Formation of Amorphous Materials Causes Parallel Brittle-viscous Flow of
Crustal Rocks: Experiments on Quartz - Feldspar Aggregates
The brittle – viscous transition in the lithosphere occurs in a region
where many large earthquakes nucleate. To study this transition, we
sheared bi-mineralic aggregates with varying ratio of quartz and
potassium feldspar at temperature, T=750oC and pressure, Pc = 800 MPa
under either constant displacement rate or constant load boundary
conditions. Under constant displacement rate, samples reach high shear
stress (τ = 0.4−1 GPa) depending on mineral ratio) and then weaken.
Under constant load, the strain rate shows low sensitivity to stress
below τ ≈ 400 MPa, followed by a high stress sensitivity (stress
exponent, n = 9 − 13) at higher stresses irrespective of mineral ratio.
Strain is localized along “slip zones” in a C and C’ orientation. The
material in the slip zones shows extreme grain size reduction and flow
features. At peak strength, 1-2 vol% of the sample is composed of slip
zones that are straight and short. With increasing strain, the slip
zones become anastomosing and branching and occupy up to 9 vol%; this
development is concomitant with strain-weakening of the sample. Slip
zones delimit larger cataclastic lenses, which develop a weak foliation.
Our results suggest that strain localization leads to microstructural
transformation of the rocks from a crystalline solid to an amorphous,
fluid-like material in the slip zones. The measured rheological response
is a combination of viscous flow in the slip zones and cataclastic flow
in coarser-grained lenses and can be modeled as a frictional slider
coupled in parallel with a viscous dashpot.