Thermal Shielding by the Contrast in Thermal Conductivity: Implications for Heat Flow Paradox

Major faults on the Earth should have a heat source generated by a long-term frictional slip. However, geophysical surveys have reported that surface heat flow anomalies along the faults are unpredictably low (i.e., heat flow paradox). To resolve the paradox, many authors have suggested various hypotheses related with weakening mechanisms of mechanical strength such as a thermal decomposition. In our work, we focus on the contrast in thermal conductivity to explain the paradox. Once viscous dissipation or fictional heating initiate, then there is a localized zone with high temperature. Since the conductivity depends on temperature (i.e.,k(T)∝(1/T)^b,0<b≤1) in shallow crust, the conductivity contrast is induced. This contrast suppresses heat diffusion from the warm shear zone to the outside. This shielding effect influences the system by lowering surface heat flow anomalies. Comparing with the case of the uniform conductivity, there is ~30 % reduction of the surface anomaly when there is about a ~50% contrast in the conductivity in the variable conductivity case. Moreover, the conductivity drop owing to grain-size reduction through dynamical formation of fault gouge can also cause the contrast in the conductivity in the crust. This dynamically induced phenomenon is analogous to the conductivity drop in temperature-dependent thermal conductivity. In these models, we have not encountered any temperature rising above the melting point. Our results would argue that the thermal shielding represents a distinctly new approach for resolving the heat flow paradox, posed more than 40 years ago.