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

摘要

New Guinea forms the northern margin of the Australian Plate which is nowudcharacterised by a zone crustal deformation and accreted terranes. The present dayudconfiguration is the result of global tectonics in the southwestern Pacific since theudTriassic. The Papuan Fold Belt is located within Papua New Guinea, the eastern half ofudNew Guinea, and comprises deformed basement, platformal and basinal Mesozoic andudTertiary sediments. Deformation within the fold belt commenced possibly as early asudMiddle to Late Miocene and is currently continuing.udThe structure of the western part of the Papuan Fold Belt is characterised by thin skinnedudthrusting and basement involved structures, the latter attributed to inversion ofudextensional faults active in the Tertiary and the Mesozoic. Inversion is thought to haveudpost-dated the initiation of thin skinned thrusting by approximately 5 Ma. Continuedudbasement shortening may be due to the current high convergence rate between theudAustralian and Pacific Plates.udThe Alice Anticline formed due to inversion of a Tertiary extensional fault system.udThree-dimensional restoration of the Alice Anticline makes use of a series of balancedudcross-sections and is based on a line length method. Paradoxically, this restorationudreveals non-plane strain in the balanced cross-sections upon which it relies. However, theudrestoration also reveals and quantifies a component of rotation about vertical axes whichudwould not be obvious by application of conventional methods of structural analysis. Twoudtransfer zones associated with the original extensional geometry acted as obstructions touddeformation and have effectively pinned contractional structures during their formationudcausing the rotations about vertical axes.udA general fracture system is developed in rocks in the Alice Anticline area. This typicallyudcomprises two sets of conjugate shear fractures and a third set, interpreted as extensional,udwhich is sub-nonnal to the acute bisector of the two conjugate sets. Unfolding of beddingudusing the three-dimensional restoration results in a symmetrical geometric relationshipudbetween the general fracture system and folds. The mechanical interpretation ofudfractures, their geometric relationships and the timing constraints on their formation areudconsistent with folding.udThe structure of the Ok Tedi mine area is complicated by the presence of approximatelyudsyn-tectonic intrusive bodies. The development of the Parrots Beak and Taranaki Thrustsudas 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 withudthe regional shortening direction. Reduced stress tensors determined for the FubilanudMonzonite Porphyry are related to emplacement processes and would be consistent with development of radial and concentric fracture sets.