Rock bursts and associated neotectonic forms at Minnipa Hill, northwestern Eyre Peninsula, South Australia

Abstract

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.

On the origin of A-tents (pop-ups), sheet structures, and associated forms

“…First, and most telling, the formation of A-tents during brief phases of horizontal compressive stress generated in earthquakes (Terzaghi, 1950) is demonstrated by their recorded formation during such events (eg, United States Geological Survey Staff, 1971: 67;Jennings, 1978b;Denham et al, 1979;Twidale and Bourne, 2000;2003). Thus, an A-tent consisting of slabs of granite 2-3 cm thick was formed during a January 1999 seismic event that affected Minnipa Hill, northwestern Eyre Peninsula, South Australia.…”

“…Some of the explanations offered are readily rebutted, while others are still under consideration. For example, books of laminae were formed by compressive stresses adjacent to fractures during an earthquake that affected Minnipa Hill, a low bornhardt located on northwestern Eyre Peninsula in January 1999 (Twidale and Bourne, 2000). It is not clear whether they were caused entirely by direct compression, or whether the stress found expression through some pre-existing system of latent lamination or planes of strain aligned parallel to the land surface.…”

“…It is known that some postdate the last glaciation in various parts of North America (Cushing et al, 1910;Peterson, 1975;Ericson and Olvmo, 2004) while others are known to have developed during the last few decades and, in some instances, the last few years. Thus, some on Minnipa Hill on northwestern Eyre Peninsula formed at 1137 hours local time on 19 January 1999 during a small earthquake estimated at 2.3 on the Richter Scale and with its epicentre 80 km to the east (Twidale and Bourne, 2000;2003; see also Twidale, 1986).…”

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

“…It is developed in the Minnipa Red variety of the Hiltaba Suite Granite (Ferris et al, 1998). A weak earthquake (estimated strength 2.3 on the Richter Scale) struck the site at 1137 CSST on 19 January 1999 (Twidale and Bourne, 2000).…”

The Lochiel Landslip in 2010 and the Significance of Reinforcement Mechanisms

“…Frequent visits to various quarries and bornhardts on the north-western Eyre Peninsula have allowed the identification of various neotectonic forms as well as passive but significant structural features revealed and eliminated by quarrying (e.g. Twidale et al 1996;Twidale & Bourne 2000, 2003.…”