Influence of basin architecture on the style of inversion and fold-thrust belt tectonics—the southern Adelaide Fold-Thrust Belt, South Australia

Abstract

The southern Adelaide Fold-Thrust Belt involves Neoproterozoic (Adelaidean) strata deposited during protracted rift and sag phases, and an overlying Cambrian (Kanmantoo) sequence represented by a thick wedge of turbidites, rapidly deposited in an eastwards-deepening basin, deeply incised into the underlying rocks. The belt was deformed during the Delamerian orogeny, developed at the southeastern margin of the Australian craton during early Palaeozoic NW-directed displacement. During this shortening event the margin of the Kanmantoo Basin was reactivated and formed a significant strain guide that controlled key features of the fold-thrust belt. Strain-integrated and restored sections reveal contrasting geometric and kinematic styles along the strike of the belt. A transpressional zone is developed at the steep basin margin in the south, oriented oblique to the structural grain. The central parts of the belt are dominated by strong buttressing, growth fault reactivation and basement-involved footwall shortcut thrusting. In the northern part of the belt, shortening is accommodated by homogeneous folding above a master de´collement. These variations in regional structural style and internal strain partitioning reflect significant variations in the original Kanmantoo Basin margin geometry. Rheological parameters control the partitioning of strain associated with pervasive fabrics, for example cleavage and transposed bedding. Folding is accompanied by negative line-length changes (shortening) in the northern part of the belt. In its thrusted central part, pelites commonly show positive line-length changes. Psammites were least affected by line-length changes and thus provide key beds for section balancing. Pervasive strain makes a full restoration of cross-sections impossible. However, incorporating regional strain data assists establishment of balanced and restorable sections, which provide a powerful tool in understanding both pre-deformational basin geometry and its shortening.