Open University, United Kingdom

Large-scale crustal thinning of the Alpine orogen in the westernmost Mediterranean (Alboran Sea and surrounding regions) was rapid, simultaneous over an area of 60,000 km (super 2) , and took place approximately 25 m.y. after the main crustal thickening event. Crustal thinning, which locally exposed subcontinental mantle, was accompanied by high-grade metamorphism and partial melting. Both U-Pb dating of zircon and thermal modeling of cooling histories indicate that the thermal peak was reached between 23 and 21 Ma over the entire region. Final exhumation and cooling followed immediately. Possible explanations for this dramatic late orogenic extension include subduction rollback, slab detachment, lithospheric delamination, and convective removal of subcontinental lithosphere. Of these processes, only convective removal of subcontinental lithosphere predicts a significant time lapse between crustal thickening and the onset of extension, the simultaneity of extension over the whole region, and heating of rock during rapid exhumation to produce the high-grade metamorphic event.