P. Upton, Geology Department, University of Otago, P.O. Box 56, Dunedin,
phaedra.upton@stonebow.otago.ac.nz
Strain localisation onto discrete structures during continental collision or other plate interactions dynamically links geological processes, such as faulting and fluid flow, to material characteristics, such as inherited structure and rheology of the deforming plates. The Pacific/Australian plate boundary zone through New Zealand, which shows considerable along strike variation as it changes from subduction, through oblique continental collision back to subduction, provides an ideal setting in which to investigate various aspects of strain localisation.
Field studies, aimed at integrating the observations of the locus of deformation east of the Main Divide of the Southern Alps, the effect of inherited structure and consideration of the material properties of the crust are combined with numerical models in an attempt to provide insight into the relationship between rheological constraints and strain localisation within a deforming plate boundary region. For example, the Mackenzie Basin, South Canterbury, is the largest undeformed region within the Southern Alps and is adjacent to the narrowest region of the orogen, the Mt Cook region, where nearly all of the convergent motion of the plate collision is taken up on the Alpine Fault (Norris and Cooper 2001). Numerical models test two hypotheses; that some part of the crust underlying the Mackenzie Basin is considerably stronger than that surrounding it or that concentrated uplift on the Alpine Fault in this region has weakened the deeper parts of the Alpine Fault sufficiently to focus all convergent deformation onto this one structure in this region of the collision zone (Koons et al. 1998).
Also of inherent importance to the question of strain localisation through
all depths of the crust is how strain is transferred between the brittle, friction
dominated upper crust and the ductile, thermally controlled lower crust. Numerical
experiments have been carried out to investigate transfer of strain from a weak
shear zone within the lower crust to the upper crust, including the effect of
strain softening rheologies. Further work will investigate the possibility of
a localised zone in the upper crust leading to localisation within a strain
softening lower crust.