Shear heating in continental strike-slip shear-zones: model and field examples.
Leloup Philippe Hervé*, Yanick Ricard&, Jean Battaglia*
and Robin Lacassin*
* Laboratoire de tectonique et mécanique de la lithosphère,
C.N.R.S.- U.M.R. 7578, Institut de physique du globe de Paris,
4 place Jussieu, 75252 Paris CEDEX 05.
& Laboratoire de Sciences de la Terre, URA 726, Ecole Normale
Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon
Cedex 07.
A two-layer (crust and upper mantle), finite difference steady-state
thermomechanic model of a long-lived (several millions years),
lithospheric strike-slip fault is presented, and its predictions
compared to field observations from various major fault zones.
In order to estimate the maximum amount of shear heating, all
mechanical energy is assumed to be dissipated in heat, in ductile
as well as in brittle layers. Deformation follows a friction law
in the brittle layer(s), and a power flow law in the ductile one(s).
Variations of several independent parameters and their influence
on the thermo-mechanical state of the fault zone and on shear
heating are systematically explored. Shear heating is found to
be more important in fault zones affecting an initially cold lithosphere,
and increases with slip rate, friction coefficient and stiffness
of materials. In extreme cases (slip rate of 10 cm/yr, stiff lithosphere)
shear heating could lead to temperature increases close to 590°C
at the Moho, and 475°C at 20 km depth. For more common cases,
shear heating leads to smaller temperature increases, but can
still explain high-grade metamorphic conditions encountered in
strike-slip shear-zones. However, modelled temperature conditions
often fall short of those observed. This could be due to heat
transport by mechanisms more efficient than conduction. Common
syntectonic emplacement of granitic melts in ductile strike-slip
shear-zones can be explained by lower crust partial melting induced
by shear heating in the upper mantle. Beside slip rate, the possibility
of such melting depends mostly on the upper mantle rheology and
on the fertility of the lower crust: for hard upper mantle and
highly fertile lower crust, partial melting could occur at rates
of 1 cm/yr, while in most cases it would result from micas break-down
for slip rates over 3 cm/yr. Due to shear heating, partial melting
of the upper mantle could occur in the presence of small amonts
of fluids. Rise of magmas and/or hot fluids in the shear-zone
will further enhance the temperature increase in shallower parts
of the fault zone. In nature shear heating would inevitably cause
strain localization in the deeper parts of strike-slip faults
as it is often observed in the field for crustal shear-zones.
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