Earthquakes in the Mantle? Insights From Rock Magnetism of Pseudotachylytes

Abstract

Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host peridotites contain only small amounts of coarse‐grained magnetite. In contrast, the ultramafic pseudotachylytes contain variable amounts of significantly finer magnetite that formed coseismically through melting. Among each locality, magnetite abundance in the pseudotachylytes ranges over several orders of magnitude (4–2,000 ppm), and magnetic grain size varies considerably (from single domain to multidomain). Because the host peridotites are compositionally similar, the pseudotachylyte magnetic properties are interpreted to primarily reflect the physical and cooling conditions prevailing during seismic slip. Further, the examination of laboratory‐produced ultramafic pseudotachylytes shows that quenching does not produce superfine magnetite. We hypothesize that the magnetic properties of ultramafic pseudotachylytes are controlled by fO2 and in consequence vary systematically with depth of formation. Therefore, these properties can be used to assess if the ruptures producing the earthquakes that these pseudotachylytes represent nucleated at actual mantle depths or at shallow depths during exhumation of mantle rocks.

Faulting Processes Unveiled by Magnetic Properties of Fault Rocks

“…As mentioned earlier, pseudotachylytes may occur within seismic slip zones (e.g., Ferré et al, 2012Ferré et al, , 2017Hirono, Ikehara, et al, 2006;A. Lin, 2008;A.…”

“…In contrast, pseudotachylytes formed at even greater depths tend to be ilmenite‐dominated (e.g., Ferré, Geissman, et al, 2014; Moecher & Steltenpohl, 2009). Therefore, rock magnetic properties are informative on the ambient conditions of the earthquake rupture (Ferré et al, 2017; Ferré, Geissman, et al, 2014).…”

The Ductile‐to‐Brittle Transition Recorded in the Balmuccia Peridotite Body, Italy: Ambient Temperature for the Onset of Seismic Rupture in Mantle Rocks

“…The B-type pseudotachylyte of Souquière and Fabbri (2010), however, corresponds to Group 3 (F. Souquière, personal communication, August, 2009). The Balmuccia A-and B-type pseudotachylytes of Ferré et al (2017) were classified according to the scheme of Souquière and Fabbri (2010), because plagioclase-bearing pseudotachylyte (i.e., Group 2 pseudotachylyte) had not been reported from the Balmuccia peridotite at that time.…”

Focal Mechanisms of Intraslab Earthquakes: Insights From Pseudotachylytes in Mantle Units

“…The values of P ′ are consistent with the AMS being carried by a ferromagnetic phase. The upper boundary of paramagnetic susceptibility is 559 × 10 −6 [SI] (Ferré et al., 2017). (b) K m versus T showing that the symmetry of the AMS is neither strongly prolate nor oblate (except in the case of COR24‐05), which is also consistent with magnetite being the dominant AMS carrier.…”

“…(c) T versus P ′ showing that the anisotropic specimens tend to have more oblate fabrics than other specimens. The paramagnetic field is defined by the maximum paramagnetic susceptibility (Ferré et al., 2017).…”