Movement of the lithosphere‐asthenosphere interface in response to erosion of thickened continental lithosphere: a moving boundary approach

SUMMARY Models of retrograde metamorphism in many orogenic belts are based on crustal thickening and erosion of continental lithosphere. The pressure and temperature evolutions in such models are generally obtained by solving the advection diffusion equation with the lithosphere‐asthenosphere boundary (LAB) either fixed at a specified depth or moving downwards at the same rate as erosion from the surface. However, this boundary is defined as a solid‐partial‐melt boundary in many geophysical interpretations. In the present work, we discuss a solution of the above problem considering the LAB as a phase boundary that moves. This solution is obtained by the Fourier series approach for a general case of surface erosion and basal heat transport. The results obtained for different models of crustal thickening show a significant movement of the LAB in response to erosion from the surface. A corresponding variation in the lithospheric thickness is also a significant result of this analysis. The earlier notion of a fixed‐base lithosphere seems to be a good approximation when analysing metamorphic data, as we obtain nearly the same temperature profile in the crust as after including the LAB motion effect. However, the erosion of thickened crust is found to affect the lithospheric growth. These results indicate that metamorphic data (sampling the thermal structure of the upper lithosphere) do not preserve the signatures of such boundary motion.