In the 1 July 2012 issue of the Earth and Planetary Science Letters journal an article has been published on a suspected 100 km sized impact structure that on verification would document the oldest cosmic collision on Earth so far known.
Adam A. Garde, Iain McDonald, Brendan Dyck, Nynke Keulen: Searching for giant, ancient impact structures on Earth: The Mesoarchaean Maniitsoq structure, West Greenland. – Earth Planet. Sci. Let., vol. 337-338, 197-210.