Can self-organization of shear zones control the scale and structure of plate boundaries?

John P. Platt, Louis Moresi, & Thorsten W. Becker

Published September 2012, SCEC Contribution #1715

Several interrelated concepts suggest that ductile shear zones cutting the main body of the lithosphere can selforganize in such a way that they control their spacing and cumulative width at any given depth. Microstructural weakening involves damage processes such as grainsize reduction by dynamic
recrystallization, the mixing and dispersal of phases in polymineralic rocks, and the development of lattice preferred orientations. These processes require that at any given depth and temperature (a) the yield strength of the undeformed material be exceeded, and (b) sufficient permanent strain occurs that these processes can cause weakening. The development of shear zones therefore requires that the ambient stress remain at the yield stress, and hence the shear zones are likely to develop a cumulative width such that they can accommodate the imposed plate boundary velocity at that level of stress. We examine the way shear zones evolve under these boundary conditions, and the controls on shear zone spacing and continuity under different temperature conditions and in different materials. Dissipative heating complicates this picture because it has a strongly scale-dependent effect on the evolution of shear zones. If shear zones lose heat by conduction, they will reach an average temperature differential from the ambient value that depends on the square of their thickness and on the depth below the surface. Dissipative heating and temperature-dependent
weakening will therefore favor small numbers of thick shear zones. We examine the interrelationships between these two groups of effects.

Platt, J. P., Moresi, L., & Becker, T. W. (2012, 9). Can self-organization of shear zones control the scale and structure of plate boundaries?. Poster Presentation at 2012 SCEC Annual Meeting .