A note on the discovery of two new nunatacks in the southernmost Heimefrontfjella

Two new nunataks were discovered in southern Tottanfjella during our geological expedition to Heimefrontjella in early 1994. The exposed rocks are typical for the Sivorg Terrane. One explanation that these rocks were not recorded earlier is that they were only relatively recently exposed due to ice retreat

Geology of the Kottas terrane, Heimefrontfjella (East Antarctica)

The Kottas Terrane is the northernmost crustal block in the Heimefrontfjella. It is composed of juvenile Mesoproterozoic igneous rocks of calcalkaline composition and minor sedimentary rocks, which underwent amphibolite-facies metamorphism in late Mesoproterozoic times (Grenvillianage orogeny). Whereas central Heimefrontfjella was strongly affected by the East-African – Antarctic Orogeny in Late Neoproterzoic to Cambrian times, the Kottas Terrane shows only a weak overprint at brittle/ductile transition conditions, mostly manifested as northward-directed thrusts. The boundary between the Kottas and Sivorg terranes of central Heimefrontfjella is marked by the prominent Heimefront Shear Zone, which is exposed in two small nunataks immediately south of the main massif of Kottasberge

Geology of the Sivorg Terrane, Heimefrontfjella, (East Antarctica), and new U-Pb zircon provenance analyses of metasedimentary rocks.

The Sivorg Terrane is the largest crustal block of the Heimefrontfjella. It consists of a thick supracrustal sequence of metavolcanic and metasedimentary rocks that are intruded by a wide range of predominantly granitic plutonic rocks. The protolith ages of the metavolcanic rocks have been dated at ~1170-1140 Ma and the granitoid intrusions at ~1110-1050 Ma. The best estimate for Grenville-age metamorphism in the Sivorg Terrane is 1090-1060 Ma. Unlike the other two terranes in the Heimefrontfjella, the Sivorg Terrane records intense reworking of Mesoproterozoic rocks during the Late Neoproterozoic- Cambrian East African – Antarctic Orogeny. U-Pb detrital zircon provenance analyses from two samples indicate that at least two age-groups of different supracrustal sequences crop out in the Sivorg Terrane. The older, preorogenic sequence gave youngest detrital ages of ~1140 Ma, which are interpreted as dating the maximum deposition age of the original sediment. These rocks also provide evidence of a Palaeoproterozoic to Archaean foreland. The second sample is dominated by Mesoproterozoic to late Neoproterozoic detrital zircons, with a significant proportion of ages ranging from 1100 to 980 Ma. The youngest ages significantly postdate the Grenville-age metamorphism, so the sediments must have been deposited after or during the Late Mesoproterozoic orogenesis and, as such, might represent remnants of a molasse deposit of the orogen