Age, Origin and Vertical Deformation of a Previously Interpreted Regional Erosion Surface: A Multi-Disciplinary Analysis of the Wellington K Surface

Abstract

Measuring and describing horizontal motion in plate boundary zones is relatively straightforward. Vertical movements are, on the other hand, more difficult to measure but they are important as they provide key insights to crustal and upper mantle dynamics.

This thesis is directed towards learning about the vertical movements of a previously interpreted regional erosion surface, termed the K Surface. Sir Charles Cotton (1912) was the first to draw attention to this physiographic feature that dominates the landscape of the western Wellington region from Makara in the south to Paraparaumu in the north. Little information is known about the age, origin and uplift history of this feature, due to a lack of overlying Neogene sediments in the region. A multidisciplinary approach is applied here, using a combination of geological, geophysical and topographic methods to document the age, origin and uplift history of the K Surface in a local and regional context.

A depth profile of cosmogenic ¹⁰Be exposure ages from a 300 m high K Surface remnant suggest that the K Surface was last exposed 238.1− +141 149..3 7 ka (2σ), which correlates with marine isotope stage (MIS) 7. Erosion rates are 1.79 − +0 0..83 59 cm ka ⁻¹, indicative of ˜4 m of total erosion since exposure.

On a regional scale, timing, magnitude, and wavelength of K Surface uplift is consistent with a similar regional uplift in the Wairarapa, while at the same time subsidence is seen offshore, southwest and northwest of Wellington. This coeval uplift and subsidence on a ˜ 70 – 80 km spatial scale is explicable within the context of a simple plate flexure model with a free edge. The flexure is proposed to have been enhanced in the last ˜ 0.3 my when the freely subducting Pacific plate interacts with a barrier that produces a consequent bending moment, on the end of the plate. This barrier is proposed to be mantle lithosphere of the overriding Australian plate that rapidly thickened during the Pliocene. Combining the Isotopic and geologic evidence with geophysical modelling, suggests that the “K Surface” is not a singular regional erosion surface, as previously suggested (e.g. Cotton, 1953; Ota et al., 1981). Rather it is composed of time transgressive marine platforms differentially formed and uplifted by both flexure of the underlying Pacific Plate and smaller scale crustal fault movements.