Structural Style of Subglacial Shear Zones

Structures in the deforming bed are produced by progressive simple shear, disregarding relatively minor deviations from this ideal deformation model. This deformation history leads to a monoclinic symmetry in three dimensions (Passchier and Trouw, 1996). This shows in two-dimensional view (in the plane of shearing) as a typically asymmetric structural geometry, which is the most reliable indicator of tectonic movement and therefore ice movement direction. Structural styles of brittle and ductile shear zones are summarized below. They are treated in greater detail in Van der Wateren et al. (2000).

Brittle- and ductile-style structures usually coexist in shear zones in unlithified sediments. Brittle-style structures usually form in the more competent coarse-grained sediments, while clays show more ductile behaviour. Generally this is a matter of scale, as macroscopically, ductile deformation may be the cumulative effect of numerous microscale discrete displacements. Figure 8.10 explains the ductile and brittle shear zone structures, which are common in tills on all scales varying from microscale to outcrop scale. The fabric is similar to that of many mylonites and cataclastic shear zones and a matching terminology to describe these features (Passchier and Trouw, 1996) is adopted here.

A ductile shear zone contains shear band surfaces with a characteristic asymmetry in the plane of shearing (Fig. 8.10a). Parallel-oriented platy and prismatic mineral grains define a penetrative cleavage of S surfaces, which is oriented roughly parallel to the direction of maximum finite extension. In thin section and in crossed polarized light, the S cleavage shows a dominant birefringence direction (illumination) of clay minerals. Rotation of skeletal grains (sand-to-pebble size) leads to preferred orientations toward the shear direction. The common method of measuring clast orientations to determine the till fabric is based on this principle. C and C' planes are more widely spaced shear bands also known as extensional crenulation cleavages (ECC), with the C surfaces oriented parallel to the boundaries of the shear zone. As in mylonites (e.g. Passchier and Trouw, 1996) the two most common combinations of cleavages are termed S-C fabric (Fig. 8.10b) and S-C' fabric (Fig. 8.10c).

Figure 8.10d explains the geometry of Riedel shear surfaces (Y, P, R and R') in a brittle shear zone. Tension veins (T) are oriented perpendicular to the direction of maximum extension. They are filled with either fluidized sorted sediment or till and are equivalent with injection veins, till wedges and clastic dikes described in numerous till sections (e.g. Âmark, 1986; Dreimanis, 1992; Larsen and Mangerud, 1992; Dreimanis and Rappol, 1997; Rijsdijk et al., 1999; Van der Wateren, 1999). The characteristic asymmetry in the plane of shearing makes it relatively easy to determine the shear direction — in this case dextral (clockwise) shearing. The fabric has a markedly higher degree of symmetry in sections perpendicular to the direction of shearing. Figure 8.11

Reidel Strain Elpsiod

Figure 8.10 Ductile (A-C) and brittle (D) shear zone structures, which are common in tills on a microscale as well as on outcrop scale. Simple shear produces structures with monoclinic symmetry (asymmetric in the plane of shearing - see strain ellipse), which is an important indicator of the direction of shearing and, therefore, in tills of the ice flow direction. The fabric is similar to that of many mylonites and cataclastic shear zones: A) Geometry of shear band surfaces and symmetry of a dextral ductile shear zone. C and C' planes also known as extensional crenulation cleavages (ECC). B) S-C fabric. C) S-C' fabric. D) Geometry and shear direction of Riedel shears (Y, P, R and R') in a dextral brittle shear zone. T indicates tension veins oriented perpendicular to the direction of maximum extension, which are filled with fluidized sediment. The diagram also shows a strain ellipse produced by brittle fracture.

Figure 8.10 Ductile (A-C) and brittle (D) shear zone structures, which are common in tills on a microscale as well as on outcrop scale. Simple shear produces structures with monoclinic symmetry (asymmetric in the plane of shearing - see strain ellipse), which is an important indicator of the direction of shearing and, therefore, in tills of the ice flow direction. The fabric is similar to that of many mylonites and cataclastic shear zones: A) Geometry of shear band surfaces and symmetry of a dextral ductile shear zone. C and C' planes also known as extensional crenulation cleavages (ECC). B) S-C fabric. C) S-C' fabric. D) Geometry and shear direction of Riedel shears (Y, P, R and R') in a dextral brittle shear zone. T indicates tension veins oriented perpendicular to the direction of maximum extension, which are filled with fluidized sediment. The diagram also shows a strain ellipse produced by brittle fracture.

summarizes structures produced by progressive simple shear, indicating how they can be used as kinematic indicators. For ease of comparison, all structures indicate dextral simple shearing. The cross section of sheath folds — strongly attenuated tube- or sock-shaped structures parallel to the shear direction — are in a plane perpendicular to the shear direction.

Shear sense indicators (dextral shearing)

Asymmetry in plane of shearing, symmetry in plane _Lshear direction:

Riedel shears

Tension (injection) veins

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