Shear heating in continental strike-slip shear zones: model and field examples

A two-layer (crust and upper mantle), finite difference steady-state thermomechanical model of a long-lived (several million years) lithospheric strike-slip fault is presented, and its predictions compared with 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 thermomechanical 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(-1), stiff lithosphere), shear heating could lead to temperature increases close to 590 degrees C at the Moho, and 475 degrees 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. Besides 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(-1), while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr(-1). As a result of shear heating, partial melting of the upper mantle could occur in the presence of small amounts 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 is often observed in the field for crustal shear zones.