In this paper we study the behavior of thermo-viscoplastic fault materials under steady shear. It is shown that during creep and at lower temperatures, the rate- and thermally dependent friction laws most commonly used could present similar mathematical and physical behavior. Under certain conditions, a fault may creep in a stable or unstable manner, expressing this instability as localization of dissipation and abrupt temperature rise. We show that this unstable manifestation is significant enough to trigger certain endothermic chemical reactions, and by the use of numerical bifurcation methods we verify that such a reaction determines the later stages of sliding, at elevated temperatures. With this formulation a qualitative description of the quasi-static problem is attempted, with the model admitting different regimes of frictional sliding, namely steady creep, transition from aseismic to seismic slip and seismic slip alone, depending on the material parameters, the nature of the reaction and the boundary and loading conditions. © 2010 Elsevier Ltd. All rights reserved.