MOVEMENT OF THE LITHOSPHERE-ASTHENOSPHERE INTERFACE IN RESPONSE TO EROSION OF THICKENED CONTINENTAL LITHOSPHERE - A MOVING BOUNDARY APPROACH

Models of retrograde metamorphism in many orogenic belts are based on crustal thickening and erosion of continental lithosphere. The pressur e and temperature evolutions in such models are generally obtained by solving the advection diffusion equation with the lithosphere-asthenos phere boundary (LAB) either fixed at a specified depth or moving downw ards at the same rate as erosion from the surface. However, this bound ary ris defined as a solid-partial-melt boundary in many geophysical i nterpretations. In the present work, we discuss a solution of the abov e problem considering the LAB as a phase boundary that moves. This sol ution is obtained by the Fourier series approach for a general case of surface erosion and basal heat transport. The results obtained for di fferent models of crustal thickening show a significant movement of th e LAB in response to erosion from the surface. A corresponding variati on 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 near ly the same temperature profile in the crust as after including the LA B motion effect. However, the erosion of thickened crust is found to a ffect 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.