Abstract
Abstract. This study characterizes the elastic and fluid flow properties systematically across a ductile–brittle fault zone in crystalline rock at the Grimsel Test Site underground research laboratory. Anisotropic seismic velocities and permeability measured every 0.1 m in the 0.7 m across the transition zone from the host Grimsel granodiorite to the mylonitic core show that foliation-parallel P- and S-wave velocities systematically increase from the host rock towards the mylonitic core, while permeability is reduced nearest to the mylonitic core. The results suggest that although brittle deformation has persisted in the recent evolution, antecedent ductile fabric continues to control the matrix elastic and fluid flow properties outside the mylonitic core. The juxtaposition of the ductile strain zone next to the brittle zone, which is bounded inside the two mylonitic cores, causes a significant elastic, mechanical, and fluid flow heterogeneity, which has important implications for crustal deformation and fluid flow and for the exploitation and use of geothermal energy and geologic waste storage. The results illustrate how physical characteristics of faults in crystalline rocks change in fault zones during the ductile to brittle transitions.
Highlights
Brittle faults and ductile shear zones and their associated damage and high-strain zones have a localized yet influential impact on crustal mechanics and fluid flow
These models suggest that the fault zone consists of single or multiple high-strain cores surrounded by a damage zone where the physical properties are a function of the rock matrix, fracture density, and fault core
The seismic velocity of the foliation-parallel samples increases towards the mylonitic core, while the velocity perpendicular to the foliation remains fairly constant
Summary
Brittle faults and ductile shear zones and their associated damage and high-strain zones have a localized yet influential impact on crustal mechanics and fluid flow (see reviews by Sibson, 1994; Faulkner et al, 2010). Previous studies from the laboratory (cm) to field outcrop (km) scale have developed into generalized models for the mechanical and hydraulic behavior of fault zones (e.g., Chester and Logan, 1986; Caine et al, 1996; Faulkner et al, 2003, 2010). These models suggest that the fault zone consists of single or multiple high-strain cores surrounded by a damage zone where the physical properties are a function of the rock matrix, fracture density, and fault core.
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