The role of tectonics, structures and etch processes for the present relief in glaciated Precambrian besement rocks of SW Sweden

Sammanfattning: In high latitudes, landscapes of low relief are known to contain residues of pre-glacial deep weathering, or etching, which testify to limited modification of the pre-glacial relief by the Pleistocene ice sheets. In this thesis landforms in all scales are classified according to their process of formation. Etched landforms are identified and used as a basis to describe the subsequent glacial reshaping. Large- and medium-scale landform characteristics and patterns of spatial distribution are mapped using digital elevation data on a 50-m grid and air-photos. Medium- and small-scale landforms are studied and documented in field. It is shown that landforms within the glacially scoured Precambrian basement of SW Sweden are closely related to the principal structural pattern of the bedrock. It is concluded that this pattern is more compatible with differential etching than glacial erosion, even if multiple ice flows in different directions are considered. Etched rock basins occur in all levels along the Swedish west coast. Their shape and distribution are clearly adjusted to the joint pattern as shown by their location along joints or joint intersections. The basins coalesce as they grow and start to form a maze-like pattern of topographic depressions separated by more compact upland surfaces. Basin development is identified as an important component of long-term landform evolution and the transformation of a primary upland into a surface of low relief with residual hills. Other forms are directly related to the development of basins. High cliffs occur where master joints form marginal scarps to the basin. Basement areas in between master joints form hills. Deep narrow clefts have developed through etching and in their lower parts small-scale etchforms like corestones, corestone initiations, deeply weathered sheet slabs and thick exfoliation occasionally have been preserved throughout the glaciations. Glacial reshaping of the ancient landforms is studied along a profile crossing three different types of palaeorelief, ranging from a pediplain to an etchsurface with relative relief of 20-135 m. It is shown that increased relief promotes enhanced glacial reshaping, but the magnitude of glacial erosion is considerably less than the amplitude of the palaeorelief. The pre-glacially etched relief is decisive for the efficiency of glacial erosion and limited glacial reshaping results when the ice flow crosses this structural relief. The thesis emphasises the importance of using a long-term evolutionary approach to glacial landscapes in high latitudes, as well as landform analysis, to separate the sequence of exogenic processes that have formed the landscape.