A linkage criterion for segmented normal faults

A detailed field study of 39 centimetre- to metre-scale relay ramps from two outcrops was performed to investigate the development of a linkage criterion for segmented normal faults. We analysed the displacement distribution and the geometry of fault arrays containing three types of relay ramp: open, linked, and fully breached, in order to identify which parameters are relevant to fault linkage, and to establish a linkage criterion. Each relay ramp geometry has a specific graphical field on a relay displacement–separation diagram. The field including all the linked geometries (initiation of linkage) separates open and fully breached relay ramps and is interpreted as a value of relay displacement to separation ratio for which faults link during their overlap. A ‘linkage threshold’, in each studied fault system, is defined as the best-fit linear trend of linked relays. We discuss the scaling and the variability of the linkage criterion using published datasets from a wide variety of settings and scales. The observed linkage threshold is linear, with a slope value varying less than one order of magnitude. This suggests that linking relay ramps have self-similar geometries from centimetre- to kilometre-scale and that normal fault linkage is governed by similar fault interaction across a broad range of scales. The linkage criterion, which can be an effective tool to estimate relay ramp geometry at depth or at the earth surface, could therefore be used to improve investigations in determining fluid entrapment or in the evaluation of potential surface of seismic ruptures.     

Nine Fallacies of Floods

A number of important misconceptions or fallacies stand in the way of a better understanding of the nation's flood problem. The fallacies are not universal, with many flood experts, decision makers, and sectors of the public escaping their seductive logic. But enough people do fall prey to these fallacies of floods so as to create obstacles to improved utilization of the lessons of experience. This paper uses three of these lessons to organize presentation of the nine fallacies:We know the wrong things about the nature of the problem.Fallacy 1: Flood frequencies are well understood.Fallacy 2: Damaging flooding in recent years is unprecedented because of global warming.Fallacy 3: Levees prevent damages.Fallacy 4: Flood forecasts are universally available.We don't know enough about why and with what intensity we should act.Fallacy 5: Societal vulnerability to floods is well understood.Fallacy 6: Data on flood casualties is a proxy for flood risk.Fallacy 7: Data on flood damages is a proxy for flood risk.We know enough about what might be done. Fallacy 8: Knowledge leads to action.Fallacy 9: The U.S. flood problem can be addressed without Federal leadership and support.The purpose of raising the fallacies in this paper is to contribute to a systematic definition of the nation's flood problem.     

Lake Manly(?) Shorelines in the Eastern Mojave Desert, California

Near Mesquite Spring on the southern edge of the Soda Lake basin in the Mojave Desert, there is a shoreline of an ancient lake at an elevation of 340 m above sea level. At present, Soda Lake would overflow at 280 m; a lake surface at 340 m would extend 240 km northward, to the northern end of Death Valley. Shorelines and lacustrine deposits near the Salt Spring and Saddle Peak Hills, 75 km north of Mesquite Spring, are at 180 m; a lake surface at this elevation today would also extend to the northern end of Death Valley. The most prominent shoreline of the pluvial lake that occupied Death Valley during the Pleistocene, Lake Manly, is that of the Blackwelder stand which ended 120,000 yr ago. This shoreline is 90 m above sea level. The Mesquite Spring and Salt Spring Hills shorelines were probably formed by the Blackwelder stand and subsequently displaced with respect to one another, tectonically, due to transpression in the northeastern Mojave Desert and NW–SE extension across Death Valley. This tectonic regime would result in subsidence of Death Valley and the Salt Spring Hills relative to Mesquite Spring. A reconstruction suggests that the topography at the time of the Blackwelder stand would have had a sill near the level of the highest lake, and also one 20 m lower, corresponding to the next most prominent shoreline in Death Valley. Expansion of the lake over these sills would have increased evaporation, thus possibly stabilizing the lake level.     

Un type de minéralisations épigénétiques familières: les filons à antimoine du Massif Central français. Hypothèse de la sécrétion latérale

Résumé Les filons antimonifères étudiés sont encaissés dans les terrains métamorphiques du socle hercynien (district cévenol, district de Brioude-Massiac), ou dans les grès autuniens formant la base de la couverture sédimentaire (Buzeins). La plupart sont contrôlés par des failles de direction NE. Ils sont aussi sous la dépendance de métallotectes lithostratigraphiques plus ou moins stricts. D'autre part, le socle et les dépôts détritiques de base de la couverture sédimentaire (Stéphanien, Autunien) sont caractérisés par une forte anomalie géochimique antimonifère. Les roches encaissant les filons représentent la source possible de l'antimoine, mais aussi du soufre et de tous les éléments des minéraux de gangue. Ces résultats tendent à montrer la familiarité des concentrations filoniennes par rapport à leur encaissant. L'hypothèse d'une concentration par «sécrétion latérale» est étayée par les profils de distribution de l'antimoine en traces à proximité des filons (mise en évidence de zones de drainage probables). Le contenu des filons antimonifères serait donc hérité, par remobilisations successives, d'un stock métallique déjà présent dans les roches avant le métamorphisme.

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The antimoniferous lodes studied in this paper are formed either in the metamorphic rocks of the Hercynian basement (Cévennes and Brioude-Massiac districts), or in the Autunian sandstones which form the lowest part of the sedimentary cover (Buzeins). Most of them are controlled by faults of a NE trend. They also depend upon more or less strict lithostratigraphic metallotects. Moreover, a high antimoniferous geochemical anomaly characterises the rocks of the basement and of the lowest detrital deposits of the sedimentary cover (Stephanian, Autunian). The host rocks of the lodes are a possible source for antimony, and also for sulfur, as for all the elements of the gangue minerals. These results tend to show the consanguinity of the lode concentrations with their enclosing rocks. The hypothesis of a concentration by lateral secretion is supported by the curves of Sb distribution in the rocks near the lodes (revealing probable areas of leaching). The contents of Sb lodes would consequently be inherited, by successive remobilizations, from the metal contents which were already included in the rocks before metamorphism.

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Cette publication résume une Thèse d'Etat, dirigée par M. le Professeur P. Routhier, présentée le 2 Avril 1971 à la Faculté des Sciences de l'Université de Paris VI (11 quai Saint-Bernard, Paris 5°).     

Composition of olivine,silica activity and oxygen fugacity in kimberlite

Two generations of primary olivine are present in kimberlite, rounded phenocrysts (Fo₉₄Fo₉₁) and euhedral groundmass olivines (Fo₉₁Fo_88-5). Rounded phenocrysts less magnesian than Fo₈₈ are considered to be mantle derived xenocrysts; such crystals comprise up to 40% of the phenocrysts material in kimberlite. Calculated silica activity ranges from 10^?1,5 at 1200 °C, 50 kbs, to between 10^?1,6 and 10^?2.4 at 600 °C, 0.5–1.0 kb. Silica buffers involving olivines, pyroxenes and garnet are considered. Oxygen fugacities on the order of 10^?20 bars indicate that kimberlite magmas are highly reduced at the time of groundmass formation.     

The roles of bryozoans in modern coral reefs

Contrary to impressions gained from the literature, collecting in the living reefs of Bermuda, Eniwetok, and Eleuthera implies that bryozoans are common and diversified in at least some modern coral reefs.In all the reefs examined, encrusting cheilostomes are the most abundant and diversified bryozoans. Tuft-like cheilostomes, and lichenoporid and idmoneid cyclostomes, frequently are moderately important. Reteporid and aeteid cheilostomes, and crisiid cyclostomes, are also occasionally encountered.Never the principal frame-builders, the bryozoans do reinforce the reef mass by encrusting the undersides of coral heads, rock ledges, and rock fragments, and by locally partially filling cavities deep within the reef. These animals do not form significant amounts of loose carbonate sediment, nor do they trap or bind such sediment. A few bryozoans drift epiplanktonically through the surface waters around some reefs. Some cyclostome bryozoans may participate in the cryptic sclerosponge-brachiopod community of modern reefs. Correlation of bryozoan morphologic variations with environmental-factor fluctuations may eventually be possible.Of interest to students of living reefs and bryozoans, clarification of the roles played by bryozoans in modern reefs will also be valuable for paleontologists studying both fossil reefs and bryozoan evolutionary history.

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Zusammenfassung Sammlungen an lebenden Korallenriffen in der Nähe Bermudas, Eniwetoks und Eleutheras weisen darauf hin, daß Bryozoen hier häufig und differenziert in den Korallenriffen vorkommen, was bisher in der Literatur wenig beachtet wurde.In all den hier untersuchten Riffen sind die inkrustierenden Cheilostomen die häufigsten und vielfältigsten Bryozoen. Büschel-ähnliche Cheilostome sowie lichenoporide und idmoneide Cyclostome besitzen häufig einige Bedeutung. Außerdem werden dort ab und zu auch reteporide und aeteide Cheilostome und crisiide Cyclostome gefunden.Obwohl sie niemals das primäre Riffgerüst bilden, unterstützen die Bryozoen den Aufbau des Riffes erheblich dadurch, daß sie die Unterseite der Korallenstöcke sowie Felsenkanten und -fragmente inkrustieren. Teilweise füllen die Bryozoen auch Hohlräume, die tief im Riff eingelagert sind. Diese Tiere bilden keine bedeutenden Mengen von Karbonatablagerungen; sie können auch kein loses Sediment einfangen und binden. Manche Bryozoen treiben epiplanktonisch in den oberen Wasserschichten in der Umgebung von Riffen. Einige cyclostome Bryozoen sind in geschützten Bereichen zusammen mit Sclerospongien und Brachiopoden vergesellschaftet. Wahrscheinlich lassen sich einmal Beziehungen zwischen der Morphologie der Bryozoen und der Veränderungen im Biotop herstellen.Die Klärung der Rolle, welche Bryozoen in rezenten Riffen spielen, wird nicht nur das Interesse der Forscher wecken, die die lebenden Riffe und Bryozoen untersuchen, sondern es wird auch den Paläontologen anziehen, der sich mit fossilen Riffen oder der Bryozoen-Evolution befaßt.

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Résumé Contrairement aux impressions données par la littérature, il apparait d'après des collections faites dans les récifs vivants de Bermuda, Eniwetok, et Eleuthera que les bryozoaires s'y présentent souvent et de façons diverses.Dans tous les récifs examinés, les cheilostomes incrustés sont des bryozoaires se présentant le plus souvent et sous des aspects les plus différents. Les cheilostomes comme des touffes, les cyclostomes lichenoporides et idmoneides sont fréquemment de quelque importance. Par ailleurs les cheilostomes rétéporides et aétéides, et les cyclostomes crisides n'y sont rencontrées qu'occasionnellement.Bien qu'ils n'aient jamais bâti le cadre du récif, les bryozoaires en renforcent la masse en s'incrustant au-dessous des têtes coralliennes, des bords et fragments de rochers; ils remplissent aussi en partie des cavités profondes du récif. Ces animaux ne forment pas de quantités significatives de carbonate et ils ne peuvent ni capter ni lier des sédiments détachés. Quelques bryozoaires flottent epiplanktoniquement dans les eaux de surface de quelques récifs. Quelques bryozoaires cyclostomes participent à la communauté cryptique des sclérosponges et brachiopodes dans les récifs modernes. Des corrélations entre les variations morphologiques des bryozoaires et les fluctuations environnantes seront éventuellement possibles.Intéressante pour les chercheurs étudiant les récifs vivants et les bryozoaires, la clarification des rôles joués par les bryozoaires dans les récifs modernes aura aussi de la valeur pour le paléontologue s'occupant de récifs fossiles et d'évolution des bryozoaires.

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Bermuda, Eniwetok Eleuthera , , . ( Cheilostoma, Lichenoporida Idmoneida — Cyclostomata -, Reteporida Aeteida — Cheilostomata -, Cyclostomata. , , , . , . , . . . . , Cyclostomata, Sclerospongia chiopoda. , . - , , , .

     

The sustainability of Australian agricultural production systems: a realistic objective or simply a desirable aim?

Whilst the term sustainability has undergone considerable transformation from its original intended meaning, in the context of agriculture it is considered to have relevance to four aspects of the activity; the maintenance of production, financial returns, employment opportunities, and the stability of the ecosystem. This article's appraisal of market trends suggests that the key issue of substantially improved financial returns for Australian agriculturalists is at present unlikely for some enterprises. What is particularly discouraging is that downward trends have persisted, despite the many efficiency gains in agriculture. Further gains from technological developments and trade liberalisation will be limited for many agricultural sectors. Whilst initiatives to reduce land degradation have achieved a high public profile, greater effectiveness can be achieved in this area. There is some scope for significant improvements in the overall performance of agriculture, but the most achievable gains will probably be confined to those specific enterprises that can use geography to greatest advantage.