Abstract
AbstractThe final lithospheric breakup of the Australian‐Antarctic rift system remains controversial due to sparse geological constraints on the nature of the basement along the ocean‐continent transition (OCT) zones. We present new interpretations of multichannel seismic reflection transects and new petrological data of dredged mantle rocks along the East Antarctic margin (Seamount B, offshore Terre Adélie). By combining both data sets, we show that a 50–100 km wide domain of cold and fertile subcontinental mantle was exhumed along the magma‐poor Antarctic margin. This study represents only the second locality, along with the Iberia‐Newfoundland margins, where the importance of exhumed mantle domains along OCTs can be clearly identified. The dredged peridotites preserve characteristics similar to mantle xenoliths found in syn‐ to post‐rift volcanism at the eastern end of the Australian margin (Victoria and Tasmania), indicating the exhumation of fertile subcontinental mantle during rifting between Australia and Antarctica. Seamount B represents the initial stages of exhumation of cold subcontinental lithosphere along an OCT during rifting. This thick mantle domain was likely affected by melt impregnation at high pressure (8 kbar), leading to the formation of plagioclase‐pyroxenites. The combination of continental rifted blocks, a wide domain of volcanic‐poor subcontinental mantle and (ultra‐) slow spreading is analogous to OCTs from the Jurassic Western Tethys and Iberia‐Newfoundland rifted margins. Additionally, evidence of melt stagnation at high pressure suggests that magmatism along the Australian‐Antarctic rifted margins was sufficient to form magnetic anomalies that can be used as isochrons despite their formation in lithosphere other than mature, steady‐state ocean crust.
Highlights
IntroductionThe Australian‐Antarctic conjugate margins represent a locality where the transition from rifting to spreading has been characterized as a magma‐poor system (e.g., Gillard et al, 2015; Sayers et al, 2001, Figure 1)
The transition from continental rifting to steady‐state ocean spreading is generally characterized by two contrasting end‐member models of rifting, corresponding to a magma‐poor system with exhumation of subcontinental mantle (Iberia‐Newfoundland and Jurassic Western Tethys, e.g., Manatschal, 2004; Whitmarsh et al, 2001), a magma‐rich model characterized by a rapid transition from rifting to ocean spreading and extensive magmatism (e.g., South Atlantic; Franke, 2013) with an intermediate model characterized by the rapid production of MORB magmatism during crustal extension (e.g., South China Sea; Larsen et al, 2018)
Two end‐member hypotheses have proposed that the ocean‐continent transition (OCT) is either (i) composed of a wide domain of highly stretched continental crust interspersed with magmatic intrusions and only minor exhumed mantle (e.g., Colwell et al, 2006; Sayers et al, 2001) or (ii) a wide domain of exhumed subcontinental mantle followed by non‐steady‐state proto‐oceanic lithosphere (e.g., Beslier et al, 2004; Gillard et al, 2015)
Summary
The Australian‐Antarctic conjugate margins represent a locality where the transition from rifting to spreading has been characterized as a magma‐poor system (e.g., Gillard et al, 2015; Sayers et al, 2001, Figure 1). Interpretations of geophysical data suggest up to 150 km of discrepancy when defining the initiation of unequivocal oceanic crust (e.g., Close et al, 2009; Colwell et al, 2006; Gillard et al, 2015; Leitchenkov et al, 2007) This leads to conflicting interpretations regarding the nature of magnetic anomalies, whether they can be used as isochrons for plate tectonic reconstructions, and the timing of the rift to drift transition This leads to conflicting interpretations regarding the nature of magnetic anomalies, whether they can be used as isochrons for plate tectonic reconstructions, and the timing of the rift to drift transition (c.f. Williams et al, 2019; Figure 1)
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