Detailed mapping of Fraser Formation has revealed a different relationship between mylonitic and gneissic rocks than found by previous workers. Mylonitic zones envelop areas of amphibolite facies gneisses and granitoid rocks, ranging up to several square kilometres, throughout the area. Mylonitisation does not increase in intensity towards either the Fraser Fault or the Alpine Fault. Numerous lamprophyric and trachytic dikes intrusive into Fraser Formation have not been observed to cut the foliation in the mylonite zones, and several of the dikes have been truncated by mylonitic fabric. These dikes therefore give a maximum age of mylonitisation, of probably middle Cretaceous, rather than a minimum age as previously believed. Structural data from the mylonites are consistent with dextral strike-slip movement.