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Origin of Foliation in Glaciers

  • Roger Leb. Hooke (a1) and Peter J. Hudleston (a1)

Abstract

Laboratory studies suggest that neither bubbles nor dirt particles migrate rapidly enough in glacier ice to be responsible for the alternating layers of bubbly and clear ice or dirty and clean ice which constitute foliation. We therefore suggest that these variations in bubble or dirt content are inherited from primary inhomogeneities such as may occur in sedimentary stratification in the accumulation region, in crevasse fillings, or during debris entrainment at the base of the glacier: the appearance of these inhomo-geneities is later modified by strain during flow to produce foliation. We consider six types of inhomogeneity, or components of foliation, and show that, at the very large total strains expected in glaciers, all are eventually flattened, stretched out, and rotated to form a layered structure roughly perpendicular to the direction of maximum total shortening. Most characteristics of observed foliation can be explained by this hypothesis. For example, in the marginal zones of polar ice sheets the rapid decrease in dip of foliation with depth and with distance up-glacier from the margin can he explained by a model in which the foliation is assumed to be nearly parallel to the base of the glacier some distance from the margin, and is deformed passively with the ice thereafter. However, some observations of cross-cutting foliations may require localized inhomogeneous shear parallel to the "new" foliation.

Résumé

Des études de laboratoire suggerènt que ni les bulls ni les particules de poussière ne migrent assez rapidement dans la glace de glacier pour être responsables de l'alternance de couches de glace bulleuse et claire ou de glace propre et sale qui constituent la foliation. Nous suggérons donc que ces variations dans la teneur en bulles ou en poussière sont héritées des irrégularités primitives comme il peut s'en produire dans la stratification de la zone d'accumulation par les remplissages de crevasses ou pendant l'entraînement des sédiments à la base des glaciers : l'apparence de ces irrégularités est plus tard modifiée par la déformation durant l'écoulement pour produire la foliation. Nous considérons six types d'irrégularités ou de composantes de la foliation et montrons que, au long de très fortes, déformations totales que l'on doit s'attendre à trouver dans les glaciers, elles sont toutes susceptibles d'être laminées, étirées et retournées jusqu'à former une structure stratifiée grossièrement perpendiculaire à la direction du retrécissement total maximum. La plupart des caraetéristiques des foliations observées peuvent être expliquées par ces hypothèses. Par exemple, dans les zones marginales des calottes glaciaires polaires la rapide diminu-tion de l'épaisseur de la foliation avec la profondeur et la distance du sommet du glacier a la bordure peut être expliquée par un modèle dans lequel on fait l'hypothèse que la foliation est presque parallèle à la base du glacier à quelque distance du bord, et se trouve déformé passivement avec la glace par la suite. Cependartt, quelques observations de foliations recoupant les autres en croix peuvent plakler en faveur de cisaillements localisés irréguliers parallèles à la "nouvelle" foliation.

Zusammenfassung

Aus Laborversuchen geht hervor, dass weder Blasen noch Schmutzpartikel aids schnell genug im Gletschereis bewegen, urn die Wechselschichtung von blasenreichem und klarern Eis bzw. verschmutztem und reinem Eis bewirken zu können, aus der die Bänderung besteht. Wir nehmen deshalb an, dass diese Schwankungen im Blasen- oder Schmutzgehalt von primären Inhomogenitäten herrühren, wie sie in der sedimentären Lagerung im Akkumulationsgebiet, in Spaltenfüllungen oder bei der Schuttaufnahme am Grunde des Gletschers vorkommen: das Erscheinungsbild dieser Inhomogenitaten wird spater durch die Fliessspannungen so verändert, dass die Bänderung entsteht. Wir betrachten 6 Arten von als Komponenten der Bänderung und zeigen, dass alle unter den sehr hohen Gesamtspannungen, die in Gletschern auftreten, im Laufe der Zeit ausgewalzt, gestreckt und gedreht werden, so dass sie eine geschichtete Struktur annähernd senkrecht zur Richtung der maximalen Gesamtkürzung bilden. Die meisten Eigenschaften beobachteter Bänderungen können mit dieser Hypothese erklärt werden. Zum Beispiel lässt sich die rasche Abnahme des Einfallswinkels der Bänderung mit der Tiefe und mit dem Randabstand gletscheraufwarts am Rande polarer Eisdecken durch ein ModeIl erklären, in dens die Bänderung annähernd parallel zum Untergrund des Gletschers in einigern Abstand vom Rand angenomrnen wird, aber mit dem Eis darnach eine passive Deformation erleidet. Einige Beobachtungen von gekreuzten Bänderungen allerdings können es erforderlich machen, eine lokale, inhomogene Scherung parallel zu der "neuen" Bänderung anzunehmen.

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References

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Origin of Foliation in Glaciers

  • Roger Leb. Hooke (a1) and Peter J. Hudleston (a1)

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