The structural geometry of the Onkaparinga Gorge region, southern Adelaide Fold Belt, South Australia

Abstract

The Onkaparinga Gorge, situated on the Onkaparinga River southwest of Clarendon contains exposed middle Adelaidean rocks of the southern Adelaide Fold Belt. The structural geometry displayed exists as the result of a Cambro-Ordovician compressional deformation, the Delamarian Orogeny. Folds are gentle in competent strata and close to tight and west vergent in fine grained incompetent strata. The majority of faulting is thrusting oriented subparallel to bedding with one high angle thrust, the Onkaparinga Fault that cross­cuts all other tectonic elements. Low angle thrusting caused the repetition of competent beds and the duplexing or imbrication of incompetent units. Total displacement calculated by the addition of all minimum displacements measured on individual thrusts is in the order of 1.8 kilometres. Strain patterns indicate that thickening of bedding has occurred in the Sturt Formation diamictite with considerable shortening and volume loss due to compaction. The style of deformation in the Belair Subgroup is one of stretching parallel to bedding. The difference in strain patterns in the two above mentioned formations suggests that there is a structural discontinuity between the Belair Subgroup and the Sturt Formation. Evidence from geological mapping and subsequent stereographic projections of field data indicates that thrusting subparallel to bedding has been the major deformational influence at the levels of the Sturt and Tapley Hill Formations with folding less influential. This evidence may support the existence of a roof thrust zone at this level but such features can also be attributed to other deformational models. Two models of tectonic evolution can explain the current structural geometry in the Onkaparinga Gorge. The first model associates the thrusting observed with fold development during compression. The second model proposes a two-part deformation of thrusting followed by folding of the thrusted strata during continued compression.