GRAVITY-ANOMALIES AND CRUSTAL LOADING AT AND ADJACENT TO THE ALPINE FAULT, NEW-ZEALAND

An analysis of Bouguer and isostatic gravity anomalies from the conver gent plate boundary, central South Island, New Zealand, is developed u sing constraints from seismic refraction data, plate tectonics, and an elastic flexure model. Between 50 and 80 km of late Tertiary plate co nvergence is assumed so that relatively dense, subducted mantle is inc luded in the gravity model. An elastic plate with a free edge and an e ffective elastic thickness (T-e) 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 topog raphy of the Southern Alps and the excess mass of subducted Pacific ma ntle. Far a weak plate with T-e = 10 km, only 50 km of subducted plate , and hence convergence, is required to explain the Bouguer gravity an omalies. For a stronger plate model, where T-e varies between 15 and 3 5 km, c. 80 km of convergence is required. The latter model is preferr ed as it is more consistent with a seismic determination for dip on to p of the Pacific plate. Neither model gives an ideal fit to the observ ed Bouguer gravity anomalies, thus underscoring the limitations and as sumptions inherent in a two-dimensional elastic plate analysis. The mo st important departure from a simple elastic plate model occurs over t he eastern South Island, where a zone of 30 mgal isostatic gravity ano malies exist. These gravity highs are, instead, accounted for by crust al loading of mobile greywacke-schist rocks that have been thrust upwa rds and sideways by the pressure of the growing orogen in the central South Island.