Transition from compression to strike-slip tectonics revealed by miocene-pleistocene volcanism west of the karliova triple junction (east anatolia)

Abstract

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 Bingöl provinces). Here the Eurasia-Arabia convergence resulted in collision, marked by the Bitlis-Zagros suture (~13 Ma), followed by activation of the dextral transform North Anatolian Fault (NAF). At ~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 40Ar-39Ar 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 Karakoçan (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 87Sr/86Sr (0.7052-0.7065) and low 143Nd/144Nd isotopic ratios (0.51246-0.51262). The late Miocene basalts display variable geochemical characteristics, including large variations in 87Sr/86Sr (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 87Sr/86Sr (0.7033- 0.7038) and high 143Nd/144Nd (0.51270-0.51290), with compositions similar to those of oceanic intraplate magmas. Pb isotopes vary slightly: 206Pb/204Pb ranges from 18.66 to 19.11, 207Pb/204Pb from 15.64 to 15.72 and 208Pb/204Pb from 38.67 to 39.24, with the calc-alkaline early-middle Miocene rocks characterized by higher 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb. 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.