Layer parallelisation: An unrecognised mechanism for palaeomagnetic rotations in fold belts

We report a new driving mechanism for vertical-axis rotation in fold belts: during directed folding, there always are layers which are tilted oblique to the regional shortening direction. These layers are geometrically incompatible with fold closure and must become accommodated towards parallelism with the other layers in the course of further shortening. Accommodation is achieved through vertical-axis rotations of the layers towards the shortening direction. A revision of palaeomagnetic data sets from fold belts shows that (1) incompatible layers occur in almost all data sets, reflecting a certain degree of non-cylindrical folding, and (2) a parallelisation of the layers by true vertical-axis rotations occurred when folding became intense. Layer parallelisation is a potential source of disturbance for palaeomagnetic and tectonic interpretation. It can be the explanation for some of the frequent outliers in palaeomagnetic data sets, and a basic model for the rotation pattern of palaeomagnetic directions.