Alexey G. Petrunin

Diverse Continental Subduction Scenarios Along the Arabia‐Eurasia Collision Zone

It is generally accepted that convergence of the Arabian and African plates is a subduction-dominated process. However, the subduction scenarios are still debatable. Here we present a 3-D model of the mantle to a depth of 700 km, based on a joint interpretation of the seismic tomography, residual topography, residual mantle gravity field, and seismicity. At the northwestern edge of the collision zone, we only observe partial underthrust of the Eurasian plate under the West Greater Caucasus. We suggest that this is the initial stage of subduction polarity reversal after the break off of the plate formerly subducted northward. To the southeast, counteracting subduction zones are found beneath northwest Zagros in the south and beneath the East Greater Caucasus and Alborz in the north. This scenario is likely the result of highly buoyant and weak blocks of the Lesser Caucasus, Alborz, and northwest Zagros, which are underlain in the south by the Arabian plate and the Scythian plate and South Caspian from the opposite side. Further to the southeast, a delaminated lithospheric slab is observed under southeast Zagros, while the Arabian and Eurasian plates only partially underthrust East Zagros and Kopet Dagh, respectively. In the southern part of the collision zone under Makran, only remnants of the formerly subducted slabs are found below 200 km. Plain Language Summary Continental collision zones are a result of plate tectonics on the planet Earth, when moving lithospheric plates collide at convergent boundaries. This process causes enormous concentration of deformations and stresses leading to increased seismic activity and large earthquakes, which can destroy entire cities. On the other hand, the collision process may also lead to formation of large sedimentary basins bearing ore and hydrocarbon deposits. Therefore, understanding of the mechanisms governing this process is crucial for human habitat. It is generally accepted that convergence of the Arabian and Eurasian plates is a subduction-dominated process, when one plate penetrates under another along an inclined subduction zone. However, the subduction scenarios are still debatable. Here, we present a 3-D model of the mantle to a depth of 700 km, based on a joint interpretation of various geophysical data. For the first time, we demonstrate an image of the counter-acting subduction zones beneath Zagros in the South and beneath the East Greater Caucasus and Alborz in the North. This observation contradicts the conventional view of the northward subduction at the northern flank of the Arabia-Eurasia plate boundary. These results have important implications for the debates concerning evolution and dynamics of continental collision zones.

Reconsidering Effective Elastic Thickness Estimates by Incorporating the Effect of Sediments: A Case Study for Europe

In the present study we analyzed the influence of density heterogeneity in the sedimentary cover on estimates of the effective elastic thickness (EET) of the lithosphere based on a cross‐spectral analysis of gravity and topography data. The fan wavelet coherence technique was employed to calculate EET for most of Europe and adjoining southern mountain belts. We employed Bouguer gravity anomalies and topography corrected for the effect of density variations within sediments. Correcting for sediments considerably suppresses the effect of unexpressed subsurface loads and substantially reduces EET estimates in areas with negligible topography variations as it was demonstrated for North Europe and East European Platform. The results show a good correspondence between the EET patterns and tectonic fragmentation of Europe and better agree with independent estimates based on the strength model of the lithosphere. Therefore, considering of the effect of sediments is essential for correct determinations of EET in flat areas.

Effects of the postperovskite phase change on the observed geoid

In the lowermost mantle, seismic velocity variations beneath Pacific margins have been related to the perovskite to postperovskite (pPv) phase transition. We investigate the influence of this phase transformation on the geoid using 3-D spherical mantle circulation models based on a seismic tomography model and strong lateral viscosity variations in the lower mantle. We demonstrate that the geoid anomalies are strongly affected by the presence of pPv because of phase-dependent viscosity changes relative to the surrounding mantle. Whereas geoid heights above subduction zones are increased for high-viscosity pPv, the presence of weak pPv reduces them, thereby improving the fit to the observed geoid. An investigation using two different tomography models, different pPv density contrasts, and the presence or absence of a global thermal boundary layer and of lateral viscosity variations in the lower mantle demonstrates the various effects of weak pPv on the geoid.