Basement control on the deformation of cover basins: an example from the Cobar district in the Lachlan Fold Belt, Australia

Early Carboniferous deformation of ensialic Early Devonian to ?Early Carboniferous sedimentary basins in the Cobar district was controlled by movement on reactivated basement faults. Two cover zones, each with different structures, reflect differences in basement geometry, particularly the frequency and orientation of pre-existing faults. Zone 1, a high-strain zone on the eastern edge of the deformed basins, has a vertical cleavage and down-dip lineation characteristic of slate belts. Zone 2, a low-strain zone to the west, bears the imprint of wrench tectonics, at least in the early part of the history of its deformation. The boundary between Zones 1 and 2 is a sharply defined cleavage front and is localized along a major fault. In Zone 2, NW-trending F₁ folds developed initially as a response to left-lateral movement on WNW-trending basement faults. Later F₁ folds trending west suggest rotation of the direction of local maximum principal stress, perhaps as resistance to sliding of basement blocks developed. Further rotation of this local stress direction led to the formation of NE-trending F₂ folds and to left-lateral movement on bounding N- and NNE-trending faults. In Zone 1, deformation was more intense and controlled by high-angle reverse movement (with some left-lateral displacement) on N- and NW-trending basement faults, leading to D₁ folding, cleavage formation and vertical extension in the cover. Shortening was both oblique and parallel to the eastern margin of the basin. At an early stage of deformation, before cleavage formation, Zone 1 may have been locally coupled to Zone 2. Subzone boundaries defined by abrupt changes in strike of S₁ and F₁ in Zone 1, probably represent reactivated faults which acted as hinge lines separating areas deforming in response to different directions of shortening. Except for possible folds contributing to abrupt changes in strike of D₁ structures, regional D₂ structures are absent from Zone 1. This contrasts with the well-developed F₂ folds in Zone 2.