Estimation Of Effective Elastic Thickness And Mechanical Anisotropy Of The Lithosphere, And Its Tectonic Implications

The lithospheric strength is a key parameter to link stress in the plate boundaries and intraplate deformation, and to evaluate the correlation between the upper mantle convection and tectonic activities at shallow levels. Effective elastic thickness of the lithosphere (Te), gained from coherence analysis of gravity anomaly and topography data, is a good proxy for the long-term (105year) lithospheric strength. Load-deconvolution Bouguer coherence (LDC) method has been widely used to estimate Te in continental regions. Unfortunately, the Te values are strongly biased in the flat topography area subjected to significant erosion or sedimentation, which is the "noise" in the Te estimation. Based on forward modeling, we found that all internal loads could be affected by the surficial erosion or sedimentation and lose some topography expression, which would lead to Te overestimation using the LDC method To resolve this problem, we developed a new coherence forward modeling (CFM) method that is not based on bad-deconvolution. The CFM method can recover reliable Te using synthetic and real data with noise.We used the CFM method to calculate Te and its anisotropy in the Eurasian area. The values of Te vary from2km to>100km, and show a strongly bimodal distribution. In the stable cratons, such as the East European craton, the Arabian craton, the Siberian craton, and the Indian craton, Te is>80km and larger than the crustal depth, suggesting a strong lithospheric mantle beneath these cratons. In active plateaus and mountain belts, Te is<25km and smaller than the crustal depth, implying a weak lithospheric mantle. In basins and small cratons, Te is around30-50km, which is similar with the crustal depth. The positive correlation between Te and S-wave velocity at a depth of100km demonstrates the controlling influence of temperature on the lithospheric strength. Comparison between Te and intraplate earthquake distribution indicates strong seismicity in the low Te regions, suggesting that strain concentration is prone to occur in the weak part of the lithosphere. Te anisotropy reflects mechanical anisotropy of the lithosphere, which is closely related to the tectonic activity.