PARTIAL MELTING OF THE MID-TO LOWER CRUST AND STRUCTURES IN MIGMATITES: INSIGHTS INTO TYPE OF FLOW DURING LATE-OROGENIC EXTENSIONAL COLLAPSE (E. RHODOPE, BULGARIA) N. BONEV Department of Geology and Paleontology, Sofia University "St. Kliment Ohridski", 15 Tzar Osvoboditel Bd, 1504 Sofia, Bulgaria Abstract: High-grade gneisses and migmatites are exposed in the core of the Kesebir dome in Eastern Rhodope. Structural analysis of migmatites indicate that partial melting in the mid-to lower crust was coeval with NE-directed shearing in the core gneisses and upper crustal ductile-brittle extension. Geometry and kinematics of shear zones and structures in migmatites define partitioned internal deformation in the footwall of the detachment between coaxial strain in the core and localized non-coaxial shear in detachment zone. Keywords: partial melting, migmatites, shear zones, late-orogenic collapse, Rhodope Introduction The formation of high-temperature/low-pressure migmatite-granite terranes have been attributed to the processes of crustal anatexis and migmatite production in the late stages in evolution of orogenic belts. Petrological studies have shown that the normal metamorphic consequence of crustal thickening is an increase of temperature after the maximum pressures were attained (England & Thompson, 1984), that caused thermal relaxation of overthickened crust. Thermal equilibration and subsequent partial melting of the middle to lower part of crust have been suggested as an efficient driving mechanism, operating in renewed syn-to post-thickening crustal extension in the collapse stage (Dewey, 1988). The Kesebir dome in Eastern Rhodope displays structural section, which provide an unique opportunity to direct examination of strain pattern in metamorphic domes associated with its emplacement from deep to shallow structural levels beneath low-