In this study, we present a recently developed methodology, based on Finite Element Approach, which does not require a priori assumptions to resolve the regional and residual gravity components. We take the North China Craton (NCC) as a case-study, where previous geological and geophysical studies have demonstrated extensive lithospheric thinning and craton destruction. The residual gravity anomaly map that we computed based on this novel method clearly demarcates the geotectonic boundaries and compare with those from other methods. We identify three prominent gravity high zones located over the Central Asian Orogenic Belt and the Inner Mongolia Suture Zone (Khondalite Belt) in the west, and over the Jizhong depression in the east. The nearly circular anomaly obtained over the Jizhong depression covers an area of ~200,000 km2, which may represent extensive magmatism generated by the hot asthenospheric upwelling. This anomaly cannot be explained by only subduction related melting phenomenon. The 21⁄2D residual gravity modeling across an E-W profile covering the entire North China Craton reveals shallow depth to the Moho and lithosphere-asthenosphere boundary (LAB) at 35 km and 70 km respectively below the Eastern Block, and 50 km and 140 km respectively, below the Western Block. This would suggest almost 100–150 km removal of the lithospheric keel of the NCC. Consequently, a 15 km thick layer of lower crustal and upper-mantle magma, which may be differentiated, is considered to have been emplaced below the Jizhong depression, which is well reflected by the presence of a highly positive residual gravity anomaly (+ 90 mGal) over this region. The average crustal density computed from the present gravity model is found to be relatively higher (2.87 g/cm3) in the Western Block, compared to that in the Eastern Block (2.80 g/cm3) of the NCC. Our study provides further insights into the lithospheric architecture of the North China Craton and the extensive cratonic destruction.
Figure: Cartoon showing the relationship between extensive lithospheric destruction and crust-mantle magma underplating caused by both tectonic erosion and mantle upwelling
A. Vasanthi, M. Santosh (2021) Earth-Science Reviews, Volume 215, 103548, ISSN 0012-8252, https://doi.org/10.1016/j.earscirev.2021.103548.