Friday, February 1, 2013

Fault Gouge

It would be nice to show this photo and simply say this is what fault gouge looks like and be done with it. Yes, the photo is fault gouge, but as in all things scientific, there are as many kinds of gouge as there are kinds of faults (perhaps more), and, as science necessitates, there’s also a complicated classification scheme to describe all the nuances.

Fault gouge or breccia is the material within a fault zone that is immediately affected by the movement of the fault around it. Gouge evolves during the lifespan of the fault from something that looks much like the country rocks surrounding the fault to something that looks like petrologic mush: as the fault continues moving, the gouge is further ground up, dragged, pulverized and stretched out, often joined by fresh rocks to make new gouge components. Fault gouge can be coarse, gritty and dry; it can have a mix of sizes of rocky material (a breccia); it can be milled down to a powdery-looking substance; it can have water in it encouraging the old ground up rock to react and create new minerals. In some faults, the pressure in the fault zone is so great or frictional heating high enough to create an entirely new collection of minerals as gouge. After being fractured in an earthquake, in many circumstances the broken gouge can “heal” or anneal itself back to being solid rock again.

The ingredients within a gouge are extremely variable: the wall rocks could be granite, the gouge could be talc, or mica, or clay or many kinds of minerals, all depending on the variables present within and around the fault zone, including:
--What kinds of rock surround the fault and contribute the material for forming the gouge?
--What is the force involved in moving the fault that can crush the gouge; what is the direction across the fault that this force is applied?
--Does the fault move slowly and continuously, or as sporadic events on, for example, a time scale of every thousand years or so.
--How deep within the earth is the gouge when the fault moves; what are its ambient temperatures and pressures?
--Is water available to alter the gouge to a different, more fault-friendly, material?

The thickness of gouge, that is, the thickness within the moving part of a fault, has little meaning for the importance of the fault itself. Some very thin fault zones are capable of moving rocks over an immense distance: thrust faults with tens of kilometer of displacement sometimes have centimeter-scale zones of gouge; an entire tectonic plate can be moved about on a zone sometimes less than a meter thick. On the other extreme, the gouge within some faults that don’t actually displace things very much can be hundreds of meters thick and look impressively crushed.

In addition to being formed by the movement of the fault blocks around it, the gouge itself can act as a lubricant or as a brake on the movement of the fault. Coarse crystalline gouge material such as quartzite or limestone is, as you can imagine, more difficult to move than fine-grained clays. If water invades a fault zone, the gouge can become even more slippery and can facilitate movement at much lower stress on the fault. This is one of the many potential problems with “fracking:” unknowingly injecting water into old, supposedly inactive faults can decrease the resistance in the gouge and potentially reactivate the fault.

While earthquakes are inevitable, the type of gouge in a fault zone can aid in reducing their frequency and catastrophic effects. The San Andreas fault, where the North American Plate and Pacific Plate are grinding alongside each other in opposite directions, includes segments where the fault seems to constantly “creep” along without the sudden displacements that cause massive jolts. In these areas, the gouge at depth is so slippery (serpentine as predicted and lately verified) that the fault seems to move far more easily than the strength of its host rocks would predict.

Photo: Gouge in fault within shallow oceanic section preserved in the Vourinos Ophiolite, Greece.

Further reading on gouge and what happens to it in faults:

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