REGIONAL GRAVITY-ANOMALIES, DEPTH OF THE FORELAND BASIN AND ISOSTATIC COMPENSATION OF THE NEW-GUINEA HIGHLANDS

The New Guinea Highlands form an active thrust belt advancing southward over the Australian continental margin. Surface geology indicates thin‐skinned deformation of sediments and continental basement, building mountains 2–4 km in average elevation. The depth of the adjacent foreland basin is no more than 1500 m in Papua New Guinea and in many places only 100–200 m, indicating little deflection of the Australian lithosphere beneath the load of the mountains. Calculations of elastic loading, in which the topographic load of the entire mountain range is supported by Australian lithosphere of uniform flexural rigidity (D), predict a basin 2–10 times deeper than observed in eastern New Guinea for a wide range of flexural rigidities (1021–1025 Nm). Bouguer gravity anomalies in the eastern highlands between 143°E and 144°E are consistent with local Airy compensation of the mountains, and preclude a continuous plate with D > 1022 Nm underlying any significant portion of the mountains. In contrast, the observed broad foredeep flexure in central New Guinea near 141°–142°E requires a strong lithosphere (D = 1024–1025 Nm), and limited gravity and sediment thickness observations farther west indicate similar flexural rigidities. Bouguer gravity measurements and observed foreland basin depths throughout New Guinea can be explained by a strong and continuous lithosphere beneath the foreland basin extending no more than 20–60 km north of the range front, implying that the plate has weakened beneath the thrust belt. Collateral evidence for plate weakening beneath the highlands may be found in earthquakes at 10–25 km depth with high‐angle thrust mechanisms and in widespread Quaternary volcanism throughout the eastern thrust belt. A 50‐mGal positive residual anomaly over the northern 100 km of the eastern highlands cannot be explained by local compensation or mechanical support from the south, indicating either a large excess mass at depth or some additional form of support for the northeastern highlands. The gravity observations in eastern New Guinea require 10–15 km of crustal thickening, but observed sediment thicknesses can only account for 0–5 km, necessitating 5–15 km of thickening in the basement. Copyright 1990 by the American Geophysical Union.