Relationship between fault growth mechanism and permeability variations with depth of siliceous mudstones in northern Hokkaido, Japan

In order to assess the influence of remote mean stress correlated with depth of burial on the principal mode of failure at fault tips during fault slip in a lithologically homogeneous, fractured rock mass, the growth mechanisms of strike-slip faults have been studied at outcrop-scale in the siliceous mudstones of northern Hokkaido, Japan. We take a multifaceted approach combining i) geological characterization of fractures by fracture mapping in outcrop and fracture logging of boreholes (drilling depth: ≤1020 m), ii) rock mechanical characterization by laboratory tests on core samples, and iii) theoretical analyses using the Griffith–Coulomb criterion. These suggested that the principal mode of failure in the mudstones is dependent, not only on rock strength, but also on remote mean stresses. During and/or after uplift and erosion the faults grew mainly by linking with adjacent faults via numerous splay cracks, formed by tensile failure above roughly 400 m depth. In contrast, below this depth, the faults grew predominantly by shear failure. Such growth mechanisms are consistent with the fact that hydraulic tests performed in boreholes show that highly permeable sections (hydraulic transmissivity: >10−5 m2/s) are restricted to depths of less than 400 m.