The gravity field in New Guinea

Interpretation of a regional gravity survey, of eastern Newi Guinea has been carried out with the intention of determining the geologically most probable structures consistent with the anomalies. This approach has emphasized several upper-crustal features not obvious from geological studies: a mantle-thrust origin for the Papuan Ultrabasic Belt; a deep Mesozoic sedimentary basin in the Southern Highlands; upthrusting of very deep sediments in the Adelbert, Finisterre and Saruwaged Ranges; and northeast-trending transcurrent faulting in the Madang and Goropu Mountains areas. Qualitatively, the anomaly maps indicate that the northern coastal strip of eastern New Guinea is isostatically unstable, while the central cordillera and the southern platform are broadly compensated. The following evolutionary sequence, based on gravity interpretations over the island of New Guinea, is proposed for that type of geosyncline developed on the northeastern Australian continental margin: (a) Development of, and sedimentation in, a deep geosynclinal trough, with concomitant thinning of the crust - an apparently tensional feature (b) Vertical uplift of the sediments by expansion within the upper mantle, accompanied by the addition of some light material to the base of the crust (c) Continued uplift by mantle expansion; intrusion into the geosyncline of granodioritic magma from the mantle, and continued addition of this magma to the base of the crust resulting in the eventual stabilization of the uplifted area by a broad isostatic root. Gravity anomalies over other parts of Melanesia emphasize that changes within the upper mantle are of fundamental importance in a wide range of tectonic processes. A rigorous procedure for topographic reduction of the observations, demanded by the mountainous nature of the region, involves the expression of the topography as vertical square prisms of the correct density, and the aggregation of their gravitational effects at the observation point; the computation being carried out to a distance of 540 kilometres on a spheroidal earth. Pratt and Airy isostatic anomalies are computed on the basis of local compensation of each topographic prism, and the topographic and isostatic correction procedures are combined in a comprehensive computer program. A running-means smoothing technique enhances the low-spatial-frequency components of free-air, Bouguer and isostatic anomaly fields