Healing microstructures of experimental and natural fault gouge

Abstract

[1] The healing of fault gouge was studied by examining microstructures of naturally and experimentally produced granitoid fault rock. We performed deformation experiments on intact granitoid rock samples at T = 300-500°C, P c = 500 MPa, and _ e = 1.2 Â 10 À4 -1.3 Â 10 À7 s À1 with 0.2 wt% H 2 O added. Healing experiments were carried out on deformed samples at T = 200-500°C, P c = 500 MPa, for 4 h to 14 days under hydrostatic and nonhydrostatic conditions. The grain size distributions (GSD) of the deformed samples were quantified using the D > value (slope of log(frequency) -log(radius) of the GSD) for quartz and feldspar fault gouge. Healing causes a decrease in the D > value from >2.0 to $1.5. The time dependence of the D > decrease is described by a hydrostatic healing law of the form DD = D > (t) À D f = A Á e (ÀlÁt) . The results of the laboratory experiments were compared to three natural fault systems, (1) Nojima Fault Zone (Japan), (2) fault zones in the Black Forest (Germany), and (3) Orobic Thrust (Italian Alps). Natural and experimental gouges have similar D > values. Healing is only observed in monomineralic aggregates; polymineralic (i.e., mixed) fault gouges retain their high D > value after extended healing times because grain growth is inhibited. Healing under nonhydrostatic conditions is more rapid than hydrostatic healing. The low strain rates, which were measured during nonhydrostatic healing, are temperature-dependent and suggest that diffusive mass transfer processes take place during deformation. Thus, fault rocks at upper to midcrustal depth may deform by combined cataclasis and diffusive mass transfer.

The south-western Black Forest and the Upper Rhine Graben Main Border Fault: thermal history and hydrothermal fluid flow

“…This value is close to the value measured for parts of the San Andreas Fault (D = 1.60; Sammis et al 1987), the Qin-Ling Mountain, China (D = 1.59; Shao and Zou 1996), and the Nojima Fault Zone, Japan (D = 1.59; Boullier et al 2004;Keulen et al 2008). Keulen et al (2008) showed that for granitoid samples, experimentally deformed at a rate of 10 -4 s -1 , the D values of 1.5 to 1.6 are the result of healing after heat treatment after deformation of the samples in presence of a fluid. The observed grain size distribution of Cataclasite-II in the Kandern Fault Zone is, therefore, most likely the result of healing of the cataclasite after deformation.…”

“…Post-fracture healing of cataclasites reduces the relative amount of small grains with respect to large grains and causes a decrease in the D-value. The decrease in D values may provide a measure for the consolidation of the cataclasite (Keulen et al 2008).…”

On the role of phyllosilicates on fault lubrication: Insight from micro‐ and nanostructural investigations on talc friction experiments

“…Depending on the prevailing fluid flow properties and permeability structure of the fault zone [e.g., Wibberley et al , 2008; Mitchell and Faulkner , 2008], mechanical and chemical healing processes occur during the interseismic period [ Sibson , 1989; Bos and Spiers , 2000, 2002; Nakatani and Scholz , 2004; Gratier and Gueydan , 2007; Keulen et al , 2008]. These processes may have the capacity to wipe out pre‐existing fabric of clay‐rich fault gouge [ Rutter et al , 1986].…”

Effects of interfacial energy on compaction creep by intergranular pressure solution: Theory versus experiments on a rock analog (NaNO3)

“…The present results may have significant implications for making predictions of pressure solution healing, sealing and strength recovery in very fine‐grained materials, such as fault gouges in both major fault systems [ Jefferies et al , 2006; Wibberley and Shimamoto , 2003] and in faults sealing hydrocarbon reservoirs [ Fisher and Knipe , 1998]. Rates of compaction [ Niemeijer et al , 2010], healing/sealing [ Keulen et al , 2008; Niemeijer et al , 2008] and restrengthening [ Tenthorey et al , 2003; Yasuhara et al , 2005] of active fault systems may be overestimated if pressure solution rates are extrapolated to very fine grain sizes. In turn, this may lead to underestimates of the repeat frequency of seismogenic activity in such fault zones, as well as to underestimates of their permeability.…”