Variation in fold axis geometry and slaty cleavage microfabric associated with a major fold arc, Fleurieu Peninsula, South Australia

The slaty cleavage in the sigmoidal Adelaide Fold Belt of South Australia is a predominantly planar fabric with only a weak down‐dip lineation. However, in a gradation towards the Rapid Bay area of south‐west Fleurieu Peninsula, the linear element of the slaty cleavage becomes more pronounced. This increase in fabric linearity is accompanied by a progressive change in fold axis orientation, from approximately horizontal fold axes pitching at a high angle to the mineral lineation (which is typical for the fold belt) to reclined folds with axes at a low angle to the lineation. The variation in fold axis orientation is similar to that calculated by Sanderson (1973) for increasing X:Y strain. The region of more linear fabrics and reclined folds is on the inner arc of a major change in‐ the fold‐belt orientation. The inner segment of this fold arc has undergone a more constrictional strain history than the outer segment, thereby producing both the rotation of the fold axes and the more linear slaty cleavage fabrics. This geometry is consistent with an overall flattening of the mobile belt against a curved cratonic margin during the slaty cleavage and fold forming event, and with the accommodation of some further bending of the already arcuate fold belt by a tangential longitudinal strain mechanism.