CONVECTIVE REMOVAL OF LITHOSPHERE BENEATH MOUNTAIN BELTS - THERMAL AND MECHANICAL CONSEQUENCES

Shortening and thickening of continental lithosphere cause an increase in surface elevation, a decrease in the thermal gradient and, unless the crust is initially quite thick, a decrease in potential energy wit h respect to a mid-ocean-ridge lithospheric column. Continental conver gence could therefore be self-sustaining. The lower part of the lithos phere (whether continental or oceanic) is probably removed intermitten tly by convection, however, and this process may be triggered by thick ening, though delayed a few million to a few tens of million years aft er the start of thickening. Convective removal of the lithospheric roo t below regions of continental convergence will cause a rapid increase in surface elevation and potential energy and create a step in the ge otherm. The large excess in potential energy with respect to its surro undings may result in indefinite extension of the continental column. In addition to extension, convective removal of the lowermost lithosph ere may lead to characteristic igneous and metamorphic events. The ste p in the geotherm at the base of the convectively thinned lithosphere will cause partial melting if the lithospheric mantle has been previou sly enriched by metasomatic processes. This melting could occur before any appreciable extension of the crust took place. Extension of litho sphere immediately following convective removal of the root will decre ase the thermal time constant for decay of the step in the geotherm, a llowing a transient heating event to affect the crust. The increase in temperature will be synchronous with extension and decompression in u pper mantle and lower crustal rocks, but if extension is rapid, it ma reach the upper crust after extension has ceased and be detectable the re as a post-tectonic low P/T ratio metamorphic event. Extension may b e accompanied by decompression-related magmatism.