Quaternary Deformation Along the Wharekauhau Fault System, North Island, New Zealand: Implications for an Unstable Linkage Between Active Strike-Slip and Thrust Faults

The southern Wairarapa region of the North Island of New Zealand preserves a variably deformed late Quaternary stratigraphic sequence that provides insight into the temporal variability in the partitioning of contraction onto faults in the upper plate of an obliquely convergent margin. Detailed mapping, stratigraphic data, and new radiocarbon and optically stimulated luminescence ages from Quaternary units reveal the interaction between tectonics and sedimentation from ∼125 ka to(i.e., Wharekauhau fault system) at the southern end of the Wairarapa fault zone, a major oblique-slip fault in the upper plate of the Hikurangi Margin. The Wharekauhau thrust accommodated a minimum of 280 ± 60 m of horizontal shortening from ∼70 to 20 ka. The inferred shortening rate, 3.5–8.4 mm/yr, may have accounted for 11–30% of margin-normal plate motion. By ∼20 ka, the thrust was abandoned. Subsequent deformation at shallow levels occurred on a segmented fault system that accommodated/yr shortening. Active deformation in the region is partitioned between slip on (1) the more western Wairarapa-Mukamuka fault system (dominantly dextral slip but also causing local uplift of the coast); (2) a series of discontinuously expressed strike-slip faults and linking blind oblique-reverse thrusts near the trace of the inactive Wharekauhau thrust; and (3) a blind thrust fault farther to the east. The spatial and temporal complexity of the Wharekauhau fault system and the importance it has had in accommodating upper plate deformation argue for an unsteady linkage between upper plate faults and between these faults and the plate interface