Gravity measurements and terrane tectonics in the New Guinea region

Abstract

Composite terranes consist of amalgamations of crustal blocks with differeing geological histories and crustal thicknesses which can usually be identified by conventional geological techniques. Times of amalgamation may be deduced from sedimentological and seismic stratigraphic studies in successor basins and palaeomagnetism can often provide some control on translational history. However, information on the geometries of terrane boundaries at depth is all too often lacking and cannot always be obtained by seismic reflection techniques. A considerable degree of control on this third dimension can be obtained from gravity measurements, especially where oceanic or ophiolitic terranes are in contact with continental blocks. The Papuan Ultramafic Belt of eastern New Guinea provides one excellent example of the ways in which terrane models can be evaluated by considering the gravity fields they would produce. Comparisons of the Bouguer anomaly patterns at various points along the Belt with those observed around geologically less well mapped ophiolites in other parts of New Guinea provide insights into structural settings which cannot be obtained from any of the other data sets available at present. The differences between types of ophiolitic terranes are clearly demonstrated. In some areas, as in the Weyland Terrane of western New Guinea, gravity results, instead of suggesting solutions to existing problems, raise new ones, complicating still further the task of formulating satisfactory structural models. Geological and geophysical studies must be closely integrated, and geologists and geophysicsts must structural closely together, if such problems are to be solved.