DATING OF SIRIUS GROUP TILLITES IN THE ANTARCTIC DRY VALLEYS WITH COSMOGENIC HE-3 AND NE-21

We measured in situ produced cosmogenic He-3 and Ne-21 in boulder and bedrock surface samples in and near Sirius Group tillites from two loc alities in Southern Victoria Land, Antarctica. Pyroxene quantitatively retains cosmogenic He-3 for millions of years, The deposit at Mt. Fle ming has a minimum exposure age of similar to 6.5 Ma, while similar to 6 Ma is a probable minimum age for the Table Mountain tillite, These lower limits (not taking into account erosion of the sampled surfaces) are based on the samples with the highest concentrations of cosmogeni c noble gases and currently accepted production rates. Since the plagi oclase-bearing Mt. Fleming samples almost certainly lost part of their cosmogenic Ne, the minimum exposure age at this location is presumabl y even approximate to 50% higher than the stated value, The concentrat ions of cosmogenic Ne in our samples constrain uplift rates of the Tra nsantarctic Mountains to < 170 m/Ma. These data contradict the hypothe sis of a collapse of the East Antarctic ice sheet in the Pliocene coup led with high uplift rates and support the model of a stable ice sheet since the middle Miocene.