Crustal architecture of the oblique-slip conjugate margins of George V Land and southeast Australia

Abstract

A conceptual, lithospheric-scale cross-section of the conjugate, oblique-slip margins of George V Land, East Antarctica, and southeast Australia (Otway Basin) has been constructed based on the integration of seismic and sample data. This cross-section is characterised by asymmetry in width and thickness, and depth-dependent crustal extension at breakup in the latest Maastrichtian. The broad Antarctic margin (~360 km apparent rift width) developed on thick crust (~42 km) of the Antarctic craton, whereas the narrow Otway margin (~220 km) developed on the thinner crust (~31 km) of the Ross-Delamerian Orogen. The shallow basement (velocities ~5.5 km.s -1 ) and the deep continental crust (velocities >6.4 km.s -1 ) appear to be largely absent across the central rift, while the mid-crustal, probably granitic layer (velocities ~6 km.s -1 ) is preserved. Comparison with published numerical models suggests that the shallow basement and deep crust may have been removed by simple shear, whereas the mid-crust has been ductilely deformed.

Dominant symmetry of a conjugate southern Australian and East Antarctic magma-poor rifted margin segment

“…The high reflection amplitude suggests a large acoustic impedance contrast; the most obvious explanation of this phenomenon is that low‐velocity material of the middle crust is in tectonic contact with mantle at velocities of 8 km/s; however, other alternatives, such as a mafic sill, are possible, if less likely. The Moho interpretation presented here is similar to Moore et al [2000] from the Otway margin, east of the GAB, and to Stagg and Reading [2007] from the Otway‐George V conjugate margins.…”

“…Previous studies using these data sets have focused either solely on the Australian [ Sayers et al , 2001; Direen et al , 2007] or Antarctic margin architecture [ Stagg et al , 2004, 2005; Colwell et al , 2006; Stagg and Reading , 2007; Close et al , 2009]; or on specific aspects of the seismic sequence stratigraphy on either margin [ Totterdell et al , 2000; Krassay and Totterdell , 2003; Totterdell and Bradshaw , 2004; Close et al , 2007]. The present contribution is significant in that it is the first to integrate the seismic, dredging, gravity and magnetic data sets between conjugate lines on both margins.…”

“…These rocks included granite, slate and diorite [ Tanahashi et al , 1997] and peridotite that chemical analysis indicated was a fragment of subcontinental fertile mantle exhumed from shallow crustal levels during rifting [ Yuasa et al , 1997]. Stagg and Reading [2007] presented a seismic profile from the George V Land margin (approximately 141°E) that shows the general structural location of these dredges on the continental margin.…”

Early opening of Australia and Antarctica: New inferences and regional consequences

“…Bruce Rise, a marginal plateau located off Antarctica (and north of pseudo-isochron F), is bounded by a gravity anomaly that seems to mimic the southern margin of the Naturaliste Plateau. It may be formed of stretched continental crust (Stagg and Reading, 2007) and would represent the conjugate of the Naturaliste Plateau.…”

“…Using this interpretation, he dates the breakup at 95+-5 Ma. Veevers's (1986) COB for the South Australian margin and its Antarctic conjugate delineated from seismic data by Colwell et al (2006) has been used in many subsequent works (Direen et al, 2011;Espurt et al, 2009;Gohl, 2007;O'Brien and Stagg, 2007;Powell et al, 1988;Stagg and Reading, 2007;Veevers and Li, 1991;Williams et al, 2011). A revised interpretation of the COB was proposed by Ball et al (2013) who moves it~100 km-further oceanward.…”