Brittle Shear Zone Structures

Shear zones in unlithified sediments rarely contain either only brittle- or only ductile-style deformations, usually the two are associated. Clay, which typically deforms in a ductile way, is sometimes cut by brittle structures due to strain hardening associated with loss of pore water. This association of brittle structures and ductile structures is illustrated by a shear zone beneath one of the nappes in the Dammer Berge thrust moraine (style D; Van der Wateren, 1995). The sands and gravels of the footwall are cut by sets of Riedel shear planes, while the silts and clays at the base of the overriding nappe contain a transposed foliation of boudins, sand lenses and intrafolial folds (Fig. 8.13). This is a mixture of footwall and hangingwall materials.

This sub-nappe shear zone shows characteristic upward increasing shear strain, which is also common in many tills. At the interface between the glacier sole and the bed, strain rates are at maximum (Boulton, 1987; Boulton and Hindmarsh, 1987). The intensity of subglacial deformation therefore increases from the in situ footwall sediments to the top of the deforming layer, which carries the most allochthonous elements. Following Van der Wateren (1987), and similar to Hart and Boulton (1991), the shear zone can be subdivided in units Sr, Sb and Sh, grading from relatively intact to completely homogenized sediment (Fig. 8.13). Homogenized unit Sh at the base of the overriding nappe contains boudins of a Cretaceous clay. They contain a

Brittle Structures

Figure 8.13 Association of brittle structures (Riedel shears) and ductile structures (boudins, transposed foliation, intrafolial folds) in a shear zone beneath the uppermost nappe in the Dammer Berge thrust moraine (Van der Wateren, 1995). The shear strain increases upward, which is also representative of many tills. Unit Sr contains rooted structures (top of lower nappe). Sb in the lower half of the shear zone consists of a transposed foliation of boudins and detached intrafolial folds of footwall material in a more allochthonous matrix. Sh, the unit with the highest finite strain, is a completely homogenized mixture of local (footwall, nappe III) and far-travelled material (hangingwall, nappe IV). (After Van der Wateren, 1987). This unit contains boudins of a Cretaceous clay which has been transported from a level —150 m below the outcrop.

Figure 8.13 Association of brittle structures (Riedel shears) and ductile structures (boudins, transposed foliation, intrafolial folds) in a shear zone beneath the uppermost nappe in the Dammer Berge thrust moraine (Van der Wateren, 1995). The shear strain increases upward, which is also representative of many tills. Unit Sr contains rooted structures (top of lower nappe). Sb in the lower half of the shear zone consists of a transposed foliation of boudins and detached intrafolial folds of footwall material in a more allochthonous matrix. Sh, the unit with the highest finite strain, is a completely homogenized mixture of local (footwall, nappe III) and far-travelled material (hangingwall, nappe IV). (After Van der Wateren, 1987). This unit contains boudins of a Cretaceous clay which has been transported from a level —150 m below the outcrop.

Campanian-Maastrichtian fauna and originate from Cretaceous beds subcropping in the glacial basin north of the thrust moraine, ~150 m below the level of the outcrop.

Observations of till sections in Germany and The Netherlands indicate that the intensity of deformation quite commonly increases upwards from undeformed sediments to a strongly sheared and homogenized diamict at the top (Van der Wateren, 1987, 1995; Kluiving et al., 1991). This produces a petrographic layering where higher levels contain clasts of distant provenance, whereas those from lower levels have a more local origin (Rappol and Stoltenberg, 1985; Rappol et al., 1989; Boulton, 1996a). Subglacially deforming sediments are incorporated into the shear zone from below, and consequently particles higher up in the subglacial shear zone have travelled greater distances than have those in the bottom layers.

In a cliff section on the Baltic coast near Heiligenhafen, northern Germany, three Pleistocene tills can be distinguished on the basis of structural and petrographic characteristics. The Lower and Middle Tills had previously been ascribed to the Saalian, and the Upper Till to the Late Weichselian (Stephan et al., 1983). In a more recent evaluation Stephan (2002) recognized three stratigraphically different tills below the Weichselian Upper Till.

An alternative explanation holds that the Lower and Middle Tills constitute one single subglacial shear zone with relatively autochthonous sediments in the bottom part and more exotic far-travelled material at the top (Van der Wateren, 1999, 2002b). This is corroborated by a parallel trend towards increasing deformation (finite strain) from the bedrock to the top of the section. The most obvious aspect of this latter trend is the massive appearance of the Middle Till, which can be interpreted as the result of homogenization by repeated folding and attenuation of sediment lenses that have been incorporated into the till.

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