Petrological and Geophysical Characterization of Exhumed Subcontinental Mantle from the Australian-Antarctic Ocean-Continent Transition Zone

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 zones. We present new interpretations of multichannel seismic reflection transects, as well as new petrological data of dredged mantle rocks along the East Antarctic margin (Seamount B, offshore Terre Adélie). By combining both datasets, we show that a 50–100 km wide domain of cold (900°C), fertile subcontinental mantle was exhumed along the non-volcanic Antarctic margin. 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 sampling of a common fertile subcontinental mantle during rifting between Australia and Antarctica. Seamount B represents the initial stages of exhumation of cold subcontinental lithosphere along an ocean-continent transition during rifting. This thick mantle domain was likely affected by syn-rift melt impregnation at high-pressure (8 kbar), leading to the formation of plagioclase-pyroxenites. Overall, the combination of continental rifted blocks, a 50-100 km wide domain of volcanic-poor subcontinental mantle and (ultra)-slow spreading implies that ocean-continent transition zones along the Australian-Antarctic margins represent a recent analogue to ocean continent transition zones from the Jurassic Western Tethys. Additionally, evidence of syn-rift 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.

澳大利亚-南极裂谷系统（Australian-Antarctic rift system）的岩石圈最终破裂（lithospheric breakup）问题仍存争议，原因在于洋陆过渡带（ocean-continent transition zones）基底性质的地质约束较为稀疏。我们提出了对多道地震反射剖面（multichannel seismic reflection transects）的新解释，以及东南极边缘（East Antarctic margin）阿德利地近海B海山（Seamount B, offshore Terre Adélie）拖网采集的地幔岩石（dredged mantle rocks）的新岩石学数据（petrological data）。通过整合这两个数据集，我们发现，沿非火山型南极边缘（non-volcanic Antarctic margin）剥露了一个宽度为50-100公里的低温（900°C）富集型亚大陆地幔（fertile subcontinental mantle）域。拖网采集的橄榄岩（dredged peridotites）保留了与澳大利亚边缘东端（维多利亚和塔斯马尼亚）裂谷同期至裂谷后火山活动（syn- to post-rift volcanism）中发现的地幔包体（mantle xenoliths）相似的特征，表明澳大利亚与南极裂谷作用（rifting）期间对共同的富集型亚大陆地幔进行了取样。B海山代表了裂谷作用期间沿洋陆过渡带剥露低温亚大陆岩石圈（cold subcontinental lithosphere）的初期阶段。该厚地幔域（thick mantle domain）可能受到高压（8千巴）下裂谷同期熔体浸染（syn-rift melt impregnation）的影响，从而形成斜长石-辉石岩（plagioclase-pyroxenites）。总体而言，大陆裂块（continental rifted blocks）、50-100公里宽的贫火山亚大陆地幔（volcanic-poor subcontinental mantle）域以及（超）慢速扩张（(ultra)-slow spreading）的组合表明，澳大利亚-南极边缘的洋陆过渡带是侏罗纪西特提斯（Jurassic Western Tethys）洋陆过渡带的近期类似物（recent analogue）。此外，高压下裂谷同期熔体滞留（syn-rift melt stagnation）的证据表明，尽管磁异常（magnetic anomalies）形成于成熟、稳态洋壳（mature, steady-state ocean crust）以外的岩石圈中，但澳大利亚-南极裂谷边缘的岩浆作用（magmatism）足以形成可作为等时线（isochrons）的磁异常。