Deformation history of a Mesozoic forearc basin sequence on Alexander Island, Antarctic Peninsula
The Mesozoic Fossil Bluff Group of Alexander Island contains trench-slope and forearc basin deposits that formed during the eastward subduction of oceanic crust beneath the Antarctic Peninsula region. The oldest rocks of the group were deposited in the Early to Middle Jurassic on the upper trench slope, with the forearc basin (sensu stricto) forming in Late Jurassic times. Deposition continued until at least Early Cretaceous (Albian) times. The Fossil Bluff Group was affected by three phases of deformation, as follows:
D1 — Movement on a major fault (the LeMay Range Fault) in the accretionary complex during the Middle Jurassic. Structural evidence suggests that the fault had a strike-slip component, and that the dip-slip component varied along strike from normal to reverse.
D2 — Basin inversion in the late Early Cretaceous, whilst deposition was still occurring. Fold patterns and fault movement directions indicate that inversion occurred in a dextral transpressional setting.
D3 — Late Cretaceous or Tertiary post-inversion extension. This caused the opening of a linear graben, George VI Sound, in a dextral transtensional setting. Phases (1) and (3) can be most easily explained by an oblique subduction model, with SE-directed subduction causing strike-slip motion on large-scale N-S trending structures in both arc and forearc. Dextral transtensional structures, such as those resulting from D3, formed when the forearc was in extension; whereas in a compressional forearc setting, sinistral transpressional structures would have resulted. As basin inversion (D2) occurred by dextral transpression, it cannot be explained by the above geometric model. It is likely, therefore, that the subduction direction was different during the D2 inversion event and that it corresponded to a Pacific-wide mid-Cretaceous compressional event. The D3 transtensional event corresponded with a dramatic decrease in spreading and subduction rates in early Tertiary times