Late Holocene beach ridges displaced by the Wellington Fault in the Lower Hutt area, New Zealand

The Wellington Fault cuts a prograded, late Holocene shoreline at the southern end of the Hutt Valley. The fault scarp crosses a suite of five beach ridges (B-F; youngest to oldest). Two steps in scarp height of 2 m, and 3-4 m, between southwest Petone and north Petone/central Lower Hutt occur over a distance of about 1 km, whereas, southeast of the fault, there are no steps, and the topography is more or less level (2-2.5 m altitude) for about 2 km upvalley. Uplift of c. 6 m has occurred on the northwest side of the Wellington Fault in the last few thousand years (vertical deformation rate of c. 1 mm/yr, determined from vertical offset of beach ridge D). Net uplift southeast of the fault is no more than 2-2.5 m, and could have occurred solely during the 1855 Wairarapa earthquake. A dextral slip rate of c. 5 mm/yr is suggested for the Wellington Fault from possible lateral offset of beachridge F. Near-surface stratigraphy and radiocarbon dating in eastern Petone suggest that swampy conditions developed c. 470 years ago upvalley from beach ridge C. This may have been associated with the 300-450 year last-displacement event suggested for the Wellington Fault in the Wellington Hutt Valley area (i.e., subsidence southeast of the fault). The 1855 uplift probably was anomalously large: repeated late Holocene uplifts of this magnitude, and of the frequency observed at Turakirae Head, would have elevated the Petone area well above what is observed today.