Investigation of the crustal and upper-mantle structure of the eastern Pontides orogenic belt (NE, Turkey): a receiver-function study

Abstract

In this study, we use teleseismic P and S receiver functions (i.e., S-to-P and P-to-S converted signals) and their joint inversions to determine seismic discontinuities in the crust and upper mantle. Eight permanent broadband stations from the KOERI (Kandilli Observatory and Earthquake Research Institute) that are distributed along the eastern Pontides orogenic belt (EPOB), NE, Turkey, comprise our database. Inversion is performed by using a simulated annealing technique with and without travel time residuals. Our inversion results reveal the Moho depth, a high S velocity lid, a low-velocity zone, and the underlying upper mantle layer. The studied area is divided into two regions based on the station locations: (a) a northern region and (b) a southern region. The inversion results from the northern area produce crustal models that indicate that the uppermost crust is represented by a low P and S wave velocity (Vp = ~ 5.0 km/s and Vs = ~ 2.8 km/s). These velocities are clear evidence of (1) the sedimentary and volcanic rocks that widely crop out in the region and (2) a thinner uppermost crust, whereas the velocities of the southern region (Vp = ~ 6.0 km/s and Vs = ~ 3.1 km/s) indicate a thicker uppermost crust (~ 7 km). Our calculated Vp/Vs velocity ratio in the lower crust is approximately 1.90 and 1.80 in the northern and southern regions, respectively. These ratios are generally attributed to mafic rocks. Beneath the northern stations, the crustal thicknesses are 30, 33, 37, and 40 km from east to west, while the depths of the Moho are 46, 42, 39, and 44 km beneath the southern stations. Some velocity histograms show a transition from the high S velocity mantle lid to the low-velocity zone, which is known as the lithosphere–asthenosphere boundary. A representative value of the boundary’s depth is around 83 km in the north and ~ 88 km in the south. The hypothesis of some researchers regarding the existence of southward subduction beneath the eastern Pontides orogenic belt during the Late Mesozoic–Cenozoic is supported by our 2-D and 3-D velocity-depth models.