Deformation microfabrics of clay gouge, Lewis Thrust, Canada: a case for fault weakening from clay transformation

Abstract

A sequence of bentonite and shale samples in a gouge zone of the Lewis Thrust (Alberta, Canada) that display increasing degree of transformation of clay minerals toward the hanging wall of the thrust has been studied by X-ray diffraction (XRD), X-ray texture goniometry (XTG), scanning electron microscopy (SEM), and transmission and analytical electron microscopy (TEM-AEM), to examine the relations among mineral transformations, microfabrics and fault zone properties. TEM images of authigenic clays show abundant smectite in shale away from the hanging wall, characterized by anastomosing layers with an average orientation that parallels bedding, coexisting with uncommon R1 illite-smectite (I-S). In the sample nearest the hanging wall, by contrast, the dominant clay is mixed-layered, illite-rich ilite-smectite (R1 I-S), coexisting with discrete illite, occurring in individual packets of relatively straight layers with well-defined boundaries. Deformed clay packets are common. Pore space, where packets intesect at high angles to one another and to bedding, is abundant (c. 25%). The microfabric and proportion of illite of intermediate samples are transitional to these end-members. Inter-layered bentonite samples show properties that are similar to those of shale. TEM observations are supported by quantification of the fabrics using XTG, which shows that the intensity of clay preferred orientation decreases significantly with increasing illitization. These relations imply that faulting was the cause of mineral transformations and formation of secondary pore space. The illitization reaction rate was enhanced both by stress-induced defects in clays, and by increased water/rock ratio resulting from deformation-related pore space, resulting in lowering of the effective stress. The deformation-enhanced reaction thus created a positive feedback for further faulting in clay gouge, leading to enhanced weakening of the fault zone.

The role of tectonic shear strain on the illitization mechanism of mixed-layers illite–smectite. A case study from a fault zone in the Northern Apennines, Italy

“…13) indicating, therefore, a progressive illitization of I-S particles in the scaly cleavage domain. The growth and thickening of illite usually takes place by the dissolution-crystallisation process and progressive transition from smectite-rich I-S to illite-rich I-S, and eventually to newly formed illite as a consequence of tectonic stress (Ho et al 1999;Yan et al 2001).…”

“…Tectonic strain can also influence the clay mineral assemblage indirectly by increasing the lattice defects and lowering of the activation temperature of recrystallisation (Teichmuller et al 1979) and enhancing permeability and fluid migration along shear surfaces favouring the illitization and the growth of hydrothermal illite (McCaig and Knipe 1990;Dong et al 1997;Ho et al 1999;Yan et al 2001). However, few data concern the illitization of mixedlayers I-S in low P-T conditions in which Casciello et al (2004) disclosed an increased illite content in clay gouges of a fault zone and suggested that shear-stress-induced dehydration makes smectite highly reactive and prone to transform into illite.…”

Microstructural evolution of an incipient fault zone in Opalinus Clay: Insights from an optical and electron microscopic study of ion-beam polished samples from the Main Fault in the Mt-Terri Underground Research Laboratory

“…13 and 14) has been described in nature and experiments (e.g. Sibson, 1977;Yan, 2001;Zulauf et al, 1990;Logan et al, 1979;Rutter et al, 1986;Haines et al, 2013). The total offset of the gouge bands might lay in the range of several cm to dm, with the thin shear zones at the gouge boundaries likely accommodating the majority of this offset, acting as Y-shears (cf.…”

Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating

“…Depending on the mineralogy of the host rock and availability and chemistry of fluids, illite can grow by illitisation of smectite or dissolution and reprecipitation of pre-existing clays (in clay-bearing host rocks; e.g., Vrolijk & van der Pluijm, 1999;Solum et al, 2005;Haines & van der Pluijm, 2008), by retrograde hydration reactions of feldspar and mica (mainly in crystalline host rocks; e.g., Zwingmann & Mancktelow, 2004;Siebel et al, 2010;Zwingmann et al, 2010b) or even by direct neocrystallisation from a fluid phase. The growth of illite during fault activity is promoted by a number of factors, including temperature (frictional heating and advective heating by hydrothermal fluids), grain comminution (increased surface area), strain (increase of crystal defects) and changes in fluid composition (mainly availability of potassium) and fluid/rock ratio (Vrolijk & van der Pluijm, 1999;Yan et al, 2001). …”