Alexey Goncharov

Eocene to Miocene geometry of the West Antarctic Rift System

Tectonic models for the Late Cretaceous/Tertiary evolution of the West Antarctic Rift System range from hundreds of kilometres of extension to negligible strike-slip displacement and are based on a variety of observations, as well as kinematic and geodynamic models. Most data constraining these models originate from the Ross Sea/Adare Trough area and the Transantarctic Mountains. We use a new Antarctic continental crustal-thinning grid, combined with a revised plate-kinematic model based on East Antarctic – Australia – Pacific – West Antarctic plate circuit closure, to trace the geometry and extensional style of the Eocene – Oligocene West Antarctic Rift from the Ross Sea to the South Shetland Trench. The combined data suggest that from chron 21 (48 Ma) to chron 8 (26 Ma), the West Antarctic Rift System was characterised by extension in the west to dextral strike-slip in the east, where it was connected to the Pacific – Phoenix – East Antarctic triple junction via the Byrd Subglacial Basin and the Bentley Subglacial Trench, interpreted as pullapart basins. Seismic-reflection profiles crossing the De Gerlache Gravity Anomaly, a tectonic scar from a former spreading ridge jump in the Bellingshausen Sea, suggest Late Tertiary reactivation in a dextral strike-slip mode. This is supported by seismic-reflection profiles crossing the De Gerlache Gravity Anomaly in the Bellingshausen Sea, which show incised narrow sediment troughs and vertical faults indicating strike-slip movement along a north – south direction. Using pre-48 Ma plate circuit closure, we test the hypothesis that the Lord Howe Rise was attached to the Pacific Plate during the opening of the Tasman Sea. We show that this plate geometry may be plausible at least between 74 and 48 Ma, but further work especially on Australian – Antarctic relative plate motions is required to test this hypothesis.

Linking active margin dynamics to overriding plate deformation: Synthesizing geophysical images with geological data from the Norfolk Basin

The Tonga-Kermadec subduction system in the southwest Pacific preserves a series of crustal elements and sediments which have recorded subduction initiation, rift, and back-arc basin formation. The Norfolk Basin is the farthest landward of all back-arc basins formed in the Tonga-Kermadec region and may preserve the earliest record of subduction initiation regionally. For the Norfolk Basin, we use a set of multibeam bathymetry, magnetic, and seismic reflection and refraction data to constrain basin structure and the mode and timing of formation. A structural interpretation reveals a two-stage tectonic evolution: (1) a nconvergent tectonic regime until 38–34 Ma, alternatively related to island arc collision or subduction initiation, and (2) lithospheric extension after 34 Ma. These observations may help to constrain mechanical models that predict rapid extension following convergence of the overriding plate during subduction initiation or arc reversals.

Basement and crustal controls on hydrocarbon maturation : Lessons from bremer sub-basin for other frontier exploration areas

A consistent approach to the assessment of basement and crustal controls on hydrocarbon maturation in the Bremer Sub-basin, offshore southwest Australia, has been undertaken as part of the Australian Government’s Big New Oil initiative. Geoscience Australia acquired marine reflection seismic survey in this area during late 2004 in conjunction with recording of refraction seismic data by sonobuoys at sea and by land stations in the onshore/offshore observation scheme. One of the key findings of the refraction seismic study is that velocities in the basement are generally in the 5.0–5.7 km/s range, indicating that, contrary to prior expectations, basement in the area is mostly not granitic in composition. Results from the conjugate margin in Antarctica also show low velocities in the basement on the inner side of Antarctic continent-ocean boundary, consistent with results from the Australian margin. It appears that a ~400-km-wide zone in Gondwana prior to break up had basement velocities significantly lower than the normal continental values of 6.0–6.2 km/s most commonly associated with granites and gneisses. Low-grade metasediments of the Albany-Fraser Orogen and its Antarctic equivalent is the preferred interpretation of this observation. Granites, dredged from the sea floor in the Bremer area, may represent only a small fraction of the basement, as within the basement highs where higher velocities have been detected by refraction work. As metasediments produce substantially less heat than granites, a different scenario for hydrocarbon maturation in the Bremer Sub-basin is possible. To quantify possible heat production in the Bremer basement and crust below it we have used contents of radioactive elements in rock samples taken from outcrops of Yilgarn Craton and Albany-Fraser Orogen onshore, as well as in rock samples dredged from the sea floor in the Bremer Sub-basin. Advanced burial and thermal geohistory modelling in this area was carried out using Fobos Pro modelling software for the first time in Australia without relying on default or inferred values (such as heat flow or geothermal gradient). Modelling showed that subsidence curves can be matched in various basement composition scenarios, but the high heat-producing granitic scenario leads to a present-day surface heat flow of 68 mW/m2 predicted by the model—unrealistically high given the context of heat flow measurements on the Australian Southern Margin. Other basement compositions (low heat- producing granite, metasediments, basalts) lead to a present-day surface heat flow of 46–57 mW/m2 and cannot be ruled out on the basis of heat flow modelling and data alone. This work details a methodologically consistent approach to burial and thermal geohistory modelling for other frontier areas where appropriate geophysical data have been collected.

Southbound migration corridor of pygmy blue whales off the northwest coast of Australia based on data from ocean bottom seismographs

A line array of 14 ocean bottom seismographs was deployed on the Exmouth Plateau northwest of the North West Cape in Western Australia in December 2014-January 2015. Acoustic data collected with this array were used to evaluate the corridor of the southbound migration of pygmy blue whales of the eastern Indian Ocean population. It is found that pygmy blue whales tended to travel southward much further away from the Western Australian coast, at distances of up to 400u2009km from shore, than that expected from data on their northbound migration. This is an important observation providing additional information on the migration pattern of pygmy blue whales, which is crucial for assessing their population and migration by passive acoustic means.

Australian national ocean bottom seismograph fleet advances conventional exploration

A fleet of new Australian ocean bottom seismographs (OBSs) have broadband frequency range, and similar instruments are available at only five or six institutions globally. These OBSs are multi-purpose devices able to record passive-source seismic data (earthquakes, ambient noise) as well as active-source (airgun generated) data and, at the same time, to monitor seismic survey noise and whale calls for environmentally responsible exploration. OBS data collected during commercial seismic surveys in Australian waters prove that it is possible to image the velocity distribution of the whole crust and upper mantle from analysis of both reflected and refracted phases generated by an industry-standard broadband airgun array. This means that valuable information on a regional scale can be obtained as a by-product of commercial seismic surveys. Three-component recording capability of OBSs allows analysis of S-waves in addition to the P-waves that are conventionally used in marine reflection surveys.