CRUST AND UPPER MANTLE DYNAMICS OF TURKEY INFERRED FROM PASSIVE SEISMOLOGY: IMPLICATIONS OF SEGMENTED SLAB GEOMETRY

Turkey lies within the Alpine–Himalayan orogenic belt and is shaped by continent-continent collision in the east, slab rollback and back arc extension in the west and westward extrusion of Anatolian plate in between. In this study, passive seismic data are used to analyze the crust and upper mantle structure of the region to test different tectonic models proposed for Turkey and its surrounding.

In Eastern Turkey, continent-continent collision resulted in a topographic high, the East Anatolian Plateau which exhibits a widespread Neogene volcanism. The plateau is characterized by lack of subcrustal earthquakes and predominantly strike-slip crustal seismicity suggesting that the collision is accommodated by tectonic escape. Receiver functions indicate thin crust and lithosphere across the plateau that is inconsistent with a simple Airy compensational mechanism and an anomalously high Vp/Vs corridor along the North Anatolian Fault and near the youngest volcanic units supporting the presence of partial melt in the crust. Tomographic images of upper mantle reveal anomalously slow upper mantle velocities where slab-like fast-velocity anomalies only visible at or below mantle transition zone and support the slab detachment and emplacement of the hot asthenosphere. The western limit of this slab window located most likely along the palaeotectonic Inner-Tauride Suture in central Turkey where sharp mantle transitions are visible.

Across Anatolian plate and Aegean Sea, the most prominent upper mantle structures are related to the northward subducting African oceanic lithosphere. New tomographic results clearly indicate segmented slab geometry beneath Anatolia and these segments define Cyprus and Aegean slabs separated by a left-lateral tear that is occupied by slow upper mantle velocities. The Aegean slab which defines a roughly northward-concave configuration has its eastern termination at the north part of the Pliny and Strabo transforms. The Cyprus slab terminates at the tip of the Isparta Angle in the west and near the gulf of Iskenderun in the east. The lateral continuity of the Cyprus slab is also disrupted by a minor right lateral tear at shallow depths aligning with the Paphos transform fault west of the Cyprus Island. Low seismic activity along the Cyprus trench and the sub-vertical down-dip segmented slab geometry suggest a stagnant Cyprus slab that might have started to detach. We believe presence of continuous cold Cyprus slab underneath the Central Anatolia has a shielding effect on the region and provides important contribution to the relatively undeformed crustal characteristics of this province.

Tomographic images show that the lateral extent of the tear spans nearly the entire width of the Western Anatolia and the trends of the upwelling hot asthenosphere are located directly below late Miocene to Quaternary alkaline Kirka-Afyon-Isparta and Kula volcanic fields. Based on the resolved configuration of slab segments, we favor a STEP model for the segmentation and hence tear formation between the Aegean and the Cyprus slabs. The generation of this STEP geometry is probably related with differential retreat rates between the Cyprus and the Aegean trenches that initiated when a locally thicker continental fragment of African margin, similar to the Eratosthenes Seamount further west, entered in to the Cyprus trench and obstructed the subduction beneath the Isparta Angle.