The Pasterze glacier, Austria: an analogue of an extensional allochthon
Published:January 01, 2000
Paul Herbst, Franz Neubauer, 2000. "The Pasterze glacier, Austria: an analogue of an extensional allochthon", Deformation of Glacial Materials, Alex J. Maltman, Bryn Hubbard, Michael J. Hambrey
Download citation file:
Structures of the Pasterze glacier (Austria) have been studied in detail and interpreted as representing a natural model of an extensional allochthon formed on top of an orogenic wedge, and also a model for raft tectonics at passive continental margins. Structures of the lower, ablation-controlled portion of the glacier, including ductile structures at the base, are forming close to the melting point of ice, with predominantly brittle structures close to the surface of the glacier. Formation of these structures results from self-weight, gravity-driven spreading, similar to extensional allochthons.
S-planes are common and clearly related to three individual flow units. They show a typical, spoon-like arrangement. These three flow units are bordered by centimetre- to decimetre-wide shear zones which also include shear folds. Brittle structures include ice-mineralized tension gashes, thrust and normal faults, and hybrid, shear-extensional fractures. Together, these structures show that glacial flow is more rapid in middle to upper sectors of the glacier than along the lateral, and lower margins. The coherent upper sheet is behaving in a brittle manner and is elongating slightly along the flow direction by tensional deformation.
The distribution of structures allows three structural domains within the lower, ablation-controlled sector of the glacier to be distinguished: (1) an upper sector with predominant extensional structures due to rapid flow; (2) a lower sector with ductile and brittle thrust faults, penetrating from the ground and dipping strictly opposite to the local flow direction; (3) a few normal faults at the terminus that developed by rapid melting along the steep lower frontal margin of the glacier. These three structural domains are also found within extensional allochthons as exemplified by the Neogene Alpine–Carpathian system where a huge allochthon, partly driven by gravity, extruded from the Eastern Alps towards the Carpathian arc. Three similar structural domains are also found in recent analogue models and field examples of passive continental margins.
Figures & Tables
Deformation of Glacial Materials
The flow of glacier ice can produce structures that are striking and beautiful. Associated sediments, too, can develop spectacular deformation structures, and examples are remarkbly well preserved in Quaternary deposits. Although such features have long been recognized, they are now the subject of new attention from glaciologists and glacial geologists.
This collection of papers addresses how the methods for unravelling deformation structures evolved in recent years by structural geologists can be used for glacial materials, and the opportunities offered to structural geologists by glacial materials for studying deformation in rocks.