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Figure 5. Examples of brecciation and fault-
related melting of subduction-zone metamor-
phic rocks (the photographs in [A] and [C] are
from Samuel Angiboust, personal commun.,
2017; the photograph in [B] is from Michele
Locatelli, personal commun., 2017). (A) Pseudo-
tachylite (result of fault-related frictional heat-
ing) from the Arosa Zone St. Moritz (Switzer-
land) exposure of ~35 km depths of an ancient
subduction zone interface investigated by
Bachmann et al. (2009). The horizontal dimen-
sion is 5 cm. In this photograph, we see the
result of plastic deformation (clasts are
mylonitized) and evidence of melt injection
during the seismic event. (B) Eclogite-facies
breccia from the Monviso ultra-high pressure
(UHP) metamorphic locality (Italy) representing
a major rupture event at ~80 km depth in an
ancient subduction zone (the horizontal dimen-
sion is ~1.0 m; see Angiboust et al., 2012a).
(C) Another example of intensely fractured/
brecciated eclogite-facies metabasaltic rock
from the UHP metamorphic Monviso locality,
showing the mineralization of these fractures
by omphacite, a Na-rich pyroxene stable at
extremely high pressure-temperature (P/T)
conditions (the horizontal dimension is 15 cm).
The mineralized matrix of the breccia in (B), and
the veins in (C), reflect high-P/T element mobil-
ity, likely in H2O-rich metamorphic fluids.
anonymous reviewer, and to Steve Whitmeyer for
his efforts as GSA Today editor.
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