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Disturbance of the surface of Minnipa Hill Southeast, a low granite dome located on the northwestern Eyre Peninsula, on 19 January 1999, coincided with a shallow earth tremor, and it is suggested that the two events are causally related. The disturbance, which affected an area 170X100 m, took the form of rock bursts, reverse fault scarps, low angle planes of dislocation and overriding sheets, A-tents, dislocated slabs and blocks and widespread surficial flaking. Evidence of two earlier disturbances was noted. The event directly confirms the association of A-tents and dislocated slabs with compressive stress and by implication links bornhardts and sheet fractures with similar strain environments.

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No abstract

Most of the granitic residuals of the Wheat Belt of southwestern Western Australia are bornhardts, with some nubbins developed at the western margin and occasional poorly developed castellated forms. Their origin and age can be deduced from their structure and their relationship to a weathered (lateritic) land surface and various palaeochannels. The bornhardts are massive and most stand lower than local palaeosurface remnants. They are best interpreted as having formed by differential fracture density controlled weathering beneath the weathered land surface in pre-Eocene times. They were exposed by the stripping of the regolith beginning in the Eocene. Many are clearly stepped, indicating that their exposure took place not all at once, but episodically. A few bornhardts stand higher than the weathered land surface. They pre-date the Eocene and the stepped morphology preserved on some suggests that their crests are much older.