The Anatolia (Eurasia), Arabia, and Africa tec­tonic plates intersect in southeast Turkey, near the Gulf of İskenderun, forming a tectonically active and unstable triple junction (the A³ triple junction). The plate boundaries are marked by broad zones of major, dominantly left-lateral transform faults including the East Anatolian fault zone (the Anato­lia-Arabia boundary) and the Dead Sea fault zone (the Arabia-Africa boundary). Quaternary basalts occur locally within these “leaky” transform fault zones (similar to those observed within oceanic transform faults), providing evidence that mantle melting, basalt genesis, and eruption are linked to crustal deformation and faulting that extends into the upper mantle. We investigated samples of alkaline basalt (including basanite) from the Toprakkale and Karasu volcanic fields within a broad zone of transtension associated with these plate-boundary faults near the İskenderun and Amik Basins, respectively.

Toprakkale basalts and basanites have ⁴⁰Ar/³⁹Ar plateau ages ranging from 810 ± 60 ka to 46 ± 13 ka, and Karasu volcanic field basalts have ⁴⁰Ar/³⁹Ar plateau ages ranging from 2.63 ± 0.17 Ma to 52 ± 16 ka. Two basanite samples within the Toprak­kale volcanic field have isotopic characteristics of a depleted mantle source, with ⁸⁷Sr/⁸⁶Sr of 0.703070 and 0.703136, ¹⁴³Nd/¹⁴⁴Nd of 0.512931 and 0.512893, ¹⁷⁶Hf/¹⁷⁷Hf of 0.283019 and 0.282995, ²⁰⁶Pb/²⁰⁴Pb of 19.087 and 19.155, and ²⁰⁸Pb/²⁰⁴Pb of 38.861 and 38.915. The ¹⁷⁶Hf/¹⁷⁷Hf ratios of Toprakkale basalts (0.282966–0.283019) are more radiogenic than Karasu basalts (0.282837–0.282965), with some overlap in ¹⁴³Nd/¹⁴⁴Nd ratios (0.512781–0.512866 vs. 0.512648–0.512806). Toprakkale ²⁰⁶Pb/²⁰⁴Pb ratios (19.025 ± 0.001) exhibit less variation than that observed for Karasu basalts (18.800–19.324), and ²⁰⁸Pb/²⁰⁴Pb values for Toprakkale basalts (38.978– 39.103) are slightly lower than values for Karasu basalts (39.100–39.219). Melting depths estimated for the basalts from both volcanic fields gener­ally cluster between 60 and 70 km, whereas the basanites record melting depths of ~90 km. Depth estimates for the basalts largely correspond to the base of a thin lithosphere (~60 km) observed by seismic imaging. We interpret the combined radio­genic isotope data (Sr, Nd, Hf, Pb) from all alkaline basalts to reflect partial melting at the base of the lithospheric mantle. In contrast, seismic imaging indicates a much thicker (>100 km) lithosphere beneath southern Anatolia, a substantial part of which is likely subducted African lithosphere. This thicker lithosphere is adjacent to the surface loca­tions of the basanites. Thus, the greater melting depths inferred for the basanites may include par­tial melt contributions either from the lithospheric mantle of the attached and subducting African (Cyprean) slab, or from partial melting of detached blocks that foundered due to convective removal of the Anatolian lithosphere and that subsequently melted at ~90 km depth within the asthenosphere.

The Quaternary basalts studied here are restricted to a broad zone of transtension formed in response to the development of the A³ triple junction, with an earliest erupted age of 2.63 Ma. This indicates that the triple junction was well established by this time. While the current posi­tion of the A³ triple junction is near the Amik Basin, faults and topographic expressions indicate that inception of the triple junction began as early as 5 Ma in a position farther to the northeast of the erupted basalts. Therefore, the position of the A³ triple junction appears to have migrated to the southwest since the beginning of the Pliocene as the Anatolia-Africa plate boundary has adjusted to extrusion (tectonic escape) of the Anatolia plate. Establishment of the triple junction over the past 5 m.y. was synchronous with rollback of the Afri­can slab beneath Anatolia and associated trench retreat, consistent with Pliocene uplift in Cyprus and with the current positions of plate boundaries. The A³ triple junction is considered to be unstable and likely to continue migrating to the southwest for the foreseeable geologic future.