Structural collapse of a transpressive hanging‐wall fault wedge, Charwell region of the Hope Fault, South Island, New Zealand

Abstract

The northeast‐trending dextral‐reverse oblique slip Hope Fault is one of the major structures of the Marlborough Fault System 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 to understand the near‐surface temporal and spatial structural style of deformation and fault zone kinematics along a 10 km section of the fault. 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 subhorizontal piedmont terrace remnants that indicate the range front has repeatedly propagated to the southeast into the footwall block. We recognise four distinct types of fault scarps: (1) the main range‐front trace of the Hope Fault defines a releasing bend geometry, with a projected step‐over width of c. 1000 m; (2) at the foot of the range front, two thrust faults ramp over the aggradational surfaces in the footwall block; (3) in the toe of the hanging‐wall block, c. 20 normal fault scarps are mapped near‐parallel to the main Hope Fault; and (4) more than 100 late normal faults are oblique to the main Hope Fault and cut 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 and 1 km at its widest point. A secondary wedge defined by the late normal faults is 7 km in length, 2 km at its widest, and it 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 range front, and is recorded by the tectonic geomorphic landforms there.