The deformation of Earth's lithosphere in orogenic belts is largely forced externally by the sinking slab, but can also be driven by internal delamination processes caused by mechanical instabilities. Here we present an integrated analysis of geophysical and geological data to show how these processes can act contemporaneously and in close proximity to each other, along a lithosphere scale discontinuity that defines the lateral boundary between the Hellenide and Anatolide segments of the Tethyan orogen in western Turkey. The Hellenides and Anatolides have experienced similar rates of convergence, but display remarkable differences in the structure of Earth's crust and lithospheric mantle across the Aegean coast of the Anatolian peninsula. We review the tectonics of southwest Turkey in the light of new and published data on crustal structure, cooling history, topography evolution, gravity, Moho topography, earthquake distribution and seismic tomography. Geological data constrain that one of Earth's largest metamorphic core complexes, the Menderes Massif, experienced early Miocene tectonic denudation and surface uplift in the footwall of a north-directed extensional detachment system, followed by late Miocene to recent fragmentation by E-W and NW-SE trending graben systems. Gravity data, earthquake locations and seismic velocity anomalies highlight a north-south oriented boundary in the upper mantle between a fast slab below the Aegean and a slow asthenospheric region below western Turkey. Based on the interpretation of geological and geophysical data we propose that the tectonic denudation of the Menderes Massif and the delamination of its subcontinental lithospheric mantle reflect the late Oligocene/early Miocene onset of transtension along a lithosphere scale shear zone, the West Anatolia Transfer Zone (WATZ). We argue that the WATZ localised along the boundary of the Adriatic and Anatolian lithospheric domains in the Miocene, when southward rollback of the Aegean slab started to affect the central Aegean-Menderes portion of the Tethyan orogen. Transtension across the West Anatolia Transfer Zone affected the entire Menderes Massif in the Early Miocene. The current crustal expression of this boundary is a NNE-trending, distributed brittle deformation zone that localised at the western margin of the denuded massif. Here, sinistral transtension accommodates the continuing velocity difference between relatively slow removal of lithospheric mantle below western Anatolia and trench retreat in the rapidly extending Aegean Sea region. Our review highlights the significance of lateral variations of the lower plate in subduction-collision systems for evolving structure and surface processes in orogenic belts, particularly in relation to the formation of continental plateaux and metamorphic core complexes.