Unusual Strike-Slip Related Structures within the Obliquely Transpressive Australia-Pacific Plate Boundary Zone, NE South Island, New Zealand

J. Pettinga, J. Campbell, N. Litchfield, H. Cowan, and A. Nicol

The transition from oblique subduction to continental collision of the Australia-Pacific plate boundary zone as it transfers across the northeastern South Island is also accompanied by a flip in subduction polarity. Tectonic shortening, crustal thickening and uplift across the rapidly widening plate boundary zone is accommodated by a complex array of active faults and folds grouped into three fundamentally separate structural domains.

The complexity of present-day structures is primarily caused by two factors, including: i) the temporal increase in oblique motion produced by the changing relative convergence vector during the upper Cenozoic; and ii) the inherited less favourably oriented major faults in basement. Oblique-slip has modified many of the early stage conventional secondary scale structures (restraining and releasing bends and step- overs; en echelon folds) into complex geometries producing characteristic styles indicative of oblique slip motion. One suite of structures includes those initially developed by strike-slip motion, but now evolving under oblique-slip, so allowing us to view the way in which restraining and releasing bends and step-overs are progressively modified. A second suite of structures develop by progressively forcing strike-slip on early phase imbricate thrust systems, represented by en echelon folds with sigmoidal axial traces propagating over oblique imbricate thrust fans.

We have developed models based on detailed field observations which typically form at major segment boundaries as well as at internal variations in segment geometry. These models illustrate the influence of individual, as well as the interaction between major structures, and the way that slip is partitioning in a complex 3D deformation.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California