Structure and topology of a brittle-ductile fault swarm at Crawford Knob, Franz Josef, New Zealand

ABSTRACT

We present surface and structural models of a swarm of dm-scale subparallel faults exposed in a ∼2000 m² glaciated outcrop near Franz Josef Glacier, in the Southern Alps of New Zealand. These structures are inferred to have slipped at ∼20 km depth in the hanging-wall Alpine Schist of the Alpine Fault under conditions that were variably brittle to ductile as the Pacific Plate was tilted and uplifted. Using field mapping, real-time kinematic GPS and digital images from a remotely piloted aircraft system, we have created a digital surface model of the outcrop and orthophotographs at a ground resolution of ∼1 cm to map compositional layering in the metasediments and the array of brittle-ductile faults displacing them. In order of decreasing relative age (and average thickness), displaced markers in the schist include primary psammite and pelite beds, quartz veins, and a deformational foliation. The surface models have been used to create 2-D transects and a 3-D model where faults are projected down-dip, to determine the connectivity, topology and intersection types of the fault swarm. Lithological variations, particularly the interface between pelitic and psammitic schist, were a primary control on the topology of the fault network and the spacing between faults.

KEYWORDS:

• Brittle-ductile rheology
• fault networks
• mapping
• RPAS
• drone
• RTK
• Southern Alps
• Alpine Schist

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials. The appendices are available for download at https://doi.org/10.6084/m9.figshare.22492936. Two supplementary data collections containing fault geometry and lithology (Supplementary Dataset 1) and the 3-D fault model (Supplementary Dataset 2) supporting the findings of this study are openly available at https://doi.org/10.21420/8VT4-2J73.

Acknowledgements

This study was funded by core funding to GNS Science and Victoria University of Wellington. The authors would like to thank the Department of Conservation for the help in permitting and providing radios; the Local Air Users Group at Franz Josef for their cooperation in providing permit conditions that allowed us to fly a remotely piloted aircraft in an area used by many helicopter companies; Neville Palmer and Garth Archibald for RTK GPS equipment preparation and training; and Te Runanga o Makaawhio (Komiti Taiao) for permission to access the field area. We acknowledge fruitful discussions with Simon Cox, David Prior, Virginia Toy, Steve Kidder, Andrew Cross, Cecile Massiot, Ruth Wightman, Susanne Grigull, and many others over the years. Mark Rattenbury, Cecile Massiot, Jack Williams and an anonymous reviewer provided constructive comments on the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the GNS Te Riu-a-Māui/Zealandia Programme (Strategic Science Investment Fund, contract C05X1702).