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Structures and Ice Deformation in the White Glacier, Axel Heiberg Island, Northwest Territories, Canada

  • M.J. Hambrey (a1) and F. Müller (a1)

Abstract

The major structures in the long, narrow tongue of a sub-polar valley glacier are described: namely, longitudinal foliation, crevasses, clear-ice layers related to crevasses, debris-rich layers (frequently referred to as thrust or shear planes in the past), and folds. The foliation is vertical, is as well-developed in the centre of the glacier as at the margins, and does not, apparently, form perpendicular to the principal compressive strain-rate axis, nor exactly parallel to a line of maximum shearing strain-rate, although it sometimes approximately coincides with the latter. The intensity of foliation development is not related to the magnitude of the strain-rates, but the structure consistently lies parallel to flow lines through the glacier. There is no critical extending strain-rate, as such, associated with the development of new crevasses. Some crevasses have formed where the principal extending strain-rate is as low as 0.004 a-1 while, in other areas, extending strain-rates of 0.163 a-1 have not always resulted in fracturing. Prominent clear-ice layers, referred to as crevasse traces as displayed at the glacier surface, have formed in crevasse belts parallel to the main fracture directions. These are interpreted either as tensional veins or as the result of the freezing of water in crevasses. Extension parallel to the layering occurs during flow and, near the snout, the surface dip decreases rapidly. The fact that the crevasse traces can be followed to the snout implies that fracture occurs almost to the bottom of the glacier in the source area of the traces. Near the snout, debris-rich layers have developed parallel to the crevasse traces; frequently these are marked by prominent ridge-like ice-cored moraines. It is suggested that these structures are formed by a combination of basal freezing and thrusting. Isoclinal and tight similar folds on all scales are present. Some may be formed by the passive deformation of clear-ice layers as a result of differential flow; others may arise from the lateral compression of the original stratification in areas where ice flow becomes constricted by the narrowing of the valley. An axial plane foliation sometimes is associated with these folds.

Résumé

On décrit les principales structures rencontrées sur la langue étroite et longue d'un glacier de vallée subpolaire, à savoir la foliation longitudinale, les crevasses, les niveaux de glace claire liés aux crevasses, les niveaux riches en sédiments (souvent considérés comme des traces d'anciens plans de cisaillement ou de poussée) et des ogives. La foliation est verticale et aussi bien développée au centre du glacier que sur ses bords. Elle ne se forme pas, apparemment, perpendiculairement à I'axe du principal effort de compression, ni exactement parallèlement à une ligne de cisaillement maximum, bien qu'elle coïncide quelquefois approximativement avec cette dernière. L'intensité de la foliation n'est pas en rapport avec l'ordre de grandeur de la vitesse de déformation, mais cette structure reste fidèlement parallèle aux lignes de courant dans le glacier. Il n'y a pas de vitesse critique de déformation en tant que telle liée au développement de nouvelles crevasses. Des crevasses se sont formées avec des vitesses de déformation principales aussi faibles que 0,004 a-1 tandis que, en d'autres lieux, des vitesses de déformation de 0.163 a-1n’ ont Pas encore abouti à une rupture. Des niveaux de glace claire en relief interprétés comme des traces de crevasses ayant disparu de la surface du glacier se sont formés parallèlement aux principales directions de fracturation des ceintures de crevasses. Celles-ci sont considérées soit comme des filons de tension, soit comme le résultat du regel d'eau dans les crevasses. Au cours de l écoulement, des tractions parallèles aux stratifications se produisent et, près du front, le pendage superficiel décroît rapidement. Le fait que les traces des crevasses puissent être suivies jusqu'au front implique que la fracture se produit presque au fond du glacier dans la zone d'accumulation. Près du front, des niveaux riches en sédiments se sont développés parallèlement aux traces des crevasses; souvent ils sont marqués par des cordons morainiques à coeur de glace. On suggère que ces structures se forment grâce à une combinaison de gel basal et de poussée. Des sortes de plissements isoclines el serrés de toutes échelles sont présents. Certains peuvent résulter de la déformation passive de niveaux de glace claire résultant d'écoulement différentiels; d'autres peuvent venir de la compression latérale d'une stratification originelle dans les secteurs où le courant de glace se trouve comprimé par le rétrécissement de la vallée. Une foliation sur le plan axial est quelquefois associée à ces plis.

Zusammenfassung

Es werden die wesentlichen Strukturen in der langen. schmalen Zunge eines subpolaren Talgletschers,nàmlich Làngsbânderung, Spalten, Blankeisschichten in Verbindung mit Spalten, schuttreiche Schichten (oft gedeutet als fruhere Schub- oder Scherebenen) und Falten beschrieben. Die Banderung verlauft vertikal und ist in der Mitte des Gletschers ebenso gut entwickelt wie an den Randern. Sie bildet sich offensichtlich weder senkrecht zur Hauptachse der Druckverformungsrate noch parallel zur Linie der maximalen Scherver-formungsrate. obwohl sie manchmal annâhernd mit letzterer zusammenfallt. Die Intensitat der Bânderungs-entwicklung steht nicht in Beziehung zur Grosse der Verformungsraten, aber die Struktur liegt durchwegs parallel zu Stromlinien durch den Gletscher. Es gibt keine kritische Zugverformungsrate als solche, die mit der Bildung neuer Spalten verknupft ware. Einige Spalten haben sich an Stellen geofflnet, wo die Hauptzug-spannungsrate nur 0,004 a-1 betrug, wahrend in anderen Gebieten Zuspannungsraten von 0,163 a-1nicht immer zum Bruch fuhrten. Deutliche Blankeisschichten, die als Spaltenspuren an der Gletscheroberflâche anzusehen sind, haben sich parallel zu den Hauptbruchrichtungen in Spaltenzonen gebildet. Sie werden entweder als Spannungsadern oder als Ergebnis des Gefrierens von Wasser in Spalten gedeutet Wahrend des Fliessens tritt eine Ausdehnung parallel zur Schichtung ein; nahe der Zunge nimmt die Oberflachen-neigung rasch ab. Die Tatsache, dass die Spaltenspuren bis zur Zunge verfolgt werden konnen, lasst darauf schliessen, dass der Bruch im Entstehungsgebiet der Spalten nahezu bis auf den Grund des Gletschers reicht. Nahe der Zunge haben sich schuttreiche Schihten parallel zu den Spaltenspuren entwickelt; oft sind sie durch hervortretende, ruckenahnliche Moranen mit Eiskern markiert. Es lasst sich vermuten, dass diese Strukturen durch eine Kombination von Gefrieren am Untergrund und Schub gebildet werden. Isoklinale und dichte, ahnliche Falten sind vorhanden. Einige davon konnen sich durch die passive Deformation von Blankeisschichten infolge unterschiedlicher Fliessverhaltnisse gebildet haben ; andere konnen aus der seitlichen Kompression der ursprunglichen Schichtung in Gebieten, wo der Eisfluss infolge der Talverengung konver-giert, stammen. Mit diesen Falten ist manchmal eine achsiale, ebene Bânderung verbunden.

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References

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Structures and Ice Deformation in the White Glacier, Axel Heiberg Island, Northwest Territories, Canada

  • M.J. Hambrey (a1) and F. Müller (a1)

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