Geometry of the Hikurangi subduction thrust and upper plate, North Island, New Zealand

Abstract

We use 2800 line km of seismic reflection data to map the offshore character and geometry of the Hikurangi subduction thrust and outer forearc wedge to depths of ̃15 km. For 200 km along-strike south of Hawke Bay, the subduction thrust is relatively smooth, dips less than 8°, and the wedge is characterized by accretion of young sediment and topographic slopes of less than 3°. In Hawke Bay and north for 200 km, a kink in the subduction thrust is apparent, with a downdip increase in dip to angles greater than 8° at depths of 10-15 km; there is a corresponding steepening of the topographic slope to greater than 3° outboard of the kink and the wedge is characterized by lithified sedimentary rock and slope failure. The kink in the subduction thrust is a locus of inherent weakness in the subducting slab; we suggest its occurrence relates to a northward increase in subduction rate that controls initial slab dehydration and fluid release rates and hence intraslab deformation patterns. The subduction thrust geometry, in combination with a northward increase in subducting plate roughness and decrease in the amount of sediment accreted, causes the observed spatial change in character of the subduction thrust and forearc wedge. © 2009 by the American Geophysical Union.