Pyrite alteration and neoformed magnetic minerals in the fault zone of the Chi‐Chi earthquake (M_w7.6, 1999): Evidence for frictional heating and co‐seismic fluids

Abstract

During an earthquake, physical and chemical transformations lead to alteration and formation of minerals in the gouge layer. Altered and neoformed minerals can be used as tracers of some earthquake processes. In this study, we investigate pyrite and magnetic minerals within the host Chinshui siltstone and the 16‐cm‐thick gouge. This gouge hosts the principal slip zone of Chi‐Chi earthquake (Mw7.6, 1999). In the Chinshui siltstone, pyrite framboids of various sizes and euhedral pyrite are observed. The magnetic mineral assemblage comprises stoichiometric magnetite, greigite, and fine‐grained pyrrhotite. The pyrite content is generally reduced in the gouge compared to the wall rock. The magnetic mineral assemblage in the gouge consists of goethite, pyrrhotite, and partially oxidized magnetite. The pyrrhotite, goethite and some magnetite are neoformed. Pyrrhotite likely formed from high temperature decomposition of pyrite (>500°C) generated during co‐seismic slip of repeated earthquakes. Goethite is inferred to have formed from hot aqueous co‐seismic fluid (>350°C) in association with the 1999 Chi‐Chi event. Elevated fluid temperatures can also explain the partial alteration of magnetite and the retrograde alteration of some pyrrhotite to pyrite. We suggest that characterization of neoformed magnetic minerals can provide important information for studying earthquake slip zones in sediment‐derived fault gouge.

Preservation of amorphous ultrafine material: A proposed proxy for slip during recent earthquakes on active faults

“…Although the pH of interstitial fluid along the slip zone was not measured, it was reported that pyrite was absent in the slip zone35. Because pyrite dissolution would lower the pH of the immediately surrounding sediment36, we used pH values of 3.0 and 6.0.…”

Rock record and magnetic response to large earthquakes within Wenchuan Earthquake Fault Scientific Drilling cores

“…The presence of new‐formed magnetic minerals is also an indicator of a certain degree of temperature elevation [ Chou et al ., ]. Pan et al .…”

“…Magnetic studies conducted by Chou et al . [] indicated that pyrrhotite probably formed at high temperatures (>500°C) at the expense of pyrite in gouge within the principal slip zone of the Chi‐Chi earthquake. These studies indicate that rock magnetism can potentially detect frictional heating in earthquake slip zones.…”

Quantitative modeling of the newly formed magnetic minerals in the fault gouge of 1999 Chi‐Chi earthquake (Mw 7.6), Taiwan

“…[] and Chou et al . [] have already suggested that magnetite formed by frictional process. If it seems clear that the friction can cause new magnetic minerals, it is surprising that there is a concentration gradient between the PSZ and the rest of the gouge.…”

“…[] and Chou et al . [] (Figure a). The Verwey transition (120 K) is well identified in the deformed sediments and is not visible in the gouge.…”