Clay fabric intensity in natural and artificial fault gouges: Implications for brittle fault zone processes and sedimentary basin clay fabric evolution

Abstract

[1] The role of phyllosilicate fabrics in fault gouge is a poorly understood component of the mechanical and hydrologic behavior of brittle fault zones. We present 90 fabric intensity measurements using X-ray texture goniometry on 22 natural clay-rich fault gouges from low-angle detachment faults (Death Valley area detachments, California; Ruby Mountains, Nevada; West Salton Detachment Fault, California) and the Peramola thrust in NE Spain. Natural fault gouges have uniformly weak clay fabrics (multiples of a random distribution (MRD) = 1.7-4.5, average MRD = 2.6) when compared to phyllosilicate-rich rocks found in other geologic settings. Clay fabric intensities in natural gouges do not vary significantly either as a function of tectonic environment or of dominant clay mineralogy in the gouge. We compare these natural samples with 69 phyllosilicate fabric intensities measured in laboratory experiments on synthetic clay-quartz mixtures. Clay fabric intensities from laboratory samples are similar to those in natural gouges (MRD = 1.7 -4.6), but increase systematically with increasing shear strain and normal stress. Total phyllosilicate content does not significantly affect clay fabric intensity. Shear strain is important for developing stronger fabrics; samples subjected solely to compression exhibit uniformly weak fabrics (MRD = 1.6 -1.8) even when compressed at high normal stresses (150 MPa). The weak fabrics found in natural fault gouge indicate that if anisotropic and overall low fault zone permeability allow elevated pore fluid pressures and fault weakening, this anisotropy must be a transient feature that is not preserved. Our data also reinforce the idea that clay fabric development in sedimentary rocks is primarily a function of authigenic mineral growth and not of compaction-induced particle rotation.

Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)

“…4-7). It shows a distinctive, well organized P foliation (orientation nomenclature sensu Logan et al, 1979) with a high fabric intensity (sensu Yan, 2001;Haines et al, 2009). Correspondingly, ultrathin sections of gouge display a uniform extinction pattern along the P foliation under crossed polars (not shown here; refer to Laurich et al, 2014).…”

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

“…Predictions of mechanical and transport properties over long timescales are essential for the evaluation of subsurface integrity. For this, it is generally agreed that a multiscale experimental approach that combines measurement of bulk mechanical and transport properties with microstructural study to identify deformation mechanisms is required to develop microphysics-based constitutive equations, which can be extrapolated to timescales not available in the laboratory, after comparison with naturally deformed specimens (Morgenstern and Tchalenko, 1967;Tchalenko, 1968;Lupini et al, 1981;Rutter et al, 1986;Logan et al, 1979Logan et al, , 1987Logan et al, , 1992Marone and Scholz, 1989;Evans and Wong, 1992;Katz and Reches, 2004;Niemeijer and Spiers, 2006;Colletini et al, 2009;Haines et al, 2009Haines et al, , 2013French et al, 2015;Crider, 2015;Ishi, 2016).…”

Preferred mineral orientation of a chloritoid-bearing slate in relation to its magnetic fabric

“…Experimental studies of fabric formation in fine-grained sediments have shown that mechanical rotation of pre-existing grains is only capable of producing fabrics up to about 10 m.r.d. (Tullis, 1976;Haines et al, 2009). Hence, the extremely strong muscovite and chloritoid texture in the sample of the MASB has to result from a preferred metamorphic mineral growth in response to a differential stress (cf.…”