We explore the sensitivity of extensional systems to the thermal structure of the lithosphere using numerical simulations that fully couple the energy, momentum and continuity equations. The theology of the lithosphere is controlled by weakening processes, such as shear heating, that localises strain into shear zones and faults. Numerical models show that during extension of an initially unpatterned lithosphere, structures develop spontaneously out of basic thermodynamic energy fluxes, and without the imposition of ad hoc rules on strain localisation. This contrasts with the classical Mohr-Coulomb theory for brittle localisation, which prescribes the angles of faults by a mathematical rule. Our results show that the mode of extension is sensitive to subtle changes in theology, heat flux and geometry of the system. This sensitivity lies at the core of the variety and complexity observed in extensional systems. Localisation processes make the lithosphere weaker than previously estimated from the Brace-Goetze quasi-static approach. Consequently, typical estimates for plate tectonic forces are capable of splitting the lithosphere under extension, even without the role of active magmatism. (C) 2008 Elsevier B.V. All rights reserved.