School of Earth Sciences - Theses

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    The structure and evolution of the northern Australian margin: Insights from the Papuan Fold and Thrust Belt, Papua New Guinea
    Mahoney, Luke George ( 2021)
    The Papuan Fold and Thrust Belt (PFTB) in Papua New Guinea (PNG), located on the leading edge of the northern Australian continental margin, has been subject to complex tectonism as a result of its location throughout much of the Cenozoic between the obliquely converging Australian and Pacific plates. The remoteness and inhospitable terrain characterising the PFTB make it one of the least well-known fold and thrust belts on Earth. The architecture of the northern Australian continental margin has been affected by both extensional and compressional tectonic forces, which first formed, and subsequently deformed, the Papuan Basin in the period from the early Mesozoic through to the present-day. Defining the geology, structure and evolution of the PFTB and Papuan Basin is central to our understanding of the geological and tectonic evolution of the northern Australian margin. In this thesis, a multidisciplinary approach is used to investigate the evolution of the PFTB, Papuan Basin and northern Australian continental margin. Field mapping and structural analysis within the remote Western Fold and Thrust Belt (WFTB) provide significantly improved constraints on the geology, structure and evolution of the fold belt. New geological constraints acquired over > 100 km of traverses suggest that the exposed Cenozoic Darai Limestone has very low shortening between ~ 12-22% yet structures in the Muller Range are elevated up to 7 km above regional. Structural work utilising regional-scale geological observations suggest that the inversion of pre-existing rift architecture on the northern Australian continental margin is the primary influence on the evolution of the area. The huge structural relief is produced by both tectonic inversion on deep-rooted normal faults and their linkage to the surface via triangle zones that form within the incompetent Mesozoic passive margin sedimentary sequence. Local- and regional-scale heterogeneities within the northern Australian continental margin, such as accommodation-zones and transfer structures are now expressed in the fold belt structure as discontinuities and cross-cutting structural features that are recognised throughout the PFTB. The 2018 Mw 7.5 PNG Highlands earthquake and aftershock sequence has provided an unprecedented opportunity to observe and analyse the crustal processes that have ultimately controlled the evolution of the PFTB. Seismological, GPS and remote sensing data offer constraint on the complex nature and spatiotemporal distribution of crustal deformation during the event, revealing that the PFTB experienced up to 1.2 m of uplift and ground deformation over 7,500 km2. Remarkable spatial and morphological similarities exist between the distribution of coseismic ground deformation associated with the event, and the less-inverted and uninverted extensional architecture that is well-constrained in the foreland across the Stable Platform. This suggests that the 2018 Highlands earthquake sequence was related to tectonic inversion along a previously unidentified extensional fault system beneath the PFTB, indicating the northern Australian passive margin has had a primary control on the evolution of structural styles observed throughout the PFTB. New low-temperature thermochronology data from extensive field surveys in the Muller Range were combined with legacy data in modern thermal history modelling tools to investigate the thermotectonic evolution of the WFTB and Papuan Basin. In particular, the Late Cretaceous to Oligocene history of the region is largely unknown due to the absence of a continuous stratigraphic record. Thermal history models based on these data suggest two major Cenozoic cooling episodes. The youngest, and best constrained, is clearly recorded in the stratigraphic record and relates to Neogene collision at the northern margin of the Australian continent. An older episode of comparable or greater magnitude occurred in the Eocene to Oligocene and may relate to the removal of 1,500-3,000 m of Late Cretaceous to Eocene stratigraphic section across the Muller Range prior to the widespread deposition of the shelfal Darai Limestone. It is suggested that extension along major faults beneath the Muller Range accommodated sedimentation from the Late Cretaceous to the Eocene, consistent with long-lived extensional structures observed in the foreland across the Stable Platform. The selective removal of this sequence across the Muller Range suggests it was uplifted in the Eocene to Oligocene, possibly in part facilitated by the inversion of extensional faults in the Muller Range area. This inversion is interpreted to have resulted from the Eocene to Oligocene collision of the expansive Sepik Terrane to the northwest of the PNG margin, an interpretation that has significant implications for the tectonic evolution of PNG and Southeast Asia. The studies presented in this thesis provide several key insights that significant advance our understanding of the geological, structural and tectonic evolution of the PFTB, Papuan Basin and northern Australian margin. An ongoing theme relates to the complex interplay between spatial variations in the architecture of the margin and spatial and temporal variations in the compressional stress field associated with an evolving tectonic setting between the Australian and Pacific plates.