The Influence of Mechanical Stratigraphy on the Evolution of the Papua New Guinea Fold and Thrust Belt

Abstract

Fold and thrust belts (FTB) form as the crust accommodates shortening due to compressional tectonic forces. They are ubiquitous in orogenic systems and have formed throughout geological times. FTB are studied extensively due to their significance in both hydrocarbon and mineral exploration. The structural architecture of FTB varies widely and is influenced by a multitude of factors including the driving boundary conditions, the amount of shortening and the level of basement involvement (i.e. thin-skinned vs thick-skinned tectonics). Here, we focus on understanding the role mechanical stratigraphy plays in controlling the structural style of FTB. Mechanical stratigraphy refers to the mechanical layering, involving the succession of competent and less competent lithologies, present in a stratigraphic column. The mechanical stratigraphy is influenced by the thickness, the competence contrast and the degree of coupling between sedimentary layers. For a given set of boundary conditions and amount of shortening, the mechanical stratigraphy is expected to influence the partitioning of shortening between folds and thrusts, their respective distribution pattern, their respective wavelengths, and possibly the nature and expression of their interaction. An understanding of the mechanical stratigraphy is crucial for assessing structural traps, fluid flow, and refining geologic interpretations. We aim to understand to what extent the position and depth of the competent layers within a multilayer system controls the overall structural style of FTB. We also investigate how décollement horizons control the accommodation of convergence via strain partitioning between décollement and detachment faults. We run two-dimensional, coupled thermal and mechanical, numerical experiments using the Underworld framework to explore the effects of mechanical stratigraphy on the structural evolution of FTBs. The numerical domain is 192 km in length and 38 km in depth, with a grid resolution of 500 m. Boundary conditions involve pushing a rigid wedge into a stack of layers with contrasting mechanical properties. This preliminary study has allowed us to test a range of rheologies and weakening mechanisms enabling the formation of realistic sets of structures. These insights are then used to run numerical simulations of the Papua New Guinea (PNG) Fold and Thrust belt. The PNG-FTB is a result of the ongoing oblique collision between Australia's northern margin and the Pacific plate since ca. 45 Ma. The main collisional phase began in the Miocene, inverting a rift sequence of sedimentary rock deposited on an extended basement. Shortening of the Muller ranges was in part accommodated through the inversion of pre-existing extensional faults in the basement. These pre-existing extensional faults and the mechanical stratigraphy are key controlling factors in the development of the PNG-FTB. The cover sequence is influenced by the presence of strong layers including the thick Miocene Darai limestone that caps underlying much weaker mudstones and shales. Using a set of well-imaged structures along PNG-FTB, we attempt to calibrate the mechanical stratigraphy. We aim to propose a coherent set of structures in parts of the FTB where the structural geology is less understood. Our models demonstrate the control mechanical stratigraphy has on multi-wavelength deformation of the PNG-FTB. The effect of mechanical stratigraphy is seen at both the outcrop and landscape scale. At the outcrop scale, deformation of the competent units contrasts from the deformation style of the incompetent units, creating a set of small-scale structures comparable to those seen at the landscape scale. The competent units, such as the darai limestone deform via faulting, whereas the incompetent Mesozoic sediments tend to fold. Models with a calibrated mechanical stratigraphy are able to reproduce structures similar to those found in the New Guinea Fold Belt, reveal the structure of the lesser constrained areas of the FTB, and illuminate subsurface geometry.

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