Are feldspar-to-mica reactions necessarily reaction-softening processes in fault zones?

Abstract

Reaction-softening by mineralogical changes from feldspars to sericite has been documented from many fault zones. During external crystalline basement deformation in the Alpine orogeny, the Ser Barbier thrust and splay faults in the Pelvoux Massif experienced ultracataclasis and sericitisation. Microstructural information and geochemical data from the fault rocks suggest that different muscovitisation reactions occurred at different times within the evolution of the fault zone, and each reaction had its own impact on fault rheology. Early cataclasis aided chemical breakdown of orthoclase feldspars to muscovite, yet quartz release accompanying this process resulted in local cementation and consequent hardening of the ultracataclasite. Continued deformation was accompanied by muscovitisation of the albite feldspar, and resulted in the formation of mica-rich fault rocks which experienced progressive silica removal by the fluid with increasing deformation. At this stage, reaction-enhanced ductility dominated. Much of the early cemented ultracataclasites escaped later deformation, and their low permeability allowed preservation of their early geochemical characteristics by preventing later fluid access. Such findings demonstrate how the complex interplay between deformation processes and geochemical reactions may result in a changing rheology during fault zone evolution.

Introduction

Geochemical reactions in fault zones are generally fluid induced and are commonly suggested to result in changes in fault rock rheology. Most of these changes are reaction-softening effects (e.g. White and Knipe, 1978, Brodie and Rutter, 1985, Rubie, 1990) and the breakdown of relatively strong feldspars to easily deformable phyllosilicates is one of the most commonly reported. At lower greenschist facies conditions, feldspar breakdown to white mica commonly occurs. This process has often been suggested to be an important reaction-softening step in granitic fault zones (e.g. O'Hara, 1988, Evans, 1990, Mitra, 1992, Evans and Chester, 1995, Wintsch et al., 1995), and has also been suggested to be a explanation for anomalously low friction on the San Andreas Fault Evans and Chester, 1995, Wintsch et al., 1995.

The breakdown reactions of albite and orthoclase to fine-grained muscovite (often called sericite) may be represented by the following equations (Hemley and Jones, 1964), written to conserve aluminium:

$$\text{3NaAlSi}{}_3\text{O}{}_8\text{+K}{}^{\mathrm{+}}\text{+2H}{}^{\mathrm{+}}\text{→KAl}{}_3\text{Si}{}_3\text{O}{}_{10}\text{(OH)}{}_2\text{+6SiO}{}_2\text{+3Na}{}^{\mathrm{+}}$$

$$\text{albite→muscovite+silica}$$

$$\text{3KAlSi}{}_3\text{O}{}_8\text{+2H}{}^{\mathrm{+}}\text{→KAl}{}_3\text{Si}{}_6\text{O}{}_{10}\text{(OH)}{}_2\text{+6SiO}{}_2\text{+2K}{}^{\mathrm{+}}$$

$$\text{orthoclase→muscovite+silica}$$

These equations demonstrate the requirement of an acidic fluid for muscovitisation to occur. Temperature, pressure and fluid composition may govern the reactions and phase stability in the albite–orthoclase–muscovite system, and this has been demonstrated with the use of (a_Na⁺/a_H⁺) vs (a_K⁺/a_H⁺) activity plots (e.g. Wintsch, 1975a).

Feldspar muscovitisation reactions play an important role in deformation processes (e.g. White and Knipe, 1978, Knipe and Wintsch, 1985). The most noted way is by reaction-enhanced ductility (White and Knipe, 1978), where relatively strong feldspars [at temperatures less than, say, 500°C (e.g. Voll, 1976, Tullis, 1983 and many others)] are replaced by micas, which are much more easily deformed (e.g. Shea and Kronenberg, 1993). This has been documented in basement rocks both in cataclastic fault zones (e.g. Janecke and Evans, 1988, Evans, 1990, Mitra, 1992) and in mylonitic shear zones (e.g. Knipe and Wintsch, 1985, O'Hara, 1988, Imber et al., 1997). Wintsch (1978) describes evidence for syntectonic mica growth perpendicular to σ₁ by dissolution of quartz and feldspar, and later by recrystallisation of earlier formed phyllosilicates. Micas deform relatively easily by grain boundary sliding, cleavage plane slip, and dislocation glide, and the (001) plane in micas is important in crystal `stacking faults' (Bell and Wilson, 1981). Hence the formation of an aligned mica foliation weakens the tectonite (e.g. Wintsch et al., 1995). Given the common occurrence of aligned sericite in basement fault zones (e.g. Wibberley, 1995), this makes muscovitisation and consequent reaction-softening an especially important process. In prograde situations, the reactions in , may operate in reverse (Wintsch, 1975b) resulting in reaction hardening (Wintsch et al., 1995).

Hence the behaviour of a fault zone subsequent to reaction is a direct result of the rheology of the products of the particular reactions taking place. However, deformation can control the occurrence and location of these reactions in the first place. For example, Knipe and Wintsch (1985) describe how deformation influences the operation and reaction direction of feldspar↔muscovite reactions by enhancing (1) source solubility (e.g. by generating new reaction surfaces by fracturing), (2) pathways (e.g. by grain-size reduction) and (3) sinks (e.g. by void formation).

In this way, muscovitisation reactions can both facilitate, and be facilitated by, deformation. I aim to illustrate possible relationships between muscovitisation and the evolution of deformation mechanisms operating in granitic basement fault zones, using data from the Ser Barbier thrust sheet. Relevance to consequent hardening and softening will be emphasised.