STRAIN PARTITIONING AND BLOCK ROTATION OF THE CLARENCE-ELLIOTT WEDGE AS A PRODUCT OF DEXTRAL-REVERSE MOTION IN THE OBLIQUE COMPRESSIONAL MARLBOROUGH FAULT ZONE, NEW ZEALAND

Detailed structural mapping via DEM, aerial photography, and field observations was carried out within the Clarence – Elliott wedge (CEW), defined by the Clarence and Elliott dextral-strike slip reverse faults of the Marlborough Fault Zone (MFZ). Three N-NE striking cross faults link the Clarence to the Elliott fault. From west to east the cross faults are the Acheron, Dillon and Tweed faults, all displaying sinistral normal or sinistral reverse motion. The Acheron Fault is marked by subtly offset geomorphic features and an array of en echelon, right stepping segments with an overall sigmoidal shape. The Dillon has extensive fault gouge, well-developed scarps with offset geomorphic features and is defined by 4 parallel, linear, right stepping strands with slight sigmoidal shape. The Tweed shows a series of sinistral normal and potentially sinistral reverse segments in a roughly sigmoidal arrangement. These three cross faults define the boundaries of four clockwise-rotated fault blocks within the CEW. Regionally, the CEW shows progressively variable degrees of uplift/thrusting and collapse/extension with low topographic relief in the west giving way to progressively higher relief in the east corner. Each individual block also shows higher relief to the SE, along the Elliott fault, and lower relief to the NW along the Clarence fault. This is likely related to a progressive eastward decrease in the plunge of the axis of block rotation. Evidence for similar rotational systems displaying strain partitioning can be seen throughout the Marlborough Fault Zone suggesting that fault-bounded rotational blocks are a governing structural feature for the entire region. Evidence from the 1996 Arthur’s Pass earthquake and resulting aftershocks suggests that block rotation along cross faults linking major faults plays a key factor in the southward development of the MFZ. The CEW has a similar geometry to that of the block involved in the Arthur’s Pass event which connects the currently active Hope fault to the incipient Porter’s Pass fault to the south. This may address the question of how the fundamental strain boundary between thrust fault dominated deformation to the south of the MFZ and dextral-reverse oblique dominated faulting within the MFZ develops over time.