LITHOLOGIC HETEROGENEITY, MECHANICAL ANISOTROPY AND THE FORMATION OF DEEP CRUSTAL SHEAR ZONES IN FIORDLAND, NEW ZEALAND

We present structural and U-Pb geochronologic data on zircon from two deep-crustal shear zones exposed in Doubtful Sound, Fiordland, New Zealand. Fiordland is a unique setting where rocks deformed in the middle to lower continental crust during the Cretaceous are exposed at the surface with a well-constrained exhumation history. Our data illustrate the role of pre-existing lithologic and structural heterogeneity in controlling the formation of steep and gently dipping structures during the initiation and evolution of extensional and transpressional shear zones. The data also constrain the ages of the different phases of deformation. Field observations show that deformation is localized within rheologically weak layers, such as marble, as well as along the boundaries between different lithologies. Spatial variations in structures within one lithology show that the primary mechanism for the formation of steep transpressional foliations was folding of pre-existing, flat-lying foliations and compositional layering. We interpret the following progression: (1) open, upright folding (2) monocline development (3) rotation of axial planes to vertical, tight folds (4) development of an axial planar shear zone foliation. This process resulted in a network of steeply dipping shear zones superimposed on flat-lying foliations in a ~10km wide region. Deformation during extension produced a network of interconnected low-angle shear zones that form antithetic/synthetic pairs. Extensional deformation was focused within a ~500m thick marble-rich layer along a major lithologic boundary. U-Pb analyses of zircon from syn-tectonic dikes show that extensional commenced by 102.1 +/- 1.2 Ma and ceased by ~88.4 ± 1.2 Ma. Transpression began after 88.4 Ma. These age constraints allow us to place the formation of these shear zones in a regional tectonic context. Transpressional shear zones developed in locations where the structural weakness of flat-lying foliations facilitated the formation of steep foliations through folding. The formation of extensional shear zones along lithologic boundaries demonstrates how compositional heterogeneity controls shear zone formation. These processes are important at various scales ranging from meters to tens of kilometers within the entire lower crustal section.