THERMOCHRONOLOGIC HISTORY OF SUBGLACIAL GEOLOGY OF WILKES SUBGLACIAL BASIN SECTOR OF EASTERN ANTARCTICA THROUGH MULTI-DATING OF DROPSTONES

Ice-rafted dropstones provide insights into ice sheet evolution and the geologic history of the subglacial bedrock. In Antarctica, offshore ice-rafted dropstones open a window into the continent, and provide important context for ice-rafted sand grains. Dating dropstones with multiple thermochronometers provides a more complete picture of the source-rock cooling history, from crystallization to surface exposure, than is possible from single mineral grains. We dated 11 dropstones from the International Ocean Discovery Program Cruise 318 near the Adélie Coast of the Wilkes Subglacial Basin. The dropstones were retrieved from sites 1356, 1358, and 1360 and have early Oligocene to mid-Pliocene depositional ages. We dated each pebbled with four thermochronometers where possible (U/Pb zircon, Ar/Ar biotite, Ar/Ar feldspar, and (U-Th)/He apatite), thus providing ages from crystallization to cooling through ~60 °C to interrogate their geologic histories. The dropstones fall into three crystallization age populations: 1.4-1.7 Ga - interpreted as an Adélie Coast and Mertz Shear Zone source (n=7), 1.0 Ga - interpreted as a Grenville orogeny source (n=2), and 500 Ma (n=1), interpreted as a Ross orogeny source. The small difference between U/Pb zircon and Ar/Ar biotite ages within each population, implies all three age populations cooled rapidly through 300 °C and then slowly cooled until reaching the surface. Implications for the ice sheet dynamics can be drawn from the dropstones provenance recorded by crystallization age and low-temperature thermochronologic history. The provenance of the pebbles consistently matches the Adélie Craton and Mertz Shear Zone (terrains located in the west margin of Wilkes Subglacial Basin) from the Oligocene to mid-Miocene, until the mid-Pliocene when it shifts to sources of Grenville and Ross Orogeny age (terrains located on the eastern margin of Wilkes Subglacial Basin). Apatite (U-Th)/He ages decrease with depositional age, showing the influence of glacial erosion on denudation rates. Generally, apatite (U-Th)/He ages are very old (100-400 Ma), implying that enhanced glacial erosion was much less than 2 km in the rocks tapped by the dropstones. Two Pliocene dropstones with young apatite (U-Th)/He ages (<34 Ma) indicate that glacial erosion exceeded 2 km locally.