Elsevier

Marine Geology

Volume 269, Issues 1–2, 15 February 2010, Pages 1-17
Marine Geology

Cenozoic deformation in the George V Land continental margin (East Antarctica)

https://doi.org/10.1016/j.margeo.2009.12.001Get rights and content

Abstract

This investigation is based on the analysis of multichannel seismic data collected in the continental shelf of the George V Land, between 140°E and 155°E on the East Antarctic margin.

Most of the East Antarctic continental shelf is covered by permanent, thick sea and terrestrial ice and it is therefore still unexplored. This is the reason why the tectonic deformation affecting the Antarctic margin during the Mesozoic rift from the Australian plate and the Cenozoic post-rift phase is poorly known. The few coastal polynyas (such as the Mertz polynya, in the George V Land continental shelf) are the only places where the oldest sedimentary section can be studied with the existing technology.

The data presented were not collected to address tectonic questions, however the relevance of this study is to document for the first time the occurrence of rift and post-rift tectonic structures in the sedimentary section near the coast, where the oldest sediment section is shallowest. These considerations are particularly relevant as Mertz-Ninnis trough, also known as George V Basin is located near the area of transition (around the Spencer Fracture Zone) between the extensionally-dominated Wilkes Land–Great Australian Bight Basin conjugate segment of the Australian–Antarctic Rift and the transtensional, strike-slip kinematics of the Otway Basin–South Tasman Rise–Oates Land segment.

The tectonic structure in the George V Land sector presented in this study is two-fold with two rift phases: one being connected to the breakup process and the later one associated to a change in plate rotation. A former extensional phase opened structural grabens, with axis oriented WNW–ESE and possibly NE–SW. A latter transpressional phase reactivated previous structures and tilted, faulted and folded sedimentary strata, located in the inner continental shelf. The first tectonic phase is likely related with the Cretaceous rifting between the Antarctic and Australian plates. The second tectonic phase might be related to the onset of the fast spreading phase of Pacific–Indian Ocean, that caused uplift, inversion and folding of post-rift strata in a narrow east–west oriented region, near coastal basement outcrop, in Paleocene–Eocene times.

Introduction

George V Land is located between approximately 140°E and 155°E on the East Antarctic margin (Fig. 1, Fig. 2), adjacent to the Wilkes–Adelie Land segment in the west and Oates Land to the east.

The depth of the continental shelf off the George V Land margin (Fig. 1) averages 500 m and generally increases shoreward from the shelf break. The inner-middle shelf contains the > 1000 m deep Mertz–Ninnis trough, also known as George V Basin. It is NE–SW oriented, and dips towards to the Mertz Glacier floating ice tongue. The basin is asymmetric in cross section and is steep-sided towards the coast, shoaling towards the northwest.

The landward side of the shelf near the coast is cut by three small topographic basins, seaward of Buchanan, Watt and Commonwealth Bays, oriented perpendicularly to the Mertz–Ninnis Trough (Fig. 1). The sedimentary sequence overlying the acoustic basement is generally dipping toward the north.

The continental slope extends from the shelf break down to about 2000 m water depth and it is deeply incised by canyons and wide channels (Fig. 1). In the continental rise, between 2500 and 3500 m of water depth, these channels are bound by north–south elongated, asymmetrical, sediment mounds, with relief of up to 1000 m, decreasing toward the ocean (Escutia et al., 2000, De Santis et al., 2003, Donda et al., 2003).

The George V continental margin area has been of general interest to several international surveys (USA, France, Japan and Australia). However previous survey coverage in the area is mostly restricted to widely spaced regional transects. The first offshore program was undertaken through the so called “Operation Deep Freeze” in 1979 (Domack et al., 1980). Several other surveys followed: the TH82 cruise, conducted by the Japan National Oil Corporation (Tsumuraya et al., 1985, Tanahashi et al., 1987); the IFP cruise, conducted by Institut Francais du Petrol (Wannesson et al., 1985), the USGS cruise, conducted by the US Geological Survey (Eittreim and Smith, 1987) and the AASOPP cruises, conducted by Geoscience Australia (Stagg et al., 2004).

The region of the George V Land margin was the target of the WEGA (WilkEs basin GlAcial history) multichannel seismic survey (Fig. 1). A coring program, together with a 3.5 kHz and Chirp subbottom survey was also conducted in the frame of the WEGA cruise, to image and sample sediment from the continental shelf, slope and rise of the George V Land continental margin. The survey aimed to study the sedimentary sequences of this sector of the East Antarctic margin, which appears to be strictly related to the glacial history of the Wilkes Basin, one of the largest East Antarctic subglacial basins (Drewry, 1976; Fig. 2). An additional outcome of the WEGA survey is that it extended seismic data coverage into the shallow continental shelf for the first time in this region. The coastal polynya of the George V Land is one of the few places of the East Antarctic margin where the oldest sedimentary section can be studied with the existing technology, because most of the East Antarctic continental shelf is covered by permanent, thick sea and terrestrial ice. Therefore even though the WEGA survey was not designed for investigating tectonic setting, the multichannel seismic data collected near the George V Land coast, where the oldest sediment section is shallowest, provide new evidence to highlight the structural and stratigraphic style of the rift phases between Australia and Antarctica and post-rift tectonic processes in the inner continental shelf.

These considerations are particularly relevant as the Mertz-Ninnis trough is located near the area of transition between the extensionally-dominated Wilkes Land–Great Australian Bight Basin conjugate margins and the transtensionally-dominated Otway Basin–South Tasman Rise–Oates Land conjugate margins.

The main parameters related to data acquisition and processing are listed in Table 1, Table 2. Key profiles W04, W05 and W06 were also processed by applying a wave equation migration, in order to best image the geometry of the tectonic and morphologic features.

Section snippets

Antarctica–Australia geological and geodynamic setting

Australia and Antarctica were once connected and, before breakup, they represented the conjugate parts of the northwest–southeast Southern Rift System (Stagg et al., 1990). It was active from the Late Jurassic to the Late Cretaceous, with the Wilkes Land located near the centre of the rift (Willcox & Stagg, 1990, Colwell et al., 2003). Several evidences of geological correlations of the Australia–Antarctica pre breakup fit have been provided, some of which are based on the age and style of

The George V Land continental shelf tectonic features

The analysis of the multichannel seismic profiles collected in the framework of the WEGA cruise highlights that two main structural phases affected the basement and its sediment cover in this sector of the George V Land continental shelf: a first tectonic phase that caused the formation of extensional grabens; a second tectonic phases that caused partial inversion of previous structures and sediment deformation.

The George V Land continental rise tectonic features

Dyke-like intrusion faults characterise the acoustic basement beneath the continental rise between 143°E and 144°E, as shown by multichannel seismic profiles collected in previous cruises (Eittreim and Smith, 1987) and the Australian AASOPP cruise (Stagg et al. 2004) with deeper penetration than the WEGA data. The WEGA seismic profiles also reveal the occurrence of cone-like structures characterised by convex reflectors and diffractions (Fig. 14). The reflectors onlapping and overlying these

Structural interpretation

  • 1)

    The extensional grabens that formed in the first tectonic phase are interpreted to be related to the Mesozoic phase of rifting between Antarctica and Australia. This is consistent with the lower Cretaceous (Aptian) age of the in-situ siltstone sample recovered from the oldest strata infilling these graben system and onlapping the basement along the coast, between the Mertz and Ninnis glacier (Domack et al., 1980). Brown lignites of Cretaceous age were also dredged west of the Mertz Glacier (

Relationship between the Southern Ocean geodynamic evolution and the superposed post-rift tectonics in the George V Land margin

The sector from Wilkes Land to Oates Land (105°–160°E) was formed during the separation of Australia and East Antarctica, culminating with the sea floor spreading in the late Cretaceous. Geologically the margin can be divided in two broad zones: one from the western Wilkes Land to the Terre Adélie, which formed predominantly in an orthogonal extensional setting involving the margins of western central Australia and East Antarctica; and the other off George V Land and Oates land, which formed in

Conclusions

The analysis of the multichannel seismic data collected in the George V Land margin highlights the importance of this sector of the Southern Ocean as it is located near the area of transition between the extensionally-dominated Wilkes Land–Great Australian Bight Basin conjugate segment of the Australian–Antarctic Rift and the transtensional, strike-slip kinematics of the Otway Basin–South Tasman Rise–Oates Land segment. For the first time the occurrence of rift and post-rift tectonic structures

Acknowledgments

The WEGA cruise was funded by Italian Programma Nazionale di Ricerche in Antartide (PNRA) and Australian National Antarctic Research Expeditions (ANARE) agencies (Brancolini and Harris, 2000). This work was funded by the PNRA under the WEGA and MOGAM projects. We thank the Institute Française du Pétrole and United Geological Survey for kindly providing access to the seismic data throughout the Antarctic Seismic Data Library System. We would like to thank Belinda Brown for her input in an early

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