Open AccessFrictional‐viscous flow of phyllosilicate‐bearing fault rock: Microphysical model and implications for crustal strength profiles
Author(s) -
Bos Bart,
Spiers Christopher J.
Publication year - 2002
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2001jb000301
Subject(s) - halite , geology , brittleness , shear (geology) , fault gouge , fault (geology) , geotechnical engineering , strain rate , overburden pressure , deformation (meteorology) , pore water pressure , deformation mechanism , shear zone , petrology , materials science , seismology , tectonics , microstructure , composite material , paleontology , oceanography , gypsum
It is widely believed that around the brittle‐ductile transition, crustal faults can be significantly weaker than predicted by conventional two‐mechanism brittle‐ductile strength envelopes. Factors contributing to this weakness include the polyphase nature of natural rocks, foliation development, and the action of fluid‐assisted processes such as pressure solution. Recently, ring shear experiments using halite/kaolinite mixtures as an analogue for phyllosilicate‐rich rocks for the first time showed frictional‐viscous behavior (i.e., both normal stress and strain rate sensitive behavior) involving the combined effects of pressure solution and phyllosilicates. This behavior was accompanied by the development of a mylonitic microstructure. A quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model. In this paper, a microphysical model for shear deformation of foliated, phyllosilicate‐bearing fault rock by pressure solution‐accommodated sliding along phyllosilicate foliae is developed. The model predicts purely frictional behavior at low and high shear strain rates and frictional‐viscous behavior at intermediate shear strain rates. The mechanical data on wet halite + kaolinite gouge compare favorably with the model. When applied to crustal materials, the model predicts major weakening with respect to conventional brittle‐ductile strength envelopes, in particular, around the brittle‐ductile transition. The predicted strength profiles suggest that in numerical models of crustal deformation the strength of high‐strain regions could be approximated by an apparent friction coefficient of 0.25–0.35 down to depths of 15–20 km.