Collisional Fold and Thrust Belts Involving Pre-Existing Salt Diapirs and Mini-Basins: Example of the Flinders Ranges, South Australia

Rowan, Mark G., Rowan Consulting, Inc, Boulder, CO

Salt-detached fold-and-thrust belts take two forms: those with an undeformed prekine­matic section and those with diapirs and minibasins that were established prior to the onset of shortening. The geometries are typically simple in the former case, with highly elongate folds with a regular wavelength. In the latter case, however, the preexisting structural archi­tecture and the consequent variable strength of the overburden control the deformation and yield much more complex geometries. Modelling of minibasins separated by a polygonal pattern of salt ridges, with diapirs at the ridge intersections, shows that the weak diapirs are squeezed, the ridges form a polygonal pattern of contractional structures, and the mini­basins simply translate and rotate (Vendeville, 1999). The result is a polygonal array of folds, thrusts, strike-slip faults, and even extensional structures.

The Flinders Ranges of South Australia provide a beautiful example of the influence of established diapirs and minibasins in a fold-and-thrust belt. Exposures clearly show that salt withdrawal and diapirism started shortly after the evaporites were deposited in the Proterozoic, but regional shortening of the Delamerian Orogeny did not begin until the Middle Cambrian. By that time, there were already numerous diapirs located between ellip­tical minibasins close to 10 km in thickness. Analysis of published maps shows a polygonal pattern of contractional folds, thrusts, and thrusted diapirs bounding a series of relatively undeformed, tilted minibasins. Deformation resulting from the regional shortening can be distinguished from the smaller-scale, near-diapir deformation associated with halokinesis.