Pliocene exhumation and landscape evolution of central North Island, New Zealand: The role of the upper mantle

Abstract

Mudstone porosity analyses show that over 2 km of exhumation has occurred in central North Island, New Zealand, since 5 Ma. This, combined with a coeval surface uplift of ∼500 m, indicates a maximum 2.5 km of rock uplift in central North Island. The magnitude and wavelength (∼400 km) of the rock uplift is similar to some topographic doming observed in continental areas associated with mantle upwelling, yet central North Island is recognized as a region of back arc extension. Marine terraces 100–150 km from the center of maximum exhumation show uplift rates of ∼0.2–0.3 mm yr−1, consistent with about 1 km of exhumation if averaged over 4–5 Myr, which compares well with mudstone porosity analyses. Seismic attenuation and slow Pn velocities indicate that a thermal anomaly in the upper mantle is the source of this rock uplift. The rock uplift may be explained by the replacement of a 100‐km‐thick mantle lid by asthenosphere with a density ∼66 kg m−3 less than the lid. About half this density contrast can be explained by thermal expansion, and the rest is ascribed to partial melts in the upper mantle. Southward migration for the locus of maximum rock uplift is consistent with the zone of maximum incision erosion, being 50 km south of the southernmost volcano. Such a migration is in keeping with other geological markers of volcanic and thermal phenomena and further strengthens the link of landscape evolution at active continental margins with processes in the mantle.