Locating hidden terrane boundaries in East Antarctica with detrital feldspar Pb isotopes

The assembly of eastern Gondwana involved the amalgamation of Indo-Antarctica and Australo-Antarctica along the Kuunga orogen. The location of the suture or sutures between Indo-Antarctica and Australo-Antarctica within the Kuunga orogen remains debated as key geological relationships are concealed beneath ice. The most recent interpretation identifies the ‘Mirny Fault’ as a key suture between crust of Indo-Antarctic and Australo-Antarctic affinity. However, sparse exposure of bedrock in this region precludes traditional geological characterisation of the crust on both sides this structure.

Here, we test the Mirny Fault as a key suture between Indo-Antarctica and Australo-Antarctica using Pb isotopes of detrital feldspars from offshore sediments referenced to new and compiled feldspar Pb isotopic data from onshore exposures of Indo-Antarctica and Australo-Antarctica. The feldspar Pb-isotope signature of Indo-Antarctica forms a broad population with ²⁰⁷Pb/²⁰⁴Pb ratios of 15.5—16.1 and ²⁰⁶Pb/²⁰⁴Pb of 17—19. In contrast, Australo-Antarctica has a feldspar Pb-isotope signature characterised by a tight cluster of ²⁰⁷Pb/²⁰⁴Pb (15.5—15.8) and ²⁰⁶Pb/²⁰⁴Pb (16.5—17) ratios, which is distinct from Indo-Antarctica. Locally-derived Holocene sediments offshore of the Mirny region contain detrital feldspars with Pb-isotope compositions recording a contribution from both Indo-Antarctica and Australo-Antarctica.

The presence of Australo-Antarctic and Indo-Antarctic crust near the Mirny Fault supports the interpretation of this structure as a fundamental terrane boundary. We interpret the Mirny Fault as the key suture between Indo-Antarctica and Australo-Antarctica and trace this boundary into the interior of East Antarctica to connect with an inferred Gondwanan-aged suture within the Gamburtsev Subglacial Mountains.