THE GRAVITY FIELD AND STRUCTURE OF PAPUA AND NEW GUINEA

Abstract

Interpreted in the light of existing geological knowledge a regional gravity survey of eastern New Guinea defines the major tectonic units of the area and clarifies its tectonic history. At least 7000 metres of Mesozoic sediments are present in a basin extending from the Aure Trough to West Irian, but reaching its maximum development in the Southern Highlands area. To the south and west of tlfis basin the Fly River platform exhibits an undulating basement resulting from northwest-towest-trending faults or flexures of Lower Cretaceous age. The Kubor Range, forming part of the northern margin of the Southern Highlands basin, appears to have been an intermittently-positive area since the early Mesozoic. Anomalies over the Papuan Ultrabasic Belt are consistent with the origin of this body as a thrust of subcrustal material from the northeast. Originating during the Cretaceous the thrust uplifted metamorphics overlying the ultrabasics, and this rising land northeast of the present Owen Stanley Range provided a source for thick Tertiary sedimentation in the Aure Trough. Transcurrent faulting in the Cape Vogel area separates the small and rising Cape Vogel Basin from the Northern Papuan Basin over the subsiding northeastern edge of the ultra-basic belt. More than 8000 metres of Miocene and younger sediments are present in a basin extending from the Markham valley through to the lower Sepik, the sediments being uplifted by thrusting in the Adelbert, Finisierre and Saruwuged Ranges. The sedimentary section through the Sepik valley is generally less than 3000 metres, with local increases to about 5000 metres in the Nuku Basin and to a possibily greater depth in the Upper Sepik August Rfver Basin.(somatic and free-air anomalies indicate that a zone of isostatic and tectonic instability extends the length of the northern coast of eastern New Guinea. To the south and east of this zone, which is generally less than 150 kilometres wide, the Owen Stanley block, the central highlands and the southern platform are in a state of much greater stability. This present distribution of stable and unstable regions lends support to the concept of the evolution of New Guinea as a series of orogenic zones developing on, and becoming incorporated in, the northeastern margin of the Australian continent.