At a meeting of the Geological Society of America Oct. 26, van der Pluijm presented data from his analysis of gouge deposits in major thrust faults in the southern Canadian Rockies near Banff. Based on our new analysis, we now believe it occurred about 51 million years ago. We can show that 80 percent of gouge deposits are actually created by chemical reactions, rather than from rock grinding. The change involves a dehydration process and the transformation of original clays called smectite into other forms of clay called illite. These changes have significant mechanical and hydrologic consequences.
Clays are inherently weak, making it possible for a fault to move more easily.
This is not necessarily a bad thing, if you live near one. In subduction zones and other major faults, gouge may work like vaseline between bricks, greasing the fault and making it easier for blocks to slide. All van der Pluijm and his colleagues need to unlock the secrets stored in gouge is advanced X-ray equipment, sophisticated radiogenic dating technology and six months of painstaking processing time. If the grains are stacked up like playing cards, they slip easily. To date fault activity, the scientists use argon isotopic dating technology to date grains of newly grown clays in gouge deposits.