STRAIN PARTITIONING AND LOCALIZATION IN GRANITOID FAULT ROCKS DEFORMED AT ELEVATED PRESSURES AND TEMPERATURES (Invited Presentation)

Strain localization into narrow fault zones is one of the pre-requisites for plate tectonics to operate. Within a single fault zone, strain localization and partitioning occur at different depths and hence, a number of deformation mechanisms accommodate the imposed movement. The dominant deformation mechanism will determine the strength of the fault zone and may also determine whether a fault will creep steadily or generate earthquakes.

Here, I focus on deformation mechanisms active in synthetic granitoid fault rocks deformed in experiments at P-T conditions corresponding to the base of the seismogenic layer (P = 500 MPa, T = 300 – 600˚C, i.e. ~15 km depth). Only two experiments (performed at the fastest strain rates, 10^-3 s^-1, and lowest temperatures) failed abruptly and audibly right after reaching peak strength (τ ~ 1500 MPa). All other samples reach high shear stresses (τ ~ 570–1500 MPa) then weaken slightly (by Δτ ~ 10–190 MPa) and continue to deform at a steady state stress level. Clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels are observed.

Microstructures show widespread comminution, strain partitioning, and localization into slip zones. The slip zones contain at first nanocrystalline (d_equ ≈ 30 nm) and partly amorphous material. Later, during continued deformation, fully amorphous material develops in some of the slip zones. Despite the mechanical steady state conditions, the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below γ = 5. Within the slip zones, the fault rock material progressively transforms from a crystalline solid to an amorphous material. The amorphous material displays many similarities with naturally occurring pseudotachylites. However, heat diffusion calculations suggest that this material is not the result of friction induced melting, and microstructural observations show that it clearly pre-dates abrupt failure. Thermodynamic considerations demonstrate that comminution to nanometric grain size is sufficient to vitrify feldspars in agreement with experimental observations. The material transformation form a crystaline solid to an amorphous fluid is accompanied by a profound change in rheological behavior and may serve as a nucleation point for abrupt failure.