Collapse in a Quaternary shelf basin off East Cape, New Zealand: evidence for passage of a subducted seamount inboard of the Ruatoria giant avalanche

The Ruatoria margin indentation and its associated giant avalanche off East Cape, New Zealand, have been inferred to result from margin instability following oblique subduction of a large seamount. The earlier studies hypothesise that a diachronous seamount-wake trough formed the northern pan of the indentation, and collapse between the oblique trough and an oversteepened margin front formed the southern indentation and giant avalanche. If correct, then the impacting seamount must now be landward of the indentation. New seismic profiles, supported by multibeam bathymetry and core samples, from landward of the Ruatoria Indentation, provide support for the passage of a large seamount deep beneath the continental shelf. The continental margin around the head of the indentation is underlain by Quaternary basins that are inferred to result from transpression associated with oblique plate convergence. One basin underlies the shelf landward of the indentation, its seaward edge having collapsed into the indentation. There, faults and a graben with gravitational collapse structures that are transverse to the northeast-southwest regional trends may be evidence of passage of a seamount. A tentative sequence stratigraphy, based on Quaternary unconformities and shelf-edge prograding units, suggests that the main phase of collapse occurred on the outer shelf before the penultimate, major, glacial sea-level lowering c. 155-135 000 yr ago. Depths to the post-lastglacial erosion surface indicate that the basin is subsiding rapidly at >4 m/ka. If a significant part of the subsidence relates to wake collapse behind a subducting seamount, then the seamount must now underlie the adjacent land. Onshore, doming hills, with coastal terraces indicating uplift of 2.6 m/ka, suggest that the seamount is now c. 10 km west of East Cape.