Gravity anomalies and main tectonic units of the southwest Pacific

Abstract

Thirteen new gravity profiles, 16,000 km in total extent, cover all the major tectonic regions of Melanesia. The isostatic balance of individual features is judged from comparison of free air to Bouguer anomalies. Isostatic anomalies are also shown for two representative profiles. Topography is the key factor producing free air anomalies over the trenches and adjacent ridges despite variations in tectonics. The Manus, South Mussau, and Vityaz trenches along the northern border of Melanesia are less deep than the Tonga and New Hebrides trenches and accordingly show smaller amplitude free air anomaly minima. Long‐wavelength free air highs south of the New Hebrides Trench, over Solomon Sea, and probably west of North Mussau Trench, support the idea of lithospheric uparching suggested by A. B. Watts and M. Talwani. A large amount of our data delineates predominantly positive free air anomalies over the Bismarck and Fiji plateaus, the result being consistent with earlier measurements. A representative profile across the Lord Howe Rise indicates a significant correlation of free air and isostatic anomalies with topography. Gravity anomalies over some local topographic features are discussed.

Geophysics and tectonic development of the Caroline basin / Dewey R. Bracey ; prepared by Commanding Officer, Naval Oceanographic Office ; prepared for Commander, Naval Oceanography Command.

“…There is some indication that the Oligocene magnetic lineations may extend northward beyond the eastern limit of the South of 3.5°N, the Caroline-Pacific Plate boundary, while still compressional, apparently changes from overthrusting of the Caroline Plate to underthrusting of the plate along the Mussau Trench, This trench is generally considered to be the site of incipient subduction (Bracey and Andrews, 1974;Bracey, 1975;Kogan, 1976;Weissel and Anderson, 1978). Bracey and Andrews (1974) felt that the former Pacific-Philippine plate boundary, marked by the Yap-Palau-Ayu Trough may now be extinct, or may be in the process of becoming inactive.…”