Trench triple junction off Central Japan—preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2

Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.     

Magma and hydrothermally driven sector collapses: The 3100 and 11,500 y. B.P. eruptions of la Grande Decouverte (la Soufrière) volcano, Guadeloupe, French West Indies

We recently reported (Boudon et al., 1984) on an eruption similar to that of May 18, 1980 at Mount St. Helens, that took place about 3100 years ago at la Soufrière, Guadeloupe. During the course of detailed geological mapping of the deposits of this event, older debris flow and blast deposits were recognized in the northern sector of the mapped area. Uncarbonized wood fragments in the debris flow have yielded ages ca. 11,500 y. B.P. The deposits extend from an amphitheater crater westward to the caribbean shore about 10 km downslope from the volcano. The deposits and crater structure suggest that they are the result of catastrophic flank failure like the event 3100 years ago. Unlike the latter activity, however, no magmatic component is found in the deposits.     

A 10,000 yr B.P. extensive ice shelf over Viscount Melville Sound,Arctic Canada

Late Wisconsinan age glacial landforms and deposits indicate that an ice shelf of at least 60,000 km² flowed northwestward into Viscount Melville Sound, probably from the M'Clintock Dome of the Laurentide Ice Sheet. The ice shelf overlapped coastal areas and laid Winter Harbour Till up to 125 m above present sea level on the southern coast of Melville Island, to 135 m on Byam Martin Island, to possibly 90 m on the northeast tip of Banks Island, and to 150 m on the north coast of Victoria Island. The contemporary sea level was 50 to 100 m higher than present (it now rises eastward). A maximum age of 10,340 ± 150 yr B.P. for the till, and thus the ice-shelf advance, is provided by shells in marine sediments which underlie it, whereas a minimum age of 9880 ± 150 yr B.P. is provided by overlying shells that postdate the ice advance. The major advance of shelf ice into Viscount Melville Sound may be the result of the rapid disintegration of the M'Clintock Dome while the climate ameliorated in the western Arctic.     

Jet streams,thunderstorms, and Project Skyfire

Summary More than 70% of forest fires in the northwestern United States are caused by lightening. The Project Skyfire is an attempt to learn more about the atmospheric conditions leading to thunderstorms in this area and to investigate the possibilities of an artificial influence on their development by cloud seeding. The experiences gained with the training of amateur observers as well as the first scientific results of the project are briefly discussed. During the first year of observations, for instance, there was found to exist an intimate relationship between a certain type of fast moving thunderstorms and the intensity and position of the jet stream. Finally, the great economical value of researches in the field of applied meteorology is stressed.

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Zusammenfassung Über 70% der Waldbrände im Nordwesten der Vereinigten Staaten werden durch Blitze entzündet. Das Project Skyfire hat es sich zum Ziel gesetzt, die atmosphärischen Bedingungen, welche zur Bildung von Gewittern in diesem Gebiet führen, zu untersuchen und die Möglichkeiten einer künstlichen Beeinflussung durch seeding zu erforschen. Die gesammelten Erfahrungen mit der Schulung von Amateur-Beobachtern, sowie die bisherigen wissenschaftlichen Ergebnisse werden kurz diskutiert. Schon im ersten Jahr der Beobachtungen ergaben sich interessante Zusammenhänge zwischen einer bestimmten Art von rasch ziehenden Gewittern und dem Jet-Stream. Schließlich wird auf den weitreichenden wirtschaftlichen Nutzen großzügiger Forschungen auf dem Gebiet der angewandten Meteorologie hingewiesen.

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Résumé Plus de 70% des incendies de forêts dans le Nord-Ouest des Etats-Unis sont dûs à la foudre. Le «Project Skyfire» a pour but d'étudier les conditions atmosphériques conduisant dans ces régions à la formation d'orages ainsi que les possibilités d'agir sur cette formation au moyen d'inséminations artificielles. On discute brièvement les expériences faites en instruisant des observateurs amateurs ainsi que les résultats scientifiques. Dès la première année de recherches, on a constaté d'intéressantes relations entre un certain type d'orages rapides et le jet stream. Enfin on attire l'attention sur le grand intérêt économique d'études étendues dans le domaine de la métérologie appliquée.

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With 7 Figures.

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The Munitalp Foundation, Inc.     

Ordnance Survey Policy and Practical Implications for The Revision of Rural Mapping

During 1994 a project was initiated by Ordnance Survey to investigate the requirement for completing the multi-million pound digital revision of 1:2500 scale mapping within Great Britain. This paper, although concentrating on this scale of mapping, discusses the revision policy issues that faced Ordnance Survey and the practical implications arising from implementing the policy for its 1:2500 and 1:10 000 basic scale mapping. Use of contractors was seen as a prime requirement as the resources available within Ordnance Survey were not sufficient to implement the full revision programme. This paper discusses the background to the task and considers the range of photogrammetric options and contractual and quality procedures required for the revision of the mapping over the next few years.     

Regular spacing of drainage outlets from linear fault blocks

Outlets of river basins located on fault blocks often show a regular spacing. This regularity is most pronounced for fault blocks with linear ridge crests and a constant half-width, measured perpendicular to the ridge crest. The ratio of the half-width of the fault block and the outlet spacing is used in this study to characterize the average shape (or spacing ratio) of 31 sets of drainage basins. These fault-block spacing ratios are compared with similar data from small-scale flume experiments and large-scale mountain belts. Fault-block spacing ratios are much more variable than those measured for mountain belts. Differences between fault-block spacing ratios are attributed to variability in factors influencing the initial spacing of channel heads and subsequent rates of channel incision during the early stages of channel network growth (e.g. initial slope and uplift rate, precipitation, runoff efficiency and substrate erodibility). Widening or narrowing of fault blocks during ongoing faulting will also make spacing ratios more variable. It is enigmatic that some of these factors do not produce similar variability in mountain belt spacing ratios. Flume experiments in which drainage networks were grown on static topography show a strong correlation between spacing ratios and surface gradient. Spacing ratios on fault blocks are unaffected by variations in present-day gradients. Drainage basins on the Wheeler Ridge anticline in central California, which have formed on surfaces progressively uplifted by thrust faulting during the last 14 kyr, demonstrate that outlet spacing is likely to be determined during the early stages of drainage growth. This dependency on initial conditions may explain the lack of correlation between spacing ratios of fault blocks and slopes measured at the present day. Spacing ratios determine the location of sediment supply points to adjacent areas of deposition, and hence strongly influence the spatial scale of lateral facies variations in the proximal parts of sedimentary basins. Spacing ratios may be used to estimate this length scale in ancient sedimentary basins if the width of adjacent topography is known. Spacing ratio variability makes these estimates much less precise for fault blocks than for mountain belts.     

Biological record of added manganese in seawater: a new efficient tool to mark in vivo growth lines in the oyster species Crassostrea gigas

The biological response of increased manganese in seawater was tested experimentally with the oyster species Crassostrea gigas by adding, once per day, a fixed quantity of MnCl₂ to the container where the oysters were living. Uptake of Mn²⁺ in the shell was traced with cathodoluminescence and quantified with a high spatial resolution proton microprobe. The daily addition of MnCl₂ resulted in the visualization of distinct growth increments seen simultaneously in both the calcitic shell and the aragonitic ligament. A relation was observed between the addition of Mn²⁺ to the seawater and incorporation of Mn in the mineral part of the shell. Thus, addition of MnCl₂ to seawater is an efficient tool to mark in vivo growth increments in bio-mineralised carbonates.