Basement Structure and Boundaries of the Hikurangi Plateau

The Hikurangi Plateau is a Large Igneous Province, possibly rifted from the Manihiki Plateau about a spreading centre which stalled along the present-day Osbourn Trough. Spreading may have ceased following the entry of the Hikurangi Plateau into a postulated North Chatham Rise subduction trench. The eastern plateau boundary features the curved Wishbone Scarp lineation, apparent in satellite gravity data, which merges into the northeastern Chatham Rise. The Wishbone Scarp is interpreted as either a strike-slip plate boundary or alternatively a paleo-spreading ridge. Recent low-fold and deep crustal seismic surveys conducted across the Hikurangi Plateau margins east of 180 degrees, the latter in support of the New Zealand UNCLOS continental shelf delineation, provide supporting evidence for the above models and fresh insights into plateau structure. To the west of the Wishbone Scarp/ Chatham Rise intersection the seismic reflection data reveals stacked sedimentary thrust wedges, consistent with accretionary prism development, and similar to structures observed accompanying present day plateau subduction beneath the North Island, New Zealand. The Wishbone Scarp images as 2-3 major southeast-facing tilted-block ridges which climb to the Chatham Rise crest. The southeast facing scarp orientation matches the dominant orientation of half-graben fault scarps throughout the Chatham Rise and Bounty Trough generally associated with New Zealand-Antarctic extension and rifting. Across the northern plateau boundary extensional rotated block structures, consistent with a rifted origin, are interpreted in the plateau stratigraphy and oceanic crust further north. Well-defined, although fault-disrupted, layers within the Hikurangi Plateau crust are interpreted including the possible upper surface of the basal oceanic crust upon which later volcanics rest.