The ups and downs of inversion faults

Professor Rick Sibson says Christchurch Cathedral is advancing towards the Haast pub at the rate of about 40 mm a year, with the crust in-between faulting, crumpling and shortening to give us the Southern Alps.

He ought to know. Sibson has been researching fault zones and earthquakes for nearly four decades, lately with a particular interest in compressional inversion faults.

"These are steep faults that formed around 90 million years ago during the stretching of New Zealand's crust, but they have reactivated as reverse faults during the current period of crustal shortening," Sibson says.

"Such inherited structures present peculiar problems. First, activity is difficult to assess because of the structures' changing offset history. Second, mechanical analysis suggests their continued activity depends on the presence of highly pressurised fluids at depth."

Sibson's current research on compressional inversion earthquakes in Honshu, Japan, may support this theory. There have been at least four such events in Honshu over the past four years, the last in July 2007 occurring directly below the world's biggest nuclear power plant.

"The interesting thing about these Japanese earthquakes is that there is strong geophysical evidence for the existence of over-pressured fluids in the rupture nucleation sites, as suggested by the fault mechanics analysis."

It is this characteristic that may be responsible for the association of some compressional inversion faults with gold-quartz veining, and others with related structures that contain oil-gas fields, as is the case in the Taranaki Basin.

Sibson says this is caused by inversion faults functioning as "valves", bleeding off over-pressured fluids immediately after earthquake rupture when there is greater permeability along the fault.

The resulting on-again-off-again fluid discharge helps to explain the mineralisation sometimes found in exhumed ancient fault zones - the mineralisation at Macraes Mine, for example, is hosted along an ancient fault structure uplifted from a depth of 10-15 kilometres.

Sibson also suggests that earthquake activity on inversion structures may help oil and gas migrate episodically into the dome structures associated with compressional inversion.

Earthquake activity, it turns out, may have its uses!

"What unknown affinity
Lies between mountain and sea
In country crumpled like an unmade bed"
- Denis Glover