TECTONIC EVOLUTION OF THE IZMIR ANKARA SUTURE ZONE IN NORTHWEST TURKEY USING ZIRCON U-PB GEOCHRONOLOGY AND ZIRCON LU-HF ISOTOPIC TRACERS

Detrital zircons from a Cretaceous forearc basin and Tertiary foreland basin located along the Sakarya Zone of the Western Pontides were analyzed to better understand the closure history of the Tethyan oceans. The Variscan Orogeny is characterized by abundant 350-300 Ma U-Pb ages and vertical εHf arrays, consistent with a mature magmatic arc that emplaced plutons through a southward growing accretionary margin. The Cimmerian Orogeny is characterized by a 250-230 Ma εHf pull-down, followed by a 230-200 Ma magmatic gap consistent with underthrusting of the Karakaya Complex. From 200-120 Ma another magmatic lull is observed. The Alpine Orogeny is characterized by an εHf pull-down from 120-85 Ma within Cretaceous forearc sediments and a 100 Ma deviant εHf vertical array within Tertiary foreland basin sediments. Minor zircon U-Pb age peaks and contrasting inter-basinal εHf evolution are interpreted to represent onset of Andean-style subduction along the southern margin of the Sakarya Zone at 120 Ma followed by crustal thickening until 85 Ma. The deviant 100 Ma εHf vertical array within foreland basin detritus is interpreted as initiation of intra-oceanic subduction within the Izmir Ankara Ocean. An 85-75 Ma εHf pull-up from forearc basin sediments is interpreted as slab roll-back along the southern margin of the Sakarya Zone, responsible for final rifting of the Western Black Sea. At ~80 Ma, a vertical εHf array from Tertiary foreland basin deposits is interpreted to represent synchronous melting of the Tavsanli Zone and intra-oceanic slab break-off. A single 66 Myr pre-collisional grain defines a sharp εHf pull-down immediately prior to total arc shut-off, interpreted to represent incipient collision between the Sakarya and Tavsanli zones. A ~52 Ma syn-collisional tuff yields minimally intermediate εHf values followed by a slight ~48 Ma εHf pull-down, interpreted as a second episode of slab break-off followed by crustal thickening, a result of renewed underthrusting.