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Chapter 2 – Enderby Basin, East Antarctica

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Chapter 2 – Enderby Basin, East Antarctica The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica Chapter 2 - A regional tectonic model for the Enderby Basin, East Antarctica, suggests a later breakup between India and Antarctica but an earlier breakup for India and Madagascar 57 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica ABSTRACT Before East Gondwana broke up, the Enderby Basin, off East Antarctica, was the southward continuation of the ocean basin between the West Australia and Greater India. We link the spreading corridors of the Enderby Basin and West Australian abyssal plains, using rigorous regional plate tectonic constraints. These include the minimisation of overlap/compression between Madagascar and India, and between Greater India and Australia along the Wallaby-Zenith Fracture zone, off West Australia. We integrate magnetic and gravity anomaly data from the Enderby Basin within a regional Indian Ocean plate kinematic framework to identify a conjugate series of eastwest-trending magnetic anomalies, M4 to M0 (~126.7-120.4 Ma). The spreading that rifted Greater India away from Australia-Antarctica progressed from north to south, starting ~136 Ma northwest of Australia, and reaching the southern tip of India at ~126 Ma. Seafloor spreading in the Enderby Basin was abandoned at ~115 Ma, when the spreading ridge relocated to the north, transferring the Elan Bank and the continental portions of the South Kerguelen Plateau to the Antarctic plate. Our revised plate kinematic model for the Enderby Basin, including a later onset of opening, helps resolve the problem of successive two-way strike-slip motion between Madagascar and India, inherent in previously published reconstructions. Our reconstructions suggest that seafloor spreading between India and Madagascar progressed from south to north from 94-84 Ma (previous studies date this separation to a maximum of ~91 Ma). This Indian motion is essential to replicate the coeval curved fracture zones of the Wharton Basin, and the West Enderby Basin (ours is the first study to accurately model them). We model the onset of these fracture zones bends at ~98 Ma, as Greater India gradually began to unzip 58 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica from Madagascar, initially via slow right-lateral transtensional rfting. This is a good match to the well-documented 100 Ma seafloor spreading reorganisation. The 85°E Ridge and Kerguelen Fracture Zone formed as conjugate flanks of a ‘leaky’ transform fault following the ~98 Ma spreading reorganisation. Our model also identifies the Afanasy Nikitin Seamounts as products of the Conrad Rise hotspot following ~80 Ma. INTRODUCTION East Gondwana, comprising India, Antarctica, Australia, Madagascar, the Seychelles and other micro-continental blocks, separated from Africa in the Mid Jurassic (2008; Konig and Jokat, 2010; Royer and Coffin, 1992). Australia and Antarctica were then left behind as the remainder of East Gondwana diverged from the Early Cretaceous (e.g. Johnson et al., 1980; Li et al., 1996; Mihut, 1998). The complexity of East Gondwana breakup, volcanic overprinting from the Kerguelen plume, and a lack of geophysical data particularly offshore southwest Australia and East Antarctica, leaves the early Indian Ocean opening history as a source of controversy in Mesozoic plate tectonic models. The migration of Greater India away from East Gondwana, created the abyssal plains offshore East Antarctica (Fig. 2.1) and West Australia (Fig. 2.2), which was a single margin. Relative motion between Madagascar and Africa around this time also created the Somali basin yet no plate tectonic reconstructions have been published that incorporate all available potential field constraints from the abyssal plains created during this early dispersal. This has left various unresolved issues, including successive left- and right-lateral strike-slip motion between Madagascar and India inherent in many 59 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica reconstructions (e.g. Gaina et al., 2007; Ramana et al., 2001), and magnetic anomaly picks that suggest different motions for Greater India against West Australia and East Antarctica, which were a continuous margin (e.g. Williams et al., 2011). Published regional plate models were also unable to accurately reproduce the sharp bend in the Kerguelen and Wharton Basin fracture zones off Antarctica and Australia, respectively (Acharyya, 2000; Besse and Courtillot, 1988; Norton and Sclater, 1979), or account for the geometry of the enigmatic 85-East Ridge off the East Indian coast (85°E Ridge, Fig 2.3), and confirm the source of the Afanasy Nikitin Seamounts further south (ANS, Fig. 2.3), all of which are clearly visible in the 1-minute satellite free-air gravity (Sandwell and Smith, 2009). Fitting Sri Lanka into the pre-breakup configuration has also proven difficult (e.g. Eagles and Konig, 2008; Lawver et al., 1998). The Enderby Basin (Fig. 2.1) retains vital information to constrain the early motion of India, but no study had yet combined the seafloor spreading record in the Enderby and Perth basins offshore East Antarctica and southwest Australia, respectively. Enderby Basin investigations suffer from limited data with regards to interpreting seafloor- spreading anomalies and the older fracture zones. Volcanic output from the Kerguelen Plume masks nearly half the basin while the Cretaceous Normal Superchron (CNS), which commenced soon after opening, restricts the use of seafloor spreading anomalies. 60 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica Figure 2.1. The Sandwell and Smith (2009) 1-degree satellite free-air derived gravity along the East Antarctic margin. Black dashed line represents the onset of rapid seafloor spreading at ~43.8 Ma (Müller et al., 2000a). The Kerguelen Plateau is outlined in red. Black stars indicate ODP leg 183 sites 1137 (Elan Bank) and 1136 (South Kerguelen Plateau), with minimum ages indicated. 61 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica Figure 2.2. The Sandwell and Smith (2009) 1-degree satellite free-air derived gravity of the West Australian margin. Seafloor along the West Australian margin was the northward continuation of Enderby Basin seafloor, prior to Australia-Antarctica rifting. ZP is the Zenith Plateau, WP is the Wallaby Plateau, NP is the Naturaliste Plateau, WZFZ is the Wallaby-Zenith Fracture Zone. An accurate model of East Gondwana breakup must satisfy geophysical constraints from all abyssal plains involved. A tectonic model for the relative motion between 62 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica Greater India and Australia (Gibbons et al., submitted) incorporates potential data from all the abyssal plains along the West Australian margin. Here we construct a revised motion history for Greater India, which is compatible with data in the Enderby Basin, which was the early continuation of the West Australian abyssal plains and also records Greater India’s early motion history. STUDY AREA AND PREVIOUS WORK The Enderby Basin is situated along East Antarctica’s margin between Gunnerus Ridge in the west, Princess Elizabeth Trough, in the east, and the Kerguelen Plateau, in the northeast (Fig. 2.1). The basin contains several northwest-trending fracture zones originating from the Antarctic margin. Some terminate abruptly at the Kerguelen Fracture Zone (KFZ, Fig. 2.1), while others terminate ~500 km south of the Conrad Rise. The only discernible fracture zone further east is the Vincennes Fracture Zone, located at the eastern edge of the Bruce Rise. Running northwest of the Bruce Rise and Kerguelen Plateau is a tectonic boundary marking the onset of rapid seafloor spreading between Australia and Antarctica (thick dashed line, Fig. 2.1). The Kerguelen Plateau (Fig. 2.1), at the northeast extent of the Enderby Basin, is identified as a plume-related igneous province active from ~120 Ma (Coffin et al., 2002). Mantle plumes are associated with continental breakup (Richards et al., 1989) and the Kerguelen Plume has been linked to the breakup of East Gondwana, and the 132 Ma Bunbury Basalts of southwest Australia (Frey et al., 1996a), the 118 Ma Rajmahal Traps in northeast India (Kent et al., 2002), and the ~115 Ma ultramafic lamprophyres from the East Antarctic margin (Coffin et al., 2002). The Kerguelen Plateau was initially 63 The University of Sydney, PhD Thesis, Ana Gibbons, 2012 – Chapter 2 – Enderby Basin, East Antarctica identified as a LIP (Duncan, 2002; Watkins et al., 1974) but there is also evidence of geochemical contamination by continental material (Frey et al., 2002). A recent study summarizing all available data for the Kerguelen area concluded that at least the southern portion of the Kerguelen Plateau is underlain by stretched continental crust (Bénard et al., 2010). The Elan Bank (Fig. 2.1) is a promontory jutting out west from the tip of the South Kerguelen Plateau. ODP and seismic data indicate it is a micro-continental fragment, underlying the Kerguelen volcanic carapace (Borissova et al., 2003). The Elan Bank rifted from Antarctica as part of India before being transferred to the Antarctic plate during a northward ridge jump, which left a conjugate set of seafloor spreading anomalies reflected about an extinct ridge, which was dated to ~124.1 Ma/M2 (Gaina et al., 2007). Magnetic anomalies in the Enderby Basin have previously been identified as a single flank formed from 134 Ma/M11 to 120.4 Ma/M0, forming at a half-spreading rate of 6.5 to 2.8 cm/yr (Ramana et al., 2001). These were a good match to anomalies identified by the same study in the Bay of Bengal, conjugate to the Enderby Basin, but this reconstruction did not consider constraints from beyond the Antarctic and Indian margins, or include Madagascar. Another study identified a conjugate set of Enderby spreading anomalies from 130 Ma/M9 to 120.4 Ma/M0, at half-spreading rates of 3.9 to 0.8 cm/yr (Gaina et al., 2007).
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