Rock and mineral transformations in a fault zone leading to permanent creep: Interactions between brittle and viscous mechanisms in the San Andreas Fault

Abstract

Creep processes may relax part of the tectonic stresses in active faults, either by continuous or episodic processes. The aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space. Results are presented from microstructural studies of natural samples collected from San Andreas Fault Observatory at Depth borehole drilled through the San Andreas Fault, which reveal the chronology of the deformation within three domain types. (i) A relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution. (ii) Shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition: abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others, (iii) zones with mainly dissolution trended toward the present-day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite. This study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep. In parallel, the authors discuss how the creeping mechanism, mainly controlled by the very low friction of the saponite in the first 3-4 km depth, may evolve with depth.

Cohesion versus friction in controlling the long‐term strength of a self‐healing experimental fault

“…However, equation ignores the cohesive component of the Coulomb's equation , with the implicit assumption that large lithostatic pressures rule Coulomb's failure. However, this is not necessarily true for shallow faults, for mineralogical conditions favoring a low friction coefficient along the fault [ Carpenter et al ., ; Lockner et al ., ; Richard et al ., ], or for sea ice where the measured in situ isotropic stresses are of the order of or below the differential stresses [ Weiss et al ., ]. Therefore, an important goal is the development of an empirical formulation for the fault shear strength that takes into account both friction and cohesion of the form | τ | = τ 0 ( V , T , p , t , …) + μ ( V , θ , D c , T , t , …) × ( σ n − p ), where T is the temperature and the frictional term would come from the rate‐and‐state formalism.…”

Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

“…Experiments on cuttings from the CDZ document a low coefficient of friction (μ < 0.25), low (near-zero) rates of frictional healing, and strong localization of these mechanical properties to the creeping gouge zone [Carpenter et al, 2011]. These behaviors are correlated with the presence of magnesium-rich clays, suggesting that clay mineralogy is an important control on the strength and healing behavior of the fault, at least at these depths [e.g., Schleicher et al, 2010;Bradbury et al, 2011;Holdsworth et al, 2011;Hadizadeh et al, 2012;Richard 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

“…The microstructural geology community studied microstructures in deformed mudrocks to infer deformation mechanisms (Dehandschutter et al, 2004;Gratier et al, 2004;Klinkenberg et al;Renard, 2012;Robinet et al, 2012;Richard et al, 2015;Kaufhold et al, 2016), but this was limited by problems with sample preparation for highresolution electron microscopy. Conversely, the mechanical properties and related microstructures of natural and experimental high-strain fault rocks have been studied extensively (Bos and Spiers, 2001;Faulkner et al, 2003;Marone and Scholz, 1989).…”