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Glaciotectonic Structures |
1 Faculty of Earth Sciences, Vrije Universiteit, de Boelelaan 1085, 1081 HV Amsterdam, Netherlands wateren{at}xs4all.nl
2 Department of Geology, Box 870338, University of Alabama, Tuscaloosa, AL 35487-0338, USA
3 Department of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315, USA
Criteria to distinguish between sediments that have been subglacially deformed and those that are undeformed, or deformed by other mechanisms, are sparse. In this paper we develop structural criteria to reconstruct the deformation history of glacial sediments that can be readily applied in the field as well as to analyses of thin sections of tills and related materials.
Progressive simple shear is the simplest model to describe the deformation history of subglacially deformed sediments. It includes most of their characteristic structural aspects and provides tools for the kinematic analysis of subglacially deformed sediments. Progressive simple shear generates asymmetric structures, in which the principal direction of finite extension is subparallel to the direction of shearing. This is the simple shear fabric’s most distinctive characteristic, and that which most reliably defines the palaeo-ice flow direction. At a moderately strong intensity of deformation a typical shear zone in unlithified sediments may contain folded and strongly attenuated sediment layers, producing a transposed foliation which must not be mistaken for a sedimentary layering. Original sedimentary and deformation structures may completely disintegrate in the most intensely deformed sediments leading to its homogenization, although the typical shear zone fabric may still be identified in thin section.
