CENOZOIC MAGMATISM AND EXTENSIONAL DEFORMATION IN WESTERN ANATOLIA AS A RESULT OF MANTLE RESPONSE TO COLLISION, SLAB BREAK-OFF, AND LITHOSPHERIC TEARING

The Cenozoic geology of western Anatolia is a great example of how the mantle responds to collision-driven thickening, slab break-off, delamination and lithospheric tearing in the latest stages of orogenic belt evolution. The collision of the Sakarya and Tauride blocks in the late Paleocene was followed by Eocene magmatism along a curvilinear belt that produced granitoid plutons and mafic to felsic volcanic suites. The next magmatic phase involved the formation of volcanic and plutonic rocks with medium- to high-K calc-alkaline compositions in the Oligo–Miocene. Asthenospheric heat flow, driven by collision-induced slab breakoff resulted in partial melting of the previously subduction-metasomatized mantle lithosphere beneath the suture zone, producing these Eocene and Oligo-Miocene igneous suites. Magmatism in the middle Miocene developed mildly alkaline, bimodal volcanic rocks, which display decreasing effects of crustal contamination and subduction influence in time. Melt sources of these rocks included both subduction-modified lithospheric mantle and asthenospheric mantle. Partial delamination of the lithospheric root beneath the western Anatolian orogenic belt was responsible for the asthenospheric upwelling, which in turn thermally weakened the orogenic crust causing region-wide extension, lower crustal exhumation and core complex development. Late Miocene–Quaternary alkaline to super-alkaline volcanic rocks with OIB-like geochemical features farther south in western Anatolia represent the last major phase of magmatism; they show progressively more potassic compositions towards south with time. Decompression melting of the asthenospheric mantle, which flowed in beneath the attenuated continental lithosphere in the Aegean extensional province, was the main source of their magmas with little or no subduction component. Major extensional fault systems and slab tears within the subducting African lithosphere acted as natural conduits for the transport of uncontaminated alkaline magmas to the surface. This time-progressive, spatial evolution of Cenozoic magmatism and extensional deformation in western Anatolia provides a realistic template for the mode and nature of post-collisional magmatism and crustal extension in many Tethyan–type orogenic belts.