A marine geophysical study of the Wilkes Land rifted continental margin, Antarctica

Thesis

The Wilkes Land margin of East Antarctica, conjugate to the southern Australian margin, is a non-volcanic rifted margin that formed during the Late Cretaceous. During 2000-01 and 2001-02, Geoscience Australia (GA) acquired ~10,000 line km of seismic reflection, magnetic anomaly, and gravity anomaly data, on the Wilkes Land margin. Seismic reflection and sonobuoy refraction data provide the first constraints on sediment thickness and images of the deep crustal structure for the extent of the Wilkes Land margin. Two major post-rift seismic-stratigraphic sequences are recognised, separated by a regionally correlatable unconformity. The unconformity is interpreted as Early- to Middle-Eocene (~50 Ma). This unconformity has previously been interpreted to represent the onset of continent-wide glaciation at ~34 Ma. A major unconformity at the base of post-rift sediments is interpreted as a breakup unconformity, of approximately Turonian (85-90 Ma) age. Timing the onset of seafloor spreading using lineated magnetic anomalies within the Australia-Antarctic Basin (AAB) is extremely difficult due to uncertainties in correlating anomalies to the geomagnetic reversal time scale. Modelling indicates that the anomaly commonly correlated to Chron 34y may, in some cases, be associated with high level intrusions and/or serpentinisation of exhumed upper-mantle peridotites. Process-oriented gravity modelling indicates that the Wilkes Land margin lithosphere is characterised by a relatively high effective elastic thickness (Te). Isostatic anomalies are most effectively reduced for models utilising Te = 30 km. Although the margin is broadly characterised by a high Te, zones of low Te are inferred from modelling. Spectral analysis of isostatic anomalies indicates that the power of the flexural isostatic anomalies is lower than the free air gravity anomalies. The margin does not appear to be segmented, at least in regard to its long-term strength. However, a change in initial, zero-elevation crustal thickness (Tc) is inferred from west to east. A Tc of ~35 km is inferred for western Wilkes Land, whereas eastern Wilkes Land is characterised by Tc = 29 to 31 km. Limited seismic refraction data from the conjugate margin indicates a similar trend from southwest to southeast Australia.

Authors: Close, D; David Close

• Publication date: 2005
• DOI: 10.5287/ora-eovvkydnn
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Wilkes Land; subsidence; geophysics; flexure; Antarctica; sedimentology
• Subjects: Marine geology and geophysics