Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: The North Mernoo Fault Zone, offshore New Zealand

Abstract

A complex transition from subduction at the Hikurangi margin to transpressive, continental collision across the Southern Alps occurs in the NE South Island region of New Zealand. Offshore, the southern limit of oceanic subduction is constrained by the Chatham Rise, a submerged continental plateau on the Pacific Plate, which is presumably too buoyant for normal subduction. In the NW corner of the rise is the North Mernoo Fault Zone (NMFZ), a 100 by 300 km region of basement‐involved, active normal faulting of late Miocene (circa 8–6 Ma) to Recent age. The NMFZ evolved contemporaneously with the rotation and development of the adjacent plate boundary zone. Growth faulting is characteristic within the sedimentary cover, which is generally less than 2 km thick, but the distribution of faulting has varied temporally. The amount of mid Pliocene‐Recent extension is estimated to be not more than a few percent. The NMFZ consists of an array of overlapping, south dipping normal faults, which are typically 2–5 km apart and which trend roughly E–W at a high angle to the plate boundary zone. Late Quaternary surface traces are widely distributed on the mid to upper continental slope, but many surface scarps are poorly preserved due to extensive erosion of the seafloor. The western end of the fault zone crosses submarine canyons at the southern end of the Hikurangi Trough and almost abuts two fold and thrust fault systems on the NE South Island continental margin. Although few data constrain the kinematics of extension, a single MW 6.1 earth quake at the western end of the NMFZ suggests planar rotating faults with a component of left‐lateral oblique slip. Two possible models of regional extension are considered; (1) lateral buckling of the upper crust, in which faulted slivers of crust are peeled off as the Chatham Rise slides past the transpressive Marlborough fault system toward the Southern Alps and (2) flexure of the edge of continental Pacific Plate as the Chatham Rise is bent downward into the southern end of the Hikurangi subduction zone. Improved seismological data and deep crustal seismic reflection profiles may help to constrain the kinematics of deformation.