Silica gel formation during fault slip: Evidence from the rock record

J.D Kirkpatrick,J.C White,C.D Rowe,E.E Brodsky

doi:10.1130/g34483.1

Abstract

Dynamic reduction of fault strength is a key process during earthquake rupture. Many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments, including silica gel lubrication. However, few have been observed in nature. Here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface. The Corona Heights fault slickenside in San Francisco, California, is covered by a shiny layer of translucent silica. Microstructures in this layer show fl ow banding, armored clasts, and extreme comminution compared to adjacent cataclasites. The layer is composed of ~100 nm to 1 µm grains of quartz, hydrous crystalline silica, and amorphous silica, with 10‐100 nm inclusions of Fe oxides and ellipsoidal silica colloids. Kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip. The layer therefore represents a relict silica gel that formed during fault motion, and which could have resulted in frictional instability. These observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions.

Highlights

Dynamic reduction of fault strength as a function of slip or slip rate is fundamental to earthquake propagation and slip (Rice, 2006)
Silica gel lubrication (Goldsby and Tullis, 2002) is one of a variety of mechanisms that have been proposed to cause coseismic weakening based on theoretical and experimental work (e.g. Sibson, 1973; Brodsky and Kanamori, 2001; Di Toro et al, 2006; Rice, 2006; Han et al, 2007; Brantut et al, 2008), but only a handful of these mechanisms are documented in nature (e.g., Di Toro et al, 2006; Rowe et al, 2012)
We describe the microto nano-scale structure of this layer and evaluate the potential that it represents a natural example of silica gel formation

Summary

INTRODUCTION

Dynamic reduction of fault strength as a function of slip or slip rate is fundamental to earthquake propagation and slip (Rice, 2006). Experiments on silica-rich rocks (chert and quartzite) show that steady-state friction values decrease with slip rate from ~0.6 at 10−6 m/s to 10−3 m/s, and show time-dependent strengthening (Goldsby and Tullis, 2002; Di Toro et al, 2004; Hayashi and Tsutsumi, 2010; Nakamura et al, 2012). No clear natural examples of gels formed in situ along natural fault surfaces have been documented, so the significance of silica gel lubrication as a dynamic weakening mechanism remains unknown. We describe the microto nano-scale structure of this layer and evaluate the potential that it represents a natural example of silica gel formation

COMPOSITION AND MICROSTRUCTURE OF THE SHINY LAYER

RELICT GEL The shiny slickenside layer on the Corona