Revisiting the Australian‐Antarctic Ocean‐Continent Transition Zone Using Petrological and Geophysical Characterization of Exhumed Subcontinental Mantle

Abstract

The 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.

Compositional Attributes of the Deep Continental Crust Inferred From Geochemical and Geophysical Data

“…On the other hand, the highly localized, extremely felsic borders around Antarctica and between Japan and China likely indicate a misclassification of crust type and/or Moho depth. Oceanic‐type crust has been documented in both regions (Cho et al., 2004; Gohl, 2008; Hirata et al., 1992; McCarthy et al., 2020). Better Moho and temperature resolution of the ocean‐continent transition should increase the accuracy of compositional models in these regions.…”

Crustal Structure of the Camamu‐Almada Margin Along the Northeastern Rift Segment of Brazil From an Integration of Deep‐Penetration Seismic Reflection Profiles, Refraction, and Gravity Modeling

“…(2015), McCarthy et al. (2020), Tugend et al. (2018)), this model is not consistent across the entire transitional zone.…”

A Song of Ice and a Warm Southern Ocean

“…We present seismo-stratigraphic interpretations of two multichannel seismic re ection pro les crossing (line SO36-47) and closely passing (line N417) Site 1168 (Figure 1c, d). We follow the seismic interpretation of Sauermilch et al (2019a) and Hill et al (1997) for the sedimentary units and continental crust, and the exhumed subcontinental mantle part is our own (following the characteristics of McCarthy et al, 2020). Key re ections are linked to lithological units, largely following interpretations by Sauermilch et al (2019a) and the Shipboard Scienti c Party (Exon et al, 2001b).…”

“…We interpreted the exhumation of subcontinental mantle material o shore of western Tasmania, along its oceancontinent transition zone (OCT; Figure 1). e interpretation of the mantle domain followed the analytical scheme of Gillard et al (2015) and McCarthy et al (2020), who investigated exhumed mantle domains of the o shore central southern Australian and conjugate East Antarctic margins (Seamount B, east of Adélie Ri Block) respectively.…”

“…A high-amplitude, continental dipping re ection is detected at ~10 s TWT depth, which rises up to about 9 s TWT and disappears (Figure 1c; shot points, SP, 2500-3200). is sharp rising feature is interpreted as the Moho boundary and is characteristic of regions with exhumed subcontinental mantle along the ocean-continent transition (OCT; Gillard et al, 2015;McCarthy et al, 2020). e continental basement and pre-/syn-ri sediments contain closely spaced fault structures, indicating major deformation in the region during mantle exhumation.…”