The tomography of the Poisson ratio structures based on the Vp/Vs ratio provides much tighter constraints on the crust and uppermost mantle than Vp or Vs alone. In this study, high-resolution crustal tomography beneath tectono-magmatic lake regions (Lakes Van and Ercek) of Eastern Anatolia (Turkey) was used to estimate Poisson's ratio from the Vp and Vs structures through the joint inversion analyses of the travel times of Vp and Vs using the station records of the permanent network of Kandilli Observatory and Earthquake Research Institute (KOERI, Turkey). The results of Poisson's ratio anomaly reveal distinct and substantial variations in the different tectonic units and show a prominent, continuous and dense high Poisson's ratio anomalies in the range of 0.27 to 0.30 for the lowermost crust/the uppermost mantle beneath the regions of Lakes Van and Ergek and comparably low average Poisson's ratio (similar to 0.23) anomalies for the brittle crust in several local areas. High Poisson's ratios (over similar to 0.29) in the basins of Lakes Van and Ercek and in the vicinity of their boundary faults appear to be due to partial melts. Low Poisson's ratios (similar to 0.23) near seismogenetic faults at similar to 14 km depth beneath the lake regions suggest weaker and more easily deformable crust. The high and low Poisson's ratio structures suggest a strongly fault-controlled and highly heterogeneous crustal architecture beneath the lake regions. High Poisson's ratios at variable depths suggest that the mafic content beneath the lake regions is higher than has ever been expected, thus indicating the existence of intrusive materials of possible mantle origin, interacting with faulted crustal rocks. The regional extent of the high Poisson's ratios (0.27-0.30) of the lowermost crust/the uppermost mantle proposes strong interaction of partial melts with active tectonics in the basins of Lakes Van and Ercek. Key constraints from the estimated Poisson's ratios and V-anomalies indicate the diverse and interactive nature of crustal deformations beneath the lake regions where the underplating of the mantle magma scenario is proposed to exert a key control on tectono-magmatic evolution of the regions of Lakes Van and Ergek and serves as a working hypothesis for further studies.