Abstract - The Rangitaiki Plains extend across one of the world’s fastest spreading areas, the Taupo Rift near where it is intersected by the active strike-slip North Island Fault System in the Bay of Plenty. The tectonic activity of the region was emphatically illustrated by the 1987 Edgecumbe earthquake (Mw 6.3), when a number of northeast-striking normal faults ruptured. However, the scarcity of surface fault traces within the plains in comparison with other parts of the rift, onshore and offshore, has long been puzzling.
High resolution LiDAR data (vertical resolution of <0.1 m) collected recently across the plains by Environment Bay of Plenty Regional Council reveal vivid topographical images of tectonic features that conventional datasets (i.e., topographic maps, aerial photograph analysis, field mapping) can not resolve. Derivative images reveal numerous previously unrecognised fault scarps in a density that matches that of faults in the Taupo Rift elsewhere. Active faults previously mapped to the southwest and northeast (offshore) can now be aligned with identifiable scarps across the plains.
The Edgecumbe Fault, which is the most significant tectonic feature on the plains, can now be extended both southwest and northeast of the 1987 rupture. Scarp height measurements obtained at various locations along the fault exceed slip values of the 1987 earthquake, supporting the inference that the scarp records multiple surface rupturing events since deposition of the Taupo pumice alluvium (< 1.8 ka).
A series of discontinuous beach ridges are preserved between the coast at Whakatane and the stranded early Holocene sea cliffs near Awakeri (9.5 km to the south). The Edgecumbe Fault truncates these beach ridges. Topographic profiles obtained along and across the beach ridge crests indicate that this block of land has suffered only minor vertical deformation during the last 5.2-7.5 ka.
Similar beach ridge sequences are not present northwest of the Edgecumbe Fault, where they are probably buried beneath a blanket of redeposited pumiceous alluvium and peat. The average elevation of beach ridge crests deposited at the coast today is 5-7 m. The minimum elevation of the Rangitaiki Plains is 1.5 m below sea level, just 2.6 km south of the coast near the Rangitaiki River. Combining these data, we derive a net value for subsidence since deposition of Kaharoa pumice alluvium.
Abstract of talk given to:
New Zealand Geological Society Annual Meeting, Tauranga, New Zealand, December 2007.