Three‐stage massive gravitational collapse of the emergent imbricate frontal wedge, Hikurangi Subduction Zone, New Zealand

Abstract

A large‐scale east‐facing Quaternary gravitational collapse is affecting the otherwise thrust‐faulted terrain of coastal southern Hawke's Bay on the east coast of North Island of New Zealand, involving an area >30 km long and up to 6 km inland from the coast, but also extending offshore. The western limit of the collapse structure onshore is represented by a footwall escarpment forming the eastern margin of the Maraetotara Plateau (av. elevation 500 m a.s.l.). To the east of this escarpment, the disposition of the Pliocene Te Aute Limestone within a complex extensional zone reflects numerous northeast‐southwest‐trending normal faults (some Holocene active) with the development of horst and graben, fault‐angle depressions, synclinal folds, fissured zones, and large parasitic landslides. Listric normal faulting is reflected by the back‐tilting of large blocks up to several kilometres in length, near the centre of the gravitational collapse. Horizontal extension across the gravitational collapse domain is calculated at between 15 and 20%. West of the footwall escarpment, the Maraetotara Plateau defines a separate structural domain with horizontal extension of between 2 and 5%, inferred to be related to bending moment faults driven by regional arching accompanying uplift. A fundamental contrast exists between active extensional structures onshore and the mainly contractional structures (asymmetrical anticlines, reverse and thrust faults) offshore. A tectonically driven model involving underplating of trench fill best explains the large‐scale gravitational collapse of this part of the emergent imbricate frontal wedge of the Hikurangi Subduction Zone margin.