Generation of billow-like wavy folds by fluidization at high temperature in Nojima fault gouge: microscopic and rock magnetic perspectives

Hiroyuki Nagahama,Norihiro Nakamura,Tomohiko Fukuzawa,Minoru Uehara,Hirokuni Oda

doi:10.1186/s40623-017-0638-y

Abstract

Microscopic billow-like wavy folds have been observed along slip planes of the Nojima active fault, southwest Japan. The folds are similar in form to Kelvin–Helmholtz (KH) instabilities occurring in fluids, which implies that the slip zone underwent “lubrication” such as frictional melting or fluidization of fault gouge materials. If the temperature range for generation of the billow-like wavy folds can be determined, we can constrain the physical properties of fault gouge materials during seismic slip. Here, we report on rock magnetic studies that identify seismic slip zones associated with the folds, and their temperature rises during ancient seismic slips of the Nojima active fault. Using a scanning magneto-impedance magnetic microscope and a scanning superconducting quantum interference device microscope, we observed surface stray magnetic field distributions over the folds, indicating that the folds and slip zones are strongly magnetized. This is due to the production of magnetite through thermal decomposition of antiferromagnetic or paramagnetic minerals in the gouge at temperatures over 350 °C. The presence of micrometer-sized finely comminuted materials in the billow-like wavy folds, along with our rock magnetic results, suggests that frictional heating-induced fluidization was the driving mechanism of faulting. We found that the existence of the magnetized KH-type billow-like wavy folds supports that the low-viscosity fluid induced by fluidization after frictional heating decreased the frictional strength of the fault slip zone.Graphical abstract.

Highlights

Li and Yamazaki (2001) observed from their fluid experiments that the power law slope at low wavenumbers is close to −2, rather than −5/3, which agrees with our plotting of data from Dead Sea sediments and billow-like wavy folds in Nojima fault gouge (Fig. 3)
Our study shows that the power spectra of the Dead Sea sediment deformations and the billow-like wavy folds in Nojima fault gouge follow a similar trend with a power law fit of about −2
In Nojima fault gouge, we found billow-like wavy folds which are similar in form to Kelvin–Helmholtz (KH) instabilities

Summary

Introduction

1942; McKenzie and Brune 1972; Sibson 1975; Cardwell et al 1978; Allen 1979), thermal pressurization (Sibson 1973; Lachenbruch 1980; Mase and Smith 1984, 1987), and/or fluidization (Monzawa and Otsuki 2003; Ujiie et al 2007; Boullier et al 2009) of the fault gouge As these lubrication mechanisms might be recorded in fault gouge as microtextures, it is important to find direct evidence from natural gouge to evaluate ancient frictional faulting mechanisms. The wavy folds are similar to the Kelvin–Helmholtz (KH) instability that normally occurs in fluids (Helmholtz 1868; Kelvin 1871).

Methods

Results

Conclusion