A DEEP-CRUSTAL SHEAR ZONE EXPOSED IN WESTERN FJORDLAND, NEW-ZEALAND

An approximately 180-km-long, deep crustal shear zone in western Fiord land, New Zealand, has been shown to have formed during continental ex tension between 116 and 100 Ma. The western Fiordland shear zone forms a major tectonic boundary between two sets of gneisses with unrelated deformation histories. A detailed study of the northern part of the s hear zone has revealed that the shear zone formed at depths of similar to 40 km (approximately 12 kbar and 680 degrees C) and was originally very gently dipping or subhorizontal prior to subsequent upright fold ing. The current thickness (postfolding) of the shear zone is similar to 3-4 km. Abundant asymmetric structures in the shear zone indicate n oncoaxial deformation and a top-to-the-NE sense of shear. Unlike ducti le shear zones in metamorphic core complexes from elsewhere in the wor ld, there is no evidence that decompression of the shear zone rocks or footwall rocks occurred during the active life of the shear zone, sug gesting that displacement along the shear zone by noncoaxial deformati on must have been largely horizontal. This shear zone provides importa nt evidence for the nature of deep crustal deformation occurring durin g continental extension.