Coseismic magnetization of fault pseudotachylytes: 1. Thermal demagnetization experiments

Abstract

Fault pseudotachylytes form by quenching of silicate liquids produced through coseismic frictional melting. Here we show that in natural pseudotachylytes the main carrier of magnetic remanence blocked in during cooling of the frictional melt is fine-grained magnetite. This confirms previous studies on friction melt experiments. Stoichiometric magnetite, produced during earthquakes by the breakdown of ferromagnesian silicates, records the ambient magnetic field during seismic slip. We find that most fault pseudotachylytes exposed in the Santa Rosa Mountains, southern California, a classic pseudotachylyte locality, acquired their natural remanent magnetization (NRM) upon cooling of the frictional melt through the range of magnetization blocking temperatures of the magnetite grains and this primarily constitutes a thermal remanent magnetization. NRM intensities typical of most pseudotachylyte veins range from 1 to 60·10 À4 Am 2 /kg. A few specimens, however, contain magnetizations significantly higher than that caused by the Earth's field as well as magnetization directions that are highly variable over short distances. Other magnetization processes, possibly related to coseismic electric currents, may be involved during the seismogenic process to control NRM acquisition.

Earthquakes in the Mantle? Insights From Rock Magnetism of Pseudotachylytes

“…The magnetic properties of the studied ultramafic pseudotachylytes, similar to pseudotachylytes formed from protoliths of other compositions (e.g., Ferré et al, ; Ferré, Friedman, et al, and Ferré, Geissman, et al, ), display a remarkable homogeneity. This consistency reflects the fact that despite the microstructural complexity of pseudotachylyte veins, frictional melts are produced all along the slip plane and their compositions average that of a relatively large volume of host rock.…”

“…For these localities, the mean magnetic coercivity ( H c ) falls in the 20 to 50 mT range, with a mean around 30 mT, in agreement with values obtained from the major hysteresis loops (Table ). These ultramafic pseudotachylytes show FORC similarities to pseudotachylytes derived from quartzo‐feldspathic/tonalitic host rocks (e.g., Ferré et al, ; Ferré, Friedman, et al, and Ferré, Geissman, et al, ). The H c parameter varies in a range typical for stoichiometric magnetite and further confirms the absence of highly oxidized phases such as hematite.…”

“…A postseismic origin for magnetite can be ruled out for three main reasons: (i) magnetite grains are elongated parallel to the viscous flow fabric of the pseudotachylyte and carry an AMS fabric that is kinematically consistent with coseismic slip, at least in the case of Cima di Gratera (Ferré et al, ); (ii) in contrast to other pseudotachylytes (Ferré, Friedman, et al, and Ferré, Geissman, et al, ), the ultramafic pseudotachylyte veins that we sampled, away from fractures, do not display any color variation from margin to center, a characteristic demonstrating the lack of alteration/serpentinization in our samples; and (iii) the magnetite grains observed in thin section in our samples are not spatially associated with any hydrous phases, which precludes a serpentinization origin (Figure ).…”

“…We consider likely origins of the magnetite grains in section 5. At room temperature, ferromagnesian silicates and chrome spinel are paramagnetic, while plagioclase is diamagnetic (e.g., Lagroix & Borradaile, 2000;Dunlop & Özdemir, 2009;Ferré, Zechmeister, et al, 2005;Ferré, Friedman, et al, 2014;Ferré, Geissman, et al, 2014). Magnetite, the only ferromagnetic (s.l.)…”

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

“…First, ferrimagnetic minerals in the fault slip zone may acquire a thermal remanent magnetization (TRM) upon cooling (Piper and Poppleton 1988;Ferré et al 2014). Second, earthquake lightning may constitute an additional magnetization process (Enomoto and Zheng 1998;Ferré et al 2005).…”