion. The hotspot became active with rapid eruption of the Deccan flood basalts, western India, and subsequent volcanic products record the northward motion of the Indian and African plates over the hotspot through Tertiary time. The radiometric ages are in general accord with basal biostratigraphic age estimates, although some adjustments in current magnetobiostratigraphic time scales may be required.

Recalculated petrographic parameters and normalized microprobe analyses of volcanic glasses for DSDP Sites 63-467, 63-468 and 63-469

The southward passage of the Rivera triple junction and its effect on the North American plate are primary controls on the Miocene tectonic evolution of the outer borderland of California. Detrital modes of sand shed off the Patton Ridge and cored by the Deep Sea Drilling Project provide evidence of progressive tectonic erosion of the Patton accretionary prism and neartrench volcanism. Volcanic glass in the sediment is predominantly calcalkaline rhyolite and andesite, typical of subductionrelated volcanism, but also includes minor low-K2O tholeiitic basalt. We attribute these compositional features to interaction with a spreading ridge associated with a possible trench-ridge-trench triple junction along the Patton Escarpment from 18 to 16 Ma. This study suggests that evidence of ridge-trench interaction may be commonly preserved along submerged plate margins, in contrast to its more limited recognition and discussion in the literature based on exposed examples in Chile, Japan and Alaska.

Composition of rocks, minerals, water, and gases from uplift areas of the Central Atlantic

Original geological, geophysical, lithological, mineralogical data on uplifts of the Central Atlantic are given in the book based on materials of Cruise 1 of the R/V Akademik Nikolaj Strakhov. Geological and geophysical studies include description of the obtained material and analysis of structural and morphological elements of the ocean floor. Results of lithological, petrochemical and geochemical studies were extremely innovative and develop a conceptual model. The latter include studies of petrochemical evolution of tholeiitic alkaline plate volcanism, large-scale hydrothermal transformation of basement rocks - palygorskitization, phosphatization and ferromanganese mineralization. Showing imposition Superposition of hydrogenic alteration on hydrothermally altered rocks and its role in Cenozoic history of sedimentation is shown.

Magnetic properties and geochemistry in DSDP Holes 69-504B an d 69-505B

More than 60 basalt samples from two Deep Sea Drilling Project holes on the Costa Rica Rift were studied for magnetic properties and were found to have no properties significantly different from other DSDP basalts. Opaque mineralogical and thermomagnetic properties of these samples, however, to some extent show differences from normal submarine basalts; a new type of thermomagnetic curve and wide range of chemical compositions were recognized. Oxidized samples possibly containing incipient ilmenite exsolution lamellae were reduced and re-equilibrated during heating. The Curie temperatures of the re-equilibrated titanomagnetites are interpreted to be those of the original crystallized phase before oxidation.