Journal of Geography (Chigaku Zasshi)
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
Review Articles
A Finding of Ultramafic Mylonitic Pseudotachylyte and Its Seismogenic Significance
Toward a Better Understanding of Seismogenic Processes in the Mantle
Masaaki OBATATadamasa UEDA
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2011 Volume 120 Issue 3 Pages 439-451

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Abstract

 This paper reviews the occurrence of pseudotachylyte having a mylonitic texture of a ultramafic composition, originally reported by Ueda et al. (2008a), as a new type of pseudotachylyte—mylonitic pseudotachylyte—and discusses its seismogenic significance in the context of deep to intermediate-depth earthquakes in the mantle. Referring to widely accepted definitions of pseudotachylyte and mylonite, it is emphasized that mylonitic fault-vein pseudotachylytes represent frozen melts generated by frictional melting of host rocks during earthquakes. It is also emphasized that the mylonitic pseudotachylyte is a complex, integrated product of a whole sequence of seismogenic processes including pre-seismic (ductile) shear localization and recrystallization, followed by a (brittle) seismic rupture and comminution and frictional heating-and-melting, and further, by rapid cooling and crystallization, with or without co-seismic or post-seismic shear deformation. Although unraveling and accurate analyses of the whole processes is a difficult task because of the extremely fine-grained nature of the pseudotachylytes and the recrystallization overprints therein, careful petrographic analyses of such rocks should provide useful insights into the seismogenic processes bridging the gap between conventional seismological, geological and petrological methods. A need for a multidisciplinary approach is emphasized for a better understanding of seismogenic processes in the mantle.

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© 2011 Tokyo Geographical Society
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