Variations of the effective elastic thickness of the lithosphere suggest a broad diffusive boundary between the North American and Eurasian plates in Siberia

Northeastern Eurasia is one of the least studied regions in the world, with limited geophysical data available due to its inaccessibility. The exact location of the plate boundary between Eurasia and North America is still under debate. The effective elastic thickness (EET) of the lithosphere serves as an indicator of lithospheric strength and provides valuable information on thermal conditions and tectonic activity. We have produced a high-resolution EET map for northeastern Eurasia using the fan wavelet coherence method applied to Bouguer gravity anomalies and topography/bathymetry data, with appropriate adjustments for density variations within sediments. The resulting EET variations provide valuable insights into the different tectonic regimes of this largely unexplored region. In particular, we identify the boundary between the Eurasian and North American plates in Siberia as a rheologically weak, diffusive zone extending from the Verkhoyansk and Sette-Daban Ranges to the eastern edge of the Chersky Range. In contrast to the Sette-Daban and Verkhoyansk Ranges, which were formed by plate collisions and have an EET of 30-50 km, other mountainous regions have much lower EET values, often less than 15 km, indicating recent tectonic activity that has weakened the lithosphere. This is fully consistent with the distribution of earthquakes and focal mechanisms. The majority of earthquakes are concentrated on the western and eastern boundaries of this zone (the eastern slope of the Verkhoyansk Range on one side and the eastern slope of the Chersky Range on the other). In the center of the diffuse zone only weak earthquakes of small depth occur.