Stress Field at a Transcurrent Plate Boundary in the Presence of Frictional Heat Production at Depth.

A model is proposed to study the modification of the stress field at a transcurrent plate boundary due to frictional heat production at depth. Two cases are considered a stable and a stretched lithosphere. The model is applied to those weak faults where the dynamic friction is small compared to a static one; if the deformation along the brittle portion of the fault is entirely accommodated by a series of seismic ruptures in a quasi-static state where the fault has been moving for millions of years, the long-term thermal field perturbation due to these ruptures results in only a few degrees and can be neglected. The boundary zone is considered as a viscoelastic body subject to a constant strain rate. The lower section of the boundary is assumed to slip aseismically along a vertical transcurrent fault and to completely accommodate the plate motion, while the upper section is locked. The slipping zone is divided into a semi-brittle zone, placed between the isothermal surfaces of 300°C and 450°C, and a ductile zone beneath. The frictional heat is calculated by assuming a linearly decreasing friction in the semi-brittle and a constant friction in the ductile zones. The heat modifies the temperature field, producing an upward movement of the semi-brittle and ductile fault sections. As a consequence, the thickness of the brittle fault section is reduced and friction at the base of this section is less. The stress field in the boundary zone is calculated as a function of time for different friction profiles and slip rates on the fault. Owing to heat production, a greater stress concentration is produced on the brittle fault section, while shear stress is lowered in regions occupied by the uplifted semi-brittle layer. These effects are found to be remarkable only in the case of a stable zone, with a standard unperturbed geotherm, while they are irrelevant in a stretched zone with a high geothermal gradient. In any case, the role of the semi-brittle layer appears to be more prominent in the case of boundaries with higher slip rates, due to the presence of higher stress values.