Subseismic to Seismic Slip in Smectite Clay Nanofoliation.

S Aretusini,E Spagnuolo,G Di Toro,O Plümper

doi:10.1029/2019jb017364

Abstract

Smectite clays are the main constituent of slipping zones found in subduction zone faults at shallow depth (e.g., <1‐km depth in the Japan Trench) and in the decollements of large landslides (e.g., 1963 landslide, Vajont, Italy). Therefore, deformation processes in smectite clays may control the mechanical behavior from slow creep to fast accelerations and slip during earthquakes and landslides. Here, we use (1) laboratory experiments to investigate the mechanical behavior of partly water‐saturated smectite‐rich gouges sheared from subseismic to seismic slip rates V and (2) nanoscale microscopy to study the gouge fabric. At all slip rates, deformation localizes in volumes of the gouge layer that contain a “nanofoliation” consisting of anastomosing smectite crystals. “Seismic” nanofoliations produced at V = 0.01, 0.1, and 1.3 m/s are similar to “subseismic” nanofoliations obtained at V = 10−5 m/s. This similarity suggests that frictional slip along water‐lubricated smectite grain boundaries and basal planes may occur from subseismic to seismic slip rates in natural smectite‐rich faults. Thus, if water is available along smectite grain boundaries and basal planes, nanofoliations can develop from slow to fast slip rates. Still, when nanofoliations are found highly localized in a volume, they can be diagnostic of slip that occurred at rates equal or larger than 0.01 m/s. In such a case, they could be markers of past seismic events when found in natural fault rocks.

Highlights

Scientific drilling evidenced that smectite clay minerals typically constitute the cores of the shallow sections of mature crustal faults and subduction zone megathrust faults
Based on novel high‐resolution structural and chemical analysis obtained from analytical transmission electron microscopy (TEM) we show that a very similar nanofoliation microstructure, possibly produced by frictional sliding along water‐lubricated smectite basal planes and grain boundaries and rotation of grains, is found in the gouges sheared from subseismic to seismic slip rates
Stage 0 was followed by stage 1, lasting up to 0.08 m of slip, which included the decay of the friction coefficient to a minimum value

Summary

Introduction

Scientific drilling evidenced that smectite clay minerals typically constitute the cores of the shallow sections of mature crustal faults (e.g., the creeping section of the San Andreas Fault; Carpenter et al, 2011) and subduction zone megathrust faults (e.g., the Japan Trench megathrust; Kameda et al, 2015). In subduction zone megathrust faults, smectite clay minerals may control the frictional behavior of natural faults at shallow depths ( 10−4 m/s) for the additional transient fluid pressure increase due to the temperature increase by frictional heating that induces

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Discussion

Conclusion