Interaction of the Cyprus/Tethys slab with the mantle transition zone beneath Anatolia

The interaction of subducted oceanic lithosphere with the discontinuities of the mantle transition zone (MTZ) provides insight into the composition and temperature of the subducted slab as well as potential melting of the slab or the surrounding mantle and loss of volatiles from the slab. Detailed mapping of the structure of the MTZ will help to better understand how slabs transport material and volatiles into the mantle and how phase transitions affect the slab dynamics. Here we use a dense network of seismic stations in northern Anatolia to image the structure of the MTZ discontinuities in detail using P-wave receiver functions. With a station spacing of about 7 km and a surface footprint of ~35 km by ~70 km, analysing receiver functions calculated from teleseismic earthquakes that occurred during an ~18 month deployment produced clear images of where the mantle transition zone interacts with the Tethys/Cyprus slabs that either lie flat on the 660-km discontinuity or pass into the lower mantle. We observe an undulating 660-km discontinuity depressed by up to 30 km and a slightly depressed (1 – 2 km) 410-km discontinuity, apparently undisturbed by the slab. The MTZ is thickened to ~270 km as result of the cool slab in the MTZ influencing the 660-km discontinuity and includes an arrival at ~520-km depth likely from the top of a flat lying slab or a discontinuity related to a solid-solid phase transition in the olivine component of the mantle. We find evidence for low-velocity zones both above and below the 410- km discontinuity and above the 660-km discontinuity. The low velocity zones around the 410-km discontinuity might be the result of hydration of the MTZ from the slab and upward convection of MTZ material into the upper mantle. The origin of the low velocity zone around the 660-km discontinuity is less clear and could be related to sedimentation of subducted mid-ocean ridge basalts. The small footprint of the seismic array provides accurate information on the structure of the MTZ in an area influenced by subduction and shows small-scale changes in MTZ structure that might be lost in studies covering larger areas with sparser sampling.