Faulting and Extension Rate over the last 20,000 Years in the Offshore Whakatane Graben, New Zealand Continental Shelf.

Abstract

Oblique rifting in the offshore Taupo Volcanic Zone, New Zealand, is expressed in widely distributed active normal faulting in the 20 km-wide Whakatane Graben. Active faults are identified along seafloor scarps and displacements of the post-last glacial transgressive ravinement surface (<20ka), using a network of seismic reflection data and multibeam bathymetry. The rifting involves basement blocks back-tilted by 12-16°, controlled by large NW-dipping faults with intersecting antithetic faults within the 3 km-thick sedimentary sequence. Faults along the graben border parallel the rift axis, while those in the centre are moderately oblique to it. We present a novel method to estimate the age of a post-glacial surface (7.5 to 20.5 ka), with consideration to spatially varying subsidence and uplift, and measure fault throw across >400 faults. We derive an extension rate at seismogenic depths (6-10 km) across the graben of 13 ± 6 mm/yr, by summing surface measurements, assuming an average crustal fault dip of 45±15º, and correcting for the discrepancies between surface and deep crustal extension estimates. Structural and kinematic data implies an extension direction 20º oblique to the rift axis, resulting in up to 4.6 ± 2.1 mm/yr of dextral motion parallel to the rift axis. The strike-slip motion is accommodated by dip-slip displacements on oblique faults in the centre of the graben, and oblique-slip faulting along the rift margins. Pure dip-slip in the graben centre represents >50% of the total slip, with the Rangitaiki Fault accommodating 25% of the total extension in the graben