Oligocene to Holocene erosion and glacial history in Marie Byrd Land, West Antarctica, inferred from exhumation of the Dorrel Rock intrusive complex and from volcano morphologies

The Dorrel Rock intrusive complex in\n\nMarie Byrd Land, West Antarctica, consists of a coarse-grained gabbro cut by fine grained benmoreite and trachyte dikes, all exposed in a single nunatak. It is the only exposed plutonic body related to late Cenozoic\n\nvolcanism in this part of the West Antarctic rift system. Our 40Ar-39Ar age determinations indicate emplacement of the gabbro\n\ntook place ca. 34 Ma, followed by dike injection at ca. 33.5 Ma. Marie Byrd Land volcanoes are all younger than ca. 27–29 Ma, and lie on a low-relief Late Cretaceous erosion\n\nsurface that has been disrupted by block faulting and dome uplift since late Oligocene time. The erosion surface and overlying volcanoes\n\nare well preserved, but in contrast,\n\nwe estimate that at least 3 km of overburden has been eroded away to expose the gabbro.\n\nThis anomaly is most easily explained\n\nif most of the exhumation took place during a period of rapid erosion between ca. 34 Ma and 27–29 Ma and was followed by a pronounced\n\ndecrease in erosion rate in the late\n\nOligocene.\n\nTemporal anomalies in the degree of dissection of volcanic edifices, together with evidence from hydrovolcanic deposits, suggest there was an ice cap in Marie Byrd Land in\n\nthe late Oligocene and that inland (200+ km) volcanoes were being actively eroded by glaciers\n\nuntil ca. 15 Ma. This is consistent with\n\nseismic and stratigraphic work in the Ross Sea, which documents at least two expansions of the West Antarctic Ice Sheet in the early mid-Miocene. We fi nd that rates of glacial erosion in Marie Byrd Land increase significantly with nearness to the coast, and in\n\nnonresistant rock. Thus, the observation that inland volcanoes younger than ca. 15 Ma\n\nshow no effects of glacial erosion, except for one with a basal section of weak tuffs, suggests that a transition from warm-based to cold-based glaciers took place around 15 Ma.\n\nThese fi ndings are similar to many of those reported from well-studied McMurdo Sound and Ross Sea localities, so they provide a wider regional picture of middle to late Cenozoic climatic and surficial geologic events in Antarctica.