Abstract:

The orientations and ages of polygenetic volcano chains, elongate volcano edifices, and elongate summit calderas in Marie Byrd Land, Antarctica, are analysed to determine stress directions during volcanism and coeval glaciation across the West Antarctic Rift. Initiation of volcanism c. 36–34 Ma and major polygenetic volcanism beginning c. 14 Ma are broadly coeval with establishment of significant ice volumes in West Antarctica. Middle–Late Miocene volcanism occurred primarily along roughly north–south volcano chains, whereas latest Miocene–Pleistocene volcanism had a strong east–west preferred orientation across the spatial extent of the province. Anisotropic stress conditions controlled both phases of volcanism, and a rapid rotation in the maximum horizontal stress direction from north–south to east–west occurred as early as c. 6 Ma. Glacial loading and unloading appears to have facilitated volcanism in Marie Byrd Land, but ice margin reconstructions make it unlikely that stress field rotation is a result of flexural stresses imposed by glacial loading cycles. The east–west orientation of latest Miocene to Pleistocene maximum horizontal stress is parallel to the absolute motion of the Antarctic plate for the past 6 Ma, and the north–south to east–west stress field rotation coincides with changes in Antarctic–Pacific Ridge spreading and Pacific basin tectonic events associated with plate reorganization.

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