Transition from Compression to Strike-slip Tectonics Revealed by Miocene-Pleistocene Volcanism West of the Karliova Triple Junction (East Anatolia)

We report the results of a study on early Miocene to Pleistocene volcanic rocks cropping out west of the Karliova Triple Junction in Eastern Anatolia (Elazig, Tunceli, and Bingol provinces). Here the Eurasia-Arabia convergence resulted in collision, marked by the Bitlis-Zagros suture (similar to 13 Ma), followed by activation of the dextral transform North Anatolian Fault (NAF). At similar to 6 Ma the formation of the sinistral transform East Anatolian Fault (EAF) marked the separation of the Anatolian block, which became a kinematically independent plate. On the basis of petrographic, geochemical and Sr-Nd-Pb isotopic characteristics, as well as new Ar-40-Ar-39 age determinations, we distinguish three phases of activity: (1) early-middle Miocene (16.3-15.5 Ma) production of calc-alkaline basaltic trachyandesite to dacites in the Pertek and Mazgirt districts; (2) emplacement of late Miocene (11.4-11.0 Ma) transitional basalts in the Tunceli area; (3) emplacement of Plio-Pleistocene Na-alkali basalts in Karakocan (4.1Ma) and Elazig (1.7 Ma). The oldest samples are characterized by large ion lithophile element (LILE) enrichment (e.g. Ba/Nb = 32-76) with high Sr-87/Sr-86 (0.7052-0.7065) and low Nd-143/Nd-144 isotopic ratios (0.51246-0.51262). The late Miocene basalts display variable geochemical characteristics, including large variations in Sr-87/(86) Sr (0.7039-0.7068) and LILE/HFSE (high field strength element) ratios (e.g. Ba/Nb = 13-36). The Plio-Pleistocene alkali basalts have higher LILE and HFSE contents and lower LILE/HFSE ratios (Ba/Nb = 8-21) compared with the two previous groups, low Sr-87/Sr-86 (0.70330 0.7038) and high Nd-143/Nd-144 (0.51270-0.51290), with compositions similar to those of oceanic intraplate magmas. Pb isotopes vary slightly: Pb-206/Pb-204 ranges from 18.66 to 19.11, Pb-207/Pb-204 from 15.64 to 15.72 and Pb-208/Pb-204 from 38.67 to 39.24, with the calc-alkaline early-middle Miocene rocks characterized by higher Pb-207/Pb-204 and Pb-208/Pb-204 at a given Pb-206/Pb-204. The evolution of volcanic activity is strictly linked to the geodynamic scenario. The early-middle Miocene magmas, emplaced in a convergent setting, indicate derivation from mantle sources modified by subduction components, whereas the late Miocene Tunceli transitional basalts mark the change from compressional to strikeslip tectonics. During the development of the NAF and EAF, passive upwelling of the sub-slab mantle, favoured by the formation of small pull-apart basins, led to the onset of Na-alkali basaltic activity.