High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: Nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from Hole B, TCDP

Abstract

The Taiwan Chelungpu‐fault Drilling Project (TCDP) was undertaken in 2002 to investigate the faulting mechanism of the 1999 Taiwan Chi‐Chi earthquake. Hole B penetrated the Chelungpu fault, and recovered core samples from between 948.42 m and 1352.60 m depth. Three zones, marked 1136mFZ, 1194mFZ and 1243mFZ, were recognized in the core samples as active fault‐zones within the Chelungpu fault. Multi‐Sensor Core Logger measurements revealed lower densities and higher magnetic susceptibilities within the black gouge zones in all three fault zones. Even though the fault zone that slipped during the 1999 earthquake has not been identified, higher magnetic susceptibilities indicate that frictional heating has taken place in the Chelungpu fault.

Magnetic properties of fault pseudotachylytes in granites

“…However, this increase, generally attributed to the breakdown of ferromagnesian silicates and subsequent formation of fine-grained magnetite, is not systematic [Zechmeister et al, 2007;Molina Garza et al, 2009]. An increase in magnetic susceptibility, up to 35 times the host rock, has also been reported in fault gouges and ultracataclasites [e.g., Enomoto et al, 2001;Hirono et al, 2006;Tanikawa et al, 2007Tanikawa et al, , 2008. A number of reactions, driven by frictional heating, may account for the observed increases of magnetic susceptibility in fault rocks.…”

“…In general, the most common primary ferromagnetic (s.l.) minerals in fault pseudotachylytes are magnetite (Fe 3 O 4 ), and maghemite, (g-Fe 2 O 3 ) [Nakamura and Nagahama, 2001;Fukuchi, 2003;Ferré et al, 2005;Hirono et al, 2006;Zechmeister et al, 2007;Molina Garza et al, 2009]. Post-seismic alteration of these minerals generally leads to formation of more oxidized phases: At Santa Rosa, opaque grains in the pseudotachylyte, identified using optical and electron microscopy, are dominated by euhedral magnetite grains (<5 mm).…”

“…[56] In general, fault pseudotachylytes have a magnetic susceptibility that is significantly higher (up to 250 times) than their immediate host rock [e.g., Nakamura and Nagahama, 2001;Fukuchi, 2003;Ferré et al, 2005;Hirono et al, 2006]. However, this increase, generally attributed to the breakdown of ferromagnesian silicates and subsequent formation of fine-grained magnetite, is not systematic [Zechmeister et al, 2007;Molina Garza et al, 2009].…”

“…Two examples of such reactions are: (R 1 ) siderite ⇒ pyrite or pyrrhotite ⇒ magnetite, under low oxygen fugacity [ f O 2 ] (Chelungpu Fault [Tanikawa et al, 2008]); (R 2 ) goethite ⇒ hematite, under higher f O 2 (Nojima Fault [Fukuchi et al, 2005] Fukuchi, 2003;Petrík et al, 2003]. However, other factors such as host rock composition and degree of melting also affect f O 2 and thus the magnetic properties of pseudotachylytes [Henkel and Reimold, 2002;Nakamura and Iyeda, 2005;Zechmeister, 2005;Hirono et al, 2006].…”

“…[4] Nevertheless, recent investigations on borehole cores through the Nojima Fault, Japan, and the Chelungpu Fault, Taiwan [Fukuchi, 2003;Fukuchi et al, 2005;Hirono et al, 2006] revealed that magnetization acquisition processes could be more complex than a simple TRM and could also include a chemical remanent magnetization (CRM) as a major component of the NRM. Although TRM acquisition is inevitably coseismic due to the quasi instantaneous cooling of the pseudotachylyte melt [Nakamura et al, 2002;Ferré et al, 2005], the timing of CRM acquisition is less well understood.…”