Foraminiferal evidence for Holocene synclinal folding at Porangahau, southern Hawkes Bay, New Zealand

Abstract Porangahau, southern Hawkes Bay, lies above the Hikurangi Subduction Zone in the transition zone between strong and weak plate interface coupling. This coastal area is unusual in the absence of uplifted Holocene coastal terraces or active fault traces. Holocene estuarine foraminiferal faunas in five short cores in the southern part of the Porangahau coastal plain were used to estimate palaeo-tidal elevations of sediment accumulation. These show that the western side of the plain, deposited c. 6 ka, has been uplifted by 100–250 cm, after allowing for mid-Holocene higher sea level. Faunas in the estuarine sediment infill forming the southeastern part of the plain indicate compaction-corrected subsidence of c. 80–400 cm since 3 ka. We infer that the Fingerpost Syncline extends northwards beneath the Porangahau coastal plain and is actively deforming with a Holocene tilt rate of 0.017°–0.05° kyr−1 on the western flank.