Rheology of the continental lithosphere

The deformation of the continental lithosphere is associated with a combination of ductile and brittle processes. Ductile deformation is analyzed utilizing creep (Newtonian, non-Newtonian) and plastic rheologies. These can be extremely sensitive to composition and temperature. In some cases elastic stresses are relaxed, but in other cases elastic stresses are preserved over 108-109 years. The distinction between renewable stress (plate tectonic, bending) and nonrenewable (thermal, membrane) must be made. The concept of a yield stress is blurred. Brittle deformation tends to be much more complex. Displacements on faults certainly play an important role, but faults are present at all scales. Under some circumstances it is appropriate to treat these deformations in a continuum manner. An avenue for doing this is damage mechanics. The concept of damage mechanics have been utilized widely in engineering problems. We show that when damage mechanics is applied to the brittle deformation of the upper continental crust, a non-Newtonian, power-law viscous rheology is derived. There is a well defined yield stress that can be associated with the dynamic coefficient of friction. Below this stress the upper crust behaves elastically and can act as a stress guide. Above the yield stress the continuum deformations can be modeled as a power-law viscous fluid (with exponent,l 10). This behavior is associated with aftershock sequences. A main shock suddenly increases the stress in regions of the upper crust. Stress relaxation is accomplished by the aftershock sequence and Omori­Ýs law for the decay of aftershocks quantifies the relevant fluid rheology.