EROSIONAL CONTROL ON THE STRUCTURAL EVOLUTION OF A TRANSPRESSIONAL THRUST COMPLEX ON THE ALPINE FAULT, NEW-ZEALAND

The Waikukupa thrust is a 4-km long oblique thrust segment of the New Zealand Alpine Fault that has developed over the last 65,000 years, em placing mylonite and cataclasite over fluvioglacial gravel. During the last 20,000 years or so, a 2-km long 'out-of-sequence' imbricate, the Hare Mare thrust, has formed on the east side of the Waikukupa River valley. The average rate of slip on the two thrusts is estimated al 22 -30 mm/year. Analysis of the fault structure in terms of critical wedg e theory is consistent with imbrication resulting from a reduction in wedge taper below the critical value due to rapid river erosion. The i nternal structure of the Waikukupa thrust sheet consists of reverse fa ults and annular folds forming duplex like systems, associated with mi nor strike-slip faults Within the limits of uncertainty, the structure s are consistent with strain accumulating during internal transpressio nal shear of the thrust sheet while the taper of the latter was subcri tical prior to the transfer of fault displacement to the Hare Mare imb ricate. The thrust complex forms part of a serrated range Front fault system consisting of oblique thrust sections linked by strike-slip fau lts. We introduce the term 'serial partitioning' for this type of part itioned transpressional fault system. Its development is linked to the erosional processes at the range front. An alternative system consist ing of parallel thrust and strike-slip faults we here term 'parallel p artitioning'. While angle of obliquity is an important parameter in de termining which partitioning model develops, we suggest that erosion r ate is also an influential factor. (C) 1997 Elsevier Science Ltd.