The last million years of deformation in part of the New Zealand plate-boundary zone

Abstract

Tectonic deformation of Pleistocene sediments is occurring within the plate-boundary zone in the North Island of New Zealand. Folds, associated with an active fault, are forming mainly by flexural-slip, and contain comformable and unconformable relations which show that the folding started ca 1.0 Ma. Simple geometric models of fold growth indicate that the fold structure has shortened at an average rate of 0.76 ± 0.1 mm a −1 across 4.5 km, though with an increase in the last 100 Ka of about 1.8 times the average rate, which is entirely compatible with the rate of shortening observed in the last 50 years from retriangulation studies. Areas for 15 km to the southeast of the fold structures show significantly lower horizontal shortening rates. The strain rates across the plate-boundary zone vary both with position and time, with a negative correlation between the uplift pattern and the inferred geological shortening rates. This can be explained by the presence of large semi-rigid blocks within the plate-boundary zone which are back-tilting and overriding weaker more rapidly deforming areas. The available evidence suggests that a large proportion of the deformation is occurring seismically.