Paleozoic siliciclastic rocks from northern Victoria Land (Antarctica): Provenance, timing of deformation, and implications for the Antarctica-Australia connection
Paleozoic sequences exposed along the Transantarctic Mountains in Antarctica and in southeastern Australia are segments of a formerly contiguous accretionary orogen that developed along the eastern margin of Gondwana. The margin underwent amalgamation and eastward accretion in the early Cambrian to Early Ordovician Ross-Delamerian orogen and in the Ordovician to Carboniferous Lachlan orogen. Northern Victoria Land plays a key role in many geodynamic reconstructions because it has long been considered the along-strike continuation of Australia in Antarctica; however, the correlation between lithotectonic units in Antarctica (Wilson, Bowers, and Robertson Bay terranes) and those in southeastern Australia (Glenelg, Grampians-Stavely, and Stawell zones), as well as the presence of Lachlan-aged tectono-metamorphic events in northern Victoria Land, are still uncertain. 40 Ar- 39 Ar laser experiments on detrital and syndeformational white micas from low-grade siliciclastic rocks of northern Victoria Land, in conjunction with mineral-textural analysis and whole-rock geochemical and Nd isotope data, are used to constrain provenance and the timing of deformation, and to assess analogies with correlative structural zones in southeastern Australia. Detrital white micas of the western lithotectonic unit (Wilson terrane) yielded an age pattern dominated by late Cryogenian to Ediacaran ages (650–550 Ma), closely matching those of turbidites from the Australian Kanmantoo Group. Detrital white micas from the easternmost lithotectonic units (Bowers terrane and Robertson Bay terrane) yield indistinguishable age patterns, strikingly in agreement with those available for the western subprovince of the Lachlan orogen in Australia, which are dominated instead by younger ages with a dominant Ross orogen fingerprint (550–480 Ma). Deposition of siliciclastic detritus in the three lithotectonic units most likely occurred synchronously in the early–middle Cambrian, and the different signatures suggest that detritus was supplied from different source areas, with sediment supply from the west-southwest (East African orogen or ice-covered regions of East Antarctica) in the Wilson terrane and from the south (Ross orogen) in the Bowers and Robertson Bay terranes. Results also provide evidence for post-Ross orogen (ca. 462 Ma) contractional tectonics at the boundary between the Bowers and the Robertson Bay terranes, suggesting that available Ar data from the literature are variably affected by the presence of detrital micas. This finding establishes a cause-effect relationship between compressional tectonics at the plate margin and Middle–Late Ordovician intraplate reactivation processes in the western Wilson terrane. The accretion of northern Victoria Land was polyphase; it began with the amalgamation of the Bowers and Wilson terranes in the middle–late Cambrian and was followed by the docking of the Robertson Bay and Bowers terranes in the Middle–Late Ordovician. Results further support the link between northern Victoria Land and southeastern Australia. Together with a careful examination of data from the literature, they also suggest that the Bowers terrane and the across-strike contiguous Robertson Bay terrane are correlatives of the whole Stawell zone.