NON-STEADY BOUNDARY CONDITIONS AND THE EFFECTS OF TECTONIC ENVIRONMENT ON THE EVOLUTION OF HIGH STRAIN ZONES

Studies of deformation in orogenic belts have shown that kinematic patterns at mid-lower crustal depths are strongly influenced by local variations in crustal structure, strength and rheology. These observations have led to interpretations that many aspects of deformation kinematics in the deep crust are insensitive to tectonic environment. In exposures of the mid-lower crust in Fiordland, New Zealand we show that most of the parameters used to describe ductile deformation in shear zones were highly variable as both local conditions and plate motions changed. During the period 126-108 Ma the orogen was characterized by an evolving mid-lower crustal rheology that accompanied convergence and arc magmatism. The period 108-90 Ma was characterized by decompression and extension as convergence stopped and nearby plate boundaries reorganized. We examined the kinematics of superposed shear zones from each period at multiple length scales ranging from a few meters to tens of kilometers. The data show that as local conditions such as crustal temperature, structure, rheology and composition changed, the following kinematic parameters changed repeatedly: the orientation of shear zone boundaries, the degrees of non-coaxiality and kinematic partitioning, vorticity, strain symmetry, and whether shear zones were thickening or thinning in different planes. However, despite this variability, several characteristics of the deformations were unique to each tectonic setting. All shear zones that formed during the 126-108 Ma period recorded bulk horizontal (layer-parallel) shortening and vertical (layer-perpendicular) thickening. In contrast, all shear zones that formed during the 108-90 Ma period recorded bulk vertical thinning and subhorizontal stretching. These patterns are correlative with regional contraction and regional extension, respectively. Our data show that although non-steady flows characterize deformation in the deep crust of orogens, a careful choice of length and time scales allows investigation of deformation using techniques that require steady-state conditions. In addition, at appropriate length scales the effects of changing plate boundary dynamics on the kinematics of lower crustal shear zones can be distinguished quantitatively from the effects of changing local conditions.