Structural evolution and landscape development of a collapsed transpressive duplex on the Hope Fault, North Canterbury, New Zealand

Abstract

This study examined the transpressional Conway segment of the Hope Fault in North Canterbury, the fastest moving fault of the Marlborough Fault Zone in northern South Island of New Zealand, in an attempt to reconstruct, via air photograph interpretation, detailed field mapping, and theoretical constraints, the styles of structural geometry of the late Quaternary deformation. We relate the evolving landscape to the development and modification of this fault in an active tectonic setting. The section of the fault zone studied is a 13 km long, 1.3 km wide, asymmetric transpressional reverse fault duplex, bounded to the southwest by the Lottery River and to the northeast by the Mason River, which tapers down to a single fault trace to the northeast. Between the bounding faults of the duplex are approximately 100 subsidiary fault scarps that initially formed an imbricate set of footwall propagating reverse/thrust faults. These faults became inactive and topographically unsupported when younger footwall propagating reverse faults migrated northeast along the main trace of the Hope Fault as the duplex also migrated to the northeast. The unsupported duplex structurally collapsed back into the dilating fault zone, causing reversal of slip on the imbricate reverse faults so that they became normal faults. As the duplex collapsed, the adjacent hanging wall was uplifted triggering landsliding, rapid incision by streams, formation of large alluvial fans, and minor normal fault gravity collapse structures. The footwall block outside the duplex became rapidly incised by streams and experienced widespread topographic slumping and associated structurally controlled, shallow‐level ridge renting. Stream dissection of flights of late Pleistocene aggradation/degradation surfaces also occurred at this time, leaving remnant flat‐topped hills between the dissected and slumped valleys. Our analysis of the interrelationships between the structural geology and the landscape over time is a new approach for the Hope Fault and reconfirms the necessity of integrating detailed structural geology with geomorphology in areas of active tectonism.