Gravity anomalies and crustal loading at and adjacent to the Alpine Fault, New Zealand

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Abstract
An analysis of Bouguer and isostatic gravity anomalies from the convergent plate boundary, central South Island, New Zealand, is developed using constraints from seismic refraction data, plate tectonics, and an elastic flexure model. Between 50 and 80 km of late Tertiary plate convergence is assumed so that relatively dense, subducted mantle is included in the gravity model. An elastic plate with a free edge and an effective elastic thickness (Te) that varies laterally between 10 and 30 km is used to simulate the deflection of the Pacific plate beneath the central South Island. Loading on the plate is in the form of topography of the Southern Alps and the excess mass of subducted Pacific mantle. For a weak plate with Te = 10 km, only 50 km of subducted plate, and hence convergence, is required to explain the Bouguer gravity anomalies. For a stronger plate model, where Te varies between 15 and 35 km, c. 80 km of convergence is required. The latter model is preferred as it is more consistent with a seismic determination for dip on top of the Pacific plate. Neither model gives an ideal fit to the observed Bouguer gravity anomalies, thus underscoring the limitations and assumptions inherent in a two‐dimensional elastic plate analysis. The most important departure from a simple elastic plate model occurs over the eastern South Island, where a zone of 30 mgal isostatic gravity anomalies exist. These gravity highs are, instead, accounted for by crustal loading of mobile greywacke‐schist rocks that have been thrust upwards and sideways by the pressure of the growing orogen in the central South Island.