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Formation of Nanocrystalline and Amorphous Materials Causes Parallel Brittle‐Viscous Flow of Crustal Rocks: Experiments on Quartz‐Feldspar Aggregates
Author(s) -
Pec Matej,
Al Nasser Saleh
Publication year - 2021
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2020jb021262
Subject(s) - cataclastic rock , strain rate , quartz , slip (aerodynamics) , brittleness , geology , overburden pressure , mineralogy , differential stress , shear stress , materials science , deformation (meteorology) , composite material , fault (geology) , geotechnical engineering , thermodynamics , seismology , physics
The brittle‐viscous transition in the lithosphere occurs in a region where many large earthquakes nucleate. To study this transition, we sheared bimineralic aggregates with varying ratio of quartz and potassium feldspar. We deformed the samples in a solid‐medium deformation apparatus at temperature, T = 750°C and pressure, P c = 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 higher 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 a 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.