Extended Data Fig. 8: Thermal-kinematic calculations of a lithospheric column undergoing depth-dependent extension.

Each panel shows the lithospheric geotherm (red line) and PTt paths (black markers and lines) for rocks at initial depths of 25, 30, 35 and 40 km, undergoing different combinations of mantle (δ) and crustal thinning (β). The duration of each model calculation is 30 Myr and geotherms and marker nodes are plotted at 5 Myr increments. White markers are PT estimates from a global compilation of exhumed granulites⁷⁴. All calculations assume an initial lithospheric thickness of 125 km, a crustal thickness of 40 km, a mantle potential temperature of 1330 °C, an exponential distribution of heat production throughout the crust with a surface value of 3 μW.m^-3 and an e-folding length of 8 km, crustal and mantle densities of 2.9 and 3.3 kg.m^-3 respectively, and crustal and mantle and thermal diffusivities of 10^-6 and 8x10^-7 m².s^-1, respectively. Calculations do not take into account the effects of latent heat or the temperature-dependence of thermal conductivity. These models illustrate that replacement of the lowermost lithosphere by hotter, less dense asthenospheric mantle drives conductive heating of the remaining, overlying lithosphere; the timescale of metamorphism associated with this heating is controlled by the thickness of, and rate of extension within, the overlying lithosphere. Provided that the rate of crustal thinning is less than the rate of heat conduction through the crust, peak metamorphic T may occur during decompression. Many exhumed granulite terranes record a high-T clockwise-sense PT segment, and the tectonic sequence represented by these models (crustal thickening, lithospheric removal, and extension) is recognized in a number of modern orogens: (1) Tibet⁷⁵, (2) Sierra Nevada⁷⁶, (3) Anatolian plateau⁷⁷, (4) south-central Appalachians⁷⁸, (5) Alboran Sea and Betic-Rif mountains⁷⁹.