3-D electrical structure of the lithosphere beneath the Yanshan Belt and its adjacent areas, North China

Abstract

The North China Craton is one of the oldest continental nuclei in the world. However, extreme lithospheric modification has occurred since the Mesozoic, of which the most typical area is the Yanshan orogenic belt. In this research, a 3-D lithospheric electrical structure beneath the Yanshan belt and adjacent areas was obtained using magnetotelluric array data, which reveals the regional heterogeneity. Notably, the crust and upper mantle (< 100 km) within the Yanshan belt, especially in its southern part, are characterized by high resistivity (> 1000 Ωm). These features are interpreted to be a relic cratonic lithosphere mainly composed of highly depleted olivine. A lithosphere-scale relatively high-conductivity zone (~ 100 Ωm, near EW-trending), which roughly lies beneath the Miyun-Xifengkou-Qinglong fault coincides with the boundary of Archean greenstone belt and also the Mesozoic granitic zone, probably indicating a reactivated Precambrian lithospheric weak/shear zone within Yanshan belt. In contrast, the lower crust and upper mantle of the Bohai Bay basin show large areas of high conductivity, which may result from the modification of the crust and mantle by the upwelled asthenospheric materials. Additionally, the epicenters of several historic destructive earthquakes, including the 1976 Tangshan earthquake (M7.8), are mainly located near the high- and low-resistivity boundary at the conjunction of the Yanshan belt and the Bohai Bay basin. The fragmentation of brittle crustal rocks mainly happened in where the tectonic stresses concentrated, which might result from the rheological difference between the high-resistivity ancient and the high-conductivity modified lithosphere. In these places, intense seismicity would be induced.