Constraining the Regional Slope of Western Papua New Guinea: A Study of Lithospheric Flexure

Abstract

A successful balanced and restored section in a fold-thrust belts (FTB) requires definition of the regional slope. This is assumed to be the same as the structural dip of the basal décollement. This study presents 2D and 3D models of lithospheric flexure to constrain the regional slope of Western Papua New Guinea (PNG). The regional slope is a primary influence to the geometry of the structures and affects the prospectivity of a region. In PNG FTB it is challenging to determine regional slope from the seismic reflectors due to the poor-quality 2D seismic data. The incoherent near basement reflections and the artifacts produced during the depth conversion of the over-thrust structures lead to inaccurate predictions of this slope. Assessment of the regional slope involved modelling the lithospheric flexure along four 2D regional transects in the direction of maximum shortening. Load geometries were defined as a composite of individual columns of defined height, width and density. Well data from 20 onshore locations across the PNG foreland basin and highlands gave an average density for the sedimentary cover and basement. A literature review provided the average density for the mantle and elastic properties of the lithosphere. The elastic thickness (Te) was adjusted for each section to achieve the best fit model. Transects show a variable Te, decreasing from the foreland basin into the hinterland. There are also noticeable changes to the flexure profiles, including an increase in depth of flexure, decrease in Te and narrowing of the flexure wavelength in an eastward direction. A 3D flexure model generated using the concepts and methodology of the 2D modelling allowed construction of 3D load and Te surfaces for the region. Te and load profiles were interpolated between 2D flexure models to generate a 3D gridded surface for both parameters. The resultant model showed a good correlation with the 2D flexure profiles. Through the process of modelling flexure, a consistent trend of decrease in the Te was observed, varying from 40km in the northwest to 20km in the southeast. This feature is attributed to the strength of the lithosphere. Lithospheric strength decreases significantly along the axis of the central New Guinea highlands with elastic thickness as low as 5km. The trends have been attributed to both the thermal conditions and the mechanical weakening of the lithosphere. The dip of the regional flexure was computed for the main structures of PNG. The regional slope is an integral parameter of structural modelling. The uncertainties associated with regional slope are reduced through modelling the 3D flexure of the lithosphere. The process of flexure modelling revealed trends in strength properties of the lithosphere relating to structural style and the tectonic model in PNG.

AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020