Structural evolution of the Western Papuan Fold Belt , Papua New Guinea

Abstract

New Guinea forms the northern margin of the Australian Plate which is now characterised by a zone crustal deformation and accreted terranes. The present day configuration is the result of global tectonics in the southwestern Pacific since the Triassic. The Papuan Fold Belt is located within Papua New Guinea, the eastern half of New Guinea, and comprises deformed basement, platformal and basinal Mesozoic and Tertiary sediments. Deformation within the fold belt commenced possibly as early as Middle to Late Miocene and is currently continuing. The structure of the western part of the Papuan Fold Belt is characterised by thin skinned thrusting and basement involved structures, the latter attributed to inversion of extensional faults active in the Tertiary and the Mesozoic. Inversion is thought to have post-dated the initiation of thin skinned thrusting by approximately 5 Ma. Continued basement shortening may be due to the current high convergence rate between the Australian and Pacific Plates. The Alice Anticline formed due to inversion of a Tertiary extensional fault system. Three-dimensional restoration of the Alice Anticline makes use of a series of balanced cross-sections and is based on a line length method. Paradoxically, this restoration reveals non-plane strain in the balanced cross-sections upon which it relies. However, the restoration also reveals and quantifies a component of rotation about vertical axes which would not be obvious by application of conventional methods of structural analysis. Two transfer zones associated with the original extensional geometry acted as obstructions to deformation and have effectively pinned contractional structures during their formation causing the rotations about vertical axes. A general fracture system is developed in rocks in the Alice Anticline area. This typically comprises two sets of conjugate shear fractures and a third set, interpreted as extensional, which is sub-nonnal to the acute bisector of the two conjugate sets. Unfolding of bedding using the three-dimensional restoration results in a symmetrical geometric relationship between the general fracture system and folds. The mechanical interpretation of fractures, their geometric relationships and the timing constraints on their formation are consistent with folding. The structure of the Ok Tedi mine area is complicated by the presence of approximately syn-tectonic intrusive bodies. The development of the Parrots Beak and Taranaki Thrusts as floor and roof thrusts respectively constitutes shortening estimates in the mine area which are consistent with those determined regionally. Striation analysis of rnesoscale faults from country rocks in the mine area reveals a reduced stress tensor compatible with the regional shortening direction. Reduced stress tensors determined for the Fubilan Monzonite Porphyry are related to emplacement processes and would be consistent with development of radial and concentric fracture sets.