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Active dislocations in granitic terrains of the Gawler and Yilgarn cratons, Australia, and some implications

Department of Geology and Geophysics, University of Adelaide, Adelaide 5005, South Australia e-mail: rowl.twidale{at}adelaide.edu.au

Examination and monitoring of bornhardts and of quarry exposures on the Gawler Craton exposed on northwestern Eyre Peninsula, South Australia, and in the Yilgarn Craton of the southwest of Western Australia have shown, first, that the bornhardts of those regions are complex features, with each topographic form including several structural domes; second, that the rock masses are in compression along one axis so that one set of steeply-inclined fractures has not been penetrated by water and hence is not weathered; third, and in contrast, sheet fractures, which are tectonic features, continue to be arched and thus are readily infiltrated by groundwaters so that the adjacent rock is consistently weathered; fourth, the rock masses continue to be stressed so that neotectonic forms are still developing. These observations confirm that bornhardts are developed on rock compartments which are in compression and that they are two-stage or etch forms, and that sheet fractures and A-tents, or popups, are tectonic in origin.