Models for the deposition and simple shear deformation of a turbidite sequence in the South Georgia portion of the southern Andes back-arc basin
The Cumberland Bay Formation on South Georgia (South Atlantic) is an Upper Jurassic–Lower Cretaceous andesitic turbidite sequence >8 km thick, derived from a contemporary volcanic are and deposited in part of the southern Andes back-arc basin. 4 main lithofacies are recognized which show coarsening and thickening upward cycles. A Lower Cretaceous palinspastic reconstruction shows that lobes of sand-rich material were banked against the (?) fault-bounded SW margin of the basin, adjacent to the volcanic arc. The basin had a northerly-directed palaeoslope and evidence for both axial and lateral palaeocurrents is preserved. Sedimentation of the formation cannot be explained by the classical fan model but resembles that described from linear, tectonically-controlled troughs (cf. Grenada Basin, Lesser Antilles). Folding resulted from mid-Cretaceous closure of the basin and increases in intensity to the NE: open, upright folds pass into trains of inclined chevron folds with progressive decrease in interlimb angle, axial plane dip and wavelength. The turbidites were affected by up to 55% shortening and had a minimum original cross-strike width of about 60 km. The geometry of the structures is consistent with deformation above a gently inclined simple shear zone resulting from limited underthrusting or subduction of the basin floor beneath the arc terrane. Extension of both sedimentary and tectonic models to the southern Andes is discussed.