THERMAL FEEDBACKS BETWEEN STRAIN HEATING AND MELT PRODUCTION IN OROGENIC BELTS

The thermal evolution of orogenic belts is largely controlled by the rate of heat transfer by conduction. The governing physical properties are thermal diffusivity (κ) and conductivity (k = κρC_P), where ρ denotes density and C_P denotes specific heat capacity at constant pressure. Although for crustal rocks both κ and k decrease above ambient temperature, most thermal models of the Earth's lithosphere assume constant values for κ (~1 mm²s^-2) and/or k (~3 to 5 Wm^-1K^-1). Recent advances in laser-flash analysis permit accurate measurements of κ to upper mantle temperatures. Laser-flash data for several different crustal rocks indicate that κ strongly decreases from 1.5–2.5 mm²s^-2 at ambient conditions, approaching 0.5 mm²s^-2 at mid-crustal temperatures. The latter value is approximately half that commonly assumed, and hot middle to lower crust is therefore a much more effective thermal insulator than previously thought. Above the quartz α–β phase transition, crustal κ is nearly independent of temperature, and similar to that of mantle materials. Calculated values of k indicate that it also diminishes by 50% from the surface to the quartz α–β transition.

Thermal models of lithospheric evolution during continental collision show that the temperature dependence of k and C_P leads to positive feedback between strain heating in shear zones and more efficient thermal insulation, removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. Once melting begins, rock strength decreases and the strain-heating mechanism should cease to be effective, unless melt can be extracted rapidly. Melts have lower thermal diffusivity than their crystalline counterparts, so the onset of crustal melting may therefore lead to the production of a more insulating layer than the surrounding crust. This positive feedback between melting and thermal insulation may promote increased melt fraction, and allows strain heating to play a significant role in triggering crustal anatexis even though it must become negligible once melting is achieved.