SUBDUCTING GONDWANAN-AFFINITY PASSIVE MARGIN TRIGGERED LATE CRETACEOUS BACK-ARC EXTENSION OF THE BLACK SEA

Back-arc extension often preceded intercontinental collision across much of the Alpine-Himalaya orogenic system. However, the number and timing of subduction zones active prior to intercontinental collision and their role in back-arc extension is unclear. We address this gap in knowledge by investigating greater Anatolia’s late Cretaceous pre-collisional tectonic evolution to determine the sequence of geodynamic events that occurred prior-to and during extension of the archetypal Black Sea back-arc basin. We analyzed Hf isotopes from Jurassic and late Cretaceous detrital zircons deposited within the late Cretaceous Central Sakarya forearc basin, located north of Turkeys principal Tethyan suture zone. Subduction of the Gondwanan-affinity passive margin Tavşanlı Zone occurred from 95-80 Ma. We demonstrate that subduction of the Tavşanlı Zone resulted in a significant decrease in detrital zircon age-abundances and εHf(t) values, consistent with a decrease in magmatism and partial melting of passive margin siliciclastic rocks. Back-arc extension of the Black Sea occurred from 85-75 Ma, coeval with the latter stages of high-pressure low-temperature metamorphism of the subducted passive margin. We explain the decrease in detrital zircon age-abundances and negative-shift in εHf(t) values as constriction of the mantle wedge and reduced melt-fertility as the Tavşanlı Zone ‘clogged’ a north-dipping subduction zone. We propose that episodes of back-arc extension observed across much of the Alpine-Himalaya orogenic system may have been triggered by clogging north-dipping Neotethyan subduction zones with buoyant Gondwanan-affinity passive margin.