STRUCTURE AND ORIGIN OF THE FLEURIEU AND NACKARA ARCS IN THE ADELAIDEFOLD-THRUST BELT, SOUTH AUSTRALIA - SALIENT AND RECESS DEVELOPMENT INTHE DELAMERIAN OROGEN

The regional map pattern of structural trends in the Early Paleozoic A delaide fold-thrust belt of South Australia resembles an upright lette r '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 rece ss. The map-view axial trace of the Nackara Arc coincides with the thi ckest portion of the Adelaidean (Late Proterozoic) basin, and the map- view axial trace of the Fleurieu Arc coincides with the intersection b etween 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 wi th that of the Fleurieu Arc. In the Nackara Arc, the belt consists of open detachment folds that probably formed above an evaporite-hosted d ecollement, whereas in the Fleurieu Arc, the belt consists of an imbri cate fan of basement-involved thrusts. Further, the foreland edge of t he Nackara Arc coincides with a blind ramp whose strike parallels fold axes of the arc, whereas the foreland edge of the Fleurieu Arc coinci des with emergent thrusts that cut obliquely across earlier formed fol ds 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 Arc s indicate that the two curves did not form in the same way. We presen t a model for development of these curves based on a comparison of the ir 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 curvatur e of the Fleurieu Arc reflects oroclinal bending of the fold-thrust be lt when it impinged on the southeastern corner of the Gawler craton. O ur model explains the structural contrast between the two arcs and ill ustrates correlations between fold-thrust belt curve formation and pre -deformational basin geometry. Copyright (C) 1996 Elsevier Science Ltd .