Structure and origin of the Fleurieu and Nackara Arcs in the Adelaide fold-thrust belt, South Australia: Salient and recess development in the Delamerian Orogen

Stephen Marshak, T. Flöttmann

Research output: Contribution to journalArticlepeer-review

Abstract

The regional map pattern of structural trends in the Early Paleozoic Adelaide fold-thrust belt of South Australia resembles an upright letter 'S'. In general, structures in the fold-thrust belt verge toward the Gawler craton to the west, so the northern curve (Nackara Arc) of the 'S' is a salient, whereas the southern curve (Fleurieu Arc) is a recess. The map-view axial trace of the Nackara Arc coincides with the thickest portion of the Adelaidean (Late Proterozoic) basin, and the map-view axial trace of the Fleurieu Arc coincides with the intersection between the Adelaidean and Kanmantoo (Cambrian) basins. Our structural analysis of the Adelaide fold-thrust belt demonstrates that the cross-sectional structural geometry of the Nackara Arc contrasts markedly with that of the Fleurieu Arc. In the Nackara Arc, the belt consists of open detachment folds that probably formed above an evaporite-hosted décollement, whereas in the Fleurieu Arc, the belt consists of an imbricate fan of basement-involved thrusts. Further, the foreland edge of the Nackara Arc coincides with a blind ramp whose strike parallels fold axes of the arc, whereas the foreland edge of the Fleurieu Arc coincides with emergent thrusts that cut obliquely across earlier formed folds of the arc. In the transition region between the two arcs, the fold-thrust belt consists of an intensely sheared basal duplex overlain by open folds. Structural contrasts between the Nackara and Fleurieu Arcs indicate that the two curves did not form in the same way. We present a model for development of these curves based on a comparison of their structural features to those of curves in other fold-thrust belts, and based on previous studies of curve formation using sandbox analogs. We suggest that curvature of the Nackara Arc reflects the control of basin stratigraphy on the width of a fold-thrust belt, while curvature of the Fleurieu Arc reflects oroclinal bending of the fold-thrust belt when it impinged on the southeastern corner of the Gawler craton. Our model explains the structural contrast between the two arcs and illustrates correlations between fold-thrust belt curve formation and pre-deformational basin geometry.

Original languageEnglish (US)
Pages (from-to)891-908
Number of pages18
JournalJournal of Structural Geology
Volume18
Issue number7
DOIs
StatePublished - Jul 1996

ASJC Scopus subject areas

  • Geology

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