Irina Borissova

Structure and development of a microcontinent: Elan Bank in the southern Indian Ocean

Microcontinents appear to commonly form on young continental margins close to hot spots, but difficulties in understanding their geology and evolution have inhibited assessment of their global distribution and significance. Thick volcanic accumulations in areas affected by hot spot magmatism only complicate the issue. Elan Bank, a large western salient of the Kerguelen Plateau, is a microcontinent that originally lay between India and Antarctica in Gondwana. Recent regional plate tectonic reconstructions suggest that during Gondwana breakup, Elan Bank and India initially separated from Antarctica, and Elan Bank became isolated in the Southern Ocean via a ridge jump to the north between Elan Bank and India. In Albian time (∼108 Ma), voluminous magmatism attributed to the Kerguelen hot spot overprinted and radically altered the original microcontinent and its surroundings. Recent ODP investigations, deep seismic reflection data, and a wide-angle seismic line on Elan Bank allow us to gain the first insight into the features integrated crustal structure and geological evolution and the adjacent continent-ocean transition zone. Our analysis shows that Elan Banks crust is at least 16 km thick. The upper igneous crust consists of a 2–3 km thick layer with seismic velocities ranging from 4.4 to 5.9 km/s that can be interpreted as the result of accumulation of lava flows originating from the Kerguelen hot spot. Seismic velocities at the base of the crust are as low as 6.6 km/s, which is consistent with a fragment of thinned continental crust ∼14 km thick. A high velocity body, located at depths of 5 to 10 km, could be interpreted as plutonic rocks emplaced during the major regional magmatic episode. On the basis of deep seismic reflection data, we interpret extensional structures beneath the volcanic flows. In Albian time, when the area was affected by the Kerguelen hot spot, volcaniclastic material and lava flows accumulated in faulted grabens and basins both on the bank and within the continent-ocean transition zone to the south, creating the appearance of flat, unstructured basement. The seismic structure and inferred composition of Elan Bank revealed by this study contribute to our understanding of microcontinent formation as well as provide a template for identifying microcontinents in accreted terranes and mountain belts.

Naturaliste Plateau, offshore Western Australia: A submarine window into Gondwana assembly and breakup

The origin of the submarine Naturaliste Plateau off the southwestern coast of Australia is controversial; previous work supports both oceanic and continental affinities for the basement to volcanic and sedimentary sequences. We report the first evidence of reworked Mesoproterozoic (ca. 1230–1190 Ma) continental crust, based on laser ablation–inductively coupled plasma–mass spectrometry analysis of zircons from granite and orthogneiss samples dredged from the southern margin of the plateau. Thermobarometry of peak metamorphic minerals and electron microprobe chemical dating of monazite reveal that these igneous rocks were metamorphosed to ~700 °C and ~6.5 kbar during the Cambrian Pinjarra Orogeny at ca. 515 Ma. These data confirm a continental origin for a significant swathe of the southern Naturaliste Plateau, and suggest that the protoliths may have affinities to Mesoproterozoic crust within the Albany-Fraser-Wilkes Orogen (Australia-Antarctica). The present Naturaliste Plateau basement beneath its volcanic carapace probably represents a middle-to lower-crustal extensional allochthon exhumed during Cretaceous hyperextensional breakup between Australia and Antarctica.

Nature of the continent–ocean transition zone along the southern Australian continental margin: a comparison of the Naturaliste Plateau, SW Australia, and the central Great Australian Bight sectors

Abstract We document the interpretation of three crustal sections from coincident deep seismic reflection, gravity and magnetic data acquired on Australias southern margin: one section from the Naturaliste Plateau and the Diamantina Zone; and two in the Great Australian Bight (GAB). Interpretations are based on an integrated study of deep multichannel seismic, gravity and magnetic data, together with sparse sonobuoy and dredging information. All interpreted sections of the margin show a transition from thinned continental crust, through a wide continent ocean transition zone (COTZ). In the GAB the transition is to slow sea-floor spreading oceanic crust that dates from breakup in the Campanian (c. 83 Ma); in the Naturaliste–Diamantina margin the earliest oceanic crust is undated. The COTZ on these margins is geologically and geophysically complex, but interpretation of all data, including dredge hauls, is consistent with the presence of a mixture of modified continental lower crust, breakup related volcanics and exhumed continental mantle. Serpentinized detachment faults are not well imaged, but have been inferred from high-amplitude magnetic signatures interpreted to arise from magnetite associated with the hydration of peridotites. Alternative models for the structure of the COTZ, involving either mafic underplating or aborted sea-floor spreading, have been explored, but are considered unlikely on this margin. Similarity in the final architecture of these margins has major implications for the nature of rifting in the Southern Rift System, and may point to the entire 4000 km-long system being non-volcanic in character. Second-order differences in geometry and morphology of the two areas studied are unlikely to be a function of strain rate. Instead, they probably reflect complexities owing to the multiple tectonic events that occurred during final Gondwanide fragmentation. The most dramatic of these is the impact of hotspot activity in the Kerguelen Plateau, which commenced some 50 Ma prior to final breakup in that sector.

The Structure of the Continental Margin off Wilkes Land and Terre Adélie Coast, East Antarctica

In 2001 and 2002, the Australian Government acquired approximately 9 000 km of high-quality geophysical data over the margin of East Antarctica between 110–142° E that provide a sound framework for understanding the geology of the region. The data comprise 36-fold deep-seismic, gravity and magnetic data and non-reversed refraction/wide-angle reflection sonobuoys recorded along transects that extend from the lower continental slope out to oceanic crust at a spacing along the margin of approximately 90 km. The continental slope is underlain by a major rift basin beneath which the crust thins oceanwards through extensive faulting of the rift and pre-rift sedimentary section and by mainly ductile deformation of the crystalline crust. Outboard of the margin rift basin, the 90 to 180 km wide continent-ocean transition zone is interpreted to consist primarily of continental crust with magmatic components that can account for the lineated magnetic anomalies that have been interpreted in this zone. The thick sedimentary section in the COT zone is floored by dense lower crustal or mantle rocks indicating massive (>10 km) thinning of the lower and middle crust in this zone. The boundary between the margin rift basin and the COT is marked by a basement ridge which potential field modelling indicates is probably composed of altered/serpentinised peridotite. This ridge is similar in form and interpreted composition to a basement ridge located in a similar structural position at the inboard edge of the COT on the conjugate margin of the Great Australian Bight. On both margins, the ridge is probably the product of mantle up-welling and partial melting focussed at the point of maximum change/necking of crustal thickness. Integrated deep-seismic and potential field interpretations point very strongly to the boundary between unequivocal oceanic crust and largely continental crust of the continent-ocean transition as lying in very deep water, and considerably seaward of most previous interpretations (often based on inadequate seismic data or magnetic data only). We consider the continent-ocean boundary to be well-constrained from 124–131° E and unequivocal from 131–140° E, but open to debate in the sector from 110–124° E. There is a strong degree of pre-breakup symmetry between the conjugate margins of southern Australia and East Antarctica east of about 120° E. In addition to the crustal symmetry, there is also a strong correlation in seismic character between the margins, which allows us to date the major unconformities as probably of base Turonian, Maastrichtian and early Middle Eocene age.

Age and geochemistry of magmatism on the oceanic Wallaby Plateau and implications for the opening of the Indian Ocean

The temporal relationship between tectonic and volcanic activity on passive continental margins immediately before and after the initiation of mid-ocean ridge spreading is poorly understood because ...

Geology and hydrocarbon prospectivity of the northern Houtman Sub-basin

New 2D seismic data acquired by Geoscience Australia in the northern Houtman Sub-basin of the Perth Basin provides important information on the prospectivity of this frontier area. To date, lack of quality seismic data and limited geological understanding have led to the perception that the hydrocarbon potential of the area is very low. However, interpretation of newly collected data suggests that the northern Houtman depocentre contains up to 15 km of pre-breakup sediments comprised of Permian, Triassic and Jurassic successions, which potentially contain multiple source rock, reservoir and seal intervals. The Permian syn-rift succession is confined to a series of large half-graben that are controlled by basement-involved faults separating the Houtman depocentre from the Bernier Platform. This succession is up to 10 km thick and is mapped throughout the inboard part of the new seismic grid. A prominent unconformity at the top of the Permian syn-rift sequence is overlain by a thick (up to 1800 m) and regionally extensive seismic sequence interpreted as the Lower Triassic Kockatea Shale. The thickness of the overlying Triassic succession ranges from approximately 1 km in the inboard part of the basin to up to 5 km further outboard. The Jurassic succession is thickest (up to 4 km) in the outboard part of the basin and is interpreted to contain sequences corresponding to the Cattamarra, Cadda and Yarragadee formations. Our study integrates new results from regional mapping, geophysical modelling and petroleum systems analysis, which enables a more accurate prospectivity assessment of this frontier basin.

Petroleum systems analysis of the northern Houtman Sub-basin

Interpretation of newly acquired seismic data in the northern Houtman Sub-basin (Perth Basin) suggests the region contains potential source rocks similar to those in the producing Abrolhos Sub-basin. The regionally extensive late Permian–Early Triassic Kockatea Shale has the potential to contain the oil-prone Hovea Member source interval. Large Permian syn-rift half-graben, up to 10 km thick, are likely to contain a range of gas-prone source rocks. Further potential source rocks may be found in the Jurassic–Early Cretaceous succession, including the Cattamarra Coal Measures, Cadda shales and mixed sources within the Yarragadee Formation. This study investigated the possible maturity and charge history of these different source rocks. A regional pseudo-3D petroleum systems model was constructed using new seismic interpretations. Heat flow was modelled using crustal structure and possible basement composition determined from potential field modelling, and subsidence analysis was used to investigate lithospheric extension through time. The model was calibrated using temperature and maturity data from nine wells in the Houtman and Abrolhos sub-basins. Source rock properties are assigned based on an extensive review of total organic carbon, Rock Eval and kinetic data for the offshore northern Perth Basin. Petroleum systems analysis results show that Permian, Triassic and Early Jurassic source rocks may have generated large cumulative volumes of hydrocarbons across the northern Houtman Sub-basin, whereas the Middle Jurassic–Cretaceous sources remain largely immature. However, the timing of hydrocarbon generation and expulsion with respect to trap formation and structural reactivation is critical for the successful development and preservation of hydrocarbon accumulations.

The role of seal integrity in the Vlaming Sub-basin (Perth Basin) for preservation of hydrocarbon accumulations

The offshore Vlaming Sub-basin, located in the southern part of the Perth Basin, is a Mesozoic depocentre estimated to contain over 12 km of sediments. It has several potential source rock intervals, good reservoir and seal pairs and an active petroleum system. The reasons for a lack of exploration success in this basin have been re-assessed by analysing fault reactivation and signs of hydrocarbon seepage. A recently completed study integrated structural mapping with analysis of fluid inclusion results. New data and interpretations show that a number of synrift faults with signs of reactivation in seismic data also have Fluid Inclusion Stratigraphy (FIS) anomalies above the regional seal. Many previously identified plays rely on the post-rift South Perth Shale for a seal. Our analysis suggests that many faults were reactivated after the deposition of the South Perth Shale, with some showing signs of present-day reactivation. Reactivated faults provided migration pathways for generated hydrocarbons; therefore, no accumulations were formed at these locations. The study provides insight into the location of leaky structures and areas with potentially valid plays in the Vlaming Sub-basin.

Geomorphology and seismic stratigraphy of the early Cretaceous delta in the Vlaming Sub-basin and implications for seal quality

The early Cretaceous South Perth Shale has been previously identified as the regional seal in the offshore Vlaming Sub-basin. The South Perth Shale is a deltaic succession, which infilled a large palaeotopographic low in the Early Cretaceous through a series of transgressive and regressive events. A study undertaken at Geoscience Australia has shown that the seal quality varies greatly throughout the basin and in places has very poor sealing properties. A re-evaluation of the regional seal based on seismic mapping determined the extent of the pro-delta shale facies within the South Perth Shale succession, which provides effective sealing capacity. New sequence stratigraphic interpretation, seismic facies mapping, new and revised biostratigraphic data and well log analysis were used to produce palaeogeographic reconstructions which document the distribution of depositional facies within the South Perth Shale and reveal the evolution of the early Cretaceous deltas. Our study documents spatial variations in the seal quality and re-defines the extent and thickness of the regional seal in the offshore central Vlaming Sub-basin. It provides an explanation for the lack of exploration success at some structural closures and defines constraints on the possible location of valid plays.