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Formation of Amorphous Materials Causes Parallel Brittle-viscous Flow of Crustal Rocks: Experiments on Quartz - Feldspar Aggregates
  • Matej Pec,
  • Saleh Al Nasser
Matej Pec
Massachusetts Institute of Technology

Corresponding Author:mpec@mit.edu

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Saleh Al Nasser
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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.