Growth of contractional structures during the last 10 m.y. at the southern end of the emergent Hikurangi forearc basin, New Zealand

Abstract

Growth histories of contractional structures at the southern end of New Zealand's Hikurangi forearc basin have been analysed for the last c. 10 m.y. Growth data are from outcrop and seismic‐reflection profiles that contain syntectonic strata and angular unconformities, and from deformed fluvial terrace surfaces. Deformation is described for up to eight intervals of time, spanning c. 12 000 yr to 5 m.y., the ages of which were determined by biostratigraphy and tephrochronology. Reverse faults and related asymmetric folds, which strike parallel to the subduction margin and verge troughwards, experienced variable rates of shortening through time. The current period of deformation commenced at c. 1.8 Ma with displacement rates of c. 0.1–0.7 mm/yr on the main faults (i.e., Martinborough, Huangarua, and Mangaopari Faults). Before this time there were periods of accelerated deformation during the mid Pliocene (c. 3.4–2.4 Ma) and latest Miocene (c. 8.0–6.0 Ma). Therefore, shortening since 10 Ma accumulated mainly during three periods of 1–2 m.y., with structures active in the Quaternary forming in the late Miocene or earlier. Local intervals of accelerated deformation are coincident with the timing of intervals of uplift and faulting along much of the emergent forearc and cannot be attributed to local transfer of displacements between faults. Instead, these intervals of deformation appear to reflect regional changes in the kinematics of the upper plate. These changes could arise due to margin‐normal migration of strain to regions outside the forearc basin or may indicate temporal variations in the dynamics of subduction.

Keywords:

• subduction
• reverse faults
• folds
• syntectonic growth strata
• angular unconformities
• episodic deformation