Publication Date:
2017-04-04
Description:
The evolution of the frictional strength along a fault at seismic slip rates (about 1 m/s) is
a key factor controlling earthquake mechanics. At mantle depths, friction-induced melting
and melt lubrication may influence earthquake slip and seismological data. We report on
laboratory experiments designed to investigate dynamic fault strength and frictional melting
processes in mantle rocks. We performed 20 experiments with Balmuccia peridotite in a
high-velocity rotary shear apparatus and cylindrical samples (21.8 mm in diameter) over a
wide range of normal stresses (5.4–16.1 MPa), slip rates (0.23–1.14 m/s), and displacements
(1.5–71 m). During the experiments, shear stress evolved with cumulative displacement in
five main stages (stages 1–5). In stage 1 (first strengthening), the coefficient of friction m
increased up to 0.4–0.7 (first peak in friction). In stage 2 (abrupt firstweakening), m decreased
to about 0.25–0.40. In stage 3 (gradual second strengthening), shear stress increased toward
a second peak in friction (m = 0.30–0.40). In stage 4 (gradual second weakening), the
shear stress decreased toward a steady state value (stage 5) with m = 0.15. Stages 1 and 2 are
of too short duration to be investigated in detail with the current experimental configuration.
By interrupting the experiments during stages 3, 4, and 5, microstructural (Field Emission
Scanning Electron Microscope) and geochemical (Electron Probe Micro-Analyzer and
Energy Dispersive X-Ray Spectroscopy) analysis of the slipping zone suggest that second
strengthening (stage 3) is associated with the production of a grain-supported melt-poor
layer, while second weakening (stage 4) and steady state (stage 5) are associated with the
formation of a continuous melt-rich layer with an estimated temperature up to 1780 C.
Microstructures formed during the experiments were very similar to those found in natural
ultramafic pseudotachylytes. By performing experiments at different normal stresses
and slip rates, (1) the ‘‘thermal’’ (as it includes the thermally activated first and second
weakening) slip distance to achieve steady state from the first peak in strength decreased
with increasing normal stress and slip rate and (2) the steady state shear stress slightly
increased with increasing normal stress and, for a given normal stress, decreased with
increasing slip rate. The ratio of shear stress versus normal stress was about 0.15, well below
the typical friction coefficient of rocks (0.6–0.8). The dependence of steady state shear stress
with normal stress was described by means of a constitutive equation for melt lubrication.
The presence of microstructures similar to those found in natural pseudotachylytes and the
determination of a constitutive equation that describes the experimental data allows
extrapolation of the experimental observations to natural conditions and to the study of
rupture dynamics in mantle rocks.
Description:
FIRB-MIUR
project ‘‘Sviluppo Nuove Tecnologie per la Protezione e Difesa del
Territorio dai Rischi Naturali.
Progetti
di Eccellenza Fondazione Cassa di Risparmio di Padova e Rovigo
(CARIPARO)
The European Research Council Starting Grant Project
205175 (USEMS)
Description:
Published
Description:
B06306
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
JCR Journal
Description:
open
Keywords:
Frictional melting
;
Pseudotachylyte
;
Peridotite
;
Slip
;
04. Solid Earth::04.06. Seismology::04.06.99. General or miscellaneous
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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