STRUCTURAL COLLAPSE OF A TRANSPRESSIVE HANGING WALL FAULT WEDGE, CHARWELL SECTION OF THE HOPE FAULT, SOUTH ISLAND NEW ZEALAND

The NE trending dextral-reverse oblique slip Hope fault is one of the major structures of the Marlborough Fault Zone and the Australia-Pacific plate boundary zone in the South Island of New Zealand. This study presents an analysis of the structural and tectonic geomorphic development of the Hope fault zone in the vicinity of the Charwell River. Our key objective is to understand the near-surface temporal and spatial structural style of deformation and fault zone kinematics along a 10 km section of the fault. Paradoxically, the transpressive rangefront setting includes a collapsing internal fault zone wedge in the lower hanging wall, superbly expressed in the structurally-driven geomorphic landscape.

Significant fault-related landscape units include: 1) flights of aggradation-degradation terraces in the footwall forming an extensive piedmont; 2) fault dissected, sloping topography in the hanging wall containing 95% of all the faults; and 3) eroded sub-horizontal piedmont terrace remnants that indicate the rangefront has repeatedly propagated to the SE into the footwall block.

We recognise four distinct types of fault scarps: 1). The main rangefront trace of the Hope fault defines a releasing bend geometry, with a projected step-over width of ~1000m; 2). At the foot of the rangefront two thrust faults ramp over the aggradational surfaces in the footwall block; 3). In the toe of the hanging wall block, ~20 normal fault scarps are mapped near-parallel the main Hope fault; and 4). More than 100 late normal faults oblique to the main Hope fault, and cutting obliquely across all other faults. The overall fault pattern outlines an initial fault wedge between the thrust and early normal faults that is 5 km in length, 1 km at its widest point and tapers in a westerly direction to 200 m in width. A secondary wedge defined by the late normal faults is 7 km in length, 2 km at its widest and overprints the initial wedge. The structural/geomorphic interactions between the initial and secondary fault wedges developed in a series of at least four successive stages.

These spatial and temporal changes in the structural geometry and style of deformation of the Hope fault zone at Charwell are interpreted to reflect the role of topographic loading in adjusting the fault zone break-out along the rangefront, and is recorded by the world-class tectonic geomorphic landforms there.