Dipartimento di Scienze della Terra, Ambiente e Vita, University of Genova, Genoa, Italy.
Dipartimento di Geoscienze, University of Padova, Padua, Italy.
Dipartimento di Scienze della Terra e dell’Ambiente, University of Pavia, Pavia, Italy.
GeoZentrum Nordbayern, Friedrich-Alexander University of
Erlangen-Nürnberg, Erlangen, Germany.
Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands.
ver a total length of ∼ 55,000 km, subduction zones at con-
vergent plate margins are the main setting for earthquakes
globally. In such environments, seismicity is caused by accu
mulation and release of stress from shallow levels down to interme-
diate depths (50–300 km) (refs
). Intermediate-depth earthquakes
are inaccessible to direct investigation and much knowledge relies
on seismic data, rock-deformation experiments and modelling.
Geophysical data show that the seismic activity in subduction zones
concentrates either inside the subducting lithosphere, or in thick
(kilometres) layers along the plate interface. These layers consist of
hydrated rocks that host pressurized pore fluids and show low seis
. Experimental work and numerical models suggest
that subduction-zone seismicity is triggered by a thermal runaway
, dehydration embrittlement
, phase transforma-
or the reactivation of earlier discontinuities
. To date, inves-
tigations have focused prevalently on seismicity in the subducting
oceanic crust and in the low-velocity plate interface
the seismic potential of the lithospheric mantle of subducting oce
anic plates remains poorly understood.
Compared with the above studies, field-based investigations of
exhumed high-pressure rocks have so far been underutilized to
study fossilized earthquake phenomena directly. Pseudotachylytes,
the solidified friction-induced melts produced during seismic
slip along a fault, are unique indicators of palaeo-earthquakes in
exhumed faults. Unfortunately, pseudotachylytes are rarely pre
served in the rock record and the examples related to subduction
settings are limited to findings within exhumed blueschist- and
eclogite-facies continental and oceanic crust sections
Here we investigate pseudotachylytes in a gabbro–peridotite
body from the Lanzo Massif (Italian Western Alps), a tectonic slice
of oceanic mantle involved in Alpine subduction. These pseudot
achylytes were previously attributed to a pre-subduction oceanic
, but here we conclude they formed under
eclogite-facies conditions. Although similar to pseudotachylytes
from Corsica, related to blueschist-facies metamorphism at shal
lower subduction depths
, our case study provides a unique
record of oceanic slab eclogitization in the Wadati–Benioff seismic
zone, in analogy with the intermediate-depth seismicity that affects
the lithospheric mantle in present-day subducting slabs.
The host rocks of fossil seismic faults
The Alpine Lanzo Massif is a 20 × 9 km sliver of oceanic mantle
peridotite with subordinated 160 Ma old gabbro dykes
embedded in serpentinite and metagabbro (Supplementary Fig. 1).
It records oceanic serpentinization around unaltered peridotite
and later subduction-related Alpine metamorphism under
eclogite-facies conditions (2–2.5 GPa and 550–620 °C at 55–46 Ma)
). The southernmost body of the Lanzo Massif (Moncuni
(Supplementary Fig. 1)) has a core of poorly hydrated to anhy
drous mantle peridotite and pyroxenite intruded by dykes (centi-
metres to tens of centimetres thick) of preserved dry gabbro. Such
unaltered gabbro and peridotite are predominant in Moncuni, and
contain minor volumes (∼ 5 vol%) of hydrated metaperidotite and
metagabbro that record static eclogite-facies metamorphism. The
heterogeneous water distribution can be related to limited oceanic
hydration prior to subduction. Unaltered and hydrated eclogitized
domains form a coherent body that underwent the same subduc
tion-zone evolution, but the predominant peridotite and gabbro
metastably escaped eclogitization. Hence, most oceanic lithosphere
in Moncuni, as in the whole Lanzo Massif, metastably preserved the
Fossil intermediate-depth earthquakes in
subducting slabs linked to differential
*, Giorgio Pennacchioni
, Mattia Gilio
, Michel Bestmann
, Oliver Plümper
and Fabrizio Nestola
The cause of intermediate-depth (50–300 km) seismicity in subduction zones is uncertain. It is typically attributed either to
rock embrittlement associated with fluid pressurization, or to thermal runaway instabilities. Here we document glassy pseudo-
tachylyte fault rocks—the products of frictional melting during coseismic faulting—in the Lanzo Massif ophiolite in the Italian
Western Alps. These pseudotachylytes formed at subduction-zone depths of 60–70 km in poorly hydrated to dry oceanic gab-
bro and mantle peridotite. This rock suite is a fossil analogue to an oceanic lithospheric mantle that undergoes present-day
subduction. The pseudotachylytes locally preserve high-pressure minerals that indicate an intermediate-depth seismic envi-
ronment. These pseudotachylytes are important because they are hosted in a near-anhydrous lithosphere free of coeval ductile
deformation, which excludes an origin by dehydration embrittlement or thermal runaway processes. Instead, our observations
indicate that seismicity in cold subducting slabs can be explained by the release of differential stresses accumulated in strong
dry metastable rocks.
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NATURE GEOSCIENCE | VOL 10 | DECEMBER 2017 | 960–966 | www.nature.com/naturegeoscience