Thermal Alteration of Pyrite to Pyrrhotite During Earthquakes: New Evidence of Seismic Slip in the Rock Record

Abstract

Seismic slip zones convey important information on earthquake energy dissipation and rupture processes. However, geological records of earthquakes along exhumed faults remain scarce. They can be traced with a variety of methods that establish the frictional heating of seismic slip, although each has certain assets and disadvantages. Here we describe a mineral magnetic method to identify seismic slip along with its peak temperature through examination of magnetic mineral assemblages within a fault zone in deep‐sea sediments cored from the Japan Trench—one of the seismically most active regions around Japan—during the Integrated Ocean Drilling Program Expedition 343, the Japan Trench Fast Drilling Project. Fault zone sediments and adjacent host sediments were analyzed mineral magnetically, supplemented by scanning electron microscope observations with associated energy dispersive X‐ray spectroscopy analyses. The presence of the magnetic mineral pyrrhotite appears to be restricted to three fault zones occurring at ~697, ~720, and ~801 m below sea floor in the frontal prism sediments, while it is absent in the adjacent host sediments. Elevated temperatures and coseismic hot fluids as a consequence of frictional heating during earthquake rupture induced partial reaction of preexisting pyrite to pyrrhotite. The presence of pyrrhotite in combination with pyrite‐to‐pyrrhotite reaction kinetics constrains the peak temperature to between 640 and 800°C. The integrated mineral‐magnetic, microscopic, and kinetic approach adopted here is a useful tool to identify seismic slip along faults without frictional melt and establish the associated maximum temperature.

High‐Velocity Friction Experiments Indicate Magnetic Enhancement and Softening of Fault Gouges During Seismic Slip

“…The lower E a allows the decomposition or reaction of minerals to occur at relatively low temperatures (Hirono et al, ; Masumoto et al, ). This favors the release of iron ion from Fe‐bearing minerals, and thus the neoformation of ferrimagnetic minerals (i.e., magnetite, maghemite, and pyrrhotite) in slip zones (e.g., Chou, Song, Aubourg, Lee, et al, ; Chou, Song, Aubourg, Song, et al, ; Hirono et al, ; Pei et al, ; Tanikawa et al, ; Yang et al, , , and references therein), as also demonstrated by the present HVF experiments. These newly formed ferrimagnetic minerals, which would carry either a stable thermochemical or even a thermoremanent magnetization (Chou, Song, Aubourg, Lee, et al, ; Yang et al, ), could generally remain stable over geological times.…”

“…This mixing limits our understanding of the intricacies of thermal transformations induced by frictional heating. Such transformations have been reported to occur pervasively in seismic slip zones (e.g., Chou, Song, Aubourg, Song, et al, ; Han et al, ; Tanikawa et al, ; Yang et al, , ).…”

“…Anomalous rock magnetic properties, often magnetic enhancement, have been reported in earthquake slip zones, for example, from the Nojima Fault (Japan), which ruptured during the 1995 Kobe Mj 7.3 earthquake (e.g., Enomoto & Zheng, ; Ferré et al, ), the Chelungpu Fault (Taiwan) that hosted the 1999 Mw 7.6 Chi‐Chi earthquake (e.g., Hirono et al, ), and the Yingxiu‐Beichuan Fault (YBF; Sichuan, China) that accommodated the 2008 Wenchuan Mw 7.9 earthquake (e.g., Li et al, ; Pei et al, ). Rock magnetic measurements thus have been proposed as a means to locate fault slip (e.g., Chou, Song, Aubourg, Lee, et al, ; Chou, Song, Aubourg, Song, et al, ; Ferré et al, ; Han et al, ; Hirono et al, ; Yang et al, , and references therein) and to constrain the maximum temperature rise caused by the frictional heating (e.g., Chou, Song, Aubourg, Song, et al, ; Hirono et al, ; Mishima et al, ; Yang et al, , ). However, a full appreciation of the rock magnetic approach for earthquake slip diagnosis and concomitant temperature rise requires a thorough understanding of the mechanisms of such magnetic changes.…”

Earthquake slip surfaces identified by biomarker thermal maturity within the 2011 Tohoku-Oki earthquake fault zone

“…We use these concentrations to calculate the n-alkane parameters used in the analysis (Supplementary Data 2). The CPI is calculated by dividing the summed concentrations of odd chain-length n-alkanes by the summed concentrations of even chain-length n-alkanes between chain lengths of 26-35 (ΣC odd,27-35 /ΣC even, [26][27][28][29][30][31][32][33][34] ). The ADI, defined by Rabinowitz et al 33 , is calculated as (C 27 + C 31 )/(C 28 + C 29 + C 30 ).…”

Tōhoku-oki Fault Zone Frictional Heat Measured During IODP Expeditions 343 and 343T

“…Additional evidence of frictional heating in the JFAST core samples comes from other faults identified at depths around 697, 720, and 801 mbsf. Using mineral magnetic methods, electron microscopy, and X-ray spectroscopy, Yang et al (2018) identified the presence of pyrrholite, a mildly magnetic iron sulfide mineral, exclusively within these faults zones. Ruling out other potential mechanisms for pyrrholite formation at this site, such as prolonged diagenetic reaction of magnetite or thermochemical sulfate reduction, they interpret their analyses as evidence of thermally driven pyrite-to-pyrrholite reactions at temperatures between 640°C and 800°C.…”