The dammed Hikurangi Trough: a channel‐fed trench blocked by subducting seamounts and their wake avalanches (New Zealand–France GeodyNZ Project)

The Hikurangi Trough, off eastern New Zealand, is at the southern end of the Tonga–Kermadec–Hikurangi subduction system, which merges into a zone of intracontinental transform. The trough is mainly a turbidite‐filled structural trench but includes an oblique‐collision, foredeep basin. Its northern end has a sharp boundary with the deep, sediment‐starved, Kermadec Trench.

Swath‐mapping, sampling and seismic surveys show modern sediment input is mainly via Kaikoura Canyon, which intercepts littoral drift at the southern, intracontinental apex of the trough, with minor input from seep gullies. Glacial age input was via many canyons and about an order of magnitude greater. Beyond a narrow, gravelly, intracontinental foredeep, the southern trench‐basin is characterized by a channel meandering around the seaward edge of mainly Plio‐Pleistocene, overbank deposits that reach 5 km in thickness. The aggrading channel has sandy turbidites, but low‐backscatter, and long‐wavelength bedforms indicating thick flows. Levées on both sides are capped by tangentially aligned mudwaves on the outsides of bends, indicating centrifugal overflow from heads of dense, fast‐moving, autosuspension flows. The higher, left‐bank levée also has levée‐parallel mudwaves, indicating Coriolis and/or boundary currents effects on dilute flows or tail plumes.

In the northern trough, basin‐fill is generally less than 2 km thick and includes widespread overbank turbidites, a massive, blocky, avalanche deposit and an extensive, buried, debris flow deposit. A line of low seamounts on the subducting plate acts as a dam preventing modern turbidity currents from reaching the Kermadec Trench. Major margin collapse probably occurred in the wake of a large subducting seamount; this seamount and its wake debris flow probably dammed the trench from 2 Ma to 0.5 Ma. Before this, similar dams may have re‐routed turbidity currents across the plateau.