School of Earth Sciences - Theses
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ItemNeogene vertical motions in intraplate settings: case studies from Australia and Korea( 2012)The lithosphere is dynamically linked with the large-scale dynamics in the convective mantle beneath. While the horizontal lithospheric movements are now well understood in a framework of plate tectonics, it is still a challenge to explain the vertical motions of the Earth’s surface, especially in intraplate regions. Such movements may arise from a variety of causes, such as upper mantle flow and lithospheric responses to plate boundary forces. This thesis explores the unique records of vertical motion in two settings. In Southeast Australia, tectonic modes and rates during the Late Neogene are established from uplifted marine terraces and other geomorphic constraints. Uplift of southern Victoria has occurred at rates of up to ~55 m Myr-1 during the Quaternary, and the rate of uplift has increased progressively since the Middle Pliocene. Non-uniform uplift of coastal Tasmania varies from ~40 m Myr-1 to ~112 m Myr-1. Spatial variations of vertical motions in southern Victoria and Tasmania is order 100’s kms, and is largely explicable in terms of lithospheric flexure, in response to contemporary plate boundary forces coupled with localised lithospheric failures, superimposed on a longer wavelength dynamic topography. The eastern margin of NE Asia, including Korea, sits in the back-arc setting to the Western Pacific Subduction Zone, in the vicinity of a complex trench triple junction of the Philippine, Pacific, and Eurasian plate. Geochronology of the Quaternary marine terraces and fission track dating results of upper Cretaceous granites imply maximum uplift rates of 45.3±1.1 m Myr-1 during the Neogene and 168±16 m Myr-1 in the Late Quaternary. Maximum uplift rates correspond in a general sense with a belt of Quaternary mafic intraplate volcanism suggesting the uplift can be attributed to asthenospheric upwelling around the edges of the sinking Western Pacific slab. The edges of the subducting slab are currently located beneath the eastern margin of the peninsula at a depth of 600 km. In addition, plate boundary forces arising from interactions between the Pacific, Asian and Philippine plate contribute to lithospheric failure relating with faulting movements in the peninsula since the Late Neogene. This research on the long- term vertical deformation field of the continents provides new insights into the way in which the internal dynamics of the Earth modify the surface with the various scales driven by both lithospheric and sub-lithospheric processes.
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ItemLate Neogene stratigraphy and sedimentation across the Murray Basin, southeastern Australia( 2007)The Late Neogene sedimentary sequence of the Murray Basin provides an excellent opportunity to examine paleoenvironmental change across southeastern Australia. A detailed stratigraphic analysis of sediments deposited within the basin in the last 10 Ma was undertaken to assess the influence of tectonic and eustatic processes on deposition. Stratigraphic observations and radiogenic isotope analysis reveals the onset of deposition by 7.2 Ma with a transgressive episode that deposited the marine marls of the Bookpurnong Beds. Deposition was restricted to the central and eastern parts of the basin due to the Hamley Fault. In the west, subsurface elevation contours indicate the presence of incised paleodrainage channels above Miocene limestones, which facilitated the formation of a large estuary system at 5.3 Ma. The sediments of the Norwest Bend Formation were deposited within this western region, while further east, the Loxton-Parilla Sands strandplain deposited over 214 coastal ridges. The basal parts of this unit occur as lateral equivalents to the sediments of the Lower Norwest Bend Formation (in the west). Topographic and magnetic data reveal that tectonism was active during this period and resulted in the erosion and truncation of strandlines. Tectonic evidence and an estimated minimum 28,037 year cyclicity between strandline sets, suggests that the Loxton-Parilla Sands strandlines do not represent an unbroken record of glacioeustatic change. The subaerial exposure of these sediments at approximately 3.0 Ma caused the formation of a calcareous karst above the Norwest Bend Formation and a ferruginous and/or silicious cap (the Karoonda Surface) above the Loxton-Parilla Sands. The stratigraphic position of these surfaces are indicative of a regional widespread unconformity. The Douglas-Blackburn paleodrainage system in western Victoria was dammed during the Mid-Late Pliocene by uplift associated with the Padthaway High, which caused the formation of a 400,000 km2 lacustrine system, known as Lake Bungunnia. Topographic analysis indicates that Lake Bungunnia comprised at least four distinct sub-basins with water depths of up to 30 metres, with lake shorelines indicating that active tectonism occurred during this period. The resulting lack of sediment input to the coast caused the formation of the Kanawinka Escarpment, a large erosional scarp along the southern margin of the Padthaway High. The geomorphology of the modern Murray Basin can be directly attributed to the demise of the Lake Bungunnia system. Movement along the Morgan Fault in the west at approximately 700 Ka, resulted in the draining and progressive drying of Lake Bungunnia as a breach was created along the Padthaway High. The Murray River gorge as observed today was incised following this episode. The modern Murray River (and playa lakes such as Lake Tyrell) occupy the lowest elevations along the former sub-basins of Lake Bungunnia. The Late Neogene sedimentary sequence across the Murray Basin illustrates a complex interaction of eustatic and tectonic processes on deposition. Sedimentation within strandline, estuarine and lacustrine systems, particularly in the western Murray Basin, display evidence of significant tectonic control. This highlights the important role that neotectonic processes have played in shaping southeastern Australia.