Structure of the Alice anticline, Papua New Guinea: serial balanced cross-sections and their restoration

Abstract

The structure of the western part of the Papuan Fold Belt is characterised by both thin-skinned thrusting and basement involved structures. Basement involvement is attributed to inversion of earlier formed extensional faults. The Alice anticline is a frontal structure of the Papuan Fold Belt and formed due to inversion of a Tertiary extensional fault system. Complex forelimb geometry of the Alice anticline changes markedly along strike and suggests varying amounts of shortening in this part of the structure. Balanced cross-sections quantify differential shortening along the anticline, whereas palinspastic restoration shows that the differential shortening is compatible between cross-sections. 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, the 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 detected by application of conventional methods of structural analysis. Rotations about vertical axes are attributed to pinning of progressive, foreland propagating deformation. The distribution of rotations about vertical axes suggest that zones of pinning are coincident with relay zones in the early extensional fault geometry. Two relay zones associated with the original geometry acted as obstructions to deformation and have effectively pinned contractional structures during their formation causing the rotations about vertical axes. Such rotations are not manifested in changing fold axis orientations and have only been derived from a consideration of material balance during deformation.