R is the Universal Gas Constant that turns up everywhere in physical chemistry. T is the temperature in degrees Kelvin. This does not happen, though, at geologically realistic temperatures. At high temperatures the temperature dependence is weak because rocks are ductile enough to deform as fast as strain can be applied. Another way to look at it is that ambient thermal energy is nearly enough to overcome the activation energy barrier.
Some everyday materials obey power-law creep or some similar behavior.
Mayonnaise in a jar flows very sluggishly. You can tilt the jar on its side without it spilling. On the other hand, it offers no resistance to a knife and spreads easily on bread. This is another material whose viscosity drops as shear stress is applied. Materials such as these are commonly described as shear-thinning fluids because they become less viscous with increasing shear stress.