Normal fault growth and linkage in the Whakatane Graben, New Zealand, during the last 1.3 Myr

Abstract

Determination of fault growth rates and fault network evolution at timescales from 10 4 to 10 6 years has been hampered by a lack of a well‐constrained stratigraphic succession that provides a high‐fidelity record of fault development over these time periods. Here we show how seismic reflection data of different spatial resolutions can be used to constrain the linkage history and displacement rate variations of a single major fault. We present data collected in the offshore Whakatane Graben, Bay of Plenty, New Zealand, where intense normal faulting occurs as a result of active back extension. The focus of our study is the Rangitaiki Fault, a linked segmented normal fault which is the dominant active structure in the graben. The total linked fault length is ∼20 km and has a displacement of up to 830 ± 130 m in the top 1.5 km of sediments. The fault has been actively growing for the last 1.34 ± 0.51 Myr and has developed from isolated fault segments to a fully linked fault system. Initially, the dominant process of fault growth was tip propagation, with an average and maximum displacement rates of 0.52 ± 0.18 and 0.72 ± 0.23 mm yr −1 , respectively. Interaction and linkage became more significant as the fault segments grew toward each other, resulting in the fault network becoming fully linked between 300 and 18 ka. Following fault segment linkages, the average displacement rate of the fault network increased by almost threefold to 1.41 ± 0.31 mm yr −1 , while the maximum displacement rate increased to 3.4 ± 0.2 mm yr −1 . This is the first time that the growth rate of unlinked fault segments has been resolved and has been shown to be slower than in the subsequent linked fault system.