Significant shortening by pressure solution creep in the Dwyka diamictite, Cape Fold Belt, South Africa

• Study of anastomosing cleavage developed at subgreenschist conditions in diamictite.
• The cleavage is defined by fine grained phyllosilicates and minor oxides.
• The cleavage is a pressure solution cleavage with spacing controlled by dropstone size.
• The cleavage is axial planar and its formation contributed to horizontal shortening.

The Dwyka diamictite preserves a record of horizontal shortening related to the development of the Cape Fold Belt at subgreenschist conditions. This shortening was accommodated by folding and thrust faulting, but pressure solution may also have contributed significantly to bulk deformation. Cleavage within the Dwyka group is, in the studied part of the Karoo Basin, subvertical to moderately south dipping, and approximately axial planar to regional folds. The cleavage is anastomosing, leading to the development of ‘tombstone cleavage’, and defined microscopically by thin seams of fine grained dark material. X-ray diffraction analyses show that the diamictite matrix is made up of quartz, feldspars, muscovite and chlorite. Element maps further indicate that the cleavage is defined predominantly by phyllosilicates and minor oxides, implying that it is made up of relatively insoluble material and hydrothermal alteration products. Overall, the cleavage therefore formed by dissolution and removal of mobile elements. This indicates that pressure solution likely accommodated a significant component of shortening during the Cape Orogeny, and provides an example of low temperature cleavage development during orogenesis.