An approximately 180-km-long, deep crustal shear zone in western Fiordland, New Zealand, has been shown to have formed during continental extension 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 shear zone has revealed that the shear zone formed at depths of ~40 km (approximately 12 kbar and 680 ¿C) and was originally very gently dipping or subhorizontal prior to subsequent upright folding. The current thickness (postfolding) of the shear zone is ~3--4 km. Abundant asymmetric structures in the shear zone indicate noncoaxial deformation and a top-to-the-NE sense of shear. Unlike ductile shear zones in metamorphic core complexes from elsewhere in the world, there is no evidence that decompression of the shear zone rocks or footwall rocks occurred during the active life of the shear zone, suggesting that displacement along the shear zone by noncoaxial deformation must have been largely horizontal. This shear zone provides important evidence for the nature of deep crustal deformation occurring during continental extension. ¿ American Geophysical Union 1995.