East Gondwana is generally thought to have assembled through the amalgamation of Indo-Antarctica and Australo-Antarctica along the Ediacaran–Cambrian Kuunga orogen. The location of a boundary between Indo-Antarctica and Australo-Antarctica within this key Gondwana-forming orogen remains controversial because extensive ice cover in East Antarctica precludes traditional characterization of terranes. Here, we integrated Pb-isotope analysis of detrital feldspar grains with U-Pb dating of detrital monazite and zircon grains from offshore sediments to infer the location of the onshore boundary between Indo-Antarctica and Australo-Antarctica. New and compiled data from onshore basement exposures highlight the different age and Pb-isotope signatures of Indo-Antarctica and Australo-Antarctica. Holocene sediments offshore from Mirny Station (Queen Mary Land, East Antarctica) have detrital feldspar Pb-isotope signatures and detrital monazite and zircon U-Pb ages that reflect contributions from both Indo-Antarctica and Australo-Antarctica. The presence of both Indo-Antarctic and Australo-Antarctic crust beneath ice cover near Mirny Station implies proximity to a fundamental terrane boundary within the Kuunga orogen, which could coincide with a geophysical lineament at ∼94°E (Mirny fault). The geophysical expression of this boundary extends into the subglacial interior of East Antarctica, where, prior to more recent rifting, it may have connected with one or more previously inferred Gondwana-forming sutures. The revised geometry of the Kuunga orogen suggests that the assembly of East Gondwana involved dominantly strike-slip motion in the Mirny region coupled with high-angle convergence between Indo-Antarctica and Australo-Antarctica to the west.