Deformation processes in accretionary wedges—examples from the SW margin of the Southern Uplands, Scotland

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Abstract
Summary: The deformation which accompanies the evolution of accretionary complexes at active margins is discussed using examples from the Southern Uplands of Scotland. The area was part of a northward-dipping subduction complex during Ordovician and Silurian times before the final closure of the Iapetus Ocean in the Early Devonian. The structures present in the SW margin of the Southern Uplands may be assigned to four different processes typical of accretionary-prism evolution. These are (a) downslope gravity-driven movement, (b) accretion-related folding and thrusting, (c) late accretion or collision processes, and (d) post-collision adjustments. The problems of recognizing and separating these processes in the Southern Uplands are discussed as well as the similarities which occur between overthrusting processes in foreland thrust-and-fold belts and the underthrusting processes taking place in accretionary complexes. In addition an attempt is made to assess the effects of the changes in deformation behaviour occurring during lithification which accompanies accretion. A model is presented here which relates the evolution of folds and thrusts to the changing material properties of sediments during accretion, and emphasizes the importance of the deformation behaviour of the hanging wall to accreting packages in these processes.