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: 1. 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. 2. 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. 3. 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 time

Sedimentology and structure of the trench-slope to forearc basin transition in the Mesozoic of Alexander Island, Antarctica

The Mesozoic forearc of Alexander Island, Antarctica, is one of the few places in the world where the original stratigraphic relationship between a forearc basin and an accretionary complex is exposed. Newlydiscovered sedimentary rocks exposed at the western edge of the forearc basin fill (the Kimmeridgian–Albian Fossil Bluff Group) record the events associated with the basin formation. These strata are assigned to the newly defined Selene Nunatak Formation (?Bathonian) and Atoll Nunataks Formation (?Bathonian-Tithonian) within the Fossil Bluff Group.The Selene Nunatak Formation contains variable thicknesses of conglomeratesand sandstones, predominantly derived from the LeMay Group accretionary complex upon which it is unconformable. The formation marks emergence and subsequent erosion of the inner forearc area. It is conformably overlain by the1 km thick Atoll Nunataks Formation, characterized by thinly-bedded mudstones and silty mudstones representing a marine transgression followed by trench-slope deposition. The Atoll Nunataks Formation marks a phase of subsidence, possibly in response to tectonic events in the accretionary prism that are known to have occurred at about the same time.The Atoll Nunataks Formation is conformably overlain by the Himalia Ridge Formation, a thick sequence of basin-wide arc-derived conglomerates. This transition from fine- to coarse-grained deposition suggests that a well-developed depositional trough (and hence trench-slope break) had formed by that time. The Atoll Nunataks Formation therefore spans the formation of the forearc basin, and marks the transition from trench-slope to forearc basin deposition

On the origin of fore-arc basins: new evidence of formation by rifting from the Jurassic of Alexander Island, Antarctica

The Middle Jurassic–Lower Cretaceous Fossil Bluff Group of Alexander Island, Antarctica represents the fill of a fore-arc basin unconformably overlying an accretionary complex. Like most fore-arc basins, this example had been considered to have a passive origin, as a topographic hollow between the arc and the trench-slope break. Recent discoveries of igneous rock coeval with sedimentation have altered this view. Oxfordian–Kimmeridgian basaltic and rhyolitic sills and lava flows are found in a restricted area at the north of the basin, within a single formation. Chemically, most basalts are high-Nb types, which cannot have originated in a supra-subduction zone setting. Since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the Antarctic Peninsula volcanic arc, it is concluded that a late Jurassic pause in subduction led to active rifting to form the fore-arc basin

First Carboniferous and ?Permian marine macrofaunas from Antarctica and their tectonic implications

The first Carboniferous and ?Permian marine macrofaunas from the Antarctic continent are described from three sites near Mount King, Alexander Island, Antarctic Peninsula. They include bivalves, brachiopods, bryozoans, crinoids, gastropods, a possible monoplacophoran, nautiloids and a possible serpulid or microconchid. Overall the faunas of two localities are Carboniferous in age and compare well with the Levipustula levis Zone of Argentina and eastern Australia, and are of Namurian (Serpukhovian–Bashkirian) age, based mainly on the brachiopod and bryozoan faunas. Less positive brachiopod evidence from a third locality indicates the presence of a linoproductid fauna of possible Carboniferous or Permian (Gzhelian–Artinskian) age, having affinities with the Argentinian Cancrinella fauna. The lithological and structural characteristics of the Mount King beds are comparable to the accretionary complex of the LeMay Group (hitherto of only proven Jurassic–Cretaceous age) of Alexander Island, in which they are provisionally placed. However, the beds may also correlate with the Trinity Peninsula Group (Carboniferous–Triassic) of the northern Antarctic Peninsula. The features of the Mount King beds are consistent with the presence of an accretionery complex related to an island arc in the Late Palaeozoic, but are not necessarily conclusive proof of the presence of such a terrane at that time in what is now Alexander Island

Ridge-trench collision-induced switching of arc tectonics and magma sources: clues from Antarctic Peninsula mafic dykes

Compositions and distributions of mafic dykes in the Antarctic Peninsula continental arc show that tapping of several mantle sources was tectonically controlled. In the Cretaceous to Tertiary, between 135 Ma and 55 Ma, calc-alkaline dykes intruded the arc. In the late Cretaceous, however, between 95 Ma and 65 Ma, there was a pulse of compositionally diverse magmatism. This change resulted from collision of an ocean spreading centre with the trench. As a consequence, non-partitioned dextral transtensional shear in the overriding plate became partitioned into strike-slip and extensional domains. Calc-alkaline magmatism was, therefore, replaced by strike-slip-related shoshonitic magmatism towards the rear-arc and extensionrelated tholeiitic magmatism towards the fore-arc. OIB-like dykes were emplaced because of the break in otherwise continuous subduction. During the early Tertiary subduction continued but ceased after a late Tertiary ridge-trench collision