Holocene marine terraces on the northeast coast of North Island, New Zealand, and their tectonic significance

Abstract

Abstract As many as seven Holocene marine terraces are preserved between Raukokore River and Gisborne on the northeast coast, North Island, New Zealand. Six terraces up to 20 m above present mean sea level (a.m.s.l.) are dated at c. 300, 600–700, 900–1200, 1600–2000, 4500, and 6000 radiocarbon yr B.P. to the west of East Cape. Seven terraces are preserved up to 27 m a.m.s.l. near Pakarae River mouth, and the higher six terraces have radiocarbon ages of c. 1000, 1600, 2500, 3900, 5500, and 7000 yr B.P. The coastal region from Waiapu River to Tolaga Bay has only two to three marine terraces, the highest attaining a maximum height of c.8m. Sponge Bay Terrace is generally the highest preserved marine terrace, and it is underlain by more than 10 m of estuarine deposits that record the rapid rise of postglacial sea level. The terrace surface records the culmination of this sea‐level rise at 5500 yr B.P. or slightly younger in areas of low average uplift rate (<1.5 m/1000 yr) and c. 7000 yr B.P. in areas of high average uplift rate (>2 m/1000 yr). Lower terraces are usually abrasion platforms with a veneer of intertidal marine deposits, and are interpreted to have been preserved as a result of uplift accompanying large earthquakes. Three tectonic regions are identified on the basis of the height and the deformation of Sponge Bay Terrace. Region A in the northern Raukumara Peninsula is characterised by rapid Holocene uplift in the east and a westerly tilt toward the eastern Bay of Plenty. Much of the study area lies within Region B that is marked by a low (<1.5 m/1000 yr) average uplift rate and the preservation of only one or two terraces below Sponge Bay Terrace. Region C is confined to a small area 20 km northeast of Gisborne and is defined by one of the highest known average rates of coastal uplift (c. 4 m/1000 yr) in New Zealand. Boundaries between regions are poorly defined but suggest the locations of major, active geologic structures.