The Rapuhia Scarp (northern Hikurangi Plateau) — its nature and subduction effects on the Kermadec Trench

Abstract

Multibeam bathymetric data, seismic reflection and magnetic data are used to examine the nature of the 1-km-high Rapuhia Scarp and the tectonic effects of its subduction on the Kermadec margin. The Rapuhia Scarp is the boundary between the Hikurangi Plateau and >85Ma oceanic crust. Its intersection with the Kermadec Trench moves south at 4cm/yr. Magnetic modelling indicates that the northern Hikurangi Plateau may be formed of 5km of relatively strongly-magnetised extrusive and intrusive volcanics rocks. Uplift of the Rapuhia Scarp near the trench is consistent with vertical motion on a tear-fault in the subducting plate.The coincident intersection of the subducting Rapuhia Scarp with a NNW-trending volcanic ridge (Intersection Ridge) and the Kermadec Trench results in an abrupt 1.5km increase in the trench depth and inner margin topography. The Rapuhia Scarp boundary sweeps south along the margin. Slumping and subsidence of the inner margin resulting from this transition occur on a variety of scales, from 5-km-wide, 1-km-high arcuate frontal erosion slumps to regional subsidence of up to 4km across the 50–100-km-wide mid-upper inner trench margin as it adjusts to the new subduction geometry. The pattern of inner margin subsidence involving the entire forearc is broadly similar to that observed 1200km further north in the wake of subduction of the Louisville Ridge seamount chain.Subsidence of the inner trench margin results in the loss of approximately 400km3 per kilometre of margin as the subducting Hikurangi Plateau support is removed. We estimate however only one fifth of the above subsidence is frontal or basal erosion highlighting the risks of estimating tectonic erosion from margin subsidence if the cause of subsidence is mis-identified. The remaining 320km2 of subsidence is due to the steeper dip of the subducting plate associated with subduction of the comparatively dense >85Ma oceanic crust.