--> Extensional Structures in Foreland Basin Development: Insights From Northwestern Bonaparte Basin, Australia

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Extensional Structures in Foreland Basin Development: Insights From Northwestern Bonaparte Basin, Australia

Abstract

The concept and our understanding of foreland basin systems has largely advanced over the last four decades through studies focused on: 1) foreland-associated fold and thrust belts, 2) stratigraphy-driven basin evolution research, 3) rigidity and flexural profiles of lithosphere, and 4) geodynamics. In comparison, few studies have concentrated on extensional structures in these compression-dominated systems. This is probably because flexural extension is 1) a transient phase during foreland system development, 2) easily superimposed and modified by subsequent compressional structures, 3) usually covered by thick sediments of the foreland sequence, which makes the out-crop exposure limited and/or recognition in seismic challenging, and 4) accompanied by significant involvement of pre-existing fabrics. Extensional structures are crucial in understanding the early stage evolution of foreland basin development. Based on newly released industrial subsurface data, our study centers on characteristics of a Neogene normal fault system in NW Bonaparte Basin, which is the foredeep part of the underfilled Timor foreland basin. 2D regional cross-sections stretching from Timor to Australian shelf clearly document the spatial configuration of the thrust front, crustal down-warping and flexural normal faulting. Neogene normal faults developed on the Australian shelf during the crustal downwarp, with the rapid subsidence of present-day foredeep from −20m to −2500m in late Pliocene to Pleistocene. These Neogene normal faults include newly formed normal faults and reactivation of Jurassic rift-related normal faults, with their fault density gradually decreasing landward. Instant fault throw analysis on regional sections shows the along-dip variation of the normal fault activity during crustal down-warping. In Bonaparte Basin, en echelon normal fault arrays with different stepping patterns are observed in several places with different oriented underlying Jurassic structural grains, which altogether indicate a WSW–ENE Timor foreland flexural wave. In addition, based on our study and synthesis of the current understanding of Timor foreland system, we propose that 1) Neogene flexural extension in NW Bonaparte Basin is decoupled from the sinistral transpressive nature of Timor fold and thrust belt by the thrusting décollement, and 2) flexural extension of Timor foreland is mainly derived from tectonic loading and subduction, instead of lithospheric buckling.