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 parall el to the west coast of northern Fiordland and is marked by amphibolit e facies mylonites and intensely deformed gneisses. This shear zone ma rks a tectonic boundary between the metasediments and metagranites of the Tuhua Sequence and the gneisses of the Arthur River Complex and We stern Fiordland Orthogneiss. Detailed structural analysis indicates th at the Anita Shear Zone has been folded and reoriented into its curren t 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 sha llowly dipping. The Anita Shear Zone is very similar in many aspects t o a middle Cretaceous shear zone in Doubtful Sound which has formed al ong the same lithological boundary. Both shear zones separate the Tuhu a Sequence from the Western Fiordland Orthogneiss, indicating that the re is no intrusive contact between these two units as has been previou sly suggested.