THE ANITA SHEAR ZONE - A MAJOR, MIDDLE CRETACEOUS TECTONIC BOUNDARY IN NORTHWESTERN FJORDLAND

A 3-4 km wide shear zone, here named the Anita Shear Zone, runs parallel to the west coast of northern Fiordland and is marked by amphibolite facies mylonites and intensely deformed gneisses. This shear zone marks a tectonic boundary between the metasediments and metagranites of the Tuhua Sequence and the gneisses of the Arthur River Complex and Western Fiordland Orthogneiss. Detailed structural analysis indicates that the Anita Shear Zone has been folded and reoriented into its current steep northeast trend by late folds associated with steeply dipping retrograde shear zones. Removing the effects of the overprinting folds reveals that the Anita Shear Zone was originally subhorizontal or shallowly dipping. The Anita Shear Zone is very similar in many aspects to a middle Cretaceous shear zone in Doubtful Sound which has formed along the same lithological boundary. Both shear zones separate the Tuhua Sequence from the Western Fiordland Orthogneiss, indicating that there is no intrusive contact between these two units as has been previously suggested.