Rates of deformation, uplift, and landscape development associated with active folding in the Waipara area of North Canterbury, New Zealand

Abstract

Analysis of the geometry and ages of faulted and tilted late Quaternary fluvial terraces and their associated cover beds provide evidence of active folding at three localities in the Waipara area of North Canterbury, New Zealand. Terrace survey data, the occurrence of the approximately 22.6-kyr-old Aokautere Ash, and examination of soil profiles indicate that folding has continued into the late Holocene but that the amounts and rates of deformation are locally variable. Rates of uplift in the Waipara area are compared with those derived from marine terraces preserved at the Pacific coast, east of the study area. Results indicate that rates of measurable deformation reach a maximum along the Waipara range front, where bedrock deformation is most intense and shortening rates of up to 5.57±0.69%/100 kyr occur. Across the coastal ranges the average rate of shortening is 0.8±0.4%/100 kyr, which corresponds with an absolute shortening rate of 1.4±0.6 m/kyr and represents only a small proportion of the predicted plate motion vector in this region. Uplift rates range from 0–1.83 m/kyr for a late last glacial fluvial terrace and from 1.36–2.16 m/kyr for three marine terraces. Fluvial and marine terrace uplift rates vary in accord with the geometries of the folds in bedrock, and the spatial pattern of uplift directly reflects fold growth. The structure contour pattern of folded surfaces provides a first approximation to the spatial pattern of uplift. Differential uplift due to folding accounts for up to approximately 55–75% of the total uplift and has produced folds with structural relief of about 1300 m (i.e., amplitudes of 600–700 m). These folds have formed over the last 0.8±0.4 m.y. since the onset of Quaternary deformation in the Waipara region.